Solution-processed semiconducting transition metal dichalcogenides (TMDs) are at the centre of an ever-increasing research effort in printed (opto)electronics. However, device performance is limited by structural defects resulting from the exfoliation process and poor inter-flake electronic connectivity.Here, we report a new molecular strategy to boost the electrical performance of TMD-based devices via the use of dithiolated conjugated molecules, to simultaneously heal sulfur vacancies in solutionprocessed transition metal disulfides (MS2) and covalently bridge adjacent flakes, thereby promoting percolation pathways for the charge transport. We achieve a reproducible increase by one order-ofmagnitude in field-effect mobility (µFE), current ratios (ION / IOFF), and switching times (τS) of liquid-gated transistors, reaching 10 -2 cm 2 V -1 s -1 , 10 4 , and 18 ms, respectively. Our functionalization strategy is an universal route to simultaneously enhance the electronic connectivity in MS2 networks and tailor on demand their physicochemical properties according to the envisioned applications.
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.
Over the past 15
years, two-dimensional (2D) materials have been
studied and exploited for many applications. In many cases, 2D materials
are formed by the exfoliation of layered crystals such as transition-metal
disulfides. However, it has recently become clear that it is possible
to exfoliate nonlayered materials so long as they have a nonisotropic
bonding arrangement. Here, we report the synthesis of 2D-platelets
from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS2), using liquid-phase exfoliation. The resultant 2D platelets
exhibit the same crystal structure as bulk pyrite but are surface
passivated with a density of 14 × 1018 groups/m2. They form stable suspensions in common solvents and can
be size-selected and liquid processed. Although the platelets have
relatively low aspect ratios (∼5), this is in line with the
anisotropic cleavage energy of bulk FeS2. We observe size-dependent
changes to optical properties leading to spectroscopic metrics that
can be used to estimate the dimensions of platelets. These platelets
can be used to produce lithium ion battery anodes with capacities
approaching 1000 mAh/g.
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