Halide Ligands to Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness

International audienceZinc blende II-VI semiconductor nanoplatelets (NPLs) are defined at the atomic scale along the thickness of the nanoparticle and are initially capped with carboxylates on the top and bottom [001] facets. These ligands are exchanged on CdSe NPLs with halides that act as X-L-type ligands. These CdSe NPLs are costabilized by amines to provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. After ligand exchange, the optical features are redshifted. Thus, ligand tuning is another way, in addition to confinement, to tune the optical features of NPLs. The improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. However, for chloride and iodide, the surface coverage is incomplete, and thus, the fluorescence quantum efficiency is lower. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs

All-quantum dot based Förster resonant energy transfer: key parameters for high-efficiency biosensing.

peer reviewedWhile colloidal quantum dots (QDs) are commonly used as fluorescent donors within biosensors based on Förster resonant energy transfer (FRET), they are hesitantly employed as acceptors. On the sole basis of Förster theory and the well-known behaviour of organic dyes, it is often argued that the QD absorption band over the UV-visible range is too wide. Discarding these preconceptions inherited from classical fluorophores, we experimentally examine the FRET process occurring between donor and acceptor CdTe QDs and provide a mathematical description of it. We evidence that the specific features of QDs unexpectedly lead to the enhancement of acceptors' emission (up to +400%), and are thus suitable for the design of highly efficient all-QD based FRET sensors. Our model enables us to identify the critical parameters maximizing the contrast between positive and negative biosensing readouts: the concentrations of donors and acceptors, their spectral overlap, the densities of their excitonic states, their dissipative coupling with the medium and the statistics of QD-QD chemical pairing emerge as subtle and determinant parameters. We relate them quantitatively to the measured QD-QD FRET efficiency and discuss how they must be optimized for biosensing applications

l-Methionine adsorption on Cu(110), binding and geometry of the amino acid as a function of coverage

International audienceThe adsorption of l-methionine on Cu(110) has been characterized by combining in situ Polarization Modulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS) and X-ray Photoelectron Spectroscopy (XPS). Both the chemical state of the molecule, and its anchoring points were determined at various coverage values. Adsorbed methionine is anionic and first interacts with the copper surface via its sulfur and/or oxygen atoms, likely lying flat on the surface; at higher coverage, a stronger interaction of oxygen and nitrogen atoms with copper, evidenced by slight shifts of the XPS peaks, together with an angular dependence of the peak ratios, suggests that the molecule stands up on the surface, interacting with the surface via the N and O atoms but almost not anymore via its S atom. Last but not least, no multilayers were evidenced, and this was explained by the geometry of the molecules which leaves no groups accessible for intermolecular interactions

An electrical percolation model for tin dioxide polycrystalline thin films

International audienceThe physical properties of tin dioxide and its electrical performance largely depend on its morphological properties. As mentioned by several authors, the size of crystallites relative to the Debye length appears to be a determining factor for the sensitivity to gas. In a previous paper concerning thin films elaborated by reactive evaporation, it has been established that the electrical conductivity value depends on the relative density of the film

Assessing carbon or tungstates coverage of ZrO2 nanoparticles supported on MWCNT via NOx-TPD

International audienceMulti-walled carbon nanotubes (MWCNT) promoted by oxides, that are far from trivial to characterize, have emerged as promising candidates in several fields such as biosensors and catalysis. In this study the coverage of ZrO2 nanoparticles supported on MWCNT, by carbonaceous deposits or tungstates was investigated by NO temperature-programmed desorption (NO-TPD). ZrO2 coverage by carbonaceous deposits was found to be about 30 %, whereas that by tungstates varied from 40 to 77 % depending on the W loading of the sample. The size of the ZrO2 particles estimated by NO-TPD (2.3 nm) was found to be in excellent agreement with that determined by TEM. Remarkably, it was found that W interacted preferentially with the ZrO2 nanoparticles for a W surface density of 1.9 W/nm (half monolayer coverage of the ZrO2 nanoparticles by tungstates). Monolayer coverage of ZrO2 by tungstates was nearly complete when introducing a W loading twice as high as that theoretically needed. These insights into the surface coverage of ZrO2 by carbon or tungstates could not be obtained by any means other than NO-TPD, which makes it a unique method to provide advanced characterization of the surface of oxides supported on MWCNT, in particular, and carbon supports, to a broader general interest

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we anlayze and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis

Electron Transfer at the Metal Oxide/Electrolyte Interface: A Simple Methodology for Quantitative Kinetics Evaluation

International audienceWhile quantitative models exist for measuring rate constants for electron exchange at the surface of semiconductors to or from redox couples in solution, their use is very limited for studying transition metal oxide electrodes. Taking advantage of the possibility of tuning the doping level of WO 3 by thermal treatment, we measured the rate constants for electron injection from soluble probes in the so-called reverse mode. Hence, we could demonstrate that intermediate states located below the conduction band take part to the oxidation process and that the rate constant is bimolecular with a first order dependence on the concentration of redox probes and the concentration of intermediate states. A kinetics model was thereby developed to describe this reverse mode and a zone diagram established which can then be used to quickly assess rate constants for interfacial electron transfer on the surface of transition metal oxides semiconductors that are critical for applications such as solar energy conversion

Biomineralization in barnacle base plate in association with adhesive cement protein

Barnacles strongly attach to various underwater substrates by depositing and curing a proteinaceous cement that forms a permanent adhesive layer. The protein MrCP20 present within the calcareous base plate of the acorn barnacle Megabalanus rosa (M. rosa) was investigated for its role in regulating biomineralization and growth of the barnacle base plate, as well as the influence of the mineral on the protein structure and corresponding functional role. Calcium carbonate (CaCO3) growth on gold surfaces modified by 11-mercaptoundecanoic acid (MUA/Au) with or without the protein was followed using quartz crystal microbalance with dissipation monitoring (QCM-D), and the grown crystal polymorph was identified by Raman spectroscopy. It is found that MrCP20 either in solution or on the surface affects the kinetics of nucleation and growth of crystals and stabilizes the metastable vaterite polymorph of CaCO3. A comparative study of mass uptake calculated by applying the Sauerbrey equation to the QCM-D data and quantitative X-ray photoelectron spectroscopy determined that the final surface density of the crystals as well as the crystallization kinetics are influenced by MrCP20. In addition, polarization modulation infrared reflection-absorption spectroscopy of MrCP20 established that, during crystal growth, the content of β-sheet structures in MrCP20 increases, in line with the formation of amyloid-like fibrils. The results provide insights into the molecular mechanisms by which MrCP20 regulates the biomineralization of the barnacle base plate, while favoring fibril formation, which is advantageous for other functional roles such as adhesion and cohesion

Adsorption of functionalised thiols on gold surfaces: how to build a sensitive and selective sensor for a nitroaromatic compound?

Adsorption of thiols of varying structures bearing amine, acid or aromatic functionalities was characterised by Polarisation Modulation Infrared Absorption Spectroscopy (PM-IRRAS). The so functionalised surfaces were then submitted to atmospheres containing traces of 2,4-dinitrotrifluoromethoxybenzene and gas capture was evaluated by Quartz Crystal Microbalance (QCM). The selectivity of these new sensors was also evaluated thanks to exposures to solvents: ethanol, methylethylketone, toluene and dichloromethane. The QCM response appeared to be the best one, high and specific to the nitroaromatic, for aromatic amine functions