n-type diamond substrates are highly desired for diamond electronics and quantum technology based on NV centers. In this work, we investigate the high growth rates obtained using plasma with high-power densities in a commercial bell-jar chemical vapor deposition reactor equipped with a gas panel for diamond doping with phosphorus impurities. We evidence that the diamond growth rate is increased by adding phosphorus precursor in the gas mixture, up to 21.6 mu m/h. We found conditions for growing extremely thick (up 580 mu m) phosphorus doped (100) diamond homoepilayers keeping a n-type character as shown by optical spectroscopy. From such films we succeed in slicing a free-standing phosphorus-doped diamond plate, showing the feasibility of n-type diamond substrate fabrication for further use in optoelectronics and quantum devices.

We present a novel approach for synthesizing few-layered two-dimensional boron nitride (FL-BN) using low pressure chemical vapor deposition (LPCVD). In our approach, a radio frequency (RF) magnetic field was generated to inductively heat copper foils. The process involved placing the copper foil on a tungsten substrate and applying inductive heating until the copper melted within about two minutes. Subsequently, boron-nitrogen precursors were generated via the decomposition of ammonia borane at approximately 80 degrees C, and these species were then transported by an argon/hydrogen flow to the liquid Cu, promoting the nucleation and growth of hBN. Thermochemical calculations performed by using ChemkinPro software indicated the prevalence of B-N-H species within the temperature range of 20 degrees C to 1100 degrees C, surpassing the melting point of copper. The resulting hBN layers exhibited exceptional structural integrity, as confirmed by Raman and X-ray photoelectron spectroscopy (XPS) analyses. Furthermore, room-temperature cathodoluminescence (CL) measurements revealed a prominent excitonic peak at the photon energy position 5.77 eV (215 nm), which is a consequence of the deep UV optical gap of the h-BN crystal. Thus, the inductive heating process demonstrates significant potential for large-scale and rapid manufacturing of h-BN, offering enhanced energy efficiency. Additionally, we introduce a new conceptual design enabling precise control of the synthesis parameters, thus facilitating scalable production of high-quality h-BN films.

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons.

We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm^2/(Vs) at a carrier density of |n| = 10^12 cm^-2, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm^2/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm^2/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm^-1. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices.

We present a novel experimental protocol using Cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of 2D materials. The protocol is then applied at room and at cryogenic temperatures to four bulk hexagonal boron nitride crystals grown by different synthesis routes. The exciton diffusivity depends on the sample quality but not on the temperature, indicating it is limited by defect scattering even in the best quality crystals. The lower limit for the diffusivity by phonon scattering is 0.2 cm$^{2}$.s$^{-1}$. Diffusion lengths were as much as 570 nm. Finally, the surface recombination velocity exceeds 10$^{5}$ cm$^{2}$.s$^{-1}$, at a level similar to silicon or diamond. This result reveals that surface recombination could strongly limit light-emitting devices based on 2D materials.

We investigate the photon statistics of the light emitted by single self-assembled hybrid gold-CdSe/CdS/CdZnS colloidal nanocrystal supraparticles through the detailed analysis of the intensity autocorrelation functiong(2)(τ). We first reveal that, despite the large number of nanocrystals involved in the supraparticle emission, antibunching can be observed. We then present a model based on non-coherent Förster energy transfer and Auger recombination that well captures photon antibunching. Finally, we demonstrate that some supraparticles exhibit a bunching effect at short time scales corresponding to coherent collective emission.

The ultimate purity of synthetic diamond crystals is currently limited by traces of boron and nitrogen. Here we study diamond crystals grown at high-pressure high-temperature, which are made of 3D growth sectors with variable residual impurity contents. The boron concentration is found in the 0.5-6.4 ppb range thanks to continuous cathodoluminescence analysis. Time-resolved cathodoluminescence experiments complete the impurity analysis with measurements of free exciton lifetimes. From them, we deduced an estimate of the nitrogen concentration at the ppb level, from 0.6 to 30 ppb depending on the growth sectors. We identified n-type, p-type and highly compensated regions, which illustrates the potential of cathodoluminescence as a local characterization tool for qualifying diamond for electronic and quantum applications.

We report on the synthesis of hybrid light emitting particles with a diameter ranging between 100 and 500 nm, consisting in a compact semiconductor CdSe/CdS/CdZnS nanocrystal aggregate encapsulated by a controlled nanometric size silica and gold layers. We first characterize the Purcell decay rate enhancement corresponding to the addition of the gold nanoshell as a function of the particle size and find a good agreement with the predictions of numerical simulations. Then, we show that the contribution corresponding to Förster resonance energy transfer is inhibited.

Coupling an ensemble of fluorescent emitters with a plasmonic resonator opens opportunities to control their emission properties at the nanoscale. However, synthesis of such hybrid colloidal nanostructures is challenging. In this manuscript, we report the synthesis of compact assemblies of semiconductor quantum dots encapsulated into a gold nanoshell. We discuss how the synthesis parameters enable full control of the architecture of the nanostructure. Optical properties are discussed and compared to theoretical models. Coupling of the emitters to the gold layer is evidenced by an acceleration of the photoluminescence decay and results in an improvement of the photostability.

Optical properties of nanocrystals have the potential to drive the next generation of optoelectronic devices. However, a number of technological limitations remain to be overcome. Nanocrystals' emission is strongly impacted by the chemical environment and high excitation power. In this paper, we present a preparation route to hybrid core-shell nanoplatelets encapsulated in a gold silica shield. The induced plasmon coupling offers a higher brightness at high power excitation. We also detail the highly increased fluorescence stability offered by the silica-gold-shell protection against photobleaching processes.

Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect excitons in hBN is equal to 27 ns, which is much shorter than in other indirect bandgap semiconductors. This is explained by the close proximity of the electron and the hole in the exciton complex, and also by the small energy difference between indirect and direct excitons. The unusually high luminescence efficiency of hBN for an indirect bandgap is therefore semi-quantitatively understood.

Controlling the facet orientation and polarity of semiconductor nanostructures is a major issue for achieving devices requiring specific surface properties. Herein, new facets are created during the growth of ZnO nanowires (NWs) by metal‐organic chemical vapor deposition, as a consequence of the trimethylgallium flow introduced in the gas phase. This in situ Ga‐doping induces original shapes such as “Christmas tree”‐like and “Taipei tower”‐like nanostructures, developed along the C‐axis direction. In particular, lateral facets exhibit polar surfaces normal to this growth direction, which can be seen either as overhangs (for Christmas trees) or as terraces (for Taipei towers). In both cases, convergent beam electron diffraction reveals that those surfaces are O‐polar (000 1¯ ) planes, so that the two kinds of nano‐objects grow with opposite polarities, i.e., Zn‐polar (+C) for Christmas trees and O‐polar (−C) for Taipei towers. By confirming previously published theoretical calculations, this work provides an experimental evidence that Ga doping favors the growth of O‐polar ZnO facets instead of the nonpolar M‐surfaces usually developed for undoped ZnO NWs. Nano‐cathodoluminescence studies emphasize the intense near band edge emission of the nanostructures in the ultraviolet range, demonstrating their high optical quality.

An auxetic behaviour is evidenced in CoFe2O4 thin films grown by pulsed laser deposition on (100) MgO substrates under various O-2/N-2 pressures. This rare behaviour for an intrinsic material is observed for intermediate oxidation conditions, in between two non auxetic domains delimited by high (>0.05 mbar) and low (<0.03 mbar) O-2/N-2 deposition pressures. Combining X-ray resonant diffraction and Mossbauer spectroscopy, we experimentally prove that the auxetic behaviour is related to the presence of cobalt ions in the tetrahedral sites. The impact of the structural modifications caused by the various oxidation conditions on the electronic and magnetic properties are studied for the various oxidation domains. Variations as important as a transition from a p-type semiconducting to an insulating behaviour, or from an in-plane to an out-of-plane magnetization are observed when spanning from the lowest (0.01 mbar) to the highest (1 mbar) studied oxidation pressures. (C) 2020 Elsevier B.V. All rights reserved.

We first report the synthesis of supraparticles with a mean diameter of 130 nm consisting in a compact self-assembly of colloidal CdSe/CdS/ZnS nanocrystals encapsulated in a silica shell. This provides a system with robust optical properties such as a high quantum efficiency, a stable and Poissonian emission at room temperature. Additionally, enhancement of the photoluminescence decay rate through Forster resonance energy transfer is observed.

Controlling the facet orientation and polarity of semiconductor nanostructures is a major issue for achieving devices requiring specific surface properties. Herein, new facets are created during the growth of ZnO nanowires (NWs) by metal‐organic chemical vapor deposition, as a consequence of the trimethylgallium flow introduced in the gas phase. This in situ Ga‐doping induces original shapes such as “Christmas tree”‐like and “Taipei tower”‐like nanostructures, developed along the C ‐axis direction. In particular, lateral facets exhibit polar surfaces normal to this growth direction, which can be seen either as overhangs (for Christmas trees) or as terraces (for Taipei towers). In both cases, convergent beam electron diffraction reveals that those surfaces are O‐polar (000) planes, so that the two kinds of nano‐objects grow with opposite polarities, i.e., Zn‐polar (+ C ) for Christmas trees and O‐polar (− C ) for Taipei towers. By confirming previously published theoretical calculations, this work provides an experimental evidence that Ga doping favors the growth of O‐polar ZnO facets instead of the nonpolar M ‐surfaces usually developed for undoped ZnO NWs. Nano‐cathodoluminescence studies emphasize the intense near band edge emission of the nanostructures in the ultraviolet range, demonstrating their high optical quality.

s Poster Session P8 NANOPHOTONICS AND NANO-OPTICS Thematic Session: Nanophotonics & nano-optics

Strongly compensated Ga2O3 is shown to be an intrinsic (or native) p-type conductor with the largest bandgap for any reported p-type transparent semiconductor oxide which may shift the frontiers in fields such as power electronics and photonics.

Les cristaux moleculaires de perovskites hybrides de formule generale (R-NH 3) p PbX n (R groupement organique, X: halogene) representent une nouvelle classe de semiconducteurs qui a effectue une percee spectaculaire dans le monde du photovoltaique depuis 5 ans. En effet, le developpement des cellules solaires contenant les perovskites hybrides comme materiau absorbeur de lumiere a connu un developpement extremement rapide qui a permis d'atteindre des rendements records aussi eleves que 22,7% en 2017, rentrant en competition avec les records des cellules solaires a base de silicium. En plus des applications photovoltaiques, ces cristaux moleculaires se revelent tout a fait pertinents dans le domaine de l'emission de lumiere: diodes electroluminescentes et lasers. Ce succes fulgurant est du au fait que les perovskites hybrides possedent des proprietes optiques et electroniques remarquables, combinant les avantages des semiconducteurs organiques et inorganiques. Ces proprietes excitoniques peuvent etre ajustees en jouant sur la composition des molecules et la mise en forme des cristaux (couches minces, monocristaux). En particulier, nous etudierons par des experiences d'absorption, de photoluminescence et de micro-photoluminescence resolue en temps et en fonction de la temperature, les proprietes intrinseques des excitons dans des monocristaux de perovksites hybrides .

We investigate the influence of the temperature (between 4 K and room temperature) on the fluorescence of colloidal thick-shell CdSe/CdS nanocrystals coupled with a flat gold film. First, we deduce the complex dielectric function (SIC) from the spectrophotometric measurements on the flat gold film. Then, when compared to a nanocrystal deposited on a glass coverslip, calculations show that the enhancement of the photoluminescence decay rate is divided by a factor of 2.3 between 11 and 293 K due to the reduction of optical losses. Our analysis well accounts for the experiments performed on single CdSe/CdS nanocrystals.