Theoretical Investigations Of Adsorption And Switching Phenomenon Of Organic Molecules On Metal Surfaces

The flourish of the contemporary information society subsequently arsies from the continuous microminiaturization of silicon-based electronic devices.When the thickness of the insulating layer of silicon dioxide in the silicon wafer process is less than 0.7 nm,the quantum tunneling effect would lead to the failure of insulation,and the silicon-based device fabrication would be problematic.The traditional top-down fabrication process cannot meet requirements of further microminiaturization of the device.Feynman,the Nobel laureate and famous physicist,has pointed out in the 1950s: “Humans in the future may build molecular-sized microdevices that use molecules or even individual atoms as building blocks to constitute electronic components”.Molecular devices have higher storage capacity and faster response than siliconbased ones,and may even exhibit new effects and physical phenomena that are not available with traditional materials.Furthermore,the molecular device enables lower production costs.Among numerous applications of molecular devices,molecular switch is the most basic functions in functional molecular devices,which can be utilized in information storage and logic gates.The interface between the molecule and the electrode has an important influence on the transport performance of the molecular switch.A large number of studies have shown that the modification of the contact type can effectively control the interfacial coupling strength and transport properties,and consequently optimize the performance of the molecular device.However,the interfacical interactions are complicated,including covalent bonds,van der Waals forces,Pauli repulsive forces,hydrogen bonds,and charge transfer.It is difficult to provide quantitative characterization by experimental methods.Traditional density functional theory methods often lead to erroneous predictions of interfacical structure and stability due to the lack of accurate description of van der Waals forces.In this dissertation,the density functional theory,coupled with van der Waals force correction method including screening effect in metal,and non-equilibrium Green's function method were used to design molecular switches based on coexistent precursor state and chemisorbed state of organic molecules on metal surfaces.Single-molecule switches,ensemble molecular switches and then magnetic molecular switches have been systematically studied.The key scientific issues,i.e.,switching mechanism and modification method,involved in this field were investigated.We have revealed the switching mechanism of molecular switches based on bistable adsorption behaviors.The modification method of the stability and bistability was proposed.A novel strategy was proposed to realize fully controllable collective switching in ensemble molecular switches.The bistable ferromagnetic metal-organic spinterface was designed based on bistable adsorption states.It mainly includes the following research contents:(1)Since the switching mechanism of single molecule switch is unclear,the mechanism of switching transition is revealed.The structure and stability of metal-organic interface are modified by molecular functionalization and external field.The interfacial structure,stability and electronic properties of the physisorbed and chemisorbed states of antradithiophene/Cu(111)were studied.The delicate interplay between the strong and weak interaction forces was discussed,and the role of van der Waals forces in the structure and stability were clarified.The switching effects is substantially derived from the acene moiety while the sulfur atoms constrain the molecular position during the switching process.By molecular substitution,the anthradiselenophene/Cu(111)molecular switch with higher diffusion barrier and stability was obtained.The molecular functionalization and application of the external field can effectively regulate the structure,stability and electronic properties of metal-organic interface.The switching characteristics substantially depend on the interfacial charge redistribution and energy level matching.In order to obtain a bistable adsorption molecular switch and improve the interfacial structural stability and bistability,a molecule with a suitable HOMO-LUMO energy gap and a metal surface with a moderate activity should be selected.(2)To solve the problem that traditional techniques fail to realize precise and efficient switching of ensemble molecular switches,the surface strain strategy is proposed due to its tunning of the adsorption stability of molecules on metal surfaces.The metal surface strain has a significant influence on the stability of the chemisorbed molecule,whereas the stability of the physisorbed state remains almost constant.Therefore,the single-potential well can transist between the physisorbed and chemisorbed states,which consequently leads to fully controllable collective switching between physisorbed and chemisorbed states.Since the strain is uniformly applied to the entire surface,all molecules adsorbed on the metal surface can undergo fully controllable switching.The change of the metal-organic interface contact type significantly tailors the energy level matching between the molecular orbital and the metal states.When wired into circuits,the bistable interface can functioned as a conductance switch.The straindriven molecular switching is feasible for adsorption systems with different coverage and can be realized with different loading types.The zero point vibration energy presents slight influence on relative stability the two states.As the temperature increases,the contribution of entropy would be larger,and the relative stability of the two states may be inversed beyond 400 K.Remarkably,the strain-driven molecular switching phenomenon is general,which is further demonstrated in the molecular switches,such as pyridine,isoquinoline,anthracene,dithiophene and diselenophene on Cu(111).(3)To solve the problem that the bistability of traditional bistable metal organic spinterfaces is easily expired,a metal-organic spinterface based on bistable states of halogeno benzene on ferromagnetic metal surfaces is proposed.The halogenated benzene derivatives have bistable adsorption states on Ni(111).The atomic magnetic moment of Ni atoms on surface decreases monotonously with decreased adsorption height of the adsorbate.For the physisorbed state,the hybridization between the molecular orbitals and the metal states is weak,while for the chemisorbed state,the spin-polarized density of state of the surface atoms is significantly broadened.The magnetic transition of the metal atoms is substantially derived from the contribution of the d-? orbital interactions.The reversible control of the magnetic properties can thus be achieved by controlling the adsorption state of organic molecules on the ferromagnetic surface.The two states of tetrafluoropyrazine/Ni(111)system exhibit opposite spin filtering effects,which provides a new territory for memory devices.Moreover,the bistable adsorption metal-organic spin interface is a general phenomenon.By changing the molecular size,some more bistable spinterface systems have been obtained.By applying strain and element substitution,the stability of the chemisorbed and physisorbed states of spinterface is effectively modified,and spinterfaces with excellent bistability and suitable energy barrier can be consequently obtained.