Dongxue Chen

Two-dimensional (2D) transition metal dichalcogenides hold exceptional promise for next-generation light-emitting applications owing to their distinctive electronic configuration and exceptional optoelectronic characteristics. Their vertically stackable architecture not only reduces contact resistance but also enables higher current densities and enhanced extrinsic quantum efficiency (EQE). Nevertheless, the performance of current 2D electroluminescent (EL) devices remains constrained by low operating voltages and weak electric fields, limiting their broader adoption in light-emitting diode technologies. In this work, we demonstrate a high-electric-field-driven (>109 V/m) vertical tunneling EL mechanism in 2D heterostructures. By applying such intense fields, we achieve field emission of electrons from Bi2Sr2CaCu2O8 and holes from gold electrodes, resulting in diploid enhancement in EQE compared to conventional approaches in the low electric-field regime. Our findings in EL emission technology offer alternative perspectives for cavity-integrated exciton modulation and on-chip integrated optoelectronic devices.

Moiré superlattices of semiconducting transition metal dichalcogenides enable unprecedented spatial control of electron wavefunctions, leading to emerging quantum states. The breaking of translational symmetry further introduces a new degree of freedom: high symmetry moiré sites of energy minima behaving as spatially separated quantum dots. We demonstrate the superposition between two moiré sites by constructing a trilayer WSe₂/monolayer WS₂ moiré heterojunction. The two moiré sites in the first layer WSe₂ interfacing WS₂ allow the formation of two different interlayer excitons, with the hole residing in either moiré site of the first layer WSe₂ and the electron in the third layer WSe₂. An electric field can drive the hybridization of either of the interlayer excitons with the intralayer excitons in the third WSe₂ layer, realizing the continuous tuning of interlayer exciton hopping between two moiré sites and a superposition of the two interlayer excitons, distinctively different from the natural trilayer WSe₂.

Transition metal dichalcogenide (TMDC) moiré superlattices, owing to the moiré flatbands and strong correlation, can host periodic electron crystals and fascinating correlated physics. The TMDC heterojunctions in the type-II alignment also enable long-lived interlayer excitons that are promising for correlated bosonic states, while the interaction is dictated by the asymmetry of the heterojunction. Here we demonstrate a new excitonic state, quadrupolar exciton, in a symmetric WSe₂-WS₂-WSe₂trilayer moiré superlattice. The quadrupolar excitons exhibit a quadratic dependence on the electric field, distinctively different from the linear Stark shift of the dipolar excitons in heterobilayers. This quadrupolar exciton stems from the hybridization of WSe₂valence moiré flatbands. The same mechanism also gives rise to an interlayer Mott insulator state, in which the two WSe₂layers share one hole laterally confined in one moiré unit cell. In contrast, the hole occupation probability in each layer can be continuously tuned via an out-of-plane electric field, reaching 100% in the top or bottom WSe₂under a large electric field, accompanying the transition from quadrupolar excitons to dipolar excitons. Our work demonstrates a trilayer moiré system as a new exciting playground for realizing novel correlated states and engineering quantum phase transitions.

Moiré coupling in transition metal dichalcogenides (TMDCs) superlattices introduces flat minibands that enable strong electronic correlation and fascinating correlated states, and it also modifies the strong Coulomb-interaction-driven excitons and gives rise to moiré excitons. Here, we introduce the layer degree of freedom to the WSe₂/WS₂ moiré superlattice by changing WSe₂ from monolayer to bilayer and trilayer. We observe systematic changes of optical spectra of the moiré excitons, which directly confirm the highly interfacial nature of moiré coupling at the WSe₂/WS₂ interface. In addition, the energy resonances of moiré excitons are strongly modified, with their separation significantly increased in multilayer WSe₂/monolayer WS₂ moiré superlattice. The additional WSe₂ layers also modulate the strong electronic correlation strength, evidenced by the reduced Mott transition temperature with added WSe₂ layer(s). The layer dependence of both moiré excitons and correlated electronic states can be well described by our theoretical model. Our study presents a new method to tune the strong electronic correlation and moiré exciton bands in the TMDCs moiré superlattices, ushering in an exciting platform to engineer quantum phenomena stemming from strong correlation and Coulomb interaction.

In order to avoid the detrimental effects of excessive noise in the phase fringe patterns of a laser digital interferometer over the accuracy of phase unwrapping and the successful detection of mechanical fatigue defects, an effective method of adaptive multidirectional frequency domain filtering is introduced based on the characteristics of the energy spectrum of localized wrapped phase patterns. Not only can this method automatically set the cutoff frequency, but it can also effectively filter out noise while preserving the image edge information. Compared with the sine and cosine transform filtering and the multidirectional frequency domain filtering, the experimental results demonstrate that the image filtered by our method has the fewest number of residues and is the closest to the noise-free image, compared to the two aforementioned methods, demonstrating the effectiveness of this adaptive multidirectional frequency domain filter.

It is well established that absorbate-absorbate interactions play a key role in determining the distribution of adsorbates on surfaces. In cases where these interactions are repulsive adsorbates frequently arrange so as to minimize these unfavorable interactions. This simple picture, however, neglects the influence of adsorption on the properties on the underlying substrate. Here, using STM, we show that on Si(100) many intrinsically repulsive adsorbates cluster to form surface patches even at low surface coverages. With the aid of density functional theory calculations and Monte Carlo simulations, we show that patch formation is an intrinsic property of the Si(100) surface that is driven by the energy lowering associated with the formation of extended regions of bare dimers. The enhanced attraction between anticorrelated tilted bare dimers is sufficient to offset the repulsions between adsorbates.