STRUCTURE AND PROPERTIES OF SUPER THIN
LIQUID CRYSTAL FILMS GROWN BY LENGMUIR METHOD
V. B. Zaitseva,c, A. V. Zaitsevab, N. L. Levshina,
P. A. Forsha,c, and S. G. Yudind, S. V. Khlybova
a Department of Physics, Moscow State University, Russia
b OOO NET, Moscow Region, Skolkovo, Russia
c Russian Research Centre Kurchatov Institute, Moscow
dShubnikov Institute of Crystallography, RAS, Moscow
email:
Abstract
Ultrathin films were prepared by Langmuir technology from smecticliquid crystal materialpara-tetradecyloxybenzyliden-amino-2-methylbutyl-cyanocinnamate. A thorough investigation of the films properties was performed. The results were compared to the experimental data obtained on thicker films of the same material produced by a spin coating method. The films structure was studied using atomic force microscopy The adsorption isotherms of water moleculesrevealed a structural phase transition near 75°C in ultrathin films. To clarify the nature of this transition, optical and electrophysical properties of the films were investigated. Temperature dependences of diffuse reflection spectra and the polarization of light reflected from the films were studied. Specific features of the reflection spectra at the phase transition temperature were found. Temperature dependences of the electric capacity and conductance of metal–film–metal structures were measured. The maximum capacity for ultrathin films lies near 75°C, indicating the ferroelectric–paraelectricnature of the phase transition.An existence of mesomorphous phase is proved in ultrathin films prepared by Langmuir technique. Similarity of the results obtained forultrathinand thicker (spin coated) samples was noticed. The phase transition is shifting towards lower temperatures with the increase of the film thickness.The observed differences could be associated with either dimensional effects or differences in the structures of the films.The ultrathin films structure imperfection explains the peculiarities of the temperature dependences of conductivity and capacity, in particular, the temperature range in which the phase transition is observed.
INTRODUCTION
Liquid crystals are very promising materials for optoelectronicsapplications. The modern optoelectronicsfrequently has a need in the thin and super thin film materialspossessing active optical properties.Methods for producing Langmuir films from materials that are capable of existing in a mesophase liquidcrystal phase have been developed in the last severalyears. The problem concerning variations in the properties of these substances in the process of obtaininghyperfine layers based on the Langmuir techniqueas well as the question of whether theultrathin Langmuir films are liquid crystals remain open. If the answer is yes, then we must examine thesequence of phase changes in the samples, along withthe temperature ranges in which these phases exist.Differences in the techniques for manufacturing films(Langmuir and spin coating) will inevitably lead to differences in their structure. Increases in film thickness willbe accompanied by reductions in the influence of thesubstrate. In addition, there could exist size effects.Reducing the mobility of molecules in ultrathin filmscan influence the presence or absence of certainphases, and the temperatures of phase transitions. As arule, a wide variety of phase transitions is observed inliquid crystal over a narrow temperature range.Reducing the mobility of the molecules should therefore lead to greater differences between the temperatures of the same phase transition observed during theheating and cooling of a sample. Under these circumstances, some of the mesomorphic phases in the areaof such thermal hysteresis may not occur.
Thus, such films need a thorough analysis. Several papers devoted to the investigation ofLangmuir–Blodgett (LB) films fabricated based onthe materials that form smectic liquid crystals havebeen published. It was pointed out that some properties of LC films vary as the film thickness decreases. Because of this, a change in the properties of these substances should be expected when passing from the bulk LC layers to LB films. This change may be related primarily to the decrease in the LC molecule mobility in an LB film. Moreover, it was not possible to say with certainty that an ultrathin film has a mesomorphic phase.The limited body of the experimental data does not allow making any conclusion onhow the properties of a liquid crystal mesophasechange in the process of fabricating hyperfine films ofthese materials deposited on a substrate. The influence of the structure and thickness offilms on their electrophysical and optical propertiesremains an open question. Solving this problem wasthe goal of this study.
As an object of study, we chose paratetradecyloxybenzylideneamino-2-methylbutylcyanocinnamate (TDOBAMBCC) LC, the bulk properties of which were studied quite explicitly. As a rule, in hyperfine films that have structuraldefects, the change in their properties during a phasetransition is of the “blurred” character. To detect thistransition and understand the nature and mechanismof this process, it is necessary to comprehensivelystudy changes in different properties of the material ina wide temperature range inclusive of the transitionpoint. To study phase transitions in hyperfine films, aset of independent techniques was applied. The structure of films fabricated based on a liquidcrystal material by the Langmuir technology was studied using atomic forcemicroscopy, and the correlation of structural and electrical properties of these films was determined. To detect the phase transitions (PTs) in LB films, we used a method that consists of measuring the isotherms of molecule adsorption from the gas phase at different temperatures. Themethod of adsorption isotherms wasshown to be highly suitable for determining structural transformations of finefilms.Passage through the structural PT temperature was accompanied by a sharp increase in the number of molecules adsorbed on the film surface.The study ofthe films optical properties at different temperatureswithin the range including the phase transition temperature can give rather valuable information.For this reason, we studied the temperature dependences of the light absorption and diffuse scattering spectra of LB films as well as the polarization of reflected light. The results were compared with the data obtained from thicker TDOBAMBCC films prepared by spin coating. To clarify the nature of the detected phase transition, the temperature dependence of the electrical capacitance of the metal–film–metal structure was studied. To study the conductivity mechanism in more detail, the current–voltage characteristics were investigated for direct and alternating currents at different frequencies in the temperature range including the phase transition temperature.