Scanning Tunneling Microscopy Studies and First-Principles Calculations of Molecular Rotors

Abstract

A molecular machine is an assembly of molecular components that can perform machine-like movements due to external stimulation. A molecular rotor, an important building block towards functional artificial molecular machines, is a molecular structure that can rotate via a predefined axis, with or without external stimuli.

Scanning tunneling microscope (STM) is capable of studying molecular machine/molecular rotor systems. It is an instrument that operates based on the quantum tunneling effect. Apart from imaging atom-scale surface structure, an STM can also detect and control the movement of a molecular machine. A number of molecular machine/molecular rotor systems have been studied using STM.

In this thesis, we study two examples of molecular rotors on the metal substrate using STM: a kind of originally synthesized standing rotor molecules adsorbed on Cu(111) surface and Au(111) surface, and ferrocenecarboxylic acid (FcCOOH) molecules adsorbed on Au(111) and Ag(111) surface.

In Chapter 1, the background of the research field of molecular machine and molecular rotor is briefly reviewed. Inspired by natural molecular machines, scientists have been working on creating artificial molecular machines that have potential applications in the field of biomedicine and nanoelectronics. These previous efforts have led us to the project of molecular rotors investigations using STM.

In Chapter 2, the working mechanism of STM is introduced, and one more example of molecular machines studied by STM is given to explain the methodology and research protocol. The large-array of molecular rotors and standing molecular rotors on the metal surface are of significant concern for future studies on potential STM-controlled molecular rotors. The STM system we used in the experiment is also introduced.

In Chapter 3, we introduce the calculation methodology related to our projects. We used density functional theory and time-dependent density functional theory to explain the experimental finding. A unique intramolecular torque-based approach is introduced.

In Chapter 4, we study a molecular rotor system. A newly synthesized molecule is observed to stand on the Cu(111) surface and Au(111) surface. Rod-like and ring-like shapes of molecules are found. DFT calculations prove that the rod-like shape is the STM image of the molecule standing still on the surface. While investigating the conformation for ring-like shape STM image, the torque approach is used to study the tendency of rotation qualitatively. A self-assembly array of standing rotor molecules is also found on the Ag(111) surface.

In Chapter 5, the self-assembly of the FcCOOH molecule on the metal surface is reported. A novel honeycomb shape self-assembly is found on the Ag(111) surface. A self-assembly mechanism is proposed, and DFT calculation is carried out to verify it.

In Chapter 6, we conclude the two projects and propose future work regarding the projects and the topic of STM-controlled molecular rotors.

Date of Award	8 Feb 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Ruiqin ZHANG (Supervisor)