奈米結構熱電材料之組裝與介面工程

Abstract

熱電發電機(Thermoelectric power generation)可將天然或人工產生之廢熱直接轉換成電能，因此被視為減少石化燃料損耗，並從而抑制全球溫化之根本解決方案之ㄧ。然而，要製作具實際應用價值之高轉換效率熱電元件，至今仍是一項極為艱鉅的挑戰。熱電材料之奈米結構化(Nanostructuring)，已經於理論與實驗上同時被證明確實為一有效強化熱電效率之技術策略。奈米化所造成之材料尺度下降，經證明能直接引發量子侷限效應(Quantum confinement effect)，有利於Seebeck係數(Seebeck coefficient, S)與導電率(Electrical conductivity, σ)之提昇。此外，奈米化大量產生各種維度與尺度之介面或表面，除了可以有效散射聲子, 達到降低熱傳導係數(Thermal conductivity, κ)之目的，在特定的介面結構下，甚至可能進一步選擇性影響帶電載子之傳導行為，進而提升熱電功率因子(Power factor, 定義為S2σ)，故最終將有機會獲得期望之高熱電優值(Thermoelectric figure of merit, ZT,定義為S2σ / к)。 因此，在奈米化的框架下，本研究論文提出四種嶄新之組裝及介面工程，用以修飾或取代傳統之表面及介面，以期同時優化載子與聲子在熱電材料中之傳輸行為，達到最大幅度提升熱電優值之目標。 不同維度奈米晶體之規則組裝：在無模板與觸媒輔助下，利用脈衝雷射沉積(Pulsed laser deposition, PLD)技術，製備一系列碲化鉍(Bi2Te3)奈米結構薄膜，分別是由零維、一維、二維、乃至三維奈米晶體垂直規則組裝而成。藉由單一物理沉積技術同時製備如此多樣化維度之奈米晶體及其組裝薄膜為全球之首例。值得一提的是零維奈米粒子組裝薄膜展現極高之功率因子，高於至今已報導之各式碲化鉍奈米結構薄膜1至3個數量級。探究其原理主要是無氧化物及化學殘留物之介面與表面，加上高規則排列之奈米晶體，導致奈米薄膜之導電率大幅提升。 階層式奈米結構之超組裝：透過精確調控沉積參數，在自發成長碲化鉍磊晶層之上，碲化鉍奈米晶體進行多層次組裝形成一系列高度規則之階層式(Hierarchical)奈米結構。此新穎奈米結構具有極大之比表面積與比介面積。大量增加之表面結構可有效散射聲子，進而降低熱傳導係數，甚而有助於載子之傳輸。此外，磊晶底層則有助於更進一步提升導電率。因此，所得之導電率約高於各文獻報導約2至3個數量級。 具整合介面之高異向性奈米晶體組裝：高度整合之雙晶介面(Twin interface)對熱電材料來說是一理想的介面，這是由於其可根本解決一般高角度非整合(Incoherent)介面所引起之高阻抗，同時卻不致降低對聲子之散射。在此概念下，我們成功製備各種具有高度方向性及雙晶介面之碲化銻鉍(BixSb2-xTe3)奈米自組裝薄膜，並首次以實驗證明於碲化銻鉍中，基底面(Basal plane)與非基底面雙界介面之存在。相較於一般介面之碲化銻鉍薄膜，此些具有雙晶介面之奈米結構薄膜不僅具有極高導電率(~2700 Scm-1)與功率因子(~25 μWcm-1K-2)，同時亦具備較低之熱傳導係數(~1.1 Wm-1K-1)。 具異質介面之側向組裝奈米複合薄膜：相較於上述之同質(Homogeneous)介面，此主題是利用異質(Heterogeneous)介面來取代一般介面。在此嶄新概念下，利用簡易之製程製備各種側向組裝之碲化鎵/碲(Ga2Te3/Te)奈米複合薄膜，其中二維類週期性之碲化鎵奈米組裝薄膜被具有高導電率且相互連結之碲單晶所環繞，其擁有次奈米至微米尺度之多功能性介面可以全面地散射不同平均自由徑之聲子。此獨特碲化鎵/碲奈米複材薄膜之功率因子，大約高於傳統碲化鎵及碲材料約60倍，主要是來自於2至3個數量級之導電率提升。特別值得一提的是，密集且側向交錯異質介面可以有效地強化熱載子過濾及聲子散射效應，從而獲得可接受之Seebeck係數及極低之熱傳導係數。

Directly and efficiently converting natural or artificial waste heat to electricity by thermoelectric power generators is one of the fundamental solutions for reducing the consumption of fossil fuel and thus suppressing the deterioration of the climate due to global warming. Thus far, realizing practical thermoelectric devices with sufficiently high conversion efficiency, however, remains an enormous challenge. Nanostructuring has been theoretically and experimentally demonstrated as a promising strategy to potentially induce quantum effects on specifically benefiting the Seebeck coefficient (S) and electrical conductivity (σ). The abundantly formed interfaces involving various dimensions, topographies, structures and scales would not only significantly suppress the thermal conductivity (к) via enhanced phonon scattering, but even selectively modulate charge carrier transporting in matters under specific conditions and thus finally achieve an applicable level of thermoelectric figure of merit ZT defined as S2σTк-1. In this dissertation, under the frame of nanostructuring, we proposed four innovative assembling and interfacial engineering to modify or alternate conventional surfaces and interfaces for simultaneously optimizing the transporting behaviors of charged carriers and phonons in maters and thus approached a greatest enhancement in ZTs. Orderedly assembling of nanocrystals with various dimensions: We have successfully fabricated a series of bismuth telluride (Bi2Te3) nanostructured films respectively and uniformly composed of orderly aligned zero-dimensional (0-D) nanoparticles, 1-D nanorods, 2-D nanoflakes, and 3-D nanocanyons by using pulsed laser deposition (PLD) with the absence of any template and catalyst. Such many highly-uniform nanostructured Bi2Te3 films are the first time prepared using single facile deposition technique. It is worth noting that the 0-D nanoparticles assembled film exhibits an extremely high power factor which is about 1 to 3 orders of magnitude higher than the reported values. The very clean interfaces and surfaces as well as the orderly aligned nanocrystals play as key roles for the present phenomenon. Superassembling of hierarchical nanostructures: By precisely controlling the deposition parameters, 0-D to 2-D Bi2Te3 nanocrystals are secondly assembled into a series of hierarchical nanostructures on the epitaxial Bi2Te3 bottom layers. The spontaneously formed innovative superassemblies-on-epitaxy structures exhibit an extremely high surface-to-volume ratio and interface-to-volume ratio. The greatly created surface can effectively scatter phonons to have a relatively low thermal conductivity whereas its effects on the carrier transport are limited. In addition, the bottom single crystalline layer further enhances the electrical conductivity to reach an unexpected value. Orientedly assembling of nanocrystals with coherent interfaces: An innovative concept of twin-enhanced thermoelectricity was proposed to fundamentally resolve the high electrical resistance while not degrading the phonon scattering of the thermoelectric nanoassemblies. Under this frame, a variety of highly oriented and twinned bismuth antimony telluride (BixSb2-xTe3) nanocrystals were successfully fabricated. The significant presence of the nonbasal- and basal-plane twins across the hexagonal BiSbTe nanocrystals, which were experimentally and systematically observed for the first time, evidently contributes to the unusually high electrical conductivity of ~2700 Scm-1 and the power factor of ~25 μWcm-1K-2 as well as the relatively low thermal conductivity of ~1.1 Wm-1K-1 found in these nanostructured films. Laterally assembling of nanocomposites with hetero-interfaces: We described an innovative concept and facile approach in fabricating laterally assembled Ga2Te3/Te binary nanocomposite films which comprise 2-D quasi-periodic Ga2Te3 nanoassemblies surrounded by interlocking highly-conductive Te single crystals for comprehensively establishing subnano- to micro-scaled multi-style versatile interfaces. The distinct Ga2Te3/Te nanocomposite film exhibits a power factor about 60 times higher than the conventional Ga2Te3 and Te materials reported mainly due to the 2 to 3 orders improved electrical conductivity. Especially noteworthy, the dense lateral heterogeneous interfaces effectively strengthen hot carrier filtering and phonon scattering for obtaining the comparable Seebeck coefficient and ultralow thermal conductivity.