Solution-processed Nanomaterials Research Articles

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.

Stimuli-Responsive Surface Ligands for Direct Lithography of Functional Inorganic Nanomaterials.

ConspectusColloidal nanocrystals (NCs) have emerged as a diverse class of materials with tunable composition, size, shape, and surface chemistry. From their facile syntheses to unique optoelectronic properties, these solution-processed nanomaterials are a promising alternative to materials grown as bulk crystals or by vapor-phase methods. However, the integration of colloidal nanomaterials in real-world devices is held back by challenges in making patterned NC films with the resolution, throughput, and cost demanded by device components and applications. Therefore, suitable approaches to pattern NCs need to be established to aid the transition from individual proof-of-concept NC devices to integrated and multiplexed technological systems.In this Account, we discuss the development of stimuli-sensitive surface ligands that enable NCs to be patterned directly with good pattern fidelity while retaining desirable properties. We focus on rationally selected ligands that enable changes in the NC dispersibility by responding to light, electron beam, and/or heat. First, we summarize the fundamental forces between colloidal NCs and discuss the principles behind NC stabilization/destabilization. These principles are applied to understanding the mechanisms of the NC dispersibility change upon stimuli-induced ligand modifications. Six ligand-based patterning mechanisms are introduced: ligand cross-linking, ligand decomposition, ligand desorption, in situ ligand exchange, ion/ligand binding, and ligand-aided increase of ionic strength. We discuss examples of stimuli-sensitive ligands that fall under each mechanism, including their chemical transformations, and address how these ligands are used to pattern either sterically or electrostatically stabilized colloidal NCs. Following that, we explain the rationale behind the exploration of different types of stimuli, as well as the advantages and disadvantages of each stimulus.We then discuss relevant figures-of-merit that should be considered when choosing a particular ligand chemistry or stimulus for patterning NCs. These figures-of-merit pertain to either the pattern quality (e.g., resolution, edge and surface roughness, layer thickness), or to the NC material quality (e.g., photo/electro-luminescence, electrical conductivity, inorganic fraction). We outline the importance of these properties and provide insights on optimizing them. Both the pattern quality and NC quality impact the performance of patterned NC devices such as field-effect transistors, light-emitting diodes, color-conversion pixels, photodetectors, and diffractive optical elements. We also give examples of proof-of-concept patterned NC devices and evaluate their performance. Finally, we provide an outlook on further expanding the chemistry of stimuli-sensitive ligands, improving the NC pattern quality, progress toward 3D printing, and other potential research directions. Ultimately, we hope that the development of a patterning toolbox for NCs will expedite their implementation in a broad range of applications.

Room-Temperature Semiconductor Gas Sensors: Challenges and Opportunities.

Our demand for ubiquitous and reliable gas detection is spurring the design of intelligent and enabling gas sensors for the next-generation Internet of Things and Artificial Intelligence. The desire to introduce gas sensors everywhere is fueled by opportunities to create room-temperature semiconductor gas sensors with ultralow power consumption. In this Perspective, we provide an overview of the recent achievement of room-temperature gas sensors that have been translated from the advances in the design of the chemical and physical properties of low-dimensional semiconductor nanomaterials. The emergence of solution-processable nanomaterials opens up remarkable opportunities to integrate into high-performance and flexible room-temperature gas sensors by using low-temperature, large-area, solution-based methods instead of costly, high-vacuum, high-temperature device manufacturing processes. We review the fundamental factors which affect the receptor and transducer functions of semiconductor gas sensors. We also discuss challenges that must be addressed in the move to the continuous miniaturization and evolution of semiconductor gas sensors.

Direct in situ photolithography of perovskite quantum dots based on photocatalysis of lead bromide complexes

Photolithography has shown great potential in patterning solution-processed nanomaterials for integration into advanced optoelectronic devices. However, photolithography of perovskite quantum dots (PQDs) has so far been hindered by the incompatibility of perovskite with traditional optical lithography processes where lots of solvents and high-energy ultraviolet (UV) light exposure are required. Herein, we report a direct in situ photolithography technique to pattern PQDs based on the photopolymerization catalyzed by lead bromide complexes. By combining direct photolithography with in situ fabrication of PQDs, this method allows to directly photolithograph perovskite precursors, avoiding the complicated lift-off processes and the destruction of PQDs by solvents or high-energy UV light, as PQDs are produced after lithography exposure. We further demonstrate that the thiol-ene free-radical photopolymerization is catalyzed by lead bromide complexes in the perovskite precursor solution, while no external initiators or catalysts are needed. Using direct in situ photolithography, PQD patterns with high resolution up to 2450 pixels per inch (PPI), excellent fluorescence uniformity, and good stability, are successfully demonstrated. This work opens an avenue for non-destructive direct photolithography of high-efficiency light-emitting PQDs, and potentially expands their application in various integrated optoelectronic devices.

All‐Solution‐Processed Van der Waals Heterostructures for Wafer‐Scale Electronics (Adv. Mater. 12/2022)

2D Nanomaterials In article number 2106110, Joohoon Kang and co-workers demonstrate various electronic devices such as field-effect transistors, photodetectors, diodes, and logic gates in wafer-scale based on building-block assembly for van der Waals heterostructure formation. The building blocks consist of solution-processed nanomaterials including metallic graphene, semiconducting MoS2 and CNT, and oxidized HfS2 (i.e., HfO2) as an insulator.

Guest Editorial Special Issue on Papers From the IEEE FLEPS Conference 2020

Printed devices on flexible and disposable substrates are receiving considerable interest for the realization of low-cost electronic and sensing systems. As a result, new methods such as resource-efficient printing and multimaterial additive manufacturing have been explored to develop a wide range of easily deployable systems such as displays, sensors, and smart tags. The technologies developed for silicon-based planar electronics and solid-state sensors are also being re-purposed to meet the demands in the emerging field of flexible and printed sensors. This trend will continue as all market indicators point towards significant growth in this area. At the same time, the rapidly growing e-waste issue and its adverse socioeconomic impact have highlighted the need for sustainable electronics and a circular economy, where naturally degradable electronics, design for sustainability, and recycle and reuse are critical factors. As a result, flexible, printed, and disposable sensor technology, solution-processable nanomaterials, and novel printing techniques are likely to bring about a new wave of green technologies. Considering these developments, it became necessary to provide a forum to discuss the latest developments in the field and to bring together the academic and non-academic sectors to carefully lay the future roadmap. The IEEE International Conference on Flexible, Printable Sensors and Systems (IEEE FLEPS) provides such a forum and aims to be the premier conference in the world. The high standards of the conference are ensured by the directions set by the steering, organizing, and technical committees formed by world-leading experts in the field.

Direct Optical Lithography of CsPbX3 Nanocrystals via Photoinduced Ligand Cleavage with Postpatterning Chemical Modification and Electronic Coupling.

Microscale patterning of solution-processed nanomaterials is important for integration in functional devices. Colloidal lead halide perovskite (LHP) nanocrystals (NCs) can be particularly challenging to pattern due to their incompatibility with polar solvents and lability of surface ligands. Here, we introduce a direct photopatterning approach for LHP NCs through the binding and subsequent cleavage of a photosensitive oxime sulfonate ester (-C═N-OSOO-). The photosensitizer binds to the NCs through its sulfonate group and is cleaved at the N-O bond during photoirradiation with 405 nm light. This bond cleavage decreases the solubility of the NCs, which allows patterns to emerge upon development with toluene. Postpatterning ligand exchange results in photoluminescence quantum yields of up to 79%, while anion exchange provides tunability in the emission wavelength. The patterned NC films show photoconductive behavior, demonstrating that good electrical contact between the NCs can be established.

Accelerated Development of Colloidal Nanomaterials Enabled by Modular Microfluidic Reactors: Toward Autonomous Robotic Experimentation.

In recent years, microfluidic technologies have emerged as a powerful approach for the advanced synthesis and rapid optimization of various solution-processed nanomaterials, including semiconductor quantum dots and nanoplatelets, and metal plasmonic and reticular framework nanoparticles. These fluidic systems offer access to previously unattainable measurements and synthesis conditions at unparalleled efficiencies and sampling rates. Despite these advantages, microfluidic systems have yet to be extensively adopted by the colloidal nanomaterial community. To help bridge the gap, this progress report details the basic principles of microfluidic reactor design and performance, as well as the current state of online diagnostics and autonomous robotic experimentation strategies, toward the size, shape, and composition-controlled synthesis of various colloidal nanomaterials. By discussing the application of fluidic platforms in recent high-priority colloidal nanomaterial studies and their potential for integration with rapidly emerging artificial intelligence-based decision-making strategies, this report seeks to encourage interdisciplinary collaborations between microfluidic reactor engineers and colloidal nanomaterial chemists. Full convergence of these two research efforts offers significantly expedited and enhanced nanomaterial discovery, optimization, and manufacturing.

Guest Editorial Special Issue on Papers From the IEEE FLEPS Conference 2019

In recent years, material printing on flexible and disposable substrates has received substantial interest for realizing low cost electronic and sensing systems on large-areas. As a result, new methods such as printing and additive manufacturing, etc. have received significant attention to develop a wide range of easily deployable systems such as displays, sensors, and RFID smart tags. The technologies developed for silicon-based planar electronics and solid-state sensors is also being repurposed to meet the demands in the emerging field of flexible and printed sensors. This trend will continue as all market indicators point towards significant growth in this area. The exponential growth in flexible, printed and disposable sensor technology, the development of novel printable and solution processable nanomaterials, and the development of novel printing techniques have brought about a new wave of novel printed sensor technologies and applications. Considering this growth, it became necessary to provide a forum to discuss latest developments in the field and to bring together academia and industry to lay the future roadmap. The IEEE International Conference on Flexible and Printable Sensors and Systems (IEEE FLEPS) aims to be such a forum. The 2019 conference was the first edition, started by IEEE Sensors Council, with an aim to be a premier conference in the world. The high standards of the conference will be ensured by the steering committee and the track chairs formed by world-leading experts in the field.

MXene Composite and Coaxial Fibers with High Stretchability and Conductivity for Wearable Strain Sensing Textiles

AbstractThe integration of nanomaterials with high conductivity into stretchable polymer fibers can achieve novel functionalities such as sensing physical deformations. With a metallic conductivity that exceeds other solution‐processed nanomaterials, 2D titanium carbide MXene is an attractive material to produce conducting and stretchable fibers. Here, a scalable wet‐spinning technique is used to produce Ti3C2Tx MXene/polyurethane (PU) composite fibers that show both conductivity and high stretchability. The conductivity at a very low percolation threshold of ≈1 wt% is demonstrated, which is lower than the previously reported values for MXene‐based polymer composites. When used as a strain sensor, the MXene/PU composite fibers show a high gauge factor of ≈12900 (≈238 at 50% strain) and a large sensing strain of ≈152%. The cyclic strain sensing performance is further improved by producing fibers with MXene/PU sheath and pure PU core using a coaxial wet‐spinning process. Using a commercial‐scale knitting machine, MXene/PU fibers are knitted into a one‐piece elbow sleeve, which can track various movements of the wearer's elbow. This study establishes fundamental insights into the behavior of MXene in elastomeric composites and presents strategies to achieve MXene‐based fibers and textiles with strain sensing properties suitable for applications in health, sports, and entertainment.

Growth of quantum dot coated core-shell anisotropic nanowires for improved thermal and electronic transport

Anisotropic nanowires are promising candidates for electronic thermal management due to their unique electrical and thermal properties. However, eco-friendly solution-processed nanomaterials with an elaborate morphology and microstructure for modulating thermal and charge transfer are still a considerable challenge. Herein, we present a simple but effective approach for synthesizing pseudo core-shell nanowires through quantum dot (QD)-like nanostructure coating (p-NW@QD) to generate exceptional electron-phonon transport properties. With the assistance of diphenyl ether as a coordination solvent, high crystallinity lead sulfide NWs can be fabricated with a large aspect ratio together with uniform QD coating. This p-NW@QD exhibits high electronic mobility (30.65 cm2/Vs) as well as a diameter independent low thermal conductivity (1.53 ± 1 W/m K). Direct charge/heat carrier flow measurements and computational simulations demonstrate that the unusual electrical and thermal transport phenomenon is strongly dependent on the fast charge transport through the QD shell, and a slow phonon migration across the Umklapp process dominated NW cores. These findings indicate a significant step toward colloidal synthesis nanostructures for future high-performance nanoelectronics and thermal energy devices.

Tailoring the Porosity and Microstructure of Printed Graphene Electrodes via Polymer Phase Inversion

Phase inversion is demonstrated as an effective method for engineering the microstructure of graphene films by exploiting the well-defined solubility characteristics of polymer dispersants. Drying of a tailored phase inversion ink containing a nonvolatile nonsolvent leads to gelation and subsequent pore formation, providing a promising strategy to tailor the porosity of the resulting graphene films. Graphene films with tunable porosity and electrical conductivity ranging from ∼1000 to ∼22 000 S/m are fabricated by this method. Moreover, this dry phase inversion technique is compatible with conventional coating and printing methods, allowing direct ink writing of porous graphene microsupercapacitor electrodes for energy storage applications. Overall, this method provides a straightforward and versatile strategy for engineering the microstructure of solution-processed nanomaterials.

Hierarchical Multicomponent Nanoheterostructures via Facet-to-Facet Attachment of Anisotropic Semiconductor Nanoparticles

As performance and functionality requirements for solution-processed nanomaterials become more stringent and demanding, there is an ever-growing need for hierarchical nanostructures with sophisticated architecture and complex composition. However, the production of structurally complex nanomaterials is often not possible by direct synthesis. In this work, we describe synthetic methodology to covalently link presynthesized anisotropic semiconductor nanoparticles of different composition in a stoichiometrically controlled manner via specific facet sites at room temperature. We demonstrate that CdSe nanorods can be cojoined with CdTe tetrapods via a competitive cation-exchange process with Ag+ that results in linking between the tips of the tetrapod arms with only one end of each nanorod via a Ag2Se–Ag2Te interface. This selective linking was engineered by having a large fraction of CdSe nanorods present in the reaction, which sterically hindered homolinking between Ag2Se-tipped CdSe nanorods and Ag2Te-tippe...

Dry Sintering Meets Wet Silver-Ion "Soldering": Charge-Transfer Plasmon Engineering of Solution-Assembled Gold Nanodimers From Visible to Near-Infrared I and II Regions.

Achieving highly tunable and localized surface plasmon resonance up to near infrared (NIR) regions is a key target in nanoplasmonics. In particular, a self-assembly process capable of producing highly uniform and solution-processable nanomaterials with tailor-made plasmonic properties is lacking. We herein address this problem through a conjunctive use of wet Ag+ soldering and dry thermal sintering to produce nanodimer-derived structures with precisely engineered charge-transfer plasmon (CTP). The sintered dimers are water soluble, featuring gradually shifted CTP spanning an 800 nm wavelength range (up to NIR II). Upon silica removal, the products are grafted by DNA to offer surface functionality. This process is also adaptable to DNA-linked AuNP dimers toward plasmonic meta-materials via DNA-guided soldering and sintering.

Excitonic Lasing in Solution-Processed Subwavelength Nanosphere Assemblies.

Lasing in solution-processed nanomaterials has gained significant interest because of the potential for low-cost integrated photonic devices. Still, a key challenge is to utilize a comprehensive knowledge of the system's spectral and temporal dynamics to design low-threshold lasing devices. Here, we demonstrate intrinsic lasing (without external cavity) at low-threshold in an ultrathin film of coupled, highly crystalline nanospheres with overall thickness on the order of ∼λ/4. The cavity-free geometry consists of ∼35 nm zinc oxide nanospheres that collectively localize the in-plane emissive light fields while minimizing scattering losses, resulting in excitonic lasing with fluence thresholds at least an order of magnitude lower than previous UV-blue random and quantum-dot lasers (<75 μJ/cm(2)). Fluence-dependent effects, as quantified by subpicosecond transient spectroscopy, highlight the role of phonon-mediated processes in excitonic lasing. Subpicosecond evolution of distinct lasing modes, together with three-dimensional electromagnetic simulations, indicate a random lasing process, which is in violation of the commonly cited criteria of strong scattering from individual nanostructures and an optically thick sample. Subsequently, an electron-hole plasma mechanism is observed with increased fluence. These results suggest that coupled nanostructures with high crystallinity, fabricated by low-cost solution-processing methods, can function as viable building blocks for high-performance optoelectronics devices.

Enhanced thermoelectric properties in bulk nanowire heterostructure-based nanocomposites through minority carrier blocking.

To design superior thermoelectric materials the minority carrier blocking effect in which the unwanted bipolar transport is prevented by the interfacial energy barriers in the heterogeneous nanostructures has been theoretically proposed recently. The theory predicts an enhanced power factor and a reduced bipolar thermal conductivity for materials with a relatively low doping level, which could lead to an improvement in the thermoelectric figure of merit (ZT). Here we show the first experimental demonstration of the minority carrier blocking in lead telluride-silver telluride (PbTe-Ag2Te) nanowire heterostructure-based nanocomposites. The nanocomposites are made by sintering PbTe-Ag2Te nanowire heterostructures produced in a highly scalable solution-phase synthesis. Compared with Ag2Te nanowire-based nanocomposite produced in similar method, the PbTe-Ag2Te nanocomposite containing ∼5 atomic % PbTe exhibits enhanced Seebeck coefficient, reduced thermal conductivity, and ∼40% improved ZT, which can be well explained by the theoretical modeling based on the Boltzmann transport equations when energy barriers for both electrons and holes at the heterostructure interfaces are considered in the calculations. For this p-type PbTe-Ag2Te nanocomposite, the barriers for electrons, that is, minority carriers, are primarily responsible for the ZT enhancement. By extending this approach to other nanostructured systems, it represents a key step toward low-cost solution-processable nanomaterials without heavy doping level for high-performance thermoelectric energy harvesting.

Low temperature plasma sintering of silver nanoparticles

The fabrication of flexible electronics using the deposition of solution-processed nanomaterials generally requires low-temperature post-processing to optimize functionality. We studied sintering of silver nanoparticle (AgNP) films on glass substrates by applying argon (Ar) plasma to achieve improved electrical conductivity. This process meets the low temperature processing requirements for standard low-cost polymeric flexible substrates. The relationship between plasma parameters (such as power and sintering time) versus sintering results (such as electrical sheet resistance, sintered structure depth, materials composition variation, and film nanostructure) is reported for 23 and 77nm diameter AgNPs. In addition, plasma processing typically induces a small surface thermal effect. We monitored the surface temperatures of the AgNP films in-situ during plasma sintering. By sintering control groups at these monitored surface temperatures using a vacuum oven, we confirmed that the resistivity due to plasma sintering is less than that produced by thermal sintering. Our data show that, the measured lowest resistivities for plasma sintered AgNP films are about only 5 and 12 times greater than the bulk Ag resistivity for 23 and 77nm, respectively.

Charge Transport Dilemma of Solution-Processed Nanomaterials

A large variety of nanoparticles were synthesized during the last 25 years and are used now as “building blocks” for a variety of materials. Bottom-up solution processing of devices emerged as a promising direction of their technological applications because this method can (a) utilize intrinsic ability of nanocolloids to self-organize, (b) reduce high energy and equipment cost of device manufacturing, and (c) impart new functionalities to electronic devices. However the technological impact of solution processable semiconductor materials—although potentially considerable—has been so far limited because of the long-standing dilemma between the need for effective colloidal stabilization of nanoparticles and effective charge transport. Surfactants and other organic materials being used to synthesize and/or disperse nanocolloids introduce a barrier for charge transport between the particles. Although these barriers do make it impossible to use them in electronic devices, they certainly make it more difficult. In this review, we look into the latest progress in the solution processable devices and methods to produce electrically conductive thin films from nanoscale dispersions. We are specifically interested in the understanding of the prospects of self-assembly to facilitate charge transport and nanoscale connectivity during solution processing. The updated theoretical description of charge transport in nanoparticle solids and similar nanomaterials is also given. It includes consideration of the key mechanisms such as tunneling and cotunneling, as well as key electrical parameters characterizing transport of electrons through the surfactant-related barriers, such as coupling energy and Coulombic charging energy. Manifestations of these mechanisms in different electronic materials made from nanoparticles, nanowires, nanotubes, and nanosheets and their relative advantages and disadvantages are also discussed. We conclude the topic with a brief description of new opportunities and approaches to improve charge transport in solution processed materials from nanoscale dispersions.

Influence of the Annealing Atmosphere on the Performance of ZnO Nanowire Dye-Sensitized Solar Cells

Postsynthesis thermal treatments are key to promote crystallinity and reduce the defect density in solution-processed nanomaterials. In particular, the annealing atmosphere strongly influences the ...

Solution-processed transparent electrodes

Abstract