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In graphene-based electronic devices like in transistors, the field effect applied thanks to a gate electrode allows tuning the charge density in the graphene layer and passing continuously from the electron to the hole doped regime across the Dirac point. Homogeneous doping is crucial to understand electrical measurements and for the operation of future graphene-based electronic devices. However, recently theoretical and experimental studies highlighted the role of the electrostatic edge due to fringing electrostatic field lines at the graphene edges [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008); F. T. Vasko and I. V. Zozoulenko, Appl. Phys. Lett. 97, 092115 (2010)]. This effect originates from the particular geometric design of the samples. A direct consequence is a charge accumulation at the graphene edges giving a value for the density, which deviates from the simple picture of a plate capacitor and also varies along the width of the graphene sample. Entering the quantum Hall regime would, in principle, allow probing this accumulation thanks to the extreme sensitivity of this quantum effect to charge density and the charge distribution. Moreover, the presence of an additional and counter-propagating edge channel has been predicted [P. Silvestrov and K. Efetov, Phys. Rev. B 77, 155436 (2008)] giving a fundamental aspect to this technological issue. In this article, we investigate this effect by tuning a high mobility graphene wire into the quantum Hall regime in which charge carriers probe the electrostatic potential at high magnetic field close to the edges. We observe a slight deviation to the linear shift of the quantum Hall plateaus with magnetic field and we study its evolution for different filling factors, which correspond to different probed regions in real space. We discuss the possible origins of this effect including an increase of the charge density towards the edges. [ABSTRACT FROM AUTHOR]

This research work demonstrates an innovative technique to measure the thermal conductivity of a small volume of non-volatile liquids. The method utilizes a micro-pipette thermal sensor (MPTS) (tip diameter < 2 μm) and is based on laser point heating thermometry and transient heat transfer. A laser beam is irradiated at the sensor tip immersed in a few microliters of the test fluid and the transient temperature change is recorded with the sensor. This temperature change is dependent on the surrounding fluid's thermal properties, such as thermal conductivity and diffusivity. The numerical solution for transient temperature profile for a point source is obtained using the finite element method in the COMSOL software. To determine the optimizing parameters such as thermal conductivity and power absorbed at the sensor tip, the multi-parameter fitting technique is used in MATLAB, which will fit the COMSOL simulation result with the experimental data. Three liquids with known thermal conductivity were tested to verify that the technique can be used to determine the thermal conductivity with high accuracy, and in addition, the thermal conductivity of growth media and serum used for culturing cancer cells is estimated. With the sensor size of 1-2 μm, we demonstrate the possibility of using this described method as the MPTS technique for measuring the thermal properties of microfluidic samples and biological fluids. [ABSTRACT FROM AUTHOR]

A detailed analysis of the tunability of a radio-frequency quantum point contact setup using a C - LCR circuit is presented. We calculate how the series capacitance influences resonance frequency and charge-detector resistance for which matching is achieved as well as the voltage and power delivered to the load. Furthermore, we compute the noise contributions in the system and compare our findings with measurements taken with an etched quantum point contact. While our considerations mostly focus on our specific choice of matching circuit, the discussion of the influence of source-to-load power transfer on the signal-to-noise ratio is valid generally. [ABSTRACT FROM AUTHOR]

A gate-defined quantum dot (QD) in an InAs nanowire is fabricated on top of a quantum point contact realized in a two-dimensional electron gas. The strong coupling between these two quantum devices is used to perform time-averaged as well as time-resolved charge detection experiments for electron flow through the quantum dot. We demonstrate that the Fano factor describing shot noise or time-correlations in single-electron transport depends in the theoretically expected way on the asymmetry of the tunneling barriers even in a regime where the thermal energy kBT is comparable to the single-particle level spacing in the dot. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy is used to characterize the electronic structure of 1 ML films of c(2×2)N/Cu(100) (i.e., Cu2N). By varying nitrogen coverage, a variety of morphologies are prepared, including (1) isolated ∼25 nm2 islands, (2) close-packed arrays of islands, and (3) quasicontinuous monolayer films. In all three regimes, the authors find that Cu2N acts as an insulator, with a band gap that exceeds 4 eV. The insulating Cu2N films are used to control the coupling of adsorbed Co atoms to the Cu(100) surface electron density. Tunneling spectroscopy of Co on Cu2N reveals an unoccupied atomic resonance, Kondo effect, and spin-flip excitation. These features depend on binding site within the Cu2N film, and are distinctly different than corresponding spectra for Co on Cu(100). [ABSTRACT FROM AUTHOR]

The field electron energy distributions were investigated for field emission from semiconductors as well as metals. The average energy <[variant_greek_epsilon]> and the standard deviation Δ[variant_greek_epsilon] of the distributions were obtained analytically using the WKB transmission coefficient. The obtained analytic forms of <[variant_greek_epsilon]> and Δ[variant_greek_epsilon] are found to be useful to characterize field emission. In particular, 2Δ[variant_greek_epsilon] represents the width of field electron energy distribution. As the field increases, it increases for metals and p-type semiconductors while it decreases for n-type semiconductors. [ABSTRACT FROM AUTHOR]

In order to employ solid state quantum dots as qubits, both a high degree of control over the confinement potential as well as sensitive charge detection are essential. We demonstrate that by combining local anodic oxidation with local Schottky-gates, these criteria are nicely fulfilled in the resulting hybrid device. To this end, a quantum dot with adjacent charge detector is defined. After tuning the quantum dot to contain only a single electron, we are able to observe the charge detector signal of the quantum dot state for a wide range of tunnel couplings. [ABSTRACT FROM AUTHOR]

Cryogenic-temperature technology is a promising and practical solution for the intense demand for high performance electronics. By utilizing the unique hysteretic behavior of perovskite (La,Pr,Ca)MnO3 observed either as a function of temperature and/or applied electric fields, we discovered low-temperature phases with either metallic and/or insulating resistance values. Low temperature resistivity state can be repeatedly switched by applying various voltage pulses. Resistance value of each phase is stable in time and reproducible after many switching cycles. Phases with intermediate resistance values can also be repeatedly stabilized. The behaviors reported here can be harnessed for cryogenic-temperature phase-change random access memory applications. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy is used to characterize the electronic structure of 1 ML films of CuN on Cu(100). We find that CuN acts as an insulator with a band gap that exceeds 4 eV. Measurements of the tunneling barrier height and image potential states indicate that the CuN work function is ∼0.9 eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory. Our results indicate that CuN films are useful for controlling the electronic coupling between adsorbates and surface electron density on the nanometer scale. [ABSTRACT FROM AUTHOR]

A modification of the thickness of the low-growth-temperature component of the GaAs spacer layers in multilayer 1.3 μm InAs/GaAs quantum-dot (QD) lasers has been used to significantly improve device performance. For a p-doped seven-layer device, a reduction in the thickness of this component from 15 to 2 nm results in a reduced reverse bias leakage current and an increase in the intensity of the spontaneous emission. In addition, a significant reduction of the threshold current density and an increase of the external differential efficiency at room temperature are obtained. These improvements indicate a reduced defect density, most probably a combination of the selective elimination of a very low density of dislocated dots and a smaller number of defects in the thinner low-growth-temperature component of the GaAs spacer layer. [ABSTRACT FROM AUTHOR]

Ultrashort pulsed-laser radiation is an effective method for precision materials processing and surface nano-/micromodification because of minimal thermal and mechanical damage. This study demonstrates that controllable surface nanomachining can be achieved by femtosecond laser pulses through local field enhancement in the near-field of a sharp probe tip. Nanomachining of thin gold films was accomplished by coupling 800-nm femtosecond laser radiation with a silicon tip in ambient air. Finite-difference time-domain numerical predictions of the spatial distribution of the laser field intensity beneath the tip confirmed that the observed high spatial resolution is due to the enhancement of the local electric field. Possible structuring mechanisms and factors affecting this process are discussed. The present process provides an intriguing means for massive nanofabrication due to the flexibility in the substrate material selection, high spatial resolution of ∼10 nm (not possible with standard nanomachining techniques), and fast processing rates achievable through simultaneous irradiation of multiarray tips. [ABSTRACT FROM AUTHOR]

In this paper, we describe the thermal conductivity measurement of single-walled carbon nanotubes thin film using a laser point source-based steady state heat conduction method. A high precision micropipette thermal sensor fabricated with a sensing tip size varying from 2 μm to 5 μm and capable of measuring thermal fluctuation with resolution of ±0.01 K was used to measure the temperature gradient across the suspended carbon nanotubes (CNT) film with a thickness of 100 nm. We used a steady heat conduction model to correlate the temperature gradient to the thermal conductivity of the film. We measured the average thermal conductivity of CNT film as 74.3 ± 7.9 W m-1 K-1 at room temperature. [ABSTRACT FROM AUTHOR]

Many biochemical processes at the sub-cellular level involve a small number of molecules. The local numbers of these molecules vary in space and time, and exhibit random fluctuations that can only be captured with stochastic simulations. We present a novel stochastic operator-splitting algorithm to model such reaction-diffusion phenomena. The reaction and diffusion steps employ stochastic simulation algorithms and Brownian dynamics, respectively. Through theoretical analysis, we have developed an algorithm to identify if the system is reaction-controlled, diffusion-controlled or is in an intermediate regime. The time-step size is chosen accordingly at each step of the simulation. We have used three examples to demonstrate the accuracy and robustness of the proposed algorithm. The first example deals with diffusion of two chemical species undergoing an irreversible bimolecular reaction. It is used to validate our algorithm by comparing its results with the solution obtained from a corresponding deterministic partial differential equation at low and high number of molecules. In this example, we also compare the results from our method to those obtained using a Gillespie multi-particle (GMP) method. The second example, which models simplified RNA synthesis, is used to study the performance of our algorithm in reaction- and diffusion-controlled regimes and to investigate the effects of local inhomogeneity. The third example models reaction-diffusion of CheY molecules through the cytoplasm of Escherichia coli during chemotaxis. It is used to compare the algorithm's performance against the GMP method. Our analysis demonstrates that the proposed algorithm enables accurate simulation of the kinetics of complex and spatially heterogeneous systems. It is also computationally more efficient than commonly used alternatives, such as the GMP method.

Ripples in graphene monolayers deposited on SiO(2)/Si wafer substrates were recently shown to give rise to friction anisotropy. High friction appears when the AFM tip slides in a direction perpendicular to the ripple crests and low friction when parallel. The direction of the ripple crest is, however, hard to determine as it is not visible in topographic images and requires elaborate measurements of friction as a function of angle. Here we report a simple method to characterize ripple crests by measuring the cantilever torsion signal while scanning in the non-conventional longitudinal direction (i.e., along the cantilever axis, as opposed to the usual friction measurement). The longitudinal torsion signal provides a much clearer ripple domain contrast than the conventional friction signal, while both signals show respective rotation angle dependences that can be explained using the torsion component of the normal reaction force exerted by the graphene ripples. We can also determine the ripple direction by comparing the contrast in torsion images obtained in longitudinal and lateral scans without sample rotation or complicated normalization.

Deterministic models of biochemical processes at the subcellular level might become inadequate when a cascade of chemical reactions is induced by a few molecules. Inherent randomness of such phenomena calls for the use of stochastic simulations. However, being computationally intensive, such simulations become infeasible for large and complex reaction networks. To improve their computational efficiency in handling these networks, we present a hybrid approach, in which slow reactions and fluxes are handled through exact stochastic simulation and their fast counterparts are treated partially deterministically through chemical Langevin equation. The classification of reactions as fast or slow is accompanied by the assumption that in the time-scale of fast reactions, slow reactions do not occur and hence do not affect the probability of the state. Our new approach also handles reactions with complex rate expressions such as Michaelis-Menten kinetics. Fluxes which cannot be modeled explicitly through reactions, such as flux of Ca(2+) from endoplasmic reticulum to the cytosol through inositol 1,4,5-trisphosphate receptor channels, are handled deterministically. The proposed hybrid algorithm is used to model the regulation of the dynamics of cytosolic calcium ions in mouse macrophage RAW 264.7 cells. At relatively large number of molecules, the response characteristics obtained with the stochastic and deterministic simulations coincide, which validates our approach in the limit of large numbers. At low doses, the response characteristics of some key chemical species, such as levels of cytosolic calcium, predicted with stochastic simulations, differ quantitatively from their deterministic counterparts. These observations are ubiquitous throughout dose response, sensitivity, and gene-knockdown response analyses. While the relative differences between the peak-heights of the cytosolic [Ca(2+)] time-courses obtained from stochastic (mean of 16 realizations) and deterministic simulations are merely 1%-4% for most perturbations, it is specially sensitive to levels of G(βγ) (relative difference as large as 90% at very low G(βγ)).