The Auger process in multilayer WSe2 crystals

Control of Hybrid Exciton Lifetime in MoSe2/WS2 Moiré Heterostructures

Hybrid excitons, characterized by their strong oscillation strength and long lifetimes, hold great potential as information carriers in semiconductors. They offer promising applications in exciton-based devices and circuits. MoSe2/WS2 heterostructures represent an ideal platform for studying hybrid excitons, but how to regulate the exciton lifetime has not yet been explored. In this study, layer hybridization is modulated by applying electric fields parallel or antiparallel to the dipole moment, enabling us to regulate the exciton lifetime from 1.36 to 4.60 ns. Furthermore, the time-resolved photoluminescence decay traces are measured at different excitation power. A hybrid exciton annihilation rate of 8.9 × 10-4 cm2 s-1 is obtained by fitting. This work reveals the effects of electric fields and excitation power on the lifetime of hybrid excitons in MoSe2/WS2 1.5° moiré heterostructures, which play important roles in high photoluminescence quantum yield optoelectronic devices based on transition-metal dichalcogenides heterostructures.

Ultrafast Dynamics of Defect-Assisted Auger Process in PdSe2 Films: Synergistic Interaction between Defect Trapping and Auger Effect

By using optical pump and terahertz probe spectroscopy, we have investigated the photocarrier dynamics in PdSe₂ films with different thicknesses. The experimental results reveal that the photocarrier relaxation consists of two components: a fast component of 2.5 ps that shows the layer-thickness independence and a slow component that has typical lifetime of 7.3 ps decreasing with the layer thickness. Interestingly, the relaxation times for both fast and slow components exhibited both pump fluence and temperature independence, which suggests that synergistic interactions between defect trapping and Auger effect dominate the photocarrier dynamics in PdSe₂ films. A model involving a defect-assisted Auger process is proposed, which can reproduce the experimental results well. The fitting results reveal that the layer-dependent lifetime is determined by the defect density rather than carrier occupancy rate after photoexcitation. Our results underscore the interplay between the Auger process and defects in two-dimensional semiconductors.

Defect-related dynamics of photoexcited carriers in 2D transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit enormous potential in the field of optoelectronics. The high performance of TMD materials and optoelectronic devices significantly depends on processes involved in photoelectric conversion, including photo-excitation, relaxation, transportation, and recombination. Remarkably, inevitable defects in materials prolong or shorten the characteristic time of these processes and even bring about new photoelectric conversion channels, namely, the defect-related relaxation pathways of photoexcited carriers tailor the performance of photoelectric applications. In recent years, there have been numerous investigations in exploring the variant transient signals caused by defects in TMDs utilizing ultrafast spectroscopies. They have the capability in providing an accurate and overall representation of ultrafast processes owing to the subtle temporal resolution. The defect-related mechanisms occurring in different time scales (from femtosecond (fs) to microsecond (μs)) play influential roles throughout the relaxation process of photoexcited species. Herein, we review the defect-related relaxation mechanisms of photoexcited species in TMDs according to the time scale utilizing ultrafast spectroscopy techniques. By interpreting and summarizing the defect-related transient signals, we furnish the direction in material design and performance optimization.

Ultrafast Auger process in few-layer PtSe2

Enhanced many-body interactions due to strong Coulomb interactions and quantum confinement are one of the most prominent features of two-dimensional systems. The Auger process is a representative many-body interaction typically observed in two-dimensional semiconductors, determining important physical properties of materials, such as carrier lifetime, photoconductivity, and emission quantum yield. Recently, platinum dichalcogenides, represented by PtSe2 and PtS2, have attracted great attention due to their superior air stability, thickness-dependent semimetal-to-semiconductor transition, and exotic magnetic characteristics. However, the Auger process in platinum dichalcogenides has not been investigated to date. Here, we utilized ultrafast optical-pump terahertz-probe spectroscopy to explore carrier dynamics in few-layer semiconducting PtSe2. Most of the excited carriers are trapped by defects within ∼10 ps after excitation due to high defect density. We overcome this challenge by raising the excitation intensity to saturate trap sites with carriers, and observed a many-body process involving the carriers that survived the rapid trapping. This process is not band-to-band Auger recombination, but rather defect-assisted Auger recombination in which free carriers interact with trapped carriers at defects. Theoretical simulations show that this three-body Auger process can be approximated as bimolecular recombination at the rate of ∼3.3 × 10-3 cm2 s-1. This work provides insights into the interplay between ultrafast many-body processes and defects in two-dimensional semiconductors.

The highly-efficient light-emitting diodes based on transition metal dichalcogenides: from architecture to performance

Transition metal dichalcogenides (TMDCs) with layered architecture and excellent optoelectronic properties have been a hot spot for light-emitting diodes (LED). However, the light-emitting efficiency of TMDC LEDs is still low due to the large size limit of TMDC flakes and the inefficient device architecture. First and foremost, to develop the highly-efficient and reliable few-layer TMDC LEDs, the modulation of the electronic properties of TMDCs and TMDC heterostructures is necessary. In order to create efficient TMDC LEDs with prominent performance, an in-depth understanding of the working mechanism is needed. Besides conventional structures, the electric (or ionic liquid)-induced p-n junction of TMDCs is a useful configuration for multifunctional LED applications. The significant performances are contrasted in the four aspects of color, polarity, and external quantum efficiency. The color of light ranging from infrared to visible light can be acquired from TMDC LEDs by purposeful and selective architecture construction. To date, the maximum of the external quantum efficiency achieved by TMDC LEDs is 12%. In the demand for performance, the material and configuration of the nano device can be chosen according to this review. Moreover, novel electroluminescence devices involving single-photon emitters and alternative pulsed light emitters can expand their application scope. In this review, we provide an overview of the significant investigations that have provided a wealth of detailed information on TMDC electroluminescence devices at the molecular level.

WSe2 Homojunction p-n Diode Formed by Photoinduced Activation of Mid-Gap Defect States in Boron Nitride

A single nanoflake lateral p-n diode (in-plane) based on a two-dimensional material can facilitate electronic architecture miniaturization. Here, a novel lateral homojunction p-n diode of a single WSe₂ nanoflake is fabricated by photoinduced doping via optical excitation of defect states in an h-BN nanoflake upon illumination. This lateral diode is fabricated using a mechanical exfoliation technique by stacking the WSe₂ nanoflake partially on the h-BN and Si substrates. The carrier type in the part of the WSe₂ film on the h-BN substrate is inverted and a built-in potential difference is formed, ranging from 5.0 to 4.50 eV, which is measured by Kelvin probe force microscopy. The contact potential difference across the junction of p-WSe₂ and n-WSe₂ is found to be ∼492 mV. The lateral diode shows an excellent rectification ratio, up to ∼3.9 × 10⁴, with an ideality factor of ∼1.1. A typical self-biased photovoltaic behavior is observed at the p-n junction upon the illumination of incident light, that is, a positive open-circuit voltage (V_oc) is generated, that is, voltage obtained (at I_ds = 0 V), and also a negative short-circuit current (I_sc) is generated, that is, current obtained (at V_ds = 0 V). The presence of built-in potential in the proposed homojunction diode establishes I_sc and V_oc upon illumination, which can be implemented for a self-powered photovoltaic system in future electronics. The proposed doping technique can be effectively applied to form planar homojunction devices without a photoresist for future electronic and optoelectronic applications.

Neutral and defect-induced exciton annihilation in defective monolayer WS2

As defects and exciton-exciton annihilation (EEA) frequently govern the properties of nanoscale optoelectronic devices based on monolayer transition metal dichalcogenides (TMDCs), understanding the interaction between defects and EEA is of fundamental importance. Here we perform a systematic investigation of the effect of defects on EEA of neutral excitons and defect-bound excitons in monolayer WS2, using fluorescence lifetime imaging technology. Scanning transmission electron microscopy confirms the creation of atomic-scale defects introduced by argon plasma treatment in defective WS2. Defects can bind neutral excitons or trions to form defect-bound excitons. And defects have a slight effect on the lifetime of neutral excitons. However, owing to the impeded exciton diffusion caused by defects, the EEA rate of neutral excitons reduces from 0.26 cm2 s-1 in the pristine monolayer to 0.16 cm2 s-1 in the defective monolayer. For defect-bound excitons, the EEA rate of 0.068 cm2 s-1 is obtained, which results from the localized nature of defect-bound excitons and suppressed exciton diffusion. Our results reveal the important role of defect-EEA interactions in tailoring the properties of monolayer TMDCs.