Epitaxial growth of atomically thin two-dimensional crystals such as transition metal dichalcogenides remains challenging, especially for producing large-size transition metal dichalcogenides bilayer crystals featuring high density of states, carrier mobility and stability at room temperature. Here we achieve in epitaxial growth of the second monolayer from the first monolayer by reverse-flow chemical vapor epitaxy and produce high-quality, large-size transition metal dichalcogenides bilayer crystals with high yield, control, and reliability. Customized temperature profiles and reverse gas flow help activate the first layer without introducing new nucleation centers leading to near-defect-free epitaxial growth of the second layer from the existing nucleation centers. A series of bilayer crystals including MoS2 and WS2, ternary Mo1−xWxS2 and quaternary Mo1−xWxS2(1−y)Se2y are synthesized with variable structural configurations and tunable electronic and optical properties. The robust, potentially universal approach for the synthesis of large-size transition metal dichalcogenides bilayer single crystals is highly-promising for fundamental studies and technological applications.

In vivo optical imaging shows that a fluorescent imaging probe, comprised of a near-infrared fluorophore attached to an affinity group containing two zinc(II)-dipicolylamine (Zn-DPA) units, targets prostate and mammary tumors in two different xenograft animal models. The tumor selectivity is absent with control fluorophores whose structures do not have appended Zn-DPA targeting ligands. Ex vivo biodistribution and histological analyses indicate that the probe is targeting the necrotic regions of the tumors, which is consistent with in vitro microscopy showing selective targeting of the anionic membrane surfaces of dead and dying cells.There is a major ongoing research effort to identify oligonucleotide and protein biomarkers of malignant disease. 1 Phospholipid biomarkers are less common, however, there is increasing evidence that the membrane surfaces of certain cells and particles of biomedical significance, smith.115@nd.edu. Supporting Information Available: Experimental details and additional imaging data. The information is available free of charge via the Internet at http://pubs.acs.org. Synthetic zinc(II)-dipicolylamine (Zn-DPA) coordination complexes are known to associate with multianionic phosphorylated biomolecules, 14 and we have discovered that they can be converted into optical imaging probes that target the outer surfaces of anionic vesicle and cell membranes.15 Fluorescent Zn-DPA probes can distinguish dead and dying mammalian cells from healthy cells in a cell culture,16 and also selectively target bacteria in heterogeneous biological media.17 Furthermore, we have recently demonstrated that the near-IR fluorescent probe 1 can be used to image bacterial infections in living mice, 18 indicating that probe 1 has a notable ability to selectively target anionic cells over other anionic sites in the bloodstream and extracellular matrix. Here, we greatly expand the animal imaging capability of probe 1 by showing that it can also target the anionic dead and dying cells within xenograft tumors in rat and mouse models. The structure of probe 1 includes a near-IR carbocyanine fluorophore whose absorption and emission wavelengths of 794 and 810 nm, respectively, are within the optimal window for maximum penetration through skin and tissue. 19 The high tumor selectivity of 1 is demonstrated by comparison to the less-selective imaging that is achieved by using control near-IR fluorophores 2 and 3 whose structures do not have Zn-DPA targeting ligands. The expected ability of probe 1 to selectively target dead and dying cells with exposed anionic phosphatidylserine was confirmed with in vitro fluorescence microscopy studies of mammalian cells treated with a cytotoxic agent. 16 Specifically, treatment of Jurkat cells (T lymphocytes) with camptothecin induced significant amounts of cell death, and as shown in Figure 1, the near-IR probe 1 stained the same cells as fluorescently labeled Annexin V. Using procedures that were approved by the appropriate institutional animal care and use committee, two tumor...

This communication reports a unique example of water-soluble and fluorescent-switchable amphiphilic diarylethene. This compound performs stable vesicle aggregation in water and shows aggregation-dependent emission in its open form. The fluorescence can be effectively switched by alternating between UV and visible light irradiation. This compound thus can stain KB cells for switchable living cell imaging with excellent resistance to fatigue.

A palladium phosphide electrocatalyst supported on carbon black (PdP2@CB) shows efficient water splitting in both alkaline and neutral electrolytes. Significantly lower overpotentials are required for PdP2@CB (27.5 mV in 0.5 m H2SO4; 35.4 mV in 1 m KOH; 84.6 mV in 1 m PBS) to achieve a HER electrocatalytic current density of 10 mA cm−2 compared to commercial Pt/CB (30.1 mV in 0.5 m H2SO4; 46.6 mV in 1 m KOH; 122.7 mV in 1 m PBS). Moreover, no loss in HER activity is detectable after 5000 potential sweeps. Only 270 mV and 277 mV overpotentials are required to reach a current density of 10 mA cm−2 for PdP2@CB to catalyze OER in 1 m KOH and 1 m PBS electrolytes, which is better OER activity than the benchmark IrO2 electrocatalyst (301 mV and 313 mV to drive a current density of 10 mA cm−2). 1.59 V and 1.72 V are needed for PdP2@CB to achieve stable water splitting catalytic current density of 10 mA cm−2 in 1 m PBS and 50 mA cm−2 in 1 m KOH for 10 h, respectively.

Transition from multi-layer to monolayer and sub-monolayer thickness leads to the many exotic properties and distinctive applications of two-dimensional (2D) MoS2. This transition requires atomic-layer-precision thinning of bulk MoS2 without damaging the remaining layers, which presently remains elusive. Here we report a soft, selective and high-throughput atomic-layer-precision etching of MoS2 in SF6 + N2 plasmas with low-energy (<0.4 eV) electrons and minimized ion-bombardment-related damage. Equal numbers of MoS2 layers are removed uniformly across domains with vastly different initial thickness, without affecting the underlying SiO2 substrate and the remaining MoS2 layers. The etching rates can be tuned to achieve complete MoS2 removal and any desired number of MoS2 layers including monolayer. Layer-dependent vibrational and photoluminescence spectra of the etched MoS2 are also demonstrated. This soft plasma etching technique is versatile, scalable, compatible with the semiconductor manufacturing processes, and may be applicable for a broader range of 2D materials and intended device applications.