The Phase Dependent Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals and Their Synthesis

Abstract

Colloidal semiconductor nanocrystals have become one of the most versatile systems for studying the fundamental properties of nanoscale materials and their applications. The ternary I-III-VI2 semiconductors hold particular promise for applications due to their flexible stoichiometry, low toxicity constituent elements, and range of desirable band gap energies (0.5 – 3.5 eV). Furthermore, I-III-VI2 nanocrystals can be isolated in metastable, anisotropic crystal structures not seen in the bulk. This structural anisotropy can be exploited to produce nanostructures with asymmetric morphology and electronic structure, which can enhance their performance in optoelectronic applications.

In this dissertation, metastable, anisotropic crystal structures of I-III-VI2 materials are synthesized and their optoelectronic properties are investigated. CuInS2 has been widely explored for use in solar energy capture due to its band gap near the visible spectral region. Here, a direct synthesis to luminescent CuInS2 nanocrystals with the anisotropic wurtzite phase is developed and the mechanism of their formation is identified. A combined experimental and theoretical approach is then used to identify the radiative defect responsible for the luminescence observed. Furthermore, hybrid wurtzite CuInS2-Pt nanocrystals are prepared and their photoelectrical properties characterized to determine the efficacy of this system in photocatalytic applications.

The knowledge obtained from the CuInS2 system is then applied to additional I-III-VI2 materials, CuFeS2 and AgFeS2. Wurtzite CuFeS2 is prepared using three distinct synthetic routes and the resultant nanocrystals are compared to each other and the In-containing analogues. Anisotropic, orthorhombic nanocrystals of AgFeS2 are also synthesized and characterized for the first time. The presence of Fe in both these systems leads to the observation of broad multimodal absorbance features at low energy, which can be utilized in thermoelectric and photothermal applications. Experimental measurements and density functional theory calculations indicate that this unique absorbance originates from changes in the composition of the nanocrystals.