Center for Nanoscale Science and Technology, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
Maryland Nanocenter, University of Maryland , College Park, Maryland 20742, United States.
Anal Chem. 2017 Dec 19;89(24):13524-13531. doi: 10.1021/acs.analchem.7b03878. Epub 2017 Dec 6.
Photothermal induced resonance (PTIR), also known as AFM-IR, is a scanning probe technique that provides sample composition information with a lateral resolution down to 20 nm. Interest in PTIR stems from its ability to identify unknown samples at the nanoscale thanks, in first approximation, to the direct comparability of PTIR spectra with far-field infrared databases. The development of rapidly tuning quantum cascade lasers has increased the PTIR throughput considerably, making nanoscale hyperspectral imaging within a reasonable time frame possible. Consequently, a better understanding of PTIR signal generation and of the fine details of PTIR analysis has become of paramount importance for extending complex IR analysis methods developed in the far-field, e.g., for classification and hyperspectral imaging, to nanoscale PTIR spectra. Here we calculate PTIR spectra via thin-film optics, to identify subtle changes (band shifts, deviation from linear approximation, etc.) for common sample parameters in the case of PTIR with total internal reflection illumination. Results show signal intensity linearity and small band shifts as long as the sample is prepared correctly, with band shifts typically smaller than macroscale attenuated total reflection (ATR) spectroscopy. Finally, a generally applicable algorithm to retrieve the pure imaginary component of the refractive index (i.e., the chemically specific information) is provided to overcome the PTIR spectra nonlinearity.
光热诱导共振（PTIR），也称为原子力显微镜红外光谱（AFM-IR），是一种扫描探针技术，可提供具有 20nm 以下横向分辨率的样品成分信息。PTIR 的应用兴趣源于其在纳米尺度上识别未知样品的能力，这主要得益于 PTIR 光谱与远场红外数据库的直接可比性。快速调谐量子级联激光器的发展大大提高了 PTIR 的吞吐量，使得在合理的时间内进行纳米级高光谱成像成为可能。因此，更好地理解 PTIR 信号的产生以及 PTIR 分析的细微细节对于将远场中开发的复杂 IR 分析方法扩展到纳米级 PTIR 光谱变得至关重要，例如分类和高光谱成像。在这里，我们通过薄膜光学计算 PTIR 光谱，以识别在总内反射照明下 PTIR 中常见样品参数的细微变化（带移动、偏离线性近似等）。结果表明，只要样品制备正确，信号强度就具有线性性，并且带移动很小，通常比宏观衰减全反射（ATR）光谱小。最后，提供了一种通用的算法来检索折射率的纯虚部（即化学特异性信息），以克服 PTIR 光谱的非线性。
