Local symmetry-driven interfacial magnetization and electronic states in (ZnO)n/(w-FeO)n superlattices.

Superlattices constructed with the wide-band-gap semiconductor ZnO and magnetic oxide FeO, both in the wurtzite structure, have been investigated using spin-polarized first-principles calculations. The structural, electronic and magnetic properties of the (ZnO)n/(w-FeO)n superlattices were studied in great detail. Two different interfaces in the (ZnO)n/(w-FeO)n superlattices were identified and they showed very different magnetic and electronic properties. Local symmetry-driven interfacial magnetization and electronic states can arise from different Fe/Zn distributions at different interfaces or spin ordering of Fe in the superlattice. The local symmetry-driven interfacial magnetization and electronic states, originating either from different Fe/Zn distribution across interfaces I and II, or by spin ordering of Fe in the superlattice, can be identified. It was also found that, in the case of the ferromagnetic phase, the electrons are more delocalized for the majority spin but strongly localized for the minority spin, which resulted in interesting spin-dependent transport properties. Our results will pave the way for designing novel spin-dependent electronic devices through the construction of superlattices from semiconductors and multiferroics.

Enhancing N uptake and reducing N pollution via green, sustainable N fixation-release model.

The overuse of N fertilizers has caused serious environmental problems (e.g., soil acidification, excessive N2O in the air, and groundwater contamination) and poses a serious threat to human health. Improving N fertilizer utilization efficiency and plant uptake is an alternative for N fertilizers overuses. Enterobacter cloacae is an opportunistic pathogen, also used as plant growth-promoting rhizobacteria (PGPR), has been widely presented in the fields of bioremediation and bioprotection. Here we developed a new N fixation-release model by combining biochar with E. cloacae. The efficiency of the model was evaluated using a greenhouse pot experiment with maize (Zea mays L.) as the test crop. The results showed that biochar combined with E. cloacae significantly increased the N content. The application of biochar combined with E. cloacae increased total N in soil by 33% compared with that of N fertilizers application. The N-uptake and utilization efficiency (NUE) in plant was increased 17.03% and 14.18%, respectively. The activities of urease, dehydrogenase and fluorescein diacetate hydrolase (FDA) was improved, the catalase (CAT) activity decreased. Analysis of the microbial community diversity revealed the abundance of Proteobacteria, Actinobacteria, Firmicutes, and Gemmatimonadetes were significantly improved. The mechanism under the model is that E. cloacae acted as N-fixation by capturing N2 from air. Biochar served as carrier, supporting better living environment for E. cloacae, also as adsorbent adsorbing N from fertilizer and from fixed N by E. cloacae, the adsorption in turn slower the N release. Altogether, the model promotes N utilization by plants, improves the soil environment, and reduces N pollution.

X-ray Absorption Spectroscopy Study of Iron Site Manganese Substituted Yttrium Orthoferrite.

In this work, manganese (Mn)-doped YFeO3, i.e., YFMxO powders with 0 ≤ x ≤ 0.1, was synthesized by a hydrothermal method to study the influences of doping on its structural, morphological, optical, magnetic, and local electrical properties. The experimental results show that all the samples exhibit an orthorhombic structure with space group Pnma. Refined structure parameters are presented. Morphology images show the shape evolution from layered to multilayered with increasing Mn content. Infrared spectra reveal the characteristic vibrations of the obtained YFMxO samples. From the magnetic study, an increased magnetic moment in the range of 0 ≤ x ≤ 0.075 is observed. The Fe and Y K-edge local structure studies indicate that the valency of Fe and Y is mainly found in the trivalent state, which also indicates that the substitution of Mn ions not only affects the nearest neighbor atomic shell of Fe but also affects the nearest neighbor's local structure of Y atoms. Our results show that the addition of Mn exhibits an evident influence on the local structural and magnetic properties.

A super-stretchable boron nanoribbon network.

We have studied the mechanical properties of a two-dimensional (2D) boron nanoribbon network (BNRN) subjected to a uniaxial or a biaxial tensile strain using first principles calculations. The results show that the 2D BNRN is super-stretchable. The critical tensile strains of the BNRN in the χ-h1 phase along the a- and b-directions are 0.51 and 0.41, respectively, and that for the biaxial strain reaches an ultrahigh value of 0.84. By analyzing the B-B interatomic distance, coordination number and charge distribution, it is found that with increasing biaxial tensile strain, the χ-h1 BNRN undergoes two structural phase transitions, which are characterized by breaking of the B-B bonds and the partial transformation of the nanoribbon-like structures into chain-like structures. The strain-induced phase transitions significantly reduce the strain energy. We also discuss the elastic constants, Young's modulus, shear modulus, and Poisson's ratios. The super-stretchable and flexible mechanical properties of the BNRNs, together with their superior transport properties, make BNRNs useful in a wide range of applications in nanoscale electronic devices.

Structural and optical characteristics of the hexagonal ZnO films grown on cubic MgO (001) substrates.

In this Letter, we report on the structural and optical characteristics of ZnO films with a wurtzite structure grown on MgO (001) substrates with cubic structures. The ZnO films were prepared through the molecular beam epitaxy method, and growth orientation transformation from [0001] to [10-10] direction was observed with the change of growth temperature and thickness. The x-ray diffraction pole figures and in situ RHEED patterns demonstrated that the rotational relationship among grains within the ZnO films appeared in a typical two-fold rotation of about 30° for the [0001] growth orientation and four-fold rotation of about 30° or 60° for the [10-10] growth orientation, respectively. Last, we investigated their optical properties through measuring the transmission and photoluminescence spectra of the ZnO films, which showed the bulk-like bandgap feature of the ZnO films in spite of the existing growth orientation transformation.

High anisotropy of fully hydrogenated borophene.

We have studied the mechanical properties and phonon dispersions of fully hydrogenated borophene (borophane) under strains by first principles calculations. Uniaxial tensile strains along the a- and b-direction, respectively, and biaxial tensile strain have been considered. Our results show that the mechanical properties and phonon stability of borophane are both highly anisotropic. The ultimate tensile strain along the a-direction is only 0.12, but it can be as large as 0.30 along the b-direction. Compared to borophene and other 2D materials (graphene, graphane, silicene, silicane, h-BN, phosphorene and MoS2), borophane presents the most remarkable anisotropy in in-plane ultimate strain, which is very important for strain engineering. Furthermore, the phonon dispersions under the three applied strains indicate that borophane can withstand up to 5% and 15% uniaxial tensile strain along the a- and b-direction, respectively, and 9% biaxial tensile strain, indicating that mechanical failure in borophane is likely to originate from phonon instability.

Interfacial electronic state between hexagonal ZnO and cubic NiO.

The interface of two dissimilar materials gives rise to a myriad of interesting structural, magnetic, and electronic properties that may be utilized to produce novel materials with unique characteristics and functions. In particular, growing a cubic oxide film on top of a hexagonal oxide substrate results in such unique properties due to the conflict of their respective stabilization mechanisms within the interface layer. This study aims to elucidate the electronic properties of the interface between hexagonal ZnO and cubic NiO by analyzing the interface electronic states within epitaxial NiO films grown on ZnO substrates, expressed in the form of ultraviolet photoemission spectroscopy (UPS) for valence band structure and X-ray absorption spectroscopy (XAS) spectra for conduction band structure. This is accomplished through a modeling approach in which the film, substrate, and interface signals are assumed to be related to each other by a set of mathematical equations, and then rearranging and modulating the equations to obtain unique UPS and XAS spectra that depict the interface electronic states.

Comprehensive Transcriptome and Proteome Analyses Reveal the Drought Responsive Gene Network in Potato Roots.

The root system plays a decisive role in the growth and development of plants. The water requirement of a root system depends strongly on the plant species. Potatoes are an important food and vegetable crop grown worldwide, especially under irrigation in arid and semi-arid regions. However, the expected impact of global warming on potato yields calls for an investigation of genes related to root development and drought resistance signaling pathways in potatoes. In this study, we investigated the molecular mechanisms of different drought-tolerant potato root systems in response to drought stress under controlled water conditions, using potato as a model. We analyzed the transcriptome and proteome of the drought-sensitive potato cultivar Atlantic (Atl) and the drought-tolerant cultivar Qingshu 9 (Q9) under normal irrigation (CK) and weekly drought stress (D). The results showed that a total of 14,113 differentially expressed genes (DEGs) and 5596 differentially expressed proteins (DEPs) were identified in the cultivars. A heat map analysis of DEGs and DEPs showed that the same genes and proteins in Atl and Q9 exhibited different expression patterns under drought stress. Weighted gene correlation network analysis (WGCNA) showed that in Atl, Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG)-enriched pathways were related to pyruvate metabolism and glycolysis, as well as cellular signaling and ion transmembrane transporter protein activity. However, GO terms and KEGG-enriched pathways related to phytohormone signaling and the tricarboxylic acid cycle were predominantly enriched in Q9. The present study provides a unique genetic resource to effectively explore the functional genes and uncover the molecular regulatory mechanism of the potato root system in response to drought stress.

Ferromagnetic Insulating Ground-State Resolved in Mixed Protons and Oxygen Vacancies-Doped La0.67Sr0.33CoO3 Thin Films via Ionic Liquid Gating.

The realization of ferromagnetic insulating ground state is a critical prerequisite for spintronic applications. By applying electric field-controlled ionic liquid gating (ILG) to stoichiometry La0.67Sr0.33CoO3 thin films, the doping of protons (H+) has been achieved for the first time. Furthermore, a hitherto-unreported ferromagnetic insulating phase with a remarkably high Tc up to 180 K has been observed which can be attributed to the doping of H+ and the formation of oxygen vacancies (VO). The chemical formula of the dual-ion migrated film has been identified as La2/3Sr1/3CoO8/3H2/3 based on combined Co L23-edge absorption spectra and configuration interaction cluster calculations, from which we are able to explain the ferromagnetic ground state in terms of the distinct magnetic moment contributions from Co ions with octahedral (Oh) and tetrahedral (Td) symmetries following antiparallel spin alignments. Further density functional theory calculations have been performed to verify the functionality of H+ as the transfer ion and the origin of the novel ferromagnetic insulating ground state. Our results provide a fundamental understanding of the ILG regulation mechanism and shed light on the manipulating of more functionalities in other correlated compounds through dual-ion manipulation.

Role of sodium doping in lead chalcogenide thermoelectrics.

The solubility of sodium and its effects on phonon scattering in lead chalcogenide PbQ (Q = Te, Se, S) family of thermoelectric materials was investigated by means of transmission electron microscopy and density functional calculations. Among these three systems, Na has the highest solubility limit (~2 mol %) in PbS and the lowest ~0.5 mol %) in PbTe. First-principles electronic structure calculations support the observations, indicating that Na defects have the lowest formation energy in PbS and the highest in PbTe. It was also found that in addition to providing charge carriers (holes) for PbQ, Na introduces point defects (solid solution formation) and nanoscale precipitates; both reduce the lattice thermal conductivity by scattering heat-carrying phonons. These results explain the recent reports of high thermoelectric performance in p-type PbQ materials and may lead to further advances in this class of materials.

Morphology control of nanostructures: Na-doped PbTe-PbS system.

The morphology of crystalline precipitates in a solid-state matrix is governed by complex but tractable energetic considerations driven largely by volume strain energy minimization and anisotropy of interfacial energies. Spherical precipitate morphologies are favored by isotropic systems, while anisotropic interfacial energies give energetic preference to certain crystallographically oriented interfaces, resulting in a faceted precipitate morphology. In conventional solid-solution precipitation, a precipitate's morphological evolution is mediated by surface anchoring of capping molecules, which dramatically alter the surface energy in an anisotropic manner, thereby providing exquisite morphology control during crystal growth. Herein, we present experimental evidence and theoretical validation for the role of a ternary element (Na) in controlling the morphology of nanoscale PbS crystals nucleating in a PbTe matrix, an important bulk thermoelectric system. The PbS nanostructures formed by phase separation from a PbI(2)-doped or undoped PbTe matrix have irregular morphologies. However, replacing the iodine dopant with Na (1-2 mol %) alters dramatically the morphology of the PbS precipitates. Segregation of Na at PbTe/PbS interfaces result in cuboidal and truncated cuboidal morphologies for PbS. Using analytical scanning/transmission electron microscopy and atom-probe tomography, we demonstrate unambiguously that Na partitions to the precipitates and segregates at the matrix/precipitate interfaces, inducing morphological anisotropy of PbS precipitates. First-principles and semiclassical calculations reveal that Na as a solute in PbTe has a higher energy than in PbS and that Na segregation at a (100) PbTe/PbS interface decreases the total energy of matrix/precipitate system, resulting in faceting of PbS precipitates. These results provide an impetus for a new strategy for controlling morphological evolution in matrix/precipitate systems, mediated by solute partitioning of ternary additions.

Growth, structure, and morphology of van der Waals epitaxy Cr1+δTe2 films.

The preparation of two-dimensional magnetic materials is a key process to their applications and the study of their structure and morphology plays an important role in the growth of high-quality thin films. Here, the growth, structure, and morphology of Cr1+δTe2 films grown by molecular beam epitaxy on mica with variations of Te/Cr flux ratio, growth temperature, and film thickness have been systematically investigated by scanning tunneling microscopy, reflection high-energy electron diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy. We find that a structural change from multiple phases to a single phase occurs with the increase in growth temperature, irrespective of the Cr/Te flux ratios, which is attributed to the desorption difference of Te atoms at different temperatures, and that the surface morphology of the films grown at relatively high growth temperatures (≥ 300 °C) exhibits a quasi-hexagonal mesh-like structure, which consists of nano-islands with bending surface induced by the screw dislocations, as well as that the films would undergo a growth-mode change from 2D at the initial stage in a small film thickness (2 nm) to 3D at the later stage in thick thicknesses (12 nm and 24 nm). This work provides a general model for the study of pseudo-layered materials grown on flexible layered substrates.

Deciphering Adverse Detrapped Hole Transfer in Hot-Electron Photoelectric Conversion at Infrared Wavelengths.

Hot-carrier devices are promising alternatives for enabling path breaking photoelectric conversion. However, existing hot-carrier devices suffer from low efficiencies, particularly in the infrared region, and ambiguous physical mechanisms. In this work, the competitive interfacial transfer mechanisms of detrapped holes and hot electrons in hot-carrier devices are discovered. Through photocurrent polarity research and optical-pump-THz-probe (OPTP) spectroscopy, it is verified that detrapped hole transfer (DHT) and hot-electron transfer (HET) dominate the low- and high-density excitation responses, respectively. The photocurrent ratio assigned to DHT and HET increases from 6.6% to over 1133.3% as the illumination intensity decreases. DHT induces severe degeneration of the external quantum efficiency (EQE), especially at low illumination intensities. The EQE of a hot-electron device can theoretically increase by over two orders of magnitude at 10 mW cm-2 through DHT elimination. The OPTP results show that competitive transfer arises from the carrier oscillation type and carrier-density-related Coulomb screening. The screening intensity determines the excitation weight and hot-electron cooling scenes and thereby the transfer dynamics.

Anisotropy Engineering of ZnO Nanoporous Frameworks: A Lattice Dynamics Simulation.

The anisotropy engineering of nanoporous zinc oxide (ZnO) frameworks has been performed by lattice dynamics simulation. A series of zinc oxide (ZnO) nanoporous framework structures was designed by creating nanopores with different sizes and shapes. We examined the size effects of varying several features of the nanoporous framework (namely, the removal of layers of atoms, surface-area-to-volume ratio, coordination number, porosity, and density) on its mechanical properties (including bulk modulus, Young's modulus, elastic constant, and Poisson ratio) with both lattice dynamics simulations. We also found that the anisotropy of nanoporous framework can be drastically tuned by changing the shape of nanopores. The maximum anisotropy (defined by Ymax/Ymin) of the Young's modulus value increases from 1.2 for bulk ZnO to 2.5 for hexagon-prism-shaped ZnO nanoporous framework structures, with a density of 2.72 g/cm3, and, even more remarkably, to 89.8 for a diamond-prism-shape at a density of 1.72 g/cm3. Our findings suggest a new route for desirable anisotropy and mechanical property engineering with nanoporous frameworks by editing the shapes of the nanopores for the desired anisotropy.

Strain engineering of thermal conductivity in graphene sheets and nanoribbons: a demonstration of magic flexibility.

Graphene is an outstanding material with ultrahigh thermal conductivity. Its thermal transfer properties under various strains are studied by reverse nonequilibrium molecular dynamics. Based on the unique two-dimensional structure of graphene, the distinctive geometries of graphene sheets and graphene nanoribbons with large flexibility and their intriguing thermal properties are demonstrated under strains. For example, the corrugation under uniaxial compression and helical structure under light torsion, as well as tube-like structure under strong torsion, exhibit enormously different thermal conductivity. The important robustness of thermal conductivity is found in the corrugated and helical configurations of graphene nanoribbons. Nevertheless, thermal conductivity of graphene is very sensitive to tensile strain. The relationship among phonon frequency, strain and thermal conductivity are analyzed. A similar trend line of phonon frequency dependence of thermal conductivity is observed for armchair graphene nanoribbons and zigzag graphene nanoribbons. The unique thermal properties of graphene nanoribbons under strains suggest their great potentials for nanoscale thermal managements and thermoelectric applications.

Determination of the embedded electronic states at nanoscale interface via surface-sensitive photoemission spectroscopy.

The fabrication of small-scale electronics usually involves the integration of different functional materials. The electronic states at the nanoscale interface plays an important role in the device performance and the exotic interface physics. Photoemission spectroscopy is a powerful technique to probe electronic structures of valence band. However, this is a surface-sensitive technique that is usually considered not suitable for the probing of buried interface states, due to the limitation of electron-mean-free path. This article reviews several approaches that have been used to extend the surface-sensitive techniques to investigate the buried interface states, which include hard X-ray photoemission spectroscopy, resonant soft X-ray angle-resolved photoemission spectroscopy and thickness-dependent photoemission spectroscopy. Especially, a quantitative modeling method is introduced to extract the buried interface states based on the film thickness-dependent photoemission spectra obtained from an integrated experimental system equipped with in-situ growth and photoemission techniques. This quantitative modeling method shall be helpful to further understand the interfacial electronic states between functional materials and determine the interface layers.

Growth kinetic processes of AlN molecules on the Al-polar surface of AlN.

We studied growth kinetic processes of AlN molecules on the Al-polar surface of AlN using ab initio and Monte Carlo simulations. Molecular processes were presented and analyzed during the nucleation, ripening, and coalescence stages. The results show that the nucleus number decreases as temperature rises due to the increasing diffusion of the molecules. By analyzing the growth time dependence of average cluster size, interface-limited Ostwald ripening is found to be the main ripening mechanism when the temperature is lower than 1773 K. As cluster-corner crossing diffusion is limited, the growth is fractal-like extension, and the coalescence is achieved through adhesion of clusters, leading to a generally continuous morphology with some vacancies and closure failures, which is in good agreement with our experimental results. Moreover, coverage/temperature kinetic phase diagrams under different deposition rates are presented (from 0.025 to 0.1 ML/s). Our finding suggests that a temperature higher than 1800 K is crucial for growth of an ideal atomic-scale Al-polar AlN surface.

Reversing abnormal hole localization in high-Al-content AlGaN quantum well to enhance deep ultraviolet emission by regulating the orbital state coupling.

AlGaN has attracted considerable interest for ultraviolet (UV) applications. With the development of UV optoelectronic devices, abnormal carrier confinement behaviour has been observed for c-plane-oriented AlGaN quantum wells (QWs) with high Al content. Because of the dispersive crystal field split-off hole band (CH band) composed of p z orbitals, the abnormal confinement becomes the limiting factor for efficient UV light emission. This observation differs from the widely accepted concept that confinement of carriers at the lowest quantum level is more pronounced than that at higher quantum levels, which has been an established conclusion for conventional continuous potential wells. In particular, orientational p z orbitals are sensitive to the confinement direction in line with the conducting direction, which affects the orbital intercoupling. In this work, models of Al0.75Ga0.25N/AlN QWs constructed with variable lattice orientations were used to investigate the orbital intercoupling among atoms between the well and barrier regions. Orbital engineering of QWs was implemented by changing the orbital state confinement, with the well plane inclined from 0° to 90° at a step of 30° (referred to the c plane). The barrier potential and transition rate at the band edge were enhanced through this orbital engineering. The concept of orbital engineering was also demonstrated through the construction of inclined QW planes on semi- and nonpolar planes implemented in microrods with pyramid-shaped tops. The higher emission intensity from the QWs on the nonpolar plane compared with those on the polar plane was confirmed via localized cathodoluminescence (CL) maps.

Tuning the Magnetism in Boron-Doped Strontium Titanate.

The magnetic and electronic properties of boron-doped SrTiO3 have been studied by first-principles calculations. We found that the magnetic ground states of B-doped SrTiO3 strongly depended on the dopant-dopant separation distance. As the dopant-dopant distance varied, the magnetic ground states of B-doped SrTiO3 can have nonmagnetic, ferromagnetic or antiferromagnetic alignment. The structure with the smallest dopant-dopant separation exhibited the lowest total energy among all configurations considered and was characterized by dimer pairs due to strong attraction. Ferromagnetic coupling was observed to be stronger when the two adjacent B atoms aligned linearly along the B-Ti-B axis, which could be associated with their local bonding structures. Therefore, the symmetry of the local structure made an important contribution to the generation of a magnetic moment. Our study also demonstrated that the O-Ti-O unit was easier than the Ti-B-Ti unit to deform. The electronic properties of boron-doped SrTiO3 tended to show semiconducting or insulating features when the dopant-dopant distance was less than 5 Å, which changed to metallic properties when the dopant-dopant distance was beyond 5 Å. Our calculated results indicated that it is possible to manipulate the magnetism and band gap via different dopant-dopant separations.