Comment:
The sample was annealed at 850 K for 90 min in Ar after being degreased in acetone, methanol and deionized water, was polished in HNO3 and HF mixture, and immersed in deionized water. The final cleaning was Ar+ ion bombardment (Ep = 4 keV, j = ~ 1.0 microampere cm-2, time = 2 - 3 h).