S. Grafström

Friction force microscopy of heavy‐ion irradiated mica

The effects of energetic heavy ions passing through a solid are reflected in the size, shape, and structure of individual latent tracks. Their detailed study thus provides information on the primary processes of energy deposition, which results in bond breaking, lattice disruption, and increased chemical reactivity. Furthermore, ion track cross sections, being objects very limited in size, are well suited for studies of material properties such as friction on a nanometer scale. The present work displays results of scanning force microscopy of mica, irradiated with 11.4 MeV/u Pb28+ and Xe21+ ions at the UNILAC heavy‐ion accelerator of the Gesellschaft fur Schwerionenforschung. Their track diameters were determined to be 10.6 and 7.5 nm, respectively. Recording topographical and lateral force data simultaneously allows one to quantify the contribution of friction to topographic images. In this way the kinetic friction coefficient of the sensor (Si3N4) on the sample (mica) was measured. It was found that the...

Analysis and compensation of thermal effects in laser‐assisted scanning tunneling microscopy

As an indispensable step towards detection of selective, resonant light absorption in laser‐assisted STM, we analyzed the thermal expansion produced by laser irradiation of the tunneling junction. The expansion as a function of the modulation frequency of the light power could be related to the specific heat and thermal conductivity of the materials involved. A scheme is presented, aiming for detection of small tunneling current changes induced by resonant optical excitation of an adsorbate. It is based on the idea of irradiating the tunneling gap with two laser beams of different wavelengths—only one being in resonance with the adsorbate—and suppressing the thermal contribution to the tunneling current by an amplitude modulation of both beams with a 180° phase shift.

Analysis of lateral force effects on the topography in scanning force microscopy

The mechanism of how lateral forces can produce artifacts in topographical images obtained by scanning force microscopy is analyzed theoretically. It is shown that due to the detailed elastic behavior of the force sensing cantilever, a scanning force microscope based on the light beam deflection method is particularly subject to such artifacts. They are produced by lateral forces, arising not only from friction but also from the topography itself. Consequently, the images are modified by a self‐distortion of the topography. The various effects contributing to the image contrast are demonstrated experimentally by images of a tribologically heterogeneous sample. The topographical contrast is compared with lateral force images. A quantitative analysis shows that both friction‐induced and topography‐induced topographical artifacts can be described reasonably well by the theoretical formulas.

Fast laser beam position control with submicroradian precision

Abstract Precise control of spatial position and pointing stability of the output beam of a cw free-jet ring dye laser was realized. The fast opto-electronic feedback device, based on four-quadrant photodiode detectors and piezo-driven mirrors, limits beam angle variations to 4 × 10 -7 rad. The importance of the method is discussed and the efficiency is illustrated.

Scanning tunneling microscopy of liquid crystals, perylene‐tetracarboxylic‐dianhydride, and phthalocyanine

Scanning tunneling microscopy measurements conducted on pure liquid crystals and liquid crystals in combination with the dyes perylene‐tetracarboxylic‐dianhydride (PTCDA) and phthalocyanine (Pc), all adsorbed on highly oriented pyrolytic graphite (HOPG), are being reported on. The liquid crystal octylcyanobiphenyl (8CB) was used as a solvent for the dyes, allowing the preparation of ordered dye layers in a very simple way. A combination of vapor deposition of PTCDA and liquid deposition of 8CB provided films, where 8CB and PTCDA are embedded in each other on a small length scale. The samples were prepared in two different ways yielding either 8CB islands within a PTCDA monolayer or PTCDA islands surrounded by 8CB. The Pc films on HOPG exhibit a hexagonal molecular lattice that could be related to the lattice of the graphite substrate and for which a structural model is proposed. For dodekylcyanobiphenyl (12CB) on HOPG the ordering in unit cells containing 10 rather than 8 molecules is reported for the fir...

A tunneling atomic force microscope with inertial tip‐to‐sensor approach

An atomic force microscope is presented which uses the tunneling effect for signal detection, the tunneling tip‐to‐sensor approach being based on an inertial sliding mechanism. Concentric tube piezo arrangements of both the sample and slider mounts serve for thermal drift compensation. Particular emphasis lies on the description of the sliding device, sawtooth voltage generation and control for the slider motion, and tip feedback electronics. The image of a 4.7×4.7 μm2 surface section of an optical storage disk illustrates the large‐scale scanning performance of the instrument.

Contrast mechanisms in photothermal scanning tunneling microscopy

By irradiation of the tunneling junction of a scanning tunneling microscope with intensity-modulated laser light a gap-width modulation due to thermal expansion of tip and sample was produced. Photothermal images were obtained by spatial mapping of the resulting modulation of the tunneling current or its logarithm. The various mechanisms responsible for the observed contrast are discussed quantitatively. In case of a highly corrugated gold film on mica the contrast arises mainly from either the current variations caused by the non-zero reaction time of the current control loop or from a geometry factor. In both cases the images reflect certain properties of the sample topography. On the other hand, for a liquid-crystal film adsorbed on graphite a contrast on a molecular scale was found which is attributed to variations of the effective barrier height.

Heterodyne laser spectroscopy of lithium ions: 23P fine and hyperfine structure of 7Li+

The fine structure (fs) splittings in the 1s2p3PJ=1,2,0 multiplet of the helium-like7Li+ ion were measured with a laser spectrometer. The results with 3σ errors are: Δv01 (3P0,F=3/2−3P1,F=5/2)=152081.6(2.0) MHz and Δv02 (3P0,F=3/2−3P2,F=7/2)=82704.3(1.9) MHz. Combining the new precise fs measurements with earlier hyperfine structure (hfs) results from laser-microwave spectroscopy provided improved fs constants:D1=−155709.0(2.1) MHz,D2=−93049.2(2.0) MHz, and hfs constants:Ac=3679.0(6) MHz,A0=51.0(4) MHz, andAd=−11.3(1) MHz, thus allowing for a stringent test of available theoretical data. The spectroscopic method used in this work opens up the possibility of determining Li+, 23S−23P absolute transition frequencies with a precision of ∼2·10−9.

Scanning tunneling microscopy studies of diazo dye monolayers on HOPG

We report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-[4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C20H19N5) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.

A new precise value of the absolute \(\), \(\), \(\) transition frequency in \(\)

Abstract:The absolute frequency of the optical electric-dipole transition between the metastable 1s2s3S1, F=5/2 and the short-lived 1s2p3P2, F=7/2 hyperfine structure sublevels of the helium-like ion in its rest frame was determined by combining Doppler-free saturation spectroscopy and laser heterodyning. One of two cw single-mode dye lasers was locked to the calibrated w-component of the [R(85) 26-0]-line in the (B-X)-system of the -molecule at MHz. While crossing two counterpropagating beams of the other laser perpendicularly (lab angle ) with a low-velocity ion beam and tuning its frequency over the transition under study, the distance of the Lamb-dip center frequency from was obtained by mixing both laser frequencies with an avalanche diode and registering their beat frequency at about 4 GHz with a quartz-stabilized high-frequency counter. The Lamb-dip position was measured for different ion velocities (in units of the velocity of light) in the range of . The parameters and of the relativistic Doppler formula were extracted via a fit to the experimental data set , providing MHz and . This result of differs noticeably from MHz, measured by another group in a collinear-beam configuration at an ion beam propagating with .