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Shift current is a direct current generated from nonlinear light–matter interaction in a noncentrosymmetric crystal and is considered a promising candidate for next-generation photovoltaic devices. The mechanism for shift currents in real materials is, however, still not well understood, especially if electron–hole interactions are included. Here, we employ a first-principles interacting Green’s-function approach on the Keldysh contour with real-time propagation to study photocurrents generated by nonlinear optical processes under continuous wave illumination in real materials. We demonstrate a strong direct current shift current at subbandgap excitation frequencies in monolayer GeS due to strongly bound excitons, as well as a giant excitonic enhancement in the shift current coefficients at above bandgap photon frequencies. Our results suggest that atomically thin two-dimensional materials may be promising building blocks for next-generation shift current devices.

The framework of many-body perturbation theory led to deep insight into electronic structure and optical properties of diverse systems and, in particular, many semiconductors. It relies on an accurate approximation of the screened Coulomb electron-electron interaction W, that in current implementations is usually achieved by describing electronic interband transitions. However, our results for several oxide semiconductors indicate that for polar materials it is necessary to also account for lattice contributions to dielectric screening. To clarify this question in this work, we combine highly accurate experimentation and cutting-edge theoretical spectroscopy to elucidate the interplay of quasiparticle and excitonic effects for cubic bixbyite In2O3 across an unprecedentedly large photon energy range. We then show that the agreement between experiment and theory is excellent and, thus, validate that the physics of quasiparticle and excitonic effects is described accurately by these first-principles techniques, except for the immediate vicinity of the absorption onset. Finally, our combination of experimental and computational data clearly establishes the need for including a lattice contribution to dielectric screening in the screened electron-electron interaction, in order to improve the description of excitonic effects near the absorption edge.

Piezocatalytic therapy (PCT) based on 2D layered materials has emerged as a promising non‐invasive tumor treatment modality, offering superior advantages. However, a systematic investigation of PCT, particularly the mechanisms underlying the reactive oxygen species (ROS) generation by 2D nanomaterials, is still in its infancy. Here, for the first time, biodegradable piezoelectric 2D bilayer nickel‐iron layered double hydroxide (NiFe‐LDH) nanosheets (thickness of ≈1.86 nm) are reported for enhanced PCT and ferroptosis. Under ultrasound irradiation, the piezoelectric semiconducting NiFe‐LDH exhibits a remarkable ability to generate superoxide anion radicals, due to the formation of a built‐in electric field that facilitates the separation of electrons and holes. Notably, the significant excitonic effect in the ultrathin NiFe‐LDH system enables long‐lived excited triplet excitons (lifetime of ≈5.04 µs) to effectively convert triplet O2 molecules into singlet oxygen. Moreover, NiFe‐LDH exhibited tumor microenvironment (TME)‐responsive peroxidase (POD)‐like and glutathione (GSH)‐depleting capabilities, further enhancing oxidative stress in tumor cells and inducing ferroptosis. To the best of knowledge, this is the first report on piezoelectric semiconducting sonosensitizers based on LDHs for PCT and ferroptosis, providing a comprehensive understanding of the piezocatalysis mechanism and valuable references for the application of LDHs and other 2D materials in cancer therapy. Ultrathin NiFe‐LDH nanosheets with tumor microenvironment‐responsive and GSH‐depletion ability are developed for the first time for enhanced piezocatalytic therapy and ferroptosis. The piezoelectric semiconductor showed outstanding 1O2 generation ability arising from the large excitonic effect in NiFe‐LDH. This study pioneers the development of piezocatalytic tumor therapy in two‐dimension ultrathin LDH materials, effectively inducing the generation of reactive oxygen species.

We conducted a systematic investigation using state-of-the-art techniques on the electronic and optical properties of two crystals of alkaline earth metal fluorides, namely rutile MgF2 and cubic SrF2. For these two crystals of different symmetry, we present density functional theory (DFT), many-body perturbation theory (MBPT), and Bethe–Salpeter equation (BSE) calculations. We calculated a variety of properties, namely ground-state energies, band-energy gaps, and optical absorption spectra with the inclusion of excitonic effects. The quantities were obtained with a high degree of convergence regarding all bulk electronic and optical properties. Bulk rutile MgF2 has distinguished ground-state and excited-state properties with respect to the other cubic fluoride SrF2 and the other members of the alkaline earth metal fluoride family. The nature of the fundamental gaps and estimates of the self-energy and excitonic effects for the two compounds are presented and discussed in detail. Our results are in good accordance with the measurements and other theoretical–computational data. A comparison is made between the excitation and optical properties of bulk rutile MgF2, cubic SrF2, and the corresponding clusters, for which calculations have recently been published, confirming strong excitonic effects in finite-sized systems.

We present ab initio a study on linear and nonlinear optical properties of topological semimetal Tantalum arsenide and Sodium bismuthate. The real and imaginary part of the dielectric function in addition to the energy loss spectra of TaAs and Na 3 Bi have been calculated within random phase approximation (RPA); then, the electron–hole interaction is included by solving the Bethe–Salpeter equation for the electron–hole Green’s function. In spite of being in the single category of topological materials, we have found obvious distinction between linear optical responses of TaAs and Na 3 Bi at a high energy region where, in contrast to Na 3 Bi, Tantalum arsenide has excitonic peaks at 9 eV and 9.5 eV. It is remarkable that the excitonic effects in the high energy range of the spectrum are stronger than in the lower one. The dielectric function is overall red shifted compared with that of RPA approximation. The resulting static dielectric constants for Na 3 Bi are smaller than corresponding ones in TaAs. At a low energy region, the absorption intensity of TaAs is more than Na 3 Bi. The calculated second-order nonlinear optical susceptibilities χ ijk (2) ( ω ) show that Tantalum arsenide acts as a Weyl semimetal, and has high values of nonlinear responses in the low energy region which makes it promising candidate for the second harmonic generation in the terahertz frequency region. In the low energy regime, optical spectra are dominated by the 2 ω intra-band contributions.

Nanocrystals (NCs) of perovskite materials have recently attracted great research interest because of their outstanding properties for optoelectronic applications, as evidenced by the increasing number of publications on laboratory scale devices. However, in order to achieve the commercial realisation of these devices, an in-depth understanding of the charge dynamics and photo-physics in these novel materials is required. These dynamics are affected by material composition but also by their size and morphology due to quantum confinement effects. Advances in synthesis methods have allowed nanostructures to be produced with enhanced confinement and structural stability, enhancing the efficiency of energy funnelling and radiative recombination and so resulting in more efficient light emitting devices. In addition, photovoltaics could greatly benefit from the exploitation of these materials not only through their deployment in tandem cell architectures but from the use of multiple exciton generation in these NCs. These systems also offer the opportunity to study quantum effects relating to interactions of excited states within and between NCs. Properties and behaviour that includes an enhanced Rashba effect, superfluorescence, polariton lasing, Rydberg exciton polariton condensates, and antibunched single photon emission have been observed in a single metal halide perovskite NC. The further study of these in NC systems will shed new light on the fundamental nature of their excited states, their control and exploitation. In this perspective, we give an overview of these effects and provide an outlook for the future of perovskite NCs and their devices.

Ab initio calculations based on many-body perturbation theory have been used to study the electronic and optical properties of AgGaO2 in rhombohedral, hexagonal, and orthorhombic phases. GW calculations showed that AgGaO2 is an indirect-bandgap semiconductor in all three phases with energy bandgap of 2.35 eV, 2.23 eV, and 2.07 eV, in good agreement with available experimental values. By solving the Bethe–Salpeter equation (BSE) using the full potential linearized augmented plane wave basis, optical properties of the AgGaO2 polymorphs were calculated and compared with those obtained using the GW-corrected random phase approximation (RPA) and with existing experimental data. Strong anisotropy in the optical absorption spectra was observed, and the excitonic structures which were absent in the RPA calculations were reproduced in GWBSE calculations, in good agreement with the optical absorption spectrum of the rhombohedral phase. While modifying peak positions and intensities of the absorption spectra, the GWBSE gave rise to the redistribution of oscillator strengths. In comparison with the z-polarized response, excitonic effects in the x-polarized response were dominant. In the x- (and y-) polarized responses of r- and h-AgGaO2, spectral features and excitonic effects occur at the lower energies, but in the case of o-AgGaO2, the spectral structures of the z-polarized response occur at lower energies. In addition, the low-energy loss functions of AgGaO2 were calculated and compared using the GWBSE approach. Spectral features in the energy loss function components near the bandgap region were attributed to corresponding excitonic structures in the imaginary part of the dielectric function.

Photosynthetic species evolved to protect their light-harvesting apparatus from photoxidative damage driven by intracellular redox conditions or environmental conditions. The Fenna–Matthews–Olson (FMO) pigment–protein complex from green sulfur bacteria exhibits redox-dependent quenching behavior partially due to two internal cysteine residues. Here, we show evidence that a photosynthetic complex exploits the quantum mechanics of vibronic mixing to activate an oxidative photoprotective mechanism. We use two-dimensional electronic spectroscopy (2DES) to capture energy transfer dynamics in wild-type and cysteine-deficient FMO mutant proteins under both reducing and oxidizing conditions. Under reducing conditions, we find equal energy transfer through the exciton 4–1 and 4–2-1 pathways because the exciton 4–1 energy gap is vibronically coupled with a bacteriochlorophyll-a vibrational mode. Under oxidizing conditions, however, the resonance of the exciton 4–1 energy gap is detuned from the vibrational mode, causing excitons to preferentially steer through the indirect 4–2-1 pathway to increase the likelihood of exciton quenching. We use a Redfield model to show that the complex achieves this effect by tuning the site III energy via the redox state of its internal cysteine residues. This result shows how pigment–protein complexes exploit the quantum mechanics of vibronic coupling to steer energy transfer.

Dye molecules, arranged in an aggregate, can display excitonic delocalization. The use of DNA scaffolding to control aggregate configurations and delocalization is of research interest. Here, we applied Molecular Dynamics (MD) to gain an insight on how dye–DNA interactions affect excitonic coupling between two squaraine (SQ) dyes covalently attached to a DNA Holliday junction (HJ). We studied two types of dimer configurations, i.e., adjacent and transverse, which differed in points of dye covalent attachments to DNA. Three structurally different SQ dyes with similar hydrophobicity were chosen to investigate the sensitivity of excitonic coupling to dye placement. Each dimer configuration was initialized in parallel and antiparallel arrangements in the DNA HJ. The MD results, validated by experimental measurements, suggested that the adjacent dimer promotes stronger excitonic coupling and less dye–DNA interaction than the transverse dimer. Additionally, we found that SQ dyes with specific functional groups (i.e., substituents) facilitate a closer degree of aggregate packing via hydrophobic effects, leading to a stronger excitonic coupling. This work advances a fundamental understanding of the impacts of dye–DNA interactions on aggregate orientation and excitonic coupling.