有機半導體薄膜在金(111)與其修飾表面上的成長、結構與電晶體之應用

Organic Semiconducting Thin Films on Au(111) and Au(111)–benzenethiolate Surface : Growth, Structure, and Transistor Application

呂健瑋

指導教授 ： 楊耀文

國立清華大學/理學院/化學系/博士(2014年)

https://doi.org/10.6843/NTHU.2014.00257

摘要

目前許多以併環噻吩為主體的有機半導體衍生物被開發出來，因其有較佳的分子間硫-硫作用力與較大之游離能而使其具有空氣穩定性。而在有機場效電晶體元件中，有機薄膜與金屬電極之介面為其中一種影響元件效能之重要因素。而本研究主要於Au(111)及苯硫醇修飾之Au(111)基材上，分別成長2-phenylbenzo [d,d’]thieno-[3,2-b;4,5-b’] dithiophene (P-BTDT)分子與其衍生物monofluorine- substituted 2–phenylbenzo [d,d_]thieno[3,2-b;4,5-b_] –dithiophene (m-FP-BTDT)有機半導體薄膜，並由XPS、NEXAFS與UPS來探討其成長機制、分子位向與電子結構，且結合XRD、AFM之分析結果，來解釋不同薄膜特性對電晶體性能之影響。由TPD圖譜得知P-BTDT分子在Au(111)上其化學吸附層穩定至750 K，而在Au(111)-benzenethiolate (Au(111)-BT)上得知此分子以物理吸附形式存在而抑制其於基材上之化學吸附。由XPS訊號強度隨P-BTDT分子薄膜厚度變化量之改變，得知在Au(111)上為Stranski-Krastanov成長模式，先以形成二維單層結構於基材上，再轉變成三維結構的成長;而P-BTDT在Au(111)-BT基材上其成長模式則傾向於二維結構成長。藉由觀察UPS光譜中，功函數及價帶與膜厚之間的關係，可以推測薄膜的成長趨勢，而此趨勢與在XPS圖譜中觀察到的結果相符。而由NEXAFS光譜得知厚層P-BTDT分子其芳香環與Au(111)-BT基材表面成66 º，高傾角允許分子的電子更有效從事分子間的躍遷提升電洞傳輸效率。對於成長於Au(111)上之厚層P-BTDT薄膜，由出平面XRD顯示其具有不同之晶面組成；而成長於Au(111)-BT基材上之薄膜則主要以(002)晶面所組成。因此我們可以藉由對基材做不同修飾來改變薄膜之晶體結構，並控制其成長行為，來設計並改善臨場底閘極底接觸式有機場效電晶體之效能。由量測結果得知分子P-BTDT在BT-Au /OTS/SiO2基材上有較佳之載子遷移率，薄膜厚度於120 nm時載子遷移率為3.0 × 10-2 cm2/Vs。
為使分子之電子雲密度下降，進而降低LUMO之能階，並有利於對其成長位向與成長模式的探討。因此，於本研究中，亦使用具氟原子取代之有機半導體分子m-FP-BTDT來進行薄膜之成長，並探討其薄膜特性。由XPS圖譜得知m-FP-BTDT薄膜成長於Au(111)時為Stranski-Krastanov成長模式。此外，於此實驗中亦觀察到一有趣現象，即當分子m-FP-BTDT在Au(111)上形成單層薄膜時，其XPS圖譜中之Au 4f訊號突然增加，此乃因Au(111)重構解除時會釋放出額外的金原子。然而，m-FP-BTDT分子在Au(111)-BT是以近似二維成長模式來形成薄膜。由NEXAFS結果得知，在Au(111)-BT上成長厚層薄膜時，m-FP-BTDT分子傾向以較垂直於基材的方向成長，其與基材之傾角為62.5°，因此分子間具有較佳的π-π堆疊。由XRD得知m-FP-BTDT薄膜成長於Au(111)-BT上具有結晶相並沿著晶格c軸成長。而由UPS結果得知，在Au(111)-BT上所成長之厚層P-BTDT薄膜，其功函數為4.62 eV而電子注射能障為0.94 eV。由上述結果得知此分子在Au(111)-BT上較利於載子傳輸。

關鍵字

金(111) ； 併環噻吩 ； 成長模式 ； 光電子光譜 ； 有機場效電晶體

並列摘要

Recent years have seen an increasing number of studies on the utilization of fused thiophene-based materials for organic filed effect transistor (OFET) applications due to their strong intermolecular S-S interaction, large ionization potential energy, and better ambient stability. In OFETs, the interface between the electrodes and the organic semiconductor plays a critical role in affecting the device performance. In our work, the 2-phenylbenzo [d,d’]thieno-[3,2-b;4,5-b’] dithiophene (P-BTDT) and its derivative, monofluorine-substituted 2–phenylbenzo [d,d_]thieno [3,2-b;4,5-b_]–dithiophene (m-FP-BTDT), were grown on the Au (111) and modified Au (111) substrate as the organic semiconductor thin films, respectively. The growth mechanisms, the molecular orientations and the electronic structure of the thin films were analyzed via XPS, NEXAFS and UPS. In addition, the above results were combined with the XRD and AFM data to reach a thorough understanding of the performances of various OFETs. The thermal desorption data show that the chemisorbed P-BTDT layer remains thermally stable up to 750 K on Au(111). However, P-BTDT desorption on Au-BT substrate is found to derive exclusively from physisorption state, and chemisorption desorption is entirely suppressed. Based on the change of XPS intensity with the amount of P-BTDT deposited, it is concluded that the growth of P-BTDT film on Au(111) follows the Stranski-Krastanov mechansim, i.e., a completion of one monolayer (2D growth) followed by a 3D crystallite growth. In comparison, for the growth of P-BTDT on Au(111)-benzenethiolate, a pseudo 2D-like growth is observed. From the results of UPS, the change of work function and valence band spectrum with the film-thickness is in accord with this growth behavior. The results obtained by XPS are also in agreement with UPS observations. The molecules of the multilayer grown on Au(111)-BT have their aromatic rings inclined toward the surface by an average 66°, as determined by NEXAFS data. For the thick film grown on Au(111), out-of-plane x-ray diffraction reveals the presence of several crystallite alignment schemes. In contrast, the XRD data of the thicker layer grown on Au(111)-BT show (002) diffraction peaks only, indicating that the crystal orientation of the thin film is (002) preferred. Therefore, we can change the crystal structure and control the growth behavior of the thin film through different modification of substrates, leading to an improved performance of the OFETs. The results show that P-BTDT thin film that is grown on BT-Au can yield a better OFET mobility up to 3.0 × 10-2 cm2/Vs with thickness of 120 nm.
In order to shift energy level of LUMO toward lower value, we also prepared m-FP-BTDT organic semiconducting thin films and investigated their properties. XPS intensity analysis shows that on Au(111), m-FP-BTDT film grows according to Stranski–Krastanov (SK) mode. It is interesting to note an abrupt increase of Au 4f signal at around one ML of m-FP-BTDT, which is explained by the production of excess Au adatoms, accompanied by the lifting of the herringbone reconstruction of Au(111). In comparison, the initial growth of m-FP-BTDT on Au-BT proceeds via a pseudo layer-by-layer growth mechanism. NEXAFS data show that m-FP-BTDT molecules on Au-BT adopt a more erected configuration, the angle between the molecule and substrate is 62.5°, resulting in a better cofacial π-stacking. The XRD pattern reveals that the crystal growth orientation of m-FP-BTD thin film is along c axis. Work function for the thick m-FP-BTDT film on Au-BT is determined with UPS as 4.62 eV and the hole injection barrier as 0.94 eV. According to above results the thin film of m-FP-BTDT grown on Au(111)-BT is expected to produce better carrier transport phenomenon.

並列關鍵字

Au(1 1 1) ； Fused thiophene derivative ； Growth mode ； XPS ； NEXAFS ； UPS ； OFET

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