SnSe Nanocrystals: Synthesis, Structure, Optical Properties, and Surface Chemistry William J. Baumgardner,* ,† Joshua J. Choi, ‡ Yee-Fun Lim, ‡ and Tobias Hanrath* ,§ Department of Chemistry and Chemical Biology, School of Applied and Engineering Physics, and School of Chemical and Biomolecular Engineering, Cornell UniVersity, Ithaca, New York 14853 Received February 16, 2010; E-mail: wjb87@cornell.edu; th358@cornell.edu Abstract: The colloidal synthesis of SnSe nanoparticles is ac- complished through the injection of bis[bis(trimethylsilyl)amino]tin(II) into hot trioctylphosphine:selenium in the presence of oleylamine. Through the manipulation of reaction temperature particles are grown with the average diameter reliably tuned to 4-10 nm. Quantum confinement is examined by establishing a relationship between particle size and band gap while the in depth growth dynamics are illuminated through UV-vis-NIR spectroscopy. Sur- face chemistry effects are explored, including the demonstration of useful ligand exchanges and the development of routes toward anisotropic particle growth. Finally, transient current-voltage proper- ties of SnSe nanocrystal films in the dark and light are examined. Colloidal IV-VI compound semiconductor nanocrystals (NCs) have garnered immense interest from fundamental and applied research communities alike. NC size, shape, and surface states have an important influence on confinement of electron and hole wave functions within the NC. 1-5 On the other hand, the influence of crystal symmetry is less understood. Lead chalcogenide NCs (PbX; X ) S, Se, Te) have been a successful model system for the fundamental study of quantum confinement since their narrow energy gap and electronic structure provide access to one of the most strongly quantum confined systems, which have recently been exploited in prototype optoelectronic devices. 6-9 To investigate the relationship between quantum confinement and NC symmetry however, alternative systems are required. The entire lead chalcogenide family exhibits cubic NaCl symmetry (space group: Fm3 j m); by contrast tin chalcogenides exhibit a richer diversity of crystal symmetries. 10 For example, SnTe exhibits cubic NaCl symmetry, while SnO forms a tetragonal lattice (space group: P4/nmm) with a van der Waals gap separating the (100) planes. 11 SnSe adopts features of each, crystallizing into an orthorhombic structure (space group: Pnma) which may be viewed as a highly distorted NaCl lattice. 12 Alongside spatial confinement, these structural aspects, in particular the lack of dangling bonds and the stereochemical activity of the Se (5s) lone pair in SnSe, are expected to strongly influence the electronic nature of the (100) surface and the NC as a whole. 13,14 This unique combination of crystallographic and optical properties, including slow carrier relaxation rates and a mid-IR band gap (0.9 eV indirect and 1.3 eV direct), render SnSe NCs as a promising, lead-free building block for a variety of optoelectronic applications. 15 Despite this promise, progress toward developing the potential of SnSe NCs has been slow. Recently, a synthesis of elongated, anisotropic nanocrystals of fairly large size (19 nm wide, 50-100 nm long) was reported, yet robust control over size, shape, and surface chemistry remains a significant hurdle. 16 To address this challenge, we studied the hot-injection colloidal synthesis of SnSe NCs. Beyond illustrating the relationship between NC size and the quantum confined energy gap, we discovered that SnSe NCs exhibit orthorhombic symmetry with a mixture of Pnma and Cmcm space groups, which has previously only been observed in bulk phases at high temperatures. 17,18 We analyzed fundamental NC nucleation and growth aspects and found that the SnSe NC shape is sensitive to the type of surface ligand employed during the synthesis. While we were able to synthesize crude SnSe QDs with other tin precursors (such as tin oleate), we found the most success utilizing bis[bis(trimethylsilyl)amino]tin(II), the same precursor employed in previous reports of SnTe and SnS NC syntheses. 19,20 The synthetic method for SnSe NCs was as follows: 2 mL of 0.1 M trioctylphospine: selenium were injected into 14 mL of degassed oleylamine under an inert atmosphere. 780 μL of bis[bis(trimethylsilyl)amino]tin(II) were diluted in 6 mL of oleylamine and injected into the reaction mixture at the adjusted temperature in the range of 65 to 175 °C. Shortly after nucleation, 3 mL of oleic acid were injected into the mixture, which was quenched after 2 min. The particles were cleaned and precipitated using hexane and isopropanol as a solvent and antisolvent respectively. Energy dispersive X-ray spectroscopy confirms the nearly stoichio- † Department of Chemistry and Chemical Biology. ‡ School of Applied and Engineering Physics. § School of Chemical and Biomolecular Engineering. Figure 1. (a) Low resolution image of SnSe NCs synthesized at 155 °C. (b) High resolution image showing (200) and (111) lattice fringes (inset) fast Fourier transform of (b). (c) (001) and (010) projections of a model SnSe unit cell with Pnma symmetry. Published on Web 06/28/2010 10.1021/ja1013745  2010 American Chemical Society J. AM. CHEM. SOC. 2010, 132, 9519–9521 9 9519