Date of Award
Doctor of Philosophy (PhD)
Committee Member 1
Committee Member 2
Committee Member 3
Today's world is frequently going through fossil energy shortage and environmental consequences brought by the over-emission of greenhouse gas from burning fossil fuels. Therefore, it is urgent now more than ever to discover or develop clean and sustainable power generation approaches. Among various approaches, photovoltaics and thermoelectrics have been more and more attentive both in academia and industry. Photovoltaic power generators can significantly decrease carbon dioxide emission by directly converting sunlight into electricity, and thermoelectric power generators can increase energy use efficiency by recycling waste heat into electricity. This research seeks to gain a better understanding of the mechanism that influences the energy conversion process in photovoltaic and thermoelectric materials and meanwhile use nano-engineering approaches to improve the performance of thermoelectric materials.
For photovoltaic nanomaterials, we have first made progress in simulations of electron-phonon coupling, which is a major mechanism for efficiency loss, in CdSe quantum dots. Time-domain non-adiabatic ab initio simulations have been performed to study the phonon-assisted hot electron relaxation dynamics in CdSe nanocrystals. It is found that the shape of the nanocrystals has a strong impact on the electron decay dynamics. The electron-phonon coupling is generally stronger in elongated nanocrystal than in the spherical nanocrystal. The relaxation of hot electrons proceeds faster in the elongated nanocrystal than in the spherical nanocrystal, and it also shows stronger temperature dependence in the elongated nanocrystal. The hot electron decay rates calculated from non-adiabatic molecular dynamics show weaker temperature dependence than the T-1 trend in both elongated and spherical nanocrystals, which can be attributed to the thermal expansion effect.
We then performed experiments to synthesize and characterize semiconductor nanocrystals. Monodisperse CdSe, PbSe, and PbTe nanocrystals of various morphologies have been synthesized by using different combinations of surfactant and solvent in the refined phosphonic-acid-assisted organometallic method. XRD spectra have confirmed the formation of desired crystal phase and size. SEM and TEM images have confirmed the morphology and crystallinity. UV-visible absorption spectra show that the bandgap decreases with increasing crystal size. With collaborators, we have characterized the hot electron relaxation dynamics using transient absorption spectroscopy. The results show that the hot electron relaxation can result from both electron-phonon coupling and the Auger process
Raman spectroscopy has also been used to investigate the size, shape and temperature dependence of phonon vibrational modes, for the interest of Raman thermometry using NCs. For spherical CdSe NCs of diameters 2.8 nm, 3.6 nm, and 4.4 nm, the temperature sensitivities are -0.0131, -0.0171, and -0.0242 cm^-1/K, respectively. This trend indicates that as the diameter increases, the effect of increasing phonon anharmonicity dominates over the effect of the decreasing thermal expansion coefficient. On the other hand, triangular NCs with a size of 4.2 nm and elongated NCs of a dimension of 4.6 nm by 14 nm show temperature sensitivities of -0.0182 and -0.0176 cm^-1/K, respectively. This trend indicates that in non-spherical shape NCs, the effect of decreasing thermal expansion coefficient dominates over the effect of slightly increasing phonon anharmonicity.
For thermoelectric nanomaterials, both material synthesis and device fabrication have been conducted. For the material synthesis part, Bi2Te3-based nanocrystals have been made using both pyrolysis of organometallic method and ball milling method. Experimental parameters have been optimized to make impurity-free Bi2Te3 nanocrystals of various morphologies. Raman spectroscopy has been used to investigate the morphology dependence of phonon modes. The A1u mode is invisible in bulk Bi2Te3, but becomes visible in Bi2Te3 nanocrystals no matter whether they are synthesized by wet-chemistry method or ball milling method. Furthermore, for wet-chemistry synthesized Bi2Te3 nanocrystals, the 2D nanostructure shows similar Raman features as those of few-quintuple-thick Bi2Te3 layers, while the 0D and 1D nanostructures show a blue-shifted A1g2 mode and a much stronger A1u mode, which is the first report regarding the morphology impact on the Raman modes of Bi2Te3 nanocrystals.
We have also used ball milling and hot pressing to obtain nanostructured bulk and improve the figure of merit of Bi2Te3 based alloys. Nanostructured bulk pellets are fabricated by densifying nanocrystal powders into bulk using hot pressing method. Due to the increased phonon scattering at the grain boundaries introduced in nanostructured bulk process, significantly reduced thermal conductivities have been obtained on nanostructured bulk Bi2Te3 pellet samples. It is also observed that thermal conductivity decreases with decreasing average grain size. Several post-fabrication treatments, like removing surface oxide layer by Ar plasma and improving crystallinity by thermal annealing, have been used to further improve the thermoelectric properties of the samples. Ion bombardment by Ar plasma is found to improve the contact between the metal electrodes and the material. Thermal annealing is found to not only increase the electrical conductivity but also the increase the Seebeck coefficient. The improved figure of merit at room temperature is around 1.23 on the p-type Bi0.5Sb1.5Te3 sample and 0.32 on the n-type Bi2Te2.7Se0.3 sample. The value from the p-type sample is close to the state-of-the-art value and still has room for improvement.
Chen, Liangliang, "Semiconductor Nanomaterial Development For Photovoltaic And Thermoelectric Applications" (2013). Open Access Dissertations. 205.