Effects of quantum confinement on the doping limit of semiconductor nanowires

We have calculated the effects of quantum confinement on maximum achievable free carrier concentrations in semiconductor nanowires. Our calculations are based on the amphoteric defect model, which describes the thermodynamic doping limit in semiconductors in terms of the compensation of external dopants by native defects. We find that the generation of amphoteric native defects strongly limits maximum achievable carrier concentrations for nanowires with small widths where quantum confinement is appreciable. The magnitude of this effect in a given material is found to be determined by two material properties: the effective mass of the free carriers, and the position of the conduction (n-type) or valence band (p-type) edge on the absolute energy scale. These results offer a simple, predictive guideline for designing nanostructure devices and contacts where high doping levels are needed.     

Energy band alignment at interfaces of semiconducting oxides: A review of experimental determination using photoelectron spectroscopy and comparison with theoretical predictions by the electron affinity rule, charge neutrality levels, and the common anion rule

The energy band alignment at interfaces of semiconducting oxides is of direct relevance for the electrical function of electronic devices made with such materials. The most important quantities of the interface determined by band alignment are the barrier heights for charge transport given by the Fermi level position at the interface and the band discontinuities. Different models for predicting energy band alignment are available in literature. These include the vacuum level alignment (electron affinity rule), branch point or charge neutrality level alignment governed by induced gap states, and an alignment based on the orbital contributions to the density of states (common anion rule). The energy band alignment at interfaces of conducting oxides, which have been experimentally determined using photoelectron spectroscopy with in situ sample preparation, are presented. The materials considered include transparent conducting oxides like In₂O₃, SnO₂, ZnO, and Cu₂O, dielectric and ferroelectric perovskites like (Ba,Sr)TiO₃ and Pb(Zr,Ti)O₃, and insulators like Al₂O₃. Interface formation with different contact partners including metals, conducting and insulating oxides are addressed. The discussion focuses on the energy band alignment between different oxides. A good estimate of the band alignment is derived by considering the density of states of the materials involved.     

Superior Hydrogen Evolution Reaction Performance in 2H‐MoS2 to that of 1T Phase

In the hydrogen evolution reaction (HER), energy‐level matching is a prerequisite for excellent electrocatalytic activity. Conventional strategies such as chemical doping and the incorporation of defects underscore the complicated process of controlling the doping species and the defect concentration, which obstructs the understanding of the function of band structure in HER catalysis. Accordingly, 2H‐MoS₂ and 1T‐MoS₂ are used to create electrocatalytic nanodevices to address the function of band structure in HER catalysis. Interestingly, it is found that the 2H‐MoS₂ with modulated Fermi level under the application of a vertical electric field exhibits excellent electrocatalytic activity (as evidenced by an overpotential of 74 mV at 10 mA cm^?2 and a Tafel slope of 99 mV per decade), which is superior to 1T‐MoS₂. This unexpected excellent HER performance is ascribed to the fact that electrons are injected into the conduction band under the condition of back‐gate voltage, which leads to the increased Fermi level of 2H‐MoS₂ and a shorter Debye screen length. Hence, the required energy to drive electrons from the electrocatalyst surface to reactant will decrease, which activates the 2H‐MoS₂ thermodynamically.     

Atomic geometry and electronic structure of defects in Zn3N2

Atomic geometry, electronic structure and formation energy of native defects in Zn₃N₂ films have been studied by means of density functional theory to interpret the different behaviors of defective Zn₃N₂. The effects of the vacancy and self-interstitial N on electronic and optical properties of zinc nitride were investigated, from which we conclude that N vacancy is responsible for n-type conduction character. Various defects may cause energy shift or gap change, which explains different optical band gap detected in Zn₃N₂ samples.     

On the origin of blue-green luminescence in spray pyrolysed ZnO thin films

Origin of the well-known blue-green emission of spray pyrolysed ZnO thin films has been discussed on the basis of variation of the properties due to different treatment of the samples such as ion beam irradiation and doping. 120 MeV Au ions and 80 MeV Ni ions were used for ion beam irradiation while indium was used for doping studies. It was assumed that, Au ions might have produced huge amount of defects while the defects might be less in Ni irradiated films due to the difference in energy and mass number. Photoluminescence studies of pristine sample showed only one emission at 517 nm at room temperature while the irradiated films showed a decrease in intensity of this emission. Indium doping also reduced the intensity of this emission. But additional emissions were also observed in these films. Based on the observations, we proposed that the blue-green emission was due to the transition from conduction band to the level due to oxygen antisite (O_Zn).     

Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films

Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiC_x:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiC_x:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV-Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiC_x:H thin films.     

Ellipsometry and optical measurements on amorphous thin films of As2Se3

Thin films of amorphous AsSe_3/2 have been prepared by thermal evaporation of the material under a vacuum of 1.33×10^?3 Pa. Reflectivity, transmission and ellipsometric measurements of the films have been carried out. The optical energy gap and the absorption coefficient as a function of wavelength were obtained. Two absorption bands were observed and interpreted in terms of defects in the AsSe_3/2 system (homopolar bonds). Analysis of reflection and transmission spectra shows that the electron density at band tails of both conduction and valency bands follows N(E)?E^1/2 (Taue plots). No considerable variations were observed on changing the film thickness.     

Tuning the electronic structure of the transparent conducting oxide Cu2O

The electronic structure of Cu₂O is important for its application as a p-type transparent conducting oxide (TCO). To be useful as a TCO, a material needs to show enhanced transparency in the visible range (band gap > 3 eV) as well as good conduction properties. While Cu₂O has too small a band gap, alloys of Cu₂O and Al₂O₃ or Cu₂O and alkaline earth oxides are known to display enhanced transparency, with little degradation of electrical properties. It is of interest to consider how to dope Cu₂O p-type, e.g. Cu vacancies (oxidation) or cationic dopants. We present a study of the electronic structure and effective hole masses of stoichiometric and oxidised Cu₂O and study metal cation doping, using density functional theory (DFT), to analyse p-type doping scenarios. We show that formation of a Cu vacancy is relatively facile, introducing delocalised hole states, with a light hole present. Substitutional cation doping with Al and Au/Ag is found to decrease the band gap but maintains a light hole effective mass necessary for p-type conduction.     

Study of Band Structure Properties of Pnictide LaO1−xF x FeAs (x = 0, 0.2) Superconducting Compound

This paper reports on the band structure properties and changes in band structure of fluorine-doped LaO_1?x F _x FeAs (x = 0, 0.2) compound, measured using X-ray photoemission spectroscopy (XPS). The band structure of the superconducting compound is compared with nonsuperconducting parent compound LaOFeAs. With fluorine doping, a shift of the shallow core level is observed in XPS spectra, which may be a response of the band structure due to fluorine doping in the system. The balance of the chemical potential shift with the screening effect of conduction electrons near the Fe and As ions is discussed using nearly unchanged Fe 2p and As 3d core-level spectra. The La 3d core-level spectra shift towards the high energy, ～0.36 eV, may be due to the chemical potential shift caused by fluorine doping. In our valence band spectra, a small peak at around 0.2 eV is observed, which disappeared with the fluorine doping in the system, indicating a change of Fe 3d state from low spin to high spin states and also confirming the nature of Fe 3d electrons as itinerant, which is responsible for superconductivity in these compounds.     

Influence of substitutional doping on the electronic properties of carbon nanotubes with Stone Wales defects: density functional calculations

Abstract

In this work, we implemented density function theory to investigate the structural and the electronic properties of nitrogen doped single walled carbon nanotube under different orientations of Stone Wales defect. We have found that, the doped defected structures are more stable than the non-doped defected structures. Furthermore, doping defected carbon nanotubes with a nitrogen atom has significantly narrowed the band gap and slightly shifted the Fermi level toward the conduction band. Moreover, nitrogen substitution creates new band levels just above the Fermi level which exemplifies an n-type doping. However, the induced band gap is indirect band gap compared to direct band gap as in pristine carbon nanotubes. Furthermore, the electronic and structural properties of nitrogen doped carbon nanotube with Stone Wales defects is crucially affected by the dopant site as well as the orientations of Stone Wales defects.     

掺杂单层MoS_2电子结构的第一性原理计算

采用第一性原理研究Cu,Ag,Au掺杂单层MoS_2的键长畸变、能带结构和态密度。探讨Cu,Ag,Au掺杂对单层MoS_2电子结构的影响。结果表明:Cu,Ag,Au在S位掺杂的杂质能都低于在Mo位掺杂的杂质能,其在S位掺杂的体系的稳定性强于在Mo位掺杂的体系。在S位掺杂时,杂质与最近邻的Mo,S原子的键长都发生了畸变,畸变率最大的是dAu-Mo,达23.8%。与单层MoS_2的超胞相比,掺杂体系的禁带中出现了4条新能级,导带和价带的能量向低能区移动。杂质原子周围存在着电荷聚集,同时也存在电荷损失。     

Origin of Magnetism from Native Point Defects in ZnO

Based on the first-principle calculations by using the Korringa?CKohn?CRostoker coherent potential approximation (KKR-CPA) method in connection with the local density approximation (LDA), we study theoretically the electronic and magnetic properties of different point defects in ZnO, which are Zinc interstitials (Zn_i), Zinc antisites (Zn_O), Oxygen interstitials (O_i) and Oxygen antisites (O_Zn) defects in ZnO. The supercell calculations were also performed using the full potential local-orbital (FPLO) band structure scheme. This work presents detailed information about total and local density of states at some concentrations of these defects; the stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing calculating their total energy. The results show on one hand that Zn_i and Zn_O produce a shallow donor bellow the bottom of the conduction band (CB), while O_i and O_Zn produces the shallow acceptors above the top of the valence band (VB), and moment magnetic; on other hand that the ferromagnetic state is more stable than the spin-glass in Oxygen interstitials (O_i) and vice versa for oxygen antisites (O_Zn) of native point defects in ZnO. The other native point defects (Zn_i, Zn_O, V_O, and V_Zn) have a zero magnetic moment. The results show that the Curie temperature increases with the concentration of interstitial oxygen.     

Solar photocatalysts from Gd–La codoped TiO2 nanoparticles

Gd–La codoped TiO₂ nanoparticles with diameter of 10 nm were successfully synthesized via a sol–gel method. The photocatalytic activity of the Gd–La codoped TiO₂ nanoparticles evaluated by photodegrading methyl orange has been significantly enhanced compared to that of undoped or Gd or La monodoped TiO₂. Ti⁴⁺ may substitute for La³⁺ and Gd³⁺ in the lattices of rare earth oxides to create abundant oxygen vacancies and surface defects for electron trapping and dye adsorption, accelerating the separation of photogenerated electron–hole pairs and methyl orange photodegradation. The formation of an excitation energy level below the conduction band of TiO₂ from the binding of electrons and oxygen vacancies decreases the excitation energy of Gd–La codoped TiO₂, resulting in versatile solar photocatalysts. The results suggest that Gd–La codoped TiO₂ nanoparticles are promising for future solar photocatalysts.     

Electrical transport mechanisms in a-Se95M5 films (M = Ga, Sb, Bi)

The temperature dependence of direct current (dc) conductivity has been reported in thin films of a-Se₉₅M₅ (where M = Ga, Sb, Bi), in the temperature range 219-375 K, in order to identify the conduction mechanism and to observe the doping effect of different metals on amorphous selenium. It is found that the conduction in high temperature range (314-375 K) is due to thermally activated tunneling of charge carriers in the band tails of localized states; and in the low temperature range (219-314 K) conduction takes place through variable range hopping in the localized states near the Fermi level. Current-voltage (I-V) measurements at high electric fields (the field dependence of dc conductivity) have also been carried out for the samples of present system. The analysis of data shows the existence of space charge limited conduction (SCLC) in these glassy alloys. The density of localized states near the Fermi level is calculated for these alloys using dc conductivity (Mott parameters) and SCLC measurements data. The properties have been found to be highly composition dependent.     

Research Progress on Photocatalytic CO2 Reduction Based on Perovskite Oxides

Photocatalytic CO₂ reduction to valuable fuels is a promising way to alleviate anthropogenic CO₂ emissions and energy crises. Perovskite oxides have attracted widespread attention as photocatalysts for CO₂ reduction by virtue of their high catalytic activity, compositional flexibility, bandgap adjustability, and good stability. In this review, the basic theory of photocatalysis and the mechanism of CO₂ reduction over perovskite oxide are first introduced. Then, perovskite oxides' structures, properties, and preparations are presented. In detail, the research progress on perovskite oxides for photocatalytic CO₂ reduction is discussed from five aspects: as a photocatalyst in its own right, metal cation doping at A and B sites of perovskite oxides, anion doping at O sites of perovskite oxides and oxygen vacancies, loading cocatalyst on perovskite oxides, and constructing heterojunction with other semiconductors. Finally, the development prospects of perovskite oxides for photocatalytic CO₂ reduction are put forward. This article should serve as a useful guide for creating perovskite oxide-based photocatalysts that are more effective and reasonable.     

Defects and electrical conduction in mixed lanthanum transition metal oxides

Lanthanum and transition metal mixed oxides form an interesting series of compounds with a general formula LaMO₃ (with M = Ti, V, Mn, Cr, Co, Ni and Fe). These compounds have been investigated by many workers as regards to their various physical parameters including the electrical conductivity. The new data reported in this paper refer to the Seebeck coefficient (S) in the temperature interval 300 to 1250 K. Electrical conductivity as a function of temperature has also been reported in the same temperature range and this is quite consistent with the values reported in the literature. Using these and other relevant data reported in the literature it has been concluded that native defects are mainly responsible for the electrical conduction in these solids. The nature of defect centres, charge carrier and their electrical transport mechanism is briefly discussed.     

Ab-initio calculations of synergistic chromium–nitrogen codoping effects on the electronic and optical properties of anatase TiO2

Electronic and optical properties of compensated and noncompensated (Cr, N) codoped TiO₂ have been investigated using density functional theory with plane wave basis set and pseudopotential. To investigate the formation of defect pair in the codoped models, defect pair binding energy was calculated. Compensated codoped model has two Cr atoms doped at Ti sites, one N atom at O sites along with an oxygen vacancy that gave stable configuration, better electronic and optical properties. Defect pair binding energy of this model showed that, individual defects would bind each other leading to stable configuration compared to mono-doped models. Band structure results showed that compensated (Cr, N) codoping introduced substantially broaden intermediate states in the forbidden band along with narrowed band gap. Furthermore, the Fermi level was shifted from top of the valence band to middle of the forbidden band describing half metallic character. Cr doping changed the nature of N 2p states from unoccupied to occupied which will improve electron–hole pair separation. Optical properties comparison showed that all doped models effectively shifted the absorption edge of TiO₂ towards visible light. Compensated (Cr, N) codoped TiO₂ has better optical properties and covered wide absorption band in the visible light region, attributed to the stable configuration, narrowed band gap and widely distributed states in the band gap. Our results provide reasonable explanation of the experimental findings.     

Electrical conductivity,Seebeck coefficient,and defect structure of oxygen nonstoichiometric Nd2−xSrxNiO4+δ

To elucidate the electronic state and the conduction mechanism of Nd₂NiO_4+δ series oxides at high temperatures, the electrical conductivity, Seebeck coefficient, and nonstoichiometric oxygen content of Nd_2−xSr_xNiO_4+δ (x = 0, 0.2, 0.4) were measured as a function of the Sr content, temperature, and oxygen partial pressure. The hole mobility is estimated from the electrical conductivity and the hole concentration which is defect chemically determined. The mobility slightly decreases as temperature increases as in metals at high temperatures. The relationships between the Seebeck coefficient, electrical conductivity, and hole concentration can be explained by Mott's equation, which expresses the Seebeck coefficient for metals. Semi-quantitative analyses strongly indicate that the electron or hole is itinerant in Nd_2−xSr_xNiO_4+δ, and the conduction mechanism is metal-like band conduction at high temperatures. Based on the experimental results, schematics for energy level and band structure are proposed. At high temperatures, free holes in the σ_x2−y2 band composed of d_x2−y2 orbitals contribute to metallic conduction.     

Effects of Fe-ion irradiation and annealing on the optical absorption in sapphire

We have studied the optical absorption in sapphire after Fe-ion irradiation and subsequent vacuum heat treatment. The absorption parameters and the nature of the transitions between localized states and allowed bands have been shown to be influenced by substitutional defects, intrinsic radiation-induced defects, and their complexes. We have evaluated the contributions of individual substitutional defects, their clusters, and complexes of substitutional defects with native vacancies and the effect of the iron oxides forming during annealing on the absorption in sapphire. The nature of the substitutional defect clusters and impurity-native defect complexes has been analyzed in detail.     

The study of optical properties of In2O3 and of mixed oxides In2O3−MoO3 system deposited by coevaporation

A discussion of the optical properties of two systems of dielectric films i.e. In₂O₃ and of mixed oxides In₂O₃−MoO₃ system is presented. Film thickness, substrate temperature, annealing and composition (in molar%) have a profound effect on the structure and optical properties of these films. The decrease in optical band gap with the increase in film thickness of In₂O₃ is interpreted in terms of incorporation of oxygen vacancies in the In₂O₃ lattice. The decrease in optical band gap with the increase in substrate temperature and annealing of In₂O₃ thin films is ascribed to the release of trapped electrons by thermal energy or by the outward diffusion of the oxygen-ion vacancies, which are quite mobile even at low temperature. For the mixed oxides In₂O₃−MoO₃ system the results are found to be compatible with the reduction in the value of optical band gap of these materials as the molar fraction of MoO₃ increases in the In₂O₃ thin films and is attributed to the incorporation of Mo(VI) ions in an In₂O₃lattice that causes the indium orbital to become a little less tightly bound. The decrease in optical band gap of mixed oxides In₂O₃−MoO₃ system, with increasing film thickness is interpreted in terms of incorporation of oxygen vacancies in both In₂O₃ and MoO₃ lattice which are also believed to be the source of conduction electrons in In₂O₃–MoO₃ complex. The decrease in optical band gap with increasing substrate temperature and annealing of mixed oxides In₂O₃−MoO₃ system is due to the increasing concentration of oxygen vacancies, formation of indium and molybdenum species of lower oxidation state and indium interstitials. The blue colouration of mixed oxides In₂O₃–MoO₃ samples is due to the inter-electron transfer from oxygen 2p to molybdenum 4d level due to which Mo species of lower oxidation states are formed.