Kyoung E. Kweon

Giant Zeeman splitting in nucleation-controlled doped CdSe:Mn2+ quantum nanoribbons

Doping of semiconductor nanocrystals by transition-metal ions has attracted tremendous attention owing to their nanoscale spintronic applications. Such doping is, however, difficult to achieve in low-dimensional strongly quantum confined nanostructures by conventional growth procedures. Here we demonstrate that the incorporation of manganese ions up to 10% into CdSe quantum nanoribbons can be readily achieved by a nucleation-controlled doping process. The cation-exchange reaction of (CdSe)(13) clusters with Mn(2+) ions governs the Mn(2+) incorporation during the nucleation stage. This highly efficient Mn(2+) doping of the CdSe quantum nanoribbons results in giant exciton Zeeman splitting with an effective g-factor of approximately 600, the largest value seen so far in diluted magnetic semiconductor nanocrystals. Furthermore, the sign of the s-d exchange is inverted to negative owing to the exceptionally strong quantum confinement in our nanoribbons. The nucleation-controlled doping strategy demonstrated here thus opens the possibility of doping various strongly quantum confined nanocrystals for diverse applications.

Charge disproportionation and the pressure-induced insulator–metal transition in cubic perovskite PbCrO3

Significance The steric activity of the lone pair electrons of Pb2+-containing compounds distorts the crystal structure and produces exotic physical properties. In ferroelectric PbTiO3 and PbVO3, the lone-pair electrons hybridizing with the oxygen lead to polarized MO6 octahedra. In PbRuO3, the hybridization induces unprecedented Pb-Ru bonds at high pressure. The sterochemical effect in PbCrO3 makes Pb bond with oxygen without a long-range periodicity. Under the influence of displaced Pb2+, Cr4+ undergoes a charge disproportionation that opens up a gap. In contrast to the pressure effect on PbTiO3 and PbRuO3, pressure restores the undistorted perovskite structure in PbCrO3. This result implies that the sterochemical effect of Pb2+ in a perovskite depends sensitively on the number and energy of the d electrons. The perovskite PbCrO3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr4+ → 2Cr3+ + Cr6+ in association with the 6s-p hybridization on the Pb2+ is responsible for the insulating ground state of PbCrO3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations.

Surface structure and hole localization in bismuth vanadate: A first principles study

The monoclinic and tetragonal phases of bismuth vanadate (BiVO4) have been found to exhibit significantly different photocatalytic activities for water splitting. To assess a possible surface effect on the phase-dependent behavior, we calculate and compare the geometries and electronic structures of the monoclinic and tetragonal BiVO4 (001) surfaces using hybrid density functional theory. The relaxed atomic configurations of these two surfaces are found to be nearly identical, while an excess hole shows a relatively stronger tendency to localize at the surface than the bulk in both phases. Possible factors for the phase-dependent photocatalytic activity of BiVO4 are discussed.

Formation, nature, and stability of the arsenic-silicon-oxygen alloy for plasma doping of non-planar silicon structures

We demonstrate stable arsenic-silicon-oxide film formation during plasma doping of arsenic into non-planar silicon surfaces through investigation of the nature and stability of the ternary oxide using first principles calculations with experimental validations. It is found that arsenic can be co-mingled with silicon and oxygen, while the ternary oxide exhibits the minimum energy phase at x ≈ 0.3 in AsxSi1−xO2−0.5x. Our calculations also predict that the arsenic-silicon-oxide alloy may undergo separation into As-O, Si-rich As-Si-O, and Si-O phases depending on the composition ratio, consistent with experimental observations. This work highlights the importance of the solid-state chemistry for controlled plasma doping.

Anomalous perovskite PbRuO3 stabilized under high pressure

Significance Perovskites had the highest density in oxides and fluorides with the formula ABX3 before the postperovskite structure was found in MgSiO3 under high temperature and high pressure. The densification of a perovskite under pressure can be realized by shortening A—X and B—X bond lengths and cooperative rotations of octahedra. In most cases, the densification is within the same space group. The behavior of PbRuO3 under high pressure offers a case where the perovskite structure can be densified by significantly shortening the A–B distance and distorting the octahedra. Forming such a highly unusual structure by Pb—Ru bonding shows the flexibility of the perovskite structure to avoid collapsing into the postperovskite structure. Perovskite oxides ABO3 are important materials used as components in electronic devices. The highly compact crystal structure consists of a framework of corner-shared BO6 octahedra enclosing the A-site cations. Because of these structural features, forming a strong bond between A and B cations is highly unlikely and has not been reported in the literature. Here we report a pressure-induced first-order transition in PbRuO3 from a common orthorhombic phase (Pbnm) to an orthorhombic phase (Pbn21) at 32 GPa by using synchrotron X-ray diffraction. This transition has been further verified with resistivity measurements and Raman spectra under high pressure. In contrast to most well-studied perovskites under high pressure, the Pbn21 phase of PbRuO3 stabilized at high pressure is a polar perovskite. More interestingly, the Pbn21 phase has the most distorted octahedra and a shortest Pb—Ru bond length relative to the average Pb—Ru bond length that has ever been reported in a perovskite structure. We have also simulated the behavior of the PbRuO3 perovskite under high pressure by first principles calculations. The calculated critical pressure for the phase transition and evolution of lattice parameters under pressure match the experimental results quantitatively. Our calculations also reveal that the hybridization between a Ru:t2g orbital and an sp hybrid on Pb increases dramatically in the Pbnm phase under pressure. This pressure-induced change destabilizes the Pbnm phase to give a phase transition to the Pbn21 phase where electrons in the overlapping orbitals form bonding and antibonding states along the shortest Ru—Pb direction at P > Pc.

Defect‐Assisted Covalent Binding of Graphene to an Amorphous Silica Surface: A Theoretical Prediction

We propose a mechanism for defect-assisted covalent binding of graphene to the surface of amorphous silica (a-SiO(2)) based on first-principles density functional calculations. Our calculations show that a dioxasilirane group (DOSG) on a-SiO(2) may react with graphene to form two Si-O-C linkages with a moderate activation barrier (≈0.3 eV) and considerable exothermicity (≈1.0 eV). We also examine DOSG formation via the adduction of molecular O(2) to a silylene center, which is an important surface defect in a-SiO(2) , and briefly discuss modifications in the electronic structure of graphene upon the DOSG-assisted chemical binding onto the a-SiO(2) surface.