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Mechanical cleaning of graphene
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Image of FIG. 1.

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FIG. 1.

(Color online) Tapping mode image of sample A after annealing at 440 °C and contact mode scanning (both with a Veeco Nanoscope IIIa AFM). Only the part within the marked window was scanned with the CM AFM. We chose to show this device because it was much more contaminated than other devices before scanning, so that the effect of the CM AFM scan is easily visible. Wrinkles and some tears on the upper right side of the graphene are induced by the tip but were not observed in other devices. On the left and right of the bounding box, walls of deposited residue are visible. The contacts of the device are not visible in this image.

Image of FIG. 2.

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FIG. 2.

(Color online) (a) Backgate traces of sample B at room temperature in vacuum ( nA). The lower curve is before CM AFM imaging and the upper curve after. Using the geometric capacitance, we convert the backgate axis into carrier density. Then we extract the field effect mobility by fitting a straight line to the steepest part of the backgate trace: , where and , as calculated from Fig. 3. We extracted the thickness from TM AFM images of the devices.

Image of FIG. 3.

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FIG. 3.

(Color online) Measurements on a double gated bilayer graphene transistor fabricated out of sample A. The 4-probe resistance at T = 50 mK is plotted as a function of backgate and topgate voltage ( and , respectively). From the slope of the diagonal line, we calculated the relative dielectric constant of the hBN to be 3.0 assuming a parallel plate capacitor model and . The thickness of the bottom hBN flake was 14 nm and the top hBN flake 50 nm, values extracted from AFM images. Lowerleft inset: schematic of the device. Blue colored regions are hBN, green is bilayer graphene, and yellow are the contacts and gate. Upper right inset: resistance as a function of at V. The dip at V is caused by the uncovered graphene part.

Image of FIG. 4.

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FIG. 4.

(Color online) Force-distance curve of sample D, measured by holding the tip of the AFM in a fixed lateral position and approaching and retracting the tip in the vertical direction. While making these vertical movements, the deflection of the tip is recorded. Assuming that when the tip is in contact with the surface, the tip deflects the same distance as the piezo moves, we can calibrate the deflection scale. With the spring constant of the tip, we convert that deflection to a force. The horizontal axis has an arbitrary offset. The blue region indicates the range of forces that we used for cleaning the samples. Sample D was scanned at a force of −22 nN as indicated by the arrow. The illustrations picture the pulling and pushing regime.


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Table I.

Results summary for four different samples (measurements in vacuum).


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Contamination of graphene due to residues from nanofabrication often introduces background doping and reduces electron mobility. For samples of high electronic quality, post-lithography cleaning treatments are therefore needed. We report that mechanical cleaning based on contact mode atomic force microscopy removes residues and significantly improves the electronic properties. A mechanically cleaned dual-gated bilayer graphene transistor with hexagonal boron nitride dielectrics exhibited a mobility of ∼36 000 cm2/Vs at low temperature.


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Scitation: Mechanical cleaning of graphene