Molecular interaction is an essential process in various fields such as chemistry, biology and materials science. The interaction of molecules includes: 1) the local electron transfer process, 2) local chemical bonding and 3) assembling to form a structure. All three occur essentially from each molecule’s “recognition” ability. We are trying to extend the molecular interaction system onto inorganic electronic materials to harness the highest-performance of inorganic systems. In this presentation, I will present our recent results of molecular interaction on a semiconducting 2D material, transition metal dichalcogenides (TMDCs).^1,2,3

TMDCs are constructed by stacking atomically-thin layers (thickness ~0.7 nm) with van der Waals interaction, and they are expected to generate tiny optoelectronic devices due to their thin nature. Since monolayer TMDC has a direct bandgap, it is a good platform for realizing atomically thin optoelectronic devices. However, an issue is their low luminescence brightness. For example, a representative TMDC, MoS­₂, shows photoluminescence (PL) quantum yield of ~1% or less. In this presentation, I will show our recent results of improving the brightness of monolayer MoS₂ dramatically (more than 100 times enhancement from the original) via molecular treatments. One method is using a superacid (bis(trifluoromethane)sulfonimide; TFSI) molecule.³ The superacid molecular treatment onto a monolayer MoS₂ changes the PL quantum yield of MoS₂ from 1% to near-unity. I will discuss the PL enhancement factor in the TFSI treatment on MoS₂. The second method is using a redox-active molecule, fluoranil. The fluoranil molecule can reduce the electron concentration of monolayer MoS₂, resulting in enhancement of the PL intensity.⁴ Furthermore, we found that the solvent used for the fluoranil treatment is critical to obtain bright monolayer MoS₂. Solvent molecules would screen the charges on fluoranil molecules, as a result, which stabilizes the electron transfer-interaction between fluoranil and MoS₂.

Keywords: Superacid, redox-active molecules, Transition metal dichalcogenides, Photoluminescence, Transistors

References:

¹D. Kiriya et al., J. Am. Chem. Soc. 2014, 136, 7853.

²H. Ichimiya, et al., ACS NANO, 2018, 12, 10123.

³M. Amani^*, D.H. Lien^*, D. Kiriya^* et al., Science, 2015, 350, 1065. (* equal contribution)

⁴H. Ichimiya, et al. Appl. Phys. Express, 2019, in press.