I just realized the paper is open access

https://pubs.acs.org/doi/10.1021/acs.nanolett.5c05319

Figure 3. Optically resolving atomic-scale features using MIR cw radiation. (a) Topography of a vicinal gold surface imaged with atomic force microscopy (AFM) (oscillation amplitude A = 100 pm, frequency shift set point Δνset = −1.7 Hz), revealing characteristic monatomic steps. (b) Simultaneously acquired map of the scattered intensity I2, demodulated at the second harmonic of the tip oscillation frequency. A clear contrast emerges with enhanced intensity at the atomic step edges. The inset shows a linecut of I2 along x. Fitting a step in the signal with an error function (black dashed line), we find a spatial modulation on the length scale of 1.3 Å using a 90%–10% criterion. © Simultaneously acquired map of the tunneling current J, recorded under minimized d.c. bias, where a close resemblance to the I2 map is found. (d) Two-dimensional histogram representing the pixelwise correlation between the current J and scattered intensity I2, indicating a linear correlation between the two quantities. The color map indicates the number of pixels whose value pairs fall into each bin. The black spheres depict the data points binned exclusively along the J-axis, while the error bars represent one standard deviation. For clarity, the AFM topography was line-leveled and smoothed using a Gaussian filter.

Figure 4. Optical imaging at the mechanical limit. (a) Topography of the vicinal gold surface, measured in AFM feedback mode (A = 100 pm, Δνset = −1.8 Hz). (b) Simultaneously recorded scattered intensity I2, where a clear modulation can be seen. © Simultaneously acquired map of the mechanical tip oscillation amplitude A2, demodulated at the second harmonic of the oscillation frequency. The scan shows local peaks of A2, indicating anharmonicity of the tip oscillation. (d) Two-dimensional histogram representing the pixelwise correlation between I2 and A2, where the color map indicates the number of value pairs that fall into each bin. High values of A2 tend to coincide with high values of I2. The highlighted areas in b and c show the data points to the right of the gray dashed line, where A2 > 4 pm. (e) Comparison of the distance dependence of the intensity I2 and the second harmonic oscillation amplitude A2, revealing different decay lengths for both signals. The dashed lines show exponential fits of the data. The experimental data in panels d and e were averaged over the forward and backward direction of the scan with the shaded area representing one standard deviation.