Aiwei Tang

Synthesis and Shape-Tailoring of Copper Sulfide/Indium Sulfide-Based Nanocrystals

Heterostructured Cu2S-In2S3 nanocrystals with various shapes and compositions were synthesized by a high-temperature precursor-injection method using the semiconductor nanocrystal Cu1.94S as a catalyst. The intrinsic cationic deficiencies formed at high temperature by Cu ions made the Cu1.94S nanocrystal a good candidate for catalyzing the nucleation and subsequent growth of In 2S3 nanocrystals, eventually leading to the formation of heterostructured Cu2S-In2S3 nanocrystals. Gelification of the reaction systems, which were composed of different types of nanocrystal precursors and solvent, was found to be a very effective measure for controlling the growth kinetics of the heterostructured particles. Consequently, matchsticklike Cu2S3-In2S3 heterostructured nanorods, teardroplike quasi-core/shell Cu2S@In2S3 nanocrystals, and pencil-like In2S3 nanorods were successfully obtained by manipulating the gelification of the reaction system; this formed a solid experimental basis for further discussion of the growth mechanisms for differently shaped and structured nanocrystals. By reaction with 1,10-phenanthroline, a reagent that strongly and selectively binds to Cu(+), a compositional transformation from binary matchsticklike Cu2S-In2S3 nanorods to pure In2S3 nanorods was successfully achieved.

Hybrid polymer-CdSe solar cells with a ZnO nanoparticle buffer layer for improved efficiency and lifetime

We report the use of a solution-processed ZnO nanoparticle buffer layer in hybrid solar cells based on blends of poly(3-hexylthiophene) (P3HT) and CdSe quantum dots. Depending on the size of the CdSe nanocrystal, these devices exhibit 20–70% higher photocurrent output than similar devices without the ZnO nanoparticle layer, which is attributed to a combination of electronic and optical effects. With negligible change in open-circuit voltage and a small increase in the fill factor, the power conversion efficiency of these P3HT:CdSe hybrid solar cells was increased by 30–80% with incorporation of the ZnO nanoparticle layer. The presence of the ZnO nanoparticle layer also drastically improves the stability of these hybrid solar cells. Less than 40% loss in efficiency was observed for such devices after 70 days of exposure to the laboratory ambient without any type of encapsulation.

One-pot synthesis and self-assembly of colloidal copper(I) sulfide nanocrystals

A simple one-pot method is developed to prepare size- and shape-controlled copper(I) sulfide (Cu(2)S) nanocrystals by thermolysis of a mixed solution of copper acetylacetonate, dodecanethiol and oleylamine at a relatively high temperature. The crystal structure, chemical composition and morphology of the as-obtained products are characterized by powder x-ray diffraction (PXRD), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The morphology and size of the Cu(2)S nanocrystals can be easily controlled by adjusting the reaction parameters. The Cu(2)S nanocrystals evolve from spherical to disk-like with increasing reaction temperature. The spherical Cu(2)S nanocrystals have a high tendency to self-assemble into close-packed superlattice structures. The shape of the Cu(2)S nanodisks changes from cylinder to hexagonal prism with prolonged reaction time, accompanied by the diameter and thickness increasing. More interestingly, the nanodisks are inclined to self-assemble into face-to-face stacking chains with different lengths and orientations. This one-pot approach may extend to synthesis of other metal sulfide nanocrystals with different shapes and sizes.

Seed-Mediated Growth of Anatase TiO2 Nanocrystals with Core–Antenna Structures for Enhanced Photocatalytic Activity

We demonstrate that anatase TiO2 nanocrystals composed of a nanocrystal core and nanorod antennas can be produced via a nonaqueous colloidal seed-mediated growth method. Anatase TiO2 nanocrystals with defined morphologies were first prepared as seeds, and then secondary anatase TiO2 nanorods were grown on the defined facets of the seeds under appropriate conditions. Systematic studies on the growth mechanism reveal that the formation of core-antenna nanocrystals involves an epitaxial growth process with specific orientational preference governed by both thermodynamic and kinetic factors. By manipulating the reaction conditions including the precursor amount and introduction rate, the epitaxial growth behavior can be well controlled. By further varying the morphology of seed nanocrystals, we have also been able to produce core-antenna anatase TiO2 nanocrystals with complex spatial configurations in a highly predictable manner. The high structural configurability and predictability offered by this seed-mediated growth method may provide great opportunities in enhancing the performance of TiO2-based nanostructures in many energy-related applications. As a demonstration, we show by simply manipulating the core-antenna structures that the photocatalytic activity of the anatase nanocrystals can be improved from the relatively less active seed nanocrystals or pure nanorods to the extent that exceeds the activity of the commercial P25 titania.

Synthesis, optical properties, and superlattice structure of Cu(I)-doped CdS nanocrystals

High quality Cu(I)-doped CdS [CdS:Cu(I)] nanocrystals were synthesized by thermolysis of metal acetylacetonate complexes in n-dodecanethiol. The optical emission of the doped nanocrystals was observed to change from surface trap-dominant emission to Cu(I)-dominant emission with respect to the reaction time and the doping levels of Cu(I) ions in CdS particles. The maximum photoluminescence quantum yields of the CdS:Cu(I) nanocrystals could reach 15.8%. More interestingly, these doped nanocrystals could self-assemble into highly ordered superlattices depending on the doping levels of the Cu(I) ions. A possible explanation for the formation of the CdS:Cu(I) nanocrystal superlattices is dipole–dipole interactions between the adjacent nanocrystals.

Synthesis and self-assembly of Cu1.94S–ZnS heterostructured nanorods

Cu1.94S–ZnS heterostructured nanorods are prepared by injecting Zn(acac)2-dodecanethiol solution into hot reaction systems containing Cu1.94S nanocrystals which catalyze the pyrolysis of Zn(acac)2 and the following epitaxial growth of ZnS on Cu1.94S seeds. Crystallographic analysis suggests that the built-in p–n type heterostructures allow the dipole moment vectors of both the Cu1.94S “head” and ZnS “tail” to overlap, consequently giving rise to unexpected zipper-like self-assembled structures formed by the heterostructured Cu1.94S–ZnS nanorods.

Optical properties and electrical bistability of CdS nanoparticles synthesized in dodecanethiol

We reported the synthesis of CdS semiconductor nanoparticles using a simple one-pot reaction by thermolysis of cadmium acetylacetonate in dodecanethiol. Optical measurements of the as-obtained CdS nanoparticles revealed that their optical properties were closely related to surface effects. Based upon the cocktail of poly(N-vinylcarbazole) (PVK) and CdS nanoparticles, a bistable device was fabricated by a simple solution processing technique. Such a device exhibited a remarkable electrical bistability, which was attributed to the electric field-assisted charge transfer between PVK and the CdS nanoparticles capped by dodecaethiol. The conduction mechanism changed from an injection-controlled current to a bulk-controlled one during switching from OFF-state to ON-state.

Facile One-Step Synthesis and Transformation of Cu(I)-Doped Zinc Sulfide Nanocrystals to Cu1.94S–ZnS Heterostructured Nanocrystals

A facile one-pot heating process without any injection has been developed to synthesize different Cu-Zn-S-based nanocrystals. The composition of the products evolves from Cu(I)-doped ZnS (ZnS:Cu(I)) nanocrystals into heterostructured nanocrystals consisting of monoclinic Cu1.94S and wurtzite ZnS just by controlling the molar ratios of zinc acetylacetonate (Zn(acac)2) to copper acetylacetonate (Cu(acac)2) in the mixture of n-dodecanethiol (DDT) and 1-octadecene (ODE). Accompanying the composition transformation, the crystal phase of ZnS is changed from cubic zinc blende to hexagonal wurtzite. Depending on the synthetic parameters including the reaction time, temperature, and the feeding ratios of Zn/Cu precursors, the morphology of the as-obtained heterostructured nanocrystals can be controlled in the forms of taper-like, matchstick-like, tadpole-like, or rod-like. Interestingly, when the molar ratio of Cu(acac)2 to Zn(acac)2 is increased to 9:1, the crystal phase of the products is transformed from monoclinic Cu1.94S to the mixed phase composed of cubic Cu1.8S and tetragonal Cu1.81S as the reaction time is further prolonged. The crystal-phase transformation results in the morphological change from quasi-spherical to rice shape due to the incorporation of Zn ions into the Cu1.94S matrix. This method provides a simple but highly reproducible approach for synthesis of Cu(I)-doped nanocrystals and heterostructured nanocrystals, which are potentially useful in the fabrication of optoelectronic devices.

Upconversion multicolor tuning: Red to green emission from Y2O3:Er, Yb nanoparticles by calcination

In this paper, we report the color tuning of Er and Yb codoped Y2O3 upconversion materials by calcination within a fixed doping concentration under the excitation of 980 nm semiconductor laser diode. By introducing the defects through use of a surfactant (cetyltrimethylammonium bromide) and removing the defects by changing the calcination temperature, the green emission (2H11/2, 4S3/2→4I15/2) and red emission (4F9/2→4I15/2) of Er ions were enhanced selectively, thus the color output could be tuned from red to green. It is expected that these color-tuned materials have great potential for applications in multiplexed labeling.

Investigation on Photovoltaic Performance based on Matchstick-Like Cu 2 S–In 2 S 3 Heterostructure Nanocrystals and Polymer

In this paper, we synthesized a novel type II cuprous sulfide (Cu2S)–indium sulfide (In2S3) heterostructure nanocrystals with matchstick-like morphology in pure dodecanethiol. The photovoltaic properties of the heterostructure nanocrystals were investigated based on the blends of the nanocrystals and poly(2-methoxy-5-(2′-ethylhexoxy)-p-phenylenevinylene) (MEH-PPV). In comparison with the photovoltaic properties of the blends of Cu2S or In2S3nanocrystals alone and MEH-PPV, the power conversion efficiency of the hybrid device based on blend of Cu2S–In2S3and MEH-PPV is enhanced by ~3–5 times. This improvement is consistent with the improved exciton dissociation or separation and better charge transport abilities in type II heterostructure nanocrystals.