Browsing by Author "Hofer, Werner A."

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Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO 2 ( 110 ) surface
(2008) Enevoldsen, Georg H.; Pinto, Henry P.; Foster, Adam S.; Jensen, Mona C. R.; Kühnle, Angelika; Reichling, Michael; Hofer, Werner A.; Lauritsen, Jeppe V.; Besenbacher, Flemming
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The atomic-scale contrast in noncontact atomic force microscopy (nc-AFM) images is determined by thegeometry 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 (It) images on TiO2(110) surface. The exact state of the SPM tip must, therefore, bedetermined 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.
Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of the TiO2 (110) surface
(2008) Enevoldsen, Georg H.; Pinto, Henry P.; Foster, Adam S.; Jensen, Mona C.R.; Kühnle, Angelika; Reichling, Michael; Hofer, Werner A.; Lauritsen, Jeppe V.; Besenbacher, Flemming
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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 (It) 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.
Imaging of the Hydrogen Subsurface Site in Rutile TiO2
(2009) Enevoldsen, Georg H.; Pinto, Henry P.; Foster, Adam S.; Jensen, Mona C. R.; Hofer, Werner A.; Hammer, Bjørk; Lauritsen, Jeppe V.; Besenbacher, Flemming
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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.
Reply to comment on Imaging of the hydrogen subsurface site in rutile TiO2
(2010) Enevoldsen, Georg H.; Pinto, Henry P.; Foster, Adam S.; Jensen, Mona C.R.; Hofer, Werner A.; Hammer, Bjørk; Lauritsen, Jeppe V.; Besenbacher, Flemming
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
A Reply to the Comment by M. Calatayud et al.
Reply to the comment on "Imaging of the Hydrogen Subsurface Site in Rutile TiO2''
(2010) Enevoldsen, Georg H.; Pinto, Henry P.; Foster, Adam S.; Jensen, Mona C. R.; Hofer, Werner A.; Hammer, Bjørk; Lauritsen, Jeppe V.; Besenbacher, Flemming
School of Science | B1 Kirjoitus tieteellisessä aikakauslehdessä
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.
Theories of scanning probe microscopes at the atomic scale
(2003) Hofer, Werner A.; Foster, Adam S.; Shluger, Alexander L.
School of Science | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Significant progress has been made both in experimentation and in theoretical modeling of scanning probe microscopy. The theoretical models used to analyze and interpret experimental scanning probe microscope (SPM) images and spectroscopic data now provide information not only about the surface, but also the probe tip and physical changes occurring during the scanning process. The aim of this review is to discuss and compare the present status of computational modeling of two of the most popular SPM methods—scanning tunneling microscopy and scanning force microscopy—in conjunction with their applications to studies of surface structure and properties with atomic resolution. In the context of these atomic-scale applications, for the scanning force microscope (SFM), this review focuses primarily on recent noncontact SFM (NC-SFM) results. After a brief introduction to the experimental techniques and the main factors determining image formation, the authors consider the theoretical models developed for the scanning tunneling microscope (STM) and the SFM. Both techniques are treated from the same general perspective of a sharp tip interacting with the surface—the only difference being that the control parameter in the STM is the tunneling current and in the SFM it is the force. The existing methods for calculating STM and SFM images are described and illustrated using numerous examples, primarily from the authors' own simulations, but also from the literature. Theoretical and practical aspects of the techniques applied in STM and SFM modeling are compared. Finally, the authors discuss modeling as it relates to SPM applications in studying surface properties, such as adsorption, point defects, spin manipulation, and phonon excitation.