Here at the University of Wisconsin-Platteville, we are researching technology that could ultimately impact the development of smaller and more powerful laptops, tablets, smartphones and similar handheld electronics by building nanoscale electronic devices –  transistors and sensors – using carbon nanotubes (CNTs).

Carbon nanotubes are long “tubes” of carbon atoms, and can be thought of as rolled-up “sheets” of carbon.  Their diameters are on the order of a small number of nanometers (billionths of a meter), and their lengths can be on the order of micrometers – thousands of times longer than they are wide.

Description of Research

Our research project idea is actually based on an antiquated concept: the earliest computers used vacuum tubes that contained electrodes for controlling electron flow, allowing them to be used as a switch or amplifier. These were eventually replaced by transistors.

In the nanoscale vacuum channel transistor, a typical vacuum tube operation is mimicked, using carbon nanotubes as the electrodes.

Carbon Nanotubes have exceptional material properties, including mechanical strength and electron transport. Their electronic properties range from semiconducting ("silicon-like") to metallic ("copper-like"), resulting from differences in the size and helicity. Helicity can be thought of as the "direction" in which the carbon sheets are rolled up to create the CNTs.

Recently, there have been breakthroughs in both the purification of CNTs as well as in the creation of aligned single-layer films of CNTs. This advance in purification means that in contrast to recent history, we can produce films entirely of semiconducting CNTs, and can consider their use in next-generation electronics.

Additionally, aligned monolayers are a key building block for advanced electronics. A published paper co-authored in 2016 by Dr. Evensen, demonstrated the first CNT transistor that out-performed traditional transistors, built upon these aligned films.*

Carbon nanotubes are also expected to be valuable as sensors.  This is because every atom in a CNT is exposed to the surrounding environment, and therefore their electronic properties can be very sensitive to interactions with the environment. Because CNTs are on the same size scale as molecules and biological structures, it is expected that they will be an ideal means to detect trace amounts of gases or various proteins and enzymes in solution.

Our group works in collaboration with the Advanced Materials for Energy and Electronics Group at UW-Madison, which provides purified CNTs and aligned CNT films. UW-Platteville students have:

  • Constructed working transistors on their own, using equipment entirely in our nano lab in Engineering Hall
  • Created a proof-of-principle oxygen sensor
  • designed a “nano-scale vacuum tube,” or vacuum-channel FET

The nanoscale vacuum-channel FET (VCFET) is an updated version of the “classic” vacuum-tube:  electrons travel through space between two electrodes, in a chamber (bulb) from which the air has been removed. At the nano-scale size, the gap between the electrodes can be made significantly smaller than the distance between adjacent molecules found in the air, effectively enabling the device to operate under vacuum without the challenge of actually creating a vacuum! 

There are other advantages: the nano-scale size means the device can operate at low voltages without getting hot, thus surmounting two other shortcomings of macro-scale vacuum tubes.

What are the advantages of a nano-scale vacuum tube? For one, they are expected to be radiation-resistant and therefore to find a niche in space applications. For another, they also have promise for high-power and high-frequency operation, as compared with traditional transistors.

The design, fabrication, and testing effort of the VCFET has been led by students, who again are doing the entire process in our nano lab in Engineering Hall.

All of this work is built upon a common set of skills that students have learned:

  • Preparation of surfaces that are clean at the nano-scale
  • Deposition of thin films of CNTs, polymers, and metals
  • Design of patterns for these films
  • Creation of these patterns via selective etching (removal) of the materials
  • Electronic testing of the completed devices. 

This effort has required additional work to be done in instrumentation: building test chambers, writing software for data collection and analysis, etc.

*[Brady, G.J., Way, A.J., Safron, N.S., Evensen, H.T., Gopalan, P., & Arnold, M.S. (2016). Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs. Science Advances 2 (9), e1601240 (2016).]

Application and Career Opportunities

There are many career opportunities for students with experience in microfluidics-based medical devices, as well as in sensor development. More broadly, however, the thin-film industry encompasses companies in electronics, from computing to displays and sensors, and the skills used in our group are broadly transferable.

Importantly, it should be noted that past students have found success even if they did not seek employment in these specific fields. They made themselves stand out by virtue of their experience in "leading" their own aspect of a project, and by demonstrating an ability to not only apply what they're learning in their classes, but to also gain additional knowledge and skills and apply it to the work described here.

Academic Areas of Focus

Engineering
This project requires an understanding of fabrication methods, electronics, and electrical measurements. It also utilizes design skills for apparatus and software. In addition, future work in biochemical sensors would likely involve microfluidics: understanding fluid handling and motion at the micro-scale.

Engineering Physics & Electrical Engineering
Understanding why the CNTs behave as they do when in transistors and sensors which involves an understanding of semiconductor electronics. Additionally, the common fabrication methods are building blocks used in any micro-scale or nano-scale fabrication.

Chemistry & Biology
Building a sensor that detects only “Molecule A” or “Protein B” requires an understanding of chemistry or biochemistry, which is used to modify the CNT and to determine a valid means of detection.

Join the Research Group

There are many opportunities for students in this area: constructing measurement appartus, devising chemical treatments of CNTs, improving or developing the fabrication process, and even designing novel electronic devices.

This research work has involved students from engineering physics, chemistry, and electrical engineering. Collaboration with other program with interests in sensing (i.e., biology or biochemical sensing) are certainly possible.

We are always looking for highly motivated students. If you are interested in a more in-depth research experience such as independent study credit, please contact Dr. Hal Evensen if you are in engineering. If you are in chemistry or biology, Dr. Evenson would be happy to collaborate with you and one of your professors.

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