Preparation of Molecular Rotors and Motors on Semi-metallic Surfaces and Their Rotational Properties Investigatedby STM

Author:Sun Kai

Supervisor:wang jun zhong


Degree Year:2019





Nanomaterials can be defined as physical substances of which a single unit is sized(in at least one dimension)between 1 and 100 nanometers.Due to their unique physical and chemical properties different from the bulk materials,nanomaterials have attracted considerable attention of scientists in recent years.In these nanomaterials,molecular machines with specific functions at the molecular scale have aroused wide interest of researchers.In spite of its small size,it can perform the same functions as a traditional machine.So far,scientists have successfully developed a series of molecular machines,such as molecular elevator,molecular muscle,molecular motor,molecular car,etc.Molecular motor,a molecular machine that can convert external energy into directional motion or rotation,have become an important research branch of molecular machines.Scanning tunneling microscope is a suitable tool for studying the structure and rotational property of molecular motors on solid surface because it can not only monitor the rotation of a single molecule,but also study the relationship between molecular rotation and temperature,external field,energy of tunneling electrons,and investigate the effect of neighboring molecules on rotation.However,up to now,most of the research results about molecular motors on solid surface are single-molecule motors,and surface-mounted supramolecular motors have rarely been addressed.Water is the most common substance in nature and also the essential substance for life.Therefore,lucubration of the interaction between water and solid surface is benificial to solving a number of important issues closely related to our life.In the past few years,the dissolution mechanism of hydrated electrons in water structure has become a research hotspot.Hydrated electron has unique advantages in dealing with water pollution and air control due to its strong chemical activity.As an important intermediate product in the biological system of radiation,hydrated electron can also pose a great threat to human health in the process of high-energy radiation.Therefore,the research on hydrated electron will poses profound impact on physical chemistry,radiation biology,environmental science and other fields.Although scientists have achieved many important successes in studying the dissolution mechanism of hydrated electrons in different water structures,few hydrated electronic structures have been directly observed by means of STM.There have been no reports on molecular motors induced by hydrated electrons.In this paper,the off-center rotation and the adsorption orientation transition of copper phthalocyanine(CuPc)on semi-metallic Bi(111)surface,ethanol supramolecular rotors and motors induced by hydrogen bonds and charged state and water supramolecular rotors and motors induced by hydrated electrons are systematically investigated by a low temperature scanning tunnel microscopy(LT-STM)system.The specific research content and results are as follows:1.Off-center rotation and adsorption orientation transition of CuPc on Bi(111)The self-assembled structures of CuPc was prepared on semi-metallic Bi(111)substrate by means of molecular beam epitaxy.At liquid nitrogen temperature,an individual CuPc molecule can make an off-center rotation on the substrate until it falls into a defect or encounter another CuPc molecule.In the sub-monolayer films,each achiral CuPc molecule exhibits a flat-lying orientation with chiral feature.The chiral feature can be attributed to the combination of molecular-substrate asymmetric charge transfer and molecular-intermolecular van der Waals interaction.In the multilayer films,CuPc molecules adopt a standing-up adsorption orientation.More importantly,a molecular orientation transition from flat-lying in sub-monolayer to standing-up in multilayer films is observed due to the inhanced intermolecular interactions.2.Ethanol supramolecular rotors and motors induced by hydrogen bonds and charge state on graphite surfaceEthanol molecules can self-assembled into nanoclusters with different sizes and molecular configurations on graphite surface.The individual clusters can be charged by tunneling electrons from a scanning tunneling microscopy tip,and then trap the ethanol chains with permanent dipole moment.Serving as a rotator,the trapped ethanol chain rotate around a charged cluster randomly driven by the inelastic tunneling electrons,forming a supramolecular rotor.Directional rotation can be realized by introducing a chiral branch to the near end of ethanol chains to suppress the chiral flipping with steric hindrance.3.Water supramolecular rotors and motors induced by hydrated elections on graphite surfaceWater clusters and molecular chains with different sizes and configurations are prepared on graphite surface by variable temperature deposition.After accepting a hydrated electron from STM tip,the water cluster transfers into a negatively charged clusters which can trapping a water molecular chain with a permanent dipole.The molecular chain can rotate around the charged cluster randomly with either clockwise or anti-clockwise direction,resulting a surface surface-mounted water supramolecular rotor.Directional rotations of supramolecular motors have been achieved as the chirality of the molecular chain increases.Our results demonstrate that the configuration of the water molecular chain is one of the key elements required for directional rotation because of the increased rotational energy barrier brought by chirality.