Effect of grain boundaries on paraconductivity of YBa2Cu3Ox

To investigate the effect of grain boundaries on paraconductivity of YBa₂Cu₃O_x, melt-textured and c-axis oriented thin films with controlled grain boundaries (superconducting transition width, ΔT, varying between 0.54 and 2.85 K) were prepared, and dc-conductivity has been measured as a function of temperature. In the logarithmic plots of excess-conductivity (Δσ) and reduced temperature (?), starting from low values of ?, we have observed three different regions namely critical region, mean field region and short wave fluctuation region. A correlation is observed between the range of critical region and ΔT, which is found to increase with ΔT. While for ΔT values smaller than 2.5 K only static critical region is observed, for higher ΔTs both static and dynamic critical regions are observed. In the mean field region a crossover from 3D to 2D was observed for all the samples. At ? values larger than 0.24, the excess-conductivity decreased sharply as ?⁻³, which suggested the existence of the short wave fluctuations.     

Finite temperature effect on mechanical properties of graphene sheets with various grain boundaries

The mechanical properties of graphene sheets with various grain boundaries are studied by molecular dynamics method at finite temperatures.The finite temperature reduces the ultimate strengths of the graphenes with different types of grain boundaries.More interestingly,at high temperatures,the ultimate strengths of the graphene with the zigzagorientation grain boundaries at low tilt angles exhibit different behaviors from those at lower temperatures,which is determined by inner initial stress in grain boundaries.The results indicate that the finite temperature,especially the high one,has a significant effect on the ultimate strength of graphene with grain boundaries,which gives a more in-depth understanding of their mechanical properties and could be useful for potential graphene applications.     

The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon–heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.     

Modulation of the second-harmonic generation in MoS_2 by graphene covering

Nonlinear optical frequency mixing,which describes new frequencies generation by exciting nonlinear materials with intense light field,has drawn vast interests in the field of photonic devices,material characterization,and optical imaging.Investigating and manipulating the nonlinear optical response of target materials lead us to reveal hidden physics and develop applications in optical devices.Here,we report the realization of facile manipulation of nonlinear optical responses in the example system of MoS₂ monolayer by van der Waals interfacial engineering.We found that,the interfacing of monolayer graphene will weaken the exciton oscillator strength in MoS₂ monolayer and correspondingly suppress the second harmonic generation(SHG)intensity to 30%under band-gap resonance excitation.While with off-resonance excitation,the SHG intensity would enhance up to 130%,which is conjectured to be induced by the interlayer excitation between MoS₂ and graphene.Our investigation provides an effective method for controlling nonlinear optical properties of two-dimensional materials and therefore facilitates their future applications in optoelectronic and photonic devices.     

Band renormalization and spin polarization of MoS2 in graphene/MoS2 heterostructures

Transition metal dichalcogenides exhibit spin–orbit split bands at the K‐point that become spin polarized for broken crystal inversion symmetry. This enables simultaneous manipulation of valley and spin degrees of freedom. While the inversion symmetry is broken for monolayers, we show here that spin polarization of the MoS₂ surface may also be obtained by interfacing it with graphene, which induces a space charge region in the surface of MoS₂. Polarization induced symmetry breaking in the potential gradient of the space charge is considered to be responsible for the observed spin polarization. In addition to spin polarization we also observe a renormalization of the valence band maximum (VBM) upon interfacing of MoS₂ with graphene. The energy difference between the VBM at the Γ‐point and K‐point shifts by ～150 meV between the clean and graphene covered surface. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of grain boundaries on the conductivity of high-purity ZrO2Y2O3 ceramics

Grain boundary conductivities are determined by complex impedance measurements (1–10⁶ Hz) on high-purity ceramics prepared by the alkoxide synthesis and on less pure ceramics obtained from a commercial powder. The grain size was varied systematically in the region 0.36–55 μm. The grain boundary conductivity is strongly influenced by the grain size, impurities and cooling procedure. The grain boundary conductivity increases linearly with the grain size for small grain sizes (0.3 to 2–4 μm) and is constant for larger grain sizes. The calculated specific conductivity of the grain boundary for pure materials is about 100 times smaller than that of the bulk. The grain boundary thickness was estimated to be 5.4 nm. The activation energy of the grain boundary conductivity is 7 kJ mole^?1 higher than of the bulk.     

Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.     

Effect of copassivation of Cl and Cu on CdTe grain boundaries

Using a first-principles method, we investigate the structural and electronic properties of grain boundaries (GBs) in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this Letter, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs.     

Facile synthesis of two-dimensional MoS_2/WS_2 lateral heterostructures with controllable core/shell size ratio by a one-step chemical vapor deposition method

Heterostructures based on two-dimensional(2D) transition-metal dichalcogenides(TMDCs) possess unique electronic and optical properties, which open up unprecedented opportunities in nanoscale optoelectronic devices. Synthesizing high-quality 2D TMDC heterostructures with different core/shell size ratios is of great significance for practical applications. Here, we report a simple one-step chemical vapor deposition(CVD) method for fabricating MoS_2/WS_2 lateral heterostructures with controllable core/shell size ratio. An ultrathin MoO_3/WO_3 film prepared by thermal evaporation was used as the precursor, and a step-like heating process was adopted to separately grow MoS_2 and WS_2 monolayers by taking advantage of the different melting points of MoO_3 and WO_3 sources. High-quality MoS_2/WS_2 lateral heterostructures with sharp interfaces were fabricated by optimizing the key growth parameters. Furthermore, the core/shell size ratio of heterostructures could be easily controlled by changing the thickness ratio of MoO_3/WO_3 film, and an approximately linear dependence between them is revealed. Compared with MoS_2 or WS_2 monolayers, the MoS_2/WS_2 heterostructure exhibited a shortened exciton lifetime owing to the type-Ⅱ energy band alignment, which is conducive to the application of high-performance devices. This work provides a facile strategy for the synthesis of 2D lateral heterostructures with controllable size ratio.     

Effect of grain boundaries on the electron work function of nanocrystalline nickel

The electron work function of nickel with various grain sizes has been studied. It has been shown that the work function decreases as the specific length of grain boundaries in nickel increases with decreasing average grain size. It has been found that the transformation of grain boundaries from a nonequilibrium to equilibrium state leads to an increase in the electron work function by 0.15 eV.     

Spin transport properties in Fe-doped graphene/hexagonal boron-nitride nanoribbons heterostructures

By combining non-equilibrium Green's function (NEGF) with density functional theory (DFT), we systematically study the spin-related transport properties of the heterostructures composed of graphene and hexagonal boron-nitride (h-BN) when the metal Fe is doped different positions of the heterostructures interface. The results show that the heterostructures exhibit obvious spin-filtering effect (SFE) and negative differential resistance (NDR) due to the different absorbing positions of the metal Fe. And the spin filtering ratio can reach more than 90% in a specific bias voltage range. Moreover, spin-rectifying behaviors are detected in the heterostructures. Whether it is for the design of multifunctional devices or the synthesis of spintronic devices, these findings will have some reference value.     

Resonant states in heterostructures of graphene nanoribbons

We study the transport properties of heterostructures of armchair graphene nanoribbons (AGNR) forming a double symmetrical barrier configuration. The systems are described by a single-band tight-binding Hamiltonian and Green's functions formalism, based on real-space renormalization techniques. We present results for the quantum conductance and the current for distinct configurations, focusing our analysis on the dependence of the transport with geometrical effects such as separation, width and transverse dimension of the barriers. Our results show the apparition of a series of resonant peaks in the conductance, showing a clear evidence of the presence of resonant states in the conductor. Changes in the barrier dimensions allow the modulation of the resonances in the conductance, making possible to obtain a complete suppression of electron transmission for determined values of the Fermi energy. The current–voltage curves show the presence of a negative differential resistance effect with a threshold voltage that can be controlled by varying the separation between the barriers and by modulating its confinement potential.     

Facilitative effect of graphene quantum dots in MoS_2 growth process by chemical vapor deposition

The substrate treatment with seeding promoter can promote the two-dimensional material lateral growth in chemical vapor deposition(CVD) process. Herein, graphene quantum dots(GQDs) as a novel seeding promoter were used to obtain uniform large-area MoS_2 monolayer. The obtained monolayer MoS_2 films were confirmed by optical microscope,scanning electron microscope, Raman and photoluminescence spectra. Raman mapping revealed that the MoS_2 monolayer was largely homogeneous.     

N、P掺杂硼烯/石墨烯异质结的密度泛函理论研究

采用基于密度泛函理论的第一性原理计算方法系统研究了氮、磷掺杂对硼烯/石墨烯异质结的几何结构和电子性质的影响.结果表明,相较完整硼烯/石墨烯异质结的金属特性,氮、磷掺杂的硼烯/石墨烯异质结均表现为半导体特性.室温下的分子动力学模拟进一步论证了相关体系的动力学稳定性.研究结果能够为硼烯/石墨烯异质结在新型二维半导体材料中的应用提供参考价值.     

Observation of magnetoresistance in CrI3/graphene van der Waals heterostructures

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI₃/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.     

Infrared photodetectors based on graphene van der Waals heterostructures

We propose and evaluate the graphene layer (GL) infrared photodetectors (GLIPs) based on the van der Waals (vdW) heterostructures with the radiation absorbing GLs. The operation of the GLIPs is associated with the electron photoexcitation from the GL valence band to the continuum states above the inter-GL barriers (either via tunneling or direct transitions to the continuum states). Using the developed device model, we calculate the photodetector characteristics as functions of the GL-vdW heterostructure parameters. We show that due to a relatively large efficiency of the electron photoexcitation and low capture efficiency of the electrons propagating over the barriers in the inter-GL layers, GLIPs should exhibit the elevated photoelectric gain and detector responsivity as well as relatively high detectivity. The possibility of high-speed operation, high conductivity, transparency of the GLIP contact layers, and the sensitivity to normally incident IR radiation provides additional potential advantages in comparison with other IR photodetectors. In particular, the proposed GLIPs can compete with unitravelling-carrier photodetectors.