Probing the Size- and Environment-Induced Phase Transformation in CdSe Quantum Dots

The structural and electronic properties of CdSe quantum dots were investigated by in situ micro X-ray diffraction, X-ray photoelectron spectroscopy, and UV-visible absorption and emission spectroscopy. The in situ microdiffraction data show that the CdSe quantum dots capped with trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) in toluene exhibit predominantly wurtzite crystal structure, which undergoes a phase transformation to zinc blende crystal structure following drop-casting on Si. Furthermore, decreasing the size of the CdSe quantum dots enhances this phase transformation. The X-ray photoelectron spectra show a systematic increase in the core level binding energies of Cd 3d and Se 3d and the band gap with decreasing size of CdSe QDs exhibiting quantum confinement. In addition, the Cd/Se ratio increases as the size of the quantum dots decreases from 6.6 to 2.1 nm, thereby increasing Se vacancies that cause the phase transformation. Drop-casting CdSe quantum dots on Si alter the arrangement of capping ligands and facilitate significantly more phase transformation.