Simultaneous observation of atomic step and domain wall structure of ultrathin Co films by magnetic force microscopy in ultrahigh vacuum

We have applied magnetic force microscopy in ultrahigh vacuum to study the correlation between the atomic step and magnetic domain wall structure of ultrathin Co films prepared in situ on Au(111) substrates. For the first time we were able to achieve high-resolution images showing simultaneously a clear domain wall contrast and the underlying atomic step structure. Although for in-plane magnetized Co films the domain walls were found to run preferentially in a direction perpendicular to the steps, no such correlation could be observed for out-of-plane magnetized Co films. Received: 3 June 1999 / Accepted: 4 June 1999 / Published online: 29 July 1999     

True atomic resolution under ambient conditions obtained by atomic force microscopy in the contact mode

2 ) has been investigated by contact-mode atomic force microscopy (AFM) in air. Both the terraces and the monolayer step itself were reproducibly imaged at atomic resolution in the repulsive-force regime at forces between tip apex and sample of the order of 10^-9 N. Several kinks were also imaged at atomic resolution. Details of the atomic registry of subsequent Se-Nb-Se sandwich layers as well as the arrangement of the individual atoms at the kink sites were resolved. The results are in perfect quantitative agreement with the lattice structure known from X-ray analysis and indicate that true atom-by-atom lateral resolution of microscopic defects is feasible by AFM in the contact mode and under ambient conditions. Published online: 10 February 1999     

Thickness-dependent magnetic domain structures of ultrathin Co/Au(111) films studied by means of magnetic force microscopy in ultrahigh vacuum

Received: 10 December 1997/Accepted: 11 December 1997     

Strain relaxation at cleaved surfaces studied by atomic force microscopy

Atomic force microscopy (AFM) in air is used to study the (110) cleaved surface of strained (100) In_xGa_1-xAs/ InP heterostructures for different compositions and thicknesses of the ternary compound layers. We find that the elastic strain relaxation induces a surface undulation of a few ? amplitude, even for very small misfits, provided the layers are thick enough. Using finite-element calculations of the strain relaxation near the cleaved edge, we reproduce quantitatively the AFM observations for compressive- as well as for tensile-strained layers with an accuracy better than 0.1 nm. This demonstrates the ability of AFM to quantify strain distributions by making use of surface profile measurements. Received: 9 November 1998 / Accepted: 11 March 1999 / Published online: 7 July 1999     

Magnetostatic interaction studied by force microscopy in ultrahigh vacuum

3 (FeGe)₅ O₁₂ magnetic garnet, Co/Pt multilayer sample and the high coercivity BaFe₁₂O₁₉ single crystal. We have prepared in UHV by electron beam evaporation magnetic sensors/tips which allowed us to reach high sensitivity and high resolution of our microscope. Ideal UHV conditions (pressure 5×10^-11 \text{Torr}) helped us to observe domain wall contrast on all of studied samples by using a dynamic mode of operation of the force microscope. Received: 28 October 1996/Accepted: 5 November 1996     

Atomic force microscopy study of lymphoblastoid cells under 50-Hz 2-mT magnetic field irradiation

Received: 18 December 1997/Accepted: 5 January 1998     

Atom force microscopy study on HTHP as-grown diamond single crystals

For understanding the mechanism of diamond growth at high temperature–high pressure (HTHP) from a metallic catalyst–graphite system, it is of great interest to perform atomic force microscopy (AFM) experiments, which provide a unique technique different from that of normal optical and electronic microscopy studies, to study the topography of HTHP as-grown diamond single crystals. In the present paper, we report first AFM results on diamond single crystals grown from a Fe-Ni-C system at HTHP to reveal the growth mechanism of diamond single crystals at HTHP. AFM images for as-grown diamond samples show dark etch pits on the (111) surface, indicating dislocations. Some fine particles about 100–300 nm in dimension were directly observed on the (100) diamond surface. These particles are believed to have been formed through transition of graphite to diamond under the effect of the catalyst and to have been transported to the growing diamond surface through a metallic thin film by diffusion. The roughness of the (100) diamond surface is found to be about several tens of nanometers through profile analysis. The diamond growth at HTHP, in a sense, could be considered as a process of unification of these fine diamond particles or of carbon-atom-cluster recombination on the growing diamond crystal surface. Successive growth interlayer steps on the (111) diamond surface were systemically examined. The heights of the growth interlayer steps were measured by sectional analysis. It was shown that the heights of the growth interlayer steps are quite different and range from about 10 to 25 nm. The source of the interlayer steps might be dislocations. The diamond-growth mechanism at HTHP could be indicated by the AFM topography of the fine diamond particles and the train-growth interlayer steps on the as-grown diamond surfaces. Received: 29 March 2001 / Accepted: 20 August 2001 / Published online: 2 October 2001     

Fast imaging method combining cantilever and feedback signals in contact-mode atomic force microscopy

A fast imaging method in a contact-mode atomic force microscope (AFM) is examined for its principle and performance, where the image is acquired by combining a cantilever signal and a feedback signal applied to a piezotube. The frequency component of the feedback signal is restricted in the lower frequency region to keep the linear relationship between the feedback signal and the displacement of the piezotube. It is shown that the image is basically independent of the feedback details since a wide detection bandwidth is certified by the cantilever response much faster than by the feedback response, allowing a fast scanning. The fast scanning, however, enhances the distortion in the transient region where surface height changes abruptly. This influence can be reduced by choosing the scan line direction for the data acquisition. The combination procedure also reduces the low-frequency noise in the feedback signal. A 512×512-pixel image was obtained in 90 s without sacrificing the resolution. Received: 24 November 1998 / Accepted: 31 March 1999 / Published online: 2 June 1999     

Surface morphology of metal films deposited on mica at various temperatures observed by atomic force microscopy

Thin films of eight metals with a thickness of 150 nm were deposited on mica substrates by thermal evaporation at various temperatures in a high vacuum. The surface morphology of the metal films was observed by atomic force microscopy (AFM) and the images were characterized quantitatively by a roughness analysis and a bearing analysis (surface height analysis). The films of Au, Ag, Cu, and Al with the high melting points were prepared at homologous temperatures T/T_m = 0.22-0.32, 0.40, and 0.56. The films of In, Sn, Bi, and Pb with the low melting points were prepared at T/T_m = 0.55-0.70, where T and T_m are the absolute temperatures of the mica substrate and the melting point of the metal, respectively. The surface morphology of these metal films was studied based on a structure zone model. The film surfaces of Au, Ag, and Cu prepared at the low temperatures (T/T_m = 0.22-0.24) consist of small round grains with diameters of 30-60 nm and heights of 2-7 nm. The surface heights of these metal films distribute randomly around the surface height at 0 nm and the morphology is caused by self-shadowing during the deposition. The grain size becomes large due to surface diffusion of adatoms and the film surfaces have individual characteristic morphology and roughnesses as T increases. The surface of the Al film becomes very smooth as T increases and the atomically smooth surface is obtained at T/T_m = 0.56-0.67 (250-350 °C). On the other hand, the atomically smooth surface of the Au film is obtained at T/T_m = 0.56 (473 ± 3 °C). The films of In, Sn, Bi, and Pb prepared at T/T_m = 0.55-0.70 also show the individual characteristic surface morphology.     

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate. Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002 RID="*" ID="*"This work is part of the thesis of H. M?ltgen RID="**" ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de     

Investigation of ripple structures of thin polycrystalline Co films by magnetic force microscopy

Received: 14 August 1997/Accepted: 14 August 1997     

Silver-island films probed by hyper-Rayleigh scattering and atomic force microscopy

Incoherent second-harmonic generation, or hyper-Rayleigh scattering (HRS), and atomic force microscopy (AFM) are proposed as a combined probe of nonlinear optical and structural properties of silver-island films. HRS and linear (Rayleigh) scattering indicatrices are measured. The correlation function, deduced from the HRS indicatrices and characterising spatial fluctuations of the total polarisation at second-harmonic frequency, has a length scale significantly larger than that of the correlation functions that are obtained from the AFM data and characterise spatial fluctuations of quadratic optical susceptibilities of the dipole and quadrupole types. This difference is interpreted as indicating that the HRS indicatrix shape is determined by the long-range fluctuations of the local-field factors. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

Structure reorganization in polymer films of nafion due to swelling studied by scanning force microscopy

The surface morphology of dry and swollen Nafion films was investigated on the micrometer and nanometer scale by scanning force microscopy. The results show that the disordered network structure of dry films undergoes a reorganization process in the course of swelling and transforms into an ordered structure of parallel fibrils. No substantial changes in the fibril dimensions were found in the swollen state. This can be an indicator that swelling occurs at the supramolecular level.     

Investigation of heteroepitaxial diamond films by atomic force and scanning tunneling microscopy

We report on Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) investigations on chemical vapour deposited heteroepitaxial diamond films. Besides the good macroscopic crystal morphology a statistical tilt up to ±5.2° of the oriented crystallites has been found relative to the silicon substrates. By optimizing the process conditions, however, the crystal tilt of the films can be reduced, resulting in an improved film perfection. On crystallite (001)-surfaces a substructure of growth facets or islands has been found and high resolution STM images have established a 2×1 surface reconstruction on these growth facets. AFM and SEM were applied to study the morphology of diamond nuclei initially grown on the silicon substrate. Strong island like (Volmer-Weber) growth has been found, with a nucleus height to diameter ratio of 1:1. While the islands are growing in size with respect to time of nucleation, its aspect ratio does not change, due to the high surface free energy of the diamond relative to silicon.     

Investigation of precipitated colloidal particles by scanning force microscopy: silver-halide microcrystals

Scanning Force Microscopy (SFM) is presented as an alternative to electron microscopy for the investigation of precipitated colloidal particles. The information which can be gained by SFM is illustrated by the example of silver-halide microcrystals with different shapes and sizes. Sample preparation and scan-related problems are discussed extensively from general aspects.     

Application of scanning force microscopy in nanotube science

Recent developments in the application of scanning force microscopy in nanotube science are reviewed. The non-destructive character of this technique allows the structural characterisation of (chemically modified) single- and multi-wall nanotubes deposited on substrates for further investigations such as electrical transport measurements. Furthermore, SFM is now an established tool for manipulation of nanotubes, which allows position control and determination of elastic constants such as the Young’s modulus. Finally it is shown that very sharp and stable probes for scanning force microscopy can be made from nanotubes due to their excellent stability and aspect ratio. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

Magnetic force microscopy studies of domain walls in nickel and cobalt films

A magnetic force microscopy is used to examine the domain walls in nickel and cobalt films deposited by argon ion sputtering. Thin nickel films deposited at high substrate temperatures exhibit coexistent Bloch and Neel walls. Films grown at room temperature display alternative Bloch lines with cap switches. These films agglomerate to form grains after annealed at high temperatures. The film composed of larger grains behaves better nucleation implying magnetic domains of closure, while the film composed of smaller grains exhibits more defects implying alternative Bloch lines. We have also observed domain displacements and cap switches, which occur due to precipitation of particles in small grain size films. Stripe domains are observed for film thicknesses larger than 100 nm. They become zigzag cells when an external field of 1.5 T is applied perpendicular to the surface of the films. This experiment indicates that the domain sizes in thin films and the strip widths for thick films both depend on the square-root of the film thickness, which varies from 5 to 45 nm and from 100 to 450 nm, respectively.     

Monolayers of graphite rotated by a defined angle: hexagonal superstructures by STM

Received: 27 March 1998     

Liquid-crystal alignment of phthalocyanine-derived Langmuir-Blodgett films

Twisted nematic liquid crystals were uniformly aligned on Phthalocyanine (Pc)-derived Langmuir-Blodgett (LB) films. By scanning tunneling microscopy imaging, it was found that the Pc-LB film has a well-ordered in-plane column-like structure oriented in the dipping direction, based on which the alignment mechanisms are discussed.     

Frictional and atomic-scale study of C60 thin films by scanning force microscopy

Scanning force microscopy (SFM) was employed to characterize C₆₀ island films in an ultra-high vacuum (UHV). The initial growth stage of C₆₀ on NaCl cleavage faces and nanotribological properties of this solid lubricant are investigated. In comparison to the NaCl(001) face, higher friction is measured on the C₆₀ islands, resulting in a ratio of friction of 13 for NaClC₆₀. The friction coefficient of the (111) oriented C₆₀ island is determined to be 0.15±0.05. High-resolution SFM images reveal the hexagonal lattice of the unreconstructed (111) top surfaces and the overgrowth relationships of the C₆₀ islands.