Recently Highlighted Research - Strouse Group Research

September 23, 2002
Volume 80, Number 38
CENEAR 80 38 p. 90
ISSN 0009-2347

The dope on magnetic quantum dots (read full article)

Scientists anticipate that adding magnetic dopants to quantum dots should generate a panoply of interesting quantum effects, which could prove useful for applications such as electronic and magnetic data storage or spintronics. Until recently, however, doping of quantum dots produced inconsistent results, making it difficult to study the dots' properties. Now, Geoffrey F. Strouse, chemistry professor at the University of California, Santa Barbara, and graduate students Khalid M. Hanif and Robert W. Meulenberg report that their synthetic strategy--a single-source precursor method using inorganic clusters--produces quantum dots with controlled, random doping, making them good systems in which to study magnetic spin-spin interactions [J. Am. Chem. Soc., 124, 11495 (2002)]. The researchers find that cobalt-ion doping of cadmium selenide quantum dots has large effects on the magnetic properties of these dilute magnetic quantum dot systems. For example, the temperature of the magnetic phase transition from a paramagnetic state to an antiferromagnetic "spin glass" state (in which magnetic spins freeze in random orientations) increases by an order of magnitude, compared with analogously doped bulk materials.

DNA maintains its functionality when it's attached to nanoparticles, and enzymes can be used to manipulate the conformation of the conjugates, according to chemists at the University of California, Santa Barbara. Chemistry professor Geoffrey F. Strouse and his coworkers--students C. Steven Yun, Gregory A. Khitrov, and Danielle E. Vergona, and chemistry professor Norbert O. Reich--use enzymes to manipulate DNA attached to 1.4-nm gold particles [J. Am. Chem. Soc., 124, 7644 (2002)]. These enzymes interact with the DNA and cause conformational changes. Using transmission electron microscopy, the team demonstrates that the distance separating the gold nanoparticles corresponds to that expected from a properly functioning DNA-protein interaction. Strouse and coworkers are now moving ahead to apply the DNA-gold constructs to "nanoFRET" (fluorescence resonance energy transfer) in which they can double the length of the DNA strands used in FRET.

The second talk in the session was delivered by Khalid Hanif (UC Santa Barbara) who presented a study of electron spin interactions and magnetic behavior in Co doped CdSe nanocrystals. The study of electronic and magnetic properties of quantized systems is of great interest considering the continuing miniaturization of electronic and magnetic data storage components. Nanocrystals (5.5 nm diameter) with doping levels of 0.04 to 0.30 % Co were prepared by controlled decomposition of single source precursor clusters in a coordinating solvent. The doping level was measured by elemental analysis. A uniform distribution of dopant atoms in the CdSe core was confirmed by Vegard's Law, which correlates lattice spacing changes (measured by XRD) with Co doping levels. This previously unreported uniform Co distribution in CdSe nanocrystals made these materials an excellent system to study spin-spin interactions. Nanocrystals with all doping levels exhibited a paramagnetic to antiferromagnetic behavior transition around 4 K, compared to less than 1 K for the bulk system. The transition occurred at a higher temperature for more extensively doped materials because of more prevalent Co-Co interactions. These extended interactions arise from changes in the radial extent of the Co spin interaction, which is found to be 20 times larger than that observed in the bulk system. These results shed light on the effects of quantization on the magnetic behavior of semiconductor systems and the ability to control doping levels in quantum confined materials.

Also appears in the MRS Bulletin, July 2002, Vol. 27, No. 7, Page 551