A Thesis Submitted to Chongqing University in Partial Fulfillment of the Requirement for the Degree of Master of Science
By Beidou Guo Supervised by Prof. Liang Fang Associate Prof. Jianru Gong Major: Condensed Matter Physics
College of Physics of Chongqing University Chongqing, China May 2011
中文摘要
摘
要
石墨烯是由 sp2 杂化碳原子构成的一种具有蜂窝状六方点阵结构的二维纳米材 料，是构成其它维度碳材料的基础。石墨烯的长程 π 共轭电子，使其具有优异的 热学、机械和电学性能。因此，研究者对石墨烯未来在纳米电子学、材料科学、 凝聚态物理以及低维物理方面的应用产生了广泛的兴趣，但石墨烯在电子领域的 应用受限于它的零带隙特性。为了打开石墨烯的带隙，研究者探索了许多方法， 比如剪裁石墨成量子点、纳米带、纳米网格或者把石墨烯铺到特殊的衬底上，其 中一个可行的方法就是通过掺杂来调控石墨烯的电学性质，但本征石墨烯具有完 美的蜂窝状结构，很难通过杂质原子的掺杂来调控其电学性能，为此，本文重点 对 N+离子注入实现石墨烯的掺杂进行了探索。 本文制备了机械剥离和还原氧化两类石墨烯，利用光学显微镜、AFM、拉曼 光谱、傅里叶红外光谱、XPS、AES 等手段对石墨烯进行了表征；对两类石墨烯 分别进行了 N+离子注入和随后的退火处理，成功实现了两类石墨烯的掺杂；并制 备了相应的石墨烯场效应晶体管，研究了其电学性能。 得出的主要结果如下： ① 利用表征石墨烯的重要工具——拉曼光谱， 研究了石墨烯缺陷的变化与离 子注入剂量之间的关系。得到了合适的离子注入的剂量：1× 1014 cm-2，在此剂量 下，石墨烯表面会有适当的缺陷用来掺杂，并且这些缺陷可以通过退火来消除； ② 发现在氮气中退火样品的拉曼光谱谱峰的蓝移比在氨气中的明显。这可能 是因为在氨气中退火后氮原子掺杂进石墨烯，使得石墨烯产生较小的应变； ③ 采用 XPS 和 AES 检测了在氮气和氨气气氛中退火的离子注入石墨烯样 品。结果显示，在氮气中退火的样品中没有 N 信号，而在氨气退火的样品中有 N 信号。结果说明了离子注入的石墨烯通过在氨气中退火，实现了 N 掺杂； ④ 为了研究不同石墨烯样品的电学性质，制备了背栅石墨烯场效应晶体管。 结果显示，本征石墨烯场效应晶体管是双极晶体管，它的电导最小值位于正栅压 位置，说明石墨烯是 p-型掺杂。用在氮气中退火的离子注入石墨烯制备的场效应 晶体管的双极特性消失了，电导最小值仍然处于正栅压位置，还是 p-型掺杂。用 在氨气中退火的离子注入石墨烯制备的场效应晶体管显示出了双极特性，在真空 中它的电导最小值位于负栅压位置，表明是 n-型掺杂。 此外，本文还研究了还原石墨烯的掺杂，利用拉曼光谱和傅里叶红外光谱表 征了 N+离子注入后的还原石墨烯的结构，制备了场效应晶体管，并测试了其电学 性能。结果发现，N+离子注入还原石墨烯和本征石墨烯不同，N+离子与还原后的
III
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重庆大学硕士学位论文
③ XPS and AES were carried out on the irradiated graphene after annealing in N2 and NH3. The results of XPS and AES of samples annealed in NH3 showed N single, while there had no N single after annealing in N2. These results revealing that controllable N-doping in graphene was realized by NH3 annealing after N+ ion irradiation. ④ To investigate the electronic properties of different graphene samples, the graphene-based back-gate FETs were fabricated. Results showed that pristine graphene field effect transistor was bipolar transistor, and its minimum conductance point of FET located at positive voltage range, indicating that the garphene was p-type property. The biolar property of FET mabe by irratiated graphene after annealing in N2 disappeared, and the minimun conductance located at positive votltages, indicating p-type property. While the bipolar paoperty of FET made by irradiated graphene after annealing in NH3 restored, and the minimum conductance located at negitive voltage range, indicating n-type property. We also studied the doping of reduced graphene oxide. We investigated the structure of reduced graphene oxide after N+ ions irradiation using Raman specroscopy and FTIR. At last, we fabricated FET using reduced graphene oxide bafore and after N+ ions irradiation, and measured its electrical properties. We found that N+ reacted with the functional group on reduced graphene oxide. We also found that the threshold voltage could be controlled by N+ ions irradiation. And the carrier was electron when the gate voltage was zero, indicating that the sample was n-type property. Keywords: Graphene, doping, Raman spectroscopy, reduced, Field-Effect Transistor.
I
重庆大学硕士学位论文
氧化石墨烯表面的官能团发生了反应，从而起到了掺杂的效果。通过场效应晶体 管的测试发现 N+离子注入具有调节晶体管阈值电压的功能。 关键词：石墨烯，掺杂，拉曼光谱，还原，场效应晶体管
II
英文摘要
ABSTRACT
Graphene, a two-dimensional (2D) network of sp2 hybridized carbon atoms packed into hexagonal structure, is a basic building block for graphitic materials of all other dimensionalities. Since long-range π-conjugation in graphene yields extraordinary thermal, mechanical, and electrical properties, an enormous effort has been devoted to exploration of its many applications in nanoelectronics, materials science, condensed-matter physics, and low-dimensional physics. However, most electronic applications are handicapped by the absence of a bandgap in the intrinsic material. In the quest to opening and tuning an energy gap in graphene, various approaches have been developed to improve the semiconducting properties, exemplified by forming confined geometries of quantum dots, nanoribbons, and nanomesh, or binding graphene to particular substrates. One of the most feasible methods to control the semiconducting properties of graphene is by doping, which is a process intentionally used to tailor the electrical properties of intrinsic semiconductors. In this paper, we studied the doping of graphene and reduced graphene oxide by N ions irradiation. We studied the N-doping graphene firstly. As we know, pristine graphene has perfect honeycomb structure, and it is difficult to introduce hetero atoms into graphene and control the electrical properties of graphene. In this paper, we carried out N+ ions irradiation on mechanical exfoliated single-layer pristine graphene to induce the defects. Then, controllable N-doping in graphene was realized by NH3 annealing after ion irradiation. Taman spectroscopy is a much more important tool for the characterization of graphene. We studied the evolution of the defects of graphene with the increasing fluence of ions irradiation using Raman spectroscopy. At last, we got the proper influence of ions irradiation: 1× 1014 cm-2. Under this fluence, there were enough defects in graphene for doping, and these defects can be also restored after annealing. ② We compared the Raman spectroscopy of irradiated graphene after annealing in N2 and NH3, respectively. The peaks of Raman spectroscopy of irradiated graphene after annealing in N2 showed more blue shift than that annealed in NH3. The resaon was that the N atoms were substitutional doping in graphene after annealing in NH3, resulting in lower stress and strain in graphene.