Unoccupied Band Research Articles

Many-body effects in an MXene Ti2CO2 monolayer modified by tensile strain: GW-BSE calculations.

MXenes, two-dimensional (2D) layered transition metal carbide/nitride materials with a lot of advantages including high carrier mobility, tunable band gap, favorable mechanical properties and excellent structural stability, have attracted research interest worldwide. It is imperative to accurately understand their electronic and optical properties. Here, the electronic and optical response properties of a Ti2CO2 monolayer, a typical member of MXenes, are investigated on the basis of first-principles calculations including many-body effects. Our results show that the pristine Ti2CO2 monolayer displays an indirect quasi-particle (QP) band gap of 1.32 eV with the conduction band minimum (CBM) located at the M point and valence band maximum (VBM) located at the Γ point. The optical band gap and binding energy of the first bright exciton are calculated to be 1.26 eV and 0.56 eV, respectively. Under biaxial tensile strains, the lowest unoccupied band at the Γ point shifts downward, while the lowest unoccupied band at the M point shifts upward. Then, a direct band gap appears at the Γ point in 6%-strained Ti2CO2. Moreover, the optical band gap and binding energy of the first bright exciton decrease continuously with the increase of the strain due to the increase of the lattice parameter and the expansion of the exciton wave function. More importantly, the absorbed photon flux of Ti2CO2 is calculated to be 1.76–1.67 mA cm−2 with the variation of the strain, suggesting good sunlight optical absorbance. Our work demonstrates that Ti2CO2, as well as other MXenes, hold untapped potential for photo-detection and photovoltaic applications.

Charge-transfer dynamics in anion collisions with vicinal nanostepped surfaces

SynopsisWe investigate the electron dynamics at various monocrystalline metal surfaces with stepped vicinal nanostructures. The unoccupied bands of the surface superlattice are resonantly excited via the charge transfer interaction of the surface with a H− ion which flies by at grazing angles. The ion survival exhibits modulations as a function of the vicinal-terrace size and shows peaks at energies that access the image state subbands.

A modified energy detection based dynamic spectrum sharing technique and its real time implementation on wireless platform for cognitive radio networks

Cognitive radio offers a flexible and efficient utilization of radio frequency resources by dynamic spectrum sharing as required in next gen (5G) architecture of wireless communication. The channel allocation time, probability of false alarm detection and spectral efficiency are the major performance parameters to characterize a spectrum sharing technique. This paper presents modified energy detection based dynamic channel allocation technique based on sensing the power spectral density of idle spectrum bands i.e. spectrum hole. Receiver operating characteristics (ROC) curves have been used to analyze the detector performance of sensing with respect to probability false alarm at different values of SNR. Allocation of unoccupied bands to the SUs has been done by coalition based cooperative game, which provides SUs with an incentive to cooperate. Based on their worth, SUs get payoffs which have been computed using Shapely values as a one-point solution. Vickrey–Clarke–Groves (VCG) auction mechanism has been used to allocate the spectrum resources fairly to each user. On the basis of allocation time, the present model for dynamic spectrum access appears to be more efficient as compared to the conventional opportunistic spectrum access model.

Formation and regulation of unoccupied hybridized band with image potential states at perylene/graphite interface.

Occupied and unoccupied electronic structures of submonolayer perylene (C20H12) on a graphite surface have been investigated using two-photon photoemission (2PPE) spectroscopy for two phases at room and low temperatures. Low energy electron diffraction measurements indicated that the molecules are disordered at room temperature and form a well-ordered superstructure below 180 K. In 2PPE, a specific unoccupied peak (Lx) was observed at around room temperature (>180 K) but not at low temperature (<180 K). The temperature-dependence of the excitation probability was attributed to a contribution of a diffuse unoccupied state, which is characterized by the molecular orbital extending outside the perylene molecular framework. At around room temperature, perylene adopts a flat-lying molecular orientation so that the diffuse state can hybridize with a free-electron-like unoccupied surface state, image potential states (IPS). As a result, the hybridized Lx state can be excited from the occupied bulk band through the IPS-mediated process. In contrast, hybridization is not efficient in the low-temperature phase due to the standing molecular orientation, which decouples the molecule away from the image plane of the substrate. The size of molecular islands also affects hybridization between the diffuse states and IPS because the two states encounter each other at the edge part of molecular aggregates. The temperature-dependent 2PPE results indicate that the molecular orientation and island size of perylene are directly linked to the formation of hybridized states, and thus, the excitation probability at the interface can be regulated by the morphology on the surface.

Phonon-assisted processes in the ultraviolet-transient optical response of graphene

Many recent experiments investigated potential and attractive means of modifying many-body interactions in two-dimensional materials through time-resolved spectroscopy techniques. However, the role of ultrafast phonon-assisted processes in two-dimensional systems is rarely discussed in depth. Here, we investigate the role of electron–phonon interaction in the transient optical absorption of graphene by means of first-principles methods. It is shown at equilibrium that the phonon-assisted transitions renormalize significantly the electronic structure. As a result, absorption peak around the Van-Hove singularity broadens and redshifts by ~100 meV. In addition, temperature increase and chemical doping are shown to notably enhance these phonon-assisted features. In the photoinduced transient response, we obtain spectral changes in close agreement with the experiments, and we associate them to the strong renormalization of occupied and unoccupied pi bands, which predominantly comes from the coupling with the zone-center {E}_{2g} optical phonon. Our estimation of the Coulomb interaction effects shows that the phonon-assisted processes can have a dominant role even in the subpicosecond regime.

Resource-partitioned spectrum assignment to realize efficient multicasting for flexible grid optical networks

Abstract With the increasing demand of some novel point-to-multi-point services, such as live video delivering and interactive distance learning, efficient multicasting is highly desirable in flexible grid optical networks (FGONs). Thus, many sophisticated multicast-enabled routing and resource allocation algorithms have been proposed to realize efficient multicasting in FGONs. These algorithms, known as multicast routing and spectrum assignment (MRSA) algorithms, optimized the light-trees and the allocated spectrum resources for multicast services in FGONs. But, most of them ignored some small-sized, isolated, and un-occupied spectrum bands, which are remained after multicast services being accommodated. Due to the continuity and the contiguity constraints in service accommodation, these remained spectrum bands can hardly be allocated to subsequent multicast services and are thus known as spectrum fragments. Their existence and accumulation may exhaust available spectral resources and affect the networking performance (e.g. service blocking performance) for the multicast traffic in FGONs. In this way, the efficiency of FGONs in supporting multicast traffic may be greatly deteriorated. Therefore, the spectrum fragments generated in accommodating multicast services has become an important issue in FGONs. In this paper, we propose a resource-partitioned interval-maximized (RPIM) MRSA algorithm to restrain the generation of spectrum fragments for the multicast traffic in FGONs. The proposed RPIM MRSA algorithm does not utilize the metric, “cut”, to reduce spectrum fragments via avoiding splitting the whole spectrum band into too many band pieces as in fragmentation-aware MRSA algorithms. Instead, the proposed RPIM MRSA algorithm partitions the spectrum resources on each sought light-tree into several elastic groups according to the types of multicast services, and maximizes the group interval between two adjacent groups on the sought light-trees. Since the spectrum resources in each group can always be utilized by the multicast services of the same type, the proposed algorithm restricts the generation of spectrum fragments in group intervals. In this way, the proposed RPIM MRSA algorithm can remarkably reduce spectrum fragments generated in accommodating multicast services. Note that, different from previous group-based algorithms, the proposed RPIM algorithm allows one elastic group for the multicast services of a certain type to contain different amount of spectrum resources on different light-trees. This helps guarantee the flexibility in constructing elastic groups on diverse light-trees. Additionally, the proposed RPIM MRSA algorithm maximizes the group intervals on the sought light-trees, which helps reserve more vacant spectrum resources between two adjacent groups on the light-trees. Since these vacant spectrum resources can be merged into their adjacent groups if needed, the proposed algorithm can reduce not only the spectrum fragments generated in group intervals but also the blocked multicast services in groups by maximizing group intervals on the light-trees. Simulation results verify that the proposed RPIM algorithm can realize efficient multicasting with significantly reduced spectrum fragments and low service blocking probability in FGONs.

Experimental and DFT Studies on Thermochromism Induced Binary HBLC Mixture

Novel linear double hydrogen-bonded liquid crystals (HBLCs) were derived from diglycolic acid (DGA, a non-mesogenic compound) and 4-n-alkoxybenzoic acids (nOBA, n = 7 and 8 mesogenic compounds). Hydrogen bond (H bond) formation and its vibrational stretching frequencies had been calculated by experimental and theoretical IR spectroscopy. The calculated band gap energy (4.96 eV) using a UV-Vis spectrum clearly reveals the coincidence of highest occupied molecular orbital-lowest unoccupied molecular orbital band energy of the present HBLC mixture. Further, X-ray diffraction (XRD) studies at room temperature confirm the monoclinic nature of the HBLC mixture. Mesophases and their transition temperature were studied by a polarized optical microscope (POM) and differential scanning calorimetry (DSC). The order of the phase transition was evaluated by the thermal analysis. Due to the rotary motion of molecules, nematic phase (threaded texture) with thermochromic effect was observed. The induced thermochromism in the present HBLC and its possible color recording applications were discussed. Molecular descriptors (using computational density functional theory (DFT)) of the present mixture indicate the hardness and softness of the HBLC mixture. Natural bond orbital (NBO) studies revealed the O–H…O stabilization energy in the present HBLC mixture. Also, the lone pair (LP)-to-π* transition confirms the existence of intermolecular hydrogen bonding in the HBLC mixture. The calculated band gap energy of the DGA + nOBA HBLC mixture is a more useful parameter to identify suitable hydrogen-bonded liquid crystal material for photonic applications. Mulliken analysis shows clear evidence of the charge distribution in different molecules of the HBLC system.

Synthesis and properties of bay unilaterally extended and mono-substituted perylene diimides

The synthesis of two sulfur-decorated perylene diimides, the five-membered S-heterocyclic annulated perylene diimide (1) and 1-propanethiol- N,N′-dicyclohexylperylene-3,4,9,10-tetracarboxylic diimide (2), and a novel sulfoxide-containing perylene diimide, 1-propyl sulfoxide- N,N′-dicyclohexylperylene-3,4,9,10-tetracarboxylic diimide (3), are reported. The photophysical, electrochemical, aggregation, and thermal properties of these compounds are investigated by ultraviolet visible absorption, fluorescence, cyclic voltammetric, X-ray diffraction, and thermogravimetric analysis techniques. The geometries of the compounds are optimized at the 6-31G* level of theory using density functional theory, and their potentials are correlated with molecular orbitals. The prepared perylene diimide derivatives exhibit narrow the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals band gaps, and they have quite different absorptions and emissions in dichloromethane solutions, which are in agreement with the density functional theory–calculated results.

Investigation of Sodium Plating and Stripping on a Bare Current Collector with Different Electrolytes and Cycling Protocols.

In the present study, stable sodium plating/stripping has been achieved on a bare aluminum current collector, without any surface modifications or artificial SEI deposition. The crucial role of predeposited sodium using cyclic voltammetry on bare aluminum as a matrix for plating/stripping has been highlighted using different protocols for cycling. The predeposition strategy ensures stable and efficient cycling of sodium in anode-free sodium batteries without dendritic formations. The study highlights the difference of sodium plating/stripping in carbonate and glyme solvent electrolytes on the bare aluminum current collector. Contrary to the carbonate solvent electrolyte, the cell with the tetraglyme solvent electrolyte and sodium loading of 1 mA h/cm2 has an overpotential under 20 mV during the sodium plating/stripping cycles at 0.5 mA/cm2 for a testing period of 650 h. Overpotentials under 40 and 100 mV have been achieved at current densities up to 1 and 2 mA/cm2 for loadings up to 5 and 10 mA h/cm2, respectively, for a testing time up to 1500 h. Density functional theory simulations have been performed to obtain the solvation energies, and the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap of the solvent-sodium ion complexes for the glyme solvent electrolytes and their trends have been correlated with the experimental observations.

Resonant Charge-Transfer in Grazing Collisions of H− with Vicinal Nanosurfaces on Cu(111), Au(100) and Pd(111) Substrates: A Comparative Study

We compare the electron dynamics at monocrystalline Cu(111), Au(100) and Pd(111) precursor substrates with vicinal nanosteps. The unoccupied bands of a surface superlattice are populated via the resonant charge transfer (RCT) between the surface and a H − ion that flies by at grazing angles. A quantum mechanical wave packet propagation approach is used to simulate the motion of the active electron, and time-evolved wave packet densities are used to visualize the dynamics through the superlattice. The survived ion fraction in the reflected beam generally exhibits modulations as a function of the vicinal terrace size and shows peaks at those energies that access the image state subband dispersions. Differences in magnitudes of the ion-survival as a function of the particular substrate selection and the ion-surface interaction time, based on the choice of two ion-trajectories, are examined. A square well model, producing standing waves between the steps on the surface, explains the energies of the maxima in the ion survival probability for all the metals considered. This indicates that the primary process of confinement induced subband formation is robust. The work may motivate measurements and applications of shallow-angle ion-scattering spectroscopy to access electronic substructures in periodically nanostructured surfaces.

Effects of non-stoichiometry on electronic structure of CuxSy compounds studied by first-principle calculations

Non-stoichiometric CuxSy (1 ≤ x/y ≤ 2) is a potential photocatalytic material. In this article, the non-stoichiometric models are constructed by Cu2S with copper vacancies and CuS with sulfur vacancies. The crystal structure, electronic properties, and optical properties of different stoichiometries of CuxSy were studied by First-principle calculations. Their similarities and differences in electronic structure and optical properties and the changing trends have been systematically compared and analyzed. From Cu2S to CuS, the metallic Cu–Cu bonds gradually disappear; the covalent S–S bonds gradually appear. Most important, the presence of S–S layer is enhancing the covalentity of CuxSy compound, as the x/y ratio is increasing. Comparison to the stoichiometric Cu2S and CuS, both copper vacancy and sulfur vacancy can induce obvious unoccupied energy band on the top of valence band. Namely, the Fermi energy level enters the valence band, resulting in the degenerate semiconductor. The little unoccupied band is the key factor to determine the photo-electronic properties of CuxSy, in which the most important effect is the potential local surface plasmon resonance. All of the above factors lead to changes in optical properties and ultimately affect the photocatalytic properties of non-stoichiometric CuxSy. Therefore, the photocatalytic performance of CuxSy can be adjusted by the stoichiometries. These calculated results can provide reliable data and support for the development of CuxSy and copper sulfide-based photocatalysts.

Geometric orbital magnetization in adiabatic processes

We consider periodic adiabatic processes of gapped many-body spinless electrons. We find an additional contribution to the orbital magnetization due to the adiabatic time evolution, dubbed \textit{geometric} orbital magnetization, which can be expressed as derivative of the many-body Berry phase with respect to an external magnetic field. For two-dimensional band insulators, we show that the geometric orbital magnetization generally consists of two pieces, the topological piece that is expressed as third Chern-Simons form in $(t,k_x,k_y)$ space, and the non-topological piece that depends on Bloch states and energies of both occupied and unoccupied bands.

Origin of Charge Trapping in TiO2/Reduced Graphene Oxide Photocatalytic Composites: Insights from Theory.

Composites of titanium dioxide (TiO2) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO2 alone. However, little attention has been paid so far to the chemical structure of TiO2/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO2 rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti–O–C covalent bonds and strong hydrogen bonds between TiO2 and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO2 conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron–hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO2/RGO composites and the enhancement of photocatalytic efficiency of these composites.

Predicted Efficient Visible-Light Driven Water Splitting and Carbon Dioxide Reduction Using Photoredox-Active UiO-NDI Metal Organic Framework

The structural and electronic properties of metal–organic frameworks (MOFs) constructed from stable Zr(IV) oxide-based nodes and naphthalenediimide-based linkers (UiO(Zr)-NDI) can be tuned to make them useful for specific photoredox processes. The NDI linker presents outside, inside, and core positions where functionalization can influence its optical properties, and such substitution in combination with suitable optical band gaps and band edge positions in UiO(Zr)-NDI offers a platform for the design of efficient MOF-based photocatalysts for water splitting and CO2 reduction. The band gaps and edge positions remain similar for UiO nodes when Zr is substituted with Ti, Th, or Ce. However, in contrast to the case for Zr-, Ti-, and Th-based UiO-NDIs, where photoexcitation remains localized on the NDI linkers owing to the very high energies of unoccupied node-based bands, for UiO(Ce)-NDI, the availability of low-lying empty f orbitals in the metal node offers the potential for energy transfer and exciton migration, which could further boost photocatalytic performance by extending exciton lifetimes.

Exploring the role of Zn doping on the structure, morphology, and optical properties of LaFeO3

The structural and optical properties of nanocrystalline LaFe1−xZnxO3 (0 ≤ x ≤ 0.3) samples, synthesized through sol–gel auto-combustion technique, have been studied. Transmission electron microscopy concomitant with Williamson–Hall analysis elucidates the nanocrystalline nature of the samples. High-resolution transmission electron microscopy in combination with selected area electron diffraction patterns reveals the interplanar spacing and poly-crystalline nature of samples, respectively. LFO being charge transfer insulator, its direct optical bandgap corresponding to O2p band, and the unoccupied Fe-3d band is determined. The red shift in bandgap is found with the increase in Zn doping and may be attributed to the formation of localized energy levels near the conduction band and also to the change in crystal field with the creation of oxygen vacancies. Higher order transition energy corresponding to multiple transitions has also been determined and is found to decrease with Zn doping. Urbach tail which measures the disorder in the system has been found in the pristine and doped samples and is found to increase with zinc doping. Optical constants including complex refractive index and optical conductivity have been determined from the reflectance data. The pristine sample has a refractive index of maximum value 3 which decreases with Zn doping making the sample more transparent and hence conductive. Zn doping enhances the conductivity of the pristine sample by creating the charge imbalance and thereby promoting the hopping motion of charge carriers, making the sample more transparent in the optical region.

Electron‐ and X‐Ray Spectroscopies of Organic Charge‐Transfer Complexes

Recent work aiming at a deeper understanding of the electronic properties of classical and novel charge‐transfer (CT) complexes is reviewed herein. From a variety of studied CT salts, the prototypical examples TTF‐TCNQ, (TMTTF)2SbF6, (TMTSF)2PF6, and TMP/HMP‐TCNQ have been selected. Three different types of electron and X‐ray spectroscopies were applied, namely ultraviolet photoelectron spectroscopy (UPS), hard X‐ray photoelectron spectroscopy (HAXPES), and near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. Using UPS, the development of the valence band structure during co‐deposition of the donor and acceptor moieties TMP/HMP and TCNQ has been studied. The spectra reveal characteristic level shifts and a strong reduction of the HOMO–LUMO gap upon formation of the complex. HAXPES probes the core levels and reveals characteristic shifts of the binding energies in (TMTTF)2SbF6 at the phase transition to the low‐temperature charge‐ordered state. NEXAFS is a local, element‐specific probe for the unoccupied bands above the Fermi level and reveals changes in the population of specific orbitals when the CT complex is formed. As an outlook, the novel method of photoelectron momentum microscopy is presented, giving access to the valence bands and core‐levels and in particular to high‐resolution X‐ray photoelectron diffraction patterns.

Unoccupied conduction band of chloroaluminum phthalocyanine

The electronic structure of the unoccupied conduction band of polycrystalline chloroaluminum phthalocyanine was investigated by X-ray absorption spectroscopy (XAS) near the Cl K-edge. XAS spectra were recorded in total electron yield (TEY) and partial Auger electron yield (AEY) modes. Partial AEY spectra were measured using normal and spectator Cl KL2,3L2,3 Auger yields as a function of photon energy. Although the TEY spectrum showed a broad shape; that was difficult to deconvolute curve fitting, the normal and spectator AEY spectra clearly showed that Cl 1s → σ*(Cl–Al) transition splits into two competitive peaks attributed to the core excitons (localized with a core hole) and conduction band (delocalized without a core hole). The existence of a band-like peak in the Cl K-edge XAS is evidence that the Cl axial ligand participates in forming the conduction band. Thus, band-like electron transport is expected to occur in the oligomer-like (–Al–Cl–Al–) site.

Electronic Bands and Dielectric Functions of In0.5Tl0.5I Solid State Solution with Structural Defects

We investigate an influence of the crystal structure imperfections on the electronic properties and dielectric functions of the In0.5Tl0.5I semiconductor in the frame of the density functional theory calculations. The tensor of electron effective mass m^{*}_{ij} for the InI, In0.5Tl0.5I and TlI crystals has been calculated for the valence and conduction bands at different K-points of the Brillouin zone. The dielectric functions ε(hν) of the imperfect crystals based on In0.5Tl0.5I solid state solution with iodine vacancy and a thallium interstitial atom were calculated taking into consideration the inter-band and intra-band electron transitions. The studies of the imperfect crystals reveal increased low-frequency and stationary electron conductivity with anisotropy resulted from the anisotropy of the electron effective mass tensor. Our findings explain the origin of crucial changes in the band structure by formation of the donor half-occupied levels close to the unoccupied conduction bands due to the crystal structure defects, i.e., iodine vacancy or a thallium interstitial atom. It has been shown that in the case of real crystals, in particular metal-halides, the proper consideration of defects in quantum-chemical calculations results in a better matching with experimental data and, opposite to the perfect structure calculations, gives opportunities to explain the observed phenomena.

DFT+U in Dudarev's formulation with corrected interactions between the electrons with opposite spins: The form of Hamiltonian, calculation of forces, and bandgap adjustments.

Hubbard corrected density functional theory (DFT) methods, such as the DFT+U approach in Dudarev's approximation, are widely used for the description of energetics and electronic structure of strongly correlated materials, providing higher level of accuracy than local DFT calculations (e.g., local density approximation or generalized gradient approximation). However, the DFT+U method in Dudarev's formulation limits the introduced corrections to interactions between the electrons within the same spin channel, whereas interactions between the electrons with opposite spins are still treated using local DFT functional (e.g., Perdew-Burke-Ernzerhof). In recent years, the need for correction of these interactions between the electrons with opposite spins has been recognized and additional terms have been added to the Hubbard term to reflect it. Although such extended DFT+U functionals have been proposed, the form of respective Hamiltonian operator, defined as a total energy derivative over density with appropriate treatment of double counting corrections due to additional Hubbard terms, has not been explicitly presented. In this work, we provide an expression for such a type of Hamiltonian, which contains the respective double counting correction contributions. This formulation also allows evaluation of atomic forces, using computational settings discussed herein. In addition, we also introduce adjustments for too narrow theoretical bandgaps, using scissor operator technique. This allows for a greater level of corrections of energetics and magnetic properties of studied transition metal compounds, avoiding possible unphysical overlap between occupied and unoccupied electronic bands.

A theoretical approach of the ethanol adsorption on Au(100), Au(110) and Au(111) surfaces

The adsorption of ethanol on Au(100), Au(110) and Au(111) surfaces was studied using electronic structure calculations under the scheme of the density functional theory (DFT) with van der Waals corrections, vdW-DF2 and PBE functionals, using numerical atomic orbitals (NAO) basis set implemented in the SIESTA code and plane-waves basis set in the CASTEP code. The lowest physisorption energies using the plane-wave basis set in neutral media were observed at the TOP position in all the surfaces, with values in between −53.35 kJ.mol−1 and −61.80 kJ.mol−1. Structurally, the method based on linear combination of atomic orbitals (LCAO) using NAO as basis set gives the best result respect to the ideal bulk. However, the relaxation of the slab is better defined by the plane-wave scheme. Band structure has shown no change in the sp-like bands and the 5d occupied bands with and without the ethanol molecule adsorbed. However, a slight variation was obtained in the unoccupied bands due to the interaction with the lone pair of electrons from the oxygen (non-bonding orbitals).