Organometallic Synthesis and Spectroscopic Characterization of Manganese Doped CdSe Nanocrystals
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eMe)2(CO)8 was synthesized by adapting Coleman's procedure for making the analogous tellurium compound [5]. The product (75% yield) has an IR spectrum (carbonyl region) and a 1H NMR spectrum that match literature values for Mn2(µ-SeMe)2(CO)8 [6]. Undoped CdSe quantum dots were synthesized according to previously described literature methods [7]. Doped nanocrystals were prepared in a similar fashion by dissolving (sonication) a small amount of precursor (2-200 mg) into the TOP injection solution. For example, the injection solution used to prepare the FLN/PLE sample consisted of 200 µL CdMe2, 3 mL 1M TOPSe, 30 mg Mn2(µSeMe)2(CO)8, and 16 mL TOP. Surface ligand exchange is performed as previously described [7b]. Electron Paramagnetic Resonance (EPR). Room temperature (300 K, 9.77 GHz) and low temperature (4.2 K, 9.44 GHz) EPR spectra were obtained using a Bruker ESP 300 instrument. Microwave powers employed were below saturation levels. Quantitative standards were prepared by adding known amounts of a manganese (II) cyclohexanebutyrate stock solution to undoped CdSe QDs in toluene. The concentrations of nanocrystals in the standard and the unknown were approximately equal. The amount of manganese added to the reaction which ended up doping the nanocrystals was generally 1% (at.) or less. Wavelength Dispersive X-ray Spectroscopy (WDS). A JEOL SEM 733 electron microprobe operating at 15 kV was used to determine manganese concentrations. The takeoff angle to the detector was 40 ° and the magnification was 1000×. Samples were prepared by precipitating TOPO/TOPSe capped nanocrystals from solution and washing the material three times with methanol. One drop of a very concentrated pyridine solution of QDs was placed on a silicon (100) wafer, dried under vacuum at 80 °C for 24 hours, and then coated with a thin layer of carbon. Solid-state NMR. Magic angle spinning (MAS) NMR experiments were performed using a homebuilt 211 MHz NMR spectrometer (47 MHz for 113Cd) and a custom designed, two-channel transmission line probe. The 113Cd π/2 pulse width was 5 µs. Hahn echo pulse sequences (π/2-τ-π-τ-Acq) were used for all spectra, with the interpulse delay τ equal to the rotor period. A 5 mm Chemagnetics spinning system (Otsuka Electronics USA) was used for MAS, with spinning frequencies of the samples varied from 6.5 to 9 kHz. Spin lattice relaxation times (T 1 values) were estimated from the recycle delays. Optical Spectroscopy . Variable temperature fluorescence line narrowing (FLN) and photoluminescence excitation (PLE) experiments were conducted on doped and undoped QDs using a SPEX Fluorolog-2 spectrofluorometer. Samples were prepared by loading hexane solutions of TOPO/TOPSe capped nanocrystals between two sapphire flats separated by a 0.5 mm Teflon spacer. The absorbance of the lowest energy feature in all samples was kept below 1.0 to minimize reabsorption of the luminescence. The FLN experiment was conducted by choosing five energies over the full luminescence profile of the nanocrystals. In the PLE experiment the detector
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