大面積定向性奈米碳管之成長機制及性質—觸媒輔助電子迴旋共振化學氣相沉積法

Abstract

本研究是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為反應氣源成功地合成大面積(4吋直徑)且具定向性之奈米碳管。所使用之觸媒材料為是過渡金屬元素包括Fe粒、Ni粒、Co粒、CoSix膜和Ni 膜。合成的奈米結構利用掃描式電子微顯鏡(SEM)、穿透式電子微顯鏡(TEM)、拉曼光譜儀(Raman spectroscopy)和I-V量測儀來分析其特性。 結果顯示，沉積的奈米結構材料包括 : 奈米碳管、藤蔓狀碳管、海草狀奈米碳片、花瓣狀奈米碳片及碳膜。形成不同奈米結構材料的關鍵因素如下列所示 : 觸媒種類及其施加方式、偏壓、溫度、反應氣體中氫氣的含量和沉積的時間等。 關於形成奈米碳管之必要條件包括 : ( 1 )需有適當形狀且具溶碳能力的觸媒、( 2 )需加負偏壓、( 3 )沉積溫度＞560 ℃及( 4 )反應氣體中之氫氣之添加，可形成結構更純化之碳管。至於其它奈米結構材料之成長機制如下所示 : 奈米碳管成長過程中，當觸媒失去其活性時(約15 min沉積時間)，藤蔓狀奈米碳管即開始形成。此即為觸媒停止溶入氣體中碳原子且不再析出碳原子至碳管(亦稱為觸媒毒化)。造成觸媒毒化的可能原因有( 1 )觸媒和碳原子反應形成碳化物、( 2 )觸媒表層全部被碳管結構所覆蓋。在這兩種情況下，氣相中的碳原子將被迫沿著管子的周圍擴散，形成其它不具觸媒輔助之奈米結構。其次海草狀奈米碳片需當基材在較高溫且需有負偏壓的施加和沒施加觸媒情況下形成之奈米結構。而花瓣狀奈米碳片需基材在較高溫且無加負偏壓的條件才能形成。進而言之， 製程溫度較低時，只能在基材上形成碳膜。 奈米碳管形貌(例如 : 管長、管徑、管形及管數密度)會受觸媒種類與其施加方式影響。不同觸媒對管長增長程度的影響依序為 : Ni膜＞Co粒＞Ni粒＞CoSix膜 ; 對於形成之最大管徑方面，它與觸媒之最大粒徑有密切關係，依序為 : Co粒＞Ni膜＞CoSix膜＞Ni粒 ; 至於觸媒對管數密度之影響方面，結果顯示在較小的觸媒顆粒尺寸與較密的分佈兩者組合下可得較高的管數密度，管數密度依序為 : Ni粒＞Co粒＞CoSix膜＞Ni膜 ; 在管形方面，以SEM觀察的結果，除了以Ni觸媒所成長的碳管呈波浪狀外，其餘觸媒所成長的碳管大致均相當筆直。其原因可能是Ni觸媒具有獨特的晶面，碳原子析出速率受不同晶面影響而相異所導致。 在奈米碳管的抗氧化方面，以Ni粒為觸媒所形成的奈米碳管比Co粒為觸媒者佳，這意味著鈷觸媒成長的奈米碳管具有較多的結構缺陷，此結果可由TEM証實。值得注意的是以Co粒為觸媒所成長的奈米碳管在氬氣的保護氣氛下400℃以上退火時，碳管的頂端之觸媒顆粒有膨脹呈香菇狀現象。其原因推測可能是在此沉積溫度下形成之飽合狀態的觸媒固溶體，在退火時析出多餘的碳原子或是觸媒的氧化效應所造成。 在奈米碳管的場效發射性質方面，結果顯示其場效發射與觸媒種類及施加方式有關。其場效發射優劣性為 : Co粒＞Ni粒＞CoSix膜＞Ni膜。比較以Co及Ni觸媒成長之碳管，前者具有較佳的場發射特性的原因，應與碳管形貌之綜合效應有關。以Co粒為觸媒所形成的碳管，因具有適當的管徑和碳數密度組合，可使碳管間之屏蔽效應(screen effect)最小化，使得有效場發射面積提高，且可補償碳管長細比較低所造成之電場增強因子之不足。換言之，最佳的場效發射特性可由降低管徑及控制適當的管數密度而得到。同理，此成法亦可適用於其它觸媒所成長的奈米碳管。Co為觸媒所成長的奈米碳管，在電場5.3 V/μm時，場效發射電流密度可達32 mA/cm2，其臨限電場Eth為4.2 V/mm。總而言之，此製程所合成大面積定向性奈米碳管擁有極優的場效性質，相當具有開發場效發射平面顯示器之潛力。

Large area (4-inch in diameter) well-aligned carbon nanotubes were successfully synthesized by using catalyst-assisted ECR-CVD method with CH4 and H2 as gas sources. The transition metals acted as catalysts include Fe, Ni and Co particles, CoSix and Ni thin films. The deposited nano-structures were characterized by SEM, TEM, Raman spectroscopy and I-V measurements. The results show that the deposited nano-structures include carbon nanotubes, rattan-like carbon nanotubes, seaweed-like carbon nano-sheets, petal-like carbon nano-sheets and carbon film. The following key factors determine which nano-structure prevails: catalyst type and its application method, bias, temperature, hydrogen content in the reaction gases and deposition time. About formation of carbon nanotubes, the required conditions include : ( 1 ) catalyst in proper forms and able to dissolve carbon atoms, ( 2 ) negative substrate bias, ( 3 ) substrate temperature > 560℃, and ( 4 ) hydrogen addition to form more purer carbon nanotubes. As to the growth mechanisms of other nano-structures, the rattan-like carbon nanotubes start to form after the catalyst loses its activity (about 15-min. deposition time). Where the catalyst stops to dissolve carbon atoms from the gas phase and ceases to precipitate carbon atoms to the tubes (also called “the poisoned catalyst”). The possible reasons may be: ( 1 ) the catalyst reacts with carbon atoms to form carbides, and ( 2 ) the catalyst surface is entirely covered by carbon nanotube structure. Under these two conditions, carbon atoms from the gas phase may be forced to diffuse along the tube surrounding to form other nano-structures without catalyst assistance. Regarding the seaweed-like carbon nano-sheets, they are formed under negative substrate bias and higher substrate temperature, but without catalyst applications. The petal-like carbon nano-sheets are obtained under higher substrate temperature but without negative bias application. Further more, the lower deposition temperatures may cause formation of carbon films. Regarding the morphologies of carbon nanotubes (i.e. tube length, diameter, shape and tube-number density), they are dependent on catalyst type and its preparation methods. Effect on lengthening tube length is in order of: Ni film > Co particle > Ni particle > CoSix film. Effect on the biggest diameter of tubes is in order of: Co particle > Ni film > CoSix film > Ni particle, which is closely related to the maximum size of catalyst particles. As to tube-number densities of nanotubes, a smaller particle size combining with a denser distribution of the catalyst particles on the substrate surface can result in a higher tube-number density. Under the present deposition conditions, the order of tube-number densities is Ni particle > Co particle > CoSix film > Ni film. All carbon nanotubes are relatively straight in shape under SEM examination except Ni catalytic-grown nanotubes, which are wavier in shape. This may relate to the fact that the Ni-catalyst possesses a unique feature of crystallographic plane dependence of carbon precipitation rate. As to oxidation resistance of carbon nanotubes, the resistance of the Ni catalytic-grown nanotubes is greater than the Co catalytic-grown nanotubes. This may imply that the Co catalytic-grown nanotubes contain more structure defects. This was supported by TEM examination. It is noted that the caps of tubes are expanded and become mushroom-like after oxidation above 400℃. This may relate to precipitation effect of extra carbon from the saturated catalyst solid solution formed at deposition temperature, or oxidation effect of catalyst. As regards field emission properties of the nanotubes, the results show that the emission properties depend on the catalyst type and its application methods. The order of emission current density of the nanotubes for various catalyst applications is Co particle > Ni particle > CoSix film > Ni film. By comparing the Co and Ni catalytic-grown nanotubes, the better emission properties for the former one must relate to a combination effect of the tube morphologies. The Co catalytic-grown nanotubes possess a proper combination of tube diameter and tube-number density to minimize the screen effect among neighbor tubes to enhance the effective emission area, which may compensate insufficient in field enhancement factor due to a lower aspect ratio of tube length-to-diameter. In other words, the best field emission properties can be improved by decreasing the tip radius of the tubes and manipulating a proper tube-number density. The same reasoning can be applied to other catalytic-grown nanotubes. For Co catalytic grown nanotubes, the current density can reach 32 mA/cm2 at 5.28 V/mm; and the threshold voltage can go down to 4.2 V/mm. In summary, this large area nanotube process and its property seem to indicate a promising basis for future researches in field emission display applications.