Organic Single Crystalline Micro-& Nano-Sized Devices Of Tetrathiafulvalene (TTF)

It's very important to develop micro- and nano-sized organic single crystallinedevices for investigating the intrinsic transport property of organic semiconductor. Wehave fabricated high performance organic single crystalline field-effect transistors andanalyzed molecular structure, growth direction and molecular packing of organicsingle crystals, which will help us understand the relationship between molecularstructure and transport properties and the transport anisotropy of organic singlecrystals, and obtain ideal molecular configurations with high device performance tocontrollably synthesize organic functional molecules.As the widely studied heterocyclic system, TTF and its derivatives have beenapplied into various fields because of outstanding physical and chemical properties.This paper has studied organic single crystalline field-effect transistor andphototransistor of TTF and its derivatives, and the main results are presented asfollows:(1) The crystals of TTF existed in two polymorphic modifications, and we haveadopted different solvents to grow TTF single crystals with different phases. Inour experiments, one-dimensional pureαphase TTF single crystals could becontrollably obtained by the selection of n-heptane while one-dimensional pureβphase could be provided by chlorobenzene. The size of these crystals could beaccurately controlled by the parameters of crystal growth process. TTF singlecrystalline field-effect transistors with two phases have been in situ fabricated bydrop cast. The mobilities of different phases were different. The maximumfield-effect mobility ofαphase was near 1.20 cm2V^-1s^-1 whileβphase showedthe maximum mobility only about 0.23 cm2V^-1s^-1. We modeled the chargetransport of TTF molecule as a Brownian motion process, and the results oftheoretical calculations have been highly accorded with our experimental results.(2) One of TTF derivatives, TCTT-TTF have been synthesized based on theintroduction of four groups of cyanoethylthio at the instable positions of TTF.Single crystals of TCTT-TTF with various patterns and dimensions could beobtained by solution self-assembly. The comparisons of electrochemical stability,thermal stability and vacuum stability combined with the analysis of molecularstructure and packing between TCTT-TTF and TTF have showed that the stabilityof TCTT-TTF would be greater than that of TTF. TCTT-TTF single crystalline field-effect transistors have been in situ fabricated by the drop cast and theinsulators have been modified by the combination of polyvinyl alcohol (PVA) andoctadecyltrichlorosilane (OTS). The field-effect mobility, on/off ratio andthreshold voltage of the device by the selection of chloroform would be 0.01cm²V^-1s^-1, 1.1×10³ and 26.7 V, respectively.3) Single crystal ofβphase TTF and TCNQ have been responding to the lightirradiation. The response time ofβphase TTF single crystal would exceed 60 sand on/off ratio between dark and light irradiation would be very low while theresponse time of TCNQ single crystal would be below 0.5 s and on/off ratiowould be greater than 100. The introduction of gate voltage would have no effectin the responsibility of the two kinds of crystals and just modify on/off ratio.Comparison of crystalline structure and packing betweenβphase TTF and TCNQhave showed that more perfect packing ofπ···πinteraction between molecules,including the packing of"side-by-side"and"face-to-face", the more rapid thetransfer speed of the photo-excited carriers would be, at the same time the chanceand speed of recombination of electrons and holes would be further enhanced.Relatively loose packing would result in the low speed of the transfer of thephoto-excited carriers and reduction of the chance and speed of recombination ofelectrons and holes, which would show very high persistent conductivity andcould not be improved by the adjustment of gate voltage. TCNQ single crystallinephototransistor showed good function of light guide and great potentials ofapplication.