Event
Ph.D. Research Proposal Exam: Leo (Yi-Jan) Sun
Wednesday, December 18, 2024
10:30 a.m.
AVW 1146 (ISR)
Maria Hoo
301 405 3681
mch@umd.edu
ANNOUNCEMENT: Ph.D. Research Proposal Exam
Name: Leo (Yi-Jan) Sun
Committee:
Professor Thomas Murphy (Chair)
Professor Cheng Gong
Professor Kevin Daniels
Date/time: Wednesday, Dec 18, 10:00 am-12:00 pm
Location: AVW 1146 (ISR)
Title: Optical and electrical properties of MoSe2 monolayer nanostructure
Abstract:
Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are promising candidates for creating nano-electronic and photonic devices. TMD nanoribbons have been theoretically shown to possess metallic edge states along the zigzag edges, and the edge states have potential for applications in electronic or spintronic devices. In this proposal, we investigated MoSe2 nanoribbons grown by a substrate direct synthesis method that naturally forms pristine zigzag edges.
First, we implemented tip-enhanced photoluminescence (TEPL) measurement on as-grown MoSe2 nanoribbons to analyze the surface and edge features with photoluminescence signal. The most interesting feature we observed from the TEPL map is the dimmer line on the “spine” of most nanoribbons, indicating a twin grain boundary on the middle of the nanoribbons. If the twin boundary is a zigzag edge shared by 2 nanoribbons, this is possible evidence of the existence of metallic edge states on the zigzag edge.
Second, we developed a high-yield gold-transfer technique for as-grown MoSe2 nanoribbons. Typical dry transfer techniques employ a polymer stamp to transfer 2D flakes from a targeted region, however, we found that the polymer stamp transfer is inefficient for transferring as-grown MoSe2 nanoribbons. We adapted the method of gold exfoliation, originally employed to exfoliate TMD monolayers, to the transfer of synthesized MoSe2 nanoribbons. The yield of the Au transfer was found to be much higher and more efficient than the polymer stamp transfer for as-grown MoSe2 nanoribbons
Finally, we measured the field-effect transistor (FET) transfer and output characteristics along the edges and bulk of the nanoribbons to investigate the conduction from edge states. To date, it has been difficult to resolve the edge and bulk FET mobility because the electrical contacts include a rectifying Schottky barrier. To solve the contact issue, we tested different configurations and treatments to improve the performance of the MoSe2 contacts. Results show that Au contacts give the smallest contact resistance, and that vacuum annealing at 250 °C can improve the current level by an order of magnitude and reduce the charging effect at the contact interface.
For the future work, we will explore different strategies to detect edge states on nanoribbons. Because the improvement of contacts has proven to be challenging, we will implement other high-resolution scanning techniques including conductive atomic force microscopy (CAFM) and near-field scanning optical microscopy (NSOM) to identify other features that could prove the existence of the edge states. We will also compare the basic properties between as grown 2D MoSe2 layers, and mechanically etched MoSe2 nanoribbons.
