L. Howald

Sled-Type Motion on the Nanometer Scale: Determination of Dissipation and Cohesive Energies of C60

The tribological properties of C60 on the mesoscopic scale were investigated with a scanning force microscope, which allowed simultaneous measurements of normal and lateral forces under ultrahigh-vacuum conditions. Islands of C60, deposited on NaCl(001), could be moved by the action of the probing tip in a controlled way. Different modes of motion, such as translation and rotation, were observed. An extremely small dissipation energy of about 0.25 millielectron volt per molecule and a cohesive energy of 1.5 electron volts were determined in these nanometer-scale experiments. The corresponding shear strength of 0.05 to 0.1 megapascal was smaller by one order of magnitude than typical values of boundary lubricants. For C60 on graphite, disruption of the islands was observed and collective motion of the islands could not be achieved. These results could find use in the field of nanotechnology; for example, C60 islands could be developed into a sled-type transport system on the nanometer scale.

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.

Nanotribology: an UHV-SFM study on thin films of C60 and AgBr

We performed scanning force microscopy (SFM) in ultrahigh vacuum (UHV) on C60 and AgBr thin films deposited on NaCl(001) substrates. The morphology of the initial growth stage and the nanotribological properties of these thin films are characterized and discussed. A novel experimental approach is presented where local friction coefficients are determined: the lateral (frictional) forces are measured as a function of normal load, controlled by an external ramp generator. The local friction coefficient can be extracted by means of the two-dimensional histogram technique. In the low load regime, friction coefficients of 0.15 +/- 0.02, 0.33 +/- 0.07 and > 0.03 were found between probing SiOx tip and C60, AgBr and NaCl, respectively. The two-dimensional histogram reveals significant details about the force regime of wear-less friction and the initial stage of wear on these thin films.

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.

Ultrahigh vacuum atomic force microscopy : true atomic resolution

In this note we report the first observation of salient features of the Si(111)(7 x 7) reconstructed surface across monatomic steps by dynamic atomic force microscopy (AFM) in ultrahigh vacuum (UHV). Simultaneous measurements of the resonance frequency shift Delta f of the Si cantilever and of the mean tunneling current (I) over bar(t) from the cleaned Si tip indicate a restricted range for stable imaging with true atomic resolution. The corresponding characteristics vs. distance reveal why feedback control via Delta f is problematic, whereas it is as successful as in conventional STM via (I) over bar(t). Furthermore, local dissipation (energy loss of 10(-14) W) through individual atoms is observed and explained by the coupling of the surface atoms to phonons.

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.

Friction force microscopy on clean surfaces of NaCl, NaF, and AgBr

With a bidirectional atomic force microscope, measurements are performed on insulating surfaces of ionic crystals, such as NaF, NaCl, and AgBr. Atomic‐scale friction is observed on surfaces, and is being prepared and studied in ultrahigh vacuum, where contaminants can be excluded. Comparative measurements show the favorable frictional properties of NaCl.