synthesis and optical spectroscopy of a hybrid cadmium sulfide-dendrimer nanocomposite

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3064 J. Opt. Soc. Am. B/Vol. 24, No. 12 /December 2007 Gayen et al.

Synthesis and optical spectroscopy of a hybridcadmium sulfide–dendrimer nanocomposite

S. K. Gayen,1,* M. Brito,1 B. B. Das,1 G. Comanescu,1 X. C. Liang,1 M. Alrubaiee,1 R. R. Alfano,1 C. Gonzalez,2

A. H. Byro,2 D. L. V. Bauer,2 and V. Balogh-Nair2

1Department of Physics, Department of Defense (DoD) Center for Nanoscale Photonic Emitters and Sensors,Institute for Ultrafast Spectroscopy and Lasers, City College of New York, 138th Street at Convent Avenue,

New York, New York 10031, USA2Department of Chemistry, City College of New York, 138th Street at Convent Avenue, New York,

New York 10031, USA*Corresponding author: [email protected]

Received August 16, 2007; revised October 19, 2007; accepted October 23, 2007;posted November 1, 2007 (Doc. ID 86068); published November 30, 2007

Hybrid nanocomposites of cadmium sulfide �CdS� quantum dots and poly(propyleneimine) dendrimer having a1,4-diaminobutane core have been produced by colloidal synthesis in degassed methanol at room temperatureusing third-, fourth-, and fifth-generation (G5.0) dendrimers, and their spectroscopic properties have been in-vestigated. The nanoparticles fluoresced from 375 to 650 nm under near-ultraviolet excitation, and their ab-sorption spectra exhibited a strong blueshift of the band edge compared to that of the bulk CdS. The stabilityof nanocomposites depended significantly, while the size and spectroscopic properties exhibited a weaker de-pendence, on the dendrimer generation. Most compact and stable nanoparticles were obtained with G5.0 den-drimers. Average diameter was estimated to be 2.2±0.3 nm, assuming nanoparticles of spherical shape withinan infinite well potential. The room-temperature luminescence has a fast component with 165±5 ps lifetimeand a slow component with a 40±2 ns lifetime. The luminescence is partially polarized with an initial aniso-tropy of 0.39±0.02. © 2007 Optical Society of America

OCIS codes: 160.4236, 300.6280, 300.6530, 300.1030, 160.4670, 160.5470.

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. INTRODUCTIONybrid inorganic–organic nanocomposites, such as semi-

onductor quantum dots (QDs) in organic hosts, are ofarticular interest since the resonant coupling betweenhe Frenkel excitons of the organic molecules and theannier–Mott excitons of the inorganic semiconductorDs may lead to interesting effects, including higher os-

illator strength, enhanced optical nonlinearity, and effi-ient Forster energy transfer [1]. These effects make theanocomposites versatile for light emitting (lasers, light-mitting diodes, displays), nonlinear optical devices, andiological and intracellular probe applications [1–6]. Hy-rid nanocomposites build on the quantum size effects inemiconductor QDs first investigated in the early 1980s7,8].

Synthesis of stable semiconductor QD-organic hybridseeds to satisfy the following requirements for optimiza-ion of desired optical and spectroscopic characteristics:a) the QD size should be controllable to derive the ben-fits of quantum confinement; and (b) the QDs should beynthesized with a protective layer to endow them withtability to ambient environment. Dendrimers are nano-ize, highly branched, treelike monodisperse organic mac-omolecules that emanate from a central core with aranch occurring at each monomer unit [9] and are suit-ble for incorporation of semiconductor QDs. In contrastith other polymers, the inherently high degree of orga-ization [9], the ease of site-specific chemical modification10], and multivalency [11] endow dendrimers with anique potential to serve concomitantly as “nanoreac-

0740-3224/07/123064-8/$15.00 © 2

ors,” stabilizers, and interfaces in hybrid organic–norganic materials. Dendrimer encapsulation conveystability, controls QD size, which in turn influences emis-ion wavelengths, offers some resistance to photobleach-ng, and facilitates bioconjugation chemistry. The firsttudies used starburst polyamidoamine (PAMAM) den-rimers to stabilize and template QDs [12,13].This paper reports on the synthesis and optical spectro-

copic characteristics of hybrid cadmium sulfide �CdS�Ds encapsulated in poly(propyleneimine) dendrimeraving 1,4-diaminobutane (DAB) core. We chose third-G3.0)-, fourth (G4.0), and fifth-generation (G5.0) DABendrimers because the periphery of these dendrimers islled with primary amine groups available for further de-ivatizations, while all the branching points in the inte-ior of the dendrimers are occupied by tertiary nitrogen torovide nucleation sites for formation of small QDs.The remainder of the paper is organized as follows. Sec-

ion 2 describes the synthesis of the CdS-DAB QDs in so-ution, and luminescence properties. Section 3 presents

easurements of the absorption and fluorescence spectra,uorescence lifetime, and fluorescence anisotropy of theynthesized CdS-DAB nanocomposites. Experimental re-ults are discussed in Section 4.

. MATERIALS AND METHODS. MaterialsAB-Am-16 (polypropyleneimine hexadeca-amine den-rimer, G3.0), DAB-Am-32 (polypropyleneimine

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Gayen et al. Vol. 24, No. 12 /December 2007 /J. Opt. Soc. Am. B 3065

otriaconta-amine dendrimer, G4.0), DAB-Am-64polypropyleneimine tetrahexaconta-amine, G5.0) con-aining 16, 32, and 64 surface amino groups, respectively,ere purchased from Aldrich and were used without fur-

her purification. Cd�NO3�2 ·4H2O (Baker), Na2S (Ald-ich), and the solvents employed were of reagent grade.or each experiment, the stock solutions were freshly pre-ared and stored under nitrogen prior to their immediatese.

. Methods for Nanocomposite Synthesisybrid nanocomposites of CdS QDs and G5.0 DAB wererepared according to the following standard, optimizedrocedure. Two stock solutions, each in 50 ml of degassedethanol, were prepared from 59.3 mg �0.192 mmol� ofd�NO3�2 ·4H2O and 15.0 mg �0.192 mmol� of Na2S. To aeaction vial containing 8 mg �1.12�10−3 mmol� of G5.0AB dendrimer 0.5 ml of the Cd2+ stock solution wasdded and the mixture was stirred under nitrogen untilll the DAB dissolved. To this solution was added inrops, at room temperature, under nitrogen, and with vig-rous stirring, 0.5 ml of the S2− solution followed by 0.5 mld2+ solution and 0.5 ml S2− solution over a period of30 min. The final concentrations of DAB, Cd2+, and S2−

ere 0.56, 1.92, and 1.92 mM, respectively. The reactionixture was stirred vigorously during these additions

nd for �1 h after the last addition.The methanol solution of CdS-DAB QDs was colorless

nd showed strong blue fluorescence, displayed in Fig. 1,hen irradiated with ultraviolet (UV) light. The fluores-

ence extended from 375 to 650 nm with a peak of470 nm. The solutions were stored at −20°C and were

table for weeks with no formation of a precipitate,hange in color, or changes in their emission properties.owever, the solutions were unstable to strong acids and

trong bases, as demonstrated by complete loss of fluores-ence when the pH was reduced to 5 or increased to 9.eating the methanol solutions above 80°C also de-

troyed the fluorescence completely.To evaluate the effect of ion concentrations on the fluo-

escence intensity of G5.0 CdS-DAB QDs, a set of experi-ents were carried out according to the above-mentioned

ig. 1. (Color online) Room-temperature fluorescence spectra ofybrid CdS-DAB nanoparticles in methanol synthesized using5.0, of a DAB dendrimer, and showing the effect of incrementaldditions of Cd2+ and S2− ions on the fluorescence emissioncurves S1, S2, S3, S4, and S5). The concentration of Cd2+ and S2

ons in sample 1 (curve S1) was 3.84�10−4 M. Ions were added inhots of this 3.84�10−4 M solution to prepare other samples, sohat ion concentrations in samples 2, 3, 4, and 5 were two, three,our, and five times, respectively, of that in sample 1. The concen-ration of the dendrimer in all the samples was 2.23�10−4 M.

tandard procedure, but the concentrations of the Cd2+

nd S2− ions were varied from 3.84�10−4 to 1.9210−3 M while maintaining the DAB concentration at

.23�10−4 M. As displayed in Fig. 1, curves S1, S2, S3,4, and S5, fluorescence intensity increased with increas-

ng ion concentration, peaking at a Cd2+ and S2− concen-ration of 1.92�10−3 M each, which corresponded to theCd2+� / �DAB� molar concentration ratio of 8.61. The iononcentration dependent measurements indicated thathere is a range of this ratio for which the nanocompositesre stable, but at a very high molar ratio, e.g., 27.6, bulkdS precipitates out within hours with concomitant lossf fluorescence intensity. Figure 1 also shows that chang-ng the ion concentrations causes fluctuations in the emis-ion peak position between 465 and 480 nm.

In another set of experiments the concentrations ofd2+ and S2− ions were maintained constant at 1.9210−3 M, and the concentration of G5.0 DAB dendrimeras varied from 1.395�10−5 to 5.58�10−4 M to assess

he effect of DAB concentration on fluorescence emissionntensity. As Fig. 2 shows, the fluorescence intensity in-reased with dendrimer concentration initially, buttarted to level off at 5.58�10−4 M. This indicates that aCd2+� / �DAB� molar ratio of at least 3.44 is required to ob-ain substantial fluorescence intensity.

To investigate the effect of the dendrimer generation onhe performance and stability of the nanocomposites, syn-heses were carried out using three different generationsG3.0, G4.0, and G5.0) of the dendrimer using a procedureimilar to that used for the G5.0 dendrimer describedbove. The final concentrations of dendrimer, Cd2+, and2− for all three generations were 3.72�10−4, 1.92�10−3,nd 1.92�10−3 M, respectively. While the nanocompositesrepared using different generations of the dendrimerisplayed similar fluorescence properties (Fig. 3), a veryignificant difference was observed in their stability. Theolutions containing nanocomposites with the G3.0 DABormed a precipitate within a few hours. The nanocompos-tes with the G4.0 dendrimer were stable for approxi-

ately 3 weeks at −20°C, while those with the G5.0 den-rimer lasted longer, indicating that larger dendrimersrovided better protection of these nanocomposites. Over-

ig. 2. Dependence of fluorescence signal on the dendrimer con-entration, which was varied from 1.395�10−5 M �1/4 C� to.58�10−4 M �10 C�, while the concentration of Cd2+ and S2− ionsas maintained constant at 1.92�10−3 M.

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. Spectroscopic Investigation Methodshe absorption and fluorescence spectra, fluorescence life-ime, and fluorescence anisotropy of the CdS-DAB nano-omposite samples were measured. Optical absorptionpectra of nanocomposites in methanol were measured atoom temperature in the 200–700 nm spectral range us-ng a PerkinElmer Lambda-9 dual beam UV-visible-NIRpectrophotometer, 5 mm path length quartz cuvettes,nd reagent grade methanol in the reference cell. Fluores-ence spectra were then measured using a PerkinElmerpectrophotometer (Model LS50) using the front face ex-itation and collection geometry.

The time evolution of the fluorescence from thedS-DAB nanocomposites was measured in two different

ime domains using two different experimental arrange-ents. The fast component was measured by exciting the

amples using the 375 nm radiation generated by fre-uency doubling the output of a model locked tunablei:sapphire laser (Spectra-Physics Tsunami) generating00 fs pulses at 82 MHz repetition rate. The typical en-rgy of a 375 nm pulse was approximately 60 pJ. A streakamera (Hamamatsu Model C5680) with a typical tempo-al resolution of 6 ps measured the time evolution of fluo-escence. This experimental arrangement was also usedor fluorescence anisotropy measurement with additionalse of oriented polarizers in the collection path.Fluorescence dynamics of the slow component waseasured by exciting the samples with 266 nm fourth

armonic, 50 ps, 0.3 �J, 10 Hz repetition rate pulses frommode locked Nd:YAG laser (Continuum, Model PY-

1C), collecting the fluorescence through high pass col-red glass filters to block the scattered light at excitationavelength, dispersing the fluorescence through a mono-

hromator, sensing it using a S-20 response photomulti-lier tube, and analyzing the fluorescence temporal pro-le with a 50 � terminated digital oscilloscope.

ig. 3. Room-temperature fluorescence spectra of CdS-DABanocomposites in methanol synthesized using G3.0, G4.0, and5.0 of DAB dendrimers. The excitation wavelength was66 nm.

. RESULTS. Absorption Spectraoom-temperature absorption spectra of CdS-DAB nano-articles in methanol synthesized with G3.0, G4.0, and5.0 of DAB are shown in Fig. 4. The salient feature of

he spectra is the first absorption peak that is ascribed tohe bandgap absorption by the CdS QDs. The peak posi-ion of this bandgap absorption in CdS-DAB synthesizedith G5.0, G4.0, and G3.0 DAB are at 314±2, 324±2, and40±2 nm, respectively. The absorption band edge isuch blueshifted in all three cases compared to the band-

ap of 2.42 eV �512 nm� at 300 K for bulk CdS of both theurtzite (hexagonal) and zincblende (cubic) structures

14–18]. This blueshift of the absorption band edge is aonsequence of quantum size effect that may be used tostimate the size of the nanoparticles. For spherical nano-articles the size dependence of the absorption edge dueo the lowest excited state can be evaluated using therus effective mass model [19]

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he experimental data points extracted from the absorp-ion spectra of CdS-DAB nanocomposites synthesized us-ng G3.0, G4.0, and G5.0 dendrimers, as displayed in Fig.. The inset shows the data points and the model predic-ions on an expanded scale. The dashed line at 512 nmorresponds to the room-temperature bandgap energy2.42 eV� of bulk CdS [14–18]. Table 1 summarizes the es-imated diameters of the nanoparticles synthesized usingifferent generations of DAB. Nanocomposites synthe-ized with G5.0 DAB seem to be more compact than thatynthesized with G4.0 and G3.0 DAB.

ig. 4. Room-temperature absorption spectra of CdS-DABanocomposites in methanol synthesized using G3.0, G4.0, and5.0 of DAB dendrimers. Spectra are corrected for the solventbsorption. Baselines of the absorption spectra of nanocompos-tes with G3.0 and G4.0 dendrimers are shifted vertically forlarity.

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. Fluorescence Spectra and Quantum Efficiencyhe room-temperature fluorescence spectra of nanocom-osites excited by UV light centered on 266 nm are shownn Figs. 1 and 3. The broadband emission extends from75 to 650 nm with a peak of �470 nm, which is typical ofolloidal CdS QDs, as well as their nanocomposites withrganic molecules reported by other researchers12,13,20–23]. Fluorescence spectra were also measuredy exciting separately at 300, 340, 355, and 375 nm intohe absorption band of the nanocomposites. Except forome relative changes in the overall intensity, the shapend spectral characteristics of the fluorescence spectraid not change appreciably with excitation wavelength.luorescence spectral shape did not change significantlyith the dendrimer generation either. While CdS QDs

ynthesized with G3.0 dendrimer had a consistently loweruorescence yield, those encapsulated with the G4.0 and5.0 dendrimers had comparable yields.The relative quantum yield of the G5.0 CdS-DAB nano-

articles in methanol was estimated by comparing theirntegrated fluorescence intensity with that of a standardolution of Coumarin 460 laser dye in methanol having anquivalent optical density at the excitation wavelength of50 nm. The relative quantum yield was found to be 0.03.his value falls within the range of 0.001 reported fordS colloids [20], and 0.097 and 0.22 for CdS nanopar-

icles stabilized by G4.0 PAMAM starburst dendrimer12,21]. It is to be noted that this value of the relative

ig. 5. Dependence of the band-edge absorption peak positionin terms of wavelength in nanometers) on the diameter of thedS QD estimated using the Brus effective mass model discussed

n the text (solid curve). The three solid squares are experimentalbsorption band-edge peak positions of CdS-DAB nanocompos-tes in methanol synthesized using G3.0, G4.0, and G5.0 den-rimers displayed in Fig. 4. The dashed line corresponds to theoom-temperature bandgap energy �2.42 eV� of bulk CdS. Insetooms on the section of the curve with experimental data points.

Table 1. Estimation of the Diameter of CdS-DABNanoparticles

AB GenerationPeak Wavelength

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5.0 314±2 2.2±0.34.0 324±2 2.3±0.33.0 340±2 2.5±0.3

uantum yield indicates the relative brightness of theanoparticles as compared to a well-known fluorophorend is not a molecular quantum yield because molar con-entration of the nanoparticles is not being consideredere.

. Fluorescence Kineticstypical fluorescence temporal profile is displayed in Fig.

(a). The fluorescence decayed to approximately 30% of itseak value within 1000 ps, and appeared to attain auasi-steady state on the picosecond time scale, indicat-ng that the fluorescence has a fast lifetime component,nd a much slower decaying component. A single expo-ential fit to the initial part of the temporal profile as-uming a relationship of the form

I�t� = I1 exp�− t/�1� + Is�t�, �2�

ields a decay time of 165±5 ps for the fast component. I1s the peak value, and Is�t� is the quasi-steady (on the pi-osecond time scale) value of the fluorescence intensity,espectively.

The time evolution of fluorescence at 465 nm on theanosecond time scale is presented in Fig. 6(b). The fluo-escence temporal profile of this slow component, Is�t�,ould be fit by a single exponential with a decay time of1±3 ns at longer times ��10 ns�, as shown in Fig. 6(b).imilar fluorescence temporal profiles were measured for

ig. 6. Room-temperature fluorescence decay dynamics of adS-DAB nanocomposite in methanol synthesized using G5.0AB dendrimer: (a) fast component measured using a streak

amera, and (b) slow component measured using a photomulti-lier tube and a 50 � terminated oscilloscope. The smooth curves a single exponential fit to the experimental data representedy the wiggly curve in each of the two profiles.

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xcitation at other wavelengths. The decay time valuesid not depend substantially on the fluorescence wave-ength and were consistently in the 41±3 ns range. Theresence of the fast and slow components indicates thatore than one initial state may be involved in fluores-

ence transitions. Probable nature of the initial states wille discussed in Section 4.

. Fluorescence Anisotropyhe room-temperature fluorescence of CdS-DAB nanopar-icles was found to be partially polarized. To estimate thextent of polarization of emission we measured the fluo-escence anisotropy, r, at several emission wavelengthscross the fluorescence spectrum. We also investigatedhe time evolution of r�t�. The fluorescence anisotropy isefined as [24]

r�t� = �I��t� − I�t��/�I��t� + 2I�t��, �3�

here I��t� and I�t� are the components of fluorescencentensity measured parallel and perpendicular, respec-ively, to the polarization of the excitation beam. Aniso-ropy may be obtained from time-resolved measurementss well as from steady-state measurements.The measurement of fluorescence anisotropy was car-

ied out by exciting the sample with vertically polarizedight and collecting the fluorescence at right angles to theirection of the incident beam through an analyzer. Atreak camera measured I��t� and I�t� with the polariz-ng axis of the analyzer oriented parallel and perpendicu-ar, respectively, to the polarization of the excitationeam. The steady-state values of I� and I were measuredith the streak camera in the focus mode. Figure 7 pre-

ents the time evolution of r�t� (denoted by open squares)ver the first 800 ps, estimated using Eq. (3), from thetreak camera measurements of I��t� (filled squares) and�t� (open circles). The peak value of r�t� is �0.39, whichecays to �0.33 within the first 800 ps.The steady-state value of r=0.14±0.02 obtained from

he measured steady-state values of I� and I did nothange with emission wavelength. The results of time-esolved and steady-state measurements indicate that

ig. 7. Time evolution of fluorescence anisotropy of CdS-DAB inethanol (open squares) obtained using Eq. (3) and the temporal

rofiles of fluorescence parallel and perpendicular to the polar-zation of the excitation beam. The profiles of fluorescence polar-zed parallel (filled square) and perpendicular (open circles) arelso shown. The measurements were carried out at roomemperature.

he anisotropy decays from an initial high value of �0.4 tosteady value of 0.14. Implication of this substantial fluo-

escence anisotropy will be discussed in Section 4.

. DISCUSSIONpatial confinement of excitations controls the photo-hysical properties of semiconductor QDs. Control of theanoparticle size and stability to ambient environmentre key considerations in the study of these nanocompos-tes and their potential applications. The results of ourynthesis of CdS QDs in the presence of G3.0, G4.0, and5.0 of DAB dendrimer demonstrate that the dendrimers

nteract with CdS QDs in early stages of the synthesisrocess and control their size and stability. While the ex-ct location and mechanism of stabilization of CdS QDsithin the DAB molecular architecture are not knownet, it is evident that DAB-Am-64 containing 64 surfacemino groups provides nanoparticles with better stabilityhan that afforded by the G3.0 and G4.0 DAB. The ob-erved enhancement of fluorescence with DAB concentra-ion further indicates that the molecular architecture ofhe dendrimer plays an important role in determining thetability and optical spectroscopic properties of thedS-DAB nanoparticles. While it is highly desirable to beble to obtain predefined QD sizes by manipulating theynthesis procedure and parameters, the factors andechanism that determine the QD size are not yet under-

tood enough to develop predictive ability. It remains anntriguing problem for further investigation.

The blueshift of the band edge with increasing den-rimer generation, being strongest for nanoparticles syn-hesized with G5.0 DAB, indicates that the inorganic CdSD interacts with the organic dendrimer. The relativelueshifts of the absorption edge (and the change in thedS QD radius estimated from the blueshift) with den-rimer generation are not large but consistent. Furtherxperimental investigation, such as Raman scattering,ill be needed to identify modes that may arise from QD-endrimer coupling. The Brus model [19] that we use, andas been used by others [12,21], for estimating the QD ra-ius is rather elementary, as it assumes the QD to be aphere with an infinite potential barrier and ignores thetructure of the QD surface, and the coupling between vi-rations and electronic states. To model the interactionsetween the dendrimer and the CdS-QD, a more sophis-icated approach is needed. Since excitons determine op-ical processes that occur close to the bandgap, Huongnd Birman [25] recently proposed a theoretical modelhat considers the hybrid exciton formed in a QD-endrimer system with a QD at the core of the dendrimer.n this model, the Wannier exciton in the QD interactsith the Frenkel exciton in the dendrimer throughipole–dipole interaction to form the hybrid exciton,hich they study using a real space Green’s functionethod and a diagrammatic technique to obtain the en-

rgy of the hybrid exciton for different QD-dendrimer sys-ems. Since the location of the QD in the CdS-DAB sys-em is not known, this approach may not be used directlyn this case.

The 375–650 nm broadband emission with a full-idth-at-half-maximum bandwidth ��6400 cm−1 and a

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arge Stokes shift of over 10 300 cm−1 indicate that bothomogeneous and inhomogeneous spectral line broaden-

ng mechanisms are operative in these CdS-DAB nano-articles. The spectral broadening mechanism and thedentity of states involved in luminescence transitions areot yet well understood. However, the large Stokes shiftnd broadband emission indicate that the observed fluo-escence is not due to bandgap recombination. The mea-ured low luminescence quantum efficiency of 0.03 indi-ates that nonradiative processes play a dominant role inhe relaxation of the excited nanoparticles. Any model foruminescence transitions in these nanocrystals should ac-ount for the observed large Stokes shift, broadband emis-ion, low fluorescence quantum efficiency, and the fastomponent ��165 ps� and the slow component ��40 ns� ofhe luminescence decay.

The bandgap in quantum confined QDs depends bothn the size and the shape of the particles. During colloidalrowth of the nanoparticles, both size distribution andhape distribution may change until the system reachesptimal fluorescence at a particular size/shape distribu-ion. The slight fluctuations in the emission maximum be-ween 465 and 480 nm with ion concentration, as seen inig. 1, could be caused by stabilization of slightly differ-nt size/shape distributions by the varying ion concentra-ions that sustain the growth. Plausibility of smallhanges in shape is also supported by observed polarizedmission and emission anisotropy discussed later in thisection.

The nature of the states involved in luminescence tran-itions in II-VI semiconductor QDs has been a matter ofontention [26–33]. The issue is complicated by a numberf factors that include the dependence of luminescence onanocomposite synthesis method, surface composition,tructure, and presence of traps. One of the systems thateceived a great deal of attention is CdSe. Emissiontrongly redshifted from the absorption in many earlierdSe nanoparticle samples synthesized from colloidal so-

utions was ascribed mainly to “deep trap” emission [26].ater advent of the high temperature synthesis methodrovided highly monodisperse nanocrystallites with di-meter tunable between 2 and 10 nm [29,30]. Details ofpectroscopic characteristics of these nanocrystallitesere first attempted to be explained in terms of a quali-

ative “surface model,” [31,32] and then by a more quan-itative model that takes into consideration the effect ofhe crystal shape asymmetry, the intrinsic crystal field,nd the electron-hole exchange interaction [33].While the above-mentioned interactions and effects are

ikely to be present in high quality CdS-DAB nanocrystal-ites, the room-temperature luminescence spectra and dy-amics of our colloidal nanocomposite samples exhibiteatures that are significantly different from that of theear band-edge emission in CdSe nanocrystallites dis-ussed above. For example, while the size-dependenttokes shift of the luminescence in CdSe ranges from100 to �25 meV (for nanocrystallite radius varying from

.6 to 5.6 nm, respectively), we observe an order-of-agnitude higher Stokes shift of �1.4 eV in our samples.he large Stokes shift, broadband emission, and fluores-ence kinetics in our sample have closer resemblance tohose observed by O’Neil et al. [22] and Spanhel et al. [20].

ccordingly, along the line of their analysis we use ahree-level structure involving thermal repopulation torovide a plausible, qualitative explanation of the ob-erved luminescence properties. In this scheme [22], illus-rated schematically in Fig. 8, the absorption of a photoneads to the formation of an exciton, that is, an electron inhe conduction band (level 2) and a hole in the valenceand (level 1). The exciton rapidly decays into a trappedlectron-hole pair. The electron traps to a narrow distri-ution of states that are close in energy to level 3, whilehe hole traps to a wide distribution of traps near level 1.ubsequent relaxation of the trapped electron may pro-eed along either of the following two routes. It may ther-ally repopulate level 2 and combine with one of theoles trapped at a number of possible sites and depths.lternatively, it may recombine from level 2 with a

rapped hole near level 1. The recombination followinghermal repopulation of level 2 is responsible for the fastecay component, while the slow component is due to re-ombination from level 3. A significant fraction of the totaluorescence quantum yield is contributed by the fastomponent, which requires that the shallow electronraps be on the order of kT, where k is the Boltzmann con-tant and T is the temperature. Level 3 may be lookedpon as a tightly spaced distribution of very shallowraps. Energetically, a trapped electron can readily detrapnd make a transition back to level 2 at room tempera-ure. The delocalized nature of the conduction band en-bles strong overlap between the trap states and the con-uction band states in the configuration coordinate space.onsequently the decay mechanism involving thermal re-opulation is efficient and provides an explanation of whyfast component is observed at all emission wavelengths.he other relaxation mechanism involving the recombina-

ion of a localized trapped electron in level 3 with a local-zed trapped hole near level 1 requires that the electronouple strongly to lattice modes and a favorable nuclearonfiguration be available for the Franck–Condon inte-ral to be substantial. Detailed temperature dependenteasurements of fluorescence yield and lifetime areeeded to substantiate this probable model and extractey relaxation parameters [20,22].

ig. 8. Proposed partial energy-level diagram of the CdS-DABanocomposite in methanol synthesized using the G5.0 DABendrimer. (NR is the nonradiative relaxation.)

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Observation of polarized emission following polarizedxcitation of CdS-DAB nanocomposite, and a high valuef emission anisotropy, r�0�=0.39 (close to the theoreti-ally predicted [24] value of 0.4) at t=0 suggests that: (a)here may be some structural feature that promotes direc-ional electronic transitions, (b) the absorption and emis-ion dipoles are parallel, (c) the nanoparticles are ran-omly oriented in the solvent (methanol), and (d) theanoparticles may have potential application as fluores-ent biophysical probes [34–39]. While constant positivemission anisotropy is not that common, anisotropy of0.22 was previously reported in CdS QDs composites byakowicz et al. [21,34]. They noted that similar direc-

ional transition dipoles are observed in excited organicolecules and suggested that the emission anisotropy in

heir CdS-PAMAM nanoparticles [21] may be due to anlongated shape for the quantum-confined state. More re-ent work on the size and shape dependence of emissionrom colloidal QDs demonstrated that even slight elonga-ion of the QD might result in substantial polarizationnd Stokes shift of emission [37]. Synthesis methods areow being developed to control the elongation to obtainquantum rods” with desired physical and spectroscopicroperties [38,39]. It is conceivable that the CdS QDs inAB may have a somewhat elongated shape, in deviation

rom the strict spherical shape that was assumed to esti-ate the diameter using the Brus model. Emission aniso-

ropy at t=0 of our CdS-DAB nanoparticles is �0.39,hich is in excellent agreement with the theoretical valuef r�0�=0.4 for random orientation of the nanoparticles inhe solvent with the absorption and emission dipoles be-ng parallel to each other [34,35]. The steady-state valuef r=0.14±0.02 is indicative of depolarization from thenitial value of 0.39, presumably due to rotational motionf the entire CdS-DAB nanoparticle, as well as indepen-ent motion of the CdS QD within the nanoparticle24,36]. Detailed measurements of the time evolution ofnisotropy, extraction of correlation times, and indepen-ent determination of the size, shape, and flexibility ofhe nanoparticles are needed for characterization of ob-erved polarized emission.

Fluorescent probes with nonzero emission anisotropyan act as biophysical probes of hydrodynamics, such asotational diffusion, where the fluorescence lifetime isomparable to the correlation time of interest21,24,28,34,37]. These probes can be used to label pro-eins and other macromolecules, and as internal cellulararkers [21,28]. The good chemical stability, the high

hotobleaching threshold, size-dependent tunable fluores-ence emission, and characteristic fluorescence decayime, combined with the advantages of the dendrimerlatform that may enable bioconjugation to antibodies,eceptors, and DNA, endow these hybrid nanoparticlesith a potential to be useful in bioimaging and medicaliagnostics.

CKNOWLEDGMENTShe authors thank J. L. Birman, J. I. Gersten, and N.uong for useful discussions, and A. Shukla for technicalelp. The work was supported in part by the Departmentf Defense (DoD) through its Center for Science, Math-

matics, and Engineering Research Program (grant911NF-04-1-0023) and an Enhanced Center for Ad-

anced Technology (E-CAT) grant from the New Yorktate Office of Science, Technology, and Academic Re-earch (NYSTAR). V. Balogh-Nair acknowledges partialupport from National Institutes of Health/National Cen-er for Research Resources (NIH/NCRR) grant G12R03060.

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synthesis and optical spectroscopy of a hybrid cadmium sulfide-dendrimer nanocomposite

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