Highlights
- Best ammonia sensing found for high-power ultrasonically treated (6,5)-SWNTs annealed into bundled nanotube networks.
- Resistance rose with NH3 from 100-10,000 ppm, then dropped by three orders of magnitude above about 2500 ppm.
- Explained the dual response through p-type carrier neutralization plus Schottky barrier and interface-dipole effects at the gold contacts.
Abstract
This study examines networks of semiconducting single-walled carbon nanotubes (SWNTs) as ammonia (NH3) sensors, focusing on how ultrasonic treatment and thermal annealing into bundled nanotube networks affects sensing behavior. The best sensing performance was found for high-power ultrasonically treated (6,5)-chirality SWNTs subsequently annealed into a bundled network. These networks showed a multidirectional chemiresistive response: increased resistance at ammonia concentrations from 100 to 10,000 ppm, followed by a dramatic three-orders-of-magnitude drop in resistance at higher concentrations (above roughly 2500 ppm). The results are explained by a combination of partial and full neutralization of the nanotubes' natural p-type conductivity, plus changes in the Schottky barrier and interface dipoles at the gold-nanotube contacts.
Research summary
Carbon nanotube networks are a well-studied platform for ammonia gas sensing, but exactly how their electrical resistance responds to ammonia — and why — has remained only partly understood. This study looks specifically at bundled networks of single-walled carbon nanotubes (SWNTs), asking how ultrasonic treatment and thermal annealing change their sensing behavior and what physical mechanisms drive the response.
What the study examined
- Single-walled carbon nanotubes of (6,5) chirality, ultrasonically treated at high power and then thermally annealed into bundled nanotube networks
- Chemiresistive response of these networks to ammonia across a concentration range of roughly 100 to 10,000 ppm
- The physical mechanisms behind the sensing response, including nanotube doping state and the gold-nanotube electrode interface
Main findings
The best sensing performance came from high-power ultrasonically treated (6,5)-chirality SWNTs that were subsequently annealed into a bundled network. These networks showed a two-part chemiresistive response: resistance increased at lower ammonia concentrations, then dropped sharply — by about three orders of magnitude — once concentrations rose above roughly 2500 ppm.
The authors explain this multidirectional response through two combined mechanisms: partial and then full neutralization of the nanotubes’ intrinsic p-type conductivity by ammonia, and separately, ammonia-driven changes to the Schottky barrier and interface dipoles at the gold-nanotube contacts. Both effects had been proposed individually in prior work; this study argues they act together.
Why it matters
A more complete mechanistic picture — rather than treating the resistance response as an unexplained curve — gives a stronger foundation for engineers to optimize future carbon-nanotube ammonia sensors, for example by tuning nanotube treatment or electrode material to favor whichever mechanism gives the most useful response in a given concentration range.
Citation
Petro M. Lutsyk, Prabakaran Shankar, Alex G. Rozhin, Sergei A. Kulinich. Surface Sensitivity of Ultrasonically Treated Carbon Nanotube Network towards Ammonia. Surfaces and Interfaces 17 (2019) 100363.
