Effect Of Surface States Research Articles

Bulk and surfaces half-metallicity of RbSe with zinc-blende structure: first-principles study

In this paper, the electronic, magnetic, and half-metallicity characteristics of the bulk as well as (111) and (001) surfaces of zinc-bland RbSe compound are explained using the first-principles calculations. All of calculations are performed through GGA approximations, and the results reveal that both the bulk and ends of the surfaces preserve the half-metallic property. In addition, the stability of the surfaces is computed. It is discovered that the Rb-terminated (001) is the most stable one compared to other terminates. Based on the magnetic characteristic computations, the spin magnetic moment of bulk is equal to 1 µB, while this value has changed at the surfaces due to the effects of surface states. All of the above-mentioned properties mark this compound as a suitable promising candidate for spintronics applications.

Quantum Transport in Nanostructures of 3D Topological Insulators

Quantum transport measurement is an efficient tool to unveil properties of topological surface states in 3D topological insulators. Herein, experimental and theoretical results are reviewed, presenting first some methods for the growth of nanostructures. The effect of the disorder and the band bending is discussed in details both experimentally and theoretically. Then, the focus is put on disorder and quantum confinement effect in topological surface states of 3D topological insulators narrow nanostructures. Such effect can be revealed by investigating quantum interferences at very low temperature such as Aharonov–Bohm oscillations or universal conductance fluctuations.

Effect of surface states on scattering from a slab made of topological insulator material with rough interface

ABSTRACTUsing the perturbation theory (PT), reflected field both in specular and non specular directions from a slab made of topological insulator (TI) material with rough interface is studied. The specular reflections are described by zeroth order terms of the perturbation series while non specular reflections are associated with higher order terms. To describe the surface states on the TI, γ-model characterized by the surface admittance is utilized. It is observed that the co-polarized specular reflection increases for large values of surface admittance which results in vanishing transmission. The main objective is to pronounce distinctions between the results for a dielectric slab and TI slab. It is observed that the co polarized diffraction efficiency of the sinusoidal grating is enhanced for p-polarization as the surface admittance increases. Moreover, the scattering behavior of the TI slab with Gaussian rough interface as a function of surface admittance differs significantly from that of the dielectric slab.

Giant photoinduced anomalous Hall effect of the topological surface states in three dimensional topological insulators Bi2Te3

Topological insulators (TIs) are considered as ideal spintronic materials due to the spin-momentum-locked Dirac surface states. The photoinduced anomalous Hall effect (PAHE) is a powerful tool to investigate the spin Hall effect of topological insulators even at room temperature. In this Letter, the PAHE has been observed in three dimensional topological insulator Bi2Te3 thin films grown on Si substrates at room temperature. As the thickness of the Bi2Te3 films increases from 3 to 20 quintuple layer (QL), the PAHE first increases and then decreases, and it reaches a maximum at 7 QL. The sign reversal of the PAHE of the 3 QL sample after oxidation reveals that the PAHE of the Bi2Te3 thin films is dominated by the top surface states, which is further confirmed by the circular photogalvanic effect under front and back illuminations. The photoinduced anomalous Hall conductivity excited by 1064 nm light is as large as 5.28 nA V−1 W−1 cm2 in the 7 QL sample, much larger than that observed in InGaAs/AlGaAs quantum wells (0.445 nA V−1 W−1 cm2) and GaN/AlGaN heterostructures (0.143 nA V−1 W−1 cm2). By comparing the PAHE current excited by 1064 nm with that excited by 1342 nm, we reveal that the tremendous PAHE excited by 1064 nm light is due to the modulation effect of spin injection from Si substrates. The giant PAHE value observed in TI Bi2Te3 may offer spintronic applications of TIs such as high-efficient light-polarization-state detectors.

Numerical modelling of the potential inside the cylindrical shaped nanocrystallite metal oxide semiconductors and its effect on gas sensor response

The surface of metal oxide semiconductors exhibits great gas sensing response when the size of the grains becomes much smaller, i.e. in the range of the Debye length. In the presented work, the gas sensing properties of nanocrystalline cylindrical shaped metal oxide semiconductors have been studied. The effect of crystal size and surface states on gas sensor response has been investigated. The electric potential inside the grains has been calculated by solving the Poisson’s equation coupled with electroneutrality condition. The relationship between the electric potential and the size of the nanograins has been established as a function of available surface states, surface temperature and occupied surface states. It has been found that the critical value of available surface state density increases with an increase in the size of nanograins. Total carrier concentration inside the grain volume has been simulated as a function of available surface state density and grain size. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals. From the simulated studies, it is clear that the smaller cylindrical-shaped nanostructured grains could result in better gas sensing performances. Presented model has universal application for all metal oxides (both n-type and p-type) based semiconductors and can be extended for both oxidising and reducing gases with essential modifications.

Relationship of the electric potential inside the nano crystallite with the chemisorbed gas molecules at metal oxide semiconductor surface and its effect on gas sensor response

The metal oxide semiconductors surface exhibit great gas sensing response when the size of the grains becomes much smaller i.e. in the range of debye length. In the presented study, the effect of crystal size and surface states has been studied on gas sensing properties for cylindrical shaped nanostructures of metal oxide semiconductors. To calculate the grain potential Poisson’s equation coupled with electroneutrality condition in cylindrical shaped nanostructures has been solved. The shape of the potential profile inside the nanograins has been calculated as a function of available surface states, surface temperature and occupied surface states. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals.

Вплив стану поверхні електроіскрових частинок на процес формування твердих розчинів при ультразвуковій обробці високодисперсних порошкових сумішей міді з залізом та кобальтом

Вплив стану поверхні електроіскрових частинок на процес формування твердих розчинів при ультразвуковій обробці високодисперсних порошкових сумішей міді з залізом та кобальтом

Surface states on (001) oriented β -Ga2O3 epilayers, their origin, and their effect on the electrical properties of Schottky barrier diodes

Surface states on (001) oriented halide vapor phase epitaxy (HVPE) grown β-Ga2O3 epilayers were explored through the determination of the Schottky barrier height (SBH) as a function of the metal work function using Cr, Cu, Ni, and Au Schottky barrier diodes. SBH is found to be nearly pinned between 1.2 and 1.35 eV in the HVPE grown epilayers. The position of the Fermi level pinning is closely matched with the energy level of the oxygen vacancy [VO(III)] state (EV + 3.57 eV) in the energy bandgap of β-Ga2O3, indicating that Fermi level pinning is due to oxygen vacancy type surface states on (001) oriented β-Ga2O3 epitaxial layers. The Fermi level is found to be relatively unpinned on the bulk β-Ga2O3 (001) substrate, suggesting the presence of lower density of oxygen vacancy states on its surface. Hence, the HVPE growth process was found to be responsible for the presence of oxygen vacancy states [VO(III)] in the epilayer. Moreover, this work highlights the role of these surface states in determining the SBH on β-Ga2O3 (001) epilayers and also explains the reason behind the scattered data of SBH values reported in the literature. In addition to these results, we also showed an increment in the built-in potential and the reduction of reverse leakage current for the epilayer with lower surface state density, which gives a direct evidence of the effect of surface states on the properties of β-Ga2O3 (001) Schottky barrier diodes.

Engineering interfacial modification on nanocrystalline hematite photoanodes: A close look into the efficiency parameters

This work describes an approach to direct evaluate the role of selected modifier added over hematite photoanode interfaces for light assisted water splitting. Hematite photoanodes with thickness varying from 25 up to 130 nm were designed via spin coating deposition of polymeric precursor that provides mesoporous planar morphology. The photoelectrochemical data analyzed in terms of global efficiency parameter (ηglobal) revealed a close dependence with hematite thickness for unmodified photoanode, which large number of interfaces associated with high charge recombination rate led to drop the electrode performance. To address this issue and manipulate the large number of interfaces due to the planar morphology, Sn4+ ions (1 and 3% molar ratio) were deposited over the hematite surface leading to enhance the electronic transport with a cost of creation of surface states at the solid-liquid interface. Then, a second modification process was done by photoelectrodeposition of Ni/Fe based material to suppress or minimizing the effect of surface state created by the Sn4+ ions addition. The results show that the interfacial traps were mitigated for the thinner hematite photoanodes, where the presence of Sn4+ and Ni/Fe significantly increases the ηglobal. The present find, highlights the importance of morphology design that allows a combination of modifier insertion to work in synergy for enhancing the overall photoanode performance.

Role of surface states on luminescence shift of caramelised sugar carbon dots for color conversion emitting devices

Carbon dots made of various sources have shown unique and excellent optical properties for many potential applications, including light emitting devices. The luminescence origin of carbon dots is crucial in colour conversion emitting device applications. In this work, an intensive study on surface state effects of caramelised sugar carbon dots for colour conversion applications was done. Using caramelised carbon dots synthesised by microwave-assisted technique, we studied optical properties of carbon dots using photoluminescence, time resolved photoluminescence, Fourier-transform infrared spectroscopies, absorbance spectroscopy and Raman spectroscopy. We found that different concentrations of caramelised sugar dissolved in water caused absorbance and emission peak shift of carbon dots. However, life-time, crystal structure and functional groups of carbon dots were not significantly affected by carbon dot concentration. We have utilised this shifting luminescence effect in colour conversion light emitting devices using blue and green light emitting diodes. Caramelised sugar carbon dots, which were placed on top of light emitting diodes, were able to generate new colours. This work is useful for developing advanced lighting devices using carbon dots.

Effect of Surface State on Acid Rain Corrosion Resistance of T6 6005A Aluminum Alloy by BT-FSW Joint

The test sample was 5 mm thick artificial aging 6005A aluminium alloy profile, which was butted by BT-FSW. This work aimed to analyse the effect of surface state on corrosion resistance of welded joints in acid rain environment based on acetate ssalt spray test (CASS), polarization curve and EIS impedance test, combined with SEM scanning and EDS energy spectrum. The results showed that the corrosion rate of pre-welded polished joints was high at the initial stage of salt spray corrosion, and the corrosion rate of both joints were equal at the later stage of corrosion, higher than that of base metal. The best preservation of corrosion morphology was THAZ, followed by nugget zone and HAZ, and the pitting degree of base metal was the most serious. The polarization curves of the polished joints before 30 min corrosion were located at the lower right of the image, and its self-corrosion current density in the nugget zone showed the worst corrosion resistance. The corrosion resistance of the pre-welded polished joint increased after 336 hours of corrosion, the corrosion breakdown potential and self-corrosion current were not significantly different from those of the unpolished joints. The fitting curves of the two kinds of joints had similar dual capacitance arc resistance characteristics and the impedance parameters in EIS, and the electrochemical corrosion resistance of the two joints were weaker than that of the base metal.

Temperature dependent characteristics of graphene/silicon Schottky junction

Graphene/silicon Schottky junction has been reported as a promising device for chemical sensor, biological sensor and photodetector applications. However, abnormal characteristics are often reported and these ambiguities are explained inconsistently in the literature. This work aims at characterising the temperature effects on the current-voltage characteristics so as to explore the physical mechanisms leading to the abnormal behaviours of graphene/silicon Schottky junction. Particular attention is on the effect of silicon surface states which have been studied quite comprehensively in various silicon devices but have not been given enough attention for the graphene/Si structure. Results show that a graphene/Si Schottky junction could have quite different temperature dependences on the barrier height, ideality factor and reverse characteristics as compared with metal/semiconductor contacts. The dangling bonds on silicon surface, isolated by the ultrathin graphene layer, are still electrically active and play an important role in the carrier transport, photonic and chemical sensing capabilities of the graphene/Si junction. Graphene/Si contact prepared by transfer method cannot be a good Schottky junction from the electronic property and stability points of view.

Computational Modelling-Based Device Design for Improved mmWave Performance and Linearity of GaN HEMTs

In this work, a comprehensive, TCAD based design approach for mmWave (mmW) GaN HEMTs is presented. Unique trade-offs between epi-layer design and HEMT's mmW performance are discussed. Effect of surface states on cut off frequency is modeled and presented. We have found that carrier trapping by the donor type interface states causes RF performance drift at high drain fields, which particularly leads to the non-linear behavior of mmW HEMTs at high drain bias. Moreover, we have observed that channel electrostatics, barrier layer, and UID GaN channel design govern the linearity and scaling behavior of such GaN HEMTs. To improve channel electrostatics, which improves the linearity and cut-off frequency, a partially recessed barrier under the gate is studied. A relative study of AlN/GaN HEMT and AlGaN/GaN HEMTs is performed to investigate the nonlinearity behavior. In addition, the dependence of cut-off frequency on contact resistance and lateral scaling is studied for partially-recessed barrier and conventional design for both AlN and AlGaN barrier types. The mmW performance is found to be a strong function of barrier design in the gate and recess regions. Unique design trends and physical behavior was observed for AlN and AlGaN barriers, which signifies that design guidelines derived for one epi-stack can't be deployed to the other.

Effect of metal surface state on injection joining strength of aluminum-rubber composite part

Metal–rubber composite parts with characteristics of high elasticity and compressibility are widely used in seal and vibration resistance engineering fields. The injection technology was proposed to mold metal – rubber composite parts. With pretreatments of anodization and silane coupling agent, the morphology and composition of Al surface were changed, and then Al - rubber composite parts were fabricated by injection technology. The effects of Al surface morphologies and compositions induced by different conditions of anodization and coupling agent were discussed. The scanning electron microscopy (SEM) and X-ray energy spectrum analyser (XPS) of Al surface examination proved that microstructures and hydroxyl groups formed on the metal surface contribute significantly to the bonding strength of the Al - rubber interface. Furthermore, the effect of metal temperature on the bonding strength between the rubber and the Al was investigated. By increasing the metal temperature to 120 °C, the bonding strength improved by more than twenty folds compared to the lower temperature case. Results showed that the molded composite parts exhibit excellent tensile shear strength, exceeding the shearing strength of the rubber itself. The presented method can be promoted in related engineering fields.

Landau Levels of Topologically-Protected Surface States Probed by Dual-Gated Quantum Capacitance.

Spectroscopy of discrete Landau levels (LLs) in bulk-insulating three-dimensional topological insulators (3D TIs) in perpendicular magnetic field characterizes the Dirac nature of their surface states. Despite a number of studies demonstrating the quantum Hall effect (QHE) of topological surface states, quantitative evaluation of the LL energies, which serve as fundamental electronic quantities for study of the quantum states, is still limited. In this work, we explore the density of states of LLs by measuring quantum capacitance (CQ) in a truly bulk insulating 3D TI via a van der Waals heterostructure configuration. By applying dual-gate voltages, we access the individual surface states' LLs and extract their chemical potentials to quantify the LL spacings of each surface. We evaluate the LLs' energies at two distinguished QH states, namely, dissipationless (ν = ±1) and dissipative (ν = 0) states in the 3D TI.

Effects of surface states on salt-frost scaling resistance of cement concrete

This study discussed influences of surface states of concrete on salt-frost durability. By using the Capillary suction of Deicing solution and Freeze-thaw test method(CDF), the effects of factors, such as solution saturation in capillary, surface property and load coupling were compared and analyzed. It is found that drying shrinkage exerts greater effects on salt-frost resistance of the concrete in comparison with solution saturation in capillary. According to the CDF test standard recommended by RILEM, the salt-frost test results of the surface formed with the partitions in the laboratory overestimate salt-frost resistance of the concrete in the actual situation. Scaled mass of the top surface of the molded concrete due to salt frost is about 50% larger than that of the surface formed with the partitions. Coupling effects of repeated load and salt-freezing cycles can aggravate salt-frost damages of the concrete.

Interface engineering on ZnO/Au based Schottky junction for enhanced photoresponse of UV detector with TiO2 inserting layer

Metal-oxide semiconductor ZnO shows enormous potential in the field of photoelectric detection. However, the adsorption of H2O/O2 on its surface unavoidably results in degradation in the photoresponse. Herein, Au–TiO2–ZnO (ATZ) ultraviolet (UV) photodetectors were designed to benefit the photosensing performance by weaken the surface state effect through tuning the Schottky junction interface with TiO2 insertion layer. As expect, 97% enhancement of IPh response together with three orders of magnitude decrease of dark current was triggered. This enhancement is owing to the efficiently separation and extraction of photoexcites under the effect of the stronger and expanding built-in field, which is a result of reduction of space charge density in barrier region induced by inhibiting H2O/O2 adsorption at the ZnO/Au interface. These findings here provide a promising method to boost performances of optoelectronic devices by interface engineering and could be extended to other semiconductor devices.

(Europe Section Heinz Gerischer Award) The Physical Electrochemistry of Semiconductors

Semiconductor photoelectrodes have remarkable differentiating properties from metal electrodes, including photo effects, simultaneous charge transfer from two different bands, different double layer structures, and effects of surface atoms and states. These differences play a profound role in the behavior of semiconductors as electrodes, as first recognized and developed by H. Gerischer in many instances. This presentation will discuss the use of contemporary methods of surface science including operando techniques to directly measure the band edge positions of semiconductor electrodes as proposed by Gerischer, will characterize mechanisms and rates of interfacial charge transfer within the model advanced by Gerischer, and will define the role of surface states as proposed by Gerischer in influencing the development of the double layer of the semiconductor/liquid interface, as well as the influence of monolayers of two-dimensional materials on interfacial energetics and kinetics of charge-transfer processes. The use of semiconductors to produce photo-driven fuel formation will also be described in detail for both oxidative and reductive processes in aqueous and non-aqueous solvents for both simple, one-electron outer sphere redox reactions as well as more complex multi-electron charge transfer processes associated with the direct production of fuels from sunlight.

The investigation of effects of (Fe2O4-PVP) organic-layer, surface states, and series resistance on the electrical characteristics and the sources of them

In this work, Au/n-Si (MS) structures with and without (Fe2O4-doped PVP) interlayer were prepared with the same conditions to see effects of organic layer on the electrical characteristics and conduction mechanisms. For this aim, I–V and Z–V measurements of them were carried out at room temperature. The saturation current (Is), ideality factor (n), barrier height (ΦB(I–V)), series (Rs) and shunt (Rsh) resistances, and rectifying rate (RR = IF/IR) of them were extracted from the I–V data as 2.90 × 10−8 A, 1.699, 0.741 eV, 1.58 kΩ, 25.7 MΩ, 1.45 × 104 for MS and 2.30 × 10−9 A, 1.634, 0.806 eV, 1.17 kΩ, 103 MΩ, 9.01 × 104 for MPS, respectively. The values of interface states (Nss) were also extracted from the I–V data at forward bias by considering voltage dependent BH and n, and it is found that they increase from the mid-gap of semiconductor towards the conductance band. The values of doping atoms (ND), Fermi-energy (EF), and (ΦB(C–V)) were also acquired from the C−2–V plots at reverse bias as 9.08 × 1014 cm−3, 0.258 eV, 0.914 eV for MS and 7.650 × 1014 cm−3, 0.263 eV, 0.981 eV, for MPS structure, respectively. It is clear that the (Fe2O4-PVP) interlayer leads to decreases in Rs, Nss, leakage current and increase in rectifying rate (RR), Rsh and BH, so that it can used in place of the conventional oxide or insulator layer.

Quantum Hall effect of Dirac surface states of as-grown single crystal flakes in Sn0.02-Bi1.08Sb0.9Te2S without gate control

The quantum spin Hall effect (QSHE) in three-dimensional topological insulators (3D-TIs) is an important hallmark of realization of ultralow dissipative electrical transport originating from nontrivial topological surface Dirac states (TSDSs). The observation of QSHE, however, had been limited at low temperatures for thin film crystals prepared by epitaxial growth or ultrathin (nanometer-level) flakes, and the finite Fermi energy tuning was required using field effect transistor (FET) construction. Here, we show that a clear observation of QSHE with a quantized level of e2/h can be possible for high quality single crystal flakes of Sn-(Bi,Sb)2(Te,S)3 (Sn-BSTS) prepared by an exfoliation method without any gate control under FET construction. The QSHE is observed in an extremely large sample area of millimeter square at the largest thickness of micrometer and up to the highest temperature of 20 K, which is greatly superior to those reported for other 3D-TIs so far. The accurate band picture of both top and bottom topological surface states of Sn-BSTS is determined from both the QSHE and the Shubnikov-de Hass (SdH) oscillations. High quality Sn-BSTS of 3D-TIs will effectively be used for experiments targeting on science as well as many technological applications expected in the future.