Towards Unveiling The Chain Molecular Structure Of Amorphous Selenium By Single-molecule Studies

There are many allotropies of selenium,they can be divided into two categories based on crystal structure: crystalline selenium and amorphous selenium.The structure of crystal selenium has been fully studied by X-ray diffraction and other researching methods.Predecessors also tried to study the molecular structure of amorphous selenium by the above methods,and possible structures of amorphous selenium molecules(molecular chain,molecular ring,etc.)have been proposed by them.But the exact molecular structure of amorphous selenium cannot be obtained by these methods because of its non-crystalline structure,so it has not been determined so far.In recent years,amorphous selenium has received widespread attention,due to its unique photosensitive,photoelectric and semiconductor properties.It has received extensive attention in the field of new materials(especially in the fields of nanomaterials and semiconductor materials).Therefore,the macroscopic research results of amorphous selenium can’t provide the intrinsic properties of amorphous selenium molecules.In order to further improve the properties of new materials,it is necessary to study them from the microscopic level,especially to obtain the exact molecular structure of amorphous selenium.In this paper,the single-chain elasticity and structure of amorphous selenium have been studied by single-molecule force spectroscopy(SMFS)technique based on atomic force microscopy and quantum mechanics(QM)calculations.First,the characteristics of amorphous selenium have been studied in nonane by SMFS.The results show that the amorphous selenium has a long linear molecular structure and the single-chain elasticity of different amorphous selenium samples prepared by dissolving in different solvents are the same.Then,the theoretical single-chain elasticity of several possible molecular structures of amorphous selenium have been obtained by QM calculations.The corresponding theoretical elasticity fitting curves have been generated by integrating the QM calculations into the freely jointed chain model,respectively.Comparing the fitting curve with the experimental curve,it is found that the fitting curve of the current planar zig-zag single-chain superposes well with the experimental curve.The experimental results and simulation results are independent and verified each other.The single-chain elasticity of the amorphous selenium molecules obtained from the experiments can be described by the theoretical freely jointed chain model,and the theoretical model is the bridge between the two results.It is determined that the molecular structure of amorphous selenium is a zig-zag structure.Based on the above discussions,the main conclusions are as follows:(1)Amorphous selenium has a long chain molecular structure,and amorphous selenium is a kind of inorganic polymer.The results of single-molecule force spectroscopy show that amorphous selenium has a wide molecular weight distribution;(2)Amorphous selenium dissolved in oxidizing solvent,reducing solvent and organic solvent has the same single-chain elasticity in the nonane experiment environment.It determines that amorphous selenium maintains the same molecule structure in different samples without any change.So,the different solutions don’t cause the changes of amorphous selenium.Therefore,the single-chain elasticity of amorphous selenium obtained in nonane experimental environment is the inherent elasticity of amorphous selenium molecule;(3)We constructure the repeating unit models,including planar zig-zag model,stereo helical model,and stereo crown model based on the previous research.The theoretical singlechain elasticity of these structures is calculated by using QM calculations.Combining the freely jointed chain model with the theoretical single-chain elasticity can further obtain the theoretical elasticity fitting curve of each model.By comparison,the fitting curve of the planar zig-zag 4 atoms repeating unit superposes well with the experimental force spectroscopy of amorphous selenium;(4)We calculate the average deviation between the fitting curve of the planar zig-zag 4atoms repeating unit and the experimental curve.The result shows that the optimal Kuhn length of the model is 0.23 nm,which is very close to the bond length of Se-Se bond(0.23nm).The planar zig-zag 4 atoms repeating unit can be used to describe the real molecule structure of amorphous selenium.