Engineering monolithic nanoscopic tunnel junctions for molecular electronics using atomic layer deposition

In his classic talk "There's plenty of room at the bottom" in 1959, Richard Feynman proclaimed several visionary statements on working at the nanometer scale, which gave birth to the field of nanotechnology. He imagined devices that would involve manipulation of atoms and work on the principles of quantum mechanics. As a result, much interest has developed for the study of nanoparticles that possess unique physical properties that do not exist in bulk materials. They may provide unprecedented benefits in various fields such as electronics, medicine, and chemistry.;For electronics applications, nanoparticles provide several performance advantages over silicon microelectronics such as reduced size, higher sensitivity, and faster response time. Further, principles of selfassembly may provide economical solutions for designing future logic circuits. Most of the electronics research on nanoparticles has focused on materials such as quantum dots, carbon nanotubes, and fullerenes, for example, but less work has been conducted on molecule-based applications. The primary reason is the difficulty in making chemical contacts to single molecules or small molecular ensembles, which are only a few nanometers in size. However, single molecule electronic applications provide greater possibilities of control by tailoring the functionality of chemical groups. Most moleculeelectronic based investigations have used scanning tunneling microscopy to probe single molecule electrical transport properties. STM studies of molecules have revealed many interesting and potentially useful phenomena, but the slow speed and economics of the STM based measurements are not scalable for engineering practical molecular devices.;In this thesis, a novel design is presented wherein nanoscopic tunnel junctions are embedded as monolithic structures in a silicon wafer and used for investigating the electronic properties of small ensembles down to single molecules. Molecules are adsorbed (i.e. trapped) under high electric fields at room temperature and detected using inelastic electron tunneling spectroscopy (IETS) at cryogenic temperatures. The peaks in IETS spectra provide critical information for the engineering of electrode-molecule interfaces and the study of vibrational features of molecules present in tunnel junctions. The design uses advanced reaction engineering principles including atomic layer deposition (ALD) and selective area growth to fabricate molecule-size electrodes with the required 1-2 nm spacing. ALD has been selected because of its excellent control over the growth rate (typically ≤0.1 nm/cycle), and the electrode microstructure.;Successful nanofabrication of monolithic metal-vacuum-metal tunnel junctions has been achieved with demonstrated critical feature sizes of 1-2 nm. In-situ field emission and electron tunneling measurements are used to characterize the electrode properties and electrode spacing during the ALD processing. We demonstrate that molecular adsorption into tunnel junctions can be precisely controlled using electric fields for adsorption, and detection via IETS spectra. The IETS features are shown to distinguish between physisorbed and chemisorbed states of the adsorbed molecules. Moreover, we demonstrate that the molecules can be desorbed and re-adsorbed reversibly without affecting the electrode properties, thereby demonstrating the reusability of these tunnel junctions for sensor applications.;This work is a significant advance over previous nanofabrication designs that lacked sub-nanometer feature size control or control of the electrode structure, and were unable to engineer nanoscopic tunnel junctions. In addition, we demonstrate the nanofabrication of multiple pairs of tunnel junctions operating in parallel for the first time. These nanostructures are promising for the scaling of molecule-based devices for developing future applications such as logic circuits, memory elements, and sensors.