Naihua Miao

Theoretical investigation on the transition-metal borides with Ta3B4-type structure: A class of hard and refractory materials

Conference Name:19th International Workshop on Computational Mechanics of Materials (IWCMM 19). Conference Address: Constanta, ROMANIA. Time:SEP 01-04, 2009.

Strain-mediated type-I/type-II transition in MXene/Blue phosphorene van der Waals heterostructures for flexible optical/electronic devices

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Ab initio study of the structure and chemical bonding of stable Ge3Sb2Te6

The atomic arrangements and chemical bonding of stable Ge(3)Sb(2)Te(6), a phase-change material, have been investigated by means of ab initio total energy calculations. The results show that an ordered stacking of Ge-Te-Ge-Te-Sb-Te-Te-Sb-Te-Ge-Te- is the most stable configuration in respect that the -Sb-Te-Te-Sb- configuration enhances the structure stability as analyzed by electron localization function (ELF) and bond energies. Ge(3)Sb(2)Te(6) shows the character of a p-type semiconductor as seen from the density of states. The chemical bonding of Ge(3)Sb(2)Te(6) is rather inhomogeneous; strong and weak covalence coexist between Te and Sb atoms, while the strength of the covalent bonding between Te and Ge atoms of various Te-Ge bonds is very close, whereas the interaction between the neighboring Te layers is a van der Waals-type weak bond. The bonding character of Ge(3)Sb(2)Te(6) is assumed to be applied to the other pseudobinary nGeTe.mSb(2)Te(3) phase-change materials.

2D Intrinsic Ferromagnets from van der Waals Antiferromagnets

Intrinsically ferromagnetic 2D semiconductors are essential and highly sought for nanoscale spintronics, but they can only be obtained from ferromagnetic bulk crystals, while the possibility to create 2D intrinsic ferromagnets from bulk antiferromagnets remains unknown. Herein on the basis of ab initio calculations, we demonstrate this feasibility with the discovery of intrinsic ferromagnetism in an emerging class of single-layer 2D semiconductors CrOX (CrOCl and CrOBr monolayers), which show robust ferromagnetic ordering, large spin polarization, and high Curie temperature. These 2D crystals promise great dynamical and thermal stabilities as well as easy experimental fabrication from their bulk antiferromagnets. The Curie temperature of 2D CrOCl is 160 K, which exceeds the record (155 K) of the most-studied dilute magnetic GaMnAs materials, and could be further enhanced by appropriate strains. Our study offers an alternative promising way to create 2D intrinsic ferromagnets from their antiferromagnetic bulk counterparts and also renders 2D CrOX monolayers great platform for future spintronics.

Pressure-induced semimetal-semiconductor transition and enhancement of thermoelectric performance in α-MgAgSb

Comparable to bismuth telluride, α-MgAgSb-based materials (α-MAS) have been investigated recently as promising candidates for room-temperature thermoelectric energy harvesting and thus various efforts have been devoted to the enhancement of their thermoelectric performance. By utilizing first-principles density functional calculations and Boltzmann transport theory, we report that the thermoelectric properties of α-MAS can be dramatically improved with the application of hydrostatic pressure. This is attributed to a pressure-induced semimetal to semiconductor transition in α-MAS. With the benefit of this pressure-tunable behaviour, the Seebeck coefficient of α-MAS can be manipulated flexibly. Furthermore, we found that, through the combination of applying pressure and p-type doping, the optimal thermoelectric power factor and figure of merit of α-MAS can be enhanced remarkably by 110% at 550 K compared with the intrinsic case. Our results provide an interesting insight and a feasible guideline for the imp...

Mechanical properties and electronic structure of the incompressible rhenium carbides and nitrides: A first-principles study

By means of first-principles calculations, the structural stability, mechanical properties and electronic structure of the newly synthesized incompressible Re(2)C, Re(2)N, Re(3)N and an analogous compound Re(3)C have been investigated. Our results agree well with the available experimental and theoretical data. The proposed Re(3)C is shown to be energetically, mechanically and dynamically stable and also incompressible. Furthermore, it is suggested that the incompressibility of these compounds is originated from the strong covalent bonding character with the hybridization of 5d orbital of Re and the 2p orbital of C or N, and a zigzag topology of interconnected bonds, e.g., Re-Re, Re-C or Re-N bonding.

Polyhedral transformation and phase transition in TcO2

By using ab initio random structure search, we have predicted a stable low pressure phase (phase-LP) of TcO2, which shares the same space group with the ambient condition phase (phase-AC) of TcO2 but exhibits very different crystallographic features. A novel polyhedral transformation from the TcO6-octahedrons in phase-AC to TcO5-hexahedrons in phase-LP has been observed in the phase transition. Large voids along the (100) and (001) directions in phase-LP protect its low density stability, which can serve as the transport channel for protons and β particles. We show that the TcO5-hexahedrons in phase-LP are more distorted and show stronger anisotropy than the TcO6-octahedrons in phase-AC. According to the analysis on the electronic structures and chemical bonding of TcO2, the various origins of the metallic nature of phase-AC and phase-LP have been explored and discussed as well.

The pressure induced twisted distortion in the flexible oxide Tc2O7

The pressure induced twisted distortion of the diagonal-double tetrahedrons (DDTs) in Tc2O7 and the corresponding anomalous lattice variation under pressure were unraveled based on van der Waals (vdW) corrected density functional theory calculations. We show that the optPBE functional precisely represents the crystal structure of Tc2O7 with less than 0.5% volume overestimation. The flexibility of the Tc2O7 cell originating from the weak vdW interactions between the Tc2O7 DDTs is confirmed. On the contrary, we have found that the strong ionic bonding in the Tc2O7 DDT leads to the rigidity of the Tc2O7 DDT. According to the analysis of chemical bonding characterization, the pressure induced distortion of the Tc2O7 DDT leads to inhomogeneous Tc–O bonding distributions.

Structural stability and thermoelectric property optimization of Ca2Si

By using an ab initio evolutionary algorithm structure search, low enthalpy criterion as well as stability analysis, we have found that cubic Fmm Ca2Si can be achieved under a negative external pressure, and the cubic phase is dynamically and mechanically stable at ambient conditions and high pressure. From first-principle hybrid functional calculations, we have unraveled the direct bandgap nature and bandgap variation of cubic Fmm Ca2Si with respective to pressure. Moreover, by combining with Boltzmann transport theory and the phonon Boltzmann transport equation, we have predicted that the figure of merit ZT for the cubic Fmm Ca2Si reaches the maximum value of 0.52 by p-type doping. Our results provide an interesting insight and feasible guidelines for the potential applications of cubic Fmm Ca2Si and related alkaline-earth metals silicides as the thermoelectric materials for heat-electricity energy convertors.

Reduction of thermal conductivity in YxSb2–xTe3 for phase change memory

Thermal conductivity (κ) is one of the fundamental properties of materials for phase change memory (PCM) application, as the set/reset processes strongly depend upon heat dissipation and transport. The κ of phase change materials in both amorphous and crystalline phases should be quite small, because it determines how energy-efficient the PCM device is during programming. At a high temperature, the electronic thermal conductivity (κe) is always notable for semiconductors, which is still lacking for antimony telluride under doping in the literature as far as we know. In this paper, using density functional theory and Boltzmann transport equations, we report calculations of lattice thermal conductivity κL and electronic thermal conductivity κe of the yttrium doped antimony telluride. We show that the average value of thermal conductivity decreases from ∼2.5 W m−1 K−1 for Sb2Te3 to ∼1.5 W m−1 K−1 for Y0.167Sb1.833Te3. This can be attributed to the reduced κL and κe, especially the κe at high temperature (nea...