Dielectric Effects on Exciton Emission of MoS2/P(VDF-TrFE) Heterostructure—A Correlated Mapping Study

Abstract

Abstract Body: Semiconducting two-dimensional (2D) materials possess intriguing optoelectronic properties owing to the unique layered confined structure, such as strong light-matter interaction and large exciton binding energies. Especially, their excitonic binding energies far exceeding thermal energy will allow for room-temperature optoelectronic applications, such as excitonic computing applications and optical interconnects. It is therefore of significant importance to engineer excitonic properties and dynamics. The most widely used approaches to modulate the semiconducting 2D materials' excitonic dynamics include modifying the dielectric environment,[1] applying strain,[2] and gate-tuning.[3] In this study, we study modulation of exciton dynamics in monolayer MoS2 via dielectric Coulomb engineering and local strain by correlating photoluminescence (PL) mapping and scanning probe microscopy characterization. A mixed-dimensional heterojunction (MDHJ) was fabricated consisting of monolayer MoS2, sub-μm 0-dimensional P(VDF-TrFE) copolymer islands, and hBN. An optical microscope image, AFM topography map, and side view schematic of the MDJS are showing in Figures 1a, 1b, and 1c, respectively. Photoluminescence (PL) maps were obtained; A and B exciton emissions were fitted to Gaussians for parameter extraction (Figure 2a and 2b) and correlated with the topography data (Figure 2c). A clear blueshift of A exciton emission energy was observed on 1L-MoS2 on P(VDF-TrFE) islands with respect to 1L-MoS2 on hBN (Figure 2b). MoS2 on the polymer islands (red scatter plot; draped over polymer islands) showed a noticeable blueshift of ~3 meV compared to that of planar MoS2 on hBN (blue scatter plot) (Figure 2c). The bandgap increase can be attributed to the band structure renormalization and binding energy decrease due to the strong dielectric screening of the P(VDF-TrFE) substrate (εr ≈ 13).[1] Effects of dielectric environment and local strain on excitonic dynamics were examined by analyzing the A and B exciton intensity ratio; a high A-to-B intensity ratio is a sign of low nonradiative recombination.[4] Regions of increased A/B ratio (up to 2x) correlate well with the P(VDF-TrFE) islands locations (Figure 3a). Suppressed B exciton from the MoS2/P(VDF-TrFE) region contributes to the majority of the A/B ratio enhancement. Moreover, the B intensity showed a negative correlation with height, suggesting that large substrate screening combined with higher strain from the morphology enhances the intravalley scattering from the B exciton state to the A exciton state (Figure 3b). The A and B exciton energy difference map (Figure 3c) demonstrates that in contact with P(VDF-TrFE), the spin-orbit coupling-induced valence band splitting reduces, which may lead to an increased intravalley scattering rate. The correlation of PL and topography map revealed that the substrate dielectric screening and local strain affect the exciton dynamics of monolayer MoS2. [1] A. Raja et al., "Coulomb engineering of the bandgap and excitons in two-dimensional materials," Nat. Commun., vol. 8, no. May, p. 15251, May 2017. [2] A. Castellanos-Gomez et al., "Local Strain Engineering in Atomically Thin MoS 2," Nano Lett., vol. 13, no. 11, pp. 5361–5366, Nov. 2013. [3] Z. Qiu et al., "Giant gate-tunable bandgap renormalization and excitonic effects in a 2D semiconductor," Sci. Adv., vol. 5, no. 7, p. eaaw2347, Jul. 2019. [4] K. M. McCreary, A. T. Hanbicki, S. V. Sivaram, and B. T. Jonker, "A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers," APL Mater., vol. 6, no. 11, p. 111106, Nov. 2018.