Graphene-based solar cells and LEDs take step closer
"With this new technique, we can grow large sheets of electronic-grade graphene in much less time and at much lower temperatures," explained Caltech staff scientist David Boyd, who developed the method.
Boyd is the first author of a study, published in the journal Nature Communications, which details the manufacturing process and the novel properties of the graphene it produces.
Graphene has a tensile strength 200 times stronger than steel and an electrical mobility that is two to three orders of magnitude better than silicon.
But achieving these properties on an industrially relevant scale has proven to be challenging. Existing techniques require temperatures that are much too hot – 1,000 degrees Celsius – for incorporating graphene fabrication with current electronic manufacturing. Additionally, high-temperature growth of graphene tends to induce large, uncontrollably distributed strain – deformation – in the material, which severely compromises its intrinsic properties.
"Previously, people were only able to grow a few square millimeters of high-mobility graphene at a time, and it required very high temperatures, long periods of time, and many steps," explained Caltech physics professor Nai-Chang Yeh, the Fletcher Jones Foundation Co-Director of the Kavli Nanoscience Institute and the corresponding author of the study. "Our new method can consistently produce high-mobility and nearly strain-free graphene in a single step in just a few minutes without high temperature. We have created sample sizes of a few square centimeters, and since we think that our method is scalable, we believe that we can grow sheets that are up to several square inches or larger, paving the way to realistic large-scale applications."
Boyd hit upon the new manufacturing process by deciding to use a system first developed in the 1960s to generate a hydrogen plasma to remove the copper oxide at much lower temperatures. The initial experiments revealed not only that the technique worked to remove the copper oxide, but that it simultaneously produced graphene as well.
At first, Boyd could not figure out why the technique was so successful but later discovered that two leaky valves were letting in trace amounts of methane into the experiment chamber. "The valves were letting in just the right amount of methane for graphene to grow," explained Boyd.
Images of early-stage growth of graphene on copper. The lines of hexagons are graphene nuclei, with increasing magnification from left to right, where the scale bars from left to right correspond to 10 μm, 1 μm, and 200 nm, respectively. The hexagons grow together into a seamless sheet of graphene. (Courtesy of Nature Communications)
The ability to produce graphene without the need for active heating not only reduces manufacturing costs, but also results in a better product because fewer defects – introduced as a result of thermal expansion and contraction processes – are generated. This in turn eliminates the need for multiple postproduction steps.
"Typically, it takes about ten hours and nine to ten different steps to make a batch of high-mobility graphene using high-temperature growth methods," explained Yeh. "Our process involves one step, and it takes five minutes."
Work by Yeh’s group and international collaborators later revealed that graphene made using the new technique is of higher quality than graphene made using conventional methods: The material is stronger because it contains fewer defects that could weaken its mechanical strength, and it has the highest electrical mobility yet measured for synthetic graphene.
The team thinks one reason their technique is so efficient is that a chemical reaction between the hydrogen plasma and air molecules in the chamber’s atmosphere generates cyano radicals – carbon-nitrogen molecules that have been stripped of their electrons. Like tiny superscrubbers, these charged molecules effectively scour the copper of surface imperfections providing a pristine surface on which to grow graphene.
The scientists also discovered that their graphene grows in a special way. Graphene produced using conventional thermal processes grows from a random patchwork of depositions. But graphene growth with the plasma technique is more orderly. The graphene deposits form lines that then grow into a seamless sheet, which contributes to its mechanical and electrical integrity.
According to Yeh a scaled-up version of the plasma technique could open the door for new kinds of electronics manufacturing. For example, graphene sheets with low concentrations of defects could be used to protect materials against degradation from exposure to the environment. Another possibility would be to grow large sheets of graphene that can be used as a transparent conducting electrode for solar cells and display panels. "In the future, you could have graphene-based cell-phone displays that generate their own power," suggested Yeh.
Another possibility, she says, is to introduce intentional imperfections into graphene’s lattice structure to create specific mechanical and electronic attributes. "If you can strain graphene by design at the nanoscale, you can artificially engineer its properties. But for this to work, you need to start with a perfectly smooth, strain-free sheet of graphene," explained Yeh. "You can’t do this if you have a sheet of graphene that has uncontrollable defects in different places."
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