M. Lampimäki

Oxygen adsorption-induced nanostructures and island formation on Cu{100}: Bridging the gap between the formation of surface confined oxygen chemisorption layer and oxide formation

Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O(2) pressure (8.0x10(-7) and 3.7x10(-2) mbar) at 373 K. Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of Cu(100)-c(2x2)-O and Cu(100)-(2sqrt[2]xsqrt[2])R45 degrees -O domains, (2) the growth of well-ordered (2sqrt[2]xsqrt[2])R45 degrees-O islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered Cu(2)O. We demonstrate that the (2sqrt[2]xsqrt[2])R45 degrees-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu-O islands between two (2sqrt[2]xsqrt[2])R45 degrees-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu(2)O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.

Nanoscale oxidation of Cu(100): Oxide morphology and surface reactivity

Surface oxidation of Cu(100) in O(2) has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (T(S)=303-423 K) and O(2) pressure (p(O(2) )=3.7 x 10(-2)-213 mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu(2)O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O(2) and the onset of Cu-(2 square root 2 x square root 2)R45 degrees -O (theta(O)=0.5 ML) surface reconstruction, (2) initial formation of Cu(2)O and the appearance of 1.8 A high elongated islands that also adopt the Cu-(2 square root 2 x square root 2)R45 degrees -O structure, and (3) formation of highly corrugated Cu-O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu(2)O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.

Instrumentation and analytical methods of an x-ray photoelectron spectroscopy–scanning tunneling microscopy surface analysis system for studying nanostructured materials

The design and performance of an x-ray photoelectron spectroscopy (XPS)–scanning tunneling microscopy (STM) surface analysis system for studying nanostructured materials are described. The analysis system features electron spectroscopy methods (XPS and Auger electron spectroscopy) in addition to a variable temperature STM. With the analytical methods of the system, surface chemical analysis as well as surface morphology down to atomic resolution can be obtained. The system also provides facilities for sample cleaning, annealing, gas dosing, depth profiling, and surface modifications by sputtering and evaporation. Controlled gas exposures from ultrahigh vacuum to atmospheric pressures in the adjustable temperature range of 120–1100K can be carried out in different chambers. A fast entry air lock allows the transfer of samples and STM tips into the system without air exposures. The surface analysis system uses a common sample holder in all five chambers which are independently pumped and separated from each...

Kinetic hindrance during the surface oxidation of Cu(100)–c(10×2)-Ag

The influence of c(10x2)-Ag superstructure on the oxidation kinetics and oxygen adsorption-induced nanostructures on Cu(100) has been investigated as a function of O(2) exposure at 373 K by employing scanning tunneling microscopy and x-ray photoelectron spectroscopy. The oxygen adsorption-induced segregation of Cu through the Ag overlayer is found to trigger agglomeration of Ag and subsequent formation of ordered oval-shaped nanosize metallic Ag islands separated by Cu(100)-(2 radical2x radical2)R45 degrees -O surface phase. As oxygen exposure is further increased, all Ag is eventually covered by oxidized Cu. The presence of Ag delays the completion of the fully reconstructed (2 radical2x radical2)R45 degrees -O surface and the nucleation and growth of Cu(2)O islands by limiting Cu diffusion toward the surface. Once Cu(2)O grows into the bulk deeper than buried Ag, the oxidation kinetics follow that of the unalloyed clean Cu(100) surface. Similar kinds of Cu-O nanostructures are found on both clean Cu(100) and Cu(100)-c(10x2)-Ag surfaces. Details of the morphology of the Ag structures and kinetic control of the surface oxidation mechanism on Cu(100)-c(10x2)-Ag are discussed.

Substrate-induced effects in the creation and decay of potassium 2p core excitations in ultrathin films of KCl on copper

Substrate-induced effects on the transport properties in thin KCl films on the Cu(100) surface have been studied using K + 2p photoelectron, photoabsorption and resonant Auger spectra. The measurements were performed at different KCl coverages ranging from a partial monolayer to a thick bulk-like film. The morphology and layer thickness were estimated from an analysis of the electron energy loss structure of the K + 2p and Cl − 2p photoelectron peaks, and from the variations in the photoelectron peak fine structure. The resonant Auger spectator decay spectra measured at the photoabsorption resonances show that the significant differences between the spectra of the solid and of thin layers are related to the charge delocalization from the K + 3d excited state into the metal substrate. The core-hole-clock approach yields an estimate of 1.5 fs for the corresponding charge transfer time at a single monolayer KCl coverage. (Some figures in this article are in colour only in the electronic version)

Ag/Cu(100) Surface Alloy and Polycrystalline Cu(Ag) Alloy Studied by XPS

In this work, polycrystalline Cu(Ag) bulk alloy (1 wt.% Ag) and Ag/Cu(100) surface alloy (0.9 ML Ag) have been characterized by x-ray photoelectron spectroscopy (XPS) employing Al Kα and Mg Kα radiation. XPS spectra of the principal core levels (Cu, Ag) are presented together with XAES (x-ray induced Auger electron spectroscopy) spectra of the Cu LMM transition. The samples were prepared in situ by argon ion sputtering at room temperature and subsequent annealing at 700 K. Ag overlayer was deposited on the Cu(100) surface by a resistively heated Ag evaporator. The absence of contaminants such as C or O was confirmed by XPS. Together, the industrial Cu(Ag) alloy and the well defined Ag/Cu(100) model system serve as a template for studies of nanoscale surface oxidation and segregation phenomena.