Giant Chemical Surface Enhancement of Coherent Raman Scattering on MoS₂

Abstract

Raman spectroscopy is a powerful tool for molecular chemical analysis and bioimaging, which shows an astonishing sensitivity when combined with a huge enhancement by the coherence and surface effects. Noble metal nanoparticles have been commonly used for the spontaneous surface-enhanced Raman scattering (SERS) and for the surface-enhanced coherent anti-Stokes Raman scattering (SECARS) spectroscopies, as they provide large enhancement factors via the electromagnetic and chemical mechanisms. Recently, two-dimensional (2D) semiconductors, such as monolayer molybdenum disulfide (MoS2), were used for potential SERS applications as cheaper substrates compared to noble metal nanoparticles. However, the coherent enhancement of SECARS on 2D materials has not been previously explored. Here we present the experimental SECARS measurements of pyridine–ethanol solutions containing 2D MoS2 nanocrystals with the giant chemical enhancement factor of 109 over coherent anti-Stokes Raman scattering (CARS), which is attributed to the charge transfer states and resonant MoS2 excitation. As a comparison, the SERS signals on MoS2 using incoherent nonresonant excitation show at least 2 orders of magnitude smaller enhancement. Time-resolved SECARS measurements directly reveal the increased vibrational dephasing rates, which provide strong evidence for the charge transfer in the pyridine–ethanol–MoS2 system.

Multi-pulse laser-induced bubble formation and nanoparticle aggregation using MoS2 nanoparticles

“…The presence of a direct excitonic transition 32 in MoS 2 allows for the enhancement of nonlinear optical phenomena, making it an attractive candidate as a substrate for optical studies. MoS 2 nanoparticles in solution have also been used to enhance nonlinear optical phenomena as they were employed to enhance the coherent anti-Stokes Raman scattering intensity of the surrounding pyridine medium, with a calculated nine orders of magnitude enhancement of the Raman excitation 33 . MoS 2 is also nontoxic in bulk form, shown previously by Weng et al 34 , making it suitable for optical study of biological samples.…”

Synergy of B4C@MoS2 hybrid for significantly improved mechanical and tribological properties of PI/PTFE fabric composites

“…31 The A 1g mode at 707 and 1071 cm À1 were ascribed to the icosahedral breathing modes of the B 4 C. 32 For MoS 2 , the peak at 378 cm À1 corresponded to 2g vibrational mode, which involved in-layer vibration of Mo and S atoms; and the 403 cm À1 peak represented A 1g vibrational mode, which revealed the out-of layer vibrations of S atoms along the c axis. 33,34 As for the B 4 C-PDA/PEI and B 4 C@MoS 2 , there were two wide peaks about 1375 and 1590 cm À1 , which corresponding to the stretching vibrations of indole and pyrrole rings in PDA. 35 Both of the peaks about B 4 C-PDA/PEI and MoS 2 could also be discovered in Raman curve of B 4 C@MoS 2 hybrid, indicating the B 4 C@MoS 2 hybrid was exactly prepared.…”

Spatially Resolving the Enhancement Effect in Surface-Enhanced Coherent Anti-Stokes Raman Scattering by Plasmonic Doppler Gratings

“…This is particularly important for plasmon-enhanced nonlinear light–matter interactions involving coherent interaction of multiple beams at different frequencies. Coherent anti-Stokes Raman scattering (CARS) is one such nonlinear light–matter interaction. − In the simplest case of two-color CARS, coherent interaction of two input beams at the frequencies of the pump and the Stokes results in one output beam at the anti-Stokes frequency. In a classical model, the enhancement in the CARS signal compared to spontaneous Raman scattering is due to the coherent enhancement, i.e.…”

“…, the coherent population of molecules to a specific vibrational state defined by the energy difference between the pump and Stokes beams . In addition to this coherent enhancement effect, the CARS signal can be further increased by chemical enhancement of the substrate and surface enhancement effect from random or well-designed nanostructures. ,− ,− In this work, we focus on the surface enhancement effect of the plasmonic nanostructures in surface-enhanced coherent anti-Stokes Raman scattering (SECARS). The coherent enhancement and chemical enhancement involved in the CARS process are not directly relevant to the main observation and thus not discussed in this work.…”