Preparation of the Si( )-7x7 surface A Si(111) substrate n-doped with phosphorus (0.001-0.002 Ω.cm) mounted on a molybdenum sample holder is used for the preparation of Si(111)-7x7 surface. Under UHV, sample was degassed subsequently until 800 °C by direct current heating where the pressure is maintained below 3x10 -9 mbar in the chamber. Then, sample was flashed many times at 1200 °C until maintaining this temperature for 10 s without exceeding 10 -10 mbar pressure range was feasible. After each flash-cycle, we let the time for the pressure to normalize before we proceed to the next flash. Following the last flash, temperature was dropped instantly to around 950 °C, then decreased gradually by adapting a rate of 10 °C/min until 750 °C. Below 750 °C, we cut the current rapidly and let the sample cool down. Since we use pyrometer to control sample temperature, this method insures samples exposure at 850 °C, where the Si(111)-7x7 surface reconstruction occurs. Sample was then moved to the STM head and scanned to control the state of the surface as well as its cleanliness.