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Smart Optical Materials by Sol-Gel Method
by R. Reisfeld
Dept. of Inorganic and Analytical Chemistry , The Hebrew University, Jerusalem 91904, Israel


Tunable Laser samples prepared by the sol-gel method

Nanoparticles in sol-gel glasses

Nanoparticles of metals, semiconductors and dye aggregates have atomic arrangements in aggregates of the size ranging between molecular clusters and infinite solid state arrays of atoms. Their properties are determined by the extent of confinement of highly delocalized valence electrons. When introduced into glasses their spectroscopic properties can be measured and the shifting of the discrete energy levels compared to the continuous band energies determined. 

 

These energy states of the quantum dots are positioned between the discrete energy levels of the atoms or bands of molecules and the broad band of the condensed phase.
Glass films can be prepared by AgCl, CdS, CdSe,CdTe, PbS, CuCl, CuBr, PbIe and Bi2S3 quantum dots. The relation between the energies of the electronic states and size of the particles have been discussed and their nonlinear properties reported. Dye aggregates having characteristic electronic spectra have also properties of quantum dots.

Semiconductor nanocrystals (NCs) in transparent media have, especially in sol-gel glasses, received recently a large attention due to their promising applications in non-linear optics and optical switches. Those nanometer-sized crystals, which are comparable with the bulk exciton Bohr radius, exhibit an intermediate behavior between a bulk crystal and an isolated molecule. For example, CdS and CdSe NCs and CdTe exhibit a large optical non-linearity. Lead sulfide (PbS) is suitable for the telecommunication application, because its bulk band gap is located at 0.41eV at 300K. The latter is blue shifted to the spectral regime of 0.7-1.5μm (1.77-0.82 eV) upon the decrease of the PbS NCs diameter below the size of the exitonic Bohr radius (16nm), due to a quantum confinement effect.

The nanometer sized semiconductor crystallites of CdS, CdSe, CdTe, and PbS were formed by chemical procedure in glass films prepared by the sol gel method. The films are either pure silica, silica ormosils, zirconia or combined zirconia with ormosils.

As the sizes of the nanocrystallites decrease, controlled by the preparation method the band gap shifts to higher energies due to quantum size effect. Spectroscopic method of absorption and luminescence allow to determine the shift and Atomic Force microscope (AFM) and Transmission Electron Microscope (TEM) the size of the nanoparticles.  X-ray diffraction for larger nanoparticles provides information on the crystallographic structure of the particles. 

PbS nanocrystals embedded in zirconia sol-gel films

As an example PbS nanocrystals were formed by sol-gel technology in zirconia (ZrO2) thin films and their optical and electrical properties in the temperature range 5-450K. The size of the nanocrystals (2-8nm) was determined by transmission electron microscopy and by a blue shift of the absorption edge and photoluminescence (PL) spectra. The size of NCs increased with an increase in the synthesis temperature between 200-300C and PbS volume fraction (0.05-0.3). The PL spectra were Stokes shifted from the corresponding absorption edge by about 0.4eV. The farther increase in PbS volume fraction from 0.3 to 0.5 opened up possibilities to achieve thin layer of PbS solid solution (bulk) on PbS NCs- ZrO2 substrates. I-V characteristic were measured by deposition of the films on ITO/glass substrate, followed by their coverage with gold contact by photolithographically techniques. 
The I-V characteristic of one layer structure exhibit nearly symmetric nonlinear characteristics, while two layer heterostructure exhibit rectifying behavior with rectifying ratio 100-300 at 1V. These non–linear phenomena in combination with the small size of the device make them useful for application in micro-opto-electronics. The technique can be extended to other heterostructures .

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