ABSTRACT
The ability to assemble nanoparticles (NPs) into desired patterns in a controlled fashion is crucial for the study of collective properties and for the fabrication of a variety of NP-based devices. Drying-mediated assembly directed by a template provides a facile route for organizing NPs in predefined patterns. We utilize the branched topographical landscapes displayed by partially crystallized poly(ethylene glycol) (PEG) films as a generic template for studying the drying-mediated organization of dodecanethiol- and polystyrene thiol-protected gold NPs (Au-DT and Au-PS), and explore the combined effects of NP size and ligand, concentration, and spin rate on the distribution of NPs inside the channels. We show how NP concentration and the spin rate applied during NP deposition can be used to influence the tendency of NPs either to fill the channel uniformly or to localize near the channel edges, explain the important role of the enhanced aggregation tendency of larger NPs on the resulting morphologies, and demonstrate how this tendency can be tuned by the proper choice of ligands. The different effects are explained in the context of possible scenarios of drying-mediated assembly by analyzing the relevant interactions and forces acting on the NPs during solvent evaporation.
ABSTRACT
We describe herein a valence bond (VB) study of 27 triply bonded molecules of the general type X≡Y, where X and Y are main element atoms/fragments from groups 13-15 in the periodic table. The following conclusions were derived from the computational data: (a) Single π-bond and double π-bond energies for the entire set correlate with the "molecular electronegativity", which is the sum of the X and Y electronegativites for X≡Y. The correlation with the molecular electronegativity establishes a simple rule of periodicity: π-bonding strength generally increases from left to right in a period and decreases down a column in the periodic table. (b) The σ frame invariably prefers trans bending, while π-bonding gets destabilized and opposes the trans distortion. In HC≡CH, the π-bonding destabilization overrides the propensity of the σ frame to distort, while in the higher row molecules, the σ frame wins out and establishes trans-bent molecules with 2(1)/2 bonds, in accord with recent experimental evidence based on solid state (29)Si NMR of the Sekiguchi compound. Thus, in the trans-bent molecules "less bonds pay more". (c) All of the π bonds show significant bonding contributions from the resonance energy due to covalent-ionic mixing. This quantity is shown to correlate linearly with the corresponding "molecular electronegativity" and to reflect the mechanism required to satisfy the equilibrium condition for the bond. The π bonds for molecules possessing high molecular electronegativity are charge-shift bonds, wherein bonding is dominated by the resonance energy of the covalent and ionic forms, rather than by either form by itself.
ABSTRACT
Two-dimensional, hierarchical assemblies of nanorods were obtained by exploiting the structures afforded by block copolymers in ultrathin films. Under the appropriate conditions, the nanorods segregate to the film surface already upon casting the composite film, and organize with the block copolymer through phase separation. In this paper we compare the structures formed by CdSe nanorods of three different lengths and two polystyrene-block-poly(methyl methacrylate) copolymers with different nanorods/copolymer ratios, and study the temporal evolution of the structure in each case. It is found that the initial morphology of the film largely dictates the resulting structure. The combination of short nanorods and/or short copolymers is shown to be more prone to morphological defects, while assembling long nanorods with long copolymers leads to highly organized nanorod morphologies. These phenomena are explained by a combination of kinetic and thermodynamic factors.
