Mechanically reconfigurable van der Waals devices via low-friction gold sliding

Abstract

Interfaces of van der Waals (vdW) materials, such as graphite and hexagonal boron nitride (hBN), exhibit low-friction sliding due to their atomically flat surfaces and weak vdW bonding. We demonstrate that microfabricated gold also slides with low friction on hBN. This enables the arbitrary post-fabrication repositioning of device features both at ambient conditions and in situ to a measurement cryostat. We demonstrate mechanically reconfigurable vdW devices where device geometry and position are continuously tunable parameters. By fabricating slidable top gates on a graphene-hBN device, we produce a mechanically tunable quantum point contact where electron confinement and edge-state coupling can be continuously modified. Moreover, we combine in situ sliding with simultaneous electronic measurements to create new types of scanning probe experiments, where gate electrodes and even entire vdW heterostructure devices can be spatially scanned by sliding across a target.

Fig. 1.. AFM friction measurements for gold squares sliding on hBN.

Scale bars, 3 μm. (A and B) Optical images of ~170-nm-tall gold squares on hBN before and after manipulation with an AFM tip. (C) AFM height image of a 3-μm-wide gold square on atomically flat hBN surface with contaminants swept aside by sliding. The root mean square roughness of the swept hBN is 1 Å, and the height of the surface contaminants is roughly 8 nm. A nonlinear color scale is used to highlight features across a wide range of heights. (D) Schematic illustrating AFM lateral friction measurement: before contact (left), during static friction (middle), and during kinetic friction (right). (E) AFM friction linetrace for a 3-μm-wide square with the tip moving at 1 nm/s. The peak voltage corresponds to the Au-hBN static friction, and the subsequent constant voltage corresponds to the kinetic friction. (F) Lateral deflection voltage versus interface area between gold and hBN for 0.5-, 0.75-, 1-, 2-, and 3-μm-wide squares before and after annealing at 350°C for 30 min. Each data point is the average of multiple measurements for each size. Error bars show SD. Lines are linear fits through zero, fitting only the 4 and 9 μm² data points; this excludes smaller deflection data points that have variable AFM sensitivity (see the “Friction measurements” section in Materials and Methods for more details).

Fig. 2.. Measurement of a mechanically reconfigurable QPC device.

(A) Schematic of an hBN-encapsulated graphene device with a local graphite back gate and flexible serpentine leads connected to the movable QPC top gates (metal contacts to the graphene and graphite not shown). (B and C) QPC edge mode schematic for ν_d = 0 and ν_bg = −2. (B) For a large QPC separation, all edge modes are completely transmitted, as in the 1110-nm separation. (C) Reduced QPC separation such that the innermost edge mode is completely backscattered, while the outer edge mode is partially backscattered (indicated by the dotted lines), as in the 10-nm separation. (D) Linecuts of full 2D conductance color plots taken at 9 T and 1.5 K along ν_d = 0 for each of the four separations. The vertical dashed line at V_bg = −1.65 V indicates the ν_bg = −2 filling. (E to H) Conductance color plots versus graphite back-gate and QPC top-gate voltages at separations of 1110, 170, 80, and 10 nm, respectively. Dashed lines correspond to ν_d = 0 linecuts presented in (D). Insets are false color AFM amplitude images of QPC gates. Scale bar in (E) is 500 nm and applies to all AFM images.

Fig. 3.. In situ mechanically reconfigurable devices.

(A) Optical images showing gold sliding on hBN (pink) at 7.6 K, actuated by an AFM tip (gray). Arrows denote the range and direction of motions. Dark purple on the left is the SiO₂ substrate. See movie S1 for a video recording of the motions. (B) AFM image of hBN surface after cryogenic scanning motions from (A) showing that the hBN surface is left undamaged with only swept-up surface contaminants. AFM area is the solid red boxed area in (A). (C) Optical images of gold serpentine electrodes on hBN (pale yellow/green) showing ~2-μm longitudinal and transverse motions. Red dotted lines outline the initial position. Video recordings of oscillating motion shown in movies S2 and S3. (D) Side profile schematic of a sliding top-gate hBN-encapsulated graphene device, actuated with an AFM tip. Top-gate slides over stationary graphene to change local gating and device resistance. (E) Top-down optical image of the same device. The graphene is outlined with a dotted white line, and the light purple background is hBN. Red rectangle is 2.4 × 9.5 μm. (F) Graphene resistance versus top-gate position. Scanning range shown as the red rectangle in the optical image. Dashed lines indicate graphene edges. (G) Side profile schematic of a slidable graphene-hBN device on a stationary hBN substrate, actuated with an AFM tip. The slidable features in the schematic are outlined in black. (H) Top-down optical image of the same device. The pale green background is hBN. (I) Two-probe resistance of the slidable graphene device versus sliding position (see movie S4 for the video recording). 0 μm corresponds to the initial, transferred position. The increase in resistance over subsequent motions is likely due to photodoping from the light source used for imaging.