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

Abstract

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.