Understanding how junction resistances impact the conduction mechanism in nano-networks.

Networks of nanowires, nanotubes, and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the resistance between the particles, often referred to as the junction resistance. Minimising the junction resistance has proven to be challenging, partly because it is difficult to measure. Here, we develop a simple model for electrical conduction in networks of 1D or 2D nanomaterials that allows us to extract junction and nanoparticle resistances from particle-size-dependent DC network resistivity data. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ω for silver nanosheets to 24 GΩ for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be obtained simultaneously from AC impedance spectra of semiconducting nanosheet networks. Through our model, we use the impedance data to directly link the high mobility of aligned networks of electrochemically exfoliated MoS2 nanosheets (≈ 7 cm2 V-1 s-1) to low junction resistances of ∼2.3 MΩ. Temperature-dependent impedance measurements also allow us to comprehensively investigate transport mechanisms within the network and quantitatively differentiate intra-nanosheet phonon-limited bandlike transport from inter-nanosheet hopping.

Knot Architecture for Biocompatible and Semiconducting 2D Electronic Fiber Transistors.

Wearable devices have generally been rigid due to their reliance on silicon-based technologies, while future wearables will utilize flexible components for example transistors within microprocessors to manage data. Two-dimensional (2D) semiconducting flakes have yet to be investigated in fiber transistors but can offer a route toward high-mobility, biocompatible, and flexible fiber-based devices. Here, the electrochemical exfoliation of semiconducting 2D flakes of tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) is shown to achieve homogeneous coatings onto the surface of polyester fibers. The high aspect ratio (>100) of the flake yields aligned and conformal flake-to-flake junctions on polyester fibers enabling transistors with mobilities µ ≈1 cm2 V-1 s-1 and a current on/off ratio, Ion/Ioff ≈102-104. Furthermore, the cytotoxic effects of the MoS2 and WSe2 flakes with human keratinocyte cells are investigated and found to be biocompatible. As an additional step, a unique transistor 'knot' architecture is created by leveraging the fiber diameter to establish the length of the transistor channel, facilitating a route to scale down transistor channel dimensions (≈100 µm) and utilize it to make a MoS2 fiber transistor with a human hair that achieves mobilities as high as µ ≈15 cm2 V-1 s-1.

Near Room Temperature Production of Segregated Network Composites of Carbon Nanotubes and Regolith as Multifunctional, Extra-Terrestrial Building Materials.

Constructing a semi-permanent base on the moon or Mars will require maximal use of materials found in situ and minimization of materials and equipment transported from Earth. This will mean a heavy reliance on regolith (Lunar or Marian soil) and water, supplemented by small quantities of additives fabricated on Earth. Here it is shown that SiO2-based powders, as well as Lunar and Martian regolith simulants, can be fabricated into building materials at near-ambient temperatures using only a few weight-percent of carbon nanotubes as a binder. These composites have compressive strength and toughness up to 100 MPa and 3 MPa respectively, higher than the best terrestrial concretes. They are electrically conductive (>20 S m-1) and display an extremely large piezoresistive response (gauge factor >600), allowing these composites to be used as internal sensors to monitor the structural health of extra-terrestrial buildings.

Quantitative analysis of printed nanostructured networks using high-resolution 3D FIB-SEM nanotomography.

Networks of solution-processed nanomaterials are becoming increasingly important across applications in electronics, sensing and energy storage/generation. Although the physical properties of these devices are often completely dominated by network morphology, the network structure itself remains difficult to interrogate. Here, we utilise focused ion beam - scanning electron microscopy nanotomography (FIB-SEM-NT) to quantitatively characterise the morphology of printed nanostructured networks and their devices using nanometre-resolution 3D images. The influence of nanosheet/nanowire size on network structure in printed films of graphene, WS2 and silver nanosheets (AgNSs), as well as networks of silver nanowires (AgNWs), is investigated. We present a comprehensive toolkit to extract morphological characteristics including network porosity, tortuosity, specific surface area, pore dimensions and nanosheet orientation, which we link to network resistivity. By extending this technique to interrogate the structure and interfaces within printed vertical heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

Transparent Conductors Printed from Grids of Highly Conductive Silver Nanosheets.

Transparent conductors (TCs) represent key components in many applications from optoelectronic devices to electromagnetic shielding. While commercial applications typically use thin films of indium tin oxide, this material is brittle and increasingly scarce, meaning higher performing and cheaper alternatives are sought after. Solution-processible metals would be ideal owing to their high conductivities and printability. However, due to their opacity to visible light, such films need to be very thin to achieve transparency, thus limiting the minimum resistance achievable. One solution is to print metallic particles in a grid structure, which has the advantages of high tunable transparency and resistance at the cost of uniformity. Here, we report silver nanosheets that have been aerosol jet printed into grids as high-performance transparent conductors. We first investigate the effect of annealing on the silver nanosheets where we observe the onset of junction sintering at 160 °C after which the silver network becomes continuous. We then investigate the effect of line width and thickness on the electrical performance and the effect of varying the aperture dimensions on the optical performance. Using these data, we develop simple models, which allow us to optimize the grid and demonstrate a printed transparent conductor with a transmittance of 91% at a sheet resistance of 4.6 Ω/sq.

High-Mobility Flexible Transistors with Low-Temperature Solution-Processed Tungsten Dichalcogenides.

The investigation of high-mobility two-dimensional (2D) flakes beyond molybdenum disulfide (MoS2) will be necessary to create a library of high-mobility solution-processed networks that conform to substrates and remain functional over thousands of bending cycles. Here we report electrochemical exfoliation of large-aspect-ratio (>100) semiconducting flakes of tungsten diselenide (WSe2) and tungsten disulfide (WS2) as well as MoS2 as a comparison. We use Langmuir-Schaefer coating to achieve highly aligned and conformal flake networks, with minimal mesoporosity (∼2-5%), at low processing temperatures (120 °C) and without acid treatments. This allows us to fabricate electrochemical transistors in ambient air, achieving average mobilities of µMoS2 ≈ 11 cm2 V-1 s-1, µWS2 ≈ 9 cm2 V-1 s-1, and µWSe2 ≈ 2 cm2 V-1 s-1 with a current on/off ratios of Ion/Ioff ≈ 2.6 × 103, 3.4 × 103, and 4.2 × 104 for MoS2, WS2, and WSe2, respectively. Moreover, our transistors display threshold voltages near ∼0.4 V with subthreshold slopes as low as 182 mV/dec, which are essential factors in maintaining power efficiency and represent a 1 order of magnitude improvement in the state of the art. Furthermore, the performance of our WSe2 transistors is maintained on polyethylene terephthalate (PET) even after 1000 bending cycles at 1% strain.

Chiroptically Active Multi-Modal Calcium Carbonate-Based Nanocomposites.

The development of multimodal nano- and micro-structures has become an increasingly popular area of research in recent years. In particular, the combination of two or more desirable properties within a single structure opens multiple opportunities from biomedicine, sensing, and catalysis, to a variety of optical applications. Here, for the first time, we report the synthesis and characterization of multimodal chiroptically active CaCO3 nanocomposites. These composites have been prepared by a modified microemulsion method in the presence of an amino acid (cysteine). Following this, additional modalities have been introduced by loading the composites with luminescent nanoparticles or doping with Eu3+ ions. The luminescent composites have been produced by the incorporation of CuInZnS/ZnS or CdSe@ZnS/ZnS core/shell quantum dots, or via doping with trivalent europium. In this manner, we have produced chiroptically active composites with orange, green, and red luminescence. Overall, this work demonstrates the unique advantage and potential of our approach and new class of chiroptically active CaCO3 nanocomposites, which display tunable functionality to specific requirements via the incorporation of desired ions, nanoparticles, and chirality of the structure.

Quantifying the Piezoresistive Mechanism in High-Performance Printed Graphene Strain Sensors.

Printed strain sensors will be important in applications such as wearable devices, which monitor breathing and heart function. Such sensors need to combine high sensitivity and low resistance with other factors such as cyclability, low hysteresis, and minimal frequency/strain-rate dependence. Although nanocomposite sensors can display a high gauge factor (G), they often perform poorly in the other areas. Recently, evidence has been growing that printed, polymer-free networks of nanoparticles, such as graphene nanosheets, display very good all-round sensing performance, although the details of the sensing mechanism are poorly understood. Here, we perform a detailed characterization of the thickness dependence of piezoresistive sensors based on printed networks of graphene nanosheets. We find both conductivity and gauge factor to display percolative behavior at low network thickness but bulk-like behavior for networks above ∼100 nm thick. We use percolation theory to derive an equation for gauge factor as a function of network thickness, which well-describes the observed thickness dependence, including the divergence in gauge factor as the percolation threshold is approached. Our analysis shows that the dominant contributor to the sensor performance is not the effect of strain on internanosheet junctions but the strain-induced modification of the network structure. Finally, we find these networks display excellent cyclability, hysteresis, and frequency/strain-rate dependence as well as gauge factors as high as 350.