Theoretical Studies Of Molecular Adsorption On Nano-Systems And Surfaces

With the progress in computational methods and the enhancement of computational ability,density functional theory(DFT)based first-principles calculation has been widely used in condensed matter physics,quantum chemistry,and material science. Nowadays,due to their peculiar structures and properties,nanomaterials have attracted a broad research interest.Modifying the electronic structures of nanotubes by organic molecule encapsulation is very important to nano device applications.Meanwhile, molecule adsorption on surfaces leads to novel structures and properties.Studies on these systems are important for development of new low-dimensional materials and for understanding the surface catalysis.The dissertation is devoted to the study of physical and chemical properties of nanotubes and surfaces with molecule adsorptions from first-principles calculations.In the first chapter,we introduce the basic concept of DFT and review its recent progress.People have been making great efforts to find good approximation for the exchange-correlation functional in DFT.With the development of modern functionals, we can obtain more and more accurate results.In addition,extension of DFT to the time dependent region and combination of DFT with the dynamic mean field theory (DMFT)are also active topics.All these progresses in DFT lead to a broad range for application.This is illustrated by some examples at the end of this chapter.In the second chapter,we study the boron nitride nanotubes with organic molecule encapsulation.Boron nitride nanotubes have uniform geometries and electronic structures, and show pronounced resistance to oxidation with high thermal stability.Therefore, boron nitride nanotubes may play an important role in nano-device application. However,the band gap of boron nitride nanotubes is very large(about 5.5eV).It is desirable to modify the band structure to obtain a metallic or a p-type/n-type semiconducting behavior.Recent experiments show that,the band structures of carbon nanotubes can be modified through organic molecule encapsulation.Here,we perform first-principles calculations to study the electronic structure properties of boron nitride nanotubes with organic molecules encapsulated.We fred that,we can obtain a p-type semiconductor by doping electrophilic molecules,while for typical nucleophilic organic molecule doped BNNTs,their gap is too large to be considered as a good n-type semiconductor.Since applying transverse electric field may change the band gap of nanotubes according to the giant stark effect,we also study the effect of a transverse electric field on these doped systems.It is shown that electric fields greatly modify the band structure,the charge transfer,and the optical properties.In addition,boron nitride nanotubes provide a relatively good electrostatic shielding for the inside organic molecular chain.In the third chapter,we study the C₆₀monolayer adsorbed on Ag(100)surface. The bulk fulleride A₃C₆₀(A=K,Cs)are confirmed to be superconductors with high transition temperatures.Thus it is very interesting to check whether there is possible two-dimensional superconducting state in the C₆₀monolayer on Ag(100).Cooperating with the experiment group leaded by Prof.X.D.Xiao at Chinese University of Hong Kong,we study the geometries and electronic structure properties of this system via combination of STM/STS experiment and DFT calculations.Our results reveal that the bright-dim contrast has a definite geometric origin and there are two types of dim C₆₀molecules,one is a monomer and the other is a dimer.We make comprehensive investigation on the various electronic properties and charge transfers in the differently adsorbed C₆₀molecules,and discuss the possibility of superconducting states in this system.In the fourth chapter,we study the defect states at the hydroxylated TiO₂(110) surface.TiO₂ has been paid a great attention due to its broad applications on photocatalysis, heterogeneous catalysis,and solar energy conversion.Recently it is revealed that,the reduced TiO₂ surface can be easily hydroxylated by reacting with the adsorbed water molecule.However,the electronic property of the OH defects is only very poorly understood.Cooperating with the experiment group leaded by Prof.J.G.Hou and Prof.B.Wang,we study the defect stats introduced by single OH group or OH pair groups on TiO₂(110)via STM and DFT calculations.The electronic state of OH group defects is found to be delocalized on the surface through STM experiment.Our DFT calculation with a vary large superceU c(1x12)give reasonable band structures with de- fect state separated from the conduction band bottom,and successfully reproduce our STM experiment observations.The remarkable agreement between theoretical and experimental results in this work makes it necessary to reconsider the previously doubted applicability of the generalized gradient approximation(GGA)for describing the TiO₂ (110)system.