Optically Induced Coherent Phonons in Bismuth Oxyiodide (BiOI) Nanoplatelets

Abstract

Bismuth oxyiodide (BiOI) is a promising material for photocatalysis combining intriguing optical and structural properties. We show that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. We find that the two underlying frequencies originate from lattice vibrations along the [001] crystallographic axis, the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Further, we determine the two major dephasing mechanisms that lead to the loss of vibronic coherence: (i) the anharmonic decay of an optical phonon into two acoustic phonons and (ii) phonon-carrier scattering. Our results provide a direct demonstration of the presence of an electric field in BiOI along the [001] axis and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

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

“…[45] However, it has long been known that excimer is diffusive if such an entity can be described by an exciton relaxed to an excimer geometry (i.e., the so-called excimer exciton). [42,46,47] We propose here that the "excimer exciton" scenario is consistent with the existence of a built-in dipolar electric field in Bi 2 O 2 Se nanosheets and its ultrafast screening by the optically injected charge carriers will result in a sudden interlayer structural change, [19] causing photoexcited electrons relaxed into the excimer geometry.…”

“…[7,10] Besides these two absorption features, the 18-layer Bi 2 O 2 Se nanosheet also shows a broad absorption spectrum in the range of 550-721 nm due to the close-lying electronic states in the band structure. [19] In principle, efficient photoluminescence (PL) in 2D semiconductors is expected to originate from the direct optical transition with a small Stokes shift, [23,24] as supported by the narrow emission band from the much thinner Bi 2 O 2 Se nanosheet (e.g., 6-layer sample in which the emission band peaks at ≈559 nm and the Γ−Γ direct transition locates at ≈547 nm, see Figure S4 Adv. Mater.…”

“…[ 7,10 ] Besides these two absorption features, the 18‐layer Bi 2 O 2 Se nanosheet also shows a broad absorption spectrum in the range of 550–721 nm due to the close‐lying electronic states in the band structure. [ 19 ] In principle, efficient photoluminescence (PL) in 2D semiconductors is expected to originate from the direct optical transition with a small Stokes shift, [ 23,24 ] as supported by the narrow emission band from the much thinner Bi 2 O 2 Se nanosheet (e.g., 6‐layer sample in which the emission band peaks at ≈559 nm and the Γ−Γ direct transition locates at ≈547 nm, see Figure S4 for details, Supporting Information). However, in the 18‐layer Bi 2 O 2 Se nanosheet, we have surprisingly observed a broad and structureless emission band peaking at ≈738 nm with a huge Stokes shift of 142 nm, which is almost one order of magnitude larger than the value in typical 2D materials.…”

Templating Unidirectional Bismuth Oxyiodide Crystal Growth with Layered Silicates for Enhanced Photocatalysis

“…2D materials in particular are of great interest due to their tunable band gaps, a unique electronic structure, and applications in optoelectronics, energy conversion, and storage. − Recently, bismuth-based materials such as Bi 2 WO 6 , Bi 2 MoO 6 , bismuth oxyhalides (BiOX, X = I, Br, and Cl), and Bi 2 O 2 CO 3 have been investigated for their charge separation and transport ability, leading to improvements in photocatalytic performance. − Among these, bismuth oxyhalides exhibit tunable band gaps, good photostability, and an intrinsic internal electric field. Among the bismuth oxyhalides, BiOI shows broader light absorption than BiOCl and BiOBr, indicating a higher potential utilization of solar energy. − Various synthetic strategies, including hydrothermal/solvothermal methods, , anion exchange methods, microwave methods, and templated methods, have been applied to synthesize BiOX; however, most reports focused on the adjustment of the morphology and construction of a heterostructure. ,− There are few reports involving manipulating the crystalline growth direction to tune both the internal electric field and band gap of BiOI, which can have a great effect on electron–hole pair separation and transport for improved photocatalytic activity …”

Second-Harmonic-Generation-Active Oxyhalides: CuSb2O3X (X = Cl, Br)

“…Over the past decades, interest in metal oxyhalides has rapidly increased, which is attributed to their diverse functionalities such as nonlinear-optical (NLO), photocatalytic, magnetic, piezoelectric, and ferroelectric properties. − For the NLO application, metal oxyhalides may have the advantage of combining the merits of halides and oxides and have recently attracted broad interest.…”