Single molecules under high pressure

Single-molecule Raman spectra were studied at high pressure (1-4 GPa) in a diamond-anvil cell (DAC) with an Ar hydrostatic pressure medium, with the intent of resolving the different pressure-induced vibrational blueshifts of individual molecules. The molecules were two isotopologues of the dye rhodamine 6G (R6G and d₄-R6G), adsorbed on colloidal Ag particles immobilized in poly(vinyl alcohol) (PVA). Surface-enhanced Raman (SERS) ensemble measurements were compared to single-molecule surface-enhanced Raman (SMSERS) measurements made in a confocal Raman microscope. Spectra of mixed isotopologues in the 610 cm^-1 region (the "isotope-sensitive" transition) allowed us to identify when the majority of spectra came from single-isotope sites, and were thereby statistically likely to arise from single molecules. There was a dramatic drop in SERS intensity when samples were pressurized in the DAC. SMSERS measurements revealed the intensity drop was caused by a pressure-induced destruction of SMSERS-active hot spots. A hot spot is a site with ultrahigh Raman enhancement containing at least one R6G molecule. The hot spots that were not destroyed had large enhancement factors. The disappearance of hot spots was attributed to deoptimization of the gap junctions between Ag nanoparticles due to pressure-induced strain. Because the isotope-sensitive transition had little pressure-induced blueshift (<5 cm^-1 between 0 and 6 GPa), we also studied a transition near 1650 cm^-1 (the "pressure-sensitive" transition), which had a >30 cm^-1 blueshift and an approximate doubling of line width in the 0-6 GPa pressure range. The single-molecule spectra of this transition did not broaden as pressure was increased to 4.1 GPa. However, there was a variation in the blueshift of different molecules. The fwhm of the blueshift variation was able to account for most or all of the observed pressure-induced spectral broadening. The pressure-induced broadening of this R6G vibrational transition is due to the different blueshifts of different molecules.