Giant Chemical Surface Enhancement of Coherent Raman Scattering on MoS2
Additional Document Info
Copyright 2018 American Chemical Society. 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 (MoS 2 ), 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 MoS 2 nanocrystals with the giant chemical enhancement factor of 10 9 over coherent anti-Stokes Raman scattering (CARS), which is attributed to the charge transfer states and resonant MoS 2 excitation. As a comparison, the SERS signals on MoS 2 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-MoS 2 system.