Chiral Self-assembly Of Pentacene And Structural Transition Of Single-layer Film

In recent years,there has been great interest in polyacenes,as organic semiconducting materials.In particular,pentacene,as a typical organic semiconductor molecule,whose thin films show promising high charge-carrier mobility in organic devices,has important application value in organic electronics.As an advanced surface analysis equipment,scanning tunneling microscope(STM)can not only obtain the material space information on the atomic scale,but also can manipulate the single atom,the molecule and the nanostructure of the surface.In this paper,we used organic molecular beam deposition(OMBD),low-temperature scanning tunneling microscopy(LT-STM)and density functional theory to investigate the adsorption mode,growth and self-assembly structure of pentacene on the Cd(0001)surface.Three structural transitions induced by temperature were found.Under the room-temperature deposition conditions,when the coverage of pentacene molecules is much less than 1 ML,pentacene molecules are difficult to be imaged clearly by STM because the molecules exhibit high mobility and molecules collide frequently with the STM tip.Instead,many fuzzy features appeared on the substrate terraces like a 2D gas.It indicates that the interaction between the molecules and the substrate is weak.With the increase of coverage,however,the distance between molecules decreases and the mobility of molecules on the substrate surface decreases gradually.When the coverage reaches 1 ML,pentacene molecules form a disordered molecular monolayer on Cd(0001)surface.In order to obtain two-dimensional self-assembly and long-range order of pentacene molecules,the disordered monolayer was annealed.Thermal annealing can not only relieve the stress in the film but also increase the vibrational energy of the molecules,and then decrease the interaction between the molecules and the substrate,and strengthen the van der Waals force between the molecules,which is beneficial to the ordered growth of the film.In the first place,for the disorder monolayer,annealing the sample at 320 K leads to the transition from disordered phase to a well ordered monolayer,in which two kinds of molecular chains with different molecular orientations are alternately arranged,constituting the herringbone structure.In the second place,when the annealing temperature is increased to 350 K,the herringbone packing transforms into a unique chiral clover structure that has never been reported: three same pentacene dimers arrange into a chiral pinwheel structure.Finally,further increasing the annealing temperature to 400 K,the chiral clover structure transforms into a common and ordered brick-wall structure.The scanning tunneling spectrum showed that the energy gap,between the highest occupied molecular orbital(HOMO,-1.3 eV)and the lowest unoccupied molecular orbital(LUMO,1.4 eV)of the self-assembled monolayer,is 2.7 eV.With the combination of density functional theory(DFT)calculation,it is found that the change of charge density at both ends of individual pentacene molecule is opposite,which indicates that the each pentacene molecule has the net dipole moment.It is particularly interesting that the organizational chirality of chiral clover structure can be switched from one enantiomer to another driven by the electric filed from a STM tip.Although the voltage pulse was applied only at one point,the chirality of the whole domain region has been changed.By statistical analysis,it is found that the two isomers have the same chiral exchange probability.Our findings are significant for the understanding and controlling of the chirality in the self-assembled structure.