Monolithic Tunnel Junctions for Single Molecule Detection

Rahul Gupta and Brian G. Willis. Chemical Engineering, University of Delaware, Colburn Laboratory, 150 Academy Street, Newark, DE 19716

In his classic talk on "There's plenty of room at the bottom" in 1959, Richard Feynman quoted several visionary statements on working at nanometer scale, which gave birth to the field of nanotechnology.1 He imagined devices that will involve manipulation of atoms and work on the principles of quantum mechanics. As a result, a lot of interest has developed on studying nanoparticles for sensor applications, which possess unique physical properties that do not exist in bulk materials. Nanosensors provide several performance advantages such as reduced size, higher sensitivity, and faster response time. Further, they can be self-assemble to provide economical solutions for designing logic circuits. They can provide unprecedented benefits in various fields, such as electronics, medicine, and chemistry. Most of the research on nanosensors has focused on nanometer scale materials such as quantum dots, carbon nanotubes, and fullerenes, for example, but less work has been conducted on molecule based sensor. The primary reason is the difficulty in making chemical contact to single molecules, which are only a few angstroms in length. However, single molecule sensors provide larger possibilities of control by tailoring the attachment chemical groups. Scanning tunneling microscopy (STM) studies on molecules have revealed interesting information, but the slow speed and economics do not favor mass production of STM-based sensors.

In this work, a novel design is presented where nanoscopic tunnel junctions are embedded in a thermally oxidized silicon wafer and used for single molecule detection. Molecules are adsorbed (i.e. trapped) under high electric field 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 the tunnel junction (see figure). 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. This technique has the potential to fabricate parallel tunnel junctions with same electrode spacing at nanometer scale for practical molecule based sensors.

Successful nanofabrication of monolithic metal-vacuum-metal tunnel junctions has been achieved with demonstrated nanojunctions of 1-2 nm spacing. In-situ field emission and electron tunneling measurements are used to characterize the electrode properties and electrode spacing during the ALD of copper. We demonstrate that molecular adsorption into tunnel junctions can be precisely controlled using electric field and detection via IETS spectra. The IETS features are shown to distinguish between physisorbed and chemisorbed state of these 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 spacing and electrode structure control, and hence were unable to engineer nanoscopic tunnel junctions. Work is currently in progress to demonstrate parallel fabrication of these tunnel junctions for a viable sensor application.

1 http://www.zyvex.com/nanotech/feynman.html