Electric Field and Contact Interactions of Tip with Adenine Molecules on SrTiO₃(100)-√ 5×√ 5 Surfaces

We have performed tip-assisted migration of adenine molecules adsorbed on SrTiO 3 (100)-√ 5×√ 5 surfaces with scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions at room temperature. The migration with stepwise current control reveals the difference in binding energy of adenine molecules adsorbed on various sites on √ 5×√ 5 surfaces. The adenine molecules easily migrate on the √ 5×√ 5 surface at room temperature and settle down at the trap sites of Sr atoms remaining on the surface. Quantitative analysis of migration probability as a function of tunneling current (I) and tip bias voltage ( V t ) shows that the electric field effect is dominant at V t ≧4 V or I ≧70 pA, while contact interaction is dominant at 0 V< V t ≦0.1 V for the tip-assisted migration in this system.