Vibrational spectroscopy of single molecules/single entities without plasmonic enhancement

Single-molecule vibrational spectroscopy is typically carried out using surface-enhanced Raman scattering and related techniques that rely on plasmonic enhancement.While these techniques are powerful, the need for the molecule of interest to interact with a nanostructured metal surface is a drawback for many applications.This review summarizes methods that have been developed to enable vibrational spectroscopy at the single-molecule level without plasmonic enhancement, which fall into two general categories.Techniques that resolve the vibrational structure in electronic emission spectra are well established and can produce high quality spectra, but require cryogenic temperatures.Newer methods that utilize more than one excitation ield to couple vibrational excitation to a strong electronic transition are potentially applicable to roomtemperature materials and biological samples, but single-molecule sensitivity has been demonstrated in only a few systems to date.Potential future applications of both classes of techniques are discussed.