Near-Field Vibrational Spectroscopy and Imaging of Chemical Species on Nanoparticles during Catalytic (de)Hydrogenation Michael J. Gordon, Dept. of Chemical Engineering University of California, Santa Barbara Interaction of laser light with free electrons in metallic nanostructures (also known as plasmonics) can be used to focus optical fields to dimensions far below the diffraction limit. These spatially-confined optical fields can be used to excite and detect molecular vibrations of chemical species on a surface when the field-enhancing optical antenna or “tip” is brought in close proximity (few nm). Excitation of molecular bonds by the probe light results in Raman scattering [an inelastic process] where some of the light leaving the surface has less energy; if one detects this energy deficit, chemical species on the surface can be unambiguously identified at nanoscale spatial resolutions. In this work, we combine near-field vibrational spectroscopy with plasma-based synthesis of nanoparticles (Pd, Pt, and Pt alloys) to investigate reaction mechanisms on nanoscale catalytic materials. Plasmonic coupling causes “enhanced” Raman scattering
laser light
Rayleigh scattering (elastic)
80
(a)simulation
70
Au tip
Raman scattering
60 50
inelastic = chemical info
40
E
30
k
optical field laser light
Microplasma synthesis of catalytic nanoparticles
z
enhancement
Enhanced vibrational signature of graphitic species on substrate surface
20
2 nm 10
x
glass substrate
16.3 nm
Pd nanoparticles
3200
Intensity (counts)
optical antenna
experiment
graphitic carbon
3000
G-band D-band
2800
tip engaged
2600 tip 1 µm away
2400 2200 500
1000
1500
2000 -1
Raman shift (cm )
1 mm
0.0 nm σ = 1.86 x 10 particles/cm
