Extreme Ultraviolet Reflection-Absorption (XUV-RA) Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective
Extreme Ultraviolet Reflection-Absorption (XUV-RA) Spectroscopy: Probing Dynamics at Surfaces from a Molecular Perspective
复制标题
极紫外反射吸收 (XUV-RA) 光谱：从分子角度探测表面动力学
DOI：
10.1021/acs.accounts.1c00765
复制
发表时间：
2022
影响因子：
18.3
通讯作者：
Baker, L. Robert
中科院分区：
文献类型：
--
作者：
Biswas, Somnath;Baker, L. Robert
ConcurrentusExtreme紫外光源的基础上高次谐波发生是使新的光谱方法的发展，以帮助推进超快科学和技术的前沿。在这个帐户中，我们讨论了极紫外反射-吸收（XUV-RA）光谱在近掠入射反射几何形状的发展，并强调这种方法在表面研究超快电子动力学的最新应用。用宽带飞秒脉冲XUV光测量核-价跃迁将X射线吸收光谱的优势扩展到实验室桌面，提供了材料的化学指纹，包括能够分辨对氧化态、自旋态、载流子极性和配位几何结构敏感的单个元素。将这种化学状态灵敏度与飞秒时间分辨率相结合，为复杂材料中控制电荷载流子动力学的材料特性提供了新的见解。众所周知，表面动力学与体材料中的等效过程显著不同，并且在表面处唯一发生的电荷分离、捕获、传输和复合控制跨越电子学、光电子学以及信息存储和处理的许多技术相关过程的效率。重要的是，XUV-RA光谱在近掠射角也是表面敏感的，探针深度为103 nm，提供了一个新的窗口，在表面和界面的电子和结构动力学。在这里，我们强调了独特的能力和最近的应用XUV-RA光谱研究光致表面动力学的金属氧化物半导体，包括光催化氧化物（Fe 2 O3，Co 3 O 4 NiO和CuFeO 2）以及光开关磁性氧化物（CoFe 2 O 4）。我们首先比较了超快电子自陷率通过小极化子形成在表面和散装的Fe 2 O3，我们注意到，这个过程的能量和动力学显着不同的表面。此外，我们证明了通过分子功能化系统地调节这种动力学的能力，从而提供了一种控制表面载流子传输的途径。我们还测量了一系列过渡金属氧化物半导体（Fe_2O_3，NiO，Co_3O_4）中电子和空穴的电荷转移激子的光谱特征。在氧L1边缘探测的价带空穴的存在证实了金属-氧键的共价性和水的氧化效率之间的直接关系。XUV-RA结果表明，CuFeO 2的O 2 p态到Cu 3d态的亚皮秒热释导致了电子和空穴的空间分离，从而使该材料具有优异的光催化放氢和还原CO2的性能.最后，我们提供了一个例子，以显示的能力XUV-RA探测自旋态特定的动力学在光开关亚铁磁体，钴铁氧体（CoFe 2 O 4）。这项研究提供了一个详细的understating在一个复杂的磁性材料与特定位置的分辨率的超快自旋开关。总之，这里展示的XUV-RA光谱的应用说明了这种方法的当前能力和未来前景，将分子水平的理解从定义明确的光化学复合物扩展到复杂的材料，以便表面的电荷和自旋动力学可以用分子光化学的精度进行调整。
ConspectusExtreme ultraviolet light sources based on high harmonic generation are enabling the development of novel spectroscopic methods to help advance the frontiers of ultrafast science and technology. In this Account, we discuss the development of extreme ultraviolet reflection–absorption (XUV-RA) spectroscopy at near grazing incident reflection geometry and highlight recent applications of this method to study ultrafast electron dynamics at surfaces. Measuring core-to-valence transitions with broadband, femtosecond pulses of XUV light extends the benefits of X-ray absorption spectroscopy to a laboratory tabletop by providing a chemical fingerprint of materials, including the ability to resolve individual elements with sensitivity to oxidation state, spin state, carrier polarity, and coordination geometry. Combining this chemical state sensitivity with femtosecond time resolution provides new insight into the material properties that govern charge carrier dynamics in complex materials. It is well-known that surface dynamics differ significantly from equivalent processes in bulk materials and that charge separation, trapping, transport, and recombination occurring uniquely at surfaces govern the efficiency of numerous technologically relevant processes spanning photocatalysis, photovoltaics, and information storage and processing. Importantly, XUV-RA spectroscopy at near grazing angle is also surface sensitive with a probe depth of ∼3 nm, providing a new window into electronic and structural dynamics at surfaces and interfaces. Here we highlight the unique capabilities and recent applications of XUV-RA spectroscopy to study photoinduced surface dynamics in metal oxide semiconductors, including photocatalytic oxides (Fe2O3, Co3O4NiO, and CuFeO2) as well as photoswitchable magnetic oxide (CoFe2O4). We first compare the ultrafast electron self-trapping rates via small polaron formation at the surface and bulk of Fe2O3where we note that the energetics and kinetics of this process differ significantly at the surface. Additionally, we demonstrate the ability to systematically tune this kinetics by molecular functionalization, thereby providing a route to control carrier transport at surfaces. We also measure the spectral signatures of charge transfer excitons with site specific localization of both electrons and holes in a series of transition metal oxide semiconductors (Fe2O3, NiO, Co3O4). The presence of valence band holes probed at the oxygen L1-edge confirms a direct relationship between the metal–oxygen bond covalency and water oxidation efficiency. For a mixed metal oxide CuFeO2in the layered delafossite structure, XUV-RA reveals that the sub-picosecond hole thermalization from O 2p to Cu 3d states of CuFeO2leads to the spatial separation of electrons and holes, resulting in exceptional photocatalytic performance for H2evolution and CO2reduction of this material. Finally, we provide an example to show the ability of XUV-RA to probe spin state specific dynamics in a photoswitchable ferrimagnet, cobalt ferrite (CoFe2O4). This study provides a detailed understating of ultrafast spin switching in a complex magnetic material with site-specific resolution. In summary, the applications of XUV-RA spectroscopy demonstrated here illustrate the current abilities and future promise of this method to extend molecule-level understanding from well-defined photochemical complexes to complex materials so that charge and spin dynamics at surfaces can be tuned with the precision of molecular photochemistry.