Title: Defect Passivation in 2D Semiconducting Material for Optoelectronic Applications

Speaker: Zhaojun Li, Department of Materials Science and Engineering, Uppsala University

Email: zhaojun.li@angstrom.uu.se

Time: July 3rd, 2023, 3:30 PM

Venue: Lecture Room 324, AISMST

Abstract:

The discovery of 2D materials based on transition metal dichalcogenides (TMDs), has opened up new interesting possibilities in optoelectronic devices, as monolayer TMDs possess direct bandgaps with absorption in the visible to near-infrared (NIR) spectral region. However, monolayer TMDs often exhibit poor photoluminescence quantum yields (PLQEs) and mobilities, which are signs of poor-quality semiconductor material. While there have been advances in materials growth in the past years, our understating of defects and how they degrade performance is still unsatisfactory. Thus, while many defect passivation strategies have been discussed in the literatures, most achieve only moderate PL enhancement. Even the most successful treatment to date, based on the super-acid H-TFSI, leads to trap-limited PL, and the mechanism is still under debate. Importantly, the harsh nature of this superacid limits its application in devices. Therefore, there is an active search for new chemical treatments that might provide better passivation and be compatible with device fabrication. And the treatment of liquid exfoliated materials is, to date, hardly explored. Here, I will present new chemical functionalization approaches to greatly enhance the PL intensity of mechanically exfoliated monolayer molybdenum disulfide (MoS2) and tungsten disulfide (WS2), while simultaneously enhancing the charge and exciton transport properties. I will also illustrate the passivation mechanisms, and probe the exciton dynamics of mechanically exfoliated and liquid-phase exfoliated 2D WS2 with ultrafast optical spectroscopies. In addition, I will present a family of ionic salts with superior PL enhancement effect compared to the widely discussed “super acid” H-TFSI treatment. More importantly, the ionic salts used in chemical treatments are compatible with a range of greener solvents and are easier to handle compared to super acid, which provides the possibility of directly treating TMDs for device fabrication.