First-principles Study Of Surface Adsorption On Ni And GaP

Surface adsorption is an important character of the solid surface.The force field of atom or molecular is unsaturated on the solid surface and the remanent force field exists. Clean solid surface is unstable,and it tends to adsorb other atoms.It is proved that even ultrathin layers of adsorbate material affects strongly the stable geometry,electronic property of surface and the performance of heterojunction devices.Therefore,the surface adsorption is studied with particular concern.In this paper,ab initio calculations based on density functional theory have been performed to study the nonmagnetic atoms of Sn and Pb,and the magnetic atom of Mn were adsorbted on the Ni surface.The surface reconstruction and surface passivation of GaP(001) were also investigated.1.We applied first-principles calculations to investigate the structural and electronic properties of the tin adsorption on Ni(100),(110) and(111) surfaces.Total energy calculations show that substitutional surface alloys in the outermost layer are preferable for all surfaces and the Ni(3^1/2×3^1/2)R30°-Sn model has no stacking fault.Rumpling magnitudes of Sn adsorption on Ni(100),(110) and(111) surfaces are 0.47(?),0.49(?) and 0.55(?),respectively.It is in the order of(111)＞(100)＞(110),agreeing well with experiments.We found the rumpling amplitude is significantly smaller than the expectation from a simple hard-sphere model due to the valence electron depletion and the reduction of the surface stress.On the other hand,Pb adsorption on the Ni(111)(3^1/2×3^1/2)R30°surface was also studied.It is indicated that the results are similar to that of Sn adsorption on this surface.2.Using first-principles total energy method,we study the adsorption of Mn on a Ni(110)-c(2x2) surface.Paramagnetic,ferromagnetic,and antiferromagnetic surfaces in the top and second layer have been taken into account.It turns out that the substitutional alloy in the outermost layer with ferromagnetic surface is the most stable in all cases, and the substitutional subsurface position for the Mn is the most favorable position provided that only paramagnetic solutions are considered.It is proved that the magnetism of Mn can stabilize this surface alloy.The buckling of the Mn-Ni(110)c(2×2) surface alloy in the top layer is as large as 0.26(?) and the weak rippling is 0.038(?) in the third layer,in excellent agreement with experiments.Density of states(DOS) analysis shows a large magnetic splitting for the Mn atom,which is slightly higher than that of Mn-Ni(100)c(2×2) surface alloy(3.41eV) due to the higher magnetic moment.A strong hybridization between 3d states of Mn and Ni is also observed.The work-function change due to Mn adsorption is found to be-0.59eV,and charge transfer from the adsorbate to the substrate is confirmed.The magnetic moment of Mn atom is calculated to be 3.81μ_B.The large magnetic moment of Mn is responsible for the large buckling of the surface alloy and the work-function change.In addition,the adsorption of Mn on a Ni(111)-p(2×2) surface was also investigated.It turns out that the substitutional alloy in the second layer with antiferromagnetic surface is the most stable. The magnetic moment of Mn atoms is calculated to be 3.15μ_B and-3.03μ_B.The electronic analysis shows that the localization of Mn-3d states is smaller than that for Mn-Ni(110)c(2×2) surface alloy,which is due to the smaller magnetic moment of Mn. Also,the comparison of our results with similar adsorption systems suggests that the buckling of the magnetic surface alloy is affected by the geometrical effect, magneto-volume effect and the vacancy formation energy.3.We presented a comprehensive study of the surface reconstruction of GaP(001) surface,and the disordered structure during phase transitions was determined.The structural,energetic and electronic properties of GaP(001)(2×1),(2×2) and(2×4) surfaces are obtained.The geometrical parameters agree well with available calculational and experimental results.It is suggested that during phase transition between(2×1) and(2×4) structures the disordered phase should be the mixture ofβ2(2×4),δ(2×4) and Ga(2×1)md Structures.4.The structural and electronic properties of 0.5ML and 1ML sulfur adsorption on the Ga-and P-terminated GaP(001)(1×2) surfaces were investigated.Our calculations show that at the coverage of 0.5 ML S adatoms prefer to be adsorbed on the Ga-terminated surface at the bridge sites.For the coverage of 1 ML,the most stable configuration is a combination of the T4 and HB sites on the Ga-terminated surface,and all Ga dimers are broken with the periodicity changing from(1×2) to(1×1).The S-Ga bond is stronger than the S-P bond.The phase transition from(1×2) to(1×1) pattern was modeled in this work.It turns out that the(1×1) structure is more stable than(1×2) phase when-1.34eV＜△μ(S)＜0,and the(1×2) phase is transformed into the(1×1) phase at the△μ(S) of-1.34eV.These results agree well with the experimental findings. The electronic band structure and DOS analysis showed that the surface state density in the mid-gap region of the Ga-terminated GaP(001) surface was noticeably reduced by the sulfur adsorption,while such reduction did not occur on the P-terminated surface due to the S-P antibonding state within the energy gap.In addition,the nearly filled S dangling bonds on the Ga-terminated surface make this surface resistant to contamination.Therefore,the passivation of GaP(001) with S is effective.5.We applied first-principles calculations to discuss the structural and electronic properties of indium adsorption on the GaP(001) surface at the coverages((?)) of 0.5 and 1 ML.It is found that at(?)=0.5 ML In adatoms prefer to adsorb at the pedestal(HH) site,followed by the bridge site.This is similar to the In/GaAs(001) adsorption system. For the coverage of 1 ML,the most stable configuration is a combination of the HH and valley bridge(T3) or cave(T4) sites,with the energy difference of only 30 meV between HH-T3 and HH-T4 sites adsorption.The density of states(DOS) analysis showed that the deposition of pure indium on GaP substrate can be employed as surface passivation of underlying buffered layers.The similar adsorption behaviors for the coverages of 0.5 and 1 ML were obtained.The work function change due to In adsorption was lowered,indicating that some charges are transferred from the adsorbate to the substrate.