Inorganic cluster syntheses of TM2+-doped quantum dots (CdSe, CdS, CdSe/CdS): physical property dependence on dopant locale.

A series of colloidal transition-metal-doped chalcogenide semiconductor nanocrystals (TM2+:CdSe, TM2+:CdS, etc.) has been prepared by thermal decomposition of inorganic cluster precursors. It is shown through extensive spectroscopic and structural characterization that the nanocrystals prepared following literature procedures for synthesis of TM2+:CdSe nanocrystals actually possess an unintended CdSe/TM2+:CdS core/shell morphology. The conditions required for successful formation of TM2+:CdSe and TM2+:CdS by cluster decomposition have been determined. Magneto-optical and photoluminescence spectroscopic results for this series of doped nanocrystals reveal major physical consequences of dopant localization within the shell and demonstrate the capacity to engineer dopant-carrier exchange interactions via core/shell doping strategies. The results presented here illustrate some of the remarkable and unexpected complexities that can arise in nanocrystal doping chemistries and emphasize the need for meticulous characterization to avoid false positives.

Bimodal bond-length distributions in cobalt-doped CdSe, ZnSe, and Cd1-xZnxSe quantum dots.

Electronic absorption spectroscopy has been used to study changes in Co2+ ligand-field parameters as a function of alloy composition in Co2+-doped Cd(1-x)Zn(x)Se nanocrystals. A shift in the energy of the 4T1(P) excited-state with alloy composition is observed. Analysis reveals that Co2+-Se2- bond lengths change relatively little as the host is varied continuously from CdSe to ZnSe, generating a large difference between microscopic and average cation-anion bond lengths in Co2+-doped CdSe nanocrystals but not in Co2+-doped ZnSe nanocrystals. The bimodal bond-length distributions observed here are shown to cause a diameter-dependent enthalpic destabilization of doped semiconductor nanocrystals.

Direct Observation of sp-d exchange interactions in colloidal Mn2+- and Co2+-doped CdSe quantum dots.

The defining attribute of a diluted magnetic semiconductor (DMS) is the existence of dopant-carrier magnetic exchange interactions. In this letter, we report the first direct observation of such exchange interactions in colloidal doped CdSe nanocrystals. Doped CdSe quantum dots were synthesized by thermal decomposition of (Me4N)2[Cd4(SePh)10] in the presence of TMCl2 (TM2+ = Mn2+ or Co2+) in hexadecylamine and were characterized by several analytical and spectroscopic techniques. Using magnetic circular dichroism spectroscopy, successful doping and the existence of giant excitonic Zeeman splittings in both Mn2+- and Co2+-doped wurtzite CdSe quantum dots are demonstrated unambiguously.

Activation of high-TC ferromagnetism in Co2+:TiO2 and Cr3+:TiO2 nanorods and nanocrystals by grain boundary defects.

Colloidal Co(2+)- and Cr(3+)-doped TiO(2) nanorods and nanocrystals were synthesized and studied by X-ray powder diffraction, electronic absorption spectroscopy, magnetic circular dichroism spectroscopy, magnetic susceptibility, and transmission electron microscopy. The nanorods were paramagnetic as colloids but showed room-temperature ferromagnetism when spin-coated aerobically into films. Crystalline domain size, thermal annealing, and dopant or defect migration are not the dominating factors converting the doped TiO(2) nanocrystals from the paramagnetic state to the ferromagnetic state. The most important factor for activating ferromagnetism is found to be the creation of grain boundary defects, proposed to be oxygen vacancies at nanocrystal fusion interfaces. These defects are passivated and the ferromagnetism destroyed by further aerobic annealing. These results not only help elucidate the origins of the TM(n+):TiO(2) DMS ferromagnetism but also represent an advance toward the controlled manipulation of high-T(C) DMS ferromagnetism using external chemical perturbations.