When generating force curves with atomic force microscopy (AFM), the conventional assumption is that the silicon tip’s apex is infinitely stiffer than the force gradient acting between the apex and test object. Although true for measurements in vacuum or at long distances, we show this assumption fails badly at short distances in aqueous environments. In this case, the effective apex is an adsorbed water molecule, bound by a weak O–H···O–H H-bond. At short distances, the magnitude of the force gradient exceeds the stiffness of this bond. This causes conventional AFM measurements to be dominated by the mechanical H-bond stiffness, instead of the force gradient. Here, we introduce a new multifrequency technique that is able to measure the surface force gradient independently from the H-bond. We compare our results to conventional FM-AFM and show that due to the H-bond, FM-AFM can give extremely erroneous measurements and even the wrong force polarity.

中文翻译：

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水合原子力显微镜头的有效刚度

当用原子力显微镜 (AFM) 生成力曲线时，传统的假设是硅尖端的顶点比作用在顶点和测试对象之间的力梯度更硬。虽然在真空或长距离测量中是正确的，但我们表明这种假设在水环境中的短距离内严重失败。在这种情况下，有效的顶点是一个吸附的水分子，由弱O-H···O-H H键结合。在短距离内，力梯度的大小超过了该键的刚度。这导致传统的 AFM 测量以机械 H 键刚度而不是力梯度为主。在这里，我们介绍了一种新的多频技术，该技术能够独立于 H 键测量表面力梯度。