We report unusual spectral features in the resonant Raman scattering spectra of colloidal CdSe nanoparticles as small as 2–3 nm. High-frequency shoulders of the longitudinal optical phonon peak and its overtones were observed and their dependence on the excitation wavelength, temperature, nanoparticle size, and surface passivation with ZnS shell studied. As the probable origin of the uncommon spectral feature the participation of acoustic phonons and manifestation of the density of surface-related vibrational states is discussed.

Semiconductor core–shell nanocrystals (NCs) have greatly improved luminescent properties including better resistance to photobleaching and ligand exchange. It was suggested that compound alloying at the core/shell interface could play an important role in obtaining bright and stable NCs. Here, we investigate the interface composition and strain evolution in spherical and dot-in-plate CdSe/CdS nanocrystals with shell thickness ranging from 1 to 3 nm, using a combination of Raman and infrared spectroscopy. A slower rate of strain accumulation in the core is observed for dot-in-plate nanocrystals and is linked to the anisotropic shape of the plate-like shell. We resolved the respective contributions of the core, shell, and alloyed interface and observed a drastic change of the shell-related Raman feature with the appearance of the bulk-like optical phonon in the shell thicker than 1 nm. The average composition of the alloy interface is estimated using the frequencies of the alloy modes. Because of the high crystallinity of the samples, up to fourth-order optical phonon processes are observed and analyzed. This work confirms the presence of an alloyed interface in core/shell CdSe/CdS structures of different geometries and establishes a precise roadmap for its quantitative analysis using vibrational spectroscopy.

CdSe nanocrystals (NCs) were obtained from cadmium sulfate and sodium selenosulfate in aqueous gelatin solutions. A near-bandgap emission of CdSe NCs was noticeably enhanced after passivation with CdS or ZnS. Resonant Raman scattering spectra of the passivated NCs revealed new peaks attributed to the formation of the sulfide shells around CdSe cores. The peaks observed for the CdSe/CdS core-shell NCs near 280 cm −1 were attributed to LO vibrations within a thin CdS passivating layer. Observation of the peak in the same frequency range for CdSe/ZnS is discussed within an assumption of alloying at the core-shell interface. Notable changes in the Raman spectra at different excitation wavelengths and shell parameters were attributed to the resonant and size-selective nature of the Raman process.

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidal CdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS is demonstrated by analysing the NP ensembles with different average size [Formula: see text] and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of [Formula: see text] and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS on surface optical phonons. From the experimentally observed small variation of the I(2LO)/I(LO) ratio for 2-5 nm NPs a minor effect of [Formula: see text] on the electron-phonon coupling strength in this [Formula: see text] range is supposed.

The effect of shells of various thicknesses on the vibrational resonant Raman spectra of CdSe/ZnS core-shell nanoparticles is studied. The dependence of the core-shell structure on the method of shell deposition is derived from a comparison of the vibrational and photoluminescence spectra of nanoparticles. Along with the appearance of peaks attributed to the shell, the phonon spectrum of the core undergoes significant changes upon shell growth. The change of the CdSe LO peak lineshape in core-shell nanoparticles is discussed with respect to possible changes in the spectrum of both optical and acoustical phonons upon shell formation. Based on the observed decrease of the CdSe 2LO/LO peak intensity ratio, a weakening of exciton coupling to the CdSe LO phonon upon ZnS shell deposition is supposed. The change in the carrier localization volumes upon shell formation is discussed as a possible reason for the reduced coupling.

We present a detailed analysis of Raman and infrared (IR) phonon spectra of strongly luminescent nonstoichiometric M−In−S (M = Cu, Ag, Hg) and core/shell M−In-S/ZnS nanocrystals (NCs) of small size (d ≈ 2−4 nm), formed by means of aqueous colloidal chemistry under mild conditions. Despite presumably similar factors determining position and broadening of the Raman and X-ray diffraction (XRD) peaks, phonon spectra are shown to be more sensitive to NC composition and crystals structure. The spectral Raman pattern of these strongly Mdeficient M−In-S NCs is different from that of the corresponding stoichiometric phases, e.g., CuInS 2 or AgIn 5 S 8 , and excludes its assignment to relevant binary sulfides, e.g., In 2 S 3 . Resonant behavior of relative peak intensities in Raman spectra is different from that of larger-size stoichiometric NCs and bulk samples studied before, while the temperature dependence reveals an analogous enhancement of the highest-frequency LO modes supporting an unambiguous assignment of the latter. Therefore, we conclude that the Raman spectra observed are characteristic of the specific structure of these highly nonstoichiometric small NCs. IR modes of these NCs occur in the same frequency range as the Raman ones but at higher frequencies than the IR phonons in bulk material. The IR spectra are less characteristic, compared to Raman ones, revealing more similarity among the three NC compounds and with the bulk counterparts.

We report on the synthesis and optical and structural characterization of ultrasmall (<2 nm) CdS nanoparticles with a narrow size distribution of 10% prepared in aqueous and alcohol solutions of polyethyleneimine (PEI). The PEI-stabilized CdS nanoparticles reveal a structured absorption band with the first excitonic maximum at 3.5 eV and broad-band photoluminescence with quantum yields of 12−14% in water, 18−20% in ethanol (80 vol %), and up to 60−70% in solid PEI films at room temperature. The nature of the photoluminescence was studied by using the time- and wavelength-dependent emission measurements. The role of precursor cadmium(II)−PEI complex in the formation of uniform and ultrasmall luminescent CdS nanoparticles, as well as the dynamic emission quenching by water, are discussed. A study of the photochemical properties of PEI-stabilized CdS nanoparticles both under continuous and nanosecond pulse illumination showed excellent stability of solid PEI films incorporating CdS nanoparticles toward UV illumination and reductive character of the CdS nanoparticle photocorrosion in aqueous solutions.

Recently developed two-dimensional colloidal semiconductor nanocrystals, or nanoplatelets (NPLs), extend the palette of solution-processable free-standing 2D nanomaterials of high performance. Growing CdSe and CdS parts subsequently in either side-by-side or stacked manner results in core-crown or core/shell structures, respectively. Both kinds of heterogeneous NPLs find efficient applications and represent interesting materials to study the electronic and lattice excitations and interaction between them under strong one-directional confinement. Here, we investigated by Raman and infrared spectroscopy the phonon spectra and electron-phonon coupling in CdSe/CdS core/shell and core-crown NPLs. A number of distinct spectral features of the two NPL morphologies are observed, which are further modified by tuning the laser excitation energy E between in- and off-resonant conditions. The general difference is the larger number of phonon modes in core/shell NPLs and their spectral shifts with increasing shell thickness, as well as with E. This behaviour is explained by strong mutual influence of the core and shell and formation of combined phonon modes. In the core-crown structure, the CdSe and CdS modes preserve more independent behaviour with only interface modes forming the phonon overtones with phonons of the core.