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Water-Based Route to Ligand-Selective Synthesis of ZnSe and

Cd-Doped ZnSe Quantum Dots with Tunable Ultraviolet A to Blue

Photoluminescence

Zhengtao Deng,†,‡ Fee Li Lie,† Shengyi Shen,§ Indraneel Ghosh,§ Masud Mansuripur,‡ andAnthony J. Muscat†,*

Department of Chemical and EnVironmental Engineering, College of Optical Sciences and Department of Chemistry, The UniVersity of Arizona, Tucson, Arizona 85721

ReceiVed July 17, 2008. ReVised Manuscript ReceiVed October 7, 2008

A water-based route has been demonstrated for synthesizing ZnSe and Cd-doped ZnSe (Zn x Cd1-x Se, 0 < x < 1)quantum dots (QDs) that have tunable and narrow photoluminescence (PL) peaks from the ultraviolet A (UVA) tothe blue range (350-490 nm) with full-width at half-maximum (fwhm) values of 24-36 nm. Hydrazine (N2H4) wasused to maintain oxygen-free conditions, allowing the reaction vessel to be open to air. The properties of the QDswere controlled using the thiol ligands, 3-mercaptopropionic acid (MPA), thiolglycolic acid (TGA), and L-glutathione(GSH). On the basis of optical spectra, linear three-carbon MPA attenuated nucleation and growth, yielding smallZnSe QDs with a high density of surface defects. In contrast, TGA and GSH produced larger ZnSe QDs with lowersurface defect densities. The absorption spectra show that growth was more uniform and better controlled with lineartwo-carbonTGA thanbranched bifunctional GSH. After 20 min of growth TGA-capped ZnSehad an average diameter

of 2.5 nm based on high-resolution transmission electron microscopy images; these nanocrystals had an absorbancepeak maximum of approximately 340 nm (3.65 eV) and a band gap PL emission peak at 372 nm (3.34 eV). Highlyfluorescent Zn x Cd1-x Se QDs were fabricated by adding a Cd-thiol complex directly to ZnSe QD solutions; PL peakswere tuned in the blue range (400-490 nm) by changing the Zn to Cd ratio. The Cd-bearing nanocrystals containedproportionally more Se based on X-ray photoelectron spectroscopy, and Cd-Se bonds had ionic character, in contrastto primarily covalent Zn-Se bonds.

Introduction

Quantum dots (QDs) or semiconductor nanocrystals havepotential applications as biological labels, biosensors, light-emitting diodes (LEDs), and lasers.1-6 Synthesis of highlyfluorescent QDs has been accomplished by pyrolyzing organo-metallic reagents in hot coordinating solvents;7-9 this approach

utilizes oil-soluble ligands and high temperatures (typically200-360 °C).Materials that arecompatible with water facilitateusing QDs in biological systems. The exchange of hydrophobicligands, such as trioctylphosphine (TOP) or oleic acid (OA),with hydrophilic ligands, such as thiols, and subsequent transferof QDs from oil to aqueous solutions require complicatedprocesses and can significantly reduce photoluminescence (PL)quantum yield.2 Alternatively QDs and their primary ligandscan be encapsulated with another coating containing hydrophiliclipids or polymers to preserve PL quantum yield, but the size

will invariably increase (e.g., larger than 20 nm), which may notbe suitable for biological applications.10,11

Direct synthesisof thiol-capped II-VI semiconductor QDs inwaterisapromisingalternativeroutetoorganometallicreactionsand offers the following advantages: (1) lower reaction tem-perature (80-100 °C) with comparable PL quantum yield and

size-tunable fluorescence; (2) functionalization during synthesiswithout further treatment, such as using L-glutathione (GSH) orL-cysteine (Cys) biomolecules; (3) comparatively smaller sizes(3-8 nm).3,12,13 In addition, a recent report revealed that directfabrication of core-shell-shell (CdTe/CdS/ZnS) quantum dotsexhibitedexcellentphotostability in water.14Alloftheseattributesare useful characteristicsfor biological sensing,e.g., applicationsinvolving the pH-sensitive property of QDs.4,15 Advances havebeen made in the aqueous synthesis of highly fluorescent(40-60%), thiol-capped CdTe QDs with tunable emission fromthe green to the near-infrared (500-800 nm).12 Several groups

* To whom correspondence should be addressed. E-mail: [email protected].

† Department of Chemical and Environmental Engineering.‡

College of Optical Sciences.§ Department of Chemistry.(1) Bruchez, M.; Morrone, P.; Gin, S.; Weiss, S.; Alivisatos, A. P. Science

1998, 281, 2013–2016.

(2) Chan, W. C.; Nie, S. Science 1998, 281, 2016–2018.

(3) Zheng, Y. G.; Gao, S.; Ying, J. Y. Ad V. Mater. 2007, 19 , 376–380.

(4) Deng, Z.; Zhang, Y.; Yue, J.; Tang, F.; Wei, Q. J. Phys. Chem. B 2007,111, 12024–12031.

(5) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth,J. A.; Leatherdale, C. A.; Eisler, H. J.; Bawendi, M. G. Science 2000, 290, 314–317.

(6) Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D. Y.; Zhu, T.; Xu, J.; Yang, C. H.;Li, Y. F. Nat. Photon. 2007, 1 , 717–722.

(7) Murray, C. B.;Norris, D. J.;Bawendi, M. G. J. Am. Chem. Soc. 1993, 115,8706–8715.

(8) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183–184.

(9) Deng, Z. T.; Cao, L.; Tang, F. Q.; Zou, B. S. J. Phys. Chem. B 2005, 109,16671–16675.

(10) Wu, X. Y.; Liu, H. J.; Liu, J. Q.; Haley, K. N.; Treadway, J. A.; Larson,J. P.; Ge, N. F.; Peale, F.; Bruchez, M. P. Nat. Biotechnol. 2003, 21, 41–46.

(11) Zhou, M.; Nakatani, E.; Gronenberg, L. S.; Tokimoto, T.; Wirth, M. J.;Hruby, V. J.; Roberts, A.; Lynch, R. M.; Ghosh, I. Bioconjugate Chem. 2007,18, 323–332.

(12) (a) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.;Lesnyak, V.;Shavel,A.; Eychmuller, A.;Rakovich,Y. P.;Donegan, J.F. J. Phys.Chem. C 2007, 111, 14628–14637. (b) Li, L.; Qian, H. F.; Ren, J. C. Chem.Commun. 2005, 528–530. (c) Zhang, H.; Zhou, Z.; Yang, B.; Gao, M. Y. J. Phys.Chem. B 2003, 107 , 8–13. (d) Zhang, H.; Wang, L. P.; Xiong, H. M.; Hu, L. H.;Yang, B.; Li, W. Ad V. Mater. 2003, 15, 1712–1715. (e) Yang, Y. H.; Wen, Z. K.;Dong, Y. P.; Gao, M. Y. Small 2006, 2, 898–901. (f) Li, L. L.; Chen, D.; Zhang,Y. Q.; Deng, Z. T.; Ren, X. L.; Meng, X. W.; Tang, F. Q.; Ren, J.; Zhang, L. Nanotechnology 2007, 18, 405102.

(13) Lei, Y.; Jiang, C. Y.; Liu, S. J.; Miao, Y. M.; Zou, B. S. J. Nanosci. Nanotechnol. 2006, 6 , 3784–3788.

(14) He, Y.; Lu, H. T.; Sai, L. M.; Su, Y. Y.; Hu, M.; Fan, C. H.; Huang, W.;Wang, L. H. Ad V. Mater. 2008; DOI: 10.1002/admma.200701166.

(15) Zhang, Y.; Deng, Z. T.; Yue, J. C.; Tang, F. Q.; Wei, Q. Anal. Biochem.2007, 364, 122–127.

434 Langmuir 2009, 25, 434-442

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have explored aqueous routes to fabricate thiol-capped ZnSeQDs with tunable PL emission from the UVA to the blue range,but problems wereencountered with defect-relatedemission andtuning the near band-gap emission peaks.16-19 Ying and co-workers recently reported the synthesis of GSH-stabilized ZnSeand Zn x Cd1-x Se QDs in an oxygen-free water solution undernitrogen, which represents the first direct synthesis of bluefluorescent QDs in aqueous solution.20 In contrast to that work,whichwasperformedundernitrogen,thispaperdescribesawater-

based route that utilizeshydrazinein solution to maintainoxygen-free conditions; this allows the reaction mixture to be kept opento air without oxidizing the sodium hydroselenide (NaHSe)precursor. Highly fluorescent thiol-capped ZnSe and Zn x Cd1-x SeQDs were fabricated with finely tunable PL from the UVA tothe blue range (350-490 nm). The thiols, 3-mercaptopropionicacid (MPA), thiolglycolic acid (TGA), and GSH were selectedto systematically examine the effect of ligandchemistry and sizeon the growth and optical properties of QDs.

Experimental Section

The chemicals were used as purchased from Sigma-Aldrich.All reactions were carried out in ultrapure deionized (DI) water,

open to air. Hydrazine (N2H4, 35 wt % in H2O) [Safety notice:hydrazine should be handled carefully] was introduced to keepthe reaction in a reducing atmosphere, enabling highly oxygen-sensitive sodium hydroselenide (NaHSe) to react with Zn-thiolcomplexes forming QDs. Sodium hydroselenide (NaHSe) wasprepared by mixing sodium borohydride (NaBH4, reagent grade,g98.5%) and selenium (Se, 99.99%) powder in DI water. A darkred solution was obtained after the Se powder was completelyreduced by NaBH4. A typical synthetic procedure for ZnSe QDsfollows: 1 mmol of Zn(NO3)2 (reagent grade, 98%), a calculatedamount of a thiol molecule, and 2 mL of N 2H4 were dissolvedin 100 mL of DI water in a three-neck flask. The thiol moleculesused were 3-mercaptopropionic acid [MPA, HS(CH2)2COOH](g98%), thiolglycolic acid [TGA, HSCH2COOH] (g99%), andL-glutathione [GSH, HSCH2CH(NHCdO(CH2)2CHNH2COOH)-

(CdONHCH2COOH)] (g98%). Two milliliters of as-preparedNaHSe solution was added to the flask with vigorous stirring.The pH of the mixed solution was adjusted to 11 with additionof 1 M NaOH. The molar ratio of Zn/thiol/Se was 1/1.3/0.5.During refluxing of thereactionmixture in airwith thetemperatureclose to 100 °C the red color gradually faded, resulting in ahomogeneous, colorless solution, whichsubsequently turned lightblue as the ZnSe QDs grew. Aliquots were removed from thereaction vessel at regular intervals, rapidly cooled to roomtemperature, and stored at 4 °C in the dark.

A typical synthetic procedure for Zn x Cd1-x Se QDs follows:Cd-thiolcomplexeswerepreparedinathree-neckflaskbydissolving1mmolofCd(NO3)2 (reagent grade, 99%) with a calculated amountof a thiol-bearing molecule in 50 mL of DI water. The molar ratio

ofCd/thiolwas1/1.3.ACd-

thiolsolutionwasaddedduringrefluxingto a Zn-thiol-Se solution, which was prepared as described. TheZn/Cd ratio was varied from 1/0.1 to 1/2. The solution turned lightyellow as Zn x Cd1-x Se QDs grew. Aliquots were removed from thereaction vessel at regular time intervals, rapidly cooled to roomtemperature, and stored at 4 °C in the dark.

Aliquots were diluted with DI water for characterization withoutany size sorting. UV-vis absorption spectra of QD samples inaqueous solution were obtained using an Agilent 8453 UV-visspectrometer. Room temperature photoluminescence (PL) and

photoluminescence excitation (PLE) spectra were measured usinga PTI fluorescence spectrometer (814 photomultiplier detectionsystem and LPS-220B power supply). The PL quantum yield forboth ZnSe (emission in the UVA range) and Zn x Cd1-x Se (emissionin the blue range) QDs was obtained by referencing to a standard(2-aminopyridine (g99%) in 0.1 M H2SO4, QY ) 60%) followinga procedure reported in the literature.21 The PL quantum yield wascalculated using the following equation

φ)φ′ ×

(

I

I ′) ×

(

A′

A ) ×

(

n

n′)

2(1)

where φ and φ′ are the PL QY for the sample and standard,respectively; I (sample) and I ′ (standard) arethe integrated emissionpeak areas at a given wavelength; A (sample) and A′ (standard) arethe absorption intensities at the same wavelength used for PLexcitation; n (sample) and n′ (standard) are the refractive indices of the solvents.

High-resolution TEM (HRTEM) wasperformed on a conventionalHitachi H8100 electron microscope operating at 200 kV. Samplesfor HRTEM were prepared by putting a few drops of a solutioncontaining QDs on an amorphous carbon substrate supported on acoppergridandallowingthesolventtoevaporateatroomtemperature.A purified quantum dot sample wascrushed finely for X-ray powderdiffraction (XRD) analysis. XRD measurements were made usinga Philips X’Pert MPD diffractometer with Cu KR radiation ( λ )1.5418 Å) at a scanning rate of 0.02° /s with a resolution limit of ∼5%. The composition of supported QDs was obtained with anX-ray photoelectron spectroscopy (XPS) system consisting of aKratos Axis Ultra X-ray photoelectron spectrometer and a mono-chromatic Al KR source (1486.6 eV). Powder QD samples weredeposited on carbon tape supported by an aluminum sample holder.Survey spectra were acquired with a pass energy of 160 eV andhigh-resolution spectra with a pass energy of 20 eV. Quantitativeanalysis was performed as follows: (1) correct for surface chargingby aligning to the Se 3d3/2 peak, which was assigned to a bindingenergy of 54.5 eV on the basis of data from ZnSe thin films;22 (2)subtract an elastic/inelastic background using a Shirleyalgorithm;23

(3) fit using a Voigt line shape with peak width and position as free

parameters; (4) compute atomic ratios using the peak areas of theZn 2p3/2, Cd 3d, and Se 3d states corrected by relative sensitivityfactors.24

Results and Discussion

The ligand chosen to complex metal atoms not only stabilizesa quantum dot in water but also significantly affects the growthkinetics.Thetemporal evolution of ZnSe QDscapped with threedifferent ligands is shown by the UV-vis absorption spectra inFigure 1. When the ligand was MPA, small ZnSe QDs formedafter10minwithadistinctabsorptionpeakat315nmthatshiftedcontinuously to 321 nm after 20 min and 337 nm after 30 min(Figure 1a). The absorption peak attenuated sharply for longergrowth times, becoming a shoulder at approximately 360 nm.When TGA and GSH were used as capping ligands, similarresults were obtained where a distinct absorption peak shiftedto longer wavelengths and attenuated with increasing growthtime (Figure 1b and 1c). The well-resolved peaks at shortwavelengths suggesthomogeneousnucleationof smallcrystallitesthatgrewbyanOstwaldripeningprocess,forminglargercrystalsat the expense of smaller ones. The relatively pronouncedabsorption peaks show that at early stages the size distributionof ZnSe QDs was nearly monodispersed with growth stopping

(16) Karanikolos, G. N.; Alexandridis, P.; Itskos, G.; Petrou, A.; Mountziaris,T. J. Langmuir 2004, 20, 550–553.

(17) Shavel, A.; Gaponik, N.; Eychmuller, A. J. Phys. Chem. B. 2004, 108,5905–5908.

(18) Qian, H.;Qiu, X.;Li, L.; Ren, J. J. Phys. Chem. B 2006, 110, 9034–9040.

(19) Xiong, S.; Huang, S. H.; Tang, A. W.; Teng, F. Mater. Lett. 2007, 61,5091–5094.

(20) Zheng, Y. G.; Yang, Z. C.; Ying, J. Y. Ad V. Mater. 2007, 19, 1475–1479.

(21) (a)Valeur,B. MolecularFluorescence:Principlesand Applications;Wiley-VCH: NewYork,2001. (b) Deng, Z. T.; Peng, B.;Chen, D.;Tang, F. Q.; Muscat,A. J Langmuir 2008; ASAP, 10.1021/la800984g.

(22) Chaparro,A. M.;Maffiotte, C.;Herrero, J.;Gutierrez,M. T. Surf. Interface

Anal. 2000, 30, 522–526.(23) Shirley, D. A. Phys. ReV. B 1972, 5 , 4709.(24) Wagner, C. D. J. Electron Spectrosc. Relat. Phenom. 1983, 32, 99–102.

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in the “focusing of size distribution” regime.9 At later stages theabsorptionpeaks were no longer distinct or disappeared entirely,and absorption tails appeared showing that the quality of theQDs degraded. These results are consistent with those reportedby Zheng et al.,20 where the highest quality ZnSe nanocrystalswere obtained after 30 min.

The chain length, shape, and terminal functional groups of thiol ligands influenced the number and size of the ZnSe nucleiformed, which determined the growth rate of the QDs. Theabsorption peaks of the ZnSe QDs in the early stage of growthdiffered for the three ligands studied. After 10 min, for example,absorption peaks were observed at 315, 320, and 345 nm forMPA, TGA, and GSH ligands, respectively. Under identicalsynthetic conditions, the smallest ZnSe QDs were formed usingMPA, while the largest were obtained with GSH. MPA has onemore methylene group than TGA, and both are terminated by

a carboxylic acid, which becomes carboxylate COO- at the highpH conditions in the reactor during growth. These ligands canbe packed efficiently on the surface of a cluster since they areshort, linear molecules. The negative charge imposed a kineticbarrierforgrowth dueto Coulombic repulsion betweentheligandshells of neighboring QDs, favoring many, small, stablecrystallites. Growth was stabilized, resulting in the pronouncedpeaks in the absorption spectra in the first growth regime atshorter times. An absorption peak was observed even after 40min in the case of TGA. GSH is not only bulky compared toMPA and TGA but also contains one amine and two carboxylicacid terminal functional groups. Since the pH of the reactionsolution was above the pK a’s, these groups were negatively

charged.25,26

In contrast to MPA and TGA, GSH is branched,which lowered the packing density and hence the charge densityon the surface of clusters as well as reduced the water solubility,enhancingthegrowthrate, andproducinglarger QDs. Thegrowthwas not as controlled since the absorption peak intensity for theGSH-capped ZnSe QDs was the least distinct and attenuatedmore quickly than for either MPA or TGA.

Thiol ligands not only produced different growth regimes andmodes but also played a major role in determining thephotoluminescence properties of ZnSe QDs. Figure 2 shows thePL and PLE spectra measured for MPA-capped ZnSe QDs atthe same reaction times from 10-50 min that absorption data

were obtained. The spectra of samples grown for 10 min areshown in Figure 2a-c for different excitation and monitorwavelengths. Exciting at 310 nm produced one sharp emissionpeak at 350 nm in the PL spectrum and another broad peak at420nm(EX:310nm).Excitingat340nmproducedoneemissionpeak at 420 nm (EX: 340 nm). Monitoring the emission at 350nm produced a sharp upward response but no PLE peak (EM:350 nm). Monitoring at 420 nm, however, produced a narrowPLEpeakcenteredat340nm(EM:420nm).Theseresultsindicatethatthepeakat350nmisduetonearband-gapemission,whereasthe peak at 420 nm is related to surface-defect emission. Statescreated in the middle of the band gap by crystal imperfectionsat the surface, such as dislocations and vacancies, act as efficientelectron-hole recombination centers. Se defects are the mostlikely since the starting molar ratio of Zn/thiol/Se was 1/1.3/0.5,which is Se lean, and the surface of nanocrystals is capped byZn-thiol complexes. To corroborate the PL properties of thesample, the excitation wavelength was varied in discrete stepsin the range 290-325 nm as shown in Figure 2c. The band-gapemission peak at 350 nm remained fixed for different excitationwavelengths, reflecting an intrinsic characteristic of the QDs;the surface-defect-induced emission peak red shifted slightly(from 414 to 420 nm), which is due to a weak dependence of sub-band transitions on excitation energy. It is also worth notingthat the intensity ratio of near band-gap emission and surface-defect-induced emission depended on excitation wavelength.Surface-defect-induced emission was dominant at 290 nm, butas theexcitation wavelength approached theband gapat 350nm,the near band-gap emission was more intense. The photolumi-

nescencepropertiesof ZnSeQDs consequentlydependedstronglyon the excitation wavelength, and capping with MPA produceda relatively high density of surface states.

Similar results were obtained from the MPA-capped ZnSeQDsamplesatgrowthtimesof20and30minasshowninFigure2d and 2e, respectively. Here the spectra are dominated by nearband-gap emission, and surface-defect-induced emission wasnot as distinct as at shorter times. At growth times of 40 and 50min (Figure 2f), however, the near band-gap emission peakattenuated strongly, and surface-defect-induced emission domi-nated but was broad. The highest quality QDs were obtained atgrowth times of 20-30 min, which is consistent with the resultsobtainedfromUV-vis absorption spectra. Comparing the results

as a function of time indicatesthat thesmallest crystals containedthe most structural defects.

(25) Perrin,D.D., Dissociation Constantsof Organic Basesin Aqueous Solution;

Butterworths: London, 1965; Supplement, 1972.(26) Serjeant, E. P.; Dempsey, B. Ionization Constants of Organic Acids in Aqueous Solution; Pergamon: Oxford, 1979.

Figure 1. UV-vis absorption spectra of (a) MPA-capped, (b) TGA-capped, and (c) GSH-capped ZnSe quantum dots.

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The PL spectra of TGA-capped ZnSe QDs at growth timesof 10, 20, and 30min and for GSH-cappedQDs at a growthtimeof 20 min are shown in Figure 3. In contrast to MPA capping,band-gap emission dominated for linear but shorter TGA andmore branched GSH. Nanocrystal growth with these ligandsoccurred with low defect densities, indicating that ligands withshorter main chain and more or different types of functionalgroups endow the surface with fewer structural flaws.16,19 Bysystematically changing the ligands and reaction times, the size

and PL properties of ZnSe QDs were tuned, producing emissionin the UVA range (350-390 nm) with narrow peaks (fwhm24-32 nm) and exhibiting quantum yields up to ∼20%.

ZnSe is a wide-gap material, allowing the band gap to betuned by introduction of other metal ions.The water-synthesizedZnSe QDs could serve as the host matrix for dopants such as Cd,Mn, and Cu.27-29 This has been demonstrated using the thiol-capped ZnSe QDs as a starting material to dope with Cd atomsand synthesize Zn x Cd1- x Se QDs, where 0 < x < 1. Temporal

evolution of Zn1- x Cd x Se QD growth is shown by the UV-visabsorption spectra in Figure 4a, where the red-shifted absorptionshoulder indicates that Cd atoms were successfully introducedintoaZnSematrix.InjectingCdintoagrowthsolutioncontainingZnSe nanocrystals gradually formed Cd-rich outer layersdepending on the Zn/Cd molar ratio in the feed. The red shiftindicative of CdSe, which has a smaller band gap than ZnSe, isconsistent with incorporation of Cd. Figure 4b and 4c shows thePLandPLEspectraofTGA-cappedZn1- x Cd x SeQDsaftergrowth

times of 50 min. These samples behaved similarly to the TGA-capped ZnSe QDs. Figure 4d summarizes the PL spectra of fourtypical TGA and GSH-capped Zn1- x Cd x Se samples. The startingZnSe QDs were TGA capped (emission peak ≈ 372 nm) fortraces 1 and 2 and GSH capped (emission peak ≈ 390 nm) fortraces3and4.ThePLpeaksoftheZn 1- x Cd x Se nanocrystals weretuned in the blue range (400-490 nm) by changing both theinitial Cd to Zn ratio andthe capping ligand; narrow peaks (fwhm30-36 nm) were obtained with quantum yields up to 40%.

Figure 5 shows high-resolution TEM images of TGA-cappedZnSe QDs with near band-gapemission at 372 nm and the TGA-capped Zn x Cd1- x Se QDs samples with near band-gap emissionat 425 nm. As shown by the size distribution histograms in the

insets of Figure 5a and 5c the as-synthesized TGA-capped ZnSeand Zn x Cd1- x Se QDs had average diameters of 2.5 and 3.0 nm,

(27) (a) Zhong, X. H.; Feng, Y. Y.; Zhang, Y. L.; Gu, Z. Y.; Zou, L. Nanotechnology 2007, 18, 385606. (b) Ge, J. P.; Xu, S.; Zhuang, J.; Wang, X.;Peng, Q.; Li, Y. Q. Inorg. Chem. 2006, 45, 4922–4927. (c) Yang, C. C.; Li, S. J. Phys. Chem. C 2008, 112, 2851–2856.

(28) Zhong, X.; Han, M.; Dong, Z.; White, T. J.; Knoll, W. J. Am. Chem. Soc.

2003, 125, 8589–8594.(29) Pradhan, N.; Goorskey, D.; Thessing, J.; Peng, X. J. Am. Chem. Soc.

2005, 127 , 17586–17587.

Figure 2. PL and PLE spectra of MPA-capped ZnSe quantum dots synthesized at different times: (a-c) 10, (d) 20, (e) 30, and (f) 40 and 50 min.(sEM: 350) Photoluminescence excitation spectrum of the sample with the emission monitored at 350 nm. (sEX: 310) Photoluminescence emissionspectrum of the sample with excitation at 310 nm.


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respectively. Figure 5b and 5d shows magnified high-resolutionTEM images of individual ZnSe and Zn x Cd1- x Se QDs, respec-tively. The existence of distinct lattice planes confirmed thecrystallinity of QDs synthesized in water. The lattice spacing of the ZnSe QD was about 0.32 nm, corresponding to the (111)plane of a cubic zinc blende ZnSe structure (Joint Committeeon Powder Diffraction Standards, JCPDS, Card No. 80-0021),which increased to about 0.33 nm due to Cd doping, which isconsistent with the literature.20,30

Powder XRD patterns of the same TGA-capped ZnSe andZn x Cd1- x Se QD samples are shown in Figure 6. The XRD patternof the ZnSe QDs had characteristic features at 27.5°, 45.6°, and54.3° corresponding to the (111), (220), and (311) planes of cubic zinc blende ZnSe (JCPDS Card No. 80-0021). The XRDpatternoftheZn x Cd1- x SeQDsamplehadfeaturesat26.9 °,44.6°,and 51.9°, which are between those for ZnSe and those at 25.3°,42.0°, and 49.7° for zinc blende CdSe (JCPDS Card No. 19-0191).9 The dependence of the diffraction peak positions of theZn x Cd1- x Se QDs on the Zn/Cd ratio is in accord with Vegard’sLaw31 andshowsthatlayerscontainingCdwereaddedtoaZnSecore. As expected, the XRD peaks of the ZnSe and Zn x Cd1- x Se

QD samples were considerably broadened compared to bulkmaterialsduetothesmallsizeoftheQDs.Thesizeofnanocrystalswas estimated from the (111) reflection using the Scherrerformula32

Lhkl) 0.94 λ

βhkl cos θhkl

(2)

where Lhkl is the coherence length, βhkl is the full-width at half-

maximum (fwhm) of the peak, λ(1.5418 Å) is the wavelengthof the X-ray radiation, and θhkl is the angle of diffraction. ForZnSe, 2θhkl is 27.5°, βhkl is 4.4°, and the calculated average sizeis 2.0 nm, while for Zn x Cd1- x Se, 2θhkl is 26.9°, βhkl is 3.2°, andthe calculated average size is 2.7 nm. Inherent stress inside ananocrystal could contribute to broadening of the XRD peaks,making the size estimate from the Scherrer formula a lowerbound.

The absorption and PL band-gap emission peaks of ZnSenanocrystals were below 400 nm (∼3.1 eV), which are shiftedtoward shorter wavelengths in comparison to the 2.67 eV(corresponding to 465 nm) band gap of the bulk material. Thelargeblueshiftsintheabsorptionandemissionspectraarecausedby quantum confinement of an electron-hole pair (exciton). The

(30) Cozzoli,P. D.;Manna,L.; Curri, M. L.;Kudera,S.; Giannini, C.;Striccoli,M.; Agostiano, A. Chem. Mater. 2005, 17 , 1296–1306.

(31) Vegard, L.; Schjelderup, H. Z. Phys. 1917, 18, 93–96. (32) Langford, J. I.; Wilson, A. J. C. J. Appl. Crystallogr. 1978, 11, 102.

Figure 3.

PL spectra of TGA- and GSH-capped ZnSe quantum dots synthesized for a fixed period of time: (a) TGA-capped, 10 min; (b) TGA-capped,20 min; (c) TGA-capped, 30 min; (d) GSH-capped, 20 min. (sEX: 315) Photoluminescence emission spectra of the sample with excitation at 315nm. (sEX: 350) Photoluminescence emission spectra of the sample with the emission peak at 350 nm. Band gap emission at 360-390 nm and surfacedefect emission at 450-500 nm are marked by dashed lines.


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spherical-dielectric continuum model based on the effectivemass approximation explains strong exciton confinement whenthe particle radius is less than aB*, the Bohr exciton radius of the bulk material. In this regime there is a progressive increasein excitonic transition (band-gap) energy withdecreasingparticlesize given by the equation33

E (r )) E g

bulk+p

2π 2

2r 2

( 1

me*+

1

mh*)-

1.8e2

4πεε0

1

r + smaller terms

(3)

where E gbulk is the bulk band gap (2.67 eV), r is the nanocrystal

radius, me* is the effective mass of electrons (0.157 me), mh

* is theeffective mass of holes (0.64 me), and ε is the dielectric constant(8.7); the values in parentheses are averages for cubic ZnSe atroom temperature.34 The Bohr exciton radius aB* is 3.65 nm forZnSe.Thesecondtermineq2isquantumconfinementofelectronsand holes and produces a blue shift; the third term is due to

Coulomb interactionsand produces a red shift; the smaller termsare spatial correlation energy and can be neglected. Substitutingthe nanoparticle diameter d and using the values for ZnSe, thisequation becomes

E (d )) 2.67+11.9

d 2 -

0.596d

eV (4)

where d is in nanometers. Using the absorption peak atapproximately 340 nm (3.65 eV) for TGA-capped ZnSe after 20min of growth yields a nanocrystal diameter of 3.2 nm. Thisappears to be an upper bound on the size of these ZnSenanocrystals, complementing the 2.0 nm lower bound fromXRDmeasurements;themostaccuratediameteristhe2.5nmmeasuredfrom TEM images.

Elemental analysis of three QD samples containing differentamounts of Cd was made using XPS. The survey spectra shownin Figure 7 are for supported TGA-capped ZnSe (with a 372 nmnear band-gap emission peak), TGA-capped Zn x Cd1- x Se (425nm), and GSH-capped Zn x Cd1- x Se (490 nm) QDs. Both corelevel photoemission and Auger peaks were observed forthe constituentatoms. Thecarbon tape support contributed to the

strong C and O signals. The chemicals in the reaction mixtureproduced residual N due to hydrazine and Na due to sodium

(33) (a) Brus,L. J. Phys. Chem. 1986, 90, 2555–2560. (b)Brus,L. E. J. Chem.Phys. 1983, 79,5566–5571.(c)Kayanuma,Y. Phys.ReV. B 1988, 38, 9797–9805.(d) Norris, D. J.; Bawendi, M. G. Phys. ReV. B 1996, 53, 16338–16346.

(34) Madelung, O.; Schulz M.; Weiss, H. Landolt Bornstein, Numerical Dataand Functional Relationships in Science and Technology. New Series, Group III:Crystal and Solid State Physics; Springer: Berlin, 1996; Vol. III/17b.

Figure 4. (a) UV-vis absorption spectra of TGA-capped ZnSe and Zn x Cd1- x Se quantum dots samples at different intervals. (b) PL and PLEspectra of a typical TGA-capped Zn x Cd1- x Se QDs sample with PL peak at 400 nm. (c) PL and PLE spectra of a typical TGA-capped Zn x Cd1- x SeQDs sample with PL peak at 425 nm. (d) PL spectra of four different Zn x Cd1- x Se samples exhibiting different emission wavelengths; the initial

Cd/Zn ratio in solution was 0.1/1 for curve labeled 1 (excited at 360 nm), 0.3/1 for curve 2 (excited at 390 nm), 0.6/1 for curve 3 (excitedat 410 nm), and 1.6/1 for curve 4 (excited at 450 nm). The starting QDs are TGA-capped ZnSe QDs (emission peak ≈ 372 nm) for curves1 and 2 and GSH-capped ZnSe QDs (emission peak ≈ 390 nm) for curves 3 and 4. (sEM: 425 nm) PL excitation spectra of the sample withemission monitored at 425 nm. (sEX: 370) PL emission spectra of the sample with excitation at 370 nm. (sEM: 400) PL emission spectraof the sample with the emission peak at 400 nm.


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hydroselenide andsodium hydroxide.Thethiolligandsproducedthe S signals. The amount of QD powder used to prepare asample for XPS analysis was not controlled and caused sample-to-sample variations in peak heights.

EvidenceforZn-Se,Cd-Se,andSe-Se bondingwas presentin theSe 3d regionbased on high-resolutionXPS spectra (Figure

8). The Se 3d peak is a doublet because of spin-orbit couplingwith a branching ratio of 1.5 and a binding energy separation of

0.9 eV.24,35 The Se 3d5/2 peak at a binding energy of 53.6 eVand the Se 3d3/2 peak at 54.5 eV were assigned to Se bonded toeither Zn or Cd. The high-resolution data were measured usingthesameQDsamplesthatweresurveyedinFigure7.Thedoubletat higher binding energies in Figure 8 is the Se 3d5/2 peak at 54.9eV and the Se 3d3/2 peak at 55.8 eV of elemental Se0. Althoughthere may have been a contribution from Se-Se bonding inQDs, the elemental Se signal was due primarily to Se-bearingmolecules in solution that were assimilated in the powder. Apeak at 56.0 eV was assigned to Se0 metal that was producedby decomposition of unreacted Se precursors in ZnSe filmsdeposited from solution.22 The chemical shift ∆ E chem betweenelemental Se and Se in a Zn or Cd bonding environment was 1.3eV.High-resolution XPS spectra of theZn 2p region(not shown)revealed a single peak at a binding energy of 1020 eV, which

was assigned to the Zn 2p3/2 state in ZnSe and Zn1- x Cd x Se.Similarly in the Cd 3d region (not shown) there were peaks atbinding energies of 404.2 and 411 eV, which were assigned tothe Cd 3d5/2 and 3d3/2 spin-orbit split states. There were peaks(not shown) due to the S 2s state at a binding energy of 225.4eV, which was assigned to S bonded to either Zn or Cd, and dueto the Se 3s state at 228.8 eV, which was broader (fwhm 3.2-3.9eV) than for other peaks, indicating that more than one bondingenvironment was present.

Doping ZnSe QDs with Cd increased the proportion of SeboundtoZnandCdrelativetothatinelementalform.Comparingthe ratio of two peak areas in an XPS spectrum measured on one

(35) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Eden Prairie,MN, 1995.

Figure 5. TEM images of (a, b) TGA-capped ZnSe quantum dots sample with a near band-gap emission peak at 372 nm, and (c,d) TGA-cappedZn x Cd1- x Se QDs sample with a near band-gap emission peak at 425 nm. The insets in a and c are the size distribution histograms. The scale barsin b and d are 2 nm.

Figure 6. XRD patterns of (a) TGA-capped ZnSe quantum dots samplewith an emission peak at 372 nm, and (b) TGA-capped Zn x Cd1- x Se QDssample with an emission peak at 425 nm. The green vertical linescorrespond to cubic ZnSe (JCPDS card No. 80-0021), and the blue barscorrespond to cubic CdSe (JCPDS card No. 19-0191).


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samplewiththeratioofthesamepeakareasfromanothersampleis one way to correct for variations in the amount of QD powderused in sample preparation. When QDs were capped with TGA,addition of Cd significantly increased theproportion of Se boundtoZnandCdsincetheratio3d 5/2(Se-Zn+Se-Cd)/3d5/2(Se-Se)increased from 1.6 to 4 based on the peak areas in Figure 8a and8b. Changing the ligand from TGA to GSH, however, yieldedapproximately the same proportion of Se when Cd was present.Although Cd could serve as a counterion to either Se2- or HSe-

in solution, one interpretation of these results is that the Cd

reacted with Se and promoted incorporation of Se in QDs. If Cdreacted with Se but did not incorporate into a cluster, then QDs

produced under similar conditions should have been ap-proximately the same size. For a given reaction time, increasingthe Cd concentration produced larger QDs as shown by the redshift of the photoluminescence peaks (Figure 4d). Zn and Cdhave comparable electronegativity coefficients and ionization

energies; however, a Cd2+

ion is about 30% larger than Zn2+

.36

The size made Cd better able to accommodate multiple, bulkythiol ligands, but they were held less strongly than by Zn dueto a weaker Coulomb force. The more labile ligands increasedthe reactivity between a Cd-thiol complex and Se ions. Thehigher concentration of CdSe moieties in solution was a drivingforce for increasing the proportion of Se in QDs. An alternativeto adding Cd in a two-step process is the direct insertion of Cdinto Se-Se sites on the surface of a quantum dot. These sitescouldhavebeenpresentontheZnSeQDsusedtoseedthegrowthof Zn x Cd1- x Se.

The ratio of Zn and Cd to Se for a given sample was greaterthan one, which suggests that unreacted Zn and Cd precursorswere present as well. The Zn/Cd/Se/S ratio based on XPS peakareas corrected for elementsensitivity was1/0/0.68/1.9for TGA-capped ZnSe QDs (372 nm), which was close to the molar ratioof 1/0/0.5/1.3 of the starting mixture. The Zn/Cd/Se/S ratio was1/0.22/0.73/2.4 for TGA-capped Zn x Cd1- x SeQDs (425 nm) and1/1.5/1.2/3.7 for GSH-capped Zn x Cd1- x Se (490 nm); however,the Zn to Cd ratioin the starting mixture was also increased from1/0.3 when TGA was used to 1/1.6 for GSH. For the smallerTGA ligand, the S to Zn and Cd ratio was approximately 2,whereas for the larger, bifunctional GSH ligand the ratio wasabout 1.5. A portion of the unreacted Zn and Cd atoms waspresentafterformingapowderprobablyattachedtothiolligands.

Thebinding energyseparation between XPSpeaksis a measureof thechargetransfer andchemical environment of theconstituent

(36) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements, 2nd ed.;Butterworth-Heinemann: Oxford, England, 1997; p 1205.

Figure 7. Survey X-ray photoelectron spectra of supported (a) TGA-capped ZnSe QDs (emission peak at 372 nm), (b) TGA-capped Zn x Cd1- x Se QDs(emission peak at 425 nm), and (c) GSH-capped Zn x Cd1- x Se QDs (emission peak at 490 nm). The Cd 3d region is shown in the inset magnified ×2.5.XPS peaks are labeled by orbitals and Auger peaks by transitions.

Figure 8. High-resolution X-ray photoelectron spectra of the Se 3d linefor (a) TGA-capped ZnSe QDs (emission peak at 372 nm), (b) TGA-

cappedZn x Cd1- x Se QDs(emissionpeakat 425nm),and (c)GSH-cappedZn x Cd1- x Se QDs(emissionpeakat 490nm). Experimental data are shownas open circles, least-squares peak fit components by red lines, and thesum of the peak components by lines through the data.


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atoms. The difference between the Zn 2p3/2 and the Se 3d5/2

peaks was 966.4 eV for ZnSe QDs; this value compares closelyto 966.2 eV calculated for the pure elements35 but is 1.4 eV lessthan 967.8 eV measured for ZnSe thin films.22 There was moreelectron density on Zn atoms in QDs compared to a thin film,andZn-Sebondingwasmorelikethatofamolecule.Theenergylevelsinananocrystalarediscretestateshavingatomiccharacterthat are grouped into two bands separated by a gap that dependson crystal size. Using an atomic description of the levels Se2-

ions via filled sp-hybridized 4-shell valence orbitals sharedelectron density with the outermost 4s shell on Zn ions that werenominally 2+; the similar energies and shapes lent significantcovalent character to the bond. In contrast, the binding energyseparationbetweentheCd3d5/2 andSe3d5/2 peaksforZn x Cd1- x SeQDs was 350.6 eV, which is 1.1 eV greater than 349.5 eVcalculated for the pureelements35 butcomparesclosely with 351eV measured for CdSe thin films and nanocrystals.37,38 Therewas less electron density on Cd atoms in QDs compared to thepure element, and Cd-Se bonding was more like that of a bulksolid. The size of a Cd ion and the relatively good shielding of a complete 4d shell weaken the force of attraction of electronsin the outermost 5s orbital. Moreover, the difference in energybetweenaCd5sorbitalandtheSe4sp-hybridizedorbitalsreducesoverlap;thenetresultisCd-Se bondsthat have significant ioniccharacter. The electrostatic nature of the bonding and ionic sizesuggest that Cd could coordinate with more than one Se atom.Multiple Cd-Se bonds are borne out by the XPS peak area datashowing that more Se atoms were coordinated in nanoclustersthat contained Cd atoms.

Conclusion

A ligand-selective synthesis of ZnSe andZn x Cd1- x SeQDswasdemonstrated using a water-based route that was reproducible

and open to air. The photoluminescence peaks of the QDs weretunable over the range from UVA to blue (350 to 490 nm) andexhibited fwhm in the range from 24 to 36 nm. Use of hydrazinecircumvented the problem of air oxidation of highly oxygen-sensitive sodium hydroselenide, thus making the synthesisprocedure reproducible and easy to handle and promoting PLquantum yields as high as 40%. The optical properties of theas-synthesized QDs could be tuned through the use of MPA,TGA, andGSH ligands. WithMPA ligands, QDsexhibitedstrong

near band-gap emission in the UVA range as well as a broad,surface-defect-induced emission peak in the blue. Use of TGAor GSH suppressed surface-defect-induced emission, leaving astrong near band-gap UVA emission. Zn x Cd1- x Se QDs werefabricated by direct addition of a Cd-thiol complex to solutionscontaining ZnSe nanocrystal seeds. Inclusion of Cd increasedthe proportion of Se in QDs and changed the type of bonding;Zn-Se bonds were primarly covalent, but Cd-Se bonds wereionic in character. Simple variations of this approach will enabledirect synthesis of nanocrystals with a variety of compositionsand properties. Methods must be developed to separate QDsfrom reaction constituents and link them with other materials toutilize QDs for applications ranging from biological labels andbiosensors to light-emitting diodes,lasers,and electronic devices.

Acknowledgment. Thanks are due to Prof. Mary Wirthin theDepartmentofChemistryattheUniversityofArizonaforgrantingaccess to the UV-vis absorption spectrometer in her laboratoryand Prof. Supapan Seraphin in the Department of MaterialsScience and Engineering at the University of Arizona for theTEM work. We had fruitful discussions with Prof. Oliver Montiin the Department of Chemistry at the University of Arizona onthe data and analysis. Funding for this project was provided byScience Foundation Arizona (StrategicResearchGroupProgram),the Arizona Technology and Research Initiative Fund (TRIF)Water Sustainability Program (WSP)at theUniversity ofArizona,and the National Science Foundation (CHE-0548264).

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(37) Xu, D.; Guo, G.; Guo, Y.; Zhang, Y.; Gui, L. J. Mater. Chem. 2003, 13,360–364.

(38) Bowen-Katari, J. E.; Colvin, V. L.; Alivisatos, A. P. J. Phys. Chem. 1994,98, 4109–4141

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