Charge-transfer interaction between few-layer MoS2 and tetrathiafulvalene.

Graphene has emerged to be a material of great interest because of its unique electronic structure and properties associated with its two-dimensional structure. Single-layer graphene is well known for properties such as quantum Hall effect, ambipolar electric field effect, and ballistic conduction of charge carriers. Graphene exhibits significant changes in electronic structure and properties on introduction of electrons or holes by electrochemical means. Such doping is reported to stiffen the Raman G band (frequency of the Raman band increases). Electron and hole doping can also be achieved by molecular charge transfer through interaction with electron donor and acceptor molecules, respectively. Molecular charge transfer with graphene has been investigated in detail by using Raman spectroscopy and other techniques. Charge-transfer interaction with an electron donor molecule like tetrathiafulvalene (TTF) softens the G band of graphene, whereas stiffening occurs upon interaction with an electron acceptor like tetracyanoethylene (TCNE), the Raman G band frequency will increase and decrease due to interaction with TCNE and TTF, respectively. These changes in the Raman G band are different from those found with electrochemical doping. We were interested to explore the interaction of electron donor and acceptor molecules with a two-dimensional layered material such as MoS2 to explore the occurrence of charge transfer, if any. With this purpose, we have studied the interaction of few-layer MoS2 material with TTF and TCNE. It is to be noted that MoS2 layers consist of Mo atoms sandwiched between two layers of chalcogen atoms, where the adjacent sheets are stacked by van der Waals interactions. We have observed the occurrence of charge transfer of fewlayer MoS2 material with TTF, but not with TCNE. Electronic absorption spectroscopic measurements indicate the formation of TTF radical cation by the interaction of TTF with few-layer MoS2 material, accompanied by the stiffening of the A1g mode of MoS2 in the Raman spectrum. This shift in the Raman A1g mode is opposite to that found in electrochemical doping. We have carried out detailed first-principle calculations to understand the results. The XRD pattern of the few-layer MoS2 material does not exhibit the (002) reflection, thus confirming the presence of only a few layers and the graphene-like nature of the material. The AFM images and the corresponding height profiles also confirm the presence of two to three layers with an average thickness of 2.44 nm. Figure 1 shows