Force spectroscopy in noncontact mode

We have analyzed the possibility of using noncontact scanning force microscopy (NCAFM) to detect variations in surface composition, i.e., to detect a ‘spectroscopic image' of the sample. This ability stems from the fact that the long-range forces, acting between the AFM tip and sample, depend on the composition of the AFM tip and sample. The long-range force can be magnetic, electrostatic, or van der Waals forces. Detection of the first two forces is presently used in scanning force microscopy technique, but van der Waals forces have not been used. We demonstrate that the recovery of spectroscopic image has a unique solution. Furthermore, the spectroscopic resolution can be as good as lateral one.     

Surface adhesion properties of graphene and graphene oxide studied by colloid-probe atomic force microscopy

Surface adhesion properties are important to various applications of graphene-based materials. Atomic force microscopy is powerful to study the adhesion properties of samples by measuring the forces on the colloidal sphere tip as it approaches and retracts from the surface. In this paper we have measured the adhesion force between the colloid probe and the surface of graphene (graphene oxide) nanosheet. The results revealed that the adhesion force on graphene and graphene oxide surface were 66.3 and 170.6 nN, respectively. It was found the adhesion force was mainly determined by the water meniscus, which was related to the surface contact angle of samples.     

An AFM study of the effect of chemical treatments on the surface microstructure and adhesion properties of flax fibres

The mechanical properties of fibre-reinforced polymer composites are largely dependant on the adhesion between the matrix and the fibre. In order to enhance the interaction between flax fibres and unsaturated polyester resins, raw fibres were chemically modified using sodium hydroxide, sodium hydroxide plus acetic anhydride and formic acid-based treatments. The physical properties of the modified fibres were investigated by means of the atomic force microscopy. At first, the morphological analysis of the surfaces shows that after the chemical treatments, the fibres surface appear to be less heterogeneous in topology and smoother. Nonetheless, no significant roughness difference was found between the different treatments. Secondly, adhesion forces measurements were performed between a standard AFM silicon nitride tip and the fibres. The adhesion forces were found to vary according to the chemical treatment. The sodium hydroxide-based treatment was found to increase the adhesion force between the fibre and the AFM tip whereas the lowest adhesion force was found for the formic acid- based treated fibre. These results were attributed to the different hydrophilic character of the modified fibres. Due to the importance of the water layer adsorbed on the fibres, the adhesion forces between the AFM tip and the different samples are found to be mainly dominated by capillary forces in relation with the fibre's surface hydrophilicity.     

Sublattice identification in scanning force microscopy on alkali halide surfaces

We propose and apply to the KBr(001) surface a new procedure for species recognition in scanning force microscopy (SFM) of ionic crystal surfaces which show a high symmetry of the charge arrangement. The method is based on a comparison between atomistic simulations and site-specific frequency versus distance measurements. First, by taking the difference of force-distance curves extracted at a few judiciously chosen surface sites we eliminate site-independent long-range forces. The obtained short-range force differences are then compared with calculated ones assuming plausible tip apex models. This procedure allows for the first time identification of the tip apex polarity and of the positive and negative sublattices in SFM images of the (001) cleavage surface of an ionic crystal with the rock salt structure.     

纳米沟槽表面黏着接触过程的分子动力学模拟研究

采用大规模分子动力学方法研究了刚性球型探头与具有不同纳米沟槽基体表面的黏着接触过程,探讨了表面沟槽结构对载荷-位移曲线、接触引力和拉离力以及材料转移的影响规律.研究结果表明:在相同的压入深度下,与原子级光滑表面的黏着接触过程相比,刚性探头与具有纳米沟槽结构基体表面的接触压力较小,接触加载过程中的引力作用范围较大,并伴随载荷的多次跳跃,且接触引力和拉离力均有减小;当沟槽深度相同时,随着沟槽宽度的增大,接触引力和拉离力逐渐减小,当沟槽宽度逐渐接近探头与光滑表面的接触直径时,接触引力和拉离力又逐渐增大,趋于接近探头与光滑表面的接触过程;当沟槽宽度相同时,随着沟槽深度的增大,接触引力相对减小,拉离力变化不大.     

Thermodynamic analysis of liquid bridge for fixed volume in atomic force microscope

In ambient condition,capillary forces are the major contributors to the adhesive forces between the tip of an atomic force microscope(AFM) and the sample.In general,capillary forces are thought to be related to water film thickness,contact time and relative humidity and so on.In this paper,an original analysis regarding the liquid bridge,based on the surface and interface thermodynamic theory,is proposed.The cases covered in the study include the capillary forces and temperature of liquid bridge for quickly drawn liquid bridge,and for nonvolatile liquid bridge.The study results show that variation in temperature may occur in the liquid bridge when it is stretched.     

Atomic-scale force-vector fields

The magnitude and direction of forces acting between individual atoms as a function of their relative position can be described by atomic-scale force-vector fields. We present a noncontact atomic force microscopy based determination of the force fields between an atomically sharp tip and the (001) surface of a KBr crystal in conjunction with atomistic simulations. The direct overlap of experiment and simulation allows identification of the frontmost tip atom and of the surface sublattices. Superposition of vertical and lateral forces reveals the spatial orientation of the interatomic force vectors.     

Atomistic simulations of fluid structure and solvation forces in atomic force microscopy

We describe results of atomistic molecular dynamics simulations modelling an atomic force microscope (AFM) tip immersed in a fluid. Both the tip and the surface are modelled by rigid arrays of atoms. The tip is pyramidal and the surface is the (100) face of a fcc crystal. The focus is on the solvation forces acting on the tip and on the surface and their relation to the structural and dynamic properties of the fluid. Fluid particles in the neighborhood of the tip-surface junction are found to be highly ordered compared to the bulk, as shown by localized variations in the average fluid density. The atomistic nature of the model gives rise to several effects related to the discrete sizes of the fluid, tip, and surface particles which are not observed in continuum-based theories. A number of simulated force-distance curves are presented, along with an analysis of the effect of changing fluid particle size, tip (lateral) position, tip shape, and the lyocompatability of the tip and surface materials. The atomic-scale distribution of fluid-surface forces is examined for various positions of the tip, and the extent to which the fluid can act as a “cushion” by increasing the effective area of the tip-surface interaction is studied. The effect of a fluid on AFM imaging is investigated by generating “fluid images”, which are shown to be comparable in magnitude to the direct tip-surface interaction in the noncontact mode. We compare images generated by defective and defect-free surfaces, and analyse the fluid-tip forces acting in a lateral direction. An image formed from fluid forces acting in the direction of the surface normal does not show the presence of a vacancy, but an image formed from lateral fluid forces does.     

Description of force surfaces in atomic force microscopy with allowance for the mobility of the lattice atoms

The surfaces of constant force and the profiles of the horizontal component of the force during scanning of the tip of an atomic force microscope above the surface of a close-packed lattice in the contact mode are calculated taking account of the mobility of the lattice atoms. It is shown that when the mobility is taken into account, the previously observed discontinuities on the surface of constant force arise at smaller scanning forces on the tip above the surface than in the immobile-atom approximation. The force surfaces arising when scanning above vacancies are obtained. The possibility of using atomic force microscopy data for diagnostics of point defects on a solid surface is discussed.     

Non-Contact to Contact Transition： Direct Measurements of Interaction Forces between a Solid Probe and a Planar Air-Water Interface

The interaction force between a solid probe and a planar air-water interface is measured by using an atomic force microscope. It is demonstrated that during the approach of the probe to the air-water interface, the force curves decline all the time due to the van der Waals attraction and induces a stable profile of water surface raised. When the tip approaches very close to the water surface, force curves jump suddenly, reflecting the complex behaviour of the unstable water surface. With a theoretical analysis we conclude that before the tip touches water surface, two water profiles appear, one stable and the other unstable. Then, with further approaching, the tip touches water surface and the non-contact to contact transition occurs.     

Tip-surface interactions at redox responsive poly(ferrocenylsilane) (PFS) interface by AFM-based force spectroscopy

Poly(ferrocenylsilanes) (PFS) belong to the class of redox responsive organometallic polymers. Atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS) was used earlier to study single chain PFS response and redox energy driven single chain PFS molecular motors. Here we present further AFM investigations of force interactions between tip and a grafted PFS surface under potential control in electrochemical redox cycles. Typical tip-Au interaction is considered as reference in the force measurements. First the electrostatic component in the diffused double layer (DL) in NaClO₄ electrolyte environment was considered for a “grafted to” PFS, which dominated the interplay between the tip and sample surface. The DL forces can also hinder the physisorption of PFS chain onto the tip when the voltage was applied at −0.1 V. On the other hand, if the tip contacted the PFS surface prior to the electrochemical process, physisorption of PFS chains governed the overall interaction regardless of subsequently applied surface potential. In addition, prolonged contact time, t_c, may also contribute to the stability of tip-PFS bridging and detection of electrostatic forces between the tip-PFS interface. The results showed that tip-substrate interaction forces without PFS grafts have negligibly small force contributions under similar, electrochemically controlled, conditions used in single PFS chain based molecular motors.     

Investigation into layers formed by selective transfer CMP mechanisms with atomic force microscope

To understand mechanisms of chemical mechanical planarization (CMP), an atomic force microscope (AFM) was used to characterize polished layer surfaces formed by selective transfer after a set of polishing experiments. It is know that in the process of friction of two materials and in the presence of own lubricants, wear phenomenon itself manifests as a transfer of material from an element of a friction couple on the other, this phenomenon being characteristic to the selective transfer process. A selective transfer can be safely achieved in a friction couple, if there is a favorable energy, and in the presence of relative movement, if in the friction area is a material made by copper and the lubricant is adequate (glycerin or special lubricant). The forming selective layer on the contact surfaces makes that the friction force to be very low because of the structure formed by selective transfer. To optimize the CMP process, one needs to obtain information on the interaction between the slurry abrasive particles (with the size range of about 30–70 nm) and the polished surface. To study such interactions, we used AFM. Surface analysis of selective layer using the AFM revealed detailed surface characteristics obtained by CMP. Studying the selective layer CMP, of which the predominated one is copper (in proportion of over 85%), we found that the AFM scanning removes the surface oxide layer in different rates depending on the depth of removal and the pH of the solution. Oxide removal happens considerably faster than the copper CMP removal from the selective layer. This is in agreement with generally accepted models of copper CMP. It was found that removal mechanisms depend on the slurry chemistry, potential per cent of oxidizer, and the applied load. This presentation discusses these findings. Both load force and the friction forces acting between the AFM tip and surface during the polishing process were measured. One big advantage of using the AFM tip (of radius about 50 nm) as abrasive silica particle is that we can measure forces acting between the particle-tip and the surface being polished. Here, we report measurement of the friction force while scratching and polishing. The correlation between those forces and removal rate is discussed.     

Single atomic contact adhesion and dissipation in dynamic force microscopy

By combining dynamic force microscopy experiments and first-principles calculations, we have studied the adhesion associated with a single atomic contact between a nanoasperity--the tip apex--and a semiconductor surface--the Ge(111)-c(2 x 8). The nanoasperity's termination has been atomically characterized by extensive comparisons of the measured short-range force at specific sites with the chemical forces calculated using many atomic models that vary in structure, composition, and relative orientation with respect to the surface. This thorough characterization has allowed us to explain the dissipation signal observed in atomic-resolution images and force spectroscopic measurements, as well as to identify a dissipation channel and the associated atomic processes.     

Theoretical Study on the Capillary Force between an Atomic Force Microscope Tip and a Nanoparticle

Considering that capillary force is one of the most important forces between nanoparticles and atomic force microscope （AFM） tips in ambient atmosphere, we develop an analytic approach on the capillary force between an AFM tip and a nanoparticle. The results show that the capillary forces are considerably affected by the geometry of the AFM tip, the humidity of the environment, the vertical distance between the AFM tip and the nanoparticle, as well as the contact angles of the meniscus with an AFM tip and a nanoparticle. It is found that the sharper the AFM tip, the smaller the capillary force. The analyses and results are expected to be helpful for the quantitative imaging and manipulating of nanoparticles by AFMs.     

Self-consistent calculation of electrostatic force for two kinds of mesoscopic surface structures

The scanning electrostatic force microscopy (SEFM) can acquire information of surface structures in a non-contact way. We calculate the electrostatic force between the charged tip and polarized surface structure in SEFM in the framework of self-consistent integral equation formalism (SCIEF), incorporating the image method to treat the electrostatic coupling of substrate and tip. We consider two kinds of surface structures, one is the topographic structure on the surface, the other is the dielectric structure embedded in the substrate. The force pattern of the topographic structure shows a protrusion around the surface structure. However, the force pattern displays a hollow around an embedded structure with a dielectric constant less than that of substrate medium. For an embedded structure with a larger dielectric constant, the force pattern exhibits a protrusion, and the force signal is much weaker than that of the topographic structure. Therefore, it is expected that one may identify these surface structures from the pure electrostatic force information in SEFM. The force signal of the densely arranged dielectric pads is simply the superposition of force signal of each pad individually, the interference effect of electric field is not remarkable. Received: 26 March 1998 / Accepted: 9 June 1998     

Capillary Interactions between a Probe Tip and a Nanoparticle

To understand capillary interactions between probe tips and nanoparticles under ambient conditions, a theoretical model of capillary forces between them is developed based on the geometric relations. It is found that the contribution of surface tension force to the total capillary force attains to similar order of magnitude as the capillary pressure force in many cases. It is also shown that the tip shape and the radial distance of the meniscus have great influence on the capillary force. The capillary force decreases with the increasing separation distances, and the variance of the contact angles may change the magnitudes of capillary forces several times at large radial distances. The applicability of the symmetric meniscus approximation is discussed.     

Friction anisotropy of the surface of organic crystals and its impact on scanning force microscopy

The transverse component of the friction forces acting on the tip of an atomic force microscope scanning on the surface of an organic crystal was monitored as a function of the scan direction. The relation between friction and the crystallographic system is disclosed, revealing that the symmetry of the friction phenomenon is dictated by the direction of the prominent corrugations of the crystal surface. It is also illustrated that molecular-resolution images can be collected through the monitoring of the motion of the tip in a transverse direction with respect to the scan direction.     

Effect of chain length on the adhesion behaviour of n-alkanethiol self-assembled monolayers on Au(111): An atomic force microscopy study

The effect of chain length on the adhesion behaviour of n-alkanethiols CH₃(CH₂)_nSH, wheren = 5, 6, 7, 9, 10, 11, 14 and 15 has been followed by carrying out pull-off force measurement using atomic force microscopy (AFM). The self-assembled monolayers on Au(111) surface has been characterized by reflection absorption infra-red spectroscopy (RAIRS) and contact mode AFM. It is observed that the work of adhesion is independent of thiol chain length though the standard deviation is high for short chain length thiol-based monolayers. This may be attributed to the relatively more deformable nature of the short chain thiol films due to their heterogeneity in the monolayer structure than the long chain ones. This, in turn, increases the contact area/volume between the AFM tip and the monolayer, and hence the force of adhesion. However, in the presence of water, the force of adhesion is lower than that observed in air reflecting the effects of capillary forces/polar components associated with the surface energy.     

High resolution atomic force microscopic imaging of the Si(111)-(7 x 7) surface: contribution of short-range force to the images

Observation of the rest-atom layer of the Si(111)-(7 x 7) surface is performed by atomic force microscopy. By detecting the force due to the single chemical covalent bond formed between the tip and the sample surface, individual atoms on the layer were clearly resolved. Unprecedented high spatial resolution was achieved by setting the detection force at a small value and by reducing background forces due to the long-range interactions with the small oscillation amplitude of the cantilever and sharp probe tip.     

Scratch direction and threshold force in nanoscale scratching using atomic force microscopes

The nanoscaled tip in an AFM (atomic force microscope) has become an effective scratching tool for material removing in nanofabrication. In this article, the characteristics of using a diamond-coated pyramidal tip to scratch Ni-Fe thin film surfaces was experimentally investigated with the focus on the evaluation of the influence of the scratch or scan direction on the final shape of the scratched geometry as well as the applied scratch force. Results indicated that both the scratched profile and the scratch force were greatly affected by the scratch direction. It has been found that, to minimize the formation of protuberances along the groove sides and to have a better control of the scratched geometry, the tip face should be perpendicular to the scratching direction, which is also known as orthogonal cutting condition. To demonstrate the present findings, three groove patterns have been scratched with the tip face perpendicular to the scratching direction and very little amount of protuberances was observed. The threshold scratch force was also predicted based on the Hertz contact theory. Without considering the surface friction and adhesive forces between the tip and substrate, the threshold force predicted was twice smaller than the measurement value. Finally, recommendations for technical improvement and research focuses are provided.