We present here a summary of some recent techniques used for atomistic studies of thin film growth and morphological evolution. Specific attention is given to a new kinetic Monte Carlo technique in which the usage of unique labeling schemes of the environment of the diffusing entity allows the development of a closed data base of 49 single atom diffusion processes for periphery motion. The activation energy barriers and diffusion paths are calculated using reliable manybody interatomic potentials. The application of the technique to the diffusion of 2-dimensional Cu clusters on Cu(111) shows interesting trends in the diffusion rate and in the frequencies of the microscopic mechanisms which are responsible for the motion of the clusters, as a function of cluster size and temperature. The results are compared with those obtained from yet another novel kinetic Monte Carlo technique in which an open data base of the energetics and diffusion paths of microscopic processes is continuously updated as needed. Comparisons are made with experimental data where available.
Optical and electron microscopies reveal complexity in the multilayered chiral coatings that produce green metallic-like reflections from manuka (scarab) beetles. In particular the reflectors are shown to have the form of small concave pits and troughs that are filled with contouring chiral material. Each chiral micro-reflector presents a range of pitch and tilt to an incident beam of light. The presentation attempts to relate these physical properties to optical properties such as spectral reflectance, angle of spread and perceived color of the beetles.
Composite films of copper and aluminum were deposited by reactive direct current co-sputtering of copper and aluminum. Varying the number of copper pieces during deposition made it possible to vary the composition of the films. In order to understand the optical behavior of the coatings, the transmittance and the near normal reflectance spectra in the 300-2500nm wavelength range are fitted with model dielectric functions. Valuable information regarding the surface roughness layer and its thickness are obtained. The thicknesses of the layers obtained by modeling were confirmed by surface profilometry. The extracted optical constants by the model were compared with calculations from Maxwell-Garnett, Bruggeman and incremental Maxwell-garnett effective medium theories. The XPS studies done on the samples reveal that the number of copper pieces used during co-sputtering control the formation of copper oxide.
A defining characteristic of metamaterials is that they exhibit
behaviour which is not exhibited--either to the same extent or
not at all--by their component materials. Prime examples of
such metamaterial behaviour may be conceptualized through the
process of homogenization. A survey of five different types of
metamaterial--each envisaged as a homogenized composite medium
(HCM)--is presented. The unusual structures and properties of
these HCM-metamaterials are discussed. The constitutive
parameters of HCMs may be estimated through the implementation of
well-established homogenization formalisms. In particular, those
of Maxwell Garnett and Bruggeman, as well as the more comprehensive
strong-property-fluctuation theory, are considered. Firstly, we
explore bianisotropic HCMs as metamaterials. While bianisotropy
is rarely observed in naturally-occurring materials, bianisotropic HCMs may be readily conceptualized, arising from component phases with comparatively simple constitutive properties and simple particulate geometries. Secondly, the issue of Voigt wave propagation is examined. It is demonstrated that this degenerate mode of
plane wave propagation can develop in an HCM, even though its component phases do not support such propagation. Thirdly, the topic of plane wave propagation with negative phase velocity is discussed for HCMs. Strategies for achieving negative phase velocities are considered. Fourthly, we illustrate how homogenization can lead to the group velocity in certain HCM-metamaterials exceeding the group velocity in the component materials. Fifthly, the phenomenon of nonlinearity enhancement in weakly nonlinear HCMs is presented.
This paper presents a short overview of the methods used for the study of mechanics at small scales. The key issue to be tackled is the presence of multiple scales starting from the atomic scale. The methods outlined include continuum, atomistic and mixed methods.
Tilted Cavity Laser (TCL) is developed that combines advantages of a high power operation of an edge-emitting semiconductor diode laser and wavelength-stabilized operation of a surface emitting laser. A TCL emits laser light in a tilted optical mode that propagates effectively at a certain tilt angle to the p-n junction. Designed TCL comprises a high-finesse cavity into which an active region is placed and at least one multilayer interference reflector (MIR). The cavity and the MIR are designed such that the spectral position of the reflectivity dip of the cavity and the position of the stopband reflectivity maximum of the MIR coincides at one tilt angle of a tilted optical mode, and draw apart as the angle deviates from the optimum value. As a result, the leakage loss of the optical modes to the substrate is minimum at the optimum wavelength and increases dramatically as the wavelength deviates from the optimum one. This ensures the stabilization of the wavelength of the emitted laser light. Both quantum well (QW) and quantum dot (QD) TCLs have been fabricated on the basis of GaAs/GaAlAs waveguides. QW TCL using InGaAs QW as the active region and operating at 1000-1100 nm reveals the temperature shift of the lasing wavelength 0.2 nm/K. QW TCL operates up to and above 210°C with the differential efficiency 20%. QD TCL using InAs QD overgrown by InGaAs alloy as the active region and operating at 1100-1200 nm reveals the temperature shift of the lasing wavelength 0.165 nm/K. These shifts are significantly slower than the shift for a conventional edge-emitting semiconductor diode laser. The QD TCL shows an output power 2W in a pulsed mode. Characteristic temperature of the threshold current measured at and below room temperature (T0) is 150 K.
The influence of local fields on the excitonic Rabi oscillations
in isolated, arbitrary shaped quantum dot (QD) has been theoretically investigated. Hamiltonian of the system "QD+electromagnetic field" has been obtained. Both QD interaction with classical electromagnetic field and ultrashort optical pulse has been considered. As a result, the bifurcation and anharmonism in the Rabi oscillations in a QD exposed to the monochromatic field have been predicted. The dependence of Rabi oscillations period on the QD depolarization parameter, which characterize local field has been obtained. It has been shown, that for the Gaussian pulse the final state of inversion as a function of peak pulse strength demonstrates step-like transitions.
We study the shaping of pulsed two-dimensional optical beams by chiral sculptured thin films (STFs) in the time domain, so that the spatiotemporal evolution of the light is elucidated as it propagates through the film. The time-domain manifestation of the circular Bragg phenomenon is examined in two dimensions, and we comment on its implications for pulsed beam shaping by STF devices. Several crucial differences separate the time-domain manifestation of the circular Bragg phenomenon for pulsed plane waves and pulsed beams. Specifically, the beam waist is an important parameter for prediction of how reflection from or transmission through a chiral STF will shape the edges of a pulsed beam with respect to its central portion. In addition, control of the position of chiral STFs with respect to pulsed beam sources is important for proper pulse shaping. We expect that further advances in STF fabrication will allow for the development of STF-based pulsed beam shaping devices.
The piecewise homogeneity approximation is used to investigate the
optical response of chiral sculptured thin films with perturbations
in the angle of rise. Changes in the reflectances and transmittances of normally incident, circularly polarized light are calculated for
both sinusoidal and step perturbations for a range of wavelengths, using experimentally determined material parameters. The feasibility of using a single manufacturing process to fabricate a normal dielectric mirror as well as a selective circular Bragg effect mirror is demonstrated.
The control of charged particles at sub micrometer and nanometer length scales presents an intrinsically interesting challenge, as well as being a rich field for the study of trapped ions and plasmas. Motivated by this, we obtain the exact solution for the
vector potential for a wire of finite length and of arbitrary form. Closed form solutions can then be deduced describing the electromagnetic waves propagating from the wire. This allows us to investigate design parameters, so that we may produce spiral wire shapes which, when injected with oscillatory currents, produce effects similar to conventional magnetic mirrors, except at the submicron and nanometre scale.
Nanoscale devices present an added complication: very closely placed surfaces can exchange heat through the tunneling of evanescent radiation modes. This can augment the local heating effect when compared to blackbody emission, so any fabrication defects on the surface of the wire spirals could be problematic. We show that the evanescent contributions scale as a function of separation and dominate the heat exchange process when the spacing is much less than the characteristic wavelength of a given temperature. We expect that excess material might be deposited erroneously during fabrication of the spiral wires, so the transfer of heat from one wire coil to the defect will be higher than the rate due to uniform blackbody radiation. In the case of tungsten, for our typical spiral geometry, the heating rate is enhanced by a factor of 15. In the case of a carbon or other high conductivity composite material this rate can be raised by as much as 106, which is evidently not appropriate.
The mechanical properties of nanoscale helical structures have become the subjects of great research interest lately. These helical structures include natural helices like the α-helical polypeptide and man-made helices such as nanosprings or nanocoils. Based on a common belief that a nanoscale helical structure would behave like a spring, much attention is devoted to obtaining its spring-constant, or stiffness. For nanocoils, however, whether a single stiffness value exists is questionable. Very often, a nanospring structure experiences a large deformation with respect to its dimension, and its coil radius decreases when it is in tension and increases when in compression. According to the classical equation by Ancker and Goodier, the stiffness of a coil is inversely proportional to the coil radius to the third order. Thus, a single stiffness value does not exist for nanosprings: the stiffness should increase when it is in tension and decrease when in compression. To investigate the mechanical characteristics of nanoscale helical coils
undergoing large deformations, nonlinear finite element analysis (both elastic and plastic) modeling was performed. Nanocoils behave linearly with single stiffness values only when their deformation, either extension or shortening, is very small. When the deformation is large, nanocoils will exhibit stiffening behavior in tension and softening behavior in compression. The stiffening and softening behavior of the nanocoils is mainly attributed to the geometric nonlinearity, which is caused by the change in the geometric configuration of the nanocoils. Geometric nonlinearity is elastic in nature, and the deformation in the nanocoils will disappear when the applied load is removed. It differs from material nonlinearity, with which plastic permanent deformation will develop in the nanocoils.
A theoretical investigation of the electromagnetic responses of a slanted chiral sculptured thin film (STF) to both plane waves and finite sources (such as optical beams and dipolar sources) was carried out. First, a rigorous coupled--wave analysis was implemented with algorithmic stability to obtain the planewave response; second, the angular--spectrum decomposition of the incident field was exploited to represent the reflected and the transmitted fields. The most prominent feature of the planewave responses is the circular Bragg phenomenon which is partially specular and angularly asymmetric for slanted, but nor for unslanted, chiral STFs. Correspondingly, the far--field radiation pattern of a Beltrami source configuration is spatially asymmetric in the presence of a slanted chiral STF. Also, optical beams are laterally shifted on reflection by chiral STFs. Two types of lateral shifts of Gaussian beams were studied: one is related to the Bragg reflection of co-handed beams, and the other is the Goos-Hanchen shift on total reflection.
Materials surfaces mimic cell like architecture and proteins can be encapsulated by these material surfaces (e.g. a porous glass or gold). Depending on the number and types of surface interactions, this confine environment could destroy the protein or help it maintain its bioactivity. We developed computer models and simulation tools for the understanding of surface-protein interaction at the atomistic levels. At the molecular level, molecular dynamics simulations are very powerful, but the high computational cost of molecular simulations is a drawback. A viable alternative method to study protein-surface interactions is the coarse-grained molecular simulations of simplified models, such as elastic network model. At the atomic interaction level, we used ab initio simulations to calculate the potential between surface and protein atoms.
Electron motion in an (n,1) carbon nanotube is shown to correspond to a de Broglie wave propagating along a helical line on the nanotube wall. This helical motion leads to periodicity of the electron potential energy in the presence of an electric field normal to the nanotube axis. The period of this potential is proportional to the nanotube radius and is greater than the interatomic distance in the nanotube. As a result, the behavior of an electron in an (n,1) nanotube subject to a transverse electric field is similar to that in a semiconductor superlattice. In particular, Bragg scattering of electrons from the long-range periodic potential results in the opening of gaps in the energy spectrum of the nanotube. Modification of the bandstructure is shown to be significant for experimentally
attainable electric fields, which raises the possibility of applying this effect to novel nanoelectronic devices.
The paper reviews current theoretical methods to study quasi-electrostatic phenomena in single-wall nanotube systems. Several models are presented to demonstrate importance of selfconsistent calculation of the electric fields for electronic device applications. The quantum mechanical formalism of the dielectric function is chosen to obtain the selfconsistent solution. It gives a unified approach to calculate exciton binding energy, to obtain transverse and longitudinal polarization in the nanotube, to study symmetry breaking and band gap engineering in electric fields, and to perform modelling of ballistic transport in a light-operated switches.
Profiles of the electric field strength |E|2/|E0|2 for spherical metallic shells on a dielectric core are presented both inside the particle and outside. The dependence of the near-field strength and extent on shell thickness and total particle size is discussed qualitatively. Although the internal fields inside the shell and in the core are larger than for homogeneous particles, for not too thick shells, this does not translate into a stronger near-field away from the surface of the shell. The fields inside the shell, at the low energy resonance and close to it, are rotated by π/2 with respect to fields inside homogeneous particles, which means that the maximum field strengths in the shell are perpendicular to the incident polarisation. This follows from the fact that the low energy resonance for a shell is for the largest dipole moment of the whole system, which compensates the incident field. The largest moment is created when the same charges are collected at both interfaces (shell/medium and core/shell) along the incident polarisation. This creates regions of low field densities at the poles along the incident polarisation, because same charge fields repel each other. Following from that, the field lines are bunched up at the perpendicular poles, creating large field line densities and hence large fields at these points. The case for opposite charges across the interfaces creates the high energy, antisymmetric resonance.
Some optical properties of the multilayer system: a helical periodical medium layer (1) (HPML(1))-an isotropic dielectric layer (IDL)-HPML(2) are discussed. HPML is in external magnetic field and due to this it becomes nonreciprocal. The electromagnetic wave energy distribution peculiarities inside the system are investigated as well. It is shown that wave energy accumulation takes place in certain spectral regions. Multilayer optical systems based on realistic materials with periodical structure are also discussed under the scope of applications for liquid or gas-heaters and for using as a solar energy and fiber optics converters.
We have carried out semiempirical molecular orbital calculations of C Z+/60 cations and their elongated isomers-C20+40 barrelenes. The highest possible metastable charge state of a fullerene ion was found to be C 14+/60. For this charge state the lowest energy isomer is not a spherical buckyball, but an elongated barrelene. For Z≥8, the fission channels C Z+/20+40 - C (Z-2)+/20+20 + C 2+/20 are open, the energy difference between fullerene and barrelene isomers being less than 10 eV. We have calculated the energy releases in this type of fission reactions for positive charge states of barrelenes Z=8,10,12,14 and dissociation energy thresholds for Z=4,6. The reaction coordinate calculations of C 6+/20+40 - C 4+/20+20 + C 2+/20 fission and estimate of fission Coulomb barrier have been done.
We study an electron-hole system in double quantum wells theoretically using the powerful Green's function technique. We demonstrate that there is a temperature interval over which an abrupt jump in the value of the ionization degree occurs with an increase of the carrier density or temperature. The opposite effect, the collapse of the ionized electron-hole plasma into an insulating exciton system, should occur at lower densities. In addition, we predict that under certain conditions there will be a sharp decrease of the ionization degree with increasing temperature-the anomalous Mott transition. We discuss how these effects could be observed experimentally.
Certain aspects of spin-dependent kinetic theory of conductivity for Fe-containing graphite composites have been considered, in particular those concerning charge transport along carbon nanotubes. Band structure simulations have been performed taking into account spin polarization and assumption of ferromagnetic ordering of Fe atoms in graphite composite. It has been demonstrated that infinite log-like barriers which corresponded to Coulomb screening potential in 1D wires are partially penetrable for stochastically driven particle in diffusion approximation.
In present investigation the function of average value of drift velocity versus electric field strength in GaAs quantum wires with various dimensions at temperature T=77 K at electric quantum limit is studied. In the framework of the eveloped model the nonparabolicity is taken into account. The scattering rates in the considered structures are calculated with account both noncollisional and collisional broadening of energy levels.
Under diminishing size of semiconductor nanoparticles (quantum dots) they change properties dramatically due to familiar quantum size effects. The species of the lowest possible size corresponding semiconductor crystal lattices (clusters) possess the maximum quantum confined features. They are of interest to follow the transition between molecular properties of few-atomic precursors and nuclei of a solid phase. We consider quantum chemical modeling of cadmium chalcogenide clusters containing tens of atoms. They may be the fragments of bulk lattices and nave arbitrary geometries. Ab initio geometry optimization (at the restricted Hartree-Fock level) together with electronic structure analysis allows to find possible structures of the CdnXm-clusters (X=S,Se,Te) terminating with hydrogen atoms. The clusters considered (up to n=17, m=32) reveal the dramatic variation of properties with size, interband gap is much higher than for the bulk counterparts for all chalcogenides, and simulate well some experimental findings on spectral properties of (CdX)n produced in colloids with organic ligand capping. The results obtained are important as a theoretical base of small semiconductor clusters those do not described within the effective mass approximation and as pathways to discover a growth sequence of these species towards bigger nanoparticles.