Student Project Abstracts 2005 - Pluto - University of Washington

OPTIMIZATION OF SEMICONDUCTOR NANOPARTICLE SYNTHESIS AND INTEGRATION INTO SOL-GEL MONOLITHSFigure 3. Absorbance and Fluorescence Spectra of CdS-1 and CdSe-16 (24minutes), the nanoparticle samples used in sol-gel inclusion experiments.Figure 3: Absorbance and Fluorescence Spectraof CdS-1 and CdSe-16 (24 minutes), thenanoparticle samples used in sol-gel inclusionexperiments.108 CMDITR Review of Undergraduate Research Vol. 2 No. 1 Summer 2005Once the reaction was complete the mixture was separatedanticipated, proving the nanoparticles hadusing a separatory funnel and the aqueous phase discarded.the desired optical property ofThe remaining organic layer was then washed with ethanolphotoluminescence while the second wasand centrifuged three times. The resulting solid was dispersedmuch smaller and red-shifted. This wasin dichloromethane and allowed to dry overnight yielding yellowtodueorangetocrystallineunpassivatedCdS nanoparticles.sites on theIt wassurfacelater notedofthatthecarryingnanoparticlesout these washingscalledresulted“surfacein a substantialtrapsloss”of product that trapped and diminished photons the desired and optical released properties them of the atSC-NP’s. higher wavelengths. This second low-Cadmium energy peak selenide increased nanoparticles in were size synthesized as the size using ofthe same the procedure CdS nanoparticles as the cadmium increased. sulfide NP’s with Surface slightdifferences. traps were The molar not quantities a problem of the in reagents the spectra were scaled fordown the by a cadmium factor of five selenide and selenourea though, (0.016g, indicating0.12mmol)replaced that thiourea these in the NP’s aqueous had phase. more In replacing thoroughly the sulfuratom passivated with selenium surfaces. it became necessary The to intensity degas the aqueous of thephase CdSe with argon particles prior to was the reaction. also much This prevented greater oxidationthat of the of precursors. CdS at Only the oleic same acid concentration was used as the capping givingthanligand them during greater CdSe formation. efficiency. Also, the formation of CdSewas considerably quicker than that of CdS due to the fasterdecomposition rate of selenourea, resulting in a reaction timeSol-Gel Formationof only three minutes at 100˚C. Because of this, the synthesiswas also performed several times at 80˚C, resulting in aSol-gels are formed by the hydrolysisof an alkoxide followed by condensationrequired time of 16-20 minutes to reach the desired nanoparticlediameter. Due to the product loss that occurred during(Figure 4). Deionized water (0.214 mL,12mmol) and hydrochloric acid (0.56the ethanol washing/centrifuging process of the CdS nanoparticles,only an extraction was performed on the CdSe reactionmol) were added to tetramethylorthosilicate (TMOS, 1 mL, 6.7mmol) in amixture and the organic layer containing the nanoparticles wastransferredsmalldirectlyvialtoanda storagestirredvial.for 15 minutes toThehydrolyzeSC-NP’s werethe methylcharacterizedterminatedby UV-Visendsabsorbanceof theand alkoxide, fluorescence spectra yielding (Figure the 3). precursor SC-NP’s typically solution. havea high Once molar completely absorptivity, which hydrolyzed was observed (evidenced in the analyses bytaken the of the evolution samples. Due of to the the liquids incredibly to small one size phase) of thenanoparticles the precursor (5-7 nm for was CdS & added 2-3 nm for to CdSe), a disposablethe absorbanceacrylate spectra were cuvette blue shifted containing relative to phosphate the bulk material. bufferThe (20mM, fluorescence pH spectra 7, 2 mL) for the and cadmium deionized sulfide nanoparticlesin contained a 1:1 two ratio peaks. with The the first volume high-energy of peak solvent waswateranticipated, containing proving nanoparticles.the had the desired opticalproperty of photoluminescence while the second was muchsmaller and red-shifted. This was due to unpassivated siteson the surface of the nanoparticles called “surface traps” thattrapped photons and released them at higher wavelengths.This Figure second 4: low- Schematic energy peak representation increased in size of as hydrolysis the sizeof the CdS and nanoparticles condensation increased. occurring Surface during traps were sol-gel not aproblem in the spectra monolith for the cadmium formation. selenide though, indicatingthat these NP’s had more thoroughly passivated surfaces.The intensityThe firstof thevariableCdSe particlesoptimizedwas alsowasmuchthegreaterpHthanofthat oftheCdS atbufferthe same concentrationsolutiongivingused.them greaterTheefficiency.polymerization (solidifying) rate of the solgelincreased in direct proportion with pH.This was initially a problem as the sol-gelwould polymerize quickly, entrapping airbubbles and weakening the matrix. It was