The Manipulation Of Spin Polarization On Organic Charge Transfer Complexes

In 1976,the emergence of polyacetylene,an organic conductive material,overturned the inherent concept of organic materials as insulators and opened up the research as well as application of organic materials in the field of electrical properties.In the 1990s,the optical properties of organic semiconductor materials began to attract much attention.With the development of photoelectric properties of organic semiconductor materials,the novel spin properties have also received increasing interest.The rich magnetic,optical and electronic properties of organic semiconductors are derived from the ?-conjugated structure and strong electron-lattice coupling.In the ?-conjugated system,the overlap of electron cloud provides a channel for ? electrons transmission,providing the opportunity of conducting;The transition of ? electrons between the energy levels of HOMO and LUMO makes its absorption and emission spectrum distributed between near-ultraviolet to near-infrared;The strong electron lattice coupling in organic semiconductor materials leads to a complicated charge-spin relationship of the carriers,which is the basis of the novel spin properties.Based on this,people have designed and synthesized numerous organic semiconductor materials,which are practically used in various fields,such as organic photovoltaics,organic field effect transistors,organic light-emitting diodes,organic spin valves,organic spin currents,organic multiferroic,and organic magneto-chiral dichroism detection.At present,organic semiconductor materials have gained considerable attention and emerged as a promising research interest because of their magnetic,optical and electronic properties.One of the most promising topics is design,synthesis,optimization or manipulation of spin-polarized materials,such as organic ferromagnetic or ferrimagnetic materials.In 1988,Ovchinnikov et al.synthesized the organic polymer magnetic poly-BIPO for the first time.A series of open-shell free radicals suspend on the zigzag-shaped carbon main chain,and there is strong spin exchange interaction between the free radicals,which makes the system exhibit obvious spin polarization.In 1993,Sessoli et al.discovered the first single-molecule magnet[Mn12O12(O2CCH3)16(H2O)4]-4H2O·2CH3CO2H,it behaved superparamagnet-ism at low temperatures,in which one molecule can be seen as a magnetic domain.In addition,fullerene may also exhibit excellent ferromagnetism after oxidation or hydrogenation.In 1996,Murakami and Suematsu et al.found that the saturation magnetization of oxidized C60 can reach 10-3?B/C60 in the experiment,Kvyatkovskii pointed out that the saturation magnetization of each C60Hx was between 0.001 to 0.16?B.Moreover,many studies have proved that organic charge transfer complexes formed by multiple components are also a typical representative for organic ferromagnets.For example,Allemand et al.revealed that the organic charge transfer complex TDAE-C60 was ferromagnetic,and the Curie temperature was only 16.1 K.Further research by Tanaka et al.found that electron spin polarization was mainly located on the acceptor C60 molecule.The exchange interaction between spins was about 5.03×10-4 eV.The orientation of C60 molecules also has an important influence on the ferromagnetism.In the TTF-TCNQ charge transfer complex,spin polarization is not only distributed on the acceptor TCNQ,but also existes on the donor TTF.In TTF-BA,it displays ferromagnetism,ferroelectricity and magneto-electric coupling effect,when the temperature is lower than 84 K.The molecular configuration...D+A-D+A-...can be seen as a Heisenberg chain with 1/2 spin,so it behaves ferromagnetism.When the temperature drops to 53 K,the space group changes from nonpolar P(?)to polarity of P1,due to its spin-Peierls instability,the polarization and magneto-electric coupling effect could be induced under its Curie temperature.In recent years,the Curie temperature of ferromagnetic organic charge transfer complexes has been raised above room temperature.One of typical examples in organic charge transfer systems is based on polythiophene nanowires,such as P3HT-nw-C60,P3HT-nw-gold nanoclusters.In the case of P3HT-nw-C60 charge transfer complex,not only the magnetic anisotropy is observed experimentally,but also its sensitive light and force responses are found.At the same time,the system also shows obvious magneto-electric coupling effect.In addition,the theoretical studies also find that the system has a net spin polarization when the pentagon of thiophene facing the pentagon of C60,however,the spin polarization of the system disappears when pentagon of thiophene facing the hexagon of C60.After that,on the basis of P3HT-nw and C60 charge transfer complexes,chiral P3HT-nw and C60 charge transfer complexes are fabricated,whose magnetic properties have different responses to linearly and circularly polarized light.PNNBNO is the first organic ferrimagnet,which fills the blank of organic ferrimagnets,but there are few reports of organic chiral ferrimagnets.It should be noted that organic charge transfer complex does not have 3d electrons,which would be different from the sources of ferromagnetism or ferrimagnetism for inorganic materials.Based on computational modeling,the origin of ferromagnetism is from localized spin polarization induced by electron in organic acceptor.However,the other theoretical study shows that holes spin alignment in donor induce localized spin polarization,which is responsible for the room temperature ferromagnetism.The theoretical explanations for the magnetism of the organic charge transfer complexes are not uniform.Explaining the ferromagnetism or ferrimagnetism of organic charge transfer systems experimentally is even more challenging.In addition,what are the key factors for the generation of organic magnetism,how to synthesize ferromagnets and ferrimagnet.Based on the existence of spin polarization in organic charge transfer system,it is a question that how to realize and explain the manipulation of light field,electric field and lattice distortion on the performance of organic charge transfer system in the experiment.Additionally,for the charge transfer magnets with chirality,it is not clear whether there is a difference in spin polarization between two chiral isomers.Given the analysis above,further research on spin polarization manipulation of organic charge transfer systems is needed.In view of the above mentioned problems,this paper has done a detailed investigation and carried out the corresponding work:In the first part of our work,we fabricate P3HT-nw/CH3NH3PbBr3 charge transfer heterojunction and find that the system has room temperature ferromagnetism.The spin polarization intensity can be regulated by different light frequencies,power and different electric field intensity.Separated optical double beam excitations on P3HT-nw/CH3NH3PbBr3 charge-transfer heterojunction are studied to reveal the effects of optically generated charge transfer on organic magnetism and magnetoelectric coupling.The double beam 355 nm and 607 nm lights are selected to separately excite the CH3NH3PbBr3 and P3HT-nw.Spin polarization and magnetoelectric coupling could be effectively tuned through the photoexcitation of CH3NH3PbBr3,rather than photoexcitation of P3HT-nw phase.Closed-shell structure broken of P3HT-nw-based charge-transfer complex is the main reason to induce room temperature magnetism.In other words,the charge transfer between different components in the crystallization system is an important reason for ferromagnetism.With increasing 355 nm light intensity,room-temperature magnetism,ferroelectricity,and magnetoelectric coupling effects are enhanced in P3HT-nw/CH3NH3PbBr3 charge transfer heterojunction.Studying light frequency and intensity dependent spin polarization and magnetoelectric coupling in organic charge transfer heterojunction not only help us to understand optically controlled organic magnetization and magnetoelectric coupling,but also bring us more information of organic spintronics.Based on the conclusions from the first part of the work,in the second part of the work,we purposefully prepare organic charge transfer coronene-TCNQ crystals.As expected,they display ferromagnetism at room temperature.In addition,because the unit cell volumes or lattice constants of these crystals are different,the ferromagnetism has significant difference.At the same time,the external magnetic field can also change the distance between the donor and acceptor in the crystal,so there is spin-lattice-interaction dependent magnetoelastic coupling in the coronene-TCNQ crystals.Combined with theoretical calculations,charge transfer induced energy level splitting leads to ferromagnetism of coronene-TCNQ crystal.When the unit cell volume changes,the exchange interaction between electrons and holes also changes,which affects the spin polarization of the system.In addition,the change of unit cell volume leads to unequal changes of hole and electron lattice coupling coefficient in the donor and acceptor,which also changes the localization of hole and electron out-of-phase,causing the spin up and spin down magnetic moment produces different change,which ultimately changes the spin polarization of the system.In addition,under the external magnetic field,the distance between the donor and acceptor reduces slightly,which is also attributed to the magnetoelastic coupling effect in the coronene-TCNQ charge transfer crystals.Therefore,in order to realize the magnetoelastic coupling in the organic crystal,the following two factors need to be considered:1)The energy level splitting induced by the charge transfer in the organic crystal leads to the spin polarization;2)The spin-lattice coupling coefficient is different,and the spin-lattice coupling is strong.In the third part of the work,organic charge transfer crystals Bper-FTCNQ contain the donor Bper and acceptor FTCNQ,and the structures of the crystal are characterized through single crystal X-ray diffraction,two kinds of chiral Bper-FTCNQ crystals are obtained.Combined with circular dichroism spectroscopy,it is further verified.We find that the two kinds of chiral Bper-FTCNQ charge transfer crystals are both ferrimagnetic,but the magnetization,antiferromagnetic phase transition point and ferromagnetic phase transition point of these two kinds of chiral crystals are different.In addition,we get the temperature dependent PL characteristic peak intensity and capacitance,which are consistent with the temperature transition point of in the temperature dependent magnetization curves.This work shows that the spin polarization in these two kinds of Bper-FTCNQ chiral materials is inequable due to the different configurations of the left handed and right handed chiral,and there maybe a certain relationship between the time inversion symmetry and the space inversion symmetry,which will also provide a new idea for the preparation of diverse organic ferrimagnets.