When high-density charge carriers are introduced into a crystal, its electronic state changes from an insulator to a metal. For clarity, the Fermi level is confined in a band gap in insulators, whereas it is inside a band in metals. “Metal–insulator transition (MIT)” is a natural consequence of the band picture based on the periodic lattice of atoms in crystals with noninteracting or weakly interacting electron systems, and it can be triggered predominantly by a precise control of band filling. For the last half century, MIT has been extensively investigated in condensed matter not only for in-depth understanding of their electronic properties, but also for possible application to electronic devices (M. Imada, et al., Review of Modern Physics, 70, 4 (1998)). The transistor, invented by Shockley, Brattain, and Bardeen, is one of the most representative achievements.
Since the early 1990s, the journey to explore MIT and related electronic phase transitions has exploded into molecular solids. Presumably, this has been triggered by the discovery of superconductivity of alkali-doped fullerenes (K. Tanigaki, et al., Nature, 352, 222 (1992)). Subsequently, many researchers have attempted to observe MIT in solid crystals of p-conjugated organic semiconductors, in acenes such as pentacene and rubrene. I was exposed to extensive research in the early days of MIT studies back in the 2010s, and have been fascinated by the beauty of physics in MIT ever since. During the past two decades, unfortunately, no apparent MIT signature has been observed in organic semiconductors. MIT presupposes a periodic electrostatic potential in extremely high quality crystals; therefore, the observation of MIT in organic semiconductors is largely limited by the presence of unavoidable structural disorders in them.
We focused on the experimental observation of MIT in organic semiconductors. Most recently, we successfully demonstrated that one-shot printing of organic semiconductor ink allows for the ideal production of self-assembled molecular nanosheets with an extremely large areal coverage of up to 100 cm2 (Figure 1, S. Watanabe, J. Takeya et al., Nat. Phys. 13, 994 (2017), Sci. Adv. 4, 2 (2018)). Surprisingly, the obtained molecular monolayer formed a perfect single crystal with a molecularly flat surface, which further motivated us to explore our MIT study. We were also lucky to supervise Mr. Naotaka Kasuya, one of the most passionate Ph.D. students in our team (Figure 2), who conducted and revisited this study. His deep understanding of condensed matter physics and constant efforts over the past four years realised the first experimental observation of MIT in organic semiconductors. Here, a positive temperature coefficient of resistance with a reasonably low resistance of 10 kW was demonstrated. We realised that the key behind this discovery was the precise construction of two-dimensionally confined electronic systems at the interface between the electric double layer and monolayer organic single crystal. Unlike conventional field-effect transistors with a solid gate dielectric, the remarkably high charge carriers, corresponding to 0.25 holes per molecule, can be induced by the electric double layer, which is almost ten times larger than that with the solid gate transistors. The experimental observation of MIT in organic semiconductors demonstrates that the periodic electrostatic potential is established even in solution-processed organic thin films.
Another surprising fact is that the sheet resistance (in the order of 10 kW) obtained for our organic two-dimensional hole gas (2DHG) is comparable to that of inorganic 2DHGs (R. Chaundhuri, et al., Science 365, 1454 (2019)). We believe that our findings represent a distinct paradigm shift with respect to the production of 2DHGs and also enable advancement in the field of condensed matter physics and device engineering. Molecular self-assembly can be responsible not only for the construction of molecular orientations favourable for electron transport, but also for the exotic electronic phase in organic semiconductors. Clearly, tunable carrier filling by the electric double layer in conjunction with a wide variation of organic semiconductor materials will help further experimental and theoretical studies.
For more information, please refer to our recent publication in Nature Materials, “Two-dimensional hole gas in organic semiconductors”