This article surveys recent developments in the rational synthesis of single‐crystalline zinc oxide nanowires and their unique optical properties. The growth of ZnO nanowires was carried out in a simple chemical vapor transport and condensation (CVTC) system. Based on our fundamental understanding of the vapor–liquid–solid (VLS) nanowire growth mechanism, different levels of growth controls (including positional, orientational, diameter, and density control) have been achieved. Power‐dependent emission has been examined and lasing action was observed in these ZnO nanowires when the excitation intensity exceeds a threshold (∼40 kW cm–2). These short‐wavelength nanolasers operate at room temperature and the areal density of these nanolasers on substrate readily reaches 1 × 1010 cm–2. The observation of lasing action in these nanowire arrays without any fabricated mirrors indicates these single‐crystalline, well‐facetted nanowires can function as self‐contained optical resonance cavities. This argument is further supported by our recent near‐field scanning optical microscopy (NSOM) studies on single nanowires.

over timescales of hundreds of seconds (0.00 lm 2 s ±1 ). It is interesting that in the neutral MEA experiment, in which some of the surface amine groups are protonated, the diffusion coefficient was slightly lower than in the other cases.A surprising result was obtained when measuring the diffusion coefficients of the three sizes of MESA particles on the neutral surface derivatized with hexadecanethiol (C 16 H 33 SH).Comparing D values to those found on the MESA surface, we found near agreement for all but the small-diameter particles (6 lm 90 nm), which reproducibly stuck to the substrate immediately on contact. For larger particles, the long-range electrostatic force (presumably between the negatively charged sulfonate groups and an image charge in the substrate) is expected to dominate, [22] but for smaller ones the short-range van der Waals force is more important. Apparently, the two are closely balanced for the C 16 H 33 SH functionalized surface with rods of the size investigated here.While the scaling of these interactions is not completely understood at present, these initial studies provide some guidance as to the conditions that are desirable for nanorod assembly experiments. The observation of pH-dependent diffusion may, for example, be useful for affixing particles to specific areas or in specific conformations on basic surfaces. More importantly, the particle tracking method described here provides a simple and convenient method for quantifying the surface diffusion of non-spherical particles under arbitrary conditions. ExperimentalWhatman Al 2 O 3 filter membranes that contain 300±350 nm diameter internal pores were used as a template material. For 90 nm internal pore diameter, Al 2 O 3 membranes were prepared in-house by the electrochemical anodization of an Al plate [23]. In both cases, one face of the membranes was coated with approximately 150 nm of thermally evaporated Ag. More Ag was electrodeposited (Silver 1024, Technic, Inc.) directly onto the evaporated Ag in order to close any open pores. This Ag layer was then used as the back contact in the electrochemical cell, and more Ag was deposited, further filling-in the pores. The membrane and cell were rinsed with deionized H 2 O, and Au solution was added (Orotemp, Technic, Inc.). Plating was stopped with the desired rod length was reached. The Ag backing was removed by dissolving in 2 mL of 50 % HNO 3 , and the Al 2 O 3 template was dissolved in 2 mL of 5 M NaOH. The rods were sedimented using a laboratory centrifuge. The supernatant was removed and water was added. The rods were resuspended by physical agitation and brief immersion in an ultrasonic water bath. This rinsing process was repeated a total of five times in order to prepare the rods for addition of self-assembled monolayers (SAMs). Rods were derivatized with 2-mercaptoethanesulfonic acid, sodium salt (MESA, Aldrich) by suspending them in 1 mL of 1 mM MESA(aq) for 1 h. The MESA rods were then rinsed as described above, suspended in a total volume of 1 mL, and diluted 1000-fold f...

Interactions among β-sheets occur widely in protein quaternary structure, protein-protein interaction, and protein aggregation and are central in Alzheimer’s and other amyloid-related diseases. This Perspective looks at the structural biology of these important yet under-appreciated interactions from a supramolecular chemist’s point of view. Common themes in the supramolecular interactions of β-sheets are identified and richly illustrated though examples from proteins, amyloids, and chemical model systems. β-Sheets interact through edge-to-edge hydrogen bonding to form extended layers and through face-to-face hydrophobic or van der Waals interactions to form layered sandwich-like structures. Side chains from adjacent layers can fit together through simple hydrophobic contacts or can participate in complementary interdigitation or knob-hole interactions. The layers can be aligned, offset, or rotated. The right-handed twist of β-sheets provides additional opportunities for stabilization of edge-to-edge contacts and rotated layered structures.

ZnO nanoribbons with pseudo‐rectangular cross‐sections (see Figure) are demonstarted to be excellent microcavities with a high quality factor (∼ 3000). The lasing threshold is shown to be inversely proportional to the length of the ribbon for pumping intensities lower than the saturation region. Analysis of the emission spectra points to the possibility of the existence of both pure axial modes and “bow‐tie” cavity modes.