‘All-inclusive’ imaging of the rutile TiO₂(110) surface using NC-AFM

Abstract

Non-contact atomic force microscopy (NC-AFM) at true atomic resolution is used to investigate the (110) surface of rutile TiO 2. We are able to simultaneously resolve both bridging oxygen and titanium atoms of this prototypical oxide surface. Furthermore, the characteristic defect species, i.e. bridging oxygen vacancies, single and double hydroxyls as well as subsurface defects, are identified in the very same frame. We employ density functional theory (DFT) calculations to obtain a comprehensive understanding of the relation between the tip apex structure and the observed image contrast. Our results provide insight into the physical mechanisms behind atomic-scale contrast, indicating that electrostatic interaction can lead to a far more complex contrast than commonly assumed.

Understanding image contrast formation in TiO2with force spectroscopy

“…Based either on the contrast they produce at typical imaging distances 11 or the values at the force minima, our calculations rule out the contaminated Si tips. Among the TiO 2 tip models, the structure which best reproduces the measured forces is a cluster rotated to expose a singly coordinated oxygen atom at the apex [see model in Fig.…”

“…This same effect has been observed previously in a Si-based, OH-terminated tip model used to explain the neutral nc-AFM contrast in TiO 2 . 11 For the Ti site, the bending of the H atom allows a stronger O-Ti interaction at close distances, but the OH distance is small and the electrostatic screening quite effective, reducing the maximum force.…”

“…5,6 At variance with STM, atomic force microscopy with true atomic resolution [known as noncontact (nc)-AFM 7 ] has revealed a number of qualitatively different contrast modes. [8][9][10][11][12] Most images show either a contrast similar to STM with Ti 5c rows and defects imaged bright and O b rows imaged dark (protrusion mode), or its complete reversal (O b bright and Ti 5c rows and defects appearing dark), called hole mode. In a third but rather rare mode called neutral, OH groups and the O b atoms are imaged bright while O b vacancies and Ti 5c rows appear dark.…”

“…In a third but rather rare mode called neutral, OH groups and the O b atoms are imaged bright while O b vacancies and Ti 5c rows appear dark. Other noncommon image contrasts include the "all-inclusive mode" where both O b and Ti 5c are simultaneously resolved as bright features 11 and the "hidden mode" which does not exhibit OH groups. 12 This contrast variability in nc-AFM represents both an opportunity and a challenge.…”

Characterization of thin silicon overlayers on rutileTiO2(110)−(1×1)

“…This reduced basis represents a simpler approach with respect to the use of plane-waves, but it is known to produce good results at a moderated computing cost. 30 …”