Herein, we report an original adaptation of an XRD set-up in order to measure the miscut angle values in our 4H-SiC on axis substrates with a high precision of ± 0.02°. This study also reveals a correlation between the formation of wide steps on 4H-SiC(0001) and the relative orientation of the SiC crystalline planes versus the gas flow direction. These two results paves the way towards the reproducible growth of graphene over wide SiC steps.

Single-layer MoS2 has been widely studied due to its promising electronic and optoelectronic properties, which can be efficiently tuned by electrostatic doping in a transistor configuration [1]. To consider actual applications, a perfect understanding and control of the contacts and transport regime in the devices is required [2-3]. We present a systematic study of the transport characteristics of monolayer MoS2 as a function of electron density and temperature. High quality MoS2 flakes are grown by CVD on a Si/SiO2 substrate, then plasma etched into the desired shape and top-contacted with Ti/Au through electron beam lithography (Fig. 1). More than 10 devices were studied by electrical measurements after in-situ annealing at 600 K under vacuum. The 2- and 4-probe resistance was systematically recorded as a function of temperature (down to 20 K), gate voltage and source-drain voltage. In this presentation, we will focus on the intrinsic conductivity of the MoS2 channel (4-probe measurements). For all the devices the conductivity increases with gate voltage and the apparent threshold voltage decreases with temperature. Insulating or metallic-type temperature dependencies are observed (without the need of top-gating or high-k dielectric capping) depending on carrier density (see Fig. 2), with a transition occurring around 2x1012 cm-2. The devices exhibit high mobilities at low temperature (for CVD MoS2 on SiO2) on the order of 200 cm2/Vs, with a nontrivial dependence between the gate voltage and the density of occupied states above the mobility edge (i.e, extended states). A detailed analysis with different models fitting these results [3,4] will be presented to clearly elucidate the dominant transport mechanism for each doping and temperature range. For low gate voltages, thermally activated and variable-range hopping transport of in-gap states explain the drastic change in threshold voltage and the insulating transport [e.g. Ref. 2]. For higher gate voltages, the metallic behavior is well explained by phonon and static defects scattering, in agreement with a theoretical model done through a full energy- and momentum-dependent Boltzmann transport equation [5]. All these results are consistent with other device characterization such as Raman spectroscopy. We will show that this metal-insulator transition also matches more elaborate models involving quantum phase transitions. We will tentatively discuss how this approach can be reconciliated with the previous models, which are more consistent with the temperature and mobility ranges observed.

Ionosilica ionogels have been lately introduced as emerging all-ionic designer materials. They consist of an ionic liquid (IL) guest trapped within a solid ionosilica support host. In this work, we investigate the influence of the (i) ionosilica nature and (ii) the ionosilica/IL ratio on the mobility of the confined IL. We report the elaboration of various ionosilica ionogels via a nonhydrolytic sol-gel process, using namely tris(3-(trimethoxysilyl)propyl)amine (TTA), Methyl-tris(3-(trimethoxysilyl)propyl)ammonium iodide (MTTA) and tetrakis(3-(trimethoxysilyl)-propyl)ammonium iodide (TKTA) as ionosilica precursors, with the IL butyl-methyl imidazolium bis-triflimide ([BMIM] TFSI). Various ionogels were prepared from different ratios between the ionosilica host and the IL guest. The host/guest interactions i.e. the change in the conformational contribution of the IL counter-anion were explored by means of Raman spectroscopy. In addition, the transport properties of the confined species were probed via spin echo solid-state NMR experiments and Complex Impedance Spectroscopy (CIS) measurements. Raman experiments revealed different conformational equilibrium for the TFSI anion in the various ionosilica ionogels, with an increase in the cisoid form compared to the bulk IL due to confinement effects. The TFSI anion seems to experience different degree of confinement and thus different interactions with the ionosilica scaffold as a function of the investigated parameters. Concomitantly, the 1H spin echo NMR and CIS measurements revealed a significantly higher ionic mobility in the materials synthesized in the presence of the tris-trialkoxysilylated ammonium precursor compared to the one synthesized in the presence of tetra-trialkoxysilylated ammonium precursor. These results may reflect stronger host-guest interactions in the latter case related to the higher degree of reticulation of the network. All these techniques give concordant results and highlight an effect of the chemical constitution of the ionosilica scaffold and the amount of confined IL on its dynamics within the network.

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.

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.