Negative Charge State Research Articles

Charged dust in the D-region incoherent scatter spectrum

We investigate the influence of charged dust on the incoherent scatter from the D-region ionosphere. Incoherent scatter is observed with high-power, large aperture radars and results from electromagnetic waves scattering at electrons that are coupled to other charged components through plasma oscillations. The influence of charged dust can hence be considered an effect of dusty plasma. The D-region contains meteoric smoke particles that are of nanometre size and form from incoming ablating meteors. Detection of such charged dust in the incoherent scatter spectrum from the D-region has previously been proposed and studied to some degree. We here present model calculations to investigate the influence of the charged dust component with a size distribution, instead of the one size dust components assumed in other works. The developed code to calculate the incoherent scatter spectrum from the D-region including dust particles with different sizes and different positive and negative charge states is made available (https://doi.org/10.18710/GHZIIY). We investigate how sizes, number density and charge state of the dust influence the spectrum during different ionospheric conditions. We consider the ionospheric parameters for the location of the EISCAT VHF radar during a year and find that conditions are most suitable for dust detection in winter below 80 km at times with increased electron densities. The prospects to derive dust parameters increase, when the incoherent scatter observations are combined with those of other instruments to provide independent information on electron density, neutral density and temperature.

Increasing Iridium Oxide Activity for the Oxygen Evolution Reaction with Hafnium Modification.

Synthesis and implementation of highly active, stable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is a major challenge in developing energy efficient and economically viable energy conversion devices such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. The current benchmark electrocatalyst for OER is based on iridium oxide (IrOx) due to its superior performance and excellent stability. However, large scale applications using IrOx are impractical due to its low abundance and high cost. Herein, we report a highly active hafnium-modified iridium oxide (IrHfxOy) electrocatalyst for OER. The IrHfxOy electrocatalyst demonstrated ten times higher activity in alkaline conditions (pH = 11) and four times higher activity in acid conditions (pH = 1) than a IrOx electrocatalyst. The highest intrinsic mass activity of the IrHfxOy catalyst in acid conditions was calculated as 6950 A gIrOx-1 at an overpotential (η) of 0.3 V. Combined studies utilizing operando surface enhanced Raman spectroscopy (SERS) and DFT calculations revealed that the active sites for OER are the Ir-O species for both IrOx and IrHfxOy catalysts. The presence of Hf sites leads to more negative charge states on nearby O sites, shortening of the bond lengths of Ir-O, and lowers free energies for OER intermediates that accelerate the OER process.

Structural resolution of disaccharides through halogen anion complexation using negative trapped ion mobility spectrometry

Carbohydrates are an indispensable part of early life evolution. The determination of their structures is a key step to analyze their critical roles in biological systems. A variation of composition, glycosidic linkage, and (or) configuration between carbohydrate isomers induces structure diversity and brings challenges for their structural determination. Ion mobility spectrometry (IMS), an emerging gas-phase ion separation technology, has been considered as a promising tool for performing carbohydrate structure elucidation. In this work, eight disaccharides were analyzed by trapped ion mobility spectrometry-mass spectrometry (TIMS-MS) in the negative ion mode as the complexed form of [M + X]-, where M = disaccharide, and X = Cl, Br, and I. As compared to the positive ion analysis of the selected disaccharide in a sodiated form, a reversal charge state provided the ability to eliminate or even reverse the collision cross section (CCS) difference between disaccharide isomers. By the combination of TIMS analysis and the calculation of density functional theory, the only observed two conformers of ions [lactulose + I]- may result from different adduction sites for an iodide anion. Based on the comparison of different halogen adducts, the [M + I]- ion form exhibited more powerful ability for isomeric disaccharide differentiation with an average resolution (RP-P) of 1.17, which results in a 34.5% improvement as compared to the corresponding chloride adducts. This result indicates that the use of negative charge states, especially the complexation of an iodide anion, could be a supplemental strategy to commonly used positive ion analysis for carbohydrate separation.

Thermal behaviors of the sharp zero–phonon luminescence lines of NV center in diamond

The nitrogen–vacancy (NV) center in diamond has quickly emerged as a promising solid–state candidate for single-photon emitters, quantum bits or qubits, and ultrasensitive sensors in recent years. At the same time, it may also provide an excellent molecule–like platform for exploiting the fundamental principles of quantum mechanics, such as quantum entanglement mechanism. Therefore, it is of both scientific and technological significance to have an in-depth investigation on NV center, especially on the optoelectronic processes and optical properties of NV center. In this study, the temperature dependence of the sharp zero–phonon lines (ZPL) of luminescence of NV center with neutral and negative charge states in a CVD–grown diamond crystal is measured in the range from 5 K to 300 K. It is shown that the experimental data, including the temperature induced broadening, peak shift and even intensity quenching of the two sharp ZPL lines of NV0 and NV−1, all on the whole support a generalized quantum theory in spite of in which Franck–Condon, harmonic and Debye approximations were assumed. In particular, the thermal quenching tendency of the integrated intensities can be described by Debye–Waller factor in the theory of the Mössbauer effect. The study sheds some light on the challenging issue, the temperature dependence of the so–called zero-phonon luminescence of NV center in diamond.

Interplay of Anionic Quasi-Atoms and Interstitial Point Defects in Electrides: Abnormal Interstice Occupation and Colossal Charge State of Point Defects in Dense fcc-Lithium.

Electrides are an emerging class of materials with highly localized electrons in the interstices of a crystal that behave as anions. The presence of these unusual interstitial quasi-atom (ISQ) electrons leads to interesting physical and chemical properties and wide potential applications for this new class of materials. Crystal defects often have a crucial influence on the properties of materials. Introducing impurities has been proved to be an effective approach to improve the properties of a material and to expand its applications. However, the interactions between the anionic ISQs and the crystal defects in electrides are yet unknown. Here, dense fcc-Li was employed as an archetype to explore the interplay between anionic ISQs and interstitial impurity atoms in this electride. This work reveals strong coupling among the interstitial impurity atoms, the ISQs, and the matrix Li atoms near to the defects. This complex interplay and interaction mainly manifest as the unexpected tetrahedral interstitial occupation of impurity atoms and the enhancement of electron localization in the interstices. Moreover, the Be impurity occupying the octahedral interstice shows the highest negative charge state (Be8-) discovered thus far. These results demonstrate the rich chemistry and physics of this emerging material and provide a new basis for enriching their variants for a wide range of applications.

Luminescence of brown CVD diamond: 468 nm luminescence center

Detailed study of the luminescence of multiple brown CVD diamonds was performed. It has been found that the well-known optical center with zero-phonon line at 468 nm is a characteristic of brown color. It has been found that the defects responsible for 468 nm center are located within brown striations suggesting close relation of the 468 nm center and the vacancy clusters. Simultaneous reduction of the intensity of 468 nm center and brown color during annealing support the assumption of their close relation. Identical spectroscopic parameters of the 468 nm center and the radiation center with ZPL at 492 nm suggest that the former relates to an intrinsic defect probably containing vacancies. The distribution of intensity of the 468 nm center in some brown diamonds follows the distribution of the NV− center while being opposite to that of the NV0 center and the dislocation-related A-band. This observation suggests the negative charge state of the 468 nm center. Due to its high luminescence efficiency, the 468 nm center can be used as a highly sensitive indicator of the traces of vacancy clusters. We found that the 468 nm center is detected practically in every as-grown CVD diamond including colorless CVD diamonds of high structural perfection and high purity.

Effects of oxidized Ketjen Black as conductive additives on electrochemical performance of the LiMn2O4@Al2O3 cathode in lithium-ion batteries

Metal oxide modified LiMn2O4 (LMO) is an effective method to improve the cycling performance. However, the metal oxide usually has low electronic conductivity, which lead to poor rate performance. Herein, Al2O3 modified LMO (LMO@AO) with enough oxygen vacancies have been prepared, and further constructed a highly conductive network of LMO@AO/KBO electrode though a self-assembly method using oxidized Ketjen Black (KBO) as conductive carbon additives. The KBO with a large number of oxygen-containing groups and show a negative charge state, while LiMn2O4@Al2O3 particles have a positive charge state. Therefore, the KBO can uniformly adsorption on the surface of LMO@AO particles, forming a continuously conductive network by self-assembly process. In this constructed architecture, a continuously conductive network can not only enhance the electronic conductivity but also facilitate the Li ions transport. As a result, the LMO@AO/KBO-3 electrode still shows excellent rate capability (a high reversible capacity of 83.3 mAh g−1 can be achieved at ultrahigh rate of 10 C) and superior long-term cycling stability (with a high capacity retention of 85.2% at 10 C even after 900 cycles).

Prediction of LinCd compounds with unusual stoichiometry and valence states

Cadmium (Cd) and its compounds are a family of materials exhibiting diverse physical and chemical properties that find wide-ranging applications in many science and technology fields. In all known Cd compounds, Cd ions behave like a typical $d$-block element, donating electrons and adopting positive $(+2$ or $+1)$ charge states. To explore the possibility of forming compounds with Cd in negative charge states, would greatly expand this material family and enrich understanding of the associated physical and chemical properties and the underlying mechanisms. Here, we report on an exemplary case study via a systematic search and examination of Li-Cd binary compounds over a wide range of stoichiometry. We have employed an unbiased crystal structure search method in conjunction with first-principles energetic calculations to predict viable crystal structures under ambient and high-pressure conditions. Our investigations have identified a series of stable ${\mathrm{Li}}_{n}\mathrm{Cd}$ compounds with an unusually wide range $(n=1$--6) of stoichiometry at both ambient and high pressures. Strikingly, our results reveal that Cd attracts electrons from Li in forming a rich variety of bonding configurations with the Cd ions adopting a range of negative charge states, producing highly tunable structural and electronic properties. Moreover, calculated elastic parameters show that the mechanical properties of LiCd compound can be enhanced considerably over those of Li and Cd metals and other Li-Cd binary compounds. We analyze the charge transfer from Li to Cd and the resulting electron distribution patterns and find a significant and variable amount of partial occupation of the Cd-$5p$ states in this traditional $d$-block element. The present findings provide insights into fundamental knowledge and practical processes in valence state control for tuning transitions from $d$- to $p$-block behaviors via a proper choice of compound formation and application of high pressure. The results expand conventional definition and understanding of charge states in the compound formation and enrich structure-property relations that have broad implications for Li-rich binary compounds.

Simultaneous assessment of energy, charge state and angular distribution for medium energy ions interacting with ultra-thin self-supporting targets: A time-of-flight approach

We demonstrate simultaneous measurements of the charge state, energy and angular distribution of keV ions in transmission experiments through self-supporting foils. Using a time-of-flight approach we have introduced an electrostatic deflection apparatus as an extension to existing medium energy ion scattering (MEIS) instrumentation. Different positive, neutral and negative charge states have been discriminated and quantified for initially singly charged beams of He, N, O and Ne in the energy range from 25 to 250 keV. In parallel, the ion energy after interaction with the target has been assessed for all detected particles, while particles can be discriminated by deflection angle. Self-supporting thin carbon foils were used as samples to benchmark our experiments with literature data where available.

Core-level shifts in x-ray photoelectron spectroscopy of arsenic defects in silicon crystal: A first-principles study

We systematically investigated the arsenic (As) 3d core-level x-ray photoelectron spectroscopy (XPS) binding energy and formation energy for As defects in silicon by first-principles calculation with a high accuracy of 0.1 eV by careful evaluation of the supercell size. For As, we adopt a pseudopotential with 3d states as the valence and the spherical hole approximation to ensure the convergence of self-consistent calculation for the XPS binding energy with large size systems. Some of the examined model defects have threefold coordinated As atoms. The XPS binding energies of these As atoms are distributed in the narrow region from −0.66 eV to −0.73 eV in neutral charge states. Such defects in negative charge states have a lower XPS binding energy by about 0.1 eV. From the XPS binding energy and electrical activity, negatively charged defects of a vacancy and two adjacent substitutional As atoms (As2V) are the most probable candidates for the experimentally observed peak at −0.8 eV called BEM from the reference substitutional As peak. Under the experimental condition, we find that As2V−,2− do not deeply trap electrons and are electrically inactive. We also demonstrate the surface effect that surface states near the bandgap decrease the XPS binding energy, which may generate defects with low binding energies similarly to the experimental peak at −1.2 eV called BEL.

Visible-Light Activation of Photocatalytic for Reduction of Nitrogen to Ammonia by Introducing Impurity Defect Levels into Nanocrystalline Diamond.

Nitrogen impurity has been introduced in diamond film to produce a nitrogen vacancy center (NV center) toward the solvated electron-initiated reduction of N2 to NH3 in liquids, giving rise to extend the wavelength region beyond the diamond’s band. Scanning electron microscopy and X-ray diffraction demonstrate the formation of the nanocrystalline nitrogen-doped diamond with an average diameter of ten nanometers. Raman spectroscopy and PhotoLuminescence (PL) spectrum show characteristics of the NV0 and NV− charge states. Measurements of photocatalytic activity using supraband (λ < 225 nm) gap and sub-band gap (λ > 225 nm) excitation show the nitrogen-doped diamond significantly enhanced the ability to reduce N2 to NH3 compared to the polycrystalline diamond and single crystal diamond (SCD). Our results suggest an important process of internal photoemission, in which electrons are excited from negative charge states into conduction band edges, presenting remarkable photoinitiated electrons under ultraviolet and visible light. Other factors, including transitions between defect levels and processes of reaction, are also discussed. This approach can be especially advantageous to such as N2 and CO2 that bind only weakly to most surfaces and high energy conditions.

Density functional investigation of boron incorporation in silicon-vacancy complexes

The neutral silicon-vacancy complex in diamond is of interest for quantum applications due to its favourable optical properties relative to both its negative charge state and the nitrogen-vacancy centre. To establish an uncharged form, co-doping with electrically active impurities has been suggested, and although complexes with hydrogen or nitrogen have been identified, complexes with boron are largely unstudied. This report presents results from density-functional modelling of SiB, SiVB and SiVBH complexes, some of which are expected to produce highly characteristic magnetic signatures. Critically for the neutral silicon-vacancy complex, we find that boron binds less strongly than nitrogen or hydrogen.

Relative stability of planar clusters B11, B12, and B13 in neutral- and charged-states

Theoretically, within the diatomic model, the relative stability of most abundant boron clusters B11, B12, and B13 with planar structures in neutral, positive and negative charged-states is studied. According to the specific (per atom) binding energy criterion, B12+ (6.49 eV) is found to be the most stable boron cluster, while B11– + B13+ (5.83 eV) neutral pair is expected to present the preferable ablation channel for boron-rich solids. Obtained results would be applicable in production of boron-clusters-based nanostructured coating materials with super-properties such as lightness, hardness, conductivity, chemical inertness, neutron-absorption, etc., making them especially effective for protection against cracking, wear, corrosion, neutron- and electromagnetic-radiations, etc.

Electron paramagnetic resonance and theoretical study of gallium vacancy in β -Ga2O3

Unintentionally doped n-type β-Ga2O3 becomes highly resistive after annealing at high temperatures in oxygen ambient. The annealing process also induces an electron paramagnetic resonance (EPR) center, labeled IR1, with an electron spin of S = 1/2 and principal g-values of gxx = 2.0160, gyy = 2.0386, and gzz = 2.0029 with the principal axis of gzz being 60° from the [001]* direction and gyy along the b-axis. A hyperfine (hf) structure due to the hf interaction between the electron spin and nuclear spins of two equivalent Ga atoms with a hf splitting of ∼29 G (for 69Ga) has been observed. The center can also be created by electron irradiation. Comparing the Ga hf constants determined by EPR with corresponding values calculated for different Ga vacancy-related defects, the IR1 defect is assigned to the double negative charge state of either the isolated Ga vacancy at the tetrahedral site (V2−Ga(I)) or the VGa(I)–Gaib–VGa(I) complex.

Diffusion properties of n-type dopants diffused from spin on glass into Ge

Solid phase diffusion properties of three typical n-type dopants (P, As, and Sb) in Ge have been studied through experimental depth profiles of dopants diffused from spin on glass (SOG) films. Numerical calculations of dopant profiles in Ge are performed to extract the diffusion constant through comparison with the experimental profiles. It is pointed out that experimental results of diffusion at low temperature are important to correctly examine the influence of negative charge states of a vacancy on dopant diffusion. It is found that vacancies with the charge states of −2 mediate the diffusion of P and As without implantation-induced defects. The diffusion constants of P and As, obtained in this study, are larger than those for intrinsic diffusion, while that of Sb is almost the same as that for intrinsic diffusion. The present larger diffusion constants of P and As can be explained by the supply of vacancies from Ge surfaces via E-centers during diffusion from SOG.

Optical Gating of Photoluminescence from Color Centers in Hexagonal Boron Nitride.

We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2 eV. Resonant, or quasi-resonant, excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect-dependent threshold between 2.25 and 2.6 eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4 eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center.

Hydrogenation behaviors in passivated emitter and rear silicon solar cells with variously hydrogenated SiNx films

From the point of view in sustainable energy, photovoltaic (PV) technology has been recognized as one of the most suitable solutions to replace fossil fuel technologies for electrical generation. However, in PV modules, 95% of solar cells made from P-type boron doped CZ-grown silicon substrates have a reliability issue, called light induced degradation (LID), which strongly affects their practically long-term use. Carrier induced hydrogenation (CIH) is one of the solutions through injection of excess carrier into Si solar cells with moderate thermal treatment. The mechanism of CIH process is the passivation of B-O related defects by hydrogen atoms with negative charge state. In the report, we can find the efficiency enhancement of commercial passivated emitter and rear cells (PERC) by hydrogenated silicon nitride (SiNx:H) films which were prepared by plasma-enhanced chemical vapor deposition and released hydrogen for Si passivation. In order to understand the behaviors of hydrogenation in Si solar cells, we manipulate the amount of hydrogen effused from the rear SiNx:H layers into the silicon substrate. A quadratic trend of average conversion efficiency gain for solar cells was observed after CIH treatment process. The mechanism of hydrogenation for PERC, including carrier induced hydrogenation and excess-hydrogen induced degradation, was investigated.

Charge stability of nitrogen-vacancy color centers in organic nanodiamonds

The negatively-charged nitrogen-vacancy (NV) color center in diamond undergoes stochastic charge state transitions between the negatively charged state (NV−) and the neutral charged state (NV o ) upon optical illumination. While the negative charge state is normally preferred for magnetic sensing, optically-controlled switching between charges states is often desirable, for example in super-resolution imaging. The concentration of electron donor impurities in (bulk/nano) diamond crystals determine how much optical control can be exercised over the NV− and NV o charge states. Here we report how the growth speed of nanodiamonds (NDs) can control the concentration of substitutional nitrogen (P1) donors, ranging from highly pure to highly doped diamond. Hence by growth temperature, it is possible to tune the stability of the NV charge state to optimally match the intended application. This work has many promising bio-sensing applications, especially for super-resolution magnetic-sensing with the NV color center.

Optical spectroscopy of muon/hydrogen defects in 6H-SiC

Positive muons can be implanted into silicon carbide (SiC), where they model the isolated hydrogen defect in the negative, neutral, or positive charge states and act as either an acceptor or a donor with midgap energy levels [Lichti et al., Phys. Rev. B 70, 165204 (2004); Lichti et al., Phys. Rev. Lett. 101, 136403 (2008)]. The charge states evolve after implantation depending on the temperature and material doping. We have measured optically induced effects on muons implanted in 6H-SiC using a pulsed, tunable laser [Yokoyama et al., Rev. Sci. Instrum. 87, 125111 (2016)]. In n-type 6H-SiC at 85 K and 40 K, with a laser pulse of energy below the bandgap, we observe photoionization of the doubly occupied level (Mu−) to the neutral defect Mu0 and also ionization of Mu0 to Mu+. Varying the timing of the laser pulse relative to muon arrival confirms that the laser interacts directly with the muons in a stable or metastable state. There is no evidence of any interaction when the laser pulse is timed to arrive before the muons, so either few free carriers are generated by absorption at other dopant sites or the excess carriers have a very short lifetime (≪100ns). Wavelength scans show absorption edges corresponding to the possible interstitial sites within the structure. Combining these data and the existing muon results shows that at low temperature, we have co-existence of Mu0 and Mu− charge states, with the muon or hydrogen acting as a deep compensating impurity. The technique can be applied to many other semiconductors where the muon has been observed to be electrically active, modeling hydrogen.

CO adsorption, reduction and oxidation on Pb(Zr,Ti)O3(001) surfaces associated with negatively charged gold nanoparticles

Gold is deposited on atomically clean, inwards polarized, ferroelectric lead zirco-titanate deposited by pulsed laser deposition on strontium titanate (001) single crystal, then carbon monoxide adsorption and desorption experiments are investigated by in situ fast photoelectron spectroscopy using synchrotron radiation. Atomic force microscopy and high resolution photoelectron spectroscopy are consistent with the formation of 50–100 nm nanoparticles, and their Au 4f core levels point to a negative charge state of gold. As compared with a similar experiment performed on ferroelectric lead zirco-titanate with similar polarization state and without gold, the saturation coverage after exposure to carbon monoxide increases by about 68 %, and also most of the additional carbon is found in oxidized state. Desorption experiments with in situ follow-up by photoelectron spectroscopy are performed as function of temperature, and the neutral carbon intensity decreases when the ferroelectric polarization decreases, while the components corresponding to oxidized carbon remain unchanged. It looks that neutral carbon adsorption is strictly related to the polarization of the ferroelectric film, while carbon still found in molecular form is related to its carbonyl bonding on metal nanoparticles, independent of the polarization state of the substrate. Desorbed carbon at higher temperature uptakes oxygen from the substrate.