Research Article

Received: 13 January 2008, Revised: 24 January 2009, Accepted: 26 February 2009, Published online in Wiley Interscience: 29 May 2009

(www.interscience.wiley.com) DOI 10.1002/bio.1134

Copyright © 2009 John Wiley & Sons, Ltd. Luminescence 2010; 25: 14–18

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John Wiley & Sons, Ltd.

Synthesis of CdSe quantum dots with luminescence in the violet region of the solar spectrumSynthesis of CdSe quantum dots with luminescence in the violet region

Nisha Shuklaa,c* and Michael M. Nigrab

ABSTRACT: We have designed a simple, one-step synthesis of CdSe quantum dots with photoluminescence frequencies rangingfrom the red through to the violet region of the solar spectrum. The photoluminescence peaks have FWHM of 30 nm indicatingabsorption over a narrow range of wavelengths. The effect of solvent type and solvent boiling point on the physical and pho-toluminescence properties of the quantum dots has been studied. High boiling point, non-polar solvents shift the photolumi-nescence peak to longer wavelengths and low boiling point, polar solvents shift the photoluminescence peak to shorterwavelengths. Copyright © 2009 John Wiley & Sons, Ltd.

Keywords: synthesis CdSe; quantum dots; luminescence

Introduction

Semiconductor nanoparticles exhibit unique properties that aredependent on their size and composition. Control of the nano-particle size and shape allows control of their bandgap over acontinuous range and, thus, the preparation of nanoparticlesthat can absorb and emit light over the entire frequency rangeof the solar spectrum. Chemically synthesized nanoparticleshave an advantage over physically prepared nanoparticlesin that they are more easily tailored to have different shapesand sizes, characteristics that are challenging for conventionalmethods such as epitaxial growth or sputtering. The synthesisconditions and reagents can be used to control the sizes andmorphologies of nanoparticles. As a result, syntheses of mono-dispersed II–VI semiconductor nanoparticles have attracted agreat deal of interest due to the potential applications of thesenanoparticles as non-linear optical materials,[1] in devices, forbiomedical imaging[2] and as photovoltaics.[3] There are manystudies which have reported wet chemical syntheses of CdSe.[4–7]

These syntheses have produced CdSe nanoparticles with photo-luminescence peaks in the green to orange region of the solarspectrum. Various studies have also shown that CdSe nanoparti-cles coated with shells of ZnSe or ZnS[8–12] photoluminesce in theblue region of the solar spectrum. These core–shell structuredCdSe–ZnS[9] and CdSe–ZnSe[10] nanoparticles are synthesizedusing a two–step process which involves synthesis of CdSe nano-particle seeds followed by the addition of Zn and Se precursorsthat form the outer coating of the CdSe nanoparticles. Thesyntheses of the core–shell structure are relatively complex andinvolve multiple steps.

This paper reports the development of a fast, one-step synthe-sis of CdSe nanoparticles with narrow photoluminescence peaksin the blue to violet region of the solar spectrum. The focus ofthis work has been to understand the factors which influencethe synthesis of nanoparticles and their resulting photolumines-cence properties. We have studied the use of different types ofCd precursors and the effect of using solvents with differentboiling points on the synthesis of CdSe nanoparticles with photo-

luminescence at the shorter wavelengths of the solar spectrum.In addition, the molar ratio of Cd and Se was varied in the syn-thesis. This work demonstrates that the solvent plays an impor-tant role in the synthesis of CdSe nanoparticles. The high boilingsolvents shift the photoluminescence peaks to longer wave-lengths while lower boiling point solvents shift the photolumi-nescence peaks to shorter wavelengths. This understanding willaccelerate the development of nanoparticles with improvedoptical properties.

Experimental

The synthesis of CdSe nanoparticles was conducted in a moisture-and oxygen-free atmosphere. CdSe nanoparticles were synthesizedusing a one-step process involving reduction of cadmium (II)acetylacetonate or CdO, in polar solvents of varying boiling pointand in the presence of surfactants. The Cd(acac)2 (0.17 mmol)was first dissolved in a heated solvent (10 mL) in the presenceof oleic acid (0.6 mL). The selenium stock solution was prepared bydissolving 0.38 mmol of Se powder in 5 mL of 1,2-dichlorobenzeneand 0.4 mL of trioctylphosphine (TOP). Then, 1 mL of the solu-tion of Se in 1,2-dichlorobenzene and trioctylphosphine (TOP)was added to the reaction mixture. The reaction mixture wasallowed to reflux and aliquots of the reaction mixture were with-drawn at various intervals. The aliquots were diluted in toluene

* Correspondence to: N. Shukla, Institute for Complex Engineered Systems,Carnegie Mellon University, Pittsburgh, PA 15213, USA. E-mail:nisha@andrew.cmu.edu

a Institute for Complex Engineered Systems, Carnegie Mellon University,Pittsburgh, PA 15 213, USA

b Department of Chemical Engineering, Carnegie Mellon University,Pittsburgh, PA 15 213, USA

c National Energy Technology Laboratory, 620 Cochrans Mill Road, Pittsburgh,PA 15 236, USA

Synthesis of CdSe quantum dots with luminescence in the violet region

Luminescence 2010; 25: 14–18 Copyright © 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/bio

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in order to obtain photoluminescence spectra. Similar experimentswere performed using solvents with a range of boiling points(1,2-dichlorobenzene, b.p. 180°C; dioctylether, b.p. 286°C; and 1-octadecene, b.p. 315°C). Following synthesis, the nanoparticleswere washed using polar ethanol (CH3CH2OH) and then non-polarhexane (C6H14) as solvents. The original solution of surfactantcoated (TOP and oleic acid) nanoparticles and excess surfactantin solvent was first mixed with 10 parts of ethanol and centrifuged,causing the particles to precipitate. The supernatant solutionconsisting of solvent and the smaller nanoparticles was thendiscarded and the nanoparticles were redispersed in non-polarhexane. This method removes the bimodal distribution of thequantum dots and yields a narrow size distribution. This sequenceof precipitation in ethanol, centrifuging to separate the solventand redispersion in hexane was then repeated twice. Finally, thenanoparticles were dispersed in hexane.

XRD spectra were obtained from a Panalytical X’Pert Pro x-raydiffractometer. A Cu radiation source was used along with a poly-capillary lens and an Ni filter to provide the incident beam. A flatgraphite crystal was used on the diffracted beam. The powersettings on the X-ray generator were 45 kV and 40 mA. Thesamples of CdSe used for the XRD measurements were preparedby placing a colloidal solution of CdSe nanoparticles on a glassside and the solvent was removed by drying in a vacuum ovenat 30°C for several hours.

The photoluminescence spectra were taken using a PTI Time-master series spectrofluorometer fitted with a PMT sensor. Theexcitation source was a Xe lamp emitting at 350 nm. The scanrange chosen for each sample was 375–650 nm using a 1 nm stepand a 0.5 s integration time. All resulting waveforms depict rawdata with no normalization performed. Photoluminescence spec-tra were captured using the FeliX32™ Advanced FluorescenceAnalysis Software Package. The samples used for photolumines-cence were prepared by diluting 0.5 mL of the reaction mixturewith 3 mL of toluene. The diluted solution was placed in a Fisherbrand quartz fluorescence cell with a 10 mm pathlength.

All the solvents (hexane, ethanol, 1,2-dichlorobenzene, 1-octadecene and dioctylether) used for synthesis and wash-ing of the CdSe nanoparticles were obtained from the AldrichChemical Co. and were used as obtained without further purifi-cation. Ethanol was obtained in Sureseal™ bottles with very lowwater content (<0.05%). The hexane was 99% pure with lowwater content. Oleic acid, trioctylphosphine, cadmium acetylac-etonate (99%), CdO and Se powder were also obtained from theAldrich Chemical Co. and were used without any purification.

Results

Many parameters influence the synthesis of CdSe nanoparticles.In this work, we have used two different types of cadmium pre-cursors and three types of solvents with different boiling pointsand we have varied the molar ratio of Cd:Se precursors used inthe synthesis mixture of the CdSe nanoparticles. The cadmiumprecursors used were cadmium acetylacetonate and cadmiumoxide, CdO. The boiling point of the solvent has a significantimpact on the synthesis because it dictates the temperatureat which the reaction occurs during refluxing. We used 1,2-dichlorobenzene (b.p. 180°C), dioctylether (b.p. 286°C) andoctadecene (b.p. 315°C) as solvents for the synthesis of CdSenanoparticles. These three solvents were chosen because theirboiling points are relatively high and cover a broad range ofvalues. The goal of the study was to understand the impact of

the synthesis temperature on the photoluminescence proper-ties of the quantum dots.

The syntheses were all performed for durations of 30 min withaliquots of the synthesis solution being taken periodically. In allcases, the photoluminescence peak maximum increased inwavelength with increasing reaction time. As the reaction timeincreases, the size of nanoparticles increases and they photo-luminesce at longer wavelengths.[13] Figure 1 shows the photo-luminescence spectra of CdSe nanoparticles synthesized usingCd(acac)2 as the cadmium precursor and 1,2-dichlorobenzene asthe solvent. In this synthesis the Cd:Se molar ratio in the reactionmixture was 2.2. Cd(acac)2 (0.17 mmol) was first dissolved in 10 mLof 1,2-dichlorobenzene and 0.6 mL of oleic acid and slowlyheated to 180°C. Then a 1.0 mL solution of Se powder in 1,2-dichlorobenzene and TOP (trioctylphosphine) (0.08 mmol/mL)was added to the Cd(acac)2 solution. The reaction mixture wasthen refluxed at 180°C and 0.5 mL aliquots of the reaction mixturewere removed after 1, 2, 4 and 20 min. The aliquots were dilutedin toluene (~0.5 mL of CdSe nanoparticle solution in 3 mL oftoluene) in order to obtain the photoluminescence spectra. Thephotoluminescence spectra in Fig. 1 shows peaks at wavelengthsof 433, 455, 468 and 473 nm after reaction times of 1, 2, 4 and20 min, respectively. These wavelengths lie in the violet to blueregion of the visible part of the solar spectrum. The photolumi-nescence spectra of the CdSe nanoparticles synthesized underall other conditions showed the same increase in wavelengthwith increasing reaction time.

The structures of the CdSe nanoparticles were studied usingXRD. Figure 2 shows the XRD spectrum of CdSe nanoparticlesobtained after a reaction time of 20 min. In order to obtain theXRD spectra the CdSe nanoparticles were precipitated fromsolution by dilution in polar ethanol with dissolved surfactant(1:1 ratio of oleic acid and oleylamine). The CdSe nanoparticleswere then dispersed in a non-polar hexane. The CdSe nanoparti-cles were deposited on a glass slide and dried in a vacuum oven

Figure 1. Photoluminescence spectra of CdSe quantum dots synthesized usingCd(acac)2 in 1,2-dichlorobenzene with a Cd:Se molar ratio of 2.2. PL peak maximumshift to increasing wavelength with increasing reaction time.

N. Shukla and M. M. Nigra

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at 50°C for 30 min. The XRD spectra were obtained using a Cu Kα

source. The XRD spectra showed three prominent peaks at 2θvalues of 26° (111), 42° (220) and 49.5° (311), suggesting that theCdSe nanoparticles have a cubic structure. A rough estimationof the particle size using the Scherer equation puts their diame-ter at 3 nm.[14] The XRD spectra of chemically synthesized CdSenanoparticles reported in literature[15] all indicate growth withthe wurzite structure (hexagonal structure). The CdSe growthtemperature used in our experiments is much lower than thosereported in literature. Peng et al.[15] synthesized CdSe in TOPO(trioctylphosphine oxide) at 300°C. Synthesis of cubic CdSe hasbeen reported by Sasamoto et al.[16] and Palchik et al.[17] Sasamotoet al. reported that CdSe synthesized at lower temperatures suchas 60°C gave cubic CdSe particles while hexagonal CdSe wereformed at a synthesis temperature of 80°C. Palchik et al.[15]

observed the opposite behavior but using different solvents andhigher reaction temperatures. They observed formation of cubicCdSe at 287°C when the solvent used was triethylene glycol andformation of hexagonal CdSe at 196°C in ethylene glycol. Qianet al. synthesized CdSe[12] at room temperature in aqueous solu-tion and observed formation of cubic CdSe. The structure ofthese particles is very sensitive to the synthesis conditions and itis not clear what factors exert the dominant control.

Similar syntheses of CdSe were conducted using various molarratios of cadmium and selenium precursors. Figure 3 plots thephotoluminescence peak wavelengths vs the synthesis reactiontime of CdSe nanoparticles synthesized with varying molar ratiosof Cd:Se precursors. The molar ratios of Cd:Se in the reactionmixture were 0.7, 1.3, 2.0 and 2.2 and the solvent used in thesesyntheses was 1,2-dichlorobenzene. As observed from Fig. 3, it isevident that the photoluminescence peak maximum increaseswith increasing reaction time under all synthesis conditions.Figure 3 also shows that the wavelengths of the photolumines-cence peak maximum increase rapidly over the initial period of

reaction and then reach a plateau after a period of ~10 min. Inaddition, these experiments indicate that, as the molar ratio ofCd:Se precursors in the reaction mixture is increased from 0.7 to2.2 the wavelengths of the photoluminescence peak maximumdecrease. This observation is independent of synthesis reactiontime. The CdSe nanoparticles synthesized with a molar ratioof 2.2 in the reaction mixture have photoluminescence peakmaxima at the shortest wavelength and in the violet region ofthe spectrum. This experiment indicates that increasing the Cdcontent of the CdxSe1−x nanoparticles causes a blue shift in thephotoluminescence peaks. CdSe quantum dots synthesized witha Cd:Se molar ratio of 2.2 in the reaction mixture show photo-luminescence peaks at lowest wavelength of 433 nm (violet) after1 min of reaction.

In addition, we investigated that use of different types ofsolvents in the synthesis of CdSe. The solvents used for this syn-thesis are dioctylether (b.p. 286°C), 1-octadecene (b.p 315°C)and 1,2 dichlorobenzene (b.p. 180°C). One of the main differencesin these solvents is their boiling point; as a result the synthesistemperature of CdSe quantum dots will occur at different tem-peratures. Figure 4 shows the PL peak maximum wavelength ofCdSe of similar molar ratio of Cd:Se vs the reaction time in threedifferent types of solvent. The details of precursor ratios, reactiontime and type of solvent are also listed in Table 1. The data pointsmissing from Table 1 could not be measured. Figure 4 showsthat the PL peak maximum for CdSe quantum dots synthesizedin 1,2-dichlorobenzene is at a lower wavelength in comparisonto 1-octadecene and dioctylether at similar reaction times. Thehigher temperature either increases the size of the quantumdots or changes the composition of the quantum dots. Figure 4also shows that PL peak maximum of CdSe quantum dots syn-thesized in 1-octadecene is lower than that of dioctylether. Sincethe boiling point of 1-octadecene is higher than dioctylether it is

Figure 2. XRD spectra of CdSe nanoparticles using Cd(acac)2 in 1,2 dichlorobenzene. Figure 3. Shows PL peak maximum wavelength of CdSe quantum dots vs thesynthesis reaction time for various molar ratios of Cd and Se in the reactionmixture. The solvent used was 1,2-dichlorobenzene.

Synthesis of CdSe quantum dots with luminescence in the violet region

Luminescence 2010; 25: 14–18 Copyright © 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/bio

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expected that the PL peaks of CdSe quantum dots synthesizedin 1-octadecene will be at higher wavelengths in comparison toCdSe synthesized in dioctylether. This is because the molar ratioof Cd:Se precursors in 1,2-dichlorobenzene and dioctylether is1.3 and in the case of 1-octadecene it is slightly lower, at 1.1.

In the next experiment, the effect of different types of cadmiumprecursors on the synthesis of CdSe was studied. In these exper-iments the ratio of Cd:Se was kept constant and dichlorobenzenewas used as the solvent. Figure 5 shows the PL peak maximumof CdSe synthesized using CdO and Cd(acac)2 as cadmium pre-cursors at different intervals of synthesis reaction time. CdO iswidely reported in the literature as a cadmium precursor for syn-thesizing CdSe quantum dots. In this work, CdO was used toprovide a comparison with the results obtained using Cd(acac)2.It was observed from this experiment that the type of cadmium

precursor did not play an important role in the synthesis of theCdSe quantum dots.

Conclusion

We demonstrate synthesis of CdSe quantum dots with photolu-minescence in the violet–green region of the solar spectrum.This work shows that the solvent and the molar ratio of Cd:Seplay an important role in the synthesis of the CdSe quantumdots. Depending on the type of the solvent and the molarratio of the CdSe, it is possible to tailor the synthesis of CdSequantum dots to show photoluminescence in a wide rangeof solar spectrum. As reported in the literature, most of theCd based quantum dots show photoluminescence peaks inthe blue region only in the presence of sulfur or zinc. Our syn-thesis method allows the shift of the photoluminescence peakin the violet region of the solar spectrum without the use ofsulfur or zinc. This is an important step in expanding the photo-luminescence wavelength of CdSe quantum dots.

Table 1. PL peak maximum wavelength (nm) of CdSe quantum dots synthesized in different solvents using various cadmiumand selenium precursor molar ratio.

Cd Precursor Cd:Se molar ratio

Solvent b.p.(°C)

1 min 2 min 4 min 15 min 20 min 25 min

Cd(acac)2 2.2 DCB 180 433 455 468 473Cd(acac)2 2.0 DCB 180 455 468 483 486Cd(acac)2 1.3 DCB 180 493 503 506 510Cd(acac)2 0.7 DCB 180 483 496 512 518Cd(acac)2 1.3 DOE 286 583 617 625Cd(acac)2 1.1 OCTAD 315 569 593 595CdO 1.7 DCB 180 505 518 524 528

DCB = 1,2 dichlorbenzene; DOE = dioctylether; and OCTAD = 1-octadecene.

Figure 4. Shows the PL peaks maximum wavelength of CdSe nanoparticlessynthesized in three different types of solvents at different reaction time. The molarratio of Cd:Se precusors in 1,2-dichlorobenzene and dioctylether is 1.3 while in 1-octadecene it is 1.1.

Figure 5. The PL peaks maximum wavelength of CdSe nanoparticles synthe-sized using two different types of cadmium precursors. The solvent used is 1,2-dichlorobenzene.

N. Shukla and M. M. Nigra

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