Olga V. Ershova

Fast diffusion of a graphene flake on a 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.

Diffusion and drift of graphene flake on graphite surface

Diffusion and drift of a graphene flake on a graphite surface are analyzed. A potential energy relief of the graphene flake is computed using ab initio and empirical calculations. Based on the analysis of this relief, different mechanisms of diffusion and drift of the graphene flake on the graphite surface are considered. A new mechanism of diffusion and drift of the flake is proposed. According to the proposed mechanism, rotational transition of the flake from commensurate to incommensurate state takes place with subsequent simultaneous rotation and translational motion until a commensurate state is reached again, and so on. Analytic expressions for the diffusion coefficient and mobility of the flake corresponding to different mechanisms are derived in wide ranges of temperatures and sizes of the flake. The molecular dynamics simulations and estimates based on ab initio and empirical calculations demonstrate that the proposed mechanism can be dominant under certain conditions. The influence of structural defects on the diffusion of the flake is examined on the basis of calculations of the potential energy relief and molecular dynamics simulations. The methods of control over the diffusion and drift of graphene components in nanoelectromechanical systems are discussed. The possibility to experimentally determine the barriers to relative motion of graphene layers based on the study of diffusion of a graphene flake is considered. The results obtained can also be applied to polycyclic aromatic molecules on graphene and should be qualitatively valid for a set of commensurate adsorbate-adsorbent systems.

Can density functional theory describe the NO(X2Π)-Ar and NO(A2Σ+)-Ar van der Waals complexes?

The interaction of nitric oxide (NO) in its ground state X(2)Π and the first excited Rydberg state A(2)Σ(+) with an argon (Ar) atom has been studied using density functional theory. A number of exchange-correlation functionals that account for dispersion interactions have been considered, including functionals with both empirical and non-empirical treatments of dispersion. To study NO in the excited state, the recently developed maximum overlap method was used. Potential energy surfaces for interaction of NO with Ar have been constructed and parameters describing their minima, such as NO-Ar distance, orientation angle, and binding energy, have been determined. A comparison with combined experimental and accurate theoretical data has been made in terms of these parameters and the overall shape of the surfaces. For the ground state, several of the functionals give very good results. Treatment of the excited state is more problematic. None of the functionals considered provides completely satisfactory results. Several reasons for this failure have been identified: an incorrect description of the non-dispersion component of the interaction and the damping of the dispersion interaction at small interatomic distances.

Nanotube-based nanoelectromechanical systems: Control versus 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.

Ab initio calculations of the walls shear strength of carbon nanotubes

The dependence of the energy of interwall interaction in double-wall carbon nanotubes (DWNT) on the relative position of walls has been calculated using the density functional method. This dependence is used to evaluate forces that are necessary for the relative telescopic motion of walls and to calculate the shear strength of DWNT for the relative sliding of walls along the nanotube axis and for their relative rotation about this axis. The possibility of experimental verification of the obtained results is discussed.

Control of the motion of nanoelectromechanical systems based on carbon nanotubes by electric fields

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube. The electric dipole moments of carbon nanotubes with chemically modified ends are calculated by the molecular orbital method. These nanotubes can be set in motion under the effect of a nonuniform electric field. The possibility of controlling the motion of nanoelectromechanical systems with the proposed method is demonstrated using a nanotube-based gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator are analyzed, and its operation is simulated by the molecular dynamics method. The controlling parameters and characteristics corresponding to the controlled operating conditions at a constant frequency for the system under investigation are determined.

Nanoresonator Based on Relative Vibrations of the Walls of Carbon Nanotubes

A new type of ultrahigh frequency resonator based on the relative vibrations of carbon nanotubes walls is proposed. Microcanonical molecular dynamics simulations performed for a model resonator based on the (9,0)@(18,0) double-walled carbon nanotube with the movable outer wall give the value of frequency equal to 130 GHz and the quality factor up to 500. Possible applications of the resonator are discussed. The estimated mass sensitivity of the mass sensor based on the proposed resonator reaches the atomic mass limit at liquid helium temperature. The possibility of resonator based on relative vibrations of graphene layers is also considered.

Control of the motion of nanoelectromechanical systems based on carbon nanotubes

A new method is proposed for controlling the motion of nanoelectromechanical systems based on carbon nanotubes. In this method, a single-walled nanotube acquires an electric dipole moment owing to the chemical adsorption of atoms or molecules at open ends of the nanotube and, then, the electric dipole moment thus induced can be set in motion under the effect of a nonuniform electric field. The electric dipole moments of chemically modified nanotubes are calculated for the first time. The possibility of controlling the motion of nanotube-based nanoelectromechanical systems with the proposed method is demonstrated using a gigahertz oscillator as an example. The operating characteristics of the gigahertz oscillator and the controlling electric field are calculated.

Interaction of the NO 3pπ (C 2Π) Rydberg state with RG (RG = Ne, Kr, and Xe): Potential energy surfaces and spectroscopy

We present new potential energy surfaces for the interaction of NO(C (2)Π) with each of Ne, Kr, and Xe. The potential energy surfaces have been calculated using second order Møller-Plesset perturbation theory, exploiting a procedure to converge the reference Hartree-Fock wavefunction for the excited states: the maximum overlap method. The bound rovibrational states obtained from the surfaces are used to simulate the electronic spectra and their appearance is in good agreement with available (2+1) REMPI spectra. We discuss the assignment and appearance of these spectra, comparing to that of NO-Ar.