A study of adhesion forces by atomic force microscopy

An atomic force microscope (AFM) was used to investigate Si₃N₄ tip interactions with various materials in four different liquid media (water, ethanol, ethylene glycol, and formamide). The adhesion forces calculated using surface energies and the values measured experimentally were compared. For all materials, the calculated adhesion force closely correlated with AFM measurements, except in water. In the case of water, the AFM experiments showed strong adhesion, whereas theoretically (van Oss-Chaudhury-Good model) repulsion is predicted. The difference observed is discussed in terms of the chemical interactions between Si₃N₄ and water.     

Surface morphology of Nafion 117 membrane by tapping mode atomic force microscope

Surface morphology of Nafion 117 membrane was studied by tapping mode atomic force microscopy. Three different samples were analyzed and correspond respectively to dry membrane and wet membrane equilibrated either with water or with tributylphosphate. These studies show the supermolecular structure of the membrane, which is made of nodules or spherical grains of a mean diameter of 11 nm, and are surrounded by interstitial regions of a mean thickness of 50 Å. Roughness analysis of the samples shows the influence of the swelling properties of the membrane on its surface morphology. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 503–508, 1998     

Adhesion force measurement of a DPI size pharmaceutical particle by colloid probe atomic force microscopy

The method to determine the adhesion characteristics of fine drug particles for dry powder inhalation (DPI) was established using a colloid probe which mounted a 1-3 μm drug particle on a commercial atomic force microscope (AFM) cantilever. A new preparation system of colloid probes for fine particles smaller than 2.5 μm in diameter was developed with the aid of a micromanipulator and a video microscope. Using this colloid probe, adhesion force distribution between a spherical polycrystalline drug particle and a plate of lactose monohydrate representing for DPI carrier materials or stainless steel for device wall materials was measured. Atmospheric humidity as well as the material and surface roughness of a target plate affected the determined adhesion force. With increasing surface roughness of a lactose plate, the adhesion force between a drug particle and the plate distributed more widely and their mean value decreased. Adhesion force increased meaningfully with atmospheric humidity. Adhesion force for stainless steel was higher than that for lactose.     

The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: A comparative atomic force microscope and scanning electron microscope study

Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to investigate the surface structure and morphology of 10,000, 30,000, and 100,000 dalton molecular weight cutoff (MWCO) polyethersulfone (PES) ultrafiltration membranes, and the results are compared. Although both approaches reveal the pore structure in the 30,000 and 100,000 MWCO membranes, the pore diameters derived from SEM are smaller than those measured by AFM. This discrepancy is a result of the diminution in pore dimensions during the sample preparation for SEM, that is, the solvent exchange procedure needed to remove the water from the membrane prior to the high vacuum gold coating deposition step. In contrast to SEM, which requires a high vacuum both during heavy metal coating and during examination, AFM can be performed on wet ultrafiltration membranes. Consequently, the potential of altering the membranes' pore structures during sample preparation is eliminated. Therefore, the pore diameters obtained from AFM are more accurate than those derived from SEM.     

原子力显微镜技术研究重组人内皮抑素对内皮细胞的作用

目的应用原子力显微镜(atomic force microscope,AFM)技术研究重组人内皮抑素(recombinant human endostatin,rhES)对内皮细胞的作用。方法采用MTT法检测不同浓度的rhES(0.05~2.4μg/ml)对人脐静脉内皮细胞ECV304增殖活力的影响;分别用0.8和2μg/ml的rhES处理ECV304细胞,应用AFM观察内皮细胞整体形貌的变化,SPI 3800 New DFM动力显微镜观察ECV304细胞表面局部形貌的变化。结果 rhES可明显抑制ECV304细胞增殖,且呈剂量效应(P0.001);rhES可降低贴壁的ECV304细胞的厚度,且呈剂量依赖效应,使较光滑的细胞表面变粗糙,产生了一些微小的突起;经rhES处理的ECV304细胞表面结构呈现不规则的变化。结论 AFM技术具有样品制备简便和分辨率较高等优点,适合贴壁培养细胞的原位观察。     

Local photoconductivity on diamond metal-semiconductor-metal photodetectors measured by conducting probe atomic force microscopy

Local photoconductivity characterizations have been carried out on planar interdigitated metal-semiconductor-metal (MSM) devices fabricated on homoepitaxial CVD diamond for UV application. For this purpose a deuterium light source with an integrated UV fibre was combined and adapted to a conducting probe atomic force microscope (CP-AFM) tool. In this study, photocurrent was evidenced by local electrical resistance mapping measurements and analyzed as a function of the applied voltage and time.     

Patterned growth and field emission properties of vertically aligned carbon nanotubes

Vertically aligned carbon nanotubes were grown selectively on patterned Ni thin films by microwave plasma-enhanced chemical vapor deposition and their field emission properties were investigated using a diode-structure. Ni thin films patterned with a form of dot-arrays were prepared using a shadow mask having an array of holes. The nanotubes were found to be well-graphitized with multiwalled structures. The measurements of field emission properties revealed that the carbon nanotube tips emitted high current density at low macroscopic electric field. The Fowler–Nordheim (F–N) plot clearly showed two characteristic regions where the current saturates at the high electric field region. It was found that the saturation behavior was caused by the adsorbates-enhanced field emission mechanism. Eliminating the adsorbates resulted in no saturation behavior, increasing turn-on field, decreasing current, and increasing field enhancement factor. Using ZnS/Cu,Al phosphor, very bright and uniform emission patterns were obtained.     

Sample rotation angle dependence of graphene thickness measured using atomic force microscope

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.     

Dynamic response of a cracked atomic force microscope cantilever used for nanomachining

ABSTRACT: The vibration behavior of an atomic force microscope [AFM] cantilever with a crack during the nanomachining process is studied. The cantilever is divided into two segments by the crack, and a rotational spring is used to simulate the crack. The two individual governing equations of transverse vibration for the cracked cantilever can be expressed. However, the corresponding boundary conditions are coupled because of the crack interaction. Analytical expressions for the vibration displacement and natural frequency of the cracked cantilever are obtained. In addition, the effects of crack flexibility, crack location, and tip length on the vibration displacement of the cantilever are analyzed. Results show that the crack occurs in the AFM cantilever that can significantly affect its vibration response.PACS: 07.79.Lh; 62.20.mt; 62.25.Jk.     

原子力显微镜在聚氨酯材料性能分析中的应用

简述了原子力显微镜（AFM）探测物体表面形貌的工作原理和操作模式，介绍了AFM在观察聚氨酯（PU）材料微相分离、复合树脂的相容性的应用情况，综述了AFM应用于PU材料研究的新进展。     

An atomic force microscope study of calcium carbonate adhesion to desalination process equipment: effect of anti-scale agent

Whilst carbon dioxide is water soluble the system is somewhat complex and results in the presence of carbonate anions which interact with cations such as Ca²⁺ and Mg²⁺ present in seawater to form insoluble carbonates, especially at high temperatures. In multistage flash (MSF) desalination plants CO₂ gas becomes less soluble in the brine as a result of the brines high temperature and high salinity which causes the pH to be in the range of 8–9. The presence of these conditions causes the release of CO₂, simultaneous to the formation of scale deposits since its solubility is a function of the solution pH.

The formation of scale deposits, such as CaCO₃ causes fouling in the MSF distillers which has previously been studied by many researchers. A great amount of work has been carried out and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. The deposits may serve as an adsorbing film raising the speed of the loss of crystals or promoting the formation of scale deposits and therefore further adhesion on the wall surfaces of the MSF distillers and other process plant equipment leading to deterioration in the performance and efficiency of the whole desalination plant.

This paper shows direct quantification of the adhesion forces between CaCO₃ crystals and different process equipment surfaces under different conditions. This was carried out using an atomic force microscope (AFM) with an attached CaCO₃ crystal as a colloid probe to bring the CaCO₃ directly into and out of contact with the surfaces and measuring the resultant adhesion. This involved using surfaces different grades of roughness and carrying out measurements in synthetic sea water solutions of differing ionic strengths as well as with real seawater samples. Furthermore, the effect on measured adhesion of adding anti-scalant to the solutions was examined.     

A morphological study of melt‐spun polypropylene filaments by atomic force microscopy

Surface morphology of melt‐spun polypropylene (PP) filaments, spun from an additive‐free PP powder and from a commercial‐grade PP with different draw ratios, were examined with atomic force microscopy (AFM). The surface morphology of as‐spun filaments was spherulitic. The gradual transformation of the surface structure from a spherulitic morphology to a fibrillar morphology during stretching was studied. In the filaments spun from the commercial‐grade PP, the transformation was initiated by deformation of spherulites with a draw ratio of 1.2 and continued with association of lamellar stacks into fibrillar chains with a draw ratio between 1.2 and 2.0. A hierarchical morphological microstructure of fibrils, microfibrils, and nanofibrils was developed with a draw ratio of 4.0. In the filaments spun from the additive‐free PP, the association of lamellar stacks into fibrillar morphology occurred considerably later, between draw ratios of 2.0 and 4.0. An oriented lamellar structure was found in these filaments, still with a draw ratio of 4.0. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1242–1249, 1999     

Atomic force microscope study of the rejection of colloids by membrane pores

An atomic force microscope (AFM) in conjunction with the colloid probe technique has been used to study the electrical double layer interactions between a 0.75 μm silica sphere and a polymeric microfiltration track etch Cyclopore membrane (nominally 1 μm) in aqueous solutions. The silica colloid probe was used to image the membrane surface (using the double layer mode) at different imaging forces in high purity water and at constant imaging force in sodium chloride solutions of different ionic strengths at pH 8. Force-distance measurements show clearly how the sphere detects the membrane surface. Quality of images produced from scanning the 0.75 μm silica particle across the surface deteriorates with increasing distance between the silica sphere and membrane surface. Such images were compared with those obtained from scanning a sharp silicon nitride tip over the membrane surface.     

Local probing of the field emission stability of vertically aligned multi-walled carbon nanotubes

Metallic cantilever in high vacuum atomic force microscope has been used as anode for field emission experiments from densely packed vertically aligned multi-walled carbon nanotubes. The high spatial resolution provided by the scanning probe technique allowed precise setting of the tip-sample distance in the submicron region. The dimension of the probe (curvature radius below 50 nm) allowed to measure current contribution from sample areas smaller than 1 μm². The study of long-term stability evidenced that on these small areas the field emission current remains stable (within 10% fluctuations) several hours (at least up to 72 h) at current intensities between 10⁻⁵ and 10⁻⁸ A. Improvement of the current stability has been observed after performing long-time conditioning process to remove possible adsorbates on the nanotubes.     

Crystallographic plane-orientation dependent atomic force microscopy-based local oxidation of silicon carbide

The effect of crystalline plane orientations of Silicon carbide (SiC) (a-, m-, and c-planes) on the local oxidation on 4H-SiC using atomic force microscopy (AFM) was investigated. It has been found that the AFM-based local oxidation (AFM-LO) rate on SiC is closely correlated to the atomic planar density values of different crystalline planes (a-plane, 7.45 cm^-2; c-plane, 12.17 cm^-2; and m-plane, 6.44 cm^-2). Specifically, at room temperature and under about 40% humidity with a scan speed of 0.5 μm/s, the height of oxides on a- and m-planes 4H-SiC is 6.5 and 13 nm, respectively, whereas the height of oxides on the c-plane increased up to 30 nm. In addition, the results of AFM-LO with thermally grown oxides on the different plane orientations in SiC are compared.     

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.     

Continuous electrospinning of polymer nanofibers of Nylon-6 using an atomic force microscope tip

An atomic force microscopy (AFM) probe is successfully utilized as an electrospinning tip for fabricating Nylon-6 nanofibers. The nanometre-size tip enabled controlled deposition of uniform polymeric nanofibers within a 1 cm diameter area. Nylon-6 nanofibers were continuously electrospun at a solution concentration as low as 1 wt% Nylon-6 in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results of the AFM electrospun fibers indicated that the nanofibers predominantly display the meta-stable γ crystalline form suggesting rapid crystallization rate during the process. In addition to precise control over fiber deposition and diameter, some of the drawbacks of conventional electrospinning such as large volume of solutions and clogging of needles can be overcome using this AFM based electrospinning technique. Lastly, a comparison of electrospun fibers from syringe-needle based electrospinning and AFM probe-tip based electrospinning indicated significant morphological and microstructural differences in the case of AFM based electrospinning.     

Pulsed laser deposition-assisted patterning of aligned carbon nanotubes modified by focused laser beam for efficient field emission

Patterned carbon nanotube (CNT) arrays have been synthesized on patterned substrates created via pulsed laser deposition (PLD) of the precursor catalyst films with a mask. Arrays of CNTs in square and hexagonal patterns with tube lengths of 8 μm and 16 μm were created on silicon or quartz substrates, respectively. Using the method of laser cutting, as-grown CNT patterns were pruned by focused He-Ne laser beam. It is found that after pruning, CNTs tend to cluster together and form welded junctions. The comparison of field emission properties of CNTs before and after pruning shows that laser modification of CNT morphologies effectively enhanced the emission currents.     

Mechanical properties of porous methyl silsesquioxane and nanoclustering silica films using atomic force microscope

Mechanical properties of porous methyl silsesquioxane samples with dielectric constant 2.4 and 2.0 and a recently developed nanoclustering silica film samples with dielectric constants 2.3 and 2.0 were evaluated using an atomic force microscope based nanoindentation. It was found that the Young’s modulus and the hardness decrease while the fracture toughness increases with a decrease in the dielectric constant in the same type of material. Moreover, the Young’s modulus and the hardness of the nanoclustering silica films were observed to be at least twice and fracture toughness values ~1.3–1.5 higher than those for methyl silsesquioxane films with similar dielectric constants. The high resolution topographic imaging capability of atomic force microscope was shown to be particularly useful in the measurement of cracks generated by the ultra-low indentation loads, and the evaluation of the fracture toughness of the nanoscale volumes of materials.     

Apparent contrast reversal in tapping mode atomic force microscope images on films of polystyrene-b-polyisoprene-b-polystyrene

Summary Thin films of phase separated polystyrene-b-polyisoprene-b-polystyrene block copolymers were studied by tapping mode atomic force microscopy. The relative contrast in height and phase mode images of the phase separated regions was found to be very sensitive to changes in the operating conditions of the microscope. Contrast variations and reversals were observed for height and phase mode images as a function of the set-point amplitude ratio and drive frequency. No unique height or phase contrast was observed for the the tri-block copolymer system examined in this study. Received: 30 December 1997/Revised version: 14 January 1998/Accepted: 15 January 1998