2,889 research outputs found
Electronic response of graphene to linelike charge perturbations
The problem of electrostatic screening of a charged line by undoped or weakly
doped graphene is treated beyond the linear-response theory. The induced
electron density is found to be approximately doping independent, n(x)~(log
x)^2/x^2, at intermediate distances x from the charged line. At larger x, twin
p-n junctions may form if the external perturbation is repulsive for graphene
charge carriers. The effect of such inhomogeneities on conductance and quantum
capacitance of graphene is calculated. The results are relevant for transport
properties of graphene grain boundaries and for local electrostatic control of
graphene with ultrathin gates.Comment: Fixed typos and added reference
Dynamic Behavior in Piezoresponse Force Microscopy
Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM)
implemented on a beam-deflection atomic force microscope (AFM) is analyzed
using a combination of modeling and experimental measurements. The PFM signal
comprises contributions from local electrostatic forces acting on the tip,
distributed forces acting on the cantilever, and three components of the
electromechanical response vector. These interactions result in the bending and
torsion of the cantilever, detected as vertical and lateral PFM signals. The
relative magnitudes of these contributions depend on geometric parameters of
the system, the stiffness and frictional forces of tip-surface junction, and
operation frequencies. The dynamic signal formation mechanism in PFM is
analyzed and conditions for optimal PFM imaging are formulated. The
experimental approach for probing cantilever dynamics using frequency-bias
spectroscopy and deconvolution of electromechanical and electrostatic contrast
is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques
Fermi-level alignment at metal-carbon nanotube interfaces: application to scanning tunneling spectroscopy
At any metal-carbon nanotube interface there is charge transfer and the
induced interfacial field determines the position of the carbon nanotube band
structure relative to the metal Fermi-level. In the case of a single-wall
carbon nanotube (SWNT) supported on a gold substrate, we show that the charge
transfers induce a local electrostatic potential perturbation which gives rise
to the observed Fermi-level shift in scanning tunneling spectroscopy (STS)
measurements. We also discuss the relevance of this study to recent experiments
on carbon nanotube transistors and argue that the Fermi-level alignment will be
different for carbon nanotube transistors with low resistance and high
resistance contacts.Comment: 4 pages, 3 ps figures, minor corrections, accepted by Phys. Rev. Let
Scanning Quantum Dot Microscopy
Interactions between atomic and molecular objects are to a large extent
defined by the nanoscale electrostatic potentials which these objects produce.
We introduce a scanning probe technique that enables three-dimensional imaging
of local electrostatic potential fields with sub-nanometer resolution.
Registering single electron charging events of a molecular quantum dot attached
to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we
quantitatively measure the quadrupole field of a single molecule and the dipole
field of a single metal adatom, both adsorbed on a clean metal surface. Because
of its high sensitivity, the technique can record electrostatic potentials at
large distances from their sources, which above all will help to image complex
samples with increased surface roughness.Comment: main text: 5 pages, 4 figures, supplementary information file: 4
pages, 2 figure
Physical origin underlying the entropy loss upon hydrophobic hydration
The hydrophobic effect (HE) is commonly associated with the demixing of oil
and water at ambient conditions and plays the leading role in determining the
structure and stability of biomolecular assembly in aqueous solutions. On the
molecular scale HE has an entropic origin. It is believed that hydrophobic
particles induce order in the surrounding water by reducing the volume of con-
figuration space available for hydrogen bonding. Here we show with computer
simulation results that this traditional picture is not correct. Analyzing
collective fluctuations in water clusters we are able to provide a
fundamentally new picture of HE based on pronounced many-body correlations
affecting the switching of hydrogen bonds between molecules. These correlations
emerge as a non-local compensation of reduced fluctuations of local
electrostatic fields in the presence of an apolar solute
Site-selective adsorption of naphthalene-tetracarboxylic-dianhydride on Ag(110): First-principles calculations
The mechanism of adsorption of the
1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) molecule on the Ag(110)
surface is elucidated on the basis of extensive density functional theory
calculations. This molecule, together with its perylene counterpart, PTCDA, are
archetype organic semiconductors investigated experimentally over the past 20
years. We find that the bonding of the molecule to the substrate is highly
site-selective, being determined by electron transfer to the LUMO of the
molecule and local electrostatic attraction between negatively charged carboxyl
oxygens and positively charged silver atoms in [1-10] atomic rows. The
adsorption energy in the most stable site is 0.9eV. A similar mechanism is
expected to govern the adsorption of PTCDA on Ag(110) as well.Comment: 8 pages, 4 figures, high-quality figures available upon reques
An interpretation of fluctuations in enzyme catalysis rate, spectral diffusion, and radiative component of lifetimes in terms of electric field fluctuations
Time-dependent fluctuations in the catalysis rate ({delta}k(t)) observed in single-enzyme experiments were found in a particular study to have an autocorrelation function decaying on the same time scale as that of spectral diffusion {delta}{omega}0(t). To interpret this similarity, the present analysis focuses on a factor in enzyme catalysis, the local electrostatic interaction energy (E) at the active site and its effect on the activation free energy barrier. We consider the slow fluctuations of the electrostatic interaction energy ({delta}E(t)) as a contributor to {delta}k(t) and relate the latter to {delta}{omega}0(t). The resulting relation between {delta}k(t) and {delta}{omega}0(t) is a dynamic analog of the solvatochromism used in interpreting solvent effects on organic reaction rates. The effect of the postulated {delta}E(t) on fluctuations in the radiative component ({delta}{gamma}Formula(t)) of the fluorescence decay of chromophores in proteins also is examined, and a relation between {delta}{gamma}Formula(t) and {delta}{omega}0(t) is obtained. Experimental tests will determine whether the correlation functions for {delta}k(t), {delta}{omega}0(t), and {delta}{gamma}Formula are indeed similar for any enzyme. Measurements of dielectric dispersion, {varepsilon}({omega}), for the enzyme discussed elsewhere will provide further insight into the correlation function for {delta}E(t). They also will determine whether fluctuations in the nonradiative component {gamma}Formula of the lifetime decay has a different origin, fluctuations in distance for example
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