On‐Chip Rolling Design for Controllable Strain Engineering and Enhanced Photon–Phonon Interaction in Graphene

On‐chip strain engineering is highly demanded in 2D materials as an effective route for tuning their extraordinary properties and integrating consistent functionalities toward various applications. Herein, rolling technique is proposed for strain engineering in monolayer graphene grown on a germanium substrate, where compressive or tensile strain could be acquired, depending on the designed layer stressors. Unusual compressive strains up to 0.30% are achieved in the rolled‐up graphene tubular structures. The subsequent phonon hardening under compressive loading is observed through strain‐induced Raman G band splitting, while distinct blueshifts of characteristic peaks (G⁺, G^?, or 2D) can be well regulated on an asymmetric tubular structure with a strain variation. In addition, due to the strong confinement of the local electromagnetic field under 3D tubular geometry, the photon–phonon interaction is highly strengthened, and thus, the Raman scattering of graphene in rolled‐up tubes is enhanced. Such an on‐chip rolling approach leads to a superior strain tuning method in 2D materials and could improve their light–matter interaction in a tubular configuration, which may hold great capability in 2D materials integration for on‐chip applications such as in mechanics, electronics, and photonics.     

Highly Confined and Tunable Hyperbolic Phonon Polaritons in Van Der Waals Semiconducting Transition Metal Oxides

2D van der Waals (vdW) layered polar crystals sustaining phonon polaritons (PhPs) have opened up new avenues for fundamental research and optoelectronic applications in the mid‐infrared to terahertz ranges. To date, 2D vdW crystals with PhPs are only experimentally demonstrated in hexagonal boron nitride (hBN) slabs. For optoelectronic and active photonic applications, semiconductors with tunable charges, finite conductivity, and moderate bandgaps are preferred. Here, PhPs are demonstrated with low loss and ultrahigh electromagnetic field confinements in semiconducting vdW α‐MoO₃. The α‐MoO₃ supports strong hyperbolic PhPs in the mid‐infrared range, with a damping rate as low as 0.08. The electromagnetic confinements can reach ≈λ₀/120, which can be tailored by altering the thicknesses of the α‐MoO₃ 2D flakes. Furthermore, spatial control over the PhPs is achieved with a metal‐ion‐intercalation strategy. The results demonstrate α‐MoO₃ as a new platform for studying hyperbolic PhPs with tunability, which enable switchable mid‐infrared nanophotonic devices.     

High Efficiency Organic Solar Cells Achieved by the Simultaneous Plasmon‐Optical and Plasmon‐Electrical Effects from Plasmonic Asymmetric Modes of Gold Nanostars

The plasmon‐optical effects have been utilized to optically enhance active layer absorption in organic solar cells (OSCs). The exploited plasmonic resonances of metal nanomaterials are typically from the fundamental dipole/high‐order modes with narrow spectral widths for regional OSC absorption improvement. The conventional broadband absorption enhancement (using plasmonic effects) needs linear‐superposition of plasmonic resonances. In this work, through strategic incorporation of gold nanostars (Au NSs) in between hole transport layer (HTL) and active layer, the excited plasmonic asymmetric modes offer a new approach toward broadband enhancement. Remarkably, the improvement is explained by energy transfer of plasmonic asymmetric modes of Au NS. In more detail, after incorporation of Au NSs, the optical power in electron transport layer transfers to active layer for improving OSC absorption, which otherwise will become dissipation or leakage as the role of carrier transport layer is not for photon‐absorption induced carrier generation. Moreover, Au NSs simultaneously deliver plasmon‐electrical effects which shorten transport path length of the typically low‐mobility holes and lengthen that of high‐mobility electrons for better balanced carrier collection. Meanwhile, the resistance of HTL is reduced by Au NSs. Consequently, power conversion efficiency of 10.5% has been achieved through cooperatively plasmon‐optical and plasmon‐electrical effects of Au NSs.     

Tunable Fano Resonance and Plasmon–Exciton Coupling in Single Au Nanotriangles on Monolayer WS2 at Room Temperature

Tunable Fano resonances and plasmon–exciton coupling are demonstrated at room temperature in hybrid systems consisting of single plasmonic nanoparticles deposited on top of the transition metal dichalcogenide monolayers. By using single Au nanotriangles (AuNTs) on monolayer WS₂ as model systems, Fano resonances are observed from the interference between a discrete exciton band of monolayer WS₂ and a broadband plasmonic mode of single AuNTs. The Fano lineshape depends on the exciton binding energy and the localized surface plasmon resonance strength, which can be tuned by the dielectric constant of surrounding solvents and AuNT size, respectively. Moreover, a transition from weak to strong plasmon–exciton coupling with Rabi splitting energies of 100–340 meV is observed by rationally changing the surrounding solvents. With their tunable plasmon–exciton interactions, the proposed WS₂–AuNT hybrids can open new pathways to develop active nanophotonic devices.     

Model Calculation of Distance-Dependent Electron–Phonon Coupling Parameters Derived from One- and Two-Body Interactions for a Dimer

We evaluate all the electron–phonon coupling terms derived from one and two-body electronic interactions, in both the adiabatic and the extreme nonadiabatic limit, for a dimer with a nondegenerate orbital built from atomic wave functions of Gaussian shape. Different forms of the Hamiltonian contributions result in the two limits.     

Formation of Air Stable Graphene p–n–p Junctions Using an Amine‐Containing Polymer Coating

An ultrathin layer of a polymer containing simple aliphatic amine groups, polyethylenimine ethoxylated (PEIE), is deposited on a back‐gated field effect graphene device to form graphene p–n–p junctions. Characteristic I–V curves indicate the superposition of two separate Dirac points, which confirms an energy separation of neutrality points within the complementary regions. This is a simple approach for making graphene p–n–p junctions without a need for multiple lithography steps or electrostatic gates and, unlike, the destructive techniques such as substitutional doping or covalent functionalization, it induces a minor defect, if any, as there is no discernible D peak in the Raman spectra of the graphene films after creating junctions and degradation in the charge carrier mobilities of the graphene devices. This method can be easily processed from dilute solutions in environmentally‐friendly solvents such as water or methoxyethanol and does not suffer any change upon exposure to air or heating at temperatures below 100 °C.     

Hierarchical Graphene Foam for Efficient Omnidirectional Solar–Thermal Energy Conversion

Efficient solar–thermal energy conversion is essential for the harvesting and transformation of abundant solar energy, leading to the exploration and design of efficient solar–thermal materials. Carbon‐based materials, especially graphene, have the advantages of broadband absorption and excellent photothermal properties, and hold promise for solar–thermal energy conversion. However, to date, graphene‐based solar–thermal materials with superior omnidirectional light harvesting performances remain elusive. Herein, hierarchical graphene foam (h‐G foam) with continuous porosity grown via plasma‐enhanced chemical vapor deposition is reported, showing dramatic enhancement of broadband and omnidirectional absorption of sunlight, which thereby can enable a considerable elevation of temperature. Used as a heating material, the external solar–thermal energy conversion efficiency of the h‐G foam impressively reaches up to ≈93.4%, and the solar–vapor conversion efficiency exceeds 90% for seawater desalination with high endurance.     

The development of hybrid axisymmetric elements based on the Hellinger–Reissner variational principle

We present a general procedure for the development of hybrid axisymmetric elements based on the Hellinger–Reissner principle within the context of linear elasticity. Similar to planar elements, the stress interpolation is obtained by an identification of the zero‐energy modes. We illustrate our procedure by designing a lower‐order (four‐node) and a higher‐order (nine‐node) element. Both elements are of correct rank, and moreover use the minimum number of stress parameters, namely seven and 17. Several examples are presented to show the excellent performance of both elements under various demanding situations such as when the material is almost incompressible, when the thickness to radius ratio is very small, etc. When the variation of the field variables is along the radial direction alone, when the mesh is uniform, and the loading is of pressure type, the developed elements are superconvergent, i.e. they yield the exact nodal displacement values. Copyright © 2005 John Wiley & Sons, Ltd.     

Thermal Exfoliation of Layered Metal–Organic Frameworks into Ultrahydrophilic Graphene Stacks and Their Applications in Li–S Batteries

2D nanocarbon‐based materials with controllable pore structures and hydrophilic surface show great potential in electrochemical energy storage systems including lithium sulfur (Li–S) batteries. This paper reports a thermal exfoliation of metal–organic framework crystals with intrinsic 2D structure into multilayer graphene stacks. This family of nanocarbon stacks is composed of well‐preserved 2D sheets with highly accessible interlayer macropores, narrowly distributed 7 Å micropores, and ever most polar pore walls. The surface polarity is quantified both by its ultrahigh water vapor uptake of 14.3 mmol g^?1 at low relative pressure of P /P ₀ = 0.4 and ultrafast water wetting capability in less than 10.0 s. Based on the structural merits, this series hydrophilic multilayer graphene stack is showcased as suitable model cathode host for unveiling the challenging surface chemistry issue in Li–S batteries.