Bond-level imaging of organic molecules using Q-controlled amplitude modulation atomic force microscopy

Abstract

The bond imaging atomic force microscopy (AFM) technique has become an invaluable tool for studying organic molecules on surfaces. The key feature of this technique is the functionalization of the AFM-tip with a single CO molecule, which improves the lateral resolution and allows us to visualize the chemical structure of adsorbed organic molecules. Such experiments are usually performed at low temperatures in an ultrahigh vacuum environment in the frequency modulation (FM) mode. Here, we use the Q-controlled amplitude modulation (AM) mode for imaging organic molecules with CO-functionalized tips in constant-height mode. By reducing the effective quality factor (Qeff) of the sensor from about 20 000–30 000 to 1500–4000, we are able to image molecules with atomic resolution. Detailed instructions for determining the optimum Qeff and oscillation amplitude are given. To compare the phase and frequency shift images of the Q-controlled AM and the FM mode, we define an effective signal-to-noise ratio (SNR) that relates the observed contrast between the bonds and centers of imaged carbon rings to the noise in the respective image regions. This effective SNR is systematically analyzed in different regions of the imaged molecule for different oscillation amplitudes and average tip-substrate distances. By using appropriate imaging parameters, an increased effective SNR is achieved in the Q-controlled AM mode (on the order of 30–60%). This advantage over the conventional FM mode might, e.g., be used for increasing the experimental throughput.

Chemical bond imaging using torsional and flexural higher eigenmodes of qPlus sensors

“…It was recently reported that the signal-to-noise performance can be increased by approximately 30-60% by actuating the fundamental eigenmode in AM mode, which is related to the simpler electronics setup (without a phase-locked loop) and the slightly higher slope of the phase shift vs. distance curves. 63 In case of the fundamental eigenmode this usually requires a moderate reduction of the effective Q-factor via the Q-control method. [63][64][65] This is not needed in case of the 2 nd flexural eigenmode due to the smaller absolute frequency shift values (which lead to minor changes of the oscillation amplitude during scanning).…”

“…63 In case of the fundamental eigenmode this usually requires a moderate reduction of the effective Q-factor via the Q-control method. [63][64][65] This is not needed in case of the 2 nd flexural eigenmode due to the smaller absolute frequency shift values (which lead to minor changes of the oscillation amplitude during scanning).…”

Molecular Identification with Atomic Force Microscopy and Conditional Generative Adversarial Networks

“…The final goal of our CGAN model is to identify molecules from their experimental AFM images. As discussed above, the range of AFM operational parameters used to simulate the images generated for each of the molecules and the use of gas-phase configurations introduce enough To test the performance of the model with experimental results, we have selected sets of AFM images originally published in refs (43,44,45,46,47,48). In general, fewer than ten images corresponding to different tip-sample distances were published in these papers, so we have linearly interpolated the images two by two to extract additional images to complete the input, the stack of 10 images, required for the CGAN model.…”

“…Figure 5: Experimental AFM images, prediction performed with the CGAN and structure for (a) 1-azahexacyclo[11.7.1.1 3,19 .0 2,7 .0 9,21 .0 15,20 ]docosa-2,4,6,9(21),10,12,15,17,19-nonaene-8,14,22-trione, (b) 2-iodotriphenylene, (c) 21,23-dihydroporphyrin, (d) dibenzothiophene and (e) [19]dendriphene. Experimental images in(a, c, d, e)(43,45,47,46) were taken in the FM operation mode at constant height, while, in (b)(44), a novel Q-control Amplitude Modulation Atomic Force Microscopy (AM-AFM) mode was used. The color code for the balls representing the chemical species is: carbon (grey), hydrogen (white), oxygen (red), iodine (purple) and nitrogen (blue).…”