A Systematic Study on Preparing CdS Quantum Dots

Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, pp. 3293∼3299

A Systematic Study on Preparing CdS Quantum Dots

B. T. Huy, Min-Ho Seo, Jae-Min Lim, Dong-Soo Shin and Yong-Ill Lee∗

Department of Chemistry, Changwon National University, Changwon 641-773, Korea

Sang-Su Kim and Kiwan Jang

Department of Physics, Changwon National University, Changwon 641-773, Korea

Taekwon Song

Department of Ceramic Engineering, Changwon National University, Changwon 641-773, Korea

(Received 31 December 2010, in final form 14 July 2011)

Quantum dots (QDs) have been attracting researchers’ attention because of their potentials foruse as nanotechnology key components. In this work, we report a systematic study of the synthesisprocess for making compound semiconductor QDs in 1-octadecene (ODE). The synthetic QDswere characterized by using the photoluminescence and absorption spectra, and TEM and XRDanalysis. The average size of the nanocrystals was 3.7 – 4.5 nm from their absorption spectra andTEM images. ODE facilitates nucleation by providing an available source of sulfite, and an excessof oleic acid may lead to cationic monomer excess. The samples show excitonic luminescence andbrightness in the range 420 – 460 nm. These results indicate that we can control the diameter ofCdS QDs synthesized in 1-octadecene (ODE). The synthesized semiconductor QDs can be used asfluorescence markers.

PACS numbers: 42.40.My, 78.55.-mKeywords: Quantum dots, Cd:S ratio, 1-octadecene, Growth timeDOI: 10.3938/jkps.59.3293

I. INTRODUCTION

Semiconductor nanocrystals, so called quantum dots(QDs), are emerging nanomaterials, which have at-tracted much interest in nanoscience and optical sci-ence. Having excitons confined in all three spatial di-mensions, quantum dots have properties that are inter-mediate between those of bulk semiconductors and dis-crete molecules. When excited by photons or an electricfield with energy higher than the band gap, quantumdots can emit photons, releasing the absorbed energy.Quantum dot nanocrystals have many important opticalproperties. In particular, their energy is related to size,implying that the optical properties can be finely tunedby the size. The emission energy is adjustable by con-trolling the composition, and the particle size of quan-tum dots leads to the quantum confinement effect [1].The ability to change the optical properties by tuningthe size of the QDs is advantageous for many applica-tions. Basically QDs have found applications in solarcells, diodes, lasers, detectors, tele-communication, andfluorescent biological labels [2–4].

∗E-mail: [email protected]; Fax: +82-55-213-3439

Colloidal QD dispersion in a solvent is quite interest-ing to fluorescence probes for all types of labeling stud-ies because of their reduced tendency to photobleach-ing. Control of the chemistry at the nanocrystal sur-face is an important fundamental when QDs are usedfor biological applications. Quantum dots are typicallysynthesized at a high boiling point and with nonpolarorganic solvents to obtain high-quality monocrystallineparticles with a narrow size distribution. Inorganic QDsare usually bright and stable under relatively harsh envi-ronments. The absorption spectra of quantum dots arecontinuous, and the emission spectra are narrow, typ-ically 20-30 nm full width at half maximum (FWHM)[5]. The excitation spectra of QDs are broad, but theiremission spectra are narrow, strong and tunable [6], al-lowing simultaneous observation of multiple probes withdifferent fluorescent colors by using a single light source.Therefore, different colors of the entire emission spec-trum from QDs can be measured at the same time byusing only one laser excitation source.

Modern colloidal techniques allow high-precision fab-rication of semiconductor nanocrystals with a varietyof compositions and shapes [6-8]. By combining differ-ent materials in a single nanocrystal, one can also pro-duce various types of heterostructures such as core/shell

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or multicomponent structures, hetero-nanorods andtetrapods [10, 11]. Depending on the bandgaps andthe relative position of the electronic energy levels ofthe involved semiconductors, the shell can have differ-ent functions in the core/shell nanocrystal. In type-II heterostructures, either the valence-band edge or theconduction-band edge of the shell material is located inthe bandgap of the core. The staggered band alignmentleads to a smaller effective bandgap than those of the coreand the shell materials. Many researchers have reportedon the synthesis of CdSe/CdS [12–14], CdTe/CdSe [15],ZnTe/CdS [16], and ZnSe/CdS [17] quantum dots whilereports on CdS/ZnSe QDs nanocrystal are still limited[18–20].

The purpose of this study is a systematic developmentof the preparation for the core of a core/shell structure,which would correspond to a true type-II heterostruc-ture. The CdS QDs were prepared successfully in 1-octadecene (ODE). The diameter of CdS QDs was ma-nipulated by changing parameters such as the growthtemperature or the concentration of 1-octadecene.

II. MATERIALS AND EXPERIMENTALPROCEDURE

The CdS cores were prepared by using wet chemistryand a bottom up technique. Technical grade (90%) 1-octadecene, sulfite powder (99.99%), oleic acid (90%),and cadmium oxide were obtained from Sigma-Aldrich(St. Louis, MO, USA). Analytical grade hexane and 2-propanol were purchased from Daejung Chemicals Co.(Siheung, Korea). The sizes and shapes of these CdScores were determined using transmission electron mi-croscopy (TEM) with the JEM2100F-JEOL system (op-erated at 200 kV). Sample preparation for the TEM mea-surements was done by placing a drop of ethanol disper-sion of quantum dots on amorphous carbon-coated cop-per grids. The structure of the powder quantum dots wasinvestigated by using X-ray diffraction (XRD) with CuKα radiation. Optical absorption spectra were collectedon a Carry UV-1650 spectrometer, and the photolumi-nescence spectra were recorded upon excitation at 355nm by using a FP-6500 (Jasco, Japan) fluorescence spec-trometer. All measurements were carried out at roomtemperature.

III. RESULTS AND DISCUSSION

1. Preparation of the CdS Core

The noncoordinating solvent used in this study was1-octadecene (ODE), which is a liquid at room temper-ature and boils at about 320 ◦C. Oleic acid (OA) waschosen as the ligand for stabilizing the nanocrystals. The

Fig. 1. (Color online) Photo of CdS quantum dots in so-lutions with different Cd:S ratios.

Fig. 2. Photoluminescence spectra of CdS nanocrystals fordifferent Cd:S ratios.

cadmium stock solution was produced by mixing cad-mium oxide with OA and ODE in a three-neck flask.The mixture was heated to 290 ◦C under nitrogen flowwhile stirring. Separately, the sulfur solution was pre-pared by heating sulfur powder in 1-octadecene at 250 ◦Cin a three-neck flask under nitrogen flow while stirring.Both the mixtures were heated until the solution becameto colorless. For the synthesis of CdS nanocrystals, ata certain temperature, the sulfite solution was quicklyinjected into the cadmium solution. Subsequently, thetemperature was cooled to a lower temperature for grow-ing quantum-dot crystals. After a duration of time, themixture was cooled again to room temperature, and theparticles were precipitated with 2-propanol. The mixturewas centrifuged for 15 minutes to get CdS nanoparticles.The resulting CdS nanoparticles were purified severaltimes by using the 2-propanol/hexane technique. TheCdS nanopartilces were dispersed in hexane and placedin the dark for storage. We studied the influence of differ-ent Cd:S ratios, reaction temperatures, growth temper-atures, reaction times, concentrations of OA and ODEand speeds of centrifugation on the optical properties ofthe CdS QDs. Some samples of CdS quantum dots inthe solution are illustrated in Fig. 1.

2. Effect of the Cd:S Ratio

In this experimental series, the reaction and growthtemperatures were fixed at 290 ◦C and 260 ◦C, and the

A Systematic Study on Preparing CdS Quantum Dots – B. T. Huy et al. -3295-

Fig. 3. Core diameters for different Cd:S ratios.

reaction and growth time were 15 minutes. All reac-tions were performed under identical conditions, exceptfor the Cd:S ratio. The effect of Cd:S ratio on photolu-minescence intensity are shown in Fig. 2. It was revealedthat the PL intensity strongly depends on the Cd:S ra-tio. An excess of sulfite decreases the PL intensity of theQDs. However, the PL intensity is clearly stronger whenthe concentration of cadmium increases. This is relatedto the improved CdS particles with respect to size andquantity. The maximum wavelengths of the peaks areshifted to higher wavelengths (red shift) when the Cd:Sratio increases. The red shift is attributed to the diam-eter of the core. The empirical equation for estimatingthe core diameter is provided as follows [18], where thediameter of core is direct proportional to the emissionwavelength:

D(nm) = (−6.6521× 10−8)λ3 + (1.9557× 10−4)λ2

−(9.2352x10−2)λ + (13.29), (1)

where D (nm) is the size of a given nanocrystal sam-ple, and λ (nm) is the wavelength of the first excitonicabsorption peak of the corresponding sample.

The diameters of the cores were investigated from theabsorption spectra. The excitonic absorption peak is al-ready known to be associated with the lowest opticaltransition, and it provides a simple way to determine thenanocrystal bandgap. The wavelengths of the absorptionpeaks are in the range 442 - 490 nm, corresponding tobandgap values in the range 2.80 – 2.53 eV. A compari-son with the value of bulk CdS (2.42 eV) shows that thebandgap values of our CdS were higher than that, whichindicates quantum confinement in the CdS quantum dots[21,22]. The diameters of CdS QDs with different Cd:Sratios is shown in Fig. 3. An increase of the concentra-tion of cadmium leads to the appearance of more cationicmonomers so the nucleation process will quickly finish.It was resulted in a decreased particle size and an in-creased PL intensity. Excessive cadmium will be clungon the surfaces of particles, and this causes increases inthe size and PL intensity of the quantum dots.

Fig. 4. Effect of OA concentration on the PL intensity forCd:S ratios of 3:1 and 4:1 (the last index in the legend showsthe normalized concentration of oleic acid).

3. Effect of Oleic Acid

Oleic acid was chosen as the ligand for stabilizing thenanocrystals and the cationic precursors. Based on es-timates of the effect of Cd:S ratio on the PL intensity,the effect of oleic acid was investigated by using the PLintensities for Cd:S ratio of 3:1 and 4:1. The results areshown in Fig. 4, and the last index in the legend of thefigure shows the normalized concentration of oleic acid.

The influence of the concentration of oleic acid on thephotoluminescence intensity is clear. As the concentra-tion of oleic acid is increased, the PL intensity is de-creased. On the contrary, a “red shift” occurs with in-creasing concentration of oleic acid. This red-shift orig-inates from the increase in the diameter of the core,which was calculated from the absorption spectra. Es-pecially, this influence is dramatic for a Cd:S raio of4:1. This influence is the result of the changed reac-tivity of the cationic monomers, which refers to the cad-mium in solution. More cationic monomers will be cre-ated when the concentration of oleic acid increases. Anincreased monomer concentration leads to too many nu-clei being formed during the initial nucleation period, sothe remaining monomers will not be sufficient to grownanocrystals. However, excessive cadmium will cling onthe surfaces of the particles, so the sizes of the QDs will


Fig. 5. Photoluminescence (a) and absorption (b) spectrafor different ODE concentration.

be increased, and a red-shift appears in the PL and theabsorption spectra. Therefore, oleic acid plays the roleof a ligand of cadmium, and an excess of OA may leadto an excess of cationic monomers or the destruction ofCdS particles in the growth process for the quantum-dotcore.

4. Effect of 1-octadecene

1-octadecene as a noncoordinating solvent providestunable reactivity of the monomer, making a necessarybalance between nucleation and growth [23]. This isthe main factor for controlling the optical properties ofthe nanoparticles. During the experiments, 1-octadecenewas found to affect strongly the PL intensity and the ab-sorption spectra of CdS nanocrystals. The results fromFig. 5 show that increasing ODE leads to increased PLintensity and decreased nanoparticle size. The most com-mon allotrope of sulfur is well known to be the cyclicS8 compound. This particular allotrope undergoes aring-opening polymerization when the nanocrystals areheated at temperatures between 140 - 170 ◦C. The nu-cleate particles from ODE-S were prevented due to thepolymerization of S into long and unranked chains. Inaddition, ODE plays the role of an intermediate in chemi-

Fig. 6. Photoluminescence spectra of CdS nanocrystals fordifferent nucleation temperatures.

cal reactions, and its viscosity prevents the particles fromaggregating, so its size will decrease and the PL intensitywill increases when the concentration of ODE increases.This increase of PL intensity indicates that ODE facili-tates nucleation by providing an available source of sul-fite.

5. Effect of Nucleation Temperature

For the study of the effect of nucleation temperatureon the formation of CdS quantum dots, CdS sampleswere prepared at three different temperatures. Figure6 shows the effect of nucleation temperature on the PLintensity. Lamer and Dinegar [24] reported study on thenucleation and the growth of colloidal particles; thesepreparative routes involved a temporally discrete nucle-ation event followed by a relatively rapid growth fromsolution-phase monomers and slower growth by Ostwaldripening. Nucleation is achieved by quick injection ofa precursor into the hot solvents, resulting in thermaldecomposition of the precursor reagents and supersat-uration of the formed “monomers”, which is partiallyrelieved by particle generation [25].

Recently, Xie et al. [26] presented a new concept forthe nucleation kinetics and the particle formation rate.Clearly, more monomers will be created when the nucle-ation temperature increases. However, if too many nucleiare formed in the initial nucleation period, the remainingmonomers will not be sufficient to promote focusing ofthe size distribution for a sufficient time. This will resultin an undesired Ostwald ripening or defocusing of the sizedistribution. In the case of too few nuclei being formed,the growth reaction will be too fast to be controlled toreach the desired size and size distribution. The resultsfrom Fig. 6 agree with the above theory. There is adramatic jump of the photoluminescence intensity in therange of 230 - 290 ◦C. The nucleation temperatures in-creases leading to an increase in the monomers, so more


Fig. 7. Change in the core diameter with growth temperature after (a) 5, (b) 10, and (c) 15 min.

Fig. 8. (Color online) Effect of reaction time on PL inten-sity for a growth temperature of ∼260 ◦C

particles will be grown. Therefore, the photolumines-cence intensity will increase.

6. Effect of Growth Time - Temperature

The temperature dependence was investigated system-atically for five different growth temperatures. The ex-perimental steps are (i) preparing the cadmium sulfiteby injecting the sulfite solution into the cadmium solu-tion at 290 ◦C (reaction temperature), (ii) lowering thetemperature to 150 ◦C and (iii) heating the mixture toa specific growth temperature (160, 200, 230, 260, and290 ◦C) for growing the quantum dots.

Figure 7 presents the change in the diameter of theCdS core with growth temperature, each after 5, 10, and15 minutes from the moment of “seed creation”. Clearly,a lower growth temperature causes a smaller core diam-eter. The core diameter did not change when the growthtemperature was lower than 200 ◦C. When the growntemperature was 260 ◦C, the PL intensity reached to amaximum value after 13 minutes (Fig. 8).

The time dependence was investigated systematicallyfor four different grown temperatures 160, 200, 230, and260 ◦C. The time trends for each temperature are shown

Fig. 9. Wavelength of the maximum peak and PL intensityvs growth time at (a) 160 ◦C, (b) 200 ◦C, (c) 230 ◦C, and (d)260 ◦C.

in Fig. 9. The results of these experiments indicate thatwhen the grown temperatures are equal to or lower than200 ◦C, the photoluminescence intensity and the peakposition decrease with increasing time. This means a“blue shift” of the spectrum shown in Figs. 9(a) and 9(b).On the contrary, a “red shift” occurs with increasing timewhen the grown temperature is above 200 ◦C (Figs. 9(c)and 9(d)). At any grown temperature, the photolumi-nescence intensity will be decreased if the grown timeincreases because the particles may be re-dissolved orbleached by heating.

7. Effect of the Speed of Centrifugation

After the reaction finished, the solution was dividedinto five parts, and each of them was centrifuged with adifferent speed. The centrifugation speed dependences ofthe PL intensity are displayed in Figs. 10(a) and 10(b).The wavelength of the absorption peak changes slightlywhen the centrifuged speed increases while the photolu-


Fig. 10. Effect of the centrifugation speed on (a) the PLintensity and (b) the wavelength of the absorption for sampleswith Cd:S ratio of 3:1.

Fig. 11. TEM images of quantum dots with Cd:S ratios of(a) 2:1, (b) 3:1, and (c) 4:1.

minescence intensity dramatically changes and reaches amaximum value at a speed around 8000 rpm. When thecentrifugation speed is over 8000 rpm, excess chemicalsor byproduct can be accumulated because of decreas-ing the size of QDs particles. This causes a decrease inthe photoluminescence intensity while keeping the wave-length of the absorption peak unchanged.

8. Morphologic Characteristics (TEM, XRD)

The TEM images of quantum dots in Fig. 11 clearlyshow an unclumping of CdS. The reason for this un-clumping might be the fact that OA is a strong lig-and that prevents the CdS from self-binding. In thesame preparative conditions, the particles have a per-

Fig. 12. SAED pattern of the quantum dot with Cd:S ratioof 2:1.

Fig. 13. XRD pattern of CdS QDs.

Fig. 14. Distribution of the diameters of the CdS QDs.

fect spherical form when the Cd:S ratio is in the range of2:1 to 3:1. With a higher Cd:S ratio, the surfaces of theparticles can be dirtied by byproducts produced duringthe synthesis process.

The selected area electron diffraction (SAED) patternof the TEM image of the CdS sample with a of CdS ratioof 2:1 is shown in Fig. 12. The interplanar distance calcu-lated from the TEM and the SAED is 0.30 nm, which fitswell with the (111) planes of the zinc-blende structure.To confirm the crystal phase of the CdS nanocrystal, wecarried out X-ray diffraction measurement. Figure 12displays the XRD pattern of the CdS QDs, which is in


good agreement with recent reports. The CdS diffrac-tion pattern shows obviously broad peaks because CdSnanocrystals with small sizes were formed. The peaksmatched well with the Bragg reflections for the cubicstructure with no indication of a hexagonal phase. TheXRD pattern of the CdS nanocrystals was perfectly iden-tified with the zinc-blende structure.

The size distribution was evaluated by measuring thediameter of more than one hundred quantum dots ran-domly selected from the TEM images. Figure 14 showsthat the distribution of particle sizes is concentrated inthe range 3.6 – 4.5 nm for a Cd:S ratio of 2:1.

IV. CONCLUSION

In this report, the optical properties of CdS nanocrys-tals are studied systematically. The roles of ligands,chemical intermediates, nucleation temperature, growntemperature, speed of centrifugation, etc. strongly af-fect the quality of the nanocrystals. An increasing con-centration of cadmium leads to the appearance of morecationic monomers, so the nucleation process will quicklyfinish. The result is decreased particle size and increasedPL intensity. Excessive cadmium will cling on the sur-faces of the nanoparticles, and this causes increases inthe size and the photoluminescence intensity. An excessof oleic acid may leads to a cationic monomer excess ordestroys CdS particles in the growth process of quantumdots. The TEM images and XRD analyses revealed azinc-blende structure. The average diameter of the CdSQDs were in the range 3.8 – 4.5 nm with a narrow dis-tribution when the nucleation and growth temperatureswere 290 and 260 ◦C, respectively.

ACKNOWLEDGMENTS

This work was supported by the Priority ResearchCenters Program through the National Research Foun-dation of Korea (NRF) funded by the Ministry of Edu-cation, Science and Technology (NRF 2010-0029634).

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A Systematic Study on Preparing CdS Quantum Dots

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