Abstract
In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe2. The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 1012/cm2. The exciton capture time extracted from experimental data is around ~ 1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygen-associated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.
Highlights
Two-dimensional transition metal dichalcogenides (TMDs) have gained extensive research interests in recent years due to their extraordinary properties, such as direct band gap in monolayers,[1] strong spin-valley coupling,[2] and stable excitons or trions at room temperature.[3]
Can be ruled out in our experiment for several reasons: bandgap renormalization (BGR) should cause a red-shift of the absorption spectrum of the probe
This shift should lead to a reduced absorption rather than an increased absorption because the probe wavelength is set to resonant with the A exciton absorption peak
Summary
Two-dimensional transition metal dichalcogenides (TMDs) have gained extensive research interests in recent years due to their extraordinary properties, such as direct band gap in monolayers,[1] strong spin-valley coupling,[2] and stable excitons or trions at room temperature.[3]. The direct band gap in monolayer TMDs makes them excellent candidates for light-emitting diodes,[6] photodetectors,[7] and lasers.[8] The spin-valley coupling opens the possibility of realizing valleytronics, where the hole spin as a quantum information carrier can be manipulated through the interplay between spin and valley in spin quantum gates.[9]. Chemical vapor deposition (CVD) grown monolayer TMDs are known to have many defects caused by thermal strain and local variations in the precursor concentration during the growth process.[10] Defects can severely jeopardize the performance of TMD-based devices. The mid-gap defects are believed to serve as either effective recombination centers or effective carrier traps, depending on whether the defects possess a small or large difference in capture rates of electrons and holes, respectively.[16]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
