We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe force microscopy, working in frequency modulation mode ͑FM-KPFM͒, performed in ultrahigh vacuum. To study the limits of potential and lateral resolutions in FM-KPFM, we have investigated clean surface of compound semiconductor InSb͑001͒ and the same surface with some submonolayer coverages of KBr and Au. It was found that long-and short-range bias-dependent interactions, acting between the tip and the surface, could be detected and that both interactions contribute to the measured contact potential difference ͑CPD͒ signal. On the one hand, when only the long-range electrostatic interactions between the tip and the surface are active, the CPD map provides the distribution of the local surface potential on the imaged sample with the lateral resolution and the correctness of the measured values depending on the measurement conditions. For this case, the experimental findings were compared with the predictions of theoretical calculations based on a realistic model for the cantilever-sample geometry. On the other hand, when the short-range and bias-dependent interactions are detected, FM-KPFM provides even the sub-nanometer contrast in the CPD signal. In this situation, however, the measured CPD signal is not related to the sample surface potential but reflects the properties of the front tip atom-surface atom interactions.

The surface structure of an individual metal-organic framework (MOF) microcrystal grown on a functionalized surface has been successfully investigated for the first time in air and vacuum using high-resolution atomic force microscopy. Moreover, this detailed surface analysis has been utilized to optimize the MOF formation procedure to obtain a defect-free surface structure. Comparison of obtained data with recent microscopic studies performed on the same MOF crystal but grown by a conventional procedure clearly shows a much higher quality of crystals produced by surface oriented growth. Importantly, this method of preparing crystals suitable for microscopic analysis is also much faster (3 days compared to 2 years) and, in contrast to the conventional method, produces material suitable for in situ study. These results thus demonstrate for the first time the possibility of nanoscale investigation/modification of MOF surface structure.

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