Material World Network: SWCNT Sensors: Interplay Between Schottky Barrier and Gas Adsorption

Georgetown University, Washington DC

Investigators

Abstract

This is a joint project between Georgetown University (GU), USA, and the Russian Research Centre (RRC) Kurchatov Institute in Moscow, with the goal to tackle a very challenging problem: identifying the sensing mechanism in carbon nanotube (CNT) devices. Carbon nanotube field effect transistors (CNT-FETs) have extraordinary potential as nanoscale, highly-sensitive chemical sensors. However, to date, no systematic study has been done to understand the underlying response mechanism. An intuitive and widely accepted explanation is that molecules bind to the surface of the nanotube and charge transfer occurs between the nanotube and the molecules. A second possibility is a change in the Schottky barrier at the interface between the nanotube and the electrical contacts. This project will conduct a thorough experimental and theoretical investigation of the interaction between carbon nanotube devices and analyte molecules for several chemicals, including hole dopants such as NO2, and electron dopants, such as NH3, by using experimental methods specifically designed to determine the response mechanism. This study will include isolated carbon nanotubes, carbon nanotube networks and nanotubes decorated with metal nanoparticles, as well as the use of a Kelvin Probe system to measure the change in the metal work-function for the materials forming the electrodes or the nanoparticles, upon exposure to a controlled concentration of gases. The sample fabrication, the preliminary testing and the measurements of the change in the work-function of the metal electrodes will be performed at GU, where a Gas-cell Kelvin probe system will be acquired. The characterization of the sample response as a function of concentration and humidity level for different gases will be performed only in part at GU and mainly at RRC. A custom built gas sensor testing facility at the RRC Kurchatov Institute allows control of analyte concentrations from several tens of ppb and variation of the sample temperature up to 400 degrees C. By performing measurements as a function of temperature on some of the fabricated devices the team will measure both the Schottky barrier height and the doping level of the CNT conducting channel. The modeling to analyze the experimental results will be done by the RRC group. This study will impact all carbon nanotube device applications, since the difficulty controlling the nanotube/electrode interface and its sensitivity to the environment is one of the major stumbling blocks for the realization of nanotube-based integrated circuits. Moreover, if different mechanisms are involved for different chemicals, it will be possible to design selective sensors by allowing only a specific mechanism, corresponding to a targeted chemical, to occur. The US graduate students will travel to Russia to participate in testing of the samples with the equipment in the Russian laboratory. The senior investigators will also visit Russia for a yearly meeting with the Russian counterpart. This award is co-funded by the Division of Materials Research and the Office of International Science and Engineering.