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Solution Synthesis and Optical Properties of SnTe Nanocrystals Ying Xu,† Najeh Al-Salim,‡ Justin M. Hodgkiss,† and Richard D. Tilley*,† †
School of Chemical and Physical Sciences and the MacDiarmid Institute of Advance Materials and Nanotechnology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand ‡ Industrial Research, Ltd., P.O. Box 31-310, Lower Hutt, New Zealand
bS Supporting Information ABSTRACT: A facile approach has been developed for the synthesis of SnTe nanocrystals using triethanolamine as the stabilizing agent. The size of SnTe nanoparticles can be readily tuned from 2.7 to 32 nm. Powder X-ray diffraction and selective-area electron diffraction (SAED) indicate that the nanoparticles adopt the cubic rock-salt crystal structure. The direct band gap of the resulting nanoparticles can be significantly tuned and blue-shifted relative to the bulk value by changing the nanocrystal size.
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in chalcogenide nanoparticles SnS, SnSe, and SnTe have received considerable attention because of their interesting optoelectronic properties.1 SnTe is an important narrow band gap semiconductor material. The direct band gap of bulk SnTe is 0.18 eV at 300 K, which is narrower than that of the bulk SnS (1.1 eV) and SnSe (1.3 eV).2,3 It has been used in near-infrared photodetectors, light-emitting diodes, and solar cells.4 More recently, it has been considered as a promising material for biomedical applications such as hyperthermal therapy.5 These applications would benefit from an increased optical bandgap compared with the case of bulk SnTe. Although several synthetic routes, including mechanical alloying, molecular beam epitaxy (MBE), thermal evaporation, and metal organic chemical vapor deposition (MOCVD),6 have been developed for the deposition of thin films of SnTe, to the best of our knowledge, there is only one report for the synthesis of well-formed SnTe nanoparticles.7 Kovalenko et al. demonstrated a method of forming SnTe nanoparticles of less than 10 nm in size. The synthesis of Kovalenko and co-workers uses the air sensitive organometallic complex Sn[N(SiMe3)2]2 (bis[bis(trimethylsilyl)amino]tin(II)) as the tin precursor and expensive trioctylphosphine telluride (TOPTe) as a tellurium precursor.7 Additionally, the nanoparticles were hydrophobic and so require addition steps to make them suitable for biomedicine.8 Recently, we have developed a novel, greener chemical route to obtain hydrophilic SnS nanoparticles with controllable size using ethanolamine stabilizing ligands.9 Triethanolamine (TEA) has been proven to be beneficial in controlling the particle growth.9 Here, we have modified the method to make size-controlled SnTe nanoparticles. SnTe nanoparticles were synthesized by the addition of a freshly prepared telluride solution to a mixture of SnBr2 and TEA in N,N-dimethylformamide (DMF) at 50 °C. The size of the SnTe nanoparticles can be controlled by further heat treatment or by using different amounts of TEA (see the Supporting Information for full synthesis details). r 2011 American Chemical Society
Figure 1a shows a typical transmission electron microscopy (TEM) image of SnTe nanoparticles obtained without further heat treatment. Nearly monodispersed SnTe nanoparticles were obtained, which were approximately spherical in shape. The inset in Figure 1b shows a HRTEM image of an individual nanoparticle with the atomic lattice fringes clearly visible. A histogram showing the nanoparticle size distribution taken from over 200 nanoparticles is shown in Figure 1c and indicates an average nanoparticle size of 2.7 ( 0.6 nm. Figure 1d shows a typical TEM image of SnTe nanoparticles obtained after further heat treatment (refluxing at 154 °C for 2 min). The inset in Figure 1e shows a HRTEM image of an individual nanoparticle, showing that the nanoparticles are highly crystalline and single crystals. The average nanoparticle size increased to 6.5 ( 1.2 nm (Figure 1f). The results show that the size of the SnTe nanoparticles can be controlled by further heat treatment. The increase in particle size is most likely due to Ostwald ripening.10 The powder X-ray diffraction (XRD) pattern (Figure 2) was taken using Co radiation and could be readily indexed to the cubic rock-salt crystal structure, matching that is expected for SnTe. Neither elemental tin nor tellurium were observed, as was further confirmed by selective-area electron diffraction (SAED) (see Figure SI 1 of the Supporting Information). The average crystallite size of nanoparticles calculated using the Scherrer equation from the broadening of the XRD peaks was consistent with those obtained from the TEM images, indicating that the SnTe nanoparticles are single crystals (Figure 2). Energy-dispersive X-ray (EDX) analysis of the composition of the SnTe nanoparticles showed an atomic ratio of Sn to Te of 1:1, consistent with SnTe (see Figure SI 2 of the Supporting Information). Received: May 24, 2011 Published: May 30, 2011 2721
dx.doi.org/10.1021/cg200660y | Cryst. Growth Des. 2011, 11, 2721–2723
Crystal Growth & Design
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Figure 2. XRD patterns of SnTe nanoparticles prepared in the presence of 2 mL (a), 4 mL (b), and 0.5 mL (c) of TEA, respectively.
Scheme 1. Schematic Illustration of the Formation of SnTe Nanoparticles
Figure 1. (a and b) TEM images of SnTe nanoparticles obtained without further heat treatment. (c) Size distribution histogram of SnTe nanoparticles obtained without further heat treatment. (d and e) TEM images of SnTe nanoparticles obtained with further heat treatment. (f) Corresponding histogram of the nanoparticle size distribution.
Control of SnTe nanoparticle size can also be achieved by using different amounts of TEA (see Figure SI 2 of the Supporting Information). When 0.5 mL of TEA was used in the reaction, the SnTe nanoparticles became irregular in shape and were relatively large in size, being typically over 30 nm. This is most likely due to there being insufficient TEA to stabilize the nanocrystals as they grow. The smallest size and size distribution is achieved with 2 mL of TEA. When 4 mL of TEA was used, the nanoparticles are nearly spherical, with an average size of 12.5 ( 2.0 nm, and larger than is the case with 2 mL of TEA. This is most likely due to the competing reaction shown in Scheme 1 being more dominant with the higher TEA concentrations.11 The [Sn(TEA)n]2þ complex will be more stable with higher amounts of TEA, and so when the Te source is added, only a small amount of SnTe will nucleate, which can grow over time as Sn2þ is released from the complex. The IR absorption measurements were made on drop-cast films of nanoparticles deposited on a CaF2 disk. Figure 3 shows the IR absorption data for the films of the three nanoparticle
Figure 3. IR absorption spectra of SnTe nanoparticle films. The horizontal lines indicate zero-absorption. Inset: a photo of 6.5 nm SnTe nanoparticles in ethanol.
samples with the average sizes 6.5 nm, 12.5 nm, and over 30 nm, respectively. An absorption peak at 2270 nm (0.54 eV) was observed in the spectrum for the sample with the smallest particle size (6.5 nm). This value is higher than the 0.18 eV of the bulk band gap of SnTe and is similar to reports for 7.2 nm SnTe nanoparticles.7 A blue shift in the optical band gap with reduced particle size can be attributed to quantum confinement effects and is strongest for these smaller, 6.5 nm particles. Fourteen nanometer SnTe nanoparticles have been found to have an absorption peak around 3150 nm (0.39 eV), which is in the same position of the OH stretch of TEA.7 An absorption peak around 3000 nm with a shoulder about 3400 nm can be seen on each spectrum and is due to the vibration modes of the TEA and is likely to be obscuring absorption peaks from the 12.5 and 32.4 nm nanocrystals. 2722
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Crystal Growth & Design The smaller 6.5 nm SnTe nanoparticles could be readily dispersed in ethanol to give a brown-black colloidal solution, as shown in the photograph inset in Figure 3. The nanocrystals were stable in air for up to 1 month. This study demonstrates a facile solution-phase synthesis of SnTe nanoparticles soluble in polar solvents of well-defined sizes and size distributions. By altering the size of the SnTe nanoparticles, the optical properties can be tuned and blue-shifted compared to the case of the bulk. The size of the product is relatively easy to control. Our approach can also be adopted for other tin chalcogenide nanoparticles and unlock their use in a number of applications including solar cells and photodetectors.
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(11) Pramanik, P.; Basu, K.; Biswas, S. Thin Solid Films 1987, 150, 269.
’ ASSOCIATED CONTENT
bS
Supporting Information. Details of the synthesis and EDX, SAED, and TEM analysis. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Y.X., N.A., and R.D.T. thank FRST for funding through Grants IIOF VICX0601 and PROJ-13733-NMTS. ’ REFERENCES (1) (a) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Chem. Rev. 2010, 110, 389–458. (b) Leitsmann, R.; Bechstedt, F. ACS Nano 2009, 11, 3505–3512. (c) Baumgardner, W. J.; Choi, J. J.; Lim, Y.-F.; Hanrath., T. J. Am. Chem. Soc. 2010, 132 (28), 9519–9521. (d) Vaughn, D. D., II; Patel, R. J.; Hickner, M. A.; Schaak, R. E. J. Am. Chem. Soc. 2010, 132 (43), 15170–15172. (2) (a) Hickey, S. G.; Waurisch, C.; Rellinghaus, B.; Eychm€uller, A. J. Am. Chem. Soc. 2008, 130, 14978. (b) Franzman, M. A.; Schlenker, C. W.; Thompson, M. E.; Brutchey, L. R. J. Am. Chem. Soc. 2010, 132, 4060–4061. (3) (a) Fouad, S. S.; Morsy, A. Y.; Soliman, H. S.; Ganainy, G. A. J. Mater. Sci. Lett. 1994, 13, 82. (b) Thangaraju, B.; Kaliannan, P. J. Phys. D: Appl. Phys. 2000, 33, 1054. (c) Pejova, B.; Tanusevski, A. J. Phys. Chem. C 2008, 112, 3525. (4) (a) Prince, M. B. J. Appl. Phys. 1955, 26, 534. (b) Loferski, J. J. J. Appl. Phys. 1956, 27, 777. (5) Son, S. J.; Bai, X.; Lee, S. B. Drug Discovery Today 2007, 12, 657–663. (6) (a) Saini, R.; Pallavi; Singh, M.; Kumar, R.; Jain, G. Chalcogenide Lett. 2010, 7, 197. (b) Abramof, E.; Ferreira, S. O.; Rappl, P. H. O.; Closs, H.; Bandeira, I. N. J. Appl. Phys. 1997, 82, 2405. (c) Subba Rao, T.; Ray Samanata, B. K.; Chaudhuri, A. K. Thin Solid Films 1988, 165, 257. (d) Burrafato, G.; Troja, S. O.; Turrisi, E.; Marletta, G. N. Torrisi, Il Nuovo Cimento D 1988, 10, 463. (7) Kovalenko, M. V.; Heiss, W.; Shevchenko, E. V.; Lee, J. S.; Schwinghammer, H.; Alivisatos, A. P.; Talapin, D. V. J. Am. Chem. Soc. 2007, 129, 11354–11355. (8) Cheong, S.; Ferguson, P.; Feindel, K. W.; Hermans, I. F.; Callaghan, P. T.; Meyer, C.; Slocombe, A.; Su, C.-H.; Cheng, F.-Y.; Yeh, C.-S.; Ingham, B.; Toney, M. F.; Tilley, R. D. Angew. Chem., Int. Ed. 2011, 50, 42064209. (9) Xu, Y.; Al-Salim, N.; Bumby, C. W.; Tilley, R. D. J. Am. Chem. Soc. 2009, 131, 15990–15991. (10) Jun, Y.-w.; Lee, J.-H.; Choi, J.-s.; Cheon, J. J. Phys. Chem. B 2005, 109, 14795–14806. 2723
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