Local force gradients on Si(111) during simultaneous scanning tunneling/atomic force microscopy

Abstract

The authors report simultaneous scanning tunneling and force imaging of Si(111) 7×7 with sub-angstrom oscillation amplitudes. Both constant height and constant current scans with tungsten tips/levers always showed larger attractive stiffness over corner holes than over adatoms, the opposite of theoretical expectations. Constant height scans show that this cannot be explained by interaction of tip motion with long range forces. Silicon levers, however, sometimes exhibited inversions of force contrast following local tip changes. The authors suggest that there may be charge variations between atomic sites on the surface, which produce electrostatic tip forces additional to the covalent forces usually regarded as dominant.

Detailed scanning probe microscopy tip models determined from simultaneous atom-resolved AFM and STM studies of theTiO2(110)surface

“…9 From recording numerous images, it is, however, our experience that constant height imaging is somewhat unstable on the rather corrugated TiO 2 ͑110͒ surface to sustain stable atomic resolution for a longer period.…”

“…Nevertheless, recent results have demonstrated the great potential of simultaneous AFM/STM imaging. 8,9 In the present study we overcome the problem associated with the lack of information about the imaging tip by analyzing a large set of simultaneously recorded atom-resolved topographic nc-AFM and tunneling current ͑I t ͒ images of the TiO 2 ͑110͒ surface recorded with a number of different tip terminations. We make use of the fact that rather detailed knowledge on the nature of the tip apex is already available from previous nc-AFM studies and simulations of this surface.…”

Repulsive interaction and contrast inversion in noncontact atomic force microscopy imaging of adsorbates

“…7,9,10 Besides protruding features, contrast inversion has also been observed on bare surfaces such as Au͑111͒ 11 and Si͑111͒-͑7 ϫ 7͒. [12][13][14] A comprehensive analysis of the mechanisms behind contrast inversion requires means for differentiating whether the tip enters the repulsive regime. Moreover, feedback loop effects need to be considered when imaging in the constant-detuning mode as the feedback loop response may heavily influence contrast formation.…”

“…Moreover, feedback loop effects need to be considered when imaging in the constant-detuning mode as the feedback loop response may heavily influence contrast formation. 13,15 In this paper, we present an analysis clarifying whether or not the cantilever tip enters the repulsive regime based on the measured frequency shift, ⌬ƒ͑z͒, and the actual oscillation amplitude. This analysis is used for elucidating physical mechanisms behind contrast inversion observed in NC-AFM imaging.…”

Imaging of the Hydrogen Subsurface Site in RutileTiO2

“…The sensitivity to atomic defects may therefore be distinctly different in AFM and STM due to a larger range of electronic states pinned at defects. When applied simultaneously [7][8][9], the complementary information obtained from STM and AFM may be used in a unique way to deconvolute geometric and electronic effects and, as demonstrated here for a TiO 2 ð110Þ model system, discriminate between two types of atomic H species in TiO 2 -H in surface OH groups, which appear in both channels, and subsurface H atoms as OH sub groups, which exclusively appear in the STM images as an electronic perturbation. DFT total energy calculations and STM image simulations implementing the accurate nanoscale structure of the imaging tip verify the energetic stability and the characteristic STM I t signature of both sOH and OH sub species.…”