Dr. Bilal Gökce

Technical Chemistry
University of Duisburg-Essen

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  • Comparing the Activity of Complex Solid Solution Electrocatalysts Using Inflection Points of Voltammetric Activity Curves as Activity Descriptors
    Löffler, T. and Waag, F. and Gökce, B. and Ludwig, Al. and Barcikowski, S. and Schuhmann, W.
    ACS Catalysis 11 (2021)
    Complex solid solution (CSS) (often denoted as high-entropy alloy) electrocatalysts enable access to unique possibilities for tailoring active sites while overcoming ever-existing limitations in electrocatalysis by unique interactions of various elements in direct neighborhood. The challenge lies in the development of strategies, which allow for systematic design of element combination and composition optimization in the multinary composition space. This challenge is accompanied by a lack of a suitable analysis method of experimental activity measurements, which can cope with the complex surface structure of this catalyst class. In this work, we propose the advantageous use of inflection points of voltammetric activity curves as activity descriptors enabling to correlate the potential of individual surface site groups to the respective peaks in the adsorption energy distribution pattern. This concept allows to methodologically gather information about the importance of each element in a CSS with respect to activity and stability of the relevant active sites and provides the basis for a guideline for systematic composition optimization. Further, the effect of phase stability on specific surface site groups as induced by degradation of the CSS phase or oxidation can be monitored. These concepts are experimentally evaluated using Cr-Mn-Fe-Co-Ni as a model system. Nanoparticles are synthesized with systematically varied compositions by means of scalable laser ablation synthesis using a multinary target. The composition is optimized with respect to the electrocatalytic activity for the oxygen reduction reaction (ORR) by varying its Mn content via laser ablation synthesis in ethanol. Subsequently, the concept is applied using rotating disk electrodes for ORR analysis in alkaline media. © 2021 American Chemical Society. All rights reserved.
    view abstract10.1021/acscatal.0c03313
  • Discrimination of ablation, shielding, and interface layer effects on the steady-state formation of persistent bubbles under liquid flow conditions during laser synthesis of colloids
    Kalus, M.-R. and Lanyumba, R. and Barcikowski, S. and Gökce, B.
    Journal of Flow Chemistry (2021)
    Over the past decade, laser ablation in liquids (LAL) was established as an innovative nanoparticle synthesis method obeying the principles of green chemistry. While one of the main advantages of this method is the absence of stabilizers leading to nanoparticles with “clean” ligand-free surfaces, its main disadvantage is the comparably low nanoparticle production efficiency dampening the sustainability of the method and preventing the use of laser-synthesized nanoparticles in applications that require high amounts of material. In this study, the effects of productivity-dampening entities that become particularly relevant for LAL with high repetition rate lasers, i.e., persistent bubbles or colloidal nanoparticles (NPs), on the synthesis of colloidal gold nanoparticles in different solvents are studied. Especially under batch ablation conditions in highly viscous liquids with prolonged ablation times both shielding entities are closely interconnected and need to be disentangled. By performing liquid flow-assisted nanosecond laser ablation of gold in liquids with different viscosity and nanoparticle or bubble diffusivity, it is shown that a steady-state is reached after a few seconds with fixed individual contributions of bubble- and colloid-induced shielding effects. By analyzing dimensionless numbers (i.e., Axial Peclet, Reynolds, and Schmidt) it is demonstrated how these shielding effects strongly depend on the liquid’s transport properties and the flow-induced formation of an interface layer along the target surface. In highly viscous liquids, the transport of NPs and persistent bubbles within this interface layer is strongly diffusion-controlled. This diffusion-limitation not only affects the agglomeration of the NPs but also leads to high local densities of NPs and bubbles near the target surface, shielding up to 80% of the laser power. Hence, the ablation rate does not only depend on the total amount of shielding matter in the flow channel, but also on the location of the persistent bubbles and NPs. By comparing LAL in different liquids, it is demonstrated that 30 times more gas is produced per ablated amount of substance in acetone and ethylene glycol compared to ablation in water. This finding confirms that chemical effects contribute to the liquid’s decomposition and the ablation yield as well. Furthermore, it is shown that the highest ablation efficiencies and monodisperse qualities are achieved in liquids with the lowest viscosities and gas formation rates at the highest volumetric flow rates. © 2021, The Author(s).
    view abstract10.1007/s41981-021-00144-7
  • How the Physicochemical Properties of the Bulk Material Affect the Ablation Crater Profile, Mass Balance, and Bubble Dynamics During Single-Pulse, Nanosecond Laser Ablation in Water
    Kalus, M.-R. and Barcikowski, S. and Gökce, B.
    Chemistry - A European Journal 27 (2021)
    Understanding the key steps that drive the laser-based synthesis of colloids is a prerequisite for learning how to optimize the ablation process in terms of nanoparticle output and functional design of the nanomaterials. Even though many studies focus on cavitation bubble formation using single-pulse ablation conditions, the ablation efficiency and nanoparticle properties are typically investigated under prolonged ablation conditions with repetition rate lasers. Linking single-pulse and multiple-pulse ablation is difficult due to limitations induced by gas formation cross-effects, which occur on longer timescales and depend on the target materials’ oxidation-sensitivity. Therefore, this study investigates the ablation and cavitation bubble dynamics under nanosecond, single laser pulse conditions for six different bulk materials (Au, Ag, Cu, Fe, Ti, and Al). Also, the effective threshold fluences, ablation volumes, and penetration depths are quantified for these materials. The thermal and chemical properties of the corresponding bulk materials not only favor the formation of larger spot sizes but also lead to the highest molar ablation efficiencies for low melting materials such as aluminum. Furthermore, the concept of the cavitation bubble growth linked with the oxidation sensitivity of the ablated material is discussed. With this, evidence is provided that intensive chemical reactions occurring during the very early timescale of ablation are significantly enhanced by the bubble collapse. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202005087
  • Influence of sub-monolayer quantities of carbon nanoparticles on the melting and crystallization behavior of polyamide 12 powders for additive manufacturing
    Sommereyns, A. and Hupfeld, T. and Gann, S. and Wang, T. and Wu, C. and Zhuravlev, E. and Lüddecke, A. and Baumann, S. and Rudloff, J. and Lang, M. and Gökce, B. and Barcikowski, S. and Schmidt, M.
    Materials and Design 201 (2021)
    In this paper, the influence of 0.005 vol% and 0.05 vol% of carbon nanoparticles on the surface of polyamide 12 powder particles by dry coating and colloidal additivation is evaluated in great detail concerning thermal and microstructural properties. The dispersion of the nanoparticles on the polymer surface influences the flowability of the feedstock powder already during the additivation process. When analyzing the composite powders dynamically and isothermally with fast scanning and differential scanning calorimetry, carbon nanoparticles influence the crystallization behavior of the feedstock material significantly by acting as nucleation seeds, already at a few percent of a monolayer coating, while showing no effect on the fast heating process. The difference in calorimetric properties and crystallization behavior between the additivation methods of different abrasive forces is discussed. The surface-additivated carbon nanoparticles significantly increase the crystalline area by up to a threefold and the crystallization rate by up to a hundredfold. Furthermore, they change the crystal growth from a typical two- to three-dimensional growth of spherulites to a one- to two-dimensional growth of ellipsoidal impinged lamellar structures. Between 0.005 vol% and 0.05 vol% of well-dispersed carbon nanoparticles should be added to polyamide 12 to trigger an anisotropic heterogeneous nucleation while avoiding agglomerates. © 2021 The Authors
    view abstract10.1016/j.matdes.2021.109487
  • Laser powder bed fusion of polymers: Quantitative research direction indices
    Kusoglu, I.M. and Doñate‐buendía, C. and Barcikowski, S. and Gökce, B.
    Materials 14 (2021)
    Research on Laser Powder Bed Fusion (L‐PBF) of polymer powder feedstocks has raised over the last decade due to the increased utilization of the fabricated parts in aerospace, automotive, electronics, and healthcare applications. A total of 600 Science Citation Indexed articles were published on the topic of L‐PBF of polymer powder feedstocks in the last decade, being cited more than 10,000 times leading to an h‐index of 46. This study statistically evaluates the 100 most cited articles to extract reported material, process, and as‐built part properties to analyze the research trends. PA12, PEEK, and TPU are the most employed polymer powder feedstocks, while size, flowability, and thermal behavior are the standardly reported material properties. Likewise, process properties such as laser power, scanning speed, hatch spacing, powder layer thickness, volumetric energy density, and areal energy density are extracted and evaluated. In addition, material and process properties of the as‐built parts such as tensile test, flexural test, and volumetric porosity contents are analyzed. The incorporation of additives is found to be an effective route to enhance mechanical and functional properties. Carbon‐based additives are typically employed in applications where mechanical properties are essential. Carbon fibers, Ca‐phosphates, and SiO2 are the most reported additives in the evaluated SCI‐expanded articles for L‐PBF of polymer powder feedstocks. A comprehensive data matrix is extracted from the evaluated SCI‐index publications, and a principal component analysis (PCA) is performed to explore correlations between reported material, process, and as‐built parts. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14051169
  • Microstructure formation and mechanical properties of ODS steels built by laser additive manufacturing of nanoparticle coated iron-chromium powders
    Doñate-Buendia, C. and Kürnsteiner, P. and Stern, F. and Wilms, M.B. and Streubel, R. and Kusoglu, I.M. and Tenkamp, J. and Bruder, E. and Pirch, N. and Barcikowski, S. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and Gault, B. and Gökce, B.
    Acta Materialia 206 (2021)
    Oxide dispersion strengthened (ODS) steels are known for their enhanced mechanical performance at high temperatures or under radiation exposure. Their microstructure depends on the manufacturing process, from the nanoparticle addition to the base steel powder, to the processing of the nanoparticle enriched powder. The optimization and control of the processing steps still represent a challenge to establish a clear methodology for the additive manufacturing of ODS steels. Here, we evaluate the microstructure, nanoparticle evolution, and mechanical properties of ODS steels prepared by dielectrophoretic controlled adsorption of 0.08 wt% laser-synthesized yttrium oxide (Y2O3) on an iron-chromium ferritic steel powder (PM2000). The influence of the ODS steel fabrication technique is studied for two standard additive manufacturing techniques, directed energy deposition (DED) and laser powder bed fusion (LPBF). The compressive strength of the ODS steels at 600 °C is increased by 21% and 29% for the DED and LPBF samples, respectively, compared to the DED and LPBF steels manufactured without Y2O3 nanoparticle addition. The Martens hardness is enhanced by 9% for the LPBF ODS steel while no significant change is observed in the DED ODS steel. The microstructure and nanoparticle composition and distribution are evaluated by electron backscatter diffraction, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and atom probe tomography, to compare the microstructural features of DED and LPBF manufactured parts. Smaller grain size and more homogeneous distribution with lower agglomeration of Y-O nanoparticles in the LPBF sample are found to be key factors for enhanced mechanical response at 600 °C. The enhanced mechanical properties of the LPBF-processed sample and the more homogeneous nanoparticle dispersion can be linked to results obtained by finite element methods simulations of the melt pool that show two orders of magnitude faster cooling rates for LPBF than for DED. Therefore, this work presents and validates a complete laser-based methodology for the preparation and processing of an ODS steel, proving the modification of the microstructure and enhancement of the high-temperature strength of the as-built parts. © 2020
    view abstract10.1016/j.actamat.2020.116566
  • Nanoparticle tracing during laser powder bed fusion of oxide dispersion strengthened steels
    Yang, Y. and Doñate-Buendía, C. and Oyedeji, T.D. and Gökce, B. and Xu, B.-X.
    Materials 14 (2021)
    The control of nanoparticle agglomeration during the fabrication of oxide dispersion strengthened steels is a key factor in maximizing their mechanical and high temperature reinforcement properties. However, the characterization of the nanoparticle evolution during processing represents a challenge due to the lack of experimental methodologies that allow in situ evaluation during laser powder bed fusion (LPBF) of nanoparticle-additivated steel powders. To address this problem, a simulation scheme is proposed to trace the drift and the interactions of the nanoparticles in the melt pool by joint heat-melt-microstructure–coupled phase-field simulation with nanoparticle kinematics. Van der Waals attraction and electrostatic repulsion with screened-Coulomb potential are explicitly employed to model the interactions with assumptions made based on reported experimental evidence. Numerical simulations have been conducted for LPBF of oxide nanoparticle-additivated PM2000 powder considering various factors, including the nanoparticle composition and size distribution. The obtained results provide a statistical and graphical demonstration of the temporal and spatial variations of the traced nanoparticles, showing ∼55% of the nanoparticles within the generated grains, and a smaller fraction of ∼30% in the pores, ∼13% on the surface, and ∼2% on the grain boundaries. To prove the methodology and compare it with experimental observations, the simulations are performed for LPBF of a 0.005 wt % yttrium oxide nanoparticle-additivated PM2000 powder and the final degree of nanoparticle agglomeration and distribution are analyzed with respect to a series of geometric and material parameters. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133463
  • On the selection and design of powder materials for laser additive manufacturing
    Doñate-Buendía, C. and Gu, D. and Schmidt, M. and Barcikowski, S. and Korsunsky, A.M. and Gökce, B.
    Materials and Design 204 (2021)
    view abstract10.1016/j.matdes.2021.109653
  • Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease
    Dittrich, S. and Barcikowski, S. and Gökce, B.
    Opto-Electronic Advances 4 (2021)
    Understanding shielding cross-effects is a prerequisite for maximal power-specific nanosecond laser ablation in liquids (LAL). However, discrimination between cavitation bubble (CB), nanoparticle (NP), and shielding, e.g., by the plasma or a transient vapor layer, is challenging. Therefore, CB imaging by shadowgraphy is performed to better understand the plasma and laser beam-NP interaction during LAL. By comparing the fluence-dependent CB volume for ablations performed with 1 ns pulses with reports from the literature, we find larger energy-specific CB volumes for 7 ns-ablation. The increased CB for laser ablation with higher ns pulse durations could be a first explanation of the efficiency decrease reported for these laser systems having higher pulse durations. Consequently, 1 ns-LAL shows superior ablation efficiency. Moreover, a CB cascade occurs when the focal plane is shifted into the liquid. This effect is enhanced when NPs are present in the fluid. Even minute amounts of NPs trapped in a stationary layer decrease the laser energy significantly, even under liquid flow. However, this local concentration in the sticking film has so far not been considered. It presents an essential obstacle in high-yield LAL, shielding already the second laser pulse that arrives and presenting a source of satellite bubbles. Hence, measures to lower the NP concentration on the target must be investigated in the future. © 2021 Institute of Optics and Electronics, Chinese Academy of Sciences.
    view abstract10.29026/oea.2021.200072
  • Synthesis of gold, platinum, and gold-platinum alloy nanoparticle colloids with high-power megahertz-repetition-rate lasers: the importance of the beam guidance method
    Waag, F. and Streubel, R. and Gökce, B. and Barcikowski, S.
    Applied Nanoscience (Switzerland) 11 (2021)
    Nanoparticles of noble metals and their alloys are of particular interest for biomedicine and catalysis applications. The method of laser ablation of bulk metals in liquids gives facile access to such particles as high-purity colloids and is already used in industrial research. However, the method still lacks sufficient productivity for industrial implementation into series production. The use of innovative laser technology may help to further disseminate this colloid synthesis method in the near future. Ultrashort-pulsed lasers with high powers and megahertz-repetition-rates became available recently, but place high demands on the accurate optical laser pulse delivery on the target. Full lateral pulse separation is necessary to avoid a reduction of nanoparticle productivity due to pulse shielding. In this study, we compare flexible but rather slow galvanometer scanning with much faster but more expensive polygon-wheel scanning in their performance in the production of colloidal nanoparticles by laser ablation in liquid. Both beam guidance technologies are applied in the laser ablation of gold, platinum, and a gold-rich platinum alloy in micromolar saline water. We found that the dimensions of the scan pattern are crucial. A threshold pattern length exists, at which one scan technology becomes more productive than the other one. In addition, a much lower productivity was found for the ablation of gold compared to that of platinum. Alloying gold with only 10 at.% of platinum improved the productivity nearly to the level of platinum, reaching 8.3 g/h. © 2021, The Author(s).
    view abstract10.1007/s13204-021-01693-y
  • Towards laser printing of magnetocaloric structures by inducing a magnetic phase transition in iron-rhodium nanoparticles
    Nadarajah, R. and Landers, J. and Salamon, S. and Koch, D. and Tahir, S. and Doñate-Buendía, C. and Zingsem, B. and Dunin-Borkowski, R.E. and Donner, W. and Farle, M. and Wende, H. and Gökce, B.
    Scientific Reports 11 (2021)
    The development of magnetocaloric materials represents an approach to enable efficient and environmentally friendly refrigeration. It is envisioned as a key technology to reduce CO2 emissions of air conditioning and cooling systems. Fe-Rh has been shown to be one of the best-suited materials in terms of heat exchange per material volume. However, the Fe-Rh magnetocaloric response depends on its composition. Hence, the adaptation of material processing routes that preserve the Fe-Rh magnetocaloric response in the generated structures is a fundamental step towards the industrial development of this cooling technology. To address this challenge, the temperature-dependent properties of laser synthesized Fe-Rh nanoparticles and the laser printing of Fe-Rh nanoparticle inks are studied to generate 2D magnetocaloric structures that are potentially interesting for applications such as waste heat management of compact electrical appliances or thermal diodes, switches, and printable magnetocaloric media. The magnetization and temperature dependence of the ink’s γ-FeRh to B2-FeRh magnetic transition is analyzed throughout the complete process, finding a linear increase of the magnetization M (0.8 T, 300 K) up to 96 Am2/kg with ca. 90% of the γ-FeRh being transformed permanently into the B2-phase. In 2D structures, magnetization values of M (0.8 T, 300 K) ≈ 11 Am2/kg could be reached by laser sintering, yielding partial conversion to the B2-phase equivalent to long-time heating temperature of app. 600 K, via this treatment. Thus, the proposed procedure constitutes a robust route to achieve the generation of magnetocaloric structures. © 2021, The Author(s).
    view abstract10.1038/s41598-021-92760-5
  • 3D printing of magnetic parts by laser powder bed fusion of iron oxide nanoparticle functionalized polyamide powders
    Hupfeld, T. and Salamon, S. and Landers, J. and Sommereyns, A. and Doñate-Buendía, C. and Schmidt, J. and Wende, H. and Schmidt, M. and Barcikowski, S. and Gökce, B.
    Journal of Materials Chemistry C 8 (2020)
    The development of new feedstock materials is a central prerequisite for advances in Additive Manufacturing (AM). To increase the breadth of potential applications for 3D and 4D printing of polymers, micro- and nano-additives incorporated into the feedstock material play an important role. In this context, magnetic materials are of great interest. Our study describes a way to fabricate polymer powders for laser powder bed fusion (PBF-LB) with a homogeneous, well-dispersed coating of iron oxide nanoparticles. Without the addition of chemical precursors, spherical superparamagnetic FeOxnanoparticles with monomodal size distribution below 10 nm are generated from FeOxmicropowder by laser fragmentation in liquid. The adsorption of the nanoparticles on polyamide (PA12) powder is conducted directly in an aqueous dispersion after laser fragmentation, followed by drying, powder analysis and PBF-LB processing.ViaMössbauer spectroscopy and magnetometry, we determined that the saturation magnetization and structure of the iron oxide nanoparticles were not influenced by PBF-LB processing, and the magnetic properties were successfully transferred to the final 3D-printed magnetic part. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0tc02740e
  • Analysis of the nanoparticle dispersion and its effect on the crystalline microstructure in carbon-additivated PA12 feedstock material for laser powder bed fusion
    Hupfeld, T. and Sommereyns, A. and Riahi, F. and Doñate-Buendía, C. and Gann, S. and Schmidt, M. and Gökce, B. and Barcikowski, S.
    Materials 13 (2020)
    Driven by the rapid development of additive manufacturing technologies and the trend towards mass customization, the development of new feedstock materials has become a key aspect. Additivation of the feedstock with nanoparticles is a possible route for tailoring the feedstock material to the printing process and to modify the properties of the printed parts. This study demonstrates the colloidal additivation of PA12 powder with laser-synthesized carbon nanoparticles at >95% yield, focusing on the dispersion of the nanoparticles on the polymer microparticle surface at nanoparticle loadings below 0.05 vol%. In addition to the descriptors "wt%" and "vol%", the descriptor "surf%" is discussed for characterizing the quantity and quality of nanoparticle loading based on scanning electron microscopy. The functionalized powders are further characterized by confocal dark field scattering, differential scanning calorimetry, powder rheology measurements (avalanche angle and Hausner ratio), and regarding their processability in laser powder bed fusion (PBF-LB). We find that heterogeneous nucleation is induced even at a nanoparticle loading of just 0.005 vol%. Finally, analysis of the effect of low nanoparticle loadings on the final parts' microstructure by polarization microscopy shows a nanoparticle loading-dependent change of the dimensions of the lamellar microstructures within the printed part. © 2020 by the authors.
    view abstract10.3390/ma13153312
  • Composition and structure of magnetic high-temperature-phase, stable Fe-Au core-shell nanoparticles with zero-valent bcc Fe core
    Kamp, M. and Tymoczko, A. and Popescu, R. and Schürmann, U. and Nadarajah, R. and Gökce, B. and Rehbock, C. and Gerthsen, D. and Barcikowski, S. and Kienle, L.
    Nanoscale Advances 2 (2020)
    Advanced quantitative TEM/EDXS methods were used to characterize different ultrastructures of magnetic Fe-Au core-shell nanoparticles formed by laser ablation in liquids. The findings demonstrate the presence of Au-rich alloy shells with varying composition in all structures and elemental bcc Fe cores. The identified structures are metastable phases interpreted by analogy to the bulk phase diagram. Based on this, we propose a formation mechanism of these complex ultrastructures. To show the magnetic response of these magnetic core nanoparticles protected by a noble metal shell, we demonstrate the formation of nanostrands in the presence of an external magnetic field. We find that it is possible to control the lengths of these strands by the iron content within the alloy nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0na00514b
  • Controlling the oxidation of magnetic and electrically conductive solid-solution iron-rhodium nanoparticles synthesized by laser ablation in liquids
    Nadarajah, R. and Tahir, S. and Landers, J. and Koch, D. and Semisalova, A.S. and Wiemeler, J. and El-Zoka, A. and Kim, S.-H. and Utzat, D. and Möller, R. and Gault, B. and Wende, H. and Farle, M. and Gökce, B.
    Nanomaterials 10 (2020)
    This study focuses on the synthesis of FeRh nanoparticles via pulsed laser ablation in liquid and on controlling the oxidation of the synthesized nanoparticles. Formation of monomodal γ-FeRh nanoparticles was confirmed by transmission electron microscopy (TEM) and their composition confirmed by atom probe tomography (APT). For these particles, three major contributors to oxidation were analysed: (1) dissolved oxygen in the organic solvents, (2) the bound oxygen in the solvent and (3) oxygen in the atmosphere above the solvent. The decrease of oxidation for optimized ablation conditions was confirmed through energy-dispersive X-ray (EDX) and Mössbauer spectroscopy. Furthermore, the time dependence of oxidation was monitored for dried FeRh nanoparticles powders using ferromagnetic resonance spectroscopy (FMR). By magnetophoretic separation, B2-FeRh nanoparticles could be extracted from the solution and characteristic differences of nanostrand formation between γ-FeRh and B2-FeRh nanoparticles were observed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10122362
  • Dynamics of laser-induced cavitation bubbles at a solid-liquid interface in high viscosity and high capillary number regimes
    Hupfeld, T. and Laurens, G. and Merabia, S. and Barcikowski, S. and Gökce, B. and Amans, D.
    Journal of Applied Physics 127 (2020)
    No unified model is available yet to explain the dynamics of laser-induced cavitation bubbles during laser ablation of solid targets in liquids, when an extremely high capillary number is achieved (>100), i.e., when the viscous forces strongly contribute to the friction. By investigating laser-induced bubbles on gold and yttrium-iron-garnet targets as a function of the liquid viscosity, using a nanosecond laser and an ultrafast shadowgraph imaging setup, we give a deeper insight into what determines the bubble dynamics. We find that the competition between the viscous forces and the surface tension (capillary number Ca), on the one hand, and the competition between the viscous forces and inertia (Reynolds number Re), on the other hand, are both key factors. Increasing the viscous forces, and hereby Ca up to 100 has an impact on the bubble shape and results in a very pronounced rim, which separates the bubble in a spherical cap driven by inertia and an interlayer. The temporal evolution of the footprint radius of the interlayer can be addressed in the framework of the inertiocapillary regime. For an intermediate viscosity, the thickness of the interlayer is consistent with a boundary layer equation. Interestingly, our data cannot be interpreted with simplified hydrodynamic (Cox-Voinov) or molecular-kinetic theory models, highlighting the originality of the dynamics reported when extremely high capillary numbers are achieved. Upon bubble collapse, spherical persistent microbubbles are created and partly dispersed in water, whereas the high-viscous polyalphaolefines lead to long-standing oblate persistent bubbles sticking to the target's surface, independent of the ablated target. Overall, liquid's viscosity determines laser ablation-induced cavitation. © 2020 Author(s).
    view abstract10.1063/1.5116111
  • Effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels produced by laser powder bed fusion and directed energy deposition
    Doñate-Buendia, C. and Streubel, R. and Kürnsteiner, P. and Wilms, M.B. and Stern, F. and Tenkamp, J. and Bruder, E. and Barcikowski, S. and Gault, B. and Durst, K. and Schleifenbaum, J.H. and Walther, F. and Gökce, B.
    Procedia CIRP 94 (2020)
    In this contribution, the effect of nanoparticle additivation on the microstructure and microhardness of oxide dispersion strengthened steels (ODS) manufactured by laser powder bed fusion (L-PBF) and directed energy deposition (DED) additive manufacturing (AM) is studied. The powder composites are made of micrometer-sized iron-chromium-alloy based powder which are homogenously decorated with Y2O3 nanoparticles synthesized by pulsed laser fragmentation in water. Consolidated by L-PBF and DED, an enhanced microhardness of the AM-built ODS sample is found. This increase is related to the significant microstructural differences found between the differently processed samples. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.009
  • Evaluation of essential powder properties through complementary particle size analysis methods for laser powder bed fusion of polymers
    Sommereyns, A. and Hupfeld, T. and Gökce, B. and Barcikowski, S. and Schmidt, M.
    Procedia CIRP 94 (2020)
    The resolution of complex parts produced by laser powder bed fusion of polymers (PBF-LB/P) is defined significantly by the shape and size distribution of the feedstock powder. Its analysis is usually performed by optical measurement systems such as laser diffraction or image analysis. In this study, the most relevant particle size parameters will be extracted from a set of different measuring methods, as well as the Hausner ratio and finally discussed regarding safe and successful processability. Extracted data include the sphericity, fine fraction, volume- and number-weighted diameter distributions, and statistical significance analysis, including comparison of PA12 and carbon-black-additivated PA12. The presented results should give researchers a first impression about the suitability of polymer powders for PBF-LB/P, based on powder feedstock characterization. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.023
  • How colloidal surface additivation of polyamide 12 powders with well-dispersed silver nanoparticles influences the crystallization already at low 0.01 vol%
    Hupfeld, T. and Sommereyns, A. and Schuffenhauer, T. and Zhuravlev, E. and Krebs, M. and Gann, S. and Keßler, O. and Schmidt, M. and Gökce, B. and Barcikowski, S.
    Additive Manufacturing 36 (2020)
    As Additive Manufacturing (AM) is fast-growing, properties adaption of feedstock materials for AM is becoming more and more relevant due to high quality standards in industrial applications. Compared to traditional manufacturing techniques like injection molding, laser powder bed fusion (PBF-LB) of polymers has a very limited variety of processable materials, which is a major obstacle for future growth. Nanocomposites are an established material class for addressing the limitations in PBF-LB but often show poor dispersion of the nanomaterial in/on the polymer powder. Especially in the context of plasmonic nanomaterials and composites, where the state of aggregation considerably influences the optical properties, dispersion plays an important role. Our study presents a deeper understanding of the colloidal surface additivation of polyamide 12 (PA12) powders with laser-generated plasmonic silver nanoparticles, leading to high dispersion of the nanoparticles on the micropowder surface with good reproducibility. The additivation is ruled by colloidal stability and control of electrostatic forces between particles and resulted in powders that could successfully be processed on a PBF-LB machine to generate plasmonic-functionalized parts. Finally, we introduce the surface specific nanoparticle dose (surf%) as scaling key parameter complementary to the commonly used mass specific dose (wt%) to appropriately describe nanoparticle load, proving the effect of such surface additivation on the recrystallization behavior of PA12. Via flash calorimetry, already at 0.01 vol% silver load, significant nanoparticle-induced heterogeneous nucleation effects are evident, whereas the thermal properties analyzed by conventional calorimetry remain unaffected. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101419
  • In situ speciation and spatial mapping of Zn products during pulsed laser ablation in liquids (PLAL) by combined synchrotron methods
    Reich, S. and Göttlicher, J. and Ziefuss, A. and Streubel, R. and Letzel, A. and Menzel, A. and Mathon, O. and Pascarelli, S. and Baumbach, T. and Zuber, M. and Gökce, B. and Barcikowski, S. and Plech, A.
    Nanoscale 12 (2020)
    Pulsed laser ablation in liquids is a hierarchical multi-step process to produce pure inorganic nanoparticle colloids. Controlling this process is hampered by the partial understanding of individual steps and structure formation. In situ X-ray methods are employed to resolve macroscopic dynamics of nanosecond PLAL as well to analyse the distribution and speciation of ablated species with a microsecond time resolution. High time resolution can be achieved by synchrotron-based methods that are capable of 'single-shot' acquisition. X-ray multicontrast imaging by a Shack-Hartmann setup (XHI) and small angle X-ray scattering (SAXS) resolve evolving nanoparticles inside the transient cavitation bubble, while X-ray absorption spectroscopy in dispersive mode opens access to the total material yield and the chemical state of the ejecta. It is confirmed that during ablation nanoparticles are produced directly as well as reactive material is detected, which is identified in the early stage as Zn atoms. Nanoparticles within the cavitation bubble show a metal signature, which prevails for milliseconds, before gradual oxidation sets in. Ablation is described by a phase explosion of the target coexisting with full evaporation. Oxidation occurs only as a later step to already formed nanoparticles. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0nr01500h
  • Increasing the size-selectivity in laser-based g/h liquid flow synthesis of pt and ptpd nanoparticles for co and no oxidation in industrial automotive exhaust gas treatment benchmarking
    Dittrich, S. and Kohsakowski, S. and Wittek, B. and Hengst, C. and Gökce, B. and Barcikowski, S. and Reichenberger, S.
    Nanomaterials 10 (2020)
    PtPd catalysts are state-of-the-art for automotive diesel exhaust gas treatment. Although wet-chemical preparation of PtPd nanoparticles below 3 nm and kg-scale synthesis of supported PtPd/Al2 O3 are already established, the partial segregation of the bimetallic nanoparticles remains an issue that adversely affects catalytic performance. As a promising alternative, laser-based catalyst preparation allows the continuous synthesis of surfactant-free, solid-solution alloy nanoparticles at the g/h-scale. However, the required productivity of the catalytically relevant size fraction <10 nm has yet to be met. In this work, by optimization of ablation and fragmentation conditions, the continuous flow synthesis of nanoparticles with a productivity of the catalytically relevant size fraction <10 nm of >1 g/h is presented via an in-process size tuning strategy. After the laser-based preparation of hectoliters of colloid and more than 2 kg of PtPd/Al2 O3 wash coat, the laser-generated catalysts were benchmarked against an industry-relevant reference catalyst. The conversion of CO by laser-generated catalysts was found to be equivalent to the reference, while improved activity during NO oxidation was achieved. Finally, the present study validates that laser-generated catalysts meet the size and productivity requirements for industrial standard operating procedures. Hence, laser-based catalyst synthesis appears to be a promising alternative to chemical-based preparation of alloy nanoparticles for developing industrial catalysts, such as those needed in the treatment of exhaust gases. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10081582
  • Laser fragmentation synthesis of colloidal bismuth ferrite particles
    Siebeneicher, S. and Waag, F. and Castillo, M.E. and Shvartsman, V.V. and Lupascu, D.C. and Gökce, B.
    Nanomaterials 10 (2020)
    Laser fragmentation of colloidal submicron-sized bismuth ferrite particles was performed by irradiating a liquid jet to synthesize bismuth ferrite nanoparticles. This treatment achieved a size reduction from 450 nm to below 10 nm. A circular and an elliptical fluid jet were compared to control the energy distribution within the fluid jet and thereby the product size distribution and educt decomposition. The resulting colloids were analysed via UV-VIS, XRD and TEM. All methods were used to gain information on size distribution, material morphology and composition. It was found that using an elliptical liquid jet during the laser fragmentation leads to a slightly smaller and narrower size distribution of the resulting product compared to the circular jet. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10020359
  • Manipulation of the size and phase composition of yttrium iron garnet nanoparticles by pulsed laser post-processing in liquid
    Hupfeld, T. and Stein, F. and Barcikowski, S. and Gökce, B. and Wiedwald, U.
    Molecules 25 (2020)
    Modification of the size and phase composition of magnetic oxide nanomaterials dispersed in liquids by laser synthesis and processing of colloids has high implications for applications in biomedicine, catalysis and for nanoparticle-polymer composites. Controlling these properties for ternary oxides, however, is challenging with typical additives like salts and ligands and can lead to unwanted byproducts and various phases. In our study, we demonstrate how additive-free pulsed laser post-processing (LPP) of colloidal yttrium iron oxide nanoparticles using high repetition rates and power at 355 nm laser wavelength can be used for phase transformation and phase purification of the garnet structure by variation of the laser fluence as well as the applied energy dose. Furthermore, LPP allows particle size modification between 5 nm (ps laser) and 20 nm (ns laser) and significant increase of the monodispersity. Resulting colloidal nanoparticles are investigated regarding their size, structure and temperature-dependent magnetic properties. © 2020 by the authors.
    view abstract10.3390/molecules25081869
  • Nanoparticles engineering by pulsed laser ablation in liquids: Concepts and applications
    Fazio, E. and Gökce, B. and De Giacomo, A. and Meneghetti, M. and Compagnini, G. and Tommasini, M. and Waag, F. and Lucotti, A. and Zanchi, C.G. and Ossi, P.M. and Dell’aglio, M. and D’urso, L. and Condorelli, M. and Scardaci, V. and Biscaglia, F. and Litti, L. and Gobbo, M. and Gallo, G. and Santoro, M. and Trusso, S. and Neri, F.
    Nanomaterials 10 (2020)
    Laser synthesis emerges as a suitable technique to produce ligand-free nanoparticles, alloys and functionalized nanomaterials for catalysis, imaging, biomedicine, energy and environmental applications. In the last decade, laser ablation and nanoparticle generation in liquids has proven to be a unique and efficient technique to generate, excite, fragment and conjugate a large variety of nanostructures in a scalable and clean way. In this work, we give an overview on the fundamentals of pulsed laser synthesis of nanocolloids and new information about its scalability towards selected applications. Biomedicine, catalysis and sensing are the application areas mainly discussed in this review, highlighting advantages of laser-synthesized nanoparticles for these types of applications and, once partially resolved, the limitations to the technique for large-scale applications. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10112317
  • Picosecond laser-induced surface structures on alloys in liquids and their influence on nanoparticle productivity during laser ablation
    Nadarajah, R. and Barcikowski, S. and Gökce, B.
    Optics Express 28 (2020)
    The productivity of nanoparticles formed by laser ablation of gold-silver and iron-gold alloy as well as copper and iron-nickel alloy targets in water is correlated with the formation of laser-induced surface structures. At a laser fluence optimized for maximum nanoparticle productivity, it is found that a binary alloy with an equimolar ratio forms laser-induced periodic surface structures (LIPSS) after ablation, if one of the constituent metals also form LIPSS. The ablation rate of nanoparticles linearly depends on the laser fluence if LIPSS is not formed, while a logarithmic trend and a decrease in productivity is evident when LIPSS is formed. To cancel LIPSS formation and recover from this decrease, a change to circularly polarized light is performed and an increase in nanoparticle productivity of more than 30% is observed. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
    view abstract10.1364/OE.28.002909
  • Plasmonic Seasoning: Giving Color to Desktop Laser 3D Printed Polymers by Highly Dispersed Nanoparticles
    Hupfeld, T. and Wegner, A. and Blanke, M. and Doñate-Buendía, C. and Sharov, V. and Nieskens, S. and Piechotta, M. and Giese, M. and Barcikowski, S. and Gökce, B.
    Advanced Optical Materials 8 (2020)
    Material development is key for continuing the exponential growth in the field of 3D printing. However, 3D printing of polymers by laser powder bed fusion (PBF-LB) still is limited to a few polymer powder materials, which restricts the range of applications. Tailoring the chemical, rheological, mechanical, or optical properties of the feedstock powder to the requirements of the laser printing process poses a significant challenge. In order to meet global trends in the commercialization of desktop 3D printers, the use of inexpensive and compact diode lasers for PBF-LB in the visible or near-infrared range is highly desired. However, at present, only black objects can be printed by desktop laser printers since only commercial carbon black-based composite powders meet their laser absorption requirements. In this study, a route for tuning the absorption properties of thermoplastic polyurethane polymers and incorporating color into printed objects by using minute amounts (i.e., 0.01 vol%) of highly dispersed plasmonic silver nanoparticles is reported, presenting a new way for colored parts to be produced through laser 3D printing. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adom.202000473
  • Recent progress in laser materials processing and synthesis
    Gökce, B. and Filipescu, M. and Barcikowski, S.
    Applied Surface Science 513 (2020)
    view abstract10.1016/j.apsusc.2020.145762
  • Research trends in laser powder bed fusion of Al alloys within the last decade
    Kusoglu, I.M. and Gökce, B. and Barcikowski, S.
    Additive Manufacturing 36 (2020)
    Research on Laser Powder Bed Fusion (L-PBF) of Al alloy powders increased exponentially over the last decade, and Al powders have become the third most studied metal alloy composition for L-PBF. This study evaluates the trends of the literature in this field to understand which material properties, process properties, and as-built part properties were decisive and of interest for L-PBF processing within the last decade. Even though the effect of the powder properties is often not discussed in detail within the studied 100 referenced Sci-Index articles, it was found that the use of particulate additives in Al powders overcame the main processing and metallurgic problems, such as anisotropic microstructure, segregations, and crack formation in the as-built parts during L-PBF. By quantifying related statistics and extracting data from the studied articles, it is found that powder size ranges of Al feedstock powders overlap in a narrower size range. AlSi10Mg is the most studied alloy; it is a eutectic Al composition that avoids the problems associated with resolidification in L-PBF. The latest research trends focused on additives for grain refinement to improve the mechanical properties and to avoid crack sensitivity in the as-built Al-Zn-Mg-(Cu) and Al-Cu-Mg parts. Additives are found to be promising to industrialize new Al alloy compositions in L-PBF. A Principle Component Analysis is carried out to determine the correlation between reported material, process, and as-built part properties. © 2020 Elsevier B.V.
    view abstract10.1016/j.addma.2020.101489
  • Scaling up colloidal surface additivation of polymer powders for laser powder bed fusion
    Hupfeld, T. and Doñate-Buendía, C. and Krause, M. and Sommereyns, A. and Wegner, A. and Sinnemann, T. and Schmidt, M. and Gökce, B. and Barcikowski, S.
    Procedia CIRP 94 (2020)
    Nanoadditivation of polymer materials has high potential to meet the needs of material modification for laser powder bed fusion (PBF-LB/P), e.g. by tuning optical or mechanical properties. Colloidal additivation of polymer powders has proven to avoid aggregation of nanofillers on the polymer surface during additivation. In our study, we demonstrate kg-scale, continuous colloidal surface additivation of polymer powders to generate sufficient amounts for PBF-LB/P process development and manufacturing of test specimens. Furthermore, colloidal additivation achieves a high surface coverage even at low wt% and allows PBF-LB/P with CO2 and diode lasers to form parts preserving the superior nanoparticle dispersion within TPU and PA12. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.022
  • Second harmonic generation as a probe of broken mirror symmetry
    Fichera, B.T. and Kogar, A. and Ye, L. and Gökce, B. and Zong, A. and Checkelsky, J.G. and Gedik, N.
    Physical Review B 101 (2020)
    The notion of spontaneous symmetry breaking has been used to describe phase transitions in a variety of physical systems. In crystalline solids, the breaking of certain symmetries, such as mirror symmetry, is difficult to detect unambiguously. Using 1T-TaS2, we demonstrate here that rotational-anisotropy second harmonic generation (RA-SHG) is not only a sensitive technique for the detection of broken mirror symmetry, but also that it can differentiate between mirror symmetry-broken structures of opposite planar chirality. We also show that our analysis is applicable to a wide class of different materials with mirror symmetry-breaking transitions. Lastly, we find evidence for bulk mirror symmetry-breaking in the incommensurate charge density wave phase of 1T-TaS2. Our results pave the way for RA-SHG to probe candidate materials where broken mirror symmetry may play a pivotal role. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.241106
  • The effect of downstream laser fragmentation on the specific surface area and photoelectrochemical performance of barium tantalum oxynitride
    Haydous, F. and Waag, F. and Si, W. and Li, F. and Barcikowski, S. and Gökce, B. and Lippert, T.
    Applied Surface Science 510 (2020)
    One approach to improve the photoelectrochemical solar water splitting performance of photoanodes based on oxynitride perovskite particles is through increasing the active surface area which allows the generation of more electron-hole pairs that contribute in the water reduction and oxidation reactions. In this study, we explore the pros and cons of downstream laser fragmentation as a method to increase the specific surface area of oxynitride particles and highlight the important issues that must be considered for effective solar water splitting. The synthesis of particles with a high surface area of up to 32.4 m2 g−1 is demonstrated. Furthermore, the fragmented oxynitrides revealed lower absorbance values, a blue shift in the absorption edge and a higher background absorbance. These observations, in addition to the lower crystalline quality of the fragmented oxynitrides, were attributed to the loss of N content during fragmentation and the formation of secondary phases. The photoanodes based on the fragmented particles showed lower photocurrents than those prepared from the un-fragmented particles even though the surface area was increased. The decrease in photoactivity was ascribed to the presence of more grain boundaries in the fragmented oxynitride photoanodes which leads to more recombinations of the photogenerated carriers. Interestingly, after seven fragmentation passages, the photocurrent starts to increase again due to the formation of an amorphous layer which improves the transport of the photogenerated carriers. © 2020
    view abstract10.1016/j.apsusc.2020.145429
  • Use of (nano-)additives in Laser Powder Bed Fusion of Al powder feedstocks: Research directions within the last decade
    Kusoglu, I.M. and Gökce, B. and Barcikowski, S.
    Procedia CIRP 94 (2020)
    An exponential increase for the number of Scientific Citation Index (SCI) expanded articles in the field of Laser Powder Bed Fusion (L-PBF) of metal powder feedstocks are generated within the last decade. Al powder feedstocks have become one of the most cited metal alloys in this field. By in-depth analyzing the experimental research data provided within SCI-expanded articles, it is obvious that material properties and laser process properties have a high impact to produce as-built parts with crack free microstructure, high density, high mechanical properties and high repetition rates of as-built parts. As a future research trend of using additive powders in Al powder feedstocks are found to be promising candidates to develop L-PBF of as-built Al parts. This study quantitatively evaluates the effect of compositions and mass fractions on several Al alloy powder feedstocks by extracting research data given in the SCI-expanded articles. The latest research directions showed that several additives are useful to refine microstructure and to enhance the mechanical properties of as-built Al parts. The effect of additives on processability, as-built density, tensile properties, and flexural properties of several Al alloy parts are discussed. © 2020 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.procir.2020.09.003
  • Ablation target cooling by maximizing the nanoparticle productivity in laser synthesis of colloids
    Waag, F. and Gökce, B. and Barcikowski, S.
    Applied Surface Science 466 (2019)
    Even if ultrashort laser pulses are used during the laser synthesis of colloids, a significant amount of laser energy is converted into thermal energy, which results in heating the ablation target and the colloid. To date, little attention has been paid to these heating effects in the literature. This study was focused on measurements of the process temperature during the high-power, ultrashort-pulsed laser ablation of a nickel target in a continuous water flow setup. Time-resolved monitoring of the temperature of the ablation target and of the colloid indicated that there was an initial rapid uptake of thermal energy followed by a thermally-stable state in which there was very little additional heating. Shifting the focal plane from behind the target onto its surface and further into the fluid provided insight concerning the different mechanisms of heat generation, dissipation, and transfer in the laser synthesis of colloids. It even was possible to distinguish the fluence effects and the colloid re-irradiation effects. New possibilities of process control were identified by correlating the productivity of laser ablation at different focal plane shifts with the measured thermal data. Counterintuitively, the temperature of the target was minimized via ablation cooling when the productivity of the process was maximized. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.10.030
  • Comparison of the productivity and ablation efficiency of different laser classes for laser ablation of gold in water and air
    Dittrich, S. and Streubel, R. and McDonnell, C. and Huber, H.P. and Barcikowski, S. and Gökce, B.
    Applied Physics A: Materials Science and Processing 125 (2019)
    In this study, we compare different laser systems used for the synthesis of nanoparticles. The productivity and ablation efficiency of laser ablation of gold in water and in air are determined for three pulsed laser systems with comparable pulse energy but different pulse duration and repetition rate. All experiments are performed in a fluence range of up to 20J/cm2. The highest productivity among the considered lasers is found for a high-power picosecond laser, which shows 12 times higher ablation rate for the ablation in air compared to ablation in liquid. Further, we find that the threshold fluence for ablation in air is up to 1.9 times higher than for ablation in water. The highest ablation efficiency, which is defined as an energy specific ablation volume by the ablation rate divided by the laser power, is found for the low power, compact nanosecond laser system. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00339-019-2704-8
  • Correction: Determining the role of redox-active materials during laser-induced water decomposition (Physical Chemistry Chemical Physics (2019) 21 (18636-18651) DOI: 10.1039/C9CP02663K)
    Kalus, M.-R. and Lanyumba, R. and Lorenzo-Parodi, N. and Jochmann, M.A. and Kerpen, K. and Hagemann, U. and Schmidt, T.C. and Barcikowski, S. and Gökce, B.
    Physical Chemistry Chemical Physics 21 (2019)
    The authors would like to correct Fig. 6d. The values on the y-axis were incorrectly scaled by a factor of 100 which makes no sense from chemical point of view. The corrected figure is shown below. Note that the correction of this figure will not change the core message of the manuscript,1 as the statements in the text refer to the qualitative trend. (Figure Presented). The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. © 2019 the Owner Societies.
    view abstract10.1039/c9cp90263e
  • Determining the role of redox-active materials during laser-induced water decomposition
    Kalus, M.-R. and Lanyumba, R. and Lorenzo-Parodi, N. and Jochmann, M.A. and Kerpen, K. and Hagemann, U. and Schmidt, T.C. and Barcikowski, S. and Gökce, B.
    Physical Chemistry Chemical Physics 21 (2019)
    Laser ablation in liquids (LAL) drives the decomposition of the liquid inducing the formation of a large number of different redox equivalents and gases. This not only leads to shielding effects and a decrease of the nanoparticle (NP) productivity but also can directly affect the NP properties such as the oxidation degree. In this study, we demonstrate that liquid decomposition during laser ablation in water is triggered by the redox activity of the 7 different bulk materials used; Au, Pt, Ag, Cu, Fe, Ti and Al, as well as by the reactivity of water with the plasma. Laser ablation of less-noble metals like aluminum leads to a massive gas evolution up to 390 cm3 per hour with molar hydrogen to oxygen ratios of 17.1. For more noble metals such as gold and platinum, water splitting induced by LAL is the dominant feature leading to gas volume formation rates of 10 up to 30 cm3 per hour and molar hydrogen to oxygen ratios of 1.2. We quantify the material-dependent ablation rate, shielding effects as well as the amount of hydrogen peroxide produced, directly affecting the yield and oxidation of the nanoparticles on the long-time scale. © 2019 the Owner Societies.
    view abstract10.1039/c9cp02663k
  • Discrimination of effects leading to gas formation during pulsed laser ablation in liquids
    Kalus, M.-R. and Reimer, V. and Barcikowski, S. and Gökce, B.
    Applied Surface Science 465 (2019)
    Pulsed laser ablation of a bulk target in liquid induces the formation of cavitation bubbles and persistent gas bubbles, which both shield subsequent laser pulses leading to a decrease in nanoparticle productivity. A further shielding entity and a source for gas formation when post-irradiated are the synthesized nanoparticles. In this study, an experimental setup is developed, which allows quantitative measurement of the gas volume produced by these shielding entities. It can be shown that 1 cm3 gas is produced in 10 min ablation time when 8 W picosecond-laser power is applied. By a combined experimental and mathematical approach, the gas volumes induced by silver bulk ablation and post-irradiation effects of the produced colloids are discriminated. It is shown that a characteristic nanoparticle mass concentration threshold exists, where post-irradiation effects mostly dominate gas formation. In a synergistic process, the effective laser fluence available for bulk ablation decreases with increasing nanoparticle mass concentration and up to 80% of the laser power is coupled into the nanoparticles. At the same time, the interparticle distance between the nanoparticles decreases favoring the laser-induced breakdown of the liquid. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.09.224
  • Early appearance of crystalline nanoparticles in pulsed laser ablation in liquids dynamics
    Reich, S. and Letzel, A. and Menzel, A. and Kretzschmar, N. and Gökce, B. and Barcikowski, S. and Plech, A.
    Nanoscale 11 (2019)
    The size and crystallinity of gold and silver nanoparticles during the process of pulsed laser ablation in water (PLAL) is investigated with microsecond and sub-microsecond time resolution. While basic observations have already been established, such as detection of particles inside the cavitation bubble, trapping of ablated matter by the bubble or the action of size quenching on a sub-millisecond time scale, the structure formation mechanism is still a matter of debate. Quantifying the nanoparticle release and crystallinity close to the irradiated metal target by wide and small angle X-ray scattering reveals the presence of nanoparticles ahead of the developing vapour bubble and inside the bubble. While the (temporal) distribution is in agreement with a homogeneously particle-filled bubble, solid particles are detected at the advancing bubble front. Wide-angle X-ray scattering confirms the crystalline nature of these large particles. This reveals that for picosecond ablation the expulsion of condensed phases of material during the ablation process adds significantly to the bimodal size distribution, relating to recent models of film lift-off and liquid metal Rayleigh instabilities. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9nr01203f
  • How the re-irradiation of a single ablation spot affects cavitation bubble dynamics and nanoparticles properties in laser ablation in liquids
    Letzel, A. and Santoro, M. and Frohleiks, J. and Ziefuß, A.R. and Reich, S. and Plech, A. and Fazio, E. and Neri, F. and Barcikowski, S. and Gökce, B.
    Applied Surface Science 473 (2019)
    Fundamental theoretical and experimental studies on the formation of nanoparticles and cavitation during laser synthesis of colloids usually employ single-pulse conditions, whereas studies of the properties of nanoparticles naturally require prolonged ablation. We explored how a defined number of pulses changes a silver target's surface geometry and thereby the dynamics of the laser-induced cavitation bubble and the resulting properties of the nanoparticles. The shape of the cavitation bubble transforms from hemispherical to almost spherical. The indirectly calculated mass concentration in the cavitation bubble follows a decay with the number of laser pulses. Surprisingly, the ablated mass does not set the volume of the extended cavitation bubble, as one would expect, because of the linear dependency of both the volume of the bubble and the ablation mass per pulse on the laser fluence. No influence of the altered cavitation bubble on the nanoparticles was identified. Instead, clear evidence of a high share of silver nanoclusters (d < 3 nm) with improved instrumentation (ultracentrifuge) was found in all samples at our low concentration conditions. The influence of these reactive species on the final particle size was found to be much larger than the cavitation bubble variations caused by prolonged surface ablation. In addition, no correlation was observed between the size of the primary particles (∼8 nm) and the mass concentration in the cavitation bubble. © 2018
    view abstract10.1016/j.apsusc.2018.12.025
  • Incubation Effect of Pre-Irradiation on Bubble Formation and Ablation in Laser Ablation in Liquids
    Reich, S. and Letzel, A. and Gökce, B. and Menzel, A. and Barcikowski, S. and Plech, A.
    ChemPhysChem 20 (2019)
    Pulsed laser ablation in liquids (PLAL) is a multi-scale process, which is widely studied either in batch ablation with prolonged target irradiation as well as mechanistic investigations, in a defined (single-shot) process. However, fundamental studies on defined pulse series are rare. We have investigated the effect of a developing rough morphology of the target surface on the PLAL process with nanosecond pulses and, partially, picosecond pulses. At low fluence the cavitation bubble growth as well as the ablation yield depend on the irradiation history of the target. The bubble size increases with repeated irradiation on one spot for the first 2–30 pulses as well as with the applied dose. This is discussed within the framework of incubation effects. Incubation is found to be important, resulting in a bubble volume increase by a factor of six or more between pristine and corrugated targets. The target surface, changing from smooth to corrugated, induces a more efficient localization of laser energy at the solid-liquid interface. This is accompanied by a suppressed reflectivity and more efficient coupling of energy into the laser-induced plasma. Thus, the cavitation bubble size increases as well as ablation being enhanced. At high fluence, such incubation is masked by the rapid development of surface damage within the first shots, which eventually would lead to a reduction of bubble sizes. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cphc.201900075
  • Kinetically-controlled laser-synthesis of colloidal high-entropy alloy nanoparticles
    Waag, F. and Li, Y. and Ziefuß, A.R. and Bertin, E. and Kamp, M. and Duppel, V. and Marzun, G. and Kienle, L. and Barcikowski, S. and Gökce, B.
    RSC Advances 9 (2019)
    The single-step incorporation of multiple immiscible elements into colloidal high-entropy alloy (HEA) nanoparticles has manifold technological potential, but it continues to be a challenge for state-of-the-art synthesis methods. Hence, the development of a synthesis approach by which the chemical composition and phase of colloidal HEA nanoparticles can be controlled could lead to a new pool of nanoalloys with unparalleled functionalities. Herein, this study reports the single-step synthesis of colloidal CoCrFeMnNi HEA nanoparticles with targeted equimolar stoichiometry and diameters less than 5 nm by liquid-phase, ultrashort-pulsed laser ablation of the consolidated and heat-treated micropowders of the five constituent metals. Further, the scalability of the process with an unprecedented productivity of 3 grams of colloidal HEA nanoparticles per hour is demonstrated. Electrochemical analysis reveals a unique redox behavior of the particles' surfaces in an alkaline environment and a potential for future application as a heterogeneous catalyst for the oxygen evolution reaction. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9ra03254a
  • Oxynitride thin films versus particle-based photoanodes: A comparative study for photoelectrochemical solar water splitting
    Haydous, F. and Döbeli, M. and Si, W. and Waag, F. and Li, F. and Pomjakushina, E. and Wokaun, A. and Gökce, B. and Pergolesi, D. and Lippert, T.
    ACS Applied Energy Materials 2 (2019)
    The solar water splitting process assisted by semiconductor photocatalysts attracts growing research interests worldwide for the production of hydrogen as a clean and sustainable energy carrier. Because of their optical and electrical properties, several oxynitride materials show great promise for the fabrication of efficient photocatalysts for solar water splitting. This study reports a comparative investigation of particle-and thin-film-based photocatalysts using three different oxynitride materials. The absolute comparison of the photoelectrochemical activities favors the particle-based electrodes because of the better absorption properties and larger electrochemical surface area. However, thin films surpass the particle-based photoelectrodes because of their more suitable morphological features that improve the separation and mobility of the photogenerated charge carriers. Our analysis identifies what specific insights into the properties of materials can be achieved with the two complementary approaches. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.8b01811
  • Time and Mechanism of Nanoparticle Functionalization by Macromolecular Ligands during Pulsed Laser Ablation in Liquids
    Letzel, A. and Reich, S. and Dos Santos Rolo, T. and Kanitz, A. and Hoppius, J. and Rack, A. and Olbinado, M.P. and Ostendorf, A. and Gökce, B. and Plech, A. and Barcikowski, S.
    Langmuir 35 (2019)
    Laser ablation of gold in liquids with nanosecond laser pulses in aqueous solutions of inorganic electrolytes and macromolecular ligands for gold nanoparticle size quenching is probed inside the laser-induced cavitation bubble by in situ X-ray multicontrast imaging with a Hartmann mask (XHI). It is found that (i) the in situ size quenching power of sodium chloride (NaCl) in comparison to the ablation in pure water can be observed by the scattering contrast from XHI already inside the cavitation bubble, while (ii) for polyvinylpyrrolidone (PVP) as a macromolecular model ligand an in situ size quenching cannot be observed. Complementary ex situ characterization confirms the overall size quenching ability of both additive types NaCl and PVP. The macromolecular ligand as well as its monomer N-vinylpyrrolidone (NVP) are mainly effective for growth quenching of larger nanoparticles on later time scales, leading to the conclusion of an alternative interaction mechanism with ablated nanoparticles compared to the electrolyte NaCl, probably outside of the cavitation bubble, in the surrounding liquid phase. While monomer and polymer have similar effects on the particle properties, with the polymer being slightly more efficient, only the polymer is effective against hydrodynamic aggregation. © 2019 American Chemical Society.
    view abstract10.1021/acs.langmuir.8b01585
  • Tribological properties of laser-generated hard ceramic particles in a gear drive contact
    Jendrzej, S. and Gondecki, L. and Debus, J. and Moldenhauer, H. and Tenberge, P. and Barcikowski, S. and Gökce, B.
    Applied Surface Science 467-468 (2019)
    The lubricant is a central element in the transmission design. It primarily separates the two contact partners through a pressure-induced solidification in the lubrication gap, thus enabling the operation of heavily loaded sliding-rolling contacts. On the one hand, the quality and properties of a lubricant depend on the base oils, which differ by their viscosity and process-technological parameters. The addition of particulate additives gives the lubricants further functional properties that are not contained in the base oil. In this study, the influence of laser-synthesized yttria-stabilized zirconia nano- or submicrometer spheres as dispersed functional elements in the lubricant is studied, and their impact on wear and fatigue on the tooth flank is investigated. The work includes systematic investigations on the influence of the particle's shape and size by running tests on a FZG gear test rig. Finally, the potential of the laser-generated particles as a lubricant additive is evaluated in a first conclusion. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.10.060
  • A new approach to coat PA12 powders with laser-generated nanoparticles for selective laser sintering
    Hupfeld, T. and Laumer, T. and Stichel, T. and Schuffenhauer, T. and Heberle, J. and Schmidt, M. and Barcikowski, S. and Gökce, B.
    Procedia CIRP 74 (2018)
    The modification of selective laser sintering (SLS) powder materials by nanoadditives offers the possibility to adapt the powder properties to the laser sintering process or the resulting part properties. To avoid agglomeration of the nanofiller, a new approach in which surfactant-free laser-generated colloidal nanoparticles are adsorbed onto the polymer surface directly in an aqueous solution is demonstrated. Based on this novel approach, polyamide 12 (PA12) powders are decorated with metal and oxide nanoparticles and processed via SLS. Electron microscopy and confocal laser scanning imaging are utilized to analyze the dispersion of the filler. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.procir.2018.08.103
  • Catalyst Support Effect on the Activity and Durability of Magnetic Nanoparticles: Toward Design of Advanced Electrocatalyst for Full Water Splitting
    Davodi, F. and Mühlhausen, E. and Tavakkoli, M. and Sainio, J. and Jiang, H. and Gökce, B. and Marzun, G. and Kallio, T.
    ACS Applied Materials and Interfaces 10 (2018)
    Earth-abundant element-based inorganic-organic hybrid materials are attractive alternatives for electrocatalyzing energy conversion reactions. Such material structures do not only increase the surface area and stability of metal nanoparticles (NPs) but also modify the electrocatalytic performance. Here, we introduce, for the first time, multiwall carbon nanotubes (MWNTs) functionalized with nitrogen-rich emeraldine salt (ES) (denoted as ES-MWNT) as a promising catalyst support to boost the electrocatalytic activity of magnetic maghemite (γ-Fe2O3) NPs. The latter component has been synthesized by a simple and upscalable one-step pulsed laser ablation method on Ni core forming the core-shell Niγ-Fe2O3 NPs. The catalyst (Niγ-Fe2O3/ES-MWNT) is formed via self-assembly as strong interaction between ES-MWNT and Niγ-Fe2O3 results in NPs' encapsulation in a thin C-N shell. We further show that Ni does not directly function as an active site in the electrocatalyst but it has a crucial role in synthesizing the maghemite shell. The strong interaction between the NPs and the support improves notably the NPs' catalytic activity toward oxygen evolution reaction (OER) in terms of both onset potential and current density, ranking it among the most active catalysts reported so far. Furthermore, this material shows a superior durability to most of the current excellent OER electrocatalysts as the activity, and the structure, remains almost intact after 5000 OER stability cycles. On further characterization, the same trend has been observed for hydrogen evolution reaction, the other half-reaction of water splitting. Copyright © 2018 American Chemical Society.
    view abstract10.1021/acsami.8b08830
  • Depositing laser-generated nanoparticles on powders for additive manufacturing of oxide dispersed strengthened alloy parts via laser metal deposition
    Streubel, R. and Wilms, M.B. and Doñate-Buendía, C. and Weisheit, A. and Barcikowski, S. and Schleifenbaum, J.H. and Gökce, B.
    Japanese Journal of Applied Physics 57 (2018)
    We present a novel route for the adsorption of pulsed laser-dispersed nanoparticles onto metal powders in aqueous solution without using any binders or surfactants. By electrostatic interaction, we deposit Y2O3 nanoparticles onto iron-chromium based powders and obtain a high dispersion of nano-sized particles on the metallic powders. Within the additively manufactured component, we show that the particle spacing of the oxide inclusion can be adjusted by the initial mass fraction of the adsorbed Y2O3 particles on the micropowder. Thus, our procedure constitutes a robust route for additive manufacturing of oxide dispersion-strengthened alloys via oxide nanoparticles supported on steel micropowders. © 2018 The Japan Society of Applied Physics.
    view abstract10.7567/JJAP.57.040310
  • Laser additive manufacturing of oxide dispersion strengthened steels using laser-generated nanoparticle-metal composite powders
    Wilms, M.B. and Streubel, R. and Frömel, F. and Weisheit, A. and Tenkamp, J. and Walther, F. and Barcikowski, S. and Schleifenbaum, J.H. and Gökce, B.
    Procedia CIRP 74 (2018)
    A new route for the synthesis of powder composites suitable for processing with laser additive manufacturing is demonstrated. The powder composites, consisting of micrometer-sized stainless steel powder, homogenously decorated with nano-scaled Y2O3 powder particles, are manufactured by laser processing of colloids and electrostatic deposition. Consolidated by laser metal deposition and selective laser melting, the resulting specimens show superior mechanical properties at elevated temperatures, caused by the nano-sized, homogenously distributed dispersoids. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.
    view abstract10.1016/j.procir.2018.08.093
  • Oxide dispersion-strengthened alloys generated by laser metal deposition of laser-generated nanoparticle-metal powder composites
    Doñate-Buendía, C. and Frömel, F. and Wilms, M.B. and Streubel, R. and Tenkamp, J. and Hupfeld, T. and Nachev, M. and Gökce, E. and Weisheit, A. and Barcikowski, S. and Walther, F. and Schleifenbaum, J.H. and Gökce, B.
    Materials and Design 154 (2018)
    A new method is proposed for producing nanoparticle-metal composite powders for laser additive manufacturing of oxide-dispersion strengthened (ODS) alloys. Different composite powders containing laser-generated Y2O3 and yttrium iron garnet (YIG) nanoparticles were produced and consolidated by Laser Metal Deposition (LMD). The structural properties of the manufactured ODS alloys were analyzed, and their hardness, remnant porosity, and temperature-dependent compression behavior were characterized to study the effect of the composition and size of the nanoparticles on the structural and mechanical properties. While the structural analyses did not show significant differences between the processed samples within the limits of the characterization methods that were used, the temperature-dependent compression behavior showed an increase of up to 22 ± 11% in the high-temperature strength of the specimens that contained only 0.08 wt% of laser-generated nanoparticles. This increase is attributed to the dispersed and deagglomerated nature of the nanoparticles that were used during the powder-preparation step. © 2018 Elsevier Ltd
    view abstract10.1016/j.matdes.2018.05.044
  • Primary particle diameter differentiation and bimodality identification by five analytical methods using gold nanoparticle size distributions synthesized by pulsed laser ablation in liquids
    Letzel, A. and Gökce, B. and Menzel, A. and Plech, A. and Barcikowski, S.
    Applied Surface Science 435 (2018)
    For a known material, the size distribution of a nanoparticle colloid is a crucial parameter that defines its properties. However, measured size distributions are not easy to interpret as one has to consider weighting (e.g. by light absorption, scattering intensity, volume, surface, number) and the way size information was gained. The radius of a suspended nanoparticle can be given as e.g. sphere equivalent, hydrodynamic, Feret or radius of gyration. In this study, gold nanoparticles in water are synthesized by pulsed-laser ablation (LAL) and fragmentation (LFL) in liquids and characterized by various techniques (scanning transmission electron microscopy (STEM), small-angle X-ray scattering (SAXS), analytical disc centrifugation (ADC), dynamic light scattering (DLS) and UV–vis spectroscopy with Mie-Gans Theory) to study the comparability of different analytical techniques and determine the method that is preferable for a given task related to laser-generated nanoparticles. In particular, laser-generated colloids are known to be bimodal and/or polydisperse, but bimodality is sometimes not analytically resolved in literature. In addition, frequently reported small size shifts of the primary particle mode around 10 nm needs evaluation of its statistical significance related to the analytical method. Closely related to earlier studies on SAXS, different colloids in defined proportions are mixed and their size as a function of the nominal mixing ratio is analyzed. It is found that the derived particle size is independent of the nominal mixing ratio if the colloid size fractions do not overlap considerably. Conversely, the obtained size for colloids with overlapping size fractions strongly depends on the nominal mixing ratio since most methods cannot distinguish between such fractions. Overall, SAXS and ADC are very accurate methods for particle size analysis. Further, the ability of different methods to determine the nominal mixing ratio of sizes fractions is studied experimentally. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.11.130
  • Two mechanisms of nanoparticle generation in picosecond laser ablation in liquids: The origin of the bimodal size distribution
    Shih, C.-Y. and Streubel, R. and Heberle, J. and Letzel, A. and Shugaev, M.V. and Wu, C. and Schmidt, M. and Gökce, B. and Barcikowski, S. and Zhigilei, L.V.
    Nanoscale 10 (2018)
    The synthesis of chemically clean and environmentally friendly nanoparticles through pulsed laser ablation in liquids has shown a number of advantages over conventional chemical synthesis methods and has evolved into a thriving research field attracting laboratory and industrial applications. The fundamental understanding of processes leading to the nanoparticle generation, however, still remains elusive. In particular, the origin of bimodal nanoparticle size distributions in femto- and picosecond laser ablation in liquids, where small nanoparticles (several nanometers) with narrow size distribution are commonly observed to coexist with larger (tens to hundreds of nanometers) ones, has not been explained so far. In this paper, joint computational and experimental efforts are applied to understand the mechanisms of nanoparticle formation in picosecond laser ablation in liquids and to explain the bimodal nanoparticle size distributions. The results of a large-scale atomistic simulation reveal the critical role of the dynamic interaction between the ablation plume and the liquid environment, leading to the generation of large nanoparticles through a sequence of hydrodynamic instabilities at the plume-liquid interface and a concurrent nucleation and growth of small nanoparticles in an expanding metal-liquid mixing region. The computational predictions are supported by a series of stroboscopic videography experiments showing the emergence of small satellite bubbles surrounding the main cavitation bubble generated in single pulse experiments. Carefully timed double pulse irradiation triggers expansion of secondary cavitation bubbles indicating, in accord with the simulation results, the presence of localized sites of laser energy deposition (possibly large nanoparticles) injected into the liquid at the early stage of the bubble formation. © The Royal Society of Chemistry.
    view abstract10.1039/c7nr08614h
  • X-ray spectroscopic and stroboscopic analysis of pulsed-laser ablation of Zn and its oxidation
    Reich, S. and Göttlicher, J. and Letzel, A. and Gökce, B. and Barcikowski, S. and dos Santos Rolo, T. and Baumbach, T. and Plech, A.
    Applied Physics A: Materials Science and Processing 124 (2018)
    Pulsed laser ablation in liquids (PLAL) as an attractive process for ligand-free nanoparticle synthesis represents a multiscale problem to understand the mechanisms and achieve control. Atomic and nanoscale processes interacting with macroscale dynamics in the liquid demand for sensitive tools for in-situ and structural analysis. By adding X-ray methods, we enlarge the available information on millimeter-scale bubble formation down to atomic-scale nanoparticle reactions. X-ray spectroscopy (XAS) can resolve the chemical speciation of the ablated material during the ablation from a zinc wire target showing a first oxidation step from zinc to zinc oxide within some 10 min followed by a slower reaction to hydrozincite. X-ray imaging investigations also give additional information on the bubble dynamics as we demonstrate by comparing the microsecond radiography and optical stroboscopy. We show different features of the detachment of the ablation bubble from a free wire. The location of the first collapse occurs in front of the target. While a first rebound bubble possesses an homogeneous interior, the subsequent rebound consists merely of a cloud of microbubbles. © 2017, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00339-017-1503-3
  • Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose
    Waag, F. and Gökce, B. and Kalapu, C. and Bendt, G. and Salamon, S. and Landers, J. and Hagemann, U. and Heidelmann, M. and Schulz, S. and Wende, H. and Hartmann, N. and Behrens, M. and Barcikowski, S.
    Scientific Reports 7 (2017)
    Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm-2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec-1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials. © 2017 The Author(s).
    view abstract10.1038/s41598-017-13333-zf
  • Colloidal Stability of Metal Nanoparticles in Engine Oil under Thermal and Mechanical Load
    Jendrzej, S. and Gökce, B. and Barcikowski, S.
    Chemical Engineering and Technology 40 (2017)
    Extended colloidal stability and high dispersion degree of nanolubricants are required to avoid nanoparticle deposition in combustion engines and to reduce friction and wear. The simple and rapid one-step technique of pulsed laser ablation in liquids is employed to synthesize precursor-free and highly dispersed gold nanoparticles in lubricants while the colloidal stability is measured by optical spectroscopy and transmission electron microscopy. A remarkable colloidal stability is determined under engine-like and ambient conditions for nine months in terms of constant primary particle size. In contrast to additive-free oils, almost no agglomeration is observed, which might be attributed to the attachment of engine oil additives or pyrolyzed/oxidized molecules to the nanoparticles preventing attractive nanoparticle interactions. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ceat.201600541
  • Fluence Threshold Behaviour on Ablation and Bubble Formation in Pulsed Laser Ablation in Liquids
    Reich, S. and Schönfeld, P. and Letzel, A. and Kohsakowski, S. and Olbinado, M. and Gökce, B. and Barcikowski, S. and Plech, A.
    ChemPhysChem 18 (2017)
    The ablation yield and bubble-formation process during nanosecond pulsed-laser ablation of silver in water are analysed by stroboscopic videography, time-resolved X-ray radiography and in situ UV/Vis spectroscopy. This process is studied as function of lens–target distance and laser fluence. Both the ablation yield and the bubble-cavitation process exhibit threshold behaviour as a function of fluence, which is linked to the efficiency of coupling of energy at the water/target interface. Although ablation happens below this threshold, quantitative material emission is linked to bubble formation. Above the threshold, both bubble size and ablation show linear behaviour. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cphc.201601198
  • Formation Mechanism of Laser-Synthesized Iron-Manganese Alloy Nanoparticles, Manganese Oxide Nanosheets and Nanofibers
    Zhang, D. and Ma, Z. and Spasova, M. and Yelsukova, A.E. and Lu, S. and Farle, M. and Wiedwald, U. and Gökce, B.
    Particle and Particle Systems Characterization 34 (2017)
    Laser ablation in liquids (LAL) has emerged as a versatile approach for the synthesis of alloy particles and oxide nanomaterials. However, complex chemical reactions often take place during synthesis due to inevitable atomization and ionization of the target materials and decomposition/hydrolysis of solvent/solution molecules, making it difficult to understand the particle formation mechanisms. In this paper, a possible route for the formation of FeMn alloy nanoparticles as well as MnOx nanoparticles, -sheets, and -fibers by LAL is presented. The observed structural, compositional, and morphological variations are clarified by transmission electron microscopy (TEM). The studies suggest that a reaction between Mn atoms and Fe ions followed by surface oxidation result in nonstoichiometric synthesis of Fe-rich FeMn@FeMn2O4 core-shell alloy particles. Interestingly, a phase transformation from Mn3O4 to Mn2O3 and finally to Ramsdellite γ-MnO2 is accompanied by a morphology change from nanosheets to nanofibers in gradually increasing oxidizing environments. High-resolution TEM images reveal that the particle-attachment mechanism dominates the growth of different manganese oxides. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ppsc.201600225
  • Germanium Sub-Microspheres Synthesized by Picosecond Pulsed Laser Melting in Liquids: Educt Size Effects
    Zhang, D. and Lau, M. and Lu, S. and Barcikowski, S. and Gökce, B.
    Scientific Reports 7 (2017)
    Pulsed laser melting in liquid (PLML) has emerged as a facile approach to synthesize submicron spheres (SMSs) for various applications. Typically lasers with long pulse durations in the nanosecond regime are used. However, recent findings show that during melting the energy absorbed by the particle will be dissipated promptly after laser-matter interaction following the temperature decrease within tens of nanoseconds and hence limiting the efficiency of longer pulse widths. Here, the feasibility to utilize a picosecond laser to synthesize Ge SMSs (200∼1000 nm in diameter) is demonstrated by irradiating polydisperse Ge powders in water and isopropanol. Through analyzing the educt size dependent SMSs formation mechanism, we find that Ge powders (200∼1000 nm) are directly transformed into SMSs during PLML via reshaping, while comparatively larger powders (1000∼2000 nm) are split into daughter SMSs via liquid droplet bisection. Furthermore, the contribution of powders larger than 2000 nm and smaller than 200 nm to form SMSs is discussed. This work shows that compared to nanosecond lasers, picosecond lasers are also suitable to produce SMSs if the pulse duration is longer than the material electron-phonon coupling period to allow thermal relaxation. © The Author(s) 2017.
    view abstract10.1038/srep40355
  • High productive and continuous nanoparticle fabrication by laser ablation of a wire-target in a liquid jet
    Kohsakowski, S. and Santagata, A. and Dell'Aglio, M. and de Giacomo, A. and Barcikowski, S. and Wagener, P. and Gökce, B.
    Applied Surface Science 403 (2017)
    To scale-up pulsed laser ablation in liquids for nanoparticle synthesis, we combine two promising approaches, a wire-shaped target and a small liquid layer, in one setup. Using thin liquid layers a significant increase in nanoparticle productivity (up to 5 times) is obtained. This increase is attributed to the dynamics, shape of the cavitation bubble and the spring-board like behavior of the wires in the small liquid filament. It is found that despite the increase in productivity, the particle size is independent of the productivity-related ablation parameters such as repetition rate, liquid layer thickness and wire diameter. In addition to the cavitation bubble, further shielding effects have been related to both, the laser ablated material and the presence of generated small vapor bubbles. The obtained results show that this setup can provide a good strategy to realize a continuous and process-stable (particle size and quality) ablation process without the need of target replacement. © 2017 Elsevier B.V.
    view abstract10.1016/j.apsusc.2017.01.077
  • How persistent microbubbles shield nanoparticle productivity in laser synthesis of colloids - Quantification of their volume, dwell dynamics, and gas composition
    Kalus, M.-R. and Bärsch, N. and Streubel, R. and Gökce, E. and Barcikowski, S. and Gökce, B.
    Physical Chemistry Chemical Physics 19 (2017)
    During laser synthesis of colloids, cavitation bubbles with lifetimes in the microsecond-scale form and shield the laser pulse leading to a decrease in nanoparticle output. A second type of productivity-limiting bubble that severely affects the productivity of the process is often neglected. With lifetimes from milliseconds to seconds, these persistent bubbles are systematically studied in this work by quantifying their composition, amount, size and dwell time in liquids with different viscosities and by relating the results to the nanoparticle productivities. It is found that during synthesis in water, water splitting occurs leading to persistent bubbles consisting of hydrogen and oxygen. In glycols, hydrogen and molecular carbon species containing microbubbles are formed. These persistent microbubbles shield up to 65% of the incoming laser beam depending on the liquid as well as the laser fluence and require attention by means of reducing their dwell time in the ablation zone and enhancing the nanoparticle output by liquid flow. The highest productivities and monodisperse quality are achieved in liquids with the lowest viscosities. © the Owner Societies 2017.
    view abstract10.1039/c6cp07011f
  • How Size Determines the Value of Gold: Economic Aspects of Wet Chemical and Laser-Based Metal Colloid Synthesis
    Jendrzej, S. and Gökce, B. and Epple, M. and Barcikowski, S.
    ChemPhysChem 18 (2017)
    Gold is one of the most valuable materials, and its monetary value is enhanced by size reduction from bullions to colloidal nanoparticles by a factor of 450. Wet-chemical reduction with subsequent centrifugation and pulsed laser ablation in liquids are frequently used for pure colloidal gold synthesis. Both methods provide similar physicochemical nanoparticle properties, but are very different synthesis techniques. However, the costs inherent to these methods are surprisingly seldom discussed. Both methods have in common that the labor effort poses the majority of synthesis costs. Besides an increase in batch size and mass concentration, especially an increase of the nanoparticle productivity via higher laser power and centrifugation capacity reduces synthesis costs if pilot- or industrial-scale applications are intended. In this case, laser-based synthesis is more economical if its productivity exceeds a break-even value of 550mgh-1, where the costs arising are limited by the metal costs. In contrast to industrial scale production, wet-chemical synthesis is more feasible for laboratory-scale applications, especially if the advantageous nanoparticle properties provided by laser ablation in liquids are not needed. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201601139
  • Laser Synthesis and Processing of Colloids: Fundamentals and Applications
    Zhang, D. and Gökce, B. and Barcikowski, S.
    Chemical Reviews 117 (2017)
    Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials, thereby enabling the fabrication of bioconjugates and heterogeneous catalysts. Accurate size control of LSPC-synthesized materials ranging from quantum dots to submicrometer spheres and recent upscaling advancement toward the multiple-gram scale are helpful for extending the applicability of LSPC-synthesized nanomaterials to various fields. By discussing key reports on both the fundamentals and the applications related to laser ablation, fragmentation, and melting in liquids, this Article presents a timely and critical review of this emerging topic. © 2017 American Chemical Society.
    view abstract10.1021/acs.chemrev.6b00468
  • Laser-induced growth of YVO4:Eu3+ nanoparticles from sequential flowing aqueous suspension
    Wang, H. and Lau, M. and Sannomiya, T. and Gökce, B. and Barcikowski, S. and Odawara, O. and Wada, H.
    RSC Advances 7 (2017)
    Ligand-free lanthanide ion-doped oxide nanoparticles have critical biological applications. An environmentally friendly and chemically green synthesis of YVO4:Eu3+ nanoparticles with high crystallinity is achieved using a physical method, laser irradiation from sequential flowing aqueous suspension in a free liquid reactor. The fabricated nanoparticles have an ovoid or spindle shape depending on the number of laser irradiation cycles. A transmission electron microscopy study showed that spindle-like particles are single-crystalline with high crystallinity, which is beneficial for high luminescence efficiency. Strong light emission even from a single particle was confirmed by cathodoluminescence mapping. A possible mechanism of nanoparticle formation was proposed as follows. Primary nanocrystals were produced from the plasma plume and self-assembled into ovoid-like nanoparticles via oriented attachment. After several cycles of laser irradiation, we observed spindle-like nanoparticles that were much longer than the ovoid-like particles. The spindle-like nanoparticles grew as a result of the diffusion and coalescence of the ovoid-like nanoparticles during repetitive laser irradiation. These findings provide useful information for the formation of ligand-free luminescent nanoparticles with different sizes based on YVO4. © The Royal Society of Chemistry.
    view abstract10.1039/c6ra28118d
  • Opportunities and Challenges for Laser Synthesis of Colloids
    Gökce, B. and Amendola, V. and Barcikowski, S.
    ChemPhysChem 18 (2017)
    view abstract10.1002/cphc.201700310
  • Perspective of laser-prototyping nanoparticle-polymer composites
    Zhang, D. and Gökce, B.
    Applied Surface Science 392 (2017)
    Nanoparticle synthesis by laser ablation in liquids has attracted attention from researchers worldwide the past few years and the integration of these nanoparticles in functional materials such as nanoparticle-polymer composites, represents a natural next step. Such “nanointegration” into polymers can be achieved by the ex situ dispersion of laser-synthesized inorganic nanoparticles in polymer matrices and the in situ encapsulation/grafting of nanoparticles with polymers/monomers during synthesis. Because the nanoparticle shell and the polymer matrix may be identical, this method often does not require the use of dispersants or matrix binders and constitutes a new avenue for direct particle-polymer coupling. In this perspective review, we summarize the methodologies for in situ and ex situ laser prototyping of nanoparticle-polymer composites (LaNPC) and downstream bulk-processing techniques. The determinants of polymer-solvent-laser parametrization for aimed physical and chemical properties of the composites are discussed. By highlighting representative works related to a variety of promising applications, the advantageous features of this technique are demonstrated. Finally, the challenges and prospects of LaNPC are outlined and a perspective is given regarding how the recent research findings reviewed changed the research direction in the field. © 2016
    view abstract10.1016/j.apsusc.2016.09.150
  • Process Chain for the Fabrication of Nanoparticle Polymer Composites by Laser Ablation Synthesis
    Maurer, E. and Barcikowski, S. and Gökce, B.
    Chemical Engineering and Technology 40 (2017)
    Nanoparticle polymer composites are of growing interest due to their unique properties. However, conventional composite synthesis methods usually require several process steps including steps for cleaning and improving the particle-matrix dispersion. As an alternative, laser ablation synthesis can be used to prepare tunable composite materials. This method enables an easy process chain, without the need of additional steps. In this status report, the process chain of laser-based pre-series fabrication of nanocomposites is visualized, and the increase of the method's technology readiness level is demonstrated. The process steps are demonstrated from the synthesis of the colloid to applicable functional products. The advantages of using laser ablation for nanocomposite synthesis are highlighted. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ceat.201600506
  • Pulsed laser ablation in liquids: Impact of the bubble dynamics on particle formation
    Reich, S. and Schönfeld, P. and Wagener, P. and Letzel, A. and Ibrahimkutty, S. and Gökce, B. and Barcikowski, S. and Menzel, A. and dos Santos Rolo, T. and Plech, A.
    Journal of Colloid and Interface Science 489 (2017)
    Pulsed laser ablation in liquids (PLAL) is a multiscale process, involving multiple mutually interacting phenomena. In order to synthesize nanoparticles with well-defined properties it is important to understand the dynamics of the underlying structure evolution. We use visible-light stroboscopic imaging and X-ray radiography to investigate the dynamics occurring during PLAL of silver and gold on a macroscopic scale, whilst X-ray small angle scattering is utilized to deepen the understanding on particle genesis. By comparing our results with earlier reports we can elucidate the role of the cavitation bubble. We find that symmetry breaking at the liquid-solid interface is a critical factor for bubble motion and that the bubble motion acts on the particle distribution as confinement and retraction force to create secondary agglomerates. © 2016 Elsevier Inc.
    view abstract10.1016/j.jcis.2016.08.030
  • Size Quenching during Laser Synthesis of Colloids Happens Already in the Vapor Phase of the Cavitation Bubble
    Letzel, A. and Gökce, B. and Wagener, P. and Ibrahimkutty, S. and Menzel, A. and Plech, A. and Barcikowski, S.
    Journal of Physical Chemistry C 121 (2017)
    Although nanoparticle synthesis by pulsed laser ablation in liquids (PLAL) is gaining wide applicability, the mechanism of particle formation, in particular size-quenching effects by dissolved anions, is not fully understood yet. It is well-known that the size of small primary particles (d ≤ 10 nm), secondary particles (spherical particles d > 10 nm), and agglomerates observed ex situ is effectively reduced by the addition of small amounts of monovalent electrolyte to the liquid prior to laser ablation. In this study, we focus on the particle formation and evolution inside the vapor filled cavitation bubble. This vapor phase is enriched with ions from the afore added electrolyte. By probing the cavitation bubbles' interior by means of small-angle X-ray scattering (SAXS), we are able to examine whether the size quenching reaction between nanoparticles and ions starts already during cavitation bubble confinement or if these reactions are subjected to the liquid phase. We find that particle size quenching occurs already within the first bubble oscillation (approximately 100 μs after laser impact), still inside the vapor phase. Thereby we demonstrate that nanoparticle-ion interactions during PLAL are in fact a gas phase phenomenon. These interactions include size reduction of both primary and secondary particles and a decreased abundance of the latter as shown by in situ SAXS and confirmed by ex situ particle analysis (e.g., static SAXS and TEM). (Figure Presented). © 2017 American Chemical Society.
    view abstract10.1021/acs.jpcc.6b12554
  • Ultrasmall Yttrium Iron Garnet Nanoparticles with High Coercivity at Low Temperature Synthesized by Laser Ablation and Fragmentation of Pressed Powders
    Schmitz, T. and Wiedwald, U. and Dubs, C. and Gökce, B.
    ChemPhysChem 18 (2017)
    Pulsed laser ablation of pressed yttrium iron garnet powders in water is studied and compared to the ablation of a single-crystal target. We find that target porosity is a crucial factor, which has far-reaching implications on nanoparticle productivity. Although nanoparticle size distributions obtained by analytical disc centrifugation and transmission electron microscopy (TEM) are in agreement, X-ray diffraction and energy dispersive X-ray analysis show that only nanoparticles obtained from targets with densities close to that of a bulk target lead to comparable properties. Our findings also show why the gravimetrical measurement of nanoparticle productivity is often flawed and needs to be complemented by colloidal productivity measurements. The synthesized YIG nanoparticles are further reduced in size by laser fragmentation to obtain sizes smaller than 3nm. Since the particle diameters are close to the YIG lattice constant, these ultrasmall nanoparticles reveal an immense change of the magnetic properties, exhibiting huge coercivity (0.11 T) and irreversibility fields (8 T) at low temperatures. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201601183
  • Upconversion Nanoparticles Synthesized by Ultrashort Pulsed Laser Ablation in Liquid: Effect of the Stabilizing Environment
    Gemini, L. and Schmitz, T. and Kling, R. and Barcikowski, S. and Gökce, B.
    ChemPhysChem 18 (2017)
    The distinctive feature of upconverting compounds to absorb and emit light in the near-infrared region has made upconverting nanoparticles of great interest in various application fields. Nevertheless, these colloids show a highly hydrophobic behavior, and therefore, the use of a proper stabilizing agent is necessary in most cases. Although few chemical techniques for colloid stabilization are available, it is still difficult to achieve a fully reproducible synthesis method for stable upconverting nanoparticle colloids. In this work, upconversion 18 %Yb:1 %Er:NaYF4 nanoparticles were produced by ultrafast pulsed laser ablation in a water and 2-[2-(2-methoxyethoxy)- ethoxy]acetic acid (MEEAA) environment to assess the stabilization effect of the surfactant on the nanoparticle colloid properties. The effects of the laser fluence and MEEAA concentration on the nanoparticles′ properties were investigated by TEM, EDS, and emission spectra analyses. The results show that ultrashort pulsed laser ablation in liquid allows generating highly spherical nanoparticles with conserved stoichiometry and optical properties. Moreover, it is possible to obtain colloids with significantly higher stability and preserved optical properties by one-step PLAL processes directly in the MEEAA environment. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cphc.201601266
  • Barrierless growth of precursor-free, ultrafast laser-fragmented noble metal nanoparticles by colloidal atom clusters - A kinetic in situ study
    Jendrzej, S. and Gökce, B. and Amendola, V. and Barcikowski, S.
    Journal of Colloid and Interface Science 463 (2016)
    Unintended post-synthesis growth of noble metal colloids caused by excess amounts of reactants or highly reactive atom clusters represents a fundamental problem in colloidal chemistry, affecting product stability or purity. Hence, quantified kinetics could allow defining nanoparticle size determination in dependence of the time. Here, we investigate in situ the growth kinetics of ps pulsed laser-fragmented platinum nanoparticles in presence of naked atom clusters in water without any influence of reducing agents or surfactants. The nanoparticle growth is investigated for platinum covering a time scale of minutes to 50 days after nanoparticle generation, it is also supplemented by results obtained from gold and palladium. Since a minimum atom cluster concentration is exceeded, a significant growth is determined by time resolved UV/Vis spectroscopy, analytical disc centrifugation, zeta potential measurement and transmission electron microscopy. We suggest a decrease of atom cluster concentration over time, since nanoparticles grow at the expense of atom clusters. The growth mechanism during early phase (<1. day) of laser-synthesized colloid is kinetically modeled by rapid barrierless coalescence. The prolonged slow nanoparticle growth is kinetically modeled by a combination of coalescence and Lifshitz-Slyozov-Wagner kinetic for Ostwald ripening, validated experimentally by the temperature dependence of Pt nanoparticle size and growth quenching by Iodide anions. © 2015.
    view abstract10.1016/j.jcis.2015.10.032
  • Continuous multigram nanoparticle synthesis by high-power, high-repetition-rate ultrafast laser ablation in liquids
    Streubel, R. and Barcikowski, S. and Gökce, B.
    Optics Letters 41 (2016)
    Utilizing a novel laser system consisting of a 500 W, 10 MHz, 3 ps laser source which is fully synchronized with a polygon scanner reaching scanning speeds up to 500 m/s, we explore the possibilities to increase the productivity of nanoparticle synthesis by laser ablation in liquids. By exploiting the high scanning speed, laser-induced cavitation bubbles are spatially bypassed at high repetition rates and continuous multigram ablation rates up to 4 g/h are demonstrated for platinum, gold, silver, aluminum, copper, and titanium. Furthermore, the applicable, ablation-effective repetition rate is increased by two orders ofmagnitude.The ultrafast ablation mechanisms are investigated for different laser fluences, repetition rates, interpulse distances, and ablation times, while the resulting trends are successfully described by validating a model developed for ultrafast laser ablation in air to hold in liquids as well. © 2016 Optical Society of America.
    view abstract10.1364/OL.41.001486
  • Debris-free rear-side picosecond laser ablation of thin germanium wafers in water with ethanol
    Zhang, D. and Gökce, B. and Sommer, S. and Streubel, R. and Barcikowski, S.
    Applied Surface Science 367 (2016)
    In this paper, we perform liquid-assisted picosecond laser cutting of 150 μm thin germanium wafers from the rear side. By investigating the cutting efficiency (the ability to allow an one-line cut-through) and quality (characterized by groove morphologies on both sides), the pros and cons of this technique under different conditions are clarified. Specifically, with laser fluence fixed, repetition rate and scanning speed are varied to show quality and efficiency control by means of laser parameter modulation. It is found that low repetition rate ablation in liquid gives rise to a better cut quality on the front side than high repetition rate ablation since it avoids dispersed nanoparticles redeposition resulting from a bubble collapse, unlike the case of 100 kHz which leads to large nanorings near the grooves resulting from a strong interaction of bubbles and the case of 50 kHz which leads to random cutting due to the interaction of the former pulse induced cavitation bubble and the subsequent laser pulse. Furthermore, ethanol is mixed with pure distilled water to assess the liquid's impact on the cutting efficiency and cutting quality. The results show that increasing the ethanol fraction decreases the ablation efficiency but simultaneously, greatly improves the cutting quality. The improvement of cut quality as ethanol ratio increases may be attributed to less laser beam interference by a lower density of bubbles which adhere near the cut kerf during ablation. A higher density of bubbles generated from ethanol vaporization during laser ablation in liquid will cause stronger bubble shielding effect toward the laser beam propagation and therefore result in less laser energy available for the cut, which is the main reason for the decrease of cut efficiency in water-ethanol mixtures. Our findings give an insight into under which condition the rear-side laser cutting of thin solar cells should be performed: high repetition, pure distilled water and high laser power are favorable for high-speed rough cutting but the cut kerf suffers from strong side effects of ripples, nanoredeposition occurrence, while low laser power at low repetition rate (10 kHz), mixed solution (1 wt% ethanol in water) and moderate scanning speed (100 μm/s) are preferable for ultrafine high-quality debris-free cutting. The feasibility of high-quality cut is a good indication of using rear laser ablation in liquid to cut thinner wafers. More importantly, this technique spares any post cleaning steps to reduce the risk to the contamination or crack of the thin wafers. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2016.01.071
  • Effect of various dispersing agents on the stability of silver microparticle dispersion and the formulation of uniform silver film by laser melting
    Letzel, A. and Maurer, E. and Meixner, M. and Poprawe, R. and Stollenwerk, J. and Hessner, S. and Lehmann, K. and Gökce, B. and Barcikowski, S.
    Journal of Laser Applications 28 (2016)
    We report on the substitution of silver nanoparticles' inks by silver microparticle dispersions as a material for the production of printable silver tracks by laser melting. This approach is promising, because it helps to reduce the production costs of such silver tracks. Though silver dispersions used as materials for laser melting mostly contain polyvinylpyrrolidone as a stabilizer, which results in the appearance of an undesired balling effect of silver during laser melting, the authors test stabilizers differing in molecular weight and functionality. The resulting differences in colloidal and physicochemical properties are investigated and related to the final silver layer quality. © 2016 Laser Institute of America.
    view abstract10.2351/1.4955011
  • Pilot-scale synthesis of metal nanoparticles by high-speed pulsed laser ablation in liquids
    Streubel, R. and Bendt, G. and Gökce, B.
    Nanotechnology 27 (2016)
    The synthesis of catalysis-relevant nanoparticles such as platinum and gold is demonstrated with productivities of 4 g h-1 for pulsed laser ablation in liquids (PLAL). The major drawback of low productivity of PLAL is overcome by utilizing a novel ultrafast high-repetition rate laser system combined with a polygon scanner that reaches scanning speeds up to 500 m s-1. This high scanning speed is exploited to spatially bypass the laser-induced cavitation bubbles at MHz-repetition rates resulting in an increase of the applicable, ablation-effective, repetition rate for PLAL by two orders of magnitude. The particle size, morphology and oxidation state of fully automated synthesized colloids are analyzed while the ablation mechanisms are studied for different laser fluences, repetition rates, interpulse distances, ablation times, volumetric flow rates and focus positions. It is found that at high scanning speeds and high repetition rate PLAL the ablation process is stable in crystallite size and decoupled from shielding and liquid effects that conventionally occur during low-speed PLAL. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/20/205602
  • Plasmon assisted 3D microstructuring of gold nanoparticle-doped polymers
    Jonušauskas, L. and Lau, M. and Gruber, P. and Gökce, B. and Barcikowski, S. and Malinauskas, M. and Ovsianikov, A.
    Nanotechnology 27 (2016)
    3D laser lithography of a negative photopolymer (zirconium/silicon hybrid solgel SZ2080) doped with gold nanoparticles (Au NPs) is performed with a 515 nm and 300 fs laser system and the effect of doping is explored. By varying the laser-generated Au NP doping concentration from 4.8 • 10-6 wt% to 9.8 • 10-3 wt% we find that the fabricated line widths are enlarged by up to 14.8% compared to structures achieved in pure SZ2080. While implicating a positive effect on the photosensitivity, the doping has no adverse impact on the mechanical quality of intricate 3D microstructures produced from the doped nanocompound. Additionally, we found that SZ2080 increases the long term (∼months) colloidal stability of Au NPs in isopropanol. By discussing the nanoparticle-light interaction in the 3D polymer structures we provide implications that our findings might have on other fields, such as biomedicine and photonics. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/27/15/154001
  • Target geometry and rigidity determines laser-induced cavitation bubble transport and nanoparticle productivity-a high-speed videography study
    Kohsakowski, S. and Gökce, B. and Tanabe, R. and Wagener, P. and Plech, A. and Ito, Y. and Barcikowski, S.
    Physical Chemistry Chemical Physics 18 (2016)
    Laser-induced cavitation has mostly been studied in bulk liquid or at a two-dimensional wall, although target shapes for the particle synthesis may strongly affect bubble dynamics and interfere with particle productivity. We investigated the dynamics of the cavitation bubble induced by pulsed-laser ablation in liquid for different target geometries with high-speed laser microsecond videography and focus on the collapse behaviour. This method enables us observations in a high time resolution (intervals of 1 μs) and single-pulse experiments. Further, we analyzed the nanoparticle productivity, the sizes of the synthesized nanoparticles and the evolution of the bubble volume for each different target shape and geometry. For the ablation of metal (Ag, Cu, Ni) wire tips a springboard-like behaviour after the first collapse is observed which can be correlated with vertical projectile motion. Its turbulent friction in the liquid causes a very efficient transport and movement of the bubble and ablated material into the bulk liquid and prevents particle redeposition. This effect is influenced by the degree of freedom of the wire as well as the material properties and dimensions, especially the Young's modulus. The most efficient and largest bubble movement away from the wire was observed for a thin (500 μm) silver wire with velocities up to 19.8 m s-1 and for materials with a small Young's modulus and flexural rigidity. We suggest that these observations may contribute to upscaling strategies and increase of particle yield towards large synthesis of colloids based on targets that may continuously be fed. © 2016 the Owner Societies.
    view abstract10.1039/c6cp01232a
  • Layered Seed-Growth of AgGe Football-like Microspheres via Precursor-Free Picosecond Laser Synthesis in Water
    Zhang, D. and Gökce, B. and Notthoff, C. and Barcikowski, S.
    Scientific Reports 5 (2015)
    Hybrid particles are of great significance in terms of their adjustable optical, electronic, magnetic, thermal and mechanical properties. As a novel technique, laser ablation in liquids (LAL) is famous for its precursor-free, "clean" synthesis of hybrid particles with various materials. Till now, almost all the LAL-generated particles originate from the nucleation-growth mechanism. Seed-growth of particles similar to chemical methods seems difficult to be achieved by LAL. Here, we not only present novel patch-joint football-like AgGe microspheres with a diameter in the range of 1 ~ 7 ìm achievable by laser ablation in distilled water but also find direct evidences of their layered seed growth mechanism. Many critical factors contribute to the formation of AgGe microspheres: fast laser-generated plasma process provide an excellent condition for generating large amount of Ge and Ag ions/atoms, their initial nucleation and galvanic replacement reaction, while cavitation bubble confinement plays an important role for the increase of AgGe nuclei and subsequent layered growth in water after bubble collapse. Driven by work function difference, Ge acts as nucleation agent for silver during alloy formation. This new seed-growth mechanism for LAL technique opens new opportunities to develop a large variety of novel hybrid materials with controllable properties.
    view abstract10.1038/srep13661
  • Ripening kinetics of laser-generated plasmonic nanoparticles in different solvents
    Gökce, B. and Van't Zand, D.D. and Menéndez-Manjón, A. and Barcikowski, S.
    Chemical Physics Letters 626 (2015)
    Abstract Pulsed laser ablation in liquid is considered to be a fast nanoparticle-synthesis method taking place on ps to μs timescale. Here, we report the comparably slow ripening kinetics of laser-generated plasmonic nanoparticles (copper, silver, and gold) immediately after ablation. The growth dynamics is studied in situ by following the surface plasmon resonance and correlated to known models. We thereby identify a two-step diffusion-controlled coalescence and growth mechanism, quantify their kinetic constants and show the effect of different solvents (water, acetone, ethanol, and ethyl acetate). © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.cplett.2015.03.010
  • Solid solution magnetic FeNi nanostrand-polymer composites by connecting-coarsening assembly
    Barcikowski, S. and Baranowski, T. and Durmus, Y. and Wiedwald, U. and Gökce, B.
    Journal of Materials Chemistry C 3 (2015)
    An approach to assemble high aspect ratio nanostrands consisting of magnetic nanowires and their incorporation in a polymer with the aim of tailoring transparent FeNi nanostrand-PMMA-composites is presented. These nanostrands are controllable in length (<600 μm) and width (<12 μm) via process parameters and have an ultra-high aspect ratio (∼160). This rapid and universal method provides flexible and transparent magnetic materials with tunable transparency and magnetic attraction force by adjusting the density of nanoparticles in the composite. These composites can be used as a window coating for shielding radio frequency electromagnetic waves while being transparent in the optical range. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5tc02160j
  • Femtosecond pulse shaping using the geometric phase
    Gökce, B. and Li, Y. and Escuti, M.J. and Gundogdu, K.
    Optics Letters 39 (2014)
    We demonstrate a femtosecond pulse shaper that utilizes polarization gratings to manipulate the geometric phase of an optical pulse. This unique approach enables circular polarization-dependent shaping of femtosecond pulses. As a result, it is possible to create coherent pulse pairs with orthogonal polarizations in a 4f pulse shaper setup, something until now that, to our knowledge, was only achieved via much more complex configurations. This approach could be used to greatly simplify and enhance the functionality of multidimensional spectroscopy and coherent control experiments, in which multiple coherent pulses are used to manipulate quantum states in materials of interest. © 2014 Optical Society of America.
    view abstract10.1364/OL.39.001521
  • Control of the oxidation kinetics of H-terminated (111)Si by using the carrier concentration and the strain: A second-harmonic-generation investigation
    Gökce, B. and Gundogdu, K. and Aspnes, D.E.
    Journal of the Korean Physical Society 60 (2012)
    We discuss recent results regarding the effects of strain, carrier type and concentration on the oxidation of H-terminated (111)Si. Second-harmonic-generation data show that this is a two-stage process where the H of the "up" bonds of the outermost Si layer is replaced by OH, followed by O insertion into the "back" bonds. These data provide additional detailed information about both stages. In particular, directional control of the in-plane surface chemistry by using the applied uniaxial stress provides new opportunities for interface control. © 2012 The Korean Physical Society.
    view abstract10.3938/jkps.60.1685
  • Effect of p-type doping on the oxidation of H-Si(111) studied by second-harmonic generation
    Gökce, B. and Dougherty, D.B. and Gundogdu, K.
    Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 30 (2012)
    Atomic force microscopy and second-harmonic generation data show that boron doping enhances the rate of oxidation of H-terminated silicon. Holes cause a greater increase in the reactivity of the Si-H up bonds than that of the Si-Si back bonds. © 2012 American Vacuum Society.
    view abstract10.1116/1.4721329
  • Back-reflection second-harmonic generation of (111)Si: Theory and experiment
    Gökce, B. and Gundogdu, K. and Adles, E.J. and Aspnes, D.E.
    Journal of the Korean Physical Society 58 (2011)
    We consider second-harmonic generation (SHG) from a (111) surface of a tetrahedrally bonded semiconductor illuminated at normal incidence by a focused pump beam of Gaussian cross section as a model of SHG by focused beams. Calculations are done in the anisotropic bond model (ABM) and the results are applied to Si. The unit-cell configuration is simple enough for the calculations to be done analytically, so the results can be compared directly to similar calculations done for amorphous material. Although the differences in unit-cell symmetry occur on the atomic scale, they lead to large differences in the spatial distribution of the emerging radiation. Lateral focusing, which might be expected to increase the bulk contribution to SHG by increasing the lateral field gradient, has little effect; the spatial-dispersion contribution remains dominated by the phase term. Focusing does not inhibit backscattered SHG from the bulk, although our data on the oxidation of H-terminated (111)Si clearly show that in some cases the interface contribution dominates by a wide margin.
    view abstract10.3938/jkps.58.1237
  • Bond-specific reaction kinetics during the oxidation of (111) Si: Effect of n-type doping
    Gökce, B. and Aspnes, D.E. and Lucovsky, G. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    It is known that a higher concentration of free carriers leads to a higher oxide growth rate in the thermal oxidation of silicon. However, the role of electrons and holes in oxidation chemistry is not clear. Here, we report real-time second-harmonic-generation data on the oxidation of H-terminated (111)Si that reveal that high concentrations of electrons increase the chemical reactivity of the outer-layer Si-Si back bonds relative to the Si-H up bonds. However, the thicknesses of the natural oxides of all samples stabilize near 1 nm at room temperature, regardless of the chemical kinetics of the different bonds. © 2011 American Institute of Physics.
    view abstract10.1063/1.3537809
  • Effect of strain on bond-specific reaction kinetics during the oxidation of H-terminated (111) Si
    Gökce, B. and Aspnes, D.E. and Gundogdu, K.
    Applied Physics Letters 98 (2011)
    Although strain is used in semiconductor technology for manipulating optical, electronic, and chemical properties of semiconductors, the understanding of the microscopic phenomena that are affected or influenced by strain is still incomplete. Second-harmonic generation data obtained during the air oxidation of H-terminated (111) Si reveal the effect of compressive strain on this chemical reaction. Even small amounts of strain manipulate the reaction kinetics of surface bonds significantly, with tensile strain enhancing oxidation and compressive strain retarding it. This dramatic change suggests a strain-driven charge transfer mechanism between Si-H up bonds and Si-Si back bonds in the outer layer of Si atoms. © 2011 American Institute of Physics.
    view abstract10.1063/1.3567528
  • Measurement and control of in-plane surface chemistry during oxidation of H-terminated (111)Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    AIP Conference Proceedings 1399 (2011)
    We demonstrate both directional control and measurement of the oxidation of H-terminated (111)Si. Control is achieved through externally applied strain, with strained back bonds oxidizing faster than unstrained ones. Real-time measurement is achieved by second-harmonic generation (SHG), with SHG anisotropy data analyzed with the anisotropic bond-charge model of nonlinear optics. Anisotropic oxidation also results in structural changes, which appear as rotations of the average orientations of the back bonds from their unperturbed directions. © 2011 American Institute of Physics.
    view abstract10.1063/1.3666321
  • Measurement and control of in-plane surface chemistry during the oxidation of H-terminated (111) Si
    Gökce, B. and Adles, E.J. and Aspnes, D.E. and Gundogdu, K.
    Proceedings of the National Academy of Sciences of the United States of America 107 (2010)
    In-plane directional control of surface chemistry during interface formation can lead to new opportunities regarding device structures and applications. Control of this type requires techniques that can probe and hence provide feedback on the chemical reactivity of bonds not only in specific directions but also in real time. Here, we demonstrate both control and measurement of the oxidation of H-terminated (111) Si. Control is achieved by externally applying uniaxial strain, and measurement by second-harmonic generation (SHG) together with the anisotropic-bond model of nonlinear optics. In this system anisotropy results because bonds in the strain direction oxidize faster than those perpendicular to it, leading in addition to transient structural changes that can also be detected at the bond level by SHG.
    view abstract10.1073/pnas.1011295107
  • alloys

  • electronics

  • laser ablation

  • lasers

  • magnetism

  • nanoparticles

  • nanoparticles

  • optics

  • photocatalysis

  • pulsed lasers

  • synthesis

  • tribology

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