Marc Chaigneau

Electrostatic Grafting of Diamond Nanoparticles: A Versatile Route to Nanocrystalline Diamond Thin Films

Nanodiamond (ND) seeding is a well-established route toward the CVD (chemical vapor deposition) synthesis of diamond ultrathin films. This method is based on the deposition onto a substrate of diamond nanoparticles which act as pre-existing sp(3) seeds. Here, we report on a straightforward method to disperse diamond nanoparticles on a substrate by taking advantage of the electrostatic interactions between the nanodiamonds and the substrate surface coated with a cationic polymer. This layer-by-layer deposition technique leads to reproducible and homogeneous large-scale nanoparticle deposits independent of the substrates nature and shape. No specific functionalization of the nanoparticles is required, and low concentrated solutions can be used. The density of NDs on the substrate can be controlled, as shown by in situ ATR-FTIR (attenuated total reflection Fourier transform infrared) analysis and QCM (quartz crystal microbalance) measurements. Highly dense and compact ND deposits can be obtained, allowing CVD growth of nanocrystalline diamond ultrathin films (70 nm) on various substrates. The synthesis of 3D structured and patterned diamond thin films has also been demonstrated with this method.

Recent advances in germanium emission [Invited]

The optical properties of germanium can be tailored by combining strain engineering and n-type doping. In this paper, we review the recent progress that has been reported in the study of germanium light emitters for silicon photonics. We discuss the different approaches that were implemented for strain engineering and the issues associated with n-type doping. We show that compact germanium emitters can be obtained by processing germanium into tensile-strained microdisks.

Control of tensile strain in germanium waveguides through silicon nitride layers

Germanium ridge waveguides can be tensilely strained using silicon nitride thin films as stressors. We show that the strain transfer in germanium depends on the width of the waveguides. Carrier population in the zone center Γ valley can also be significantly increased when the ridges are oriented along the 〈100〉 direction. We demonstrate an uniaxial strain transfer up to 1% observed on the room temperature direct band gap photoluminescence of germanium. The results are supported by 30 band k·p modeling of the electronic structure and the finite element modeling of the strain field.

High quality tensile-strained n-doped germanium thin films grown on InGaAs buffer layers by metal-organic chemical vapor deposition

We show that high quality tensile-strained n-doped germanium films can be obtained on InGaAs buffer layers using metal-organic chemical vapor deposition with isobutyl germane as germanium precursor. A tensile strain up to 0.5% is achieved, simultaneously measured by x-ray diffraction and Raman spectroscopy. The effect of tensile strain on band gap energy is directly observed by room temperature direct band gap photoluminescence.

Billion-Fold Increase in Tip-Enhanced Raman Signal

A billion-fold increase in the Raman signal over conventional tip-enhanced Raman spectroscopy/microscopy (TERS) is reported. It is achieved by introducing a stimulating beam confocal with the pump beam into a conventional TERS setup. A stimulated TERS spectrum, closely corresponding to its spontaneous TERS counterpart, is obtained by plotting the signal intensity of the strongest Raman peak of an azobenzene thiol self-assembled monolayer versus the stimulating laser frequency. The stimulated TERS image of azobenzene thiol molecules grafted onto Au ⟨111⟩ clearly shows the surface distribution of the molecules, whereas, when compared to the simultaneously recorded surface topography, it presents an image contrast of different nature. The experimentally obtained stimulated gain is estimated at 1.0 × 10(9), which is in reasonable agreement with the theoretically predicted value. In addition to the signal increase, the signal-to-noise ratio was 3 orders of magnitude higher than in conventional spontaneous TERS. The proposed stimulated TERS technique offers the possibility for a substantially faster imaging of the surface with respect to normal TERS.

Effect of increasing thickness on tensile-strained germanium grown on InGaAs buffer layers

We have investigated the optical properties of tensile-strained germanium grown on InGaAs buffer layers as a function of film thickness and buffer layer composition. We study the dependence of the photoluminescence as a function of the strain amplitude and degree of relaxation which are also monitored by X-ray diffraction and Raman spectroscopy. We show that 0.75% biaxially strained germanium can be obtained up to a thickness of 150 nm, a value sufficiently high to allow confinement of the spontaneous emission in a guiding structure. For large thicknesses (>200 nm) and large indium content in the buffer layer, a partial relaxation of the film is observed characterized by a large in-plane anisotropy of the germanium lattice. In this case, a difference of strain magnitude deduced either by microphotoluminescence spectra or by X-ray or Raman measurements is reported. We explain this difference by the sensitivity of microphotoluminescence to the local properties of the material. This study provides guidelines...

Dual graphene films growth process based on plasma-assisted chemical vapor deposition

Graphene has been given great attention to overcome current physical limits in electronic devices and its synthesis routes are developing rapidly. However, graphene film manufacturing is still hindered by either low throughput or low material quality. Here, we present a low temperature PE-CVD assisted graphene growth process on nickel thin films deposited on silicon oxide. Furthermore, our process leads to the formation of two separated graphene films, one at the nickel surface and the other at the Ni/SiO2 interface. A mixture of methane and hydrogen was employed as carbon precursor and activated by DC plasma. We found that the number of graphene layers on top of nickel can be controlled by carbon exposure time, from 1 to around 10 layers. Further annealing process of samples allowed us to achieve improved graphene films by the dissolution and segregation-crystallization process.

Synthesis of conducting transparent few-layer graphene directly on glass at 450??C

Post-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq⁻¹. Finally the samples equipped with printed contacts appear to be efficient humidity sensors.

Synthesis, characterization, morphological behaviour, and photo- and electroluminescence of highly blue-emitting fluorene-carbazole copolymers with alkyl side-chains of different lengths

Four novel phenyl-end-capped π-conjugated polymers comprising alkyl chains of different lengths were synthesized: the poly(9,9-dihexyl-2,7-fluorene-alt-9,9-dioctylfluorene) (P0), the alternating fluorene-carbazole copolymers P1 and P2 comprising 20% and 50% of carbazole, respectively, and the homopolymer poly(9-hexyl-9H-carbazole) (P3). The non-end-capped alternating fluorene-carbazole copolymer P4 comprising 20% of carbazole was also synthesized for the sake of comparison with P1. P0-P2 and P4 were synthesized by a Pd-catalyzed Suzuki-Miyaura coupling in good yields (80-89%), while P3 was obtained by a Ni-catalyzed Yamamoto coupling reaction from the 3,6-dibromocarbazole monomer in a moderate yield (52%). P0-P4 polymers were characterized by NMR, elemental analysis, and GPC. The molecular weights are 40.30, 23.42, 14.33, 3.92, and 37.49 kDa, with polydispersity indices of 2.5, 1.7, 1.8, 1.3, and 2.6, for P0, P1, P2, P3, and P4, respectively. These polymers were found to show a high thermal stability, with decomposition temperatures in the range of 395-420 °C, and the glass transition temperature was found to regularly increase with the amount of carbazole inserted in the conjugated backbone. AFM images obtained on thin films (thickness of about 90 nm) of P0-P2 revealed films with surfaces of good quality, being homogeneous with low roughness (0.2 nm for the smaller ones). These polymers were found to be blue-emitting both in diluted dichloromethane and chlorobenzene solutions as well as in thin films and exhibit relatively high values of the absolute quantum yields in the range of 100-5.5% in dichloromethane and 51.4-7.7% in thin films. Blue-emitting electroluminescent devices were obtained with P0 and P1 as emitting layers, respectively. The device built with P1 showed improved performances (EQE of 1.32%) with respect to the one built with the parent polyfluorene material (EQE of 0.75%).

Molecular arrangement in self-assembled azobenzene-containing thiol monolayers at the individual domain level studied through polarized near-field Raman spectroscopy.

6-[4-(phenylazo)phenoxy]hexane-1-thiol self-assembled monolayers deposited on a gold surface form domain-like structures possessing a high degree of order with virtually all the molecules being identically oriented with respect to the surface plane. We show that, by using polarized near-field Raman spectroscopy, it is possible to derive the Raman scattering tensor of the ordered layer and consequently, the in-plane molecular orientation at the individual domain level. More generally, this study extends the application domain of the near-field Raman scattering selection rules from crystals to ordered organic structures.