Bond-order discrimination by atomic force microscopy.

Area of Science:

• * Surface science
• * Chemical physics
• * Materials science

Background:

• * Polycyclic aromatic hydrocarbons (PAHs) and fullerenes are crucial carbon-based materials with diverse electronic properties.
• * Characterizing the precise nature of chemical bonds within these molecules is essential for understanding their reactivity and functionality.
• * Atomic Force Microscopy (AFM) offers high-resolution surface imaging but distinguishing subtle electronic differences, like bond order, remains challenging.

Purpose of the Study:

• * To demonstrate the capability of noncontact atomic force microscopy (AFM) with a functionalized tip to differentiate carbon-carbon bond orders.
• * To elucidate the physical mechanisms behind the observed contrast variations in AFM images related to bond order.
• * To validate experimental findings using theoretical calculations.

Main Methods:

• * Noncontact atomic force microscopy (AFM) utilizing a carbon monoxide (CO)-functionalized tip.
• * High-resolution imaging of polycyclic aromatic hydrocarbons (PAHs) and fullerenes.
• * Density functional theory (DFT) calculations for theoretical validation.

Main Results:

• * AFM imaging with a CO-functionalized tip successfully distinguished varying carbon-carbon bond orders.
• * Two contrast mechanisms were identified: increased Pauli repulsion at higher bond orders enhanced image brightness.
• * Apparent bond length in AFM images decreased with increasing bond order due to CO tip apex tilting.

Conclusions:

• * CO-functionalized AFM is a powerful tool for probing electronic structure at the nanoscale.
• * The interplay of Pauli repulsion and tip-molecule geometry dictates AFM contrast for bond order differentiation.
• * This method provides new insights into the chemical bonding of complex carbon materials.