DOI: 10.1002/cphc.200500428 Alignment of Colloidal CdS Nanowires Embedded in Polymer Nanofibers by Electrospinning M. Bashouti, [a] W. Salalha, [b] M. Brumer, [a] E. Zussman,* [b] and E. Lifshitz* [a] Semiconductor nanocrystals (NCs) exhibit size-controlled spec- tral tunability and chemical flexibility, making them attractive materials for use in new and emerging applications, such as fluorescent tagging, [1] lasing, [2] light-emitting diodes [3] and nanoelectronics. [4] One-dimensional quantum wires (QWs) and quantum rods (QRs) have become a class of attractive materi- als 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 fluores- cence Stokes shift with an increase in the aspect ratio (length/ width), exhibiting a linearly polarized photoluminescence. [5] Semi-empirical pseudo-potential calculations of CdSe rods pre- dicted a crossover of the electronic states at a certain aspect ratio, leading to a transition from plane-polarized to linearly polarized light emission. [5] Recent work [6] on QRs shows a re- duced lasing threshold, compared with spherical NCs, an in- crease in the absorption cross-sections, a reduced Auger re- combination rate and an increase in the optical gain lifetime (all of which are properties that improve the lasing perform- ance). The manipulation of the shape of nanoscale materials has been achieved in the past mainly by growth on a static tem- plate; for example, pyramidal InAs dots and wires are obtained by strain growth on an epitaxial GaAs substrate. [7] GaN QWs were recently prepared inside carbon nanotubes. [8] Hollowed polystyrene and silica nanotubes were prepared by the deposi- tion of these materials on Au nanorods, followed by dissolu- tion of the Au core. [9] Liang et al. [10] produced arrays of CdS rods in the nanopores of anodized aluminum oxide (AAO) fol- lowed by the removal of the AAO matrix. Nanocables of SiC nanowires sheathed with an amorphous SiO 2 coating were pre- pared by a carbothermal reduction of SiO 2 xerogel containing carbon nanoparticles. [11] Other coaxial nanocables, containing silicon carbide and silicon oxide sheathed with boron nitride and carbon, were prepared by laser ablation. [12] Semiconduc- tor/polymer cables were produced by the formation of hol- lowed polyvinylacetate polymer tubes, with hydrophilic inner surfaces that permit the formation of inorganic CdSe wires. [13] There have been several attempts to prepare free-standing QWs. Duan and Lieber [14] developed laser-assisted catalytic growth of GaAs QWs. Chang et al. [15] used an electrochemical method which yielded anisotropic gold particles. Alivisatos and co-workers [16] developed a controlled colloidal synthetic method for the formation of rods and multipod-shaped CdSe nanoscale structures. Lifshitz etal. [17] produced PbSe rods, wires and multipods using template-coordinating surfactants during a colloidal growth at 10–60 8C. Lee et al. [18] produced QWs and QRs of MnS and Cd x Mn 1x S, using template ligands, controlled by a delicate balance between kinetic and thermo- dynamic 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 aniso- tropic 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 nano- wires (NWs) and nanotubes into one- or two-dimensional or- dered arrays have recently been successful. Electric [19] and magnetic [20] fields have been used to manipulate dielectric NWs that are suspended in liquid media. These methods re- quire extensive lithography to fabricate the microelectrodes. Fluid-based methods for aligning NWs have proved successful in generating parallel and cross-bar NW assemblies. Whang etal. [21] aligned NWs with nanometer-to-micrometer-scale con- trol using the Langmuir–Blodgett technique and then transfer- red the NWs to planar substrates using the layer-by-layer proc- ess. Alignment through Marangoni convection of NWs within microchannels was demonstrated by Salalha and Zussman. [22] The alignment of multiwalled carbon nanotubes (MWCNT) and single-walled carbon nanotubes (SWCNT) embedded in poly- mer nanofibers was recently demonstrated using electrospin- ning. [23] By manipulating the electrostatic field in the electro- spinning process, [24] cross-bar structures of NWs embedded in nanofibers could be fabricated. Artemyev et al. [25] showed a unique one-dimensional ordering of CdSe nanorods by attach- ment of the nanorods to the cleaved edge of an epitaxially grown ZnS nanolayer, leading to 70% polarization of the QW emission. Tang etal. [26] aligned CdTe nanocrystals into a one-di- mensional assembly. Viral assembly of oriented quantum dot NWs was discussed recently by Mao et al. [27] This Communication describes a promising route for unidir- ectional alignment of free-standing CdS QWs and the forma- tion of a semiconductor–polymer core-shell fiber by electro- spinning 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. [28] When the electric Maxwell stresses overcome the polymer sol- ution 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 electro- spinning can be used for various applications, such as in drug- release 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:ssefrat@tx.technion.ac.il [b] Dr. W. Salalha, Prof. E. Zussman Faculty of Mechanical Engineering Technion, Haifa 32000 (Israel) Fax:(+ 972)4-8228931 E-mail:meeyal@tx.technion.ac.il 102 # 2006 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemPhysChem 2006,7,102–106