Huijuan Zhou

Behind the weak excitonic emission of ZnO quantum dots: ZnO/Zn(OH)2 core-shell structure

The structure of ZnO quantum dots prepared via the wet chemical method was studied. By introducing an annealing treatment (150 °C–500 °C), we also investigated the effect of the change in the structure of the dots on their luminescence properties. Our studies revealed that the surface of the as-prepared dots is passivated by a thin layer of Zn(OH)2, thus, the dots consist of a ZnO/Zn(OH)2 core-shell structure. We present evidence that the weak excitonic transition of ZnO quantum dots is strongly correlated with the presence of the surface shell of Zn(OH)2. When Zn(OH)2 is present, the excitonic transition is quenched.

Photoluminescence of indium–oxide nanoparticles dispersed within pores of mesoporous silica

Photoluminescence study was performed on indium–oxide (INO) nanoparticles dispersed within pores of mesoporous silica, annealed at different temperatures (from 500 to 850 °C). It was found that, for the dispersed INO nanoparticles, there exists a broad luminescence band, consisting of three peaks at about 430, 480, and 520 nm, spanning the whole visible region, in contrast to the aggregates of INO nanoparticles which exhibit no luminescence. This band increases with rise of annealing temperature up to 650 °C, at which the band reaches maximum. When annealing temperature is 700 °C, the whole band redshifts and peaks at about 545 nm. Annealing at higher temperatures leads to additional redshift and decrease of the luminescence band. It has been shown that the luminescence in this study is associated with the size and the structure of the INO particles within pores of porous silica. The peaks at 430, 480, and 520 nm originate from amorphous INO particles about 2, 4, and 6 nm in diameter. The 545 nm peak is a...

Magnetic resonance investigation of Mn2+ in ZnO nanocrystals

Electron paramagnetic resonance measurements were carried out to probe the structure of Mn2+ in ZnO nanocrystals with different surface conditions, modified by an annealing process. Changes in the spectra by the annealing treatment indicate the existence of three Mn2+ centers. The first is a sextet signal SI with g=2.0028 and a hyperfine coupling constant |A|=74.6×10−4 cm−1 and is present in all samples. Its hyperfine splitting is similar to Mn-doped ZnO single crystals, and therefore it is assigned to isolated substitutional Mn2+ ions. The second spectrum SII with g=2.001 and |A|=89×10−4 cm−1 decreases upon annealing. It is attributed to Mn2+ in strongly distorted environment, i.e., Mn2+ in Zn(OH)2 which is present as a surface shell of the ZnO nanoparticles. The superimposed broad background is associated with Mn–Mn clusters or dipole interactions, and it increases in intensity upon annealing.

Synthesis and structure of indium oxide nanoparticles dispersed within pores of mesoporous silica

Abstract Nanosized indium oxide (In 2 O 3 ) particles, dispersed within pores of mesoporous silica, were synthesized by soaking and thermal decomposition of indium sulfate. The particles were characterized by transmission electron microscopy (TEM) and nitrogen sorption isotherms. Photoluminescence spectra of the samples annealed at different temperatures were measured. It was shown that the In 2 O 3 nanoparticles were isolated from each other and highly uniformly dispersed inside the pores of silica host, which were less than 8 nm in diameter. New photoluminescences were observed for the samples annealed at different temperatures.

Magnetic resonance studies on ZnO nanocrystals

ZnO nanocrystals with diameters ranging from 4 to 50 nm were prepared via a wet chemical method and post-growth annealing treatments. The electron paramagnetic resonance (EPR) spectra of the nanocrystals show the resonance of electron centers with g-value close to that of the shallow donors in bulk ZnO. The temperature dependence of the resonance, which can be measured in the nanocrystals up to room temperature, shows the behavior expected from K ċ P theory for shallow donors, i.e. it decreases with increasing temperature due to the shrinkage of the bandgap. With decreasing diameter of the nanocrystals, we observe an increase of the g-values which is explained in terms of the quantum size effect.

Correlation of Mn local structure and photoluminescence from CdS:Mn nanoparticles

The structure and luminescence properties of Mn2+ in CdS nanoparticles are studied. Electron paramagnetic resonance measurements show the existence of three distinct Mn2+ centers with different local structures in CdS nanocrystals: Mn ions substitutionally incorporated on Cd sites [Signal SI: g=2.0025 and a hyperfine interaction constant A of 64.6×10−4cm−1 (6.9mT)], Mn located near the surface [Signal SII: g=2.0013 and A=89×10−4cm−1 (9.5mT)], and a broad background signal (SIII) correlated to interacting Mn ions or Mn clusters. Luminescence studies indicate that only substitutional ions located in the core of the nanocrystals contribute to the typical T14 to A16 (2.12eV) emission. The colloidal preparation process of the nanoparticles allows selective incorporation of the Mn ions, i.e., core/shell doping of the nanoparticles. It shows that Mn located in the shell quenches the total luminescence via nonradiative processes.

Effect of the (OH) Surface Capping on ZnO Quantum Dots

ZnO quantum dots were prepared by the reaction ofZn 2+ with OH – – in alcoholic solution. By annealing (150–500 � C), both the particle size and the surface conditions could be changed intentionally. The structural properties of the dots were characterized by X-ray diffraction and Raman measurements, and the optical properties by photoluminescence. For the as-prepared dots, the excitonic recombination is very weak, while it becomes dominant for the annealed samples. We present evidence that the weak excitonic transition ofZnO quantum dots is strongly correlated with the presence ofthe surf ace (OH) groups.

Rapid deposition of high temperature YBa2Cu3O 7−x superconducting thin films directly on silver substrates

The superconducting YBa2Cu3O7−x (YBCO) films have been deposited directly on silver (Ag) substrates by metalorganic chemical vapor deposition. Critical current density (Jc) as high as 1.4×104 A/cm2 (0 T, 77.3 K) was obtained. The high Jc was attributed to the small, compact grains of the YBCO films.