Laboratory for Physical Sciences, College Park, Maryland 20740, United States.
Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States.
Anal Chem. 2022 Sep 27;94(38):13126-13135. doi: 10.1021/acs.analchem.2c02612. Epub 2022 Sep 13.
Photothermal induced resonance (PTIR), also known as AFM-IR, enables nanoscale infrared (IR) imaging and spectroscopy by using the tip of an atomic force microscope to transduce the local photothermal expansion and contraction of a sample. The signal transduction efficiency and spatial resolution of PTIR depend on a multitude of sample, cantilever, and illumination source parameters in ways that are not yet well understood. Here, we elucidate and separate the effects of laser pulse length, pulse shape, sample thermalization time (τ), interfacial thermal conductance, and cantilever detection frequency by devising analytical and numerical models that link a sample's photothermal excitations to the cantilever dynamics over a broad bandwidth (10 MHz). The models indicate that shorter laser pulses excite probe oscillations over broader bandwidths and should be preferred for measuring samples with shorter thermalization times. Furthermore, we show that the spatial resolution critically depends on the interfacial thermal conductance between dissimilar materials and improves monotonically, but not linearly, with increasing cantilever detection frequencies. The resolution can be enhanced for samples that do not fully thermalize between pulses (i.e., laser repetition rates ≳ 1/3τ) as the probed depth becomes smaller than the film thickness. We believe that the insights presented here will accelerate the adoption and impact of PTIR analyses across a wide range of applications by informing experimental designs and measurement strategies as well as by guiding future technical advances.
光热诱导共振（PTIR），也称为原子力显微镜红外光谱（AFM-IR），通过原子力显微镜的针尖来转换样品的局部光热膨胀和收缩，从而实现纳米级红外（IR）成像和光谱学。PTIR 的信号转换效率和空间分辨率取决于许多样品、悬臂梁和照明源参数，这些参数的影响方式尚未得到很好的理解。在这里，我们通过设计分析和数值模型，阐明并分离了激光脉冲长度、脉冲形状、样品热化时间（τ）、界面热导和悬臂梁检测频率的影响，这些模型将样品的光热激发与悬臂梁动力学联系起来，涵盖了很宽的带宽（10 MHz）。这些模型表明，较短的激光脉冲会激发探针在更宽的带宽内振动，并且应该优先用于测量具有较短热化时间的样品。此外，我们表明，空间分辨率严重依赖于不同材料之间的界面热导，并且随着悬臂梁检测频率的增加而单调但非线性地提高。对于在脉冲之间不能完全热化的样品（即，激光重复率≳1/3τ），分辨率可以提高，因为探测深度变得小于薄膜厚度。我们相信，这里提出的见解将通过为实验设计和测量策略提供信息以及指导未来的技术进步，加速光热诱导共振分析在广泛应用中的采用和影响。
