A novel soft hydrothermal (SHY) route to crystalline PbS and CdS nanoparticles exhibiting diverse morphologies{ Deborah Berhanu, a Kuveshni Govender, a David Smyth-Boyle, a Martin Archbold, b Douglas P. Halliday b and Paul O’Brien* a Received (in Cambridge, UK) 6th September 2006, Accepted 11th October 2006 First published as an Advance Article on the web 25th October 2006 DOI: 10.1039/b612934j We report a simple and rapid aqueous route to crystalline nanoparticles of PbS and CdS using single-source precursors and a conventional household pressure cooker. There is considerable interest in developing simple, inexpensive and environmentally benign protocols (with control over issues such as phase purity, crystallinity and size distribution of products) to grow metal chalcogenide nanomaterials for practical use in e.g. light emitting diodes, non-linear optics, lasers and solar cells. 1 Semiconductor nanoparticles and quantum dots (QD’s) of CdS and PbS are particularly well investigated. 2,3 Telecommunication and biological applications requires QD’s that luminesce primarily in the NIR in the range 1300–1550 nm and 700–900 nm, consequently QD’s based on semiconductors of Group IV–VI materials such as lead chalcogenides are particularly appealing. Factors include their small bulk bandgap (PbS 0.41 eV), ease of size tenability covering the NIR region and, in contrast to most II–VI and II–V semiconductors, lead chalcogenide QD’s exhibit strong quantum size effects due to the large Bohr radii of both electrons and holes, leading to large confinement energies. Consequently, non-linear optical (NLO) behaviour within the confinement regime is expected to be significantly greater than for II–VI materials, thus attracting attention for optical switching and photonic devices. More recently, multiple exciton generation for PbS and PbS QD’s has been reported, with potential to lead to an entirely new paradigm in high efficiency and low cost solar cell technology. 4 Although both size and morphology determine the properties (and ultimately the potential applications) of semiconductor nanoparticles, the ability to direct or tailor the latter during nanoparticle growth processes remains comparatively under- developed in comparison to the former. 5 Addressing this issue would greatly facilitate further fundamental and interdisciplinary studies (e.g. comparison of the optoelectronic properties of nano- rods, tubes, prisms, and cubes of similar dimensions for a single compound). In addition to the obvious benefits to nano- technologists, new insights into crystal growth at the nano- dimensional level would be expected. A variety of synthetic routes to semiconductors nanocrystals have been employed including solvothermal, arrested precipitation (via injection of organometallic precursors in hot coordinating solvents), chemical bath and sol–gel methods, 6 leading to different nanoparticle morphologies for CdS (e.g. nanorods, tetrapods, nanoprisms, etc.) 7 and PbS (nanocubes, nanorods, spherical, dendritic etc.). 8 Protocols that employ single-source (SS) pre- cursors are attractive (i.e. air-stable, non-toxic and easy to handle) as they incorporate all the elementary constituents required to synthesise the final product in a single reagent and often employ clean, low-temperature decomposition routes to yield crystalline nanomaterials with minimal impurity incorporation. 9 A non- aqueous solution route to PbS nanoparticles involving amine- catalysed decomposition of SS precursors under ambient conditions has been reported recently. 10 Ostensibly environmen- tally-friendly approaches using aqueous solvents to deposit metal chalcogenide nanomaterials, such as ‘‘soft-chemical’’ solvothermal, or chemical bath methods often employ toxic chalcogenide precursors (e.g. thioacetamide, a potent hepatotoxin and carcino- gen or sodium sulfide, which is harmful to aquatic species). In addition, the crystallinity of solution deposited materials is often poor, resulting in inferior optical and electronic characteristics. We have previously grown CdS and PbS nanoparticles via the arrested precipitation–‘‘hot-injection’’ method using metal bis- (dialkyldithiocarbamato) single source compounds in hot coordi- nating solvents. 11 In this Communication, we demonstrate a rapid, simple ‘‘soft-hydrothermal’’ (hereafter termed SHY) route to nanocrystalline PbS and CdS, using air stable crystalline complexes as SS precursors. The precursors chosen in this work (A: [2,29-bipyridyl(Pb(SC(O)(C 6 H 5 ) 2 )]; B: [Pb(S 2 (P(C 6 H 5 ) 2 ) 2 N)]); C: [2,29-bipyridyl(Cd(SC(O)(C 6 H 5 ) 2 )]) readily decompose in aqueous media at low temperatures, which facilitates rapid nanoparticle synthesis using a conventional household steam pressure cooker. The as-demonstrated SHY procedure (e.g. non-reflux, air ambient, etc.) differs strongly from solvothermal approaches, which typically entail heterogeneous chemical reactions in the presence of a solvent at supercritical or near-supercritical conditions (and where varying solvents are required to lower temperature–pressure conditions). Transmission electron micrographs of as-grown PbS nanocrys- tals are shown in Fig. 1. In the reactant concentrations and stoichiometries employed herein ({ESI Table A{), it appears that morphological evolution of smooth faceted polyhedral nano- crystallites of PbS operates largely under kinetic control at low levels of supersaturation, thus avoiding emergence of dendritic morphologies (faceted polyhedral PbS nanoparticles with narrow a School of Chemistry, University of Manchester, Manchester, UK M13 9PL. E-mail: paul.obrien@manchester.ac.uk; Fax: + 44 161 275 4616; Tel: +44 161 275 4652 b Department of Physics, University of Durham, Durham, UK DH1 3LE. E-mail: d.p.halliday@durham.ac.uk; Fax: +44 191 334 3585; Tel: +44 191 334 3571 { Electronic supplementary information (ESI) available: Tables of experimental conditions, XRD and FT-IR. See DOI: 10.1039/b612934j COMMUNICATION www.rsc.org/chemcomm | ChemComm This journal is ß The Royal Society of Chemistry 2006 Chem. Commun., 2006, 4709–4711 | 4709