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44294-G7
Terahertz Spectroscopy of Photo-Conducting Liquid Crystals
The goal of this PRF G project is to investigate the photoconductive properties of liquid crystalline (LC) materials using the technique of terahertz time-domain spectroscopy. LCs are a relatively new class of photoconductors. Carrier mobilities in LCs are often limited by the impurities, defects, interfaces and domain boundaries while studied by conventional transport techniques. The THz time-domain spectroscopy that measures the conductivity at THz (1012 Hz) frequencies could be an ideal approach to study the intrinsic transport properties of organic materials, high quality large crystalline samples of which are often difficult to grow.
In this first year of the project we have successfully tested the feasibility of the THz technique in probing charge transport in organic photoconductors. As a model system, we investigated phthalocyanine (PC) derivatives that belong to a family of discotic LCs. These disk-like molecules self-assemble into columns with significant overlap of the delocalized π-electrons of the neighboring molecules, thereby creating quasi-one-dimensional channels for efficient charge transport. High carrier mobilities (> 100 cm2/Vs) in have been previously reported in PC crystals [Hellmeier and Harrison, Phys. Rev. 132, 2010 (1963)].
We studied Zn-PC thin films (~ 10 micron thickness) on quartz substrates in an ordered polycrystalline phase with micron-sized domains of various column orientations at room temperature. They were prepared by slow cooling from a liquid crystalline phase. THz conductivity was observed in PC films immediately after photoexcitation by a femtosecond optical pulse at either the fundamental (800 nm) or the second harmonic (400 nm) of our laser system. The conductivity was found to persist for 100's picoseconds. The decay dynamics is well described by a double exponential dependence with a fast and slow time constants of a few ps and 10's ps, respectively. In addition, the photoconductivity was found to be linearly dependent on the pump fluence. The decay time constants, however, are independent of the pump fluence.
A closer examination of the frequency dependence of the complex conductivity reveals that in the range of 0.2 – 1 THz the real part of the conductivity is positive and the imaginary part is negative. The absolute values of the real and imaginary part of the photoconductivity are comparable and they increase with frequency. This behavior is drastically different from the Drude conductivity, phenomenon often observed in band transport in crystalline inorganic and organic materials [for example, Shan et al., Phys. Rev. Lett. 90, 247401 (2003)]. The observed conductivity resembles some of the features of ac hopping conductivity that was observed in photoconductive polymers [Hendry et al., Phys. Rev. Lett. 92, 196601 (2004)]. Comparison of the experimental result with detailed charge transport models and extraction of the carrier mobility are underway.
A phase transition from the polycrystalline to the liquid crystalline phase for Zn-PC was observed near 370 K and the photoconductivity was observed in both phases. Our preliminary investigation showed a pronounced change in the shape of the spectral dependence. A more systematic and quantitative study of the two different phases is planned for the second year of the project.
