Controlled layer-by-layer removal of graphene. (A) Schematic illustration of the method: (Step 1) The bilayer graphene on top of a Si/SiO2 substrate. (Step 2) A patterned layer of zinc metal is sputtered atop the graphene. (Step 3) The zinc is removed by aqueous HCl (0.02 M) in 3 to 5 min with simultaneous removal of one graphene layer. (Step 4) Patterning of a second zinc stripe. (Step 5) HCl treatment removes the second stripe of zinc plus the underlying carbon layer. (B to D) SEM image of the same bilayer GO flake: (B) original, (C) after the first, and (D) after the second Zn/HCl treatment. (E) SEM image of a monolayer GO flake patterned in the image of an owl. (F and G) SEM images of a continuous GO film patterned with horizontal and vertical stripes in two consecutive Zn/HCl treatments. The lightest squares (an example is marked with “n-2”), where the horizontal and vertical stripes overlap, represent areas exposed to two treatments. Areas exposed to one treatment (examples are marked with “n-1”) are with a shade between the lightest and darkest squares. The darkest squares (examples are marked with “n”) represent the areas with the original untreated GO film.
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.
A Rice University owl about 15 millionths of a meter wide was patterned in a single layer of graphene by Ayrat Dimiev, a postdoctoral researcher in the Rice lab of Professor James Tour. The work is part of a new paper in this week’s edition of the journal Science. (Credit: Tour Lab/Rice University
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.”