J. Phys. Chem. B 1997, 101, 189-197
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Pressure/Temperature Phase Diagrams and Superlattices of Organically Functionalized Metal Nanocrystal Monolayers: The Influence of Particle Size, Size Distribution, and Surface Passivant James R. Heath,* Charles M. Knobler, and Daniel V. Leff UCLA Department of Chemistry and Biochemistry, 405 Hilgard AVenue, Los Angeles, California 90095-156905 ReceiVed: April 22, 1996; In Final Form: August 23, 1996X
The phase behavior of organically passivated 20-75 Å diameter Ag and Au nanocrystals is investigated by examining surface-area isotherms of Langmuir monolayers and transmission electron micrographs of Langmuir-Blodgett (LB) films. The effects of temperature, organic passivant chain length, and nanocrystal size and composition are studied. Three distinct types of phase behavior are observed and may be classified in terms of the extra (conical) volume (Ve) available to the alkyl capping group as it extends from a nearly spherical metal core. For Ve > 350 Å3, the phase diagram is dominated by extended, low-dimensional structures that, at high pressures, compress into a two-dimensional foamlike phase. This behavior is rationalized as originating from the interpenetration of the ligand shells of adjacent particles. For Ve < 350 Å3, dispersion attractions between the metal cores dominate particle condensation. For 350 Å3 > Ve > 150 Å3, the particles condense to form closest packed structures, which, for sufficiently narrow particle size distributions, are characterized by crystalline phases. For Ve ≈ 30 Å3, the particles irreversibly aggregate into structures similar to those expected from a diffusion-limited-aggregation (DLA) model. Optical properties of certain LB films of the closest packed phases are reported.
Introduction Chemical techniques for the preparation and size separation of organically soluble semiconductor and metal nanocrystals have advanced rapidly over the past several years. Narrow size distributions for particles with diameters in the range of 1-20 nm of a wide variety of metal and semiconductor particles may now be prepared.1-3 Randomly dispersed semiconductor, metal, and metal oxide nanoparticles have historically found applications in areas such as reprography, photocatalysis, catalysis, and optical materials.4 Recent advances in directing the selfassembly of (disordered) semiconductor nanocrystal thin films5 have enabled researchers to electrically address certain nanocrystals,6 making them viable materials for optoelectronics applications as well. Even more recent, however, has been the development of techniques for directing the self-assembly of nanocrystals into ordered aggregates, or quantum dot superlattices.7 These assemblies present some very exciting possibilities. In principle, interparticle separations, particle size, and particle stoichiometry may be individually manipulated to produce a macroscopic solid with a tailored band structure, similar to the well-known case of one-dimensional quantumwell superlattices. For metal particles in particular, quantum dot superlattices might be engineered to exhibit unique magnetic, electronic, and/or superconducting behavior. In addition, they should provide nearly ideal models for the well-studied granular metals. For the case of II-IV semiconductor quantum crystallites, Bawendi’s group has explored various techniques to investigate the formation of quantum dot superlattice structures.8 In one study, they utilized the Langmuir-Blodgett procedure to prepare ordered arrays of trioctylphosphine oxide-capped CdSe nanocrystals.9 These particles may be prepared with extremely narrow (σ < 4%) size distributions,10 and experiments with five different size particles between 25 and 53 Å were performed at * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 15, 1996.
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a single temperature. In all cases, the particles self-assembled upon compression to form closest packed two-dimensional arrays. The only size-dependence observed was related to the pressure per particle area necessary to compress the particles into an array. An alternative technique for the fabrication of ordered monolayers of ligand-stabilized cluster compounds has recently been reported by Schmid’s group.11 In that work, they utilize strong attractive interactions between polyelectrolytes, in which imino groups on a substrate and sulfonic groups on cluster surfaces were used to promote the self-assembly of the acid of [Au55{PPh2(m-C6H4SO3Na)12}Cl6] on a mica surface coated with poly(ethylenimine). We have also recently reported on the formation of metal nanocrystal superlattices, or “opals,”12 from an initially broad size distribution of dodecanethiol-capped 20-55 Å diameter Au particles.13 In that paper, we demonstrated that dispersion interactions between particles can lead to size-dependent phase separations followed by size-selective 2-D and 3-D opal formation. For any nanocrystal system, interparticle dispersion attractions are expected to scale geometrically with increasing particle size.14 For metal nanocrystals in particular, these attractions may be quite strong15 and exhibit a profound and often organizing influence on particle aggregation. The nature of these interactions implies that, for metal particles, superlattice formation should depend on particle size and stoichiometry, the nature and size of the organic passivants on the particle surfaces, and the particle size distribution. Such expectations contrast sharply with what is observed in the fabrication of more traditional two-dimensional colloidal arrays, e.g. arrays of polystyrene, latex, or silica spheres.16 Such particles are typically designed to behave as hard spheres, and superlattice formation and/or size separation is thought to proceed through either hydrodynamic or capillary forces17 or through entropic depletion interactions.18 In this paper, we investigate the role that the variables of particle size, size distribution, and size of the passivating ligand © 1997 American Chemical Society
190 J. Phys. Chem. B, Vol. 101, No. 2, 1997 play in determining the structures resulting from metal particle condensation in two dimensions. Surface-pressure isotherms of films spread at the air/water interface were utilized to determine the phase diagrams corresponding to the organization of various organically passivated Au and Ag nanocrystal systems. The structural nature of certain phases was interrogated by transmission electron microscopy (TEM) of LangmuirBlodgett (LB) films. Certain of the LB films were also optically characterized using UV/vis absorption spectroscopy. We find that the phase diagrams depend strongly upon the amount of excess (conical) volume (Ve) available to the passivating ligands as they extend from the nearly spherical metal nanocrystal surface. For Ve > 350 Å3 (case I), the phase diagrams are dominated by extended, 1-D structures that, at high pressures, compress into a 2-D foamlike phase. These structures result from the interpenetration of the ligand shells of adjacent particles. For Ve < 350 Å3 (case II and case III), attractive dispersion forces between the metal cores dominate particle condensation. For Ve > 150 Å3, these forces are sufficiently weak that, upon compression, the particles condense to form a closest packed 2-D solid. For Ve ≈ 30 Å3 (case III), the particles irreversibly aggregate into nonequilibrium structures similar to those expected from a diffusion-limited-aggregation model. Experimental Section A. Particle Synthesis. All particles were synthesized using chemicals obtained from Aldrich Chemical Co. without further purification except where noted. Particles were structurally and optically characterized by some or all of the following: TEM, X-ray powder diffraction (Cu KR radiation), and UV/vis absorption spectroscopy. More extensive characterizations of these particles are described elsewhere.19 A.1. Synthesis of Dodecanethiol and Nonanethiol-Capped Au Particles. These particles were prepared in the manner developed by Brust et al.20 Briefly, a weighed amount of HAuCl4‚3H2O is dissolved in distilled H2O. The gold salt is then quantitatively transferred into toluene using (octyl)4N+Bras a phase transfer reagent. A measured amount of alkylthiol is added to the mixture. In a separate beaker, a measured amount of NaBH4 reducing agent is dissolved in H2O. While the Au-containing mixture is rapidly stirred, the reducing agent is added, and the reaction is allowed to proceed for several hours. Typical molar ratios for HAuCl4‚3H2O/(octyl)4N+Br-/alkylthiol/ NaBH4 are 1:3:3:10, and a typical initial weight of the gold salt is 200 mg. Upon completion of the reaction, the aqueous layer is removed and the toluene solution is rotary evaporated to a volume of ∼5 mL. The particles are precipitated with either methanol, ethanol, or acetone. The initial size distribution is broad and may be arbitrarily narrowed (depending on the amount of product available) using the technique of size-selective precipitation.10 All particles used here were selectively precipitated up to six times. A.2. Synthesis of Oleylamine-Capped Au Nanocrystals. The amine-capped Au nanocrystals represent an unusual chemical species since the amine-Au interaction is considered to be quite weak. Nevertheless, the amine-Au interaction in these particles is charge neutral, and the particles are actually kinetically, rather than thermodynamically, stabilized. Other than the fact that these particles possess an 18-carbon atom surface cap (rather than a 12 or 9 atom one), their chemical behavior and stability are identical to that of their thiol-capped counterparts. The synthesis and chemical characterization of these particles is presented elsewhere.21 Their preparation is similar to that for dodecanethiol- or nonanethiol-capped Au nanocrystals, except that the (octyl)4N+Br- phase transfer reagent is not included in the reaction. This has the effect of lowering the product yield.
Heath et al. A.3. Synthesis of Dodecanethiol-Capped Ag Nanocrystals. The dodecanethiol-capped Ag nanocrystals are prepared in a manner similar to the dodecanethiol-capped Au nanocrystals. AgNO3 or AgClO4 is used as a substitute for HAuCl4‚3H2O. B. Measurement of Pressure/Area Isotherms. Pressure/ area isotherms were measured using a Nima Technology Type 611 Langmuir Trough. All measurements were done with 18 MΩ water (pH 5.7), purified with a Millipore Milli-Q UV Plus system. A variable-temperature water circulator cycled water through the body of the trough and allowed for the determination of phase diagrams through the temperature range 15-38 °C. For each isotherm, the temperature of the trough was held constant to (0.2 °C. The trough was cleaned after each measurement, and fresh material was deposited. Particles were prepared for Langmuir isotherm measurements in the following manner. After synthesis and size selection, a powder of the desired particles was dispersed by sonication in an acetone/ethanol solution and filtered in order to remove any residual organic material. The resulting dry powder was weighed and then dissolved in a known amount of chromatographed hexane to a concentration of ∼1 mg/mL. The solubility limit of the particles in hexane is somewhere between 10 and 30 mg/mL, depending on particle size and surface passivant. The solution was passed through a 0.2 µm pore size filter and stored in clean glassware. The best results are obtained on very fresh particles. Apparently, the particles “ripen” over time (a few days), resulting in a slight change in the particle size distribution and the release of some surfactant molecules into solution. The result is that isotherms from freshly prepared particles tend to exhibit sharper features than do isotherms from older particles. Practical considerations mandated that some measurements were made on solutions that had been prepared 2-3 days earlier. In that case, the particles were stored in a refrigerator (-20 °C) during the period between preparation and measurements. All phase diagrams, however, were measured over the course of single 3-5 h time periods. A glass 100 µL syringe was utilized to disperse a known amount of the particles uniformly across the water surface. Depending on the nature of the particles and the concentration of the solution, between 5 and 200 µL of material was used. Pressure/area isotherm measurements were carried out using double-barrier compression at a compression rate of 50 mm/ min. C. TEM and UV/vis Characterization. TEM analysis was performed on an Akashi EM002β TEM operating at 200 keV with either 0.17 or 0.30 nm point-to-point resolution (depending on the installed tilt-axis) at the University of Southern California’s Center for Electron Microscopy. UV/vis spectra were measured on a Perkin-Elmer Lambda 3B spectrophotometer. Langmuir monolayers were transferred to TEM grids in two different ways. For certain of the low-density particle films, the edge of a grid was clamped with tweezers and lowered through the clean water surface at right angles to the surface. The LB nanoparticle monolayers were transferred onto the grid by vertical lifting through the interface at constant surface pressure and at a constant speed of 1 mm/min. TEM grids were purchased from Ladd Research Industries, Inc. and used as supplied. Transfers were attempted to Formvar substrates on carbon-coated 200 mesh copper grids and bare-carbon-coated 200 mesh copper grids. The transfers to the carbon-coated grids proved to be the most successful. A second method for transferring the monolayers onto grids and optical (glass) substrates was employed for the high-density films only. In this procedure, which is similar to the LangmuirSchaeffer technique, a grid (carbon coat down) or glass coverslip
Metal Nanocrystal Monolayers was attached to the base of a small flat surface and brought parallel to the surface of the trough by hand. The surface of the grid or substrate was briefly contacted with the particle layer and lifted off. These films are not properly LB films. Nevertheless, for simplicity’s sake, we will refer to these films as LB films throughout this article. Because of the high optical density of small Au and Ag particles, the high-density films are quite easily observed by eye. Thus, when this second technique was utilized to transfer particle phases to glass substrates, the efficiency of the transfer is readily apparent. Cracks, fissures, holes, and other density gradients in the Langmuir monolayer are transferred intact. The vertical lifting technique described above did not successfully transfer such features. The compressed, transferred films are apparently unstable over extended time periods (many hours or days). Thus, all TEM data were collected ∼1-4 h after film preparation, and all UV/vis spectra were taken within minutes of film preparation. Surface coverages were estimated using TEM. A hexane solution of particles that was used to prepare a particular isotherm was diluted by a known amount, and a 3.5 µL drop of that solution was evaporated onto the carbon-coated side of a 3 mm diameter TEM grid. Two-dimensional particle densities were measured directly and used to calibrate isotherm surface coverages. The particle sizes used for the coverage estimates include both the metallic cores and the ligand shells. We estimate the accuracy of these calibrations to be between 20 and 30%. Results and Discussion In previous work, we demonstrated through both simulations and experiment the critical size-dependent role that attractive dispersion (van der Waals) forces can play in particle aggregation.13 Of particular importance for the formation of nanocrystal superlattices is the case for which such forces are strong enough to cause particle aggregation and yet sufficiently weak that particle aggregates can “anneal” under ambient conditions to form ordered structures. The important physical parameter that determines the nature (power law scaling) of the dispersion forces between two identical nanocrystals is the ratio of the size of the metal core to the interparticle separation distance. For the present case of nonionic, organically functionalized metal nanocrystals, the separation distance is largely determined by the length of the organic surfactants. This treatment of the dispersion attractions implies that the organic surfactants constitute a spatially uniform dielectric about the metallic core. Such an approximation is, in general, not valid. For the case of linear alkylthiols and alkylamines, the volume available to a surfactant molecule increases as the ligand extends from the metal surface. This is demonstrated graphically in Figure 1. In this illustration the excess volume (Ve) available to the ligand as it extends from the surface of the particle is shown as a hollow cone surrounding the ligand. The volume of the cone is determined by the length of the ligand (L), the footprint of the ligand (f), and the size of the metal core (R). The included angle, θ/2, is equal to tan-1 (f/2R). For thiols on Au, the area of the ligand footprint is 21.4 Å2.22 Since f is a constant, the dominant term in Ve scales as L3/R2.23 Clearly, the lack of facets is going to also play a large role in determining Ve. All of the particles discussed here are assumed to be spherical. Although this is certainly an approximation, most of our particles do appear spherical by TEM, even when imaged at high resolution. The magnitude of Ve gives some indication of the amount that a ligand shell associated with one particle may interpenetrate into the ligand shell of a second particle. Such interpenetration
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Figure 1. Illustration of the excess volume (referred to as Ve in the text) available to a ligand as it extends out from the surface of a particle.23 The included angle contained in the cone is determined by the footprint size of each ligand (assumed to be independent of particle size) and the radius of the metal core. This ligand is shown as an extended structure for the purposes of this diagram only. The actual ligands are likely to sample many geometries, both compact and extended in nature.
TABLE 1: Alkyl Group Chain Length C18 metal core size Ve (Å3), case distb’n width metal core size Ve (Å3), case distb’n width metal core size Ve (Å3), case distb’n width metal core size Ve (Å3), case distb’n width metal core size Ve (Å3), case distb’n width metal core size Ve (Å3), case distb’n width
C12
C9
20 Å 20 Å 20 Å 1550; case Ia 560; case Ic 270; case IId 20% 20% 20% 18 Å 670; case Id 10% 40 Å 40 Å 460; case Ib 150; case IIa 20% 20% 35 Å 200; case IIb 70% coverage. These phases were sampled as LB films using TEM. B. Case II Particles (150 Å3 < Ve < 350 Å3). Particles that fall into case II are clearly going to be the most important particles for preparing ordered two-dimensional metal quantum dot superlattice structures. The Langmuir isotherms of case II particles are very similar to those reported by Dabbousi and co-workers for CdSe quantum dots. Only two inflection points are observed, corresponding to the transition from the gas phase to the 2-D closest packed phase, and from the 2-D phase to the collapsed 2-D (i.e., 3-D) phase. In addition, the gas phase to closest packed phase transition occurs at very nearly 100% surface coverage. A phase diagram constructed from compression isotherms of monolayers of 40 Å Au nanocrystals capped with dodecanethiol is shown in Figure 7. LB films were transferred onto TEM grids for all case IItype particles listed in Table 1. The major difference observed between the various case II particles was in the degree of crystallinity of the closest packed 2-D phase, and this was related to the both the width of the size distribution and the nature of the solvent. Narrower distributions obviously lead to higher crystallinity. In addition, more slowly evaporating solvents (such as heptane vs hexane) also increase the crystallinity of the phase (presumably an annealing affect). All phases shown here were prepared from hexane solutions. The most crystalline
Heath et al.
Figure 7. π/temperature phase diagram for case IIa particles (4 nm diameter Au cores with C12 caps). Although the slope of the phase boundaries resembles those presented in Figure 3, the particle coverage is much greater, and the phases are closest packed, rather than extended. Data above 35 °C were extrapolated from lower temperature data. The “gas phase” region constitutes the region of the phase diagram in which the measured pressure is not different from that of a clean water surface.
phases were observed for dodecanethiol-capped 35 and 28 Å Ag nanocrystals (case IIb and IIc), both of which were characterized by a fwhm of the size distribution of
