Shift Of Valence Band Research Articles

Structural, elastic, mechanical, and electronic properties of chalcogenide perovskite SnZrS3 under pressure

In this paper, we have presented the structural, elastic, mechanical, and electronic properties of the transition metal chalcogenide perovskite SnZrS3 under different pressures by using first-principles method. Our calculated lattice parameters at ambient pressure are in good agreement with the experimental and previous theoretical results. The elastic constants were evaluated numerically for orthorhombic SnZrS3 using the strain-stress approach. Orthorhombic SnZrS3 shows a strong anisotropic behavior of the elastic and structural properties. According to the calculations of the electronic properties, we find the states near the valence band top are derived from S 3p, Zr 4d, Sn 5p, and Sn 5s orbitals, and the lowest conduction band is composed of Zr 4d, S 3p, and Sn 5p orbitals. As the pressure increases, the conduction and valence band shift to lower and higher energies, respectively. These results indicated that lattice constants and band gap decrease with the increase of pressure.

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.

MnOx Co-Catalysts Improved the Photoelectrochemical Properties of Fe2O3

The associated use of limited fossil fuels has also aroused an urgent need to seek for alternative, renewable energy sources. In this regard, hydrogen is the potential and green alternative to replace fossil fuels. Photo-electrochemical (PEC) water splitting towards the production of hydrogen is one of research hotspots, which has received significant attention due to its easy setup and great flexibility for further integrating with energy conversion and storage systems. Hematite (Fe2O3) has been widely utilized in photoelectrochemical (PEC) cells due to its suitable band gap and high electrochemical stability. However, its efficiency is limited by poor charge dynamics and slow reaction kinetics. Surface modification with an amorphous MnOx co-catalyst has been adopted to improve the PEC efficiency of Fe2O3. In this work, the mechanistic origin of introducing amorphous nano-fibrous MnOx as co-catalysts on the PEC performance of Fe2O3 photoanodes has been explored. Introduction of MnOx over Fe2O3 with an optimized thickness results in enhanced efficiency towards PEC water splitting, showing four-fold enhanced photocurrent over bare Fe2O3. The improved PEC performance of Fe2O3/MnOx can be assigned to the synergistic effect between Fe2O3 and MnOx, which led to improved charge dynamics at interface, anodic shift of valence band and increased electrochemical active sites.

DFT computation of rare earth element doped TiO2 anatase: Tunable absorption spectra for water splitting application

The electronic band structure, density of states and optical properties of pure TiO2 and TiO2 doped with different elements (La, Ce, Hf and Pr) have been calculated using the first-principles density general function theory. The calculated results show that the La and Ce doped materials show a negative shift in the conduction band and an insignificant valence band shift. the band energy band gap values of La (Eg = 1.904 eV), Ce (Eg = 1.756 eV), Hf (Eg = 2.005 eV) and Pr (Eg = 1.970 eV) doped TiO2 are smaller than those of undoped TiO2 (Eg = 2.035 eV), among which, Ce doped anatase TiO2 has a narrower band width, smaller band gap value, high redshift and higher photocatalytic activity in the visible region.

Effect of Heat Treatment on Zirconium Oxide High‐k Gate Dielectric in Silicon‐Based Metal Oxide Semiconductor Capacitors

Herein, zirconia thin films are successfully prepared on silicon substrates using the sol–gel method, and the microstructure, optical, and electrical properties of zirconia high‐k gate dielectric films are analyzed at different annealing temperatures. X‐ray diffraction results show that ZrO2 films crystallize above 500 °C and the grain size increases with the increase of annealing temperature; X‐ray photoelectron spectroscopy analysis confirms that metal–oxygen bonds can be effectively formed and defects in the films are eliminated by annealing treatment, with significant changes in the valence band shift (ΔEv) and conduction band shift (ΔEc) with increasing annealing temperature. Analysis of the optical properties of the films by UV‐Vis confirms that all zirconia films have high transmittance, with the bandgap increasing from 5.54 to 5.74 eV with increasing annealing temperature. Atomic force microscope and field emission scanning electron microscope show that the film quality is better and the root mean square roughness of the zirconia films increases from 0.551 to 1.190 nm and the film thickness decreases from 176.19 to 58.73 nm with increasing annealing temperature. Electrical performance tests show that proper annealing is effective in improving electrical properties, such as obtaining a larger dielectric constant (k) and a lower leakage current density.

Mechanistic insights into the origin of MnOx co-catalysts for the improved photoelectrochemical properties of Fe2O3

Fe2O3 is a promising photoanode towards photoelectrochemical (PEC) reactions, but its efficiency is hampered by poor charge transfer dynamics and sluggish surface reaction kinetics. To resolve these issues, surface modification with co-catalysts has been widely adopted. Among different co-catalysts, MnOx has been considered as a particularly appealing candidate for PEC performance optimization of Fe2O3. However, the origin of MnOx co-catalysts has been ambiguous, making its widespread employment stagnating. Herein, the influence of introducing amorphous MnOx co-catalysts on the PEC performance of Fe2O3 has been systematically investigated. Analytical results reveal that the introduced MnOx not only promotes charge transfer dynamics of Fe2O3, induces anodic shift of valence band, but also functions as electrochemical active sites to facilitate surface reaction kinetics. The present study has successfully delivered mechanistic understanding of the function of MnOx co-catalysts, and provided a rational design strategy to maneuver Mn-based co-catalysts for improving PEC performance of semiconductor photoanodes.

Enhancing photothermal VOCs removal in ZnO-based catalysts via transition metal-based band edge tuning and oxygen vacancy engineering

In this study, the band edge tuning and oxygen vacancy optimization of ZnO-based catalysts were explored via transition metal doping for application to photothermal aromatic and halogenated VOCs degradation. Initial minute doping (0.5 wt %) experiments were conducted to determine the dopant with the highest potential, and it was revealed that Zr4+ doping was able to improve chlorobenzene removal to 74.14 % after 4 h of simulated solar light irradiation at 320 K. Further optimization of Zr (Zr0.05ZnOx; 5 % Zr doping) resulted to even greater reduction of toluene and chlorobenzene, from 300 ppmv to 0.4 ppmv (99.9 %) and 52.9 ppmv (82.4 %), respectively. Additional comparative removal experiments also showed the superior performance of Zr0.05ZnOx in contrast to ZnO. The extent of removal highly depended on the functional group attached, with the removal efficiency following the trend: o-xylene ≥ toluene > benzene > bromobenzene > chlorobenzene. Based from the comprehensive analysis of the EPR, XPS, and UV-DRS results, the observed boost in the photocatalytic activity from pristine ZnO can be attributed to 1) the increase in the concentration of oxygen vacancies, resulting in improving the hole-carrier separation, and 2) the valence band shifts associated to the doping of Zr4+ into the ZnO lattice for an enhanced potential difference towards ·OH generation. Comparison with commercially available photocatalysts (TiO2 and ZnO) also revealed that the VOC removal over sol-gel derived Zr0.05ZnOx was much higher, suggesting immense potential for industrial application.

Efficiency of electron doping to monolayer hexagonal boron nitride by alkali metals

We investigated electron doping of monolayer hexagonal boron nitride (hBN) on metallic substrates by doping alkali metals (AMs). The valence band maximum (VBM) of hBN/Rh after doping with Li and Cs was directly observed using angle-resolved photoemission spectroscopy in a wide wavenumber space. The valence band shift resulting from doping confirms the vacuum level alignment model. Furthermore, when the same AM is used, the resultant binding energy of VBM was almost identical regardless of the substrate, even if it differs by ∼1 eV before doping, which we found by comparison with the literature. This independence from the substrate is explained by an extension of the vacuum level alignment model, wherein the VBM is determined by the work function of the AM when it intercalates to the interface as well as adsorbs on surfaces of hBN. It means that the doping of Cs, which has the lowest work function, gives the deepest binding energy at VBM. This is evaluated as 5.7 eV in our experiments.

Fe Substitutions Improve Spectral Response of Bi2WO6-Based Photoanodes

The quest for a scalable solar fuel technology has stimulated a concerted effort to develop a metal oxide semiconductor for solar-driven photoelectrochemical water oxidation (oxygen evolution reaction) in an efficient and durable manner. So far, the search for such a metal oxide photoanode has highlighted the promise of Bi-based oxides, which have also been extensively studied for other photocatalyst applications. Bi2WO6 is a durable photocatalyst whose primary shortcoming is a 2.8 eV band gap that limits utilization of the solar spectrum. Improvements in visible photoresponse upon incorporation of Fe have been reported in the photocatalysis literature, motivating our use of high-throughput synthesis and photoelectrochemistry to determine the spectral photoresponse for Bi–W–Fe oxides synthesized under nonequilibrium conditions based on thermal oxidation of metallic films. Photoactivity down to 2 eV was achieved over a broad range of compositions, with detailed characterization of optimal compositions revealing that Fe incorporation increases the valence band position by 0.75 eV. Density functional theory calculations of Fe substitutions on W sites in Bi2WO6 are consistent with this valence band shift, providing a plausible explanation for the experiments. This Fe-mediated band tuning yields a ca. 2 eV band gap while retaining a turn-on potential for photoanodic current near 0.4 V versus RHE, which combined with the operational durability motivates continuous study and development of this promising class of metal oxide photoanodes.

Influence of self-doping on band-edges and Fermi energy of CsPbBr3

The electronic properties of CsPbBr 3 thin films cast through a solution route are known to depend on the stoichiometry of the precursors. In this work, with the aid of Kelvin probe force microscopy (KPFM) and scanning tunneling spectroscopy (STS) studies, a complete energy landscape has been derived for the all-inorganic perovskite formed with a wide range of precursor stoichiometries. While the optical band gap of the perovskite is known to remain unchanged with a variation in the stoichiometry, we show the manner in which their Fermi energy, valence band (VB), and conduction band (CB) edges shift. The variation in the energy levels has been correlated to the presence of defects. Since the formation energy of different intrinsic point defects depends on the growth environment, the nature of defects has depended on the precursor stoichiometry used during the formation of the perovskites. The presence of defects has thereby tuned the Fermi energy and the type of electronic conductivity in CsPbBr 3 as well. Here, the CB and VB edges have responded due to the self-doping process. Finally, we have presented the energy landscape of the perovskite with a variation in the precursor stoichiometry, since the energy levels are indeed crucial in designing any optoelectronic devices based on CsPbBr 3 . • Fermi level and band edges of CsPbBr 3 are affected by the precursor stoichiometry. • Energy landscape of the perovskite derived from a combination of KPFM and STS. • CsPbBr 3 formed with CsBr-rich precursors has p -type conductivity. • CsPbBr 3 formed with PbBr 2 -rich growth condition exhibits an n -type behavior. • Bands moved away from vacuum level in perovskites formed in PbBr 2 -rich environment.

Molecular Doping of CVD-Graphene Surfaces by Perfluoroalkyl-Substituted Perylene Diimides Derivatives.

Non-covalent π-π and dipolar interactions with small aromatic molecules have been widely demonstrated to be a valid option to tune graphene work functions without adding extrinsic scattering centers for charge carriers. In this work, we investigated the interaction between a CVD-graphene monolayer and a thermally evaporated sub-monolayer and the following few-layer thin films of similar perylene diimide derivatives: PDI8-CN2 and PDIF-CN2. The molecular influence on the graphene work function was estimated by XPS and UPS analysis and by investigating the surface potentials via scanning Kelvin probe force microscopy. The perfluorinated decoration and the steric interaction in the early stages of the film growth determined a positive work function shift as high as 0.7 eV in the case of PDIF-CN2, with respect to the value of 4.41 eV for the intrinsic graphene. Our results unambiguously highlight the absence of valence band shifts in the UPS analysis, indicating the prevalence of dipolar interactions between the graphene surface and the organic species enhanced by the presence of the fluorine-enriched moieties.

Effect of Annealing Temperature on the Structural, Optical, and Electrical Properties of Al-Doped ZrO2 Gate Dielectric Films Treated by the Sol–Gel Method

In this article, we report the preparation of Al-doped ZrO2 (AZO) thin films by the sol–gel method. The electrical properties, microstructure, and optical properties of AZO high-k gate dielectric films at different annealing temperatures were systematically investigated. XRD results confirm that the AZO film was amorphous at a temperature of 600 °C, and Al doping raises the crystallization temperature of ZrO2. AFM and FESEM show a smooth surface of AZO film without cracks. As the annealing temperature increased from 600 to 900 °C, all of the films had similar RMS roughness under 1 nm, the film thickness decreased accordingly, the bandgap value increased from 5.77 to 5.82 eV, the valence band shift (ΔEv) increased, and the conduction band shift (ΔEc) decreased. XPS analysis showed that silicates were formed at the AZO/Si interfacial layer. Electrical performance tests showed that high-temperature annealing can effectively improve the electrical properties, as shown by the increase in dielectric constant (k) and the decrease in flat-band voltage (ΔVfb). However, an increase in leakage current density with increasing annealing temperature was observed due to the decrease in ΔEc and the appearance of crystallization caused by the increase in annealing temperature.

Band alignment of Cs2BX6 double halide perovskites and TiO2 using electron affinity rule

Cs2BX6 double halide perovskites have attracted broad attention in the field of materials science, physics, and optoelectronics. They are promising as potential candidates for light-absorber layers in solar cells and ultrafast scintillators. This is due to their remarkable preferences such as low-cost and simple processability, tunable bandgaps, large-absorption coefficient, high mobility and fast respond under excitation. To provide an initial information for designing multijunction devices made from Cs2BX6, the band alignment of Cs2BX6 is necessary to determine. Thus, we investigated the surface physics of Cs2BX6, where B = Ti, Zr, Hf and X = Cl, Br, I, by using electron affinity rule and extracted all the natural band offsets among Cs2BX6 and TiO2. We found that the band alignment of Cs2BX6 is type-I, whether they were grouped by the same transition-metal or same halide. The positions of conduction band minimum of Zr – and Hf – compounds are relatively close to each other due to the position of Zr – and Hf – d states. The shift in valence band is more influential cause of bandgap variation in Ti – compounds than the change in conduction band. We hope that the band alignment of Cs2BX6 will be benefit not only for the heterostructure design but also for understanding the behavior of tuning bandgaps of alloys among Cs2BX6 either by mixing B or X atoms.

K Improves Cu(In,Ga)Se2 Surface Band Alignment through Reconstruction

K, Rb, and Cs improve Cu(In,Ga)Se2 (CIGS) solar cell performance, but the mechanism remains unclear. Here we use air-free transfer of multiple samples to study KF post-deposition treatments (PDTs) by X-ray photoelectron spectroscopy. The KF PDTs do not change the majority carrier concentration or Cd in-diffusion, but they boost efficiency by 6.1% absolute, improve minority carrier lifetime, and shift the surface valence band further from the Fermi energy. Unlike former reports, the valence-band shift is not a result of lower Cu/(Ga+In) or higher K/(K+Cu) composition ratios. We propose that instead, KF PDTs alter the surface valence-band alignment through a reconstructive phase transformation from chalcopyrite CIGS to K2CuIn3Se6 or KInSe2, which have layered structures. These compounds can leave Cu-free cation planes after K is rinsed away, unlike the structure of chalcopyrite CIGS or CuIn3Se5.

A novel Bi2WO6/Si heterostructure photocatalyst with Fermi level shift in valence band realizes efficient reduction of CO2 under visible light

In this paper, a novel Bi2WO6/Si heterostructure photocatalyst with Fermi level shift in valence band was developed for efficient reduction of CO2 under visible light. The shift of the Fermi level to the valence band can effectively reduce the difficulty of electron holes generation. According to a series of characterizations, it is found that the heterostructure photocatalysts with special branch structure achieve an increase in BET surface area and the photocatalysts’ ability to absorb visible light. In terms of photoelectric performance, the existence of heterojunction in photocatalysts enhances the ability of photo-excited electron-hole pair separation and prolongs the electron lifetime. Among the samples in this paper, WSi1-1 (Bi2WO6:Si = 1:1) reduces CO2 into the only product ethanol, and the ethanol yield reached 402.76 μmol/L. Therefore, the Bi2WO6/Si heterostructure photocatalyst is a low-cost, efficient and promising photocatalyst with broad application prospects.

Temperature dependence of electronic bands in Al/GaN by utilization of invariant deep defect transition energies

We show experimentally that deep point defect levels in GaN, AlN, and AlGaN are constant with respect to the vacuum level and can be used as invariant internal energy references. This offered a convenient and quick way to assess band shifts and impurity levels as a function of temperature via photoluminescence. For AlGaN, we determined that the band shift in the temperature range of 3–600 K occurred primarily in the valence band and that the lowering of the conduction band edge was comparatively small. The valence band shift (as a fraction of the Varshni bandgap shift) in AlGaN varies from ∼70% in AlN to ∼90% in GaN.

Impact of Al-doping on structural, electrical, and optical properties of sol-gel dip coated ZnO:Al thin films

Aluminum-doped ZnO (AZO) thin films were coated on glass substrates using the sol-gel dip-coating technique. We investigated the effect of Al-doping level on the surface morphology, crystal structure, atomic bonding, and optical and electrical properties of the AZO films. The Al-doping levels in the sol-gel solution were 0, 0.25, 0.50, 1.0, 1.5, and 3.0% with a Zn precursor concentration of 0.50 M. The results show that the grain boundary increased with the doping level, while the crystallite size decreased from 27.2 to 14.2 nm. The AZO films were subjected to tensile stress. The Al-doping levels ≤ of 1.5% encouraged the intensity of the (002) x-ray diffraction peak. The center of the (002) peak shifted from 34.46° to 34.51°, and that of the Raman mode shifted from 435.0 to 432.4 cm−1. Doping with 1.5% Al resulted in a maximum electron concentration of 4.7 × 1018 cm−3 with a minimum resistivity of 2.6 × 10−1 Ω cm and a mobility of 5.14 cm2/V.s. The Urbach energy increased from 88 to 120 meV with increasing doping level. For the AZO films doped with Al (≤1.5%), the analysis of the UV–vis spectra reveals that the position of the conduction band (CB) minimum of the films shifted from −0.16 to −0.21 eV and shifted outward from the valence band (VB). Further Al doping to 3.0% resulted in a VB shift toward CB. The optical band gap reached a maximum of 3.33 eV at 1.5% Al. The combination of tensile stress and electron density due to Al doping influences the shift in the optical band gap of the AZO films. The crystal structure, atomic bonding, and electronic band structure of ZnO films can be modified by Al doping.

Quality assessment of GaN epitaxial films: Acidification scenarios based on XPS-and-DFT combined study

X-ray photoelectron spectroscopy (XPS) of electronic structure and surface states of GaN epitaxial films grown on Al2O3 and SiC substrates established the presence of only traces of surface oxidation after three years storage in standard air media conditions. No additional treatment was applied during this storage process. The slight shift (~1 eV) of Valence Band (VB) spectra was found which occurs without any visible contribution of gallium oxide electronic states to the valence band area. Additionally, GaN-typical N 2s band is present in VB-spectra of studied epitaxial films what is confirming their high quality grade. First-principle modelling demonstrates favorability of GaN surface acidification because of humidity and much slower oxidation process in the dry-air conditions. Theoretical models of acidification scenarios also demonstrate that oxidation of defect-free and defective surfaces (N-vacancies) are a good leveler: electronic structures of all considered surfaces became almost identical that is in a perfect agreement with results of XPS measurements and analytics.

Surface oxygenous groups modified graphitic carbon nitride with significant positive shift of valence band for efficient photocatalytic oxidation

Graphitic carbon nitride (g-C3N4) is a remarkable metal-free photocatalyst which is widely used for photocatalytic reductive reaction due to its suitable potential of conduction band, but also suffers from intrinsically weak photocatalytic oxidation ability. Here, by simply refluxing g-C3N4 in H2O2, oxygenous groups are introduced into g-C3N4. The resultant oxygenous groups modified g-C3N4 (g-C3N4(H2O2)) exhibits significantly improved hydrophilic ability and photocatalytic oxidative ability. It is well evidenced that valence band position is positive shifted significantly by introduction of surface oxygenous groups. However, the photocatalytic activity for oxygen evolution is not improved obviously, suggesting that the driving force for water oxidation is not increased simultaneously. Luckily, the photocatalytic oxidative activity of the modified g-C3N4 (include oxygen evolution, pollutant degradation and antibacterial property) could be further improved after forming TiO2-containing nanocomposites during the H2O2 reflux treatment with the addition of tertrabutyl titanate (g-C3N4/TiO2(H2O2) due to much increased driving force and accelerated charge migration. Herein, a facile and simple method is proposed to engineer the band structure of g-C3N4 for improving its photocatalytic oxidative ability, which expand the application of carbon nitride.

Nitrogen-Ion Implantation Induced Bandgap Tailoring in Multifunctional Brownmillerite KBiFe2O5

In the present work, KBiFe2O5 (KBFO) films were implanted with 1 MeV N+-ions at different fluencies i.e., 1015, 1016 and 1017 ions cm−2 and their structural, optical and electronic properties were analyzed. From the X-ray diffraction studies, it is evident that the crystal structure of these films belong to that of brownmillerite. Effective bandgaps of these implanted films, as observed from the UV–visible absorption spectroscopy, are tuned from 1.72 eV to 1.60 eV with increasing the ion fluences. X-ray photoelectron spectroscopy reveals that the valence states of Bi and Fe ions in both pristine and implanted films are in trivalent states and also confirm the presence of nitrogen in the implanted films. A clear shift in valence band (VB) edge position towards lower binding energy in N+-ion implanted films is observed due to the occupation of N 2p states above O 2p states in the VB.