Observation of room‐temperature out‐of‐plane switchable electric polarization in supported 3R‐MoS_(2) monolayers

Two‐dimensional(2D)ferroelectrics have attracted considerable attention due to their potential in the development of devices of miniaturization and multifunction.Although several van der Waals(vdW)‐layered materials show ferroelectricity,the experimental demonstrations of ferroelectric behavior in monolayers are very limited.Here we report the observation of room‐temperature out‐of‐plane switchable electric polarization in supported MoS_(2) monolayers exfoliated from 3R‐stacked bulk crystals under ambient conditions.Using in situ piezoelectric force microscopy and Kelvin probe force microscopy in a glovebox,we reveal that trapped water/ice molecules are responsible for this switchable electric polarization and this conclusion is strongly supported by theoretical simulations.It is worth noting that the water/ice trapping in the monolayers exfoliated from 2H‐stacked MoS_(2) crystals is not as much as that in 3R monolayers and,consequently,the out‐of‐plane electric polarization is missing there.Our findings indicate that monolayers with a trapped single layer of polar molecules might be emerging alternatives to 2D ferroelectrics.Furthermore,the stacking sequences may bring new properties and applications to 2D vdW materials not only when we stack them up but also when we thin them down.

Direct observation of ultrafast plasmonic hot electron transfer in the strong coupling regime

Achieving strong coupling between plasmonic oscillators can significantly modulate their intrinsic optical properties.Here,we report the direct observation of ultrafast plasmonic hot electron transfer from an Au grating array to an MoS_(2) monolayer in the strong coupling regime between localized surface plasmons(LSPs)and surface plasmon polaritons(SPPs).By means of femtosecond pump-probe spectroscopy,the measured hot electron transfer time is approximately 40 fs with a maximum external quantum yield of 1.65%.Our results suggest that strong coupling between LSPs and SPPs has synergetic effects on the generation of plasmonic hot carriers,where SPPs with a unique nonradiative feature can act as an‘energy recycle bin’to reuse the radiative energy of LSPs and contribute to hot carrier generation.Coherent energy exchange between plasmonic modes in the strong coupling regime can further enhance the vertical electric field and promote the transfer of hot electrons between the Au grating and the MoS_(2) monolayer.Our proposed plasmonic strong coupling configuration overcomes the challenge associated with utilizing hot carriers and is instructive in terms of improving the performance of plasmonic opto-electronic devices.

Dynamical evolution of anisotropic response of type-Ⅱ Weyl semimetal TalrTe_(4) under ultrafast photoexcitation

Layered type-ⅡWeyl semimetals,such as WTe_(2)/MoTe_(2),and TalrTe_(4)have been dem on strated as a supreme photodetection material with topologically enhanced responsivity and specific sensitivity to the orbital angular momentum of light.Toward future device applications with high performance and ultrafast response,it is necessary to understand the dynamical processes of hot carriers and transient electronic properties of these materials under photoexcitation.In this work,mid-infrared ultrafast spectroscopy is performed to study the dynamical evolution of the anisotropic response of TalrTe_(4).The dynamical relaxation of photoexcited carriers exhibits three exponential decay components relating to optical/acoustic phonon cooling and subsequent heat transfer to the substrate.The ultrafast transient dynamics imply that TalrTe_(4)is an ideal material candidate for ultrafast optoelectronic applications,especially in the Iong-wavelength region.The angle-resolved measurement of transient reflection reveals that the reflectivity becomes less anisotropic in the quasi-equilibrium state,indicating a reduction in the anisotropy of dynamical conductivity in presence of photoexcited hot carriers.The results are indispensable in material engineering for polarization-sensitive optoelectronics and high field electronics.