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List of Publications

27. Promoter effect of hydration on the nucleation of nanoparticles: direct observation for gold and copper on rutile TiO2 (110)

Mathilde Iachella, Axel Wilson, Ahmed Naitabdi, et al.
Nanoscale, volume: 8, pages: 16475-16485, (2016)

Abstract :
Direct observation of the promoting effect of hydration on the nucleation of gold and copper nanoparticles supported on partially reduced rutile TiO2 (110) is achieved by combined scanning tunneling microscopy experiments and density functional theory calculations. The experiments show a clear difference between the two metals. Gold nanoparticles grow at the vicinity of the surface hydroxyl domains, whereas the nucleation of copper is not substantially affected by hydration. The nucleation of gold on surface oxygen vacancies is observed although this is not the only preferential site. Theoretical calculations of the coadsorbed phases of gold, copper and hydroxyl species on stoichiometric and reduced TiO2 (110) surfaces under relevant conditions of temperature and pressure support the experimental interpretation. Surface hydration tends to stabilize significantly gold adsorption on the stoichiometric support, while its influence on copper adsorption is not pronounced. The theoretical analysis shows that the early stages of the nucleation on hydrated stoichiometric surfaces correspond to mono-hydroxylated metallic species co-chemisorbed with hydroxyl species, whereas those on hydrated reduced surfaces are metallic atoms bound to oxygen vacancies and weakly perturbed by surface hydration.


26. Real-Time X-ray Photoemission Spectroscopy Study of Si(001)-2×1 Exposed to Water Vapor: Adsorption Kinetics, Fermi Level Positioning, and Electron Affinity Variations

D. Pierucci, J.-J. Gallet, F. Bournel, F. Sirotti, M. G. Silly, H. Tissot, A. Naitabdi, and F. Rochet.
Journal of Physical Chemistry C, volume: 120, pages: 21631–21641, , (2016)

Abstract :
The great advantage of X-ray photoemission spectroscopy, when performed in real time, e.g., during the reaction of a gas with a surface, is the possibility of monitoring in a single experiment both the chemical aspects (adsorption kinetics, bond formation) and the physical ones (Fermi level positioning, variations in the electron affinity). In the present study we examine the reaction of water with Si(001)-2×1 at room temperature in real time not only because water, ubiquitous in (ultra) high-vacuum systems, is the main source of surface defects controlling the surface Fermi level, but also because water-saturated silicon may become an interesting starting surface in the atomic layer deposition of dielectrics on silicon. The question of water adsorption on silicon Si(001)-2×1 is renewed under the following four perspectives: (1) We propose an original kinetic analysis of the water uptake using an integrated form of the precursor model differential equations, underlying a dependence on pressure. (2) We perform a thorough analysis of the Fermi positioning within the band gap due to water-related surface defects as a function of water coverage and for four different doping types and levels. (3) We follow the changes in the surface dipole as a function of coverage, with considerations of the dissociation channels. (4) Using seven different n and p doping levels, we extract the electron affinity at saturation, a useful parameter to know if heterostructures are built upon the water-covered surface. In addition to an applicative view, the present data can be a benchmark for theoretical calculations such as molecular dynamics, surface defect energy, and work function calculations.


25. Room temperature differential conductance measurements of triethylamine molecules adsorbed on Si(001)

Ahmed Naitabdi, François Rochet, Stéphane Carniato et al.
Physical Chemistry Chemical Physics, volume: 18, pages: 23231-23237, (2016)

Abstract :
We have measured the differential conductance of the triethylamine molecule (N(CH2CH3)3) adsorbed on Si(001)-2 × 1 at room temperature using scanning tunneling spectroscopy. Triethylamine can be engaged in a dative bonding with a silicon dimer, forming a Si–Si–N(CH2CH3)3 unit. We have examined the datively bonded adduct, either as an isolated molecule, or within an ordered molecular domain (reconstructed 4 × 2). The differential conductance curves, supported by DFT calculations, show that in the explored energy window (±2.5 near the Fermi level) the main features stem from the uncapped dangling bonds of the reacted dimer and of the adjacent unreacted ones that are electronically coupled The formation of a molecular domain, in which one dimer in two is left unreacted, is reflected in a shift of the up dimer atom occupied level away from the Fermi level, likely due to an increased π-bonding strength. In stark contrast with the preceding, pairs of dissociated molecule (a minority species) are electronically decoupled from the dimer dangling bond states. DFT calculation show that the lone-pair of the Si–N(CH2CH3)2 is a shallow level, that is clearly seen in the differential conductance curve.


24. Oxidation of Small Supported Platinum-based Nanoparticles Under Near-Ambient Pressure Exposure to Oxygen

Ahmed Naitabdi, Robert Fagiewicz, Anthony Boucly, Giorgia Olivieri, et al.
Topics in Catalysis, volume: 59, pages: 550-563, (2016)

Abstract :
The investigation of nanocatalysts under their working conditions of pressures and temperatures represents a real strategy toward a realistic understanding of their chemical reactivity and related issues. Additionally, the reduction of Pt load in the catalysts while maintaining their optimum performances is essential to large scale practical applications. Here, we show that small PtZn bimetallic nanoparticles (NPs) supported on the rutile and reduced TiO2(110)-(1 × 1) surface can be prepared by a two step consecutive deposition process where Pt was deposited first and followed by Zn. In situ synchrotron- based near ambient pressure photoemission spectroscopy experiments are used to monitor the evolution of the oxidation states and surface elemental composition of pure Pt and PtZn NPs under high exposure to O2 pressure. The formation of stable Pt surface oxide was evidenced for both pure and PtZn NPs. While a sizeable encapsulation of pure Pt NPs by TiOx was seen after annealing at 440 K under 1 mbar of O2, no such effect was noticed for PtZn NPs. The formation of a zinc oxide layer on PtZn NPs enhances the stability of the NPs and induces a partial reduction of the TiO2(110) surface. Spontaneous formation of a Pt–Zn alloy phase at room temperature was seen in PtZn NPs.


23. Water adsorption on TiO2 surfaces probed by soft X-ray spectroscopies: bulk materials vs. isolated nanoparticles

Safia Benkoula, Olivier Sublemontier, Minna Patanen, Christophe Nicolas, Fausto Sirotti, Ahmed Naitabdi, et al.
Scientific Reports, 5, Article number: 15088(2015)

Abstract :
We describe an experimental method to probe the adsorption of water at the surface of isolated, substrate-free TiO2 nanoparticles (NPs) based on soft X-ray spectroscopy in the gas phase using synchrotron radiation. To understand the interfacial properties between water and TiO2 surface, a water shell was adsorbed at the surface of TiO2 NPs. We used two different ways to control the hydration level of the NPs: in the first scheme, initially solvated NPs were dried and in the second one, dry NPs generated thanks to a commercial aerosol generator were exposed to water vapor. XPS was used to identify the signature of the water layer shell on the surface of the free TiO2 NPs and made it possible to follow the evolution of their hydration state. The results obtained allow the establishment of a qualitative determination of isolated NPs’ surface states, as well as to unravel water adsorption mechanisms. This method appears to be a unique approach to investigate the interface between an isolated nano-object and a solvent over-layer, paving the way towards new investigation methods in heterogeneous catalysis on nanomaterials.


22. Walking peptide on Au(110) surface: Origin and nature of interfacial process

V. Humblot, A. Tejeda, J. Landoulsi, A. Vallée, A. Naitabdi, et al.
Surface Science, volume: 628 pages: 21-29, (2014)

Abstract :
IGF tri-peptide adsorption on Au(110)-(1 × 2) under Ultra High Vacuum (UHV) conditions has been investigated using surface science techniques such as synchrotron based Angle Resolved X-ray Photoemission Spectroscopy (AR-PES or AR-XPS), Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM). The behaviour of IGF molecules has been revealed to be coverage dependent; at low coverage, there is formation of islands presenting a chiral self-organised molecular network with a (4 2,−3 2) symmetry as shown by Low Energy Electron Diffraction (LEED) and Scanning Tunnelling Microscopy (STM) on the unaltered Au(110)-(1  2) reconstruction, suggesting significant intermolecular interactions. When the coverage is increased, the islands grow bigger, and one can observe the disappearance of the self- organised network, along with a remarkable destruction of the (1  2) substrate reconstruction, as shown by STM. The effect of IGF on the surface gold atoms has been further confirmed by angle-resolved photoemission measurements which suggest a modification of the electronic states with the (1  2) symmetry. The resulting molecular organisation, and overall the gold surface disorganisation, prove a strong surface- molecule interaction, which may be probably be explained by a covalent bonding.


21. Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated Silicon Dangling Bonds versus Concerted Hydrosilylation

D. Pierucci, A. Naitabdi, F. Bournel, J.-J. Gallet, et al.
Journal of Physical Chemistry C, volume: 118, pages: 10005-10016, (2014)

Abstract :
Despite strong similarities due to the common presence of silicon monohydrides and isolated silicon dangling bonds (silicon radicals), the water- saturated Si(001)-2  1 surface and the hydrogen-terminated Si(001)-2  1 surface show very different reactivities with respect to benzaldehyde. By using real-time scanning tunneling microscopy, synchrotron radiation photoemission, X-ray absorption, and high-resolution electron energy loss spectroscopies in combination, we demonstrated that benzaldehyde reacts with the silicon dangling bonds of water- saturated Si (001). As we found no evidence for the abstraction of a nearby H leading to the formation of a new dangling bond, the formation of a stable radical adduct is a plausible explanation. This observation contrasts with the H-terminated case for which benzaldehyde grafting occurs via a radical chain reaction that can propagate after abstraction of a nearby H by the radical adduct. Also at odds with the H-terminated case, a second chemisorption channel is observed [i.e., a concerted hydrosilylation reaction between a surface monohydride (SiH) and the carbonyl moiety] without any participation of the silicon dangling bond. We discuss how the presence of hydroxyls on water-saturated Si(001)-2  1 could make its reactivity markedly different from that of H-terminated Si(001)-2  1.

http://pubs.acs.org/doi/abs/10.1021/jp4077678?prevSearch=%255BContrib%2 53A%2BNaitabdi%255D&searchHistoryKey=

20. Silicon Monomer Formation and Surface Patterning of Si(001)-2 x 1 Following Tetraethoxysilane Dissociative Adsorption at Room Temperature

H. Tissot, J.-J. Gallet, F. Bournel, A. Naitabdi, et al.
Journal of Physical Chemistry C, volume: 118, pages: 1887-1893, (2014)

Abstract :
The adsorption of tetraethoxysilane (TEOS, Si[OC2H5]4) on the Si(001)-2  1 surface at 300 K is studied through a joint experimental and theoretical approach, combining scanning tunneling microscopy (STM) and synchrotron radiation X-ray photoelectron spectroscopy (XPS) with first-principles simulations within the density functional theory (DFT). XPS shows that all Si-O bonds within the TEOS molecules are broken upon adsorption, releasing one Si atom per dissociated molecule, while the ethoxy (-OC2H5) groups form new Si-O bonds with surface Si dimers. A comparison between experimental STM images and DFT adsorption configurations shows that the four ethoxy groups bind to two second-neighbor silicon dimers within the same row, while the released silicon atom is captured as a monomer on an adjacent silicon dimer row. Additionally, the surface displays alternate ethoxy- and Si adatom- covered rows as TEOS coverage increases. This patterning, which spontaneously forms upon TEOS adsorption, can be used as a template for the nanofabrication of one- dimensional self-organized structures on Si(001)-2  1.


19. Triethylamine on Si(001)-(2 x 1) at 300 K: Molecular Adsorption and Site Configurations Leading to Dissociation A. Naitabdi, F. Bournel, J.-J. Gallet, A. Markov

A. Naitabdi, F. Bournel, J.-J. Gallet, A. Markovits, et al.
Journal of Physical Chemistry C, volume: 116, pages: 16473-164486, (2012)

Abstract :
We provide a comprehensive investigation of the adsorption of a tertiary amine molecule, triethylamine (TEA, N(CH2CH3)3), on the Si(001)-(2  1) surface at room temperature (RT), using real-time synchrotron radiation X-ray photoemission (XPS) and scanning tunneling microscopy (STM), in combination with density functional theory (DFT). calculations. Real-time XPS measurements point to two consecutive reactions, the first one leading to the formation of a dative adduct, the second one to the conversion of the latter into a dissociated adduct via N C bond cleavage (to give Si-N(CH2CH3)2 and Si-CH2CH3 moieties). The kinetic model, that fits the experimental data, distinguishes unreactive dative bond molecules (forming a reservoir) from reactive ones, exchanged in a two-way reaction. At low coverage, the STM images show that the adsorption of a dative-bond TEA molecule induces a static buckling of the Si dimers, leading to the c(4  2) templating of the silicon surface. Approaching saturation coverage, dative bonded TEA molecules self-organize into c(4  2) domains, with an occupancy of one molecule per two dimers, and exhibit a ternary symmetry. While DFT points to two possible molecular conformations (staggered and eclipsed) with almost the same adsorption energy, only the eclipsed conformer is observed at RT. STM helps also in identifying sites and experimental conditions (pressure) that make the dissociation of TEA possible. At high dilution, dissociated molecules are systematically sitting as pairs on two adjacent dimers in a row. Remarkably, dissociated adducts do not induce static buckling, in stark contrast with the case of dative bonding. Close to surface saturation, the dative-to-dissociated conversion starts from the boundaries of dative-bond self-assembled c(4  2) domains, emphasizing the role of these domains as reservoirs of stable molecular adducts. Our data show also that the conversion reaction needs the presence of the gas phase and is accelerated by an increase of pressure.


18. Drastic Au(111) Surface Reconstruction upon Insulin Growth Factor Tripeptide Adsorption

V. Humblot, A. Vallée, A. Naitabdi, F. Tielens, et al.
Journal of the American Chemical Society, volume: 134, pages: 6579-6583, (2012)

Abstract :
Adsorption of biomolecules at metal surfaces often creates two-dimensional ordering of the adlayers. However, metal substrate reconstruction is less commonly observed, unless upon annealing of the molecule-surface system. Here, we report on the drastic room-temperature reconstruction of the Au(111) surface, driven by the adsorption of insulin growth factor tripeptide molecules. Scanning tunneling microscopy images show that the surface reconstruction, which takes place without annealing the system, is dynamic and evolves over time. It is initiated at kinks and steps edges, but the reconstruction also takes place within defect-free terraces. Theoret


17. Chiral self-assemblies of amino-acid molecules: D- and L-methionine on Au(111) surface

A. Naitabdi, and V. Humblot
Applied Physics Letters, volume: 97, article number: 223112, (2010)
Abstract :
Spontaneous self-assembly into chiral well-ordered two- dimensional molecular film at room temperature has been evidenced using the scanning tunneling microscopy for the growth of two enantiomerically pure L- and D-methionine on Au(111) surface. From the x-ray photoelectron spectroscopy measurements, the growth of methionine occurs through the formation of zwitterionic species. Our results demonstrate that this self-assembly mechanism is crucially driven by the interchain interactions via zwitterionic hydrogen bonding between neighboring negatively charged carboxylate groups and positively charged amino groups. Moreover, the molecular pair formation is incidental and results from the o


16. Sixton rectangles in the structure of alumina ultrathin films on metals

G. Prévot, A. Naitabdi, R. Bernard, and Y. Borensztein
Physical Review B, volume: 81, article number: 085405, (2010)

Abstract :
In situ room temperature scanning tunneling microscopy (STM) observations combined with low energy electron diffraction and Auger electron spectroscopy were performed to investigate the structure of a thin aluminum oxide film grown on Ni(111). Well-ordered alumina films were obtained after the deposition of 2.5 ML of aluminum on a clean Ni(111) surface, followed by its oxidation under O2 flow and subsequent annealing at 1000 K. Whereas an hexagonal unit cell corresponding to a (53  53) reconstruction with respect to the Ni(111) surface had been previously ascribed to this superstructure, our results indicate that the unit cell corresponds to a sixton rectangle, i. e., a rectangle with a root 3 ratio between the lengths of the two sides of the mesh (18.2  10.5 Å). We attribute this specific ratio to the presence of the hexagonal arrangement of an oxygen plane in the layer. From the size and aspect ratio of the mesh and from the STM observations, we also conclude that the atomic organization observed for alumina/Ni(111) is very similar to the organization observed for alumina grown on FeAl(110), NiAl(110), Cu-9 at. % Al(111), and Cu(111), which provides strong argument that this alumina structure is not specific of aluminum-based substrates but could be the equilibrium state of a two-layers-thick alumina film on a metal.


15. Structure and phonon density of states of supported size-selected nanoclusters: A nuclear resonant inelastic x-ray scattering study

B. Roldan Cuenya, L. K. Ono, J. Croy, A. Naitabdi, et al.
Applied Physics Letters, volume: 95, article number: 143103, (2009)

Abstract :
We have measured the phonon density of states (PDOS) of isolated bcc and fcc FexAu1-x alloy nanoclusters (NCs) by nuclear resonant inelastic x-ray scattering. Drastic deviations were observed with respect to the PDOS of bulk Fe-Au alloys. Important information on the structure and thermodynamic properties of these NCs was obtained.


14. Phonon density of states of self-assembled isolated Fe-rich Fe-Pt alloy nanoclusters

B. Roldan Cuenya, J. Croys, L. K. Ono, A. Naitabdi, et al.
Physical Review B, volume: 80, article number: 125412, (2009)

Abstract :
The Fe-projected phonon density of states (PDOS) of monolayer-thick films of self-assembled, size-selected, isolated (57)Fe1-xPtx alloy nanoclusters (NCs) (height: similar to 2 nm, diameter: similar to 8 nm) supported on flat SiO2/Si(111) substrates has been measured by nuclear resonant inelastic x-ray scattering. The samples were characterized by atomic force microscopy (AFM), transmission electron microscopy, and x-ray photoelectron spectroscopy (XPS). Surface segregation of Pt and PtSi formation at the NC surface due to the sample-preparation process is inferred from the XPS data. As compared to the bulk, pronounced modifications of the PDOS beyond the bulk cut-off energy are observed in bcc 57Fe0.8Pt0.2 (core)/PtSi(shell) NCs. By contrast, the PDOS of fcc 57Fe0.75Pt0.25 (core)/PtSi(shell) NCs retains features of bulk ordered Fe3Pt Invar alloys (presumably due to a thicker PtSi shell), in particular, the transverse-acoustic [110]TA1 mode near 9 meV. Apparently, this mode is not affected by size effects. The existence of the [110]TA1 phonon mode is a prerequisite for the persistence of Invar-related effects in Fe3Pt NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom.


13. Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)

A. Naitabdi, F. Behafarid, and B. Roldan Cuenya
Applied Physics Letters, volume: 94, article number: 083102, (2009)

Abstract :
This letter reports (i) the enhanced thermal stability (up to 1060 degrees C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced.


12. Thermal Stability and Segregation Processes in Self-Assembled Size-Selected AuxFe1-x Nanoparticles Deposited on TiO2(110): Composition Effects

A. Naitabdi, L. K. Ono, F. Behafarid, and B. Roldan Cuenya
Journal of Physical Chemistry C, volume: 113, pages: 1433-1446, (2009)

Abstract :
In-situ scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) measurements have been performed to investigate the formation and thermal stability of mono- and bimetallic AuxFe1-x (x = 1, 0.8, 0.5, 0.2, 0) nanoparticles (NPs) supported on TiO2(110). Nearly hexagonal arrangements of size- selected Au, Fe, and Au-Fe NPs with well-defined interparticle distances have been achieved by diblockcopolymer encapsulation. Upon stepwise annealing from 300 to 1060 degrees C, a remarkable thermal stability of the Au-Fe NPs was observed, maintaining their original spatial arrangement on the TiO2 surface up to 900 degrees C. A majority phase of a gold-iron alloy (solid solution) was achieved for our Au0.5Fe0.5 NPs in the temperature range of 700 degrees C - 800 degrees C, and for Au0.2Fe0.8 N.Ps at 800 degrees C, while a phase mixture of bcc Fe and Au-Fc alloy was observed for the Au0.2Fe0.2 System at 800 degrees C-900 degrees C. For all samples the segregation of Au atoms toward the NP surface was detected upon high temperature annealing (800 degrees C) in vacuum. Nearly complete Au desorption was observed by XPS at 900 degrees C for Au0.2Fe0.8 NPs, at 1000 degrees C for Au0.5Fe0.5 NPs, and at 1060 degrees C for Au0.8Fe0.2 NPs. The enhanced thermal stability of Au in the Au0.8Fe0.2 NPs is believed to be related to the formation of core(Fe)/shell(Au) structures. Furthermore, contrary to the case of pure Fe or Fe-rich NPs where nearly complete Fe desorption or Fe diffusion into TiO2 was observed at 1000 degrees C, an Fe signa


11. Nanostructured zinc oxide films synthesized by successive chemical solution deposition for gas sensor applications

O. Lupan, L. Chow, S. Shishiyanu, E. Monaico, et al.
Materials Research Bulletin, volume: 44, pages: 63-69, (2009)

Abstract :
Nanostructured ZnO thin films have been deposited using a successive chemical solution deposition method. The structural, morphological, electrical and sensing properties of the films were studied for different concentrations of Al-dopant and were analyzed as a function of rapid photothermal processing temperatures. The films were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron and micro- Raman spectroscopy. Electrical and gas sensitivity measurements were conducted as well. The average grain size is 240 and 224 Å for undoped ZnO and Al-doped ZnO films, respectively. We demonstrate that rapid photothermal processing is an efficient method for improving the quality of nanostructured ZnO films. Nanostructured ZnO films doped with Al showed a higher sensitivity to carbon dioxide than undoped ZnO films. The correlations between material compositions, microstructures of the films and the properties of the gas sensors are discussed.


10. Grafting and thermal stripping of organo-bimetallic clusters on Au surfaces: Toward controlled Co/Ru aggregates

A. Naitabdi, O. Toulemonde, J.-P. Bucher, J. Rosé et al.
Chemistry - A European Journal, volume: 14, pages: 2355-2362, (2008)

Abstract :
The controlled stoichiometry of heterometallic carbonyl clusters make them attractive precursors for the stabilization of bare metal alloy clusters for magnetic applications. The mixed-metal molecular cluster [RuCo3(H)(CO)12] has been functionalized with the phosphane–thiol ligand Ph2PCH2CH2SH to allow subsequent anchoring on a gold surface. The resulting tetrahedral cluster [RuCo3(H)(CO)11(Ph2PCH2CH2SH)] (1) has been characterized by X-ray diffraction and the P-monodentate ligand is axially bound to a cobalt center and trans to the ruthenium cap. This synthesis also yielded the product of oxidative coupling, in which two SH groups were coupled intermolecularly to give a disulfide ligand that links two tetrahedral cluster units in [{RuCo3(H)(CO)11(Ph2PCH2CH2S)}2] (2). This cluster has also been characterized by X-ray diffractions studies. After deposition of 1 on a Au(111) surface by self-assembly, the carbonyl ligands were stripped off by thermal annealing in ultra-high vacuum (UHV) to form a metallic species. X-ray photoelectron spectroscopic measurements performed as a function of the annealing temperature show that the cobalt and ruthenium centers converge towards metallic character and that the stoichiometry of the alloy is retained during the annealing process. Preliminary X-ray absorption spectroscopy (XAS) synchrotron experiments indicate that clusters 1 and 2 behave similarly, which is consistent with the retention of their tetrahedral units on the gold surface after transformation of the thiol function or breaking of the disulfide bond to form Au S bonds, respectively, has occurred.


9. Nanofabrication and characterization of ZnO nanorod arrays and branched microrods by aqueous solution route and rapid thermal processing

O. Lupan, L. Chow, G. Y. Chai, B. Roldan-Cuenya, A. Naitabdi, et al.
Materials Science and Engineering B, volume: 145, pages: 57-66, (2007)

Abstract :
This paper presents an inexpensive and fast fabrication method for one-dimensional (1D) ZnO nanorod arrays and branched two-dimensional (2D), three-dimensional (3D) - nanoarchitectures. Our synthesis technique includes the use of an aqueous solution route and post-growth rapid thermal annealing. It permits rapid and controlled growth of ZnO nanorod arrays of 1D - rods, 2D - crosses, and 3D - tetrapods without the use of templates or seeds. The obtained ZnO nanorods are uniformly distributed on the surface of Si substrates and individual or branched nano/microrods can be easily transferred to other substrates. Process parameters such as concentration, temperature and time, type of substrate and the reactor design are critical for the formation of nanorod arrays with thin diameter and transferable nanoarchitectures. X- ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and Micro-Raman spectroscopy have been used to characterize the samples.


8. Atomic vibrations in iron nanoclusters: Nuclear resonant inelastic x-ray scattering and molecular dynamics simulations

B. Roldan-Cuenya, A. Naitabdi, J. Croy, W. Struhahn, et al.
Physical Review B, volume: 76, article number: 195422, (2007)

Abstract :
The lattice vibrational dynamics of supported, self-assembled, isolated 57Fe nanoclusters was studied by nuclear resonant inelastic x-ray scattering and molecular dynamics calculations. The morphological and structural properties and the chemical state of the experimental nanoclusters were investigated by atomic force microscopy, high resolution transmission electron microscopy, and x-ray photoelectron spectroscopy. The measured and calculated vibrational densities of states (VDOSs) reveal an enhancement of the low- and high-energy phonon modes and provide experimental and theoretical proof of non-Debye-like behavior in the low-energy region of the VDOS. Experimentally, this effect was found to depend on the nature of the surface shell (oxide or carbide) of the core/shell nanoclusters. According to the calculations for supported isolated pure Fe nanoclusters, the non-Debye-like behavior appears not only in the surface shell but also in the bcc-Fe core of the nanocluster due to the hybridization of surface and bulk modes.


7. Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles

A. Naitabdi, and B. Roldan-Cuenya
Applied Physics Letters, volume: 91 article number: 113110, (2007)

Abstract :
The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300 degrees C. A subsequent thermal treatment at 700 degrees C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs' surface. Annealing at 900 degrees C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surfac


6. Epitaxial growth, magnetic properties, and lattice dynamics of Fe nanoclusters on GaAs(001)

B. Roldan-Cuenya, A. Naitabdi, E. Schuster, R. Peters, et al.
Physical Review B, volume: 76, page number: 094403, (2007)

Abstract :
Epitaxial bcc-Fe(001) ultrathin films have been grown at similar to 50 degrees C on reconstructed GaAs(001)-(4x6) surfaces and investigated in situ in ultrahigh vacuum (UHV) by reflection high-energy electron diffraction, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and 57Fe conversion electron Mossbauer spectroscopy (CEMS). For tFe=1 ML (monolayer) Fe coverage, isolated Fe nanoclusters are arranged in rows along the [110] direction. With 17 increasing tFe the Fe clusters first connect along the [-110], but not along the [110] direction at 2.5 ML, then consist of percolated Fe clusters without a preferential orientation at 3 ML, and finally form a nearly smooth film at 4 ML coverage. Segregation of Ga atoms within the film and on the Fe surface appears to occur at t(Fe)=4 ML, as evidenced by XPS. For coverages below the magnetic percolation, temperature- dependent in situ CEMS measurements in zero external field provided superparamagnetic blocking temperatures TB of 62  5, 80 +/- 10, and 165 - 5 K for tFe=1.9, 2.2, and 2.5 ML, respectively. At T < T-B, freezing of superparamagnetic clusters is inferred from the observed quasilinear T dependence of the mean hyperfine magnetic field < Bhf >. By combining the STM and CEMS results, we have determined a large magnetic anisotropy constant of similar to 5105 and similar to 8105 J/m3 at tFe=1.9-2.2 and 2.5 ML, respectively. For tFe≤ 2.5 ML, our uncoated "free" Fe clusters exhibit intrinsic magnetic ordering below T-B, contrary to literature reports on metal-coated Fe clusters on GaAs. Our present results demonstrate that the nature of the percolation transition for free Fe nanoclusters on GaAs(001) in UHV is from superparamagnetism to ferromagnetism. From the Mossbauer spectral area, a very low Debye temperature D of 196  4 K is deduced for these uncoated Fe nanoclusters in UHV, indicating a strong phonon softening in the clusters.


5. Local investigation of the electronic properties of size-selected Au nanoparticles by scanning tunneling spectroscopy

A. Naitabdi, and B. Roldan-Cuenya
Applied Physics Letters, volume: 89 article number: 043101, (2006)

Abstract :
The relationship between the structural/morphological and electronic properties of size-selected gold nanoparticles was investigated using scanning tunneling microscopy and spectroscopy. The nanoparticles were synthesized by inverse micelle encapsulation and were dip-coated on TiO2/Ti(15 nm)/Si(111). Annealing in vacuum to 500 degrees C resulted in the removal of the polymer and the formation of an ultrathin TiC support. Significant changes in the electronic local density of states (LDOS) of the nanoparticles, in particular, the onset of nonmetallic behavior, were observed with decreasing particle size. The nanoparticle-support interactions were studied and evidence for substrate-induced modifications in the LDOS of interfacial gold atoms is found.


4. Self-assembly and magnetism of Mn12 nanomagnets on native and functionalized gold surfaces

A. Naitabdi, J.-P. Bucher, Ph. Gerbier, P. Rabu et al.
Advanced Materials, volume: 17, pages: 1612-1616, (2005)
Abstract :
Mn12 single molecule magnets (SMMs) survive organized grafting on Au(111) surfaces as shown by a magnetic study in the monolayer range (see Figure and inside cover). Although the magnetic features are still reminiscent of ferromagnetic SMMs, effects due to the reduced dimensionality are clearly visible. Individual molecules are identified by scanning tunneling microscopy, opening new opportunities in the fiel


3. Magnetic anisotropies of ultrathin Fe films on Au(111) as function of iron thickness and gold capping

O. Toulemonde, V. Petrov, A. Naitabdi, J.-P. Bucher
Journal of Applied Physics, volume: 95, pages: 6565-6567, (2004)

Abstract :
Iron thin films between 1.5 and 3.75 ML grown on a reconstructed Au(111) surface have been studied by in situ magneto-optic Kerr effect in the polar and longitudinal geometries. In films with Fe coverage less than 2.8 ML, we observe a perpendicular magnetic anisotropy tendency at room temperature. With further iron deposition, the film exhibits a continuous spin reorientation from out-of- plane to in-plane. Distinct changes in the magnetic properties as a function of substrate quality and gold capping layer thickness may be related to different state of the film structures.


2. Magnetic properties of bulk Mn12Pivalates16 single molecule magnets and their self assembly on functionnalized gold surface

A. Naitabdi, J.-P. Buchere, P. Rabu, O. Toulemonde, et al.
Journal of Applied Physics, volume: 95, pages: 7345-7347, (2004)

Abstract :
Magnetic properties of oriented Mn12Pivalates16 (Mn12Piv16) single molecule crystals have been studied. Steps in the hysteresis loops, which are characteristic of quantum tunneling of magnetization, are observed at regular intervals of the magnetic field. Self-assembling of monolayers of this molecular material on a 19 surface is then achieved. A process is described by which Mn12Piv16 clusters are grafted onto the crystalline Au(111) surface by means of organosulfur SH-(CH2)10-COOH ligands in order to hold them firmly. This self-assembled monolayer was then characterized by Scanning tunneling microscopy and X-ray photoelectron spectroscopy so as to provide the topographic structure, the quality, and the stability during annealing to high temperature.


1. Magnetoresistance and switching of electrochemically etched Ni wires

A. Naitabdi, and J.-P. Bucher
Applied Physics Letters, volume: 82, pages: 430-432, (2003)

Abstract :
Nickel wires, a few 100 nm in diameter, with a resistance up to 20 , have been prepared by controlled electrochemical etching of macroscopic wires. Although the wires are polycrystalline and the presence of domain walls can be inferred directly from the magnetoresistance curves, the switching field, H-SW, for complete reversal is insensitive to details of the wire diameters provided their aspect ratio is larger than 100. Order of magnitude arguments show evidence for spin accumulation at domain walls, while the angular dependence H-SW(Theta) with respect to the wire axis is well described by the analytical form of the curling model, indicating a wider range of applicability of this model than previously thought.