Publications

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    ABSTRACT: The effects of surface and interface on the thermodynamics of small particles require a deeper understanding. This step is crucial for the development of models that can be used for decision-making support to design nanomaterials with original properties. Based on experimental results for phase transitions in compressed ZnO nanoparticles, we show the limitations of classical thermodynamics approaches (Gibbs and Landau). We develop a new model based on the Landau-Ginzburg theory that requires the consideration of several terms, such as the interaction between nanoparticles, pressure gradients, defect density, and so on. This phenomenological approach sheds light on the discrepancies in the literature as it identifies several possible parameters that should be taken into account to properly describe the transformations. For the sake of clarity and standardization, we propose an experimental protocol that must be followed during high-pressure investigations of nanoparticles in order to obtain coherent, reliable data that can be used by the scientific community.
    Nano Letters 12/2013; · 13.03 Impact Factor
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    ABSTRACT: Nanoparticles have more propensities for amorphization than their bulk counterparts, opening the opportunity to achieve the amorphous states in well-known poor glass-former compounds. Classical size effects are often invoked to explain such phenomenon. However, this argument is not sufficient to cover all experimental results such as functionalization effect. In this work, Y2O3 nanoparticles of 7 nm diameter are investigated under pressure. Special care is taken on the surface state of the particles by comparing the pressure-induced transformation of nanoparticles stored under different conditions (atmospheric vs argon environment). A clear difference is reported as one batch shows a transformation to an amorphous state, whereas the second undergoes a crystal-to-crystal transition. These results are discussed in terms of interface energy taking into account not only the usual surface energy but also the surface state contribution. The link between different types of amorphization (pressure-, mechanical-, and radiation-induced amorphization) in nanoparticles is underlined as a critical defect density is required to achieve the crystal-to-amorphous transformation. Defect creation may arise from multiple sources: irradiation, functionalization, and reconstructive transition.
    The Journal of Physical Chemistry C 05/2013; 117(21):11133-11140. · 4.81 Impact Factor
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    Denis Machon
    The Journal of Physical Chemistry C 04/2013; 117:11133. · 4.81 Impact Factor
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    ABSTRACT: Pressure-induced amorphization (PIA) is a phenomenon that involves an abrupt transition between a crystalline material and an amorphous solid through application of pressure at temperatures well below the melting point or glass transition range. Amorphous states can be produced by PIA for substances that do not normally form glasses by thermal quenching. It was first reported for ice Ih in 1984 following prediction of a metastable melting event associated with the negative initial melting slope observed for that material. The unusual phenomenon attracted intense interest and by the early 1990’s PIA had been reported to occur among a wide range of elements and compounds. However, with the advent of powerful experimental techniques including high resolution synchrotron X-ray and neutron scattering combined with more precise control over the pressurization environment, closer examination showed that some of the effects previously reported as PIA were likely due to formation of nanocrystals, or even that PIA was completely bypassed during examination of single crystals or materials treated under more hydrostatic compression conditions. Now it is important to understand these results together with related discussions of polyamorphic behavior to gain better understanding and control over these metastable transformations occurring in the low temperature range where structural relaxation and equilibration processes are severely constrained. The results will lead to useful new high-density amorphous materials or nanocrystalline composites containing metastable crystalline varieties and the experiments have driven new theoretical approaches to modeling the phenomena. Here we review the incidence and current understanding of PIA along with related phenomena of density- and entropy-driven liquid-liquid phase transitions (LLPT) and polyamorphism. We extend the discussion to include polymeric macromolecules and biologically-related materials, where the phenomena can be correlated with reversible vs irreversible unfolding and other metastable structural transformations.
    Progress in Materials Science 01/2013; · 23.19 Impact Factor
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    ABSTRACT: The vibrational and structural properties of modified double-wall carbon nanotubes (DWNTs) were investigated by high-pressure resonance Raman scattering. We studied bromine-intercalated DWNTs grown by chemical vapor deposition (CVD) and 13 C 60 peapod-derived DWNTs in comparison with pristine CVD-grown DWNTs. The effects of chemical modification, carbon interwall geometry, and inhomogeneous filling on the high-pressure evolution of the DWNTs have been investigated. We find that the mechanical resistance of the DWNT system is affected both in the case of bromine-intercalated CVD-DWNTs and also for the 13 C 60 peapod-derived DWNTs, thus lowering the onset of collapse pressure P (onset) c compared with pristine CVD-DWNTs. For bromine CVD-DWNTs, P (onset) c was observed to be 13 GPa, well below the 21 GPa found for pristine CVD-DWNTs. Uniaxial constrains in the interstitial regions of the DWNT bundle due to the presence of bromine arrangements explains this mechanical instability rather than a charge transfer process. Isotopic 13 C enrichment of the inner tube reduces the frequency of its tangential contribution to the G-band Raman spectrum, which appears to be an effective method to separate the contribution of inner-and outer-tube G + components during pressure evolution. P (onset) c was found to be 12 GPa for the 13 C 60 -derived DWNT system. In this case, the instability of the DWNT is mainly due to the high inhomogeneous filling of the outer tube, as a consequence of the conversion method used to produce the inner-wall nanotube from the peapods, which produces inner tubes which are usually shorter than the outer tubes, leading to the outer tube not being completely filled.
    Physical review. B, Condensed matter 11/2012; 86(19):195410. · 3.77 Impact Factor
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    The Journal of Physical Chemistry C 09/2012; 116:22043-22050. · 4.81 Impact Factor
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    ABSTRACT: In this work, we study the adhesion forces between atomic force microscopy (AFM) tips and superficial dentin etched with phosphoric acid. Initially, we quantitatively analyze the effect of acid etching on the surface heterogeneity and the surface roughness, two parameters that play a key role in the adhesion phenomenon. From a statistical study of the force-distance curves, we determine the average adhesion forces on the processed substrates. Our results show that the average adhesion forces, measured in water, increase linearly with the acid exposure time. The highest values of such forces are ascribed to the high density of collagen fibers on the etched surfaces. The individual contribution of exposed collagen fibrils to the adhesion force is highlighted. We also discuss in this paper the influence of the environmental medium (water/air) in the adhesion measurements. We show that the weak forces involved require working in the aqueous medium.
    Journal of Colloid and Interface Science 03/2012; 376(1):262-8. · 3.17 Impact Factor
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    ABSTRACT: Through a systematic structural search we found an allotrope of carbon with Cmmm symmetry which we predict to be more stable than graphite for pressures above 10 GPa. This material, which we refer to as Z-carbon, is formed by pure sp(3) bonds and it provides an explanation to several features in experimental x-ray diffraction and Raman spectra of graphite under pressure. The transition from graphite to Z-carbon can occur through simple sliding and buckling of graphene sheets. Our calculations predict that Z-carbon is a transparent wide band-gap semiconductor with a hardness comparable to diamond.
    Physical Review Letters 02/2012; 108(6):065501. · 7.94 Impact Factor
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    ABSTRACT: We investigate the interface energy impact on phase stability using the shining example of TiO2 nanoparticles under pressure. We revisit the previously reported phase diagram of this system and propose a new mechanism allowing the control of pressure-induced amorphization of TiO2 ultrafine particles. We demonstrate that the size effect is necessary for stabilizing the amorphous state but is not sufficient in the sense that surface chemical functionalization of nanoparticles is determinant. This discovery opens the possibility to select the high-pressure phase in nanomaterials and, consequently, the recovered structure under ambient conditions.
    The Journal of Physical Chemistry C 11/2011; 115(45):22286-22291. · 4.81 Impact Factor
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    ABSTRACT: Exfoliated graphene and few layer graphene samples supported on SiO(2) have been studied by Raman spectroscopy at high pressure. For samples immersed on a alcohol mixture, an electron transfer of ∂n/∂P ∼ 8 × 10(12) cm(-2) GPa(-1) is observed for monolayer and bilayer graphene, leading to giant doping values of n ∼ 6 × 10(13) cm(-2) at the maximum pressure of 7 GPa. Three independent and consistent proofs of the doping process are obtained from (i) the evolution of the Raman G-band to 2D-band intensity ratio, (ii) the pressure coefficient of the G-band frequency, and (iii) the 2D band components splitting in the case of the bilayer sample. The charge transfer phenomena is absent for trilayer samples and for samples immersed in argon or nitrogen. We also show that a phase transition from a 2D biaxial strain response, resulting from the substrate drag upon volume reduction, to a 3D hydrostatic compression takes place when going from the bilayer to the trilayer sample. By model calculations we relate this transition to the unbinding of the graphene-SiO(2) system when increasing the number of graphene layers and as function of the surface roughness parameters. We propose that the formation of silanol groups on the SiO(2) substrate allows for a capacitance-induced substrate-mediated charge transfer.
    Nano Letters 08/2011; 11(9):3564-8. · 13.03 Impact Factor
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    ABSTRACT: The vibrational properties of double-walled carbon nanotubes (DWNTs) is investigated by high-pressure resonance Raman scattering up to 30 GPa in two different pressure-transmitting media (PTM): paraffin oil and NaCl. The protection effect on the outer tube during compression is verified .The collapse of DWNTs is experimentally observed for the first time, showing to be two-step: the onset of the outer 1.56 nm diameter tube collapse at 21 GPa is followed by the collapse of the inner 0.86 nm diameter tube at a higher pressure of 25 GPa. This observation is supported by calculations. We show that filling a tube with another tube leads to a pressure stabilization against collapse, in strong opposition to what is observed when filling a tube with fullerenes or iodine. The collapse pressure in DWNTs appears to follow a 1/dtav3 law, where dtav is the average diameter from the inner and outer tubes, in agreement with predictions [Yang, X.; Appl. Phys. Lett. 2006, 89, 113101]. Contrary to SWNTs and peapods, for DWNTs, the observed collapse pressure is independent of the PTM nature. Those differences are discussed in terms of tube filling homogeneity and of the separate roles of inner and outer tubes: the outer tube offers chemical screening to the inner tube, whereas the inner tube guarantees mechanical support to the outer one. This leads to high collapse pressure independent of the DWNT environnment: a characteristic that makes DWNTs ideal fillers for composite nanomaterials for high load mechanical support.
    The Journal of Physical Chemistry C. 03/2011; 115(13).
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    ABSTRACT: The high pressure stability of the silicon type-III clathrate Ba24Si100 has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba8Si46. An isostructural volume collapse is observed at ~23 GPa, a value higher than for Ba8Si46 (13-15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu , Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical.
    Physical review. B, Condensed matter 01/2011; 83. · 3.77 Impact Factor
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    ABSTRACT: We identify a mechanism responsible for amorphization of rare-earth molybdate isostructural compounds by means of x-ray absorption spectroscopy (XAS) and Raman spectroscopy as well as first-principles calculations. Our present Raman spectra show that both β′ and α-Eu2(MoO4)3 undergo a pressure-induced amorphization. Both first-principles calculations and XAS measurements explain the process of amorphization by a spatial self-reorganization of the oxygen clouds around the Mo and Eu subnetworks as the pressure is increased inducing a change in the coordination number of the Mo atoms. This latter transforms the eg crystal field of the tetrahedra into a series of stabilization-degenerated peaks, which energetically favors a disordered overlapping of the former MoO4 tetrahedra instead of a mere ordered compression and rotation of these fragments.
    Physical review. B, Condensed matter 01/2011; 83(21). · 3.77 Impact Factor
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    ABSTRACT: The magnetostructural bcc-to-hcp phase transition in iron is analysed theoretically in the framework of the Landau theory of phase transitions. In contrast to recent interpretations which emphasize the driving role of magnetism at the transition, the collapse of the ferromagnetic order in ε-Fe is interpreted as resulting from the large spontaneous strains and the magnitude of the displacive order-parameter involved in the Burgers reconstructive transition mechanism. It yields a direct first-order transition from the ferromagnetic α-phase to the non-magnetic ε-phase, without going across an intermediate magnetic structure.
    Journal of Physics Condensed Matter 11/2010; 22(46):466002. · 2.36 Impact Factor
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    ABSTRACT: Two different nanometric (6 nm) TiO2 compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13–16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation HDA2→HDA1 at ∼30 GPa. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.
    Physical Review B 10/2010; 82(14). · 3.77 Impact Factor
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    ABSTRACT: Angle-resolved synchrotron radiation diffraction was used to investigate lithium potassium sulfate (LiKSO(4)) crystals under high pressure. We confirm that the title compound undergoes three phase transitions, α →β, β → γ and γ →δ, observed at around 0.8 GPa, 4.0 GPa and 7.0 GPa, respectively. Two competitive structures are proposed for the β-phase after powder diffraction data Rietveld refinements: an orthorhombic (space group Cmc 2(1)) or a monoclinic (space group Cc) structure. These structures correspond to the models of the low temperature phases. The γ-phase is indexed by a monoclinic structure. Finally, the δ-phase is found to be highly disordered. No evidence of any pressure-induced amorphous phase was observed up to 24 GPa, even under imposed highly non-hydrostatic conditions, contrary to previous propositions.
    Journal of Physics Condensed Matter 08/2010; 22(31):315401. · 2.36 Impact Factor
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    ABSTRACT: The power of Raman spectroscopy for the study of the high-pressure evolution of carbon nanotubes is shown. After an introduction to carbon nanotubes and its resonance Raman scattering signal, we discuss the high-pressure Raman studies on single-wall carbon nanotubes with particular emphasis on the identification of pressure-induced structural and electronic transitions. KeywordHigh pressure-Raman spectroscopy-carbon nanotubes
    06/2010: pages 435-446;
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    ABSTRACT: We investigate the cubic to tetragonal phase transition in the pressure-temperature phase diagram of strontium titanate SrTiO3 (STO) by means of Raman spectroscopy and x-ray diffraction on single-crystal samples. X-ray diffraction experiments are performed at room temperature, 381 and 467 K up to 53 GPa, 30 GPa, and 26 GPa, respectively. The observation of the superstructure reflections in the x-ray patterns provides evidence that the crystal undergoes at all investigated temperatures a pressure-induced transition from cubic to the tetragonal I4/mcm phase, identical to the low-temperature phase. No other phase transition is observed at room temperature up to 53 GPa. Together with previously published data, our results allow us to propose a linear phase boundary in the pressure-temperature phase diagram. The data are analyzed in the framework of the Landau theory of phase transitions. With a revised value of the coupling coefficient between the order parameter and the volume spontaneous strain, the model built from pressure-independent coefficients reproduces satisfactorily the boundary in the phase diagram, but fails at reflecting the more pronounced second-order character of the pressure-induced phase transition as compared to the temperature-induced transition. We propose a Landau potential suitable for the description of the pressure-induced phase transition. Finally, we show that particular attention has to be paid to hydrostatic conditions in the study of the high-pressure phase transition in STO.
    Physical review. B, Condensed matter 02/2010; 81(5). · 3.77 Impact Factor
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    ABSTRACT: The high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species.
    Physical review. B, Condensed matter 01/2010; · 3.77 Impact Factor

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