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Register a new userStructural, Vibrational and Electronic Properties of Single Layer Hexagonal Crystals of Groups IV and V
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Using first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula $A_2B_2$, where $A$ and $B$ belong to groups IV and V, respectively ($A$ = C, Si, Ge, Sn, Pb; $B$ = N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices $Pbar{6}m2$ and $Pbar{3}m1$ are shown to be dynamically stable, named as $alpha$- and $beta$-phases correspondingly. Both phases have similar cohesive energies, and the $alpha$-phase is found to be energetically favorable for structures except CP, CAs, CSb and CBi, for which the $beta$-phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange-correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a Mexican hat shape. SnAs and PbAs have purely quartic valence band edges, i.e. $E{sim}{-}alpha k^4$, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.
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The magnetic and electronic properties of metal phthalocyanines (MPc) and fluorinated metal phthalocyanines (F$_{16}$MPc) are studied by means of spin density functional theory (SDFT). Several metals (M) such as Ca, all first d-row transition metals and Ag are investigated. By considering different open shell transition metals it is possible to tune the electronic properties of MPc, in particular the electronic molecular gap and total magnetic moment. Besides assigning the structural and electronic properties of MPc and F$_{16}$MPc, the vibrational modes analysis of the ScPctextendash ZnPc series have been studied and correlated to experimental measurements when available.
GeSe and SnSe monochalcogenide monolayers and bilayers undergo a two-dimensional phase transition from a rectangular unit cell to a square unit cell at a temperature $T_c$ well below the melting point. Its consequences on material properties are studied within the framework of Car-Parrinello molecular dynamics and density-functional theory. No in-gap states develop as the structural transition takes place, so that these phase-change materials remain semiconducting below and above $T_c$. As the in-plane lattice transforms from a rectangle onto a square at $T_c$, the electronic, spin, optical, and piezo-electric properties dramatically depart from earlier predictions. Indeed, the $Y-$ and $X-$points in the Brillouin zone become effectively equivalent at $T_c$, leading to a symmetric electronic structure. The spin polarization at the conduction valley edge vanishes, and the hole conductivity must display an anomalous thermal increase at $T_c$. The linear optical absorption band edge must change its polarization as well, making this structural and electronic evolution verifiable by optical means. Much excitement has been drawn by theoretical predictions of giant piezo-electricity and ferroelectricity in these materials, and we estimate a pyroelectric response of about $3times 10^{-12}$ $C/K m$ here. These results uncover the fundamental role of temperature as a control knob for the physical properties of few-layer group-IV monochalcogenides
The electronic and thermoelectric properties of one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, and WSe$_{2}$ are calculated. For few layer thicknesses,the near degeneracies of the conduction band $K$ and $Sigma$ valleys and the valence band $Gamma$ and $K$ valleys enhance the n-type and p-type thermoelectric performance. The interlayer hybridization and energy level splitting determine how the number of modes within $k_BT$ of a valley minimum changes with layer thickness. In all cases, the maximum ZT coincides with the greatest near-degeneracy within $k_BT$ of the band edge that results in the sharpest turn-on of the density of modes. The thickness at which this maximum occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses.
We systematically explore chemical functionalization of monolayer black phosphorene via chemisorption of oxygen and fluorine atoms. Using the cluster expansion technique, with vary- ing concentration of the adsorbate, we determine the ground states considering both single- as well as double- side chemisorption, which have novel chemical and electronic properties. The nature of the bandgap depends on the concentration of the adsorbate: for fluorination the direct bandgap first decreases, and then increases while becoming indirect, with increasing fluorination, while for oxidation the bandgap first increases and then decreases, while mostly maintaining its direct nature. Further we find that the unique anisotropic free-carrier effective mass for both the electrons and holes, can be changed and even rotated by 90 degrees, with controlled chemisorption, which can be useful for exploring unusual quantum Hall effect, and novel electronic devices based on phosphorene.
Unusual physical properties of single-wall carbon nanotubes have started a search for similar tubular structures of other elements. In this paper, we present a theoretical analysis of single-wall nanotubes of silicon and group III-V compounds. Starting from precursor graphene-like structures we investigated the stability, energetics and electronic structure of zigzag and armchair tubes using first-principles pseudopotential plane wave method and finite temperature ab-initio molecular dynamics calculations. We showed that (n,0) zigzag and (n,n) armchair nanotubes of silicon having n > 6 are stable but those with n < 6 can be stabilized by internal or external adsorption of transition metal elements. Some of these tubes have magnetic ground state leading to spintronic properties. We also examined the stability of nanotubes under radial and axial deformation. Owing to the weakness of radial restoring force, stable Si nanotubes are radially soft. Undeformed zigzag nanotubes are found to be metallic for 6 < n < 11 due to curvature effect; but a gap starts to open for n > 12. Furthermore, we identified stable tubular structures formed by stacking of Si polygons. We found AlP, GaAs, and GaN (8,0) single-wall nanotubes stable and semiconducting. Our results are compared with those of single-wall carbon nanotubes.
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