【作者】 郭莉；

【导师】 宋俊峰； 孙洪波； 张永来；

【作者基本信息】 吉林大学 ， 微电子学与固体电子学， 2013， 博士

【摘要】 石墨烯（Graphene）是指单层的碳原子以六边形排列而成的二维碳晶体，自从石墨烯在实验上被发现以来，其独特的电、光、热、磁和机械等性能，使其成为了相关学科的研究热点。然而石墨烯在电子器件领域广泛应用目前仍存在一些难题，尤其在大规模制备，电学性质调控，器件制备和集成等方面仍然面临巨大的挑战。目前制备石墨烯的方法主要有机械剥离法、金属表面化学气相沉积（CVD）法、SiC衬底外延生长法以及氧化石墨烯（GO）还原法。相比而言，石墨氧化法具有独特的优势，如大规模制备，溶液工艺兼容性并且易于调控。然而大量含氧基团很大程度上限制了GO在电子器件中的应用，因此,对GO进行还原处理是必须的工序。最初的还原方法主要包括化学方法和热处理，前者不仅要用到有毒的还原剂（如肼），还会引起化学残留；而后者需要在惰性气体中进行高温煅烧（如>1000℃）。因此这两种方式显然存在与器件制备不兼容的问题。为了解决这一问题，在本文中，我们采用飞秒激光直写（FsLDW）和纳秒激光干涉的新方法还原GO。在还原过程中，激光加工不仅可以精确含氧量、电导率、禁带宽度、氮掺杂浓度，甚至可以制备各种微图案、形成分层的纳米结构。激光加工氧化石墨烯开发了石墨烯器件制造和集成的新方法。主要研究内容如下：1.采用飞秒激光直写技术还原GO薄膜，通过预先设计的程序控制激光焦点的扫描轨迹，成功制作石墨烯微电路。该方法不仅可以实现任意的图案化加工，还可以调变激光功率精确控制还原石墨烯的电阻率。这种微纳加工技术为石墨烯微电子器件的制备和集成提供了技术支持。2.细致研究了飞秒激光功率对GO禁带宽度的影响。由于GO的禁带宽度受控于含氧量，在实验中，我们通过调节激光功率来调控GO的还原程度，进而实现对其禁带宽度的调谐。通过第一性原理计算，解释了含氧量与还原石墨烯带隙的依赖关系，当GO中含氧量从0变化到100%，禁带宽度由0逐渐增大到2.74eV。实验上，我们对不同激光功率还原的GO进行了漫反射光谱(DRS)测试，证实了当飞秒激光功率从0增加到23mW，GO的禁带宽度在2.4eV到0.9eV范围内可调。结合飞秒激光直写技术,在预先准备好的电极之间直写导电沟道制作底栅石墨烯场效应晶体管（FET）。得到了具有一定开关比的p型FET。3.在NH3环境下，我们采用飞秒激光对GO进行还原的同时实现了N原子掺杂。利用X射线光电子能谱（XPS）分析了激光功率对氮原子掺杂效率和掺杂类型的影响。结果发现，最高掺杂浓度可达10.3%，掺杂形式包括吡啶，吡咯以及石墨化的氮原子。对于不同的激光功率，不同掺杂类型的氮比例会发生变化。利用第一性原理计算分析了不同种类氮原子相应的形成能。通过测试FET的输出特性证实了掺杂使沟道的导电类型从p型向n型发生了转变。4.采用纳秒激光双光束干涉（TBLI）技术对GO进行还原与图案化，在柔性衬底上制备湿敏器件。激光干涉GO薄膜形成石墨烯微米类光栅结构/纳米层状结构的多级微纳结构，大大促进了气体分子的吸附、传输和扩散，因此显著提高了器件性能。通过激光功率调控含氧基团含量，进而实现了对响应/恢复时间的调控。采用第一性原理进一步解释了湿敏器件响应恢复性能的调节机制。此方法无需任何表面活性剂，无掩膜，并且可以实现大面积制备石墨烯分层纳米结构，在石墨烯结构化器件制备方面显示出巨大的优势。综上所述，本文以研究激光对GO的还原为基础，开发了一系列GO激光微纳加工与器件制备的方法。采用飞秒激光对GO薄膜进行直写还原和图案化制备石墨烯微电路；通过改变激光功率实现其禁带宽度的精确调节；在氨气中对GO进行飞秒激光直写实现氮原子掺杂，引起RGO导电类型从p型向n型的转变；通过纳秒激光干涉GO加工分层纳米结构。我们利用以上方法分别进行了器件的制作，把氧化石墨烯在微电器件领域的应用推向了新高度。更多还原

【Abstract】 Graphene, a single-atom-thick two-dimensional carbon crystal, has be consideredas a rising star on the horizon of material science, modern physics and relateddisciplines due to its unique and excellent electrical, optical, thermal, magnetic andmechanical properties. However, some problems with respect to their applications inthe field of electronics, such as mass production, band gap opening, device fabricationand integration are still challenging. Up to now, there have been four typical methodsthat could be used to prepare graphene, including mechanical exfoliation, chemicalvapour deposition(CVD) on metal surface, epitaxial growth on SiC substrate and thereduction of graphene oxide. In comparison, chemical oxidation of graphite (thepreparation of graphene oxide, GO), shows some unique advantages over otherapproaches, such as large-scale preparation, solution processing compatibility andtractable modification. However, the presence of abundant oxygen groups makes GOa isolator, and significantly restricts its applications, and therefore, the demands onthe methods used for GO reduction become critical. In the beginning, methods for GOreduction mainly include chemical and thermal treatments, the former approachmakes use of toxic reducing reagent (e.g., hydrazine), whereas the later methodresorts to high temperature annealing in inert gases (e.g.,>1000°C). However, thesereduction processes suffer from poor compatibility with the device fabrication. Tosolve these problems, in this paper, femtosecond laser direct writing(FsLDW) andnanosecond laser interference have been adopted for the reduction and patterning ofGO towards the development of graphene-based micro-devices. With the help of laserprocessing, exquisite control of GO films over the oxygen content, conductivity,bandgap, N-doping concentration, micropatterns and even the formation ofhierarchical could be achieved easily. In this regard, new methods that could be used for the fabrication and integration of graphene-based devices have been successfullydeveloped. The main research contents are listed as follows:1. Graphene microcircuits have been successfully created on graphene oxide filmsby femtosecond laser direct writing (FsLDW) induced reduction of GO according tothe preprogrammed patterns. Any desired patterns could be directly created bycontrolling the trajectory of the laser focus; the resistivity of these microcircuits canbe precisely controlled by tuning the laser power. This micro-nanofabricationtechnology laid the foundation for the preparation and integration of graphene-basedmicro-electronic devices.2. We have carefully studied the influence of femtosecond laser power on thereduction degree of GO. Since the bandgap of GO depends on the residual oxygencontents, in our experiments, by tuning the reduction degree of GO through thecareful control of laser power, band gap of the the reduced GO (RGO) could bemodulated in a certain range. Through the first-principle study, the origin of GO bandgap tailoring is explained. Theoretically, the variation of bandgap from0to100%oxygen coverage in GO is in the range of0to2.74eV. Experimentally, we measuredthe diffuse reflectance spectra (DRS) of RGO reduced by different laser powers，which confirmed that the band gap of GO has been modulated in the range of2.4to0.9eV by tuning the femtosecond laser power from0to23mW. Combined withFsLDW technology, a RGO channel could be post-fabricated between two pre-coatedelectrodes for the fabrication of bottom-gate graphene FETs, and p type FETsbehavior with a certain on-off ratio are obtained.3. We have demonstrated the N doping and simultaneous reduction of GO byFsLDW in NH3atmosphere. We investigated the effect of laser power on theN-doping efficiency and N-bonding types by XPS. Experimental results showed that atotal nitrogen concentration as high as10.3%in the form of pyridinic, pyrrolic andgraphitic has been achieved. The ratio of different N-bonding types changes withdifferent laser power. We confirmed that doping can change the conductive type ofchannels from p to n, and finally, n type FETs behavior has been observed based onour N-doped RGO.. 4. We have fabricated humidity sensing device on flexible substrate bytwo-beam-laser interference (TBLI) reduction and patterning of GO. HierarchicalRGO nanostructures were formed after laser interference treatment of GO, whichholds great promise for gaseous molecular adsorption, and thereby significantlyincreases their sensing performance. By tuning the laser power, the content of oxygenfunctional groups could be changed within a certain range. The modulation ofoxygen-group content gives the feasibility for controlling response/recovery time. Toget further insight into the mechanism of the tunable response/recovery property ofour humidity sensors, first principle study has been carried out to give an essentialexplanation. This method is a surfactant-free, mask-free and facile approach to theproduction of large-area hierarchical micro-nanostructures on RGO films, and thusshows great potential for fabrication of future graphene-based microdevices.In summary, we have successfully developed a series of laser-related methods forthe reduction, patterning and nanostructuring of GO towards the fabrication andintegration of graphene-based devices. FS laser has been used to fabricate graphenemicrocircuits by direct reduction and patterning of GO films. Band gaps of reducedGO could be precisely modulated by controlling the laser power. N-doping can beachieved by FsLDW of GO in ammonia atmosphere, caused the transition from thep-type to n-type of RGO. Hierarchical nanostructures were formed after nanosecondlaser interference treatment of GO. We fabricated graphene-based devices using theabove-mentioned methods, they hold great promise for the wide application of GO inmicro-electronic devices.更多还原