Epitaxial Growth And Optical Properties Of Twodimensional Organic Semiconducting Thin Films

Organic semiconductors have the advantages of flexibility,light weight,low cost,and molecular diversity.Organic semiconductors are composed of organic molecules that can be synthesized through molecular design to have tunable functions.This has enabled the successful development of organic solid-state lasers(OSSLs),organic light-emitting diodes(OLEDs),organic field effect transistors(OFETs),organic light-emitting transistors(OLETs),etc.Two-dimensional(2D)materials exhibit distinct band structures due to the quantum confinement of electrons in 2D.Monolayer WS₂ is a typical 2D material with unique properties.From bulk to monolayer,the band gap of WS₂ exhibits an indirect-to-direct transition.Meanwhile,monolayer WS₂ emits bright fluorescence.To figure out whether there are similar 2D properties in organic semiconductors,we need to grow2D organic semiconducting films with different layers.Organic semiconductors have the properties of high solubility and low sublimation temperature.Thence main methods to grow 2D organic semiconducting films are divided into liquid phase and gas phase method.Comparatively,the gas phase method gives the best results for organic semiconducting films with high quality and high purity.However,it is difficult for the gas phase method to grow ultrathin organic semiconducting films.We report the vapor phase epitaxial growth method based on van der Waals epitaxy technology for production of ultrathin 2D organic films.Based on the vapor phase epitaxial growth method,we develop two methods named high temperature epitaxial growth method and low temperature epitaxial growth method to grow organic thin films.The high-temperature epitaxial growth method obtains monolayer films with high uniformity,high crystallinity,high stability,high luminous intensity,high quality.The low-temperature epitaxial growth method grows multi-layer films with advantages of large area,high flatness,high crystallinity.These two methods can be applied to the growth of various organic materials.Further,single-crystal organic thin films and organic-inorganic heterostructures with high quality can be obtained,using in high-performance organic optoelectronic devices.In this paper,monolayer and multi-layer Me-PTCDI films with different optical properties are obtained.We further explore the 2D properties of the Me-PTCDI organic materials.The main works are list as follows.In Chapter 1,after reviewing the properties of organic semiconductors and the main methods to grow 2D organic thin films,the advantages of vapor phase epitaxial growth method are introduced.In Chapter 2,we first select organic materials and substrate used in our experiments.Moreover,the details of vapor phase epitaxy growth method and the principles of common instruments are also demonstrated.In Chapter 3,we report the high-temperature epitaxial growth method.The film thickness estimated from AFM observations was determined to be approximately 0.3nm,which is equivalent to the height of monolayer.Through fluorescence microscope test,we find that the monolayer Me-PTCDI organic film emits green fluorescence.Through polarized microscopy test,we find that the monolayer Me-PTCDI film has high crystallinity.Through stability experiments,it shows that the monolayer Me-PTCDI film is stable in the air.In order to obtain monolayer Me-PTCDI film with bright luminescence and high quality,we adjust the growth temperature,air flow,substrate thickness and other factors.In Chapter 4,we report the low-temperature epitaxial growth method.Through the control of growth temperature and substrate location,different layers of Me-PTCDI films are obtained.Through optical tests,we find that monolayer and multi-layer Me-PTCDI organic thin films have completely different light-emitting properties.In Chapter 5,we summarize our works and put the reported two methods into further applications.