S. Di Nardo

Thin and ultra-thin films of nickel phthalocyanine grown on highly oriented pyrolitic graphite : an XPS, UHV-AFM and air tapping-mode AFM study

Abstract Thin and ultra-thin films of nickel phthalocyanine have been deposited in ultra-high vacuum on highly oriented pyrolitic graphite. The growth mode and the interaction with the substrate have been studied in situ by X-ray photoelectron spectroscopy, scanning tunneling microscopy and atomic force microscopy, and ex situ by means of tapping-mode atomic force microscopy. We present photoelectron spectroscopy measurements at various film thicknesses which show no detectable interaction of the adsorbed organic layers with the substrate, but, varying the deposited film thickness by in-situ annealing the substrate, we show desorption effects semiquantitatively described. The XPS spectrum of the C 1s multiplet structure of NiPC obtained using a monochromatized source is also presented. The scanning probe images presented address morphological issues like the growth mode at initial stages of deposition and at higher coverages. Moreover, upon annealing of the PC films, we show high-resolution measurements consistent with the low-size stable α crystalline phase of phthalocyanine molecules. The potential of tapping-mode atomic force microscopy in imaging soft adlayers deposited over soft substrates is addressed throughout the paper.

Thermally induced phase transition in crystalline lead phthalocyanine films investigated by XRD and atomic force microscopy

Lead phthalocyanine films have been deposited at room temperature on to silicon substrates and than annealed in high vacuum at different temperatures. The samples have been analysed by X-ray diffraction (XRD) as well as atomic force microscopy (AFM) in order to get structural and morphological information on the samples. The as deposited film shows a monoclinic structure with a (320) orientation. Increasing the annealing temperature up to 170°C, crystallites with both triclinic and monoclinic phases are observed, although the triclinic phase seems unstable. Annealing the sample at 190°C the film consists mainly of monoclinic phase. The AFM topographic images suggest that at this temperature the triclinic crystallites evaporate, leaving wide holes in the films.

PbPC growth on Si surfaces studied with XPS and various SPM techniques

Abstract We have studied the growth mode of lead-phthalocyanine sublimated in ultrahigh vacuum conditions over Si(100)-(2 × 1) and Si(111)-(7 × 7) clean substrates. The samples have been studied in situ by means of X-ray photoelectron spectroscopy using non-monochromatized and monochromatized radiation, and also with a combined atomic force microscope (AFM)/scanning tunneling microscopy (STM) instrument. Occasionally their morphology has been also determined in air by means of a tapping-mode AFM. We give strong evidences for a planar adsorption of the shuttle-cock shaped PbPC molecule over the two substrates at the very initial stages of deposition. We report some original results of a combined STM/AFM experiment with measurements showing simultaneously taken height and tip-sample interaction force maps.

Preparation and characterization of bulk ZnGa2O4

High-quality bulk ZnGa2O4 has been synthesized from equimolar mixtures of ZnO and Ga2O3 by the conventional solid-state method. For the first time, the sample has been characterized in detail to confirm the formation of pure single phase of spinel ZnGa2O4. The formation of ZnGa2O4 has been confirmed by sintering the mixtures of ZnO and Ga2O3 at different temperatures, ranging from 900–1200 °C. It is observed that the single phase of ZnGa2O4 has been formed at and above 1000 °C sintering temperature for 24 h. The crystallinity and phase formation of this single phase has been confirmed by X-ray diffraction. X-ray photoelectron spectroscopic studies have been carried out for bulk ZnGa2O4 sintered at 1000 °C for 24 h which showed 14% Zn, 28% Ga and 58% O, indicating stoichiometric ZnGa2O4. A new parameter, the energetic separation between the Zn 2p3/2 and Ga 2p3/2 peaks, has been used as a sensitive tool to distinguish between a complete formation of ZnGa2O4 compound and a mixture of ZnO and Ga2O3 powders. Surface morphology studies by scanning electron microscopy reveal that the formation of ZnGa2O4 takes place in mosaic rod-like structure. The purity of the compound has also been checked by the energy dispersive X-ray method, indicating the absence of foreign ions and the ratio of zinc to gallium has been calculated and found to be 1 : 2, indicating stoichiometric ZnGa2O4.

Growth of Te thin films deposited at room temperature on the Si(100)2 × 1 surface

Abstract The electronic properties and the growth mode of very thin films of tellurium deposited in ultra-high vacuum, at room temperature, on the Si(100)2 × 1 surface have been studied by means of XPS, AES and LEED spectroscopy. Additionally the measured AES weighted peak to peak intensity ratio between Te and Si was compared with calculated layer by layer and Stransky-Krastanov growth mode curves. Our results indicate that the interaction between tellurium and silicon is very weak like the characteristic behaviour of simple metals near Si in the Periodic Table. Tellurium grows on large 1 × 1 domains reconstructed on Si(100)2 × 1 until a layer is completed, at ≈ 2 A of nominal coverage. Once the substrate is completely covered by the first Te layer, and up to ≈ 10 A, the deposited tellurium grows, forming islands which have a 1 × 1 order on Si(100), and covers ≈ 40% of the underlying continuous layer. With increasing thickness the islands begin to coalesce and the Te atoms do not follow the lattice order of the Si(100) substrate any more. The behaviour of Te on Si(100) is found to be very similar to that of antimony because of the similar electronic core level structure and atomic radius. However, the higher mobility of Te atoms with respect to Sb atoms allows the formation of a continuous layer of Te on Si(100)2 × 1 also at room temperature, whereas Sb needs higher temperatures. The observed weak absorbate-substrate interaction, together with a strong adsorbate-adsorbate interaction, leads to the saturation of the dangling bonds of the silicon substrate, which shows a bulk-like behaviour, and are responsible for the growth mode of the tellurium deposited on Si(100)2 × 1.

UPS and XPS studies of Cu clusters on graphite

Abstract The electronic structure of small copper clusters deposited on polycrystalline graphite has been studied by means of UPS and XPS spectroscopies as a function of the cluster size. The Cu d-band, Fermi edge and 2p core levels have been carefully analyzed. As the cluster dimension decreases, the Fermi edge shift is very small, less than 0.1 eV for the smallest clusters, while the d-band and the 2p core level peaks move towards higher binding energy of 1 and 0.4 eV, respectively. These results show a strong variation of the electronic structure of Cu clusters when their dimension decreases. The effect of the decreased screening of the valence band electrons and of the charge left on the cluster after the photoemission process seem to be negligible.

XPS, LEED and AFM investigation of the Si(100) surface after the deposition and annealing of tellurium thin films

Abstract In this study the structural properties and the morphology of the Te Si (100) 2 × 1 system after an annealing process have been investigated. Several Te Si (100) samples have been annealed at different temperatures. Up to about 150°C no changes are observed in the samples. At about 200°C the tellurium films evaporate from the silicon substrate leaving one monolayer of Te on the Si(100) surface indicating a higher strength in the TeSi bonds with respect to the TeTe one. The remaining monolayer of Te on Si(100) induces a 1 × 1 reconstruction of the surface. The annealing procedure performed at higher temperatures (more than about 400–500°C), leads to the complete removal of the tellurium. The resulting silicon surface is 2 × 1 reconstructed with a different morphology with respect to the one observed before the Te film deposition. Repeating several times the deposition of Te and the annealing procedure, at temperatures higher than about 400°C, a picturesque morphology of the silicon surface is observed.

Electron spectroscopy investigation of Te thin films deposited at room temperature on Si(100) 2 × 1

Abstract Ultraviolet and X-ray photoelectron spectroscopies, Auger electron spectroscopy and low energy electron diffraction were used in order to investigate the electronic properties and the growth mode of very thin films of tellurium with mean thickness between 0.5 and 1000 A deposited at room temperature on a Si(100)2 × 1 surface. The adsorbate-substrate interaction is found to be weak. In the initial stage of the growth the tellurium atoms are chemisorbed on the silicon surface and form a continuous monolayer. The Te 5p valence electrons are involved in order to saturate the Si(100) dangling bonds inducing a 1 × 1 reconstruction of the surface. Tellurium deposited after the formation of the first continuous layer and up to 10 A of average thickness, forms 1 × 1 ordered islands, that is Te deposited at room temperature on Si(100) follows a Stranski-Krastanov growth mode (one layer plus islands). Increasing the amount of Te the islands coalesce and the deposited atoms show a bulk-like behaviour.

NiPC/Si(111)(7 × 7) STUDIED WITH XPS, STM AND TAPPING MODE AIR AFM

We evaporated a few angstroms of nickel phthalocyanine (NiPC) in ultrahigh vacuum (UHV) on clean single crystal substrates of Si(111)(7×7) and studied in situ the structural and electronic properties of the interface with X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). The mesoscopic morphology of the samples has also been studied in air with a tapping mode atomic force microscope (TM-AFM). XPS measurements with variation of the NiPC thickness suggest planar adsorption of the first layer of admolecules; in particular, we found evidences for stronger chemisorption at the outer benzene rings of the PC molecule. UHV STM measurements confirm the XPS results; despite the lack of intermolecular resolution we show an image suggesting chemisorption commensurate with the substrate lattice and planar stacking consistent with the intermolecular stacking in the known crystalline phases of metal phthalocyanines. TM-AFM shows a growth mode in terms of flat islands of 20–35 A typical height and a few hundreds of nm width. The potential of imaging with TM-AFM elastic sample properties of soft materials deposited on hard substrates is addressed.

XPS analysis on SiO2 sol-gel thin films

Abstract The X-ray Photoelectron Spectroscopy has been applied to study the composition and the electronic structure of thin SiO 2 films prepared using the sol-gel technique on silicon substrates. Samples prepared using different amount of tetraethoxysilane and methyltriethoxysilane have been annealed at various temperatures. The valence band, the Si 2p and KL 23 L 23 Auger features have been followed as a function of the preparation conditions. The Si Auger parameter has been used to determine the attendance of different CH and OH groups. The presence of SiC bonds, observed in the valence band spectra, strongly decreases as the annealing temperature grows, showing the loss of methyl groups.