Contact Mode AFM Research Articles

Grain-dependent anodic dissolution of iron

The influence of different dissolution techniques (electropolishing or chemical polishing and electrochemical machining (ECM)) on the topography of grains and grain boundaries of polycrystalline iron was analyzed by a combination of electron backscatter diffraction (EBSD) and contact-mode AFM. For electrochemical dissolution at large current densities, small electrode areas were addressed by a capillary microcell to specify the influence of grain orientation on the anodic behaviour, especially the dissolution in sodium nitrate solutions.

Lattice-resolution imaging of the sapphire (0 0 0 1) surface in air by AFM

Lattice-resolution images of single-crystal α-alumina (sapphire) (0 0 0 1) surfaces have been obtained using contact-mode AFM under ambient conditions. It was found that the hexagonal surface lattice has a periodicity of 0.47 ± 0.11 nm, which is identical to that reported previously when the same surface was imaged in water. Large lattice corrugations (as high as 1 nm) were observed, but were concluded to be imaging artifacts because of the strong friction which causes additional deflection of the cantilever. The additional deflection of the cantilever is registered by the detector of the optical beam-deflection AFM resulting in an overestimation of the height at each lattice point. Abrupt changes were also resolved in the topography including honeycomb patterns and a transition from 2D lattices to 1D parallel stripes, with scanning direction. These phenomena can be explained by the commensurate sliding between the tip and sapphire surface due to the strong contact force.

Vacuum-ultraviolet photochemically initiated modification of polystyrene surfaces: morphological changes and mechanistic investigations

Vacuum-ultraviolet (VUV) irradiation (lambda(exc): 172 +/- 12 nm) of polystyrene films in the presence of oxygen produced not only oxidatively functionalized surfaces, but generated also morphological changes. Whereas OH- and C=O-functionalized surfaces might be used for e.g. secondary functionalization, enhanced aggregation or printing, processes leading to morphological changes open new possibilities of microstructurization. Series of experiments made under different experimental conditions brought evidence of two different reaction pathways: introduction of OH- and C=O-groups at the polystyrene pathways is mainly due to the reaction of reactive oxygen species (hydroxyl radicals, atomic oxygen, ozone) produced in the gas phase between the VUV-radiation source and the substrate. However, oxidative fragmentation leading to morphological changes, oxidation products of low molecular weight and eventually to mineralization of the organic substrate is initiated by electronic excitation of the polymer leading to C-C-bond homolysis and to a complex oxidation manifold after trapping of the C-centred radicals by molecular oxygen. The pathways of oxidative functionalization or fragmentation could be differentiated by FTIR-ATR analysis of irradiated polystyrene surfaces before and after washing with acetonitrile and microscopic fluorescence analysis of the surfaces secondarily functionalized with the N,N,N-tridodecyl-triaza-triangulenium (TATA) cation. Ozonization of the polystyrene leads to oxidative functionalization of the polymer surface but cannot initiate the fragmentation of the polymer backbone. Oxidative fragmentation is initiated by electronic excitation of the polymer (contact-mode AFM analysis), and evidence of the generation of intermediate C-centred radicals is given e.g. by experiments in the absence of oxygen leading to cross-linking (solubility effects, optical microscopy, friction-mode AFM) and disproportionation (fluorescence).

Atomic force microscopy studies of twins in yttrium-doped barium titanate

Barium titanate is the main constituent of PTC materials and their electric properties are sensitive to microstructure and defects, in atomic scale, that are significantly affected by processing parameters. The microstructure of barium titanate doped with yttrium was investigated using topographic images obtained by AFM in contact mode. The AFM images of barium titanate doped with yttrium showed the effect of large grains with double twins at different (1 1 1) planes.

Research on Field-Induced Oxidation Processing Technology by Contact-Mode AFM in Ambient Air

The mechanism responsible for scanning probe field-induced oxidation in ambient air is attributed to an electrochemical process, i.e., anodic oxidation or anodization,after the analyses is given of a surface of a sample exposed to air. The effects of biases, tip speeds on morphology of field-induced oxidation, are introduced and deduced in the form of kinetics formula of oxidation growth. The field-induced oxidation of hydrogen-passivated Si (Si:H)using contact-mode AFM in air at room temperature is investigated. The result achieved from the experiment and that drawn from theoretical analysis are identical, which indicates the accuracy of the experimental operation.This experiment suggested that it may be used for further investigation of field-induced oxidation technology.

Microscopic Studies of the GC/Poly-NiTMHPP/Nafion Electrochemical Nitric Oxide Sensor

Glassy carbon (GC) discs and platinum microcylindrical electrodes were modified with a layer of nickel(II) tetrakis 3-methoxy-4 hydroxyphenyl porphyrin (poly-TMHPP-Ni) of different loadings (Γ) between 0.6 and 10 nmol/cm2, and subsequently with a Nafion layer. The topography and in some cases the thickness of films were measured by means of contact mode AFM. SEM and optical microscopy observations were used for the comparative studies on a larger scale. The useful range of the coverage with modifying polymer was investigated. The optimal value of loading for the sensor was found to be in the range of 6-8 nmole of poly-TMHPP-Ni per cm2. We observed the Nafion layer uniformity in nanometric scale.

An experimental study of the contact mode AFM scanning capability of polyimide cantilever probes

This paper presents a preliminary exploration of high-speed contact mode performed with a polyimide probe. The probe is batch micromachined by a lithographic manufacturing process. It offers a spring constant of <0.1 N/m, a resonance frequency of about 50 kHz, and a tip diameter of 50–100 nm. The probe is particularly suitable for scanning soft specimens such as biological and polymeric samples. Topographical contact mode imaging at high scanning rates of 48 Hz (1.47 mm/s) has been demonstrated, detecting features <100 nm over a 15 μm scan, yielding >7 bit resolution at 48 Hz. Scanning rates of 16 Hz (0.5 mm/s) have been demonstrated for lateral force imaging with spatial resolution of 100 nm over a 15 μm scan, which translates into >7 bit resolution at 16 Hz. These results suggest that the probe can be used in high throughput applications.

DNA immobilization onto electrochemically functionalized Si(1 0 0) surfaces

N-type Si(1 0 0) surfaces were modified by reduction of 4-nitrobenzenediazonium through cyclic voltammetry. Contact mode AFM was employed to produce holes in the deposited layers and cross-sectional profiles were obtained to determine their thicknesses. Layer thickness was found to increase with the number of cyclic potential scans in both aqueous and non-aqueous media. In acetonitrile, the single scan thickness was determined to be approximately 15 nm, whereas for three scans the layer thickness was found to be approximately 35 nm. These thicknesses were also measured and confirmed by ellipsometry. Both thicknesses are indicative of multilayer formation on the silicon surface. Layers formed in acetonitrile were more uniform and of better quality (without holes), compared to those prepared in water. This type of functionalized surface, after further cyclic voltammetric reduction of the nitro groups and treatment with glutaraldehyde, was then used to immobilize single strand DNA-C 6H 12NH 2 probe sequences for hybridization with complementary DNA sequences. Fluorescein-labeled probe and target oligonucleotide sequences were used to validate the immobilization of the probe layer and hybridization with the complementary sequence. No binding was observed when using a non-complementary sequence as probe.

AFM study of conducting polymer polypyrrole nanoparticles formed by redox enzyme – glucose oxidase – initiated polymerisation

Redox enzyme – glucose oxidase E.C. 1.1.3.4 from Penecillum vitale (GOx) – initiated polypyrrole (Ppy) synthesis was applied for the formation of polypyrrole based nanoparticles. The increase in optical absorbance at λ = 460 nm was exploited for the monitoring of polypyrrole polymerisation process. The shape and size of the formed Ppy nanoparticles was also monitored by means of contact mode AFM. The highest increase in the diameter of the formed Ppy nanoparticles was detected during 15-day period. AFM imaging was performed in contact mode to investigate the shape and flexibility of particles deposited on the SiO 2 and Pt surfaces. Contact mode AFM investigations allowed us to conclude that after drying at 50 °C the formed Ppy particles are more flexibly deposited on the Pt electrode if compared to those deposited on the SiO 2 substrate. The application of well-shaped Ppy nanoparticles in biomedicine, chromatography and bioanalysis may be predicted.

Cell mimetic lateral stabilization of outer cell mimetic bilayer on polymer surfaces by peptide bonding and their blood compatibility

The biological lipid bilayer membranes are stabilized laterally with the help of integral proteins. We have simulated this with an optimized ternary phospholipid/glycolipid/cholesterol system, and stabilized laterally on functionalized poly methyl methacrylate (PMMA) surfaces, using albumin, heparin, and polyethylene glycol as anchors. We have earlier demonstrated the differences due to orientation and packing of the ternary phospholipid monolayers in relation to blood compatibility (Kaladhar and Sharma, Langmuir 2004;20:11115-11122). The structure of albumin is changed here to expose its interior hydrophobic core by treating with organic solvent. The interaction between the hydrophobic core of the albumin molecule and the hydrophobic core of the lipid molecules is confirmed by incorporating the molecule into bilayer membranes. The secondary structure of the membrane incorporated albumin is studied by CD spectral analysis. The structure of the altered albumin molecule contains more beta-sheet as compared to the native albumin. This conformation is also retained in membranes. The partitioning of the different anchors based on its polarity and ionic interactions in the monolayer is studied from the pressure-area (pi-A) isotherm of the lipid monolayers at the air/water interface using Langmuir-Blodgett (LB) trough facility. Such two monolayers are deposited onto the functionalized PMMA surface using LB trough and crosslinked by carbodiimide chemistry. The structure of the deposited bilayer is studied by depth analysis using contact mode AFM in dry conditions. The stabilized bilayer shows stability up to 1 month by contact angle studies. Preliminary blood compatibility studies reveal that the calcification, protein adsorption, as well as blood-cell adhesion is significantly reduced after the surface modification. The reduced adsorption of ions, proteins, and cells to the modified surfaces may be due to the fluidity of the microenvironment along with the contribution of the mobile PEG groups at the surface and the phosphorylcholine groups of the phospholipids. The stability of the anchored bilayer under low shear stress conditions promises that the laterally stabilized supported bilayer system can be used for low shear applications like small diameter vascular graft and modification of biosensors, and so forth.

Synthesis, preparation, and conformation of stimulus-responsive end-grafted poly(methacrylic acid-g-ethylene glycol) layers.

Here we report the formation of stimulus-responsive chemically end-grafted "brush-brushes" by synthesizing, mono thiol(end)-functionalized poly(methacrylic acid--ethylene glycol) or poly(MAA--EG) comb-type graft copolymers a combination of protecting group chemistry and atom transfer radical polymerization using the initiator 2-(2,4-dinitrophenylthio)ethyl 2-bromo-2-methyl propionate. The polymers were synthesized with three different molecular weights (15 k, 17 k and 27 k), PEG side chain graft densities (EG/MAA mole ratio = 2.2, 0.4 and 1.9, respectively), and a PEG molecular weight = 1100 and then chemically end-grafted to gold substrates chemisorption, resulting in molecular separation distances of ∼3-4 nm. pH-Dependent swelling was confirmed to take place gradually above pH 4-5 and quantified by heights measured by contact mode AFM imaging of microcontact printed (µCP) samples. Swelling factors (maximum height/minimum height) were fairly large (3.6-7.3) and a decrease in molecular weight by ∼2× and side chain graft density by ∼4× resulted in a decrease in swelling factor by ∼2×. Layer height normal force for all three polymers measured by contact mode atomic force microscope imaging on µCP samples at pH 9 showed a nonlinearly decreasing relationship and complete compression ∼<2 nm for forces >10 nN. At pH 4, all polymer layers were largely collapsed (heights ∼<4 nm) and incompressible (, heights were independent of normal force).

Formation of Aminosilane-Functionalized Mica for Atomic Force Microscopy Imaging of DNA

Factors affecting the functionalization of mica with aminosilanes, in particular, aminopropyltriethoxysilane (APTES-mica), formed from the vapor phase have been systematically studied. The relative humidity (RH) during vapor deposition has been varied, and postdeposition treatment through baking has been used, as well as the comparison of mono and trifunctionality, to investigate how optimal surfaces for AFM imaging of DNA are formed. It is found that the stability of the APTES layers is a consequence of lateral polymerization and not covalent attachment to the mica substrate. At low RH (<25%), DNA adopts an open, well-resolved conformation, whereas at >25% RH, DNA surface-induced condensation occurs. Contact mode AFM scratching experiments show that two main structures of the silane layer exist at different humidity: a monolayer exists at RH < 25%, and a bilayer structure exists at RH > 25%. Finally, structural changes that these two layer types undergo after baking at 150 degrees C were investigated by AFM and X-ray photoelectron spectroscopy (XPS), and these now prevented DNA from binding to the APTES-mica, except in the presence of Mg(II) ions.

Molecular dimensions of dried glucose oxidase on a Au(1 1 1) surface studied by dynamic mode scanning force microscopy

We have investigated the molecular dimensions of a dried single glucose oxidase (GO) molecule adsorbed on a Au(1 1 1) surface with the UHV non-contact atomic force microscopy (NC-AFM) and tapping mode atomic force microcopy (TMAFM). The smallest air-dried GO particles in a TMAFM-measured size distribution are found to be 10–11 nm wide and 0.3–0.4 nm high. We find each collapsed ellipsoidal feature with a groove in a NC-AFM image, which measured 12 nm × 10 nm × 0.5 nm. The lateral dimensions (12 nm × 10 nm) of the observed feature is close to those of a GO monomer measured by scanning tunneling microscopy (STM) [Quijin et al., 12.2 nm × 8.9 nm as the size of one wing of an opening butterfly (dimer) appeared in a STM image] and by contact mode AFM [Quinto et al., 14 nm × 8 nm]. Our value of the vertical dimension (0.5 nm) is consistent with AFM results and molecular dynamics simulations that suggest a surface-induced complete unfolding, showing the average diameter of amino acid residues.

Structural Characterization of Si0.75Ge0.25 Alloy Layers with Sb/Ge-Mediated Low Temperature-grown Si Buffers

SiGe has become a natural choice for the fabrication of low cost and high performance electronic devices. 2000-Å-thick Si0.75Ge0.25 alloy layers were grown on Si (001) substrate using (1 ML Sb)/(∼10 ML Ge)-mediated low temperature-grown Si (LT-Si) buffers by MBE process. After formation of 1 ML Sb on Si (001) a very thin Ge layer (∼10 ML) was grown just before the growth of LT-Si layer, which may be favorable for the creation of strain relaxing dislocation sites. High-resolution X-ray diffraction was used to determine the residual strain in the Si0.75Ge0.25 alloy layers. From the XRD data, it was seen that the residual strain of the alloy layers decreased with decreasing the growth temperature of the LT-Si buffer layers and reached to a lower value of about -0.06% at 300°C. Contact mode AFM observation of the same sample showed a smooth surface with rms roughness of about 13 Å.

AFM Characterization of Electroactive Polymer Nanocomposites

ABSTRACTAn AFM-based method is used to probe the electromechanical response of a polyimide-SWNT nanocomposite at the nanoscale level. Our previous investigations on bulk polyimide-SWNT nanocomposites using a fiber optic sensor-based method have shown that the nanocomposites are electroactive and exhibit a quadratic relationship between output displacement and input voltage. In the current study, the composites are probed at the nanoscale using the AFM in contact mode. One goal of the current study is to assess the AFM-based technique in measuring our nanocomposites and verifying their actuation. A second goal is to improve the methodology itself by characterizing a commercially available piezoelectric ceramic, PZT, and a commercially available piezoelectric polymer, PVDF. The experiments are conducted in contact mode under the application of a DC voltage. Values for the strain piezoelectric coefficients d33 are widely reported in the literature and consequently permit to calibrate the methodology applied in the present work.These findings, coupled with bulk film characterization will be used to shed light on the polyimide-SWNT interaction and its effect on the electroactive mechanism displayed by the nanocomposites.

Spatial and Temporal Variations in Electronic Transport Through a CdTe-Based Schottky Barrier

AbstractWe study the electric current through metal-semiconductor junctions of a type used in thin-film PV for back contacts. To concentrate on one type of junction we used the symmetric structures of rf-sputtered CdTe layer sandwiched between two Cr contacts. Along with the conventional measurements, the current-sensing contact mode AFM was employed to measure the current-voltage characteristics and current variations with time under fixed voltage. We found that (i) the electric current flow is laterally strongly nonuniform; (ii) it chaotically varies over time; (iii) this behavior did not correlate with surface topography. We interpret our observations in terms of defect assisted tunneling through time-dependent defect pathways.

InP patterning using contact mode and non-contact AFM lithography for quantum dot localization

AFM contact and non-contact mode have been used to oxidize InP(001) surface at the nanoscale in order to create, after oxide etching, nano-holes used as nucleation sites for self-assembled quantum dots (QDs). Results obtained in contact mode are similar to those obtained with the local anodization of silicon, and principally explained by the effect of space charges that occurs during the initial stages of the oxide growth. Using a negative pulse permits the experimenter to reduce the space charge effects in the oxide and thus to enhance both homogeneity and resolution. Non-contact mode coupled with a modulated voltage brings even more specific results. For more than two a decade variation of probe velocity (0.01–5 μm s −1), the AFM oxidation introduces no significant changes in the features. Studies on the influence of oxidation time have identified two regimes. With the first one, a high growth rate was found for oxidation times shorter than 100 ms. Second, for oxidation times longer than 100 ms, we observe an oxide height saturation and a lower lateral growth rate. These results provide a way to control separately both depth and diameter of holes. The achievement of non-contact oxidation confirms the compatibility of this technique in both resolution and homogeneity for QD nucleation sites.

The early stages of silicon surface damage induced by pulsed CO2 laser radiation: an AFM study

The early stages of the surface microstructural modification of silicon, induced by single pulses of CO2 laser irradiation (λ=10.6μm), have been studied, in both vacuum and air, by contact mode AFM. The laser pulse was found to be absorbed at the front surface of the sample, facing the laser; this was shown to be due to the presence of native oxide, which absorbs at this wavelength. We found that this absorption of energy caused the stress-induced formation of vertically oriented, square-shaped fragments, 400–700nm in length, often with short branches, that form a wall around the impact site; they oriented toward the plane of the sample with distance from the impact site, aligning more in the electric field direction of the pulse. In addition, electrically charged, branched fragments were redeposited at the outer extremities of the pulse site.

Properties of titanium oxide synthesized by pulsed metal vacuum arc deposition

In this work, TiO 2 films were deposited on Ti6Al4V and silicon(100) substrates using pulsed metal vacuum arc deposition. The as-deposited samples were annealed at 800 °C for 30 min at a pressure of 8.0×10 −4 Pa. The film composition was determined by XPS, RBS and the surface morphology were studied by AFM. XRD shows that the as-deposited films may consist of anatase, brookite or rutile TiO 2. After annealing at 800 °C for 30 min at a pressure of 8.0×10 −4 Pa, the films become rutile TiO 2(110)(101)(200)(211)(002) and the preferred orientation changes from (110) to (101) and (002) at high oxygen pressure. Thus, it can be concluded that the growth on the (002) and (101) plane parallel to the surface becomes more dominant in samples deposited at higher oxygen partial pressures when annealed at 800 °C for 30 min. XRD and contact mode AFM disclose that the as-deposited films are quite dense and composed of grains 80–100 nm in size, whereas the annealed films have larger grains exceeding 400 nm. The RBS results show that O/Ti ratio of the titanium oxide deposited at different oxygen pressure become nearly the same after annealing. The results suggest that all of the ‘low valence’ titanium oxide phases in the film have been oxidized to be rutile TiO 2, but the crystalline orientation of the annealed films is different. The wear resistance of titanium oxide increased approximately 10-fold compared with Ti6Al4V.

Synthesis and Characterization of Nanostructured BSTO Thin-Films for Microwave Applications

Nanophase synthesis of ferroelectric thin-films of Ba0.6Sr0.4TiO3 (BSTO) was studied systematically for applications in tunable microwave components. Synthesis of nanostructured BSTO was performed using a pulsed-laser deposition system with real-time in-situ process control. The main research goal was to utilize the pulsed laser deposition parameters to control the grain growth for low microwave loss nanostructured BSTO thin-films on crystalline substrates such as LaAlO3. These parameters include the energy density of the laser pulses, wavelength, oxygen partial pressure, distance between the target and the substrate, and the substrate temperature. The nanostructural characterization was performed using XRD, SEM and AFM. Microwave characterization was done using coplanar waveguide lines to characterize the frequency dependent dielectric properties (ϵr and tan δ). BSTO films were grown at the same measured temperature and energy density but in different oxygen ambient pressures from 19 mTorr through 300 mTorr. Using contact mode AFM, the grain size was found to decrease as the oxygen ambient pressure was reduced from 150 mTorr to 38 mTorr. The growth process changed when the pressure was increased above 150 mTorr. Nanocluster structures rather than nanoparticles were found at 225 mTorr. Average grain sizes less than 100 nm were obtained to oxygen pressures below 75 mTorr. The XRD spectra indicate the highly crystalline nature of the film. Microwave measurements, performed between 9–18 GHz, suggest the nano-structured BSTO thin-films on LaAlO3 (LAO) substrates are highly tunable (up to 25%).