Residual Electron Research Articles

Microwave-heating for in-situ Ag NPs preparation into viscose fibers

In the current approach, the microwave heating and polyol method are applied for first time to facilitate silver nanoparticles (Ag NPs) in-situ incorporated into viscose fibers. The Ag NPs gave a distinctive color as well as multifunctional properties to viscose fibers. The Ag+ ions in the viscose fiber surfaces are reduced to Ag NPs with ethyleneglycol as reducing agent and polyvinylpyrrolidone (PVP) as stabilizing agent. This takes less period and energy than that consumed during the conventional heating method (exo-situ incorporated). The silver nanoparticles were examined through UV–Vis spectrum of the residual solutions and transmission electron microscopy (TEM). The Ag NPs incorporated viscose fiber surfaces were characterized by using scanning electron microscopy coupled with an energy dispersive spectrometer (SEM-EDX), Fourier transform infrared spectroscopy (FTIR), and thermo-gravimetric analysis (TGA) measurements. The effects of silver nanoparticles on the multifunctional properties of the fibers including coloration, antibacterial activity and catalytic properties were evaluated. The overall results point out that, the Ag NPs incorporated viscose fibers exhibited excellent color fastness as well as good antibacterial and catalytic properties. In straightforward, the procedure adopted for fabricating these multifunctional viscose fibers is environmental friendly beside time and energy-saving.

Multicarrier transport in InGaSb/InAs superlattice structures

The electrical properties of In0.25Ga0.75Sb/InAs superlattices designed for use as very long wavelength infrared detectors were studied with magnetic field dependent transport measurements and multicarrier analysis. Two electron channels and one hole channel were identified. The low concentration, high mobility electron channel was identified with conduction within the superlattice. Residual electron concentrations in the superlattice are found to be in the high 1010 cm-2 range with mobilities on the order of 40 000 cm2/V s. The other electron and hole channels have significantly lower mobilities with concentrations in the 1011 cm-2 range and can have a significant impact on the resistivity, carrier concentration, and mobility as measured at a single magnetic field value in conventional transport measurements.

Electron hybridization and anharmonic thermal vibration effect on structure transition of SrTiO3 at high-pressure and low-temperature

Abstract We execute electron density analysis of SrTiO3 at low temperatures up 80 K and high pressures up to 11.88 GPa using X-ray single-crystal diffraction and ab initio quantum chemical molecular orbital (MO) calculation. By changing pressures, the cubic SrTiO3 with perovskite structure goes through a antiferroelastic distortion to tetragonal symmetry above the critical pressure Pc=7 GPa with c/a 1 and increasing with lowering temperature. Difference Fourier (D-F) synthesis experimentally proves the residual electron densities Δρ(xyz) are associated with two different effects: electron hybridization bonding electron and anharmonic thermal vibration atoms. The d–p–π hybridization between Ti(3d) and O(2p) orbitals is confirmed in the residual electron density, which is deformed from the ideal spherical density conducted by the atomic scattering factor fi using Hartree–Fock (HF) approximation. MO calculation also reveals the electron hybridization. Anharmonic thermal vibration of atoms yields a large effect to the structure transition. Mulliken charges analysis of MO calculation indicates much smaller charges than their formal ionic charges. Their ionicity increases from cubic to tetragonal above Pc and below Tc.

High-Pressure Chemistry of a Zeolitic Imidazolate Framework Compound in the Presence of Different Fluids.

Pressure-dependent structural and chemical changes of the zeolitic imidazolate framework compound ZIF-8 have been investigated using different pressure transmitting media (PTM) up to 4 GPa. The unit cell of ZIF-8 expands and contracts under hydrostatic pressure depending on the solvent molecules used as PTM. When pressurized in water up to 2.2(1) GPa, the unit cell of ZIF-8 reveals a gradual contraction. In contrast, when alcohols are used as PTM, the ZIF-8 unit cell volume initially expands by 1.2% up to 0.3(1) GPa in methanol, and by 1.7% up to 0.6(1) GPa in ethanol. Further pressure increase then leads to a discontinuous second volume expansion by 1.9% at 1.4(1) GPa in methanol and by 0.3% at 2.3(1) GPa in ethanol. The continuous uptake of molecules under pressure, modeled by the residual electron density derived from Rietveld refinements of X-ray powder diffraction, reveals a saturation pressure near 2 GPa. In non-penetrating PTM (silicone oil), ZIF-8 becomes amorphous at 0.9(1) GPa. The structural changes observed in the ZIF-8-PTM system under pressure point to distinct molecular interactions within the pores.

Controlling charge quantization with quantum fluctuations

In 1909, Millikan showed that the charge of electrically isolated systems is quantized in units of the elementary electron charge e. Today, the persistence of charge quantization in small, weakly connected conductors allows for circuits in which single electrons are manipulated, with applications in, for example, metrology, detectors and thermometry. However, as the connection strength is increased, the discreteness of charge is progressively reduced by quantum fluctuations. Here we report the full quantum control and characterization of charge quantization. By using semiconductor-based tunable elemental conduction channels to connect a micrometre-scale metallic island to a circuit, we explore the complete evolution of charge quantization while scanning the entire range of connection strengths, from a very weak (tunnel) to a perfect (ballistic) contact. We observe, when approaching the ballistic limit, that charge quantization is destroyed by quantum fluctuations, and scales as the square root of the residual probability for an electron to be reflected across the quantum channel; this scaling also applies beyond the different regimes of connection strength currently accessible to theory. At increased temperatures, the thermal fluctuations result in an exponential suppression of charge quantization and in a universal square-root scaling, valid for all connection strengths, in agreement with expectations. Besides being pertinent for the improvement of single-electron circuits and their applications, and for the metal-semiconductor hybrids relevant to topological quantum computing, knowledge of the quantum laws of electricity will be essential for the quantum engineering of future nanoelectronic devices.

Visualizing chaperone-assisted protein folding.

Challenges in determining the structures of heterogeneous and dynamic protein complexes have greatly hampered past efforts to obtain a mechanistic understanding of many important biological processes. One such process is chaperone-assisted protein folding, where obtaining structural ensembles of chaperone:substrate complexes would ultimately reveal how chaperones help proteins fold into their native state. To address this problem, we devised a novel structural biology approach based on X-ray crystallography, termed Residual Electron and Anomalous Density (READ). READ enabled us to visualize even sparsely populated conformations of the substrate protein immunity protein 7 (Im7) in complex with the E. coli chaperone Spy. This study resulted in a series of snapshots depicting the various folding states of Im7 while bound to Spy. The ensemble shows that Spy-associated Im7 samples conformations ranging from unfolded to partially folded and native-like states, and reveals how a substrate can explore its folding landscape while bound to a chaperone.

Charge-density analysis of an iron-sulfur protein at an ultra-high resolution of 0.48 Å.

The fine structures of proteins, such as the positions of hydrogen atoms, distributions of valence electrons and orientations of bound waters, are critical factors for determining the dynamic and chemical properties of proteins. Such information cannot be obtained by conventional protein X-ray analyses at 3.0-1.5 Å resolution, in which amino acids are fitted into atomically unresolved electron-density maps and refinement calculations are performed under strong restraints. Therefore, we usually supplement the information on hydrogen atoms and valence electrons in proteins with pre-existing common knowledge obtained by chemistry in small molecules. However, even now, computational calculation of such information with quantum chemistry also tends to be difficult, especially for polynuclear metalloproteins. Here we report a charge-density analysis of the high-potential iron-sulfur protein from the thermophilic purple bacterium Thermochromatium tepidum using X-ray data at an ultra-high resolution of 0.48 Å. Residual electron densities in the conventional refinement are assigned as valence electrons in the multipolar refinement. Iron 3d and sulfur 3p electron densities of the Fe4S4 cluster are visualized around the atoms. Such information provides the most detailed view of the valence electrons of the metal complex in the protein. The asymmetry of the iron-sulfur cluster and the protein environment suggests the structural basis of charge storing on electron transfer. Our charge-density analysis reveals many fine features around the metal complex for the first time, and will enable further theoretical and experimental studies of metalloproteins.

Surface defect passivation of MoS2 by sulfur, selenium, and tellurium

Few-layer MoS2 field-effect transistors often show an n-type conduction behavior due to the presence of high-density sulfur vacancies. Here, we investigated the possibility of surface defect passivation of MoS2 by sulfur treatment in (NH4)2S solution or coating with an ultrathin layer of selenium or tellurium. It was found that all three elements investigated are able to induce a p-doping effect through suppressing the residual electron concentration by an amount exceeding 0.5 × 1012 cm−2 in few-layer MoS2. Among them, the sulfur-treatment exhibits the most superior thermal stability that survives thermal annealing at temperatures ≥120 °C for at least 10 h. Tellurium exhibits the strongest p-doping effect due to electron trapping by physisorption-induced gap states near the valence band edge. On the other hand, selenium is highly volatile on MoS2; it evaporates and desorbs easily due to Joule heating during electrical measurements in vacuum. The results of first-principles calculations support the experimental observations.

Effects of N2/O2Additives on the Repeatability of the Dynamics of an Atmospheric-Pressure He Plasma Jet

To better understand the role of residual charges in the repeatability of an atmospheric-pressure He plasma plume, the effects of N2/O2 additives on the minimum pulse repetition frequency $f_{\textrm {min}}$ to sustain the repeatable discharge are investigated for the first time. First, when the percentage of N2 gas in He/N2 mixture increases from 0% to 1.7%, $f_{\textrm {min}}$ increases from 130 to 900 Hz. While when the percentage of O2 gas in He/O2 mixture increases from 0.38% to 1.7%, $f_{\textrm {min}}$ keeps almost constant at 300 Hz. Second, a charge collection system is used to measure the residual charge density. The results show that the residual charge density in the plasma core for He/(1%) O2 mixture is much higher than that for He/(1%) N2 mixture. Furthermore, the simulation results on the time-resolved distribution of residual charge density show that the residual electron density just before the following voltage pulse decreases with the increase in N2 percentage, while the residual O2− ion density is independent on the O2 percentage. Finally, the residual charge density of $10^{9}$ cm $^{-3}$ or higher is needed for the repeatable discharge mode.

Pressure-induced ferroelectric to paraelectric transition in LiTaO3 and (Li,Mg)TaO3

X-ray powder diffraction and Raman scattering of LiTaO3 (LT) and (Li,Mg)TaO3 (LMT) have been measured under pressure up to 46 GPa. Above 30 GPa, the ferroelectric rhombohedral phase (R3c, Z = 6) of LiTaO3 transforms to a paraelectric orthorhombic phase (Pnma with Z = 4) with a large hysteresis. Rietveld profile fitting analysis shows that the Li-O bond is compressed and approaches that of Ta-O with pressure. The cation distribution analysis of the orthorhombic perovskite structure shows that Li and Ta are located in the octahedral 8-fold coordination sites. Difference Fourier |Fobs(hkl)| - |Fcal(hkl)| maps of LiTaO3 and (Li,Mg)TaO3 indicate polarization in the c axis direction and a more distinct electron density distribution around the Ta position for (Li,Mg)TaO3 compared to LiTaO3. The observed effective charges indicate that for (Li,Mg)TaO3 without vacancies Ta5+ becomes less ionized as a function of Mg substitution. Considering both site occupancy and effective charge analysis, Ta5+ is reduced to Ta4.13+. Mg2+ and O2- change to Mg1.643+ and O1.732 -, respectively. The space- and time-averaged structures of the dynamical vibration of atoms can be elucidated from the electron density analysis by difference Fourier and temperature factors T(hkl) in the structure refinement. The refinement of the temperature factor is consistent with the cation distribution assuming full stoichiometry. The residual electron density induced from the excess electron in (Li,Mg)TaO3 indicates more electrons around the Ta site, as confirmed by the effective charge analysis. Raman spectra of LiTaO3 and (Li,Mg)TaO3 show notable changes over the measured pressure range. Raman peaks centered at 250 cm−1 and 350 cm−1 at ambient pressure merge above 8 GPa, which we associate with the diminishing of difference in distances between Li-O and Ta-O bonds with pressure in both materials. Raman spectra show significant changes at 28 GPa and 33 GPa for LT and LMT, respectively, due to the structural transition from R3c to Pnma consistent with the x-ray diffraction results.

Experimental and theoretical charge-density analysis of 1,4-bis(5-hexyl-2-thienyl)butane-1,4-dione: applications of a virtual-atom model.

The experimental and theoretical charge densities of 1,4-bis(5-hexyl-2-thienyl)butane-1,4-dione, a precursor in the synthesis of thiophene-based semiconductors and organic solar cells, are presented. A dummy bond charges spherical atom model is applied besides the multipolar atom model. The results show that the dummy bond charges model is accurate enough to calculate electrostatic-derived properties which are comparable with those obtained by the multipolar atom model. The refinement statistics and the residual electron density values are found to be intermediate between the independent atom and the multipolar formalisms.

Method for determining the residual electron- and hole-densities about the neutrality point over the gate-controlled n ↔ p transition in graphene

The Hall effect and the diagonal resistance, which indicates a residual resistivity ρxx≈h/4e2, are experimentally examined over the p ↔ n transition about the nominal neutrality point in chemical vapor deposition (CVD) grown graphene. A distribution of neutrality potentials is invoked in conjunction with multi-carrier conduction to model the experimental observations. From the modeling, we extract the effective residual electron- and hole-densities around the nominal neutrality point. The results indicate mixed transport due to co-existing electrons and holes in large area zero-band gap CVD graphene devices, which indicates domain confined ambipolar currents broadly over the gate-induced n ↔ p transition.

Photoreflectance investigation of exciton-acoustic phonon scattering in GaN grown by MOVPE

In this paper, we report a systematic investigation of the near band edge (NBE) excitonic states in GaN using low temperature photoluminescence (PL) and photoreflectance (PR) measurements. For this purpose, GaN films of different thicknesses have been grown on silicon nitride (SiN) treated c-plane sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (MOVPE). Low temperature PR spectra exhibit well-defined spectral features related to the A, B and C free excitons denoted by FXA FXB and FXC, respectively. In contrast, PL spectra are essentially dominated by the A free and donor bound excitons. By combining PR spectra and Hall measurements a strong correlation between residual electron concentration and exciton linewidths is observed. From the temperature dependence of the excitonic linewidths, the exciton-acoustic phonon coupling constant is determined for FXA, FXB and FXC. We show that this coupling constant is strongly related to the exciton kinetic energy and to the strain level.

Transformation of the structure in a series of mixed K2Ni x Co1–x (SO4)2 · 6H2O single crystals

The structure of crystals of solid solutions K2Ni x Co1–x (SO4)2 · 6H2O has been studied. The substitution of nickel atoms for cobalt atoms in the K2Co(SO4)2 · 6H2O structure leads to a decrease in all interatomic distances (Co,Ni)–O. However, the decrease in one of them with the increase in the Ni concentration is smaller than that in the other two. As a consequence, CoO6 octahedra are more distorted than NiO6. Syntheses of the difference electron density show that the structures of crystals of solid solutions K2Co(SO4)2 · 6H2O contain more uninterpreted peaks of the residual electron density than the K2Ni(SO4)2 · 6H2O structure, indicating a change in the character of the hydrogen bond.

InN nanocolumns grown by molecular beam epitaxy and their luminescence properties

InN nanocolumns (NCs) without vertical diameter variation have been grown on GaN templates by molecular beam epitaxy. Growth temperature is the key parameter to obtain such shaped InN NCs. The density and surface distribution can be controlled by the V/III ratio and initial nucleation process. Intense photoluminescence (PL) is observed for the optimal InN NCs, about two orders of magnitude stronger than the InN epilayers. Superior excitation power and temperature dependence of the PL indicate significantly reduced defect density and residual electron density, non-degenerate property, and possibly suggest that there is a small or negligible level of electron accumulation at the smooth non-polar lateral surfaces.

Compressibility and crystal–fluid interactions in all-silica ferrierite at high pressure

The high-pressure behavior of a synthetic siliceous ferrierite has been studied by in situ single-crystal and powder synchrotron X-ray diffraction with a diamond anvil cell, using four different P-transmitting fluids: the non-penetrating silicone oil and the potentially pore-penetrating methanol:ethanol:H2O = 16:3:1 mixture, ethylene glycol and 2methyl-2propen-1ol. The high-pressure experiment in silicone oil shows a remarkable flexibility of the FER framework. Two displacive phase transitions, following the path Pmnn-to-P121/n1-to-P21/n11 with pressure, were observed. The three polymorphs were found to share a virtually identical bulk compressibility, though showing a different anisotropic pattern. The experiments with potentially penetrating media enhanced the occurrence of a complex scenario, from which the P-induced intrusion of fluid molecules into the FER structural voids can be assumed by the different phase-transition paths and compressibility patterns, by the calculated residual electron density and by the different deformation mechanisms at the atomic scale, observed as a function of the used medium. The starting orthorhombic polymorph was always restored upon decompression in all the experiments. The roles of the different surface area in single crystal and polycrystalline samples, and of the process kinetics on the compressibility and crystal–fluid interactions, are discussed.

Selective activators of protein phosphatase 5 target the auto-inhibitory mechanism.

Protein phosphatase 5 (PP5) is an evolutionary conserved serine/threonine phosphatase. Its dephosphorylation activity modulates a diverse set of cellular factors including protein kinases and the microtubule-associated tau protein involved in neurodegenerative disorders. It is auto-regulated by its heat-shock protein (Hsp90)-interacting tetratricopeptide repeat (TPR) domain and its C-terminal α-helix. In the present study, we report the identification of five specific PP5 activators [PP5 small-molecule activators (P5SAs)] that enhance the phosphatase activity up to 8-fold. The compounds are allosteric modulators accelerating efficiently the turnover rate of PP5, but do barely affect substrate binding or the interaction between PP5 and the chaperone Hsp90. Enzymatic studies imply that the compounds bind to the phosphatase domain of PP5. For the most promising compound crystallographic comparisons of the apo PP5 and the PP5-P5SA-2 complex indicate a relaxation of the auto-inhibited state of PP5. Residual electron density and mutation analyses in PP5 suggest activator binding to a pocket in the phosphatase/TPR domain interface, which may exert regulatory functions. These compounds thus may expose regulatory mechanisms in the PP5 enzyme and serve to develop optimized activators based on these scaffolds.

The stability of microbially reduced U(IV); impact of residual electron donor and sediment ageing

The stimulation of microbial U(VI) reduction to precipitate insoluble U(IV) has been proposed as a means of remediating mobile uranium groundwater contamination. Crucial to the success of such a remediation strategy is determining the longevity of U(IV) biominerals in the subsurface, particularly if the groundwater becomes oxidising. Here we describe experiments to assess the susceptibility of microbially-reduced U(IV) to oxidative remobilisation both via aeration and by the addition of nitrate at environmentally-relevant conditions. Additional factors examined include the possibility of biogenic U(IV) becoming more crystalline (and potentially more recalcitrant) during a period of ageing, and the role played by residual electron donor in controlling the long-term fate of the uranium. Biogenic U(IV) was precipitated as a non-crystalline U(IV) or “monomeric” phase, with a small but increasing contribution to the EXAFS spectra from nanocrystalline uraninite occurring during 15months of ageing. Despite this, no evidence was observed for an increase in recalcitrance to oxidative remobilisation. However, the presence of residual electron donor post-biostimulation was shown to exert a strong control on U(IV) reoxidation kinetics, highlighting the importance of maintaining the presence of electron donor in the subsurface, in order to protect biogenic U(IV) from oxidative remobilisation.

Efficiency of gyrotrons working at the second harmonic of gyrofrequency with multistage systems for recuperation of residual electron energy

Comprehensive optimization of the output efficiency of the gyrotron with a multistage system for energy recuperation is performed for the first time with allowance for the electron-beam interaction with the high-frequency field in the resonator and the subsequent deceleration of the beam in the presence of the electric field of the collector immersion lenses. The energy approach makes it possible to estimate the upper-bound limit of the efficiency. The effect of the ohmic loss on the efficiency of the gyrotron with recuperation is taken into account. The efficiency of the gyrotron working at the harmonics of the gyrofrequency with the two-stage recuperation can be increased to the levels typical of the magnetron technological complexes with a simultaneous increase in the working frequency of the sample heating by at least an order of magnitude.

Cholesterol oxidase: ultrahigh-resolution crystal structure and multipolar atom model-based analysis.

Examination of protein structure at the subatomic level is required to improve the understanding of enzymatic function. For this purpose, X-ray diffraction data have been collected at 100 K from cholesterol oxidase crystals using synchrotron radiation to an optical resolution of 0.94 Å. After refinement using the spherical atom model, nonmodelled bonding peaks were detected in the Fourier residual electron density on some of the individual bonds. Well defined bond density was observed in the peptide plane after averaging maps on the residues with the lowest thermal motion. The multipolar electron density of the protein-cofactor complex was modelled by transfer of the ELMAM2 charge-density database, and the topology of the intermolecular interactions between the protein and the flavin adenine dinucleotide (FAD) cofactor was subsequently investigated. Taking advantage of the high resolution of the structure, the stereochemistry of main-chain bond lengths and of C=O···H-N hydrogen bonds was analyzed with respect to the different secondary-structure elements.