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

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 (Q_eff) 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 Q_eff 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.