Acoustic Imaging Frequency Dynamics of Ferroelectric Domains by Atomic Force Microscopy

We report the acoustic imaging frequency dynamics of ferroelectric domains by low-frequency acoustic probe microscopy based on the commercial atomic force microscopy. It is found that ferroelectric domain could be firstly visualized at lower frequency down to 0.5 kHz by AFM-based acoustic microscopy. The frequency-dependent acoustic signal revealed a strong acoustic response in the frequency range from 7kHz to 10 kHz, and reached maximum at 8.1 kHz. The acoustic contrast mechanism can be ascribed to the different elastic response of ferroelectric microstructures to local elastic stress fields, which is induced by the acoustic wave transmitting in the sample when the piezoelectric transducer is vibrating and exciting acoustic wave under ac electric fields due to normal piezoelectric effects.     

Transient Hole Trapping in Individual GeSi Quantum Dot Grown on Si（001） Studied by Conductive Atomic Force Microscopy

Electrical properties of individual self-assembled GeSi quantum dots grown on Si substrates are investigated by using conductive atomic force microscopy at room temperature. By controlling the bias voltage sweep in a certain fast sweep rate range, a novel current peak is observed in the current-voltage characteristics of the quantum dots. The current peaks are detectable only during the backward voltage sweep immediately after a forward sweep. The current peak position and intensity are found to depend strongly on the voltage sweep conditions. This kind of current-voltage characteristic under fast sweep is very different from the ordinary steady state current behaviour of quantum dots measured previously. trapping in the potential well formed bottom Si substrate. The origin of this phenomenon by the quantum dot sandwiched can be attributed to the transient hole between the native oxide layer and the     

A Thin Liquid Film and Its Effects in an Atomic Force Microscopy Measurement

Recently, it has been observed that a liquid film spreading on a sample surface will significantly distort atomic force microscopy （AFM） measurements. In order to elaborate on the effect, we establish an equation governing the deformation of liquid film under its interaction with the AFM tip and substrate. A key issue is the critical liquid bump height yoc, at which the liquid film jumps to contact the AFM tip. It is found that there are three distinct regimes in the variation of yoc with film thickness H, depending on Hamaker constants of tip, sample and liquid. Noticeably, there is a characteristic thickness H^＊ physically defining what a thin film is; namely, once the film thickness H is the same order as H^＊, the effect of film thickness should be taken into account. The value of H^＊ is dependent on Hamaker constants and liquid surface tension as well as tip radius.     

Determination of the hydrophilic character of membranes by pulsed force mode atomic force microscopy

Hydrophilic polysulphone (PSU) membranes were modified with hydrophilic polyethylene oxide (PEO) to obtain membranes less susceptible to fouling. Pulsed force atomic force microscopy was employed to determine the hydrophilic character of the different membranes and to acquire quantitative values that can be compared easily. This technique proved to be extremely valuable in the characterisation and quantification of membrane hydrophilicity.     

Electronic Structure and Elastic Properties of Ti3AlC from First-Principles Calculations

We perform a first-principles study on the electronic structure and elastic properties of TiaA1C with an antiperovskite structure. The absence of band gap at the Fermi level and the finite value of the density of states at the Fermi energy reveal the metallic behavior of this compound. The elastic constants of Ti3AlC are derived yielding c11 = 356 GPa, c12 = 55 GPa, c44 = 157 GPa. The bulk modulus B, shear modulus G and Young＇s modulus E are determined to be 156, 151 and 342 GPa, respectively. These properties are compared with those of Ti3AlC2 and Ti2AlC with a layered structure in the Ti-Al-C system and FeaAlC with the same antiperovskite structure.     

Influence of the oxidation temperature on the fabrication process of silicon dioxide aperture tips

The thermal oxidation of structured silicon surfaces was successfully used to reproducibly define apertures of approximately 100 nm in silicon dioxide tips at reduced oxidation temperatures. In this paper we theoretically investigate the oxidation process in more detail, describing the rheological behavior of silicon dioxide as a Maxwell fluid with non-linear viscosity. For this purpose numerical calculations of the oxidation process of trench-like silicon structures were performed. Contrary to former assumptions, our theoretical results indicate that oxide-growth retardation is more effective at raised oxidation temperatures. This is experimentally confirmed in the case of trench structures. The more pronounced oxide retardation at elevated temperatures is exploited to obtain apertures in silicon dioxide tips of 60 nm for oxidation temperatures of 1100 °C. Received: 8 April 2002 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-561/804-4136, E-mail: oester@physik.uni-kassel.de     

Quantification issues in the identification of nanoscale regions of homopolymers using modulus measurement via AFM nanoindentation

Since 1989, AFMs have been used to map the nanomechanical properties of surfaces using measurements such as force-distance curves. Quantification of the force and elastic parameters are critical to the nanomechanical analysis and positive identification of materials at the nanoscale, as well as for assessing behaviour at surfaces. In recent years, there have been AFM papers publishing “quantitative” values for the indentation modulus, however, many involved large uncertainties arising from the lack of calibration of key components, the use of manufacturers’ nominal values for these components or the use of incorrect models. This paper addresses the quantification issues in modulus measurement at surfaces for homogeneous materials using force-distance curves and how to do this with sufficient accuracy to identify materials at the nanoscale. We review the available theory and describe two routes to quantitative modulus measurement using both the AFM on its own and the AFM combined with a nanoindenter. The first involves the direct measurement of modulus using a fully calibrated instrument and allows depth analysis. The second uses indirect measurement through calibration by reference materials of known reduced modulus. For depth analysis by this second route, these reference moduli need to be known as a function of depth. We show that, using the second route, an unknown polymer may be analysed using the nanoindenter, its modulus determined and, providing the moduli of the polymers to be identified or distinguished differ by more than 20%, identified with 95% confidence. We recommend that users evaluate a set of reference samples using a traceable nanoindenter via the first route, and then use these to calibrate the AFM by the second route for identification of nano-regions using the AFM.     

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.     

Chemical Synthesis and Characterization of Flaky h-BCN at High Pressure and High Temperature

Hexagonal boron carbonitrogen （h-BCN） compound is synthesized from a mixture of boron powder and CNH compound prepared by pyrolysis of melamine （CaH6N6） under high temperature （1400-1500℃） and high pressure （5.0-5.5 GPa）. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spec- troscopy are used to determine the chemical composition and bonds of the product. The results show that the product has composition of B0.18C0.64N0.16 （near BC4N） and atomic-level hybrid. X-ray diffraction analysis indicates that the powder has a hexagonal network structure. Scanning and transmission electron microscopy results suggest that h-BCN compound morphology is mainly flaky in width about 1 μm and thickness 200nm.     

A Novel Ni/Ag/Pt Ohmic Contact to P-Type GaN for Flip-Chip Light-Emitting Diodes

We present a high-quality Ni/Ag/Pt Ohmic contact to p-type GaN. After the sample is annealed at 500℃ in 02 ambient for 3min, a specific contact resistance as low as 2.6 × 10^-5 Ωcm^2 and an optical reflectivity of 82% at 460 nm are obtained. The Auger electron spectroscopy analysis shows that the Pt layer can improve the surface morphology and thermal reliability of the annealed Ag-based electrode, Ag plays a key role in achieving good ohmic contact due to the outdiffusion of Ga into Ag forming Ga vacancies which increase the hole concentration, while the surface contamination of p-type GaN is reduced by Ni.     

A cryogenic scanning force microscope for the characterization of frozen biological samples

We present the design of a new scanning force microscope specially suited for the investigation of soft matter, particularly biological, in an ultrahigh vacuum. The key point is that the samples are immobilized by shock freezing in order to maintain their native structure before they are introduced into the vacuum system. The vacuum system itself consists of a transfer chamber, which allows an exchange of the cold sample with cryo-electron microscopes, a preparation chamber including a stage for in-situ freeze drying, freeze etching, or freeze fracturing, and the analysis chamber with the microscope. Sample cooling is maintained in all chambers. The microscope is mounted on a commercially available vibration isolation system; a flow cryostat cools the sample to the temperature of liquid nitrogen, while the tip is scanned. Besides measurements on test samples, which demonstrate the imaging capabilities of the instrument, first results on T4-bacteriophages (viruses) are shown. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-6188, E-mail: wiesendanger@physnet.uni-hamburg.de     

Delineation of a p–n junction using dC/dX imagery produced by shear-mode scanning capacitance microscopy

Using a laterally oscillating all-metallic probe, a scanning capacitance microscope (SCM) has been used to yield an image of the spatial derivative of the local capacitance, dC/dX, where C and X are the local capacitance and the axis of the probe tip locus on the sample surface, respectively. Bias fields, except for the ultra-high-frequency fields used for sensing the capacitance, are not necessary to detect the dC/dX signal, which yields an image delineating clearly the depletion region due to the p–n junction. Simultaneously with the dC/dX image, the new SCM can give images of topography and dC/dV if an alternating field V is applied between the probe and sample. Received: 19 March 2001 / Accepted: 22 March 2001 / Published online: 27 June 2001     

Resonant Response of Rectangular/kFM Cantilever in Liquid

Dynamic characteristics of atomic force microscopy （AFM） cantilevers can be influenced by their working media. We perform an experimental study on the resonant responses of rectangular AFM cantilevers with different sizes immersed in various viscous fluids. The measured resonance frequencies in liquids are used to valldate several theoretical models. Comparison shows the analytical model proposed by Sader [J. Appl. Phys. 84 （1998） 64] can give the best agreement with the experimental results with the maximum relative error nearly 16% for all the cantilevers in different liquids. The ratio between the resonant frequencies in air and water is almost independent of the cantilever length, which is consistent with the theoretical analyses.     

Modification of AFM Tips for Facilitating Picking-up of Nanoparticles

The radius of atomic force microscope (AFM) tip is a key factor that influences nonspecific interactions between AFM tip and nanoparticles. Generally, a tip with larger radius contributes to a higher efficiency of picking up nanoparticles. We provide two methods for modifying the AFM tip: one is to wear a tip apex on a solid substrate and the other is to coat a tip with poly (dimethylsiloxane) (PDMS). Both the approaches can enhance the adhesion force between the tip and nanoparticles by increasing tip radius. The experimental results show that a modified tip, compared to an unmodified one, achieves six-fold efficiency improvement in the capture of targeted colloidal gold nanoparticles.     

Molecular dynamics simulation of AFM studies of a single polymer chain

Single polymer chain force-extension behavior measured by Atomic Force Microscopy (AFM) was interpreted by molecular dynamics (MD) simulation performed by applying a bead-spring (coarse-graining) model in which the bond potential function between adjacent beads is described by a worm-like chain (WLC) model. Simulation results indicate that caution should be applied when interpreting experimental AFM data, because the data vary depending on the point of AFM tip-polymer chain attachment. This approach offers an effective way for eventual analysis of the mechanical behavior of complex polymer networks.     

Effects of Substrate Hydrophobicity/Hydrophilicity on Height Measurement of Individual DNA Molecules

Effects of substrate hydrophobicity/hydrophilicity on height measurement of individual ds-DNA molecules are investigated with tapping mode atomic force microscopy （TMAFM） and vibrating mode scanning polarization force microscopy （VSPFM）. The measured heights of ds-DNA on hydrophobic highly oriented pyrolytic graphite （HOPG） are remarkably less than those on hydrophilic bare mica and Ni^2＋ treated mica in both TMAFM and VSPFM. By analysing the results, we propose that the hydrophobicity/hydrophilicity of substrate can greatly influence the height measurement of DNA molecules.     

Etch Damage Evaluation in Integrated Ferroelectric Capacitor Side Wall by Piezoresponse Force Microscopy

The etch damage in integrated ferroelectric capacitors side wall fabricated by the typical integrated process (TIP-FeCAP) and the innovated integrated process (IIP-FeCAP) are investigated by piezoresponse force microscopy (PFM). The IIP-FeCAP side wall exhibits fine and clear nanoscale domain images and the same piezoresponse signal as the thin film, and the domains can also be easily switched by an external voltage. In the TIP-FeCAP side wall, owing to the effect of etch damage, the very weak piezoresponse signal and some discrete domains can be observed, and the discrete domains cannot be switched by the applied 9V and -9V dc voltage. The PFM results reflect the etch damage in the integrated ferroelectric capacitors and also suggest that the PFM can be used as an efficacious tools to evaluate the etch damage at nanoscale and spatial variations.     

Characterization of gaseous phase and nanoparticles produced in ultra-short pulsed laser ablation of transition metal borides

The plasma produced by ultra-short laser ablation of ZrB₂ and ReB₂ has been studied by ICCD imaging and time and space resolved optical emission spectroscopy. The aim was to clarify the mechanism of deposition leading to the morphology and composition found in the deposited films. The results indicate that for all systems the film characteristics are compatible with a deposition mechanism involving a growth from nanoparticles, ejected directly from the target, whose composition can be interpreted in terms of equilibrium vaporization during the flight from the target to the substrate.     

Tribological behavior of diamond-like carbon film with different tribo-pairs: A size effect study

A friction force microscope (FFM) with different probes and a ball-on-disk (BOD) tribo-meter were used to investigate the tribological properties of diamond-like carbon (DLC) films. DLC films were prepared by chemical vapor deposition (CVD) method by altering the deposition parameters, and their morphologies and structural information were examined with an atomic force microscope (AFM) and the Raman spectrum. The wear traces of the DLC films after frictional tests were analyzed by an optical microscope. It is found that surface roughness and adhesion play important roles in characterizing the tribological properties of DLC films using FFM. Moreover, the debris accumulation is another significant factor affecting the frictional behavior of DLC films, especially for the sharp tip. The difference in coefficients of friction (COFs) obtained by the BOD method among different DLC films under water lubrication is much smaller than the case without water lubrication. The variation trends in COF for the flat tip and the BOD test are similar in comparison with the result obtained with the sharp tip. The wear traces after frictional tests suggest that DLC films under water lubrication are prone to be damaged more readily.