A theoretical treatment of deeply supercooled liquids is difficult because their properties emerge from spatial inhomogeneities that are self-induced, transient, and nanoscopic. I use computer simulations to analyse self-induced static and dynamic heterogeneity in equilibrium systems approaching the experimental glass transition. I characterise the broad sample-to-sample fluctuations of salient dynamic and thermodynamic properties in elementary mesoscopic systems. Findings regarding local lifetimes and distributions of dynamic heterogeneity are in excellent agreement with recent single molecule studies. Surprisingly broad thermodynamic fluctuations are also found, which correlate well with dynamics fluctuations, thus providing a local test of the thermodynamic origin of slow dynamics.

Today, most of electronic devices are based on heterostructures made by molecular beam epitaxy. The impact of van der Waals heterostructures made of 2D materials, already significant at the academic level [1], could generate a technological breakthrough in this field of electronics. Indeed, due to the weak van der Waals interlayer interactions, several materials can be assembled with a precise control of their thicknesses or their twist angles and with high quality interfaces.Recently, hybrid van der waals heterostructures, obtained by combining 2D materials and self-organized organic layers has been developed [2]. Such approach is promising to push further the versatility in terms of materials and thus a wide range of properties are expected.In this poster, we present recent results obtained on field effect transistor based on an hybrid van der Waals heterostructure made of molybdenum disulfide (MoS2) for the inorganic part and quinoidal zwitterions (QZ) for the organic part. The organic layer deposition is achieved under high vacuum, on top of the MoS2 channel of the transistor and the evolution of the transfer characteristic of the transistor is measured in situ.We observed a n-type doping and an increase of the charge carrier density of two orders of magnitudes which is comparable to the doping obtained with optimized charge transfer molecules [3][4]. These results which demonstrate the high compatibility of the QZ with MoS2 open the way for future investigations on functionalized hybrid QZ-2D materials heterostructures[1] A. K. Geim & I. V. Grigorieva Nature 499, 419–425 (2013)[2] Gobbi, M., Orgiu, E., Samorì, P., Adv. Mater. 2018, 30, 1706103.[3] Kiriya, D., Tosun, M., Zhao, P., Kang, J. S., & Javey, A. (2014). Air-Stable Surface Charge TransferDoping of MoS2 by Benzyl Viologen. Journal of the American Chemical Society, 136(22)[4] Dey, S., Matte, H. S. S. R., Shirodkar, S. N., Waghmare, U. V., & Rao, C. N. R. (2013). Charge-Transfer Interaction between Few-Layer MoS2and Tetrathiafulvalene. Chemistry - An Asian Journal,8(8)

Dispersionless energy bands in k space are a peculiar property gathering increasing attention for the emergence of novel electronic, magnetic, and photonic properties. Here, we explore the impact of electronic flat bands on the light-matter interaction. The van der Waals interaction between the atomic layers of hexagonal boron nitride induces flat bands along specific lines of the Brillouin zone. The macroscopic degeneracy along these lines leads to van Hove singularities with divergent joint density of states, resulting in outstanding optical properties of the excitonic states. For the direct exciton, we report a giant oscillator strength with a longitudinal-transverse splitting of 420 meV, a record value, confirmed by our ab initio calculations. For the fundamental indirect exciton, flat bands result in phonon-assisted processes of exceptional efficiency, that compete with direct absorption in reflectivity, and that make the internal quantum efficiency close to values typical of direct band gap semiconductors.

We report on the response of semi-dilute shear-banding wormlike micelles to time-dependent flow protocols such as step stress and shear startup. We use global rheology coupled to direct optical visualisations in Taylor-Couette flow to provide a detailed 2D description of the spatio-temporal dynamics of the shear banding flow under imposed stress. We compare these dynamics to their extensively studied counterpart under imposed shear rate. We show that, for a given asymptotic state, the onset of banding is delayed and the transients are longer when the stress is used as control parameter. The way the banding structure is built up strongly depends on the control parameter. In the two cases, secondary flows develop on top of the shear-banding instability: after shear startup, the interface between bands is formed in the bulk of the system and migrates inwards to its stationary position in the gap of the Taylor-Couette device, before being destabilized due to elastic instability; under creep flow, the interface between bands forms at the inner wall and becomes wavy while it is still migrating outwards to its steady position, leading to permanent rearrangements of the flow structure until steady state is reached, potentially explaining longer transients. We also outline the possible mechanical signature of the onset of shear banding in terms of shape of the bulk mechanical signals, which is the subject of the second part of this work [Briole et al., submitted].

The recent measurement of a very low dielectric constant, $\epsilon$, of water confined in nanometric slit pores leads us to reconsider the physical basis of ion partitioning into nanopores. For confined ions in chemical equilibrium with a bulk of dielectric constant $\epsilon_b>\epsilon$, three physical mechanisms, at the origin of ion exclusion in nanopores, are expected to be modified due to this dielectric mismatch: dielectric exclusion at the water-pore interface (with membrane dielectric constant, $\epsilon_m<\epsilon$), the solvation energy related to the difference in Debye-H\"uckel screening parameters in the pore, $\kappa$, and in the bulk $\kappa_b$, and the classical Born solvation self-energy proportional to $\epsilon^{-1}-\epsilon_b^{-1}$. Our goal is to clarify the interplay between these three mechanisms and investigate the role played by the Born contribution in ionic liquid-vapor (LV) phase separation in confined geometries. We first compute analytically the potential of mean force (PMF) of an ion of radius $R_i$ located at the center of a nanometric spherical pore of radius $R$. Computing the variational grand potential for a solution of confined ions, we then deduce the partition coefficients of ions in the pore. Phase diagrams of the LV transition are established for various parameter values and we show that a signature of this phase transition can be detected by monitoring the total osmotic pressure. For charged nanopores, these exclusion effects compete with the electrostatic attraction that imposes the entry of counterions into the pore to enforce electro-neutrality. This study will therefore help in deciphering the respective roles of the Born self-energy and dielectric mismatch in experiments and simulations of ionic transport through nanopores.

We present a theoretical study of near-field radiative thermal rectification combining phase-transition and high-infrared-transmittance materials. The phase-transition material vanadium dioxide (VO2), with a metal–insulator transition near 341 K, is utilized under a reasonable temperature. Four types of high-infrared-transmittance materials, including potassium bromide, sodium chloride, polyethylene, andmagnesium fluoride, are introduced as thin film substrates under a VO2 grating on one side of the near-field rectifier. We explore the effects of various high-infrared-transmittance thin-film substrates and relevant geometric parameters on the thermal rectification of the device. The results show that thermal rectification can be greatly enhanced by using a one-dimensional VO2 grating backed with a high-infraredtransmittance thin-film substrate. With the introduction of a high-infrared-transmittance substrate, the rectification ratio is dramatically boosted due to the enhancement of the substrate transmittance. This work predicts a remarkable rectification ratio as high as 161—greater than the recently reported peak values for comparable near-field radiative thermal rectification. The results outlined herein will shed light on the rapidly expanding fields of nanoscale thermal harvesting, conversion, and management.

Complex systems often exhibit shear banding-the coexistence of two different states characterized by their internal structuring and local shear rates. For some of them, the heterogeneous flow corresponds to the final steady state response while for others, shear banding can only be transient, the banding structure healing back to homogeneous flow in the ultimate steady state after long-lived periods. In order to explain the diversity of observations, Moorcroft and Fielding have established general criteria for the onset of banding in time-dependent flows of complex systems, ranging from polymeric fluids to soft glassy materials [Moorcroft et al., Phys. Rev. Lett., 2013, 110, 086001]. The proposed criteria are based on the time evolution of the bulk rheological response function of the system to a given time-dependent flow protocol and are associated with a specific signature in the mechanical response. In this contribution, we test the validity of these criteria in the case of two common time-dependent flow protocols: a step stress and a shear startup. Two types of fluids are examined. On the one hand, a wormlike micelles system exhibiting steady shear banding is studied experimentally, using rheometry coupled with direct visualisations and particle image velocimetry. On the other hand, we analyse previous literature on yield stress fluids exhibiting transient shear banding. Under creep flow, for both types of fluids the onset of banding arises in a time window compatible with the Moorcroft-Fielding criterion. However, the mechanical signature, i.e. the shape of the bulk mechanical signal as a function of time is not the one expected within some of the specific models with which the general Moorcroft-Fielding criteria were tested numerically. Under shear startup, both types of fluids behave differently. The criterion holds for yield stress fluids, the onset of banding arising just after the stress overshoot, as expected. On the contrary, for wormlike micelles the window of instability is delayed, even if the overshoot clearly plays a crucial role in the nucleation of the shear-induced structures. Regardless of the flow protocol or the system, wall slip seems to go hand in hand with banding indicating that it is a key ingredient to take into account.