467 research outputs found
Size-dependent effects on electrical contacts to nanotubes and nanowires
Metal-semiconductor contacts play a key role in electronics. Here we show
that for quasi-one dimensional (Q1D) structures such as nanotubes and
nanowires, side contact with the metal only leads to weak band realignment, in
contrast with bulk metal-semiconductor contacts. Schottky barriers are much
reduced compared with the bulk limit, and should facilitate the formation of
good contacts. However, the conventional strategy of heavily doping the
semiconductor to obtain Ohmic contacts breaks down as the nanowire diameter is
reduced. The issue of Fermi level pinning is also discussed, and it is
demonstrated that the unique density of states of Q1D structures makes them
less sensitive to this effect. Our results agree with recent experimental work,
and should apply to a broad range of Q1D materials.Comment: Physical Review Letters 97, 026804 (2006
Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires
We present electronic transport measurements of GaAs nanowires grown by
catalyst-free metal-organic chemical vapor deposition. Despite the nanowires
being doped with a relatively high concentration of substitutional impurities,
we find them inordinately resistive. By measuring sufficiently high
aspect-ratio nanowires individually in situ, we decouple the role of the
contacts and show that this semi-insulating electrical behavior is the result
of trap-mediated carrier transport. We observe Poole-Frenkel transport that
crosses over to phonon-assisted tunneling at higher fields, with a tunneling
time found to depend predominantly on fundamental physical constants as
predicted by theory. By using in situ electron beam irradiation of individual
nanowires we probe the nanowire electronic transport when free carriers are
made available, thus revealing the nature of the contacts
Study of charge-charge coupling effects on dipole emitter relaxation within a classical electron-ion plasma description
Studies of charge-charge (ion-ion, ion-electron, and electron-electron)
coupling properties for ion impurities in an electron gas and for a two
component plasma are carried out on the basis of a regularized electron-ion
potential without short-range Coulomb divergence. This work is motivated in
part by questions arising from recent spectroscopic measurements revealing
discrepancies with present theoretical descriptions. Many of the current
radiative property models for plasmas include only single electron-emitter
collisions and neglect some or all charge-charge interactions. A molecular
dynamics simulation of dipole relaxation is proposed here to allow proper
account of many electron-emitter interactions and all charge-charge couplings.
As illustrations, molecular dynamics simulations are reported for the cases of
a single ion imbedded in an electron plasma and for a two-component
ion-electron plasma. Ion-ion, electron-ion, and electron-electron coupling
effects are discussed for hydrogen-like Balmer alpha lines.Comment: 13 figures, submitted to Phys. Rev.
A MEMS Light Modulator Based on Diffractive Nanohole Gratings
We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V
Exact expression of the impact broadening operator for hydrogen Stark broadening
International audienceAims. Recent measurements on the Stark broadening of radio recombination lines show values and trends in disagreement with conventional theories. Different attemps to explain those disagreements have not been successfull for any of the employed theoretical models. In particular, the impact model that describes well the physical conditions at which the studied broadenings occur, shows a functional trend upon the principal quantum number of the studied transitions that does not correspond to the experimental observations. Methods. High values of the principal quantum number require computable formulas for the calculation of transition probabilities. Some of those expressions have been published, leading to approximate formulas on the dependence of the line width versus the principal quantum number of the upper level of the transition. Results. In this work an exact expression for the hydrogen Stark width in the frame of impact approximation is given
Slow and fast micro-field components in warm and dense hydrogen plasmas
The aim of this work is the investigation of the statistical properties of
local electric fields in an ion-electron two component plasmas for coupled
conditions. The stochastic fields at a charged or at a neutral point in plasmas
involve both slow and fast fluctuation characteristics. The statistical study
of these local fields based on a direct time average is done for the first
time. For warm and dense plasma conditions, typically , , well controlled molecular dynamics (MD)
simulations of neutral hydrogen, protons and electrons have been carried out.
Relying on these \textit{ab initio} MD calculations this work focuses on an
analysis of the concepts of statistically independent slow and fast local field
components, based on the consideration of a time averaged electric field. Large
differences are found between the results of these MD simulations and
corresponding standard results based on static screened fields. The effects
discussed are of importance for physical phenomena connected with stochastic
electric field fluctuations, e.g., for spectral line broadening in dense
plasmas.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Unveiling Stability Criteria of DNA-Carbon Nanotubes Constructs by Scanning Tunneling Microscopy and Computational Modeling
We present a combined approach that relies on computational simulations and scanning tunneling microscopy (STM) measurements to reveal morphological properties and stability criteria of carbon nanotube-DNA (CNT-DNA) constructs. Application of STM allows direct observation of very stable CNT-DNA hybrid structures with the well-defined DNA wrapping angle of 63.4° and a coiling period of 3.3 nm. Using force field simulations, we determine how the DNA-CNT binding energy depends on the sequence and binding geometry of a single strand DNA. This dependence allows us to quantitatively characterize the stability of a hybrid structure with an optimal π-stacking between DNA nucleotides and the tube surface and better interpret STM data. Our simulations clearly demonstrate the existence of a very stable DNA binding geometry for (6,5) CNT as evidenced by the presence of a well-defined minimum in the binding energy as a function of an angle between DNA strand and the nanotube chiral vector. This novel approach demonstrates the feasibility of CNT-DNA geometry studies with subnanometer resolution and paves the way towards complete characterization of the structural and electronic properties of drug-delivering systems based on DNA-CNT hybrids as a function of DNA sequence and a nanotube chirality
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