J. Phys. Chem. B 2001, 105, 8911-8916
8911
Formation and Electron Diffraction Studies of Ordered 2-D and 3-D Superlattices of Amine-Stabilized Gold Nanocrystals† Leif O. Brown and James E. Hutchison* Department of Chemistry and Materials Science Institute, UniVersity of Oregon, Eugene, Oregon 97403-1253 ReceiVed: April 2, 2001; In Final Form: June 20, 2001
Two- and three-dimensional superlattices formed from a family of amine-stabilized gold nanoparticles were investigated by transmission electron microscopy and selected area diffraction studies. The samples were prepared by ligand-exchange reactions between a phosphine-stabilized 1.5 nm precursor and pentadecylamine and exhibit metal core diameters ranging from 1.8 to ∼8 nm. Several of the observed specimens have surprisingly narrow core size dispersity and, as a consequence, form highly organized two- and threedimensional superlattices. TEM imaging and electron diffraction studies are used to determine both the packing arrangement and the type and degree of order present in these superlattices. Smaller (dCORE ∼ 1.8 nm) nanoparticles form three-dimensional fcc superlattices, a surprising finding in light of the fact that the 1.4 nm precursors have not previously been observed to form such highly ordered superlattices. Narrow dispersity samples of larger nanoparticles (dCORE >5 nm) form organized superlattices in both two and three dimensions. All of these samples exhibited at least translational ordering of the metal cores. In one class of nanoparticle (dCORE ∼ 8 nm) electron diffraction studies provide evidence that the atomic lattices within neighboring nanoparticles are oriented in the same fashion (orientational ordering). The high degree of order found with these superlattices suggests it may be possible, with sufficiently monodisperse samples, to obtain single crystals of this family of nanoparticles.
Introduction Metal nanoparticles have attracted much attention in the past few years due to their potential for use as device elements in nanoelectronic and optical applications. Successful fabrication of useful devices depends on the ability to prepare ordered assemblies of nanoparticles in a rapid and inexpensive manner.1 Optical devices often rely on the collective properties of nanoparticle arrays and may therefore require relatively simple assembliessordered monolayers and multilayers.2 Both passive and active methods of creating ordered monolayers have now been investigated.3 The use of (sub)monolayer nanoparticle assemblies in a polarization-dependent color filter was recently demonstrated by Dirix et al.4 In the case of electronic devices, the composition and dimensionality of the arrays will directly impact device performance because electron transport usually depends on the existence of a specific pathway through a material.5 Recent attempts to direct the ordering of nanoparticles have included the use of synthetic or biomolecular polymer backbones to act as templates in assembly.6 Metal nanoparticle systems previously shown to give ordered two- and three-dimensional assemblies include the Ag and Au nanoparticles studied by Wang and Whetten.7 The degree of order in the superlattice correlates with core size dispersity. Samples of larger nanoparticles tend to have narrower size dispersities and thus form more ordered superlattices. Ordered superlattices of nanoparticles with core sizes 1.
either by dropcasting or by aerosol deposition onto carboncoated (no polymer) Ni grids. Prior to diffraction studies, the camera length was checked by measuring d-spacings on a sample of Au shadow-evaporated onto latex spheres. Particle Size Measurements. NIH Image 1.6212 was used to measure particle sizes. The following steps were taken to prepare the data for analysis: (i) A bright field TEM image was acquired with even illumination. An image is chosen to be as representative of the sample as possible.13 (ii) The image was scanned into a computer (using either a transparency scanner for the negative, or a flatbed scanner for a print of the negative). (iii) Using Adobe PhotoShop, the contrast and channel curves were adjusted such that particles stand out clearly from the background carbon film. This is most difficult for small particles that inherently have less contrast. Visibly overlapping or touching particles are deleted from the image at this stage. (iv) In NIH Image 1.62, the “Analyze Particles” feature was used to generate a table of particle diameters (major and minor axes). A map of particle outlines is also generated, and this is compared to the original image. In difficult cases, the outline map may be printed on a transparency and overlaid on the original image to check for discrepancies. For each particle, the diameter was taken as the mean of the major and minor diameters. Microsoft Excel 98 was used for generating histograms and for further analyses. UV-Visible Spectroscopy. UV-visible spectroscopy was performed either on a Perkin-Elmer Lambda 6 dual beam instrument using a slit width of 2 nm and scan speed of 600 nm min-1 or on a Hewlett-Packard HP 8453 diode array instrument with a fixed slit width of 1 nm. Microsoft Excel 98 was used for any extended analysis of the data. Synthesis of PPh3-Stabilized Au Nanoparticles (Au-TPP). The AuI precursor, AuCl(PPh3), was prepared from HAuCl4‚xH2O and PPh3 as previously described.14 The PPh3stabilized Au nanoparticles, Au-TPP, were prepared following the procedure reported by Schmid for Au55(PPh3)12Cl6.15 The material we obtained possessed a bulk relative atomic composition of Au1.0(PPh3)0.20Cl0.16 from quantification of the XPS spectrum. This is slightly poorer in PPh3 and slightly richer in
Cl than Schmid’s published formula. Measurement of 369 particles in a TEM image gives the particle size as 1.4 ( 0.4 nm. Synthesis of Pentadecylamine-Stabilized Au Nanocrystals (Au-PDA). Au-PDA was prepared as described previously.9 Typically, Au-TPP (10 mg) is dissolved in CH2Cl2 (10 mL) and stirred at room temperature in the presence of 1-pentadecylamine (40 mg, 4 mass equivalents) to effect a ligand exchange. Samples were removed and examined by TEM at several stages from within 1 h up to several weeks from initiation of the ligand exchange. Larger nanocrystal precipitates can be removed by filtration through a medium porosity fritted funnel. These can then be redissolved in CHCl3 for TEM analysis. Results and Discussion Assemblies of Small Nanoparticles. The ligand exchange process converts small phosphine-stabilized Au nanoparticles into large alkylamine-stabilized nanocrystals, with retention of a narrow dispersion of particle sizes. As part of our study of the growth process, we made the surprising observation that TEM samples prepared from the reacting mixtures reveal areas of high order ∼24-48 h into the reaction. Examples are shown in Figure 1. Although panels a and c of Figure 1 may initially suggest the presence of close packed monolayers of nanoparticles, careful analysis of the interparticle spacing leads to the conclusion that these structures are actually three-dimensional superlattices possessing a face-centered cubic packing geometry. For the [111] projection shown in Figure 1a, this is easily confirmed by tilting the sample stage and obtaining a second image, Figure 1b. A monolayer would be expected to maintain an ordered appearance at higher stage angles. Tilting the sample back to its original position again reveals the ordered pattern (Figure 1a), indicating that the island is not being destroyed by the electron beam within the time frame (several minutes) of the experiment. Further evidence for a 3D superlattice comes from the structure shown in Figure 1f. This structure can be assigned as a [hk0] projection, in which h and k > 1.7a In the
Superlattices of Amine-Stabilized Gold Nanocrystals
J. Phys. Chem. B, Vol. 105, No. 37, 2001 8913
Figure 2. Possibilities for long-range order in a superlattice. Left: A noncrystalline superlattice caused by positioning centers of mass to produce an ordered assemblystranslational order only. Right: A crystalline superlattice in which the atomic lattices of individual particles are aligneds translational and orientational order.
absence of the observed [111] and [100] projections (Figure 1a,c), it would have been tempting to label Figure 1f as a wiretype structure such as that reported by Fitzmaurice et al.16 caused by unidirectional aggregation of particles. Three-dimensional superlattices such as those shown in Figure 1 are unexpected for a system such as the one under study. Generally, small particles do not self-assemble well when compared to larger particles17,18 due to the greater number atom distribution between individual small particles (this distribution may be especially broad during a particle growth time frame). For instance, a 1.4 nm particle such as Au-TPP requires only ∼100 atoms to increase in core diameter by 0.5 nm, whereas the equivalent tolerance in a 10 nm particle is ∼5000 atoms.19 The net result is that small deviations in core size easily disrupt the assembly of small particles but have very little effect on the assembly of large particles. Analysis of the available projections and comparison of the interparticle spacings reveals that the three-dimensional superlattice is of a standard fcc type with a unit cell of side l ) 3.8 nm. An obvious question is whether the nanoparticles contained in these arrays are stabilized by PPh3 (the Au-TPP starting material) or by PDA. There are two observations that point toward the particles being stabilized by PDA. The first is that we have never been able to observe high degrees of particle packing in ordinary (nonexchanging) samples of Au-TPP. This would suggest that the packed particles are at least under the influence of the exchange process. The second and more significant observation is that the nearest-neighbor distance in the superlattice is ∼2.3 nm. This is sufficiently long to incorporate interdigitated PDA ligands (∼2.1 nm, extended chain conformation). A PPh3 ligand by contrast only requires ∼0.7 nm, which would leave a large amount of space in the lattice, even if no interdigitation were to occur. Thus, we would expect that the particles taking part in the packing contain at least a significant proportion of PDA ligands. Whether tight packing of particles is possible with a mixed (PPh3/PDA) ligand sphere is not clear.20 The finding of such highly ordered superlattices in an evolving system suggests that one particle size is unusually stable (either geometrically or electronically). A stable particle size might represent a local energy minimum or a slow step in the growth process. Accumulation of such stable, monodisperse particles provides an opportunity for assembly of highly ordered superlattices due to the localized reduction in the bulk size distribution. It further represents good evidence that with a suitable sample, the formation of large crystalline arrays of small (
