The Dynamic Process Of Single Molecule And Applications Based On Single Molecular Junctions

Single molecule junctions are the devices that sandwich a single molecule on two separated electrodes.On the one hand,we can obtain the information of the structure and the transformation between the different conformations from the value of current.On the other hand,the devices with remarkable functionalities,like single molecule switches,can be achieved resulting from the isomerization of the functional molecule.As the size of the traditional FET(Field Effect Transistor)becomes smaller,the quantum effect should be noticed.The researchers are seeking for new devices or materials to resolve the problem.Single molecule junctions as a potential candidate have been payed much attention to realize functional single molecule FET,because of the smaller size of it and the variety of functional molecules.This paper studied the dynamic process of azobenzene based on the platform of single molecule junctions.Besides,the dynamic process of phenyl ring on the backbone is influenced by the isomerization of azobenzene unit.We investigated ability of sidechain to control the rotation barrier of phenyl rings on backbone.Furthermore,we introduced the third electrode as gate into the original device and realized single molecule FET.As the typical photochromic molecule,azobenzene can isomerize between cis and trans forms under the irradiation of UV(ultra violet)and vis(visible).With the help the single molecule junctions,the researchers found that the inelastic scattering of electrons and electric field can cause the isomerization of azobenzene as well.We sandwiched the azobenzene derivative on the nano-gaped electrodes of graphene and found that the molecule can transform form trans to cis at the negative bias.The dipoles of the different conformations are obtained based on the simulation,and the projection of the dipoles on the direction of backbone is different.As a result,the ability of electric field to control the energy of the trans are differ from cis.When increasing the electric field intensity at one certain direction,the energy different between trans and cis form are narrow down to 0 e V,then the energy of cis form is lower than trans form and become the most stable state.While on the other direction of electric field,the energy of trans form is always higher than cis form and the trans form is always the most stable state.The properties of side chain on biphenyl molecule can affect the torsional angle between the phenyls.The two conformations of azobenzene in our device can influence the rotation of phenyls on the backbone.Combined with the simulation,the azobenzene unit can hardly affect the torsional angle of the phenyl ring far away from it,while the torsional angle of the phenyl ring adjacent to azobenzene unit is larger in trans form than that in cis form.This phenomenon imply that the steric hindrance of the trans is higher than cis.The dynamic process of the rotation can verify the conclusion.The rotation barrier of the phenyl ring far away from azobenzene show no difference among the isomerization of azobenzene,while the rotation barrier of the phenyl ring adjacent to azobenznen is higher in trans form than that in cis form.This dynamic process has been confirmed in experiment by warming the temperature.Except the dynamic process of molecule,a third electrode of gate was introduced in single molecule junctions to investigate the influence of gate on molecular conductance.The ultimate objective is the realization of single molecule FET.To produce the dielectric layer which has to satisfy the advantages,like a smooth surface,the thickness of nm and compact,is the most difficult part.After trying several materials and preparation methods,a dielectric layer with thickness of 10 nm was achieved which can stand the gate voltage of 10 V.The capability of the gate voltage in our devices is comparable with the lonic liquid gate.The gate voltage can modulate the molecule conductance,and the on/off ratio can reach 10~4.We realized the graphene based single molecule FET for the first time.This paper study the dynamic process of azobenzene and phenyl rings in singlemolecule scale,reveal the mechanism of isomerization driven by electric field and inherent rotation rules of phenyl,which offering a perspective to design functional molecule in future.Furthermore,we realized the single molecule FET in experiment.