PUMP-PROBE MICROSCOPY INVESTIGATIONS OF ANISOTROPIC CARRIER DYNAMICS IN RES2 NANOFLAKES AND EFFECTS OF SURFACE PASSIVATION IN SI NANOWIRES

Pump-probe microscopy is a powerful tool for investigating the optical and electronic properties of nanomaterials. With this technique, individual nanowires or nanoflakes can be studied, thereby avoiding ensemble averaging of signals from structures of different morphologies and composition. A high energy pump laser pulse optically excites electrons and holes above the conduction band and below the valence band. Measuring the change in transmission of a probe laser pulse, whose arrival time at the sample varies with respect to the pump, provides insight on how the excited charge carriers behave. For example, an electron and hole can Coulombically bind together to form a quasi-particle called an exciton. After a characteristic time period, called a lifetime, charge carriers relax, i.e., recombine, through a variety of pathways, including Shockley-Read-Hall recombination through surface defect states. This dissertation will discuss the pump-probe microscopy technique as well as experiments performed on rhenium disulfide (ReS2) and silicon nanowires (Si NWs). ReS2 is anisotropic, meaning its optical and electronic properties have preferred physical orientations. Three exciton states have preferred orientations relative to the b-axis in the material. Polarization-resolved pump-probe experiments here illustrate a cascade from higher to lower energy excitons upon pump excitation. This cascade manifests as a rotation in the transmitted probe polarization as the pump-probe time delay increases. For Si NWs, the effects of dopant density and surface passivation on the surface recombination velocity are explored. As the dopant density increases, the carrier-carrier scattering rate increases, thus shortening the effective lifetime and increasing the surface recombination velocity. Two different surface passivation treatments that were hypothesized to decrease surface recombination velocity were also explored. Pump-probe kinetic traces and images show that field effect passivation by atomic layer deposition of aluminum oxide increased both the lifetime and ambipolar diffusion of charge carriers. The findings in this dissertation contribute to our understanding of nanomaterials and pave the way for future advancements in their design and application.