Application Optical

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Reviews

N a n o c o m p o s i t e M a t e r i a l s f o r O p t i c a l A p p l i c a t i o n s

Laur a L. Beecroft and Christopher K. Ober*

Departm ent of M aterials S cience and Engineering, Cornell University, Ith aca, New York 14850

Received August 20, 1996. Revised Manu script R eceived May 6, 1997 X

A substant ial amount of work h as been carried out in th e area of na nocomposite mat erialsfor optical applications. Composites ar e t ypically constru cted by embedding an opticallyfunctiona l phase into a processable, tra nspa rent ma trix mat erial. By doing so, the opticalproperties can be utilized in more technologically important forms such as films and fibers.This review covers ma ny ar eas of optical composite resea rch to date. Composites with second-an d th ird-order n onlinear ities and laser a mplification properties ar e discussed with exam plesfrom the recent litera tu re. Other composites, including tra nspa rent m agnets, may be madeusing similar st ructures. The principles u sed to constru ct these composites may h aveimportan t t echn ological ap plications soon an d a re t herefore su mma rized in th is review.

1 . I n t r o d u c t i o n

As optical ma terials a pplications ar e expanding, theneed for novel optically functional and tran sparentmaterials increases. These needs range from highperforma nce, all optical switches for fut ur e use in opticalcomputing, to hard transparent coatings as protectiveor bar rier layers. In a ddition to optical needs such asswitching and amplification, the materials must beintegrat ed into existing stru ctures such as wa veguidesan d optical fibers. As such, films and fibers ar e of grea tinterest as the f inal form of these novel materials .Nanocomposite materials show great promise as they

can provide the necessary stability and processabilityfor t hese importan t a pplications.The gener al pr inciples in the constr uction of optical

composites involve the intimate mixing of opticallyfunctional mater ials within a processable matr ix. Fig-ur e 1 shows th is type of composite schema tically, wherethe small particles possess the desirable optical proper-ties an d th e enclosing ma trix impar ts pr ocessability infilm or fiber forms . Examples of incorporat ed pha sesinclude quan tum -confined semiconductors, solid-sta telasers, small molecules, and polymers. Matr ix mat erialscan be polymers, copolymers, polymer blends, glasses,or ceramics. Using such a composite str uctur e, nano-composites have been formed with nonlinear optical

(NLO) and la ser a mplification pr operties, among others.Optical scatter ing must be avoided in th ese types of composites, resu lting in a t ra deoff between par ticle sizeand refractive index (RI) mismatch (the difference inRI between the ma trix and the part icles). For verysmall particles (typically <25 nm), the RI mismatch isnot important, but for larger particles the RI of thematrix and the particles must be carefully matched toavoid scattering. In our work, calculations based onRayleigh scatt ering have shown tha t par ticles as largeas 100 nm require matrix materials with closely matchedRI (within 0.02).1

Nanocomposite structures have been used to createoptically functiona l mat erials. By incorporat ing semi-condu ctor nan opart icles into polymer, glass, or ceram icmatrix materials , many of thei r interest ing opt ical

properties including absorption, fluorescence, lumines-cence, and nonl ineari ty may be studied. In thesesystems, very small particle sizes enhance the opticalproperties while the matr ix materials a ct to stabilizethe par ticle size and growth. Ceram ic nan oparticles of solid-state laser materials can be incorporated intopolymer matrix materials resulting in laser-active com-posites. This str uctur e allows the forma tion of solid-stat e laser amplifying films which would t ra ditiona llybe very difficult to ma ke. Optically functiona l sma llmolecules a nd polymers m ay also be included in p olymerand glass matrixes and retain their optical properties.Other applications of nanocomposite structures haveresulted in tr anspar ent ma terials with unu sually highRI, magnet ic properties, an d excellent mechan ical pr op-erties.

Nanocomposite structures provide a new method toimprove the processability and stability of materialswith interest ing optical properties. The applications of such composites are extremely broad, ranging fromsolid-sta te amplifier films to tra nspa ren t magnets . Thisreview focuses on recent d evelopmen ts in th e synth esisand applications of optical composite mat erials.

2 . N o n l i n e a r O p t i c a l C o m p o s i t e s

Nonlinear optical materials can be u seful for al loptical switching and wavelength man ipulation. χ2, orX Abstra ct published in Advance ACS Abstracts, Ju ne 15, 1997.

F i g u r e 1 . Schemat ic of optical composite.

1302 Chem. Mater. 1997, 9, 1302-1317

S0897-4756(96)00441-3 CCC: $14.00 © 1997 American Chem ical Society

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second-order nonlinear optical materials, can be usefulfor frequency doubling a nd optoelectronic switching. Byincorporating these materials into composites the sta-bility of χ2 processes can be improved as shown insection 2.2.3 χ3, or third-order nonlinear optical materi-als, can be useful for all optical switching and signalprocessing. The strength of the n onlinearity can beenhanced by using particles exhibiting quantum con-finem ent effects which can be st abilized by a composite

stru cture. Much work in th e ar ea of colloidal semicon-ductors has focused on taking advantage of this en-hancement.

The third-order nonlinear properties of solids aretypically reported as the nonlinear susceptibility ( χ3,esu), the nonlinear refractive index (n 2, cm 2 /W), or thenonlinear absorption coefficient (R2 / R0, cm 2 /W). Table1 summarizes nonlinear optical properties for the opticalcomposites t ha t will be discus sed in this section. A widerange of χ3 values have been reported, and largesusceptibilities have been shown for several systems.

The nonlinear susceptibility ( χ3) is a complex nu mber(both real and imaginary parts), making comparisonsbetween different measurement techniques difficult.

The best stra tegy is to compare similar m easur ements,as the conversion between r eal and imaginar y χ3 canbe complicated. Additiona lly, th e susceptibility willchange depending on th e measur ement wavelength. Thewavelength may be near or far from resonance, and itwill affect the amount of absorption (imaginary contri-bution to χ3) in the sa mple.

Entries 1-3 in Table 1 can be compa red t o each oth eras they are al l based on nonl inear absorpt ion mea-surements.2-4 They can also be compared to the non-linear absorption of bulk CdS (3.2 × 10 -9 cm/W at 610nm), which is considered to have a strong nonlinearresponse.5 Entries 4-7 were all measured using de-genera te four -wave mixing experimen ts, which give the

susceptibility as t he squa re sum of the r eal and ima gi-nary parts .6-8 The experiment used to measure entr y8 resulted in th e nonlinear r efractive index, or real par tof χ3.9 Finally, entries 9-11 in Table 1 show the realpart of χ3 for several commonly studied nonlinear opticalmaterials.5,10 Quartz h as a weak nonlinear response,while bulk CdS is considered t o be str ongly nonlinear.

Al l composite values in Table 1 show larger orcompa ra ble nonlinear su sceptibilities to bulk CdS. Thebest results have been seen with quantum-confinedCdS xSe 1- x (entr ies 1-4). CdS grown in situ in an ion-exchange resin (section 2.1.2), CdSe grown ex situ usinga capping method (section 2.1.4.2), and CdSSe in com-mercial glass (section 2.1.3.1) all show excellent non-

linear behavior. Composites conta ining nonlinear op-tical polymer inclusions have also shown strong non-linear ity (entr ies 6 and 7, section 2.2.1). Other com-posites st udied show nonlinea rities similar to bulk CdS.

Many reviews concerning the synthesis, nonlinearoptical pr operties, an d applications of colloidal semi-conductors have been writ ten.11 -17 Because such awealth of information exists for colloidal semiconductormaterials, this review will focus only on examples where

composite structures are uti l ized to synthesize andprocess quan tum -confined semiconductors. Wang hascoauthored several reviews discussing optical compos-ites and m entioning the potential of these mater ials asoptical devices.18 -20

Boyd and Sipe published several papers t ha t sh owedtheoretically that the composite st ru cture itself shouldenha nce nonlinear optical properties. Calculations forboth spherical inclusions 21 as well as layered compos-ites22 were presented and demonstrated that the en-hancement can come when the NLO material is eitherthe inclusion or the host . The theories have beenconfirmed experimenta lly using a dense a tomic potas -sium vap or 23 as well as a layered TiO2 /poly( p-phenyle-

neben zobisth iazole) composite.24

A significant amount of experiment al work h as beendone with semiconductor containing composites in avariety of matr ix materials. Polymers, including ho-mopolymers, block and random copolymers, and poly-mer blends, as well as glasses and ceramics such asporous glass , zeolite, and sol-gel glass, have improvedthe stability and processability of quantum-confinedsemiconductors. Sma ll molecules an d polymers withnonlinear optical properties have also been found to beuseful for n onlinea r optics in composite form . Compos-ites have been chara cterized for their str ucture u singtechniques such a s X-ra y diffraction a nd t ra nsmissionelectron microscopy (TEM). More relevant to t his

review, th e optical properties of these nan ocompositesincluding absorption, fluorescence, lum inescence, an doptical nonlinear ity ha ve been investigated.

2 .1 . N a n o c o m p o s i t e s C o n ta i n i n g S e m i c o n d u c -

tors. 2.1.1. Optical Absorption as a Measu re of Particle

S ize and Distribution. Curr ently, semiconductor na noc-rysta ls are pr imarily studied for their enh anced opticalpropert ies. These optical propert ies ar e directly relat edto the size and size distribution of the nanocrystalswhich can be extracted from the optical absorptionspectru m. If crystallite sizes are below the Bohr ra diusof both the holes and electrons in the semiconductor,strong quantum confinement occurs.25 Table 2 lists th emaximum diameter for strong confinement for several

T a b l e 1 . T h i r d -O r d e r N o n l i n e a r O p ti c a l P r o p e r t i e s i n N a n o c o m p o s i t e M a t e r i a l s

composite measd NLO st rength unit s pa r t of χ3 measd a λ (nm ) ref

1 CdS in Na fion -6.1 × 10 -7 cm 2 /W Im ( χ3)/ R 480 22 C dS in N afion /N H3 -8.3 × 10 -7 cm 2 /W Im ( χ3)/ R 450 33 capped CdSe in PMMA 1.2 × 10 -5 cm/W Im ( χ3) 544-560 44 CdS xSe 1- x gla ss 1.3 × 10 -8 esu [Re( χ3)2 + Im ( χ3)2]0.5 532 65 CdS in sol-gel gla ss 5 × 10 -12 es u ‚cm [Re( χ3)2 + Im ( χ3)2]0.5 / R 380 76 P PV in SiO2 3 × 10 -10 esu [Re( χ3)2 + Im ( χ3)2]0.5 602 87 P PV in V2O5 6 × 10 -10 esu [Re( χ3)2 + Im ( χ3)2]0.5 602 88 G aAs i n Vycor gl as s -5.6 × 10 -12 cm 2 /W Re( χ3) 1064 9

Standard NLO Materials

9 fused qua r tz 8.5 × 10-

14 esu Re( χ3) 1064 1010 SF 6 8 × 10 -13 esu Re( χ3) 1064 1011 CdS -5 × 10 -11 esu Re( χ3) 610 5

a R is the absorption coefficient.

R eview s Ch em . M ater., V ol. 9, N o. 6, 1997 1303



common semiconductors.26,27 Weaker confinement ef-fects can be seen at somewhat larger crystal diameters.The confinement effect appears as a shift to lowerwavelengths in th e a bsorption spectrum representinga changing bandgap. Ideally this shift sh ould be ac-compa nied by exciton featu res in th e spectrum , whichshow that the pa rt icle size distr ibution is very nar row.The absorption shift and spectra l featu res can act as ameasu re of par ticle size and size distribution. Contr olof these parameters i s ext remely important in theenhancement of χ3 effects.

The shift in absorption is i l lustrated in Figure 2a,which shows a blue-shift with decreasing PbS nanoc-rystallite size as well as a steepening of the absorptionedge for a PbS/polymer composite.28 Spectra of crysta lsabove 25 Å ar e featur eless, but th ose below 13 Å showsome structur e indicating a na rr ow size distr ibution inthe sam ple. The smallest par ticles exhibit a bandgapof 2.3 eV, considera bly shifted from the bulk va lue of 0.41 eV. Figure 2b displays a n absorption spectrumwith s tr ong exciton feat ur es of PbS in poly(vinyl alcohol)made in our laboratories following a modified procedureof Nenadovic et al .29 The strong exciton featur esindicated a narrow par t icle size dist r ibut ion. The

magnitu de of the bandgap sh ift ha s been corr elated withpar ticle sizes of 4 nm ,29 alth ough the shift is at a slightlylower wavelength tha n th at shown for 4.5 nm pa rticlesin Figure 2a.

Alivisatos et a l. studied th e h omogeneous and inho-mogeneous contributions to the optical spectrum of CdSe nanocrystals using optical hole burning.30 Th eCdSe clust ers were synt hesized using inverse micellesand redispersed in polystyrene for the optical measure-ments. The inhomogeneous contributions, caused by

even small variat ions in size distribution, dominat ed thespectra . Becaus e of this inhomogeneous broadening, thesize distribution mus t be carefully controlled t o maxi-mize qua nt um effects. As will be shown thr oughout thesemiconductor examples, size distribution control isoften quite difficult with t hese qu an tum -confined ma -terials.

Early absorption measurements showed either littleshift in absorption edges 31 or no exciton features whenthe edges were shifted.32 Copolymers cont ain ing poly-eth ylene (85%) an d poly(met ha crylic acid) (15%) aidedth e synth esis of PbS-cont aining composites prepa red byWang et al.28,33 The pa rticle sizes could be altered bychanging the concentra tion of Pb 2+ in th e films and by

subsequent heat tr eatments. Bandgap and absorptionmeasurements were made over a range of crystall i tesizes (Figure 2a), and theoretical models were used toexplain t he tr ends of the dat a.

Work with poly(vinyl alcohol) as a stabilizer andmat rix mater ial for PbS ha s consistent ly shown shiftedand featur ed absorption spectra (Figure 2b). This wasfirst observed by Gallardo et al. in 1989 in a study of the absorption an d fluorescence properties.34 Nenadovicet al . also studied similarly prepared materials withparticular interest in the effects of surface propertieson th e bleaching of PbS.29 Our group has also carriedout some research in this area, improving the fi lm-forming nat ur e of the composites.35

2.1.2. S emiconductor Composites with Polymer Ma-

trix Materials. Early work with quantu m-confinedsemicondu ctors was performed in colloidal solutions,36-38

which could often be sta bilized with small a mounts of polymer.39 After several years of work with polymericstabilizers, these stabilizers were discovered to beexcellent ma tr ix mat erials yielding processable polymerfilms with semiconductor optical properties. The resu lt-ing films often were more stable than colloidal solutionsand were useful for many optical measurements.

The first group to formally recognize semiconductorpolymer composites such as these in terms of engineeredoptical media was Akimov et al. in 1992.40 In this work,

CdS na nocrysta ls from 2 to 50 nm in s ize were prepa redin poly(vinyl alcohol), poly(vinylpyridine), and photo-graph ic gelatin. Other polymers th at did not stabilizethe particles were also mentioned. Remarkably highCdS concentra tions, u p t o 50 wt %, could be prepar edwithout a gglomer at ion. The composites exhibited goodphotosensitivity and photoconductivity. Par ticular ap-plications of interest to the authors included dispersiveoptical element s, ban dpass and cutoff filters, an d elec-trophotographic and photothermoplastic materials.

The ea rliest sem iconductor composites wer e used fortheir catalyt ic propert ies rather than their opt icalproperties.41 Early in the catalysis work, in situ syn-thesis meth ods were developed by Krishnan et al.42 By

F i g u re 2 . (a) Absorbance spectra shifts as a function of par ticle size. Reprinted with permission from ref 28. (b)Absorban ce spectru m with exciton featur es from PbS in P VA.

T a b l e 2 . M a x i m u m C r y s t a l l i te S i z e f o r S t r o n g

C o n fi n e m e n t i n S e v e r a l S e m i c o n d u c t o r s

semiconductormax diam

(n m ) s em icon du ct ormax diam

(nm)

CdS 0.9 PbSe 46PbS 20 GaAs 2.8CdSe 2 CuCl 0.3

1304 Ch em . M ater., V ol. 9, N o. 6, 1997 R eview s



ion excha nging a Nafion mat rix (perfluoroethylene-

sulfonic acid ion-exchange polymer) with Cd2+

, thenexposing to H 2S, submicron particles were produced,which could photocatalytically drive chemical reactions.Mau et a l . used a similar method that created 20 nmsize part icles which were u sed for photocat alytic hydro-gen generation in the presence of platinum catalyst.43

These in situ meth ods were later u tilized for the noveloptical properties t hat could be genera ted.

The genera l schem e used t o prepare po l ym er -

semiconductor composites in situ is shown in Figure 3.In a typical experiment, the ma trix mater ial and metalions a re mixed in solution an d ar e then exposed to thecounterion (S2-, Se 2-) in the form of gas or as ionsdissolved in solution. The composite may be cast a s a

fi lm before or after exposure to the counterion. Forexample, if poly(vinyl alcohol) (PVA) were the matrixmat erial for PbS n anopar ticles, a solution of PVA andPb2+ could be prepar ed in water (step 1a). This solutioncould th en be exposed to H 2S gas in order to form thePbS and th en cast as a film (step 2a). In this part icularrea ction, th e crystalline semicondu ctor forms extr emelyquickly yielding a wine-red solution. Similarly, if ablock copolymer were to be used as the matr ix, Pb 2+

could be complexed with t he copolymer in solution an dth en cast a s a film (step 1b). The phas e-separa ted film,which m ight require an nealing, would th en be exposedto H 2S gas in a closed container (step 2c). Over a periodof several hours, the film would turn brown, indicating

PbS format ion. Many variations in this general syn-thesis scheme can be imagined.2.1.2.1. N LO Properties of Polym er Composites. While

most authors praise the enhanced nonlinear opticalproperties of quantum confined semiconductors, fewactually have measured them. Wang et al . at DuPonthave been th e leaders in ma king these measurementson polymeric systems. In 1987, Wang and Mahlerreported the first study of NLO properties in polymerstabilized qua ntu m-confined semiconductors u sing th edegenera te four-wave mixing (DFWM) resu lts of a CdS/ Nafion composite.44 The pa rt icles stu died were 50 Å insize, which would cause moderate confinement (Table2) an d resu lted in a ban dgap of 2.55 eV (the ban dgap of

bulk CdS is 2.5 eV). DFWM measu rement s were takenat 505 nm (2.43 eV) as shown in F igure 4, wher e a slopeof 1.9 and saturation at about 2 MW/cm 2 were found ,indicating a third-order process. The signal strengthwas a bout ha lf tha t seen in semicondu ctor-doped glasssamples, and a 10 ns r esponse time was measu red. Thesaturation indicated that a three-level saturable ab-sorber model was the most likely mechanism for non-linearity in these composites. A later paper reportedDFWM experiments and absorption saturation experi-ment s tha t confirmed the interest ing nonlinear opticalproperties of these composites.45

Hillinski et al. presented work concerning 5.5 nm CdS

clusters grown in Nafion, with a focus on the NLOproperties.2 Their films showed large χ3 nonlinearity(R2 / R0 ) -6.1 × 10 -7 cm 2 /W) which they attributed toth e bleaching of th e exciton absorpt ion. This bleachingwas enh anced due to th e high concentra tion of surfacedefects on the small part icles. They caut ioned tha t anunderstan ding of the surface chemistry of these par-t icles will be very important in the interpretation of their properties.

Continu ing this work, Wan g et al. fur ther discussedbleaching of qua nt um -confined CdS p ar ticles in Na fionfilms.3 Crystallites in a ra nge of sizes up t o 40 Å weresynthesized and passivated with am monia. The effectsof surface-trapped, electron -hole pairs were examined

using absorption, photoluminescence, and pump-probeexperiments. An even higher χ3 nonlinearity (R2 / R0 )

-8.3 × 10 -7 cm 2 /W) was measured in the surfacepassivated samples. A short review paper by the samegroup reported the NLO propert ies for the CdS inNafion samples.46

Nonlinear optical measurements were also made onCdS gr own in a swollen, cross-linked, copolymer ma-trix.47 A third harmonic generation experiment wasused, compar ing intensities to a quart z reference. Nearresonance at 1.45 µm, an increase in signal as mu ch as11.2 times th e reference was observed. Off resonan ceat 1.06 an d 1.5 µm sm all increases from 1.2 to 1.6 timesthe quart z standard were shown.

F i g u r e 3 . Schemat ic of in situ polymer synth esis methods.F i g u re 4 . Nonlinear optical properties in a CdS/polymernanocomposite. Reprinted with permission from ref 44.





2.1.2.2. Using Polymer Architecture To Control Par-

ticle Size. As has been men tioned, part icle size and sizedistribut ion m ust be carefully controlled in order t o takeadvan tage of NLO enhan cement effects. The ar chitec-ture of the polymer matrix can be used to help incontr olling th ese par amet ers. Both block copolymersand polymer blends exhibit phase separation, which

may help isolate the semiconductor clusters as theyform. In some cases th is controlled phase separ ationcan result in superlat tice structur es. Most work in thear ea of super latt ices ha ve been with colloidal solids;48,49

however, similar str uctures may be possible with com-posites.

Researchers at MIT have done extensive work withblock copolymers prepar ed u sing th e r ing-opening m e-tathesis polymerization (ROMP) synthesis technique. Bytaking advantage of the phase separation of the blockcopolymers, good stabilization of the semiconductor wasfoun d. STEM studies of PbS 50 as well as CdS a nd ZnS51

composites sh owed tha t sm all part icles can be synth e-sized ut ilizing spher ical pha se separ ation of the m etal-

conta ining pha se; however, the optical pr operties werenot measured. The bandgap of 30 Å ZnS part iclesprepared by this technique was 5.7 eV, which is con-siderably higher than the bulk bandgap of 3.5 eV.

Similar copolymers were synthesized which were usedto prepa re ZnS- and Zn-conta ining composites in bothlamellar and spherical morphologies.52 As seen inFigure 5, both lamellar (Figur e 5a) and spherical (Figure5b) morphologies effectively isolated the ZnS particles(dark regions) in a contr olled mann er. Both m orphol-ogies r esulted in clusters less tha n 20 Å in size with abandgap of 6.3 eV. X-ray and X-ray photoelectronspectroscopy were a lso used to chara cterize th ese ma -terials. In 1994, members from the same r esearch

group again reported PbS grown in a ROMP blockcopolymer.53 These composites displayed a featuredabsorption spectru m, indicating a nar row size distribu-tion, but were primarily characterized by TEM andX-ray diffra ction.

Moller r eported th e growth of na nocrysta ls of ma nysemicondu ctors, includin g CuS, CdS, and PbS, in blockcopolymer s of polystyr ene an d poly(vinylpyridine) withspherical morphology.54 Shifted absorption spectrawere shown indicating small particle sizes, but theabsorbance spectra were featureless, indicating that thepar ticles were not monodisperse. More recent work bythe sa me group on gold nanoparticles h as shown thatthe stabilization of the ionic block is very important if a single particle per micelle is desired.55 When a poly-(ethylene oxide) block was used as a matrix for goldparticles, heat treatment caused the particles to cometogether, resulting in a system where the particle sizecould be controlled by the composition of the copolymerand the concentra tion of ions. However, when poly-(vinylpyr idine) was u sed a s th e sta bilizing block for gold(and several semiconductors) the particles did n otcoalesce dur ing heat t rea tmen t because th e stabilizationwas mu ch st ronger. The st rategy for forming onepar ticle per micelle using a weakly sta bilizing block an dheat treatment might be very important if i t can beextended t o semiconductor par ticles.

The first work uti l izing polymer blends as matrixmaterials was reported by Yuan et al .56,57 Quantum-confined CdS was pr epar ed in poly(styren e phosphonat edieth yl ester) (PSP ) and cellulose acetat e (CA) polymerblends, resulting in structur ed absorption spectra . Thephosphonate ester chelates metal nitrates, isolating

them before reacting with H 2S. The fluorescence re-sults, sh own in F igur e 6 for several P SP:CA composi-tions, showed a shift to lower energies with increasingamounts of PSP. This behavior indicated increasingpar ticle sizes with increasing PSP composition. Theparticle sizes measured by absorbance edge positionwere 44 Å (Figure 6a, 2.81 eV), 58 Å (Figure 6b, 2.63eV), and 82 Å (Figure 6d, 2.48 eV). The higher energypeak in Figure 6a may be due to excitonic fluorescenceor due to crystalline cellulose acetate being present.Cont inued work sh owed size quant ization by th e chang-ing bandgap an d str uctured absorption spectra.

2.1.3. S emicond uctor Com posites with Inorganic Ma-

trix Materials. A significant a mount of work in th e area

F i g u r e 5 . TEM micrographs of ZnS in a block copolymer. (a)Lamellar morphology, (b) spherical morphology. Reprintedwith permission from ref 52.

F i g u r e 6 . Fluorescence spectra of CdS in several polymerblend compositions. PSP:CA ) (a) 1:4, (b) 1:1, (c) 2:1, a nd (d)4:1. λex ) 400 nm. Reprinted with perm ission from r ef 57.


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of quan tum -confined semicondu ctors ha s been carriedout with h igh-tempera tur e glass, porous glass, zeolite,and sol-gel derived matrix materials. The small andregular pore sizes of these materials are useful incontrolling the particle sizes and distributions of thesemiconductor nan oparticles. Additionally, semicon-ductor/semiconductor composites have been found tohave enhan ced optical properties. Early work in thisarea was done on commercially available CdS xSe 1- x

glasses which are used as sharp-cutoff color fi l ters.Large size distributions and varying sulfur concentra-tions made these materials difficult to study quantita-tively. More r ecent work dea ls with precisely synth e-sized compositions. Figure 7 summarizes the synthesisof nan ocomposites in various glass a nd cera mic stru c-tures.

2.1.3.1. High-Tem perature Glasses. Synthesis of semicondu ctors in glasses in volves the incorpora tion of the necessary ions in the glass at high temperature,casting to form a m onolith, ann ealing to remove str esses,and h eat t reat ment t o crysta llize the semicondu ctor asshown in Figure 7a. Nonlinear optical measurements

were made very ear ly in t he work with glass na nocom-posites. In 1983, J ain and Lind studied degenera te four-wave mixing in commercial CdS xSe 1- x glass, and com-pared the r esults t o single-crystal CdS.6 They showedtha t th e glass could be used as an aberr ation corrector,with χ3 values as high as 1.3 × 10 -8 esu and a fastresponse time. Warnock and Awscha lom sh owed con-finement in these glasses using luminescence experi-ments.58 Cullen et al. fabricated directional couplersusing ion-excha nged waveguides from similar semicon-ductor-doped glasses.59

Roussignol et al. continued experiments on the glassesused by Ja i n and Li nd .60,61 In an extensive study,researchers at Corn ing Inc. reported resu lts concerning

commercially available filter glasses (CdSe xS1- x) as wellas experimental CdS and CdSe glasses.62 The experi-ment al glasses th ey studied showed featu red absorpt ionand luminescence spectra, indicating small monodis-perse particle sizes, which were confirmed by TEM.Other ph ases such as AgCl and CuCl were grown bysimilar techniques and resulted in quantum-confinedmaterials.

More recently, the first observation of resonatorlessbistability and nonlinear switching due to increasing

absorption in a semiconductor-doped glass was re-ported.63 A nondegenerate pu mp-probe technique wasused to measure these effects in CdS xSe 1- x-doped glass .Perfect switching behavior could be demonstrated bychanging the pump intensity a s shown in F igure 8.Input power above 167 kW/cm 2 resulted in a region of high absorption, while below this value the m ater ial wasvery transparent, defining two distinct regions of switch-ing and depha sing. Absorpt ion induced bista bility doesnot require ext ra resonat ors such as Fabry-Perotresonators or phase m atching elements t o create non-linear switching.

2.1.3.2. Porous Glasses. Porous glasses conta in well-defined pores which can assist in confinement of the

semiconductor clusters. They are attr active as hostmaterials because low-temperatur e solution and gas-phase synthesis techniques can be used. Figure 7bshows a typical synth esis procedure wh ere th e glass isinfi l trated with the metal ions and subsequently ex-posed to H 2S (or th e appr opriate coun ter ion) to form th esemiconductor n an oparticles. The work of Kuczynskiand Thomas discussed CdS prepared in porous Vycorglass.64 The CdS was confined in only one dimension,giving some excitonic str uctur e t o its a bsorption s pec-tru m, but ban dgap properties were similar to bulk CdS.The effects of methylviologen and water on the spec-troscopic properties including emission and quenchingwere studied.

F i g u r e 7 . Schematic of semiconductor nanocomposite syn-th esis in glass and ceram ic mat rixes. (a) Traditiona l glass, (b)porous glass or zeolite, (c) sol-gel glass.

F i g u re 8 . Switching behavior in commercially availableCdSSe glass. Reprinted from ref 63.






Luong discussed the growth of Cd, Pb, and Zn sulfidesand selenides in Vycor glass.65 For Zn- and Cd-conta in-ing crystallites, only a small shift from bulk propertieswas seen, due to broad particle size distributions and

par ticles larger th an th e Bohr r adius of th e exciton. PbSand PbSe showed quantum confinement in the glasswith blue-shifted absorption spectra which becamestructured upon heat treatment.

J ustu s et al. measu red th e optical properties of 50 ÅGaAs particles grown in Vycor glass.9 The nonlinearmeasurements showed n 2 of -5.6 × 10 -12 cm 2 /W, anorder of magnitude stronger than the bulk material .This is shown in th e Z-scan da ta pr esented in Figure 9,where th e nonlinear response increases with increasingpower. The valley which develops with increasingintensity indicates n onlinear absorption. The differencein transmission between the maxima and minima isdirect ly r elated to the change in refract ive index.

Because the peak of transmission precedes the valley,the response is negative. Measur ements were taken inthe technologically importa nt near IR r egion, althoughthis region is far from resonance for GaAs.

2.1.3.3. Zeolites. Zeolites are crystalline ceramicswith well-defined pores of uniform size and size distri-bution. The synthesis of composites based on t hesematerials i s quite simi lar to th at of porous glasscomposites shown in Figure 7b. Unlike porous glasses,the zeol ite pores are situated on a lat t ice so thatmater ials substituted in th em might form a superstr uc-ture as well as have quantum semiconductor properties.A survey of synthesis t echniques is present ed by Ozinet al.26

Wang and H erron presen ted absorption spectra fromCdS and PbS grown in two zeolites.66 A blue-shiftedabsorption was found for both semiconductors whichshifted to red a s th e concentr at ion of semiconductor wasincreased, shown in Figure 10 for a CdS/Zeolite Ycomposite. Above concentr at ions of about 4 wt %, th eoptical shift was abruptly arrested because the percola-tion thr eshold of the zeolite was reached, an d no cha ngewas observed with loading up to 18 wt % and 100 °Cheating. No intermediate absorption levels were ob-served between those shown in the figure. The thr ee-dimensional lattice allowed the synthesis of clusterswith solid-state behavior which differs from th e bulk.

Later some of th e same authors described workconcerning CdS clusters grown in other zeolite hosts.67

As had been seen earlier, the CdS particles remaineddiscreet within t he host a t low concentr at ion, but a boveconcentra tions of 4% a super cluster str uctur e developed.The supercluster structure had optical properties in-termediate between the individual clusters and thebulk. The aut hors indicated the poten tial of cont rollingthe stru cture a nd electronic properties by the choice of zeolite host m ater ial.

Wang and Herron reported the luminescence andexcited-state dynamics of CdS grown in zeolites X, Y,and A.68 Three emission bands were found in theyellow-green, r ed, an d blue, wh ich could be att ributed

to defects. The yellow-green em ission was at tribut edto Cd atoms, while the red a nd blue were attr ibuted toS-related defects.

Lat er Moller et a l. compar ed th e str uctu re of PbS intwo zeolites to PbS in a polymer m at rix using exten dedX-ray absorption fine st ructur e.69 The PbS was foundto be more confined in the zeolite ma trix tha n in th epolymer, with higher order at higher concentra tions.Considera bly larger PbS pa rt icles were genera ted in thepolymer m atr ix.

2.1.3.4. S ol-Gel Derived Glasses. Sol-gel type syn-theses allow for low-tempera tur e pr ocessing, high pu-rity, an d more flexibility in t he componen ts of the glass .Additionally, sol-gel precursors lend th emselves to film

Figure 9. Z-scan data for GaAs-doped Vycor glass. Reprintedwith permission from ref 9.

F i g u r e 1 0 . Absorption spectra for CdS in zeolite Y. (a) 1.1wt % CdS/Y, (b) 7.4 wt % CdS/Y, (c) m icron-sized CdS.Reprinted with permission from ref 66.




an d fiber applicat ions more read ily th an other glass a ndceram ic composites that h ave been discussed. Figure7c shows th is type of synth esis schemat ically. Typicallythe sol-gel glass is prepa red with t he meta l ions pr esentand exposed to S 2- (or other counterions) after glassform at ion. Levy an d Esqu ivas give a nice review of th euse of sol-gel matr ix materials t o design optical ma teri-als.70 They discuss CdSe a s wel l as laser dyes an dliquid-crystalline m ater ials as incorpora ted ph ases, alltak ing advant age of the sma ll pore sizes of the sol-gelderived materials.

Several sem iconductors were incorporat ed int o sol-

gel sil icate glasses by Rajh et al .71 The synthesisinvolved preparing the colloidal semiconductor with astabilizer (20-40 Å) an d then a dding tetram ethyl orth o-silicate t o form the m atrix. The samples were dried,but not heat treated, and sh owed shifted and featuredabsorpt ion for severa l compositions sh own in Figur e 11.Some solut ions lost exciton featu res du rin g drying whichcould be recovered by exposure to H 2S.

Stru ctured an d shifted absorban ce spectra were seenin CdS nan oparticles in sol-gel glass depend ing on h eattrea tment of the glass.72,73 The crystalline str uctur e of the na nopart icles was confirmed by X-ra y diffra ction.Othm an i et al. discussed the pr epar at ion of CdS in sol-

gel derived SiO2 at concentra tions up t o 20%.74 X-rayand Rama n st udies were combined to predict a par ticlesize of about 7 nm . Nogami et al. report ed the cont rolledprepar ation of CdS xSe1- x in sol-gel derived SiO2 glass.75

Shifted absorption edges a nd bandgaps varying withparticle size were shown, but the absorption spectrawere featur eless due to large size distributions.

Glasses derived from sol gel precursors for SiO 2 and1.4Na2O-20.8ZrO2-77.8SiO2 were u sed to st abilize in

situ growth of CdS crystall i tes.7

The sodium glassshowed much higher stability to CdS oxidation at hightemperatur es than the simple SiO2 glass. Absorptionedges were featu reless and χ3 parameters of about 5 ×10 -12 esu were measu red for several conditions. Thissynthesis of a mixed oxide glass took a dvanta ge of thecompositional flexibility of t he sol-gel route.

2.1.3.5. S emicondu ctor Matrix Com posites. Thin-filmcomposites have been constructed which contain semi-conductor nanoparticles in a semiconductor matrix.76

The synt hesis involves prepa ra tion of quan tum -confinedCdSe an d CdSe coated with ZnSe by a colloidal met hod.The nanoparticles are then dispersed by an electrospraydur ing the orga nometa llic chemical vapor deposition of

a ZnSe thin film. Becau se the ban dgap of ZnSe is muchhigher than CdSe, the nanopart icles are very wel lisolated from each other. The photoluminescence be-havior was measured and showed that nanoparticlestha t wer e coated with ZnSe before film format ion h admuch higher efficiency. Composites such as these maybe more easily integrated into electronic devices becausethe conductivity of the matrix can be controlled bydoping.

2.1.4. Controlling Particle S ize. In all of the semi-

conductor work discussed here, the size and size distri-bution of the particles have been of great interest .Par ticle size an d size distribution ar e critical in m axi-mizing enha ncement in qua ntu m-confined systems. Thepart icle size affects the magnitude of the shift inabsorption (cha nge in bandgap), while the distr ibutionaffects the strength of the quantu m effect at a givenwavelength. These properties can be contr olled by thepolymer matrix architecture (2.1.2.2), porous matrixmaterials (2.1.3), additives and heat treatments (2.1.4.1),an d ex situ pa rt icle-capping meth ods (2.1.4.2). Polymerarchitecture and porous matrix materials have beendiscussed and typically result in a fixed particle sizedetermined by th e matr ix st ructure. Other methodsdiscussed in this section can result in tunable particlesizes which might be par ticularly useful in chan ging thewavelength of operation in optical devices.

2.1.4.1. Heat Treatm ents and Add itives. Heat treat-ments and additives have been coupled with compositesynth esis stra tegies to control particle size and distribu-tion. In t he ear ly work of Wang et al., shown in Figure2a, PbS particle size was controlled by varying theconcentra tion of the Pb2+ and heat treating the samples.28

Similarly, work from the same group on CdS in Nafion(section 2.1.2) also found t hat heat tr eatm ent could beused t o contr ol par ticle size.3 Sankar an et al. found th atZnS cluster size was increased when the block copoly-

mer films wer e exposed to H2S at higher temperaturesor in the presence of solvent vapor (section 2.1.4).52

Annea ling after ZnS form at ion, however, did not resu ltin particle growth probably because the matrix actedas a diffusion barr ier. Heat tr eatm ents ha ve also beenused in ma ny of the glass an d cera mic mat rix materialsdiscussed previously.72,73

Kyprian idou-Leodidou et a l. var ied th e size of PbS inpoly(ethylene oxide) from 4 to 80 nm using acid andsurfactant additives.77 With out a dditives, part icle sizeswere about 29 nm. Acetic acid increased t he pa rticlesizes, while sodium dodecyl sulfate, a surfactant, low-ered th em. Alth ough the cont rol of average part icle sizewas demonstrated, the size distributions were quitelarge.

In our work with PbS synthesized in poly(vinylalcohol) (PVA), attempts were made to change particlesizes using similar treatments.35 In a sta ndar d reactionof PbS in PVA, 4 nm part icles were reproduciblysynthesized as shown in the absorbance spectrum inFigur e 2b. The concentr at ion of th e PVA was increasedfrom th e stan dar d 0.1% up to 5% in wat er an d found t oha ve no effect on th e par ticle sizes while improving thefilm-forming abilities and increasing photostability.Acetic acid was added t o the synth esis and s howed anincreas e in par ticle size by X-ray line broadening seenin Figure 12. As shown, th e standa rd rea ction produced

F i g u r e 1 1 . Absorption spectra for several semiconductorsprepared in sol-gel glasses. Reprinted with permission fromref 71.




a particle size (4 nm) which resulted in a featureless

spectrum (Figure 12a). The a ddition of acid clearlycaused an increase in par ticle size which ma de the P bSpeak s visible (Figure 12b-f), indicating crysta llite sizesbetween 10 and 15 nm (calculated using the Scherrerequation). The a bsorption spectra lost their excitonfeatures upon addit ion of acid, probably due to abroadening of the size distribution. Surfactant s a ndhydrochloric acid were also added t o the synthesis, butthey did not resu lt in par ticle size cha nges or absorptionedge shifts.

2.1.4.2. Ex S itu S emicondu ctor Capping. In furtherefforts to control semiconductor particle sizes and sizedistributions, capping methods have been developed.Capping agents are typically th iols or mercaptans,

which can compete with sulfur (or other counterions)for Pb2+ or Cd2+ sur faces. In doing so, the capping agentto sulfur r atio chemically determines t he pa rticle size.In addition to contr olling t he pa rticle size, the cappingagents a ct as a surface treat ment for the par ticles whichmay aid th eir dispersion into various ma trix mat erials.In th ese experiments, the par ticles are prepar ed ex situand then dispersed in a polymer matrix to form nano-composites.

Figur e 13 shows the form at ion of th e capped pa rt iclesschema tically. Typically, th e sulfide (or oth er coun te-rions) and capping agent (RS-) i ons a re m i xed i nsolution. A solution conta ining the met al ions is addedan d semicondu ctor pa rt icles form with sizes determ ined

from th e S2-

/RS-

ra tio. The resulting particles can bestudied in solution or can be dried to a powder. Byredispersing the capped particles in a polymer solution,nanocomposites can be formed.

Ea rly work in th is ar ea focused on colloidal synth eseswith no effort to form composites.78 -82 Several thiolswere studied and found to control particle size withrespect to the ratio of capping agent to counterion inthe system. Researchers at DuPont synthesized CdSpar ticles which were capped with th iophen ol.46,83 UsingX-ray diffraction and absorbance measurements, theyshowed th at the rat io of capping agent to sulfur chemi-cally controlled the par ticle size. The n onlinearity of these clusters was measur ed by third har monic genera-

tion.84 χ3 increased from 4.7 × 10 -12 esu for a 7 Åmolecular cluster to 3.2 × 10 -10 esu for 30 Å cluster s.

Further characterization of these particles includedcapped precursor synthesis85 and inelastic neutron-scattering studies.86

Some work has been done incorporating the cappedparticles in polymer mat rix materials. Majetich a ndCart er reported the synthesis of colloidal CdSe cappedwith six different term inating ligan ds to study su rfaceeffects.4 Samples were prepared for optical measure-men ts by dispers ing th e colloids in solid epoxy or poly-(methyl metha crylate) matrix mat erials. The bandgap,absorption oscillator stren gth, an d spectral h ole widthand t rapping t ime were unaffected by the varioussurfaces tested. However, the optical hole-burn ing andluminescence experimen ts were a ffected by the sur faces

of the par ticles. Absorption spectra showed excitonicfeatures, indicating a small size distribution in theparticles. Nonlinear optical measu rements indicatedvery promising χ3 values ranging from 6.5 × 10 -8 to 2.9× 10 -7 esu (R2, 8.7 × 10 -6 to 1.2 × 10 -5 cm/W).

Our group has studied capping syntheses for PbSpar ticles for composite a pplicat ions. 35 Thiophenol an dseveral other t hiols ha ve been investigated as possiblecapping agents to control particle size as well as tailorthe pa rticle sur faces for dispersion into a mat rix. Allcapping systems show the expected Scherr er line broad-ening in X-ray diffraction stu dies. Pr eliminar y workhas begun on redispersing the capped part icles inpolymer mat rix ma terials for composite applications.

Figure 12. X-ra y line broadenin g due t o added acetic acid inth e forma tion of PbS in PVA. (a) No ad ded a cid, (b) 0.015 M,(c) 0.076 M, (d) 0.38 M, (e) 0.46 M, (f) 0.53 M.

F i g u r e 1 3 . Schemat ic of ex situ capped semiconductor (MS)synthesis. RS - is the capping agent , S2- is the sulfur, and M 2+

is the met al ion.





2 .2 . N a n o c o m p o s i t e s Co n t a i n i n g P o l y m e r s a n d

Smal l Molecul es . Many polymers and small moleculesexhibit interesting optical effects including second ( χ2)and third ( χ3) order nonlinear ities and laser pr operties.Using the nanocomposite structure, polymers and smallmolecules such as these can be incorporated into glass,ceramic, and polymer matrix materials as shown inFigure 14. The m at r ix m at er ia l can s t abi lize t hecompounds , improve the optical pr operties, and improvethe processability as well.

2.2.1. Th ird-Order Nonlinear Optical Polym ers in

Glasses. Pra sad a nd co-workers have presented severalpapers concerning poly( p-phenylenevinylene) (PPV) insilica and V2O5 matrix materials. PPV is a conjugatedpolymer with a fast χ3 nonlinear optical response;however, it shows high optical losses when used in thebulk. Sil ica and V2O5 both show excellent photonicproperties, in particular low losses, and improve theoptical properties of the PPV when they a re combinedin a composite. The composite prepar at ion is shown

schematically in F igur e 14a.Initial work with silica glass matrixes showed thatcomposites exhibited good optical qu ality.87 The UV-

vis spectrum of the PPV composite showed a slight blue-shift compared to bulk PPV indicating that the conju-gation length was redu ced in the composite. The samegroup performed a study of the N LO response in PPV-

silica composites using degenerate four-wave mixingand optical Kerr gate techniques.88

Using similar composites, with V2O5 inst ead of silica,two-dimensional gra tings were pr oduced by introducingrefractive index changes or sur face relief patt erns witha laser.89 Gratings such as these could be useful inlaser-arr ay systems and multichann el optical com-

munications. A later report on V2O5 composites showedtha t th is matr ix resulted in a longer conjugation lengththan seen in the silica composites.8 The measured χ3

values were 3 × 10 -10 esu for PPV/silica and 6 × 10 -10

esu for PPV/V2O5, showing a slight improvement in theV2O5 system.

2.2.2. Sm al l Mol ecul es i n Glasses and Cerami cs.

Glass and ceramic matr ixes can improve the optical ,mechanical, and t herm al pr operties of small moleculesas discussed in th e review by Levy and E squivas.70 Th e

synthesis of small molecule glass composites is verysimilar to that shown in 14a, except that the polymeris replaced with sma ll molecules. Ear ly work in th earea of small molecules in glass was reported by Avniret al. for rhodamine 6G/silica composites.90 Rhodamine6G is a laser dye which displays fluorescence, absorp-tion, and em ission in t he visible region bet ween 500 an d600 nm. This dye is typically used in solution a nd ha sproblems with concentration quenching and photosta-bility. Many virtu es of glass-based composites oversolut ions include th e rem oval of int ermolecular inter ac-tions between dye molecules, isolation of impurities,isolation from surrounding atmosphere, good thermaland photostability, and good optical properties am ong

others. Several other laser dyes have also been suc-cessfully incorporated into sol-gel derived matr ix ma-terials an d reta ined their optical properties.91 The effectof the dyes on th e sol-gel synthesis has been studied.

A significant amount of work has been done incorpo-rating enzymes and other proteins in sol-gel materi-als.92,93 Several pa pers ha ve focused on optical a ppli-cations of these composites. Bacteriorh odopsin is alight-transducing protein which could be useful as anactive component in optically coupled devices. Afterencapsulation in sol-gel silica glass, the optical andphotocycle behavior was ret ained .94 This material mayalso be useful as an optically based ion-sensor. Similarcomposites were r eported which were potent ially usefulas a real-time hologra phic medium.95

Another light-tra nsdu cing protein, phycoeryth rin, hasalso been incorporat ed int o sol-gel silica glass.96 Ab-sorption and fluorescence measurements of the com-posite showed that the protein retained i t s opt icalproperties in the matrix and even showed enhancedstabili ty toward photodegradation. Figure 15 showsthat the optical properties were retained in the com-posite by compar ing the a bsorption a t severa l stages inthe sol-gel process (Figure 15a -c) to the optical proper-ties in solution (Figur e 15d). Only a slight change inthe intensity of the peaks at 565 and 495 nm can beidentified. The fluorescence spectr a ar e also un changed

with the addition of the ma trix, and th e mat erial canexhibit two-photon fluorescence. Possible applicat ionsof the material include biosensors, 3D biomolecularimaging, and 3D optical st orage.

Zeolites have been examined as potential hosts forsmall molecules with second-order nonlinearit ies.97

Composites were prepared as shown in Figure 14b,where t he sma ll molecule was vaporized to infiltra te th ezeolite. These studies have shown th at n oncentr osym-metr ic hosts were necessar y to reta in the χ2 propertiesin t he sm all m olecules and in som e cases t he χ2

properties were enha nced by the zeolite ma trix. Thestructure of the small molecules in the zeolite host wasdiscussed. The composites can be tu ned by changing

F i g u r e 1 4 . Schematic of composite synthesis with opticallyactive polymers and small molecules. (a) Glass matrix, (b)zeolite matrix, (c) polymer matrix.





the loading level, changing the small-molecule structure,

and changing the host framework.2.2.3. S m all Molecules with S econd-Order Nonlin-

earities in Polymer Matrixes. Tripath y et al. have beenleaders in the study of sma ll molecules with χ2 nonlin-earity embedded in various high-T g, cross-linkablepolymer mat rixes. Several var iations of mat rixes andsma ll molecules have been studied. As shown in Figure14c, after mixing the polymer and small molecules, thecomposites were poled during the cross-l inking andcooling procedures to lock in alignment of the χ2

molecules. In some cas es the NLO chr omophore couldreact with the matrix to further lock in orientation.While these h ybrids are n ot tru e composites since theyare largely single-phase materials, they share many

features of optical composites.One example of a system in which the matrix andchromophore intera ct involves an epoxy-based NLOpolymer a nd a small-molecule NLO chromophore tha thas been functionalized to be chemically reactive withthe epoxy.98 By photo-cross-linking in the poled state,the optical nonlinearity is stabilized, even at 100 °C.Similar results were found for an alkoxysilane dye,which was incorporated into a si loxane-based hostpolymer.99,100 In this case, the host polymer could becross-linked a nd could incorporat e th e dye u sing sol-

gel chemistry. The sta bility of the χ2 response a t 100°C was discussed.

Polyimides were also studied as mat rix mat erials for

NLO dyes because of their low dielectric consta nt , easeof processing, and h igh-temper at ur e sta bility.101 Figure16 shows the stability of the second harmonic coefficientof such a composite at both r oom tem perat ur e and 120°C. In th is case th e NLO dye was incorpora ted into thepolyimide u sing s ol-gel chemistry. The composite wasquite stable at room temperature; however, it showeda slight loss of χ2 strength at 120 °C. This result issimilar t o all of the composites discussed in th is section.Finally, NLO a ctive chromophores ha ve been incorpo-rated into hexakis(methoxymethyl)melamine matrixmaterials.102 The melamine matrix shows good trans-paren cy an d high T g an d can be cross-linked us ing sol-

gel chemistr y. The resu lting composites ar e slightly less

stable than those shown in Figure 16 but show lowoptical losses.

3 . N a n o c o m p o s i t e s w i t h L a s e r A m p l i f i c a t i o n

P r o p e r t i e s

Using th e optical composite pr inciples, our group h asdeveloped a method to use the nanocomposite structureto crea te films with s olid-sta te laser properties.103,104 Bysynthesizing composites, the cumbersome process of single-crystal growth typically used for solid-state laserscan be avoided. Additiona lly, th ey can be processed astechnologically useful fi lms or fibers instead of asmonolithic materials. This type of structure may bevery importa nt for optical commu nications applications.

The solid-sta te laser mat erial our group h as focusedon is chromium -doped forst erite (Cr -Mg2SiO4). In theform of a single crystal, this mat erial ha s shown tu na blelasing in the technologically att ra ctive nea r-IR r egion

(1167-1345 nm).105

1300 nm is a particularly impor-tan t wavelength in optical commu nications because itis a dispersion minimu m for silica waveguide ma teria ls.However, no solid-sta te laser am plifiers for 1300 n m a recurrent ly available. Using the nanocomposite structur e,we ha ve developed laser amplifying films t ha t conta inCr forsterite.

Unlike th e bulk of th e work with semicondu ctors, ourcomposites are prepared ex si tu, with much largerparticle sizes (∼100 nm), which require tailored refrac-tive index matrix materials to avoid significant scat-tering. Three steps used in this nan ocomposite synthe-sis are outlined in Figure 17. First, sma ll par ticles of forst erite were pr epar ed using a d ispersion-polymerized

prepolymer.106

The polymer was based on silicon- andmagnesium-conta ining meth acrylate m onomers, whichwere ra ndomly copolymerized resu lting in 100-500 nmsize beads. The beads acted as size templates as th eywere heated to 1000 °C to remove the organics andcrystallize the forsterite. The resulting forst erite par-ticles were about 100 nm in size.

Second, a polymeric matr ix material was preparedwith the average refractive index of forsterite (1.652 at589.3 nm).107 Aromat ic and br omina ted monomers wereused to atta in this high RI. Several copolymer systemswere studied as potent ial matr ix ma terials, but the mostsuccessful was the tribromostyrene/naphthyl methacry-late syst em. The refra ctive index could easily be fixed

F i g u r e 1 5 . Absorption spectra of phycoerythrin in sol-gelglass a t several stages of the sol-gel process (a -c) and insolution (d). Reprinted with permission from ref 96.

F i g u r e 1 6 . Time behavior of the second harmonic coefficientof a polyimide-small molecule composite. Reprinted withpermission from ref 101.





to 1.65 by the composition of the copolymer, and thematerial showed good near-IR transparency and goodfilm-forming properties. Finally, the ma trix and par -t icles were mixed, and a f i lm was cast on a glasssubstr ate. Typical film dimens ions were 2 cm long, 1cm wide, and 5 µm thick.

Optical measurements were performed on the com-posite films to exam ine amplification behavior.1 Th eexperiment consist ed of collinea r bea ms of a Cr forst er-ite reference at 1.24 µm and a Nd -yittrium aluminumgarnet pum p at 1.06 µm which were passed thr ough th efi lm as shown in Figure 18a. The pump could beblocked independently from the reference, and wasremoved from th e signa l using a monochr omator. Bycomparing the detected signal at the reference wave-length before and during pumping of the sample, a2-fold increase in signal was observed. Figure 18bshows the relat ionsh ip between amplificat ion an d pum ppower for these composites. The gain of 300 dB/mexceeds th e gain of single-crysta l Cr forster ite a s well

as Er-doped silica fibers used in optical communicationsat 1.55 µm, which show a gain of 3 dB/m. Oth er solid-stat e laser mat erials an d optically functional cera micsare expected to benefit from composite structures suchas these.

Recently our group h as extended t he par ticle synthe-sis an d composite constr uction to another system.1 Crdiopside (Cr -CaMgSi2O6) is a m ater ial that fluorescesin the n ear -IR (700-1200 nm); however, a s ingle crysta lof this mat erial cann ot be prepared du e to incongru entmelting. Thus t he potentially useful solid-state laserproperties of this mat erial have not been stu died. Wehave synth esized this mat erial using a method similarto the Cr forsteri te synthesis and embedded the par-

ticles in a h igh RI mat rix. Optical am plification of 95dB/m h as been m easur ed in th ese composites from 760to 810 nm using a Ti sapphire laser as the referencesignal and an a rgon laser as a pump. This particularcomposite syst em is a n excellent examp le of how nan o-composites can be used to improve the processabilityand functionali ty of a material which otherwise wasimpossible to use.

4 . P r a c t i c a l A p p l i c a t i o n s a n d I s s u e s

4 .1 . R e a l D e v ic e s . A limited amount of work hasbeen published on devices based on polymer semicon-ductor composites including two papers that discusselectroluminescence effects. Colvin et al. prepared light-emitt ing diodes from a layered CdSe/ p-phenylenevi-nylene (PPV) composite.108 The effects fr om th e polymercould be separated from the CdSe in the diode operation.The very low t hr eshold voltage of 4 V, lower tha n forPPV alone, caused luminescence from the CdSe fromred to yellow (dependin g on pa rt icle size), while lar gervoltages caused t he PP V to luminesce in the green . Thisis seen in Figure 19 as a change in the shape of theelectr olum inescence curve with chan ging voltage. Thisvoltage-tun able light source could h ave importan t a p-plications in display technology where the appliedvoltage could determine th e color of a pixel. Threeparticle diameters were examined.

Dabbousi et al. sh owed electroluminescence using ahomogeneous composite of 5-10% CdSe in poly(vinyl-carbazole) and an oxadiazole derivative.109 Experimentswith th ree pa rticle sizes were performed, with em issionsfrom 530 to 650 nm. The tur n-on voltage for thesecomposites was a bit higher , at 13 V. These compositesalso displayed a voltage dependence on light emission,going from red to white with increas ed voltage.

F i g u r e 1 7 . Schematic of ex situ optical composite synthesisfor la ser a mplifying films.

Figure 18. (a) Schema tic of the a mplification experiment . (b)Amplification in a Cr forsterite nanocomposite as a functionof pump power. Reprinted with permission from ref 104.






4 .2 . I s s u e s i n N o n l in e a r O p t i c a l Co m p o s i t e s .

Traditional nonlinear optical materials such as ba riumtitan at e ar e difficult to process an d integra te into opticalapplications as they are typically used in single-crystalform. Additionally, stronger nonlinear optical proper-ties are desirable to make these materials technologi-cally useful. Commonly stu died semiconductors suchas CdS and PbS must be used in the nanocrystal formin order to tak e a dvantage of their optical pr opertiesas they are opaque in the bulk and the nonlinear opticalproperties are enha nced at small crystal sizes. Usingnanocomposite structures the processability and stabil-ity of these mater ials can be great ly improved. Severalissues rem ain to be solved before these mat erials willbecome integrated into mainst ream devices.

In quan tum -confined sem iconductor composites, t heparticle size an d size distributions st i ll need greatercontrol. Enha ncement in th e optical properties cancome only when a very na rr ow distr ibution of cont rolledpar ticle size is used. Additiona lly, th e ability to gener -ate a ra nge of par ticle sizes in one system is desirableas this would allow operation at many wavelengths.Current techniques ar e showing improvement in thisarea , bu t m ore i s needed t o m ake t hese m at er i a l stechnologically useful. In system s with polymers andsmall molecules, stability and processability are key tomak ing u seful devices.

4 .3 . I s s u e s i n O p t i c a l Am p l i f i c a t i o n w i t h C o m -

p o s i t e s . The area of optically amplifying composites

is quite new, an d ma ny issues n eed to be solved beforethese composites can be integrated into real opticalsystems. The size of the part icles used is very importan tto the scattering char acterist ics of t he composites.Current techniques discussed h ere pr ovide laser par-ticles that are larger than would be desirable to avoidall scatt ering. Additionally the u niformity of the com-posites must be studied to ensur e that the par ticles aredispersed evenly and without agglomeration in thecomposites. Finally, the s tability of the composites,part icularly the polymer matrix materials, must beevaluated. Both pr olonged exposur e to laser r adiationand heat evolution from the laser particles may beissues.

5 . O t h e r O p t i c a l A p p l i c a t i o n s

5 .1 . C o m p o s i t e s f o r H i g h R e f ra c t i v e I n d e x A p -

p l i c a t i o n s . High refractive index polymers h ave man yapplications ranging from antireflection coatings forsolar cells to high RI lenses. Our group ha s synth esizedhigh RI copolymers (up to 1.7) as index-matched ma-

tr ixes for solid-stat e laser conta ining composites (section3).107 Semiconductor polymer composites can exhibiteven higher refractive indices due to the high RI of thesemiconductor pha se.

In the work by Zimmerman et al., composites of PbSand gelat in were synthesized and cont inuously in-creased th e r efra ctive index of the gelatin from 1.5 to2.5.110 These materials represented some of the highestrefractive index polymers a vailable and were ta rgetedfor a nt ireflection coatings for solar cell mater ials. Kyp-rian idou-Leodidou et a l., from th e same r esear ch group,var ied th e size of PbS in poly(eth ylene oxide) from 4 to80 nm by adding surfactants a nd a cids.77 Again opticalmeasurements were used to evaluate refractive index,

showing composite RI values a s high as 3.9. As shownin Figure 20, the extrapolated RI of particles greatertha n 25 nm in size was found to agree with the bulk RIof PbS (4.3). However, the RI was highly dependen t onparticle size below 20 nm, with 4 nm particles showinga RI of about 2.5. These effects may be due to qua nt umconfinement as strong confinement should becomeevident at particle sizes near and below 20 nm (Table2). The absorpt ion of t he composites at 632.8 nmdecreased as the particle size decreased, also indicatingconfinement effects. TEM an d X-ray were used tocharacterize the composites and showed broad sizedistributions.

5 .2 . T r a n s p a r e n t Ma g n e t i c C o m p o s i t e s . Trans-

parent magnet ic materials have many applicat ionsincluding microwave m agnet ooptical modulat ion of vis-ible lasers with very low modulation power per unitban dwidth, optical deflection an d isolation, magn etoop-tic displays, and h olograms. Usable transparent mag-nets must have high Curie temperatures, avoid bire-fringence, and have fundamental absorption edges inthe nea r UV region. The composite stru ctures discussedhere m ay m ake t ransparent m agnetic m at er ia ls areality.

Several groups have prepared magnetic compositeswith both polymer and glass matr ixes. Okada et a l .reported t he p roduction of sub-100-nm Fe2O3 particlesin a multibilayer film.111 Opaque composite films that

Figure 19. Electroluminescence from a CdSe/PPV composite.Reprinted with permission from ref 108.

Figure 20. Effects of PbS particle size on the refractive indexat 632.8 n m. Na nocomposite values (O), extra polated valuesfor PbS (b). Reprinted with permission from ref 77.





contained magnetite behaved as ferromagnets withcoercivities of 20 Oe. Papa efthymiou et al. reportedmagnet ic studies of 9-20 nm Fe particles coated withFe2O3 embedded in wax, but th e optical properties werenot ment ioned.11 2 Shull et al . prepared F e2+ and Fe3+

confined to 20 nm regions in a silica matrix using a sol -

gel method.113 Again the magnetic properties wereexamined with n o ment ion of tra nspar ency. Zhao et al.discussed work embedding colloidal Fe2O3 magneticparticles in bilayer lipid membranes.114 The sa mples

were thin enough to be studied in transmission, butgeneral absorption properties were not measured. Re-flective properties and t he Kerr a nd Fa ra day rotationswere examined.

Ziolo et al. report ed th e production of optically tra ns-parent magnetic composites for the first time.115 γ-Fe2O3

was grown in a polymeric ion-exchange resin by intro-ducing the Fe3+ and then exposing to H2O2 at 60 °C,very similar t o th e process for prepa rin g CdS in Nafionshown in Figure 3 (steps 1b, 2c). The par ticles creat edranged from 50 to 250 Å in size, which resulted insuperparam agnetic properties. If the ionomer resinwere loaded multiple times, the particle size did notincrease but the concentra tion of particles did. Thecomposites showed appreciable magnetization at andbelow room t emperature. The highest sa tura tion m o-men t observed wa s 46 emu /g for 250 Å par ticles, whichis considerably stronger than transparent crystals of FeBO3 (3 emu/g) and FeF 3 (1 emu/g). The absorptioncoefficient was n early an order of magnitude less th anthe bulk crystal , and the composite had a refractiveindex of 1.6.

Clement et a l. present ed very promising resu lts for amat erial which exhibits both χ2 nonlinearity and mag-netic properties.116 This was accomplished by int erca-lating a χ2 small molecule into a ferr imagnetic layeredmaterial . The χ2 res ponse sh owed very high efficiency,

and the magnet ic properties could be seen below 40 K.Miller ha s also out lined man y poten tial ap plicat ions formolecular/organ ic magnet s.117

5 .3 . H a r d Tr a n s p a r e n t C o a t i n g s . By combiningthe strength and hardness of ceramics with the pro-cessability and ductility of polymers, novel transparentmat erials have been synth esized. Typically these ma-terials are prepa red using an in situ synth esis like thatshown in Figure 14a. The polymer (or polymer precur-sor) and silica precursor (or other alkoxide) are m ixedin solution and cast as a fi lm or monolith which issubsequent ly heated to convert the pr ecur sors to theirfinal form. Hard t ransparent coat ings and barrierlayers are the primary applications of polymer n ano-composites prepar ed with reinforcing cera mic phases.A significant a moun t of work in t his ar ea ha s been withpolyimide-silica hybrid materials which have highthermal stabili ty, toughness, and hardn ess.

Our group at Cornell has investigated silica/polyimidematerials that show improved har dness and moduluswhile retaining tra nspar ency.118 The hybrids are basedon two different silicon-containing poly(amic acids)(PAA) which are mixed with the silica in situ using sol -

gel chemistr y. The poly(amic acids) are t ailored t o bechemically reactive with th e silica n etwork. One PAAcont ains a short siloxane segm ent wh ich can be cleavedto chemically bond with the silica a s it form s. The other

PAA is end functionalized with ethoxy groups which canparticipate in the sol-gel chemistry.

The hybrids formed extremely small silica domainsas sh own by SEM studies.119 As expected, the ha rdn essof both types of hybrid increased with increasing silicaconten t a s shown in F igure 21. A 17-fold increase inhar dness was observed for the si loxane-containinghybrids. The modulus of th e hybrids was also improvedwith increasing silica content. Other work in the ar eaof polyimide-silica hybrids ha s confirmed th at chemicalrea ctivity between ph ases imp roves homogeneity of th eresult ing materials.120,121

6 . Co n c l u s i o n s a n d P r o s p e c t s

Nan ocomposite str uctures based on embedded fun c-tional materials in processable matrixes have manyoptical applications. Pa rticulate phases can be prepa redin situ or ex situ a nd ma y have properties ran ging fromlaser amplification to improved har dness. Matr ix ma-terials can be polymers, glasses, or ceramics. Compos-ites such as these have been used to prepare materialswith nonlinear optical properties, laser properties,magnetic properties, and those with high refractiveindex.

Semiconductor particles have a wealth of opticalfunctions and can be prepared in polymers, ceramics,and glasses. Of part icular interest ar e their third-ordernonlinear optical properties, which may be useful forall optical switches. By processin g in film or fiber form s,utilizing a composite structure, these devices will be

more easily integra ted into targeted applicat ions. Com-posites contain ing optical polymers a nd sma ll moleculesfind advantages over the bulk materials in stability andprocessability. Solid-sta te laser ma terials show greatpromise as amplifying films in the near-IR and beyond.Finally, novel transparent magnetic materials have alsobeen synthesized using nanocomposite principles andhave prospects in printing, data storage, and magne-tooptics.

A c k n o w l e d g m e n t . L.L.B. is grat eful to th e Na tiona lScience F oundation, Department of Education, andCornell Materials Science Center for support of thisresearch. Eric Kutcher participated in the PbS/PVA

F i g u re 2 1. Hardness of siloxane containing (b) a n d e n dfunctionalized (2) hybrids as a function of si l ica content.Reprinted with permission from ref 119.




composite studies. Todd Krauss a nd Inu k Kang h avecontr ibuted to u seful discussions.

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