A Survey for Infall Motions toward Starless Cores. II. CS(2−1)CS (2-1) and N2H+(1−0)N_2H^+ (1-0) Mapping Observations

We present the results of an extensive mapping survey of 53 `starless' cores in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0 and C18O 1-0. The purpose of this survey was to search for signatures of extended inward motions. This study finds 10 `strong' and 9 `probable' infall candidates, based on $\delta V_{CS}$ analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the $\delta V_{CS}$ parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, half of whose values lie in the range of 0.05-0.09 km s$^{-1}$. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly sub-critical core. In addition, the observed infall regions are too extended to be consistent with the `inside-out' collapse model applied to a very low-mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the $\rm N_2H^+$ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.Comment: Two tex files for manuscript and tables, and 38 figures. To appear in ApJ

Electromechanical Piezoresistive Sensing in Suspended Graphene Membranes

Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical (NEMS) devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nano-electromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nano-electromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.Comment: 20 pages, 3 figure

Edge-functionalized and substitutional doped graphene nanoribbons: electronic and spin properties

Graphene nanoribbons are the counterpart of carbon nanotubes in graphene-based nanoelectronics. We investigate the electronic properties of chemically modified ribbons by means of density functional theory. We observe that chemical modifications of zigzag ribbons can break the spin degeneracy. This promotes the onset of a semiconducting-metal transition, or of an half-semiconducting state, with the two spin channels having a different bandgap, or of a spin-polarized half-semiconducting state -where the spins in the valence and conduction bands are oppositely polarized. Edge functionalization of armchair ribbons gives electronic states a few eV away from the Fermi level, and does not significantly affect their bandgap. N and B produce different effects, depending on the position of the substitutional site. In particular, edge substitutions at low density do not significantly alter the bandgap, while bulk substitution promotes the onset of semiconducting-metal transitions. Pyridine-like defects induce a semiconducting-metal transition.Comment: 12 pages, 5 figure

Large Scale Integration of Graphene Transistors for Potential Applications in the Back End of the Line

A chip to wafer scale, CMOS compatible method of graphene device fabrication has been established, which can be integrated into the back end of the line (BEOL) of conventional semiconductor process flows. In this paper, we present experimental results of graphene field effect transistors (GFETs) which were fabricated using this wafer scalable method. The carrier mobilities in these transistors reach up to several hundred cm$^2$V$^{-1}$s$^{-1}$. Further, these devices exhibit current saturation regions similar to graphene devices fabricated using mechanical exfoliation. The overall performance of the GFETs can not yet compete with record values reported for devices based on mechanically exfoliated material. Nevertheless, this large scale approach is an important step towards reliability and variability studies as well as optimization of device aspects such as electrical contacts and dielectric interfaces with statistically relevant numbers of devices. It is also an important milestone towards introducing graphene into wafer scale process lines

Non-volatile switching in graphene field effect devices

The absence of a band gap in graphene restricts its straight forward application as a channel material in field effect transistors. In this letter, we report on a new approach to engineer a band gap in graphene field effect devices (FED) by controlled structural modification of the graphene channel itself. The conductance in the FEDs is switched between a conductive "on-state" to an insulating "off-state" with more than six orders of magnitude difference in conductance. Above a critical value of an electric field applied to the FED gate under certain environmental conditions, a chemical modification takes place to form insulating graphene derivatives. The effect can be reversed by electrical fields of opposite polarity or short current pulses to recover the initial state. These reversible switches could potentially be applied to non-volatile memories and novel neuromorphic processing concepts.Comment: 14 pages, 4 figures, submitted to IEEE ED

DFT study of graphene doping due to metal contacts

The experimental results of Metal\u2013graphene (M\u2013G) contact resistance (RC) have been investigated in\u2013depth by means of Density Functional Theory (DFT). The simulations allowed us to build a consistent picture explaining the RC dependence on the metal contact materials employed in this work and on the applied back\u2013gate voltage. In this respect, the M\u2013G distance is paramount in determining the RC behavior

Dense Cores in Dark Clouds. XIV. N2H+(1-0) maps of dense cloud cores

We present results of an extensive mapping survey of N2H+(1-0) in about 60 low mass cloud cores already mapped in the NH3(1,1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution about 1.5 times finer than the previous ammonia observations. Cores with stars typically have map sizes about a factor of two smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Velocity gradients range between 0.5 and 6 km/s/pc, similar to what has been found with NH3 data. ``Local'' velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complexmotions not interpretable as simple solid body rotation. Integrated intensity profiles of starless cores present a ``central flattening'' and are consistent with a spherically symmetric density law n ~ r^{-1.2} for r < ~0.03 pc and n ~ r^{-2} at larger r. Cores with stars are better modelled with single density power laws with n ~ r^{-2}. Line widths change across the core but we did not find a general trend. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~ 10 coherence lengths. The corresponding value of the coherence length, ~ 0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased ``coherence'' of line widths on small scales. Despite of the finer angular resolution, the majority of N2H+ and NH3 maps show a similar ``simple'' structure, with single peaks and no elongation.Comment: 62 pages, 11 figures, ApJ, in pres

A Spherical Model for "Starless" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains

In the standard picture of isolated star formation, dense ``starless'' cores are formed out of magnetic molecular clouds due to ambipolar diffusion. Under the simplest spherical geometry, I demonstrate that ``starless'' cores formed this way naturally exhibit a large scale inward motion, whose size and speed are comparable to those detected recently by Taffala et al. and Williams et al. in ``starless'' core L1544. My model clouds have a relatively low mass (of order 10 $M_\odot$) and low field strength (of order 10 $\mu$G) to begin with. They evolve into a density profile with a central plateau surrounded by a power-law envelope, as found previously. The density in the envelope decreases with radius more steeply than those found by Mouschovias and collaborators for the more strongly magnetized, disk-like clouds. At high enough densities, dust grains become dynamically important by greatly enhancing the coupling between magnetic field and the neutral cloud matter. The trapping of magnetic flux associated with the enhanced coupling leads, in the spherical geometry, to a rapid assemblage of mass by the central protostar, which exacerbates the so-called ``luminosity problem'' in star formation.Comment: 27 pages, 4 figures, accepted by Ap