M. Luna

Lock‐in technique for measuring friction on a nanometer scale

A method for measuring friction forces on a nanometer scale is described. This method combines a lock‐in technique with scanning force and friction microscopy. Essentially, a lock‐in amplifier is used to determine the amplitude of the friction loop, which is measured at high frequency. To demonstrate the capability of this method, the dependence of the friction force with normal load is measured and a two dimensional image is presented.

Intermittent contact scanning force microscopy: The role of the liquid necks

We have studied the intermittent contact mode for small oscillation amplitudes and soft cantilevers with sharp tips. For appropriate working conditions the tip does not touch the mica surface. Instead, a high dissipative tip–sample interaction takes place which reduces the oscillation amplitude of the cantilever. Our experiments show that this dissipative interaction is related to the relative humidity and we believe that it is induced by a nanometer sized liquid neck between tip and sample. A phase contrast image on different surface materials is then due mainly to the wetting properties of the corresponding material rather than to their elastic or viscoelastic properties. The intermittent contact mode described in this work is especially advantageous since the tip is extremely gentle with the surface.

Application of non-contact scanning force microscopy to the study of water adsorption on graphite, gold and mica

The tip of an oscillating cantilever experiences a strong variation of energy dissipation with the distance just before contacting the surface. This allows scanning force microscopy (SFM) imaging in non-contact mode with high resolution. The potential of this mode is demonstrated by the study of an extremely soft and easily detachable sample such as water adsorbed on graphite, gold and mica substrates. Flat layers of water, ranging in height from molecular dimensions to several bilayers, are observed on the different surfaces studied.

Study of tip-sample interaction in scanning force microscopy

Abstract The study of the tip–sample interaction has been achieved combining the simultaneous measurement of force, resonance frequency, oscillation amplitude and quality factor vs. distance curves. From the analysis of these experiments performed in air with cantilevers of low force constant (

Interaction forces and conduction properties between multi wall carbon nanotube tips and Au(111).

We have studied the interaction forces and electrical conduction properties arising between multiwall carbon nanotube tips and the Au(111) surface in air, by means of amplitude modulation scanning force microscopy, also called intermittent contact. We have centered our work on tips with metallic electronic structure and for the specific parameters used we have found a preliminary interaction range where there is no contact between tip and surface. Stable imaging in this non-contact range is possible with multiwall carbon nanotube tips. These tips have also been used to obtain simultaneous topographic and current maps of the surface. They show excellent properties as tips due to their high aspect ratio and durability, as a result of their elastic and non-reactive properties. Correspondingly, multiwall carbon nanotube tips allow high resolution local analysis of electrical conductivity on a nanometer scale.

Adsorption of Thin Liquid Films on Solid Surfaces and its Relevance for Tribology

In many applications the sliding surfaces are exposed to ambient conditions and thus to humid air. It is well known that in air a liquid film adsorbs on surfaces, which will severely influence its tribological properties. In the present paper, a brief introduction to the physics of surfaces in equilibrium with vapor will be presented and experiments related to water adsorption on surfaces using Scanning Force Microscopy will be described. Tip-sample interaction has been measured and capillary condensation of water between tip and sample has been observed. By careful adjustment of tip-sample interaction a surface can be imaged extremely gently and the effect of water adsorption on solid surfaces can be visualized.

Modulation Technique for Measuring Friction on a Nanometer Scale

Surface Science has experienced a new impulse since the invention of the Scanning Tunneling Microscope and the further development of a whole Family of Scanning Probe Microscopes. A variety of physical properties can now be measured on surfaces with nanometer or even atomic resolution. Topography and forces normal as well as lateral to the surface can be measured with atomic resolution using a Scanning Force and Friction Microscope (SFFM)[1, 2, 3, 4]. Since these three physical quantities are among the most important in tribological processes, the SFFM is the ideal instrument for tribological studies on a nanometer scale, and correspondingly SFFM is a fast growing field. Moreover, two main features are unique to a SFFM tip-sample system compared to other tribological systems. On the one hand, a SFFM is operated in the wearless regime. On the other hand, since a typical SFFM tip has a very sharp apex - 25 nm or less -, tip and sample represent a single point contact. This is in contrast with macroscopic tribological systems, where two surfaces slide against each other over multiple, ill-defined contacts.