Vertically aligned carbon based varactors

Abstract

This paper gives an assessment of vertically aligned carbon based varactors and validates their potential for future applications. The varactors discussed here are nanoelectromechanical devices which are based on either vertically aligned carbon nanofibers or vertically aligned carbon nanotube arrays. A generic analytical model for parallel plate nanoelectromechanical varactors based on previous works is developed and is used to formulate a universal expression for their voltage-capacitance relation. Specific expressions for the nanofiber based and the nanotube based varactors are then derived separately from the generic model. This paper also provides a detailed review on the fabrication of carbon based varactors and pays special attention to the challenges in realizing such devices. Finally, the performance of the carbon based varactor is assessed in accordance with four criteria: the static capacitance, the tuning ratio, the quality factor, and the operating voltage. Although the reported performance is still far inferior to other varactor technologies, our prognosis which stems from the analytical model shows a promise of a high quality factor as well as a potential for high power handling for carbon based varactors.

Mechanical properties of selected nanostructured materials and complex bio-nano, hybrid and hierarchical systems

“…None of the examples reviewed offer realised NEMS to the inspirational degree imagined by many. There are obvious, natural roles for CNTs, 323 shape-memory materials and other primary nanostructures and molecules in this field, 45,237,292,293,[324][325][326] but the marriage of two or more immature areas of investigation will be laborious. True nanoscale mechanical devices have been essentially restricted to bending and resonating beams, largely because of the lack of adequate substrates and surface properties (mostly limited to silicon-based materials) to allow the desired motions, much like their larger counterparts.…”

“…Nevertheless, several devices that incorporate similar switches have long since been realised, including RAM, actuators and nanotweezers, to name a few. 323,[338][339][340][341][342][343][344] Particular to the resonant properties of CNTs (as well as other nanostructures, as discussed later) in NEMS are nanomechanical antennas. 316,345,346 While there are major challenges associated with this use (i.e.…”

“…These switches are built following a handful of design archetypes generally based on a CNT suspended between electrodes335 or an array of vertically aligned CNTs;316, 336, 337 however, it should be noted that both of these approaches are prone to RF self-actuation and RF latching failures. Nevertheless, several devices that incorporate similar switches have long since been realised, including RAM, actuators and nano-tweezers, to name a few 323,338344…”

Direct measurement of bending stiffness and estimation of Young's modulus of vertically aligned carbon nanofibers

“…As a result of their synthesis process and their resemblance to carbon nanotubes (CNTs), VACNFs have been proposed to be used in a number of applications, where some have already been demonstrated including electron emitters, 2 gene delivery arrays, 3 and nanoelectromechanical systems. [4][5][6][7] CNTs are similar to VACNFs and their mechanical properties have been thoroughly studied. The Young's modulus, E, for tubes with a high crystallinity has been found to be about 1 TPa.…”