Vacancy Impurity Research Articles

Crystal Growth and Spectroscopy of Yb2+-Doped CsI Single Crystal

The single crystals of CsI-Yb2+ were grown, and their spectroscopic studies were conducted. The observed luminescence in CsI-Yb2+ is due to 5d–4f transitions in Yb2+ ions. Using time-resolved spectroscopy, spin-allowed and spin-forbidden radiative transitions of ytterbium ions at room temperature were found. The excitation spectra of Yb2+ luminescence bands were obtained in the range of 3–45 eV. The mechanism of charge compensation of Yb2+ ions in a CsI crystal was also studied, the spectrum of the thermally stimulated depolarization current was measured, and the activation energies of the two observed peaks were calculated. These peaks belong to impurity–vacancy complexes in two different positions. The charge compensation of Yb2+ occurs via cation vacancies in the nearest-neighbor and next-nearest-neighbor positions.The Yb2+ ions are promising dopants for CsI scintillators and X-ray phosphors in combination with SiPM photodetectors.

The Defects Genome of Janus Transition Metal Dichalcogenides.

2D Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarization field within the monolayer. While efforts are made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through high-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (VS and VSe) and double chalcogen vacancy (VS -VSe), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

Hydrogen permeation of single layer graphene with substitutional metal impurity defects

Two-dimensional materials have recently emerged as promising candidates for blocking hydrogen isotopes, with pristine graphene showing the most promising attributes. Nevertheless, synthesized graphene typically harbors various defects. In this study, we conducted a systematic investigation employing a first-principles approach to assess the permeation of hydrogen through single-layer graphene featuring a single-atom vacancy and diverse substitutional metal impurity defects. Our findings indicate that most substitutional metal impurities in a carbon vacancy effectively enhance graphene's ability to obstruct hydrogen when it harbors vacancy defects, resulting in elevated hydrogen penetration barriers in contrast to graphene with a single-atom vacancy. The hydrogen penetration barriers are contingent upon both the atomic size and the number of outermost electrons in the metal defects. Transition metals characterized by larger atoms and more outermost electrons exhibit heightened penetration barriers, while non-transition metals exhibit a relatively diminished impact of outermost electrons on hydrogen penetration barriers due to reduced charge transfer to graphene. This study underscores the pivotal role played by metal defects in obstructing the infiltration of hydrogen and its isotopes into graphene.

Fabrication of oxygen-vacancy-adjustable Yb3+/Er3+ co-doped BiOBr 3D flowerlike hierarchical architectures with enhanced photocatalytic activity under broad spectrum (UV/vis/NIR) excitation

A novel Yb3+/Er3+ co-doped BiOBr photocatalyst with 3D flowerlike hierarchical architectures were successfully fabricated through self-assembled process without any templates under hydrothermal conditions, and the structure and properties of the photocatalysts were characterized. The results indicate that Yb3+ ions and Er3+ ions are successfully doped into the lattice of BiOBr, and the as-obtained photocatalyst can respond the NIR light and emit red and green light due to the up-conversion effect of the rare-earth ions. Meanwhile, with co-doped Yb3+/Er3+ ions content increasing, oxygen vacancy concentration in the photocatalyst rises. The oxygen vacancy will form the intermediate energy band, which can decrease the photon energy required for electronical transition, and thus help to utilize the luminous energy generated by up-conversion luminescence. Moreover, the appropriate introduction of defects (oxygen vacancy defect and impurity atom) can further promote the transfer and separation efficiency of photo-generated carriers. Hence, Yb3+/Er3+ co-doped BiOBr photocatalyst exhibits enhanced photocatalytic properties under broad spectrum (UV/vis/NIR) excitation, which is about 4 times as high as that of pure BiOBr.

Hybrid density functional theory simulation of sodium impurity and impurity–vacancy defect complexes in germanium: perspectives of defect engineering for activation of shallow donors

At present, germanium (Ge) as a high-mobility material is considered as a possible replacement for Si in microelectronics. The main obstacle in the method is the large vacancy concentration obtained after the implantation of shallow donors. This prevents the formation of n+ regions and has a negative impact on device performance. One way to eliminate the detrimental effects of such defects is through the codoping approach. In this study, the behavior of Na impurities in germanium together with the formation of defect complexes with vacancies, divacancies and shallow donors (P and As) were investigated using hybrid density functional theory calculations. The site preference, diffusion barriers, charge states and binding energies of various complexes, as well as the activation of donor–vacancy complexes with both F and Na codoping were investigated. For this purpose, two extreme cases were considered. Firstly, we calculate the concentrations of substitutional and interstitial Na together with monovacancies in the case of quasi-equilibrium conditions realized for doping from melt. Another extreme case is Na codoping during ion implantation. Our calculations show that Na passivates vacancies, divacancies and donor–vacancy complexes (E-centers) with a large energy gain, which prevents rapid donor diffusion similar to the case of doping with fluorine. We have shown that the passivation of E-centers with both F and Na impurities leads to the back transformation of such complexes into shallow donors. This means that codoping with Na may neutralize the harmful effects of vacancies and is a perspective for defect engineering.

Crystal-field analysis of photoluminescence from orthorhombic Eu centers and energy transfer from host to Eu in GaN co-doped with Mg and Eu

Gallium nitride co-doped with magnesium and europium shows great potential for active layers in red light emitting diode structures due to strong and sharp luminescence emission around 620 nm. In this work, sharp and intense Eu3+ luminescence lines from the excited states of the 5DJ (J = 0, 1) multiplets to the ground states of the 7FJ (J = 0, 1, 2) multiplets have been analyzed using a C2v crystal-field equivalent operator Hamiltonian. A model of Eu centers with the C2v symmetry has been proposed to be an Eu3+ complex accompanied by either a pair of nitrogen and gallium vacancies (VN-VGa) or a pair consisting of a nitrogen vacancy and magnesium impurity (VN-MgGa) in the vicinity of the Eu ion based on the crystal-field analysis, the selection rules and the observed polarization of the Eu3+ luminescence lines. Energy transfer from the host to the Eu ions under band-to-band excitation occurs through electron-hole recombination between VN with the electron-like state and VGa or MgGa with the hole-like state; these may be associated with the shallow-trapped or deep-trapped states, respectively, proposed as the energy transfer mechanism in previous literature.

Influence of Channel Surface with Ozone Annealing and UV Treatment on the Electrical Characteristics of Top-Gate InGaZnO Thin-Film Transistors.

The effect of the channel interface of top-gate InGaZnO (IGZO) thin film transistors (TFTs) on the electrical properties caused by exposure to various wet chemicals such as deionized water, photoresist (PR), and strippers during the photolithography process was studied. Contrary to the good electrical characteristics of TFTs including a protective layer (PL) to avoid interface damage by wet chemical processes, TFTs without PL showed a conductive behavior with a negative threshold voltage shift, in which the ratio of Ga and Zn on the IGZO top surface reduced due to exposure to a stripper. In addition, the wet process in photolithography increased oxygen vacancy and oxygen impurity on the IGZO surface. The photo-patterning process increased donor-like defects in IGZO due to organic contamination on the IGZO surface by PR, making the TFT characteristics more conductive. The introduction of ozone (O3) annealing after photo-patterning and stripping of IGZO reduced the increased defect states on the surface of IGZO due to the wet process and effectively eliminated organic contamination by PR. In particular, by controlling surface oxygens on top of the IGZO surface excessively generated with O3 annealing using UV irradiation of 185 and 254 nm, IGZO TFTs with excellent current-voltage characteristics and reliability could be realized comparable to IGZO TFTs containing PL.

Local structure of an oxygen impurity and fluorine vacancy complex in LiYF4

Crystal-growth defects and impurities are among the key factors limiting the performance of fluorine-based optical materials. This study reports the local structure of an oxygen impurity centre in LiYF4 single-crystal. Experimental electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy investigations were combined with first principle theoretical modelling to elucidate the defect structure. It was determined that, after irradiation with X-rays, an electron spin S=1/2 paramagnetic centre is generated. It is stable at room temperature and anneals above 150°C. The alignment of the g-tensor and magnitudes of hyperfine interactions suggest that the centre is an oxygen impurity and fluorine vacancy complex. Highly anisotropic hyperfine interaction with three fluorine atoms within 3 Å of the oxygen defect is observed. The results provide novel insights into oxygen ion incorporation in LiYF4 and are crucial to understanding and minimising radiation-induced degradation of the material.

THE STRUCTURE OF THERMOLUMINESCENCE TRAPS IN AQUEOUS GROWN KCl

The results of TL studies in aqueous grown undoped KCl crystals are reported. A de-convolution of TL curve into five component peaks has been performed and trap depths have been obtained. The importance of bi-vacancy has been established as a fundamental trap and its anion vacancy end has also been proposed as a suitable trap for thermoluminescence in KCl crystals. The traps at anion vacancies linked to impurity vacancy I-V pairs have also been envisaged and the shift in the depths of traps is attributed to the electric field of I-V pairs in the lattice caused by inherent divalent ions. Some traps linked to hydroxyl groups have also been identified.

Research Progress on the Point-defect Relaxation in β-Ti Alloys

The anelastic relaxation is measured at a low and high frequency from low to high temperature in some Ti alloys, most of which are usually recorded in the internal friction-temperature spectra. In this paper, the point-defect relaxation in β-Ti alloys is described and discussed. The point defects are the interstitial impurity atoms, substitution atoms and vacancies in β-Ti alloys. The reorientation of interstitial impurity O atoms around matrix Ti and substitute atoms produces Snoek-type peaks. The various interactions between the interstitial impurity atoms (O,N,et al.) and matrix atoms/substitute atoms (Ti, Nb,Mo,V,et al. ) can also produce a complex relaxation spectra consisting of several Snoek-type peaks. The concentrations of interstitial impurity atoms and the substitute atoms have important roles on the height and width of the Snoek-type peaks. The reorientation of defect pairs consisting of the interstitial impurity atoms and vacancies is also involved in the relaxation processes. At high temperatures, the Mo-Mo atom-pairs can be reoriented under the inducing of cycle stress and produce similar Zener relaxation peak.

Микроспектроскопия единичных центров окраски в щелочно-галоидных кристаллах

Single color centers in the LiF crystal were studied by confocal fluorescence microscopy with analysis and statistical processing of the time dependences of their fluorescence intensity. The possibility of identifying quantum systems whose selection by luminescence excitation (absorption) spectra, luminescence spectra, and luminescence decay time is difficult due to their overlap or due to the coincidence of the corresponding characteristics of different centers is shown. An additional identification feature in this case is the flicker frequency and other characteristics of the time dependences of the luminescence intensity. The efficiency of the method is demonstrated by the example of two color centers of different nature (F2 and F3, stabilized by an impurity–vacancy dipole) in a lithium fluoride crystal.

Unexpected features in the optical vibrational spectra of δ-UO3

Uranium trioxide displays a complex chemical phase space, with at least six structurally distinct polymorphs accessible via different synthetic routes. Remarkably, despite its technological importance, full structural and electronic characterization of these polymorphs remains an open area of study. δ-UO3 in particular has attracted significant theoretical attention due to its high point group and space group symmetries, having U (VI) in octahedral coordination with polyhedra interconnected through corner-sharing to build a 3-D cubic lattice with space group symmetry Pm-3m and Z = 1. Critical experimental information, such as its optical vibrational spectra, are not known. Here, we study the Raman and infrared (IR) spectra of δ-UO3 together with the support of density functional theory (DFT) calculations for spectral interpretation. A symmetry analysis of the DFT-predicted phonon eigenmodes indicates that δ-UO3 should have two IR active modes and no Raman active modes. Experimental results, however, indicate significant Raman scattering from δ-UO3. We therefore propose four potential explanations for this apparent contradiction: a possible tetragonal distortion to the cubic cell, the existence of a surface impurity layer, vacancy scattering, and structural activation of Raman signal. We use powder X-ray diffraction and confocal Raman spectroscopy with depth profiling to investigate these possibilities and suggest future experiments to explore this phenomenon in more detail. Understanding the lattice dynamics of δ-UO3 is important for identification of technogenic U phases via Raman and infrared spectroscopy and our results indicate that the simple understanding of δ-UO3 as a high-symmetry cubic structure should be reconsidered.

Influence of confinement and surface functionalization effects on fluorescent properties of the impurity–vacancy complexes in ultrasmall nanodiamonds

We have done a thorough ab-initio research of vacancy–impurity complexes (including nitrogen, silicon and germanium) in 0.65 nm thick diamond nanocrystal and compared the results with those obtained for the complexes, located in diamond nanofilms of the same thickness and bulk crystals. While we consider only hydrogen-terminated and bare surfaces of nanoparticles and nanofilms , we are able to investigate the influence of confinement effects onto formation energy and electron structure of the impurity levels in the vicinity of band gap. It was found, that formation energy of these centers in nanoobjects (nanofilms and nanoparticles) is 2 eV less than that of bulk crystals. This implies preferable formation of these centers during growth close to the surface or in ultrasmall nanodiamonds. Confinement effects also significantly affect the band structure of nanoobjects. Beside the well-known effect of the band gap widening, they also lead to “popping up” of filled impurity levels over the valence band maximum. For negatively charged NV− center, located in nanodiamond, this results in interference of the upper, partially filled impurity level with the surface states, bringing about the splitting of this impurity level. We predict, that this effect would manifest itself in intermittent luminescence of NV− doped ultrasmall nanodiamonds with size below 5 nm.

(Digital Presentation) Pinpointing the Potential-of-Zero-Charge in a Alumina-Coated Aluminum/Water Interface Model for Corrosion Applications

The surfaces of most metals immersed in electrolytes feature a several nanometer-thick oxide/hydroxide layer in aqueous electrolytes. This leads to the existence of both a metal/oxide and an oxide/liquid electrotlyte interface. The resulting complexity, and the uncertainty about the structure of and the charges on the surface films, make it challenging to predict from first principles the potential-of-zero-charge (PZC) which has been correlated to the pitting potential [1]. In this work, we use large-scale Density Functional Theory and ab initio molecular dynamics to calculate the PZC of a Al(111)|gamma-Al(2)O(3)|water model within the context of aluminum corrosion. By partitioning the multiple, complex interfaces involved into binary components with additive contributions to the overall work function and voltage, we calculate the PZC in liquid water for this model. Simultaneously, we calculate the orbital energy levels of defects like oxygen vacancies in gamma-Al(2)O(3), which are critical parameters in theories associated with pitting corrosion onset. Our predictions align the Fermi level at PZC with oxygen vacancy impurity defect levels, and estimate the voltage needed to generate charged oxygen vacancies in the flat band approximation. The similarities and differences in results and assumptions used between our model calculations and experiments will be discussed.[1] J.~O'M. Bockris and Y.~Kang, J. Solid Electrochem. 1:17 (1997).Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the document do not necessarily represent the views of the U.S. Department of Energy or the United States Government. Figure 1

Recovery, recrystallization and diffusion in cold-rolled Ni

Abstract Recovery and recrystallization processes in cold-rolled Ni are investigated. While recrystallization and grain growth at temperatures above 700 K lead to a significant (threefold) decrease in micro-hardness, recovery at 600 K is shown to cause a measurable micro-hardness increase. This increase in micro-hardness is confirmed by atomic force microscopy observations of Vickers indents, and it is correlated with the vacancy-annihilation peak of the calorimetric signal. Diffusion of impurity–vacancy complexes to the dislocation cores and the resulting pinning of dislocations are surmised to contribute to the measured micro-hardness increase. Self-diffusion along interfaces in cold-rolled and partially recrystallized Ni samples is measured employing the radiotracer serial sectioning technique. Diffusion rates similar to those of general high-angle grain boundaries in well-annealed coarse-grained Ni polycrystals are observed. The results imply that the diffusion rate along the stationary recrystallization front, i. e. the interfaces separating the recrystallized and the as-deformed regions of the material, is similar to that along a general high-angle grain boundary.

The effect of Cr impurity and Zn vacancy on electronic and magnetic properties of ZnSe crystal

The spin-polarized electronic energy spectra of the ZnCrSe crystal were obtained based on calculations for a supercell containing 64 atoms. First, calculation is performed with an impurity of Cr atom, replacing the Zn atom. In the second variant, the Cr impurity and the vacancy at the Zn atom site are considered simultaneously. The results obtained in the first variant are as follows. It was found that the presence of the Cr atom leads to significant changes in the electronic energy bands, showing a large difference for different spin moments. The density curves of electronic states with opposite spins show an asymmetry, the consequence of which is the existence of a nonzero magnetic moment of the supercell. It was found that in the ZnCrSe crystal electronic 3d states with spin up are present at the Fermi level, i.e. the material is a metal. For spin-down states, the material is a semiconductor in which the Fermi level is inside the band gap. The value of the direct interband gap for electronic states with spin up is equal to 1.56 eV, and the magnetic moment of the supercell is 4.00 . The results obtained by the second variant of the calculation show a significant effect of the vacancy on the zinc site on the electronic structure of the ZnCrSe crystal. The Fermi level now intersects the dispersion curves of the upper part of the valence band for both spin orientations. The magnetic moment of the supercell is 2.74 .

Regulate chemical environment to control the formation of defects on Ta3N5 (1 1 0) surface: From theoretical perspectives

The defect properties of Ta3N5 (110) surface are studied by first-principles calculation. We focus on the formation of three typical defects, oxygen impurity, nitrogen vacancy, and magnesium impurity and further emphasize their roles on changing the electronic structure and band edge position of the surface. Our results show that Mg is more easily doped on (110) surface in nitrogen-rich condition, in which the formation of co-defects is energetically more favorable. These co-defects can reduce the influence of single defects on the electronic structure and band edge position of the (110) surface.

Assessment of positrons for defect studies in CeO2 materials

Disposal of spent nuclear fuel poses significant challenges, as the UO2 and fission products are under constant irradiation and must be safely stored for millennia. CeO2 is a non-radioactive analog for UO2 for studying microstructure and its evolution. Many techniques have been applied to uranium and cerium oxides to investigate point defects. Positron annihilation spectroscopies (PAS) are sensitive to neutral and negatively charged vacancy-like point defects and impurity vacancy complexes. PAS has been applied previously to UO2+x to investigate nuclear fuels, but virtually no PAS work exists on CeO2. Here, the basics of positron annihilation spectroscopy is reviewed, and preliminary work on undoped and doped CeO2 is shown and compared to the literature results from UO2. To simulate fission product incorporation in spent nuclear fuels, CeO2 samples were doped at different concentration with yttrium. Select samples were irradiated with heavy ions at different doses. Doping and irradiation are shown to give rise to different defect characteristics.

Photoluminescence line shapes for color centers in silicon carbide from density functional theory calculations

Silicon carbide with optically and magnetically active point defects offers unique opportunities for quantum technology applications. Since interaction with these defects commonly happens through optical excitation and de-excitation, a complete understanding of their light-matter interaction in general and optical signatures, in particular, is crucial. Here, we employ quantum mechanical density functional theory calculations to investigate the photoluminescence lineshapes of selected, experimentally observed color centers (including single vacancies, double vacancies, and vacancy impurity pairs) in 4H-SiC. The analysis of zero-phonon lines as well as Huang-Rhys and Debye-Waller factors are accompanied by a detailed study of the underlying lattice vibrations. We show that the defect lineshapes are governed by strong coupling to bulk phonons at lower energies and localized vibrational modes at higher energies. Generally, good agreement to the available experimental data is obtained, and thus we expect our theoretical work to be beneficial for the identification of defect signatures in the photoluminescence spectra and thereby advance the research in quantum photonics and quantum information processing.

A quantum-logic gate between distant quantum-network modules

The big challenge in quantum computing is to realize scalable multi-qubit systems with cross-talk-free addressability and efficient coupling of arbitrarily selected qubits. Quantum networks promise a solution by integrating smaller qubit modules to a larger computing cluster. Such a distributed architecture, however, requires the capability to execute quantum-logic gates between distant qubits. Here we experimentally realize such a gate over a distance of 60 meters. We employ an ancillary photon that we successively reflect from two remote qubit modules, followed by a heralding photon detection, which triggers a final qubit rotation. We use the gate for remote entanglement creation of all four Bell states. Our nonlocal quantum-logic gate could be extended both to multiple qubits and many modules for a tailor-made multi-qubit computing register.