Strong mechanical anisotropy and an anisotropic Dirac state in 2D C5N3.

Two-dimensional (2D) carbon nitride materials have emerged as a versatile platform for the design of high-performance nanoelectronics, but strong anisotropy in 2D carbon nitrides has rarely been reported. In this work, a 2D carbon nitride with strong anisotropy composed of tetra-, penta-, and hexa-rings (named as TPH-C5N3) is proposed. This TPH-C5N3 exhibits both dynamical and mechanical stability. Furthermore, it also showcases remarkable thermal stability, reaching up to 2300 K, as evidenced by AIMD simulations conducted in an NVT environment utilizing the Nosé-Hoover thermostat. Significantly, TPH-C5N3 demonstrates high anisotropic ratios in its mechanical properties, positioning it as the frontrunner in the current carbon nitride systems. In addition, a Dirac cone with an anisotropic ratio of 55.8% and Fermi velocity of 7.26 × 105 m s-1 is revealed in TPH-C5N3. The nontrivial topological properties of TPH-C5N3 are demonstrated by a non-zero Z2 invariant and topologically protected edge states. Our study offers theoretical insights into an anisotropic 2D carbon nitride material, laying the groundwork for its design and synthesis.

Enhanced OER catalytic activity of single metal atoms supported by the pentagonal NiN2 monolayer: insight from density functional theory calculations.

Two-dimensional material-supported single metal atom catalysts have been extensively studied and proved effective in electrocatalytic reactions in recent years. In this work, we systematically investigate the OER catalytic properties of single metal atoms supported by the NiN2 monolayer. Several typical transition metals with high single atom catalytic activity, such as Fe, Co, Ru, Rh, Pd, Ir, and Pt, were selected as catalytic active sites. The energy calculations show that transition metal atoms (Fe, Co, Ru, Rh, Pd, Ir, and Pt) are easily embedded in the NiN2 monolayer with Ni vacancies due to the negative binding energy. The calculated OER overpotentials of Fe, Co, Ru, Rh, Pd, Ir and Pt embedded NiN2 monolayers are 0.92 V, 0.47 V, 1.13 V, 0.66 V, 1.25 V, 0.28 V, and 0.94 V, respectively. Compared to the 0.57 V OER overpotential of typical OER noble metal catalysts IrO2, Co@NiN2 and Ir@NiN2 exhibit high OER catalytic activity due to lower overpotential, especially for Ir@NiN2. The high catalytic activity of the Ir embedded NiN2 monolayer can be explained well by the d-band center model. It is found that the adsorption strength of the embedded TM atoms with intermediates follows a linear relationship with their d-band centers. Besides, the overpotential of the Ir embedded NiN2 monolayer can be further reduced to 0.24 V under -2% biaxial strain. Such findings are expected to be employed in more two-dimensional material-supported single metal atom catalyzed reactions.

Enhanced optical absorption in two-dimensional Ruddlesden-Popper (C6H5CH2NH3)2PbI4 perovskites via biaxial strain and surface doping.

Two-dimensional Ruddlesden-Popper (2D RP) perovskites can form layered protective materials using long organic cations as "barrier" caps, which is expected to solve the problem of instability of perovskites in the working environment. In this work, we systematically studied the 2D Ruddlesden-Popper (C6H5CH2NH3)2PbI4 hybrid perovskites using density functional theory. The results reveal that the 2D (C6H5CH2NH3)2PbI4 perovskites are semiconductors with band gaps of 2.22 eV. The optical absorption peak of the 2D (C6H5CH2NH3)2PbI4 perovskite structure is located at 532 nm in the visible region. Interestingly, the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure enhanced under suitable strains. The highest optical absorption peak appears in 2D (C6H5CH2NH3)2PbI4 under a -2% strain, and its theoretical photoelectric conversion efficiency is 28.5%. More interestingly, the replacement of surface I atoms with Br is another ways to enhance the optical absorption spectrum of the 2D (C6H5CH2NH3)2PbI4 perovskite structure. The optical absorption peak blue-shifts to the high energy region, which has higher solar energy flux density than the low energy region. The good stability, tuneable band gap and excellent theoretical photoelectric conversion efficiency of the 2D (C6H5CH2NH3)2PbI4 perovskite structure make it a promising candidate for novel 2D hybrid perovskite based photoelectronic devices and solar cells.

How Hydrodynamic Phonon Transport Determines the Convergence of Thermal Conductivity in Two-Dimensional Materials.

The phonon Boltzmann transport equation combined with first-principles calculation has achieved great success in exploring the lattice thermal conductivity (κ) of various materials. However, the convergence of the predicted κ is a critical issue, leading to quite scattered results recorded in the literature, even for the same material. In this paper, we explore the origin for the convergence of thermal conductivity in two-dimensional (2D) materials. Two kinds of typical 2D materials, graphene and silicene, are studied, and the bulk silicon is also compared as a control system for a three-dimensional material. The effect of the cutoff radius (rc) in the third-order interatomic force constants on κ is studied for these three materials. It is found that that κ of these three materials exhibits diverse convergence behaviors with respect to rc, which coincides very well with the strength of hydrodynamic phonon transport. By further analyzing the phonon lifetime and scattering rates, we reveal that the dominance of the normal scattering process gives rise to the hydrodynamic phonon transport in both graphene and silicene, which results in long-range interaction and a large lifetime of low-frequency flexural acoustic phonons, while the same phenomenon is absent in bulk silicon. Our study highlights the importance of long-range interaction associated with hydrodynamic phonon transport in determining the thermal conductivity of 2D materials.

Reducing Kapitza resistance between graphene/water interface via interfacial superlattice structure.

The control of thermal transport across solid/liquid interface has attracted great interests for efficient thermal management in the integrated devices. Based on molecular dynamics simulations, we study the effect of interfacial superlattice structure on the Kapitza resistance between graphene/water interface. Compared to the original interface, introducing interfacial superlattice structure can result in an obvious reduction of Kapitza resistance by as large as 40%, exhibiting a decreasing trend of Kapitza resistance with the decrease of superlattice period. Surprisingly, by analyzing the structure of water block and atomic vibration characteristics on both sides of the interface, we find the interfacial superlattice structure has a minor effect on the water structure and overlap in the vibrational spectrum, suggesting that the improved interfacial heat transfer is not mainly originated from the liquid block. Instead, the spectral energy density analysis reveals that phonon scattering rate in the interfacial graphene layer is significantly enhanced after superlattice decoration, giving rise to the increased thermal resistance between the interfacial graphene layer and its nearest neighboring layer. As this thermal resistance is coupled to the Kapitza resistance due to the local nature of interfacial superlattice decoration, the enhanced thermal resistance in the solid segment indirectly reduces the Kapitza resistance between graphene/water interface, which is supported by the enhancement of the spectral interfacial thermal conductance upon superlattce decoration at microscopic level. Our study uncovers the physical mechanism for controlling heat transfer across solid/liquid interface via interfacial superlattice structure, which might provide valuable insights for designing efficient thermal interfaces.

The Impact of Interlayer Rotation on Thermal Transport Across Graphene/Hexagonal Boron Nitride van der Waals Heterostructure.

Graphene/hexagonal boron nitride (h-BN) van der Waals (vdW) heterostructure has aroused great interest because of the unique Moiré pattern. In this study, we use molecular dynamics simulation to investigate the influence of the interlayer rotation angle θ on the interfacial thermal transport across graphene/h-BN heterostructure. The interfacial thermal conductance G of graphene/h-BN interface reaches 509 MW/(m2K) at 500 K without rotation, and it decreases monotonically with the increase of the rotation angle, exhibiting around 50% reduction of G with θ = 26.33°. The phonon transmission function reveals that G is dominantly contributed by the low-frequency phonons below 10 THz. Upon rotation, the surface fluctuation in the interfacial graphene layer is enhanced, and the transmission function for the low-frequency phonon is reduced with increasing θ, leading to the rotation angle-dependent G. This work uncovers the physical mechanisms for controlling interfacial thermal transport across vdW heterostructure via interlayer rotation.

Effect of boundary chain folding on thermal conductivity of lamellar amorphous polyethylene.

Thermal transport properties of amorphous polymers depend significantly on the chain morphology, and boundary chain folding is a common phenomenon in bulk or lamellar polymer materials. In this work, by using molecular dynamics simulations, we study thermal conductivity of lamellar amorphous polyethylene (LAPE) with varying chain length (L 0). For a short L 0 without boundary chain folding, thermal conductivity of LAPE is homogeneous along the chain length direction. In contrast, boundary chain folding takes place for large L 0, and the local thermal conductivity at the boundary is notably lower than that of the central region, indicating inhomogeneous thermal transport in LAPE. By analysing the chain morphology, we reveal that the boundary chain folding causes the reduction of both the orientation order parameter along the heat flow direction and the radius of gyration, leading to the reduced local thermal conductivity at the boundary. Further vibrational spectrum analysis reveals that the boundary chain folding shifts the vibrational spectrum to the lower frequency, and suppresses the transmission coefficient for both C-C vibration and C-H vibration. Our study suggests that the boundary chain folding is an important factor for polymers to achieve desirable thermal conductivity for plastic heat exchangers and electronic packaging applications.