Phonon Confinement and Related Effects in Three-Dimensional Assemblies of Cubic Cadmium Selenide Quantum Dots Synthesized by Conventional Chemical and Sonochemical Routes

Abstract

The effects of phonon confinement and exciton−phonon coupling (EPC) in three-dimensional (3D) assemblies of CdSe quantum dots with zincblende structure, synthesized by colloidal chemical and sonochemical routes as well as in postdeposition thermally treated samples, were studied by resonant Raman scattering (RRS) technique. Combining RRS results with those from structural analysis based on X-ray diffraction and optical spectroscopy, insights into the factors determining the intricate trends in the positions of bands due to firstand higher-order vibrational transitions upon crystal size reduction were gained. The band shifts with respect to the bulk material were found to result from subtle balance between phonon confinement effects and lattice contraction due to internal strain. Accurate assignments of the surface optical (SO) phonon modes were made on the basis of the dielectric continuum model of Ruppin and Englman. The magnitude of EPC (measured, e.g., through the Huang−Rhys parameter) and its size dependence in the synthesized 3D QD assemblies were studied as well. It was found that the EPC in the studied low-dimensional material appears to be much weaker than in the case of the corresponding bulk specimen, but a decreasing trend was observed upon postdeposition thermal annealing treatment, which is accompanied by an average crystal size increase. In strongly confined nanocrystal assemblies, it is larger than in those with weaker confinement effects. The relative importance of the two EPC mechanisms, the long-range one (Froḧlich interaction) vs the short-range one (due to the optical deformation potential) was discussed and related to the appearance of bands due to transverse optical (TO) phonon modes in the studied material. Combination modes involving acoustic phonons and the 1LO mode were observed for the first time in 3D QD assembly deposited in thin film form.

Sonochemically Synthesized Quantum Nanocrystals of Cubic CuInS2: Evidence for Multifractal Surface Morphology, Size-Dependent Structure, and Particle Size Distribution

“…Another notable feature of the measured spectral dependencies of the absorption coefficients in the cases of both asdeposited and thermally treated materials is the absence of clearly resolved excitonic peaks. Similar observations have been reported in previous studies by our [60][61][62][63][64][65][66]117,118 and other groups. 11−18 We rationalize these observations by the following reasoning.…”

“…In the present study, we have utilized a colloidal chemical approach − to synthesize thin films of band gap tunable self-assembled zinc blende CuInS 2 nanocrystals (NCs) onto glass substrates (i.e., microscope glasses with standard dimensions). Prior to the deposition process, the substrates were subjected to appropriate treatment − in order to improve the heterogeneous nucleation process and, eventually, the adhesion of film to the substrate surface.…”

“…In the present study, we have utilized a colloidal chemical approach − to synthesize thin films of band gap tunable self-assembled zinc blende CuInS 2 nanocrystals (NCs) onto glass substrates (i.e., microscope glasses with standard dimensions). Prior to the deposition process, the substrates were subjected to appropriate treatment − in order to improve the heterogeneous nucleation process and, eventually, the adhesion of film to the substrate surface. The deposition process was carried out in a single pot with a multicomponent aqueous reaction system, containing CuSO 4 and In 2 (SO 4 ) 3 of ultrapure analytical quality as metal ionic species precursors; Na 2 S 2 O 3 used in a double role, as a reducing and complexion agent; and finally thiourea, which has been utilized as sulfide anion source.…”

Effect of Phosphine-Free Selenium Precursor Reactivity on The Optical and Vibrational properties of Colloidal CdSe Nanocrystals

“…5a. These two components can be assigned to surface optical (SO) and longitudinal optical (LO) phonons in agreement with previous observations for NCs [16]. To evaluate the LO and the SO phonons, the elemental Raman bands are fitted by a sum of several Lorentzian peaks as shown in Fig.…”

Whispering Gallery Mode Enabled Efficiency Enhancement: Defect and Size Controlled CdSe Quantum Dot Sensitized Whisperonic Solar Cells

“…In order to estimate the lattice contraction induced blue shift ] in LO frequency, the details obtained from XRD is fed in the following equation 35 , 39 , 43 , where γ is the Gruneisen parameter and is the lattice contraction which also depends on size and temperature. By substituting the value of Gruneisen parameter γ = 1.1 for CdSe, = 210 cm −1 at 300 K temperature 35 , 43 , and the = −0.0010 estimated from XRD for 5.2 nm size QD, we find that the is ~0.7 cm −1 . Such finding implies that contribution to the shift is −3.5 cm −1 with overall red shift −2.8 cm −1 (Table 2 ).…”