Unconventional acoustic wave propagation transitions induced by resonant scatterers in the high-density limit.

Experiments on ultrasound propagation through a gel doped with resonant encapsulated microbubbles provided evidence for a discontinuous transition between wave propagation regimes at a critical excitation frequency. Such behavior is unlike that observed for soft materials doped with non-resonant air or through liquid foams, and disagrees with a simple mixture model for the effective sound speed. Here, we study the discontinuous transition by measuring the transition as a function of encapsulated microbubble volume fraction. The results show the transition always occurs in the strong-scattering limit (l/λ < 1, l and λ are the mean free path and wavelength, respectively), that at the critical frequency the effective phase velocity changes discontinuously to a constant value with increasing microbubble volume fraction, and the measured critical frequency shows a power law dependence on microbubble volume fraction. The results cannot be explained by multiple scattering theory, viscous effects, mode decoupling, or a critical density of states. It is hypothesized the transition depends upon the microbubble on-resonance effective properties, and we discuss the results within the context of percolation theory. The results shed light on the discontinuous transition's physics, and suggest soft materials can be engineered in this manner to achieve a broad range of physical properties with potential application in ultrasonic actuators and switches.

Acoustic cavities in 2D heterostructures.

Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high quality factor (Q) 2D acoustic cavities operating in the 50-600 GHz frequency (f) range with f × Q up to 1 × 1014. Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS2/h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons.

Critical Role of a Nanometer-Scale Microballoon Shell on Bulk Acoustic Properties of Doped Soft Matter.

A material's acoustic properties depend critically upon porosity. Doping a soft material with gas-filled microballoons permits a controlled variation of the porosity through a scalable fabrication process while generating well-tailored spherical cavities that are impermeable to liquids. However, evidence is lacking of how the nanometer-scale polymeric shell contributes to the overall effective material properties in the regime where the wavelength is comparable to the sample thickness. Here, we measure ultrasound transmission through a microballoon-doped soft material as a function of microballoon and impurity concentration, sample thickness, and frequency. The measured longitudinal wave speeds are an order of magnitude larger than those in similar systems where no shell is present, while the transverse wave speed is found to linearly increase with microballoon concentration, also in contrast to systems with no shell. Furthermore, we find the results are independent of the soft material's elastic moduli as well as a lesser contribution of the microballoon shell on material attenuation. The results are validated with a multiple scattering model and suggest the shell contributes significantly to the material's bulk acoustic properties despite its thickness being 4 orders of magnitude smaller than the acoustic wavelength. Our results demonstrate how a nanometer-scale interface between a gas cavity and a soft polymer can be used in the submicrometer design of acoustic materials, and are important for observations of such phenomena as strong interference effects in soft matter.

Demonstration of a quasi-monostatic autonomous underwater vehicle based high duty cycle active sonar.

The feasibility of resolving target returns within receive signals collected by a continuously transmitting quasi-monostatic, broadband, autonomous underwater vehicle (AUV) based sonar is explored. Theoretical studies supported by experimental results suggest that it is possible to capture the source-to-receiver coupling response and target scattering with sufficient fidelity during the continuous transmission to enable detection and (potentially) classification processing. Demonstrations focused upon the detection of a bottomed target object at sea using transmit signals with duty cycles of 60% and 100% indicate that such an approach is feasible for a representative AUV-based side looking sonar system operating in shallow water.

Electronic Transport in Bilayer MoS2 Encapsulated in HfO2.

The exact nature of the interface between a two-dimensional crystal and its environment can have a significant impact on the electronic transport within the crystal, and can place fundamental limitations on transistor performance and long-term functionality. Two-dimensional transition-metal dichalcogenides are a new class of transistor channel material with electronic properties that can be tailored through dielectric engineering of the material/environmental interface. Here, we report electrical transport measurements carried out in the insulating regime of bilayer molybdenum disulfide, which has been encapsulated within a high-κ hafnium oxide dielectric. Temperature- and carrier-density-dependent measurements show that for T < 130 K the transport is governed by resonant tunneling, and at T = 4.2 K the tunneling peak lineshape is well-fitted by a Lorentzian with an amplitude less than e2/h. Estimates of tunneling time give τ ∼ 1.2 ps corresponding to a frequency f ∼ 0.84 THz. The tunneling processes are observable up to T ∼ 190 K (more than a factor of 6 higher than that previously reported for MoS2 on SiO2) despite the onset of variable range hopping at T ∼ 130 K, demonstrating the coexistence of the two transport processes within the same temperature range. At constant temperature, varying the Fermi energy allows experimental access to each transport process. The results are interpreted in terms of an increase in charge carrier screening length and a decrease in electron-phonon coupling induced by the hafnium oxide. Our results represent the first demonstration of the intermediate tunneling-hopping transport regime in a two-dimensional material. The results suggest that interface engineering may be a macroscopic tool for controlling quantum transport within such materials as well as for increasing the operating temperatures for resonant-tunneling devices derived from such materials, with applications in high-frequency electronics and logic devices.

Energy Dissipation Pathways in Few-Layer MoS2 Nanoelectromechanical Systems.

Free standing, atomically thin transition metal dichalcogenides are a new class of ultralightweight nanoelectromechanical systems with potentially game-changing electro- and opto-mechanical properties, however, the energy dissipation pathways that fundamentally limit the performance of these systems is still poorly understood. Here, we identify the dominant energy dissipation pathways in few-layer MoS2 nanoelectromechanical systems. The low temperature quality factors and resonant frequencies are shown to significantly decrease upon heating to 293 K, and we find the temperature dependence of the energy dissipation can be explained when accounting for both intrinsic and extrinsic damping sources. A transition in the dominant dissipation pathways occurs at T ~ 110 K with relatively larger contributions from phonon-phonon and electrostatic interactions for T > 110 K and larger contributions from clamping losses for T < 110 K. We further demonstrate a room temperature thermomechanical-noise-limited force sensitivity of ~8 fN/Hz1/2 that, despite multiple dissipation pathways, remains effectively constant over the course of more than four years. Our results provide insight into the mechanisms limiting the performance of nanoelectromechanical systems derived from few-layer materials, which is vital to the development of next-generation force and mass sensors.

Evidence for Spin Glass Ordering Near the Weak to Strong Localization Transition in Hydrogenated Graphene.

We provide evidence that magnetic moments formed when hydrogen atoms are covalently bound to graphene exhibit spin glass ordering. We observe logarithmic time-dependent relaxations in the remnant magnetoresistance following magnetic field sweeps, as well as strong variances in the remnant magnetoresistance following field-cooled and zero-field-cooled scenarios, which are hallmarks of canonical spin glasses and provide experimental evidence for the hydrogenated graphene spin glass state. Following magnetic field sweeps, and over a relaxation period of several minutes, we measure changes in the resistivity that are more than 3 orders of magnitude larger than what has previously been reported for a two-dimensional spin glass. Magnetotransport measurements at the Dirac point, and as a function of hydrogen concentration, demonstrate that the spin glass state is observable as the zero-field resistivity reaches a value close to the quantum unit h/2e(2), corresponding to the point at which the system undergoes a transition from weak to strong localization. Our work sheds light on the critical magnetic-dopant density required to observe spin glass formation in two-dimensional systems. These findings have implications to the basic understanding of spin glasses as well the fields of two-dimensional magnetic materials and spintronics.

Acoustic identification of buried underwater unexploded ordnance using a numerically trained classifier (L).

Using a finite element-based structural acoustics code, simulations were carried out for the acoustic scattering from an unexploded ordnance rocket buried in the sediment under 3 m of water. The simulation treated 90 rocket burial angles in steps of 2°. The simulations were used to train a generative relevance vector machine (RVM) algorithm for identifying rockets buried at unknown angles in an actual water/sediment environment. The trained RVM algorithm was successfully tested on scattering measurements made in a sediment pool facility for six buried targets including the rocket at 90°, 120°, and 150°, a boulder, a cinderblock, and a cinderblock rolled 45° about its long axis.

Engineering graphene mechanical systems.

We report a method to introduce direct bonding between graphene platelets that enables the transformation of a multilayer chemically modified graphene (CMG) film from a "paper mache-like" structure into a stiff, high strength material. On the basis of chemical/defect manipulation and recrystallization, this technique allows wide-range engineering of mechanical properties (stiffness, strength, density, and built-in stress) in ultrathin CMG films. A dramatic increase in the Young's modulus (up to 800 GPa) and enhanced strength (sustainable stress ≥1 GPa) due to cross-linking, in combination with high tensile stress, produced high-performance (quality factor of 31,000 at room temperature) radio frequency nanomechanical resonators. The ability to fine-tune intraplatelet mechanical properties through chemical modification and to locally activate direct carbon-carbon bonding within carbon-based nanomaterials will transform these systems into true "materials-by-design" for nanomechanics.

Shear modulus of monolayer graphene prepared by chemical vapor deposition.

We report shear modulus (G) and internal friction (Q(-1)) measurements of large-area monolayer graphene films grown by chemical vapor deposition on copper foil and transferred onto high-Q silicon mechanical oscillators. The shear modulus, extracted from a resonance frequency shift at 0.4 K where the apparatus is most sensitive, averages 280 GPa. This is five times larger than those of the multilayered graphene-based films measured previously. The internal friction is unmeasurable within the sensitivity of our experiment and thus bounded above by Q(-1) ≤ 3 × 10(-5), which is orders-of-magnitude smaller than that of multilayered graphene-based films. Neither annealing nor interface modification has a measurable effect on G or Q(-1). Our results on G are consistent with recent theoretical evaluations and simulations carried out in this work, showing that the shear restoring force transitions from interlayer to intralayer interactions as the film thickness approaches one monolayer.

Surface doping and band gap tunability in hydrogenated graphene.

We report the first observation of the n-type nature of hydrogenated graphene on SiO(2) and demonstrate the conversion of the majority carrier type from electrons to holes using surface doping. Density functional calculations indicate that the carrier type reversal is directly related to the magnitude of the hydrogenated graphene's work function relative to the substrate, which decreases when adsorbates such as water are present. Additionally, we show by temperature-dependent electronic transport measurements that hydrogenating graphene induces a band gap and that in the moderate temperature regime [220-375 K], the band gap has a maximum value at the charge neutrality point, is tunable with an electric field effect, and is higher for higher hydrogen coverage. The ability to control the majority charge carrier in hydrogenated graphene, in addition to opening a band gap, suggests potential for chemically modified graphene p-n junctions.

Ultrathin single crystal diamond nanomechanical dome resonators.

We present the first nanomechanical resonators microfabricated in single-crystal diamond. Shell-type resonators only 70 nm thick, the thinnest single crystal diamond structures produced to date, demonstrate a high-quality factor (Q ≈ 1000 at room temperature, Q ≈ 20 000 at 10 K) at radio frequencies (50-600 MHz). Quality factor dependence on temperature and frequency suggests an extrinsic origin to the dominant dissipation mechanism and methods to further enhance resonator performance.

Zero-power autonomous buoyancy system controlled by microbial gas production.

A zero-power ballast control system that could be used to float and submerge a device solely using a gas source was built and tested. This system could be used to convey sensors, data loggers, and communication devices necessary for water quality monitoring and other applications by periodically maneuvering up and down a water column. Operational parameters for the system such as duration of the submerged and buoyant states can be varied according to its design. The gas source can be of any origin, e.g., compressed air, underwater gas vent, gas produced by microbes, etc. The zero-power ballast system was initially tested using a gas pump and further tested using gas produced by Clostridium acetobutylicum. Using microbial gas production as the only source of gas and no electrical power during operation, the system successfully floated and submerged periodically with a period of 30 min for at least 24 h. Together with microbial fuel cells, this system opens up possibilities for underwater monitoring systems that could function indefinitely.

Wafer-scale reduced graphene oxide films for nanomechanical devices.

We report a process to form large-area, few-monolayer graphene oxide films and then recover the outstanding mechanical properties found in graphene to fabricate high Young's modulus ( =185 GPa), low-density nanomechanical resonators. Wafer-scale films as thin as 4 nm are sufficiently robust that they can be delaminated intact and resuspended on a bed of pillars or field of holes. From these films, we demonstrate radio frequency resonators with quality factors (up to 4000) and figures of merit ( f x Q>10(11)) well exceeding those of pure graphene resonators reported to date. These films' ability to withstand high in-plane tension (up to 5 N/m) as well as their high Q-values reveals that film integrity is enhanced by platelet-platelet bonding unavailable in pure graphite.

The vibro-acoustic response and analysis of a full-scale aircraft fuselage section for interior noise reduction.

The surface and interior response of a Cessna Citation fuselage section under three different forcing functions (10-1000 Hz) is evaluated through spatially dense scanning measurements. Spatial Fourier analysis reveals that a point force applied to the stiffener grid provides a rich wavenumber response over a broad frequency range. The surface motion data show global structural modes (approximately < 150 Hz), superposition of global and local intrapanel responses (approximately 150-450 Hz), and intrapanel motion alone (approximately > 450 Hz). Some evidence of Bloch wave motion is observed, revealing classical stop/pass bands associated with stiffener periodicity. The interior response (approximately < 150 Hz) is dominated by global structural modes that force the interior cavity. Local intrapanel responses (approximately > 150 Hz) of the fuselage provide a broadband volume velocity source that strongly excites a high density of interior modes. Mode coupling between the structural response and the interior modes appears to be negligible due to a lack of frequency proximity and mismatches in the spatial distribution. A high degree-of-freedom finite element model of the fuselage section was developed as a predictive tool. The calculated response is in good agreement with the experimental result, yielding a general model development methodology for accurate prediction of structures with moderate to high complexity.

On the feasibility of elastic wave visualization within polymeric solids using magnetic resonance elastography.

In this paper, the feasibility of extending previously described magnetic resonance elastography (MRE) dynamic displacement (and associated elasticity) measurement techniques, currently used successfully in tissue, to solid materials which have much higher shear rigidity and much lower nuclear spin densities, is considered. Based on these considerations, the MRE technique is modified in a straightforward manner and used to directly visualize shear wave displacements within two polymeric materials, one of which is relatively stiff.

At-sea measurements of sound penetration into sediments using a buried vertical synthetic array.

Acoustic bottom penetration experiments were carried out in a medium-grain sandy bottom at a site in St. Andrews Bay, Florida. These investigations used a new buried, vertical, one-dimensional synthetic array system where a small hydrophone was water-jetted into the sediment to a depth of approximately 2 m. Once buried, this hydrophone was mounted to a vertical robotics stage that translated the hydrophone upward in 1-cm increments. A broadband (3 to 80 kHz) spherical source, positioned 50 cm above the sediment-water interface, was used to insonify the sediment. Measurements were made with insonification angles above and below the critical angle by changing the horizontal distance of the source relative to the insertion point. This new measurement system is detailed, and results are presented that include temporal, frequency, and wavenumber analysis for natural and roughened interfaces. The measured compressional sound speed and attenuation are shown to be self-consistent using the Kramers-Kronig relation. Furthermore, only a single fast compressional wave was observed. There was no observation of a second slower compressional wave as predicted by some applications of the Biot model to unconsolidated water-saturated porous media.

Fast Fourier transform and singular value decomposition formulations for patch nearfield acoustical holography.

Nearfield acoustical holography (NAH) requires the measurement of the pressure field over a complete surface in order to recover the normal velocity on a nearby concentric surface, the latter generally coincident with a vibrator. Patch NAH provides a major simplification by eliminating the need for complete surface pressure scans-only a small area needs to be scanned to determine the normal velocity on the corresponding (small area) concentric patch on the vibrator. The theory of patch NAH is based on (1) an analytic continuation of the patch pressure which provides a spatially tapered aperture extension of the field and (2) a decomposition of the transfer function (pressure to velocity and/or pressure to pressure) between the two surfaces using the singular value decomposition (SVD) for general shapes and the fast Fourier transform (FFT) for planar surfaces. Inversion of the transfer function is stabilized using Tikhonov regularization and the Morozov discrepancy principle. Experimental results show that root mean square errors of the normal velocity reconstruction for a point-driven vibrator over 200-2700 Hz average less than 20% for two small, concentric patch surfaces 0.4 cm apart. Reconstruction of the active normal acoustic intensity was also successful, with less than 30% error over the frequency band.

The effect of internal point masses on the radiation of a ribbed cylindrical shell.

A finite element analysis is performed on a submerged cylindrical shell with internal frames and point masses attached to the frames. A point force on the shell is shown to excite resonances of the frames. Without the point masses, most of these resonances are evanescent and do not radiate to the far field. With the point masses, the high circumferential order resonances couple with those at low circumferential order with the result of increasing radiation to the far field. In this investigation, the radiation increase is about 10 dB over a broad frequency range (1.5 to 3 times the ring resonance).