Surface physics

- Molecular 2D materials

We seek for collective electronic states in 2D materials assembled from molecular building blocks. The diversity of the possible building blocks opens the way to rich electronic properties, with sizeable spin-orbit interaction via the presence of metallic centres. To observe these electronic states we currently develop various strategies leading to 2D polymerisation reactions. This includes reactions performed on ultra-clean crystalline surface, under ultra-high vacuum, in solutions, or at the interface between immiscible liquid phases. In close collaboration with our colleagues in Institut FEMTO-ST (Besançon), we have developed 2D organometallic polymers, formed by so-called (arene)ruthenium-sulfur chemistry remaining soluble in solution owing to ambivalent physicochemical properties [1], all-carbon conjugated polymers on gold surfaces by means of a novel UHV-implemented condensation reaction [2] and are currently working on a few other kinds of reactions. We explore the structural and vibrational properties of these objects, which poses important challenges for microscopies and spectroscopies.

 

[1] J. Coraux et al., Chem. Eur. J. 21, 10969 (2017) -> doi.org/10.1002/chem.201700054
[2] J. Landers et al., 2D Mater. 1, 034005 (2014) doi.org/10.1088/2053-1583/1/3/034005

 

Assembling a 2D polymer from individual building blocks

 

Contact : johann.coraux@neel.cnrs.fr


- Physics of order in 2D lattices

When deposited on a substrate, 2D lattices host a variety of structural phases, with sometimes non-intuitive phase transitions in between them. This richness is inherent to the competition between two length-scales in the system (the lattices parameter of the 2D lattice and that of the substrate), which gives rise to frustration phenomena. We are exploring both highly cohesive 2D lattices, in the form of 2D materials such as graphene, ultra-thin oxides, and transition metal dichalcogenides, but also weakly cohesive layers, such as alkali metal monolayers on metals. We have established classifications of the different structural phases of 2D lattices on substrates, proposing an extended Wood’s notation [1,2], and devote strong efforts to understand the nature and formation of defects in 2D materials, some of which are or topological nature [3]. Recently we have discovered long-predicted unconventional structural phases and focus on structural phase transitions. This work is performed in collaboration with Pascal Pochet and Claude Chapelier from CEA-INAC and with Harley Johnson from Urbana Champaign University.

 

[1] A. Artaud et al., Sci. Rep. 6, 25670 (2016) -> doi.org/10.1038/srep25670
[2] P. Pochet et al., Appl. Mater. Today 9, 240-250 (2017) -> doi.org/10.1016/j.apmt.2017.07.007
[3] S. Mathur et al., Phys. Rev. B 92, 161410(R) (2015) -> doi.org/10.1103/PhysRevB.92.161410

 

6 nm x 4 nm 3D STM image of a single layer silicon oxide on Ru(0001),
with a line of topological defects

 

Contact : johann.coraux@neel.cnrs.fr


- Membrane properties of 2D materials

Graphene has been the first highly cohesive, atomically-thin membrane with an ordered lattice. As such it is prone to unique mechanical and thermal properties. It for instance exhibits mechanical instabilities, for instance a tendency to scrolling and when deposited on a substrate, a buckling in the form of wrinkles. A substrate also favours the formation of periodic nanoripples in graphene. We investigate these mechanical properties, which often relate them to the deformations of the atomic lattice [1,2,3]. This work is a collaborative work with several groups in the lab, colleagues at CEA-INAC, ESPCI Paris, and ELETTRA.

 

[1] M. S. Bronsgeest et al., Nano Lett. 15, 5098-5104 (2015) -> doi.org/10.1021/acs.nanolett.5b01246
[2] F. Jean et al., Phys. Rev. B 91, 245424 (2015) -> doi.org/10.1103/PhysRevB.91.245424
[3] S. Vlaic et al., J. Phys. Chem. Lett., 9, 2523−2531 (2018) -> doi.org/ 10.1021/acs.jpclett.8b00586

 

Graphene forming a wrinkle

 

Contact : johann.coraux@neel.cnrs.fr ; nedjma.bendiab@neel.cnrs.fr


- Epitaxial hybrid systems

Graphene readily grows on metal surfaces, and this is a straightforward route to epitaxial hybrid systems, in which the two materials in contact (the metal and graphene) are characterised by very different chemical bonds. In addition, the choice of the metal substrate may allow to induce properties in graphene, and this is the route we have chosen to induce superconductivity in graphene, on a rhenium substrate [1]. More recently we are building more advanced systems that will include intercalated layers allowing to tune the graphene-metal interaction, and decoration with foreign species onto graphene to create original electronic states. This work is performed in close collaboration with Claude Chapelier from CEA-INAC. We also address the opposite case, when graphene is the active material that influences the properties of the material put in contact with it. Using intercalated ferromagnetic layers we explore the way graphene strongly alters magnetic energy anisotropy. To this respect graphene’s effect is remarkably strong, pushing magnetisation perpendicular to the surface for unusually “thick” films (a few nanometers) [2,3] and inducing non conventional spin textures [4]. This work is performed in close collaboration with Nicolas Rougemaille from the MNM group.

 

[1] C. Tonnoir et al., Phys. Rev. Lett. 111, 246805 (2013) -> doi.org/ 10.1103/PhysRevLett.111.246805
[2] N. Rougemaille et al., Appl. Phys. Lett. 101, 142403 (2012) doi.org/10.1063/1.4749818
[3] H. Yang et al., Nano Lett. 16, 145-151 (2016) -> doi.org/10.1021/acs.nanolett.5b03392
[4] A. D. Vu et al., Sci. Rep. 6, 24783 (2016) -> doi.org/10.1038/srep24783

 

Magnetic domain structure of 8 atomic layers of cobalt sandwiched
between graphene and Ir(111), as seen with spin-polarised low-energy
electron microscopy (field of view is 20 µm)

 

Contact : johann.coraux@neel.cnrs.fr

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