氧化鈦添加對氧化鎢奈米結構及其光電特性之影響

Effect of Titanium Oxide Addition on the Nanostructure and Electro-Optical Properties of Tungsten Oxide Nanomaterials

林烜慶(Hsuan-Ching Lin)

指導教授 ： 蘇程裕

國立臺北科技大學/機電學院/機電科技研究所/博士(2009年)

https://doi.org/10.6841/NTUT.2009.00283

摘要

本研究系利用電漿電弧氣凝合成法製備非化學計量(non-stoichiometric)氧化鎢奈米材料，並探討氧化鈦摻雜對氧化鎢奈米材料之形貌、結構及其光電特性之影響做一研究。研究過程利用FESEM、HAADF、 XRD、HRTEM、EDS、XPS 以及Raman光譜儀等技術分析其形貌、相態、結構以及化學成分分析。光電特性方面，針對場發射與螢光光譜儀探討其場發射及光致發光特性。最後本製程合成之奈米材料提出氣-固 (vapor-solid) 成長機制。
根據實驗結果，在本文中透過參數及靶材的設計，可以製備出W18O49/TiO2之核殼結構奈米顆粒、W18O49奈米棒所建構之奈米束以及氧化鈦添加之W18O49奈米棒。研究結果顯示所製備出的W18O49/TiO2核殼奈米顆粒其直徑大小為43.5 ± 8.0 nm。由拉曼光譜儀中可發現到相較於氧化鈦以及氧化鎢之特性峰，W18O49/TiO2核殼奈米顆粒的特性峰位置皆有偏移；除此之外在光致發光光譜儀中在483 nm處可發現明顯峰值。推測造成此結果的可能性是因為其特殊的殼核結構導致奈米顆粒內部晶格的扭曲以及由氧缺陷亦或是殼核材料介面的缺陷所導致。在熱學性質方面透過熱重分析儀可以發現，W18O49/TiO2核殼奈米顆粒的確具有提升氧化鎢的高溫熱穩定性以及防止氧化鎢在高溫下進一步的氧化。
氧化鎢奈米棒研究方面，透過實驗參數的調整並在一缺氧的環境下利用電漿電弧直接蒸發鎢靶材直接製作直徑約在25∼200 nm 的奈米棒所構成之束狀結構。透過XRD及HRTEM分析可知奈米棒的相態為W18O49，且奈米棒是沿著[010]方向成長之單斜晶結構。而W18O49奈米棒當中大量的的氧缺陷造成其在藍光波段 (約在420 nm) 具有一明顯峰值。另外一維奈米結構具有良好的場發射特性，場發射量測結果可知其起始電壓以及臨界電壓各為3.5 and 4.6 V/μm。透過Fowler-Nordheim (F-N) 公式換算其在高低不同電場作用其電場增強因子各為2269 以及2131。
為了提升W18O49 奈米棒的特性以及應用層面，本研究也探討微量氧化鈦對於W18O49 奈米棒結構及其特性之影響。結果顯示：本研究製備的奈米棒其直徑約在20 ~100 nm之間。相較於W18O49奈米棒，微量氧化鈦的添加會使得W18O49奈米棒的拉曼峰值有紅移現象，並且在螢光光譜儀當中可以發現在接近綠光波段 (~497 nm) 有一明顯峰值。而在場發射性質方面，微量氧化鈦的添加可以使得W18O49 奈米棒的起始電壓以及臨界電壓降低至2.2 V/μm 以及3.4 V/μm，而且電場增強因子提升至4578，明顯地提升W18O49 奈米棒的場發射特性。相較於W18O49 奈米棒，添加微量氧化鈦可以提升其場發射特性可以歸因為下列幾項因素 (1) 具有較小的直徑 (2) 導電性以及電子遷移率的增加 (3) 由氧缺陷以及鈦含量所造成的缺陷導致較多電子的產生 (4) 材料功函數的降低。

關鍵字

核殼奈米顆粒 ； 奈米棒 ； 氣凝合成法 ； 氧化鎢 ； 氧化鈦 ； 場發射性質 ； 光致發光

並列摘要

In this dissertation, a modified plasma arc gas condensation has been successfully developed to fabricate non-stoichiometric tungsten oxide materials and investigate the effect of titanium oxide addition on the morphologies, structures and electro-optical properties of tungsten oxide nanomaterials. All the samples were characterized by field emission scanning electron microscope (FESEM), high-angle annular dark field (HAADF), high-resolution transmission electron microscope (HRTEM), X-ray diffractormeter (XRD), X-ray photoelectron spectrometer (XPS) and Raman spectra for the morphological and structural investigation. Practical field emission (FE) property and photoluminescence (PL) measurement will reveal the electro-optical properties of the as-prepared samples. The growth mechanism for the as-prepared materials has been proposed for vapor-solid (VS) process.
Three as-prepared products, namely, W18O49/TiO2 core-shell nanoparticles, W18O49 nanorod bundles and Ti-modified W18O49 nanorods, can be obtained by the experiment and target design. The results show that the diameter of the W18O49/TiO2 core-shell nanoparticles is estimated to be as 43.5 ± 8.0 nm. In Raman spectrum, the characteristic peaks of the core-shell nanoparticles are also shifted; in addition, green emission peak at 483 nm is also observed in the PL spectrum. The possible explanation for these unique phenomena can be attributed to the lattice distortion induced by the defects from the oxygen vacancies or the interface between the core and shell. The stability at high temperature of W18O49 nanoparticles can be also enhanced by TiO2 shell and prevent from the further oxidization.
Unique bundle-like structure with crystalline phase can be prepared by directly evaporating a tungsten bulk in an oxygen-deficient environment and a diameter of 25 nm~200 nm via FESEM and TEM observations. Meanwhile, XRD and HRTEM results confirm that the nanorod bundles are in a single crystalline monoclinic W18O49 phase with growth direction along [010] direction. Also, oxygen deficiencies within the nanostructures induce the band-to-band transition emission and blue emission at 350 nm and 420 nm observed in the PL spectrum. The FE measurement shows that the tungsten oxide nanorod bundles exhibits low turn-on and threshold voltages, which are about 3.5 and 4.6 V/μm, respectively. The corresponding field enhancement factor β values at high and low field regions are estimated as 2269 and 2131, which are high enough for various FE applications.
To enhance the applications of W18O49 nanorods, the effects of titanium oxide addition on the structures and properties W18O49 nanorods have been investigated. The results show that the average diameter of the as-prepared Ti-modified W18O49 nanorods is ranged from 20 nm to 100 nm. In Raman spectrum, the peaks for Ti-modified W18O49 nanorods are shifted compared those of pure W18O49 nanorods. Meanwhile, Ti modification results in the green emission peak at 497 nm observed in PL spectrum. Better FE performance of W18O49 nanorods can be obtained by introducing titanium element. The turn-on and threshold voltages of Ti-modified W18O49 nanorods can be as low as 2.2 V/μm and 3.4 V/μm, respectively. The corresponding field enhancement factor β value is estimated as 4578. The reasons for the FE enhancement can be attributed to Ti-modified W18O49 nanorods can be attributed to (1) smaller average diameter (2) conductivity and electron mobility enhancement (3) more electron carriers generated by oxygen defects for Ti modification, and (4) reduced work function.

並列關鍵字

core-shell nanoparticles ； nanorods ； gas condensation ； tungsten oxide ； titanium oxide ； field emission property ； photoluminescence

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