Yanning Zhang

Iron Pyrite Thin Films Synthesized from an Fe(acac)3 Ink

Iron pyrite (cubic FeS2) is a promising candidate absorber material for earth-abundant thin-film solar cells. Here, we report on phase-pure, large-grain, and uniform polycrystalline pyrite films that are fabricated by solution-phase deposition of an iron(III) acetylacetonate molecular ink followed by sequential annealing in air, H2S, and sulfur gas at temperatures up to 550 °C. Phase and elemental compositions of the films are characterized by conventional and synchrotron X-ray diffraction, Raman spectroscopy, Auger electron spectroscopy, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy (XPS). These solution-deposited films have more oxygen and alkalis, less carbon and hydrogen, and smaller optical band gaps (E(g) = 0.87 ± 0.05 eV) than similar films made by chemical vapor deposition. XPS is used to assess the chemical composition of the film surface before and after exposure to air and immersion in water to remove surface contaminants. Optical measurements of films rich in marcasite (orthorhombic FeS2) show that marcasite has a band gap at least as large as pyrite and that the two polymorphs share similar absorptivity spectra, in excellent agreement with density functional theory models. Regardless of the marcasite and elemental impurity contents, all films show p-type, weakly activated transport with curved Arrhenius plots, a room-temperature resistivity of ~1 Ω cm, and a hole mobility that is too small to measure by Hall effect. This universal electrical behavior strongly suggests that a common defect or a hole-rich surface layer governs the electrical properties of most FeS2 thin films.

Increasing the band gap of iron pyrite by alloying with oxygen

Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS(2)) can be increased from ~1.0 to 1.2-1.3 eV by replacing ~10% of the sulfur atoms with oxygen atoms (i.e., ~10% O(S) impurities). O(S) formation is exothermic, and the oxygen atoms tend to avoid O-O dimerization, which favors the structural stability of homogeneous FeS(2-x)O(x) alloys and frustrates phase separation into FeS(2) and iron oxides. With an ideal band gap, absence of O(S)-induced gap states, high optical absorptivity, and low electron effective mass, FeS(2-x)O(x) alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies.

Porous BN for hydrogen generation and storage

Hydrogen is a highly appealing renewable energy resource, while hydrogen generation and storage for practical applications remain a great challenge at present. Herein, porous monolayer boron nitride, named p-BN, is proposed based on first-principles calculations. Compared with the perfect h-BN, the band gap of p-BN is decreased by about 0.7 eV. Interestingly, the band gap of p-BN can be easily modulated, and the C-doped p-BN possesses a moderate band gap of 1.8 eV, which can exhibit strong adsorption in the visible light region. Additionally, p-BN exhibits higher ability in hydrogen storage than h-BN, due to its large specific surface area. The adsorption energy of hydrogen on p-BN can be further improved by Li decoration. The hydrogen storage on one side of the Li-decorated p-BN reaches a maximum of 7.5 wt%, with an adsorption energy of 160 meV. Consequently, p-BN has great potential to be utilized in both hydrogen generation and storage for practical applications.

Novel heterostructures by stacking layered molybdenum disulfides and nitrides for solar energy conversion

Two-dimensional graphene-like materials have attracted considerable attention for the further development of nanoscale devices. In this work, the structural, electronic and optical properties of free-standing, graphene-like nitrides XN (X = B, Al and Ga) are studied by density functional calculations with the inclusion of the nonlocal van der Waals correction. The results show that all the studied nitrides are thermodynamically stable and their electronic structures can be easily tuned by forming the heterostructure with MoS2 monolayer. Although GaN and AlN monolayers retain the indirect band gap of bulk, MoS2–AlN and MoS2–GaN heterostructures have suitable direct gaps, complete electron–hole separation and fascinating visible light adsorption, which is promising for solar energy applications. Moreover, the MoS2–AlN heterostructure is a good candidate for enhanced photocatalytic activity of hydrogen generation from water.

Rigid band model for prediction of magnetostriction of iron-gallium alloys

Using the highly precise full potential linearized augmented plane wave method, we determined atomic structure and the magnetostriction of Fe100−xGax with x<19%. It is demonstrated that the extraordinary enhancement of magnetostrictive coefficient of Fe100−xGax at low concentration stems from intrinsic electronic origins. Moreover, we recognized the potential of using a rigid band model for the prediction of magnetostriction of intermetallic alloys, through studies of ternary alloys with Zn substitution into Fe87.5Ga12.5.Using the highly precise full potential linearized augmented plane wave method, we determined atomic structure and the magnetostriction of Fe100−xGax with x<19%. It is demonstrated that the extraordinary enhancement of magnetostrictive coefficient of Fe100−xGax at low concentration stems from intrinsic electronic origins. Moreover, we recognized the potential of using a rigid band model for the prediction of magnetostriction of intermetallic alloys, through studies of ternary alloys with Zn substitution into Fe87.5Ga12.5.

Real-Space Imaging of Kondo Screening in a Two-Dimensional O2 Lattice

Rows of paramagnetic oxygen molecules adsorbed on a gold surface exhibit both deconfined and localized Kondo screening. Kondo lattice systems can exhibit unusual many-body behaviors that result from the interplay between onsite Kondo screening and intersite coupling. We used scanning tunneling microscopy to image the Kondo resonance in a nonconventional Kondo lattice formed by self-assembled oxygen (O2) molecules, which are paramagnetic, on the gold reconstructed surface [Au(110)-1×2]. The interplay between the intermolecular coupling for molecules adsorbed along chains and the onsite Kondo effect leads to the coexistence of both local and nonlocal Kondo screening at the atomic level. The latter provides evidence for collective deconfinement of magnetization induced in Au, whereas the former shows local “hybridization” between the Kondo clouds of nearest-neighbor O2 molecules.

Understanding of large auxetic properties of iron-gallium and iron-aluminum alloys

Large auxetic properties of iron-gallium and iron-aluminum alloys have been investigated with both theoretical and experimental approaches. Tensile tests of single-crystal iron-gallium alloys with compositions of 12%–25% gallium were conducted to determine the composition dependent values of the Poisson’s ratio. Systematic density functional calculations revealed a simple correlation between the Poisson’s ratio and tetragonal shear modulus. We attribute the auxetic properties of these intermetallic alloys to the drastic reduction in C′ with the presence of metalloid atoms in the DO3-type structures.

Correlating Electronic Transport to Atomic Structures in Self-Assembled Quantum Wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.

The stability and electronic properties of novel three-dimensional graphene-MoS2 hybrid structure

Three-dimensional (3D) hybrid layered materials receive a lot of attention because of their outstanding intrinsic properties and wide applications. In this work, the stability and electronic structure of three-dimensional graphene-MoS2 (3DGM) hybrid structures are examined based on first-principle calculations. The results reveal that the 3DGMs can easily self-assembled by graphene nanosheet and zigzag MoS2 nanoribbons, and they are thermodynamically stable at room temperature. Interestingly, the electronic structures of 3DGM are greatly related to the configuration of joint zone. The 3DGM with odd-layer thickness MoS2 nanoribbon is semiconductor with a small band gap of 0.01–0.25 eV, while the one with even-layer thickness MoS2 nanoribbon exhibits metallic feature. More importantly, the 3DGM with zigzag MoS2 nanoribbon not only own the large surface area and effectively avoid the aggregation between the different nanoribbons, but also can remarkably enhance Li adsorption interaction, thus the 3DGM have the great potential as high performance lithium ion battery cathodes.

Magnetostriction, elasticity, and D03 phase stability in Fe–Ga and Fe–Ga–Ge alloys

The contrast between the saturation tetragonal magnetostriction, λγ,2 = (3/2)λ100, of Fe1−xGax and Fe1−yGey, at compositions where both alloys exhibit D03 cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2 x + 3 y + 1, to an e/a value of ∼1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe1−x−y GaxGey alloys with e/a ∼1.55 and gradually increasing y/x were investigated experimentally (magnetostriction, elasticity, powder XRD) and theoretically (density functional calculations). It was found that a small amount of Ge (y = 1.3) replacing Ga in the Fe–Ga alloy has a profound effect on the measured λγ,2. As y increases, the drop in λγ,2 is considerable, reaching negative values at y/x = 0.47. The two shear elastic constants c′ = (c11− c12)/2 and c44 measured for four compositions with 0.06 ≤ y/x ≤ 0.45 at 7 K range from 16 to 21 GPa and from 133 to 138 GPa, respectively. Large temperature dependence was...