Guangfu Luo

Dynamic layer rearrangement during growth of layered oxide films by molecular beam epitaxy

The A(n+1)B(n)O(3n+1) Ruddlesden-Popper homologous series offers a wide variety of functionalities including dielectric, ferroelectric, magnetic and catalytic properties. Unfortunately, the synthesis of such layered oxides has been a major challenge owing to the occurrence of growth defects that result in poor materials behaviour in the higher-order members. To understand the fundamental physics of layered oxide growth, we have developed an oxide molecular beam epitaxy system with in situ synchrotron X-ray scattering capability. We present results demonstrating that layered oxide films can dynamically rearrange during growth, leading to structures that are highly unexpected on the basis of the intended layer sequencing. Theoretical calculations indicate that rearrangement can occur in many layered oxide systems and suggest a general approach that may be essential for the construction of metastable Ruddlesden-Popper phases. We demonstrate the utility of the new-found growth strategy by performing the first atomically controlled synthesis of single-crystalline La3Ni2O7.

Nanometre-thick single-crystalline nanosheets grown at the water-air interface.

To date, the preparation of free-standing 2D nanomaterials has been largely limited to the exfoliation of van der Waals solids. The lack of a robust mechanism for the bottom-up synthesis of 2D nanomaterials from non-layered materials has become an obstacle to further explore the physical properties and advanced applications of 2D nanomaterials. Here we demonstrate that surfactant monolayers can serve as soft templates guiding the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. One- to 2-nm-thick, single-crystalline free-standing ZnO nanosheets with sizes up to tens of micrometres are synthesized at the water–air interface. In this process, the packing density of surfactant monolayers adapts to the sub-phase metal ions and guides the epitaxial growth of nanosheets. It is thus named adaptive ionic layer epitaxy (AILE). The electronic properties of ZnO nanosheets and AILE of other materials are also investigated.

GaAs1−y−zPyBiz, an alternative reduced band gap alloy system lattice-matched to GaAs

The growth and properties of alloys in the alternative quaternary alloy system GaAs1−y−zPyBiz were explored. This materials system allows simultaneous and independent tuning of lattice constant and band gap energy, Eg, over a wide range for potential near- and mid-infrared optoelectronic applications by adjusting y and z in GaAs1−y−zPyBiz. Highly tensile-strained, pseudomorphic films of GaAs1−yPy with a lattice mismatch strain of ∼1.2% served as the host for the subsequent addition of Bi. Lattice-matched alloy materials to GaAs were generated by holding y ∼ 3.3z in GaAs1−y−zPyBiz. Epitaxial films with both high Bi content, z ∼ 0.0854, and a smooth morphology were realized with measured band gap energies as low as 1.11–1.01 eV, lattice-matched to GaAs substrates. Density functional theory calculations are used to provide a predictive model for the band gap of GaAs1−y−zPyBiz lattice-matched to GaAs.

Understanding and reducing deleterious defects in the metastable alloy GaAsBi

Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods, we investigate the defect thermodynamics and phase segregation in the technologically important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with those from numerous previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects that degrade the electronic and optical performances are predicted to be AsGa, BiGa, AsGa+BiAs, BiGa+BiAs, VGa and VGa+BiAs, of which the first four and last two defects are minority-electron and minority-hole traps, respectively. VGa is also observed to have a critical role in controlling metastable Bi supersaturation by mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from an As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. We expect this work to aid in the applications of GaAsBi for novel electronic and optoelectronic devices and to illuminate the control of deleterious defects in other metastable materials. Crystal defects can stop metastable alloys from working in novel devices, but a new study predicts ways to minimize such faults. When alloyed with bismuth atoms, gallium arsenide semiconductors could endow devices with unprecedented properties, such as high efficiency for high-power infrared laser diodes and temperature-insensitive performance for electronic devices. To reduce the defects caused by bismuth alloying including unwanted bismuth clusters that arise in typical fabrications, Guangfu Luo and Dane Morgan from the University of Wisconsin-Madison in the USA studied various possible alloy defects in the bismuth-containing gallium arsenide using density functional theory calculations. Their investigations revealed the dominant defect trap states and helped the team propose new ideas, including hydrogen passivation and new growth conditions, to reduce the populations of deleterious defects. Ab initio calculations reveal that defects AsGa, BiGa, AsGa+BiAs and BiGa+BiAs are the dominant minority-electron traps and defects VGa and VGa+BiAs are the dominant minority-hole traps in the metastable alloy GaAsBi grown under As-rich condition. Changing the growth away from the As-rich condition and/or using hydrogen passivation are suggested to reduce the deleterious effects of these defects.

Ab Initio Modeling of Electrolyte Molecule Ethylene Carbonate Decomposition Reaction on Li(Ni,Mn,Co)O2 Cathode Surface

Electrolyte decomposition reactions on Li-ion battery electrodes contribute to the formation of solid electrolyte interphase (SEI) layers. These SEI layers are one of the known causes for the loss in battery voltage and capacity over repeated charge/discharge cycles. In this work, density functional theory (DFT)-based ab initio calculations are applied to study the initial steps of the decomposition of the organic electrolyte component ethylene carbonate (EC) on the (101̅4) surface of a layered Li(Nix,Mny,Co1-x-y)O2 (NMC) cathode crystal, which is commonly used in commercial Li-ion batteries. The effects on the EC reaction pathway due to dissolved Li+ ions in the electrolyte solution and different NMC cathode surface terminations containing adsorbed hydroxyl -OH or fluorine -F species are explicitly considered. We predict a very fast chemical reaction consisting of an EC ring-opening process on the bare cathode surface, the rate of which is independent of the battery operation voltage. This EC ring-opening reaction is unavoidable once the cathode material contacts with the electrolyte because this process is purely chemical rather than electrochemical in nature. The -OH and -F adsorbed species display a passivation effect on the surface against the reaction with EC, but the extent is limited except for the case of -OH bonded to a surface transition metal atom. Our work implies that the possible rate-limiting steps of the electrolyte molecule decomposition are the reactions on the decomposed organic products on the cathode surface rather than on the bare cathode surface.

Strain-compensated GaAs1−yPy/GaAs1−zBiz/GaAs1−yPy quantum wells for laser applications

GaAs1−zBiz/GaAs1−yPy strained-compensated quantum well (QW) structures for laser applications were grown by metalorganic vapor phase epitaxy. The band offsets for the GaAs1−zBiz/GaAs1−yPy heterojunction were calculated by the density functional theory, and the design of strain-compensated structures was undertaken by the zero stress analysis. The post-growth thermal annealing of the structures dramatically increases the photoluminescence intensity compared to that from as-grown GaAs1−zBiz QW samples. Transmission electron microscopy studies verified layer thicknesses as well as the presence of abrupt interfaces in the annealed GaAs1−zBiz/GaAs1−yPy QW structure. Electroluminescence measurements from ridge-waveguide devices show broad spectral emission characteristics and lasing was not observed up to a current injection of 4 kA cm−2.

First-principles predictions of electronic properties of GaAs1-x-yPyBix and GaAs1-x-yPyBix-based heterojunctions

Significant efficiency droop is a major concern for light-emitting diodes and laser diodes operating at high current density. Recent study has suggested that heavily Bi-alloyed GaAs can decrease the non-radiative Auger recombination and therefore alleviate the efficiency droop. Using density functional theory, we studied a newly fabricated quaternary alloy, GaAs1-x-yPyBix, which can host significant amounts of Bi, through calculations of its band gap, spin-orbit splitting, and band offsets with GaAs. We found that the band gap changes of GaAs1-x-yPyBix relative to GaAs are determined mainly by the local structural changes around P and Bi atoms rather than their electronic structure differences. To obtain alloy with lower Auger recombination than GaAs bulk, we identified the necessary constraints on the compositions of P and Bi. Finally, we demonstrated that GaAs/GaAs1-x-yPyBix heterojunctions with potentially low Auger recombination can exhibit small lattice mismatch and large enough band offsets for stro...