Thin Film Growth Of Organic Semiconductors On The Two Dimensional Materials

Organic semiconductors have become one of the popular researches in recent decades because of their advantages of adjustable chemical structure,flexibility and large area processing.However,compared with inorganic semiconductors,the electrical transport performance of organic films is still lower,where the optoelectronic properties of organic semiconductors are directly related to molecular stacking,crystallinity,growth modes and purity in their films.In typical organic semiconductor devices,organic films are usually grown on amorphous silicon oxide substrates,and the quality of their thin film is usually poor.Recently,with the rapid development of two-dimensional materials,this novel material with atomic level flat and interlayer van der Waals(van der Waals)interaction provides a good basis for the high-quality growth of organic thin films.As a two-dimensional material with wide band gap,hexagonal boron nitride(h-BN)has shown great value in recent years as a substrate for organic thin film devices for the growth of high ordered organic crystals and the fabrication of high mobility transistors.However,the van der Waals(vdW)epitaxial growth relationship remains unclear in the growth of this organic-two-dimensional hybrid heterojunctions.This paper investigated the growth behavior and mechanism of small organic molecules on hexagonal boron nitride(hBN)in detail,where Rubrene and fullerene(C60)are research objects,as well as the interaction between small molecular organic layer and two-dimensional material induced layer.Besides this,the corresponding high performance organic semiconductor devices were also prepared.The three main results of this paper are as follows.1.The high-quality layer growth of organic small molecule Rubrene films as well as the crystal films with large size continuity can be obtained through regulating the substrate temperature.The epitaxial relationship between Rubrene and h-BN is also analyzed using high-resolution atomic force microscopy(Atomic Force Microscope,AFM)and transmission radio microscopy(Transmission Electron Microscope,TEM).That is,the stripped single crystal h-BN substrate has excellent orientation induction effect on Rubrene molecules,solves the problem of traditional vacuum evaporation to prepare high-quality crystalline Rubrene film,and provides a foundation for further building high-performance organic electronic devices.Based on this,we construct high-mobility thin film transistors based on the h-BN/Rubrene heteroj unction with a device carrier average mobility of up to 0.41 cm2/Vs,up to 1.20 cm2/Vs.By irradiation the transistor with 365 nm ultraviolet light,it is found that the transistor is responsive to the light of 365 nm wavelength,providing a basic theoretical study for the development of optical transistor based on h-BN/Rubrene heterojunction.2.The growth behavior of C60 on h-BN is systematically studied on this thesis.The transition of C60 molecules from a sub monolayer of fractal growth to an island growth film was observed at high substrate temperatures by selecting a high-quality h-BN sheet substrate.The large size,terraced C60 crystal films are obtained.The accumulation structure of C60 molecules growing layer upon layer on the stripped single crystal h-BN surface is also obtained.Furthermore,the highest mobility of the constructed h-BN/C60 field effect transistor body reached 3.20 cm2/Vs,compared to the highest C60 films deposited on CVD h-BN(1.60 cm2/Vs),and well exceeded most of the type n field effect transistors,broadening the path of type n field effect transistors.3.The growth of organic p-n heterojunctions on h-BN is further explored by exploring the growth laws of organic small molecules on h-BN in two-dimensional material respectively.It is found that Rubrene can grow epitaxial on h-BN/C60 films to obtain highquality C60/Rubrene heterostructures,whose heterojunction morphology preserves the terraced structure.While the h-BN/C60/Rubrene based devices,carrier mobility increased to 3.3 cm2/Vs.compared with the h-BN/C60 devices On the other hand,the C60 film obtained on the covered Rubrene film presents not a continuous flat growth form,but a spherical crystalline state,there is no obvious epitaxial effect between Rubrene and C60,probably due to completely different crystal structure,low symmetry and large lattice mismatch of C60 and the underlying Rubrene,while the growth temperature of 120℃ is lower than the C60 optimal growth temperature of 180 ℃,making C60 difficult to grow along the underlying lattice epitaxy.Its devices exhibit typical bipolar transistor properties.