Transition metal dichalcogenides (TMD) monolayers, are the worlds thinnest semiconductors. At just three atoms thick, this relatively new breed of materials has captured the attention of the low dimensional materials research community both theoretically and experimentally.
Unlike graphene, a direct band gap in the optical rage can be found at the K and K’ valleys. Not only are these near 2-dimensional materials semiconductors, the cornerstone material upon which a lot of modern technology rests, but they are also particularly desirable semiconductors, with intriguing optical response.
A number of applications have been proposed and demonstrated for TMD monolayers including electrically tuneable mirrors, qubits and single photon emitters. It is in the pursuit of creating single photon emitters in TMD monolayers that the notion of strain engineering has climbed to prominence.
Single photon emitters (SPEs), ideally should deterministically create only 1 photon at a specified time, place and wavelength. This is incredibly difficult to achieve and a number of various methods have been tried over the years. In TMDs, single photon emission from the material has been shown by optically exciting the monolayer either resonantly or above the band gap.
The problem with demonstrations of this effect in TMDs has been the seemingly random nature of the locations of the SPEs. SPEs tend to be located at local band gap minima in K-space, where some potential tends to localise the excitons before the recombine, emitting their photon. In TMDs, these tend to occur in real space around lattice impurities or at the edge of the TMD flakes. At this point in the isolation techniques of TMD monolayers, lattice impurities are to be expected. The SPEs at the edge of the flakes, however, indicates that lattice strain may induce sufficient potential for useful applications.
In this group, one of our colleagues (Alejandro Rodriguez) has shown that this principal may be exploited, creating strain potentials in WS2 and WSe2 flakes by tenting the monolayer about glass nanopillars, showing SPE locations at the top of the pillar.
This principal, of strain induced bandgap renormalisation in TMDs, is my current research interest. I am currently working on developing a theoretical framework which may be used to usefully describe the results of my colleague’s experiment. This, can be done by a combination of the plate theory of continuum mechanics and a tight binding approach to TMD curvature, with parameters from previous DFT strain studies of the materials.
Once a reasonable description of the nanopillar SPE is developed, I hope to see how far this principal can be extended. Can bending the monolayers be used to confine electron in strain induced dots? Can this be used for quantum information? Can wires in the monolayer be defined this way? What are the possible spintronic applications of such wires? Can a similarly large effect be achieved with sagged monolayers? Can this allow for printable TMD circuitry by etching the substrate?
I believe, and hope to soon show, that strain engineering will truly add a new dimension to TMD applications.
By Matthew Brooks, PhD Student of the Burkard Group at Univeristät Konstanz, Germany.
Kern, Johannes, et al. “Nanoscale Positioning of Single‐Photon Emitters in Atomically Thin WSe2.” Advanced Materials 28.33 (2016): 7101-7105.