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CNTs make good transistor sensors


Single-walled nanotubes could be ideal for detecting trace amounts of analytes in solution, according to new work by researchers at Stanford University in the US. The nanomaterials can be used both as the sensing component and electrical read-out part of a detector and their sensitivity depends on various factors, such as how closely packed the nanotube bundles are, their chirality and in which direction the tubes are aligned. The sensors might come in useful for applications like monitoring environmental pollutants and in homeland security.

"We have shown that single-walled carbon nanotubes (SWCNTs) with different chiralities respond differently to target analytes of interest," team member Mark Roberts told "For example, semiconducting nanotubes are very sensitive to polar analytes while metallic tubes are not." 

Highly sensitive
The team, led by Zhenan Bao of the Department of Chemical Engineering, also demonstrated that thin-film transistors based on semiconducting SWCNT networks can be used to rapidly detect extremely low concentrations (2 ppb) of analytes such as the nerve agent dimethyl methylphosphonate (DMMP) and the explosive trinitrotoluence (TNT) in aqueous solution. The transistor's response to the analytes depends on how the analyte interacts with the nanotubes, which depends directly on the nanotubes' properties. 

SWCNTs are ideal for use in sensors thanks to their extremely high surface area and excellent charge transport properties. These allow a SWCNT network to function as both the sensor layer and read-out signal, explains Roberts. "The high surface area allows many analytes to be adsorbed along the tube, potentially resulting in a large concentration detection window. SWCNTs can also respond to a wide range of analytes and are easily functionalized to selectively detect different compounds." 

The researchers made their thin-film transistors by coating silicon or ITO/PET substrates with a 25 nm cross-linked polymer insulating layer, subsequently modified with an amine-terminated silane layer. Next, they placed the SWCNT network on top of this substrate using spin coating. Gold top-contact electrodes were then deposited by thermal evaporation. 

The bias to the gate electrode modulates the conductivity of the SWCNT layer, and therefore the current between source-drain electrodes when a bias is applied between them," said Roberts. The devices operate as sensors because the conductivity of the SWCNTs changes depending on what is absorbed on their surfaces. 

Easy to make and inexpensive
"Such electronic sensors are easy to design and fabricate and can be printed or deposited with other solution methods to make a large number of low-cost devices," he added. "Compared to other nanotube or nanowire devices, our network system provides a simple way of depositing a well defined nanotube system on polymer substrates." 

Using a polymer gate dielectric in this way allows the devices to operate at voltages as low as 500 mV and less. 

The devices made in this work were tested in water but they function in air too, say the researchers. "We specifically investigated water-stable electronic sensors to monitor large bodies of water in numerous locations for detecting environmental pollutants and toxins, or discarded warfare materials," said Roberts. 

The team now plans to better control the conditions under which the SWCNTs are deposited on the polymer substrates, and then further functionalize the nanotubes within the network for selectively detecting different analytes. "Then, we will try to understand how the network properties (nanotube density and alignment, for example) influence sensor characteristics." 

The work was reported in ACS Nano.

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