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

Abstract

The layered semiconductor Bi2O2Se is a promising new‐type 2D material that holds layered structure via electrostatic forces instead of van der Waals (vdW) attractions. Aside from the huge success in device performance, the non‐vdW nature in Bi2O2Se with a built‐in interlayer electric field has also provided an appealing platform for investigating unique photoexcited carrier dynamics. Here, experimental evidence for the observation of excimers in multilayer Bi2O2Se nanosheets via transient absorption spectroscopy is presented. It is found that the excimer formation is the primary decay pathway of photoexcited excitons and three‐stage excimer dynamics with corresponding time scales are established. Excitation‐fluence‐dependent excimer dynamics further suggest that the excimer is diffusive and its formation can be simply described as excitons relaxed to an excimer geometry. This work indicates the outstanding promise of unique excitonic processes in Bi2O2Se, which may motivate novel device designs.

Room temperature exciton formation and robust optical properties of CVD-grown ultrathin Bi2O2Se crystals on arbitrary substrates

“…Due to the close-lying valleys in the band structure, the possible optical transitions/exciton formation are at Γ VB (outer)-Γ CB (outer), Γ VB (inner)-Γ CB (outer), Γ VB (inner)-Γ CB (outer), M CB -M VB (outer/inner), X VB -Γ CB (inner), R VB -Γ CB (inner), R VB -M CB (outer/inner), and Γ VB (outer/inner)-M CB (outer/inner), in the range of 400 to 725 nm according to the theoretical electronic band structure. 11,35 In our case, the broad absorption peak in the range from ∼590 to 750 nm could be assigned to the combined effect of R-M, X-M, and Γ-Γ point transitions. The peak at 486 nm (2.55 eV) is possibly caused by M VB -M CB (outer) and Γ VB (inner)-Γ CB (inner) transitions, and the peak at 456 nm (2.72 eV) is mainly contributed by M VB -M CB (inner) point transitions.…”

“…To validate our experimental observation, we have calculated the electronic band structure of bulk Bi 2 O 2 Se by density functional theory (DFT) (see Fig. 4d 35 the internal geometry of Bi 2 O 2 Se changes during the excited state due to the built-in dipolar electric field along the stacking direction, making us rescale the conduction band valleys. Interestingly, after rescaling (an upshift of 0.3835 eV and 0.1704 eV of outer and inner CB valleys, respectively), the energy gaps between the VB and CB at different symmetry points match very well with the experimental results of different exciton formation.…”

“…Yu et al reported that the excimer formation causes a broad absorption band (550-740 nm) due to the stacking of positively charged [Bi 2 O 2 ] 2+ and negatively charged Se 2− layers with covalent bonds creating a built-in dipolar electric field along the stacking direction in ultrathin Bi 2 O 2 Se crystals, which basically modifies the excited states under light irradiation. 35 The light-modified excited state was termed the excimer state. Interestingly, the close-lying electronic states in the band structure can give rise to broad absorption with multiple broad/narrow peaks, which is con-sistent with our observations (Table S2 †) explained below.…”

“…The broad PL feature of Bi 2 O 2 Se may be partly associated with the excimer geometry (i.e., internal structural change during the excited state). 35 For the quantitative analysis of different excimer excitons and to corroborate the exciton formation with PL contribution, we deconvoluted the differential reflectance and PL spectra with three Gaussian peaks, as shown in Fig. 5b.…”

Defect-induced photogating effect and its modulation in ultrathin free-standing Bi2O2Se nanosheets with a visible-to-near-infrared photoresponse

“…However, further investigations are necessary to bridge the gap between synthesis and applications in an orderly manner by unraveling the hidden properties of Bi 2 O 2 Se nanostructures. Many inherent properties, such as the bolometric effect, 24 low lattice thermal conductance, [25][26][27] high carrier mobility, 28,29 room-temperature ferroelectricity, 18,30 and excimer formation, 31 have been reported in semiconducting Bi 2 O 2 Se.…”