The properties of two‐dimensional (2D) material stacks critically depend on the number of monolayers (m) in the stack. It is therefore important to quantify this number, which is a local quantity since 2D stacks are essentially heterogeneous. Optical interferential techniques based on contrast‐enhancing surfaces may be sensitive enough to visualize m variations but the experimental determination of m requires heavy and unstable comparisons with multiparameter numerical models. Focusing on the recent backside absorbing layer microscopy, the most sensitive to date among interferential techniques, a self‐calibrating method is demonstrated allowing instantaneous monolayer counting all over the sample surface which does not require the knowledge of the instrumental parameters, the sample or ambient refractive indices or the detailed structure of the contrast‐enhancing layer. This method is introduced step by step using examples of hexagonal boron nitride (hBN) stacks with increasing complexity. Exact monolayer counting up to 36 hBN monolayers is obtained using basic image analysis.

Tethered bilayer lipid membranes (tBLMs) are artificial membranes largely used for the in situ study of biological membranes and membrane-associated proteins. To date, the formation of these membranes was essentially monitored by surface averaging techniques like surface plasmon resonance (SPR) and quartz crystal microbalance with dissipation monitoring (QCM-D), which cannot provide both local and real-time information in a single approach. Here, we report an original application of backside absorbing layer microscopy (BALM), a novel white-light wide-field optical microscopy, to study tBLMs. Thanks to the combination of sensitivity and resolution, BALM not only allowed the real-time quantitative monitoring of tBLM formation but also enabled the high-resolution visualization of local fluxes and matter exchanges taking place at each step of the process. Quantitative BALM measurements of the final layer thickness, reproduced in parallel with SPR, were consistent with the achievement of a continuous lipid bilayer. This finding was confirmed by BALM imaging, which additionally revealed the heterogeneity of the bilayer during its formation. While established real-time techniques, like SPR or QCM-D, view the surface as homogeneous, BALM showed the presence of surface patterns appearing in the first step of the tBLM formation process and governing subsequent matter adsorption or desorption steps. Finally, matter fluxes persisting even after rinsing at the end of the tBLM formation demonstrated the lasting presence of dispersed vesicular pockets with laterally fluctuating positions over the final single and continuous lipid bilayer. These new mechanistic insights into the tBLM formation process demonstrate the great potential of BALM in the study of complex biological systems.

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.

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.

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.

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.

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.

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 \(aN^{1/3}\) that we consider is different from the one \(aN^{1/2}\) 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.

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.

Two-dimensional nanomaterials and their association into 2-D heterostructures have been extensively studied for the last couple of years. Thus, the development of new, versatile, high-resolution visualization and placement techniques is highly desirable. It is well known that a single layer of graphene can be observed under optical microscopy on Si/SiO2 substrates because of interference effects when the thickness of silicon oxide is optimized.1,2However, differentiating monolayers from bilayers remains a challenge and advanced techniques like Raman mapping, Atomic Force or Scanning Electron Microscopy (AFM, SEM) are more suitable to observe monolayers of graphene. Raman mapping and AFM are relatively slow and SEM is operated in vacuum so that in all cases real-time experiments including chemical modifications are not accessible. Therefore, the development of techniques that combine the large scale and speed of SEM, the topological information of AFM and the simplicity optical microscopy may greatly facilitate the study of nanomaterials. In our group, we are interested in particular forms of graphene derivatives which are graphene oxide (GO) and reduced graphene oxide (r-GO). While a single layer of graphene absorbs 2.3% of the incident light,3 graphene oxide monolayer exhibits a weaker light absorption that makes it almost impossible to distinguish on Si/SiO2 surface. In 2007, Ruoff and co-workers were able to observe directly GO flakes using confocal microscopy; they even obtained higher contrast for GO than for graphene on Si/SiO2. For that they optimized a substrate by depositing silicon nitride layers from 60 to 70 nm on silicon.4 Here we introduce a novel optical microscopy technique based on the use of Anti-Reflecting and Absorbing (ARA) layers yielding to ultra-high contrast reflection imaging of graphene monolayers in air or in water. We name this technique “Backside Absorbing Layer Microscopy” (BALM) and we illustrate its efficiency by in-situ imaging graphene oxide (see Figure) and its chemical modification. Reference: (1) Blake, P.; Hill, E. W.; Castro Neto, A. H.; Novoselov, K. S.; Jiang, D.; Yang, R.; Booth, T. J.; Geim, A. K. Appl. Phys. Lett. 2007, 91, 063124. (2) Roddaro, S.; Pingue, P.; Piazza, V.; Pellegrini, V.; Beltram, F. Nano Lett. 2007, 7, 2707-2710. (3) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Science 2008, 320, 1308. (4) Jung, I.; Pelton, M.; Piner, R.; Dikin, D. A.; Stankovich, S.; Watchrotone, S.; Hausner, M.; Ruoff, R. S. Nano Lett. 2007, 7, 3569-3575. Figure 1

The single Anti-Reflecting (AR) layer is a classical problem in optics. When all materials are pure dielectrics, the solution is the so-called lambda/4 layer. Here we examine the case of absorbing layers between non absorbing media. We find a solution for any layer absorption coefficient provided that the light goes from the higher towards the lower index medium, which characterizes backside layers. We describe these AR absorbing (ARA) layers through generalized index and thickness conditions. They are most often ultrathin, and have important applications for high contrast imaging in fluid cells.

Anti-reflecting layers deposited on flat surfaces make molecular films visible in reflecting light microscopy. For centuries, single Anti-Reflecting layers have been implicitly associated with dielectric materials. We recently demonstrated that anti-reflecting layers could be achieved out of absorbing materials such as metals as well, but only when used as backside layers where illumination and detection are performed through a supporting window. Fortunately, this corresponds to the best geometry when envisaging biophotonic or electrochemical applications at the solid/liquid interface. Here we explain how single absorbing anti-reflecting layers can serve each of these applications, and both simultaneously.

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.

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.

Amphiphilic molecules such as phospholipids, smectic liquid crystals or diblock copolymers are chemical dipoles which, under appropriate conditions, spontaneously line up side by side in a ferro-like arrangement to form stable monolayers. In three dimensions, these monolayers stack in a head-to-tail or antiferro-like arrangement, which favors monopolar contact between similar species. It results in a symmetric bilayered lamellar material. Here we show that dipolar chemical interactions can be extended in the direction normal to the lamellae by the use of triphilic rather than amphiphilic molecules, so that stable self-assembled smectic stacks may be designed with a fully polar, hence noncentrosymmetric, ordering. We describe in details a generic model of such materials. It is made of a mixture of three triblock copolymers aBc, bCa, and cAb with their end blocks twice sorter than the corresponding middle block. Because each constitutive molecule extends over three different chemical layers, all pairs of adjacent layers are linked and oriented via dipolar interactions. As a consequence, a polar structure with remarkable thermodynamic and mechanical stabilities is expected. We name these materials "ferrochemicals" because any given triblock molecule of the blend is oriented the same way through the whole sample due to chemical dipole interactions with all neighboring molecules. In the polymer case, the polar lamellar stacks can also be used as organic matrices hosting and orienting inorganic Janus particles in order to get a large variety of hybrid polar materials with interesting nonlinear optical, ferromagnetic, or ferroelectric properties.

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.