Theoretical Investigations On Two-Dimensional Photoelectric Responsive Materials

Low-dimensional materials,such as graphene and graphene analogs have attracted extensive attention in fields of physics,chemistry and material science,due to their outstanding physical and chemical properties.Searching for high-performance graphene-based optical absorption and photoelectric-responsive nanomaterials is crucial to the practical applications of photoelectric-conversion devices.Topological insulators have also aroused interest due to their scientific importance as a novel quantum state and promising potential applications in information technology and spintronics.To expand practical applications of topological insulator at room temperature,many efforts have been dedicated to searching for two-dimensional topological insulators with stable structures and large enough gaps to overcome the thermal disturbance.In this dissertation,by using density functional theory calculations,we investigated(1)the photoelectric modulation and energy storage as well as electrocatalytic performance of multifunctional graphene-based complexes;and(2)the topological electronic states and the Rashba splitting effect of two-dimensional arsenene-based nanomaterials.Our results are summarized as follows:1.The photoelectric modulation and electrocatalytic performance of multifunctional graphene-based compositesDensity functional theory calculations with long-range van der Waals corrections were performed to study the electronic structures and energy storage/release of graphene through sulfur(S)-doping and physisorption of ?-conjugated photoresponsive molecules,trans/cis-azobenzene(AB)derivatives with electron-donating substituent group and trans/cis-stilbene(ST),respectively.With the increase of the S dopingconcentration,the bandgap of graphene exhibits the enhanced metallic characteristics with a direct-to-indirect transition.Although AB and ST molecules have different unsaturated bridge bonds,-N=N-versus-CH=CH-,physisorption of these two photoresponsive molecules onto the graphene can both broaden band gap to about 0.02 eV,as a result of the ?-? interfacial interactions.Under the exposure to the solar light,the facile trans-to-cis isomerization of AB(ST)molecule adsorbed onto graphene renders the energy storage of about 1.04 eV(0.49 eV)in each molecule.The noncovalent physisorption of trans/cis-AB molecules onto graphene is unexpectedly more favorable to energy storage than that of covalent binding.In addition,a multifunctional graphene-based material,with the combination of both S-doping and physisorption of photoresponsive moelcules,could not only open a bandgap of about 0.27eV,but also induce energy storage of 0.84 eV per molecule via the conformational change from trans to cis AB isomer.Strong charge localization at S dopant may become the active sites for catalysis and energy storage,and meanwhile,photoactive adsorbates could further promote the energy conservation and release.Moreover,we show by density functional theory calculations that the conductance and optical properties of the graphene-based photo-responsive nanocomposites can be modulated through noncovalent adsorption of azobenzene(AB)derivative onto pristine,Si-doped and pyridine-like N3-vacancy graphene,respectively.The optical absorbance properties of three hybrid nanostructures were found to exhibit visible-light response within the low-frequency region,accompanying with the trans-to-cis isomerizations of the nanocomposites under the excitation of the solar light.The excellent solar light absorption performances can be used to modulate the conductance of both N3-vacancy-graphene and Si-doped-graphene AB hybrids effectively through the reversible change of the tunneling barrier length of AB molecule in thetrans-to-cis isomerization.In addition,the solar thermal energy up to 1.53 eV per molecule can be stored in the designed nanocomposite via the conformational changeduring trans/cisAB isomerization.In addition,the nitrogen doped graphene quantum dots(NGQDs)anchored on the Ni3S2 nanosheets(Ni3S2-NGQDs/NF)can favorably improve the conductivity and add more active edge sites.The calculated free energy diagram in the oxygen evolution reaction(OER)shows that almost all the steps are energetically downhill and the chemisorption energy for each reaction is more energetically favorable as compared to the pure Ni3S2 and N-doped graphene.These results imply that the existence of the NGQDs onto Ni3S2 can highly accelerate the OER process.The chemisorption free energies of hydrogen in the hydrogen evolution reaction(HER)is much closer to zero than those with sole component of Ni3S2or NGQDs,suggesting the higher HER catalytic activity of the Ni3S2-NGQDs.The synergistic effects between NGQDs with Ni3S2 nanosheetsendow the Ni3S2-NGQDs/NF with remarkable electrocatalytic performance toward OER,HER as well as the overall water splitting reaction.This work provides a new route to construct noble-metal-free nanocomposites through coupling the heteroatom doing graphene quantum dots with transition metals-based nanomaterials.2.The topological electronic states and the Rashba splitting effect of two-dimensional arsenene-based nanomaterials Our first-principles calculations predicted three candidates for two-dimensional topological insulators,arsenene functionalized with F,OH and CH3 groups(AsX,X=F,OH and CH3),which preserved large bulk band gaps from 100 to 160 meV(up to 260 meV)derived from the spin-orbit coupling within the px,y orbitals.The topological electronic bandgap of AsF monolayer can be effectively modulated by biaxial tensile strain and vertical external electric field.In addition,pronounced light absorption in the near-infrared and visible range of solar spectrum was expected for AsX(X = H,F)monolayers from the adsorption peaks at 0.45?1.6 eV,which is attractive for light harvesting.The nontrivial quantum spin Hall insulators AsX could be promising candidates for practical room-temperature applications in dissipationless transport devices and photovoltaics.Through changing stacking order,a unique bilayered fluorinated arsenene AsF system is demonstrated to simultaneously possess Rashba spin splitting(RSS)and non-trivial topological electronic states.We show by first-principles calculations that tunable RSS can be realized in bilayered AsF.Intrinsically RSS of 25 meV(?R:1.67 eV·A)is found in the AA-stacked AsF bilayer by considering the spin-orbit coupling effect.The RSS is tunable in a range from 0 to 50 meV by applying biaxial strains.RSS is significantly enhanced up to 186 meV in the presence of an external electric field.The AB-stacked AsF bilayer is shown to be a two-dimensional topological insulator with a sizable bulk band gap of 140 meV,originated from the spin-orbit coupling within the Px,Y-pz band inversion.The AsF bilayers with tunable RSS and nontrivial bandgap with AA-and AB-stacking orders could pave the way of designing spin field-effect transistors and new quantum spin Hall devices.Considering that the exfoliation of gray arsenic into single or few layered arsenene remains a challenge,through the conformational change in photo-isomerization of azobenzene-based photochromes,we propose an effective method to promote the exfoliation of multilayered arsenene.Density functional theory calculations show hat the trans-to-cis conformational change can lead to an increase of the adjacent interlayer distance of arsenene from 6.59 A to 11.07 A.Reactive molecular dynamics(RMD)simulations also show that about 5 A interlayer distance change can be induced by the cis-to-trans photo-isomerization of an azobenzene-OC10H21 molecule.Thus,we can expect that the predicted template will promote the exfoliation of multilayered arsenene into single or few-layer arsenene through the light-induced isomerization process.