Development of Metal Selenide Semiconductor Nanocomposites through Cation Exchange at Room Temperature

Abstract

Semiconductor nanocomposites represent a class of semiconductor nanomaterials characterized by the combination of multiple nano-sized phases. Currently they are widely applied in various fields, including light emitting devices, photocatalysts, displays, photovoltaics, etc. The synthesis of these nanocomposites requires controllable methods to manipulate their composition, morphology, and properties. Cation exchange reactions provide an elegant strategy to achieve tunable modifications of pre-synthesized semiconductor nanomaterials, enabling multifunctional designs. In the typical cation exchange process, the guest cations in the solution can partially or fully replace the host cations in the original crystal. As a result, it can either form nanocomposites or a completely new nanocrystal, depending on the degree of exchange. However, most current cation exchange processes often necessitate harsh conditions, including high temperatures, the use of complex and costly organic solvents/ligands and inert gas atmospheres, posing challenges for industrial applications. The imperative is to develop an energy-efficient, cost-effective methodology to apply cation exchange reactions in semiconductor nanocomposite synthesis. Overcoming these challenges is essential to unlock the full potential of this strategy for the benefit of the semiconductor industry. This thesis aims to develop a novel cation exchange reaction methodology to synthesize semiconductor nanocomposites in an energy-efficient and cost-effective way. Specifically, we focus on the metal selenide system. We demonstrate the synthesis of metal selenide nanocomposite through cation exchange reactions at room temperature using three systems: Ag2Se/SnSe, Cu2Se/ZnSe, and Cu2Se/SnSe. Ag2Se/SnSe nanocomposites were successfully synthesized at room temperature in methanol solution through gradual cation exchange of Sn2+ in SnSe nanorods precursor by Ag+, and systematically investigated with the Ag2Se fraction varying in the range from 0% to 100%. We found that by increasing the Ag2Se content in the nanocomposites, the morphology of the synthesized nanocomposites gradually transformed from nanorod to smaller nanoparticles. Simultaneously, the phase distribution of Ag2Se and SnSe changed from core-shell structure to random distribution and finally to phase separation. Spectroscopy measurements revealed the tunability of the direct optical bandgap values in the range between 0.653 eV for 100% Ag2Se and 1.093 eV for SnSe with low Ag2Se content. Additionally, the cation exchange reactions were employed to synthesize Cu2Se/ZnSe nanocomposites starting from ZnSe nanoparticles precursor, which also revealed tunable direct optical bandgaps ranging between 1.573 eV and 2.692 eV. Surprisingly, we discovered that the initial Cu/Zn molar ratio can determine the structure of copper selenide phase in the synthesized nanocomposites. When the Cu/Zn ratio was low, the tetragonal Cu3Se2 formed, whereas a high Cu/Zn ratio led to cubic Cu1.82Se phase. Lastly, we also explored the synthesis of Cu2Se/SnSe nanocomposites through cation exchange strategy starting from SnSe nanorods precursor to demonstrate the conversion of Se2- sublattice from hcp to ccp structure and the change in the cation coordination geometry from octahedral to tetragonal sites simultaneously under room temperature. Our future plans include the expansion of cation exchange reactions to a new nanocomposite system (Ag2Se/ZnSe), the synthesis of nanocomposites with ternary component systems, as well as modifying the electrical properties of semiconductor thin film samples. In conclusion, we have demonstrated the capability of applying cation exchange strategy to synthesize semiconductor nanocomposites in an energy-efficient way, which can pave the path to future applications in the semiconductor industry.