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  以有機半導體為代表的有機多功能材料的物理/化學結構和形貌本質上決定了其電學和光學性能。液晶態（liquid crystal, LC）分子自組裝（self-assembly） 和自取向 （self-alignment）機理對實現有機半導體物理形貌的有效調控提供了新的機遇；進而期待可以被利用去有效地控制其光電性能的異向性（例如電致發光，熒光反應，電子和空穴傳導）和產生全新的制造先進功能光電結構的加工技術。然而，不同于常規使用的低分子模量液晶材料，液晶有機共軛高分子 (conjugated polymers)半導體的相變溫度會非常高（通常高于200攝氏度）。在這種高溫下，工業界和實驗室常用的Rubbed Polyimide取向薄膜會出現結構不穩定，進而失效。另外，現階段可以有效地對共軛高分子半導體材料做高精度取向排列控制的技術也缺失，這對于有機發光二極管 (OLEDs)，激光 (Organic Semiconductor Lasers)，太陽能電板 (Organic Solar Cells)和場效應管 (Field-Effect Transistors)的制造和性能提高是非常不利的。

  為了解決上述難題，本文首次采用雙光子激光直寫技術，并結合具有高溫穩定性的光控取向材料（SD1）和偏正紫外光照射，成功實現了對于光控取向納米薄膜取向排列的亞微米精度控制。然后通過在激光直寫完畢后的光控取向薄膜上旋涂熒光液晶有機共軛聚合物薄膜，跟進在惰性氣氛里進行優化后的熱處理工藝（具體加工工藝參考圖一），實現了空間取向排列圖形從光控取向薄膜到熒光液晶有機共軛聚合物薄膜的有效轉移。通過系統的優化激光能量密度和直寫速度，本文報道了在熒光液晶有機共軛F8BT薄膜里≈750 nm的二維控制精度。基于Micro-PL spectra和polarised confocal fluorescence imaging結果，在光致聚合鏈取向的F8BT薄膜里觀察到了全新的，可以利用不同偏正方向分離的兩個不同的發光激發態(emissive intra/interchain species),并歸因與紫外光激發和雙光子吸收產生的迥異的分子取向結構。

圖一. (1) SD1 photoalignment layer is spin‐coated and (2) aligned initially through uniform exposure to linearly polarized UV illumination. (3) Regions of the photoalignment layer are then realigned using two‐photon laser writing. The SD1 molecules orient perpendicular to the polarization of the incident light; here the reorientation is shown to be orthogonal to that of the UV‐aligned background but it is possible to tune the relative angle by adjusting the laser beam polarization direction. An F8BT film is then spin‐coated onto the SD1 layer (4) and heated to isotropic melt before slow cooling into the nematic LC phase, where the polymer chains become oriented along a direction defined by the SD1 molecular orientation. The polymer chain and associated optical transition dipole moment orientations are then frozen‐in by rapid quenching to room temperature (5) to form a nematic glass film.

圖二.(a - d) Polarized optical micrographs and (e – h) polarized fluorescence intensity images demonstrating laser patterned orientation in quenched nematic glass F8BT films. The micrographs are recorded with the samples placed between the crossed polarizer and analyzer pairs; double‐headed white arrows in (a) and (c) delineate the polarization directions, labeled, respectively, P and A. The square patterns ((a) and (b)) were two‐photon laser written with a bi‐directional raster on a non‐aligned SD1 layer using different optical polarizations and the F8BT films were then spin‐coated on top before thermotropic alignment and quenching. The resulting F8BT chain axis orientation directions are indicated by double‐headed yellow, red, and pink arrows for the three separate squares. The crest pattern ((c) and (d)) based on the St. Cross College Oxford crest (see inset) was laser rewritten on a UV‐aligned SD1 film with a polarization direction at 90° relative to the UV‐oriented background. The double headed blue and dashed red arrows show, respectively, the orientation of the polymer chains in the crest and the background. A dark state is observed when the orientation of the polymer chains is parallel to either the polarizer or analyzer whereas a bright state is observed when it is at 45° to the axes of the polarizer/analyzer. For the crest, the visibility of the pattern only arises because of a transition region between the two orthogonal orientations which shows up bright for (c) and dark for (d), consistent with a local orientation that is intermediate between the two main orientations. Polarized PL intensity images are shown for the squares in (e) and (f) and the crest pattern in (g) and (h). The doubled headed green arrows show the orientation of the analyzer used to select the polarization of the collected emission light. There is a clear correlation between the polarized PL intensity and the designed polymer chain alignment. The scale bars in all images are 20 μm.

圖三.Spectrally integrated Micro‐PL intensity maps and selected spectra for different locations on F8BT films with laser (re)written SD1 alignment patterns. Intensity maps, which have been normalized to the maximum emission intensity in each case, are shown where the background was (a) aligned with UV illumination and (b) nonaligned. For (a), the excitation polarization and the PL collection polarization were aligned along the x‐axis, which is orthogonal to the orientation of the F8BT polymer chains in the laser written crest and parallel to the UV‐aligned background. For (b), the excitation was along the y‐axis (chain alignment direction in the crest) and the PL collection polarization was along the x‐axis. PL spectra are shown for: (c) location #1 in (a); (d) location #2 in (a); and (e) location #3 in (b) along with the spectrum from a nonaligned F8BT region highlighted by the white dot (bottom corner in the 2‐D intensity map of (b)). For each location, the excitation polarization was rotated to lie along the local chain orientation direction and PL spectra were collected with the analyzer both parallel and perpendicular to this direction. For the nonaligned spectrum, the excitation was along the y‐axis and the analyzer polarization was along the x‐axis.

圖四.F8BT laser‐pattern resolution optimization. (a) Crossed‐polarizer optical microscope images for F8BT oriented on groups of four laser rewritten SD1 lines at 5 μm separation intervals on a UV‐aligned uniform SD1 background. Double‐headed white arrows represent the polarization directions for the incident light polarizer, P, and the transmitted light analyzer, A. The top two panes (each with dark and bright states for the background) were rewritten using the 0.5 NA and the bottom two the 0.95 NA objective lens. The writing (substrate displacement) speed used for each set of lines is shown as a graded scale at the bottom of the sub‐figure. Results are presented for polarizations of the rewriting laser that are either at 90° (solid double‐headed blue arrow; 1st and 3rd panes) or 45° (dashed double‐headed blue arrow; 2nd and 4th panes) relative to the uniform UV‐aligned background (dashed double‐headed red arrows). These images were used to extract the FWHM linewidth of the patterned F8BT orientation stripes. (b) The FWHM is plotted as a function of laser writing speed and (c) laser power, parametric in each case in the objective lens numerical aperture (NA = 0.5, red squares; 0.95, blue circles) and laser polarization relative to the background uniform UV‐alignment polarization (90° writing, filled symbols; 45° writing, open symbols). The horizontal dash‐dotted line in (b) delineates a 1 μm FWHM.

  相關研究成果以“Two-photon Laser-written Photoalignment Layers for Patterning Liquid Crystalline Conjugated Polymer Orientation”為題發表在材料學頂刊Advanced Functional Materials上。牛津大學史玉平博士為論文第一作者兼通訊作者，Patrick Salter研究員，Steve Morris和Donal Bradley (FRS)教授為本文的共同通訊作者。

  論文信息：Yuping Shi*, Patrick S. Salter*, Mo Li, Robert A. Taylor, Steve J. Elston, Stephen M. Morris*, Donal D.C. Bradley*, Two-photon Laser-written Photoalignment Layers for Patterning Liquid Crystalline Conjugated Polymer Orientation, Advanced Functional Materials 2020, DOI: 10.1002/adfm.202007493.

  原文鏈接：https://onlinelibrary.wiley.com/doi/10.1002/adfm.202007493