Raman spectrum of graphene and graphene layers.
TL;DR: This work shows that graphene's electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers, and allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area.
Abstract: Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality We show that its electronic structure is captured in its Raman spectrum that clearly evolves with the number of layers The D peak second order changes in shape, width, and position for an increasing number of layers, reflecting the change in the electron bands via a double resonant Raman process The G peak slightly down-shifts This allows unambiguous, high-throughput, nondestructive identification of graphene layers, which is critically lacking in this emerging research area
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TL;DR: This work intentionally creates atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and studies the evolution of their Raman and photoluminescence spectra to determine the defectiveness of 2D semiconducting nanosheets.
Abstract: It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1g ring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2 or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ∼0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications.
390 citations
TL;DR: A face-to-face transfer method for wafer-scale graphene films that is so far the only known way to accomplish both the growth and transfer steps on one wafer, and which is amenable to batch processing in a semiconductor production line, and thus will speed up the technological application of graphene.
Abstract: Graphene has attracted worldwide interest since its experimental discovery, but the preparation of large-area, continuous graphene film on SiO2/Si wafers, free from growth-related morphological defects or transfer-induced cracks and folds, remains a formidable challenge. Growth of graphene by chemical vapour deposition on Cu foils has emerged as a powerful technique owing to its compatibility with industrial-scale roll-to-roll technology. However, the polycrystalline nature and microscopic roughness of Cu foils means that such roll-to-roll transferred films are not devoid of cracks and folds. High-fidelity transfer or direct growth of high-quality graphene films on arbitrary substrates is needed to enable wide-ranging applications in photonics or electronics, which include devices such as optoelectronic modulators, transistors, on-chip biosensors and tunnelling barriers. The direct growth of graphene film on an insulating substrate, such as a SiO2/Si wafer, would be useful for this purpose, but current research efforts remain grounded at the proof-of-concept stage, where only discontinuous, nanometre-sized islands can be obtained. Here we develop a face-to-face transfer method for wafer-scale graphene films that is so far the only known way to accomplish both the growth and transfer steps on one wafer. This spontaneous transfer method relies on nascent gas bubbles and capillary bridges between the graphene film and the underlying substrate during etching of the metal catalyst, which is analogous to the method used by tree frogs to remain attached to submerged leaves. In contrast to the previous wet or dry transfer results, the face-to-face transfer does not have to be done by hand and is compatible with any size and shape of substrate; this approach also enjoys the benefit of a much reduced density of transfer defects compared with the conventional transfer method. Most importantly, the direct growth and spontaneous attachment of graphene on the underlying substrate is amenable to batch processing in a semiconductor production line, and thus will speed up the technological application of graphene.
388 citations
TL;DR: The false-color (3D type) image of the intensity of the Raman spectra of monolayer MoS2 versus both peak positions and polar angles is plotted and shows that the strongest E2g (1+) and E2G (1-) peaks appear at different angles.
Abstract: The false-color (3D type) image of the intensity of the Raman spectra of monolayer MoS2 versus both peak positions and polar angles is plotted. It shows that the strongest E2g (1+) and E2g (1-) peaks appear at different angles, reflected as the alternation of the maxima of the intensity within the frequency range of the E2g (1) mode, which is the consequence of the crystallographic orientation relevant to the strain direction as predicted by theoretical analysis.
387 citations
TL;DR: In this article, a photocatalytic reduction of the graphene oxide sheets by the TiO2 nanoparticles in ethanol was shown to increase the graphitized sp2 structure over the disorders in the reduced graphene oxides.
Abstract: TiO2 nanoparticles were physically attached to chemically synthesized single-layer graphene oxide nanosheets deposited between Au electrodes in order to investigate the electrical, chemical, and structural properties of the TiO2/graphene oxide composition exposed to UV irradiation. X-ray photoelectron spectroscopy showed that after effective photocatalytic reduction of the graphene oxide sheets by the TiO2 nanoparticles in ethanol, the carbon content of the reduced graphene oxides gradually decreased by increasing the irradiation time, while no considerable variation was detected in the reduction level of the reduced sheets. Raman spectroscopy indicated that, at first, the photocatalytic reduction resulted in a significant increase in the graphitized sp2 structure over the disorders in the graphene oxides. After that, as the carbon content decreased by UV irradiation, further disorders appeared in the reduced graphene oxide sheets, confirming degradation of the reduced sheets after the photocatalytic redu...
386 citations
TL;DR: This comprehensive review describes some important recent advances with respect to 3DGMs, including their preparation methods, characterization and applications inCatalysis, e.g., photocatalysis, electrocatalysis, organic catalysis, and CO oxidation.
Abstract: Over the past few decades, two-dimensional graphene based materials (2DGMs) have piqued the interest of scientists worldwide, and the exploration of their potential applications in catalysis, sensors, electronic devices and energy storage due to their extraordinary physical and chemical properties has rapidly progressed. As for these 2DGMs, there is a complementary need to assemble 2D building blocks hierarchically into more complicated and hierarchical three-dimensional graphene-based materials (3DGMs). Such a capability is vitally crucial in order to design sophisticated and multi-functional catalysts with tailorable properties. This comprehensive review describes some important recent advances with respect to 3DGMs, including their preparation methods, characterization and applications in catalysis, e.g., photocatalysis, electrocatalysis, organic catalysis, and CO oxidation. The importance of the relationship between the structure and catalytic performance, a topic which has become a central focus of research in order to develop high-performance catalytic systems, is discussed. Likely future developments and their associated challenges are proposed and discussed.
386 citations
References
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28 Jul 2005
TL;DR: PfPMP1）与感染红细胞、树突状组胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作�ly.
Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。
18,940 citations