| 摘要: | 石墨烯作為第一個被成功製備出的二維材料，它的各項優異性質，從極高的表面積比到電子遷移率，吸引了廣大領域研究者們的目光。然而，缺乏能隙使得石墨烯的應用受限。近年，有許多研究致力於調控石墨烯的能隙。其中，藉由調控石墨烯的氧化與還原程度，進而調控電性是常見的方式之一。目前，已有許多還原氧化石墨烯的方式，包含化學還原劑的作用、熱還原、微波還原、光還原。還原程度對許多電子特性，例如：電子遷移率影響甚大，因此，能否有效移除石墨烯上的氧化官能基和了解氧化石墨烯的還原機制實為重要，若能直接觀測到還原過程，將不只對石墨烯的應用有所幫助，也能了解在二維材料上官能基的交互作用。
在我們團隊之前的研究顯示了微米級局部氧化石墨烯在二氧化矽基板上的還原動態學。首先以掃描探針顯影術為基底的局部陽極氧化術，在以化學氣相成積法成長出的石墨烯上，製造出微米級的局部氧化圖樣。接著以X光聚焦在這些氧化圖樣上，由X光所激發出的光電子提供能量，還原氧化的石墨烯。在還原的過程中，各種化學鍵的濃度由X光光電子能譜監控，藉以直接監測局部氧化石墨烯的還原動態。並且經由我們所觀測到，各鍵結的還原動態，我們提出一組相互耦合之微分方程作為還原機制的模型及擬合的速率方程，認為還原是由高鍵結能量的氧化相關的鍵結逐步轉換成較低鍵結能量的化學組成，其中最後一步驟作為還原反應中的速率限制階段為來自sp^3的修復sp^2結構，我們也能推測此步驟需要較大的能量去達成。
此研究想得知來自基板由X光所激發出的光電子對還原過程的貢獻，因此我們揀選了基板元素的鍵結能量略大於石墨烯且光學性質與原本使用的二氧化矽相近的氟化鈣為基板，用適當能量的X光進行還原，發現即使沒有來自基板的光電子，石墨烯本身的光電子也會觸發還原，但還原的途徑發生改變，我們推測是由於光電子的動能不足以達成由sp^3修復sp^2，而是以氧相關的鍵結直接修復。因此，我們提高X光能量以激發出同樣擁有原本實驗中來自二氧化矽的光電子相似的動能，進一步了解在不同基板上，由於X光能量不同，激發出不同動能的光電子，對氧化石墨烯的還原途徑與動態學。並且在還原前後，我們藉由微拉曼光譜，觀測這些局部石墨烯缺陷的結構；化學組成則以掃描式光電子顯影術與X光光電子能譜測得。
;Graphene, as the first two-dimensional material successfully produced, its excellent properties, from extremely high specific surface to electron mobility, has attracted the attention of researchers in the various field. However, the lack of band gap limits applicability in graphene device. Graphene oxide (GO) engineers band gap through introduction of sp3 bonding. Tuning the electric properties by controlling the degree of oxidation and reduction is one of the most popular method. So far, there are many methods evolving to reduce GO, e.g., chemical, thermal, and optical methods. The degree of reduction affects many electronic properties much, such as electron mobility. Therefore, it is important to effectively remove the oxidized functional groups on graphene. Hence, understanding the mechanism of reduction of GO is necessary for both the application of graphene and interaction of functional groups on a 2D material.
In our previous work, we have shown the reduction dynamics of micron-scaled defective oxidized graphene patterns done on CVD-grown graphene on SiO_2 by scanning probe lithography (SPL), which provides localized functionalization of graphene. These patterns were subsequently reduced by the irradiation of photoelectrons induced by a focused beam of soft x-ray. By in-situ monitoring the chemical configuration of the irradiated defects during the reduction process, the evolution of each oxygen functional group is resolved by scanning photoelectron microscope (SPEM) and x-ray photoelectron spectra (XPS). From the coupled reduction behavior of each bond, we proposed a model that describes the dynamics of reduction with a step-by-step process. That is C=O→C-OH→C-sp^3→C-sp^2. By the result of least-squares fitting, we find the rate constant of each step of reaction and we consider that C-sp^3→C-sp^2, the recovery of sp^2 is the limiting step of the whole process. We can also speculate that this step requires more energy to achieve.
In this study, we want to know the contribution of photoelectrons excited by X-rays from the substrate for the reduction process. Therefore, we select calcium fluoride (CaF_2) which has the slightly larger binding energy than graphene and similar optical properties of SiO_2. Using the appropriate energy of X-ray, it is found that the reduction still happens even without the photoelectrons excited from the substrate. The photoelectrons of graphene itself trigger the reduction, but the approach of reduction changes. We speculate that the kinetic energy of photoelectrons is not enough to achieve repairing sp^2 by sp^3, but directly repaired by oxygen-related bonds. So that, we increase the X-ray energy to excite the photoelectron from CaF_2 with the similar kinetic energy of the photoelectron from the SiO_2 in our original experiment, and investigate the reduction pathway and dynamics of oxidized graphene on different substrates by the photoelectron of different kinetic energy owing to the different X-ray energy. |
