Moiré superlattices in van der Waals (vdW) heterostructures could trap long-lived interlayer excitons. These moiré excitons could form ordered quantum dot arrays, paving the way for unprecedented optoelectronic and quantum information applications. Here, we perform first-principles simulations to shed light on moiré excitons in twisted MoS2/WS2 heterostructures. We provide direct evidence of localized interlayer moiré excitons in vdW heterostructures. The interlayer and intralayer moiré potentials are mapped out based on spatial modulations of energy gaps. Nearly flat valence bands are observed in the heterostructures. The dependence of spatial localization and binding energy of the moiré excitons on the twist angle of the heterostructures is examined. We explore how vertical electric field can be tuned to control the position, polarity, emission energy, and hybridization strength of the moiré excitons. We predict that alternating electric fields could modulate the dipole moments of hybridized moiré excitons and suppress their diffusion in moiré lattices.
Surface-illuminated GeSn p-i-n photodetectors (PDs) with Ge0.964Sn0.036 active layer on Ge substrate were fabricated. Photodetection up to 1.95 μm is achieved with a responsivity of 0.13 A/W. High responsivities of 0.56 and 0.71 A/W were achieved under a reverse bias voltage of 3 V at 1640 and 1790 nm, respectively. A low dark current of 1.08 μA was obtained at a reverse bias of 1 V with a diameter of 150 μm, which corresponds to a current density of 6.1 mA/cm2. This value is among the lowest dark current densities reported among GeSn PDs.
Recent experiments revealed stacking-configuration-independent and ultrafast charge transfer in transition metal dichalcogenides van der Waals (vdW) heterostructures, which is surprising given strong exciton binding energies and large momentum mismatch across the heterojunctions. Previous theories failed to provide a comprehensive physical picture for the charge transfer mechanisms. To address this challenge, we developed a first-principles framework which can capture exciton−phonon interaction in extended systems. We find that excitonic effect does not impede, but actually drives ultrafast charge transfer in vdW heterostructures. The many-body electron−hole interaction affords cooperation among the electrons, which relaxes the constraint on momentum conservation and reduces energy gaps for charge transfer. We uncover a two-step process in exciton dynamics: ultrafast hole transfer followed by much longer relaxation of intermediate "hot" excitons. This work establishes that many-body excitonic effect is crucial to the ultrafast dynamics and provides a basis to understand relevant phenomena in vdW heterostructures.
Formation of heterostructures is often inevitable in two-dimensional (2D) halide perovskites and band alignment in 2D perovskite heterostructures is of central importance to their applications. However, controversies abound in literature on the band alignment of the 2D perovskite heterostructures. While external factors have been sought to reconcile the controversies, we show that the 2D perovskite heterostructures are in fact intrinsically prone to band "misalignment", driven by thermal fluctuations. Owing to the "softness" of inorganic layers in the perovskites, electron−phonon coupling at room temperature could be strong enough to override band offsets at zero temperature, leading to oscillatory band alignment between type-I and type-II at 300 K. We further demonstrate that by tuning the inorganic layers, one can increase the band offsets and stabilize the band alignment, paving the way for optoelectronic applications of the 2D perovskite heterostructures.
Two-dimensional
boron monolayers (i.e., borophene) hold promise
for a variety of energy, catalytic, and nanoelectronic device technologies
due to the unique nature of boron–boron bonds. To realize its
full potential, borophene needs to be seamlessly interfaced with other
materials, thus motivating the atomic-scale characterization of borophene-based
heterostructures. Here, we report the vertical integration of borophene
with tetraphenyldibenzoperiflanthene (DBP) and measure the angstrom-scale
interfacial interactions with ultrahigh-vacuum tip-enhanced Raman
spectroscopy (UHV-TERS). In addition to identifying the vibrational
signatures of adsorbed DBP, TERS reveals subtle ripples and compressive
strains of the borophene lattice underneath the molecular layer. The
induced interfacial strain is demonstrated to extend in borophene
by ∼1 nm beyond the molecular region by virtue of 5 Å
chemical spatial resolution. Molecular manipulation experiments prove
the molecular origins of interfacial strain in addition to allowing
atomic control of local strain with magnitudes as small as ∼0.6%.
In addition to being the first realization of an organic/borophene
vertical heterostructure, this study demonstrates that UHV-TERS is
a powerful analytical tool to spectroscopically investigate buried
and highly localized interfacial characteristics at the atomic scale,
which can be applied to additional classes of heterostructured materials.
The electrochemical reduction of CO 2 (CO 2 RR) is a promising alternative to achieve carbon neutrality and the sustainable development of human civilization. Rational design of electrocatalysts for CO 2 RR requires thorough understanding of catalytically active sites and corresponding reaction mechanisms from the atomic level, where theoretical computations and simulations are indispensable. In this perspective, we summarize the recent progress in simulating CO 2 RR from both thermodynamic and kinetic aspects, including different methods for describing solvent−ion effects and applied potentials. In addition, a brief overview of applications of machine learning (ML) to the design of CO 2 RR catalysts is presented, and the simulation of electrocatalytic processes is also discussed, considering the solvent model under a constant potential by ML. Finally, an outlook is provided for the future development of computational models and their applications to CO 2 RR.
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