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16/09/16 - Shashank Mathur will defend his PhD works about "Growth and atomic-scale structure of novel 2D material based on silicon and oxygen" in room Remy Lemaire (K223), at Institut Néel, 2pm on the 16th of September

12/08/16 - Radiatively Limited Dephasing and Exciton Dynamics in MoSe2 Monolayers Revealed with Four-Wave Mixing Microscopy
By implementing four-wave mixing (FWM) microspectroscopy, we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe2. We reveal their dephasing times T2 and radiative lifetime T1 in a subpicosecond (ps) range, approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature of 6 K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At the nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse toward the dark states but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer intervalley exciton dynamics, revealing an initial polarization degree of around 30%, constant during the initial subpicosecond decay, followed by the depolarization on a picosecond time scale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing us to investigate the neutral exciton, the charged one, or both transitions at the same time. In the latter, we observe the exciton–trion beating in the coherence evolution indicating their coherent coupling. Nano Lett., ASAP, 2016

05/08/16 - Teasing out chirality in graphene
A chiral elementary particle has its spin pointing in either the same or the opposite direction as its momentum. In graphene, electrons have an analogous chirality, but observing it in electrical transport experiments is tricky. To do this, Wallbank et al. studied how electrons tunnel between two slightly misaligned graphene sheets separated by a layer of insulating hexagonal boron nitride. The chiral nature of the electrons imposed restrictions on the tunneling, which made it possible to discern the signatures of chirality in the data. Science 353, 575, 2016

29/07/16 - Equal variations of the Fermi level and work function in graphene at the nanoscale
If surface effects are neglected, any change of the Fermi level in a semiconductor is expected to result in an equal and opposite change of the work function. However, this is in general not observed in three-dimensional semiconductors, because of Fermi level pinning at the surface. By combining Kelvin probe force microscopy and scanning tunneling spectroscopy on single layer graphene, we measure both the local work function and the charge carrier density. The one-to-one equivalence of changes in the Fermi level and the work function is demonstrated to accurately hold in single layer graphene down to the nanometer scale. Nanoscale, ASAP, 2016

09/06/16 - A critical thickness condition for graphene and other 2D materials
A critical thickness analysis for dislocation formation in 2D lateral heterostructures is reported. The analysis applies to graphene, h-BN, and other 2D materials. We compare a fully atomistic approach to a continuum critical thickness theory, for the limiting cases of thin "films" on much thicker "substrates", and for cases in which the "film" and "substrate" are of comparable thickness -- also known as the compliant substrate case. By comparing atomistic and continuum formulations, we compute the effective dislocation core cutoff radii for graphene and h-BN, as well as the dislocation core energies. The work provides a comprehensive critical thickness analysis for 2D materials, and paves the way for defect-free growth of strained 2D lateral heterstructures. Phys. Rev. B 93, 214103, 2016

20/05/16 - Tuning valley polarization in a WSe2 monolayer with a tiny magnetic field
We create dark excitons in a chosen valley using circularly polarized nonresonant optical excitation. We probe the dynamics of the valley pseudospin relaxation of these dark excitons using polarization and time-resolved measurements of the emission from low-energy localized excitons, which we show to be a product of the dark exciton relaxation. We determine the pseudospin depolarization time of the dark excitons to be roughly 100 ps, which is significantly longer than that of bright excitons. This dark exciton depolarization channel can be completely switched off by a tiny magnetic field less than 100 mT in strength. Our findings reveal the hidden potential of dark excitons in the emerging field of opto-valleytronics. We anticipate that this potential will be exploited in future investigations of valley pseudospin in quantum information storage. Phys. Rev. X 6, 021024, 2016

10/05/16 - Rhombohedral Multilayer Graphene: A Magneto-Raman Scattering Study
Graphene layers are known to stack in two stable configurations, namely, ABA or ABC stacking, with drastically distinct electronic properties. Unlike the ABA stacking, little has been done to experimentally investigate the electronic properties of ABC graphene multilayers. Here, we report on the first magneto optical study of a large ABC domain in a graphene multilayer flake, with ABC sequences exceeding 17 graphene sheets. ABC-stacked multilayers can be fingerprinted with a characteristic electronic Raman scattering response, which persists even at room temperatures. Tracing the magnetic field evolution of the inter Landau level excitations from this domain gives strong evidence for the existence of a dispersionless electronic band near the Fermi level, characteristic of such stacking. Our findings present a simple yet powerful approach to probe ABC stacking in graphene multilayer flakes, where this highly degenerated band appears as an appealing candidate to host strongly correlated states. Science 16, 3710, 2016

26/04/16 - Unconventional magnetisation texture in graphene/cobalt hybrids
Magnetic domain structure and spin-dependent reflectivity measurements on cobalt thin films intercalated at the graphene/Ir(111) interface are investigated using spin-polarised low-energy electron microscopy. We find that graphene-covered cobalt films have surprising magnetic properties. Vectorial imaging of magnetic domains reveals an unusually gradual thickness-dependent spin reorientation transition, in which magnetisation rotates from out-of-the-film plane to the in-plane direction by less than 10° per cobalt monolayer. During this transition, cobalt films have a meandering spin texture, characterised by a complex, three-dimensional, wavy magnetisation pattern. In addition, spectroscopy measurements suggest that the electronic band structure of the unoccupied states is essentially spin-independent already a few electron-Volts above the vacuum level. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism. Sci. Rep. 6, 24783, 2016

02/02/16 - Impact of crystalline quality on neuronal affinity of pristine graphene
Due to its outstanding mechanical and electrical properties as well as chemical inertness, graphene has attracted a growing interest in the field of bioelectric interfacing. Herein, we investigate the suitability of pristine, i.e. without a cell adhesive coating, chemical vapor deposition (CVD) grown monolayer graphene to act as a platform for neuronal growth. We study the development of primary hippocampal neurons grown on bare graphene (transferred on glass coverslip) for up to 5 days and show that pristine graphene significantly improves the neurons adhesion and outgrowth at the early stage of culture (1–2 days in vitro). At the later development stage, neurons grown on coating free graphene (untreated with poly-L-lysine) show remarkably well developed neuritic architecture similar to those cultured on conventional poly-l-lysine coated glass coverslips. This exceptional possibility to bypass the adhesive coating allows a direct electrical contact of graphene to the cells and reveals its great potential for chronic medical implants and tissue engineering. Moreover, regarding the controversial results obtained on the neuronal affinity of pristine graphene and its ability to support neuronal growth without the need of polymer or protein coating, we found that the crystallinity of CVD grown graphene plays an important role in neuronal attachment, outgrowth and axonal specification. In particular, we show that the decreasing crystalline quality of graphene tunes the neuronal affinity from highly adhesive to fully repellent. Biomaterials 86, 33, 2016

07/12/15 - Anatomy and Giant Enhancement of the Perpendicular Magnetic Anisotropy of Cobalt–Graphene Heterostructures
We report strongly enhanced perpendicular magnetic anisotropy (PMA) of Co films by graphene coating from both first-principles and experiments. Our calculations show that graphene can dramatically boost the surface anisotropy of Co films up to twice the value of its pristine counterpart and can extend the out-of-plane effective anisotropy up to unprecedented thickness of 25 Å. These findings are supported by our experiments on graphene coating on Co films grown on Ir substrate. Furthermore, we report layer-resolved and orbital-hybridization-resolved anisotropy analysis, which help understanding of the physical mechanisms of PMA and more practically can help design structures with giant PMA. As an example, we propose superexchange stabilized Co–graphene heterostructures with a robust constant effective PMA and linearly increasing interfacial anisotropy as a function of film thickness. These findings point toward possibilities to engineer graphene/ferromagnetic metal heterostructures with giant magnetic anisotropy more than 20-times larger compared to conventional multilayers, which constitutes a hallmark for future graphene and traditional spintronic technologies. Nano Lett. 16, 145, 2016

19/10/15 - Chair of Excellence from LABEX Lanef granted to Prof. Athanasios Dimoulas (NCSR Demokritos)
Prof. Dimoulas will spend research stays in Grenoble between 2016 and 2018 working on "Two-dimensional transition metal dichalcogenide materials" (see examples of his research on this link) in close collaborations with colleagues from CEA-INAC and Institut Néel.

15/10/15 - Atomic-level birthmarks in 2D sand
S. Mathur and co-workers reveal the mechanism for defect generation in silica, the most common constituent of sand and a key ingredient in catalysts and microelectronic devices, formed with the ultimate thinness of about 0.3 nm. Unlike bulk silica which displays various crystallographic structures, this ultra-thin form adopts a characteristic honeycomb atomic lattice reminiscent of that of other ultimately thin materials such as graphene. This similarity qualifies two-dimensional silica as a building block in multifunctional stacks of distinctive two-dimensional materials. Based on atomic scale-imaging with unprecedented resolution and simulations, a sequence is proposed which describes the atomic processes during the growth of silica onto a metallic substrate. During the sequence, collective atomic movements imposed by symmetries occur with few-nanometer-sized silica domains moving like the pieces of a sliding puzzle. The boundaries that patch these pieces together are line defects, which are found to be unavoidable birthmarks for silica. Phys. Rev. B 92, 161410 (2015)

01/07/15 - Quality heterostructures from two-dimensional crystals unstable in air by their assembly in inert atmosphere
Many layered materials can be cleaved down to individual atomic planes, similar to graphene, but only a small minority of them are stable under ambient conditions. The rest react and decompose in air, which has severely hindered their investigation and potential applications. Here we introduce a remedial approach based on cleavage, transfer, alignment, and encapsulation of air-sensitive crystals, all inside a controlled inert atmosphere. To illustrate the technology, we choose two archetypal two-dimensional crystals that are of intense scientific interest but are unstable in air: black phosphorus and niobium diselenide. Our field-effect devices made from their monolayers are conductive and fully stable under ambient conditions, which is in contrast to the counterparts processed in air. NbSe2 remains superconducting down to the monolayer thickness. Starting with a trilayer, phosphorene devices reach sufficiently high mobilities to exhibit Landau quantization. The approach offers a venue to significantly expand the range of experimentally accessible two-dimensional crystals and their heterostructures. Nano Lett. 15, 4914, 2015

04/05/15 - Single photon emitters in exfoliated WSe2 structures
Crystal structure imperfections in solids often act as efficient carrier trapping centres, which, when suitably isolated, act as sources of single photon emission. The best known examples of such attractive imperfections are well-width or composition fluctuations in semiconductor heterostructures (resulting in the formation of quantum dots) and coloured centres in wide-bandgap materials such as diamond. In the recently investigated thin films of layered compounds, the crystal imperfections may logically be expected to appear at the edges of commonly investigated few-layer flakes of these materials exfoliated on alien substrates. Here, we report comprehensive optical micro-spectroscopy studies of thin layers of tungsten diselenide, a representative semiconducting dichalcogenide with a bandgap in the visible spectral range. At the edges of the flakes we discover centres that, at low temperatures, give rise to sharp emission lines. These narrow emission lines reveal the effect of photon antibunching, the unambiguous attribute of single photon emitters. The optical response of these emitters is inherently linked to the two-dimensional properties of the monolayer, as they both give rise to luminescence in the same energy range, have nearly identical excitation spectra and have very similar, characteristically large Zeeman effects. With advances in the structural control of edge imperfections, thin films of WSe2 may provide added functionalities that are relevant for the domain of quantum optoelectronics. Nat. Nanotech. 10, 503, 2015

Centre National de la Recherche Scientifique Commissariat à l'énergie atomique et aux énergies alternatives
Laboratoire d'alliances nanosciences-énergies du futur
Université Grenoble Alpes
Institut polytechnique de Grenoble