Yinchang Zhao

Phonon-mediated superconductivity in borophenes

We use first-principles calculations to systematically investigate electronic, vibrational, and superconducting properties in borophenes (boron monolayer sheets). Remarkably, superconducting transition temperature Tc is a V-like function of hexagon hole density and has a similar tendency to the variations of the total energy and density of states at the Fermi level, which shows that the larger density of states at the Fermi level corresponds to the higher Tc. In consideration of substrate, the Ag(111) surfaces weaken the superconductivity in borophenes, which results in Tcμ*=0.1 of about 5.2 K in the buckled triangular sheet. As synthesis of borophenes was reported, superconducting boron sheets are feasible.

Spin-semiconducting properties in silicene nanoribbons

We have investigated the relative stabilities and electronic properties of silicene nanoribbons with sawtooth edges (SSiNRs) by first-principles calculations. The SSiNR is more stable than the zigzag silicene nanoribbon (ZSiNR) and has a ferromagnetic ground state with an intrinsic energy gap between majority and minority spin-polarized bands, which shows that SSiNR is a spin-semiconductor. Under an external transverse electric field, the energy gap decreases and even vanishes. Meanwhile, the charge densities of the two edge bands near the Fermi level become spatially separated at different edges. We find also that the electric field-induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. At last, the electronic structures of SSiNRs tuned by electric field and strain together are studied, showing that a small tensile strain makes the SSiNRs more sensitive to the electric field. These results suggest that the electric field or/and strain modulated SSiNRs have potential applications in silicon-based spintronic nanodevices.

Exotic thermoelectric behavior in nitrogenated holey graphene

Combining first-principles approaches with the Boltzmann transport equation and semi-classical analysis we investigate the thermal conductivity κ, thermopower S, electrical conductivity σ, and carrier mobility μ of newly synthesized nitrogenated holey graphene (NHG). Strikingly, the NHG possesses exceedingly high S and σ but a fairly low lattice thermal conductivity κL, and therefore extraordinary thermoelectric properties, with a figure of merit zT even exceeding 5.0, are obtained in the n-type doped NHG. The outstanding thermoelectric behavior of NHG is attributed to its exotic atomic and electronic structure: (i) strong anharmonic phonon scattering results in a very low κL; (ii) flat bands around the Fermi level together with a large band gap cause high S; and (iii) conduction band dipping at the Brillouin zone center leads to a high electron mobility μ and thus high σ.

Spin Excitation Under Electron Delocalization of Side Radicals in Quasi-One-Dimensional Organic Ferromagnet

Spin excitation in poly(1,4-bis(2,2,6,6-tetramethyl-4-oxy-piperidyl-1-oxyl)-butadiin) (poly-BIPO), a quasi-one-dimensional organic ferromagnet, was investigated based on the extended Su–Schriffer–Heeger model by considering electron hopping and the spin correlation between the main chain and side radicals. The lattice, charge density, and spin density configurations of the single spin as well as spin domain excited states of the organic ferromagnet poly-BIPO were systematically studied. The side radical spin excitation gives rise to lattice distortion, charge density localization, and a spin density defect on the main chain. A peak induced by spin excitation is predicted to appear in the density of states of the organic ferromagnet poly-BIPO based on calculations for different spin electron states. These results expand knowledge on elementary excitation in organic materials and have significant implications for the design of spintronic devices.

Versatile electronic and magnetic properties of chemically doped 2D platinum diselenide monolayers: A first-principles study

First-principles calculations have been performed to study the chemically doped platinum diselenide (PtSe2) monolayers. We examine the stability of different doping sites by calculating the formation energy. The different electronic and magnetic characters originate from hybridization between the dopants and nearest local atoms. Exceptional electronic and magnetic characters are observed in the B-, P-, Li-, and Ca-doped cases because of doping site independence. The magnetic behavior of the dopant atoms is found to be complex because of interplay between strong structural relaxation, spin lattice coupling, and crystal field splitting. More interestingly, the ferromagnetic half metallic character obtained in B- and N-doped cases, expected to be very useful because of large half metallic energy bandgap. The interaction between dopants is analyzed as a function of their separation, showing that substitution typically counteracts spin polarization. The long range ferromagnetic behavior can be established with...

Magnetic properties of X-C2N (X=Cl, Br and I) monolayers: A first-principles study

The electronic and magnetic properties of X-C2N (X=F, Cl, Br and I) monolayers have been systematically investigated from first-principles calculations. The F atom can be strongly adsorbed on the top of the host carbon atoms, while the Cl, Br and I atoms favor the top of the host nitrogen atoms of C2N monolayers. These functionalized X-C2N (X=F, Cl, Br and I) monolayers exhibit interesting electronic and magnetic features. The F-C2N monolayer system shows a nonmagnetic metallic state, while the X-C2N (X=Cl, Br and I) monolayer systems exhibit the magnetic semiconducting ground state. Moreover, the ferromagnetic state is energetically more stable configuration for the X-C2N (X=Cl, Br and I) monolayer systems. Magnetic analysis further elaborates that the induced magnetism in the X-C2N (X=Cl, Br and I) monolayer systems mainly arises from the local magnetic moments of the halogen adatoms. Thus, the chemical functionalization of nitrogenated honey graphene through halogen atoms adsorption has promising applications in electronic devices.

Band Gap Adjustment of SiC Honeycomb Structure through Hydrogenation and Fluorination

Previous calculations show that the two-dimensional (2D) silicon carbide (SiC) honeycomb structure is a structurally stable monolayer. Following this, we investigate the electronic properties of the hydrogen and fluorine functionalized SiC monolayer by first-principles calculations. Our results show that the functionalized monolayer becomes metallic after semi-hydrogenation or semi-fluorination, while the semiconducting properties are obtained by the full functionalization. Compared with the bare SiC monolayer, the band gap of the fully hydrogenated system is increased, in comparison with the decrease of the gap in the fully fluorinated case. As a result, the band gap can be tuned from 0.73 to 4.14 eV by the functionalization. In addition to the metal–semiconductor transition, hydrogenation and functionalization also realize a direct-indirect semiconducting transition in the 2D SiC monolayer. These results provide theoretical guidance for design of photoelectric devices based on the SiC monolayer.