Electronic States In Band Gap Research Articles

Influence of S vacancy and O doping in MoS2/GaN heterostructure on charge carrier dynamics: A time-domain ab initio study

Van der Waals heterostructure MoS2/GaN is proposed as a promising candidate material for optoelectronic and photovoltaic applications, and we have studied the influence of S vacancy and O doping on charge carrier dynamics in MoS2/GaN, by means of ab initio nonadiabatic molecular dynamics simulations. It is found that these defects have negligible effect on charge carrier separation, and electron transfer is faster than hole transfer, owing to the stronger nonadiabatic coupling, originating from the more intermediate states and out-of-plane high frequency A1g mode. The presence of S vacancy in MoS2/GaN significantly accelerates the electron-hole recombination by introducing additional electronic states in band gap. It is notable that doping O atom in S vacancy in MoS2 layer induces stronger out-of-plane N atom vibration mode at 250 cm−1, resulting in shorter coherence between CBM and VBM states, which prolongs the carrier lifetime compared to that of the pristine MoS2/GaN.

An overview on the use of adsorption and reactions of NO to probe (photocatalytic) TiO2

Abstract Studies of the adsorption and reactions of appropriate molecules are a valuable means to probe active surfaces. This overview deals with the use of nitrogen monoxide – a molecule that has less been utilized than some others as a surface probe – to investigate TiO2 and TiO2-containing materials, with or without photo-excitation. It shows that diverse information can be derived from: (1) the infrared spectra of adsorbed NO; (2) the isotopic exchange of N18O; (3) the formation of N2O and N2; (4) the use of NO (or N18O) in oxidation reactions in place of O2; and (5) the evaluation of the electron transfer to NO through density functional theory calculations, ultraviolet photoemission spectroscopy, and photoconductance measurements. Valuable knowledge may thus be potentially acquired on: the accessibility and reactivity of surface OH groups, the lability of surface O atoms, the self-cleaning efficacy, the environment and dispersion of TiO2 in composite materials, the existence of bandgap electronic states, and the electron transfer capacity, depending on the investigation technique employed, the conditions and the type of TiO2 sample.

Curved surface effect and manipulation of electronic states in nanosilicon

It is interesting in low-dimensional nanostructures of silicon that the two quantum effects play different roles in nanosilicon emission, in which the quantum confinement (QC) effect opens band gap and makes emission shift into shorter wavelengths (blue-shift) as the size of the nanocrystals is reduced; however the breaking symmetry originating from impurities on nanosilicon produces the localized electronic states in band gap and makes emission shift into longer wavelengths (red-shift). The results of experiment and calculation demonstrated that the energy levels of nanosilicon can be manipulated through these quantum effects, where the curved surface (CS) effect of impurity atoms bonding on nanosilicon is important in breaking symmetry of nanosilicon system. Here, the CS effect plays an important role on impuritied nanosilicon in smaller scale with larger surface curvature, in which a few characteristic parameters have been found to describe the breaking symmetry of nanosilicon system, such as bonding angle and projecting length of bonds on curved surface. More interesting, the coupling ways between the QC effect and the CS effect determinate the levels position of localized states in band gap and manipulate emission wavelength, where a few new phenomena were explored.

Prospects of Chalcopyrite-Type Nanocrystals for Energy Applications

Colloidal chalcopyrite-type nanocrystals M+M3+E2 (M+ = Cu, Ag; M3+ = In, Fe; E = S, Se) are an emerging class of materials used in energy conversion systems, owing to their high absorption coefficients, nontoxicity, and appropriate band gaps. Their properties can be tuned by varying their size and composition, and they are characterized by peculiar photophysical properties due to a high density of intra band gap electronic states, which arise from defects in the crystal structure and from trap states at the surface. In this Perspective we first discuss optical and electrochemical studies, which give insight into the electronic structure of chalcopyrite nanocrystals. In the second part, their exploitation for energy conversion is highlighted, in particular their use in photovoltaics (with power conversion efficiencies already exceeding 10%) as well as in luminescent solar concentrators, photocatalytic systems, and thermoelectrics. Finally, we discuss current challenges in the chemistry and applications of ...

Photoluminescence and Photoelectrochemical Properties of ZnTe-AgInTe2 Solid Solution Nanocrystals

Introduction Much attention has been paid to near-infrared light (NIR)-absorbing and -emitting semiconductor nanocrystals (NCs) due to their unique physicochemical properties, such as tunable energy structure depending on their size and multiple exciton generation. The most widely studied NIR responsive NCs are based on II-VI and IV-VI semiconductors such as CdTe, PbS, and PbSe. However, their high toxicity is an obstacle to practical application of these NCs, because of potential risks for the human health and the environment. Recently, increasing attention has been paid to I-III-VI2 semiconductor NCs such as CuInS2, AgInSe2, and AgInTe2 [1 -2 ] for the application to in vivo imaging or photoenergy conversion system, because they contain no highly toxic heavy metals and have intense photoluminescence (PL) and absorption in near-infrared region. Furthermore their absorption and emission wavelength can be varied by making solid solutions between I-III-VI2 and II-VI semiconductors. We have successfully prepared nanocrystals of ZnS-AgInS2 [2] and ZnSe-AgInSe2 [3] solid solutions and controlled the optical properties by changing their chemical composition. In this study, we report a solution phase synthesis of ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1 - x)Te2, ZAITe) NCs having a tunable optical properties in near-infrared light wavelength region and their photoelectrochemical properties. Experimental ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. Photoelectrochemical properties of ZAITe NCs were measured in an aqueous solution containing 0.2 mol dm-3 Eu(NO3)3as an electron scavenger. The Ag/AgCl electrode and Pt wire were used as reference and counter electrodes respectively. Results and discussions TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. Thus-obtained NCs exhibited broad peaks in their XRD patterns, each peak position being gradually shifted to higher diffraction angle from the corresponding peak of hexagonal AgInTe2 with a decrease in the x value. Figure 1 shows the absorption and PL spectra of ZAITe NCs. The absorption onset was blue-shifted from 1100 to 810 nm with a decrease in x from 1.0 to 0.25. These results indicated that the obtained NCs were composed of ZAITe solid solution, the composition of which could be controlled by varying the fraction of Zn precursor in preparation, and that their energy gap could be varied depending on their chemical composition. The NCs exhibited near-band-edge PL peak located near the corresponding absorption onset except for x = 0.25, the peak wavelength being blue-shifted with a decrease in x value. The main PL peak observed for NCs with x = 0.25 was largely red-shifted from their absorption onset, suggesting that electronic states in band gap, such as donor and acceptor sites or surface defect sites, were formed at the xvalue smaller than 0.5. Layer-by-layer deposition enabled the accumulation of ZAITe NCs on an ITO electrode. The thus-obtained particle films exhibited a photoresponse similar to p-type semiconductor electrode regardless of x value. Action spectra of the photocurrent were in good agreement with absorption spectra of ZAITe NCs immobilized. The potentials of the valence band and conduction band edges were successfully determined from the onset potentials of cathodic photocurrents and the energy gaps estimated from absorption spectra. ZAITe NCs had a tunable electronic energy structure depending on xvalue. These demonstrate the potential use of ZAITe NCs as photovoltaic materials in near-infrared region. Reference [1] M. Langevin et al., Nanoscale Res. Lett., 2015, 10, 255. [2] T. Kameyama, et al., Nanoscale, 2016, 8, 5435. [3] T. Torimoto, et al., J. Am. Chem. Soc., 2007, 129, 12388. [4] T. Kameyama, T. Torimoto, et al., J. Phys. Chem. C, 2015, 119, 24740. Figure 1

Ti3+ states induced band gap reduction and enhanced visible light absorption of TiO2 nanotube arrays: Effect of the surface solid fraction factor

In this work, a simple strategy was used to prepare Ti3+ surface defect states through the simple UV irradiation of TiO2 nanotubes (TiNT) formed in ethylene glycol/NH4F/H3PO4/H2O mixture at different anodizing potentials. The band gap electronic states created by Ti3+ are mostly responsible for the effective narrowing of band gap.The effect of anodizing potential on surface states of TiO2 nanotubes/electrolyte interface has been studied under UV irradiation. A quantitative and qualitative analysis of surface states of TiNT formed at different anodizing potential was performed using capacitance measurements. These measurements allow us to provide evidence for the presence of monoenergetic surface states in the band gap. The energy levels and the density of surface states have been determined. Both density and energetic location are very dependent on the geometric characteristics. Relatively wide distribution of these states in a range of electric energy of 0.17–0.35eV below the edge of the conduction band was obtained according to the anodizing potential. The optical absorption at 630nm may be attributed to deep level states originated from oxygen vacancies associated with Ti3+. The presence of energetic level states and elevated surface solid fraction factor of TiNT are the determining factors in enhancing the photoactivity in visible light as evidenced by high photocurrent densities up to 16mAcm−2.

Thermodynamically driven one-dimensional evolution of anatase TiO2 nanorods: one-step hydrothermal synthesis for emerging intrinsic superiority of dimensionality.

In photoelectrochemical cells, there exists a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombine with oxidized species on the semiconductor-electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the density and distribution of electronic states in band gap and architectures of the semiconductor electrodes. One-dimensional (1D) anatase TiO2 nanostructures are promising to improve charge transport in photoelectrochemical devices. However, the conventional preparation of 1D anatase nanostructures usually steps via a titanic acid intermediate (e.g., H2Ti3O7), which unavoidably introduces electronic defects into the host lattice, resulting in undesired shielding of the intrinsic role of dimensionality. Here, we manage to promote the 1D growth of anatase TiO2 nanostructures by adjusting the growth kinetics, which allows us to grow single-crystalline anatase TiO2 nanorods through a one-step hydrothermal reaction. The synthesized anatase nanorods possess a lower density of trap states and thus can simultaneously facilitate the diffusion-driven charge transport and suppress the electron recombination. Moreover, the electronically boundary free nanostructures significantly enhance the trap-free charge diffusion coefficient of the anatase nanorods, which enables the emergence of the intrinsic superiority of dimensionality. By virtue of these merits, the anatase nanorods synthesized in this work take obvious advantages over the conventional anatase counterparts in photoelectrochemical systems (e.g., dye-sensitized solar cells) by showing more efficient charge transport and collection and higher energy conversion efficiency.

CS effect and characteristic emission on silicon nanostructures prepared in various atmospheres

The curved surface (CS) effect breaks symmetrical shape of nanosilicon on which some bonds can produce localized electron states in band gap. The different curvature can form the characteristic electron states for some special bonding on nanosilicon surface, which are related to a series peaks in photoluminecience (PL), such as LN, LNO, LO1 and LO2 line in PL spectra due to Si–N, Si–NO, Si=O and Si–O–Si bond on curved surface, respectively. Si–Yb bond on curved surface of Si nanostructures can manipulate the emission wavelength into the window of optical communication by the CS effect, which is marked as LYb line.

Shape and curved surface effect on silicon quantum dots

Curviform surface breaks the symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The calculation results show that the bonding energy and electronic states of silicon quantum dots are different on various curved surfaces, for example, an Si–O–Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. The red-shifting of PL spectrum on smaller silicon quantum dots can be explained by curved surface effect. Experiments demonstrate that silicon quantum dots are activated for emission due to the localized levels formed in the band gap.

Electronic states and curved surface effect of silicon quantum dots

The calculation results show that the bonding energy and electronic states of silicon quantum dots (Si QDs) are different on various curved surfaces (CS), for example, a Si-O-Si bridge bond on curved surface provides the localized levels in band gap and its bonding energy is shallower than that on facet. Curved surface breaks symmetrical shape of silicon quantum dots on which some bonds can produce localized electronic states in band gap. The red-shifting of photoluminescence spectra on smaller silicon quantum dots can be explained by CS effect. In CS effect, surface curvature is determined by the shape of Si QDs or silicon nanostructures, which is independent of their sizes. The CS effect has the interesting fundamental physical properties in nanophysics as that of quantum confinement effect.

Investigation of Band-Gap States in AlGaN/GaN Hetero-Structures with Different Growth Conditions of GaN Buffer Layers

ABSTRACTWe have investigated electronic band-gap states in AlGaN/GaN hetero-structures with different growth conditions of GaN buffer layers from a viewpoint of Carbon impurity incorporation into GaN, using photoluminescence (PL), capacitance-voltage (C-V) and steady-state photo-capacitance spectroscopy (SSPC) techniques. The Carbon incorporation was found to be enhanced with decreasing the growth temperature of the GaN buffer layer between 1120 and 1170 °C. Acting in concert, three specific deep levels located at ~2.07, ~2.70, and ~3.23 eV below the conduction band were found to become dense significantly at the low growth temperature. Therefore, these levels are probably attributable to Ga vacancies and/or Carbon acceptors produced by the Carbon impurity incorporation, and are likely in conjunction with each other.

First-principles study of electronic structures and optical properties of Cu, Ag, and Au-doped anatase TiO2

We perform first-principles calculations to investigate the band structure, density of states, optical absorption, and the imaginary part of dielectric function of Cu, Ag, and Au-doped anatase TiO2 in 72 atoms systems. The electronic structure results show that the Cu incorporation can lead to the enhancement of d states near the uppermost of valence band, while the Ag and Au doping cause some new electronic states in band gap of TiO2. Meanwhile, it is found that the visible optical absorptions of Cu, Ag, and Au-doped TiO2, are observed by analyzing the results of optical properties, which locate in the region of 400–1000nm. The absorption band edges of Cu, Ag, and Au-doped TiO2 shift to the long wavelength region compared with the pure TiO2. Furthermore, according to the calculated results, we propose the optical transition mechanisms of Cu, Ag, and Au-doped TiO2. Our results show that the visible light response of TiO2 can be modulated by substitutional doping of Cu, Ag, and Au.

Wetting Angles on Illuminated Ta2O5 Thin Films with Controlled Nanostructure

Ta2O5 thin films with different nanostructure and surface roughness have been prepared by electron evaporation at different angles between the evaporation source and the substrates. Large variation of refraction indexes (n) from 1.40 to 1.80 were obtained by changing the geometry of evaporation and/or by annealing the evaporated films at increasing temperatures up to 1000 °C to make them crystalline. Very flat and compact thin films (n = 2.02) were also obtained by assisting the growth by bombardment with O2+ ions of 800 eV kinetic energy. A similar correlation has been found between the wetting contact angle of water and the roughness of the films for the evaporated and evaporated + annealed samples, irrespective of their procedure of preparation and other microstructural characteristics. When the films were illuminated with UV light of h > Eg = 4.2 eV (Eg, band gap energy of Ta2O5), their surface became superhydrophilic (contact angle < 10°) in a way quite similar to those reported for illuminated TiO2 thin films. The rate of transformation into the superhydrophilic state was smaller for the crystalline than for the amorphous films, suggesting that in Ta2O5 the size of crystal domains at the surface is an important parameter for the control of this kinetics. Changes in the water contact angle on films illuminated with visible light were also found when they were subjected to implantation with N2+ ions of 800 eV kinetic energy. The origin of this photoactivity is discussed in terms of the electronic band gap states associated with the nitrogen-implanted atoms. The possibility of preparing antireflective and self-cleaning coatings of Ta2O5 is discussed.

Intrinsic Degradation in Alq3-Based OLEDs Probed by Deep-Level Optical Spectroscopy

AbstractWe have successfully investigated degradation-induced variations in electronic band-gap states in the emissive region of the Alq3-based OLEDs by a deep-level optical spectroscopy technique. Through the intrinsic degradation, both deep-level traps and near-band-edge transitions in the Alq3 emissive zone are found to be red-shifted significantly towards their corresponding bulk levels of the Alq3 single layer. These variations in the interfacial electronic states are probably induced by the intrinsic degradation and indicate that initial molecular structures characteristic of the Alq3 emissive zone are transformed into the bulk-like relaxed ones through the degradation.

Comparative photoemission study of the electronic properties of L-CVD SnO 2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO 2 thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position. From the centre of gravity of binding energy of the main XPS Sn 3d 5/2 line the interface Fermi level position E F − E v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO 2 thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.

Dislocation-induced electronic states and point-defect atmospheres evidenced by electron energy loss imaging

Electronic band gap states connected with individual dislocations in diamond and GaN are revealed, using highly spatially resolved electron energy loss (EEL) spectrum mapping. Comparison with calculations of low EEL spectra from first-principle methods allows the identification of the joint density of states of different dislocation core types. Also presented is evidence for instances where point defects/impurities have accumulated in the strain field or segregated to the core of dislocations.

The electronic structure of intrinsic defects at TiO2 (110) surfaces: An ab initio molecular orbital study

We report on an isolated cluster approach to determine local electronic structures of TiO2 surfaces before and after formation of intrinsic defects, i.e., oxygen vacancies, at different crystallographic sites. In particular, isolated oxygen vacancies at bridging sites, isolated oxygen vacancies at in-plane sites and aggregated oxygen vacancies at bridging sites have been treated which lead to changes in the coordination of the adjacent Ti atoms. We find that electronic band gap states are only formed in the presence of fourfold coordinated Ti surface atoms. © 1996 John Wiley & Sons, Inc.

Theory of deep electronic levels of large chalcogen (S, Se, and Te) complexes in Si.

Calculations of the deep band-gap electronic states due to large substitutional complexes of the chalcogen impurities S, Se, and Te are described. The complexes considered are those consisting of neighboring impurities of up to five-atom clusters. It is found that the number of gap levels for a given charge state of the cluster is equal to the number of impurities in the complex. In addition, it is found that the electronically active level for a complex of three or more impurities is quite different in wave-function character from that of the isolated impurity, but rather similar to that of a nearest-neighbor pair. The theory predicts a continuous change from deep-level character to nearly-shallow effective-mass-like character as the size of the complex increases. In addition, the theory predicts that several new intracenter transitions between localized bound states are possible. Results are compared with recent experimental observations involving identified and unidentified chalcogen complexes.