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Black phosphorus interlayer coupling in heterostructures with boron nitride for photonics

Periodic Reporting for period 1 - BrightPhoton (Black phosphorus interlayer coupling in heterostructures with boron nitride for photonics)

Reporting period: 2016-04-01 to 2018-03-31

Almost after 30 years after the discovery of nanotubes and 20 years after the presentation of graphene, the diversity of the 1D and 2D nanomaterials to stack and assembly become important.
In this wide horizon, the project BrightPhoton focuses on the association of dielectric or wide band gap semiconducting structures (5-7 eV) with optically or electronically active semiconductor with bandgap energies in the range of 0.5-3 eV. More precisely, the studies realized during the project investigated the stability and the dielectric properties of 2D black phosphorus (BP) thin layer as well as 1D chains of organic dyes. These materials, very different in nature at first glance, have common denominators that represent a huge bottle necks for their fundamental study as well as their diffusion in applicative fields. Indeed, their outstanding properties such as the strong light-matter interactions and or high mobility of the carriers through a direct bandgap, are hugely modulated by the quantum confinement effects, dielectric screening, and worse, disturbed or annihilated by photoinduced processes in ambient conditions. Photooxidation of Black phosphorus in air and the well known photobleaching for the organic dyes to just name a few. In this context the concept of heterostructure is apply to these two types of materials, to control and stabilize their properties.

One of the main achievements, performed in the BrightPhoton project framework, is the first measurement of the layer dependent band gap energy of exfoliated BP in free standing conditions for the monolayer, bilayer and trilayer. A second study, performed in collaboration with the Thales UMPhy group, shows that Al203 ultrathin dielectric capping layer, as thin as 1 nm, acts as efficient passivation layer to protect BP from oxidation in ambient condition.
In a same way we capitalize on the high crystallinity of Boron nitride nanotubes wall to encapsulated and stabilize the optical properties 1D chain of dyes. The resulting nanohybrids (Dyes@BNNT) display high resistance to harsh environment under photoexcitation, with an enhanced emission lifetime of the encapsulated dye by a factor 10^4. This stability over days, represents a paradigm shift in the use of organic dyes and semiconductors, that we apply to create a new library of nanoprobes for bio-imaging with multimodal capability, in collaboration with Pr. R. Martel group at the University of Montreal.
Dedicated setups and sample preparation.

The first 4-6 months of the project were focused on the design and installation of a dedicated glovebox with controlled atmosphere (<1ppm of O2 and H2O) to fabricate BP sample in safe conditions. The glovebox was thus equipped with an atomic force microscope (AMF), exfoliation and 2D materials transfer set up coupled with an optical microscope. Additionally, a custom sample sealing system have been implemented to transfer the sample from the glove box to the other apparatus set up such as our TEM, the passivation chamber (Thalès group) or the hyperspectral imager at the University of Montreal.

The next years were dedicated on 3 studies, selected by the PI as a function of their fundamental results potential and timeliness regarding the other internationnal competitive groups,

1-Results on BP passivation:

In order to effectively protect the thinnest flakes, we confirm that 1 nm of Al2O3 efficiently passivates exfoliated black phosphorus (below 5 layers) on Si/SiO2 substrates. Such an ultrathin and transparent passivation layer can act as a tunnel barrier allowing for black phosphorus devices processing without passivation layer removal.

2-Results on BP dielectric response

During the project, we probed the dielectric response of 1 to 3 layers and bulk exfoliated BP layers suspended in vacuum using Valence Energy Electron Loss Spectroscopy (V-EELS) in a TEM operated at 80 and 40 kV The results obtained indicates a significant shift of the band gap to energy higher than previously observed in other environment such as substrates or solvents. By capitalizing on an in-column energy filter of our TEM apparatus at LEM, we also analysed the energy dispersion of the volume plasmon in 1-3 BP layer.

3-Results on Dyes@BNNT optical response and photo-stabilité.

Using encapsulation and 1D confinement processes in liquid, we show that organic dyes placed inside small diameter boron nitride nanotubes (BNNT) (dinner < 3 nm) are resistant to bleaching and provide stable luminescence properties for imaging applications. By capitalizing on the wide bandgap of BNNTs, which is around 5.5 eV, our experiments on various molecules encapsulated inside a boron nitride nanotube show drastically improved photo-stability even under high light intensity with a enhanced emission lifetime by a factor 0.9x104. Finally we demonstrate that the hybrids Dyes@BNNT can be use as multimodal nanoprobes for bio-imaging in living organisms with reduced toxicity, from the VIS to NIR I optical ranges.
Progress beyond the state of the art:

Some highlights:
- The 1nm passivation layer developed for BP, in collaboration with Thalès group, is the thinnest layer at full wafer scale developed up to know. A thickness mandatory to enable carrier injections through the passivation layer, as well as efficient transparency of optoelectronic devices.
- First measurement of the band gap energy of the black phosphorus monolayer and bilayer in free standing conditions.
- The works on1D hybrids presents the first confinement of dyes inside BNNT having this dimeter distribution (1-3 nm). The stabilization of the 1D aggregation state and of the optical properties of Dyes represent a new way to revisit the fundamental properties of molecular aggregate and open original perspectives in bio-imaging applications.

International visibility of the project:

The PI have been invited in international conferences to present the results obtained during the BrightPhoton

Fading-Free Fluorescent Imaging Using Dyes Nano-Hybrids
Wonton’18, Hakone, Japan, 8-12 of July 2018

Optical properties of dyes confined into carbon and boron nitride nanotubes for multimodal bio-imaging
GDRinternationnal conference, Sète, France, Oct 2018

Optical properties of dyes confined into carbon and boron nitride nanotubes
Wonphys 2017, Varadero, Cuba, 26-29 of Sept 2017

Probing the dielectric response of exfoliated black phosphorus in free standing conditions2028
233rd Electro-Chemical Society Meeting, New-Orleans, 28-31 of May 2017

1D and 2D hybrid nanomaterials for nano-optics
GDRinternationnal conference, Oleron island, France, Oct 2016

Spectroscopy on 1D and 2D heterostructures
NPO conference, Sainaa lake, Finland Aug 2016

Reinforcement and sustainability of the PI research activity in the French and EU framework:

There no doubt that the entire liberty of action coupled to the powerful scientific conditions provide during the MSCA action to the PI during 2 years have been a direct impact on its career. Indeed, the PI has just got a permanent position at CNRS, with a research thematic directly inspired on the research conducted in BrightPhoton project.
Moreover, the collaborative consortium developed during the action, especially with Thalès group obtained funding (700kE) to study black phosphorus from the French National Agency for Research (ANR), in the continuity of the project BrightPhoton.
"Combining ""wide and small"" band gap materials (5-7 eV vs 0.5-2eV)"