AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS1-TuM

Paper NS1-TuM5
Metallic Adhesion and Tunneling at the Atomic Scale

Tuesday, October 26, 1999, 9:40 am, Room 612

Session: Nanomechanics
Presenter: A. Schirmeisen, McGill University, Canada
Authors: A. Schirmeisen, McGill University, Canada
G. Cross, McGill University, Canada
A. Stalder, McGill University, Canada
P. Grutter, McGill University, Canada
U. Durig, IBM Research Division, Zurich Research Laboratory
Correspondent: Click to Email
We have measured forces and currents between atomically defined W(111) tips and a Au(111) sample in ultra high vacuum at 150 K. The W tips are manipulated and characterized on an atomic scale both before and after the experiment by field ion microscopy (FIM). The force-distance curve shows a peak of the attractive, adhesive metallic force of 5 nN for a three atom tip. Unexpected for a metallic system, there is no spontaneous jump-to-contact (Cross et al., PRL 80. 4685 (1998)). An analysis of the tip by FIM after the approach reveals an atomically unchanged tip apex even for repulsive forces of up to several nN. From a fit of our data to the Maugis-Dugdale theory we can determine that our system is close to the rigid body Bradley limit. The experimental data is described very well with a scaled Rydberg function with an unexpected large decay length of 0.2 nm. The simultaneously measured tunneling current has the expected exponential dependence on tip-sample separation, giving a reasonable barrier height of 3.7 eV. After a model proposed by Chen (J. Phys. Cond. Mat 3, 1227 (1991)) there should be a direct correlation between the tunneling current and adhesion force. Using our experimental data we extract a value for the LDOS of the W trimer tip of 1 state/eV/atom at the Fermi energy, which is in agreement with theoretical predictions. Finally, we have measured the evolution of the tip-sample contact for a tip radius of 3 nm and repulsive loadings as large as 200 nN. Half of these measurements show little or no hysteresis, whereas in the remaining indications of reversible slip behavior was observed.