N. B. Leonov

Fine structures and switching of electrical conductivity in labyrinth silver films on sapphire

Changes in electrical resistance of silver films were measured in the range from 1013 to 103 Ω during thermal deposition on sapphire in a high vacuum, after the deposition over time, and under an applied voltage. The dependences of the electrical resistance of the films on their thickness and deposition rate were determined. It was established that, with an increase in the film thickness from 2 to 10 nm during the deposition at rates of 0.6 and 0.1 Å/s, the resistance decreases by 7.5 and 4 orders of magnitude, respectively. The measured dependences of the resistance on the deposition time were found to be close to exponential. The room-temperature resistance of 10-nm-thick films deposited at different rates changed spontaneously by 3–4 orders of magnitude in different ways: the resistance of the slowly deposited films spontaneously increased, whereas in the rapidly deposited films, it spontaneously decreased. After fine annealing, the steady-state resistance changed also differently: it increased by 2 orders of magnitude in the former case and by 9 orders of magnitude in the latter case. Under voltages above 5 V, the resistance of the rapidly deposited films abruptly decreased from ∼1012 to ∼106 Ω, and these films became ohmic. After fine annealing, they became again high-ohmic. Under voltages above 5 V, the high-ohmic films thus obtained became again low-ohmic. This cycle of electrical conductivity switching was reproduced many times. The observed phenomena were explained in the framework of the hypothesis of the formation of fine metastable structures in channels of labyrinth films, namely, protrusions and bridges that bring together the boundaries of islands and connect them into conducting clusters, respectively.

Mutual modification of silver-nanoparticle plasmon resonances and the absorptive properties of polymethine-dye molecular layers on a sapphire surface

Extinction spectra of a hybrid material comprised of molecular layers of polymethine dyes and granulated silver films preliminarily deposited onto sapphire substrates are studied experimentally. Specific features of the dye layer composition and broadening of plasmon absorption bands are discussed. In hybrid samples, both an increase and a decrease in the silver-nanoparticle plasmon resonance frequency upon spreading of dyes have been observed, as well as a considerable increase in the dye molecule absorption. Possible mechanisms of the phenomena are discussed.

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.

Evolution of the optical properties and morphology of thin metal films during growth and annealing

The formation kinetics of an island silver film on a dielectric substrate and changes in the film morphology during heating have been investigated using optical methods. The results obtained are interpreted within a model relating the film optical properties with the shape and size of the nanoparticles forming this film. The proposed interpretation is confirmed by electron microscopy data.

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.

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.

Changes in morphology and optical properties of silver island films on transparent dielectric substrates under exposure to laser radiation

The influence of a single nanosecond laser pulse on the morphology of an island silver film was studied. It is shown that, during the pulse time span, the islands can change their shape from an arbitrary one to nearly spheroidal. The islands change their shape due to diffusion of silver atoms on the islands’ surface (self-diffusion), as well as on the substrate surface.

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.

Application of nondiffracting Bessel beams for shaping of surface metal microstructures

Abstract A novel method of laser-controlled shaping of metal microstructures based on the processes of metal atoms adsorption on the surface of crystalline substrate and simultaneous control of photostimulated desorption of atoms by spatially modulated nondiffracting laser beam illumination is presented. The experiments were performed for sodium atoms deposition to the sapphire substrate, which was illuminated by Bessel beam at 532 nm wavelength and 2 W/cm2 intensity. Experiments showed that the optical pattern was well reproduced in the sodium deposits thus creating the annularly microstructured metal film with few tens nanometre thickness.