Giblin-Burnham Joshua, Javanmardi Yousef, Moeendarbary Emad, Hoogenboom Bart W
Department of Engineering Science, University of Oxford, Wellington Square, Oxford OX1 2JD, UK.
London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London WC1H 0AH, UK.
Soft Matter. 2024 Dec 4;20(47):9483-9492. doi: 10.1039/d4sm01084a.
Atomic force microscopy (AFM) provides a three-dimensional topographic representation of a sample surface, at nanometre resolution. Computational simulations can aid the interpretation of such representations, but have mostly been limited to cases where both the AFM probe and the sample are hard and not compressible. In many applications, however, the sample is soft and therefore deformed due to the force exerted by the AFM tip. Here we use finite element modelling (FEM) to study how the measured AFM topography relates to the surface structures of soft and compressible materials. Consistent with previous analytical studies, the measured elastic modulus in AFM is generally found to deviate from the elastic modulus of the sample material. By the analysis of simple surface geometries, the FEM modelling shows how measured mechanical and topographic features in AFM images depend on a combination of tip-sample geometry and indentation of the tip into the sample. Importantly for the interpretation of AFM data, nanoparticles may appear larger or smaller by a factor of two depending on tip size and indentation force; and a higher spatial resolution in AFM images does not necessarily coincide with a more accurate representation of the sample surface. These observations on simple surface geometries also extend to molecular-resolution AFM, as illustrated by comparing FEM results with experimental data acquired on DNA. Taken together, the FEM results provide a framework that aids the interpretation of surface topography and local mechanics as measured by AFM.
原子力显微镜（AFM）能以纳米分辨率提供样品表面的三维形貌图。计算模拟有助于对这类图像进行解读，但大多局限于AFM探针和样品均为硬质且不可压缩的情况。然而，在许多应用中，样品是软质的，因此会因AFM针尖施加的力而发生变形。在此，我们使用有限元建模（FEM）来研究测量得到的AFM形貌与软质可压缩材料表面结构之间的关系。与先前的分析研究一致，通常发现AFM中测得的弹性模量与样品材料的弹性模量存在偏差。通过对简单表面几何形状的分析，有限元建模展示了AFM图像中测得的力学和形貌特征如何取决于针尖 - 样品几何形状以及针尖对样品的压痕。对于AFM数据的解读而言重要的是，纳米颗粒根据针尖尺寸和压痕力的不同，其显示出的大小可能会有两倍的差异；并且AFM图像中更高的空间分辨率并不一定与样品表面更准确的表征相一致。对简单表面几何形状的这些观察结果也适用于分子分辨率的AFM，通过将有限元建模结果与在DNA上获取的实验数据进行比较即可说明这一点。综上所述，有限元建模结果提供了一个有助于解读AFM所测量的表面形貌和局部力学的框架。
