First-principals Investigations Of Surface And Interface Properties Of Several Two-dimension Materials And Their Applications

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.