J. Am. Chem. Soc. 2001, 123, 12207-12214
12207
Electronic Absorption Spectroscopy of Cobalt Ions in Diluted Magnetic Semiconductor Quantum Dots: Demonstration of an Isocrystalline Core/Shell Synthetic Method Pavle V. Radovanovic and Daniel R. Gamelin* Contribution from the Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700 ReceiVed June 22, 2001
Abstract: This paper reports the application of ligand-field electronic absorption spectroscopy to probe Co2+ dopant ions in diluted magnetic semiconductor quantum dots. It is found that standard inverted micelle coprecipitation methods for preparing Co2+-doped CdS (Co2+:CdS) quantum dots yield dopant ions predominantly bound to the nanocrystal surfaces. These Co2+:CdS nanocrystals are unstable with respect to solvation of surface-bound Co2+, and time-dependent absorption measurements allow identification of two transient surface-bound intermediates involving solvent-cobalt coordination. Comparison with Co2+:ZnS quantum dots prepared by the same methods, which show nearly isotropic dopant distribution, indicates that the large mismatch between the ionic radii of Co2+ (0.74 Å) and Cd2+ (0.97 Å) is responsible for exclusion of Co2+ ions during CdS nanocrystal growth. An isocrystalline core/shell preparative method is developed that allows synthesis of internally doped Co2+:CdS quantum dots through encapsulation of surface-bound ions beneath additional layers of CdS.
I. Introduction Semiconductor quantum dots (QDs) have emerged as an attractive class of materials for photonics applications.1-3 Recently, attention has turned to the unusual optical, magnetic, and photophysical phenomena observed when QDs are doped with paramagnetic impurities to form diluted magnetic semiconductor QDs (DMS-QDs).4-14 Among bulk DMS materials, Mn2+-doped II-VI semiconductors have received extraordinary attention due to their giant Faraday rotation and magnetoresistance effects, both of which arise from strong sp-d exchange * Corresponding author. E-mail:
[email protected]. (1) Special Issue on Semiconductor Quantum Dots. MRS Bull. 1998, 23, No. 2. (2) Special Issue on Nanoscale Materials. Acc. Chem. Res. 1999, 32, No. 5. (3) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314-317. (4) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 2532-2540. (5) Norris, D. J.; Yao, N.; Charnock, F. T.; Kennedy, T. A. Nano Lett. 2001, 1, 3-7. (6) Bhargava, R. N. J. Lumin. 1996, 70, 85-94. (7) Hoffman, D. M.; Meyer, B. K.; Ekimov, A. I.; Merkulov, I. A.; Efros, A. L.; Rosen, M.; Counio, G.; Gacoin, T.; Boilot, J.-P. Solid State Commun. 2000, 114, 547-550. (8) Counio, G.; Esnouf, S.; Gacoin, T.; Boilot, J.-P. J. Phys. Chem. 1996, 100, 20021-20026. (9) Feltin, N.; Levy, L.; Ingert, D.; Pileni, M.-P. J. Phys. Chem. B 1999, 103, 4-10. (10) Chen, W.; Sammynaiken, R.; Huang, Y.; Malm, J.-O.; Wallenberg, R.; Bovin, J.-O.; Zwiller, V.; Kotov, N. A. J. Appl. Phys. 2001, 89, 11201129. (11) Bhargava, R. N.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72, 416-420. (12) Tanaka, M.; Qi, J.; Masumoto, Y. J. Lumin. 2000, 87-89, 472474. (13) Bol, A. A.; Meijerink, A. Phys. ReV. B 1998, 58, 15997-16000. (14) Soo, Y. L.; Ming, Z. H.; Huang, S. W.; Kao, Y. H.; Bhargava, R. N.; Gallagher, D. Phys. ReV. B 1994, 50, 7602-7607.
interactions between the semiconductor band electrons and the unpaired spin density localized on the dopant ion.15 Additionally, Mn2+ is an efficient luminescence activator in these materials.16 There is growing evidence that some of these properties may be enhanced by quantum confinement in high-quality DMSQDs.5,7,11,17 A major obstacle in the preparation of high-quality DMSQDs is the rapid increase in surface-to-volume ratios as particle diameters are reduced to strong quantum confinement dimensions (e.g
