Force Spectroscopy of Iron Tetraphenylporphyrin Molecules with Cl Tips

Abstract

At low sample voltages, Cl ions may be bidirectionally transferred between a Fe tetraphenylporphyrin (FeTPP) molecule on Au(111) and the tip of a low-temperature scanning tunneling microscope that also operates as an atomic force microscope. Via this effect, Cl-terminated gold tips are prepared. Chlorinated and pristine metal tips are used to probe the forces and conductances of contacts to Au(111), FeTPP, and chlorinated FeTPP. The interaction between the substrate and the Cl ion at the tip may be modeled by a point charge of ≈−0.3 e interacting with its image. The maximal attraction experienced by the tip in junctions with one Cl ion is much larger when the ion is located at the molecule. Junctions with two Cl ions exhibit two maxima of the attractive force. Simultaneous imaging of the tunneling current and the frequency shift of the force sensor with Cl tips reveals intriguing contrast changes that are tentatively interpreted in terms of tip-induced distortions of the molecules. Density functional theory calculations reproduce important aspects of the experimental data.

Tracking the Interaction between a CO-Functionalized Probe and Two Ag-Phthalocyanine Conformers by Local Vertical Force Spectroscopy

“…Extrema in the vertical force variation between a CO-terminated tip and a surface can also reflect the bending of the CO probe . Similar effects were reported for Cl-terminated tips, where the Cl atom may translate at the tip apex . The flexibility of CO was previously identified as the cause for the AFM contrast mechanism ,− as well as for the modification of apparent bond lengths , in the Pauli repulsion range of probe–adsorbate separations.…”

“…17 Similar effects were reported for Cl-terminated tips, where the Cl atom may translate at the tip apex. 18 The flexibility of CO was previously identified as the cause for the AFM contrast mechanism 10,19−21 as well as for the modification of apparent bond lengths 11,22 in the Pauli repulsion range of probe−adsorbate separations. The close proximity of the probe was likewise shown to impact the geometry of the adsorbate complex, such as the tilting of molecular adsorbates in the vicinity of a metal tip 23 or different preferred orientations of an adsorbed molecule depending on the distance to a CO-terminated probe.…”

Extraction of Chemical Reactivity and Structural Relaxations of an Organic Dye from the Short-Range Interaction with a Molecular Probe

“…7,8 With these tools at hand, the chemical bond order was determined 9 and intramolecular changes upon chemical reactions were identified. 10 The subpicometer control of the distance between an intentionally terminated AFM tip and the surface further enabled the observation of the interaction between reaction partners at the single-atom and singlemolecule level, such as the coupling between two CO molecules, 11 the bending of molecular adsorbates in the vicinity of a CO-terminated tip, 12 the probing of physisorbed and chemisorbed states in a CO−Fe contact, 13 relaxations 14 and nonequilibrium bond forces 15 in C 60 −C 60 junctions, the identification of chemically reactive Fe cluster sites, 16 the probing of acidity of surface hydroxyl groups with a OHterminated tip, 17 the motion of atomic 18 and molecular 19 terminations of the tip apex in the vicinity of the surface as well as forces and energies at the verge of a chemical reaction. 20 Moreover, insights into the force fields atop adsorbed molecules, 21 the resolution of H atoms at the onset of Pauli repulsion between the functionalized probe and the imaged molecule 22 as well as the anisotropic charge distributions at surfaces 23 were reported.…”

Black Phosphorus for Directed Molecular Assembly with Weak Electronic Coupling

“…We focus our study on FeTPP only, since the spectroscopic measurements on FeTPP-Cl often induce removal of the Cl ion and tip modification. [29] The unit cell vectors of the molecular lattice have lengths 1.25 ± 0.01 nm along the [100] direction and 1.31 ± 0.01 nm along the [010] direction (see more examples in Figure S1, Supporting Information) which are smaller than the intermolecular distance obtained in FeTPP layers (and other TPP molecules) on other surfaces (see Figure S2, Supporting Information). Indeed, on Au(111) FeTPP arrange in a rectangular lattice with a periodicity of 1.40 ± 0.05 nm and 1.55 ± 0.05 nm, [27] on Ag(111) it forms a square lattice with a period of 1.43 ± 0.01 nm, [30] on graphene [28] we have found a square lattice with a periodicity of 1.4 ± 0.02 nm (see Figure S3, Supporting Information).…”

“…Those with a dark center correspond to FeTPP. We focus our study on FeTPP only, since the spectroscopic measurements on FeTPP−Cl often induce removal of the Cl ion and tip modification [29]. The unit cell vectors of the molecular lattice have lengths 1.25 ± 0.01 nm along the [100] direction and 1.31 ± 0.01 nm along the [010] direction (see more examples in supporting information Figure S1) which are smaller than the intermolecular distance obtained in FeTPP layers (and other TPP molecules) on other surfaces (see supporting information Figure S2).…”