Goals

Overview

Vibrational properties of optoelectronics devices

Quantum circuits based on two dimensional materials

Thermal properties of 2D systems

Surface physics with 2D materials

Vibrational properties of optoelectronics devices

Optical phonons couple to electrons in most of the materials we investigate. Thus by studing optical phonons using spatially resolved Raman spectroscopy , we access their structure and electronic properties. Moreover, by combinig such a technique with photoluminescence / cathodoluminescence experiments, local probe measurements, and first principles calculations, we can address most of their physical properties . This analysis can be further enriched with Kelvin probe microscopy (coll. SyMMES) high magnetic field optical spectroscopy (col . LNCMI), or our-wave mixing spectroscopy ( coll. NPSC).

For example, we have addressed the prototypical dichalcogenide MoS2 (poorly understood due to strong disorder), developed encapsulation on strategies to eliminate this disorder and access the intrinsic excitonic properties, and thus understand better charge/energy transfers on others 2D materials.

We also study the optical gating of 1D and 2D materials using adsorbed chromophore molecules (coll. DCM) in order to  address charge transfer down to the single electron tunneling at low temperature, and energy transfers related to excitonic effects.

Finally, electron-phonon coupling in some of the 1d and 2D Materials has a strong influence on their electronic and excitonic properties. Thus, we investigate the mechanism which enhances the phonon response and specially the role of the excitons localization in 1D systems. By probing the interplay between phonons and Fermi electrons we will be able to detect weak light signals at low temperature. This approach is generalized to VDW stacks of 2D materials with direct bandgaps.

The role of electron-phonon coupling in Charge density waves TaS2 family is also a part of our activity on 2D Materials. Some outstanding questions related to the coexistence of such an order with superconductivity at low dimensionality are one of challenging issues that we want to address. To reach this goal we use electronic transport and Raman spectroscopy which show strong signatures of the transitions.

Quantum circuits based on two dimensional materials

2D materials are potential building-blocks for electron-based (graphene) and photon-based (transition metal dichalcogenides) quantum circuits.

In superconducting circuits, the integration of graphene into a Josephson junction brings electrical tunability. In the team, we develop quantum circuits based on ultra-high quality graphene Josephson junctions.

In quantum photonics, exploiting strong light-matter coupling is a route to non-classical photon states. We study, in collaboration with the NPSC team in Néel Institute, the potential of single-layer dichalcogenides, aiming at polariton quantum blockade effects, that critically depend on the unique Coulomb interaction between excitons in these materials. Our efforts focus on the fabrication of heterostructures based on dichalcogenides and on the investigation of the exciton-exciton interaction before addressing quantum blockade effects.

A monolayer transition metal dichalcogenides inserted in an optical microcavity

A graphene based Josephson junction can be inserted in a microwave cavity to allow gate tunability

For more information, contact Julien Renard (julien.renard@neel.cnrs.fr) or visit this website.

Thermal properties of 2D systems

Graphene exhibit an outstanding thermal transport properties. However, the mechanism of heat propagation is still lively discussed. Usually, the Fourier law is invoked to rationalize the observed properties, which assumes that the mean free path of heat carrier is much smaller that the sample size. Recent theoretical works show that this assumption is not correct in 2D materials.

We address the thermal properties of 2D membranes via versatile, non-invasive optical techniques, especially Raman spectroscopy in which intensity and energy of the Raman modes is temperature dependent. The originality of our approach is to use two lasers beams, one as a heater and the other one as a probe which allows us to spatially map the local temperature, doping and strain. The theory developed by our collaborator at IMPMC in Paris predicts signature pf ballistic transport at room temperature in Graphene. We are currently testing experimentally such prediction and work on phonon engineering using patterned 2D materials from C2N. collaborator.

Surface physics with 2D materials

Experimental techniques

Sample fabrication and nano-characterization

We devote significant efforts to the controlled elaboration of 2D materials, hybrid systems (with molecules, metallic clusters or thin films) based on these materials, and van der Waals heterostructures. For that purpose we use CVD and MBE UHV reactors, some equipped with in situ STM and electron diffraction apparatuses (in collaboration with the EpiCM technological group of the lab) deterministic micro-transfer setup (in air and inside a glove box, collaboration with Experimental Engineering and Automation groups), chemical functionalisation. Device fabrication based on these materials is done in the Nanofab cleanroom using state of the art lithography techniques.

Raman spectroscopy

The team shares two Raman micro-spectrometers with the Optics and Microscopy technological group. Two other setups are used for in situ Raman measurements, one coupled to a cryogenic electronic transport measurement stage down to 10 K, and the other  to a UHV cluster, which allows synthesizing 2D materials, and characterizing them with electron diffraction and scanning tunneling microscopy.

Cryogenics and electronics

The team used two 3He-4He dilution refrigerators (one inverted dilution and one dilution stick with 6 T dual axis vector magnet) dedicated to electron transport measurements from DC to microwave frequencies (~20 GHz). Another inverted dilution refrigerator with optical access is shared with the NOF team, and allow combined electronic/optical measurement in the sub-Kelvin regime. Besides, a cryogenic probe station under controlled environment is available for rapid characterizations.

Calculations

DFT calculations are performed using the VASP code on local, regional and national computers. Most are now performed on ADA, one of the IDRIS computers. Tight-binding calculations are performed on local computers using home-made codes.

Large facilities

We also make extensive use of unique large facilities outside the laboratory, especially surface science probes: low-energy electron microscopy/spectroscopy (ELETTRA, ALBA), angle-resolved photoemission spectroscopy (LPEM ESPCI, IJL, SOLEIL, ISA synchrotron facility), X-ray magnetic circular dichroism (SOLEIL, ELETTRA, ALBA), X-ray scattering (ESRF).

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