Sample preparation
This protocol is extracted from research article:
Internal strain tunes electronic correlations on the nanoscale
Sci Adv, Dec 14, 2018; DOI: 10.1126/sciadv.aau9123

α-(BEDT-TTF)2I3 single crystals were grown by D. Schweitzer using standard electrochemical methods (45). Single crystals are plate like, with lateral dimensions of several millimeters and thickness d ≈ 10 to 100 μm. We first preselected samples with clean surfaces, judging from their visual appearance under an optical microscope. In a next step, AFM images of the crystal surfaces were taken, yielding a typical roughness of 5 to 10 nm for good samples. The surface topography does not coincide with the characteristic phase boundary detected by near-field optics. This fact is taken as another strong evidence that our optical measurements probe the bulk properties; these are affected by electronic correlations under inhomogeneous strain but are not related to surface impurities or defects. In situ AFM maps of the crystal surfaces displayed in Figs. 2 and 3 are presented in fig. S1.

The selected high-quality specimens were then mounted on sapphire substrates using a conductive carbon paste. Apart from electrical transport and its mechanical properties, this type of glue has a good thermal conduction that effectively dissipates heat away from the sample during the gold evaporation procedure. Shadow masks were individually manufactured from a thin metallic foil and arranged close to the sample surface for the gold deposition. The entire substrate was fixed in a vacuum chamber about 50 cm above the boat. Such a long distance is necessary for two reasons: Placing the specimen far away from the boat effectively reduces heating effects, and it geometrically supports the deposition of sharp gold edges with a step height of 30 to 40 nm and a width of <1 μm (fig. S1), as required for the near-field experiments. Evaporation was performed at a slow rate of 2 Å to reduce heating of the crystal. Monitoring and regulating the sample temperature are crucial because organic charge-transfer salts become unstable just a few hundred kelvins above room temperature. When exposed to elevated temperatures for a longer period of time, the compound is known to lose iodine; α-(BEDT-TTF)2I3 may also change phase upon moderate tempering around 80°C (46). For that reason, we again checked the optical properties subsequent to the gold evaporation and verified that the material properties were not altered.

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