Optical Materials xxx (2014) xxx–xxx

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Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Low-cost and large-scale synthesis of CuInS2 and CuInS2/ZnS quantumdots in diesel

http://dx.doi.org/10.1016/j.optmat.2014.09.0160925-3467/� 2014 Published by Elsevier B.V.

⇑ Corresponding author.E-mail address: chittk@ims.vast.ac.vn (T.T.K. Chi).

Please cite this article in press as: N.T.M. Thuy et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.09.016

Nguyen Thi Minh Thuy a,b, Tran Thi Kim Chi a,⇑, Ung Thi Dieu Thuy a, Nguyen Quang Liem a

a Institute of Materials Science (IMS), Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Namb Thai Nguyen University of Education, Luong Ngoc Quyen, Thai Nguyen, Viet Nam

a r t i c l e i n f o a b s t r a c t

Article history:Received 2 July 2014Received in revised form 10 September2014Accepted 10 September 2014Available online xxxx

Keywords:CuInS2

Quantum dotsDieselHeating-upRaman

In this paper, we present the results of the syntheses of CuInS2 (CIS) and CIS/ZnS core/shell quantum dots(QDs) by heating-up method using diesel as the high boiling-point reaction solvent. The influences of thesynthesis parameters, namely the reaction temperature, growth time and the Cu:In molar ratio to thestructure and optical properties of the obtained QDs were systematically investigated. CIS QDs were syn-thesised at the reaction temperatures of 200–230 �C for 5–45 min and the Cu:In molar ratios of 0.5:1–1.5:1. The optical characteristics from absorption and photoluminescence spectra have been used as indi-cators to the quality of the synthesised QDs, showing clearly that the highest quality CIS QDs wereobtained at the reaction temperature of 210 �C for 15 min with the Cu:In molar ratio of 1:1. For suchQDs, their mean size of 3.5 nm was determined directly from the transmission electron microscopy(TEM) image and calculated from their XRD pattern.

� 2014 Published by Elsevier B.V.

1. Introduction

CuInS2 (CIS), like CuInSe2, CuGaS2 and CuAlS2, crystallizes in thetetragonal system similar to the chalcopyrite and the zinc-blendestructure of the II–VI (e.g. ZnS and ZnSe) compound in which Cuand In replace two sites of Zn in the crystalline lattice. Bulk CIShad found many applications in optoelectronic devices, especiallyfor solar cells because it is a direct bandgap semiconductor withthe bandgap energy of 1.53 eV very suitable for absorbing visiblelight [1–4]. In recent years, along with concerns on nanocrystals(NCs) or semiconductor QDs, CIS semiconductor QDs have gainedmuch interest; therefore a number of studies both in the synthesistechnology and their optical properties have been performed[5–11]. For this kind of the ternary semiconductor QDs, the studiesof characteristics related to the constituent element ratios, latticeimperfections and to the surface-atoms with their dangling bondsare necessary to be carried out. To improve the crystalline qualityand tune the bandgap energy for varying the absorption spectralrange, CIS was doped with Zn (Zn–CIS) [11,12,31] or modified withGa and/or Se(CIGS or CIGSe) to become the effective light-absorb-ing materials in solar cells [13–16,32]. Because of the significantthird-order nonlinearity, CIS NCs may also have applications inultrafast optical telecommunication [17]. Besides, CIS/ZnS QDs

[18,19] and CuAlS2 QDs [18,20] have showed to be good examplesfor the Cd-free luminescent biolabeling or biosensor. Our CIS/ZnSQDs used for labeling in live mouse were synthesised in octade-cane (ODE) having the luminescence quantum efficiency of 60%[18]. There have been publications concerning the synthesis ofCIS QDs using different methods: solvothermal synthesis[21,22,23,24,25], thermolysis [1,2,15,26], thermal decompositionfrom single-source precursors [3,10], thermal decomposition ofmetal-thiolate [17], photochemical decomposition [27], heating-up precursors in one-pot [18,28], hot-injection using various highboiling-point solvents [2,8,33]. For the synthesis of CIS and CIS-based QDs, ODE was widely used as the reaction medium becauseit is a high boiling-point solvent. To the best of our knowledge, noexample using diesel as solvent in the syntheses has been reportedto date. Diesel, a popular low-cost fuel, could serve well as a highboiling-point solvent to replace the expensive ones like ODE or tri-octyl phosphine oxide (TOPO) in synthesizing CdSe QDs [34]. Inaddition, diesel contains a mixture of hydrocarbons so that its boil-ing point varies between 177 and 343 �C. By testing directly thediesel purchased from gasoline stations we determined the boil-ing-point of diesel to be around 220–240 �C that makes the reac-tion to nucleate CIS QDs very efficient.

In this paper we present the results on the syntheses and char-acterizations of CIS QDs and CIS/ZnS core/shell QDs. The CIS QDswere synthesized at 200–230 �C by heating-up method using die-sel as the high boiling-point reaction solvent and then shelled with

2 N.T.M. Thuy et al. / Optical Materials xxx (2014) xxx–xxx

ZnS at lower temperatures. The influences of the synthesis param-eters, namely the growth time, reaction temperature and the Cu:Inmolar ratio on the structure and optical properties of the obtainedQDs were systematically investigated for determining the optimalconditions to get the high quality QDs. XRD and Raman scatteringmeasurements were used to determine the structure showing thatCIS QDs had the tetragonal (chalcopyrite) phase. The optical char-acteristics from the absorption and photoluminescence (PL) spec-tra were used to estimate the quality of synthesized NCs,showing clearly the quantum confinement effect and the highestquality CIS QDs could be synthesised at the reaction temperatureof 210 �C for 15 min with the Cu:In molar ratio of 1:1. Our studyon the function of diesel as a good/suitable solvent in the synthesisof CIS QDs is necessary to support to the large-scale, low-cost syn-theses of QDs for applications in which large amounts of QDs areneeded, e.g. in solid-state lighting or in photovoltaic devices.

2. Experimental

2.1. Chemicals

Copper (I) iodide (CuI, 98%) and dodecanethiol (DDT, 97%) werepurchased from Merck while indium(III) acetate (In(Ac)3, 99.99%),zinc stearate (90%), oleic acid and other chemicals were obtainedfrom Sigma–Aldrich. Zinc ethylxanthate was prepared from zincchloride and potassium ethylxanthogenate according to themethod reported previously [35]; diesel was bought from gasolinestations without further purification.

2.2. Synthesis of CIS and CIS/ZnS NCs

CIS NCs were prepared using heating-up method in diesel sol-vent. In a typical procedure, In(Ac)3 (0.1 mmol), CuI (0.1 mmol),1.0 ml (4.175 mmol) of DDT, 0.4 mmol oleic acid and 8 ml of dieselwere mixed in a three-neck flask using a magnetic stirrer. DDT wasused as the sulfur precursor, which was much excessive for thereaction to nucleate CIS seed nanocrystallites. Then, the mixturewas very fast heated to a certain reaction temperature in the rangeof 200–230 �C with a heating rate of 150–200 �C/min. The mixturewas kept at the reaction temperature for a certain duration timebetween 5 and 45 min to develop the nanocrystal size. Along withthe growing of NCs, the color of the mixture changed from color-less to yellow, red and dark red showing the increase of the NCssize. The CIS QDs were then isolated by adding acetone and centri-fuging. CIS QDs could be easily redispersed in organic solvents suchas hexanes, toluene or chloroform. To further demonstrate thelarge-scale synthesis of CIS and CIS/ZnS QDs, we performed thesynthesis in 320 ml of diesel with 4 mmol of In(Ac)3, 4 mmol CuI,40 ml DDT, 0.16 mol of oleic acid. All the above agents were mixedin a 1000 ml three-neck flask using a magnetic stirrer. The finalgood-quality CIS QDs of more than 500 mg was collected after cen-trifuging and precipitation. We did not see any problem to scale-upthe synthesis in the gram scale.

For shelling ZnS onto CIS QDs to make the CIS/ZnS core/shellstructures, the ZnS shell precursor solution was prepared by dis-solving 0.8 mmol of zinc stearate and 1 mmol of zinc ethylxanthatein a mixture of 3 ml of diesel, 300 ll of dimethylformamide (DMF)and 1 ml of toluene at room temperature for 10 min. The clear pre-cursor solution for the ZnS shelling was then added drop-wise bymeans of a syringe to the CIS NCs containing reaction mixture at200 �C for 30 min. It should be noted that, shelling ZnS onto CISQDs was performed at 200 �C, lower than that of the synthesis ofcore CIS NCs, to intentionally avoid the growing up in the size ofthe CIS core. Purification of the CIS/ZnS core/shell QDs was carriedout in the same way as described for the core CIS QDs, namely by

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adding acetone and centrifuging. The obtained CIS and CIS/ZnS QDscould be used in the colloidal form by redispersing in organic sol-vents or in the close-packed form by evaporating the residual sol-vents out from the QDs.

2.3. Characterisation

Morphology of QDs was taken using high-resolution transmis-sion electron microscopy (HR-TEM, Tecnai G2 F20). The atomic fac-ets could be observed with a high-magnified HR-TEM image. Thestructures of the obtained QDs were investigated by X-ray diffrac-tion (XRD, Siemens D5000) and Raman scattering (iHR550, Horiba,using the 532-nm laser excitation). The size distribution of the syn-thesised QDs was determined using dynamic light scattering mea-surement (Zetasizer Ver 6.20, Mal1062724). The constituentelements of the CIS QDs products were analyzed using Energy-dis-persive X-ray spectroscopy (EDX). Optical characterizations wereperformed with the UV–Vis absorption (using an UV-1800-Shim-azu) and PL measurements. For the PL measurement, the PL signalwas dispersed by a 150-grating monochromator (Horiba iHR550)and then detected by a thermoelectrically cooled Si-CCD camera(Synapse). Samples for the optical measurements were preparedin the colloidal form of CIS and CIS/ZnS QDs dispersed in toluene.

3. Results and discussion

We first synthesised CIS QDs following the proceduresdescribed elsewhere [18,28] with some modifications: using dieselinstead of ODE as the reaction environment and just taking a cer-tain set of the technological parameters as the reaction tempera-ture of 210 �C, growth time for 15 min, and the Cu:In:S molarratio of 1:1:40. This first trial was really decisive to check if CISQDs could be synthesised in diesel solvent at temperatures of210 �C, compared to those already well synthesised in ODE at220–230 �C. Using diesel as the reaction solvent in the synthesisof NCs, one could see some advantages as follows: first, it is notan expensive organic solvent, easy to buy from any gasoline sta-tion; and second, 200–230 �C is very close to the boiling tempera-ture of diesel that makes the chemical reaction kinetics increasedmuch. Fig. 1 shows the XRD pattern from the CIS QDs and CIS/ZnS core/shell QDs (CIS QDs shelled with ZnS at 200 �C). Three dis-tinct broaden diffraction peaks with 2h values of 27.8�, 46.5�, and54.8� could be well indexed to (112), (204)/(220), and (116)/(312) planes of the standard pattern of the chalcopyrite structure(PDF card No. 25-0159), respectively. These XRD results are consis-tent with those published in the literature for CIS NCs[5,15,29,30,36,37]. The XRD peaks are broaden confirming thatCIS nanoparticles sizes are very small. The average grain size of3.8 nm was calculated from the Scherrer equation, which wasclosed to that obtained from the TEM images (Fig. 2). After theCIS NCs were coated with the ZnS shell, all peaks exhibited a smallshift to the larger angles, closer to the peak positions of bulk cubicZnS (PDF card No. 5-0566). This behavior is similar to that observedfor other core/shell systems [18,38,39]. Fig. 2 shows the low mag-nification TEM image indicating the shapes of the synthesised CISNCs are varied being microspherical and square with the mean sizeof about 3.5 mm. The HR-TEM image (the inset in figure) shows thelattice planes, confirming the good crystalline structure of the syn-thesised CIS QDs. It is noticed that synthesised CIS QDs could be indifferent shapes depending on the synthesis conditions, namelythe reaction solvent and/or growth temperature. In our syntheses,most samples showed cubic shape or quasi-spherical ones. For CISNCs synthesised using OLA as the reaction solvent, the morpholo-gies showed the tune from pyramid-shaped to nanospheres byvarying the concentrations of OLA were observed [17]. Using the

//dx.doi.org/10.1016/j.optmat.2014.09.016

Fig. 1. XRD patterns of CIS and CIS/ZnS QDs.

Fig. 2. HR-TEM image and size distribution of CIS QDs.

Fig. 3. Deconvolution of Raman spectra of CIS by fitting with the Voigt function.

N.T.M. Thuy et al. / Optical Materials xxx (2014) xxx–xxx 3

dynamic light scattering measurement, the size distribution wasevaluated showing the low-dispersed particle size with a peak�3.6 nm (the inset in Fig. 2). The good results from our first exper-iment with a certain set of the technological parameters indicatedabove have confirmed that diesel could be used as the high boiling-point reaction solvent for successfully synthesizing CIS QDs andCIS/ZnS core/shell QDs. In the following, we present the results ofthe experiments on the influences of different synthesis conditionsto the formation and quality of CIS QDs.

In the synthesis of NCs or QDs, the reaction temperature,growth time and the molar ratio of precursors are the importantfactors to determine the structural quality and optical propertyof the synthesised products [28,40–42]. Therefore, we did studyin detail on such problems. The effect of the reaction temperaturewas checked by changing the reaction temperature from 200 �C to

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230 �C while maintaining the duration of growth time unchangedof 15 min and the same Cu:In:S molar ratio of 1:1:40. The XRD pat-terns taken from all the synthesised samples indicate that the CISQDs have the good crystalline structure, same as shown above.With increasing reaction temperature from 200 �C to 230 �C, theQDs size was increased from 3.2 nm to 3.6 nm, respectively. Fordetermining the effect of the growth duration time, the varioussyntheses of CIS QDs were carried out at the same 210 �C for differ-ent times between 5 and 45 min. The experimental result showedthe particle size of QDs became bigger with increasing the growthduration time, to be of 3.2 nm and 3.6 nm by the 5-min and 45-mingrowths, respectively.

Before carrying out the last experiment on the influence of themolar ratio of the precursors to the final QDs product, we didRaman scattering measurement on our synthesised samplesbecause this technique is considered to be a powerful tool to studythe microstructure [43,44] through the lattice vibrations which arerelated to the local crystalline imperfections or non-stoichiometryin the structures. Fig. 3 shows a typical Raman spectrum of CIS

//dx.doi.org/10.1016/j.optmat.2014.09.016

Fig. 4. Raman spectra of CIS QDs as a function of the Cu:In molar ratio.

Fig. 5. XRD patterns of CIS QD as a function of the Cu:In molar ratio.

4 N.T.M. Thuy et al. / Optical Materials xxx (2014) xxx–xxx

QDs, which can be deconvoluted into components by fitting withthe Voigt function. Table 1 shows the main Raman scattering linesfrom the CIS crystals [47,49]. The A1 peak has been used to discussthe crystalline of CuInS2 thin film [45,46,50]. Though one couldanalyses the Raman spectra taken from the synthesised CIS QDsinto the Raman scattering lines similar to those taken from theCIS thin film or crystals, we see that the two Raman lines at293 cm�1 and 331 cm�1 corresponding to the symmetric vibrationmode A1 and the longitudinal optical mode B2

1LO, respectively, could

be enough to estimate the change in the microstructure in relationwith the Cu:In molar ratio. In chalcopyrite CIS, the Cu–S bond isweak consequently the Cu-vacancy is preferably generated. There-fore, the concentration of the Cu-vacancy could be changed bychanging the Cu:In molar ratio from the precursors. In our experi-ments, the Cu:In molar ratio of 0.5–1.5 were applied in the synthesisof CIS QDs (at 210 �C for 15 min), while the S precursor was kept verymuch excessive. Fig. 4 presents the Raman spectra of CIS QDs as afunction of the Cu:In molar ratio. Making the Raman intensity ratioof the 331-cm�1 line and the 293-cm�1 line the result comes out asthis Raman intensity ratio is increased with increasing the Cu:Inmolar ratio. In other words, the Cu deficiency could be observed inthe Raman spectrum by the less intense of the lines around335 cm�1, similar to the elicitation of this shoulder in the Ramanspectra observed in CIS thin film [45,50,51]. With increasing theCu:In molar ratio, along with the Raman intensity ratio changed,we also observed the shift of the A1 peak to the higher frequency.Note that because the Raman scattering measurement is sensitiveto the microstructure, we could observe the change in the Ramanspectra with the change of the Cu:In molar ratio; while as an averagedetermination of the phase structure the XRD patterns showed nochange with the mentioned CIS QDs (Fig. 5). In fact, in order to havethe direct data on the elemental composition of the synthesised QDs,we have performed the XRF and EDX analyses but the resultsshowed confusively. However, we could rationalise from the XRDpatterns and the Raman scattering spectra to confirm the crystallinestructure; and more reasonably based on the nice excitonic absorp-tion and PL spectra (Fig. 6) to estimate the final high quality of ourCIS QDs.

For luminescence materials, after necessary checks on thecrystalline structure, the final quality must be monitored by theiroptical properties, generally taken from the absorption and PLspectra. The systematic experimental results show clearly thehighest quality CIS QDs were obtained by the synthesis at the reac-tion temperature of 210 �C for 15 min, with the Cu:In molar ratio of1:1. At these reaction temperature and growth time, the absorptionspectrum shows most clearly the excitonic structure at �2.61 eVand the PL spectrum (taken with the 532-nm laser excitation)shows a broad band peaking at 1.96 eV with full width at half max-imum of 315 meV. The emission energy from the synthesised CISQDs is therefore much larger than the bandgap energy (1.53 eV)of bulk crystalline CIS, indicating the quantum confinement effectin the synthesised CIS QDs (this is normal because the bulk exciton

Table 1The main Raman scattering lines from the CIS crystals.

Vibrationalmode

Raman shift(cm�1)[theory]

Ramanshift(cm�1) [47]

Ramanshift (cm�1)[49]

Raman shift(cm�1)[present work]

A1 294 294 290 293B2

1TO 323 323 323 331

B21

LO 352 352 345 350E3

TO 244 244 240E2

LO – 314 – 314E1

TO – 323 –

Bold values indicate the main Raman lines of CIS.

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diameter of CIS is of 8.1 nm while our synthesised CIS QDs have thesize of 3.5 nm only). The stoke shift of 650 meV and the decay timeof 220 ns show that the observed PL could not be resulted from theexcitonic transition. As far as we know, until now there has beenno paper reported the excitonic PL from synthesised CIS QDs. Thisis understandable because the photogenerated electrons and holescould easily relax to the donor and acceptor levels and then theyrecombine to emit PL with the proper characteristics (e.g., the longdecay time, red-shift in time-resolved PL measurement with thedelay time from the excitation moment). We observed all thedonor–acceptor recombination characteristics from the PL of ourCIS and CIS/ZnS QDs, similar to the PL from recombination of theelectrons and holes in defect states reported in the literature[5,18,41,42,52,53]. Defects involved in the electron–hole recombi-nation processes could be Cu-vacancy or S-vacancy, In–Cu substi-tution [28,48]. These defects exhibit in different electronicstructures and related directly to the non-stoichiometry of the ter-nary compound. The optical properties of CIS QDs as a function ofCu:In molar ratio showed that with increasing the Cu:In molarratio to be 0.5, 0.8, 1.0, 1.2, 1.5 the peak energy of luminescence

//dx.doi.org/10.1016/j.optmat.2014.09.016

Fig. 6. The absorption and normalized PL spectra of CIS QDs synthesised at 210 �Cfor 15 min, with the Cu:In molar ratio of 1:1.

N.T.M. Thuy et al. / Optical Materials xxx (2014) xxx–xxx 5

became lower, namely the absorption peaks shifted to the lowerenergy of 1.85, 1.84, 1.83, 1.79, 1.76, respectively. A paper on thedetailed optical characteristics from the CIS and CIS/ZnS QDs withdifferent Cu:In molar ratios is currently under preparation.

4. Conclusion

In conclusion, CIS and CIS/ZnS core/shell QDs were synthesizedby heating-up the precursors in diesel. The use of diesel as the highboiling-point reaction solvent is very good from the practical pointof view because it provides a low-cost and large-scale procedurefor the synthesis of CIS and CIS/ZnS core/shell QDs. The absorptionand photoluminescence spectra have been used as indicators to thequantum confinement effect on the charge carriers and to the qual-ity of the synthesized CuInS2 QDs. The highest quality CuInS2 QDswere obtained at the reaction temperature of 210 �C for 15 min andthe Cu:In molar ratio of 1:1.

Acknowledgments

This work was supported by the National Foundation forScience and Technology Development, Vietnam (NAFOSTED, code103.06-2011.53, 09-Physics). We thank the National KeyLaboratory for Electronic Materials and Devices (VAST/IMS) forthe use of facilities.

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