Showing papers on "Electronic structure published in 2011"
TL;DR: This work addresses the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moiré pattern periodicity leads to moirÉ Bloch bands.
Abstract: A moire pattern is formed when two copies of a periodic pattern are overlaid with a relative twist. We address the electronic structure of a twisted two-layer graphene system, showing that in its continuum Dirac model the moire pattern periodicity leads to moire Bloch bands. The two layers become more strongly coupled and the Dirac velocity crosses zero several times as the twist angle is reduced. For a discrete set of magic angles the velocity vanishes, the lowest moire band flattens, and the Dirac-point density-of-states and the counterflow conductivity are strongly enhanced.
2,323 citations
TL;DR: In this article, it was shown that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale, and the properties of related TmS2 nanolayers (Tm = W, Nb, Re) were studied.
Abstract: Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its size to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this experimental observation by first principles calculations and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure at the nanoscale. We further studied the properties of related TmS2 nanolayers (Tm = W, Nb, Re) and show that the isotopological WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent on size.
1,532 citations
TL;DR: In this article, a combination of high-resolution in situ X-ray photoemission and Xray absorption spectroscopies was used to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process.
Abstract: Despite the recent developments in graphene oxide due to its importance as a host precursor of graphene, the detailed electronic structure and its evolution during the thermal reduction remain largely unknown, hindering its potential applications. We show that a combination of high-resolution in situ X-ray photoemission and X-ray absorption spectroscopies offer a powerful approach to monitor the deoxygenation process and comprehensively evaluate the electronic structure of graphene oxide thin films at different stages of the thermal reduction process. It is established that the edge plane carboxyl groups are highly unstable, whereas carbonyl groups are more difficult to remove. The results consistently support the formation of phenol groups through reaction of basal plane epoxide groups with adjacent hydroxyl groups at moderate degrees of thermal activation (∼400 °C). The phenol groups are predominant over carbonyl groups and survive even at a temperature of 1000 °C. For the first time, a drastic increase...
1,265 citations
TL;DR: This Review focuses on four classes of thienoacenes, which are classified in terms of their chemical structures, and elucidates the molecular electronic structure of each class, and provides insight into new molecular design strategies for the development of superior organic semiconductors.
Abstract: Thienoacenes consist of fused thiophene rings in a ladder-type molecular structure and have been intensively studied as potential organic semiconductors for organic field-effect transistors (OFETs) in the last decade. They are reviewed here. Despite their simple and similar molecular structures, the hitherto reported properties of thienoacene-based OFETs are rather diverse. This Review focuses on four classes of thienoacenes, which are classified in terms of their chemical structures, and elucidates the molecular electronic structure of each class. The packing structures of thienoacenes and the thus-estimated solid-state electronic structures are correlated to their carrier transport properties in OFET devices. With this perspective of the molecular structures of thienoacenes and their carrier transport properties in OFET devices, the structure-property relationships in thienoacene-based organic semiconductors are discussed. The discussion provides insight into new molecular design strategies for the development of superior organic semiconductors.
813 citations
TL;DR: Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, the electronic structure is investigated and the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons is predicted.
Abstract: Using density functional theory coupled with Boltzmann transport equation with relaxation time approximation, we investigate the electronic structure and predict the charge mobility for a new carbon allotrope, the graphdiyne for both the sheet and nanoribbons. It is shown that the graphdiyne sheet is a semiconductor with a band gap of 0.46 eV. The calculated in-plane intrinsic electron mobility can reach the order of 10(5) cm(2)/(V s) at room temperature, while the hole mobility is about an order of magnitude lower.
791 citations
TL;DR: It is shown, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades.
Abstract: As silicon is the basis of conventional electronics, so strontium titanate (SrTiO(3)) is the foundation of the emerging field of oxide electronics. SrTiO(3) is the preferred template for the creation of exotic, two-dimensional (2D) phases of electron matter at oxide interfaces that have metal-insulator transitions, superconductivity or large negative magnetoresistance. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO(3) (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of ∼0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO(3)-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO(3) lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO(3)-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.
594 citations
TL;DR: In this article, the structural, vibrational and electronic properties of the monolayer graphene-like transition-metal dichalcogenide (MX2) sheets were investigated using first principles calculations.
Abstract: Using first principles calculations, we investigate the structural, vibrational and electronic structures of the monolayer graphene-like transition-metal dichalcogenide (MX2) sheets. We find the lattice parameters and stabilities of the MX2 sheets are mainly determined by the chalcogen atoms, while the electronic properties depend on the metal atoms. The NbS2 and TaS2 sheets have comparable energetic stabilities to the synthesized MoS2 and WS2 ones. The molybdenum and tungsten dichalcogenide (MoX2 and WX2) sheets have similar lattice parameters, vibrational modes, and electronic structures. These analogies also exist between the niobium and tantalum dichalcogenide (NbX2 and TaX2) sheets. However, the NbX2 and TaX2 sheets are metals, while the MoX2 and WX2 ones are semiconductors with direct-band gaps. When the Nb and Ta atoms are doped into the MoS2 and WS2 sheets, a semiconductor-to-metal transition occurs. Comparing to the bulk compounds, these monolayer sheets have similar structural parameters and properties, but their vibrational and electronic properties are varied and have special characteristics. Our results suggest that the graphene-like MX2 sheets have potential applications in nano-electronics and nano-devices.
593 citations
TL;DR: The electronic structure of (E)-4-methoxy-2-[(p-tolylimino)methyl]phenol has been characterized by the B3LYP/6-31G(d) level by using density functional theory and the non-linear optical properties have been computed with the same level of theory.
552 citations
TL;DR: Two unexpected orthorhombic high-pressure structures Aba2-40 and Cmca-56 are reported, by using a newly developed particle swarm optimization technique on crystal structure prediction, and it is predicted that a local trigonal planar structural motif adopted by CmCA-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.
Abstract: Under high pressure, "simple" lithium (Li) exhibits complex structural behavior, and even experiences an unusual metal-to-semiconductor transition, leading to topics of interest in the structural polymorphs of dense Li. We here report two unexpected orthorhombic high-pressure structures Aba2-40 (40 atoms/cell, stable at 60-80 GPa) and Cmca-56 (56 atoms/cell, stable at 185-269 GPa), by using a newly developed particle swarm optimization technique on crystal structure prediction. The Aba2-40 having complex 4- and 8-atom layers stacked along the b axis is a semiconductor with a pronounced band gap >0.8 eV at 70 GPa originating from the core expulsion and localization of valence electrons in the voids of a crystal. We predict that a local trigonal planar structural motif adopted by Cmca-56 exists in a wide pressure range of 85-434 GPa, favorable for the weak metallicity.
472 citations
TL;DR: In this paper, Bollinger et al. reported the synthesis of epitaxial films of La2− xSr x CuO4 that are one unit cell thick and fabrication of double-layer transistors.
Abstract: High-temperature superconductivity in copper oxides arises when a parent insulator compound is 'doped' by adding or removing valence electrons, usually by inserting atoms into the lattice structure. This would be better achieved by tuning the carrier density using the electric field effect, as it removes ambiguity about whether the electronic properties change because of alterations in the crystal structure or in the electronic structure. Such tuning is difficult to achieve because it requires perfect, ultrathin films and a huge local field. Bollinger et al. report the synthesis of one-cell-thick epitaxial films of La2xSrxCuO4, and the use of the films to make double-layer transistors. The transistors have very large fields, and by changing the surface carrier density, the critical temperature can be shifted by up to 30 K. The resistance varies as predicted for a two-dimensional superconductor–insulator transition. High-temperature superconductivity in copper oxides arises when a parent insulator compound is doped beyond some critical concentration; what exactly happens at this superconductor–insulator transition is a key open question1. The cleanest approach is to tune the carrier density using the electric field effect2,3,4,5,6,7; for example, it was learned in this way5 that weak electron localization transforms superconducting SrTiO3 into a Fermi-glass insulator. But in the copper oxides this has been a long-standing technical challenge3, because perfect ultrathin films and huge local fields (>109 V m−1) are needed. Recently, such fields have been obtained using electrolytes or ionic liquids in the electric double-layer transistor configuration8,9,10. Here we report synthesis of epitaxial films of La2− xSr x CuO4 that are one unit cell thick, and fabrication of double-layer transistors. Very large fields and induced changes in surface carrier density enable shifts in the critical temperature by up to 30 K. Hundreds of resistance versus temperature and carrier density curves were recorded and shown to collapse onto a single function, as predicted for a two-dimensional superconductor–insulator transition11,12,13,14. The observed critical resistance is precisely the quantum resistance for pairs, RQ = h/(2e)2 = 6.45 kΩ, suggestive of a phase transition driven by quantum phase fluctuations, and Cooper pair (de)localization.
443 citations
TL;DR: It is anticipated that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.
Abstract: Angle-resolved photoelectron spectroscopy (ARPES) is widely used to study the electronic structure of crystalline solids such as high-temperature superconductors, topological insulators and graphene-based materials. Time-resolved ARPES has opened the door to the study of the response of such electronic features on ultrafast timescales. Now Rohwer et al. add a new dimension. Using high photon energies, they are able to study ultrafast dynamics at high momenta, at which some of the most interesting fundamental phenomena occur. Applying the technique to the charge density wave material 1T-TiSe2, they obtain stroboscopic images of the electronic band structure at high momentum and show that atomic-scale periodic long-scale order collapses on a surprisingly short timescale of 20 femtoseconds. This work reveals rapid response times in photoinduced properties that could stimulate research into new types of ultrafast switching device. Angle-resolved photoemission spectroscopy (ARPES) is widely used to study the electronic structure of a wide range of correlated materials. Time-resolved ARPES allows the study of the response of such electronic features on ultrafast timescales; this paper now adds an exciting new dimension by using high photon energies that allow the study of ultrafast dynamics at high momenta, where often the most interesting fundamental phenomena occur. The technique is applied to the charge density wave material 1T-TiSe2 and it is shown with stroboscopic imaging of the electronic band structure at high momentum that atomic-scale periodic long-range order collapses on a surprisingly short timescale of 20 femtoseconds. Intense femtosecond (10−15 s) light pulses can be used to transform electronic, magnetic and structural order in condensed-matter systems on timescales of electronic and atomic motion1,2,3. This technique is particularly useful in the study4,5 and in the control6 of materials whose physical properties are governed by the interactions between multiple degrees of freedom. Time- and angle-resolved photoemission spectroscopy is in this context a direct and comprehensive, energy- and momentum-selective probe of the ultrafast processes that couple to the electronic degrees of freedom7,8,9,10. Previously, the capability of such studies to access electron momentum space away from zero momentum was, however, restricted owing to limitations of the available probing photon energy10,11. Here, using femtosecond extreme-ultraviolet pulses delivered by a high-harmonic-generation source, we use time- and angle-resolved photoemission spectroscopy to measure the photoinduced vaporization of a charge-ordered state in the potential excitonic insulator 1T-TiSe2 (refs 12, 13). By way of stroboscopic imaging of electronic band dispersions at large momentum, in the vicinity of the edge of the first Brillouin zone, we reveal that the collapse of atomic-scale periodic long-range order happens on a timescale as short as 20 femtoseconds. The surprisingly fast response of the system is assigned to screening by the transient generation of free charge carriers. Similar screening scenarios are likely to be relevant in other photoinduced solid-state transitions and may generally determine the response times. Moreover, as electron states with large momenta govern fundamental electronic properties in condensed matter systems14, we anticipate that the experimental advance represented by the present study will be useful to study the ultrafast dynamics and microscopic mechanisms of electronic phenomena in a wide range of materials.
TL;DR: In this paper, an angle-resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe1-xCox)2As2 in the underdoped region was performed.
Abstract: Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high-temperature superconductors. Similarly, the new iron-based high-temperature superconductors exhibit a tetragonal-to-orthorhombic structural transition (i.e., a broken C4 symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here, we present an angle-resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant dxz and dyz character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (TS) precedes the magnetic transition (TSDW), an anisotropic splitting is observed to develop above TSDW, indicating that it is specifically associated with TS. For unstressed crystals, the band splitting is observed close to TS, whereas for stressed crystals, the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.
TL;DR: A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS(2)/graphene hybrid accompanied by a small band-gap opening due to the variation of on-site energy induced by MoS (2).
Abstract: The geometric and electronic structures of graphene adsorption on MoS2 monolayer have been studied by using density functional theory. It is found that graphene is bound to MoS2 with an interlayer spacing of 3.32 A and with a binding energy of −23 meV per C atom irrespective of adsorption arrangement, indicating a weak interaction between graphene and MoS2. A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS2/graphene hybrid accompanied by a small band-gap (2 meV) opening due to the variation of on-site energy induced by MoS2. These findings are useful complement to experimental studies of this new synthesize system and suggest a new route to facilitate the design of devices where both finite band-gap and high carrier mobility are needed.
TL;DR: In this paper, the femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots were conducted to provide the first quantitative measure of excitonic state-to-state transition rates.
Abstract: The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...
TL;DR: It is predicted that LaAuO(3) bilayers have a topologically non-trivial energy gap of about 0.15 eV, which is sufficiently large to realize the quantum spin Hall effect at room temperature.
Abstract: Topological insulators are a class of materials with an unusual band structure that makes them metallic at the surface and insulating in the bulk. Okamoto and co-workers use electronic structure calculations to predict a new family of possible topological insulators based on transition-metal oxides.
TL;DR: A state-resolved spectroscopic approach is developed which has yielded precise measurements of the electronic structural dynamics of quantum dots and has made inroads toward creating a unified picture of many of the key dynamic processes in these materials.
Abstract: The quantum dot, one of the central materials in nanoscience, is a semiconductor crystal with a physical size on the nanometer length scale. It is often called an “artificial atom” because researchers can create nanostructures which yield properties similar to those of real atoms. By virtue of having a size in between molecules and solids, the quantum dot offers a rich palette for exploring new science and developing novel technologies. Although the physical structure of quantum dots is well known, a clear understanding of the resultant electronic structure and dynamics has remained elusive. However, because the electronic structure and dynamics of the dot, the “excitonics”, confer its function in devices such as solar cells, lasers, LEDs, and nonclassical photon sources, a more complete understanding of these properties is critical for device development. In this Account, we use colloidal CdSe dots as a test bed upon which to explore four select issues in excitonic processes in quantum dots. We have deve...
TL;DR: Overall defect energetics suggests a preference for the native donor-type defects over acceptor- type defects in ZnO, which is likely to play essential roles in electrical properties.
TL;DR: Density functional theory calculations were adopted in the present work, and it was found that its slightly distorted crystal structure enhances the lone-pair impact of Bi 6s states, leading to the special optical properties and excellent photocatalytic activities.
Abstract: Monoclinic clinobisvanite bismuth vanadate is an important material with wide applications. However, its electronic structure and optical properties are still not thoroughly understood. Density functional theory calculations were adopted in the present work, to comprehend the band structure, density of states, and projected wave function of BiVO4. In particular, we put more emphasis upon the intrinsic relationship between its structure and properties. Based on the calculated results, its molecular-orbital bonding structure was proposed. And a significant phenomenon of optical anisotropy was observed in the visible-light region. Furthermore, it was found that its slightly distorted crystal structure enhances the lone-pair impact of Bi 6s states, leading to the special optical properties and excellent photocatalytic activities.
TL;DR: The synthesis of a Fe-porphyrin-like carbon nanotube from conventional plasma-enhanced chemical vapor deposition exhibits an excellent oxygen reduction catalytic activity with the extreme structural stability over 0.1×10(6) cycles, vastly superior to the commercial Pt-C catalyst.
Abstract: We report the synthesis of a Fe-porphyrin-like carbon nanotube from conventional plasma-enhanced chemical vapor deposition. Covalent but seamless incorporation of the 5-6-5-6 porphyrinic $\mathrm{Fe}\mathrm{\text{\ensuremath{-}}}{\mathrm{N}}_{4}$ moiety into the graphene hexagonal side wall was elucidated by x-ray and ultraviolet photoemission spectroscopies and first-principles electronic structure calculations. The resulting biomimetic nanotube exhibits an excellent oxygen reduction catalytic activity with the extreme structural stability over $0.1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ cycles, vastly superior to the commercial Pt-C catalyst.
TL;DR: The self-consistent continuum solvation (SCCS) model proposed by Fattebert and Gygi as discussed by the authors is reformulated, overcoming some of the numerical limitations encountered and extending its range of applicability.
Abstract: The solvation model proposed by Fattebert and Gygi [Journal of Computational Chemistry 23, 662 (2002)] and Scherlis et al [Journal of Chemical Physics 124, 074103 (2006)] is reformulated, overcoming some of the numerical limitations encountered and extending its range of applicability We first recast the problem in terms of induced polarization charges that act as a direct mapping of the self-consistent continuum dielectric; this allows to define a functional form for the dielectric that is well behaved both in the high-density region of the nuclear charges and in the low-density region where the electronic wavefunctions decay into the solvent Second, we outline an iterative procedure to solve the Poisson equation for the quantum fragment embedded in the solvent that does not require multi-grid algorithms, is trivially parallel, and can be applied to any Bravais crystallographic system Last, we capture some of the non-electrostatic or cavitation terms via a combined use of the quantum volume and quantum surface [Physical Review Letters 94, 145501 (2005)] of the solute The resulting self-consistent continuum solvation (SCCS) model provides a very effective and compact fit of computational and experimental data, whereby the static dielectric constant of the solvent and one parameter allow to fit the electrostatic energy provided by the PCM model with a mean absolute error of 03 kcal/mol on a set of 240 neutral solutes Two parameters allow to fit experimental solvation energies on the same set with a mean absolute error of 13 kcal/mol A detailed analysis of these results, broken down along different classes of chemical compounds, shows that several classes of organic compounds display very high accuracy, with solvation energies in error of 03-04 kcal/mol, whereby larger discrepancies are mostly limited to self-dissociating species and strong hydrogen-bond forming compounds
TL;DR: In this paper, a multiband photovoltaic device was designed, fabricated and tested using GaNxAs1� x alloys and demonstrated an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum.
Abstract: Using the unique features of the electronic band structure of GaNxAs1� x alloys, we have designed, fabricated and tested a multiband photovoltaic device. The device demonstrates an optical activity of three energy bands that absorb, and convert into electrical current, the crucial part of the solar spectrum. The performance of the device and measurements of electroluminescence, quantum efficiency and photomodulated reflectivity are analyzed in terms of the band anticrossing model of the electronic structure of highly mismatched alloys. The results demonstrate the feasibility of using highly mismatched alloys to engineer the semiconductor energy band structure for specific device applications.
TL;DR: Gold and silver nanoclusters have unique molecule-like electronic structure and a nonzero HOMO-LUMO gap, which may lead to an understanding of nanoparticle luminescence, excited-state dynamics, and the metallic to molecular transition.
Abstract: Gold and silver nanoclusters have unique molecule-like electronic structure and a nonzero HOMO-LUMO gap. Recent advances in X-ray crystal structure determination have led to a new understanding of the geometric structure of gold nanoparticles, with significant implications for electronic structure. The superatom model has been effectively employed to explain properties such as one- and two-photon optical absorption, circular dichroism, EPR spectra, and electronic effects introduced by nanoparticle doping. Future investigations may also lead to an understanding of nanoparticle luminescence, excited-state dynamics, and the metallic to molecular transition.
TL;DR: Results indicate that the present NDT cores, in particular the linear-shaped, centrosymmetric naphtho[2,3-b:6,7-b']dithiophene, are promising building blocks for the development of organic semiconducting materials.
Abstract: A straightforward synthetic approach that exploits linear- and angular-shaped naphthodithiophenes (NDTs) being potential as new core structures for organic semiconductors is described. The newly established synthetic procedure involves two important steps; one is the chemoselective Sonogashira coupling reaction on the trifluoromethanesulfonyloxy site over the bromine site enabling selective formation of o-bromoethynylbenzene substructures on the naphthalene core, and the other is a facile ring closing reaction of fused-thiophene rings from the o-bromoethynylbenzene substructures. As a result, three isomeric NDTs, naphtho[2,3-b:6,7-b′]dithiophene, naphtho[2,3-b:7,6-b′]dithiophenes, and naphtho[2,1-b:6,5-b′]dithiophene, are selectively synthesized. Electrochemical and optical measurements of the parent NDTs indicated that the shape of the molecules plays an important role in determining the electronic structure of the compounds; the linear-shaped NDTs formally isoelectronic with naphthacene have lower oxida...
01 Jan 2011
TL;DR: Gold and silver nanoclusters have unique molecule-like electronic structure and a nonzero HOMO-LUMO gap, which has led to a new understanding of the geometric structure of gold nanoparticles, with significant implications for electronic struc- ture as discussed by the authors.
Abstract: Gold and silver nanoclusters have unique molecule-like electronic structure and a nonzero HOMO-LUMO gap. Recent advances in X-ray crystal structure determination have led to a new understanding of the geometric structure of gold nanoparticles, with significant implications for electronic struc- ture. The superatom model has been effectively employed to explain properties such as one- and two-photon optical absorption, circular dichroism, EPR spectra, and electronic effects introduced by nanoparticle doping. Future investigations may also lead to an understanding of nanoparticle luminescence, excited-state dynamics, and the metallic to molecular transition.
TL;DR: In this paper, the authors investigated In2O3 and Ga2o3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs.
Abstract: Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.
TL;DR: It is found that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting.
Abstract: Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi2Se3 as a function of water vapor exposure We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting The surface is thus not chemically inert, but the topological state remains protected The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface Because of the presence of water vapor, a similar effect takes place when Bi2Se3 crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi2Se3 band structure
TL;DR: Room-temperature calculations of the excited-state dynamics in FMO are reported on using a combination of molecular dynamics simulations and electronic structure calculations to obtain trajectories for the Hamiltonian of this system which contains time-dependent vertical excitation energies of the individual bacteriochlorophyll molecules and their mutual electronic couplings.
Abstract: The experimental observation of long-lived quantum coherences in the Fenna-Matthews-Olson (FMO) light-harvesting complex at low temperatures has challenged general intuition in the field of complex molecular systems and provoked considerable theoretical effort in search of explanations. Here we report on room-temperature calculations of the excited-state dynamics in FMO using a combination of molecular dynamics simulations and electronic structure calculations. Thus we obtain trajectories for the Hamiltonian of this system which contains time-dependent vertical excitation energies of the individual bacteriochlorophyll molecules and their mutual electronic couplings. The distribution of energies and couplings is analyzed together with possible spatial correlations. It is found that in contrast to frequent assumptions the site energy distribution is non-Gaussian. In a subsequent step, averaged wave packet dynamics is used to determine the exciton dynamics in the system. Finally, with the time-dependent Hamiltonian, linear and two-dimensional spectra are determined. The thus-obtained linear absorption line shape agrees well with experimental observation and is largely determined by the non-Gaussian site energy distribution. The two-dimensional spectra are in line with what one would expect by extrapolation of the experimental observations at lower temperatures and indicate almost total loss of long-lived coherences.
TL;DR: In this article, the structure and energy of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO2 were investigated using hybrid functional electronic structure calculations.
Abstract: Using hybrid functional electronic structure calculations, we have investigated the structure and energetics of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO2. Excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature-dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. Electron and hole polarons are localized at Ti3+ and O– lattice sites, respectively. At the surface, the trapping sites generally correspond to undercoordinated Ti3+5c and O–2c surface atoms or to isolated OH species in the case of a hydroxylated surface. The polaron trapping energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for the polarons to travel from the bulk to the surface. Computed one-electron energy levels in the gap and hyperfine coupling constants compare favorably with oxidation potential and EPR measurements.
TL;DR: In this article, first-principles calculations in the framework of adiabatic connection fluctuation-dissipation theory in the random-phase approximation were performed to investigate the adsorption of graphene sheets on h-BN substrates.
Abstract: We investigate the adsorption of graphene sheets on h-BN substrates by means of first-principles calculations in the framework of adiabatic connection fluctuation-dissipation theory in the random-phase approximation. We obtain adhesion energies for different crystallographic stacking configurations and show that the interlayer bonding is due to long-range van der Waals forces. The interplay of elastic and adhesion energies is shown to lead to stacking disorder and moir\'e structures. Band-structure calculations reveal substrate induced mass terms in graphene, which change their sign with the stacking configuration. The dispersion, absolute band gaps, and the real-space shape of the low-energy electronic states in the moir\'e structures are discussed. We find that the absolute band gaps in the moir\'e structures are at least an order of magnitude smaller than the maximum local values of the mass term. Our results are in agreement with recent scanning tunneling microscopy experiments.
TL;DR: In this article, the electronic structure of vanadium pentoxide (V2O5), a transition metal oxide with an exceedingly large work function of 7.0 eV, was studied via ultraviolet, inverse and x-ray photoemission spectroscopy.
Abstract: The electronic structure of Vanadium pentoxide (V2O5), a transition metal oxide with an exceedingly large work function of 7.0 eV, is studied via ultraviolet, inverse and x-ray photoemission spectroscopy. Very deep lying electronic states with electron affinity and ionization energy (IE) of 6.7 eV and 9.5 eV, respectively, are found. Contamination due to air exposure changes the electronic structure due to the partial reduction of vanadium to V+4 state. It is shown that V2O5 is a n-type material that can be used for efficient hole-injection into materials with an IE larger than 6 eV, such as 4,4′-Bis(N-carbazolyl)-1,1′-bipheny (CBP). The formation of an interface dipole and band bending is found to lead to a very small energy barrier between the transport levels at the V2O5/CBP interface.