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Two-dimensional hole gas in organic semiconductors

Abstract

A highly conductive metallic gas that is quantum mechanically confined at a solid-state interface is an ideal platform to explore non-trivial electronic states that are otherwise inaccessible in bulk materials. Although two-dimensional electron gases have been realized in conventional semiconductor interfaces, examples of two-dimensional hole gases, the counterpart to the two-dimensional electron gas, are still limited. Here we report the observation of a two-dimensional hole gas in solution-processed organic semiconductors in conjunction with an electric double layer using ionic liquids. A molecularly flat single crystal of high-mobility organic semiconductors serves as a defect-free interface that facilitates two-dimensional confinement of high-density holes. A remarkably low sheet resistance of 6 kΩ and high hole-gas density of 1014 cm−2 result in a metal–insulator transition at ambient pressure. The measured degenerate holes in the organic semiconductors provide an opportunity to tailor low-dimensional electronic states using molecularly engineered heterointerfaces.

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Fig. 1: Formation of an EDL at the single-crystalline bilayer of C8-DNBDT-NW.
Fig. 2: High carrier density achieved with EDLTs.
Fig. 3: Metal–insulator transition of C8-DNBDT-NW.
Fig. 4: Hall effect measurement of C8-DNBDT-NW.

Data availability

We declare that all data supporting the findings of this study are included within the paper and its Supplementary Information files. Source data are provided with this paper.

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Acknowledgements

S.W. acknowledges the support from the Leading Initiative for Excellent Young Researchers of JSPS. T.O. acknowledges the support from PRESTO-JST through the project ‘Scientific Innovation for Energy Harvesting Technology’ (grant no. JPMJPR17R2). This work was supported by Kakenhi Grants-in-Aid (nos. JP17H06123, JP17H06200, 20H00387) from JSPS, and JST FOREST Program, grant no. JPMJFR2020.

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Contributions

N.K. conceived, designed and performed the experiments and analysed the data. N.K. and J. Tsurumi performed the density functional theory calculations. T.O. synthesized and purified C8-DNBDT-NW. N.K., S.W. and J. Takeya wrote the manuscript. S.W. and J. Takeya supervised the work. All authors discussed the results and reviewed the manuscript.

Corresponding authors

Correspondence to Shun Watanabe or Jun Takeya.

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The authors declare no competing interests.

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Peer review information Nature Materials thanks Mario Caironi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Figs. A1–D1, Notes A–D and references.

Source data

Source Data Fig. 2

Current–voltage characteristics, gate voltage dependent carrier density and Hall mobility derived from the Hall effect measurements.

Source Data Fig. 3

Temperature dependence of sheet resistance.

Source Data Fig. 4

Temperature dependence of Hall mobility and carrier density.

Source Data in Supplementary information

Sample variation in current–voltage characteristics and magnetotransports.

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Kasuya, N., Tsurumi, J., Okamoto, T. et al. Two-dimensional hole gas in organic semiconductors. Nat. Mater. 20, 1401–1406 (2021). https://doi.org/10.1038/s41563-021-01074-4

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