Ultrafast charge carrier and coherent phonon dynamics study of semiconductor nanostructures using femtosecond transient absorption spectroscopy
Date of Issue2017
School of Physical and Mathematical Sciences
Semiconductor nanomaterials are poised to revolutionize the field of electronics. Their low-cost synthesis and unique optoelectronic properties are aligned to the needs of modern industry. The miniaturization of semiconductor electronic components can achieve higher performance in a smaller volume, and the easily tailored electronic structure as a consequence of quantum confinement effect gives life to a myriad of semiconductor devices. Accompanying the emergence of a large number of promising applications is the challenge to monitor, understand and eventually manipulate the physical properties of these semiconductor nanostructures. Beyond the extensive research on the quantum confinement effect of semiconductor nanostructures, another striking quantum mechanical phenomenon in nanoscale systems that has attracted intense attention is that of electronic coherence. To investigate coherent electronic motion in low-dimensional semiconductors, we employ transient absorption spectroscopy with few-cycle laser pulses. The broadband spectra overlap with the two lowest-lying excitonic states of colloidal CdSe quantum dots (QDs), thus preparing a superposition of 1S_e 1S_h and 1S_e 2S_h states. The excitonic coherence, with a 15-fs dephasing time, is clearly discerned from our optical pump-probe data at 77 K. An ultrafast charge migration over a 1-nm-length scale with rates that can potentially exceed 1 Å/fs is reconstructed. The temperature-dependent dephasing time established that the electronic decoherence is partially induced by acoustic phonon scattering, with the dominant contribution arising from either exciton-defect scattering and/or exciton-exciton scattering. The inherent size distribution of the ensemble QDs contributes to inhomogeneous dephasing of electronic coherence. Fortunately, quasi-two-dimensional colloidal nanoplatelets (NPLs) have recently emerged as a new class of materials with monodisperse thicknesses of atomic precision, leading to relatively narrower absorption and emission bands compared with QDs. After photoexcitation at the bandgap of CdSe/CdS core/shell NPLs, band selective probing is used to separate the electron and hole dynamics. Pump fluence-dependent measurements reveal a sub-picosecond Auger-mediated hole trapping process with an effective second-order rate constant of 3.5±1.0 cm2/s. At high excitation intensity, where the initial exciton number N_0>1, the spectral blue shift in the first-moment time traces indicates a concomitant Auger hole heating process, along with electron cooling dynamics, which is especially evident as a time-dependent red-shift when N_0<1. In addition to the Auger-like charge carrier dynamics, the oscillatory signals resolved in the first-moment time trace are the fingerprints of coherent phonons. Four types of coherent phonons are observed, including the well-known longitudinal optical (LO) and surface optical (SO) phonon modes of CdSe. At the same time, two newly resolved frequencies of ~5 – 10 cm–1 and ~20 cm–1 are assigned to the in-plane and out-of-plane breathing-mode acoustic phonons, respectively. The increase in the frequency of the in-plane phonon mode and the variation of the initial oscillation phase with exciton density both suggest that the in-plane coherent acoustic phonon is impulsively generated by displacive excitation when N_0<1 and by sub-picosecond Auger hole trapping process when N_0>1. This observation is consistent with the absence of carrier quantum confinement in the lateral plane of NPLs. Analysis of the experimental data within a classical mechanical framework for wave packet propagation on the excited and trap state potentials yields the relative dimensionless displacement of the two states. On the other hand, the initial phase of ~π rad observed for the out-of-plane coherent phonon, independent of N0, suggests displacive excitation as the mechanism for generation of the out-of-plane coherent acoustic phonon. The coherent and incoherent dynamics of carriers and phonons described in this thesis provide further insight into the photophysical behavior of nanostructured CdSe QDs and NPLs.