Charge Puddles and Edge Effect in a Graphene Device Studied by Scannin…

"Despite the recent progress in understanding the geometric structures of defects and edges and their role in the transport property in a graphene sheet, there has been no report showing direct correlation between them. That is because the structural studies were performed using microscopic tools such as optical microscopy, electron microscopy and scanning tunneling microscopy, while the transport properties have been measured macroscopically in a two or four terminal device with a back gate. Scanning Gate Microscope (SGM) is a unique microscopic tool with which the local geometric and electronic structures and the transport property of a device can be measured simultaneously. A SGM uses a conducting tip to apply an electric field locally and measures the transport current through two or four contacts. It can use the same tip to measure the geometric structure in Atomic Force Microscopy (AFM) mode.

Our SGM is operated under ultrahigh vacuum at 20 K, 95K, and at room temperature. An ultra-sharp AFM tip can be positioned over the graphene device using home-made vacuum motors. As we place the conducting AFM over the area of interest in a graphene device, the transport current vary as we move around the device. We have observed the conductance change originated from the spatial distribution of charge puddles with a length scale of ~ 100nm in a graphene device, very similar to the previously reported results measured with AFM with a single electron transistor tip as shown in the following figure. We also discovered that there is strong conductance enhancement when the tip is placed along the edges of a graphene device.
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