Qiming Zhang

pH-dependence of conduction type in cuprous oxide synthesized from solution

Supported by the U.S. Department of Energy, Office of Basic Energy Sciences, and Division of Materials Sciences and Engineering under Award No. DESC0002062.

First-principles prediction of enhanced magnetic anisotropy in FeCo alloys

The structural, electronic, and magnetic properties of FeCo alloys were studied by first-principles calculations. It has been found that the alloys prefer chemically noncubic geometries in a wide composition range. This produces appreciable uniaxial magnetic anisotropy, which facilitates interphase magnetic interaction and enhances the overall magnetization in exchange-coupled nanocomposite systems. Large magnetostrictive coefficients provide another venue for manipulations of magnetic anisotropy energies.

n-type doping in Cu2O with F, Cl, and Br: A first-principles study

The n-type doping in the cuprous oxide (Cu2O) synthesized from an acidic solution environment has been studied by the first-principles calculations. The dopants of F, Cl, and Br in substitution of an O atom have low formation energies. They present an n-type conduction behavior with donor levels below but close to the bottom of the conduction bands. The Cl dopant has the shallowest donor level among the three halogens. The results are in good agreement with the very recent experimental results.

DFT+U study of ultrathin α-Fe2O3 nanoribbons from (110) and (104) surfaces

We present a systematic study of the geometric, electronic, and magnetic properties of hematite nanoribbons (α-Fe2O3NRs) using density functional theory. The hematite nanoribbons were generated by cutting atomically thin hematite nanosheets from the (110) and (104) surfaces along their [100] and [010] directions. All nanoribbon types are energetically feasible to synthesize. Nanoribbons obtained from the (110) surface show definite tunable semiconducting character. One type of nanoribbons obtained from the (104) surface shows surface modifications or bending nature indicating pseudo-Jahn-Teller effect, while the other type showed built-in oxygen vacancy on one edge despite preserving the stoichiometry, wherein the built-in oxygen vacancy introduces a half-metallicity into the nanoribbons at larger widths. The results indicate that α-Fe2O3NRs with the appropriate width and type are promising future materials in solar energy conversions and spintronics-based devices.

First-principles electronic structure and formation energies of group V and VII impurities in the α-Fe2O3 alloys

Based on density functional theory, the electronic structures, formation energy, and transition level of the selected group V and VII impurities in α-Fe2O3 are investigated by means of first-principles methods. Numerical results show that the group V and VII atoms-doped α-Fe2O3 can be energetically favorable under the Fe-rich condition. Group V atom substituting O atom can induce the acceptor impurity level, while the deep donor impurity states are formed inside the band gap when group VII atom substitute O atom in the α-Fe2O3. Moreover, our results show that halogen atom F substituting O atom should be very easy in the α-Fe2O3. In addition, our results also show that for both group V and VII atom-doped α-Fe2O3, the upper sides of valence band are modified obviously, while the conduction band edge does not change.

Ab initio studies of Ge addimers on the Si(100) surface

The atomic and electronic structures of Ge dimers adsorbed on top of a Si(100) flat surface has been investigated by first-principles molecular dynamics method. Four high-symmetry configurations have been considered and thoroughly relaxed. The most stable configuration for Ge dimers is found to be on the trough, in the middle of, and parallel to the substrate Si dimers. These results are consistent with recent experimental studies of the system using the scanning tunneling microscopy.

Electronic structures and optical properties of α-Fe2O3-xSex alloys for solar absorber

The band structures and optical properties of α-Fe2O3-xSex alloys are studied by means of first-principles methods, considering different Se contents x. Numerical results show that Se content has an obvious influence on band structures and optical properties of α-Fe2O3-xSex alloys. The band gap values of α-Fe2O3-xSex alloys decrease monotonically when Se concentrations increase, resulting in an obvious increase of the optical absorption edge in the visible range. In particular, our results show that α-Fe2O3-xSex alloys have the direct band gap properties with band gap values when Se content x ≈ 0.17, which is beneficial to solar cell applications.

First-principles studies of p-type nitrogen-doped α-Fe2O3-xSx alloys

Based on spin-polarized density functional theory, the characteristics of p-type doping are investigated in the N-doped α-Fe2O3−xSx alloys by means of first-principles methods. Numerical results show that when N substitutes O atom in pure α-Fe2O3, N impurity level is a deeper acceptor state. However, the unoccupied level is much shallower when N doped in the α-Fe2O3−xSx alloys, which indicates N impurity can provide good and effective p-type carriers. These predicted numerical results are interesting and useful to understand the α-Fe2O3−xSx alloys as a new low-cost solar cell material.