Investigation of Etching Behavior of Single-Walled Carbon Nanotubes Using Different Etchants

As gas-phase etching has become an important method to obtain single-walled carbon nanotubes (SWCNTs) with specific electronic structures, numerous etchants were studied in the past decades. However, the understanding of the dynamic process as well as the etching state is still limited. Herein, we investigated the etching process of SWCNTs at the level of the individual tube in real time using an improved polarized optical microscope equipped with a miniature chemical vapor deposition (CVD) system. Experiments showed a universal etching behavior in a SWCNT oxidation process under different etchants (O₂, H₂O, and CO₂), that is, the random appearance of etching sites and the self-terminating phenomenon. We built a new etching model at the level of the individual tubes to describe the etching rate. Based on this model, a new constant <i>R</i>, the ratio of etching rates between metallic and semiconducting SWCNTs, was defined to describe the etching selectivity. Optical spectroscopy was used to identify the chirality of SWCNTs, and it was found that the etching selectivities followed the order: H₂O (<i>R</i> = 17.1, 850 °C) > CO₂ (<i>R</i> = 3.6, 850 °C) > O₂ (<i>R</i> = 1.2, 610 °C). Field effect transistor (FET) performance of the SWCNT arrays also verified the statistical results

Diameter-Specific Growth of Semiconducting SWNT Arrays Using Uniform Mo₂C Solid Catalyst

Semiconducting single-walled nanotube (s-SWNT) arrays with specific diameters are urgently demanded in the applications in nanoelectronic devices. Herein, we reported that by using uniform Mo₂C solid catalyst, aligned s-SWNT (∼90%) arrays with narrow-diameter distribution (∼85% between 1.0 and 1.3 nm) on quartz substrate can be obtained. Mo₂C nanoparticles with monodisperse sizes were prepared by using molybdenum oxide-based giant clusters, (NH₄)₄₂[Mo₁₃₂O₃₇₂(H₃CCOO)₃₀(H₂O)₇₂]·10H₃CCOONH₄·300H₂O­(Mo₁₃₂), as the precursor that was carburized by a gas mixture of C₂H₅OH/H₂ during a temperature-programmed reduction. In this approach, the formation of volatile MoO₃ was inhibited due to the annealing and reduction at a low temperature. As a result, uniform Mo₂C nanoparticles are formed, and their narrow size-dispersion strictly determines the diameter distribution of SWNTs. During the growth process, Mo₂C selectively catalyzes the scission of C–O bonds of ethanol molecules, and the resultant absorbed oxygen (O_ads) preferentially etches metallic SWNTs (m-SWNTs), leading to the high-yield of s-SWNTs. Raman spectroscopic analysis showed that most of the s-SWNTs can be identified as (14, 4), (13, 6), or (10, 9) tubes. Our findings open up the possibility of the chirality-controlled growth of aligned-SWNTs using uniform carbide nanoparticles as solid catalysts for practical nanoelectronics applications

Anomalous Polarized Raman Scattering and Large Circular Intensity Differential in Layered Triclinic ReS₂

The Raman tensor of a crystal is the derivative of its polarizability tensor and is dependent on the symmetries of the crystal and the Raman-active vibrational mode. The intensity of a particular mode is determined by the Raman selection rule, which involves the Raman tensor and the polarization configurations. For anisotropic two-dimensional (2D) layered crystals, polarized Raman scattering has been used to reveal the crystalline orientations. However, due to its complicated Raman tensors and optical birefringence, the polarized Raman scattering of triclinic 2D crystals has not been well studied yet. Herein, we report the anomalous polarized Raman scattering of 2D layered triclinic rhenium disulfide (ReS₂) and show a large circular intensity differential (CID) of Raman scattering in ReS₂ of different thicknesses. The origin of CID and the anomalous behavior in polarized Raman scattering were attributed to the appearance of nonzero off-diagonal Raman tensor elements and the phase factor owing to optical birefringence. This can provide a method to identify the vertical orientation of triclinic layered materials. These findings may help to further understand the Raman scattering process in 2D materials of low symmetry and may indicate important applications in chiral recognition by using 2D materials

Ultrasensitive Size-Selection of Plasmonic Nanoparticles by Fano Interference Optical Force

In this paper, we propose a solution for the ultrasensitive optical selection of plasmonic nanoparticles using Fano interference-induced scattering forces. Under a Gaussian beam excitation, the scattering of a plasmonic nanoparticle at its Fano resonance becomes strongly asymmetric in the lateral direction and consequently results in a net transverse scattering force, that is, Fano interference-induced force. The magnitude of this transverse scattering force is comparable with the gradient force in conventional optical manipulation experiments. More interestingly, the Fano scattering force is ultrasensitive to the particle size and excitation frequency due to the phase sensitivity of the interference between adjacent plasmon modes in the particle. Utilizing this distinct feature, we show the possibility of size-selective sorting of silver and gold nanoparticles with an accuracy of about ±10 nm and silica-gold core–shell nanoparticles with shell thickness down to several nanometers. These results would add to the toolbox of optical manipulation and fabrication

Light-Concentrating Plasmonic Au Superstructures with Significantly Visible-Light-Enhanced Catalytic Performance

Noble metals are well-known for their surface plasmon resonance effect that enables strong light absorption typically in the visible regions for gold and silver. However, unlike semiconductors, noble metals are commonly considered incapable of catalyzing reactions via photogenerated electron–hole pairs due to their continuous energy band structures. So far, photonically activated catalytic system based on pure noble metal nanostructures has seldom been reported. Here, we report the development of three different novel plasmonic Au superstructures comprised of Au nanoparticles, multiple-twinned nanoparticles and nanoworms assembling on the surfaces of SiO₂ nanospheres respectively via a well-designed synthetic strategy. It is found that these novel Au superstructures show enhanced broadband visible-light absorption due to the plasmon resonance coupling within the superstructures, and thus can effectively focus the energy of photon fluxes to generate much more excited hot electrons and holes for promoting catalytic reactions. Accordingly, these Au superstructures exhibit significantly visible-light-enhanced catalytic efficiency (up to ∼264% enhancement) for the commercial reaction of p-nitrophenol reduction

Synthesis of Ultrathin Graphdiyne Film Using a Surface Template

Graphdiyne is predicted to have a natural band gap and simultaneously possesses superior carrier mobility, which makes it potential for electronic devices. Synthesis of ultrathin graphdiyne film is highly demanded. In this work, we proposed an approach for synthesis of ultrathin graphdiyne film using graphene as a surface template, which can induce confined reaction on substrate. With all-carbon, conjugated, atomically flat structure, graphene has a strong interaction with the graphdiyne system, resulting the formation of continuous flat ultrathin graphdiyne film with thickness less than 3 nm. Raman spectra, grazing incidence X-ray diffraction, and transmission electron microscopy characterization all confirmed the features of graphdiyne. Furthermore, this strategy was also extended to the hexagonal boron nitride (hBN) surface with resembling structure, serving as a perfect dielectric layer. Field-effect transistor devices based on graphdiyne film grown on hBN were fabricated directly, and electrical transport measurements demonstrate the good conductivity with <i>p</i>-type characteristics of the as-obtained graphdiyne film

Synthesis of Hierarchical Graphdiyne-Based Architecture for Efficient Solar Steam Generation

Synthesis of Hierarchical Graphdiyne-Based Architecture for Efficient Solar Steam Generatio

Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials

Surface-enhanced Raman scattering (SERS) on two-dimensional (2D) layered materials has provided a unique platform to study the chemical mechanism (CM) of the enhancement due to its natural separation from electromagnetic enhancement. The CM stems from the charge interactions between the substrate and molecules. Despite the extensive studies of the energy alignment between 2D materials and molecules, an understanding of how the electronic properties of the substrate are explicitly involved in the charge interaction is still unclear. Lately, a new group of 2D layered materials with anisotropic structures, including orthorhombic black phosphorus (BP) and triclinic rhenium disulfide (ReS₂), has attracted great interest due to their unique anisotropic electrical and optical properties. Herein, we report a unique anisotropic Raman enhancement on few-layered BP and ReS₂ using copper phthalocyanine (CuPc) molecules as a Raman probe, which is absent on isotropic graphene and h-BN. According to detailed Raman tensor analysis and density functional theory calculations, anisotropic charge interactions between the 2D materials and molecules are responsible for the angular dependence of the Raman enhancement. Our findings not only provide new insights into the CM process in SERS, but also open up new avenues for the exploration and application of the electronic properties of anisotropic 2D layered materials

Template Synthesis of an Ultrathin β‑Graphdiyne-Like Film Using the Eglinton Coupling Reaction

β-Graphdiyne (β-GDY) is a two-dimensional carbon material with zero band gap and highly intrinsic carrier mobility and a promising material with potential applications in electronic devices. However, the synthesis of continuous single or ultrathin β-GDY has not been realized yet. Here, we proposed an approach for ultrathin β-GDY-like film synthesis using graphene as a template because of the strong π–π interaction between β-GDY and graphene. The as-synthesized film presents smooth and continuous morphology and has good crystallinity. Electrical measurement reveals that the film presented a conductivity of 1.30 × 10^–2 S·m^–1 by fabricating electronic devices on β-GDY grown on a dielectric hexagonal boron nitride template

Hydrogen Radical-Induced Electrocatalytic N₂ Reduction at a Low Potential

Realizing efficient hydrogenation of N2 molecules in the electrocatalytic nitrogen reduction reaction (NRR) is crucial in achieving high activity at a low potential because it theoretically requires a higher equilibrium potential than other steps. Analogous to metal hydride complexes for N2 reduction, achieving this step by chemical hydrogenation can weaken the potential dependence of the initial hydrogenation process. However, this strategy is rarely reported in the electrocatalytic NRR, and the catalytic mechanism remains ambiguous and lacks experimental evidence. Here, we show a highly efficient electrocatalyst (ruthenium single atoms anchored on graphdiyne/graphene sandwich structures) with a hydrogen radical-transferring mechanism, in which graphdiyne (GDY) generates hydrogen radicals (H•), which can effectively activate N2 to generate NNH radicals (•NNH). A dual-active site is constructed to suppress competing hydrogen evolution, where hydrogen preferentially adsorbs on GDY and Ru single atoms serve as the adsorption site of •NNH to promote further hydrogenation of NH3 synthesis. As a result, high activity and selectivity are obtained simultaneously at −0.1 V versus a reversible hydrogen electrode. Our findings illustrate a novel hydrogen transfer mechanism that can greatly reduce the potential and maintain the high activity and selectivity in NRR and provide powerful guidelines for the design concept of electrocatalysts