Investigating Defects in the Sensing Mechanism of Carbon Nanotube Field-Effect Transistors
In nanotechnology, understanding the effect of interfaces and defects becomes criticallyimportant in determining a material’s properties and device performance. It is well known thatone-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 andthe interfaces they form with other materials commonly used in semiconductor devicefabrication. 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 thefield-effect transistor. To fabricate a carbon nanotube field-effect transistor, metal contactelectrodes must be deposited on either side of a semiconducting carbon nanotube. The resultinginterface between the metal and the nanotube form a Schottky barrier, which can have animportant role in establishing transistor characteristics. Modifications to this interface by theenvironment can modulate the barrier and produce a commensurate change in the overallperformance of the device. Transistor operation may also be modified by the presence of defectsin the carbon nanotube structure. The role of defects and the interplay between the Schottkybarrier at the interface and the defects present in the carbon nanotube represent critical areas ofinterest when studying sensors based on carbon nanotube field-effect transistors. Therefore, theivpurpose of this study is to explore how defects in carbon nanotubes can affect the sensingmechanism, and to assess its relative importance when compared with modulations of theSchottky 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 detectammonia (an electron donor) and nitrogen dioxide (an electron acceptor). Gas exposuremeasurements were performed on near pristine (low defect) nanotube devices and compared withhighly-defective nanotube devices. By utilizing selective passivation of critical device areas toisolate the sensing mechanism, results show that the presence of defects has a critical role in thesensing mechanism of carbon nanotube field-effect transistor gas sensors. Results also suggest aresolution to the long-standing debate over the sensing mechanism of these devices. Theseresults represent an important step toward understanding the effect of both interfaces and defectsfor carbon nanotube sensor development and adds a critical piece of understanding necessary forthe development of future nanoscale sensors.
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Dube, Isha (Georgetown University, 2014)Gas sensors based on carbon nanotubes in the field effect transistor configuration have exhibited impressive sensitivities compared to the existing technologies. However, the lack of an understanding of the gas sensing ...