Synthesis of complex two-dimensional heterostructures with tailored optoelectronic properties

• Applicants: Privatdozent Dr. Tobias Kipp; Professor Dr. Alf Mews
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510528670

Complex semiconductor/semiconductor heteronanostructures or metal/semiconductor hybrid structures are of great interest for various applications in optoelectronics. Semiconductor heterostructures can be used in photovoltaics by separating photo-generated electron-hole pairs across the hetero-interface. Conversely, charge carriers injected into the heterostructure can recombine in the interface region, emitting light. Metal/semiconductor hybrid structures are ideally suited for photocatalysis, e.g., by transferring electrons optically generated in the semiconductor material to attached metal particles, which can reduce protons to hydrogen. In this project, chemical synthesis methods for various two-dimensional hetero- and hybrid nanostructures will be developed. By controlled epitaxial growth of one semiconductor material onto another, SnSe/SnS core/shell nanosheets are to be synthesized, for example, in which the valence band edge progression can be selectively adjusted via anion variation. Furthermore, cation exchange reactions will be used to synthesize Janus-like CuS/CdS hetero-nanosheets starting from CuS nanosheets, in which the conduction band energy and thus the electron localization is controlled via the cation gradient. Finally, the targeted growth of metal nanocrystals on the edges of the nanosheets should lead to hybrid structures in which the photo-generated electrons are localized in the edge metal particles, resulting in particularly efficient photocatalysts. In addition to the chemical synthesis of the nanosheets, the research of their optoelectronic properties is in the foreground of this project. An important goal here is to directly follow the localization of the photogenerated charge carriers in individual hybrid and heterostructures. The central experiment combines a confocal laser scanning microscope with a scanning force microscope, which allows the spatially resolved measurement of electrostatic forces under local illumination of the nanostructure. On uncontacted hetero-nanosheets, both the potential profile across the heterointerfaces and the redistribution of photogenerated charge carriers will be quantitatively determined with high spatial resolution. Time-resolved measurements will also be developed here to directly follow the charging and discharging dynamics. On electrically contacted nanosheets, additional scanning photocurrent measurements will be combined with the electrostatic atomic force measurements to study the potential profile, especially at the contacts. These experiments will also be performed using focused X-rays from synchrotron radiation sources to significantly increase the spatial resolution of the photocurrent measurements and to combine them with versatile X-ray spectroscopy methods.

• DFG Programme: Research Grants