First-principles Studies On The Electronic Properties And Photocatalytic Performance Of Low-dimensional Nanocomposites

Besides the carbon nanomaterials,there are a wide variety of other low-dimensional（LD）nanomaterials.Among them,g-C₃N₄,hBN,phosphorene（BP）and transitionmetal dichalcogenides（TMDs）have attracted widespread concerns over the past decade,owing to their extraordinary physical properties and successful fab-rications in experiments.In this thesis,combing first-principles calculations with frontier technologies,we mainly investigate the electronic structures,interfacial ef-fects,magnetism,and optical properties of the above several LD nanomaterials and their nanocomposites.We not only aim to modulate or improve the relevant properties of these heterostructures,but also try to reveal the underlying mechanism for their enhanced performance.The main contents of this thesis are presented as follows.（1）Using first-principles calculations,we investigate the electronic and magnetic properties of zigzag graphene nanoribbons（ZGNRs）with side-attached trans-polyacetylene（CnHn+i）.The gap of the system with even n is much bigger than that with odd n+1,whereas the parity dependence of n is gradually weakened with an increasing n.Moreover,the magnetic moment oscillation between 1 and 0 p,B,with n changing between odd and even,which reveals a parity effect due to the electron transfer from the ZGNRs to polyacetylene chain with odd n.（2）Using first-principles calculations,the interfacial effects of C60/g-C₃N₄ nanocomposites on the electronic properties,charge transfer and optical response have been explored in detail.For different stacking patterns,the two constituents are al-ways linked by van der Waals（vdW）forces without any exception,and form type-Ⅱheterojunctions in most cases.The valence band maximum and conduction band minimum of these heterostructures are dominated by the unsaturated nitrogen（N2）atoms and C60 molecule,respectively,and strongly interact with each other,resulting in strong charge transfer between the two involved constituents and an obvious bend-ing of the g-C₃N₄ sheets.The unsaturated N2 atoms included in the interfaces have a significant influence on promoting the photocatalytic performance,while the exist-ence of saturated nitrogen（Ni and N3）atoms lying in the interfaces will weaken the interfacial interactions between C60 molecules and the g-C₃N₄ monolayers.Moreover,the sensitive optical response and satisfactory type-II band alignment clearly show that the C60/g-C₃N₄ heterostructure is an outstanding photocatalyst for hydrogen pro-duction.We proposed a deep insight（the role of nitrogen）into understanding the im-proved photocatalytic ability of the C60/g-C₃N₄ nanocomposites,which may contrib-ute to the rational design of both C60/g-C₃N₄ and g-C₃N₄-based nanocomposite pho-tocatalysts.（3）The first-principles calculations demonstrate that widely tunable direct and indirect band gaps both can be obtained in hBN/MoS₂ vertical heterostructures,under a finite vertical electric field（E-field）.In hBN/MoS₂ bi-and multi-heterostructures,the interactions between the two individuals produces a very special y-band.Then,an enhancing forward E-field shifts this y-band down and makes its lowest point become the conduction band minimum（CBM）of hBN/MoS₂ bilayer at 0.47 V/A,leading to a continuously tunable direct band gap.In contrast,an enhancing backward E-field shifts the valence band maximum（VBM）of hBN up and makes it be the VBM of hBN/MoS₂ bilayer at-0.07 V/A,resulting into a highly tunable indirect band gap.Moreover,the magnitude of the two critical E-field is obviously reduced with in-creasing the layer number of hBN flakes,offering multiple choices to devise band-gap tunable MoS₂-based device only under a weak E-field,which may be a significant breakthrough in MoS₂-based field-effect transistors and photodetectors.（4）Both the SiC/MoS₂ heterostructure（SiC monolayer coupled with MoS₂ mon-olayer）and SiC/BL-MoS₂ heterostructure（SiC monolayer combined with MoS₂ bi0 layer）possess narrower band gaps than the two involved constituents,with the energy gaps of 1.46 eV and 1.36 eV,respectively,which mainly due to the formation of type-II heterojunctions and the sensitive electronic properties of SiC to lattice con-stant variations.Moreover,for the two vdW heterostructures,the electron transition mainly occurs between different constituents,which greatly facilitates effective sep-aration for photogenerated charge carrier.The considerable built-in potential well in interfaces is another key factor to improve the photocatalytic activity and structural stability.This work reveals that the SiC/MoS₂ and SiC/BL-MoS₂ vdW heterostructure are both promising photocatalysts.