The construction and structure-property manipulation of “small-molecule machines”

Abstract

Constructing low-dimensional quantum devices and manipulating their properties is one of the key research directions in physics. Due to their intrinsic structural diversities and ingenious functions as small building blocks, small molecules are a significant resource to build quantum machines for various applications, ranging from nano-rotors to ultrahigh-density information storage. The research in this project focuses on the design, construction, and structure-property manipulation of “small molecule machines” using scanning tunneling microscopy/spectroscopy (STM/STS). The primary discoveries include the following: (1) We construct a large-scale, highly-ordered array of anchored, single-molecule rotors with well-defined axis on gold surfaces using tetra- tert -butyl zinc phthalocyanine (( t -Bu)4-ZnPc) molecules. Experiments and theoretical calculations reveal the role of gold adatoms as the well-defined contact between the molecule and the surface. We further verified the manipulation of molecular rotors by putting the molecule at different regions of the surface and by changing the functional groups of the molecules. We also develop a new methodology to determine molecular configurations of a large molecular complex in a dynamical process on a metal surface by combining time-resolved tunneling spectroscopy ( I - t spectra) and density functional theory calculations (DFT). (2) We demonstrate a reversible conductance transition on the molecular scale (0.6 nm) in a complex of 3-nitrobenzal malononitrile and 1,4-phenylenediamine, by the application of local electric field pulses. Macroscopic and local current-voltage ( I / V ) measurements confirm an excellent electrical-bistability behavior. We also suggest that the conductance transition may arise from the disorder-to-crystalline transition in the thin film. (3) We investigate the structure and conductance of rotaxane molecules. For the first time, we reveal that the conductance switching in rotaxane-based devices is an inherent property of rotaxane molecules. The results suggest that the conductance switching should be attributed to movements of the cyclobis ring along the rod section of the dumbbell-shaped backbone of the rotaxane molecule. (4) By using hydrogen-atom adsorption on and desorption from manganese phthalocyanine (MnPc) molecules on Au(111) surfaces, we demonstrate reversible control of a Kondo resonance and a single-spin state in individual magnetic- molecule machines. We also reveal that hydrogen-atom adsorption leads to redistribution of charge within the 3d orbitals, which directly contributes to the Kondo resonance disappearance. This process is reversed by thermal annealing or applying local voltage pulse to desorb the hydrogen atom. A prototype of extremely high-density information storage (>40 TB/cm2) is achieved. (5) By applying an “atomic operation” on MnPc molecules, we create a new quantum system, dehydrogenated MnPc. In dehydrogenated MnPc molecules on Au(111) substrates, by using low-temperature, high-magnetic-field scanning tunneling microscopy, we achieved measurements of the Lande g -factor with intramolecular resolution. The magnetic-field dependence of the extended Kondo effect at different sites of the molecule reveals an inhomogeneous distribution of Lande g -factors and opens up a new route to access local spin properties within a single molecule. The above achievements provide insights for physics in quantum systems and have significant scientific value for the design of new-generation small-molecule machines.