Study On Synthesis And Properties Of Cadmium-based Semiconductor Heterostructures

As one of the semiconductor nanomaterials, cadmium-based semiconductorheterostructures have many excellent optical properties, for example, the adjustableabsorption spectrum, narrow and symmetric emission spectrum, which endowed them withimportant applications in areas, such as biological imaging, LED devices and solar cells. Thisthesis mainly aiming at the study of semiconductor heterostructures, puts forward its ownresearch content, and makes contribution to the sduty of semiconductor heterostructures.In order to obtain near infrared emitting matetials, ternary alloy of CdTeSe and quaternaryalloy of CdZnTeSe nanocrystals (NCs) were synthesized. Changing the composition of the NCs,the photoluminescence (PL) peak position can be adjusted from650nm to800nm. So,multiple NCs with near infrared emitting spectrum can be obtained through our method. Inorder to enhance the PL efficiency and stability of the NCs, we coated the NCs with a layer ofCdZnS shell. We also found that the morphology of NCs and its growth kinetics are associatedwith the ligands used in the reaction. Using oleic acid and oleylamine as ligands, tetrapod shapeand rod shape can be obtained, respectively. These NCs have important potential applicationsin the solar cell and LED device.To get high quality CdTe-based core-shell quantum dots (QDs), which have high stabilityand low toxicity, we explored the encapsulation of CdTe QDs with CdZnS and ZnS shell.After the encapsulation of CdTe QDs with CdZnS and ZnS shell, the PL efficiency of thecore-shell QDs are greatly improved, higher than60%. With the increase of the shell thickness,the core-shell QDs showed the red shift of the emission spectrum. In contrast to the large redshift of the CdTe/CdZnS QDs, CdTe/ZnS QDs showed smaller red shift. We find that opticalproperties of the core-shell QDs have a lot relationship with the size of the nucleus and thethickness of the shell. X-ray diffraction spectrum and fluorescent decay spectrum were usedto prove the epitaxial growth of shell material on the surface of the QDs. Because of theirhigh PL efficiency, adjustable emission wavelength and low toxicity, these QDs hold greatpotential in the biological imaging and fluorescence marker area.In order to obtain water-soluble alloyed CdTeSe QDs, in chapter4, we introduced a kindof facile hydrothermal method. In the preparation of water-soluble CdTeSe QDs, we used two different ligands, mercaptopropionic acid (MPA) and L-cysteine (L-Cys) respectively. Wefound that the stability and PL intensity of the L-Cys coated CdTexSe1-xQDs were superior tothe MPA coated CdTexSe1-xQDs. In order to study the relationship between composition andits luminescent properties, we adjusted the Se/Te ratio in the preparation process. Meanwhile,we studied the influence of pH on the growth kinetics of the QDs. Coated with an inorganicCdS shell, the PL intensity and stability of CdTexSe1-x/CdS core-shell QDs had been greatlyimproved, accompanied by a red shift of the emission peak. Because these QDs possessexcellent optical properties, they hold great potential in aspects of fluorescent tags and LEDdevice.As to explore the growth mechanism of the cadmium-based heterostructures, CdS/CdTeheterostructure were prepared. In chapter5, CdS/CdTe heterostructure with tetrapodmorphology was synthesized through seeded growth approach using cubic CdS seeds. Thegrowth of the tetrapod experienced a stage of rapid growth and subsequent Ostwald ripeninggrowth process. Ostwald ripening growth led to the melt of the end of the legs, thus a ’spindle’structure with a cuspidal head and thick waist formed. Subsequently, Au nanoparticles weredeposited on the tetrapod, and photocatalytic experiment was carried out. The existence of Aunanoparticles on the nanorods make it easier for the electronic conduction, thus welldegradation of methyl orange and other organic matter. Photoelectric properties of CdS/CdTeheterostructures are excellent, worthy of in-depth study.In order to explore the growth mechanism of CdS nanorods, in chapter6, CdS nanorodswere prepared using a facile seeded growth approach. Subsequently, Au and CuSnanoparticles were deposited on the nanorods. Through the study, it is found that differentcrystal facets have different reactivity. Reactivity of the±{0001} facets are higher than thelateral facets of the nanorods. Therefore, Au and CuS nanoparticles would selectively depositat the end of the nanorods. CdS-Au and CdS-CuS heterostructures hold excellentphotoelectric properties, and have potential application in the field of photocatalysis and solarcells, worthy of further research.