Optically Detected Magnetic Resonance Study of Core-Shell and Alloy Nanocrystals of HgTe and CdS
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Optically Detected Magnetic Resonance Study of Core-Shell and Alloy Nanocrystals of HgTe and CdS L. Fradkin, L. Langof and E. Lifshitz Dept. of Chemistry and Solid State Inst. Technion, Haifa 32000, Israel, A.Rogach, N. Gaponik, H. Weller, and A. Eychmüller Institute of Physical Chemistry, University of Hamburg, 20146 Hamburg, Germany ABSTRACT The synthesis of HgTe nanocrystals (NCs), coated with a CdS shell, presumably results in the formation of HgTe/CdS, HgTe/CdHgS core-shell structures or separated CdHgS alloys. Photoluminescence (PL), continuous-wave (cw) and time-resolved optically detected magnetic resonance (ODMR) spectroscopy examined the magneto-optical properties of the dominating resonance aforementioned products. The cw ODMR measurements indicated that the NCs exhibit a band, centered at 0.39 Tesla, corresponding to an excited state electron (e) and hole (h) spin manifold, with total angular momentum (F=S+L) Fe=1/2 and Fh=3/2, respectively. Theoretical simulation of the ODMR band revealed an anisotropy of the gfactor, indicating the existence of trapped carriers’ at a mixed Cd-Hg tetrahedral site, confirming the formation of an alloy component. The time-resolved ODMR measurements reveal a characteristic radiative decay time and spin-lattice relaxation time of these trapped carriers of hundreds of microseconds.
INTRODUCTION The chemical synthesis of colloidal HgTe NCs in aqueous solution was recently reported [1,2]. This novel material exhibits an emission band in the near infrared spectral regime, with giant intensity and a tunable energy, varying with the NCs' size. Therefore, these NCs are currently of great technological interest as emitting materials for thin-film electroluminescence devices, and as optical amplifier media for telecommunication networks. These recent studies [2] indicated that the bare HgTe NCs show “aging” processes and temperature instability, associated with the surface reactivity, which deteriorates the luminescence quantum efficiency. Therefore, the authors of ref. [1] and [2] suggested to improve the surface quality by capping the HgTe NCs with epitaxial layers of another semiconductor (e.g., CdS, CdHgS), to form a core-shell structure [3]. The current work shows our attempts to chemically identify carriers’ trapping sites either at the core, the shell or at interface defects, by the use of photoluminescence (PL), continuous-wave and timeresolved optically detected magnetic resonance (ODMR) spectroscopy.
EXPERIMENTAL The “bare” thioglycerol stabilized HgTe colloidal NCs were prepared in aqueous solution, at room temperature, using the method described previously [1]. The starting solution contained a 4:1 ratio of the Hg2+ and Te2- ions. The core-shell structures were prepared by the addition of Cd-perchlorate and addition of H2S to form a shell of CdS (> 2 nm thick) according to the synthetic conditions mentioned in reference [3]. However, the existence of excess Hg2+ ions in the original solution may lead to the formation of a CdHgS alloy as the coating over the “bare” NCs.
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