Alignment of colloidal CdS nanowires embedded in polymer nanofibers by electrospinning.

Semiconductor nanocrystals (NCs) exhibit size-controlled spectral tunability and chemical flexibility, making them attractive materials for use in new and emerging applications, such as fluorescent tagging, lasing, light-emitting diodes and nanoelectronics. One-dimensional quantum wires (QWs) and quantum rods (QRs) have become a class of attractive materials as their dimensional anisotropic behaviour gives rise to unique physical properties ; for example, recent observations of CdSe QRs showed a non-monotonic change of the fluorescence Stokes shift with an increase in the aspect ratio (length/ width), exhibiting a linearly polarized photoluminescence. Semi-empirical pseudo-potential calculations of CdSe rods predicted a crossover of the electronic states at a certain aspect ratio, leading to a transition from plane-polarized to linearly polarized light emission. Recent work on QRs shows a reduced lasing threshold, compared with spherical NCs, an increase in the absorption cross-sections, a reduced Auger recombination rate and an increase in the optical gain lifetime (all of which are properties that improve the lasing performance). The manipulation of the shape of nanoscale materials has been achieved in the past mainly by growth on a static template; for example, pyramidal InAs dots and wires are obtained by strain growth on an epitaxial GaAs substrate. GaN QWs were recently prepared inside carbon nanotubes. Hollowed polystyrene and silica nanotubes were prepared by the deposition of these materials on Au nanorods, followed by dissolution of the Au core. Liang et al. produced arrays of CdS rods in the nanopores of anodized aluminum oxide (AAO) followed by the removal of the AAO matrix. Nanocables of SiC nanowires sheathed with an amorphous SiO2 coating were prepared by a carbothermal reduction of SiO2 xerogel containing carbon nanoparticles. Other coaxial nanocables, containing silicon carbide and silicon oxide sheathed with boron nitride and carbon, were prepared by laser ablation. Semiconductor/polymer cables were produced by the formation of hollowed polyvinylacetate polymer tubes, with hydrophilic inner surfaces that permit the formation of inorganic CdSe wires. There have been several attempts to prepare free-standing QWs. Duan and Lieber developed laser-assisted catalytic growth of GaAs QWs. Chang et al. used an electrochemical method which yielded anisotropic gold particles. Alivisatos and co-workers developed a controlled colloidal synthetic method for the formation of rods and multipod-shaped CdSe nanoscale structures. Lifshitz et al. produced PbSe rods, wires and multipods using template-coordinating surfactants during a colloidal growth at 10–60 8C. Lee et al. produced QWs and QRs of MnS and CdxMn1 xS, using template ligands, controlled by a delicate balance between kinetic and thermodynamic growth. Thus, the chemical synthesis of free-standing or template semiconductor QWs and QRs has shown substantial progress in recent years. In contrast, a well-controlled one-dimensional arrangement of QWs and QRs remains a problem. These anisotropic structures could be very important for polarized light sources, for high-resolution detection of polarized light, and for nano-electronic circuitry. A few attempts to assemble nanowires (NWs) and nanotubes into oneor two-dimensional ordered arrays have recently been successful. Electric and magnetic fields have been used to manipulate dielectric NWs that are suspended in liquid media. These methods require extensive lithography to fabricate the microelectrodes. Fluid-based methods for aligning NWs have proved successful in generating parallel and cross-bar NW assemblies. Whang et al. aligned NWs with nanometer-to-micrometer-scale control using the Langmuir–Blodgett technique and then transferred the NWs to planar substrates using the layer-by-layer process. Alignment through Marangoni convection of NWs within microchannels was demonstrated by Salalha and Zussman. The alignment of multiwalled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) embedded in polymer nanofibers was recently demonstrated using electrospinning. By manipulating the electrostatic field in the electrospinning process, cross-bar structures of NWs embedded in nanofibers could be fabricated. Artemyev et al. showed a unique one-dimensional ordering of CdSe nanorods by attachment of the nanorods to the cleaved edge of an epitaxially grown ZnS nanolayer, leading to 70% polarization of the QW emission. Tang et al. aligned CdTe nanocrystals into a one-dimensional assembly. Viral assembly of oriented quantum dot NWs was discussed recently by Mao et al. This Communication describes a promising route for unidirectional alignment of free-standing CdS QWs and the formation of a semiconductor–polymer core-shell fiber by electrospinning QW colloids with a polymer solution. Electrospinning is a generic method attained when an appropriate electrostatic field is applied to a pendant droplet of a polymer solution. When the electric Maxwell stresses overcome the polymer solution surface tension, a jet is injected from the droplet, which is stretched by bending instabilities and eventually solidified into an ultrathin fiber. Polymer nanofibers prepared by electrospinning can be used for various applications, such as in drugrelease systems, in protective clothing, as a load substrate for [a] M. Bashouti, M. Brumer, Prof. E. Lifshitz Department of Chemistry and Solid State Institute Technion, Haifa 32000 (Israel) Fax: (+972)4-8235107 E-mail : [email protected] [b] Dr. W. Salalha, Prof. E. Zussman Faculty of Mechanical Engineering Technion, Haifa 32000 (Israel) Fax: (+972)4-8228931 E-mail : [email protected]