Quantifying the Influence of Nanosheet Aspect Ratio on Network Morphology and Junction Resistance in Solution-Processed Nanosheet Networks.

On the relationship between morphology and conductivity in nanosheet 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.

Liquid phase exfoliation is a powerful and scalable technique to produce defect-free mono- and few-layer graphene. However, samples are typically polydisperse and control over size and thickness is challenging. Notably, high throughput techniques to measure size and thickness are lacking. In this work, we have measured the extinction, absorption, scattering and Raman spectra for liquid phase exfoliated graphene nanosheets of various lateral sizes (90 ≤ 〈L〉 ≤ 810 nm) and thicknesses (2.7 ≤ 〈N〉 ≤ 10.4). We found all spectra to show well-defined dependences on nanosheet dimensions. Measurements of extinction and absorption spectra of nanosheet dispersions showed both peak position and spectral shape to vary with nanosheet thickness in a manner consistent with theoretical calculations. This allows the development of empirical metrics to extract the mean thickness of liquid dispersed nanosheets from an extinction (or absorption) spectrum. While the scattering spectra depended on nanosheet length, poor signal to noise ratios made the resultant length metric unreliable. By analyzing Raman spectra measured on graphene nanosheet networks, we found both the D/G intensity ratio and the width of the G-band to scale with mean nanosheet length allowing us to establish quantitative relationships. In addition, we elucidate the variation of 2D/G band intensities and 2D-band shape with the mean nanosheet thickness, allowing us to establish quantitative metrics for mean nanosheet thickness from Raman spectra.

Printed networks of 2D nanosheets have found a range of applications in areas including electronic devices, energy storage systems and sensors. For example, the ability to print graphene networks onto flexible substrates enables the production of high-performance strain sensors. The network resistivity is known to be sensitive to the nanosheet dimensions which implies the piezoresistance might also be size-dependent. In this study, the effect of nanosheet thickness on the piezoresistive response of nanosheet networks has been investigated. To achieve this, we liquid-exfoliated graphene nanosheets which were then subjected to centrifugation-based size selection followed by spray deposition onto flexible substrates. The resultant devices show increasing resistivity and gauge factor with increasing nanosheet thickness. We analyse the resistivity versus thickness data using a recently reported model and develop a new model to fit the gauge factor versus thickness data. This analysis allowed us to differentiate between the effect of strain on inter-nanosheet junctions and the straining of the individual nanosheets within the network. Surprisingly, our data implies the nanosheets themselves to display a negative piezo response.

Abstract2D material inks have the potential to strongly impact printed electronics, offering exciting opportunities for flexible and wearable devices. However, their electrical performance is often hindered by the resistive nature of inter‐nanosheet junctions within randomly assembled nanosheet networks, limiting their efficiency compared to individual nanosheets. Overcoming this challenge necessitates a comprehensive understanding of the conduction mechanisms governing charge transport in these networks. In this study, a water‐based graphene ink is prepared via liquid‐phase exfoliation (LPE), deposited onto Si/SiO₂ substrates through inkjet printing, and electrically characterized over a wide temperature range (80–400 K) following thermal annealing at different temperatures. To interpret the temperature‐dependent conductivity, a Random Resistor Network (RRN) model is employed that accounts for spatial and energetic variability among nodes. With this approach low and high temperature transport regimes are effectively studied, identifying inter‐flake and intra‐flake hopping mechanisms and providing valuable insights into the factors governing charge transport. Using Monte Carlo simulations, the RRN model delivers statistically robust predictions while capturing temperature‐dependent transitions and annealing effects, achieving excellent agreement with experimental observations.

Production and processing of graphene and related materials

ConspectusTwo-dimensional sp2-hybridized graphene has been seriously considered and applied in various fields, such as materials science, energy storage/conversion, catalysis, and biomedicine, on account of its unique long-range-ordered and π-conjugated structure as well as excellent thermal and electric conductivity. At present, the adopted methods for graphene synthesis cover micromechanical exfoliation, epitaxial growth, chemical reduction of graphite oxide, chemical vapor deposition, and liquid-phase exfoliation. Nonetheless, a number of issues and challenges still existed in these employed methods in terms of sustainable and green energy chemistry and environmental friendliness. Electrochemical exfoliation is one of the most promising methods for the large-scale production of graphene by virtue of simple/eco-friendly operation and high-efficiency production. Depending on different exfoliation methods (anodic, cathodic, and dual-electrode) and electrochemical exfoliation conditions such as the reaction device, raw material, electrolyte, and power-supply mode, the exfoliated graphene features various and versatile characteristics. The relatively perfect graphene with intrinsic long-range π-conjugated character is accompanied by the fast capability of electron transfer. The defective graphene features the adjustable size and defect density and tunable amphipathicity. As a result, the corresponding graphene has been employed in energy storage/conversion, biology, and medicine fields. In this Account, we summarize the recent progress in the fundamentals of electrochemical exfoliation to produce the graphene and the derivative exfoliation method and their applications in energy-related fields, and the corresponding perspectives are also highlighted. First, we describe the fundamentals for different exfoliation methods and electric-field-induced effects that can help to guide us to prepare and produce graphene with various properties, covering the size, defect density, and surface chemistry. Subsequently, three electric-field-triggered methods (anodic, cathodic, and dual-electrode exfoliation) are discussed in detail. In particular, the effects of various electrolytes and power-supply modes on electrochemically exfoliated graphene (EEG) are systematically underlined in terms of the exfoliation principles and process, which are the significant factors affecting the structure and properties of EEG and the efficiency of intercalation and exfoliation. In addition, the recent progress in multilevel applications of EEG in energy storage/conversion fields such as supercapacitors, secondary batteries, the oxygen evolution reaction, the hydrogen evolution reaction, the carbon dioxide reduction reaction, and so forth is also summarized. Finally, the expectations and future research directions for the EEG in terms of controllably technological parameters and conditions, precise monitoring and in-depth comprehension of the exfoliation processes, multiscale and diverse products (carbon quantum dots, graphene quantum dots, and single-atom catalysts), and large-scale preparation and application are discussed, which hope to provide significant guidance for future research.

Entropy-driven liquid-phase exfoliation of non-Van-Der-Waals crystals into nanoplatelets

This study explores the challenges associated with translating electrical characteristics of individual two-dimensional semiconductor nanosheets into a network of partially overlapping sheets. Such systems typically suffer from high-energy barriers required to overcome the junctions formed between the adjacent nanosheets, and consequently quench the current passing through the network. We use in-operando Kelvin probe force microscopy to image electrostatic potential profiles during the operation of MoS2 nanosheet network transistors. Direct imaging of the potential drops allows us to distinguish contributions from individual nanosheets and those from junctions, correlated by the junction-related potential drops with the network morphology. A diagram-based model is developed to describe the system numerically and to estimate the current path formation probabilities. Finally, a correlation with the integral electrical characteristics of the nanosheet-based transistors is made using a robust Y-function approach. It is shown that the total junction resistance is well estimated by the proposed equivalent circiut model.

Exfoliated graphene and its derivatives from liquid phase and their role in performance enhancement of epoxy matrix composite

Transition metal dichalcogenides (TMDs) are promising 2D materials which are attracting significant interest because of their distinctive physicochemical properties. The possibility of being exfoliated and dispersed in liquid solutions offers a viable pathway to scalable production. This personal account focuses on recent advancements in 2D TMD inks produced by liquid‐phase exfoliation (LPE) methods and intercalation‐based electrochemical exfoliation. In particular, different LPE production strategies, like ultrasonication LPE, high‐shear mixing exfoliation, and microfluidization, are introduced alongside a broad range of liquid media employed to provide functionalized TMD inks. The main advantage of TMD inks is its scalability, for practical applications in printed optoelectronics, energy storage, and conversion. Furthermore, the chemical functionalization of TMD inks can solve the poor electrical conductivity attributed to edge defects inherent in TMD inks. Finally, the ultimate orientations for future applications of chemically functionalized TMD devices are forecasted, with a specific focus on wearable and flexible printed electronics.

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Antimonene with a honeycomb layered structure has great application prospects in a wide spectrum of domains due to its high carrier mobility, high thermal conductivity, and layer-dependent electrical properties. Since the first successful synthesis of antimonene by epitaxy in 2015, various fabrication methods have been proposed successively. Herein, several representative synthetic methods are described in detail, including mechanical exfoliation, epitaxial growth, liquid-phase exfoliation, electrochemical exfoliation, etc. In addition, band engineering via modification strategies of antimonene, particularly intercalation and doping, is discussed based on available theoretical studies. By comparing the achieved structure characteristics and performances of these different synthesis and modification strategies, we present promising future developments and critical challenges for antimonene.

Nanotechnology and nanoscience have emerged out as one of the most exciting areas of research today. As an inimitable morphological 2-D carbon material, graphene has triggered a gold rush in the nanomaterial society by introducing controlled functional building blocks. Besides, the mechanical, electrical, and optical properties of graphene make it an attractive contender for applications in solar energy conversion and electrochemical energy devices. Graphene based nanocomposites have been preferred greatly due to their low cost, low density, high electron mobility, exceptional optical transmittance, versatile process ability and excellent thermal conductivity. Subsequently, to encounter the global needs, energy scavenging has become an ultimate part of pervasive sensor network. A gold rush has been prompted all over the world for exploiting the possible applications of graphene-based nanomaterials. The best solution to this problem is to improve the photoconversion efficiencies by optimizing materials and device fabrication.In this chapter, first of all we have discussed different synthesis methods of graphene nanocomposites like mechanical exfoliation, chemical vapor deposition, liquid phase exfoliation, electrochemical exfoliation and reduction of graphene oxide. For the elucidation of their structural and morphological characteristics, different techniques including SEM, TEM, Energy dispersive X-ray spectroscopy (EDX), UV/Vis absorption spectrum, Raman Spectroscopy, Photoluminescence spectroscopy (PL), X-ray photoelectron spectroscopy (XPS), XRD, Cyclic voltammetry, Impedance spectroscopy, DSC, FTIR and TGA have been discussed. Application potential analysis for graphene-based nanocomposites is discussed based on means including flexible and stretchable electronics, photocatalysis and electrochemical sensing, use in Li-ion batteries, supercapacitors, photovoltaic applications and hydrogen production. Some of the future concerns have also been discussed related to their feasibility, controlled device fabrication of composites, stability and life span of composite, multistep heterogeneous catalysis and safe dumping of environmental contaminants.KeywordsGraphene based nanocompositesEnergyHydrogen productionSolar cells

Graphene is fundamentally a two-dimensional material with extraordinary optical, thermal, mechanical, and electrical characteristics. It has a versatile surface chemistry and large surface area. It is a carbon nanomaterial, which comprises sp2 hybridized carbon atoms placed in a hexagonal lattice with one-atom thickness, giving it a two-dimensional structure. A large number of synthesis techniques including epitaxial growth, liquid phase exfoliation, electrochemical exfoliation, mechanical exfoliation, and chemical vapor deposition are used for the synthesis of graphene. Graphene prepared using different techniques can have a number of benefits and deficiencies depending on its application. This study provides a summary of graphene preparation techniques and critically assesses the use of graphene, its derivates, and composites in environmental applications. These applications include the use of graphene as membrane material for the detoxication and purification of water, active material for gas sensing, heavy metal ions detection, and CO2 conversion. Furthermore, a trend analysis of both synthesis techniques and environmental applications of graphene has been performed by extracting and analyzing Scopus data from the past ten years. Finally, conclusions and outlook are provided to address the residual challenges related to the synthesis of the material and its use for environmental applications.