terminating the tip of an atomic force microscope with a co molecule allows data to be acquired with a well-known and inert apex. Previous studies have shown conflicting results regarding the electrostatic interaction, indicating in some cases that the negative charge at the apex of the co dominates, whereas in other cases the positive charge at the end of the metal tip dominates. to clarify this, we investigated CaF 2 (111). CaF 2 is an ionic crystal and the (111) surface does not possess charge inversion symmetry. far from the surface, the interaction is dominated by electrostatics via the negative charge at the apex. closer to the surface, pauli repulsion and co bending dominate, which leads to an unexpected appearance of the complex 3-atom unit cell. We compare simulated data in which the electrostatics are modeled by point particles versus a charge density calculated by Dft. We also compare modeling pauli repulsion via individual Lennard-Jones potentials versus a total charge density overlap. In doing so, we determine forcefield parameters useful for future investigations of biochemical processes.

Traditionally, atomic force microscopy (AFM) experiments are conducted at tip–sample distances where the tip strongly interacts with the surface. This increases the signal-to-noise ratio, but poses the problem of relaxations in both tip and sample that hamper the theoretical description of experimental data. Here, we employ AFM at relatively large tip–sample distances where forces are only on the piconewton and subpiconewton scale to prevent tip and sample distortions. Acquiring data relatively far from the surface requires low noise measurements. We probed the CaF2(111) surface with an atomically-characterized metal tip and show that the experimental data can be reproduced with an electrostatic model. By experimentally characterizing the second layer of tip atoms, we were able to reproduce the data with 99.5% accuracy. Our work links the capabilities of non-invasive imaging at large tip–sample distances and controlling the tip apex at the atomic scale.

Functionalizing a metal tip with a single CO molecule (CO tip) leads to an unprecedented spatial resolution of small organic molecules by frequency-modulation atomic force microscopy (FM-AFM) at low temperatures. O-terminated Cu tips (CuOx tips) show comparable imaging capabilities as CO tips but exhibit a much stiffer apex. So far, to verify tip functionalization with oxygen (i.e., CuOx tips), scanning tunneling microscopy and AFM images, together with force spectroscopy curves of copper oxide domains, have been compared with calculated data for different tip models. Here, we apply the carbon-monoxide front atom identification (COFI) method and additional force spectroscopy to characterize CuOx tips in-situ on a Cu(110) surface. In COFI, a single CO molecule adsorbed on a Cu surface is imaged to atomically resolve the tip apex. Based on our findings, we suggest accompanying tip fingerprinting with COFI and force spectroscopy to identify the atomic and chemical compositions of the apex of CuOx tips for high-resolution AFM experiments.

No abstract

Topological insulators are a class of materials that are semiconducting or insulating in their bulk but possess topologically protected gapless states at their boundaries. Bi 2 Se 3 is a promising material for applications due to its large band gap and single surface state Dirac cone. Pure electric conduction exclusively via the topological surface state is, however, hampered due to an n-type doping caused by the presence of native point defects, especially Se vacancies. Here, we apply highresolution atomic force microscopy for real-space imaging and determination of the polarity of surface defects in Bi 2 Se 3 . We observe surface defects ranging from a single missing Se atom to defects composed of multiple missing Se atoms in the surface layer and find a positive polarity for all Se vacancies, confirming them as electron donors. Our work links to existing STM findings and adds precise structural information provided by the additional AFM channel, opening the possibility to more accurately determine the physical properties of defects in topological insulators.