Room Temperature Enhancement of Electronic Materials by Superacid Analogues

Treatment with the superacid bis(trifluoromethanesulfonyl)amide (sometimes known as TFSA, TFSI, or HNTf₂) enhances the properties of a wide range of optoelectronic materials, resulting in longer effective carrier lifetimes and higher photoluminescence quantum yields. We have conducted a multimaterial study treating both crystalline silicon and transition metal dichalcogenide (TMDC) monolayers and few-layer flakes with solutions formed from TFSA and a range of compounds with related chemical structures with different Lewis acidities, in order to elucidate the factors underpinning the TFSA-related class of enhancement treatments. We adopt dichloromethane (DCM) as a common solvent as it provides good results at room temperature and is potentially less hazardous than TFSA-dichloroethane (DCE) heated to ∼100 °C, which has been used previously. Kelvin probe experiments on silicon demonstrate that structurally similar chemicals give passivating films with substantially different charge levels, with the higher levels of charge associated with the presence of CF₃SO₂ groups resulting in longer effective lifetimes due to an enhancement in field-effect passivation. Treatment with all analogue solutions used results in enhanced photoluminescence in MoS₂ and WS₂ compared to untreated controls. Importantly we find that MoS₂ and WS₂ can be enhanced by analogues to TFSA that lack sulfonyl groups, meaning an alternative mechanism to that proposed in computational reports for TFSA enhancement must apply.