摘要
随着石墨烯（graphene）的成功剥落，在过去的十年里我们见证了二维材料的快速发展。二维层状材料如金属硫族化合物，过渡族金属氧化物，拓扑绝缘体，以及其它的二维材料复合物获得了人们的广泛关注。由于具有独特的性能以及高的比表面积，这些二维材料在许多方面都具有应用潜力，例如光电设备，自旋设备，催化剂，化学和生物传感器，超电容，太阳能电池以及锂离子电池。当材料的维度降低到二维，其表面体积比迅速增大，表面对于材料性能的影响变得越来越大。特别是对于graphene，单层六方氮化硼（h-BN)等材料，其基本上完全由表面构成。从这个角度上来说，研究二维材料的表面对于我们理解以及应用二维材料都非常的重要。除此之外，随着研究的深入，为了获得所需要的性能，人们已经逐渐从研究二维材料本身转向研究二维材料异质结构，即按一定的顺序堆垛不同的二维材料。将不同的二维材料堆垛在一起时，异质界面的出现对所获得的二维异质材料的性能有着重要的影响。因此对于二维材料而言，表面功能化和界面设计逐渐成为调节它们电子性能的重要手段。
Graphene是碳原子通过sp2杂化的方式排列形成的一种二维单层材料。相比于传统的材料，graphene具有许多优异的性能，如在白光图谱中2.3%的吸光率，高的表面积、杨氏模量以及极佳的热导。此外，由于其线性的能带散布关系，理想的graphene具有超高的载流子迁移率。其常温下的载流子迁移率（15000cm2.V-1.s-1）约是商业硅片的10倍。这使得graphene在高速电子设备领域具有重大的潜力，如场效应晶体管（FET）。但是，原始的graphene本身不能被用来制造室温下运行的FET，这是因为graphene中没有能隙。能隙的缺失使得基于graphene的FET无法有效的关闭，其开关电流比很小。因此，打开graphene的能隙便成为实现其在逻辑电路中运用的重要的一步。除此之外，在graphene表面集成FET或者其它电子光学设备，还依赖于我们对其局部载流子类型和浓度的控制能力，即在graphene表面可控的实现p-n节/超点阵的能力。
基于金属铂（Pt）的催化剂对许多重要的化学反应都具有良好的催化活性及选择性，因此在商业中被广泛地应用。不过在实际的催化过程中Pt的利用率却很低，这是因为在催化反应中只有Pt纳米颗粒表面的原子参与了催化，其内部的Pt原子并没有得到有效的利用。降低催化剂粒子的尺寸可以有效地提高Pt的利用率，当使用单原子Pt作为催化剂时它的利用率达到最大，但是在实际的使用过程中单个的Pt原子容易烧结并团聚在一起。这就需要找到一种适当的基底材料，它必须可以有效地固定住催化剂原子，抑制其在反应中的团聚并能够保持其高的催化活性。二维材料具有大的表面体积比，并且其电子性能囊括了绝缘体，半导体以及金属，因此是一类潜在的可以作为单原子Pt催化剂基底的材料。
基于以上考虑，我们研究了graphene与其它二维材料之间的异质界面对于调节石墨烯的能隙以及其载流子类型和浓度的作用，并详细地阐明了能隙和载流子调节的物理机制。由于二维材料具有极高的表面积，我们还探讨了二维单层金属酞菁片（金属原子从Sc到Zn）在作为催化剂基底方面的应用。其具体内容主要分为以下三个部分：
首先，研究了在氢化的graphene/h-BN中，异质界面的存在对体系能隙的影响，并阐述了能隙调节的物理机制。研究结果表明在氢化的graphene/h-BN中，界面处电荷转移诱导产生一个由graphene层指向h-BN层的内建势差。该内建势差对体系的能隙值起着决定性的作用。通过外加电场或者改变氢化的graphene/h-BN中h-BN的层数可以调节这个势差，从而实现对系统能隙值的线性调控。
其次，研究了通过界面设计的方法在MoSe2基底和graphene之间插入一层功能化的双面材料作为缓冲层（H-G-F），以实现对graphene中载流子类型和浓度的控制。研究发现，取决于H-G-F，MoSe2以及graphene的相对排列，电子将自发地从MoSe2基底以隧道的方式转移到最上层的graphene中或从graphene中转移到基底。Graphene p-n节/超点阵的形成便可以通过选择性地功能化缓冲层使其形成graphene纳米带包围的H-G-F和F-G-H岛来实现。这种隧道掺杂现象的物理机制源于：H-G-F中的势差与由于它的引入而产生的界面偶极矩共同作用，最终导致MoSe2基底的价带顶或导带底高于（低于）graphene的狄拉克点。
最后，研究了二维金属酞菁片作为单原子Pt催化剂的基底的可能性，并使用CO氧化反应测试了其催化反应的活性。研究结果表明Ti-Pc是一个较好的可实现单原子Pt负载的基底，它可以有效的防止单个Pt原子的团聚并保留其高的催化活性。
With the successful exfoliation of single layer graphene, we have witnessed a rapiddevelopment of two-dimension (2D) materials during the passed decades.2D layeredmaterials such as metal chalcogenides, transition metal oxides, topological insulator andother2D compounds have gained comprehensive interest. Because of their remarkableproperties, they have shown great potential in a wide range of areas, including optical andelectronic devices, spin equipment, catalyst, chemical and biosensor, supercapacitor, solarand Li-ion battery. In view of the rapid increase of specific surface area in2D materials, thesurfaces become very important to their properties, especially for graphene and monolayerhexagonal boron nitride (h-BN) which consist of entirely surfaces. In this respect, it isessential to study the surface properties in order to better understand or use the2D materials.Additionally, the attentions have shifted from2D materials themselves to theheterostructures fabricated by stacking different2D crystals on top of each other to obtainthe required properties. When stacking different2D crystals into heterostructures, theemergence of heterointerface plays a significant role in determining the properties of theobtained system. Consequently, surface functionalization as well as interface designationgradually becomes a vital way to modulate the electronic properties of the2D materials.
Graphene is a single layer of sp2-hybridized2D carbon atoms. Compared withtraditional materials, graphene exhibits unique and fascinating electronic properties, such asits2.3%absorption in the white light spectrum, high surface area, excellent thermalconductivity and high young’s modulus. In addition, in view of its linear band dispersion,ultrahigh carrier mobility (15000cm2.V-1.s-1in room temperature) has been observed, almost10times larger than that in commercial sillion wafers, holding great potential in high-speedelectronic devices, such as filed effect transitor (FET). However, pristine graphene can notbe used to fabricate FET operated in room temperature with the absence of band gap. One ofthe important steps toward the applications of graphene in logic circuit is to open a band gap.On the other hand, integrations of FET or other electronic and photonic devices on grapheneare highly dependent on our ability to locally control the carrier type and concentration ingraphene, namely the ability of controllable formation of graphene p-n junctions/superlattice.
Supported Pt-based catalysts are widely used in industry by virtue of their high activityand/or selectivity for many important chemical reactions, while their efficiency is extremely low because only the surface active-sites are used. Reducing the size of catalyst couldsteadily improve its efficiency and the highest one could be achieved when it is down tosingle atom scale. However, the application of Pt single atom catalysts has been largelyhampered by their easiness to sinter and aggregation under realistic reaction conditions，which call for a suitable substrate mateiral with the ability to fasten the single catalyst atomtightly and to prevent their aggregation in reaction while retaining their high catalyticactivity. On account of the huge surfaces and variety of electronic properties ranging frominsulator, semiconductor to metal in2D materials, they hold great potential as the substratematerials for single atom catalysts.
Based on the above considerations, the effects of heterogeneous interfaces betweengraphene and other2D materials on its band gap and carrier type and concentration havebeen studied. The corresponding physical mechanisms have also been discussed. In terms ofthe high surface area in2D materials, we also investigated the application of2D monolayerTM-Phthalocyanine (TM from Sc to Zn) as a substrate material for catalyst. The three partsof our research contents are listed as below:
Firstly, the effect of heterogeneous interface on the band gap of hydrogenated graphene/h-BN and the corresponding physical mechanism have been investigated. The resultsdemonstrate that the emergence of dipoles at the interface induced by the charge transferbetween the graphene and the h-BN layer introduces a built-in potential difference, whichplays a critical role in determining the energy gap of the resulting system. Tuning thisbuild-in potential difference through changing the number of h-BN layers or an external biasallows linear modulation of the gap.
Secondly, on the basis of interface designation, we propose to modulate the carrier typeand concentration in graphene through inserting a functionalized janus material in betweenMoSe2substrate and graphene as the buffer layer (defined as H-G-F). It is found that after itsintroduction, electrons would transfer from the MoSe2substrate to graphene or reversethrough a tunneling effect depending on their detailed arrangements. Appropriatefunctionalization of the janus material would open the possibility of creating well orderedand atomically sharp graphene p–n junctions/superlattices in a single layer of graphene. Thephysical origin of the tunneling phenomena is determined by a net potential differencebetween the valence band maximum/conduction band minimum in substrate and the Diracpoint in graphene layer caused by the competition between the dipole of the janus materialand the concomitant interface dipoles after its insertion.
Lastly, the potential of TM-Phthalocyanine as a substrate material for single Pt catalysthas been studied. CO oxidation is selected as a probe reaction to test the catalytic activity ofthe resulting system. It is found that Ti-Pc is the most appropriate compound which preventsthe aggregation of Pt atoms and retains its high catalytic activity.
引文
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