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Advanced Biomedical Engineering Part 9

Tham khảo tài liệu 'advanced biomedical engineering part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Semiconductor II-VI Quantum Dots with Interface States and Their Biomedical Applications 151 superlattices and in core shell QDs by more than 20 cm-1 Dinger et al. 1999 . The authors supposed that the Cd-S vibration related Raman peak appeared at 270 cm-1 which downward shifted on 30 cm-1 from the bulk value of the CdS LO phonon 305 cm-1 due to the formation of an alloyed layer at the interface between CdSe core and CdS shell. The interdiffusion during the ZnS shell growth was also assumed for CdSe ZnS QDs which revealed a similar Cd-S mode Dzhagan et al. 2008 . The role of sulfur as an initiator of the interdiffusion was supported by the fact that the CdS-like peak was observed to be stronger Fig.7 for the samples where sulfur atoms were deposited first CdSe ZnS2 . The larger lattice mismatch for the CdSe ZnS interface can further stimulate interdiffusion. The red shift of the CdSe LO phonon peak after passivation Fig. 7 was explained by the formation of an intermixed core shell interface as well. The late effect is accompanied by the quenching of QD emission intensity. 4. Theoretical analysis of the emission spectra of QDs using mirror boundary conditions in the quantum mechanical description To explain the emission spectra of QDs observed the corresponding system of electronic energy levels for them should be known. Theoretical analysis of the energy spectra and optical properties of nanosized semicoinductor sphere was published first in 1982 Efros Efros 1982 and the discussed core-shell II-VI semiconductor QDs present ideal material for comparison of theory with experiment. Fig. 7. Normalized Raman spectra of CdSe and core-shell QDs. Inset Absorption and PL of CdSe and CdSe ZnS QDs Dzhagan et al. 2008 . 152 Advanced Biomedical Engineering However during almost two decades of investigation of these objects no one publication appeared related to such a comparison. As we shall see the reason for that is simple the calculations performed in Efros Efros 1982 .