Z-Contrast Stem Imaging and Eels of CdSe Nanocrystals: Towards the Analysis of Individual Nanocrystal Surfaces
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assigned directly from the Z-Contrast image without resorting to extensive image simulations. Furthermore, since the Z-Contrast image is a direct projection of the atomic positions, the technique is sensitive to aperiodic detail at the surface unlike phase-contrast HRTEM. [4, 5] Using the sub-nanometer STEM probe for EELS analysis allows chemical analysis on the length scale of a few A. In principle it should thus be possible to obtain both chemical and structural information from individual nanocrystal surfaces in the STEM. [6] EXPERIMENT Sample Preparation CdSe nanocrystals were prepared by the method of Murray [7, 8] as modified by Peng [9] for size-focussing. The TOPO surface ligands were exchanged with pyridine by heating in anhydrous pyridine for several hours. The nanocrystals were subsequently precipitated with hexanes and dissolved in chloroform. Poly (2-methoxy,5-(2'-ethyl-hexyloxy)-pphenylenevinylene) (MEH-PPV) was prepared by the method of Wudl [10] and dissolved in chloroform. CdSe samples were stored in a glovebox until use, MEH-PPV was stored under argon in brown glass vials. TEM samples were prepared by mixing the MEH-PPV and CdSe solutions and spin-coating onto single-crystal NaC! substrates (100 surfaces). Typical parameters were 20pl of 2 mgeml' MEH-PPV/0.05 mg-ml' CdSe solution, spun at 2000 rpm. The films were removed by dipping into a water surface, whereupon the film floats onto the surface as the NaCl dissolves away. The floating films were picked up with lacey carbon coated copper TEM grids (Ted Pella Co.). Film thicknesses were typically in the range from 150-200 A as judged
503 Mat. Res. Soc. Symp. Proc. Vol. 581 ©2000 Materials Research Society
from the optical absorption of identical films spun onto glass slides. Attempts to directly measure the film thickness using AFM have so far not yielded reliable results but indicate significant roughness on the order of 50A consistent with ellipsometry measurements. Specimens for EELS analysis were prepared in a glovebag (Aldrich Atmosbag) purged with dry nitrogen and stored under nitrogen or argon. Specimens were loaded with a glovebag attached to the microscope under nitrogen flow directly from a standard gas cylinder. Specimens for Z-contrast imaging were prepared in air, stored under argon, and loaded in air. STEM EELS analysis was performed in a Vacuum Generators (VG) model HB501 STEM operating at 100kV with an ultimate resolution of 2.2A. However, to optimize EELS data collection, the probe was run with low excitation of the condenser lens and the exact probe size was not measured. A parallel EELS system using a CCD detector with near single-electron sensitivity was employed. The system is described in more detail in [6]. Data were analyzed using Wavemetrics Igor Pro 3.13. Pre-edge backgrounds were fitted to a power law function and subtracted. Integration of stripped edges used Igor Pro's algorithm based on the trapezoidal
method. For scanning individual nanocrystals, the EELS signal was collected with the STEM in area imaging mode at
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