| One of the major tasks of molecular electronics is to construct prototype molecular devices to mimic the electronic behaviors of today’s semiconductor components for developing smaller and faster electronic devices.Negative differential resistance(NDR)effect,which is characterized by the phenomenon of decreasing current with increasing voltage in the current-voltage(I-V)curves,and which is the essence of electronic devices such as Esaki diode and resonant tunneling diode has been observed in different molecular devices.The scanning tunneling microscopy(STM)based measurements,using molecular resolution images prior to I-V measurements to identify the NDC from individual molecules,have made great contribution in measuring the NDC behavior of various molecules and understanding the underlying cause.STM based NDC was found for C60 monolayer islands deposited on thio-monolayers self-assembled on Au(111)surface.Similar effect was also found for C60 bilayer islands deposited on Au(111)surface.In both work,a buffering layer,the thio-monolayers or the C60 monolayers in contact with the metal surface,is utilized to weaken the interactions of the measured C60 molecules with the metal substrates.This is to keep the sharp molecular electronic states of the measured C60 to guarantee the realization of the NDR behaviors when electrons tunnel through the STM junction.For NDR observed in various molecules with the STM setup,similar buffering layers are needed.In many cases,a unique electronic state of the STM tip is also required to realize NDR.In order to develop reliable NDR protype molecular electronics devices,repeatable and controllable NDR behaviors are expect in a STM-molecular setup,i.e.the NDR should be independent on the electronic structure of the STM tip and the buffering layers will not be needed for simplifying the complexity of the devices.In our recent work,the electronic properties of C60 molecules on semiconducting black phosphorus(BP)single crystal surface were studied.It shows that there is no charge transfer between C60 and BP and the interactions between them are van der Walls(vdWs)interactions.As the C60-C60 intermolecular interactions are also vdWs,these make the molecules within the monolayers keeping the original sharp molecular orbitals as those found for C60 adsorbed on buffering layers on metal surfaces.This provides a model system without buffering layers to study the C60 related NDR behaviors in a STM setup.In current work,by utilizing BP as the substrate,repeatable and controllable tip-independent NDR behaviors are observed in STM current-voltage(I-V)measurement for C60 monolayers.Combing STM measurement,model simulations and density functional theory(DFT)calculations,it is found that the NDR appears when electrons tunnel from the continuous metallic tip state to the sharp lowest unoccupied molecular orbitals(LUMOs)of C60.However,the NDR disappears when electrons tunnel from the sharp highest occupied molecular orbitals(HOMOs)of C60 to the continuous unoccupied metallic tip state.The variations of the tunneling barriers during the tunneling process in the two cases explain the different behaviors.As only a continuous metallic tip state,a vacuum barrier and a sharp molecular electronic state are involved to produce the solid NDR behaviors in our work,this reveals a universal mechanism to develop NDR molecular electronic devices without designing complicating device structures.In summary,this work realizes a reproducible and stable negative differential resistance phenomenon in a simple system including a metallic tip,a C60 monolayer and a semiconducting substrate.The phenomenon is independent of the electronic state of the STM tip and occurs when the electrons tunnel from a continuous metallic state to an isolated,sharp molecular state.The understanding provides a clear picture for building simple,reproducible NDR prototype device. |
