Alexander Tikhonov

2-D Array Photonic Crystal Sensing Motif

We have developed the first high-diffraction-efficiency two-dimensional (2-D) photonic crystals for molecular recognition and chemical sensing applications. We prepared close-packed 2-D polystyrene particle arrays by self-assembly of spreading particle monolayers on mercury surfaces. The 2-D particle arrays amazingly diffract 80% of the incident light. When a 2-D array was transferred onto a hydrogel thin film showing a hydrogel volume change in response to a specific analyte, the array spacing was altered, shifting the 2-D array diffraction wavelength. These 2-D array photonic crystals exhibit ultrahigh diffraction efficiencies that enable them to be used for visual determination of analyte concentrations.

Calculating electron current in a tight-binding model of a field-driven molecular wire: Application to xylyl-dithiol

A recently developed Floquet theory-based formalism for computing electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic ac radiation field is applied to an experimentally relevant system, namely a xylyl–dithiol molecule in contact at either end with gold electrodes. In this treatment, a nondissipative tight-binding model is assumed to describe the conduction of electric current. Net current through the wire is calculated for two configurations of the electrode–wire–electrode system. In one, symmetric, configuration, the electrodes are close (∼2 A) and equidistant from the bridge molecule. In the other, asymmetric configuration, one electrode is farther away (∼5 A), representing the tip of a scanning tunneling microscope located at this distance from the bridge molecule (the other end being chemisorbed to a gold substrate). For both configurations, electron current is calculated for a range of experimental inputs, including dc bias and the intensit...

Calculating electron transport in a tight binding model of a field-driven molecular wire: Floquet theory approach

This paper considers electron transport through a molecular bridge coupled to two metal electrodes in the presence of a monochromatic radiation field. Current flow through the wire is calculated within a nondissipative one-electron tight binding model of the quantum dynamics. Using Floquet theory, the field-driven molecular wire is mapped to an effective time-independent quantum system characterized by a tight-binding Hamiltonian with the same essential structure as the nondriven analog. Thus, Green’s Function methods for computing current flow through the wire, which have been profitably applied to the molecular wire problem in the absence of driving, can also be used to analyze the corresponding field-driven system. Illustrative numerical calculations on a simple model system are presented.

Reflectivity enhanced two-dimensional dielectric particle array monolayer diffraction

Very high diffraction efficiencies (>80%) were observed from two-dimensional (2-D) photonic crystals made of monolayers of ∼490 nm diameter dielectric polystyrene spheres arranged in a 2-D hexagonal lattice on top of a liquid mercury surface. These almost close packed 2-D polystyrene particle arrays were prepared by a self-assembly spreading method that utilizes solvent evaporation from the mercury surface. Two-dimensional arrays transferred onto a dielectric glass substrate placed on top of metal mirrors show diffraction efficiencies of over 30%, which is 6- to 8-fold larger than those of the same 2-D monolayers in the absence of mirrors. A simple single particle scattering model with refraction explains the high diffraction efficiencies in terms of reflection of the high intensity forward diffraction.

Monodisperse, high refractive index, highly charged ZnS colloids self assemble into crystalline colloidal arrays

We developed an efficient synthesis of monodisperse, highly surface charged, high refractive index ZnS spherical particles by using a gel-sol method. Concentrated solutions of zinc-ammonia-NTA (nitrilotriacetic acid) were reacted with thioacetamide in the presence of gelatin which stabilized the growing particles. We dramatically increased the particle surface charge density by condensing silica and silylated phosphonate groups on the particle surface. These monodisperse highly charged ZnS particles are somewhat porous and have a refractive index of 1.868. These are the highest refractive index, monodisperse, highly charged spherical particles to self assemble into non-close-packed crystalline colloidal arrays which Bragg diffract light.

Charge stabilized crystalline colloidal arrays as templates for fabrication of non-close-packed inverted photonic crystals

We developed a straightforward method to form non-close-packed highly ordered fcc direct and inverse opal silica photonic crystals. We utilize an electrostatically self assembled crystalline colloidal array (CCA) template formed by monodisperse, highly charged polystyrene particles. We then polymerize a hydrogel around the CCA (PCCA) and condense silica to form a highly ordered silica impregnated (siPCCA) photonic crystal. Heating at 450 degrees C removes the organic polymer leaving a silica inverse opal structure. By altering the colloidal particle concentration we independently control the particle spacing and the wall thickness of the inverse opal photonic crystals. This allows us to control the optical dielectric constant modulation in order to optimize the diffraction; the dielectric constant modulation is controlled independently of the photonic crystal periodicity. These fcc photonic crystals are better ordered than typical close-packed photonic crystals because their self assembly utilizes soft electrostatic repulsive potentials. We show that colloidal particle size and charge polydispersity has modest impact on ordering, in contrast to that for close-packed crystals.

Colloidal Crystal Growth Monitored By Bragg Diffraction Interference Fringes

We monitored the crystal growth kinetics of crystallization of a shear melted crystalline colloidal array (CCA). The fcc CCA heterogeneously nucleates at the flow cell wall surface. We examined the evolution of the (1 1 1) Bragg diffraction peak, and, for the first time, quantitatively monitored growth by measuring the temporal evolution of the Bragg diffraction interference fringes. Modeling of the evolution of the fringe patterns exposes the time dependence of the increasing crystal thickness. The initial diffusion-driven linear growth is followed by ripening-driven growth. Between 80 and 90 microM NaCl concentrations the fcc crystals first linearly grow at rates between 1.9 and 4.2 microm/s until they contact homogeneously nucleated crystals in the bulk. At lower salt concentrations interference fringes are not visible because the strong electrostatic interactions between particles result in high activation barriers, preventing defect annealing and leading to a lower crystal quality. The fcc crystals melt to a liquid phase at >90 microM NaCl concentrations. Increasing NaCl concentrations slow the fcc CCA growth rate consistent with the expectation of the classical Wilson-Frenkel growth theory. The final thickness of wall-nucleated CCA, that is determined by the competition between growth of heterogeneously and homogenously nucleated CCA, increases with higher NaCl concentrations.

Silica Crystalline Colloidal Array Deep Ultraviolet Narrow-Band Diffraction Devices

We developed a facile method to fabricate deep ultraviolet (UV) photonic crystal crystalline colloidal array (CCA) Bragg diffraction devices. The CCAs were prepared through the self-assembly of small, monodisperse, highly surface charged silica particles (~50 nm diameter) that were synthesized by using a modified Stöber process. The particle surfaces were charged by functionalizing them with the strong acid, non-UV absorbing silane coupling agent 3-(trihydroxylsilyl)-1-propane-sulfonic acid (THOPS). These highly charged, monodisperse silica particles self assemble into a face-centered cubic CCA that efficiently Bragg diffracts light in the deep UV. The diffracted wavelength was varied between 237 nm to 227 nm by tilting the CCA orientation relative to the incident beam between glancing angles from 90° to ~66°. Theoretical calculations predict that the silica CCA diffraction will have a full width at half-maximum (FWHM) of 2 nm with a transmission of ~10−11 at the band center. We demonstrate the utility of this silica CCA filter to reject the Rayleigh scattering in 229 nm deep UV Raman measurements of highly scattering Teflon.