Syntheses and magnetic properties of Cr2Te3 and CuCr2Te4 nanocrystalsw

Nanoscale magnetic materials are being intensively investigated because of both their unique properties and potential applicability in different fields, including spin-based electronics, recording media, magneto-optics, magneto-caloric refrigeration, biomedical imaging, etc. Chromium-based tellurides in the bulk are known to be ferromagnetic with variable Curie temperatures (TCs). Chromium telluride itself exists in various stoichiometries, including Cr1 xTe, Cr2Te3, Cr3Te4, Cr5Te8 and Cr5Te6. All these compositions crystallize in the hexagonal NiAs-type structure with ordered Cr vacancies. Ferromagnetism in these compounds, with TCs ranging from 180 to 340 K, results primarily from ordering of the magnetic moment localized at the Cr sites. Because of its unusual magnetic properties, metallic Cr2Te3 (TC B 180 K) is one of the better studied systems in this family, but a detailed understanding of its complex magnetic structure is still lacking. Ferromagnetic ordering also occurs in a remarkable class of Cr-based chalcogenide spinels (chalcospinels) that display metallic, semiconducting, or insulating characteristics. In particular, the compounds CuCr2S4, CuCr2Se4 and CuCr2Te4 are metallic with TCs of 377, 430 and 360 K, respectively. 6 Because of their high Curie temperatures, these materials are potential candidates for spin-based electronics and other applications. Furthermore, recent band structure calculations suggest the exciting possibility of inducing half-metallicity, i.e., a material in which there is a gap in one spin band at the Fermi level and no gap in the other spin band, in some of the mixed Cu,Cd chalcospinels. Thus far only a few tellurospinels (MCr2Te4, M=Cu, Ag, Fe) have been identified, among them the Cu andAg analogues are reported to be ferromagnetic at room temperature. Thus, metallic behavior and ferromagnetism with TC above room temperature are the significant characteristics of CuCr2Te4. The material has also been reported to exhibit a pronounced room temperature magneto-optical Kerr effect. Unlike other semiconducting and metallic tellurides, the synthesis of Cr-based tellurides is limited. Conventional solid state methods have been developed for the bulk synthesis of Cr2Te3 and CuCr2Te4 using elemental precursors. 4,10 However, solution-based synthesis of these compounds in any form (either bulk or nano-sized) remains largely unexplored. This can be attributed in part to difficulties in controlling the oxidation state of chromium and limited known Te precursors for phase selective synthesis of Cr2Te3 and CuCr2Te4. The utility of these unique materials can undoubtedly be augmented if they can be synthesized with highly controlled dimensions, such as colloidal nanocrystals. We recently reported chemical routes for the synthesis of phase-pure CuCr2S4 and CuCr2Se4 nanocrystals. 11 Building on our experience with solution-phase synthesis of these magnetic chalcospinels, herein we report the first colloidal syntheses of Cr2Te3 and CuCr2Te4 nanocrystals. The syntheses involve reduction of tellurium with sodium borohydride in boiling trioctylamine to produce telluride ions, followed by reaction with chloride salts of the cation(s). For the synthesis of Cr2Te3 nanocrystals, 0.4 mmol of Te powder and 0.4 mmol of NaBH4 in 25 mL of TOA (98%) were heated to 170–180 1C under vacuum and then to 350–355 1C under N2, and maintained at this temperature for 1 h. In another vessel, 0.4 mmol of CrCl3 6H2O and 2 mL of oleylamine (OLA) were placed under vacuum for 10 min and then in a N2 atmosphere for 10 min at room temperature. The vessel was heated in stages to 80–90 1C under vacuum and then to 180–190 1C under N2. The CrCl3 6H2O and oleylamine mixture was then rapidly injected into the Te-containing vessel. The vessel was then quickly reheated to the original temperature (350–355 1C), and the resulting mixture continually stirred for 30 min. Following this, the mixture was cooled to 60–70 1C and a 1 : 3 mixture of hexane and ethanol was added to precipitate the nanocrystals. A similar procedure was used for the synthesis of CuCr2Te4 nanocrystals but using excess of tellurium powder (0.8 mmol) and NaBH4 (0.8 mmol), and amixture of CuCl2 and CrCl3 6H2O (with 1 : 2 stoichiometry of Cu : Cr) in oleylamine was injected into the Te-containing vessel. Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487, USA. E-mail: [email protected]; Fax: +1 205-348-2346; Tel: +1 205-348-3822 w Electronic supplementary information (ESI) available: Reaction schemes; TEM, EDX images; Raman spectra; isothermal and Arrott plots. See DOI: 10.1039/c2cc32021e ChemComm Dynamic Article Links