Thickness-Dependent Interlayer Charge Transfer in MoSe2/MoS2 Heterostructures Studied by Femtosecond Transient Absorption Measurements

Room Temperature, Twist Angle Independent, Momentum Direct Interlayer Excitons in van der Waals Heterostructures with Wide Spectral Tunability

Interlayer excitons (IXs) in van der Waals heterostructures with static out of plane dipole moment and long lifetime show promise in the development of exciton based optoelectronic devices and the exploration of many body physics. However, these IXs are not always observed, as the emission is very sensitive to lattice mismatch and twist angle between the constituent materials. Moreover, their emission intensity is very weak compared to that of corresponding intralayer excitons at room temperature. Here we report the room-temperature realization of twist angle independent momentum direct IX in the heterostructures of bulk PbI2 and bilayer WS2. Momentum conserving transitions combined with the large band offsets between the constituent materials enable intense IX emission at room temperature. A long lifetime (∼100 ns), noticeable Stark shift, and tunability of IX emission from 1.70 to 1.45 eV by varying the number of WS2 layers make these heterostructures promising to develop room temperature exciton based optoelectronic devices.

Defect and interface/surface engineering synergistically modulated electron transfer and nonlinear absorption properties in MoX2 (X = Se, S, Te)@ZnO heterojunction

Systematic interface and defect engineering strategies have been demonstrated to be an effective way to modulate the electron transfer and nonlinear absorption properties in semiconductor heterojunctions. However, the role played by defects and interfacial strain in electron transfer at the interface of the MoX2 (X = Se, S, Te)@ZnO heterojunction remains poorly understood. Herein, using the MoX2@ZnO heterojunction, we reveal that vacancies play a critical role in the interfacial electron transfer of heterojunctions. Specifically, we present the defect and interface engineering of the MoX2@ZnO heterojunction for controlled charge transfer and electron excitation-relaxation. The experimental characterization combined with first-principles calculations showed that the presence of defects promoted the transport of photogenerated carriers at the heterojunction interface, thereby inhibiting their rapid recombination. The DFT calculation confirmed that the electron band structure, density of states and charge density distribution in the system changed after the formation of Mo-O bonds. On the basis of defects and interfacial stress and the effective charge transfer, the MoX2@ZnO heterojunction exhibited excellent excitation and emission behaviors. The nonlinear optical regulation behavior of TMDs is expected to be realized with the help of the defects and interface/surface synergistically modulated effect of ZnO nanoparticles. The local strain generation on the MoX2@ZnO heterojunction boundary provides a new method for the design of new heterogeneous materials and will be of great significance to investigate the contact physical behavior and application of metals and two-dimensional (2D) semiconductors. This work provides some inspiration for the construction of heterojunctions with rich defects and surface/interface charge transfer channels to promote tunable electron transfer dynamics, thereby achieving a good nonlinear optical conversion efficiency and efficient charge separation in optoelectronic functional materials.

Construction of electrochemical biosensors based on MoSe2@1T-MoS2 heterojunction for the sensitive and rapid detection of miRNA-155 biomarker in breast cancer

MiRNA-155 is a typical biomarker for breast cancer. Since its low concentration in the physiological environment and the limitations of conventional miRNA detection methods like Northern imprinting and RT-qPCR, convenient, real-time, and rapid detection methods are urgently needed. In this work, an electrochemical biosensor was constructed based on the flower-like MoSe2@1T-MoS2 heterojunction electrode material and specific RNA recognition probes, which can realize the rapid determination of miRNA-155 content with a wide detection range from 1 fM to 1 nM and a limit of detection (LOD) as low as 0.34 fM. Furthermore, the contents of miRNA-155 in blood samples of tumor-bearing mice and normal mice were measured as 724.93 pM and 21.42 pM, respectively by this biosensor, demonstrating its strong identification ability and miRNA-155 can be regarded as an ideal diagnostic marker. On this basis, a portable sensor platform was designed for on-site detection simulation and showed good recovery efficiency from 95.80% to 98.69%. Meanwhile, compared with the standard detection method RT-qPCR, the accuracy and reliability of the biosensor were verified, indicating that the biosensor has the potential to provide point-of-care testing (POCT) for the early diagnosis of breast cancer.

Ultrasensitive Near-Infrared InAs Colloidal Quantum Dot-ZnON Hybrid Phototransistor Based on a Gradated Band Structure

Amorphous metal oxide semiconductor phototransistors (MOTPs) integrated with colloidal quantum dots (QDs) (QD-MOTPs) are promising infrared photodetectors owing to their high photoconductive gain, low off-current level, and high compatibility with pixel circuits. However, to date, the poor mobility of conventional MOTPs, such as indium gallium zinc oxide (IGZO), and the toxicity of lead (Pb)-based QDs, such as lead sulfide and lead selenide, has limited the commercial applications of QD-MOTPs. Herein, an ultrasensitive QD-MOTP fabricated by integrating a high-mobility zinc oxynitride (ZnON)-based MOTP and lead-free indium arsenide (InAs) QDs is demonstrated. A new gradated bandgap structure is introduced in the InAs QD layer that absorbs infrared light, which prevents carriers from moving backward and effectively reduces electron-hole recombination. Chemical, optical, and structural analyses confirm the movement of the photoexcited carriers in the graded band structure. The novel QD-MOTP exhibits an outstanding performance with a responsivity of 1.15 × 10⁵ A W^-1 and detectivity of 5.32 × 10¹⁶ Jones at a light power density of 2 µW cm^-2 under illumination at 905 nm.

Temporally probing the thermal phonon and charge transfer induced out-of-plane acoustical displacement of monolayer and bi-layer MoS2/GaN heterojunction

Acoustical behavior of semiconducting transition metal dichalcogenides determines the heat transfer pathway, and thus plays a crucial role in the electronics and optoelectronics design. In this research, van der Waals heterojunctions (vdWHs) consisting of transferred monolayer and bi-layer MoS₂ on GaN substrate were studied. We observed an asymmetric bipolar acoustic strain wave with ∼5 ps duration, which describes the surface of substrate undergoing strong compressive deformation after weak tensile deformation in the out-of-plane direction. We developed a theory to explain the mechanisms responsible for the observed strain waveform in the vdWHs elastic system, and obtained the critical parameters of the carrier dynamics by temporal fitting. Our results not only report a coherent acoustic phonon generated in the vdWHs, which will complement our understanding of the thermal transfer at the 2D/substrate interface, but also provide information about the intrinsic properties in the vdWHs, which would benefit the design of the 2D-based devices in the future.

Ultrafast Interlayer Charge Transfer between Bilayer PtSe2 and Monolayer WS2

Interlayer charge transfer (CT) between PtSe₂ and WS₂ is studied experimentally. Layer-selective pump-probe and photoluminescence quenching measurements reveal ultrafast interlayer CT in the heterostructure formed by bilayer PtSe₂ and monolayer WS₂, confirming its type-II band alignment. The CT facilitates the formation of the interlayer excitons with a lifetime of several hundred ps to 1 ns, a diffusion coefficient of 0.9 cm² s^-1, and a diffusion length reaching 200 nm. These results demonstrate the integration of PtSe₂ with other materials in van der Waals heterostructures with novel charge-transfer properties and help develop fundamental understanding on the performance of various optoelectronic devices based on heterostructures involving PtSe₂.

Observation of robust charge transfer under strain engineering in two-dimensional MoS2-WSe2 heterostructures

Strain is one of the effective ways to modulate the band structure of monolayer transition metal dichalcogenides (TMDCs), which has been reported in theoretical and steady-state spectroscopic studies. However, the strain effects on the charge transfer processes in TMDC heterostructures have not been experimentally addressed thus far. Here, we systematically investigate the strain-mediated transient spectral evolutions corresponding to excitons at band-edge and higher energy states for monolayer MoS₂ and monolayer WSe₂. It is demonstrated that Γ and K valleys in monolayer WSe₂ and monolayer MoS₂ present different strain responses, according to the broadband femtosecond pump-probe experimental results. It is further observed that the resulting band offset changes tuned by applied tensile strains in MoS₂-WSe₂ heterostructures would not affect the band-edge electron transfer profiles, where only monolayer WSe₂ is excited. From a flexible optoelectronic applications perspective, the robust charge transfer under strain engineering in TMDC heterostructures is very advantageous.