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Langmuir 2004, 20, 978-983
Metal Nanocrystal Superlattice Nucleation and Growth Michael B. Sigman, Jr., Aaron E. Saunders, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712-1062 Received July 31, 2003. In Final Form: October 9, 2003 Thin films of dodecanethiol-passivated Au and Ag nanocrystals drop cast from different solvents were examined by high-resolution scanning electron microscopy (HRSEM). C12-coated Au and Ag nanocrystals, 5-7 nm in diameter, form face-centered cubic (fcc) superlattices oriented with the (111)s planes (subscript s denoting superlattice) parallel to the substrate when deposited from good solvents, such as hexane, chloroform, and toluene. The gross morphology of the films depended on the solvent: hexane produced rough superlattice films whereas chloroform deposited smooth films. The difference in interparticle attraction, which is approximately 20% higher in hexane, appears to give rise to the difference in film morphology. Addition of a poor solvent to the dispersion prior to drop casting led to superlattices with decreased order. Although the superlattices always orient with (111)s as the basal plane on the substrate, superlattices deposited from chloroform grow preferentially in the [110]s direction, whereas hexane deposits superlattices that grow primarily in the [111]s direction.
Introduction Small-angle X-ray scattering has revealed that sizemonodisperse (σ j (10%) organic monolayer-coated silver nanocrystals spontaneously self-assemble into superlattice thin films when drop cast from concentrated dispersions.1-3 Superlattice crystallization is a thermodynamically driven process, with the lattice structure depending primarily on the size distribution and interparticle interactions.2-5 For crystallization to occur during the time allowed by solvent evaporation, the energetic barrier to superlattice nucleation must be overcome and the nanocrystals must have enough diffusional freedom to reorient into their lowest energy lattice positions after condensing from solution. This appears to be the case for monodisperse nanocrystals drop cast from good solvents. Nonetheless, the deposition conditions (i.e., the solvent polarity, temperature, capping ligand, etc.) can significantly affect the crystallization kinetics and the corresponding superlattice film morphology. This is analogous to the case of epitaxial thin films grown by molecular beam epitaxy or chemical vapor deposition, where the growth conditions affect the nature of the nucleation and growth processes that determine the morphology of the film. Here, we present high-resolution scanning electron microscopy (HRSEM) images of superlattice thin films deposited from various solvents and conditions to examine the underlying crystallization kinetics of nanocrystal organization. Deposition of nanocrystals by evaporating a good solvent leads to heterogeneous superlattice nucleation and thin film growth on the (111)s plane. Significant differences in superlattice morphology are observed with deposition from different solvents, such as toluene, hexane, and chloroform, and it appears that subtle differences in interparticle attraction lead to significant differences in * Corresponding author. E-mail:
[email protected]. Telephone: (512) 471-5633. Fax: (512) 471-7060. (1) Connolly, S.; Fullam, S.; Korgel, B.; Fitzmaurice, D. J. Am. Chem. Soc. 1998, 120, 2969. (2) Korgel, B. A.; Fitzmaurice, D. Phys. Rev. Lett. 1998, 80, 3531. (3) Korgel, B. A.; Fitzmaurice, D. Phys. Rev. B 1999, 59, 14191. (4) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379. (5) Ge, G.; Brus, L. J. Phys. Chem. B 2000, 104, 9573.
superlattice nucleation and growth kinetics. We also show that antisolvent addition increases interparticle attraction but does not lead to homogeneous superlattice nucleation during the evaporative drying process, only to the deposition of thin films of spatially disordered nanocrystal films. However, an input of energy by sonication during evaporative drying does promote homogeneous superlattice nucleation during the drop casting process. Experimental Section Dodecanethiol-stabilized gold and silver nanocrystals were produced by arrested precipitation.4,6 All chemicals were used as supplied from Aldrich Chemical Co. Gold Nanocrystal Synthesis. An aqueous gold tetrachloroaurate solution (0.38 g of HAuCl4‚3H2O in 36 mL of D-H2O) is combined with an organic solution of the phase transfer catalyst tetraoctylammonium bromide (2.7 g of [CH3(CH2)7]4NBr in 24.5 mL of toluene). The mixture is stirred for approximately 1 h before discarding the aqueous phase. The organic solution containing the metal ion/surfactant molecules is then combined with an aqueous solution of the reducing agent sodium borohydride (0.5 g of NaBH4 in 30 mL of D-H2O). After stirring for 4 to 12 h, the aqueous phase is discarded. 240 µL of dodecanethiol (C12H25SH) is then added to the nanocrystal dispersion. Silver Nanocrystal Synthesis. An aqueous solution of silver ions (0.19 g of AgNO3 in 36 mL of D-H2O) was added to an organic solution containing a phase transfer catalyst, tetraoctylammonium bromide (2.7 g of [CH3(CH2)7]4NBr in 24.5 mL of chloroform). The mixture is stirred for approximately 1 h before discarding the aqueous phase. 240 µL of dodecanethiol (CH3(CH2)11SH) is added to the organic phase and stirred for 5-10 min before mixing with an aqueous solution of the reducing agent, sodium borohydride (0.5 g of NaBH4 in 30 mL of D-H2O). The two-phase mixture is stirred for 4-12 h before collecting the organic phase for nanocrystal purification. Postsynthesis Preparation. After synthesizing the gold and silver nanocrystals, ethanol is added as an antisolvent to precipitate the hydrophobic nanocrystals. The particles are isolated from the supernatant by centrifugation. After this cleaning step to remove residual phase transfer catalyst and other reaction byproducts, the nanocrystal size distribution is narrowed (
