Showing papers by "Wolf Gero Schmidt published in 2012"

The electronic structure and optical response of rutile, anatase and brookite TiO2.

Abstract: In this study, we present a combined density functional theory and many-body perturbation theory study on the electronic and optical properties of TiO2 brookite as well as the tetragonal phases rutile and anatase. The electronic structure and linear optical response have been calculated from the Kohn‐Sham band structure applying (semi)local as well as nonlocal screened hybrid exchange‐correlation density functionals. Single-particle excitations are treated within the GW approximation for independent quasiparticles. For optical response calculations, two-particle excitations have been included by solving the Bethe‐Salpeter equation for Coulomb correlated electron‐hole pairs. On this methodological basis, gap data and optical spectra for the three major phases of TiO2 are provided. The common characteristics of brookite with the rutile and anatase phases, which have been discussed more comprehensively in the literature, are highlighted. Furthermore, the comparison of the present calculations with measured optical response data of rutile indicate that discrepancies discussed in numerous earlier studies are due to the measurements rather than related to an insufficient theoretical description. (Some figures may appear in colour only in the online journal)

Activation of surface hydroxyl groups by modification of H-terminated Si(111) surfaces.

Abstract: Chemical functionalization of semiconductor surfaces, particularly silicon oxide, has enabled many technologically important applications (e.g., sensing, photovoltaics, and catalysis). For such processes, hydroxyl groups terminating the oxide surface constitute the primary reaction sites. However, their reactivity is often poor, hindering technologically important processes, such as surface phosphonation requiring a lengthy postprocessing annealing step at 140 °C with poor control of the bonding geometry. Using a novel oxide-free surface featuring a well-defined nanopatterned OH coverage, we demonstrate that hydroxyl groups on oxide-free silicon are more reactive than on silicon oxide. On this model surface, we show that a perfectly ordered layer of monodentate phosphonic acid molecules is chemically grafted at room temperature, and explain why it remains completely stable in aqueous environments, in contrast to phosphonates grafted on silicon oxides. This fundamental understanding of chemical activity an...

Fingerprints of order and disorder in the electronic and optical properties of crystalline and amorphous TiO 2

Abstract: We have investigated the structural and electronic properties as well as the linear optical response of amorphous TiO${}_{2}$ within density functional theory and a numerically efficient density functional based tight-binding approach as well as many-body perturbation theory. The disordered TiO${}_{2}$ phase is modeled by molecular dynamics. The equivalence to experimentally characterized amorphous phases is demonstrated by atomic structure factors and radial pair-distribution functions. By density functional theory calculations, using both the semilocal Perdew-Burke-Ernzerhof functional and the nonlocal Heyd-Scuseria-Ernzerhof screened hybrid functional, the electronic energy gap is found to be larger than in the crystalline TiO${}_{2}$ phases rutile and brookite but close to the anatase band gap. The quasiparticle energy gap of amorphous TiO${}_{2}$ is determined to be $\ensuremath{\gtrsim}$3.7 eV, while the optical gap is estimated to $\ensuremath{\lesssim}$3.5 eV. The disorder-induced formation of localized electronic states has been analyzed by the information entropy of the charge density distributions. The frequency-dependent optical constants, calculated from the complex dielectric function, have been determined in independent particle approximation. Besides similar absorption characteristics between the most common crystalline phases and amorphous TiO${}_{2}$, we find distinct differences in the optical spectra in the energy region between 5 eV and 8 eV. These differences can be assigned to the loss of symmetry in the local atomic structure of the disordered material. While the composition of the crystalline phases rutile, anatase, and brookite is well described by periodic arrangements of distorted TiO${}_{6}$ octahedra building blocks, the amorphous phase is characterized by partial loss of this octahedral coordination and the disorder-induced formation of under- and over-coordinated Ti ions. This leads to the absence of the characteristic crystal-field splitting of unoccupied Ti${}_{3d}$ states into ${e}_{g}$ and ${t}_{2g}$ like subbands. The optical characteristics of the amorphous phase are interpreted as a superposition of optical transitions that reflect the various local symmetries of the manifold of synthesizable crystalline TiO${}_{2}$ phases. The linear optical properties, calculated within the independent-particle approximation, are found to be in good agreement with the available experimental data.

Atomistic picture of charge density wave formation at surfaces.

Abstract: We used ultrafast electron diffraction and density-functional theory calculations to gain insight into the charge density wave (CDW) formation on In=Sið111Þ. Weak excitation by a femtosecond-laser pulse results in the melting of the CDW. The immediate freezing is hindered by a barrier for the motion of atoms during the phase transition: The melted CDW constitutes a long-lived, supercooled phase and is strong evidence for a first-order transition. The freezing into the CDW is triggered by preexisting adsorbates. Starting at these condensation nuclei, the CDW expands one dimensionally on the In=Sið111Þ surface, with a constant velocity of more than 80 m=s. DOI: 10.1103/PhysRevLett.109.186101

Identification of the nitrogen split interstitial (N-N)(N) in GaN.

Abstract: Combining electron paramagnetic resonance, density functional theory, and positron annihilation spectroscopy (PAS), we identify the nitrogen interstitial defect in GaN. The isolated interstitial is unstable and transforms into a split interstitial configuration $(\mathrm{N}\mathrm{\text{\ensuremath{-}}}\mathrm{N}{)}_{\mathrm{N}}$. It is generated by particle irradiation with an introduction rate of a primary defect, pins the Fermi level at ${E}_{C}\ensuremath{-}1.0\text{ }\text{ }\mathrm{eV}$ for high fluences, and anneals out at $400\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The associated defect, the nitrogen vacancy, is observed by PAS only in the initial stage of irradiation.

Polarization-dependent water adsorption on the LiNbO 3 (0001) surface

Abstract: The effect of ferroelectric poling on the adsorption characteristics of water at lithium niobate surfaces is investigated by ab initio calculations. Thereby we model the adsorption of H${}_{2}$O monomers, small water clusters, and water thin films on the LiNbO${}_{3}$(0001) surface. The adsorption configuration and energy are determined as a function of the surface coverage on both the positive and negative (0001) surfaces. Confirming the results of temperature programed desorption measurements [Garra, Vohs, and Bonnell, Surf. Sci. 603, 1106 (2009)], polarization-dependent adsorption energies, geometries, and equilibrium coverage are found. Our calculations predict the adsorption to occur mainly nondissociatively, independently of the coverage. The water structures formed at the surface are coverage-dependent, though. The different affinity of water to the two surfaces is explained in terms of electrostatic interactions between the substrate and polar molecules. Water adsorption accentuates the surface relaxation, thus increasing the microscopic surface roughness.

Atomic-resolution imaging of the polar (000 1 ¯ ) surface of LiNbO 3 in aqueous solution by frequency modulation atomic force microscopy

Abstract: S. Rode,1 R. Holscher,2 S. Sanna,2 S. Klassen,1 K. Kobayashi,3 H. Yamada,3 W. G. Schmidt,2 and A. Kuhnle1,* 1Institut fur Physikalische Chemie, Fachbereich Chemie, Johannes Gutenberg-Universitat Mainz, Jakob-Welder-Weg 11, 55099 Mainz, Germany 2Lehrstuhl fur Theoretische Physik, Universitat Paderborn, 33095 Paderborn, Germany 3Department of Electronic Science and Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan (Received 31 March 2012; revised manuscript received 12 June 2012; published 29 August 2012)

Substrate Induced Thermal Decomposition of Perfluoro-Pentacene Thin Films on the Coinage Metals

Abstract: The thermal and chemical stability of perfluoropentacene (PFP) thin films grown by organic molecular beam deposition onto the (111)-oriented surfaces of the coinage metals copper, silver, and gold have been studied by means of temperature dependent X-ray photoelectron spectroscopy (XPS) and Near-Edge X-ray absorption fine structure spectroscopy (NEXAFS). Under vacuum conditions, PFP multilayers are completely desorbed at 425 K while molecules in contact with the Au(111) surface remain intact up to 500 K. By contrast, PFP that is in contact with Cu(111) is distinctly distorted and becomes partially defluorinated already upon thermal desorption of multilayers. A pronounced defluorination of PFP also takes place on Ag(111) at temperatures around 440 K, while further heating causes a complete cracking and defluorination. Additional measurements carried out on a regularly stepped silver surface demonstrate that steps are active sites that promote defluorination already at lower temperatures. van der Waals corr...

In-Si(111)(4 × 1)/(8 × 2) nanowires: Electron transport, entropy, and metal-insulator transition†

Abstract: Department of Chemistry, University of California, One Shields Avenue, Davis, CA 95616, USAReceived 20 July 2011, revised 16 September 2011, accepted 26 September 2011Published online 28 December 2011Dedicated to Thomas Frauenheim on the occasion of his 60th birthdayKeywords charge-density wave, electron transport, entropy, in adsorption, nanowire, Peierls transition, Si(111) surface

Polarization-dependent methanol adsorption on lithium niobate Z-cut surfaces

Abstract: The adsorption of single methanol molecules on the lithium niobate (0001) surface, commonly referred to as Z-cut, is investigated using first-principles calculations. It is found that the binding energy for molecular adsorption on the negative surface ($\ensuremath{\sim}$1 eV) is about twice as large as for the positive surface. This difference is related to different bond strengths rather than electrostatics. Depending on the reaction path, lithium extraction from the negative surface may occur. This leads to an additional energy lowering by a few tenths of an eV. Larger energy gains are realized by dissociative adsorption, which is an activated process, however.

Ferroelectric phase transition in LiNbO 3 : Insights from molecular dynamics

Abstract: Ab initio molecular dynamics within density functional theory is used for the first time to model the ferroelectric-paraelectric phase transition in LiNbO3. Our calculations show that the structural phase transition is not an abrupt event, but rather a continuous process occurring over a range of about 100K and involving different ionic species at different temperatures. Because of the different behavior of the Li and Nb sublattices, the ferroelectric transition displays both displacive and order-disorder character.

2-Aminopyrimidine-silver(I) based organic semiconductors: Electronic structure and optical response

Abstract: Calculations based on (occupation constrained) density functional theory using local as well as hybrid functionals to describe the electron-electron exchange and correlation are combined with many-body perturbation theory in order to determine and rationalize the electronic and optical excitation properties of 2-aminopyrimidine-silver(I) based organic semiconductors and their parent molecules Large quasiparticle shifts and exciton binding energies of about 4 eV are found in the aminopyrimidine molecules Both the quasiparticle blueshift and the excitonic redshift are reduced upon crystal formation They cancel each other partially and thus allow for a meaningful description of the molecular and crystal optical response within the independent-particle approximation We find a surprisingly strong influence of local-field effects as well as resonant-nonresonant coupling terms in the electron-hole Hamiltonian on the optical properties The calculations reproduce well measured data and allow for identifying chemical trends with respect to the organic building blocks of the crystals

Adsorption of OH and H at the LiNbO3(0001) surface

Abstract: The adsorption of single hydrogen atoms and hydroxyl radicals (OH) at the polar (0001) surface is simulated by means of first-principles total energy calculations, in order to investigate the influence of ferroelectric poling on the surface physics and chemistry of LiNbO3. H and OH are found to adsorb at a similar site both at the positive and at the negative (0001) surface. Despite this, the adsorption energy is for both adsorbates strongly polarization dependent, with adsorption energy differences as high as 2 eV. This striking contrast is traced back to electrostatic effects, which lead to a different bonding scenario at the two LN(0001) sides. The polar radical OH lies relatively flat on the positive surface, with the adsorbate oxygen forming a covalent bond with the surface oxygen. At the negative face, OH adsorbs roughly perpendicular and the adsorbate oxygen forms a bond of covalent nature with the surface cations. The adsorbate dipole moment is directed against the spontaneous polarization of the substrate. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vibrational fingerprints of LiNbO3-LiTaO3 mixed crystals

Abstract: The structural and vibrational properties of lithium niobate (LN) – lithium tantalate (LT) mixed crystals (LNT, LiNb1-xTaxO3) are investigated over the whole composition range by first-principles simulations. The crystal volume grows roughly linearly from LT to LN, whereby the lattice parameters a and c show minor deviations from the Vegard behavior between the end compounds, LiNbO3 and LiTaO3. Our calculations in the framework of the density functional theory show the TO1, TO2 and TO4-modes to become harder with increasing Nb concentration. TO3 becomes softer with increasing Nb content, instead. The frequency shifts of the zone center A1-TO phonon modes for crystals with different compositions are found to be as large as 30 cm-1. Raman spectroscopy, which is sensitive to the A1 modes, can be therefore employed to determine the crystal composition.

Entropy and Metal-Insulator Transition in Atomic-Scale Wires: The Case of In-Si(111)(4×1)/(8×2)

Abstract: Density functional theory (DFT) calculations are performed to determine the mechanism and origin of the intensively debated (4×1)–(8×2) phase transition of the Si(111)-In nanowire array. The calculations (i) show the existence of soft phonon modes that transform the nanowire structure between the metallic In zigzag chains of the room-temperature phase and the insulating In hexagons formed at low temperature and (ii) demonstrate that the subtle balance between the energy lowering due to the hexagon formation and the larger vibrational entropy of the zigzag chains causes the phase transition.

Vibrational fingerprints of LiNbO 3 -LiTaO 3 mixed crystals

Abstract: The structural and vibrational properties of lithium niobate (LN) — lithium tantalate (LT) mixed crystals (LNT, LiNb 1−x Ta x O 3 ) are investigated over the whole composition range by first-principles simulations. The crystal volume grows roughly linearly from LT to LN, whereby the lattice parameters a and c show minor deviations from the Vegard behavior between the end-compounds, LiNbO 3 and LiTaO 3 . Our calculations in the framework of the density functional theory show the TO 1 , TO 2 and TO 4 -modes to become harder with increasing Nb concentration. TO 3 becomes softer with increasing Nb content, instead. The frequency shifts of the zone center A 1 -TO phonon modes for crystals with different compositions are found to be as large as 30 cm−1. Raman spectroscopy, which is sensitive to the A 1 modes, can be therefore employed to determine the crystal composition.

Copper Substrate Catalyzes Tetraazaperopyrene Polymerization

Abstract: The polymerization of tetraazaperopyrene (TAPP) molecules on a Cu(111) substrate, as observed in recent STM experiments, has been investigated in detail by first principles calculations. Tautomerization is the first step required for the formation of molecular dimers and polymers. The substrate is found to catalyze this tautomerization.

Lithium niobate-tantalate mixed crystals electronic and optical properties calculated from first principles

Abstract: The electronic and optical properties of the mixed-crystal system lithium niobate-tantalate has been calculated from first principles. Based on density functional theory we calculate the electronic properties including quasiparticle effects within the GW approach. The optical response including electron-hole attraction effects is obtained from the solution of the Bethe-Salpeter equation. While the band gap increase with increasing tantalum content and shows some bowing, a nearly linear dependence of the optical birefringence on the stochiometry is calculated.

First-principles and empirical potential simulation study of intrinsic and carbon-related defects in silicon

Abstract: Results of atomistic simulations aimed at understanding precipitation of the highly attractive wide band gap semiconductor material silicon carbide in silicon are presented. The study involves a systematic investigation of intrinsic and carbon-related defects as well as defect combinations and defect migration by both, quantummechanical first-principles as well as empirical potential methods. Comparing formation and activation energies, ground-state structures of defects and defect combinations as well as energetically favorable agglomeration of defects are predicted. Moreover, accurate ab initio calculations unveil limitations of the analytical method based on a Tersoff-like bond order potential. A work-around is proposed in order to subsequently apply the highly efficient technique on large structures not accessible by first-principles methods. The outcome of both types of simulation provides a basic microscopic understanding of defect formation and structural evolution particularly at non-equilibrium conditions strongly deviated from the ground state as commonly found in SiC growth processes. A possible precipitation mechanism, which conforms well to experimental findings and clarifies contradictory views present in the literature is outlined (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)