Design and fabrication of electron energy filters for room temperature inelastic electron tunneling spectroscopy

Abstract

Odor detection has wide range of applications in a variety of industries, including the agricultural, clinical diagnosis, pharmaceutical, cosmetics, food analysis, environmental and defense fields. Spectroscopic techniques such as FTIR and Raman are commonly used for electronic nose application. However, their application is limited by factors such as poor sensitivity, selectivity and non-portability. Inelastic electron tunneling spectroscopy (IETS) is an all electronic spectroscopy that has been extensively used to measure the vibrational modes of molecules and can be used for electronic nose application. It has several advantages such as ultra-high sensitivity and compact size. However, IETS requires cryogenic temperature to resolve molecular spectra, which limits its use in electronic nose application. A new theory of biological olfaction postulates that the odorant detectors inside a nose recognize an odorant's vibrations via inelastic electron tunneling (Turin, 1996). However, a biological system works at room temperature but conventional IET spectroscopy requires cryogenic temperatures. Thus posing the following question: Is it possible to resolve molecular vibrational spectra using inelastic electron tunneling spectroscopy at room temperature? IET spectroscopy involves the tunneling of electrons through an insulating barrier that is situated between two conducting metal electrodes. At room temperature, tunneling electrons possess thermal energy and occupy broad distribution of energy levels available in metals. This thermal distribution of electrons drastically reduces the resolution of IET spectroscopy. By reducing the thermal distribution of tunneling electrons at room temperature, we can increase the resolution of IET spectroscopy. The objective of this work is to develop electron energy filters to narrow down the thermal energy distribution of electrons at room temperature. I further evaluate the application of these electron energy filters to increase the resolution of IET spectroscopy at room temperature. Some recent advancements in nanomaterials, such as quantum dots with discrete electron energy levels are an excellent choice as electron energy filters. In metals, the continuous distribution of available energy states causes broad thermal distribution of electrons at room temperature. In contrast, quantum dots have discrete energy levels due to their small size. So even though electrons might possess thermal energy at room temperature, they can only occupy the discrete energy levels available in quantum dots. Hence, the thermal energy distribution of electrons can be narrowed down to the energy levels available in quantum dots. The electron energy filter designed in this work, consists of a 2-dimensional array of CdSe quantum dots of sizes around 2.5nm sandwiched between metal electrodes. Through electrical characterization of these devices, we can conclude that they can narrow down thermal distribution of electrons from 25meV down to around 10meV. However, to resolve the molecular vibrational energy level at room temperature, thermal energy distribution of electrons should be less than 6.6meV. Since array of quantum dots results in formation of energy minibands, this work suggests that single quantum dot should be used instead of array of dots to improve the performance of electron energy filters. Moreover, the study of electron transport through single quantum dots done in this work suggests that the size of the dot should be less than 2.5nm to be used in room temperature IET spectroscopy. Interestingly, this length scale is consistent with the size of donor and acceptor sites in odorant receptors potentially explaining how these receptors could be able to resolve molecular spectra at room temperatures.