Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from those of their bulk counterparts. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S–W–S layers is also reported. The PL weakens with increasing number of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet’s center. The structure and chemical composition of the platelet edges appear to be critical for PL enhancement.

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.

The rice Xa21 gene, which confers resistance to Xanthomonas oryzae pv. oryzae race 6, was isolated by positional cloning. Fifty transgenic rice plants carrying the cloned Xa21 gene display high levels of resistance to the pathogen. The sequence of the predicted protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response. Characterization of Xa21 should facilitate understanding of plant disease resistance and lead to engineered resistance in rice.

Graphene nanosheets were produced in large quantity via a soft chemistry synthetic route involving graphite oxidation, ultrasonic exfoliation, and chemical reduction. X-ray diffraction and transmission electron microscopy (TEM) observations show that graphene nanosheets were produced with sizes in the range of tens to hundreds of square nanometers and ripple-like corrugations. High resolution TEM (HRTEM) and selected area electron diffraction (SAED) analysis confirmed the ordered graphite crystal structure of graphene nanosheets. The optical properties of graphene nanosheets were characterized by Raman spectroscopy.

Nanog, Sox2, and Oct4 are transcription factors all essential to maintaining the pluripotent embryonic stem cell phenotype. Through a cooperative interaction, Sox2 and Oct4 have previously been described to drive pluripotent-specific expression of a number of genes. We now extend the list of Sox2-Oct4 target genes to include Nanog. Within the Nanog proximal promoter, we identify a composite sox-oct cis-regulatory element essential for Nanog pluripotent transcription. This element is conserved over 250 million years of cumulative evolution within the eutherian mammals. A Nanog proximal promoter-EGFP (enhanced green fluorescent protein) reporter transgene recapitulates endogenous Nanog mRNA expression in embryonic stem cells and their differentiated derivatives. Sox2 and Oct4 interaction with the Nanog promoter was confirmed through mutagenesis and in vitro binding assays. Electrophoretic mobility shift assays indicate that the Sox2-Oct4 heterodimer forms more efficiently on the composite element within Nanog than the similar element within Fgf4. Using chromatin immunoprecipitation, we show that Oct4 and Sox2 bind to the Nanog promoter in living mouse and human embryonic stem cells. Furthermore, by specific knockdown of Oct4 and Sox2 mRNA by RNA interference in embryonic stem cells, we provide genetic evidence for a link between Oct4, Sox2, and the Nanog promoter. These studies extend the understanding of the pluripotent genetic regulatory network within which the Sox2-Oct4 complex are at the top of the regulatory hierarchy.Nanog is a homeobox-containing transcription factor with an essential function in maintaining the pluripotent cells of the inner cell mass and in the derivation of embryonic stem cells (ESCs) 1 from these (1). Furthermore, overexpression of Nanog is capable of maintaining the pluripotency and self-renewing characteristics of ESCs under what normally would be differentiation-inducing culture conditions (2). Concomitant with this essential function in pluripotent cell maintenance is its restricted expression pattern. Nanog transcripts first appear in the inner cells of the morula prior to blastocyst formation (1, 2), are restricted to the inner cell mass in the blastocyst (3), and are no longer detectable at implantation. Expression of Nanog reappears in the proximal epiblast at embryonic day 6 and remains restricted to the epiblast as development progresses (4). The factors controlling expression of this gene have yet to be described. The POU domain-containing Oct4 and the HMG domaincontaining Sox2 are two other transcription factors known to be essential for normal pluripotent cell development and maintenance (5, 6). Although both have independent roles in determining other cell types (6, 7), at least part of their function in pluripotent cells is via a synergistic interaction between the two to drive transcription of target genes. Currently known targets of Sox2-Oct4 synergy are Fgf4, Utf1, and Fbx15, as well as Sox2 and Pou5f1 (the gene encoding Oct4) themselves (8 -13). Each of these targe...