Y. Y. Shan

Surfactant-assisted alignment of ZnO nanocrystals to superstructures.

Self-organization of ZnO nanoparticles into various superstructures (sheet, platelet, ring) has been achieved with the assistance of micelles formed by surfactant cetyltrimethylammonium bromide (CTAB) under one-pot condition. The CTAB-modified zinc hydroxy double salt (Zn-HDS) mesocrystals act as intermediates to form ZnO hexagonal superstructures at temperatures as low as 50 degrees C. The decomposition temperature of Zn-HDS mesocrystals is much lower than that of the corresponding bulk crystals because the organic additive CTAB effectively decreases the degree of crystallinity. Taking advantage of temperature-induced phase transformation of micelles, two-stage self-organization can form ZnO platelets and ring mesocrystals, that is, ZnO ellipsoidal superstructures formed through vertical attachment on (0001) facets of basic units can further assemble to form ZnO platelets and rings through vertical attachment on (0001) facets of ZnO ellipsoidal superstructures. The structural transformation of micelles as shape templates can offer a new route for self-assembly of nonspherical colloids into three-dimensional photonic crystals. ZnO sheet, ring, and platelet mesocrystals with a high population of polar Zn-(0001) plane are expected to have high photocatalytic activity.

One-dimensional zigzag gallium nitride nanostructures

Two one-dimensional (1D) single-crystalline gallium nitride (GaN) nanostructures with periodic zigzag (type I) and diameter-modulated (type II) shapes have been synthesized by passing through ammonia over a mixture of gallium and gallium oxide (Ga2O3) powders held at elevated temperature. The process was catalyzed by the dispersion of thio-capped Au nanoparticles on the substrate onto which GaN nanostructures were condensed. The transformation between these two nanostructure morphologies was also observed. A possible growth model for the zigzag-shaped nanostructures is proposed, in which the formation of the zigzag nanostructures results from the construction of two different nanoscale unit cells. This work provides an avenue to a group of 1D nanostructures with a zigzag shape. The possibility to form 1D nanostructures yet to be discovered by changing the stacking direction of the (0001) plane will facilitate the fabrication of nanoscale functional devices as well as our understanding of the growth behavi...

Wavelength-tunable lasing in single-crystal CdS1-XSeX nanoribbons

Alloyed ternary CdS1−XSeX nanoribbons of variable composition X were synthesized by the combination of thermal evaporation and laser ablation. High-resolution transmission electron microscopy and x-ray diffraction showed that the ternary CdS1−XSeX nanoribbons were single phase and highly crystalline. Room-temperature optical measurements showed that band-gap engineering could be realized in CdS1−XSeX nanoribbons via modulation in composition X. Lasing emission between the band-gap energy of CdS (512 nm) and that of CdSe (710 nm) was observed for composition 0<X<1 under optical pumping (266 nm) at power densities of 35–60 kW cm−2. Cathodoluminescence imaging and spectroscopy of single CdS1−XSeX nanoribbons reveal the uniform optical properties of the nanoribbons, which supports the absence of phase segregation within the nanoribbon. Fine tuning of the lasing wavelength via composition changes is shown to be smaller than 0.1 nm, and is capable of overlapping thermally induced tuning, demonstrating the possibility of continuous tuning in the lasing wavelength. The broad and fine tunable lasing properties of ternary nanoribbons have potential applications in color-tuned nanolasers, biological labels, and nano-optoelectronics.

Continuous near-infrared-to-ultraviolet lasing from II-VI nanoribbons

The authors show that II-VI nanoribbons are capable of room-temperature lasing covering the complete spectral range from near infrared (NIR) to ultraviolet (UV). This is accomplished by simply using nanoribbons of two ternary compositions, namely, CdSXSe1−X and ZnYCd1−YS. Under optical pumping, CdSXSe1−X nanoribbons lase from NIR (710nm) to green (510nm) as X changes from 0 to 1, while ZnYCd1−YS nanoribbons lase from green (510nm) to UV (340nm) as Y varies from 0 to 1. Furthermore, lasing control shows fine-tuning via composition changes that overlap thermally induced tuning. This demonstrates that II-VI materials can enable lasing at any selected wavelength between 710 and 340nm with continuous tuning capabilities.

Heteroepitaxial growth and optical properties of ZnS nanowire arrays on CdS nanoribbons

The authors present the results of heteroepitaxial growth of single-crystalline ZnS nanowire arrays on CdS nanoribbon substrates by the metal-catalyzed vapor-liquid-solid growth method. ZnS nanowire arrays were vertically or crosswise aligned to the surface of CdS nanoribbon substrates. Room-temperature lasing from ZnS nanowire arrays was demonstrated. The present synthesis provides a new approach to the rational design of building blocks for nanodevices.

418cm−1 Raman scattering from gallium nitride nanowires: Is it a vibration mode of N-rich Ga–N bond configuration?

A Raman-active vibration mode at 418cm−1 is observed in wurtzite gallium nitride (GaN) nanowires synthesized by different growth methods. In particular, Raman scattering measurements of a number of GaN nanowires systematically prepared by nitriding β-Ga2O3 nanowires at different temperatures show an interesting evolution of the mode, revealing that it is most likely the vibration mode of N-rich octahedral Ga–N6 bonds. This idea is further supported by the high-resolution transmission electron microscopic observation.

Single mullite nanoribbon as a glucose sensor

Mullite (2SiO(2).3Al(2)O(3)) nanoribbons, millimetres in length and with a high width-to-thickness ratio, were synthesized at temperatures as low as 1150 degrees C. This high ratio made it easy to fabricate a single nanoribbon sensor. The I-V relation of the sensor versus concentration of glucose was recorded with a pico-ammeter. The sensor shows good reproducibility and long-term stability. This single nanoribbon sensor may be used as an in situ monitor. The nanoribbons were also characterized by x-ray diffraction, scanning electron microscopy, transmission electron microscopy and x-ray photoemission spectroscopy.