Distance and Voltage Dependence of Orbital Density Imaging Using a CO-Functionalized Tip in Scanning Tunneling...

Area of Science:

• Surface Science
• Scanning Probe Microscopy
• Molecular Imaging

Background:

• Scanning tunneling microscopy (STM) is crucial for visualizing molecular orbitals, often termed orbital density images.
• Understanding the influence of tip-sample distance and bias voltage on STM imaging is key to interpreting molecular electronic structures.

Purpose of the Study:

• To investigate the appearance of frontier molecular ion resonances using STM with a CO-functionalized tip.
• To analyze the dependence of these resonances on bias voltage and tip-sample distance for specific molecular systems.

Main Methods:

• Utilized scanning tunneling microscopy (STM) with a CO-functionalized tip to image single molecules.
• Employed pentacene and naphthalocyanine on bilayer NaCl on Cu(111) as model systems.
• Determined absolute tip-sample distances using atomic force microscopy (AFM) and performed simulations.

Main Results:

• Observed a transition from predominant p-wave to s-wave tip contrast as the tip-sample distance increased.
• Noted minimal changes in contrast as a function of applied bias voltage.
• Simulations successfully reproduced the experimental data, including the distance-dependent contrast transition.

Conclusions:

• The distance-dependent contrast change in STM imaging is attributed to the differing decay rates of tunneling matrix elements for p-wave versus s-wave states.
• The study provides insights into the nature of tunneling contributions and their dependence on tip-sample separation.