Temperature‐Dependent Electronic Ground‐State Charge Transfer in van der Waals Heterostructures

Abstract

A single layer of a 2D material may be insulating (e.g., hexagonal boron nitride), semiconducting (e.g., MoS 2 ), or conducting (e.g., graphene), and thus all electrical material properties required for the construction of an electronic or optoelectronic device are available in the monolayer limit. [3][4][5][6] Stacks of such mono layers are typically bound by weak van der Waals (vdW) interlayer interactions, and vdW heterostructures have emerged as prime candidates for realizing electronic and optoelectronic functions in the smallest possible volume. The type and efficiency of the functionality that can be achieved depends critically on the electronic energy level alignment across the heterostructure, [7] and substantial charge density rearrangement upon contact can occur, depending on the electronic structure of each component. Consequently, charge rearrangement and also charge transfer (CT) phenomena that define the electronic ground state of vdW heterostructures must be thoroughly understood Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of state distribution of the components governs the amount of charge transfer, but a notable dependence on temperature is not yet considered, particularly for weakly interacting systems. Here, it is experimentally observed that the amount of ground-state charge transfer in a van der Waals heterostructure formed by monolayer MoS 2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of 3 when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that accounts for nuclear thermal fluctuations reveal intracomponent electron-phonon coupling and intercomponent electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multicomponent van der Waals heterostructures.

Photoemission Study of Charge Transfer between ET (BEDT-TTF) and Acceptors F6TCNNQ and F2TCNQ

“…In a previous study of F6 deposited on gold, a similar broad feature at the same BE was found. 25 Other studies of F6 on graphene, 26,27 MoS 2 , 28 and halide perovskite 29 found two peaks with a BE difference of 0.5 to 1.3 eV that were assigned to the relaxed former HOMO (higher BE) and the singly occupied former LUMO (lower BE). It is possible to fit the present spectra with two Voigt peaks with a BE separation of 0.6 eV which would be in agreement with the aforementioned studies.…”

Donors, acceptors, and a bit of aromatics: electronic interactions of molecular adsorbates on hBN and MoS2 monolayers

“…With the electron density being its central quantity, DFT grants immediate access to the charge redistribution induced by adsorption. 13,14,21,27 This way, it is possible to assess the type of ground-state doping and to gain insight into the spatial extension of the electron cloud at the interface. Furthermore, DFT calculations are able to deliver work functions, level alignments, band structures, and (projected) density of states, among other important properties.…”

Donors, Acceptors, and a Bit of Aromatics: Electronic Interactions of Molecular Adsorbates on hBN and MoS$_2$ Monolayers

“…Furthermore, DFT calculations are able to deliver work functions, level alignments, band structures, and (projected) density of states, among other important properties. 2,[13][14][15]27 While state-of-the-art first-principles methods to obtain the electronic structure of solid-state materials are currently based on many-body perturbation theory, 18,28 the choice of range-separated hybrid functionals to approximate the exchange-correlation potential in DFT offers the optimal trade-off between accuracy and computational costs. 15,27 Proper inclusion of van der Waals interactions improves the prediction of structural arrangements and hence the description of electronic properties.…”

“…2,[13][14][15]27 While state-of-the-art first-principles methods to obtain the electronic structure of solid-state materials are currently based on many-body perturbation theory, 18,28 the choice of range-separated hybrid functionals to approximate the exchange-correlation potential in DFT offers the optimal trade-off between accuracy and computational costs. 15,27 Proper inclusion of van der Waals interactions improves the prediction of structural arrangements and hence the description of electronic properties. 29 The level of accuracy currently achieved by such ab initio calculations ensures reliable results complementary to experiments.…”

“…29 The level of accuracy currently achieved by such ab initio calculations ensures reliable results complementary to experiments. 2,27,30 In this work, we present a DFT study on the structural, energetic, and electronic properties of five representative organic molecules, including donor and acceptor compounds as well as a purely aromatic moiety, adsorbed on freestanding hexagonal boron nitride (hBN) and molybdenum disulfide (MoS 2 ) monolayers. The former is a known insulator, widely used as a substrate and/or as an encapsulating material in low-dimensional heterostructures, 17 which has been receiving increasing attention in surface and interface science, [31][32][33][34][35][36][37] for instance to sustain the growth of well-defined organic thin films.…”