Observation of inter-layer charge transmission resonance at optically excited graphene–TMDC interfaces

Abstract

The transfer of charge carriers across the optically excited hetero-interface of graphene and semiconducting transition metal dichalcogenides (TMDCs) is the key to convert light to electricity, although the intermediate steps from the creation of excitons in TMDC to the collection of free carriers in the graphene layer are not fully understood. Here, we investigate photo-induced charge transport across graphene–MoS2 and graphene–WSe2 hetero-interfaces using time-dependent photoresistance relaxation with varying temperature, wavelength, and gate voltage. In both types of heterostructures, we observe an unprecedented resonance in the inter-layer charge transfer rate as the Fermi energy ( E F) of the graphene layer is tuned externally with a global back gate. We attribute this to a resonant quantum tunneling from the excitonic state of the TMDC to E F of the graphene layer and outline a new method to estimate the excitonic binding energies ( E b) in the TMDCs, which are found to be 400 meV and 460 meV in MoS2 and WSe2 layers, respectively. The gate tunability of the inter-layer charge transfer timescales may allow precise engineering and readout of the optically excited electronic states at graphene–TMDC interfaces.

High‐Efficiency Infrared Sensing with Optically Excited Graphene‐Transition Metal Dichalcogenide Heterostructures

“…This can be attributed to phonon assisted or thermally activated intrinsic back transfer of electrons from Gr to the TMDC layer followed by recombination with the trapped holes. [ 7 ] We confirm this by recording the recovery photo current, Δ I rec = I rec − I ON , where I rec is the steady state current after turning OFF the visible light, as a function of V G − V D in Figure 1f and observing it to be proportional to d I /d V G , just like initial photoresponse Δ I P (= I ON − I OFF ). This leads us to conclude that Δ I P and Δ I rec are respectively caused by photogating due to TMDC → Gr and Gr → TMDC charge transfer and establishes the existence of a bidirectional charge transfer mechanism.…”

“…[ 1,17 ] This photo‐generated charge transfer across the vdW layers leads to an effective change in back gate voltage V G and results in a net photo‐response in the channel. [ 1,7,17 ]…”

“…Photodetection based on 2D van der Waals (vdW) materials and their heterostructures have recently gained significant interest from both fundamental and applied research sectors. [ 1–9 ] Dirac cone‐like band structure of graphene (Gr) [ 10 ] enables the generation of photo‐carriers over a wide range of the electromagnetic spectrum, [ 11,12 ] but the zero bandgap and ultra‐fast electron‐hole recombination time‐scales [ 13,14 ] leads to poor photoresponsivity. [ 12,15,16 ] Placing Gr in vdW proximity of light absorbing nano‐materials, for example, quantum dots, [ 17,18 ] chromophores [ 19 ] or nano‐wires [ 20,21 ] is a popular method to overcome this problem.…”

Engineering sensitivity and spectral range of photodetection in van der Waals materials and hybrids

“…This has been attributed to defects/traps in MoS 2 lying between conduction band level and excitonic (E X ) levels (see the schematic of figure 4(e)) since control samples of Gr/ hBN show no NIR response. Using a similar model, Khasid et al [112] showed through time-dependent photo relaxation measurements, resonant quantum tunnelling occurs from E X of a TMDC to E F of Gr in Gr/MoS 2 hybrids.…”

Anomalous electrical transport in orientationally controlled trinary hybrids of graphene and twisted bilayer molybdenum disulphide

“…Sandwiching a hexagonal boron nitride (hBN) layer, in between the Gr/MoS 2 photodetector, shows an increase in the charge decay time, which can be controlled by the hBN thickness [17]. Recently, a resonant inter-layer electron transfer has been observed in these heterostructures, by externally tuning the fermi level ðE F Þ of graphene to match the excitonic state of the TMDC [18]. It provides an insight into the exciton dissociation process and outlines a new way to determine the exciton binding energy in TMDCs that can be obtained by tuning E F of graphene.…”