Introduction

My research is focused on the development and the use of X-ray scattering (mostly using synchrotron radiation) for structural determination with new experimental methods and new algorithms for data analysis. The purpose of this research is to explore the frontiers of X-ray diffraction in order to determine the structure of new materials that cannot be studied using established techniques.

X-ray Coherent Diffraction Imaging on semiconductor nanowires (2007-)

• Coherent Diffraction Imaging is a technique developed in the last 10 years, using X-ray nanobeams to yield the 3D structure of single objects with a size between 50 nm to 1 μm. We have worked specifically on the development of Coherent Bragg Imaging: in this method the scattering is recorded around a Bragg peak, and the 3D data can be used to recover the shape of the object, its deformation field and its strain, as well as well a sequence of stacking faults. This work involves both experimental developments (in collaboration with the ID01 beamline of the ESRF), as well as algorithmic ones, for the analysis of coherent diffraction data in the case of strongly strained or faulted structures.

• Selected publications:

  • Favre-Nicolin, V, F Mastropietro, J Eymery, D Camacho, Y M Niquet, B M Borg, M E Messing, et al. “Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging.”

  • Favre-Nicolin, V., J. Eymery, R. Koester, & P. Gentile. “Coherent-diffraction imaging of single nanowires of diameter 95 nanometers.”

Grazing incidence anomalous scattering and Diffraction Anomalous Fine Structure on semiconductor nanostructures (2002-)

• Semiconductor nano-structures (quantum dots, nanowires,...) are generally produced using epitaxial growth, e.g. using the Stranski-Krastanow growth mode for quantum dots, or the Vapor-Liquid-Solid (VLS) mode for nanowires. This leads to the existence of strain fields as well as interdiffusion in these materials. The structural characterization of these materials requires separating the diffraction from the nano-objects and the substrate. For this we use resonant (anomalous) scattering – measuring the scattered intensity around the absorption edge of one element (e.g. Ga for GaN quantum dots grown on AlN). We have been developing two methods:

• Grazing-Incidence Multiwavelength Anomalous Diffraction (GI-MAD), collecting reciprocal space maps at ~10 energies, and extracting partial structure factors corresponding to the resonant atoms, yielding the average composition and strain in the object.

• Grazing-Incidence Diffraction Anomalous Fine Structure (GI-DAFS): in this technique the intensity is collected at a single point in reciprocal space, but for ~500 energies spanned over 500-1000eV. The extracted oscillations (similar to EXAFS) can yield information about the environment of the resonant atoms (type and number of neighbours) This work also involves experimental developments (ensuring reliable I/I0 data collection over 500-1000eV) as well as the development of software programs such as NanoMAD (http://nanomad.sf.net).

• Selected publications:

  • Coraux, J., V. Favre-Nicolin, M. G. Proietti, B. Daudin, & H. Renevier. “Grazing-incidence diffraction anomalous fine structure: Application to the structural investigation of group-III nitride quantum dots.”

  • Coraux, J., M. G. Proietti, V. Favre-Nicolin, H. Renevier, et B. Daudin. “Step-by-step capping and strain state of GaN/AlN quantum dots studied by grazing-incidence diffraction anomalous fine structure.”

  • Létoublon, A., V. Favre-Nicolin, H. Renevier, M.G. Proietti, C. Monat, M. Gendry, O. Marty & C. Priester. “Strain, Size, and Composition of InAs Quantum Sticks Embedded in InP Determined via Grazing Incidence X-Ray Anomalous Diffraction.”

Grazing-incidence X-ray Scattering (2002-)

• Grazing-incidence X-ray scattering is the golden method for in-situ and ex-situ studies of epitaxially grown structures. We have used this technique for the study of free-standing heterogeneous nanowires and quantum dots, and buried nanocolumns. While the technique by itself is not new, we have developed original analysis in several cases:

• Taking into account multiple scattering effects to correctly interpret grazing incidence scattering maps of free-standing quantum dots

• Combining atomistic simulations on large assemblies of atoms to interpret the scattering from buried nanocolumns in a strained matrix

• Developing software to calculate scattering fro large atomistic models (see http://arxiv.org/abs/1010.2641 “Fast computing of scattering maps of nanostructures using graphical processing units” - submitted to J. Appl. Cryst. Also: http://pynx.sf.net)

Selected publications:

• Tardif, S., V. Favre-Nicolin, F. Lançon, E. Arras, M. Jamet, A. Barski, C. Porret, et al. “Strain and correlation of self-organized Ge_{1-x}Mn_{x} nanocolumns embedded in Ge (001).” Physical Review B82 (2010): 104101

• Richard, M.-I., V. Favre-Nicolin, G. Renaud, T. U. Schulli, C. Priester, Z. Zhong, & T.-H. Metzger. “Multiple scattering effects in strain and composition analysis of nanoislands by grazing incidence x rays.”

• Eymery, J, F. Rieutord, V. Favre-Nicolin, O. Robach, Y.-M. Niquet, L. Froberg, T. Martensson, & L. Samuelson. “Strain and Shape of Epitaxial InAs/InP Nanowire Superlattice Measured by Grazing Incidence X-ray Techniques.” http://dx.doi.org/10.1021/nl070888q

Ab initio structure solution from powder diffraction (2000-)

• While refining structures (once a model is known) from powder diffraction is an old technique, ab initio structure solution from powder diffraction is relatively recent. In this context I wrote the “Fox” program, which can be used to solve any type of structure (inorganic and organic) using a direct space (reverse Monte-Carlo) approach. This software was the first to fully take into account special positions, and allow joint optimizations of neutron and X-ray diffraction data. More recently it allowed the optimization of large flexible restraints through the use of smart restraints. In 2009, this program was used to solve ~50 new structures ab initio from powder diffraction (full list at http://vincefn.net/Fox/BiblioStructures), i.e. a significant percentage of new published structures (estimations of new published structures from powder diffraction ranges between 100 to 200/year).

Selected publications: * Favre-Nicolin, V. & R. Černý. “FOX, `free objects for crystallography': a modular approach to ab initio structure determination from powder diffraction.”

• Favre-Nicolin, V. & R. Černý. “A better FOX: using flexible modelling and maximum likelihood to improve direct-space ab initio structure determination from powder diffraction.”

Development of anomalous diffraction for incommensurate structures & macromolecular compounds (PhD)

• During my PhD I worked on original developments of resonant X-ray scattering:

• determination of the 1D incommensurate structure of (TaSe4)2I using resonant scattering on satellite reflections

• using a “dispersive diffraction” experimental set-up to allow the simultaneous diffraction of multiple energies for incommensurate and macromolecular crystals Selected publications: * Favre-Nicolin, V., S. Bos, J. E. Lorenzo, J-L. Hodeau, J-F. Berar, P. Monceau, R. Currat, F. Levy, et H. Berger. “Structural Evidence for Ta-Tetramerization Displacements in the Charge-Density-Wave Compound (TaSe4)2I from X-Ray Anomalous Diffraction.”

  • Hodeau, Jean-Louis, Vincent Favre-Nicolin, Sandra Bos, Hubert Renevier, Emilio Lorenzo, et Jean-Francois Berar. “Resonant Diffraction.”

  • Favre-Nicolin, V., S. Bos, J. E. Lorenzo, P. Bordet, W. Shepard, et J. L. Hodeau. “Integration procedure for the quantitative analysis of dispersive anomalous diffraction.”