We investigated the orientations of interface dipole moments of individual non-planar titanyl phthalocyanine(TiOPc)molecules on Cu(111)and Cu(100)substrates using scanning tunneling microscope(STM)and noncontact atomic force microscope(NC-AFM).The dipole moment orientations corresponding to two different configurations of individual TiOPc molecules were determined unambiguously.The correlation between the actual molecular structures and the corresponding STM topographies is proposed based on the sub-molecular resolution imaging and local contact potential difference(LCPD)measurements.Comparing with the pristine substrate,the LCPD shift due to the adsorption of non-planar molecule is dependent on the permanent molecular dipole,the charge transfer between the surface and the molecule,and the molecular configurations.This work would shed light on tailoring interfacial electronic properties and controlling local physical properties via polar molecule adsorption.
It has been demonstrated that intermolecular interaction,crucial in a plenty of chemical and physical processes,may vary in the presence of metal surface.However,such modification is yet to be quantitatively revealed.Here,we present a systematical density functional theory study on adsorbed bis（para-pyridyl）acetylene（BPPA） tetramer on Au（111） surface.We observed unusually high electron density between two head-to-head N atoms,an intermolecular ＂non-bonded＂ region,in adsorbed BPPA tetramer.This exceptional electron density originates from the wavefunction hybridization of the two compressed N lone-electron-pair states of two BPPA,as squeezed by a newly revealed N-Au-N threecenter bonding.This bond,together with the minor contribution from N...H-C intermolecular hydrogen bonding,shortens the N-N distance from over 4 A to 3.30 A and offers an attractive lateral interacting energy of 0.60 eV,effectively to a surface-confined in-plane pressure.The overlapped non-bonding vvavefunction hybridization arising from the effective pressure induced by the N-Au-N three-center bonding,as not been fully recognized in earlier studies,was manifested in non-contact Atomic Force Microscopy.
Individual titanyl phthalocyanine(TiOPc)molecules on ultrathin sodium chloride striped films grown on Cu(110)exhibit two different topographies with 8-lobes and 6-lobes when imaged by scanning tunneling microscopy(STM).Direct images of the molecular orbitals of the molecules with 8-lobes are obtained,indicating that the electronic structure of the TiOPc molecule are decoupled from the metallic substrate.For the TiOPc molecule with 6-lobes,the STM images at negative and positive bias polarities show the same structures as 2-fold symmetry except for the 90°rotation with respect to each other.This phenomenon may be attributed to the splitting of the two former degenerate lowest unoccupied molecular orbitals due to the negative charging of the molecule.The identification of the molecular orbital splitting on the ultrathin insulating layer could deepen the understanding of the intrinsic properties of semi-conducting molecules.
