Two-dimensional Perylene Derivative Molecular Crystals: Controlled Synthesis And Optical Properties

The conductive organic materials were discovered in the 1970s.After half a century of development,organic materials have been widely used due to the flexible chemical modification and large-area solution growth.There is a class of local states that is rarely disturbed by disorder in J-aggregates,which is called coherent states.However,the coherent state is limited in most cases due to the extremely fast charge transfer process between molecules.So breaking the interaction of electronic states between molecules was necessary.Inspired by two-dimensional atomic crystals,many organic semiconductors stack layer-by-layer and have a two-dimensional structure.A variety of solution methods and vapor deposition technologies have developed in recent years,which can be used to prepare 2D organic molecular crystals with controllable layers.In organic molecular crystals,excitons are dominant by Frenkel excitons,and the dipole-dipole interaction is stronger.The control of the dimension of molecular semiconductor is expected to greatly change the exciton coupling and generate new optoelectronic devices.We explore a class of perylene derivatives,namely dimethyl-3,4,9,10-perylenetetracarboxilic diimide?Me-PTCDI?,3,4,9,10-perylene-tetracarboxylic dianhydride?PTCDA?,3,4,9,10-perylene-tetracarboxylic diimide?PTCDI?and N,N?-Dioctyl-3,4,9,10-perylenedicarboximide?PTCDI-C8?with interesting optical properties and develop two methods for preparing 2D organic molecular crystals based on physical vapor deposition and van der Waals epitaxy.The controllable growth is achieved by adjusting temperature and the relative distance between the organic source.We find that monolayer organic molecular crystal has different optical phenomena compared with that in bilayer and multi-layer.We systematically take optical measurements and analyzations.Finally,we carry out the preliminary exploration of LED devices and heterojunctions based on perylene derivatives.The main contents of this thesis are as follows:1.Two methods based on van der Waals epitaxial have been developed to prepare controllable perylene derivatives thin films on boron nitride?BN?substrates,namely high-temperature method and low-temperature method.The molecules are packed in a“herringbone”shape,and the thickness of each layer is around 0.3 nm.The transmission electron microscope?TEM?,high-resolution atomic force microscopy and polarized photoluminescence indicate that these films are highly crystallized.2.We further find that the monolayer exhibited a very strong fluorescence effect.At resonance energy,excitons show a large oscillator strength and absorption?30%?.In addition,the quantum yield could reach 60%-100%.From the systematical experiments at room temperature and low temperature,the phenomena of superradiance are observed,such as bathochromic,reduced absorption band and the enhancement of oscillator strength,because of the suppression of charge transfer process and strong dipole-dipole interaction in the“limited two-dimension”condition.Finally,the LED device based on perylene derivatives monolayer is achieved,showing 30GHz speed.The controllable preparation of perylene derivative films and the unique optical properties in monolayer provide a new chance for highly efficient light-emitting devices with coherent dipole interactions3.A preliminary exploration has been made on charge transfer and energy transfer of 2D organic-inorganic heterojunctions.The heterojuction based on organic monolayer?Me-PTCDI?and transition metal dichalcogenides materials?Mo Se₂?is fabricated.It was found that when organic and TMDCs are in direct contact,the PL is quenched,indicating a strong charge transfer process happening at interface due to the disassociation of excitons in these two materials.When they are separated by a thinner boron nitride,the PL of Mo Se₂ will be enhanced,indicating that boron nitride can block charge transfer.