Showing papers by "Wolf Gero Schmidt published in 2021"
TL;DR: In this article, a comprehensive study on the adsorption of N-Heterocyclic carbenes on silicon surfaces was presented, where covalently bound NHC molecules were found to exhibit good thermal stability and strong work function reductions.
Abstract: N-Heterocyclic carbenes (NHCs) are promising modifiers and anchors for surface functionalization and offer some advantages over thiol-based systems. Because of their strong binding affinity and high electron donation, NHCs can dramatically change the properties of the surfaces to which they are bonded. Highly ordered NHC monolayers have so far been limited to metal surfaces. Silicon, however, remains the element of choice in semiconductor devices and its modification is therefore of utmost importance for electronic industries. Here, a comprehensive study on the adsorption of NHCs on silicon is presented. We find covalently bound NHC molecules in an upright adsorption geometry and demonstrate the formation of highly ordered monolayers exhibiting good thermal stability and strong work function reductions. The structure and ordering of the monolayers is controlled by the substrate geometry and reactivity and in particular by the NHC side groups. These findings pave the way towards a tailor-made organic functionalization of silicon surfaces and, thanks to the high modularity of NHCs, new electronic and optoelectronic applications. Although monolayers of N-heterocyclic carbenes (NHCs) readily form on metals, surface reactivity usually hinders their self-assembly on semiconductors. Now, it has been shown that thermally stable, well-ordered monolayers of NHCs can be formed on silicon surfaces. A large reduction in work function is observed and steric effects enable sufficient diffusivity of the NHCs.
19 citations
13 May 2021
TL;DR: In this article, a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy was performed.
Abstract: Lithium niobate (LiNbO3), a material frequently used in optical applications, hosts different kinds of polarons that significantly affect many of its physical properties. In this study, a variety of electron polarons, namely free, bound, and bipolarons, are analyzed using first-principles calculations. We perform a full structural optimization based on density-functional theory for selected intrinsic defects with special attention to the role of symmetry-breaking distortions that lower the total energy. The cations hosting the various polarons relax to a different degree, with a larger relaxation corresponding to a larger gap between the defect level and the conduction-band edge. The projected density of states reveals that the polaron states are formerly empty Nb 4d states lowered into the band gap. Optical absorption spectra are derived within the independent-particle approximation, corrected by the GW approximation that yields a wider band gap and by including excitonic effects within the Bethe–Salpeter equation. Comparing the calculated spectra with the density of states, we find that the defect peak observed in the optical absorption stems from transitions between the defect level and a continuum of empty Nb 4d states. Signatures of polarons are further analyzed in the reflectivity and other experimentally measurable optical coefficients.
10 citations
TL;DR: The nitrogen-vacancy (NV) center in 3C-SiC has recently emerged as a solid-state qubit with competitive properties and significant technological advantages as mentioned in this paper.
Abstract: The nitrogen-vacancy (NV) center in 3C-SiC, the analog of the NV center in diamond, has recently emerged as a solid-state qubit with competitive properties and significant technological advantages. Combining first-principles calculations and magnetic resonance spectroscopy, we provide thorough insight into its magneto-optical properties. By applying resonantly excited electron paramagnetic resonance spectroscopy, we identified the zero-phonon absorption line of the 3A2 → 3E transition at 1289 nm (within the telecom O-band) and measured its phonon sideband, the analysis of which reveals a Huang-Rhys factor of S = 2.85 and a Debye-Waller factor of 5.8%. The low-temperature spin-lattice relaxation time was found to be exceptionally long (T1 = 17 s at 4 K). All these properties make NV in 3C-SiC a strong competitor for qubit applications. In addition, the strong variation of the zero-field splitting in the range 4-380 K allows its application for nanoscale thermal sensing.
10 citations
TL;DR: In this article, the authors employed synchrotron-based core-level and angle-resolved photoemission spectroscopy measurements in combination with density functional theory (DFT) calculations to address the inconsistencies with regard to the Si $2p$ core level and valence-band dispersion.
Abstract: The $\mathrm{Si}(111)\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}\text{\ensuremath{-}}\mathrm{B}\phantom{\rule{0.28em}{0ex}}[\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}]$ surface has evolved into a particularly interesting surface in the context of on-surface molecular self-assembly. Photoemission spectroscopy is a powerful tool to understand the interaction between the surface and the adsorbates. Previous studies of $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ contain many inconsistencies with regard to the Si $2p$ core level and valence-band dispersion. Here we employ synchrotron-based core-level and angle-resolved photoemission spectroscopy measurements in combination with density functional theory (DFT) calculations to address these issues. DFT calculations of the Si $2p$ core-level spectra accurately identify contributions from one bulk and five surface components, which allows us to obtain a comprehensive understanding of the spectra recorded at different photon energies. As an archetypal example, this refined decomposition is employed to understand the changes in Si $2p$ spectra upon the adsorption of cobalt phthalocyanine molecules. Regarding the valence-band dispersion of the clean $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}$ surface, our comprehensive DFT and photoemission investigations are able to reconcile the inconsistencies appearing in previous studies and reveal several yet unreported surface states. Furthermore, we are able to theoretically and experimentally resolve the distribution of these surface states in constant energy plots.
6 citations
TL;DR: In this article, the vibrational properties of the Si(553)-Au surface were studied by Raman spectroscopy and ab initio calculations, and a multitude of surface localized phonon modes were experimentally observed, along with two modes at about 400
Abstract: The vibrational properties of the Si(553)-Au surface are studied by Raman spectroscopy and ab initio calculations. A multitude of surface localized phonon modes with wave number below 200 ${\mathrm{cm}}^{\ensuremath{-}1}$ is experimentally observed, along with two modes at about 400 ${\mathrm{cm}}^{\ensuremath{-}1}$. Atomistic models within density functional theory allow to assign the low-energy spectral features to vibrations within the Au chain, while the Raman signatures at higher energies are mostly localized at the Si step edge. The Raman activity of nominally silent modes associated with a large charge transfer between Au chain and Si step edge states is explained by scattering at charge density fluctuations. Temperature-dependent measurements reveal specific mode shifts that are discussed in terms of a recently proposed order-disorder phase transition. The presence of model-specific displacement patterns allows us to identify the structural models compatible with the measured spectra at low and at room temperature.
6 citations
25 Feb 2021
TL;DR: In this paper, the atomic structure and electronic properties of the InP and Al0.5In 0.5P(001) surfaces at the initial stages of oxidation are investigated via density functional theory.
Abstract: The atomic structure and electronic properties of the InP and Al0.5In0.5P(001) surfaces at the initial stages of oxidation are investigated via density functional theory. Thereby, we focus on the mixed-dimer (2 × 4) surfaces stable for cation-rich preparation conditions. For InP, the top In-P dimer is the most favored adsorption site, while it is the second-layer Al-Al dimer for AlInP. The energetically favored adsorption sites yield group III-O bond-related states in the energy region of the bulk band gap, which may act as recombination centers. Consistently, the In p state density around the conduction edge is found to be reduced upon oxidation.
5 citations
TL;DR: In this paper, the second-order susceptibility χ(2) of LiNbO3 was determined using density-functional theory within a Berry-phase formulation of the dynamical polarization.
Abstract: Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility χ(2) of lithium niobate (LiNbO3). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at NbV–VLi defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced χ(2) coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.
5 citations
TL;DR: In this article, the stoichiometry-dependent band alignment at the GaxIn1 xP=AlyIn1 yP alloy interface is investigated. But, to the best of our knowledge, there are neither experimental nor theoretical data available on the relative positions of the valence band maxima (VBM) and conduction band minima (CBM) at the gaxIn 1 xP = AlyIn 1 yP interface, apart from valence bands offsets at the binary end points.
Abstract: The III–V compound semiconductors play an important role for a variety of electronic and optoelectronic devices such as high electron mobility and heterostructure bipolar transistors, diode lasers, light-emitting diodes, photodetectors, electro-optic modulators, and frequency-mixing components. Apart from the binary compounds, ternary and quaternary alloys also may be combined within a countless variety of heterostructure configurations. The efficient exploitation of this flexibility requires, however, knowledge not only of the electronic properties of single compounds and alloys and their surfaces but also information on the band alignment of the respective interfaces. This motivates this article, which addresses the stoichiometrydependent band alignment at the GaxIn1 xP=AlyIn1 yP alloy interface. This interface occurs, e.g., in heterostructure solar cells and tandem absorber structures for the direct solar-tohydrogen conversion. GaP and AlP have indirect bandgaps of 2.34–2.35 and 2.5 eV, respectively, at zero temperature. In contrast, InP is a direct-gap semiconductor with a lowtemperature bandgap of 1.42 eV. According to photoluminescence data, the Ga content of GaxIn1 xP at the direct–indirect bandgap crossover is at about x1⁄4 0.71. Measurements for AlyIn1 yP indicate the direct–indirect crossover for y1⁄4 0.41, whereas recent density functional calculations place it at y1⁄4 0.48. To the best of our knowledge, there are neither experimental nor theoretical data available on the relative positions of the valence band maxima (VBM) and conduction band minima (CBM) at the GaxIn1 xP=AlyIn1 yP alloy interface, apart from valence band offsets at the binary end points. This article aims at closing this gap by providing band alignment values for the complete composition range. We focus here on the “natural” band lineups between unstrained materials obtained from the branch-point energies of the respective materials. They are determined here within hybrid density functional theory (DFT).
4 citations
4 citations
Posted Content•
TL;DR: In this paper, the second-order susceptibility of lithium niobate (LiNbO$_3$ ) was determined using density-functional theory within a Berry-phase formulation of the dynamical polarization.
Abstract: Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility $\chi^{(2)}$ of lithium niobate (LiNbO$_3$). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at Nb$_\mathrm{V}$-V$_{\mathrm{Li}}$ defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced $\chi^{(2)}$ coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.
TL;DR: In this article, the ground-state atomic structures of KTP surfaces, frequently termed Z and Y cuts, respectively, have been determined using ground state density-functional theory total energy calculations.
Abstract: While potassium titanyl phosphate (KTP) is widely used for various optics applications, essentially nothing is known about its surfaces and electronic properties. Here the ground-state atomic structures of KTP [001] and [010] surfaces, frequently termed Z and Y cuts, respectively, have been determined using ground-state density-functional theory total-energy calculations. The calculated surface phase diagrams in dependence on the chemical potentials of the materials constituents show several stable nonstoichiometric terminations. A tendency to form oxygen-rich surfaces is observed. The ${\mathrm{Z}}^{+}$ and ${\mathrm{Z}}^{\ensuremath{-}}$ surfaces, discriminated by oppositely orientated internal electric fields, are found to differ with respect to their stoichiometry and structure. Occupied O-derived and in some cases unoccupied Ti-derived surface states appear in the lower and upper part of the bulk band gap, respectively.