Metallic adhesion forces and tunneling between atomically defined tip and sample

Elements of metallic adhesion are discussed in light of issues in non-conta ct force microscopy. We have measured forces between an atomically defined W(111) tip and a Au(lll) sample in ultrahigh vacuum at 150 K. The W tips ar e manipulated and characterized on an atomic scale both before and after sa mple approach by field ion microscopy. We observe strong attractive adhesio n forces, which turn repulsive upon further approach of the tip towards the Au surface. Unexpected for a metallic system, there is no spontaneous jump -to-contact, although we find such wetting is inducible with undefined poly crystalline W tips. The force vs, tip-sample distance curve shows only mode st hysteresis, and the field ion microscopy images reveal an atomically unc hanged tip apex. The tunneling current between tip and sample is found to i ncrease exponentially in a regime of strong adhesive forces. (C) 2000 Publi shed by Elsevier Science B.V. All rights reserved.