Fluctuational-electromagnetic interaction of a moving nanoparticle with walls of a flat dielectric gap

General expressions for the tangential and normal components of the force of interaction and the rate of heat exchange between a neutral spherical nanoparticle (atom) and dielectric walls of a flat vacuum gap are obtained for the first time in a nonrelativistic nonretarded approximation of the fluctuational electromagnetic field theory. In the general case, the walls have a temperature different from that of the particle and possess arbitrary permittivities.     

Ion accelerator with space-time modulation of a relativistic electron beam

An analysis is made of a possible method of collective ion acceleration by combining spatial modulation of the electron beam potential with temporal modulation of the current. A conceptual design is proposed. Pis’ma Zh. Tekh. Fiz. 24, 74–78 (September 26, 1998)     

Efficient charge separation in multidimensional nanohybrids

We report unidirectional charge transfer in multidimensional nanohybrids, consisting of a quantum dot, an electronically active molecular linker, and a carbon nanotube. After covalent attachment to the nanotube, only emission consistent with the negatively charged quantum dot exciton ion rather than the neutral exciton is observed, showing nearly monoexponential recombination kinetics and an average lifetime of 3.5 ns. Using kinetic models, we explain how charge transfer is biased at the expense of other decay pathways.     

Laser-induced charge separation in CdSe nanowires

A combination of electrostatic force microscopy and optical microscopy was used to investigate the charge state of individual CdSe nanowires upon local illumination with a focused laser beam. The nanowires were found to be positively charged at the excitation spot and negatively charged at the distant end(s). For high laser powers, the amount of accumulated charges increases logarithmically with the laser power. These effects are described by a diffusion-based model where the results are in good agreement with the experimentally observed effects. On the basis of this model the charge imbalance along the nanowire should establish in the course of nanoseconds. The net charge separation within homogeneous nanowires upon local illumination is of importance for several electronic devices.     

Plasmonic photosensitization of a wide band gap semiconductor: converting plasmons to charge carriers

A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.(1-8) Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron-hole pairs in the gold.(9-15) We propose that a significant fraction of these electrons tunnel into the semiconductor's conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor's band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ～50% of the device current measured with UV (?ω>Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications.     

Ion beam modification of exchange coupling to fabricate patterned media

For bit-patterned media, media with low remanent magnetization (M(r)) and high M(r) regions are needed for storing information, which is usually achieved by lithographically defining magnetic and non-magnetic regions. In this work, we have investigated the use of ion beam modification of media surface to define the low and high M(r) states using a medium that is at a low M(r) state to start with. The low M(r) state is achieved by the use of synthetic antiferromagnetic coupling obtained in Co-alloy/Ru/Co-alloy trilayer structured film. Local ion beam modification at 30 keV energy using Ga+ ions was used to create high M(r) regions. AFM and MFM observations indicated that patterned regions of low and high M(r) can be observed with ion beam irradiation. This technique is a potential method to achieve patterned media without the need of planarization techniques.     

Limitation of runaway electron beam duration in air-filled gap with inhomogeneous field

Alternative factors that account for a limitation of the period of injection of picosecond runaway electron bunches in air-filled diode with inhomogeneous electric field are analyzed. Experimental data on the characteristics of such electron beams have been obtained under the conditions with variable emissive properties of the cathode, time of the voltage prepulse action, and electric field strength in the region of electron injection. Based on these data, a hypothesis is formulated and justified that the mechanism of limitation related to a transition from the field electron emission to the explosion of microinhomogeneities is less probable than the mechanism of current limitation by a screening plasma cloud formed over the point electron emitters.     

Analysis of the effects of the residual charge and gap size on electrospun nanofiber alignment in a gap method

In this paper, the effects of residual charges on nanofiber alignment in a gap method are studied and presented. The gap method was presented by Li and Xia (2003?Nano Lett.?3 1167); in it, a gap is introduced into a traditional collector. Due to the non-perfect conductivity of electrospun nanofibers, they carry residual charges after deposition across the gap. These residual charges will interact with the charges carried by the upcoming jet/fiber, that will also deposit across the gap. The effects of these charge interactions on nanofiber alignment were studied numerically at various gap sizes. Results showed that alignments of nanofibers improve substantially with the gap size increasing from 3 to 8?mm. Numerical studies on the effect of residual charges in already deposited nanofibers on the alignment of nanofibers deposited afterwards were also conducted. Studies showed that the residual charges result in worse alignment, with a 10%-25% decrease in orientation parameters.     

Ultrafast charge separation at a polymer-single-walled carbon nanotube molecular junction

We have investigated the charge photogeneration dynamics at the interface formed between single-walled carbon nanotubes (SWNTs) and poly(3-hexylthiophene) (P3HT) using a combination of femtosecond spectroscopic techniques. We demonstrate that photoexcitation of P3HT forming a single molecular layer around a SWNT leads to an ultrafast (～430 fs) charge transfer between the materials. The addition of excess P3HT leads to long-term charge separation in which free polarons remain separated at room temperature. Our results suggest that SWNT-P3HT blends incorporating only small fractions (1%) of SWNTs allow photon-to-charge conversion with efficiencies comparable to those for conventional (60:40) P3HT-fullerene blends, provided that small-diameter tubes are individually embedded in the P3HT matrix.     

DNA separation methodology based on charge neutralization in a polycationic gel matrix

A novel method for separation of DNA fragments is here reported, based on migrating the polyanionic DNA fragments in a polycationic polyacrylamide gel, made by incorporating positively charged monomers (the Immobilines used for creating immobilized pH gradients) into the neutral polyacrylamide backbone. Separations can be operated under two working conditions: either against a gradient of positive charges, to allow the various DNA fragments to reach a steady-state position along the migration path and condense (focus) in an environment inducing charge neutralization, or in a plateau gel (i.e., in a gel containing a constant level of positive charges from anode to cathode). In this last case, separation is still obtained due to differential charge modulation of the various DNA fragments. In the 100-1000-bp length, it is shown that separation can be obtained even for fragments differing in length by <0.5%, as shown in the splitting of a 656- and 659-bp doublet, that could not be resolved by conventional polyacrylamide gels. In the 10-100-bp range, it is shown that the present method can resolve single nucleotide polymorphisms, i.e. fragments of identical number of nucleotides but differing by one base substitution. In this last case, separations are obtained only in gradient gels containing a much steeper gradient of charges (0-20 mM Immobiline pK 10.3 and pK 12, as opposed to gradients of only 2-4 mM positive charges for larger size fragments). This novel methodology represents a marked improvement over existing techniques and appears to hold promises for applications in diverse fields, such as molecular biology, forensic medicine, and genetic screening.     

Magnetoelastic wave tunneling via a gap between ferroelectric crystals with relative longitudinal displacement

The tunneling of a plane monochromatic acoustic wave via a gap between two ferromagnetic crystals exhibiting relative longitudinal displacement has been studied. It is shown that, at a gap width comparable with the wavelength, the acoustic wave can exhibit complete transmission at the Damon-Eshbach mode frequency. If the gap width is much smaller than the wavelength, the complete transmission takes place at two resonance frequencies. Allowance for the longitudinal displacement of one crystal leads in all cases to violation of the resonance conditions, which results in a significant decrease in the transmission coefficient. The greater the velocity of crystal displacement, the stronger the decrease in the acoustic wave transmission though the gap between ferromagnetic crystals.     

Axial separation of orthogonally polarized focal field components due to a radially polarized beam

In this paper, we investigate the field distribution in the focal volume of an aberrated radially polarized beam. Using two different forms of the vectorial diffraction theory, we show that the presence of defocus in the beam displaces both the axially and the radially polarized fields parallel to the optical axis of the focusing lens, while the presence of spherical aberration primarily shifts the longitudinally polarized field only. This facilitates axial separation of the two orthogonally polarized field components, resulting in a significant boost to the ratio of the peak longitudinally polarized field to the peak laterally polarized field in the focal plane. We further show that with an appropriate combination of oppositely signed defocus and spherical aberration, the energy density in the focal volume due to the longitudinally polarized field can be caused to peak at the focal plane. The results obtained are expected to be beneficial to the applications requiring a stronger longitudinally polarized focal field relative to the laterally polarized focal field component.     

Increasing the negative charge of a macroanion in the gas phase via sequential charge inversion reactions

Protonated and deprotonated biological molecules in the gas phase play an important role in life sciences research. The structural information accessible from the ions is highly dependent upon their charge states. Therefore, it is desirable to develop means for increasing absolute charge states, particularly for ionization methods, such as MALDI, that yield relatively low charge ions. The work presented here demonstrates the formation of a doubly deprotonated polypeptide or oligonucleotide ion (dianion) from a singly deprotonated analogue via two sequential ion/ion proton-transfer reactions involving charge inversion. The high exoergicity and the large cross section arising from the long-range attractive Coulomb potential of ion/ion reactions make this process plausible. In this example, an overall efficiency of conversion of singly charged ions to doubly charged ions of roughly 8% for polypeptide was noted while lower efficiency (roughly 2%) observed with an oligonucleotide is likely due to a greater degree of neutralization. No other approach to increasing the net negative charge of an anion in the gas phase has as yet been reported.     

Promoting the charge separation and photoelectrocatalytic water reduction kinetics of Cu_2O nanowires via decorating dual-cocatalysts

Developing high activity and low-cost materials to produce hydrogen by the sustainable way of photoelectrochemical is key to social development.The abundance and inexpensive Cu₂O has been received increasing research as its suitable energy level for photocatalytic water reduction.However,the fast charge recombination rate and the sluggish catalytic kinetics are the huge challenges facing the Cu₂O photoreduction.Here,the highly reactive Cu₂O@C-MoS₂photocathode is constructed by depositing dual-cocatalysts of the carbon layer and MoS₂nanosheets on Cu₂O nanowires to realize efficient water reduction.An impressive carrier concentration of 6.59×10²³cm^-3is received,which is 2.78 times of the bare Cu₂O,resulting in remarkable enhancement in photocurrent density of 3.34 times for the Cu₂O@CMoS₂photocathode.Moreover,the applied bias photon-to-current conversion efficiency of the bare Cu₂O enhanced 4.5 times from 0.16%to 0.72%in the Cu₂O@C-MoS₂photocathode.The analysis shows that the Cu₂O as light absorber,the carbon layer as electron transfer promoter,and MoS₂nanosheets as catalytic sites,thus facilitating chrage separation and enhancing catalytic kinetics.This system paves a feasible strategy for designing other photoelectrodes to realize efficient charge separation and high catalytic activity.     

Asymmetric superstructure formed in a block copolymer via phase separation

Self-assembly of amphiphilic block copolymers into well-ordered structures has attracted significant interest over the past decade. An especially attractive application of block-copolymer self-assembly is the formation of isoporous membranes. A major problem in this process is the lack of sufficient long-range order and the difficulty of up-scaling due to the time-consuming preparation steps. Here, we report an innovative and simple method to prepare isoporous membranes with nanometre-sized pores. The combination of the industrially well-established membrane formation method by non-solvent-induced phase separation with the self-assembly of a block copolymer is demonstrated. The result is the creation of an integral asymmetric membrane of a block copolymer with a highly ordered thin layer on top of a non-ordered sponge-like layer. This straightforward and very fast one-step procedure for membrane formation is reported for the first time. The developed membrane has the potential for highly selective separation.     

Application of a model of a quasiequlibrium plasma and methods of lie algebra to calculations of parameters of a transported high-current relativistic electron beam

A method for calculation of the parameters of an intense beam of charged particles transported by the magnetic field of the focusing elements is described, and the results of numerical modeling are presented. Scientific Research Institute for Nuclear Problems, Belarusian State University, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 69, No. 1, pp. 33–38, January–February, 1996.