We report the quasiparticle band gap, excitons, and highly anisotropic optical responses of fewlayer black phosphorous (phosphorene). It is shown that these new materials exhibit unique manyelectron effects; the electronic structures are dispersive essentially along one dimension, leading to particularly enhanced self-energy corrections and excitonic effects. Additionally, within a wide energy range, including infrared light and part of visible light, few-layer black phosphorous absorbs light polarized along the structure's armchair direction and is transparent to light polarized along the zigzag direction, making them viable linear polarizers for applications. Finally, the number of phosphorene layers included in the stack controls the material's band gap, optical absorption spectrum, and anisotropic polarization energy-window across a wide range.PACS numbers:

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17–xNi0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish oxygen evolution reaction (OER) in pH-neutral media. We took the view that generating transition-metal sites with high valence at low applied bias should improve the activity of neutral OER catalysts. Here, using density functional theory, we find that the formation energy of desired Ni sites is systematically modulated by incorporating judicious combinations of Co, Fe and non-metal P. We therefore synthesized NiCoFeP oxyhydroxides and probed their oxidation kinetics with in situ soft X-ray absorption spectroscopy (sXAS). In situ sXAS studies of neutral-pH OER catalysts indicate ready promotion of Ni under low overpotential conditions. The NiCoFeP catalyst outperforms IrO and retains its performance following 100 h of operation. We showcase NiCoFeP in a membrane-free CO electroreduction system that achieves a 1.99 V cell voltage at 10 mA cm, reducing CO into CO and oxidizing HO to O with a 64% electricity-to-chemical-fuel efficiency.

The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

Despite much interest in engineering new topological surface (edge) states using structural defects, such topological surface states have not been observed yet. We show that recently imaged tilt boundaries in gated multilayer graphene should support topologically protected gapless edge states. We approach the problem from two perspectives: the microscopic perspective of a tight-binding model and an ab initio calculation on a bilayer, and the symmetry-protected topological (SPT) state perspective for a general multilayer. Hence, we establish the tilt-boundary edge states as the first concrete example of the edge states of symmetry-protected Z-type SPT, protected by no-valley mixing, electron-number conservation, and time-reversal T symmetries. Further, we discuss possible phase transitions between distinct SPTs upon symmetry changes. Combined with a recently imaged tilt-boundary network, our findings may explain the long-standing puzzle of subgap conductance in gated bilayer graphene. This proposal can be tested through future transport experiments on tilt boundaries. In particular, the tilt boundaries offer an opportunity for the in situ imaging of topological edge transport.

Abstract:We report a systematic study of the optical conductivity of twisted bilayer graphene (tBLG) across a large energy range (1.2 eV to 5.6 eV) for various twist angles, combined with first-principles calculations. At previously unexplored high energies, our data show signatures of multiple van Hove singularities (vHSs) in the tBLG bands, as well as the nonlinearity of the single layer graphene bands and their electron-hole asymmetry. Our data also suggest that excitonic effects play a vital role in the optical spectra of tBLG. Including electron-hole interactions in first-principles calculations is essential to reproduce the shape of the conductivity spectra, and we find evidence of coherent interactions between the states associated with the multiple vHSs in tBLG.Keywords: graphene, twisted bilayer graphene, optical spectroscopy, GW-BSE In two-dimensional materials with van der Waals interlayer coupling, the rotation angle (θ) between layers has emerged as an important degree of freedom with significant effects on these materials' electronic and optical properties. Twisted bilayer graphene (tBLG), a prototypical bilayer system, has been the subject of many recent theoretical and experimental studies [1][2][3][4][5][6] . In tBLG, the interlayer interactions perturb the band structure of each graphene layer to create new, θ-dependent van Hove singularities (vHSs), which have been observed by scanning tunneling spectroscopy 5,7,8 and optical spectroscopy [9][10][11][12][13][14] . However, these previous studies focused on relatively low energies where single layer graphene (SLG) has a unique linear band structure. On the contrary, the band structure of SLG becomes more complex at higher energies: the bands lose their linearity, electrons and holes are no longer symmetric, and a saddle point vHS occurs at the M point in the SLG Brillouin zone 15,16 . To date, little is known about how this asymmetry and nonlinearity affects the θ-dependent vHSs and associated optical properties in tBLG.In addition, while it is known that there are resonant excitons associated with the M point vHS in SLG [17][18][19] , the excitonic effects associated with the interlayer vHSs in tBLG are poorly understood. Furthermore, new excitonic states could form as coherent combinations of the multiple intralayer and interlayer vHSs in tBLG, particularly those closest in energy. Understanding these effects in tBLG would also aid our understanding of other stacked and twisted two-dimensional materials, such as hexagonal boron nitride and transition metal dichalcogenides, whose single layers have intrinsic band gaps and other higher energy vHSs.In this work, we perform optical absorption spectroscopy of tBLG with known θ to explore the tBLG band structure and many-body states. For the first time, we present the full optical absorption spectra of tBLG with various θ over a large energy range (1.2 -5.6 eV), which encompasses multiple θ-dependent vHSs in tBLG as well as the absorption peak associated with the M point vHS in SLG. We find that the

The scarce inventory of compounds that allow for diffusion of multivalent cations at reasonable rates poses a major impediment to the development of multivalent intercalation batteries. Here, we contrast the thermodynamics and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically accessible metastable phases of V2O5, ζ- and ε-V2O5, with the relevant parameters for the thermodynamically stable α-phase of V2O5 using density functional theory calculations. The metastability of the frameworks results in a higher open circuit voltage for multivalent ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted within these structures encounter suboptimal coordination environments and expanded transition states, which facilitate easier ion diffusion. Specifically, a nudged elastic band examination of ion diffusion pathways suggests that migration barriers are substantially diminished for Na- and Mg-ion diffusion in the metastable polymorphs: the predicted migration barriers for Mg ions in ζ-V2O5 and ε-V2O5 are 0.62–0.86 and 0.21–0.24 eV, respectively. More generally, the results indicate that topochemically derived metastable polymorphs represent an interesting class of compounds for realizing multivalent cation diffusion because many such compounds place cations in “frustrated” coordination environments that are known to be useful for realizing low diffusion barriers.

We report the quasiparticle band-edge energy of monolayer of molybdenum and tungsten dichalcogenides, MX 2 (M=Mo, W; X=S, Se, Te). Beyond calculating bandgaps, we have achieved converged absolute band-edge energies relative to the vacuum level. Compared with the results from other approaches, the GW calculation reveals substantially larger bandgaps and different absolute quasiparticle energies because of enhanced many-electron effects. Interestingly, our GW calculations ratify the band-gap-center approximation, making it a convenient way to estimate band-edge energy. The absolute band-edge energies and band offsets obtained in this work are important for designing heterojunction devices and chemical catalysts based on monolayer dichalcogenides.