Liquid Interface Deposition of Thin Films and van derWaals Heterostructures of Two-Dimensional Solids

The scalable production of thin films of graphene and other two-dimensional materials is a key research challenge which needs to be addressed if the unique and desirable properties of these materials is to be exploited beyond the laboratory. In this report a brief overview of two-dimensional layered materials, their production and of heterostructures produced from them is presented. Experiments exploring the time and temperature dependence of shear exfoliation for the production of graphene suspensions, a key precursor for thin film and heterostructure fabrication are reported. It is concluded that, for shear exfoliation of graphene in an aqueous surfactant solution of Triton X-100 and ultra-high purity water, the graphene platelets formed consist predominantly of three layers (trilayer graphene). Data indicate that there is an initial decrease in platelet laterial size with exfoliation time, with the size then remaining constant (albeit with considerable scatter in the data). Preliminary measurements indicate that temperature is also found to influence shear exfoliation through viscosity of the solvent. Increasing viscosity is found to initially increase the concentration of shear exfoliated suspensions until the viscosity increases to a point where the minimum shear rate required for exfoliation cannot be met by the mixer employed, at which point exfoliation is suppressed and concentration decreases. Raman spectroscopy was employed to demonstrate that the shear exfoliation approach used for graphene was also applicable to MoS2, MoSe2 and WS2. Langmuir-Blodgett deposition was used to deposit thin films of graphene, but the resultant films were found to be non-uniform and it was not possible to produce films from few layer transition metal dichalcogenides (TMDCs) by this route. To address this issue, a new technique was developed, termed ‘Liquid Interface Deposition’ (LID), which is described. The generic nature of the LID approach is demonstrated through the presentation of data from thin films derived from few layer suspensions of graphene, TMDCs and hexagonal boron nitride. A mechanism of thickness control is applied to films derived from suspensions of graphene trilayers, and films ranging from 1 to 5 trilayer units produced. The use of LID for the fabrication of heterostructures is demonstrated through the production of a graphene/MoS2 heterostructure characterised by Raman and optical spectroscopies.