Titolo della tesi: Carrier dynamics in semiconductor nanowires
This Ph.D. thesis presents results on the ultrafast spectroscopy of semiconductor nanowires with the aim of studying the carrier dynamics in these quasi-one-dimensional nanostructures. Six different semiconductor nanowire systems were studied using optical measurement techniques in the span of the last three (2017-2020) years and their results are discussed here.
Fast transient absorption spectroscopy with a femtosecond laser source was the primary experimental technique used throughout this thesis. With the use of a femtosecond laser system, the time evolution of photoexcited carriers in the nanowire structures was probed, giving insights into several fundamental physical phenomena of the photoexcited carriers. Several other optical measurement techniques such as photoluminescence, cathodoluminescence, Raman spectroscopy and UV-Vis steady state spectroscopy were also used.
The first material investigated for this thesis was Si nanowires grown through plasma enhanced chemical vapor deposition. These nanowires were grown on a transparent quartz substrate, and the as-grown samples were used for studying the optical response to light excitation using a femtosecond laser with energy less than the direct band gap (3.3 eV) energy of Si. Even when excited below the direct band gap energy, an absorption signal was observed at 3.3 eV in the transient absorption measurements. By comparing the results obtained in this thesis with those obtained by the excitation above the direct band gap energy, this work has enabled me to disentangle the electron and hole dynamics.
The second material under study was InP nanowires. InP nanowires of both zincblende and wurtzite structures were studied using ultrafast transient absorption spectroscopy. The samples were probed both in the visible and in the NIR spectral region. The changes in the band structure due to changes in the crystal structure were observed in the form of different energy transitions in different crystal structures. The transient absorption response was systematically studied to understand both the spectral and kinetic properties of these electronic transitions. Carrier temperature of photoexcited carriers as a function of delay times were also extracted for the highest energy transition in the wurtzite InP with the help of these measurements. The energy loss rate by the hot carriers were also calculated as a function of carrier temperature giving insights into the occurrence of a phonon-bottleneck.
The third material under study was GaAs nanowires. This short study investigated the photoinduced changes in the visible spectral region. This study was done with a high pump energy with the aim of observing the two critical points in the band structure of GaAs namely, E1 and E1 + ∆. The most common experimental technique to observe the critical points is ellipsometric studies, however, in this thesis their observation using ultrafast spectroscopic techniques are presented.
The NWs of ternary alloy semiconductor GaAsP, with about 20 % phosphide and 80% arsenide content were studied next. This study was aimed at investigating the rate of hot carrier cooling as a function of the diameter of the nanowires after photoexcitation using an ultrafast laser pulse. Carrier temperatures and energy loss rates were extracted from the analysis of the transient absorption spectra. The experimental data provided direct evidence that NWs with smaller diameter sustain higher carrier temperatures compared to NWs with larger diameter for longer periods of time.
The fifth system under study was ZnSe nanowires decorated with Ag-nanoparticles. This study was aimed at understanding the modifications in the optical properties and carrier dynamics of ZnSe nanowires when Ag plasmonic nanoparticles were deposited on their sidewalls. Ag-nanoparticles were deposited on the sidewalls of ZnSe nanowires through thermal dewetting, creating a physical contact between the two. The energy of the local surface plasma resonance of these nanoparticles was very close to the optical band gap of ZnSe nanowires. Low temperature photoluminescence measurements showed significant changes in the line shape of donor acceptor pair bands of ZnSe, with enhanced phonon replicas in the presence of Ag-nanoparticles. Ultrafast spectroscopy measurements showed changes in the rise time and decay time of transient absorbance signal in the presence of Ag-nanoparticles. As a comparison, ZnSe nanowires were also decorated with Au-nanoparticles, in which case there was no overlap between the energy of local surface plasmon resonance of Au-nanoparticles and the optical band gap of ZnSe. In this latter case there were no significant changes in the optical properties of ZnSe. This comparison enabled us to understand the importance of resonant interactions between plasmonic nanoparticles and semiconductor nanowires.
The final section of this thesis presents doping induced changes in the optoelectronic properties of ZnO nanorods. ZnO nanorods were synthesized using a cheap, and scalable seed mediated chemical bath deposition method. Doping with cobalt was done simultaneously by introducing Co2+ ions in the growth solution and the doping concentration was determined by the amount of Co2+ introduced in the growth solution. Co-doped ZnO nanorods were prepared in order to study their usability as a photoanode material for photoelectrochemical water splitting. Through cathodoluminescence and ultrafast spectroscopic measurements the improvements in the optoelectronic properties of Co-doped ZnO nanorods were explored. All the measurements pointed to the formation of more surface defects in the presence of Co-doping and their role in the modification of the optoelectronic properties of the nanorods. These were then characterized using photoelectrochemical measurements such as incident photon to current efficiency and voltammetry measurements to quantify photogenerated current density. This allowed the determination of the ideal value of Co2+ in growth solution for photoelectrochemical applications, which was found to be 1%. These nanorods were further improved by functionalizing their surfaces with a metal organic framework, the zeolitic imidazolate framework – 8 (ZIF-8). Further optical characterization of these ZIF-8 coated Co-doped ZnO nanorods were also discussed, demonstrating further improvement in photoelectrochemical performance.