Nanotubes Fluoresce within Biological Cells
- PDF / 1,365,632 Bytes
- 2 Pages / 612 x 792 pts (letter) Page_size
- 54 Downloads / 217 Views
RESEARCH/RESEARCHERS
Optical Limiting Displayed by Polymer Films with Ag Nanoparticles Grown In Situ Assemblies of metal nanoparticles within polymer or sol-gel films have nonlinear optical applications such as optical limiting. Typically, such films are cast from a mixture containing preformed nanoparticles, but with current concerns about the health hazards of inhalable nanoparticles, the growth of nanoparticles in thin films in situ is preferable. T.P. Radhakrishnan of the University of Hyderabad, S. Singh of the Centre for Cellular and Molecular Biology in India, and their colleagues have developed such a method that uses chemical reduction to form metal nanoparticles within a polymer matrix. As reported in the January 11 issue of Chemistry of Materials (p. 1; doi: 10.1021/cm0485963), the researchers fabricated a poly(vinyl alcohol) (PVA) film with silver nanoparticles grown in situ. The simple fabrication process, which uses environmentally friendly materials, has three steps. First, an aqueous mixture of silver nitrate and PVA is spin-coated to a thickness of 400–500 nm onto quartz substrates or glass slides previously coated with polystyrene (PS). In the second step, silver nanoparticles are generated by heating the silver nitrate/PVA films in ambient atmosphere at temperatures ranging from 50°C to 110°C for 5–60 min. PVA acts as both the reducing agent for the silver nitrate and the matrix for homogeneous distribution and immobilization of the silver nanoparticles. In the third step, freestanding films are peeled off the PS substrate. The films are transparent and colorless, but increasing amounts of silver nanoparticles result in a yellow color. Radhakrishnan and co-researchers performed optical absorption studies (on films formed on glass substrates) to show that nanoparticle production increases with heating times up to a saturation point of ~1 h. The researchers said that a decrease in particle size is shown by a steady but slow blueshift in the absorption maxima with increasing heating time. In addition, the decrease in linewidth observed with heating time demonstrates that particle size becomes more monodisperse with heating. Transmission electron micrographs (with electron diffraction) revealed that films heated for 60 min at 90°C have small particles (with a mean diameter of 2.6 nm) and a tight distribution (estimated standard deviation of 0.2 nm). The researchers said that the particle size can be fine-tuned with temperature and that the average diameter and size distribution of the particles both increase as the silver nitrate concentration increases. Z-scan measurements at 532 nm demonstrated that the films show strong reverse-saturable absorption at high laser intensities. The researchers also observed appreciable optical limiting with a threshold (defined as the input fluence at which the transmittance reduces to half of the linear transmittance) of 0.83 J cm–2 and output clamped at 0.35 J cm–2. The researchers said their method can be easily extended to other metal–polymer and semiconductor–p
Data Loading...
