Special roles of two-dimensional octahedral frameworks in photodynamics of Cs3Bi2Br9 nanoplatelets: Electron and lattice-wave localization

Abstract

The low-dimensional cesium bismuth halides are intriguing wide-bandgap semiconductors with fruitful photophysics. However, their photodynamics is rather intricate and remains debated. We study the optical properties of the Cs3Bi2Br9 nanoplatelets (NPLs) by using the combined experimental and first-principles calculation methods. The results indicate that the exhibited dominant blue emission band and weak green band arise from two kinds of shallow color centers. The Cs3Bi2Br9 NPLs exhibit Raman active and inactive vibrational modes that are separately ascribed to the localized lattice waves propagating along the edges and interiors of the quantum well-like bromide–bismuth octahedral frameworks in Cs3Bi2Br9. These findings improve our understanding of the unique photodynamics of these multiple quantum well-like semiconductor nanocrystals.

Critical Roles of Octahedron Bilayer Surface/Interior Bromide Defects in Photodynamics of Multi-Quantum-Well-Structured Cesium Bismuth Bromide

“…The localized lattice relaxation around the defect leads to different equilibrium positions of the crystal before and after the transition; as a result, there is a significant Stokes shift of 0.47 eV between the photon absorption and emission energies (Figure 4a). It is close to the measured Stokes shift of 0.62 eV, 35 supporting the defect origin of the blue luminescence in Cs 3 Bi 2 Br 9 considering that the interband luminescence usually has only a small Stokes shift. The photon absorption energy is 3.13 eV, being close to the bandgap value, suggesting that the defect (Br i1 ) absorption will be mixed with the interband absorption, and this is also consistent with the experimental result.…”

“…This value is close to the roughly estimated experimental value of 3.03 – 2.75 = 0.28 eV by assuming the first dip in the absorption spectrum also as the interband absorption onset. In practice, the fit of the measured absorption spectrum by taking into account a more accurate position of the absorption onset gives a more accurate binding energy of 0.15 eV . The calculation shows that Cs 3 Bi 2 Br 9 has a large direct bandgap of 3.10 eV (Figure c), which agrees with the experimental value.…”

“…The SRH recombination lifetime of the CBM electron can be calculated using τ normale = 1 k normaln N normalD , where N D is the corresponding defect concentration. As shown in the inset of Figure c, τ e equals 184 ns when N D of Br i2 equals a reasonable value of 1 × 10 15 cm –3 , and the order of magnitude of this lifetime agrees with the measured value in Cs 3 Bi 2 Br 9 within the errors. ,, It suggests that Br i2 has a significant contribution to the non-radiative recombination in Cs 3 Bi 2 Br 9 , and this efficient multiphonon recombination process explains the low quantum yield of Cs 3 Bi 2 Br 9 . The experimental study had indicated that the photoinduced lattice expansion can affect the optical properties of Cs 3 Bi 2 Br 9 .…”

“…In the presence of Br i1 , the excited system makes a vertical transition (Franck–Condon principle) from the equilibrium excited state to the lower energy state, during which the excited electron in the CBM with a delocalized orbital jumps to the localized orbital bound by Br i1 (Figure a). The energy released is 2.66 eV (466 nm), being close to that of the dominant blue emission peak (∼474 nm) observed in the Cs 3 Bi 2 Br 9 nanoplatelets, which was supposed to arise from some defect . Therefore, the Br interstitials localized on the surfaces of the octahedral bilayers very likely cause blue luminescence in Cs 3 Bi 2 Br 9 .…”

“…15,31,32 It suggests that Br i2 has a significant contribution to the non-radiative recombination in Cs 3 Bi 2 Br 9 , and this efficient multiphonon recombination process explains the low quantum yield of Cs 3 Bi 2 Br 9 . 35 The experimental study had indicated that the photoinduced lattice expansion can affect the optical properties of Cs 3 Bi 2 Br 9 . 33 Our DFT calculation shows that, when the lattice of Cs 3 Bi 2 Br 9 expands by 1%, the electron capture barrier E b decreases from 1.48 to 1.38 eV (Figure S5 of the Supporting Information), and this suggests that the lattice expansion is favorable for carrier capture in Cs 3 Bi 2 Br 9 .…”