A. Shik

Polarization-sensitive optical phenomena in thick semiconducting nanowires

For semiconducting nanowires with the dielectric constant e differing from that of their environment, the distribution of ac electric fields, created in a nanowire by an external lightwave and emitted outside by an effective dipole in a nanowire, is found. The results are used for calculating the spectra and polarization properties of optical absorption, luminescence, and photoconductivity in such nanowires. For relatively thick nanowires, with diameter a comparable to the light wavelength, all polarization characteristics exhibit strong oscillations with the light frequency ω and even change the sign over some intervals of ωa∕c. Some of these phenomena have already been observed experimentally. In a system of randomly oriented nanowires, they result in a polarization memory where polarization of photoluminescence is determined by the polarization of the exciting light, by a factor strongly dependent on the frequencies of both the exciting and emitted light.

Universal description of channel conductivity for nanotube and nanowire transistors

A theory of drift-diffusion transport in a low-dimensional field-effect transistor is developed. Two cases of a semiconductor nanowire and a single-wall nanotube are considered using self-consistent electrostatics to obtain a general expression for the transconductance. This quantum-wire channel device description is shown to differ from a classical device theory because of the specific nanowire charge density distribution.

Giant anisotropy of second harmonic generation for a single ZnSe nanowire

The effect of second harmonic generation was experimentally investigated in ZnSe nanowires grown by the vapor-liquid-solid method. The effect dramatically depended on the angle between the nanowire axis and the linear polarization of the excitation light. The magnitude of the effect was 20 times stronger for the parallel polarization in comparison with the perpendicular one. The results were theoretically explained in terms of a large difference in dielectric constants between the nanowire and the environment, resulting in a strong orientation-dependent optical electric field in the nanowire.

Luminescent properties and electronic structure of conjugated polymer-dielectric nanocrystal composites

We report results of experimental investigations of the luminescent properties of two different conjugated polymers in which we embedded nanocrystals of Al2O3, Y2O3, ZnO, and SnSbO. The dielectric nanocrystals result in a blueshifting and broadening of luminescence spectra of poly(p-phenylene vinylene) with a simultaneous disappearance of its vibronic structure. The same nanocrystals in a poly[2-(6-cyano-6′-methylheptyloxy)-1,4-phenylene] matrix cause redshifting and spectral broadening. These observations are explained by referring to a model that accounts for the change in the polarization component of the carrier and exciton energy in the vicinity of inclusions.

Nonlinear optical phenomena in nanowires

The theory of second harmonic generation (SHG) in nanowires with different dielectric constants from their environment is developed. A nonuniform distribution of the optical electric field created by image forces results in strong SHG even in materials with inversion symmetry. Integral intensity, spatial distribution, and the polarization of the SHG depend dramatically on the light polarization, as well as on the mutual orientation of the light wave vector and nanowire axis. In semiconducting or dielectric nanowires, the SHG is maximal when the light polarization is parallel to the nanowire axis. In metallic nanowires, the SHG is a function of the light frequency ω being maximal when ω or 2ω coincide with the surface plasmon frequency. At the first resonant frequency, the SHG is caused mostly by light polarized perpendicularly and at the second frequency—parallel to the nanowire axis.

Influence of contacts on the conductivity of thin wires

The conductivity of thin wires was shown to depend noticeably on the properties and, particularly, dimensionality of contacts. Two- and one-dimensional contacts create a strongly nonuniform electric field in a wire which, in turn, causes the redistribution of electron density along a wire and nonlinear current–voltage characteristic. The particular shape of the latter has been calculated for both Ohmic and Schottky contacts of different dimensionality. The case of ac applied voltage is also considered. Spatial distribution of nonequilibrium carriers is found for different signal frequencies ω. The wire conductivity is shown to increase with ω for any type of low-dimensional contacts.

Negative capacitance in polymer-nanocrystal composites

The effect of negative capacitance (NC) was observed and investigated in conducting polymers with embedded PbS nanocrystals. NC occurred for signal frequencies up to 1MHz and was absent in control samples without nanocrystals. The amplitude of NC increased with the applied bias and was accompanied by a superlinear current–voltage characteristic. A theoretical description of NC is presented based on a voltage-induced redistribution of carriers in a nonuniform sample with nanocrystals playing the role of potential wells for carriers. The characteristic time of carrier capture by nanocrystals, determined from the frequency dispersion of NC, was ∼3×10−5s.

Electron screening in nanostructures

Distributions of electrical potential and carrier concentration, the contact capacity, and its voltage dependence are calculated for Schottky contacts to various types of nanostructures, including nanolayers and nanowires of different thickness, as well as their arrays. The results demonstrate a dramatic dependence on the nanostructure geometry. Single nanostructures and planar arrays of nanowires cannot provide effective screening of the contact potential, so that the total stored charge and the structure capacity depend on the separation between external contacts. On the contrary, for nanolayer and two-dimensional nanowire arrays, the mutual electrostatic interaction between different elements provides effective screening with the screening length equal to the interelement distance, which determines the contact capacity and its voltage dependence.

Orientation dependent nonlinear optical effects in ZnSe nanowires

Three nonlinear optical phenomena, two-photon induced luminescence, second-harmonic generation, and third-harmonic generation, were studied experimentally in ZnSe nanowires. All three effects demonstrate a strong dependence on the orientation of linearly polarized excitation, being maximal for the polarization parallel to the nanowire axis. The phenomenon is caused by the anisotropic nanowire depolarization due to the difference in dielectric constants between the nanowires and the environment and has different amplitude for nanowires embedded in different dielectrics. The amplitude depends also on nanowire bending and nonuniformity.

Conductivity and photoconductivity in undoped ZnSe nanowire array

Arrays of free-standing ZnSe nanowires with the length of 8–10μm and diameters of 80–150nm were fabricated by Au-catalyzed vapor-liquid-solid growth. Current-voltage characteristics of the arrays over the temperature interval of 90–400K showed a superlinear character. The differential conductance varied between two saturating regimes at low and high biases, respectively. This behavior was explained using a model of nonuniform wires with concentration fluctuations along them. The nanowire photoconductivity had a spectral edge corresponding to the ZnSe band gap and a strong frequency dispersion, presumably due to carrier capture by deep centers.