Implications of the formation of small polarons in Li2O2 for Li-air batteries

Lithium-air batteries (LABs) are an intriguing next-generation technology due to their high theoretical energy density of similar to 11 kWh/kg. However, LABs are hindered by both poor rate capability and significant polarization in cell voltage, primarily due to the formation of Li2O2 in the air cathode. Here, by employing hybrid density functional theory, we show that the formation of small polarons in Li2O2 limits electron transport. Consequently, the low electron mobility mu = 10(-10)-10(-9) cm(2)/Vs contributes to both the poor rate capability and the polarization that limit the LAB power and energy densities. The self-trapping of electrons in the small polarons arises from the molecular nature of the conduction band states of Li2O2 and the strong spin polarization of the O 2p state. Our understanding of the polaronic electron transport in Li2O2 suggests that designing alternative carrier conduction paths for the cathode reaction could significantly improve the performance of LABs at high current densities.open20

Metamorphic Ga0.76In0.24As/GaAs0.75Sb0.25 tunnel junctions grown on GaAs substrates

Lattice-matched and pseudomorphic tunnel junctions have been developed in the past for application in a variety of semiconductor devices, including heterojunction bipolar transistors, vertical cavity surface-emitting lasers, and multijunction solar cells. However, metamorphic tunnel junctions have received little attention. In 4-junction Ga0.51In0.49P/GaAs/Ga0.76In0.24As/Ga0.47In0.53As inverted-metamorphic solar cells (4J-IMM), a metamorphic tunnel junction is required to series connect the 3rd and 4th junctions. We present a tunnel junction based on a metamorphic Ga0.76In0.24As/GaAs0.75Sb0.25 structure for this purpose. This tunnel junction is grown on a metamorphic Ga0.76In0.24As template on a GaAs substrate. The band offsets in the resulting type-II heterojunction are calculated using the first-principles density functional method to estimate the tunneling barrier height and assess the performance of this tunnel junction against other material systems and compositions. The effect of the metamorphic growth on the performance of the tunnel junctions is analyzed using a set of metamorphic templates with varied surface roughness and threading dislocation density. Although the metamorphic template does influence the tunnel junction performance, all tunnel junctions measured have a peak current density over 200 A/cm2. The tunnel junction on the best template has a peak current density over 1500 A/cm2 and a voltage drop at 15 A/cm2 (corresponding to operation at 1000 suns) lower than 10 mV, which results in a nearly lossless series connection of the 4th junction in the 4J-IMM structure.The authors thankfully acknowledge the invaluable support by W. Olavarria and M. Young growing and processing the semiconductor devices. I. Garcıa holds an IOF grant from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/ 2007-2013) under REA grant agreement No. 299878. This work is supported by the U.S. Department of Energy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory

Metal-free N2-to-NH3 thermal conversion at the boron-terminated zigzag edges of hexagonal boron nitride: Mechanism and kinetics

Nitrogen fixation is essential for all life and various industrial processes. Recent developments in the growth techniques of hexagonal boron nitride (hBN) enable controlled termination of hBN with zigzag edges. Here, we show that the B-terminated zigzag (B-ZZ) edge of hBN, which is hydrogenated and thus “defect-free”, can act as a metal-free catalyst for thermal conversion from N2 and H2 to NH3 at high temperatures. Using density functional calculations, we identify the catalytic cycle of the NH3 production, which involves simultaneous N2 binding and hydrogenation at the one-dimensional edge of hBN. Further hydrogenation of the N2-binding B-ZZ is facilitated by the H2-induced local conversion between the sp2 B and sp3 B sites at the B-ZZ edge. The NH3 synthesis at the metal-free, defect-free B-ZZ edges, although less practical compared to the conventional Haber-Bosch process that uses transition metals, offers important insights into how the chemical flexibility of boron can be used for the challenging nitrogen transformations. © 2019 Elsevier Inc.1

Electric-Field-Tunable Bandgaps in the Inverse-Designed Nanoporous Graphene/Graphene Heterobilayers

The recent bottom-up synthesis of atomically precise nanoporous graphene (NPG) offers a way of tuning graphene's properties by forming NPG/graphene (Grp) bilayers. Depending on the size, shape, and periodicity of the nanopores in NPG, the heterobilayers can exhibit various functionalities. This theoretical work presents an inverse design of NPG/Grp bilayers with electric-field-tunable bandgaps as a target property. The interlayer interaction in such heterobilayers can induce a bandgap in graphene either by breaking inversion symmetry (type I) or by moving and merging Dirac points of graphene (type II). The bandgap opening also requires electron-hole symmetry breaking induced by an applied perpendicular electric field, leading to two distinct, linear versus nonlinear, field dependences of the bandgap for the type-I and type-II cases, respectively. To translate the underlying physics of the bandgap opening in graphene into real atomic structures, the authors develop an inverse design method and find NPG/Grp bilayers with the target functionality. The field-tunable bandgap in graphene, supported by first-principles calculations for the inverse-designed systems, holds promise for new types of graphene transistors. © 2022 Wiley-VCH GmbH.FALS

Enhanced Reactivity of Magic-Sized Inorganic Clusters by Engineering the Surface Ligand Networks

The carboxylate-ligated In37P20 is an intriguing magic sized cluster (MSC) whose high stability (i.e., magic size) stems from a delicate balance between the energy cost and gain associated with its partially disordered, In-rich core and its passivation by the bidentate ligands. In order to use such MSCs as intermediates for non-classical nucleation and growth of quantum dots, it is essential to control the reactivity (or stability) of MSCs by disrupting the energetic balance. Here, using ab initio molecular dynamics simulations, we reveal the destabilization process of the InP MSC induced by a modification of the surface ligand network beyond a critical limit. When three In(O2CR)3 subunits are released from the cluster at high temperatures, the remaining In34P20 core suddenly loses its stability and undergoes a structural transformation through In-P bond breaking and rearrangement. The net effect of the isomerization is an In-P bond exchange between a pair of In atoms, thereby leading to a rupture on the cluster surface. We elucidate the mechanism for the MSC instability by studying the intricate interactions between the surface ligand network and the inorganic core. Finally, we discuss the similarity and fundamental differences in the cluster isomerization of group III-V InP and group II-VI CdS MSCs.FALS

Subband-enhanced carrier multiplication in graphene nanoribbons

Carrier multiplication (CM), which generates multiexcitons from a single photon absorption, is advantageous for increasing the optoelectronic device efficiency. However, CM is intrinsically inefficient in conventional semiconductors, and enhancing CM has been a long-standing challenge. Here, we propose that multisubbands in nanostructures can significantly enhance CM by opening up the intersubband CM transitions, which circumvent the strict restrictions enforced by the energy and momentum conservations. Using real-time time-dependent density functional theory, we demonstrate the mechanism in graphene nanoribbons as an example of a multisubband system. The CM mechanism provides a pathway for developing efficient optoelectronic devices.FALS

Charge-induced magnetic instability of atomically thin ferromagnetic semiconductors: The case of CrI3

Recent studies have shown that the ferromagnetic intralayer order in CrI3 is weakened by electrostatic electron doping, with magnetization and Curie temperature decreasing linearly with doping density. The linear doping dependence observed in n-type CrI3 is puzzling because it requires a "fine-tuned"nonlinear decrease of the spin-wave gap upon electron doping. Here, using first-principles-based simulations, we reveal that electron doping of CrI3 induces a quantum phase transition to a magnetic state characterized by spontaneous spin-flip formation at the atomic scale. The electron localization in the presence of the spin-flips "renormalizes"the energy gap for collective spin excitations, which explains the puzzling doping effect on the two-dimensional magnetism of CrI3. © 2021 American Physical Society.1

A unified understanding of the direct coordination of NO to first-transition-row metal centers in metal-ligand complexes

The binding of nitric oxide (NO) to heme-proteins is an important biochemical process involved in a variety of physiological functions. Here, using hybrid density-functional calculations, we systematically investigate the adsorption of NO to first-transition-row metal centers in metal-ligand complexes. Through the comparative study for different transition metal (TM) centers, we provide a unified understanding of the microscopic interactions of NO with the TM centers and related chemical trends. We found that as the atomic number of the TM center increases, the binding strength of NO is largely reduced from 207 kJ mol-1 to near zero due to the low d-orbital energies for late TM centers. The intermolecular spin coupling between the localized spins at the TM center and the NO molecule is generally antiferromagnetic, except for the case of Sc. The spin-spin coupling is determined in such a way to avoid the energy penalty associated with the electron occupation in the antibonding states of the NO-bound complex. The adsorption strength of NO is generally larger than of CO because the unpaired electron of NO occupies the associated bonding state. © This journal is the Owner Societies.1