[en] Full text: Graphene nanoribbons (GNRs) promise high potential for future nanoscale electronic devices. While 2-dimensional graphene is semimetallic, electron confinement and edge effects in narrow (<10nm) GNRs can result in the opening of a band gap. Only recent advances in the bottom-up fabrication of atomically precise GNRs have enabled reliable experimental investigations of well-defined GNRs. We have studied the optical properties of GNRs grown on Au(788) using reflectance difference spectroscopy (RDS), taking advantage of the optical anisotropy due to uniaxial alignment of the GNRs parallel to the step edges of the vicinal Au(788) surface. (author)

[en] Despite many excellent properties, the synthesis of high quality graphene with low-cost way is still a challenge, thus many different factors have been researched. In this work, the effect of surface roughness to the graphene quality was studied. Graphene was synthesized by plasma enhanced chemical vapor deposition (PECVD) method on copper substrates with different roughness from 0.074 μm to 0.339 μm, which were prepared via annealing, corrosion or polishing, respectively. Ar⁺ plasma cleaning was applied before graphene growth in order to accommodate similar surface chemical reactivity to each other. Scanning electron microscope and Raman spectroscope were employed to investigate the effect of surface roughness, which reveals that the graphene quality decrease first and then increase again according to the ratio of I_D/I_G in Raman spectroscopy. When the ratio of I_D/I_G reaches the largest number, the substrate roughness is 0.127 μm, where is the graphene quality changing point. First principle calculation was applied to explain the phenomenon and revealed that it is strongly affected by the graphene grain size and quantity which can induce defects. This strategy is expected to guide the industrial production of graphene. (paper)

[en] The fascinating Dirac cone in honeycomb graphene, which underlies many unique electronic properties, has inspired the vast endeavors on pursuing new two-dimensional (2D) Dirac materials. Based on the density functional theory method, a 2D material Zn₃Si₂ of honeycomb transition-metal silicide with intrinsic Dirac cones has been predicted. The Zn₃Si₂ monolayer is dynamically and thermodynamically stable under ambient conditions. Importantly, the Zn₃Si₂ monolayer is a room-temperature 2D Dirac material with a spin–orbit coupling energy gap of 1.2 meV, which has an intrinsic Dirac cone arising from the special hexagonal lattice structure. Hole doping leads to the spin polarization of the electron, which results in a Dirac half-metal feature with single-spin Dirac fermion. This novel stable 2D transition-metal-silicon-framework material holds promises for electronic device applications in spintronics. (paper)

[en] A single ZnO nanowire with intrinsic oxygen vacancies is utilized to fabricate four-contact device with focus ion beam lithography technique. Cathodoluminescent spectra indicate strong near-UV and green emission at both room temperature and low temperatures. Experimental measurement shows the temperature-dependent conductivity of the ZnO nanowire at low temperatures (below 100 K). The further theoretical analysis confirms that weak localization plays an important role in the electrical transport, which is attributed to the surface states induced by plenty of oxygen vacancies in ZnO nanowire. (condensed matter: electronic structure, electrical, magnetic, and optical properties)

[en] Full text: Graphene Nanoribbons (GNRs) offer great potential for future electronic and optoelectronic devices due to sizable electronic band gaps that are expected for narrow (<10 nm) GNRs. Recently, we reported a bottom-up fabrication protocol based on on-surface chemical routes to colligate and dehydrogenate specifically designed precursor monomers to form N=7 armchair GNRs (7-AGNRs) with atomic precision, a prerequisite for the control of the band gap magnitude. Here, we report scanning tunneling spectroscopy (STS) and angle-resolved photoelectron spectroscopy (ARPES) results allowing for the determination of band gap and exact band dispersion of 7-AGNRs. Furthermore, ARPES results are compared to Fourier-transformed STS (FT-STS) results, which additionally give direct access to the unoccupied band dispersion. (author)

[en] An electric field-assisted Hummers method for the synthesis of graphene oxide (GO) is reported which can enhance the efficiency of oxidation processes, while decreasing destructive effects on the resultant graphite lattice. The electric field varies from 0 ∼ 167 V cm⁻¹ was applied outside the reaction system and operated on three temperature stages of Hummers method which use a low dose of oxidizer. The as-prepared GO nanoplate has a layer spacing of 1 nm. This improved method may be important for preparing high-quality GO with low cost and light pollution. Furthermore, it is a promising improvement to the efficiency of reduction processes and optimization of the structure of GO or reduced graphene oxide (rGO), allowing them to be applied to various graphene-based functional materials. (paper)

[en] Using first-principle calculations, we predict a new family of stable two-dimensional (2D) topological insulators (TI), monolayer Be₃ X ₂ (X = C, Si, Ge, Sn) with honeycomb Kagome lattice. Based on the configuration of Be₃C₂, which has been reported to be a 2D Dirac material, we construct the other three 2D materials and confirm their stability according to their chemical bonding properties and phonon-dispersion relationships. Because of their tiny spin–orbit coupling (SOC) gaps, Be₃C₂ and Be₃Si₂ are 2D Dirac materials with high Fermi velocity at the same order of magnitude as that of graphene. For Be₃Ge₂ and Be₃Sn₂, the SOC gaps are 1.5 meV and 11.7 meV, and their topological nontrivial properties are also confirmed by their semi-infinite Dirac edge states. Our findings not only extend the family of 2D Dirac materials, but also open an avenue to track new 2DTI. (paper)

[en] Heat conducting silica gel sheets with graphene nanoplatelets (GNPs) filler prepared by high pressure homogenization were fabricated. The dispersed GNPs filler in silica gel significantly affects the thermal conductivity of GNPs silica gel sheets (GNPs-SGS). The thermal conductivity of GNPs-SGS with 5 wt% GNPs reaches 0.43 W(m · k)⁻¹ which increased by 110% and 50% comparing to the pure silica gel sheets (Pure-SGS) and graphite silica gel sheets (GP-SGS) with the same mass fraction. The efficient of heat conduction of heat-sink device which made of GNPs-SGS with 5 wt% is higher than the one which made of Pure-SGS. Besides, The temperature of the thermal plate is 22 °C lower when using 5 wt% GNPs-SGS compared to the bare one measured by thermal management simulator (TMS), proving its good heat radiation ability. FE-SEM was used to observe the fillers and the section of gel sheets, it can be clearly observed the layered and the uniform distribution of GNPs in the matrix. The facile process of high pressure homogenization to exfoliate GNPs is a feasible program for industrial production. (paper)

[en] The growth of high quality graphene on an insulator substrate such as SiO₂/Si broadens the way for the development of graphene-based electronic devices, which still remains significant attraction. Here, we present an approach to directly synthesize high quality single layer graphene (SLG) on SiO₂ substrate with Cu-vapor-assisted chemical vapor deposition (CVD). Because Cu is in vapor state, not only it catalyzes the pyrolysis of hydrocarbon precursors and accelerates the nucleation, but also it won’t remain on the graphene surface nor under the graphene. Different testing methods confirm that large scale, high quality of SLG is successfully synthesized. Furthermore, the graphene has good conductivity and transparency. Therefore, this method bares great potential in future electronic applications. (paper)

[en] Cu₂Se monolayer (ML) synthesized experimentally is a member of transition metal chalcogenides materials, which has attracted significant attention due to its diversity and unique properties. However, the feature of an indirect band gap of Cu₂Se ML in the low-temperature phase limits its’ application in electronics devices. Our study results based on the first principle calculations show that indirect-direct band gap transitions can occur in Cu₂Se ML under appropriate uniaxial or biaxial strains. The band gap of Cu₂Se ML is controllable due to the different responses of the edge-states near the Fermi level to the strain. The phonon dispersion suggests that the semiconducting Cu₂Se ML can maintain dynamic stability in a wide range of strains. With the tunable electronic structure, semiconducting Cu₂Se ML would become a promising candidate for electronic devices. (paper)