Tiny Gap Research Articles

Van der Waals graphene/g-GaSe heterostructure: Tuning the electronic properties and Schottky barrier by interlayer coupling, biaxial strain, and electric gating

Graphene-based van der Waals heterostructures are expected recently to design and fabricate many novel electronic and optoelectronic devices. The combination of the electronic structures of graphene and graphene-like GaSe monolayer (g-GaSe) in an ultrathin heterostructure has been realized experimentally, such as graphene/g-GaSe field effect transistor and dual Schottky diode device. In the present work, we investigate the electronic properties of the graphene/g-GaSe heterostructures under the applied electric field, in-plane strains, and interlayer coupling. Our results show that the electronic properties of the graphene/g-GaSe heterostructures are well preserved owing to a weak vdW interaction. Especially, a tiny band gap of 13 meV has opened in the presence of the g-GaSe monolayer. We found that the n-type Schottky contact is formed in the graphene/g-GaSe heterostructure with a Schottky barrier height of 0.86 eV, which can be efficiently modulated by applying the electric field, in-plane strains, and interlayer coupling. Furthermore, a transformation from the n-type to p-type Schottky contact is observed when the applied electric field is larger than 0.1 V/Å or the interlayer distance is smaller than 3.2 Å. Our results may provide helpful information to design and fabricate the future graphene-based vdW heterostructures, such as graphene/g-GaSe heterostructure and understand the physics mechanism in the graphene-based 2D vdW heterostructures.

Investigation of the Axial Gap Clearance in a Hydrodynamic-Passive Magnetically Levitated Rotary Blood Pump Using X-Ray Radiography.

The HeartWare HVAD is a radial rotary blood pump with a combination of passive magnetic and hydrodynamic bearings to levitate the impeller. The axial gap size between impeller and housing in this bearing and its sensitivity to speed, flow, and pressure difference is difficult to assess. Shear stresses are exceptionally high in this tiny gap making it important for blood damage and related adverse events. Therefore, the aim of this study was to measure the axial gap clearance in the HVAD at different operating conditions employing radiography. To quantify the gap size in the HVAD, the pump was positioned 30 mm in front of the X-ray source employing a microfocus X-ray tube with an acceleration voltage up to 300 kV. Beams were detected on a flat panel detector (Perkin Elmer XRD 1611-CP3). The pump was connected to a tubing circuit with a throttle to adjust flow (0, 5, 10 L/min) and a water glycerol mixture to set the desired viscosity (1, 4, 8 mPas). Rotational speed was varied between 1800 and 3600 rpm. In this study, for clinically relevant conditions at 5 L/min and 2700 rpm, the axial gap was 22 µm. The gap size increased with rotational speeds dependent on the viscosity (2.8, 6.9, and 9.4 µm/1000 rpm for 1, 4, and 8 mPas, respectively), but was independent from the volume flow and the pressure head at constant speeds. In summary, using X-ray radiographic imaging small gaps in a rotary blood pump during operation can be measured in a nondestructive contact-free way. The axial hydrodynamic bearing gap in the HVAD pump was determined to be in the range of about three times the diameter of a red blood cell. Its dependence on operating volume flow and generated pressure head across the pump is not pronounced.

Interlayer coupling and electric field tunable electronic properties and Schottky barrier in a graphene/bilayer-GaSe van der Waals heterostructure.

In this work, using density functional theory we investigated systematically the electronic properties and Schottky barrier modulation in a multilayer graphene/bilayer-GaSe heterostructure by varying the interlayer spacing and by applying an external electric field. At the equilibrium state, the graphene is bound to bilayer-GaSe by a weak van der Waals interaction with the interlayer distance d of 3.40 Å with the binding energy per carbon atom of -37.71 meV. The projected band structure of the graphene/bilayer-GaSe heterostructure appears as a combination of each band structure of graphene and bilayer-GaSe. Moreover, a tiny band gap of about 10 meV is opened at the Dirac point in the graphene/bilayer-GaSe heterostructure due to the sublattice symmetry breaking. The band gap opening in graphene makes it suitable for potential applications in nanoelectronic and optoelectronic devices. The graphene/bilayer-GaSe heterostructure forms an n-type Schottky contact with the Schottky barrier height of 0.72 eV at the equilibrium interlayer spacing. Furthermore, a transformation from the n-type to p-type Schottky contact could be performed by decreasing the interlayer distance or by applying an electric field. This transformation is observed when the interlayer distance is smaller than 3.30 Å, or when the applied positive external electric field is larger than 0.0125 V Å-1. These results are very important for designing new electronic Schottky devices based on graphene and other 2D semiconductors such as a graphene/bilayer-GaSe heterostructure.

Microstructure and properties of FeSiCr/PA6 composites by injection molding using FeSiCr powders by phosphating and coupling treatment

FeSiCr/PA6 composites were prepared by injection molding using the FeSiCr powders modified by different phosphating agents and KH550 coupling agent. The resistivity, impact strength, magnetic permeability and magnetic loss of the FeSiCr/PA6 composites were measured. The morphologies of different FeSiCr powders and the FeSiCr/PA6 composites were also observed by scanning electron microscope (SEM). The results showed that 1-Hydroxyethylidene-1,1-diphosphonic acid, phytic acid and H3PO4 could improve the electrical resistivity of FeSiCr powders by forming the dense phosphating layer except diphenylphosphinic acid. However, the resistivity of FeSiCr/PA6 composites using the FeSiCr powders treated by all the four phosphating agents had no obvious increase though the phosphating layer on the surface of FeSiCr powder came into being. The nylon insulation layer had much stronger influence than the phosphating layer on electrical resistivity of the composites. After adding appropriate KH550 coupling agent, the impact strengths of FeSiCr/PA6 composites were significantly improved, which may be associated with the tiny gap between FeSiCr powder and PA6 matrix. The effects of the phosphating agents on the magnetic permeability and loss of the FeSiCr/PA6 were small and the mechanism were also discussed.

Geometric and electronic structures of a two-dimensional covalent network of sp2 and sp3 carbon atoms

Based on first-principles total-energy calculations, we investigated the geometric, electronic, and spin structures of two-dimensional honeycomb networks of polymerized C40. Our calculations showed that the polymerized C40 possesses two polymeric structures that are statically and dynamically stable, retaining their covalent topologies up to 3000K. The C40 polymers are semiconductors with a tiny band gap and small dispersion bands near the gap, which causes the magnetic ordering of both ferromagnetic and antiferromagnetic states as their stable states. In addition to spin polarization, the C40 polymer possesses the electronic polarity normal to the two-dimensional covalent network because of the inhomogeneous charge density arising from the arrangement of pentagonal and hexagonal rings in the networks.

Computational study for noise reduction and characteristic of unsteady flow field/flow-induced noise generated in a small radial fan

This study is an attempt to characterize unsteady flow field and predict flow noise produced by a radial fan installed in a system that has a tiny gap between the fan casing and upper wall (similar conditions found in actual products). The large-eddy simulation turbulence model was used to simulate unsteady flow conditions, and an impeller was rotated 13 times in total to sufficiently develop unsteady flow and obtain data for Computational aeroacoustics (CAA). Unsteady flow field due to the radial fan structure and narrow flow channel in a system was verified, and a location wherein flow properties rapidly changed over time due to the complicated flow field was identified. The noise spectrum obtained through CAA was compared with that measured through experiment. Both tonal noise components consisting of blade passing frequency with its harmonic frequencies and broadband noise were matched. The major source of noise of the radial fan was on the inner surface of the casing. However, the sources of noise in the upper and lower sides of the casing were not equivalent to each other due to the interaction between the flow coming through an inlet located in the upper part of the casing and the flow discharged through an outlet installed at its side, as well as the interaction between impeller rotation and cut-off. Sources of noise located at the upper casing are caused by the flow around impeller tips, whereas sources on the lower casing are related to the interaction of the flow between the impeller and casing. A low noise model with a modified impeller tip was proposed, and its noise reducing effects were evaluated. The modified model reduced overall sound pressure level by 0.8 dB compared with the base model.

Oxygen vacancy chain and conductive filament formation in hafnia

The stability and aggregation mechanisms of oxygen vacancy chains are studied for hafnia using self-energy corrected density functional theory. While oxygen vacancies tend not to align along the c-axis of monoclinic HfO2, oxygen vacancy chains along a-axis and b-axis are energetically favorable, with cohesive energies of 0.05 eV and 0.03 eV per vacancy, respectively. Nevertheless, with an increase of the cross section area, intensive oxygen vacancy chains become much more stable in hafnia, which yields phase separation into Hf-clusters and HfO2. Compared with disperse single vacancy chains, intensive oxygen vacancy chains made of 4, 6, and 8 single vacancy chains are energetically more favorable by 0.17, 0.20, and 0.30 eV per oxygen vacancy, respectively. On the other hand, while a single oxygen vacancy chain exhibits a tiny electronic energy gap of around 0.5 eV, metallic conduction emerges for the intensive vacancy chain made of 8 single vacancy chains, which possesses a filament cross section area of ∼0.4 nm2. This sets a lower area limit for Hf-cluster filaments from metallic conduction point of view, but in real hafnia resistive RAM devices the cross section area of the filaments can generally be much larger (>5 nm2) for the sake of energy minimization. Our work sets up a bridge between oxygen vacancy ordering and phase separation in hafnia, and shows a clear trend of filament stabilization with larger dimensions. The results could explain the threshold switching phenomenon in hafnia when a small AFM tip was used as the top electrode, as well as the undesired multimode operation in resistive RAM cells with 3 nm-thick hafnia.

Excellent valleytronic properties and nontrivial topological phase in germanene heterostructure

In comparison with transition metal dichalcogenides (TMDs), germanene is not an ideal valleytronic material due to its very weak spin–orbit coupling (SOC) and tiny band gap. By performing density functional calculations, we studied the heterostructures of germanene/Sb and germanene/SbF in order to augment the valleytronic properties of germanene to a level similar to that of TMDs. Although germanene/Sb has large spin splittings, the system is found to be gapless. By proximity with the fluorine-adsorbed Sb layer, germanene can acquire large SOC. Germanene/SbF has a sizable direct band gap at Dirac points and the Dirac valleys of SbF and germanene are preserved in the heterostructure. The Ge–SbF interaction breaks the inversion symmetry, leading to large spin splittings and Berry curvatures, which are valley contrasting due to time reversal symmetry. We studied the interband optical transitions and found that valley polarization is achievable via valley-selective circular dichroism. The calculated Z2 topological invariant further confirms that Ge/SbF is a topological insulator. Our studies illustrate that weak SOC materials can be converted into good valleytronic material and that nontrivial topological phase and valleytronic properties can be simultaneously realized.

Short Term Clinical and Radiographic Results of the Salto Mobile Version Ankle Prosthesis

Category: Ankle, Ankle Arthritis Introduction/Purpose: Total ankle arthroplasty (TAA) has become an increasingly attractive option for advanced tibiotalar arthritis treatment. The Salto prosthesis (Tornier SA, Saint Ismier, France) was reported to have midterm and long term promising outcomes. However, to date, the clinical reports of this prosthesis are few and most of the papers were published by the inventors and disclosed consultants. The published areas are also limited to Europe, and no results were documented from other region. Methods: We retrospectively reviewed 62 patients in total. All of underwent unilateral Salto TAA. The average clinical follow-up was 34.3 months. Clinical and radiologic results were evaluated and enrolled. Clinical follow-up included medical records review, visual analogue scale (VAS) pain score, American Orthopeadic Foot and Ankle Society (AOFAS) ankle-hindfoot score, Ankle Osteoarthritis Scale (AOS) pain and disability score and range of motion (ROM). Apart from these, subsequent surgeries and perioperative events that occurred at the first 3 months after the operation were recorded simultaneously. In the radiographic evaluation, we measured the tibial angle (TA), talar angle (TAL), tibial slope, and talocalcaneal angle (TCA) described by Bonnin. Distributive regions of osteolysis and radiolucency were reviewed. In addition, heterotopic ossification also be observed and graded. Endpoint of the follow-up was either any component removal or reoperation for any reason. Results: Date to last follow-up, the survival rate with any reoperation as the endpoint was 88.7% (7/62). Using any component removal as the endpoint of followup, the survival rate was 95.2% (3/62). The mean VAS score, AOFAS ankle-hindfoot score, and AOS pain and disability score were improved postoperatively (p < 0.001). Postoperative ROM of ankle was also improved (p = 0.003). Within 6 weeks of postoperation, radiolucent line arose around tibial implant in 39 cases and disappeared at mean 5.5 ± 2.6 months. 28 patients with postoperative mean 8.5 ± 4.5 months appeared radiolucent area. Osteolysis was found in 28 patients with mean 18.1 ± 8.2 months postoperatively. 13 patients suffered heterotopic ossification which appeared 9.4 ± 4.3 months after operation. Conclusion: At the last followup, obvious pain relief and satisfactory results were achieved in most of the patients. Different from previous studies on Salto prosthesis, we considered that the early radiolucent line around the tibial implant is likely due to the tiny gap between the implant and bone contacted interface, and it would disappear at the time of osseous integration. The lucent area and osteolysis occurred frequently around the tibial keel and tray due to the stress shielding of the tibial component. However, longer follow-up is necessary to determine the durability of this implant.

Two-dimensional topological nodal line semimetal in layeredX2Y(X=Ca, Sr, and Ba;Y=As, Sb, and Bi)

In topological semimetals the Dirac points can form zero-dimensional and one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and topological nodal line semimetals, respectively. Here, based on first-principles calculations, we predict a topological nodal line semimetal phase in the two-dimensional compounds $X_2Y$ ($X$=Ca, Sr, and Ba; $Y$=As, Sb, and Bi) in the absence of spin-orbit coupling (SOC) with a band inversion at the M point. The mirror symmetry as well as the electrostatic interaction, that can be engineered via strain, are responsible for the nontrivial phase. In addition, we demonstrate that the exotic edge states can be also obtained without and with SOC although a tiny gap appears at the nodal line for the bulk states when SOC is included.

DFT design of polyguanidine – a unique two-dimensional material with high-energy density

ABSTRACTWe report herein a theoretical prediction and characterisation of a new two-dimensional (2D) material based on energetic polyguanidine. The structure represents a hexagonal type lattice of the P6/m space group. The material is dynamically and mechanically stable. Highly accurate band structure calculation with hybrid functional HSE06 reveals a tiny direct band gap being equal to 0.181 eV. We provide an additional spectral characterisation of the 2D polyguanidine substance including UV-vis, nuclear magnetic resonance and nuclear quadrupolar resonance parameters. The electron transport properties of a 26.6 Å wide polyguanidine ribbon are calculated in terms of tight-binding density functional theory approach. The predicted 2D material is also analysed by means of Quantum Theory of Atoms in Molecules and the aromatic character of the formed rings is estimated using nucleus-independent chemical shifts quantities.

Pronounced Fano Resonance in Single Gold Split Nanodisks with 15 nm Split Gaps for Intensive Second Harmonic Generation.

Single metallic nanostructures supporting strong Fano resonances allow more compact nanophotonics integration and easier geometrical control in practical applications such as enhanced spectroscopy and sensing. In this work, we designed a class of plasmonic split nanodisks that show pronounced Fano resonance comparable to that observed in widely studied plasmonic oligomer clusters. Using our recently developed "sketch and peel" electron-beam lithography, split nanodisks with varied diameter and split length were fabricated over a large area with high uniformity. Transmission spectroscopy measurements demonstrated that the fabricated structures with 15 nm split gap exhibit disk diameter and split length controlled Fano resonances in the near-infrared region, showing excellent agreement with simulation results. Together with the plasmon hybridization theory, in-depth full-wave analyses elucidated that the Fano resonances observed in the split nanodisks were induced by mode interference between the bright antibonding dipole mode of split disks and the subradiant mode supported by the narrow split gap. With the giant near-field enhancement enabled by the intensive Fano resonance at the tiny split gap, strong wavelength-dependent second harmonic generation was observed under near-infrared excitation. Our work demonstrated that single split nanodisks could serve as important building blocks for plasmonic and nanophotonic applications including sensing and nonlinear optics.

Goldstone origin of black hole hair from supertranslations and criticality

Degrees of freedom that carry black hole entropy and hair can be described in the language of Goldstone phenomenon. They represent the pseudo-Goldstone bosons of certain supertranslations, called [Formula: see text]-transformations, that are spontaneously broken by the black hole metric. This breaking gives rise to a tower of Goldstone bosons created by the spontaneously-broken generators that can be labeled by spherical harmonics. Classically, the number of charges is infinite, they have vanishing vacuum expectation values (VEVs) and the corresponding Goldstone modes are gapless. The resulting hair and entropy are infinite, but unresolvable. In quantum theory, the two things happen. The number of legitimate Goldstone modes restricted by requirement of weak-coupling, becomes finite and scales as black hole area in Planck units. The Goldstones generate a tiny gap, controlled by their gravitational coupling. The gap turns out to be equal to the inverse of black hole half-life, [Formula: see text]. Correspondingly, in quantum theory the charges are neither conserved nor vanish, but non-conservation time is set by [Formula: see text]. This picture nicely matches with the idea of a black hole as critical system is composed of many soft gravitons. The [Formula: see text]-Goldstones of geometric picture represent the near-gapless Bogoliubov–Goldstone modes of critical soft-graviton system.

Bulk-edge correspondence for topological photonic continua

Here, building on our previous work [Phys. Rev. B, 92, 125153, (2015)], it is shown that the propagation of unidirectional gapless edge states at an interface of two topologically distinct electromagnetic continua with a well-behaved asymptotic electromagnetic response is rigorously predicted by the bulk-edge correspondence principle. We work out detailed examples demonstrating that when the spatial-cut off of the nonreciprocal part of the material response is considered self-consistently in the solution of the relevant electromagnetic problem, the number of unidirectional gapless edge modes is identical to the difference of the Chern numbers of the bulk materials. Furthermore, it is shown how the role of the spatial cut-off can be imitated in realistic systems using a tiny air gap with a specific thickness. This theory provides a practical roadmap for the application of topological concepts to photonic platforms formed by nonreciprocal electromagnetic continua.

Magnetic interactions between vacancy-induced intrinsic magnetic impurities in single-wall carbon nanotubes

Spin-half paramagnetism induced by point detects was found in graphene recently, micromechanism of this magnetic response can be explained well by the intrinsic magnetic impurity theory. In this paper, we apply this theory to two types of single-walled carbon nanotubes (SWCNs) and calculate the properties of various magnetic interactions for comparison. Interestingly, magnetic interactions have different behaviors in these systems. Following our calculation, within a short length, the interactions can be suppressed by ether size effect or a tiny band gap, and then exhibit exponentially decaying. However, in the absence of a band gap, the RKKY interaction could leave a tiny tail at long range, which determines long range magnetic order. Further more, when a tiny band gap exist in the systems, the Heisenberg coupling is the dominate one due to the expanded wavefunction. According to these result, vacancy states in different types of SWCNs could form different magnetic order, bringing abundant candidates for application.

Giant spin splitting, strong valley selective circular dichroism and valley-spin coupling induced in silicene

Silicene is a potential candidate for valleytronics. However, in comparison with the transition metal dichalcogenides (TMDs), silicene has a tiny energy gap and zero spin splitting at its Dirac valleys, being unfavorable for valleytronic applications. Based on first principles calculations, we find that by proximity with Bi(111) bilayer, the Dirac valleys of silicene acquire a sizable energy gap and giant spin splittings, which are even larger than the splittings of $\mathrm{Mo}{\mathrm{S}}_{2}$. Our calculations show that the silicene over Bi layer exhibits a strong valley-contrasting circular dichroism, enabling selective optical pumping of valley carriers. Due to the time reversal symmetry and the breaking of inversion symmetry, the Berry curvatures and the spin splittings are opposite at the $K$ and ${K}^{\ensuremath{'}}$ valleys of silicene, and hence the valley and spin are locked and can be simultaneously polarized. In this way, silicene and likely other similar Dirac materials can be comparable to TMDs in valleytronics, which not only adds a new dimension to the properties of silicene but also expands the members of the valleytronic family.

Experimental investigation of CO2 dry-ice assisted jet impingement cooling

The novel concept of dry-ice assisted jet impingement cooling is proposed in this study. When carbon-dioxide (CO2) passes through a tiny orifice gap or jet nozzle, it experiences a rapid temperature drop as well as a pressure decrease via the Joule-Thomson effect. This temperature drop causes the formation of small CO2 dry-ice particles. In addition to the enhanced cooling performance caused by lowered bulk-jet temperature, heat transfer is improved by the additional sublimation effect between the dry-ice particles and the cooling target surface. A comparison of the cooling performance between the suggested CO2 solid-gas two-phase jet and a single-phase nitrogen (N2) jet was performed experimentally. In the experiment, both jet fluids are expanded through a circular nozzle and impinged on an electrically heated flat heater surface, and their heat transfer coefficients are measured. The performances of the impinging jet for both fluids are also evaluated via the variance of flow parameters, for example, the Reynolds number, and the jet geometry configurations.

Inversion response to optimize lumped‐element circuits for metamaterial

SUMMARYEnhancing electromagnetic research on prior light bend mechanisms is a significant issue for composite material phenomena, and one that is not available in nature. The unit cell of metamaterial can be revised, such that it is significantly smaller than the free space of the wavelength. It is essential to extend the appropriate retrieval process to measure electrical and magnetic responses to the backward wave propagation and dispersion diagram characteristics. In this paper, we propose a retrieval process inversion response between known and unknown parameters of the lumped unit‐cell circuits that optimizes the best fit parameters. By utilizing the optimization of particle swarm techniques, we provide appropriate values of periodic unit cells of CRLH transmission line circuits that matched with the microstrip implementation model. The group velocity of the wave is indicated in the parallel direction, whereas the phase velocity is in the anti‐parallel direction. The dispersion diagram displayed a specifically left‐handed and right‐handed medium, and a tiny band gap appeared in the attenuation medium. The constitutive parameters of dielectric permittivity, permeability, and refractive index were obtained from the periodic composite transmission line circuits.

Study on Glow Discharge Plasma Used in Polyester Surface Modification

To achieve an atmospheric pressure glow discharge (APGD) in air and modify the surface of polyester thread using plasma, the electric field distribution and discharge characteristics under different conditions were studied. We found that the region with a strong electric field, which was formed in a tiny gap between two electrodes constituting a line-line contact electrode structure, provided the initial electron for the entire discharge process. Thus, the discharge voltage was reduced. The dielectric barrier of the line-line contact electrodes can inhibit the generation of secondary electrons. Thus, the transient current pulse discharge was reduced significantly, and an APGD in air was achieved. We designed double layer line-line contact electrodes, which can generate the APGD on the surface of a material under treatment directly. A noticeable change in the surface morphology of polyester fiber was visualized with the aid of a scanning electron microscope (SEM). Two electrode structures - the multi-row line-line and double-helix line-line contact electrodes - were designed. A large area of the APGD plasma with flat and curved surfaces can be formed in air using these contact electrodes. This can improve the efficiency of surface treatment and is significant for the application of the APGD plasma in industries.

Corrosion behaviour of welds and Ta in liquid lead

Four specimens, P91 welded by friction stir welding with and without post heat treatment, P91 electromagnetic pulse welded (EMP) and 14Cr ODS (explosive welding) were exposed at 550 °C for up to 2131 h to Pb containing 10−6 wt% oxygen. After the exposure none of the samples showed dissolution attack, all were protected by an oxide layer at the surface. Nearly no effect on the oxidation due to welding was found in both friction stir welded specimens. Severe deformation and partial melting during explosive welding result in a slower oxide layer growth within the welding zone. The EMP sample was tested as delivered without post-heat treatment. No Pb penetrated into the tiny gap between the welded parts. After the test, the gap is filled up with oxides.Additionally, Ta, discussed as a pump impeller material, was exposed to Pb and PbBi at different temperatures (400–900 °C) and oxygen concentrations in liquid metal (saturated, 10−6 wt%, 10−8 wt% and reduced (<<10−8 wt%). Only the Ta specimens exposed to Pb with highly reduced oxygen content showed nearly no attack. All the others exhibited oxide scale formation that becomes severe above 400 °C test temperature.