行政院國家科學委員會補助專題研究計畫 ▓成果報告 □ 期 中 進 度 報 告
過渡性金屬氧化物及硫屬化合物之奈米結構與單晶製備及其 特性研究
計畫類別：
▓
個別型計畫 □整合型計畫 計畫編號：NSC 97-2112-M-011-001-MY3 執行期間：97 年 8 月 1 日至 100 年 10 月 31 日 執行機構及系所：國立台灣科技大學電子工程所計畫主持人：黃鶯聲 計畫參與人員： 陳麒安
國立台灣科技大學電子工程所博士後研究員 陳宜民、王逸平、詹景翔
國立台灣科技大學電子工程所博士班研究生 潘家頎
國立台灣科技大學應用科技研究所博士班研究生 魏宇宣、楊子弦、陳品宏、劉湘君
國立台灣科技大學電子工程所碩士班研究生
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□赴國外出差或研習心得報告
□赴大陸地區出差或研習心得報告
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一、 中文摘要
本 計 畫 目 地 從 事 （ 一 ） 黃 鐵 礦 結 構 （ Pyrite Structure ） 、 （ 二 ） 層 狀 結 構
（ Layered Structure ） 之 過 渡 性 金 屬 硫 屬 化 合 物 （ Transition Metal Dichalcogenides ） 、 （ 三 ） 奈 米 結 構 之 過 渡 性 金 屬 硫 屬 化 合 物 之 單 晶 及 奈 米 結 構 、
（四）Cu2ZnC(IV)X4[C(IV)=Si,Ge,Sn; X=S,Se] 及（五）過渡性金屬氧化物奈米晶體
製備，物性研究並探討其可能之應用。將全力利用各種長晶技術（溶劑法、化學汽化 傳導法、物理汽化傳導法、有機金屬化學汽相沈積法、磁反應式濺鍍法）成長質優且 大 的 單 晶 ， 同 時 將 嘗 試 各 種 不 同 方 法 成 長 奈 米 結 構 之 過 渡 性 金 屬 氧 化 物 及 硫 屬 化 合 物。利用 X-光繞射技術量測各種單晶與奈米結構之晶格常數並進行電學特性量測，同 時 運 用 各 種 調 制 反 射 光 譜 技 術 研 究 其 能 帶 結 構 ， 使 用 拉 曼 散 射 （ Raman Scattering ） 研 究 與 其 晶 格 振 動 相 關 的 現 象 、 光 激 發 光 譜 (Photoluminescence) 、 時間解析光激螢光 光譜系統(Time-resolved photoluminescence) 以 及 利 用 同 步 輻 射 光 源 進 行 ， X 射 線 光 電子光譜(X-ray Photoelectron Spectroscopy，XPS)，歐傑電子光譜(Auger Electron Spectroscopy， AES) 量測，研究其光學相關特性及探討其基本的電子組態結構。並利 用 AFMM、FESEM 及 HRTEM 觀察過渡性金屬氧化物及硫屬化合物奈米晶體並探討 其可能應用。
關鍵詞：黃鐵礦結構、層狀結構、硫屬化合物、Cu2ZnIVX4單晶、金紅石結構、奈米晶
體、無機奈米管、無機類富勒稀。
二、 英文摘要
Growth and characterization of (1) Quaternary Cu2-II-IV-VI4 chalcogenides, (2) nanocrystals and single crystals of transition metal–dioxides and dichalcogenides with pyrite, layered or rutile structures by using flux, chemical vapor transport, physical vapor transport methods, metal-organic chemical vapor deposition, and radio frequency reactive magnetron sputtering. A detailed characterization program include X-ray diffraction, Energy-dispersive X-ray spectroscopy, resistivity and Hall measurements, C-V, I-V, modulation spectroscopy, photoconductivity, SEM, TEM, atomic force microscopy, Raman scattering, photoluminescence, time-resolved photoluminescence, X-ray photoelectron spectroscopy, and Auger electron spectroscopy will be carried out. In addition, the possible application studies of these materials will also be carried out.
三、 計畫緣由與目的
過渡金屬硫屬化合物（Dichalcogenides）， 屬層狀結構者：如二硫化鉬（MoS2）、二硒
化鉬（MoSe2）、 二硫化鎢（WS2）、 二硒化鎢（WS2）、 二硫化錸（ReS2）、 二硒化錸
（ReSe2）等。屬黃鐵礦結構者：如二硫化釕（RuS2）、 二硒化釕（RuSe2）、 二碲化釕
（RuTe2）、 二硫化鐵（FeS2）等。 此兩類結構截然不同的化合物，皆具有半導體之特性。
過渡金屬氧化物( Dioxides )，屬金紅石結構（Rutile structure）者，如：二氧化（RuO2），二氧
化銥（IrO2）與二氧化鋨（OsO2），雖為氧化物，但具有金屬般的導電性。近幾年來，在光電
化學及能源工業應用方面，頗受重視。其中二硫化鉬的用途甚廣，因其本身質軟，潤滑性佳，
對高溫高壓的穩定性好，是其他潤滑劑所無法比擬的。而用於碳氫化合物中，則是加氫、去氫 的良好催化劑。近年來更被用為作石油中加氫去硫（Hydrodesulfurization）的主要觸媒。而在 照光後產生d-d 能帶間的轉換，不須破壞化學鍵，即不產生光腐（Photocorrosion）的現象，可
作為光電化學太陽電池的理想電極。近年來,二氧化鈦（TiO2）也有廣大研究團對投入研究，
二氧化鈦具金紅石結構（Rutile structure; 簡稱R-TiO2）與銳鈦礦結構（Anatase structure; 簡稱A-
TiO2）。此兩不同結構具有不同的物理與化學特性。近幾年來，在光電、化學及綠色能源工業
應用頗受歡迎。其中當為A-TiO2，因其化學較強特性，常被用來作為光觸媒應用。R-TiO2，因
為有高的介電常數與反射係數，常被用來當作光學元件或是光電轉換應用。近年來奈米結構之 二氧化鈦，因其獨特構造與及光電、電化學特性使其成為熱門研究主題之一此外層狀結構，層 與層之間屬於微弱的凡得瓦爾力（Van der Waal's force），可在其中插入有機分子，或鹼金屬 原子類的物質，而改變其電學及光學的特性。除了上述的應用外，由於近似平面（二維）的結 構，容易出現穩定而明顯的激子（excitons），因此可作為研究激子特性的良好材料。另外，
釕金屬的化性穩定，具有催化作用，是公認良好的電化學材料。其硫屬化合物，近年來在能源 應用上也備受重視。硫屬化合物是自然界存量相當豐富的礦物，但是對其特性了解卻是有限 的，主要原因是難以長出較大之單晶來作量測，以及雜質濃度較難控制，以致不同實驗室的量 測結果，有不少差異。所以，有待成長出成份、雜質濃度可控制的單晶來作研究。近年來過渡 性金屬之硫屬奈米結構包括類富稀勒結構及奈米管，因其獨特構造及其可能利用為熱門研究主 題之一，詳細參考文獻如附錄二。擬在本計劃中著手製備奈米微細結構之過鍍性金屬氧化物及 層狀過渡性金屬硫屬化合物（包括Micro- and Nano- tubes）。並探討其結構光學，電傳輸及機 械等特性和其可能之利用（如潤滑劑，能源，場發射等）。
四、 研究方法
1. 利用有機金屬化學汽相沈積法成長 R-TiO2,A-TiO2 奈米晶體。
2. 利用有機金屬化學汽相沈積法成長 IrO2,RuO2 奈米晶體。
3. 利用化學汽相傳輸法成長 ReS2, ReSe2, MoS2, MoSe2, WS2, WSe2層狀結構單晶。
4. 利用壓電調制反射式光譜探討層狀單晶之半導體特性。
5. 利用 X 光繞射與拉曼散射實驗檢測 R-TiO2,A-TiO2, RuO2, IrO2 奈米晶體之結構特性及 結晶相變
6. 利用掃瞄式電子顯微鏡檢視 R-TiO2,A-TiO2, RuO2, IrO2 奈米晶體之結晶形貌與大小厚 度等相關參數。
7. 利用 X 光光電子發射譜檢測 R-TiO2,A-TiO2, RuO2, IrO2奈米晶體之成分與固態鍵結狀 態。
8. 利用穿透式電子顯微鏡檢視R-TiO2,A-TiO2, RuO2, IrO2奈米晶體之微結構。
五、 參考文獻
1. D. B. Rogers, R. D. Shannon, A. W. Sleight and J. L. Gillson, Inorg. Chem. 8 (1969) 841.
2. D. D. Sarma and C. N. R. Rao, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 25.
3. B. R. Chalamala, Y. Wei, R. H. Reuss, S. Aggarwal, B. E. Gnade, R. Ramesh, J. M. Bernhard, E.
D. Sosa, D. E. Golden, Appl. Phys. Lett. 74 (1999) 1394.
4. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, Y. Yang, Science 292 (2001) 1897.
六、 研究計畫相關成果：
1. Chen, Y. M., J. H. Cai, Y. S. Huang, K. Y. Lee, D. S. Tsai, and K. K. Tiong, “Deposition and characterization of IrOx nanofoils on carbon nanotube templates by reactive magnetron sputtering” Thin Solid Films, accepted for publication (2011).
http://dx.doi.org/10.1016/j.tsf.2011.09.074
2. Levcenco, S., D. Dumcenco, Y. S. Huang, K. K. Tiong, and C. H. Du, “Anisotropic optical properties study of wurtzite-stannite Cu2ZnGeS4 single crystals”, Opt. Mater., Vol. 34, no. 1, pp.183~188(2011).
3. Chen, Y. M., J. H. Cai, Y. S. Huang, K. Y. Lee, D. S. Tsai, and K. K. Tiong, “A nanostructured electrode of IrOx foil on the carbon nanotubes for supercapacitors”, Nanotechnology, Vol. 22, no.
35, 355708 (7pp) (2011).
4. Chen, Y. M., Y. S. Huang, K. Y. Lee, D. S. Tsai, K. K. Tiong, “Characterization of IrO2/CNT nanocomposites”, J. Mater. Sci. - Mater. Electron., Vol.22, no.7, pp.890~894 (2011).
5. Levcenco, S., D. Dumcenco, Y. S. Huang, E. Arushanov, V. Tezlevan, K. K. Tiong, C. H. Du,
“Polarization-dependent electrolyte electroreflectance study of Cu2ZnSiS4 and Cu2ZnSiSe4 single crystals”, J. Alloys Compd., Vol. 509, no. 25, pp. 7105-7108 (2011).
6. D. O. Dumcenco, Y. S. Huang, Y. P. Wang, C. H. Ho, and K. K. Tiong, “Piezoreflectance and Raman characterization of the Mo1−xWxS2 mixed layered crystals”, Solid State Phenomena, Vol.
170, pp. 55-59 (2011).
7. Y. M. Chen, Y. S. Huang, K. Y. Lee, and K. K. Tiong, “Deposition and characterization of IrO2
nanocrystals on vertically aligned carbon nanotubes by MOCVD”, Solid State Phenomena, Vol.
170, pp. 70-73 (2011).
8. H. P. Hsu, Y. S. Huang, Y. M. Chen, C. N. Yeh, D. S. Tsai, and K. K. Tiong, “Growth and characterization of well-aligned RuO2/R-TiO2 heteronanostructures on sapphire (100) substrates by reactive magnetron sputtering”, Solid State Phenomena, Vol. 170, pp.78-82 (2011).
9. Levcenco, S., D. Dumcenco, Y. S. Huang, E. Arushanov, V. Tezlevan, K. K. Tiong, C. H. Du,
“Absorption-edge anisotropy of Cu2ZnSiQ4 (Q=S, Se) quaternary compound semiconductors”, J.
Alloys Compd., Vol. 509, no. 15, pp. 4924-4928 (2011).
10.
Chen, Y. M., J. H. Cai, Y. S. Huang, K. Y. Lee, and D. S. Tsai, “Preparation and characterization of iridium dioxide-carbon nanotube nanocomposites for supercapacitors”, Nanotechnology, Vol.
22, no. 11, 115706 (7pp) (2011).
11. Dumcenco, D. O., Y. C. Su, Y. P. Wang, K. Y. Chen, Y. S. Huang, C. H. Ho, and K. K. Tiong,
“Polarization dependent Raman active modes study of the Mo1-xWxS2 mixed layered crystals”, Chin. J. Phys., Vol. 49, no. 1, pp. 270-277 (2011).
12. Su, Y. C., C. A. Chen, Y. M. Chen, Y. S. Huang, and K. Y. Lee, “Characterization of RuO2
nanocrystals deposited on carbon nanotubes by reactive sputtering”, J. Alloys Compd., Vol. 509, no. 5, pp. 2011-2015 (2011).
13. Levcenco, S., D. Dumcenco, Y. S. Huang, E. Arushanov, V. Tezlavan, K. K. Tiong, and C. H. Du,
“Near-band-edge anisotropic optical transitions in wide band gap semiconductor Cu2ZnSiS4”, J.
Appl. Phys., Vol. 108, no. 7, 073508 (5pp) (2010).
14. Wang, Y. P., C. H. Ho, and Y. S. Huang,“The study of surface photoconductive response in indium sulfide crystals”, J. Phys. D: Appl. Phys., Vol. 43, no. 41, 415301(5pp) (2010).
15. Dumcenco, D. O., K. Y. Chen, Y. P. Wang, Y. S. Huang, and K. K. Tiong, “Raman study of the Mo1−xWxS2 mixed layered crystals”, J. Alloys Compd., Vol. 506, no. 2, pp. 940-943 (2010).
16. Levcenco, S., D. Dumcenco, Y. S. Huang, E. Arushanov, V. Tezlavan, K. K. Tiong, and C. H. Du,
“Temperature dependent study of the band-edge excitonic transitions of Cu2ZnSiS4 single crystals by polarization dependent piezoreflectance”, J. Alloys Compd., Vol. 506, no. 1, pp. 46-50 (2010).
17. Yeh, C. N., Y. M. Chen, C. A. Chen Y. S. Huang, D. S. Tsai, and K. K. Tiong, “Growth and characterization of well-aligned densely-packed rutile TiO2 nanocrystals on sapphire (100) and (012) substrates via reactive magnetron sputtering”, Thin Solid Films, Vol. 518, no. 15, pp. 4121- 4125 (2010).
18. Chen, Y. M., C. A. Chen, Y. S. Huang, K. Y. Lee, and K. K. Tiong, “Characterization and enhanced field emission properties of IrO2-coated carbon nanotube bundle arrays”, Nanotechnology, Vol. 21, no. 3, 035702 (7pp) (2010).
19. Chen, Y. M., C. A. Chen, Y. S. Huang, K. Y. Lee, and K. K. Tiong, “Synthesis of IrO2
nanocrystals on carbon nanotube bundle arrays and their field emission characteristics”, J. Alloys Compd., Vol. 487, no. 1-2, pp. 659-664 (2009).
20. Chen, C. A.,Y. M. Chen, Y. S. Huang, D. S. Tsai, K. K. Tiong, and P. C. Liao, “Synthesis and characterization of well-aligned anatase TiO2 nanocrystals on fused silica via metal organic vapor deposition”, CrystEngComm, Vol. 11, pp. 2313-2318 (2009).
21. Chen, C. A., Y. M. Chen, K. Y. Chen, J. K. Chi, Y. S. Huang, and D. S.Tsai, “Growth and characterization of the coexistence of vertically aligned and twinned V-shaped RuO2 nanorods on nanostructural TiO2 template”, J. Alloys Compd., Vol. 485, no .1-2, pp. 524-528 (2009).
22. Chen, Y. M., C. A. Chen, C. N. Yeh, J. K. Chi, Y. S. Huang, D. S. Tsai, and K. K. Tiong, “(301) and (101) RuO2 twins on nanostructural rutile TiO2 template”, Mater. Chem. Phys., Vol. 117, no.
2-3, pp. 544-549 (2009).
23. Liao, P. C., C. A. Chen, J. G. Chi, Y. S. Huang, D. S. Tsai, and K. K. Tiong, “Growth and structural characterization of well-aligned RuO2 nanorods on LiNbO3 (1 0 0) via MOCVD”, J.
Alloys Compd., Vol. 480, no. 1, pp. 100-103 (2009).
24. Dumcenco, D. O., W. Y. Huang, Y. S. Huang, and K. K. Tiong, “Anisotropic optical characteristics of Au-doped rhenium diselenide single crystals”, J. Alloys Compd., Vol. 480, no. 1, pp. 104-106 (2009).
25. Chen, C. A., Y. M. Chen, Y. S. Huang, D. S. Tsai, P. C. Liao, and K. K. Tiong, “Synthesis and structural characterization of twinned V-shaped IrO2 nanowedges on TiO2 nanorods via MOCVD”, J. Alloys Compd., Vol. 480, no. 1, pp. 107-110 (2009).
26. Chen, C. A., A. Korotcov, Y. S. Huang, W. H. Chung, D. S. Tsai, and K. K. Tiong, “Growth and characterization of well-aligned TiO2 nanocrystals on sappire substrates via metal organic vapor deposition”, J. Mater. Sci. - Mater. Electron., Vol. 20, no. 1, pp. S332-S335 (2009).
27. Chen, C. A., Y. S. Huang, W. H. Chung, D. S. Tsai, and K. K. Tiong, “Raman scattering study of the phase transformation on nanostructured titania films prepared via metal organic vapor deposition”, J. Mater. Sci. - Mater. Electron., Vol. 20, no. 1, pp. S303-S306 (2009).
28. Chen, C. A., Y. M. Chen, Y. S. Huang, D. S. Tsai, K. K. Tiong, and C. H. Du, “Growth and characterization of V-shaped IrO2 nanowedges via metal-organic vapor deposition”, Nanotechnology, Vol. 19, no. 46, 465607 (5pp) (2008).
29. Dumcenco, D. O., Y. S. Huang, C. H. Liang, and K. K. Tiong, “Optical characterization of Au- doped rhenium diselenide single crystals”, J. Appl. Phys., Vol. 104, no. 6, 063501 (6pp) (2008).
30. Chen, C. A., K. Y. Chen, Y. S. Huang, D. S. Tsai, K. K. Tiong, and F. Z. Chien, “X-ray diffraction and Raman scattering study of thermal-induced phase transformation in vertically- aligned TiO2 nanocrystals grown on sapphire(100) via metal organic vapor deposition”, J. Cryst.
Growth, Vol. 310, no. 15, pp. 3663-3667 (2008).
31. Chen, C. A., A. Korotcov, Y. M. Chen, Y. S. Huang, D. S. Tsai, K. K. Tiong, and P. C. Liao,
“Growth and characterization of vertically aligned densely packed TiO2 nanocrystals on sapphire(100) via metal-organic chemical vapor deposition”, ECS Transactions, Vol.11, no..11, pp. 19-25, doi:10.1149/1.2889404 (2008).
32. Chen, C. A., Y. M. Chen, A. Korotcov, Y. S. Huang, D. S. Tsai, and K. K. Tiong, “Growth and characterization of well-aligned densely-packed rutile TiO2 nanocrystals on sapphire substrates via metal-organic chemical vapor deposition”, Nanotechnology, Vol. 19, no. 7, 075611 (5pp) (2008).
七、 計畫成果自評
在此三年中，除實驗室本身發表上述研究成果之外也與其它研究團對合作，總共發表與計 劃相關 SCI 論文 32 篇，成果豐碩。
Deposition and characterization of IrO
xnanofoils on carbon nanotube templates by reactive magnetron sputtering
Yi-Min Chena, Jhen-Hong Caia, Ying-Sheng Huanga,
⁎
, Kuei-Yi Leea,b, Dah-Shyang Tsaic, Kwong-Kau TiongdaDepartment of Electronic Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106, Taiwan
bGraduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106, Taiwan
cDepartment of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106, Taiwan
dDepartment of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
a b s t r a c t a r t i c l e i n f o
Article history:
Received 2 May 2011
Received in revised form 20 September 2011 Accepted 28 September 2011
Available online xxxx
Keywords:
Electron microscopy Raman scattering
Reactive magnetron sputtering Nanocomposites
Oxides
Large surface area IrOxnanofoils (IrOxNF) were deposited on multi-wall carbon nanotube (MWCNT) templates, forming IrOx/MWCNT nanocomposites, by reactive radio frequency magnetron sputtering using Ir metal target.
The structural and spectroscopic properties of IrOxNF were characterized. The micrographs offield emission scanning electron microscopy showed the formation of foil-like structure for the as-deposited samples. Trans- mission electron microscopy analysis revealed the contiguous presence of glassy iridium oxide, iridium metal, and iridium dioxide nanocrystals in the foil. X-ray photoelectron spectroscopy analysis provided the information of the oxidation states and the stoichiometry of IrOxNF. Raman spectra revealed the amorphous-like phase of the as-deposited IrOxNF. The nanofoil structure provided ultra-high surface area for electrical charge storage which made the IrOx/MWCNT nanocomposites as an attractive candidate for the supercapacitor applications.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Iridium oxide (IrO2) is a well-known metallic oxide of high chemical stability with important applications in durable electrode materials for optical switching layers in electrochromic displays[1], microsensors for gas detection[2], ferroelectric capacitors for nonvolatile memories [3]and functional neurostimulation[4,5]. From the practical point of view, a porous composite structure with large surface area nanocrystals (NCs) should be highly desirable for increasing the surface-to-volume ratio for high performance of devices[6]. The porous structure of the high-density multi-wall carbon nanotubes (MWCNT) provides an ideal template for synthesizing IrOx-coated MWCNT (IrOx/MWCNT) nanocomposites[7]. Recently, we have grown nanostructural IrO2on MWCNT templates by cold wall metal organic chemical vapor deposi- tion (MOCVD)[8]. However, MOCVD generally requires multiple pro- cessing steps to fabricate the nanostructures. Proper control of these processing steps can be difficult. For example, the properties of the precursor may change due to oxidation after a few running of the growth process. To overcome such deficiency, we have used the tech- nique of reactive radio frequency magnetron sputtering to synthesize IrOx/MWCNT nanocomposite. The technique is a simple method to fabricate large area structures and has several advantages including
better control of the growth conditions and a single deposition step to obtain the nanostructures.
In this report, thinflakes of curved IrOxnanofoils (NF) are synthe- sized by radio frequency reactive magnetron sputtering on top of the MWCNTfilm which is grown on a stainless steel (SUS) substrate. The structural and spectroscopic properties of the IrOx/MWCNT were char- acterized usingfield emission scanning electron microscopy (FESEM), field emission transmission electron microscopy (FETEM), X-ray photo- electron spectroscopy (XPS), and Raman scattering spectroscopy. The preliminary study concerning the possible application of the IrO-
x/MWCNT nanocomposites for the supercapacitor application was car- ried out. The results were presented and discussed.
2. Experimental details
IrOxNF were deposited on the MWCNT templates to form IrO-
x/MWCNT nanocomposites. The MWCNT templates were synthesized on stainless steel substrates (SUS) using thermal chemical vapor deposi- tion[9]. The IrOxNF were grown using a home-made high vacuum radio frequency magnetron sputtering system. The sputtering gun consists of a standard circular planar magnetron. The sputtering target was a high purity (N99.95%) metal disk of 2.54 cm in diameter. Sputtering operation was carried out in the main chamber of a radio frequency magnetron sputtering system, equipped with a turbo-molecular pump. The pressure of main chamber was first pumped down to a base pressure of
~3×10−3Pa. A gas mixture of 2 sccm oxygen and 10 sccm argon was Thin Solid Films xxx (2011) xxx–xxx
⁎ Corresponding author. Tel.: +886 2 27376385; fax: +886 2 27376424.
E-mail address:[email protected](Y.-S. Huang).
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then introduced near the substrate surface. The working pressure during sputtering was regulated at 16 Pa. Distance between the target and the substrate was set at 45 mm, with a RF power of 30 W. The sputtering lasted for 2 h, with the substrate temperature was controlled at 250 °C.
A JEOL-JSM6500F FESEM was used to study the morphology of IrO-
xNF/MWCNT/SUS sample. TEM images were recorded to study the fine structural details of the IrOxNF and carbon nanotubes by a Phillips Tecnai G2 F20 FE-TEM at working voltage of 200 kV. The chemical bind- ing states of the IrOxsamples were investigated from Ir 4f and O 1s spec- tra obtained by XPS using a Thermo VG Scientific Theta Probe system under the base pressure of 1.3 × 10−7Pa. The Al Kα 1486.68 eV line was the X-ray source and the Ag 3d5/2line at 368.26 eV was the calibra- tion reference before measurement. In the XPS analysis, surface Ar ion etching was used to remove the contamination layer on the surface.
XPS peak positions and integrated intensities were obtained through the curvefitting, using Thermo VG Scientific: Avantage v3.2 software.
Raman scattering spectroscopy was used to extract crystal information of the IrOxNFs. Raman spectra were recorded at room temperature utilizing the back-scattering mode on a Renishaw inVia micro-Raman system with 1800 grooves/mm grating and an optical microscope with a 50× objective. The same microscope was used to collect the sig- nal in a backscattering geometry. The Ar-ion laser beam of the 514.5 nm excitation line with a power about 1.5 mW was focused on a spot size of ~5μm in diameter. Prior to the measurement, the system was calibrated by means of the 520 cm−1Raman peak of a polycrystalline Si.
The electrochemical properties were measured using a three-elec- trode setup in a thermostat cell at 22 ± 1 °C, with a 4 cm2platinum plat as the counter electrode and an Ag/AgCl (saturated KCl) reference electrode. The supporting electrolyte was 1 M KOH solution. Current- potential data were recorded with a potentiostat/galvanostat station (PGSTAT100, AutoLab). The working electrode was prepared by gluing the IrOxNF/MWCNT/SUS sample with a copper wire along the sample
edge with a conducting silver paste and then insulated with Miccrostop lacquer (Pyramid Plastics, Inc., Tolber Division), such that the surfaces of the IrOxNF and MWCNT were the sole surfaces exposed to the support- ing electrolyte. For comparison purpose, a working electrode of MWCNT/SUS was also prepared and its properties were measured.
Values of electrode potential, in this work, refer to an Ag/AgCl (saturat- ed KCl) reference electrode. Total mass of the electrode was measured with a precision balance before and after the growth of IrOxNF and MWCNT. The deposited mass of CNT or IrOxNF was obtained from the mass difference.
3. Results and discussion
The morphological features of MWCNT and nanocrystalline IrOxare illustrated inFig. 1. As shown inFig. 1(a), the SEM image of the 2μm thick MWCNTfilm viewing from the top displays the highly populated and entangled nanotubes. The number density of nanotubes is estimat- ed to be ~107cm−2. The cross-sectional view of this MWCNTfilm,Fig. 1 (b), reveals the alignment of the underlying nanotubes supporting the entangled upper nanotubes. The inset shows the tube diameter of the MWCNT is about 40 nm.Fig. 1(c) and (d) presents the morphological features of IrOxNF at different angles. The tilted view of Fig. 1(c) shows the geometrical shape of thin curved foil for the deposited IrOx. These foils stand on nanotubes of ~110 nm high with the winding nano- tube serving as the template. The cross-sectional view ofFig. 1(d) shows the connection between the nanofoils and CNT and more refined details of the nanofoils. Each IrOxfoil is so thin that secondary electrons seem to have sufficient energy to tunnel across the curved foil, especial- ly near the edges. The inset ofFig. 1(d) providesfine details of the foil edge, showing tiny grains and isolated pores.
Fig. 2(a) presents the high-resolution TEM image of a nanotube taken from the upper section of the MWCNT template. Wall of a
Fig. 1. (a) The SEM image of top view of MWCNT; (b) the cross view of MWCNT. The inset of (b) is a magnified image of the upper section. (c) The SEM image of top view of IrOxNF on the MWCNT/SUS template; (d) the cross view of IrOxNF on MWCNT/SUS template. The inset of (d) is a magnified image of a leaf of IrOxNF.
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typical nanotube consists of 16–18 layers of graphene sheet, separat- ed by an interplanar spacing of 0.34 nm. A fast Fourier transform (FFT) of the high-resolution TEM image of this MWCNT is illustrated in the inset. This FFT pattern exhibits a pair of strong arcs for (002), corresponding to the predominant (002) plane orientation for the nanotube[10]. A bright-field image of IrOxnanofoils together with their residing nanotube is shown inFig. 2(b), showing several petals of winding IrOxnanofoils protruding out of a single MWCNT.Fig. 2 (c) depicts the HRTEM image of an IrOxnanofoil.Fig. 2subpanels (d), (e) and (f) are the magnified HRTEM images corresponding to regions (1), (2) and (3), respectively, inFig. 2(c). The insets are the FFT patterns of different regions. Careful examination of the various locations in this HRTEM images reveals the heterogeneous nature of the iridium oxide foil. Each FFT pattern displays the diffraction pattern of a unique structure. As can be seen in the inset ofFig. 2 (d), no sharp spots or rings are observed at location (1) suggesting a non-crystalline structure in this area. Inset ofFig. 2(e) is the FFT
pattern of location (2), exhibiting the [111] zone axis diffraction pat- tern of iridium face center cubic metal. Inset ofFig. 2(f) shows the FFT pattern of location (3), corresponding to the [010] zone axis diffrac- tion pattern of the tetragonal iridium dioxide crystal.
Further evidence of a mixed phase of metal and oxide can be found in the XPS spectra. Slow scan on the Ir 4f doublet (7/2 and 5/2) and oxygen O 1s features of IrOxNF were carried out in the binding energy range of 58–72 eV and 526–540 eV, as illustrated inFig. 3(a) and (b), respectively. The Gaussian and Lorentzian mixing line shapes were employed after the background treatment by the Shirley function in fitting to ensure accurate evaluation of the peak positions. The Ir 4f scan inFig. 3(a) is deconvoluted into two pairs of peaks. The main pair of 4f7/2and 4f5/2at 61.1 and 64.1 eV is assigned to Ir0, in agreement with the literature report on Ir single crystal[11,12]. The minor pair at 61.9 and 65.1 eV is attributed to Ir4+, very close to those of the IrO2
single crystal[13]. As shown inFig. 3(b), the O 1s signal appears to be a triplet, the sharp peak at 529.7 eV is close to that of the IrO2single
Fig. 2. (a) The HRTEM image of a MWCNT; the inset is the FFT pattern. (b) The low magnification TEM image of IrOxNF/MWCNT; (c) the HRTEM image a leaf of IrOxnanofoil. (d), (e) and (f) The magnified HRTEM images corresponding to regions (1), (2) and (3) in the (c); the insets are the FFT patterns.
Y.-M. Chen et al. / Thin Solid Films xxx (2011) xxx–xxx 3
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crystal at 529.7 eV[13]. The other two oxygen broad peaks are at 531 and 533 eV could be attributed to oxygen in Ir-OH[14]and indicate the presence of chemisorbed oxygen on the surface or/and higher oxidation states[15].
Fig. 4 shows the Raman spectrum of IrOxNF in the range of 300–900 cm−1. For comparison purposes, the Raman spectrum IrO2
single crystal is also depicted inFig. 4. Owing to the nanometric size effect, the scattering features of IrOxNF are substantially shifted and broadened. The strongest Eg feature of IrOxNF is located at ~549 cm− 1, which deviates significantly with respect to that of IrO2
single crystal at 559 cm−1[16]. The other two minor scatterings of A1gand B2gof IrOxNF are barely distinguishable from the background noise. These three Raman features show considerable redshifts and the results concur well with the TEM observation. An additional weaker scattering of IrOxNF, not observed in the single crystal spectrum, is marked at ~360 cm− 1 inFig. 4. The additional scattering was also observed by Thanawala and coworkers, who attributed the feature to
vibration of the molecular units in localized non-stoichiometric phases [17].
A preliminary study has also been carried out to understand the potential application of IrOx/MWCNT nanocomposites in electrochemical capacitors as electrode materials.Fig. 5presents the cyclic voltammo- grams (CV) for MWCNT/SUS and IrOx/MWCNT/SUS, plotted as response current versus potential at sweep rate of 30 mV s−1. The response current of IrOx/MWCNT/SUS is considerably higher than that of MWCNT/SUS, suggesting a higher capacitance. Meanwhile CV ofFig. 5 is featured with a pair of redox peaks at−0.8 V which could be attributed to the surface reaction of iridium oxide[18]. The other pair of broad peaks around−0.4 V belongs to hydrogen and hydroxide adsorption of iridium metal surface[19,20]. The capacitance of each scan has been calculated from the CV curves using Eq.(1)as follows[21]:
CS¼ ∫IdV
2×m×V×S; ð1Þ
where CSis the specific capacitance, ∫I dV is the integrated area of the CV curve, m is the mass of active material,ΔV is the potential range, and S is the scan rate. The CSof IrOxNF/MWCNT/SUS is estimated to be 310 Fg−1, significantly higher than the values of 18 Fg−1 for MWCNT/SUS. It is known that MWCNT exhibits the double-layer capac- itance only[22], while the capacitive property of IrOxsurface is domi- nated by its pseudocapacitance. The enhancement of the specific capacitance can be attributed to the presence of IrOxNF on the surface of MWCNT. This in turn modifies the structure and morphology of MWCNT, allowing the IrOxNF to be available for the electrochemical reactions and improves the efficiency of the nanocomposites. The progressive redox reactions occurring at the surface and bulk of IrOx
through Faradiac charge transfer between electrolyte and electrode results in the enhancement of the specific capacitance of IrOx/MWCNT nanocomposites from that of the MWCNT. The nanofoil structure provides ultra-high surface area for electrical charge storage which makes the IrOx/MWCNT nanocomposites as an attractive candidate for the supercapacitor application.
4. Summary
We have deposited IrOxNF on MWCNT templates by reactive radio frequency magnetron sputtering. The micrographs of FESEM showed growth of IrOxNF with thicknesses of a few nanometers on carbon nanotubes. TEM analysis revealed the contiguous presence of glassy iridium oxide, iridium metal, and iridium dioxide nanocrystals in the foil. XPS showed a mixed phase of metal and oxide, while Raman spec- tra revealed the co-existence of amorphous-like phase of the as-depos- ited nanocrystalline IrOx. The nanofoil structure provided ultra-high surface area for electrical charge storage which made the IrOx/CNT Fig. 3. The peakfitted slow scan XPS spectra of (a) Ir 4f and (b) O 1s of IrOxNF on the
MWCNT/SUS template.
Fig. 4. The Raman spectra of IrOxNF on the MWCNT/SUS template and IrO2single crystal.
Fig. 5. The cyclic voltammograms at a scan rate of 30 mV s−1for the IrOxNF/MWCNT/SUS and MWCNT/SUS.
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nanocomposites as an attractive candidate for the supercapacitor application.
Acknowledgments
This work was supported by the National Science Council of Taiwan under Contract Nos. NSC-97-2112-M-001-MY3, NSC-98-2221-E-011- 021 and NSC99-2112-M-019-001.
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Anisotropy of the spectroscopy properties of the wurtz-stannite Cu
2ZnGeS
4single crystals
S. Levcencoa,1, D. Dumcencoa,1, Y.S. Huanga,⇑, K.K. Tiongb, C.H. Duc
aDepartment of Electronic Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan
bDepartment of Electrical Engineering, National Taiwan Ocean University, Keelung 202, Taiwan
cDepartment of Physics, Tamkang University, Tamsui 251, Taiwan
a r t i c l e i n f o
Article history:
Received 20 June 2011
Received in revised form 27 July 2011 Accepted 10 August 2011
Available online 9 September 2011
Keywords:
Crystal growth Semiconductors Optical properties Optical spectroscopy
a b s t r a c t
In this study, the near band edge anisotropic optical properties of wurtz-stannite (WS) Cu2ZnGeS4single crystals were characterized using polarization-dependent transmittance and electrolyte electroreflec- tance (EER) techniques. Single crystals of Cu2ZnGeS4were grown by chemical vapor transport method using iodine as a transport agent. Analysis of absorption spectra revealed indirect allowed transitions for Cu2ZnGeS4with the band gaps of 2.02 (2.07) and 2.08 (2.14) eV for Ekb and Eka polarization config- urations at 300 (10) K. The room-temperature EER spectra in the vicinity of the direct band edge showed anisotropic transitions at around 2.38, 2.44 and 2.45 eV for Ekb, Eka and Ekc polarizations, respectively.
Based on the experimental observations and recent band-structure calculations a plausible band diagram near band edge of WS-Cu2ZnGeS4was constructed.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Recently, quaternary chalcogenides I2–II–IV–VI4have been ac- tively investigated for their anisotropic structure [1], nonlinear optical properties [2,3]and potential for application in the field of energy, environment and optoelectronics[4–7]. Cu2ZnGeS4be- longs to the family of Cu-based quaternary chalcogenide com- pounds, Cu2-II–IV–VI4. It is known to crystallize in two types of structural forms: low-temperature tetragonal (stannite) structure and high-temperature orthorhombic (wurtz-stannite (WS)) struc- ture [8–13]. The low temperature zinc blende derived structure is based on a cubic close packing of the anions, while the wurtzite derived structure is based on a hexagonal close packing of the an- ions (S)[8,11]. In both structures the coordination of the cations is tetrahedral so that two Cu atoms, one Zn atom and one Ge atom surround each sulphur atom [10–12]. The material is of interest for its photocatalytic properties [9]and potential for application in optoelectronics [13]. Recently, it has been reported that solar cells based on Cu2Zn(SnxGe1x)S4nanocrystals with a Ge/(Ge + Sn) ratio 0.7 have reached efficiencies of up to 6.8% [14]. In the past few years a number of theoretical studies on the structure and electronic properties of the quaternary chalcogenide semiconduc- tors have been performed[15–20]. Several optical measurements (absorption[12,21,22], diffuse reflection[9], far infrared spectros-
copy[23], and spectroscopic ellipsometry[24]) were carried out on Cu2ZnGeS4bulk samples at room temperature. However, relatively few experimental results have been reported on the anisotropic optical properties related to the electronic band structures of Cu2ZnGeS4, due to the difficulty of preparing suitable size, compo- sitionally homogeneous and high purity single crystals.
In this paper, we report a detailed study of the near band edge anisotropic optical transition properties of WS-CuZnGeS4 single crystals by polarization-dependent transmittance and electrolyte electroreflectance (EER) techniques. Good quality single crystals of WS-CuZnGeS4 were grown by chemical vapor transport using iodine as the transport agent. Analysis of the absorption spectra in polarized light along a and b axis revealed that the interband transitions in Cu2ZnGeS4 are indirect and their band gaps are polarization-dependent. The polarized EER spectra measured with an electric field polarization parallel to a, b, and c axes in the vicinity of the direct band edge of Cu2ZnGeS4displayed dis- tinct structures associated with transitions from three upper- most valence bands to the conduction band minimum atCpoint.
Based on the experimental results and the recent band-structure calculations [16–17], a plausible band diagram near the band edge of WS-Cu2ZnGeS4was constructed. In addition, the polariza- tion-dependent indirect energy gaps of the material at various temperatures were determined by the absorption data and their temperature dependences were analyzed by the Varshni equation [25] and an expression containing the Bose–Einstein occupation factor for phonons [26]. The parameters that describe the temperature dependence of band gaps were evaluated and discussed.
0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.optmat.2011.08.005
⇑Corresponding author. Tel.: +886 2 27376385; fax: +886 2 27376424.
E-mail address:[email protected](Y.S. Huang).
1Permanent address: Institute of Applied Physics, Academy of Sciences of Moldova, Chisinau, MD 2028, Moldova.
Optical Materials 34 (2011) 183–188
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2. Experimental
2.1. Crystal growth
Single crystals of Cu2ZnGeS4were grown using the chemical va- por transport method with iodine as a transport agent. Prior to the crystal growth, a quartz ampoule (22 mm OD, 17 mm ID, 20 cm length) containing 10 g of stoichiometric amounts of the elements (Cu, 99.99% pure; Zn, 99.99%; Ge, 99.9999%; S, 99.999%) and iodine (5 mg/cm3) was cooled with liquid nitrogen, evacuated to 1 105mbar and sealed. Two ampoules were placed in a three- zone furnace and the charge prereacted for 24 h at 800 °C with the growth zone at 850 °C, preventing the transport of the product.
The temperature of the furnace was increased slowly to avoid any possibility of explosion due to sulfur pressure. The furnace was then equilibrated to give a constant temperature across the reac- tion tube, and was programmed over 24 h to produce the temper- ature gradient at which single-crystal growth took place. Optimum crystal growth was achieved with the charge zone maintained at 850 °C and the growth zone at 800 °C. After four weeks, the furnace was allowed to cool down slowly (40 °C/h). Reddish single crystals Cu2ZnGeS4up to 5 mm 3 mm 2 mm were obtained.
The stoichiometric compositions of selected Cu2ZnGeS4samples were determined by using energy dispersive X-ray analysis (EDX).
EDX measurements showed some variations between the ratio of Cu and Zn among the examined samples. However, the average atomic ratio of Cu:Zn:Ge:S was found to be closed to 2:1:1:4. No effective residue of iodine was detected with the detection limit of 0.1%[27], showing Cu2ZnGeS4single crystals have been grown without the incorporation iodine during the chemical vapor trans- port process. The grounded crystals were examined by slow X-ray scan using the Cu Kacharacteristic line (1.5406 Å) and compared with the X-ray powder data reported by Schäfer and Nitsche [28]. The as-synthesized crystals were identified to crystallize in WS structure. Lattice parameters calculated with the aid of a com- puter using a least squares refinement program are a = 7.506 Å, b = 6.467 Å and c = 6.178 Å. These values are found to agree reason- ably well with those of the previous reports[10,21,28]. The crystal orientation was determined by rotating orientation X-ray diffrac- tion method. The crystals showed p-type conductivity as verified through hot point probe method. The typical value of resistivity was determined to be about 12–20Xcm at room temperature.
2.2. Optical characterization
The polarization-dependent of optical measurements were mainly carried out on the as-grown plane (0 0 1) with the axis b parallel to the long edge of the crystal platelets. In order to perform the EER measurements in c direction, the crystals were cut and pol- ished parallel to (0 1 0) plane. For the transmittance measure- ments, single crystals were mechanically polished to a thickness of about 70lm using 0.05lm aluminum oxide (Al2O3) powder and then rinsed with acetone and deionized water. A 150 W xenon arc lamp filtered by a 0.25 m grating monochromator provided the source for optical measurements with the resolution of 1 nm.
Model PRH 8020 CASIX Rochon prisms were employed for polari- zation-dependent measurements. An UV enhanced Si detector was used to detect the transmitted or reflected signals. Measure- ments of the reflectivity and transmission at near-normal inci- dence configuration with chopped light were carried out. A closed-cycle cryogenic refrigerator equipped with a digital ther- mometer controller was used for the low temperature measure- ments with a temperature stability of 0.5 K or better.
An electrolyte of the tartaric acid (3 wt.%) in ethylene alcohol was used for EER measurements. A 200 Hz, 3 V peak-to-peak,
square wave with a 0.5 V (vs Pt electrode) DC bias was used to modulate the electric field. A dual-phase lock-in amplifier was used to measure the detected signals. The entire data acquisition procedure was performed under computer control. Multiple scans over a given photon energy range was programmed until a desired signal-to-noise level has been obtained.
3. Results and discussion
3.1. Absorption
The absorption coefficientawas calculated from the measured transmittance (T) and reflectance (R) through the relation[29,30]
T ¼ð1 RÞ2ead
1 R2e2ad ; ð1Þ
where and d is the sample thickness. The absorption coefficientaat 10 and 300 K in the vicinity of band edge of Cu2ZnGeS4crystals is presented inFig. 1. The dash-dotted curves inFig. 1correspond to the Eka polarization, while the solid curves represent Ekb polariza- tion. It is seen that a notable shift towards lower energy for the Ekb polarization is observed as compared to the Eka polarization. The obtained polarization dependence of the absorption spectra pro- vides conclusive evidence that both the absorption edges are asso- ciated with the interband transitions from different origins. As expected, with increasing temperature from 10 to 300 K the band edges exhibit a redshift characteristic. Analysis of the experimental data showed that the absorption coefficientato be proportional to (hm Eg)nwith n 2 over an appreciable energy range, revealing the character indirect band gap. The plotted (ahm)1/2 spectra of Cu2ZnGeS4 with their best fits are shown in Fig. 2, where the open-circles and triangles are representative experimental points deduced from Ekb and Eka polarizations absorption spectra and the solid lines are their best fits to the expression[31]
ahm¼Aðhm Eindg þ EpÞ2
expðEp=kTÞ 1 þBðhm Eindg EpÞ2
1 expðEp=kTÞ ð2Þ
where hmis the energy of the incident photon, Eindg is the indirect band gap, Epis the energy of the phonon assisting the transition, and A and B are constants. The obtained values of the indirect band gaps and phonon energies at 10 and 300 K are listed inTable 1. The results strongly indicate that WS-Cu2ZnGeS4 is indirect transition semiconductor, in which Ekb polarization exhibits a smaller band gap and a single phonon makes important contributions in assisting the indirect transitions. The nonuniform thicknesses and unsmooth sample surfaces will cause some deviations of the incident angles
Fig. 1. The Eka polarization and Ekb polarization absorption spectra of Cu2ZnGeS4
at 10 and 300 K. The dash-dotted curves in correspond to the Eka polarization, while the solid curves represent Ekb polarization.
184 S. Levcenco et al. / Optical Materials 34 (2011) 183–188
