Investigating Defects in the Sensing Mechanism of Carbon Nanotube Field-Effect Transistors

Abstract

In nanotechnology, understanding the effect of interfaces and defects becomes critically

important in determining a material’s properties and device performance. It is well known that

one-dimensional and two-dimensional materials exhibit excellent physical, electrical, thermal,

and optical properties, making them highly desirable for a wide array of applications. However,

their low dimensionality also means they can be heavily affected by defects in the material and

the interfaces they form with other materials commonly used in semiconductor device

fabrication. Carbon nanotubes are one such material that is often used in sensing applications.

The best and most commonly used device configuration for carbon nanotube-based sensors is the

field-effect transistor. To fabricate a carbon nanotube field-effect transistor, metal contact

electrodes must be deposited on either side of a semiconducting carbon nanotube. The resulting

interface between the metal and the nanotube form a Schottky barrier, which can have an

important role in establishing transistor characteristics. Modifications to this interface by the

environment can modulate the barrier and produce a commensurate change in the overall

performance of the device. Transistor operation may also be modified by the presence of defects

in the carbon nanotube structure. The role of defects and the interplay between the Schottky

barrier at the interface and the defects present in the carbon nanotube represent critical areas of

interest when studying sensors based on carbon nanotube field-effect transistors. Therefore, the

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purpose of this study is to explore how defects in carbon nanotubes can affect the sensing

mechanism, and to assess its relative importance when compared with modulations of the

Schottky barrier in carbon nanotube field-effect transistor-based sensors. To explore this effect,

carbon nanotube field-effect transistors have been fabricated as gas sensors, specifically to detect

ammonia (an electron donor) and nitrogen dioxide (an electron acceptor). Gas exposure

measurements were performed on near pristine (low defect) nanotube devices and compared with

highly-defective nanotube devices. By utilizing selective passivation of critical device areas to

isolate the sensing mechanism, results show that the presence of defects has a critical role in the

sensing mechanism of carbon nanotube field-effect transistor gas sensors. Results also suggest a

resolution to the long-standing debate over the sensing mechanism of these devices. These

results represent an important step toward understanding the effect of both interfaces and defects

for carbon nanotube sensor development and adds a critical piece of understanding necessary for

the development of future nanoscale sensors.