Atomic force microscopy-infrared (AFM-IR) spectroscopic imaging offers non-perturbative, molecular contrast for nanoscale characterization. The need to mitigate measurement artifacts and enhance sensitivity, however, requires narrowly-defined and strict sample preparation protocols. This limits reliable and facile characterization; for example, when using common substrates such as Silicon or glass. Here, we demonstrate a closed-loop (CL) piezo controller design for responsivity-corrected AFM-IR imaging. Instead of the usual mode of recording cantilever deflection driven by sample expansion, the principle of our approach is to maintain a zero amplitude harmonic cantilever deflection by CL control of a subsample piezo. We show that the piezo voltage used to maintain a null deflection provides a reliable measure of the local IR absorption with significantly reduced noise. A complete analytical description of the CL operation and characterization of the controller for achieving robust performance are presented. Accurate measurement of IR absorption of nanothin PMMA films on glass and Silicon validates the robust capability of CL AFM-IR in routine mapping of nanoscale molecular information.

原子力显微镜-红外 (AFM-IR) 光谱成像为纳米级表征提供非微扰的分子对比度。然而，要减少测量伪影并提高灵敏度，就需要定义严格且严格的样品制备方案。这限制了可靠和容易的表征；例如，当使用硅或玻璃等常见基材时。在这里，我们展示了用于响应度校正的 AFM-IR 成像的闭环 (CL) 压电控制器设计。我们的方法的原理是通过子样本压电的 CL 控制来保持零振幅谐波悬臂偏转，而不是记录由样本扩展驱动的悬臂偏转的常用模式。我们表明，用于保持零偏转的压电电压提供了对局部红外吸收的可靠测量，同时显着降低了噪声。提供了 CL 操作的完整分析描述和实现稳健性能的控制器特性。精确测量玻璃和硅上的纳米 PMMA 薄膜的红外吸收，验证了 CL AFM-IR 在纳米级分子信息的常规映射中的强大能力。