Jakob Devon S, Wang Haomin, Xu Xiaoji G
Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015, United States.
ACS Nano. 2020 Apr 28;14(4):4839-4848. doi: 10.1021/acsnano.0c00767. Epub 2020 Apr 16.
Measurement of the contact potential difference (CPD) and work functions of materials are important in analyzing their electronic structures and surface residual charges. Kelvin probe force microscopy (KPFM), an imaging technique of atomic force microscopy, has been widely used for surface potential and work function mapping at the nanoscale. However, the conventional KPFM variants are often limited in their spatial resolution to 30-100 nm under ambient conditions. The continuingly decreasing size and increasing complexity of photoactive materials and semiconductor devices will present future challenges in uncovering their nanometer-scale electrical properties through KPFM. Here, we introduce a KPFM technique based on the pulsed force mode of atomic force microscopy. Our technique, named pulsed force Kelvin Probe Force Microscopy (PF-KPFM), is a single-pass technique that utilizes the intrinsic Fermi level alignment between the AFM tip and the conductive sample without the need for an external oscillating voltage. Induced cantilever oscillations generated by a spontaneous redistribution of electrons between tip and sample are extracted and used to obtain the cantilever oscillation amplitude and to derive the surface potential. The spatial resolution of PF-KPFM is shown to be <10 nm under ambient conditions. The high spatial resolution surface potential mapping enables in situ determination of ohmic and nonohmic contacts between metals and semiconductors, mapping boundaries of ferroelectric domains of BaTiO, as well as characterization of protein aggregates. High spatial resolution measurements with PF-KPFM will facilitate further studies directed at uncovering electrical properties for emerging photoactive materials, biological samples, and semiconductor devices.
测量材料的接触电势差（CPD）和功函数对于分析其电子结构和表面残余电荷至关重要。开尔文探针力显微镜（KPFM）作为原子力显微镜的一种成像技术，已被广泛用于纳米尺度的表面电势和功函数映射。然而，在环境条件下，传统的KPFM变体的空间分辨率通常限制在30 - 100纳米。光活性材料和半导体器件的尺寸不断减小且复杂性不断增加，这将给通过KPFM揭示其纳米级电学性质带来未来挑战。在此，我们介绍一种基于原子力显微镜脉冲力模式的KPFM技术。我们的技术名为脉冲力开尔文探针力显微镜（PF - KPFM），是一种单通道技术，它利用原子力显微镜（AFM）探针与导电样品之间的固有费米能级对齐，无需外部振荡电压。提取由探针和样品之间电子的自发重新分布产生的诱导悬臂振荡，并用于获得悬臂振荡幅度并推导表面电势。在环境条件下，PF - KPFM的空间分辨率显示小于10纳米。高空间分辨率表面电势映射能够原位确定金属与半导体之间的欧姆和非欧姆接触，绘制钛酸钡铁电畴的边界，以及表征蛋白质聚集体。使用PF - KPFM进行高空间分辨率测量将有助于进一步研究，以揭示新兴光活性材料、生物样品和半导体器件的电学性质。
