Theory of tip‐dependent imaging of adsorbates in the STM: CO on Cu(111)

Abstract

ABSTRACT:The processes of local electron injection or extraction in the scanning tunneling microscopy (STM) and spectroscopy (STS) lead to the creation of short-lived excited states localized at the electrode surfaces. The dynamic relaxation of the transient negative or positive ion resonances, due to both local and long-range interactions, is the clue to the understanding of numerous phenomena in STM/STS ranging from the "anomalously" large tip height corrugation amplitudes on clean metal surfaces to the observation of quantum mirages and features in the STS, which are not observed with the help of other surface spectroscopies. Quantum nanodynamics theory (QND) has been applied to calculate the interaction potential of a single CO molecule with the Cu(111) surface, with a transient negative ion resonance formed when an electron is injected from the tip, and the tunneling conductance on the clean and CO covered Cu(111) surface using a clean metal tip Al/Al(111) and a Pt(111) tip with an adsorbed CO molecule at the apex. Within QND and three-dimensional scattering theory, regarding the tunneling as an excited-state problem, we provide the explanation of the tip-dependent STM image of a single CO molecule on Cu(111). The appearance of the CO molecule as an indentation, using a clean metal tip and as a protrusion with a tip terminated by a CO molecule, is understood as a result of tunneling through two competing channels. Tunneling via adsorbate-induced ion resonances enhances the tunneling conductance. In contrast, tunneling via metal ion resonances only leads to attenuation of the conductance in the presence of the adsorbate. The current in the vicinity of the adsorbed CO molecule is reduced when a clean metal tip is used; i.e., CO appears dark in the STM image, because metal ion resonances on Cu(111) derive from the surface states with image state components coupling to plasmons and are therefore very diffuse. With a CO-terminated tip, the major current channel is, for symmetry reasons, from the 2π-derived orbital of the tip CO molecule, via the diffuse 2π-derived orbital of the CO molecule on the sample, hence adsorbed CO appears bright.

Adsorption and Diffusion of CO on Clean and CO2-Precovered ZnO(101̅0)

“…These species were assigned to CO molecules as further confirmed by in situ dosing experiments (see Figure S1 in the Supporting Information). It is well acknowledged that the STM topography of an isolated CO molecule on metal surfaces can sensitively depend on the termination of the tip, and it has been also observed in the current case of the ZnO surface. As illustrated in Figure b, CO exhibits as a normal pit in the top part of the image when the tip is metallic, while it presents a specific sombrero feature when the tip switched to a CO-termination after picking up CO molecules as marked by the yellow arrow.…”

Observations of image contrast and dimerization of decacyclene by low temperature scanning tunneling microscopy

“…6,42 Comparing the observations of DC shown in Figs. 6,42 Comparing the observations of DC shown in Figs.…”

“…23 Tip-dependent chemical contrast behavior has been explained through the variety of computational methods described previously. 41,42 In the CO/Cu͑111͒ system, electronic coupling between the sp surface band edge at −0.44 eV and adsorbate MOs results in surface state quenching upon adsorption. This leads to CO molecules appearing as depressions when imaged with a bare tip.…”

How Precisely Can Individual Molecules Be Analyzed? A Case Study on Locally Quantifying Forces and Energies Using Scanning Probe Microscopy

“…For the tip to pick up a CO molecule from the Ag(111) surface, consecutive scans in STM mode with a bias of a few mV and various currents up to 50 pA were carried out over a CO molecule. This caused the tip to be in very close proximity to the CO and eventually resulted in the transfer of the CO molecule to the very end of the tip apex, as determined by the disappearance of a CO molecule in the STM images and a change in STM contrast . As detailed in the experimental apparatus section, the tip is prepared by controlled nanoindentation resulting in Ag at the tip apex; thus, CO is considered to be bound to Ag on the tip.…”

“…This caused the tip to be in very close proximity to the CO and eventually resulted in the transfer of the CO molecule to the very end of the tip apex, as determined by the disappearance of a CO molecule in the STM images and a change in STM contrast. 96 As detailed in the experimental apparatus section, the tip is prepared by controlled nanoindentation resulting in Ag at the tip apex; thus, CO is considered to be bound to Ag on the tip. The CO−Ag interaction is a weak physisorption interaction.…”