Sensors and actuators based on carbon nanotubes and their composites: A review

We discovered that many of the commonly studied two-dimensional monolayer transition metal dichalcogenide (TMDC) nanoscale materials are piezoelectric, unlike their bulk parent crystals. On the macroscopic scale, piezoelectricity is widely used to achieve robust electromechanical coupling in a rich variety of sensors and actuators. Remarkably, our density-functional theory calculations of the piezoelectric coefficients of monolayer BN, MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2 reveal that some of these materials exhibit stronger piezoelectric coupling than traditionally employed bulk wurtzite structures. We find that the piezoelectric coefficients span more than 1 order of magnitude, and exhibit monotonic periodic trends. The discovery of this property in many two-dimensional materials enables active sensing, actuating, and new electronic components for nanoscale devices based on the familiar piezoelectric effect.

Intrinsic Piezoelectricity in Two-Dimensional Materials

The idea of wireless power transfer (WPT) has been around since the inception of electricity. In the late 19th century, Nikola Tesla described the freedom to transfer energy between two points without the need for a physical connection to a power source as an ?all-surpassing importance to man? [1]. A truly wireless device, capable of being remotely powered, not only allows the obvious freedom of movement but also enables devices to be more compact by removing the necessity of a large battery. Applications could leverage this reduction in size and weight to increase the feasibility of concepts such as paper-thin, flexible displays [2], contact-lens-based augmented reality [3], and smart dust [4], among traditional point-to-point power transfer applications. While several methods of wireless power have been introduced since Tesla?s work, including near-field magnetic resonance and inductive coupling, laser-based optical power transmission, and far-field RF/microwave energy transmission, only RF/microwave and laser-based systems are truly long-range methods. While optical power transmission certainly has merit, its mechanisms are outside of the scope of this article and will not be discussed.

Harvesting Wireless Power: Survey of Energy-Harvester Conversion Efficiency in Far-Field, Wireless Power Transfer Systems

This paper presents a study of reception and rectification of broad-band statistically time-varying low-power-density microwave radiation. The applications are in wireless powering of industrial sensors and recycling of ambient RF energy. A 64-element dual-circularly-polarized spiral rectenna array is designed and characterized over a frequency range of 2-18 GHz with single-tone and multitone incident waves. The integrated design of the antenna and rectifier, using a combination of full-wave electromagnetic field analysis and harmonic balance nonlinear circuit analysis, eliminates matching and filtering circuits, allowing for a compact element design. The rectified dc power and efficiency is characterized as a function of dc load and dc circuit topology, RF frequency, polarization, and incidence angle for power densities between 10/sup -5/-10/sup -1/ mW/cm/sup 2/. In addition, the increase in rectenna efficiency for multitone input waves is presented.

Recycling ambient microwave energy with broad-band rectenna arrays

The present invention relates generally to sub-microelectronic circuitry, and more particularly to nanometer-scale articles, including nanoscale wires which can be selectively doped at various locations and at various levels. In some cases, the articles may be single crystals. The nanoscale wires can be doped, for example, differentially along their length, or radially, and either in terms of identity of dopant, concentration of dopant, or both. This may be used to provide both n-type and p-type conductivity in a single item, or in different items in close proximity to each other, such as in a crossbar array. The fabrication and growth of such articles is described, and the arrangement of such articles to fabricate electronic, optoelectronic, or spintronic devices and components. For example, semiconductor materials can be doped to form n-type and p-type semiconductor regions for making a variety of devices such as field effect transistors, bipolar transistors, complementary inverters, tunnel diodes, light emitting diodes, sensors, and the like.

Nanoscale wires and related devices

High Q resonators are a critical component of stable, low-noise communication systems, radar, and
precise timing applications such as atomic clocks. In electronic resonators based on Si integrated circuits,
resistive losses increase as a result of the continued reduction in device dimensions, which decreases their Q
values. On the other hand, due to the mechanical construct of bulk acoustic wave (BAW) and surface
acoustic wave (SAW) resonators, such loss mechanisms are absent, enabling higher Q-values for both BAW
and SAW resonators compared to their electronic counterparts. 1 The other advantages of mechanical
resonators are their inherently higher radiation tolerance, a factor which makes them attractive for NASA's
extreme environment planetary missions, for example to the Jovian environments where the radiation doses
are at hostile levels. 2 Despite these advantages, both BAW and SAW resonators suffer from low resonant
frequencies and they are also physically large which precludes their integration into miniaturized electronic
systems.

AC modeling of mechanical resonators based on carbon nanotubes

We provide an overview of our work where carbon-based nanostructures have been applied to two-dimensional (2D) planar and three-dimensional (3D) vertically-oriented nano-electro-mechanical (NEM) switches. In the first configuration, laterally oriented single-walled nanotubes (SWNTs) synthesized using thermal chemical vapor deposition (CVD) were implemented for forming bridge-type 2D NEMS switches, where switching voltages were on the order of a few volts. In the second configuration, vertically oriented carbon nanofibers (CNFs) synthesized using plasma-enhanced (PE) CVD have been explored for their potential application in 3D NEMS. We have performed nanomechanical measurements on such vertically oriented tubes using nanoindentation to determine the mechanical robustness of the CNFs. Electrostatic switching was demonstrated in the CNFs synthesized on refractory metallic nitride substrates, where a nanoprobe was used as the actuating electrode inside a scanning-electron-microscope. The switching voltages were determined to be in the tens of volts range and van der Waals interactions at these length scales appeared significant, suggesting such structures are promising for nonvolatile memory applications. A finite element model was also developed to determine a theoretical pull-in voltage which was compared to experimental results.

Nano-Electro-Mechanical Switches Derived from Carbon-Based Nanomaterials

Electrostatically actuated nanoelectromechanical switches based on carbon nanotubes have been fabricated and tested in a continuing effort to develop high-speed switches for a variety of stationary and portable electronic equipment. As explained below, these devices offer advantages over electrostatically actuated microelectromechanical switches, which, heretofore, have represented the state of the art of rapid, highly miniaturized electromechanical switches. Potential applications for these devices include computer memories, cellular telephones, communication networks, scientific instrumentation, and general radiation-hard electronic equipment. A representative device of the present type includes a single-wall carbon nanotube suspended over a trench about 130 nm wide and 20 nm deep in an electrically insulating material. The ends of the carbon nanotube are connected to metal electrodes, denoted the source and drain electrodes. At bottom of the trench is another metal electrode, denoted the pull electrode (see figure). In the off or open switch state, no voltage is applied, and the nanotube remains out of contact with the pull electrode. When a sufficiently large electric potential (switching potential) is applied between the pull electrode and either or both of the source and drain electrodes, the resulting electrostatic attraction bends and stretches the nanotube into contact with the pull electrode, thereby putting the switch into the "on" or "closed" state, in which substantial current (typically as much as hundreds of nanoamperes) is conducted. Devices of this type for use in initial experiments were fabricated on a thermally oxidized Si wafer, onto which Nb was sputter-deposited for use as the pull-electrode layer. Nb was chosen because its refractory nature would enable it to withstand the chemical and thermal conditions to be subsequently imposed for growing carbon nanotubes. A 200- nm-thick layer of SiO2 was formed on top of the Nb layer by plasma-enhanced chemical vapor deposition. In the device regions, the SiO2 layer was patterned to thin it to the 20-nm trench depth. The trenches were then patterned by electron- beam lithography and formed by reactive- ion etching of the pattern through the 20-nm-thick SiO2 to the Nb layer.

https://ntrs.nasa.gov/api/citations/20090017546/downloads/20090017546.pdf

Fast Electromechanical Switches Based on Carbon Nanotubes

High Q resonators are a critical component of stable, low-noise communication systems, radar, and precise timing applications such as atomic clocks. In electronic resonators based on Si integrated circuits, resistive losses increase as a result of the continued reduction in device dimensions, which decreases their Q values. On the other hand, due to the mechanical construct of bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators, such loss mechanisms are absent, enabling higher Q-values for both BAW and SAW resonators compared to their electronic counterparts. The other advantages of mechanical resonators are their inherently higher radiation tolerance, a factor that makes them attractive for NASA s extreme environment planetary missions, for example to the Jovian environments where the radiation doses are at hostile levels. Despite these advantages, both BAW and SAW resonators suffer from low resonant frequencies and they are also physically large, which precludes their integration into miniaturized electronic systems. Because there is a need to move the resonant frequency of oscillators to the order of gigahertz, new technologies and materials are being investigated that will make performance at those frequencies attainable. By moving to nanoscale structures, in this case vertically oriented, cantilevered carbon nanotubes (CNTs), that have larger aspect ratios (length/thickness) and extremely high elastic moduli, it is possible to overcome the two disadvantages of both bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators. Nano-electro-mechanical systems (NEMS) that utilize high aspect ratio nanomaterials exhibiting high elastic moduli (e.g., carbon-based nanomaterials) benefit from high Qs, operate at high frequency, and have small force constants that translate to high responsivity that results in improved sensitivity, lower power consumption, and im - proved tunablity. NEMS resonators have recently been demonstrated using topdown, lithographically fabricated ap - proaches to form cantilever or bridgetype structures. Top-down approaches, however, rely on complicated and expensive e-beam lithography, and often require a release mechanism. Reso - nance effects in structures synthesized using bottom-up approaches have also recently been reported based on carbon nanotubes, but such approaches have relied on a planar two-dimensional (2D) geometry. In this innovation, vertically aligned tubes synthesized using a bottom- up approach have been considered, where the vertical orientation of the tubes has the potential to increase integration density even further. The simulation of a vertically oriented, cantilevered carbon nanotube was performed using COMSOL Multi - physics, a finite element simulation package. All simulations were performed in a 2D geometry that provided consistent results and minimized computational complexity. The simulations assumed a vertically oriented, cantilevered nanotube of uniform density (1.5 g/cu cm). An elastic modulus was assumed to be 600 GPa, relative permittivity of the nanotube was assumed to be 5.0, and Poisson s ratio was assumed to be 0.2. It should be noted that the relative permittivity and Poisson s ratio for the nanotubes of interest are not known accurately. However, as in previous simulations, the relative permittivity and Poisson s ratios were treated as weak variables in the simulation, and no significant changes were recognized when these variables were varied.

/pdf/carbon-nanofiber-based-high-frequency-high-q-miniaturized-2k2w4g483d.pdf

Carbon Nanofiber-Based, High-Frequency, High-Q, Miniaturized Mechanical Resonators

The finite element method (FEM) is used in conjunction with the method of moments (MoM) and the mode matching technique (MM) to calculate reflection coefficients and radiation patterns for axisymmetric waveguide fed horns. The coupling of FEM to the MoM, on one hand, and the coupling of the FEM to the MM, on the other, are performed by using boundary integrals. One advantage of this approach is that it allows for the presence of inhomogeneous materials to be included in the modelling domain as this poses no special problems for the FEM. In this respect, this method differs from the work of Berthon and Bills [1989] who use only the MoM with a single waveguide mode serving as the excitation. After describing the basic theory, the method is applied to a horn antenna on the Cassini spacecraft. The two dimensional modelling domain for this horn clearly shows where the FEM/MoM and the FEM/MM boundary surfaces are located. Comparisons of measured and calculated far field radiation patterns and reflection coefficients are then shown. It is noted that this method generates a sparse, diagonally dominant, complex-symmetric system matrix which may be solved with standard library routines. Moreover, this FEM formulation has been shown to be free of spurious solutions.

Larry Epp

Papers

AC modeling of mechanical resonators based on carbon nanotubes

Nano-Electro-Mechanical Switches Derived from Carbon-Based Nanomaterials

Fast Electromechanical Switches Based on Carbon Nanotubes

Carbon Nanofiber-Based, High-Frequency, High-Q, Miniaturized Mechanical Resonators

Radiation from axisymmetric waveguide fed horns