Adsorption states of NO on the Pt(1 1 1) step surface

Using infrared reflection absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM), we investigated the adsorption states of NO on the Pt(9 9 7) step surface. At 90 K, we observe three N-O stretching modes at 1490 cm⁻¹, 1631 cm⁻¹ and 1700 cm⁻¹ at 0.2 ML. The 1490 cm⁻¹ and 1700 cm⁻¹ peaks are assigned to NO molecules at fcc-hollow and on-top sites of the terrace, respectively. The 1631 cm⁻¹ peak is assigned to the step NO species. In the present STM results, we observed that NO molecules were adsorbed at the bridge sites of the step as well as fcc-hollow and on-top sites of the terrace. To help with our assignments, density functional theory calculations were also performed. The calculated results indicate that a bridge site of the step is the most stable adsorption site for NO, and its stretching frequency is 1607 cm⁻¹. The interactions between NO species at different sites on Pt(9 9 7) are also discussed.     

Insulating properties of ultrathin KF layers on Cu(1 0 0): Resonant Auger spectroscopy

Solid-state effects in the creation and decay of K 2p core excitations in thin KF films on Cu(1 0 0) surface have been studied in resonant Auger spectra, excited using synchrotron radiation. The spectra of films of various thickness starting from a single monolayer were measured.The photoabsorption spectra reveal crystal field splitting already at film thickness of about 1 monolayer. The Auger decay spectra of the K 2p⁻¹3d core excitations in films of thickness up to 2 monolayers exhibit a band characteristic of the decay of core ionised states, showing that the excited electron delocalises into substrate before the core hole decays. In thicker films the coexistence of the decay of excited states in the bulk of the KF crystalline film and of ionised states at the KF-metal interface is observed, indicating that the charge transfer probability from the upper layers of the film into the metallic substrate is strongly reduced.     

Aromatic interactions in the close packing of phenyl-imides at Cu(1 1 0) surfaces

The oxidation of aniline at Cu(1 1 0) surfaces at 290 K has been studied by XPS and STM. A single chemisorbed product, assigned to a phenyl imide (C₆H₅N(a)), is formed together with water which desorbs. Reaction with preadsorbed oxygen results in a maximum surface concentration of phenyl imide of 2.8 × 10¹⁴ mol cm⁻² and a surface dominated by domains of three structures described by , and unit meshes. However, concentrations of phenyl imide of up to 3.3 × 10¹⁴ mol cm⁻² were obtained from the coadsorption of aniline and dioxygen (300:1 mixture) resulting in a highly ordered biphasic structure with and domains. Comparison of the STM and XPS data shows that only half the phenyl imides at the surface are imaged. Pi-stacking of the phenyl rings is proposed to account for this observation.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

Intracavity laser absorption spectroscopy of HDO between 12 145 and 13 160 cm

The high resolution absorption spectrum of monodeuterated water, HDO, has been recorded by Intracavity Laser Absorption Spectroscopy (ICLAS) in the 12 145-13 160 cm⁻¹ region. The achieved sensitivity (noise equivalent absorption on the order of α_min ∼ 10⁻⁹ cm⁻¹) allowed detecting transitions with line strengths as weak as 10⁻²⁷ cm/molecule which is about 50 times lower than the weakest line intensities previously detected in the considered region.The rovibrational assignment of the 1179 lines attributed to the HDO isotopologue was based on the results of the variational calculations of Schwenke and Partridge as well as the recent calculations based on a new HDO potential energy surface refined from the fitting to the available experimental data. The overall agreement between these new calculations and the observed spectrum is very good, the rms deviation of the differences between the calculated and observed energy values being 0.05 cm⁻¹. A set of 304 new experimental HDO energy levels was obtained. In particular, band origins for the (1 2 2), (2 0 2), and (3 1 1) vibrational states, at 12 568.190, 12 644.652, and 12 919.938 cm⁻¹, respectively, and their rotational sublevels are derived for the first time. A detailed HDO database of 1337 transitions was constructed and is provided as Supplementary Material.     

Room-temperature electrosynthesized ZnO thin film with strong (0 0 2) orientation and its optical properties

ZnO thin film with strong orientation (0 0 2) and smooth surface morphology was electrosynthesized on ITO-coated glass substrate at room temperature under pulsed voltage. Photoluminescence (PL) shows two obvious peaks: violet band and strong green band. The former is due to the free-excitonic transition and the latter is believed to arise from the single ionized oxygen vacancy (V_O⁺). Raman scattering reveals that the 580 cm⁻¹ mode and the shoulder peak mode at 550 cm⁻¹ originate from the N-related local vibration mode (LVM) and E₁ (LO) mode, respectively.     

Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 × 1) NaCl atomic structure for sample bias voltage V_s < −2.5 V and the bright protrusions should be assigned to the Cl⁻ ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)-p(2 × 2) surface as the result of weak interface interaction between the dangling bonds coming from valence π states of the Ge substrate and the 3p states of the interfacial Cl⁻ ion. These results are consistent with the experiments of STM, LEED and EELS.     

Formation and hydrogenation of p(2 × 2)-N on Pt(1 1 1)

The formation of a well-ordered p(2 × 2) overlayer of atomic nitrogen on the Pt(1 1 1) surface and its reaction with hydrogen were characterized with reflection absorption infrared spectroscopy (RAIRS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The p(2 × 2)-N overlayer is formed by exposure of ammonia to a surface at 85 K that is covered with 0.44 monolayer (ML) of molecular oxygen and then heating to 400 K. The reaction between ammonia and oxygen produces water, which desorbs below 400 K. The only desorption product observed above 400 K is molecular nitrogen, which has a peak desorption temperature of 453 K. The absence of oxygen after the 400 K anneal is confirmed with AES. Although atomic nitrogen can also be produced on the surface through the reaction of ammonia with an atomic, rather than molecular, oxygen overlayer at a saturation coverage of 0.25 ML, the yield of surface nitrogen is significantly less, as indicated by the N₂ TPD peak area. Atomic nitrogen readily reacts with hydrogen to produce the NH species, which is characterized with RAIRS by an intense and narrow (FWHM ∼ 4 cm⁻¹) peak at 3322 cm⁻¹. The areas of the H₂ TPD peak associated with NH dissociation and the XPS N 1s peak associated with the NH species indicate that not all of the surface N atoms can be converted to NH by the methods used here.     

Gd adsorption on the Mo(2 1 1) surface

The adsorption of Gd thin layers on the Mo(2 1 1) face was investigated by using Auger electron spectroscopy (AES), low electron energy diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and measurements of the work function changes (Δφ). It was found that at 300 K Gd does not form any dilute chain structures and from the very beginning of the adsorption process Gd forms a densely packed layer. The dilute p(4 × 1) chain structure was observed by LEED after annealing thin layers (θ < 1 ML) to temperatures above 770 K. STM images confirm the existence of the p(4 × 1) structure islands. The intermixing of the substrate and adsorbate atoms takes place.     

Characterization of crystal lattice constant and dislocation density of crack-free GaN films grown on Si(1 1 1)

GaN have sphalerite structure (Cubic-GaN) and wurtzite structure (hexagonal GaN). We report the H-GaN epilayer with a LT-AlN buffer layer has been grown on Si(1 1 1) substrate by metal-organic chemical vapor deposition (MOCVD). According to the FWHM values of 0.166° and 14.01 cm⁻¹ of HDXRD curve and E₂ (high) phonon of Raman spectrum respectively, we found that the crystal quality is perfect. And based on the XRD spectrum, the crystal lattice constants of Si (a = 5.3354 ?) and H-GaN (a_epi = 3.214 ?, c_epi = 5.119 ?) have been calculated for researching the tetragonal distortion of the sample. These results indicate that the GaN epilayer is in tensile strain and Si substrate is in compressive strain which were good agreement with the analysis of Raman peaks shift. Comparing with typical values of screw-type (D_screw = 7 × 10⁸ cm⁻²) and edge-type (D_edge = 2.9 × 10⁹ cm⁻²) dislocation density, which is larger than that in GaN epilayers growth on SiC or sapphire substrates. But our finding is important for the understanding and application of nitride semiconductors.     

Carbon monoxide adsorption on Pd-deposited Cu(1 1 0) surface: Infrared reflection absorption and temperature programmed desorption studies

We investigated carbon monoxide (CO) adsorption and desorption behaviors on 0.1-nm-, 0.15-nm-, and 0.3-nm-thick-Pd-deposited Cu(1 1 0) surfaces using infrared reflection absorption (IRRAS) and temperature-programmed desorption (TPD) spectroscopic methods. CO was exposed to the 0.1-nm-thick-Pd/Cu(1 1 0) surface at the substrate temperature of 90 K. The IR band attributable to CO bonded to Cu atoms emerged at 2092 cm⁻¹: the band was located at 2100 cm⁻¹ at saturation coverage, with a shoulder at 2110 cm⁻¹. In addition to these bands, weak absorptions attributable to the PdCO bonds appeared at 2050 and 1960 cm⁻¹. With increasing Pd thickness, the Pd related-bands became increasingly prominent. Particularly at the early stage of exposure, the band at 2115 cm⁻¹ became visible. The band at 2117 cm⁻¹ dominated the spectra all through the exposures for the 0.3-nm-thick-Pd surface. The TPD spectra of the surfaces showed two remarkable features at around 220-250 and 320-390 K, ascribable ,respectively, to CuCO and PdCO. The desorption peaks shifted to higher temperatures with increasing Pd thickness. Based on the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the Pd/Cu(1 1 0) surfaces.     

Growth of Ag thin films on ZnO(0 0 0 −1) investigated by AES and STM

The growth of Ag films on ZnO(0 0 0 −1) has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). A high density of islands is nucleated at the earliest stages of the growth. An upstepping mechanism causes these islands to coalesce while the uncovered fraction of the ZnO surface remains constant (30%).     

Rate coefficients measurement for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F)

We report experimental rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). In the experiment the Cs(5D) state was populated via photodissociation of Cs₂ molecules using an argon-ion laser at wavelength 488.0 nm. We also consider the competing process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F) that might also populate 9D, 11S and 7F. An intermodulation technique was used to select the fluorescence contributions due only to the process 6P_1/2 + 7S → 6S + (nl = 9D, 11S, 7F). The excited atom (nl_J) density and spatial distribution were mapped by monitoring the absorption of a counterpropagating probe laser beam tuned to various transitions. The measured excited atom densities are combined with measured fluorescence ratios to yield rate coefficients for the energy-pooling collisions Cs(5D) + Cs(5D) → Cs(6S) + Cs(nl = 9D, 11S, 7F). The rate coefficients for nl = 9D, 11S, 7F are (4.1 ± 2.0) × 10⁻¹⁰ cm³ s⁻¹, (1.6 ± 0.8) × 10⁻¹⁰ cm³ s⁻¹ and (3.6 ± 1.8) × 10⁻¹⁰ cm³ s⁻¹, respectively. The contributions to the rate coefficients from other energy transfer processes are also discussed.     

Focused ion beam induced swelling in MgO(0 0 1)

The interplay between swelling and milling phenomena in determining the morphology of Focused Ion Beam (FIB) -processed MgO(0 0 1) was investigated by atomic force microscopy. At the early stages of ion irradiation, before milling erosion is observed, MgO shows a relevant swelling behaviour with protrusion of the bombarded areas up to 6 nm for a dose of 5 × 10¹⁶ ions cm⁻². The effect is mainly ascribed to subsurface defect accumulation, while the low Ga ions concentration, as measured by in-depth Auger analysis, seems to exclude a contribution from ion implantation. In order to explain and control the morphology of Fe/NiO FIB patterned sub-micron structures on MgO substrates, we have also investigated FIB effects on Fe(0 0 1) and NiO(0 0 1) single crystals. Absent or negligible swelling has been observed on these materials.     

The structures of physisorbed and chemisorbed formic acid on Si(1 1 1)-7 × 7

The reaction of formic acid on Si(1 1 1)-7 × 7 was investigated using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and high-resolution electron energy loss spectroscopy (HREELS). The hydroxyl and carbonyl O 1s core levels of chemisorbed formic acid display chemical shifts of 2.4 and 0.2 eV respectively, compared with those of physisorbed molecules. The HREELS spectra of chemisorbed formic acid show the absence of stretching and bending modes of the O-H bond, the appearance of Si-H (2089 cm⁻¹) and the Si-O (680 cm⁻¹) stretching modes and the retained stretching mode of CO at 1703 cm⁻¹. Our results clearly suggest that formic acid dissociates to form monodentate formate species and H-atom on the adatom-rest atom pair of Si(1 1 1)-7 × 7.     

Electrical characterisation of ruthenium Schottky contacts on n-Ge (1 0 0)

Ruthenium (Ru) Schottky contacts were fabricated on n-Ge (1 0 0) by electron beam deposition. Current–voltage (I–V), deep level transient spectroscopy (DLTS), and Laplace-DLTS techniques were used to characterise the as-deposited and annealed Ru/n-Ge (1 0 0) Schottky contacts. The variation of the electrical properties of the Ru samples annealed between 25 °C and 575 °C indicates the formation of two phases of ruthenium germanide. After Ru Schottky contacts fabrication, an electron trap at 0.38 eV below the conduction band with capture cross section of 1.0×10⁻¹⁴ cm⁻² is the only detectable electron trap. The hole traps at 0.09, 0.15, 0.27 and 0.30 eV above the valence band with capture cross sections of 7.8×10⁻¹³ cm⁻², 7.1×10⁻¹³ cm⁻², 2.4×10⁻¹³ cm⁻² and 6.2×10⁻¹³ cm⁻², respectively, were observed in the as-deposited Ru Schottky contacts. The hole trap H(0.30) is the prominent single acceptor level of the E-centre, and H(0.09) is the third charge state of the E-centre. H(0.27) shows some reverse annealing and reaches a maximum concentration at 225 °C and anneals out after 350 °C. This trap is strongly believed to be V–Sb₂ complex formed from the annealing of V–Sb defect centre.     

Carbon monoxide adsorption on Co deposited Pt(1 0 0)-hex: IRRAS and LEED investigations

Infrared reflection absorption spectroscopy (IRRAS) was used to investigate carbon monoxide (CO) adsorption on Pt(1 0 0) surfaces deposited with Co layers with different thicknesses. Pt(1 0 0) surfaces cleaned in ultrahigh vacuum showed surface reconstruction, i.e., Pt(1 0 0)-hex: two absorption bands ascribable to adsorbed CO on the 1 × 1 surface and hex domains emerge at 2086 and 2074 cm⁻¹, respectively, after 1.0 L CO exposure. Deposition of a 0.3-nm-thick-Co layer on Pt(1 0 0)-hex at 333 K changes the low-energy electron diffraction (LEED) pattern from hex to p(1 × 1), indicating that the deposited Co lifts the reconstruction. The IRRAS spectrum for 1.0-L-CO-exposed Co_0.3 nm/Pt(1 0 0)-hex fabricated at 333 K yields a single absorption band at 2059 cm⁻¹. For Co_0.3 nm/Pt(1 0 0)-hex fabricated at 693 K, the LEED pattern shows a less-contrasted hex and the pattern remains nearly unchanged even after CO exposure of 11 L, although only 1.0 L CO exposure to Pt(1 0 0)-hex lifts the surface reconstruction. A Co_0.3 nm/Pt(1 0 0)-hex surface fabricated at 753 K exhibits an absorption band at 2077 cm⁻¹, which is considered to originate from CO adsorbed on the Pt-enriched surface alloy. Co_0.3 nm/Pt(1 0 0)-hex surfaces fabricated above 773 K show a clear hex-reconstructed LEED pattern, and the frequencies of the adsorbed CO bands are comparable to those of Pt(1 0 0)-hex, indicating that the deposited Co atoms are diffused near the surface region. The outermost surface of the 3.0-nm-thick-Co-deposited Pt(1 0 0)-hex is composed of Pt-Co alloy domains even at a deposition temperature of 873 K. Based on the LEED and IRRAS results, the outermost surface structures of Co_x/Pt(1 0 0)-hex are discussed.     

Local chemical transformations in poly(dimethylsiloxane) by irradiation with 248 and 266 nm

Poly(dimethylsiloxane) (PDMS) has been irradiated with a frequency quadrupled Nd:YAG laser and a KrF^*-excimer laser at a repetition rate of 1 Hz. The analysis of ablation depth versus pulse number data reveals a pronounced incubation behavior. The thresholds of ablation (266 nm: 210 mJ cm⁻², 248 nm: 940 mJ cm⁻²) and the corresponding effective absorption coefficients α_eff (266 nm: 48900 cm⁻¹, 248 nm: 32700 cm⁻¹, α_lin = 2 cm⁻¹) were determined. The significant differences in the ablation thresholds for both irradiation wavelengths are probably due to the different pulse lengths of both lasers. Since the shorter pulse length yields a lower ablation threshold, the observed incubation can be due to a thermally induced and/or a multi-photon absorption processes of the material or impurities in the polymer.Incubation of polymers is normally related to changes of the chemical structure of the polymer. In the case of PDMS, incubation is associated with local chemical transformations up to several hundred micrometers below the polymer surface. It is possible to study these local chemical transformations by confocal Raman microscopy, because PDMS is transparent in the visible. The domains of transformation consist of carbon and silicon, as indicated by the appearance of the carbon D- and G-bands between 1310 and 1610 cm⁻¹, a band appearing between 502 and 520 cm⁻¹ can be assigned to mono- and/or polycrystalline silicon.The ablation products, which are detected in the surroundings of the ablation crater consist of carbon and amorphous SiO_x (x ≈ 1.5) as detected by infrared spectroscopy.     

Growth and oxidation of a Ni3Al alloy on Ni(1 0 0)

The growth and oxidation of a thin film of Ni₃Al grown on Ni(1 0 0) were studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). At 300 K, a 12 Å thick layer of aluminium was deposited on a Ni(1 0 0) surface and subsequently annealed to 1150 K resulting in a thin film of Ni₃Al which grows with the (1 0 0) plane parallel to the (1 0 0) surface of the substrate. Oxidation at 300 K of Ni₃Al/Ni(1 0 0) until saturation leads to the growth of an aluminium oxide layer consisting of different alumina phases. By annealing up to 1000 K, a well ordered film of the Al₂O₃ film is formed which exhibits in the EEL spectra Fuchs-Kliewer phonons at 420, 640 and 880 cm⁻¹. The LEED pattern of the oxide shows a twelvefold ring structure. This LEED pattern is explained by two domains with hexagonal structure which are rotated by 90° with respect to each other. The lattice constant of the hexagonal structure amounts to ∼2.87 Å. The EELS data and the LEED pattern suggest that the γ^′-Al₂O₃ phase is formed which grows with the (1 1 1) plane parallel to the Ni(1 0 0) surface.