Single-crystalline vanadium dioxide nanowires with rectangular cross sections

We report the synthesis of single-crystalline VO2 nanowires with rectangular cross sections using a vapor transport method. These nanowires have typical diameters of 60 (+/-30) nm and lengths up to >10 mum. Electron microscopy and diffraction measurements show that the VO2 nanowires are single crystalline and exhibit a monoclinic structure. Moreover, they preferentially grow along the [100] direction and are bounded by the (01) and (011) facets. These VO2 nanowires should provide promising materials for fundamental investigations of nanoscale metal-insulator transitions.     

Head-to-tail self-assembly of a calix[4]arene inclusion polymer controlled by a pendant arm

A calix[4]arene functionalized at one phenolic group with a pendant ethoxy acetate group, forms an inclusion complex that is stable even in the presence of other potential guest molecules.     

A new supramolecular isomer of [Zn(nicotinate)2]n: a novel 4(2).8(4) network that is the result of self-assembly of 4-connected nodes

Self-assembly of Zn(NO3)2 with m-pyridinecarboxylate (nicotinate) under mild conditions affords several products including a novel three-dimensional network [Zn(nicotinate)2]n that has connectivity defined by the circuit symbol 4(2).8(4).     

Qubit channels can require more than two inputs to achieve capacity

We give examples of qubit channels that require three input states in order to achieve the Holevo capacity.     

Bead-spring model of a polymer chain: A transient solution

With the Rouse-Zimm differential equation of the spring-bead model, the distribution function ofN + 1 beads(x, y, z,t) [here x denotes x₀, x₁,..., x_N, and similarly for y and z] is explicitly solved with the two different initial conditions: the Gaussian and delta distribution functions. We find that although the mean end-to-end distances obtained from the two initial conditions are the same, the expressions of the mean square end-to-end distances are different. We also obtain the expression for the mean and mean square end-to-end distances analytically from the Langevin equation with the delta initial distribution function. With this analytic expression, we show that the statistical quantities obtained from the Monte Carlo calculation are consistent with those obtained from the Rouse-Zimm differential equation if a suitable length is chosen for the time increment.NIH-Special Fellow (1F03GM43885-01) with a partial support by USPHS GM11916.     

Miscibility behavior and reequilibration of binary poly(amic acid) mixtures

Five poly (amic acid) solutions based on PMDA-PDA, PMDA-ODA, PMDA-6F, ODPA-ODA, and 6FDA-ODA were prepared in N-methylpyrrolidone at a polymer concentration of ca. 10 wt %. From these five poly (amic acid) solutions, six different binary blends were prepared: PMDA-PDA/PMDA-ODA, PMDA-PDA/PMDA-6F, PMDA-ODA/6FDA-ODA, PMDA-ODA/ODPA-ODA, PMDA-PDA/ODPA-ODA, and PMDA-PDA/6FDA-ODA. These blends were then characterized with respect to miscibility in the ternary state (polyamic acid-1/polyamic acid-2/N-methylpyrrolidone), the condensed state (ca. 70 wt % polymer), and the fully imidized state. All blends except for PMDA-PDA/PMDA-6F yielded homogeneous mixtures in the ternary solution of 10 wt % polymer concentration. The PMDA-PDA/PMDA-6F mixture eventually became homogeneous after 10 days of mixing at room temperature. Upon solvent evaporation (condensed state) and full cure (imidized state) two blends incorporating ODPA-ODA as one of the components exhibited apparent miscibility as evidenced by optical microscopy. The remaining blends exhibited large-scale phase separation upon solvent evaporation with no significant differences in the overall morphology between the condensed and imidized state. However, as in the case of the PMDA-PDA/PMDA-6F ternary system, the morphology in the condensed and imidized state was strongly dependent on the mixing time of the precursor poly(amic acid) components; the phase-separated domain size decreased with increasing mixing time, eventually leading to complete miscibility. These results are discussed with respect to the proposed “polymer-monomer” reequilibration reaction as well as its implications with respect to the preparation of polyimide molecular composites.     

Surfactant layering on mixed monolayer-protected gold clusters

Positively-charged monolayer protected gold clusters (MMPCs) were mixed with sodium dodecyl sulfate (SDS). At lower SDS concentration, the initially water-soluble particles became organic-soluble while remaining discrete. Upon further addition of SDS, the particles aggregate and become water-soluble. NaCN decomposition, TEM, and DLS characterization reveal the morphology and properties of these encapsulated assemblies.     

Assembling of amphiphilic highly branched molecules in supramolecular nanofibers

We found that the amplification of weak multiple interactions between numerous peripheral branches of irregular, flexible, polydisperse, and highly branched molecules can facilitate their self-assembly into nanofibrillar micellar structures at solid surfaces and the formation of perfect long microfibers in the course of crystallization from solution. The core-shell architecture of the amphiphilic dendritic molecules provides exceptional stability of one-dimensional nanofibrillar structures. The critical condition for the formation of the nanofibrillar structures is the presence of both alkyl tails in the outer shell and amine groups in the core/inner shell. The multiple intermolecular hydrogen bonding and polar interactions between flexible cores stabilize these nanofibers and make them robust albeit flexible. This example demonstrates that one-dimensional supramolecular assembling at different spatial scales (both nanofibers and microfibers) can be achieved without a tedious, multistep synthesis of shape-persistent molecules.     

A 3D metal-organic network, [Cu2(glutarate)2(4,4'-bipyridine)], that exhibits single-crystal to single-crystal dehydration and rehydration

The reaction of Cu(NO3)2, glutaric acid and 4,4'-bipyridine in water affords a novel 3D coordination polymer which exhibits reversible desorption and adsorption of water molecules with retention of single crystallinity.