Phonon Spectra of Small Colloidal II-VI Semiconductor Nanocrystals

Abstract

Resonant Raman spectroscopy has been employed to explore the first-and higher-order phonon spectra of several kinds of II-VI nanocrystals (NCs), with the aim of better understanding of the nature of phonon modes and forming a unified view onto the vibrational spectrum of semiconductor NCs. Particularly, besides the previously discussed TO, SO, LO, and 2LO modes, the combinational modes of TO+LO and SO+LO can be assumed to account for the lineshape of the spectrum below 2LO band. No trace of 2TO or 2SO band was detected, what can be the result of the dominance of Fröhlich mechanism in electron-phonon coupling in II-VI compounds. The resonant phonon Raman spectrum of NCs smaller than 2 nm is shown to be dominated by a broad feature similar to the SO mode of larger NCs or phonon density of states of a bulk crystal.The understanding of the phonon spectrum of semiconductor nanocrystals (NCs), electron-phonon coupling (EPC), dependence of the phonon spectra on the NC size, surfacebound moieties, and extended environment is of a large fundamental and application significance [1][2][3][4][5][6][7][8][9][10][11] due to the determinant role phonons play in the optical and electrical properties of semiconductor nanostructures [12,13]. Though a number of in-depth studies have addressed the phonon spectra of colloidal [14][15][16][17][18][19][20], glass-embedded [21][22][23] NCs, and epitaxial nanostructures [24][25][26], a significant divergence of the results exists concerning both the nature of the modes and their response to such factors as size, surface conditions, properties of the hosting medium and so forth. In particular, the apparently similar Raman spectra of a wide range of compounds (e.g., II-VI, III-V, and IV) and NC morphologies (dots, rods, tetrapods, wires, and discs) have been assigned differently.Recently, attempts were made to build a general picture of the phonon spectrum of various semiconductor NCs, based on analysis of the experimental Raman data and appropriate models [14,27,28]. This paper further explores the first-and higher-order phonon spectra of several kinds of II-VI NCs, as well as their similarity to other compounds, with the aim of better understanding of the vibrational spectrum of small NCs. High-quality NC samples were selected for this study in order to resolve Raman features, which are usually smeared by size dispersion, structural disorder, or low signal-to-noise ratio.The NC samples studied in this work have been prepared by means of colloidal chemistry according to protocols reported elsewhere [29][30][31][32], and CdSe, CdS, and CdTe NC samples of the same mean diameter (∼4 nm) were selected. Another sort of NCs studied were ultrasmall NCs of about 1.8 nm mean diameter, which revealed distinct Raman spectra. The attempts to measure the size and shape of these NCs directly by electron microscopy were unsuccessful [19], obviously due to ultrasmall NC size and charging of the stabilizing polymer. The broad X-ray features of these ultrasmall NCs [3] indicated their small size or/and n...

Morphology-induced phonon spectra of CdSe/CdS nanoplatelets: core/shell vs. core–crown

“…27 This could be a plausible assumption for CdSe NCs, because the strongest features of elemental or amorphous Se is known to peak near 250 cm −1 . 16 However, an analogous shoulder was reported for CdS and CdTe NCs, 50 but the Raman features of Te are not above but below the LO feature of CdTe, 51 and the elemental sulphur peaks are also far from the CdS LO position. 52 The lower-frequency shoulder of the LO peak can be fitted with two components (Fig.…”

“…3a), reported recently also for spherical CdSe NCs. 50 The combination modes will be discussed in more detail also later in this manuscript.…”

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

“…Besides electric field-enhancement, the reorientation of the electric field vector perpendicular to the surface allows access to highly confined phonon modes not visible in standard Raman spectroscopy. Despite extensive research on Raman scattering of nanocrystals [10][11][12][13][14][15], there has been little focus on the surface-enhanced Raman scattering (SERS) of colloidal quantum dots [16][17][18]. Until now no reports exist of SERS of individual semiconducting nanoparticles.…”

Copper Incorporation into CdS Thin Films by Ionic Exchange in an Aqueous Solution Process at Room Temperature

“…On the contrary, Raman bands located at 344, 377, and 560 cm −1 are the results from the active participation of the multiphonon scattering processes E 2 (m) or some combination these modes. In this sense, the asymmetry in the high frequency side of the LO optical mode can be denoted as HFS (high-frequency shoulder) which assumes the combination of the Raman signals located at 344 and 377 cm −1 , respectively, as proposed in the earlier literature [31,35,36]. Similarly, the low-frequency asymmetry of the 2LO mode (344 cm −1 ) can be assigned to combination of (LO + E 2 ) and/or (LO + E 1 /TO) modes, and similar assignments have been found in the literature for the SO mode, derived from the sum of frequency and shape [36].…”