Fermi Energy Edge Research Articles

Novel synthesis of BiVO4 using homogeneous precipitation and its enhanced photocatalytic activity

The improvement of the photocatalyst performance with reducing the production cost is still the most important concern for finding target material properties and the possibility of mass production. Herein, we report a novel synthesis of BiVO4 photocatalyst by homogeneous precipitation. Different surfactants such as polyvinylpyrrolidone (PVP), ethylene glycol (EG), cetyltrimethylammonium bromide (CTAB), and sodium dodecylbenzene sulphonate (SDBS) were added to assist the crystal orientation. XRD, SEM, Raman, and DRS were used to evaluate the crystal orientation of the as-synthesized BiVO4 samples. The XRD revealed the purity of the produced samples except for SDBS that produced a mixture (tetragonal /monoclinic) phase. The Raman analysis confirmed that the PVP surfactant has the smallest V–O bond length that led to the lowest bandgap 2.17 eV. The photoelectrochemical results revealed that there is a decrease in the electron-hole recombination rate as shown from electrochemical impedance spectroscopy (EIS) data, as well as swelling of band pinning of the Fermi energy edge toward the negative direction according to the Mott-Schottky (MS) curve. These impacts are due to the different facets possessing truncated bipyramid-like shape which is created via PVP surfactant. These variations enhanced the photodegradation of methylene blue (MB) dye as a wastewater module.

Study of valence band electronic states of near-surface atoms of niobium used for superconducting cavity

The results of investigations conducted on the valence band structure of the near-surface atoms of niobium (Nb) sample, taken from a sheet that is used for fabricating superconducting radio frequency (SRF) cavity, are presented. The valence band electronic states were excited using low energy (≤50 eV) ultraviolet radiation. It was observed that a) the valence band spectrum primarily consists of oxygen 2p-derived states, b) a typical feature expected near the Fermi edge in the valence band of Nb is conspicuously absent. Analysis of the wide band profile provides a number of electronic states peaked at ∼6.3, 7.6, 9.2, and 10.9 eV. After sputtering the surface, a small peak appeared at ∼1.4 eV near the Fermi energy edge. The Nb 4d states showed a resonance involving intra-atomic transition and also hybridization with the states of O 2p. The present study will be useful in identification of the electronic states of Nb that are accountable for superconductivity related issues in an SRF cavity.

Sputtering Effect on Amorphous Ga–In–Zn–O Thin-Film Surface: Occurrence of Subgap and Metallic States

The chemical states of Ne+-ion-sputtered amorphous Ga-In-Zn-O (a-GIZO) thin films were investigated by high-resolution X-ray photoelectron spectroscopy. The sputtering reduced the Zn and In contents relative to that of Ga and generated a subgap state above the valence band maximum. Further sputtering resulted in metallic states at the In3d and In4d orbitals and at the Fermi energy edge, more so for the lower Zn- and In-content film. Locally generated metallic In is suggested to contribute to the metallic states.

Temperature-dependent contactless electroreflectance and photoluminescence study of GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures

Temperature-dependent contactless electroreflectance (CER) and photoluminescence (PL) measurements in the range of 30 K<T<300 K on two GaAlAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with different well widths fabricated by molecular-beam epitaxy on (100) GaAs substrates are presented. For the CER measurement, the 11H transition is completely screened out by the two-dimensional electron gas and the prominent feature related to the Fermi energy edge singularity transition showed a Stokes shift to higher energy with respect to the 21H transition of the PL measurements at low temperature. From the Stokes shifts, the Fermi energy level of the system is evaluated, and hence, the density of the two-dimensional electron gas. The temperature-dependent PL measurements revealed two features, identified to be the 11H and 21H transitions. The relative intensities of the 11H and 21H transitions were analyzed taking into account the effect of the subband filling and the wave-function overlap factors. A good agreement is found between experimental data and theoretical calculation results.

Use of a narrow-gap prewell for the optical study of charge buildup and the Fermi-energy edge singularity in a double-barrier resonant-tunneling structure.

A strained ${\mathrm{In}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As layer is incorporated adjacent to the emitter barrier of an ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As double-barrier resonant-tunneling structure (DBRTS), so that it forms a prewell for the electrons that accumulate prior to tunneling into the GaAs quantum well (QW). Observation of photoluminescence (PL) and photoluminescence excitation (PLE) from the prewell then enables a direct optical determination of the charging behavior in the emitter-accumulation region to be achieved. We show in addition that an optical probe of the prewell can, by consideration of the electrostatics, provide a reliable determination of the charge distribution in the whole DBRTS at the peak of the tunneling resonance. These results are shown to be in agreement with separate determinations of the charge in the GaAs QW by direct PL measurements, and of the charge in the emitter-accumulation layer from magnetotransport studies. The electron density in the prewell can be varied continuously over a wide range, from 0 to 9\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The second subband is populated at high applied bias, with the density varying from 0 to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. This structure is then very well suited to the study of the many-body nature of the excitonic enhancement near the absorption threshold, over a much wider range of electron densities than previously studied on an individual sample. Utilizing temperature-dependent PLE measurements, we have monitored the variation from an atomic exciton in n=1, to a Fermi-energy edge singularity (FEES) in n=1, through to a FEES in n=2, in the same sample.

Fermi-energy edge singularity and excitonic enhancement associated with the second subband in asymmetric modulation-doped quantum wells.

A study of the Fermi-energy edge singularity (FEES) in the second (n=2) subband of asymmetric modulation-doped (AlGa)As-(InGa)As-GaAs quantum wells (AMDQW's) is reported. In one of the AMDQW's a Schottky gate is employed to vary the electron density in the n=2 subband (${\mathit{n}}_{\mathit{s},2}$) from 0 to 1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Temperature-dependent photoluminescence excitation (PLE) measurements clearly show that the n=2 PLE feature has FEES character for ${\mathit{n}}_{\mathit{s},2}$\ensuremath{\gtrsim}0.4\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. In contrast to PLE, photoluminescence (PL) intensity is not a true measure of oscillator strength, since PL intensity can be affected by competing recombination pathways. Temperature-dependent PL measurements have been performed on two types of AMDQW. One type has ${\mathit{n}}_{\mathit{s},2}$\ensuremath{\sim}0, with the Fermi energy close to the n=2 subband energy. The other type has ${\mathit{n}}_{\mathit{s},2}$=1\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ and a FEES associated with n=2 observed in PLE. We demonstrate that the very similar broadening and reduction in peak height of the n=2 PL peak with temperature for the two types of samples can be accounted for in terms of spreading of the electron or exciton populations near the n=2 subband edge. Therefore, we conclude that temperature-dependent PL does not provide unequivocal evidence for a many-body enhancement of the n=2 PL transition, in contrast to that reported by Chen et al. [Phys. Rev. Lett. 64, 2434 (1990)].

Carrier density dependent effects on the Fermi energy edge singularity in quantum wells containing two occupied subbands

Optical transitions into the second electronic subband of a wide modulation doped quantum well (QW) are shown to exhibit behaviour characteristic of a Fermi edge singularity for upper subband densities ⪆ 0.4 × 10 11 cm −2. Variations in oscillator strength of such transitions with lower subband Landau level filling are demonstrated to be small.

Many-electron edge singularity at the Fermi cutoff in the photoluminescence spectrum of modulation-doped quantum wells

We have studied the low-temperature photoluminescence (PL) of n-type modulation doped In 0.53 Ga 0.47 As InP quantum wells. The photocreated holes are localised and can recombine with any of the electrons. The PL lineshape is strongly skewed to higher energies, on account of the many-body Fermi energy edge singularity. This interpretation is supported by calculations of the lineshape that include the screened electron-hole interaction, and by the temperature-dependence of the spectrum.

Fermi energy edge singularity in luminescence spectra from modulation-doped quantum wells

The observation of a Fermi energy edge singularity in the luminescence spectra of InGaAs/InP quantum wells is discussed. The importance of hole localization for the observation of the phenomenon is emphasized from the contrast between spectra obtained from samples with either strong or weak hole localization.

Observation of a many-body edge singularity in quantum well luminescence spectra.

The observation of a many-body, Fermi-energy edge singularity in the low-temperature photoluminescence spectra of InGaAs-InP quantum wells is reported. Strong enhancement of the photoluminescence intensity towards the electron Fermi energy (${E}_{\mathrm{F}}^{e}$) is observed, due to multiple electron-hole scattering processes to states above ${E}_{\mathrm{F}}^{e}$. Recombination of electrons in states up to ${E}_{\mathrm{F}}^{e}$ is allowed by hole localization. The many-body processes are analogous to the core-hole phenomena in the soft-x-ray emission spectra of metals.