Self-Assembly of PbTe Quantum Dots into Nanocrystal Superlattices and Glassy Films Jeffrey J. Urban,* ,²,‡ Dmitri V. Talapin, ² Elena V. Shevchenko, ²,§ and Christopher B. Murray* ,² Contribution from the IBM T.J. Watson Research Center, Nanoscale Materials and DeVices Group, 1101 Kitchawan Road, Yorktown Heights, New York 10598, Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48823, and Department of Applied Physics & Applied Mathematics, Columbia UniVersity, 200 SW Mudd Building, 500 West 120th Street, New York, New York 10027 Received December 6, 2005; E-mail: urban@post.harvard.edu; cbmurray@us.ibm.com Abstract: Monodisperse lead telluride (PbTe) nanocrystals ranging from ∼4 to 10 nm in diameter are synthesized to provide quantum dot building blocks for the design of novel materials for electronic applications. Two complementary synthetic approaches are developed that enable either (1) isolation of small quantities of nanocrystals of many different sizes or (2) the production of up to 10 g of a single nanocrystal size. PbTe nanocrystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical absorption. Assembly of PbTe nanocrystals is directed to prepare nanocrystal solids that display either short-range (glassy solids) or long-range (superlattices) packing order by varying deposition conditions. Film order and average interparticle spacing are analyzed with grazing-incidence small-angle X-ray scattering (GISAXS) and high-resolution scanning electron microscopy (HRSEM). We perform the first optical and electronic studies of PbTe solids and demonstrate that chemical activation of these films enhances conductivity by ∼9-10 orders of magnitude while preserving their quantum dot nature. Introduction Nanomaterials display unique size-tunable physical and chemical properties distinct from their parent bulk compounds. 1,2 Specifically, in semiconductor nanocrystals, the size-tunable electronic and optical properties have their origins in “quantum confinement”, the fact that quantum dots may be synthesized whose radius (r) is comparable to or smaller than the Bohr exciton radius (a B ). 3 Harnessing this nanoscale tunability on a macroscopic length scale could provide exciting new classes of materials for scientific and technological applications. One method to produce these “quantum dot solids” involves the self- organization of individual quantum dot building blocks into two- (2D) and three-dimensional (3D) macroscopic assemblies. 4-6 These nanocrystal solids can have their properties further modified by chemical treatments that optimize the interparticle spacing, passivate electronic traps, or introduce electronic dopants. Constructing semiconductor nanocrystal solids requires highly tunable nanocrystal building blocks whose properties may be rationally designed. Therefore, it is critical to synthesize and study materials existing in the limit of strong-quantum confine- ment (r , a B ), as exemplified by the IV-VI lead chalcogenide quantum dots. These materials, whose excitonic Bohr radii (a B ) range from 20 (PbS) to 46 nm (PbTe), 7 present a degree of confinement not possible in many traditional II-VI and III-V systems whose a B ’s typically range from ∼1 to 10 nm. 7 Synthetic, 8,9 optical, 7,10 and electronic 11,12 experiments on two members of this family, PbS 13 and PbSe, 11,12 have generated significant scientific and technological interest. IV-VI quantum dots could provide useful materials for solar cell, 13 light- harvesting, 14 and telecommunication applications 15 because their large quantum confinement and small band gaps provide materials with size-tunable properties in the near-IR. This family of materials also shares flat, multivalley bandstructures and phonon dispersion relationships that enable fundamental studies of the size dependence of vibrational modes and phonon ² IBM T.J. Watson Research Center. ‡ Michigan State University. § Columbia University. (1) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (2) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (3) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (4) Redl, F. X.; Cho, K.-S.; Murray, C. B.; O’Brien, S. Nature 2003, 423, 968. (5) Shevchenko, E. V.; Talapin, D. V.; O’Brien, S.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 8741. (6) Springholz, G.; Holy, V.; Pinczolits, M.; Bauer, G. Science 1998, 282, 734. (7) Wise, F. W. Acc. Chem. Res. 2000, 33, 773. (8) Cho, K.-S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (9) Murray, C. B.; Sun, S.; Gaschler, W.; Doyle, H.; Betley, T. A.; Kagan, C. R. IBM J. Res. DeV. 2001, 45, 47. (10) Du, H.; Chen, C.; Krishnan, R.; Krauss, T. D.; Harbold, J. M.; Wise, F. W.; Thomas, M. G.; Silcox, J. Nano Lett. 2002, 2, 1321. (11) Talapin, D. V.; Murray, C. B. Science 2005, 310, 86. (12) Romero, H. E.; Drndic, M. Phys. ReV. Lett. 2005, 95, 156801. (13) McDonald, S. A.; Konstantatos, G.; Zhang, S.; Cyr, P. W.; Klem, E. J. D.; Levina, L.; Sargent, E. H. Nat. Mater. 2005, 4, 138. (14) Schaller, R. D. a. K.; V. I. Phys. ReV. Lett. 2004, 92, 186601. (15) Harrison, M. T.; Kershaw, S. V.; Burt, M. G.; Rogach, A. L.; Kornowski, A.; Eychmuller, A.; Weller, H. Pure Appl. Chem. 2000, 72, 295. Published on Web 02/14/2006 3248 9 J. AM. CHEM. SOC. 2006, 128, 3248-3255 10.1021/ja058269b CCC: $33.50 © 2006 American Chemical Society