Properties Of Charge Carriers Research Articles

ON the Nature of Ionic Liquid Gating of La2−xSrxCuO4

Ionic liquids have recently been used as means of modulating the charge carrier properties of cuprates. The mechanism behind it, however, is still a matter of debate. In this paper we report experiments on ionic liquid gated ultrathin La2−xSrxCuO4 films. Our results show that the electrostatic part of gating has limited influence in the conductance of the cuprate in the gate voltage range of 0 to V. A non-electrostatic mechanism takes over for gate voltages below V. This mechanism most likely changes the oxygen concentration of the film. The results presented are in line with previous X-ray based studies on ionic liquid gating induced oxygenation of the cuprate materials YBa2Cu3O7−x and La2−xSrxCuO4.

Advanced Terahertz Frequency-Domain Ellipsometry Instrumentation for In Situ and Ex Situ Applications

We present a terahertz (THz) frequency-domain spectroscopic ellipsometer design that suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements geometry , coating , and modulation . The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, in situ gas cells, or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1–1 THz (3.3–33 cm $^{-1}$ or 0.4–4 meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect (OHE), resonant-cavity enhanced OHE, and in situ OHE experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two-dimensional electron gas in a group III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.

Light-induced current mapping in oxide based solar cells with nanoscale resolution

Transport properties of photo-induced charge carriers through different grains in the polycrystalline photovoltaic devices strongly depend on the microstructural pattern of the active layers. Therefore, photocurrent mapping with nanoscale resolution is important to know about the electrical responses of the different grains in the polycrystalline photovoltaic devices. Here, we have used photoconductive atomic force microscopy for mapping the photocurrent with nanoscale resolution of two types of ZnO nanorods/Cu2O based solar cells. The morphology and current have been measured simultaneously with nanoscale resolution from the top surfaces of the devices at different applied voltages. It is demonstrated that the nanostructure of the active layers is one of the most important variables determining device performances. Different local photovoltaic performances have been observed from these two devices due to various microstructural and electrical phenomena of their seed layers. On the other hand, significant variations in short-circuit current have been observed from different grains of the devices which appeared more alike in the micrograph owing to various transport properties of photocarriers. It is observed that the grain boundaries are more preferable for charge collection over the grain interiors. It shows a higher short circuit current close to the boundary than the grain inside. This study illustrates an important area for future fundamental research to enhance the performances of the polycrystalline photovoltaic devices through better control of morphology and improving the inherent properties of the active layers.

Hydrothermal synthesis of nanostructured Cr-doped hematite with enhanced photoelectrochemical activity

Using the easily applicable hydrothermal method Cr-doped hematite thin films have been deposited polycrystalline on conductive glass substrates. The hydrothermal bath consisted of an aqueous solution containing a mixture of FeCl3·6H2O and NaNO3 at pH = 1.5. The samples were introduced in an autoclave and heated for a fixed time at a fixed temperature and then annealed in air at 550 °C. The concentration of the incorporated Cr atoms (Cr4+ ions) was controlled by varying the concentration of the Cr(ClO4)3 precursor solution, varied from 0% to 20%. All samples followed morphological and structural studies using field-emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray diffraction. Chronoamperometry measurements showed that Cr-doped hematite films exhibited higher photoelectrochemical activity than the undoped films. The maximum photocurrent density and incident photon conversion efficiencies (IPCE) were obtained for 16 at.% Cr-doped films. This high photoactivity can be attributed to both the large active surface area and increased donor density caused by Cr-doping in the α-Fe2O3 films. All samples reached their best IPCE at 400 nm. IPCE values for 16 at.% Cr-doped hematite films were thirty times higher than that of undoped samples. This high photoelectrochemical performance of Cr-doped hematite films is mainly attributed to an improvement in charge carrier properties.

The Scattering of Massive Holes by Supercritical Impurity

The properties of charge carriers in doped graphene are considered. The closed set of explicit equations determining the spectrum and wavefunctions of charge carriers for total angular momentum J = M + 1/2 = 0, ±1/2, ±1… is obtained for the case of the Coulomb potential modified at small distances. The critical values Zcr of the dopant charge at which the energy level with the given quantum numbers crosses the lower continuum boundary are determined. For Z < Zcr for several values of the orbital angular momentum dependence of the position of the energy level as a function of charge Z is obtained. For Z > Zcr, the position ε0 and width γ of the lowest quasidiscrete state, which may manifest itself as a resonance in hole-dopant scattering, are calculated. It’s shown that there is no electron-hole pairs creation because of unitarity of the partial scattering matrix.

Structural and electronic properties of oxygen defective and Se-doped p-type BiVO4(001) thin film for the applications of photocatalysis

Monoclinic BiVO4 is being used as a photocatalyst due to its stability, cost-effectiveness, ease of synthesis, and narrow band gap. Although, the valence band maximum, VBM (∼−6.80eV vs vacuum) of BiVO4 is well below the redox potential of water but having less positive conduction band minimum, CBM (−4.56eV vs vacuum), responsible for its low efficiency. We have carried out a comprehensive periodic density functional theory (DFT) simulations for the pristine, Oxygen defective (Ov) and Se doped BiVO4, to engineer not only its CB edge position but the overall photocatalytic and charge carrier properties. Our theoretical method has nicely reproduced the experimental data of pristine BiVO4, which encouraged us to elaborate further its Ov and Se-doped characteristics. It is found that both the Ov (1% Oxygen vacancy) and Se-doped BiVO4 (1–2% Se) have ideal band edges, band gaps, and small effective masses of electrons and holes, responsible for high photocatalytic activities. Moreover, Se-doped BiVO4 behave as p-type semiconductor. Finally, the photocatalytic water-splitting behaviour of the selected surfaces were counterchecked with water interaction, where the strong water adsorption energy of about ∼−38 to −50kcal/mol, confirms and predicts their higher efficiencies compared to that of parent BiVO4.

The Kubo-Greenwood spin-dependent electrical conductivity of 2D transition-metal dichalcogenides and group-IV materials: A Green’s function study

The spin-dependent electrical conductivity of counterparts of graphene, transition-metal dichalcogenides (TMDs) and group-IV nanosheets, have investigated by a magnetic exchange field (MEF)-induction to gain the electronic transport properties of charge carriers. We have implemented a k.p Hamiltonian model through the Kubo-Greenwood formalism in order to address the dynamical behavior of correlated Dirac fermions. Tuning the MEF enables one to control the effective mass of carriers in group-IV and TMDs, differently. We have found the Dirac-like points in a new quantum anomalous Hall (QAH) state at strong MEFs for both structures. For both cases, a broad peak in electrical conductivity originated from the scattering rate and entropy is observed. Spin degeneracy at some critical MEFs is another remarkable point. We have found that in the limit of zero or uniform MEFs with respect to the spin-orbit interaction, the large resulting electrical conductivity depends on the spin sub-bands in group-IV and MLDs. Featuring spin-dependent electronic transport properties, one can provide a new scenario for future possible applications.

Band Transport and Trapping in Didodecyl[1]benzothieno[3,2-b][1]benzothiophene Probed by Terahertz Spectroscopy.

Terahertz electromodulation spectroscopy provides insight into the material-inherent transport properties of charge carriers in organic semiconductors. Experiments on didodecyl[1]benzothieno[3,2-b][1]benzothiophene (C12-BTBT-C12) devices yield for holes an intraband mobility of 9 cm2 V-1 s-1. The short duration of the THz pulses advances the understanding of the hole transport on the molecular scale. The efficient screening of Coulomb potentials leads to a collective response of the hole gas to external fields, which can be well described by the Drude model. Bias stress of the devices generates deep traps that capture mobile holes. Although the resulting polarization across the device hinders the injection of mobile holes, the hole mobilities are not affected.

Study on the photoconductive properties of charge carriers in donor-acceptor copolymer P(EDOT-FL) and P(EDOT-FL):PCBM blend

The photoinduced charge transfer properties of low band gap donor-acceptor π-conjugated copolymer poly(2,5-(3,4-ethylenedioxythiophene)-alt-2,7-(9,9-dioctylfluorene)) labelled as P(EDOT-FL) has been investigated through solvatochromic and photoconductivity studies. The effect of strong acceptor, Phenyl-C61-butyric acid methyl ester (PCBM) on the photogeneration of free carriers is studied by comparing the fluorescence spectra and photoconductive properties of the polymer-PCBM blend [P(EDOT-FL):PCBM] films with the pristine polymer films. The positive solvatochromism exhibited by the P(EDOT-FL) revealed its photoinduced charge transfer nature of the absorption and emission bands. The blend films exhibited quenching of the fluorescence intensity and showed increased photoconductivity. Internal photocurrent efficiency and photoconductive sensitivity of the pristine P(EDOT-FL) films and P(EDOT-FL):PCBM blend films were calculated as a function of electric field, by measuring the photocurrent generated in the sample. Blend films exhibited higher internal photocurrent efficiency of 86% and photoconductive sensitivity of 5.44×10−7 SW−1cm at 70V/μm. The obtained photoconductive sensitivity at low electric field (<10V/μm) is sufficient for photorefractive applications.

In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

Analysis of Nanoprecipitates in a Na-Doped PbTe-SrTe Thermoelectric Material with a High Figure of Merit.

The dimensionless figure of merit, ZT, of bulk thermoelectric materials depends mainly on the transport properties of charge carriers and heat-carrying phonons. PbTe-4 mol % SrTe doped with 2 mol % Na (Pb0.94Na0.02Sr0.04Te) is a nanostructured material system that exhibits a ZT higher than 2. The precipitate size distribution of SrTe precipitates is believed to play a key role. This raises the question of whether its performance is limited by precipitate coarsening (Ostwald ripening) at elevated temperatures. Herein, we utilize an atom-probe tomography (APT) to study the number density and mean radii of precipitates in concert with partial radial distribution functions (RDFs) of individual atoms. We find that the SrTe precipitates actually contain oxygen: SrTe1-xOx. We correlate this information with the overall ZT performance, specifically focusing on the electrical and lattice thermal conductivities after isothermal heat treatments at 300 and 400 °C for 7 days, followed by furnace cooling. Comparison of the samples annealed at 400 and 300 °C demonstrates significant coarsening of SrTe1-xOx precipitates as well as strong segregation of oxygen impurities in the SrTe1-xOx precipitates. Additionally, on the basis of the partial RDFs, the Na dopant atoms cluster with other Na atoms as well as with Pb, Te, and Sr atoms; clustering depends strongly on the annealing temperature and concomitantly affects the overall ZT values. We found that the coarsening slightly increases the lattice thermal conductivity and also increases the electrical conductivity, thereby having little or even a beneficial effect on the ZT values. Importantly, these findings demonstrate that APT enables quantitative analyses in three dimensions of the PbTe-4 mol % SrTe samples in addition to correlation of their properties with the thermoelectric performance.

Squaraine‐Based Polymers: Toward Optimized Structures for Optoelectronic Devices

A novel polymer semiconductor based on squaric acid is successfully prepared through polycondensation reaction. Physicochemical properties of polysquaraine are explored by Fourier transform infrared (FT‐IR) and ultraviolet‐visible (UV‐vis) absorption spectroscopy, thermogravimetric/modulated temperature differential scanning calorimetry (TGA/MTDSC) analyses, and cyclic voltammetry (CV). Next, the charge carrier properties are investigated through the fabrication and characterization of field‐effect transistors (FETs) using solution‐processed polymeric films. It is found that the polysquaraine is FET active and exhibits decent p‐type mobilities (up to 5 × 10−4 cm2 V−1 s−1), which illustrates the promising properties of this semiconductor in optoelectronics. Notably, there is no precedent for the use of squaraine‐based polymers in field‐effect transistors. Bulk heterojunction solar cells (BHJ‐OSCs) are finally prepared from the blends of polysquaraine with the fullerene derivative [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM). Power conversion efficiencies up to 0.86% are achieved for nonoptimized system under simulated air mass 1.5 (AM1.5) conditions. image

In situ synthesis of polymetallic Co-doped g-C3N4 photocatalyst with increased defect sites and superior charge carrier properties

Preparation of Fe and Pd co-doped g-C3N4 was described, using dicyandiamide monomer, and [1, 1′-Bis (diphenylphosphino) ferrocene] dichloropalladium (II) as precursor. X-ray powder diffraction, Fourier transform infrared spectra, Raman spectra and transmission electron microscopy were used to confirm the formation of bulk Fe-Pd/C3N4, X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy were used to analysis the chemical states and element content of Fe-Pd/C3N4. N2 adsorption and desorption isotherms, UV–vis diffuse reflectance spectroscopy, electrochemical impedance spectroscopy, photoluminescence and Mott-Schottky capacitance measurement were used to further investigate the semiconducting properties of Fe-Pd/C3N4. The results indicated that the addition of dopants enriched the nitrogen content of graphitic carbon nitride, induced more structural defects, expanded the surface area, decreased the band gap energy, reduced the charge-transfer resistance, raised the flat band potential and restrained the recombination of photogenerated electrons and holes. Consequently, Fe and Pd co-doped g-C3N4 exhibited higher Rhodamine B photodegradation rate than raw g-C3N4. The most possible coordination sites of Fe and Pd were inferred as the CNHC defect sites.

Effect of side‐chain functionality on the organic field‐effect transistor performance of oligo(p‐phenylenevinylene) derivatives

ABSTRACTIn order to observe the effects of the substitution of electronegative flourine with aromatic groups in oligo(p‐phenylenevinylene) compounds on their packing, morphology, and charge carrier mobility, we have synthesized napthol‐substituted oligo(p‐phenylenevinylene) compounds and examined their solubility, redox properties, thin film morphologies, and charge carrier properties. To date, very few examples of conjugated oligomers bearing napthol side groups have been reported in the literature. After annealing at 150 °C, the mobility of S1, S2, and S3 was 4.0 × 10−2 cm2 V−1 s−1, 1.2 × 10−2 cm2 V−1 s−1, and 2.6 × 10−3 cm2 V−1 s−1, respectively. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44825.

Band tunneling through double barrier in biased graphene bilayer

We calculate the transport properties of charge carriers in graphene bilayers across symmetric and asymmetric double potential barrier considering energies exceeding the inter-layer coupling where two transport modes exist. Evaluating the transmission and reflection probabilities and corresponding conductances, we show that the transport is sensitive to the distance between the two barriers. Moreover, we explain the characteristic features observed in the numerical calculations, such as resonance tunneling at normal incidence, based on the Febry–Pèrot oscillations and ballistic transmission carried out by the evanescent waves. Finally, we compute the conductance of each mode separately and investigate contributions from inter-mode scattering and show that some geometric potential parameters can be used to control the total conductance of the system.

On the nature of ionic liquid gating of Nd2−xCexCuO4 thin films

Recently, ionic liquid gating has been used to modulate the charge carrier properties of metal oxides. The mechanism behind it, however, is still a matter of debate. In this paper, we report experiments on doped and undoped Nd2CuO4. We find major resistance drops of the bilayer coupled to observations of the presence of a considerable Faradeic component in the gate current and of the appearance of charge transfer peaks in the cyclic voltammetry data. This leads us to propose a mechanism of gating based on irreversible electrochemical reactions, likely due to trace amounts of contaminations present in the ionic liquid. This work is therefore in line with previous reports confirming the presence of irreversible electrochemistry in ionic liquid gated electron- doped cuprates.

Coulomb Problem for Graphene with Supercritical Impurity

The properties of charge carriers in graphene with dopants with charge Z within two-dimensional effective Dirac equation with non-zero gap between upper and lower continuums are analyzed. The closed set of explicit equations determining the spectrum of charge carriers are obtained for the case of the Coulomb potential modified at small distances. The critical values Zcr of the dopant charge at which the energy level with the given quantum numbers crosses the lower continuum boundary are determined. For Z < Zcr for several values of the orbital angular momentum, which may take half-integer values in 2D case, dependence of the position of the energy level as a function of charge Z is obtained. For Z > Zcr, the position ε0 and width γ of the lowest quasidiscrete state are calculated. For such values of Z screening of the impurity charge and resonance scattering of holes are also considered.

Multilayer core–shell MoS2/CdS nanorods with very high photocatalytic activity for hydrogen production under visible-light excitation and investigation of the photocatalytic mechanism by femtosecond transient absorption spectroscopy

Understanding the structural features and the dynamics and properties of charge carriers in photocatalysts is critical to develop them for practical applications.

Bismuth Iodide Perovskite Materials for Solar Cell Applications: Electronic Structure, Optical Transitions, and Directional Charge Transport

Cesium and methylammonium bismuth iodides (Cs3Bi2I9 and MA3Bi2I9) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs3Bi2I9 (2.17 and 2.0 eV) and MA3Bi2I9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs3Bi2I9 and MA3Bi2I9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors...

Cavity-enhanced optical Hall effect in epitaxial graphene detected at terahertz frequencies

Cavity-enhanced optical Hall effect at terahertz (THz) frequencies is employed to determine the free charge carrier properties in epitaxial graphene (EG) with different number of layers grown by high-temperature sublimation on 4H-SiC(0001). We find that one monolayer (ML) EG possesses p-type conductivity with a free hole concentration in the low 1012cm−2 range and a free hole mobility parameter as high as 1550cm2/Vs. We also find that 6ML EG shows n-type doping behavior with a much lower free electron mobility parameter of 470cm2/Vs and an order of magnitude higher free electron density in the low 1013cm−2 range. The observed differences are discussed. The cavity-enhanced THz optical Hall effect is demonstrated to be an excellent tool for contactless access to the type of free charge carriers and their properties in two-dimensional materials such as EG.