Ligand-Induced Gold Nanocrystal Superlattice Formation in Colloidal Solution

Introduction. During the past decade, nanocrystal research has been focused on two major properties of finite size materials, namely, quantum size effects and surface/interface effects.1 A new trend, however, has emerged in the past few years to arrange the nanocrystals into two or three-dimensional superlattices.2 The ability to synthesize nanocrystal superlattice (NCS) structures will provide a new horizon to study the collective properties due to the particle interaction and develop future optical3 and information storage devices.4 Several groups are striving to control nanocrystal size through size selective precipitation methods and create NCSs through evaporation of colloids containing lyophobic nanocrystals on a substrate. Face-centered cubic CdSe NCSs were synthesized through reducing the solubility of particles by slowly evaporating one component of a solvent mixture.5 Oxide6 and sulfide7 NCSs were spontaneously formed by evaporating the colloid on a substrate. Linking molecules have also been used to form two-dimensional gold NCSs.8 Narrow size latex particles and nanometer ferritin molecules were selfassembled in thin liquid films with thickness comparable to the size of the particles.9 All these methods of forming NCSs rely on modifying the nature of the nanocrystal surface and its surrounding environment and therefore control the interaction between different particles. Other physical means such as electrophoretic deposition were also explored to form NCSs on a substrate10a,b and on chemically modified silica surfaces.10c Recently, Whetten et al.11 used a fractional crystallization technique to create gold and silver nanoparticles with a very narrow size distribution, and NCSs with different structures were formed spontaneously after evaporating the solvent. However, exactly how important the condition of narrow size distribution in forming NCSs remains a question. Furthermore, except for the recent work involving CdSe NCSs by Murray et al.,5 the other NCSs that have been reported so far were formed on a small substrate by evaporating the solvent. Forming NCSs of different materials directly in the colloid still remains a challenge. Although several important properties of NCSs, including dipolar coupling12 and long wavelength shifting of the fluorescence band5 in CdSe superlattice, have been reported, properties of NCSs remain largely unexplored. In this paper, we describe an inverse micelle synthesis13 to create a gold nanocrystal colloid of relatively narrow size distribution. Several ligands which contain thiol, sulfide, and amino functional groups were chosen to further modify the surface of the existing gold nanocrystals. These ligands were chosen because the functional groups they contain have fairly good affinity for metal surfaces. We found that certain ligands added to the colloid will induce a spontaneous and self-size-selective NCS formation. Bulk quantities of gold NCS structures were directly synthesized in the colloid and their optical properties were studied. Experimental Section. Didodecyldimethylammonium bromide (DDAB) was purchased from Fluka and used as received. Sodium borohydride, gold chloride (99.99%), dioctyl sulfide, and dodecanethiol were ordered from Aldrich. Gold chloride (99%) was purchased from Fisher. Toluene was purchased from Fisher and was further purified by distillation. Hexylamine was purchased from Alfa and used as received. Deionized distilled water was obtained from a Barnstead Nanopure system. Toluene and distilled water were first degassed by bubbling with dry argon gas for 2 h prior to the experiment. The degassing process is a necessary step for a well-controlled growth of nanoparticles, as pointed out by Wilcoxon et al.13 All the synthetic steps were carried out in a dry argon environment in order to prevent chemicals from absorbing moisture. A 0.75 g portion of DDAB was dissolved in 5.2 mL of toluene to form a 0.35 M micelle solution. A 15 mg sample of gold chloride was then dissolved in 5 mL of the micelle solution by sonication to form a dark orange solution. A 17 μL aliquot of 9.4 M aqueous NaBH4 solution ([BH4]:[Au] ) 3:1) was added while the solution vigorously stirred. The solution first decolored and then turned to red after about 20 s. The mixture was stirred for 15 min to ensure a complete reaction. The gold (1) Alivisatos, A. P. Science 1996, 271, 933. Shi, J.; Gider, S.; Babcock, D.; Awschalom, D. D. Science 1996, 271, 937. (2) Weller, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1079. (3) Xia, Y.; Kim, E.; Mrksich, M.; Whitesides, G. M. Chem. Mater. 1996, 8, 601. Kamar, A.; Whitesides, G. M. Science 1994, 263, 601. (4) Jin, B. Y.; Ketteren, J. B. Adv. Phys. 1988, 38, 189. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (6) Yin, J. S.; Wang, Z. L. Phys. Rev. Lett. 1997, 79, 2570. Yin, J. S.; Wang, Z. L. J. Phys. Chem. B 1997, 101, 8979. (7) Motte, L.; Billoudet, F.; Lacaze, E.; Pileni, M. P. Adv. Matter. 1996, 8, 1018. Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. 1997, 101, 138. (8) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; James, D. B.; Lolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (9) Nagayama, K. Colloids Surf. 1996, 109, 363. (10) (a) Trau, M.; Saville, D. A.; Aksay, I. A. Science 1996, 272, 706. (b) Trau, M.; Saville, D. A.; Aksay, I. A. Langmuir 1997, 13, 6375. (c) Tian F.; Klabunde, K. J. New J. Chem. Invited paper, in press. (11) Whetten, R. L.; Khoury, J. 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