Department of Physics of the University of  Trento		Institute of Photonics and Nanotechnologies, National Research Counsil - Section CeFSA, Istituto Trentino di Cultura of  Trento
X-Ray Synchroton Radiation Laboratory

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Nano-scale chemical mapping and surface structural modification by joined use of X-ray microbeams and tip assisted local
European  Specific Targeted Research Project (STRP) (VI European framework project) (2004 - 2006)
 

Structural and dynamical properties of disordered systems : ionic glasses
Scientific Research Programs of Prominent National Interest (Italian Ministry of Education and Scientific Research) (2203 - 2004)

Development of the XEOL technique for XAFS spectroscopy on quantum confined systems
Project for the Utilization of Syncrotron Radiation (PURS) (National Institute of Matter Physics, INFM) (2002 -2003)

Research proposals accepted by the LURE Syncrotron Radiation facilities  (Orsay-Paris)
 

Research proposals accepted by  ESRF  (Grenoble)

Nano-scale chemical mapping and surface structural modification by joined use of X-ray microbeams and tip assisted local detection.

Project summary
We aim to deliver instrumentation and techniques that merge the ability of Synchrotron Radiation spectroscopies in providing elemental composition, chemical status and structural information with the lateral resolution of Local Probe Microscopes that already occupy an important place in nanotechnology in providing detailed surface morfology and handling of nanosystems.
The instrument that we propose to deliver can be seen as the natural extension of a microprobe instrument to which it adds chemical sensitivity, morphology recognition, nanoposition and nanomanipulation.  This new instrument will be of direct interest to nanoscience but also to many others areas of applied science.
The core of the instrument consists in a multi head local probe microscope (AFM-STM-SNOM) integrated in a synchrotron radiation beamline that will provide a fine focused X-ray beam on the area explored by the probe tip.
The head will provide three different functionalities:

·          XAS-SNOM: Element-Specific Contrast in Local Probe Microscopy via X-Ray Excited Optical Luminescence (XEOL) detection by optical probe in SNOM mode.

·          XAS-TEY - Element-Specific Contrast in Local Probe Microscopy via X-Ray excited photoelectrons detection by conductive tip in Total Electron Yield collection mode [TEY].

·          XAS-AFM - Element-Specific Contrast in Local Probe Microscopy via X-Ray induced changes in capacitance by conductive tip in AFM mode.

All the three funtionalities include the basic feature of standard imaging of surface morphology and the provision of mechanically modifying the structure of surface species by direct tip-surface interaction.
The duration of this project is scheduled in three years. However, we expect to have substantial results already after first the first eighteen months with the test of a first prototype. The subsequent industrialisation of the prototypes will require the intervention of subcontractors that have been targeted but are not present in this proposal.

Project objectives
The objectives that the project pursues can be listed as follows:
- Development of a new instrument integrating the specificities of X-ray spectroscopies with the lateral resolution of Local Probe Microscopy.
- Demonstrate the performances and limitation of the instrument by performing extensive tests in well characterised samples.
- Establish a strong co-ordination among scientists issued of different instrumental cultures for furthering the possibilities opened by integration of different techniques.
- Enhancing the collaboration between scientists from NAS with western European central facilities
- Disseminating the results of the project for encouraging further developments.
 - In case of success, investigate the possibility of commercialising new laboratory equipment based on X-Ray/LPM combinations.

Partecipants list

Project Co-ordinator: Fabio COMIN, ESRF Grenoble

The Trento group (UNITN, IFN-CNR) is involved in the work packages covering different possibilities for experimental detection of X-ray absorption signal via X-ray excited optical luminescence (XEOL-SNOM), secondary electrons (XAS-TEY), and change in capacitance (XAS-AFM).

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Structural and dynamical properties of disordered systems : ionic glasses

Project summary
Many properties of ionic conductor glasses, in principle amenable to the ion diffusivity within the glassy matrix, can be better understood through a thorough knowledge of the influence of disorder on the ion dynamics. Aim of the present project is to investigate on the relationships connecting the structural properties of the host matrix to the dynamics of the mobile ions. Within this broad research field, the attention will be mainly focused, both theoretically and experimentally, on the effects of the local thermal contraction, which has been recently evidenced, on the ion dynamics in glasses doped with silver halides.To this aim, several glasses usually considered as prototypes of glassy ionic conductors will be produced on purpose. Their complete physico-chemical characterisation will be achieved through a careful control of the thermal history, say of the annealing and aging temperature. A systematic investigation will then be performed on these glasses: the local structure will be studied through XAFS (X-ray Absorption Fine Structure) measured with Synchrotron Radiation as a function of temperature; the dynamics will be studied by acoustic and dielectric spectroscopy in broad ranges of temperature (1.2 to 700 K) and frequency (10 Hz to 40 GHz).
 

Project objective
Within the broad research field concerning the fast ion conducting glasses (see below, par. 2.2), the present project is aimed to study structural and thermal disorder in oxide glasses doped with silver halides, and to correlate it to the dynamics of mobile ions. The research project is articulated according to the following steps.

• a) Preparation of silver-molybdate and silver-borate glassy matrices both with binary composition and containing silver halide dopants. The physico-chemical characterisation will be performed on glasses as prepared and also annealed at different temperatures. The glass preparation and characterisation will be done by one of the proposing research units, utilising instrumentation already available, which will be upgraded with the funds of the present project.
   

• b) Investigation of thermal and structural disorder through EXAFS. The possibilities of EXAFS as a probe of thermal disorder (see par. 2.2 below) have already been exploited by one of the proposing units to study the local thermal expansion in silver-borate glasses, with and without AgI dopant. The glass samples studied to date show a contraction of the Ag-O distance as the temperature increases, contrary to the behaviour of the Ag2O crystal, for which the lattice parameter decreases but the Ag-O distance increases. Also the distance I-Ag is reduced in glasses when the temperature increases, while in AgI crystal is nearly constant. The present research program contemplates the extension of these investigations to other silver-borate and silver-molybdate glasses, as well as the search for a connection between local thermal expansion as measured by EXAFS, and ionic conductivity. A further relevant point concerns the effects of structural disorder on EXAFS, due to the possible presence of several different inter-atomic distances around the absorber atom. The basic problem is to understand wether EXAFS effectively selects only the nearest-neighbours atoms linked to the absorber atom by a well defined chemical bond, or is equally sensitive to all the atoms present in the environment. The solution of this problem, which is part of the research program of one of the proposing units, requires the comparison with other experimental techniques, e.g. x-ray or neutron diffusion, and the use of modelisation techniques, both phenomenological, like reverse Monte Carlo, and ab-initio, like Path-Integral Monte Carlo or Molecular Dynamics.
   

• c) Analysis of the mechanical and dielectric response. The comparative study of the response of the system to a stress field (mechanical response) and to an electric field (dielectric response) allowed to obtain useful information on diffusion and localised hopping processes characterising the motion of mobile ions in glasses. Remarkable differences in mechanical response as a function of temperature have been recently observed in silver-molybdate glasses with different thermal histories. Within the present project, a systematic mechanical and dielectric analysis will be done on glasses with different compositions and thermal histories, previously characterised from the structural point of view. The analysis of activation energies and of characteristic times for dielectric relaxation processes (from a few Hz up to 40 GHz) will be compared with results from mechanical relaxation in the frequency range from 10^3 to 10^7 Hz. The temperature will be varied from 10 to 700 K. By this procedure, the accurate evaluation of hopping frequencies and distances of mobile ions within the glassy matrices will be possible.
  The mechanical and dielectric techniques, although giving information on local properties, are based on the macroscopic response of the whole system. The structural and thermal information obtainable from EXAFS and diffraction (X rays and/or neutrons) is thus particularly important for the interpretation of the universality of the dynamical behaviour observed in these systems.
  The main peculiarity of the present research project is the well established will of the participants to investigate the same systems by different techniques, in order to get unified microscopic information. This will allow a better understanding of the mechanism of ionic transport in these materials, as well as of the dynamic behaviour of glassy systems on a large frequency range.

Partecipants list

Research group of the University of Messina headed by Prof. Maria Cutroni
Research group of the University of Trento headed by Prof. Giuseppe Dalba

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Development of the xeol technique for xafs spectroscopy on quantum confined systems
 

Introduction to the Project
X-ray absorption Fine Spectroscopy (XAFS) has been established as a suitable tool for the determination of the atomic structure of condensed matter, independently of its aggregation state. Since many years, the X-ray Laboratory of Trento, whose members are listed in the above participant table, has been involved in the development of XAFS fundamentals, in applications of XAFS to the study of amorphous systems and in the collaboration to the construction of experimental XAFS apparatuses (PWA project, Laboratori Nazionali di Frascati; GILDA beamline, ESRF-Grenoble).
Generally, the standard XAFS detection modes (transmission, x-ray fluorescence, or total electron yield (TEY)) are sensitive to all the absorbing atoms in a given sample. This implies that if the same atomic species is present in different structural configurations, conventional XAFS techniques give an average information about the local environment of all the absorbing atoms.
In the last years, we have developed a new instrumentation, able to monitor the x-ray excited optical luminescence (XEOL) in the visible range. By means of such an apparatus, installed on international synchrotron radiation facilities (ESRF, LURE), we have recorded XEOL-XAFS spectra. Measurements on porous silicon and c-Si nanodots have shown that this technique may be particularly suitable to the study of light emitting quantum confined systems. Among all Si sites distributed in a layer of porous Si, XEOL-XAFS has proved to be sensitive exclusively to the luminescent ones, thus giving original information on their specific local structure Our recent measurements on c-Si nanodots embedded in a silica matrix have shown that the light emission of these very promising systems  is strongly influenced by the presence of a modified SiO₂ region surrounding the c-Si nanoparticles.

Project objectives
The main aim of the present proposal is the development of a new XEOL apparatus optimised for the Infrared region (900-1600 nm). The apparatus will allow to perform PLY-XAFS measurements on different Synchrotron Beamlines (SA32-LURE; GILDA-ESRF; ELETTRA).
Continuing our current research programme, we plan to extend the application of the XEOL technique to the study of quantum confined systems and, more generally, to materials for optoelectronics having IR emission properties. In particular, we hope to perform our first PLY-XAFS studies on the following systems:

• a)      Semiconductors quantum nanodots growth by MBE on monocrystals or embedded in amorphous matrices (at GILDA for K-edges and in the future at ELETTRA for L-edges).

• b)      Er-doped c-Si nanodots. (at GILDA for Er, and at LURE for Si environment)

• c)      Rare earths-doped silica gels in bulk and film (mainly at GILDA).

For all these systems we have already established scientific cooperations with other INFM and CNR partners in Italy; we can guarantee the availability of the new apparatus to other groups interested in cooperate to the local characterisation of light emitting materials. This Program may be of primary interest of INFM Sect. E (semiconductors – new materials for optoelectronics) and, partially of Sect. C (local structure in amorphous and low-dimensional systems; Int. Facility activity support).

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Research proposals accepted by the lure syncrotron radiation facilities  (Orsay-Paris)

Research proposals accepted by  ESRF  (Grenoble)

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