Bending-Induced Symmetry Breaking of Lithiation in Germanium Nanowires

From signal transduction of living cells to oxidation and corrosion of metals, mechanical stress intimately couples with chemical reactions, regulating these biological and physiochemical processes. The coupled effect is particularly evident in the electrochemical lithiation/delithiation cycling of high-capacity electrodes, such as silicon (Si), where on the one hand lithiation-generated stress mediates lithiation kinetics and on the other the electrochemical reaction rate regulates stress generation and mechanical failure of the electrodes. Here we report for the first time the evidence on the controlled lithiation in germanium nanowires (GeNWs) through external bending. Contrary to the symmetric core–shell lithiation in free-standing GeNWs, we show bending the GeNWs breaks the lithiation symmetry, speeding up lithaition at the tensile side while slowing down at the compressive side of the GeNWs. The bending-induced symmetry breaking of lithiation in GeNWs is further corroborated by chemomechanical modeling. In the light of the coupled effect between lithiation kinetics and mechanical stress in the electrochemical cycling, our findings shed light on strain/stress engineering of durable high-rate electrodes and energy harvesting through mechanical motion

Identification of an Intrinsic Source of Doping Inhomogeneity in Vapor–Liquid–Solid-Grown Nanowires

The vapor–liquid–solid (VLS) process of semiconductor nanowire growth is an attractive approach to low-dimensional materials and heterostructures because it provides a mechanism to modulate, in situ, nanowire composition and doping, but the ultimate limits on doping control are ultimately dictated by the growth process itself. Under widely used conditions for the chemical vapor deposition growth of Si and Ge nanowires from a Au catalyst droplet, we find that dopants incorporated from the liquid are not uniformly distributed. Specifically, atom probe tomographic analysis revealed up to 100-fold enhancements in dopant concentration near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. We hypothesize that radial and azimuthal inhomogeneities arise from a faceted liquid–solid interface present during nanowire growth, and we present a simple model to account for the distribution. As the same segregation behavior was observed in two distinct semiconductors with different dopants, the observed inhomogeneity is likely to be present in other VLS grown nanowires

Overall Water Splitting with Room-Temperature Synthesized NiFe Oxyfluoride Nanoporous Films

A room-temperature synthesis of NiFe oxyfluoride (NiFeOF) holey film, using electrochemical deposition and anodic treatments, has been developed in this work. The developed room-temperature synthetic route can preserve the fine nanoporous structure inside the holey film, providing high surface area and abundant reaction sites for electrocatalytic reactions. Both computational and experimental studies demonstrate that the developed NiFeOF holey film with highly porous structure and metal residuals can be used as a high-efficiency and bifunctional catalyst for overall water splitting. Simulation result indicates that the exposed Ni atom on the NiFeOF surface serves as the active site for water splitting. Fe doping can improve the catalytic activity of the Ni active site due to the partial charge-transfer effect of Fe³⁺ on Ni²⁺. Electrochemical performance of the NiFeOF catalyst can be experimentally further enhanced through improved electrical conductivity by the residual NiFe alloy framework inside the holey film. The synergistic combination of NiFeOF holey film properties results in a highly efficient electrochemical catalyst, showing overall water splitting

Overall Water Splitting with Room-Temperature Synthesized NiFe Oxyfluoride Nanoporous Films

A room-temperature synthesis of NiFe oxyfluoride (NiFeOF) holey film, using electrochemical deposition and anodic treatments, has been developed in this work. The developed room-temperature synthetic route can preserve the fine nanoporous structure inside the holey film, providing high surface area and abundant reaction sites for electrocatalytic reactions. Both computational and experimental studies demonstrate that the developed NiFeOF holey film with highly porous structure and metal residuals can be used as a high-efficiency and bifunctional catalyst for overall water splitting. Simulation result indicates that the exposed Ni atom on the NiFeOF surface serves as the active site for water splitting. Fe doping can improve the catalytic activity of the Ni active site due to the partial charge-transfer effect of Fe³⁺ on Ni²⁺. Electrochemical performance of the NiFeOF catalyst can be experimentally further enhanced through improved electrical conductivity by the residual NiFe alloy framework inside the holey film. The synergistic combination of NiFeOF holey film properties results in a highly efficient electrochemical catalyst, showing overall water splitting

Catalyst Composition and Impurity-Dependent Kinetics of Nanowire Heteroepitaxy

The mechanisms and kinetics of axial Ge–Si nanowire heteroepitaxial growth based on the tailoring of the Au catalyst composition <i>via</i> Ga alloying are studied by environmental transmission electron microscopy combined with systematic <i>ex situ</i> CVD calibrations. The morphology of the Ge–Si heterojunction, in particular, the extent of a local, asymmetric increase in nanowire diameter, is found to depend on the Ga composition of the catalyst, on the TMGa precursor exposure temperature, and on the presence of dopants. To rationalize the findings, a general nucleation-based model for nanowire heteroepitaxy is established which is anticipated to be relevant to a wide range of material systems and device-enabling heterostructures

Probing the Origin of Interfacial Carriers in SrTiO₃–LaCrO₃ Superlattices

Emergent phenomena at complex oxide interfaces could provide the basis for a wide variety of next-generation devices, including photovoltaics and spintronics. To date, detailed characterization and computational modeling of interfacial defects, cation intermixing, and film stoichiometry have helped to explain many of the novel behaviors observed at a single heterojunction. Unfortunately, many of the techniques employed to characterize a single heterojunction are less effective for a superlattice made up of a repeating series of interfaces that induce collective interfacial phenomena throughout a film. These repeating interfaces present an untapped opportunity to introduce an additional degree of complexity, such as confined electric fields, that cannot be realized in a single heterojunction. In this work, we explore the properties of SrTiO₃–LaCrO₃ superlattices to understand the role of defects, including variations in cation stoichiometry of individual layers of the superlattice, intermixing across interfaces, and interfacial oxygen vacancies. Using X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy electron energy-loss spectroscopy (STEM-EELS), we quantify the stoichiometry of individual layers of the superlattice and determine the degree of intermixing in these materials. By comparing these results to both density functional theory (DFT) models and STEM-EELS measurements of the Ti and Cr valence in each layer of the superlattice, we correlate different types of defects with the associated materials properties of the superlattice. We show that a combination of ab initio modeling and complementary structural characterization methods can offer unique insight into structure–property relationships in many oxide superlattice systems