The TiO(2)(110)-(1 x 1) surface is investigated using non-contact atomic force microscopy (nc-AFM) at 80 K. We successfully obtained a distinct type of image contrast mode which does not exhibit hydroxyl (OH) impurity defects that mostly appear in common nc-AFM images. We named the obtained distinct type of image contrast as the 'hidden mode'. The assignments of surface atomic rows in this contrast mode are not easy in the absence of defects. By recording different contrast modes in the same region of the surface, we identified the atomic rows obtained in the 'hidden mode' image contrast as bridging oxygen atoms (O(b)). The mechanism of contrast formation was attributed to tip-induced displacement of H atoms over oxygen atoms in the OH groups on the O(b) rows. This interpretation was supported by dissipation measurements. A possible candidate for the tip-generating hidden-mode image contrast was interpreted to be a positively terminated tip apex with a dimer-like structure, revealing an attractive interaction with oxygen and a repulsive force on H atom sites. In addition, with a different tip state at close tip-sample distances, we were able to successfully resolve a high resolution image of the in-plane oxygen atoms.

Site-specific force measurements on a rutile TiO 2 (110) surface are combined with first-principles calculations in order to clarify the origin of the force contrast and to characterize the tip structures responsible for the two most common imaging modes. Our force data, collected over a broad range of distances, are only consistent with a tip apex contaminated with clusters of surface material. A flexible model tip terminated with an oxygen explains the protrusion mode. For the hole mode we rule out previously proposed Ti-terminated tips, pointing instead to a chemically inert, OH-terminated apex. These two tips, just differing in the terminal H, provide a natural explanation for the frequent contrast changes found in the experiments. As tip-sample contact is difficult to avoid while imaging oxide surfaces, we expect our tip models to be relevant to interpret scanning probe studies of defects and adsorbates on TiO 2 and other technologically relevant metal oxides.

The effects of Pb intercalation on the structural and electronic properties of epitaxial single-layer graphene grown on SiC(0001) substrate are investigated using scanning tunneling microscopy (STM), noncontact atomic force microscopy, Kelvin probe force microscopy (KPFM), X-ray photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (ARPES) methods. The STM results show the formation of an ordered moiré superstructure pattern induced by Pb atom intercalation underneath the graphene layer. ARPES measurements reveal the presence of two additional linearly dispersing π-bands, providing evidence for the decoupling of the buffer layer from the underlying SiC substrate. Upon Pb intercalation, the Si 2p core level spectra show a signature for the existence of PbSi chemical bonds at the interface region, as manifested in a shift of 1.2 eV of the bulk SiC component toward lower binding energies. The Pb intercalation gives rise to hole-doping of graphene and results in a shift of the Dirac point energy by about 0.1 eV above the Fermi level, as revealed by the ARPES measurements. The KPFM experiments have shown that decoupling of the graphene layer by Pb intercalation is accompanied by a work function increase. The observed increase in the work function is attributed to the suppression of the electron transfer from the SiC substrate to the graphene layer. The Pb intercalated structure is found to be stable in ambient conditions and at high temperatures up to 1250 °C. These results demonstrate that the construction of a graphene-capped Pb/SiC system offers a possibility of tuning the graphene electronic properties and exploring intriguing physical properties such as superconductivity and spintronics.

We have used noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) to directly visualize the presence of charged subsurface impurities on rutile TiO2(110). The subsurface charges add an additional electrostatic force between the sample and tip so that they appear as hillocks in the NC-AFM topography. Analysis of several subsurface defects in the same NC-AFM image reveals that the hillocks have discrete heights, which means that defects at different subsurface levels can be detected and distinguished. H adatoms, which are positively charged at the TiO2(110) surface, were found to be repelled by the buried positive charge, so that they form a ∼80–120 Å wide hydrogen-free zone around the charge. Thus, there is an opportunity to deliberately add dopants in order to exclude or perhaps even to confine certain adsorbates to a local region at the surface.

We report force mapping experiments on Si(111)-(7×7) surfaces with adsorbed hydrogen, using atomic force microscopy at room temperature supported by density functional theory (DFT) simulations. On the basis of noncontact atomic force microscopy (NC-AFM) images as well as force versus distance curves measured over both hydrogen-passivated and bare Si adatoms, we identified two types of tip termination, which result in different modes of interaction with the surface. The statistics of the tip dependence of the measured forces, which are effectuated using various tip states with different cantilevers, reveal the typical values of the force and their distribution in the two characteristic interaction modes. The experimental results are corroborated by DFT calculations performed for different tip structures. As a reactive tip, the dimer-terminated Si tip yields results in satisfactory agreement with experimental force curves for hydrogen-passivated and nonpassivated Si adatom sites. An oxidized Si dimer tip that bears a hydroxyl group on its apex reproduces well the experimental force curves acquired by nonreactive tips. This tip model could thus be used to interpret the experimentally obtained weak image contrast for the Si(111)-(7×7) surface. The forces are thought to arise as a result of a weak electrostatic interaction involving a permanent dipole at the tip apex enhanced by the charge density redistribution due to the interaction with surface adatoms.