PHOTOCONDUCTIVITY AND DIELECTRIC PROPERTIES OF (C60+C70) FERROELECTRIC LIQUID CRYSTAL COMPOSITE

Физика/8. Молекулярная физика

Shevchuk O.F., Gel P.V.

Vinnitsa State Agricultural University, Ukraine

PHOTOCONDUCTIVITY AND DIELECTRIC PROPERTIES OF (C60+C70) FERROELECTRIC LIQUID CRYSTAL COMPOSITE

1. The purpose of work. In the present work we report on the formation of fullerene-LC systems stable in time. Frequency dependence of complex dielectric permittivity and photoelectric properties was studied to analyze how the properties of each component of the composite can influence dielectric parameters of the whole system.

2. Materials and methods. We used in our studies a mixed fullerenes (F) from MER Corporation with following composition: 76% C60, 22% C70, and 2% higher-order fullerenes. A ferroelectric liquid crystal (FLC) was used, which is an eutectic mixture of 4-n-hexyloxyphenyl 4'-n-decyloxybenzoate and 4-n-hexyloxyphenyl 4'-n-octyloxybenzoate with LUCH-15 polar chiral additive. Pure FLC has the following changes of phases and phase temperatures (K):

Fullerene content of 43 wt.% was selected for all our experiments. The studies were carried out with the use of sandwich cells. A thickness of composite layer d in the range of 30-50 mm was obtained by introducing Teflon film between the glass plates over the protecting electrode.

The measurements were performed in the temperature range of 315-370 K. Sample capacity C and resistance R were measured in the frequency range of
10^-3-10⁶ Hz by means of the oscilloscopic method. The signal measured had the triangular shape. The peak voltage value was 0.25 V.

3. Experimental results. F-FLC composite exhibits a linear dependence of e² on ¦ in a rather wide frequency interval. According to the following inter-relation

where e₀ is the dielectric permittivity in vacuum, w = 2p¦ is circular frequency.


Fig. 1. Frequency dependence of e² for homeotropically oriented FLC at Т = 357 K (1), planar oriented FLC at Т = 354 K (2), composite F-FLC at Т = 355 K (3). The straight lines cover the areas where the conductance does not depend on frequency	Fig. 2. Temperature dependences of s_АС for homeotropically oriented FLC (1) and composite F-FLC (2). The arrows mark the temperatures of FLC phase changes.

The composite conductance equals 3.2 ^. 10^-8 W ^-1 .m^-1at Т = 344 K, and activation energy for this temperature dependence is DE_s = 0.99 eV. The latter value turned to be higher than DE_s for FLC. Such a large difference in DE_s values for the composite components results in a change in the slope of temperature dependence of s_АС (at temperatures close to 329 K).

The dispersion area is observed on a general ‘background’ where e² values change due to the higher conductance. Therefore we analyzed the frequency dependences of e¢ and e² accounting for substantial influence of the ionic conductivity. The dispersion equation (Debye equation) in this case is as follows:

where e_¥ and e_s are the values of dielectric permittivity for the frequencies
¦ = µ and ¦ = 0, respectively.

Fig. 3. Frequency dependence of e² for homeotropically oriented FLC at
Т = 316 K (1), and composite F - FLC at Т = 314 K (2).

The values are close to each other and almost equal to the relaxation time for planar-oriented molecules. Since the relaxation time for Goldstone mode is determined by the inter-relation

where h is the rotary viscosity, К₃₃ is Frank’s elastic modulus, р₀ is the length for equilibrium step of a spiral.


Fig. 4. Temperature dependences of t for homeotropically oriented FLC (1) and composite F-FLC (2).	Fig. 5. Spectral dependence of a photovoltage for composite F-FLC at Т = 316 K.

4. Conclusions

1. We obtained stable in time F-FLC composite with F:FLC ratio of 43:57 (by weight). Comparison of the frequency dependence of e² for the composite and for FLC with different orientation showed that the orientation of FLC molecules in the composite is close to homeotropic.

2. Analysis of the temperature dependence of s_АС showed that at Т > 329 K the composite conductance is determined by the conductance of F. At Т = 344 K the composite conductance is 3.2^.10^-8 W ^{- 1.}m^-1, and the activation energy is DE_s = 0.99 eV. Spectral dependences of composite photoconductivity were obtained and compared to those for C₆₀.

3. Dielectric properties of F-FLC composite at Т < 329 K are determined by the properties of FLC, as confirmed by the existence of dispersion area due to rotation of dipole moments around the helicoid (Goldstone mode) axis. Almost equal values for the relaxation times of such dispersion for homeotropically oriented FLC and for F-FLC composite lead us to the conclusion that the presence of fullerene does not substantially change the FLC parameters. In turn this means that the formation of a highly ordered state of FLC molecular dipoles (spontaneous polarization) can take place in the composite as well. In this case, the composite photoconductivity increases by two orders of magnitude.

4. Our results might be important for creating new materials having a wide range of functionalities, such as photoconductive and ferroelectric properties simultaneously.