Prof. Dr. Peter Kratzer

Theoretical Physics
University of Duisburg-Essen

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  • A fresh look at the structure of aromatic thiols on Au surfaces from theory and experiment
    Hekele, J. and Linke, M. and Keller, T. and Jose, J. and Hille, M. and Hasselbrink, E. and Schlücker, S. and Kratzer, P.
    Journal of Chemical Physics 155 (2021)
    A detailed study of the adsorption structure of self-assembled monolayers of 4-nitrothiophenol on the Au(111) surface was performed from a theoretical perspective via first-principles density functional theory calculations and experimentally by Raman and vibrational sum frequency spectroscopy (vSFS) with an emphasis on the molecular orientation. Simulations—including an explicit van der Waals (vdW) description—for different adsorbate structures, namely, for (3×3), (2 × 2), and (3 × 3) surface unit cells, reveal a significant tilting of the molecules toward the surface with decreasing coverage from 75° down to 32° tilt angle. vSFS suggests a tilt angle of 50°, which agrees well with the one calculated for a structure with a coverage of 0.25. Furthermore, calculated vibrational eigenvectors and spectra allowed us to identify characteristic in-plane (NO2 scissoring) and out-of-plane (C-H wagging) modes and to predict their strength in the spectrum in dependence of the adsorption geometry. We additionally performed calculations for biphenylthiol and terphenylthiol to assess the impact of multiple aromatic rings and found that vdW interactions are significantly increasing with this number, as evidenced by the absorption energy and the molecule adopting a more upright-standing geometry. © 2021 Author(s).
    view abstract10.1063/5.0053493
  • All-electron real-time and imaginary-time time-dependent density functional theory within a numeric atom-centered basis function framework
    Hekele, J. and Yao, Y. and Kanai, Y. and Blum, V. and Kratzer, P.
    Journal of Chemical Physics 155 (2021)
    Real-time time-dependent density functional theory (RT-TDDFT) is an attractive tool to model quantum dynamics by real-time propagation without the linear response approximation. Sharing the same technical framework of RT-TDDFT, imaginary-time time-dependent density functional theory (it-TDDFT) is a recently developed robust-convergence ground state method. Presented here are high-precision all-electron RT-TDDFT and it-TDDFT implementations within a numerical atom-centered orbital (NAO) basis function framework in the FHI-aims code. We discuss the theoretical background and technical choices in our implementation. First, RT-TDDFT results are validated against linear-response TDDFT results. Specifically, we analyze the NAO basis sets’ convergence for Thiel’s test set of small molecules and confirm the importance of the augmentation basis functions for adequate convergence. Adopting a velocity-gauge formalism, we next demonstrate applications for systems with periodic boundary conditions. Taking advantage of the all-electron full-potential implementation, we present applications for core level spectra. For it-TDDFT, we confirm that within the all-electron NAO formalism, it-TDDFT can successfully converge systems that are difficult to converge in the standard self-consistent field method. We finally benchmark our implementation for systems up to ∼500 atoms. The implementation exhibits almost linear weak and strong scaling behavior. © 2021 Author(s).
    view abstract10.1063/5.0066753
  • First-principles computational exploration of ferromagnetism in monolayer GaS via substitutional doping
    Khan, R. and Rahman, A.U. and Zhang, Q. and Kratzer, P. and Ramay, S.M.
    Journal of Physics Condensed Matter 33 (2021)
    Using first-principles calculations, functionalization of the monolayer-GaS crystal structure through N or Cr-doping at all possible lattice sites has been investigated. Our results show that pristine monolayer-GaS is an indirect-bandgap, non-magnetic semiconductor. The bandgap can be tuned and a magnetic moment (MM) can be induced by the introduction of N or Cr atomic anion/cation doping in monolayer GaS. For instance, the intrinsic character of monolayer GaS can be changed by substitution of N for the S-site to p-type, while substitution of Cr at the S-site or Ga-site induces half-metallicity at sufficiently high concentrations. The defect states are located in the electronic bandgap region of the GaS monolayer. These findings help to extend the application of monolayer-GaS structures in nano-electronics and spintronics. Since the S-sites at the surface are more easily accessible to doping in experiment, we chose the S-site for further investigations. Finally, we perform calculations with ferromagnetic (FM) and antiferromagnetic (AFM) alignment of the MMs at the dopants. For pairs of impurities of the same species at low concentrations we find Cr atoms to prefer the FM state, while N atoms prefer the AFM state, both for impurities on opposite surfaces of the GaS monolayer and for impurities sharing a common Ga neighbor sitting at the same surface. Extending our study to higher concentrations of Cr atoms, we find that clusters of four Cr atoms prefer AFM coupling, whereas the FM coupling is retained for Cr atoms at larger distance arranged on a honeycomb lattice. For the latter arrangement, we estimate the FM Curie temperature T C to be 241 K. We conclude that the Cr-doped monolayer-GaS crystal structure offers enhanced electronic and magnetic properties and is an appealing candidate for spintronic devices operating close to room temperature. © 2021 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/ac04ce
  • Magnetic exchange interactions in bilayer CrX3 (X=Cl,Br,and I): A critical assessment of the DFT+U approach MAGNETIC EXCHANGE INTERACTIONS in BILAYER ... SOUMYAJIT SARKAR and PETER KRATZER
    Sarkar, S. and Kratzer, P.
    Physical Review B 103 (2021)
    We perform calculations with the DFT+U approach for the three Cr trihalides CrI3, CrBr3, and CrCl3 with the aim to determine magnetic exchange interactions and magnetic ordering temperatures. A comprehensive investigation is carried out to assess the role of the Hubbard parameter U as well as of the schemes used to correct for the double counting (DC) of the Coulomb interaction. For the bilayer systems, both with low-temperature (LT) or high-temperature (HT) stacking, magnetic exchange parameters and ordering temperatures are calculated within the random-phase approximation for spin waves. Our results show that the most commonly used DC scheme, the "fully localized limit"(FLL) of DFT+U, erroneously favors ferromagnetic coupling between the layers of a bilayer structure, and yields Curie temperatures in excess of the experimental values. With the help of a perturbative model for superexchange, we are able to trace the source of this error back to the too small band gap in the majority spin channel in the FLL calculations. In contrast, when using the "around-mean-field"(AMF) DC scheme and a realistic value of U=1.7 eV for the Cr-3d orbitals, we find that the magnetic interlayer coupling is antiferromagnetic (AFM) in all three materials, in agreement with recent experiments for CrI3. We calculate the antiferromagnetic ordering temperature for both LT and HT stacking to be 22 K and 29 K for bilayer CrI3. Thus, according to our calculations, the AFM order can be observed independently of the crystal structure of the film, while the LT structure remains to be the ground state. Bilayer CrBr3 films are predicted to have a Néel temperature similar to that of CrI3, whereas the CrCl3 films prefer antiparallel in-plane magnetization. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.224421
  • Chemisorption and Physisorption at the Metal/Organic Interface: Bond Energies of Naphthalene and Azulene on Coinage Metal Surfaces
    Kachel, S.R. and Klein, B.P. and Morbec, J.M. and Schöniger, M. and Hutter, M. and Schmid, M. and Kratzer, P. and Meyer, B. and Tonner, R. and Gottfried, J.M.
    Journal of Physical Chemistry C 124 (2020)
    Organic/inorganic hybrid interfaces play a prominent role in organic (opto)electronics, heterogeneous catalysis, sensors, and other current fields of technology. The performance of the related devices and processes depends critically on the nature and strength of interfacial interaction. Here, we use the molecular isomers naphthalene (Nt) and azulene (Az) on the Ag(111) and Cu(111) surfaces as model systems that cover different bonding regimes from physisorption to chemisorption. Az also serves as a model for nonalternant molecular electronic materials and for topological 5-7 defects in graphene. The interaction energies are determined from the quantitative analysis of temperature-programmed desorption data. On both surfaces, Az binds more strongly than Nt, with zero-coverage desorption energies (in kJ/mol) of 120 for Az/Ag and 179 for Az/Cu, compared to 103 for Nt/Ag and 114 for Nt/Cu. The integrated experimental energies are compared with adsorption energies from density-functional theory (DFT) calculations, which include van der Waals contributions using four different correction schemes for the PBE functional: (1) the DFT-D3 scheme with Becke-Johnson damping, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion correction scheme, and (4) the D3surf scheme. Differences in the performance of these methods are discussed. Periodic energy decomposition analysis reveals details of the surface chemical bond and confirms that Az/Cu forms a chemisorptive bond, while the other systems are physisorbed. The variation of the adsorbate-substrate interaction with the topology of the Ï-electron system and the type of surface can be employed to modify the interface properties in graphene-based and organic electronic devices. © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c00915
  • Electronic correlation, magnetic structure, and magnetotransport in few-layer Cr I3
    Sarkar, S. and Kratzer, P.
    Physical Review Materials 4 (2020)
    Using density functional theory combined with a Hubbard model (DFT+ U), the electronic band structure of CrI3 multilayers, both freestanding and enclosed between graphene contacts, is calculated. We show that the DFT+ U approach, together with the "around-mean-field"correction scheme, is able to describe the vertical magnetotransport in line with the experimental measurements of magnetoresistance in multilayered CrI3 enclosed between graphene contacts. Moreover, by interpolating between different double-counting correction schemes, namely the around-mean-field correction and the fully localized limit, we show their importance for consistently describing both the band structure and the ground-state total energy. Our description of the magnetic exchange interaction is compatible with the experimentally observed antiferromagnetic ground state in the bilayer CrI3 and the transition to a ferromagnetic arrangement in a small external magnetic field. Thus, using spin-polarized DFT+ U with an around-mean-field correction, a consistent overall picture is achieved. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.104006
  • Erratum: Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer (Journal of Physical Chemistry C (2020) 124: 4 (2689−2697) DOI: 10.1021/acs.jpcc.9b10120)
    Pollmann, E. and Morbec, J.M. and Madauß, L. and Bröckers, L. and Kratzer, P. and Schleberger, M.
    Journal of Physical Chemistry C 124 (2020)
    The scale bar in the abstract graphic was unintentionally inserted incorrectly. The corrected abstract graphic is provided here. We kindly apologize for any possible irritation while reading the publication. (Figure Presented). © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c01297
  • Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer
    Pollmann, E. and Morbec, J.M. and Madauß, L. and Bröckers, L. and Kratzer, P. and Schleberger, M.
    Journal of Physical Chemistry C 124 (2020)
    Two-dimensional molybdenum disulfide on graphene grown by chemical vapor deposition is a promising van der Waals system for applications in optoelectronics and catalysis. To extend the fundamental understanding of growth and intrinsic properties of molybdenum disulfide on graphene, molybdenum disulfide on highly oriented pyrolytic graphite is a suitable model system. Here, we show experimentally and by density functional theory calculations that molybdenum disulfide flakes grow in two orientations. One of the orientations is energetically preferred, the other one is rotated by 30°, but both orientations are found to be stable at room temperature. Combined Kelvin probe microscopy and Raman spectroscopy measurements show that the flakes with a typical size of a few hundred nanometers are electron doped in the order of 1012/cm2, while the doping of a molybdenum disulfide single layer exfoliated on silicon dioxide is on the order of 1013/cm2. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b10120
  • Ab initio simulation of the structure and transport properties of zirconium and ferromagnetic cobalt contacts on the two-dimensional semiconductor WS2
    Kahnouji, H. and Kratzer, P. and Hashemifar, S.J.
    Physical Review B 99 (2019)
    Using density-functional theory calculations, the atomic and electronic structure of single-layer WS2 attached to Zr and Co contacts are determined. Both metals form stable interfaces that are promising as contacts for injection of n-type carriers into the conduction band of WS2 with Schottky barriers of 0.45 eV and 0.62 eV for Zr and Co, respectively. With the help of quantum transport calculations, we address the conductive properties of a freestanding WS2 sheet suspended between two Zr contacts. It is found that such a device behaves like a diode with steep I-V characteristics. Spin-polarized transport is calculated for such a device with a floating-gate Co electrode added. Depending on the geometrical shape of the Co gate and the energy of the carriers in WS2, the transmission of spin majority and minority electrons may differ by up to an order of magnitude. Thus the steep I-V characteristics of the nanoscale device makes it possible to realize a spin filter. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.035418
  • Adsorption and dissociation of iron phthalocyanine on H/Si(111): Impact of van der Waals interactions and perspectives for subsurface doping
    Geisler, B. and Kratzer, P.
    Physical Review B 99 (2019)
    The adsorption of iron phthalocyanine (FePc) on the passivated H/Si(111) surface is explored from first principles. We find that the organic molecule is predominantly physisorbed with a distance to the surface of 2.6±0.1Å and an adsorption energy of 1.55±0.1 eV but also exhibits sizable resonance with the underlying substrate. This establishes the present system as interesting mixed covalent-van der Waals-bound test case, which we use to compare the impact of different approaches to van der Waals interactions. Spin-polarized scanning tunneling microscopy (SP STM) images are simulated, selectively accessing different molecular orbitals via the applied bias voltage in the spirit of scanning tunneling spectroscopy. Comparison with experimental STM images reveals very good agreement. We report a significant magnetic contrast exceeding ±1Å in the SP STM images for -2 and +1.5 V. Aiming for a magnetic functionalization of Si for possible spintronics applications, magnetic moments and binding energies of different (transition-metal) atoms in the center of the Pc ring are presented, which particularly show that Fe is strongly bound in the molecule (about 9.6 eV). Finally, we discuss different mechanisms for subsurface Fe doping by room-temperature FePc deposition and point out two feasible reactions. Concomitantly, we identify the crucial role of a preceding destabilization of FePc, for instance, by preadsorbed H atoms, which subsequently strongly stabilize the final state of the reaction. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.155433
  • Boltzmann relaxation dynamics of strongly interacting spinless fermions on a lattice
    Queisser, F. and Schreiber, S. and Kratzer, P. and Schützhold, R.
    Physical Review B 100 (2019)
    Motivated by the recent interest in nonequilibrium phenomena in quantum many-body systems, we study strongly interacting fermions on a lattice by deriving and numerically solving quantum Boltzmann equations that describe their relaxation to thermodynamic equilibrium. The derivation is carried out by inspecting the hierarchy of correlations within the framework of the 1/Z expansion. Applying the Markov approximation, we obtain the dynamic equations for the distribution functions. Interestingly, we find that in the strong-coupling limit, collisions between particles and holes dominate over particle-particle and hole-hole collisions-in stark contrast to weakly interacting systems. As a consequence, our numerical simulations show that the relaxation timescales strongly depend on the type of excitations (particles or holes or both) that are initially present. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.245110
  • Molecule-Metal Bond of Alternant versus Nonalternant Aromatic Systems on Coinage Metal Surfaces: Naphthalene versus Azulene on Ag(111) and Cu(111)
    Klein, B.P. and Morbec, J.M. and Franke, M. and Greulich, K.K. and Sachs, M. and Parhizkar, S. and Bocquet, F.C. and Schmid, M. and Hall, S.J. and Maurer, R.J. and Meyer, B. and Tonner, R. and Kumpf, C. and Kratzer, P. and Gottfried, J.M.
    Journal of Physical Chemistry C 123 (2019)
    Interfaces between polycyclic π-electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the π-electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagon-heptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopies (UPS and XPS), and density functional theory (DFT). Coverage-dependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using (1) the DFT-D3 scheme, (2) the vdWsurf correction based on DFT-TS, (3) a many-body dispersion (MBD) correction scheme, and (4) the D3surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 »Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b08824
  • Phonon-induced electronic relaxation in a strongly correlated system: The Sn/Si(111) (3 × 3) adlayer revisited
    Zahedifar, M. and Kratzer, P.
    Physical Review B 100 (2019)
    The ordered adsorbate layer Sn/Si(111) (3×3) with coverage of one third of a monolayer is considered as a realization of strong electronic correlation in surface physics. Our theoretical analysis shows that electron-hole pair excitations in this system can be long lived, up to several hundred nanoseconds, since the decay into surface phonons is found to be a highly nonlinear process. We combine first-principles calculations with help of a hybrid functional (HSE06) with modeling by a Mott-Hubbard Hamiltonian coupled to phononic degrees of freedom. The calculations show that the Sn/Si(111) (3×3) surface is insulating and the two Sn-derived bands inside the substrate band gap can be described as the lower and upper Hubbard band in a Mott-Hubbard model with U=0.75 eV. Furthermore, phonon spectra are calculated with particular emphasis on the Sn-related surface phonon modes. The calculations demonstrate that the adequate treatment of electronic correlations leads to a stiffening of the wagging mode of neighboring Sn atoms; thus, we predict that the onset of electronic correlations at low temperature should be observable in the phonon spectrum, too. The deformation potential for electron-phonon coupling is calculated for selected vibrational modes and the decay rate of an electron-hole excitation into multiple phonons is estimated, substantiating the very long lifetime of these excitations. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.125427
  • Relaxation of electrons in quantum-confined states in Pb/Si(111) thin films from master equation with first-principles-derived rates
    Kratzer, P. and Zahedifar, M.
    New Journal of Physics 21 (2019)
    Atomically thin films of Pb on Si(111) provide an experimentally tunable system comprising a highly structured electronic density of states. The lifetime of excited electrons in these states is limited by both electron-electron (e-e) and electron-phonon (e-ph) scattering. We employ the description by a master equation for the electronic occupation numbers to analyze the relative importance of both scattering mechanisms. The electronic and phononic band structures, as well as the matrix elements for electron-phonon coupling within deformation potential theory were obtained from density functional calculations, thus taking into account quantum confinement effects. For the relaxation dynamics, the contribution of impact ionization processes to the lifetime is estimated from the imaginary part of the electronic self-energy calculated in the GW approximation. By numerically solving rate equations for the occupations of the Pb-derived electronic states coupled to a phononic heat bath, we are able to follow the distribution of the electronic excitation energy to the various modes of Pb lattice vibrations. While e-e scattering is the dominant relaxation mechanism, we demonstrate that the e-ph scattering is highly phonon-mode-specific, with a large contribution from surface phonons. At electron energies of about 0.3 eV above the Fermi surface, a 'phonon bottleneck' characteristic of relaxation in nanostructures with well-separated electronic states is observed. The time scales extracted from the simulations are compared to data from pump-probe experiments using time-resolved two-photon photoemission. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/ab5c76
  • Spin caloric transport from density-functional theory
    Popescu, V. and Kratzer, P. and Entel, P. and Heiliger, C. and Czerner, M. and Tauber, K. and Töpler, F. and Herschbach, C. and Fedorov, D.V. and Gradhand, M. and Mertig, I. and Kováčik, R. and Mavropoulos, P. and Wortmann, D. and Blügel, S. and Freimuth, F. and Mokrousov, Y. and Wimmer, S. and Ködderitzsch, D. and Seemann, M. and Chadova, K. and Ebert, H.
    Journal of Physics D: Applied Physics 52 (2019)
    Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer-Büttiker methods. These are showcased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin-orbit torques. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aae8c5
  • Surface structural phase transition induced by the formation of metal-organic networks on the Si(111) - In surface
    Suzuki, T. and Lawrence, J. and Morbec, J.M. and Kratzer, P. and Costantini, G.
    Nanoscale 11 (2019)
    We studied the adsorption of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on the Si(111)- √7 × √3-In surface, a known surface superconductor. Scanning tunneling microscopy shows the development of a surface-confined metal-organic network (SMON) where TCNQ molecules coordinate with indium atoms from the underlying √7 × √3 reconstruction. The formation of the SMON causes a surface structural phase transition from the √7 × √3 reconstruction to a previously unknown 5 × 5 reconstruction of the Si(111)-In surface. Scanning tunneling spectroscopy measurements indicate that the 5 × 5 reconstruction has a stronger insulating character than the √7 × √3 reconstruction. Density-functional-theory calculations are used to evaluate the atomic arrangement and stability of the 5 × 5 and √7 × √3 reconstructions as a function of In coverage, and suggest that the structural phase transition is driven by a slight reduction of the In coverage, caused by the incorporation of indium atoms into the SMON. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9nr07074e
  • The basics of electronic structure theory for periodic systems
    Kratzer, P. and Neugebauer, J.
    Frontiers in Chemistry 7 (2019)
    When density functional theory is used to describe the electronic structure of periodic systems, the application of Bloch's theorem to the Kohn-Sham wavefunctions greatly facilitates the calculations. In this paper of the series, the concepts needed to model infinite systems are introduced. These comprise the unit cell in real space, as well as its counterpart in reciprocal space, the Brillouin zone. Grids for sampling the Brillouin zone and finite k-point sets are discussed. For metallic systems, these tools need to be complemented by methods to determine the Fermi energy and the Fermi surface. Various schemes for broadening the distribution function around the Fermi energy are presented and the approximations involved are discussed. In order to obtain an interpretation of electronic structure calculations in terms of physics, the concepts of bandstructures and atom-projected and/or orbital-projected density of states are useful. Aspects of convergence with the number of basis functions and the number of k-points need to be addressed specifically for each physical property. The importance of this issue will be exemplified for force constant calculations and simulations of finite-temperature properties of materials. The methods developed for periodic systems carry over, with some reservations, to less symmetric situations by working with a supercell. The chapter closes with an outlook to the use of supercell calculations for surfaces and interfaces of crystals. © 2019 Kratzer and Neugebauer.
    view abstract10.3389/fchem.2019.00106
  • Band structure and thermoelectric properties of half-Heusler semiconductors from many-body perturbation theory
    Zahedifar, M. and Kratzer, P.
    Physical Review B 97 (2018)
    Various ab initio approaches to the band structure of ANiSn and ACoSb half-Heusler compounds (A= Ti, Zr, Hf) are compared and their consequences for the prediction of thermoelectric properties are explored. Density functional theory with the generalized-gradient approximation (GGA), as well as the hybrid density functional HSE06 and ab initio many-body perturbation theory in the form of the GW0 approach, are employed. The GW0 calculations confirm the trend of a smaller band gap (0.75 to 1.05 eV) in ANiSn compared to the ACoSb compounds (1.13 to 1.44 eV) already expected from the GGA calculations. While in ANiSn materials the GW0 band gap is 20% to 50% larger than in HSE06, the fundamental gap of ACoSb materials is smaller in GW0 compared to HSE06. This is because GW0, similar to PBE, locates the valence band maximum at the L point of the Brillouin zone, whereas it is at the Γ point in the HSE06 calculations. The differences are attributed to the observation that the relative positions of the d levels of the transition metal atoms vary among the different methods. Using the calculated band structures and scattering rates taking into account the band effective masses at the extrema, the Seebeck coefficients, thermoelectric power factors, and figures of merit ZT are predicted for all six half-Heusler compounds. Comparable performance is predicted for the n-Type ANiSn materials, whereas clear differences are found for the p-Type ACoSb materials. Using the most reliable GW0 electronic structure, ZrCoSb is predicted to be the most efficient material with a power factor of up to 0.07 W/(K2 m) at a temperature of 600 K. We find strong variations among the different ab initio methods not only in the prediction of the maximum power factor and ZT value of a given material, but also in comparing different materials to each other, in particular in the p-Type thermoelectric materials. Thus we conclude that the most elaborate, but also most costly GW0 method is required to perform a reliable computational search for the optimum material. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.035204
  • Commensurate versus incommensurate heterostructures of group-III monochalcogenides
    Rahman, A.U. and Morbec, J.M. and Rahman, G. and Kratzer, P.
    Physical Review Materials 2 (2018)
    First-principles calculations based on density-functional theory were performed to investigate heterostructures of group-III monochalcogenides (GaS, GaSe, InS, and InSe) and the effects of incommensurability on their electronic structures. We considered two heterostructures: GaS/GaSe, which has a lattice mismatch of 4.7%, and GaSe/InS, with a smaller mismatch of 2.1%. We computed the cost of having commensurate structures, and we also examined the potential energy landscape of both heterostructures in order to simulate the realistic situation of incommensurate systems. We found that a commensurate heterostructure may be realized in GaSe/InS as the interaction energy of this system with the monolayers assuming the average lattice constant is smaller than the interaction energy of an incommensurate system in which each layer keeps its own lattice constant. For GaS/GaSe, on the other hand, we found that the incommensurate heterostructure is energetically more favorable than the commensurate one, even when taking into account the energetic cost due to the lack of proper registry between the layers. Since the commensurate condition requires that one (or both) layer(s) is (are) strained, we systematically investigated the effect of strain on the band gaps and band-edge positions of the monolayer systems. We found that, in all monolayers, the conduction-band minimum is more than two times more sensitive to applied strain than the valence-band maximum; this was observed to strongly affect the band alignment of GaS/GaSe, as it can change from type-I to type-II with a small variation in the lattice constant of GaS. The GaSe/InS heterostructure was found to have a type-II alignment, which is robust with respect to strain in the range of -2% to +2%. © 2018 American Physical Society.
    view abstract10.1103/PhysRevMaterials.2.094002
  • Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-648X/aab8b8
  • Single-atom vacancy in monolayer phosphorene: A comprehensive study of stability and magnetism under applied strain
    Morbec, J.M. and Rahman, G. and Kratzer, P.
    Journal of Magnetism and Magnetic Materials 465 (2018)
    Using first-principles calculations based on density-functional theory we systematically investigate the effect of applied strain on the stability and on the electronic and magnetic properties of monolayer phosphorene with single-atom vacancy. We consider two types of single vacancies: the symmetric SV-55|66, which has a metallic and non-magnetic ground state, and the asymmetric SV-5|9, which is energetically more favorable and exhibits a semiconducting and magnetic character. Our results show that compressive strain up to 10%, both biaxial and uniaxial along the zigzag direction, reduces the formation energy of both single-atom vacancies with respect to the pristine configuration and can stabilize these defects in phosphorene. We found that the magnetic moment of the SV-5|9 system is robust under uniaxial strain in the range of −10 to +10%, and it is only destroyed under biaxial compressive strain larger than 8%, when the system also suffers a semiconductor-to-metal transition. Additionally, we found that magnetism can be induced in the SV-55|66 system under uniaxial compressive strain larger than 4% along the zigzag direction and under biaxial tensile strain larger than 6%. Our findings of small formation energies and non-zero magnetic moments for both SV-5|9 and SV-55|66 systems under zigzag uniaxial compressive strain larger than 4% strongly suggest that a magnetic configuration in monolayer phosphorene can be easily realized by single-vacancy formation under uniaxial compressive strain. © 2018 Elsevier B.V.
    view abstract10.1016/j.jmmm.2018.06.016
  • Surface vibrations in the T4 and H3 Pb phases on Si(111)
    Speiser, E. and Baumann, A. and Chandola, S. and Esser, N. and Zahedifar, M. and Kratzer, P. and Tegenkamp, C.
    Physical Review B 98 (2018)
    We present here a combined experimental Raman spectroscopy and ab initio theoretical study of the vibrational modes of the (3×3) reconstructed SIC phase of Pb on Si(111) and discuss their relation to the atomic surface structure. The Raman response of the surface localized vibrational modes, in particular, is identified in the low-frequency spectral range (down to 15cm-1). We demonstrate that Raman spectroscopy is a very powerful approach to test atomic structures of surfaces and a valuable complement to standard surface analytics. While the calculated spectra of H3 and T4 are too similar to allow a discrimination of these phases, the good overall agreement to the measured Raman spectra enables a classification of the observed vibrational modes. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.195427
  • Coupling of quantum well states and phonons in thin multilayer Pb films on Si(111)
    Zahedifar, M. and Kratzer, P.
    Physical Review B 96 (2017)
    Density functional theory calculations for the electronic and phononic band structures of Pb/Si(111) thin films with a thickness of 4 and 5 monolayers (ML) are performed. We employ a Si(111)(3×3) unit cell to model the Pb films including the Si substrate, and identify quantum well (QW) states for film thicknesses between 3 and 6 ML. The calculations show that the quantum-confined state closest to the Si band gap acquires the character of a quantum well resonance with a major part of its wave function extending into the Si(111) substrate. This finding explains the unusually low dispersion of this state and its lacking sensitivity to phonons in the 5 ML Pb film. Moreover, several unoccupied QW states are identified in the calculations and are assigned to previously observed features in structurally simpler freestanding Pb films. The calculated phonon band structures of the Pb/Si(111)(3×3) films display stiff surface phonon modes in the 2.3-2.5 THz range. The electron-phonon coupling strength in the quantum-confined states is addressed by means of deformation-potential theory using the calculated atomic displacements of Γ-point phonons. It is found that both the acoustic shear deformation potential as well as the optical deformation potentials of unoccupied QW states are sizable. Comparing the results for 4 and 5 ML Pb films, we conclude that the optical deformation potentials are generally larger for the 4 ML film. The occupied QW resonance in the 5 ML Pb film shows weak electron-phonon coupling, in qualitative agreement with the small experimentally observed lifetime broadening of this state. Our results form the basis for addressing the role of electron-phonon scattering for the lifetime of unoccupied QWs acting as intermediate states in two-photon photoemission from Pb/Si(111) films. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.115442
  • Crystal Structure Induced Preferential Surface Alloying of Sb on Wurtzite/Zinc Blende GaAs Nanowires
    Hjort, M. and Kratzer, P. and Lehmann, S. and Patel, S.J. and Dick, K.A. and Palmstrøm, C.J. and Timm, R. and Mikkelsen, A.
    Nano Letters 17 (2017)
    We study the surface diffusion and alloying of Sb into GaAs nanowires (NWs) with controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal phases. Using atomically resolved scanning tunneling microscopy, we find that Sb preferentially incorporates into the surface layer of the {110}-terminated Zb segments rather than the {1120}-terminated Wz segments. Density functional theory calculations verify the higher surface incorporation rate into the Zb phase and find that it is related to differences in the energy barrier of the Sb-for-As exchange reaction on the two surfaces. These findings demonstrate a simple processing-free route to compositional engineering at the monolayer level along NWs. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b00806
  • Detection of adsorbed transition-metal porphyrins by spin-dependent conductance of graphene nanoribbon
    Kratzer, P. and Tawfik, S.A. and Cui, X.Y. and Stampfl, C.
    RSC Advances 7 (2017)
    Electronic transport in a zig-zag-edge graphene nanoribbon (GNR) and its modification by adsorbed transition metal porphyrins is studied by means of density functional theory calculations. The detachment reaction of the metal centre of the porphyrin is investigated both in the gas phase and for molecules adsorbed on the GNR. As most metal porphyrins are very stable against this reaction, it is found that these molecules bind only weakly to a perfect nanoribbon. However, interaction with a single-atom vacancy in the GNR results in chemical bonding by the transition metal centre being shared between nitrogen atoms in the porphyrin ring and the carbon atoms next to the vacancy in the GNR. For both the physisorbed and the chemisorbed geometry, the inclusion of van der Waals interaction results in a significant enlargement of the binding energy and reduction of the adsorption height. Electronic transport calculations using non-equilibrium Greens functions show that the conductivity of the GNR is altered by the chemisorbed porphyrin molecules. Since the metal centers of porphyrins carry an element-specific magnetic moment, not only the net conductance, but also the spin-dependent conductance of the GNR is affected. In particular, the adsorption of Ru-porphyrin on the single-atom vacancy results in a very large spin polarization of the current of 88% at small applied source-drain voltages. Based on our results, we suggest that a spin valve constructed from a GNR with ferromagnetic contacts could be used as a sensitive detector that could discriminate between various metal porphyrins. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ra04594h
  • Indium coverage of the Si(111)- 7 × 3 -In surface
    Suzuki, T. and Lawrence, J. and Walker, M. and Morbec, J.M. and Blowey, P. and Yagyu, K. and Kratzer, P. and Costantini, G.
    Physical Review B 96 (2017)
    The indium coverage of the Si(111)-7×3-In surface is investigated by means of x-ray photoelectron spectroscopy and first-principles density functional theory calculations. Both experimental and theoretical results indicate that the In coverage is a double layer rather than a single layer. Moreover, the atomic structure of the Si(111)-7×3-In surface is discussed by comparing experimental with simulated scanning tunneling microscopy (STM) images and scanning tunneling spectra with the calculated density of states. Our structural assignment agrees with previous studies, except for the interpretation of experimental STM images. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.035412
  • Magnetic monolayer Li2N: Density Functional Theory calculations
    Rahman, G. and Ur Rahman, A. and Kanwal, S. and Kratzer, P.
    EPL 119 (2017)
    Density functional theory (DFT) calculations are used to investigate the electronic and magnetic structures of a two-dimensional (2D) monolayer Li2N. It is shown that bulk Li3N is a non-magnetic semiconductor. The non-spin-polarized DFT calculations show that p electrons of N in 2D Li2N form a narrow band at the Fermi energy due to a low coordination number, and the density of states at the Fermi energy () is increased as compared with bulk Li3N. The large shows instability towards magnetism in Stoner's mean-field model. The spin-polarized calculations reveal that 2D Li2N is magnetic without intrinsic or impurity defects. The magnetic moment of in 2D Li2N is mainly contributed by the p z electrons of N, and the band structure shows half-metallic behavior. Dynamic instability in planar Li2N monolayer is observed, but a buckled Li2N monolayer is found to be dynamically stable. The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms is also investigated to access the exchange field strength. We found that planar (buckled) 2D Li2N is a ferromagnetic material with Curie temperature T c of 161 (572) K. © EPLA, 2017.
    view abstract10.1209/0295-5075/119/57002
  • Native defects in the Co2TiZ (Z=Si,Ge,Sn) full Heusler alloys: Formation and influence on the thermoelectric properties
    Popescu, V. and Kratzer, P. and Wimmer, S. and Ebert, H.
    Physical Review B 96 (2017)
    We have performed first-principles investigations on the native defects in the half-metallic, ferromagnetic full Heusler alloys Co2TiZ (Z one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that the Co vacancies (VcCo) and the TiSn as well as the CoZ or CoTi antisites exhibit the smallest formation energies. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced changes in the electronic structure above the half-metallic gap. Two of the systems, VcCo and CoZ, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co2TiZ compounds as being due to unintentionally created native defects. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.054443
  • The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface
    Morbec, J.M. and Kratzer, P.
    Journal of Chemical Physics 146 (2017)
    Using first-principles calculations based on density-functional theory (DFT), we investigated the effects of the van der Waals (vdW) interactions on the structural and electronic properties of anthracene and pentacene adsorbed on the Ag(111) surface. We found that the inclusion of vdW corrections strongly affects the binding of both anthracene/Ag(111) and pentacene/Ag(111), yielding adsorption heights and energies more consistent with the experimental results than standard DFT calculations with generalized gradient approximation (GGA). For anthracene/Ag(111) the effect of the vdW interactions is even more dramatic: we found that “pure” DFT-GGA calculations (without including vdW corrections) result in preference for a tilted configuration, in contrast to the experimental observations of flat-lying adsorption; including vdW corrections, on the other hand, alters the binding geometry of anthracene/Ag(111), favoring the flat configuration. The electronic structure obtained using a self-consistent vdW scheme was found to be nearly indistinguishable from the conventional DFT electronic structure once the correct vdW geometry is employed for these physisorbed systems. Moreover, we show that a vdW correction scheme based on a hybrid functional DFT calculation (HSE) results in an improved description of the highest occupied molecular level of the adsorbed molecules. © 2017 Author(s).
    view abstract10.1063/1.4973839
  • Towards a standardized setup for surface energy calculations
    Kaminski, J.W. and Kratzer, P. and Ratsch, C.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    High-throughput design of new materials with desired electronic properties, based on screening of large collections of crystal structures organized in the from of libraries or databases require fast, widely applicable, consistent and unsupervised methods to calculate the property of interest. In this work we present an approach for the calculation of surface energies of two-dimensional periodic crystal lattices which meets all these requirements. For materials slabs which are terminated with two identical surfaces, the task of calculating the surface energy is trivial. More problematic are the cases where both terminating surfaces are different, as there is no single established method allowing for equal treatment of a wide range of surface morphologies and orientations. Our proposed approach addresses this problem. It relies on appropriately chosen capping atoms, whose bonding energy contributions are used to approximate the total energy of the surface. The choice of the capping atoms is governed by a set of simple guidelines that are applicable for surfaces with different terminations. We present the results for different semiconductor materials and show that our approach leads to surface energies with errors that are below 10%, and that are as low as 2% in many cases. We show that hydrogen is not always the best choice for a capping atom if accurate surface energies are the target of the calculations. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.085408
  • Ternary semiconductors NiZrSn and CoZrBi with half-Heusler structure: A first-principles study
    Fiedler, G. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The ternary semiconductors NiZrSn and CoZrBi with C1b crystal structure are introduced by calculating their basic structural, electronic, and phononic properties using density functional theory. Both the gradient-corrected PBE functional and the hybrid functional HSE06 are employed. While NiZrSn is found to be a small-band-gap semiconductor (Eg=0.46 eV in PBE and 0.60 eV in HSE06), CoZrBi has a band gap of 1.01 eV in PBE (1.34 eV in HSE06). Moreover, effective masses and deformation potentials are reported. In both materials ABC, the intrinsic point defects introduced by species A (Ni or Co) are calculated. The Co-induced defects in CoZrBi are found to have a higher formation energy compared to Ni-induced defects in NiZrSn. The interstitial Ni atom (Nii) as well as the VNiNii complex introduce defect states in the band gap, whereas the Ni vacancy (VNi) only reduces the size of the band gap. While Nii is electrically active and may act as a donor, the other two types of defects may compensate extrinsic doping. In CoZrBi, only the VCoCoi complex introduces a defect state in the band gap. Motivated by the reported use of NiZrSn for thermoelectric applications, the Seebeck coefficient of both materials, both in the p-type and the n-type regimes, is calculated. We find that CoZrBi displays a rather large thermopower of up to 500μV/K when p doped, whereas NiZrSn possesses its maximum thermopower in the n-type regime. The reported difficulties in achieving p-type doping in NiZrSn could be rationalized by the unintended formation of Nii2+ in conjunction with extrinsic acceptors, resulting in their compensation. Moreover, it is found that all types of defects considered, when present in concentrations as large as 3%, tend to reduce the thermopower compared to ideal bulk crystals at T=600 K. For NiZrSn, the calculated thermodynamic data suggest that additional Ni impurities could be removed by annealing, leading to precipitation of a metallic Ni2ZrSn phase. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.075203
  • Thermoelectric properties of Ge/Si heterostructures: A combined theoretical and experimental study
    Reith, H. and Nielsch, K. and Fiedler, G. and Nausner, L. and Hu, Y. and Chen, P. and Rastelli, A. and Kratzer, P.
    Physica Status Solidi (A) Applications and Materials Science 213 (2016)
    We present a combined experimental and theoretical investigation of the thermoelectric properties of p-doped Ge/Si superlattices grown on Si(001) substrates by molecular beam epitaxy. Electrical conductivity is measured both in the direction parallel and perpendicular to the interfaces by means of a modified transfer length method. Electronic transport is strongly anisotropic, with the cross-plane conductivity being about five times lower than in plane. This result is in very good agreement with the theoretical predictions based on the tight-binding method combined with the Boltzmann equation applied to the experimentally investigated structure. The cross-plane thermal conductivity of doped superlattices is measured with the differential 3ω method and compared with that of undoped superlattices and alloys with similar average Ge content. The comparison reveals that superlattices have strongly reduced thermal conduction compared to alloys, and that doping increases their thermal conductivity by about 50%. Considering the used doping level, this increase appears surprising. The Seebeck coefficient of the structures is addressed theoretically and displays a less pronounced anisotropy compared to the electric conductivity. Combined with the knowledge of the other thermoelectric parameters, we conclude that, while p-doped Si/Ge superlattices may be used as model systems for the investigation of thermoelectric transport in nanostructured materials, their relevance for application is limited. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532486
  • Thermoelectric Properties of Half-Heusler Heterostructures from Ab Initio Calculations
    Fiedler, G. and Kratzer, P.
    Journal of Electronic Materials 45 (2016)
    Semiconducting half-Heusler alloys have recently emerged as a class of thermoelectric materials with outstanding performance in the medium- to high-temperature range. Heterostructures promise a further reduction of thermal conductivity as a result of phonon scattering at interfaces. Here, both the electronic and phononic spectra of half-Heusler compounds based on Ti, Zr, and Hf are calculated using density functional theory. With this input, thermoelectric properties are obtained, and the thermal conductivity of a heterostructure superlattice is estimated by extending the diffuse mismatch model of interface conductance. We find that a high power factor (Formula presented.) can be retained in a short-period superlattice, while thermal conductivity is reduced compared to that in single-phase half-Heusler crystals. © 2015, The Minerals, Metals & Materials Society.
    view abstract10.1007/s11664-015-4205-7
  • Atomic-scale detection of magnetic impurity interactions in bulk semiconductors
    Geisler, B. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    We demonstrate on the basis of ab initio simulations how passivated semiconductor surfaces can be exploited to study bulklike interaction properties and wave functions of magnetic impurities on the atomic scale with conventional and spin-polarized scanning tunneling microscopy. By applying our approach to the case of 3d transition metal impurities close to the H/Si(111) surface, we show exemplarily that their wave functions in Si are less extended than for Mn in GaAs, thus obstructing ferromagnetism in Si. Finally, we discuss possible applications of this method to other dilute magnetic semiconductors. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.100407
  • Large morphological sensitivity of the magneto-thermopower in Co/Cu multilayered systems
    Popescu, V. and Kratzer, P.
    New Journal of Physics 17 (2015)
    We present results of first-principles calculations on the transport properties, both under an electric field or a temperature gradient, in Co/Cu multilayered systems. The various effects brought about by the changes in the morphological parameters, such as the number of repeats and the layer thickness, are discussed in a systematic way. Our calculations show that the Seebeck coefficient and the magneto-thermopower (MTP) converge rather rapidly with the number of Co repeats. In the range of thin Co layers, we find strong variations in the amplitude and sign of both the Seebeck coefficient and the MTP. These large variations, which have no correspondent in the (magneto)conductance, are shown to be the result of quantum well states present in the minority spin channel of thin Co layers. ©2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
    view abstract10.1088/1367-2630/17/3/033036
  • Reduced thermal conductivity of TiNiSn/HfNiSn superlattices
    Hołuj, P. and Euler, C. and Balke, B. and Kolb, U. and Fiedler, G. and Müller, M.M. and Jaeger, T. and Chávez Angel, E. and Kratzer, P. and Jakob, G.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Diminution of the thermal conductivity is a crucial aspect in thermoelectric research. We report a systematic and significant reduction of the cross-plane thermal conductivity in a model system consisting of dc sputtered TiNiSn and HfNiSn half-Heusler superlattices. The reduction of κ is measured by the 3ω method and originates from phonon scattering at the internal interfaces. Heat transport in the superlattices is calculated based on Boltzmann transport theory, including a diffusive mismatch model for the phonons at the internal interfaces. Down to a superlattice periodicity of 3 nm the phonon spectrum mismatch between the superlattice components quantitatively explains the reduction of κ. For very thin individual layers the interface model breaks down and the artificial crystal shows an enhanced κ. © 2015 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
    view abstract10.1103/PhysRevB.92.125436
  • Spincaloric properties of epitaxial Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions
    Geisler, B. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic Co2MnSi Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. An approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it accounts for temperature-dependent chemical potentials in the electrodes and finite-bias effects. We find that especially the former are important for obtaining qualitatively correct results, even if the variations of the chemical potentials are small. It is shown how the spincaloric properties can be tailored by the choice of the growth conditions. We find a large effective and spin-dependent Seebeck coefficient of -65μV/K at room temperature for the purely Co-terminated interface. We suggest to use such interfaces in thermally operated magnetoresistive random access memory modules, which exploit the magneto-Seebeck effect, to maximize the thermally induced readout voltage. © 2015 American Physical Society. ©2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.144418
  • Unoccupied electronic structure and momentum-dependent scattering dynamics in Pb/Si(557) nanowire arrays
    Syed, A.S. and Trontl, V.M. and Ligges, M. and Sakong, S. and Kratzer, P. and Lükermann, D. and Zhou, P. and Avigo, I. and Pfnür, H. and Tegenkamp, C. and Bovensiepen, U.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    The unoccupied electronic structure of quasi-one-dimensional reconstructions of Pb atoms on a Si(557) surface is investigated by means of femtosecond time- and angle-resolved two-photon photoemission. Two distinct unoccupied electronic states are observed at E-EF=3.55 and 3.30 eV, respectively. Density functional theory calculations reveal that these states are spatially located predominantly on the lead wires and that they are energetically degenerated with an energy window of reduced electronic density of states in Si. We further find momentum-averaged lifetimes of 24 and 35 fs of these two states, respectively. The photoemission yield and the population dynamics depend on the electron momentum component perpendicular to the steps of the Si substrate, and the momentum-dependent dynamics cannot be described by means of rate equations. We conclude that momentum- and direction-dependent dephasing of the electronic excitations, likely caused by elastic scattering at the step edges on the vicinal surface, modifies the excited-state population dynamics in this system. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.134301
  • Atomistic calculation of the thermoelectric properties of Si nanowires
    Bejenari, I. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The thermoelectric properties of 1.6-nm-thick Si square nanowires with [100] crystalline orientation are calculated over a wide temperature range from 0 K to 1000 K, taking into account atomistic electron-phonon interaction. In our model, the [010] and [001] facets are passivated by hydrogen and there are Si-Si dimers on the nanowire surface. The electronic structure was calculated by using the sp3 spin-orbit-coupled atomistic second-nearest-neighbor tight-binding model. The phonon dispersion was calculated from a valence force field model of the Brenner type. A scheme for calculating electron-phonon matrix elements from a second-nearest-neighbor tight-binding model is presented. Based on Fermi's golden rule, the electron-phonon transition rate was obtained by combining the electron and phonon eigenstates. Both elastic and inelastic scattering processes are taken into consideration. The temperature dependence of transport characteristics was calculated by using a solution of the linearized Boltzmann transport equation obtained by means of the iterative orthomin method. At room temperature, the electron mobility is 195 cm2 V-1 s-1 and increases with temperature, while a figure of merit ZT=0.38 is reached for n-type doping with a concentration of n=1019 cm-3. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.045429
  • Electronic and structural differences between wurtzite and zinc blende inas nanowire surfaces: Experiment and theory
    Hjort, M. and Lehmann, S. and Knutsson, J. and Zakharov, A.A. and Du, Y.A. and Sakong, S. and Timm, R. and Nylund, G. and Lundgren, E. and Kratzer, P. and Dick, K.A. and Mikkelsen, A.
    ACS Nano 8 (2014)
    We determine the detailed differences in geometry and band structure between wurtzite (Wz) and zinc blende (Zb) InAs nanowire (NW) surfaces using scanning tunneling microscopy/spectroscopy and photoemission electron microscopy. By establishing unreconstructed and defect-free surface facets for both Wz and Zb, we can reliably measure differences between valence and conduction band edges, the local vacuum levels, and geometric relaxations to the few-millielectronvolt and few-picometer levels, respectively. Surface and bulk density functional theory calculations agree well with the experimental findings and are used to interpret the results, allowing us to obtain information on both surface and bulk electronic structure. We can thus exclude several previously proposed explanations for the observed differences in conductivity of Wz-Zb NW devices. Instead, fundamental structural differences at the atomic scale and nanoscale that we observed between NW surface facets can explain the device behavior. © 2014 American Chemical Society.
    view abstract10.1021/nn504795v
  • First-principles study of spin-dependent thermoelectric properties of half-metallic Heusler thin films between platinum leads
    Comtesse, D. and Geisler, B. and Entel, P. and Kratzer, P. and Szunyogh, L.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The electronic and magnetic bulk properties of half-metallic Heusler alloys such as Co2FeSi, Co2FeAl, Co2MnSi, and Co2MnAl are investigated by means of ab initio calculations in combination with Monte Carlo simulations. The electronic structure is analyzed using the plane-wave code quantum espresso and the magnetic exchange interactions are determined using the Korringa-Kohn-Rostoker (KKR) method. From the magnetic exchange interactions, the Curie temperature is obtained via Monte Carlo simulations. In addition, electronic transport properties of trilayer systems consisting of two semi-infinite platinum leads and a Heusler layer in-between are obtained from the fully relativistic screened KKR method by employing the Kubo-Greenwood formalism. The focus is on thermoelectric properties, namely, the Seebeck effect and its spin dependence. It turns out that already thin Heusler layers provide highly spin-polarized currents. This is attributed to the recovery of half-metallicity with increasing layer thickness. The absence of electronic states of spin-down electrons around the Fermi level suppresses the contribution of this spin channel to the total conductance, which strongly influences the thermoelectric properties and results in a spin polarization of thermoelectric currents. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.094410
  • Interplay of growth mode and thermally induced spin accumulation in epitaxial Al/Co2TiSi/Al and Al/Co2TiGe/Al contacts
    Geisler, B. and Kratzer, P. and Popescu, V.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between Al leads was investigated by means of ab initio calculations of the thermodynamic stability and electronic transport. We have focused on two main issues and found that (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co2TiZ (Z = Si or Ge) is stable under various growth conditions; and (ii) the conventional and spin-dependent Seebeck coefficients in such heterojunctions exhibit a strong dependence on both the spacer and the atomic composition of the Al/Heusler interface. The latter quantity gives a measure of the spin accumulation and varies between +8 and ∼3 V/K near 300 K, depending on whether a Ti-Ge or a Co-Co plane makes the contact between Al and Co2TiGe in the trilayer. Our results show that it is in principle possible to tailor the spin-caloric effects by a targeted growth control of the samples. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.184422
  • Interplay of hydrogen treatment and nitrogen doping in ZnO nanoparticles: A first-principles study
    Gutjahr, J. and Sakong, S. and Kratzer, P.
    Nanotechnology 25 (2014)
    With the help of density functional calculations using the HSE and PBE functionals, it is shown that incorporation of nitrogen into ZnO nanoparticles is energetically less costly compared to ZnO bulk, due to charge transfer between Zn dangling bonds and the NO impurity. Neutral NO results after full passivation of the doped nanoparticles by a treatment with atomic hydrogen. A nanocomposite made from such ZnO particles could show thermally activated p-type hopping conductivity. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/25/14/145204
  • As vacancies, Ga antisites, and Au impurities in zinc blende and wurtzite GaAs nanowire segments from first principles
    Du, Y.A. and Sakong, S. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    In this paper some specific issues related to point defects in GaAs nanowires are addressed with the help of density functional theory calculations. These issues mainly arise from the growth of nanowires under conditions different from those used for thin film or bulk GaAs, such as the coexistence of zinc blende and wurtzite polytypes, the use of gold particles as catalyst, and the arsenic-limited growth regime. Hence, we carry out density functional calculations for As vacancies, GaAs antisites, and Au impurities in zinc blende (ZB) and wurtzite (WZ) GaAs crystals. Our results show that As vacancies can diffuse within a ZB GaAs crystal with migration barriers of ∼1.9 eV. Within WZ GaAs, As vacancy diffusion is found to be anisotropic, with low barriers of 1.60 up to 1.79 eV (depending on doping conditions) in the ab plane, while there are higher barriers of 2.07 to 2.44 eV to diffuse along the c axis. The formation energy of Au impurities is found to be generally much lower than those of arsenic vacancies or GaAs antisites. Thus, Au impurities will be the dominant defects formed in Au-catalyzed nanowire growth. Moreover, we find that it is energetically more favorable by 1 to 2 eV for a Au impurity to replace a lattice Ga atom than a lattice As atom in GaAs. A Au substitutional defect for a lattice Ga atom in ZB GaAs is found to create a charge transfer level in the lower half of the band gap. While our calculations locate this level at Ev+0.22 eV, taking into account the inaccuracy of the density functional that ought to be corrected by a downshift of E v by about 0.2 eV, results in good agreement with the experimental result of Ev+0.4 eV. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.075308
  • Atomistic modeling of the Au droplet-GaAs interface for size-selective nanowire growth
    Sakong, S. and Du, Y.A. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    Density functional theory calculations within both the local density approximation and the generalized gradient approximation are used to study Au-catalyzed growth under near-equilibrium conditions. We discuss both the chemical equilibrium of a GaAs nanowire with an As2 gas atmosphere and the mechanical equilibrium between the capillary forces at the nanowire tip. For the latter goal, the interface between the gold nanoparticle and the nanowire is modeled atomically within a slab approach, and the interface energies are evaluated from the total energies of the model systems. We discuss three growth regimes, one catalyzed by an (almost) pure Au particle, an intermediate alloy-catalyzed growth regime, and a Ga-catalyzed growth regime. Using the interface energies calculated from the atomic models, as well as the surface energies of the nanoparticle and the nanowire sidewalls, we determine the optimized geometry of the nanoparticle-capped nanowire by minimizing the free energy of a continuum model. Under typical experimental conditions of 10-4 Pa As2 and 700 K, our results in the local density approximation are insensitive to the Ga concentration in the nanoparticle. In these growth conditions, the energetically most favored interface has an interface energy of around 45 meV/Å2, and the correspondingly optimized droplet on top of a GaAs nanowire is somewhat larger than a hemisphere and forms a contact angle around 130° for both pure Au and Au-Ga alloy nanoparticles. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.155309
  • Comparison of density functionals for nitrogen impurities in ZnO
    Sakong, S. and Gutjahr, J. and Kratzer, P.
    Journal of Chemical Physics 138 (2013)
    Hybrid functionals and empirical correction schemes are compared to conventional semi-local density functional theory (DFT) calculations in order to assess the predictive power of these methods concerning the formation energy and the charge transfer level of impurities in the wide-gap semiconductor ZnO. While the generalized gradient approximation fails to describe the electronic structure of the N impurity in ZnO correctly, methods that widen the band gap of ZnO by introducing additional non-local potentials yield the formation energy and charge transfer level of the impurity in reasonable agreement with hybrid functional calculations. Summarizing the results obtained with different methods, we corroborate earlier findings that the formation of substitutional N impurities at the oxygen site in ZnO from N atoms is most likely slightly endothermic under oxygen-rich preparation conditions, and introduces a deep level more than 1 eV above the valence band edge of ZnO. Moreover, the comparison of methods elucidates subtle differences in the predicted electronic structure, e.g., concerning the orientation of unoccupied orbitals in the crystal field and the stability of the charged triplet state of the N impurity. Further experimental or theoretical analysis of these features could provide useful tests for validating the performance of DFT methods in their application to defects in wide-gap materials. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4810862
  • Interface defects and impurities at the growth zone of Au-catalyzed GaAs nanowire from first principles
    Sakong, S. and Du, Y.A. and Kratzer, P.
    Physica Status Solidi - Rapid Research Letters 7 (2013)
    The defects and impurities at the interface of a Au-catalyzed GaAs nanowire have been studied by the first-principles method. The interface is modeled by Au layers on the ${\rm GaAs}(\bar 1\bar 1\bar 1)$ substrate with both Ga- and As-terminations. From the energetics of interface defects and impurities, we find that a highly ordered As-terminated interface is expected under As-rich growth, but mixed Ga- and As-terminations are expected under Ga-rich growth. Comparing the interface defects and impurities to their bulk species, we expect the interface to be a sink for Au impurities in the GaAs nanowire. Based on DFT results, we estimate that materials transport by impurity diffusion through a liquid nanoparticle is sufficient for sustained GaAs growth. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201307210
  • Large Seebeck magnetic anisotropy in thin Co films embedded in Cu determined by ab initio investigations
    Popescu, V. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The longitudinal thermopower of a Cu/Co/Cu trilayer system exhibits an oscillatory dependence on the thickness of the Co layer, a behavior related to the formation of quantum well states in the minority spin channel. In addition, it is found to be very sensitive to a switching between an in-plane and out-of-plane magnetization. The resulting magnetothermopower (MTP) is therefore much larger than anticipated from the conventional anisotropic magnetoresistance (AMR). Our calculations establish a direct connection between the magnitude of the MTP signal and the asymmetry of the AMR around the Fermi energy. An enhancement of MTP based on this understanding may offer the possibility of implementing an efficient spin read-out thermoelectric device based on a single ferromagnetic layer. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.104425
  • Mode conversion and long-lived vibrational modes in lead monolayers on silicon (111) after femtosecond laser excitation: A molecular dynamics simulation
    Sakong, S. and Kratzer, P. and Wall, S. and Kalus, A. and Horn-Von Hoegen, M.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The dynamics of vibrations localized in a monolayer of Pb on Si(111) is studied using density functional theory and molecular dynamics methods. Recently, a pump-probe experiment has given direct access to the relaxation dynamics of the vibrational excitations in Pb, revealing two distinct time scales which are separately related to the directions of the Pb vibrational motions. We analyze the experimental findings theoretically using atomistic modeling. After excitation of two-dimensional optical phonons, equilibration between parallel and perpendicular modes is observed before dissipation into the Si substrate. Even after several nanoseconds, equilibration with the substrate is not achieved due to energy trapped in acoustic Pb modes. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.115419
  • Strain stabilization and thickness dependence of magnetism in epitaxial transition metal monosilicide thin films on Si(111)
    Geisler, B. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    We present a comprehensive study of different 3d transition metal monosilicides in their ground state crystal structure (B20), ranging from equilibrium bulk over biaxially strained bulk to epitaxial thin films on Si(111), by means of density functional theory. The magnetic properties of MnSi and FeSi films are found to be considerably modified due to the epitaxial strain induced by the substrate. In MnSi bulk material, which can be seen as a limit of thick films, we find a strain-induced volume expansion, an increase of the magnetic moments, and a significant rise of the energy difference between different spin configurations. The latter can be associated with an increase of the Curie temperature, which is in accordance with recent experimental results. While a ferromagnetic spin alignment is found to be the ground state also for ultrathin films, we show that for films of intermediate thickness a partially compensating magnetic ordering is more favorable; however, the films retain a net magnetic moment. Furthermore, we analyze the orbital structure in FeSi around the band gap, which can be located somewhere in the density of states for all studied B20 transition metal monosilicides, and find that FeSi becomes metallic and ferromagnetic under epitaxial strain. Finally, the influence of on-site electronic correlation and the reliability of ab initio calculations for 3d transition metal monosilicides are discussed. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.115433
  • Surface morphology of MnSi thin films grown on Si(111)
    Suzuki, T. and Lutz, T. and Geisler, B. and Kratzer, P. and Kern, K. and Costantini, G.
    Surface Science 617 (2013)
    The surface morphology of MnSi thin films grown on Si(111)-7 × 7 substrates was investigated by systematically changing the amount of deposited Mn. A new 3 × 3 surface reconstruction was found at the very initial growth stages, whose atomic configuration was analyzed both experimentally and theoretically. At a coverage of 0.1 monolayers, the formation of nanometer-sized MnSi islands was observed in coexistence with Mn nanoclusters that fit within the 7 × 7 half unit cell. With increasing Mn deposition, the MnSi islands grow, develop extended flat tops and eventually coalesce into an atomically flat film with a high corrugated 3×3 reconstruction punctuated by several holes. The successive film growth mode is characterized by the formation of MnSi quadlayers with a low corrugated 3×3 reconstruction. © 2013 Elsevier B.V.
    view abstract10.1016/j.susc.2013.08.005
  • Theoretical prediction of improved figure-of-merit in Si/Ge quantum dot superlattices
    Fiedler, G. and Kratzer, P.
    New Journal of Physics 15 (2013)
    A detailed theoretical model for thermoelectric transport perpendicular to the multilayers of a Si-Ge heterostructure is presented. The electronic structure of a three-dimensional superlattice, consisting of a regular array of Ge quantum dots in each layer, capped by Si layers, is calculated using an atomistic tight-binding approach. The Seebeck coefficient, the electric conductivity and the contribution of the electrons to the thermal conductivity for n-doped samples are worked out within Boltzmann transport theory. Using experimental literature data for the lattice thermal conductivity, we determine the temperature dependence of the figure of merit ZT. A nonlinear increase of ZT with temperature is found, with ZT > 2 at T = 1000 K in highly doped samples. Moreover, we find an enhanced thermoelectric power factor already at room temperature and below, which is due to highly mobile electrons in strain-induced conductive channels. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/15/12/125010
  • Anisotropic ferromagnetism in carbon-doped zinc oxide from first-principles studies
    Nayak, S.K. and Gruner, M.E. and Sakong, S. and Sil, S. and Kratzer, P. and Behera, S.N. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the nonspinpolarized C Zn impurity is under almost all conditions thermodynamically more stable than the C O impurity which has a magnetic moment of 2μ B, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize d0 ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C O-C O impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal ab plane of wurtzite ZnO than along the c axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C O impurities in ZnO. From the calculated results, we derive that a C O concentration between 2% and 6% should be optimal to achieve d0-ferromagnetism in C-doped ZnO. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.054441
  • Catalytic role of gold nanoparticle in GaAs nanowire growth: A density functional theory study
    Kratzer, P. and Sakong, S. and Pankoke, V.
    Nano Letters 12 (2012)
    The energetics of Ga, As, and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density functional calculations. Apart from formation of the compound Au 7Ga 2, Ga is found to form a surface alloy with gold with comparable ΔH ∼ -0.5 eV for both processes. Dissociative adsorption of As 2 is found to be exothermic by more than 2 eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle in vacuo in contact with a GaAs substrate. The results of the calculations are interpreted in the context of Au-catalyzed growth of GaAs nanowires. We argue that arsenic is supplied to the growth zone of the nanowire mainly by impingement of molecules on the gold particle and identify a regime of temperatures and As 2 partial pressures suitable for Au-catalyzed nanowire growth in molecular beam epitaxy. © 2012 American Chemical Society.
    view abstract10.1021/nl204004p
  • Growth mode and atomic structure of MnSi thin films on Si(111)
    Geisler, B. and Kratzer, P. and Suzuki, T. and Lutz, T. and Costantini, G. and Kern, K.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    Thin films of MnSi(111) in B20 structure formed by reactive epitaxy on Si(111) are studied using scanning tunneling microscopy (STM) and density functional theory calculations. Coexisting √3×√3 structures with high or low corrugation are observed and assigned to different Mn coverage by using a detailed analysis of simulated STM images. Comparison with our interpretation of STM images of films previously grown by codeposition of Mn and Si provides us with evidence that the stacking sequence of Mn and Si lattice planes depends on the growth protocol. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.115428
  • Role of sidewall diffusion in GaAs nanowire growth: A first-principles study
    Pankoke, V. and Sakong, S. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    The molecular processes during the growth of GaAs nanowires in molecular beam epitaxy (MBE) are studied from first principles. For the wurtzite crystal structure of GaAs, which is formed exclusively in nanowire growth, potential energy surfaces for sidewall diffusion of Ga, As, and GaAs surface species are calculated using density functional theory. We compare materials transport on type-I and -II nanowires (with {101̄0} and {112̄0} facets of wurtzite GaAs, respectively) and discuss its role for materials supply to the growth zone at the nanowire tip. On the sidewalls of type-II nanowires, the diffusion barrier for Ga along the growth direction is particularly low, only 0.30 eV compared to 0.60 eV on type-I nanowires. For As adatoms, the corresponding diffusion barriers are 0.64 eV and 1.20 eV, respectively, and hence higher than for Ga adatoms. The GaAs molecule formed by the chemical surface reaction of Ga and As finds very stable binding sites on type-II sidewalls where it inserts itself into a chemical bond between surface atoms, triggering radial growth. In contrast, on type-I nanowires the GaAs molecule adsorbed with the As end towards the surface has a low diffusion barrier of 0.50 eV. Together with our previous finding that the gold particle at the nanowire tip is efficient in promoting dissociative adsorption of As 2 molecules, we conclude that the influx of Ga adatoms from sidewall diffusion is very important to maintain stoichiometric growth of GaAs nanowires, in particular when a large V-III ratio is used in MBE. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.085425
  • Analytic many-body potential for GaAs(001) homoepitaxy: Bulk and surface properties
    Fichthorn, K.A. and Tiwary, Y. and Hammerschmidt, T. and Kratzer, P. and Scheffler, M.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    We employ atomic-scale simulation methods to investigate bulk and surface properties of an analytic Tersoff-Abell type potential for describing interatomic interactions in GaAs. The potential is a modified form of that proposed by Albe and colleagues [Phys. Rev. BPRBMDO1098-012110.1103/PhysRevB.66. 035205 66, 035205 (2002)] in which the cut-off parameters for the As-As interaction have been shortened. With this modification, many bulk properties predicted by the potential for solid GaAs are the same as those in the original potential, but properties of the GaAs(001) surface better match results from first-principles calculations with density-functional theory (DFT). We tested the ability of the potential to reproduce the phonon dispersion and heat capacity of bulk solid GaAs by comparing it to experiment and the overall agreement is good. In the modified potential, the GaAs(001) β2(2×4) reconstruction is favored under As-rich growth conditions in agreement with DFT calculations. Additionally, the binding energies and diffusion barriers for a Ga adatom on the β2(2×4) reconstruction generally match results from DFT calculations. These studies indicate that the potential is suitable for investigating aspects of GaAs(001) homoepitaxy. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.195328
  • Calculation of the diameter-dependent polytypism in GaAs nanowires from an atomic motif expansion of the formation energy
    Pankoke, V. and Kratzer, P. and Sakong, S.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The formation energies of GaAs nanowires have been calculated from a structural motif approach supported by first-principles data for small-diameter nanowires. GaAs bulk material has zincblende (ZB) structure, but the ground state of nanowires may be either ZB or wurtzite (WZ), possibly depending on energetic contributions from their surfaces and edges. The calculated nanowires are cut from the bulk material in the 111 direction and the 0001 direction for ZB and WZ structure, respectively. We consider nanowires with hexagonal cross sections and (112̄0) and (101̄0) facets in the case of WZ, and (101̄) and (112̄) for ZB nanowires. It is found that the WZ formation energy of small nanowires is lower than the ZB one due to the lower WZ surface energy. This holds if edge energies are neglected. The role of additional dangling bonds at the edges and its effect on the WZ-ZB transition is discussed. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.075455
  • Density functional study of carbon doping in ZnO
    Sakong, S. and Kratzer, P.
    Semiconductor Science and Technology 26 (2011)
    The formation energy and charge states of substitutional and interstitial C impurities and their complexes in ZnO have been studied using density functional theory calculations. While single CZn defects have the highest absolute stability, interstitial C in n-type ZnO prefers to form interstitial C2 pairs or CZn-Ci complexes, thereby lowering the defect formation energy. Moreover, those atomic C impurities that have low formation energy are found to be nonmagnetic in their stable charge states. However, both in p-type and n-type ZnO, certain charge states of C2 complexes possessing a spin magnetic moment are identified. This might give a clue why both p-type and n-type magnetism have been reported for C-doped ZnO samples. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0268-1242/26/1/014038
  • Isotopic effect on the vibrational lifetime of the carbon-deuterium stretch excitation on graphene
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 135 (2011)
    The relaxation of vibrational energy in the H and D stretch modes has been studied on the graphene surface using ab initio calculations. The dissipation of the vibrational energy stored in the stretching modes proceeds through vibration-phonon coupling, while the dissipation through electronic excitations makes only minor contributions. Recently, we reported the fast relaxation of the H stretch energy on graphene [S. Sakong and P. Kratzer, J. Chem. Phys. 133, 054505 (2010)]10.1063/1.3474806. Interestingly, we predict the lifetime of the D stretch to be markedly longer compared to the relaxation of the H stretch. This is unexpected since the vibrational amplitudes at carbon atoms in the joint C-D vibrational modes are larger than in the joint C-H modes, due to the mass ratio mDmC mHmC. However, the vibrational relaxation rate for the D stretch is smaller than for the H stretch, because the energy is dissipated to an acoustic phonon of graphene in the case of C-D rather than an optical phonon as is the case in C-H, and hence, the corresponding phonon density of states is lower in the C-D case. To rationalize our findings, we propose a general scheme for estimating vibrational lifetimes of adsorbates based on four factors: the density of states of the phonons that mediates the transitions, the vibration-phonon coupling strength, the anharmonic coupling between local modes, and the number of quanta involved in the transitions. Mainly the first two of these factors are responsible for the differences in the lifetimes of the C-H and C-D stretches. The possible role of the other factors is illustrated in the context of vibrational lifetimes in other recently studied systems. © 2011 American Institute of Physics.
    view abstract10.1063/1.3637040
  • Effects of wetting layer structure on surface phase stability and on indium surface diffusion
    Rosini, M. and Kratzer, P. and Magri, R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 7 (2010)
    We study the effects of surface reconstruction and step formation on the surface phase stability, of an InAs wetting layer on GaAs(001). In particular we focus our attention on the α2 and β2 (2×4) surface reconstructions. The two investigated reconstructions have been shown to be formed at an high In coverage, at the onset of the 2D→3D transition. The analysis of the connection between the step stability and the strain distribution around the step edges leads to the conclusion that the favoured step geometries are those minimising the strain. Finally, In diffusion on the flat reconstructed wetting layers has been investigated.We find: (i) the elements of the surface reconstructions favouring In diffusion; (ii) that In diffusion on these surfaces is strongly anisotropic, favoring the [-110] direction; (iii) that the As surface dimers introduce additional adsorption sites with high barriers for In escape. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.200982501
  • Electronic excitations in magnesium epitaxy: Experiment and theory
    Hagemann, U. and Timmer, M. and Krix, D. and Kratzer, P. and Nienhaus, H.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The nonadiabatic response of the electronic system during growth of Mg films is investigated both experimentally by measuring chemicurrents in Mg/p-Si (001) Schottky diodes, and theoretically by time-dependent perturbation theory applied to first-principles electronic-structure calculations. Reverse currents are detected in the diodes when they are exposed to thermally evaporated Mg atoms. Dissipation of condensation energy to the electronic system as well as absorption of infrared photons due to heat radiation are the current-generating mechanisms. They can be distinguished by studying the dependence of the currents on the evaporator temperature and on the Mg film thickness. In contrast to the photocurrents, the chemicurrent is proportional to the Mg atom flux as it reproduces the enthalpy of Mg sublimation in an Arrhenius diagram. Independent measurements of photocurrents by use of an empty evaporator as a source of heat radiation provide further evidence for a chemicurrent contribution to the overall signal. The presence of chemicurrents in Mg epitaxy is further supported by simulations of monolayer growth and calculations of the pertinent rates for nonadiabatic electronic transitions in Mg adsorption. The simulations show that the grown surface is atomically rough with many step and kink sites. Adsorption at these sites is sufficiently exothermic to induce energetic electron-hole pairs that give rise to a detectable current across the Schottky barrier of the diode. The calculated spectra of the excited electrons and holes are found to display high-energy tails above 0.4 eV. While the contribution of the electronic channel to the dissipation of condensation energy is very small (less than 1%), the calculated probability for high-energy electronic excitations in Mg epitaxy is compatible with the chemicurrent contribution extracted from the experimental data. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.155420
  • Erratum: Analytic many-body potential for InAs/GaAs surfaces and nanostructures: Formation energy of InAs quantum dots (Physical Review B (2008) 77 (235303)
    Hammerschmidt, T. and Kratzer, P. and Scheffler, M.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    view abstract10.1103/PhysRevB.81.159905
  • Hydrogen vibrational modes on graphene and relaxation of the C-H stretch excitation from first-principles calculations
    Sakong, S. and Kratzer, P.
    Journal of Chemical Physics 133 (2010)
    Density functional theory (DFT) calculations are used to determine the vibrational modes of hydrogen adsorbed on graphene in the low-coverage limit. Both the calculated adsorption energy of a H atom of 0.8 eV and calculated C-H stretch vibrational frequency of 2552 cm-1 are unusually low for hydrocarbons, but in agreement with data from electron energy loss spectroscopy on hydrogenated graphite. The clustering of two adsorbed H atoms observed in scanning tunneling microscopy images shows its fingerprint also in our calculated spectra. The energetically preferred adsorption on different sublattices correlates with a blueshift of the C-H stretch vibrational modes in H adatom clusters. The C-H bending modes are calculated to be in the 1100 cm-1 range, resonant with the graphene phonons. Moreover, we use our previously developed methods to calculate the relaxation of the C-H stretch mode via vibration-phonon interaction, using the Born-Oppenheimer surface for all local modes as obtained from the DFT calculations. The total decay rate of the H stretch into other H vibrations, thereby creating or annihilating one graphene phonon, is determined from Fermi's golden rule. Our calculations using the matrix elements derived from DFT calculations show that the lifetime of the H stretch mode on graphene is only several picoseconds, much shorter than on other semiconductor surfaces such as Ge(001) and Si(001). © 2010 American Institute of Physics.
    view abstract10.1063/1.3474806
  • Indium-gallium segregation in CuInxGa1-xSe2: An Ab initio-based Monte Carlo study
    Ludwig, C.D.R. and Gruhn, T. and Felser, C. and Schilling, T. and Windeln, J. and Kratzer, P.
    Physical Review Letters 105 (2010)
    Thin-film solar cells with CuInxGa1-xSe2 (CIGS) absorber are still far below their efficiency limit, although lab cells already reach 20.1%. One important aspect is the homogeneity of the alloy. Large-scale simulations combining Monte Carlo and density functional calculations show that two phases coexist in thermal equilibrium below room temperature. Only at higher temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which contributes to the observed low efficiency of Ga-rich CIGS solar cells. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.105.025702
  • Influence of the substrate lattice structure on the formation of quantum well states in thin in and Pb films on silicon
    Dil, J.H. and Hülsen, B. and Kampen, T.U. and Kratzer, P. and Horn, K.
    Journal of Physics Condensed Matter 22 (2010)
    The substrate lattice structure may have a considerable influence on the formation of quantum well states in a metal overlayer material. Here we study three model systems using angle resolved photoemission and low energy electron diffraction: indium films on Si(111) and indium and lead on Si(100). Data are compared with theoretical predictions based on density functional theory. We find that the interaction between the substrate and the overlayer strongly influences the formation of quantum well states; indium layers only exhibit well defined quantum well states when the layer relaxes from an initial face-centred cubic to the bulk body-centred tetragonal lattice structure. For Pb layers on Si(100) a change in growth orientation inhibits the formation of quantum well states in films thicker than 2ML. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/22/13/135008
  • Magnetism in C- or N-doped MgO and ZnO: A density-functional study of impurity Pairs
    Wu, H. and Stroppa, A. and Sakong, S. and Picozzi, S. and Scheffler, M. and Kratzer, P.
    Physical Review Letters 105 (2010)
    It is shown that substitution of C or N for O recently proposed as a way to create ferromagnetism in otherwise nonmagnetic oxide insulators is curtailed by formation of impurity pairs, and the resultant C2 spin=1 dimers as well as the isoelectronic N22+ interact antiferromagnetically in p-type MgO. For C-doped ZnO, however, we demonstrate using the Heyd-Scuseria-Ernzerhof hybrid functional that a resonance of the spin-polarized C2 ppπ* states with the host conduction band results in a long-range ferromagnetic interaction. Magnetism of open-shell impurity molecules is proposed as a possible route to d0-ferromagnetism in oxide spintronic materials. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.105.267203
  • Modeling of minibands and electronic transport in one-dimensional stacks of InAs/GaAs quantum dots
    Fomin, V.M. and Kratzer, P.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    We investigate the effect of the electron miniband energy spectrum of periodic 1D stacks of disk-shaped InAs quantum dots in GaAs on their electronic transport characteristics. The widely used approximation of a constant relaxation time is not adequate for a stack of semiconductor quantum dots. Certain windows of concentration are revealed, where arrays of quantum dots possess a geometry-controlled enhanced efficiency as thermoelectric converters. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2009.10.033
  • Stable structure and magnetic state of ultrathin CrAs films on GaAs(001): A density functional theory study
    Hashemifar, S.J. and Kratzer, P. and Scheffler, M.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Density functional theory calculations using the pseudopotential-plane-wave approach are employed to investigate the structural and magnetic properties of epitaxial CrAs thin films on GaAs(001). Motivated by recent reports of ferromagnetism in this system, we compare zinc-blende CrAs films (continuing the lattice structure of the GaAs substrate) and CrAs films with a bulklike orthorhombic structure epitaxially matched to three units of the GaAs(001) lattice. We find that even for very thin films with three Cr layers the bulklike crystal structure is energetically more favorable than zinc-blende CrAs on GaAs(001). CrAs films with orthorhombic structure, even if under epitaxial strain, preserve the antiferromagnetic order of CrAs bulk. In the light of our calculations, it appears likely that the magnetic hysteresis loop measured in ultrathin CrAs/GaAs(001) films originates from uncompensated antiferromagnetic moments near the CrAs/GaAs interface. In conclusion, our results do not support earlier proposals that thick CrAs films could be employed as perfectly matched spin-injection electrode on GaAs. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.214417
  • Theoretical investigation of the influence of isotope mass on chemicurrents during adsorption of H on K(110)
    Timmer, M. and Kratzer, P.
    Surface Science 604 (2010)
    Using our recently developed method for calculation of electron-hole (e-h) spectra in adsorption on metal surfaces [Phys. Rev. B 19:165407, 2009], we investigate the system H/K(110). Comparing to our previous results for H/Al(111), we show that the narrower conduction band of K in contrast to Al leads to notable differences in the excitation spectra of electrons and holes. We also find that our results do not obey the scaling of the number of excited charge carriers above a certain energy barrier with the particle's velocity, which is in our case mainly depending on the isotope mass. Instead, we find a different (approximately m1/6 rather than m1/2) scaling. Extrapolating our results to adsorbates with large masses, we expect larger electronic excitations than from the "forced oscillator" approach. This makes the electronic dissipation channel for energy more important even for heavy adsorbates. Our results are in qualitative agreement with other theoretical and experimental results. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.susc.2010.05.008
  • Thermoelectric transport in periodic one-dimensional stacks of InAs/GaAs quantum dots
    Fomin, V.M. and Kratzer, P.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    We investigate the effect of the narrow electronic minibands of periodic one-dimensional stacks of disk-shaped InAs quantum dots (QDs) in GaAs on their electronic transport characteristics by employing an empirical tight-binding calculation and a continuum model of the electronic structure. Our model includes both the minibands and the continuum of the host conduction band. The rate of the electron-acoustic-phonon scattering is found using Boltzmann's semiclassical transport theory. The electric conductivity, the Seebeck coefficient and the thermoelectric figure-of-merit for n -doped QD arrays are then analyzed as a function of the donor concentration and temperature. For QDs several nanometers in height, the figure-of-merit at temperatures below 100 K as a function of doping is richly structured, reflecting the miniband electron energy spectrum of a QD stack. Certain windows of concentration are revealed, where QD arrays display a geometry-controlled enhanced efficiency as thermoelectric converters. For optimizing the peak values of the figure-of-merit attainable for donor concentrations within the experimentally accessible range, a very thin spacer layer between the QDs (≤ 5 nm) is found to be most suitable. Assuming that the lattice thermal conductivity can be reduced below 0.5 W/ (m K), a figure-of-merit larger than 2 appears within reach. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.045318
  • ab initio calculations

  • first-principles calculation

  • nanocrystals

  • quantum dots

  • spincaloritronics

  • statistics

  • surfaces

  • thermoelectrics

  • thin films

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