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Register a new userExceptional piezoelectricity, high thermal conductivity and stiffness and promising photocatalysis in two-dimensional MoSi2N4 family confirmed by first-principles
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Chemical vapor deposition has been most recently employed to fabricate centimeter-scale high-quality single-layer MoSi2N4 (Science; 2020;369; 670). Motivated by this exciting experimental advance, herein we conduct extensive first-principles based simulations to explore the stability, mechanical properties, lattice thermal conductivity, piezoelectric and flexoelectric response, and photocatalytic and electronic features of MA2Z4 (M = Cr, Mo, W; A = Si, Ge; Z = N, P) monolayers. The considered nanosheets are found to exhibit dynamical stability and remarkably high mechanical properties. Moreover, they show diverse electronic properties from antiferromagnetic metal to half metal and to semiconductors with band gaps ranging from 0.31 to 2.57 eV. Among the studied nanosheets, the MoSi2N4 and WSi2N4 monolayers yield appropriate band edge positions, high electron and hole mobilities, and strong visible light absorption, highly promising for applications in optoelectronics and photocatalytic water splitting. The MoSi2N4 and WSi2N4 monolayers are also predicted to show outstandingly high lattice thermal conductivity of 440 and 500 W/mK, respectively. For the first time we show that machine learning interatomic potentials trained over small supercells can be employed to examine the flexoelectric and piezoelectric properties of complex structures. As the most exciting finding, WSi2N4, CrSi2N4 and MoSi2N4 are found to exhibit the highest piezoelectric coefficients, outperforming all other-known 2D materials. Our results highlight that MA2Z4 nanosheets not only undoubtedly outperform the transition metal dichalcogenides family but also can compete with graphene for applications in nanoelectronics, optoelectronic, energy storage/conversion and thermal management systems.
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Experimental realization of single-layer MoSi2N4 is among the latest groundbreaking advances in the field of two-dimensional (2D) materials. Inspired by this accomplishment, herein we conduct first-principles calculations to explore the stability of MC2N4 (M= Cr, Mo, W, V, Nb, Ta, Ti, Zr, Hf) monolayers. Acquired results confirm the desirable thermal, dynamical and mechanical stability of MC2N4 (M= Cr, Mo, W, V) nanosheets. Interestingly, CrC2N4, MoC2N4 and WC2N4 monolayers are found to be semiconductors with band gaps of 2.32, 2.76 and 2.86 eV, respectively, using the HSE06 functional, whereas VC2N4 lattice shows a metallic nature. The direct gap semiconducting nature of CrC2N4 monolayer results in excellent absorption of visible light. The elastic modulus and tensile strength of CrC2N4 nanosheet are predicted to be remarkably high, 676 and 54.8 GPa, respectively. On the basis of iterative solutions of the Boltzmann transport equation, the room temperature lattice thermal conductivity of CrC2N4 monolayer is predicted to be 350 W/mK, among the highest in 2D semiconductors. CrC2N4 and WC2N4 lattices are also found to exhibit outstandingly high piezoelectric coefficients. This study introduces CrC2N4 nanosheet as a novel 2D semiconductor with outstandingly high mechanical strength, thermal conductivity, carrier mobility and piezoelectric coefficient.
In a latest experimental advance, graphene-like and insulating BeO monolayer was successfully grown over silver surface by molecular beam epitaxy (ACS Nano 15(2021), 2497). Inspired by this accomplishment, in this work we conduct first-principles based simulations to explore the electronic, mechanical properties and thermal conductivity of graphene-like BeO, MgO and CaO monolayers. The considered nanosheets are found to show desirable thermal and dynamical stability. BeO monolayer is found to show remarkably high elastic modulus and tensile strength of 408 and 53.3 GPa, respectively. The electronic band gap of BeO, MgO and CaO monolayers are predicted to be 6.72, 4.79, and 3.80 eV, respectively, using the HSE06 functional. On the basis of iterative solutions of the Boltzmann transport equation, the room temperature lattice thermal conductivity of BeO, MgO and CaO monolayers are predicted to be 385, 64 and 15 W/mK, respectively. Our results reveal substantial decline in the electronic band gap, mechanical strength and thermal conductivity by increasing the weight of metal atoms. This work highlights outstandingly high thermal conductivity, carrier mobility and mechanical strength of insulating BeO nanosheets and suggest them as promising candidates to design strong and insulating components with high thermal conductivities.
The low thermal conductivity of piezoelectric perovskites is a challenge for high power transducer applications. We report first principles calculations of the thermal conductivity of ferroelectric PbTiO$_3$ and the cubic nearly ferroelectric perovskite KTaO$_3$. The calculated thermal conductivity of PbTiO$_3$ is much lower than that of KTaO$_3$ in accord with experiment. Analysis of the results shows that the reason for the low thermal conductivity of PbTiO$_3$ is the presence of low frequency optical phonons associated with the polar modes. These are less dispersive in PbTiO$_3$, leading to a large three phonon scattering phase space. These differences between the two materials are associated with the $A$-site driven ferroelectricity of PbTiO$_3$ in contrast to the $B$-site driven near ferroelectricity of KTaO$_3$. The results are discussed in the context of modification of the thermal conductivity of electroactive materials.
Solid solution BiFe1-xCoxO3 shows anti-ferromagnetic order and pyroelectric order, simultaneously. It has been known that BiFe1-xCoxO3 exhibits a structural phase transition between monoclinic and tetragonal phases as x increases. This kinds of transition is often called morphotoropic phase boundary, which is well known to take place in a representative piezoelectric oxide, PbZr1-xTixO3. In order to theoretically understand the piezoelectric property in BiFe1-xCoxO3, we performed ab-initio electronic-structure calculations and studied the structural stability, the magnetic property, and the electronic polarization by means of super-cell approach. It turns out that the large electric polarization and the particular pyramidal coordination suppress the response of the electric polarization under strain. A way to enhance the piezoelectric effect in BiFe1-xCoxO3 is proposed.
In a recent preprint Kong et al, arXiv:0902.0642v1 (2009) claimed to calculate the lattice thermal conductivity of single and bi-layer graphene from first principles. The main findings were that the Umklapp-limited thermal conductivity is only slightly higher than that of high-quality bulk graphite along the basal plane, and that it does not strongly depend on the number of atomic layers. Here we explain that the calculation of Kong et al used a truncation procedure with a hidden parameter, a cut-off frequency for the long-wavelength acoustic phonons, which essentially determined the final result. Unlike in bulk graphite, there is no physical justification for introducing the cut-off frequency for the long wavelength phonons in graphene. It leads to substantial underestimation of graphenes lattice thermal conductivity and a wrong conclusion about the dependence on the number of atomic layers. We outline the proper way for calculating the lattice thermal conductivity of graphene, which requires an introduction of other scattering mechanisms to avoid a logarithmic divergence of the thermal conductivity integral.
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