10636 DOI: 10.1021/la100866z Langmuir 2010, 26(13), 10636–10644 Published on Web 05/18/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Synthesis, Characterization, and Self-Organization of Dendrimer-Encapsulated HgTe Quantum Dots Amiya Priyam, Daniel E. Blumling, and Kenneth L. Knappenberger, Jr.* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306 Received March 1, 2010. Revised Manuscript Received May 3, 2010 Mercury telluride (HgTe) quantum dots (QDs) were synthesized in methanol at 5 °C using generation 5 (G5) and 7 (G7) polyamidoamine (PAMAM) dendrimers, which function both as nucleation sites and as nanoparticle stabilizers. Transmission electron microscopy (TEM) data indicate these particles were slightly oblate, with an average aspect ratio of 1.3 ( 0.1 and a minor axis of 2.6 ( 0.3 nm. The crystal phase was determined to be coloradoite (cubic system) by analysis of the electron diffraction pattern. Absorption maxima for HgTe QDs ranged from 950 to 970 nm, depending on the dendrimer generation and concentration. QD size distribution was optimized by careful variation of the Hg 2þ : dendrimer surface group molar ratio for both G5 and G7 dendrimers. An increase in molar ratio from 1:0.5 to 1:4 resulted in a decrease in the half-width at half-maximum (HWHM) of the HgTe bandgap absorption from 68 ( 3 to 52 ( 2 nm, indicating a size distribution focusing of 23 ( 4%. Second-derivative analysis of HgTe QD FTIR absorption spectra suggested that the quantum dots were fully encapsulated by a single G7 dendrimer, whereas multiple G5 dendrimers were necessary to stabilize a single nanoparticle. TEM and FTIR data revealed that the HgTe QDs form two-dimensional necklace-type arrays through a self-organization process, which proceeds through interpenetration of dendritic arms. TEM data further indicated that the average nanonecklace contained 10-15 QDs with an average inter- QD separation of 1.3 ( 0.7 nm and a total chain length of 46 ( 6 nm. 1. Introduction Mercury chalcogenides represent a technologically important class of near-infrared (NIR) materials. Bulk mercury telluride (HgTe) is a semimetal with a negative band gap of -0.15 eV at room temperature and has one of the largest Bohr exciton diameters (80 nm) among II-VI semiconductors. 1-3 Nanocrys- talline HgTe undergoes a semimetal to semiconductor transfor- mation for particle sizes smaller than 18 nm, 2 providing an opportunity to study quantum confinement effects in small to large nanoparticles. Devices based on the NIR quantum dot (QD) platform are expected to impact areas such as solar-to-electric energy conversion, 4 optoelectronic and light-emitting devices, 1,5-9 photonics, 10 and several sensing applications. 11 Recent interest in developing photovoltaics that utilize NIR QDs stems from findings that indicate that solar light harvesting efficiency for these particles is significantly enhanced relative to that of similar devices that operate in the visible frequency range. 6,12 In addition, NIR nanoparticles may allow for improved biological imaging methods since they have potential to provide greater image penetration depths and function in the higher-transparency window of water. 6,13,14 Creation of devices based on these technologies requires development of controlled and cost-effective methods to produce NIR QDs that are characterized by narrow size distributions and long-term stability. Many challenges still remain in achieving size- and shape-controlled synthesis of HgTe QDs. To date, the most successful route to HgTe QDs employs thiol capping in an aqueous medium. 3,5,15 However, the opaqueness of water at wavelengths of >1300 nm makes the optical characterization in aqueous solutions difficult. In general, mercury chalcogenides, as a class of materials, have been found to be difficult to prepare. 2,5,16 Therefore, a new method for HgTe QD synthesis is needed, a method that can provide improved control over the nucleation and growth processes while still allowing for optical characteriza- tion. Here, we present low-temperature (5 °C) dendrimer-mediated *To whom correspondence should be addressed. E-mail: klk@chem.fsu. edu. Phone: (850) 645-8617. Fax: (850) 644-8281. (1) Harrison, M. T.; Kershaw, S. V.; Burt, M. G.; Rogach, A. L.; Kornowski, A.; Eychm€ uller, A.; Weller, H. Pure Appl. Chem. 2000, 72, 295. (2) Green, M.; Wakefield, G.; Dobson, P. J. J. Mater. 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