We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm thick bilayers were found to be present in regions of the interface of graphene/mica heterostacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene with hole densities of (9 ± 2) × 10(12) cm(-2). The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, which is in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.

Atomically thin binary two-dimensional (2D) semiconductors exhibit diverse physical properties depending on their composition, structure, and thickness. By adding another element in these materials, which will lead to formation of ternary 2D materials, the property and structure would greatly change and significantly expanded applications could be explored. In this work, we report structural and optical properties of atomically thin chromium thiophosphate (CrPS), a ternary antiferromagnetic semiconductor. Its structural details were revealed by X-ray and electron diffraction. Transmission electron microscopy showed that preferentially cleaved edges are parallel to diagonal Cr atom rows, which readily identified their crystallographic orientations. Strong in-plane optical anisotropy induced birefringence that also enabled efficient determination of crystallographic orientation using polarized microscopy. The lattice vibrations were probed by Raman spectroscopy and exhibited significant dependence on thickness of crystals exfoliated down to a single layer. Optical absorption determined by reflectance contrast was dominated by d-d-type transitions localized at Cr ions, which was also responsible for the major photoluminescence peak at 1.31 eV. The spectral features in the absorption and emission spectra exhibited noticeable thickness dependence and hinted at a high photochemical activity for single-layer CrPS. The current structural and optical investigation will provide a firm basis for future study and application of this kind of atomically thin magnetic semiconductors.

The process of oxidation of a copper surface coated by a layer of graphene in water-saturated air at 50 °C was studied where it was observed that oxidation started at the graphene edge and was complete after 24 h. Isotope labeling of the oxygen gas and water showed that the oxygen in the formed copper oxides originated from water and not from the oxygen in air for both Cu and graphene-coated Cu, and this has interesting potential implications for graphene as a protective coating for Cu in dry air conditions. We propose a reaction pathway where surface hydroxyl groups formed at graphene edges and defects induce the oxidation of Cu. DFT simulation shows that the binding energy between graphene and the oxidized Cu substrate is smaller than that for the bare Cu substrate, which facilitates delamination of the graphene. Using this process, dry transfer is demonstrated using poly­(bisphenol A carbonate) (PC) as the support layer. The high quality of the transferred graphene is demonstrated from Raman maps, XPS, STM, TEM, and sheet resistance measurements. The copper foil substrate was reused without substantial weight loss to grow graphene (up to 3 cycles) of equal quality to the first growth after each cycle. It was found that dry transfer yielded graphene with less Cu impurities as compared to methods using etching of the Cu substrate. Using PC yielded graphene with less polymeric residue after transfer than the use of poly­(methyl methacrylate) (PMMA) as the supporting layer. Hence, this dry and clean delamination technique for CVD graphene grown on copper substrates is highly advantageous for the cost-effective large-scale production of graphene, where the Cu substrate can be reused after each growth.

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.

Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D 'on-stack' chemical conversion of single-layer crystalline MoS 2 into MoO 3 with a precise layer control that enables truly 2D MoO 3 and MoO 3 /MoS 2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O 2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO 3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS 2 . The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.