Yingnan Wang

Size-dependent composition and molar extinction coefficient of PbSe semiconductor nanocrystals.

Atomic compositions and molar extinction coefficients of PbSe semiconductor nanocrystals were determined by atomic absorption spectrometry, UV-vis-NIR spectrophotometry, and transmission electron microscopy. The Pb/Se atomic ratio was found to be size-dependent with a systematic excess of Pb atoms in the PbSe nanocrystal system. Experimental results indicated that the individual PbSe nanocrystal was nonstoichiometric, consisting of a PbSe core and an extra layer of Pb atoms. For these nonstoichiometric PbSe semiconductor nanocrystals, we proposed a new computational approach to calculate the total number of Pb and Se atoms in different sized particles. This calculation played a key role on the accurate determination of the strongly size-dependent extinction coefficient, which followed a power law with an exponent of approximately 2.5.

Facile fabrication of faceted copper nanocrystals with high catalytic activity for p-nitrophenol reduction

This article reports a reproducible and facile approach to synthesize faceted copper nanocrystals (Cu NCs) using an inexpensive copper oxide as a precursor. By simply prolonging the reaction time, Cu cubes and polyhedrons were successfully produced, and the mean size could be effectively controlled in the range of 9 to 21 nm. The catalytic activities of the Cu cubes and polyhedrons were investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4. The kinetics of the reduction reaction at different temperatures were investigated to determine the activation parameters. Our investigations indicate that Cu nanocubes exhibit higher catalytic activity than Cu polyhedrons, which can be ascribed to three features: the higher surface-to-volume ratio, the higher surface energy of the {100} facet, and the lower redox potential. In addition, these catalysts can be easily recycled with a slight decrease of the catalytic activities, and are stable in the air. Therefore, this facile route provides a useful platform for the fabrication of Cu catalysts which have the potential to replace noble metals for certain catalytic applications.

Stability study of PbSe semiconductor nanocrystals over concentration, size, atmosphere, and light exposure.

Infrared-emitting PbSe nanocrystals are of increasing interest in both fundamental research and technical application. However, the practical applications are greatly limited by their poor stability. In this work, absorption and photoluminescence spectra of PbSe nanocrystals were utilized to observe the stability of PbSe nanocrystals over several conventional factors, that is, particle concentration, particle size, temperature, light exposure, contacting atmosphere, and storage forms (solution or solid powder). Both absorption and luminescence spectra of PbSe nanocrystals exposed to air showed dependence on particle concentration, size, and light exposure, which caused large and quick blue-shifts in the optical spectra. This air-contacted instability arising from the destructive oxidation and subsequent collision-induced decomposition was kinetically dominated and differed from the traditional thought that smaller particles with lower concentrations shrank fast. The photoluminescence emission intensity of the PbSe nanocrystal solution under ultraviolet (UV) exposure in air increased first and then decreased slowly; without UV irradiation, the emission intensity monotonously decreased over time. However, if stored under nitrogen, no obvious changes in absorption and photoluminescence spectra of the PbSe nanocrystals were observed even under UV exposure or upon being heated up to 100 degrees C.

A one-step green route to synthesize copper nanocrystals and their applications in catalysis and surface enhanced Raman scattering

A nontoxic, simple, inexpensive, and reproducible strategy, which meets the standard of green chemistry, is introduced for the synthesis of copper nanocrystals (Cu NCs) with olive oil as both reducing agent and capping agent. By changing the reaction parameters, the shape, size and surface structure of the Cu NCs can be well controlled. The obtained Cu nanocubes show excellent catalytic properties for the catalytic reduction of dyes and CO oxidation. Moreover, the prepared Cu nanocubes as substrates exhibit surface enhanced Raman scattering (SERS) activity for 4-mercaptopyridine (4-Mpy). Therefore, this facile route provides a useful platform for the fabrication of Cu NCs which have the potential to replace noble metals for certain applications.

Recent advances in IV–VI semiconductor nanocrystals: synthesis, mechanism, and applications

This review is focused on the recent developments of the synthesis, mechanism and applications of IV–VI semiconductor nanocrystals (NCs), including germanium-, tin- and lead-based chalcogenides NCs. First of all, we systematically introduce a series of investigations on the preparation with controllable size and shape via a wide variety of methods. Corresponding growth mechanisms are also discussed. Moreover, the promising potential of IV–VI semiconductor NCs as building blocks with respect to energy, sensors and catalysis is highlighted. For the purpose of enhancing the performance to satisfy the practical applications, tailored nanocomposites by combining noble metals or graphene etc. are further developed. Finally, we present some concluding remarks and perspectives for future developments. We hope this article can provide researchers with the key snapshots of the recent advances and the future challenges, thus achieving a great progress in IV–VI semiconductor NCs.

Size-Dependent Temperature Effects on PbSe Nanocrystals

An investigation show that the temperature-induced band gap (E(g)) variation of PbSe nanocrystals is strongly size-dependent. The temperature coefficients (dE(g)/dT) evolve from negative to zero and then to positive values, with the increase of PbSe nanocrystal sizes. Such phenomena imply that PbSe nanocrystals may be the potential candidate as sensitive temperature markers. Additional analyses disclose that the molar extinction coefficients of PbSe nanocrystals remain unchanged in the investigated temperature range (25-120 degrees C).

One-step solution synthesis of bismuth sulfide (Bi2S3) with various hierarchical architectures and their photoresponse properties

In this paper, we introduce a facile and phosphine-free one-step solution method to synthesize size- and shape-controlled bismuth sulfide (Bi2S3) with hierarchical architectures. Changing variables, such as the reaction temperature, the ratio of precursors, and the concentration of oleic acid were observed to influence the resultant shape of Bi2S3 microstructures. For the formation of Bi2S3 hierarchical architectures, the crystal splitting growth mechanism played the dominant role. The absorption spectra were recorded at room temperature, which revealed that the obtained Bi2S3 product was a direct band gap semiconductor and the band gap Eg was estimated to be about 1.9 eV. Furthermore, the I–V characteristics of the Bi2S3-based device show a significant increase by ca. 1 order of magnitude compared with the dark state, indicating an enhanced conductivity and high sensitivity. The response and decay times are estimated to be about 0.5 and 0.8 s, respectively, which are short enough for it to be an excellent candidate for high-speed and high-sensitivity photodetectors or optical switches. Thus the Bi2S3 hierarchies as building blocks may offer the potential for monolithic, low-cost and large-scale integration with CMOS electronics.

Solution synthesis of copper selenide nanocrystals and their electrical transport properties

In this paper, we developed a one-pot solution strategy to synthesize copper selenide NCs with controllable shape and structure. By changing the precursors in the reaction, copper selenide NCs (Cu2−xSe nanoparticles, nanorods and CuSe nanoplates) with various morphologies could be achieved. We proposed a possible mechanism to explain the influence of precursors on the shape of copper selenide NCs and we found that the chemical activities of precursors played key roles in the morphologies and crystal structures of the final products. Moreover, the electrical transport properties of as-prepared products were investigated. The morphologies of copper selenide NCs have a great influence on the electrical transport properties. The copper selenide NCs with nanorods display the best electrochemical performance compared with the other two types. We believe that copper selenide NCs would be promising candidates for electrical transport materials.

Facile Assembly of Size- and Shape-Tunable IV—VI Nanocrystals into Superlattices

In comparison to the previous lengthy approaches, we described a general and simple strategy for engineering the superlattice assembly of IV-VI semiconductor nanocrystals (NCs) with tunable sizes and morphologies. Not only the well-studied spherical NCs but also some special-shaped NCs, such as the quasi-cubic, cubic, truncated octahedral, and octahedral, could self-assemble into well-ordered patterns, as demonstrated in PbS, PbSe, and PbTe. These results extended our proposed model about the configuration of ligand chains in the superlattice assembly. This powerful capability of assembling superlattices was dominated by a heat-treatment process, providing a significant and extensive direction in the engineering of morphology-tunable NC superlattices.

Size-Controlled Synthesis of Bifunctional Magnetic and Ultraviolet Optical Rock-Salt MnS Nanocube Superlattices

Wide-band-gap rock-salt (RS) MnS nanocubes were synthesized by the one-pot solvent thermal approach. The edge length of the nanocubes can be easily controlled by prolonging the reaction time (or aging time). We systematically explored the formation of RS-MnS nanocubes and found that the present synthetic method is virtually a combination of oriented aggregation and intraparticle ripening processes. Furthermore, these RS-MnS nanocubes could spontaneously assemble into ordered superlattices via the natural cooling process. The optical and magnetic properties were investigated using measured by UV-vis absorption, photoluminescence spectra, and a magnetometer. The obtained RS-MnS nanocubes exhibit good ultraviolet optical properties depending on the size of the samples. The magnetic measurements suggest that RS-MnS nanocubes consist of an antiferromagnetic core and a ferromagnetic shell below the blocking temperatures. Furthermore, the hysteresis measurements indicate these RS-MnS nanocubes have large coercive fields (e.g., 1265 Oe for 40 nm nanocubes), which is attributed to the size and self-assembly of the samples.