Yunye Liang

TiSe monolayer possesses good thermal dynamic stability and conductivity. It shows high specific capacities of 423 mA h g⁻¹ for Li/Na/K. Small ion diffusion barriers indicate a high-rate capability, making it a good candidate as anode material.

The family of III–V element compounds (i.e., XY compounds; X = B, Al, Ga, In, or Tl; Y = N, P, As, or Sb) have been intensively investigated for several decades because of their enormous applications for many optoelectronic devices. Here, by employing first‐principles calculations, the electronic structures of bulk XY haeckelite compounds are examined. It is identified that InSb (TlN and TlP) is Dirac semimetal (are strong topological insulators). The other fifteen XY compounds are semiconducting. The effect of biaxial and uniaxial tensile and compressive strains on the electronic structures are studied. These materials offer diverse topological orders. The semiconducting band gaps are mainly found between the bonding and antibonding states of the mixed X(p)–Y(p) orbitals at the top of the valence band and the bottom of the conduction bands, respectively. The topological insulating nature of the XY compounds is explained based on the degenerate p_x + p_y orbitals and their orbital energies relative to the p_z orbitals near the Fermi energy. The nontrivial band topologies of TlN and TlP are confirmed by calculating the Z₂ (1;000) index, surface states, and Wilson loop calculations. The bands split into two branches by including spin‐orbit interaction. The results demonstrate that haeckelite compounds are fascinating materials with broad potential applications in optoelectronics and possessing the possibility of hosting emergent physical phenomena.

MXenes have attracted lots of attention because of the potential applications in electronic devices and energy storage. A variety of transition metals in MXenes give rise to distinct properties and trigger more interests. Depending on the exfoliation processes from the MAX phase, the surfaces of MXenes can be terminated by O, F, Cl, OH groups. Theoretical calculations reveal that the electronic properties of MXenes can be tuned by different surface terminations. For example, some F and O terminated MXenes are predicted to be topological insulators with the quantum spin hall states. In OH terminated MXene multilayers, the image potential states are close to the Fermi level. The energies of these states are sensitive to the interlayer distances. Consequently, the topology of the energy bands can be modulated. Here, based on the density functional theory, we study the electronic structures of the ordered double transition metal MXenes M′ ₂ M″ ₂ C ₃ T ₂ (where M′ = V, Nb, Ta, M″ = Ti, Zr, Hf and T = F, Cl). We propose that these materials are topologically nontrivial insulators or semimetals. The topological Z 2 index is 1 and the presence of the conducting helical edge states is demonstrated. Their dynamical stabilities are confirmed by the phonon spectra. We expect that our prediction can facilitate the future application of MXenes as the topological insulating devices.

Using the first-principles calculations, we explore the nearly free electron (NFE) states in the transition-metal dichalcogenides MX ₂ ( M = Mo, W; X = S, Se, Te) monolayers. It is found that both the external electric field and electron (not hole) injection can flexibly tune the energy levels of the NFE states, which can shift down to the Fermi level and result in novel transport properties. In addition, we find that the valley polarization can be induced by both electron and hole doping in MoTe ₂ monolayer due to the ferromagnetism induced by the charge injection, which, however, is not observed in other five kinds of MX ₂ monolayers. We carefully check band structures of all the MX ₂ monolayers, and find that the exchange splitting in the top of the valence band and the bottom of conduction band plays the key role in the ferromagnetism. Our researches enrich the electronic, spintronic, and valleytronic properties of MX ₂ monolayers.

MXenes, a family of two-dimensional transition metal carbides and nitrides, have various tunable physical and chemical properties. Their diverse prospective applications in electronics and energy storage devices have triggered great interests in science and technology. MXenes can be functionalized by different surface terminations. Some O and F functionalized MXenes monolayers have been predicted to be topological insulators (TIs). However, the reported OH functionalized MXenes TIs are very few and their electronic structures need to be investigated in more detail. It has been revealed that the work functions of MXenes are reduced significantly by OH termination and the image potential (IP) states move close to the Fermi level. The wave functions of these IP states are spatially extensive outside the surfaces. By stacking the OH-functionalized MXenes, the energies of the IP states can be modulated by the interlayer distances of multilayers, because the overlap and hybridization of the wave functions between the neighboring layers are significant. Therefore, these stacking layers are interacted and coupled with IP states. Here, based on first-principles calculations, we demonstrate that the stacking of two-dimensional topologically trivial OH-functionalized MXenes, such as V$_2$HfC$_2$(OH)$_2$, possibly gives rise to the topologically nontrivial energy bands. In other words, the topological properties of V$_2$HfC$_2$(OH)$_2$ multilayers can be modulated by its interlayer distance. An energy band inversion involving IP states is proposed. We expect that these results can advance the future application of MXenes or other low work function multilayer materials as controllable TI devices.

The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics and offer many potential applications in electronic devices and sensors. Using density functional theory (DFT), it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH) functionalized MXenes. Most of the OH-terminated MXene are metallic, but some of them, e.g. Sc2C(OH)2, are semiconductors and the NFE states are conduction bands. In this paper, to investigate the NFE states in MXenes, an attractive image-potential well model is adopted. Compared the solutions of this model with the DFT calculations, it is found that due to the overlap of spatially extensive wave functions of NFE states and their hybridization between the artificial neighboring layers imposed by the periodical boundary conditions (PBCs), the DFT results represent the properties of multiple layers, intrinsically. Based on the DFT calculations, it is found that the energy gap widths are affected by the interlayer distances. We address that the energetics of the NFE states can be modulated by the external electric fields and it is possible to convert semiconducting MXenes into metals. This band-gap manipulation makes the OH-terminated semiconducting MXenes an excellent candidate for electronic switch applications. Finally, using a set of electron transport calculations, I-V characteristics of Sc2C(OH)2 devices are investigated with the gate voltages.

Two-dimensional transition metal carbides and nitrides (named as MXenes) and their functionalized ones exhibit various physical and chemical characteristics. For example, it has been reported that the nearly free electron (NFE) states can be energetically found near the Fermi levels in hydroxyl functionalized MXenes. Most of these OH-terminated MXene are metallic, but some of them, e.g. Sc₂C(OH)₂, are semiconductors with NFE state conduction bands. In a variety of low dimensional materials, such as graphene, BN nanotubes and fullerenes, NFE states have been theoretically predicted and/or observed experimentally. In these systems, NFE states play less important roles in chemical reactions or electronic device applications because they appear at energies several electron-volts above the Fermi level. Here, based on the density functional theory (DFT) calculations and the image-potential well model, we show that the wave functions of these NFE states are spatially extensive outside the surface. We propose that the energy gap width is affected by the interlayer distance because of the significant overlap and the hybridization between the wave functions of NFE states from the neighboring layers. We also demonstrated that the energetics of the NFE bands can be engineered by external electric fields. This results in semiconducting to metallic transition in Sc₂C(OH)₂. The band-gap manipulation makes Sc₂C(OH)₂ an excellent candidate for electronic switch applications. Finally, by performing a set of electron transport calculations, the I–V characteristics of the Sc₂C(OH)₂ device are investigated at various gate voltages. It is illustrated that the NFE states in Sc₂C(OH)₂ contribute to the transport properties significantly.