Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3
Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3
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DOI：
10.1038/nmat4936
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发表时间：
2017-08-01
期刊：
影响因子：
41.2
通讯作者：
Leone, Stephen R.
中科院分区：
文献类型：
--
作者：
Carneiro, Lucas M.;Cushing, Scott K.;Leone, Stephen R.
已知小极化子形成限制金属氧化物光催化剂中的基态迁移率。然而，小极化子形成在光激发态中的作用以及这如何影响光转换效率还有待确定。在这里，瞬态飞秒极紫外测量表明，小极化子本地化负责超快捕获的光激发载流子在赤铁矿（α-Fe 2 O3）。小极化子的形成是由Fe M-2，M-3边缘的Fe 3 p核心轨道的亚100 fs分裂证明的。小极化子形成动力学再现了通常归因于陷阱态的三指数弛豫。然而，测得的光谱签名类似于只有一个小极化子的光谱预测，而不是预期的边缘前功能的中间间隙陷阱状态。小极化子的形成几率、跳跃半径和寿命随激发波长的变化而变化，在t（2g）导带中随能量的增加而减小。小极化子的形成对载流子的激发波长依赖性定位是赤铁矿光转换效率的潜在限制因素。
Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion effciency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (alpha-Fe2O3). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M-2,M-3 edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t(2g) conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite's photoconversion effciency.