Atomic Force Microscopy Studies On Structural And Physical Properties Of Functional Molecules On Metal Surfaces

Molecule–substrate interactions play an important role in determining the structures and physical properties of functional molecules adsorbed on solid surface,and hence influencing the applicational performance of functional molecules.Studying the structures and physical properties of functional molecules adsorbed on solid surface are essential research directions of molecular electronics and surface physics,which can provide useful information for potential applications of functional molecules.Owing to its high spatial resolution and single atom/molecule manipulation capabilities at atomic scale,scanning probe microscopy(SPM)has proven to be the powerful tool for single molecule studies.As an important branch of SPM,non-contact atomic force microscopy(NC-AFM)detects forces between the tip and under-tip atoms/molecules rather than tunneling current,and therefore able to identify molecular adsorption configurations and image intermolecular chemical bonds.Moreover,NC-AFM is capable of manipulating single atoms/molecules the same way a scanning tunneling microscopy(STM)does,therefore becomes an effective approach to study the structures,properties,and most importantly,manipulation and tuning of molecules adsorbed on solid surfaces.In this thesis,we used NC-AFM as the tool to study the adsorption configurations,structure manipulation and physical properties tuning of functional molecules adsorbed on metal substrates,which has yielded the following original research results.For the first work in this thesis,we mainly studied the adsorption configuration of a small organic molecule,phenylacetylene,adsorbed on Cu(111)substrate.STM constant current topography showed featureless bright spot for phenylacetylene molecules adsorbed on Cu(111).Using nc-AFM,however,the line features representing the C–H and C–C bonds in phenyl rings are evident,implying a possible standing configuration in which the phenyl ring plane is almost perpendicular to the substrate plane.Further density functional theory(DFT)calculations reveal multiple optimized adsorption configurations of standing phenylacetylene on Cu(111)substrate.Comparing nc-AFM simulations with experimental observation,we identify the adsorption configuration of PA on Cu(111)to be a standing styrene derivative,in which the acetylenic bond opens one ? bond and forms bonds with the underlying copper atoms,resulting in a standing configuration on substrate.This work combines nc-AFM real-space imaging and DFT calculations to identify adsorption configurations of functional molecules adsorbed on substrates,and lay a foundation for further study of physical properties and tuning physical properties of functional molecules.For the second work in this thesis,we mainly studied on-surface isomerization and structure manipulation of functional molecules adsorbed on substrates.Using NC-AFM,we studied a polycyclic aromatic hydrocarbon with dimethylamino groups,dimethylamino-2,6-di(2-anthryl)-benzene(DMADAB)molecule,by imaging its adsorption configurations on Ag(100)substrate,and triggered its on-surface isomerization by tip lateral manipulation.We found that DMADABs can isomerize between all three on-surface isomers reversibly and repeatedly by tip lateral manipulation.With the help of DFT calculations,we show that the interaction force between the tip and the out-of-plane dimethylamino groups in DMADABs as well as the torque applied on dimethylamino groups by the tip is the key in this process.To do a control experiment,we detached the dimethylamino groups in DMADABs by substrate annealing and transfer them into planar 1,3-di(2-anthryl)-benzene(DAB)molecule.DABs,however,cannot isomerize under the same manipulation conditions,which further emphasize the crucial role of dimethylamino groups in the process of onsurface isomerization.This work emphasizes the tip–molecule interaction force in onsurface molecular isomerization,and opens up opportunities to manipulate molecular configuration with new force mechanism.Moreover,this work also shows that out-ofplane functional groups have the feasibility to act as an “ON/OFF” switch to isomerization of molecules on surface,and provide new ideas to further study of tuning physical properties of functional molecules by STM tip.For the third work in this thesis,we mainly studied the adsorption behaviors and physical properties tuning of molecular rotors on solid surfaces.Using a tetra-tert-butyl nickel phthalocyanine molecule adsorbed on Au(111)substrate,we demonstrate that both tip-molecule distance and sample bias can modify the rotational potential of molecular rotors,and hence tuning their rotational behaviors.Further experiments by control variate method indicates that the mechanism of modifying the rotational potential is a combination of the van der Waals interaction and the interaction between the molecular dipole and an electric field.We achieved the potential energy difference variations of ?0.3 meV/pm and ?18 meV/V between two configurations of this tetratert-butyl nickel phthalocyanine molecular rotor on Au(111)substrate.This work provides insight into the methods used to modify the effective rotational potential energy of molecular rotors,and offers important references for studies of bistable molecular devices.To sum up,this thesis utilized NC-AFM as a key approach to study the behaviors of certain functional molecules adsorbed on metal substrates in detail and in depth,and studied the adsorption configurations,structure manipulation and physical properties tuning of functional molecules adsorbed on metal substrates,which has revealed certain intrinsic regulations in the field of molecules adsorbed on metal substrates,and therefore offer important references and new ideas for relevant studies on molecular electronics and surface physics.