One-Step Preparation of Strongly Luminescent and Highly Loaded CdSe QuantumDot-Silica Films

Sanghwa Jeong,† JinSik Lee,† Jutaek Nam,† Kyuhyun Im,‡ Jaehyun Hur,‡ Jong-Jin Park,‡

Jong-Min Kim,‡ Bonghwan Chon,† Taiha Joo,† and Sungjee Kim*,†

Department of Chemistry, Pohang UniVersity of Science and Technology, San 31, Hyojadong,Namgu, Pohang, 790-784, South Korea, and Samsung AdVanced Institute of Technology, Mt. 14-1,Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do 446-712, South Korea

ReceiVed: April 22, 2010; ReVised Manuscript ReceiVed: July 12, 2010

Strongly luminescent and highly loaded CdSe quantum dot (QD)-silica composite films are prepared by asimple one-step method. QDs are grown in situ as the silica matrix forms from spin-on-glass organosiloxanepolymers. The nucleation and growth of QDs in the silica matrix is directly observed using TEM. The QDcomposites are comprehensively characterized, which includes photoluminescence(PL) decay, PL efficiency,and QD volume fraction. The QD emission can span the entire visible range and can be tuned by the preparationcondition. The PL of QDs is very strong, reaching the quantum efficiency up to 35%. Very high QD loadingis achieved with the volume fraction as high as 13%, which makes our composite the highest loaded QD-silicacomposite material. We further showcase the versatile applicability of our method by a QD-silica compositecapped down-conversion pseudo white LED.

1. Introduction

Colloidal semiconductor nanocrystal quantum dots (QDs) arebeing extensively studied to exploit their unique properties,including the zero-dimensional discrete electronic energy feature,high photostability, tunable and bright emission, and solutionprocessability. They can promise many potential applications,including biological fluorescent probes,1,2 light-emitting diodes(LEDs),3 optical modulators,4 lasers,5 and solar cells.6,7 BeforeQDs can be actively applied to many device applications, theyoften need to be stably embedded in an appropriate matrix.Polymers8-12 or inorganic matrixes13-25 can be used to fabricateQD-embedded composites. QD-silica composites through vari-ous synthetic routes have been reported because silica isattractive as a transparent and nonscattering matrix and is well-suited for many optical components of waveguides, lasers,optical filters, and modulators.13-18,20,21,23-27 QD-silica com-posites can be typically prepared by compatibilizing QDs withsilica sol precursors, which is followed by gelation andannealing.13-18,20,21,23-25 Recently, Seo and co-workers havereported bright QD-silica monoliths by using their simultaneousligand-exchange and shell formation method.23 Brock and co-workers have also demonstrated highly luminescent QD mono-liths using an arrested precipitation strategy.24 In general,stabilizing QDs into solid matrixes typically requires multistepprocesses. The compatibilization needs surface modification ofQDs and/or addition of surfactants to avoid aggregation in thesilica sol precursor medium. This step can risk losing the highphotoluminescense (PL) quantum efficiency (QE) of as-preparedQDs. In addition, surface molecules on QDs or encapsulatingsurfactants limit obtaining high QD loading fractions assignificantly occupying volumes in the QD composite medium.The QD volume fraction can be a critical factor in many

applications, including inventory control and lasing devices. Forexample, multicolor QD beads can only guarantee their fullmultiplexing power when they can reach a high QD loadingfraction.28 The QD volume fraction is pivotal for lasingapplications due to the fast Auger process.29 On the other hand,QD-silica composites can be also prepared by melting semi-conductor precursors in a glass host.15,16 However, this melting-pot process has an inherent limitation to reach a high volumefraction of QDs because of the low solubility of semiconductorsin glass. In addition, the melting-pot process is energy-inefficientas typically demanding a very high temperature of ∼1600 °C.We report herein a simple and rapid method to prepareQD-silica composites by using QD precursors and spin-on-glass (SOG) organosiloxanes. Highly fluorescent QD-silicafilms can be obtained by mixing QD precursor solutions withSOG, spin-casting, and baking at a relatively low temperatureof ∼240 °C. QDs are in situ synthesized concurrently with thesilica matrix formation. The QD-silica composites are stronglyluminescent with the QE as high as reaching up to 35%. TheQD loading fraction can be also very high, reaching up to 13%,which makes our composite the highest loaded QD matrix, asfar as the authors know. This QD loading fraction is extremelyhigh, at least an order of magnitude larger than conventionalQD composites.

2. Experimental Section

2.1. Procedures for QD Precursors. All chemicals wereused without further purifications. Precursor preparations werecarried out under an inert atmosphere. The Cd precursor wasprepared by dissolving cadmium acetate dihydrate (98%, Sigma-Aldrich) in methanol (Mallinckrodt). The Cd concentration wasadjusted to be 2 M. The Se precursor was prepared bycomplexing selenium (pellets, 99.999+%, Sigma-Aldrich) withexcess tris(3-hydroxypropyl)phosphine (THPP) (HISHICOLINP-540, Nippon Chemical Industrial) in ethanol (anhydrous,Sigma-Aldrich). The Se precursor solution contained 2 Mselenium in 1:1 (vol/vol) THPP and ethanol.

* To whom correspondence should be addressed. E-mail: sungjee@postech.ac.kr.

† Pohang University of Science and Technology.‡ Samsung Advanced Institute of Technology.

J. Phys. Chem. C 2010, 114, 14362–1436714362

10.1021/jp103617m 2010 American Chemical SocietyPublished on Web 08/09/2010

2.2. Fabrication of QD-Silica Composite Films. Glassslides (Microslide 2947-3X1, Corning) were cut to 2.5 cm by2.5 cm for the substrates. The glass substrate was washed withAlconox and sonicated in acetone for 30 min and in ethanolfor another 5 min. It was rinsed with distilled water and driedunder a nitrogen blow. For a typical fabrication, the Cd and Seprecursor solutions prepared above were added and mixed wellin SOG solution (Accuglass T-512B from Honeywell, Morris-town, NJ). The volume ratio of Cd precursor solution/Seprecursor solution/SOG solution was made to be 1:1:3. Themixture was spin-cast onto the glass substrate with 2000 rpmfor 30 s. The substrate was baked at ∼240 °C in a tube furnaceunder a continuous nitrogen flow. The baking time was adjustedfor a few minutes up to hours depending on the desired emissioncolor.

2.3. Optical Measurements. Absorption spectra were ob-tained using an Agilent UV-vis spectrophotometer 8453. PLspectra were taken by an Ocean Optics USB4000-UV-visspectrophotometer using a 385 nm LED (SU50-RUP501AT385,Seoul Semiconductor Co., LTD) as the excitation source. PLQEs were obtained by an absolute measurement method usingan integrating sphere (4P-GPS-033-SL, Labsphere) connectedby a Jobin Yvon Horiba Fluorolog-2 system with a SymphonySi CCD. Mello’s method was used for the QE measurements.30

The QE measurement apparatus setting was built by followingthe publication by Beeby and co-workers.30 All the componentsinside of the integrating sphere were made of white Teflon tominimize the light absorption.

2.4. TEM, EDS, and AFM Characterization. A JEOLJEM-1011 was used for transmission electron microscopy(TEM) images and energy-dispersive spectroscopy (EDS), and400 mesh copper grids (Ted Pella 01822, Inc., Redding, CA)were used. For TEM sample preparation, the TEM grid wasplaced on top of a glass slide center. The QD precursor SOGsolution was spin-cast on the glass slide, and the substrate wasbaked in the oven. The TEM grid with a very thin QD-silicacomposite layer on top was retrieved from the substrate usinga tweezer. For AFM characterization, the film thickness of theQD-silica composites on the glass slides was determined usinga Dimension 3100 (VEECO) in tapping mode. To reveal thefilm thickness by AFM, QD-silica composite films werescratched by a razor.

2.5. Time-Resolved Photoluminescence Experiment. Fortime-resolved PL experiments, the light source was by home-built cavity-dumped mode-locked Ti:sapphire laser pulses. Theexcitation wavelength was 400 nm, which was doubled by thefundamental light (800 nm, 380 kHz, 20 fs) with BBO 100 µm.The 101.6 mm parabolic mirror was used for collecting theemission in the “backscattering” geometry. The detecting systemconsisted of a CCD (Andor, DU401A-BV) for time-integrated

PL (TIPL) and an MCP-PMT (Hamamatsu, R3809-U51) forPL. Time-resolved photoluminescence (TRPL) was monitoredvia time-correlated single-photon counting, which provides thesub-10 ps time resolution with deconvolution. The above-mentioned setup was previously reported.31

2.6. Fabrication of a Down-Conversion Pseudo WhiteLED. A 385 nm emitting LED (SU50-RUP501AT385, SeoulSemiconductor Co., LTD) was dip-coated into the QD precur-sors and SOG mixture solution for 10 s. After the dip-coating,the LED was dried in air for a minute and baked at ∼200 °C ina tube furnace under a continuous nitrogen flow.

3. Results and Discussion

QD-silica composites can be prepared by a simple and mildone-step mix-and-bake procedure. Figure 1a shows representa-tive CdSe QD-silica composite films on glass substrates. Thefilms are flat and smooth and look glossy with strong fluores-cence under a UV lamp. Figure 1b shows a size series ofemission spectra from typical CdSe QD-silica composite films.The emissions cover almost the entire visible range, with thePL bandwidth as narrow as 60 nm. For a typical preparation ofthe QD-silica composite films, QD precursors are mixed withSOG, spin-cast, and baked in an oven. For the Cd precursor,Cd salts, such as cadmium acetate, are dissolved in methanol.For the Se precusor, a Se-THPP complex was made anddissolved in ethanol. The QD precursors are dissolved inalcohols because the SOG solution is well miscible with them.THPP has a phosphine functional group that can easily form aSe-P complex bond. It also has three hydroxy functional groupsthat can ensure alcohol solubility and the potential ability toincorporate into SOG silica networks by forming cross-linkedSi-O-C bonds. QD precursor solutions and SOG solutionmixture can be spin-cast or molded by necessity and baked toobtain the QD-silica composites. QDs are being synthesizedconcurrently as the SOG matrix forms. The QD precursors arereadily soluble in the SOG solution. The Cd acetate andSe-THPP complex can decompose at relatively low tempera-tures. As the pyrolysis of the QD precursors proceeds, thechemical changes of the QD precursors result in a rapidreduction in solubility. The abrupt solubility drops triggernucleations of QDs as the precursor concentration overshootsthe minimum concentration for nucleation. At the initial growthstage, the QD growth occurs concurrently with nucleation untilthe effective precursor concentration drops below the concentra-tion for nucleation. QDs continue to grow presumably mostlyby Ostwald ripening. The growth is usually quenched by coolingas the substrate is removed from the oven. By using the pyrolysismethod, our process can be performed in remarkably lowertemperatures when compared with the glass melting process.

Figure 1. (a) Representative QD-silica composite films on glass substrates under UV illumination. (b) Size series of normalized emission spectra.

Preparation of CdSe QD-Silica Films J. Phys. Chem. C, Vol. 114, No. 34, 2010 14363

As shown in Figure 1, the QD-silica composite emissioncan span the entire visible range and can be tailored by thepreparation conditions. Figure 2 shows absorption and normal-ized emission spectra of QD-silica composites with changingthe preparation conditions, such as baking temperature, bakingtime, and precursor ratio between Cd and Se. Figure 2a showsthe dependency upon the baking temperature. Other parameters,such as baking time and precursor ratio, are maintainedidentically. As the baking temperature increases, we find theabsorption and emission profiles shift to longer wavelengths.The emission peak shifted from ∼500 to ∼700 nm as the bakingtemperature increased from 210 to 270 °C. Higher temperaturescauses enhanced mobility of the QD precursors and SOGsiloxane polymers. This can promote faster growth of QDs,which results in the red shift in absorption and emission.However, the absorption peak becomes significantly broader asit red shifts. It is believed that emission from trap sites increasesat higher baking temperatures. The absorption shift is mostlydue to the increased QD size. On the other hand, the emissionshift is contributed by the increased QD size and increased trapemissions. The broad PL profile results from the convolutionof trap emissions and the inhomogeneity from the QD sizedistribution. Insufficient surface passivating organic ligands mayhave contributed to the trap emissions. The low mobility of theQD precursors in the SOG matrix may have limited the QDs togrow more uniformly in size. Figure 2b shows samples that areprepared using different baking times. As the baking extends,absorption and emission shift to longer wavelengths. Theemission peak shifted from ∼500 to ∼600 nm when the bakingtime was extended from 10 to 30 min. The precursor ratiobetween Cd and Se was investigated in Figure 2c. The absoluteconcentration of the limiting reagent was kept constant. Wefound that a 3:2 Cd-to-Se precursor ratio produced larger QDsthan the cases using a 1:1 or 2:3 ratio. As the ratio of Cd to Seincreases, absorption appears at longer wavelengths, whichindicates faster particle growth (Figure 2c). A cadmium excesscondition seems to promote rapid QD growth. This has been

reported for solution-based colloidal CdSe QD syntheses assupposedly Cd precursors have a lower activation energy forQD surface binding than Se precursors.32 When the preparationconditions are modified, different colored QD-silica compositescan be obtained. To elucidate the origin of the PL, time-resolvedPL measurements were performed for different colored QD-silicacomposites (Figure 2d). Intensity-weighted average lifetimeswere obtained using quadro-exponential fittings.31 Lifetimes andamplitudes for each element are listed in Table 1. Four sampleswith their emission peaks at 514, 535, 556, and 608 nm wereused. They showed mean decay lifetimes of 4.0, 7.5, 13.4, and40.0 ns, respectively (Table 1). The decay lifetimes arecomparable to those of typical colloidal QDs synthesized insolution or in glass.21,33 In addition, our samples have the PLQEs that are comparable to those of colloidally synthesized QDs.This corroborates the hypothesis that our sample PL mostlyoriginates from the band-edge excitonic emissions. We find thatthe decays become slower for the longer emission wavelengthsamples. The prolonged decay also accords with colloidallysynthesized QDs, which is due to the well-known QD sizeeffect.34,35

Our QD-silica composites glow brightly when excited undera UV lamp. Their PL QEs were measured by an absolute methodof Mello’s equation using an integrating sphere.30,36 They showstrong fluorescence with a typical PL QE of ∼10%. Thebrightest sample reaches the PL QE up to 35%. Yellow emittingsamples typically showed the highest PL QEs. PL QEs fromdifferent sized QDs fall in the same range. Small or large QDstend to show lower PL QEs, presumably due to the lack ofenough annealing for QD crystallinity or increased trap siteson the larger QD surface area. The PL QEs were moderatelyreproducible for samples prepared by an identical recipe,showing the typical deviation of (5% point. In-situ formed QDstypically show low PL QEs, presumably because of the lack ofproper surface passivation. The strong fluorescence from ourQD-silica composites can be ascribed to THPP acting as theQD surface ligand. Phosphine is a well-known QD passivating

Figure 2. (a) Absorption and normalized emission spectra of QD-silica composites: samples prepared at different baking temperatures of 210 °C(solid lines), 240 °C (dashed lines), and 270 °C (dotted lines) using the baking time of 15 min and a 1:1 Cd-to-Se precursor ratio. (b) Absorptionand normalized emission spectra of QD-silica composites: samples prepared for different baking times of 10 min (solid lines), 15 min (dashedlines), and 30 min (dotted lines) using the baking temperature of 240 °C and a 1:1 Cd-to-Se precursor ratio. (c) Absorption and normalized emissionspectra of QD-silica composites: samples prepared using different Cd-to-Se precursor ratios of 1:1 (solid lines), 3:2 (dotted lines), and 2:3 (dashedlines) using the baking temperature of 240 °C and the baking time of 15 min. (d) PL decays of QD-silica composites emitting at 514 nm (black),535 nm (green), 556 nm (blue), and 608 nm (red).

TABLE 1: PL Decay Life Times τ and the Corresponding Amplitude Percentages A for Samples in Figure 2d.a �2 is theReduced Chi-Square Value for Each τavg

sample no.emission peak

[nm] A1 τ1 [ns] A2 τ2 [ns] A3 τ3 [ns] A4 τ4 [ns] τavg [ns] �2

1 514 64.2 0.075 21.9 1.32 10.9 10.4 3.0 83.4 3.98 1.292 535 76.8 0.051 9.0 1.80 7.9 17.3 6.4 93.1 7.49 1.193 556 60.4 0.096 19.7 2.49 10.7 20.1 9.2 117 13.4 1.184 608 35.3 0.19 20.3 4.09 21.6 32.0 22.8 141 40.0 1.18

a Each decay was fitted using a quadro-exponential function.

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surface ligand, efficiently keeping QDs with high PL QEs.37

THPP has three terminal hydroxyl groups that may haveparticipated in the silica gelation. It can be postulated that THPPsform stable networks between QD surfaces and the host silicamatrix. The brightest QD-silica composites prepared fromsolution-synthesized QDs typically show the PL QEs of3-10%.21,23-25 It is noted that our QD-silica composites arecomparably bright when compared to the previously reportedbest samples.

The lifetime measurements indicate that the PL of ourQD-silica composites originates from QD excitonic emission.To obtain direct evidence of QD growth in our composite, thedirect observation of QD growth in the composite was madeby TEM. We have observed very thin QD-silica compositefilms that were baked for different times using TEM. For theTEM sample, a grid was placed on the center of the glasssubstrate. Spin-casting was made using a 1:1 Cd-to-Se ratio QDprecursor-SOG mixture solution, and the baking temperaturewas set for 240 °C. The grid on top of the glass slide acts as anascended substrate, which accommodates very thin film. Tofollow the time evolution of the QD growth, TEM images weretaken for the samples using different baking times. Figure 3ashows the TEM image after 5 min of baking, where small andblurred particles of approximately 2 nm in size begin to appear(Figure 3a). At 7 min, QDs are visualized in a more distinctivefashion. They have grown to an average diameter of 3.4 nm in(Figure 3b). After 15 min, QDs have grown to an averagediameter of 5.5 nm with the size distribution of 27% (Figure

3c). The size distribution is rather large when compared withcolloidally synthesized QDs. However, the one-step process ofQD composites (concurrently with the matrix solidification)inherently has to trade some degree of uniformity in QD sizefor the simple and rapid preparation. It is noted that the growthof QDs accompanies a decrease in the number density of theparticles. This indicates that larger QDs are growing uponsacrifices of smaller ones. We further confirmed the existenceof CdSe QDs by using EDS measurements. Figure 3d showsthe TEM image of a QD silica composite that was synthesizedusing the condition of a 1:1 Cd-to-Se precursor ratio, bakingtemperature of 240 °C, and the baking time of 10 min. In thefigure, points A and B were assigned for a QD-rich area and aQD-sparse area, respectively. EDS spectra can be found inFigure 3e with the relative atomic compositions for the majorelements in Figure 3f. It is noted that Cd and Se are colocalizedin the QD-rich area A. This indicates that the dark particlesunder TEM are indeed CdSe and not elemental Cd or Se. TheQD-sparse area B has much less relative atomic compositionsfor Cd and Se than area A. The TEM images and EDS datashow direct evidence for in situ formations of QDs in the silicamatrix. Higher-magnification QD TEM images were not ac-cessible due to the limitations from the silica matrix electronscattering.

For QD composite applications, the loading fraction is acritical factor. The QD loading fraction plays an important role,especially in inventory control and lasing applications. QDcomposites often have low loading fractions with the volume

Figure 3. TEM images of quantum dot-silica composite films baked at 240 °C for 7 (a), 10 (b), and 15 min (c). Scale bars represent 50 nm. (d)TEM image of a QD-silica composite film baked at 240 °C for 10 min by using a 1:1 Cd-to-Se precursor ratio. Point A and point B are chosenfor QD-rich and QD-sparce areas, respectively. (e) For points A and B, elemental analysis is performed using EDS. (f) For points A and B, therelative atomic compositions for major elements are tabled.

Preparation of CdSe QD-Silica Films J. Phys. Chem. C, Vol. 114, No. 34, 2010 14365

fraction less than 1%. When presynthesized QDs are added ina matrix resin for composite fabrication, a high loading fractionis limited by the phase separations or segregations. Typically,significantly volume occupying additives are necessary toovercome the difficulties, which inherently limit achieving ahigh QD fraction.9 QDs can be also embedded in a premadehost matrix. For example, a polymer matrix can be swollen andinvite QDs in.28 In this case, QDs usually reside near the surfaceof the matrix and fail to reach deep in the matrix, which makesthe QD loading fraction very low. In our case, QDs are in situsynthesized with the matrix formation. QD precursors areprovided in high concentrations because they are very misciblewith the host SOG. By using in situ QD preparations, noadditives are required to prevent phase separations. Therefore,our QD-silica composite preparation is suited for high QDloading. Loading fractions of our QD composite films wereinvestigated using the QD absorption and the film thickness.Figure 4a shows absorption spectrum of a QD-silica compositefilm, where a high precursor concentration was used for maximalQD loading. The first excitonic absorption peak can be foundat 416 nm, which corresponds to the average QD diameter of1.1 nm. The QD concentration in the composite was determinedusing a CdSe QD molar extinction coefficient of 24 000 M-1

cm-1 at 350 nm. The extinction coefficient was borrowed froma previous report.38 For CdSe QDs, 350 nm is relatively highin energy when compared with the band gap. The CdSeelectronic states converge close to the bulk continuum at 350nm. As a result, the oscillator strength at 350 nm is quiteinsensitive to the QD size. In addition, QD precursors and SOGdo not have a notable absorption at 350 nm. The film wasmeasured to have the absorbance of 0.39 at 350 nm and theQD molar concentration of 0.30 mol L-1. The average thicknessof the composite film was measured to be 550 nm using tappingmode AFM as swiping across a razor scratched line. Figure 4bshows the AFM image with the scratched area on the left andthe film on the right. The depth profile across the white in Figure4b is shown in Figure 4c. On the basis of the QD concentration,QD average size, and film thickness, the QD volume fractionwas determined as 13%. This is a massive amount of QDsloaded in the silica matrix. In the case of close-packed QDswithout any protective host matrix, the volume fraction wasreported as ∼20%.10 A very high QD loading fraction that iscomparable to closely-packed QD films can be achieved by ourQD-silica composite preparation. The volumes occupied byorganic molecules are minimized because no excess surfacemolecules or surfactants are used. Unlike the glass meltingprocess, our method does not suffer from the limited solubility

of QD precursors. Large amounts of precursors can be dissolvedin SOG solution, which results in the high QD packing density.However, when QDs occupy more than 5% volume fractions,PL QEs begin to significantly decrease because of the enhancedself-absorption and energy transfers between QDs.

The low temperature of our process can enable QD-silicacomposite fabrication onto preprocessed devices. To demonstratethe versatility of our process, a down-conversion pseudo whiteLED was made by simply coating a QD-silica composite on a385 nm GaN LED (see the Experimental Section for details).The epoxy packaged blue LED was dip-coated in the QDprecursor SOG mixture solution and baked at 200 °C. A pseudowhite color emission was clearly observed from the QD-silicacapped LED (Figure 5a). A broad QD emission from ∼500 to∼700 nm renders the white light illumination. Figure 5b is thepseudo white LED emission spectrum. This demonstrates thatour QD-silica composite preparation can be easily applied tononflat substrates.

4. Conclusions

In conclusion, CdSe QD-silica composites were made by asimple one-step mixing and baking preparation. The QDemission can span the entire visible range and can be tuned bythe preparation condition. The PL of QDs was very strong,reaching the QE of up to 35%. The QD volume fraction canreach as high as 13%. Growth of QDs in the silica matrix wasconfirmed by PL decay lifetime and TEM measurements. Wefurther showcased the excellent applicability of our method bythe QD-silica composite capped down-conversion pseudo whiteLED.

Acknowledgment. This work was supported by the KoreaScience and Engineering Foundation (KOSEF) grant funded by

Figure 4. (a) Absorption spectrum of a QD-silica composite film. (b) AFM field scan across a scratched region for film thickness measurement.(c) The depth profile of QD-silica film along the white line in (b) by AFM.

Figure 5. (a) A QD-silica composite capped down-conversion whiteLED. (b) Emission spectra from a 385 nm LED (black) and from aQD-silica composite capped down-conversion pseudo white light LED(red).

14366 J. Phys. Chem. C, Vol. 114, No. 34, 2010 Jeong et al.

the Korea Government (MOST) (M10703001036-08M0300-03610, R0A-2008-000-20114-0(2008), and KRF-2008-331-C00140) and the Priority Research Centers Program throughthe National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (2009-0094037 and 20090090897).

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