The Study On The Modulation Of Optical Bandgap In Two-dimensional Perovskites

Two-dimensional perovskites are recently attracting a growing attention for their better environmental stability,naturally formed quantum well structures,layered nature and unique optical properties.Two-dimensional perovskites exhibit rich and tunable optoelectronic properties and might have promising potential applications in solar cells,light emitting devices,photodetectors and lasers.It is crucial to understand the fundamental photophysics of two-dimensional perovskites with pure phase for further device designing and performance improvement.To this end,we present systematic study of the bandgap evolution with the temperature and external strain in two-dimensional perovskites.First,two-dimensional perovskite(n-BA)₂(MA)_n-1Pb_nI_3n+1(n=1-5)and(iso-BA)₂(MA)_n-1Pb_nI_3n+1(n=1-3)were successfully synthesized by hydrothermal method and a series of characterizations have been carried out to ensure the pure phase and stability of sample.Subsequently,we have systematically studied how the organic species and layer number n affect the temperature evolution of the bandgap for 2D perovskites through temperature-dependent photoluminescence(PL)spectroscopy.By establishing a simplified thermal expansion interaction and electron-phonon interaction model,the temperature-dependent bandgap behavior can be attributed to the the competition between the thermal expansion interaction and electron-phonon interaction,both of which depend on the organic species and layer number n.In(n-BA)₂(MA)_n-1Pb_nI_3n+1(n=1-5)series,the thermal expansion interaction and electron-phonon interaction are both gradually enhanced and the former progressively plays a dominant role over the latter from n=1 to n=5,resulting in the bandgap versus temperature changing from a red-shift to a blue-shift.In contrast,both those two factors show a weaker layer thickness dependence,leading to the monotonous blue-shift in(iso-BA)₂(MA)_n-1Pb_nI_3n+1(n=1-3)series.In addition to temperature,strain engineering also provides an efficient strategy to modulate the fundamental properties of semiconductor.The strain was introduced in the two-dimensional perovskite(n-BA)₂(MA)_n-1Pb_nI_3n+1(n=1-3)sheets by the substrate with nanopillars.Then,the effects of strain on its band gap,anisotropy,stability and phase transition were systematically studied by PL spectra and fluorescence images.Upon applying the external strain,the bandgap decreases with a rate of-5.60/-2.74/-1.38 me V/%for n=1,2,3 two-dimensional perovskite,respectively.This change of the bandgap can be ascribed to the distortion of the octahedrons in two-dimensional perovskites,supported by the study of emission anisotropy which increases with the increase of strain.In addition,the external strain can significantly deteriorate the stability of two-dimensional perovskites due to the strain induced distortion which would make it easier for moisture and oxygen to be penetrated into the perovskite microplates,resulting in the much faster degradation rates.Finally,through the study of temperature-dependent PL spectra of(n-BA)₂Pb I₄ under strain,it was found that the changed lattice caused by strain results in the increased ratio of the low temperature phase to the high temperature phase,thereby inhibiting the(n-BA)₂Pb I₄ phase transition.In summary,we systematically studied the modulated bandgap in two-dimensional perovskites by temperature and strain,revealing its modulated mechanism and micro-physical processes.Our research not only played a significant role in understanding the basic optical properties of two-dimensional perovskites,but also provided a new perspective for improving the device design and performance improvement of two-dimensional perovskite.