Bulk Impurities Research Articles

Growth of twisted bilayer graphene through two-stage chemical vapor deposition

We investigate growth of twisted bilayer graphene through two-stage chemical vapor deposition (CVD). Exploiting the synergetic nucleation and growth dynamics involving carbon sources from the residual carbon impurities in Cu bulk and gaseous CHx, sub-millimeter-sized single crystalline graphene grains with multiple merged adlayer grains formed underneath are grown on Cu substrate. The distribution of the twist angles is investigated through a computer algorithm utilizing spectral features from micro-Raman mapping. Besides the more thermodynamically stable AB-stacking (AB-BLG) or large angle (>15°) decoupled bilayer graphene (DC-BLG) configurations, there are some bilayer regions that contain specific twist angles (3–8°, 8–13°, and 11–15°) (termed as TBLG). The statistics show no TBLG formation for BLG with single nucleation center. The formation probability of TBLG is strongly dependent on the relative orientation of merging adlayer grains. Significant defects are found at the grain boundaries formed in AB-DC merging event without creating TBLG domain. The areal fraction of TBLG increases as H2/CH4 ratio increases. The growth mechanism of TBLG is discussed in light of the interactions between the second layer grains with consideration of strain generation during merging of adlayers.

Mechanical properties of compositionally-rich body-centered cubic VNbTaMoW alloy

We determined the mechanical properties, microstructure, and composition of a VNbTaMoW alloy enriched with carbon, oxygen, and nitrogen using a combination of nano- and micro-indentation, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. We show that the alloy is nearly equiatomic, V0.20Nb0.20Ta0.20Mo0.22W0.18, body-centered cubic polycrystal, with depth-dependent C, N, and O concentrations, whose elastic moduli and hardnesses decrease with increasing indentation depth due primarily to depth-dependent changes in the bulk impurity concentration.

Evaluation and mitigation of trace 210Pb contamination on copper surfaces

Clean materials are required to construct and operate many low-background physics experiments. High-purity copper has found broad use because of its physical properties and availability. In this paper, we describe methods to assay and mitigate 210Pb contamination on copper surfaces, such as from exposure to environmental radon or coming from bulk impurities. We evaluated the efficacy of wet etching on commercial samples and observed that 210Po contamination from the copper bulk does not readily pass into solution. During the etch, the polonium appears to trap at the copper–etchant boundary, such that it is effectively concentrated at the copper surface. We observed a different behavior for 210Pb; high-sensitivity measurements of the alpha emissivity versus time indicate the lowest level of 210Pb contamination ever reported for a commercial copper surface: 0±12nBq/cm2 (1σ). Additionally, we have demonstrated the effectiveness of mitigating trace 210Pb and 210Po surface backgrounds using custom, high-purity electroplating techniques. These approaches were evaluated utilizing assays performed with an XIA UltraLo-1800 alpha spectrometer.

Doping of MoTe2 via Surface Charge Transfer in Air.

Doping is a key process by which the concentration and type of majority carriers can be tuned to achieve desired conduction properties. The common way of doping is via bulk impurities, as in the case of silicon. For van der Waals bonded semiconductors, control over bulk impurities is not as well developed, because they may either migrate between the layers or bond with the surfaces or interfaces becoming undesired scattering centers for carriers. Herein, we investigate by means of Kelvin probe force microscopy (KPFM) and density functional theory calculations (DFT) the doping of MoTe2 via surface charge transfer occurring in air. Using DFT, we show that oxygen molecules physisorb on the surface and increase its work function (compared to pristine surfaces) toward p-type behavior, which is consistent with our KPFM measurements. The surface charge transfer doping (SCTD) driven by adsorbed oxygen molecules can be easily controlled or reversed through thermal annealing of the entire sample. Furthermore, we also demonstrate local control of the doping by contact electrification. As a reversible and controllable nanoscale physisorption process, SCTD can thus open new avenues for the emerging field of 2D electronics.

Defect engineering of p‐type silicon heterojunction solar cells fabricated using commercial‐grade low‐lifetime silicon wafers

AbstractIn this work, we integrate defect engineering methods of gettering and hydrogenation into silicon heterojunction solar cells fabricated using low‐lifetime commercial‐grade p‐type Czochralski‐grown monocrystalline and high‐performance multicrystalline wafers. We independently assess the impact of gettering on the removal of bulk impurities such as iron as well as the impact of hydrogenation on the passivation of grain boundaries and B‐O defects. Furthermore, we report for the first time the susceptibility of heterojunction devices to light‐ and elevated temperature–induced degradation and investigate the onset of such degradation during device fabrication. Lastly, we demonstrate solar cells with independently verified 1‐sun open‐circuit voltages of 707 and 702 mV on monocrystalline and multicrystalline silicon wafers, respectively, with a starting bulk minority‐carrier lifetime below 40 microseconds. These remarkably high open‐circuit voltages reveal the potential of inexpensive low‐lifetime p‐type silicon wafers for making devices with efficiencies without needing to shift towards n‐type substrates.

Impurity-induced resonant states in topological nodal-line semimetals

Nodal-line semimetals are characterized by a kind of topologically nontrivial bulk-band crossing, giving rise to almost flat surface states. Yet, a direct evidence of the surface states is still lacking. Here we study theoretically impurity effects in topological nodal-line semimetals based on the T-matrix method. It is found that for a bulk impurity, some in-gap states may be induced near the impurity site, while the visible resonant impurity state can only exist for certain strength of the impurity potentials. For a surface impurity, robust resonant impurity states exist in a wide range of impurity potentials. Such robust resonant states stem from the topological protected weak dispersive surface states, which can be probed by scanning tunneling microscopy, providing a strong signature of the topological surface states in the nodal-line semimetals.

Development and Validation of Stability Indicating HPLC Method for the Determination of Impurities in the Sterile Mixture of Cefoperazone and Sulbactam

Background: Cefoperazone Sulbactam injection is a cephalosporin antibiotic with a β- lactamase inhibitor used in the treatment for intra abdominal infections, Urinary track infections, surgical infections, etc. The combination is not official in any of the pharmacopeia for their content and impurities determination. Introduction: The present study involves the development of a simple, rapid, accurate, sensitive and stability indicating RP-HPLC method for the quantitative estimation of Cefoperazone Sulbactam mixture and its impurities in bulk and pharmaceutical dosage forms. Methods: 0.005 M Tetrabutyl ammonium hydroxide buffer solution pH adjusted to 6.80 and Acetonitrile combination has been used in a gradient programme with a flow rate of 1.0 ml/min. The retention time of Cefoperazone and Sulbactam were observed at around 8.5 and 19.5 minutes respectively. The UV detection was carried out at a wavelength of 230 nm. The chromatographic separation was achieved using Waters xbridge C18-150*4.6 mm, 3.5 µm HPLC column. The method has been validated according to the current International Council for Harmonization (ICH) guidelines for the method validation parameters such as Specificity, linearity, range, accuracy, precision, robustness and sensitivity. Results: The validation results indicate that the method is specific, as the known impurities and other impurities formed during the forced degradation studies were not co-eluting with the main components. Moreover, all these impurities were found to be spectrally pure, proving the stability indicating power of the method. The linearity and range of the method is in the range of 0.01-150%, highly accurate (100.2%), precise (<1%) and robust. Conclusion: The proposed method was accurate and specific for the quantitative analysis of Cefoperazone and Sulbactam and their related impurities in the sterile mixture. Hence the proposed method can be used for the quantification of impurities in routine as well as stability analysis in the development as well as quality control laboratories.

Opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) film-coated tandem-90°-twisted nematic liquid crystal cell

The opto-electrically and electro-optically controllable diaphragm aperture in a poly(N-vinylcarbazole) (PVK) film-coated tandem-90°-twisted nematic liquid crystal (TNLC) cell is investigated. The tandem-90°-TNLC cell assembled by two common 45°-TNLC cells can be adopted for incident linearly polarized lights at any linear polarization direction. The variations of diaphragm aperture made by a PVK film-coated tandem-90°-TNLC cell are demonstrated opto-electrically and electro-optically. The differences in the degree of linear polarization between the tandem-90°-TNLC and single-90°-TNLC cells for demonstrating diaphragm apertures are investigated. The dynamic transmittance, related to orientation of LCs resulting from the built-in electric fields induced by the diffused positive and negative ions in LC bulk, the induced electric fields generated by the diffused ions in LC bulk and conductive PVK films, and the ion-moving induced force, of the PVK film-coated tandem-90°-TNLC cell is studied, and the possible mechanism based on dynamic distribution of ions (impurities) in LC bulk is also proposed. The proposed mechanism of the dynamic transmittance phenomenon can be a useful reference to the developments of other PVK film-coated LC cells. Moreover, the PVK film-coated tandem-90°-TNLC cell can be considered a controllable diaphragm aperture shutter if the above ion-induced issues are overcome.

Titanium-Dioxide-Based Visible-Light-Sensitive Photocatalysis: Mechanistic Insight and Applications

Titanium dioxide (TiO2) is one of the most practical and prevalent photo-functional materials. Many researchers have endeavored to design several types of visible-light-responsive photocatalysts. In particular, TiO2-based photocatalysts operating under visible light should be urgently designed and developed, in order to take advantage of the unlimited solar light available. Herein, we review recent advances of TiO2-based visible-light-sensitive photocatalysts, classified by the origins of charge separation photo-induced in (1) bulk impurity (N-doping), (2) hetero-junction of metal (Au NPs), and (3) interfacial surface complexes (ISC) and their related photocatalysts. These photocatalysts have demonstrated useful applications, such as photocatalytic mineralization of toxic agents in the polluted atmosphere and water, photocatalytic organic synthesis, and artificial photosynthesis. We wish to provide comprehension and enlightenment of modification strategies and mechanistic insight, and to inspire future work.

Investigating the Effects of Magnesium Doping in Various Ni-Rich Positive Electrode Materials for Lithium Ion Batteries

This work studies the effect of Mg doping in LiMO2 (M = Ni, Ni+Al, Ni+Co+Al, Ni content >0.8) at a dopant level of less than 5%. Synthesized materials were all single phase and contained no appreciable amount of surface or bulk impurities. Structural and electrochemical characterization of the materials was carried out to understand how Mg affects the materials and whether the presence of Al or Co influences the dopant effects of Mg. All synthesized materials, even those without Co, were found to contain a small amount of Ni in the Li layer. Increases in the Mg content of the material reduced the initial capacity of the materials but improved the capacity retention. The capacity reduction was related to the amount of Li trapped by electrochemically inactive Al and Mg. The capacity retention was related either to the number of Ni ions substituted by electrochemically inactive dopants or to the amount of Li trapped by those dopants. Mg helped improve capacity retention by reducing the growth of polarization during cycling. No synergistic effects were found to exist from co-doping materials containing Co and/or Al with Mg.

Density and T1 of Surface and Bulk Spins in Diamond in High Magnetic Field Gradients

We report on surface and bulk spin density measurements of diamond, using ultra-sensitive magnetic force microscopy with magnetic field gradients up to 0.5 T/$\mu$m. At temperatures between 25 and 800 mK, we measure the shifts in the resonance frequency and quality factor of a cantilever with a micromagnet attached to it. A recently developed theoretical analysis allows us to extract a surface spin density of 0.072 spins/nm$^2$ and a bulk spin density of 0.4 ppm from this data. In addition, we find an increase of the $T_1$ time of the surface spins in high magnetic field gradients due to the suppression of spin diffusion. Our technique is applicable to a variety of samples other than diamond, and could be of interest for several research fields where surface, interface or impurity bulk spin densities are an important factor.

Dendritic Oxide Growth in Zerovalent Iron Nanofilms Revealed by Atom Probe Tomography

Atom probe tomography (APT) analysis of chemically pure nanofilms of zerovalent iron (Fe(0), or ZVI) and their thermal oxide nano-overlayers reveals the presence of dendritic iron oxide features that extend from the oxide nano-overlayer surface into the ZVI bulk. The dendrites are observed by APT to be in the 5 nm × 10 nm size range and form quickly under natural atmospheric conditions. Their growth into the ZVI layer is, within the limit of our three month long study, self-limiting (i.e., their initial growth appears to quickly discontinue). The atomistic views presented here shed first light on the atmospheric corrosion process of Fe(0)-bearing engineered nanostructures and their surfaces in the limit of low bulk impurities. Possible roles of the newly identified oxidized iron dendrites are also discussed in the context of passivation processes limiting technological applications of Fe(0).

Local, electronic and surface structure of multi-component Fe-doped CdTe(S) systems

Local structural and electronic properties around Fe in multi-component Cd0.99Fe0.01Te0.97S0.03 system were studied by means of X-ray absorption fine structure (XAFS). Composition of non-polar (110) surfaces of Cd0.97Fe0.03Te and Cd0.99Fe0.01Te0.97S0.03 systems and mechanism of their oxidation in ambient conditions were studied by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS). It has been found that Fe preferentially substitutes Cd, but due to much smaller covalent radius and preferences for paring with S, it causes local distortion of the host CdTe lattice. The distortion is confined to the Fe-immediate surrounding and the second and third coordination shell atoms are (inside experimental uncertainties) placed at distances expected in CdTe. Although local structure around Fe is well defined in the bulk of both samples, their near-surface region is completely depleted from Fe, and in case of Cd0.99Fe0.01Te0.97S0.03 somewhat enriched in S. Special attention is, therefore, paid to characterization of the near-surface region and evaluation of its composition and structure. To that end we have introduced a general standard-free algorithm for XPS data analysis of the two-layer surface structure (bulk, oxide layer, and the impurity layer). Results of the in-depth composition analysis revealed that despite different bulk composition and impurity layer thickness, underneath the topmost impurity layer lays approximately one monolayer of CdTeO3 which passivates the surface.

Bubble states of atoms in liquid helium

The solvation of impurity atoms {Na, K, Rb, Cs, Mg, Ca, Sr, Ba} with small ionization energy in liquid helium is studied. It is shown that allowance for unmatched interaction of impurity atoms and helium atoms is important for determining the parameters of the atomic state. The size of the bubble state R0 is determined by the strong repulsion between helium atoms and valence electrons of impurity atoms. The R0 value is universal, and the expression 2R0/ν = const ≈ 11–13 depends weakly on the place of the impurity atom in the periodic table. The parameter ν is given by the impurity ionization energy J = 1/2ν2 (in atomic units); the ν value indicates the size of the impurity atom. An energy barrier that separates the bulk and surface atomic impurity states is determined.

Surface-displayed porcine reproductive and respiratory syndrome virus from cell culture onto gram-positive enhancer matrix particles.

Vaccine immunization is now one of the most effective ways to control porcine reproductive and respiratory syndrome virus (PRRSV) infection. Impurity is one of the main factors affecting vaccine safety and efficacy. Here we present a novel innovative PRRSV purification approach based on surface display technology. First, a bifunctional protein PA-GRFT (protein anchor-griffithsin), the crucial factor in the purification process, was successfully produced in Escherichia coli yielding 80 mg/L of broth culture. Then PRRSV purification was performed by incubation of PA-GRFT with PRRSV and gram-positive enhancer matrix (GEM) particles, followed by centrifugation to collect virions loaded onto GEM particles. Our results showed that most of the bulk impurities had been removed, and PA-GRFT could capture PRRSV onto GEM particles. Our lactic acid bacteria-based purification method, which is promising as ease of operation, low cost and easy to scale-up, may represent a candidate method for the large-scale purification of this virus for vaccine production.

Saturable impurity in an optical array: Green function approach

We examine a one-dimensional linear waveguide array containing a single saturable waveguide. By using the formalism of lattice Green functions, we compute in closed form the localized mode and the transmission across the impurity in closed form. For the single saturable impurity in the bulk, we find that an impurity state is always possible, independent of the impurity strength. For the surface saturable impurity case, a minimum nonlinearity strength is necessary to create a bound state. The transmission coefficient across the impurity shows a sub-linear behavior with an absence of any resonance. The dynamical selftrapping at the bulk impurity site shows no selftrapping transition, and it resembles the behavior of a weak linear impurity. For the surface impurity however, there is a selftrapping transition at a critical nonlinearity value. The asymptotic propagation of the optical power shows a ballistic character in both cases, with a speed that decreases with an increase in nonlinearity strength.

Nonlinear Optical and Laser Damage Properties of KDP Crystal with Trace Impurities in Bulk

AbstractSmall‐sized potassium dihydrogen phosphate (KDP) samples cut from the pyramid sector of a large KDP boule are observed to have different impurity content. The nonlinear absorption properties of two KDP samples containing different impurities are characterized by Z‐scan technique with picosecond laser at 355 nm wavelength. Reverse saturable absorption effects are found in samples, with nonlinear absorption coefficients of 7.35 × 10−2 cm GW−1 and 15.91 × 10−2 cm GW−1. The sample with more impurity contents exhibits stronger nonlinear absorption. Such behavior is considered to be a result of nonlinear energy coupling under 355 nm laser irradiation assisted by impurity defects. Nanosecond laser‐induced damage test at 355 nm of the two KDP samples is further investigated. The bulk damage thresholds of the two samples are 4.7 and 0.423 J cm−2 respectively, indicating that impurity‐induced nonlinear absorption might play an important role in laser‐induced damage processes.

Fast Pulling of n-Type Si Ingots for Enhanced Si Solar Cell Production

Reducing the manufacturing costs of silicon substrates is an important issue in the silicon-based solar cell industry. In this study, we developed a high-throughput ingot growth method by accelerating the pulling speed in the Czochralski process. By controlling the heat flow of the ingot growth chamber and at the solid-liquid interfaces, the pulling speed of an ingot could be increased by ~ 15% compared to the conventional method, while retaining high quality. The wafer obtained at a high pulling speed showed an enhanced minority carrier lifetime compared with conventional wafers, due to the vacancy passivation effect, and also demonstrated comparable bulk resistivity and impurities. The results in this work are expected to open a new way to enhance the productivity of Si wafers used for Si solar cells, and therefore, to reduce the overall manufacturing cost.

Self-organized carbon-rich stripe formation from competitive carbon and aluminium segregation at Fe0.85Al0.15(1 1 0) surfaces

By combining Scanning Tunnelling Microscopy, Low Energy Electron Diffraction and X-ray Photoelectron Spectroscopy, it was found that the surface of A2 random alloy Fe0.85Al0.15(110) is significantly influenced by the segregation of aluminium but also of carbon bulk impurities. Below ∼900 K, carbon segregates in the form of self-organized protruding stripes separated by ∼5 nm that run along the [001]B bulk direction and cover up to 34% of the surface. Their C 1s spectroscopic signature that is dominated by graphitic carbon peaks around 900 K. Above this temperature, the surface carbon concentration decays by redissolution in the bulk, whereas an intense aluminium segregation is observed giving rise to a hexagonal superstructure. The present findings is interpreted by a competitive segregation between the two elements.

Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow to determine the room temperature carrier mobilities in the surface states despite the presents of thermally activate residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30{\div}60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.