Probing Confined Phonon Modes in Individual CdSe Nanoplatelets Using Surface-Enhanced Raman Scattering

The phonon modes of individual ultrathin CdSe nanoplatelets are investigated using surface-enhanced Raman scattering in a tightly confined plasmonic geometry. The surface-enhanced Raman scattering spectra, taken on single nanoplatelets sandwiched between a gold nanoparticle and a gold surface, reveal a phonon doublet arising from oscillations perpendicular to and within the platelet plane. The out-of-plane mode cannot be observed with conventional Raman spectroscopy. The resulting strong electric field enhancements and the field vector reorientation within such nanometer-sized plasmonic gaps reveal otherwise hidden information deep into the Brillouin zone illuminating the vibrational properties of ultrathin materials.

Figure 1

(a) Individual CdSe platelet in a nanogap. (b)–(d) TEM of CdSe platelets with (b) three, (c) four, and (d) five atomic layers.

Figure 2

(a) SERS spectrum from single NPOM geometry (top, yellow) and corresponding bulk Raman spectrum (bottom, blue) of four-layer-thick CdSe platelets (SERS trace is offset and multiplied by 40 for clarity). (b) Field components $|{E_z|}^2$ (yellow, solid line) and $|E_x{|}^2$ (blue, dashed line) in the nanogap. Left graph is for 58° incidence (maximum illumination angle for $\mathrm{NA}=0.85$), right for normal incidence. Black dashed line indicates Raman excitation. (c) Magnified spectra of LO phonon mode for bulk Raman (blue, empty circles) and SERS (yellow, solid circles) with intensities normalized.

Figure 3

(a) SERS of CdSe nanoplatelets with different thickness: three, four, and five atomic layers. (b) Conventional Raman scattering of a bulk film of the same platelets. Spectra are averaged over $\sim \mathrm{ten}$ spectra from different nanoparticles and offset for clarity.

Figure 4

(a) Peak positions and (b) linewidths (FWHM) of SERS and bulk Raman spectra. (c) Phonon energies versus reduced wave vector (in units of the lattice constant). Yellow points are the out-of-plane mode with black curve fit to Eq. (1). Blue circles show the in-plane mode. Black triangles and dashed black curve are data points and fit for spherical nanocrystals from Hwang et al. [13].