X-ray spectroscopy study of local microstructures in CdSe quantum dots prepared by UV photolithography

Abstract

Colloidal Quantum Dots (QD) are inexpensive inorganic semiconductor nanoparticles, which are suspended in a solution formed by simple wet chemical methods. However, a major challenge in developing practical optoelectronics devices using these QDs is the difficulty of incorporating them into device structures. Cadmium Selenide (CdSe) QDs are synthesized using cadmium perchlorate hexahydrate and selenourea salts as precursors and 1-thioglycerol as a capping agent and patterned using UV photolithographic techniques on glass substrates. By changing UV light exposure time between 30 – 60 minutes, the size of QDs is found to be formed around 10 nm to 14 nm. With this technique, CdSe QDs and CdSe QDs/polymer matrix systems are grown on glass substrate and their local microstructure is investigated by using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methods. For the polymer, polyvinylcarbazole (PVK) is used. Three QD samples investigated are CdSe/Glass with 60 minutes exposure time, CdSe/Glass with 30 minutes exposure time and the third is on a polymer matrix of CdSe/PVK with 30 minutes exposure time. The chemical nature or electron density at the vicinity of Se atoms in the CdSe QDs samples patterned with exposure times of 30 and 60-minute is the same. The CdSe QDs patterned on PVK matrix clearly exhibits a shift in position of the absorption edge and peak absorption position toward higher energy, resulting from the modification of the electronic structure. The shift of energy position of this sample relative to the energy position of SeO2 indicates the chemical nature is different from the SeO2. The EXAFS results indicate that in the samples of CdSe deposited on PVK matrix, some of the Se atoms exist next to atoms (impurities) other than Cd. Cd-Se bond length calculation from Fourier transform EXAFS spectra is consistent with XANES results.