Quantitative determination of contact stiffness using atomic force acoustic microscopy

Atomic force acoustic microscopy is a near-field technique which combines the ability of ultrasonics to image elastic properties with the high lateral resolution of scanning probe microscopes. We present a technique to measure the contact stiffness and the Young's modulus of sample surfaces quantitatively, with a resolution of approximately 20 nm, exploiting the contact resonance frequencies of standard cantilevers used in atomic force microscopy. The Young's modulus of nanocrystalline ferrite films has been measured as a function of oxidation temperature. Furthermore, images showing the domain structure of piezoelectric lead zirconate titanate ceramics have been taken.     

Study of atomic force microscopy nanoindentation for the development of nanostructures

We have studied the fabrication of atomic force microscope (AFM) based nanotemplates using electrically controlled indentation (ECI) and a composite barrier (photoresist/alumina) that is resistant to the lithography process and presents good mechanical properties for indentation. The indentation process is affected by several factors such as the indentation speed, the trigger voltage and the barrier type. We have used the nanotemplate technique to fabricate small gold–gold nanocontacts (1–10 nm). In this limit, the size of the contacts that is obtained through the indentation process seems to be stochastic. However, low dimension, clean metallic contacts were achieved with high temporal stability and compatible with low temperature measurements. The fabricated nanotemplates are versatile and can be used in a wide range of applications, from nanojunctions to connecting a single nano-object. Small area metallic contacts can be used to study spin injection or ballistic transport.     

Local surface cleaning and cluster assembly using contact mode atomic force microscopy

Conventional contact mode atomic force microscopy (AFM) has been used for local surface cleaning and cluster alignment. By using the AFM tip to sweep and push in contact mode, we have demonstrated that Cu clusters, prepared by vacuum evaporation onto Dow Cyclotene 3022 polymer and subsequent exposure to atmosphere, can easily be moved by the AFM tip, and assembled at the outer edge of the scanned region to form a line of clusters. We have found that the force applied by the tip plays an important role in the ease of cluster motion. Cyclotene surface treatment that enhances cluster adhesion hinders this ability, and may be used as a method of nanofabrication.     

Material anisotropy revealed by phase contrast in intermittent contact atomic force microscopy

Phase contrast in intermittent-contact atomic force microscopy (AFM) reveals in-plane structural and mechanical properties of polymer monolayers. This is surprising, because measurements of nanoscale in-plane properties typically require contact mode microscopies. Our measurements are possible because the tip oscillates not just perpendicular but also parallel to the sample surface along the long axis of the cantilever. This lateral tip displacement is virtually universal in AFM, implying that any oscillating-tip AFM technique is sensitive to in-plane material properties.     

Imaging and manipulation of adatoms on an alumina surface by noncontact atomic force microscopy

Noncontact atomic force microscopy (NC-AFM) has been performed on an aluminum oxide film grown on NiAl(110) in ultrahigh vacuum (UHV) at low temperature (5 K). Results reproduce the topography of the structural model, unlike scanning tunnelling microscopy (STM) images. Equipped with this extraordinary contrast the network of extended defects, which stems from domain boundaries intersecting the film surface, can be analysed in atomic detail. The knowledge of occurring surface structures opens up the opportunity to determine adsorption sites of individual adsorbates on the alumina film. The level of difficulty for such imaging depends on the imaging characteristics of the substrate and the interaction which can be maintained above the adsorbate. Positions of single adsorbed gold atoms within the unit cell have been determined despite their easy removal at slightly higher interaction strength. Preliminary manipulation experiments indicate a pick-up process for the vanishing of the gold adatoms from the film surface.     

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     

Mechanical properties of H2Pc self-assembled monolayers at the single molecule level by noncontact atomic force microscopy

The mechanical properties of molecular self-assembled monolayers (SAMs) play an important role in understanding the interactions between molecules in the self-assembly, the interactions between molecules and substrate, and thus the formation mechanism of SAMs. Using a high-resolution noncontact atomic force microscope (NC-AFM) combined with a scanning tunneling microscope (STM), we have successfully obtained the sub-molecular resolution of a H(2)Pc self-assembled monolayer grown on a Pb(111) surface. A 2 × 2 superstructure was observed in both AFM and STM topographic images. The lateral critical force of removing a H(2)Pcmolecule from its SAM and moving a single H(2)Pc molecule on Pb(111) were measured. An oscillation of the critical force along the edge of the H(2)Pc SAM with a period of two molecular sites was observed, which can be attributed to the 2 × 2 superstructure. The lateral critical force caused by intermolecular interaction was found to be 25 pN on average and is typically two times larger than the molecule-substrate interaction.     

Thin film piezoelectric response characterisation using atomic force microscopy with standard contact mode imaging

This article introduces a technique for observing and quantifying the piezoelectric response of thin films, using standard atomic force microscopes (AFMs). The technique has been developed and verified using strontium-doped lead zirconate titanate (PSZT) thin films, which are known for their high piezoelectric response. Quantification of the electro-mechanical voltage coefficient d(33) (pm/V) is made directly based on the applied peak-to-peak voltage and the corresponding peak-to-peak displacement in the obtained scan image. Under the proposed technique the AFM is configured in contact mode, where the silicon nitride tip is set to follow the film displacement at a single point. A known sinusoidal voltage is applied across the film and the displacement determined as a function of time, rather than the typical AFM measurement of displacement versus tip position. The resulting raster image contains several bands, which are directly related to the AFM scan frequency and the applied sinusoidal voltage and its frequency. Different combinations of the AFM scan frequency and the applied sinusoid frequency have been used to characterise the PSZT thin films, with estimated values of d(33) between 109 and 205 pm/V.     

Controlled manipulation of atoms in insulating surfaces with the virtual atomic force microscope

We predict how single oxygen ions can be manipulated on the MgO (100) surface and demonstrate the possibility of detecting a single-atom event using a noncontact atomic force microscope. The manipulation process is simulated explicitly in real time with a virtual dynamic atomic force microscope including the full response of the instrumentation and demonstrates a strong dependence on temperature. The proposed new atomistic mechanism and protocols for the controlled manipulation of single atoms and vacancies on insulating surfaces may be relevant for anchoring molecules and metal clusters at these surfaces and controlling their electronic properties.     

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.     

Surface structure investigations using noncontact atomic force microscopy

Surfaces of several A_IIIB_V compound semiconductors (InSb, GaAs, InP, InAs) of the (0 0 1) orientation have been studied with noncontact atomic force microscopy (NC-AFM). Obtained atomically resolved patterns have been compared with structural models available in the literature. It is shown that NC-AFM is an efficient tool for imaging complex surface structures in real space. It is also demonstrated that the recent structural models of III-V compound surfaces provide a sound base for interpretation of majority of features present in recorded patterns. However, there are also many new findings revealed by the NC-AFM method that is still new experimental technique in the context of surface structure determination.     

Thermomechanical properties of polymer nanolithography using atomic force microscopy

The temperature-dependent mechanical properties of polyethylene terephthalate (PET) polymers are investigated using force-distance curves, adhesion force, and atomic force microscope (AFM) nanolithography combined the heating techniques. The results show that the width of grooves on the polymers at 20-60 °C were in the range of 14-363 nm. The wear depth of the polymers increased with increasing heating temperature. A volume of 251.85-2422.66 μm(3) at a load of 30-50 nN with heating to 30-60 °C was removed, as compared to that of 26.60-70.30 μm(3) obtained at room temperature. The contact forces of PET started increasing at 9 nN, whereas the size of the holes was average at a pressure. The results may be of importance in explaining the heating relationship among adhesion force, volume removal rate, and pressure.     

Tarantula hemocyanins imaged by atomic force microscopy

Individual 4 x 6-meric tarantula hemocyanins and dissociation products were imaged by AFM in the non-contact mode. Although the resolution was low, the hexamers and topological arrangement within the oligomers can be seen. However, the relative humidity seems to affect the height profiles.     

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.     

Measurement of nanomechanical properties of biomolecules using atomic force microscopy

The capabilities of atomic force microscopy (AFM) have been rapidly expanding beyond topographical imaging to now allow for the analysis of a wide range of properties of diverse materials. The technique of nanoindentation, traditionally performed via dedicated indenters can now be reliably achieved using AFM instrumentation, enabling mechanical property determination at the nanoscale using the high spatial and force resolutions of the AFM. In the study of biological systems, from biomolecules to complexes, this technique provides insight into how mesoscale properties and functions may arise from a myriad of single biomolecules. In vivo and in situ analyses of native structures under physiological conditions as well as the rapid analysis of molecular species under a variety of experimental treatments are made possible with this technique. As a result, AFM nanoindentation has emerged as a critical tool for the study of biological systems in their natural state, further contributing to both biomaterial design and pharmacological research. In this review, we detail the theory and progression of AFM-based nanoindentation, and present several applications of this technique as it has been used to probe biomolecules and biological nanostructures from single proteins to complex assemblies. We further detail the many challenges associated with mechanical models and required assumptions for model validity. AFM nanoindentation capabilities have provided an excellent improvement over conventional nanomechanical tools and by integration of topographical data from imaging, enabled the rapid extraction and presentation of mechanical data for biological samples.     

Mechanical mode dependence of bolometric backaction in an atomic force microscopy microlever

Two backaction (BA) processes generated by an optical cavity-based detection device can deeply transform the dynamical behavior of an atomic force microscopy microlever: the photothermal force or the radiation pressure. Whereas noise damping or amplifying depends on the optical cavity response for radiation pressure BA, we present experimental results carried out under vacuum and at room temperature on the photothermal BA process which appears to be more complex. We show for the first time that it can simultaneously act on two vibration modes in opposite directions: Noise on one mode is amplified, whereas it is damped on another mode. Basic modeling of photothermal BA shows that the dynamical effect on the mechanical mode is laser spot position-dependent with respect to mode shape. This analysis accounts for opposite behaviors of different modes as observed.     

Investigation of the local mechanical properties of potassium chloride single crystals by atomic force microscopy

Using an atomic force microscope (AFM), the of hardness H and Young’s modulus E are measured in near-surface layers of KCl single crystals to a depth of 300 nm at loads of 5–100 µN. The values of H and E are estimated indirectly by analyzing P(h) curves (load vs. indentation depth curves). The value of H is also estimated directly by measuring the area of an indentation with the help of an AFM with a nanoscale resolution. The effect of structural features of the surface around an indentation on the accuracy of the H and E estimates is revealed. The sharp dependence of H on the load (the nanoscale effect) is revealed. The experimental results agree qualitatively with the predictions of the geometrically necessary dislocation model developed by Nix and Gao. However, in order to quantitatively estimate mass transfer from a nanoindenter, a structural analysis is required with allowance for plastic deformation in crystals.     

Determination of silicone coating Young's modulus using atomic force microscopy

The polymerisation degree of thin polymer coatings was checked by following the variation of their local mechanical properties. Atomic force microscope (AFM) was used in an indentation mode to investigate the mechanical characteristics of silicone coatings on polycarbonate substrates. The evolution of Young's modulus of the silicone coatings was determined as a function of the polymer annealing time. We have used a relative method to measure Young's moduli, which involves a calibration step with a set of reference polymers. No variation was observed for the modulus of silicone coatings annealed during more than 40 min at 130 °C. This result indicates that over-heating does not modify the mechanical properties of the coating.     

Topographical observation of silane-treated inorganic surface using atomic force microscopy

The topography of the silane-treated layer on an inorganic surface was observed using an atomic force microscope. For this purpose, the cleaved mica plate was treated with some silane coupling agent at varying conditions. The silanes having aminopropyl or methacryloxypropyl group as organofunctional groups with di- or trialkoxyl structures were used. Three different solvents for silane solution — 2-propanol, 2-propanol/water mixture and water — were used. The pH of the aqueous solution was controlled. As a result, the most suitable solvent and pH in order to obtain smooth silane layer was clarified. The solubility of silane molecules in the solution, the wettability of silane molecule onto inorganic surface, and prevention of the mutual condensation of silane molecules in the solution were found to be important parameters for this purpose.     

Characterization of electrodeposited nickel film surfaces using atomic force microscopy

Microstructures of nickel surfaces electrodeposited on indium tin oxides coated glasses are investigated using atomic force microscopy. The fractal dimension D and Hurst exponent H of the nickel surface images are determined from a frequency analysis method proposed by Aguilar et al. [J. Microsc. 172 (1993) 233] and from Hurst rescaled range analysis. The two methods are found to give the same value of the fractal dimension D∼2.0. The roughness exponent α and growth exponent β that characterize scaling behaviors of the surface growth in electrodeposition are calculated using the height-difference correlation function and interface width in Fourier space. The exponents of α∼1.0 and β∼0.8 show that the surface growth does not belong to the universality classes theoretically predicted by statistical growth models.