4 research outputs found
Discriminating Nanoparticle Dimers from Higher Order Aggregates through Wavelength-Dependent SERS Orientational Imaging
Surface-enhanced Raman scattering (SERS) orientational imaging is a recently developed all-optical technique able to determine SERS-active silver nanoparticle dimer orientations by observing lobe positions in SERS emission patterns formed by the directional polarization of SERS along the longitudinal axis of the dimer. Here we extend this technique to discriminate nanoparticle dimers from higher order aggregates by observing the wavelength dependence of SERS emission patterns, which are unchanged in nanoparticle dimers but show differences in higher order aggregates involving two or more nanoparticle junctions. The ability of SERS orientational imaging to identify stacked nanoparticles in higher order aggregates is also demonstrated. The shape of the SERS emission patterns originating from trimers labeled with low and high concentrations of dye is investigated, showing that the emission pattern lobes become less defined as the dye concentration increases. Dynamic fluctuations in the SERS emission pattern lobes are observed in aggregates labeled with low dye concentrations, as molecules diffuse into regions of higher electromagnetic enhancement in multiple nanoparticle junctions
SERS Orientational Imaging of Silver Nanoparticle Dimers
This Article introduces surface-enhanced Raman scattering (SERS) orientational imaging as a powerful far-field optical technique for determining the in-plane and out-of-plane orientations of SERS-active nanoparticle dimers. Optical images of Rhodamine 6G (R6G) SERS emission patterns are measured and correlated with atomic force microscopy (AFM) images of the associated SERS-active silver nanoparticle dimers. The AFM is used to measure individual silver nanoparticle dimer orientations and height asymmetry, defining in-plane and out-of-plane angles associated with the dimer geometry. Theoretical emission pattern images based on these angles are generated using a simple dipole emission model and show excellent agreement with the experimental emission patterns. This technique provides a rapid all-optical technique to analyze the orientation of SERS active nanoparticle dimers
Super-Resolution SERS Imaging beyond the Single-Molecule Limit: An Isotope-Edited Approach
Super-resolution imaging of single-molecule surface-enhanced
Raman
scattering (SM-SERS) reveals a spatial relationship between the SERS
emission centroid and the corresponding intensity. Here, an isotope-edited
bianalyte approach is used to confirm that shifts in the SERS emission
centroid are directly linked to the changing position of the molecule
on the nanoparticle surface. By working above the single-molecule
limit and exploiting SERS intensity fluctuations, the SERS centroid
positions of individual molecules are found to be spatially distinct
Polarized Raman Spectroscopy of Oligothiophene Crystals To Determine Unit Cell Orientation
Raman spectra were recorded experimentally and calculated
theoretically
for bithiophene, terthiophene, and quaterthiophene samples as a function
of excitation polarization. Distinct spectral signatures were assigned
and correlated to the molecular/unit cell orientation as determined
by X-ray diffraction. The ability to predict molecular/unit cell orientation
within organic crystals using polarized Raman spectroscopy was evaluated
by predicting the unit cell orientation in a simulated terthiophene
crystal given a random set of simulated polarized Raman spectra. Polarized
Raman spectroscopy offers a promising tool to quickly and economically
determine the unit cell orientation in known organic crystals and
crystalline thin films. Implications of our methodologies for studying
individual molecule conformations are discussed