R. Lüthi

Multifunctional probe microscope for facile operation in ultrahigh-vacuum

A scanning force/tunneling microscope (SFM/STM) for remote controlled operation in ultrahigh vacuum (UHV) is described. The lateral forces, normal forces, and tunneling currents between probe tip and sample can all be measured simultaneously. The optical beam deflection detector and the sample position can be adjusted by means of three compact inertial stepping motors. An UHV‐compatible light emitting diode is introduced as a general alternative to the widely used laser diode in the detector. Images, taken at 5×10−11 mbar on Si(111) with STM and noncontact SFM, and on NaF(001) with contact SFM, are presented.

Site-specific friction force spectroscopy

The two‐dimensional histogram technique is used to determine the loading dependence of friction on terrace and step sites of NaCl(001). An extended adhesion model is used to analyze the data. Both Hertz and JKR theory are incorporated for the elastic deformation. Depending on the model, shear strengths of 100–190 MPa are found at the terrace region. Increased shear strength values of 500–1800 MPa are found at the step edges that are interpreted in terms of the increased energy barriers at the steps, known as Schwoebel barriers.

Attractive-mode imaging of biological materials with dynamic force microscopy

We have applied the non-contact dynamic force microscopy method to investigate soft biological materials such as hexagonally-packed intermediate layers, DNA, and tobacco mosaic virus under ambient conditions. This method, where a stiff cantilever is oscillated close to its resonance frequency with an amplitude of 0.3-1.5 nm above the sample, allows highly reliable investigation of soft organic matter with minimized normal and lateral forces between tip and sample. The vertical and lateral resolution are determined to be < 1 angstrom and 1-2 nm, respectively, comparing favorably to established results from repulsive-mode scanning force microscopy experiments on absorbate covered surfaces in liquids. The interaction forces are found to be attractive, dominated by damping mechanisms and attractive force gradients of capillary and van der Waals interactions.

Statics and dynamics of ferroelectric domains studied with scanning force microscopy

Scanning force microscopy for studying ferroelectric domain structures is applied. The force microscope was operated in the contact static mode (repulsive force regime) and in the noncontact dynamic mode (attractive force regime). These two techniques were applied to study cleavage faces of ferroelectric crystals of GASH (guanidinium aluminum sulfate hexahydrate) and TGS (triglycine sulfate) crystals. Using the contact mode, the positive and negative domains are revealed by opposite contrast. In the dynamic noncontact mode, the domain walls are revealed. The experimental setup allows in situ experiments to study the dynamics of ferroelectric domains. First results on the time dependence of the domains motion in TGS will be presented.

Friction on the atomic scale: An ultrahigh vacuum atomic force microscopy study on ionic crystals

We performed atomic force microscopy in ultrahigh vacuum on the ionic crystal of KBr(001). The morphology and the tribological properties of this cleavage face are characterized and discussed. The local friction coefficient was extracted by means of the two‐dimensional histogram technique. For loads below 3 nN a linear behavior was found between normal and lateral forces yielding a friction coefficient of less than 0.04. In this load regime, wearless friction is observed. For higher loads, the friction coefficient increases to a value of about 0.7–1.2. The corresponding topography images reveal the typical onset of wear. On the atomic scale a periodicity of 4.7 A was found which corresponds to the distance of equally charged ions on the KBr(001) surface. On this scale, the lateral force map exhibits the typical stick‐slip phenomenon which is discussed in terms of a novel theoretical approach.

Elasticity, wear, and friction properties of thin organic films observed with atomic force microscopy

An atomic force microscopy study is presented on lubricating systems, such as thin organic Langmuir–Blodgett films. Local elastic compliance was measured simultaneously with topography and friction on thin films of mixtures of fluorocarbons and hydrocarbons. On the phase‐separated fluorocarbon domains, higher friction and lower Young’s modulus than on the hydrocarbon domains was found. Adhesive wear has been observed to occur randomly on the stiffer hydrocarbon domains if the film is complexed with a counterion polymer of a larger cross‐sectional area. A quantitative fractal analysis of the hydrocarbon film remaining areas provided a linear adhesive wear dependence on the scan cycle number.

Progress in noncontact dynamic force microscopy

The technique of operating the scanning force microscope in the dynamic noncontact mode (dynamic force microscopy) has been improved. Home-built instruments based on an optical beam deflection scheme in two different environments were used. The two force microscopes were operated in ambient air and in ultrahigh vacuum, respectively. In order to control the oscillating cantilever different methods were applied: slope-detection (lock-in amplifier, RMS-to-DC converter) and frequency modulation (FM) technique as well. The advantages of this nondestructive technique are demonstrated on different samples, such as soft organic matter (hexagonally packed intermediate layer, Langmuir-Blodgett film), layer-structured compounds (CdI2), n-doped Si(111), and ferroelectric crystals [triglycine sulfate (TGS), guanidinium aluminum sulfate hexahydrate (GASH)]. On TGS and GASH cleavage faces, the ferroelectric domains and domain walls could be imaged. From experimental data a spatial resolution of about 1-2 nm in lateral and >0.1 nm in vertical directions could be determined.

A nanoscopic view of structure and deformation of hard elastic polypropylene with scanning force microscopy

Scanning force microscopy (SFM) was used to visualize the surface of hard elastic polypropylene (HEPP) film. The surface morphology of unstrained HEPP shows crystalline and noncrystalline rows oriented parallel to the extrusion direction. The crystalline rows are composed of lamellar blocks. The dimensions of crystalline and noncrystalline regions are determined. The structural surface changes induced by stepwise elongation of the film with a home-built stretching device are documented by SFM. Stretching of HEPP perpendicular to the extrusion direction causes cracks advancing through several crystalline rows. During elongation parallel to the extrusion direction the separation of adjacent lamellae by their translatory displacement occurs. Deformation-induced structural changes of HEPP on the nanometer scale are compared with proposed deformation models. Nanostructural changes are correlated with characteristic variations in the force-elongation curve.

BIOLOGICAL-MATERIALS STUDIED WITH DYNAMIC FORCE MICROSCOPY

Biological materials such as hexagonal packed intermediate (HPI) layers, DNA, tobacco mosaic virus and collagen deposited on various substrates with noncontact dynamic force microscopy under ambient conditions were investigated. This method is highly suited for the investigation of soft organic matter where a minimized interaction between tip and sample is needed for nondestructive and reliable operation. Hence, additional anchoring of the biological specimens was no longer found to be crucial. The vertical and lateral resolution limits of this gentle method were determined to be <0.1 nm and 1–2 nm, respectively, allowing very stable and high resolution results on all investigated systems. By taking approach curves and monitoring the dynamic properties of the cantilever (resonance frequency and Q value) during the experiment, the interaction mechanism between tip and sample was found to be dominated by attractive van der Waals interaction and capillary forces. Furthermore, initial results from a HPI layer imaged with noncontact dynamic force microscopy in a water environment are presented.

Influence of Humidity on Friction Measurements of Supported MoS2 Single Layers

We have studied friction of MoS2 single layers on mica and Al2O3 in the range of 5%–90% relative humidity. Friction coefficients have been determined simultaneously for the MoS2 platelets as well as for the support. Only a slight influence on the humidity was found for the friction coefficient of MoS2 and Al2O3. On the other hand, water adsorption on mica caused a strong increase of the friction coefficient above 40% relative humidity. The change of the friction coefficient was so strong that the contrast of the friction images inverted. At very high humidities (above 60%), the friction coefficient of mica reverted to its initial value, probably because water acted as a boundary lubricant.