A Fluorescence Spectroscopist’s Perspective of Nanovolumes and Nanomaterials

by Prof. Anindya Datta (Dept. of Chemistry, IIT Bombay)

Thursday, June 23, 2011 from 14:30 to 15:30 (Asia/Kolkata)
at Colaba Campus ( AG-66 )

TIFR< Colaba, Mumbai

Description

The advent of Nanoscience and technology, over the last couple of decades, has seen a rapid progress in techniques tailormade to “see” the tiniest of objects. These techniques are based largely on electron microscopy and X-ray diffraction. Techniques using visible light, like single molecule spectroscopy and tip-enhanced microscopy have also widened the horizons of this field. Nevertheless, the classical spectroscopic techniques continue to remain as useful as ever, in unveiling newer aspects of nanosystems. Two such examples from our laboratory will be discussed in this lecture.

The fist example is a study of nafion, using extrinsic fluorescent probes. The nafion membrane finds extensive application in fuel cells by virtue of its unique proton conductivity. In our studies, a molecular level understanding of the proton conductivity has been attempted, using the excited state proton transfer (ESPT) and solvation of the fluorescent probe, coumarin 102 (C102). The time evolution of the ESPT state from locally excited state is ultrafast in hydrated membranes, but is observed to occur over hundreds of picoseconds when the membrane is dried partially. Thus, the ESPT is found to be hindered even in the small volume probed by the dye. More remarkably, in the dried membrane, the probe molecule loses a proton from its nitrogen center in the electronically excited state, as usual, but cannot capture it at the oxygen center. This phenomenon may be rationalized in the light of an increased electrostatic interaction which is experienced by cations upon drying. Such an effect would be of utmost importance, irrespective of other, long-range morphological changes in the membrane that have been proposed earlier and are accepted widely.

The second problem to be presented concerns silica nanotubes and nanodisks prepared by the aerosol OT microemulsion- mediated sol-gel method in the presence of FeCl₃. At lower water content, nanotubes are formed, as reported in literature. At higher water content, however, the microemulsions undergo phase separation and yield not nanotubes, but nanodisks, from the lower, water-rich phase.The blue PL of these nanostructures decay biexponentially, in line with the model proposed by Uchino and co-workers, which ascribes the major, shorter component to the radiative recombination of a defect pair and the minor, longer component to surface surface trapping. Interestingly, the decays are single exponential in nonpolar solvents. The longer component in protic solvents is rationalized in the light of hydrogen bonding interactions of the surface silanol groups with the solvents. Acid treatment brings about a dramatic change in the PL maximum, presumably due to the formation of a new photoluminescent defect center. Thus, remarkable changes in spectral and temporal features of PL, as a result of simple chemical inputs, open up the possibilities of devising newer optoelectronic applications of these nanostructures.