Using Metallic Noncontact Atomic Force Microscope Tips for Imaging Insulators and Polar Molecules: Tip Characterization and Imaging Mechanisms

Abstract

We demonstrate that using metallic tips for noncontact atomic force microscopy (NC-AFM) imaging at relatively large (>0.5 nm) tip-surface separations provides a reliable method for studying molecules on insulating surfaces with chemical resolution and greatly reduces the complexity of interpreting experimental data. The experimental NC-AFM imaging and theoretical simulations were carried out for the NiO(001) surface as well as adsorbed CO and Co-Salen molecules using Cr-coated Si tips. The experimental results and density functional theory calculations confirm that metallic tips possess a permanent electric dipole moment with its positive end oriented toward the sample. By analyzing the experimental data, we could directly determine the dipole moment of the Cr-coated tip. A model representing the metallic tip as a point dipole is described and shown to produce NC-AFM images of individual CO molecules adsorbed onto NiO(001) in good quantitative agreement with experimental results. Finally, we discuss methods for characterizing the structure of metal-coated tips and the application of these tips to imaging dipoles of large adsorbed molecules.

Investigating atomic contrast in atomic force microscopy and Kelvin probe force microscopy on ionic systems using functionalized tips

“…The contributions from van der Waals forces cannot be responsible for the observed tip-dependent contrast inversions, in particular not for the large attraction above the vacancy for the Au and Cl tip. This leaves the electrostatic interactions as the most important origin of the atomic contrast for our system, in agreement with recent studies by Gao et al [41] and Schneiderbauer et al [42].…”

“…Table 1 shows that, except for Au tips, the dipole moments of the smaller five-metal-atom tips agree quite well with those of the larger 26-metal-atom tips. Note that we obtained dipole moments that are comparably large with respect to adsorbates on planar Cu surfaces, [44,45] which is an effect of the pyramidal shape of the tip apex as shown recently for metallic tips by Gao et al [41]. The measured ∆f * contrast of the Cu, Cl, and Xe tips could be understood from their respective dipole moments: The attraction is increased above sample charges of opposite sign with respect to the tip apex for these three tips.…”

Recognizing nitrogen dopant atoms in graphene using atomic force microscopy

“…The size of the tip cluster was limited to four atoms (in two layers) to balance the computational cost (considering the large unit cell needed to describe the substrate) with the accuracy (we used small oscillation amplitudes, increasing the sensitivity to short-range chemical forces). However, our calculations will underestimate the magnitude of the electrostatic contribution to the total tip-sample force [43]. During these calculations the tip-apex atom, the probed atom, and its three nearest neighbors were allowed to relax their position; the positions of all other atoms of the graphene and iridium slab were fixed.…”

“…From DFT calculations the lateral stretching in the graphene network induced by the substitutional dopant is only about 1% in compression to the nearest-neighbor distance. Thus these distortions are imaging artifacts and are most likely due to the electrostatic interaction between the dopant atom, which has a partial positive charge, and the dipole moment of the tip [42,43,47,48]. A Bader charge analysis shows that the N atom has a charge of 1.27e, where 73% of that charge is extracted from the 3 neighboring C atoms donating 0.3e each to the N atom.…”

Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy

“…3, along with a Δf image and the DFT calculated charge distribution, for comparison. Note that the dipole maps are expected to be a convolution of the local dipoles with the lateral profile of the electric field beneath the tip [40], smearing out the apparent dipole distribution. Remarkably, this map still shows pronounced intramolecular contrast without suffering from similar artifacts as the KPFS-derived map shown in Fig.…”