Growth Of Large-sized Organic Doped Crystals By Melt Method

Organic single crystals possess long-range orderly structure in three-dimensional and thermodynamic stability owing to the unique properties of conjugated organic molecules,which demonstrates an irreplaceable and important application prospect for various fields of national economy and national security.For instance,in the application of high-energy ray detection,large-sized organic scintillation crystals can directly detect fast neutrons and effectively distinguish y-rays from neutrons;In nonlinear optical terahertz band,the organic crystals have large nonlinear coefficients and high damage resistance threshold,which can obtain the maximum continuous bandwidth in the whole terahertz band.In the field of maser,pentacene doped p-terphenyl crystal was used as the gain medium to fabricate the world's first maser that can operate in the room-temperature geomagnetic environment,thereby notably reducing the preparation and operation cost.However,the poor crystallinity and high processing difficulty have refrained it from application in wider area,since it has turned out to be difficult to produce high quality and large-sized organic crystals.The technologies for growing high quality and large-sized organic crystal are supposed to be the precondition to achieve application.Therefore,it is of great theoretical significance and practical value to study the growth and properties of Large-sized organic functional crystals.In this context,this thesis focuses on the growth of large-sized organic crystals and doped crystals.Combined with the experience of our research group in inorganic crystal growth and the growth habits of organic crystals,the equipment specifically for melt growth method of organic crystals was designed to explore a more appropriate growth method for organic crystals and doped crystals,so as to grow high quality and large-sized organic functional crystals.Thermal field-elevating technique was used to carry out the research for the growth of p-terphenyl crystal,pentacene doped p-terphenyl crystal and long persistent luminescence doped organic crystals,respectively;the effect of doping on organic crystal properties and the interaction between the crystal matrix and the doped molecules were studied systematically to regulate the optical,electrical and magnetic properties of the crystals.The main research contents are organized as follows:1.Based on the physical and chemical properties of organic materials and the crystallization habits of organic melt,a melt method for the growth of organic crystals was developed.The relative movement mode between crucible and temperature field was optimized via thermal field-elevating method during crystal growth,which guaranteed the technical support for obtaining large size and high quality organic single crystals.The thermal field-elevating method provides the possibility of observing crystal growth process so that the dynamic change of solid-melt crystal interface can be grasped in real time.The crucible kept relatively static and the temperature field raised,which effectively avoided the influence of mechanical vibration in crystal growth process.The tightness of oven chamber and the unique design of crucible allowed the feasibility of adjusting growth condition,thus preventing high temperature of oxyhydrogen flame from sealing tubes and simplifying operation procedure.This advance is more appropriate for producing large-sized organic crystals.2.The study on the growth of p-terphenyl crystals was further investigated by using the thermal field-elevating.The raw materials needed for growing single crystals were purified through the multiple cycle process,including solution recrystallization,gas phase sublimation and melt zone melting.By optimizing the crystal growth process,the p-terphenyl crystal with a size of ? 10 × 35 mm was obtained.UV-Vis absorption spectra showed that the cut-off absorption wavelength of p-terphenyl is around 371 nm.A large optimal excitation wavelength range can be observed from the fluorescence excitation and emission spectra,and the maximum emission peak of fluorescence remained at 392 nm.It was found that p-terphenyl crystal exhibited a fast fluorescence decay with the lifetime 3.08 ns and the high quantum yield 52.24%.3.In this section,the growth of pentacene doped p-terphenyl crystal has been studied.A series of different size of pentacene doped p-terphenyl crystals with different doping concentrations were grown successively.All crystals exhibited good quality,high permeability and uniform doping,with the maximum doped crystal size ? 18 × 80 mm.Compared with pure crystal,the optical properties of doped crystal changed significantly.The refractive index of doped crystal integrally increased in visible light band and extinction coefficient altered.The absorption spectra and fluorescence emission spectra showed that the intensity of intrinsic absorption and emission peaks of p-terphenyl doped crystals decreased,whereas the intrinsic absorption and emission peaks of pentacene molecules appeared.Furthermore,the relative intensity of the different absorption and emission peaks changed regularly with the increase of the concentration of pentacene.Further study found that,compared solution with the same concentration and amorphous state,doped crystal can display a more sophisticated absorption and emission spectrum.The results show that p-terphenyl single crystal as an ordered matrix not only separates the guest molecules from one another to prevent quenching,but also constructs a Shpolskii-type host as an orienting medium to increase the interaction of pentacene with light.Moreover,a slightly reducing growth atmosphere is found to generate longer excitation lifetimes and a larger magnetic susceptibility of the grown doped crystal,which may be favorable for the spin-related physical process occurring in the excited state.4.The organic long persistent luminescence host-guest doped crystals systems were constructed by melt method.When the benzophenone derivatives of crystallization induced long persistent luminescence was applied as the guest with p-terphenyl,triphenylphosphine oxy(TPPO)and stilbene as the host respectively,the cm-scale doped crystals were achieved,realizing the long persistent emission in large size organic doped crystals.The effects of different doping molecules,doping substrates and doping concentrations on the long persistent luminescence properties of crystals were systematically studied,and the generation mechanism of persistent luminescence in organic doping crystal was explored.The results indicated that the long persistent luminescence lifetime of the doped crystals has reached 72.54 ms,which is 3 orders of magnitude of the pure guest molecule.The color and lifetime of long persistent luminescence can be emitted tunably by doping different guest molecules into different host crystal substrates.The relationship between defect types and long persistent luminescence mechanism of organic doped crystals were studied through theoretical calculation.The doped molecules were embedded in the crystal lattice as an alternative defect,which effectively inhibited the molecular rotation and vibration relaxation,thereby preventing the quenching of oxygen,and inhibits the non-radiative transition.Meanwhile,energy transfer can occur between host and guest,promoting the generation of more excited triplet electrons.Therefore,the long persistent luminescence performance of crystal was enhanced as a result of several synergistic factors.Host-guest organic doped crystal system is suitable for several of guest molecules of crystallization induced long persistent luminescence to enhance luminescence in host crystal,which has certain universality and provides a new strategy for long persistent luminescence.