Time-Dependent Single-Molecule Raman Scattering as a Probe of Surface Dynamics

Abstract

Large spectral fluctuations, on a time scale of many seconds, are demonstrated in the surface-enhanced Raman scattering of single rhodamine 6G molecules adsorbed on silver nanocrystals and excited at 532 nm. These fluctuations are especially evident in the case of the vibrational bands at 614 and 774 cm -1 . A definite connection between spectral fluctuations and changes in the shape of the diffuse background emitted with the SERS spectrum is seen, pointing to the possible origin of both in a molecule-surface charge transfer phenomenon. The rate of spectral fluctuations is shown to depend linearly on laser intensity through a nonthermal mechanism. The rate of fluctuations can also be modified by changing the anion concentration in the solution on top of the silver colloids. Analysis of the results suggests that the fluctuations are due to motion of the adsorbed molecule on the surface, triggered by photodesorption events that are mediated by electron tunneling between the molecule and surface. Our results show that single-molecule Raman scattering can serve as a sensitive local probe for the dynamics of adsorbed molecules under ambient conditions.

Comment on “Scanning-probe Raman spectroscopy with single-molecule sensitivity”

“…3 of a paper by Weiss et al 12 The intensity fluctuations and small band displacements that occur over a measuring time of 650 s (note the weak laser power of 10 W/cm 2 ) do not result in a loss of the characteristic spectral fingerprint of the adsorbate. To point out the large difference between Weiss' and Neacsu et al's time series, we reproduced Neacsu et al's Fig.…”

“…6,12 Changes in the band positions which have been observed in SM SERS studies include a narrowing or splitting of bands 11 and slight shifts of ± 2-5 cm −1 of band positions (which by far exceeds the resolution of the instrument of 25 cm −1 employed by Neacsu et al). 6,12 It is important to notice that these band shifts occur simultaneously for several modes, i.e. the whole spectrum shifts slightly!…”

“…In general, SM SERS literature reports spectra that are clearly characteristic for the investigated molecule and thus allow identification of the adsorbed species, also at any time during serial acquisition. [6][7][8][9][10][11][12][13] Surface diffusion may indeed account for intensity fluctuations in SERS, where a rough surface provides a variety of different adsorption sites or hot spots for the molecule. 6,12 Changes in the band positions which have been observed in SM SERS studies include a narrowing or splitting of bands 11 and slight shifts of ± 2-5 cm −1 of band positions (which by far exceeds the resolution of the instrument of 25 cm −1 employed by Neacsu et al).…”

“…the whole spectrum shifts slightly! 11,12 Adding SM spectra (i.e. averaging over a time series) must lead to a spectrum which is similar to an "ensemble" spectrum, a spatial average over many molecules, for example, an adsorbate monolayer.…”

Silver Nanodisk Growth by Surface Plasmon Enhanced Photoreduction of Adsorbed [Ag+]

“…Plasmon resonances concentrate an incident electromagnetic field via near-field enhancement; this antenna effect is the source of surface enhanced Raman scattering (SERS). The intense Ag resonance allows single molecule observation in SERS, [1][2][3][4][5] enables coupled Ag particles to form a subwavelength waveguide, 6 and enables sensitive colorimetric DNA screening. 7,8 Henglein 9,10 has shown that the physical and chemical properties of finely divided, nm-sized Ag particles are strongly modified by the adsorption of nucleophilic species and that Ag particles catalyze thermal and optical electrochemical reactions.…”