Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics

Severalhybrid halide 2D-perovskite species emit light with anemergent and controversial broadband emission Stokes-shifted downfrom the narrow band emission. This paper uncovers the sub- and above-bandgapemission and absorption characteristics of PEA(2)PbI(4) prepared with gap states introduced during single crystalgrowth. Here, gap states led to coexistent intrinsic and heterostructuredelectronic frameworks that are selectively accessible with ultraviolet(UV) and infrared (IR) light, respectively, resulting in the phenomenonof photoluminescence (PL) switching from narrowband green to broadbandred. Electron-energy dependent cathodoluminescence shows a relativeincrease in the broadband red PL intensity as the electron penetrationdepth increases from 30 nm to 2 & mu;m, confirming the heterostructuredframework is formed in the bulk of the crystal. Excitation-emissionpower slope of 2.5 and up-conversion pump transient absorption (TA)spectra suggest that the IR up-conversion excitation with red photoluminescence,peaked at 655 nm, is a multiphoton process occurring in the heterostructuredframework through a nonlinear optical response. The energetic pathwaystoward the dual emission bands are revealed by pump-probe transientabsorption spectroscopy, showing energetically broad gap states withhigh sensitivity to an IR pump are upconverted and subsequently quicklyrelax from high to low energy levels within 4 ps. Furthermore, theup-conversion red PL demonstrates a linear polarization with magneticfield effects, thus affirming that the band-like heterostructuredframework is crystallographically aligned with characteristics ofspatially extended charge-transfer states.

Automated summary

This paper investigates the emission and absorption characteristics of PEA(2)PbI(4) with gap states introduced during single crystal growth. It uncovers the switching phenomenon from narrowband green to broadband red photoluminescence, confirming the formation of a heterostructured electronic framework in the crystal.