| Low dimensional structure is a frontier topic in the field of optoelectronic materials and devices.Among them,group ?-?nanowires show broad application prospects in lasers,detectors and solar cells due to their quasi one-dimensional structure and excellent photoelectric properties,while graphene has unique advantages in transparent electrodes and high-speed and wide-band detection due to its high light transmittance and high electron mobility.Both of them have attracted extensive attention and made important progress in academia.Compared with the traditional III-V nano wires with<111>growth direction,the nanowires with<110>growth direction reported in recent years show different crystal structures and physical properties,which has become a new field of nanowire research;Compared with graphene,graphene oxide has more complex structure and novel properties,which is expected to further expand the application field of graphene.In addition,nanowire/graphene composite structure has become a research hotspot in the field of new optoelectronic devices due to the combination of the advantages of two low dimensional materials.It is of great scientific significance and application value to explore the physical mechanism.Combined with the experimental phenomena,the theoretical studies on the<110>growth direction III-V nanowires,graphene and nanowire/graphene composite structures were carried out by first principles:(1)According to the experimental results of InP nanowires with<110>growth direction,the electronic structure and optical properties were studied.The cross section of the nanowire model is rectangular,and the side is composed of two {112} and two {111} planes.The results show that the fermi surface is at the top of the valence band.The absorption spectrum analysis of nanowires shows that the optical absorption spectrum of the nanowires with increasing number of plies along the[11-2]direction has a significant blue shift in the[1-1-1]direction.(2)The electronic structure and charge distribution of GaAs nanowires with<110>growth direction are modeled and studied theoretically.The nanowires are octagonal in cross-section and consist of four {111} planes,two {110} planes and two {001} planes.The energy band structure shows that the Fermi surface of the nanowires passivated by pseudo-h atom is at the top of the valence band,and the band gap is 1.16 eV.The results of differential charge density show that the surface charge distribution is not uniform when the hydrogen atom is not completely passivated,and the charge density distribution of the valence band is uniform when the pseudo-h atom is passivated,while the charge density distribution of the conduction band is dense on the arsenic rich side.(3)The stability and electronic structure properties of graphene fumigated by nitric acid are systematically studied by the first-principles method based on ultrasoft pseudopotentials.The model of graphene oxide fumigated by nitric acid is built based on the 2×2 supercell model with orthogonal graphene unit cells,which contains 15 carbon and 2 oxygen atoms.The results show that the fumigated graphene containing a carbon atom bonded to an oxygen atom is a stable structure with lower energy,which is consistent with the experimental result.In addition,the mechanical stability analysis shows C66>0,C11>0,C11C22>C122,which satisfies the mechanical stability condition.By analyzing the reactant and product,it can be concluded that the nitric acid acts as catalyst.Moreover,the process of graphene oxidation catalyzed by nitric acid is endothermic and the reaction needs heating.By analyzing the electronic properties of the structure,the graphene oxide is determined to be an intrinsic semiconductor with a direct band gap of 1.12 eV and work function of 5.28 eV.These results provide theoretical basis for preparing the graphene oxide and its applications in the field of optoelectronic devices.In addition,the energy band structure of low concentration graphene oxide after cell expansion under uniaxial single stress was studied.The results show that the direct band gap of graphene oxide under uniaxial single stress is about 0.6 ev,and the band gap increases to 0.7 eV under X-axis compressive stress and decreases to 0.57 ev under Y-axis tensile stress.(4)In this study,the electronic structure and optical properties of graphene oxide in different structures with single point defect are studied under local density of states approximation and generalized gradient approximation by first-principles calculations based on the density functional theory.The results show that four models are mechanically stable,among which the oxide graphene containing unsaturated oxygen atoms shows an important application potential in water cracking and catalysis.The calculated band structures and partial-wave density of states show that the model containing unsaturated oxygen atoms exhibits indirect band gap,while other models exhibit direct band gap,and the doping type and band gap values vary with different models.The absorption spectrum of graphene oxide is anisotropic,and the absorption edge moves to the near-UV and visible region in the direction perpendicular to the plane.The optical absorption coefficient containing sp3 hybrid is slightly higher than that containing sp2 hybrid,suggesting that the carbon-oxygen double bond and hanging bond have important influence on the absorption spectrum.(5)The electronic structure and energy band characteristics of the composite structure of graphene and(112)A and(112)B sides of GaAs nanowires were studied theoretically.The results show that the interface spacing between graphene and GaAs nanowires(112)A surface is relatively small,and the formation energy of the interface is 3.95 ev(0.04 eV/atom),which indicates that it is easy to prepare;The conduction band of graphene moves downward,showing n-type characteristics,while the valence band of GaAs(112)A surface moves upward,showing p-type characteristics.The interface spacing between graphene and GaAs nano wires(112)B surface is relatively large,and the interface formation energy is 2.69 ev(0.03 eV/atom),which indicates that the preparation is easy;The valence band top of graphene moves up slightly,showing p-type characteristics,and the conduction band of GaAs(112)B surface moves down,showing n-type characteristics.(6)The electronic structure and energy band characteristics of the composite structure of graphene oxide and GaAs nanowires(112)A and(112)B are studied theoretically.The results show that the interface spacing between graphene oxide and GaAs(112)A surface is larger than that between graphene oxide and GaAs(112)A surface,but it is still in the range of van der Waals force;The interface formation energy is-0.05 eV(-0.0006 eV/atom).The energy band structure of graphene oxide is n-type and that of GaAs(112)A surface is p-type.The formation energy of the interface between graphene oxide and GaAs(112)B surface is 0.2 eV(0.002 eV/atom),which indicates that it is easy to prepare;The p orbitals of Ga and As on GaAs(112)B surface are slightly red shifted,showing the characteristics of n-type semiconductor;The p orbitals of C and O in graphene oxide are blue shifted,showing the characteristics of p-type semiconductor. |
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