Vibrational and thermodynamic properties of α-, β-, γ-, and 6, 6, 12-graphyne structures

Correlating negative thermal expansion and thermal conductivity in two-dimensional carbon-based materials

Negative thermal expansion (NTE) is a fascinating phenomenon wherein certain materials contract upon heating. The phonon transport properties of two-dimensional carbon-based allotropes are not yet fully understood in terms of their NTE properties. This work with a specific focus on carbon-based allotropes investigates the underlying mechanisms of the thermal conductivity (TC) and NTE of graphene, haeckelite, pentahexoctite, s-graphene, 6.6.12 and delta Graphynes (Gys). High TC is imperative for efficiently dissipating heat in electronic devices, whereas thermoelectric devices need to be thermally resistive with low TC. Delta-Gy shows the highest NTE as well as the lowest TC and vice versa is true for graphene. Graphene displays a lower degree of anisotropic TC, while s-graphene exhibits the highest level of anisotropic TC. The behaviour of their TC can be explained based on the soft-phonon modes, phonon group velocity (vg), phonon lifetime (τ) and mean free path (MFP). The acoustic and optical phonon branches play a key role in determining both the TC and NTE of the materials. Out-of-plane buckling in two-dimensional materials reduces thermal conductivity by increasing the phonon scattering. Buckling has also been shown to increase the NTE. A precise control on the pore sizes 5-7 (haeckelite), 5-6-8 (pentahexoctite), and 4-8 (s-graphene), 6-12-14 (6.6.12-Gy) and 6-14 (delta-Gy) can significantly influence their soft unit modes. This investigation not only deepens our understanding of NTE and TC but also highlights the potential of future applications of carbon-based materials with controlled thermal expansion properties in nanotechnology, composites, and beyond.

Graphynes and Graphdiynes for Energy Storage and Catalytic Utilization: Theoretical Insights into Recent Advances

Carbon allotropes have contributed to all aspects of people's lives throughout human history. As emerging carbon-based low-dimensional materials, graphyne family members (GYF), represented by graphdiyne, have a wide range potential applications due to their superior physical and chemical properties. In particular, graphdiyne (GDY), as the leader of the graphyne family, has been practically applied to various research fields since it was first successfully synthesized. GYF have a large surface area, both sp and sp² hybridization, and a certain band gap, which was considered to originate from the overlap of carbon 2p_z orbitals and the inhomogeneous π-bonds of carbon atoms in different hybridization forms. These properties mean GYF-based materials still have many potential applications to be developed, especially in energy storage and catalytic utilization. Since most of the GYF have yet to be synthesized and applications of successfully synthesized GYF have not been developed for a long time, theoretical results in various application fields should be shared to experimentalists to attract more intentions. In this Review, we summarized and discussed the synthesis, structural properties, and applications of GYF-based materials from the theoretical insights, hoping to provide different viewpoints and comments.

Strain and magnetic field effects on the electronic and transport properties of γ-graphyne

In this paper, we apply a tightly binding Hamiltonian model in the presence of a magnetic field for investigating the electronic and transport properties of γ-graphyne layers. We also consider the effects of in-plane biaxial strain on the electronic behavior of γ-graphyne layers. Moreover the impact of strain on magnetic susceptibility and specific heat of the structure is also studied. In particular, the temperature dependence of static thermal conductivity of γ-graphyne layers due to magnetic field and strain effects is studied. We exploit the linear response theory and Green's function approach to obtain the temperature behavior of thermal conductivity, electrical conductivity and the Seebeck coefficient. Our numerical results indicate that thermal conductivity increases upon increasing temperature temperatures. This effect comes from the increasing thermal energy of charge carriers and their excitation to the conduction bands. The temperature dependence of Seebeck coefficient shows that the thermopower of an undoped γ-graphyne layer is positive on the whole range of temperatures in the absence of strain effects. The effects of both electron doping and magnetic field factors on temperature behavior of the electrical conductivity of γ-graphyne are investigated in detail. Moreover the effects of biaxial strain on thermal conductivity of single layer γ-graphyne have been addressed.

Nonlinear Finite Element Analysis of γ-Graphyne Structures under Shearing

In this study, a nonlinear, spring-based finite element approach is employed in order to predict the nonlinear mechanical response of graphyne structures under shear loading. Based on Morse potential functions, suitable nonlinear spring finite elements are formulated simulating the interatomic interactions of different graphyne types. Specifically, the four well-known types of γ-graphyne, i.e., graphyne-1 also known as graphyne, graphyne-2 also known as graphdiyne, graphyne-3, and graphyne-4 rectangular sheets are numerically investigated applying appropriate boundary conditions representing shear load. The obtained finite element analysis results are employed to calculate the in-plane shear stress–strain behaviour, as well as the corresponding mechanical properties as shear modulus and shear strength. Comparisons of the present graphyne shearing response predictions with other corresponding estimations are performed to validate the present research results.

Spin-Resolved Electronic and Transport Properties of Graphyne-Based Nanojunctions with Different N-Substituting Positions

Since the rapid development of theoretical progress on the two-dimensional graphyne nanoribbons and nanojunctions, here we investigate the electronic band structures and transport properties for the junctions based on armchair-edged γ-graphyne nanoribbons (AγGYNRs) with asymmetrically nitrogen (N)-substituting in the central carbon hexagon. By employing first-principles calculation, our computational results imply that the number and the location of single or double N-doping can efficiently modulate the electronic energy band, and the N-doping hexagonal rings in the middle of the junction play a vital role in the charge transport. In specific, the effect of negative difference resistance (NDR) is observed, in which possesses the biggest peak to valley ratio reaching up to 36.8. Interestingly, the N-doped junction with longer molecular chain in the central scattering region can induce a more obvious NDR behavior. The explanation of the mechanism in the microscopic level has suggested that the asymmetrically N-doped junction by introducing a longer molecular chain can produce a more notable pulse-like current-voltage dependence due to the presence of a transporting channel within the bias window under a higher bias voltage. In addition, when the spin injection is considered, an intriguing rectifying effect in combination with NDR is available, which is expected to be applied in future spintronic devices.

Two-dimensional layered materials: from mechanical and coupling properties towards applications in electronics

With the increasing interest in nanodevices based on two-dimensional layered materials (2DLMs) after the birth of graphene, the mechanical and coupling properties of these materials, which play an important role in determining the performance and life of nanodevices, have drawn increasingly more attention. In this review, both experimental and simulation methods investigating the mechanical properties and behaviour of 2DLMs have been summarized, which is followed by the discussion of their elastic properties and failure mechanisms. For further understanding and tuning of their mechanical properties and behaviour, the influence factors on the mechanical properties and behaviour have been taken into consideration. In addition, the coupling properties between mechanical properties and other physical properties are summarized to help set up the theoretical blocks for their novel applications. Thus, the understanding of the mechanical and coupling properties paves the way to their applications in flexible electronics and novel electronics, which is demonstrated in the last part. This review is expected to provide in-depth and comprehensive understanding of mechanical and coupling properties of 2DLMs as well as direct guidance for obtaining satisfactory nanodevices from the aspects of material selection, fabrication processes and device design.

Exploring planar and nonplanar siligraphene: a first-principles study

New nonplanar g-SiC₇ and g-Si₇C have been found. g-Si₅C, though buckled, is energetically very close to its planar counterpart.

Tunable band gap of graphyne-based homo- and hetero-structures by stacking sequences, strain and electric field

A comprehensive investigation was carried out on graphyne/graphyne (Gyne/Gyne), graphyne-like BN/graphyne-like BN (BNyne/BNyne) and graphyne/graphyne-like BN (Gyne/BNyne) bilayer structures using van der Waals (vdW)-corrected density functional theory. These bilayers exhibited distinct stacking-dependent characteristics in their ground state electronic structure and also had different responses to external strain and a vertical electric field. For the Gyne/Gyne and Gyne/BNyne bilayers, the application of biaxial tensile strain led to an increase in the band gap, while the application of biaxial compressive strain in addition to uniaxial strain, either under tension or compression, induced a reduction in the band gap. However, in the case of the BNyne/BNyne bilayer, the application of biaxial tensile strain led to a decrease in the band gap, but an increase in the band gap occurred under biaxial compressive strain, which could be explained by a change in the ionic nature of the B-N bonds. Under a vertical electric field, the band gaps of the homo-bilayers (Gyne/Gyne and BNyne/BNyne) decreased and were symmetrical. However, the hetero-bilayer (Gyne/BNyne) exhibited a decreased band gap under a positive electric field, but an almost constant band gap under a negative electric field. The physical origin of the band gap variation under an electric field was unraveled using energy-band theory. Our findings pave the way for experimental research and provide valuable insight into two-dimensional vdW layered structures for use in next generation flexible nanoelectronics and optoelectronic devices.

Molecular dynamics investigation of the mechanical behavior of multi-layered graphyne and its family under tensile loading

This paper aims to study the mechanical properties of the multi-layered graphyne and other members of the graphyne family under the uniaxial tensile loading. For this purpose, molecular dynamics simulations are used. The effects of the size and number of layers on the fracture and elastic properties are studied. It is shown that Young's modulus of the zigzag multi-layered graphyne is slightly larger than armchair one. Comparing the stress-strain curves of the multi-layered graphynes with different number of layers, it is observed that the fracture stress and strain of the nanosheets are inversely related to the number of layers. Investigating the influence of the number of acetylene linkage in the structure of the graphyne-n family on their mechanical properties, it is shown increasing the number of triple bonds leads to weakening the fracture stress and Young's modulus of the nanosheet.

Implications of boron doping on electrocatalytic activities of graphyne and graphdiyne families: a first principles study

Dispersive force corrected density functional theory is used to map the oxygen reduction reaction (ORR) kinetics of six kinds of graphyne (Gy) and graphdiyne (Gdy) systems (namely αGy, βGy, γGy, δGy, 6,6,12Gy, RGy and Gdy) with substitutional boron (B) atom doping.

Unusual structural and electronic properties of porous silicene and germanene: insights from first-principles calculations

Using first-principles calculations, we investigate the geometric structures and electronic properties of porous silicene and germanene nanosheets, which are the Si and Ge analogues of α-graphyne (referred to as silicyne and germanyne). It is found that the elemental silicyne and germanyne sheets are energetically unfavourable. However, after the C-substitution, the hybrid graphyne-like sheets (c-silicyne/c-germanyne) possess robust energetic and dynamical stabilities. Different from silicene and germanene, c-silicyne is a flat sheet, and c-germanyne is buckled with a distinct half-hilled conformation. Such asymmetric buckling structure causes the semiconducting behaviour into c-germanyne. While in c-silicyne, the semimetallic Dirac-like property is kept at the nonmagnetic state, but a spontaneous antiferromagnetism produces the massive Dirac fermions and opens a sizeable gap between Dirac cones. A tensile strain can further enhance the antiferromagnetism, which also linearly modulates the gap value without altering the direct-bandgap feature. Through strain engineering, c-silicyne can form a type-II band alignment with the MoS 2 sheet. The combined c-silicyne/MoS 2 nanostructure has a high power conversion efficiency beyond 20% for photovoltaic solar cells, enabling a fascinating utilization in the fields of solar energy and nano-devices.

High thermoelectric performance in two-dimensional graphyne sheets predicted by first-principles calculations

The thermoelectric properties of two-dimensional graphyne sheets are investigated by using first-principles calculations and the Boltzmann transport equation method.

DFT and TB study of the geometry of hydrogen adsorbed on graphynes

Using density-functional calculations (DFT) and a tight-binding model, we investigate the origin of distinct favorable geometries which depend on the type of graphyne used. The change in the H geometry is described in terms of the tuning of the hopping between sp(2)-bonded C atoms and sp-bonded C atoms hybridized with the H atoms. We find that the different preferred geometry for each type of graphyne is associated with the electronic effects due to different symmetries rather than a steric effect minimizing the repulsive interaction between the H atoms. The band gaps are significantly tuned as the hopping varies, except in α-graphyne, in agreement with the result of our previous DFT study (Koo J et al 2013 J. Phys. Chem. C 117 11960). Our model can be used to describe the geometry and electronic properties of hydrogenated graphynes.