摻雜氧化石墨烯奈米微粒之高分子分散型液晶晶胞 的製備及特性研究

Abstract

高分子分散型液晶(PDLC)晶胞為一由微米大小的液晶滴與聚合物基材組成的複合型晶胞，可經由外加電場來進行液晶分子的調控，進而產生電光調制的效應，此特性可應用在與光折變晶體組成全光調控的高解析度空間光學調制器。為進一步完善此器件的功能，在本論文中，我們研究氧化石墨烯奈米粒子摻雜對PDLC晶胞電光調制特性的影響。實驗時，我們選擇三種氧化石墨烯奈米粒子，分別是：多層(記作:ML-GO)、功能基化的單層(SL-GO)、以及還原氧化(r-GO)等型態進行研究。首先，我們提出濕式球磨技術來打破粒子團聚現象，並進一步縮小粒子的大小至兩百奈米左右，使其能更均勻的摻雜到PDLC晶胞中，然後，經由拉曼光譜量測驗證這個概念。完成樣品製成後，我們量測各型態的PDLC晶胞樣品的電壓-穿透率曲線，藉以研究樣品晶胞的電光調制性質。我們發現三種奈米粒子都能有效地降低PDLC晶胞的臨界電壓與驅動電壓，且對同一系列樣品，這兩個電壓也隨摻雜的濃度增加而減小。其中，摻雜r-GO奈米微粒的PDLC樣品晶胞，電光特性的改進最為明顯，其臨界電壓與未摻雜PDLC晶胞樣品相比，約降低了三倍。為了解各型態晶胞效能改進的機制，我們以穿透式電子顯微鏡量測PDLC樣品晶胞型態，發現液晶微滴的大小隨著摻雜濃度增加，這與PDLC晶胞的理論中樣品的驅動電壓與液晶微滴大小成反比相符，另外，摻雜r-GO奈米微粒的最佳改進效果可能其可增加聚合物基材的導電率有關。未來研究若能研究摻雜奈米粒子到PDLC中對其聚合物的導電性與液晶介電率的影響程度，當可更清楚了解晶胞改進的機制。 總結來說，我們發現氧化石墨烯與還原氧化石墨烯的奈米粒子對降低PDLC的驅動電壓的成果，將可給我們更多PDLC研究與應用的可能。

Polymer-dispersed liquid crystals (PDLCs) are composite cells that consist of submicron-size droplets of liquid crystal randomly dispersed in a polymer matrix. They can modulate through application of external electric field. This e/o modulation property can apply with photorefractive crystals in all optically controlled high-resolution spatial light modulator. In order to improve this hybrid structure’s features, in this thesis, we study the effect of e/o modulation that Graphene Oxide nanoparticles doped into PDLC. In our experiment, we choose three types of Graphene Oxide nanoparticles which are multilayer (named ML-GO) and single layer (named SL-GO, functionalized, thickness almost 1 atomic layer) and Reduced Graphene Oxide respectively to study. First, we used wet ball milling method to avoid aggregation and reduce the nanoparticles size close to two hundred nanometers to achieve uniformly dispersion when they doped into PDLC cells. We use Raman analysis to ensure this method. After finished samples, we measure these samples voltage-transmittance curve to study their e/o modulation. We found that both GO and r-GO lower the threshold voltage and driving voltage in comparison with non-doped PDLC and these two voltages lower in the increasing of concentration in same series samples. The r-GO shows stronger effect by reducing the threshold voltage 3 times in comparison with non-doped PDLC. To understand the characteristics improvement mechanism, we measuring the PDLC morphology using Scanning Electron Microscopy (SEM). We found the size of LC droplets is inversely proportional to the concentration and threshold voltage, which is the same relation to PDLC theory. Furthermore, doping r-GO nanoparticles shows the best improvement may come from that it enhances conductivity of the polymer matrix. Further study of effect of nanoparticles addition on polymer conductivity and dielectric permittivity change of LC will give complete understanding of the physical phenomena inside the PCLD compound. As a conclusion, we found that both Graphene Oxide and Reduced Graphene Oxide nanoparticles reduce the driving voltage of PDLC and give perspectives for further study and practical applications.