Contact Mode Atomic Force Microscopy Research Articles

Analysis of variable scale surface roughness on Si(111): a comparative Brewster angle, ellipsometry and atomic force microscopy investigation

Brewster angle analysis (BAA) and single wavelength ellipsometry (SWE) were used to investigate the optical response of Si(111), exposed to subsequent 40% NH4F etching steps, each persisting for 20 s. During native oxide layer etchback and anisotropic etching of the silicon substrate, ultrasmall variations were accurately observed. Contact mode atomic force microscopy (CM AFM) was performed to describe the scale dependence of roughness analysis and to correlate optical and micro-topographical information. It can be shown that BAA and SWE are of comparable sensitivity with respect to variations in film thickness and surface roughness. Quantitative analysis, however, leads to new insights into the information obtainable from the optical methods. BAA data, i.e. the Brewster angle and the reflectance at the Brewster angle, change each in a unique direction, if a clean and smooth surface slightly deteriorates. Ellipsometry, however, shows more complicated variations of the data depending on the concurrent presence of small and long correlation length surface roughness. A combination of both methods allows separating and quantification of these roughness contributions.

Observation of a transitory structure during porous silicon formation: Stability of Si(1 × 1)–H-terminated surfaces and facets

The divalent dissolution of float zone (FZ) Si(1 1 1) in dilute ammonium fluoride solutions at pH 4 is followed by contact mode atomic force microscopy (AFM). Using a variety of photoelectrochemical preparation conditions rather similar structures are found. The dissolution charges range between 9 and 22 bilayer equivalents in potentiostatic and mixed potentiostatic and chronoamperometric experiments. Mesa-type structures with flat extended terraces (∼200 nm width) surrounded by strongly corrugated areas are observed. Image analysis reveals that most border lines of the mesas can be obtained by drawing lines parallel or perpendicular to a reference border line or by lines being rotated by ±30° with respect to these lines. Accordingly, the shape of the mesas can be constructed from a reduced number of crystallographic orientations, i.e., from {1 1 1}, {1 1 0} and {1 1 3} faces despite its complexity. Since the {1 1 1}, {1 1 0} and {1 1 3} faces can all be electrochemically hydrogen terminated in a (1 × 1) manner, the stability of (1 × 1)–H surfaces is thought to be responsible for inducing the structures.

Sample-and-hold atomic force microscopy for fast operation

We have proposed and demonstrated a novel method for fast imaging in intermittent contact mode atomic force microscopy (AFM) using a sample-and-hold circuit for direct monitoring of cantilever deflection signals. This method enables us to construct a quasi-topographic image from tip heights at moments when the tip taps on a sample surface. As a result, we obtained the tip height image well corresponding to the sample topography at a scanning rate above 30 Hz/line without any other customization on both a cantilever and a piezo scanner in a commercial AFM system.

Local photocurrent detection on InAs wires by conductive AFM

Local photocurrent detection has been performed by contact mode atomic force microscopy (AFM) with a conductive tip under laser illumination on InAs wires on GaAs. We adopted a piezoresistive cantilever deflection sensor to avoid the influence of an extra laser light which is conventionally used in a cantilever deflection sensor system for AFM. In this study, we measured the photocurrent from the InAs wires on a GaAs substrate and investigated its dependences on the wavelength of an incident laser light and on bias voltage.

Prototype of Atomic Force Cantilevered SNOM Based on Through-The-Lens-Type Optical Lever and Polarized Illumination and Detection System

Atomic force cantilevered scanning near-field optical microscopy (ANOM) has been proposed for observing a micromagnetic structure of a recorded magneto-optical (MO) disc. The prototype system has an ability to obtain both atomic force microscopy (AFM) and scanning near-field microscopy (SNOM) images simultaneously. This system has some special functions: (1) it keeps the sample-probe gap constant with atomic force and an AFM cantilever, (2) it generates near-field light from the small aperture formed on the tip of the cantilever, (3) it uses a polarized light in the laser beam illumination and detection systems, and (4) it adjusts laser beams incident on fixed positions of the cantilever in ANOM optics. As an experimental result, we obtained SNOM and contact mode AFM images of a commercial MO disc. We can detect a polarized plane at a minimum angle of less than 0.2 deg. and observe submicron recorded magnetic domains on a 640 MB MO disc.

Electrical damage induced by reactive ion-beam etching of lead-zirconate-titanate thin films

Ion-beam etching of sputtered Pb(Zrx,Ti1−x)O3 (PZT) thin films with x equal to 0.54 grown on Pt∕TiOx∕SiO2∕Si substrates has been performed using pure Ar gas and a varying CHF3∕Ar gas mixing ratio. The etch rate dependence on the process parameters (gas composition, current density, and acceleration voltage) has been investigated. PZT etch rate under 40% CHF3 in Ar can reach 100nm∕min with an acceleration voltage of 900 V and a current density of 0.7mA∕cm2 (in comparison to 35nm∕min in pure Ar). A selectivity ratio of 8 has been obtained between PZT and photoresist (1.3 in pure Ar). We have evaluated the PZT surface damage by contact mode atomic force microscopy. It appears that the roughness increases less under a gas mixture than under a pure argon beam, and that the preferential etching observed at the grain boundaries under a pure argon beam disappears when we increase the proportion of CHF3 in the gas mixture. For some etching parameters (current density, acceleration voltage, and gas mixing ratio), we have observed electrical damage. C(V) and hysteresis loops P(E) measurements before and after etching have demonstrated these degradations. We have noted a large decrease of the permittivity after the etching process, independently of the current density and the acceleration voltage. The ferroelectric damage was illustrated by a large increase of the average coercive field after etching in pure argon. The presence of CHF3 in the plasma partially reduces the damage.

Imaging atomically sharp crack tips in mica by contact mode AFM under ambient conditions

Using an Atomic Force Microscope (AFM) operated in contact mode under ambient conditions, we have tried to resolve atomically sharp cracks perpendicular to the basal plane of muscovite mica. However, although tip radius of probes of less than 10 nm allowed a lateral resolution of 0.7 nm and the tip-sample adhesive force has been minimized, our experimental results demonstrate that the molecular structure of crack tips can't still be resolved. Even worse, sometimes the tip-sample friction can disrupt the region around the crack tip. This thus leads to the conclusion that one will fail to resolve atomically sharp cracks under ambient conditions by contact AFM. The inability is due to low lateral resolution of contact AFM and strong tip-sample interaction force under ambient conditions. New techniques of scanning samples in liquids and UHV non-contact AFM both of which have true atomic resolution are strongly recommended for future studies.

Atomic force microscopy investigation of chemically stabilized pericardium tissue

Native and chemically stabilized porcine pericardium tissue was imaged by the contact mode atomic force microscopy (AFM), in air. Chemically stabilized pericardium is used as a tissue-derived biomaterial in various fields of the reconstructive and replacement surgery. Collagen type I is the main component of the fibrous layer of the pericardium tissue. In this study, the surface topography of collagen fibrils in their native state in tissue and after chemical stabilization with different cross-linking reagents: glutaraldehyde (GA), dimethyl suberimidate (DMS) and tannic acid (TA) was investigated. It has been found that chemical stabilization causes considerable changes in the surface topography of collagen fibrils as well as in the spatial organization of the fibrils within the tissue. The observed changes in the D-spacing pattern of the collagen fibril correspond to the formation of intrafibrilar cross-links, whereas formation of interfibrilar cross-links is mainly responsible for the observed tangled spatial arrangement of fibrils and crimp structure of the tissue surface. The crimp structure was distinctly seen for the GA cross-linked tissue. Surface heterogeneity of the cross-linking process was observed for the DMS-stabilized tissue. SDS-PAGE electrophoresis was performed in order to evaluate the stabilization effect of the tissues treated with the cross-linking reagents. It has been found that stabilization with DMS, GA or TA enhances significantly the tissue resistance to SDS/NaCl extraction. The relation between the tissue stability and changes in the topography of the tissue surface was interpreted in terms of different nature of cross-links formed by DMS, GA and TA with collagen.

Structural Evidence for α-Synuclein Fibrils Using <italic>in Situ</italic> Atomic Force Microscopy

Human alpha-synuclein is a presynaptic terminal protein and can form insoluble fibrils that are believed to play an important role in the pathogenesis of several neurodegenerative diseases such as Parkinson's disease, dementia with Lewy bodies and Lewy body variant of Alzheimer's disease. In this paper, in situ atomic force microscopy has been used to study the structural properties of alpha-synuclein fibrils in solution using two different atomic force microscopy imaging modes: tapping mode and contact mode. In the in situ contact mode atomic force microscopy experiments alpha-synuclein fibrils quickly broke into fragments, and a similar phenomenon was found using tapping mode atomic force microscopy in which alpha-synuclein fibrils were incubated with guanidine hydrochloride (0.6 M). The alpha-synuclein fibrils kept their original filamentous topography for over 1 h in the in situ tapping mode atomic force microscopy experiments. The present results provide indirect evidence on how beta-sheets assemble into alpha-synuclein fibrils on a nanometer scale.

The creation of Au nanoscale surface patterns by the low energy Ar + beam irradiation of Au clusters evaporated onto a SiO2/Si surface

Ex-situ contact-mode atomic force microscopy (C-AFM) was used to study the evolution of the topography of nanoscale Au clusters, deposited onto SiO2/Si surfaces, as a function of Ar+ beam irradiation. Various nanoscale Au surface patterns were created, at an ion beam energy of 2.5 keV, by controlling the ion beam conditions. These patterns include ordered or disordered spheres as well as a nanoporous metal layer. We found that the original (∼ 4 nm) clusters coalesced, under the ion beam, to form both larger clusters (∼ 9 nm) and a nanoporous interphase layer (∼ 6.5 nm). Under continued irradiation, both the original clusters and the nanoporous layer were absorbed into the larger clusters.

Time-resolved electrical measurements of a pulsed-dc methane discharge used in diamond-like carbon films production

Amorphous hydrogenated carbon (a-C:H) thin films were obtained at room temperature via asymmetric bipolar pulsed-dc methane glow discharge. The power frequency values were varied from 100 to 200 kHz and the maximum amplitude voltage from −600 to −1400 V. Such films present diamond-like carbon (DLC) properties [J.L. Andújar, M. Vives, C. Corbella, E. Bertran, Diamond Relat. Mater. 12 (2003) 98]. The plasma, powered by a pulse frequency of 100 kHz, was electrically studied by a Langmuir probe. The next parameters were calculated within the pulse cycle from I–V measurements with 1 μs resolution: plasma and floating potentials, electron temperature, and electron and ion densities. The presence of a population of hot electrons (10 eV) was detected at high bias voltage region. The density of cold electrons grows one order of magnitude after each negative pulse, whereas the ion density suffers a prompt increase during each positive pulse. The surface topography of DLC films was scanned by atomic force microscopy (AFM). A smoothly varying friction coefficient (between 0.2 and 0.3) was measured by AFM in contact mode. X-ray reflectivity (XRR) analysis provided a wide characterization of the films, involving density, thickness and roughness. The C/H ratio, as directly obtained by elemental analysis (EA), shows an increase at higher bias voltages. All these features are discussed in terms of process parameters varied in film growth.

Dimethyl suberimidate cross-linked pericardium tissue: Raman spectroscopic and atomic force microscopy investigations

Chemically stabilized pericardium tissue is widely used as a tissue-derived biomaterial for the preparation of bioprostheses such as heart valves or vascular grafts. The bifunctional imidoester dimethyl suberimidate (DMS) belongs to the wide class of the cross-linking reagents and is often used to cross-link a variety of proteins, including collagen matrices and collagen-based tissues. Raman spectroscopy in the wide frequency range 200–4000 cm −1 and contact mode atomic force microscopy (AFM) have been employed to investigate the structural changes and chemical bonds in DMS cross-linked porcine pericardium tissue. It has been found, that in addition to the commonly accepted reaction with the ε-amine groups of lysine or hydroxylysine residues, DMS may interact also with the carbonyl CO and amide NH groups of the peptide bond in collagen. Our paper presents for the first time spectral evidence for the peptide contribution to the formation of DMS–collagen cross-links. The results confirm also possible competition between the hydrolysis of the free imidoester group and cross-linking reaction. Products of the partial alkaline hydrolysis of DMS have been found in the spectra. The observed changes in the surface topography of the fibrils as well as in their spatial organization in the tissue support the formation of both intra- and interfibrillar cross-links in DMS-stabilized tissue.

Contact force identification using the subharmonic resonance of a contact-mode atomicforce microscopy

We propose a step-by-step experimental procedure for characterization of the nonlinearcontact stiffness on surfaces using contact-mode atomic force microscopy. Our approachdirectly estimates the first-, second-, and third-order coefficients of the contact stiffness. Itneither uses nor requires the underlying assumptions of the Hertzian contact theory. Weuse a primary resonance excitation of the probe to estimate the linear coefficient of thecontact stiffness. We use the method of multiple scales to obtain closed-form expressionsapproximating the response of the probe to a subharmonic resonance excitation of orderone-half. We utilize these expressions and higher-order spectral measurements toindependently estimate the quadratic and cubic coefficients of the contact stiffness.

Atomic Force Microscopy Contact Mode Study on Ultra High Molecular Weight Polyethylene

AbstractThis paper presents the results of contact mode atomic force microscopy (AFM) study on the nanoscale Young's modulus and work of adhesion of ultra high molecular weight polyethylene (UHMWPE). Cryoultramicrotomed surfaces of UHMWPE were scanned using the contact mode of AFM. Fibril regions are commonly found on the sample, however, a non-fibril particulate region was also found. AFM force displacement curves were obtained for the sample. The JKR theory and Maugis Dugdale model were used for the analysis. A good fitting between the theories and experimental data was found. The nanoscale Young's modulus and work of adhesion of UHMWPE extracted from the experimental data were in reasonably good agreement with the values reported in other literatures.

Thermal-induced normal grain growth mechanism in LPCVD polysilicon film

Normal grain growth mechanism due to high-temperature annealing of the polysilicon film has been shown experimentally while observing the limiting grain size to the order of the polysilicon film thickness. Polycrystalline silicon film shows grain size variation under the influence of thermal annealing. Low-voltage contact mode atomic force microscopy (AFM) has been employed to analyze the morphology of the annealed polysilicon film at four relevant different temperatures. A systematic grain growth leads to a stabilized grain size, due to clustering of grains under the influence of associated surface energy.

Effect of contact potential barrier of organic resists on atomic force microscope anodization lithography

The local oxidation on Si substrate has been studied by atomic force microscope lithography. Heights of protruded patterns were changed during the lithographic process with thin films of 2-amino-6-methoxybenzothiazole-azo (MBT-A) and 2-amino-6-methoxybenzothiazol-azo-Ni ([MBT-A] 2Ni 2+) on Si substrates. The current-value in a tip–sample junction was investigated by using scanning tunneling spectroscopy with a contact mode atomic force microscope (AFM), and it was confirmed that a change of current-values depends on applied voltages. The difference of potential barrier between [MBT-A] 2Ni 2+ and MBT-A was also confirmed by using UV–vis spectrophotometry and ultraviolet photoelectron spectroscopy. The tunneling current value of a [MBT-A] 2Ni 2+ film was larger than that of MBT-A film and the difference from threshold voltages was also observed.

Vesicle Adsorption and Lipid Bilayer Formation on Glass Studied by Atomic Force Microscopy

The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.

Microscale chemical and electrostatic surface patterning of Dow Cyclotene by N 2 plasma

Using TEM grids as masks, we have chemically modified selected areas of the surface of Dow Cyclotene, a low permittivity polymer, by a N 2 plasma (chemical surface patterning), grafting a maximum of ∼3% N; this was verified by XPS (X-ray photoelectron spectroscopy) and TOF-S-SIMS (time-of-flight static secondary ion mass spectrometry) chemical imaging. Contact mode AFM (atomic force microscopy) studies of the modified surface morphology show unexpected, initially large, values of both etch depth and friction in the treated areas, which decrease on exposure to atmosphere; similar results were absent in tapping mode images. When Cu, which forms nanoclusters on Cyclotene, was deposited by evaporation onto freshly etched Cyclotene, the large etch depth and friction in the etched areas decreased to much lower values. The depth and friction differences occurring on surface modification, which were revealed through our use of patterning, are apparent, and are, in fact, caused by enhanced electrostatic interaction of the chemically modified surface with the AFM tip, as confirmed by the tapping mode data. Some of the electrostatic surface charge, introduced by the positively charged species chemically modifying the Cyclotene surface, is reduced by subsequent charge neutralization. XPS has shown this to be due to the oxidation of these surface charges on atmospheric exposure, initially ∼70%, to form alcohol, carbonyl and carboxylic acid groups. Contact mode AFM imaging of plasma-patterned surfaces is revealed as an excellent tool for the high-resolution characterization of such surfaces.

Dynamical properties of the Q-controlled atomic force microscope

In intermittent contact mode atomic force microscopy (AFM), the quality factor (Q) of the oscillating probe is believed to account for the imaging speed and sensitivity. Q control is a method to artificially modify the quality factor of the probe. Here, we present a comprehensive study of the dynamics of the Q-controlled AFM. By comparing the analytical solutions of the force equations, we prove that the Q-controlled and non-Q-controlled systems are equivalent in the absence of surface forces. We also determine the conditions for the numerical simulation. In order to study the mechanism of contrast enhancement, we simulate the normal AFM operation including the surface forces. We found that there is a maximal probe sensitivity which cannot be exceeded even with Q control. Consistently, Q control enhances sensitivity only when imaging soft samples. Finally, we show that the phase signal of the Q-controlled system is more sensitive to the changes of the sample properties than in case of non-Q-controlled AFMs.

Hydrogel Microspheres: Influence of Chemical Composition on Surface Morphology, Local Elastic Properties, and Bulk Mechanical Characteristics

Hydrogel microspheres, beads, and capsules of uniform size, differing in their chemical composition, have been prepared by electrostatic complex formation of sodium alginate with divalent cations and polycations. These have served as model spheres to study the influence of the chemical composition on both surface characteristics and bulk mechanical properties. Resistance to compression experiments yielding the compression work clearly identified differences as a function of the composition, with forces at maximal compression in the range of 34-455 mN. The suitability and informative value of atomic force microscopy have been confirmed for the case where surface characterization is performed in a liquid environment equivalent to physiological conditions. Surface imaging and mechanical response to indentation revealed different average surface roughness and Young's moduli for all hydrogel types ranging from 0.9 to 14.4 nm and from 0.4 to 440 kPa, respectively. The hydrogels exhibited pure elastic behavior. Despite a relatively high standard deviation, resulting from both surface and batch heterogeneity, nonoverlapping ranges of Young's moduli were reproducibly identified for the selected model spheres. The findings indicate the reliability of contact mode atomic force microscopy to quantify local surface properties, which may have an impact on the biocompatibility of alginate-based hydrogel materials of different composition and conditions of preparation. Moreover, it seems that local elastic properties and bulk mechanical characteristics are subject to analogous composition influences.