Reliable Work Function Determination of Multicomponent Surfaces and Interfaces: The Role of Electrostatic Potentials in Ultraviolet Photoelectron Spectroscopy
Reliable Work Function Determination of Multicomponent Surfaces and Interfaces: The Role of Electrostatic Potentials in Ultraviolet Photoelectron Spectroscopy
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DOI：
10.1002/admi.201700324
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发表时间：
2017-10-09
影响因子：
5.4
通讯作者：
Koch, Norbert
中科院分区：
文献类型：
--
作者：
Schultz, Thorsten;Lenz, Thomas;Koch, Norbert
紫外光电子能谱（UPS）是通过测量二次电子截止（SECO）来确定表面功函数（phi）的关键技术。然而，对于多组分表面，UPS 获得的 SECO 谱的解释并不简单，并且不完全理解不均匀性的长度尺度对 SECO 的影响程度。在此，本研究通过实验和理论上确定具有定义的 phi 模式的表面上方的静电景观，揭示了控制 SECO 能量分布的物理原理。对于此类样品，测量的 SECO 光谱实际上表现出两个截止值，一个代表高 phi 表面成分，另一个对应于面积平均 phi 值。通过结合开尔文探针力显微镜和静电建模，定量证明了高 phi 区域的静电势会给低 phi 区域发射的电子带来额外的能量势垒。感应能垒对 phi 图案长度尺度和样本偏差的依赖性的理论预测得到了进一步的实验验证。这些发现为异质表面的可靠 SECO 解释奠定了坚实的基础，并提高了 UPS 实验界面能级图的可靠性。
Ultraviolet photoelectron spectroscopy (UPS) is a key technique to determine the work function (phi) of surfaces by measuring the secondary-electron cut-off (SECO). However, the interpretation of SECO spectra as obtained by UPS is not straightforward for multicomponent surfaces, and it is not comprehensively understood to what extent the length scale of inhomogeneity impacts the SECO. Here, this study unravels the physics governing the energy distribution of the SECO by experimentally and theoretically determining the electrostatic landscape above surfaces with defined patterns of phi. For such samples, the measured SECO spectra exhibit actually two cut-offs, one representing the high phi surface component and the other one corresponding to an area-averaged phi value. By combining Kelvin probe force microscopy and electrostatic modeling, it is quantitatively demonstrated that the electrostatic potential of the high phi areas leads to an additional energy barrier for the electrons emitted from the low phi areas. Theoretical predictions of the induced energy barrier dependence on the phi-pattern length scale and sample bias are further experimentally verified. These findings establish a solid base for reliable SECO interpretation of heterogeneous surfaces and improved reliability of interfacial energy-level diagrams from UPS experiments.