Single Molecules under High Pressure Yuanxi Fu and Dana D. Dlott* School of Chemical Sciences and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States ABSTRACT: 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 d 4 -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. 1. INTRODUCTION Molecular vibrations under high pressure generally exhibit blueshifts and broadenings. 1 Pressure-dependent blueshifts, which might better be thought of as density-dependent phenomena, arise from anharmonic coupling between intra- molecular vibrations and the environment. 1,2 Different vibra- tional transitions will have different anharmonic couplings to the environment, so the degree of blueshifting at a given pressure will be different for different vibrational transitions. When molecules are present in disordered media, different sites will have different anharmonic coupling strengths to the environment. For any particular vibrational transition, disorder will generate different blueshifts at every site, giving rise to pressure-dependent blueshift variations. Broadening originates from many sources, and although it is a substantial simplification, it is often satisfactory to describe broadening as homogeneous or inhomogeneous. 3,4 At ambient temper- ature, homogeneous broadening of vibrational transitions in solids arises primarily from “pure dephasing” caused by faster modulations of the vibrational frequency by the surround- ings. 3-7 Inhomogeneous broadening results from each molecule in the ensemble having different slower interactions with the surroundings; i.e., each molecule possesses a unique slowly changing or static structural environment. 3 By definition, single molecule Raman spectra should be homogeneously broadened. 8,9 In the present study, we used single-molecule surface-enhanced Raman scattering (SMSERS) 10-12 in a diamond-anvil cell (DAC) to investigate how these blueshifting and broadening processes are affected by increasing pressures up to 4 GPa, where volume compression is ∼25%. We show that individual molecules have different pressure shifts, and these differential pressure shifts are the primary cause of the ensemble-averaged pressure-induced line broadening. In these experiments, we used a well-studied system for SMSERS consisting of citrate-reduced colloidal Ag particles dosed with a probe dye molecule, rhodamine 6G (R6G). The samples were in the form of ∼200 × 200 × 20 μm 3 chips in the DAC, consisting of the dosed colloid suspended in a polymer matrix. The chips in the DAC were surrounded by supercritical Ar. This pressure medium produces hydrostatic compression up to 10 GPa. 13 The DAC was inserted either in a 532 nm Raman spectrometer for surface-enhanced Raman (SERS) ensemble studies or a 532 nm scanning Raman confocal microscope for SMSERS. A technique that helps ensure that SMSERS measurements are truly probing single molecules uses two isotopologues, R6G Received: December 24, 2014 Revised: February 14, 2015 Published: February 20, 2015 Article pubs.acs.org/JPCC © 2015 American Chemical Society 6373 DOI: 10.1021/jp512858u J. Phys. Chem. C 2015, 119, 6373-6381