Fermi Level Shift Research Articles

In-situ methyl red doped MoS2 field effect transistor made by atomically thin MoS2 channel

Here we present the effects of Methyl Red (MR) doping on the electrical properties of the MoS2 field effect transistor (FET) in in-situ experimental system where the doping and electrical measurement are carried out in a single ultra-high vacuum chamber. A single-layered MoS2 flake was used as channel material for the FET preparation. N-type doping by MR was confirmed by the left shift of threshold voltage (Vth) and the surface vibrational change measured by Raman spectroscopy. The n-doping by MR is implied by the Fermi level shift towards greater binding energy with MR coverage, as proven by an X-ray photoelectron spectroscopy (XPS) measurement. It is estimated from XPS that the MoS2-FET device may detect the doping effect from the sub-monolayer MR. MR was detected on the MoS2 surface by the Time-of-Flight Secondary Ionization Mass Spectroscopy (ToF-SIMS). The findings may pave the way for innovations in electronic device design, with potential implications for fields ranging from nanoelectronics to sensor technology.

First-principles calculations on the intrinsic resistivity of realistic metals: a case study of monolayer V2N.

The Ziman resistivity formula is extensively employed to calculate the intrinsic resistivity of realistic metals using recent works of first-principles calculations. Owing to the approximation, Allen's generalization, which relates the solution of the Boltzmann transport equation (BTE) for metals to the transport electron-phonon (e-ph) spectral function, the applicability of the Ziman resistivity formula still needs to be discussed. In this work, we perform first-principles calculations of the intrinsic resistivity of the V2N monolayer, a kind of MXene material, by employing the Ziman resistivity formula and the iterative solution of BTE. We find that in the wide temperature range of 50-1400 K, the intrinsic resistivity of the V2N monolayer obtained by means of the two approaches, has the same order of magnitude. However the Ziman resistivity formula fails to correctly describe the Fermi level dependence of the intrinsic resistivity of the V2N monolayer. The underlying reason is that the band edge of the V2N and hence Van Hove singularity (VHS), is near the shifted Fermi level. We suggest a modified Ziman resistivity formula which is valid even if there is a band edge at the Fermi level.

Enhanced Photocharacteristics by Fermi Level Modulating in Sb2 Te3 /Bi2 Se3 Topological Insulator p-n Junction.

Topological insulators have recently received attention in optoelectronic devices because of their high mobility and broadband absorption resulting from their topological surface states. In particular, theoretical and experimental studies have emerged that can improve the spin generation efficiency in a topological insulator-based p-n junction structure called a TPNJ, drawing attention in optospintronics. Recently, research on implementing the TPNJ structure is conducted; however, studies on the device characteristics of the TPNJ structure are still insufficient. In this study, the TPNJ structure is effectively implemented without intermixing by controlling the annealing temperature, and the photocharacteristics appearing in the TPNJ structure are investigated using a cross-pattern that can compare the characteristics in a single device. Enhanced photo characteristics are observed for the TPNJ structure. An optical pump Terahertz probe and a physical property measurement system are used to confirm the cause of improved photoresponsivity. Consequently, the photocharacteristics are improved owing to the change in the absorption mechanism and surface transport channel caused by the Fermi level shift in the TPNJ structure.

Study on Structural Stability of ZrO2 and YSZ: Doping-Induced Phase Transitions and Fermi Level Shift

This work summarizes the results of structural stability, electronic properties and phonon dispersion studies of biocompatible ZrO2 compound in its cubic (c-ZrO2), tetragonal (t-ZrO2) and monoclinic (m-ZrO2) phases. The authors found that the monoclinic phase of zirconium dioxide is the most stable among the three phases in terms of total energy, lowest enthalpy, highest entropy, and other thermodynamic properties. The presence of rather weak frequencies for m-ZrO2 also confirms the monoclinic phase as a stable conformation of zirconia. Our analysis of the electronic properties showed that during the m-t phase transformation of ZrO2, the Fermi level first shifts by 0.125 eV toward higher energies and then decreases by 0.08 eV in the t-c cross-section. The band gaps for c-ZrO2, t-ZrO2, and m-ZrO2 are 5.140 eV, 5.898 eV, and 5.288 eV, respectively. Calculations to study the effect of 3.23, 6.67, 10.35 and 16.15 mol%Y2O3 on the structure and properties of m-ZrO2 showed that the enthalpy of m-YSZ decreases linearly and accompanies further stabilization of zirconium dioxide. Doping-induced phase transitions of ZrO2 were discovered under the influence of Y2O3 doping, due to which the position of the Fermi level changes and the band gap decreases. It has been established that, not only for pure systems but including those doped with Y2O3, the main contribution to the formation of the conduction band is made by the p-states of electrons.

High-temperature optoelectronic transport behavior of n-MoS2 nanosheets/p-diamond heterojunction

In the past decades, even though the performance of electronic devices based on MoS2 has reached a very high level, the service life, failure rate and stability of the devices often deteriorate with the increase of temperature when they work in high temperature or harsh environment. In this work, n-MoS2 NSs/p-type boron-doped diamond (BDD) film heterojunction was fabricated by sol–gel method to investigate the temperature dependence of the optoelectronic transport behavior of the device, and the electrical properties of the heterojunction device at high temperature (RT-180 °C) were tested. The heterojunction presented all rectification characteristics from room temperature to 180 °C and exhibited good thermal stability. With the increase in temperature, the rectification ratio reached the maximum at 120 °C and turned into backward diodes at 160 °C and 180 °C. The mechanism of the current and rectification ratio of n-MoS2 NSs/p-BDD heterojunction changing with temperature was mainly attributed to the decrease of tunneling current caused by the shift of Fermi level of the two semiconductors at high temperature. The heterojunction undergoes a transition from tunneling current to thermal excitation current, and the shift of Fermi level to the valence band of n-MoS2 provides support for the transition to reverse diode. In accordance with the carrier transport behavior following ohmic laws, the current conduction of composite tunneling and the space charge limitation under different bias voltages and temperatures were also discussed, which provides theoretical and experimental basis for optimizing the design of new photoelectric devices in the field of high temperature optoelectronics.

Tailoring the gas sensing characteristics of ZnO and ZnS monolayers toward CO2, CO and N2 upon introducing defects and substitutional functionalization

This work aims to design two-dimensional (2D) catalysts based ZnO and ZnS for CO2 reduction reaction (CRR), nitrogen reduction reaction (NRR), CO2 and N2 captures and sensors. Cheap and abundant elements which are Mg, Cl, and N are doped into Zn, S, and O sites to tune the electronic properties of the ZnO and ZnS monolayers. Also, influences of O, S and Zn vacancies on the electronic properties and gas adsorption are examined as well. Adsorption energy, interaction energy, distortion energy, density of states, Bader charge, shift of Fermi level, and work function of the modified surfaces are studied via density functional theory (DFT) performed on VASP. From the results, Mg, Cl, and N doping as well as vacant sites can extremely change electronic properties, i.e., semiconductive to metallic behavior, the shift of Fermi level and the variation of work function. Some modified ZnS and ZnO monolayers can be good candidates CRR and NRR catalysts since neither too strong nor too light binding of CO2, CO and N2. The best catalysts for this study is (Mg, VZn)-ZnO and Cl-ZnS because both can be sensors and electrochemical catalysts due to the proper binding strength for CO2, CO and N2 adsorption. The change in the electronic properties leading to the change in gas adsorption shows that the ZnO and ZnS modification by cheap-element doping and vacancy-site creation are potential ways to improve their properties, extend their applications and lead to the invention promising catalysts for electrochemical reactions, gas sensors and gas storages.

Copper Phosphide Nanostructures Covalently Modified Ti3 C2 Tx for Fast Lithium-Ion Storage by Enhanced Kinetics and Pesudocapacitance.

2D layer Ti3 C2 Tx material attracts enormous attention in lithium ion energy storage field owing to the unique surface chemistry properties, but the material still suffers from restacking issue and the restriction on capacity. Herein, copper phosphide (Cu3 P) nanostructures@Ti3 C2 Tx composites are prepared by the in situ generation of Cu-BDC precursor in the bulk material followed with phosphorization. The uniformly distributed copper phosphide nanostructures effectively expand the interlayer spacing promoting the structural stability, and achieves the effective connection with the bulk material accelerating the diffusion and migration of lithium ions. The electrochemical activity of Cu3 P also provides more lithium ion active sites for lithium storage. The X-ray photoelectron spectroscopy (XPS) analysis verifies that Ti─O─P bond with strong covalency allows the upper shift of maximum valence band and Fermi level, stimulating the charge transportation between Cu3 P and the bulk Ti3 C2 Tx for better electrode kinetics. 3Cu3 P@Ti3 C2 Tx exhibits excellent rate performance of 165.4 mAh g-1 at 3000mA g-1 and the assembled 3Cu3 P@Ti3 C2 Tx //AC Lithium-ion hybrid capacitorsLIC exhibits superior energy density of 93.0Wh kg-1 at the power density of 2367.3W kg-1 . The results suggest that the interfacial modification of Ti3 C2 Tx with transition metal phosphides will be advantageous to its high energy density application in lithium-ion storage.

Carrier doping of Bi[formula omitted]Se[formula omitted] surface by chemical adsorption—A DFT study

Bi2Se3 is one of the most promising topological insulators, but it suffers from intrinsic n-doping due to Se-vacancies, which shifts the Fermi level into the bulk conduction band, leading to topologically trivial carriers. Recently it was shown that this Fermi-level shift can be compensated by a locally controlled surface p-doping process, through water adsorption and XUV irradiation. Here, the microscopic mechanism of this surface doping is studied by means of density functional theory (DFT) focusing on the adsorption of H2O, OH, O, C and CH on Bi2Se3. We find that water adsorption has a negligible doping effect while hydroxyl groups lead to n-doping. Carbon adsorption on Se vacancies gives rise to p-doping but it also strongly modifies the electronic band structure around the Dirac point. Only if the Se vacancies are filled with atomic oxygen, the experimentally observed p-doping without change of the topological surface bands is reproduced. Based on the DFT results, we propose a reaction path where photon absorption gives rise to water splitting and the produced O atoms fill the Se vacancies. Adsorbed OH groups appear as intermediate states and carbon impurities may have a catalytic effect in agreement with experimental observations.

CO adsorption on two-dimensional 2H-ZrO2 and its effect on the interfacial electronic properties: implications for sensing

Environmental pollution is commonly caused by direct and indirect greenhouse gasses and various traces of toxic gasses. CO is a tasteless, colourless and highly hazardous greenhouse gas that can bind with the hemoglobin much easier than oxygen. Materials with effective CO sensing capability are highly desirable. In this work, we have studied the repercussions of the molecular adsorption of toxic carbon monoxide (CO) on the structural, electronic, interfacial electronic and gas sensing properties of the two-dimensional (2D) 2H phase of zirconium dioxide (2H-ZrO2) using density functional theory (DFT) calculations. The most suitable adsorbent structure is screened out and interaction strengths between adsorbate molecule and adsorbent layer are provided in terms of binding energies. As a result of CO adsorption, the energy band gap (Eg) is altered and the resulting change in the conductivity of 2H-ZrO2 monolayer has implications for sensing. Energy band profiles relating to CO adsorbed 2H-ZrO2 reveal that fermi level shifts towards conduction bands exhibiting a development of n-type semiconducting character. Eg values relating to pristine and CO adsorbed 2H-ZrO2 sites such as TM, H, CB and B are 1.58, 1.98, 2.05, 2.06 and 2.03 eV, respectively. Further insights into the adsorption reveal that charge is accumulated near 2H-ZrO2 in the case of H and B adsorption sites, whereas for the case of TM and CB sites, depletion is noted. Essential gas sensing properties such as conductivity sensitivity (S), and recovery time (τ) are also reported. Notably, TM, CB, and B absorption sites depicted 17%, 2%, and 6% lower S values than H site. Furthermore, the H site achieved a shorter (32 ps) than the other adsorption sites. Response of pristine and CO adsorbed 2H-ZrO2 to that of incident photons is investigated with the help of real and imaginary parts [ and ] of complex dielectric function, electron energy loss function [] and joint density of states [] for a broader range of 0 to 10 eV. Major differences in dielectric function exist along in-plan and out-of-plan directions. After a detailed comparison, it is reported that more number of optical transitions exist between the linked states for the case of CO adsorbed 2H-ZrO2.

Fast Ce,Ca:LuAG scintillation ceramics for HEP: Fabrication, characterization, and computation

AbstractHigh‐energy physics community is looking for a hard, fast, and low‐cost scintillation material, and Ce:Lu3Al5O12 (Ce:LuAG) ceramic is one of the competitive candidates. This work presents Ce,Ca:LuAG scintillation ceramics with good optical quality, and the influence of Ce and Ca concentrations on optical and scintillation properties was fully analyzed. At relatively low level of Ce concentration, the less Ca2+ content is needed to achieve a significant intensity increase in fast scintillation component while maintaining a relatively high light yield (LY). The introduction of only 0.1 at% Ca2+ could increase the LY0.5 μs/LY3.0 μs from 79.9% to 96.1% in Ce,Ca:LuAG ceramics of 0.1 at% Ce. First‐principles investigations are further performed to reveal the tuning mechanisms of the scintillation properties of LuAG by Ce and Ca codoping. We show that the Fermi level shifts down with Ca codoping, which increases the Ce4+ content and decreases the depth of the electron traps (VO), resulting to a faster decay. Moreover, the formation preference of Ca‐VO complexes over Ce‐VO leads to the suppression of the non‐radiative decay of Ce via VO. In summary, our study demonstrates the realization of the performance tuning of LuAG via Ce and Ca codoping.

Microwave hydrothermal preparation of reduced graphene oxide-induced p-AgO/n-MoO3 heterostructures for enhanced photocatalytic activity through S-scheme mechanism and its electronic performance.

Through a powerful and modest closed system Microwave hydrothermal process, a methodological analysis is made in the rational synthesis of the reduced graphene oxide-induced p-AgO/n-MoO3 (RGAM) heterostructures. These have strong p-n junction heterostructures with considerable electron-hole recombination functioning as solar catalysts. The enhanced photocatalytic activity through the plasmonic step scheme (S-scheme mechanism) describes the effective charge recombination process. The energy band positions, bandgap, and work function are determined to understand the Fermi level shifts; this describes the S-scheme mechanism by UPS analysis which assessed an electron transfer between AgO and MoO3, yielding work function values of 6.34 eV and 6.62eV, respectively. This photocatalytic activity aids in dye removal by 94.22%, and heavy metals such as chromium (Cr) are eliminated by the surface action of sunlight on the produced material during solar irradiation. Electrochemical studies such as photocurrent response, cyclic voltammogram, and electrochemical impedance spectroscopy for RGAM heterostructures were also carried out. The study helps to broaden the search for and development of new hybrid carbon composites for electrochemical applications.

Formation of Ohmic contacts between ferromagnetic Cr2Ge2Te6 and two-dimensional metals

As a ferromagnetic semiconductor, two-dimensional (2D) Cr2Ge2Te6 holds significant implications for electronic and spintronic devices. To achieve 2D electronics, it is essential to integrate Cr2Ge2Te6 with 2D electrodes to form Schottky-barrier-free Ohmic contacts with enhanced carrier injection efficiency. Herein, by using first-principles calculations based on density-functional theory, we systematically investigate the structural, energetic, electronic and magnetic properties of 2D heterojunctions by combining Cr2Ge2Te6 with a series of 2D metals, including graphene, ZrCl, NbS2, TaS2, TaSe2, Zn3C2, Hg3C2, and Zr2N. Results show that NbS2, TaS2, TaSe2, Zn3C2, Hg3C2, and Zr2N form Ohmic contacts with Cr2Ge2Te6, in contrast to graphene and ZrCl that exhibit a finite Schottky barrier. By examining the tunneling barriers and Fermi level shift, we reveal that the heterojunctions with Zn3C2 and Hg3C2 as electrodes exhibit advantages of both high electron injection efficiency and spin injection efficiency, for which an apparent decrease of the magnetic moment of Cr atoms in Cr2Ge2Te6 can be observed. These findings not only provide physical insights into the role of interfacial interaction in regulating the physical properties of 2D heterojunctions, but also pave way for the development of high-performance spintronic nanodevices for practical implementation.

First-principles study of the rectifying properties of Au/SnO2 interface

Based on the first-principles calculations, we investigate the properties of Au contacts, surface structure, and band bending for Au/SnO2(101) and Au/SnO2(100) contacts under different conditions. Our results show that the Schottky barrier height (SBH) strongly depends on the interface structure. A stoichiometric Au/SnO2(101) interface forms a Schottky contact, while Au/SnO2(100) exhibits ohmic contact characteristics. When oxygen vacancy (VO) is present in the interfacial SnO2 layer, the Fermi level shifts up to the conduction band minimum, resulting in a smaller SBH. Although Au contact on SnO2(101) with VO has non-Schottky properties, it gains Schottky properties after doping with a low-valence element. In addition, the SBHs of each interface structure are different, indicating that the modulation effect of SBH mainly depends on the rearrangement of interfacial potentials caused by the different interface structures. Such a rearrangement causes corresponding changes in the energy band at the interface, resulting in different SBHs.

PEDOT:PSS helps to reveal the decisive role of photocurrent and photopotential on the photoinduced cathodic protection performance

Photoinduced cathodic protection (PICP) performance of a semiconductor is closely related to its photoelectric conversion performance. However, this relationship is not clear up to date. In this work, PEDOT:PSS was added during the preparation process of ZnO electrode, endowing the obtained ZnO/PEDOT:PSS electrode with enhanced charge transfer and positive shifted Fermi level. Compared with ZnO, the ZnO/PEDOT:PSS shows larger photocurrent and more positive photopotential, which have opposite effects on the final PICP performance. Thus, the effect of photocurrent and photopotential on the PICP performance can be separately investigated in this paper. ZnO/PEDOT:PSS shows increased PICP performance for Cu but reduced PICP performance for 316L SS. These disparate PICP results were further revealed by the comparison of the photoinduced Tafel plots of semiconductors and the Tafel plots of the protected metals for the first time. And the synergistic mechanism of the photoelectric conversion property of semiconductors and the corrosion rates of the metals are finally clarified. Furthermore, a series ZnO photoelectrodes with different photocurrents and constant photopotential were obtained by simply controlling the electrode area. The comparison of their PICP performance reveals the efficient way to improve the PICP performance from the perspective of both photocurrent and photopotential. Therefore, this work not only reveals the synergistic mechanism of semiconductors and metals on the PICP performance, but also plays an important role in guiding how to efficiently improve the PICP performance.

Ab-initio analysis of Li adsorption on boron doped armchair silicon carbide nanoribbon for lithium ion batteries (LIBs)

It is well known that the electrochemical storage capacity of anode materials can be modified by the doping of heteroatom. Here, first-principles approach is used to investigate the adsorption energy (Eads), open circuit voltage (OCV), and storage capacity of boron co-doped armchair silicon carbide anode (B-ASiCNR) material for Li ion battery. When B atoms are doped at the center of the armchair silicon carbide nanoribbon (ASiCNR), new energy states emerge and it results in Fermi-Level shift towards the valence band. The binding strength of a single Li atom is significantly higher in B-ASiCNR because of B atom doping. The open circuit voltage decreases from 3.76 V to 2.34 V when Li atoms adsorbed from first hexagonal position to all hole position of the substrate. Calculations reveal that the storage capacity of B-ASiCNR reached to the maximum value of 836 mAhg−1 when all hole positions are adsorbed by Li atoms. Calculated Eads, OCV, storage capacity suggest that the B-ASiCNR can be used as future anode materials for Li ion battery.

Enhancing the Rashba spin splitting at X/Au/Si(111) surface by induced intrinsic spin-polarization

Adsorbed/modified surface is usually treated as a prototype to study Rashba spin splitting. Using first-principles calculations and analyses, we first explain the Rashba spin splitting of monovalent alkali metal adsorbed on Au/Si(111) surface as realized in experiment. We clearly identify that Na adatom donates one electron to pristine Au/Si(111) surface, leading to the Fermi level shift upward to the parabolic range of Au d and Si p hybridized states and thus leading to large Rashba spin splitting in Na/Au/Si(111) system. However, for the larger intra-atomic spin-orbit coupling Cs adsorbate, it not only plays the identical role to Na adatom, but induces intrinsic spin-polarization surprisingly, which further enhances the Rashba spin splitting on the Cs/Au/Si(111) surface up to ∼200 meV. Our proposed mechanism that intrinsic spin-polarization significantly enhancing the Rashba spin splitting is generally applicable and can be extended readily to alkali-earth metal as epitaxial layer grown on Au/Si(111) surface.

Electrode Doping and Dielectric Effect in Hole Injection into Organic Semiconductors through High Work-Function Oxides.

High work-function metal oxides are common for enhancing hole injection into organic semiconductors. However, the current understanding of the electrostatic mechanism needs to be more consistent with materials' electronic properties. Here, we study the electrostatic profile of high work-function oxides by considering their dielectricity and energetic disorder. Using MoO3 as an example, we first show that the significant vacuum-level change at the electrode-oxide interface originates from electrode doping rather than the conventionally assumed interface dipole. Moreover, electrode doping is enough to explain the Fermi-level shift, so MoO3's characteristic n-type property is not necessarily due to intrinsic donors. This conclusion also applies to the n-type oxides with reduced work functions, like WO3, V2O5, and p-type NiO. Finally, the dielectricity of the oxide, either n-type or p-type, reduces the surface p-doping of the further deposited organic layer. Increasing the oxide's metallicity and energetic disorder facilitates the hole injection.

Electrostatic Gating of Monolayer Graphene by Concentrated Aqueous Electrolytes.

Electrostatic gating using electrolytes is a powerful approach for controlling the electronic properties of atomically thin two-dimensional materials such as graphene. However, the role of the ionic type, size, and concentration and the resulting gating efficiency is unclear due to the complex interplay of electrochemical processes and charge doping. Understanding these relationships facilitates the successful design of electrolyte gates and supercapacitors. To that end, we employ in situ Raman microspectroscopy combined with electrostatic gating using various concentrated aqueous electrolytes. We show that while the ionic type and concentration alter the initial doping state of graphene, they have no measurable influence over the rate of the doping of graphene with applied voltage in the high ionic strength limit of 3-15 M. Crucially, unlike for conventional dielectric gates, a large proportion of the applied voltage contributes to the Fermi level shift of graphene in concentrated electrolytes. We provide a practical overview of the doping efficiency for different gating systems.

Highly efficient graphene terahertz modulator with tunable electromagnetically induced transparency-like transmission

Graphene-based optical modulators have been extensively studied owing to the high mobility and tunable permittivity of graphene. However, weak graphene-light interactions make it difficult to achieve a high modulation depth with low energy consumption. Here, we propose a high-performance graphene-based optical modulator consisting of a photonic crystal structure and a waveguide with graphene that exhibits an electromagnetically-induced-transparency-like (EIT-like) transmission spectrum at terahertz frequency. The high quality-factor guiding mode to generate the EIT-like transmission enhances light-graphene interaction, and the designed modulator achieves a high modulation depth of 98% with a significantly small Fermi level shift of 0.05 eV. The proposed scheme can be utilized in active optical devices that require low power consumption.

Electrochemical Control over the Optical Properties of II–VI Colloidal Nanoplatelets by Tailoring the Station of Extra Charge Carriers

An electrochemical (EC) method has been successfully applied to regulate the optical properties of nanocrystals, such as reducing their gain threshold by EC doping and enhancing their photoluminescence intensity by EC filling of trap states. However, the processes of EC doping and filling are rarely reported simultaneously in a single study, hindering the understanding of their underlying interactions. Here, we report the spectroelectrochemical (SEC) studies of quasi-two-dimensional nanoplatelets (NPLs), intending to clarify the above issues. EC doping is successfully achieved in CdSe/CdZnS core/shell NPLs, with red-shifted photoluminescence and a reversal of the emission intensity trend. The injection of extra electrons (holes) into the conduction (valence) band edges needs high bias voltages, while the passivation/activation process of trap states with the shift of Fermi level starts at lower EC potentials. Then, we explore the role of excitation light conditions in these processes, different from existing SEC research studies. Interestingly, increasing the laser power density can hinder EC electron injection, whereas decreasing the excitation energy evades the passivation process of trap states. Moreover, we demonstrate that EC control strategies can be used to realize color display and anti-counterfeiting applications via simultaneously tailoring the photoluminescence intensity of red- and green-emitting NPLs.