Presence Of Intrinsic Defects Research Articles

NMR Spectra Particularities in LiNbO3 Crystals with a Near-Stoichiometric Composition

The paper studies LiNbO3 (LN) crystals with a near-stoichiometric composition (NSLN). The study establishes the possibility of different physical methods to reveal NSLN crystals’ exact composition. The main goal was to establish how precisely these methods can reveal a NSLN composition, including a defective structure. This structure determines properties that are important for the application of the crystals. Two NSLN crystals with a different Li/Nb ratio have been studied by IR and NMR spectroscopy. NSLN crystals have been grown from a congruent melt with different K2O flux contents (5.0 and 5.5 wt%). The data on NSLN have been compared with the data on congruent (CLN) crystals. CLN are the most widely used LN crystals. The study has established that analysis of the IR spectra can determine the Li/Nb ratio within [Li2O] = 48.6 – 50.0 mol% range, while the 93Nb NMR spectra has a wider range of sensibility. LN crystals’ stoichiometry or the Li/Nb ratio determine the concentration of antisite defects NbLi. Niobium substitutes lithium in its octahedron. Such defects appear up to [Li2O] = 49.9 mol%. Thus, the study shows that IR and NMR spectroscopy are sensitive methods that can complement each other when determining the precise LN composition (Li/Nb ratio) and the presence of intrinsic defects in the crystals.

Role of deposition parameters on the properties of the fabricated heterojunction ZnS/p-Si Schottky diode

Heterojunction diodes of ZnS/p-Si have been fabricated using the chemical bath deposition (CBD) technique at two different deposition durations under both stirring and non-stirring conditions. The x-ray diffraction (XRD) patterns indicate the deposited ZnS films exhibit good crystallinity with the growth direction along the (111) planes of a cubic zinc blend structure. The crystallite size of all the deposited ZnS thin films have been calculated using the Scherer formula and found to be in the range of 2.2–2.7 nm which is very close (∼4 nm) to the size estimated using transmission electron microscopy (TEM). The surface morphology of the deposited ZnS thin films were studied by scanning electron microscopy (SEM) and it was observed that spherical nanoparticles agglomerated with the increase in deposition time. Furthermore, the optical properties of the deposited ZnS thin films were studied using UV-visible (UV-VIS) and photoluminescence (PL) spectroscopy. The effective calculated band gap was found in the range from 3.7–3.82 eV for all the samples, however PL spectra shows multiple emissions in the as-deposited ZnS films, indicating the presence of intrinsic defects, The characteristics of the fabricated ZnS/p-Si heterojunction diode was studied by measuring the dark current-voltage (I–V) relation using thermionic emission model. Electrical parameters such as barrier height, saturation current, ideality factor and series resistance were extracted from the I–V characteristics of the fabricated Schottky diodes. The barrier potential for all the ZnS/p-Si heterojunction diodes range between 0.829–0.857. Moreover, the calculated ideality factor was found very close to the ideal value of the diode (1.34 and 1.43) in the devices fabricated under stirring conditions.

Molecular Functionalization of 2H-Phase MoS2 Nanosheets via an Electrolytic Route for Enhanced Catalytic Performance.

The molecular functionalization of two-dimensional MoS2 is of practical relevance with a view to, for example, facilitating its liquid-phase processing or enhancing its performance in target applications. While derivatization of metallic 1T-phase MoS2 nanosheets has been relatively well studied, progress involving their thermodynamically stable, 2H-phase counterpart has been more limited due to the lower chemical reactivity of the latter. Here, we report a simple electrolytic strategy to functionalize 2H-phase MoS2 nanosheets with molecular groups derived from organoiodides. Upon cathodic treatment of a pre-expanded MoS2 crystal in an electrolyte containing the organoiodide, water-dispersible nanosheets derivatized with acetic acid or aniline moieties (∼0.10 molecular groups inserted per surface sulfur atom) were obtained. Analysis of the functionalization process indicated it to be enabled by the external supply of electrons from the cathodic potential, although they could also be sourced from a proper reducing agent, as well as by the presence of intrinsic defects in the 2H-phase MoS2 lattice (e.g., sulfur vacancies), where the molecular groups can bind. The acetic acid-functionalized nanosheets were tested as a non-noble metal-based catalyst for nitroarene and organic dye reduction, which is of practical utility in environmental remediation and chemical synthesis, and exhibited a markedly enhanced activity, surpassing that of other (1T- or 2H-phase) MoS2 materials and most non-noble metal catalysts previously reported for this application. The reduction kinetics (reaction order) was seen to correlate with the net electric charge of the nitroarene/dye molecules, which was ascribed to the distinct abilities of the latter to diffuse to the catalyst surface. The functionalized MoS2 catalyst also worked efficiently at realistic (i.e., high) reactant concentrations, as well as with binary and ternary mixtures of the reactants, and could be immobilized on a polymeric scaffold to expedite its manipulation and reuse.

Point defects in lead sulfide: A first-principles study

The energetic and electronic properties of point defects in lead sulfide (PbS) were studied using first-principles methods. In particular, intrinsic defects including single-site and double-site defects (e.g. Schottky dimers and Frenkel pairs) were considered as well as extrinsic oxygen containing defects. A novel, stable and energetically preferred interstitial site was identified. Convergence of the calculations with supercell size was examined and found to be well-converged for most, but not all, defects in 250 atom supercells. For intrinsic defects, it was found that, after accounting for the chemical potentials of Pb and S in the environment, the lowest formation energies are associated with lead vacancies in S-rich conditions and sulfur vacancies in Pb-rich conditions and not with Schottky defects, as previously reported. Interstitials, Frenkel pairs, and antisite defects were all found to have much larger defect formation energies and are therefore unlikely to be found. The electronic band structure was affected by the presence of intrinsic defects. In particular, for lead vacancies, the Fermi level was shifted below valence band maximum, indicating p-type conductivity; and for lead interstitials, it was shifted above the conduction band minimum, indicating n-type conductivity. In contrast, sulfur vacancies were found to introduce levels deep in the band gap, which may affect the electronic properties significantly. The charged states of the point defects were examined and found to be preferred over the neutral states. The formation energies of oxygen defects were found to be highly competitive energetically with those of intrinsic defects, and therefore, oxygen point defects are expected to play a significant role in determining material properties. In particular, in Pb-rich conditions, oxygen is expected to be drawn into the material to occupy S vacancies.

On factors affecting the degradation of BaSO4 powders’ optical properties under the action of the solar spectrum quanta

It has been established that optical properties of BaSO4 powder undergo significant degradation under the action of the solar-spectrum simulating xenon lamp radiation. The cause of such degradation is the presence of intrinsic defects that absorb the solar spectrum quanta with the energy lower than the width of BaSO4 band gap. Preheating at 400 °C for 2 h leads to sintering of powder grains and to the removal of the precipitated water decomposition products, which block the defects’ relaxation. Additionally, the heating makes the radiation stability higher.

Identification of two trapping mechanisms responsible of the threshold voltage variation in SiO2/4H-SiC MOSFETs

A method based on cyclic gate bias stress followed by a single point drain current measurement is used to probe the interface or near-interface traps in the SiO2/4H-SiC system over the whole 4H-SiC bandgap. The temperature-dependent instability of the threshold voltage in lateral MOSFETs is investigated, and two separated trapping mechanisms were found. The experimental results corroborate the hypothesis that one mechanism is nearly temperature independent and it is correlated with the presence of near-interface oxide traps that are trapped via tunneling from the semiconductor. The second mechanism, having an activation energy of 0.1 eV, has been correlated with the presence of intrinsic defects at the SiO2/4H-SiC interface.

Dual topological insulator device with disorder robustness

Two-dimensional ${\mathrm{Na}}_{3}\mathrm{Bi}$ is a dual topological insulator protected by time-reversal and mirror symmetry, resulting in a promising platform for device design. However, in reality, the design of topological devices is hindered by a sensitivity against disorder and temperature. We study the topological properties of ${\mathrm{Na}}_{3}\mathrm{Bi}$ in the presence of intrinsic defects, investigating the robustness of the edge states and the resulting transport properties. We apply a recursive Green's function technique enabling the study of disordered systems with lengths comparable to experimentally synthesized materials, in the order of micrometers. We combine our findings to propose a topological insulator device, where intrinsic defects are used to filter the response of trivial bulk states. This results in a stable conductance throughout a large range of electronic temperatures, and controllable by a perpendicular electric field. Our proposal is general, enabling the design of various dual topological insulator devices.

Investigation of anisotropic π plasmon induced by the intrinsic crystallographic defects in topological crystalline insulator material—tin-substitutional lead selenide (Pb1−xSnxSe)

The presence of intrinsic defects in topological crystalline insulator materials has been predicted to improve the thermoelectric figure-of-merit values in the literature. Performing atomic-resolved high angle annular dark field imaging, momentum-resolved electron energy loss spectroscopy, and electron spectroscopic diffraction, we observed those intrinsic defects, including interstitial Se atoms and Se vacancies, to cause localized mirror symmetry breaking and further result in the anisotropic π-plasmon dispersion along the ΓX¯ and ΓM¯ directions in single-crystal Pb1−xSnxSe (x = 0 and 0.34). In addition to the anisotropic π plasmon dispersion, the splitting lines along the ΓX¯ direction were revealed with selected π plasmon energy in the electron spectroscopic diffraction pattern.

New approaches for achieving more perfect transition metal oxide thin films

This perspective considers the enormous promise of epitaxial functional transition metal oxide thin films for future applications in low power electronic and energy applications since they offer wide-ranging and highly tunable functionalities and multifunctionalities, unrivaled among other classes of materials. It also considers the great challenges that must be overcome for transition metal oxide thin films to meet what is needed in the application domain. These challenges arise from the presence of intrinsic defects and strain effects, which lead to extrinsic defects. Current conventional thin film deposition routes often cannot deliver the required perfection and performance. Since there is a strong link between the physical properties, defects and strain, routes to achieving more perfect materials need to be studied. Several emerging methods and modifications of current methods are presented and discussed. The reasons these methods better address the perfection challenge are considered and evaluated.

Tensile properties of recycled carbon fibers subjected to superheated steam treatment under various conditions

A new treatment condition involving superheated steam (SHS) was proposed to achieve optimal carbon fiber recovery with a minimal decrease in fiber strength. Unidirectional carbon fiber reinforced sheets were exposed to SHS at 650 °C for 1 h with the addition of N2 or CO2 as process gas. The recovery was assessed based on mass change and using thermogravimetry and differential scanning calorimetry. The results showed that the addition of N2 efficiently suppressed the decrease in fiber strength and achieved optimal recovery. Weibull parameters indicated that lower strength fibers tended to present a narrower strength distribution. The observation of fracture surfaces and oblique sections indicated that the presence of intrinsic defects and corrosion of surface crystallites under the oxidative condition are responsible for the deterioration of and the changes in the distribution of strength.

The role of samarium incorporated structural defects in ZnO thin films prepared by femtosecond pulsed laser deposition

Samarium, a rare-earth element is used as an impurity in ZnO to study the effects it causes in structure thereby changing the optical and electrical characteristics of ZnO thin films. Samarium zinc oxide (SmxZn1-xO, SZO) films are deposited on glass substrates employing femtosecond pulsed laser deposition (fs-PLD). XRD results show that all films are crystalline and prefer c-axis growth. 1 wt% Sm is plausible for doping in ZnO and 2 wt% samarium addition results in reaction among impurity and host atoms forming a compound ZnSm2O4. Higher concentrations lead to defect generation and decline of crystalline quality. AFM results show that surface roughness is increased with samarium incorporation in addition to affecting particle size and shape. These microstructural variations significantly affect the optical transmittance, reflectance and band gap energies of thin films which are calculated using spectroscopic ellipsometry data. Presence of intrinsic defects and extrinsic defects caused by samarium is revealed by Photoluminescence Spectroscopy. Electrical resistivity of SZO films is significantly affected with Sm concentration in nanocrystalline ZnO. These types of films are suitable for use in optical devices for UV and blue emission.

Elucidating the role of extended surface defects at Fe surfaces on CO adsorption and dissociation

The adsorption and dissociation of hydrocarbons on metallic surfaces during catalytic reactions in a steam reforming furnace often lead to the carburization of the catalysts and metallic surfaces involved. This process is greatly accelerated by the presence of intrinsic defects like vacancies and grain boundaries and is succeeded by surface to subsurface diffusion of C. We employ both density functional theory and reactive force field molecular dynamics simulations to investigate the effect of surface defects on CO dissociation rate directly related to metal dusting corrosion. We demonstrate that stable surface vacancy clusters with large binding energies accelerate the adsorption of CO molecules by decreasing the corresponding dissociation energies. In addition, we demonstrate that the appearance of multiple GBs at the surface leads to an enhancement of the CO dissociation rate. Furthermore, we demonstrate that the increase in surface roughness by emerging GBs leads to an increase in CO dissociation rate.

Study of the preparation of TiO2 powder by different synthesis methods

In this work, we present a study on the preparation of TiO2 powder employing different synthesis methods as molten salt, sol-gel, combustion and hydrothermal. The structural, morphological, optical and surface properties were studied in order to evidence the characteristic of the TiO2 powder. By the molten salt method, the TiO2 powder presented a rutile-phase crystal structure with particles of micrometric size and low superficial area. When the combustion and sol gel methods were employed, the TiO2 powder had an anatase phase with a crystallite size of ∼14 nm. Different morphologies such as nanocuboids, nanobelts, nanoparticles and interconnected nanobubbles were observed. The photocatalytic activity of TiO2 powder was assessed by the rhodamine B degradation. SG-TiO2 powder showed the highest photocatalytic efficiency in comparison with the rest of the samples. The presence of intrinsic defects and hydroxyl groups plays a fundamental role in the photocatalytic activity of the TiO2 powder.

ZnO nanorods decorated with nanocrystalline (nc) Au Particles:Electronic structure and magnetic behaviours

Electronic structures and magnetic behaviours of zinc oxide nanorods (ZnO-NRs) decorated with nanocrystalline “Au”-particles (nc-Au) have been studied using different spectroscopies, superconducting quantum interference device-type magnetometer and plane wave pseudopotential density functional theory calculations. The presence of intrinsic defects viz. zinc interstitials (Zni) and/or oxygen vacancies (VO) are established using different spectroscopies measurements. Ultraviolet photoelectron spectroscopy and valence band photoemission spectroscopy suggests that the valence band density of states of O 2p - Zn 4sp hybridized states of nc-Au/ZnO-NRs is higher than pure ZnO-NRs that could be correlated with its higher ferromagnetic behaviour. An enhanced room temperature ferromagnetism has been observed when nanocrystalline (nc) gold (Au) particles are decorated on the surface of ZnO-nanorods (ZnO-NRs). This enhanced magnetic moment in nc-Au/ZnO-NRs comes from the metallic electrons of nc-Au that induces large orbital magnetic moment at the nc-Au/ZnO-NR interface. Theoretical calculations shows the enhancement of the density of states at the valence band region of ZnO with a decoration of optimum content nc-Au particles along with the increase of oxygen vacancies in nc-Au/ZnO-NRs; that further confirms the enhancement of magnetization.

Impact of Cu doping on the structural, morphological and optical activity of V2O5 nanorods for photodiode fabrication and their characteristics.

In this paper, we report a wet chemical precipitation method used to synthesize pure and Cu-doped V2O5 nanorods with different doping concentrations (CuxV2O5 where x = 3, 5 or 7 at%), followed by annealing at 600 °C and characterizations using several techniques. Indeed, a growth mechanism explaining the morphological evolution under the experimental conditions is also proposed. The XRD patterns revealed that all of the studied samples consist of a single V2O5 phase and are well crystallized with a preferential orientation towards the (200) direction. The presence of intrinsic defects and internal stresses in the lattice structure of the CuxV2O5 samples has been substantiated by detailed analysis of the XRD. Apart from the doping level, there was an assessment of identical tiny peaks attributed to the formation of a secondary phase of CuO. SEM images confirmed the presence of agglomerated particles on the surface; the coverage increased with Cu doping level. XPS spectral analysis showed that Cu in the V5+ matrix exists mainly in the Cu2+ state on the surface. The appearance of satellite peaks in the Cu 2p spectra, however, provided definitive evidence for the presence of Cu2+ ions in these studied samples as well. Doping-induced PL quenching was observed due to the absorption of energy from defect emission in the V5+ lattice by Cu2+ ions. We have proposed a cost-effective, less complicated but effective way of synthesizing pure and doped samples in colloidal form, deposited by the nebulizer spray technique on p-Si to establish junction diodes with enhanced optoelectronic properties.

Ultrahigh-temperature conversion of biomass to highly conductive graphitic carbon

Graphitic carbon has attracted tremendous research interest in recent years owing to its exceptional thermal and electrical properties that arise from the ordered sp2 hybridized carbon structure. Due to its high activation energy, graphitization is often energy- and chemical-intensive. In addition, the electrical conductivity of graphitized materials has always been limited by the presence of intrinsic defects. In this paper, we propose a new method to convert lignin-based biomass into highly crystalline graphitic carbon by a Joule heating process. The Joule heating utilizes the internal resistance of a reduced graphene oxide/lignin (rGO-lignin) carbon film to heat the sample to up to 2500 K within 1 h. The annealing of lignin at this high temperature is found to remove impurities and intrinsic defects, as well as to initiate the graphitization process. Amorphous lignin carbon can be converted into short-range ordered and graphitic carbon with an ultrahigh electrical conductivity of 4500 S/cm, significantly higher than the original 6.4 S/cm. The microstructure change underlying this high electrical conductivity was further probed through electron microscopy and chemical analysis. This highly crystalline, electrically conductive graphitized lignin-carbon is expected to be useful for numerous applications where high conductivity and corrosion resistance are desired.

Ab initio study of luminescence in Ce-doped Lu2SiO5 : The role of oxygen vacancies on emission color and thermal quenching behavior

We study from first principles the luminescence of Lu$_2$SiO$_5$:Ce$^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (V$_O$) in the generated spectrum. The excitation energy, emission energy and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a ${\Delta}$SCF analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread of the low-energy emission band originates from a whole set of different Ce-V$_O$ complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The $4f-5d$ crossover mechanism is found to be very unlikely, with a large crossing energy barrier (E$_{fd}$) in the one-dimensional model. The alternative mechanism usually considered, namely the electron auto-ionization, is also shown to be unlikely. In this respect, we introduce a new methodology in which the time-consuming accurate computation of the band gap for such models is bypassed. We emphasize the usually overlooked role of the differing geometry relaxation in the excited neutral electronic state Ce$^{3+,*}$ and in the ionized electronic state Ce$^{4+}$. The results indicate that such electron auto-ionization cannot explain the thermal stability difference between the high- and low-energy emission bands. Finally, a hole auto-ionization process is proposed as a plausible alternative. With the already well-established excited state characterization methodology, the approach to color center identification and thermal quenching analysis proposed here can be applied to other luminescent materials in the presence of intrinsic defects.

Study of Undoped and Indium Doped ZnO Thin Films Deposited by Sol Gel Method

In this paper, we report the effects of Indium doping concentrations (from 0 to 10wt%) on the structural, morphological, and optical properties of deposited In doped ZnO (IZO) thin films prepared by the sol–gel method through the dip coating technique. X-ray diffraction (XRD) analysis indicates that all ZnO thin films have a polycrystalline nature with a hexagonal wurtzite phase with (002) as a preferential orientation. XRD results demonstrate that the particle size of ZnO decreased with the increase in Indium concentrations. Raman scattering spectra confirmed the wurtzite phase and the presence of intrinsic defects in our samples. Energy dispersive spectroscopy (EDS) and the X-ray photoelectron spectroscopy (XPS) measurements, confirmed the presence of zinc, oxygen and indium elements which is in agreement with XPS results. The photoluminescence (PL) spectra of the films exhibit defects-related visible emission peaks, with intensities differing owing to different concentrations of zinc vacancies. UV–Vis spectrometer measurements show that all the films are highly transparent in the visible wavelength region (≥ 70%) and presented two different absorption edges at about 3.21 eV and 3.7 eV, these may be correspond to the band gap of zinc oxide and indium oxide respectively.

LUMINESCENCE INVESTIGATION OF POWDER ZNO NANOPARTICLES DOPED WITH SM AND LI IONS PREPARED BY COPRECIPITATION METHOD

The present study focuses on the structural and optical properties of ZnO doped with Sm and Li at very dilute concentrations, with compounds Zn1-x-ySmxLiyO (x = 0, 0.03 and y =0, 0.01, 0.03, 0.05). These compounds have been synthesized by a chemical coprecipitation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman microscopy. The XRD pattern of all the samples showed ZnO hexagonal wurtzite structure, sharp and intense peaks with small change in lattice parameters due to doping in ZnO, indicating the substitution of Sm and Li ions for Zn sites. For Raman spectra, the strongest peak centered at about 435-437 cm-1 can be observed in all the samples, and the intensity of this peak decreases with Sm doping and increases with Li doping. The optical properties of the samples were systematically studied by ultraviolet-visible spectroscopy (UV-V) and photoluminescence (PL) spectroscopy. The maximum absorption of ZnO and Zn0.97Sm0.03O samples appears around 370 and 374 nm respectively near the absorption edge. Doping with Li affects the absorption edge and make it shift toward ultra violet and that the absorption increase with increasing Li content. The PL spectra show a broad and strong visible emission at 564 nm emission covering a large part of the visible region indicating the presence of intrinsic defects like vacancies.

Tailoring of room temperature ferromagnetism and electrical properties in ZnO by Co (3d) and Gd (4f) element co-doping

Nanoparticles of Co2+ and Gd3+ ions co-doped Zinc oxide were prepared by co-precipitation process followed by hydrothermal method. To obtain the desired crystallographic phase and also, to enhance the Oxygen vacancy, the as prepared sample was sintered at 400 °C in vaccum atmosphere. The presence of intrinsic defects and internal stress in the lattice structure of co-doped ZnO has been substantiated by the detail analysis of X-ray diffractograms. Different optical characterizations like Raman spectroscopy and photoluminescence (PL) spectra have been measured. The presence of defects/Oxygen vacancies are also critically analyzed by Raman and PL spectra of co-doped ZnO sample. Magnetic measurement such as magnetization as a function of magnetic field at different low temperatures (300-10 K) and magnetization as a function of temperature (field cooled and zero-field cooled) are measured by SQUID magnetometer. Interestingly, a very strong magnetic interaction and a large value of coercivity even at room temperature have been observed in the co-doped ZnO nanoparticles compared to the mono-doped ZnO nanoparticles. The presence of ferromagnetic ordering and large coercivity were successfully explained by the vacancy assisted bound-magnetic-polaron model and domain wall blowing theory. Negative exchange biased has been observed in our sample implies the formation and exchange interaction at the interface of FM/AFM bilayers. Dielectric measurement suggest that for both 5% Co2+ and Gd3+ ions co-doped ZnO system, the enhanced dielectric data is the contribution of large amount of space charge polarization and the intrinsic defects in the sample. Enhancement of the ac conductivity with the increase of temperature reveals the semiconducting nature of the co-doped ZnO systems.