Synthesis of Zinc Sulfide Nanocrystals and Fabrication of Nanocrystal

Materials Transactions, Vol. 45, No. 8 (2004) pp. 2650 to 2652 #2004 The Japan Institute of Metals

Synthesis of Zinc Sulﬁde Nanocrystals and Fabrication of Nanocrystal Superlattice*1

Toshihiro Kuzuya1;*2, Yutaka Tai2, Saeki Yamamuro1, Takehiko Hihara1, Dong Liang Peng1 and Kenji Sumiyama1

1Department of Materials Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan 2National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan

We report synthesis of zinc sulfide nanocrystals (NCs) via formation of polymetallic thiolate cages. Nearly monodisperse ZnS NCs with size ranging from 2.2 to 7 nm were obtained by thermolysis of S-Zn-dodecanethiol precursors. The electron diffraction pattern of zinc sulfide NCs indicates that precipitates are wurtzite or mixture of wurtzite and zincblende. TEM observation and UV-vis spectra reveal that the growth rate of ZnS NCs considerably depends on the annealing temperature. UV-vis spectra of ZnS NCs with size smaller than 3 nm show sharp excitonic features and a large blue shift from the bulk material. The photoluminescence spectra exhibit large red shift from the absorption band edges, being attributed to electron-hole recombination by surface traps. The narrow size distribution of ZnS NCs leads to formation of ordered self-assemblies with various well-defined structures, where non-closed-packing structure is predominant.

(Received April 28, 2004; Accepted June 30, 2004) Keywords: monodisperse nanocrystals, polymetallic thiolate cage, zinc sulﬁde, superlattice

1. Introduction semiconductors1,2) and transition metals.7,8) This process can provide nearly monodisperse ZnS NCs with size ranging Transition-metal chalcogenide nanocrystals (NCs) have from 2 to 7 nm. This colloid system enables us to observe been extensively investigated for the potential application to quantum confine effects and fabricate ZnS NCs superlattices. catalyst, solar cell, photoluminescence and optical devices.1,2) II-VI semiconductors such as ZnS, ZnSe, CdS, 2. Experimental Procedure CdSe and CdTe are well known to be prominent photo- and electro-luminescence materials. In the last decade, these II- 2.1 Synthesis process VI NCs have been synthesized by various solution phase All reagents were used as received. Zinc (II) chloride/ methods such as inverse micelle, arrested precipitation and methanol solution and sulfur/1-dodecanethiol solution were hot soap processes. Especially, the hot soap process can mixed with 20 ml of octyl ether in a flask, which was provide high quality NCs, which have sharp size distribution degassed by Ar gas flushing for 7.2 ks, at 333 K (Condition- and bright emission. ing). If the reaction temperature was higher than 473 K, the Zinc sulfide is a direct band gap semiconductor material, mixture solution was heated at 473 K for 30 minutes to which has a wide band gap (3.73 eV) and large bond energy eliminate hydrated water (Dehydration). Then, 1.88 cm3 of of exciton (37 meV). These properties indicate that ZnS is a oleylamine was injected into the mixture solution and heated promising candidate for UV-blue and exciton effect luminous up to set temperature (Annealing). materials. Quantum effects such as exciton confinement and size-dependent of band gap should be observed in this NC 2.2 Purification process system. Many reports detailing syntheses of ZnS quasi 2- After the mixture solution was cooled down to room dimensional system are available. In the case of quasi-zero temperature, 200 cm3 of ethanol was mixed. The white dimensional system, synthesis of monodisperse NCs is a key precipitates were separated by centrifuging the colloidal issue for investigation of ZnS quasi-zero dimensional system. solution to remove excess reaction agents and then redis- Low temperature syntheses3,4) in methanol enables us to persed in hexane. This precipitate-redispersion procedure provide very small ZnS NCs. Reverse micelle techniques5,6) was repeated several times to purify the precipitates. It was have been investigated to synthesize nearly monodisperse focused to control the size distribution of NCs (size-selective ZnS NCs in the last decade. However, to our knowledge, process). there is few published reports detailing synthesis of ZnS quasi-zero dimensional system with a narrow size distribu- 2.3 Characterizations tion, which can lead to the formation of a superlattice with a To prepare a sample for transmission electron microscope well-defined structure. (TEM) observation, a drop of hexane solution of NCs was Here, we report a solution route process of metal placed on a carbon-coated TEM grid. The grids were then chalcogenide NCs in a high boiling temperature organic examined with field emission TEM (Hitachi, HF-2000) solvent. It is based on the hot soap process, which has been operating at 200 kV. MALDI-TOF MS was performed on a developed to produce monodisperse NCs of chalcogenide TOF mass instrument (BRUKER DALTONICS, autoflex) using anthracene as the matrix. Nanocrystals were dispersed *1This Paper was Presented at the Spring Meeting of the Japan Institute of in hexane, which contained the matrix and deposited onto a Metals, held in Tokyo, on March 31, 2004 stainless steel target plate. Desorption and ionization of ZnS 2 * Corresponding author, E-mail: [email protected] NCs were achieved by irradiation of N2 laser (337 nm). Synthesis of Zinc Sulfide Nanocrystals and Fabrication of Nanocrystal Superlattice 2651

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Fig. 1 TEM image of ZnS NCs derived from Zn-dodecanethiol precursor Fig. 2 TEM image of ZnS NCs annealed at 503 K (a), 553 K (b), 553 K (S/ (a), HRTEM image of ZnS NCs(a) (b), ZnS NCs derived from S-Zn- Zn = 0.4/1) (c) and Electron diffraction pattern o ZnS NCs (d). These NCs dodecanethiol precursor (S/Zn = 1/1) (c)–(d), ZnS NCs synthesized with were annealed for 360 min. oleic acid (c) and oleylamine(d). These NCs(a)–(d) grew at 553 K for 120 h. the growth rate considerably depends on the annealing 3. Results and Discussion temperature and S/Zn ratio. In this experiment, ZnS NCs with various sizes were synthesized. The TEM images of ZnS A synthesis of zinc sulfide NCs via a thiol precursor was NCs are shown in Figs. 2(a), (b), (c) and (d). The average examined. TEM image of white precipitates obtained by size, which was estimated by the mass spectrum and TEM thermolysis of Zn-dodecanethiol precursors is shown in Figs. image, is about 2.2 nm for NCs treated at 463 K (a), 3.0 nm 1(a) and (b). These figures reveal the formation of irregular for 503 K (b) and 6.2 nm for 553 K (c). The TEM image shape NCs. The S-Zn-thiol precursors, which are so-called indicates that morphology of 6.2 and 7 nm NCs (not shown) ‘‘Herron’s or Dance’s cluster compounds’’,9,10) were also exhibits an ellipsoidal shape with aspect ratio of about 1.2. examined. To synthesize S-Zn-thiol precursors, sulfur pow- The standard deviations of 6.2 and 7 nm NC were estimated der was added to Zn/dodecanethiol system. Excess dodec- by the image analysis to be about 15% of the average anethiol reduced elemental sulfur to S2 ions, which react diameter. The formation of relatively well-defined NC lattice with Zn-dodecanethiol complexes to form SlZnm(SR)n as shown in Figs. 2(a) and (b) indicates that standard cluster compounds. The MALDI-TOF analysis reveals that deviation of 2.2 nm and 3 nm NCs is smaller than 10% of ZnS clusters with a mass number of 5000 were formed in average diameter. We can synthesize nearly monodisperse conditioning time, and these clusters compounds were ZnS NCs with size ranging from 2.2 to 7 nm. Figure 2(d) annealed in octyl ether at 463560 K. shows the SAED pattern of 7 nm ZnS NCs. The peaks When the annealing temperature exceeded 400 K, the corresponding to h(103) and weak ring pattern corresponding mixture solution slowly became cloudy. Annealing without to h(102), which are allotted to wurtzite (hexagonal symme- co-surfactant leads to the aggregation of small size NCs (not try), are also observed. Therefore, these precipitates are shown). Therefore, cationic and anionic co-surfactants were added to prevent the aggregation of ZnS NCs. The color of (b) mixture solution immediately changed to color-less, as soon 7.0nm (a) 2.2nm 3.0nm 3 3.0nm as 1.88 cm of oleylamine was injected into it at 463 K. 2.2nm 6.2nm 7.0nm However, when oleic acid was injected to the mixture Absorbance (a. u.) solution, color change of mixture solution occurred at higher Absorbance (a. u.) PL Intensity (a. u.) temperature (560 K). Figures 1(c) and (d) show that these co-surfactants play an important role to produce a monodisperse ZnS NCs. Co-surfactants such as oleic acid and oleylamine leads to the dispersion of NCs. However, oleylamine is more effective for producing monodisperse NCs, 200 250 300 350 200 300 400 500 600 Wavelength, W/nm Wavelength, W/nm because an amine group adsorbs Zn or S site of NC’s surfaces more strongly than a carboxylic group. Fig. 3 UV-vis spectra (a) and Photoluminescence (b) of ZnS NCs with size LDI-TOF mass spectra and UV-vis spectra revealed that ranging from 2.2 to 7 nm. 2652 T. Kuzuya et al.

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Fig. 4 TEM image of ZnS Superlattice. Low magnification of ZnS NC superlattice (a), Closed packing structure (b) and non-closed packing structure (c). mixture of zincblende and wurtzite or wurtzeite. larger scale using a dropping cast technique. UV-vis spectra of ZnS NCs shown in Fig. 3(a) reveal the quantum effect such as exciton confinement and broadening Acknowledgements of band gap energy. The absorption edges are shifted to higher energies from the bulk band gap of ZnS with the The authors wish to express their sincere thanks to Dr. T. decrease in the average size of NCs. The absorption spectrum Uchida and Dr. Y. Fukunaka for their use of a centrifugal of 2.2 nm ZnS NCs and 3.0 nm exhibits sharp excitonic peak separator and their insightful discussion. This work was centered at 280 and 294 nm, which also indicates the supported by a grant from the NITECH 21st Century COE formation of monodisperse NCs. The photoluminescence Program, ‘‘World Ceramics Center for Environmental Har- spectra are shown in Fig. 3(b). These spectra exhibit a large mony’’ given by the Ministry of Education, Science, Culture red shift from the absorption band edges, being attributed to and Sports, Japan, and a Grant-in-aid for Intellectual Cluster electron-hole recombination by surface traps. Project Aichi Prefecture and Aichi Science and Technology The colloidal solution drops were dispersed on a carbon- Foundation. coated grid (substrate) to prepare ZnS NC superlattices (multilayer). The TEM images are demonstrated in Figs. 4(a), (b) and (c). Figure 4(a) indicates that ZnS NCs form REFERENCES micrometer order multi-domain superlattices. As shown in 1) C. B. Murray, C. R. Kagan and M. G. Bawendi: Science 270 (1995) Figs. 4(b) and (c), well-defined arrays are observed in ZnS 1335–1338. NC assemblies, where closed packing (CP) (Fig. 4(b)) and 2) A. P. Alivisatos: Science 271 (1996) 933–937. non-closed packing (NCP) (Fig. 4(c)) structures coexist. In 3) R. Rossetti, R. Hull, J. M. Gibson and L. E. Brus: J. Chem. Phys. 82 NCP structure, capped ZnS NCs tend to occupy two fold (1985) 552–559. saddle sites of a monolayered hcp lattice. However, they are 4) H. Zhang, B. Gilbert, F. Huang and J. Banfield: Nature 424 (2003) 1025–1028. energetically less favorable than that of three-fold hollow 5) T. Kubo, T. Isobe and M. Senna: J. Lumin. 99 (2002) 39–45. sites under the interactions between van der Waals attraction 6) T. Hirai, H. Sato and I. Komasawa: Ind. Eng. Chem. Res. 33 (1994) and short-range steric repulsion of alkyl-chain. This stripe 3262–3266. array may be attributed to a surface dipole-dipole interaction 7) V. F. Puntes, K. M. Krishnan and A. P. Alivisatos: Science 291 (2001) between R-S-Mþ dipoles11) or the crystal habit of NCs.12) 2115–2117. 8) C. B. Murray, S. Sun, H. Doyle and T. Betley: MRS Bulletin 26 (2001) The packing structure of these NCs is currently under 985–991. investigation. 9) B. Krebs and G. Henkel: Angew. Chem. Int. Ed. Engl. 30 (1991) 769– 788. 4. Conclusion 10) N. Herron, J. C. Calabrese, W. E. Farneth and Y. Wang: Science 259 (1993) 1426–1428. 11) J. Fink, C. J. Kiely, D. Bethell and D. J. Schiffrin: Chem. Mater. 10 In conclusion, monodisperse and stoichiometrically well- (1998) 922–926. defined zinc sulfide NCs were successfully tailored with the 12) B. Korgel, S. Fullam, S. Connolly and D. Fitzmaurice: J. Phys. Chem. B thermolysis of transition metal–thiol precursor. They are 102 (1998) 8379–8388. expected to form make NC superlattices for optical devices in