Atomic-Scale Deformation in N-Doped Carbon Nanotubes

We present the N-doping induced atomic-scale structural deformation in N-doped carbon nanotubes by using density functional theory calculations. For substitutional N-doped nanotube clusters, the N dopant with an excess electron lone pair exhibits the high negative charge, and the homogeneously distributed dopants enlarge the tube diameter in both zigzag and armchair cases. On the other hand, in pyridine-like N-doped ones, the concentrated N atoms result in a positively curved graphene layer and, thus, can be responsible for tube wall roughness and the formation of interlinked structures

Design for Approaching Cicada-Wing Reflectance in Low- and High-Index Biomimetic Nanostructures

Natural nanostructures in low refractive index Cicada wings demonstrate ≤1% reflectance over the visible spectrum. We provide design parameters for Cicada-wing-inspired nanotip arrays as efficient light harvesters over a 300–1000 nm spectrum and up to 60° angle of incidence in both low-index, such as silica and indium tin oxide, and high-index, such as silicon and germanium, photovoltaic materials. Biomimicry of the Cicada wing design, demonstrating gradient index, onto these material surfaces, either by real electron cyclotron resonance microwave plasma processing or by modeling, was carried out to achieve a target reflectance of ∼1%. Design parameters of spacing/wavelength and length/spacing fitted into a finite difference time domain model could simulate the experimental reflectance values observed in real silicon and germanium or in model silica and indium tin oxide nanotip arrays. A theoretical mapping of the length/spacing and spacing/wavelength space over varied refractive index materials predicts that lengths of ∼1.5 μm and spacings of ∼200 nm in high-index and lengths of ∼200–600 nm and spacings of ∼100–400 nm in low-index materials would exhibit ≤1% target reflectance and ∼99% optical absorption over the entire UV–vis region and angle of incidence up to 60°

Molecular Sensing with Ultrafine Silver Crystals on Hexagonal Aluminum Nitride Nanorod Templates

A fully plasma-based technique of generating ultrafine (sub-10-nm) nanocrystalline silver particulates on wide band gap and chemically inert hexagonal aluminum nitride nanorod templates has been demonstrated. These specially prepared substrates are ready to use for molecular sensing by room-temperature surface-enhanced Raman scattering. An enhancement factor of 2 × 106 was observed for micromolar solutions of Rhodamine 6G

Electronic Supplementary Materials including models and fitting of the electrochemical impedance spectroscopy, XRD, XPS, EDX spectra, and polarization curves. from Enhanced hydrogen evolution reaction on hybrids of cobalt phosphide and molybdenum phosphide

Production of hydrogen from water electrolysis has stimulated the search of sustainable electrocatalysts as possible alternatives. Recently, cobalt phosphide (CoP) and molybdenum phosphide (MoP) received great attention due to their superior catalytic activity and stability towards the hydrogen evolution reaction (HER) which rivals platinum catalysts. In this study, we synthesize and study a series of catalysts based on hybrids of CoP and MoP with different Co/Mo ratio. The HER activity shows a volcano shape and reaches a maximum for Co/Mo = 1. Tafel analysis indicates a change in the dominating step of Volmer–Hyrovský mechanism. Interestingly, X-ray diffraction patterns confirmed a major ternary interstitial hexagonal CoMoP₂ crystal phase is formed which enhances the electrochemical activity

Functionalizing Biomaterials to Be an Efficient Proton-Exchange Membrane and Methanol Barrier for DMFCs

Biobased materials capable of transforming into selective proton-exchange composite membranes (PEMs) are highly favored for use in direct methanol fuel cells (DMFCs) because of their low cost and abundance. Here, a polysaccharide and a clay have been functionalized together to make a highly proton selective PEM. Use of chitosan and clay composites ensured limited methanol crossover and thereby high measured performance via efficient fuel convertibility. In this study, sulfonated natural nanocomposite PEMs made of chitosan and sodium–montmorillonite (CS-MMT) were characterized for their water swelling, proton conductivity and methanol permeability parameters. The CS-MMT membrane with a proton conductivity of 4.92 × 10^–2 S cm^–1 and a power density of 45 mW/cm² showed a measured methanol crossover current density (<i>J</i>) of <100 mA/cm². For higher methanol concentrations (4, 6 and 8 M), fuel loss was ∼4 times less in comparison with commercially successful PEMs, such as Nafion 117

Polymer Structure and Solvent Effects on the Selective Dispersion of Single-Walled Carbon Nanotubes

Combinations of different aromatic polymers and organic solvents have been studied as dispersing agents for preparing single-walled carbon nanotubes solutions, using optical absorbance, photoluminescence-excitation mapping, computer modeling, and electron microscopic imaging to characterize the solutions. Both the polymer structure and solvent used strongly influence the dispersion of the nanotubes, leading in some cases to very high selectivity in terms of diameter and chiral angle. The highest selectivities are observed using toluene with the rigid polymers PFO-BT and PFO to suspend isolated nanotubes. The specific nanotube species selected are also dependent on the solvent used and can be adjusted by the use of THF or xylene. Where the structure has more flexible conformations, the polymers are shown to be less selective but show an enhanced overall solubilization of nanotube material. When chloroform is used as the solvent, there is a large increase in the overall solubilization, but the nanotubes are suspended as bundles rather than as isolated tubes which leads to a quenching of their photoluminescence

Effect of Copper Oxide Oxidation State on the Polymer-Based Solar Cell Buffer Layers

Transporting buffer layers are important components of polymer-based organic photovoltaic devices. In this study, we have investigated the effects of the oxidation state in copper oxide based buffer layer in conjunction to its role in device performance. We have shown that variation in the oxidation state affects the band alignment and built-in voltage of the device, therefore leading to variation in device performance. Specifically, the fully oxidized copper oxide buffer layer has a valence band position at 5.12 eV, much closer to the highest occupied molecular orbital of poly­(3-hexylthiophene-2,5-diyl) (P3HT) (∼5.2 eV), giving a best fill factor and efficiency at 57% and 4.06%, respectively. Lastly, we also demonstrate significant enhancement in device stability, with power conversion efficiency maintained at 75% of the original value even after 40 days, and propose a strategy for recovering the device performance based on the observed property of the oxide buffer layer

Ultrasensitive Gas Sensors Based on Vertical Graphene Nanowalls/SiC/Si Heterostructure

Gas sensors, which play an important role in the safety of human life, cover a wide range of applications including intelligent systems and detection of harmful and toxic gases. It is known that graphene is an ideal and attractive candidate for gas sensing due to its high surface area and excellent mechanical, electrical, optical, and thermal properties. However, in order to fully realize its potential as a commercial gas sensor, demand for a graphene-based device of low-limit detection, high sensitivity, and fast response time needs to be met. Here, we demonstrate a metal/insulator/semiconductor (MIS) based gas sensor consisting of as-grown epitaxial graphene nanowalls (EGNWs)/silicon carbide (SiC)/silicon (Si) structure. The unique edge dominant three-dimensional (3D) EGNWs based MIS device achieved an extraordinarily low limit of detection (0.5 ppm) and unprecedented sensitivity (82 μA/ppm/cm2 for H2) with a fast response of shorter than 500 ms. These unique properties of our MIS device are attributed to the abundance of vertically oriented nanographitic edges and structural defects that act as extra-favorable adsorption sites and exhibit fast electron-transfer kinetics through the edges. Our experimental findings can pave the way for the realization of high-performance 3D graphene-based gas sensor devices

Thickness-Dependent Binding Energy Shift in Few-Layer MoS₂ Grown by Chemical Vapor Deposition

The thickness-dependent surface states of MoS₂ thin films grown by the chemical vapor deposition process on the SiO₂–Si substrates are investigated by X-ray photoelectron spectroscopy. Raman and high-resolution transmission electron microscopy suggest the thicknesses of MoS₂ films to be ranging from 3 to 10 layers. Both the core levels and valence band edges of MoS₂ shift downward ∼0.2 eV as the film thickness increases, which can be ascribed to the Fermi level variations resulting from the surface states and bulk defects. Grainy features observed from the atomic force microscopy topographies, and sulfur-vacancy-induced defect states illustrated at the valence band spectra imply the generation of surface states that causes the downward band bending at the n-type MoS₂ surface. Bulk defects in thick MoS₂ may also influence the Fermi level oppositely compared to the surface states. When Au contacts with our MoS₂ thin films, the Fermi level downshifts and the binding energy reduces due to the hole-doping characteristics of Au and easy charge transfer from the surface defect sites of MoS₂. The shift of the onset potentials in hydrogen evolution reaction and the evolution of charge-transfer resistances extracted from the impedance measurement also indicate the Fermi level varies with MoS₂ film thickness. The tunable Fermi level and the high chemical stability make our MoS₂ a potential catalyst. The observed thickness-dependent properties can also be applied to other transition-metal dichalcogenides (TMDs), and facilitates the development in the low-dimensional electronic devices and catalysts