包覆磁性合金之碳奈米結構及其性質

Abstract

為了增加奈米結構材料之應用範圍，例如製作磁記憶媒體，本研究以觸媒輔助電子迴旋共振化學氣相沉積法(ECR-CVD)利用CH4及H2為反應氣源，於矽基材上成功的合成鑲埋有磁性顆粒的碳奈米材料。而磁性之觸媒包括FePt, CoPt, Nd2Fe14B 和 Fe薄膜以及FeNi厚膜。主要之製程參數包括氣源中之氫氣含量、氫電漿前處理、基材偏壓、沉積溫度以及電漿導流板之施加。合成含磁性顆粒之奈米結構利用掃描式電子顯微技術 (SEM) 、穿透式電子顯微技術(TEM)、高倍率電子顯微技術(HRTEM)、拉曼光譜技術 (Raman spectroscpy) 、振動試樣磁量儀 (VSM) 以及磁力顯微技術 (MFM)來分析其形貌、微結構、鍵結以及磁性質。並利用超音波丙酮浴之震盪來比較其附著特性。 有關觸媒的影響方面，實驗結果顯示在相同之沉積條件下，不同之觸媒將影響管數密度、管長、碳膜之形成、觸媒和碳管之附著力以及碳管之種類。造成這些結構和性質不同之原因，推測是因為碳在不同觸媒中之溶解率以及氫電漿對碳管以及碳膜之蝕刻率的不同所造成的。依據目前之結果，最密之碳管管數密度高達134 Gtubes/inch2 (以Fe觸媒輔助成長之碳管) ，最長之碳管(或說是成長速率最快之碳管)則為2100 nm (Nd2Fe14B觸媒輔助成長15分鐘)。而對於某些領域之運用，將碳管頂端之觸媒顆粒去除是重要的，而本實驗亦成功的利用特定觸媒輔助成長之碳管，配合超音波之丙酮浴輕易的將觸媒顆粒去除。 關於電漿導流板之效應，藉由通在基材上放置具有負偏壓之導流板可以控制電漿之流向，使原本與基材夾90∘角成長之碳奈米管可藉此令其與基材夾45∘角成長。同樣的，亦可藉由電漿導流板之施加使得原本無續排列之海草狀碳片得以平行排列。 至於其它的製程參數之效應，依實驗結果指出，氫氣流量及基材偏壓是影響不同奈米結構(例如：碳管和碳膜)之蝕刻速度差異的主要因素。然而，偏壓造成之蝕刻較具方向性，而氫氣流量卻是無方向性的。在低氫氣流量之情形下將造成海草狀碳奈米結構或包覆於碳管外之碳奈米結構，而在較低之負偏壓之下則會造成碳膜厚度之增長。另外，結果也顯示氫電漿前處理之作用基本上是攻擊觸媒薄膜之表面，使產生均勻分散之奈米觸媒顆粒以作為成長碳管之觸媒。 對於磁性之影響，根據實驗結果，包在碳管中之磁性觸媒顆粒略大於最佳臨界尺寸或單磁域晶粒之尺寸(35 nm, 或10~100 nm之直徑) ，而在顆粒尺寸等於單磁域晶粒尺寸時將有利於獲得較高之矯頑磁力。在較高之沉積溫度下，碳奈米管會由於觸媒晶粒尺寸之變小而提高其矯頑力。目前經由鐵觸媒輔助成長之碳奈米管所獲得之最高矯頑磁力是750 Oe，這樣的結果可媲美於目前文獻中的相關研究。由於具有高長寬比觸媒形狀以及磁性退火步驟提供了形狀異向性以及誘發異向性的優點，因此在本製程下垂直與水平於基材方向之矯頑磁力之差值可以高達300 Oe。使用磁力顯微技術可取得這些獨立且均勻分散之磁性顆粒的影像，此結果也展示了在磁記憶媒體方面之應用潛力。

To enlarge the application areas of the nano-structured materials, such as applications in magnetic recording media, the well-aligned carbon nano-structures encapsulating with magnetic catalyst particles were successfully synthesized on Si wafer by ECR-CVD method with CH4 and H2 as gas sources. The magnetic catalysts, including FePt, CoPt, Nd2Fe14B and Fe thin films, and FeNi thick film, were studied. The main process parameters include hydrogen content in the gas sources, hydrogen plasma catalyst pretreatment, substrate bias, deposition temperature and plasma flow guiding. The magnetic properties, morphologies, microstructures and bonding structures of the magnetic catalyst-assisted carbon nanostructures were characterized by VSM, MFM, AFM, SEM, TEM, HRTEM and Raman spectroscopy. The adhesion properties of nanostructures with the substrates were qualitatively compared by ultrasonic agitation in acetone bath. Regarding effects of catalyst materials, the results show that at the same deposition conditions, different catalysts can produce carbon nanotubes (CNTs) with different tube number density, tube length, carbon film formation, bonding between catalyst and CNTs, growth mechanism and type of CNTs. These differences in structures or properties may relate to the solubility difference of carbon in catalysts, etching rate difference between CNTs and carbon films by hydrogen plasma. In the present conditions, the maximum tube number density can go up to 134 Gtubes/inch2 for Fe-assisted CNTs. For Nd2Fe14B–assisted CNTs, the longest tube length can reach 2100 nm for 15 min deposition time, which is roughly corresponding to the highest growth rate. For certain applications, if the removal of catalysts from tips of CNTs is required, it can easily be achieved by selecting proper catalyst and combining with ultrasonic agitation in acetone bath. About effect of plasma flow guiding, the 90°-inclined CNTs was successfully modified to 45°-inclined CNTs by positioning a negatively-biased metal plate above the Si substrate surface to vary the plasma flow pattern. The results also show that the plasma flow guiding may be used to modify the seaweed-like nano-sheets from random orientations to parallel alignment. For effects of other process parameters, the results indicate that the hydrogen flow rate and substrate bias are essentially the factors governing the differential etching effect to different nanostructures, e.g. carbon film and CNTs. However, the etching effect is more directional for bias, and more isotropic for hydrogen plasma. A lower hydrogen flow rate favors formation of the seaweed-like carbon nanostructures, or CNTs surrounding with other carbon nanostructures. A lower negative bias voltage favors formation of the additional thicker carbon films. The results also show that the hydrogen plasma pretreatment of the catalyst-coated substrates is basically to attack the catalyst film to become the well-distributed nano-particles to act as catalysts of CNTs. Regard to the magnetic properties of the magnetic metal-encapsulated carbon nanostructures, the grain sizes of the magnetic particles (35 nm, or 10 ~ 100 nm in diameter) are greater than but close to the critical optimum size or single domain size, which favor a higher coercive force. A higher deposition temperature of CNTs results in a greater coercive force due to a smaller catalyst size, and the greatest coercive force can go up to 750 Oe for Fe-assisted CNTs at 715℃ deposition temperature, which is comparable with the reported values in the literature. The process also takes advantages of higher shape and induced anisotropy due to its higher aspect ratio and magnetic annealing effect. The coercive force difference between vertical and horizontal direction can reach 300 Oe in the present conditions. The results also demonstrate the potential applications in magnetic recording media that the isolated and well-distributed magnetic particles in the magnetic metal-encapsulated carbon nanostructures can be imaged by MFM micrographs.