A. E. Raevskaya

Resonant Raman scattering study of CdSe nanocrystals passivated with CdS and ZnS

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.

Catalysis of the Sodium Sulfide Reduction of Methylviologene by CuS Nanoparticles

Feasibility was demonstrated for the catalysis of the sodium sulfide reduction of methylviologene (MV2+) in aqueous solution using cupric sulfide nanoparticles. The catalytic activity of the nanoparticles depends on their size. The basic features were found for the formation of the MV+· radical–cation in the reduction of MV2+ by HS− anions in the presence of CuS nanoparticles as the catalyst. This is an equilibrium reaction.

Spectral and luminescent properties of ZnO–SiO2 core–shell nanoparticles with size-selected ZnO cores

Deposition of silica shells onto ZnO nanoparticles (NPs) in dimethyl sulfoxide was found to be an efficient tool for terminating the growth of ZnO NPs during thermal treatment and producing stable core–shell ZnO NPs with core sizes of 3.5–5.8 nm. The core–shell ZnO–SiO2 NPs emit two photoluminescence (PL) bands centred at ∼370 and ∼550 nm originating from the direct radiative electron–hole recombination and defect-mediated electron–hole recombination, respectively. An increase of the ZnO NP size from 3.5 to 5.8 nm is accompanied by a decrease of the intensity of the defect PL band and growth of its radiative life-time from 0.78 to 1.49 μs. FTIR spectroscopy reveals no size dependence of the FTIR-active spectral features of ZnO–SiO2 NPs in the ZnO core size range of 3.5–5.8 nm, while in the Raman spectra a shift of the LO frequency from 577 cm−1 for the 3.5 nm ZnO core to 573 cm−1 for the 5.8 nm core is observed, which can indicate a larger compressive stress in smaller ZnO cores induced by the SiO2 shell. Simultaneous hydrolysis of zinc(II) acetate and tetraethyl orthosilicate also results in the formation of ZnO–SiO2 NPs with the ZnO core size varying from 3.1 to 3.8 nm. However, unlike the case of the SiO2 shell deposition onto the pre-formed ZnO NPs, individual core–shell NPs are not formed but loosely aggregated constellations of ZnO–SiO2 NPs with a size of 20–30 nm are. The variation of the synthetic procedures in the latter method proposed here allows the size of both the ZnO core and SiO2 host particles to be tuned.

Preparation and optical properties of highly luminescent colloidal single-layer carbon nitride

Thermal treatment of graphitic carbon nitride (g-CN) in aqueous solutions of tetraethyl ammonium hydroxide at ∼100 °C yields transparent colloidal solutions retaining stability at a CN concentration of up to 50 g L−1 and upon dilution by a factor of 103. Atomic force microscopy showed that the major part of the CN particles in diluted colloidal solutions are characterized by a lateral size of around 30–50 nm and a thickness of 0.3–0.4 nm typical of a single CN layer. Besides, a small fraction of multi-layer 2–5 nm thick g-CN particles with a size of ∼60 nm is present in the solutions. Photoexcitation of the colloidal CN near the edge of the absorption band results in emission of strong broad-band photoluminescence with a maximum at 460–470 nm and a quantum yield of 45–50%.

Optical study of CdS- and ZnS-passivated CdSe nanocrystals in gelatin films

CdSe nanocrystals were synthesized in aqueous solutions using gelatin as a stabilizer. An appreciable improvement of photoluminescence efficiency of nanocrystal-containing polymeric films was achieved by passivation of asformed nanocrystals in solutions with CdS and ZnS. The passivation-induced variation of the photoluminescence efficiency and its maximum position was found to be dependent on the total volume of the passivating material and the sequence of the reagents involved. The photoluminescence of both unpassivated and passivated nanocrystals was noticeably (∼200 meV) red-shifted from the first absorption maxima. A possible origin of the emission observed is discussed.

Non-stoichiometric Cu–In–S@ZnS nanoparticles produced in aqueous solutions as light harvesters for liquid-junction photoelectrochemical solar cells

A direct “green” aqueous synthesis of mercapto acetate-stabilized copper indium sulfide (CIS) nanoparticles (NPs) and core/shell CIS@ZnS NPs of a varied composition under ambient conditions and a temperature lower than 100 °C is reported. The CIS@ZnS NPs can be anchored to the surface of nanocrystalline FTO/TiO2 films without additional purification or ligand exchange steps yielding visible-light-sensitive heterostructures ready for using as photoanodes in the liquid-junction solar cells. The highest photoelectrochemical activity in a three-electrode cell was demonstrated by a TiO2/CIS@ZnS heterostructure with atomic Cu : In : S and Zn : Cu ratios of 1 : 5 : 10 and 1 : 1. The optimized TiO2/CIS@ZnS photoanodes were tested in two-electrode solar cells with aqueous polysulfide electrolyte and TiO2/Cu2S heterostructures produced by a photo-assisted method as counter-electrodes. Under illumination by a 30 mW cm−2 xenon lamp, the optimized cells showed the average light conversion efficiency of 8.15%, the average open-circuit voltage of −0.6 V and the average fill factor of 0.42. The cells revealed excellent stability and reproducibility of photoelectrochemical parameters with around one percent variation of the light conversion efficiency around an average value for six identical solar cells.

Interplay of factors affecting Raman scattering in cadmium chalcogenide nanocrystals in dielectric media

Micro-Raman spectra of CdS and CdSe nanocrystals embedded in different dielectric media (borosilicate glass, gelatin, polyacrylamide, polyvinyl alcohol) are studied. The observed Raman features are discussed with the account of confinement-related selection rules relaxation, scattering by surface phonons, and host matrix pressure. For nanocrystals grown in borosilicate glass a pronounced resonant dependence of surface phonon frequency on the excitation wavelength is observed. The effect of the excitation wavelength is much stronger than that of the nanocrystal size or the dielectric matrix type.

Spectral and Luminescent Characteristics of Products from Exfoliation of Graphitic Carbon Nitride Produced at Various Temperatures

It was shown that graphitic carbon nitride (CN) produced at 500–725 °C can be exfoliated in aqueous solutions of tetraethylammonium hydroxide with the formation of stable colloids characterized by photoluminescence (PL) in the visible region of the spectrum. It was established that increase of temperature leads to decrease of the intensity of PL until it disappears completely at T > 675 °C. In the case of exfoliated CN the intensity of PL increases sharply with increase of temperature. This may be due to decrease in the size of the colloidal particles of CN, to the dynamics of internal conversion of electronic excitation in colloidal CN, and to the removal of peroxy groups from the core of the bulk CN in the course of exfoliation.

Optical Characteristics of Colloidal Nanoparticles of CdS Stabilized with Sodium Polyphosphate and Their Behavior during Pulse Photoexcitation

The optical and photophysical characteristics of quantum-dimensional particles of CdS stabilized in water (the oscillator force of the first exciton transition, the forbidden band width, the conduction and valence band potentials) were determined. It was established that pulse excitation of CdS nanoparticles gives rise to nonstationary bleaching of the colloid, due to the accumulation of excess electrons in the conduction band. Their amount and their consumption rate are determined by the nature and concentration of the electron donors and acceptors present in the system.

Temperature-dependent resonant Raman scattering study of core/shell nanocrystals

The results of a temperature-dependent resonant Raman scattering on optical phonons in colloidal 2-3 nm CdSe, CdSe/CdS and CdSe/ZnS nanocrystals in gelatin matrix are reported. Both the core- and shell-related phonon peaks reveal a common blue shift, narrowing and enhancement with lowering the temperature. Negligible temperature dependence of the electron-phonon coupling is deduced from a constant 2LO/LO intensity ratio. A narrowing of the core-related phonon peak upon passivation is assigned to a diminished scattering of the phonons at the core-shell interface, as compared to the free surface of CdSe NCs.