Laser-Induced Graphene for Flexible Electronics

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There’s no need to use big lasers for making laser-induced graphene (LIG). 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 more than 60% smaller than the macro version and almost 10 times smaller than typically achieved with an infrared laser. The researchers used a very small visible beam to burn the foamy form of carbon into microscopic patterns.

James Tour, a chemist at the labs of Rice, discovered the original method to turn a common polymer into graphene in 2014, and Tennessee/ORNL materials scientist Philip Rack revealed that they can now watch the conductive material form as it makes small traces of LIG in a scanning electron microscope (SEM).

According to Tour, lower-powered lasers also make the process less expensive. That could lead to wider commercial production of flexible electronics and sensors. A key for electronics applications is to make smaller structures so that one could have a higher density, or more devices per unit area. This method allows them to make structures that are 10 times denser than they formerly made.

To prove the concept, the lab made flexible humidity sensors that are invisible to the naked eye and directly fabricated on polyimide, a commercial polymer. The devices were able to sense human breath with a response time of 250 milliseconds.

Rice postdoctoral researcher Michael Stanford stated that 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.

The smaller lasers pump light at a wavelength of 405 nanometers, in the blue-violet part of the spectrum. These are less powerful than the industrial lasers that the Tour Group and others around the world are using to burn graphene into plastic, paper, wood, and even food. The SEM-mounted laser burns only the top five microns of the polymer, writing graphene features as small as 12 microns. (A human hair, by comparison, is 30 to 100 microns wide.)

Tour, whose group recently introduced flash graphene to instantly turn trash and food waste into the valuable material, said the new LIG process offers a new 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.

Co-authors of the paper include postdoctoral researcher Cheng Zhang and graduate student Anna Hoffman of UT Knoxville, research and development scientist Ilia Ivanov of Oak Ridge National Laboratory, and staff scientist Jason Davidson Fowlkes of UT Knoxville and Oak Ridge. James Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice. Philip Rack is a professor and the Leonard G. Penland Chair and associate department head of materials science and engineering at UT Knoxville, as well as joint staff at the Center for Nanophase Materials Sciences at Oak Ridge. The research is supported by the Air Force Office of Scientific Research and the U.S. Department of Energy.