由於在有機半導體分子所形成的晶體中，其載子傳輸特性沿著晶體不同的方向，會具有方向異性。因此在製作場效電晶體時，控制半導體分子晶體在元件中排列的方向，將有助提升元件效能。根據我們之前的研究結果，蒸鍍在金表面的五環素(pentacene)晶體是以其分子長軸平行於金表面的方向生長堆疊;當蒸鍍在以自組裝分子薄膜修飾過的金表面時，五環素晶體是以其分子長軸垂直於表面的方向堆疊。在本研究中，我們進一步以導電式原子力顯微鏡，測量了生長在金表面以及在自組裝分子薄膜修飾過的金表面的五環素奈米結晶之導電特性。電流測量結果顯示，五環素奈米結晶具有整流特性。平行五環素□-□堆疊方向的電流大於垂直於□-□堆疊方向的電流約104倍。這些在表面具有不同分子方向排列的五環素奈米結晶，會對於靜電力顯微鏡的相位影像產生差異。在本研究中，我們使用了反射式紅外線光譜(reflection-absorption IR)，近邊緣X光吸收微結構光譜(near edge X-ray adsorption fine structure)以及X光低掠角繞射(grazing incidence X-ray diffraction)，來鑑定蒸鍍在經刷膜處理後的聚甲烯(polymethylene)表面之五環素以及對位六連苯(p-sexiphenyl)薄膜之結晶結構。並利用原子力顯微鏡(atomic force microscope) 研究薄膜的表面型態。聚甲烯表面在經過刷膜處理過後，碳-碳鏈的骨架會沿著刷膜的方向排列。對位六連苯蒸鍍在經刷膜處理後的聚甲烯表面上，形成長棒狀的奈米晶體，其長軸垂直於刷膜方向排列。然而在這些長棒狀晶體中的對位六連苯分子，卻是以其分子長軸平行於刷膜方向排列。當五環素分子在室溫環境下，蒸鍍在刷膜過的聚甲烯表面時。我們發現五環素是以其分子長軸垂直於基材表面生長，並以[110]晶面法線方向平行於刷膜方向排列。當五環素蒸鍍在基板溫度為50°C的聚甲烯表面時，則是以[120]晶面法線方向平行於刷膜方向排列。本研究中我們也研究了紅螢烯(rubrene)在具有方向排列的五環素分子薄膜表面的有向性生長。這層具方向性的五環素分子薄膜是蒸鍍在經刷膜處理過後的聚乙烯醇(polyvinyl alcohol)表面所產生，這層有方向性的五環素分子薄膜會影響之後蒸鍍在其上的紅螢烯薄膜，使紅螢烯薄膜產生有向性生長。 以此具有向性生長的紅螢烯/五環素薄膜製作的場效電晶體，其載子移動率(mobility)展現了方向差異性。在相對於刷膜方向45°的方向可以測得最高值(0.105 cm2/Vs) 。 此方向與X光低掠角繞射實驗測量紅螢烯[002]最大值的方向相符合。

The ability to control the packing orientation in an organic molecular crystal is of great interest in fabricating an organic field effect transistor (OFET) because of the anisotropic nature of charge transport in organic semiconducting materials. The conductance behaviors of highly oriented pentacene nano crystals with their long molecular axes parallel to the Au surface or perpendicular to the organic self-assembled monolayer (SAM) modified Au surface were measured by conductive atomic force microscopy (CAFM). Rectifier current of conduction pathway along pentacene’s □-□ stacking direction was found four orders of magnitude larger than that perpendicular to the □-□ stacking direction. Phase images by electrostatic force microscopy show that pentacene with different orientation has different response to the applied electric field from Au tip. Nanocrystals of p-sexiphenyl or pentacene with high in-plane alignment were obtained by thermally depositing these molecules on a rubbed polymethylene film surface. The structures of the films were characterized by using grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM), reflection absorption IR spectroscopy (RAIRS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The polymethylene chains near the surface were markedly aligned after the rubbing treatment, with the carbon-carbon backbone chains aligned parallel to the rubbing direction. p-Sexiphenyl formed nanometer-scale, rod-like molecular crystals, with their long crystal axes perpendicular to the rubbing direction. Nevertheless, the molecules in the crystals lay “flat” on the rubbed polymethylene surface, with their long molecular axes parallel to the rubbing direction. Large scale alignment of highly oriented molecular crystals can be achieved in this manner. In contrast, pentacene crystals deposited on the rubbed polymethylene at room temperature show a preferred orientation in that the [110] plane aligns perpendicular to the rubbing direction, whereas the [120] plane aligns parallel to the rubbing direction when deposited at 50°C. Oriented growth of polycrystalline rubrene thin film on an oriented pentacene buffer layer was also investigated. The oriented pentacene buffer layer was created by thermal evaporation of pentacene on a rubbed polyvinyl alcohol (PVA) surface. The pentacene layer in turn induced the oriented growth of rubrene crystals upon thermal deposition. With the aligned pentacene/rubrene film as the active layer of a field-effect transistor, anisotropic mobilities were observed. The highest field-effect mobility (0.105 cm2/Vs) was observed along the direction 45□ away from the rubbing direction and was ~four times higher than that for similar device prepared on unrubbed PVA. The direction was consistent with the GIXD observation that a large number of rubrene crystallites are having their [002] axes aligned in this direction.

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