Optical phonon modes in ordered core-shell CdSe/CdS nanorod arrays

We investigate the optical phonons in “dot-in-a-rod” core–shell CdSe/CdS nanorods at wavelengths resonant with the optical transitions either in the core or in the shell. At a wavelength above the CdS band gap, only CdS phonon modes were detected. In contrast, at excitation in resonance with the core transitions, we observed phonon modes of both the CdSe core and the CdS shell. In laterally ordered nanorod assemblies, the CdS longitudinal-optical phonon mode manifested a low-energy shoulder that could be related to higher-order longitudinal-optical phonon modes. Furthermore, we report on surface-optical modes that originate from the tracklike superstructure of the nanorod assemblies.

Figure 1

(a) Scanning electron microscopy (SEM) image of nanorod tracks formed by drop deposition of the nanorod solution on a Si substrate followed by solvent evaporation under ambient conditions. (b) SEM image of disordered nanorods.

Figure 2

Absorption and emission spectra of CdSe/CdS nanorods recorded from solution. Gaussian fits to the first absorption peaks from the CdSe core and CdS shell are shown by the dotted lines. The two excitation wavelengths for the Raman experiments (λ1 = 458 nm and λ2 = 514 nm) are marked by the vertical lines. The inset illustrates the dot/rod core–shell architecture and the related band structure of the nanorods. At a wavelength of 514 nm, only transitions related to the CdSe are possible.

Figure 3

Raman spectra recorded from different nanorod films at excitation wavelengths of (a) 458 nm and (b) 514 nm. (a) The excitation corresponds to transitions of the CdS shell, and solely the CdS phonon modes can be observed from the ordered, disordered, and CdS nanorod films. (b) For a laser wavelength of 514 nm, in resonance with the core transitions, the CdSe LO phonon can be observed from the core–shell rods. In particular, additional SO and LO phonon modes in the CdS band appear in the spectrum recorded from the nanorod tracks. The arrow labeled SO${}_{\mathrm{rods}}$ indicates the calculated energy for the fundamental SO phonon mode in nanorods with an aspect ratio of 1/8 (see Eq. (2) and data on CdSe in Ref. 14).

Figure 4

Lorentzian fits (red and blue dashed-dot) to the LO phonon modes and Gaussian fits (black dotted) to the Raman spectrum (open circles) recorded from core–shell nanorod tracks at an excitation wavelength of 514 nm [the same as in Fig. 3b]. The resulting fit to the data is shown by the solid (red) line. The obtained peak positions are 300 cm${}^{-1}$ for CdS(LO${}_1$), 291 cm${}^{-1}$ for CdS(LO${}_2$), 267 cm${}^{-1}$ for CdS(SO), 209 cm${}^{-1}$ for CdSe(LO${}_1$), 203 cm${}^{-1}$ for CdSe(LO${}_2$), and 191 cm${}^{-1}$ for CdSe(SO).

Figure 5

(a) Raman spectra recorded from core–shell nanorod tracks with light excitation at 514 nm for two perpendicular laser polarizations, parallel (solid line) and perpendicular (dotted line) to the long nanorod axis. The Raman spectra were normalized with respect to the amplitude of the CdS LO phonon peak. Clearly, the spectral shape depended on the polarization. The inset shows the signal amplitude at the onset of the PL peak. The displayed window corresponds to the spectral range of 557–542 nm. (b) Sketch of phonon potentials for the LO modes in nanorod homostructures with orbital quantum numbers $L$ = 2 and $L$ = 4. (c) Localization of the electrons (e, blue/light gray) and holes (h, red/dark gray) for an excitation wavelengths of 514 nm.

Figure 6

SO phonon dispersion versus the parameter $x/\pi$ that enters in Eq. (2). The two circles indicate the SO phonon energy for disordered samples (bottom left) and nanorod tracks (top right). The red wavy lines in the insets illustrate the SO phonon potential for a single rod (right) and a nanorod track (left), together with Eqs. (4) and (5) for $x$. In the tracks, $d$ + σ = $d$′.