We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low-frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.For the development of nanometer scale electronic and mechanical devices, there is an increasing need for nanometer resolution imaging method of subsurface features (groups of ions, clusters, lattice defects, crystal grains, etc.). Some relating methods have been proposed in scanning force microscopy (SFM) where the tipl'2 or the sample3,a is vibrated to modulate the force. The response to the force modulation is measured to image ion implanted layers,l embedded wires,2 carbon fiber and epoxy composites,3 and Langmuir-Blodgett films.a These methods are characterized, by a tip mounting spring with a spring constant comparable to that of the sample. It is sometimes different from the usual AFM requirement for the spring constant to be as small as possible.s In this letter we propose an alternative imaging method, ultrasonic force microscopy (UFM) that employs a tip mounting cantilever much softer than the tip-sample contact rigidity. We vibrate the sample at frequencies much higher than the resonant frequency of the cantilever6 and measure the deflection and/or torsional vibration of the cantilever. It gives nanometer resolution elastic or subsurface images, and moreoveq discriminates features of different elastic properties, by controlling the direction of vibration forces. We present a general imaging scheme extending our preliminary work,7 and an analysis to compare the elastic contrast of the force modulation mode3'a and the UFM. Then, it is verified by imaging two different subsurface features in a highly oriented pyrolytic graphite (HOPG) sample.We model the AFM with springs and the mass of tip cantilever rn as illustrated in Fig. 1. First, the cantilever is displaced by z" from its free position due to a static repulsive force. When the sample is vibrated at a frequency F lower than the cantilever resonant frequency Fs, the cantilgver is also vibrated following the sample vibration. The tip-sample contact rigidity is expressed as a spring constant s, as a slope u)Also at of the force-displacement relation.5 If s is approximated by a linear spring, the peak{o-peak cantilever vibration amplitude is given by a/zV :2211fu, where a is the sample vibration amplitude and /r is the cantilever spring constant. The amplitude V does not significantly depend upon the spring constant ratio K:t/s repre...

The electrical conduction of self-assembled monolayers (SAMs) made from conjugated molecules was measured using conductive atomic force microscopy (AFM), with a focus on the molecular structural effect on the electrical conduction. For phenylene oligomer SAMs, resistances through the monolayers increased exponentially with increases in molecular length and the decay constants of transconductance β were ca. 0.35 to 0.5 Å-1. Using an insertion technique into insulative alkanethiol SAMs, we successfully obtained single molecular resistance of terphenyl methanethiol at ca. 5.4 × 1010 Ω. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with increases in the applied load up to 15 nN. When two or three methylene spacers were introduced between the sulfur and terphenyl groups in a series of terphenyl derivatized thiols, the monolayer resistances and β values increased extraordinarily. One explanation is that the addition of methylene spacers changed the location of the molecular orbital as a result of MOPAC calculation.

We report an up-to-4-fold enhancement in the in-magnetic-field critical current density at 77 K of epitaxial YBa 2 Cu 3 O 7 films on CeO 2 -buffered SrTiO 3 substrates by 3-MeV Au 2þ irradiation. This indicates that irradiation using an industrially practical ion beam, which generally has kinetic energy less than 5 MeV, can provide a substantial increase in the in-field current performance of high-temperature superconductor films. Transmission electron microscopy results show that pointlike defects smaller than 6 nm in diameter were created in the films by the irradiation. V C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769836]One of the most significant hurdles in commercialization of electric-power devices using high-temperature superconductors (HTSC) is degraded critical-current-density (J c ) in magnetic fields. 1 In 2004, Macmanus-Driscoll et al. addressed this challenge by precipitating BaZrO 3 in YBa 2 Cu 3 O 7 (YBCO) films, 2 which work as artificial pinning centers (APCs) of vortices. 3 However, precise control of the secondary phase in shape 4 and alignment 5 has recently been recognized as vital to maximize J c and requires highly sophisticated synthesis techniques. Post-growth ion irradiation 6-12 is an easier route for such microstructure engineering, since it can control the properties of irradiation defects (size, shape, density, and alignment) by external parameters (ion mass, energy, fluence, and incident angle) without altering the growth conditions of a target material. 8,11 The following two reasons explain mainly why the irradiation approach to APC introduction has not received much attention during the recent development of second-generation HTSC wires, which consist of an epitaxial HTSC film on a metallic tape. The first reason is that previously observed J c improvements were substantial only in the bulk material but not in films, 1 which are a naturally pinning-center-rich form of the material. 13 The second is that measurable improvement in films was attained through the use of extremely high-energy ions (from hundreds MeV to GeV) generated by industrially impractical accelerators. 7,10-12 Indeed, previous ion-irradiation experiments for improving the current properties of HTSC heavily focused on the high-ion-energy range called electronic-stopping regime (>100 MeV), where incident ions lose their kinetic energy by the electronic excitation of target atoms. In that regime, continuous columnar defects can be fabricated along ion tracks 8,9,11 in perfect accordance with a long-accepted "consensus" that a cylindrical APC is most effective for increasing J c . 7,14 Conversely, despite its industrial affinity, ion irradiation in the nuclear-stopping regime (<5 MeV), where ions are decelerated by elastic collisions, has lacked an extensive investigation in the context of in-field-J c enhancement, 15-17 based on an assumption that expected point-like defects 9,11 are not as effective as continuous ones.In this letter, we propose reconsideration of these trends in the study...

We investigated an annealing effect of self-assembled monolayers (SAMs) generated from derivatives of terphenylmethanethiols and tetradecanethiol (C14) on Au(111) using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and contact angle measurement. STM data revealed that the molecules were partially desorbed by the annealing process at temperatures where the contact angle of the hexadecane began to decrease. The decreases in contact angles of both the CF3and CH3-terminated terphenyl SAMs were caused by annealing at 180 °C for 1 h, while a similar decrease began at a lower temperature of 150 °C in the C14 SAMs. From the STM data, a large amount of CH3-terminated terphenyl molecules remained after annealing at 180 °C for 3 h, while a small amount of molecules were observed in the other two SAMs. These data demonstrate that the CH3-terphenyl-derivatized thiol SAM had the highest thermal stability of the three SAMs, and the molecular backbone structure and end group were crucial for determining the thermal stability of the SAMs.

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We examined, both theoretically and experimentally, the characteristics of subsurface imaging with nanometer resolution and the effect of contact elasticity in the ultrasonic force microscope (UFM). In particular, the effect of the surface energy and effective elasticity on the maximum tip-sample force and the shift of the averaged tip-sample distance were examined. Furthermore, kink formation in the cantilever deflection (z a) against the ultrasonic frequency vibration (UFV) amplitude (a) characteristics was predicted. This model was used to explain experimental observations in UFM, such as the features of the measured z a(a) curve and the damping of the cantilever torsion vibration by the UFV. Moreover, the previously reported lateral ultrasonic force microscope image of subsurface features was explained by the response of subsurface edge dislocation to a large instantaneous force enhanced by the UFV.

Magnetic-field-angle θ dependence of critical current density Jc was measured in YBa2Cu3O7 films irradiated with 3-MeV gold ions. Such films were recently found to show large pinning force arising from point-like irradiation defects. A dimpled line-shape of Jc(θ) and its variation with magnetic field strength were revealed that could be well described by a simple model based on flux-lattice shear and Blatter scaling. Our results strongly suggest that vortex elasticity and electron-mass anisotropy coordinately produce the characteristic Jc anisotropy, called “shoulders,” in cuprate and iron-pnictide films.

We studied the oxide charges and traps within nitrided Hf-silicate (HfSiON)∕SiO2 gate stacks processed with high-temperature annealing with a spectroscopic technique by using high spatial resolution scanning capacitance microscopy. Spectroscopy was performed by detecting the static capacitance (dC∕dZ) between a conductive probe and the sample while sweeping the sample bias. The dC∕dZ image and spatially resolved dC∕dZ-V spectrum revealed the existence of positive fixed charges within HfSiON and interface trap charges between the SiO2 underlayer and Si substrate. We also observed a transient electron trap process from the conductive probe to the HfSiON film as abrupt discontinuities in the dC∕dZ-V spectrum and with bias-induced topography change of the HfSiON surface. These oxide charges and trap sites distribute inhomogeneously within HfSiON∕SiO2 gate stacks, and the origin of these charged defects is ascribable to phase separation induced by high-temperature postdeposition annealing.