Rice University researchers create single-atom lithography in graphene
Layer-by-Layer Removal of Graphene for Device Patterning
The patterning of graphene is useful in fabricating electronic devices, but existing methods do not allow control of the number of layers of graphene that are removed. We show that sputter-coating graphene and graphene-like materials with zinc and dissolving the latter with dilute acid removes one graphene layer and leaves the lower layers intact. The method works with the four different types of graphene and graphene-like materials: graphene oxide, chemically converted graphene, chemical vapor–deposited graphene, and micromechanically cleaved (“clear-tape”) graphene. On the basis of our data, the top graphene layer is damaged by the sputtering process, and the acid treatment removes the damaged layer of carbon. When used with predesigned zinc patterns, this method can be viewed as lithography that etches the sample with single-atomic-layer resolution.
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Tour said the ability to remove single layers of graphene in a controlled manner "affords the most precise level of device-patterning ever known, or ever to be known, where we have single-atom resolution in the vertical dimension. This will forever be the limit of vertical patterning -- we have hit the bottom of the scale."
Ayrat Dimiev, a postdoctoral scientist in Tour's lab, discovered the technique and figured out why graphene is so amenable to patterning. He sputtered zinc onto graphene oxide and other variants created through chemical conversion, chemical vapor deposition and micromechanically (the "Scotch-tape" method). Bathing the graphene in dilute hydrochloric acid removed graphene wherever the zinc touched it, leaving the layers underneath intact. The graphene was then rinsed with water and dried in a stream of nitrogen.
For the owl, Dimiev cut a stencil in PMMA with an electron beam and placed it on graphene oxide. He sputter-coated zinc through the stencil and then washed the zinc away with dilute hydrochloric acid, leaving the embedded owl behind.
Sputter-coating graphene with aluminum showed similar effects. But when Dimiev tried applying zinc via thermal evaporation, the graphene stayed intact.
Investigation of the sputtered surface before applying the acid wash revealed that the metals formed defects in the graphene, breaking bonds with the surrounding sheet like a cutter through chicken wire. Sputtering zinc, aluminum, gold and copper all produced similar effects, though zinc was best at delivering the desired patterning.
The researchers were able to create a 100-nanometer line in a sheet of graphene, which suggests the only horizontal limit to the resolution of the process is the resolution of the metal patterning method.
"The next step will be to control the horizontal patterning with similar precision to what we have attained in the vertical dimension," Tour said. "Then there’s no more room at the bottom at any dimension, at least if we call single atoms our endpoint -- which it is, for practical purposes."
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