Study for efficient extraction of hot electrons in plasmonic nanodiodes

Abstract

Finding strategies for high-probability of electron-hole pair generation and separation is the most important for achieving more efficient conversion of solar energy in semiconductor-based photovoltaic and photocatalytic devices. Plasmonic energy conversion has been proposed as a promising alternative to semiconductor devices that have limitations due to the band gap. This method is based on the hot electrons generated through electromagnetic decay of surface plasmon in metal nanostructures. Since such a new scheme of energy conversion can provide low manufacturing cost and potentially high conversion efficiency, development of devices capable of efficiently detecting the hot electrons has been attracting attention. Therefore, to advance this field, extensive research have to be performed on the considerations regarding materials, structures and fabrications used in hot carrier devices. In this dissertation, Chapter 1 introduces the research background. In Chapter 2, I demonstrate the detection of the hot electrons by measuring the steady-state photocurrent using the plasmonic Au/$TiO_2$ Schottky diodes. In addition, it is verified the reduction of the Schottky barrier height by the image force by applying a voltage to the diode. Furthermore, to distinguish the hot electrons generated by intraband excitation and interband transition, Cu/$TiO_2$ diode was fabricated and explored. Finally, a plasmonic bimetal/$TiO_2$ diodes composed of Au-Ag and Au-Al were fabricated and the complementary effects of the two plasmonic metals were observed. In Chapter 3, I investigate polarization dependent hot electrons detected on planar (two-dimensional) and three-dimensional (3D) tandem plasmonic Au/$TiO_2$ nanodiodes. Because, an important parameter for the efficient extraction of hot electrons is the polarization of the incident light, which can be tuned by the angle between the electric field of the incident light and the plane of the Schottky barrier. I confirm that the maximum photocurrent was obtained with the planar structure in transverse mode and with the 3D tandem structure in longitudinal mode. These results indicate that hot electrons can be extracted most efficiently when the direction of the electric field of the incident light coincides with the plane of the Schottky interface. This study sheds light on the fundamental mechanism for the polarization effect on hot electrons, with applications in the advanced design of hot electron-based photonic devices with high energy conversion efficiency. In Chapter 4, it is confirmed that the hot electron flow generated in plasmonic Au/$TiO_2$ nanodiodes by incident light can be amplified when PbS quantum dots are deposited onto the surface of the nanodiodes. The effect is attributed to efficient extraction of hot electrons via a three-dimensional Schottky barrier, thus giving new pathways for hot electron transfer. We also demonstrate a correlation between the photocurrent and Schottky barrier height when using PbS quantum dots with varying size and ligand treatments that allow us to control the electric properties (e.g., band gap and Fermi level, respectively) of the PbS quantum dots. This simple method introduces a new technique for further improving the power conversion efficiency of thin-film photovoltaic devices.