FARGO, ND — A discovery by a research team at North Dakota State University, Fargo, and the National Institute of Standards and Technology (NIST) shows that the flexibility and durability of carbon nanotube films and coatings are intimately linked to their electronic properties. The research could one day impact flexible electronic devices such as solar cells and wearable sensors. 

The NDSU/NIST research team, led by Erik Hobbie, Ph.D., is working to determine why thin films made from metallic single-wall carbon nanotubes are superior for potential applications that demand both electronic performance and mechanical durability. “One simple reason is that the metallic nanotubes tend to transport charge more easily when they touch each other,” said Hobbie. “But another less obvious reason has to do with how much the films can flex without changing their structure at very small scales.” 

Results from the study appear in “Electronic Durability of Flexible Transparent Films from Type-Specific Single-Wall Carbon Nanotubes,” published in ACS Nano. 

The team includes NDSU graduate student John M. Harris; postdoctoral researcher Ganjigunte R. Swathi Iyer; Anna K. Bernhardt, North Dakota Governor’s School attendee; and NIST researchers Ji Yeon Huh, Steven D. Hudson and Jeffrey A. Fagan. 

There is great interest in using carbon nanotube films and coatings as flexible transparent electrodes in electronic devices such as solar cells. “Our research demonstrates that the flexibility and durability of these films are intimately linked to their electronic properties,” said Hobbie. “This is a very new idea; so hopefully, it will generate a new series of studies and questions focused on the exact origins and consequences of this effect.” 

Such research could potentially result in material that reduces solar cell costs and leads to the ability to use them in clothing or foldable electronics. Electronic devices currently on the market that require transparent electrodes, like touch screens and solar cells, typically use indium tin oxide, an increasingly expensive material. “It is also very brittle, implying that it cannot be used in devices that require mechanical flexibility like wearable or foldable electronics,” said Hobbie. 

Single-wall carbon nanotubes show significant promise as transparent conductive coatings with outstanding electronic, mechanical and optical properties. “A particularly attractive feature of these films is that the physical properties can be tuned through the addition or subtraction of a relatively small number of nanotubes,” said Hobbie. “Thin films made from such materials hold tremendous potential for flexible electronics applications, including the replacement of indium tin oxide in liquid crystal displays and photovoltaic devices.” 

Thin films made from metallic single-wall carbon nanotubes show better durability as flexible transparent conductive coatings, which the researchers attribute to a combination of superior mechanical performance and higher interfacial conductivity. The research team found significant differences in the electronic manifestations of thin-film wrinkling, depending on the electronic type of the nanotubes, and examined the underlying mechanisms. 

The results of this study suggest that the metallic films make better flexible transparent conductive coatings; they have higher conductivity and are more durable. “Our results are relevant to a number of ongoing efforts in transparent conducting films and flexible electronic devices,” said Hobbie. 

The research was supported by the National Science Foundation through CMMI-0969155 and the U.S. Department of Energy through DE-FB36-08GO88160.