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Photoluminescence spectra of CdSe/ZnS quantum dots in solution

K.H. Ibnaouf a , Saradh Prasad b ,c, A. Hamdan b ,c, M. AlSalhi b ,c, , A.S. Aldwayyan c, M.B. Zaman d ,e ,V. Masilamani b ,ca Al Imam Mohammad Ibn Saud Islamic University (IMSIU), Physics Department, College of Science, P.O. Box 9095, Riyadh 11623, Saudi Arabiab Research Chair for Laser Diagnosis of Cancer, King Saud University, Saudi Arabiac Department of Physics & Astronomy, College of Science, King Saud University, Saudi Arabiad CEREM, College of Engineering King Saud University, Saudi Arabiae Advanced Medical Research Institute of Canada, Sudbury, Canada

h i g h l i g h t s

Fluorescence spectra of quantum dotscore and core shell in organicsolvents. Solvents inuence onphotoluminescence spectra of quantum dots. The longer wavelength is due to thecore of the quantum dots. The shorter wavelength is due to theinteraction between the solvent andthe shell (ZnS) in excited state.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 December 2012Accepted 3 August 2013Available online 1 November 2013

Keywords:Photoluminescence spectraQuantum dotsSolvent inuence

a b s t r a c t

The spectral properties of CdSe/ZnS coreshell quantum dots (QDs) of 3 nm size have been studied underdifferent organic solvents, concentrations and temperatures. Our results showed that the absorptionspectra of CdSe/ZnS in benzene have two humps; one around 420 nm and another at 525 nm, with asteady increase in absorption along UV region, and the absorption spectral prole under a wide rangeof concentrations did not change. On the other hand, the photoluminescence (PL) spectra of CdSe/ZnSin benzene showed two bands one around 375 nm and the other around 550 nm. It could be seen thatthe band at 375 nm is due to the interaction between the shell (ZnS) with the solvent species in highexcited state, and the band at 550 nm is due to core alone (CdSe).

2013 Elsevier B.V. All rights reserved.

Introduction

The semiconductors quantum dots (QDs) are nanoparticles orclusters-like particles with a typical size of 220 nm (nm) consist-ing of few hundreds to few thousand atoms in each particle. Quan-tum dots are new nanomaterials which have properties that areintermediate between those of bulk materials and those of isolatedor discrete molecules [1] .

Quantum dots can be made from a wide variety of material; themost common QDs are the binary semiconductor materials con-taining the IIVI elements, e.g., cadmium sulde (CdS), cadmiumselenide (CdSe), cadmium telluride (CdTe), zinc selenide (ZnSe),lead sulde (PbS), and mercury sulde (HgS) etc. These semicon-ductor QDs, in the bulk, usually have band gap energy less than3 eV. In all these nanocrystals QDs (or the clusters) the atoms arealigned periodically with certain crystal lattice structure, such asthe cubic zinc blende or hexagonal wurtzite structure of the CdSand ZnS QDs [2] . Because of these reasons, these clusters-like par-ticles display unique electronic and optical properties includingsize-tunable light emission, simultaneous excitation of multiple

1386-1425/$ - see front matter 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.saa.2013.10.089

Corresponding author at: Research Chair for Laser Diagnosis of Cancer, KingSaud University, Saudi Arabia

E-mail address: [email protected] (M. AlSalhi).

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 339345

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

j ou r na l hom e pa ge : www.e l s e v i e r. c om / l o c a t e / s a a
http://dx.doi.org/10.1016/j.saa.2013.10.089mailto:[email protected]://dx.doi.org/10.1016/j.saa.2013.10.089http://www.sciencedirect.com/science/journal/13861425http://www.elsevier.com/locate/saahttp://www.elsevier.com/locate/saahttp://www.sciencedirect.com/science/journal/13861425http://dx.doi.org/10.1016/j.saa.2013.10.089mailto:[email protected]://dx.doi.org/10.1016/j.saa.2013.10.089http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://crossmark.crossref.org/dialog/?doi=10.1016/j.saa.2013.10.089&domain=pdfhttp://-/?-

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Fig. 1. Absorption spectra of CdSe/ZnS quantum dots for different concentrations from 9.0 mg to 300 mg in 5 ml of benzene.

Fig. 2. Photoluminescence emission spectra of CdSe/ZnS quantum dots for the concentration of 9.0 mg in 5 ml.

Fig. 3. Photoluminescence emission spectra of CdSe/ZnS quantum dots for the concentration of 18 mg in 5 ml for two different wavelengths of excitation (280 nm and330 nm).

340 K.H. Ibnaouf et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 339345

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uorescence, high quantum yield and long-term photo-stability.They have attracted a lot of both theoretical and experimental re-search interest for more than two decades [37] . These propertiescan be drastically changed, while maintaining the material mor-phology, by simply varying the number of atoms in each quantum

dot. In such nanoparticles, the size of the quantum dot can be usedto tune the emission spectral range over a major part of visiblespectrum due to quantization effect: e.g., in the case of CdSe, thespectral range can be tuned from deep red 1.7 eV to green

2.4 eV by reducing the dot diameter from 20 to 2 nm [810] . Ithas been also shown that the coreshell quantum dots over coatedwith higher band gap inorganic materials exhibits high PL quan-tum yield compared to the uncoated QDs, perhaps due to the elim-ination of surface defects promoting non-radiative recombination[11] .

The photoluminescence spectra of CdSe/ZnS were found to bered shifted from the absorption maximum. This was observedmore strongly in very small size quantum dots; such a shift to-wards red in emission of CdSe/ZnS quantum dots has been attrib-

uted to the recombination of weakly overlapping surface-localizedcarriers. This effect has also been explained as a recombination of

the optically forbidden ground state exciton split from the rstoptically active state by quantum dots shape, electronhole ex-change interaction and intrinsic asymmetry of lattice B [12] .

Liu et al. studied CdSe semiconductor nanoclusters over-coatedwith CdS shell in aqueous solution. Based on their experimental re-

sults and theoretical calculation, a model of excimer formationwithin nanoclusters was proposed to explain the large Stokes shift[13] .

In our work presented here, the spectral properties of CdSe/ZnSin different concentrations and temperatures were described in aparticular solvent environment. The results showed that semicon-ductor CdSe/ZnS quantum dots could exhibit two bands of emis-sion; one due to the core and another due to the interactionbetween solvent and shell. Similar experiments done in three moredifferent types of the solvent environments showed that suchinteraction was strongly pronounced in polar solvents.

Experimental

CdSe/ZnS quantum dots were prepared following the previouslyreported procedures: TBP (tributylphosphine), TOPO (trioctylphos-

Fig. 4. Photoluminescence emission spectra of CdSe quantum dots for concentration of 300 mg in 5 ml of benzene.

Fig. 5. Photoluminescence emission spectra of ZnS quantum dots for concentration of 300 mg in 5 ml of benzene.

K.H. Ibnaouf et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 121 (2014) 339345 341
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phine oxide), HDA (hexadecylamine) or ODPA (octadecyl phos-phonic acid) capped CdSe nanocrystals were synthesized using

standard published methods [14] . We deposited about vemonolayers of ZnS around the CdSe cores by using the recently

Fig. 6. Photoluminescence emission spectra of CdSe/ZnS quantum dots for the concentration of 300 mg in 5 ml in different solvents. (a) Non-polar solvents, (b) high polarsolvents and (c) intermediate polar solvents.


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developed successive ion layer adhesion and reaction (SILAR) tech-nique [15] . To optimize the ZnS shell growth around the CdSe coreby SILAR using ZnO and S as precursors, a stock solution of 0.1 Mconcentration was prepared for ZnO, oleic acid and ODE (1-octade-cene) that were used for Zn coating and elemental sulphur andODE for S coating. The puried CdSe QDs were added to a reactionask consisting of HDA and ODE where Zn and S stock solutionswere added under argon ow to grow the ZnS shell. To optimizethe shell growth, the reaction temperaturewas controlled between200 C and 240 C.

The absorption and photoluminescence (PL) spectra of CdSe/ZnScoreshell quantum dots in various organic solutions were studiedunder wide range of concentrations and temperatures. The spectrafor the fresh solutions were measured in a small quartz cuvettewith the dimensions 1 1 4 cm with an optical path length of 1 cm.

UVVis absorption spectra were taken using a Perkin Elmerspectrophotometer (Lambda 950) and the photoluminescence(PL) was measured on a Perkin Elmer LS55 spectrouorometer.

Results and discussions

The puried quantum dots nanocrystals were dispersed in dif-ferent solvents in different concentrations and UV absorptionand photoluminescence emission spectra were taken and pre-sented below to give insight into the excited state behavior of these nanocrystals.

Fig. 1 shows the absorption spectra of CdSe/ZnS quantum dotsof different concentrations ranging from 9.0 mg to 300 mg of CdSe/ZnS in 5 ml of benzene. It shows that there were two humps;one at 420 nm and another at 525 nm with high absorbance in theUV region. When the concentration was increased, the optical den-sity increased. Note that the absorbance was not to the scale.

CdSe/ZnS in benzene at low concentration (9 mg/5 ml) was pre-pared. The photoluminescence emission spectra (PLES) depicted inFig. 2 , it could be seen that the PLES, as excited at 300 nm have aprimary band at 375 nm and small shoulder at 550 nm.

ThePLES of CdSe/ZnS in benzene at lowconcentration (18 mg in5 ml) were obtained under two different wavelengths of excitation

Fig. 7. Photoluminescence emission spectra of CdSe/ZnS quantum dots for concentrations from 9.0 mg to 300 mg in 5 ml of benzene.

Fig. 8. Photoluminescence emission spectra of CdSe/ZnS quantum dots for concentrations of 300 mg in 5 ml of benzene under different temperatures.

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(280 nm and 330 nm). In both cases one could get the same spec-tral prole (with the primary band at 375 nm and secondary at550 nm) but with different intensity as shown in Fig. 3 . This isindicative for spectral purity of the sample.

The uorescence spectrum of CdSe (core alone) of size 3 nm inbenzene at 300 mg/5 ml was recorded. It could be seen that therewas only one band at 550 nm. There was no traces of signal havebeen detected at 375 nm as shown in Fig. 3 .

Similarly, the uorescence spectrum of ZnS (shell alone) in ben-zene at 300 mg/5 ml was recorded. The result showed that therewas only one band at 375 nm. There was no other band at550 nm as shown in Fig. 4 . By comparing, Figs. 3 and 4 withFig. 2 , the bands at 375 nm and 550 nm are due to the shell(ZnS) and core (CdSe) respectively.

The PLES were recorded for three types of solvents under thesame concentration. Fig. 5 a displays the PLES spectra of CdSe/ZnScore shell quantum dots dissolved in hexane and benzene at sameconcentration (150 mg in 5 ml). It could be seen that the bands at550 nm and was strongly pronounced in non-polar solvents.

In polar solvents like dimethyl fumarate (DMF) and acetonitrile(AN), as illustrated in Fig. 5 c, the band at 550 nm for the CdSe/ZnScore shell is totally absent, and there was only one band at 375 nm.

This because the interaction between CdSe/ZnS with polar solventsin excited state is very strong. It is important to note that for CdSe/ZnS in polar solvents, the peak around 375 nm is due to the dipoledipole interaction between the solvent and shell (ZnS) in excitedstate. Here the solvent surrounds the shell and traps it as in cage.In non-polar solvents the peak at 375 nm was very weak forCdSe/ZnS; this is because the interaction between the solvent spe-cies and the quantum dots is very weak.

In some of intermediate polar solvents like ethyl acetate and n-butyl acetate, the intensity of the 375 nm and 550 nm bands of thecore shell became almost comparable (see Fig. 5 b).

Fig. 6 shows the PLES as excited at 300 nm for different concen-tration (9.0300 mg of CdSe/ZnS in 5 ml of benzene). It clearlyshows that the weak secondary band at 550 nm became strongerand stronger; and for the concentration around 300 mg of CdSe/ZnS in 5 ml of benzene the band around 550 nm became the pri-mary band and the one at 375 nm became the secondary band.

Fig. 6 also displays that the intensity of the band of 550 nm in-creased by increasing the concentration. For the same set of con-centrations; the prole of the absorption spectra remainedunchanged as shown in Fig. 1 . This could be due to increase inthe ratio between CdSe/ZnS to the solvents species, in turn de-creases the number of solvents species interacting with CdSe/ZnS. Therefore the intensity of the band at 375 nm was decreasedand the band at 550 nm increased.

The spectral proles of 300 mg of CdSe/ZnS in 5 ml of benzenewere taken under different temperatures, Fig. 7 shows the PLESas excited at 300 nm for three different temperatures. It is clearthat as the temperature was increased above room temperature,(from 298 K to 323 K) the band at 550 nm became weaker thanat 375 nm, i.e.,

I 550I 375

323 K I 550I 375

298 K Fig. 8 is a representation of I 550 /I 375 , the relativephotoluminescence(PL) emission intensity at 550 nm and 375 nm bands, as a functionof polarity (dielectric constant) of solvents. The linear t is a strongindication that polar solvent enhances the interaction between the

shell and the solvent species.The solvent environment not only changes the properties of sol-

ventshell interaction, but also the quantum efciency of photolu-minescence [16] . This is shown in Table 1 where it can be clearlyseen that quantum efciencies increase in non-polar solvents likebenzene.

Another important property of solvents inuence upon thespectral properties is Stokes shift, which is a measure of changes

Table 1

The quantum yields of (CdSe/ZnS) quantum dot in different solvents at concentrationof 300 mg in 5 ml.

Solvent Quantum yield

Hexane 0.45Benzene 0.58n-Butyl acetate 0.42Ethyl acetate 0.43Acetonitrile 0.32Dimethyl fumarate 0.35Ethanol 0.31Methanol 0.33

Fig. 9. The relative intensities of photoluminescence emission at 550 nm and 375 nm bands ( I 550 /I 375 ), as a function of polarity (dielectric constant) for different solvents.


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in the dipole moment of the species when it goes to the excitedstate from the ground state.

We observed a very small change in the uorescence spectra forCdSe/ZnS in all of the above solutions at low concentrations, theonly difference being a small shift in the band of the emissionwavelength. In cases where the polarization interaction is the major contributing factor, Metaga et al. [17] have shown that theStokes shift has a linear variation with the dipole factor.

The dipole factor is a measure of the interaction between thesolute dipole and the solvent dipoles surrounding the solute as acage. If the solute undergoes great delocalization of electron cloud(hence more polar) in excited state than the ground stat, the dipoleinteraction in becomes stronger in excited state than the groundstate. This leads to more and more red shift of absorption andemission bands, and hence the Stokes shift.

Fig. 9 gives the variation of Stokes shift as a function of dipolefactor of the solvent as dened by Mataga et al. [17] . It can be seenthat this quantum dot undergoes signicant changes in the elec-tron delocalization and becomes more polar in the excited statethan in the ground state (see Fig. 10 ) [18] .

Conclusion

In this communication, we have been able to show that thequantum dot nanocrystals of CdSe/ZnS in organic solvents exhibitstwo bands one due to the interaction between solvent and shell at375 nm, pronounced in highly polar solvents, and other one is dueto core at 550 nm. This is analogues to the exciplex behaviour of the many organic dyes. All these because of the excitons of thesenanoparticles undergo gross changes in the electronic congura-tion. These nanocrystals behave very much like organic dyemolecule.

Acknowledgement

This project was supported by King Saud University, Deanshipof Scientic Research, College of Science Research Center.

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Fig. 10. The relationship between stokes shift and the dipole factor of quantum dots in different solvents at concentration concentrations 300 mg in 5 ml.

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