Dimensionality is one of the most fundamental parameters that determine the physical characteristics of materials. Graphene, which exhibits exotic condensed matter phenomena that are absent in graphite, elegantly illustrate the effect of dimensionality. The ability to isolate and manipulate atomically thin crystals of van der Waals layer compounds offers not only an exciting opportunity to explore the unique physics of a variety of 2D systems but also a new pathway to realizing intelligently designed artificial crystals with precisely tailored functionalities.
It was recently found that 2D crystals of layered transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 can be readily obtained by exfoliation of bulk crystals or by bottom up growth by chemical vapor deposition. These materials exhibit unique excitonic features as a manifestation of strong many-body effect. In this talk, I will discuss the unusually large optical conductivity of single layer TMDs arising from band nesting effect and show how it provides an indication of dimensional crossover. I will further discuss spontaneous formation of indirect excitons as evidenced by photoluminescence (PL) spectroscopy studies. Our recent findings on the behavior of excitons in van der Waals heterostructures will also be presented.