Flinders Centre for NanoScale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park South Australia, 5042, Australia.
Ultramicroscopy. 2013 Aug;131:46-55. doi: 10.1016/j.ultramic.2013.03.009. Epub 2013 Mar 26.
Considerable attention has been given to the calibration of AFM cantilever spring constants in the last 20 years. Techniques that do not require tip-sample contact are considered advantageous since the imaging tip is not at risk of being damaged. Far less attention has been directed toward measuring the cantilever deflection or sensitivity, despite the fact that the primary means of determining this factor relies on the AFM tip being pressed against a hard surface, such as silicon or sapphire; which has the potential to significantly damage the tip. A recent method developed by Tourek et al. in 2010 involves deflecting the AFM cantilever a known distance from the imaging tip by pressing the cantilever against a sharpened tungsten wire. In this work a similar yet more precise method is described, whereby the deflection of the cantilever is achieved using an AFM probe with a spring constant much larger than the test cantilever, essentially a rigid cantilever. The exact position of loading on the test cantilever was determined by reverse AFM imaging small spatial markers that are milled into the test cantilever using a focussed ion beam. For V shaped cantilevers it is possible to reverse image the arm intersection in order to determine the exact loading point without necessarily requiring FIB milled spatial markers, albeit at the potential cost of additional uncertainty. The technique is applied to tip-less, beam shaped and V shaped cantilevers and compared to the hard surface contact technique with very good agreement (on average less than 5% difference). While the agreement with the hard surface contact technique was very good the error on the technique is dependent upon the assumptions inherent in the method, such as cantilever shape, loading point distance and ratio of test to rigid cantilever spring constants. The average error ranged between 2 to 5% for the majority of test cantilevers studied. The sensitivity derived with this technique can then be used to calibrate the cantilever spring constant using the thermal noise method, allowing complete force calibration to be accurately performed without tip-sample contact.
在过去的 20 年中，人们对原子力显微镜（AFM）悬臂梁弹性常数的校准给予了相当大的关注。不要求针尖-样品接触的技术被认为是有利的，因为成像针尖不会有损坏的风险。尽管确定这个因素的主要方法依赖于 AFM 针尖被压在坚硬的表面上，如硅或蓝宝石，这有可能显著损坏针尖，但很少有人关注测量悬臂梁的挠度或灵敏度。Tourek 等人在 2010 年开发的一种最近的方法涉及通过将悬臂梁压在尖锐的钨丝上来使 AFM 悬臂梁从成像针尖偏转已知距离。在这项工作中，描述了一种类似但更精确的方法，其中通过使用具有比测试悬臂梁大得多的弹性常数的 AFM 探针来实现悬臂梁的挠度，实质上是一个刚性悬臂梁。通过反向 AFM 成像，确定了测试悬臂梁上的加载位置，这些小的空间标记是使用聚焦离子束铣削到测试悬臂梁上的。对于 V 形悬臂梁，可以反向成像臂的交点，以确定精确的加载点，而不必一定需要 FIB 铣削的空间标记，尽管可能会增加额外的不确定性。该技术应用于无尖端、梁形和 V 形悬臂梁，并与硬表面接触技术进行了很好的比较，平均差异小于 5%。虽然与硬表面接触技术的一致性非常好，但该技术的误差取决于方法中固有的假设，如悬臂梁形状、加载点距离以及测试和刚性悬臂梁弹性常数的比值。在研究的大多数测试悬臂梁中，平均误差在 2%到 5%之间。然后可以使用该技术得出的灵敏度通过热噪声法校准悬臂梁弹性常数，从而在不进行针尖-样品接触的情况下准确地进行完整的力校准。
