Understanding image contrast formation in TiO2with force spectroscopy

Abstract

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.

Force mapping on a partially H-covered Si(111)-(7×7) surface: Influence of tip and surface reactivity

“…1,3 The short-range interaction force can be determined by force spectroscopy (FS), in which forces are measured as a function of distance. In particular, FS measurements have been performed on a variety of surfaces, including semiconductors, 4,5 metals, 6,7 metal-oxides, 8 ionic crystals, 9,10 single-walled carbon nanotubes (SWNT), 11 and graphite (0001). 12 Recently, the capability of atomic-scale imaging and spectroscopic probing of interaction forces was extended to organic molecules by using a CO-functionalized metallic tip, 13 and the observed contrast was assigned to Pauli repulsion.…”

A Preparation Method for Atomically Clean Sapphire Surfaces and High Resolution Topographic Method for Their Imaging by Non-Contact Atomic Force Microscopy

“…38) This tendency is also seen in the case of many NC-AFM measurements such as TiO 2 (110). 28,29) For imaging structures in greater detail, we need to determine the proper measurement conditions, such as the ideal tip apex material, oscillation amplitude, cantilever spring constant, and so on.…”

“…Iterations of the Ar + sputtering and annealing improved the surface condition, just as done for the TiO 2 surface. 28,29) However, it should be noted that, even when just a single iteration of the Ar + sputtering and annealing was performed, a periodic pattern emerged, as in Fig. (c) and (d) were required to obtain a clear surface.…”

“…(c) and (d) were required to obtain a clear surface. 28,29) The repeated process of Ar + sputtering and annealing removed the surface layers having defects and contaminants. The requirement of different sputtering conditions between the two surfaces indicated that the surface layers of Al 2 O 3 (0001) had fewer defects and contaminants.…”

Atomic Force Microscopy for Imaging, Identification and Manipulation of Single Atoms

“…In the image of TiO 2 surface with adsorbed H atoms ( Fig. 1(c)), individual H atoms that are adsorbed on O atoms are imaged [8][9][10]. In the image of a single PTCDA molecule, chemical structure inside the molecule can be resolved by Pauli repulsive force even at room temperature [11].…”