Resonant Raman scattering in CdS_xSe_1−xnanocrystals: effects of phonon confinement, composition, and elastic strain

Abstract

Optical phonon modes, confined in CdS x Se 1−x nanocrystal (NC) quantum dots (≈2 nm in radius) grown in a glass matrix by the melting-nucleation method, were studied by resonant Raman scattering (RRS) spectroscopy and theoretical modeling. The formation of nanocrystalline quantum dots (QDs) is evidenced by the observation of absorption peaks and theoretically expected resonance bands in the RRS excitation spectra. This system, a ternary alloy, offers the possibility to investigate the interplay between the effects of phonon localization by disorder and phonon confinement by the NC/matrix interface. Based on the concept of propagating optical phonons, which is accepted for two-mode pseudo-binary alloys in their bulk form, we extended the continuous lattice dynamics model, which has successfully been used for nearly spherical NCs of binary materials, to the present case. After determining the alloy composition for NCs (that was evaluated with only 2-3% uncertainty using the bulk longitudinal optical phonon wavenumbers) and the NC size (using atomic force microscopy and optical absorption data), the experimental RRS spectra were described rather well by this theory, including the line shape and polarization dependence of the scattering intensity. Even though the presence of a compressive strain in the NCs (introduced by the matrix) masks the expected downward shift owing to the phonons' spatial quantization, the asymmetric broadening of both Raman peaks is similar to that characteristic of NCs of pure binary materials. Although with some caution, we suggest that both CdSe-like and CdS-like optical phonon modes indeed are propagating within the NC size unless the alloy is considerably heterogeneous.

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

“…[7][8][9][10][11][12][13] Raman spectroscopy has often been employed to study semiconductor nanostructures, 14 including colloidal nanocrystals (NCs). [15][16][17][18] In addition to fundamental knowledge about the elementary excitations, 19,20 the phonon spectra provide information on chemical composition, [21][22][23][24] strain, 24 and coupling to the environment. [25][26][27] Raman spectroscopy was applied to study interdiffusion at core/shell interfaces of spherical CdSe/CdS and CdSe/ZnS NCs and nanorods.…”

Resonance Raman scattering in bulk 2H-MX2 (M = Mo, W; X = S, Se) and monolayer MoS2

“…This resonance Raman scattering creates an important avenue to studies of band/excitonic excitations in diverse nanomaterials. [34][35][36][37] It also offers unique insight into multiphonon process due to the relaxation of selection rules and a huge amplification of Raman intensity under the resonance condition. Fortunately, the direct gaps of all the above MX 2 bulk crystals and few-layer flakes ($1.5 eV-2.5 eV) 38 overlap the photon energies of visible light, which enables our studies of resonance Raman scattering in these materials by using visible laser excitations.…”

Carrier Dynamics and Magneto-Optical Properties of Cd1-xMnxS Nanoparticles

“…1a) was a consequence of the sp-d exchange interactions between electrons confined in dot states and those located in the partially filled Mn 2+ states. This explanation is reasonable since replacing Cd 2+ with Mn 2+ ions should increase the energy gap of Cd1-xMnxS QDs [18]. In addition, it is interesting to note the weak sp-d exchange interaction in the Cd1-xMnxS bulklike NCs because their OA band remains in an almost fixed position (~2.58 eV).…”

“…Figure 1a shows that the undoped CdS QDs (x = 0.000) exhibit confinement energy ( conf E ) as indicated by the OA band peak at ~3.10 eV. From this value and using a confinement model based on effective mass approximation [12,[15][16][17][18], the mean QD radius R was estimated by the expression: Econf = Eg + (ħ 2 π 2 ⁄ 2 R 2 ) -1.8(e 2 ⁄ εR), where Eg is the bulk material energy gap, is the reduced effective mass, e is the elementary charge, and ε is the dielectric constant. From this, a mean radius of about R~2.0 nm was estimated for the CdS QDs, thus confirming strong size quantum confinement [16].…”

“…The dot size increase for increasing annealing time is a well known phenomenon governing the growth kinetic of nanoparticles in glass matrices [18,27,28]. However, a recent study of thermal treatments on undoped CdSe QDs [12] embedded in this same glass matrix (SNAB) showed a much smaller redshift (~0.03 eV) when annealed for 6 h. Since CdSe and CdS structures display great similarities, as well as Cd1-xMnxS with dilute Mn-concentration, it is reasonable to assume that they have the same growth kinetic in the same glass matrix.…”