Tapping mode scanning force microscopy in water using a carbon nanotube probe

An improved technique for obtaining tapping mode scanning force microscopy (TMSFM) images of soft samples submerged in water is described. This technique makes use of a carbon nanotube several microns in length mounted on a conventional silicon cantilever as the TMSFM probe. The sample is covered by a shallow water layer and during imaging only a portion of the nanotube is submerged. This mode of operation largely eliminates the undesirable effects of hydrodynamic damping and acoustic excitation that are present during conventional tapping mode operation in liquids and leads to high-quality TMSFM images. Because of their low bending force constants, carbon nanotubes are ideal for gentle imaging of soft samples. Because of their small (5–20 nm) diameter and cylindrical shape they provide excellent lateral resolution and are ideal for scanning high aspect ratio objects.     

Calculation of the Bloch wall contrast in magnetic force microscopy

The magnetic force microscope (MFM) is a promising analytical tool for the mapping of magnetic microfield distributions on a nanometre scale. The detailed interpretation of experimentally obtained data requires a rigorous micromagnetic analysis of the underlying magnetostatic tip-sample interactions. We have performed model calculations yielding the MFM image contrast obtained for 180° Bloch walls using ferromagnetic microscope tips. The actually observed wall image is shown to depend critically upon the mesoscopic tip design. The spatial resolution and also values for the resulting interaction forces and tip-sample compliances are derived.     

Magnetic field detection for current evaluation by magnetic force microscopy

A current measurement method through magnetic field detection by magnetic force microscopy was proposed and demonstrated. We observed the magnetic field induced by an AC below 2.2 μA around a GaAs/AlGaAs mesa stripe. To achieve high sensitivity in magnetic field detection, we tuned the AC bias frequency to the torsional resonant frequency of the cantilever. As a result, the sensitivity of the magnetic field detection was much improved and specific features of the magnetic field around the mesa stripe were clearly observed at a current in sub-μA range.     

Investigation of Bloch wall fine structures by magnetic force microscopy

Using a magnetic force microscope (MFM), measurements have been performed on single crystal iron whiskers. These samples exhibit a comparatively simple magnetic domain structure and a high degree of crystallographic perfection. STM measurements yield an average surface corrugation of about 3 nm. These nearly ideal boundary conditions permit an investigation of the undistorted surface configuration of Bloch walls in a cubic bulk crystal. Interactions between the stray field configuration of the sample and a ferromagnetic microscope tip have been measured with a new MFM device which is based on direct compliance detection. Measurements have been performed in the constant-current mode, as well as in the constant-compliance, mode. Control observation by means of the Kerr magneto-optic effect permit an interpretation of the obtained MFM data in terms of isolated 180° Bloch walls. The fine structure of these walls has been analysed and compared with results obtained from model calculations.     

Advanced cantilevers for magnetic force microscopy and high frequency magnetic force microscopy

In order to improve the spatial resolution achieved by magnetic force microscopy (MFM) technique and its derivatives, we employ here advanced MFM tips fabricated by means of focused ion beam (FIB) milling. The magnetic coating applied on these tips is a CoCr film of 10 nm thickness. The MFM measurements on hard disk test samples reveal the achieved high resolution, and the measurement on a garnet film demonstrates the low invasiveness. High-frequency MFM (HF-MFM) is a development of the MFM technique to observe the HF stray fields emerging from magnetic recording writer poles at their operating conditions. By means of HF-MFM, magnetic recording writer poles are characterized in the frequency range 100-1,000 MHz. Up to now, all HF-MFM experiments conducted were using standard MFM cantilevers. From the HF-MFM images obtained using the advanced MFM cantilevers, it is clearly seen that the spatial resolution is considerably improved over the images obtained using standard MFM tips. However, the 10 nm thick magnetic coating of the cantilevers is found to work properly only at frequencies of up to about 500 MHz.     

Quantitative elastic modulus measurement by magnetic force modulation microscopy

It is experimentally demonstrated that magnetic force modulation microscopy (MFMM) is a technique allowing quantitative elastic modulus measurements. A model of the cantilever–tip–sample interaction taking into account the lateral contact stiffness (i.e., the friction effects at the level of the tip–sample contact), the position of the magnetic force applied to the cantilever with respect to the tip position, as well as the inclination of the cantilever arm with respect to the sample surface is presented. The model shows that MFMM is much less sensitive to lateral force than the other modulation techniques and thus, in contrast to the latter, that the contrast of the stiffness images can be interpreted as a true elasticity contrast and not as a mixture of friction and elasticity. Thanks to the study of the normal contact stiffness versus normal load that allows the characterization of contact between tip and sample, it is possible to perform quantitative elastic modulus measurements with a dynamic modulation method.     

Investigation of human hair fibers using lateral force microscopy

Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to investigate the morphologic and surface changes associated with various surface modifications to human hair. These included extraction with a series of solvents, bleaching, and treatment with a cationic copolymer. The study assessed the ability of these techniques to distinguish the changes in surface properties, including morphology and friction coefficient, as manifested in changes brought about by the indicated surface modifications. While topographic morphology can easily be investigated with contact AFM. LFM offers an additional tool for probing the surface distribution of oils and waxes. The removal of surface lipids from the fiber surface was accomplished using soxhlet extraction with t-butanol and n-hexane, while the free internal lipids (within the fiber structure) were removed by extraction with a mixture of chloroform and methanol (70:30, v/v). In addition, the surface of hair was modified with the cationic polymer, co(vinyl pyrrolidone-methacrylamidopropyl trimethylammonium chloride [PVP/MAPTAC]), and its distribution on the surface was monitored. Ambient AFM and LFM studies of surface modified and native fibers clearly indicate that when investigated as a function of tip loading force, the different modifications result in changes of the friction coefficient, which increase in this order: native, bleached, solvent extracted, and polymer-treated hair. Friction images show surface variations that are interpreted as areas of varying lipid film coverage. In addition, topographic images of the fibers show the presence of small pores, which become increasingly prevalent upon solvent extraction.     

Atomic and magnetic force microscopy imaging of thin-film recording heads

Investigations on the use of the scanning probe microscope (SPM) in the atomic force microscopy (AFM) mode for topography imaging and the magnetic force microscopy (MFM) mode for magnetic imaging are presented for a thin-film recording head. Results showed that the SPM is suitable for imaging the surface profile of the recording head, determining the width of the pole gap region, and mapping the magnetic field patterns of the recording head excited under current bias conditions of different polarity. For the cobalt sputter-coated tips used in MFM imaging, it was found that the magnetic field patterns obtained under different polarities of the current bias to the recording head were similar. This can be explained by the nature of the thin-film MFM tip, in which the direction of the tip magnetic moment can follow the stray magnetic field of the sample as the current bias to the recording head reverses in direction.     

Using atomic force microscopy to probe food biopolymer functionality

Atomic force microscopes (AFMs) generate images by “feeling” rather than “looking” at samples. This permits a magnification range spanning that associated with both the light and electron microscopes, but under the “natural” imaging conditions normally associated with light microscopes. Molecules and molecular interactions can be imaged at molecular or submolecular resolution in gaseous or liquid environments. By careful design of experiments it is possible to use AFM to probe how food biopolymers determine the structure and texture of food products. This approach will be illustrated through studies on food polysaccharides and proteins.     

Fabrication of carbon nanotube AFM probes using the Langmuir–Blodgett technique

Carbon nanotube (CNT)-tipped atomic force microscopy (AFM) probes have shown a significant potential for obtaining high-resolution imaging of nanostructure and biological materials. In this paper, we report a simple method to fabricate single-walled carbon nanotube (SWNT) nanoprobes for AFM using the Langmuir–Blodgett (LB) technique. Thiophenyl-modified SWNTs (SWNT-SHs) through amidation of SWNTs in chloroform allowed to be spread and form a stable Langmuir monolayer at the water/air interface. A simple two-step transfer process was used: (1) dipping conventional AFM probes into the Langmuir monolayer and (2) lifting the probes from the water surface. This results in the attachment of SWNTs onto the tips of AFM nanoprobes. We found that the SWNTs assembled on the nanoprobes were well-oriented and robust enough to maintain their shape and direction even after successive scans. AFM measurements of a nano-porous alumina substrate and deoxyribonucleic acid using SWNT-modified nanoprobes revealed that the curvature diameter of the nanoprobes was less than 3 nm and a fine resolution was obtained than that from conventional AFM probes. We also demonstrate that the LB method is a scalable process capable of simultaneously fabricating a large number of SWNT-modified nanoprobes.     

Mechanical test and friction-mapping on recycled polypropylene beads using atomic force microscopy

Mechanical tests at sub-micron scales using force microscopy are often used for the characterization of materials. Here we report the mechanical, tribologic, and morphological characterization of recycled polypropylene beads using force spectroscopy and lateral-force microscopy. The compression-elastic moduli calculated using the Hertzian model for polypropylene beads was between 0.448 ± 0.010 and 1.044 ± 0.057 GPa. The grain size analysis revealed a significant correlation between the grain size and measured compression-elastic moduli. Friction-maps of recycled polypropylene beads obtained using lateral-force microscopy were also reported for 25 μm² scanning areas.     

Quantitative analysis of surface micro-roughness alterations in human spermatozoa using atomic force microscopy

A new male contraceptive given the name RISUG^® (an acronym for Reversible Inhibition of Sperm Under Guidance) has been developed by our research group. RISUG^® is a bioactive polymer and is injected into the lumen of the vas deferens using a no-scalpel approach. The polyelectrolytic nature of this contraceptive induces a surface charge imbalance on sperm membrane system leading to its destabilization. Complete disintegration of the plasma membrane with subsequent rupture and dispersion of the acrosomal contents is observed on RISUG^® treatment. In the present study, micro-structural properties of human spermatozoa exposed to RISUG^® in vitro have been quantitatively analysed using atomic force microscopy. The parameters used to quantify these morphological changes include amplitude (peak–valley height difference, arithmetic roughness, root mean square roughness) and spatial roughness. Factor loadings (Varimax rotation) have been used to determine the parameters displaying maximum variation. Further, sperm cells have been classified in various principal-component planes using principal-component analysis. The periodic structural features of the atomic force microscopy images have also been obtained using power spectral analysis.     

Magnetic refinement of tips for magnetic force microscopy

Silicon cantilever probes with monolithically integrated tips are commercially available and are routinely used for atomic force microscopy (AFM). For such probes, amagnetic refinement of the silicon tip has been developed and results in a deposition of ferromagnetic material such as nickel or CrCoTa in the top area of the tip. The method consists of essentially three steps: (1) A broad-area sputter deposition of a ferromagnetic material; (2) a selective electron beam-induced carbon deposition at the top of the tip; (3) a broad-area ion-beam sputter etching, which removes the magnetic layer everywhere except underneath the carbon cap. The method allows to control the total amount and extension of the magnetic material left at the tip. It is applicable to all kinds of ferromagnetic materials which can be deposited as a thin layer by sputter deposition or evaporation. Experiments indicate that the method is reliable and improves the resolution of magnetic force microscopy (MFM). With such magnetically refined tips on silicon cantilevers, MFM measurements have been performed in contact mode as well as in dynamic and static noncontact modes. In this paper, the method for magnetic tip refinement is described and MFM measurements with these tips are presented.     

Scanning tunnelling and atomic force microscopy performed with the same probe in one unit

The technique demonstrated here provides features of both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). The metallic probe acts to record current variations and sense forces from the same sample area simultaneously. Thus, separate images may be recorded, in registry. The collected data allows real space correlations between some electrical properties and the geometric structure of a sample surface. The same tip is used since the geometry and condition of the tip can effect the data recordings. Platinum alloys, tungsten and graphite tips have been employed successfully. An AFM lever can respond to surface contact forces, within the elastic limits of the sample, while electric current is sensed by the tip of the lever. The usefulness of this experimental procedure is tested here by an application to semiconducting samples of Ag-doped CdTe in air and in paraffin oil media.     

On the electrical properties of dislocations in ZnS using electric force microscopy

A combination of electric force microscopy (EFM) and noncontact atomic force microscopy (AFM) was used to study microscratching-induced dislocations in sphaleritic ZnS single crystals. Dislocation bands predominantly consisting of either anion-type (S) or cation-type (Zn) dislocations were induced by scratching along either [111] or [111] on a (110) surface. A significant difference of local distortions in electrical potential between the S(g) and Zn(g) dislocation bands was observed from the EFM images. Electric charges of these dislocations were determined quantitatively and the results were compared with theoretical models.     

Adhesion and friction properties of hydrogenated amorphous carbon films measured by atomic force microscopy

Atomic force microscopy has been used to measure adhesion and friction forces at the interface between an oxidized metal probe tip and amorphous carbon films of varying hydrogen contents (12.3–39.0 atomic percent hydrogen). The interface of an oxide surface and a hard carbon coating models the unlubricated head-disk interface of current hard disk products. Adhesion forces normalized by the radius of curvature of the contacting tip range from 1.09 to 8.53 N/m. Coefficients of friction values, measured as the slope of the friction versus load plot, range from 0.33 to 0.87. A trend of increasing adhesion forces and coefficients of friction is observed for increasing hydrogen content in the films. We attribute the increase in adhesion and friction to increases in the surface free energy of the carbon films with the incorporation of hydrogen.     

Effects of probe pollutants on morphological and mechanical measurements of muscle and collagen fibers using atomic force microscopy

Because of the interaction between probes and samples, pollutants in buffer solution or in the air would easily bind to probes and make the probe polluted, which might influence the morphological and mechanical measurements with atomic force microscopy. The polluted probes might transfer the pollutants onto the samples and thus change the surface ultrastructure of samples, or collect the deviated feedback signals to make the phantasm images. The former process is irreversible even if a new probe is employed, and the latter one is a reversible process as long as changing the used/polluted probe. Effects of polluted probes on morphological and mechanical characteristics of insect flight muscle and rat tail tendon collagen I fibers had been discussed in this study, in which, we constructed a series of methods to avoid/reduce the collecting of phantasm images and deviated mechanical information, such as changing the scanning direction and scanning force, replacing the new probes, or cleaning the polluted probes. SCANNING 32:113–121, 2010. © 2010 Wiley Periodicals, Inc.     

Friction force microscopy as a probe of molecular relaxation on polymer surfaces

The scan-velocity dependence of friction force microscopy (FFM) is characterized on nominally-dry gelatin films and related to the rate dependence of dissipative molecular relaxations. For a range of scanning-parameter values the measurement itself affects the frictional characteristics of the films: imparted frictional energy populates molecular conformations from which more dissipative relaxations occur. Variations in frictional dissipation tens of nanometers in lateral size are quantified as histograms of the number of image pixels versus frictional force. Histogram breadth and symmetry apparently reflect the energy dispersion of molecular relaxations.     

Fabrication of ball-shaped atomic force microscope tips by ion-beam-induced deposition of platinum on multiwall carbon nanotubes

Ball-shaped atomic force microscope (AFM) tips (ball tips) are useful in AFM metrology, particularly in critical dimension AFM metrology and in micro-tribology. However, a systematic fabrication method for nano-scale ball tips has not been reported. We report that nano-scale ball tips can be fabricated by ion-beam-induced deposition (IBID) of Pt at the free end of multiwall carbon nanotubes that are attached to AFM tips. Scanning electron microscopy and transmission electron microscopy analyses were done on the Pt ball tips produced by IBID in this manner, using ranges of Ga ion beam conditions. The Pt ball tips produced consisted of aggregated Pt nano-particles and were found to be strong enough for AFM imaging.