A two-step chemical vapor deposition process for the growth of continuous vertical heterostructure WSe2/h-BN and its optical properties.

The expansion of two-dimensional (2D) van der Waals heterostructure materials growth and synthesis leads to impressive results in the development and improvement of electronic and optoelectronic applications. Herein, a vertical WSe2/hBN heterostructure was obtained via a dual CVD system, in which prior to the WSe2 growth a continuous monolayer hBN was obtained on a SiO2/Si substrate. Comparing growth on SiO2/Si and quartz substrates, we found that the underlayer of hBN leads to a desorption/diffusion process of tungsten (W) and selenium (Se) producing high-quality and large-area WSe2 growth. In contrast with WSe2/SiO2 and WSe2/quartz heterostructures, the photoluminescence properties of WSe2/hBN exhibit a sharp intense WSe2 peak at 790 nm with a narrow full width at half-maximum (80 meV) due to no dangling bonds and dielectric effect of the hBN interface. The photoluminescence results suggest that the WSe2/hBN heterostructure has high crystallinity with a defect-free interface.

Graphene dispersions in alkanes: toward fast drying conducting inks.

Graphene inks are becoming widely popular. However the vast majority of these inks are formulated in polar solvents with high-boiling points. Their slow evaporation is a bottleneck factor in roll-to-roll printing processes. Here, we developed a highly-conductive fast-drying graphene ink in isooctane, a non-polar and low-boiling solvent. For this purpose, a diblock copolymer containing pendant cholesterol groups was used during the exfoliation of natural graphite in isooctane. The polymer develops non-covalent supramolecular interactions with the graphene conjugated system, resulting in the formation of stable graphene dispersions (up to c = 4 mg mL-1). These dispersions were used for direct writing on a variety of substrates, and were shown to dry instantly after application. The influence of polymer concentration on graphene characteristics, on colloidal stability and on electrochemical characteristics has been studied. The lowest sheet resistance (80 Ω â–¡-1) was obtained when 23% of the graphene surface was covered by the polymer. In this case, the flakes were constituted of 2-5 graphene layers. More extensive exfoliation, down to single-layer graphene, was achieved at greater surface coverage, but led to inks with higher sheet resistance. Thus, by combining a tailored polymeric dispersant, a smooth exfoliation process and a low-boiling non-polar ink solvent, we were able to prepare highly-conductive fast-drying graphene inks which should have a high potentital for the development of roll-to-roll printed electronics.

Corrosion resistance of monolayer hexagonal boron nitride on copper.

Hexagonal boron nitride (hBN) is a layered material with high thermal and chemical stability ideal for ultrathin corrosion resistant coatings. Here, we report the corrosion resistance of Cu with hBN grown by chemical vapor deposition (CVD). Cyclic voltammetry measurements reveal that hBN layers inhibit Cu corrosion and oxygen reduction. We find that CVD grown hBN reduces the Cu corrosion rate by one order of magnitude compared to bare Cu, suggesting that this ultrathin layer can be employed as an atomically thin corrosion-inhibition coating.

Reduced graphene oxide growth on 316L stainless steel for medical applications.

We report a new method for the growth of reduced graphene oxide (rGO) on the 316L alloy of stainless steel (SS) and its relevance for biomedical applications. We demonstrate that electrochemical etching increases the concentration of metallic species on the surface and enables the growth of rGO. This result is supported through a combination of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning electron microscopy (SEM), density functional theory (DFT) calculations and static water contact angle measurements. Raman spectroscopy identifies the G and D bands for oxidized species of graphene at 1595 cm(-1) and 1350 cm(-1), respectively, and gives an ID/IG ratio of 1.2, indicating a moderate degree of oxidation. XPS shows -OH and -COOH groups in the rGO stoichiometry and static contact angle measurements confirm the wettability of rGO. SEM and AFM measurements were performed on different substrates before and after coronene treatment to confirm rGO growth. Cell viability studies reveal that these rGO coatings do not have toxic effects on mammalian cells, making this material suitable for biomedical and biotechnological applications.

Quantum Hall effect in hydrogenated graphene.

The quantum Hall effect is observed in a two-dimensional electron gas formed in millimeter-scale hydrogenated graphene, with a mobility less than 10 cm2/V·s and corresponding Ioffe-Regel disorder parameter (k(F)λ)(-1) ≫ 1. In a zero magnetic field and low temperatures, the hydrogenated graphene is insulating with a two-point resistance of the order of 250h/e2. The application of a strong magnetic field generates a negative colossal magnetoresistance, with the two-point resistance saturating within 0.5% of h/2e2 at 45 T. Our observations are consistent with the opening of an impurity-induced gap in the density of states of graphene. The interplay between electron localization by defect scattering and magnetic confinement in two-dimensional atomic crystals is discussed.