STM

Imaging and spectroscopy were carried out on an R9 model STM (RHK Technologies, USA), operating in constant current mode under ultra-high vacuum (UHV, ∼8 × 10^–10 Torr) at room temperature (∼294 K). STM probes were made in-house by carefully cutting a short length of platinum-iridium wire to mechanically form a sharp tip. Tip sharpness was verified by imaging the characteristic herringbone reconstruction on a clean Au (111) substrate (see Fig. S1a^†). Imaging was performed at a bias of –500 mV with a current setpoint of 100–200 pA. The high-resolution image in Fig. 1e was obtained at low temperature (12 K) using a bias of 1.5 V with a current setpoint of 200 pA.