Graphene Formed Under Microscope Is Small Enough for Electronics

Scientists at Rice University; the University of Tennessee, Knoxville (UT Knoxville); and Oak Ridge National Laboratory (ORNL) used a small, visible beam mounted to a scanning electron microscope (SEM) to form laser-induced graphene (LIG), a multifunctional graphene foam that is typically direct-written with an infrared (IR) laser, into a carbon-based precursor material. The researchers were able to directly observe LIG formation while the process occurred in the SEM chamber.

The researchers used a 405-nm laser to directly convert polyimide, a polymer, into LIG. The SEM-mounted laser burned only the top 5 µm of the polymer, allowing LIG features with a spatial resolution of about 12 µm and a thickness of less than 5 µm to form. The relatively small focused spot size of the 405-nm laser provided a spatial resolution that allowed more than a 60% reduction in LIG feature sizes and LIG features that were almost 10 times smaller than those typically achieved with an IR laser.

Scientists recorded the formation of laser-induced graphene made with a small laser mounted to a scanning electron microscope. Courtesy of Tour Group/Rice University.

For proof-of-concept, the researchers used LIG formed under the SEM to make flexible humidity sensors that were directly fabricated on polyimide. The devices were able to sense human breath with a response time of 250 milliseconds (ms). “This is much faster than the sampling rate for most commercial humidity sensors and enables the monitoring of rapid local humidity changes that can be caused by breathing,” researcher Michael Stanford said.

The reduced size of the LIG features could enable the direct-write formation of flexible electronics that are not visible to the unaided eye. “A key for electronics applications is to make smaller structures so that one could have a higher density, or more devices per unit area,” professor James Tour said. “This method allows us to make structures that are 10 times denser than we formerly made.” Lower-powered lasers also make the process less expensive, which could lead to wider commercial production of flexible electronics and sensors.

Scientists at Rice University and Oak Ridge National Laboratory used a small laser mounted to a scanning electron microscope to form dots and traces of conductive graphene on a polymer. The technique creates laser-induced graphene with features more than 60% smaller than the macro version and almost 10 times smaller than typically achieved with an infrared laser. Courtesy of Tour Group/Rice University.

Tour, whose group recently introduced flash graphene to instantly turn trash and food waste into graphene, said the new LIG process offers a path toward writing electronic circuits into flexible substrates like clothing. “While the flash process will produce tons of graphene, the LIG process will allow graphene to be directly synthesized for precise electronics applications on surfaces,” he said.

A scanning electron microscope image shows two traces of laser-induced graphene on a polyimide film. A laser mounted to the microscope was used to burn the patterns into the film. The technique shows promise for the development of flexible electronics. Courtesy of Tour Group/Rice University.

The research was published in ACS Applied Materials & Interfaces (www.doi.org/10.1021/acsami.0c01377).

You don’t need a big laser to make laser-induced graphene. Scientists at Rice University; the University of Tennessee, Knoxville; and Oak Ridge National Laboratory (ORNL) are using a very small visible beam to burn the foamy form of carbon into microscopic patterns. Courtesy of Tour Lab/Rice University.