Surface Review and Letters

Atomistic simulations of low coverage Sn growth on Au(111) using the Bozzolo–Ferrante–Smith (BFS) method for alloys are presented. Simulated annealing and atom-by-atom analysis of the energetics explains the close competition between the experimentally observed surface alloying and layer-by-layer growth.

The kinetics of formation of self-assembled monolayers on gold generated by the adsorption of 1-octanethiol and 1,8-octanedithiol were explored by electrochemistry measurement. The time dependence of capacitance and surface coverage supported that the adsorption of thiols typically processed with a two-step adsorption consisted of a fast initial adsorption and a slowly following reorganization. From the function of surface coverage versus time, one could get rate constants of adsorption of thiols, and the adsorption process was demonstrated to follow a diffusion-controlled Langmuir model. A comparison of the adsorption rates for different concentration between 1-octanethiol and 1,8-octanedithiol revealed that their adsorption processes were almost identical and independent on the concentration, inferring that only one sulfur atom contributed to the self-assembly of dithiol molecule.

Chalcogenide phase change material Ge₁Sb₂Te₄ (GST) nanoparticle arrays with long-range-order were fabricated by using a monolayer of self-assembled polystyrene (PS) spheres as mask. The morphology of nanoparticle arrays can be controlled via changing RIE processing conditions. Images of atomic force microscopy (AFM) and scanning electron microscopy (SEM) show that highly uniform GST nanoparticle arrays with particle density around 10⁹ cm^-2 were formed. The sizes of nanoparticles can be reduced to a tiny diameter in the range of 30–40 nm (top diameter). The GST nanoparticle arrays exhibit a prominent peak near 580 nm in reflectance spectra, which indicates that they possess a photonic band gap. These results confirm that GST nanoparticle arrays have a 2D periodicity and long-range order. The method of nanosphere lithograph may apply to manufacturing of high density memory devices based on phase change-based memory materials.

The electronic properties of a Ca bilayer deposited on Cu(111) have been investigated by high-resolution electron energy loss spectroscopy. For the as-deposited Ca bilayer, the surface plasmon was recorded at 3.3eV. Temperature induces a surface metal-insulator transition (evidenced by the shift of the surface plasmon energy), leading to the formation of CaO thin layers which are highly reactive towards water molecules. The presence of occupied d band in the calcium overlayer modifies surface plasmon frequency and strongly influences its dispersion relation. Results are well explained within the framework of the s-d polarization theory.

Fe nanowire arrays were fabricated at lower voltage by alternating current (AC) electrodeposition into the highly ordered nanoholes of the porous alumina membrane (PAM) obtained by two-step anodization in oxalic acid. The morphology, structure and magnetic properties of Fe nanowire arrays were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM), respectively. The results indicate that Fe nanowires are about 50 nm in diameter which were accorded with the pores of the PAM, and stabilized in body-centered cubic (bcc) structure with a preferred orientation along (110). The easy magnetization axis is parallel to the axis of the Fe nanowires, while corresponding coercivity and squareness ratio value is 1674.5 Oe and 0.87, respectively.

ZnO nanoneedle arrays were obtained from the transformation of ZnC₂O₄ precipitate, which was synthesized via electrodeposited method at room temperature. The diameter of those grown ZnO nanoneedles is about 80 nm. The XRD pattern and the HRTEM image indicate that the grown ZnO nanoneedles have the preferential growth toward (101) direction. The possible growth mechanism of the ZnC₂O₄ nanoneedle arrays is proposed and is different from the traditional V-L-S and V-S models. The study on the PL spectrum of those ZnO nanoneedles shows the near band-edge emission band (379 nm) and the green emission band (510 nm). Especially, a red shift of the green emission band emerges with the increase of the annealing temperature and time.

Real surfaces display defects originating either from thermodynamic principles or from local instabilities. In order to help and characterize these surfaces and their defects, infrared spectroscopy of physisorbed test molecules can be used. As an example, we study the behavior of the infrared response of CO molecules randomly deposited on ionic surfaces of MgO and NaCl with the concentration of dipolar defects randomly distributed on these surfaces. The vibrational peak is shifted and asymmetrically broadened when the defect concentration increases as a result of surface inhomogeneity, in semi-quantitative agreement with experimental data.

We have studied the superconductivity of Pb ultra-thin films with thickness from 1 monolayer (ML) to 7 ML grown on Si(111) by molecular beam epitaxy. In situ low temperature scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) measurements were performed on the films. It is suggested that the interface effect plays a critical role in enhancing the electron-phonon coupling, which consequently increases the superconducting transition temperature when a film reaches to the two-dimensional limit.