SFB 616

 

Pictures SFB616
  
  

 Project C4:
Friction of a point contact


 The goal of project C 4 is to analyze the elementary processes of friction occurring when a surface is moved with respect to another one in close contact. We are especially interested in the limiting case of a single point contact (a single atom tip apex) with a well defined surface (see Fig. 1). Using the technique of frequency modulation atomic force microscopy (FM-AFM), we are able to analyze forces and energy dissipation between tip and surface.

Fig. 1: Sketch of a macroscopic friction experiment. On the microscopic scale, the contact area is given by a number of protrusions. In our experiments, we simplify the situation by measuring the forces between the apex of a tip and a well defined surface. Thus we can study the influence of steps, defects and adsorbates.

FM-AFM

Fig 2: Setup for FM-AFM measurements. Forces between tip and sample shift the resonance frequency of the oscillating cantilever. The frequency shift signal is used for the topography feedback. Non-conservative forces cause energy dissipation (damping/friction)

 FM-AFM is a scanning probe method, i.e., a probe, in our case a cantilever beam with a micro tip, is scanned over a sample. At each point, a feedback loop adjusts the distance between tip and sample; this signal provides the topography of the surface. In FM-AFM, a dither piezo is used to excite the cantilever beam at its resonance frequency. Forces between tip and sample lead to small shifts of the resonance frequency (see Fig. 2). The topography feedback loop keeps this frequency shift at a constant value, thus FM-AFM images represent the surface of constant frequency shift.

A second feedback loop keeps the amplitude of the cantilever constant. On the one hand, this is important because the frequency shift itself is a function of the amplitude; on the other hand, this second feedback loop provides additional information: The energy that is necessary to keep the amplitude constant corresponds to the energy that is dissipated into the surface!

 

Dissipation in FM-AFM

 As a common theory of energy dissipation in FM-AFM, the concept of adhesion hysteresis has been established. When the tip approaches the surface during one oscillation cycle, a chemical bonding between the foremost tip atom and a feature of the surface may form and break when the tip is retracted. This process implies a hysteresis loop in the tip-sample force, leading to a certain amount of energy that is dissipated during one oscillation cycle.

Fig 3: Dissipation image (10 x 10 nm²) of organic PTCDA molecules on Ag(111). Enhanced energy dissipation at the ends of each molecule is caused by an adhesion hysteresis, i.e., the approaching tip forms a short time chemical bond with the functionalized groups of the molecules.

 Fig. 3 shows the dissipation image for a layer of organic molecules on a silver surface. Other experiments show that functionalized groups at the shorter ends of the molecules are bent to the silver substrate. We assume that these functionalized groups form a chemical bond with an approaching AFM tip; this adhesion hysteresis leads to enhanced energy dissipation at the ends of each molecule.

 

 

 

 

 

 This project is in close collaborations with projects A1, C1, C2, C6 and C7.

 
  Poster:

Internationaler Workshop 2008


Internationaler
Workshop 2008

C4 Lange et al.
PDF (1.0 MB)

DPG 2007


DPG 2007
C4 Fendrich et al.
 PDF (2.2 MB)

ICNT 2007


ICNT 2007
C4 Fendrich et al.
 PDF (2.4 MB)

 

 
  Publications: