Nanoelectromechanical systems were fabricated from single- and multilayer graphene sheets by mechanically exfoliating thin sheets from graphite over trenches in silicon oxide. Vibrations with fundamental resonant frequencies in the megahertz range are actuated either optically or electrically and detected optically by interferometry. We demonstrate room-temperature charge sensitivities down to 8 x 10(-4) electrons per root hertz. The thinnest resonator consists of a single suspended layer of atoms and represents the ultimate limit of two-dimensional nanoelectromechanical systems.

We demonstrate that a monolayer graphene membrane is impermeable to standard gases including helium. By applying a pressure difference across the membrane, we measure both the elastic constants and the mass of a single layer of graphene. This pressurized graphene membrane is the world's thinnest balloon and provides a unique separation barrier between 2 distinct regions that is only one atom thick.Membranes are fundamental components of a wide variety of physical, chemical, and biological systems, used in everything from cellular compartmentalization to mechanical pressure sensing. They divide space into two regions, each capable of possessing different physical or chemical properties. A simple example is the stretched surface of a balloon, where a pressure difference across the balloon is balanced by the surface tension in the membrane. Graphene, a single layer of graphite, is the ultimate limit: a chemically stable and electrically conducting membrane one atom in thickness. 1-3 An interesting question is whether such an atomic membrane can be impermeable to atoms, molecules and ions. In this letter, we address this question for gases. We show that these membranes are impermeable and can support pressure differences larger than one atmosphere. We use such pressure differences to tune the mechanical resonance frequency by ∼100 MHz. This allows us to measure the mass and elastic constants of graphene membranes. We demonstrate that atomic layers of graphene have stiffness similar to bulk graphite (E ∼ 1 TPa). These results show that single atomic sheets can be integrated with microfabricated structures to create a new class of atomic scale membrane-based devices.A schematic of the device geometry used heresa graphene-sealed microchambersis shown in Figure 1a. Graphene sheets are suspended over predefined wells in silicon oxide using mechanical exfoliation (see Supporting Information). Each graphene membrane is clamped on all sides by the van der Waals force between the graphene and SiO 2 , creating a ∼(µm) 3 volume of confined gas. The inset of Figure 1a shows an optical image of a single layer graphene sheet forming a sealed square drumhead with a width W ) 4.75 µm on each side. Raman spectroscopy was used to confirm that this graphene sheet was a single layer in thickness. [4][5][6] Chambers with graphene thickness from 1 to ∼75 layers were studied.After initial fabrication, the pressure inside the microchamber, p int , is atmospheric pressure (101 kPa). If the pressure external to the chamber, p ext , is changed, we found that p int will equilibrate to p ext on a time scale that ranges from minutes to days, depending on the gas species and the temperature. On shorter time scales than this equilibration time, a significant pressure difference ∆p ) p int -p ext can exist across the membrane, causing it to stretch like the surface of a balloon (Figure 1b). Examples are shown for ∆p > 0 in Figure 1c and ∆p < 0 in Figure 1d.To create a positive pressure difference, ∆p > 0, as shown in Figure 1c, we place a s...

Quality factors as high as 207 000 are demonstrated at room temperature for radio-frequency silicon nitride string resonators with cross sectional dimensions on the scale of 100nm, made with a nonlithographic technique. A product of quality factor and surface to volume ratio greater than 6000nm−1 is presented, the highest yet reported. Doubly clamped nanostring resonators are fabricated in high tensile-stress silicon nitride using a nonlithographic electrospinning process. We fabricate devices with an electron beam process, and demonstrate frequency and quality factor results identical to those obtained with the nonlithographic technique. We also compare high tensile-stress doubly clamped beams with doubly clamped and cantilever resonators made of a lower stress material, as well as cantilever beams made of the high stress material. In all cases, the doubly clamped high stress beams have the highest quality factors. We therefore attribute the high quality factors to high tensile stress. Potential dominant loss mechanisms are discussed, including surface and clamping losses, and thermoelastic dissipation. Some practical advantages offered by these nanostrings for mass sensing are discussed.

Type I collagen is a favorable substrate for cell adhesion and growth and is remodelable by many tissue cells; these characteristics make it an attractive material for the study of dynamic cellular processes. Low mass fraction (1.0-3.0 mg/ml), hydrated collagen matrices used for threedimensional cell culture permit cellular movement and remodeling, but their microstructure and mechanics fail to mimic characteristics of many extracellular matrices in vivo and limit the definition of fine-scale geometrical features (< 1 mm) within scaffolds. In this study, we worked with hydrated type I collagen at mass fractions between 3.0 and 20 mg/ml to define the range of densities over which the matrices support both microfabrication and cellular remodeling. We present pore and fiber dimensions based on confocal microscopy and longitudinal modulus and hydraulic permeability based on confined compression. We demonstrate faithful reproduction of simple pores of 50 µm-diameter over the entire range and formation of functional microfluidic networks for mass fractions greater than 10.0 mg/ml. We present quantitative characterization of the rate and extent of cellular remodelability using human umbilical vein endothelial cells. Finally, we present a co-culture with tumor cells and discuss the implications of integrating microfluidic control within scaffolds as a tool to study spatial and temporal signaling during tumor angiogenesis and vascularization of tissueengineered constructs.