Precise Epitaxy And Device Applications Of Two-dimensional Organic Semiconductors And Heterojunctions

Organic semiconductor materials have great promise for electronic applications in various fields,due to their unique advantages of portability,flexibility,low cost and designability,especially in the display market where organic light-emitting diode displays have been mass produced and gained significant market share.In the research of organic thin film transistors,it was found that charge carrier transport only occurs within several molecular layers of the organic semiconductor near interface.Furthermore,various two-dimensional(2D)materials like graphene and transitional metal dichalcogenides have emerged and been extensively investigated.Therefore,the concept "two-dimensional organic semiconductor" has been proposed and has raised great research interest.Up to now,significant progress has been achieved in preparation and device applications of 2D organic semiconductors.Several methods are available to grow 2D organic semiconductor crystal at present,generally divided into liquid-phase methods and vapor-phase methods.However,precise and controllable growth of high-quality 2D organic semiconductor crystal is still a difficulty,and the growth of 2D organic semiconductor heterojunction and its application has rarely been reported.In this thesis,we have developed a self-limited(SL)epitaxial growth technology to precisely grow 2D organic semiconductors and their heterojunctions,and we have also investigated ultrathin organic heterojunction devices.The main works in this thesis are listed as follows:In Chapter 1,the material structure and conductivity mechanism of organic semiconductors are introduced,and the methods to grow 2D organic semiconductor are also summarized.In Chapter 2,we developed a SL epitaxial growth technology to precisely control the molecular layer numbers of 2D organic semiconductors via exploiting the gradient of van der Waals interactions near the interface,by which we have grown monolayered and bilayered 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene(C8-BTBT)crystals,monolayered Perylene-3,4,9,10-tetracarboxylic dianhydride(PTCDA)crystals and heterojunctions of monolayered PTCDA and bilayered Cg-BTBT.Their high crystalline quality was confirmed by atomic force microscope and cross-polarized optical microscope.The SL nature of the growth was unambiguously verified by two series of experiments:growth with varied growth time and repeated growth.A reverse SL growth was also developed in which SL 2D crystals can be prepared by annealing thick crystals.We performed molecular dynamics(MD)simulations of the growth process to understand the mechanism of SL growth,and both the forward and reverse growth process were qualitatively reproduced by MD simulations.The growth technology we developed has the highest growth precision among all epitaxial technologies of organic small-molecule crystals.In Chapter 3,we have demonstrated an ultrathin organic semiconductor heterojunction device.We first used scanning Kelvin probe microscope to characterize the interfacial band properties,and then fabricated an ultrathin PTCDA/C8-BTBT heterojunction device in which the total thickness of organic layers is about 15nm.The device exhibited excellent rectifying behavior and photoelectric responses.Fitting the rectifying ?-? characteristics confirmed the high quality of organic crystals and the interface in the heterojunction device.