Atomic Force Microscopy Tip Shape Research Articles

Image reconstruction of TGZ3 grating by eliminating tip‐sample convolution effect in AFM

Accurate and meticulous measurement is an important prerequisite to obtain the real surface information of samples in atomic force microscopy (AFM). A severe problem is the frequent occurrence of measurement errors, which are mainly caused by the nonlinearity of the probe driver, the temperature drift of the system and the tip characteristics. The measurement errors caused by probe tip are the main source of errors in AFM nanoscale measurements. The shape and state of AFM tip will distort the AFM image from the actual sample morphology. If the information about the probe is known, the measurement error caused by the probe tip can be greatly reduced. In order to obtain accurate AFM images, a new method based on geometric measurement model and blind tip reconstruction is proposed to eliminate tip-sample convolution in the measurement of grating samples. The static and dynamic characteristics of the AFM tip are described by four parameters: cone angle, curvature radius, scanning inclination angle and mounting inclination angle. Finally, the feasibility and effectiveness of the new calibration method are verified by evaluating the image reconstruction quality. In conclusion, the proposed method can effectively reconstruct accurate AFM images of the grating.

Elastic moduli of lipid membranes: Reproducibility of AFM measures

Membrane elastic properties play a major role in membrane remodeling events, such as vesicle fusion and fission. They are also crucial in drug delivery by liposomes. Different experimental techniques are available to measure elastic properties. Among them, atomic force microscopy (AFM) presents the unique advantage of being directly applicable to nano-sized liposomes. Unfortunately, different AFM measures reported in the literature show little agreement among each other and are difficult to compare with measures of bending modulus obtained by other experimental techniques or by molecular simulations. In this work we determine the bending rigidity of Egg PC liposomes in terms of Young modulus via AFM measurements, using two different tip shapes and different cantilever force constants. We interpret the measures using the Hertz and Shell models, and observe a clear dependency of the Young modulus values on the tip properties and on the interpretative theory. The effect of the AFM tip shape is less important than the effect of the cantilever force constant, and the mathematical model has a major effect on the interpretation of the data. The Shell theory provides the closest agreement between AFM data and other experimental data for the membrane bending modulus. Finally, we compare the results to calculations of bending modulus from molecular dynamics simulations of membrane buckles. Simulations provide values of bending modulus consistent with literature data, but the agreement with AFM experiments is reasonable only for some specific experimental conditions.

Blind evaluation of AFM tip shape by using optical glass surface with irregular nanostructures as a tip characteriser

Atomic-force microscopy (AFM) is a type of scanning probe microscopy. It possesses nano-scale resolution which is >1000 times better than the optical diffraction limit. An image of AFM is the result of interaction between the tip and the sample. The AFM image is not accurate and distorted due to the finite size of the probe tip. To correct the AFM image and acquire accurate sample morphology information, it needs to evaluate the tip shape and status. The shape of the probe tip is calculated to determine the quality of the probe. The optical glass surface fabricated by free abrasive polishing (traditional mechanical polishing) technique is used as a new tip characteriser to calibrate the AFM probe tip. Meanwhile, an improved blind reconstruction algorithm is adopted to calculate the AFM tip shape and size. In the help of matrices, the blind reconstruction algorithm is elaborated clearly. The optical glass surface with irregular nanostructures ensures that the morphology of probe tip can be determined accurately. Without independent knowledge of the sample morphology, the three-dimensional morphology of the AFM tip can be obtained.

Nanomechanical assessment of human and murine collagen fibrils via atomic force microscopy cantilever-based nanoindentation

The nanomechanical assessment of collagen fibrils via atomic force microscopy (AFM) is of increasing interest within the biomedical research community. In contrast to conventional nanoindentation there exists no common standard for conducting experiments and analysis of data. Currently used analysis approaches vary between studies and validation of quantitative results is usually not performed, which makes comparison of data from different studies difficult. Also there are no recommendations with regards to the maximum indentation depth that should not be exceeded to avoid substrate effects. Here we present a methodology and analysis approach for AFM cantilever-based nanoindentation experiments that allows efficient use of captured data and relying on a reference sample for determination of tip shape. Further we show experimental evidence that maximum indentation depth on collagen fibrils should be lower than 10–15% of the height of the fibril to avoid substrate effects and we show comparisons between our and other approaches used in previous works. While our analysis approach yields similar values for indentation modulus compared to the Oliver–Pharr method we found that Hertzian analysis yielded significantly lower values. Applying our approach we successfully and efficiently indented collagen fibrils from human bronchi, which were about 30nm in size, considerably smaller compared to collagen fibrils obtained from murine tail-tendon. In addition, derived mechanical parameters of collagen fibrils are in agreement with data previously published. To establish a quantitative validation we compared indentation results from conventional and AFM cantilever-based nanoindentation on polymeric samples with known mechanical properties. Importantly we can show that our approach yields similar results when compared to conventional nanoindentation on polymer samples. Introducing an approach that is reliable, efficient and taking into account the AFM tip shape, we anticipate that the present work may act as a guideline for conducting AFM cantilever-based nanoindentation of collagen fibrils. This may aid understanding of collagen-related diseases such as asthma, lung fibrosis or bone disease with potential alterations of collagen fibril mechanics.

Extracting Quantitative Data from AFM Indentations on Soft, Heterogeneous Biomaterials

Atomic force microscopy (AFM) is a commonly used method for extracting mechanical information about samples ranging from soft biological matter to rigid semiconductors. AFM and other force spectroscopy techniques have been recently employed to examine how the elastic properties of metastatic cancer cells differ from their healthy counterparts and to study the contractile forces that cancer cells exert during the process of invasion through an extracellular matrix (ECM). As the samples being studied with AFM become more complex, novel analysis methods must be developed to produce meaningful and quantitative data, thus new strategies for fitting force-indentation data beyond the standard Hertz model are essential. We present a method of raw data fitting which determines the apparent Young's modulus as a function of indentation depth, providing sensitivity to sample heterogeneities such as subsurface elasticity effects. An improved AFM tip shape model is derived for a spherical apex with a smooth transition to a cone to provide a realistic representation of the experimental AFM tips used. A bonded two-layered elastic model is created to include the perturbations caused by a heterogeneous material, such as a cell embedded in ECM. This model allows for understanding the signal generated from subsurface components as well as the theoretical limits for determining the elastic properties of the underlying second layer. To validate this, we performed finite element analysis simulations and AFM indentations on polyacrylamide. We also show specific examples of how these analysis methods and finite element analysis can be employed to extract more information regarding the mechanical basis of cancer cell invasion into an ECM.

Uncertainty compensation methods for quantitative hardness measurement of materials using atomic force microscope nanoindentation technique

We suggest uncertainty compensation methods for the quantification of nanoscale indentation using atomic force microscopy (AFM). The main error factors in the force–distance curves originated from the difference between theoretical and real shape of AFM tip during nanoscale indentation measurements. For the uncertainty compensations of tip shapes and misalignment of loading axis, we applied the enhanced tip geometry function and Y-scanner moving to the AFM measurements. Three different materials such as Si wafer, glass, and Au film were characterized with these compensation methods. By applying compensation methods, our results show the decreased values from 167% to 39% below 100nm indenting depth compared with the literature values. These compensation methods applied to thin films will show the advanced quantitative analysis of hardness measurements using nanoscale indenting AFM.

Measurement of the size of spherical nanoparticles by means of atomic force microscopy

Several techniques are nowadays available to determine the size distribution of nanoparticulate matter. Among these techniques, atomic force microscopy (AFM) is especially valuable because it can provide three-dimensional information on the shape of individual nanoparticles. This paper describes a new method to determine the size distribution of a population of spherical nanoparticles deposited on a hard substrate. The method is based on the acquisition and analysis of topographical AFM images. The size of individual nanoparticles is obtained by fitting the topographical region associated with the nanoparticle with a sphere. Tests on model systems based on nanoparticle reference materials consisting of polystyrene (PS) latex suspensions show promising results. The measured mean particle size is larger than the reference value, but this is a predictable effect of the AFM tip shape. Tests on a bi-modal mixture of two PS latex reference materials show the impact of the quality of the dispersion of the nanoparticles on the results obtained with the new technique.

Tobacco mosaic virus as an AFM tip calibrator

The study of high-resolution topographic surfaces of isolated single molecules is one of the applications of atomic force microscopy (AFM). Since tip-induced distortions are significant in topographic images the exact AFM tip shape must be known in order to correct dilated AFM height images using mathematical morphology operators. In this work, we present a protocol to estimate the AFM tip apex radius using tobacco mosaic virus (TMV) particles. Among the many advantages of TMV, are its non-abrasivity, thermal stability, bio-compatibility with other isolated single molecules and stability when deposited on divalent ion pretreated mica. Compared to previous calibration systems, the advantage of using TMV resides in our detailed knowledge of the atomic structure of the entire rod-shaped particle. This property makes it possible to interpret AFM height images in term of the three-dimensional structure of TMV. Results obtained in this study show that when a low imaging force is used, the tip is sensing viral protein loops whereas at higher imaging force the tip is sensing the TMV particle core. The known size of the TMV particle allowed us to develop a tip-size estimation protocol which permits the successful erosion of tip-convoluted AFM height images. Our data shows that the TMV particle is a well-adapted calibrator for AFM tips for imaging single isolated biomolecules. The procedure developed in this study is easily applicable to any other spherical viral particles.

AFM Tip Characterizer fabricated by Si/SiO2 multilayers

An atomic force microscopy (AFM) tip characterizer with measurement ranges from 7.7 nm to 131 nm was developed using Si/SiO2 multilayers. This characterizer was constructed with isolated line structures and comb-shaped trench structures. The shape of a standard Si AFM tip was estimated using this characterizer. The result shows that this Si/SiO2 multilayer-type tip characterizer has good potential for the characterization of AFM tips with a fine radius. [DOI: 10.1380/ejssnt.2011.293]

Improving Accuracy of Sample Surface Topography by Atomic Force Microscopy

We describe method for improving the accuracy of sample surface topography by atomic force microscopy (AFM). It takes into account the effect of the AFM tip shape and image reconstruction on the acquired AFM images. The dilation effect due to the use of a finite-sized tip shape can be minimized by using a sharp AFM tip and scanning at the most symmetric direction of tip geometry. Reconstruction of AFM image could produce more accurate sample surface features. The method is useful to AFM measurement and is significant because AFM has become a fundamental tool in nanoscience and nanotechnology with multiple applications in a wide range of disciplines ranging from biology to physics and material science.

Characterizing Atomic Force Microscopy Tip Shape in Use

A new tip characterizer based on the fabrication of multilayer thin films for atomic force microscopy (AFM) was developed to analyze the effective tip shape while in use. The precise structure of this tip characterizer was measured by transmission electron microscopy. Four different types of commercial tips with various radii were characterized by the tip characterizer and by conventional scanning electron microscopy (SEM). The results were compared to obtain a relationship between the actual and effective tip shapes. A quantitative analysis was performed of apex radii measured from line profiles of comb-shaped patterns and nanometer-scale knife-edges without the problem of edge uncertainty in the SEM image. Degradation of the AFM tip induced by electron-beam irradiation was studied by using SEM and the tip characterizer. A potential technique for fabricating symmetric AFM tips based on irradiation by an electron beam and a quantitative analysis of changing the tip apex in SEM were examined with AFM using the tip characterizer.

Sputter-induced cone and filament formation on InP and AFM tip shape determination

The structures formed on the InP(100) surface by sputtering with fluences of the order of 1021 ions m−2 of 4–8 keV argon are analysed by scanning electron microscopy, atomic force microscopy (AFM) and Auger electron spectroscopy. Well-defined cones are formed if small traces of polymeric coatings are deposited on the surface. However, for clean surfaces at room temperature, the sputtered surface may be cone-free. Surfaces that are sputtered at above ambient temperature exhibit a dense filamentary growth, the heights of which increase in magnitude as the temperature rises. For the conditions used in this work, filaments 200 nm high occur at 260 °C. The mechanism for this growth is thought to arise from stress through capacitative effects arising from charging of an indium cap at the top of the filament by the ion beam. At elevated temperatures, diffusive effects occur to reduce this strain by causing elongation of the filament. Samples may be heated by the power deposited from the ion beam so that filaments occur on samples that would otherwise be at room temperature. By heating the InP in the temperature range 100–180 °C, filaments 30–80 nm high may be grown, which are excellent for imaging the structure at the ends of AFM tips. New software, which averages the images of several individual filaments, allows the tip structure to be monitored, wear effects to be diagnosed and worn tips to be rejected. The samples can be used routinely, in-between other samples, to allow diagnosis directly without the need to use other forms of microscopy. Copyright © 2000 John Wiley & Sons, Ltd.

Determining the molecular-packing arrangements on protein crystal faces by atomic force microscopy.

Previous atomic force microscopy (AFM) studies and periodic bond-chain (PBC) analyses of tetragonal lysozyme crystals have suggested that the (110) face consists of chains of molecules related to one another by 43 axes parallel to the crystal face. In this study, high-resolution AFM images of the (110) face were obtained and analyzed in order to verify this prediction. A computer program was employed which constructs the theoretical AFM image corresponding to a specific crystallographic molecular-packing arrangement and AFM tip shape. The packing arrangement and tip shape were varied in order to obtain the maximum possible correlation between experimental and theoretical images. The prediction from PBC analysis of an arrangement involving 43 helices was confirmed in this manner, while the alternate arrangement, consisting of molecules related to one another by 21 axes, was not observed. However, the surface structure was found to differ significantly even from this crystallographic arrangement. The molecules were found to pack slightly closer about what will become the 43 axes within the interior of the crystal, suggesting the occurrence of surface reconstruction or rearrangement on the tetragonal lysozyme (110) face. This study represents a new approach for more precise determination of the molecular-packing arrangements on protein crystal faces employing AFM.

Surface morphology of GaN films determined from quantitative x-ray reflectivity

The surface of two GaN films grown under Ga-rich conditions by molecular beam epitaxy was characterized using x-ray reflectivity, assuming a self-affine/fractal surface morphology. The surface height fluctuations were similar for both samples at the largest lateral length scales at which the fractal description is valid, although this lateral length was significantly greater and the “jaggedness” significantly smaller for the sample grown under higher Ga flux. Previous atomic force microscopy images revealed a higher density of large features on the surface for the sample grown under lower Ga flux. The lateral size of the features are dominated by a convolution of the atomic force microscopy tip shape and the actual features on the surface, which precludes an accurate determination of the surface structure at length scales smaller than the tip radius. This study illustrates the importance of using different techniques to evaluate the film surface morphology at different length scales.

Illite-smectite from the North Sea investigated by scanning tunnelling microscopy

AbstractTwo samples of illite-smectite (I-S) isolated from Upper Jurassic clays in well 2/7-3 (3365 m) and well 2/11-1 (4548 m) have been investigated by scanning tunnelling microscopy (STM), and the particle shape and dimensions have been correlated to results from transmission electron microscopy (TEM) on shadowed specimens and results from atomic-force microscopy (AFM). By STM, lath-like and equant particles were observed in both samples, some of the particles having sharp edges. In the I-S from well 2/11-1, spiral-shaped particles were also seen. For both samples, the most frequent particle diameter was 100–200 Å. Particle-thickness distributions from STM and TEM were similar for the I-S from well 2/7-3, being dominated by 10 Å thick particles. For I-S from 2/11-1, the STM particle-thickness distribution has a predominance of 20 Å thick particles, but the TEM particle-thickness distribution is broad, with about equal amounts of 20, 30, 40, 50 and 60 Å thick particles. The AFM particle-thickness distribution for this I-S resembles the TEM thickness distribution. It is concluded that mainly thin (10 and 20 Å) particles are seen by STM. Failure of AFM to show sharp particle edges (seen in STM and TEM) might be attributed to the AFM tip movement or tip shape. In STM, I-S particles from well 2/7-3 have peaks along edges, whereas I-S particles from well 2/11-1 have rims. These rims are also seen in AFM and are therefore real geometrical features, probably a result of two-dimensional growth, whereas the spirals in the I-S from well 2/11-1 demonstrate three-dimensional growth. The minimum thickness of most particles is 10 Å.