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Electron paramagnetic resonance study of paramagnetic centers in carbon-fumed silica adsorbent

Fumed silica A-300 was carbonized by means of pyrolysis of CH2Cl2. The obtained initial SiO2:C nanopowders of black color, with an average diameter of 14–16 nm and carbon (C) concentration 7 wt. %, subjected to the oxidation and passivation treatment were studied by electron paramagnetic resonance (EPR) in the temperature range 4–400 K. Two EPR signals of Lorentzian lineshape with nearly equal g-factors and different linewidth were observed in the initial, oxidized, and passivated SiO2:C nanopowders. The two-component EPR spectrum was explained by the presence of C in two electronic states. The intensive narrow EPR signal, which has a temperature-dependent intensity, linewidth, and resonance field position, was attributed to the carbon-related defect with non-localized electron hopping between neighboring C-dangling bonds. The striking effect is that the temperature dependence of the EPR linewidth demonstrates the motional narrowing of the EPR signal at very low temperatures from 4 K to 20 K, which is not typically for nonmetallic materials and was explained by the quantum character of C layer conductivity in the SiO2:C. The observed peaks in the temperature dependence of the conduction electron EPR signal integral intensity in the high-temperature range 200–440 K was explained by the presence of the C nanodots at the surface of SiO2 nanoparticles and the ejection of electrons from the confinement energy levels of C quantum dot when the temperature becomes comparable to the confinement energy.

Comparative electron paramagnetic resonance investigation of reduced graphene oxide and carbon nanotubes with different chemical functionalities for quantum dot attachment

Electron paramagnetic resonance (EPR) spectroscopy has been applied to different chemically treated reduced graphene oxide (rGO) and multiwalled carbon nanotubes (CNTs). A narrow EPR signal is visible at g = 2.0029 in both GO and CNT-Oxide from carbon-related dangling bonds. EPR signals became broader and of lower intensity after oxygen-containing functionalities were reduced and partially transformed into thiol groups to obtain thiol-functionalized reduced GO (TrGO) and thiol-functionalized CNT (CNT-SH), respectively. Additionally, EPR investigation of CdSe quantum dot-TrGO hybrid material reveals complete quenching of the TrGO EPR signal due to direct chemical attachment and electronic coupling. Our work confirms that EPR is a suitable tool to detect spin density changes in different functionalized nanocarbon materials and can contribute to improved understanding of electronic coupling effects in nanocarbon-nanoparticle hybrid nano-composites promising for various electronic and optoelectronic applications.

Magnetic Coupling and Optical Properties of theS6-Dodecakis(trifluoromethyl)fullerene Radical Anions in the Layered Salt (PPN+)[C60(CF3)12.−

Poly(trifluoromethyl)fullerene S6-C60(CF3)12 was reduced by sodium fluorenone ketyl in the presence of (PPN)Cl (PPN = bis(triphenylphosphine)iminium) to afford the salt (PPN)[C60(CF3)12] (1), which contains C60(CF3)12(·-) radical anions. In the crystal structure of 1, C60(CF3)12(·-) layers alternate with the PPN(+) cations. There are short F⋅⋅⋅F contacts between C60(CF3)12(·-) radical anions within the layers but no C⋅⋅⋅C contacts. DFT calculations revealed that the negative charge on C60(CF3)12(·-) is distributed mainly between sp(2) carbon and fluorine atoms, whereas spin density is localized mainly on the fullerene-cage sp(2) carbon atoms. IR and UV/Vis/NIR spectra in the solid state and solution showed characteristic changes relative to those of neutral S6-C60(CF3)12 due to the formation of radical anions. The solid-state electronic spectrum of 1 exhibits a single broad band at 738 nm attributed to C60(CF3)12(·-). Crystals of 1 show a narrow EPR signal with g = 2.0025 (ΔH = 0.45 mT) at 300 K. The temperature dependence of the integral intensity follows the Curie-Weiss law with a negative Weiss temperature of -11.8 K (30-300 K) indicating antiferromagnetic interaction of spins. This dependence was approximated by the Heisenberg model for one-dimensional chains of antiferromagnetically interacting spins with exchange interaction J/kB = -9.1 K. It was assumed that magnetic interaction between the C60(CF3)12(·-) spins in the layers is mediated by short F⋅⋅⋅F contacts.

Perspectives in magnetic resonance: NMR in the post-FFT era

Multi-dimensional NMR spectra have traditionally been processed with the fast Fourier transformation (FFT). The availability of high field instruments, the complexity of spectra of large proteins, the narrow signal dispersion of some unstructured proteins, and the time needed to record the necessary increments in the indirect dimensions to exploit the resolution of the highfield instruments make this traditional approach unsatisfactory. New procedures need to be developed beyond uniform sampling of the indirect dimensions and reconstruction methods other than the straight FFT are necessary. Here we discuss approaches of non-uniform sampling (NUS) and suitable reconstruction methods. We expect that such methods will become standard for multi-dimensional NMR data acquisition with complex biological macromolecules and will dramatically enhance the power of modern biological NMR.

Bioinspired Insights into Silicic Acid Stabilization Mechanisms: The Dominant Role of Polyethylene Glycol-Induced Hydrogen Bonding

Mono- and disilicic acids were stabilized by uncharged polyethylene glycols (PEGs) in silica-supersaturated solutions (the starting solution contained 500 ppm/8.3 mM sodium orthosilicate, Na2SiO3·5H2O, expressed as SiO2) at pH = 7, most likely by hydrogen bonding between the silanol groups and -CH2-CH2-O-ether moieties. The stabilization was monitored by measuring molybdate-reactive silica and also by a combination of liquid- and solid-state (29)Si NMR spectroscopy. It depends on PEG concentration (20-100 ppm) and molecular weight (1550-20,000 Da). Two narrow (29)Si NMR signals characteristic for monosilicic acid (Q(0)) and disilicic acid (Q(1)) can be observed in (29)Si NMR spectra of solutions containing PEG 10000 with intensities distinctly higher than the control, that is, in the absence of PEG. Silica-containing precipitates are observed in the presence of PEG, in contrast to the gel formed in the absence of PEG. These precipitates exhibit similar degrees of silica polycondensation as found in the gel as can be seen from the (29)Si MAS NMR spectra. However, the (2)D HETCOR spectra show different (1)H NMR signal shifts: The signal due to H-bonded SiOH/H2O, which is found at 6 ppm in the control, is shifted to ~7 ppm in the PEG-containing precipitate. This indicates the formation of slightly stronger H-bonds than in the control sample, most likely between PEG and the silica species. The presence of PEG in these precipitates is unequivocally proven by (13)C CP MAS NMR spectroscopy. The (13)C signal of PEG significantly shifts and is much narrower in the precipitates as compared to the pristine PEG, indicating that PEG is embedded into the silica or at least bound to its surface (or both), and not phase separated. FT-IR spectra corroborate the above arguments. The H-bonding between silanol and ethereal O perturbs the band positions attributed to vibrations involving the O atom. This work may invoke an alternative way to envision silica species stabilization (prior to biosilica formation) in diatoms by investigating possible scenarios of uncharged biomacromolecules playing a role in biosilica synthesis.

Binding mode of the activity‐modulating C‐terminal segment of NS2B to NS3 in the dengue virus NS2B–NS3 protease

The two-component dengue virus NS2B-NS3 protease (NS2B-NS3pro) is an established drug target but inhibitor design is hampered by uncertainties about its 3D structure in solution. Crystal structures reported very different conformations for the functionally important C-terminal segment of the NS2B cofactor (NS2Bc), indicating open and closed conformations in the absence and presence of inhibitors, respectively. An earlier NMR study in solution indicated that a closed state is the preferred conformation in the absence of an artificial linker engineered between NS2B and NS3pro. To obtain direct structural information on the fold of unlinked NS2B-NS3pro in solution, we tagged NS3pro with paramagnetic tags and measured pseudocontact shifts by NMR to position NS2Bc relative to NS3pro. NS2Bc was found to bind to NS3pro in the same way as reported in a previously published model and crystal structure of the closed state. The structure is destabilized, however, by high ionic strength and basic pH, showing the importance of electrostatic forces to tie NS2Bc to NS3pro. Narrow NMR signals previously thought to represent the open state are associated with protein degradation. In conclusion, the closed conformation of the NS2B-NS3 protease is the best model for structure-guided drug design.

A Proposal for a Novel Hybrid Interconnect Technology for the End of Roadmap

To suppress the impact of size effects on sub-20 nm wide wires, a novel aluminum-copper hybrid interconnect architecture is proposed and its potential performance has been quantified. Al wires offer lower resistivities at nanoscale dimensions because they do not need diffusion barriers, and size effects are less prominent in them due to their smaller bulk mean free path. However, their current conduction capacity is substantially lower than that of Cu wires. To get around this limitation, this letter proposes a hybrid interconnect technology to replace only short narrow local signal wires by Al wires. This scheme takes advantage of the fact that signal wires conduct bi-directional currents and are therefore virtually immune to electromigration. The improvement in chip clock frequency is predicted to be between 50% and 100% for the 7 nm technology node.

Acquiring narrow linewidth microwave signals based on an optical injection semiconductor laser under subharmonic microwave modulation

The performances of microwave signals generated by an optically injected semiconductor laser operated at the period-one (P1) oscillation under 1/2 subharmonic microwave modulation, are investigated experimentally. The experimental results show that under suitable injection condition, the microwave signal output from an optically injected semiconductor operated at P1 oscillation can reach a frequency of 26.5 GHz limited to the experimental conditions and may have a single sideband optical spectrum structure, but the linewidth of the microwave signal is relatively wide (on the order of MHz). After adopting 1/2 subharmonic locking technique, the linewidth of the obtained microwave signal can be reduced from tens of MHz to tens of kHz. Furthermore, we analyze the influences of the power and frequency of the subharmonic microwave on the phase noise of the generated microwave signals, and further map the subharmonic microwave locking region in the parameter space of the power and frequency of the subharmonic microwave.

Electroreflectance study of CuIn1−xGaxSe2 thin film solar cells

We have investigated the optical properties of CuIn1−xGaxSe2 (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02–1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10–2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35–2.09 eV region can be assigned to the Franz–Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency.

Microwave Signal Generation From a Dual-Wavelength Single-Frequency Highly $\hbox{Er}^{3+}/\hbox{Yb}^{3+}$ Co-Doped Phosphate Fiber Laser

Narrow linewidth microwave signals generated from a heterodyne detection configuration of a dual-wavelength (DW) single-frequency highly Er3+/Yb3+ co-doped phosphate fiber laser is presented. The oscillating cavity of the fiber laser consists of a dual-channel narrow-band fiber-Bragg-grating (DC-NB-FBG), a 0.7-cm-long Er3+/ Yb3+ co-doped phosphate fiber and a wideband FBG (WB-FBG). The wavelength selecting gratings are spatially separated to create partially separated resonant cavities. Highly Er3+/Yb3+ co-doped phosphate fiber ensures that the mode competition is relatively weak under low pump power. DW single-frequency lasing with laser linewidths of 3 kHz is achieved. A 12.014-GHz microwave signal with a 3-dB linewidth of 3 kHz is obtained from the heterodyne detection of the DW fiber laser.

Suppression of the Narrow Band Jamming Signal in the Receiver of Spread Spectrum Communication System

This paper describes the design of adaptive finite impulse response (FIR)filter with least mean square(LMS) algorithm by which we can reduce the narrow band jamming impacting the performance spreadspectrum communication system (SSS).Simulation results shows that the jamming is rejected and theoutput signals free from error.

The Realization of Random Frequency Modulation Pulse Compression Signal Based on AD9858

A scheme producing non-linearity frequency-modulated signal which can be updated on-line based on AD9858 is mentioned. A method to design NLFM signal using immediate data update is introduced. Circuit structure is designed. Major factor that impacting data is analyzed and its solution is presented. Experiments certify that this method can generate broadband and narrow pulse width non-linearity frequency-modulated signal, whose frequency spectrum can be updated on-line in real time.

Selectively dispersed isotope labeling for protein structure determination by magic angle spinning NMR

The power of nuclear magnetic resonance spectroscopy derives from its site-specific access to chemical, structural and dynamic information. However, the corresponding multiplicity of interactions can be difficult to tease apart. Complimentary approaches involve spectral editing on the one hand and selective isotope substitution on the other. Here we present a new "redox" approach to the latter: acetate is chosen as the sole carbon source for the extreme oxidation numbers of its two carbons. Consistent with conventional anabolic pathways for the amino acids, [1-(13)C] acetate does not label α carbons, labels other aliphatic carbons and the aromatic carbons very selectively, and labels the carboxyl carbons heavily. The benefits of this labeling scheme are exemplified by magic angle spinning spectra of microcrystalline immunoglobulin binding protein G (GB1): the elimination of most J-couplings and one- and two-bond dipolar couplings provides narrow signals and long-range, intra- and inter-residue, recoupling essential for distance constraints. Inverse redox labeling, from [2-(13)C] acetate, is also expected to be useful: although it retains one-bond couplings in the sidechains, the removal of CA-CO coupling in the backbone should improve the resolution of NCACX spectra.

Design of a Novel Excitation Circuit for Low Perming Error Fluxgate

An effective method for low perming error of fluxgates is to enlarge the peak value of the excitation current, but large amplitude excitation will increase the power consumption. A narrow pulse excitation current with sufficient peak value can reduce perming error with low power consumption while increase the difficulty of the signal processing with low sensitivity. The perfected excitation waveform with high ratio of peak value to RMS value by careful excitation tuning is an efficient solution. This paper presents a novel circuit which can generate an excitation current waveform with high peak value and low RMS value for fluxgates. Different from the tuned excitation circuit, the proposed circuit is a combination of an integrator and a differentiator with the same squarewave input. The excitation signal is the sum of a triangle wave and a sharp narrow pulse signal. It is easy to adjust the peak value of the narrow pulses for perming error study and no careful tune adjusting is needed. Finally, in case of the fixed peak value of the narrow pulses, an improved excitation circuit with only two resistors, two capacitors and one amplifier is also presented in the paper. The experiment results show that 0.5nT perming error can be achieved for 10mT shock.

DAC-board based X-band EPR spectrometer with arbitrary waveform control

We present arbitrary control over a homogenous spin system, demonstrated on a simple, home-built, electron paramagnetic resonance (EPR) spectrometer operating at 8–10GHz (X-band) and controlled by a 1GHz arbitrary waveform generator (AWG) with 42dB (i.e. 14-bit) of dynamic range. Such a spectrometer can be relatively easily built from a single DAC (digital to analog converter) board with a modest number of stock components and offers powerful capabilities for automated digital calibration and correction routines that allow it to generate shaped X-band pulses with precise amplitude and phase control. It can precisely tailor the excitation profiles “seen” by the spins in the microwave resonator, based on feedback calibration with experimental input. We demonstrate the capability to generate a variety of pulse shapes, including rectangular, triangular, Gaussian, sinc, and adiabatic rapid passage waveforms. We then show how one can precisely compensate for the distortion and broadening caused by transmission into the microwave cavity in order to optimize corrected waveforms that are distinctly different from the initial, uncorrected waveforms. Specifically, we exploit a narrow EPR signal whose width is finer than the features of any distortions in order to map out the response to a short pulse, which, in turn, yields the precise transfer function of the spectrometer system. This transfer function is found to be consistent for all pulse shapes in the linear response regime. In addition to allowing precise waveform shaping capabilities, the spectrometer presented here offers complete digital control and calibration of the spectrometer that allows one to phase cycle the pulse phase with 0.007° resolution and to specify the inter-pulse delays and pulse durations to ⩽250ps resolution. The implications and potential applications of these capabilities will be discussed.

Microdissection and Chromosome Painting of the Alien Chromosome in an Addition Line of Wheat - Thinopyrum intermedium

In this study, chromosome painting was developed and used to identify alien chromosomes in TAi-27, a wheat - Thinopyrum intermedium addition line, and the chromosomes of the three different genomes of Th. Intermedium. The smallest alien chromosome of TAi-27 was microdissected and its DNA amplified by DOP-PCR was used as a probe to hybridize with metaphase chromosomes of TAi-27 and Th . intermedium . Results showed that hybridization signals were observed in all regions of a pair of the smallest alien chromosomes and the pericentromeric area of another pair of alien chromosomes in TAi-27, indicating that the probe from microdissected chromosome is species specific. In Th . intermedium , 14 chromosomes had wide and strong hybridization signals distributed mainly on the pericentromere area and 9 chromosomes with narrow and weak signals on the pericentromere area. The remaining chromosomes displayed a very weak or no signal. Sequential FISH/GISH on Th . intermedium chromosomes using the DNAs of microdissected chromosome, Pseudoroegneria spicata (St genome) and pDbH12 (a Js genome specific probe) as the probes indicated that the microdissected chromosome belonged to the St genome, three genomes (Js, J and St) in Th . intermedium could be distinguished, in which there is no hybridization signal on J genome that is similar to the genome of Th . bessarabicum . Our results showed that the smallest alien chromosomes may represent a truncated chromosome and the repetitive sequence distribution might be similar in different chromosomes within the St genome. However, the repetitive sequence distributions are different within the Js genome, within a single chromosome, and among different genomes in Th . intermedium . Our results suggested that chromosome painting could be feasible in some plants and useful in detecting chromosome variation and repetitive sequence distribution in different genomes of polyploidy plants, which is helpful for understanding the evolution of different genomes in polyploid plants.

Solid-state NMR sequential assignments of the amyloid core of Sup35pNM

Sup35pNM represents the N-terminal and middle (M) domains of the yeast Saccharomyces cerevisiae prion Sup35p. This fragment is commonly used for structural and functional studies of Sup35p. We here present a solid-state NMR study of fibrils formed by this fragment and show that sequential assignments can be obtained for the rigid and well-ordered parts of the protein using 3D spectroscopy. We describe in detail the sequential assignment of the 22 residues yielding strong, narrow signals with chemical shifts that correspond mostly to β-sheet secondary-structured amino acids that form the fibril core.

Monolithically Integrated DFB Lasers for Tunable and Narrow Linewidth Millimeter-Wave Generation

A monolithic optoelectronic device for the generation of tunable and narrow linewidth millimeter-wave signals is presented. The device consists of three mutually injected distributed feedback lasers whose emission frequencies are stabilized via a four-wave mixing process. Their beating on a high-speed photodetector generates a narrow linewidth electrical signal that is continuously tunable over several tens of gigahertz.

Spin–spin interactions between π-electronic edge-localized spins and molecular oxygen in defective carbon nano-onions

π-Electronic edge-localized spin magnetism in defective carbon nano-onions (DCNOs) derived from detonation nanodiamonds was investigated in terms of molecular oxygen adsorption and microwave (MW) irradiation effect. At low MW power (PMW⩽200μW) electron paramagnetic resonance (EPR) spectra of air-contained DCNO samples show progressive broadening of the g=2.0014(2) edge-spin signal on temperature decrease. Below 140K, the edge-spin signal becomes unobservable and the only weak narrow g=2.0022(1) signal ascribed to sp3-like defects is observed. Upon higher power (PMW>20mW) MW-irradiation at T<60K, that weak signal demonstrates unusual enhancement in peak/integral intensities and line broadening accompanied with upward deviations from the conventional saturation behavior. On abrupt reduction of PMW to low (PMW<1mW) levels, a new strong narrow signal with the same g=2.0022(1) emerges. Below 60K the intensity of the strong signal shows non-Curie–Weiss behavior. This EPR signal is attributed to new magnetic entities originating from antiferromagnetically coupled dimers of edge-spin and negatively charged O2− molecule. The O2− molecules, formed by charge transfer from π-electronic reservoir of underlaid nano-graphene sheet, appear in the vicinity of nano-graphene edges in the process of desorption/adsorption of oxygen molecules caused by strong MW-irradiation and subsequent abrupt reduction of PMW.

Charge Accumulation in Organic Solar Cells during Device Operation as Investigated by Electron Spin Resonance

An electron spin resonance (ESR) method was applied to bulk-heterojunction organic thin-film solar cells to investigate the accumulated charges in such devices under simulated solar irradiation. The device structure used was an indium–tin oxide/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM)/Pd/LiF/Al. We measured light-induced ESR (LESR) signals and device characteristics simultaneously using the same device. We observed two different LESR signals, namely, a narrow signal (g = 2.0023, ΔHpp ∼0.2 mT) and a broad signal (g = 2.0034, ΔHpp ∼1.5 mT). The origin of the narrow signal was confirmed to be the charge accumulation in P3HT. The origin of the broad signal was determined to be the charge accumulation in PEDOT and/or PSS by the spectral simulation of the LESR signal. Moreover, the charge accumulation in PEDOT and/or PSS was found to be related to voltage and current.