We have been exploring the surface functionalization of group IV (Si, Ge) and III–V (In_xGa_1-xP, etc.) nanocrystals (NCs) to understand how surface chemistry influences the fundamental processes (charge generation, separation, and recombination) as well as inter-NC charge transfer. These studies are all enabled by nonthermal plasma synthesis that provides clean, highly reactive NC surfaces. Subsequent surface chemistry manipulation yields NCs largely free from competing surface defect states that are thus amenable to detailed spectroscopic studies.

For example, spectroscopic interrogation of plasma-synthesized Si NCs have provided insight into their electron-phonon interactions, quasi-direct optical transitions, and exciton formation dynamics. Doped Si NCs are easily accessible using this technique, which has allowed us to probe the degree of interaction between free carriers and photogenerated electron-hole pairs.[1,2] In addition, we have demonstrated cationic ligand exchange reactions on plasma-synthesized Ge NCs that enables effective electronic coupling and thus inter-particle charge transport in Ge NC films.[3] Finally, recent work has shown that we can use the plasma method to control the morphology of In_xGa_1-xP NCs from hollow to solid, demonstrating the viability of this method in accessing difficult-to-synthesize semiconductor nanostructures.[4]

In this presentation, we will leverage our deep understanding of the surface chemistry to reveal a new way to modulate the emission properties in Si and Ge NCs as detailed in our published[5] and ongoing work. The optical properties of Si and Ge NCs are a subject of intense study and continued debate. The photoluminescence (PL) in particular is known to depend strongly on the surface chemistry, with electron-hole recombination pathways derived from the semiconductor band-edge, surface-state defects, or combined NC-conjugated ligand hybrid states. We will describe the effect of different saturated surface functional groups—alkyls, amides, alkoxides, and alkylthiolates—on the emission properties in nonthermal plasma-synthesized Si and Ge NCs. For example, we find a systematic and size-dependent high-energy (blue) shift in the PL spectrum of Si NCs with amide and alkoxy functionalization relative to alkyl. Converse, alkylthiolate ligands result in a low-energy (red) shift in Si NCs. These results suggest that the atom bound to the Si NC surface strongly interacts with the Si NC electronic wave function and modulates the Si NC quantum confinement, revealing a potentially broadly applicable correlation between the emission energy in quantum-confined structures and the ligand binding group.