Nanoscale Belts Have Display Applications
Researchers at National Taiwan University and at Academia Sinica, both in Taipei, have come up with the latest nanofashion accessory: a nanobelt. Built of CdSe, it is strongly photoluminescent, has a high quantum yield and efficiently converts energy into light. As a result, it could have applications in a number of areas, including as an optically or electrically pumped element in displays.
Researchers fabricated CdSe nanobelts using catalytic synthesis. The structures are seen here via scanning electron microscopy (a), transmission electron microscopy (b) and two electron microscope measurement techniques (c and d). Images reprinted with permission of the Journal of the American Chemical Society. ©2005 American Chemical Society.
Unlike nanowires, nanobelts can be fabricated nearly free of dislocations and line defects, improving their electronic and optical performance. In the case of CdSe, nanostructures have been built that lase and fluoresce, which is one reason why the researchers selected it for their studies. Previous work also had indicated that nanobelts grow better in higher temperatures, and nanowires in lower ones.
“The reaction/deposition temperature plays a very important role in the growth,” explained Yit-Tsong Chen, a professor of chemistry at the university and leader of the research team. He reported that the investigators fabricated nanowires at deposition temperatures of 250 to 350 °C, nanobelts at 500 to 600 °C and nanosheets at more than 650 °C.
The spectrum of a CdSe nanobelt shows that the intensity of its photoluminescence is much higher than that of bulk CdSe source powder, suggesting that such nanobelts could be useful in displays. The data were collected using laser excitation at 514.5 nm. Inserts (left) and (right), respectively, are confocal and transmission electron microscope images of a nanobelt.
To catalytically grow CdSe nanobelts, they began with high-purity CdSe powder, which they heated and over which they flowed a carrier gas. They exposed the sample for 7 ns to 1064-nm radiation from a pulsed Spectra-Physics Nd:YAG laser operating at a repetition rate of 30 Hz. They focused the beam to a 1.5-mm spot on the CdSe, ablating the powder and leading to the deposition of the vapor on catalytic gold nanoparticles on a nearby silicon substrate. Finally, they formed the nanobelts using sequential vapor/liquid/solid and vapor/solid growth mechanisms.
The nanostructures ranged from several tens to hundreds of microns in length, and their thicknesses varied from 40 to 70 nm. Their widths were about 3 μm on one end, tapering to about 100 nm at the other, which terminated at a gold particle.
Chen noted that, if the catalytic metal were only a few nanometers in diameter, the nanobelts would taper to that size, forming fine points that would make them suitable for use as tips in atomic force microscopy.
In characterizing the nanobelts, the researchers found that the photoluminescence from a single belt was strong enough that they could measure it with 514.5-nm laser excitation. The photoluminescence was three times that of CdSe powder, which they attributed to the high crystallinity of the nanobelt and its lack of surface defects.
They plan to investigate nano-structures made of three elements, which would enable the tuning of the photoluminescence peak by varying the composition ratio of the components. Such alloyed nanostructures also show potential for display applications, Chen said.
Journal of the American Chemical Society, Aug. 17, 2005, pp. 11,262-11,268.
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