分子铁电材料调控场效应管特性研究

Molecule-based ferroelectrics, which contain inorganic acid and organic base, have attracted more and more attention since the advantages such as flexible for design all kinds of structure, controllable self-assembly of organic molecular building blocks in 3D space, low-temperature synthesis, no-lead and no pollution. It has very important significance in both basic scientific concerns and potential for practical applications to make theoretical and applied researches on moelcule-based ferroelctrics because their ferroelectric mechanism is still confusing. Recently, most organic ferroelectric field-effect transistors (FeFET) use ferroelectric polymer PVDF as the dielectric gate. However, PVDF has some disadvantages, such as large coercive field, high surface roughness, which toughly influence the performance of FeFET. The research of other molecular ferroelectrics used as insulator gate in FeFET is very scarce. We will fabricate FeFET using Diisopropylammonium Bromide and/or 4-(cyanomethyl)anilinium Perchlorate, which are molecular ferroelectrics we discovered recently, as insulator gate with different substrates and different conditions using spin coating method.The performance of FeFET is manipulated by ferroelectric properties in these films. The relationship between FeFET properties and ferroelectric will be studied by methods such as ab-init calculation, lattice parameters and depth distribution, thermodynamics analysis, Raman spectrum, magnetic coupling as well as ferro-, piezo-, pyro- and di-electric analysis. We try out best to obtain molecular FeFET with more excellent properties compared with PVDF based FeFETtheir and lay solid foundation for practical applications of molecule-base ferroelectrics.

分子基铁电体是一类由无机酸和有机碱组成的盐，因其结构易剪裁与易修饰性、有机分子构筑块在三维空间分子自组装的可控性、环保无污染等优点而受到越来越多的关注。但其铁电性的产生机理尚未十分明了，开展有机铁电体的理论和应用研究都具有非常重要的理论和实践意义。目前有机铁电场效应管都是以铁电聚合物PVDF为栅极,但是PVDF存在矫顽场大、表面粗糙度大等缺点，严重影响场效应管的性能。而其它的有机铁电材料在这方面的应用还非常缺乏。本项目拟以我们发现的分子基铁电体二异丙胺溴和对氨基苯乙腈高氯酸盐为主要研究对象，用旋涂法生长不同条件的铁电场效应管，通过不同的条件来调控其性能。运用第一性原理计算铁电性对场效应管的调控作用，通过薄膜晶格参数、深度分布、热力学分析、铁电介电压电热释电分析、拉曼光谱、磁性耦合等手段来研究其铁电性与输运性质之间的关系和调控机制。获得性能优越的有机铁电场效应器件，为其实际应用打下基础。

与无机铁电材料相比，分子铁电材料具有种类多易设计、合成易无污染、矫顽场小功耗低等优点。项目围绕如何提高分子铁电材料的自发极化值、如何提高其铁电相变温度，获得高质量的单晶和薄膜样品展开，经过四年的研究取得了一系列原创性研究成果：1、发现了两类高于室温的分子铁电体tris(2-hydroxyethyl) ammonium bromide（TAB）和(C6H5NH3)2CdCl4（CNCC）。TAB在424 K晶体结构从R-3m空间群转变为R3m，它的自发极化的大小为0.54 μC/cm2，矫顽场为0.62 kV/cm,具有介电驰豫特征。CNCC的空间群在369 K从Cmc21转变为I222，变温热释电测量发现热释电电流在370 K出现峰值。改变不同的极化电压可以改变热释电电流的方向。自发极化值为0.5 μC/cm2。2、证明甲胺铅溴在四方相和正交相都具有铁电性。由于较大漏电流的存在，常规方法很难证明其铁电性。我们通过直流偏置下的介电常数、不同极化电场下的热释电系数、双波电流法测电滞回线等三种方法证明了甲胺铅溴具有铁电性。这对研究其电池效率有着重要的理论依据。3、发现具有较高二阶倍频效应的晶体(4-amino-2-bromopyridinium) tetrafluoroborate，它的二阶倍频系数达到10 pm/V，是KDP的10倍以上，因此在激光倍频晶体上具有很高的应用价值。4、发现具有较高热释电优值晶体(4-amino-2-bromopyridinium) tetrafluoroborate，它的热释电优值系数M1和M2是目前为止分子铁电材料中最大的，因此它在热释电探测成像方面有较大的应用价值。 5、优化了DIPAB薄膜的制备，获得具有较好铁电性的薄膜，存在头对头或者尾对尾的导电畴壁。项目总共发表了SCI研究论文13篇，其中3篇1区论文The Journal of Physical Chemistry Letters。项目四年执行过程中，培养1名博士，8名硕士。目前在读5名博士生，1名硕士生。整个项目圆满完成研究目标。