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Understanding the long-term stability of MoS2 is important for various optoelectronic applications. Herein, we show that the long-term exposure to an oxygen atmosphere for up to a few months results in zigzag (zz)-directional line unzipping of the MoS2 basal plane. In contrast to exposure to dry or humid N2 atmospheres, dry O2 treatment promotes the initial formation of line defects, mainly along the armchair (ac) direction, and humid O2 treatment further promotes ac line unzipping near edges. Further incubation of MoS2 for a few months in an O2 atmosphere results in massive zz-directional line unzipping. The photoluminescence and the strain-doping plot based on two prominent bands in the Raman spectrum show that, in contrast to dry-N2-treated MoS2, the O2-treated MoS2 primarily exhibits hole doping, whereas humid-O2-treated MoS2 mainly exists in a neutral charge state with tension. This study provides a guideline for MoS2 preservation and a further method for generating controlled defects. [ABSTRACT FROM AUTHOR]

Although various applications extensively utilize polymer-assisted graphene transfer step, the role of residual polymer on graphene was not well-understood. Here, we report the effect of poly (methyl methacrylate) (PMMA) on chemical vapor deposition-grown hexagonal graphene via Raman spectroscopy. Analysis of bare-, PMMA-covered supported, and PMMA-covered suspended graphene exhibits that their G and 2D band positions are progressively downshifted in that order. Mapping of spatial G and 2D band shifts into doping and strain contributions shows that PMMA residue exerts moderate 0.15% tensile strain on graphene/substrate, as compared to that of bare graphene. During this tensile strain, residual PMMA-covered graphene maintains its doping level as much as bare graphene does. [ABSTRACT FROM AUTHOR]

We fabricated multiple-layered nonwoven multiwalled carbon nanotubes (MWCNT) sheets by means of treating MWCNTs with a spark plasma sintering process. We characterized the samples by Raman spectroscopy, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and the field effect mobility method. Each thin nonwoven MWCNT layer is mainly comprised of interlocked, intertwined, or interconnected MWCNTs. XRD and Raman spectra coincidentally revealed that the graphitization occurred in the course of transformation from pristine to nonwoven MWCNTs. The multiple-layered nonwoven MWCNTs showed a higher electrical conductance than a single-layered nonwoven MWCNTs sheet. [ABSTRACT FROM AUTHOR]

Few atom thick, twisted bilayer graphene (tBLG) possesses a rotation angle (θ) dependent van Hove singularity (vHs). Fine-tuning vHs serves a potential method to enhance charge transfer (CT) in surface enhanced Raman spectroscopy. This study shows that tBLG having a specific θ promotes as high as a 1.7 times enhancement of the Raman signals of copper phthalocyanine (CuPc) as compared to that caused by single layer graphene (SLG). The results of a combination of reflection imaging spectroscopy and widefield Raman provide spatial and spectral information about both tBLG with θ ranging from 10.9 to 13.7° and the corresponding vHs. Comparison of Raman spectra of CuPc in presence and absence of tBLG demonstrates that a significant enhancement of certain CuPc vibrational modes occurs when the underlying tBLG possesses a θ = 12.2°, showing as high as 6.8 and 1.7 times enhancements of certain vibrational mode as compared to those of CuPc on bare and SLG substrates, respectively. Theoretical calculations indicate that a match between the energies of vHs of tBLG with those of frontier orbitals of CuPc facilitates CT from the distant SLG to CuPc. [Display omitted] [ABSTRACT FROM AUTHOR]

When the width of a graphene nanoribbon (GNR) is only a few nanometers, it possesses semiconducting properties that enable various high-end electronic applications. In this study, we report that the dense and stable dispersion of a natural graphite formed using flavin mononucleotide (FMN) as a surfactant produces GNRs as small as 10 nm in width. High-resolution transmission electron microscopy reveals GNRs with various widths, along with a graphene flake containing straight-edged GNRs and cuts, depending on substrate treatments. Such nanoribbon formation originates from sonochemical graphene unzipping with a one-dimensional FMN supramolecular ribbon as a template. Raman spectroscopy demonstrates the universal intensity ratio of D over D′ bands near 4, supporting formation of continuous edge defect. Thermal annealing enhances the optical contrast and van der Waals interactions of the graphene film, resulting in increased conductivity compared to the as-prepared graphene film, which is also better than that of reduced graphene oxide. [ABSTRACT FROM AUTHOR]