Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS/WS bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.

wileyonlinelibrary.com2D materials. [8][9][10][11][12][13][14][15] As a family member of 2D materials, MoS 2 becomes an attractive hot electron acceptor due to its sizable bandgaps around 1-2 eV and internal photogain with various traps at the interfaces. [16][17][18][19][20] The light harvesting is crucial to achieve high quantum effi ciency of devices. A high plasmon to hot electron conversion effi ciency ≈35% was reported when the scanning probe technique was combined to the detection. [ 21 ] Metal nanostructures are generally regarded as ideal light acceptors, owning to the excitation of surface plasmons (SPs) which can confi ne and manipulate light at the nanoscale, and further applied for the photodetection based on the plasmonic hot electrons. [22][23][24][25][26] After SPs are excited, the energy decays by either radiatively into photons or nonradiatively into hot electrons. Besides the quantum yield, the response rate of photodetector is another vital character for devices, which depends on drift time, diffusion time, and RC time constant. Considering the atomic thickness of MoS 2 , the response rate of an MoS 2 -based photodetector is mainly depended on the drift time of photocarriers in the interface, which indicates that the dynamics of charge transfer between metal and MoS 2 plays an important role in the applications of metal-semiconductor heterojunction.Moreover, the investigation of charge transfer dynamics in the metal-semiconductor interface can be utilized to improve the performance of optoelectronic devices, while the direct experimental observation of ultrafast charge transfer in photoexcited metal nanostructures/MoS 2 heterostructures has not been reported. In addition, due to the limitation of synthesis of large-area MoS 2 , fabricating metal nanostructures on or under the surface of MoS 2 is generally completed by physical preparation techniques, such as electron beam lithography, focused ion beam lithography, and photolithography etc., which are all highcost and complicated. Template electrochemical method used for producing metal nanostructures is a low-cost, high productivity, and large-area fabrication technique. [27][28][29] Therefore, we proposed a template-based sputtering method to fabricate various metallic nanostructures such as nanorod arrays. [ 30 ] An MoS 2 photodetector based on metal nanostructures prepared by this means is supposed to be more attractive compared with other physical preparation methods.2D transition metal dichalcogenides are becoming attractive materials for novel photoelectric and photovoltaic applications due to their excellent optoelectric properties and accessible optical bandgap in the near-infrared to visible range. Devices utilizing 2D materials integrated with metal nanostructures have recently emerged as effi cient schemes for hot electron-based photodetection. Metal-semiconductor heterostructures with low cost, simple procedure, and fast response time are crucial for the practical applications of optoelectric devices. In this paper, template-based ...

Over the past years, broadband achromatic metalenses have been intensively studied due to their great potential for applications in consumer and industry products. Even though significant progress has been made, the efficiency of technologically relevant silicon metalenses is limited by the intrinsic material loss above the bandgap. In turn, the recently proposed achromatic metalens utilizing transparent, high-index materials such as titanium dioxide has been restricted by the small thickness and showed relatively low focusing efficiency at longer wavelengths. Consequently, metalens-based optical imaging in the biological transparency window has so far been severely limited. Herein, we experimentally demonstrate a polarization-insensitive, broadband titanium dioxide achromatic metalens for applications in the near-infrared biological imaging. A large-scale fabrication technology has been developed to produce titanium dioxide nanopillars with record-high aspect ratios featuring pillar heights of 1.5 µm and ~90° vertical sidewalls. The demonstrated metalens exhibits dramatically increased group delay range, and the spectral range of achromatism is substantially extended to the wavelength range of 650–1000 nm with an average efficiency of 77.1%–88.5% and a numerical aperture of 0.24–0.1. This research paves a solid step towards practical applications of flat photonics.