The newly emerging monolayer phosphorene was recently predicted to be a promising thermoelectric material. In this work, we propose to further enhance the thermoelectric performance of phosphorene using the straininduced band convergence. The effect of the uniaxial strain on the thermoelectric properties of phosphorene was investigated by using the first-principles calculations combined with the semiclassical Boltzmann theory. When the zigzag-direction strain is applied, the Seebeck coefficient and electrical conductivity in the zigzag direction can simultaneously be greatly enhanced at the critical strain of 5%, at which the band convergence is achieved. The largest ZT value of 1.65 at 300 K is then conservatively estimated by using the bulk lattice thermal conductivity. When the armchair-direction strain of 8% is applied, the room-temperature ZT value can reach 2.12 in the armchair direction of phosphorene. Our results indicate that strain-induced band convergence could be an effective method to enhance the thermoelectric performance of phosphorene.

The thermoelectric performance of the ZrS2monolayer is greatly enhanced by the biaxial tensile strain, due to the simultaneous increase of the Seebeck coefficient and decrease of the thermal conductivity.

No abstract

The structural and electronic properties of a two-dimensional monolayer bismuth are studied using density functional calculations. It is found that the monolayer forms a stable low-buckled hexagonal structure, which is reminiscent of silicene. The electronic transport properties of the monolayer bismuth are then evaluated by using Boltzmann theory with the relaxation time approximation. By fitting first-principles total energy calculations, a modified Morse potential is constructed, which is used to predicate the lattice thermal conductivity via equilibrium molecular dynamics simulations. The room temperature ZT value of a monolayer bismuth is estimated to be 2.1 and 2.4 for the n-and p-type doping, respectively. Moreover, the temperature dependence of ZT is investigated and a maximum value of 4.1 can be achieved at 500 K.

Using the nonequilibrium Green's function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications.

We report on the strain-induced switch between ferromagnetic (FM) and antiferromagnetic (AFM) orderings in 1T -CrX2 (X = Se, Te) monolayers based on the first-principles calculations. The CrSe2 and CrTe2 monolayers without strains are found to be AFM and FM, respectively. Under the biaxial tensile strain, the CrSe2 monolayer tends to be FM when the strain is larger than 2%. The FM state is further stabilized when the strain is increased. Moreover, the CrSe2 monolayer changes to be halfmetallic when the tensile strain is larger than 10%. While for the CrTe2 monolayer, the critical strain at which the transition between the FM and AFM states occurs is compressive, of −1%. Relatively small tensile strains of 4% and 2%, respectively, can enhance the Curie temperature of CrSe2 and CrTe2 monolayers above the room temperature. The strain-induced switch between the FM and AFM states in CrSe2 (CrTe2) monolayer can be understood by the competition between the AFM Cr-Cr direct exchange interaction and FM Cr-Se(Te)-Cr superexchange interaction. The tunable and attractive magnetic and electronic properties controlled by the flexible strain are desirable for the future nanoelectronic applications.

We predict by first principles calculations that the recently prepared borophene is a pristine twodimensional (2D) monolayer superconductor, in which the superconductivity can be significantly enhanced by strain and charge carrier doping. The intrinsic metallic ground state with high density of states at Fermi energy and strong Fermi surface nesting lead to sizeable electron-phonon coupling, making the freestanding borophene superconduct with Tc close to 19.0 K. The tensile strain can increase Tc to 27.4 K, while the hole doping can notably increase Tc to 34.8 K. The results indicate that the borophene grown on substrates with large lattice parameters or under photoexcitation can show enhanced superconductivity with Tc far more above liquid hydrogen temperature of 20.3 K, which will largely broaden the applications of such novel material.

-We predict by first-principles calculations that the electron-doped phosphorene is a potential BCS-like superconductor. The stretching modes at the Brillouin-zone center are remarkably softened by the electron-doping, which results in the strong electron-phonon coupling. The superconductivity can be introduced by a doped electron density (n2D) above 1.3 × 10 14 cm −2 , and may exist over the liquid helium temperature when n2D > 2.6 × 10 14 cm −2 . The superconductivity can be significantly tuned and enhanced by applying tensile strain. The maximum critical temperature of electron doped phosphorene is predicted to be higher than 10 K. The superconductivity of phosphorene will significantly broaden the applications of this novel material.The two-dimensional (2D) monolayer superconductor bears consequences for both applications and fundamental science. It can be used as the component of nanoscale superconducting devices, such as nano superconducting quantum interference devices and nano superconducting transistors [1][2][3][4][5], with the goal of achieving single-spin sensitivity for measuring and controlling. Moreover, the high-T c superconductors with the quasi-2D layered structures, such as MgB 2 [6], cuprate superconductors [7,8], and ironbased superconductors [9], can be seen as the assembly of multiple monolayers. Geim et al. [10] proposed to construct the high-T c -superconductor-like Van der Waals heterostructures using the monolayer superconductors, which may be helpful for the exploration of new high-T c superconductors. A graphene-like monolayer superconductor seems to be the natural choice of such applications. It is suggested that the carrier-doped graphene [11][12][13] and graphane [14] may exhibit superconductivity with notable T c . However, the experimental evidences are still lacking. Using the liquid-gate method, one can introduce the superconductivity into the few-layer semiconductor MoS 2 [15], which may be driven by the electron-phonon coupling (a) E-mail: wjlu@issp.ac.cn (b) E-mail: ypsun@issp.ac.cn [16]. But the thickness of such material is still far from one layer. More monolayer superconductor candidates still need to be found.Here we show a potential monolayer superconductor: electron-doped phosphorene. Based on the density functional theory (DFT) calculations, we found the electrondoping can make the stretching modes at zone center significantly softened, leading to a strong electron-phonon coupling. The superconductivity starts showing up when the carrier density (n 2D ) is 1.3 × 10 14 cm −2 . When n 2D > 2.6 × 10 14 cm −2 , the T c exceeds the liquid helium temperature. Moreover, the application of tensile strain can significantly tune and enhance the superconductivity. The maximum T c is predicted to be higher than 10 K. Our prediction can be readily verified by the liquid-gate method [15,[17][18][19], or the adsorption of the alkali/alkalineearth metal atoms. Figure 1 shows the structures of bulk black phosphorus (black-P) and monolayer black-P (which is called phosphoren...