On the current delivery limit of semiconducting carbon nanotubes

The current carrying capacity of single-walled semiconducting carbon nanotubes (CNTs) is studied by self-consistent quantum simulations using the non-equilibrium Green’s function formalism with the self-consistent Born approximation. The simulation shows that the current carrying capacity depends on the bias regime and is drastically different from that of metallic tubes. For long CNTs (with a length much longer than zone boundary and optical phonon scattering mean free path), the current saturates around 20 μA in the forward bias regime with unipolar transport due to phonon scattering. In ambipolar transport regime, the current delivery limit is still about 20 μA due to recombination of electron and hole currents. In contrast, for short semiconducting CNTs, the current delivery capacity can be above 25 μA in the unipolar transport regime and further double in the ambipolar transport regime. In reverse bias regime, the current of a long CNT can exceed 20 μA due to the second subband conduction and increased electron injection from the drain. The simulation provides a coherent explanation to the dependence of current delivery limit on bias regime and channel length, which is consistent with recent experiments.     

Mesh-offset-based method to generate a delta volume to support the maintenance of partially damaged parts through 3D printing

Journal of Mechanical Science and Technology - Three-dimensional (3D) printing technology is an excellent tool for implementing multi item, small scale production or for manufacturing objects of...     

Design of an Internal Antenna with Near‐Omnidirectional H‐Plane Radiation Pattern over Ultra‐wide Bandwidth

In this paper, an ultra‐wideband internal antenna for use in mobile applications is proposed. The proposed antenna has symmetrical bi‐arm structures printed on the top and bottom of the substrate, and it occupies a compact area of 10 mm × 10 mm × 1 mm. The designed antenna has an impedance bandwidth from 3 GHz to 12 GHz and near omnidirectional radiation patterns over the frequency band of interest. The group delay between two antennas fabricated using the proposed design is less than 0.8 ns, and the maximum gain variation is about 3.16 dB.     

Interstitial Mo‐Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin‐film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe₂) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W^?1), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe₂ TFTs is sought. A unique structural characteristic of the APCVD‐grown MoSe₂ is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe₂ phototransistors is identified, suggesting a novel route to high‐performance, multifunctional 2D material devices for future wearable sensor applications.     

Gate electrostatics and quantum capacitance of graphene nanoribbons

Capacitance-voltage (C-V) characteristics are important for understanding fundamental electronic structures and device applications of nanomaterials. The C-V characteristics of graphene nanoribbons (GNRs) are examined using self-consistent atomistic simulations. The results indicate strong dependence of the GNR C-V characteristics on the edge shape. For zigzag edge GNRs, highly nonuniform charge distribution in the transverse direction due to edge states lowers the gate capacitance considerably, and the self-consistent electrostatic potential significantly alters the band structure and carrier velocity. For an armchair edge GNR, the quantum capacitance is a factor of 2 smaller than its corresponding zigzag carbon nanotube, and a multiple gate geometry is less beneficial for transistor applications. Magnetic field results in pronounced oscillations on C-V characteristics.     

Critical Role of Cations in Lithium Sites on Extended Electrochemical Reversibility of Co‐Rich Layered Oxide

Only a very limited amount of the high theoretical energy density of LiCoO₂ as a cathode material has been realized, due to its irreversible deterioration when more than 0.6 mol of lithium ions are extracted. In this study, new insights into the origin of such low electrochemical reversibility, namely the structural collapse caused by electrostatic repulsion between oxygen ions during the charge process are suggested. By incorporating the partial cation migration of LiNiO₂, which produces a screen effect of cations in the 3b‐Li site, the phase distortion of LiCoO₂ is successfully delayed which in turn expands its electrochemical reversibility. This study elucidates the relationship between the structural reversibility and electrochemical behavior of layered cathode materials and enables new design of Co‐rich layered materials for cathodes with high energy density.     

Microparticle trapping in an ultrasonic Bessel beam

This paper describes an acoustic trap consisting of a multi-foci Fresnel lens on 127?μm thick lead zirconate titanate sheet. The multi-foci Fresnel lens was designed to have similar working mechanism to an Axicon lens and generates an acoustic Bessel beam, and has negative axial radiation force capable of trapping one or more microparticle(s). The fabricated acoustic tweezers trapped lipid particles ranging in diameter from 50 to 200?μm and microspheres ranging in diameter from 70 to 90?μm at a distance of 2 to 5?mm from the tweezers without any contact between the transducer and microparticles.     

Crew Production Rates for Contract Time Estimation: Beam Erection, Deck, and Rail of Highway Bridges

Both the importance and process of estimating highway construction time have increased in significance as roadway user costs themselves have become more significant. In estimating construction time, few parameters are more significant than work item crew production rates and factors significantly affecting those rates. A standardized data collection tool was used to acquire a total of 67 data points from 25 ongoing Texas highway projects between February 2002 and May 2004, for three selected critical work items: beam erection, bridge deck, and bridge rail. With the data, several hypothesized drivers of the crew production rates were analyzed. While the factor of shape of deck (straight versus curved) was identified as a statistically significant driver of bridge deck crew production rate, no statistically significant drivers were found for work items of beam erection and bridge rail. The study also found that both formwork and rebarwork crew sizes have significant relationships with bridge deck production rate. Findings from this study will enable highway agencies to enhance accuracy of contract time estimation for highway bridge construction. The methodology for obtaining field-based production rates will also be beneficial for future researchers.