Shift Of Valence Band Edge Research Articles

Post-Ammonia-Treated V2CTx MXene at Different Pressures: Effects on Morphology, Electronic, and Optical Properties

In the present work, vanadium carbide (V2CTx) MXene was synthesized successfully and then treated with an ammonia solution (NH4OH) under different pressures using a hydrothermal process. The sample morphology reveals that the V2CTx MXene gradually changed from uneven multilayered micrometre-size sheets to uniform few-layered nanometer-size sheets as the pressure increased. The optical emission spectra show broad emission and a blue shift due to nitrogen-related defects and quantum confinement. X-ray photoelectron spectroscopy shows the presence of nitrogen in all of the samples. The ultraviolet photoelectron spectroscopy study shows a modified density of states near the Fermi level and a maximum of ∼0.6 eV valence band edge shift towards a higher binding energy side after post-treatment with ammonia. More interestingly, the E1g Raman band becomes degenerate and the E1g band upshifts (∼15 cm–1) with increasing pressure treatment. The Raman results suggest that the induced lattice strain due to post-ammonia treatment plays a crucial role in deciding the optical and electronic properties.

Dual-defects induced band edge reconstruction of tin dioxide via cobalt and nitrogen Co-Doping for wearable supercapacitor application

Dual-defects induced band edge reconstruction of tin dioxide (SnO2) via metal and non-metal ions co-doping strategy is applied to synthesize cobalt and nitrogen co-doped tin dioxide (Co, N-SnO2) on activated carbon fiber (ACF) for supercapacitor. N-doping contributes to upward shift of valence band edge through the hybridization of N-2p and O-2p and Co-doping contributes to downward shift of conduction band edge through the hybridization of Co-3d and O-2p. The bandgap is highly narrowed from 2.29 eV for SnO2 to 0.68 eV for Co, N-SnO2. The Co, N-SnO2/ACF electrode achieves specific capacitance of 361.2 F g−1 at 1 A g−1, rate capability of 61.1% from 1 to 10 A g−1 and cycling stability of 103.3% for 10,000 cycles. The density functional theory proves that Co-doping mainly promotes the capacity due to its easier deprotonation process in acid electrolyte, whereas N-doping mainly promotes the rate capability due to its narrower bandgap and thus facile electron transport. The wearable bracelet-supercapacitor based on Co, N-SnO2/ACF electrode delivers an energy density of 70.7 Wh Kg−1 under a wide potential window of 2.0 V. This finding provides a feasible route to regulate the band edge of metal oxide electrode for the promising energy storage.

The effect of strain and spatial Bi distribution on the band alignment of GaAsBi single quantum well structure

The band line-up and band offset calculations of GaAs0.978Bi0.022/GaAs single quantum well with spatial changes of Bi composition were reported. The spatial Bi profile and a certain amount of the Bi composition in the barrier layer were determined by HR-XRD measurements. Virtual Crystal Approximation and Valence Band Anti-Crossing models were used including strain effects to obtain conduction and valence band edge shifts with Bi incorporation. Photoluminescence (PL) measurements were performed at a low temperature of 8 K as a function of excitation intensity. The PL spectra have shown asymmetric line shapes, which were fitted with different Gaussian functions. Comparing experimental PL results with calculated band edge energies, it was found that optical transition is a type I under low intensity excitation while the optical transition is switched from type I to type II due to the spatial changes in Bi concentrations. The band offsetsΔEc/ΔEv were also determined.

Revisiting La2MMnO6 (M = Co, Ni, Cu, Zn) perovskites in view of 3d-electron configuration

We investigate the impact of 3d-electron configuration on the magnetic and DC conductive properties of La2MMnO6 (M = Co, Ni, Cu, Zn) and offer evidence to an enhanced ionic ordering degree with the increase in the 3d electron number of an M ion. A key role of the 3d electronic configuration that influences the B-site ionic arrangement is revealed by changing both the ionic radii and bond covalency. The position of the valence band edge is shifted away from the Fermi level with the increase in 3d electrons, which reveals a strengthened electron localization behavior. Magnetic characterization indicates that the Co, Ni, and Zn samples present magnetic behaviors with half- and full-filled eg configurations, whereas the Cu sample shows a special Jahn-Teller configuration (t2g6eg3), giving rise to the degenerated eg states and significant change in bond parameters. Consequently, the Goodenough-Kanamori rules are not followed and the ferromagnetic coupling is weakened greatly. Further, DC resistivity measurements of samples M = Co, Ni, and Zn present thermally activated conductive behavior, in good accordance with the position shift of valence band edge. However, due to the Jahn-Teller effect arising from Cu2+ ion, a strong electron-phonon coupling is evidenced. This effect provides a suitable hopping channel for small polarons resulting in a considerable rise of its conductivity. Such changes in magnetization and DC conductivity of La2MMnO6 perovskites by modulating the M elements would inspire us to develop novel spin-electronic devices.

One step synthesis of oxygen doped porous graphitic carbon nitride with remarkable improvement of photo-oxidation activity: Role of oxygen on visible light photocatalytic activity

A novel metal-free oxygen doped porous graphitic carbon nitride (OA-g-C3N4) was synthesized by condensation of oxalic acid and urea. The 40% OA-g-C3N4 catalyst can degrade bisphenol A (15mgL−1) in 240min with a mineralization rate as high as 56%. The markedly higher visible-light-driven oxidation activity of OA-g-C3N4 is attributed to the porous morphology and unique electrical structure. The porous structure of OA-g-C3N4 provides more active sites for adsorption and degradation of pollutants. Moreover, oxygen atoms in the tri-s-triazine units help to extend sufficient light absorption range up to 700nm, improve the separation of charge-carriers and alter the position of valence band (VB) and conduction band (CB). The VB edge shifts from 1.95eV to 2.46eV due to the incorporation of O atoms, which leads to the change of active species in the photocatalytic reaction. Trapping experiment shows that superoxide radicals play the major role in the photocatalytic degradation of BPA on g-C3N4, while hydroxyl radical is the dominant active species in the photocatalytic degradation process over 40% OA-g-C3N4. This study presents a simple, economical and environment-friendly method to synthesized oxygen doped porous graphitic carbon nitride.

Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2

Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and μ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green’s function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2.

Valence Band Edge Shifts and Charge-transfer Dynamics in Li-Doped NiO Based p-type DSSCs

ABSTRACT The objective of this work is to gain deeper insight into behavior of p-type DSSCs, paying special attention to photoelectric properties of Li-doped NiO photoelectrode, and studying their effect on interfacial photoelectrochemical processes under illumination and device performance. Here we find that Li-doped NiO photoelectrodes show higher charge carrier densities, down-shift of valence band and narrowing trap energy distribution. Furthermore, the performance enhancement of p-type DSSCs is observed for the Li-doped NiO photoelectrodes with respect to the unmodified ones. These improvement is found to arise from more rapid hole transport combined with reduced recombination, contributing to improved hole collection. With regard to interfacial carriers transfer process, electrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMPS) and ultra-violet photoemission spectra (UPS) provide evidence that the decreased recombination of Li-doped NiO DSSCs is attributed to the downward shift of valence band, which increases energy barrier of interface recombination. Another reason results in narrowing trap energy distribution of Li-doped NiO photoelectrode, which can weaken energy overlap with the LUMO level of dye, and subsequently the recombination path of hole back transfer to dye is hindered. These findings suggest that it will be important to carefully optimize band edge alignment and trap states distribution of p-type oxide semiconductors film to further improve the efficiency of p-DSSCs.

Photoinduced changes in amorphous selenium

Photoinduced phenomena in thin films of amorphous selenium (a-Se) have been a subject of intensive researches so far. Thin films of a-Se were deposited on corning glass by thermal evaporation, with several thicknesses. The influence of light exposure, with different colors (blue or red or natural white light), on the optical and structural properties of a-Se thin films was carried out by using ultraviolet–visible transmittance spectroscopy. It was found that the gap of the samples illuminated with blue and natural white light at room temperature shifts to lower energies. This photodarkening is stable at room temperature and is irreversible even after several days. On the other hand the photodarkening in the samples illuminated with red light is much smaller than that for samples illuminated with blue and natural white light. The photodarkening is accompanied by an increase in the refractive index. These results are discussed with results, previously obtained, about the photoinduced changes of the negative-U centers in, T– and T+, studied using time-of-flight (TOF) traces recorded on samples prepared in the same conditions. These TOF measurements show that the defect level T– at 0.4 eV above the valence band edge shifts to 0.5 eV under illumination with white light at room temperature. This effect is not seen if the illumination is done at 35 °C. These phenomena are attributed to nanocrystallization in the a-Se films.

Size-Dependent Valence and Conduction Band-Edge Energies of Semiconductor Nanocrystals

Through the use of photoelectron spectroscopy in air (PESA), we investigate the size-dependent valence and conduction band-edge energies of CdSe, CdTe, PbS, and PbSe semiconductor quantum dots (QDs). The results are compared to those of previous studies, based on differing experimental methods, and to theoretical calculations based on k·p theory and state-of-the-art atomistic semiempirical pseudopotential modeling. To accurately map out the energy level landscapes of QDs as a function of size, the QDs must be passivated by comparable surface chemistries. This is highlighted by studying the effect of surface chemistry on the valence band-edge energy in an ensemble of 4.7 nm CdSe QDs. An energy level shift as large as 0.35 eV is observed for this system through modification of surface chemistry alone. This shift is significantly larger than the size-dependent valence band-edge shift that is observed when comparable surface chemistries are used.

Band structure of Si/Ge core–shell nanowires along the [110] direction modulated byexternal uniaxial strain

Strain modulated electronic properties of Si/Ge core–shell nanowires along the [110] directionwere reported, on the basis of first principles density-functional theory calculations. In particular,the energy dispersion relationship of the conduction/valence band was explored in detail. At theΓ point, the energy levels of both bands are significantly altered by applied uniaxialstrain, which results in an evident change of the band gap. In contrast, for theK vectors faraway from Γ, the variation of the conduction/valence band with strain ismuch reduced. In addition, with a sufficient tensile strain (∼1%), the valence bandedge shifts away from Γ, which indicates that the band gap of the Si/Ge core–shell nanowires experiences atransition from direct to indirect. Our studies further showed that effective masses ofcharge carriers can also be tuned using the external uniaxial strain. The effective mass ofthe hole increases dramatically with tensile strain, while strain shows a minimal effect ontuning the effective mass of the electron. Finally, the relation between strain and theconduction/valence band edge is discussed thoroughly in terms of site-projectedwavefunction characters.

Sputtering-induced Co formation in x-ray photoelectron spectroscopy of nanocrystalline Zn1-xCoxO spinodal enrichment models

Nanoscale enrichments resulting from spinodal decomposition have been proposed to contribute to the interesting magnetic properties of diluted magnetic oxides such as cobalt-doped ZnO (Zn1-xCoxO), but little is known experimentally about the electronic structures or physical properties of such enrichments. Here, x-ray photoelectron spectroscopy (XPS) is used to examine wurtzite Zn1-xCoxO crystallites over the full composition range (0.0≤x≤1.0) that serve as models of the proposed spinodal decomposition nanostructures within Zn1-xCoxO bulk materials. With increasing x, the valence band edge shifts to smaller binding energies and the cobalt 2p peaks shift to greater binding energies, providing spectroscopic signatures that may allow identification of spinodal decomposition in bulk Zn1-xCoxO. Reduction of Co2+ to Co0 by argon ion (Ar+) sputtering was also found to become markedly more facile with increasing x, suggesting that locally-enriched Zn1-xCoxO is at greater risk of yielding false-positive Co0 XPS signals than uniformly dilute Zn1-xCoxO with the same overall composition.

Local disorder influence on electrical phenomena of pure and Ba‐doped Pb5Ge3O11 crystals

AbstractThe relation between chemical composition, crystal structure and electrical features of lead germanate, (Pb5–x Bax)Ge3O11, pure and doped with Ba (x = 0, 0.10, 0.25) was studied. The O deficiency and GeO2 precipitation was found in the crystals studied. In the case of the Ba‐doped crystals, the XPS lines width (FWHM) variation suggests structural local disorder. The valence band is placed near 3 eV and it shows an exponential tail, which is ascribed to existence of energy levels correlated to defects. Both the valence band edge shift and the degree of disorder deduced from the change in the XPS lines width vary non‐linearly with the concentration of the Ba ions. The ac conductivity activation energy values Ea = 0.55–0.85 eV indicate electric conductivity of lead germanate is determined by local energy levels originated from point defects. The analysis of the imaginary part of the electric modulus M ″(T, f) exhibited the electronic relaxation process related to electric conductivity, for which the activation energy EM = 0.63–0.72 eV, and the characteristic times τ0 of the order of 10–13 s were evaluated. The crystal structure parameters and several of the electrical conduction parameters exhibit non‐monotonic dependence on the Ba concentration. It was deduced that Ba ions contributed to the mentioned phenomena by two opposite mechanisms. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal stability of nitrogen-doped SrTiO3 films: Electronic and optical properties studies

The thermal stability of nitrogen-doped SrTiO3 (001) films in terms of electronic and optical properties has been studied by using x-ray photoemission spectroscopy, x-ray absorption spectroscopy, and spectroscopic ellipsometry techniques. The chemical states of nitrogen in nitrogen-doped SrTiO3 films include both substitutional and interstitial states. The N 2p states localized above the O 2p-derived valence band maximum are attributed to the change of optical properties. Postannealing will induce the valence band edge shift due to the thermal instability of interstitial N states, which may degrade the photocatalysis of nitrogen-doped SrTiO3 films during applications.

Large electro-optic effect in tensile strained Ge-on-Si films

The authors report the first observation of a large, strain-enhanced, electro-optic effect in the weakly absorbing regime for Ge epitaxial films grown directly on Si substrates. The field dependence of absorption in the Ge films was measured from spectral responsivity measurements of Ge-on-Si p-i-n diodes. The experimental data were analyzed using the generalized Franz-Keldysh formalism [H. Shen and F. H. Pollak, Phys. Ref. B 42, 7097 (1990)] and the valence band edge shifts of the light- and heavy-hole energy positions were in response to biaxial stress. With measured Δα∕α∼3 and derived Δn∕F=280pm∕V, the material has significant potential for field-induced phase or electroabsorption modulator devices.

Nonionic Water-Soluble Polysilynes. Synthesis and Properties of a Novel Class of Functionalized Materials

The nonionic water-soluble polysilynes poly(4,7,10-trioxaundecylsilyne) (1) and poly(4,7,-10,13-tetraoxatetradecylsilyne) (2), which are inaccessible using the conventional Wurtz-type coupling with Na in refluxing toluene, have been prepared in reasonable yields using graphite potassium C 8 K as the reducing agent in THF at 0 °C. A water-insoluble analogue of 1, viz. poly(4,7,10-trioxahexadecylsilyne) (3), is obtained in nearly quantitative yield under similar conditions. Despite the fact that 1 and 2 possess all the characteristic polysilyne-like (photo)physical properties, aqueous solutions of 1 and 2 unexpectedly exhibit thermoresponsive behavior; i.e., at 49 °C a lower critical solution temperature (LCST) is found. The presence of an LCST, which has to originate from folding/unfolding processes of the polysilyne backbone, suggests that polysilynes have a hybrid structure with a predominantly one-dimensional overall appearance consisting of linear fragments with small branches and/or incorporated (branched) cyclics, instead of the previously proposed extended sheetlike and/or hyperbranched/dendritic structures. Additional support for a hybrid structure was given by semiempirical PM3 calculations on a variety of oligomeric model compounds. The PM3 results suggest that Si-Cl moieties incorporated in oligomers will be more reactive than monomeric Si-Cl groups. The calculations further indicate that linear chain extension is preferred over branching. Cyclic voltammetry in combination with absorption/excitation spectroscopy reveals that in going from the related polysilane to the polysilyne the valence band edge shifts ca. -0.7 V, while the conduction band edge remains virtually unchanged. Furthermore, it is demonstrated that polysilynes 1 and 2 are effective photoinitiators for radical polymerizations upon excitation at λ 400 nm. This is exemplified for the conversion of methylacrylate into poly(methylacrylate).

Semiconducting rhenium silicide thin films on Si(111)

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 Å) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x-ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 Å) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 Å) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600 °C. In the Re-Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2p and Re 4f) are the same in the compound and in the elements. As the energy shift of the Si KLL Auger is negligible also, the extra-atomic relaxation energy for Si atoms is the same in silicide as in silicon, indicating that ReSi2 has a semiconducting character. UPS results confirm this assumption: the density of states near EF decreases strongly upon ReSi2 formation and at the same time the valence band edge shifts from EF to lower binding energy. GIXD gives sharp diffraction peaks, characteristic of ReSi2 (110) in epitaxy on Si(111). This technique also reveals that the films present an additional orientation near the interface. Optical absorption measurements performed on ReSi2 films of ∼300 Å thick indicate that this silicide is a semiconductor with an indirect energy gap of 0.15 eV, in agreement with previous studies.

Conductivity Phase Transition with Shift of Valence-Band Edge and with Structural Distortion in Cu4Sn1-xGexS4

X-ray diffraction, electrical resistivity, DSC and XPS investigations were carried out on Cu 4 Sn 1- x Ge x S 4 (0≤ x ≤1.0). At room temperature, the orthorhombic structure changes to a monoclinic one at a transition concentration of x t =0.6 as x increases. This transition also exhibits the expansions in the energy width between the top of the valence band and the Fermi level and in the lattice parameter B . The transition has the characteristics of the first-order high (high temperature)-to-low (low temperature) conductivity phase transition as reported on Cu 4 SnS 4 . The transition temperature T t linearly increases with x . These features are briefly compared with the electronic and structural phase transition reported on SnTe and Pb 1- x Ge x Te.