Fabien Delpech

Surface Chemistry of InP Quantum Dots: A Comprehensive Study

Advanced (1)H, (13)C, and (31)P solution and solid-state NMR studies combined with IR spectroscopy were used to probe, at the molecular scale, the composition and the surface chemistry of indium phosphide (InP) quantum dots (QDs) prepared via a non-coordinating solvent strategy. This nanomaterial can be described as a core-multishell object: an InP core, with a zinc blende bulk structure, is surrounded first by a partially oxidized surface shell, which is itself surrounded by an organic coating. This organic passivating layer is composed, in the first coordination sphere, of tightly bound palmitate ligands which display two different bonding modes. A second coordination sphere includes an unexpected dialkyl ketone and residual long-chain non-coordinating solvents (ODE and its isomers) which interact through weak intermolecular bonds with the alkyl chains of the carboxylate ligands. We show that this ketone is formed during the synthesis process via a decarboxylative coupling route and provides oxidative conditions which are responsible for the oxidation of the InP core surface. This phenomenon has a significant impact on the photoluminescence properties of the as-synthesized QDs and probably accounts for the failure of further growth of the InP core.

InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description

Advanced (1)H, (13)C, and (31)P solution- and solid-state NMR studies combined with XPS were used to probe, at the molecular scale, the composition (of the core, the shell, and the interface) and the surface chemistry of InP/ZnS core/shell quantum dots prepared via a non-coordinating solvent strategy. The interface between the mismatched InP and ZnS phases is composed of an amorphous mixed oxide phase incorporating InPO(x) (with x = 3 and predominantly 4), In(2)O(3), and InO(y)(OH)(3-2y) (y = 0, 1). Thanks to the analysis of the underlying reaction mechanisms, we demonstrate that the oxidation of the upper part of the InP core is the consequence of oxidative conditions brought by decarboxylative coupling reactions (ketonization). These reactions occur during both the core preparation and the coating process, but according to different mechanisms.

Low-temperature FIR and submillimetre mass absorption coefficient of interstellar silicate dust analogues

Context. Cold dust grains are responsible for the far-infrared and submillimetre (FIR/submm) emission observed by Herschel and Planck. Their thermal emission is usually expressed as a modified black body law in which the FIR/submm dust opacity, or mass absorption coefficient (MAC), is described by the MAC at a given wavelength κλ0 and the temperature- and wavelength-independent emissivity spectral index β. However, numerous data from previous space and balloon-borne missions and recently from Herschel and Planck show that the dust emission is not well understood, as revealed for example by the observed anti-correlation of β with the grain temperature. Aims. The aim of this work is to measure the optical properties of interstellar dust analogues at low temperatures to give astronomers the necessary data for interpreting FIR/submm observations such as those from the Herschel and Planck satellites. Methods. We synthesised, via sol-gel methods, analogues of interstellar amorphous silicate grains, rich in Mg and Ca, and having stoichiometry of olivine and pyroxene. The samples are characterised by various techniques to determine their composition, size, amorphisation degree. All the amorphous samples are annealed at 1100 ◦ C to study the crystallised materials for comparison. We measured the MAC of all the samples in the 2–25 μm range at room temperature and in the 100–1000/1500 μm range for grain temperatures varying from 300 to 10 K. Results. The experimental results show that, for all the amorphous samples, the grain MAC decreases when the grain temperature decreases and that the local spectral index, β, defined as the slope of the MAC curve, is anti-correlated with the grain temperature. These variations, which are not observed in the crystallised samples, are related to the amorphous nature of the samples. In addition, the spectral shape of the MAC is complex and cannot be described by a single spectral index over the 100–1500 μm range. At short wavelengths (λ ≤ 500/700 μm), β is in the range 1.6–2.1 for all grain temperature and grain composition. However, at longer wavelengths (λ ≥ 500/700 μm), β ≤ 2 for samples with a pyroxene stoichiometry and β ≥ 2 for samples with an olivine stoichiometry. Conclusions. The dust properties in the FIR/submm domain and at low temperature are more complicated than expected. The simplifying asymptotic expression based on a single temperature- and wavelength-independent spectral index used by astronomers is not appropriate to describe the dust MAC, hence the dust emission, and may induce significant errors on the derived parameters, such as the dust mass and the dust physical and chemical properties. Instead, dust emission models should use the dust MAC as a function of wavelength and temperature.

Influence of a transverse static magnetic field on the magnetic hyperthermia properties and high-frequency hysteresis loops of ferromagnetic FeCo nanoparticles

The influence of a transverse static magnetic field on the magnetic hyperthermia properties is studied on a system of large-losses ferromagnetic FeCo nanoparticles. The simultaneous measurement of the high-frequency hysteresis loops and of the temperature rise provides an interesting insight into the losses and heating mechanisms. A static magnetic field of only 40 mT is enough to cancel the heating properties of the nanoparticles, a result reproduced using numerical simulations of hysteresis loops. These results cast doubt on the possibility to perform someday magnetic hyperthermia inside a magnetic resonance imaging setup.

New generation of magnetic and luminescent nanoparticles for in vivo real-time imaging

A new generation of optimized contrast agents is emerging, based on metallic nanoparticles (NPs) and semiconductor nanocrystals for, respectively, magnetic resonance imaging (MRI) and near-infrared (NIR) fluorescent imaging techniques. Compared with established contrast agents, such as iron oxide NPs or organic dyes, these NPs benefit from several advantages: their magnetic and optical properties can be tuned through size, shape and composition engineering, their efficiency can exceed by several orders of magnitude that of contrast agents clinically used, their surface can be modified to incorporate specific targeting agents and antifolding polymers to increase blood circulation time and tumour recognition, and they can possibly be integrated in complex architecture to yield multi-modal imaging agents. In this review, we will report the materials of choice based on the understanding of the basic physics of NIR and MRI techniques and their corresponding syntheses as NPs. Surface engineering, water transfer and specific targeting will be highlighted prior to their first use for in vivo real-time imaging. Highly efficient NPs that are safer and target specific are likely to enter clinical application in a near future.

Silica nanoparticles grown and stabilized in organic nonalcoholic media.

This work features an alternative approach to the well-documented preparation of silica nanoparticles in protic media. We present here the one-pot synthesis of silica nanoparticles of adjustable size (between 18 and 174 nm), prepared and stabilized in organic nonalcoholic solvents. This novel route is based on hydrolysis and condensation of tetraethoxysilane, using water as reactant and different primary amines (butylamine, octylamine, dodecylamine, hexadecylamine) as catalysts in tetrahydrofuran or dimethoxyethane. The growth rate can be finely adjusted, and the first stages of the formation are observed by transmission electronic microscopy, revealing a silicated network in which the silica particles are formed and then released in solution. The amine plays not only a catalyst role but is also implied, as well as the solvent, in the stabilization process and the size control of the particles. A detailed NMR study demonstrates a core-shell structure in which the silica core is surrounded by a layer of alkylammonium ions together with solvent.

Stoichiometry-controlled FeP nanoparticles synthesized from a single source precursor

Phase-pure FeP nanoparticles (NPs) have been synthesized through low temperature thermolysis of the single source precursor [(CO)4Fe(PH3)]. Examination of the mechanism demonstrates the central role of the labile CO ligands and the weak P-H bonds to yield stoichiometry controlled FeP materials.

New linked di-germanocenes and di-stannocenes

Abstract New bridged di-germanocenes and di-stannocenes 2 have been synthesized from a reaction between (pentamethylcyclopentadienyl)metal chloride and the dilithium salts of the corresponding linked cyclopentadienyl ligands (spacer: phenylene, biphenylene, thiophene) in good yields. These di-metallocenes react easily with catechol giving preferentially the substitution reaction. With iodine or with metal 14 dichloride (M=Ge, Sn), the oxidative products are unstable and rapidly loose the linked cyclopentadienyl ligand. Starting from SnCl2, the ionic half-sandwich compound [Cp*Ge][SnCl3] 8, so obtained, was characterized by X-ray diffraction analysis which reveals a polymeric form in the solid state. The reaction of the di-germanocenes with o-quinone leads to the expected cycloadducts, stable in the case of permethylated compounds. An unusual single electron transfer reaction takes place with [Cp2Fe][BF4] and the transient cation radical rapidly gives [Cp*Ge][BF4]. Mass spectra measurements and electrochemistry study confirm the weak stability of this cationic species.

Strawberry-like SiO2@Pd and Pt nanomaterials

Based on a synthesis strategy of silica nanoparticles (NPs) in non-alcoholic medium, a straightforward protocol leading to strawberry-like SiO2@M nanomaterials (M = Pt, Pd) has been developed. Pd and Pt NPs were immobilised onto surface modified silica NPs, by in situ decomposition of organometallic precursors, Pd2(dba)3 and Pt(dba)2 (dba = dibenzylideneacetone). The surface of silica was functionalised by aminopropyltriethoxysilane (APTES) or 2-(diphenylphosphino)ethyltriethoxysilane (PhPETES). The co-grafting of propyltriethoxysilane (PTES) and APTES was investigated to avoid the classical interfering cohydrolysis and polycondensation reactions which can lead to agglomeration phenomena between the silica NPs. It appeared that a APTES/PTES ratio of 1/3 insured a dense and homogeneous deposition of the metallic NPs onto silica. Depending on the ligand, the diameter of the metallic NPs varies from 3.6 to 6.0 nm for Pd and from 1.5 to 3.1 nm for Pt NPs. The metal loading is adjustable, at least up to 6.91 wt%, as exemplified with Pd. Preliminary catalytic tests of the SiO2@Pd nanomaterial are also presented.

Transition-metal complexes containing parent phosphine or phosphinyl ligands and their use as precursors for phosphide nanoparticles.

P-H functional transition-metal complexes were synthesized without using hazardous PH3 gas in good yields by photolysis of the transition-metal carbonyl complexes M(CO)(6-x) (M = Cr, W, Fe; x = 0, 1) in tetrahydrofuran followed by reaction with P2(SiMe3)4 and subsequent methanolysis to give the bridging complexes [(CO)(x)M(μ-PH2)]2 (M = Fe, x = 3 (1), M = Cr, x = 4 (2a), M = W, x = 4 (2b)). The photolysis of [(CO)4M(μ-PH2)]2 (M = Cr (2a), M = W (2b)) with P(SiMe3)3 was applied followed by methanolysis to synthesize the PH2 bridging transition-metal binuclear complexes with terminal PH3 groups. The products [(CO)4M(μ-PH2)2M(CO)3(PH3)] (M = Cr (3a), M = W (3b)) and [(CO)4W(μ-PH2)2W(CO)2(PH3)2] (4b) were isolated in moderate yield. Another synthetic approach to this type of compounds is the direct photolysis of the complexes [(CO)3M(PH3)3] (M = Cr (5a), M = W (5b)). The products were comprehensively characterized by (31)P NMR and IR spectroscopy as well as by X-ray structural analysis. Additionally, the relevancy of 2a as single source precursor for the synthesis of stoichiometry-controlled CrP nanoparticles has been demonstrated.