M. De Crescenzi

Copper phthalocyanine on Si(111)-7×7 and Si(001)-2×1 surfaces : an X-ray photoemission spectroscopy and synchrotron X-ray absorption spectroscopy study

Abstract The chemical bonding and molecular orientation of copper phthalocyanine (CuN 8 C 32 H 16 ), deposited on clean single crystal silicon substrates of (111) and (001) orientation, were studied using X-ray photoemission spectroscopy (XPS) of the Cu 2p, C 1s, and N 1s core-levels and synchrotron radiation X-ray absorption spectroscopy (XAS) at the Cu L 2,3 edges. In the monolayer range, the strong interaction of the molecule with the Si (111) surface is mostly evidenced by changes in the Cu 2p XPS spectra that indicate a partial reduction (up to 30%) of the Cu(II) atoms to Cu 3d 10 . The linear dichroism of the XAS spectra shows that those molecules, whose copper is still Cu(II), lie down on the surface. On the other hand, the bonding of the molecule on Si(001) is weaker, as Cu 2p XPS spectra do not exhibit Cu(II) reduction, while XAS spectroscopy indicates a random orientation of the molecules with respect to the surface. N 1s XPS spectra exhibit, on both surfaces, very intense satellites which present, on Si(111), a sharp dependence on the electron take-off angle. With the growth of thicker films, the XPS line shapes of the molecular solid are recovered, and the average tilt angle of the molecules (around 70°) does not depend on the chosen Si surface. We conclude that the structure of the first deposited layer strongly depends on the specific substrate, but does not affect the successive growth of thick layers.

Electronic and optoelectronic nano-devices based on carbon nanotubes

The discovery and understanding of nanoscale phenomena and the assembly of nanostructures into different devices are among the most promising fields of material science research. In this scenario, carbon nanostructures have a special role since, in having only one chemical element, they allow physical properties to be calculated with high precision for comparison with experiment. Carbon nanostructures, and carbon nanotubes (CNTs) in particular, have such remarkable electronic and structural properties that they are used as active building blocks for a large variety of nanoscale devices. We review here the latest advances in research involving carbon nanotubes as active components in electronic and optoelectronic nano-devices. Opportunities for future research are also identified.

Experimental imaging of silicon nanotubes

Transmission electron microscopy (TEM), electron energy loss near edge structures (EELNES) and scanning tunneling microscopy (STM) were used to distinguish silicon nanotubes (SiNT) among the reaction products of a gas phase condensation synthesis. TEM images exhibit the tubular nature with a well-defined wall. The EELNES spectra performed on each single nanotube show that they are constituted by nonoxidized silicon atoms. STM images show that they have diameter ranging from 2 to 35 nm, have an atomic arrangement compatible with a puckered structure and different chiralities. Moreover, the I-V curves showed that SiNT can be semiconducting as well as metallic in character.

EXAFS measurements on FeB metallic glasses: Asymmetry of the radial distribution function

Abstract The EXAFS structure measured above the K-edge of Fe in the disordered Fe80B20 alloy is reported. Complete agreement is obtained with X-ray diffraction results in the position of the first neighbours coordination shell when the asymmetry of the Radial Distribution Function (R.D.F.) into the EXAFS formula is included. A Finney-like R.D.F. is used to describe the pair distribution of these metallic glasses. The weak temperature dependence of the spectra is discussed in terms of the structural properties of these amorphous alloys.

Copper phthalocyanine on Si(111)-7 × 7 and Si(001)-2 × 1: an XPS/AES and STM study

Abstract Silicon surfaces with different reconstruction, Si (111)-7 × 7 and Si (001)-2 × 1, are exposed to a molecular beam of copper phthalocyanine. On Si(111) the adsorbate-substrate interaction is strong, as suggested by a partial reduction of copper shown in the XPS spectra and by the evidence provided by STM of a chemical reaction involving some of the Si adatoms. As a consequence, the molecules are not imaged at atomic scale by STM. At variance with this behaviour, on the single-domain Si (001)-2 × 1 face, the adsorbed molecules are clearly imaged by STM, suggesting an adsorption state more weakly bonded. This is the first observation of Cu phthalocyanine molecules imaged on a silicon surface, and it further illustrates the potential of STM for surface chemistry.

Extended ELS fine structures above the M2,3 edges of Cu and Ni

Abstract Very detailed electron energy loss spectra of M2,3 core levels of Cu and Ni in the reflection mode are presented. A careful analysis of the oscillations up to about 300 eV above the edges shows strict analogies to EXAFS and demonstrates that structural information can be obtained with this technique. A comparison with X-ray results above the Cu and Ni K-edges shows that very high accuracy in the radial distribution function can be obtained also with ELS spectra taken above shallow core levels.

Large photocurrent generation in multiwall carbon nanotubes

The authors demonstrate the ability of multiwall carbon nanotubes to generate photocurrents in the near ultraviolet and visible spectral ranges using electrochemical photocurrent measurements. The photogenerated current depends on the excitation wavelength similar to that for single wall carbon nanotubes. Its intensity and modulation can be related to the carbon nanotubes morphology. The maximum photon-to-current conversion efficiency is approximately 7%, about 50 times higher than that reported for single wall carbon nanotubes. This result is of particular relevance for photovoltaic nanodevices and solar energy conversion applications.

Local structure determination of the CoSi(111) interface by surface electron energy-loss fine-structure technique

Extended fine structures, similar to those observed with SEXAFS technique, have been detected in electron energy-loss spectra in the reflection mode. These structures are related to the local geometry (bond length) of the investigated system. We have applied this loss technique to study the CoSi(111) interface. Our structural results suggest the formation at room temperature of an initial silicide phase followed with a nearly pure cobalt film. Upon annealing at progressively higher temperatures, sequential silicide formation (Co2Si, CoSi, CoSi2) occurs, confirming most of our recent observations by other surface techniques.

Evidence of extended fine structures in the Auger spectra: A new approach for surface structural studies

Abstract We report for the first time on extended fine structures which we have observed above core-valence-valence Auger transitions on Cu and Co samples. We interpret these oscillating structures as originated from the same interference process which produces extended fine structures observed in the x ray absorption spectra (EXAFS). Using the same EXAFS analysis procedure, we deduce from the extended fine Auger structures (hereafter called EXFAS) the radial atomic distribution F(R) of Cu and Co samples. An excellent agreement is found as compared with results from synchrotron radiation EXAFS.

Size effects on the linewidths of the Auger spectra of Cu clusters

Abstract We discuss the role of the size distribution of Cu clusters deposited on graphite in determining the broadening and the shifting of the Auger L 3 VV Lineshapes observed when the average cluster size decreases. In our interpretation this broadening is only due to the superposition of various narrow Auger lines relative to the various cluster dimensional classes, each shifted towards lower kinetic energies because their small sizes. The Auger peak shifts are mainly due to the change in the valence band and core levels as the cluster size decreases. The model used to reproduce the Auger line broadening, following the Cini-Sawatzky approach, takes into account the broad distribution of the different diameters.