Shiwei Wu, Fudan University
Friday, March 10, 2017 - 12:30pm to 1:30pm
Understanding and controlling defects in crystalline materials is an everlasting theme in material science and engineering. For two dimensional materials at atomic limit, defects are more influential and have come to the forefront in the pursuit of electronic and optoelectronic applications. In this talk, I will present our recent studies on line defect and point defect in semiconducting transition metal dichalcogenide monolayer.
Line defects are the boundaries between grains with different crystalline orientations. The formation of line defects is determined by the growth mechanism of transition metal dichalcogenide monolayers. Hence the understanding of their formation would greatly help produce wafer-scale single crystalline materials or utilize intrinsic defects for technological applications. However, the line defects are extremely narrow with only a few atoms wide, hidden inside a polycrystalline monolayer. We introduced second harmonic generation (SHG) microscopy, a noninvasive coherent imaging technique, to visualize hundreds of grains and line defects in as-grown CVD MoS2 monolayer, and revealed the CVD growth mechanism of MoS2 monolayer. Furthermore, I will show our latest study on the electronic property of these line defects by low temperature scanning tunneling microscopy (STM).
Compared to line defects, point defects more widely spread in transition metal dichalcogenide monolayer. Their presence lowers the carrier mobility and photoluminescence quantum yield. Furthermore, point defects in transition metal dichalcogenide monolayer host localized excitons, acting as color centers within optical gap. At low temperature, these defect excitons even become single quantum emitters in WSe2 monolayer. However, the exact nature of these defects remains elusive. We recently unveiled the atomic structure of localized defect excitons in WSe2 monolayer by combinational study of low temperature STM and photoluminescence spectroscopy, providing the first real-space peek into defect excitons.