Polarity-Driven Quasi-3-Fold Composition Symmetry of Self-Catalyzed III-V-V Ternary Core-Shell Nanowires.

A quasi-3-fold composition symmetry has for the first time been observed in self-catalyzed III-V-V core-shell nanowires. In GaAsP nanowires, phosphorus-rich sheets on radial {110} planes originating at the corners of the hexagonal core were observed. In a cross section, they appear as six radial P-rich bands that originate at the six outer corners of the hexagonal core, with three of them higher in P content along ⟨112⟩A direction and others along ⟨112⟩B, forming a quasi-3-fold composition symmetry. We propose that these P-rich bands are caused by a curvature-induced high surface chemical potential at the small corner facets, which drives As adatoms away more efficiently than P adatoms. Moreover, their polarity related P content difference can be explained by the different adatom bonding energies at these polar corner facets. These results provide important information on the further development of shell growth in the self-catalyzed core-shell NW structure and, hence, device structure for multicomponent material systems.

A step closer to membrane protein multiplexed nanoarrays using biotin-doped polypyrrole.

Whether for fundamental biological research or for diagnostic and drug discovery applications, protein micro- and nanoarrays are attractive technologies because of their low sample consumption, high-throughput, and multiplexing capabilities. However, the arraying platforms developed so far are still not able to handle membrane proteins, and specific methods to selectively immobilize these hydrophobic and fragile molecules are needed to understand their function and structural complexity. Here we integrate two technologies, electropolymerization and amphipols, to demonstrate the electrically addressable functionalization of micro- and nanosurfaces with membrane proteins. Gold surfaces are selectively modified by electrogeneration of a polymeric film in the presence of biotin, where avidin conjugates can then be selectively immobilized. The method is successfully applied to the preparation of protein-multiplexed arrays by sequential electropolymerization and biomolecular functionalization steps. The surface density of the proteins bound to the electrodes can be easily tuned by adjusting the amount of biotin deposited during electropolymerization. Amphipols are specially designed amphipathic polymers that provide a straightforward method to stabilize and add functionalities to membrane proteins. Exploiting the strong affinity of biotin for streptavidin, we anchor distinct membrane proteins onto different electrodes via a biotin-tagged amphipol. Antibody-recognition events demonstrate that the proteins are stably immobilized and that the electrodeposition of polypyrrole films bearing biotin units is compatible with the protein-binding activity. Since polypyrrole films show good conductivity properties, the platform described here is particularly well suited to prepare electronically transduced bionanosensors.

Self-catalyzed GaAsP nanowires grown on silicon substrates by solid-source molecular beam epitaxy.

We realize the growth of self-catalyzed GaAsP nanowires (NWs) on silicon (111) substrates using solid-source molecular beam epitaxy. By optimizing the V/III and P/As flux ratios, as well as the Ga flux, high-crystal-quality GaAsP NWs have been demonstrated with almost pure zinc-blende phase. Comparing the growth of GaAsP NWs with that of the conventional GaAs NWs indicates that the incorporation of P has significant effects on catalyst nucleation energy, and hence the nanowire morphology and crystal quality. In addition, the incorporation ratio of P/As between vapor-liquid-solid NW growth and the vapor-solid thin film growth has been compared, and the difference between these two growth modes is explained through growth kinetics. The vapor-solid epitaxial growth of radial GaAsP shell on core GaAsP NWs is further demonstrated with room-temperature emission at ~710 nm. These results give valuable new information into the NW nucleation mechanisms and open up new perspectives for integrating III-V nanowire photovoltaics and visible light emitters on a silicon platform by using self-catalyzed GaAsP core-shell nanowires.

Surface-passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon.

Continued development of high-efficiency multi-junction solar cells requires growth of lattice-mismatched materials. Today, the need for lattice matching both restricts the bandgap combinations available for multi-junctions solar cells and prohibits monolithic integration of high-efficiency III-V materials with low-cost silicon solar cells. The use of III-V nanowires is the only known method for circumventing this lattice-matching constraint, and therefore it is necessary to develop growth of nanowires with bandgaps >1.4 eV. Here we present the first gold-free gallium arsenide phosphide nanowires grown on silicon by means of direct epitaxial growth. We demonstrate that their bandgap can be controlled during growth and fabricate core-shell nanowire solar cells. We further demonstrate that surface passivation is of crucial importance to reach high efficiencies, and present a record efficiency of 10.2% for a core-shell single-nanowire solar cell.