The influence of ripple waves on the band diagram of zigzag strained graphene nanoribbons (GNRs) is analyzed
by utilizing the finite element method. Such waves have their origin in electromechanical effects. With a novel
model, we demonstrate that electron-hole band diagrams of GNRs are highly influenced (i.e. level crossing of the
bands are possible) by two combined effects: pseudo-magnetic fields originating from electroelasticity theory and
external magnetic fields. In particular, we show that the level crossing point can be observed at large external
magnetic fields (B ≈ 100T ) in strained GNRs, when the externally applied tensile edge stress is on the order of
-100 eV/nm and the amplitude of the out-of-plane ripple waves is on the order of 1nm.
RNA molecules are very flexible in nature. This feature allows us to build various motifs which are essential in bionanotechnological applications. Based on our earlier developed models of RNA nanoclusters, in this contribution we analyze the structure and properties of RNA nanotubes in physiological solutions at different concentrations. Our major tool here is the molecular dynamics (MD) method that was implemented by using the NAMD and VMD packages, with which we study the structural and thermal properties of the nanotubes in physiological solutions. In particular, we have analyzed such characteristics as the Root Mean Square Deviation (RMSD), the radius of gyration, the number of hydrogen bonds per base pairs, and the radial distribution function (RDF) of a RNA nanotube at different concentrations of the physiological solution. Furthermore, the number of 23Na+ and 35Cl−ions around the nanotubes within the distance of 5 Å at two different concentrations has also been analyzed in detail. It has been found that the number of ions accumulated around the nanotubes within the particular distance is growing by small amount while the concentrations of the 23Na+ and 35Cl−ions are substantially increased.
We study the coupled electro-mechanical effects in the band structure calculations of low dimensional semiconductor
nanostructures (LDSNs) such as AlN/GaN quantum dots. Some effects in these systems are essentially
nonlinear. Strain, piezoelectric effects, eigenvalues and wave functions of a quantum dot have been used as
tuning parameters for the optical response of LDSNs in photonics, band gap engineering and other applications. However, with a few noticeable exceptions, the influence of piezoelectric effects in the electron wave functions
in Quantum Dots (QDs) studied with fully coupled models has been largely neglected in the literature. In this paper, by using the fully coupled model of electroelasticity, we analyze the piezoelectric effects into the band structure of cylindrical quantum dots. Results are reported for III-V type semiconductors with a major focus given to AlN/GaN based QD systems.
We discuss a relatively simple and computationally inexpensive model that has recently been developed to study phase
transformations and shape memory effects in finite nanostructures. Our major focus is given to nanowires of finite length
and other nanostructures where size effects are pronounced. The main tool used here is based on mesoscopic models
developed with the phase-field approach which we and other authors have applied before to study ferroelectrics at the
nanoscale. We study the cubic-to-tetragonal transformations in which case the 2D analogue of the model describes the
square-to-rectangle phase transformations. The actual model is based on a coupled system of partial differential equations
and is solved with a combination of the Chebyshev collocation method and the extended proper orthogonal decomposition.
The developed model and its numerical implementation allow us to study properties of nanostructures and several
representative examples of mechanical behavior of nanostructures are discussed.
We report some results on the analysis of thermo-electromechanical effects in low dimensional semiconductor
nanostructures (LDSNs). A coupled model of thermoelectroelasticity has been applied to the analysis of quantum dots
and quantum wires. Finite element solutions have been obtained for different thermal loadings and their effects on the
electromechanical properties in quantum dots and quantum wires are presented. Our model accounts for a practically
important range of internal and external thermoelectromechanical loadings. Results are obtained for typical quantum
dot and quantum wire systems with cylindrical geometry. The comparative analysis of thermoelectromechanical
effects in quantum dots and quantum wires is also presented. It is observed that the electromechanical effects in
LDSNs are noticeably influenced by thermal loadings. The influence is more significant in quantum dots as compared
to that of quantum wires.
We quantify the influence of thermopiezoelectric effects in nano-sized AlxGa1-xN/GaN heterostructures for pressure
sensor applications based on the barrier height modulation principle. We use a fully coupled thermoelectromechanical
formulation, consisting of balance equations for heat transfer, electrostatics and mechanical field.
To estimate the vertical transport current in the heterostructures, we have developed a multi-physics model
incorporating thermionic emission, thermionic field emission, and tunneling as the current transport mechanisms.
A wide range of thermal (0-300 K) and pressure (0-10 GPa) loadings has been considered. The results
for the thermopiezoelectric modulation of the barrier height in these heterostructures have been obtained and
optimized. The calculated current shows a linear decrease with increasing pressure. The linearity in pressure
response suggests that AlxGa1-xN/GaN heterostructure-based devices are promising candidates for pressure
sensor applications under severe environmental conditions.
In this paper, we propose a new design configuration for a carbon nanotube (CNT) array based pulsed field
emission device to stabilize the field emission current. In the new design, we consider a pointed height distribution
of the carbon nanotube array under a diode configuration with two side gates maintained at a negative potential
to obtain a highly intense beam of electrons localized at the center of the array. The randomly oriented CNTs are
assumed to be grown on a metallic substrate in the form of a thin film. A model of field emission from an array of
CNTs under diode configuration was proposed and validated by experiments. Despite high output, the current in
such a thin film device often decays drastically. The present paper is focused on understanding this problem. The
random orientation of the CNTs and the electromechanical interaction are modeled to explain the self-assembly.
The degraded state of the CNTs and the electromechanical force are employed to update the orientation of the
CNTs. Pulsed field emission current at the device scale is finally obtained by using the Fowler-Nordheim equation
by considering a dynamic electric field across the cathode and the anode and integration of current densities
over the computational cell surfaces on the anode side. Furthermore we compare the subsequent performance of
the pointed array with the conventionally used random and uniform arrays and show that the proposed design
outperforms the conventional designs by several orders of magnitude. Based on the developed model, numerical
simulations aimed at understanding the effects of various geometric parameters and their statistical features on
the device current history are reported.
In this contribution we propose an iterative scheme for the solution of the coupled Poisson-Schroedinger system in
a self-consistent manner. The developed methodology allows us to analyze the combined effects of piezoelectricity,
spontaneous polarization, and the charge density in low-dimensional semiconductor nanostructures. These effects
are analyzed here on an example of a wurtzite type semiconductor heterojunction. It is shown that such effects
may influence substantially the electronic states and quasi-Fermi level energies of the nanostructures, in particular
when compared to one-step calculations based on the conventional schemes. A major emphasis is given to two
different types of mechanical boundary conditions.
A Ginzburg-Landau free energy model of multivariant phase transformation in shape memory alloy has been developed. This paper is focused on linking the developed microscopic model with the atomistic reordering process which finally give rise to self-accommodating microstructure. It is analyzed how the kinetics influences the computation of stress-temperature induced dynamics of
phase transformation in microscopic and larger length-scales without attempting to solve a molecular dynamic problem in a coupled manner. A variational approach is adopted and phase transformation in Ni-Al thin film is simulated. The simulations capture a qualitative picture of the onset of microstructure formation.
A mesoscopic model to analyze various effects of electroactive and flow related properties in piezoelectric copolymer composite thin film has been developed in this paper. A three-phase composite with piezoelectric particulate phase, electroactive polymer phase and graft polymer-matrix phase is considered. The homogenized constitutive model takes into account the local transport of cations in polymer, electrostriction and anhysteretic polarization. A finite strain description is given which includes the mesoscopic dispersion of copolymer chains. Finite element simulation is carried out by considering a P(VDF-TrFE)-PZT-Araldite thin film. Analysis of the results indicate that an increasing copolymer content substantially changes the deformation pattern in the film.
Since the phase change in SMA-based devices such as actuators is accompanied by a significant heat exchange with the surroundings, different concepts to heat/cool SMAs have been proposed in the literature. Most of these concepts require the analysis of a multilayered (e.g., "sandwich"-type) structure where the SMA layer is placed between layers with another material. In this paper we propose a mathematical model and an efficient numerical method for this analysis. Although our approach can be applied to a wide range of different designs of multilayered actuators, the basic idea of the model construction is explained in this paper for a specific design based on the introduction of semiconductor "heat pump" modules into the device and the Peltier effect for the heat exchange. The dynamics of thermomechanical fields is studied with a coupled system of PDEs based on conservation laws. The system, supplemented by constitutive relationships in the Falk form, is reduced to a differential-algebraic (DA) model and solved with an effective DA solver developed in our previous works. Numerical results on thermomechanical behaviour of SMA components in multilayered actuators are presented.
In this paper we present results on numerical studies of the martensitic-austenitic phase transition mechanism in a large shape-memory-alloy rod. Three groups of experiments are reported. They include results on stress- and temperature- induced phase transformations as well as the analysis of the hysteresis phenomenon. All computational experiments are presented for Cu-based structures.