Nonlinear Optical Properties Of Two-Dimensional Bismuthene Under Space Environment

The ever-increasing space exploration activities of China's aerospace industry demand urgently for novel radiation-tolerant materials,among which nonlinear optical materials and devices are particularly scant.As a new research field,many properties of two-dimensional materials have not been fully studied,especially their nonlinear optical and quantum optical properties.Although more and more new two-dimensional materials have emerged,there are still many difficulties in synthesizing high-quality materials and the technological maturity is not high.Here we developed a hydrothermal method to obtain Bi nanosheets and exposed them to two most commonly used and representative radiation sources,and compared their third-order nonlinear optical responses before and after irradiation by using ns and fs Z-scan techniques.By monitoring the time-resolved carrier dynamics of bismuthene before and after irradiation,transient absorption spectroscopy provides new insight into the damage mechanism unavailable via conventional characterization techniques.Overall,the irradiation damage to the Bi nanosheets caused by ⁶⁰Co?-rays and electrons is small,and the Bi nanosheets have been proved to be a promising radiation-resistant material.The work mainly includes the following two parts:(1)The third-order nonlinear optical properties of two-dimensional bismuthene under space radiation environment.We synthesized Bi nanosheets with a thickness of 8.7 nm and exposed them to ⁶⁰Co?-rays and electron radiation.After radiation,Raman spectroscopy disclosed E_g and A_1g mode had slightly redshifts of about 2 cm^-1,accompanied by the broadening of full widths at the half maxima;X-ray powder diffraction patterns showed the width of the diffraction peak remained the same and neither peak shift nor intensity change was observed;X-ray photoelectron spectroscopy found although there was no displacement of respective peaks,the area ratio of Bi₂O₃/Bi decreased.These characterization techniques showed radiation only caused slight damage.Their third-order optical nonlinear responses on ns and fs time scale,only reduced by about 22.2%and 36.9%respectively before and after radiation.The excellent properties of bismuthene still maintained,indicative of promising anti-radiation use.(2)The radiation effect and damage mechanism of two-dimensional bismuthene investigated by time-resolved spectroscopy.Combining femtosecond transient absorption and laser flash photolysis spectroscopy,the carrier dynamics and the origin of the saturable absorption of Bismuthene can be disclosed.Due to the small bandgap of the bismuthene(0.96 e V),a laser pulse with the appropriate frequency pumped electrons from the VB edge to the CB edge and form photocarriers on the ultrafast fs timescale.When the pump intensity is strong enough to generate a large number of electrons and holes to fill the edges of CB and VB,respectively,further light absorption is prevented due to Pauli blocking and leads to the observed saturable absorption behaviors of bismuthene.The photogenerated carriers redistribute through several relaxation channels:carrier trapping(88 ps)and recovery time under the mediation of the trap state(528 ns and 14?s).After irradiation,⁶⁰Co?-rays and electron irradiation mainly caused ionization damage and produced considerably more defects in bismuthene.Based on the time-resolved spectra,the lifetime of carrier trapping decreased after the nanosheets were irradiated(from 88 ps to?60 ps),while the relaxation of carriers to the ground state became significantly longer(from 528 ns to>600 ns).