脈衝雷射沉積有序組裝硒化銻低維度奈米結構於熱電轉換之應用

Abstract

熱電材料之奈米化工程已被理論及實驗上証明，確實能有效提升材料熱電優值（Figure of merit, ZT定義為σS2T /κ）。熱電材料奈米化後，生成之表面或介面能大幅度降低熱傳導係數(κ)，同時因為奈米尺寸引發之量子侷限(Quantum confinement)效應，將有機會同步提升功率因子(σS2)，從而達到提升熱電優值的目標。相較於已被廣泛研究之碲化鉍(Bi2Te3)塊材，硒化銻(Sb2Se3)塊材本身具有更高之Seebeck係數(1800 μVK-1)與較低熱傳導係數(~1 Wm-1K-1)，故被視為繼碲化鉍後，另一個極具發展潛力之常溫型熱電材料系統。然而，至目前為止，硒化銻奈米結構之熱電相關報導仍屬少數。因此，硒化銻奈米結構之成長，成為一個非常具吸引力及重要之研究主題，藉此可望能達到潛在優異之熱電優值。 本研究成功利用脈衝雷射沉積技術，於不使用模板與觸媒的前提下，於SiO2/Si基板上，製備一系列有序組裝硒化銻低維度奈米結構，其組成單元之形貌多樣，包括奈米齒狀、奈米翅狀、奈米柱狀、奈米板狀、奈米鑷狀及奈米管狀等一、二維奈米結構。其中除了奈米柱狀結構為非晶結構之外，其餘皆為高品質單晶結構。另發現此些奈米結構有序組裝而成之薄膜，皆具有特殊之結晶優選方位。利用結構區帶模型(Structure-zone model, SZM) 與自我觸媒輔助氣-液-固 (Vapor-liquid-solid, VLS)之成長機制可分別合理解釋低溫(<300 oC)連續性與高溫(>300 oC)不連續性奈米有序組裝薄膜之成長。在變溫導電率 (20‒400 oC)量測方面，發現硒化銻奈米翅狀薄膜具有極佳之導電率(20‒750 Sm-1)，趨近文獻所提單根奈米線之值(852 Sm-1)。另外，在室溫下，奈米齒狀結構薄膜之導電率(~1.54 Scm-1)較多晶薄膜高4‒5個數量級，表示成功的改善硒化銻導電率。

Nanostructure engineering been theoretically and experimentally proven as a practial strategy for thermoelectric materialshas for effectively enhancing thermoelectric figure of merits, ZTs (defined as σS2T /κ). By nanostructuring, not only could the largely created surface or interfaces inhibit the thermal conductivity (κ), but the power factor (σS2) could be improved through the induced quantum confinement. Antimony selenide (Sb2Se3) has a higher Seebeck coefficient (1800 μVK-1) and lower thermal conductivity (~2.7 Wm-1K-1) compared with the widely studied bismuth telluride (Bi2Te3) bulk and is thus considered as a promising alternate of the room-temperature thermoelectric materials for the next generation. However, to date, thermoelectric data realted to the Sb2Se3 nanostructures have less been reported. Therefore, growth of nanostructured Sb2Se3 becomes a very attractive and important research topic for approaching potentially outstanding thermoelectric performance. In this work, by using pulsed laser deposition (PLD) techniques, a series of well-aligned nanostructured Sb2Se3 films was successfully prepared on insulated SiO2/Si substrates without prebuilt catalysts and templates. At least seven types of previously unreported Sb2Se3 nanostructures including nanoteeth, nanowings, nanorods, nanodecks, nanotweezers, nanocolumns, and nanotubes were reproducibly obtained via precisely controlling the substrate temperature and ambient pressures. The temperature-dependent growth seems to be reasonably explained by the well-known structure zone model (SZM) and self-catalyst enhanced vapor-liquid-solid (VLS) growth. Among these specimens, the Sb2Se3 nanowings show an excellent electrical conductivity of 750 Sm-1 at 400 oC, which is comparable to the reported value of an isolated nanowire (852 Sm-1). In addition, the room-temperature electrical conductivity of the Sb2Se3 nanoteeths is 4 to 5 orders higher that that of the non-nanostructured Sb2Se3 film. In this work we succeded in improving the electrical conductivity of Sb2Se3 film.