Optical Properties

Clusters of semiconducting materials are interesting considering their application as a photocathode. The silver chloride/silver cluster phase boundary plays a decisive role in the photocatalytic silver chloride electrode system. This system was studied by means of quantum chemical calculations [563]. The synthesis and properties of luminescent quantum-sized silver sulfide clusters in the cavities of zeolite A were reported. The color of the silver sulfide zeolite A composites ranged from colorless (low loading) to yellow-green (medium loading) to brown (high loading). A low silver sulfide content was characterized by a blue-green luminescence and distinct absorption bands, while samples with medium or high silver sulfide content showed an orange or red colored emission and a continuous absorption.

Linear (visible photoluminescence) and nonlinear (third-order susceptibility) optical properties were measured for laser-synthesized crystalline Si nanoparticles [564]. The observed luminescence was compared to that of nano-composite aerogels prepared integrating the Si nanoparticles into a continuous silica phase by sol-gel processing. The incorporation of the Si nanoparticles into a silica matrix by sol-gel technology was not detrimental to the photoluminescence emission intensity.

Two methods for the preparation of semiconductor doped sol-gel films, for applications in nonlinear optics, were studied [565]. Porous films were spun from sols containing the cation precursor, reacted with H2S gas, and then the cation was adsorbed onto the pore surfaces of passive films from aqueous solution before the gas reaction. Preliminary results were reported for CdS doping and for other semiconductor species. The effects of heat treatment of doped films and the limitation of crystallite growth by pore size were described.

C nanoparticles embedded in xAl2O3 • xP2O5 • 100SiO2 (x = 0.25 or 0.5) gel-glasses were prepared by a solgel process [566]. The gels synthesized through hydrolysis of PO(OC2H5)3, Al(NO3)3 • 9H2O and Si(OC2H5)4, were heated at 600 °C for 10 h, in which -OC2H5 was carbonized to precipitate nanosized C particles. Glasses doped with C nanoparticles showed a strong room-temperature photoluminescence with a peak at 630 nm under 532 nm Nd:YAG laser excitation. The photoluminescence spectra were caused by C nanoparticle embedded gel-glasses.

Structural, optical, electro-, and photoelectrochemical properties of amorphous and crystalline sol-gel Nb2O5 coatings were determined [567]. The coatings were «-type semiconductor with indirect allowed transition and present an overall low quantum efficiency (^ < 4%) for UV light to electric conversion. The photoconducting behavior of the coatings was discussed within the framework of the Gartner and Sodergren models. Improvement can be foreseen if Nb2O5 coatings can be made of 10-20 nm size nanoparticles.

Nanocrystalline titanium and indium oxides and TiO2-In2O3 composites were prepared by the sol-gel technique using concentrated hydrous titanium dioxide and indium hydroxide sols [568]. Structural, optical, and photoelectro-chemical properties of the composites with different molar ratios of TiO2 to In2O3 were studied. Highly transparent TiO2-In2O3 films fabricated by the spin-coating technique exhibited extreme dependencies of the various optical and photoelectrochemical properties on the film composition. The bandgap edge of the composite films was blueshifted, as compared with that of single-component films. The conductivity of TiO2-In2O3 films changed by six orders of magnitude in the range of compositions near the calculated value for percolation threshold associated with the formation of 3D infinite clusters of interconnected In2O3 nanoparticles.

A sol-gel method was employed to produce a zinc oxide colloid consisting of ZnO nanocrystalline particles with an average diameter of 3 nm subsequently mixed with a silica colloid [569]. The mixture was finally spray dried to form a powder nanocomposite. The green photoluminescence exhibited by the composite was very stable.

ZnS nanoparticles were prepared by chemical precipitation of Zn2+ with sulfur ions in aqueous solution [570]. The ultraviolet-excited samples revealed detailed structure in the luminescence spectra. A doublet pattern observed in the long wavelength region was attributed to the coexistence of the two crystalline forms in ZnS particles.

Composite particles consisting of a zinc sulfide core and a silica shell or vice versa were reported [571]. These particles were optimized for photonic applications, because ZnS had a large refractive index and did not absorb light in the visible and both ZnS and SiO2 can be easily doped with fluorophores. Both kinds of morphologies were created using a seeded growth procedure using monodisperse seeds on which homogeneous layers with a well-defined thickness were grown. The ZnS and SiO2 cores could be completely dissolved leaving SiO2 and ZnS shells, respectively, filled with solvent or air after drying.

Zinc sulphide nanoparticles were synthesized in silica matrix using a sol-gel method [572]. Silica could be loaded with zinc sulphide over a very wide range of concentration without changing the nanoparticle size. A strongly luminescent zinc sulphide-silica composite, thermally stable even up to around 700 °C, was obtained.

Synthesis and characterization of undoped and Mn2+ doped ZnS nanocrystallites (radius 2-3 nm) embedded in a partially densified silica gel matrix were presented [573, 574]. Optical transmittance, photoluminescence, ellipsomet-ric, and electron spin resonance measurements revealed manifestation of the quantum size effect [573]. A dramatic increase in cathodoluminescence emission was observed [574]. Photoluminescence spectra recorded at room temperature revealed a broad blue emission signal centered at around 420 nm and a Mn2+ related yellow-orange band centered at around 590 nm, while electron spin resonance measurements indicated that Mn in ZnS was present as dispersed impurity rather than Mn clusters [573].

The influence of the particle size on the luminescence of nanocrystalline ZnS:Pb2+ synthesised via a precipitation method was studied [575]. Depending on the excitation wavelength, nanocrystalline ZnS:Pb2+ showed a very broad white (Aexc = 380 nm) or red emission (Aexc = 480 nm). The decay kinetics of both emissions was typical for Pb2+, independent of the particle size. The quenching of luminescence with increasing temperature was explained by photoioniza-tion. The lower quenching temperature for the luminescence of larger nanoparticles was explained by quantum size effects.

ZnS nanoparticles doped with Ni2+ [576], Ti3+ and Ti4+ [577], Cu+ and Cu2+ [578], Co2+ [579], and Co3+ [580], were obtained by chemical coprecipitation from homogeneous solutions of zinc and nickel (or titanium, copper, or cobalt) salt compounds, with S2- as precipitating anion, formed by decomposition of thioacetamide. The average size of particles doped with different mole ratios, was about 2-3 nm [576-580]. The absorption spectra showed that the excitation spectra of Ni-doped ZnS nanocrystallites were almost the same as those of pure ZnS nanocrystallites (Aexc = 308310 nm) [576]. Because a Ni2+, Ti3+, or Co2+ luminescent center was formed in ZnS nanocrystallites, the photoluminescence intensity increased with the amount of ZnS nano-particles doped with these ions [576, 577, 579]. Stronger and stable green-light emission was observed from ZnS nano-particles doped with Ni2+ [576], Ti3+ [577], and Co2+ [579]. The fluorescence intensity of the ZnS nanocrystallites doped with Ti4+ was almost the same as that of the undoped ZnS nanoparticles [577]. The fluorescence intensity of Co3+-doped ZnS nanoparticles was much weaker than that of ZnS nanoparticles [580]. The emission spectrum of ZnS nano-crystallites doped with Cu+ and Cu2+ consisted of two emission peaks at 450 and 530 nm [578].

Monodispersed ZnS and Eu3+-doped ZnS nanocrystals were prepared through the coprecipitation reaction of inorganic precursors ZnCl2, EuCl3, and Na2S in a water/methanol binary solution [581, 582]. The mean particle sizes were about 3-5 nm with cubic (zinc blende) structure [582]. Photoluminescence studies showed stable room temperature emission in the visible spectrum region, with a broadening in the emission band and, in particular, a partially overlapped twin peak in the Eu3+-doped ZnS nanocrystals.

The processing, microstructure, and optical properties of CdS semiconductor nanoparticles sequestered in spin-coated polymer films were investigated [583]. A simple processing protocol was developed to form thin film structures consisting of CdS nanoparticles dispersed in the interstices created by a close-packed stacking of polystyrene spheres. Absorption, emission, and excitation spectra of CdS nano-particles embedded in sol-gel silica glasses were reported [584]. The effects of temperature on emission behavior were investigated.

Spherical CdS particles similar to 100 nm were synthesized by a hydrothermal process [585]. The particle formation and growth depended on the rate of sulfide-ion generation and diffusion-controlled aggregation of nanoparticles. Photoluminescence studies on CdS particles showed two emission bands at room temperature. The red emission at 680 nm was due to sulfur vacancies, and an infrared red emission at 760 nm was attributed to self-activated centers. A redshift of the infrared red band with the decrease of temperature was explained with a configurational coordinate model.

Highly luminescent CdTe nanocrystals were synthesized by reacting dimethylcadmium with different tellurium sources in mixtures of dodecylamine and trioctylphosphine as the coordinating and size-regulating solvent [586]. Colloids of crystalline CdTe nanoparticles with zinc blende lattice displaying mean particle sizes between 2.5 and 7 nm were prepared at temperatures from 150 to 220 °C. The particles showed strong band-edge photoluminescence shifting from green to red with increasing particle size. The photoluminescence quantum yield strongly depended on the tellurium source employed in the synthesis. The highest quantum yields were observed by using a suspension of tellurium powder in a mixture of dodecylamine and tri-octylphosphine. These CdTe samples showed a photoluminescence quantum yield up to 65% at room temperature without covering the surface of the nanoparticles with a pas-sivating inorganic shell.

A convenient and safe one-pot route was presented to capped 3 nm CdSe nanoparticles with a narrow size distribution making use of common starting materials and inexpensive, low-boiling solvents under solvothermal conditions

[587]. H2Se required for the reaction is generated in-situ through the aromatization of tetralin by Se. Figure 20 represents the powder XRD pattern of CdSe and UV-vis absorption spectrum of the nanoparticles in toluene [587].

The synthesis, characterization, and purification of GaSe nanoparticles in the size range of 2-6 nm were described

[588]. These particles had a two-dimensional single tetra-layer type structure. The particles had absorption onsets in the 360 to 450 nm region, with the smallest particles absorbing furthest to the blue. The particles were emissive, with emission quantum yields of about 10%.

20 40 60 SO

GuKa. (degrees)

Figure 20. (a) (i) Powder XRD pattern of CdSe compared with (ii) the background-subtracted Rietveld fit, (iii) DIFFaX simulation of 3 nm cubic zinc blende stacking of CdSe, and (iv) DIFFaX simulation of a 3 nm hexagonal wurtzite stacking of CdSe. The vertical lines at the top are expected peak positions for the cubic zinc blende structure. (b) UV-Vis absorption spectrum of the nanoparticles in toluene. Luminescence spectra—emission (em) following excitation at 400 nm, and excitation (ex) following emission at 540 nm are also shown. After [587], U. K. Gautam et al., Chem. Commun. 7, 629 (2001). © 2001, Royal Society of Chemistry.

Figure 20. (a) (i) Powder XRD pattern of CdSe compared with (ii) the background-subtracted Rietveld fit, (iii) DIFFaX simulation of 3 nm cubic zinc blende stacking of CdSe, and (iv) DIFFaX simulation of a 3 nm hexagonal wurtzite stacking of CdSe. The vertical lines at the top are expected peak positions for the cubic zinc blende structure. (b) UV-Vis absorption spectrum of the nanoparticles in toluene. Luminescence spectra—emission (em) following excitation at 400 nm, and excitation (ex) following emission at 540 nm are also shown. After [587], U. K. Gautam et al., Chem. Commun. 7, 629 (2001). © 2001, Royal Society of Chemistry.

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