Sophie Pain1,Nicholas Grant1,John Murphy1
School of Engineering, The University of Warwick1
Sophie Pain1,Nicholas Grant1,John Murphy1
School of Engineering, The University of Warwick1
Transition metal dichalcogenides (TMDCs) such as MoS<sub>2</sub> and WS<sub>2</sub> have potential applications in new generations of optoelectronic devices. Treatment with the superacid bis(trifluoromethanesulfonyl)amide (known as TFSA, TFSI, or HNTf<sub>2</sub>) enhances TMDC photoluminescence (PL) quantum yields greatly [1-6]. Despite extensive empirical reports on the topic, the underlying reasons for PL improvement are not clear and establishing these is key to maximizing enhancements and prolonging device stability.<br/><br/>This work aims to gain insight into the mechanisms responsible for PL enhancement following treatments of the superacid type by conducting a multi-material study using TFSA and a range of structurally related chemical analogues. Previous work identified several similarities between TFSA-based passivation of TMDCs and crystalline silicon [3, 6-8], and we first use silicon as a model system for passivation optimization using carrier lifetime and Kelvin probe measurements. The insight gained is then applied to monolayer MoS<sub>2</sub>, and MoS<sub>2</sub> and WS<sub>2</sub> flakes.<br/><br/>Firstly, we find that PL enhancement is not dependent on the superacidic nature of TFSA, as MoS<sub>2</sub> and WS<sub>2</sub> luminescence both improved considerably following treatment with non-acidic analogues. This contradicts earlier studies, which claimed that Brønsted superacidity of TFSA was crucial for good enhancement of TMDCs, particularly MoS<sub>2</sub> [1, 3, 9]. Secondly, we test the hypotheses of recent computational reports that the strong enhancement following superacid treatment is achieved through fragmentation of TFSA <i>in situ</i>, releasing SO<sub>2</sub> which either ‘repairs’ sulfur defects or fills vacancies with free oxygen or sulfur [6, 10]. Our experimental results show that sulfur vacancy occupation by sulfur or oxygen does not explain the increase in photoluminescence following TFSA treatment, as enhancements arise from treatments with analogues which lack sulfonyl functionality. Passivating solutions are investigated by NMR which shows no evidence for superacid (or analogue) dissociation in solution prior to interaction with the surface.<br/><br/>An additional aim of this work is to address practical considerations for using this class of treatments in TMDC-based optoelectronics. Previously reported treatments involve immersion in a TFSA solution, generally followed by processing at elevated temperatures [1-3, 6]. Treatment solutions primarily incorporate 1,2-dichloroethane (DCE) [1, 3-5], the use of which is problematic due to its carcinogenic, flammable, toxic and mutagenic nature, and the necessity to avoid the use of DCE at industrial scale [11]. In our study we show that it is possible to enhance TMDCs effectively with a room temperature approach, whilst also avoiding DCE. The enhancement we achieve at room temperature using alternative solvents (~43x for monolayer MoS<sub>2</sub> and up to ~53x for few-layer MoS<sub>2</sub> flakes depending on processing conditions) exceeds that of many elevated temperature studies employing TFSA-DCE [2-4], providing a more realistic route for implementation of these processes in future device manufacturing.<br/><br/><b>References</b><br/>1. M. Amani et al.,<i> Science</i> <b>350</b>, 6264, 1065-1068 (2015).<br/>2. Y. Kim et al.,<i> Nanoscale</i> <b>10</b>, 18, 8851-8858 (2018).<br/>3. D. Kiriya et al.,<i> Langmuir</i> <b>34</b>, 35, 10243-10249 (2018).<br/>4. M. R. Molas et al.,<i> Scientific Reports</i> <b>9</b>, 1989 (2019).<br/>5. M. Amani et al.,<i> ACS Nano</i> <b>10</b>, 7, 6535-6541 (2016).<br/>6. S. Roy et al.,<i> Nano Letters</i> <b>18</b>, 7, 4523-4530 (2018).<br/>7. J. Bullock et al.,<i> ACS Applied Materials & Interfaces</i> <b>8</b>, 36, 24205-24211 (2016).<br/>8. A. I. Pointon et al.,<i> Solar Energy Materials & Solar Cells</i> <b>183</b>, 164-172 (2018).<br/>9. H. Lu et al.,<i> APL Materials</i> <b>6</b>, 6, 066104 (2018).<br/>10. C. Schwermann et al.,<i> Physical Chemistry Chemical Physics</i> <b>20</b>, 25, 16918-16923 (2018).<br/>11. D. Prat et al.,<i> Green Chemistry</i> <b>18</b>, 288-296 (2016).