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The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process.

One dimensional boron nitride (BN) nanomaterials with a high aspect ratio are of great interest due to their unique properties and potential applications. However, BN nanomaterials are generally difficult to synthesize. Here, we describe the creation of arrays of vertically aligned pure BN nanofibers and BN coated carbon nanofibers, fabricated on-chip via a straightforward template-assisted chemical conversion reaction. The template, a glassy carbon nanofiber array, is produced by plasma processing of conventional photoresists. The method is highly controllable, patternable, and scalable, and the final arrays can be fabricated over large areas with a controlled fiber length. We characterize the electron field emission properties of the BN-coated carbon nanofiber array and find a large field enhancement factor, low turn-on voltage, and good stability. The outstanding field emission performance results from the small tip size and high aspect ratio of the nanofiber as well as the high chemical stability and high thermal conductivity of the BN coating.

Moiré superlattices enable the generation of new quantum phenomena in two-dimensional heterostructures, in which the interactions between the atomically thin layers qualitatively change the electronic band structure of the superlattice. For example, mini-Dirac points, tunable Mott insulator states and the Hofstadter butterfly pattern can emerge in different types of graphene/boron nitride moiré superlattices, whereas correlated insulating states and superconductivity have been reported in twisted bilayer graphene moiré superlattices1-12. In addition to their pronounced effects on single-particle states, moiré superlattices have recently been predicted to host excited states such as moiré exciton bands13-15. Here we report the observation of moiré superlattice exciton states in tungsten diselenide/tungsten disulfide (WSe2/WS2) heterostructures in which the layers are closely aligned. These moiré exciton states manifest as multiple emergent peaks around the original WSe2 A exciton resonance in the absorption spectra, and they exhibit gate dependences that are distinct from that of the A exciton in WSe2 monolayers and in WSe2/WS2 heterostructures with large twist angles. These phenomena can be described by a theoretical model in which the periodic moiré potential is much stronger than the exciton kinetic energy and generates multiple flat exciton minibands. The moiré exciton bands provide an attractive platform from which to explore and control excited states of matter, such as topological excitons and a correlated exciton Hubbard model, in transition-metal dichalcogenides.

Crystal symmetry of two-dimensional (2D) materials plays an important role in their electronic and optical properties. Engineering symmetry in 2D materials has recently emerged as a promising way to achieve novel properties and functions. The noncentrosymmetric structure of monolayer transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2), has allowed for valley control via circularly polarized optical excitation. In bilayer TMDCs, inversion symmetry can be controlled by varying the stacking sequence, thus providing a pathway to engineer valley selectivity. Here, we report the in situ integration of AA′ and AB stacked bilayer MoS2 with different inversion symmetries by creating atomically sharp stacking boundaries between the differently stacked domains, via thermal stimulation and electron irradiation, inside an atomic-resolution scanning transmission electron microscopy. The setup enables us to track the formation and atomic motion of the stacking boundaries in real time and with ultrahigh resolution which enables in-depth analysis on the atomic structure at the boundaries. In conjunction with density functional theory calculations, we establish the dynamics of the boundary nucleation and expansion and further identify metallic boundary states. Our approach provides a means to synthesize domain boundaries with intriguing transport properties and opens up a new avenue for controlling valleytronics in nanoscale domains via real-time patterning of domains with different symmetry properties.

Atomically thin MoS2 grown by chemical vapor deposition (CVD) is a promising candidate for next-generation electronics due to inherent CVD scalability and controllability. However, it is well known that the stacking sequence in few-layer MoS2 can significantly impact electrical and optical properties. Herein we report different intrinsic stacking sequences in CVD-grown few-layer MoS2 obtained by atomic-resolution annular-dark-field imaging in an aberration-corrected scanning transmission electron microscope operated at 50 keV. Trilayer MoS2 displays a new stacking sequence distinct from the commonly observed 2H and 3R phases of MoS2. Density functional theory is used to examine the stability of different stacking sequences, and the findings are consistent with our experimental observations.