Defect states play an important role in the photovoltaic performance of metal halide perovskites. Particularly, the passivation of surface defects has made great contributions to high‐performance perovskite photovoltaics. This highlights the importance of understanding the surface defects from a fundamental level by developing more accurate and operando characterization techniques. Herein, a strategy to enable the surface carriers and photocurrent distributions on perovskite films to be visualized in the horizontal direction is put forward. The visual image of photocurrent distribution is realized by combining the static local distribution of carriers provided by scanning near‐field optical microscopy with the dynamic transporting of carriers achieved via a scanning photocurrent measurement system. Taking a surface passivated molecule as an example, a comprehensive defect scene including static and dynamic as well as local and entire conditions is obtained using this strategy. The comprehensive analysis of the trap states in perovskite films is pioneered vertically and horizontally, which will powerfully promote the deep understanding of defect mechanisms and carrier behavior for the goal of fabricating high‐performance perovskite optoelectronic devices.
complex device functions, it is desirable to develop polarization-sensitive 2D materials or hybrid structures which will boost the development of smart optoelectronic devices, e.g., polarized photodetectors [7,8] or light emitting diodes. [9] Group-VI transition metal dichalcogenide (TMDC) monolayers, with the form of MX 2 (M = Mo, W and X = S, Se, etc.), possess two energy-degenerate but inequivalent valleys (K and -K) in the six corners of the Brillouin zone, which respond differently to the left-and rightcircularly polarized light. [10] However, the valley polarization (defined as circular dichroism) is typically quite low, especially at room temperature, owing to the effects such as electron-hole exchange interaction and phonon-assisted intervalley scattering. [11,12] To gain an obvious valley polarization, resonant excitation condition and cryogenic temperature are required, which largely limits the potential for practical applications. Similar requests also apply to a decent valley coherence for TMDC materials, in which linearly polarized light emission can be obtained as a result of coherent superposition of circularly polarized photon emission from K and -K valleys. [13] Different from 2H phase Group-VI TMDC, there are also 2D materials with an anisotropic crystal structure which naturally leads to polarization-sensitive absorption or emission properties. [14,15] For example, orthorhombic black phosphorus (BP) shows higher absorption and photoluminescence (PL) along the armchair direction than along the zigzag direction. [16,17] Meanwhile, germanium selenide (GeSe) also has the orthorhombic crystal structure which enables anisotropic absorption with a maximum anisotropic absorption ratio of 3.02 at 808 nm. [18] Currently, the limitations of these anisotropic 2D semiconductors mainly lie in two points: 1) Some of the materials, e.g., BP, suffers from low chemical stability so that the preparation and fabrication procedures have to be conducted in an inert atmosphere; 2) The anisotropic ratio for both absorption and PL emission is usually low and needs further enhancement towards practical applications.2D materials are famous for their flexibility and convenience for integration with other materials or nanostructures. By coupling monolayer semiconductors to plasmonic or dielectric As contemporary star materials, 2D monolayer semiconductors have drawn huge research interests owing to their striking electrical and optical properties, rendering them ideal candidates as building blocks for novel optoelectronic devices. Towards light emitting devices with extended functions, it is necessary to manipulate the polarization of light emission from monolayer semiconductors. However, most of these monolayer semiconductors exhibit no or very limited polarization sensitivity inherited from their structural anisotropy, making it challenging to develop highly polarized light sources. Herein, by embedding monolayer tungsten diselenide (WSe 2 ) in a nanowireon-film nanocavity, highly polarized light emission is demonstrated...
Vibrational strong coupling (VSC), the strong coupling between optical resonances and the dipolar absorption of molecular vibrations at mid‐infrared frequencies, holds the great potential for the development of ultrasensitive infrared spectroscopy, the modification of chemical properties of molecules, and the control of chemical reactions. In the realm of ultracompact VSC, there is a need to scale down the size of mid‐infrared optical resonators and to elevate their optical field strength. Herein, by using single quartz micropillars as mid‐infrared optical resonators, the strong coupling is demonstrated between surface phonon polariton (SPhP) resonances and molecular vibrations from far‐field observation. The single quartz micropillars support sharp SPhP resonances with an ultrasmall mode volume, which strongly couples with the molecular vibrations of 4‐nitrobenzyl alcohol (C7H7NO3) molecules featuring pronounced mode splitting and anticrossing dispersion. The coupling strength depends on the molecular concentration and reaches the strong coupling regime with only 7300 molecules. The findings pave the way for promoting the VSC sensitivity, miniaturing the VSC devices, and will boost the development of ultracompact mid‐infrared spectroscopy and chemical reaction control devices.
Transition metal dichalcogenide (TMD)‐based 2D monolayer semiconductors, with the direct bandgap and the large exciton binding energy, are widely studied to develop miniaturized optoelectronic devices, e.g., nanoscale light‐emitting diodes (LEDs). However, in terms of polarization control, it is still quite challenging to realize polarized electroluminescence (EL) from TMD monolayers, especially at room temperature. Here, by using Ag nanowire top electrode, polarized LEDs are demonstrated based on 2D monolayer semiconductors (WSe2, MoSe2, and WS2) at room temperature with a degree of polarization (DoP) ranging from 50% to 63%. The highly anisotropic EL emission comes from the 2D/Ag interface via the electron/hole injection and recombination process, where the EL emission is also enhanced by the polarization‐dependent plasmonic resonance of the Ag nanowire. These findings introduce new insights into the design of polarized 2D LED devices at room temperature and may promote the development of miniaturized 2D optoelectronic devices.
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