Dr. Jörg Debus
Experimental Physics II
TU Dortmund University
- Design of Cu- and Ag-containing amorphous carbon multilayers with improved tribo-mechanical properties
Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Matveev, S. and Arne Thomann, C. and Debus, J.
Materials Letters 284 (2021)The modification of amorphous carbon (a–C) films by adding either Cu or Ag is a common approach to tailor the film properties. These films become less hard, while they demonstrate higher friction and lower wear resistance than a–C. To enhance tribologically relevant features, multilayers of alternating a–C and a–C:Cu or a–C:Ag layers are synthetized by magnetron sputtering. The a–C/a–C:Cu and a–C/a–C:Ag multilayers possess a bilayer period of ~200 nm, a layer ratio of 1, and a bilayer number of 5. These structures are characterized by higher hardness and lower friction and wear against 100Cr6 counterparts as compared to monolayered a–C:Cu and a–C:Ag. © 2020 Elsevier B.V.
view abstract 10.1016/j.matlet.2020.128905
- Investigations of electron-electron and interlayer electron-phonon coupling in van der Waals hBN/WSe2/hBN heterostructures by photoluminescence excitation experiments
Jadczak, J. and Kutrowska-Girzycka, J. and Schindler, J.J. and Debus, J. and Watanabe, K. and Taniguchi, T. and Ho, C.-H. and Bryja, L.
Materials 14 (2021)Monolayers of transition metal dichalcogenides (TMDs) with their unique physical properties are very promising for future applications in novel electronic devices. In TMDs monolayers, strong and opposite spin splittings of the energy gaps at the K points allow for exciting carriers with various combinations of valley and spin indices using circularly polarized light, which can further be used in spintronics and valleytronics. The physical properties of van der Waals heterostructures composed of TMDs monolayers and hexagonal boron nitride (hBN) layers significantly depend on different kinds of interactions. Here, we report on observing both a strong increase in the emission intensity as well as a preservation of the helicity of the excitation light in the emission from hBN/WSe2/hBN heterostructures related to interlayer electron-phonon coupling. In combined low-temperature (T = 7 K) reflectivity contrast and photoluminescence excitation experiments, we find that the increase in the emission intensity is attributed to a double resonance, where the laser excitation and the combined Raman mode A′ 1 (WSe2) + ZO (hBN) are in resonance with the excited (2s) and ground (1s) states of the A exciton in a WSe2 monolayer. In reference to the 2s state, our interpretation is in contrast with previous reports, in which this state has been attributed to the hybrid exciton state existing only in the hBN-encapsulated WSe2 monolayer. Moreover, we observe that the electron-phonon coupling also enhances the helicity preservation of the exciting light in the emission of all observed excitonic complexes. The highest helicity preservation of more than 60% is obtained in the emission of the neutral biexciton and negatively charged exciton (trion) in its triplet state. Additionally, to the best of our knowledge, the strongly intensified emission of the neutral biexciton XX0 at double resonance condition is observed for the first time. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
view abstract 10.3390/ma14020399
- Mechanical properties and adhesion behavior of amorphous carbon films with bias voltage controlled TixCy interlayers on Ti6Al4V
Tillmann, W. and Lopes Dias, N.F. and Franke, C. and Kokalj, D. and Stangier, D. and Thomann, C.A. and Debus, J.
Diamond and Related Materials 115 (2021)Amorphous carbon is a promising functional film material to enhance the surface properties of Ti-based alloys for orthopaedic applications. However, high adhesion of the amorphous carbon film on the orthopaedic implants is essential to fully exploit its potential under high load bearings. Interlayer films are generally employed to improve the adhesion. The applied bias voltage is a decisive deposition parameter and significantly influences the structure and mechanical properties during the interlayer growth, which in turn affect the properties of the amorphous carbon film. Therefore, chemically graded titanium carbide (TixCy) interlayers were deposited using bias voltages of −50, −100, and −150 V with a subsequent hydrogen-free amorphous carbon (a-C) top layer on Ti6Al4V by magnetron sputtering. The mechanical properties and adhesion behavior of single TixCy interlayers were evaluated to analyze the interaction effect of TixCy on bilayered TixCy/a-C structures. A high bias voltage generates dense TixCy of a more disordered and defected structure with high stresses, high hardness of ~16 GPa, and high elastic modulus of ~170 GPa. However, high compressive stresses provoke a low adhesion strength, while low compressive stresses ensure a good adhesion behavior of TixCy. Highly stressed TixCy interlayers lead to overall higher stresses for the entire TixCy/a-C film. Independently of TixCy, the a-C top layer exhibits hardness and elastic modulus values of ~16 and ~160 GPa, respectively. The TixCy/a-C films with TixCy interlayers deposited at high bias voltages possess a low adhesion strength, while a lower bias voltage favors a good adhesion of TixCy/a-C on Ti6Al4V. Therefore, a moderate bias voltage is crucial to deposit lowly stressed TixCy interlayers, which ensure a high adhesion of TixCy/a-C on Ti6Al4V. Consequently, the bias voltage allows controlling the mechanical properties and adhesion behavior of the interlayer and, hence, the adhesion strength of the entire amorphous carbon film structure on Ti-based alloys for orthopaedic applications. © 2021 Elsevier B.V.
view abstract 10.1016/j.diamond.2021.108361
- Probing negatively charged and neutral excitons in MoS2/hBN and hBN/MoS2/hBN van der Waals heterostructures
Jadczak, J. and Kutrowska-Girzycka, J. and Bieniek, M. and Kazimierczuk, T. and Kossacki, P. and Schindler, J.J. and Debus, J. and Watanabe, K. and Taniguchi, T. and Ho, C.H. and Wójs, A. and Hawrylak, P. and Bryja, L.
Nanotechnology 32 (2021)High-quality van der Waals heterostructures assembled from hBN-encapsulated monolayer transition metal dichalcogenides enable observations of subtle optical and spin-valley properties whose identification was beyond the reach of structures exfoliated directly on standard SiO2/Si substrates. Here, we describe different van der Waals heterostructures based on uncapped singlelayer MoS2stacked onto hBN layers of different thicknesses and hBN-encapsulated monolayers. Depending on the doping level, they reveal the fine structure of excitonic complexes, i.e. neutral and charged excitons. In the emission spectra of a particular MoS2/hBN heterostructure without an hBN cap we resolve two trion peaks, T1and T2, energetically split by about 10 meV, resembling the pair of singlet and triplet trion peaks (TSand TT) in tungsten-based materials. The existence of these trion features suggests that monolayer MoS2has a dark excitonic ground state, despite having a 'bright' single-particle arrangement of spin-polarized conduction bands. In addition, we show that the effective excitonic g-factor significantly depends on the electron concentration and reaches the lowest value of -2.47 for hBN-encapsulated structures, which reveals a nearly neutral doping regime. In the uncapped MoS2structures, the excitonic g-factor varies from -1.15 to -1.39 depending on the thickness of the bottom hBN layer and decreases as a function of rising temperature. © 2021 Institute of Physics Publishing. All rights reserved.
view abstract 10.1088/1361-6528/abd507
- Residual stresses and tribomechanical behaviour of TiAlN and TiAlCN monolayer and multilayer coatings by DCMS and HiPIMS
Tillmann, W. and Grisales, D. and Stangier, D. and Thomann, C.-A. and Debus, J. and Nienhaus, A. and Apel, D.
Surface and Coatings Technology 406 (2021)The deposition of ternary nitrides with the incorporation of carbon atoms into its structure has demonstrated to be a promising approach in the pursuit of wear-resistant and self-lubricating coatings. Firstly, both TiAlN and TiAlCN monolayers were deposited using direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) onto quenched and tempered AISI H11 tool steel to be used as references. Acetylene was used as a carbon precursor, producing DCMS and HiPIMS TiAlCN coatings with 9.0 and 21.7 at.% C, respectively. Subsequently, TiAlN/TiAlCN multilayers of various designs were also developed as follows: 5×[10/500], 5×[50/500] and 5×[100/500] nm. Residual stresses of the coating systems were determined by X-ray radiation utilising an ETA-diffractometer with a Cu-Kα radiation source applying the sin2ψ method. Additionally, residual stresses depth gradients of the substrate before and after the deposition of the coatings were determined in a LEDDI 8-circle diffractometer equipped with a W-X-ray tube and operated in the energy-dispersive mode of diffraction. Great reduction of the compressive residual stresses in the coatings was observed after the introduction of carbon into the TiAlN coating structure, shifting from −1047 ± 149 to −307 ± 211 MPa for the DCMS and from −7035 ± 1361 to +989 ± 187 MPa for the HiPIMS coatings. In the multilayer coatings, compressive residual stresses increase along with the increment of the TiAlN interlayer. Additionally, residual stresses of the substrate in the near-surface are dragged from low compressive stresses (−218 ± 61) to tensile stresses in the range of 1000 to 2000 MPa for all the DCMS/substrate systems, a behaviour only presented in HiPIMS by the TiAlN monolayer. Wear coefficients of all the evaluated HiPIMS systems are notoriously lower than their DCMS counterparts. Compared to TiAlN, TiAlCN HiPIMS presented a lower coefficient of friction but a higher wear coefficient, which in turn was not reduced by the introduction of the multilayer systems. Finally, Scratch test and Rockwell C adhesion tests have shown higher adhesion of DCMS coatings than HiPIMS coatings, and a detriment of the monolayers adhesion by the implementation of TiAlN/TiAlCN multilayer systems. The understanding of the residual stresses, both in the coating and in the substrate, and the way they affect the tribomechanical performance of the system coating/substrate continues to be of great importance, especially for coatings deposited by new technologies such as HiPIMS and self-lubricating coatings. © 2020 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2020.126664
- Temperature-dependent tribological behavior of MoSx thin films synthesized by HiPIMS
Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
Tribology International 153 (2021)Understanding the interaction between the structure and the tribological properties of sputtered molybdenum disulfide films at elevated temperatures is essential for their use in industrial applications. Therefore, the friction and wear behavior up to of 400°C of one stoichiometric MoS2 and a sub-stoichiometric MoS1.6 film are investigated against 100Cr6 counterparts. With an increasing temperature up to 200°C, the friction decreases, which is attributed to a thermally activated water desorption and an increasing intensity of the (002) basal plane. Due to a passivation mechanism caused by the sulfur defect sites, the friction is lower for the sub-stoichiometric film. Above this temperature the friction increases for both films and failure occurs at 400°C. Therefore, the friction at elevated temperatures result from a complex interaction of re-orientation mechanisms, desorption and oxidation processes. © 2020 Elsevier Ltd
view abstract 10.1016/j.triboint.2020.106655
- Tribo-mechanical properties and biocompatibility of Ag-containing amorphous carbon films deposited onto Ti6Al4V
Tillmann, W. and Lopes Dias, N.F. and Franke, C. and Kokalj, D. and Stangier, D. and Filor, V. and Mateus-Vargas, R.H. and Oltmanns, H. and Kietzmann, M. and Meißner, J. and Hein, M. and Pramanik, S. and Hoyer, K.-P. and Schaper, M. and Nienhaus, A. and Thomann, C.A. and Debus, J.
Surface and Coatings Technology 421 (2021)The addition of Ag to amorphous carbon (a-C) films is highly effective in tailoring the tribo-mechanical properties and biocompatibility. For biomedical applications, Ag-containing a-C (a-C:Ag) represents a promising film material for improving the biofunctional surface properties of Ti-based alloys. In a sputtering process, a-C:Ag films, with Ag contents up to 7.5 at.%, were deposited with a chemically graded TixCy interlayer onto Ti6Al4V. The tribo-mechanical and biocompatible properties of a-C:Ag were evaluated. The influence of the Ag content on these properties was analyzed and compared to those of uncoated Ti6Al4V. Raman spectroscopy reveals that the amount of incorporated Ag does not induce significant structural changes in the disordered network, only a reduced number of vacancies and sp3-coordinated C bonds within the sp2-dominant a-C network is assigned to the films with high Ag concentration. With increasing Ag content, stresses, hardness, and elastic modulus decrease from (2.02 ± 0.07) to (1.15 ± 0.03) GPa, from (17.4 ± 1.5) to (13.4 ± 0.9) GPa, and from (171.8 ± 8.1) to (138.5 ± 5.8) GPa, respectively. In tribometer tests, the friction behavior against Al2O3 in lubricated condition with a simulated-body-fluid-based lubricant is not affected by the Ag concentration, but the Al2O3 counterpart wear is reduced for all a-C:Ag films compared to a-C. The friction against ultra-high-molecular-weight polyethylene (UHMWPE) decreases continuously with increasing Ag concentration and the counterpart wear is lower at higher Ag contents. Compared to a-C:Ag, Ti6Al4V demonstrates lower friction against UHMWPE and higher friction against Al2O3. The a-C:Ag films are not exposed to abrasion by Al2O3 or pronounced material transfer of UHMWPE. The hardness difference and chemical affinity between the friction partners are decisive for the tribological behavior of a-C:Ag. Compared to Ti6Al4V, the a-C:Ag films show antibacterial activity against Staphylococcus aureus, while the proliferation of osteoblast-like cells is reduced by Ag. © 2021 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2021.127384
- Asymmetric spin transitions of nonthermalized Mn2+ ions in (Zn,Mn)Se-based quantum wells
Kudlacik, D. and Kavokin, K.V. and Lüders, C. and Barthelmi, K. and Schindler, J.J. and Moldenhauer, H. and Waldkirch, P. and Sapega, V.F. and Yakovlev, D.R. and Waag, A. and Bayer, M. and Debus, J.
Physical Review B 101 (2020)In Zn1-xMnxSe/(Zn,Be)Se quantum wells with x<0.035, nonthermalized Mn2+ ions demonstrate in spin-flip scattering spectra multiple Stokes and anti-Stokes transitions whose absolute energies deviate by up to 20% from each other. This asymmetry is tuned significantly by the optical power density, magnetic field direction, and Mn ion concentration. The nonequidistant Mn2+ spin transitions are modeled by the Zeeman splitting and quadrupolar crystal-field components taking values of up to 7 GHz. We suggest that nonequilibrium carriers dynamically polarize the Mn-ion spins so that they occupy levels with positive and negative spin projection numbers giving rise to asymmetric spin transitions. © 2020 American Physical Society.
view abstract 10.1103/PhysRevB.101.155432
- Controlling the structural, mechanical and frictional properties of MoSx coatings by high-power impulse magnetron sputtering
Kokalj, D. and Debus, J. and Stangier, D. and Moldenhauer, H. and Nikolov, A. and Wittig, A. and Brümmer, A. and Tillmann, W.
Coatings 10 (2020)Tribology, as the science and technology of interacting surfaces, typically relies on liquid lubricants which reduce friction and wear. For environmentally friendly tribological purposes and applications requiring a liquid-free performance, solid lubricants, such as MoS2 coatings, play an essential role. It is crucial to understand the interplay between the parameters of the coating synthesis and the characteristics of the coating. The impact of the deposition parameters on the structural, mechanical and frictional properties of MoSx thin films, which are synthesized by high-power impulse magnetron sputtering, are studied. The morphology, topography and stoichiometry (2.02 < x < 2.22) of the films are controlled by, in particular, the bias-voltage and heating power applied during the sputtering process. In combination with a low pulse frequency the hardness and elastic stiffness of the MoSx films are enhanced up to 2 and 90 GPa, respectively. This enhancement is assigned to a shortening of the Mo-S bonding lengths and a strengthening in the interatomic coupling as well as to a formation of small-sized crystallites at the surface. The friction coefficient reduces to μ = 0.10 for films with an initial (100) orientation and the mean roughness of the MoSx films decreases below 15 nm by shortening the cathode pulses. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. All right reserved.
view abstract 10.3390/COATINGS10080755
- Effect of the bias voltage on the structural and tribo-mechanical properties of Ag-containing amorphous carbon films
Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Nienhaus, A. and Thomann, C.A. and Wittrock, A. and Moldenhauer, H. and Debus, J.
Diamond and Related Materials 105 (2020)The modification of amorphous carbon films by a mixture of Ag atoms is a promising approach to reduce the residual stresses in the coating and to improve its adhesion to the substrate. Besides the Ag concentration, the bias voltage has a crucial impact on the properties of carbon-based films. Therefore, the effect of the bias voltage on the structural and tribo-mechanical properties of hydrogen free a-C:Ag is investigated. The a-C:Ag films are sputtered from graphite targets with varying number of Ag pellets by setting the bias voltage to −100, −150, and −200 V. A non-modified a-C and two a-C:Ag film systems with different Ag content are synthetized to obtain a comprehensive understanding about the influence of the bias voltage on the properties of the a-C:Ag films. A high bias voltage leads to a reduction in the amount of Ag within the a-C:Ag films, since impinging ions remove Ag atoms during the film growth. Additionally, XRD analyses show the formation of large Ag nanocrystallites with rising bias voltage. In Raman scattering studies, an Ag-induced graphitization of the a-C films is identified. The graphitization is less pronounced at low Ag concentrations and high bias voltages. The residual stresses increase with rising bias voltage and decreasing Ag content, which also favor greater values of hardness and elastic modulus. While a high bias voltage results in a poor adhesion strength for the a-C films, a good adhesion behavior is observed for the a-C:Ag films. It is ascribed to lower stresses in the a-C:Ag films as compared to that in a-C. The friction behavior of the a-C:Ag films is not influenced by the bias voltage, since the coefficients of friction vary from 0.26 to 0.32 against a steel counterpart in tribometer tests. An agglomeration of Ag particles in the tribological contact is observed for all a-C:Ag films which contributes to the slightly higher friction when compared to non-modified a-C films. On the whole, it is demonstrated that the tribo-mechanical properties of a-C:Ag are not only affected by the Ag content, but also by the applied bias voltage. © 2020 Elsevier B.V.
view abstract 10.1016/j.diamond.2020.107803
- Effects of acetylene flow rate and bias voltage on the structural and tribo-mechanical properties of sputtered a-C:H films
Tillmann, W. and Ulitzka, H. and Lopes Dias, N.F. and Stangier, D. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
Thin Solid Films 693 (2020)The properties of sputtered a-C:H films are significantly influenced by the C2H2 flow rate and bias voltage. A suitable Design of Experiments allows to consider their effects on the mechanical and tribological properties. The a-C:H films are deposited by varying the C2H2 flow rate from 5.9 to 34.1 sccm and the bias voltage from −83 to −197 V, following the Central Composite Design. In Raman scattering studies, the presence of C[sbnd]H bands with increasing C2H2 flow rate is identified. Additionally, a decrease of the I(D)/I(G) ratio is observed with increasing C2H2 flow rate. Both observations indicate the formation of sp³-hybridized C[sbnd]H bonds. In contrast, a low C2H2 flow rate and a high bias voltage result in a higher I(D)/I(G) ratio and a lower intensity of the C[sbnd]H stretching bands, indicating a lower amount of C[sbnd]H bonds. The mechanical properties are also considerably influenced by these parameters. A higher C2H2 proportion results in a lower hardness and elastic modulus, which are related to a higher H content. However, a higher bias voltage increases the hardness and elastic modulus due to densification mechanisms, which increase the degree of distortion of the a-C:H films. Consequently, a low C2H2 flow rate and a high bias voltage ensure a high hardness of up to ~24 GPa due to a lower amount of C[sbnd]H bonds and a higher degree of distortion. In tribometer tests, most a-C:H films exhibit a low coefficient of friction against steel, ranging from 0.23 to 0.25. All a-C:H films are marked by a deformative wear, indicating a high resistance against abrasive wear when sliding against steel. © 2019 Elsevier B.V.
view abstract 10.1016/j.tsf.2019.137691
- Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2 O4
Kudlacik, D. and Ivanov, V.Y. and Yakovlev, D.R. and Sapega, V.F. and Schindler, J.J. and Debus, J. and Bayer, M. and Pisarev, R.V.
Physical Review B 102 (2020)Copper metaborate CuB2O4 crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below TN1=20 K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic Cu2+ (3d9, S=1/2) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the Cu2+ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split d-d electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range 1.7-40 K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets. © 2020 American Physical Society.
view abstract 10.1103/PhysRevB.102.035128
- Influence of the bias-voltage, the argon pressure and the heating power on the structure and the tribological properties of HiPIMS sputtered MoSx films
Tillmann, W. and Wittig, A. and Stangier, D. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
Surface and Coatings Technology 385 (2020)Understanding the growth process and its correlation to the structure of MoSx thin films is essential to control the friction behavior. Nevertheless, structural changes related to kinetic and thermal processes occurring during the deposition are not yet fully understood within the context of MoSx sputtered thin films. Therefore, MoSx films were synthesized by HiPIMS (High Power Impulse Magnetron Sputtering) technique using the one factor at a time method. By systematically changing the bias-voltage (0 to −200 V), the argon pressure (200 mPa to 600 mPa) or the heating power (0 to 3000 W) the interaction between the deposition parameters and their impact on the structure and the tribological properties was analyzed.The results show significant differences regarding the influence of kinetic and thermal effects. The investigation of the crystallographic orientation by XRD measurements reveals that a high kinetic energy induced by a high bias-voltage favors the growth of the (100) edge plane. A deposition process with a low deposition temperature and thus a low deposition rate leads to a more pronounced (002) basal plane due to the lower surface energy of the (002) surface. A high kinetic energy is also related to a densification of the morphology and a decrease in the sulfur content, which results in a thicker tribofilm and thus a lower wear and friction. Films deposited with a high heating power on the other show a low friction, but at the same time a columnar microstructure and high wear. Thus, the structure affects the amount of generated wear particles during the sliding, but more important is the ability of keeping them in the contact area during the tribo-tests. © 2020
view abstract 10.1016/j.surfcoat.2020.125358
- Nitrogen doping of MoSx thin films sputtered by reactive High Power Impulse Magnetron Sputtering
Tillmann, W. and Wittig, A. and Moldenhauer, H. and Thomann, C.-A. and Debus, J. and Aurich, D. and Bruemmer, A.
Thin Solid Films 713 (2020)Incorporating nitrogen into non-stoichiometric molybdenum disulfide (MoSx) thin films is a promising approach in order to improve the mechanical properties. Nevertheless, the adhesion between the film and the substrate is still challenging and the interaction between the mechanical and the tribological properties is not fully understood yet. Subsequently, reactive High Power Impulse Magnetron Sputtering (HiPIMS) is used to deposit nitrogen doped MoSx thin films with different nitrogen amounts on 16MnCr5 steel. The interaction between the structural changes, the mechanical properties and the tribological behavior depending on the nitrogen amount is investigated. The results prove that an increasing amount of nitrogen significantly affects the structure and the tribo-mechanical properties of the thin films. X-ray diffraction analysis reveals a transformation from crystalline to amorphous with an increasing amount of nitrogen from (7.1 ± 0.3) at.-% to (19.5 ± 0.5) at.-%. This transformation is related to a suppression of the columnar microstructure as well as an increasing hardness and Young‘s modulus from (0.14 ± 0.02) GPa, and (5.28 ± 0.32) GPa for the undoped film, to (5.12 ± 0.32) GPa and (92.5 ± 6.2) GPa, for the film with the highest nitrogen amount. The results of the Rockwell indentation tests show that the films with a small amount of nitrogen exhibit an improved adhesion behavior. The wear coefficient can be reduced to a quarter of the value of the undoped MoSx film, whereas coefficients of friction are at similar level of 0.2 in ambient air. Reactive HiPIMS has proven to be promising to deposit nitrogen doped MoSx thin films on steel substrates, which reveal improved mechanical properties and an excellent transfer film built-up during the tribo-tests without failures. © 2020 Elsevier B.V.
view abstract 10.1016/j.tsf.2020.138267
- Improved adhesion of a-C and a-C:H films with a CrC interlayer on 16MnCr5 by HiPIMS-pretreatment
Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Maus-Friedrichs, W. and Gustus, R. and Thomann, C.A. and Moldenhauer, H. and Debus, J.
Surface and Coatings Technology 375 (2019)Ensuring a high adhesion of amorphous carbon films to steel substrates remains a challenging task, sustaining continuous research efforts to improve the adhesion strength. Besides the interlayer system and the substrate material, surface pretreatments have a significant impact on the adhesion behavior. Within this context, the influence of the High Power Impulse Magnetron Sputtering (HiPIMS) pretreatment on the adhesion of magnetron sputtered hydrogenfree (a-C) and hydrogenated (a-C:H) amorphous carbon films with a chromium carbide (CrC) interlayer on 16MnCr5 steel is investigated. The plasma treatment consisted of 30 min Ar ion etching as well as a sequential 5 min of HiPIMS-pretreatment with a Cr cathode. Subsequently this pretreatment was compared to a procedure without utilizing the HiPIMS technique. The impact of the HiPIMS-pretreatment on the structure of the film was systematically analyzed by taking the CrC interlayer as well as the entire film structure into consideration. The adhesion strength of the a-C and a-C:H films is significantly improved by the formation of a Cr HiPIMS-nanolayer in the substrate/film interface. In scratch tests, the critical load Lc3 for a total film delamination increases from 43 ± 4 to 59 ± 3 N and from 48 ± 2 to 64 ± 3 N for the a-C and a-C:H film. The improved adhesion behavior of the carbon films is ascribed to the increased adhesion of the CrC interlayer, which did not delaminate when scratched with a load up to 159 ± 18 N. Complementary Rockwell indentation tests reveal that the HiPIMS-pretreatment improves the adhesion class from HF6 to HF4 and from HF5 to HF3 for a-C and a-C:H. The enhanced adhesion is essential to exploit the properties of a-C and a-C:H films in applications with high loads. In conclusion, the HiPIMS-pretreatment has proven to be a promising technique to increase the adhesion strength of carbon films. © 2019 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2019.07.076
- Interaction effects of cathode power, bias voltage, and mid-frequency on the structural and mechanical properties of sputtered amorphous carbon films
Tillmann, W. and Lopes Dias, N.F. and Stangier, D. and Bayer, M. and Moldenhauer, H. and Debus, J. and Schmitz, M. and Berges, U. and Westphal, C.
Applied Surface Science 487 (2019)The Design of Experiments is a promising method to investigate the cause-effect relation between the mid-frequency magnetron sputtering parameters on the structural and mechanical properties of amorphous carbon (a-C) films. Based on the Central Composite Design, the cathode power of two graphite targets, bias voltage, and mid-frequency were simultaneously varied from 1500 to 4000 W, −100 to −200 V, and 20 to 50 kHz, respectively. The chemical bonding state was characterized using UV and visible Raman spectroscopy with excitation wavelengths of 266 and 532 nm. Corresponding measurements were performed by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Additionally, hardness and elastic modulus of the sputtered a-C films were determined in nanoindentation tests. Multi-wavelength Raman spectroscopy identified an sp3 content below 20%, with most a-C films having an sp3 value in the range of 12 to 18%. The formation of sp3 bonded atoms is negatively influenced by a high cathode power and bias voltage, whereas the highest sp3 content is obtained with a-C films sputtered with a cathode power and bias voltage of 2750 W and −150 V. However, higher values of the cathode power and bias voltage result in a film delamination and decrease of the sp3 concentration. The bonding state affects the mechanical properties, as high hardness and elastic modulus result from a high sp3 content. Therefore, a targeted adjustment of cathode power and bias voltage is necessary to obtain a-C films with a high hardness. In contrast, the mid-frequency does not have a significant impact on the mechanical properties. In conclusion, the Central Composite Design has proven to be a suitable method to investigate the cause-effects of the sputtering parameters on the properties of the a-C film. © 2019
view abstract 10.1016/j.apsusc.2019.05.131
- Investigation of the tribofilm formation of HiPIMS sputtered MoSx thin films in different environments by Raman scattering
Tillmann, W. and Wittig, A. and Stangier, D. and Thomann, C.-A. and Moldenhauer, H. and Debus, J. and Aurich, D. and Brümmer, A.
Lubricants 7 (2019)Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoSx thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoSx films, the environment and the conditions during the Ball-on-disk tests affect tribofilms and thus the friction. Therefore, the influence of the surface pressure and sliding velocity in air, argon and nitrogen environments on the generation of the third body particles and the tribofilm formation of randomly oriented MoSx films is investigated. A high surface pressure is one major factor to achieve low friction, especially under humid conditions, which is important considering the use in industrial applications, for example dry-running screw machines. However, the mechanisms leading to that frictional behavior are still affected by the surrounding environment. While low friction is caused by a more extensive tribofilm formation in air, in argon and nitrogen, large size third body particles dispensed all over the contact area contribute to a lower friction. Raman scattering reveal a different chemistry of these particles reflected in the absence of laser- or temperature-induced surface oxidation compared to the as-deposited film and the wear track. The Raman scattering results are discussed with respect to the wear particle size, its chemical reactivity and strain-induced bonding changes. © 2019 by the authors.
view abstract 10.3390/lubricants7110100
- Porous purple glass-a cobalt imidazolate glass with accessible porosity from a meltable cobalt imidazolate framework
Frentzel-Beyme, L. and Kloß, M. and Pallach, R. and Salamon, S. and Moldenhauer, H. and Landers, J. and Wende, H. and Debus, J. and Henke, S.
Journal of Materials Chemistry A 7 (2019)We report the first microporous cobalt imidazolate glass obtained from a meltable cobalt-based zeolitic imidazolate framework, ZIF-62(Co). Crystalline ZIF-62(Co) is constructed from Co2+ cations and two different imidazolate-type linkers, namely conventional imidazolate and benzimidazolate. The microporous framework melts at ∼430 °C and converts into a glass upon cooling to room temperature. X-Ray total scattering and Raman spectroscopy reveal that the local structure of the glass and the crystalline parent material are very similar. Magnetic measurements and X-ray diffraction uncover that ZIF-62(Co) partially decomposes upon melting and glass formation resulting in the reduction of ∼3% of the Co2+ ions to metallic cobalt. Most importantly, the ZIF glass retains almost 50% of the porosity of crystalline ZIF-62(Co). Our results pave the way for the realisation of metal-organic framework glasses containing open shell metal ions, as well as the application of these porous glasses in gas separation, energy storage and catalysis. © 2019 The Royal Society of Chemistry.
view abstract 10.1039/c8ta08016j
- Resonant Raman scattering characterization of thermally annealed HiPIMS deposited MoSx coatings
Moldenhauer, H. and Wittig, A. and Kokalj, D. and Stangier, D. and Brümmer, A. and Tillmann, W. and Debus, J.
Surface and Coatings Technology 377 (2019)Raman spectroscopy is used to investigate the structural and tribological properties of HiPIMS sputtered MoSx thin films which were post-growth-annealed at different temperatures. The Raman scattering combined with X-ray diffraction determines a reduction in the residual strain within the MoSx layers with increasing annealing temperature. In the high-temperature annealed coatings a Raman signature at 40 cm−1 emerges, which results from a strengthening of the inter-layer van-der-Waals interaction. This observation indicates that the thermally annealed MoSx thin films become more resistant against shear forces, which is manifested in an increase of the coefficient of friction measured with a ball-on-disc tribometer. The coefficient of friction moreover decreases with lowering the sulfur/molybdenum ratio which, in turn, depends on the substrate and annealing temperatures. Furthermore, a Raman forbidden mode may be exploited to detect stacking faults within the sputtered coatings. Its observation is realized through resonant excitation of an MoS2 exciton at about 633 nm. © 2019 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2019.124891
- Structural and mechanical properties of carbon incorporation in DC/HiPIMS CrAlN coatings
Tillmann, W. and Stangier, D. and Roese, P. and Shamout, K. and Berges, U. and Westphal, C. and Debus, J.
Surface and Coatings Technology 374 (2019)Incorporating carbon into ternary nitride coatings to tune the mechanical and tribological properties of thin films is of great interest in order to improve the performance of tools and components. Especially, the approach to tailor CrAlN coatings by doping transition metals has been extensively studied in recent years. Nevertheless, the microstructural changes, induced by carbon incorporation, especially into Al-rich CrAlN coatings, are not yet fully understood. Thus, detailed investigations of the microstructure, performed by means of synchrotron radiation, using x-ray diffraction and x-ray photoelectron spectroscopy and Raman scattering with different laser excitation (355 nm and 532 nm), were conducted to understand the evolution of the mechanical properties of CrAlCN coatings depending on the carbon content. The results prove that an increasing carbon content significantly influences the microstructure, residual stresses, as well as the mechanical properties of the coatings. The presence of C[dbnd]C and C[dbnd]N bonds was proven by investigating the C 1s orbital. Furthermore, the increasing amount of carbon forms amorphous Cr[sbnd]C structures, which were detected by analyzing the Cr 3p orbital. These results were confirmed for the amorphous phases by Raman scattering additionally indicating the formation of nanocomposite structures due to the formation of carbon nano-onoin like structures. The investigations of the crystalline structure using XRD reveal the existence of a fcc structure for the CrAlN phase as well as small amounts of hexagonal AlN in the CrAlCN coating with the highest carbon content. © 2019 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2019.06.053
- 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 abstract 10.1016/j.apsusc.2018.10.060
- Basic Requirements of Spin-Flip Raman Scattering on Excitonic Resonances and Its Modulation through Additional High-Energy Illumination in Semiconductor Heterostructures
Debus, J. and Kudlacik, D. and Sapega, V.F. and Shamirzaev, T.S. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Waag, A. and Bayer, M.
Physics of the Solid State 60 (2018)We describe the major requirements to experimentally perform and observe resonant spin-flip Raman scattering on excitonic resonances in low-dimensional semiconductors. We characterize in detail the properties of this resonant light scattering technique and evaluate the criteria, which must be fulfilled by the experimental setup and the semiconductor sample studied to be able to observe a spin-flip scattering process. We also demonstrate the influence of additional unpolarized laser illumination with energies, which exceed considerably the band gap energy of the semiconductor nanostructure under study, on the resonantly excited electron spin-flip scattering in InAs-based quantum dot ensembles as well as on the paramagnetic Mn-ion spin-flip in (Zn,Mn)Se/(Zn,Be)Se quantum wells. © 2018, Pleiades Publishing, Ltd.
view abstract 10.1134/S1063783418080036
- Circularly polarized zero-phonon transitions of vacancies in diamond at high magnetic fields
Braukmann, D. and Glaser, E.R. and Kennedy, T.A. and Bayer, M. and Debus, J.
Physical Review B 97 (2018)We study the circularly polarized photoluminescence of negatively charged (NV-) and neutral (NV0) nitrogen-vacancy ensembles and neutral vacancies (V0) in diamond crystals exposed to magnetic fields of up to 10 T. We determine the orbital and spin Zeeman splitting as well as the energetic ordering of their ground and first-excited states. The spin-triplet and -singlet states of the NV- are described by an orbital Zeeman splitting of about 9 μeV/T, which corresponds to a positive orbital g-factor of gL=0.164 under application of the magnetic field along the (001) and (111) crystallographic directions, respectively. The zero-phonon line (ZPL) of the NV- singlet is defined as a transition from the 1E′ states, which are split by gLμBB, to the 1A1 state. The energies of the zero-phonon triplet transitions show a quadratic dependence on intermediate magnetic field strengths, which we attribute to a mixing of excited states with nonzero orbital angular momentum. Moreover, we identify slightly different spin Zeeman splittings in the ground (gs) and excited (es) triplet states, which can be expressed by a deviation between their spin g-factors: gS,es=gS,gs+Δg with values of Δg=0.014 and 0.029 in the (001) and (111) geometries, respectively. The degree of circular polarization of the NV- ZPLs depends significantly on the temperature, which is explained by an efficient spin-orbit coupling of the excited states mediated through acoustic phonons. We further demonstrate that the sign of the circular polarization degree is switched under rotation of the diamond crystal. A weak Zeeman splitting similar to ΔgμBB measured for the NV- ZPLs is also obtained for the NV0 zero-phonon lines, from which we conclude that the ground state is composed of two optically active states with compensated orbital contributions and opposite spin-1/2 momentum projections. The zero-phonon lines of the V0 show Zeeman splittings and degrees of the circular polarization with opposite signs. The magnetophotoluminescence data indicate that the electron transition from the T21 states to the A1 ground state defines the zero-phonon emission at 1.674 eV, while the T21→E1 transition is responsible for the zero-phonon line at 1.666 eV. The T21 (E1) states are characterized by an orbital Zeeman splitting with gL=0.071 (0.128). © 2018 American Physical Society.
view abstract 10.1103/PhysRevB.97.195448
- Detection of nanowatt microwave radiation by the photoluminescence of an ensemble of negatively charged nitrogen vacancies in diamond
Debus, J. and Ivanov, V.Y. and Braukmann, D. and Azamat, D.V. and Glaser, E.R. and Kennedy, T.A. and Yakovlev, D.R. and Bayer, M.
Applied Physics Letters 113 (2018)We report on detecting continuous 60-GHz microwave radiation with powers in the nanowatt range by the photoluminescence of an ensemble of negatively charged nitrogen vacancy (NV-) centers in diamond at room temperature. The high contrast of the optically detected magnetic resonance and the efficient photon collection yield a magnetic field sensitivity of 86 nT / Hz for continuous-wave laser excitation with a photon energy of 2.33 eV and a power density of 93 W/cm2. The efficiency of the microwave-power-to-magnetic-field conversion amounts to 0.54 mT / W. The microwave excitation also enhances the degree of the linear polarization of NV- photoluminescence at magnetic resonance conditions, and for linearly co-polarized NV- photoluminescence and laser light, the magnetic field sensitivity is improved by about 7%. © 2018 Author(s).
view abstract 10.1063/1.5053639
- Energy Structure of an Individual Mn Acceptor in GaAs : Mn
Dimitriev, G.S. and Krainov, I.V. and Sapega, V.F. and Averkiev, N.S. and Debus, J. and Lähderanta, E.
Physics of the Solid State 60 (2018)The energy structure of the Mn acceptor, which is a complex of Mn2+ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn2+ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor. © 2018, Pleiades Publishing, Ltd.
view abstract 10.1134/S106378341808005X
- Direct measurement of the long-range p-d exchange coupling in a ferromagnet-semiconductor Co/CdMgTe/CdTe quantum well hybrid structure
Akimov, I.A. and Salewski, M. and Kalitukha, I.V. and Poltavtsev, S.V. and Debus, J. and Kudlacik, D. and Sapega, V.F. and Kopteva, N.E. and Kirstein, E. and Zhukov, E.A. and Yakovlev, D.R. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Korenev, V.L. and Kusrayev, Y.G. and Bayer, M.
Physical Review B 96 (2017)The exchange interaction between magnetic ions and charge carriers in semiconductors is considered to be a prime tool for spin control. Here, we solve a long-standing problem by uniquely determining the magnitude of the long-range p-d exchange interaction in a ferromagnet-semiconductor (FM-SC) hybrid structure where a 10-nm-thick CdTe quantum well is separated from the FM Co layer by a CdMgTe barrier with a thickness on the order of 10 nm. The exchange interaction is manifested by the spin splitting of acceptor bound holes in the effective magnetic field induced by the FM. The exchange splitting is directly evaluated using spin-flip Raman scattering by analyzing the dependence of the Stokes shift ΔS on the external magnetic field B. We show that in a strong magnetic field, ΔS is a linear function of B with an offset of Δpd=50-100μeV at zero field from the FM induced effective exchange field. On the other hand, the s-d exchange interaction between conduction band electrons and FM, as well as the p-d contribution for free valence band holes, are negligible. The results are well described by the model of indirect exchange interaction between acceptor bound holes in the CdTe quantum well and the FM layer mediated by elliptically polarized phonons in the hybrid structure. © 2017 American Physical Society.
view abstract 10.1103/PhysRevB.96.184412
- Efficiency enhancement of the coherent electron spin-flip Raman scattering through thermal phonons in (In,Ga)As/GaAs quantum dots
Debus, J. and Kudlacik, D. and Waldkirch, P. and Sapega, V.F. and Scholz, S. and Ludwig, Ar. and Wieck, A.D. and Bayer, M.
Physical Review B 95 (2017)The spin-flip Raman scattering efficiency of the resident electron is thermally enhanced in singly charged (In,Ga)As/GaAs quantum dots, for probing the s- or p-shell trions. The Raman shift, polarization characteristics, and spectral position of the resonant scattering profile are insensitive to the sample temperature up to 50 K. This indicates a thermally robust mechanism of the coherent electron spin-flip based on exchange interaction. The background of the scattering spectra, whose intensity increases also by about one order of magnitude with temperature, is associated with acoustic phonon scattering. We propose that acoustic phonons enhance the spin-flip probability of the resident electron with growing temperature. The coherent spin-flip Raman scattering is ultimately suppressed at temperatures, which are a few times lower than that required for thermal trion dissociation. © 2017 American Physical Society.
view abstract 10.1103/PhysRevB.95.201303
- In situ chip formation analyses in micro single-lip and twist deep hole drilling
Kirschner, M. and Michel, S. and Berger, S. and Biermann, D. and Debus, J. and Braukmann, D. and Bayer, M.
International Journal of Advanced Manufacturing Technology (2017)Growing competitive pressure forces companies to optimise process productivity and shorten primary production times. At the same time, the resulting manufacturing quality must be kept on a high level. In the automotive sector, deep hole drilling with smallest tool diameters is an important process, e.g. to produce lubrication holes in crankshafts and fuel channels in injectors. A crucial criterion for the achievable productivity and manufacturing quality with respect to the dimensional and shape tolerances as well as the surface quality in smallest diameter deep hole drilling is the chip formation. Therefore, in-depth analyses regarding the mechanisms of chip formation at the cutting edge and the chip removal along the chip flutes are indispensable. To accomplish an in-depth chip formation analysis in smallest diameter deep hole drilling, a new methodology of analysis has been developed. Samples made of the particular test material are inserted into acrylic glass carriers, and the chip formation in the operating zone and the chip removal are documented by high-speed microscopy. In this paper, the experimental setup of the newly developed methodology of analysis and the experimental results for single-lip and twist deep hole drilling of high-strength bainitic steel with smallest diameters are shown. The investigations show the dependence of chip formation on the changes of the microstructure of the cutting edge due to tool wear, and form the basis for an optimization of the tools. In addition to that, a new approach to visualise machining processes running under non-transparent coolant is presented. © 2017 Springer-Verlag London Ltd., part of Springer Nature
view abstract 10.1007/s00170-017-1339-1
- Spin dynamics and magnetic field induced polarization of excitons in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
Shamirzaev, T.S. and Rautert, J. and Yakovlev, D.R. and Debus, J. and Gornov, A.Y. and Glazov, M.M. and Ivchenko, E.L. and Bayer, M.
Physical Review B 96 (2017)The exciton spin dynamics are investigated both experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells with an indirect band gap and a type-II band alignment. The magnetic field induced circular polarization of photoluminescence Pc is studied as function of the magnetic field strength and direction as well as sample temperature. The observed nonmonotonic behavior of these functions is provided by the interplay of bright and dark exciton states contributing to the emission. To interpret the experiment, we have developed a kinetic master equation model which accounts for the dynamics of the spin states in this exciton quartet, radiative and nonradiative recombination processes, and redistribution of excitons between these states as result of spin relaxation. The model offers quantitative agreement with experiment and allows us to evaluate, for the studied structure, the heavy-hole g factor, ghh=+3.5, and the spin relaxation times of electron, τse=33μs, and hole, τsh=3μs, bound in the exciton. © 2017 American Physical Society.
view abstract 10.1103/PhysRevB.96.035302
- Dynamics of exciton recombination in strong magnetic fields in ultrathin GaAs/AlAs quantum wells with indirect band gap and type-II band alignment
Shamirzaev, T.S. and Debus, J. and Yakovlev, D.R. and Glazov, M.M. and Ivchenko, E.L. and Bayer, M.
Physical Review B 94 (2016)The exciton recombination dynamics is studied experimentally and theoretically in two-monolayer-thick GaAs/AlAs quantum wells characterized by an indirect band gap and a type-II band alignment. At cryogenic temperatures, the lifetimes of the excitons that are indirect both in real and k space are in the millisecond range. The exciton recombination time and the photoluminescence (PL) intensity are strongly dependent on strength and orientation of an applied magnetic field. In contrast to the very weak influence of an in-plane field, at 2 K temperature a field applied parallel to the growth axis drastically slows down the recombination and reduces the PL intensity. With increasing temperature the magnetic field effects on PL intensity and decay time are vanishing. The experimental data are well described by a model for the exciton dynamics that takes into account the magnetic-field-induced redistribution of the indirect excitons between their bright and dark states. It allows us to evaluate the lower bound of the heavy-hole longitudinal g factor of 2.5, the radiative recombination time for the bright excitons of 0.34 ms, and the nonradiative recombination time of the bright and dark excitons of 8.5 ms. © 2016 American Physical Society.
view abstract 10.1103/PhysRevB.94.045411
- Exciton and trion dynamics in atomically thin MoSe2 and WSe2: Effect of localization
Godde, T. and Schmidt, D. and Schmutzler, J. and Aßmann, M. and Debus, J. and Withers, F. and Alexeev, E.M. and Del Pozo-Zamudio, O. and Skrypka, O.V. and Novoselov, K.S. and Bayer, M. and Tartakovskii, A.I.
Physical Review B 94 (2016)We present a detailed investigation of the exciton and trion dynamics in naturally doped MoSe2 and WSe2 single atomic layers as a function of temperature in the range 10-300 K under above band-gap laser excitation. By combining time-integrated and time-resolved photoluminescence (PL) spectroscopy, we show the importance of exciton and trion localization in both materials at low temperatures. We also reveal the transition to delocalized exciton complexes at higher temperatures where the exciton and trion thermal energy exceeds the typical localization energy. This is accompanied by strong changes in PL including suppression of the trion PL and decrease of the trion PL lifetime, as well as significant changes for neutral excitons in the temperature dependence of the PL intensity and the appearance of a pronounced slow PL decay component. In MoSe2 and WSe2 studied here, the temperatures where such strong changes occur are observed at around 100 and 200 K, respectively, in agreement with their inhomogeneous PL linewidth of 8 and 20 meV at T≈10K. The observed behavior is a result of a complex interplay between influences of the specific energy ordering of bright and dark excitons in MoSe2 and WSe2, sample doping, trion, and exciton localization and various temperature-dependent nonradiative processes. © 2016 American Physical Society.
view abstract 10.1103/PhysRevB.94.165301
- Fine structure of the Mn acceptor in GaAs
Krainov, I.V. and Debus, J. and Averkiev, N.S. and Dimitriev, G.S. and Sapega, V.F. and Lähderanta, E.
Physical Review B 93 (2016)We reveal the electronic level structure of the Mn acceptor, which consists of a valence-band hole bound to an Mn2+ ion, in presence of applied uniaxial stress and an external magnetic field in bulk GaAs. Resonant spin-flip Raman scattering is used to measure the g factor of the AMn0 center in the ground and excited states with the total angular momenta F=1 and F=2 and characterize the optical selection rules of the spin-flip transitions between these Mn-acceptor states. We determine the random stress fields near the Mn acceptor, the constant of the antiferromagnetic exchange interaction between the valence-band holes and the electrons of the inner Mn2+ shell as well as the deformation potential for the exchange energy. The p-d exchange energy, in particular, decreases significantly with increasing compressive stress. By combining the experimental Raman study with the developed theoretical model on the scattering efficiency, in which also the random local and external uniaxial stresses and magnetic field are considered, the fine structure of the Mn acceptor is determined in full detail. © 2016 American Physical Society.
view abstract 10.1103/PhysRevB.93.235202
- Heating of the Mn spin system by photoexcited holes in type-II (Zn, Mn)Se/(Be, Mn)Te quantum wells
Debus, J. and Maksimov, A.A. and Dunker, D. and Yakovlev, D.R. and Filatov, E.V. and Tartakovskii, I.I. and Ivanov, V.Y. and Waag, A. and Bayer, M.
Physica Status Solidi (B) Basic Research 251 (2016)The efficiency of the Mn-spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type-II band alignment by means of time-resolved photoluminescence and pump-probe reflectivity. An essential role in the heating is played by multiple spin-flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/pssb.201350320
- Indication of worn WC/C surface locations of a dry-running twin-screw rotor by the oxygen incorporation in tungsten-related Raman modes
Debus, J. and Schindler, J.J. and Waldkirch, P. and Goeke, S. and Brümmer, A. and Biermann, D. and Bayer, M.
Applied Physics Letters 109 (2016)By comparing the worn and untouched locations of a tungsten-carbide/carbon surface of a dry-running twin-screw rotor, we demonstrate that tungsten-oxide Raman modes become observable only at worn locations and the integral intensity of the Raman line at 680 cm-1, which is related to the incipient oxidation of the tungsten-carbide stretching mode, is enhanced. Its frequency and width moreover change significantly, thus indicating the mechanical distortion of the bonding that has been occurred during the wearing process. The shape of the tungsten-oxide Raman lines, resembling the Voigt function, hints at a surface morphology that is a characteristic for an amorphous solid environment. Our Raman scattering results may be exploited to characterize the degree of wear of coated surfaces and to identify signatures of a tribological layer. © 2016 Author(s).
view abstract 10.1063/1.4966145
- Long-range p-d exchange interaction in a ferromagnet-semiconductor hybrid structure
Korenev, V. L. and Salewski, M. and Akimov, I. A. and Sapega, V. F. and Langer, L. and Kalitukha, I. V. and Debus, J. and Dzhioev, R. I. and Yakovlev, D. R. and Muller, D. and Schroder, C. and Hövel, H. and Karczewski, G. and Wiater, M. and Wojtowicz, T. and Kusrayev, Y. G. and Bayer, M.
Nature Physics 12 (2016)Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd,Mg) Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons.
view abstract 10.1038/NPHYS3497
- Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells
Zhukov, E.A. and Kusrayev, Y.G. and Kavokin, K.V. and Yakovlev, D.R. and Debus, J. and Schwan, A. and Akimov, I.A. and Karczewski, G. and Wojtowicz, T. and Kossut, J. and Bayer, M.
Physical Review B 93 (2016)The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te diluted-magnetic-semiconductor quantum wells is investigated by means of picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the oscillatory signals from photoexcited electrons and manganese spins precessing about an external magnetic field, a surprisingly long-lived (up to 60 ns) nonoscillating spin polarization is detected. This polarization is related to optical orientation of equilibrium magnetic polarons involving resident holes. The suggested mechanism for the optical orientation of the equilibrium magnetic polarons indicates that the detected polaron dynamics originates from unexcited magnetic polarons. The polaron spin dynamics is controlled by the anisotropic spin structure of the heavy hole resulting in a freezing of the polaron magnetic moment in one of the two stable states oriented along the structure growth axis. Spin relaxation between these states is prohibited by a potential barrier, which depends on temperature and magnetic field. The magnetic polaron relaxation is accelerated with increasing temperature and in magnetic field. © 2016 American Physical Society.
view abstract 10.1103/PhysRevB.93.245305
- Resonantly enhanced spin-lattice relaxation of Mn2+ ions in diluted magnetic (Zn,Mn)Se/(Zn,Be)Se quantum wells
Debus, J. and Ivanov, V.Y. and Ryabchenko, S.M. and Yakovlev, D.R. and Maksimov, A.A. and Semenov, Y.G. and Braukmann, D. and Rautert, J. and Löw, U. and Godlewski, M. and Waag, A. and Bayer, M.
Physical Review B 93 (2016)The dynamics of spin-lattice relaxation in the magnetic Mn2+ ion system of (Zn,Mn)Se/(Zn,Be)Se quantum-well structures are studied using optical methods. Pronounced cusps are found in the giant Zeeman shift of the quantum-well exciton photoluminescence at specific magnetic fields below 10 T, when the Mn spin system is heated by photogenerated carriers. The spin-lattice relaxation time of the Mn ions is resonantly accelerated at the cusp magnetic fields. Our theoretical analysis demonstrates that a cusp occurs at a spin-level mixing of single Mn2+ ions and a quick-relaxing cluster of nearest-neighbor Mn ions, which can be described as intrinsic cross-relaxation resonance within the Mn spin system. © 2016 American Physical Society.
view abstract 10.1103/PhysRevB.93.195307
- Ground and excited states of iron centers in ZnO: Pulse-EPR and magneto-optical spectroscopy
Azamat, D.V. and Debus, J. and Yakovlev, D.R. and Ivanov, V.Yu. and Godlewski, M. and Fanciulli, M. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 92 (2015)We report on the ground- and excited-state properties of Fe3+ centers in hydrothermally and chemical-vapor-transport grown single ZnO crystals studied by continuous-wave electron-paramagnetic resonance (EPR) under dark and laser-illuminated conditions, pulsed-EPR and magneto-photoluminescence. By use of EPR experiments, the fine-structure parameters of the Fe3+ spin Hamiltonian are determined. Three types of charge-compensated Fe3+ centers are identified and the charge conversion from Fe2+ to Fe3+ is highlighted. The magneto-optical studies of the Zeeman components of the spin-forbidden electric-dipole transitions from excited T14(G) to ground A16(S6) states of the Fe3+ center indicate the trigonal symmetry of the fine structure of the lowest Γ8(T14) excited state. The energy positions of the Zeeman components are measured in the external magnetic field of 8 T rotated in (12¯10) and (0001) crystal planes. The angular variation of the Zeeman lines exhibits two magnetically nonequivalent Fe3+ centers. These features result from the contribution of high-rank Zeeman terms of dimension BJ3 in the spin Hamiltonian. For the electron spin S=5/2 system of the trigonal Fe3+ ion, we further demonstrate the tuning of one-photon Rabi oscillations by means of electron spin-echo measurements. ©2015 American Physical Society.
view abstract 10.1103/PhysRevB.92.195202
- Nuclear spin polarization in the electron spin-flip Raman scattering of singly charged (In,Ga)As/GaAs quantum dots
Debus, J. and Kudlacik, D. and Sapega, V.F. and Dunker, D. and Bohn, P. and Paßmann, F. and Braukmann, D. and Rautert, J. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 92 (2015)We study the interplay between the dynamic nuclear spin polarization and resonant spin-flip Raman scattering of the resident electron in an ensemble of singly charged (In,Ga)As/GaAs quantum dots by using a two-color laser excitation scheme. The shift of the electron spin-flip Raman line gives a direct measure of the optically induced Overhauser shift, while the linewidth indicates nuclear spin fluctuations. The dynamic nuclear spin polarization leads only to a reduction in the electron spin splitting induced by wetting-layer excitation that is copolarized with the resonant quantum dot excitation. The respective mechanism of the two-color spin-flip Raman scattering is discussed together with the electron-nuclear hyperfine interaction and Pauli exclusion principle. The temporal evolution of the Overhauser shift further demonstrates a nuclear spin depolarization within several seconds depending strongly on the temperature. © 2015 American Physical Society.
view abstract 10.1103/PhysRevB.92.195421
- Spin-flip Raman scattering of the resident electron in singly charged (In,Ga)As/GaAs quantum dot ensembles
Debus, J. and Sapega, V.F. and Dunker, D. and Yakovlev, D.R. and Reuter, D. and Wieck, A.D. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 90 (2014)Highly efficient spin-flip Raman scattering of the resident electron spin is found in singly charged (In,Ga)As/GaAs quantum dots. The applied magnetic field induces a symmetry reduction for the negatively charged exciton, which serves as intermediate scattering state, thus making the spin-flip Raman scattering of the resident electron allowed. Electron-electron exchange interaction mediates the electron spin-flip. Above a threshold magnetic field that depends on the dot size and experiment geometry, the efficiency of the scattering cross section is spectrally shifted with increasing field. This shift, which follows the electron cyclotron energy, is assigned to a hybridization of s-shell singlet and p-shell triplet states of the negatively charged exciton. © 2014 American Physical Society.
view abstract 10.1103/PhysRevB.90.235404
- Spin-flip Raman scattering of the Γ-X mixed exciton in indirect band gap (In,Al)As/AlAs quantum dots
Debus, J. and Shamirzaev, T.S. and Dunker, D. and Sapega, V.F. and Ivchenko, E.L. and Yakovlev, D.R. and Toropov, A.I. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 90 (2014)The band structure of type-I (In,Al)As/AlAs quantum dots with band gap energy exceeding 1.63 eV is indirect in momentum space, leading to long-lived exciton states with potential applications in quantum information. Optical access to these excitons is provided by mixing of the Γ- and X-conduction-band valleys, for which their spins may be oriented by resonant spin-flip Raman scattering. This access is used to study the exciton spin-level structure by accurately measuring the anisotropic hole and isotropic electron g factors. The spin-flip mechanisms for the indirect exciton and its constituents as well as the underlying optical selection rules are determined. The spin-flip intensity is a reliable measure of the strength of Γ-X-valley mixing, as evidenced by both experiment and theory. © 2014 American Physical Society.
view abstract 10.1103/PhysRevB.90.125431
- Microsecond Lifetime of Exciton Spin Polarization in (In,Al)As/AlAs Quantum Dots
Shamirzaev, T.S. and Dunker, D. and Debus, J. and Yakovlev, D.R. and Zhuravlev, K.S. and Bayer, M.
Optoelectronics, Instrumentation and Data Processing 49 (2013)The time of spin relaxation of excitons in (In,Al)As/AlAs quantum dots with an indirect bandgap and type-I band alignment is determined by measuring the dynamics of photoluminescence circular polarization induced by a magnetic field B. The spin relaxation time τ S increases with decreasing magnetic field in proportion to B -5; its value is ~40 μs in a magnetic field of 6 T at a temperature of 1.8 K. As the temperature T increases in a magnetic field of 7 T, the value of τ S decreases as T -1.1. The character of the dependences of τ S on the magnetic field and temperature evidences that spin relaxation of excitons is provided by a process with participation of one acoustic phonon. © 2013 Allerton Press, Inc.
view abstract 10.3103/S8756699013050130
- Spin-flip Raman scattering of the neutral and charged excitons confined in a CdTe/(Cd,Mg)Te quantum well
Debus, J. and Dunker, D. and Sapega, V.F. and Yakovlev, D.R. and Karczewski, G. and Wojtowicz, T. and Kossut, J. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 87 (2013)Spin-flip Raman scattering of electrons and heavy holes is studied for resonant excitation of neutral and charged excitons in a CdTe/Cd 0.63Mg0.37Te quantum well. The spin-flip scattering is characterized by its dependence on the incident and scattered light polarization as well as on the magnetic field strength and orientation. Model schemes of electric-dipole-allowed spin-flip Raman processes in the exciton complexes are compared to the experimental observations, from which we find that lowering the exciton symmetry, time of carrier spin relaxation, and mixing between electron states and, respectively, light- and heavy-hole states play an essential role in the scattering. At the exciton resonance, anisotropic exchange interaction induces heavy-hole spin-flip scattering, while acoustic phonon interaction is mainly responsible for the electron spin-flip. In resonance with the positively and negatively charged excitons, anisotropic electron-hole exchange as well as mixed electron states allow spin-flip scattering. Variations in the resonant excitation energy and lattice temperature demonstrate that localization of resident electrons and holes controls the Raman process probability and is also responsible for symmetry reduction. We show that the intensity of the electron spin-flip scattering is strongly affected by the lifetime of the exciton complex, and in tilted magnetic fields it is affected by the angular dependence of the anisotropic electron-hole exchange interaction. © 2013 American Physical Society.
view abstract 10.1103/PhysRevB.87.205316
- Spin relaxation of negatively charged excitons in (In,Al)As/AlAs quantum dots with indirect band gap and type-I band alignment
Dunker, D. and Shamirzaev, T.S. and Debus, J. and Yakovlev, D.R. and Zhuravlev, K.S. and Bayer, M.
Applied Physics Letters 101 (2012)Spin dynamics of negatively charged excitons is experimentally studied in (In,Al)As/AlAs quantum dots with indirect band gap and type-I band alignment. At low temperatures of 1.8 K, the spin relaxation time is 55 μ s in a magnetic field of 3 T. It decreases with increasing magnetic field as B -5, which evidences that the spin relaxation of the negatively charged excitons is provided by an one-acoustic-phonon process. © 2012 American Institute of Physics.
view abstract 10.1063/1.4754619
- Exciton recombination dynamics in an ensemble of (In,Al)As/AlAs quantum dots with indirect band-gap and type-I band alignment
Shamirzaev, T.S. and Debus, J. and Abramkin, D.S. and Dunker, D. and Yakovlev, D.R. and Dmitriev, D.V. and Gutakovskii, A.K. and Braginsky, L.S. and Zhuravlev, K.S. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 84 (2011)The dynamics of exciton recombination in an ensemble of indirect band-gap (In,Al)As/AlAs quantum dots with type-I band alignment is studied. The lifetime of confined excitons that are indirect in momentum space is mainly influenced by the sharpness of the heterointerface between the (In,Al)As quantum dot and the AlAs barrier matrix. Time-resolved photoluminescence experiments and theoretical model calculations reveal a strong dependence of the exciton lifetime on the thickness of the interface diffusion layer. The lifetime of excitons with a particular optical transition energy varies because this energy is obtained for quantum dots differing in size, shape, and composition. The different exciton lifetimes, which result in photoluminescence with nonexponential decay obeying a power-law function, can be described by a phenomenological distribution function G(τ), which allows one to fit the photoluminescence decay with one parameter only. © 2011 American Physical Society.
view abstract 10.1103/PhysRevB.84.155318
- Dynamical control of Mn spin-system cooling by photogenerated carriers in a (Zn,Mn)Se/BeTe heterostructure
Debus, J. and Maksimov, A.A. and Dunker, D. and Yakovlev, D.R. and Tartakovskii, I.I. and Waag, A. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 82 (2010)The magnetization dynamics of the Mn spin system in an undoped (Zn,Mn)Se/BeTe type-II quantum well was studied by a time-resolved pump-probe photoluminescence technique. The Mn spin temperature was evaluated from the giant Zeeman shift of the exciton line in an external magnetic field of 3 T. The relaxation dynamics of the Mn spin temperature to the equilibrium temperature of the phonon bath after the pump-laser-pulse heating can be accelerated by the presence of free electrons. These electrons, generated by a control laser pulse, mediate the spin and energy transfer from the Mn spin system to the lattice and bypass the relatively slow direct spin-lattice relaxation of the Mn ions. © 2010 The American Physical Society.
view abstract 10.1103/PhysRevB.82.085448
- Photo-EPR and magneto-optical spectroscopy of iron centres in ZnO
Azamat, D.V. and Debus, J. and Yakovlev, D.R. and Yu Ivanov, V. and Godlewski, M. and Fanciulli, M. and Bayer, M.
Physica Status Solidi (B) Basic Research 247 (2010)ZnO has attracted increasing interest as promising material for optoelectronics and spintronics. Magnetic properties of ZnO can be controlled by doping with transition metal ions among which Fe3+ is one of the interesting candidates. We report on the properties of Fe3+ centres in hydrothermally and chemical vapour transport grown ZnO single crystals investigated by photo-electron paramagnetic resonance (EPR) and optical spectroscopy. Detailed magneto-optical studies of Zeeman components of spin-forbidden electric dipole transitions 4T1(G) ! 6A1(6S) of Fe3+ centre in ZnO reveal the trigonal symmetry of fine structure of lowest G8 (4T1) excited state. The studies were performed in external magnetic fields up to 10 T and in the temperature range from 2 to 30 K. The energy positions of the Zeeman components at 8 T were measured as a function of the direction of applied magnetic field in (12-10) and (0001) planes. The detailed check of the angular variation of Zeeman lines shows two magnetically non-equivalent Fe3+ centres. These special features were accounted by contribution of higher rank Zeeman terms of dimension BJ3. The photo- generated EPR spectra of trigonal Fe3+ centres were detected in hydrothermally grown ZnO single crystals in addition to three types of charge compensated iron centres presented in these samples in the dark conditions. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/pssb.200983224
- Spin diffusion in the Mn2+ ion system of II-VI diluted magnetic semiconductor heterostructures
Maksimov, A.A. and Yakovlev, D.R. and Debus, J. and Tartakovskii, I.I. and Waag, A. and Karczewski, G. and Wojtowicz, T. and Kossut, J. and Bayer, M.
Physical Review B - Condensed Matter and Materials Physics 82 (2010)The magnetization dynamics in diluted magnetic semiconductor heterostructures based on (Zn,Mn)Se and (Cd,Mn)Te were studied optically and simulated numerically. In samples with inhomogeneous magnetic ion distribution, these dynamics are contributed by spin-lattice relaxation and spin diffusion in the Mn spin system. A spin-diffusion coefficient of 7× 10-8 cm2 /s was evaluated for Zn0.99 Mn0.01 Se from comparison of experiment and theory. Calculations of the exciton giant Zeeman splitting and the magnetization dynamics in ordered alloys and digitally grown parabolic quantum wells show perfect agreement with the experimental data. In both structure types, spin diffusion contributes essentially to the magnetization dynamics. © 2010 The American Physical Society.
view abstract 10.1103/PhysRevB.82.035211