Ref HAL: hal-03782459_v1
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Hybrid van der Waals heterostructures made of 2D materials and organic molecules exploit the high sensitivity of 2D materials to all interfacial modifications and the inherent versatility of the organic compounds [1-2]. In this study, we are interested in the quinoïdal zwitterion/MoS2 hybrid system in which organic crystals are grown by epitaxy on the MoS2 surface and can reorganize in other forms after thermal annealing [3]. By means of field effect transistor measurements and atomic force microscopy, we demonstrate that the charge transfer between organic molecules and 2D materials strongly depends on the conformation of the molecular film. This work shows the great sensibility of MoS2 transistors for sensing molecular events occurring at the nanoscale providing a new experimental tool in addition to the usual microscopies and spectroscopies techniques.References :[1] Late, D. J.; Huang, Y.-K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J.; Yan, A.; Charles, D.; Waghmare, U. V.; Dravid, V. P.; Rao, C. N. R. Sensing Behavior of Atomically Thin-Layered MoS 2 Transistors. ACS Nano 2013, 7 (6), 4879–4891. [2] Li, N.; Wang, Q.; Shen, C.; Wei, Z.; Yu, H.; Zhao, J.; Lu, X.; Wang, G.; He, C.; Xie, L.; Zhu, J.; Du, L.; Yang, R.; Shi, D.; Zhang, G. Large-Scale Flexible and Transparent Electronics Based on Monolayer Molybdenum Disulfide Field-Effect Transistors. Nat. Electron. 2020, 3 (11), 711–717.[3] Lockhart de la Rosa, C. J.; Phillipson, R.; Teyssandier, J.; Adisoejoso, J.; Balaji, Y.; Huyghebaert, C.; Radu, I.; Heyns, M.; De Feyter, S.; De Gendt, S. Molecular Doping of MoS 2 Transistors by Self-Assembled Oleylamine Networks. Appl. Phys. Lett. 2016, 109 (25), 253112.

Ref HAL: hal-03782396_v1
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Hybrid van der Waals heterostructures made of 2D materials and organic molecules exploit the high sensitivity of 2D materials to all interfacial modifications and the inherent versatility of the organic compounds [1-2]. In this study, we are interested in the quinoïdal zwitterion/MoS2 hybrid system in which organic crystals are grown by epitaxy on the MoS2 surface and can reorganize in other forms after thermal annealing [3]. By means of field effect transistor measurements and atomic force microscopy, we demonstrate that the charge transfer between organic molecules and 2D materials strongly depends on the conformation of the molecular film. This work shows the great sensibility of MoS2 transistors for sensing molecular events occurring at the nanoscale providing a new experimental tool in addition to the usual microscopies and spectroscopies techniques.[1] Late, D. J.; Huang, Y.-K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J.; Yan, A.; Charles, D.; Waghmare, U. V.; Dravid, V. P.; Rao, C. N. R. Sensing Behavior of Atomically Thin-Layered MoS 2 Transistors. ACS Nano 2013, 7 (6), 4879–4891.[2] Li, N.; Wang, Q.; Shen, C.; Wei, Z.; Yu, H.; Zhao, J.; Lu, X.; Wang, G.; He, C.; Xie, L.; Zhu, J.; Du, L.; Yang, R.; Shi, D.; Zhang, G. Large-Scale Flexible and Transparent Electronics Based on Monolayer Molybdenum Disulfide Field-Effect Transistors. Nat. Electron. 2020, 3 (11), 711–717. [3] Lockhart de la Rosa, C. J.; Phillipson, R.; Teyssandier, J.; Adisoejoso, J.; Balaji, Y.; Huyghebaert, C.; Radu, I.; Heyns, M.; De Feyter, S.; De Gendt, S. Molecular Doping of MoS 2 Transistors by Self-Assembled Oleylamine Networks. Appl. Phys. Lett. 2016, 109 (25), 253112.

Ref HAL: hal-03034438_v1
DOI: 10.1016/j.carbon.2020.03.027
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The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during SiC (0001) sublimation.The covalent bonds between some carbon atoms in this layer and underlying silicon atoms makes itdifferent from epitaxial graphene. We report a systematical and statistical investigation of the BLsignature and its coupling with epitaxial graphene by Raman spectroscopy. Three different BLs arestudied: bare buffer layer obtained by direct growth (BL0), interfacial buffer layer between graphene andSiC (c-BL1) and the interfacial buffer layer without graphene above (u-BL1). To obtain the latter, wedevelop a mechanical exfoliation of graphene by removing an epoxy-based resin or nickel layer. The BLsare ordered-like on the whole BL growth temperature range. BL0 Raman signature may vary from sampleto sample but forms patches on the same terrace. u-BL1 share similar properties with BL0, albeit withmore variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. Thesignal high frequency side onset upshifts upon graphene coverage, unexplainable by a simple straineffect. Two fine peaks (1235, 1360 cm1), present for epitaxial monolayer and absent for BL and transferred graphene. These findings point to a coupling between graphene and BL.

Ref HAL: hal-03040832_v1
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L’obtention de films de graphène aux caractéristiques contrôlées, comme le nombre de couches ou le dopage, anime toujours une partie de la communauté. La décomposition thermique contrôlée du carbure de silicium (SiC) permet d’envisager la croissance homogène de graphène directement sur des substrats de grande surface. Ce mode de croissance du graphène a initialement été développé à des pressions proches de la pression atmosphérique sous argon et à haute température. Dans l’étude présentée, les croissances par sublimation ont été réalisées à basse pression d’argon (10 mbar). Dans tous les cas, lors de la sublimation du SiC du côté dit « face silicium », il se forme une première couche, dite tampon, qui deviendra, par la suite, la couche de graphène sous laquelle se formera une nouvelle couche tampon. Nous discuterons de la présence et des caractéristiques de cette couche tampon. Nous mettrons notamment en évidence l’inhomogénéité de sa signature Raman, et la nécessité d’une procédure du traitement des données robuste. Puis, nous discuterons, à partir d’une étude complémentaire Raman et AFM, les premiers instants de croissance de la monocouche de graphène, par un mécanisme en bord de marches du SiC. Enfin, des mesures électriques à basse température (1.7 K) et sous champ magnétique (jusqu’à 13 T) complèteront les études menées en confirmant la continuité et la qualité de la monocouche de graphène.

Ref HAL: hal-03040807_v1
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Since 2008, epitaxial graphene growth has been developed in terms of homogeneity and scale by using a 1 ATM argon pressure at high temperature (>1650°C). Until now, it still remains challenging to obtain films with different and controlled characteristics such as the number of graphene layers or the doping by tuning the growth parameters. Here, we optimized the epitaxial growth of monolayer graphene (1LG) on 4H-SiC (0001) under a low argon pressure of 10 mbar. This intermediate pressure allows growing a continued 1LG in a short process time ~1h30. First, we discuss the initial growth stages from buffer layer (BL) to 1LG as a function of annealing temperature (same heating rate). The combined Raman spectroscopy and AFM analyses show that a BL, fully covering the Si-face of SiC, forms as the first step of growth. Subsequently, 1LG starts to grow at step edges and continue to cover the BL on terraces with a step-flow growth mechanism, as demonstrated in Fig.1. Then, we investigate the structural and electronic properties of the 1LG films. The integrated intensity of G-band in Raman spectra normalized with respect to a HOPG reference, AG/AG-HOPG, of each spectrum in Raman map of our continued graphene film is very close to the experimental value reported for a 1LG, demonstrating the good homogeneity. Regarding transport measurement, quantum Hall plateau values observed in our graphene layers confirmed both continuity and thickness of the 1LG film.

Ref HAL: hal-02100569_v1
Ref Arxiv: 1808.05173
DOI: 10.1039/c8nr09841g
WoS: WOS:000462669600047
Ref. & Cit.: NASA ADS
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4 Citations
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The growth of single-layer graphene (SLG) by chemical vapor deposition (CVD) on copper surfaces is very popular because of the self-limiting effect that, in principle, prevents the growth of few-layer graphene (FLG). However, the reproducibility of the CVD growth of homogeneous SLG remains a major challenge, especially if one wants to avoid heavy surface treatments, monocrystalline substrates and expensive equipment to control the atmosphere inside the growth system. We demonstrate here that backside tungsten coating of copper foils allows for the exclusive growth of SLG with full coverage by atmospheric pressure CVD implemented in a vacuum-free furnace. We show that the absence of FLG patches is related to the suppression of carbon diffusion through copper. In the perspective of large-scale production of graphene, this approach constitutes a significant improvement to the traditional CVD growth process since (1) a tight control of the hydrocarbon flow is no longer required to avoid FLG formation and, consequently, (2) the growth duration necessary to reach full coverage can be drastically shortened.

Résumé:

Structural and electronic properties of epitaxial graphene grown on SiC (0001) at low argon pressure