Applied Surface Science (v.188, #3-4)

CONTENTS (ix-xi).

by Masaru Tsukada (231).

Atomic corrugation in nc-AFM of alkali halides by R. Bennewitz; O. Pfeiffer; S. Schär; V. Barwich; E. Meyer; L.N. Kantorovich (232-237).
The atomic corrugation of alkali halides measured by non-contact force microscopy undergoes strong variations at low-coordinated sites like steps. We present experimental results on structured KBr surfaces and discuss the contrast mechanisms. Chemical sensitivity of the atomic corrugation is demonstrated for a mixed alkali halide crystal.
Keywords: Alkali halides; Atomic force microscopy; Atomic resolution; Nanostructures; Radiation damage; Mixed crystals;

Atomic resolution imaging and force versus distance measurements on KBr (0 0 1) using low temperature scanning force microscopy by R. Hoffmann; M.A. Lantz; H.J. Hug; P.J.A. van Schendel; P. Kappenberger; S. Martin; A. Baratoff; H.-J. Güntherodt (238-244).
Atomic scale dynamic scanning force microscopy images and force versus distance measurements at low temperatures on a cleaved KBr (0 0 1) single crystal are reported. Two distinct forms of atomically resolved contrast were observed. In one case, a nanotip was formed through a tip change. In this case, a strong corrugation of 0.07 nm was measured. It was possible to reverse this tip change intentionally. In the second case, the observed contrast was only 0.025 nm. The force–distance measurements are well modelled with a van der Waals force in the distance range of 0.5–15 nm. The residual forces at smaller tip–sample distances show a maximum attraction of 0.3 nN and decay within 0.2 nm.
Keywords: Alkali halide surfaces; Low temperature scanning force microscopy; Interaction forces;

Dynamic force microscopy with atomic resolution at low temperatures by Alexander Schwarz; Udo D. Schwarz; Shenja Langkat; Hendrik Hölscher; Wolf Allers; Roland Wiesendanger (245-251).
In this paper, we review some of the most important results obtained with our low-temperature force microscope operated in ultrahigh vacuum. In particular, we stress the resolution capabilities on the atomic scale. After describing some recent modifications of our earlier published setup, we first compare quasi-atomic resolution in the contact mode with true-atomic resolution in the non-contact mode on graphite. On xenon, we demonstrate that weak Van der Waals interactions are sufficient to achieve atomic resolution. Thereafter, atomic scale contrast with ferromagnetic tips on nickel oxide, an insulating antiferromagnet, is discussed with respect to recent theoretical calculations regarding the detection of exchange forces. Finally, tip-induced relaxation is visualized by imaging a point defect on indium arsenide at different tip–sample distances.
Keywords: Low-temperature force microscopy; Atomic resolution; HOPG; Van der Waals crystal; Exchange force; Tip-induced relaxation;

Atomic-scale structures of CeO2(1 1 1) were examined by noncontact atomic force microscopy (NC-AFM). Hexagonally arranged surface oxygen atoms at an ideally oxygen-terminated surface, oxygen point vacancies, and multiple oxygen vacancies such as triangular defects and line defects formed depending on different oxidation states of CeO2−x were visualized by NC-AFM. These observations give a new insight to understand the special oxygen storage capacity with facile oxidation–reduction cycles for CeO2−x relevant to automotive exhaust catalysis.
Keywords: Noncontact atomic force microscopy; Cerium dioxide; Surface oxygen defect; Surface diffusion; Catalysis;

Molecule-dependent topography determined by noncontact atomic force microscopy: carboxylates on TiO2(1 1 0) by Hiroshi Onishi; Akira Sasahara; Hiroshi Uetsuka; Taka-aki Ishibashi (257-264).
The feasibility of noncontact atomic force microscopy (NC-AFM) in single-molecule analysis is experimentally demonstrated. Carboxylates (RCOO) with different R’s were identified molecule-by-molecule on the TiO2(1 1 0) substrate. The constant frequency-shift topography exhibited a good correlation with the physical topography of the carboxylates with RH, CH3, C(CH3)3 and CCH. The molecule-dependent topography was interpreted with the van der Waals force pulling the tip into the adsorbates. The constant frequency-shift topography of a series of fluorine-containing acetates (RCH3, CHF2, and CF3) revealed that the intramolecular electric polarization perturbs the tip–molecule force through electrostatic couplings.
Keywords: Atomic force microscopy; Surface structure; Morphology; Chemisorption; Carboxylic acid; Titanium oxide; Low index single crystal surfaces; Semiconducting surfaces;

A needle-like organic molecule imaged by noncontact atomic force microscopy by Akira Sasahara; Hiroshi Uetsuka; Taka-aki Ishibashi; Hiroshi Onishi (265-271).
We present a noncontact atomic force microscopy (NC-AFM) image of a mixed monolayer composed of propiolate (HCCCOO) and formate (HCOO) molecules prepared on a TiO2(1 1 0)-(1×1) surface. An isolated former molecule can be regarded as a needle with a single-atom diameter perpendicular to the surface. The height difference between the propiolate and formate in the NC-AFM image was smaller than the physical height difference. A simple simulation that employed van der Waals forces between the tip and monolayer-covered surface reproduced the experimental topography.
Keywords: Atomic force microscopy; Titanium oxide; Carboxylic acid; Low index single crystal surfaces; Chemisorption; Surface structure, morphology, roughness, and topography; Semiconducting surfaces;

Atomic resolution imaging of Si(1 0 0)1×1:2H dihydride surface with noncontact atomic force microscopy (NC-AFM) by S. Araragi; A. Yoshimoto; N. Nakata; Y. Sugawara; S. Morita (272-278).
We investigated the Si(1 0 0)1×1:2H dihydride surface using the noncontact atomic force microscopy (NC-AFM). NC-AFM images showed that the pattern of the dihydride surface changed depending on the distance between the tip and the sample surface. The image with 1×1 structure appeared when the tip was a little far from the sample surface. When the tip became close to the surface, 2×1 structure where the bright lines and the dark lines were alternately located appeared. This 2×1 image was stable for retracting the tip from the surface. Furthermore, when the tip became close to the surface, 1×1 structure reappeared. It turned out that the structures of the dihydride surface changed due to the attractive force between the tip and the sample under NC-AFM measurement.
Keywords: Noncontact atomic force microscopy; Hydrogen; Si(1 0 0)1×1:2H dihydride surface; Tip–sample distance dependence; The attractive force; The repulsive force;

Observation of Si(1 0 0) surface with noncontact atomic force microscope at 5 K by T. Uozumi; Y. Tomiyoshi; N. Suehira; Y. Sugawara; S. Morita (279-284).
We observed atomic resolution images of the Si(1 0 0) surface at 5 K using low-temperature noncontact atomic force microscope (LT-NC-AFM). Si(1 0 0) surface shows almost the asymmetric dimers structure, except for the symmetric dimers structure near the defects. The asymmetric dimers are mainly c(4×2) structure with antiparallel zigzag pattern and partly p(2×2) structure with parallel zigzag pattern.
Keywords: Si(1 0 0); Noncontact atomic force microscope; Low temperature atomic force microscope; Frequency modulation technique;

We investigate the capability of single atom manipulation using the noncontact AFM operating low temperatures. Here, for the first step of atom manipulation, we try to perform the vertical manipulation. By applying the bias voltage between Si(1 1 1)7×7 surface and the tip of the conductive AFM cantilever, we succeed in removing the adatoms from the surface for the first time. This experimental result opens a new research field with the noncontact AFM. Furthermore, by analyzing the noncontact AFM images measured on Si(1 1 1)7×7 surfaces, we propose a model for the atomic arrangement of the Si tip apex with an asymmetric ad-dimer.
Keywords: Atomic force microscope (AFM); Low temperature; Atom manipulation; Field evaporation; Si(1 0 0); Si(1 1 1);

We have observed a germanium thin film grown on a Si(1 1 1)7×7 surface by noncontact atomic force microscopy (nc-AFM) while simultaneously imaging the damping of cantilever excitation. The Ge layer grown at 450 °C with a coverage of about 0.7 bi-atomic layer (BL) forms islands on the terraces of the Si(1 1 1) substrate and step flows starting from the step edges of the substrate, which partially coalesce each other. The nc-AFM images clearly exhibit 5×5, 7×7 and 9×9 reconstructions, and their domain boundaries of the Ge thin layer on an atomic scale. Simultaneously obtained damping images also show the atomic corrugation with a local minimum over adatoms. Under bias voltages beyond 1 V between a tip and a sample, strongly enhanced bright spots are observed over corner adatoms on the faulted halves of the 5×5 and 7×7 reconstructions. In damping images, a rapid decrease is found over the corner adatoms possibly due to the strong attractive interaction over the corner adatoms.
Keywords: Atomic force microscopy; Noncontact; Si(1 1 1); Germanium; Thin film growth; Damping;

Si(1 0 0)(2×1) surface is imaged using a new non-contact atomic force microscopy (nc-AFM)/STM with sub-Ångström oscillation amplitudes using stiff tungsten levers. Simultaneous force gradient and STM images of individual dimers and atomic scale defects are obtained. We measured force–distance (fd) curves with different tips. Some of the tips show long force interactions, whereas some others resolve short-range interatomic force interactions. We observed that the tips showing short-range force interaction give atomic resolution in force gradient scans. This result suggests that short-range force interactions are responsible for atomic resolution in nc-AFM.
Keywords: Non-contact atomic force microscopy; Small oscillation amplitudes; Si(1 0 0)(2×1); Force–distance spectroscopy; Short-range forces;

Quantitative modelling in scanning force microscopy on insulators by Adam S. Foster; Alexander L. Shluger; Risto M. Nieminen (306-318).
We analyse the mechanisms of contrast formation in non-contact atomic force microscopy (NC-AFM) on insulators and use comparisons of theoretical models with experiment to establish tip and surface properties. The results for the CaF2 (1 1 1) surface provide information about the character of the tip–surface interaction, tip sharpness and electrostatic potential, and the distance range of imaging. We analyse whether knowledge of the electrostatic potential at the imaging distance is enough to interpret the image and show that account of the surface deformation is crucial for quantitative understanding of the image contrast. Then we turn to a more complex CaCO3 (1 0 1 4) surface, which has a complex anion. We demonstrate that with an ionic tip the atomic structure of the CO3 2− group cannot be resolved, and we also study the dependence of imaging on the tip orientation. Again, the surface deformation induced by the tip during scanning plays a crucial role in image contrast. We argue that if the relation between the tip structures, potential and observed image could be uniquely established for some systems, these systems could serve as a reference for tip characterisation in further studies.
Keywords: AFM; Insulators; Surfaces; Modelling; Structure; Tip–surface interaction;

STM–AFM image formation on TiO2(1 1 0) 1×1 and 1×2 surfaces by S.H. Ke; T. Uda; K. Terakura (319-324).
Ultra-soft pseudopotentials plane-wave technique is used to simulate the tip–sample interaction and STM–AFM image formation on TiO2(1 1 0) 1×1 and 1×2 surfaces. It is shown that the strong tip–oxygen chemical interaction determines the main feature of the AFM image formation. By checking the effects of tip-induced surface relaxation, spin polarization, and bias voltage, we show that it should be the significant tip-induced surface relaxation on the 1×1 surface which leads to the too small image corrugation observed experimentally on the 1×1 surface.
Keywords: Non-contact atomic-force microscope; Tip–sample interaction; Image formation;

We present ab initio simulations of AFM image formation in the non-contact regime for prototypical reactive semiconductor and metal surfaces: InP(1 1 0)-1×1 and Cu(0 0 1). For the reactive surface the effect of tip morphology of the tip apex was also studied. The nature of the tip apex alters the local tip reactivity and can lead to reversal of the apparent AFM surface corrugation. We find that for both semiconductor and metal surfaces the atomic resolution is primarily mediated by a strong chemical-type of interaction between the tip and the surface. This allows for a unified interpretation of the tip–surface interactions in the non-contact AFM microscopy.
Keywords: Non-contact AFM; Tip–surface interactions; Density functional calculations;

Theoretical study on atomic and electronic structures of Ag-adsorbed Si NC-AFM tips by Yuji Motoda; Naruo Sasaki; Satoshi Watanabe (331-334).
We have studied the atomic and electronic structures of Ag-adsorbed Si NC-AFM tips using cluster models and ab initio calculations within the local spin density functional approach as the first step to clarify the tip effects on NC-AFM images. On the contrary to simple guess that the most stable position for a Ag adsorbate is just below the apex Si atom, the most favorable position was found where the Ag adsorbate bonds to both the apex and one of the second-layer Si atoms. In these two positions, the distributions of the highest occupied molecular orbitals are distinctly different from each other. Further, we have found that the energy barrier for transition between the two positions is only a few tenths of eV. This suggests that the Ag adsorbate may drift within a wide area around the tip apex at room temperature.
Keywords: Noncontact atomic force microscopy; DFT; Silicon; Silver; Tip structure;

The purpose of this paper is to carry out a theoretical analysis of the effect of temperature on atomic force microscopy images using a model of a single-atom tip on the substrate. Assuming that the effect of temperature arises from the change in phonon distribution of the substrate, we first formulate the force acting on the tip in terms of the Debye–Waller factor (DWF) within a single phonon process. Next we calculate the force acting on the tip from substrate lattice with several current types of the potential for interaction between the tip and substrate. The types of potential are: (1) Gaussian; (2) exponential; (3) Morse; (4) Lennard–Jones. The general results are: (1) the effects of temperature increase with increasing temperature; (2) at low temperatures, the effects of zero-point motion play an important role.
Keywords: Temperature effects; Debye–Waller factor; Model calculations; Computer simulations; Surface thermodynamics;

Stability analysis of an oscillating tip–cantilever system in NC-AFM by G. Couturier; L. Nony; R. Boisgard; J.-P. Aimé (341-348).
This paper is a theoretical and a numerical investigation of the stability of a tip–cantilever system used in noncontact atomic force microscopy (NC-AFM) when it oscillates close to a surface. No additional dissipative force is considered. The theoretical approach is based on a variational method exploiting a coarse grained operation that gives the temporal dependence of the nonlinear-coupled equations of motion in amplitude and phase of the oscillator. Stability criterions for the resonance peak are deduced and predict a stable behavior of the oscillator in the vicinity of the resonance. The numerical approach is based on the results obtained with a virtual NC-AFM developed by our group. The effect of the size of the stable domain in phase is investigated. These results are in particularly good agreement with the theoretical predictions. Also they show the influence of the phase shifter in the feedback loop and the way it can affect the damping signal.
Keywords: Non-linear dynamics; Dynamic force microscopy; Stability analysis;

Simulation of fluctuation and dissipation in dynamic force microscopy by H. Nanjo; L. Nony; M. Yoneya; N. Sanada; T. Iijima; J.P. Aimé (349-354).
We have simulated three possible effects of the driving force fluctuation of the cantilever, fluctuation of the surface location and contribution of an additional dissipation due to the sample, with the hope to identify effective methods of improvement of the achievable resolution during a dynamic force microscopy experiment. This study is performed through numerical simulations of the approach–retract curves. We find that in the case of soft materials, the driving force fluctuation has only a small effect on the amplitude of the cantilever oscillations, while surface fluctuation can markedly decrease the maximum amplitude. We also show that dissipation has a larger effect on the phase of oscillator than on its amplitude.
Keywords: Tapping; Oscillation; Fluctuation; Dissipation; Non-contact; Atomic force microscopy;

Dynamic atomic force microscopy using the frequency modulation technique is investigated for the case that the excitation amplitude is kept constant. This mode of operation has very unique properties. A computer simulation is used to investigate the distance dependence of the measurement signals frequency and amplitude using various conservative and non-conservative interaction forces. It is shown how the two measurement channels are interlinked and influenced by both conservative and dissipative interactions. Further, discontinuous frequency shift versus distance curves as reported in the literature were not obtained.
Keywords: AFM; Frequency modulation; Constant excitation; Dynamic force spectroscopy;

This work contains a short overview on two phenomenological models aiming to evaluate the loss of energy when a nanotip oscillates at the proximity of a surface. In one case, the surface is described as a continuous medium with a local stiffness k s and a local damping term γ s. The viscoelastic properties allows an easy evaluation of the energy loss as a function of the surface relaxation times τ s=γ s/k s. The main purpose of the second approach is to evaluate the effect of large fluctuations on the dynamic properties of the oscillator. The surface is built of atoms, each of them lying in a symmetric double-well potential and the oscillating nanotip produces a stochastic resonance process. The linear response theory is applied to evaluate fluctuations of the particle corresponding to interwell motions. Then from the amplitude, fluctuation is calculated the loss of energy. The result shows that a stochastic resonance process can increase the amount of dissipate energy of two orders of magnitude at a forcing frequency much smaller than the usual ones corresponding to atom motions.
Keywords: Stochastic resonance process; Relaxation process; Dynamic force microscopy;

Molecular dynamics study of mechanical extension of polyalanine by AFM cantilever by Kensuke Masugata; Atsushi Ikai; Susumu Okazaki (372-376).
A series of molecular dynamics calculations have been performed mimicking the mechanical extension of α-helical polyalanine molecule in water on the AFM. Force profile has been evaluated as a function of extension. In an initial stage of the extension, ∼5%, the force was found to follow the Hooke’s law up to about 140 pN. At the extension of about 10%, however, the restoring force diminishes, indicating the rupture of α-helix. Then, α-helix region and β-strand region separate from each other in one molecule. Further extension causes stepwise breakages of hydrogen bonds to give the wholly β-stranded polyalanine.
Keywords: Atomic force microscope; Polyalanine; Protein; Molecular dynamics; Mechanical extension;

Monte Carlo simulation of pyridine base adsorption on heulandite (0 1 0) by Yasuto Yokoi; Gulnihal Yelken; Yasunori Oumi; Yasunori Kobayashi; Momoji Kubo; Akira Miyamoto; Masaharu Komiyama (377-380).
Adsorption of pyridine base molecules (pyridine and α-, β- and γ-picolines) on a surface of a natural zeolite, heulandite (0 1 0), was examined by Monte Carlo simulations. Two types of adsorption areas were identified on heulandite (0 1 0) bound by surface OH arrays, and each area showed different influence on the adsorption and orientation for pyridine base molecules. The presence of methyl group and its position within the adsorbed molecule also influenced its adsorption characteristics. For pyridine adsorption, molecular dynamics simulation was also performed. The results were compared with existing experimental data obtained through atomic force microscopy.
Keywords: Zeolite; Heulandite (0 1 0) surface; Pyridine; Picoline; Adsorption simulation; Monte Carlo; Molecular dynamics;

We investigate the ‘artifact’ included in Kelvin probe force microscopy (KPFM). We theoretically show that the artifact due to the applied AC field cannot be canceled by conventional KPFM and that the artifact signal can be detected from the force signal through the second harmonic component of AC field. We made an experiment to simultaneously obtain the topography, the spatial distribution of the surface potential and the artifact signal on Si(1 1 1) surface using our noncontact atomic force microscope (NC-AFM). The result indicates that topography includes the artifact signal and that the surface potential image reflects the charge density of Si adatoms. We propose the novel method to completely eliminate the artifact due to the electrostatic force.
Keywords: Noncontact atomic force microscopy; Kelvin probe force microscopy; FM detection; Atomic resolution; Electrostatic force measurement; Ultrahigh vacuum;

Using a surface potential measurement method attached to non-contact atomic force microscope (nc-AFM), electrostatic potential at the step edges of the Si(1 1 1)7×7 surface is measured and found to be higher than that of terrace. The obtained result is opposite to the cases of Au and Cu(1 1 1) surfaces, where work function measurement with scanning tunneling microscopy (STM) revealed reduced work function at the step edges. Atomically resolved images taken in various modes with nc-AFM are also presented.
Keywords: Non-contact atomic force microscope; Surface potential; Scanning tunneling microscopy; Work function;

Local structures and electrical properties of ferroelectric molecular films on a KCl(0 0 1) surface as well as π-conjugated oligothiophene thin films on a metal substrate were investigated by non-contact atomic force microscopy (NC-AFM). The ‘ferroelectric’ molecules, vinylidene fluoride (VDF) oligomers [CF3–(CH2CF2)17–I], with large dipole moment due to the difference in the electron affinity between the fluorine and the hydrogen atoms have recently attracted much interest in terms of the dipole control of the single molecule. NC-AFM investigations of the VDF oligomer thin films revealed the initial stage of the epitaxial growth process on the KCl substrate. Furthermore, Kelvin probe force microscopy (KFM) was used to map the local surface potential of single and double molecular films of oligothiophene molecules deposited on a platinum substrate. The results indicated that the charge transfer at the molecule–substrate interface caused the surface potential of 190 mV on the molecular film against the Pt substrate in a dark environment. Photoinduced potential of the film under ultraviolet light irradiation was also investigated.
Keywords: Non-contact atomic force microscopy (NC-AFM); Organic ferroelectric materials; Kelvin probe force microscopy (KFM); π-Conjugated molecules; Charge transfer; Surface potential;

We have studied the surface potential profile of the doping superlattice by observing the cleaved edge of the doping superlattice sample with the electrostatic force microscope (EFM). The surface potential of the structure is estimated from the two frequency components of the electrostatic force between the EFM probe and the superlattice structure. Lateral resolution of the EFM measurements found to be about 200 Å. 2ω component was found to suggest an appreciable contribution in (∂C/∂z) term which might modify ω component result.
Keywords: Doping superlattice; Electrostatic force microscope (EFM); Surface potential; Scanning probe microscope (SPM); Atomic force microscope (AFM);

Surface potential microscopy for organized molecular systems by Hiroyuki Sugimura; Kazuyuki Hayashi; Nagahiro Saito; Nobuyuki Nakagiri; Osamu Takai (403-410).
Surface potentials of organosilane self-assembled monolayers (SAMs) formed on silicon substrates were measured using Kelvin-probe force microscopy (KFM) employing a SAM formed from n-octadecyltrimethoxysilane [ODS: CH3(CH2)17Si(OCH3)3] as a reference. The reference ODS surface was prepared in a micrometer scale on each of the samples based on a photolithographic technique using vacuum ultra-violet light at 172 nm. Another SAM was prepared on the same sample surface from heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane (fluoroalkylsilane with 17 fluorine atoms, FAS17), 3,3,3-trifluoropropyltrimethoxysilane (fluoroalkylsilane with three fluorine atoms, FAS3), n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) or (chloromethyl)phenyltrimethoxysilane (CMPS). Potentials of the surfaces covered with FAS17-SAM, FAS3-SAM and CMPS-SAM became more negative than ODS–SAM, while the surface covered with AHAPS-SAM showed a more positive surface potential than the reference. The acquired potential contrasts of the regions covered with FAS17, FAS3, AHAPS and CMPS with reference to ODS were −180, −150, +50 and −30 mV, respectively. These results almost agreed with potentials expected from the dipole moments of the corresponding precursor molecules estimated by ab initio molecular orbital calculation.
Keywords: Surface potential; Organosilane self-assembled monolayer; Kelvin-probe force microscopy; MO calculation; Dipole moment; Photolithography;

Investigation on hydrogen annealing effect for various ferroelectric films by electrostatic force microscope by S. Shin; U.H. Pi; D.J. Kim; B.S. Kang; T.W. Noh; Z.G. Khim (411-415).
Scanning probe microscope with a dc bias and an ac modulation signal applied to the probing tip has been quite successful for investigating the characteristics in a sub-micron scale for the high density ferroelectric memory application field. The degradation of ferroelectric films—PbZr0.4Ti0.6O3 (PZT) and Bi3.25La0.75Ti3O12 (BLT)—caused by the hydrogen forming gas annealing is investigated in a microscopic scale by using an electrostatic force microscope (EFM). From the first harmonic signal of EFM, we obtained different polarization behaviors from as-grown and hydrogen-annealed ferroelectric films. We found that the hydrogen forming gas annealing is degrading the ferroelectric film with no catalyst top electrode on top of the film. It is believed that the annealing process causes the diffusion of hydrogen into the ferroelectric film resulting in the destruction of polarization in these materials. We speculate that the different degradation behavior among these materials (PZT and BLT) is due to the different cohesivity of hydrogen in these materials.
Keywords: Hydrogen forming gas annealing; PZT; BLT; Ferroelectric film; EFM;

Constant height imaging using a custom built low temperature magnetic force microscope has been performed on patterned and unpatterned superconducting Nb films. Tip induced motion of the vortices is shown, as a function of both tip–sample separation and temperature. Constant height dissipation images of vortices suggest eddy current damping as well as vortex motion within potential wells as major sources of energy loss.
Keywords: Magnetic force microscopy; Dissipation force microscopy; Superconducting film; Vortex state;

Scanning nonlinear dielectric microscopy (SNDM) is a purely electrical method for observing the ferroelectric domain distributions. Today, its resolution is attained at the sub-nanometer. In this paper, we compare the resolution of SNDM to that of piezoresponse imaging by observing the ferroelectric domains in PZT thin film. Moreover, we observed the defects of polarization reversal with nanometer order using SNDM.
Keywords: Scanning nonlinear dielectric microscopy; Nonlinear dielectric response; Piezoelectric response; Concentration of square of electric field; Reversible and nonreversible domains; Forming nano-order dots;

Dynamic-mode AFM using the piezoelectric cantilever: investigations of local optical and electrical properties by N. Satoh; K. Kobayashi; S. Watanabe; T. Fujii; T. Horiuchi; H. Yamada; K. Matsushige (425-429).
We demonstrated applications of a microfabricated cantilever with a lead zirconate titanate (PZT) piezoelectric thin film as an integrated deflection sensor to dynamic-mode atomic force microscopy (AFM) and to the related techniques including scanning near-field optical microscopy (SNOM) and Kelvin force microscopy (KFM). In the SNOM measurement, the evanescent light scattered by the tip apex was transmitted through the pyramidal hollow tip of the cantilever and it was detected by a photodetector placed in a confocal arrangement. Furthermore, local surface potential of a ferroelectric film was mapped by KFM using the frequency modulation (FM) detection method. Local poled ferroelectric domains made by the AFM tip were imaged by both SNOM and KFM.
Keywords: PZT cantilever; SNOM; KFM; FM detection;

Dynamic force microscopy using FM detection in various environments by Kei Kobayashi; Hirofumi Yamada; Kazumi Matsushige (430-434).
High resolution imaging of the organic monolayer film in an ambient condition by dynamic force microscopy (DFM) utilizing the frequency modulation (FM) detection method was demonstrated. Although the mechanical Q-factor of the cantilever used was about 1000, we could image molecular rows in the monolayer film whose pitch was about 2 nm. The image was not clear probably due to the adsorbed water layer on the film. Furthermore, we also demonstrated the operation of the FM detection method in liquid with mechanical excitation of the cantilever by using a PLL circuit with a narrow operating range.
Keywords: Dynamic force microscopy; Q-factor; FM detection method; Liquid environment; Phase-locked loop; Copper phthalocyanine; Au(1 1 1);

High-temperature needle-sensor investigations on thin Au55 layers by G. Radu; U. Memmert; U. Hartmann (435-439).
The rearrangement of layers of ligand-stabilized Au55 clusters, deposited on graphite and mica substrates, was imaged during heating in ultrahigh vacuum. No thermally induced rearrangement of the clusters was observed below a certain decomposition temperature. Significant modifications of the layer structure were found on both substrates beyond that temperature. The observed critical temperature agrees fairly well with the decomposition temperature obtained from calorimetry measurements on solutions and pallets. At higher temperatures the cluster decomposition and aggregation processes differ significantly for the two substrates. This is attributed to a much stronger cluster–substrate interaction for mica in comparison to graphite.
Keywords: Needle-sensor microscopy; Clusters; Variable-temperature investigations;

Noncontact atomic force microscopy in liquid environment with quartz tuning fork and carbon nanotube probe by Masami Kageshima; Henriette Jensenius; Martin Dienwiebel; Yoshikazu Nakayama; Hiroshi Tokumoto; Suzanne P. Jarvis; Tjerk H. Oosterkamp (440-444).
A force sensor for noncontact atomic force microscopy in liquid environment was developed by combining a multiwalled carbon nanotube (MWNT) probe with a quartz tuning fork. Solvation shells of octamethylcyclotetrasiloxane on a graphite surface were detected both in the frequency shift and dissipation. Due to the high aspect ratio of the CNT probe, the long-range background force was barely detectable in the solvation region.
Keywords: Noncontact atomic force microscopy (NC-AFM); Carbon nanotube (CNT); Tuning fork; Solvation shell; Octamethylcyclotetrasiloxane (OMCTS);

The noise performance of the force sensor is crucial for optimizing the resolution in non-contact atomic force microscopy. Sensing forces in vacuum and low temperatures is even more demanding than at ambient conditions. Here we analyze the noise performance and the sensitivity of a force sensor based on a quartz tuning fork of which one of the prongs is fixed (qPlus sensor). The noise characteristic of the qPlus sensor, optical and piezoresistive detection schemes at room temperature are compared and the qPlus sensor is investigated at low temperatures. The frequency variation of quartz tuning forks as a function of temperature is experimentally determined for the temperature range from 4 to 300 K.
Keywords: Non-contact AFM; Low temperatures; qPlus sensor;

A non-contact atomic force microscope (NC-AFM) based on a microfabricated piezoelectric cantilever is presented. A single piezoelectric lead zirconate titanate thin film layer on the cantilever serves as deflection sensing, cantilever oscillation and feedback actuation. Since such an AFM requires neither external oscillator nor external deflection sensor, considerably simple instrumentation becomes possible even for extreme environments such as low temperature or ultra-high vacuum. Also feedback control by the integrated actuator in the cantilever makes faster scanning possible. Images of atomic steps on annealed sapphire (0001) surfaces have been observed in air atmosphere in frequency modulation mode.
Keywords: Non-contact atomic force microscope; Piezoelectric cantilever; Frequency modulation technique; Lever feedback actuation;

A study of friction by carbon nanotube tip by Makoto Ishikawa; Masamichi Yoshimura; Kazuyuki Ueda (456-459).
The carbon nanotube (CNT) probe, where a multi-walled (MW) CNT is attached onto the tip apex of a commercial silicon nitride cantilever, is applied to study friction on the nanometer scale using contact atomic force microscope (AFM). The friction versus load curve using CNT tip shows a completely different behavior from that of conventional tip, which is ascribed to the unique shape of CNT. In addition, strong scanning length dependency of the friction force is found due to the deformation of CNT.
Keywords: Friction; Nanotube; Atomic force microscopy; Tip; Friction force microscopy; Buckling force; Capillary force;

Force interactions and adhesion of gold contacts using a combined atomic force microscope and transmission electron microscope by D. Erts; A. Lõhmus; R. Lõhmus; H. Olin; A.V. Pokropivny; L. Ryen; K. Svensson (460-466).
We have investigated force interactions between two gold samples using a combination of atomic force microscope (AFM) and a transmission electron microscope (TEM) (TEM–AFM). The size and shape of the tip and sample as well as size of contact area and interactions type (elastic–plastic) is observed directly. The force was measured by direct measurement of the displacement of the AFM tip.An anomalous high value of the jump-to-contact distance was found, which we interpret as due to an enhanced surface diffusion of gold atoms towards the tip–sample gap due to the van der Waals force, leading to an avalanche situation where the gap is quickly filled until the ordinary jump-to-contact distance.The contact radius at zero applied load were measured and compared with adhesion theories. The results were in the Maugis transition region, between the limiting cases of the Derjaguin–Müller–Toporov (DMT) and the Johnson–Kendall–Roberts (JKR) models.
Keywords: TEM–STM; TEM–AFM; Jump-in-contact; Adhesion;

New AFM imaging for observing a high aspect structure by Sumio Hosaka; Takafumi Morimoto; Hiroshi Kuroda; Yasushi Minomoto; Yukio Kembo; Hirokazu Koyabu (467-473).
New imaging technique in AFM has been developed to suppress bending of the probe during scanning. The technique controls the probe such that approaching and gap-controlling are done after only one step xy-scanning is completed (only z-movement is done without scanning), and the xy-scanning is done after lifting the probe up from the sample surface. This technique permits to use very sharpened probe and to observe a steep structure such as a dry-etched groove and a hole, and a photoresist pattern with a high aspect ratio faithfully. It is possible to apply this technique to monitoring the steep structures in the LSI process without cracking the wafer.
Keywords: AFM; SPM; 3D metrology; 3D imaging; Digital probing mode; Step-in mode; High aspect; Nanometer structure; STI structure; Photoresist pattern; In-line monitor;

DNA molecules sticking on a vicinal Si(111) surface observed by noncontact atomic force microscopy by Toyoko Arai; Masahiko Tomitori; Masato Saito; Eiichi Tamiya (474-480).
The DNA molecules on a vicinal Si(111) substrate with steps of single and double bi-atomic layers are imaged by noncontact atomic force microscopy (nc-AFM) in ultrahigh vacuum. The water solution containing pBR322 plasmid DNA molecules digested by Cla I is dropped on the substrate in a pure nitrogen atmosphere in a glove box, which is connected to the introduction chamber of the AFM. The ends of DNA molecules are frequently folded and pinned at the steps on the substrate, and the DNA strings often lie along the step. The chemical and dipole interactions between the DNA and the semiconductor substrate seem to play an important role in folding, pinning and sticking on the Si(111) substrate.
Keywords: DNA; Atomic force microscopy; Noncontact; Si(111); UHV;

Cruciform structures can form in DNA sequences with inverted repeats or palindromic symmetry under unwinding torsional stress. DNA repeats are widespread in the genomes of eukaryotes and prokaryotes, and their extrusion into cruciform structures may be involved in various genetic processes, including transcription and replication. Intracellular Mg2+ may change the free energy of supercoiling to provide the driving force for cruciform extrusion. We have used atomic force microscopy (AFM) to image the effects of magnesium concentrations on cruciform extrusions in supercoiled DNA. Mg2+ concentration dependent cruciform extrusions were observed in the supercoiled topoisomer of the pPR-PARP plasmid (〈σ〉=−0.065). These data show an application of AFM imaging to demonstrate that the presence of Mg2+ results in increased extrusions and promotes compact cruciform conformation.
Keywords: Atomic force microscopy; DNA; Cruciform; Magnesium;

Exploratory Study of RNA Polymerase II Using Dynamic Atomic Force Microscopy by Thor Rhodin; Kazuo Umemura; Mohammed Gad; Suzanne Jarvis; Mitsuru Ishikawa; Jianhua Fu (486-488).
An exploratory study of the microtopological dimensions and shape features of yeast RNA polymerase II (y-poly II) on freshly cleaved mica was made in phosphate aqueous buffer solution at room temperature following previous work by Hansma and others. The molecules were imaged by stabilization on freshly cleaved mica at a limiting resolution of 10 Å and scanned using dynamical atomic force microscopy with a 10 nm multi-wall carbon nanotube in the resonance frequency modulation mode. They indicated microtopological shape and dimensional features similar to those predicted by electron density plots derived from the X-ray crystallographic model. It is concluded that this is considered primarily a feasibility study with definitive conclusions subject to more detailed systematic measurements of the 3D microtopology. These measurements appear to establish validity of the noncontact atomic force microscopy (nc-AFM) approach into defining the primary microtopology and biochemical functionality of RNA polymerase II. Further nc-AFM studies at higher resolution using dynamical nc-AFM will be required to clearly define the detailed 3D microtopology of RNA polymerase II in anaerobic aqueous environments for both static and dynamic conditions.
Keywords: Molecular imaging; Biophysical AFM; Bioinstrumentation; RNA polymersase II;

Non-destructive force measurement in liquid using atomic force microscope by Hiroshi Sekiguchi; Hideo Arakawa; Takaharu Okajima; Atsushi Ikai (489-492).
Atomic force microscopy (AFM) has been applied to measure inter- or intra-molecular forces acting to hold biological molecules and structures. For these measurements, it is important to keep the target molecules biologically active on a solid surface. Besides the strategy for immobilizing them on the surface keeping their biological activities intact, it is crucial to reduce the force applied to them through the AFM tip to avoid mechanical inactivation of the sample. In this paper, we propose a new procedure to minimize the effect of contact force. The first step of the procedure is to bring the cantilever tip close to the sample surface within less than 3 μm, but short of contact with the sample surface. The approximate distance of the tip from the sample stage is measured using the thermal fluctuation of the cantilever. The second step is a “compression-free” force spectroscopy for the measurement of protein–protein interactions only, which is possible when the piezo scanner was retracted before the cantilever starts upward deflection. The interaction force can be measured in the retraction period provided a physical contact is established between the proteins on the tip and the substrate. This procedure allowed to measure interaction forces between GroEL and a denatured protein without mechanical deformation.
Keywords: Force spectroscopy; Bio-molecule; Thermal fluctuation; Compression-free;

Time-course effects of lipopolysaccharide (LPS) with phorbol-12-myristate-13-acetate (PMA) were quantitatively examined using an atomic force microscope (AFM) cantilever with a type I collagen-coated spherical bead as a probe. LPS/PMA affects the expression level of fibronectin (FN), increasing the adhesion strength for collagen type I about 1.7 times. After 4 days, the adhesion strength for collagen increased 1.6 times. Our method is suitable for quantitative studies of cell adhesion phenomena on the single cell level.
Keywords: Atomic force microscope; Fibronectin; Collagen type I; Microbead; Separation work; LPS; PMA; C6 glioma cell;

Tapping and contact mode imaging of native chromosomes and extraction of genomic DNA using AFM tips by Yingchun Sun; Hideo Arakawa; Toshiya Osada; Atsushi Ikai (499-505).
It is very important both in medicine and biology to clarify the chromosomal structure to understand its functions. In a standard cytogenetic procedure, chromosomes are often fixed in a mixture of acetic acid and methanol. This process most likely changes the mechanical property of chromosomes. We adopted a method to prepare native and unfixed chromosomes from mouse 3T3 cells and used tapping and contact mode atomic force microscopy (AFM) to image and manipulate them. Modified AFM tips were used to image chromosomes in contact mode in air, and then the chromosome samples were immobilized on a substrate and placed in a buffer solution to pull out DNA–histone complexes from them after they were optimally treated with trypsin. From the AFM images, we could see several bands and granular structures on chromosomes. We obtained force curves indicating long fiber extensions from native chromosomes both with low (in high concentration of NaCl) and high forces (physiological conditions). The result suggested that the degree of chromosome condensation decreased in high concentration of salt. It agrees with the known fact of histone H1 dissociation in a high concentration of salt. We intend to pull out DNA–histone complexes from chromosomes for later molecular operations on them using an AFM.
Keywords: Native chromosomes; Atomic force microscope; Band patterns; DNA extraction; Force curve; Force-extension curve;

Intra- and intermolecular mechanics of proteins and polypeptides studied by AFM: with applications by Atsushi Ikai; Alimjan Idiris; Hiroshi Sekiguchi; Hideo Arakawa; Shuhei Nishida (506-512).
In order to use an atomic force microscope to measure the intramolecular mechanics of a single protein or polymer molecule, the very initial stage of stretching must be recorded without complications from nonspecific adhesions of target molecules to substrate materials. Although polyethylene glycol itself shows a certain level of adhesion to the functionalized surface of crystalline silicon wafer used as a substrate, adhesion of polypeptides and proteins can be minimized by covering the substrate surface with flexible polyethylene glycol chains. Determination of the cantilever spring constant is also important in the quantitative analysis of the result of force measurements. We introduce a hydrodynamic method of cantilever calibration as an effective method of estimating the relative spring constant of the cantilever. Finally, for mechanical manipulation of DNA, we introduce the use of a positively charged glass surface to keep DNA molecules in the stretched conformation for molecular manipulation including mechanical cutting of stretched DNA.
Keywords: Atomic force microscope; Bio-nanotechnology; Protein nano-mechanics; Spring constant calibration; DNA manipulation;

Chemical and mechanical properties of organosilane self-assembled monolayers (SAMs) terminated with methyl (CH3) or fluoromethyl (CF3) groups have been studied using lateral force microscopy (LFM), force curve measurement and dynamic force microscopy (DFM). The CF3-terminated SAM had large adhesive interactions with a Si tip surface, that is, the intrinsic adhesion and the capillary force effect caused by adsorbed water, than the CH3-terminated SAM. The origin of these larger tip/sample interactions of the CF3-terminated SAM ascribed to its permanent dipole moment arisen from a strong electron negativity of fluorine atoms. Furthermore, microstructures consisting of CH3- and CF3-terminated SAMs were fabricated and imaged by LFM. The micropatterns were clearly imaged through a difference in lateral force (LF) between the SAMs. Such micropatterns could be also imaged by DFM through a difference in phase lag. When a cantilever of 20 N/m was used, the CF3-terminated SAM showed the larger phase lag than the CH3-terminated SAM due to the adhesion difference. However, when a stiffer cantilever of 40 N/m was used, the CF3-terminated SAM showed the smaller phase lag than CH3-terminated SAM. These experimental results show that the phase lag contrasts are not always the same, that is, the contrast depends on the spring constant of cantilevers.
Keywords: Adhesion force; Organosilane self-assembled monolayer; Alkylsilane; Fluoroalkylsilane; Phase lag;

Morphology changes in thermal- and photo-polymerization were studied with a tapping-mode atomic force microscope on the (1 0 0) surface of 2,4-hexadiyne-1,6-diol-bis(p-toluenesulfonate) by careful tuning of force. In the thermal-polymerization, the surface exhibits nodules with 50 nm size which become to align as ridges along the b-axis of crystal after full polymerization. This may be resulted from the stress relaxation on the surface. In the photo-polymerization, a different feature was observed in addition to the nodules; rod-like polymer crystallites with smooth crystallographic surfaces were formed mainly at surface steps especially at low conversion. These polymer crystallites are grown at steps as preferred polymerization sites by being controlled presumably due to the surface diffusion of monomers.
Keywords: Solid-state polymerization; Diacetylene; Surface morphology; Steps;

Dynamic force microscopy analysis of block copolymers: beyond imaging the morphology by Ph. Leclère; F. Dubourg; S. Kopp-Marsaudon; J.L. Brédas; R. Lazzaroni; J.P. Aimé (524-533).
Dynamic force microscopy is known for its ability to image soft materials without inducing severe damage. For such materials, the determination of the relative contributions of the topography and the local mechanical properties to the recorded image is of primary importance. In this paper, we show that a systematic comparison between images and approach–retract curve data allows the origin of the contrast to be straightforwardly evaluated. The method provides an unambiguous quantitative measurement of the contribution of the local mechanical response to the image. To achieve this goal, experimental results are recorded on a model system, a symmetric triblock copolymer, which possesses a lamellar morphology due to nanophase separation between elastomer and glassy domains. In this particular case, we show that most of the contrast in the height and phase images is due to variations of the local mechanical properties. As a step further, the analysis of the variation of the phase is carried out as a function of the tip–surface distance. Local variations of the phase can be linked to dissipative processes between the tip and the soft sample. When the tip touches the surface, viscous forces acting against the tip motion contribute to the phase lag. Depending on the tip apex geometry and on the nature of the sample, the relationships between the phase variations and the tip–surface distance can be derived. On that basis, we propose an approach to evaluate the viscosity at the nanometer scale.
Keywords: Scanning probe microscopy; Block copolymer; Phase separation; Mechanical properties;

The phase separation of dye-labeled poly(isobutyl methacrylate) (PiBMA) and poly(octadecyl methacrylate) (PODMA) in two-dimensional monolayers was investigated by scanning near-field optical microscopy (SNOM). The energy transfer (ET) efficiency between dyes introduced to the polymers was mapped with SNOM for a PiBMA/PODMA homopolymer blend monolayer. The ET fluorescence images revealed that the phase boundary had a width of a few hundred nanometers, which was considerably larger than that expected in a three-dimensional bulk state. The micro-phase separation of a diblock copolymer PiBMA-block-PODMA was also discussed. Ribbon-like lamellar structures with a width of ca. 300 nm were observed, and the block polymer chain took a highly elongated conformation in two dimensions.
Keywords: Scanning near-field optical microscopy; Polymer; Langmuir–Blodgett film; Phase separation; Energy transfer;

The surface roughness and topography of low-pressure chemical vapor deposited (LPCVD) silicon films have been investigated by atomic force microscopy (AFM) in the non-contact mode. Deposition parameters were varied and various post-deposition treatments tested. The morphology of poly-silicon and amorphous-silicon were compared as well as morphology modification due to different amorphous-silicon deposition temperatures. The effects of post-deposition treatments: ion implantation and oxidation have been investigated.A significant difference in morphology between poly-silicon and amorphous-silicon films was revealed. Poly-silicon roughness was an order of magnitude higher than amorphous-silicon roughness. The surface morphology of amorphous-silicon layers was strongly affected by the deposition temperature. Asperity concentration and height were highly sensitive to the deposition temperature in the range of 545–560 °C.Phosphorous, argon and arsenic ion implantation decreased the roughness of amorphous-silicon films. On the other hand, the oxidation process caused an increase of roughness. Moreover, implantation followed by oxidation resulted in a drastic increase of the surface roughness, with a pronounced dose dependence.
Keywords: AFM; Poly-silicon; Roughness;

The surface morphology and the local physical properties of very thin intermediate Mg–Ti–O spinel films grown on MgO(100) were studied by the use of non-contact atomic force microscopy. When Ti was deposited in an O2 atmosphere on a MgO(100) substrate at high temperature, a long range stripe structure of Mg–Ti–O spinel was formed. Distance between the stripes is changed by the heating conditions. The stripe regions were positively charged comparing with the terrace regions, which was originated by the oxygen vacancy species.
Keywords: Non-contact atomic force microscopy; Metal oxides; Local physical property; Nano-structure; Solid state reaction;

AUTHOR INDEX (550-553).

SUBJECT INDEX (554-560).