58 (2007) 267–274

Materials Characterization

Characterization of CdS nanoparticles duringtheir growth in paraffin hot-matrix

Georgi G. Yordanov a, Eiki Adachi b, Ceco D. Dushkin a,⁎

a Laboratory of Nanoparticle Science and Technology, Department of General and Inorganic Chemistry,Faculty of Chemistry, University of Sofia, 1164 Sofia, Bulgaria

b Advanced Research Center, Nihon l'Oreal R and D Center, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan

Received 11 February 2006; accepted 5 May 2006

Abstract

This paper describes the optical and structural properties of CdS nanoparticles during their growth in paraffin hot-matrixcontaining stearic acid ligand. The nanocrystalline species are characterized with absorbance and photoluminescence spectroscopy,fluorescence microscopy, High-Resolution Transmission Electron Microscopy and X-ray diffraction. The nanoparticles size-distribution, Stokes shift and mean molar concentration are derived from the optical spectra as functions of time. Their timeevolution confirms a two-stage nanocrystal growth for CdS. The stability of aggregates of stearate-coated nanoparticles, testedagainst UV-illumination, shows that the band-edge emission is more sensitive to photo bleaching than the trap-state emission. Theobtained new quantitative results are important for the large-scale manufacturing of CdS nanoparticles and their practicalapplications.© 2006 Elsevier Inc. All rights reserved.

Keywords: CdS nanoparticles; Liquid paraffin; Stearic acid; Tributhylphosphine; Optical properties; Size-distribution; Nanocrystal growth

1. Introduction

The chemical synthesis of semiconductor nanoparticlesfrom ca. 1 to 20 nm in diameter is a rapidly extending areaof research in materials science. In this size regime ma-terials become to have special optical and electronic pro-perties, which are dramatically different from those of thebulk [1–4]. Semiconductor nanoparticles of II–VI type areunder intensive investigation, because of their quantumconfinement [2], photocatalytic [5] and linear and non-linear optical [6] properties. Such functions find applica-

⁎ Corresponding author. Tel.: +359 2 8161 387; fax: +359 2 962 5438.E-mail address: nhtd@wmail.chem.uni-sofia.bg (C.D. Dushkin).

1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.matchar.2006.05.010

tions in photovoltaic systems [4], lasers [3], LEDs [7] andfluorescent biological labels [8].

For the production of monodisperse in size nanopar-ticles of CdX (X=chalcogenide) a hot-matrix method isused. Its first variant applies TOPO (trioctylphosphineoxide) as high-boiling solvent (matrix) and organometallicprecursors like dimethyl cadmium [9]. Later researchshowed that the organometallic precursor could be succes-sfully replaced byCdO [10] or by cadmium acetate [11,12]for preparation of CdS, CdSe or CdTe nanoparticles.Recent studies replace TOPO with non-toxic solvents likeoctadecene [13] or liquid paraffin [12].

The quantum confinement effect, usually manifestedas peculiarities in the optical spectra, and the ease of theirsynthesis, developed by us, makes the CdS nanoparticles

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attractive for the practice. The purpose of this paper is toreveal the changes in the optical properties of nanocrystal-line CdS during its growth in a paraffin hot-matrix, whichare not systematically investigated so far. In order todescribe the so-called “focusing of size distribution” inthis case [14], we adopt known analytical solutions [15]for the variance of size distribution calculated from theoptical spectra. The same strategy has been successfullyapplied for studying of the size-focusing phenomenon forCdSe nanoparticles synthesized in TOPO hot-matrix [15].The mean molar particle concentration can be calculatedfrom the optical spectra as previously described [16]. Herewe apply the same procedure to trace the temporal evo-lution of the CdS particle concentration during the growthprocess. The photo bleaching of CdS nanoparticles, pre-pared in hot paraffin media, is also investigated.

The new quantitative results obtained by us can serveas the guidance for manufacturing of CdS nanoparticlesand their practical utilization.

2. Experimental section

2.1. Nanocrystal synthesis

Three syntheses are carried out at 250 °C in order tocheck the reproducibility of the nanocrystal growth. Ineach of them 50 mg (0.40 mmol) of CdO (Merck, Ger-many), 9.00 g of liquid paraffin (Valerus, Bulgaria) and1.00 g (3.52 mmol) of stearic acid (Valerus, Bulgaria) areput together in a conical flask (50 ml). The mixture be-comes transparent after heating at a temperature higherthan 150 °C. The sulfur solution is prepared from 166 mgelemental sulfur and 30.00 g paraffin and heated to 50 °Cprior to the injection. Then 1.7 g (∼2.0 ml) of 0.55-wt.%sulfur solution, equivalent to 0.28 mmol of sulfur, is fastinjected in the flask at 280 °C. After the initial temperaturedrop due to the injection, the temperature is fast adjusted at250 °C and kept constant throughout the entire synthesis.

Although we follow the synthesis procedure of CdSnanoparticles described elsewhere [12], here the amountof stearic acid is reduced twice than the reported one inorder to obtain more monodisperse in size nanocrystals.This expectation is in accord with the data from similarsynthesis of CdS nanocrystals in octadecene solution [13]where reducing the amount of the acidic ligand (oleic acid)leads to nanocrystals of narrower size distribution. Herethe molar ratio of precursors in the system, [Cd] / [S], isabout 1.4.

Preliminary experiments showed that the growth rateand the nanoparticle radius do not significantly depend onthe stirring rate of the reaction solution (the deviations arewithin the experimental error).

2.2. Aliquot sampling

Aliquots are taken at different time intervals. An aliquot(∼0.1 ml) is fast injected into 1.00 g of 5-wt.% tri-buthylphosphine (TBP) solution in toluene (TBP ispurchased from Aldrich). TBP dissolves unconsumed Cd(II) stearate. The weight of aliquot is determined bysubtracting the mass of the TBP/toluene solution from thetotal mass of the TBP/toluene solution plus reaction ali-quot. Then the sample concentration is adjusted by addingtoluene to each sample in order to be 0.0500 g of thealiquot in 2.00 g of TBP/toluene solution. This adjustmentis for accurate comparison of the exciton absorbanceintensity. From here one can obtain a precise value for themean nanoparticle concentration (see below). The time foreach sampling is 1-2 s, which can be considered as theuncertainty of time measuring. Each sample is then ana-lyzed at a room temperature by fluorescence and UV–visabsorbance spectroscopy (spectrophotometer Jenway,model 6400).

2.3. Purification and characterization

The nanoparticles are purified from the paraffin andstearic acid in the following way. The hot reactionsolution (∼250 °C) is fast poured into 100 ml of coldtoluene 40 s after completion of the CdS nanoparticlegrowth. Then the obtained precipitate is centrifuged andwashed twice with pure toluene and twice more withpure n-heptane. The obtained new precipitate containsstearate-coated CdS nanocrystals, which fluoresce underUV-illumination (365 nm). The excess Cd(II) stearate isremoved by washing the precipitate with methanol asdescribed elsewhere [13]. Absorption and photolumi-nescence (PL) spectra of the purified nanoparticles arealso measured.

The stearate-coated CdS nanoparticles are visualizedusing a fluorescent microscope (Leica DMRB, 10×–40×objective lenses, long pass filter 425 nm). The stabilityagainst photo bleaching is tested under UV-illumination(100 W Hg lamp, band pass filter 340–360 nm). Theemission longer than 425 nm is detected by a linear CCDspectrometer (USB2000, Optosirius, 350–850 nm) at-tached to the microscope.

The nanoparticles are visualized by High-ResolutionTransmission Electron Microscopy, HR-TEM (TOPCONEM 002B microscope, acceleration voltage 200 kV).They are also characterized by X-ray powder diffraction(XRD) in a step-scan mode with an automatic X-Raypowder diffractometer (DRON-3, Cu K-alpha filteredradiation, diffraction from a flat specimen, Bragg–Bren-tano geometry).

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3. Results and discussion

3.1. Optical spectra

3.1.1. AbsorbanceCdS nanoparticles absorb light of wavelengths shorter

than that determined by the band gap energy (2.49 eV forbulk CdS, which corresponds to a wavelength 512 nm). Asthe particle size declines, the band gap increases while thewavelength of absorbance onset shifts to shorter wave-lengths. Therefore, the onset wavelength is directly relatedto the nanoparticle size. The position of peak 1 in theabsorption spectrum (cf. Fig. 1A) is used for calculation ofthe mean nanoparticle diameter D=2R (in nm) using theformula [16]

D ¼ ð−6:6521� 10−8Þk3 þ ð1:9557� 10−4Þk2−ð9:2352� 10−2Þkþ 13:29

Here λ (in nm) is the maximumwavelength of the mainexciton peak 1, which accounts for the transition 1S3/2−1Se (HOMO-LUMO) between the ground states of thehole, S3/2, and of the electron, Se, respectively, as pre-viously shown for CdSe nanoparticles [15,17]. From the

Fig. 1. Absorption spectra of CdS nanocrystals dissolved in toluene.(A) definition of the characteristic peaks of absorption of purifiedsample, taken after 500 s of growth at 200 °C; explanation of the peaksis given in the text. (B) Absorption spectra of some samples from arepresentative synthesis at 250 °C taken at different time (mean radiusR=1.23 nm, time t=10 s; R=1.49 nm, t=15 s; R=1.58 nm, t=20 s).

data for peak 1 one can calculate also themean nanoparticleconcentration [16] and the variance of size-distribution asdemonstrated for CdSe [15].

Twomore absorption bands, denoted as 2 and 3 in Fig.1A, are seen in the absorbance spectrum of purified CdSnanoparticles. We assume that they account for the con-tribution from other transitions as follows: 2S3/2−1Se(peak 2) and 1P3/2−1Pe (peak 3). Here S and P are therespective orbital states with 3/2 labeling the spin-orbitalsplitting of the hole states. Peaks 2 and 3 can be detectedonly in CdS samples containing larger nanoparticles.(Such nanoparticles can be obtained at lower tempera-tures. For better-pronounced peaks the nanoparticlesamples should be purified as described in the Ex-perimental section.) That is why these two bands cannotbe recognized from the spectra in Fig. 1B showing the sizeevolution of smaller CdS nanoparticles with the time.

3.1.2. PhotoluminescenceWhen the electron decays to the ground state, it emits

light at longer wavelength, because of its energy loss—this is the so-called normal band-edge emission [18,19].This means a part of the energy of an electron is lost asatomic vibrations, which is typically converted to heat;the rest is converted to the light. The longer wavelengthof the emission does not match with the wavelengthexpected by the band gap. Nanoparticles can exhibit alsoa unique type of fluorescence resulting from a trapping ofan electron at the crystal surface [18,20]. When thecrystal has a defect, it can introduce a potential energystate in the forbidden zone in the gap. Electron decayfrom this state (called trap-state) to the ground state iscalled trap-state emission [18]. The lifetime of an excitedelectron in a trap-state is significantly longer than that ofan electron in an exciton — as much as 3 orders ofmagnitude [2]. The Stokes-shift of the trap-state emis-sion is larger than the Stokes-shift of the band-edgeemission as a result of the energy lost due to the decay tothe trap-state.

Photoluminescence spectra of CdS samples are shown inFig. 2. Fig. 2A depicts the PL spectrum of the same sample,whose UV–vis absorption spectrum is shown in Fig. 1A.The PL spectrum consists of two emission bands— sharp(band-edge) and broad (trap-state) emission. The band-edge emission (signed as 1a on Fig. 2A) is due to recom-bination of the exciton in the mostly delocalized states innanoparticles [18]. It means that this emission proveshigh-crystalline nanoparticles. Probably the trap-stateemission (signed as 2a on Fig. 2A) is due to defects onthe interface between the nanoparticle and the surround-ingmedium. It is assumed to be due to sulfur vacancies onthe nanocrystal surface [18]. Then nanoparticles having

Fig. 3. Spectral characteristics of CdS nanoparticles during their growthin paraffin at 250 °C: (A) wavelength of the maximum absorption peak1 (see Fig. 1A), used for the calculation of nanoparticle size; (B) half-width at half-maximum of the absorption peak 1; (C) wavelength at themaximum of trap-state emission; (D) half-width at half-maximum ofthe trap-state emission band; (E) Stokes-shift for trap-state emission.

Fig. 2. PL spectra of CdS nanocrystals dissolved in toluene: (A)definition of the band-edge, 1a, and the trap-state, 2a, emission bandsof purified sample, taken after 500 s of growth at 200 °C; explanationof the peaks is given in the text. (B) PL spectra of some samples from arepresentative synthesis at 250 °C taken at different time (the samesamples, whose UV–vis spectra are given in Fig. 1B).

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more amorphous interface would havemore intensive trap-state emission and less intensive band-edge emission. It ispossible to assume that the PL spectrum, consisting of twobands in our experiments, corresponds to a crystalline coreof the nanoparticle with amorphous defective interface.

We observed instability of the band-edge emissionunder UV-light illumination (see below). Our experimentsshow that bigger nanoparticles have higher intensity andstability of the band-edge emission in comparison to thetrap-state emission. This is shown in Fig. 2B, where all PLspectra are taken while illuminating of the samples for nomore than 2min (8WUV-light source of 365 nm). One cansee that the bigger nanoparticles have both PL bands, whilethe small ones have almost no detectable band-edge emis-sion. Probably the reason for this fact is that smaller na-noparticles have larger part of their atoms on the surfacethus leading tomore amorphous structure and, hence,morepronounced trap-state photoluminescence. The nanoparti-cles formed in the beginning of synthesis have moresurface defects. This assumption is supported by the resultsfrom Ref. [15], where CdSe nanoparticles with a trap-stateemission are obtained in the early stages of growth in aTOPO matrix. For this case, the band-edge luminescencebecomes dominant at later stages of growth.

3.2. Description of the optical spectra

In Fig. 3 are compared important parameters of theoptical spectra of all three experiments carried out at250 °C to ensure the reproducibility of the synthesismethod. The wavelengths λAB (Fig. 3A) and λPL (Fig.3C) show the location of the absorption maximum andthe trap-state PL maximum, respectively. Both λAB andλPL increase with the time of synthesis, which is due tothe increasing nanoparticle size (Fig. 3A,C). One candistinguish two main stages in the time evolution: arapid initial increase, due to a fast nanoparticle growth,followed by a slow increase, due to a diffusion limitedgrowth. These tendencies resemble the growth of CdSenanoparticles in TOPO matrix [15]. The mean nano-particle radii, R, are calculated from the absorptionspectra and plotted against time (Fig. 4A). As seen, the

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growth rate follows the same trend of initial increase ofthe nanocrystal radius with the time during the fastprocess followed by a plateau-like domain during theslow process. The difference Δλ=λPL−λAB defines theStokes-shift of trap-state photoluminescence. ThisStokes-shift, usually ∼180–200 nm, is proportional tothe nanoparticle size (time of synthesis), as shown inFig. 3E. The Stokes-shift of the band-edge emission(usually ∼18–20 nm) generally decreases duringsynthesis, but is not investigated in details.

The average half-widths at half-maxima of the absor-bance (ΔλAB) and PL spectra (ΔλPL) are given in Fig.3B,D. The variance of size distribution, σ, is derivedfromΔλAB andΔλPL (see Appendix). As seen from Fig.4B, σ generally increases during the growth process.However the relative dispersion of the nanoparticle sizedistribution, σ /R, shown in Fig. 4C, decreases fastwithin the first 20–30 s as a demonstration of the size-

Fig. 4. Time evolution of characteristics of CdS nanoparticles duringtheir growth in paraffin at 250 °C: (A) mean nanoparticles radiuscalculated from the absorbance spectra; (B) variance of the sizedistribution; (C) relative dispersion of the nanoparticle size distribution(given in %); (D) mean nanoparticle concentration (in micromoles ofnanoparticles per gram of matrix solution); (E) relative energy of thetrap-state, ES/EAB (see the text for details).

focusing for CdS nanoparticles. Then σ /R starts toincrease probably due to the so-called Ostwald ripeningprocess (transfer of chemical components from thesmaller to the bigger nanoparticles). These processeshave been previously demonstrated for CdSe nanopar-ticles as well [14,15,21,22].

The mean nanoparticle concentration can be tracedduring the growth process by using the absorbancespectra. For this purpose all taken samples must have aconstant aliquot/toluene mass ratio, as described in theExperimental section. One can calculate the molarnanoparticles concentration in each sample from theabsorbance intensity, the half-width at half-maximum ofthe absorbance peak and the mean nanoparticlesdiameter [16] — see Appendix. The obtained resultsare given in Fig. 4D. The nanoparticles concentration in thematrix medium, Cm, reaches a constant value (∼0.27±0.04 μmol/g) after 50–60 s from the beginning of growth.Nevertheless, the nucleation process seems very fast in allexperiments; it takes place within the first few secondsfrom the reagent mixing. The nearly constant concentra-tion, maintained during the growth, implies that thenucleation process is completed in the first few seconds.The net growth of initial nuclei is then the reason for theobserved focusing of size distribution.

The relative position of the trap-state (the one thatprovides the highest probability for radiative recombi-nation of the exciton) in the forbidden zone can becalculated from the Stokes-shift. For this purpose theenergy corresponding to the Stokes-shift, ES (usually∼1.0 eV), is divided by the respective band gap energy,EAB, and is plotted against the time for nanoparticlegrowth. The obtained dependence is rather reproduciblefor CdS nanoparticles. The respective plot is given inFig. 4E for the investigated syntheses at 250 °C. It isseen that the trap-state is closer to the conduction band.

From the calculated value of σ, the respectivelognormal size distribution (N /Nmax) can be calculatedas N /Nmax=exp[− ln2(R / R̄ ) / 2(σ / R̄ )2]. Here N is thenumber of nanoparticles with radius R; Nmax is thenumber of nanoparticles with radius R̄ (R̄ is the valuecalculated from the absorption maximum).

3.3. TEM and XRD

Fig. 5a is a representative HR-TEM image of CdSnanoparticles. Their shapes are polyhedron with crys-talline core and indistinct interface due to the cappinglayer. Fig. 5b represents the size distribution histogramestimated from TEM photographs. There is a very goodcoincidence between the experimental histogram andthe calculated lognormal distribution. The relative

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variance of size distribution is about 6–9%, determinedby TEM.

XRD indicates that the crystalline structure of the CdSnanoparticles is sphalerite, althoughwith rather amorphouspatterns. The latter are due to the vastly developed interfaceof nanoparticles, which prevails to the crystalline corevolume. The defects, inevitably existing on the interface,cause trap-state emission of the CdS nanoparticles.

3.4. Stability against photo bleaching

One big advantage of the semiconductor nanoparti-cles (quantum dots) is their reduced tendency to photo-bleach in comparison with organic fluorophores, whichmakes them useful for prolonged imaging, e.g. time-resolved studies, fluorescent labeling of cells, etc. [8].Usually photo-bleaching is due to irreversible light-induced reactions resulting in a loss of fluorescence.

Fig. 5. (a) TEM image of CdS nanoparticles synthesized at 200 °C(average diameter 3.66 nm); (b) comparison between the size-distribution histogram obtained from (a) and the calculated sizedistribution (the solid line) — see the text for details.

Fig. 6. Photo-bleaching of aggregates of CdS nanoparticles,synthesized at 200 °C, under UV-light: (a) images under fluorescentmicroscope before and after 5 min of illumination; (b) the corres-ponding PL spectra of the images from (a). (The line at 740 nm is aharmonic wavelength of the excitation lamp, 370 nm, and is an artifactdue to the apparatus).

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CdS nanoparticles, purified from the paraffin andexcess precursors and suspended in n-heptane, form asort of aggregates that are visible under fluorescentmicroscope (Fig. 6a). We found in our experiments herethat the band-edge emission of CdS nanoparticles,produced in a paraffin hot-matrix, is rather unstableunder UV-illumination in comparison to the trap-stateemission shown in Fig. 6b. The intensity of the band-edge emission (the sharp peak at 435 nm) decreases 4.6times faster than that of trap-state emission for a period of5min.We suppose that this fact is due to photo-oxidationprocesses induced by the optical excitation.

4. Conclusions

This paper is a comprehensive study on the opticalproperties and other characteristics of CdS nanoparti-cles during their growth. The synthesis is made by thehot-matrix method in a paraffin solution. It is foundthat most optical characteristics of the CdS nanopar-ticles change during the growth process in a similarway as for CdSe nanoparticles made in a TOPOmatrix. For CdS the trap-state emission prevails, whosetemporal evolution is dramatically different from thatfor the band-edge emission dominating usually forCdSe nanoparticles. The band-edge fluorescence ofCdS is more stable for the bigger nanoparticles. Thesize distribution is calculated from the absorbancespectra and its variance is plotted versus the time ofgrowth. Here we demonstrate that the calculationstrategy, previously adopted for CdSe nanoparticles inTOPO, is also valid for CdS nanoparticles made atrather different synthetic conditions. The calculatedlognormal size distribution of CdS nanoparticles isconfirmed by TEM photographs. The obtained quan-titative results open the possibility for a large-scalemanufacturing of CdS nanoparticles in a cheap, energyeconomical and environmentally safe synthesis withprecise control of the nanocrystal growth. Thedemonstrated photoluminescence of various originsmake the CdS nanoparticles suitable candidates for avariety of nanotechnological applications such asoptical devices, fluorescent biolabels, lighting sources,etc.

Acknowledgements

The financial support of the Bulgarian Ministry ofEducation and Science, project VUH-09/05, is greatlyacknowledged. We are indebted to Prof. KostadinPetrov, Bulgarian Academy of Sciences, for the X-raydiffraction study.

Appendix A

A.1. Calculation of the variance of size distribution

In Ref. [15] is derived an equation allowingcalculation of the variance of nanoparticle size distri-bution, σ, from characteristics of the UV–vis absorptionspectrum:

r ¼ lqx

21ffiffiffiffiffiffiffiffiffiffiffiffi1−vx2

p −1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ vx2p

!

Here lq ¼ h=ð2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffi2m⁎Eg

p Þ is characteristic length ofthe exciton confinement (lq=0.988 nm for CdS); h=6.60×10−34 J·s is the Planck constant; m⁎=1.397×10−31 kg is effective mass of the exciton for CdS (cal-culated as 1 /m⁎=1 /m⁎e+1 /m⁎h). The respective con-stants for CdS are: Eg=2.49 eV [23] (band gap of thebulk semiconductor); m⁎e=0.19m0 is the effective massof the electron, m⁎h=0.8 m0 is the effective mass of thehole (m0=9.10×10

−31 kg is the free electron mass). Theother parameters in the equation are: χ=ΔE /Eg, whereΔE is the half width at half maximum of the absorptionband in energy units (eV); x=(R+ l) / lq, where R is themean nanocrystal radius (nm), calculated from theabsorption spectra; l (nm) is the thickness of a sphericalshell surrounding the semiconductor core of radius R.The length l can be considered as the penetration depthof the exciton in the outer space comprising the cappingorganic layer (here for CdS we assume l=0.15 nm). Theabove equation has been applied for the case of CdSe[15]; here we demonstrate its validity also for the case ofCdS.

A.2. Calculation of the mean nanoparticle concentra-tion

In Ref. [16] is described a strategy to calculate themean nanoparticle molar concentration in solution.Using some equations from Ref. [16] one can obtainthe following formula valid for the case of CdSnanoparticles:

CM ¼ AmðhwhmÞUV2:369� 105D7=3

Here CM is the molar nanoparticles concentration inthe sample (mol/l); Am is the measured absorbance;(hwhm)UV is the half-width at half-maximum of peak1 (nm); D is the mean nanoparticles diameter (nm). Wecalculate the nanoparticle concentration in the matrix

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media, Cm (μmol/g), using the relationship Cm=103C-

MVS /ma. Here Vs is the sample volume (ml); ma is themass of aliquot (g). In our experiments, the ratio Vs /ma

is kept constant equal to 41 ml/g for all samples.

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