Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the "coffee-stain" effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles.

Spray coating is a scalable and high-throughput process for fabrication of transparent and conducting coatings (TCCs) composed of single-walled carbon nanotubes (SWNTs). Presently the fundamentals of this process are not well understood. We show that suppression of coalescence of spray droplets by sufficiently rapid heat- and mass-transfer yields homogeneous SWNT films by preventing the formation of 'coffee stains' of larger length scale. Such heat and mass transfer is driven by differential evaporation between the top and edges of the drops, whereas thermal and compositional effects on surface tension and buoyancy are weak. Ultrasonic spraying ensures that the droplets are deposited without significant splashing, and delayed splashing at higher Weber number is evidenced. We find that the performance of spray-coated TCCs made from HiPCO SWNTs is limited by bundle diameter rather than length of the constituent SWNTs and bundes. Vapor acid doping with concentrated sulfuric acid roughly doubles the conductivity of the TCCs.

Nanoscale rectennae (rectifying antennae) have been fabricated by combining rectifying organic self-assembled monolayers (SAM) with plasmonic materials because of their surface plasmon resonance (SPR) properties of capturing light. Gold antenna arrays are assembled by coating on CdSe tetrapod templates; the rectifying barrier is formed by self-assembled monolayer (SAM) of gold and electrolyte which contains alkylthiolate. Photocurrent measurements showed that electric currents can be induced at different wavelengths within visible range that strongly depend on the aspect ratio of the tetrapod. Adding gold layer can increase the generated photocurrents due to the rectification. Multiple mechanisms among the semiconductors, metals and electrolytes in response of photocurrent are indicated and analyzed.

Single-walled carbon nanotubes (SWNT) polarize readily in the presence of electromagnetic (EM) fields, enabling a variety of electrochemical reactions. Here, we study the reaction of transition metal ion salts in the presence of surfactant-stabilized SWNT individually suspended in water when activated by alternating EM fields in the radio frequency (RF), microwave (MW), and optical regimes. Atomic force microscopy (AFM) images show formation of novel SWNT nanoparticle-nanotube structures (nanoPaNTs). The resulting nanoPaNTs include SWNT with metallic nanoparticles at one or both tips ("dumbbells"), SWNT toroids, and straight SWNT "threaded" through multiple SWNT rings to form shish-kebab structures. Mixtures of surfactants and polymer apparently modify the local environment of polarized SWNT in a manner that reduces the energy needed for ring formation. We also infer that electrodeposition reactions proceed on a significantly faster time scale than ring formation. These processes,can potentially be used for self-assembly of complex 3-D structures.