טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentLilac Amirav
SubjectA Novel Technique for the Production of Semiconductor
Nanocrystals
DepartmentDepartment of Chemistry
Supervisor Full Professor Lifshitz Efrat
Full Thesis textFull thesis text - English Version


Abstract

A novel spray based technique was developed for the production of high quality semiconductor nanocrystals, which offers an attractive alternative to conventional production methods, epitaxial growth and colloidal synthesis. The novel spray technique is simple, low cost, and overcomes limitations of the indicated previous methods. According to this spray based technique, solutions of semiconductor salts are first sprayed into monodispersed droplets, which subsequently become solid nanocrystals by solvent evaporation. Each semiconductor nanocrystal is produced from a single spray droplet upon the full vaporization of the liquid.

The average diameter and size distribution of the final nanocrystals can be controlled and determined by the solute concentration of the sprayed solution and the droplet size, hence by spray production parameters. Thus, spray production parameters were fully examined and optimized for the formation of high quality semiconductor nanocrystals of the desired size and composition.

The experimental work included the design, construction and set-up of different spray systems such as electrospray, thermospray and pneumatic nebulizer. The different spray methods were characterized and compared in terms of available control over the droplet size and distribution function, simplicity of the production process, robustness of the system and reproducibility of the results. The method was demonstrated through the production of CdS nanocrystals.

The production of high quality mono-dispersed (~5% size distribution) CdS nanocrystals in the size range of 3 to 6 nm was demonstrated. The further production of MnS and PbS nanocrystals (with limited preliminary results for ZnS and MoS2) established the generalization of the method for different types of semiconductors.

The method uniquely enables the production of free standing, uncoated semiconductor nanocrystals. While utilizing the novel spray-based technique two unique phenomena were discovered: (a) the formation of spray-produced ordered clusters (~50nm diameter) of MnS nanocrystals (with grain size of 1-2nm), formed due to multi nucleation sites in a single spray droplet. These clusters represent a fascinating higher hierarchy of nanostructures; (b) The assembly of these clusters into micron-size coral-shaped fractal aggregates.

Control over the vaporization degree of the spray droplets enables the production of high quality nanocrystalline semiconductor thin films. The vaporization degree was found to affect the film production mechanism and vary its optical properties (as demonstrated for MnS thin films). 

The novel method is characterized by several new and unique features and benefits that were not produced by the currently available methods. After the successful demonstration of the novel method, the research work focuses on exploring these unique features.