Yuping Jia

Upper critical fields and thermally-activated transport of NdFeAsO(0.7)F(0.3) single crystal

We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H(c2) of NdFeAsO(0.7)F(0.3) single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H(c2) is comparable to H(c2)similar to 100 T of high-T(c) cuprates. H(c2)(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H(c2)(perpendicular to)(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H(c2)(theta) as a function of the angle theta between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter gamma(T)=(m(c)/m(ab))(1/2)=H(c2)(parallel to)/H(c2)(perpendicular to) obtained from these measurements decreases as temperature decreases from gamma similar or equal to 9.2 at 44 K to gamma similar or equal to 5 at 34 K, where parallel to and perpendicular to correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity rho(xx)(T,H) measured below T(c) but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior rho=rho(0) exp[-E(a)(T,H)/T] over 4-5 decades of rho(xx). The activation energy E(a)(T,H) has very different field dependencies for H parallel to ab and H perpendicular to ab varying from 4x10(3) K at H=0.2 T to similar to 200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of H(c2), using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H(c2)(T) in oxypnictides, which may significantly reduce H(c2)(0) as compared to H(c2)(0)similar to 200-300 T based on extrapolations of H(c2)(T) near T(c) down to low temperatures.

Graphene covered SiC powder as advanced photocatalytic material

Graphene covered SiC powder (GCSP) has been fabricated by well established method of high temperature thermal decomposition of SiC. The structural and photocatalystic characteristics of the prepared GCSP were investigated and compared with that of the pristine SiC powder. Under UV illumination, more than 100% enhancement in photocatalystic activity is achieved in degradation of Rhodamine B (Rh B) by GCSP catalyst than by pristine SiC powder. The possible mechanisms underlining the observed results are discussed. The results suggested that GCSP as a composite of graphene based material has great potential for use as a high performance photocatalyst.

Approaching the intrinsic electron field-emission of a graphene film consisting of quasi-freestanding graphene strips.

Knowledge Innovation Engineering of the Chinese Academy of Sciences [KJCX2-YW-W22, YYYJ-0701]; Ministry of Science & Technology of PR China [2007BAE34B00, 2006AA03A146, 2007CB936300, 2006AA03A107]; National Natural Science Foundation of China [50972162, 50702073]; Major State Basic Research Development Program of China (973 Program) [2011CB932700]

Mechanically exfoliated g-C3N4 thin nanosheets by ball milling as high performance photocatalysts

g-C3N4 with a layered structure has been proven as an outstanding metal-free organic photocatalyst because of its appropriate bandgap, abundant building elements, and excellent chemical stability. Here, a simple one-step ball milling method is presented for synthesis of mechanically exfoliated g-C3N4 (MECN) thin nanosheets at large scales for the first time. Characterization results showed that gradual size reduction, accompanied by a continual bandgap absorption shift, occurred with increasing grinding time. The obtained MECN thin nanosheets showed significantly enhanced simulated sun light driven photocatalytic activity toward organic degradation compared to their bulk counterpart, highlighting the crucial role of morphology and surface area on the photocatalytic performance.

Bipolar Carrier Transfer Channels in Epitaxial Graphene/SiC Core–Shell Heterojunction for Efficient Photocatalytic Hydrogen Evolution

Bipolar carrier transfer channels exist in the in situ epitaxial-graphene-wrapped 6H-SiC core-shell heterojunction due to the self-doping of graphene. Due to the special interface structure and high graphene quality, this material exhibits significant photocatalytic enhancement. Its hydrogen evolution efficiency is greater than that of the Pt/SiC composite. This micrometer-sized metal-free photocatalyst exhibits an activity comparable to that of metal-based nanophotocatalysts.

Significant enhancement in photocatalytic activity of high quality SiC/graphene core–shell heterojunction with optimal structural parameters

Structure and photocatalytic activity of high quality graphene covered SiC powder (GCSP) composites as metal-free photocatalysts with different sizes and graphene layer numbers are investigated. The results indicate that the GCSP covered with 4–9 layers of graphene reveal outstanding photocatalytic activity enhancement among the other graphene layer numbers. Moreover, it is found that smaller particles have higher activity than the larger ones and more than 730% improvement is achieved by the GCSP derived from 0.5 μm SiC powder relative to the pristine SiC powder, which is more than 6 times of that of photoreduced graphene oxide/SiC composite. Our results demonstrate that it is the high quality graphene and the perfect heterojunction interface between the graphene and SiC particles render the SiC/graphene core–shell heterojunction an outstanding photocatalytic activity, as well as potential for a low cost and metal-free photocatalyst.

Fabrication of vertically aligned graphene sheets on SiC substrates

Fabrication of unidirectional arrays of VAGSs were achieved on nonpolar (100) and (110) SiC substrates by thermal decomposition of SiC. The morphology and structural characteristics of the VAGSs were studied and compared with that of the VAGSs grown on the (000) (C-face) SiC substrate. It is found that the basal plane orientations of the VAGSs are predominantly influenced by the orientation of the SiC substrate. The mechanisms behind the phenomena were studied and discussed. As an example, the anisotropic magnetism of graphene was studied with applied magnetic field parallel and perpendicular to the graphene plane based on fully carbonized VAGSs. A nonpolar SiC substrate is a new choice for the reproducible and convenient fabrication of massive ordering graphene arrays for both fundamental research and potential applications of graphene based functional materials.

The correlation of epitaxial graphene properties and morphology of SiC (0001)

The electronic properties of epitaxial graphene (EG) on SiC (0001) depend sensitively on the surface morphology of SiC substrate. Here, 2–3 layers of graphene were grown on on-axis 6H-SiC with different step densities realized through controlling growth temperature and ambient pressure. We show that epitaxial graphene on SiC (0001) with low step density and straight step edge possesses fewer point defects laying mostly on step edges and higher carrier mobility. A relationship between step density and EG mobility is established. The linear scan of Raman spectra combined with the atomic force microscopy morphology images revealed that the Raman fingerprint peaks are nearly the same on terraces, but shift significantly while cross step edges, suggesting the graphene is not homogeneous in strain and carrier concentration over terraces and step edges of substrates. Thus, control morphology of epitaxial graphene on SiC (0001) is a simple and effective method to pursue optimal route for high quality graphene and...

Identification of dominant scattering mechanism in epitaxial graphene on SiC

A scheme of identification of scattering mechanisms in epitaxial graphene (EG) on SiC substrate is developed and applied to three EG samples grown on SiC (0001), (11 (2) over bar0), and (10 (1) over bar0) substrates. Hall measurements combined with defect detection technique enable us to evaluate the individual contributions to the carrier scatterings by defects and by substrates. It is found that the dominant scatterings can be due to either substrate or defects, dependent on the substrate orientations. The EG on SiC (11 (2) over bar0) exhibits a better control over the two major scattering mechanisms and achieves the highest mobility even with a high carrier concentration, promising for high performance graphene-based electronic devices. The method developed here will shed light on major aspects in governing carrier transport in EG to harness it effectively

Anisotropic quantum transport in a network of vertically aligned graphene sheets

Novel anisotropic quantum transport was observed in a network of vertically aligned graphene sheets (VAGSs), which can be regarded as composed of plenty of quasi-parallel, nearly intrinsic, freestanding monolayers of graphene. When a magnetic field was perpendicular to most graphene sheets, magnetoresistance (MR) curves showed a weak localization (WL) effect at low field and a maximum value at a critical field ascribed to diffusive boundary scattering. While the magnetic field was parallel to the graphene sheets, the MR maximum disappeared and exhibited a transition from WL to weak antilocalization (WAL) with increasing temperature and magnetic field. Edges as atomically sharp defects are the main elastic and inelastic intervalley scattering sources, and inelastic scattering is ascribed to electron-electron intervalley scattering in the ballistic regime. This is the first time simultaneously observing WL, WAL and diffusive boundary scattering in such a macroscopic three-dimensional graphene system. These indicate the VAGS network is a robust platform for the study of the intrinsic physical properties of graphene.