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Yin X.,Peking University | Li Y.,Columbia University | Ke F.,Peking University | Lin C.,Peking University | And 7 more authors.
Nano Research | Year: 2014

Significant changes in the Raman spectrum of single-layer graphene grown on a copper film were observed after the spontaneous oxidation of the underlying substrate that occurred under ambient conditions. The frequencies of the graphene G and 2D Raman modes were found to undergo red shifts, while the intensities of the two bands change by more than an order of magnitude. To understand the origin of these effects, we further characterized the samples by scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and atomic force microscopy (AFM). The oxidation of the substrate produced an appreciable corrugation in the substrate without disrupting the crystalline order of the graphene overlayer and/or changing the carrier doping level. We explain the red shifts of the Raman frequencies in terms of tensile strain induced by corrugation of the graphene layer. The changes in Raman intensity with oxidation arise from the influence of the thin cuprous oxide film on the efficiency of light coupling with the graphene layer in the Raman scattering process.[Figure not available: see fulltext.]. © 2014, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

Ke F.,Peking University | Yin X.-L.,Peking University | Tong N.,Peking University | Lin C.-F.,Peking University | And 6 more authors.
Chinese Physics B | Year: 2014

Few-layer graphene grown on Ni thin films has been studied by scanning tunneling microscopy. In most areas on the surfaces, moiré patterns resulted from rotational stacking faults were observed. At a bias lower than 200 mV, only one sublattice shows up in regions without moiré patterns while both sublattices are seen in regions with moiré pattens. This phenomenon can be used to identify AB stacked regions. The scattering characteristics at various types of step edges are different from those of monolayer graphene edges, either armchair or zigzag. © 2014 Chinese Physical Society and IOP Publishing Ltd.

Zhang Y.,Peking University | Han J.,Peking University | Liu S.,Peking University | Wan F.,Peking University | And 6 more authors.
Scripta Materialia | Year: 2016

Anisotropic Nd-Fe-B magnetic powders with high coercivity of 1440 kA/m as well as maximum energy products of 296 kJ/m3 were prepared. It was clearly observed that hydrogenation disproportionation desorption recombination (HDDR) process was confined in single crystal particles of Nd-Fe-B alloys. The recombined grains can be refined from 400 to 250 nm. The grain size refinement leads to a coercivity enhancement from 920 to 1440 kA/m. The coercivity enhancement is related to the grain refinement and the resultant formation of uniform Nd-rich grain boundary as strong pinning sites. © 2015 Acta Materialia Inc.

Xu X.,Peking University | Lin C.,Peking University | Fu R.,Peking University | Wang S.,Peking University | And 14 more authors.
AIP Advances | Year: 2016

Selective growth of either monolayer or bilayer graphene is of great importance. We developed a method to readily tune large area graphene growth from complete monolayer to complete bilayer. In an ambient pressure chemical vapor deposition process, we used the sample temperature at which to start the H2 flow as the control parameter and realized the change from monolayer to bilayer growth of graphene on Cu foil. When the H2 starting temperature was above 700°C, continuous monolayer graphene films were obtained. When the H2 starting temperature was below 350°C, continuous bilayer films were obtained. Detailed characterization of the samples treated under various conditions revealed that heating without the H2 flow caused Cu oxidation. The more the Cu substrate oxidized, the less graphene bilayer could form. © 2016 Author(s).

Lin C.,Peking University | Huang X.,Peking University | Ke F.,Peking University | Jin C.,Peking University | And 10 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2014

We report preparation of large area quasi-one-dimensional (1D) monolayer graphene superlattices on a prototypical high-index surface Cu(410)-O and characterization by Raman spectroscopy, Auger electron spectroscopy, low-energy electron diffraction, scanning tunneling microscopy, and scanning tunneling spectroscopy. The periodically stepped substrate gives a 1D modulation to graphene, forming a superlattice of the same superperiodicity. Consequently, the moiré pattern is also quasi-1D, with a different periodicity. Scanning tunneling spectroscopy measurements revealed new Dirac points formed at the superlattice Brillouin zone boundary as predicted by theories. © 2014 American Physical Society.

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