First Principles Study of Zinc Oxide and Graphene Based Interfacial Electronic Structures for Nanoelectrics.

摘要

Advances in experimental techniques such as nanofabrication, characterization and synthesis have resulted in the development of many novel and interesting materials and devices. Surfaces and interfaces play an indispensible role for nanoelectronics development. ZnO and graphene have drawn tremendous research interests in recent years, due to their exceptional merits in electrical, optical and magnetic applications. This thesis attempts to ferret out the current experimental research progress, particularly, the frontiers of ZnO and graphene based surfaces and interfaces, and employs first principles to explore their electronic structures, to acquire mechanistic understanding of experimental findings, and to shed light on rational design of functional devices.;First, we study the controllable modulation of the electronic structures of ZnO(10 1¯ 10) surface functionalized by various types of carboxylic acids. The calculated structural results are consistent with the experimental ones attained by the Fourier transform infrared attenuated total reflectance (FT-IR-ATR). Mercapto-acetic acid molecules are found to contribute an abundance of band gap states into ZnO. Mercapto-acetic monolayer functionalized ZnO (10 1¯ 10) is on the verge of metal-to-insulator transition, which is consistent with the experimental finding of an conductivity increase by 6 orders of magnitude. Mercapto-acetic acid functionalized ZnO (10 1¯ 10) surface shows a strong configuration-dependence for both electronic structure and adsorption energy. Moreover, mercapto-acetic acid molecule functionalized ZnO also shows facet-dependent characteristic in which the monolayer functionalized ZnO (2 1¯ 1¯ 0) does not show metal-to-insulator transition. Acetic acid does not contribute to the band gap states of ZnO (10 1¯ 10), whereas benzoic acid and 9-anthracenecarboxylic acid do contribute an abundance of band gap states to ZnO(10 1¯ 10).;Second, we study the band gap opening of graphene bilayer by F4-TCNQ doping and externally applied electric filed effects. With F4-TCNQ concentration of 8.0x1013 molecules/cm2, the electrostatic charge transfer between each F4-TCNQ molecule and graphene is 0.45 e, and the built-in electric field Ebi between the graphene layers could achieve 0.070 V/A. The charge transfer and band gap opening of the F4-TCNQ doped bilayer graphene can be further modulated by externally applied electric field (Eext ). At 0.077 eV/A, the gap opening at the Dirac point ( K) DeltaEK = 306 meV and the band gap Eg 253 meV are around 71% and 49% larger than those of the pristine bilayer under the same Eext. By combining F4-TCNQ molecular doping and Eext, the p-type semiconductor bilayer graphene are attained, with the band gap and hole concentration varied in a wide range.;Third, the self-assembly mechanism of PTCDA ultrathin films on graphene with the coverage in a range of 0.3∼3 monolayers (MLs) are interrogated by first principles method. For alpha modification mode, with critical thickness of 1 ML, the growth of PTCDA on graphene follows the Stranski-Krastanov (SK) growth mode. In contrast, for beta modification mode, the PTCDA can form two complete MLs on graphene substrate. From the thermodynamical viewpoint, alpha modification mode is more stable than beta modification mode. At 1 ML, the PTCDA follows a continuous and planar˙ packing arrangement on graphene, which is almost unperturbed by typical defects in graphene substrate. This is in consistentcy with the experimental findings. For alpha modification mode with 2 and 3 ML coverage, the bulk-like phases appear. At the same time, the total charge transfer between PTCDA and graphene per 5✓3x5 super cell at 2 MLs saturates with 0.42e, which is larger than those of 1 or 3 ML coverage.;Finally, the magnetic properties of graphene by organic molecule modification are investigated by first principles method. For the first time, we demonstrate that methoxyphenyl group can introduce a delocalized p-type ferromagnetism into graphene sheet, with the Curie temperature (T c) above room temperature. Each aryl group can totally induce 1 muB into molecule/graphene system. Moreover, an around 1.1 eV direct band gap is introduced into both majority and minority spin bands of graphene by methoxyphenyl group modification. Zigzag graphene nanoribbon (GNR) shows strong site-specific magnetism by aryl group adsorption near the edge. At specific site of GNR, each molecule could totally induce 3∼4 mu B into molecule/GNR hybrid system.;These four theoretical sub-topics stem from the experimental advances in ZnO and graphene based surfaces and interfaces. They form the mechanistic understanding of the respective surfaces and interfaces down to the molecular level.