In Situ Optical Monitoring of the Electrochemical Conversion of Dielectric Nanoparticles: From Multistep Charge Injection to Nanoparticle Motion.

By shortening solid-state diffusion times, the nanoscale size reduction of dielectric materials-such as ionic crystals-has fueled synthetic efforts toward their use as nanoparticles, NPs, in electrochemical storage and conversion cells. Meanwhile, there is a lack of strategies able to image the dynamics of such conversion, operando and at the single NP level. It is achieved here by optical microscopy for a model dielectric ionic nanocrystal, a silver halide NP. Rather than the classical core-shrinking mechanism often used to rationalize the complete electrochemical conversion and charge storage in NPs, an alternative mechanism is proposed here. Owing to its poor conductivity, the NP conversion proceeds to completion through the formation of multiple inclusions. The superlocalization of NP during such heterogeneous multiple-step conversion suggests the local release of ions, which propels the NP toward reacting sites enabling its full conversion.

Backside absorbing layer microscopy: a universal relationship between physical thickness and reflectivity.

The Backside Absorbing Layer Microscopy (BALM) is a recently introduced surface imaging technique in reflected light with an unprecedented combination of sensitivity and lateral resolution, hence very promising for the development of imaging sensors. This requires to turn BALM images into quantative analyte measurements. The usual way to analyze reflectivity is to compare the optical signal and a numerical model with many adjustable parameters. Here we demonstrate a universal relationship between the sample reflectivity and the physical thickness of the sample, ruled by three measurable quantities. Mapping the physical sample thickness becomes possible whatever the instrument setting and the sample refractive index. Application to kinetic measurements is discussed.

Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates.

The capability to observe 2D materials with optical microscopy techniques is of central importance in the development of the field and is a driving force for the assembly and study of 2D material van der Waals heterostructures. Such an observation of ultrathin materials usually benefits from antireflection conditions associated with the choice of a particular substrate geometry. The most common configuration uses a transparent oxide layer with a thickness minimizing light reflection at the air/substrate interface when light travels from air to the substrate. Backside Absorbing Layer Microscopy (BALM) is a newly proposed configuration in which light travels from glass to air (or another medium such as water or a solvent) and the antireflection layer is a light-absorbing material (typically a metal). We recently showed that this technique produces images of 2D materials with unprecedented contrast and can be ideally coupled to chemical and electrochemical experiments. Here, we show that contrast can be optimal using double-layer antireflection coatings. By following in situ and with sub-nm precision the controlled deposition of molecules, we notably establish precisely the ideal observation conditions for graphene oxide monolayers which represent one of the most challenging 2D material cases in terms of transparency and thickness. We also provide guidelines for the selection of antireflection coatings applicable to a large variety of nanomaterials. This work strengthens the potential of BALM as a generic, powerful and versatile technique for the study of molecular-scale materials and phenomena.

Combining Electrodeposition and Optical Microscopy for Probing Size-Dependent Single-Nanoparticle Electrochemistry.

Electrodeposition of nanoparticles (NPs) is a promising route for the preparation of highly electroactive nanostructured electrodes. By taking advantage of progressive electrodeposition, disordered arrays with a wide size distribution of Ag NPs are produced. Combined with surface-reaction monitoring by using highly sensitive backside absorbing-layer optical microscopy (BALM), such arrays offer a platform for screening size-dependent electrochemistry at the single NP level. In particular, this strategy allows rationalizing the electrodeposition dynamics at the single-NP level (>10 nm), up to the point of quantifying the presence of metal nanoclusters (<2 nm), and probing easier NP oxidation with size decrease, either through electrochemical or galvanic reactions.

The promise of antireflective gold electrodes for optically monitoring the electro-deposition of single silver nanoparticles.

The interest in nano-objects has recently dramatically increased in all fields of science, and electrochemistry is no exception. As a consequence, in situ and operando visualization of electrochemical processes is needed at the nanoscale. Herein, we propose a new interferometric microscopy based on an antireflective thin metal electrode layer. The technique is coupled to electrochemistry in a model example: the electro-deposition of Ag metallic nanoparticles (NPs). This challenges the current opto-electrochemical methods and even those relying on nano-impact detection. Indeed, the sensitivity allows the dynamic in situ visualization of the electrochemical growth and dissolution of individual Ag NPs, whose size was tracked dynamically down to 15 nm in diameter. The use of microelectrodes provides interesting quantitative analysis of the NPs, from optically resolved arrays of single NPs to condensed arrays of (unresolved) NPs. Particularly, the optical analysis of all the individual NPs allows the reconstruction of optical voltammograms similar to the electrochemical ones. Finally, the NP dissolution-redeposition is also investigated.

On the contribution of the chain ends to the surface tension of a polymer melt.

We propose a physical picture describing the mechanisms by which chain ends affect the surface tension of a mono-dispersed polymer melt with chain length N. The driving effect is the adsorption equilibrium of chain ends within a bulk slice adjoining the surface and acting as a confined end reservoir. The thickness of that limited space is a characteristic length of the melt. This picture conforms to a previous approach proposed years ago by de Gennes. However, the characteristic length [Formula: see text] that we consider is different from the one [Formula: see text] that he considered. Our choice is carefully argued. The resulting model correctly reflects the transition between the two N regimes reported in experimental studies, with the correct exponents. Stretching contributions are also considered, and appear small compared to the above-mentioned adsorption equilibrium effects. We think that the usefulness of the newly introduced characteristic length might exceed the specific problem addressed in the present paper. The equilibrium state of a lamellar diblock copolymer is briefly discussed for illustration.

Backside absorbing layer microscopy: Watching graphene chemistry.

The rapid rise of two-dimensional nanomaterials implies the development of new versatile, high-resolution visualization and placement techniques. For example, a single graphene layer becomes observable on Si/SiO2 substrates by reflected light under optical microscopy because of interference effects when the thickness of silicon oxide is optimized. However, differentiating monolayers from bilayers remains challenging, and advanced techniques, such as Raman mapping, atomic force microscopy (AFM), or scanning electron microscopy (SEM) are more suitable to observe graphene monolayers. The first two techniques are slow, and the third is operated in vacuum; hence, in all cases, real-time experiments including notably chemical modifications are not accessible. The development of optical microscopy techniques that combine the speed, large area, and high contrast of SEM with the topological information of AFM is therefore highly desirable. We introduce a new widefield optical microscopy technique based on the use of previously unknown antireflection and absorbing (ARA) layers that yield ultrahigh contrast reflection imaging of monolayers. The BALM (backside absorbing layer microscopy) technique can achieve the subnanometer-scale vertical resolution, large area, and real-time imaging. Moreover, the inverted optical microscope geometry allows its easy implementation and combination with other techniques. We notably demonstrate the potentiality of BALM by in operando imaging chemical modifications of graphene oxide. The technique can be applied to the deposition, observation, and modification of any nanometer-thick materials.

From permeation to pore nucleation in smectic stacks.

The last stage of the spreading of a stratified droplet in the odd wetting case is the evolution from a trilayer to a monolayer, that is, vanishing of the last bilayer in the stack. We studied it in the case of 8CB smectic liquid crystal on a hydrophilic surface. Receding of the last bilayer is accompanied by formation of pores in it, which appear in the outer part of it. From analysis of real-time experimental observations of this phenomenon, we demonstrate that the dislocation loops which border these pores are not located at the same height in the trilayer stack as the dislocation lines that border the bilayer. Also, careful analysis of our results using a recently developed theoretical approach of smectic liquid nanodrop spreading strongly suggests that pore nucleation is triggered by differences in chemical potential between adjacent layers, which contrasts with the classical scheme where it is attributed to lateral tension along the layers.

Mechanism of lipid nanodrop spreading in a case of asymmetric wetting.

Using the surface enhanced ellipsometric contrast microscopy, we follow the last stage of the spreading of egg phosphatidylcholine nanodroplets on a hydrophilic substrate in a humid atmosphere, focusing on the vanishing trilayer in terraced droplets reduced to coexisting monolayer and trilayer. We find that the line interface between them exhibits two coexisting states, one mobile and one fixed. From there, it is possible to elucidate the internal structure and the spreading mechanism of the stratified liquid in a case of asymmetric wetting, i.e., where the lipid film is made of an odd number of leaflets.

Optimal design for 100% absorption and maximum field enhancement in thin-film multilayers at resonances under total reflection.

Dielectric optical coatings are designed at resonances to reach total absorption, whatever the low value of the imaginary index. The corresponding field enhancement within the stack can be arbitrarily increased with the optimization procedure. Applications concern optical sensors and threshold lasers.

Quantitative spreading kinetics of a three molecular layer liquid patch.

The late stage kinetics of the spreading of a smectic nanodrop on a solid surface was investigated by direct and real time imaging of a three molecular layer patch using the SEEC microscopy. Experimental data do not conform to the only available theory, which covers only weakly stratified liquids. A new model is proposed, in remarkable agreement with experiments, in which the spreading mechanism appears to be a quasi-static process ruled by solid/liquid interactions, 2D Laplace pressure, and separate edge and surface permeation coefficients.

Real-time quantitative imaging of submolecular layers.

Using a recent optical contrast method, real-time and quantitative imaging of submolecular layers was performed with the help of a simple optical microscope. The measuring technique is exposed and documented by three examples. In particular, it allowed label-free detection of peptide-antibody binding interactions with 50 pg/mm2 sensitivity while keeping full optical lateral resolution.

Wide-field optical imaging of surface nanostructures.

We present a new technique that increases the sensitivity of incoherent light optical microscopy to a point where it becomes possible to directly visualize ultrathin films (approximately nanometers) and isolated nano-objects. The technique is based on the use of nonreflecting substrate surfaces for cross-polarized reflected light microscopy. These surfaces generate a contrast enhancement of about 2 orders of magnitude, extending the application field of wide-field optical microscopy toward the nanoworld. The efficiency of the method is proven experimentally on well-characterized samples. Wide-field imaging of a nonlabeled lambda-DNA molecule is also presented.