Influence Of Intrinsic Defects Research Articles

Competition between charge transfer and energy transfer: Influence of intrinsic defects in ZnO nanocrystals on rhodamine-B dye color signals

Competition between charge transfer and energy transfer: Influence of intrinsic defects in ZnO nanocrystals on rhodamine-B dye color signals

Band Structure, Phonon Spectrum and Thermoelectric Properties of Ag3CuS2.

Sulfides and selenides of copper and silver have been intensively studied, particularly as potentially efficient thermoelectrics. Ag3CuS2 (jalpaite) is a related material. However very little is known about its physical properties. It has been found that the compound undergoes several structural phase transitions, having the tetrahedral structural modification I41/amd at room temperature. In this work, its band structure, phonon spectrum and thermoelectric properties were studied theoretically and experimentally. Seebeck coefficient, electrical conductivity and thermal conductivity were measured in a broad temperature range from room temperature to 600 K. These are the first experimental data on transport properties of jalpaite. Ab initio calculations of the band structure and Seebeck coefficient were carried out taking into account energy dependence of the relaxation time typical for the scattering of charge carriers by phonons. The results of the calculations qualitatively agree with the experiment and yield large values of the Seebeck coefficient characteristic for lightly doped semiconductor. The influence of intrinsic defects (vacancies) on the transport properties was studied. It was shown that the formation of silver vacancies is the most probable and leads to an increase of hole concentration. Using the temperature dependent effective potential method, the phonon spectrum and thermal conductivity at room temperature were calculated. The measurements yield low lattice thermal conductivity value of 0.5 W/(m K) at 300 K, which is associated with the complex crystal structure of the material. The calculated room temperature values of the lattice thermal conductivity were also small (0.14-0.2 W/(m K)).

Influence of intrinsic defects on the structure and dynamics of the mixed Pb–Sn perovskite: first-principles DFT and NAMD simulations

The mixed tin (Sn) and lead (Pb) perovskite compositions have shown great potential in perovskite photovoltaic devices due to the significantly enhanced material stability and prolonged carrier lifetime, compared to the pure Sn halide perovskites.

Near infrared photon-downshifting in Yb3+-doped titanates: The influence of intrinsic defects

Powder samples of Yb3+-doped TiO2 (0.5 mol%) -exhibiting anatase and rutile phases- CaTiO3, SrTiO3 and Na0.5Bi0.5TiO3 were synthesized by the sol-gel and polymeric complex methods. The crystal structure, microstructure and optical properties of the powders annealed at 800 °C and 1000 °C for 1 h were investigated by X-ray diffraction, scanning electron microscopy, diffuse reflectance and photoluminescence spectroscopies, respectively. X-ray diffraction patterns show the presence of the expected crystalline phase for each compound. Reflectance spectra for all compounds exhibit a broad absorption band below 425 nm ascribed to the ligand-to-metal charge transfer (LMCT) O2− → Ti4+ fundamental state. The Yb3+ excitation spectra for all titanates show a broadband being also compatible with the LMCT O2− → Ti4+ state, indicating the energy transfer from the host to the Yb3+. Interestingly, the excitation band edges follow the same order as that of the optical bandgap energy of the investigated titanates. A conspicuous excitation band in the CaTiO3:Yb3+ provides evidence about the importance of intrinsic defects in the energy transfer process from the host to Yb3+. Photoluminescence emission and diffuse reflectance spectra in the visible and NIR, reveal an emission band that overlaps with the absorption band of the Yb3+, explaining reasonably well the energy transfer process from the host to Yb3+. According to each compound, the integrated intensity of the Yb3+ emission follows the order Na0.5Bi0.5TiO3 > CaTiO3 > SrTiO3 > TiO2. Finally, it is shown that titanates may downshift photons from UV to wavelengths where a crystalline-silicon photovoltaic solar cell has its higher spectral responsivity.

The improvement of thermoelectric property of bulk ZnO via ZnS addition: Influence of intrinsic defects

As in any semiconducting solids, intrinsic defects can affect the properties of ZnO, such as the electrical and thermal conductivities. Defect engineering is usually focused on optimizing the materials’ synthesis or annealing parameters, i.e., temperature, atmosphere, etc. Here we report an approach to change the intrinsic defects of ZnO by adding a small amount of ZnS. During the sintering process, ZnS was decomposed. Apart from the formation of SO2, the decomposed S and Zn can also be simultaneously doped onto O and Zn sites to change the intrinsic defects in ZnO. For instance, some of the S was converted to SO2 and led to the formation of Vo (oxygen vacancy); meanwhile, Zn may move to the VZn (Zn vacancy) site and decrease the concentration of Zn vacancy. Due to the changes in these native defects, the carrier concentration increased and the thermal conductivity decreased when the content of ZnS was increased to x = 0.01. This sample had an optimal zT value, which was twice that of undoped ZnO. However, with further increase in ZnS, the carrier concentration was reduced. These results suggest a method to tune the intrinsic defects of ZnO via doping technology and bring potential opportunities to improve the thermoelectric performance of this oxide.

Influence of intrinsic defects on the electrical and optical properties of TiO2:Nb films sputtered at room temperature

Oxide-based films and nanostructures have emerged as important materials for a wide range of applications such as photovoltaics, optoelectronics, gas sensing and electronics. To develop an appropriate understanding of the properties of these oxides, it is necessary to address the material preparation methods and defect probing issues. This work reports on the synthesis processes of TiO2 based transparent conductive films, their stoichiometry control and defect identifying, in relation with their electrical and optical properties. Un-doped and TiO2:Nb films were deposited by RF co-sputtering from TiO2 and Nb2O5 targets in Ar plasma. The chemical species present in the plasmas used in deposition process were investigated by optical emission spectroscopy, which was later on correlated with the defects structure of the films. Analysis by X-ray photoelectron spectroscopy shed more light on the nature of the vacancies and on the effect of the latter on the optical and electrical properties of the films. In terms of results, we measured electrical resistivity in the range 10−2–10−3Ωcm for the intrinsically and extrinsically doped films (films doped with oxygen vacancies and Nb+5 respectively) while the lowest resistivity was obtained for intrinsically-extrinsically co-doped TiO2 films (7.4×10−4Ωcm). The films transparency was also actively determined by the defects in the lattice and highly transparent films (65–85% in the visible range) were obtained by controlling the density of defects. The approach adopted in this work for the generation of oxygen vacancies could be useful for other oxide-based films, where the oxygen vacancies-dependent properties are crucial, for room temperature ferromagnetism and photocatalytic applications.

Room temperature ferromagnetism and gas sensing in ZnO nanostructures: Influence of intrinsic defects and Mn, Co, Cu doping

Undoped and transition metal (Cu, Co and Mn) doped ZnO nanostructures were successfully prepared via a microwave-assisted hydrothermal method followed by annealing at 500°C. Numerous characterization facilities such as X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM) were employed to acquire the structural and morphological information of the prepared ZnO based products. Combination of defect structure analysis based on photoluminescence (PL) and electron paramagnetic resonance (EPR) indicated that co-existing oxygen vacancies (VO) and zinc interstitials (Zni) defects are responsible for the observed ferromagnetism in undoped and transition metal (TM) doped ZnO systems. PL analysis demonstrated that undoped ZnO has more donor defects (VO and Zni) which are beneficial for gas response enhancement. Undoped ZnO based sensor exhibited a higher sensor response to NH3 gas compared to its counterparts owing to high content of donor defects while transition metal doped sensors showed short response and recovery times compared to undoped ZnO.

New Insights into Defect‐Mediated Heterostructures for Photoelectrochemical Water Splitting

Understanding the interfacial electronic structures of heterojunctions, a challenging undertaking, is extremely important to the design of photoelectrodes for efficient water splitting. The heterostructured interfaces in terms of crystal defects at the atomic‐level exemplified by TiO2/BiVO4 are studied. Results from both experimental observations and theoretical calculations clearly confirm the spontaneous formation of defective interfaces in the heterostructures. TiO2/BiVO4 junction with engineered interfacial defects can efficiently increase the carrier density and extend the lifetime of electrons. The inherent phenomenon of defective electronic structures in different heterostructures creates a significant impact on their photoelectrochemical performance. The synergetic effect between defect‐mediated mechanism and organic quantum dots sensitization yields significantly increased photoconversion efficiency, which is even superior to that of common metal sulfide sensitized ones. This result demonstrates an approach worthy for the design and fabrication of defect‐mediated heterostructures for water splitting, without utilizing harmful metal sulfides. Moreover, new insights into the influence of intrinsic defects on the interfacial charge transfer process between two different semiconductors for energy‐related applications have also been provided.

Tailoring the Electronic Structure of Mesoporous Spinel γ-Al2O3 at Atomic Level: Cu-Doped Case

The well crystallized mesoporous spinel gamma alumina (γ-Al2O3) has been widely studied as an important catalyst support. However, the tailoring of alumina catalyst is still an unresolved challenge when considered at the atomic level. On the basis of experimental characterization, it is found that the electronic structure of γ-Al2O3 could be continuously tailored by metal doping via one-pot route. Combined with density functional theory (DFT) calculations, the nature of geometric and electronic structure evolution upon Cu doping in γ-Al2O3 is investigated. The structure of γ-Al2O3 and the influence of intrinsic defects are studied at first. Considering the nano effect and the charge compensation effects of these intrinsic defects, the doping mechanism of Cu inside γ-Al2O3 lattice are in detail explored at the atomic level. At low doping level the excess electrons from VO or Ali would over compensate Cu dopants, leading to the formation of Cu+ species. As the doping level increases, the preferred doping si...

First-principles DFT+U modeling of defect behaviors in anti-ferromagnetic uranium mononitride

A series of point defects in uranium mononitride (UN) have been studied by first-principles DFT+U calculations. The influence of intrinsic defects on the properties of UN was explored by considering the anti-ferromagnetic (AFM) order along the [001] direction. Our results show that all the point defects lead to obvious volume swelling of UN crystal. Energetically, the interstitial nitrogen defect is the most favorable one among single-point defects in UN crystal with the formation energy of 4.539 eV, while the N-Frenkel pair becomes the most preferable one among double-point defects. The AFM order induces obvious electron spin polarization of uranium towards neighboring uranium atoms with opposite spin orientations in UN crystal.

Effect of seed layer on the growth and the consequent gas sensing characteristics of ZnO nanorod arrays using aqueous chemical growth

The ZnO nanorod arrays are grown on the sol–gel-derived seed layer through aqueous chemical growth, and then assembled as gas sensors for detecting carbon monoxide (CO). It is found that the structural and photoluminescent properties of the ZnO nanorod arrays are different as they are grown on seed layers annealed at different temperature (300–700 °C), which is ascribed to distinct growth kinetics of nanorods on the annealed seed layer. Moreover, the correlation between the exposed surface area and the defect density of those ZnO nanorod arrays points out the intrinsic (interior) defects can dominate the green emission instead of surface defects in the present study. Furthermore, the quantities of chemisorbed oxygen on ZnO nanorod arrays can be estimated through XPS analysis. Consequently, the influence of intrinsic defects and chemisorbed oxygen on the electrical properties and gas sensitivities of ZnO nanorod arrays has been clearly elucidated. It is demonstrated that the more adsorbed oxygen and an appropriate amount of intrinsic defects is advantageous to obtain superior CO gas sensitivity for ZnO nanorod arrays.

Time-resolved photoluminescence properties of CuInS2/ZnS nanocrystals: Influence of intrinsic defects and external impurities

Photoluminescence (PL) lifetime studies of CuInS2 nanocrystals (NCs) are carried out after synthesizing core-shell and compositionally variant structures using time-resolved PL spectroscopy. Long-lived excited state decay times are observed for the NCs, and decay times are very much dependent on the size of the CuInS2 NCs. The emission bands are attributed to the surface (shorter PL lifetime) and defect (longer PL lifetime) related trap states, respectively. The decay dynamics of the CuInS2 NC’s excited-state carriers is very sensitive to the surface, intrinsic defects, and extrinsic impurities. The observed large Stokes shifts and broad PL spectra also reveal the involvement of the defect-related trapping sites in the emission process.

Influence of intrinsic defects on the properties of zinc oxide

The effects of oxygen vacancies and zinc interstitials on the structure and energy of zinc oxide were studied with the semiempirical MO method MSINDO. Cyclic clusters were chosen as model systems. Single and multiple removal of oxygen atoms and zinc interstitials in zinc oxide served to determine the defect formation energy and the band gap. The interaction between two and three oxygen vacancies was investigated. The vacancies cause a decrease of the band gap, which originates from an occupied defect level. This is also found for zinc interstitials under zinc rich conditions. The defect formation energy of such zinc interstitials is found to be lower than that of oxygen vacancies at 0 K but decreases for oxygen vacancies and increases for zinc interstitials with increasing temperature.

Influence of Intrinsic Defects and Strain on Electronic Reliability of Gate Oxide films

AbstractThe effect of strain and intrinsic defects on both electronic and structural characteristics of HfO2-x used for sub-100-nm semiconductor devices was analyzed by a quantum chemical molecular dynamics analysis. The magnitude of the band gap of HfO2 decreases by about 10% under the applied strain. The stable crystallographic structure of the monoclinic HfO2 changes to a cubic-like structure under the strain. The magnitude of the band gap of the HfO2-x decreases drastically from 5.7 eV to about 1.0 eV due to the formation of an electronic state within the band gap when an oxygen vacancy is introduced to the perfect HfO2. In the HfO2-x film, oxygen atoms near the oxygen vacancy can move drastically at temperatures higher than 800 K. Therefore, it is very important to control the chemical composition of the hafnium oxide film and to optimize the annealing condition to maintain both the high reliability and performance of the gate oxide film.

Defects in half-metals and finite temperature

The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the14 cases of intrinsic defects, five affect the half-metallic properties. They are energeticallyvery unlikely to occur. Circumstances are discussed under which defects may even have abeneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects,like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contactsmay influence the magnon spectrum, improving the behaviour at finite temperature.

Nonstoichiometric Defects and Optical Properties in LiNbO3

The influence of intrinsic defects on optical properties of lithium niobate studied using an electronic-structure simulation technique. The simulation technique is based on molecular dynamics cluster geometry optimization, nonlocal pseudopotential calculation, and Green function electron−phonon interaction. Two models of defects are considered. The first one (electrostatically highly imbalanced) corresponds to mixture of oxygen vacancies and NbLi antisites. The second one corresponds to the Nb antisites charge compensated by appropriate vacancies, e.g., lithium vacancies, and corresponds to more realistic defect configuration. We present a band energy approach with a molecular dynamics cluster optimization that accounts for the various structural modifications related to the nonstoichiometry of LiNbO3 crystals. The variation of the optical properties with the deviation from the stoichiometric composition can be understood within this approach. Especially, the role of the electron−phonon contributions to t...

Influence of intrinsic point defects on the electrophysical properties of NbSe3

The temperature dependence of the electrical resistivity of quasi-one-dimensional NbSe3 is investigated in the interval 78–550 K in the thermodynamic equilibrium and nonequilibrium states. At temperatures of 300–550 K one observes an exponential deviation from the linear dependence on account of the formation of thermodynamic equilibrium Se vacancies. The influence of intrinsic defects (vacancies) on the properties of NbSe3 at 78–300 K is investigated by the quenching method. For samples with excess vacancies an anomalously large deviation from the Matthiessen rule (up to 150%) is observed.

The influence of intrinsic defects on electrophysical properties of gallium nitride films

The influence of intrinsic defects on electrophysical properties of gallium nitride films

Influence of intrinsic defects on light-induced changes in the refractive index of lithium niobate crystals

crystals at continuous-wave illumination are reported. The investigation clearly indicates an increase in the photorefraction amplitude Δns (at saturation) with light intensity J for all crystals studied. The main result is a correlation between ∂(Δns)/∂J and the density N of intrinsic defects such as NbLi. This effect is independent of the accidental iron contamination. Therefore at low J, Δns decreases with an increase in N, whereas at high J the opposite dependence is observed. The threshold intensity value of such a transformation correlates with the optical quality of the crystal. It has been established that these photorefraction changes are connected with the light induced absorption of the secondary centers, caused by the existence of the metastable small polarons at room temperature during illumination.

Influence of Intrinsic Defects on the Electronic Structure of Non-Stoichiometric CuInS2 Chalcopyrite Semiconductors

Influence of Intrinsic Defects on the Electronic Structure of Non-Stoichiometric CuInS₂ Chalcopyrite Semiconductors