Sonochemically assisted colloidal route to CdSe quantum dot assemblies: an alternative way to further fine-tune the size-dependent properties

Abstract

Continuous-wave heterogeneous sonochemical method was developed to synthesize 3D assemblies composed by QDs of metastable (cubic) modification of CdSe with sphalerite structural type. 3D QD assemblies were deposited in thin film form and as bulk precipitates. Structure, surface morphology, optical absorption as well as charge carriers' and phonon confinement effects were studied in details. The average nanocrystal radius, calculated by the Scherrer and Williamson-Hall methods, diminishes from 2.7 nm for samples synthesized by conventional colloidal chemical route to 2.0 nm for samples synthesized by sonochemical route. Upon post-deposition thermal annealing this value increases to 12.0 nm. QD size decrease is followed by increase of the uni-directional lattice strain and dislocation density (as defined by Hirsch), as well as by decrease of the lattice constant value. Previous findings were justified by analysis of X-ray diffraction peaks shape, which were found to be dominated by the particle size-distribution, instead of non-uniform strain. The energy of the fundamental direct C v 8 ! C c 6 interband electronic transition, computed from optical absorption data within parabolic approximation for the dispersion relation, shifts from 2.67 eV for sonochemically deposited samples and 2.08 eV for chemically deposited ones, to 1.77 eV upon post-deposition thermal treatment (close to the bulk value of 1.74 eV). The type of ''interband'' transitions does not change upon dimensionality reduce. Enhanced band gap energy blue shift upon average QD size decrease is followed by oscillator strength increase. Such trends were rationalized in terms of 3D confinement effects on charge carriers' motion. Experimentally observed band gap energies agree very well with the predictions of the effective mass model of Brus. The next direct interband electronic transition, from the spin-orbit split component of the valence band to the conduction band, has been detected and analyzed. The 1LO bands in the Raman spectra of chemically and sonochemically deposited samples appear at 206 and 204 cm -1 , as compared to the bulk value of 210 cm -1 . The extent of homogeneous broadening of the corresponding band is larger in sonochemically deposited samples, indicating enhanced frequency of phonon-phonon collisions in smaller QDs. The observed frequency shifts are dominated by phonon-dispersion terms, instead of lattice contraction.

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.…”

“…Although the fundamental chemical principles behind the approach utilized in the present study are known, − the implementation thereof up to achievement of the final aimbuilding up a complex array of semiconductor QD solidsis far from a standard task. The complexity of the final material that needs to be synthesized in a predefined form, with predefined and controllable (tunable) physicochemical properties, implies by definition the complexity of the reaction system.…”

“…The complexity of the final material that needs to be synthesized in a predefined form, with predefined and controllable (tunable) physicochemical properties, implies by definition the complexity of the reaction system. On the basis of our previous experience in the field, − to efficiently synthesize the title material in a form that could enable utilization of size-quantization effects to tune and control its properties, we have relied on the colloidal precipitation method. We have developed such a method by careful planning of the reactor composition and conditions, on the basis of preliminary in-depth thermodynamic and kinetic analyses.…”

“…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.…”