Exciton diffusion in TMDC monolayers

 (See an article written in french for the journal of the "Société française de physique" here about these novel 2D semiconductors)



Since the discovery of graphene in 2004, many layered materials have been thinned down to a monolayer and proven to be stable in ambient conditions. Among these, monolayers of the MX2 family, where M is a transition metal atom (typically Mo, or W) and where X is a chalcogen atom  (S, Se, Te), have been shown to be direct band-gap semiconductors with emission in the visible-near infrared range. This makes them promising candidates for ultrathin electronic and opto-electronic devices. In addition, due to the particular bandstructure and broken inversion symmetry in these atomically thin crystals, the optical selection rules are chiral and electron-hole pairs can be selectively created in one of the two non-equivallent K valleys of the Brillouin zone. This could allow to store and process information via the valley index of carriers in these ideal 2D systems.


   Spatially resolved photoluminescence of excitons in a WSe2 monolayer at cryogenic temperatures


Another striking feature of these 2D semiconductors is the very strong Coulomb interaction between carriers. Electrons and holes form tightly bound pairs, or excitons, with binding energies of the order of several hundred meV, thus orders of magnitude larger than the ones found in traditional semiconductors such as gallium arsenide or silicon. Because of this strong binding, in these materials excitons are stable even at room temperature. This opens the door for a variety of applications aiming to exploit the exciton’s properties in novel devices. For example, due to the strong light-matter interaction inherent to tightly bound excitons, efficient coupling between optical data transmission and electronic processing systems may be achieved. Very recently, devices made of MoS2–WSe2 van der Waals heterostructures encapsulated in hexagonal boron nitride have been used to demonstrate an electrically controlled transistor at room temperature based on exciton motion (Unuchek et al, Nature 2018).


Thus, knowledge on the motion and persistence of inhomogeneous excitonic distributions is central to the technologies based on the transport of excitonic properties of transition metal dichalcogenide monolayers. The relevant parameters are the averaged exciton lifetime tX and diffusion coefficient DX, which determine the time and length scale available for transport and manipulation of a nonequilibrium exciton distribution. In this work, we have combined steady-state micro-photoluminescence (µPL) with time-resolved photoluminescence (in collaboration with the LPCNO at Toulouse) to determine the exciton lifetime and diffusion coefficient in WSe2 monolayers at room and cryogenic temperatures. So far, a clean measurement of these parameters has been difficult due to the modest quality of monolayers deposited onto silicon substrates, which impacts not only the transport properties but also the optical quality of monolayers. Here, samples of ultra-high quality were prepared by mechanical exfoliation of bulk crystals and subsequent encapsulation of monolayers with thin layers of hexagonal boron nitride, provided by the K. Watanabe and T. Taniguchi group at NIMS, Japan (F. Cadiz et al, Phys. Rev. X 7, 021026 2017). 



Figure 1 (a) Optical microscope image of an WSe2 flake encapsulated in hexagonal boron nitride  (hBN). (b) Image of the diffraction-limited spot used to perform µPL measurements (left), and resulting image of the PL at 300 K (right) (c) Normalized photoluminescence intensity vs. distance to the excitation spot, revealing exciton diffusion at 300 K. The laser profile is also shown.


At room temperature, we extract an exciton lifetime tX = 90 +/- 10 ps and diffusion coefficient of DX=14.5 +/- 2 cm2/s for monolayer WSe2. This represents an order of magnitude increase in the exciton mobility with respect to several previous works in non-encapsulated samples. In addition, diffusion of different exciton complexes in WSe2 has been resolved for the first time at low temperatures due to the high optical quality of encapsulated monolayers. In the future, novel transport phenomena in these materials such as the spin/valley-Hall effect, spin/valley-Coulomb drag and valley-dependent diffusion in degenerately doped monolayers, or in dense interacting and polarized neutral exciton gas, may be demonstrated with such experiments.




See the full article here (published in Applied physics letters)




See also


Approaching the homogeneous linewidth in transition metal dichalcogenide monolayers


PhD thesis: Spin-dependent electron transport in semiconductors.