Crystallization Regulation Of Metals Catalyzed Amorphous Carbon To Graphene And Growth Mechanism

Graphene features excellent performances,such as extraordinary electron mobility,room temperature quantum Hall effect,high thermal conductivity,high strength and toughness,etc.,which makes it possess promising applications in the sensor,energy storage,semiconductor materials and other fields.However,the industrial application of graphene is limited by its preparation methods.Recently,a novel preparation method,metal-catalyzed crystallization of amorphous carbon(a-C)to graphene,draws much attention due to its potential in producing high quality,large scale and layers number controlled graphene.However,the related studies on preparation process and mechanism of transformation from a-C to graphene are lack up to now.In this work,a kind of substrate(Si)/a-C/metal catalyst(Cu/Ni)triple-layer film structure was specially designed to investigate the transformation behaviors from a-C to graphene.The a-C films and Cu films were deposited in a custom built hybrid filtered cathodic vacuum arc and magnetron sputtering equipment.The Ni films were fabricated by electron beam evaporation method.After rapid heat treatment,graphene formed on the surface of the metal layer.The deposited films and the graphene were characterized by Raman,XPS,TEM,SEM,XRD,and so on.The effects of amorphous carbon species and thickness,metal catalyst materials and thickness,annealing temperature,annealing atmosphere and annealing time on the quality of graphene were studied.The transformation mechanism was also discussed.In this paper,we optimized the ta-C/Ni system from the results of Raman and SEM.The results show that in Si/ta-C/Ni system annealing atmosphere significantly affects the quality of produced graphene at high annealing temperature(750°C~1000°C).At Ar atmosphere,the Ni film is prone to Ostwald ripening to form agglomeration,the a-C is graphitized without graphene formed.In the vacuum condition,multilayer graphene is formed with a lot of defects.With the variation of annealing temperature and thickness ratio of ta-C/Ni films,we find that Si/ta-C(10nm)/Ni(100nm)annealed at 900°C for 5min results in the best quality of graphene.The growth process of graphene at high annealing temperature follows the dissolutionprecipitation mechanism.When annealed at low temperature(200°C~600°C),both of annealing atmosphere and temperature affect the quality of produced graphene significantly.At different atmospheres,the Ni film surface keeps integrity,which is beneficial for the large scale preparation of graphene.At Ar atmosphere,the a-C is also graphitized without graphene formed.Under the vacuum condition,graphene is observed from the Raman spectra of samples annealed at 400°C for 15 min.The Si/ta-C(40nm)/Ni(100nm)sample annealed at 500°C for 15 min results in the best quality of graphene,which has 35 layers and many defects even so.The reason for so many graphene layers and defects attributes to the polycrystalline Ni catalyst film,which provides too much nucleation sites for graphene and promotes more carbon atoms diffuse along the grain boundaries and defects to the film surface.On the other hand,due to the large thickness of a-C film in the composite structure,the expanded amount of carbon is provided,which leads to too many graphene layers and a decrease in quality.At low temperature,the solid solubility of C in Ni is reduced,and the diffusion of carbon dominates.Therefore,the growth mechanism is synergistic of dissolution-precipitation and metal induction.This study provides a new idea for low temperature,large area and controllable preparation of graphene.