Transition Metal Dichalcogenides Nanostructure/Graphene van der Waals Heterostructure for Surface Enhanced Raman Spectroscopy

Abstract

Two-dimensional (2D) Van der Waals (vdW) heterostructures of graphene and transition metal dichalcogenides (TMDs/Graphene) provide a promising new material platform since it integrates the superior light–solid interaction in TMDs and ultrafast charge mobility in graphene, and therefore is promising for surface-enhanced Raman spectroscopy (SERS). This thesis explores design, fabrication and application of new SERS substrates based on novel TMD (MoS2 and WS2) nanostructure/graphene vdW heterostructures. Specifically, the design is guided by enhancing electromagnetic mechanism (EM) through development TMDs nanostructure for a strong localized surface plasmonic resonance (LSPR) upon light illumination and chemical mechanism (CM) via facilitating charge transfer between probe molecules of 2D materials. In order to achieve high EM enhancement on the TMDs/graphene vdW SERS substrates, schemes in generating nanostructures of TMDs, including 1) nanodiscs (N-discs), 2) intermixed N-discs of MoS2 and WS2, 3) nanodonuts (N-donuts), and 4) their superposition with gold nanoparticles (AuNPs) have been explored. An extraordinary SERS sensitivity has been achieved on these substrates using fluorescent Rhodamine 6G (R6G) as probe molecules. In Topic 1, Novel SERS substrate has been developed based on non-metallic TMD N-discs on single-layer CVD graphene/SiO2/Si. This Substrate provided a high-performance SERS with EM and CM enhancement, both are associated with the strong dipole-dipole interaction at the heterostructure interface as indicated by the Density Function Theory (DFT) and ab-initio molecular dynamics simulation (AMID) simulations. Using fluorescent Rhodamine 6G (R6G) as probe molecules, an extraordinary SERS sensitivity up to 5X10-12 M was obtained on TMD N-discs/graphene vdW heterostructure substrate, using 532 nm Raman excitation. This sensitivity is 4-5 orders of magnitude higher than that of the single-layer MoS2, WS2 or graphene substrates and is comparable and slightly better than the best reported plasmonic metal nanostructures/graphene SERS substrates. The enhancement factors were calculated by comparing the intensity of the Raman feature peak of R6G at 613 cm-1 obtained on different substrates, enhancement factor of 7- 9 can be achieved on the MoS2 N-discs/graphene or WS2 N-discs/graphene vdW heterostructures with respect to the single-layer MoS2, WS2 or graphene reference. Importantly, SERS enhancement and sensitivity drop significantly when the TMD N-discs are replaced with a continuous TMD layer on graphene, indicating the TMD dimension and shape are highly relevant. Besides, the peak intensity of graphene’s Raman signature with TMD N-discs is enhanced by factor of 8-10 as compared to that of graphene only, demonstrating the LSPR effect provided by the TMD N-discs. In Topic 2, we explores a rationale design of intermixed WS2 N-discs and MoS2 N-discs on graphene (WS2 N-discs s+MoS2 N-discs/graphene) for ultrasensitive SERS beyond the sensitivity limit of the SERS substrates based on metallic plasmonic nanostructures. The intermixed WS2 N-discs +MoS2 N-discs/graphene allows superposition of the LSPR effects from the two types of plasmonic NDs. The enhanced SERS sensitivity is illustrated in the boosted graphene Raman peaks by approximately 14 fold on the WS2 N-discs+MoS2 N-discs/graphene, in contrast to ~ 7.6 fold on the counterparts with single types of the NDs. Furthermore, the SERS enhancement factors of R6G of 5x10-5 M concentration Raman spectra (normalized to that on graphene) are ~ 16.4 and 8.1 considering the R6G 613 cm-1 peak intensities were sensed on the WS2 N-discs+MoS2 N-discs/graphene and MoS2-N-discs/graphene (or WS2-N-discs/graphene), respectively. In addition, the WS2N-discs+MoS2 N-discs/graphene SERS substrate exhibits remarkably high SERS sensitivity as high as 5-7×10-13 M. In Topic 3, We also further explored the effect of the TMDs nanostructure shape on electromagnetic enhancement and SERS and we reported a controllable growth of MoS2 nanodonuts (N-donuts) and nanodiscs (N-discs) on graphene and show that the MoS2 N-donuts exhibits considerably higher LSPR sensitivity than the MoS2 N-discs. Using R6G as a probe, SERS spectra were compared on MoS2 N-donuts/graphene and MoS2 N-discs/graphene vdW heterostructures substrates. The former exhibits remarkably higher R6G SERS sensitivity up to 2×10-12 M, in contrast to 5×10-12 M on the MoS2 N-discs/graphene vdW heterostructures substrate, which can be attributed to the more robust LSPR effect by the finite difference time-domain simulation. In Topic 4, we explore superposition of LSPR effect of Au nanoparticles (AuNPs) with that on WS2 N-discs. The LSPR superposition is confirmed first when Raman signatures of graphene, such as the G-peak intensity got enhanced by approximately 7.8 fold on the AuNPs/WS2 N-Discs/graphene over that of reference graphene sample, in contrast to 4.0 and 5.3 folds respectively on AuNPs/graphene and on WS2 N-discs/graphene. Furthermore, Raman spectra of probe molecules of R6G were employed to quantify the enhanced SERS on AuNPs/WS2 N-discs/graphene SERS substrates. At the R6G concentration of 5x10-5 M, enhancement factors of ~ 2.0 and 2.4 based on the R6G 613 cm-1 peak intensity are observed on the AuNPs/WS2 N-discs/graphene with respect to that on WS2 N-discs/graphene and AuNPs/graphene, respectively. The benefit of the superposition of the LSPR effects from the WS2 N-discs and AuNPs results in high SERS sensitivity up to 1×10-12 M on AuNPs/WS2 N-discs/graphene, which is more than an order of magnitude higher than that on WS2 N-discs/graphene, and several orders of magnitude higher than that on the AuNPs/graphene and metal nanostructure/TMD (continuous layer) substrates.