Monolayer Of Cu Research Articles

Transition from chemisorbed copper layers to bulk deposits on platinized platinum in sulfuric acid solution

Thin copper films which do not yet possess the properties of bulk deposits are formed on platinized platinum in 0.5 M H 2SO 4 + 0.01 M CuSO 4 · 5H 2O within a narrow potential range of about 30 mV positive to the equilibrium potential of the net reaction Cu 2+ + 2e − = Cu and at shorter times in the potential range of about 10 mV negative to it. In the latter case the presence of this film and of a monolayer of Cu ad is still visible in anodic stripping curves at constant current. Bulk deposits are non-uniform as evidenced by the appearance of reddish patches. The behavior of bulk deposits is approached by the copper films when the ratio of the anodic stripping charge to the charge for the anodic removal of a monolayer of H ad becomes larger than 10.

X-ray photoelectron forward scattering studies of surface segregation in epitaxial Ni–Cu–Ni(100) sandwich structures

The forward scattering of the Cu 2p3/2 core level photoelectrons by overlying Ni lattice atoms which occurs in the photoemission process has been used to follow the segregation of Cu to the surface of epitaxial Ni–Cu–Ni(100) sandwich structures. The results indicate that when 1 monolayer (ML) of Cu is covered by 1–2 ML of Ni, surface diffusion processes constitute the mechanism by which the segregation occurs, whereas if 1 ML of Cu is covered by 10 ML or more of Ni the segregation involves the usual bulk (vacancy) diffusion mechanism. Since these processes are quite general, it appears that the top few atomic layers of a binary alloy can equilibrate with respect to surface segregation at a temperature significantly lower than that which is required for bulk equilibration. This combination of x-ray photoelectron spectroscopy (XPS) forward scattering and epitaxial sandwich structures thus provides both more clear-cut data and a more specific understanding of segregation phenomena than is usually possible by conventional XPS and Auger studies on random alloys.

In situ studies of epitaxial growth in the low energy electron microscope

Abstract The epitaxial growth of Cu on Mo{110}, Au on Mo{110}, Cu on Si{111} and Au on Si{111} is studied in situ in the ultra high vacuum low energy electron microscope. Nucleation at atomic steps and defects is seen, and submonolayer growth is followed. Domains and domain boundaries within single terraces are resolved. The monolayer of Cu is pseudomorphic with the Mo{110} substrate, whereas the double layer is in the Nishiyama-Wassermann orientation with the Cu(111)∥Mo(110) and Cu[1 1 0]∥Mo[001]. At 700 K the Cu double layer is coherent along Mo[001] and grows with a needle-like habit in this direction. The Au monolayer grows in the Kurdjumov-Sachs orientation with Au(111)∥Mo(110) and Au[1 1 0]∥Mo[1 1 1]. At low temperatures Au forms rows along Mo〈111〉 and grows with a needle-like habit in these directions. Cu grows on Si{111}(7×7) as a silicide monolayer followed by three-dimensional islands with sides aligned along Si〈110〉. Atomic steps often bunch together into groups of three and facet along Si〈110〉. In the growth of Au on Si{111} at elevated temperature the (5×1) structure forms elongated islands oriented along Si〈110〉, which is accompanied by two-dimensional faceting of atomic steps along Si〈110〉. Step bunching, which causes three-dimensional microfaceting, leads to a loss of smoothness of the Si substrate.

In situ studies of epitaxial growth in the low energy electron microscope

The epitaxial growth of Cu on Mo{110}, Au on Mo{110}, Cu on Si{111} and Au on Si{111} is studied in situ in the ultra high vacuum low energy electron microscope. Nucleation at atomic steps and defects is seen, and submonolayer growth is followed. Domains and domain boundaries within single terraces are resolved. The monolayer of Cu is pseudomorphic with the Mo{110} substrate, whereas the double layer is in the Nishiyama-Wassermann orientation with the Cu(111)∥Mo(110) and Cu[1 1 0]∥Mo[001]. At 700 K the Cu double layer is coherent along Mo[001] and grows with a needle-like habit in this direction. The Au monolayer grows in the Kurdjumov-Sachs orientation with Au(111)∥Mo(110) and Au[1 1 0]∥Mo[1 1 1]. At low temperatures Au forms rows along Mo〈111〉 and grows with a needle-like habit in these directions. Cu grows on Si{111}(7×7) as a silicide monolayer followed by three-dimensional islands with sides aligned along Si〈110〉. Atomic steps often bunch together into groups of three and facet along Si〈110〉. In the growth of Au on Si{111} at elevated temperature the (5×1) structure forms elongated islands oriented along Si〈110〉, which is accompanied by two-dimensional faceting of atomic steps along Si〈110〉. Step bunching, which causes three-dimensional microfaceting, leads to a loss of smoothness of the Si substrate.

Interaction of O and Cu on W(110): II. The presegregation regime

Adsorption of oxygen on Cu 1/W(110) and Cu 2/W(110), as well as adsorption of Cu on O x/W(110) (0 ⩽ x ⩽ 0.85) was studied. It is concluded that at 90 K. O adsorption occurs on top of Cu 1/W(110) or Cu 2/W(110), with O diffusing to the W surface without desorption on heating to 300 K, at which point incipient segregation occurs. Cu can also be adsorbed on top of O 0.6/W(110). On heating Cu multilayer formation starts in this case at ~ 200 K. For O adsorption on Cu 1/W(110) the O dipole moment is nearly the same as on clean W(110), while it is much larger on Cu 2/W(110). A monolayer of Cu on O x/W(110) shows metallic behavior, and in particular screens the W-O dipole potential almost completely up to x = 0.6, so that φ is nearly that of Cu 1/W(110). Similar screening was also noted for a Cu monolayer on CO/W(110). The sticking coefficient s of O 2 on Cu 1]/W(110) is unity over a wide coverage range at all temperatures investigated, while it is only 0.5 on Cu 2/W(110) and 0.55 on W(110). This behavior is tentatively attributed to soft transverse vibrational modes on Cu 1/W(110) arising from W/Cu size mismatch which permits efficient energy exchange between incoming O 2 molecules and the surface. As noted in paper I there is also adsorption of O 2 on Cu 1/O 0.6/W(110). The sticking coefficient for this process is 0.58 at 90 K up to maximum θ o = 0.2. UPS measurements for Cu n /W(110) where 0.5⩽ n ⩽ 8 are presented, and show a gradual transition to Cu bulk DOS. UPS measurements for O/Cu 1/W(110), O/Cu 2/W(110) and Cu 1/O 0.6/W(110) are also presented and discussed. In particular it is noted that the spectrum for Cu 1/O 0.6/W(110) seems to resemble that of Cu[in2 /W(110), i.e. appears more Cu-like than that of O/Cu 1/W(110) or Cu 1/W(110).

ChemInform Abstract: Underpotential Deposition of Copper and Silver on Polycrystalline Ruthenium Electrodes.

AbstractThe electrodeposition of the first monolayer of Cu on Ru occurs in a potential range which is clearly separated from that of bulk Cu deposition.

AES, LEED and TDS studies of Cu on Si(111)7 × 7 and Si(100)2 × 1

We have studied Cu/Si(111)7 × 7 and Cu/Si(100)2 × 1 by means of AES, LEED and TDS. Cu films have been deposited up to a coverage of ~10 monolayers keeping the substrate at room temperature, 130°C and 200°C. Cu/Si(111) develops a well defined two-dimensional interface structure on annealing the surfaces to ~500° C, which is not commensurable with the Si lattice and contains approximately 1 monolayer of Cu. For substrate temperatures ≥130°C the evaporated Cu layers grow on top of the Cu/Si(111) interface structure in form of three-dimensional islands. For Cu/Si(100) no well defined interface was found; For substrate temperatures ≥130°C the deposited Cu atoms also condense in three dimensional islands. The binding energies of the Cu atoms in the islands were determined by TDS and agreed for both surfaces within the experimental uncertainty.

Two-dimensional surface alloys: Copper on W(100)

Structural transformations when up to two monolayers of Cu are adsorbed on W(100) and then annealed have been studied by ion scattering spectroscopy, low energy electron diffraction (LEED), Auger electron spectroscopy (AES), work function changes and thermal desorption spectroscopy (TDS). Adsorption at 300 K produces epitaxial growth up to two monolayers, but annealing the surface at ≈700 K induces coverage-dependent structural transformations. In the coverage range 0.2 4 He + collected at various scattering geometries show that this structure consists of a coplanar two-dimensional double-spaced Cu−W surface alloy. AES demonstrates that there is no significant bulk alloying. Annealing at θ=0.75 produces a p(2×2) structure which disorders at 900 K, while with higher Cu coverages the LEED patterns after annealing show disordering and domain formation. These results provide a structural basis for a coherent reinterpretation of the data produced by Bauer and coworkers for the same system.

A re-examination of the Cu/Ru(0001) system

A thick Ru(0001) film was epitaxially grown by vapor deposition on a Mo(110) surface. Using this clean Ru(0001) film as a substrate, the room temperature absorption and growth of up to 5 monolayers of Cu are studied by AES, LEED and work function change measurements. The results show two-dimensional layer-by-layer growth up to 5 monolayers. The CO chemisorption properties of the Cu/Ru(0001) bilayer at room temperature are also studied as a function of Cu coverage. The CO saturation coverage decreases linearly with Cu coverage, i.e. no ensemble effect is seen.

Growth of metastable fcc Co on Ni(001).

We have used high-energy Auger-electron diffraction and associated kinematical scattering calculations to determine that Co, which is hcp at room temperature and atmospheric pressure, can be grown as a stable, unstrained fcc film on Ni(001). Studies of the early stages of growth show that the first monolayer equivalent does not cover the surface uniformly but rather forms two-dimensional patches with limited amounts of a second layer. The second monolayer equivalent adds to the second layer of each patch and provides a limited number of third-layer atoms. When the total Co coverage exceeds two monolayer equivalents, the overlayer coalesces into a continuous fcc film with no detectable strain up to 30 monolayers (the highest coverage studied). For comparison, we have measured angular distributions for Cu/Ni(001) with 0.5 and 1.0 monolayer of Cu. From surface thermodynamic considerations, Cu is expected to wet the surface uniformly. However, Cu also tends to form two-layer-deep patches at a coverage of one monolayer equivalent. These results demonstrate that limited surface mobility is significant in determining the morphology of monolayer metal films, in agreement with theoretical models of overlayer growth behavior.

Cu adsorption on Pt(111) and its effects on Chemisorption: A comparison with electrochemistry

The growth modes and interaction of vapor-deposited Cu on a clean Pt(111) surface have been monitored by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and work function measurements. The LEED data indicate that below 475 K Cu grows in p(1 × 1) islands in the first monolayer with the interatomic Cu spacing the same as the Pt(111) substrate. The second monolayer of Cu grows in epitaxial, rotationally commensurate Cu(111) planes with the CuCu distance the same as bulk Cu. For substrate temperatures below ∼ 475 K, the variation of work function and “cross-over beam voltage” with Cu coverage show characteristic features at one monolayer that are quite useful for calibration of θ Cu. Above 525 K, Cu-Pt alloy formation was observed in AES and LEED data. Thermal desorption spectroscopy of H 2 and CO has demonstrated that simple site blocking of the Pt(111) surface by vapor-deposited Cu occurs linearly with chemisorption being essentially eliminated at θ Cu = 1.0–1.15. Conclusions drawn from this work correlate very favorably with the well-known effects of under potentially deposited copper on the electrochemistry of the H 22H + couple at platinum electrodes.

Structural and electronic trends in the growth of Cu overlayers on the Nb(110) surface

The stages of growth of thin Cu overlayers (less than 3 monolayers) on the Nb(110) clean surface have been studied by low-energy electron diffraction (LEED), Auger-electron spectroscopy, and angle-resolved photoemission spectroscopy. The LEED spectra indicated that in the early stages of growth less than about 1.5 monolayers of Cu grows in an incommensurate structure resembling a strained fcc (100) Cu surface designated Cu(I). Above that coverage the overlayer transforms to a fcc Cu(111) surfacelike structure designated Cu(II). The spectral features observed in the Cu(I) and Cu(II) photoemission spectra are compared with those of Cu(100) and Cu(111) surfaces.

LEED intensity measurements of Cu epitaxially grown on Ni(100)

LEED intensity measurements are reported for pure Ni(100), and for one monolayer, four and nine layers of Cu on Ni(100). The intensities of the (00), (10), (11), (20) and (22) sets of spots were measured using a spot photometer. The largest variation in the intensity pattern with coverage was observed for the (00) spot. Dynamical calculations were performed for the specular and non-specular beams. It is found that the clean Ni(100) surface has the same interlayer distance as bulk nickel, while one monolayer of Cu on Ni(100) is found to be expanded by about 1.7%.

STUDIES OF ELECTRODE-ELECTROLYTE INTERFACES "IN SITU" BY MIRAGE DETECTION WITH POLARIZATION MODULATION

By coupling Mirage detection and polarization modulation of the incident light, a sensitivity corresponding to one tenth of a monolayer of Cu on Pt has been achieved. Concentration gradient effects in the vicinity of the electrode are discussed.

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O 2 was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O 2, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.

Thermal desorption and UPS study of CO adsorbed on Cu‐covered Ru (0001)

We have studied the electronic structure and chemisorption properties of (0–4) monolayers of Cu evaporated on a Ru (0001) single crystal substrate. HeI UPS shows that the Cu d band initially (0–1.5 monolayers) forms as an asymmetric resonance of 0.75 eV half width, 2.75 eV below EF. When the Cu coverage is increased, the resonance broadens and shifts nearer EF until a bulk (111) Cu spectrum is attained at 4.0 monolayers [Cu(111) forms on Ru(0001)]. Thermal desorption of CO from the Cu overlayers shows that CO is more strongly bound to the first layer of Cu(?15 kcal/mol) than to the second and succeeding layers of Cu [?12 kcal/mol typical of bulk (111) Cu]. There is also evidence that more CO adsorbs on the first layer of Cu than adsorbs on bulk (111) Cu.

Chemisorption of O 2 and N 2O on Cu/Ru(001)

The interaction of O 2 and N 2O are compared on the basal plane of Ru as a function of the coverage of copper. Dissociative N 2O uptake (N 2O(g) → O(a) + N 2(g)) on clean Ru was measured using AES and transient partial pressures. The initial dissociative reaction probability is a steadily declining function of temperature while the saturation uptake of oxygen remains constant from 300–900 K. The saturation oxygen AES signal for N 2O chemisorption was one half that observed for O 2 chemisorption. The presence of 0.02 monolayers of Cu retards the initial dissociative adsorption of N 2O by 40% but has little effect on the total uptake of oxygen. The initial dissociative sticking coefficient for O 2 is not retarded significantly by small amounts of copper. Deposition of larger amounts of Cu leads to the completion of a 2D overlayer before growth in 3D begins. Surfaces covered with less than 3 monolayers of Cu exhibit larger O 2 sticking coefficients and greater oxygen stability than pure Cu.

Band structure of thin films by the linear augmented-plane-wave method

We present a linear augmented-plane-wave method for solving the band-structure problem in thin crystalline films. The potential is separated into a muffin-tin potential inside the film, a potential depending exclusively on the normal coordinate outside the film, and corrections in both regions. The method is tested on (100) and (111) monolayers of Cu using a standard muffin-tin potential.

Electronic Structure of an Ordered Monolayer of Cu on Zn(0001)

Electronic Structure of an Ordered Monolayer of Cu on Zn(0001)

Potentiodynamic desorption spectra of metallic monolayers of Cu, Bi, Pb, Tl, and Sb adsorbed at (111), (100), and (110) planes of gold electrodes

The formation of metallic adsorption layers was studied in solutions of Cu 2+, Bi 3+, Pb 2+, Tl + and Sb 3+ at (111), (100) and (110) planes of gold single crystal electrodes. Potentiodynamic desorption spectra were recorded with a sweep rate of 20 mV s −1 for all systems. Characteristic peak structures were obtained which depend strongly on the nature of the adsorbate as well as on the substrate orientation. The half width of the peaks indicates attraction and repulsion respectively for various systems. In most systems more than one peak was observed. This is explained by the formation of various ordered structures. At low coverages peak charge data obtained by integration of current/time curves yield surface concentrations which fit those of ordered structures well, e.g. c(2×2) on (100) or p( 3 × 3 ) R 30° on (111). The adsorption behaviour of the (110) plane is similar in all systems because atomic chains seem to be generally stable. Near the equilibrium potential of the correspondent metal electrode, ϵ r = 0, a “mono-molecular” adsorption layer was found for Cu 2+, Pb 2+ and Bi 3+. In the case of the small copper atom, a 1:1 adsorption was found for all planes. Larger atoms like bismuth and lead form epitactic layers at low coverages; at high coverages they form close-packed monolayers with surface concentrations independent of the substrate structure but decreasing with increasing adsorbate radius. The coulometric data for antimony and thallium are not so conclusive. Measurements with various sweep rates show that the adsorption reaction is a slow potential dependent process in various systems. The underpotential/work function correlation of Kolb, Gerischer and Przasnyski is discussed with respect to these experiments. It follows that this concept developed for polycrystalline electrodes is qualitatively valid for (110), but not clearly so for (100) and (111).