H. Haefke

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.

Surface and domain structures of ferroelectric crystals studied with scanning force microscopy

The understanding of the phenomena of ferroelectricity requires profound knowledge of the ferroelectric domain structure. In this paper we report on the progress of studying ferroelectric domains and domain walls with scanning force microscopy (SFM). Domains and domain walls of ferroelectric crystals of guanidinium aluminum sulfate hexahydrate (GASH) are imaged with SFM. Two sets of complementary results are obtained depending on the operation mode of the instrument. In the non‐contact imaging mode (attractive force regime), domain walls are imaged. In the contact imaging mode (repulsive force regime) in addition to the domain wall structure, information about the polarity of the domains is obtained. In these latter images, the opposing contrast of the ferroelectric positive and negative domains is superimposed on the GASH cleavage structure. The imaging mechanism of the contact and noncontact modes are discussed. Corroborating scanning electron microscopy images are presented as well.

Tip artefacts in scanning force microscopy

Since its invention in 1986, scanning force microscopy (SFM) has experienced great success as a characterization method for topography on small scales. In spite of the enormous potential of the method, it is limited by the quality of the tip used for probing the surface topography. Convolutions of non‐ideal tip shapes with the real topography and tip bending, flexing and jumping effects produce artefacts in the resulting images.

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.

An atomic force microscopy study of corona-treated polypropylene films

The surfaces of corona-treated isotactic polypropylene films have been investigated by atomic force microscopy. The occurrence of droplets on the film surfaces is related to the energy dose of the corona discharge. The sizes of these droplets correlate with the corona dose. The loss of adhesive strength of self-adhered polypropylene films can be explained on the basis of morphology changes during corona treatment. A comparative study of uniaxial and biaxial polypropylene films is presented.

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.

Static and dynamic structures of ferroelectric domains studied with scanning force microscopy

Abstract Scanning force microscopy has been employed to investigate static and dynamic surface structures on ferroelectric crystals of GASH and TGS. On the crystal cleavage faces two sets of complementary results are obtained depending on the operation mode of the force microscope: (1) In the contact mode, static domain structures and superimposed cleavage structures are imaged on GASH(0001), whereas on TGS(010) specific domain erosion due to water adsorption is observed. Additionally, molecular resolution has been achieved on GASH surfaces. (2) In the non-contact mode, domain walls are revealed and the motions of the walls due to heat treatment are documented.

Surface and domain structures of ferroelectric GASH crystals studied by scanning force microscopy

Abstract Scanning force microscopy (SFM) has been used to study the (0001) cleavage faces of ferroelectric crystals of GASH (guanidinium aluminium sulfate hexahydrate). For the first time, the surface and domain structures of a ferroelectric material has been imaged directly from the micrometer down to the molecular scale. On the micrometer scale SFM reveals the elementary cleavage structure with typical zig-zag-shaped steps. Superimposed on this topographic structure are ferroelectric domain structures which could be imaged with SFM operating the force microscope in the repulsive contact mode. On the nanometer scale, molecular resolution has been achieved on the atomically flat terraces. The imaged surface lattice with spacings of 0.68 nm reflects the threefold symmetry of the unreconstructed GASH (0001) surface.