【作者】 吴军；

【导师】 万建国；

【作者基本信息】 南京大学 ， 凝聚态物理， 2018， 博士

【摘要】 石墨烯的许多物理化学性质对石墨烯的层数和堆垛次序十分敏感。众所周知,单层石墨烯是零能隙的半金属,低的开关比限制了石墨烯在场效应晶体管等电子器件领域应用和发展。然而,双层或少层石墨烯的电子结构能够在垂直电场作用下调控,实现在价带和导带之间打开一个带隙或者使其能带重叠。特别是Bernal堆垛的双层石墨烯在双栅垂直电场的作用下具有绝缘态,为石墨烯在电子工业中的应用开辟了广阔的应用前景。大规模工业制备层数可控的高质量石墨烯是石墨烯在微电子和光电器件领域中应用的先决条件。然而,直接在铜箔上制备高质量的均匀双层石墨烯仍然是一个巨大的挑战。本论文以双层石墨烯为研究对象,在理解晶畴、层数、堆垛以及缺陷等石墨烯结构形成机制的基础上,实现化学气相沉积(CVD)双层石墨烯的可控制备,并进一步研究其结构对石墨烯电学输运性质的影响规律。主要的研究内容和研究结果如下:Cu表面的“自限制”效应使得直接在铜箔上快速制备大面积、高迁移率、高AB堆垛比率的双层石墨烯面临一个巨大挑战。我们报道了一种低能耗且易于快速制备Bernal堆垛的CVD双层石墨烯的新方法,采用固体碳源聚苯乙烯代替传统甲烷气体实现六角状双层石墨烯单晶到晶圆级薄膜的可控制备。通过监控双层石墨烯的生长过程,揭示了双层石墨烯的同步生长动力学机制。研究表明了固体碳源在制备同步生长双层石墨烯中的重要作用。这种同步生长动力学使得双层石墨烯的覆盖率及AB堆垛比率均达到了近100%。电子输运测量结果表明制备的双层石墨烯单晶和薄膜在室温下具有较高的载流子迁移率,分别达到了 5700和2200 cm2 V-1 s-1。高的载流子迁移率、可调的能带结构以及低能耗大规模的生产能力为双层石墨烯在纳电子学等领域应用打下了基础。然而,CVD石墨烯薄膜具有本征的多晶结构,石墨烯晶畴间形成无序的晶界,这对石墨烯应用来说可能既是优势也是挑战。一方面,石墨烯晶界通常降低多晶石墨烯薄膜的电学性质和机械强度,从而限制了 CVD石墨烯薄膜的众多应用;而另一方面,石墨烯晶界表现出增强的化学活性,成为在传感器、作为一维材料合成的模板等领域的潜在应用成为可能。因此对石墨烯晶界的结构和性质的深入研究就显得十分重要。我们研究了在多晶铜箔上利用常压化学气相沉积法制备的双层石墨烯单晶的电学性能,以及双层石墨烯晶畴之间形成的晶界对石墨烯电学性能的影响。磁输运测量观测到了双层石墨烯的线性磁电阻现象。双层石墨烯晶界处具有弱的Raman D峰,阻碍了电荷输运,降低了电子退相干长度。这一研究结果表明晶界在CVD石墨烯电荷输运中的重要影响。更多还原

【Abstract】 Most of the physicochemical properties of graphene are sensitive to its thickness and stacking order.It is well known that monolayer graphene is intrinsically a semimetal with a zero bandgap,which is unfavorable for its application in electronic devices because of the difficulty in turning off the current in graphene-based transistors In contrast,the electrical band gap opening or band overlap generated in few-layer graphene can be well modulated by vertical electric fields.In particular,the gate-induced insulating state in Bernal bilayer graphene opens a promising avenue for application of graphene in the electronic industry.Fabrication of industrial-level high-quality graphene with a controllable layer number is of fundamental importance for realizing its potential applications in nanoelectronic devices.Nevertheless,direct fabrication of high-quality large-area-uniform bilayer graphene on Cu foil is still a formidable challengeIn this work,we focused on the chemical vapor deposition(CVD)bilayer graphene.Based on the understanding the formation mechanism of domain,layer number,stacking order and defect,the controllable growth of CVD bilayer graphene was achieved,and then the effect mechanism of structure on the electrical transport properties of bilayer graphene was studied.The main research contents and results are summarized as follows.Fast growth of large-scale bilayer graphene sheets with a high AB-stacking ratio and high mobility on copper poses a tremendous challenge,which has to overcome the self-limiting effect.Here,we report a low-cost but facile method to rapidly synthesize bilayer Bernal graphene by chemical vapor deposition using polystyrene as the feedstock,which be well controllable from hexagonal single-crystal domains to wafer-scale homogeneous films.The bilayer graphene grains and continuous film obtained are of high quality and exhibit field-effect hole mobilities as high as 5700 and 2200 cm2V-1s-1 at room temperature,respectively.In addition,a synchronous growth mechanism of bilayer graphene is revealed by monitoring the growth process,resulting in a high surface coverage of nearly 100%for a near-perfect AB stacking order.In this process,the polystyrene plays a crucial role to give synchronous bilayer growth.The high carrier mobility,tunable bandgap,and ability for low-cost mass production pave the way for development of bilayer graphene nanoelectronic devices with high performance.Unfortunately,CVD graphene is intrinsically polycrystalline,with pristine graphene grains stitched together by disordered grain boundaries,which can be either a blessing or acurse.On the one hand,grain boundaries are expected to degrade the electrical and mechanical properties of polycrystalline graphene,rendering the material undesirable for many applications.On the other hand,they exhibit an increased chemical reactivity,suggesting their potential application to sensing or as templates for synthesis of one-dimensional materials.Therefore,it is important to gain a deeper understanding of the structure and properties of graphene grain boundaries.Here we study single-crystal bilayer graphene grains synthesized by ambient chemical vapour deposition on polycrystalline Cu,and show how individual boundaries between coalescing grains affect graphene’s electronic properties.Magneto-transport properties in bilayer graphene was experimentally investigated by varying magnetic-field strength and a linear magnetoresistance(LMR)was observed.Bilayer graphene grain boundaries give a significant Raman D peak,impede electrical transport,and decrease de-phasing length in graphene.This study highlights the importance of domain interfaces,especially on the carrier transport properties in CVD graphene.更多还原