The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to "anchor" the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which bandgap modulations of ∼ 0.6 eV are obtained. We propose a simple geometrical model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes.The interlayer potential landscape in layered materials is essential for understanding their mechanical and electromechanical behavior. For example, nanoelectromechanical systems (NEMS) based on low dimensional structures of such materials often rely on mechanical deformations such as twisting [1][2][3] and bending [4][5][6]. These processes involve relative sliding of the layers, which exhibits a corrugated energy landscape even in atomically flat systems, such as graphite and hexagonal boron nitride (h-BN). This corrugation arises from the non-uniform charge density distribution around the atomic positions within each layer [7][8][9]. It is well accepted that the most important factors that govern corrugation in such system are electrostatic and dispersion interactions. However, a clear picture of how the complex interplay between these factors determines the corrugated energy landscape and its manifestation in unique material properties has not emerged yet.Previous efforts towards the understanding of these phenomena have utilized density functional theory (DFT) with (semi-)local approximations [9][10][11][12]. However, these methods do not provide an appropriate description of dispersive interactions. Several approaches within DFT have been developed to overcome this problem, demonstrating successful treatment of dispersion effects in graphite [13][14][15] and h-BN [13,14,16]. However, to the best of our knowledge, such studies have not addressed the question of corrugation.In this letter, we present a theoretical study of the intricate interplay between dispersion and electrostatic interactions in layered materials. As an illustrative example we choose the case of h-BN, where both types of interactions are expected to have a considerable influence on the landscape of the interlayer potential. Using a first-principles van der Waals (vdW) corrected DFT approach, we show that dispersion interactions play the role of fixing the interlayer distance, while the electrostatic forces determine the optimal stacking mode and the interlayer sliding corrugation. In addition, we predict the existence of a nearly free-sliding path along which considerable bandgap modulations are obtained. Finally, we propose a simple geometrical model that quantifies ...

A two-axis scanning catheter was developed for 3D endoscopic imaging with spectral domain optical coherence tomography (SD-OCT). The catheter incorporates a micro-mirror scanner implemented with microelectromechanical systems (MEMS) technology: the micro-mirror is mounted on a two-axis gimbal comprised of folded flexure hinges and is actuated by magnetic field. The scanner can run either statically in both axes or at the resonant frequency (>= 350Hz) for the fast axis. The assembled catheter has an outer diameter of 2.8 mm and a rigid part of 12 mm in length. Its scanning range is ± 20˚ in optical angle in both axes with low voltages (1~3V), resulting in a scannable length of approximately 1 mm at the surface in both axes, even with the small catheter size. The catheter was incorporated with a multi-functional SD-OCT system for 3D endoscopic imaging. Both intensity and polarization-sensitive images could be acquired simultaneously at 18.5K axial scans/s. In vivo 3D images of human fingertips and oral cavity tissue are presented as a demonstration. Fujimoto, "Scanning single-mode fiber optic catheter-endoscope for optical coherence tomography," Opt. Lett. 21, 543-545 (1996). 26. B. E. Bouma and G. J. Tearney, "Power-efficient nonreciprocal interferometer and linear-scanning fiberoptic catheter for optical coherence tomography," Opt. Lett. 24, 531-533 (1999).

Millimeter-sized ferroelectric monomorph sonar transducers have been built using sol-gel PZT on micromachined silicon wafers. First generation transducer arrays with diaphragms varying in size from 0.2 to 2 mm were tested. Second generation 8 X 8 arrays have also been built and tested in water in the frequency range of 0.3 to 2 MHz.Improvements to the sol-gel process have yielded highquality, crack-free PZT films up to 12 pm in thickness, which leads directly to higher sensitivity and figure of merit for acoustic transducers. The longitudinal piezoelectric coefficient d33 is 140 to 240 pC/N, measured through a double beam laser interferometer. Remanent polarization of 28 pC/cm2, a coercive field of 30 kV/cm, and dielectric constant of 1400 were measured on 4-pm thick films. Test results are presented, including frequency response, beam patterns, and sensitivity.High-resolution acoustic images have been generated using these transducers and a four-element underwater acoustic lens. Potential applications for these transducers include high-frequency imaging sonars, medical ultrasound, ultrasonic communication links, and flaw detection (NDT).

No abstract

To investigate the use of high energy ultrasound as an alternative energy for angioplasty, an experimental ultrasonic angioplasty device was developed. The device was studied in two bioassay systems: an in vitro system for the disruption of atherosclerotic plaques and thrombi and an in vivo system for the recanalization of occluded canine femoral arteries. In vitro, sonication efficiently reduced the size of the plaques. Atheromatous plaques (n = 11) disrupted at a rate of 21 +/- 8 s/cm2; complicated plaques (n = 14) disrupted at a rate of 132 +/- 45 s/cm2 (p less than 0.001). Histologic examination revealed that the disruption of the plaques took place without concurrent damage to the media or adventitia. Ninety percent of the disrupted plaque debris had a diameter of less than 20 microns and was composed primarily of cholesterol monohydrate crystals. Solid thrombus (n = 5) weight was reduced from 1.6 +/- 0.2 to 0.4 +/- 0.1 g (p less than 0.0001) after 20 s of sonication. In vivo, sonication resulted in recanalization in all seven arteries tested in seven dogs. The obstruction was reduced from 93 +/- 11% to 18 +/- 7% (p less than 0.001). On histologic examination, the arterial wall injury index was found to be 1.56 +/- 0.42 in the test arteries compared with 1.37 +/- 0.47 in the control arteries (p = NS). The disruption of atherosclerotic plaques and thrombi, together with the efficient recanalization of the occluded arteries, demonstrates the potential of ultrasound angioplasty as a catheter-based technique for angioplasty.(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract

This article reports the fabrication of thick films of lead zirconate titanate (PZT) on platinum-buffered silicon substrates by screen printing. Crack-free films, up to 12 μm on a single pass, show a dielectric permittivity of 200, tangent losses of 0.05, remanent polarization of 2.5 μC/cm2, and coercive field of 40 kV/cm. The field-induced longitudinal piezoelectric coefficient d33 at 40 kV/cm dc bias and 4 kV/cm alternating field corresponded to 50 pC/N. The magnitude of the piezoelectric voltage coefficient g33, computed from the strain coefficient and dielectric permittivity, under the same conditions, was found to be 36×10−3 V m/N, higher than that of a poled PZT bulk ceramic in comparison. These results are promising for a broad variety of sensor applications.

Wireless neural stimulators are being developed to address problems associated with traditional lead-based implants. However, designing wireless stimulators on the sub-millimeter scale (<1 mm3) is challenging. As device size shrinks, it becomes difficult to deliver sufficient wireless power to operate the device. Here, we present a sub-millimeter, inductively powered neural stimulator consisting only of a coil to receive power, a capacitor to tune the resonant frequency of the receiver, and a diode to rectify the radio-frequency signal to produce neural excitation. By replacing any complex receiver circuitry with a simple rectifier, we have reduced the required voltage levels that are needed to operate the device from 0.5 to 1 V (e.g., for CMOS) to ~0.25–0.5 V. This reduced voltage allows the use of smaller receive antennas for power, resulting in a device volume of 0.3–0.5 mm3. The device was encapsulated in epoxy, and successfully passed accelerated lifetime tests in 80°C saline for 2 weeks. We demonstrate a basic proof-of-concept using stimulation with tens of microamps of current delivered to the sciatic nerve in rat to produce a motor response.