A New Two-Phase Route to High-Quality CdS Nanocrystals

A New Two-Phase Route to High-Quality CdS Nanocrystals
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  A New Two-Phase Route to High-Quality CdS Nanocrystals Qiang Wang, [a, b] Daocheng Pan, [a] Shichun Jiang, [a] Xiangling Ji,* [a] Lijia An,* [a] andBingzheng Jiang [a] Introduction Owing to the quantum confinement effect, semiconductornanocrystals, especially the II–VI semiconductor nanocrys-tals, exhibit remarkable size-dependent optical properties, [1] which have attracted a great deal of attention in recentyears for both fundamental research and technical applica-tions such as light-emitting diodes (LED), [2–4] solar cells, [5,6] lasers [7] and biological markers. [8–10] In the past two decades,considerable effort has been made to synthesize high-qualitysemiconductor nanocrystals. Among the methods employedfor synthesizing dot-shaped semiconductor nanocrystals, theorganometallic approach [11–13] and its variations [14–17] haveproved the most popular, although other methods have alsobeen very successful. [10,18–20] For any applications based on the optical properties of nanocrystals, it is essential to use high-quality nanocrystals.In principle, high-quality nanocrystals should possess at leasttwo characteristics, high emission color purity and high pho-toluminescence quantum yield (PL QY). The color purity of the emission is strongly dependent on the size distributionof the nanocrystals. The narrower the size distribution, thepurer the color of the emission light, which can be reflectedby the narrow PL emission bandwidth and/or the narrowUV/Vis band-edge absorption bandwidth. [10–22] The PL QYis very sensitive to the surface environment of the nanocrys-tals, and can be dramatically reduced by surface trapstates. [22] These surface trap states result from the danglingbonds and/or stacking faults of some of the surfaceatoms. [10,22,23] Surface passivation with suitable organic or in-organic materials and an increase of the crystallinity can ef-fectively remove the surface trap states, and lead to a signifi-cant increase in the quantum yield.Among the II–VI semiconductor nanocrystals, CdSenanocrystals have been most extensively investigated, andtheir size and size distribution can be controlled, whereasrelatively little work has been done on CdS nanocrystals.Recently, high-quality CdS nanocrystals with a controllablesize and a narrow size distribution were prepared successful-ly in a noncoordinating solvent. [16] It was believed that main-taining a good balance between nucleation and growth bytuning the activities of the precursors was the key to thissuccess. Very recently, Pan et al. developed a two-phase ap-proach to successfully synthesize highly luminescent andnearly monodisperse CdS nanocrystals. [24] The reaction wascarried out under mild conditions (at below 100   C) with less Abstract:  A new two-phase route hasbeen developed to synthesize high-quality CdS nanocrystals with a narrowsize distribution and a high photolumi-nescence (PL) quantum yield (QY). Inthe two-phase system, toluene andwater were used as separate solventsfor cadmium myristate (CdM 2 ) andthiourea, which served as cadmiumsource and sulfur source, respectively,and oleic acid (OA) was used as aligand for stabilizing the nanocrystals.The reactions were completed in theheated autoclaves. The initial Cd/Smolar ratio of the precursors and thereaction temperature were found to befactors that affected the growth of nanocrystals. Furthermore, a seeding-growth technique was developed tosynthesize CdS nanocrystals of differ-ent sizes, which exhibit PL peaks withquite similar full width at half-maxi-mum (FWHM) values compared tothose of the initial nanocrystal seeds inall cases. Keywords:  cadmium sulfide  · nanotechnology  ·  seeding-growthtechnique  ·  semiconductors [a] Q. Wang, D. Pan, Dr. S. Jiang, Prof. X. Ji, Prof. L. An, Prof. B. JiangState Key Laboratory of Polymer Physics and ChemistryChangchun Institute of Applied ChemistryChinese Academy of Sciences, Graduate School of Chinese Academyof SciencesChangchun, 130022 (P. R. China)Fax:(   86)431-568-5653E-mail:[b] Q. WangDepartment of Chemistry, Northeast Normal UniversityChangchun, 130022 (P. R. China) Chem. Eur. J.  2005 ,  11 , 3843–3848  DOI: 10.1002/chem.200400993   2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  3843 FULL PAPER  toxic reagents than employed traditionally. The possiblemechanism was based on slow nucleation and slow growth;it is definitely different from a mechanism based on fast nu-cleation and slow growth as reported previously. In fact, thetwo-phase approach, in which the reaction occurs at the in-terface of two liquid phases, was first applied to the synthe-sis of gold nanocrystals by Brust et al. [25] in 1994. Based onthe work by Pan et al., we developed a new two-phase ap-proach to synthesize high-quality CdS nanocrysals in an au-toclave. Compared with the earlier results, the CdS nano-crystals obtained by the new approach exhibit a narrowersize distribution and a higher PL QY. Moreover, through aseeding-growth technique, tunable size CdS nanocrystalswere obtained with PL peaks with a quite similar full widthat half-maximum (FWHM = 18–22 nm) to those of the initialnanocrystal seeds throughout the whole controllable sizerange. In this study, cadmium myristate (CdM 2 ) and thio-urea were used as cadmium source and sulfur source, re-spectively, and oleic acid (OA) was used as a ligand for sta-bilizing the nanocrystals. It was found that the resultingnanocrystals, without any size sorting, appeared to be com-parable with the CdS nanocrystals reported previously. Results and Discussion Figure 1 shows the temporal evolution of the UV/Vis andPL spectra (a–d) and the corresponding PL peak positions(e) and FWHM (f), respectively, of the CdS nanocrystalsprepared at 180   C from precursors of four different initialCd/S ratios. In these reactions, the amount of the initial cad-mium precursor was kept constant (0.1134 g, 0.2 mmol),while that of the sulfur precursor was varied from 0.24 to1.0 mmol. The other experimental conditions were also keptconstant. It is found that the initial Cd/S ratio of the precur-sors affects the growth of the CdS nanocrystals. For thesame reaction time, a larger proportion of sulfur precursorresults in a faster growth of nanocrystals, accompanied by ared shift of the PL peak. For example, when the Cd/S ratiowas 1:5 and the reaction time was 0.5 h, the band-edge PLpeak of the nanocrystals was near 409 nm, whereas underthe same reaction conditions except for a Cd/S ratio of 1:1.2, the band-edge PL peak position of the nanocrystalswas near 390 nm, and the peak near 310 nm ascribed to the“magic size” nanoclusters, which are those clusters with nomore than one unit cell of the bulk crystal and close-shellstructures in the size range between 1 and 2 nm, [14b] was stillvery strong, which implies the existence of a high monomerconcentration in the system after the reaction had occurredfor 0.5 h when less sulfur precursor was used. [26] This is a ki-netics-driven result.In addition, at a reaction time between 1.0 and 1.5 h, thesamples exhibit a relatively narrow PL FWHM, implying anarrow size distribution; a lower sulfur monomer concentra-tion favors a narrower size distribution. For example, asample with a PL FWHM of 17 nm was obtained when theCd/S ratio was 1:2 and the reaction time was 1 h. When thereaction time exceeds 1.5 h, the PL FWHM of samples in-creases for all the reactions, and this increase is more dra-matic for the samples prepared at low sulfur monomer con-centrations. Such results suggest that Ostwald ripening startsfor the reactions up to 1.5 h owing to the decrease of mono-mer concentration to a critical threshold. [12] During Ostwaldripening, the large nanocrystals continuously grow, while thesmall ones shrink and eventually disappear, inevitably lead-ing to a wide size distribution of nanocrystals. In the two-phase system, the surface of the nanocrystals is usually cov-ered by oleic acid (OA), which facilitates the dispersion of the nanocrystals in the oil phase. However, it is observedthat when the reaction time is further extended, largerorange particles are formed due to Ostwald ripening andthose drop into the water phase due to the loss of surface li- Figure 1. Temporal evolution of the UV/Vis ( c ) and PL ( g ) spectra(a–d) and the corresponding PL peak positions (e) and FWHM (f), re-spectively, of CdS nanocrystals prepared at 180   C from precursors of dif-ferent initial Cd/S ratios.  2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  Chem. Eur. J.  2005 ,  11 , 3843–3848 3844  gands, while the smaller ones are left in the oil phase, whichare used as the samples for measuring UV/Vis and PL spec-tra. This is the reason why the wavelength and/or FWHM of some of the PL peaks for the nanocrystals obtained after alonger reaction time become smaller (see Figure 1 c, d).The effect of the reaction temperature on the growth of nanocrystals is also significant. Figure 2a and b show a tem-poral evolution of the UV/Vis and PL spectra of CdS nano-crystals grown at 150 and 120   C, respectively. In these twocases the reaction conditions were the same as those for thegrowth of the nanocrystals shown in Figure 1b except forthe reaction temperature. The nanocrystals obtained atlower reaction temperature tend to show a wider PL peak.For example, the smallest PL FWHM we obtained for thenanocrystals grown at 150 and 120   C was 20 nm and 24 nm,respectively. This suggests that use of a low temperature is adisadvantage for the synthesis of the nanocrystals with anarrow size distribution. This is further supported by thetransmission electron microscope (TEM) images (Figure 3a–c) and the corresponding size distribution histograms(Figure 3 d–f) of the nanocrystals obtained under differenttemperatures. The reason may be that lower temperaturesresult in a worse mixing of the two phases. Thus, there arealways some of the small particles unable to grow into largeones due to a lower probability of their arrival at the inter-face from the oil phase. This is responsible for the broaden-ing of the size distribution at lower temperatures. On theother hand, a lower reaction temperature can postpone theoccurrence of Ostwald ripening, and the range of tunablesize can be widened. Compared with the samples obtainedat 180 and 150   C, the nanocrystals prepared at 120   C cangrow for up to 24 h, and the PL peak can shift to 473 nmwithout a significant widening of the FWHM. In fact, thedecomposition rate of thiourea is temperature-dependent; ahigher temperature can accelerate the decomposition of thi-ourea, which gives rise to a rapid formation of more nucleiat the oil/water interface, so the reaction monomers are de-pleted faster due to the growth of nuclei into nanocrystals.As a result, Ostwald ripening occurs faster at high tempera-ture. Unlike single-phase systems, in which nucleation andgrowth occur separately at two different temperatures, andthus lead to nanocrystals with a narrow size distribu-tion, [11–17] in a two-phase system, nucleation and growth arerather complicated, and their separation is likely to be de-pendent on the concentrations of the remaining monomersin both phases. In fact, during the nucleation stage, nuclea-tion and growth in the two-phase system take place simulta-neously, although the former is dominant. This is supportedby the formation of a broad PL peak and the coexistence of a first excitonic absorption peak near 368 nm and a strongabsorption peak near 310 nm attributable to “magic size”nanoclusters, a kind of stable nuclei, at short reaction times(see Figure 1a). A similar phenomenon was observed in thework by Pan et al., where the absorption peak due to magicsize nanoclusters was maintained for over 30 min. [24] Howev-er, when the concentration of the remaining monomersdrops to a critical threshold, the growth of the nuclei be-comes predominant. Owing to their higher energy and lowerstability, smaller particles always grow faster than largerones before Ostwald ripening occurs. As long as the mono-mers needed for the growth of the smaller particles can be Figure 2. Temporal evolution of the UV/Vis ( c ) and PL ( g ) spectraof CdS nanocrystals grown at 150   C (a) and 120   C (b), respectively.Figure 3. TEM images of CdS nanocrystals prepared at 180   C (a), 150   C(b), and 120   C (c), and their corresponding size distribution histograms(d–f), respectively. Chem. Eur. J.  2005 ,  11 , 3843–3848   2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  3845 FULL PAPER High-Quality CdS Nanocrystals  provided adequately at the oil/water interface, the focusingof the size distribution can be realized. [12] Of particular interest is that a high-temperature processgenerates CdS nanocrystals that exhibit a high PL QY. Forexample, as the reaction temperature increases from 120 to150 and 180   C, the maxima of the PL QYs of the resultingnanocrystals gradually increase from 34 to 48 and 57%, re-spectively. The reason for this may be that at a higher tem-perature the molecular species on the nanocrystal surfaceare more likely to readjust their position to reach an optimalsurface structure, and therefore decrease stacking faults andsurface disorder. [22] The above study shows that a high reaction temperaturefavors the synthesis of CdS nanocrystals with a narrow sizedistribution, but their tunable size range is narrow beforeOstwald ripening occurs. On the other hand, at a low reac-tion temperature, a wide-range size control can be realized,but a narrow size distribution is difficult to achieve. There-fore, it is a challenge to synthesize nanocrystals with both anarrow size distribution and a wide tunable size range.Here, a seeding-growth technique has been developed toachieve this goal. Similar to the procedure mentionedabove, an initial Cd/S ratio of 1:2 (0.2 mmol of CdM 2  and0.4 mmol of thiourea) and a reaction temperature of 180   Cand a reaction time of one hour were selected for the syn-thesis of small-size nanocrystals as initial “nanocrystalseeds”, which are denoted as “a”. A fresh mixture of theprecursors was mixed with the srcinal oil phase containingthe nanocrystal seeds “a” for the completion of the firstgrowth reaction, and the resulting nanocrystals were denot-ed as “b” and used as seeds for the next growth reaction.The reaction time used here for the growth reactions is1.5 h. This growth cycle was repeated by using nanocrystalsfrom the previous cycle as seeds, and the resulting nanocrys-tals were denoted respectively as “c, d···” in sequence (seeExperimental Section for details). The as-prepared CdSnanocrystal colloidal solution was used after dilution withtoluene for the measurement of the optical properties with-out any other post-treatment, and the temporal evolution of their UV/Vis and PL spectra is shown in Figure 4. It isfound that all the PL peaks have rather narrow FWHMvalues (18–22 nm), suggesting that our approach is very ef-fective in synthesizing nearly monodisperse nanocrystalswith different sizes. This is further demonstrated by theTEM images shown in Figure 5. In addition, the high-resolu-tion transmission electron microscopy (HRTEM) imageshown in the inset of Figure 5 confirms the highly crystallinenature of the nanocrystals, which are free from stackingfaults. The PL QYs of a solution of these nanocrystals in tol-uene were also assessed at room temperature by using 9,10-diphenylanthracence (PL QY 90% in cyclohexane) as a ref-erence. [27] A monotonic decrease is observed with the de-crease of the nanocrystal size (see the inset of Figure 4).Moreover, the average sizes of the nanocrystals were esti-mated according to the literature, [16,28] and found to rangefrom 3.2 nm to 6.0 nm. Figure 6 shows the wide-angle X-raydiffraction (WAXD) patterns of the CdS nanocrystals pre- Figure 4. UV/Vis ( c ) and PL ( g ) spectra of CdS nanocrystals ob-tained by the seeding-growth technique at 180   C. The average sizes of nanocrystals are 3.2 nm (a), 3.8 nm (b), 4.3 nm (c), 4.6 nm (d), 4.9 nm (e),5.3 nm (f), 5.5 nm (g), 5.8 nm (h) and 6.0 nm (i), respectively. Inset: PLQY of different-sized CdS nanocrystals versus their emission peak posi-tions relative to 9,10-diphenylanthracence.Figure 5. TEM and HRTEM images of CdS nanocrystals synthesized bythe seeding-growth technique at 180   C.Figure 6. WAXD patterns of CdS nanocrystals synthesized at 180   C (a),120   C (b) and 150   C (c), and by the seeding-growth technique at 180   C(d).  2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  Chem. Eur. J.  2005 ,  11 , 3843–3848 3846 X. Ji, L. An et al.  pared at different temperatures, confirming that all the sam-ples obtained belong to the cubic zinc blende structure.It is also noted in Figure 4 that the degree of red shift forthe PL emission in each growth cycle is uneven, and theshifts in the first three cycles are larger than those in theother cylces, although the new reaction monomers in termsof mass are identical in each of the seeding-growth reactions(see Experimental Section for details). When we set the re-action time to 1 h instead of 1.5 h for every growth reaction,similar results were obtained to that described in Figure 4,except that the degree of red-shift of the PL peaks in thefirst three cycles was smaller. Therefore, it is necessary toadjust simultaneously the amount of new monomers and re-action time to obtain nanocrystals with the desired wave-length of PL emission [29] (Figure 7). In contrast, in thegrowth reactions, either excessively increasing the amountof new monomers or excessively reducing the amount of added nanocrystal seeds, or using a too short reaction timeis likely to generate new nuclei, resulting in two emissionpeaks in the PL spectra of the nanocrystals. Too long agrowth reaction time can easily lead to a broadening of thenanocrystal size distribution, which is not desirable. Conclusion In conclusion, a new two-phase approach has been devel-oped for synthesizing high-quality CdS nanocrystals with arelatively narrow size distribution and a high PL QY up to57%. By using a seeding-growth technique, a relatively widecontrollable size range of nanocrystals can be achieved anda focusing of the size distribution can be maintainedthroughout the whole size range. Compared with the tradi-tional route, the approach can be performed at relativelylow temperature without the need for stirring and hot-injec-tion. In addition, the approach may be applied to the syn-thesis of other semiconductor nanocrystals; in particular, theseeding-growth technique also provides a novel and feasibleapproach for the synthesis of high-quality nanocrystals withcore/shell structure, work on which is currently underway. Experimental Section Materials : Cadmium oxide (CdO) (99.5%), myristic acid (MA) (99.5%),oleic acid (OA) (technical grade, 90%), and thiourea were purchasedfrom Aldrich. 9, 10-Diphenylanthracene was obtained from Acros, andmethanol and toluene from the Beijing chemical company in China. Cad-mium myristate (CdM 2 ) was prepared according to following method:CdO (1.926 g, 15 mmol) and MA (7.5 g, 33 mmol) were loaded into a re-action flask and heated at 220   C for 10 min to produce an optically clearsolution. The CdM 2  was obtained. The crude product was re-crystallizedtwice from toluene for use in further reactions. Synthesis : Typically, a mixture of CdM 2  (0.1134 g, 0.2 mmol), oleic acid(1 mL), and toluene (10 mL) was first loaded into the Teflon liner of a30-mL stainless steel autoclave, and then heated at 80–100   C to producean optically clear solution. After the heating had been turned off and thesolution was allowed to cool to room temperature, an aqueous solutionof thiourea (10 mL; 0.0183–0.0761 g, 0.24  ~ 1.0 mmol for different Cd/Smolar ratio) was added to the organic solution to form a two-phase reac-tion system. The Teflon liner containing the mixture was then sealed inthe stainless steel autoclave and maintained at the desired temperature(e.g. 180, 150, or 120   C) for a fixed reaction time. The autoclave wascooled naturally to ambient temperature after a given reaction time.In the reactions using the seeding-growth technique, an initial Cd/S ratioof 1:2 (0.2 mmol of CdM 2  and 0.4 mmol of thiourea), a reaction tempera-ture of 180   C, and a reaction time of one hour were selected to synthe-size small-size nanocrystals as initial “nanocrystal seeds”. Then, a freshmixture of CdM 2  (0.0113 g, 0.02 mmol), OA (0.5 mL), and toluene(1 mL) was placed into the Teflon liner of another stainless steel auto-clave and heated at 80–100   C to produce an optically clear solution, towhich 9 mL of the srcinal oil phase containing the above-mentionednanocrystal seeds was added, and the mixture was cooled to ambienttemperature before 10 mL of a new aqueous solution of thiourea(0.0183 g, 0.24 mmol) was injected into the Teflon liner. Finally, theTeflon-lined stainless steel autoclave containing the two-phase mixturewas sealed and maintained at 180   C for 1 or 1.5 h without any stirringfor the completion of the first growth reaction, and the resulting nano-crystals were used as seeds for the next one. The following growth reac-tions were carried out analogously to the first one, and the nanocrystalsobtained from the last growth reaction were always used as seeds for thenext growth reaction. The as-prepared CdS nanocrystal colloidal solutionwas diluted with toluene and then the optical properties were investigat-ed without any other post-preparative treatment. Characterization : UV/Vis absorptions were recorded on a Shimadzu UV-2450 spectrometer. The photoluminescence (PL) spectra were recordedon a Shimadzu RF-5301 PC fluorometer with an excitation wavelength of 340 nm. Room-temperature photoluminescence quantum yields (PL QY)were calculated according to Eaton et al., [27] whereby 9,10-diphenylan-thracene in cyclohexane as was used as the reference (QY = 90%). Theabsorbance of the sample and the reference at the excitation wavelength(340 nm) are similar and smaller than 0.1, thus avoiding self-absorbance.The full width at half-maximum (FWHM) of PL peaks was obtained byapplying the program “Fit Gaussian” in the software “Origin 7.0”. Thenanocrystals were precipitated with ethanol and then isolated by centrifu-gation and decantation. The purified nanocrystals were used for TEMand wide-angle X-ray diffraction (WAXD) analyses. The WAXD pat-terns were recorded on a Japan Rigaku D/max-2500 X-ray diffractometerwith Cu K a  radiation (  l = 1.5418 ). The low-resolution TEM images wererecorded on a JEM-2000FX transmission electron microscope with an ac-celerating voltage of 160 kV. High-resolution TEM (HRTEM) imageswere recorded by using a JEM-2010 microscope with an acceleratingvoltage of 200 kV. The size distribution histograms were obtained bymeasuring more than 300 individual CdS nanocrystals on enlarged photo-graphs.Figure 7. PL spectra of CdS nanocrystals grown under different experi-mental conditions by the seeding-growth technique at 180   C. [29] Chem. Eur. J.  2005 ,  11 , 3843–3848   2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim  3847 FULL PAPER High-Quality CdS Nanocrystals
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