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

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.

Structures, Properties, and Device Applications for [1]Benzothieno[3,2‐b]Benzothiophene Derivatives

“…Therefore, the type‐II heterojunction is the most conventional heterojunction to be used for promoting the spatial separation of electron–hole pairs because of the staggered structure. [ 167,168 ] Wu et al achieved the type‐II heterojunction by using PTCDA/C8‐BTBT and the photoluminescence (PL) measurements verified the charge transfer (Figure 11e). [ 169 ] He et al combined C8‐BTBT and n‐type MoS 2 to form p–n heterojunction structure.…”

Engineering the Electronic Structure of Two-Dimensional Materials with Near-Field Electrostatic Effects of Self-Assembled Organic Layers

“…We show that the effect of the molecular layer is well-approximated by a discretized planar charge density, which can be derived from the structure and the multipole moments of the molecule. We solve this model numerically and analytically for three classes of organic molecules, all previously experimentally assembled on graphene and other 2D materials: (i) 2,7-dioctyl [1]-benzothieno [3,2-b][1]benzothiophene (C8-BTBT) [41][42][43], (ii) phthalocyanine (H 2 Pc) and perfluoro-phthalocyanine (H 2 PcF 16 ) [44][45][46][47], and (iii) benzene (C 6 H 6 ) and hexafluorobenzene (C 6 F 6 ) [48][49][50]. Using our results, we elucidate the molecular descriptors controlling the magnitude of the potential fluctuations and the lengthscale of the near-field to far-field transition.…”

“…Beyond symmetry considerations, all molecules considered here have minimal charge transfer with the 2D materials, as shown in Supplementary Materials. All geometries of the self-assembled monolayers (SAM) are adopted from experiments [41][42][43][44][45][46][47][48][49][50], and optimized using DFT with the exchange-correlation potential of Perdew-Burke-Ernzerhof (PBE), and Tkatchenko-Scheffler [51] corrections for the van der Waals interactions. Computational details, computed geometries and binding energies of the molecules on 2D materials, and quadrupole moments are reported in the Supplementary Materials.…”

Tailoring the Optical Response of Pentacene Thin Films via Templated Growth on Hexagonal Boron Nitride

“…Layered van der Waals (vdW) materials have attracted significant interest as substrates for organic thin-film growth to create mixed-dimensional heterostructures, which combine the well-understood features of the component materials to engender emergent properties not possible in either isolated system. − Pentacene films have been studied on graphene , and MoS 2 ; , however, the charge transfer processes with the underlying semimetallic or semiconducting substrate complicate identification of new morphological–optical relationships within the pentacene films. In contrast, due to its electrically insulating character and exceptionally low density of trapped charges, hexagonal boron nitride (hBN) substrates have the potential to address these issues.…”