Preparation And Field Effect Properties Of Two-dimensional Molecular Crystals And Their Heterojunctions

Two-dimensional molecular crystals(2DMCs),as an emerging type of material,have attracted tremendous attentions.The main reasons contain three points:(1)Compared with two-dimensional atomic crystals,the building blocks of 2DMCs can be synthesized as needed and can be solution processed.(2)Compared with bulk organic semiconductors,2DMCs are ultra-thin and show low injection resistance in devices.(3)Compared with polycrystalline thin films,2DMCs have the advantages of long-range order and low density of defects.At present,the preparation of 2DMC arrays and heterojunctions is still challenging.This thesis focuses on the preparation of 2DMC arrays and 2DMC p-n heterojunctions and their electrical properties.The contents are as follows:1.PMMA assisted solvent vapor annealing(PASVA)was proposed as an efficient method for preparing 2DMC arrays,and high quality 2DMC arrays were obtained successfully.The method has two key points:(1)By adding a well-soluble polymer PMMA as a auxiliary layer,the fluidity of the organic semiconductor molecules was increased to promote the growth of high quality 2DMCs.(2)By tilting the substrate,the formation of ordered 2DMC arrays was induced.The effects of growth conditions such as temperature and annealing time on the growth characteristics of 2DMC arrays were studied in detail,and the growth mechanism of 2DMC arrays was given.The morphology and structure of the 2DMC arrays were characterized by atomic force microscopy(AFM),polarized light microscopy(POM),X-ray diffraction(XRD),etc.,and the high quality of the regular crystals which were more easily obtained after adding PMMA were proved.By constructing organic field-effect transistors(OFETs),it was found that the mobility of the 2DMC arrays after the addition of PMMA was twice as high as that without PMMA.This work provides a fast and efficient new method for preparing high quality 2DMC arrays.2.2DMC p-n heterojunctions were prepared by layer by layer stacking,high mobility and balanced ambipolar devices were obtained by controlling the layer numbers of p-type semiconductor and n-type semiconductor.Large-area 2DMCs of ptype and n-type semiconductors were grown on the water surface by using water surface as a liquid substrate which has a low nucleation density.Because the crystals on the water surface were easy to transfer,large-area 2DMC p-n heterojunctions can be prepared by layer by layer stacking.The high quality of the 2DMCs were confirmed by AFM,POM,transmission electron microscopy(TEM),and selected area electron diffraction(SAED).The successful preparation of the p-n heterojunctions ware confirmed by UV-vis absorption spectroscopy(UV-vis),AFM and XRD.By precisely controlling the layer number of p-type semiconductors and n-type semiconductors in the p-n heterojunction FET,a balanced high-performance ambipolar transport was obtained.2DMC p-n heterojunctions provide an efficient method for obtaining high performance,balanced ambipolar transistors based on unipolar materials.