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Register a new userBiaxial strain enhanced piezoelectric properties in monolayer g-$mathrm{C_3N_4}$
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Graphite-like carbon nitride (g-$mathrm{C_3N_4}$) is considered as a promising candidate for energy materials. In this work, the biaxial strain (-4%-4%) effects on piezoelectric properties of g-$mathrm{C_3N_4}$ monolayer are studied by density functional theory (DFT). It is found that the increasing strain can reduce the elastic coefficient $C_{11}$-$C_{12}$, and increases piezoelectric stress coefficient $e_{11}$, which lead to the enhanced piezoelectric strain coefficient $d_{11}$. Compared to unstrained one, strain of 4% can raise the $d_{11}$ by about 330%. From -4% to 4%, strain can induce the improved ionic contribution to $e_{11}$ of g-$mathrm{C_3N_4}$, and almost unchanged electronic contribution, which is different from $mathrm{MoS_2}$ monolayer (the enhanced electronic contribution and reduced ionic contribution). To prohibit current leakage, a piezoelectric material should be a semiconductor, and g-$mathrm{C_3N_4}$ monolayer is always a semiconductor in considered strain range. Calculated results show that the gap increases from compressive strain to tensile one. At 4% strain, the first and second valence bands cross, which has important effect on transition dipole moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric effect of g-$mathrm{C_3N_4}$ monolayer, which gives a useful guidence for developing efficient energy conversion devices.
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Experimentally synthesized $mathrm{MoSi_2N_4}$ (textcolor[rgb]{0.00,0.00,1.00}{Science 369, 670-674 (2020)}) is a piezoelectric semiconductor. Here, we systematically study the large biaxial (isotropic) strain effects (0.90 to 1.10) on electronic structures and transport coefficients of monolayer $mathrm{MoSi_2N_4}$ by density functional theory (DFT). With $a/a_0$ from 0.90 to 1.10, the energy band gap firstly increases, and then decreases, which is due to transformation of conduction band minimum (CBM). Calculated results show that the $mathrm{MoSi_2N_4}$ monolayer is mechanically stable in considered strain range. It is found that the spin-orbital coupling (SOC) effects on Seebeck coefficient depend on the strain. In unstrained $mathrm{MoSi_2N_4}$, the SOC has neglected influence on Seebeck coefficient. However, the SOC can produce important influence on Seebeck coefficient, when the strain is applied, for example 0.96 strain. The compressive strain can change relative position and numbers of conduction band extrema (CBE), and then the strength of conduction bands convergence can be enhanced, to the benefit of n-type $ZT_e$. Only about 0.96 strain can effectively improve n-type $ZT_e$. Our works imply that strain can effectively tune the electronic structures and transport coefficients of monolayer $mathrm{MoSi_2N_4}$, and can motivate farther experimental exploration.
A two-dimensional (2D) material system with both piezoelectricity and ferromagnetic (FM) order, referred to as a 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for intriguing physics. Inspired by experimentally synthesized Janus monolayer MoSSe from $mathrm{MoS_2}$, in this work, the Janus monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ with dynamic, mechanical and thermal stabilities is predicted, which is constructed from synthesized ferromagnetic $mathrm{CrI_3}$ monolayer by replacing the top I atomic layer with Br atoms. Calculated results show that monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ is an intrinsic FM half semiconductor with valence and conduction bands being fully spin-polarized in the same spin direction. Furthermore, monolayer $mathrm{CrBr_{1.5}I_{1.5}}$ possesses a sizable magnetic anisotropy energy (MAE). By symmetry analysis, it is found that both in-plane and out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. The calculated in-plane $d_{22}$ value of 0.557 pm/V is small. However, more excitingly, the out-of-plane $d_{31}$ is as high as 1.138 pm/V, which is obviously higher compared with ones of other 2D known materials. The strong out of-plane piezoelectricity is highly desirable for ultrathin piezoelectric devices. Moreover, strain engineering is used to tune piezoelectricity of monolayer $mathrm{CrBr_{1.5}I_{1.5}}$. It is found that compressive strain can improve the $d_{22}$, and tensile strain can enhance the $d_{31}$. A FM order to antiferromagnetic (AFM) order phase transition can be induced by compressive strain, and the critical point is about 0.95 strain. That is to say that a 2D piezoelectric antiferromagnetism (PAFM) can be achieved by compressive strain, and the corresponding $d_{22}$ and $d_{31}$ are 0.677 pm/V and 0.999 pm/V at 0.94 strain, respectively.
The realization of multifunctional two-dimensional (2D) materials is fundamentally intriguing, such as combination of piezoelectricity with topological insulating phase or ferromagnetism. In this work, a Janus monolayer $mathrm{SrAlGaSe_4}$ is built from 2D $mathrm{MA_2Z_4}$ family with dynamic, mechanical and thermal stabilities, which is piezoelectric due to lacking inversion symmetry. The unstrained $mathrm{SrAlGaSe_4}$ monolayer is a narrow gap normal insulator (NI) with spin orbital coupling (SOC). However, the NI to topological insulator (TI) phase transition can be induced by the biaxial strain, and a piezoelectric quantum spin Hall insulator (PQSHI) can be achieved. More excitingly, the phase transformation point is only about 1.01 tensile strain, and nontrivial band topology can hold until considered 1.16 tensile strain. Moreover, a Rashba spin splitting in the conduction bands can exit in PQSHI due to the absence of a horizontal mirror symmetry and the presence of SOC. For monolayer $mathrm{SrAlGaSe_4}$, both in-plane and much weak out-of-plane piezoelectric polarizations can be induced with a uniaxial strain applied. The calculated piezoelectric strain coefficients $d_{11}$ and $d_{31}$ of monolayer $mathrm{SrAlGaSe_4}$ are -1.865 pm/V and -0.068 pm/V at 1.06 tensile strain as a representative TI. In fact, many PQSHIs can be realized from 2D $mathrm{MA_2Z_4}$ family. To confirm that, similar to $mathrm{SrAlGaSe_4}$, the coexistence of piezoelectricity and topological orders can be realized by strain (about 1.04 tensile strain) in the $mathrm{CaAlGaSe_4}$ monolayer. Our works suggest that Janus monolayer $mathrm{SrAlGaSe_4}$ is a pure 2D system for PQSHI, enabling future studies exploring the interplay between piezoelectricity and topological orders, which can lead to novel applications in electronics and spintronics.
The septuple-atomic-layer $mathrm{VSi_2P_4}$ with the same structure of experimentally synthesized $mathrm{MoSi_2N_4}$ is predicted to be a spin-gapless semiconductor (SGS). In this work, the biaxial strain is applied to tune electronic properties of $mathrm{VSi_2P_4}$, and it spans a wide range of properties upon the increasing strain from ferromagnetic metal (FMM) to SGS to ferromagnetic semiconductor (FMS) to SGS to ferromagnetic half-metal (FMHM). Due to broken inversion symmetry, the coexistence of ferromagnetism and piezoelectricity can be achieved in FMS $mathrm{VSi_2P_4}$ with strain range of 0% to 4%. The calculated piezoelectric strain coefficients $d_{11}$ for 1%, 2% and 3% strains are 4.61 pm/V, 4.94 pm/V and 5.27 pm/V, respectively, which are greater than or close to a typical value of 5 pm/V for bulk piezoelectric materials. Finally, similar to $mathrm{VSi_2P_4}$, the coexistence of piezoelectricity and ferromagnetism can be realized by strain in the $mathrm{VSi_2N_4}$ monolayer. Our works show that $mathrm{VSi_2P_4}$ in FMS phase with intrinsic piezoelectric properties can have potential applications in spin electronic devices.
A two-dimensional (2D) material with piezoelectricity, topological and ferromagnetic (FM) orders, namely 2D piezoelectric quantum anomalous hall insulator (PQAHI), may open new opportunities to realize novel physics and applications. Here, by first-principles calculations, a family of 2D Janus monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) with dynamic, mechanical and thermal stabilities is predict to be room-temperature PQAHI. At the absence of spin-orbit coupling (SOC), monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) is a half Dirac semimetal state. When the SOC is included, these monolayers become quantum anomalous hall (QAH) states with sizable gaps (more than two hundred meV) and two chiral edge modes (Chern number C=2). It is also found that monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) possesses robust QAH states against biaxial strain. By symmetry analysis, it is found that only out-of-plane piezoelectric response can be induced by a uniaxial strain in the basal plane. The calculated out-of-plane $d_{31}$ of $mathrm{Fe_2ICl}$ ($mathrm{Fe_2IBr}$) is 0.467 pm/V (0.384 pm/V), which is higher than or comparable with ones of other 2D known materials. Meanwhile, using Monte Carlo (MC) simulations, the Curie temperature $T_C$ is estimated to be 429/403 K for monolayer $mathrm{Fe_2ICl}$/$mathrm{Fe_2IBr}$ at FM ground state, which is above room temperature. Finally, the interplay of electronic correlations with nontrivial band topology is studied to confirm the robustness of QAH state. The combination of piezoelectricity, topological and FM orders makes monolayer $mathrm{Fe_2IX}$ (X=Cl and Br) become a potential platform for multi-functional spintronic applications with large gap and high $T_C$. Our works provide possibility to use the piezotronic effect to control QAH effects, and can stimulate further experimental works.
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