Studies On The Growth Of Picene Molecular Film On Different Substrates

The metal-doped polycyclic aromatic hydrocarbon（PAH）superconductors have been discovered in recent years.Because of their simple molecular structure and possible high transition temperature,they have become a hot spot in current research.Polycyclic aromatic hydrocarbon organic molecules only consist of carbon and hydrogen.The pure bulk materials are usually insulators.After conducting experiments on the doping of various polycyclic aromatic hydrocarbons with alkali metals or other metals,the researchers discovered superconducting transitions that the transition temperatures varied with different moleculars and doping concentrations,ranging from4.5K（K₃pentacene）to 33.1K(K_3.45dibenzopentacene).These studies extend the scope of organic superconductors that make them have important research and application prospects.However,in the recent experiments of such superconducting materials,the samples were prepared by the vacuum thermal sintering method.Thus,the samples may contain various components.Corresponding studies have found that the proportion of superconducting components is usually only about 1%.What is the composition of the superconducting transformation?What kind of structure is it?And the interaction of metal atoms and organic molecules in the metal doping process,and the charge transfer situation,are still unclear.No corresponding explanation has been given.In order to solve these problems,a research idea is to grow a single crystal organic molecular film on a substrate and perform controlled doping in ultra high vacuum.However,with this sample preparation method,the results reported by the recent reports are also contradictory.Some articles said they have not found the superconducting transition phenomenon,but the transition have been found from insulator to metal state caused by metal doping.Others said that the metal is still an insulator after doping.Therefore,we consider that the substrate may play a crucial role in these research.Due to the influence of the substrate,the properties of the organic molecular film grown on the substrate may be different from the properties of the sample that has occurred superconducting transition before and after doping.In order to study the specific role of the substrate,we studied the growth and properties of picene molecule film on the Au（111）surface and p-SnSe surface by using the scanning tunneling microscope（STM）combined with the first principles.Among them,Au（111）is a commonly used substrate material.But there may be a strong interaction between Au and picene molecules.SnSe is a typical semiconductor,and its interaction with organic molecules adsorbed on the substrate surface may be weak.The use of two substrates allows the results to be compared.On the other hand,because of the superconducting proximity effect,doped molecular films prepared on metal substrates may be difficult to exhibit superconducting phase.It is more promising on the semiconductor substrate which has weak interaction with the molecular film to obtain a superconducting phase by doping.Therefore,the choice of SnSe as a substrate to grow picene molecules film is also prepared for the future search for superconducting phases.In this paper,we mainly studied the growth of picene molecules on Au（111）surface and p-SnSe substrate,including scanning tunneling microscopy（STM）image and scanning tunneling spectrum（STS）analysis of picene molecular film samples on Au（111）and p-SnSe substrates.For the growth of single-layer picene molecules film on Au（111）and p-SnSe substrates,we observed that the molecules are rod-shaped and have three protrusions on the long side,which correspond to the three benzene rings on the long side of the picene molecule.By superimposing the structural model of the picene molecule,it is found that the picene molecules are lying on the surface of the substrate,and there are two kind of molecules with different orientations.The difference is that when SnSe is used as the substrate,we can get a higher resolution STM image,and find a new assembly structure.We have also found that the molecules grown on the p-SnSe substrate are unstable relative to the Au（111）surface,which is attributed to the lower surface diffusion barrier of SnSe,and thus the molecules have a higher diffusion rate.For the growth of two layers of picene molecules on Au（111）surface and p-SnSe substrate,we can see the structure of the same light-dark molecules cross-arranged.It is believed that with the increase of molecular quantity,some molecular orientations will change.Some molecules lying flat will become inclined.By performing STS analysis on these two different oriented molecules,we found that there is a difference in the energy gap near the Fermi surface,which is related to the interaction between the molecule and the substrate,in which the picene molecular film grown on the SnSe surface is closer to the structure and electronic properties of the block picene.The above results indicate that the interaction between the picene molecule and the SnSe surface is weak.It is expected that the superconducting transition phenomenon will be observed on the SnSe substrate after doping.Further doping research is still in progress.