20 research outputs found
Fast diffusion of graphene flake on graphene layer
Diffusion of a graphene flake on a graphene layer is analyzed and a new
diffusion mechanism is proposed for the system under consideration. According
to this mechanism, rotational transition of the flake from commensurate to
incommensurate states takes place with subsequent simultaneous rotation and
translational motion until the commensurate state is reached again, and so on.
The molecular dynamics simulations and analytic estimates based on ab initio
and semi-empirical calculations demonstrate that the proposed diffusion
mechanism is dominant at temperatures T ~ Tcom, where Tcom corresponds to the
barrier for transitions of the flake between adjacent energy minima in the
commensurate states. For example, for the flake consisting of ~ 40, 200 and 700
atoms the contribution of the proposed diffusion mechanism through rotation of
the flake to the incommensurate states exceeds that for diffusion of the flake
in the commensurate states by one-two orders of magnitude at temperatures 50 -
150 K, 200 - 600 K and 800 - 2400 K, respectively. The possibility to
experimentally measure the barriers to relative motion of graphene layers based
on the study of diffusion of a graphene flake is considered. The results
obtained are also relevant for understanding of dynamic behavior of polycyclic
aromatic molecules on graphene and should be qualitatively valid for a set of
commensurate adsorbate-adsorbent systems.Comment: 33 pages, 6 figure
Molecular dynamics simulation of the self-retracting motion of a graphene flake
The self-retracting motion of a graphene flake on a stack of graphene flakes
is studied using molecular dynamics simulations. It is shown that in the case
when the extended flake is initially rotated to an incommensurate state, there
is no barrier to the self-retracting motion of the flake and the flake retracts
as fast as possible. If the extended flake is initially commensurate with the
other flakes, the self-retracting motion is hindered by potential energy
barriers. However, in this case, the rotation of the flake to incommensurate
states is often observed. Such a rotation is found to be induced by the torque
acting on the flake on hills of the potential relief of the interaction energy
between the flakes. Contrary to carbon nanotubes, telescopic oscillations of
the graphene flake are suppressed because of the high dynamic friction related
to the excitation of flexural vibrations of the flake. This makes graphene
promising for the use in fast-responding electromechanical memory cells.Comment: 24 pages, 8 figure
AA stacking, tribological and electronic properties of double-layer graphene with krypton spacer
Structural, energetic and tribological characteristics of double-layer
graphene with commensurate and incommensurate krypton spacers of nearly
monolayer coverage are studied within the van der Waals-corrected density
functional theory. It is shown that when the spacer is in the commensurate
phase, the graphene layers have the AA stacking. For this phase, the barriers
to relative in-plane translational and rotational motion and the shear mode
frequency of the graphene layers are calculated. For the incommensurate phase,
both of the barriers are found to be negligibly small. A considerable change of
tunneling conductance between the graphene layers separated by the commensurate
krypton spacer at their relative subangstrom displacement is revealed by the
use of the Bardeen method. The possibility of nanoelectromechanical systems
based on the studied tribological and electronic properties of the considered
heterostructures is discussed
Nanotube-Based NEMS: Control vs. Thermodynamic Fluctuations
Multi-scale simulations of nanotube-based nanoelectromechanical systems
(NEMS) controlled by a nonuniform electric field are performed by an example of
a gigahertz oscillator. Using molecular dynamics simulations, we obtain the
friction coefficients and characteristics of the thermal noise associated with
the relative motion of the nanotube walls. These results are used in a
phenomenological one-dimensional oscillator model. The analysis based both on
this model and the Fokker-Planck equation for the oscillation energy
distribution function shows how thermodynamic fluctuations restrict the
possibility of controlling NEMS operation for systems of small sizes. The
parameters of the force for which control of the oscillator operation is
possible are determined.Comment: 40 pages, 12 figure
Barriers to motion and rotation of graphene layers based on measurements of shear mode frequencies
Both van der Waals corrected density functional theory and classical
calculations show that the potential relief of interaction energy between
layers of graphite and few-layer graphene can be described by a simple
expression containing only the first Fourier components. Thus a set of physical
quantities and phenomena associated with in-plane relative vibration,
translational motion and rotation of graphene layers are interrelated and are
determined by a single parameter characterizing the roughness of the potential
energy relief. This relationship is used to estimate the barriers to relative
motion and rotation of graphene layers based on experimental measurements of
shear mode frequencies.Comment: 16 pages, 1 figur
Study of optimization options for second generation solar cell materials by multilevel modeling
Theoretical analysis of optimization options for the properties of CdTe absorber layer is an important task for increasing the efficiency of CdTe/CdS heterojunction based thin-film solar cells. Properties of the materials (e.g. the density of free carriers) often depend essentially on the parameters of the deposition process and subsequent treatment which determine the defect composition of the material. In this work a model based on the lattice kinetic Monte-Carlo method is developed to describe the process of CdTe deposition as a function of temperature and Cd and Te fluxes. To determine the effect of the treatment conditions on CdTe conductivity, we developed a quasichemical model based on the electrical neutrality equation for point defect concentrations that are described by defect formation reaction constants. Parameters obtained from the first-principles density functional calculations were used for developing the models. The developed deposition model correctly describes the transition from evaporation to precipitation as well as the increased evaporation rates in excess of Cd. To explain the observed electrical properties of CdTe after Cl-treatment, we complemented the quasichemical defect model by a deep acceptor complex defect that allowed us to describe both the high-temperature dependence of conductivity on the Cd pressure and the dependence of resistivity on Cl concentration at room temperature