Transport Property on Mercury Chalcogenide Colloidal Quantum Dot

Abstract

Beginning in the early 1980s and extending to the present, colloidal nanocrystals(NC) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Semiconductor NCs, for example, hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. Mercury chalcogenide colloidal quantum dots(QD) are of interest as solution based materials in the mid-IR spectral range. Aggregated HgTe QDs are typically undoped or lightly p-doped with size-dependent bandgaps and have shown promise as mid-IR detectors, as proved by Keuleyan et al in 2011. In contrast to aggregated HgTe, non-aggregated mercury chalcogenide QDs are usually stably n-doped in ambient conditions, which allowed the realizations of QDs intraband photodetectors. These materials have made great progress in mid-IR detection in the last few years including background limited photovoltaic (PV) devices, and are expected to lead to the simplified fabrication of high resolution mid-infrared cameras, Plasmon resonance enhanced PV devices and multi-spectral detectors. The performances of some NC-based mid-IR devices have already been comparable with the current commercial epitaxial devices. The precise engineering of the electronic interactions and wave functions with nanomaterials is a promising avenue for further improvement, which is the main project of my Ph.D. project. This thesis focuses on a deeper understanding of electrical transport in nanoparticle solids when carriers must travel via the nanocrystal states. In chapter 2, I use electrochemistry to provide the measurement of the Fermi level and the absolute measurements of the filled and empty state energies with the application of a voltage. This work helps to obtain the energy structure and doping level of those mercury chalcogenide QDs. In chapter 3, I present the work on HgTe/Se QD films which show high mobility for charges transported through discrete QD states. A hybrid surface passivation process efficiently eliminates surface states, provides tunable air-stable doping, and enables hysteresis-free filling of QD states evidenced by strong conductance modulation. In chapter 4, I show the size polydispersity effect on HgSe colloidal quantum dot. The results show mobility quite exponentially dependent on size dispersion, indicating the dispersion causes effect related to the activation energy. This effect could be from the monodispersed quantum dots narrowing the energy difference between sites, which causes the reduction in the barrier height for transport. In chapter 5, I discuss the magnetic transport properties on HgTe QDs. A positive-quadratic magnetoresistance is observed which can be several 100% at low temperature and scales like x (1- x) where x is the fractional occupation of the 1Se state. There is also a negative magnetoresistance of 1-20% from 300 K to 10 K which is rather independent of the fractional occupation, and which follows a negative exponential dependence with the magnetic field. In chapter 6, I take HgTe QD as an example, showing that the high carrier mobility is generally beneficial for QD device applications