氧化石墨烯和氫氧化石墨烯都有磁性——氫氧化石墨烯的磁性更強

【摘要】 作為一種「理想的」二維材料，石墨烯在物理學的各個領域都表現出與眾不同的特性，成為了當前最受關註的新材料之一。因為其兩套子晶格相互嵌套的特殊的對稱結構，石墨烯基面上的自旋擴散時受到的自旋-軌道相互之間的耦合作用非常微弱，從而擁有非常長的自旋擴散長度。這為石墨烯在自旋電子學器件方面的應用提供了良好的條件。

但是，完美的石墨烯本質上是完全非磁的物質，只能作為自旋傳輸的通道使用，極大限製了其在自旋電子學方面的應用前景。在石墨烯上引入局域磁矩，相互耦合的乃至磁有序的局域磁矩，將會擴大其在自旋產生、自旋註入、自旋傳輸和自旋識別等領域的應用範圍。

鐵磁性的石墨烯，作為一種新型的輕質磁性材料，也將會對磁記錄、磁存儲、磁開關等傳統領域帶來全新的革命。要在石墨烯中引入局域磁矩，本質上就是要打破其π對稱的特殊電子結構。常見的方法包括用高能粒子轟擊以產生空位、通過合適的方式進行剪裁以獲得 zigzag 邊緣、異質原子化學摻雜等等。

但迄今為止，磁性石墨烯材料的實驗研究面臨著比飽和磁化強度低、可控性和重復性差、磁性來源不明晰等諸多問題,阻礙了其進一步的研究。為了解決這個問題，在本論文中，我們從最常用到的石墨烯衍生物——氧化石墨烯 (GO) 著手，通過改變其製備條件和利用氨水對同一批次 GO 中的輕、重氧化部分進行分離，最終成功對氧化石墨烯的磁性進行了鑒別，明確了不同氧基團對石墨烯磁矩引入的差異；同時也表明，以 GO 作為初始樣品研究石墨烯磁性的實驗中，氧官能團的作用不容忽視。

接著，我們通過對氧化石墨烯進行熱退火，去除不穩定的含氧基團保留熱穩定性好的羥基，得到了羥基化石墨烯 (OHG)，並逐步確立了羥基在石墨烯磁性中扮演的重要角色。最後，我們對羥基化石墨烯薄膜進行了磁輸運方面的測試，並對石墨烯磁輸運方面的有前景的材料進行了展望。

本論文的主要研究內容及結果如下：

1）系統性研究了影響GO磁性的因素，並用氨水分離出了同一批次的GO溶液中輕、重氧化程度不同的兩部分，對其磁性進行測量，明確了氧化程度的不同是影響GO磁性的原因。另外，我們也發現超聲時間、酸處理的不同、不同轉速的分離等也能造成GO磁性的差異。通過控製氧化時間，我們能製備出比飽和磁矩高達 1.2 emu/g 的大片狀結構的 GO。

2）首次提出了一個結論：石墨烯片狀結構基面上的 sp3 型缺陷是比邊緣型缺陷更為有效的引入磁矩的手段。以GO作為研究對象，擁有大量邊緣態缺陷的小尺寸的碎片，其比飽和磁化強度要比大片狀的擁有較多 sp3 型 OH 基團的GO小得多。熱退火除去不穩定的含氧官能團，保留sp3型的OH，樣品的比飽和磁化強度會進一步升高。

3）將 OH 與其他 sp3 型缺陷引入磁矩的效率作了對比，認為 OH 是一種更好的石墨烯磁矩引入者。OH 引入磁矩效率可以達到 217μB/1000 OH ，應的羥基化石墨烯磁矩濃度能達到 4.42 μB/1000 C，比飽和磁化強度能達到 2.41 emu/g。羥基引入的部分磁矩可以耐受 900 ℃ 高溫。從磁矩引入效率和磁矩的穩定性來說，OH 遠比 H 和 F 引入的 sp3 型磁矩優越；例如F，引入磁矩效率最高～3μB/1000 OH，耐受溫度不超過 400 ℃。

4）羥基引入磁矩的行為和規律進行了研究。在GO樣品中，羥基常常以 4-5μ B 的磁團簇為基本單元來貢獻磁矩。我們建立了一個具有高度對稱性的 (OH) 7模型來解釋這種現象。這種磁團簇的穩定存在被認為與(OH) 7基團的對稱性、褶皺的存在以及石墨烯的表面拓撲結構有關，GO中的環氧基或其他基團也會提高移動勢壘而加強磁團簇的穩定性。在羥基化石墨烯中，磁團簇之間的平均距離大於 6 nm，超過了石墨烯平面上磁矩之間有明顯耦合作用距離 (3 nm) 的限製，被認為是沒有長程磁有序出現的原因，同時這也是磁團簇能各自穩定存在而沒有塌縮至低自旋態或非磁態的原因。

5）羥基化石墨烯的磁性可以通過再次羥基化或摻氮來進行調控，其中，氮摻雜的樣品可以將其比飽和磁化強度進一步提高至 4 emu/g 以上。但是，氮和羥基的共同貢獻並沒有導致長程磁有序的可控出現。

6.我們通過旋塗GO溶液，在 Si02/Si 片上獲得 GO 薄膜，再通過低溫熱退火得到完整的～100 nm 厚羥基化石墨烯薄膜。這樣的薄膜展現出強局域態的電荷輸運特性，其電阻-溫度關系符合二維變程跳躍 (2D-VRH) 機製；但我們沒有發現和稀氟化石墨烯中類似的由 sp3 型磁矩導致的巨負磁阻效應。這可能與羥基化石墨烯薄膜中導致電荷和自旋散射的晶界、孔洞、多層結構和其他含氧官能團有關。總之，我們成功對 GO 的磁性實現了鑒別，確立了 sp 型缺陷是更有前景的磁矩引入方式。而且發現，羥基能夠在石墨烯中高效穩定引入磁團簇。這些發現將會促進石墨烯作為輕質鐵磁性材料和自旋電子學器件應用的研究。

【Abstract】 As an ideal two-dimensional material, graphene is now one of the biggest concerns for its extraordinary performances in many fields. Due to the special symmetry of its bipartite structure, the interaction between the spin and orbit is very weak, and thus the spins could diffuse on the basal plane of graphene to a long relaxation length, which foundates the favorable conditions for its spintronics applications. However, pristine graphene could be only used as spin-transport channel because it is intrinsically non-magnetic and lack of local magnetic moments, and therefore,to induce localized magnetic moments in graphene is very important for spin generation, spin injection, spin transport and spin recognization, etc. The emergence of coupled magnetic moments, or even magnetic ordering would further expand its application prospect. Furthermore, light-weighted graphene ferromagnet would bring new revolution in the traditional fields of magnetic recording, magnetic memory and magnetic switch, etc. In essence, to induce localized magnetic moments in graphene is to break down its π-symmetric electronic structure. Such methods mainly include high-energy paticle irradiation to generate vacancy, zigzag edge tailoring and adatom chemisorption, etc. However so far, the weak magnetization, poor controllability and repeatability, and ambiguous magnetic source make the whole topic of magnetic graphene conflicting and hinder its deeper research. Motivated by the inherent impetus to address the situation, in this work, we choose the most commonly used starting graphene derivative, graphene oxide (GO) as the object to systemtically study its magnetic properties. Through altering the preparation conditions and seperating different oxidation-degree parts of the same-batch GO by ammonia washing, we found the oxidative degree is the key factor to influence the magnetic properties of GO, and the type, content and location of oxygen groups would as well depend on the oxidation. Next, by thermal annealing to remove unstable oxygen groups to leave stable hydroxyl groups(OH), we obtained hydroxylated graphene(OHG), and found OH groups is a superior magnetic source to induce magnetic moments on the basal plane of the graphene sheet. In the end, we measured the magnetic transport properties of OHG, and provided a glimpse on the promising graphene derivatives on the spintronics. Our results in this work are outlined as follows:

1.We systematically studied the factors which could influence the magnetic properties of GO, and divided GO into two parts with different oxidation degree by ammonia wash. The oxygen groups are identified as a primary reason to influence the magnetic properties of GO. The supersonication duration, acid treatment and centrifugal speed could exert influence on it as well. By controlling the oxidation time to prepare lightly oxidative sample, we obtained the large-sheeted GO with a magnetization as high as 1.2 emu/g.

2.We put forward a significant conclusion for the first time:sp3-type defects on the basal plane of the graphene sheet is more competitive on inducing magnetic moments in graphene than edge-type defects. For GO, the heavily oxidative debris which rich in edge-type defects has a much lower magnetization than the relative lightly oxidative large sheet, which has more sp-type OH groups. By thermal annealing to remove unstable oxygen groups to leave stable OH group, the saturated magnetization of GO could be increased further.

3.Compared OH with other sp3-type defect inducer,we considered OH is more effective to induce magnetic moments in graphene. The magnetic inducing efficiency of OH could be as high as 217 μB/1000 OH, and the corresponding spin density could be 4.42μB/1000 C.A part of magnetic moments could survive at the high temperature as 900 ℃. OH is superior to other sp3type defect inducers on both the inducing efficiency and stablity, for example,the highest inducing efficiency of F is～3μB/1000 OH and the magnetic moments could only withstand the temperature no more than 400 ℃.

4. We studied the coupling properties of the magnetic moments induced by OH in graphene. In GO,the magnetic cluster with 4-5 μB is usually induced as elementary unit by OH groups. A highly symmetric(OH)7 cluster was sketched to explain the phenomenon. The stability of magnetic cluster was enhanced by the existence of ripples, surface topology and the coincidence of the structure symmetry. Moreover,epoxy or other groups on GO would increase the migration barrier to contribute the stablity as well. In OHQ the average space between the adjacent magnetic cluster is larger than 6 nm, which exceeds the space that obvious inter-cluster interaction could apply(3 nm). This could be the main reason why no signal of magnetic ordering was observed, and at the same time,the weak interaction ensures the magnetic cluster exist independently to not collapsing into nonmagnetic or low-spin state.

5.The magnetic properties of OHG could be tuned by importing extra OH groups or nitrogen doping, and the saturated magnetization of nitrogen-doped OHG could be further increased to above 4 emu/g. However, the co-contribution of N and OH did not lead to the existence of controllable magnetic ordering.

6.By spinning GO solution on SiO2/Si substrate followed by hypothermal annealing, we obtained OHG thin film with the thickness of～100 nm. The electronsin the film is strongly localized,and the transport could be described as a two-dimensional viriable range hopping(2D-VRH) model. No colossal magnetoresistance which appears in the diluted fluorinated graphene was observed. This may relate to the scattering centers such as boundaries, vcancies, multi-layer structure and other oxygen groups in the OHG thin film. In summary,we successfully identified the magnetic properties of GO, and drew a conclusion that sp3-type defect prevail over other type defects on inducing magnetic moments in graphene. We also found that, OH groups could highly effectively contribute robust magnetic clusters to graphene. Our findings would push forward the research of graphene light-element ferromagnets and spintronics.

源自網站：http://cnki.cqgmy.edu.cn/KCMS/detail/detail.aspx?filename=1015317082.nh&dbcode=CDFD&dbname=CDFD2019