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

Abstract

We report force mapping experiments on Si(111)-(7×7) surfaces with adsorbed hydrogen, using atomic force microscopy at room temperature supported by density functional theory (DFT) simulations. On the basis of noncontact atomic force microscopy (NC-AFM) images as well as force versus distance curves measured over both hydrogen-passivated and bare Si adatoms, we identified two types of tip termination, which result in different modes of interaction with the surface. The statistics of the tip dependence of the measured forces, which are effectuated using various tip states with different cantilevers, reveal the typical values of the force and their distribution in the two characteristic interaction modes. The experimental results are corroborated by DFT calculations performed for different tip structures. As a reactive tip, the dimer-terminated Si tip yields results in satisfactory agreement with experimental force curves for hydrogen-passivated and nonpassivated Si adatom sites. An oxidized Si dimer tip that bears a hydroxyl group on its apex reproduces well the experimental force curves acquired by nonreactive tips. This tip model could thus be used to interpret the experimentally obtained weak image contrast for the Si(111)-(7×7) surface. The forces are thought to arise as a result of a weak electrostatic interaction involving a permanent dipole at the tip apex enhanced by the charge density redistribution due to the interaction with surface adatoms.

Intramolecular bonds resolved on a semiconductor surface

“…We first note that our DFT calculations do not include contributions due to local dispersion (i.e., short range van der Waals) interactions, and as such we expect them to systematically underestimate the experimentally observed forces. We also note that our simulations appear to systematically underestimate the tip-sample forces for well-studied silicon clusters compared to similar simulations performed using a different DFT code [32]. Finally we emphasise it is likely that our simplified tip models do not fully capture the complex tip structure present experimentally.…”

“…On the adatoms we see a strong attractive force, with a peak attraction of ∼2 nN. In the data acquired with the repulsive adatom contrast, we see a similar interaction over the molecule (peak force ∼100 to 150 pN), but an extremely weak interaction with the adatoms (∼10 to 50 pN), strongly suggesting a passivated tip [30][31][32].…”

“…Fixed tip atoms are grayed out while surface atoms which are free to relax are shaded in green. which observed that H or OH tip terminations suppress the chemical attraction [30,32]. Both of these tips are passivated, and, by analogy to the imaging mechanism described for CO-terminated metal tips [1], are likely to be able to produce submolecular contrast while also resulting in repulsive contrast on the silicon adatoms.…”

Measuring the reactivity of a silicon-terminated probe

“…2(a)]. The imaging, and large peak attractive forces, show that the tip is not passivated by CO or OH, since this would result in inverted imaging of the adatoms, and/or peak tip-sample tip-adatom forces of the order of ∼100 pN [31][32][33]. The combination of STM and NC-AFM characterization consequently strongly suggests a silicon-terminated tip apex, particularly in light of the data presented in Fig.…”

Electronic and Chemical Properties of Donor, Acceptor Centers in Graphene

“…In other words, the short-range forces acquired over different sites provide information about local chemical reactivity on different surface sites. 28,29 Therefore 3D maps of the short-range force can provide further understanding of local chemical reactivity of doped graphene and potential of the dopants as anchoring sites for graphene functionalization with organic molecules. [30][31][32] Here we employ 3D set of dAFM measurements to analyze the local interaction over B and N dopants.…”