Tuning of Electrical Conductivity by Photoirradiation and Electric Fields
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- 作者：Toshifumi Iimori ; Nobuhiro Ohta
- 刊名：Journal of Physical Chemistry C
- 出版年：2014
- 出版时间：April 10, 2014
- 年：2014
- 卷：118
- 期：14
- 页码：7251-7260
- 全文大小：603K
- 年卷期：v.118,no.14(April 10, 2014)
- ISSN：1932-7455
文摘
Control of electrical conductivity by using external stimuli such as photoirradiation and electric fields is one of the major subjects in materials science because of prospects for the discovery of potential optoelectronic materials. Photoexcitation of crystalline materials can lead to the change of the electronic states and the excitation of vibrational motions via the nonradiative relaxation process from the electronically excited states. Thus, the change of phases and physical properties of crystalline materials is expected with photoirradiation. The application of electric fields is also known to be able to control carrier concentration and modify electronic states in materials. Moreover, the concurrent application of these external stimuli is expected to advance for unprecedented optoelectronic functions to be revealed due to synergy effects. As a target in the exploration of the control using external stimuli, the crystalline molecular charge-transfer complex which shows metallic conductivity, that is, a molecular conductor, is one of the promising systems. Molecular conductors show unconventional properties including high-temperature superconductivity, charge ordering, and metal鈥搃nsulator transition. Moreover, there is often a competition between different phases in molecular conductors, and high susceptibility to external stimuli becomes feasible. The microscopic mechanism of unconventional properties, which is still an open question, can also be elicited by the investigation of the response to photoexcitation and electric fields. In this Feature Article, we review time-resolved measurements of electrical conductivity in molecular conductors by using photoirradiation and electric fields first. We have focused on charge-transfer complexes of the electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF). 伪-(BEDT-TTF)2I3 is in the metallic phase at room temperature and undergoes the phase transition to the insulating phase at 135 K. When the crystal in the insulating phase is photoexcited with a visible nanosecond pulsed laser light in the presence of pulsed electric fields, a switching of the electrical conductivity is observed. Moreover, the conductivity switching can show an unprecedented memory effect, of which the appearance is governed by temporal width and height of pulsed electric fields. In fully deuterated 魏-(BEDT-TTF)2Cu[N(CN)2]Br, the Mott insulating phase is converted to the metallic phase by the application of electric fields without photoirradiation. The threshold voltage which is required to induce the transition is reduced by photoirradiation. The electrical conductivity shows a bistability or a hysteresis loop as a function of electric field. Time-resolved photoresponse of organic superconductors shows a marked temperature dependence at temperatures in the vicinity of the superconducting phase transition temperature. An insight into the mechanism of the photoresponse of organic superconductors is gained from the investigation of isotope substitution effect on the photoresponse. Second, a gigantic photoinduced change of complex impedance is demonstrated in a solid-state ionic conductor AgI; a reduction of the bulk resistance by more than two orders of magnitude is found on photoexcitation. In summary, with photoirradiation, potential functionality in molecular conductors and ionic conductors can be revealed in addition to the understanding of photoexcitation dynamics. These results are obtained as a part of our strategy toward the realization of photoinduced superconductivity or photoinduced superionic conductivity, which is one of the most challenging problems in materials science.