Optical Properties and Photocarrier Dynamics of Bi₂O₂Se Monolayer and Nanoplates

Abstract

A comprehensive experimental study on optical properties and photocarrier dynamics in Bi2O2Se monolayers and nanoplates is presented. Large and uniform Bi2O2Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time‐resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm2 s−1 is obtained, corresponding to a mobility of about 180 cm2 V−1 s−1. A similar direct transition is also observed in monolayer Bi2O2Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm2 s−1.

Excimer Formation in the Non‐Van‐Der‐Waals 2D Semiconductor Bi2O2Se

“…Similar to previous reports, [43,48] we can extract the annihilation rate (γ) using a linear fitting to the N 0 dependent slope (γ × N 0 ) and an annihilation rate of about 1.11 × 10 −7 cm 3 s −1 is obtained, implying an excimer diffusion coefficient (D) of ≈0.07 cm 2 s −1 based on the relation γ = 8πD × R c (where R c is the capture radius which we assume takes on a value of ≈0.61 nm corresponding to the interlayer distance). [27,42] We have noted that the D value in 18-layer Bi 2 O 2 Se nanosheets is about one order of magnitude lower than that of excitons without signatures of excimer [22,50] since excimers are additionally dressed by interlayer structural changes.…”

“…In addition to the identification of excimers, temporal kinetics of GSB and ESA features have also been fitted by exponential functions convoluted with a Gaussian instrument response function (IRF, standard deviation σ = 100 fs) [39,40] to resolve the life span of excimers. The GSB dynamics in Figure 2c exhibit a rapid hot-carrier cooling (≈0.57 ps) to generate excitons [22] and two different recovery processes, in which the fast recovery component (≈1.46 ps) is close to the growth dynamics (≈1.23 ps) of ESA, implying the excimer formation. It is critical to note that the contribution of the fast channel is overwhelming the GSB recovery process and the PL spectrum only shows the signature of excimer emission, indicating that the primary decay channel of photoexcited excitons in the 18-layer Bi 2 O 2 Se nanosheet is excimer formation.…”

“…According to the band structure obtained by first-principles calculations, the Bi 2 O 2 Se nanosheet is an indirect semiconductor with the valence band maxima at the X point and the conduction band minima at the Γ point, [7,16,22] as shown in Figure 1d. Therefore, the optical absorption edge at ≈721 nm (i.e., the minimum transition energy in the linear absorption case) can be assigned as the X−Γ indirect transition while the strongest absorption at ≈596 nm should be attributed to the Γ−Γ direct transition, which has been theoretically predicted with the highest oscillation strength.…”

High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber

“…Therefore, it is urgent to explore the capacity of Bi 2 O 2 Se SA to generate ultrafast pulses in all-solid-state lasers. More importantly, the longer carrier lifetime (~ 200 ps) 24 will significantly hinder the realization of mode-locked lasers with high output and ultrashort pulses based on 2D Bi 2 O 2 Se SA. Therefore, a straightforward and effective approach to directly modulate the carrier dynamics and nonlinear absorption properties in 2D Bi 2 O 2 Se SA is desired.…”

Emerging two‐dimensional bismuth oxychalcogenides for electronics and optoelectronics

“…In addition, the ultrafast dynamics and optical nonlinearity behavior of atomically thin Bi 2 O 2 Se have been investigated by pump-probe technique (Figure 10(A)). 124,125 The 2D Bi 2 O 2 Se exhibits picosecond response and saturable absorption in a broadband spectral range from 1.55 to 5.0 μm, as shown in Figure 10(B, C), which makes it a promising nonlinear optical material in near-infrared and mid-infrared region. To this end, Qiu and coworkers reported a Q-switch laser at 3.0 μm by using an optical switch based on Bi 2 O 2 Se to generate pulsed laser (Figure 10(D)).…”