利用光子晶體與共振腔結構之有效光操控於二維奈米材料與光電元件之應用

光子晶體與共振腔結構具有特殊的光學特性能夠有效的操控光在結構內之行為，使得這些結構已經被廣泛應用於各光學領域中。利用這些結構我們將可以很容易的調控入射光與物質之間的交互作用，若能準確的控制光子晶體與共振腔的結構參數，透過這些結構的幫助將有助於提升光電元件或奈米材料之效能。本論文的主要目的在於利用光子晶體與共振腔之有效光操控特性來提升包含二維奈米材料元件與發光二極體元件的效能。自從石墨烯在2004年首度被製備出來後，二維奈米材料的相關研究已經越來越受到重視。由於石墨烯材料的結構品質是影響石墨烯元件效能的主要因子，為了提升石墨烯相關元件的效能，開發快速且準確的石墨烯結構品質檢測技術是一項非常重要的議題。在眾多的石墨烯檢測技術中，量測其拉曼散射光譜具有快速且非破壞性的檢測石墨烯品質之能力。雖然在過去的研究中石墨烯的拉曼散射訊號已可透過金屬奈米結構之表面增強拉曼散射技術來大幅增強，然而金屬對於石墨烯仍具有無法避免的負面效應，因其對拉曼散射訊號產生額外的干擾。因此在本論文的第一部分中，我們利用了薄膜堆的光學導納來準確的計算出基板的表面電場，用於分析石墨烯之干涉增強拉曼散射現象，進而開發出一可靠的石墨烯品質檢測方法。透過準確控制無金屬的一維光子晶體結構之光學性質，其表面電場將可經由顯著且易控制的干涉效應來達到最大化，進而最佳化石墨烯材料的拉曼散射訊號之增強因子。利用這個方式將能夠同時以相同比例大幅地增強石墨烯之拉曼散射訊號，使石墨烯的G峰與2D峰相較於在矽基板上皆以約180倍之比例提升。在特定的條件下，石墨烯的拉曼散射訊號甚至可被增強超過400倍。在我們轉置單層與數層石墨烯於各式基板上之後，我們發現單層與多層石墨烯的拉曼散射光譜於一維光子晶體基板上皆可被大幅增強，同時並不影響其主要拉曼散射訊號峰與峰之間的比例以及峰值之位置。因此，僅藉著控制基板的表面電場，石墨烯的拉曼散射訊號可被大幅地增強，同時並不會受到增強技術的影響而對拉曼光譜造成額外之改變。根據此結果，最佳化的一維光子晶體增強拉曼散射效應於檢測石墨烯的結構品質等特性上非常有幫助。另一方面，石墨烯材料本身在光收成上的能力非常弱，嚴重地影響石墨烯相關光電元件的效能，因此如何提升石墨烯的光吸收能力一直以來也是一個非常重要的議題。在本論文中，我們開發了一種具有寬波段、廣角度增強石墨烯光吸收行為之簡單的共振腔結構。利用計算出其底下不同基板的表面電場，我們提出了一個有系統地分析石墨烯材料光吸收的方法。雖然一維光子晶體以及金屬/介電材料共振腔結構皆具有使表面電場最大化的特性，利用金屬/介電材料共振腔這種兩層結構是一個相較於多層薄膜一維光子晶體而言更為簡單的方法，同時透過共振腔結構亦可達到寬波段、廣角度的增強石墨烯材料之光吸收。藉著選擇適當的背反射金屬材料以及僅需控制一層介電材料間隔層之厚度所構成的共振腔結構，於實驗上我們於紫外光、可見光以及紅外光波段皆可達到約5倍的石墨烯吸收增強，且在紫外光波段具有的吸收之半高波寬可達200奈米；在可見光波段具有的吸收之半高波寬可達400奈米；在近紅外光波段具有的吸收之半高波寬可達到超過1000奈米。此外，石墨烯轉置於共振腔上後具有明顯增強的光熱反應亦証實了其光吸收能力有所增強，同時我們也發現了石墨烯在共振腔結構上之光吸收增強光譜幾乎非常完美的與太陽光空氣質量1.5 (Air Mass 1.5; AM1.5) 光譜相吻合，使得共振腔結構可有效的應用於各式光電元件上以達到收成太陽光之能量的目的。更進一步的，我們利用了上述的簡單共振腔結構開發出了一種能大幅提高不同二維材料光外耦合(light outcoupling)的方法。由於金屬/介電材料共振腔結構具有寬波段表面電場增強的特性，不同二維奈米材料的拉曼散射光以及光致發光皆可被共振腔結構大幅地增強並準確的維持峰值與峰值之間的比例以及峰值的位置，同時二維材料的光學可見性亦可在寬波段下被顯著地增加。利用最佳化的金屬/介電材料共振腔，石墨烯的拉曼峰值皆可以約475倍之比例提升，同時亦可利用同一結構增強單層二硫化鎢 (WS2) 的拉曼散射訊號以及光致發光訊號超過350倍。值得注意的是，利用此結構來增強二維材料的光外耦合，其訊號強度皆能被大幅增強，同時對其拉曼或光致發光光譜形貌並不會產生額外的改變。更進一步的我們發現由非電漿共振金屬與介電材料所構成的共振腔結構亦具有寬波段電場增強的特性，使得二維材料的光外耦合也可以有超過200倍的增益。因此，使用此種簡單的共振腔結構，將使我們能更精確且可靠地利用拉曼以及光致發光光譜來檢測二維材料結構品質等特性。除了二維奈米材料相關光電元件外，發光二極體元件的效能仍然需要更進一步的提升以廣泛應用於照明技術上。在本論文的最後一個部份，我們開發了簡單且低成本的奈米壓印技術來同時增加雙色白光發光二極體的光萃取效率以及其演色性。首先利用嚴格耦合波分析法(Rigorous Coupled Waves Approach; RCWA) 來最佳化白光發光二極體中黃光以及藍光的光萃取效率，同時發現由於光萃取表面結構在不同波段的入射光情況下具有不同的光萃取效率，因而可增進雙色白光發光二極體的演色性。我們實際利用奈米壓印技術在白光發光二極體封裝材料的表面製作所設計出的最佳化結構後，可增強20%藍光455奈米波段的光強度。若更進一步的考慮二極體元件的演色性與色溫校正，則可分別增強15%的藍光光強度，以及4%的黃光之光強度，同時在雙色白光發光二極體發光光譜中490奈米附近的低谷光強度將增強25%，使得相對於藍光與黃光此於490奈米附近低谷的強度可被有效的提升，因此雙色白光發光二極體的演色性將可從69增加到73。因雙色白光發光二極體的光萃取效率以及演色性皆可很容易的被增強，我們於本論文中所開發的技術對於發光二極體元件未來應用於固態照明上亦具有相當的幫助。

關鍵字

光子晶體；共振腔結構；二維奈米材料；光吸收率；光外耦合；白光發光二極體；奈米壓印技術

並列摘要

Photonic crystals (PhCs) and cavity structures have been widely applied in many fields of optics due to their special optical properties which enable them to manipulate light in an efficient way. The light–matter interactions could be easily modulated through these structures; therefore, the performance of various optoelectronic devices or nanomaterials could be improved with the help of these structures if the structural parameters could be carefully controlled. Our goal of the research in this thesis is to enhance the performance of various two-dimensional (2D) materials–based devices and white light-emitting diodes (LEDs) by applying the efficient light manipulation of PhCs and cavity structures. Since the first discovery and preparation of graphene in 2004, researches on 2D materials have attracted a great deal of attention in past years. To increase the performance of graphene–based devices, first, we need to develop a rapid and accurate probing technique to characterize and investigate the structural quality of graphene. Among the several probing techniques, Raman spectroscopy is a powerful tool to characterize the quality of graphene in a fast and non-destructive manner. Although the Raman scattering signals of graphene could be enhanced by using the surface-enhanced Raman scattering (SERS) technique of metallic nanostructures, some negative effects from the metal on the Raman spectra of graphene were not avoidable. As a result, we developed a reliable method to analyze the interference-enhanced Raman scattering (IERS) effect on graphene by considering the surface electric field (E-field), which can be calculated precisely by measuring the optical admittance of the thin-film assembly. Through accurate tuning of the optical properties of metal-less one-dimensional photonic crystals (1D-PhCs), the strong and controllable interference effect allowed the surface E-field to be maximized and, thereby, to optimize the enhancement factors of the Raman scattering signals of graphene. Using this approach, we could enhance both the G and 2D bands of graphene largely, uniformly, and equally, by about 180 times relative to those obtained on a silicon substrate. Under certain conditions, the Raman peak of graphene could even be enhanced by over 400 times. After transferring single-layer graphene (SLG) and few-layer graphene (FLG) onto various substrates, we found that the Raman spectra of both SLG and FLG on the 1D-PhCs substrate were enhanced without changing the band-to-band ratio or the peak positions of the main Raman bands of graphene. Without inducing any additional signal disturbance, this enhancement technique allowed us to maintain the accurate and precise informational features from the Raman spectra. Thus, by controlling only the surface E-field, the Raman signals of graphene could be enhanced dramatically without any distortion on spectra. Accordingly, using 1D-PhCs and the optimized IERS effect is very helpful for fine structural characterization of graphene through conventional Raman spectroscopy. On the other hand, because of the intrinsically poor light harvest of graphene seriously restricts the performance of graphene–based optoelectronic devices, increasing light absorption in graphene is also a very important issue. In this study, we developed a simple nanocavity to increase the light absorption of graphene with ultra-broadband and omnidirectional behaviors. We performed a systematic study on light absorption in graphene by calculating the surface E-field of different underlying substrates. Although the surface E-field was maximized for both one-dimensional photonic crystals (1D-PhCs) and a metal/dielectric nanocavity structure, the much simpler two-layer nanocavity configuration also provided ultra-broadband and omnidirectional enhancement of absorption. By selecting a suitable metal as the back reflector and controlling the thickness of the single-layer dielectric spacer in the nanocavity structure, we found experimentally that the light absorption of graphene could be enhanced approximately fivefold, with full widths at half maximum (FWHM) of 200 nm in the ultraviolet, 400 nm in the visible, and greater than 1000 nm in the near infrared regime. Moreover, we observed a significantly enhanced photo–heat response from the greater light absorption of graphene in the nanocavity structure. The absorbance spectrum of graphene in the nanocavity structure perfectly matched the air mass 1.5 (AM 1.5) solar spectrum, suggesting that such systems might be very useful for harvesting solar energy in optoelectronic devices. Moreover, we developed a method, using a simple two-layer nanocavity structure, to significantly enhance light outcoupling from two-dimensional (2D) materials. Because the surface electric fields (E-fields) of the nanocavities were enhanced greatly over ultra-broadband regimes, the excitation of various 2D materials with laser light and their Raman and photoluminescence (PL) light emissions were all enhanced dramatically while maintaining band-to-band ratios and peak positions precisely. At the same time, the optical visibility of the 2D materials was also enhanced significantly over a broad spectral regime. Using a single type of Ag/SiO2 nanocavity structure, we obtained a 475-fold, equal enhancement in the intensities of the main Raman peaks of single-layer graphene (SLG) and more than a 350-fold increase in the intensities of both the Raman and PL signals of single-layer tungsten disulfide (WS2). Notably, the light outcouplings of these 2D materials were enhanced dramatically without any spectral distortion generated by the nanocavity. Moreover, a nanocavity structure prepared from a non-plasmonic metal reflector also enhanced the light outcoupling from 2D materials by over 200-fold. Combined with Raman and PL spectroscopy, such simple nanocavity structures appear to have great applicability for precise and reliable investigations, providing abundant structural information, of a variety of 2D materials. Despite of the 2D materials–based devices, the performance of white light-emitting diodes (LEDs) still needs to be further improved for general lighting applications. We employed a simple, inexpensive nanoimprinting process to increase both the light extraction efficiency and color rendering of dichromatic white LEDs. We employed the rigorous coupled waves approach (RCWA) to optimize the light extraction efficiency of yellow and blue light. We found that the presence of the light extracting structures could also improve the color rendering of the dichromatic white LEDs, due to the different light extraction efficiencies of the textured structures at different wavelengths. After fabricating inverted pyramid structures on the surface of the encapsulation layer, the intensity of the blue light at 455 nm increased by 20%. When we further considered the color rendering and correlated color temperature (CCT), the enhancement of blue light was 15% and that of yellow light was 4%. Meanwhile, the light extraction of the intensity dip near 490 nm was enhanced significantly (by 25%), resulting in an increased dip-intensity of light at 490 nm relative to the intensities of the blue and yellow light. Accordingly, the color rendering index (CRI) of this dichromatic white LED increased from 69 to 73. Because it improved both the light extraction efficiency and color rendering of dichromatic white LEDs, this simple method should be very helpful for enhancing their applications in solid state illumination.

並列關鍵字

photonic crystals ； cavity structures ； two-dimensional nanomaterials ； light absorption ； light outcoupling ； white light-emitting diodes ； nanoimprint lithography