, 1309 (2009); 324 Science et al. Haimei Zheng Trajectories Observation of Single Colloidal Platinum Nanocrystal Growth This copy is for your personal, non-commercial use only. clicking here. colleagues, clients, or customers by , you can order high-quality copies for your If you wish to distribute this article to others here. following the guidelines can be obtained by Permission to republish or repurpose articles or portions of articles ): October 24, 2014 www.sciencemag.org (this information is current as of The following resources related to this article are available online at http://www.sciencemag.org/content/324/5932/1309.full.html version of this article at: including high-resolution figures, can be found in the online Updated information and services, http://www.sciencemag.org/content/suppl/2009/06/03/324.5932.1309.DC1.html can be found at: Supporting Online Material http://www.sciencemag.org/content/324/5932/1309.full.html#related found at: can be related to this article A list of selected additional articles on the Science Web sites http://www.sciencemag.org/content/324/5932/1309.full.html#ref-list-1 , 4 of which can be accessed free: cites 27 articles This article 20 article(s) on the ISI Web of Science cited by This article has been http://www.sciencemag.org/content/324/5932/1309.full.html#related-urls 19 articles hosted by HighWire Press; see: cited by This article has been http://www.sciencemag.org/cgi/collection/mat_sci Materials Science subject collections: This article appears in the following

Understanding of colloidal nanocrystal growth mechanisms is essential for the syntheses of nanocrystals with desired physical properties. The classical model for the growth of monodisperse nanocrystals assumes a discrete nucleation stage followed by growth via monomer attachment, but has overlooked particle-particle interactions. Recent studies have suggested that interactions between particles play an important role. Using in situ transmission electron microscopy, we show that platinum nanocrystals can grow either by monomer attachment from solution or by particle coalescence. Through the combination of these two processes, an initially broad size distribution can spontaneously narrow into a nearly monodisperse distribution. We suggest that colloidal nanocrystals take different pathways of growth based on their size- and morphology-dependent internal energies.

We investigate the evolution of structures that result when spherical Cd nanoparticles of a few hundred nanometers in diameter react with dissolved molecular sulfur species in solution to form hollow CdS. Over a wide range of temperatures and concentrations, we find that rapid Cd diffusion through the growing CdS shell localizes the reaction front at the outermost CdS/S interface, leading to hollow particles when all the Cd is consumed. When we examine partially reacted particles, we find that this system differs significantly from others in which the nanoscale Kirkendall effect has been used to create hollow particles. In previously reported systems, partial reaction creates a hollow particle with a spherically symmetric metal core connected to the outer shell by filaments. In contrast, here we obtain a lower symmetry structure, in which the unreacted metal core and the coalesced vacancies separate into two distinct spherical caps, minimizing the metal/void interface. This pattern of void coalescence is likely to occur in situations where the metal/vacancy self-diffusivities in the core are greater than the diffusivity of the cations through the shell.

Gold/iron oxide core/hollow-shell composite nanoparticles (NPs) with controllable shell thicknesses are synthesized (see figure). The gap between the Au core and iron oxide shell is formed as a result of different outward and inward diffusion rates of Fe and 0, respectively. Control over interparticle interactions allows encapsulation of several Au cores inside one iron oxide shell. Superparamagnetic measurements of the NPs at room temperature demonstrate the plasmon resonance at 565 nm.

We report diffusion in the tunneling spectra of isolated, ligand-stabilized undecagold (Au11) clusters immobilized by attachment to alpha,omega-alkanedithiolate tethers inserted into alkanethiolate self-assembled monolayers. We use scanning tunneling microscopy and spectroscopy at cryogenic (UHV, 4 K) conditions to measure these clusters' conductance with complete control of their chemical and physical environment; additionally, thermal broadening of their electronic states as well as their mobility is minimized. At low temperature, the Au11 clusters demonstrate Coulomb blockade behavior, with zero-conductance gaps resulting from quantum size effects. Surprisingly, chemically identical and even single particles produced different families of tunneling spectra, comparable to previous results for heterogeneous distributions of particles. We hypothesize that, while these particles are chemically attached to the surface of the SAM for measurement, these assemblies may still be sufficiently dynamic to affect their transport properties significantly.

The understandings and applications of self-assembly have evolved significantly since the adsorption of n-alkyldisulfides on gold surfaces was first reported. The desire to produce features on surfaces that are placed in controlled proximity has driven study in both the chemistries and methodologies of their production. Self-assembled monolayers (SAMs) are found in applications such as molecular and biomolecular recognition, lithography resists, sensing and electrode modification, corrosion prevention, and other areas where tailoring the physicochemical properties of an interface is required. Patterned SAMs, in which specific self-assembling components have a deliberate spatial distribution on the surface (planar or otherwise), are generated to fabricate sophisticated nanoscale architectures and to provide well-characterized supports for physicochemical and biochemical processes. It is possible to introduce patterned features into both SAMs and the substrates that support them as the parameters controlling SAM formation and dynamics are better understood. As these structures are not at equilibrium once formed, one can manipulate the monolayer both during and after its formation by means of thermal, chemical, and electrochemical processing, exposure to controlled energetic beams, and scanning probe microscopes.

We report the phase separation of a self-assembled monolayer formed from a binary mixture of adsorbates, n-decanethiol, and an amide-containing alkanethiol of similar length (3-mercapto-N-nonylpropionamide), as studied by scanning tunneling microscopy. While mixtures of n-alkanethiols of similar length (i.e., n-decanethiol and n-dodecanethiol) show no phase separation, the introduction of a hydrogen-bonding functionality buried deep within the film induces the formation of single-component domains on the nanometer scale. Phase separation occurs at all relative compositions studied, and for these molecules maintains the same exposed terminal functionality across the entire film. In nonequimolar concentrations of adsorbates, we observe that the solution component present in greater concentration will dominate the composition of the adsorbed monolayer in super proportion to that in solution, consistent with enthalpic contributions from both the solvent and intermolecular interactions of adsorbates.

Hybrid self-assembled monolayers (SAMs) containing well-defined strata of different polarity enable insight into how fundamental interactions lead to higher order structure and may provide useful analogies for self-assembled multilayers, new hybrid materials, and functional biological interfaces. We report amide-containing alkanethiol SAMs with internal polar sublayers that are two amide groups thick and nonpolar overlayers comprising either dodecyl or hexadecyl chains. The assemblies have been characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and external reflective infrared spectroscopy (FTIR-ERS). XPS demonstrates the SAMs are of monolayer thickness, chemisorbed to the gold substrate, and anisotropically oriented. Contact angle data show the methyl surface for n = 16 is highly ordered, but the surface for n 12 is less well ordered. FTIR-ERS reveals that the alkyl chains for n = 16 are close packed, but that those for n = 12 are disordered. FTIR-ERS also shows that, although the two-amide sublayers are compositionally identical, they are well ordered and assume polyglycine-II-like conformations for n = 16, but they are poorly ordered for n = 12. Comparison of these two SAMs to each other in the context of previously reported one- and three-amide SAMs leads to two conclusions. (1)The threshold n for alkyl chain length ordering in two-amide SAMs is 12 less than or equal to n less than or equal to 16. Thus, in SAMs with internal amide sublayers both one and two amide groups thick, the threshold number of methylenes required to form ordered alkyl regions is significantly increased compared to alkanethiol SAMs, demonstrating destructive interference of the amide region with the hydrocarbon ordering process. (2) In two-amide SAMs the formation of a well-ordered amide region depends on the ordering of an overlying hydrocarbon region. This behavior differs with that previously demonstrated for one- and three-amide SAMs, in which the amide groups assume characteristic conformations regardless of hydrocarbon region thickness and order. For two-amide SAMs, the apparent dependence of amide ordering on complementary ordering in the alkyl region provides evidence of an energetic interplay between the two sublayers, manifested as a "reverse ordering" effect. The previously unobserved elastic-elastic character of the buried interface in two-amide SAMs is contrasted with the rigid-elastic interface found in the one-amide SAMs.