Inorganic Nanocomposites for Optical Applications

Inorganic Nanocomposites for Optical Applications

Nonlinear optical effects, such as nonlinear optical absorption and second- and thirdorderoptical nonlinearities, can be used to make optical limiters, optical modulators,etc. Although many organic materials have high optical absorption and nonlinearity,their thermal and optical stabilities are poor. Often, it helps to create hybrids or composites(organic/inorganic, inorganic/inorganic) that have acceptable optical propertiesand stability. Quantum-confined nanoparticles have been extensively used in thefabrication of such composites because of their novel optical and electronic properties.The organic-matrix nanocomposites for optical applications are described in chapter 2,which briefly discusses some of the organic materials that contain dispersed nanoparticlesin various hosts.Recent advances in controlling the fabrication and dispersion of semiconductornanoparticles in polymer and ceramic matrices have suggested possible uses forsuch nanocomposites in optical applications. A good example of an optically functionalceramic nanocomposite is GaAs nanocrystals embedded in SiO2 matrix. Theinterest in the novel optical properties of semiconductor nanocrystals [144] has resultedin strategies to package them as nanocomposites. A variety of techniques,such as colloidal synthesis, self-assembly, and electrochemistry, can be used to producethe semiconductor nanoparticles; however, ion coimplantation (Ga+, As+) is anefficient way of creating well-dispersed nanocomposite materials (e.g., GaAs/SiO2)[145]. Typically, the ions are implanted at fluxes of 1016 cm-2 into 100 nm silica filmson Si substrates and annealed at appropriate temperatures to create nanocrystals ofGaAs (several nanometers in size) in the matrix (Figure 1.27).



Photoluminescencestudies show an efficient, broad luminescence band in the visible and near-infraredspectral regions due to quantum confinement in GaAs nanocrystals and defect statesin SiO2.Nanoparticles in matrices have interesting photoluminescence properties, due tothe effects of quantum confinement on their optical properties. Indirect band semiconductorssuch as Si and Ge have very poor luminescence efficiency (hence, efficient46 1 Bulk Metal and Ceramics Nanocompositeslight emitters cannot be fabricated directly from them), because the band center transitionis optically forbidden. But, by making the particle sizes smaller than the excitonBohr radius (4.9 nm for Si, 24.3 nm for Ge), the resulting confinement produces anincrease in oscillator strength, shifting the luminescence to higher energies; this allowstailoring novel optical materials, which are otherwise impossible. To take fulladvantage of the particle size effect, the particles must be separated, and the best possibleway of accomplishing this is to disperse the particles in a matrix. Understandingand minimizing the interaction of the host matrix and the particles that create interfacestates are crucial to many of the optical applications. Embedding nanoparticles in differentmatrices and studying the optical properties provides a way to decipher interfaceeffects.



Ge quantum dots have been studied in various hosts, and one way to minimizehost interaction is to embed them in oxygen-free environments, such as AlN; AlN/Gemultilayer structures deposited by pulsed-laser deposition show a blue-shifted photoluminescencepeak [146]. Similarly, indium oxide (InO) nanoparticles dispersed (bysolvent-phase impregnation) within the pores of mesoporous silica (prepared by sol–gel technique) show multiple photoluminescence peaks related to the size and structureof the particles [147]. Amorphous InO particles (<6>


The problemwith porous Si is the structural nonuniformity, lack of reproducibility in emissionand aging, and environmental degradation problems. Incorporating light-emittingSi structures in a matrix that is chemically inert and has a wide band gap suitablefor quantum confinement can overcome this problem. Nanocomposites of Si nanoparticles( 5 nm) embedded in polycrystalline diamond matrix have been preparedFig. 1.27 Transmission electron micrograph, left,GaAs nanocrystals embedded inside SiO2 glassmatrix. Sequential ion implantation followed bythermal annealing was used to form GaAs nanocrystalsin SiO2 films. Right, efficient, broadphotoluminescence band observed in the red andnear infrared spectral regions. The efficient luminescenceis attributed to both quantum confinementstates in GaAs nanocrystals and defects inSiO2.



(Source [145] used with permission)1.9 Inorganic Nanocomposites for Optical Applications 47and studied. The room temperature photoluminescence behavior of such materials inthe 1.6–2.5 eV range shows a strong increase in emission efficiency from the Si nanostructures.








The studies conducted on this nanocomposite reveal that selecting thesize of the embedded Si particles makes it possible to tune the luminescence frequencyin the yellow–green spectral region.Nanocomposites of nanosized metal particles in transparent dielectrics can also beapplied as nonlinear optical materials in photonic devices. These materials are characterizedby large third-order optical nonlinearity (v3) and fast response times, whichare important for device applications such as optical computing, real time holography,and phase conjugators. To incorporate metal nanoparticles into dielectrics, severaltechniques such as ion implantation, sol–gel techniques, and sputtering can be employed.It is even possible to prepare graded layers of metal nanoparticle distributions(with different particle sizes and interparticle separations) by implantation, and thesestructures can be carefully tailored to produce interacting and noninteracting nanoparticlelayers that produce different optical response in the plasma resonance frequencies.


Several of these composite systems have been studied, for example, Au/SiO2, Ag/SiO2, Au/Al2O3, Au/TiO2 with metal concentrations varying from 15%–60%, for nonlinearity and plasma resonance frequency shifts. High values of susceptibilities(v3 6 10-7 esu, compared to low values of 10-12–10-11 esu for glass) havebeen reported for composites containing an optimized fraction of the metal component[148, 149]. Groups II–VI semiconducting nanocrystals (e.g., CdS) prepared inglass hosts have been studied in great detail, because of their large optical nonlinearresponse and small carrier lifetimes. Enhancement of carrier recombination rates isobserved in nanocrystals with large surface-to-volume ratios, and the enhancementresults from increases in the density of surface states and fast surface recombinationand capture due to multiphoton emission. Surface recombination in nanocrystals embeddedin glass matrices exhibits thermally activated nonradiative recombination,which is enhanced at reduced particle size.


The semiconductor/glass interfaces inthe composites may give rise to deep traps responsible for photoactive phenomena.In recent years, nanoparticles have been used to make transparent nanocompositestructures having high refractive indices. Polymers containing inorganic particles in arange of 1–100 nm (nanocomposites) are interesting in this regard. In contrast tocomposites having particles in the micron size range, nanocomposites do not scatterlight and are interesting for optical applications. Preparation of nanocomposites withrefractive indices over the entire range of 3, which is by far the lowest andhighest ever achieved for a polymer composite, has been possible. Transparent polymericmaterials can be coated with surface layers ( 100 nm thick) of UV-absorbingnanocomposites to inhibit degradation of the polymer by UV light [150]. High-refractive-index transparent materials are mainly used for improving the optical couplingefficiencies in photonic devices.


The refractive indices of polymers vary between1.3 and 1.7, those for inorganic semiconductors vary between 2 and 5, and forhigh bandgap semiconductors this value is <3.>

Asignatura: E.E.S

Saithrhu R. Gonzalez C.