有機小分子材料之ㄧ維奈米結構的製備與特性研究

Abstract

近十年來，有機小分子之奈米材料已經引起廣泛的注意，它們獨特的光學與電學特性，使得奈米結構之元件在未來發展上孕育嶄新的應用潛力。有機奈米材料相較於無機奈米材料，除了具有迥然不同的光電性質，更具備低成本、低溫製程以及結構可撓性等優勢，展現豐富的實用價值。本論文主要探討在低溫環境下所合成之一維有機奈米結構的各項影響因素並試圖將其應用於有機場發射器上，本研究使用之有機半導體材料以自我組裝的方式成長為奈米結構，最後更進一步評估所製備之一維有機奈米材料在真空系統下的場發射電子效益。

本論文第一部分主要探討以1,5-diaminoanthraquinone (DAAQ)為起始材料，經由低溫(42 °C)真空蒸鍍方式製備直立的有機奈米纖維，我們利用基板表面性質，可控制DAAQ分子自我組裝之薄膜形貌，研究中發現，直立排列之一維奈米結構傾向成長於低水接觸角基板上，如Au、Si以及Ti基板。DAAQ-Au與DAAQ-Ti元件皆展現出場發射電性特徵，在外加電場12 V/μm下，最大發射電流分別為0.31 and 0.65 mA/cm2，而起始電場則為8.5 V/μm以及8.25 V/μm。經由Fowler-Nordheim (FN)繪圖的斜率，我們可以計算出DAAQ場增強因子在Au與Ti基板上分別為447及831。

在本論文第二部分主要是設計一個簡單的方法並經由熱蒸鍍方式來製備微米級圖案定義之iron phthalocaynine (FePc)奈米纖維陣列。藉由控制基板表面能與基板溫度，我們發現到FePc薄膜在Si表面(高表面能)形成平面型態，而在Ag表面(低表面能)卻形成垂直於表面之纖維結構。這些以溫度誘發且排列性良好的FePc一維奈米纖維具備場發射特性並符合FN行為。利用此控制形貌的方法，我們可以在預先圖形定義Ag/Si之基板上，選擇性成長FePc奈米結構陣列。此外，由電性量測結果發現，具有較高之長/半徑比的元件(FE-240-P; Tsub of 240 °C)相較於較低之長/半徑比的元件(FE-180-P; Tsub of 180 °C)表現出較佳之場發射電子能力。於基板溫度240 °C之理想條件下，場發射元件的發射電流密度由無圖形定義元件之0.13 mA/cm2提升至有圖形定義元件之6.77 mA/cm2 (在外加電場12 V/μm下)，而元件之起始電場則由10.3 V/μm降低至7.7 V/μm。在場發射穩定性測試實驗中，FE-240-P元件的場發射電流密度表現出小於20%的擾動比率，顯示此元件在量測過程中具有穩定且出色的電特性。

本論文最後一部分是開發一個簡便且有效率的方法，藉由在銦錫氧化物(ITO)上覆蓋graphene作為透明導電層，進一步以熱蒸鍍方式合成29H,31H-phthalocyanine (H2Pc)之奈米纖維，並具有作為有機場發射元件的潛力。我們控制基材的表面能與基板溫度，進而使得H2Pc分子在reduced graphene oxide (rGO; 表面能: 50 mJ/m2)表面上穩定地自我組裝堆疊，形成突出於基板平面之形貌。在rGO/ITO基板上所製備之元件(FE-rGO)不僅僅展現出優秀的場發射特性，更在穩定性測試中表現出色且具有抗衰退能力。本研究以rGO塗層之簡易方式，能於透明導電層上，將有機小分子薄膜型態轉變成垂直站立之奈米結構，使得有機電子元件在多樣性應用上開啟嶄新的一頁。

In the last 10 years, nanomaterials based on small organic molecules have attracted increasing attention. Such materials have unique optical and electronic properties, which could lead to new applications in nanoscale devices. Besides the strikingly different optoelectronic properties from those of their inorganic counterparts, organic nanomaterials also have some advantages in low-cost, low-temperature processing and mechanical flexibility, which make them complementary to the inorganic materials. The dissertation describes the impact factors on the synthesis of one-dimensional (1D) organic nanostructures with low-temperture processes in an attempt to apply in organic field emitter. All of the organic semiconductor materials investigated in this thesis, display self-assembly aligned nanostructures, in which electrons can emit from the tip of nanostructures at an applied electric field using a vaccum emission measurement system.

In the first part of the dissertation, by using 1,5-diaminoanthraquinone (DAAQ) as a starting material, vertical organic nanofibers were prepared through low-temperature (42 °C) vacuum sublimation. The structural morphologies formed from the DAAQ molecules self-assembly stacking were controlled by surface properties of substrates, with vertically alighned 1D nanostructures growing preferentially on low water contact angle surfaces, such as Au, Si, and Ti. On Au and Ti substrates, the DAAQ nanofibers DAAQ-Au and DAAQ-Ti exhibited field emission characteristics, with maximum emission current densities of 0.31 and 0.65 mA/cm2, respectively, at an applied electric field of 12 V/μm. The turn-on electric fields were 8.5 V/μm for DAAQ-Au and 8.25 V/μm for DAAQ-Ti. From the slopes of Fowler-Nordheim plots, we calculated the field enhancement factor (β) of the DAAQ nanofibers on the Au and Ti substrates to be 447 and 831, respectively.

In the second part of this dissertation, we devised a simple method for micropatterned growth of iron phthalocaynine (FePc) nanofiber arrays using a thermal evaporation process. By controllong the surface energy and the temperature of the substrate (Tsub), we obtain FePc films featureing a grain-like (in-plane) morphology on Si surfaces (higher surface energy) and a fiber-like (out-of-plane) morphology on Ag surfaces (lower surface energy) within a certain range of values of Tsub. These temperature-induced and well-aligned 1D FePc nanofibers exhibited FE characterisitics and follow FN behavior. Using such morphological control, we grew patterned FePc selectively on previously patterned Ag/Si substrates; moreover, the higher AR of the devices (FE-240-P; Tsub of 240 °C) exhibited better FE performance than lower AR of devices (FE-180-P; Tsub of 180 °C). In the optimal growth condition of Tsub of 240 °C, the emission current of the device improved dramatically from 0.13 mA/cm2 for the unpatterned device to 6.77 mA/cm2 for the patterned device (when biased at an applied field of 12 V/μm), while the turn-on field of the device decreased accordingly from 10.3 to 7.7 V/μm. In FE stability tests, the current densities of FE-240-P exhibited fluctuations of less than 20%, revealing its stable and superior property whin the duration of the measurement process.

In the last part of this dissertation, we developed a simple and efficient approach—using graphene coatings on ITO as transparent electrodes—for inducing the growth of 29H,31H-phthalocyanine (H2Pc) nanofiber arrays through a thermal evaporation process, with potential use in organic field emitters (FEs). By controlling the surface energy and temperature of the electrodes during evaporation, H2Pc molecules readily self-assemble, forming an out-of-plane morphology on reduced graphene oxide surfaces (rGO; surface energy: ca. 50 mJ/m2). The devices fabricated on rGO/indium tin oxide (FE-rGO) exhibited not only excellent FE performance but also outstanding anti-degradation capability during stability tests. This facile approach toward rGO coatings opens a new avenue for the transformation of small organic molecules films into vertically standing nanostructures on transparent electrodes, with various organic electronics applications.