Surface Electronic States Research Articles

ADSORPTION-INDUCED INCREASING SPECULARITY OF CONDUCTION ELECTRONS’ SURFACE SCATTERING

We present a review of our investigations on the increasing specularity of surface scattering of conduction electrons on the W(011), Mo(011), and W(001) surfaces after covering the surfaces with well-ordered (sub)monolayers of adsorbed hydrogen or deuterium. The degree of the specularity of surface scattering was estimated on the base of measurements of magnetoresistance of single-crystalline plates, cooled with liquid He, under ultra-high vacuum conditions, and in a strong magnetic field. The structures of adsorbed hydrogen and deuterium layers were determined from LEED patterns. It is found that the magnetoresistance non-monotonically depends on submonolayer hydrogen coverage and degree of the ordering of adsorbed layers, showing a drastic increase of the specularity (which is higher than for the atomically-clean surfaces!) after the formation of the well-ordered [Formula: see text]-H structures and a pronounced increase of the specularity after the formation of the well-ordered [Formula: see text]-2H structure. Changes in the magnetoresistance induced by the ordering of structures of adsorbed H(D) layers are found to corroborate with the adsorption-induced transformation of Fermi contours of surface electronic states obtained in performed DFT calculations. The increase of the specularity of the scattering caused by the ordering of the surface structures is attributed to the decrease of the probability of electronic transitions between the surface and bulk electronic states induced by surface scattering.

Single GaN Nanowires for Extremely High Current Commutation

An array of GaN nanowires (NWs) on Si substrate is synthesized by molecular beam epitaxy. Measurements of electrical properties at room temperature of single NWs transferred to an auxiliary substrate demonstrate that the current density reaches an extremely high level of 2 MA cm−2 without NW damage. Taking into account the limited conductivity of the NW periphery due to electron surface states, that is, the conduction channel narrowing, the effective current density can reach 3 MA cm−2.

Unconventional magnonic surface and interface states in layered ferromagnets

Electronic surface, interface and edge states are well-known concepts in low-dimensional solids and have already been utilised for practical applications. It is expected that magnons–the bosonic quasiparticles representing the magnetic excitations– shall also exhibit such exotic states. However, how these states are formed in layered magnetic structures is hitherto unknown. Here we bring the topic of magnonic surface and interface states in layered ferromagnets into discussion. We provide experimental examples of synthetic layered structures, supporting our discussions and show that these states can be tailored in artificially fabricated structures. We demonstrate that the magnonic surface or interface states may show peculiar features, including "standing” or "ultrafast” states. We argue that these states can drastically change their electronic and magnonic transport properties. In this way one can design layered ferromagnets which act as magnon conductor, semiconductor and insulator of specific states.

Impact of high-dose gamma-ray irradiation on electrical characteristics of N-polar and Ga-polar GaN p–n diodes

We investigate the impact of high-dose gamma-ray irradiation on the electrical performance of Ga-polar and N-polar GaN-based p–n diodes grown by metalorganic chemical vapor deposition. We compare the current density–voltage (J–V), capacitance–voltage (C–V), and circular transfer length method characteristics of the p–n diodes fabricated on Ga-polar and N-polar orientations before and after irradiation. The relative turn-on voltage increases for the Ga-polar diodes with an increasing irradiation dose, while it increases initially and then starts to decrease for the N-polar diodes. The p-contact total resistance increases for Ga-polar and decreases for N-polar samples, which we attribute to the formation of point defects and additional Mg activation after irradiation. The J–V characteristics of most of the tested diodes recovered over time, suggesting the changes in the J–V characteristics are temporary and potentially due to metastable occupancy of traps after irradiation. X-ray photoelectron spectroscopy and photoluminescence measurements reveal the existence of different types of initial defects and surface electronic states on Ga-polar and N-polar samples. Gallium vacancies (VGa) are dominant defects in Ga-polar samples, while nitrogen vacancies (VN) are dominant in N-polar samples. The presence of a higher concentration of surface states on Ga-polar surfaces than N-polar surfaces was confirmed by calculating the band bending and the corresponding screening effect due to opposite polarization bound charge and ionized acceptors at the surface. The difference in surface stoichiometry in these two orientations is responsible for the different behavior in electrical characteristics after gamma-ray interactions.

Computing the surface electronic states on the (100), (110) and (111) surfaces of FCC monatomic crystals

In this study, we carry out a simulation of the surface band structures for face-centered cubic (fcc) leads that end up in (100), (110) and (111) surfaces. The surface Hamiltonian matrix is constructed from tight-binding approach and the secular equations of the surface eigenvalue problem. The solution of the problem is performed by integrating the Landauer–Büttiker formalism (LBF) in the phase field matching approach (PFMA). The LBF provides the quantum scattering properties and the PFMA connects the bulk modes to those of the surface based on the quantum scattering coefficients. The combination of these methods allows calculating the electronic bands in the three directions mentioned above. We report the results of ordered slabs for Ag, described as [Formula: see text]-like orbital and Ni given as [Formula: see text]-type orbitals. To show the impact of expanding the crystal wavefunction, we reveal the calculation of the localized states for Rh, Cu, Pt given as [Formula: see text]-type orbitals as first calculation then [Formula: see text]-coupling orbitals as second calculation. The results of the nonordered slabs are applied to Pd and Ir. Cutting the crystals affects the internal energy of the surface atoms, which will be subject to a relaxation effect until equilibrium is achieved.

Non-equilibrium band broadening, gap renormalization and band inversion in black phosphorus

Black phosphorous (BP) is a layered semiconductor with high carrier mobility, anisotropic optical response and wide bandgap tunability. In view of its application in optoelectronic devices, understanding transient photo-induced effects is crucial. Here, we investigate by time- and angle-resolved photoemission spectroscopy BP in its pristine state and in the presence of Stark splitting, chemically induced by Cs ad-sorption. We show that photo-injected carriers trigger bandgap renormalization, and a concurrent valence band flattening caused by Pauli blocking. In biased samples, photo-excitation leads to a long-lived (ns) surface photovoltage of few hundreds mV that counterbalances the Cs-induced surface band bending. This allows us to disentangle bulk from surface electronic states, and to clarify the mechanism underlying the band inversion observed in bulk samples.

Classical and cubic Rashba effect in the presence of in-plane 4f magnetism at the iridium silicide surface of the antiferromagnet GdIr2Si2

We present a combined experimental and theoretical study of the two-dimensional electron states at the iridium-silicide surface of the antiferromagnet ${\mathrm{GdIr}}_{2}{\mathrm{Si}}_{2}$ above and below the N\'eel temperature. Using angle-resolved photoemission spectroscopy (ARPES) we find a significant spin-orbit splitting of the surface states in the paramagnetic phase. By means of ab initio density-functional-theory (DFT) calculations we establish that the surface electron states that reside in the projected band gap around the $\overline{\mathrm{M}}$ point exhibit very different spin structures which are governed by the conventional and the cubic Rashba effect. The latter is reflected in a triple spin winding, i.e., the surface electron spin reveals three complete rotations upon moving once around the constant energy contours. Below the N\'eel temperature, our ARPES measurements show an intricate photoemission intensity picture characteristic of a complex magnetic domain structure. The orientation of the domains, however, can be clarified from a comparative analysis of the ARPES data and their DFT modeling. To characterize a single magnetic domain picture, we resort to the calculations and scrutinize the interplay of the Rashba spin-orbit coupling field with the in-plane exchange field, provided by the ferromagnetically ordered $4f$ moments of the near-surface Gd layer.

Гальваномагнитные свойства в анизотропных слоистых пленках на основе халькогенидов висмута

In anisotropic layered films of a multicomponent n-Bi1.92In0.02Te2.85Se0.15 solid solution in strong magnetic fields from 2 to 14 T at low temperatures, quantum oscillations of magnetoresistance associated with surface states of electrons in 3D topological insulators were studied. From the analysis of the spectral distribution of the amplitudes of quantum oscillations of magnetoresistance, the main parameters of the Dirac fermion surface states are determined. A comparison of the results with the data obtained by the method of scanning tunnel spectroscopy was carried out. It is shown that a high surface concentration determines the contribution of the Dirac fermion surface states to the thermoelectric properties of n-Bi1.92In0.02Te2.85Se0.15.

Influence of surface optical phonon on the electronic surface states in wurtzite group-III nitride ternary mixed crystals

An intermediate-coupling variational method is presented to investigate the surface electron states in wurtzite AxB1−xN (A, B=Al, Ga and In) ternary mixed crystals (TMCs). Corresponding effective Hamiltonian are derived by considering the surface-optical-phonon (SO-phonon) influence and anisotropic structural effect. The surface-state energies of electron, the coupling constants and the average penetrating depths of the electronic surface-state wave functions have been numerical computed as a function of the composition x and the surface potential V0 for the wurtzite AlxGa1−xN, AlxIn1−xN and InxGa1−xN, respectively. The results show that the surface-state levels of electron are reduced with the increasing of the composition x in wurtzite AxB1−xN. It is also found that the electron-surface-optical-phonon (e-SO-p) coupling lowers the surface-state energies of electron and the shifts of the electronic surface-state energy level in the wurtzite AlxGa1−xN and AlxIn1−xN increase with the increasing of the composition x. However, in the wurtzite InxGa1−xN, the case is contrary. The influence of the e-SO-p interaction on the surface electron states can not be neglected in wurtzite AxB1−xN.

Boron position-dependent surface reconstruction and electronic states of boron-doped diamond(111) surfaces: anab initiostudy

Boron-doped diamond (BDD) has attracted much attention in semi-/superconductor physics and electrochemistry, where the surface structures and electronic states play crucial roles. Herein, we systematically examine the structural and electronic properties of the unterminated and H-terminated diamond(111) surfaces by using density functional theory calculations, and the effect of the boron position on them. The surface energy increases compared to that of the undoped case when the boron is located at a deeper position in the diamond bulk, which indicates that boron near the surface can facilitate the surface stability of the BDD in addition to the H-termination. Moreover, the surface energy and projected density of state analyses suggest that the boron can enhance the graphitization of the pristine (ideal) unterminated (111) surface thanks to the alternative sp2-sp3 arrangement on that surface. Finally, we found that surface electronic states depend on the boron's position, i.e., the Fermi energy (EF) is located around the mid-gap position when the boron lies near the surface, instead of showing a p-type semiconductor behavior where the EF lies closer to the valence band maximum.

Surface electronic states mediate concerted electron and proton transfer at metal nanoscale interfaces for catalytic hydride reduction of -NO2 to -NH2.

Concerted electron and proton transfer is a key step for the reversible conversion of molecular hydrogen in both heterogeneous nanocatalysis and metalloenzyme catalysis. However, its activation mechanism involving electron and proton transfer kinetics remains elusive. With the most widely used catalytic hydride reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) as a model reaction, we evaluate the catalytic activity of noble metal nanoparticles (NPs) trapped in porous silica in aqueous NaBH4 solution. By virtue of a novel combination of catalyst design, reaction kinetics, isotope labeling, and multiple spectroscopic techniques, the real catalytic site for the conversion of -NO2 to -NH2 is identified to be the water-hydroxyl transition metal complex, which could further react with NaBH4 to form a new triangular configuration metal complex of H3B-water-hydroxyl with dynamic features. It yields an ensemble of surface electronic states (SESs) though space overlapping of p orbitals of one B and several O atoms (including the O atoms of 4-NP), which could act as an alternative channel for concerted electron and proton transfer. This work highlights the critical role of the conceptual SESs model in heterogeneous catalysis to tune the chemical reactivity and also sheds light on the intricate working of the [FeFe]-hydrogenases.

The Nature of CVC Nonlinearity in Low-Voltage Scanning Tunneling Spectroscopy of Semiconductors

A new model of field emission in a scanning tunnelling microscope was developed. The model describes the tunnelling current from a surface of semiconductor (semimetal) and allows estimating the preexponential factor in the expression for the tunneling probability. It is shown that this factor is directly related to the degree of localization of the electron density and determines the shape of the local tunnel current-voltage characteristics (LTCVCs) at low voltages. The model allows separating the contributions of surface electronic states of different symmetry (dimension) of the tunnelling current. The practical application of the model is demonstrated by the example of mathematical processing of the LTCVCs of HOPG surface containing different structural defects.

Fabrication of a porous NiFeP/Ni electrode for highly efficient hydrazine oxidation boosted H2 evolution.

Rational optimization of the surface electronic states and physical structures of non-noble metal nanomaterials is essential to improve their electrocatalytic performance. Herein, we report a facile dual-regulation strategy to fabricate NiFeP/Ni (P-NiFeP/Ni) porous nanoflowers, which involves Fe-doping and creating pores on nanosheets. The as-prepared P-NiFeP/Ni has a hierarchically porous surface, which exposes more electrochemically active sites and dramatically enhances the electron transfer rate. Thus, it exhibits excellent catalytic activity in both anodic hydrazine oxidation reaction (HzOR) and cathodic hydrogen evolution reaction (HER). Interestingly, the coupled electrolysis cell only offers a potential of 0.162 V at 10 mA cm−2 to enable HzOR boosted H2 evolution, highlighting an energy-saving hydrogen evolution strategy.

Critically Charged Superfluid 4He Surface in Inhomogeneous Electric Fields

We have studied the spatial distribution of charges trapped at the surface of superfluid helium in the inhomogeneous electric field of a metallic tip close to the liquid surface. The electrostatic pressure of the charges generates a deformation of the liquid surface, leading to a “hillock” (called “Taylor cone”) or “dimple”, depending on whether the tip is placed above or below the surface. We use finite element simulations for calculating the surface profile and the corresponding charge density in the vicinity of the tip. Typical electric fields E are in the range of a few kV/cm, the maximum equilibrium surface deformations have a height on the order of (but somewhat smaller than) the capillary length of liquid 4He (0.5 mm), and the maximum number density of elementary charges in a hillock or dimple, limited by an electrohydrodynamic instability, is some 1013 m−2. These results can be used to determine the charge density at a liquid helium surface from the measured surface profile. They also imply that inhomogeneous electric fields at a bulk helium surface do not allow one to increase the electron density substantially beyond the limit for a homogeneous field, and are therefore not feasible for reaching a density regime where surface state electrons are expected to show deviations from the classical behavior. Some alternative solutions are discussed.

原子状水素曝露によるTiO 2 (110)の表面構造変化と電子状態

原子状水素曝露によるTiO 2 (110)の表面構造変化と電子状態

Ultrafast Dynamics of Photocurrents in Surface States of Three‐Dimensional Topological Insulators

Herein, experimental work on the ultrafast electron dynamics in the topological surface state (TSS) of three‐dimensional (3D) topological insulators (TIs) observed with time‐ and angle‐resolved two‐photon photoemission (2PPE) is reviewed. The focus is laid on the generation of ultrafast photocurrents and the time‐resolved observation of their decay. 2PPE not only allows to unambiguously relate the photocurrents to the spin‐polarized electronic surface states. Probing of the asymmetric momentum distribution of the electrons carrying the current makes it possible to study the microscopic scattering processes that govern the unusual electron transport in the time domain. Ultrashort mid‐infrared pump pulses permit not only a direct optical excitation of the TSS in Sb2Te3 but also lead to a strong asymmetry of the TSS population in momentum space. Two‐dimensional band mapping of the TSS shows that this asymmetry is in fact representative of a macroscopic photocurrent, while the helicity‐dependence of the photocurrent is found to be small. The time‐resolved observation of the photocurrent decay reveals a huge mean free path of the electrons in the TSS.

Transmission Electron Microscopy: Applications in Nanotechnology

Transmission Electron Microscopy (TEM) offers multiple advanced techniques such as imaging, diffraction, and spectroscopy to learn essential information about the physical, chemical, and structural properties of materials at the microscale to nanoscale to atomic scale. It is a critical tool for studies of sizes, shapes, defects, crystal and surface structures, compositions, and electronic states of nanometer-size areas of thin films, nanoparticles, and nanostructured systems. This article first gives a brief introduction to the TEM-based techniques. It then highlights recent application examples of TEM in the studies of nanostructured heat-assisted magnetic materials, electronics, and photovoltaics to illustrate the critical role of the TEM techniques in various research and development fields of nanoscience and nanotechnology.

First-principles calculations of electronic properties, surface stability and photocatalytic potentials of seleno-germanates A[formula omitted]GeSe[formula omitted] (A = Mg; [formula omitted]-Sr) surfaces for a promising visible light photocatalytic application

The four low-index, (100), (101), (110) and (111) surfaces of the A2GeSe4 (A = Mg; γ-Sr) seleno-germanates were studied using the first principles generalised gradient density functional theory approximation. From the optimized surfaces structures, surface stabilities were established by calculating the surface energies. Our calculation reveals that the Mg2GeSe4 (101) and γ-Sr2GeSe4 (111) surfaces are the most energetically stable surfaces, for the two compounds, respectively. On the Mg2GeSe4 (101) and γ-Sr2GeSe4 (111) most stable surface, the electronic properties, charge density, electrostatic potential, work function, band gap center and band-edge potentials were calculated. Calculated electronic density of state reveal for Mg2GeSe4 (101) a metallic and γ-Sr2GeSe4 (111) a semiconductor surface. Analysis of total and partial density of state of surface compared to bulk reveals conduction band shifting toward valence band and exhibit surface electronic density state corresponding to p-orbital of Se atom contribution around the Fermi-energy. Bader charge density analysis of the Mg2GeSe4 (101) and γ-Sr2GeSe4 (111) surfaces reveals charge depletion around Mg, Sr and Ge atoms and charge accumulation around the Se atoms, which indicate covalent bonding between the Se and Ge, Mg and Sr atoms. The stable surfaces were used to calculate the electrostatic potential and thus determine the work function, band gap centre and band edges. The band edges compared to redox potential of water show that Mg2GeSe4 (101) and γ-Sr2GeSe4 (111) surfaces suggest that are promising visible light photocatalytic materials

Extracting the local electronic states of Pt polycrystalline films surface under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy

The local atomic information about the interface between the 30 nm-thick Pt polycrystalline films and the solution with and without perfluorosulfonic acid polymers (Nafion®) for the model cathode catalyst of fuel cell has been captured under electrochemical conditions using polarization-dependent total reflection fluorescence x-ray absorption near edge structure spectroscopy (PTRF–XANES). The results show that the formation of sub-monolayer-equivalent PtO or adsorbed hydrogen/oxygen species in the surface region can be successfully observed in the PTRF–XANES spectra when the thickness of the solution layer and the incidence angle are properly controlled. This capability enables us to examine the metal /(Nafion®/) solution interface structure through XANES together with other surface analysis methods, which will enhance comprehensive understanding of the nature of the interface of the fuel cell system.

Floquet Engineering of Structures Based on Gapless Semiconductors

Applying the conventional Floquet theory of periodically driven quantum systems, we developed the theory of optical control of structures based on gapless semiconductors. It is demonstrated that electronic properties of the structures crucially depends on irradiation. Particularly, irradiation by a circularly polarized electromagnetic wave lifts spin degeneracy of electronic bands and induces surface electronic states. Thus, a high-frequency off-resonant electromagnetic field can serve as an effective tool to control electronic characteristics of the structures and be potentially exploited in optoelectronic applications of them.