Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

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/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Tailoring Broadband Nonlinear Optical Characteristics and Ultrafast Photocarrier Dynamics of Bi2 O2 S 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 Bi2 O2 S is quite limited. Here, comprehensive experimental and computational investigations are conducted in the regulated band structure, nonlinear optical (NLO) characteristics, and carrier dynamics of Bi2 O2 S nanosheets via defect engineering, taking O vacancy (OV) and substitutional Se doping as examples. As the OV continuously increased to ≈35%, the optical bandgaps progressively narrow from ≈1.21 to ≈0.81 eV and NLO wavelengths are extended to near-infrared regions with enhanced saturable absorption. Simultaneously, the relaxation processes are effectively accelerated from tens of picoseconds to several picoseconds, as the generated defect energy levels can serve as both additional absorption cross-sections and fast relaxation channels supported by theoretical calculations. Furthermore, substitutional Se doping in Bi2 O2 S nanosheets also modulate their optical properties with the similar trends. As a proof-of-concept, passively mode-locked pulsed lasers in the ≈1.0 µm based on the defect-rich samples (≈35% OV and ≈50% Se-doping) exhibit excellent performance. This work deepens the insight of defect functions on optical properties of Bi2 O2 S nanosheets and provides new avenues for designing advanced photonic devices based on low-dimensional bismuth oxychalcogenides.

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 COFcNDI-OEt and COFcNDI-SEt possess higher HOMO/LUMO levels and narrower bandgaps than COFNDI-H. 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. Moreover, the femtosecond transient absorption (TA) spectra manifested that the electron donor substituents are beneficial to the charge delocalization in the π-columnar unit, resulting in a longer lifetime of charge recombination, which is one of the pivotal prerequisites for high-performance solar cells and photocatalysis.

Strain-Dependent Band Splitting and Spin-Flip Dynamics in Monolayer WS2.

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. Here, we demonstrate the effect of strain on the excitonic dynamics of monolayer WS2 via steady-state fluorescence and transient absorption spectroscopy. Combined with theoretical calculations, we found that tensile strain can reduce the spin-splitting value of the conduction band and lead to transitions between different exciton states via spin-flip mechanism. Our findings suggest that the spin-flip process is strain-dependent, provides a reference for application of valleytronic devices, where tensile strain is usually existing during their design and fabrication.

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.e., two identical charges in the different layers). Type I interlayer trions are investigated theoretically and experimentally. By contrast, Type II interlayer trions remain elusive in vdWHs, due to inadequate free charges, unsuitable band alignment, reduced Coulomb interactions, poor interface quality, etc. Here, the first observation of Type II interlayer trions is reported by exploring band alignments and choosing an atomically thin organic-inorganic system-monolayer WSe2 /bilayer pentacene heterostructure (1L + 2L HS). Both positive and negative Type II interlayer trions are electrically tuned and observed via PL spectroscopy. In particular, Type II interlayer trions exhibit in-plane anisotropic emission, possibly caused by their unique spatial structure and anisotropic charge interactions, which is highly correlated with the transition dipole moment of pentacene. The results pave the way to develop excitonic devices and all-optical circuits using atomically thin organic-inorganic bilayers.

Direct observation of contact resistivity for monolayer TMD based junctions via 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 via PL spectroscopy. Currently, electrical methods have been utilized to measure the contact resistance (RC), 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. Here, we report a simple, rapid and low-cost method to study the tunnel barrier dominated contact resistance of mTMD based junctions through PL spectroscopy. These junctions are free from chemical disorders and Fermi level pinning. Excluding the Schottky barrier component, solely tunnel barrier dominated contact resistances of 1 L MoSe2/Au and 1 L MoSe2/graphene junctions were estimated to be 147.8 Ω µm and 54.9 Ω µm, respectively. Density functional theory (DFT) simulations revealed that the larger RC of the former was possibly due to the existence of intrinsic effective potential difference (Φbarrier) between mTMD and metal. Both junctions exhibit an increasing tendency of RC as temperature decreases, which is probably attributed to the thermal expansion coefficient (TEC) mismatch-triggered interlayer spacing (d) increase and temperature-induced doping. Remarkably, a significant change of RC was observed in 1 L MoSe2/Au junctions, which is possibly ascribed to the changes of their orbital overlaps. Our results open new avenues for exploring fundamental metal-semiconductor contact principles and constructing high-performance devices.

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. Under visible-light excitation, despite being separated by few-layer boron nitride, the graphene and tungsten disulfide exhibit clear modulation of their doping level, i.e., a change of the Fermi level in graphene as large as 120 meV and a net electron accumulation in WS2. By using a combination of micro-Raman and photoluminescence spectroscopy, we demonstrate that the modulation is originated from simultaneous manipulation of charge and/or energy transfer between each of the two adjacent layers.

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. Characterization and manipulation of these complexes have become a reality due to their enhanced binding energies as a direct result from reduced dielectric screening and enhanced Coulomb interactions in the 2D regime. Furthermore, the atomic thickness and extremely large surface-to-volume ratio of 2D semiconductors allow the possibility of modulating their inherent optical, electrical, and optoelectronic properties using a variety of different environmental stimuli. To fully realize the potential functionalities of these many-body complexes in optoelectronics, a comprehensive understanding of their formation mechanism is essential. A topical and concise summary of the recent frontier research progress related to many-body complexes in 2D semiconductors is provided here. Moreover, detailed discussions covering the aspects of fundamental theory, experimental investigations, modulation of properties, and optoelectronic applications are given. Lastly, personal insights into the current challenges and future outlook of many-body complexes in 2D semiconducting materials are presented.

Excited State Biexcitons in Atomically Thin MoSe2.

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. Here, we successfully triggered the emission of biexcitons in atomically thin MoSe2, via the engineering of three critical parameters: dielectric screening, density of trions, and excitation power. The observed binding energy and formation dynamics of these biexcitons strongly support the model that the biexciton consists of a charge attached to a trion (excited state biexciton) instead of four spatially symmetric particles (ground state biexciton). More importantly, we found that the excited state biexcitons not only can exist at cryogenic temperatures but also can be triggered at room temperature in a freestanding bilayer MoSe2. The demonstrated capability of biexciton engineering in atomically thin MoSe2 provides a route for exploring fundamental many-body interactions and enabling device applications, such as bright entangled photon sources operating at room temperature.

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. 2D phosphorene with quasi-1D free excitons and 0D-like excitons provides a unique platform to investigate the fundamental phenomena in the ideal 2D-1D-0D hybrid system.

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. By varying the thickness of the spacer between the MoSe2 layer and nanoantenna, we demonstrate manipulation of the PL intensity from nearly fourfold quenching to approximately threefold enhancement. Furthermore, we show that the coupled TMDC-nanoantenna system exhibits strong polarization-dependent PL, thus offering the possibility of polarization-based emission control. Our experimental results are supported by numerical simulations as well. To the best of our knowledge, this is the first study of Au-MoSe2 plasmonic hybrid structures realizing flexible PL manipulation.

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. The effective enhancement factor could be up to ∼43. Our results open the door to manipulating the optical properties of future devices by using nanostructured 2D monolayers.

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. Moreover, in a stabilized phosphorene monolayer, we were able to precisely engineer defects for the first time, which led to efficient emission of photons at new frequencies in the near infrared at room temperature. In addition, we demonstrate the use of an electrostatic gate to tune the photon emission from the defects in a monolayer phosphorene. This could lead to new electronic and optoelectronic devices, such as electrically tunable, broadband near infrared lighting devices operating at room 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. We firstly observed highly anisotropic and layer-dependent PL quenching in the phosphorene-gold hybrid system, which is attributed to the highly anisotropic/layer-dependent interfacial charge transfer.

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. We experimentally validated the quasi-1D nature of excitonic and trionic dynamics in phospherene by demonstrating completely linearly polarized light emission from excitons and trions in few-layer phosphorene. The implications of the extraordinarily large trion binding energy in a higher-than-one-dimensional material are far-reaching. It provides a room-temperature 2D platform to observe the fundamental many-body interactions in the quasi-1D region.

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.

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.The exciton binding energy of monolayer MoTe2 was measured to be 0.58 ± 0.08 eV via photoluminescence excitation spectroscopy, which matches well with our calculated value of 0.64 eV.