Synthesis and characterization of CdSe/CdS core-shell and CdSe/CdS composites

Highly mono-dispersed CdSe/CdS core-shell and CdSe/CdS composites have been prepared by a novel route involving thermolysis in TOPO using Cd(Se2CNMe((II)Hex))(2) and Cd(S2CNMe((n)Hex))(2). The absorption band edge (652 nm, 1.90 eV) of the CdSe-CdS core-shell structure is red shifted (22 nm. 0.017 eV) as compared to the CdSe nanoparticles (630 nm, 1.96 eV) whereas the absorption spectrum of the CdSe-CdS composites shows the absorption band edge at (584 nm, 2.12 eV), blue shifted (46 nm, 0.037 eV) as expected. Photoluminescence (PL, lambda(ex). = 400 nm) of both the core-shell (622 nm) and the composites (588 nm) show values close to band edge emission. A sharper emission maximum with a considerable increase of intensity is observed for core-shell structure as compared to that of CdSe whereas the composite showed a broader emission maximum. The TEM images of the CdSe/CdS core-shell nanoparticles show crystalline, spherical particles with the average size of 53 Angstrom, (+/-7 %), a increase of 8 Angstrom than the average size of CdSe (45 Angstrom) nanoparticles, with a narrow size distribution. The High Resolution Transmission Electron Microscopy (HRTEM) showed lattice spacing intermediate between those for CdSe and CdS as is observed by Selected,Area Electron Diffraction (SAED) and X-ray patterns (hexagonal phase). As expected no interface can be observed by HRTEM between CdSe core and CdS shell. The TEM image of the CdSe-CdS composites show particles with an average size of 48.7 Angstrom (+/-10%).