【研究背景】可視蛍光を示す量子ドット(QD)蛍光体の膜は、光電子デバイスやセキュリティインクなどへ応用できる。高品質なQDは疎水性環境下で合成されるが、製膜プロセスは安全性やメンテナンスの観点から水系が望ましい。筆者の先行研究では、疎水性のCuInS2 (CIS)系 QD蛍光体の表面吸着分子を3-メルカプトプロピオン酸(MPA)に交換して親水化を行った。これを用いて水系の電気泳動堆積(EPD)法によりこのQDを導電性基板上に堆積できたが、クラックのため膜が不透明であった。本研究では透明な蛍光膜を得るため製膜条件の検討を行った。
【実験方法】ヨウ化銅と酢酸インジウムを1-ドデカンチオール(DDT)に投入し、230 ℃に加熱してCISコアを生成した。酢酸亜鉛溶液とステアリン酸亜鉛溶液を加えZnSシェルを成長させた。QD表面のDDTとリガンド交換するために、この分散液にMPAを加えた。遠心分離と真空乾燥で粉末試料を得た。MPA修飾CIS/ZnS QDsをケイ酸テトラメチルアンモニウム水溶液に分散させてから、シリコーン変性アクリル樹脂の水系エマルションを加えた。ITO被覆ガラス基板とステンレス板をそれぞれアノードとカソードに用いた。電極間の距離を10 mmに維持し、分散液へ垂直に浸漬した。4.0 Vの定電圧を1−4 min印加して、ITO被覆ガラス基板にナノコンポジット膜を堆積し、120 ℃で乾燥した。
【結果および考察】QDのリガンド交換は赤外吸収分光法により確認された。使用したQDと樹脂ナノ粒子の平均粒子径は、それぞれ3 nmおよび55 nmであった。EPD用の分散液の濃度を変化させたところ、従来よりも高濃度とすることでクラックフリーかつ透明な膜が堆積した。また、302 nmの紫外光を照射すると、QDの黄色蛍光を示した。光学顕微鏡では水の電気分解に伴い生成する気泡は膜中に観察されなかった。走査型電子顕微鏡による断面像では、基板上で密に膜が堆積していた。堆積時間を1 minから4 minへ変化させると、膜厚が14μmから64μmに増大した。ここで、堆積時間と膜厚には比例関係が見られた。透過スペクトルでは、膜厚の増大に伴い約450 nmより短い波長の透過率が減少した。これはQDによる光吸収が増大したためである。また、蛍光強度も増大した。以上より、EPDによる透明蛍光膜が得られ、堆積時間で膜厚や蛍光強度が制御できた。
<Introduction> Films of quantum dots (QDs) which show visible photoluminescence can be used in applications including photoelectronic devices and security inc. High-performance QDs are frequently synthesized in hydrophobic conditions, whereas aqueous process is desired for the film fabrication because of safety and maintenance. In the author's previous work, hydrophobic CuInS2 (CIS) QDs were hydrophilized by exchanging the surface ligands to 3-mercaptopropionic acid (MPA). Films of the QDs were deposited on a transparent conductive substrate by the aqueous electrophoretic deposition (EPD). However, they were not transparent due to cracks. In this work, experimental conditions were investigated to obtain transparent fluorescent films.
<Experimental> Copper iodide and indium acetate were mixed with 1-dodecanthiol (DDT), and then the mixture was heated to 230 ˚C for CIS core nucleation. Zinc acetate solution and Zinc stearate solutions were injected to grow ZnS shell. MPA was injected to the CIS/ZnS QDs dispersion for ligand exchange from DDT on QD surfaces. Powdered sample was obtained by centrifugation and vacuum drying. MPA-modified CIS/ZnS QDs were dispersed in a tetramethylammonium silicate aqueous solution, and then aqueous emulsion of silicone-modified acrylic resin was added. An ITO-coated glass substrate and a stainless-steel plate were used as the anode and cathode, respectively. The distance between both electrodes was maintained at 10 mm, and they were dipped vertically into the dispersion. A nanocomposite film was deposited on the ITO-coated glass substrate during the application of a constant voltage of 4.0 V for 1−4 min, followed by drying at 120 ˚C.
<Results and discussion> Ligand exchange of QDs was confirmed by infrared absorption spectroscopy. The average particle sizes of the QDs and resin nanoparticles used were 3 nm and 55 nm, respectively. When the concentration of the dispersion for EPDs was varied, crack-free and transparent films were deposited at a higher concentration than before. The QDs also exhibited yellow emission under 302 nm UV irradiation. No bubbles generated by the electrolysis of water were observed on the film by an optical microscope. Cross-sectional image by scanning electron microscopy showed dense film deposition on the substrate. When the deposition duration was changed from 1 min to 4 min, the film thickness increased from 14 µm to 64 µm. Herein, a proportional relationship between the duration and thickness was observed. In the transmission spectrum, the transmittance at wavelengths shorter than ~450 nm decreased with increasing the thickness. This was due to increased optical absorption by the QDs. The photoluminescence intensity also increased. Therefore, transparent fluorescent films were successfully obtained by the EPD, and the film thickness and photoluminescence intensity were controlled by the deposition duration.
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