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...

Optical approaches for observing the dynamics of single molecules have required pico- to nanomolar concentrations of fluorophore in order to isolate individual molecules. However, many biologically relevant processes occur at micromolar ligand concentrations, necessitating a reduction in the conventional observation volume by three orders of magnitude. We show that arrays of zero-mode waveguides consisting of subwavelength holes in a metal film provide a simple and highly parallel means for studying single-molecule dynamics at micromolar concentrations with microsecond temporal resolution. We present observations of DNA polymerase activity as an example of the effectiveness of zero-mode waveguides for performing single-molecule experiments at high concentrations.

Nanoelectromechanical systems are evolving, with new scientific studies and technical applications emerging. Mechanical devices are shrinking in thickness and width to reduce mass, increase resonant frequency, and lower the force constants of these systems. Advances in the field include improvements in fabrication processes and new methods for actuating and detecting motion at the nanoscale. Lithographic approaches are capable of creating freestanding objects in silicon and other materials, with thickness and lateral dimensions down to about 20 nanometers. Similar processes can make channels or pores of comparable dimensions, approaching the molecular scale. This allows access to a new experimental regime and suggests new applications in sensing and molecular interactions.

A nanofluidic channel device, consisting of many entropic traps, was designed and fabricated for the separation of long DNA molecules. The channel comprises narrow constrictions and wider regions that cause size-dependent trapping of DNA at the onset of a constriction. This process creates electrophoretic mobility differences, thus enabling efficient separation without the use of a gel matrix or pulsed electric fields. Samples of long DNA molecules (5000 to approximately 160,000 base pairs) were efficiently separated into bands in 15-millimeter-long channels. Multiple-channel devices operating in parallel were demonstrated. The efficiency, compactness, and ease of fabrication of the device suggest the possibility of more practical integrated DNA analysis systems.

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We studied the passage of DNA molecules, driven by an electric field, through a microfabricated channel with 90 nm size constrictions. DNA molecules were entropically trapped at the constriction and escaped with a characteristic lifetime. Counterintuitively, longer DNA were found to escape entropic traps faster than shorter ones. DNA molecules overcome the entropic barrier by stretching their monomers into the constriction, which results in the fact that the energy barrier for DNA escape is independent of the chain length.PACS numbers: 87.14. Gg, 83.10.Nn Recently, the concept of entropic trapping was introduced as a new dynamic regime in gel electrophoresis [1]. A long polymer molecule can be trapped entropically in a random restrictive environment such as a gel, and this effect becomes important when the dimension of pores in a retarding matrix is comparable to the radius of gyration (R 0 ) of the polymer. So far, entropic trapping has been demonstrated by computer simulation [2] and experiment in gel [3]. However, the lack of information on the structure of gel has hindered researchers from obtaining detailed microscopic understanding of the effect. Volkmuth and Austin [4] suggested the use of an artificial structure as a substitute for gel in electrophoresis. Aside from the application to polymer separation [5,6], microfabricated structures provide an ideal environment for studying these polymer dynamics problems, because one can easily control the dimension of obstacles or retarding matrix.The object of this Letter is to characterize the motion of long DNA polymer in an artificial channel with entropic traps. As a model pore-constriction system, we designed a channel with regions of two different depths. Figure 1(a) shows a schematic diagram of the device. The thick regions of the channel are as deep as 1 mm, comparable to the R 0 of the double stranded DNA molecules we used in this experiment. However the depth of the thin region is 90 nm, which is much smaller than R 0 . These thin and thick regions alternate along the channel, and DNA molecules in thick regions are entropically hindered from entering thin regions. Therefore at low electric fields DNA molecules are trapped at the entrance of the thin region and are unable to overcome the trapping barrier. We designed four different channels with different spatial periods of structure (4, 10, 20, and 40 mm). One period is divided by equal lengths of a thick and a thin region, which ensures that the fluid (electrical) resistance of the channel, as well as E s and E l (the electric field in the thin and thick region, respectively), are the same in all four channels. DNA molecules are free to relax while they are traveling in thick regions. By changing the period we can vary the time for DNA to relax before it meets another constriction.The channel was fabricated by standard photolithography techniques on Si substrate, and the front surface was anodically bonded to a Pyrex coverslip. The bonded channels were filled with a buffer solution containing DNA mole...