Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures

Tuning of MoS2 Photoluminescence in Heterostructures with CrSBr

Monolayers of semiconducting transition metal dichalcogenides (TMDCs) are known for their unique excitonic photoluminescence (PL), which can be tuned by interfacing them with other materials. However, integrating TMDCs into van der Waals heterostructures often results in a significant quenching of the PL because of an increased rate of nonradiative recombination processes. We demonstrate a wide-range tuning of the PL intensity of monolayer MoS2 interfaced with another layered semiconductor, CrSBr. We discover that a thin CrSBr up to ≈20 nm in thickness enhances the PL of MoS2, while a thicker material causes PL quenching, which is associated with changes in the excitonic makeup driven by the charge redistribution in the CrSBr/MoS2 heterostructure. Transport measurements, Kelvin probe force microscopy, and first-principles calculations indicate that this charge redistribution most likely causes n- to p-type doping transition of MoS2 upon contact with CrSBr, facilitated by the type II band alignment and the tendency of CrSBr to act as an electron sink. Furthermore, we fabricate an efficient AC-regime photodetector with a responsivity of 105 A/W from a MoS2/CrSBr heterostructure.

Surface structural probing and photoelectrochemical characterization of electrodeposited MoS2 nanostructured thin film

Molybdenum disulfide (MoS2) has advantageous traits and characteristics that make it suitable for a diverse array of practical applications, such as optoelectronics and gas sensing. Enhancing the surface area of the adsorbent leads to a proportional increase in its performance. Hence, the synthesized two-electrode electrodeposited MoS2 (ED-MoS2) thin films were microstructurally characterized to investigate its surface modulation for suitable enhancement in its applicative properties. The characterization shows that the surface properties of the deposited film can easily be modulated to favor its needs and can be done by simply varying its electrodeposition parameters, such as growth period and voltage supplied. In addition, persistent n-type conductivity of intrinsic MoS2 makes it challenging to achieve p-type conductivity. By simply varying the cathodic potential, the challenge of obtaining p-type MoS2 thin films was solved. The photoelectrochemical cell measurements revealed that lower cathodic potentials (1.15-1.35 V) favored the growth of p-type MoS2 layers while the growth of n-type MoS layers was achieved at the higher cathodic potential.

Self-Driven High-Performance Gate-Voltage-Tunable and Enhanced Performance Optoelectronic Device Based on FePS3/MoS2 Heterojunctions

The van der Waals heterojunctions are crucial for the development of next-generation high-performance optoelectronic devices due to their high-quality interface. In this study, FePS3/MoS2 van der Waals heterojunctions were fabricated, and their electronic and optoelectronic properties were investigated. The devices demonstrated typical rectification behavior, characterized by a low rectification ratio and electron-dominated conductivity. The suppression of electron recombination was achieved by eliminating non-heterojunction regions on the FePS3 side, leading to enhanced device performance. Notably, a large rectification ratio of 6.3 × 104 and an ideality factor of 1.24 were observed. Furthermore, the devices also exhibited self-driven photodetection performance, including a responsivity of 203 mA/W, a high-speed response/recovery time of 70.4/92 μs, and a high on/off ratio of 5.4 × 103. Responsivity and on/off ratio were further improved to higher values of 1.4 A/W and 1.2 × 105 by the modulation of Vg. The results offer valuable insights for improving and developing high-performance devices based on two-dimensional materials and heterojunctions.

Manipulation of trions to enhance the excitonic emission in monolayer p-MoS2 and its hetero-bilayer by reverse charge injection

Monolayer 2D transition metal dichalcogenides (TMDs) are known for their direct bandgaps and pronounced excitonic effects, which facilitate efficient light absorption and high photoluminescence (PL). In this study, we report a significant enhancement in PL emission from monolayers of p-type molybdenum disulfide (p-MoS2), fabricated on conductive substrates-such as indium tin oxide (ITO) and gold (Au). We attribute this behaviour to the reverse injection of charge carriers from substrates to p-MoS2 and the subsequent localization of electrons and holes in the substrate and p-MoS2, respectively. Such injection of charge carriers was suppressed when few-layer graphene (FLG) was used as a barrier layer. Further investigation of the PL emission characteristics from a vertically stacked hetero-bilayer (the p-n interface) of p-MoS2 and n-MoSe2 revealed a single resonant high-emission PL peak at 1.64 eV with the PL emission from this heterostructure significantly higher than that from free-standing monolayers. This finding contrasts sharply with the PL quenching often seen in hetero-bilayers with an n-n interface. These findings offer valuable insights into the fundamental optical and electronic properties of 2D TMDs and their heterostructures, which are essential for optimizing these materials for optoelectronic applications.

S-Scheme Interface Between K-C3N4 and FePS3 Fosters Photocatalytic H2 Evolution

In photocatalysis, photogenerated charge separation is pivotal and can be achieved through various mechanisms. Building heterojunctions is a promising method to enhance charge separation, where effective contact and charge exchange between heterojunction components remains challenging. Mostly used synthesis processes for making heterostructures require high temperatures, difficult processes, or expensive materials. Herein, a heterojunction of potassium intercalated graphitic carbon nitride (K-CN) and nanoflakes of iron phosphor trisulfide (FPS) is designed via a simple mechanical grinding process to boost the hydrogen evolution by a factor of more than 25 compared to pure K-CN. This significant improvement is rarely reached by other combinations of two semiconductors without cocatalysts, such as platinum. It can be attributed to the band alignment and band bending of an S-scheme that is validated via optical and X-ray photoelectron spectroscopy. As a consequence, strong quenching of the photoluminescence and significant H2 evolution occur for this unique heterojunction. Furthermore, the excellent durability of the designed photocatalytic heterostructure is confirmed by monitoring the catalysts' H2-evolution rate and crystal structure after 72 h under light illumination. This study opens up promising and simple pathways for constructing efficient S-scheme heterojunctions for photocatalytic water-splitting.

Giant Optical Anisotropy Induced by Magnetic Order in FePS3/WSe2 Heterostructures

Magnetic 2D materials offer a promising platform for manipulating quantum states at the nanoscale. Recent studies have underscored the significant influence of 2D magnetic materials on the optical behaviors of transition-metal dichalcogenides (TMDs), revealing phenomena such as interlayer exciton-magnon interactions, magnetization-dependent valley polarization, and an enhanced Zeeman effect. However, the controlled manipulation of anisotropic optical properties in TMDs via magnetism remains challenging. Here, the magnetic ordering in FePS3 profoundly impacts the optical characteristics of WSe2, achieving a giant linear polarization degree of 5.1 in exciton emission is demonstrated. This is supported by a detailed analysis of low-temperature photoluminescence (PL) and Raman spectra from nL-FePS3/WSe2 heterostructures. These findings indicate that a phase transition in FePS3 from paramagnetic to antiferromagnetic enhances interlayer Coulomb interactions, inducing a transition from non-polar to polar behavior in the heterostructures. Additionally, valley-polarized PL spectra under magnetic fields from -9 to 9 T reveal the influence of FePS3 on valley polarization and Zeeman splitting of excitons in monolayer WSe2. These results present a novel strategy for tailoring the optoelectronic properties of 2D magnetic van der Waals heterostructures, paving the way for advancements in nanoscale device design.

Advanced Characterization of the Spatial Variation of Moiré Heterostructures and Moiré Excitons

In this short review, an overview of recent progress in deploying advanced characterization techniques is provided to understand the effects of spatial variation and inhomogeneities in moiré heterostructures over multiple length scales. Particular emphasis is placed on correlating the impact of twist angle misalignment, nano-scale disorder, and atomic relaxation on the moiré potential and its collective excitations, particularly moiré excitons. Finally, future technological applications leveraging moiré excitons are discussed.

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Electronic Noise Spectroscopy of Quasi-Two-Dimensional Antiferromagnetic Semiconductors

We investigated low-frequency current fluctuations, i.e., electronic noise, in FePS3 van der Waals layered antiferromagnetic semiconductor. The noise measurements have been used as noise spectroscopy for advanced materials characterization of the charge carrier dynamics affected by spin ordering and trapping states. Owing to the high resistivity of the material, we conducted measurements on vertical device configuration. The measured noise spectra reveal pronounced Lorentzian peaks of two different origins. One peak is observed only near the Néel temperature, and it is attributed to the corresponding magnetic phase transition. The second Lorentzian peak, visible in the entire measured temperature range, has characteristics of the trap-assisted generation-recombination processes similar to those in conventional semiconductors but shows a clear effect of the spin order reconfiguration near the Néel temperature. The obtained results contribute to understanding the electron and spin dynamics in this type of antiferromagnetic semiconductors and demonstrate the potential of electronic noise spectroscopy for advanced materials characterization.

Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides

The absorption and emission of light in single-layer transition metal dichalcogenides are governed by the formation of excitonic quasiparticles. Strain provides a powerful technique to tune the optoelectronic properties of two-dimensional materials and thus to adjust their exciton energies. The effects of large compressive strain in the optical spectrum of two-dimensional (2D) semiconductors remain rather unexplored compared to those of tensile strain, mainly due to experimental constraints. Here, we induced large, uniform, biaxial compressive strain (∼1.2%) by cooling, down to 10 K, single-layer WS2, MoS2, WSe2, and MoSe2 deposited on polycarbonate substrates. We observed a significant strain-induced modulation of neutral exciton energies, with blue shifts up to 160 meV, larger than in any previous experiments. Our results indicate a remarkably efficient transfer of compressive strain, demonstrated by gauge factor values exceeding previous results and approaching theoretical expectations. At low temperatures, we investigated the effect of compressive strain on the resonances associated with the formation of charged excitons. In WS2, a notable reduction of gauge factors for charged compared to neutral excitons suggests an increase in their binding energy, which likely results from the effects of strain added to the influence of the polymeric substrate.

Interplay between Optical Emission and Magnetism in the van der Waals Magnetic Semiconductor CrSBr in the Two-Dimensional Limit

The van der Waals semiconductor metamagnet CrSBr offers an ideal platform for studying the interplay between optical and magnetic properties in the two-dimensional limit. Here, we carried out an exhaustive optical characterization of this material by means of temperature- and magnetic-field-dependent photoluminescence (PL) on flakes of different thicknesses down to the monolayer. We found a characteristic emission peak that is quenched upon switching the ferromagnetic layers from an antiparallel to a parallel configuration and exhibits a temperature dependence different from that of the peaks commonly ascribed to excitons. The contribution of this peak to the PL is boosted around 30-40 K, coinciding with the hidden order magnetic transition temperature. Our findings reveal the connection between the optical and magnetic properties via the ionization of magnetic donor vacancies. This behavior enables a useful tool for the optical reading of the magnetic states in atomically thin layers of CrSBr and shows the potential of the design of 2D heterostructures with magnetic and excitonic properties.

Energy transfer and charge transfer between semiconducting nanocrystals and transition metal dichalcogenide monolayers

Nowadays, as a result of the emergence of low-dimensional hybrid structures, the scientific community is interested in their interfacial carrier dynamics, including charge transfer and energy transfer. By combining the potential of transition metal dichalcogenides (TMDs) and nanocrystals (NCs) with low-dimensional extension, hybrid structures of semiconducting nanoscale matter can lead to fascinating new technological scenarios. Their characteristics make them intriguing candidates for electronic and optoelectronic devices, like transistors or photodetectors, bringing with them challenges but also opportunities. Here, we will review recent research on the combined TMD/NC hybrid system with an emphasis on two major interaction mechanisms: energy transfer and charge transfer. With a focus on the quantum well nature in these hybrid semiconductors, we will briefly highlight state-of-the-art protocols for their structure formation and discuss the interaction mechanisms of energy versus charge transfer, before concluding with a perspective section that highlights novel types of interactions between NCs and TMDs.