Kapitza-resistance-like exciton dynamics in atomically flat MoSe2-WSe2 lateral heterojunction.

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe2-WSe2) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

A general and robust analytical method for interface normal determination in TEM.

This paper presents a new analytical method to determine interface normals from a series of bright/dark field images taken from arbitrary orientations. This approach, based on a general geometrical model of interface projection, provides a generalized formulation of existing methods. It can treat an excessive number of inputs, i.e. orientation conditions. Given 6 or more sets of inputs, even with considerable experimental errors, we prove that this method is still very likely to yield results with satisfactory accuracy. The robustness of the method can thus allow its implementation in problems dealing with a large amount of data. We show that this method can also be applied to determine 1D features or to check the planarity of microstructural features.

Prediction of Co nanoparticle morphologies stabilized by ligands: towards a kinetic model.

Cobalt nanoparticles (NPs) synthesized in liquid environments present anisotropic shaped nanocrystals such as disks, plates, rods, wires or cubes. Though the synthesis parameters (precursor, reducing agent, stabilizing ligands, concentration, temperature or rate of precursor injection) controlling the final morphologies are experimentally well controlled, little is known concerning the growth mechanisms at the atomic scale. In this work, we intend to predict the morphology variation of hcp cobalt NPs as a function of the ligand concentration. To this aim, we consider two well-established thermodynamic models and develop a new kinetic one. These models require the knowledge of the adsorption behaviors of stabilizing molecules as a function of surface coverage on preferential facets of NPs. To this end, density functional theory (DFT) calculations were performed on the adsorption of a model carboxylate ligand CH3COO on different Co crystalline surfaces. The shapes of the Co NPs obtained by these models are compared to experimental morphologies and other theoretical results from the literature. While thermodynamic models are in poor agreement with experimental observations, the variety of shapes predicted by the kinetic model is much more promising. Our study confirms that the morphological control of NPs is mostly driven by kinetic effects.

Vibrational and electronic excitations in gold nanocrystals.

An experimental analysis of all elementary excitations--phonons and electron-holes--in gold nanocrystals has been performed using plasmon resonance Raman scattering. Assemblies of monodisperse, single-crystalline gold nanoparticles, specific substrates and specific experimental configurations have been used. Three types of excitations are successively analyzed: collective quasi-acoustical vibrations of the particles (Lamb's modes), electron-hole excitations (creating the so-called "background" in surface-enhanced Raman scattering) and ensembles of atomic vibrations ("bulk" phonons). The experimental vibrational density of states extracted from the latter contribution is successfully compared with theoretical estimations performed using atomic simulations. The dominant role of surface atoms over the core ones on lattice dynamics is clearly demonstrated. Consequences on the thermodynamic properties of nanocrystals such as the decrease of the characteristic Debye temperature are also considered.

Nanoscale self-organization using standing surface acoustic waves.

The diffusion of an adatom on a substrate submitted to a standing surface acoustic wave is theoretically studied. By performing large scale molecular dynamic simulations, we show that the wave dynamically structures the substrate by encouraging the presence of the adatom in the vicinity of the maximum displacements of the substrate. Using an analytical model, we explain this feature introducing an effective potential induced by the wave. Applied in an atomic deposition experiment, this dynamic structuring process should govern the nucleation sites distribution opening the route to accurately control the self-organization process at the nanoscale.