Fermi Level Effect Research Articles

Improvement in the performance of In2O3 ethanol sensor by perovskite-type LaFeO3 modification and sensitivity mechanism analysis

In this paper, LaFeO3/In2O3 composites are prepared by hydrothermal methods. The microstructure and crystal structure of LaFeO3/In2O3 composites were characterized. The results of gas-sensitive testing show that the LaFeO3/In2O3 composite gas sensor has a higher response than pure In2O3. The LaFeO3/In2O3 sensor made up of 0.01 g LaFeO3 shows a response of 147 at 220 °C to 100 ppm ethanol gas. It was found that LaFeO3 could enhance the gas-sensitive properties of In2O3. The surface properties of LaFeO3 and In2O3 are calculated and analyzed by the first principal method. The sensitivity mechanism of LaFeO3 enhancing the properties of the In2O3 sensor is analyzed from the aspects of adsorbed oxygen, heterojunction, and LaFeO3 catalyst. The LaFeO3/In2O3 sensor has a better gas-sensitive performance than the In2O3 sensor because of the large amount of oxygen adsorption, the special Fermi level effect of heterojunction, and the catalytic effect of LaFeO3 on the redox reaction.

A Readily Achieved Potentiostatic Method in Density Functional Theory Calculation for Improved Prediction of the Performance for Electrocatalytic Nitrogen Reduction Reaction.

Accurate prediction of the catalytic performance of nitrogen reduction reaction catalysts based on density functional theory (DFT) calculation is of great significance for developing catalytic materials for nitrogen fixation. However, the applied electrode potential induced the fixation of Fermi level and solvation effect are commonly ignored in the current computational hydrogen electrode method, which leads to the large deviation between the calculation predicted limit potential and the experimentally measured limit potential. In this work, the simple external iteration method is proposed to simulate the Fermi level of the catalysts that are fixed by the applied electrode potential, along with the hybrid solvent model to describe the strong interaction, such as hydrogen bond, between the solvent molecules and the intermediates. This method allowed the theoretical and experimental limit potentials to be in good agreement, indicating the significant effect of the electrode potential and solvation in the DFT calculation. These results will guide the calculation-based prediction of other reaction systems in the field of electrocatalysis.

Perfect control of spin, valley and spin-valley-coupled currents in bilayer graphene on WSe2 magnetic junction: Effect of spin-valley dependent Fermi level and energy gap

We investigate the spin-valley dependent transport properties in a gated bilayer graphene (BG) junction placed on top of WSe2. By means of layer-dependent proximity, the spin–orbit interaction (SOI) is induced in the bottom layer, while the top layer is induced into ferromagnetism by a magnetic insulator. As a result of these differing properties, the Fermi level and energy gap become spin-valley dependent, which could lead to controllable spin- and valley-dependent filtering in the BG system. Our findings predict perfect spin-valley polarization control through gate control, given specific energy and electric field conditions, as well as spin-valley-coupled and spin currents when both the electric-field-induced energy gap and the exchange energy are equal to the SOI strength. We predict that there will be very highly sensitive gate control of −100% to + 100% of spin-valley-coupled polarization for large barrier thickness. Additionally, we predict 100% valley polarization can be controlled by the gate when the exchange energy is equal to the SOI. Furthermore, we found that the performance of the junction to control the polarizations can be enhanced by increasing the thickness of the junction. This work reveals the potential of the BG/WSe2 hybrid structure for spin-valley-current-based electronics, such as spin-, valley-, and spin-valley field-effect transistors.

Colossal Nernst power factor in topological semimetal NbSb2

Today solid-state cooling technologies below liquid nitrogen boiling temperature (77 K), crucial to quantum information technology and probing quantum state of matter, are greatly limited due to the lack of good thermoelectric and/or thermomagnetic materials. Here, we report the discovery of colossal Nernst power factor of 3800 × 10-4 W m-1 K-2 under 5 T at 25 K and high Nernst figure-of-merit of 71 × 10-4 K-1 under 5 T at 20 K in topological semimetal NbSb2 single crystals. The observed high thermomagnetic performance is attributed to large Nernst thermopower and longitudinal electrical conductivity, and relatively low transverse thermal conductivity. The large and unsaturated Nernst thermopower is the result of the combination of highly desirable electronic structures of NbSb2 having compensated high mobility electrons and holes near Fermi level and strong phonon-drag effect. This discovery opens an avenue for exploring material option for the solid-state heat pumping below liquid nitrogen temperature.

Switchable triple plasmon-induced transparency in graphene sandwich metamaterial structures

In this research, a metamaterial structure composed of graphene sandwich structure is put forward to achieve triple plasmon-induced transparency. This phenomenon is generated by bright–bright modes interaction in the proposed structure, and the results calculated though couple mode theory are consistent with the simulation ones using finite difference time domain method. In addition, the effects of graphene Fermi level and scattering rate on the transmission spectrum are also discussed. It is found that the five-fold switching effects are achieved with the modulation depths (MDs) of 92%, 91%, 95%, 90% and 94%, respectively. What is more, the different graphene strip can also be adjusted by changing Fermi levels to achieve the switching effects with the MDs of 98% and 97%. This model has the prospect to be applied in multi-functional modulators and optical switches in terahertz band.

Investigation of strontium titanate spherical shell supported terahertz all-dielectric metamaterials

Based on the strontium titanate ( S r T i O 3 , STO) spherical shell all-dielectric metamaterials (ADMs) structure, the tunable propagation characteristics have been systematically studied, taking into account the effects of structural geometrical parameter, temperature, and graphene Fermi level. The results indicate that STO-based ADMs exhibit excellent thermally and electrically tunable performances, e.g., when the temperature rises from 77 to 400 K, the modulation depths (MDs) of resonant frequency and amplitude of the reflection dip reach 62% and 93%, and the absorption is tunable in the range of 0.7995–0.9017. Additionally, when the Fermi level of graphene changes from 0.1 to 1.0 eV, the amplitude MD of the reflection dip reaches 99.89%, and the absorption of the STO-based ADM increases from 0.8352 to 0.9953. These results provide a guide for understanding the resonant mechanism of STO-based ADMs and designing multifunctional terahertz devices.

Control of Water Adsorption via Electrically Doped Graphene: Effect of Fermi Level on Uptake and H2O Orientation

AbstractThe interaction of graphene with water molecules under an applied electric field is not thoroughly understood, yet this interaction is important to many thermal, fluidic, and electrical applications of graphene. In this work, the effect of electrical doping of graphene on water adsorption is studied through adsorption isotherms and current–voltage (IV) characterizations as a function of the Fermi level. The water adsorption onto graphene increases by ≈15% and the doping levels increase by a factor of three with a gate‐to‐graphene voltage of +20 or −20 V compared to 0 V for sub‐monolayer adsorption. This change in uptake is attributed to the increase in density of state of graphene upon electrical‐doping, which changes the Coulombic and van der Waals interactions. The water adsorption onto graphene is either n‐ or p‐doping depending on the applied gate‐to‐graphene voltage. The ambi‐doping nature of water onto graphene is due to the polar nature of water molecules, so the doping depends on the orientation of the water molecules.

TiO2 nanofibers decorated with monodispersed WO3 heterostruture sensors for high gas sensing performance towards H2 gas

WO3-TiO2 heterostructural nanofibers (HNFs) were prepared by a simple spin coating method. The structures, chemical constitutions and morphologies of the samples were characterized by various kinds of techniques, including XRD, XPS, EDX, BET, SEM and TEM. The results suggested that WO3 nanoparticles with a size range of 20–30 nm were decorated uniformly on the surface of TiO2 nanofibers in the WO3-TiO2 composite thin films. XPS results confirmed the existence of W5+and the increased amount of both chemisorbed oxygen species and oxygen vacancies after the WO3 nanoparticles were decorated on the surface of TiO2 nanofibers. Compared with the pure materials, the WO3-TiO2 HNFs exhibited higher response (78.21 ± 0.35) %), rapid response (20 s) and recovery time (23 s), better selectivity and long term stability towards H2 gas. The gas-sensitive properties improvement of WO3-TiO2 composite is due to high surface area, which leaded to an increase of the gas adsorption sites. Moreover, the Fermi level effect of the composite metal oxide heterojunction formed by TiO2 and WO3 also markedly enhanced the gas sensitivity properties of the composite samples.

Manipulating metal-oxygen local atomic structures in single-junctional p-Si/WO3 photocathodes for efficient solar hydrogen generation

Self-passivation in aqueous solution and sluggish surface reaction kinetics significantly limit the photoelectrochemical (PEC) performances of silicon-based photoelectrodes. Herein, a WO3 thin layer is deposited on the p-Si substrate by pulsed laser deposition (PLD), acting as a photocathode for PEC hydrogen generation. Compared to bare p-Si, the single-junctional p-Si/WO3 photoelectrodes exhibit excellent and stable PEC performances with significantly increased cathodic photocurrent density and exceptional anodic shift in onset potential for water reduction. It is revealed that the WO3 layer could reduce the charge transfer resistance across the electrode/electrolyte interface by eliminating the effect of Fermi level pinning on the surface of p-Si. More importantly, by varying the oxygen pressures during PLD, the collaborative modulation of W-O bond covalency and WO6 octahedral structure symmetry contributes to the promoted charge carrier transport and separation. Meanwhile, a large band bending at the p-Si/WO3 junction, induced by the optimized O vacancy contents in WO3, could provide a photovoltage as high as ∼ 500 mV to efficiently drive charge transfer to overcome the water reduction overpotential. Synergistically, by manipulating W-O local atomic structures in the deposited WO3 layer, a great improvement in PEC performance could be achieved over the single-junctional p-Si/WO3 photocathodes for solar hydrogen generation.

Polarization dependence of ZnO Schottky barriers revealed by photoelectron spectroscopy

In order to answer the question of whether Schottky barriers on polar ZnO surfaces are different at Zn- and O-terminated surfaces, the interface formation of $n$-type ZnO and different high work function metals and metal oxides (Pt, ${\mathrm{PtO}}_{x}$, and ${\mathrm{RuO}}_{2}$) with Schottky barrier heights of up to 1.5 eV has been studied using photoelectron spectroscopy with in situ sample preparation. The experiments are designed to exclude the effects of substrate reduction and consequent Fermi level pinning by high concentrations of oxygen vacancies. Moreover, by including the Zn LMM Auger emission in the analysis, it is demonstrated that an accurate extraction of barrier heights needs to take into account that the screening of the photoelectron core hole can change in the course of contact formation. The polarization dependence of Schottky barriers, which is important for piezotronic applications, is in most cases dominated by the influence of defects. Reducing the influence of defects, up to $\ensuremath{\sim}240$ meV higher Schottky barriers are revealed on oxygen-terminated surfaces. This is opposite to what has been reported in the literature but agrees with the dependence of barrier heights expected for an incomplete screening of the polarization of ZnO by the electrode as for ferroelectric materials.

Hydride vapor phase epitaxy of Si-doped AlN layers using SiCl4 as a doping gas

Growth of Si-doped AlN layers by hydride vapor phase epitaxy on AlN/sapphire templates prepared by metalorganic vapor phase epitaxy and bulk AlN substrates prepared by physical vapor transport was investigated using silicon tetrachloride (SiCl4) as a doping gas. On the AlN/sapphire templates, when the SiCl4 supply was low, the incorporation ratio of Si decreased due to the influence of O impurities resulting from the decomposition of the sapphire. In addition, pits were formed on the surfaces, and the pit density increased significantly at Si concentrations exceeding 2 × 1019 cm−3. In contrast, on the bulk AlN substrates, the Si concentration increased linearly with increasing SiCl4 supply, and a pit- and stress-free layer could be grown with a Si concentration as high as 6.5 × 1019 cm−3. Hall effect measurements revealed that this layer exhibited n-type conductivity with a donor activation energy of 253 meV. However, the carrier density at room temperature (RT) was as low as 3.6 × 1013 cm−3 because of the high compensation ratio due to the presence of acceptors. A broad peak centered at 3.3 eV was observed in the RT photoluminescence spectra of the Si-doped AlN layers grown on the bulk AlN substrates, indicating that Al vacancies formed by the Fermi level effect due to Si doping acted as acceptors that compensated for carriers.

Review on DFT calculation of s‐triazine‐based carbon nitride

AbstractTo improve the photocatalytic performance of pristine photocatalysts, element doping, construction of composites and fabrication of novel nanostructures are recognized as universal modification methods. These methods have been experimentally verified to be effective in manifold photocatalytic application over various photocatalysts. Density functional theory (DFT) calculation is a powerful and fundamental tool to pinpoint the intrinsic mechanism of the enhanced photocatalytic activity. And it holds the degree of precision ranging from atoms, molecules to unit cells. Herein, recent DFT calculation research progress of modified s‐triazine‐based graphitic carbon nitride (g–C3N4) systems as photocatalysts is summarized. To specify, we collected information of doping site, formation energy, geometric, and electronic properties. We also discussed the synergistic effect of work function, Fermi level and band edge position on the built‐in electric field, transfer route of photogenerated charge carriers and photocatalytic mechanism (traditional type II or direct Z‐scheme heterostructure). Moreover, we analyzed the geometric configuration, band structure, and stability of g–C3N4 nanocluster, nanoribbon, and nanotube. Finally, future perspective in the further theoretical revelation of g–C3N4‐based photocatalysts is proposed.

In situ construction and sensing mechanism of TiO2–WO3 composite coatings based on the semiconductor heterojunctions

Tungsten oxide (WO3) is one of the most promising gas-sensitivity materials owing to its high decomposition temperature and stability of microstructure. The doping of oxides can significantly improve its gas-sensitivity properties. In this paper, WO3 based coatings doped with TiO2 were prepared via plasma spraying technology by using mixed feedstock suspensions containing a precursor as WCl6 and a certain ratio of nano-TiO2. The characterization of microstructures and properties of the TiO2–WO3 composite coatings were achieved and the gas sensitivity mechanism was researched. During the suspension plasma spraying, the solvent of the precursor droplets was evaporated and then the nucleation, crystallization and growth of WO3 occurred. The crystallinity and crystal grains of WO3 increased observably as the plasma spraying power improved. With the increase of TiO2 in coatings, the average grain size of the TiO2–WO3 composite coatings decreased at first and then increased. And the gas-sensitive properties of the composite coatings were significantly better than the pure WO3 coatings. The gas-sensitive properties improvement of TiO2–WO3 composite coatings doped with TiO2 was firstly caused by the decrease of grains size, which leaded to an increase of the gas adsorption sites. And the Fermi level effect of the composite metal oxide heterojunction formed by TiO2 and WO3 also markedly enhanced the gas sensitivity properties of the composite coatings.

Schottky Barrier Height Engineering for Electrical Contacts of Multilayered MoS2 Transistors with Reduction of Metal-Induced Gap States.

The difficulty in Schottky barrier height (SBH) control arising from Fermi-level pinning (FLP) at electrical contacts is a bottleneck in designing high-performance nanoscale electronics and optoelectronics based on molybdenum disulfide (MoS2). For electrical contacts of multilayered MoS2, the Fermi level on the metal side is strongly pinned near the conduction-band edge of MoS2, which makes most MoS2-channel field-effect transistors (MoS2 FETs) exhibit n-type transfer characteristics regardless of their source/drain (S/D) contact metals. In this work, SBH engineering is conducted to control the SBH of electrical top contacts of multilayered MoS2 by introducing a metal-interlayer-semiconductor (MIS) structure which induces the Fermi-level unpinning by a reduction of metal-induced gap states (MIGS). An ultrathin titanium dioxide (TiO2) interlayer is inserted between the metal contact and the multilayered MoS2 to alleviate FLP and tune the SBH at the S/D contacts of multilayered MoS2 FETs. A significant alleviation of FLP is demonstrated as MIS structures with 1 nm thick TiO2 interlayers are introduced into the S/D contacts. Consequently, the pinning factor ( S) increases from 0.02 for metal-semiconductor (MS) contacts to 0.24 for MIS contacts, and the controllable SBH range is widened from 37 meV (50-87 meV) to 344 meV (107-451 meV). Furthermore, the Fermi-level unpinning effect is reinforced as the interlayer becomes thicker. This work widens the scope for modifying electrical characteristics of contacts by providing a platform to control the SBH through a simple process as well as understanding of the FLP at the electrical top contacts of multilayered MoS2.

Graphene patterns supported terahertz tunable plasmon induced transparency.

The tunable plasmonic induced transparency has been theoretically investigated based on graphene patterns/SiO2/Si/polymer multilayer structure in the terahertz regime, including the effects of graphene Fermi level, structural parameters and operation frequency. The results manifest that obvious Fano peak can be observed and efficiently modulated because of the strong coupling between incident light and graphene pattern structures. As Fermi level increases, the peak amplitude of Fano resonance increases, and the resonant peak position shifts to high frequency. The amplitude modulation depth of Fano curves is about 40% on condition that the Fermi level changes in the scope of 0.2-1.0 eV. With the distance between cut wire and double semi-circular patterns increases, the peak amplitude and figure of merit increases. The results are very helpful to develop novel graphene plasmonic devices (e.g. sensors, modulators, and antenna) and find potential applications in the fields of biomedical sensing and wireless communications.

Fermi-Level Unpinning Technique with Excellent Thermal Stability for n-Type Germanium.

A metal-interlayer-semiconductor (M-I-S) structure with excellent thermal stability and electrical performance for a nonalloyed contact scheme is developed, and considerations for designing thermally stable M-I-S structure are demonstrated on the basis of n-type germanium (Ge). A thermal annealing process makes M-I-S structures lose their Fermi-level unpinning and electron Schottky barrier height reduction effect in two mechanisms: (1) oxygen (O) diffusion from the interlayer to the contact metal due to high reactivity of a pure metal contact with O and (2) interdiffusion between the contact metal and semiconductor through grain boundaries of the interlayer. A pure metal contact such as titanium (Ti) provides very poor thermal stability due to its high reactivity with O. A structure with a tantalum nitride (TaN) metal contact and a titanium dioxide (TiO2) interlayer exhibits moderate thermal stability up to 400 °C because TaN has much lower reactivity with O than with Ti. However, the TiO2 interlayer cannot prevent the interdiffusion process because it is easily crystallized during thermal annealing and its grain boundaries act as diffusion path. A zinc oxide (ZnO) interlayer doped with group-III elements, such as an aluminum-doped ZnO (AZO) interlayer, acts as a good diffusion barrier due to its high crystallization temperature. A TaN/AZO/n-Ge structure provides excellent thermal stability above 500 °C as it can prevent both O diffusion and interdiffusion processes; hence, it exhibits Ohmic contact properties for all thermal annealing temperatures. This work shows that, to fabricate a thermally stable and low resistive M-I-S contact structure, the metal contact should have low reactivity with O and a low work-function, and the interlayer should have a high crystallization temperature and a low conduction band offset to Ge. Furthermore, new insights are provided for designing thermally stable M-I-S contact schemes for any semiconductor material that suffers from the Fermi-level pinning problem.

Suppression of Magnetoresistance in Thin WTe2 Flakes by Surface Oxidation.

Recent renewed interest in layered transition metal dichalcogenides stems from the exotic electronic phases predicted and observed in the single- and few-layer limit. Realizing these electronic phases requires preserving the desired transport properties down to a monolayer, which is challenging. Surface oxides are known to impart Fermi level pinning or degrade the mobility on a number of different systems, including transition metal dichalcogenides and black phosphorus. Semimetallic WTe2 exhibits large magnetoresistance due to electron-hole compensation; thus, Fermi level pinning in thin WTe2 flakes could break the electron-hole balance and suppress the large magnetoresistance. We show that WTe2 develops an ∼2 nm thick amorphous surface oxide, which shifts the Fermi level by ∼300 meV at the WTe2 surface. We also observe a dramatic suppression of the magnetoresistance for thin flakes. However, due to the semimetallic nature of WTe2, the effects of Fermi level pinning are well screened and are not the dominant cause for the suppression of magnetoresistance, supported by fitting a two-band model to the transport data, which showed the electron and hole carrier densities are balanced down to ∼13 nm. However, the fitting shows a significant decrease of the mobilities of both electrons and holes. We attribute this to the disorder introduced by the amorphous surface oxide layer. Thus, the decrease of mobility is the dominant factor in the suppression of magnetoresistance for thin WTe2 flakes. Our study highlights the critical need to investigate often unanticipated and sometimes unavoidable extrinsic surface effects on the transport properties of layered dichalcogenides and other 2D materials.

Diffusion Mechanism for Arsenic in Intrinsic and Extrinsic Conditions in HgCdTe

Due to its low diffusivity and high activation rate, arsenic has become the dopant of choice in p/n HgCdTe high operating temperature technology. Its diffusion mechanism, however, remains imprecise. In this work, arsenic diffusion was studied in molecular beam epitaxy HgCdTe structures consisting of alternatively As-doped and intrinsic layers grown on a CdZnTe substrate. The diffusion coefficient of As was extracted from secondary ion mass spectroscopy concentration profiles. Annealings were performed for different temperatures, mercury partial pressures (P Hg), annealing times and cadmium atomic fractions. Fermi-level effect on diffusion was observed, indicating extrinsic conditions for diffusion at high As concentration. Based on the variation of As diffusivity with P Hg and As concentration, we propose that As diffusion occurs on both II and VI sublattices. Our results are consistent with the fact that AsVI diffusion is assisted by the Te interstitial, introducing donor levels in the bandgap, while AsII diffusion is assisted by the cation vacancy.

Terahertz tunable graphene Fano resonance

By depositing graphene circular double rings (DR) on a SiO2/Si/polymer substrate, the tunable Fano resonance has been theoretically investigated in the terahertz regime, including the effects of the graphene Fermi level, structural parameters and operation frequency. The results demonstrate that the obvious Fano peak can be efficiently modulated because of strong coupling between the incident waves and graphene ribbons. As the Fermi level increases, the peak amplitude of the Fano curve increases, and the resonant peak position shifts to a high frequency. The amplitude modulation depth of the Fano curves is about 30% if the Fermi level changes in the scope of 0.1–1.0 eV. The optimum gap distance between the DR is about 8–12 μm, where the value of the figure of merit shows a peak. The results are very helpful in order to develop novel graphene plasmonic devices, e.g. sensors and modulators.

Influence of high-temperature processing on the surface properties of bulk AlN substrates

Deep-level luminescence at 3.3eV related to the presence of Al vacancies (VAl) was observed in room temperature photoluminescence (RT-PL) spectra of homoepitaxial AlN layers grown at 1450°C by hydride vapor-phase epitaxy (HVPE) and cooled to RT in a mixture of H2 and N2 with added NH3. However, this luminescence disappeared after removing the near surface layer of AlN by polishing. In addition, the deep-level luminescence was not observed when the post-growth cooling of AlN was conducted without NH3. Secondary ion mass spectrometry (SIMS) studies revealed that although the point defect density of the interior of the AlN layers remained low, the near surface layer cooled in the presence of NH3 was contaminated by Si impurities due to both suppression of the surface decomposition by the added NH3 and volatilization of Si by decomposition of the quartz reactor walls at high temperatures. The deep-level luminescence reappeared after the polished AlN wafers were heated in presence of NH3 at temperatures above 1400°C. The surface contamination by Si is thought to generate VAl near the surface by lowering their formation energy due to the Fermi level effect, resulting in deep-level luminescence at 3.3eV caused by the shallow donor (Si) to VAl transition.