| Compared with inorganic molecular materials,organic molecular materials have many advantages,such as easy to synthesize,low-price,having simple preparation processes,and can greatly reduce the production cost of organic electronic devices.At the same time,the organic molecular materials have good flexibility and strong plasticity.They can be deposited on flexible substrates and can be used to prepare flexible electronic devices.In addition,organic molecular materials can be chemically synthesized flexibly and precisely controlled by functional group modification,hybridization,and other methods.These features can meet the needs of organic optoelectronics and electronic devices.These advantages make organic molecular materials have important application value in the field of organic devices such as organic field-effect transistors(OFETs),organic light-emitting diodes(OLEDs),and organic solar cells(OSCs).However,the organic layers of most organic devices are mostly made by thermal evaporation,spin coating or blade coating.These films are mostly amorphous or polycrystalline structures with many impurities and defects that can cause charge and spin carrier scattering.Single crystals can significantly reduce defects,disorders,and grain boundaries.In recent years,great progress has been made in the research of organic crystals,and many novel physical properties have been discovered,such as self-healing characteristics,superelasticity,THZ characteristics,bipolar charge transfer characteristics,ferroelectric characteristics,and multiferroic characteristics.These novel properties give organic crystals great potential for new multifunctional organic devices.Recently,people have also greatly expanded the types of organic crystalline materials by combining two different organic semiconductors into a binary organic eutectic.It is believed that in the near future,organic crystalline materials will be widely used in the field of optoelectronic devices.In addition,organic semiconductors have great advantages and potential in the emerging field of organic spintronics.Organic semiconductors are mainly composed of light elements with low atomic number,and their spin relaxation time is extremely long,so they are very suitable for the application of spintronics.Although great progress has been made in the study of organic spintronics,an important issue has always plagued organic spintronics.That is,the spin diffusion length of organic materials is not long,only tens of nanometers.This is mainly caused by the low mobility of the amorphous thin film in the organic spin device.If monocrystalline organic spin devices can be fabricated to minimize defects,grain boundaries,and increase long-range orderliness,it is highly likely that the spin diffusion length of organic semiconductors will be greatly increased,and this will greatly promote the development of organic spintronics.The study of the crystal growth mechanism is the basis and key to the preparation of high-quality crystalline materials and the study of the intrinsic physical properties of the materials.With the development of modern science and technology,the research on the crystal growth mechanism has achieved considerable development,especially in the solid-state reaction,polytype,coordination polymer,eutectic and other aspects formed a series of research hotspots.The crystallization process includes nucleation and growth,which determines the material properties and preparation of various crystalline materials such as metals,semiconductors,drugs and determines their material properties.Therefore,the study of the mechanism of the crystallization process has always been one of the most concerned issues for scientists.Classical crystal growth theory assumes that the basic growth unit of a crystal is a single atom,ionor molecule.However,in the last decade,with the development of advanced electron microscope technology,the crystallization process of inorganic materials,especially inorganic nanomaterials,has achieved microscopic or even nanoscale in-situ observations.These evidences reveal the path of non-classical crystallization,the so-called Oriented Attachment.The Oriented Attachment refers to the formation of a certain shape of nanostructures in an amorphous precursor under the action of driving forces such as van der Waals forces and so on,diffusion transfer by molecules,ions or atoms controlled by Ostwald ripening,and reconstruction of the structure and growth.However,the orientation adsorption process and Ostwald ripening are two distinct mechanisms.The biggest difference between the two is that Ostwald ripening is a classical theory of crystal growth.It simply assumes that the basic unit of growth is a molecule,an ion or an atom itself,and they are deposited on a nucleus by diffusion to crystallize.The Oriented Attachment supposes that the nanoparticles can be used as the basic unit of growth,and then the oriented attachment.Compared to inorganics,the crystallization process of organic compounds is much more complex,because the vast majority of organic compounds are assembled together by non-covalent bonds that are ionic or covalent.The bond strength is much weaker,which makes the crystallization of organic molecular compounds more complicated.For a long time,scientists have used X-ray diffraction,spectroscopy,and analog calculations to understand the crystallization process of organic molecules.However,these methods lack visualization and cannot observe true microscopic dynamic processes,nor can they confirm whether there is an attachment process where nanoparticles are the basic unit of growth during the crystallization of organic molecular compounds.Through optical,electronic,scanning probe microscopy and other methods to record in situ the dynamic process of its material crystallization,combined with X-ray diffraction analysis of the internal factors of the structural changes in the crystallization process,can be fully acquired the structural transformation information during the crystallization process,more in-depth understanding of the crystal growth process,and thus more in-depth understanding of the structural transformation and crystal growth mechanism.Based on this,further precise regulation of the crystal micro-crystal structure and various physical properties can be achieved.In addition,at high temperatures,organic crystals volatilize with increasing temperature,which leads to the decrystallization of organic crystals.For the problem of degraded performance of the electron/opto-electronic device,the structural transformation of the organic crystal near the volatilization temperature is crucial.However,according to previous reports,the problem of structural transformation of organic crystals around the volatile temperature is rarely paid much attention.In the common organic small molecule materials,the 8-hydroxyquinoline metal complex has a good application prospect because of its simple structure,convenient fabrication,good stability and excellent photoelectric properties.Compared with AlQ3,ZnQ2 requires lower working voltage in OLEDs,and its carrier injection efficiency is higher,which makes ZnQ2 has a wider application prospect in OLEDs.CuQ2 is not only a photoelectric material,but also has a place in organic spin because of its own magnetic properties and spontaneous spin polarization.CuQ2 can also sterilize anti-mold agents;pesticides,pharmaceutical synthetic metal corrosion inhibitors;used as a fungicide in agriculture.And it is also used as a fungicide for ropes,threads,leather and vinyl plastics.In addition,CuQ2 has great significance in modern medicine,especially recently reported that it can treat cancer as an anticancer drug.In this paper,we study the crystallization process and crystallization mechanism of ZnQ2 and CuQ2,two common 8-hydroxyquinoline complex organic small molecule materials,by physical vapor deposition and thin film annealing.In addition to studying its crystallization process and crystallization mechanism,we have also conducted some explorations in the spintronics based on organic single crystals.The detailed contents and key results are listed as follows:1.Preparation of anhydrous(ZnQ2)4 single crystal by PVD methodHigh quality anhydrous(ZnQ2)4 single crystal was prepared by PVD method.The crystal was a triclinic single crystal.The unit cell parameters are obtained by XSCD:a=10.8537(15)A,b=11.8814(16)A,c=13.0532(18)A,?=74.119(2)°,?=73.449(2)°,?=70.999(2)°,volume= 1494.9(4)A3,Z=1.The crystallization mechanism at different temperatures was analyzed and we found:at low temperatures(300? or 320?),the crystallization mechanism is a classic crystal growth mechanism,this means the basic unit for its growth is a single molecule;at high temperatures(340?),the crystallization mechanism is the non-classical crystal growth process of particle aggregation.Finally,compared with ZnQ2 films,the luminescence properties of tetramer(ZnQ2)4 single crystals are significantly enhanced.2.Research on the structural transition of solid state to solid state of CuQ2 thin filmThe solid-to-solid crystallization process of CuQ2 thin film has been investigated by heat treatment.It is found that the film shows different crystallization behaviors at different temperatures:at low temperature(80 ?),the film grows into nanorods via classical Ostwald ripening,at a middle temperature(120 ?),the film grows into crystals via particle nucleation,particle growth,particle migration,particle attachment,and structure reconstruction.The growth of hexagonal particles is from outside to inside.Classical Ostwald ripening and non-classical particle attachment pathways have both been found at this stage,at high temperature(150 ?),the particles grow into high quality crystals via several times non-directional particle attachment and structure reconstruction processes.The crystallization process is mainly a non-classical crystallization process,which mainly includes grain nucleation,grain growth,grain migration,non-directional grain attachment and structure reconstruction.The hexagonal CuQ2 grains are formed in the order of the outer side and the inner side of the front side rear upper surface during the growth process.We have also found that the non-classical crystallization of CuQ2 grains is not known to be performed by oriented attachment,but rather by the non-orientation combination of a new micron-sized particle.We also found for the first time that microparticles can migrate in the process of merging by a distance of several tens of micrometers to complete the non-classical crystallization process.3.Structure change of volatile temperature of CuQ2 thin film:competition for crystallization and volatilizationThe structural transformation of CuQ2 at a volatilization temperature has been investigated.It is observed that the CuQ2 undergoes a fast crystallization processes from nanograins/nanorods to micro-crystals first,and then a particle-attachment crystallization process accompanying with volatilization behavior.It is found that crystals aggregated and merged during the high-temperature volatilization of CuQ2 crystals through non-classical crystal growth mechanisms.In the whole process,the classical crystal growth mechanism of molecular migration,the non-classical crystal growth mechanism of particle aggregation/combination,and the mechanism of volatilization of CuQ2 molecules compete with each other,resulting in the change of CuQ2 sample structure.We also noticed that the crystal remained solid throughout the evaporation process and did not melt into a liquid state.Our experiments provide important references for analyzing the degradation of structural properties of organic electronic and optoelectronic devices at high temperatures and studying the mechanism of high temperature failure of related devices.4.Research on organic spin valves of LSMO electrodesWe also studied the organic spin valve of the LSMO electrode.We etched the LSMO electrode and grew organic small molecule single crystals such as rubrene,CuQ2,and ZnQ2 on the etched LSMO electrode.A variety of organic single crystal spin valve devices have been tested and the spin transport characteristics of the devices have been thoroughly tested. |
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