SnTe Nanocrystals: A New Example of Narrow-Gap Semiconductor Quantum Dots Maksym V. Kovalenko,* ,² Wolfgang Heiss, ² Elena V. Shevchenko, ‡ Jong-Soo Lee, ‡ Harald Schwinghammer, ² A. Paul Alivisatos, ‡ and Dmitri V. Talapin* ,‡ Institute of Semiconductor and Solid State Physics, Johannes Kepler UniVersity Linz, A-4040 Linz, Austria, and The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received June 19, 2007; E-mail: maksym.kovalenko@jku.at; dvtalapin@lbl.gov Over the past decade a significant progress in the synthesis of narrow gap IV-VI (PbS, PbSe, PbTe), 1 II-VI (HgTe, Cd x Hg 1-x Te), 2 and III-V (InAs) 3 nanocrystals (NCs) triggered a recognition of their high potential for various optical, 4 electronic, 5 and optoelec- tronic 6 applications. Typically, the band gaps of these NCs can be tuned between 0.5 and 1.5 eV, covering the entire near-infrared (near-IR) spectral region. 7 The synthesis of colloidal NCs with band gap energy below 0.5 eV is still a challenge, with only limited information available. 8,9 At the same time, narrow-gap semiconduc- tor NCs are highly desirable for photovoltaic, thermovoltaic, and thermoelectric 10 devices as well as numerous optical applications. The recent discovery of efficient carrier multiplication in semicon- ductor quantum dots placed narrow-gap NCs among the most promising materials for thin-film photovoltaics. 11 Bulk SnTe is a IV-VI semiconductor with a direct band gap of 0.18 eV at 300 K 12 . It is used in mid-IR photodetectors 13 and thermoelectric heat converters. 14 The previous attempts to synthesize SnTe NCs did not yield uniform particles of controllable size. 15 Here we report a solution-phase synthesis of high-quality colloidal SnTe NCs with mean diameters tunable in the range of ca. 4.5- 15 nm and corresponding band gaps of 0.8-0.38 eV. As a tin(II) source, we used commercially available Sn- [N(SiMe 3 ) 2 ] 2 , bis[bis(trimethylsilyl)amino]tin(II), also known as Lappert’s stannylene. 16 Our initial attempts to synthesize tin chalcogenides by using less reactive precursors like tin oleate, acetate, or chloride failed because of an improper balance between nucleation and growth rates of the NCs. The synthesis of SnTe NCs is based on the reaction of Sn[N(SiMe 3 ) 2 ] 2 and trioctylphos- phine telluride (TOPTe) in oleylamine (OLA). In a typical synthesis, 17 0.4 mmol of Sn[N(SiMe 3 ) 2 ] 2 dissolved in 6 mL of octadecene (ODE) were injected into a three-neck flask containing a solution of 0.7 mmol of TOPTe in 14 mL of OLA, kept at 150 °C. The almost instantaneous nucleation was followed by a temperature drop to about 120 °C. The reaction was kept at this temperature for 1-2 min and rapidly cooled to room-temperature. A 3 mL aliquot of dried oleic acid (OA) was added to efficiently passivate the NC surface. The SnTe NCs were isolated and purified using the standard solvent/nonsolvent procedure. 17 The as-synthesized SnTe NCs have uniform, nearly spherical shapes (Figure 1a,b). The NCs size distribution was typically below 10% without any size-selection steps (Figure S1, Supporting Information). An analysis of the powder X-ray diffraction (XRD) patterns (Figure 1c) and high-resolution TEM images (Figure 1d,e) revealed the cubic rock-salt crystal structure, identical to that of bulk SnTe (space group Fm3m, a ) 6.235 Å). 12 The NC sizes estimated from the broadening of the XRD reflections were consistent with those deduced from TEM images, indicating a high crystallinity of the SnTe NCs. Energy dispersive X-ray spectroscopy (EDX) showed nearly stoichiometric composition of the SnTe NCs (Figure S2). The monodisperse SnTe NCs self-assembled into long-range ordered superlattices 18 upon slow drying relatively concentrated tetrachloroethylene (TCE) solutions of SnTe NCs by evaporating the solvent in a low-pressure chamber (∼3.2 kPa) at 50 °C (Figure 1a,b). The misalignment of the atomic lattice planes in the vertical rows of the SnTe NCs gives rise to rotational Moire ` fringes seen for some NC columns in Figure 1b. The size of the SnTe NCs can be controllably varied from 4.5 up to about 15 nm by adjusting the injection and growth temper- atures and the concentration of OLA in the reaction mixture (Figure S3). Generally, the NC size increased with raising injection and growth temperatures; the optimal temperature range for synthesis of monodisperse SnTe NCs was observed between 90 and 150 °C. Lowering the concentration of the stabilizing agent (OLA) in the reaction mixture resulted in a decrease of the NC size (Figure S3). Since primary amines form strong complexes with Sn 2+ ions, it is reasonable to expect that lower concentrations of OLA led to the ² University of Linz. ‡ The Molecular Foundry. Figure 1. (a,b) TEM images of a superlattice of 10.2 nm SnTe NCs capped with oleic acid. (c) Powder XRD patterns of SnTe NCs with various sizes. The sizes indicated above each curve are estimated by the Scherrer equation, applied to the width of the [100] peaks. The vertical lines indicate the corresponding reflection positions and intensities for bulk SnTe. (d,e) Representative high-resolution TEM images of SnTe NCs viewed along [001] and [111] zone axes, correspondingly. Published on Web 08/28/2007 11354 9 J. AM. CHEM. SOC. 2007, 129, 11354-11355 10.1021/ja074481z CCC: $37.00 © 2007 American Chemical Society