Interface Engineering for Efficient Photocarrier Generation and Transfer in Strongly Coupled Metallic/Semiconducting 1T'/2H MoS Heterobilayers.

Developing alternative two-dimensional (2D) metallic/semiconducting (M/S) van der Waals heterostructures (vdWHs) along with an understanding of interfacial photocarrier behavior is crucial for designing high-performance optoelectronic devices. Here, we comprehensively explored the photophysical model of photocarrier generation and interfacial transfer in as-grown 2D 1T'/2H MoS vdWHs using various spectroscopic characterizations. We demonstrated the transitions of activated photocarrier transfer trajectories by tuning the pump photon energies across the 2H MoS bandgap.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS superlattice (SnTiS) and its subsystems by comprehensive characterization and DFT calculations.

Tailoring Broadband Nonlinear Optical Characteristics and Ultrafast Photocarrier Dynamics of BiOS Nanosheets by Defect Engineering.

Low-dimensional bismuth oxychalcogenides have shown promising potential in optoelectronics due to their high stability, photoresponse, and carrier mobility. However, the relevant studies on deep understanding for BiOS is quite limited. Here, comprehensive experimental and computational investigations are conducted in the regulated band structure, nonlinear optical (NLO) characteristics, and carrier dynamics of BiOS nanosheets via defect engineering, taking O vacancy (OV) and substitutional Se doping as examples.

Design, Synthesis, and Ultrafast Carrier Dynamics of Core-Substituted Naphthalene Diimide-Based Covalent Organic Frameworks.

A series of two-dimensional polyimide covalent organic frameworks (2D COF) based on core-substituted naphthalene diimides (cNDIs) were designed and synthesized with the characteristic of tunable bandgap without global structural changes. Cyclic voltammetry (CV) and DFT calculations indicated that COF and COF possess higher HOMO/LUMO levels and narrower bandgaps than COF. Further investigation indicated that the COF bandgaps are not only related to the electron-donating substituents but also varied with respect to the interlayer distances.

Strain-Dependent Band Splitting and Spin-Flip Dynamics in Monolayer WS.

Triggered by the expanding demands of semiconductor devices, strain engineering of two-dimensional transition metal dichalcogenides (TMDs) has garnered considerable research interest. Through steady-state measurements, strain has been proved in terms of its modulation of electronic energy bands and optoelectronic properties in TMDs. However, the influence of strain on the spin-orbit coupling as well as its related valley excitonic dynamics remains elusive.

Discovery of Type II Interlayer Trions.

In terms of interlayer trions, electronic excitations in van der Waals heterostructures (vdWHs) can be classified into Type I (i.e., two identical charges in the same layer) and Type II (i.

Direct observation of contact resistivity for monolayer TMD based junctions PL spectroscopy.

Monolayer transition metal dichalcogenides (mTMDs) possess a direct band gap and strong PL emission that is highly sensitive to doping level and interfaces, laying the foundation for investigating the contact between mTMD and metal PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (), but they are complicated, time-consuming, high-cost and suffer from inevitable chemical disorders and Fermi level pinning. In addition, previously reported contact resistances comprise both Schottky barrier and tunnel barrier components.

Manipulating Charge and Energy Transfer between 2D Atomic Layers via Heterostructure Engineering.

Two-dimensional (2D) van der Waals heterostructures have attracted enormous research interests due to their emergent electrical and optical properties. The comprehensive understanding and efficient control of interlayer couplings in such devices are crucial for realizing their functionalities, as well as for improving their performance. Here, we report a successful manipulation of interlayer charge transfer between 2D materials by varying different stacking layers consisting of graphene, hexagonal boron nitride, and tungsten disulfide.

Many-Body Complexes in 2D Semiconductors.

2D semiconductors such as transition metal dichalcogenides (TMDs) and black phosphorus (BP) are currently attracting great attention due to their intrinsic bandgaps and strong excitonic emissions, making them potential candidates for novel optoelectronic applications. Optoelectronic devices fabricated from 2D semiconductors exhibit many-body complexes (exciton, trion, biexciton, etc.) which determine the materials optical and electrical properties.

Excited State Biexcitons in Atomically Thin MoSe.

The tightly bound biexcitons found in atomically thin semiconductors have very promising applications for optoelectronic and quantum devices. However, there is a discrepancy between theory and experiment regarding the fundamental structure of these biexcitons. Therefore, the exploration of a biexciton formation mechanism by further experiments is of great importance.

Exciton Brightening in Monolayer Phosphorene via Dimensionality Modification.

Exciton brightening in monolayer phosphorene is achieved via the dimensionality modification of excitons from quasi-1D to 0D. The luminescence quantum yield of 0D-like excitons is >33.6 times larger than that of quasi-1D free excitons.

Manipulation of photoluminescence of two-dimensional MoSe2 by gold nanoantennas.

Monolayer molybdenum diselenide (MoSe2), a member of the TMDCs family, is an appealing candidate for coupling to gold plasmonic nanostructures as it has smaller bandgap and higher electron mobility in comparison to frequently studied molybdenum disulfide (MoS2). The PL of MoSe2 occurs in the near-infrared spectral range where the emissive properties do not suffer from the enhanced dissipation in the gold due to inter-band transitions. Here, we study the interaction between monolayer MoSe2 and plasmonic dipolar antennas in resonance with the PL emission of MoSe2.

Strongly enhanced photoluminescence in nanostructured monolayer MoS2 by chemical vapor deposition.

Two-dimensional (2D) layered molybdenum disulfide (MoS2) has become a very promising candidate semiconducting material for future optoelectronic devices, owing to its unique properties. However, monolayer MoS2 is still a weak photon emitter, compared with other direct band gap semiconductors, which requires extra techniques or complicated steps to enhance its photon emission efficiency. Here, we demonstrated that nanostructured monolayer MoS2, produced by one-step chemical vapor deposition (CVD) growth, shows highly enhanced PL emission.

Producing air-stable monolayers of phosphorene and their defect engineering.

It has been a long-standing challenge to produce air-stable few- or monolayer samples of phosphorene because thin phosphorene films degrade rapidly in ambient conditions. Here we demonstrate a new highly controllable method for fabricating high quality, air-stable phosphorene films with a designated number of layers ranging from a few down to monolayer. Our approach involves the use of oxygen plasma dry etching to thin down thick-exfoliated phosphorene flakes, layer by layer with atomic precision.

Extraordinarily Bound Quasi-One-Dimensional Trions in Two-Dimensional Phosphorene Atomic Semiconductors.

We report a trion (charged exciton) binding energy of ∼162 meV in few-layer phosphorene at room temperature, which is nearly 1-2 orders of magnitude larger than those in two-dimensional (2D) transition metal dichalcogenide semiconductors (20-30 meV) and quasi-2D quantum wells (∼1-5 meV). Such a large binding energy has only been observed in truly one-dimensional (1D) materials such as carbon nanotubes, whose optoelectronic applications have been severely hindered by their intrinsically small optical cross sections. Phosphorene offers an elegant way to overcome this hurdle by enabling quasi-1D excitonic and trionic behaviors in a large 2D area, allowing optoelectronic integration.

Exciton and Trion Dynamics in Bilayer MoS2.

The control of exciton and triondynamics in bilayer MoS2 is demonstrated, via the comodulations by both temperature and electric field. The calculations here show that the band structure of bilayer MoS2 changes from indirect at room temperature toward direct nature as temperature decreases, which enables the electrical tunability of the K-K direct PL transition in bilayer MoS2 at low temperature.

Layer-dependent surface potential of phosphorene and anisotropic/layer-dependent charge transfer in phosphorene-gold hybrid systems.

The surface potential and the efficiency of interfacial charge transfer are extremely important for designing future semiconductor devices based on the emerging two-dimensional (2D) phosphorene. Here, we directly measured the strong layer-dependent surface potential of mono- and few-layered phosphorene on gold, which is consistent with the reported theoretical prediction. At the same time, we used an optical way photoluminescence (PL) spectroscopy to probe charge transfer in the phosphorene-gold hybrid system.

Robust Excitons and Trions in Monolayer MoTe2.

Molybdenum telluride (MoTe2) has emerged as a special member in the family of two-dimensional transition metal dichalcogenide semiconductors, owing to the strong spin-orbit coupling and relatively small energy gap, which offers new applications in valleytronic and excitonic devices. Here we successfully demonstrated the electrical modulation of negatively charged (X(-)), neutral (X(0)), and positively charged (X(+)) excitons in monolayer MoTe2 via photoluminescence spectroscopy. The binding energies of X(+) and X(-) were measured to be ∼24 and ∼27 meV, respectively.