(Invited) Electronic and Optical Properties of Quantum-Confined Nanoparticles

Abstract

We discuss the electronic and optical properties of semiconducting nanoparticles in the quantum-confinement regime and how they can be modeled accurately using first-principles calculations. We address how band-like features emerge from finite electronic states with increasing particle size and how we can control the characteristics of the electronic band structure by means of surface chemistry and alloying. In particular, we are focusing on technological important issues such as bandgap engineering of both magnitude of the bandgap as well as transition type (indirect vs. direct bandgap) and process stability of nanoparticles. We discuss this for several type of semiconducting nano particles composed of environmentally friendly materials, namely we discuss: (i) the origin of the experimentally observed change of the bandgap silicon (Si) nanoparticles upon functionalization with OH groups. We show that the decisive factor here is not as previously proposed surface-strain but charge transfer from the OH groups to the surface electronic states of the particle. (ii) How surface states control the optical properties of silicon carbide (SiC) nano particles, which can lead to in terms of quantum-confinement counter intuitive level diagrams We correlate the frontier electronic states to the measured absolute positions of the conductance band minimum and valance band maximum. (iii) Transition from indirect to direct bandgaps in ultrasmall tin (Sn) and alloyed silicon-tin (SixSn1-x) nanoparticles. Ultrasmall tin nanoparticles are found to have indeed a direct bandgap as recent measurements have proposed but its comparable small bandgap make applications difficult. We show that also for high enough Sn concentration binary silicon-ton alloyed nanoparticles can change their bandgap characteristic from indirect to direct, however the required tin concentration is larger than what one would expect from what is know for bulk silicon-tin. (iv) Lastly, we also investigate the thermal stability of silicon and tin nanoparticles which is especially important for high temperature processing necessary for in cooperating the nanoparticles into existing silicon technologies such as photovoltaic devices. We show that depending on the nanoparticles crystal structure they can remain stable up to several hundred degrees Celsius.