Because of its superior stretchability, graphene exhibits rich structural deformation behaviours and its strain engineering has proven useful in modifying its electronic and magnetic properties. Despite the strain-sensitivity of the Raman G and 2D modes, the optical characterization of the native strain in graphene on silica substrates has been hampered by excess charges interfering with both modes. Here we show that the effects of strain and charges can be optically separated from each other by correlation analysis of the two modes, enabling simple quantification of both. Graphene with in-plane strain randomly occurring between − 0.2% and 0.4% undergoes modest compression ( − 0.3%) and significant hole doping on thermal treatments. This study suggests that substrate-mediated mechanical strain is a ubiquitous phenomenon in two-dimensional materials. The proposed analysis will be of great use in characterizing graphene-based materials and devices.
Because of the dominant role of the surface of molecules and their individuality, molecules behave distinctively in a confined space, which has far-reaching implications in many physical, chemical, and biological systems. Here we demonstrate that graphene forms a unique atom-thick interstitial space that enables the study of molecular diffusion in two dimensions with underlying silica substrates. Raman spectroscopy visualized intercalation of water from the edge to the center underneath graphene in real time, which was dictated by the hydrophilicity of the substrates. In addition, graphene undergoes reversible deformation to conform to intercalating water clusters or islands. Atomic force microscopy confirmed that the interfacial water layer is only ca. 3.5 Å thick, corresponding to one bilayer unit of normal ice. This study also demonstrates that oxygen species responsible for the ubiquitous hole doping are located below graphene. In addition to serving as a transparent confining wall, graphene and possibly other two-dimensional materials can be used as an optical indicator sensitive to interfacial mass transport and charge transfer.
Even weak van der Waals (vdW) adhesion between two-dimensional solids may perturb their various materials properties owing to their low dimensionality. Although the electronic structure of graphene has been predicted to be modified by the vdW interaction with other materials, its optical characterization has not been successful. In this report, we demonstrate that Raman spectroscopy can be utilized to detect a few percent decrease in the Fermi velocity (v(F)) of graphene caused by the vdW interaction with underlying hexagonal boron nitride (hBN). Our study also establishes Raman spectroscopic analysis which enables separation of the effects by the vdW interaction from those by mechanical strain or extra charge carriers. The analysis reveals that spectral features of graphene on hBN are mainly affected by change in v(F) and mechanical strain but not by charge doping, unlike graphene supported on SiO₂ substrates. Graphene on hBN was also found to be less susceptible to thermally induced hole doping.
Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are presented. Taking advantage of the facile method of layer-by-layer (LbL) assembly, individual chemically exfoliated MoS2 layers are not only effectively isolated from interlayer coupling but also doped by functional polymeric layers. It is clearly demonstrated that MoS2 nanosheets separated by polymeric trilayers exhibit a much larger increase in photoluminescence (PL) as the number of layers is increased. The enhanced PL has been correlated to the ratio of excitons to trions with the type of polymeric spacers. Because uniform heterogeneous interfaces can be formed between various transition metal dichalcogenides and other soft materials, LbL assembly offers possibilities for further development in the solution-processable assemblies of two-dimensional materials.
⊥ These authors contributed equally. ABSTRACT:We demonstrate the large-area lithography-free ordered perforation of reduced graphene oxide (rGO) and graphene grown by chemical vapor deposition (CVD) with arrayed Pt nanoparticles (NPs) prepared by using self-patterning diblock copolymer micelles. The rGO layers were perforated by Pt NPs formed either on top or bottom surface. On the other hand, CVD graphene was perforated only when the Pt NPs were placed under the graphene layer.Various control experiments confirm that the perforation reaction of CVD graphene was catalysed by Pt NPs, where the mechanical strain as well as the chemical reactivity of Pt have lowered the activation energy barriers for the oxidation reaction of C=C bonds in graphene.Systematic AFM and Raman analysis revealed the detailed perforation mechanism. The pore size and spacing can be controlled and thus, our present work may open a new direction in the development of ordered nanopatterns on graphene using metal NPs.
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