Ligand-Dependent Nanoparticle Assembly and Its Impact on the Printing of Transparent Electrodes

Silver Nanocube Epitaxy via Nanogap-Induced Electrostatics

Silver nanostructures are highly valued in nanophotonic devices due to their appealing plasmonic properties and affordability relative to gold. Yet, fabricating high-quality, monocrystalline silver nanostructures, with full control over the shape, is challenging. A mild, liquid-phase method for the epitaxial welding of adjacent monocrystalline silver nanocubes in reductant-free conditions is introduced to prevent the formation of detrimental nuclei on the surface that can degrade the nanostructures' optical qualities. The mechanism is thoroughly investigated and it is found that the nanocubes themselves can act as reducing agents, promoting growth preferentially into the gap as a result of electrostatic interactions. By controlling experimental parameters such as temperature, pH, and the introduction of capping agents, a balance between nanocube epitaxy and shape retention is achieved. Finally, by applying this procedure to nanoparticle assembled in predefined meta-atoms, the feasibility of creating intricate silver nanostructures, that are monocrystalline as verified by transmission electron microscopy (TEM), is demonstrated. This advancement paves the way for bottom-up fabrication of optical metasurfaces that can be swiftly integrated in devices.

Coordination bonds as a tool for tuning photoconductance in nanostructured hybrid materials made of molecular antennas and metal nanoparticles

The synthesis of robust, versatile materials in which electrical conduction is enhanced by light irradiation is of prime importance for fields as varied as photodetectors, photodiodes, solar cells and light sensors. Hybrid materials offer the advantage of combining the robustness of an inorganic building block with the adaptability of a molecular subunit. Herein, we demonstrate the importance of properly investigating the nature of the chemical interactions between the constituent elements in order to optimize photoconductance within hybrid materials. To this end, platinum nanoparticle self-assemblies are synthesized in solution, including a series of zinc-porphyrins differentially functionalized with pyridine moieties in the meso position. The presence of coordinating groups on the molecular entities drastically reinforced both the structural cohesion of the system and its photoconductive properties.

Ligands of Nanoparticles and Their Influence on the Morphologies of Nanoparticle-Based Films

Nanoparticle-based thin films are increasingly being used in various applications. One of the key factors that determines the properties and performances of these films is the type of ligands attached to the nanoparticle surfaces. While long-chain surfactants, such as oleic acid, are commonly employed to stabilize nanoparticles and ensure high monodispersity, these ligands often hinder charge transport due to their insulating nature. Although thermal annealing can remove the long-chain ligands, the removal process often introduces defects such as cracks and voids. In contrast, the use of short-chain organic or inorganic ligands can minimize interparticle distance, improving film conductivity, though challenges such as incomplete ligand exchange and residual barriers remain. Polymeric ligands, especially block copolymers, can also be employed to create films with tailored porosity. This review discusses the effects of various ligand types on the morphology and performance of nanoparticle-based films, highlighting the trade-offs between conductivity, structural integrity, and functionality.

Conductive Nanosheets Fabricated from Au Nanoparticles on Aqueous Metal Solutions under UV Irradiation

Highly transparent, conductive nanosheets are extremely attractive for advanced opto-electronic applications. Previously, we have demonstrated that transparent, conductive Au nanosheets can be prepared by UV irradiation of Au nanoparticle (AuNP) monolayers spread on water, which serves as the subphase. However, thick Au nanosheets cannot be fabricated because the method is not applicable to large Au NPs. Further, in order to fabricate nanosheets with different thicknesses and compositions, it is necessary to prepare the appropriate NPs. A strategy is needed to produce nanosheets with different thicknesses and compositions from a single type of metal NP monolayer. In this study, we show that this UV irradiation technique can easily be extended as a nanosheet modification method by using subphases containing metal ions. UV irradiation of 4.7 nm AuNP monolayers on 480 µM HAuCl4 solution increased the thickness of Au nanosheets from 3.5 nm to 36.5 nm, which improved conductivity, but reduced transparency. On the other hand, the use of aqueous AgNO3 and CH3COOAg solutions yielded Au-Ag hybrid nanosheets; however, their morphologies depended on the electrolytes used. In Au-Ag nanosheets prepared on aqueous 500 µM AgNO3, Au and Ag metals are homogeneously distributed throughout the nanosheet. On the other hand, in Au-Ag nanosheets prepared on aqueous 500 µM CH3COOAg, AuNPs still remained and these AuNPs were covered with a Ag nanosheet. Further, these Au-Ag hybrid nanosheets had high conductivity without reduced transparency. Therefore, this UV irradiation method, modified by adding metal ions, is quite effective at improving and diversifying properties of Au nanosheets.

Enhancing Conductivity of Silver Nanowire Networks through Surface Engineering Using Bidentate Rigid Ligands

Solution processable metallic nanomaterials present a convenient way to fabricate conductive structures, which are necessary in all electronic devices. However, they tend to require post-treatments to remove the bulky ligands around them to achieve high conductivity. In this work, we present a method to formulate a post-treatment free conductive silver nanowire ink by controlling the type of ligands around the silver nanowires. We found that bidentate ligands with a rigid molecular structure were effective in improving the conductivity of the silver nanowire networks as they could maximize the number of linkages between neighboring nanowires. In addition, DFT calculations also revealed that ligands with good LUMO to silver energy alignment were more effective. Because of these reasons, fumaric acid was found to be the most effective ligand and achieved a large reduction in sheet resistance of 70% or higher depending on the nanowire network density. The concepts elucidated from this study would also be applicable to other solution processable nanomaterials systems such as quantum dots for photovoltaics or LEDs which also require good charge transport being neighboring nanoparticles.

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

Consolidation and performance gains in plasma-sintered printed nanoelectrodes

We report on the unusual, advantageous ageing of flexible transparent electrodes (FTEs) that were self-assembled from oleylamine-capped gold nanospheres (AuNPs) by direct nanoimprinting of inks with different particle concentrations (cAu = 3 mg mL-1 to 30 mg mL-1). The resulting lines were less than 2.5 μm wide and consisted of disordered particle assemblies. Small-Angle X-ray Scattering confirmed that particle packing did not change with ink concentration. Plasma sintering converted the printed structures into lines with a thin, electrically conductive metal shell and a less conductive hybrid core. We studied the opto-electronic performance directly after plasma sintering and after fourteen days of storage at 22 °C and 55% rH in the dark. The mean optical transmittance T̄400-800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance Rsh strongly decreased by up to ≈ 82% at all concentrations. We correlated the changes with morphological changes visible in scanning and transmission electron microscopy and identified two sequential ageing stages: (I) post-plasma relaxation effects in and consolidation of the shell, and (II) particle re-organization, de-mixing, coarsening, and densification of the core with plating of Au from the core onto the shell, followed by solid-state de-wetting (ink concentrations cAu < 15 mg mL-1) or stability (cAu ≥ 15 mg mL-1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = T̄400-800·Rsh-1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ωsq-1 reached after 7 days of ageing at cAu = 30 mg mL-1.

In Situ Self-Assembly of Nanoscale Particles into Macroscale Ordered Monolayers with Enhanced Memory Performance

In situ fabrication of macroscale ordered monolayers of nanoparticles (NPs) on targeted substrates is highly desirable for precision electronic and optical devices, while it remains a great challenge. In this study, a solution is provided to address this challenge by developing a colloidal ink formulation and employing the direct-ink-writing (DIW) technique, where on-demand delivery of ink at a targeted location and directional evaporation with controllable rate are leveraged to precisely guide the deposition of polystyrene-grafted gold NPs (Au@PS NPs) into a macroscale monolayer with an ordered Au NP array embedded in a PS thin film. A 2D steady-state diffusion-controlled evaporation model, which explains the parameter dependence of the experimental results and gives semiquantitative agreement with the experimental evaporation kinetics is proposed. The ordered monolayer is used as both nanocrystal floating gates and the tunneling layer for nonvolatile memory devices. It shows significantly enhanced performance compared with a disordered NP film prepared by spin coating. This approach allows for fine control of NP self-assembly to print macroscaleordered monolayers directly onto substrates, which has great promise for application in broad fields, including microelectronic and photoelectronic devices, sensors, and functional coatings.

Flexible and transparent electrodes imprinted from metal nanostructures: morphology and opto-electronic performance

We directed the self-assembly of nanoscale colloids via direct nanoimprint lithography to create flexible transparent electrodes (FTEs) with metal line widths below 3 μm in a roll-to-roll-compatible process. Gold nanowires and nanospheres with oleylamine shells were imprinted with soft silicone stamps, arranged into grids of parallel lines, and converted into metal lines in a plasma process. We studied the hierarchical structure and opto-electronic performance of the resulting grids as a function of particle geometry and concentration. The performance in terms of optical transmittance was dominated by the line width. Analysis of cross-sections indicated that plasma sintering only partially removed the insulating ligands and formed lines with thin conductive shells and a non-conductive core. We provide evidence that the self-assembly of high-aspect nanowires can compensate for defects of the stamp and substrate irregularities during imprinting, while spheres cannot. The wire-based electrodes thus outperformed the sphere-based electrodes at ratios of optical transmittance to sheet resistance of up to ≈ 0.9% Ω_sq ^-1, while spheres only reached ≈ 0.55% Ω_sq ^-1.

Nanosphere Lithography: A Versatile Approach to Develop Transparent Conductive Films for Optoelectronic Applications

Transparent conductive films (TCFs) are irreplaceable components in most optoelectronic applications such as solar cells, organic light-emitting diodes, sensors, smart windows, and bioelectronics. The shortcomings of existing traditional transparent conductors demand the development of new material systems that are both transparent and electrically conductive, with variable functionality to meet the requirements of new generation optoelectronic devices. In this respect, TCFs with periodic or irregular nanomesh structures have recently emerged as promising candidates, which possess superior mechanical properties in comparison with conventional metal oxide TCFs. Among the methods for nanomesh TCFs fabrication, nanosphere lithography (NSL) has proven to be a versatile platform, with which a wide range of morphologically distinct nanomesh TCFs have been demonstrated. These materials are not only functionally diverse, but also have advantages in terms of device compatibility. This review provides a comprehensive description of the NSL process and its most relevant derivatives to fabricate nanomesh TCFs. The structure-property relationships of these materials are elaborated and an overview of their application in different technologies across disciplines related to optoelectronics is given. It is concluded with a perspective on current shortcomings and future directions to further advance the field.

Hybrid Dielectric Films of Inkjet-Printable Core-Shell Nanoparticles

A new type of hybrid core-shell nanoparticle dielectric that is suitable for inkjet printing is introduced. Gold cores (dcore  ≈ 4.5 nm diameter) are covalently grafted with thiol-terminated polystyrene (Mn  = 11000 Da and Mn  = 5000 Da) and used as inks to spin-coat and inkjet-print dielectric films. The dielectric layers have metal volume fractions of 5 to 21 vol% with either random or face-centered-cubic structures depending on the polymer length and grafting density. Films with 21 vol% metal have dielectric constants of 50@1 Hz. Structural and electrical characterization using transmission electron microscopy, small-angle X-ray scattering, and impedance spectroscopy indicates that classical random capacitor-resistor network models partially describe this hybrid material but fail at high metal fractions, where the covalently attached shell prevents percolation and ensures high dielectric constants without the risk of dielectric breakdown. A comparison of disordered to ordered films indicates that the network structure affects dielectric properties less than the metal content. The applicability of the new dielectric material is demonstrated by formulating inkjet inks and printing devices. An inkjet-printed capacitor with an area of 0.79 mm2 and a 17 nm thick dielectric had a capacitance of 2.2 ± 0.1   n F @ 1   k H z .

Conductive nanosheets produced by UV irradiation of a Ag nanoparticle monolayer at the air-water interface

In a previous study, we demonstrated that conductive Au nanosheets can be prepared by UV irradiation of an Au nanoparticle monolayer spreading on water. In this study, we applied this UV irradiation technique to inexpensive Ag nanoparticles (NPs) to expand their versatility. UV irradiation of Ag NPs on water resulted in the formation of large Ag NPs and was ineffective for preparing conductive Ag films. The solubilization of additives in the water phase, however, resulted in the conversion of the large Ag NPs into a nanosheet, and the solubilization method was highly effective for preparing transparent conductive Ag films with an optical transmittance of above 70%.

Influence of core size and capping ligand of gold nanoparticles on the desorption/ionization efficiency of small biomolecules in AP-SALDI-MS

Gold nanoparticles (AuNP) are frequently used in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) for analysis of biomolecules because they exhibit suitable thermal and chemical properties as well as strong surface plasmonic effects. Moreover, the structures of AuNP can be controlled by well-established synthesis protocols. This was important in the present work, which studied the influence of the nanoparticles' structures on atmospheric pressure (AP)-SALDI-MS performance. A series of AuNP with different core sizes and capping ligands were investigated, to examine the desorption/ionization efficiency (DIE) under AP-SALDI conditions. The results showed that both the AuNP core size as well as the nature of the surface ligand had a strong influence on DIE. DIE increased with the size of the AuNP and the hydrophobicity of the ligands. Chemical interactions between ligand and analytes also influenced DIE. Moreover, we discovered that removing the organic ligands from the deposited AuNP substrate layer by simple laser irradiation prior to LDI further amplified DIE values. The optimized AuNP were successfully used to analyze a wide arrange of different low molecular weight biomolecules as well as a crude pig brain extract, which readily demonstrated the ability of the technique to detect a wide range of lipid species within highly complex samples.

Fabrication of Flexible and Transparent Conductive Nanosheets by the UV-Irradiation of Gold Nanoparticle Monolayers

Conductive films that are highly transparent and flexible are extremely attractive for emerging optoelectronic applications. Currently, indium-doped tin oxide films are the most widely used transparent conductive films and much research effort is devoted to developing alternative transparent conductive materials to overcome their drawbacks. In this work, a novel and facile approach for fabricating transparent conductive Au nanosheets from Au nanoparticles (AuNPs) is proposed. Irradiating an AuNP monolayer at the air-water interface with UV light results in a nanosheet with ≈3.5 nm thickness and ≈80% transparency in the UV-visible region. Further, the so-fabricated nanosheets are highly flexible and can maintain their electrical conductivity even when they are bent to a radius of curvature of 0.6 mm. Fourier-transform infrared and X-ray photoelectron spectroscopy characterizations reveal that the transformation of the monolayer of AuNPs into the nanosheet is induced by the photodecomposition and/or photodetachment of the dodecanethiol ligands capping the AuNPs. Further, the UV-irradiation of a hybrid monolayer consisting of AuNPs and silica particles affords the patterning of Au nanosheets with periodic hole arrays.

Quantifying Strain and Dislocation Density at Nanocube Interfaces after Assembly and Epitaxy

Nanoparticle self-assembly and epitaxy are utilized extensively to make 1D and 2D structures with complex shapes. High-resolution transmission electron microscopy (HRTEM) has shown that single-crystalline interfaces can form, but little is known about the strain and dislocations at these interfaces. Such information is critically important for applications: drastically reducing dislocation density was the key breakthrough enabling widespread implementation of light-emitting diodes, while strain engineering has been fundamental to modern high-performance transistors, solar cells, and thermoelectrics. In this work, the interfacial defect and strain formation after self-assembly and room temperature epitaxy of 7 nm Pd nanocubes capped with polyvinylpyrrolidone (PVP) is examined. It is observed that, during ligand removal, the cubes move over large distances on the substrate, leading to both spontaneous self-assembly and epitaxy to form single crystals. Subsequently, atomically resolved images are used to quantify the strain and dislocation density at the epitaxial interfaces between cubes with different lateral and angular misorientations. It is shown that dislocation- and strain-free interfaces form when the nanocubes align parallel to each other. Angular misalignment between adjacent cubes does not necessarily lead to grain boundaries but does cause dislocations, with higher densities associated with larger rotations.

Colloidal Solubility and Agglomeration of Apolar Nanoparticles in Different Solvents

We studied the concentration-dependent agglomeration of apolar nanoparticles in different solvents. Octanethiol-stabilized gold nanoparticles (AuNPs) in evaporating liquid droplets were observed in situ using small-angle X-ray scattering. Concurrent analysis of liquid volume and particle agglomeration provided time-dependent absolute concentrations of free and agglomerated particles. All dispersions underwent an initial stage where the particle concentration increased but no agglomerates formed. Subsequently, agglomeration started at concentrations that varied by several orders of magnitude for different solvents. While agglomerates grew, the concentration of the dispersed particles remained at a constant "colloidal solubility" in most solvents. We consistently found that the colloidal stability of AuNPs decreased as cyclohexane > heptane > nonane > decane > toluene and suggest that details of the molecular interactions between solvent and ligand shell set this order.

Micro-/Nanofluidics for Liquid-Mediated Patterning of Hybrid-Scale Material Structures

Various materials are fabricated to form specific structures/patterns at the micro-/nanoscale, which exhibit additional functions and performance. Recent liquid-mediated fabrication methods utilizing bottom-up approaches benefit from micro-/nanofluidic technologies that provide a high controllability for manipulating fluids containing various solutes, suspensions, and building blocks at the microscale and/or nanoscale. Here, the state-of-the-art micro-/nanofluidic approaches are discussed, which facilitate the liquid-mediated patterning of various hybrid-scale material structures, thereby showing many additional advantages in cost, labor, resolution, and throughput. Such systems are categorized here according to three representative forms defined by the degree of the free-fluid-fluid interface: free, semiconfined, and fully confined forms. The micro-/nanofluidic methods for each form are discussed, followed by recent examples of their applications. To close, the remaining issues and potential applications are summarized.