Flexible and digestible wood caused by viral-induced alteration of cell wall composition.

Woody plant material represents a vast renewable resource that has the potential to produce biofuels and other bio-based products with favorable net CO2 emissions.1,2 Its potential has been demonstrated in a recent study that generated novel structural materials from flexible moldable wood.3 Apple rubbery wood (ARW) disease is the result of a viral infection that causes woody stems to exhibit increased flexibility.4 Although ARW disease is associated with the presence of an RNA virus5 known as apple rubbery wood virus (ARWV), how the unique symptoms develop is unknown. We demonstrate that the symptoms of ARWV infections arise from reduced lignification within the secondary cell wall of xylem fibers and result in increased wood digestibility. In contrast, the mid-lamellae region and xylem ray cells are largely unaffected by the infection. Gene expression and proteomic data from symptomatic xylem clearly show the downregulation of phenylalanine ammonia lyase (PAL), the enzyme catalyzing the first committed step in the phenylpropanoid pathway leading to lignin biosynthesis. A large increase in soluble phenolics in symptomatic xylem, including the lignin precursor phenylalanine, is also consistent with PAL downregulation. ARWV infection results in the accumulation of many host-derived virus-activated small interfering RNAs (vasiRNAs). PAL-derived vasiRNAs are among the most abundant vasiRNAs in symptomatic xylem and are likely the cause of reduced PAL activity. Apparently, the mechanism used by the virus to alter lignin exhibits similarities to the RNAi strategy used to alter lignin in genetically modified trees to generate comparable improvements in wood properties.6-8.

Raman fingerprint of aligned graphene/h-BN superlattices.

Graphene placed on hexagonal-boron nitride (h-BN) experiences a superlattice (Moiré) potential, which leads to a strong reconstruction of graphene's electronic spectrum with new Dirac points emerging at sub-eV energies. Here we study the effect of such superlattices on graphene's Raman spectrum. In particular, the 2D Raman peak is found to be exquisitely sensitive to the misalignment between graphene and h-BN lattices, probably due to the presence of a strain distribution with the same periodicity of the Moiré potential. This feature can be used to identify graphene superlattices with a misalignment angle smaller than 2°.

Single- and double-sided chemical functionalization of bilayer graphene.

An experimental study on the interaction between the top and bottom layer of a chemically functionalized graphene bilayer by mild oxygen plasma is reported. Structural, chemical, and electrical properties are monitored using Raman spectroscopy, transport measurements, conductive atomic force microscopy and X-ray photoelectron spectroscopy. Single- and double-sided chemical functionalization are found to give very different results: single-sided modified bilayers show relatively high mobility (200-600 cm(2) V(-1) s(-1) at room temperature) and a stable structure with a limited amount of defects, even after long plasma treatment (>60 s). This is attributed to preferential modification and limited coverage of the top layer during plasma exposure, while the bottom layer remains almost unperturbed. This could eventually lead to decoupling between top and bottom layers. Double-sided chemical functionalization leads to a structure containing a high concentration of defects, very similar to graphene oxide. This opens the possibility to use plasma treatment not only for etching and patterning of graphene, but also to make heterostructures (through single-sided modification of bilayers) for sensors and transistors and new graphene-derivatives materials (through double-sided modification).

Probing the nature of defects in graphene by Raman spectroscopy.

Raman spectroscopy is able to probe disorder in graphene through defect-activated peaks. It is of great interest to link these features to the nature of disorder. Here we present a detailed analysis of the Raman spectra of graphene containing different type of defects. We found that the intensity ratio of the D and D' peak is maximum (∼13) for sp(3)-defects, it decreases for vacancy-like defects (∼7), and it reaches a minimum for boundaries in graphite (∼3.5). This makes Raman Spectroscopy a powerful tool to fully characterize graphene.

Raman spectroscopy of boron-doped single-layer graphene.

The introduction of foreign atoms, such as nitrogen, into the hexagonal network of an sp(2)-hybridized carbon atom monolayer has been demonstrated and constitutes an effective tool for tailoring the intrinsic properties of graphene. Here, we report that boron atoms can be efficiently substituted for carbon in graphene. Single-layer graphene substitutionally doped with boron was prepared by the mechanical exfoliation of boron-doped graphite. X-ray photoelectron spectroscopy demonstrated that the amount of substitutional boron in graphite was ~0.22 atom %. Raman spectroscopy demonstrated that the boron atoms were spaced 4.76 nm apart in single-layer graphene. The 7-fold higher intensity of the D-band when compared to the G-band was explained by the elastically scattered photoexcited electrons by boron atoms before emitting a phonon. The frequency of the G-band in single-layer substitutionally boron-doped graphene was unchanged, which could be explained by the p-type boron doping (stiffening) counteracting the tensile strain effect of the larger carbon-boron bond length (softening). Boron-doped graphene appears to be a useful tool for engineering the physical and chemical properties of graphene.

Electrochemical behavior of monolayer and bilayer graphene.

Results of a study on the electrochemical properties of exfoliated single and multilayer graphene flakes are presented. Graphene flakes were deposited on silicon/silicon oxide wafers to enable fast and accurate characterization by optical microscopy and Raman spectroscopy. Conductive silver paint and silver wires were used to fabricate contacts; epoxy resin was employed as a masking coating in order to expose a stable, well-defined area of graphene. Both multilayer and monolayer graphene microelectrodes showed quasi-reversible behavior during voltammetric measurements in potassium ferricyanide. However, the standard heterogeneous charge transfer rate constant, k°, was estimated to be higher for monolayer graphene flakes.

High-yield production and transfer of graphene flakes obtained by anodic bonding.

We report large-yield production of graphene flakes on glass by anodic bonding. Under optimum conditions, we counted several tens of flakes with lateral size around 20-30 µm and a few tens of flakes with larger size. About 60-70% of the flakes have a negligible D peak. We show that it is possible to easily transfer the flakes by the wedging technique. The transfer on silicon does not damage graphene and lowers the doping. The charge mobility of the transferred flakes on silicon is on the order of 6000 cm(2)/V s (at a carrier concentration of 10(12) cm(-2)), which is typical for devices prepared on this substrate with exfoliated graphene.