Synthesis of PbSe nanowires: the impact of alkylphosphonic acid addition Edward E. Foos, * Thomas J. Zega, Joseph G. Tischler, Rhonda M. Stroud and Janice E. Boercker Received 13th September 2010, Accepted 13th November 2010 DOI: 10.1039/c0jm03074k Reaction of Pb oleate with trioctylphosphine (TOP)/Se in 1-octadecene under appropriate conditions yields PbSe nanowires. The parameters found to be critical for the isolation of exclusively one- dimensional material are the Pb : Se ratio and the temperature profile used during synthesis. Addition of strongly coordinating tetradecylphosphonic acid (TDPA) to the system has a profound impact on the surface morphology, with increasing TDPA concentration producing narrower, smoother nanowires when all other parameters are held constant. The wire morphology and structural data are consistent with a combination of oriented attachment on h111i surfaces, yielding wires with a [100] axial orientation, and a ligand-mediated competition between growth on h111i, h110i and h100i planes. The products possess 10–20 nm diameters, with lengths generally over one micron, and have been characterized through SEM, TEM, EDS, electron diffraction, and optical spectroscopy. Introduction From a device fabrication standpoint, one-dimensional nano- wires possess many advantages over their zero-dimensional nanocrystal counterparts. Primarily, these advantages stem from the potential to access the effects of quantum confinement in a structure with one-dimension that extends into the macroscopic regime. This combination of properties can ease difficulties associated with interfacing to a given material, yet still permits transport of electrons through a continuous inorganic structure without relying on hopping or tunneling to adjacent particles. Realization of this concept would be of substantial importance to many electronic applications. A particularly compelling material to examine for this purpose is PbSe, a narrow band gap semi- conductor of interest for both photovoltaic 1 and thermoelectric 2 applications due to its large exciton Bohr radius and ability to absorb and emit infrared photons. 3 It has received significant attention recently as a potential candidate for exploiting multi- exciton generation (MEG) in photovoltaic cells, 4,5 and there are several reports of ‘‘Schottky junction’’ PV cells fabricated from PbSe nanocrystals. 1,6,7 High aspect ratio structures may lead to improvements in these devices by reducing the rate of multi- exciton Auger recombination, 8 while control over surface morphology is an important consideration for improving passivation and minimizing surface trap sites. 9 Herein, we report the synthesis of one-dimensional PbSe nanowires in a non- coordinating solvent system, and demonstrate how the morphology of the material can be impacted through the addi- tion of strongly coordinating phosphonic acid ligands. Various methods have been reported for the formation of one- dimensional nanostructures, 10,11 with the bulk of these relying on either electrochemical deposition in a pre-formed template 12 or vapor–liquid–solid (VLS) 13 growth from a catalyst droplet on a solid support. The synthesis of nanowires in solution, analo- gous to the preparation of colloidal nanocrystals, has been less widely examined despite several potential advantages to the approach. This ‘‘bottom up’’ solution synthesis offers both the potential for smaller diameter structures, as well as scalability to obtain larger quantities of material, both important features for their eventual application. In addition, the organic ligands encapsulating these materials facilitate their manipulation in a variety of solvents, expanding possibilities for self-assembly and low cost device processing. Solution methods incorporating low-melting metal catalyst particles, termed solution–liquid– solid or SLS mechanisms, have been examined for several different material systems, 14–18 but an alternative approach involves inducing one-dimensional crystallization through care- ful manipulation of the reaction conditions. One of the first systems to demonstrate asymmetric growth without addition of catalyst particles was CdSe, 19 where high temperature/high concentration conditions produced nanorod structures. Subse- quent studies have shown that wires with higher aspect ratios can also be isolated from this system. 20 The isolation of PbSe nano- wires has also been observed in a catalyst-free system through reduction of precursors in the presence of bidentate ligands to direct crystallization. 21 More recently, synthesis of PbSe nano- wires produced through the oriented attachment of nanocrystals has been reported, 22,23 a mechanism relying on nanocrystal dipole moments to drive the one dimensional growth. This mechanism is quite distinct from the axial growth mechanism invoked for the preparation of lower aspect ratio CdSe nanorods, where high monomer concentrations coupled with specific ligand Naval Research Laboratory, Washington, DC 20375, USA. E-mail: edward.foos@nrl.navy.mil 2616 | J. Mater. Chem., 2011, 21, 2616–2623 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 2616 www.rsc.org/materials PAPER Published on 05 January 2011. Downloaded by Naval Research Laboratory - Washington on 17/08/2013 20:29:38. View Article Online / Journal Homepage / Table of Contents for this issue