Huichao Zhang

Bicolor Mn-doped CuInS2/ZnS core/shell nanocrystals for white light-emitting diode with high color rendering index

We synthesized bicolor Mn-doped CuInS2 (CIS)/ZnS core/shell nanocrystals (NCs), in which Mn2+ ions and the CIS core were separated with a ZnS layer, and both Mn2+ ions and CIS cores could emit simultaneously. Transmission electron microscopy and powder X-ray diffraction measurements indicated the epitaxial growth of ZnS shell on the CuInS2 core, and electron paramagnetic resonance spectrum indicated that Mn2+ ions were on the lattice points of ZnS shell. By integrating these bicolor NCs with commercial InGaN-based blue-emitting diodes, tricolor white light-emitting diodes with color rendering index of 83 were obtained.

Effect of shell thickness on two-photon absorption and refraction of colloidal CdSe/CdS core/shell nanocrystals

Colloidal CdSe nanocrystals (NCs) are successively overcoated with CdS monolayers (MLs), and the effective two-photon absorption (TPA) coefficient and refractive index of this series of CdS/CdSe core/shell NCs are measured by a Z-scan technique at 800 nm wavelength. The TPA and nonlinear refraction are enhanced dramatically with the CdS shell growth towards 3 MLs, but decreased with the further shell growth. The effects of surface states, local field, and intrinsic piezoelectric polarization are discussed to explain the optical nonlinearity of the colloidal core/shell NCs.

Nonlinear optical response of semiconductor-nanocrystals-embedded photonic band gap structure

Colloidal CdSe/ZnS core/shell nanocrystals (NCs), which were dispersed in SiO2 sol, were utilized to fabricate a SiO2:NCs/TiO2 all-dielectric photonic band gap (PBG) structure. The third-order nonlinear refractive index (n2) of the PBG structure was nearly triple of that of the SiO2:NCs film due to the local field enhancement in the PBG structure. The photoinduced change in refractive index (Δn) could shift the PBG band edge, so the PBG structure would show significant transmission modification, whose transmission change was ∼17 folds of that of the SiO2:NCs film. Under excitation of a 30 GW/cm2 femtosecond laser beam, a transmission decrease of 80% was realized.

Temperature-dependent photovoltaic characterization of a CdTe/CdSe nanocrystal’s solar cell

An all-inorganic thin film solar cell was fabricated with colloidal CdTe and CdSe nanocrystals. Its temperaturedependent photovoltaic characterization was measured, and an open circuit voltage decay technique was used to determine the electron lifetime. The photovoltaic parameters, such as the exact carrier lifetime, were remarkably different between under low-temperature region and under temperature above 267 K. It is suggested that the temperature-dependent carrier conductivity results from a thermally-activated hopping process and this nanocrystals device is a kind of donor-acceptor solar cell.

A SERIES OF MONODISPERSE SILVER NANOPARTICLES BY A ONE-POT SYNTHESIS

A series of monodisperse Ag nanoparticles (Ag NPs), whose size can be tuned between 2 nm and 34 nm, have been successfully synthesized in one pot by controlling the growth time. The standard deviation of the small-size Ag NPs is 12% while that of the large-size ones is 15%. During the synthesis process, sodium borohydride and sodium dodecyl sulfate were used as reducing agent and surfactant, respectively, and the effect of the synthesis parameters on the size-tuning range and the size distribution of Ag NPs was studied to get the optimum growth condition. Moreover, surface enhanced Raman scattering (SERS) spectra were analyzed to explore the synthesis mechanism of Ag NPs.

Copper Acetate Etching of Colloidal CdSe Nanocrystals

Colloidal CdSe nanocrystals (NCs) were etched after Se/TBP and Zinc stearate/ODE were injected into the mixture of as-prepared CdSe NCs and Copper (II) acetate in ODE solvent. Spectroscopic and structural investigations demonstrate the etching process. Along with the etching time, both the absorption and photoluminescence (PL) spectra of etched NCs showed blue-shift while the transmission electron microscopy (TEM) images indicated that the size of the NCs became from 5.6nm to 2.6nm. X-ray diffraction (XRD) patterns suggested that no other clusters or core/shell NCs were formed in the etching process and inductively coupled plasma (ICP) data demonstrated that only selenium and cadmium comprised the etched NCs. Electronic paramagnetic resonance (EPR) spectra indicated the deoxidization of Cu2+ ions and suggested the etching mechanism through cation exchange process.

Alloying Buffer Layers in Colloidal CdSe/ZnS Core/Shell Nanocrystals

When a ZnS shell is coated onto a CdSe core, some non-radiative defects are formed with the relaxation of the strain induced by the large lattice mismatch between CdSe and ZnS even though there are Zn0.5Cd0.5Se or ZnSe buffer layers, as indicated by the decrease of photoluminescent (PL) quantum yield and the reverse evolution of temperature-dependent time-resolved PL decay. X-Ray photoelectron spectroscopy analysis reveals that these defects are induced by the formation of an interfacial alloy during the epitaxy process. These defects could be significantly suppressed if the ZnxCd1–xSeyS1–y alloy buffer layer is artificially introduced.