Nanoelectromechanical Systems: Experiments and Modeling

Nanoelectromechanical systems (NEMSs) are systems with characteristic dimensions of a few nanometers. By exploiting nanoscale effects, NEMSs present interesting and unique characteristics, which deviate greatly from their predecessor microelectromechanical systems (MEMSs). For instance, NEMSbased devices can have fundamental frequencies in the microwave range (B100GH) (Rueckes et al. 2000); mechanical quality factors in the tens of thousands (ultralow energy dissipation); active mass in the femtogram range; force sensitivity at the attonewton level; mass sensitivity up to attogram (Ilic et al. 2000) and subattogram (Davis et al. 2000) levels; heat capacities far below a ‘‘yoctocalorie’’ (Roukes 1999); power consumption in the order of 10 aw (Roukes 2004); and extreme high integration level, approaching 10 elements per cm (Rueckes et al. 2000). All these distinguishing properties of NEMS devices pave the way to applications such as force sensors, chemical sensors, biological sensors, and ultrahigh frequency resonators. The interesting properties of the NEMS devices typically arise from the behavior of the active parts, which, in most cases, are in the forms of cantilevers or doubly clamped beams with dimensions at the nanometer scale. The materials for those active components include silicon, silicon carbide, carbon nanotubes, gold and platinum, to name a few. Silicon, the basic material employed in integrated circuit (IC) technology and MEMS, has been widely used to build NEMS. However, ultrasmall silicon-based NEMS nanoresonators failed to achieve the much anticipated high-quality factors due to the dominance of surface effects, such as surface oxidation and reconstruction, and thermoelastic damping. Limitations in strength and flexibility also compromise the performance of silicon-based NEMS actuators. Instead, carbon nanotubes appear to be ideal for NEMS given their nearly one-dimensional structures with high aspect ratio and nearly perfect-terminated surfaces and excellent electrical and mechanical properties. Due to significant advances in growth, manipulation, knowledge of electrical and mechanical properties, carbon nanotubes have become the most promising building blocks for the next generation of NEMS. Several carbon-nanotube-based functional NEMS devices have been reported so far (Kim and Lieber 1999, Rueckes et al. 2000, Akita et al. 2001, Fennimore et al. 2003, Kinaret et al. 2003, Ke and Espinosa 2004, Sazonova et al. 2004). Similarly to carbon nanotubes, nanowires are a type of onedimensional novel nanostructure well suited for building NEMS because of their size and controllable electrical properties. The purpose of this article is to provide a review of NEMS devices to date and to summarize the modeling currently being pursued to gain insight into their performance. This article is organized as follows: in the first part, we review carbon-nanotubeand -nanowire-based NEMS. In the second part, we present the multiphysics modeling of NEMS based on continuum theory.