It is difficult to deposit carbon nanotube (CNT) films onto most surfaces from solution. Zhenan Bao and colleagues overcame this problem by using a "dry" technique to perform such a transfer onto surfaces containing different self-assembled monolayers. This is an ideal method, says Bao, because it ensures that the CNTs stay aligned without introducing contamination in a reproducible, quick, single-step process.

The surface onto which the transfer takes place is modified with self-assembled monolayers containing functional groups in intimate contact with the CNTs. Bao's team used several different self-assembled monolayers with different terminal chemical functional groups. These included n-octadecyltrichlorosilane (OTS), 11-cyanoundecyltrimethoxysilane (CTS) and 11-bromoundecyltrimethoxysilane (BTS).

The functional groups interact with the CNTs because they are extremely sensitive to their environment. Charge transfer can take place between the CNTs and functional groups based on their electron withdrawing or donating properties. Hydrophobic self-assembled monolayers can further improve the finished devices since less water is present on the surfaces. Water is a nuisance in such situations because it causes traps and defects in CNTs.

Making transistor devices
To understand how surface chemistry affects the electrical properties of CNTs, the researchers fabricated top-contact transistor devices from the CNT films atop the different self-assembled monolayers. To do this, they thermally evaporated gold through a shadow mask to deposit source/drain electrodes with a channel length (Lc) of 50 µm and width (Wc) of 1000 µm. The transistors' characteristics were measured using a Keithley parameter analyser.

The Stanford team compared the average on-current (Ion), mobility (u), on/off ratio and threshold voltage (Vt) between the different self-assembled monolayers. It observed a significant increase (by by a factor of 2) in the Ion and u for CNTs transferred onto BTS surfaces compared to the other functional groups. According to the researchers, this increase probably comes about thanks to the electron-withdrawing bromine group interacting with the CNTs.

"By modifying the surface chemistry of a dielectric layer, we have developed an easy and permanent way to tune the electronic properties of CNT films," Bao told nanotechweb.org. "This could potentially allow us to design novel devices, such as memories, logic circuits or sensors."

The researchers say that they will now test how different surface chemistries affect the sensor response of CNT devices. "We have already shown that CNTs are effective sensors and we can now potentially make devices with increased specificity for certain analytes thanks to their interaction with a wide variety of self-assembled monolayers on the nanotubes," said Bao.

The work was reported in ACS Nano.