Unstable amplitude and noisy image induced by tip contamination in dynamic force mode atomic force microscopy

Liquid 1-decanethiol was confined on an atomic force microscope (AFM) tip apex and the effect was investigated by measuring amplitude-distance curves in dynamic force mode. Within the working distance in the dynamic force mode AFM, the thiol showed strong interactions bridging between a gold-coated probe tip and a gold-coated Si substrate, resulting in unstable amplitude and noisy AFM images. We show that under such a situation, the amplitude change is dominated by the extra forces induced by the active material loaded on the tip apex, overwhelming the amplitude change caused by the geometry of the sample surface, thus resulting in noise in the image the tip collects. We also show that such a contaminant may be removed from the apex by pushing the tip into a material soft enough to avoid damage to the tip.     

Fabrication of sharp tungsten-coated tip for atomic force microscopy by ion-beam sputter deposition

Tungsten (W) is significantly suitable as a tip material for atomic force microscopy (AFM) because its high mechanical stiffness enables the stable detection of tip-sample interaction forces. We have developed W sputter-coating equipment to compensate the drawbacks of conventional Si cantilever tips used in AFM measurements. By employing an ion gun commonly used for sputter cleaning of a cantilever tip, the equipment is capable of depositing conductive W films in the preparation chamber of a general ultrahigh vacuum (UHV)-AFM system without the need for an additional chamber or transfer system. This enables W coating of a cantilever tip immediately after sputter cleaning of the tip apex and just before the use in AFM observations. The W film consists of grain structures, which prevent tip dulling and provide sharpness (<3 nm in radius of curvature at the apex) comparable to that of the original Si tip apex. We demonstrate that in non-contact (NC)-AFM measurement, a W-coated Si tip can clearly resolve the atomic structures of a Ge(001) surface without any artifacts, indicating that, as a force sensor, the fabricated W-coated Si tip is superior to a bare Si tip.     

Atomically resolved imaging by low-temperature frequency-modulation atomic force microscopy using a quartz length-extension resonator

Low-temperature ultrahigh vacuum frequency-modulation atomic force microscopy (AFM) was performed using a 1 MHz length-extension type of quartz resonator as a force sensor. Taking advantage of the high stiffness of the resonator, the AFM was operated with an oscillation amplitude smaller than 100 pm, which is favorable for high spatial resolution, without snapping an AFM tip onto a sample surface. Atomically resolved imaging of the adatom structure on the Si(111)-(7x7) surface was successfully obtained.     

Comparative study of lithium fluoride and graphite by atomic force microscopy (AFM)

The atomic force microscope (AFM) offers the possibility to image the topography of insulating as well as conductive surfaces. Highly oriented pyrolytic graphite (HOPG) was chosen as an example for a layered material and compared to single crystalline lithium fluoride (LiF). Both materials are easily prepared and inert at ambient pressure. Furthermore they are well characterized by Helium atom scattering experiments and other techniques. On HOPG atomic resolution has been achieved. Distortions can be observed which we interpret as a frictional effect. In addition we performed large area scans where we seldomly observed dislocations. For the first time we present measurements on LiF, showing steps of one unit cell height. On larger areas the surface of LiF showed terraces, separated by steps of variable heights, ranging from a few ångströms to 100 Å. We used a static method to get information about the distance dependence of the force between lever and sample. By slowly expanding and retracting the sample piezo and simultaneous measurement of the lever deflection, plots were recorded, showing the force as a function of sample position. The results were compared with theoretical calculations. We could determine the tip radius and found differences between LiF and HOPG being characteristic for the samples.     

Real-time atomic force microscopy in lubrication condition

We have studied frictional force and wear problem in real-time atomic force microscopy in contact-mode using a resonator type mechanical scanner allegedly reported. The fast scanning may cause wear in the sample surface or the tip, and may deteriorate the image quality. Mineral oil was used to make a lubricious surface on a polycarbonate sample, and it was found that the interfacial frictional force was decreased. A Si tip which was coated with a hydrophobic film by means of chemical modification was confirmed to diminish the frictional force in the fast scanning process. The resultant image quality was improved due to reduced friction and wear.     

Diffusion anisotropy of Ag and In on Si(1 1 1) surface studied by UHV-SEM

Anisotropic features of Ag and In electromigration on clean and Au-precovered Si(1 1 1) surfaces were studied by in situ scanning electron microscopy in ultrahigh vacuum. It was noted that the migration direction of Ag was determined by both applied direct-current direction and step orientation on the substrate surface; on an Si(1 1 1) surface with steps inclined with respect to the current direction, the electromigration direction shows an apparent deviation from the accurate current direction. On clean and Au-precovered Si(1 1 1) surfaces with various coverages of Au (within submonolayer range), the migration behaviors of Ag and In drastically changed with Au coverages and showed different diffusion anisotropy (either thermal diffusion and electromigration) depending on the adsorbate surface structures. Particularly, on a beta-square root of 3 x square root of 3-Au surface of one monolayer Au coverage, In migrated with the highest mobility across the step bands, whereas In showed only a slow movement on the 7 x 7 clean surface due to a migration barrier at step edges. This result implied that the beta-square root of 3 x square root of 3-Au surface phase served as an intermediate layer for In adatoms migration. On the contrary, Ag showed negligible migration on the beta-square root of 3 x square root of 3-Au surface, while the 7 x 7 surface was the substrate for appreciable migration of Ag atoms. The results are discussed in terms of step-edge barriers in migration and on-terrace migration.     

Chemical force microscopy of microcontact-printed self-assembled monolayers by pulsed-force-mode atomic force microscopy

A novel chemically sensitive imaging mode based on adhesive force detection by previously developed pulsed-force-mode atomic force microscopy (PFM-AFM) is presented. PFM-AFM enables simultaneous imaging of surface topography and adhesive force between tip and sample surfaces. Since the adhesive forces are directly related to interaction between chemical functional groups on tip and sample surfaces, we combined the adhesive force mapping by PFM-AFM with chemically modified tips to accomplish imaging of a sample surface with chemical sensitivity. The adhesive force mapping by PFM-AFM both in air and pure water with CH3- and COOH-modified tips clearly discriminated the chemical functional groups on the patterned self-assembled monolayers (SAMs) consisting of COOH- and CH3-terminated regions prepared by microcontact printing (microCP). These results indicate that the adhesive force mapping by PFM-AFM can be used to image distribution of different chemical functional groups on a sample surface. The discrimination mechanism based upon adhesive forces measured by PFM-AFM was compared with that based upon friction forces measured by friction force microscopy. The former is related to observed difference in interactions between tip and sample surfaces when the different interfaces are detached, while the latter depends on difference in periodic corrugated interfacial potentials due to Pauli repulsive forces between the outermost functional groups facing each other and also difference in shear moduli of elasticities between different SAMs.     

Analysis of simulated scanning of atomic-scale silicon surface by atomic force microscopy

This study constructs a contact-mode atomic force microscopy (AFM) simulation measurement model with constant force mode to simulate and analyze the outline scanning measurement by AFM. The simulation method is that when the probe passes the surface of sample, the action force of the atom of sample received by the atom of the probe can be calculated by using Morse potential. Through calculation, the equivalent force on the cantilever of probe can be acquired. By using the deflection angle equation for the cantilever of probe developed and inferred by this study, the deflection angle of receiving action force can be calculated. On the measurement point, as the deflection angle reaches a fixed deflection angle, the scan height of this simulation model can be acquired. By scanning in the right order, the scan curve of the simulation model can be obtained. By using this simulation measurement model, this study simulates and analyzes the scanning of atomic-scale surface outline. Meanwhile, focusing on the tip radii of different probes, the concept of sensitivity analysis is employed to investigate the effects of the tip radius of probe on the atomic-scale surface outline. As a result, it is found from the simulation on the atomic-scale surface that within the simulation scope of this study, when the tip radius of probe is greater than 12 nm, the effects of single atom on the scan curve of AFM can be better decreased or eliminated.     

Mapping of the receptor-associated protein (RAP) binding proteins on living fibroblast cells using an atomic force microscope

The distribution of the receptor-associated protein (RAP) binding protein and the adhesion forces between RAP and its binding protein on living fibroblast cells were examined using an atomic force microscope (AFM). The distribution of RAP binding protein was obtained on 256 (16x16) locations in 2x2 micro m sections over the surface of living cells. The adhesion forces between RAP and the binding protein were measured with an AFM tip functionalized with RAP. In the presence of RAP in the scanning solution, the number of force curves with large adhesion force decreased. These results indicate that the adhesive forces observed here represent specific binding between RAP and the binding protein. This method will be a useful application of AFM to examine receptors on cell surfaces in high resolution.     

Nanoscale potential measurements in liquid by frequency modulation atomic force microscopy

We have developed a method for local potential measurements in liquid using frequency modulation atomic force microscopy. In this method, local potential is calculated from the first and second harmonic vibrations of a cantilever induced by applying an ac bias voltage between a tip and a sample. The use of an ac bias voltage with a relatively high frequency prevents uncontrolled electrochemical reactions and redistribution of ions and water. The nanoscale resolution of the method is demonstrated by imaging potential distribution of a dodecylamine thin film deposited on a graphite surface in 1 mM NaCl solution.     

Carbon nanotubes: a promising standard for quantitative evaluation of AFM tip apex geometry

The direct contact between tip and sample in atomic force microscopy (AFM) leads to demand for a quantitative knowledge of the AFM tip apex geometry in high-resolution AFM imaging and many other types of AFM applications like force measurements and surface roughness measurements. Given, the AFM tip apex may change continuously during measurements due to wear or during storage due to oxidation, it is very desirable to develop an easy and quick way for quantitative evaluation of AFM tip radius when necessary. In this study, we present an efficient method based on Zenhausern model (Scanning 14 (1992) 212) by measuring single-wall carbon nanotubes deposited on a flat substrate to reach this goal. Experimental results show the method can be used for routine quantitative evaluation of AFM tip apex geometry for tips with effective radii down to the nanometer scale.     

Topographic effects on adhesive force mapping of stretched DNA molecules by pulsed-force-mode atomic force microscopy

Adhesive interaction between a tip and a sample surface was examined on a microscopic scale by pulsed-force-mode atomic force microscopy (PFM-AFM). The signal measured by monitoring pull-off force is influenced by various factors such as topography, elasticity, electrostatic charges, and adsorbed water on surfaces. Here, we focus on the topographic effects on the adhesive interaction. To clarify the topographic influence, the adhesive force measurement of a stretched DNA molecule with a smaller radius of curvature than that of a tip was carried out at low relative humidity (RH) with an alkanethiol-modified tip. The experimental conditions such as low RH and the use of the alkanethiol-modified tip were required to minimise the influence of water capillary force on hydrated DNA strands. The hydrophobic modification of a substrate surface was also important to minimise the adsorbed water effect. The DNA molecules were stretched on the substrate surfaces by an immobilisation process called a dynamic molecular combing method. The two-component vapour-phase surface modification with an alkylsilane mixed with a silane derivative containing an amino end group enhanced the DNA adsorption due to the electrostatic interaction. The experimental results for the topographic effects on the adhesive force mapping were reproducible.     

Velocity dependent friction laws in contact mode atomic force microscopy

Friction forces in the tip–sample contact govern the dynamics of contact mode atomic force microscopy. In ambient conditions typical contact radii between tip and sample are in the order of a few nanometers. In order to account for the large interaction area the dynamics of contact mode atomic force microscope (AFM) is investigated under the assumption of a multi-asperity contact interface between tip and sample. Thus, the kinetic friction force between tip and sample is the product of the real contact area between both solids and the interfacial shear strength. The velocity strengthening of the lateral force is modeled assuming a logarithmic relationship between shear-strength and velocity. Numerical simulations of the system dynamics with this empirical model show the existence of two different regimes in contact mode AFM: steady sliding and stick–slip where the tip undergoes periodically stiction and kinetic friction. The state of the system depends on the scan velocity as well as on the velocity dependence of the interfacial friction force between tip and sample. Already small viscous damping contributions in the tip–sample contact are sufficient to suppress stick–slip oscillations.     

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.     

Investigation of protein partnerships using atomic force microscopy

The origin of contrast in atomic force microscopy (AFM) lies in the probe's response to forces between itself and the sample. These forces most commonly result from changes in height as the tip is scanned over the surface, but can also originate in properties inherent in the sample. These have been exploited as further means of contrast and have spawned an array of similar imaging techniques, such as chemical force microscopy, magnetic force microscopy, and frictional force microscopy. All of these techniques use AFM as an extremely sensitive gauge to map forces at discrete sites on the surface. A natural extension of this approach is to map forces in an array, in order to create a force map. AFM can be used in aqueous or fluid environments, thus allowing the exploration of forces in biological systems under physiologically relevant conditions. By immobilizing one half of an interacting pair of proteins onto the tip and the other half onto the substrate, it is possible to investigate the electrostatic and hydrophobic interactions between them. We employed these techniques to examine the interaction between a pair of proteins of known affinity that are involved in exocytosis (NSF and alpha-SNAP) and separately to demonstrate how two-dimensional force mapping can be applied to the nuclear envelope to identify nuclear pore complexes.     

Influence of atomic force microscopy acquisition parameters on thin film roughness analysis

A reliable procedure for measuring parameters connected to surface roughness is needed to compare the gas sensing properties of various thin films or the effect of different fabrication procedures on the surface roughness and the sensing properties. In this article, we propose to investigate how the acquisition parameters specific to atomic force microscopy investigations such as pixel size, scan area and scan speed influence the roughness parameters, namely root mean square and surface area ratio, commonly used for characterizing the gas sensing properties of porphyrins and other materials.     

The NanoBeamBalance: a passive, tensile-test device for the atomic force microscope

An add-on device is presented, which significantly expands the force measurement capabilities of the atomic force microscope (AFM). The device consists of a completely passive mechanism, which translates the vertical motion of the AFM tip in force measurements into a horizontal motion of two sample support pads. The advantage is that it is much easier to deposit microscopic samples from suspension onto flat surfaces than to attach them reliably between tip and a surface. The working-principle and the design of the device is comprehensively described and demonstrated on the example of collagen fibres with a diameter of a few μm. Well-defined tensile measurements in longitudinal direction were performed, showing that the tensile stiffness of collagen fibres from rat tail tendon decreases by a factor of 5 when rehydrated from a dried sample and slowly increases upon cross-linking with glutaraldehyde.     

Probing local surface conductance using current sensing atomic force microscopy

We have analyzed correlations between surface morphology and current sensing images obtained using a current sensing atomic force microscope (CSAFM) and the implication of surface conductivity derived from the current sensing images. We found that in cases where the diameter of a CSAFM probe tip is much smaller than the correlation length of the surface morphological features, the current detected using the probe should have little correlation with the surface features imaged by the same probe. If the sample thickness is much larger than the tip size, the surface conductivity distribution of a sample can be derived from a current sensing image using the Holm resistance relation, and the current probed using a CSAFM reflects the conductance variations in a layer on the surface with the thickness comparable to the probe diameter. However, if the thickness of a sample is comparable to or smaller than the tip diameter, CSAFM measures the conductance across the entire portion of the sample sandwiched between the tip and the electrode.     

3D mechanical measurements with an atomic force microscope on 1D structures

We have developed a simple method to characterize the mechanical properties of three dimensional nanostructures, such as nanorods standing up from a substrate. With an atomic force microscope the cantilever probe is used to deflect a horizontally aligned nanorod at different positions along the nanorod, using the apex of the cantilever itself rather than the tip normally used for probing surfaces. This enables accurate determination of nanostructures' spring constant. From these measurements, Young's modulus is found on many individual nanorods with different geometrical and material structures in a short time. Based on this method Young's modulus of carbon nanofibers and epitaxial grown III-V nanowires has been determined.     

Application of evanescent wave optics to the determination of absolute distance in surface force measurements using the atomic force microscope

A combined scanning near field optical/atomic force microscope (AFM) is used to obtain surface force measurements between a near field sensing tip and a tapered optical fibre surface, whilst simultaneously detecting the intensity of the evanescent field emanating from the fibre. The tapered optical fibre acts as a compliant sample to demonstrate the possible use of the near field intensity measurement system in determining 'real' surface separations from normal AFM surface force measurements at sub-nanometer resolution between deformable surfaces.