Letters High-Quality Manganese-Doped ZnSe Nanocrystals

We demonstrate high-quality, highly fluorescent, ZnSe colloidal nanocrystals (or quantum dots) that are doped with paramagnetic Mn2+ impurities. We present luminescence, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements to confirm that the Mn impurities are embedded inside the nanocrystal. Optical measurements show that by exciting the nanocrystal, efficient emission from Mn is obtained, with a quantum yield of 22% at 295 K and 75% below 50 K (relative to Stilbene 420). MCD spectra reveal an experimental Zeeman splitting in the first excited state that is large (28 meV at 2.5 T), depends on doping concentration, and saturates at modest fields. In the low field limit, the magnitude of the effective g factor is 430 times larger than in undoped nanocrystals. EPR experiments exhibit a six-line spectrum with a hyperfine splitting of 60.4 × 10-4 cm-1, consistent with Mn substituted at Zn sites in the cubic ZnSe lattice. Nanometer-scale semiconductor crystallites, also referred to as nanocrystals or quantum dots, have been extensively studied to explore their unique properties and potential applications.1 Interesting behavior arises in these materials due to the confinement of optically excited electron-hole pairs by the crystallite boundary. However, while the basic explanation of this phenomenon, known as the quantum size effect, was provided early in the investigation of these materials,2-4 a detailed understanding required the advent of high-quality colloidal nanocrystals, which were uniform in size, shape, crystallinity, and surface passivation. Once such materials became available,5 tremendous progress was made in a variety of physical studies. Consequently, many of the properties of semiconductor nanocrystals are now understood in detail.1 In addition, high-quality crystallites have led to more complicated nanocrystal-based structures, such as quantum-dot solids,6 light-emitting devices,7 and even photonic crystals.8 These successes have encouraged researchers to go beyond pure nanocrystals and investigate particles that are intention* E-mail address: [email protected]. Homepage: www.neci.nj.nec.com/homepages/dnorris/. NANO LETTERS 2001 Vol. 1, No. 1 3-7 10.1021/nl005503h CCC: $20.00 © 2001 American Chemical Society Published on Web 11/28/2000 Report Documentation Page Form Approved