Printed electronics has emerged as a pathway for large scale, flexible, and wearable devices enabled by graphene and two-dimensional (2D) materials. Solution processing of graphite and layered materials demonstrated mass production of inks allowing techniques such as inkjet printing to be used for device fabrication. However, the complexity of the ink formulations and the polycrystalline nature of the thin films, together with the metal, semimetal, and semiconducting behaviour of different 2D materials, have impeded the investigation of charge transport in inkjet printed 2D material devices. Here we unveil the charge transport mechanisms of surfactant-and solvent-free inkjet-printed thin-film devices of representative few-layer graphene (semi-metal), molybdenum disulfide (MoS2, semiconductor) and titanium carbide MXene (Ti3C2, metal) by investigating the temperature (T ), gate and magnetic field dependencies of their electrical conductivity. We find that charge transport in printed few-layer MXene and MoS2 devices is dominated by the intrinsic transport mechanism of the constituent flakes: MXene devices exhibit a weakly-localized 2D metallic behavior at any T , whereas MoS2 devices behave as insulators with a crossover from 3D-Mott variable-range hopping at low T to nearest-neighbor hopping around at ∼ 200 K. The charge transport in printed few-layer graphene devices is dominated by the transport mechanism between different flakes, which exhibit 3D-Mott variable range hopping conduction at any T . These findings reveal and finally establish the fundamental mechanisms responsible for charge transport in inkjet-printed devices with 2D materials, paving the way for a reliable design of high performance printed electronics.

Hole doping can control the conductivity of diamond either through boron substitution, or carrier accumulation in a field-effect transistor. In this work, we combine the two methods to investigate the insulator-to-metal transition at the surface of nanocrystalline diamond films. The finite boron doping strongly increases the maximum hole density which can be induced electrostatically with respect to intrinsic diamond. The ionic gate pushes the conductivity of the film surface away from the variable-range hopping regime and into the quantum critical regime. However, the combination of the strong intrinsic surface disorder due to a non-negligible surface roughness, and the introduction of extra scattering centers by the ionic gate, prevents the surface accumulation layer to reach the metallic regime.

Superconductivity (SC) in the Ba-122 family of iron-based compounds can be controlled by aliovalent or isovalent substitutions, applied external pressure, and strain, the combined effects of which are sometimes studied within the same sample. Most often, the result is limited to a shift of the SC dome to different doping values. In a few cases, the maximum SC transition at optimal doping can also be enhanced. In this work, we study the combination of charge doping together with isovalent P substitution and strain, by performing ionic gating experiments on BaFe2(As0.8P0.2)2 ultrathin films. We show that the polarization of the ionic gate induces modulations to the normal-state transport properties that can be mainly ascribed to surface charge doping. We demonstrate that ionic gating can only shift the system away from the optimal conditions, as the SC transition temperature is suppressed both by electron and hole doping. We also observe a broadening of the resistive transition, which suggests that the SC order parameter is modulated non-homogeneously across the film thickness, in contrast with earlier reports on charge-doped standard BCS superconductors and cuprates.