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A reply to the Comment by M. Calatayud et al. on "Imaging of the Hydrogen Subsurface Site in Rutile TiO2" (Physical Review Letters, Volume 102, Issue 13). DOI: 10.1103/physrevlett.102.136103.Peer reviewe

Imaging of the Hydrogen Subsurface Site in Rutile TiO2

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile TiO2. Subsurface hydrogen atoms (Hsub) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.Peer reviewe

Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO2(110) surface

Enevoldsen GH, Pinto HP, Foster AS, et al. Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO2(110) surface. Physical Review B. 2008;78(4):045416.The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by the geometry and exact atomic structure of the tip apex. However, the tip state is an experimentally unknown parameter, and the lack of insight into the tip apex often limits the possibilities of extracting precise quantitative and qualitative atomistic information on the surface under inspection. From an interplay between simultaneously recorded nc-AFM and scanning tunneling microscopy (STM) data, and atomistic STM simulations based on multiple scattering theory, we demonstrate how the state of the scanning probe microscopy (SPM) tip in the experiments may be determined. The analysis of a large number of experimental SPM images recorded with different tips reveals that no general correlation exists between the contrast observed in the nc-AFM and the tunneling current (I-t) images on TiO2(110) surface. The exact state of the SPM tip must, therefore, be determined for each specific case, which is normally a very difficult endeavor. However, our analysis of the AFM contrast on TiO2(110) surface allows us to considerably reduce the number of tips to be considered in a full simulation. By carefully evaluating the contrast of a handpicked library of SPM tips, we manage to determine a very accurate model of the SPM tip used in an experiment for the first time. It is envisioned that the approach presented here may eventually be used in future studies to screen for and select a SPM tip with a special functionalization prior to imaging an unknown sample, and in that way facilitate precise modeling and chemical identification of surface species

Noncontact atomic force microscopy studies of vacancies and hydroxyls of TiO2 (110)

From an interplay of noncontact atomic force microscopy experiments and simulations, we present here a detailed account of atomic-scale contrast encountered in force microscopy of the prototypical metal-oxide surface Ti O2 (110). We have previously shown for this surface how the atomic-scale atomic force microscopy (AFM) contrast depends crucially on the tip-termination polarity. Here, we extend this finding by also taking into account the influence of the tip-surface imaging distance as controlled by the scanning parameters. Atomic-resolution imaging is shown to be possible in three distinctly different types of contrast modes corresponding to three different types of tip-apex terminations. In the two predominant modes, the AFM contrast is found to be dominated principally by the polarity of the electrostatic interactions between the tip-apex atoms and the O and Ti surface sublattices. A negatively (presumably Oδ-) terminated tip generates AFM images in which the positive sublattice (Ti) and bridging hydroxyl (OH) adsorbates are imaged as bright protrusions, whereas a positively terminated tip (Tiδ+) results in AFM images with inverted contrast. Experiments show that the qualitative details of the imaging contrast of the surface signatures are retained at all realistic tip-surface imaging distances for both tips, but a detailed comparison of AFM images recorded at different scanning parameters with calculated site-specific force-distance curves illustrates how the quantitative appearance may change as the surface is probed at closer distances. The third observed imaging mode, which, however, is obtained quite seldom, reflects a tip having a predominantly covalent interaction with the surface atoms, since the resulting imaging contrast is very close to the real topographic structure of the surface. We expect that also for other surfaces with an ionic or semi-ionic character that the atomic-scale AFM contrast depends strongly on the exact nature of the tip apex in a similar way, and the present analysis outlines how all imaging modes can be included in an atomic-scale analysis to reveal the chemical identity of defects and adsorbates on such surfaces.Peer reviewe

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