Synthesis and Characterization of Strongly Fluorescent CdTe Nanocrystal Colloids
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ABSTRACT We present a synthesis of colloidal CdTe nanocrystals whose absolute room temperature quantum yields are routinely above 60%. The preparation is based on the trioctylphosphine oxide (TOPO) method reported by Murray, with a more stalbe tellurium precursor now used as the chalcogenide source. The photoluminescence is continuously tunable over the range 590-760 nm and is as narrow as 135 meV (45 nm) FWHM. No deep trap luminescence is detected for the diameter range 4-11 nm. CdTe nanocrystals are characterized by UV/vis absorption, photoluminescence emission, transmission electron microscopy, and powder X-ray diffraction.
INTRODUCTION There exists a need for semiconductor nanocrystals which display narrow, intense band edge photoluminescence (PL), especially if such materials are to be routinely used in biological labeling experiments [1] or electroluminescent devices [2]. Recently reported (core)shell materials (CdSe)ZnS [3] and (CdSe)CdS [4) can achieve quantum yields as high as 50 and 85%, but these optimal values have only been seen for samples emitting in the green through orange (- 490-590 nm) region of the spectrum. Far red emitters (> 620 nm) would cover a portion of the spectrum for which there is presently no nanocrystalline semiconductor material - and very few molecular dyes - with very narrow, intense band edge PL. Fluorophores which posses these qualities would be extremely useful for multicolor detection schemes and in whole blood assays. In this letter we present a synthesis of colloidal CdTe nanocrystals whose PL is much brighter, narrower, and cleaner than that of previously reported samples [5]. The PL can be tuned continuously from 590 to 760 nm, thus complementing the (core)shell work described above. The absolute quantum yield of the CdTe nanocrystals reported here is as high as 70% and - attesting to the reproducibility of the method - the average quantum yield of 15 of the best samples is 60 ± 8%. Like commercial laser dyes, solutions of these nanocrystals glow brightly in room lighting. These nanocrystals are as bright as the best (core)shell composites mentioned above, even though the CdTe samples do not yet have a protective inorganic overcoating.
EXPERIMENTAL Synthesis of CdTe nanocrystals. Unless otherwise noted, all procedures are carried out under nitrogen. A stock solution of hexapropylphosphorustriamide telluride (HPPTle) is prepared by adding 6.38 g tellurium shot (99.999%, low oxide, Alfa/Aesar) to 45.00 g hexa-n-propyl phosphorous triamide (97% Lancaster; distilled and collected from 83-103 'C @ 0.55 Torr (uncorrectedI) and stirring until dissolved (1-2 days). Dimethyl cadmium (99+% Strem) (CAUTION: pyrophoric, toxic, readily absorbed
139 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
through skin) is vacuum transferred to remove impurities. 20g TOPO (90% Strem) is dried under vacuum (- 0.5 Torr) at 180 'C for 1 hour, then filled with N2 and heated to 350 'C. In a N 2 atmosphere glovebox a solution containing 50 pL dimethyl cadmium (0.69 mmol), 0.
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