We describe the application of nanoelectromechanical systems ͑NEMS͒ to ultrasensitive mass detection. In these experiments, a modulated flux of atoms was adsorbed upon the surface of a 32.8 MHz NEMS resonator within an ultrahigh-vacuum environment. The mass-induced resonance frequency shifts by these adsorbates were then measured to ascertain a mass sensitivity of 2.53 ϫ10 Ϫ18 g. In these initial measurements, this sensitivity is limited by the noise in the NEMS displacement transducer; the ultimate limits of the technique are set by fundamental phase noise processes. Our results and analysis indicate that mass sensing of individual molecules will be realizable with optimized NEMS devices. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1755417͔ Nanoelectromechanical systems ͑NEMS͒ are emerging as strong candidates for a host of important applications in semiconductor-based technology and fundamental science. The minuscule active masses of NEMS, in particular, render them extremely sensitive to added mass-a crucial attribute for a wide range of sensing applications.Resonant mass sensors with high mass sensitivities have been employed in many diverse fields of science and technology. Among the most sensitive are those based on the acoustic vibratory modes of crystals, 2,3 thin films, 4 and micron-sized cantilevers. [5][6][7][8] In all of these, the vibratory mass of the resonator, its resonance frequency, and quality factor (Q) are central in establishing its mass sensitivity. In this letter, we demonstrate attogram-scale inertial mass sensing using high-frequency NEMS, and discuss how even greater sensitivity will be obtainable with such devices.9 This provides a concrete initial demonstration of the potential that nanoscale mechanical devices offer for sensing and ultimately weighing individual molecules.These initial experiments were carried out in an ultrahigh vacuum ͑UHV͒ environment within the apparatus depicted in Fig. 1͑a͒. This system allows the operation of a NEMS resonator ͑transduced magnetomotively 10 ͒ while a pulsed, weak flux of Au atoms is directed upon it. The Au atoms are generated by a thermal evaporation source and travel ballistically toward the NEMS within the apparatus. The mass flux, F, of the evaporator is measured by a calibrated quartz crystal monitor and modulated by a shutter; both are in the vicinity of the evaporator. The resonator temperature is regulated at TϷ17 K, both to ensure unity adsorbate sticking probability 11 and to allow careful monitoring of the resonator temperature fluctuations ͑see Fig. 3͒. Then, with knowledge of the exposed NEMS surface area, S ͑determined from careful scanning electron microscopy measurements͒, we can determine the exact mass 12 of the adsorbed Au atoms on the NEMS as ⌬m(t) Ϸ͐ 0 t SF(r QCM /r NEMS ) 2 dt. In this system, the geometric factor, (r QCM /r NEMS ) 2 Ϸ5ϫ10 Ϫ3 . We employed nanomechanical doubly clamped SiC beam resonators such as the ones shown in Fig. 1͑b͒ as the sensor elements in these experiments. The beams are embedded with...

Fabrication and readout of devices with progressively smaller size, ultimately down to the molecular scale, is critical for the development of very-high-frequency nanoelectromechanical systems (NEMS). Nanomaterials, such as carbon nanotubes or nanowires, offer immense prospects as active elements for these applications. We report the fabrication and measurement of a platinum nanowire resonator, 43 nm in diameter and 1.3 μm in length. This device, among the smallest NEMS reported, has a fundamental vibration frequency of 105.3 MHz, with a quality factor of 8500 at 4 K. Its resonant motion is transduced by a technique that is well suited to ultrasmall mechanical structures.

We simultaneously determined the physical structure and optical transition energies of individual single-walled carbon nanotubes by combining electron diffraction with Rayleigh scattering spectroscopy. These results test fundamental features of the excited electronic states of carbon nanotubes. We directly verified the systematic changes in transition energies of semiconducting nanotubes as a function of their chirality and observed predicted energy splittings of optical transitions in metallic nanotubes.

The electronic properties of single-walled carbon nanotubes (SWNTs) are altered by intertube coupling whenever bundles are formed. These effects are examined experimentally by applying Rayleigh scattering spectroscopy to probe the optical transitions of given individual SWNTs in their isolated and bundled forms. The transition energies of SWNTs are observed to undergo redshifts of tens of meVs upon bundling with other SWNTs. These intertube coupling effects can be understood as arising from the mutual dielectric screening of SWNTs in a bundle.

Two-step self-assembly between polyaniline and graphene oxide leads to a porous composite with high uniformity and excellent rate performance.

SiC is an extremely promising material for nanoelectromechanical systems given its large Young's modulus and robust surface properties. We have patterned nanometer scale electromechanical resonators from single-crystal 3C-SiC layers grown epitaxially upon Si substrates. A surface nanomachining process is described that involves electron beam lithography followed by dry anisotropic and selective electron cyclotron resonance plasma etching steps. Measurements on a representative family of the resulting devices demonstrate that, for a given geometry, nanometer-scale SiC resonators are capable of yielding substantially higher frequencies than GaAs and Si resonators. Silicon carbide is an important semiconductor for high temperature electronics due to its large band gap, high breakdown field, and high thermal conductivity. Its excellent mechanical and chemical properties have also made this material a natural candidate for microsensor and microactuator applications in microelectromechanical systems ͑MEMS͒. Recently, there has been a great deal of interest in the fabrication and measurement of semiconductor devices with fundamental mechanical resonance frequencies reaching into the microwave bands. Among technological applications envisioned for these nanoelectromechanical systems ͑NEMS͒ are ultrafast, high-resolution actuators and sensors, and high frequency signal processing components and systems.2 From the point of view of fundamental science, NEMS also offer intriguing potential for accessing regimes of quantum phenomena and for sensing at the quantum limit.SiC is an excellent material for high frequency NEMS for two important reasons. First, the ratio of its Young's modulus, E, to mass density, , is significantly higher than for other semiconducting materials commonly used for electromechanical devices, e.g., Si and GaAs. Flexural mechanical resonance frequencies for beams directly depend upon the ratio ͱ (E/). The goal of attaining extremely high fundamental resonance frequencies in NEMS, while simultaneously preserving small force constants necessary for high sensitivity, requires pushing against the ultimate resolution limits of lithography and nanofabrication processes. SiC, given its larger ͱ (E/), yields devices that operate at significantly higher frequencies for a given geometry, than otherwise possible using conventional materials. Second, SiC possesses excellent chemical stability.3 This makes surface treatments an option for higher quality factors ͑Q factor͒ of resonance. It has been argued that for NEMS the Q factor is governed by surface defects and depends on the device surface-to-volume ratio.

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