Energy Gap Region Research Articles

Spectroscopic insight of low energy electron emission from diamond surfaces

Low work function materials are desirable in many applications such as electron emission and photocatalysis. We have studied low energy electron emission from low work function hydrogen terminated diamond surfaces via electron spectroscopy to gain insight into the mechanisms involved during electron excitation and emission. Electron emission was found to be dominated by electrons within the band gap energy region, allowed due to the negative electron affinity diamond surface, while sub-bandgap illumination was able to significantly increase emission current. Substantial upward surface band bending greater than 2 eV was observed for the diamond samples, which affect electron accumulation at the surfaces. Intra-bandgap states are shown to strongly influence electron emission behavior, which can have great implications for various energy conversion devices.

Tuning optical and electronic properties of graphene oxide by surface adsorption of molecular halogens (X2 = I2, Br2, Cl2, and F2) for light harvesting

Surface adsorption of small molecules modulates both optical and electronic structure, which is beneficial for different applications. The molecular adsorption of halogens X2 (X2 = I2, Br2, Cl2, and F2) was investigated using density functional theory and the dispersion (Wb97xd) module. The electronic structure and optical properties of the pristine graphene oxide (GO) and its molecular complexes with halogens (GOI2, GOBr2, GOCl2, and GOF2) are explored. The adsorption of X2 (X2 = I2, Br2, and Cl2) molecules were stabilized by non-covalent lone pair-π bonding, (O)lp… π, interactions and π-π interactions. The F2 molecule were stabilized by lone pair-π bonding, (O)lp… π, as well as halogen bonding (XB). The GOX2 complexes were energetically stable and optically red shifted in polar solvents. The simulated optical absorption spectra at TD-DFT calculations of GOX2 complexes and TDOS uncover the presence of intermediate sates in the energy gap region, which is beneficial for harvest of visible light and photocatalysis.

Realization of the Chern-insulator and axion-insulator phases in antiferromagnetic MnTe/Bi2(Se,Te)3/MnTe heterostructures

Breaking time-reversal symmetry in three-dimensional topological insulator thin films can lead to different topological quantum phases, such as the Chern insulator (CI) phase, and the axion insulator (AI) phase. Using first-principles density functional theory methods, we investigate the onset of these two topological phases in a tri-layer heterostructure consisting of a Bi$_2$Se$_3$ (Bi$_2$Te$_3$) TI thin film sandwiched between two antiferromagnetic MnTe layers. We find that an orthogonal exchange field from the MnTe layers, stabilized by a small anisotropy barrier, opens an energy gap of the order of 10 meV at the Dirac point of the TI film. A topological analysis demonstrates that, depending on the relative orientation of the exchange field at the two interfaces, the total Chern number of the system is either ${\cal C} = 1$ or ${\cal C} = 0$, characteristic of the CI and the AI phase, respectively. Non-topological surface states inside the energy-gap region, caused by the interface potential, complicate this identification. Remarkably though, the calculation of the anomalous Hall conductivity shows that such non-topological surface states do not affect the topology-induced transport properties. Given the size of the exchange gap, we estimate that gapless chiral edge states, leading to the quantum anomalous Hall effect, should emerge on the sidewalls of these heterostructures in the CI phase for widths $\ge 200$ nm. We also discuss the possibility of inducing transitions between the CI and the AI phases by means of the spin-orbit torque caused by the spin Hall effect in an adjacent conducting layer.

Theoretical investigation of the surface orientation impact on the hydrogen vacancy formation of MgH2

MgH2 is one of the most promising materials for solid-state hydrogen storage, but its slow kinetics and relatively high dehydrogenation temperature have yet to be overcome. We have theoretically investigated surface energy and surface H vacancy formation on various MgH2 surfaces, including all possible H vacancy configurations in the studied size of MgH2(110), (100), (101), and (001), up to (3 × 2), (2 × 3), (2 × 2) and (2 × 2) slabs respectively, based on density functional theory. Consequently, the hydrogen diffusion paths on the surfaces are also studied. The most unstable surface, MgH2(001), has the most drastic change in vacancy formation energy. The relative high barriers for vacancy H formation, especially on the most stable surface, MgH2(110), reflect the slow kinetics of H release. The introduction of vacancy into MgH2 surfaces often induces defect levels in the energy-gap region. In general, the release of surface H atoms exhibits a pronounced dependence on the surface stability and the concentration of surface H vacancy, in line with the remarkable effect of ball milling in experiments.

Resonant transport in Kekulé-distorted graphene nanoribbons

The formation of a superlattice in graphene can serve as a way to modify its electronic band structure and thus to engineer its electronic transport properties. Recent experiments have discovered a Kekulé bond ordering in graphene deposited on top of a copper substrate, leading to the breaking of the valley degeneracy while preserving the highly desirable feature of linearity and gapless character of its band dispersion. In this paper, we study the effects of a Kekulé distortion in zigzag graphene nanoribbons in both the subband spectrum and on its electronic transport properties. We extend our study to also investigate the electronic conductance in graphene nanoribbons composed of sequentially ordered ν=±1 Kek-Y superlattices. We find interesting resonances in the conductance response emerging in the otherwise energy gap regions, which scales with the number of Kek-Y interfaces minus one. Such features resemble the physics of resonant tunneling behavior observed in semiconductor heterostructures. Our findings provide a possible way to measure the strength of the Kekulé parameter in graphene nanoribbons.

Optical properties of (Se80Te20)100-xZnx (2 ≤ x ≤ 6) amorphous thin films

The (Se80Te20)100-xZnx (2 ≤ x ≤ 6)thin film compositions have been prepared by melt quenching technique followed by electron-beam physical vapour deposition. X-ray diffraction pattern and SEM micrographs ascertain the amorphous nature of prepared thin films. The transmission spectra from UV/VIS/NIR double beam spectrophotometer (500 ≤ λ (nm) ≤ 2600) is used for optical characterization of thin films. The absorption coefficient and energetic parameter (A) for various compositions indicate that the absorption mechanism in Se-Te-Zn glassy system occur via allowed indirect transitions. The optical band gap for thin films lies at the edge of near infrared region in A-band (λ~900 nm).The average heat of atomization has no essential influence on the optical band gap energy values. The refractive index (n) for thin films exhibit normal dispersion in the wavelength range 800 nm ≤ λ ≤ 1100 nm (IR-A), and is nearly constant at longer wavelength λ ≥ 1400 nm (IR-B). The dispersion of refractive index below the band gap energy region is studied using the Wemple-DiDomenico (WDD) single oscillator model. It is found that, to a good approximation the single-oscillator energy scales with the optical band gap (Eo≈1.4Egopt). The deduced value of parameter β = 0.24 ± 0.01 corresponds to ionic character of bonds in the Se-Te-Zn system. The dielectric parameters (ε’, ε”,ɛ∞, ɛL), optical conductivity (σ1,σ2), ratio of carrier concentration to the effective mass (N/m*) have also been deduced for thin films. The calculated values of plasma frequency ωP and Nopt are in good agreement with carrier concentration of semiconductors. The energy loss occurred in the interior of the Se-Te-Zn thin films is found to be due to the interband electronic transitions.

Magnetic-impurity-induced modifications to ultrafast carrier dynamics in the ferromagnetic topological insulators Sb2−xVxTe3

Quantum anomalous Hall effect (QAHE) is a key phenomenon for low power-consumption device applications owing to its dissipationless spin-polarized and quantized current in the absence of an external magnetic field. However, the recorded working temperature of the QAHE is still very low. Here we systematically investigate the magnetic dopants induced modifications from the view points of magnetic, structural and electronic properties and the ultrafast carrier dynamics in a series of V-doped Sb2Te3 samples of composition Sb2−xVxTe3 with x = 0, 0.015 and 0.03. Element specific x-ray magnetic circular dichroism signifies that the ferromagnetism of V-doped Sb2Te3 is governed by the p–d hybridization between the host carrier and the magnetic dopant. Time- and angle-resolved photoemission spectroscopy excited with mid-infrared pulses has revealed that the V impurity induced states underlying the topological surface state (TSS) add scattering channels that significantly shorten the duration of transient surface electrons down to the 100 fs scale. This is in a sharp contrast to the prolonged duration reported for pristine samples though the TSS is located inside the bulk energy gap of the host in either magnetic or non-magnetic cases. It implies the presence of a mobility gap in the bulk energy gap region of the host material that would work toward the robust QAHE. Our findings shed light on the material design for low-energy-consuming device applications.

Adsorption characteristics of epigenetically modified DNA nucleobases on single-layer MoS2: A first-principles study

We report a comprehensive van der Waals density functional theory study on the adsorption characteristics of four canonical and six epigenetically modified DNA nucleobases on single-layer molybdenum disulfide (SL-MoS2) substrates. All the considered nucleobases are physisorbed on SL-MoS2 with a modest electron transfer, ranging from 0.007e− to 0.041e−, from the adsorbates to SL-MoS2. Upon the physisorption of adenine, cytosine, guanine, and thymine on the SL-MoS2 sheet, the work function is decreased by 0.29, 0.09, 0.54, and 0.01 eV, respectively. It is shown that the bandgap of SL-MoS2 can be significantly reduced, by as much as 30%, through the adsorption of nucleobase molecules as the induced molecular flat bands tend to appear either in the energy gap region or in the vicinity of the valence band maximum. We unfold the emergence of distinct electron energy loss spectra upon nucleobase adsorption which may serve as potential probes for selective detection of nucleobase molecules in prospective DNA sequencing applications based on SL-MoS2.

Electronic properties of FeCl3 and CrO3 interacting with GaN nanotubes from density functional calculations.

The structural and electronic properties of FeCl3 and CrO3 interacting with (10,0) GaNNT were obtained using first principles calculations based on the density functional theory. The results show that for the CrO3 interacting with the GaNNT, the structure was locally deformed. However, in case of FeCl3 adsorbed with the GaNNT, the structure remained practically the same with the negligible deformation observed on tube surface. The projected density of states for the pristine GaNNT was modified with adsorption of FeCl3 molecule by the appearance of three strongly localized states in gap region. In case of GaNNT plus CrO3 molecule, one strongly localized level appeared in energy gap region with high contributions of molecule atoms. The analysis of the binding energy shows that the CrO3 interacting with the GaNNT is more favorable and the process occurs through chemisorption regime in both systems. Graphical Abstract Projected density of states of pristine GaNNT and FeCl3 interacting with GaNNT.

Thickness-dependent surface proton conduction in (111) oriented yttria-stabilized zirconia thin film

Polycrystalline Zr0.92Y0.08O2 (YSZ) thin films were prepared by RF magnetron sputtering. 80- and 160-nm thin films exhibited (111) orientation and a polycrystalline structure, respectively. The 80-nm thin film had larger distortion than the 160-nm thin film. While the 80-nm thin film and a 120-nm thin film exhibited proton conduction, the 160-nm thin film did not, indicating that surface proton conduction can depend on film thickness. The activation energy of the 80- and 120-nm thin films measured in a wet atmosphere (0.52eV) was about half of that measured in a dry atmosphere. The enhancement of conductivity for the thin films may be attributed to distortion, which may change the structure around an oxygen vacancy at the YSZ grain surface, accompanied by possible enhancement of H2O adsorption. H2O-annealed thin film had a hydrogen-induced level in the band gap energy region. This is the first observation of hydrogen-induced level at the surface state of the YSZ thin film obtained by X-ray absorption spectroscopy.

Ambient Carbon Dioxide Capture Using Boron-Rich Porous Boron Nitride: A Theoretical Study.

The development of highly efficient sorbent materials for CO2 capture under ambient conditions is of great importance for reducing the impact of CO2 on the environment and climate change. In this account, strong CO2 adsorption on a boron antisite (BN) in boron-rich porous boron nitrides (p-BN) was developed and studied. The results indicated that the material achieved larger adsorption energies of 2.09 eV (201.66 kJ/mol, PBE-D). The electronic structure calculations suggested that the introduction of BN in p-BN induced defect electronic states in the energy gap region, which strongly impacted the adsorption properties of the material. The bonding between the BN defect and the CO2 molecule was clarified, and it was found that the electron donation first occurred from CO2 to the BN double-acceptor state then, followed by electron back-donation from BN to CO2 accompanied by the formation of a BN-C bond. The thermodynamic properties indicated that the adsorption of CO2 on the BN defect to form anionic CO2δ- species was spontaneous at temperatures below 350 K. Both the large adsorption energies and the thermodynamic properties ensured that p-BN with a BN defect could effectively capture CO2 under ambient conditions. Finally, to evaluate the energetic stability, the defect formation energies were estimated. The formation energy of the BN defects was found to strongly depend on the chemical environment, and the selection of different reactants (B or N sources) would achieve the goal of reducing the formation energy. These findings provided a useful guidance for the design and fabrication of a porous BN sorbent for CO2 capture.

The interface of SrTiO3 and H2O from density functional theory molecular dynamics.

We use dispersion-corrected density functional theory molecular dynamics simulations to predict the ionic, electronic and vibrational properties of the SrTiO3/H2O solid-liquid interface. Approximately 50% of surface oxygens on the planar SrO termination are hydroxylated at all studied levels of water coverage, the corresponding number being 15% for the planar TiO2 termination and 5% on the stepped TiO2-terminated surface. The lateral ordering of the hydration structure is largely controlled by covalent-like surface cation to H2O bonding and surface corrugation. We find a featureless electronic density of states in and around the band gap energy region at the solid-liquid interface. The vibrational spectrum indicates redshifting of the O-H stretching band due to surface-to-liquid hydrogen bonding and blueshifting due to high-frequency stretching vibrations of OH fragments within the liquid, as well as strong suppression of the OH stretching band on the stepped surface. We find highly varying rates of proton transfer above different SrTiO3 surfaces, owing to differences in hydrogen bond strength and the degree of dissociation of incident water. Trends in proton dynamics and the mode of H2O adsorption among studied surfaces can be explained by the differential ionicity of the Ti-O and Sr-O bonds in the SrTiO3 crystal.

A comparative run for visible-light-driven photocatalytic activity of anionic and cationic S-doped TiO2 photocatalysts: A case study of possible sulfur doping through chemical protocol

Photocatalytic degradation of pollutant molecules delivered great challenges for sustainable development of green energy in the field of environmental science. For this purpose, growing interests have been made to witness the fast development of new photocatalysts with improved catalytic efficiency and to monitor the reaction mechanism at the atomic and molecular levels. Therefore, doping of semiconductor metal oxide (e.g. TiO2) with main group elements, especially with sulfur atoms (S), has gained much interest due to the introduction of a localized band between the conduction bands (CB) and valence bands (VB) that increase the absorbance in the visible light region. More interestingly, photocatalytic practices of the S-doped TiO2 material are of pronounced significance due to photon-to-carrier conversion ability of S-doping beneath the band gap energy region of pure TiO2. Therefore, visible-light-activated S-doped TiO2 photocatalysts were prepared via template free and low-temperature oxidant peroxide method (OPM) assisted hydrothermal treatments. Experimental findings have revealed the successful incorporation of sulfur atoms into TiO2 crystal lattice and, as a result, substitution of Ti4+ by S6+ to form TiOS bonds for cationic S-doping was observed. Whereas, in the case of anionic S-doping, substitution of S2− by O2− to form OTiS bonds was achieved. More evidence was observed for the presence of chemisorbed sulfate groups on the surface of S-doped TiO2 samples and inhibition of crystallite size growth by S-doping, and obviously, upsurge the absorbance in the visible light region. The photocatalytic activity of as-prepared 1-D nanorods shaped photocatalysts and the mechanism involved for the photodegradation of organic molecules (methyl orange and phenol) under visible-light irradiation were investigated by adding different scavengers into the system solution to capture active species. It was found that in cationic S-doped TiO2 photocatalysts, chemisorbed hydroxyls (OHads−) and photoinduced holes (h+) played a major role in photocatalysis. Whereas, in the case of anionic S-doped TiO2 photocatalysts, electrons (e−) and photoinduced holes (h+) played the nearly equal role in photocatalysis.

Peculiarities of photoluminescence of vertical n +/n-GaAs/Al0.25Ga0.75As MBE- and MOCVD-grown structures designed for microwave detectors

Vertical MBE- and MOCVD-grown $${n}^{+}/{n}\hbox {-GaAs}/\hbox {Al}_{0.25}\hbox {Ga}_{0.75}\hbox {As}$$ structures used for microwave electronics have been studied with continuous wave and time-correlated single photon counting dynamic photoluminescence technique. The photoluminescence spectra and light emission lifetimes are used to explain the recombination mechanisms of the excited carriers. This paper presents results showing the differences in recombination characteristics of $${n}^{+}\hbox {-Al}_{0.25}\hbox {Ga}_{0.75}\hbox {As}$$ layers grown using MBE process compared with MOCVD process. One of these differences is that the PL spectrum of the MOCVD-grown layer is shifted towards the forbidden energy gap region, as well as the characteristic recombination time is longer than for the MBE-grown sample. This peculiarity can be attributed to the formation of the localised band tails in the $${n}^{+}\hbox {-Al}_{0.25}\hbox {Ga}_{0.75}\hbox {As}$$ MOCVD-grown sample. The proposed analytical model explains the differences in microwave detection properties of the samples grown by MBE and MOCVD processes.

Cr impurity-induced electronic states in ZnTe(110) surface

The impurity states of Cr atoms, which substituted Zn sites in the topmost layer of a p-type ZnTe(110) surface, were investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and we firstly observed Cr-induced impurity states in the energy gap region of the host ZnTe including the unoccupied states by STS. Furthermore, we compared the observed energy levels and spatial distributions of the local density of states with those in the previous theoretical study [Katayama-Yoshida et al., Phys. Status Solidi A 204, 15 (2007)] and successfully identified the impurity states as the respective spin-polarized impurity states predicted by the theoretical study.

Electronic and magnetic properties of graphite quantum dots

We study the electronic and magnetic properties of multilayer quantum dots (MQDs) of graphite in the nearest-neighbor approximation of tight-binding model. We calculate the electronic density of states and orbital susceptibility of the system as function of the Fermi level location. We demonstrate that properties of MQD depend strongly on the shape of the system, on the parity of the layer number and on the form of the cluster edge. The special emphasis is given to reveal the new properties with respect to the monolayer graphene quantum dots. The most interesting results are obtained for the triangular MQD with zig-zag edge at near-zero energies. The asymmetrically smeared multipeak feature is observed at Dirac point within the size-quantized energy gap region, where monolayer graphene flakes demonstrate the highly-degenerate zero-energy state. This feature, provided by the edge-localized electronic states results in the splash-wavelet behavior in diamagnetic orbital susceptibility as function of energy.

Subgap spectroscopy of thermally excited quasiparticles in a Nb-contacted carbon nanotube quantum dot

We present electronic transport measurements of a single wall carbon nanotube quantum dot coupled to Nb superconducting contacts. For temperatures comparable to the superconducting gap peculiar transport features are observed inside the Coulomb blockade and superconducting energy gap regions. The observed temperature dependence can be explained in terms of sequential tunneling processes involving thermally excited quasiparticles. In particular, these new channels give rise to two unusual conductance peaks at zero bias in the vicinity of the charge degeneracy point and allow to determine the degeneracy of the ground states involved in transport. The measurements are in good agreement with model calculations.

Growth of TiO2−δ thin film by RF magnetron sputtering using oxygen radicals and Ti metal

TiO2−δ thin films have been deposited by RF magnetron sputtering using oxygen radicals. The deposition rate of the thin films largely increases with oxygen radical irradiation during the deposition in the metal mode. A as-deposited TiO2−δ thin film on a Pt substrate prepared at the substrate temperature of 300 °C crystallizes without postannealing. The Ti 2p X-ray absorption spectrum indicates that the Ti 3d electron enters the bottom of the conduction band. The Ti 3d states at the Fermi level (EF) and at ∼1.5 eV from EF in the band gap energy region are closely related to the electrical conductivity.

Photodecarbonylation mechanism of cyclopropenone in the gas phase: electronic structure calculation and AIMS dynamics simulation.

In this article, structures and energies of cyclopropenone in the low-lying electronic states have been determined by the CASSCF and MS-CASPT2 calculations with different basis sets. Two minimum-energy conical intersections (CI-1 and CI-2) between S0 and S1 were obtained and their topographic characters were characterized by the SA4-CAS(10,9) calculated energy gradients and nonadiabatic coupling vectors. The AIMS method was used to carry out nonadiabatic dynamics simulation with ab initio calculation performed at the SA4-CAS(10,9) level. On the basis of time evolution of wave functions simulated here, the S1 lifetime is fitted to be 125 fs with a pure exponential decay for the S1 electronic population. The CI-1 intersection is mainly responsible for ultrafast S1→S0 nonadiabatic transition and the photoinduced decarbonylation is a sequential process, where the first C-C bond is broken in the S1 state and fission of the second C-C bond occurs in the S0 state as a result of the S1→S0 internal conversion via the CI-1 region. As a minor channel through the CI-2 region, the decarbonylation proceeds in an asynchronous concerted way. Effects of the S1 excess energies and the S1-S0 energy gap on the nonadiabatic dynamics were examined, which reveals that the S1→S0 nonadiabatic transition occurs within a small energy gap and high-energy conical intersection regions can play an important role. The present study provides new insights into mechanistic photochemistry of cyclopropenones and reveals that the AIMS dynamics simulation at a high-accuracy ab initio level is a powerful tool for exploring a mechanism of an ultrafast photochemical reaction.

スピン分解光電子分光で捉えるトポロジカル絶縁体の表面電子状態

Three-dimensional topological insulators (3D TIs) with a gapless topological surface state in a bulk energy gap induced by a strong spin-orbit coupling have attracted much attention as key materials to revolutionize current electronic devices. A spin helical texture of a topological surface state, where the electron spin is locked to its momentum, is a manifestation of a 3D TI. A number of well-known thermoelectric and phase-change materials have so far been predicted to be 3D TIs. In order to experimentally confirm their topological characteristics, spin- and angle- resolved photoemission spectroscopy is one of the most powerful tools and it has actually been playing major roles in finding some real 3D TIs. In this article, after extensive introduction of topological insulators and spin- and angle- resolved photoemission spectroscopy, some of the ternary 3D TIs are shown to possess topological surface states with marked spin polarizations. It has been revealed for GeBi2Te4 that an intermixing effect in the crystal has a minor effect on the surface-state spin polarization, which is ∼70% near the Dirac point in the bulk energy gap region (∼180 meV), which promises to future technological application.