[Show abstract][Hide abstract] ABSTRACT: A low-temperature scanning force microscope (SFM) operating in ultrahigh vacuum was used to study the interaction between a sharp but unreactive oxidized silicon tip and a Si(111) 7 X 7 surface. Weakly corrugated but, nevertheless, atomically resolved images were obtained over a relatively large range of tip-sample distance. The measured short-range interaction potential and image contrast can be explained by an electrostatic interaction that results from a dipole moment induced in the tip by the distribution of electronic charge among the dangling bonds on the surface. The experiments presented here demonstrate the potential of SFM to quantitatively map even weak, physical interactions with atomic scale resolution. Such experiments should lead to a better understanding of surface reactivity and local charge-transfer effects induced by adsorbates or defects at surfaces.
Physical Review B 07/2003; 68(3-3). DOI:10.1103/PhysRevB.68.035324 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Summary form only given. The Magnetic Force Microscope (MFM) has become a major tool for characterizing and evaluating read and write heads, written tracks and material performance in magnetic data storage. However, standard MFM techniques suffer from the following problems that limit their application to image the bit structure as the storage density advances towards 1 Tbit/in<sup>2</sup>: the lack of lateral resolution and measurement sensitivity on a nanometer scale; the difficulty of relating the measured signal to the stray field; and the ability to separate the topography and magnetic signal. The techniques to obtain a high lateral resolution and achieve a complete separation of topography and magnetism, were developed with sophisticated MFMs built for fundamental research. These instruments are not suitable for industrial applications due to their complexity and restrictions regarding the sample size. In order to overcome these limitations a novel room temperature MFM (QMFM), with a focus on industrial standard applications, has been developed at the University of. Basel.
Joint NAPMRC 2003. Digest of Technical Papers [Perpendicular Magnetic Recording Conference 2003]; 02/2003
[Show abstract][Hide abstract] ABSTRACT: Atomically resolved scanning force microscopy images and force versus distance measurements at low temperatures on a cleaved NiO (001) single crystal are reported. The force-distance data are well modeled with a capacitive force in the distance range of 0.5 to 20 nm. The residual forces at smaller tip-sample distances show a maximum attraction of 2.3 nN and decay within 0.2 nm. They show a steplike behavior associated with an increase in dissipation signal. This steplike behavior can be explained by interaction of more than one atom of the tip or of the surface.
Physical Review B 02/2003; 67(8-8). DOI:10.1103/PhysRevB.67.085402 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Summary form only given. The bit size in magnetic recording has reached the resolution limit of presently available magnetic force microscopes. The next generation of magnetic force microscopes are expected to measure the magnetic field quantitatively, obtain a lateral resolution below 10 nm, and achieve a complete separation between topographical and magnetic information.
[Show abstract][Hide abstract] ABSTRACT: The dynamical structure factors of the metallic glass Ca70Mg30 and the same sample after crystallisation have been determined for momentum transfers between 0.4 and 4 AA-1 and energy transfers h(cross) omega =E0-E between -40 and 6 meV using a thermal neutron time-of-flight spectrometer. The low-frequency modes reported in a recent experiment in a different metallic glass have been studied at room temperature with improved resolution. Their dependence on momentum transfer is discussed in detail.
Journal of Physics F Metal Physics 11/2000; 11(7):1375. DOI:10.1088/0305-4608/11/7/011
[Show abstract][Hide abstract] ABSTRACT: The X-ray emission spectra of Pd and Zr in glassy Pd30Zr70 have been measured and are compared with the emissions from pure polycrystalline Pd and Zr samples. The local contributions to the total density of states are obtained from the alloy measurements and located with respect to the Fermi level by means of core-level binding energy determinations. The interesting alloying effects are analysed and discussed in relation to a new cluster approximation and previous photoemission measurements.
Journal of Physics F Metal Physics 11/2000; 11(4):L95. DOI:10.1088/0305-4608/11/4/007
[Show abstract][Hide abstract] ABSTRACT: Magnetic susceptibility and Mossbauer measurements are reported for amorphous (Fe1-xNix)77Si10B13 in a Ni concentration range 0.85<or=x<or=1, where the breakdown of magnetism occurs. The authors propose a magnetic phase diagram in which, for x<0.91, long-range magnetic order undergoes a transition to a cluster-glass state. The pronounced influence of magnetic clusters upon the temperature and field dependence of the magnetisation is reflected in strongly curved Arrott plots and gives rise to spin-wave contributions even for alloys with low Curie temperatures. The fairly high ratio of the paramagnetic moment to the spontaneous moment is discussed in terms of a recent theory by Wagner and Wohlfarth (1980).
Journal of Physics F Metal Physics 11/2000; 11(11):2429. DOI:10.1088/0305-4608/11/11/024
[Show abstract][Hide abstract] ABSTRACT: The dynamical structure factors of the glassy alloy Mg70Zn30 and the same sample after crystallisation have been determined at room temperature using neutron inelastic scattering techniques. From the time-of-flight spectra, generalised frequency distributions were determined which were used to calculate the temperature dependence of some of the thermodynamic properties of both samples. For temperatures below 70K the vibrational ('lattice') part of the specific heat of the glass is larger than that of the polycrystalline alloy. The difference between the specific heats of the two samples increases with decreasing temperature. It is shown that this larger specific heat is related to the low-frequency vibrations found in the dynamical structure factor of the metallic glass.
Journal of Physics C Solid State Physics 11/2000; 14(17):2305. DOI:10.1088/0022-3719/14/17/004
[Show abstract][Hide abstract] ABSTRACT: The dynamical structure factor of glassy Cu46Zr54 has been measured at room temperature, for momentum transfers between 1 and 8 AA-1 and energies (h(cross) omega =E0-E) between -45 and 45 meV, using neutron inelastic scattering. From the measured time-of-flight spectra a generalised frequency distribution has been determined. The experiment was repeated after crystallisation of the sample. The results show a strong increase of low-frequency modes in the amorphous sample compared with the polycrystal, and the frequency distribution of the metallic glass shows a slope approximately omega 4/3 between 4.7 and 7 meV, where the frequency distribution of the polycrystalline sample still shows a Debye type of spectrum.
Journal of Physics C Solid State Physics 11/2000; 13(8):L167. DOI:10.1088/0022-3719/13/8/006
[Show abstract][Hide abstract] ABSTRACT: The atomic dynamics of the metallic glass Mg70Zr30 and the same sample after crystallisation was investigated at room temperature using neutron inelastic scattering techniques. For the first time the dispersion of short-wavelength collective density excitations could be measured in the glassy metal for a few momentum transfers near the first maximum of the static structure factor. From the intensity distribution in the spectra the authors conclude that the modes observed are transverse collective density excitations in the glassy system (transverse phonons in the polycrystal). They compare their findings with molecular dynamics and analytical calculations on the same system.
Journal of Physics C Solid State Physics 11/2000; 13(36):L1045. DOI:10.1088/0022-3719/13/36/002
[Show abstract][Hide abstract] ABSTRACT: The electronic structure of glassy and crystalline CuxTe1-x (0<or=x<or=0.58) alloys has been investigated with ultraviolet photo-electron spectroscopy (UPS). X-ray diffraction, differential scanning calorimetry (DSC) and X-ray photo-electron spectroscopy (XPS) have been used to characterise the samples. The UPS data indicate a metal-to-semiconductor transition near x=0.45.
Journal of Physics C Solid State Physics 11/2000; 20(31):5233. DOI:10.1088/0022-3719/20/31/024
[Show abstract][Hide abstract] ABSTRACT: The atomic and electronic structures of amorphous and crystalline Mg-Zn alloys are studied by computer simulation, electronic band-structure calculations and photoemission measurements. The spectra for the metallic glasses and for pure crystalline zinc show a narrow band of Zn 3d states centred at a binding energy EB of about -9.7 eV, overlapping the bottom of a broad sp band. There are indications of a minimum in the electronic density of states at the Fermi level for the glasses and for the pure metals. Molecular dynamics and potential-energy mapping calculations based on pseudopotential-derived interatomic forces are used to construct models for the atomic structure, with no other input than the composition and the atomic numbers and atomic weights of the components. The analysis of these models-which are in reasonable agreement with X-ray and neutron diffraction data-shows that the local topology of the glassy structure is very similar to that of the stable crystalline intermetallic compounds. The glassy structure is best described as a disordered tetrahedral close packing with a weak tendency to chemical short-range order whose precise degree remains to be detailed. The linearised muffin-tin orbital method in the atomic sphere approximation is used to perform self-consistent calculations of the electronic DOS of crystalline Mg and Zn, of the hexagonal Laves phase MgZn2 and of 'amorphous' supercells (each containing 60 atoms) representing glassy MgZn2 and Mg7Zn3 alloys. In each case the authors find a minimum in the DOS at EF, and d bands centred at EB approximately=-7.5 eV. A transition-state calculation shows that the d-band position in the photoemission spectra is shifted relative to the electronic eigenvalue due to self-energy corrections. Photoemission and X-ray emission intensities are calculated from the partial local DOS and the self-consistent potentials in a single-scatterer final-state approximation. The comparison with experimental confirms the validity of the electronic structure calculations. The work represents one of the first ab initio calculations of the atomic and the electronic structure of a metallic glass, and the first confirmation of the existence of a minimum in the electronic DOS at EF. The relevance of the DOS minimum to the structure-potential relationship and to the stability of the glassy phase is discussed.
Journal of Physics F Metal Physics 11/2000; 18(12):2583. DOI:10.1088/0305-4608/18/12/010
[Show abstract][Hide abstract] ABSTRACT: The melting of isolated neutral tin cluster distributions with mean sizes of about 500 atoms has been investigated in a molecular beam experiment by calorimetrically measuring the clusters' formation energies as a function of their internal temperature. For this purpose the possibility to adjust the temperature of the clusters' internal degrees of freedom by means of the temperature of the cluster source's nozzle was exploited. The melting point of the investigated tin clusters was found to be lowered by 125 K and the latent heat of fusion per atom is reduced by 35% compared to bulk tin. The melting behavior of the isolated tin clusters is discussed with respect to the occurrence of surface premelting.
[Show abstract][Hide abstract] ABSTRACT: In a novel molecular beam experiment we have calorimetrically investigated the dependence of the formation energies of isolated Sn(N) clusters on their size. The experimentally determined size dependence of the formation energy for Sn(N) clusters consisting of between 95 and 975 atoms can be explained by the existence of two different types of cluster isomers: One class of isomers is characterized by formation energies proportional to N(-1/3), indicating compact spherical-like shapes. The other class has constant formation energies for the investigated size range, which is consistent with quasi-one-dimensional geometries.
[Show abstract][Hide abstract] ABSTRACT: A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.
[Show abstract][Hide abstract] ABSTRACT: Sliding friction between the tip of a friction force microscope and NaCl(100) was studied to deduce the velocity dependence of friction forces on the atomic scale. A logarithmic dependence of the mean friction force is revealed at low velocities. The experimental data are interpreted in terms of a modified Tomlinson model which is based on reaction rate theory.
[Show abstract][Hide abstract] ABSTRACT: We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.
[Show abstract][Hide abstract] ABSTRACT: We present a novel chemical sensor based on a microfabricated
array of silicon cantilevers. Individual cantilevers are sensitized for
the detection of analytes using metal coatings. Analyte molecules
chemisorbing or physisorbing on the cantilever coating and chemical
reactions produce a change in interfacial stress between analyte
molecules and cantilever. This leads to a nanomechanical response of the
cantilever, i.e. bending. The bending is read out using a
time-multiplexed optical beam-deflection technique. From magnitude and
temporal evolution of the bending, quantitative information on analyte
species and concentration is derived. Here, we demonstrate the detection
of ethene and water vapor with such a nanomechanical nose
[Show abstract][Hide abstract] ABSTRACT: Effects of thermal relaxation on amorphous SiAu films are investigated by means of photoelectron spectroscopy and measurements of the electrical resistivity and specific heat. Annealing of the films in the amorphous state increases the resistivity and reduces the electronic density of states at the Fermi energy. These relaxation effects can be reversed by irradiation with 5 keV argon ions at low temperature. The results are discussed in the framework of structural relaxation, clustering and a scaling theory given by McMillan (1981).