新穎性側鏈含對位取代氰基三苯胺及噁唑衍生物電子施體-受體共聚高分子:合成、性質鑑定及記憶體元件之應用

Abstract

近幾年來，高分子材料應用在記憶體元件上引起了廣泛的研究及討論，因為高分子可以透過設計不同的分子結構來改變其光電性質也易於加工，適合應用在元件上。而在高分子種類上，有幾種較具有代表性的種類，像是共軛高分子、非共軛高分子含有聚亞醯胺或是特殊的電子施體或受體基團，另外也有把高分子及奈米金屬顆粒或是有機小分子混掺的材料，這些不同種類的材料目前都有在記憶體元件應用上被深入的研究中。然而，關於不同的高分子結構所造成的影響尚未被深入研究，而在此論文的目標主要是要探討非共軛的亂排高分子使用不同的電子施體及受體對於其光電及熱性質和記憶體元件效應的影響。 在第二章，我們使用了三種噁唑衍生物(2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (A1), 2-(4-vinylbiphenyl)-5-(4-phenyl) -1,3,4-oxadiazole (A2), and 2-(4-vinylbiphenyl)-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (A3))當作電子受體，另外使用了對位取代氰基的三苯胺當作電子施體來合成高分子並且將兩者利用活性聚合法聚合成側鏈亂排共聚高分子和單聚高分子，並且利用氫譜及元素分析儀鑑定其確切其結構及施體-受體比例(8:2、5:5、2:8)。三苯胺在對位取代氰基之後，化學性質會較穩定而不易產生雙聚體化。從化學結構上來說，A2的共振長度較A1長，A3則因為多引入了一個推電子的乙氧基而增加了電子密度。在吸收光譜上，這些共聚高分子在薄膜態皆有兩根吸收鋒(353奈米、308 ~ 322奈米)分別對應於相關的單聚高分子(PD: 355奈米、PA1: 308奈米、PA2: 330奈米、PA3: 332奈米)，其吸收度則隨著不同的比例變化，另外由光學吸收極限計算可得此系列高分子之能隙在3.11 ~ 3.20電子伏特且會隨著受體含量變多而些微增大。由循環伏安法得到的結果發現這些亂排共聚高分子的HOMO及LUMO能階會與相對應的施體(-5.63電子伏特)或是受體(-2.63 ~ 2.81電子伏特)單聚高分子相同，這是由於這些高分子皆是在側鏈上有各自分離的電子施體和受體，彼此不參與軌域混成。從以上的結果可看出透過不同的化學結構及電子施體-受體的比例可以將高分子調整出不同的光電性質。 在第三章，三個系列的所有高分子都應用在元件製作及測量上，並且使用了二甲基甲醯胺和氯苯兩種溶劑來研究這些高分子所造成的記憶體效應。記憶體元件的構造是將高分子薄膜夾在兩層電極之間的三明治結構，其上層電極是鋁，而下層電極是氧化銦錫。這些元件皆表現出快閃記憶體的性質，且有著低於1伏特的起始電壓，在11000秒的維持時間測試中也有著不錯的穩定性。所有元件皆受到了燈絲理論及空間電荷極限理論的影響，在二甲基甲醯胺的系統中，我們發現到低導電態的電流在經過幾個讀寫迴圈後有變大的情形(10-5 ~ 10-2安培)，當電子受體的含量變多(施體-受體比例為5:5和2:8)或是受體的單聚高分子，此現象更加明顯，這是由於會與鋁金屬鍵結的氮原子含量變多的關係，造成燈絲效應更強烈。另外在結構方面，由於在A3上有一個推電子的乙氧基，使得在噁唑分子上的氮原子有更強的鍵結金屬的能力，使得元件更不容易被關閉，此時則需要更大的反向電壓(-3→-5伏特)才能使元件回到原始的狀態。此外，利用氯苯所製備的高分子膜會較二甲基醯胺平整，而低導電態電流上升的情形在平整的膜上就沒有再顯現出來。本研究探討了高分子之化學結構及溶劑對元件之影響，對於高分子記憶體元件的發展有很大的幫助。

In recent years, polymer systems have attracted significant research interest for memory device applications due to their tunable electrical properties through molecular design and good processibility. The representative classes of polymer materials used for fabrication on memories are conjugated polymers, non-conjugated polymers (functional polyimide systems or polymers with specific pendent donor or acceptor moiety) and polymer nanocomposites (metal nanoparticle or fullerene embedded). However, the effects of polymer structure on the memory characteristics have not been fully explored yet. In this thesis, the homopolymers pendant with donors or acceptors and related copolymers containing of both donor and accepter units were synthesized and evaluated on memory devices. In chapter 2, the homopolymers with pendant para-substituted dicyano- triphenylamine (D, as donor) or three different oxadiazole derivatives (2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (A1), 2-(4-vinylbiphenyl)-5-(4-phenyl)- 1,3,4-oxadiazole (A2), and 2-(4-vinylbiphenyl)-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (A3), as electron acceptor) and also related random copolymers consisting of pendant donor and acceptor moieties (the ratio of donor:acceptor is 8:2, 5:5 and 2:8, respectively) were synthesized by Nitroxide-Mediated Living Free Radical Polymerization (NMP) and characterized by 1H NMR spectrum and element analysis (EA). The para-substituted dicyano groups on triphenylamine improve the stability without the dimerization. The side-chain acceptor A2 shows more conjugated length relative to A1 by introducing another benzene ring, and A3 system further enhances the electronic density by adding terminal electron donating moiety (-OEt). The two thin film absorbance peaks of copolymers at 353 nm and 308 ~ 332 nm belong to the corresponding homopolymers (PD at 355 nm, PA1 at 308 nm, PA2 at 330 nm, PA3 at 332 nm, respectively) and the relative absorbance of two peaks vary with donor and acceptor ratios. The band gaps calculated from absorption edge are in the range of 3.11 ~ 3.20 eV and increase slightly with the larger acceptor content. The HOMO and LUMO energy levels of the random copolymers with different donor and acceptor ratios obtained from cyclic voltammetry (CV) are almost the same with the corresponding HOMO level of donor (-5.63 eV) and LUMO levels of acceptor (-2.63 ~ 2.81 eV), respectively, mainly due to poor hybridization of adjacent donor and acceptor which could be considered as an isolated system. The electronic properties can be well-tuned through different donor-acceptor structures and ratios of the copolymers systems. In chapter 3, the memory device of the random copolymers and homopoymers are fabricated and measured in ambient atmosphere. Different solvents (N,N-dimethyl formamide (DMF) or chlorobenzene (CB)) are chosen for polymer thin film processing. The bistable memory behavior is conducted by a simple sandwich device configuration consisted of spin-coated polymer film between indium-tin oxide (ITO) bottom electrode and aluminum (A1) top electrode. All the polymers exhibit nonvolatile flash type of switching behavior. The memory devices have low turn-on threshold voltage below 1 V and exhibits long retention time of 11000 seconds. The mechanism of the switching behavior is based on filamentary conduction with space charge limited current (SCLC) theory. As the polymer films prepare from DMF, the OFF state current during erasing scan increases from 10-5 A to 10-2 A after several WRER cycles. These phenomena are further obviously demonstrated in the case of the larger acceptor contents of the random copolymers (5:5 and 2:8 donor:acceptor ratio) and homopolymers pendant with acceptor due to the presence of the binding heteroatom (N) on acceptor with Al which contributes to filament conductions. Introducing electron donating ethoxy substituent (A3) which enhance the binding ability of N atom of oxadiazole group makes the memories difficult to turn off and also keep them on the high conductance since the stable filament channel are formed. The memory can still restore to the initial OFF state if the erasing process scan more negatively which also means the enhanced threshold voltage (from -3 V to -5 V) as the acceptor ratio increases. The polymer surface prepared from the CB solution is smooth and the current elevation of OFF state during the erasing switch could not be found. The study demonstrates that pendent donor/acceptor structures and ratio of copolymer and processing condition on the switching characteristics of memory device are fully explored.