Synthesis and Characterization of PbTe and Bi2Se3 Nanowires for Thermoelectric Application

碲化鉛和硒化鉍奈米線於熱電應用之製程與特性探討

戴迪(Dedi)

指導教授 ： 陳洋元 ； 李志浩

國立清華大學/原子科學院/工程與系統科學系/博士(2014年)

摘要

Nanoengineered thermoelectric materials used for converting waste heat into electricity have become a compelling research topic. Thermoelectric (TE) energy converters are devices that can harvest renewable energy for application in power generation. The efficiency of TE materials is determined according to the dimensionless figure of merit ZT, which is defined as S2σT/(κe + κl), where S is the thermal power or Seebeck coefficient, σ represents the electrical conductivity, κe is the electronic thermal conductivity, and κl is the lattice thermal conductivity. The quantity S2σ is defined as the power factor (PF). Theoretically, a reduction in dimensionality from three dimensions to one dimension yields a dramatically increased electronic density of states at the energy band edges and the reduction of thermal conductivity due to size effects, thereby increasing the S2σ and yielding an enhanced ZT. Slack et al. reported that semiconductors exhibiting narrow band gaps and high mobility carriers are optimal TE materials. Lead telluride (PbTe) is an AIVBVI semiconductor with an energy band gap of 0.31 eV at 300 K and PbTe has a cubic sodium chloride (NaCl) structure type. Another compound, Bismuth selenide (Bi2Se3) is a AVBVI semiconductor and topological insulator (TI) that also exhibits a narrow band gap of approximately 0.3 eV and crystallizes in a rhombohedral structure belonging to the tetradymite space group D_3d^5 (R-3m). Those compounds and its alloys can be applied in thermoelectricity, so the two compounds would be good candidates for the one dimensional study mention above.
This study reports the synthesis and the characterization of thermoelectric PbTe and Bi2Se3 nanowires (NWs). Those NWs were successful grown through the annealing (or stress-induced method), which is an alternative technique for synthesizing semiconductor NWs without a catalyst used traditionally. The growth parameters were adjusted to grow straight and high quality of single crystal NWs characterized by a transmission electron microscopy (TEM). The PbTe and Bi2Se3 NWs were found to have a wide range of diameters from 50 to 500 nm and lengths as high as several micrometers. The PbTe NWs were high-quality single crystals with a growth along the [100] direction and the formation of single crystal Bi2Se3 NWs was growing along the [11-20] direction. In this work, the thermoelectrical transport measurement of a PbTe nanowire (NW) with a diameter d of 217 nm exhibited a notable enhancement over the room-temperature S of -342 µV K-1, which was approximately 95 % larger than the bulk of PbTe.4.9 The PF of 104 µW m-1 K-2 is also higher than any previously reported PF in PbTe NW.4.12, 4.13, 4.16, 4.17 The measured thermal conductivity κ of the PbTe NW was measured by the self-heating 3 method. At room temperature, the κ of 217 nm and 75 nm NW were 0.31 W m-1 K-1 and 0.96 W m-1 K-1, which is approximately 87 % and 58 % respectively lower than the typical value of κ = 2.3 W m-1 K-1 reported for PbTe bulk.4.31 The κl values at 300 K were 0.95 W m-1 K-1 and 0.30 W m-1 K-1 for 75 nm and 217 nm, which is lower 57 % and 86 % respectively than the PbTe bulk (κl = 2.2 W m-1 K-1)4.32. The lattice contribution of 217 nm at room temperature is close with the values reported for superlattice thin films of PbSe0.98Te0.02/PbTe [κl = 0.35 W m-1 K-1]4.33. Based on the reported κ of individual PbTe NW with various diameters [with d =182, 277, and 436 nm]4.37 and a 75 nm of our work, the κ of a NW decreases as its d shrinks or shows size dependent of κ of PbTe NWs. Considering that phonon boundary scattering has a considerable effect in reducing the κ of a NW, this result clearly suggests that the enhanced boundary scattering caused by the size effect suppresses phonon transport through the PbTe NWs. Moreover, referred to the result of S, σ and κ; the experimental calculation of ZT in individual PbTe NW were ∼0.0010.004 for 75 nm at 300350 K range and ~0.090.2 for 217 nm at 300380 K range, whereas ZT of PbTe bulk is approximately ∼0.25 at 300 K and maximal ~0.8 at 700 K.4.41 However ZT value of 217 nm is still higher than of reported for PbTe NW by Lee et al. [ZT ~ 0.0054 at 300 K].4.40 Such an enhanced ZT can in part be attributed to the size effect and structure quality of NWs. Additionally, the room temperature magnetoresistance (MR) of the 142 nm NW was ~0.8% at B = 2 T, which is considerably higher than that MR [0.2% at B = 2 T]5.18 of the PbTe bulk reported.
Furthermore, the PF of a Bi2Se3 NW 200 nm in d reached 39.32 × 10-5 Wm-1K-2 at 300 K. This PF value was larger than any previously reported PF observed in polycrystalline Bi2Se3 bulk [2.8010.70 × 10-5 Wm-1K-2 in table 6.1] that were composed by microstructures/nanostructures. The enhanced PF value is likely a result of enhanced carrier concentration of the NW. The measured thermal conductivity κ value (κ is measured perpendicular to c plane) at T = 300 K of NW [2.05 W m-1 K-1 ] were 3134 % lower than those for single crystal Bi2Se3 bulk (2.96 W m-1 K-1 or 3.1 W m-1 K-1 in table 6.1). The obtained κl and κe at 300 K of NW were 29 % and 37 % lower than that of single crystal Bi2Se3 bulk (κl = 1.33 W m-1 K-1 and κe = 1.77 W m-1 K-1).6.22 It means, the κ of NW is mostly due to the electronic contribution at 290320 K range. For this NW, ZT calculated from the obtained S,  and κ increased with temperature and was about 0.06 at 300 K, It was 33 % still lower than that single crystal Bi2Se3 bulk [ZT ~ 0.17]6.21 mainly due to the low  for Bi2Se3 NW. It is likely caused by the difference in carrier concentration or mobility values. However, ZT values of this NW were still higher than any previously reported in polycrystalline Bi2Se3 bulk6.9, 6.11 and our S,  and  measurement results of Bi2Se3 NW d = 200 nm is also in reasonable agreement with theoretical study, which reveal that with decreasing diameter the thermoelectric transport in TI Bi2Se3 wires is increasingly dominated by the surface states.6.45 Our results can provide guidelines for future work on nanostructured thermoelectrics based on PbTe and Bi2Se3 compounds.

關鍵字

碲化鉛 ； 硒化鉍 ； 奈米線 ； 熱處理退火 ； 熱電

並列摘要

奈米工程熱電材料用於將廢熱轉化為電能已成為一個引人注目的研究課題。熱電能源轉換裝置可以有效率的應用可再生能源於電力的產生。熱電材料的轉換效率取決於與維度無相依的優質係數ZT,其定義為ZT = S2T/(e + l)，其中S是thermal power或Seebeck係數, 為電導率, e為電子的熱傳導, l為聲子的熱傳導, S2σ又定義為功率因子power factor (PF). 理論上，當樣品的維度縮小從三維到一維尺度,會使得能帶邊緣電子密度顯著增加以及尺寸效應所造成的熱傳導率降低,進而增加了熱電材料的功率因子 (S2) 和產生一個熱電效率ZT的提升。Slack等論點指出能帶間隙窄的半導體其具有高遷移率的載子是最佳的熱電材料。碲化鉛 (PbTe) 是一種AIVBVI半導體在300 K時的能帶間隙為0.31電子伏特並且具有立方氯化鈉 (NaCl) 的晶體結構。另一種化合物，硒化鉍(Bi2Se3)是一個AVBVI半導體也是拓撲絕緣體 (TI) 也表現出大約0.3 eV的能帶間隙其晶體結構為rhombohedral，屬於輝碲鉍礦 D_3d^5 (R3m) 空間群。這些化合物及其合金可加諸於熱電應用，所以這兩種化合物具有很大的潛力在對於上述一維熱電材料研究。
這項研究報告熱電材料PbTe和Bi2Se3奈米線的合成和基本特性描述。這些奈米線成功地通過熱處理退火(或 stress-induced method)而製備，相較於傳統生長半導體奈米線的方式它是一種不需使用到催化劑的另類的技術。通過穿透式電子顯微鏡 (TEM) 的鑑定，對生長參數進行調控得以製備長、直和高品質的單晶奈米線。所述的PbTe和Bi2Se3奈米線直徑範圍從50至500奈米，長度高達幾微米。高品質單晶PbTe奈米線沿 [100] 方向的生長，單晶Bi2Se3奈米線則沿 [11-20] 方向的成長。
在這項工作中，熱電電性量測一個直徑d為217奈米的PbTe奈米線其在室溫表現出顯著的Seebeck係數增S = -342 µV K-1，約比PbTe塊材大95 %.4.9 而且該奈米線功率因子PF = 104 µWm-1K-2也高於任何先前文獻報導中的PbTe奈米線。4.12, 4.13, 4.16, 4.17 PbTe奈米線的熱傳導率  是透過self-heating 3 方法所量測。在室溫下，直徑217 nm和75 nm奈米線他們的熱傳導率分別為0.31 W m-1 K-1 和 0.96 W m-1 K-1， 大約 87 % 和 58 % 的比例相對小於 PbTe 塊材  = 2.3 W m-1 K-1。4.31 在300 K， 直徑75 nm 和 217 nm奈米線晶格熱導率l數值分別為0.95 W m-1 K-1 和 0.30 W m-1 K-1，大約57 % 和86 %的比例相對小於PbTe塊材 (l = 2.2 W m-1 K-1) 4.32。在室溫下直徑217 nm的樣品其聲子貢獻接近與報導的超晶格薄膜PbSe0.98Te0.02/PbTe [l = 0.35 W m-1 K-1] 的值。4.33根據文獻報導熱傳導率 ，關於PbTe奈米線在各種直徑下[d =182, 277, 和436 nm]4.37和對這項工作的75 nm奈米線，PbTe奈米線其熱傳導率κ隨它的直徑縮小顯示尺寸依賴性即， 亦減小。考慮到聲子在邊界散射，其對降低一個奈米線的κ有一個相當大的影響，該結果清楚地表明，尺寸效應導致邊界散射的增加從而抑制聲子在PbTe奈米線中的傳輸。此外，綜括S， 和 的結果;在個別的PbTe奈米線實驗計算ZT，直徑75 nm ZT ∼ 0.0010.004在300350 K的溫度區間，直徑217 nm ZT ~ 0.090.2在300380 K溫度區間，而PbTe的塊材在300 K時ZT約為 ~ 0.25和最大ZT ~ 0.8於700 K。4.41 但是對直徑217 nm奈米線其ZT值仍高於Lee等人報導的PbTe奈米線 [ZT ~ 0.0054在300 K]。4.40 這樣一個增強的ZT可以部分地歸因於尺寸效應和奈米線的品質。此外，直徑142 nm奈米線的室溫磁阻 (MR) 在B = 2 T其MR ~ 0.8 %，它比所報導PbTe塊材的MR高出了許多[在B = 2 T ，MR ~ 0.2 %]。5.18
此外，直徑 d為200 nm 的 Bi2Se3 奈米線在300 K 時其功率因子 PF達到 39.32 × 10-5 W m-1 K-2。該功率因子PF值是高於任何先前的文獻報導，該文獻報導觀察在多晶Bi2Se3奈米複合材料組成的塊材 [2.8010.70 × 10-5 W m-1 K-2於表6.1] 。奈米線功率因子PF的提高可能來自於其載流子濃度增高的結果。奈米線的熱傳導率  ( 的測量垂直於c面) 在T = 300 K [ ~ 2.05 W m-1 K-1] 均低於單晶Bi2Se3塊材約3134 % (2.96 W m-1 K-1 或 3.1 W m-1 K-1於表6.1)。奈米線的l和e在300 K均低於單晶Bi2Se3塊材29 % 和37 %（l = 1.33 W m-1 K-1和 e = 1.77 W m-1 K-1）。6.22這意味著，在290320 K溫度區間，奈米線的  主要是來自於電子的貢獻。對於此d = 200 nm奈米線，計算ZT從所得到的S、 和  數據，ZT隨溫度增加而增加，在300 K時ZT大約0.06，它仍然是比單晶Bi2Se3塊材較低33 % [ZT ~ 0.17]6.21其主要是由於Bi2Se3奈米線的低導電率。這可能是由載流子濃度或遷移率的差異造成的。然而，這奈米線的ZT值仍高於任何曾經報導過的多晶Bi2Se3 塊材6.9, 6.11另外，我們對於直徑200 nm奈米線的熱電量測結果S,  及  也定量的與理論研究一致，該理論研究揭示了當減小拓撲絕緣體Bi2Se3奈米線的直徑，熱電傳輸行為會受到表面態的增加而增加。6.45我們的研究結果可以提供明確的指導方針利於未來對於PbTe和Bi2Se3化合物的納米結構熱電工作。

並列關鍵字

Lead Telluride ； Bismuth Selenide ； Nanowire ； Stress-Induced Method ； Thermoelectricity

參考文獻

References to Chapter 1:

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[1.13] H.P.R. Frederikse and W.R. Hosler, Phys. Rev. 108, 1136-1146 (1957).