Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe-Organic van der Waals Heterostructures

Shaowei Li, Chengmei Zhong, Alex Henning, Vinod K. Sangwan, Qunfei Zhou, Xiaolong Liu, Matthew S. Rahn, Spencer A. Wells, Hong Youl Park, Jan Luxa, Zdeněk Sofer, Antonio Facchetti, Pierre Darancet, Tobin J. Marks, Lincoln J. Lauhon, Emily A. Weiss, Mark C. Hersam

Research output: Contribution to journalArticle

Abstract

Layered indium selenide (InSe) is an emerging two-dimensional semiconductor that has shown significant promise for high-performance transistors and photodetectors. The range of optoelectronic applications for InSe can potentially be broadened by forming mixed-dimensional van der Waals heterostructures with zero-dimensional molecular systems that are widely employed in organic electronics and photovoltaics. Here, we report the spatially resolved investigation of photoinduced charge separation between InSe and two molecules (C70 and C8-BTBT) using scanning tunneling microscopy combined with laser illumination. We experimentally and computationally show that InSe forms type-II and type-I heterojunctions with C70 and C8-BTBT, respectively, due to an interplay of charge transfer and dielectric screening at the interface. Laser-excited scanning tunneling spectroscopy reveals a ∼0.25 eV decrease in the energy of the lowest unoccupied molecular orbital of C70 with optical illumination. Furthermore, photoluminescence spectroscopy and Kelvin probe force microscopy indicate that electron transfer from InSe to C70 in the type-II heterojunction induces a photovoltage that quantitatively matches the observed downshift in the tunneling spectra. In contrast, no significant changes are observed upon optical illumination in the type-I heterojunction formed between InSe and C8-BTBT. Density functional theory calculations further show that, despite the weak coupling between the molecular species and InSe, the band alignment of these mixed-dimensional heterostructures strongly differs from the one suggested by the ionization potential and electronic affinities of the isolated components. Self-energy-corrected density functional theory indicates that these effects are the result of the combination of charge redistribution at the interface and heterogeneous dielectric screening of the electron-electron interactions in the heterostructure. In addition to providing specific insight for mixed-dimensional InSe-organic van der Waals heterostructures, this work establishes a general experimental methodology for studying localized charge transfer at the molecular scale that is applicable to other photoactive nanoscale systems.

Original languageEnglish
Pages (from-to)3509-3518
Number of pages10
JournalACS nano
Volume14
Issue number3
DOIs
Publication statusPublished - Mar 24 2020

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Keywords

  • Kelvin probe force microscopy
  • fullerene
  • indium selenide
  • photoluminescence spectroscopy
  • scanning tunneling microscopy
  • transient absorption spectroscopy

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

Cite this

Li, S., Zhong, C., Henning, A., Sangwan, V. K., Zhou, Q., Liu, X., Rahn, M. S., Wells, S. A., Park, H. Y., Luxa, J., Sofer, Z., Facchetti, A., Darancet, P., Marks, T. J., Lauhon, L. J., Weiss, E. A., & Hersam, M. C. (2020). Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe-Organic van der Waals Heterostructures. ACS nano, 14(3), 3509-3518. https://doi.org/10.1021/acsnano.9b09661