Probing the size dependence on the optical modes of anatase nanoplatelets using STEM-EELS.
Probing the size dependence on the optical modes of anatase nanoplatelets using STEM-EELS.
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DOI：
10.1039/c5nr09264g
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发表时间：
2016-05
期刊：
影响因子：
6.7
通讯作者：
E. Liberti;R. Menzel;M. Shaffer;D. McComb
中科院分区：
文献类型：
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作者：
E. Liberti;R. Menzel;M. Shaffer;D. McComb
据报道，具有主要暴露的{001}晶面的锐钛矿二氧化钛纳米片与本体氧化钛相比具有增强的催化性能。为了了解它们的异常行为，必须在纳米尺度上充分表征它们的电子和光学性质。评估这些基本性质的一种方法是研究介电函数。使用扫描透射电子显微镜（STEM）进行的价电子能量损失谱（EELS）是唯一可以以高能量（<100 meV）和高空间分辨率（<1 nm）探测复杂介电函数的分析方法。通过将实验STEM-EELS数据与基于半经典介电理论的模拟相关联，发现薄（<5 nm）纳米片的介电响应主要由特征（光学）表面模式主导，其与纳米片的表面等离子体激元模式相关联。对于厚度小于10 nm的血小板，这些光学模式的频率根据其厚度而变化。这种独特的光学行为促进了紫外区光吸收的增强。最后，有限尺寸对介电信号的影响通过在特定晶体取向上一致地堆叠两个或更多个片晶而逐渐丧失，并且最终抑制大的片晶堆叠。
Anatase titania nanoplatelets with predominantly exposed {001} facets have been reported to have enhanced catalytic properties in comparison with bulk anatase. To understand their unusual behaviour, it is essential to fully characterize their electronic and optical properties at the nanometer scale. One way of assessing these fundamental properties is to study the dielectric function. Valence electron energy-loss spectroscopy (EELS) performed using a scanning transmission electron microscope (STEM) is the only analytical method that can probe the complex dielectric function with both high energy (<100 meV) and high spatial (<1 nm) resolution. By correlating experimental STEM-EELS data with simulations based on semi-classical dielectric theory, the dielectric response of thin (<5 nm) anatase nanoplatelets was found to be largely dominated by characteristic (optical) surface modes, which are linked to surface plasmon modes of anatase. For platelets less than 10 nm thick, the frequency of these optical modes varies according to their thickness. This unique optical behaviour prompts the enhancement of light absorption in the ultraviolet regime. Finally, the effect of finite size on the dielectric signal is gradually lost by stacking consistently two or more platelets in a specific crystal orientation, and eventually suppressed for large stacks of platelets.