Controllable Photoluminescence Modification of Monolayer Molybdenum Disulfide via Superacid Treatment and Atomic Layer Deposition of High-κ Dielectric Materials

Transition metal dichalcogenides (TMDCs) are an exciting class of two-dimensional (2D) materials that exhibit exceptional physical and chemical behaviour at the monolayer limit. ¹ Molybdenum disulfide (MoS₂) is a prototypical TMDC that has emerged as a leading candidate for inclusion in numerous optoelectronic technologies, owing to its novel optical properties. ² With a direct bandgap in the visible spectral range, monolayer MoS₂ emits a relatively strong photoluminescence (PL) signal. ³ The ability to control the PL character of MoS₂ is important for full realisation of its optoelectronic application. The PL spectrum of MoS₂ has been shown to be sensitive to a range of external treatments, including oxygen plasma exposure, ⁴ annealing, ⁵ laser irradiation, ⁶ superacid treatment, ⁷ and dielectric encapsulation. ⁸ Each treatment will induce a change in response, potentially enabling tuneable modification of the PL behaviour of MoS₂ via the choice of treatment.<br/><br/>In this work, we present controllable variation of the PL signal from chemical vapour deposition (CVD)-grown MoS₂ monolayer films. We demonstrate selective alteration of the MoS₂ PL intensity, bidirectional energy shift and spectral reshaping via superacid treatment or atomic layer deposition (ALD) of a high dielectric constant (high-<i>κ</i>) material. By submerging monolayer MoS₂ in a solution containing the superacidic bis-(trifluoromethanesulfonyl)amide (TFSA), we achieve improvement of the PL character, in agreement with previous reports.^{7, 9} We find that superacid treatment induces significant enhancement of the absolute MoS₂ PL intensity, as well as blueshift in the energy of the emission and narrowing of the dominant peak. Conversely, we reveal opposing changes to the MoS₂ PL signal result from deposition of a high-<i>κ</i> dielectric. Grown by ALD, an atop layer of hafnium oxide (HfO₂) or aluminium oxide (Al₂O₃) layer is shown to attenuate the strength of the PL emission from monolayer MoS₂, with an accompanying redshift and broadening of the PL spectrum also observed. Via PL mapping, we confirm the treatment-induced modifications of the PL intensity to be prevalent across the MoS₂ surface following both TFSA and ALD-dielectric treatments. We utilise Lorentzian deconvolution of the PL spectra, coupled with a correlative analysis of the characteristic Raman peaks, to attribute the varying PL changes to differing charge doping and strain effects.¹⁰ This work demonstrates facile control of the PL behaviour of CVD-MoS₂ monolayer films via application of an external chemical or dielectric treatment.<br/><br/>References<br/>1.X. Duan; C. Wang; A. Pan; R. Yu; X. Duan, Chemical Society Reviews <b>44</b>, 8859 (2015).<br/>2.B. Radisavljevic; A. Radenovic; J. Brivio; V. Giacometti; A. Kis, Nature Nanotechnology <b>6</b>, 147 (2011).<br/>3.A. Splendiani; L. Sun; Y. Zhang; T. Li; J. Kim; C.-Y. Chim; G. Galli; F. Wang, Nano Letters <b>10</b>, 1271 (2010).<br/>4.N. Kang; H.P. Paudel; M.N. Leuenberger; L. Tetard; S.I. Khondaker, The Journal of Physical Chemistry C <b>118</b>, 21258 (2014).<br/>5.H. Nan; Z. Wang; W. Wang; Z. Liang; Y. Lu; Q. Chen; D. He; P. Tan; F. Miao; X. Wang; J. Wang; Z. Ni, ACS Nano <b>8</b>, 5738 (2014).<br/>6.H.-J. Kim; Y.J. Yun; S.N. Yi; S.K. Chang; D.H. Ha, ACS Omega <b>5</b>, 7903 (2020).<br/>7.S.L. Pain; N.E. Grant; J.D. Murphy, ACS Nano <b>16</b>, 1260 (2022).<br/>8.S.Y. Kim; H.I. Yang; W. Choi, Applied Physics Letters <b>113</b>, 133104 (2018).<br/>9.M. Amani; D.-H. Lien; D. Kiriya; J. Xiao; A. Azcatl; J. Noh; S.R. Madhvapathy; R. Addou; S. Kc; M. Dubey; K. Cho; R.M. Wallace; S.-C. Lee; J.-H. He; J.W. Ager; X. Zhang; E. Yablonovitch; A. Javey, Science <b>350</b>, 1065 (2015).<br/>10.H. Kim; T. Lee; H. Ko; S. Kim; H. Rho, Applied Physics Letters <b>117</b>, 202104 (2020).