Electrolyte Electroreflectance Research Articles

Depletion Layer Built‐In Field at (1−100), (0001), and (000−1) GaN/Water Junction and Its Role in Semiconductor Nanowire Water Splitting

AbstractThe nature of specific GaN plane/water interfaces under external bias and illumination can influence photoelectrolysis efficiency using GaN nanowires. Studies of Ga‐polar, N‐polar, and m‐plane GaN interfaces with deionized water allow determining differences between surfaces corresponding to different nanowire facets. They are investigated under external bias conditions to reveal the profile of Fermi level localization through analysis of Franz–Keldysh oscillations using electrolyte electroreflectance (EER) technique in a specially designed measurement chamber. Calculation of the potential barrier height is also possible. EER study shows differences between surface densities of states (SDOS) at distinct GaN planes. One broad SDOS is identified near the conduction band in case of ±c‐plane and related to Ga adatom reconstruction and β‐Ga2O3 presence at the GaN electrode. Two narrow SDOS singularities are found at the m‐plane one of which is localized near the middle of the bandgap and allows to generate approximately two times higher surface potential barrier than in case of polar surfaces at zero‐bias conditions. This suggests that n‐type GaN nanowires can enhance carrier separation at sidewalls and refine the oxygen evolution rate. Additionally, a voltage‐controlled hysteresis loop of Fermi level localization is detected at the Ga‐face GaN/water interface.

High-temperature carrier density and mobility enhancements in AlGaN/GaN HEMT using AlN spacer layer

In this paper, we experimentally studied the effect of AlN spacer layer on optical and electrical properties of AlGaN/GaN high electric mobility transistors (HEMTs) grown by metal organic chemical vapor deposition method. For AlGaN layer in HEMT structure, the Al composition of the sample was determined using x-ray diffraction and photoluminescence. Electrolyte electro-reflectance (EER) measurement not only confirmed the aluminum composition of AlGaN layer, but also determined the electric field strength on the AlGaN layer through the Franz-Keldysh oscillation phenomenon. This result indicated that the electric field on the AlGaN layer could be improved from 430 to 621kV/cm when AlN spacer layer was inserted in HEMT structure, which increased the concentration of two dimensional electron gas (2DEG) and improve the mobility. The temperature dependent Hall results show that both the mobility and the carrier concentration of 2DEG would decrease abruptly causing HEMT loss of function due to phonon scattering and carrier thermal escape when temperature increases above a specific value. Meanwhile, our study also demonstrates using AlN spacer layer could be beneficial to allow the mobility and carrier density of 2DEG sustaining at high temperature region.

Growth and optical characterization of MoS2 single crystals with different dopants

Single crystals MoS2:X with different dopants X (X=Re, Nb, Fe, Co, Ni) were grown by the chemical vapor transport method using Br2 as transport agent. By analyzing the X-ray diffraction patterns, the structure of the single crystals shows rhombohedral symmetry for Re and Nb-doped and hexagonal symmetry for Fe, Co, and Ni-doped. Piezoreflectance (PzR) measurements along c-axis at 300 and 25K were carried out to confirm the origin of the broad peak as observed in electrolyte electroreflectance (EER) measurements and check the optical quality of the samples. The energies and broadening parameters of the A and B excitons of the MoS2:X single crystals have been determined accurately.

Influence of rhenium on the structural and optical properties of molybdenum disulfide

We report on the extensive structural and optical studies of Re-doped molybdenum disulfide (MoS2) grown by the chemical vapor transport (CVT) method using Br2 as a transport agent. To evaluate the influence of Re on the structural properties of crystals, we have conducted X-ray diffraction (XRD) and transmission electron microscopy (TEM) experiments. For optical characterization, we carried out piezoreflectance (PzR) and electrolyte electroreflectance (EER) measurements. The Re dopant clearly caused a structural change in MoS2 from a two-layer hexagonal (2H) structure to a three-layer rhombohedral (3R) structure, which has been clearly verified and identified herein.

Composition Dependent Band Gaps of Single Crystal Cu<sub>2</sub>ZnSn(S<sub>x</sub>Se<sub>1-x</sub>)<sub>4</sub> Solid Solutions

Single crystals of Cu2ZnSn(SxSe1-x)4(CZTSSe) solid solutions have deen grown by chemical vapor transport technique using ICl3as a transport agent. Analyzing the X-ray diffraction patterns reveal that the as-grown CZTSSe solid solutions are crystallized in kesterite structure and the lattice parameters are determined. The S contents of the obtained crystals are estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions are studied by electrolyte electroreflectance (EER) techniques. The band gaps of CZTSSe are evaluated by a lineshape fit of the EER spectra and are found to increase almost linearly with the increase of S content.

Influence of anionic substitution on the electrolyte electroreflectance study of band edge transitions in single crystal Cu2ZnSn(SxSe1−x)4 solid solutions

Single crystals of Cu2ZnSn(SxSe1−x)4 (CZTSSe) solid solutions were grown by chemical vapor transport technique using iodine trichloride as a transport agent. As confirmed by X-ray investigations, the as-grown CZTSSe solid solutions are single phase and crystallized in kesterite structure. The lattice parameters of CZTSSe were determined and the S contents of the obtained crystals were estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions were studied by electrolyte electroreflectance (EER) measurements at room temperature. From a detailed lineshape fit of the EER spectra, the band gaps of CZTSSe were determined accurately and were found to decrease almost linearly with the increase of Se content, which agreed well with the recent theoretical first-principle calculations by S. Chen, A. Walsh, J.H. Yang, X.G. Gong, L. Sun, P. X. Yang, J.H. Chu, S.H. Wei, Phys. Rev. B 83 (2011) 125201 (5pp).

Anisotropy of the spectroscopy properties of the wurtz-stannite Cu 2ZnGeS 4 single crystals

In this study, the near band edge anisotropic optical properties of wurtz-stannite (WS) Cu 2ZnGeS 4 single crystals were characterized using polarization-dependent transmittance and electrolyte electroreflectance (EER) techniques. Single crystals of Cu 2ZnGeS 4 were grown by chemical vapor transport method using iodine as a transport agent. Analysis of absorption spectra revealed indirect allowed transitions for Cu 2ZnGeS 4 with the band gaps of 2.02 (2.07) and 2.08 (2.14) eV for E‖ b and Е‖ a polarization configurations at 300 (10) K. The room-temperature EER spectra in the vicinity of the direct band edge showed anisotropic transitions at around 2.38, 2.44 and 2.45 eV for E‖ b, Е‖ a and Е‖ c polarizations, respectively. Based on the experimental observations and recent band-structure calculations a plausible band diagram near band edge of WS-Cu 2ZnGeS 4 was constructed.

Polarization-dependent electrolyte electroreflectance study of Cu 2ZnSiS 4 and Cu 2ZnSiSe 4 single crystals

Polarization-dependent electrolyte electroreflectance (EER) measurements were carried out on the oriented Cu 2ZnSiS 4 and Cu 2ZnSiSe 4 single crystals at room temperature. Thin blade single crystals of Cu 2ZnSiS 4 and Cu 2ZnSiSe 4 were grown by chemical vapor transport technique using iodine as a transport agent. Laue pattern normal to the basal plane of the as-grown crystal revealed the formation of orthorhombic structure with the normal along [2 1 0] and the c axis parallel to the long edge of the crystal platelet. The polarized EER spectra in the vicinity of the direct band edge of Cu 2ZnSiS 4 displayed distinct structures associated with transitions from two upper-most valence bands to the conduction band minimum at Γ point. In the E⊥ c configuration, the feature designated as E A ∼ 3.345 eV was detected and for Е‖ c, only E B ∼ 3.432 eV appeared. For Cu 2ZnSiSe 4, three features denoted as E A, E B, and E C at around 2.348, 2.406 and 2.605 eV, respectively, were recorded for E⊥ c polarization, whereas in the Е‖ c, only E B and E C were the allowed transitions. Based on the experimental observations and a recent band-structure calculation by Chen et al. [Phys. Rev. B 82 (2010) 195203], plausible band structures near the direct band edge of Cu 2ZnSiS 4 and Cu 2ZnSiSe 4 have been proposed.

Detection of Surface Chirality by Electrolyte Electroreflectance Rotational Anisotropy

The ability of electrolyte electroreflectance rotational anisotropy (EERRA) to detect surface chirality is demonstrated by measuring the EERRA of Au(11 7 5)L and Au(11 7 5)D. Chiral surfaces do not...

Growth and characterization of molybdenum-doped rhenium diselenide

Single crystals of molybdenum-doped ReSe 2 have been grown by chemical vapor transport (CVT) process with Br 2 as a transporting agent. XRD has been used to confirm the triclinic-layered structure of the as-grown undoped and doped samples. The optical properties are studied by anisotropic photovoltage and electrolyte electroreflectance measurements at 300 K. Both indirect and direct transition energies of the doped samples all show different degrees of red shifts for molybdenum-doped samples. The broadening parameters of the excitonic transition features broadened significantly for Mo-doped ReSe 2 due to impurity scattering in layered structures. The electrical properties are investigated by the anisotropic conductivity and Hall measurements at 300 K. The influence of optical and electrical properties from the molybdenum dopant were compared and discussed.

Electrodeposition of chalcopyrite films from ionic liquid electrolytes

An air and water stable room-temperature ionic liquid based on choline chloride/urea eutectic mixture has been investigated as a system for the electrodeposition of CuInSe 2 (CIS) and Cu(In,Ga)Se 2 (CIGS) films for photovoltaic applications. Deposition potentials and bath compositions were optimized to obtain Cu–In, Cu–In–Se and Cu–In–Ga–Se precursor films, which were selenized in a tube furnace at 500 °C for 30 min to form CIS and CI(G)S films. Photo-electrochemical measurements on these selenized films showed p-type photoconductivity with band gaps of 1.0 eV and 1.09 eV, respectively, for CIS and CIGS. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), photocurrent spectroscopy and electrolyte electro-reflectance spectroscopy (EER).

Reflectance and electrolyte electroreflectance from gold nanorod arrays embedded in a porous alumina matrix

The specular reflectance and the electrolyte electroreflectance (EER) from an array of Au nanorods embedded within an alumina matrix are reported. The nanorods were fabricated by template electrodeposition of Au into the pores of alumina formed by a two-step anodisation of Al leading to the formation of aligned nanorods with their long axes perpendicular to the surface normal. The reflectance measured as a function of the angle of incidence and polarisation state of the incident light exhibited the transverse plasmon mode with S-polarised light and both longitudinal and transverse plasmon modes with P-polarised light. The EER spectra complemented the specular reflectance data with the dominant features again being the surface plasmon modes of the Au nanoparticles. Models are presented to describe both the reflectance and electroreflectance data that show good agreement with the experimental data.

The electrical and optical anisotropy of rhenium-dopedWSe2 single crystals

Single crystals of rhenium-dopedWSe2 with large edge plane have been grown by the chemical vapour transport method usingbromine as the transporting agent. From the x-ray diffraction patterns, boththe doped and undoped crystals are found to crystallize in the 2H structure.The role of rhenium in affecting both the electrical and optical properties of theWSe2 crystals is examined.The thicker Re-doped WSe2 samples enable easier exploration of the electrical and optical anisotropiesof the materials both parallel and perpendicular to the crystalc-axis. The samples are n-type in nature from Hall measurements andthe conductivity anisotropy decreases drastically as a result of doping.Photovoltage measurements revealed that both undoped and Re-dopedWSe2 are indirect semiconductors. The indirect band gap of the doped sample showed considerable redshift compared with the undoped sample and also exhibited anisotropy along and perpendicular tothe c-axis. The anisotropy in the band gap is attributed to crystal anisotropy, which was alsostudied via the polarization-dependent electrolyte electroreflectance measurements takenalong the van der Waals plane (; ) and the as-grown edge plane (, ; ). The strong dependence of excitonic transitions A and B on polarization compared withthat of the undoped samples.

Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes

We have carried out an electrolyte electroreflectance (EER) study of the adsorption of CO on polycrystalline Ag and Au electrodes. EER proved to be a very sensitive tool for detecting the presence of chemisorbed CO on the surface of the Ag electrode, clearly distinguishing between strong adsorption (chemisorption) and weak adsorption (physisorption). In the case of Au, onto which CO chemisorbs in both alkaline and acid media, EER also detects the presence of this chemisorbed CO. However, while in alkaline medium CO adsorption provokes major changes in the EER spectrum of Au, in acid medium only minor changes are observed, since the CO coverage is 10 times lower than that in the former case. An analysis of the EER spectra of CO-covered Au provided valuable information regarding the surface electronic levels involved in the formation of the chemisorption bond.

Electrolyte electroreflectance study of TiO2 films modified with metal nanoparticles

Electrolyte electroreflectance (EER) has been used as an in situ probe for determining the distribution of energy states at the surface of naked and metal-modified TiO2 film in contact with aqueous solution. The surface modification of polycrystalline TiO2films by electrodeposition of fine (a few nanometers in size) metal particles has been shown to result in the appearance of surface states with an energy of approximately 2.15 eV for Cu, 2.40 eV for Pd, 2.50 and 2.30 eV (two peaks in the EER spectrum) for Pt, and 2.65 eV for Ag (the energy values are measured versus valence band edge of TiO2). The important role of metal-induced surface states in the enhancement of charge exchange between conduction band of semiconductor oxide and metal islets has been demonstrated using the electrocatalytic oxidation of boron hydride as a model reaction.

Electrolyte electroreflectance spectroscopy studies on the interfacial behavior of the near-surface In0.15Ga0.85As/GaAs quantum well electrode|non-aqueous electrolyte

Electrolyte electroreflectance spectra of the near-surface strained-layer In0.15Ga0.85As/GaAs double single-quantum-well electrode have been studied at different biases in non-aqueous solutions of ferrocene and acetylferrocene. The optical transitions, the Franz-Keldysh oscillations (FKOs) and the quantum confined Stark effects (QCSE) of In0.15Ga0.85As/GaAs quantum well electrodes are analyzed. Electric field strengths at the In0.15Ga0.85As/GaAs interface are calculated in both solutions by a fast Fourier transform analysis of FKOs. A dip is exhibited in the electric field strength versus bias (from 0 to 1.2 V) curve in ferrocene solution. A model concerning the interfacial tunneling transfer of electrons is used to explain the behavior of the electric field. (C) 2001 Elsevier Science B.V. All rights reserved.

Electrical and optical anisotropic properties of rhenium-doped molybdenum disulphide

Single crystals of rhenium (Re)-doped MoS 2 have been grown by chemical vapor transport method using bromine as a transport agent. Single crystalline platelets up to 10×10 mm 2 surface area and 2 mm in thickness are obtained. By analyzing the X-ray diffraction profile, the structure of the single crystals shows rhombohedral symmetry. The Hall measurements indicate that the samples are n-type in nature. The thicker Re-doped MoS 2 samples enable easier exploration of the electrical and optical anisotropies of the materials both parallel and perpendicular to the c axis of the crystals. The anisotropy of electrical conductivity perpendicular to and along the c axis for the Re-doped MoS 2 is about 10 2 and is at least one order smaller than 10 3 for the undoped samples. The optical anisotropic property was studied using electrolyte electroreflectance measurements. The excitonic transitions of A and B features parallel to c axis show different degrees of red shift in comparing with the features perpendicular to c axis and with that of the undoped sample. The role of rhenium in affecting both the electrical and optical properties of the MoS 2 crystals is examined.

Anisotropic electrolyte electroreflectance study of rhenium-dopedMoS2

Anisotropy of rhenium-doped MoS2 parallel and perpendicular to thec-axis (k∥c and k⊥c) wasstudied using electrolyte electroreflectance (EER) measurements over anenergy range from 1.75 eV to 4.5 eV. The measurements for k⊥c were made possible by the thicker samples of MoS2available through introducing a small concentration of rhenium duringgrowth. The excitonic transitions A and B for both k⊥cand k∥c configurations showed different degrees ofred shift compared with that of the undoped sample. It is found thatrhenium which is present as an intentionally doped impurity plays animportant role in affecting the observed differences.

Adsorption and electro-oxidation of carbon monoxide, methanol, ethanol and formic acid on osmium electrodeposited on glassy carbon

The activity of Os electrodeposited on glassy carbon (Os/GC) for the electro-oxidation of carbon monoxide, methanol, ethanol and formic acid has been studied. CO chemisorbed on Os/GC in its linear (on-top) form, as determined by in situ Fourier-transform infrared spectroscopy (FTIRS). The typical reflectance increase produced by the chemisorption of CO on thin metal films electrodeposited on glassy carbon was observed. Dissociative chemisorption of the above C1 organic compounds led to linearly chemisorbed CO. The electrolyte electroreflectance spectrum of Os/GC in CO-saturated 1 M HClO4 showed the same maximum at 270 nm already reported for Ru, Rh, Pd, Pt and Au. This maximum was assigned to an electronic transition from the Fermi level of the metal to the antibonding 2π* level of chemisorbed CO.

Electronic structure ofReS2andReSe2from first-principles calculations, photoelectron spectroscopy, and electrolyte electroreflectance

The electronic structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ single crystals are investigated using a first-principles density-of-states (DOS) calculation, ultraviolet photoelectron spectroscopy (UPS), and electrolyte electroreflectance (EER). The total and partial DOS were calculated by the full-potential linearized-augmented-plane-wave method. From the calculations, the main contribution near the band edge of $\mathrm{Re}{X}_{2}$ $(X=\mathrm{S},\mathrm{S}\mathrm{e})$ is determined to be dominated by the nonbonding Re d orbitals. The valence-band DOS is experimentally verified by the UPS measurements. EER measurements were performed in the energy range of 1.3\char21{}6 eV. The EER spectra exhibit sharp derivativelike structures in the vicinity of the band-edge excitonic transitions as well as higher-lying interband transitions. Transition energies are determined accurately. From the experimental and the theoretically calculated results, probable energy-band structures of ${\mathrm{ReS}}_{2}$ and ${\mathrm{ReSe}}_{2}$ are constructed.