V. V. Khromov

Granular metal films on the surfaces of transparent dielectric materials studied and modified via optical means

Granular films of alkali and coinage metals are the most popular objects for exploring plasmonic effects. They are easy to obtain via physical vapor deposition and to study via optical means. In this contribution we show several ways not only to record but also to modify the granular metal films using thermal and nonthermal optical effects.

Correlation between the morphological characteristics and extinction spectra of sodium nanoparticles and modification of nanoparticles through photoatomic emission

The morphological characteristics of ensembles of metal nanoparticles on transparent dielectric substrates have been studied by the methods of hole-burning and linear absorption spectroscopy. The size and shape of particles were changed under the exposure to the optical radiation that induced photodetachment of atoms and their photostimulated diffusion over the surface of nanoparticles. The correlation between holes burned in different absorption bands is explained by the morphological features of the island film. The relaxation times of individual modes of collective plasma oscillations and the shape parameters of particles modeled by three-axial ellipsoids have been determined.

Conductivity and photoconductivity of granular silver films on a sapphire substrate

The photoelectric properties of a high-resistance silver film on sapphire, consisting of granules 15–20 nm across with the same intervals between them, have been investigated. The ohmic conductivity of the film increased with temperature. Photoconductivity is detected in the film when optical radiation with wavelengths up to the red limit of the photoelectric effect acts on it. A sign change of the photocurrent is detected in the photoconductivity spectrum when the current through the film increases under the action of radiation with wavelength less than 460 nm, whereas it decreases when the wavelength is greater than 460 nm. A conductivity and photoconductivity model is proposed that is based on doping of the dielectric substrate due to the metallic nanoparticles placed on it and the motion of electrons over traps in the substrate. The position of the bottom of the conduction band of the dielectric relative to the Fermi level for silver is calculated in terms of the model.

Optical manifestations of self-diffusion of atoms over the surfaces of silver nanoparticles

The kinetics of the extinction spectrum of a thin island silver film heated in the temperature range 473–553 K is studied experimentally. The observed variations in the inhomogeneously broadened extinction spectrum are related to two types of changes in the morphology of islands, namely, (i) coalescence and fragmentation of the islands at the initial stage and (ii) subsequent rounding of individual islands. This conclusion is supported by the data of electron microscopy of the film at different stages of its heating. The long duration of the relaxation process is explained by specific features of mass transfer over the surfaces of granulated nanoparticles.

Fluctuations of local optical anisotropy of island films

A method for the determination of the island film structure parameters by measuring fluctuations of optical characteristics of small areas of the film is proposed and implemented. The extinction fluctuations of the laser beam are measured experimentally in a focal spot, in which the polarization of radiation is modulated in time and which is movable over the film. From the optical data thus obtained, the optical absorption anisotropy of the nanometer-size islands of the film is evaluated. It is shown that this characteristic has a considerable frequency dispersion in the region of plasmon resonances of the nanoparticles.

How illumination affects electron transport between nanoparticles in a sodium island film. Photoelectron emission from nanoparticles

This paper presents studies of the action of light on the conductivity of a metallic island film consisting of an ensemble of nanoparticles of metallic sodium, deposited on an insulating substrate and interacting with each other because of the tunneling of electrons between them. It is found that radiation has the greatest effect on the conductivity of the film when electrons are emitted from it (the photoelectric effect from the surface of metallic nanoparticles). It is established that the efficiency and spectral properties of the photoconductivity and photoelectron emission are substantially connected with the optical characteristics of the film.

Adsorption of cesium atoms at structural defects on sapphire surfaces

Results are presented from an experimental study of the adsorption of cesium atoms on sapphire surfaces and their photostimulated desorption. The adsorbed atoms are found to form chains on the surface which are localized near one-dimensional structural defects of the surface. One-dimensional adsorption is analyzed theoretically with different assumptions regarding the mobility of adsorbed atoms along a chain. A comparison of the theories with experimental data favors localized adsorption described in terms of a one-dimensional lattice gas model. The energy of adsorption for an isolated atom on a linear surface structural defect is 0.58 eV, while the energy of attraction between neighboring atoms in the chain is 26 meV.

Nonradiative transition to the ground state and superelastic scattering of exited atoms upon impact on the surface of wide-bandgap dielectrics

The impact of excited cesium atoms on sapphire and glass surfaces have been experimentally studied. It is established that the probability of electron excitation quenching upon impact of an atom on the dielectric surface is close to unity. The velocity distribution of unexcited atoms upon scattering from the surface has been determined using the time-of-flight technique. The kinetic energies of most of these atoms are several tens of times greater than the energy of thermal motion of the excited atoms impinging on the surface. Conversion of the internal energy of atoms into their kinetic energy is explained in terms of nonradiative electron transitions with simultaneous excitation of lattice vibrations in the dielectric crystal. This mechanism of atomic excitation quenching near the dielectric surface explains the significant difference between the energies of atoms upon superelastic scattering and upon photodesorption from an adsorbed state.

Optical methods of forming metallic nanostructures on the surface of insulating materials

This paper discusses nonthermal photoinduced phenomena on the surface of solids that make it possible to control the processes of depositing a new phase. It is proposed to use the phenomenon of photodesorption of metal atoms from the surface of transparent insulating materials to shift the threshold for condensing a metal on the surface of an insulator. It is shown that nonuniform surface illumination can be used to form metallic nanostructures whose shape is determined by the irradiance distribution on the substrate surface.

Shaping of surface nanostructures via non-thermal light-induced processes

Non-thermal light-induced surface processes hold out hope for the development of new approaches to surface nanostructuring. Although they do not seem to be as flexible and universal as photolithography a number of niche application are waiting for a specific cheap and easy process. The light induced atomic desorption is a reliable tool to control the surface number density of the adsorbed atoms in the course of the physical vapor deposition process. The strong enough illumination diminishes the number density of the adsorbed atoms below the threshold value needed for the beginning of the nucleation process. Hence, the deposition pattern reproduces the distribution of the illumination intensity over the surface. In some cases plasmonic nanostructures are obtained via self organization of metal deposits on dielectric substrates. Their performance is severely hindered by the broad size and shape distributions. The non-thermal light-induced detachment of surface atoms from the metal nanoparticles may be used to tune their size and shape. In particular, we find that a narrow dip in the shape distribution of the metal nanoparticles may be obtained by the laser treatment at the specific wavelengths.