Metastable states of 2D-material-on-metal-islands structures revealed by thermal cycling

Abstract

The integration of 2D materials with artificially textured substrates offers exceptional opportunities for engineering novel functional devices. A straightforward technological route towards such devices is a mechanical dry or wet transfer of 2D layer or heterostructure onto prepared patterned elements with subsequent van der Waals bonding. An issue of van der Waals bond stability is crucial for device operation but is almost unexplored. In our research we address it by studying transport properties of hBN/graphene heterostructures transferred onto metallic island arrays and subjected to thermal cycling. We reveal that heating from cryogenic to room temperature and cooling back leads to irreversible changes in electronic transport properties: the contact between metal and graphene degrades, and signatures of suspended graphene regions transport disappear. These changes are accompanied by slight movement of the flakes and atomic-force-microscope-detected breakdown of van der Waals bonds between the flake and substrate near the metal electrodes. Interestingly, a hot pressing allows to restore the metal-to-graphene contact. We relate the observed metastability to the thermal-expansion-driven flake delamination and argue that it is accompanied by redistribution of the interfacial water or organic residues. Our findings provide useful insights into the topic of interfacial stability in van der Waals heterostructures and establish constraints for low-temperature applications of transferred 2D devices. We also add up an additional control parameter for the experimentalists in the field of 2D materials - degree of quenched disorder.

Figures (9)

• Figure 1: The samples. (a) Schematics of the vertical cross-section of the samples under study. (b) SEM image of a triangular lattice of Re islands. The same geometry was used for Nb/Pt islands. (c) First (Nb/Pt) sample. (d) Second (Re) sample before top-gate electrode patterning. Two areas are seen: bare graphene (top) and graphene on islands (bottom).
• Figure 2: First (Nb/Pt) sample. (a) Gate voltage dependencies of the resistance in perpendicular magnetic field obtained during the first cooldown. Magnetic field amplitudes consequently from violet to red are 0, 0.2, 0.5, 0.8, 1 T. For each color there are three almost coincident data sets corresponding to three temperatures: 70 mK (circles), 500 mK (crosses), 10 K (triangles). Inset: four-point measurement scheme. (b) Results of the same measurements after the second cooldown. Magnetic field amplitudes from violet to red are 0, 1, 2, 3, 4, 5 T. The measurements are taken at 10 K. Inset: a direct comparison of the dependencies obtained in a first and in a second cooldown at B = 1 T, T = 10 K.
• Figure 3: Visual changes in the Nb/Pt sample. A microphotograph of the sample a) before and b) after the first cryogenic cycle of measurements.
• Figure 4: Low temperature properties of graphene-on-Re sample. (a-c)- first cooldown, (d-e) - second cooldown: (a) $R(V_g)$ dependencies of bare graphene (black) and graphene on islands (red) at T = 10 K; the Hall resistance dependence on magnetic fields at different gate voltages at T = 10 K of bare graphene (b) and graphene on islands (c); (d) $R(V_g)$ dependencies and (e) the magnetoresistance oscillations in perpendicular magnetic field at $V_g = -10 V$ for bare graphene (black) and graphene on islands (red) at T = 0.3 K.
• Figure 5: AFM scans of the graphene-on-Re sample before and after the first cooldown - panels (a) and (b), respectively. Panel (c) shows the compararison cross-section along the blue and green lines, shown in panels (a) and (b), respectively.