electromagnetic enhancement (EM) and chemical enhancement (CM). The EM involves the local electromagnetic field enhancement that is typically attributed to the localized surface plasmonic resonance (LSPR) of free charge carriers at the surface of the metal nanostructures induced by the incident light. The LSPR wavelength is determined primarily by the free charge carrier concentration of the metal with a minor effect of the dimension and shape of the metal nanostructures. Molecules positioned [2] close to the LSPR nanostructures experience an enhanced evanescent electromagnetic field as compared to the incident excitation. This EM enhancement directly depends on the morphology of the metal surface, the wavelength of the incident light, and the dielectric constant of the surrounding medium of the metal. The EM enhancement factor can reach over 10 8 to enable ultrasensitive SERS detection down to the single-molecule level. [4][5][6] The CM is induced by the charge transfer between the SERS substrate and molecule with an enhancement factor typically on the order of 10 1 to 10 3 . [7][8][9] The CM effect is dictated by the interface electronic structures between the analyte and substrate and can be optimized by selecting a substrate with favorable band alignment with the highestoccupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) at the interface where the analyte (or probe molecule) bond to the substrate. Thus, tuning of the substrate electronic structure is important to an enhanced CM effect. [10] This has prompted intensive research exploring graphene-based SERS substrates considering the unique 2D atomically flat surface with delocalized π bonds, chemical inertness, biological compatibility, superior electronic and photonic properties, and the intrinsic Fermi energy at ≈4.5 eV that is compatible, as well as tunable, for CM enhancement with a large number of probe molecules. [7,8,11,12] Therefore, graphene is an excellent SERS substrate primarily due to the CM effect with the adsorbed molecules and the enhancement factor is quantitatively affected by the alignment of the probe molecule electronic structure with the Fermi level of graphene. [3,8] The EM and CM enhancement factors may be combined by adding metal nanostructures on graphene. [7,13] Since the Two-dimensional transition metal dichalcogenides (TMDs)/graphene van der Waals (vdW) heterostructures integrate the superior light-solid interaction in TMDs and charge mobility in graphene, and therefore are promising for surface-enhanced Raman spectroscopy (SERS). Herein, a novel TMD (MoS 2 and WS 2 ) nanodome/graphene vdW heterostructure SERS substrate, on which an extraordinary SERS sensitivity is achieved, is reported. Using fluorescent Rhodamine 6G (R6G) as probe molecules, the SERS sensitivity is in the range of 10 −11 to 10 −12 m on the TMD nanodomes/ graphene vdW heterostructure substrates using 532 nm Raman excitation, which is comparable to the best sensitivity reported so far using plasmonic metal nanostructures/graphene ...

A novel substrate consisting of a 2D MoS2/graphene van der Waals (vdW) heterostructure decorated with Au nanoparticles (AuNPs) was developed for surface-enhanced Raman spectroscopy (SERS). A transfer-free chemical vapor deposition process was employed for layer-by-layer fabrication of graphene, followed with MoS2 directly on wafers of SiO2/Si without any metal catalyst. AuNPs were deposited on the MoS2/graphene via in situ electron-beam evaporation of Au at an elevated temperature in the range of 300–350 °C under high vacuum. Rhodamine 6G (R6G) was used as an SERS probe molecule with a SERS sensitivity of 5 × 10–8 M using a nonresonance 633 nm laser, which is an order of magnitude higher than that reported on the AuNPs/graphene substrate using the same excitation. A higher SERS sensitivity of 5 × 10–10 M was obtained using resonance 532 nm laser excitation. The observed SERS sensitivity enhancement can be attributed to the combination of the electromagnetic mechanism of the plasmonic AuNPs and the chemical mechanism of the AuNPs/MoS2/graphene vdW heterostructure via enhanced interface dipole–dipole interaction as compared to graphene or MoS2 only as suggested by a density functional theory calculation. Therefore, this AuNPs/MoS2/graphene vdW heterostructure is advantageous to practical applications in optoelectronics and biosensing.

This work explores superposition of the localized surface plasmonic resonance (LSPR) effect of Au nanoparticles (AuNPs) with that on transition metal dichalcogenide (TMD) WS 2 nanodomes (WS 2 −NDs) enabled by enhanced dipole−dipole interaction at van der Waals (vdW) interfaces in AuNP/WS 2 −ND/graphene heterostructures for surface-enhanced Raman spectroscopy (SERS) with high-sensitivity, The confirmation of such a superposition is first demonstrated in the enhanced graphene Raman signatures, such as the G-peak intensity by approximately 7.8 fold on the AuNP/WS 2 −ND/graphene over that of reference graphene sample, in contrast to 4.0-and 5.3-fold, respectively, on AuNP/graphene and on WS 2 −ND/graphene. Furthermore, Raman spectra of probe molecules of fluorescent Rhodamine 6G (R6G) were hired to quantify the enhanced SERS on AuNP/WS 2 − ND/graphene SERS substrates. At the R6G concentration of 5 × 10 −5 M, enhancement factors of ∼2.0 and 2.4 based on the R6G 613 cm −1 peak intensity are detected on the AuNP/WS 2 −ND/graphene with respect to that on WS 2 −ND/graphene and AuNP/ graphene, respectively. The benefit of the superposition of the LSPR effects from the WS 2 −NDs and AuNPs results in high SERS sensitivity up to 1 × 10 −12 M on AuNP/WS 2 −ND/graphene, which is about an order of magnitude better than what's on WS 2 −ND/ graphene, and several orders of magnitude better than that on the AuNP/graphene and metal nanostructure/TMD (continuous layer) substrates. This result reveals the advantage of superposition of the LSPR effects from different nanostructures through design of vdW heterostructures. In addition, considering the AuNP/WS 2 −ND/graphene vdW heterostructures can be fabricated in the layer-by-layer growth developed in this work, the high-sensitivity SERS substrates are scalable and low cost for marketable devices in optoelectronics and biosensing.

This work explores a rationale design of intermixed WS2 nanodiscs (WS2NDs) and MoS2 nanodiscs (MoS2NDs) on graphene (WS2NDs+MoS2NDs/graphene) for ultrasensitive surface-enhanced Raman spectroscopy (SERS) beyond the sensitivity limit of the SERS substrates based on metallic plasmonic nanostructures. On the individual WS2NDs and MoS2NDs, localized surface plasmonic resonance (LSPR) is enabled by the dipole–dipole interaction at the WS2NDs/graphene and MoS2NDs/graphene van der Waals (vdW) interfaces. The intermixed WS2NDs+MoS2NDs/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 ∼14-fold on the WS2NDs+MoS2NDs/graphene, in contrast to ∼7.6-fold on the counterparts with single types of the NDs. Furthermore, the SERS enhancement factors of the test molecules (fluorescent Rhodamine 6G or R6G) of 5 × 10–5 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 WS2NDs+MoS2NDs/graphene and MoS2–NDs/graphene (or WS2NDs/graphene), respectively. In addition, the WS2NDs+MoS2NDs/graphene SERS substrate exhibits remarkably high SERS sensitivity as high as (5–7) × 10–13 M, which is about 2 or more orders of magnitude greater than that reported on the AuNPs/graphene and other plasmonic metal nanostructures/2D materials SERS substrates. This result demonstrates a promising bottom-up approach in designing high-sensitivity SERS substrates based entirely on 2D atomic materials.