Growth and characterization of semiconductor nanoparticles in porous sol-gel films
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Growth and characterization of semiconductor nanoparticles in porous sol-gel films E.J.C. Dawnaya) , M. A. Fardad, Mino Green, and E. M. Yeatman Department of Electrical and Electronic Engineering, Imperial College, London SW7 2BT, England (Received 3 September 1996; accepted 17 April 1997)
Two methods for the preparation of semiconductor doped sol-gel films, for applications in nonlinear optics, have been studied and compared. In the first, porous films are spun from sols containing the cation precursor, and then reacted with H2 S gas, and in the second, the cation is adsorbed onto the pore surfaces of passive films from aqueous solution before the gas reaction. Extensive results for CdS doping are given, and preliminary results are reported for other semiconductor species. It is shown that a sputtered silica layer can seal the structure to allow further heat treatment without loss of dopant. The effects of heat treatment of doped films are described, and the limitation of crystallite growth by pore size is shown.
I. INTRODUCTION
The sol-gel method is a technique for forming glass and ceramic materials from liquid metallorganic precursors by low temperature polymerization reactions.1 Its use in the fabrication of optical glasses has been extensively studied, especially for bulk structures but also for thin films. In essence, the process begins by hydrolysis and polycondensation of the precursors in solution, using a suitable catalyst, to form a colloidal suspension of nanometer-scale particles (the “sol”). This sol is then coated onto a surface, or cast into a mold, after which further condensation produces a rigid network (the “gel”). This gel can be fired to give a final product. An important feature of this technique is that it allows the formation of materials with a great variety of structures and compositions. Homogeneous glasses can be formed, for example, with stoichiometries that would lead to crystallization if made by high temperature routes. The process also generally gives porous material, typically on a very fine scale, and this pore morphology can be widely controlled through various process parameters.2,3 Dopants can be added to the sol, either to change the nature of the homogeneous material or to give a second phase to the final product. A materials application that has attracted considerable attention relates to nonlinear optics: in particular, the doping of glasses with particles of metal or semiconductor, dispersed on a nanometer scale.4–6 The glass can provide a stable host matrix with desirable mechanical and optical properties, while the dopants will add nonlinear polarizability, making effects such as frequency shifting and all-optical switching possible. a)
Current address: Bookham Technology Ltd., 90 Milton Park, Abingdon, Oxfordshire, OX14 4RY, England. J. Mater. Res., Vol. 12, No. 11, Nov 1997
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If nanodisperse (such that the dopants do not cause excessive scattering losses), and for
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