Tunneling Photocurrent Assisted by Interlayer Excitons in Staggered van der Waals Hetero‐Bilayers

Vertically stacked van der Waals (vdW) heterostructures have been suggested as a robust platform for studying interfacial phenomena and related electric/optoelectronic devices. While the interlayer Coulomb interaction mediated by the vdW coupling has been extensively studied for carrier recombination processes in a diode transport, its correlation with the interlayer tunneling transport has not been elucidated. Here, a contrast is reported between tunneling and drift photocurrents tailored by the interlayer coupling strength in MoSe₂/MoS₂ hetero‐bilayers (HBs). The interfacial coupling modulated by thermal annealing is identified by the interlayer phonon coupling in Raman spectra and the emerging interlayer exciton peak in photoluminescence spectra. In strongly coupled HBs, positive photocurrents are observed owing to the inelastic band‐to‐band tunneling assisted by interlayer excitons that prevail over exciton recombinations. By contrast, weakly coupled HBs exhibit a negative photovoltaic diode behavior, manifested as a drift current without interlayer excitonic emissions. This study sheds light on tailoring the tunneling transport for numerous optoelectronic HB devices.     

新型有机电致荧光材料研究进展

有机发光二极管因独特的优势被看作新一代的照明及平面显示技术,引起了研究人员的广泛关注。传统的荧光材料仅能利用单重态激子发光,因而效率并不理想。近年来,能够利用三重态激子能量发光的新型荧光材料的研究实现了新的突破。按照三重态激子到单重态激子的转化机理,荧光材料可以分为三重态-三重态湮灭、热致延迟荧光和局域电荷转移杂化激发态三种特殊类型。本文围绕着这几种类型的荧光材料展开了探讨,介绍了有机电致荧光器件的概况以及不同类型荧光材料的发光机理,并从分子设计的角度说明了高性能发光器件的设计思路。     

Defect Engineering in Few‐Layer Phosphorene

Defect engineering in 2D phosphorene samples is becoming an important and powerful technique to alter their properties, enabling new optoelectronic applications, particularly at the infrared wavelength region. Defect engineering in a few‐layer phosphorene sample via introduction of substrate trapping centers is realized. In a three‐layer (3L) phosphorene sample, a strong photoluminescence (PL) emission peak from localized excitons at ≈1430 nm is observed, a much lower energy level than free excitonic emissions. An activation energy of ≈77 meV for the localized excitons is determined in temperature‐dependent PL measurements. The relatively high activation energy supports the strong stability of the localized excitons even at elevated temperature. The quantum efficiency of localized exciton emission in 3L phosphorene is measured to be approximately three times higher than that of free excitons. These results could enable exciting applications in infrared optoelectronics.     

Experimental Evidence of Anisotropic and Stable Charged Excitons (Trions) in Atomically Thin 2D ReS2

Experimentally observed, stable trions with large binding energy (≈25 meV) in atomically thin monolayer 2D transition metal dichalcogenides MX₂ (M = Mo, W, X = S, Se, and Te) with an isotropic crystal structure have been extensively studied. In contrast, the characteristics of trions in atomically thin 2D materials with an anisotropic crystal structure are not completely understood. Low‐temperature photoluminescence (PL) spectroscopy in few‐layer ReS₂ with an anisotropic crystal structure by applying a gate voltage is described. A new PL peak that emerges below the lower‐energy side of neutral excitons obtained by tuning the gate voltages is attributed to emission from negative trions. Furthermore, the trion binding energy that is strongly dependent on the layer thickness reaches a large value of ≈60 meV in 1L–ReS₂, which is ≈2 times larger than that in other isotropic 2D materials (MX₂). The enhancement of the binding energy reflects the quasi‐1D nature of the trions in anisotropic atomically thin ReS₂. These experimental observations will promote a better understanding of the optical response and applications in new categories of the anisotropic atomically thin 2D materials with a quasi‐1D nature.     

High‐Performance Monolayer MoS2 Films at the Wafer Scale by Two‐Step Growth

To realize multifunctional devices at the wafer scale, the growth process of monolayer (ML) 2D semiconductors must meet two key requirements: 1) growth of continuous and homogeneous ML film at the wafer scale and 2) controllable tuning of the properties of the ML film. However, there is still no growth method available that fulfills both of these criteria. Here, the first report is presented on the preparation of continuous and uniform ML MoS₂ films through a two‐step process at the wafer scale. Unlike in previous ML MoS₂ film growth processes, the ML MoS₂ film can be uniformly modulated across the wafer in terms of material structure and composition, exciton state, and electronic transport performance. A significant result is that the high‐quality wafer‐scale ML MoS₂ films realize superior electronic performance compared to reported two‐step‐grown films, and it even matches or exceeds reported ML MoS₂ films prepared by other processes. The transistor performance of the optimized ML film achieves a field effect mobility of 10 to 30 cm² V^?1 s^?1, an on/off current ratio of about 10⁷, and hysteresis as low as 0.4 V.     

Broadband Extrinsic Self‐Trapped Exciton Emission in Sn‐Doped 2D Lead‐Halide Perovskites

As emerging efficient emitters, metal‐halide perovskites offer the intriguing potential to the low‐cost light emitting devices. However, semiconductors generally suffer from severe luminescence quenching due to insufficient confinement of excitons (bound electron–hole pairs). Here, Sn‐triggered extrinsic self‐trapping of excitons in bulk 2D perovskite crystal, PEA₂PbI₄ (PEA = phenylethylammonium), is reported, where exciton self‐trapping never occurs in its pure state. By creating local potential wells, isoelectronic Sn dopants initiate the localization of excitons, which would further induce the large lattice deformation around the impurities to accommodate the self‐trapped excitons. With such self‐trapped states, the Sn‐doped perovskites generate broadband red‐to‐near‐infrared (NIR) emission at room temperature due to strong exciton–phonon coupling, with a remarkable quantum yield increase from 0.7% to 6.0% (8.6 folds), reaching 42.3% under a 100 mW cm^?2 excitation by extrapolation. The quantum yield enhancement stems from substantial higher thermal quench activation energy of self‐trapped excitons than that of free excitons (120 vs 35 meV). It is further revealed that the fast exciton diffusion involves in the initial energy transfer step by transient absorption spectroscopy. This dopant‐induced extrinsic exciton self‐trapping approach paves the way for extending the spectral range of perovskite emitters, and may find emerging application in efficient supercontinuum sources.     

New Light on Molecule–Nanotube Hybrids

Optoelectronics benefits from outstanding new nanomaterials that provide emission and detection in the visible and near‐infrared range, photoswitches, two level systems for single photon emission, etc. Among these, carbon nanotubes are envisioned as game changers despite difficult handling and control over chirality burdening their use. However, recent breakthroughs on hybrid carbon nanotubes have established nanotubes as pioneers for a new family of building blocks for optics and quantum optics. Functionalization of carbon nanotubes with molecules or polymers not only preserves the nanotube properties from the environment, but also promotes new performance abilities to the resulting hybrids. Photoluminescence and Raman signals are enhanced in the hybrids, which questions the nature of the electronic coupling between nanotube and molecules. Furthermore, coupling to optical cavities dramatically enhances single photon emission, which operates up to room temperature. This new light on nanotube hybrids shows their potential to push optoelectronics a step forward.     

Tuning the Excitonic States in MoS2/Graphene van der Waals Heterostructures via Electrochemical Gating

The behavior of excitons in van der Waals (vdWs) heterostructures depends on electron–electron interactions and charge transfer at the hetero‐interface. However, what still remains to be unraveled is to which extent the carrier densities of both counterparts and the band alignment in the vdWs heterostructures determine the photoluminescence properties. Here, we systematically study the photoluminescence properties of monolayer MoS₂/graphene heterostructures by modulating the carrier densities and contact barrier at the interface via electrochemical gating. It is shown that the PL intensities of excitons can be tuned by more than two orders of magnitude, and a blue‐shift of the exciton peak of up to 40 meV is observed. By extracting the carrier density of MoS₂ using an electric potential distribution model, and the Schottky barrier using first‐principle calculations, we find that the controllable carrier density in MoS₂ plays a dominant role in the PL tuning at negative gate bias, whereas the interlayer relaxation of excitons induced by the Schottky barrier has a major contribution at positive gate bias. This is further verified by controlling the tunneling barrier and screening field across MoS₂ by inserting self‐assembled monolayers (SAMs) at the interface. These findings will benefit to better understand the effect of many‐body interactions and hetero‐interfaces on the optical and optoelectronic properties in vdWs heterostructures.