Thermal-dynamic Stabilities Of Novel Twodimensional Materials And Manipulation Of Their Electronic Structures

Due to dimensional constraints, the two-dimensional nano-materials show many peculiar properties, which has become a forward field in materials science. Recently, graphyne and two-dimensional group V nano-materials represented by black-phosphorene have received considerable attention because of their unique properties. With the deepening of research, they show great potential for application. In order to broaden the application field of the above-mentioned two kinds of materials, and covenient for designing thembased nanodevices, it is very necessary to study their physical properties from a theoretical point of view. Therefore, we did several corresponding works as follows:1. Molecular dynamics simulations with the adaptive intermolecular reactive bond order(AIREBO) potential are performed to study the high-temperature(T) behavior of monolayer graphyne and graphdiyne(MGY and MGDY). During the melting process, MGY and MGDY undergo three continuous phase transitions:(i) transformation into the initial amorphous graphene phase(AGP) starting from 2800 and 2500 K for MGY and MGDY, respectively;(ii) transformation into AGP through a structural adjustment process until T reaches 3650 K. This AGP will remain relatively stable until T > 5000 K;(iii) the AGP will then gradually transform into a nearly fluid state for T > 5000 K. The sharp contraction and accompanying stretch of MGY and MGDY are decisive in the formation of the initial AGP. In contrast to the prediction of the KTHNY theory for the melting of two-dimensional materials, we don’t observe a hexatic phase but rather an AGP as an intermediate phase.2. Based on first-principles calculations, we studied the effect of hydrogen atoms and hydroxyl functional group on the stabilities and electronic structures of graphyne and graphdiyne.(1) First-principles calculations are performed to investigate dynamical stability of hydrogenated graphyne and its electronic structures. We find that the zero point energy(ZPE) is important in evaluating the stability of hydrogenated graphyne. Based on the results of formation enthalpy, the hydrogenated configuration with only sp3 carbon atoms(eHH) is more stable than that with each carbon atom passivated by single hydrogen atom(eH). However, the Helmholtz free energy as functions of temperature indicates that eH is more favorable than eHH below 670 K. Based on DFT-based phonon spectrum calculations, the dynamical stability of eHH and eH is confirmed. Of particular interest is that the band-gap feature of graphyne undergoes direct-indirect-direct transition with the increase in the concentration of hydrogen. The results indicate that eHH is favorable for the applications in the field of deep ultraviolet light-emitting devices.(2) We studied the energetically favorable geometries and corresponding electronic properties of hydroxyl functional group(-OH) decorated monolayer graphyne and graphdiyne(MGY and MGDY). The energetically favorable high coverage-OH decoration configuration shows ordered chiral-like structure on adjacent big triangle carbon rings and hexagon due to the stabilization of the hydrogen bonding between these groups. In contrast to those of hydrogenated graphyne and graphdiyne, it is an unstable geometry when all carbon bonds are saturated to single bond. It is found that-OH can introduce magnetism in the systems but the magnetic properties can only survive at low-OH concentration.-OH pair forming hydrogen bond between two-OH groups are usually nonmagnetic due to the antiferromagnetic coupling between them.3. Two novel two-dimensional(2D) carbon allotropes named Cy and Cz with large meshes are predicted based on first-principles calculations. Their formation energies are lower than that of graphdiyne. Molecular dynamics simulations indicate that Cy and Cz are stable even when the temperature is over 1000 K. Their Poisson’s ratios show their anisotropic mechanical properties. The electronic structure calculations indicate that Cy is a metal, while Cz behaves as a semiconductor. Moreover, Cz shows conductive anisotropy suggesting its potential in nanoelectronic devices. Meanwhile, their well-defined mesh structures are suitable for molecular sieves.4. Three two-dimensional phosphorus nitride(PN) monolayer sheets(named as α-, β-, and γ-PN, respectively) with fantastic structures and properties are predicted based on firstprinciples calculations. The α-PN and γ-PN have a buckled structure, whereas β-PN shows puckered characteristics. Their unique structures endow these atomic PN sheets with high dynamic stabilities and anisotropic mechanical properties. They are all indirect semiconductors and their band gap sensitively depends on the in-plane strain. Moreover, the nanoribbons patterned from these three PN monolayers demonstrate a remarkable quantum size effect. In particular, the zigzag α-PN nanoribbon shows size-dependent ferromagnetism. Their significant properties show potential in nano-electronics. The synthesis of the three phases of the PN monolayer sheet is proposed theoretically, which is deserving of further study in experiments.5. Based on a comprehensive investigation including ab initio phonon and finitetemperature molecular dynamics calculations, we find that two-dimensional tricycle-shaped arsenene(T-As) is robust and even stable under high temperature. T-As is energetically comparable to previously reported chair-shaped arsenene(C-As) and more stable than stirrup-shaped arsenene(S-As). In contrast to C-As and S-As, the monolayer T-As is a direct band gap semiconductor with an energy gap of 1.377 eV. Our results indicate that the electronic structure of T-As can be effectively modulated by stacking, strain, and patterning, which shows great potential of T-As in future nano-electronics. Moreover, by absorbing H or F atoms on the surface of T-As along a specific direction, nanoribbons with desired edge type and even width can be obtained, which is suitable for the fabrication of nano-devices.