(July 24, 2015) Keynote
presentations on the third day of Graphene Week 2015 offered an eclectic mix of
fundamental science and practical chemical engineering. Here we report briefly
on each of the talks, beginning with an introduction to optoelectronics in 2d
semiconductors and heterostructures, and concluding with an outline of a highly
promising ‘kitchen sink’ approach to graphene production.
Optoelectronics of 2D semiconductors and heterostructures
Xiaodong Xu of the University of Washington in Seattle set
the ball rolling with a look at the optoelectronics and valleytronics of 2d
semiconductors and heterostructures, such as those based on the sulphides and
selenides of molybdenum and tungsten. Xu began with a recap of the electronic
properties of MoS2, including the change in band gap with number of layers.
Of particular interest to Xu are valley-specific interlayer
excitons in monolayer WSe2-MoSe2 vertical heterostructures. Optical pumping
leads to coupled spin-valley polarisation of interlayer excitons, with measured
lifetimes of more than 30 nanoseconds. Such long-lived polarisation allows for
the visualisation of both spin and valley diffusion over length scales of
several microns.
In practical terms, the effect is important in laser
technologies with a tuneable carrier density.