Effect Of Isoelectronic Substitution Research Articles

Isoelectronically substituted group-III based monolayers: An ab initio study

Using density functional theory based electronic structure calculations, we carry out detailed investigation on geometric and electronic properties of group-III atom (B, Al, and Ga) based monolayers, namely borophene, aluminene, and gallinene, in ${\ensuremath{\beta}}_{12}$ configuration which is one of the stable structures for borophene. The results of our calculations indicate that all these monolayers form energetically stable systems and these are all metallic in nature since they possess finite density of states at the Fermi level. Analysis of optimized geometric structures reveals that while borophene in ${\ensuremath{\beta}}_{12}$ structure stabilizes in planar structure, the geometries of the remaining monolayers are nonplanar. In the case of borophene, there is a signature of crossing of two linear bands (which form Dirac cones) at about 2 eV and 0.5 eV above the Fermi level. Similar features have also been observed in electronic band structures of both aluminene and gallinene. Interestingly, one of the Dirac cones in these monolayers lies much closer to the Fermi level compared to the borophene case. The Fermi velocities around these Dirac cones are also higher and comparable to that in graphene. We further study the effect of isoelectronic substitution at one of the inequivalent sites (C site) of the ${\ensuremath{\beta}}_{12}$ configuration for these three monolayers. Our results show that borophene still retains its planar geometry following the substitution. Interestingly, in all these cases, crossing of linearly dispersing bands (Dirac cones) is observed nearer to the Fermi level compared to the pristine systems. Our study suggests that isoelectronic substitutions may be useful in tuning the electronic properties, in particular linear dispersion, of monolayer systems.

Structural evolution and electronic properties of (Sr1−xCax)2−yIrO4+z spin-orbit-assisted insulators

The effect of isoelectronic substitution within single crystals of (Sr1-xCax)2-yIrO4+z is explored. The nominal n=1 Ruddlesden-Popper phase with y=0, z=0 remains stable from x=0 until x=0.11, where the antiferromagnet spin-orbit Mott insulating state persists. An increase in the saturated moment is observed with increasing Ca-substitution, suggesting a modified coupling of the in-plane moments relative to the in-plane rotation of IrO6 octahedra. Beyond x=0.11, the x=1/4, y=0, z=1/2 structural phase Sr3CaIr2O9, consisting of a three-dimensional network of corner sharing octahedra, begins to intermix and eventually nucleates phase pure crystals at higher starting Ca-content. An insulating, nonmagnetic ground state is observed in this phase attributable to the J=0 state and is consistent with a recent powder study. At higher Ca-concentrations beyond x = 0.75, crystals begin to stabilize in the y=1/3, z=0 quasi one-dimensional Ca5Ir3O12 structure. The low temperature transport in this chain-based structure is well described via variable range hopping, and an antiferromagnetic ordering transition appears below T$_N=9$ K. Our data provide a detailed mapping of the electronic and structural properties accessible as the structural framework of the canonical spin orbit Mott insulator Sr2IrO4 is destabilized via isovalent chemical substitution.

Observation of earlier two-to-three dimensional structural transition in gold cluster anions by isoelectronic substitution: MAun− (n=8–11; M=Ag,Cu)

The effects of isoelectronic substitution on the electronic and structural properties of gold clusters are investigated in the critical size range of the two-dimensional (2D)-three-dimensional (3D) structural transition (MAu(n)(-), n=8-11; M=Ag,Cu) using photoelectron spectroscopy and density functional calculations. Photoelectron spectra of MAu(n)(-) are found to be similar to those of the bare gold clusters Au(n+1)(-), indicating that substitution of a Au atom by a Ag or Cu atom does not significantly alter the geometric and electronic structures of the clusters. The only exception occurs at n=10, where very different spectra are observed for MAu(10)(-) from Au(11)(-), suggesting a major structural change in the doped clusters. Our calculations confirm that MAu(8)(-) possesses the same structure as Au(9)(-) with Ag or Cu simply replacing one Au atom in its C(2v) planar global minimum structure. Two close-lying substitution isomers are observed, one involves the replacement of a center Au atom and another one involves an edge site. For Au(10)(-) we identify three coexisting low-lying planar isomers along with the D(3h) global minimum. The coexistence of so many low-lying isomers for the small-sized gold cluster Au(10)(-) is quite unprecedented. Similar planar structures and isomeric forms are observed for the doped MAu(9)(-) clusters. Although the global minimum of Au(11)(-) is planar, our calculations suggest that only simulated spectra of 3D structures agree with the observed spectra for MAu(10)(-). For MAu(11)(-), only a 3D isomer is observed, in contrast to Au(12)(-) which is the critical size for the 2D-3D structural transition with both the 2D and 3D isomers coexisting. The current work shows that structural perturbations due to even isoelectronic substitution of a single Au atom shift the 2D to 3D structural transition of gold clusters to a smaller size.

Investigation on the effect of isoelectronic substitution in ZnS1−xSex alloys

The composition, structure, and optical properties of thin-film ZnS1−xSex semiconductor alloys were studied by x-ray photoemission spectroscopy, x-ray diffraction, and optical transmission. Nonequilibrium growth of the ternary II–VI semiconductor alloys was accomplished using molecular beam epitaxy with simultaneous electron cyclotron resonance H2S plasma activation to form the ZnS1−xSex alloys. Photoemission measurements were used to identify changes in bulk composition and valence band electronic structure as well as changes in the Zn 3d, Se 3d, and S 2p core lines. X-ray diffraction measurements revealed that these films contain the cubic phase and that the lattice constant decreases with sulfur content. Correlation of these results with growth parameters indicates that substrate temperature, gas flow, and plasma power determine the ZnS1−xSex alloy composition and structure. In addition, optical transmission measurements showed that substitution of S into the ZnSe lattice shifted the cutoff from 460 to 380 nm.