A thermal emission microscope (THEMOS-1000, Hamamatsu Photonics) was used. The instrument has two manual probes to apply a voltage to the sample. The current-voltage properties were measured with a semiconductor parameter analyzer (B1500, Keysight). During the LIT measurement, there was no change in the current-voltage curve, which meant that LIT provided the intrinsic properties of graphene without any damage due to the applied voltage. The pulsed bias was formed by a 25-Hz external trigger. Note that the trigger had a finite time delay to shutter timing of IR camera, and this caused the LIT phase image to have the wrong phase angle. To calibrate the time delay, we used a Cu wire sample with high electrical conductivity for reference. The phase angles after calibration were used in Fig. 2 (D, E, G, and H). In the LIT measurement, objective lenses with magnifications of ×0.8, ×4.0, and ×8.0 were selectively focused onto the sample surface, and the signal from the sample was detected by an InSb CCD detector with 640 × 512 pixels. The field of view corresponded to 12 mm × 9.6 mm, 2.4 mm × 1.9 mm, and 1.2 mm × 0.96 mm for magnifications of ×0.8, ×4.0, and ×8.0, respectively. The objective lenses used for Figs. 1, 2, 3, and 4 had magnifications of ×0.8, ×4.0, ×0.8, and ×8.0, respectively. The spatial resolution was estimated to be 2 to 3 μm regarding the detected IR wavelength (3 to 5 μm) and the numerical aperture (NA) of 0.75 (in the case of ×8.0 magnification). In the case of the ×0.8 and ×4.0 magnifications, the image resolution was not determined by the spatial resolution but by the pixel resolution. All LIT images in this study were taken over the course of 10 min in ambient air. Throughout this study, the LIT measurements were performed on samples on a low thermal conductivity quartz substrate (~1.5 W/m K). The LIT images taken from a sample on a Si substrate (without an oxide layer), which is usually used in graphene research, were unclear. This problem is mainly due to the relatively high thermal conductivity of silicon (140 to 160 W/m K). The effect of the substrate on the LIT measurements will be discussed elsewhere.

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