Study On Synthesis And Photoresponse Properties Of Surface Low-dimensional Polymers Based On Schiff-base Reaction

Surface polymers are formed by covalent bonds between constituent units and directly synthesized by a chemical reaction confined on the surface.These covalent bonds between constituent units can facilitate in-plane charge transport and therefore have potential applications in the field of optoelectronics and photovoltaic devices.At present,it is challenging to synthesize surface polymers with different structures by controlling external conditions and to study the optical properties of surface polymers.In this thesis,low-dimensional polymer was selected as the research system.By selecting precursors with different structures and controlling external conditions,a series of low-dimensional polymers were prepared on the surfaces of highly oriented pyrolytic graphite,graphene and reduced graphene oxide by Schiff base reaction.The formation process and influencing factors of surface polymers were studied by scanning tunneling microscopy.Then the photoresponse properties of the obtained polymers were studied by forming composite materials with graphene and reduced graphene oxide.P-phenylenediamine(PDA)and 4,4'-dihydroxy-[1,1-biphenyl]-3,3'-dicarboxaldehye(4,4'-DHBDA)were selected as reaction precursor.By controlling the p H value,the conversion between one-dimensional polymer and closed zero-dimensional macrocycles structure was achieved.It was found that,when the reaction proceeds at the gas/solid interface,only one-dimensional polymer with a zigzag backbone was formed on the highly oriented pyrolytic graphite surface.This system showed very weak dependence on the p H.However,without surface confinement,exclusive macrocycles were synthesized by adjusting the solution p H value between 5.5and 6.0 with strong acids(hydrochloric acid and trifluoroacetic acid).When the p H value is above 6.0,one-dimensional polymer is formed.These results indicate that the macrocycles or the linear Schiff base product can be selectively obtained by adjusting the p H and surface confinement.Next,using two amines with C3 symmetry of 1,3,5-tris(4-aminophenyl)benzene(TAPB),2,4,6-tris(4-aminophenyl)-1,3,5-triazine(TAPT)and two isomers of3,3'-dihydroxy-[1,1-biphenyl]-4,4'-dicarboxaldehyde(3,3'-DHBDA)and 4,4'-DHBDA as precursor molecules,by varying the solvent,reaction temperature,structural evolution from well-ordered two-dimensional polymer to one-dimensional catenular structure has been precisely observed.It was found that reaction between 3,3'-DHBDA and TABP/TAPT leaded to long-range ordered hexagonal two-dimensional polymer.By using 4,4'-DHBDA in stead of 3,3'-DHBDA,zero-dimensional macrocyclic structure and small amount of one-dimensional catenular structure instead of ordered two-dimensional polymer were obtained.Since for the two isomeric dialdehydes,3,3'-DHBDA and 4,4'-DHBDA,the only difference is that the relative positions of the aldehyde group and the hydroxyl group are exchanged,these results indicate that the relative positions of hydroxyl and aldehyde groups on the biphenyl ring plays a determining role in the control and selection of the final products of the Schiff-base coupling reaction.After successful surface synthesis of Schiff base polymers on highly oriented pyrolytic graphite surface,we switch to the synthesis and photoresponse application of composites of two-dimensional polymers with graphene.For this purpose benzene-1,3,5-tricarbaldehyde(BTA),p-phenylenediamine(PDA),4,4”-diaminop-terphenyl(DATP)and 4-(2-(9-(2-(4-aminophenyl)ethynyl)anthracen-10-yl)ethynyl)benzenamine(AABA)were subjected to a Schiff base reaction on the graphene.The graphene was prepared by CVD and transfer to Si O2/Si surface.A series of surface two-dimensional polymer with variable structures were synthesized by changing the reaction temperature and monomer concentration.We found that the two-dimensional polymer-graphene composite devices have photoresponse performance,and the photoresponse performance of the devices is directly related to the order of the surface two-dimensional polymer structure.In order to verify that the photoresponse enhancement of light responsivity came from the sensitization effect of surface two-dimensional polymer,photoresponse performance tests were performed on both the unmodified graphene and the graphene device modified with monomers.The unmodified graphene had no detectable photoresponse,and the photoresponse of the graphene devices modified with monomers were substantially negligible compared to the two-dimensional polymer-graphene devices.In addition,the Schiff base reaction of BTA and PDA was also carried out on the surface of reduced graphene oxide.Since the dark current of reduced graphene oxide is nearly 4-5 orders of magnitude smaller than that of graphene prepared by CVD,the photocurrent gain(Ilight-Idark / Idark = ?I / Idark)of the devices based on reduced graphene oxide is nearly two orders of magnitude higher than that of the two-dimensional polymer-graphene photodetector.In addition,the response speed is nearly one order of magnitude faster than that of the two-dimensional polymer-graphene photodetector based on the CVD method.The response photocurrent increased with the increase in film thickness and the response speed decreased with the increase of the film thickness for both two-dimensional polymer-graphene and two-dimensional polymer-reduced graphene oxide devices.