Tuneable photoconductivity and mobility enhancement in printed MoS ₂ /graphene composites

Abstract

Aiming to increase carrier mobility in nanosheet-network devices, we have investigated MoS2-graphene composites as active regions in printed photodetectors.Combining liquid-exfoliation and inkjet-printing, we fabricated all-printed photodetectors with graphene electrodes and MoS2-graphene composite channels with various graphene mass fractions (0≤Mf≤16wt%). The increase in channel dark conductivity with Mf was consistent with percolation theory for composites below the percolation threshold. While the photoconductivity increased with graphene content, it did so more slowly than the dark conductivity such that the fractional photoconductivity decayed rapidly with increasing Mf. We propose that both mobility and dark carrier density increase with graphene content according to percolation-like scaling laws while photo-induced carrier density is essentially independent of graphene loading. This leads to percolation-like scaling laws for both photoconductivity and fractional photoconductivity, in excellent agreement with the data. These results imply that channel mobility and carrier density increase up to 100-fold on the addition of 16wt% graphene.Keywords: printed electronics, network devices, transistor 2The growing demand for low-cost electronics has sparked a wide investigation into printable, low-performance devices and circuits. The field has developed over the last 25 years from early demonstrations of solution processed devices 1, 2 to today's ability to print integrated circuitry. 3The most commonly studied materials in this area continue to be organic polymers and molecules which have been used to print in a range of devices, including light-emitting diodes and transistors. 3 However, these materials suffer a number of disadvantages including relatively low mobility and high cost. This has led a number of researchers to investigate the use of printed networks of inorganic nanoparticles and nanotubes in device applications. 4, 5While good device performance has been demonstrated from these materials (e.g. high mobilities and on:off ratios in printed transistors), it is not clear whether such technologies can be scaled at low cost due to difficulties in materials synthesis and processing.More recently, it has been shown that 2-dimensional nanosheets are promising candidates for electronic device applications, with single-nanosheet transistors displaying relatively high mobilities and on/off ratios. 6,7 In the context of printed electronics, fabricating printed nanosheet network-based devices will require access to nanosheet-containing inks.Critically, nanosheets can be produced cheaply in a form amenable to ink formulation by techniques such as liquid-phase exfoliation (LPE). 8,9 This method uses scalable processes, such as high shear mixing, 9 to exfoliate layered crystals into few-layer nanosheets in appropriate liquids. Using simple, centrifugation-based, post-processing techniques, it is possible to sizeselect the nanosheets while simultaneously exchanging the solvent and increasing the concent...

Percolation Effects in Electrolytically Gated WS2/Graphene Nano:Nano Composites

“…composites. 21 Here we found the carrier density data above the percolation threshold to be consistent with percolation-scaling:…”

“…21 This would be an important development which would be of use in the area of printed electronics using 2D materials. [22][23][24][25][26] A number of papers have described printed electronic devices such as photodetectors, 21 switches, 23 memory elements 25 and transistors 22,27 which are predominately based on networks of 2D nanosheets. However, nearly all such devices are limited by relatively low mobilities which tend to be ~0.1 cm 2 /Vs for printed networks of transition metal dichalcogenide nanosheets.…”

Drawing WS2 thermal sensors on paper substrates

“…22 The conductivity is also 20 times higher than that found for MoS 2 on paper 20 and 100 times larger than the higher conductivity values reported for conductive networks of liquid phase exfoliated MoS 2 . [23][24][25] Nonetheless the conductivity of the WS 2 -on-paper films is still lower than the in-plane conductivity of bulk (∼10 2 S m −1 ) 26 and mechanically exfoliated single-layer WS 2 (∼10 3 S m −1 ), 27 indicating a large contribution of the out-of-plane transport as well as the flake-to-flake hopping in the total conductivity of the WS 2 films on paper. In the ESI † a transfer length measurement on a WS 2 film on paper is described to extract the contact resistance and the conductivity.…”

Integrating superconducting van der Waals materials on paper substrates

“…For these reasons, standard device fabrication approaches, developed and optimized for the fabrication of devices on silicon substrates, cannot be directly used on paper substrates. New fabrication techniques have been developed in the last years to overcome that issue, and now vdW materials can be deposited onto paper substrates using inkjet printing of inks prepared by liquid-phase exfoliation 7,[11][12][13][14][15][16][17][18][19] or by a recently reported all-dry abrasion-induced deposition method. 20,21 Up to now these methods already demonstrated that one can fabricate heterostructures and pattern complex devices, with high spatial resolution, with conducting, semiconducting and insulating vdW materials on paper.…”