Asylum Research-An Oxford Instruments Company , Santa Barbara, California 93117, United States.
ACS Nano. 2017 Oct 24;11(10):10097-10105. doi: 10.1021/acsnano.7b04530. Epub 2017 Oct 6.
Tapping mode atomic force microscopy (AFM), also known as amplitude modulated (AM) or AC mode, is a proven, reliable, and gentle imaging mode with widespread applications. Over the several decades that tapping mode has been in use, quantification of tip-sample mechanical properties such as stiffness has remained elusive. Bimodal tapping mode keeps the advantages of single-frequency tapping mode while extending the technique by driving and measuring an additional resonant mode of the cantilever. The simultaneously measured observables of this additional resonance provide the additional information necessary to extract quantitative nanomechanical information about the tip-sample mechanics. Specifically, driving the higher cantilever resonance in a frequency modulated (FM) mode allows direct measurement of the tip-sample interaction stiffness and, with appropriate modeling, the set point-independent local elastic modulus. Here we discuss the advantages of bimodal tapping, coined AM-FM imaging, for modulus mapping. Results are presented for samples over a wide modulus range, from a compliant gel (∼100 MPa) to stiff materials (∼100 GPa), with the same type of cantilever. We also show high-resolution (subnanometer) stiffness mapping of individual molecules in semicrystalline polymers and of DNA in fluid. Combined with the ability to remain quantitative even at line scan rates of nearly 40 Hz, the results demonstrate the versatility of AM-FM imaging for nanomechanical characterization in a wide range of applications.
敲击模式原子力显微镜（AFM），也称为幅度调制（AM）或交流模式，是一种经过验证的、可靠的、温和的成像模式，具有广泛的应用。在敲击模式使用的几十年中，针尖-样品机械性能（如刚度）的定量分析仍然难以实现。双模态敲击模式保持了单频敲击模式的优点，同时通过驱动和测量悬臂的附加谐振模式扩展了该技术。该附加共振的同时测量的可观测值提供了提取关于针尖-样品力学的定量纳米力学信息所需的额外信息。具体来说，以调频（FM）模式驱动更高的悬臂共振允许直接测量针尖-样品相互作用刚度，并通过适当的建模，获得与设定点无关的局部弹性模量。在这里，我们讨论了双模态敲击（称为 AM-FM 成像）在模量映射方面的优势。结果表明，在很宽的模量范围内的样品上，从柔顺凝胶（约 100 MPa）到硬材料（约 100 GPa），使用相同类型的悬臂，都具有很好的效果。我们还展示了半结晶聚合物中单个分子和流体中 DNA 的高分辨率（亚纳米）刚度映射。结合即使在近 40 Hz 的线扫描速率下仍能保持定量的能力，结果表明 AM-FM 成像在广泛的应用中具有用于纳米力学表征的多功能性。
