Positions Of Energy Levels Research Articles

Silver loaded anodic aluminum oxide defective photonic crystals and their application for surface enhanced Raman scattering

Here, the fabrication of the Ag nanoparticles loaded one-dimensional anodic aluminum oxide defective photonic crystals (Ag@AAO DPCs) with controllable structure parameters is reported using specially designed periodic pulse anodization and vacuum thermal evaporation techniques. The constructed Ag@AAO DPCs films have ordered distributed nanopore surface, high-quality defect state and tunable photonic band gap, which are ideally suited for the surface-enhanced Raman scatting (SERS) substrates that use local electric field and defect state resonance mechanisms. These sbustrates exhibit excellent SERS performance for rhodamine B (rhB) testing with high sensitivity and reproducibility. In which the logarithmic intensity of Raman signal peak at 1646 cm−1 versus the concentration of rhB presents a linear relationship, and the detection limitation can reach to the order of 10−10 mol/L, the enhancement factor can increase to the order of 105, especially when the defect state energy level position is matched with the incident light. This means that the Ag@AAO DPCs can be used as SERS substrates for the trace detection of organic dye molecules. Compared to the common Ag@AAO substrate, the main reasons for the SERS signal enhancement of the Ag@AAO DPCs are attributed to the synergistic effect of the enhanced local electric field strengthened by the Bragg reflection of PC structure and electron transformation induced by the defect state resonance with the incident light. This would be a promising way for applications for surface enhanced Raman scattering.

Deep insights into interface engineering by buffer layer for efficient perovskite solar cells: a first-principles study

Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells (PSCs). Interface engineering is a promising route for further improving the performance of PSCs. Here we perform first-principles calculations to explore the effect of four candidate buffer materials (MACl, MAI, PbCl2 and PbI2) on the electronic structures of the interface between MAPbI3 absorber and TiO2. We find that MAX (X = Cl, I) as buffer layers will introduce a high electron barrier and enhance the electron-hole recombination. Additionally, MAX does not passivate the surface states well. The conduction band minimum of PbI2 is much lower than that of MAPbI3 absorber, which significantly limits the band bending of the absorber and open-circuit voltage of solar cells. On the other side, suitable bandedge energy level positions, small lattice mismatch with TiO2 surfaces, and excellent surface passivation make PbCl2 a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs. Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer, which shall be useful for improving the performance of PSCs and related optoelectronics.

Multiorbital electronic correlation effects of Co adatoms on graphene: An ionic Hamiltonian approach

In the present work, we propose an ionic Hamiltonian for describing the interaction of graphene with an adsorbed Co atom. In this approach, the electronic correlation effects, related to the many $d$ orbitals involved in the interaction, are taken into account by selecting appropriate electronic configurations of the adsorbed atom. The Hamiltonian parameters are calculated considering the localized and extended features of the atom-surface interacting system. The physical quantities of interest are calculated by using a Green functions formalism, solved by means of the equations of motion method closed up to a second order in the atom-band coupling term. The charge and spin fluctuations in the adsorbed Co atom are inferred from density functional theory calculations and assuming that the lower energy configurations obey Hund's rules. The calculated spectral densities and the occurrence probabilities of the different atomic configurations are analyzed as a function of the Co energy level positions and the surface temperature. In addition, the conductance spectra are calculated by using the Keldysh formalism and compared with existing measurements. We analyze the behavior, under variable bias and gate potentials, of resonancelike features in the conductance spectra which can be related to transitions between atomic configurations of low occurrence probability.

Electrochemical and catalytic assessment of peripheral bromoaryl-substituted manganese and iron porphyrazines

The cyclotetramerization reaction following the Linstead approach led to novel symmetrical magnesium(II) aminoporphyrazines with methyl(6-bromo-3-pyridylmethyl) and methyl(4-bromobenzyl) substituents. These products were applied in the demetallation reaction procedure in trifluoroacetic acid in the dark, which resulted in free-base porphyrazine derivatives. Then these macrocycles were remetallated with iron(II) bromide in DMF to give the desired iron(III) porphyrazine macrocycles. Simultaneously, the cyclotetramerization reaction of maleonitrile derivatives with manganese(II) chloride in n-pentanol and DBU led to symmetrical manganese(III) porphyrazines. New porphyrazine complexes with methyl(6-bromo-3-pyridylmethyl) and methyl(4-bromobenzyl) substituents were thoroughly characterized by the use of various analytical techniques, including electronic spectra, mass spectrometry, and FTIR. Macrocycles were subjected to electrochemical and spectroelectrochemical characterization, accompanied by preliminary catalytic study. The electrochemical properties of all obtained macrocycles were assessed with the use of cyclic and differential pulse voltammetry. The electrochemical activity of new macrocycles and their susceptibility to oxidation/reduction processes depended on the presence of nitrogen substituents in their macrocyclic periphery. The electrochemical measurements provided information on the positions of energy levels (HOMO-LUMO) of each porphyrazine, allowing estimation of the electrochemical energy gaps. The calculated electrochemical energy gaps were higher for porphyrazines with methyl(6-bromo-3-pyridylmethyl)amino peripheral substituents than methyl(4-bromobenzyl)amino ones due to the more electronegative nature of nitrogen amine atom. The calculated electrochemical HOMO-LUMO energy level gap values and the optical band gaps were found to agree within approx. 0.2 eV. Iron(III) and manganese(III) porphyrazines were assessed in terms of their catalytic properties. In these studies, DPBF was employed as a substrate of oxidation processes due to its easy UV–vis assessment. The results revealed the catalytic activity of porphyrazines with peripheral methyl(4-bromobenzyl)amino substituents following first-order kinetics.

Ab initio calculations of pure and Co+2-doped MgF2 crystals

Ab initio calculations of the atomic, electronic and vibrational structure of a pure and Co+2 doped MgF2 crystals were performed and discussed. We demonstrate that Co+2 (3d7) ions substituting for Mg is in the high spin state. In particular, the role of exact non-local exchange is emphasized for a proper reproduction of not only the band gap but also other MgF2 bulk properties. It allows us for reliable estimate of the dopant energy levels position in the band gap, and its comparison with the experimental data. Thus, the present ab initio calculations and experiment data demonstrate that the Co+2 ground state level lies at ≈2 eV above the valence band top.

MoS2 nanosheets anchored on porous ZnSnO3 cubes as an efficient visible-light-driven composite photocatalyst for the degradation of tetracycline and mechanism insight

In this study, MoS2/ZnSnO3 (MS-ZSO) composite photocatalyst with loading MS nanosheets onto the surface of porous ZSO microcubes was synthesized using a simple hydrothermal route. The prepared MS-ZSO composite can be easily excited under visible light, and 3 % MS-ZSO exhibits an outstanding photo-degradation (>80 % in 60 min) and mineralization performance (>42 % in 60 min) of the tetracycline. A remarkable improvement in the photocatalytic activity of MS-ZSO composite derived from a positive synergistic effect of well-matched energy level positions, increasement the absorption of visible light, prolonged life time decay and improved interfacial charge transfer between MS and ZSO. In-depth investigation on charge carrier separation mechanism toward MS/ZSO composite under visible light was proposed, which was further evidenced by capture experiments and electron spin resonance (ESR) techniques. Furthermore, the corresponding intermediates of tetracycline degradation over MS-ZSO composites were inspected by liquid chromatography-mass spectrometry (LC–MS) analysis, and the possible degradation paths were proposed.

Synthesis and Optical and Electrochemical Properties of Novel Random Terpolymers Based on Diketopyrrolopyrrole and Benzodithiazole/Quinoxaline Units for Polymer Solar Cells

Two new binary polymers (P16, P20) and three random terpolymers (P17, P18, P19) with different ratios of the A1 (DPP) and A2 (BTTQ) acceptors in the polymer chains were developed and synthesized. Their optical and electrochemical properties were studied. The random terpolymers (P17, P18, P19) possess wider absorption spectra than the binary D–A copolymers (P16, P20). It was shown that the position of absorption bands and energy levels can be easily tuned by varying the molar ratios of the two different acceptor units in the terpolymer backbone. The synthesized random terpolymers can be used as efficient donor materials for fullerene and non-fullerene polymer solar cells.

Morphology control of zinc oxide nanorods and its application as an electron transport layer in perovskite solar cells

<sec> ZnO is a promising electron transport material. It has not only similar energy level position and physical properties to traditional TiO<sub>2</sub>, but also excellent light transmittance, conductivity, stability, low cost and low temperature preparation. Studies have shown that the one-dimensional nanostructured electron transport layer has a higher electron transport rate, provides a direct electron transport channel and avoids its being recombined at the grain boundaries, thereby improving carrier collection efficiency. It has also been reported that the electron transport rate of ZnO nanorods is significantly better than that of TiO<sub>2</sub>, showing their great potential applications. In perovskite solar cells, the verticality of ZnO nanorods is a key factor affecting device efficiency. The AZO (ZnO∶Al) glass, as an inexpensive transparent conductive substrate, is expected to obtain the best verticality because it has no lattice mismatch with ZnO nanorods. And in the field of perovskite solar cells, the light absorbing layer is usually prepared in a glove box and it has obviously not been industralized. However, there are few reports about perovskite solar cells prepared in atmospheric environment with AZO as substrate and ZnO nanorods as electron transport layer. And it is still much less efficient than the current perovskite solar cells with TiO<sub>2</sub> as the electronic transport layer. It can be seen that further improving the efficiency of the structural battery prepared in the atmospheric environment is an urgent problem to be solved.</sec><sec> In this paper, ZnO nanorods are prepared as an electron transport layer by the hydrothermal method. The effects of hydrothermal temperature, the number of seed layer, the precursor concentration, the substrate type, the hydrothermal time, and the other process parameters on the morphology and crystalline properties of ZnO nanorods are systematically studied, and the growth mechanism is analyzed. The results show that the length of the nanorods is mainly controlled by the hydrothermal time and hydrothermal temperature, and that the radial size is mainly determined by the number of seed layers and the concentration of the precursor solution. And the results also indicate that the verticality of ZnO nanorods’ growth is closely related to the substrate, and that the ZnO nanorods on the AZO substrate have the best growth verticality. On this basis, the perovskite solar cell is prepared in the atmospheric environment, and the optimal efficiency of the photovoltaic device prepared with AZO substrate increases from 7.0% reported in the literature to 9.63%. This is of great significance for enriching the design ideas of perovskite solar cells and further reducing costs. </sec>

On the Characteristic Features of the Impurity Energy Spectrum in Arsenides

The impurity energy spectrum of undoped n-type GaAs, InAs, CdSnAs2, CdGeAs2, and CdTe, ZnO bulk crystals is studied based on a quantitative analysis of the baric and temperature dependences of the kinetic coefficients. It is found that the deep-level donor center corresponds to the intrinsic vacancy defect in the anion sublattice in the above-listed semiconductors. The conclusion on the nature of the donor center, i.e., the vacancy in the anion sublattice, is based on the fact that, in contrast to shallow impurity centers which trace the intrinsic band under uniform pressure, to which they are genetically related, the energy of deep impurity centers with respect to absolute vacuum remains constant under isotropic compression of the lattice. Therefore, it seems favorable to study the evolution of the carrier energy spectrum in semiconductors under uniform-pressure conditions. The energy-level positions with respect to the conduction-band edge and pressure coefficients of the energy gaps between them and the corresponding conduction-band bottom are determined. The shift of the energy level of the deep donor center to the conduction-band depth with decreasing band gap is observed.

Adsorption energy engineering of nickel oxide hybrid nanosheets for high areal capacity flexible lithium-ion batteries

Enriching electrode materials with definite functions is of great influence but highly challenging towards achieving high areal capacity lithium ion batteries (LIBs). Taking transition metal oxides (TMOs) as a case study, several attempts have been employed to demonstrate the large variations in lithium storage performance of TMOs, but explanation of the adsorption capability is rarely reported. Herein, the Li-ion storage chemistry of NiO nanosheets is successfully enhanced by modulating the position of the p-orbital energy level via engineering with porous N-doped carbon fiber and carbon quantum dots (CDs). The as-prepared monolithic NiO hybrid nanosheets (denoted CF/ECF/NiO/CD) exhibit high reversible areal capacity of 3.97 mA h cm−2 at 0.25 mA cm−2, excellent cyclic stability with capacity of 2.91 mA h cm−2 at 3.0 mA cm−2, as well as attractive rate capacity of 2.61 mA h cm−2 at 6.0 mA cm−2. In situ Raman analyses, XPS, and DFT calculations reveal that performance enhancement is related to the electronic modulations between NiO, porous carbon fiber and CDs that triggers the shift of the p-band towards accommodating interfacial electron transfer that helps in promoting the Li storage activity. In addition, an all-flexible lithium ion battery based on CF/ECF/NiO/CD anode is assembled and a volumetric energy density of 619.9 Wh L−1 is achieved (equivalent to an energy density of 201.7 Wh kg−1). This work opens an achievable approach for high-areal-capacity LIBs and provides relevant understanding into designing other LIB electrodes and beyond.

Electronic and Optical Properties of Size-Controlled ZnO Nanoparticles Synthesized by a Facile Chemical Approach

Facile low-temperature chemical route for the synthesis of ZnO nanoparticles is reported in this paper. Morphologically uniform and spherical shape with an average particle size of 8.8 nm and wurtzite phase with the crystalline structure of as-synthesized ZnO nanoparticles were confirmed by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The optical properties of ZnO nanoparticles were analyzed by UltraViolet Visible (UV-Vis) absorption and PhotoLuminescence (PL). The as-synthesized ZnO nanoparticles showed orange light-emitting properties when excited at 400 nm due to the well overlapping of electron and hole wave function across the compatible size of the particle of ZnO and the optical energy band gap of 3.5 eV due to quantum confinement. X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) were used for the elemental, molecular and energetic information of ZnO nanoparticles. UPS analysis depicted the energy level position of ZnO nanoparticles whereas XPS spectra showed the presence of constitute elementals with the stoichiometric atomic % of Zn and O. The elemental composition was also confirmed by the EDS analysis. The significant Raman shifts for as-synthesized ZnO nanoparticles in the typical Raman-active modes of vibration assigned to the wurtzite crystal nanostructure of ZnO.

Electronic energy levels in lyotropic chromonic liquid crystals formed by ionic perylene diimide derivatives

Lyotropic Chromonic Liquid Crystals (LCLCs) represent dispersions of disk-like organic molecules in water that form columnar aggregates bound together by weak hydrophobic interactions and aligned parallel to each other. Dry films of LCLCs preserve the aggregated molecular packing and orientational order. Our idea is that this class of materials is ideally suited for flexible electronics applications since their structure is shaped by relatively weak non-covalent molecular interactions. We report an experimental study that aims to elucidate the influence of ionic substitutes on the energy band of LCLCs based on perylene-diimide derivatives. Using cyclic voltammetry and light absorption techniques, we explore the role of functionalization of several materials of this class with ionic end groups (positive and negative) on the molecular LUMO/HOMO energy level positions. We find that both positively and negatively charged LCLC molecules show decrease in energy positions in comparison with chemically-similar non-mesogenic materials. The experimentally estimated parameters show the deep-lying LUMO energy levels (below −4.0 eV) for all studied LCLC materials with ionic substitutes.

Effect of SnF2 concentration on the optoelectronic and PV cell properties of CsSnBr3

Lead-based halide perovskites have shown a dramatic improvement in photo-conversion efficiencies in the past decade. However, the toxicity of lead (Pb2+) may present barriers or even prevent it from commercialization. On the other hand, Sn2+ is a potential candidate as a replacement of Pb2+, while maintaining the perovskite crystal structure. However, Sn2+ suffers from easy oxidation to Sn4+, resulting in Sn vacancy doping which limits the device efficiencies. It has been generally reported in earlier studies that addition of tin fluoride (SnF2) restricts the oxidation of Sn2+ to Sn4+, resulting in improvement in solar cell efficiencies. We report our findings on the role of SnF2 concentration on the photovoltaic properties and energy level positions of cesium tin bromide (CsSnBr3). The device properties were found to be affected strongly by SnF2 concentration, which could not be explained simply by filling of Sn vacancies by excess tin. Observed changes in CsSnBr3 energy levels also could not explain the improvement. Time-resolved surface photovoltage results are consistent with reduction in trap state density (assumed to be Sn4+) upon SnF2 addition, probably due to the reducing character of SnF2.

CeO2 nanocrystal-modified layered MoS2/g-C3N4 as 0D/2D ternary composite for visible-light photocatalytic hydrogen evolution: Interfacial consecutive multi-step electron transfer and enhanced H2O reactant adsorption

Developing low-cost and high-performance catalysts is significant to solar-to-fuel conversion. Here, the synthesis of zero-dimensional (0D) CeO2 nanocrystal-decorated two-dimensional (2D) layered hybrids of MoS2/g-C3N4 was reported for the first time. In the absence of noble-metal cocatalyst, the optimized ternary CeO2@MoS2/g-C3N4 still manifested high photocatalytic activity toward H2 generation, with a rate of 65.4 μmol/h, which is approximately 8.3 and 17.5-fold greater than g-C3N4 and CeO2, respectively. The corresponding apparent external quantum efficiency reached 10.35% at a wavelength of 420 nm. The superior photocatalytic behavior of CeO2@MoS2/g-C3N4 heterojunction could be ascribed to the positive synergetic effects of well-matched energy-level positions and effective charge separation arose from the multi-step electron transfer processes between Ce4+/Ce3+ reversibility pairs and heterostructures. Furthermore, the adsorption ability of reactant H2O molecules on CeO2@MoS2/g-C3N4 was investigated. Due to the interfacial electronic interaction and Ce3+ species, CeO2@MoS2/g-C3N4 presented more reaction active sites with enhanced adsorption capacity and decreased energy barrier for reactant H2O molecules adsorption, which collaboratively promoted photocatalytic water splitting. This study provides new insights into the rational design of inexpensive ternary photocatalyst with multilevel electron transfer for efficiently converting solar energy into hydrogen without noble metals.

Location and Properties of Carrier Traps in mc‐Si Solar Cells Subjected to Degradation at Elevated Temperatures

Multi‐crystalline Si (mc‐Si) solar cells subjected to carrier‐induced efficiency degradation at elevated temperatures are studied using deep‐level transient spectroscopy (DLTS), capacitance‐voltage, and electroluminescence (EL) measurements. Commercially available passivated emitter and rear cell (PERC) mc‐Si solar cells are investigated after short annealing in the dark (at T = 200 °C for 20 min), after degradation by a constant forward current at 70 °C, and after regeneration annealing at 200 °C for 20 min. The degradation is detected in situ by EL imaging of the surface of the cell. Several n+p mesa‐diodes, selectively fabricated on the front surface of the solar cell, allow the characterization of locations with various levels of degradation and density of extended defects. The degree of the degradation correlates with the local active boron concentration. At all stages, traps associated with extended defects (i.e., dislocations, grain boundaries, precipitates, etc.) are detected by DLTS, and these traps are not affected by changes in the degree of degradation. Two minority‐carrier traps with energy level positions in the bandgap at Ec −0.19 eV and Ec −0.34 eV are detected only in the degraded solar cells in concentrations that correlate with the local degree of cell degradation.

Minority Carrier Trap in n-Type 4H–SiC Schottky Barrier Diodes

We present preliminary results on minority carrier traps in as-grown n-type 4H–SiC Schottky barrier diodes. The minority carrier traps are crucial for charge trapping and recombination processes. In this study, minority carrier traps were investigated by means of minority carrier transient spectroscopy (MCTS) and high-resolution Laplace-MCTS measurements. A single minority carrier trap with its energy level position at Ev + 0.28 eV was detected and assigned to boron-related defects.

Single near-infrared-laser driven Z-scheme photocatalytic H2 evolution on upconversion material@Ag3PO4@black phosphorus

Z-scheme composite is considered as a desirable photocatalyst because of its efficient separation of electron-hole pairs and enhanced redox ability. However, single near-infrared light activated Z-scheme photocatalysts have been rarely reported. In this paper, for the first time, black phosphorus (BP) decorated three-dimensional β-NaYF4:Yb3+, Tm3+@Ag3PO4 (NYF@Ag3PO4@BP) micro-rods are successfully synthesized. Under 980 nm laser light irradiation, NaYF4:Yb3+, Tm3+ up-conversion material transforms it to visible and ultraviolet light, which could be utilized by Ag3PO4 and BP. Due to appropriate energy level positions of Ag3PO4 and BP, a direct Z-scheme photocatalytic system could be constructed. Compared to control samples, NYF@Ag3PO4@BP composites exhibit obviously enhanced photocatalytic activity in hydrogen evolution and their hydrogen production rates are about 10 times higher than that of pure BP. The improved photocatalytic activity is attributed to Z-scheme electron migration mechanism in composite, which is confirmed by surface redox reactions, and electron spin resonance spectra.

Absolute energy level positions in tin- and lead-based halide perovskites

Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

Persistent luminescence from Y3Al2Ga3O12 doped with Ce3+ and Cr3+ after X-ray and blue light irradiation

The persistent luminescence (PersL) dependence on the dopants and derived mechanism of trapping and de-trapping processes were investigated in Y3Al2Ga3O12 (YAGG) based nanophosphor, doped with Ce3+ and/or Cr3+. It is found that the presence of Cr3+ ions produce electron and hole traps and capture suitable charge after X-ray irradiation. The effect of irradiation on the carriers trapping and their pathways after excitation was studied by means of thermo luminescence technique. On the other hand, for blue light irradiation the mechanism seems to be different. In the latter case, the Ce3+ ions, having the position of energy levels in the conduction band, become sensitizers for the electrons and main emission centres for the PersL (de-trapping process goes through Ce3+).

A Three-Terminal Quantum Well Heat Engine with Heat Leakage**Supported by the National Natural Science Foundation of China under Grant No 11365015.

We propose a model for a three-terminal quantum well heat engine with heat leakage. According to the Landauer formula, the expressions for the charge current, the heat current, the power output and the efficiency are derived in the linear-response regime. The curves of the power output and the efficiency versus the positions of energy levels and the bias voltage are plotted by numerical calculation. Moreover, we obtain the maximum power output and the corresponding efficiency, and analyze the influence of the heat leakage factor, the positions of energy levels and the bias voltage on these performance parameters.