Directed Coassembly of Oriented PbS Nanoparticles and Monocrystalline Sheets of Alkylamine Surfactant

We demonstrate control over the orientation of PbS nanoparticles by way of directed assembly, which in turn affects the crystal structure of alkylamine surfactants such as octadecylamine (ODA, C₁₈H₃₇NH₂) and hexadecylamine (HDA, C₁₆H₃₃NH₂). This directed assembly method results in the arrangement of PbS nanoparticles with a well-defined epitaxial orientation on lamellar alkylamine sheets, which undertake a new crystal structure to accommodate these relations. Understanding these surfactant–nanoparticle inter-relations is very instrumental in understanding surfactant-assisted nanoparticle synthesis and assembly

Twinning and Phase Control in Template-Directed ZnS and (Cd,Zn)­S Nanocrystals

We report on the nucleation and growth of ZnS and (Cd_<i>x</i>­Zn_1–<i>x</i>)­S nanocrystals on polydiacetylene Langmuir films. It was found that the 3–5 nm nanocrystals form ordered linear arrays aligned at a constant 27° angle with respect to the conjugated direction of the polydiacetylene film, as derived from the optimal alignment between the two phases. ZnS nanocrystals were found to nucleate specifically from the zinc blende (001) face. Because of closely matched interfacial relations, twinning defects were induced on the {111} planes. These nanometer-sized twin crystals exhibit extra {111} electron diffraction reflections due to elongation of reciprocal space spots, despite their off axis orientation. The composition of solid solution (Cd,Zn)­S nanocrystals depends on the Zn²⁺\Cd²⁺ ratio in the aqueous subphase. Their structure is affected by the template mismatch both by twinning, as is the case for ZnS, for which continuous compositional shift is observed, and by phase shift to hexagonal wurtzite, with a pure CdS composition. The nanocrystals exhibited a continuous energy-gap shift, reflecting the Zn/Cd ratio in the solid solution. We demonstrate control over the nanocrystals’ crystal structure, defect structure, orientation, and composition, providing a potentially effective tool for band-gap engineering in organic–inorganic hybrid assemblies

Origin of the Contact Angle Hysteresis of Water on Chemisorbed and Physisorbed Self-Assembled Monolayers

Self-assembled monolayers (SAMs) are known to form on a variety of substrates either via chemisorption (i.e., through chemical interactions such as a covalent bond) or physisorption (i.e., through physical interactions such as van der Waals forces or “ionic” bonds). We have studied the behavior and effects of water on the structures and surface energies of both chemisorbed octadecanethiol and physisorbed octadecylamine SAMs on GaAs using a number of complementary techniques including “dynamic” contact angle measurements (with important time and rate-dependent effects), AFM, and electron microscopy. We conclude that both molecular overturning and submolecular structural changes occur over different time scales when such SAMs are exposed to water. These results provide new insights into the time-dependent interactions between surfaces and colloids functionalized with SAMs when synthesized in or exposed to high humidity or bulk water or wetted by water. The study has implications for a wide array of phenomena and applications such as adhesion, friction/lubrication and wear (tribology), surfactant–solid surface interactions, the organization of surfactant-coated nanoparticles, etc

New Nanocrystalline Materials: A Previously Unknown Simple Cubic Phase in the SnS Binary System

We report a new phase in the binary SnS system, obtained as highly symmetric nanotetrahedra. Due to the nanoscale size and minute amounts of these particles in the synthesis yield, the structure was exclusively solved using electron diffraction methods. The atomic model of the new phase (<i>a</i> = 11.7 Å, <i>P</i>2₁3<i>)</i> was deduced and found to be associated with the rocksalt-type structure. Kramers–Kronig analysis predicted different optical and electronic properties for the new phase, as compared to α-SnS

A Bottom-Up Approach toward Fabrication of Ultrathin PbS Sheets

Two-dimensional (2D) sheets are currently in the spotlight of nanotechnology owing to high-performance device fabrication possibilities. Building a free-standing quantum sheet with controlled morphology is challenging when large planar geometry and ultranarrow thickness are simultaneously concerned. Coalescence of nanowires into large single-crystalline sheet is a promising approach leading to large, molecularly thick 2D sheets with controlled planar morphology. Here we report on a bottom-up approach to fabricate high-quality ultrathin 2D single crystalline sheets with well-defined rectangular morphology via collective coalescence of PbS nanowires. The ultrathin sheets are strictly rectangular with 1.8 nm thickness, 200–250 nm width, and 3–20 μm length. The sheets show high electrical conductivity at room and cryogenic temperatures upon device fabrication. Density functional theory (DFT) calculations reveal that a single row of delocalized orbitals of a nanowire is gradually converted into several parallel conduction channels upon sheet formation, which enable superior in-plane carrier conduction

Oriented Attachment: A Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films

We have studied columnar PbSe thin films obtained using chemical bath deposition. The columnar microstructure resulted from an oriented attachment growth mechanism, in which nuclei precipitating from solution attached along preferred crystallographic facets to form highly oriented, size-quantized columnar grains. This is shown to be an intermediate growth mechanism between the ion-by-ion and cluster growth mechanisms. A structural zone model depicting the active growth mechanisms is presented for the first time for semiconductor thin films deposited from solution. The columnar films showed well-defined twinning relations between neighboring columns, which exhibited 2D quantum confinement, as established by photoluminescence spectroscopy. In addition, anisotropic nanoscale electrical properties were investigated using current sensing AFM, which indicated vertical conductivity, while maintaining quantum confinement