G.A. Rizzi

PECVD of amorphous TiO2 thin films : effect of growth temperature and plasma gas composition

Abstract Amorphous TiO 2 thin films are grown using a r.f. plasma-enhanced chemical vapour deposition process at substrate temperatures between 393 and 523 K using titanium tetraisopropoxide as a precursor, and Ar or N 2 , pure or mixed with O 2 , as the plasma gas. All films are smooth and adherent, their roughness slightly increases by increasing the substrate temperature or if oxygen is added to the plasma gas. Films grown in the presence of oxygen result transparent in the visible region and highly resistive, as expected for pure titanium dioxide. Films grown in an oxygen-free plasma appear grey–blue and fairly conductive suggesting the presence of Ti(III) species. The operating conditions provide high deposition rates, up to 37 nm/min in the presence of oxygen.

Surface carboxylate species on Cu(100) studied by angle-scanned photoelectron diffraction and LCAO-LDF calculations

Chemisorption site geometries of formate and acetate species on Cu(100) have been studied by means of angle-scanned X-ray photoelectron diffraction (XPD) and first-principle quantum-mechanical calculations on a Cu60-formate cluster in the framework of local density functional theory (LDF). According to our LDF calculations, the short bridge site results to be more stable than the cross bridge one by about 20kcalmol. The reasons for the more effective interaction of the adsorbate in the short bridge site have been clearly outlined on the basis of the analysis of the LDP wavefunctions. LDF-optimized structural parameters have been used as a starting input in a series of single-scattering cluster spherical wave (SSC-SW) simulations of the O 1s XPD curves, which have experimentally confirmed the occupation of the short bridge site proposed by LDF calculations. The SSC-SW simulations have also outlined the role of the low-frequency-hindered rotational modes of the adsorbate (librations) and have furnished an estimate of the frequency (∼ 70 cm−1) of the hindered rotation on the axis perpendicular to the surface. Furthermore, as regards the formate species, the experimental data give evidence of a Cu-O distance of 1.95 A, in agreement with the LDF calculations and with angle-resolved photoemission fine structure (ARPEFS) literature results. An estimate of the OCO angle (129° ± 5°) has been obtained from polar scans for both formate and acetate species.

Growth of NiO ultrathin films on Pd(100) by post-oxidation of Ni films: the effect of pre-adsorbed oxygen

Abstract This paper reports on the optimisation of the growth parameters of NiO ultrathin films on Pd(1xa00xa00). Growth is performed by means of UHV metal deposition and post-oxidation cycles. Chemical and structural characterisation of the deposits is achieved by means of electron spectroscopy (X-ray photoelectron spectroscopy, XPS; angle resolved XPS) and electron diffraction techniques (low energy electron diffraction, LEED; X-ray photoelectron diffraction, XPD). Three growth procedures have been investigated, which differ for the particular growth parameters adopted in each case. We demonstrate that post-oxidation is effective in order to obtain epitaxial NiO only if the initial dose of Ni evaporated on the clean Pd(1xa00xa00) substrate exceeds a critical value, corresponding approximately to two equivalent monolayers. However, the overlayer thus obtained is strongly understoichiometric in oxygen close to the metal/oxide interface and poorly ordered on the long range. When a Ni dose below this limiting first value is used, the layer evolves toward polycrystalline NiO, due to substantial oxidation of the Pd substrate promoted by the presence of Ni, very likely through a work function decrease upon direct metal/metal interface formation. On the contrary, epitaxial NiO(1xa00xa00) layers of good structural quality, with limited oxygen deficiency at the interface, with negligible substrate oxidation and with a good degree of long-range order are obtained if deposition and post-oxidation cycles are initiated on an oxygen pre-saturated Pd surface, characterised by the (√5×√5)- R 27° O/Pd(1xa00xa00) LEED pattern. We therefore demonstrate that oxygen can act either as an inhibitor or as a promoter of NiO epitaxial growth on Pd(1xa00xa00), depending on the way it is used.

A chemical vapour deposition route to MoO3–Bi2O3 thin films

Abstract MoO 3 –Bi 2 O 3 thin films are grown by CVD on SiO 2 /Si(100) and Al 2 O 3 using BiPh 3 and MoO 2 (dpm) 2 as precursors. The composition, microstructure and morphology of the samples can be suitably controlled by the proper combination of synthesis and thermal treatment conditions. The chemical characterization of the obtained films is carried out by XPS and FTIR, while the microstructure and surface morphology are investigated respectively by XRD and AFM. Preliminary measurements on Bi-doped MoO 3 films indicate that they could be used for the detection of scarcely reducing gases.

Structure of highly strained ultrathin Ni films on Pd(100)

Abstract The epitaxial growth of ultrathin Ni films on the Pd(1xa00xa00) surface was studied by means of XPS, angle-scanned photoelectron diffraction (XPD) and LEED experiments. The XPD and LEED data indicate that the early stages of deposition are characterised by the formation of a tetragonally strained Ni fcc phase, which subsequently develops into a bulk-like Ni fcc structure as the critical thickness of approximately 12 ML eq is exceeded. The highly strained structure below the critical thickness is associated with subtle electronic structure changes with respect to Ni bulk, as evidenced by the analysis of the satellite structure in the Nixa02p XPS region.

An X-ray photoelectron diffraction structural characterization of an epitaxial MnO ultrathin film on Pt(1 1 1)

Abstract An epitaxial ultrathin MnO/Pt(1xa01xa01) film (13 A thick) has been grown by reactive UHV deposition of Mn2(CO)10 on Pt(1xa01xa01) at 200°C in the presence of water. Angle scanned X-ray photoelectron diffraction (XPD) and LEED have been used to structurally characterize the film. The observed 1×1 LEED pattern is in accord with the bulk lattice parameter of a MnO(1xa01xa01) surface (3.14 A) and demonstrates that the film is ordered in the long range. Full hemispherical O1s and Mn2p XPD plots have been obtained and analysed on the basis of multiple scattering calculations. The XPD data confirm that the MnO(1xa01xa01) surface (as a single domain) is exposed and a best fit procedure based on a R-factor analysis provides a direct evidence for a relaxation of the outermost double layer, whose values are similar to those found in other similar systems (CoO and FeO).

Growth and optical properties of silver nanostructures obtained on connected anodic aluminum oxide templates

Ag nanostructures are grown by AC electrodeposition on anodic alumina oxide (AAO) connected membranes acting as templates. Depending on the thickness of the template and on the voltage applied during the growth process, different Ag nanostructures with different optical properties are obtained. When AAO membranes about 1 μm thick are used, the Ag nanostructures consist in Ag nanorods, at the bottom of the pores, and Ag nanotubes departing from the nanorods and filling the pores almost for the whole length. When AAO membranes about 3 μm thick are used, the nanostructures are Ag spheroids, at the bottom of the pores, and Ag nanowires that do not reach the upper part of the alumina pores. The samples are characterized by angle resolved x-ray photoelectron spectroscopy, scanning electron microscopy and UV-vis and Raman spectroscopies. A simple NaOH etching procedure, followed by sonication in ethanol, allows one to obtain an exposed ordered array of Ag nanorods, suitable for surface-enhanced Raman spectroscopy, while in the other case (3 μm thick AAO membranes) the sample can be used in localized surface plasmon resonance sensing.

Silver nanoprism arrays coupled to functional hybrid films for localized surface plasmon resonance-based detection of aromatic hydrocarbons.

We report the achievement of sensitive gas detection using periodic silver nanoprisms fabricated by a simple and low-cost lithographic technique. The presence of sharp tips combined with the periodic arrangement of the nanoprisms allowed the excitement of isolated and interacting localized surface plasmon resonances. Specific sensing capabilities with respect to aromatic hydrocarbons were achieved when the metal nanoprism arrays were coupled in the near field with functional hybrid films, providing a real-time, label-free, and reversible methodology. Ultra-high-vacuum temperature-programmed desorption measurements demonstrated an interaction energy between the sensitive film and analytes in the range of 55-71 kJ/mol. The far-field optical properties and the detection sensitivity of the sensors, modeled using a finite element method, were correlated to experimental data from gas sensing tests. An absorbance variation of 1.2% could be observed and associated with a theoretical increase in the functional film refractive index of ∼0.001, as a consequence to the interaction with 30 ppm xylene. The possibility of detecting such a small variation in the refractive index suggests the highly promising sensing capabilities of the presented technique.

A photoelectron diffraction study of the surface-V2O3 (2 × 2) layer on Pd(1 1 1)

Abstract X-ray photoelectron diffraction (XPD) has been applied to the study of the surface-(s)-V 2 O 3 (2xa0×xa02) layer on Pd(1xa01xa01), which is a novel interface-stabilised vanadium oxide phase with no bulk oxide counterpart. It has been detected by scanning tunnelling microscopy (STM) during the growth of ultrathin films of vanadium oxide on Pd(1xa01xa01). XPD confirms the general features of the model for s-V 2 O 3 /Pd(1xa01xa01), which has been proposed previously on the basis of STM measurements and ab initio density-functional-theory (DFT) calculations. In addition, quantitative agreement is found between the DFT model and the XPD experiment in the estimate of the average V–O interlayer spacing: the experimental result is 0.72±0.07 A, while the DFT-derived value is 0.723 A.

Pseudomorphic-to-bulk fcc phase transition of thin Ni films on Pd(100)

We have measured the transformation of pseudomorphic