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Sci. Lett. J. 2015, 4: 140
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Review Article
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Full Text
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Photoluminescent silica nanostructures
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Subhasree Banerjee, Anindya Datta
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Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
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Abstract |
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The present review deals with the preparation of silica nanostructures and their photoluminescence. The motivation for this study is three folds: Such nanostructures can be prepared by the template mediated sol-gel method. The mechanism of the blue photoluminescence (PL) of such nanostructures and especially, the effect of simple chemical inputs on this PL is not well understood. Finally, surface of silica is covered with silanol (= Si-OH) groups, which can be functionalized using simple silanol chemistry. These properties are more pronounced with silica nanostructures having large surface area. Presence of large number of surface silanol groups in silica nanostructures renders them for easy surface functionalization and their colloidal dispersion. This aspect has been exploited in the field of light harvesting nanoantenna and sensors. Optical properties of amorphous silica nanostructures are attributed to different defects. The blue photoluminescence (PL) is the most interesting and occurs when excited at ~360 nm light. This blue PL band shows biexponential behavior. This is explained by the Uchino model, in which the shorter component is assigned to the radiative recombination of a defect pair and the longer one is assigned to surface trap states. Interestingly, the spectra are more structured in nonpolar solvents. The longer component is more prominent in protic solvents, but shows up at longer emission wavelengths in nonpolar solvents as well. Using time resolved area normalized emission spectroscopy (TRANES), the PL spectrum of the surface trap states is resolved from that of the defect pair. Hydrogen bonding interactions of the surface silanol groups with the solvents appear to stabilize the trap state. Acid treatment of these nanostructures brings about dramatic modifications in the PL maximum, causes a loss of structure and a new species with a different lifetime is observed. This is ascribed to ground state and more prominently, excited state protonation of the defect pair. Once again, TRANES comes handy in elucidating the excited state dynamics. The 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.
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Keywords
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Silica nanostructures; Emissive trap states; Time resolved photoluminescence; Soft template; Fluorescence spectroscopy
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