Engineering Substrate Interactions for High Luminescence Efficiency of Transition‐Metal Dichalcogenide Monolayers

Abstract

4733wileyonlinelibrary.com the infl uence of substrates. [ 2 ] It has been reported that substrates may affect the luminescence effi ciency of the monolayers by inducing strain, doping, or dielectric screening. [ 2 a, e-g, 3 ] However, despite the recent progress, many important questions about the substrate effect have remained to be answered. For instance, while it is known that substrates could affect the luminescence effi ciency through multiple ways, there is no quantitative understanding for the effect of each mechanism and no knowledge on which mechanism could be dominant. More importantly, it is not clear how the effect of substrates might depend on the nature of the substrate and the physical features of the monolayers. Answers to these questions would provide useful guidance for the realization of optimal luminescence effi ciency through engineering the substrate effects. Here we quantitatively evaluate the effect of substrates on the luminescence effi ciency of monolayers MoS 2 , WS 2 , and WSe 2 and demonstrate strategies of substrate engineering to improve the effi ciency by orders of magnitude. We fi nd that the main effects of the substrate lie in doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations. The doping may be from substrate-borne water moisture and the substrate itself, the former of which is much stronger than the latter for WS 2 and MoS 2 but negligible for WSe 2 . Using proper substrates can substantially mitigate the doping effect on the photoluminescence (PL), such as mica for WS 2 and MoS 2 and hexagonal boron nitride (h-BN) or polystyrene (PS) for WSe 2 . The defect-assisted recombination depends on the interaction of the defects in the monolayer such as sulfur vacancies with the substrate and may be substantially suppressed by either removing the substrate or lowering the number of defects. In this work we largely ignore the optical resonance effects associated with the substrate's geometrical features. [ 4 ] Results and DiscussionWe start with comparing the PL of suspended MoS 2 , WS 2 , and WSe 2 monolayers to those of as-grown counterparts. The monolayers were synthesized on sapphire substrates using chemical vapor deposition (CVD) processes as described previously, [ 5 ] and the suspended monolayers were prepared by manually It is demonstrated that the luminescence effi ciency of monolayers composed of MoS 2 , WS 2 , and WSe 2 is signifi cantly limited by the substrate and can be improved by orders of magnitude through substrate engineering. The substrate affects the effi ciency mainly through doping the monolayers and facilitating defect-assisted nonradiative exciton recombinations, while the other substrate effects including straining and dielectric screening play minor roles. The doping may come from the substrate and substrate-borne water moisture, the latter of which is much stronger than the former for MoS 2 and WS 2 but negligible for WSe 2 . Using proper substrates such as mica or hexagonal boron nitride can substantially mitigat...

In Situ Monitoring of the Thermal-Annealing Effect in a Monolayer of MoS2

“…31 The astransferred film exhibits weak bonding with the substrate. 5 Figure 1a shows the optical image of a transferred MoS2 film on which spatially resolved Raman and PL measurements were performed in a marked area of 20 μm × 20 μm. The sample contains a visible long wrinkle (in darker color near the bottom of the marked area) and many less visible shorter ones, and a crack (in lighter color near the lower left corner of the marked area).…”

“…Numerous studies on the substrate supported transition metal dichalcogenide (TMD) films concerned the effects of different substrate types. [1][2][3][4][5] It was often implicitly assumed that the substrate effects depended solely on the substrate material and the film-substrate bonding was stationary, thus, the obtained results were representative of the specific substrate type. However, it has been shown that on the same substrate type variations in film morphology and film-substrate bonding strength can have major impacts on the material properties.…”

“…The polymer residues as well as adsorbed H2O and O2 are known to modify the optical and electrical properties of the film, such as the quenching of photo-generated excitons and the reduction of carrier mobility. 5,[23][24][25] Temperature dependent Raman scattering has been used to investigate the vibrational properties of both bulk and monolayer MoS2, and in general both E2g and A1g peaks exhibit redshift with increasing temperature. 2, [26][27][28] Besides the fundamental interest in the vibrational properties of the 2D material, we have shown that because of the expected nearly linear temperature dependence of the phonon frequencies for an idealistic 2D material in the elevated temperature region (above RT), the temperature dependent Raman study can be used as an effective probe to the film-substrate interaction.…”

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

“…Ultimately, the role of the dielectric environment and surface properties on excitons shall be unravelled in detail soon. Nevertheless, first studies indicate resonance shifts due to surface quality [51], strain and tensions in the material [38,52,53], and water moisture [54]. Others indicate that the excitonic resonance remains fixed even though the dielectric environ ment is altered [55].…”

Giant Gating Tunability of Optical Refractive Index in Transition Metal Dichalcogenide Monolayers

“…TMDC materials much 16 unless the changing may induce substantial change in the exciton spectral width or the doping to the materials. 46 More importantly, this result may open up a new age of field-effect photonics whose optical functionality can be electrically controlled in ways similar to that of stateof-art CMOS circuits. We also demonstrate by combining with the nano-photoic structure, the weak absorption nature of 2D TMDC could be overcome and lead to 40% absorption/reflection modulation.…”