Numerical Evaluation of the Elastic Moduli of AlN and GaN Nanosheets

Two-dimensional (2D) nanostructures of aluminum nitride (AlN) and gallium nitride (GaN), called nanosheets, have a graphene-like atomic arrangement and represent novel materials with important upcoming applications in the fields of flexible electronics, optoelectronics, and strain engineering, among others. Knowledge of their mechanical behavior is key to the correct design and enhanced functioning of advanced 2D devices and systems based on aluminum nitride and gallium nitride nanosheets. With this background, the surface Young's and shear moduli of AlN and GaN nanosheets over a wide range of aspect ratios were assessed using the nanoscale continuum model (NCM), also known as the molecular structural mechanics (MSM) approach. The NCM/MSM approach uses elastic beam elements to represent interatomic bonds and allows the elastic moduli of nanosheets to be evaluated in a simple way. The surface Young's and shear moduli calculated in the current study contribute to building a reference for the evaluation of the elastic moduli of AlN and GaN nanosheets using the theoretical method. The results show that an analytical methodology can be used to assess the Young's and shear moduli of aluminum nitride and gallium nitride nanosheets without the need for numerical simulation. An exploratory study was performed to adjust the input parameters of the numerical simulation, which led to good agreement with the results of elastic moduli available in the literature. The limitations of this method are also discussed.

Revealing the electronic, optical and photocatalytic properties of PN-M2CO2 (P = Al, Ga; M = Ti, Zr, Hf) heterostructures

Using DFT, the electronic structure, optical, and photocatalytic properties of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers and their PN-M₂CO₂ van der Waals heterostructures (vdWHs) are investigated. Optimized lattice parameters, bond length, bandgap, conduction and valence band edges show the potential of PN (P = Ga, Al) and M₂CO₂ (M = Ti, Zr, Hf) monolayers in photocatalytic applications, and the application of the present approach to combine these monolayers and form vdWHs for efficient electronic, optoelectronic and photocatalytic applications is shown. Based on the same hexagonal symmetry and experimentally achievable lattice mismatch of PN (P = Ga, Al) with M₂CO₂ (M = Ti, Zr, Hf) monolayers, we have fabricated PN-M₂CO₂ vdWHs. Binding energies, interlayer distance and AIMD calculations show the stability of PN-M₂CO₂ vdWHs and demonstrate that these materials can be easily fabricated experimentally. The calculated electronic band structures show that all the PN-M₂CO₂ vdWHs are indirect bandgap semiconductors. Type-II[-I] band alignment is obtained for GaN(AlN)-Ti₂CO₂[GaN(AlN)-Zr₂CO₂ and GaN(AlN)-Hf₂CO₂] vdWHs. PN-Ti₂CO₂ (PN-Zr₂CO₂) vdWHs with a PN(Zr₂CO₂) monolayer have greater potential than a Ti₂CO₂(PN) monolayer, indicating that charge is transfer from the Ti₂CO₂(PN) to PN(Zr₂CO₂) monolayer, while the potential drop separates charge carriers (electron and holes) at the interface. The work function and effective mass of the carriers of PN-M₂CO₂ vdWHs are also calculated and presented. A red (blue) shift is observed in the position of excitonic peaks from AlN to GaN in PN-Ti₂CO₂ and PN-Hf₂CO₂ (PN-Zr₂CO₂) vdWHs, while significant absorption for photon energies above 2 eV for AlN-Zr₂CO₂, GaN-Ti₂CO₂ and PN-Hf₂CO₂, give them good optical profiles. The calculated photocatalytic properties demonstrate that PN-M₂CO₂ (P = Al, Ga; M = Ti, Zr, Hf) vdWHs are the best candidates for photocatalytic water splitting.

A new high capacity cathode material for Li/Na-ion batteries: dihafnium sulfide (Hf2S)

Structural and electronic properties of the newly synthesized dihafnium sulfide (Hf₂S) monolayer were investigated in this study. The Hf₂S monolayer is a magnetic metal and it retains its metallicity despite external factors, such as tensile/compressive strain or various atom terminations. This robust metallic Hf₂S monolayer has a high storage capacity of 1377.7 mA h g^-1 for both Li and Na atoms, which is much higher than conventional battery electrodes. The minimum diffusion barrier value is 131 meV for Li, and 117 meV for Na. The average open-circuit voltage values of Li and Na were calculated as 2.37 V and 1.69 V, respectively. Investigation of the mechanical properties showed that it is softer than graphene due to the Young's modulus of 111.7 N m^-1, but comparable with the molybdenum disulfide (MoS₂) monolayer. In light of the results of this study, the Hf₂S monolayer can serve as a useful cathode material for Li-ion and Na-ion batteries. In addition, the interactions of the Hf₂S monolayer with aluminium nitride (AlN) and MoS₂ monolayers were presented. While AlN behaves like a substrate for the Hf₂S monolayer, the MoS₂ monolayer forms a heterostructure with the Hf₂S monolayer. This study is a guide for the Hf₂S monolayer and its functions in nanotechnology.

Bonding, structure, and mechanical stability of 2D materials: the predictive power of the periodic table

This tutorial review describes the ongoing effort to convert main-group elements of the periodic table and their combinations into stable 2D materials, which is sometimes called modern 'alchemy'. Theory is successfully approaching this goal, whereas experimental verification is lagging far behind in the synergistic interplay between theory and experiment. The data collected here gives a clear picture of the bonding, structure, and mechanical performance of the main-group elements and their binary compounds. This ranges from group II elements, with two valence electrons, to group VI elements with six valence electrons, which form not only 1D structures but also, owing to their variable oxidation states, low-symmetry 2D networks. Outside of these main groups reviewed here, predominantly ionic bonding may be observed, for example in group II-VII compounds. Besides high-symmetry graphene with its shortest and strongest bonds and outstanding mechanical properties, low-symmetry 2D structures such as various borophene and tellurene phases with intriguing properties are receiving increasing attention. The comprehensive discussion of data also includes bonding and structure of few-layer assemblies, because the electronic properties, e.g., the band gap, of these heterostructures vary with interlayer layer separation and interaction energy. The available data allows the identification of general relationships between bonding, structure, and mechanical stability. This enables the extraction of periodic trends and fundamental rules governing the 2D world, which help to clear up deviating results and to estimate unknown properties. For example, the observed change of the bond length by a factor of two alters the cohesive energy by a factor of four and the extremely sensitive Young's modulus and ultimate strength by more than a factor of 60. Since the stiffness and strength decrease with increasing atom size on going down the columns of the periodic table, it is important to look for suitable allotropes of elements and binaries in the upper rows of the periodic table when mechanical stability and robustness are issues. On the other hand, the heavy compounds are of particular interest because of their low-symmetry structures with exotic electronic properties.

Two-dimensional group-III nitrides and devices: a critical review

As third-generation semiconductors, group-III nitrides are promising for high power electronic and optoelectronic devices because of their wide bandgap, high electron saturation mobility, and other unique properties. Inspired by the thickness-dependent properties of two-dimensional (2D) materials represented by graphene, it is predicted that the 2D counterparts of group-III nitrides would have similar properties. However, the preparation of 2D group-III nitride-based materials and devices is limited by the large lattice mismatch in heteroepitaxy and the low rate of lateral migration, as well as the unsaturated dangling bonds on the surfaces of group-III nitrides. The present review focuses on theoretical and experimental studies on 2D group-III nitride materials and devices. Various properties of 2D group-III nitrides determined using simulations and theoretical calculations are outlined. Moreover, the breakthroughs in their synthesis methods and their underlying physical mechanisms are detailed. Furthermore, devices based on 2D group-III nitrides are discussed accordingly. Based on recent progress, the prospect for the further development of the 2D group-III nitride materials and devices is speculated. This review provides a comprehensive understanding of 2D group-III nitride materials, aiming to promote the further development of the related fields of nano-electronic and nano-optoelectronics.

2D III-Nitride Materials: Properties, Growth, and Applications

2D III-nitride materials have been receiving considerable attention recently due to their excellent physicochemical properties, such as high stability, wide and tunable bandgap, and magnetism. Therefore, 2D III-nitride materials can be applied in various fields, such as electronic and photoelectric devices, spin-based devices, and gas detectors. Although the developments of 2D h-BN materials have been successful, the fabrication of other 2D III-nitride materials, such as 2D h-AlN, h-GaN, and h-InN, are still far from satisfactory, which limits the practical applications of these materials. In this review, recent advances in the properties, growth methods, and potential applications of 2D III-nitride materials are summarized. The properties of the 2D III-nitride materials are mainly obtained by first-principles calculations because of the difficulties in the growth and characterizations of these materials. The discussion on the growth of 2D III-nitride materials is focused on 2D h-BN and h-AlN, as the developments of 2D h-GaN and h-InN are yet to be realized. Therefore, applications have been realized mostly based on the 2D h-BN materials; however, many potential applications are cited for the entire range of 2D III-nitride materials. Finally, future research directions and prospects in this field are also discussed.

Atomic-Scale Dynamics and Storage Performance of Na/K on FeF3 Nanosheet

Developing highly efficient FeF₃-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly, we designed a two-dimensional (2D) FeF₃ nanosheet to obtain highly efficient Na/K-ion batteries. Moreover, first-principles calculations were implemented to discuss systematically the Na and K storage mechanism on the FeF₃(012) nanosheet. The adsorption energies of Na and K are -3.55 and -3.98 eV, respectively, which can guarantee the Na/K loading process. Interestingly, Na and K adatoms on FeF₃(012) prefer to get together in the form of the Na dimer and K tetramer, respectively. Energy barrier of the K tetramer is lower than that of the Na dimer (0.43 eV vs 0.45 eV). As a result, the K tetramer possesses a larger diffusion coefficient than the Na dimer (4.22 × 10^-10 cm²·s^-1 vs 3.32 × 10^-10 cm²·s^-1). That is to say, good Na/K-ion mobility can be achieved. Also, the FeF₃(012) nanosheet exhibits high initial discharge voltage (4.10 V for K and 3.74 V for Na). Moreover, it has a stable discharge voltage curve in Na/K-ion batteries. Besides, the FeF₃(012) nanosheet is more favorable to be fabricated as a flexible cathode material for potassium batteries. Therefore, the 2D FeF₃ nanosheet belongs to a promising cathode material in Na/K-ion batteries.

Structural deformations of two-dimensional planar structures under uniaxial strain: the case of graphene

In the present work, a method for the study of the structural deformations of two dimensional planar structures under uniaxial strain is presented. The method is based on molecular mechanics using the original stick and spiral model and a modified one which includes second nearest neighbor interactions for bond stretching. As we show, the method allows an accurate prediction of the structural deformations of any two dimensional planar structure as a function of strain, along any strain direction in the elastic regime, if structural deformations are known along specific strain directions, which are used to calculate the stick and spiral model parameters. Our method can be generalized including other strain conditions and not only uniaxial strain. We apply this method to graphene and we test its validity, using results obtained from ab initio density functional theory calculations. What we find is that the original stick and spiral model is not appropriate to describe accurately the structural deformations of graphene in the elastic regime. However, the introduction of second nearest neighbor interactions provides a very accurate description.

Electronic, optical and mechanical properties of SrSi6N8 and SrSi6N8O via first-principles

The different properties of two structurally similar nitridosilicates, SrSi₆N₈ and SrSi₆N₈O, are attributed to the oxygen atom.

Mechanical degradation of graphene by epoxidation: insights from first-principles calculations

Oxidation is a major cause for the degradation of materials including graphene, where epoxidation (forming the C-O-C bond) is very common. In addition, graphene oxide, in which the epoxy group is one of the two major functional groups (the other is hydroxy), is an important precursor material used for the bulk synthesis of graphene sheets. Information about the mechanical stabilities, non-linear elastic properties, and elastic limits under various strain components is invaluable for application of these nanomaterials. Here, we investigate the mechanical properties of the epoxidized graphene in ordered graphene oxide, namely C6O1, C6O2, and C6O3, representing the carbon : oxygen ratios of 6 : 1, 3 : 1, and 2 : 1, respectively, using first-principles calculations within the framework of density functional theory. We predict a reduction of Young's modulus of graphene by a factor of 20%, 23%, and 27% for C6O1, C6O2, and C6O3, respectively, indicating a monotonic degradation with respect to epoxidation. However, there is no clear trend for Poisson's ratio, implying that the local atomic configurations are dominant over oxygen concentrations in determining the Poisson ratio. Our computed high order elastic constants are good for the design of graphene oxide based flexible transparent electronics.

Peculiar pressure effect on Poisson ratio of graphone as a strain damper

Hydrogenation is an effective way to modify the electronic and magnetic properties of graphene. The semi-hydrogenated graphene, known as "graphone", has promising applications in nanoelectronics including field-effect transistors. However, the elastic limit of this two-dimensional material remains unknown despite its importance in applications as well as strain engineering to tailor functions and properties. Here we report using first-principles calculations an abnormal increase in the Poisson ratio of graphone in response to an increase in pressure. This peculiar behavior is proposed to originate from the asymmetry of hydrogenation and could be used to design a nanodevice of strain damper to reduce harmful strains in graphene-based nanoelectronics.

Mechanical properties and stabilities of g-ZnS monolayers

Planar graphene-like ZnS monolayers are mechanically stable under various large strains.

Modeling of the interaction between polypropylene and monolayer sheets: a quantum mechanical study

In this paper, we performed quantum mechanical calculations to determine the best monolayer sheet for preparing polypropylene nanocomposites.

Mechanical properties and stabilities of α-boron monolayers

α-Boron monolayers are mechanically stable under various large strains.

Elastic limit of silicane

Silicane is a fully hydrogenated silicene-a counterpart of graphene-having promising applications in hydrogen storage with capacities larger than 6 wt%. Knowledge of its elastic limit is critical in its applications as well as tailoring its electronic properties by strain. Here we investigate the mechanical response of silicane to various strains using first-principles calculations based on density functional theory. We illustrate that non-linear elastic behavior is prominent in two-dimensional nanomaterials as opposed to bulk materials. The elastic limits defined by ultimate tensile strains are 0.22, 0.28, and 0.25 along armchair, zigzag, and biaxial directions, respectively, an increase of 29%, 33%, and 24% respectively in reference to silicene. The in-plane stiffness and Poisson ratio are reduced by a factor of 16% and 26%, respectively. However, hydrogenation/dehydrogenation has little effect on its ultimate tensile strengths. We obtained high order elastic constants for a rigorous continuum description of the nonlinear elastic response. The limitation of second, third, fourth, and fifth order elastic constants are in the strain range of 0.02, 0.08, and 0.13, and 0.21, respectively. The pressure effect on the second order elastic constants and Poisson's ratio were predicted from the third order elastic constants. Our results could provide a safe guide for promising applications and strain-engineering the functions and properties of silicane monolayers.

Outstanding mechanical properties of monolayer MoS2 and its application in elastic energy storage

The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS2 (g-MoS2) under various large strains are investigated using density functional theory (DFT). g-MoS2 is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N m(-1) (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS2 is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ L(-1) and 1.7 MJ kg(-1), or 476 W h kg(-1), larger than a Li-ion battery and is environmentally friendly.

Epitaxial growth of high-quality AlN films on metallic nickel substrates by pulsed laser deposition

Single-crystalline AlN films with smooth surface and abrupt interface have been grown on metallic nickel (Ni) substrates by pulsed laser deposition with an in-plane epitaxial relationship of AlN[112̄0]//Ni[11̄0].