Carbon/boron Nitride Heterostructures: Geometry Design And Property Tuning From First-principles Calculations

Carbon nanomaterials have been the focus of much research because of its numerous allotropes and unique potential applications in computer technology, environmental protection, source of energy, aviation, and nanoelectronics. Carbon nanomaterials, such as fullerene, nanotube, graphene, nanoribbon, and graphdiyne, have greatly promoted the development of nanotechnology. It is quite crucial to tune the electronic structures of carbon nanomaterials in a control way to fulfill special applications in various field. The structure resemblance and small lattice mismatch between graphene and BN nanomaterials, has motivated the synthesis of C/BN heterostructures, which opens a way to tune electronic structures of carbon nanomaterials.In this thesis, we report our first-principles calculations within density-functional-theory (DFT) on the configuration, stability, and electronic structures of C/BN nanotube heterostructures (1D), heterobiribbons (1D), and graphene/BN heterobilayers (2D). The role of BN and the tunable electronic properties for carbon nanomaterials were fully addressed. The main results are summarized as follows.(1) The nanoscaled junctions formed by carbon noanotubes (CNRs) with different chiralites and diameters have been found in experiments. However, due to the different in controlling the chirality of CNTs, the electronic properties of these CNT-junctions are uncontrollable. The C/BN heteronanotubes are promising to improve the performance of CNT-junctions. Here, we presented our systematic investigation on the geometric and electronic properties of zigzag and armchair C/BN nanotube heterostructures. Our results showed that both of them have smooth interfaces which are free from bond mismatch and vacancy defect. Interface states appear in the band gaps, due to the discontinuity ofπbonding of carbon nanotube segments. For zigzag C/BN heterostructures, the overall electron transfer occurs between the two interfaces, which set up a sawtoothlike static potential profiles and the build-in electric field is throughout the superlattice. For armchair heterostructures, the charge redistribution mainly takes place in the region near the interfaces, giving rise to a steplike profile. The nonpolar interface can be achieved by rearranging the atoms at the polar zigzag interface, thus the build-in electric field and electric potential can be modulated. Our calculations of bandoffsets indicate that all these C/BN heterostructured nanotubes have type-I band alignment features. The field-emission properties of the C/BN nanotube clusters containing a single C/N interface are remarkably enhanced as compared with that of the pristine CNT clusters.(2) One-dimensional graphene nanoribbon possess unique electronic properties due to the quantum size effect compared with the two-dimensional grapheme. For example, half-metallicity can be achieved in zigzag GNRs under a specific external transverse electric field. However, the extra electric field may hinders the application of the half-metallic feature. Here, we present two types of C/BN heterobiribbons, a laterally-heterostructured nanoribbon composing of graphene and BN nanoribbons with zigzag edges. We find that, the spin-polarization and electronic structures can be well-modulated by changing the width of carbon componet, undergoing manifold transition from semiconducting to half-metal and ferromagnetic metal. The critical points of GNR width to induce the transitions depend on the interface type (B/C or C/N) rather than the BNNR width. The ground states of metallic BNC biribbons are spin-polarized forming non-zero magnetic moments. The tunable electronic structures and magnetic ground states make the BNC biribbons promising candidate nanomaterials for building nanoscaled spintronic devices.(3) The high carrier mobility of graphene due to the nearly linear dispersion relation has attracted considerable interest, but the zero-band-gap features impede its applications in building electronic nanodevices. In this letter, we open a band gap by depositing graphene on BN monolayer, because the charge redistribution breaks the equivalence of the two graphene sublattices. The band gap and electron effective mass of graphene/BN heterobilayers can be modulated effectively by tuning the interlayer spacing and stacking arrangement. The HBLs have smaller EEM than that of graphene bilayers, and thus higher carrier mobility. For specific stacking patterns, the nearly linear band dispersion relation of graphene monolayer can be preserved in the HBLs accompanied by a small band-gap opening. The tunable band gap and high carrier mobility of these C/BN HBLs are promising for building high-performance nanodevices.