High-Concentration Synthesis of Sub-10-nm Copper Nanoparticles for Application to Conductive Nanoinks

Laser-Based Selective Material Processing for Next-Generation Additive Manufacturing

The connection between laser-based material processing and additive manufacturing is quite deeply rooted. In fact, the spark that started the field of additive manufacturing is the idea that two intersecting laser beams can selectively solidify a vat of resin. Ever since, laser has been accompanying the field of additive manufacturing, with its repertoire expanded from processing only photopolymer resin to virtually any material, allowing liberating customizability. As a result, additive manufacturing is expected to take an even more prominent role in the global supply chain in years to come. Herein, an overview of laser-based selective material processing is presented from various aspects: the physics of laser-material interactions, the materials currently used in additive manufacturing processes, the system configurations that enable laser-based additive manufacturing, and various functional applications of next-generation additive manufacturing. Additionally, current challenges and prospects of laser-based additive manufacturing are discussed.

Synthetic graphene-copper nanocomposites interact with the hACE-2 enzyme and inhibit its biochemical activity

This study demonstrates the copper nanocomposite-induced enzymatic inhibition of human angiotensin I-converting enzyme-2 (hACE-2) by complex stabilization through the formation of the enzyme nanocomposite. The immediate application of this work is related to ACE-2 as a mechanism of SARS-CoV-2 entry into cells. Moreover, ACE-2 enzyme regulation is a potential therapeutic strategy in hypertension and cardiovascular disease, diabetes, lung injury, and fibrotic disorders. Thus, inhibition of ACE-2 with nanocomposite therapy, may have pharmacologic application with regard to infectious and non-infectious diseases. Synthesized copper nanocomposites described here alone with a commercially available compound, were tested for their potential to inhibit hACE-2 activities. Following wet chemical synthesis, Cu/CuO nanoparticles and graphene-copper (GO-Cu) complexes were synthesized and characterized for their chemical integrity. Cu/CuO formed well-dispersed clusters of 390 ± 100 nm, that when complexed with the hACE-2 enzyme exhibited larger clusters of 506 ± 56 nm. The formation of the Cu/CuO and hACE-2 enzyme complex was monitored by analyzing the zeta potential, which reflected the surface charge distribution of the complex. A negatively charged Cu/CuO nanocomposite nearly becomes neutral when complexed with hACE-2 further assuring the complex formation. Formation of this complex and its inactivation of hACE-2 was evaluated using a standardized protocal for enzymatic activity. Similarly, carboxylate-functionalized graphene was complexed with copper, and its inhibitory effect was studied. Each step in the GO-Cu composite formation was monitored by characterizing its surface electrical properties, resulting in a decrease in its zeta potential and conductivity when complexed with copper. The interaction of the nanocomposites with hACE-2 was confirmed by 2D-FDS and gel electrophoresis analysis. GO-Cu was a rapid and efficacious inhibitor compared to Cu-CuO, especially at lower concentrations (2 μg ml-1). Considering the environmental friendliness of copper and graphene and their use in industries as surface coating materials, we anticipate that use of these composites once proven effective, may have future antimicrobial application. Utility of nanocomposites as antimicrobials, either as a surface antimicrobial or as an in vivo therapeutic, could be invisioned for use against current unknown and/or emergent pathogens.

Copper inks for printed electronics: a review

Conductive inks have attracted tremendous attention owing to their adaptability and the convenient large-scale fabrication. As a new type of conductive ink, copper-based ink is considered to be one of the best candidate materials for the conductive layer in flexible printed electronics owing to its high conductivity and low price, and suitability for large-scale manufacturing processes. Recently, tremendous progress has been made in the preparation of cooper-based inks for electronic applications, but the antioxidation ability of copper-based nanomaterials within inks or films, that is, long-term reliability upon exposure to water and oxygen, still needs more exploration. In this review, we present a comprehensive overview of copper inks for printed electronics from ink preparation, printing methods and sintering, to antioxidation strategies and electronic applications. The review begins with an overview of the development of copper inks, followed by a demonstration of various preparation methods for copper inks. Then, the diverse printing techniques and post-annealing strategies used to fabricate conductive copper patterns are discussed. In addition, antioxidation strategies utilized to stabilize the mechanical and electrical properties of copper nanomaterials are summarized. Then the diverse applications of copper inks for electronic devices, such as transparent conductive electrodes, sensors, optoelectronic devices, and thin-film transistors, are discussed. Finally, the future development of copper-based inks and the challenges of their application in printed electronics are discussed.

A mild aqueous synthesis of ligand-free copper nanoparticles for low temperature sintering nanopastes with nickel salt assistance

An organic ligand-free aqueous-phase synthesis of copper (Cu) nanoparticles (NPs) under an air atmosphere was successfully achieved by reducing copper(II) oxide particles with a leaf-like shape in the presence of Ni salts at room temperature. The resulting Cu NPs with a mean particle diameter of ca. 150 nm exhibited low-temperature sintering properties due to their polycrystalline internal structure and ligand-free surface. These Cu NPs were applied to obtain Cu NP-based nanopastes with low-temperature sintering properties, and the resistivities of the obtained Cu electrodes after annealing at 150 °C and 200 °C for 30 min were 64 μΩ∙cm and 27 μΩ∙cm, respectively. The bonding strength between oxygen-free Cu plates prepared using the Cu NP-based nanopastes reached 32 MPa after pressure-less sintering at 260 °C for 30 min under a nitrogen atmosphere. The developed manufacturing processes using the developed Cu nanopastes could provide sustainable and low-CO₂-emission approaches to obtain Cu electrodes on flexible films and high-strength bonding between metal plates as die-attach materials for power devices under energy- and resource-saving conditions.

Large-Scale Synthesis of Hybrid Conductive Polymer-Gold Nanoparticles Using "Sacrificial" Weakly Binding Ligands for Printing Electronics

We describe the gram-scale synthesis of hybrid gold nanoparticles with a shell of conductive polymers. A large-scale synthesis of hexadecyltrimethylammonium bromide (CTAB)-capped gold nanoparticles (AuNP@CTAB) was followed by ligand exchange with conductive polymers based on thiophene in a 10 L reactor equipped with a jacket to ensure a constant temperature of 40 °C and a mechanical stirrer. Slow and controlled reduction of the gold precursors and the presence of small amounts of silver nitrate are revealed to be the critical synthesis variables to obtain particles with a sufficiently narrow size distribution. Batches of approximately 10 g of faceted AuNP@CTAB with tunable average particle sizes from 54 to 85 nm were obtained per batch. Ligand exchange with poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) in the same reactor then yielded hybrid Au@PEDOT:PSS nanoparticles. They were used to formulate sinter-free inks for the inkjet printing of conductive structures without the need for a sintering step.

MODs vs. NPs: Vying for the Future of Printed Electronics

This Minireview compares two distinct ink types, namely metal-organic decomposition (MOD) and nanoparticle (NP) formulations, for use in the printing of some of the most conductive elements: silver, copper and aluminium. Printing of highly conductive features has found purpose across a broad array of electronics and as processing times and temperatures reduce, the avenues of application expand to low-cost flexible substrates, materials for wearable devices and beyond. Printing techniques such as screen, aerosol jet and inkjet printing are scalable, solution-based processes that historically have employed NP formulations to achieve low resistivity coatings printed at high resolution. Since the turn of the century, the rise in MOD inks has vastly extended the range of potentially applicable compounds that can be printed, whilst simultaneously addressing shelf life and sintering issues. A brief introduction to the field and requirements of an ink will be presented followed by a detailed discussion of a wide array of synthetic routes to both MOD and NP inks. Unindustrialized materials will be discussed, with the challenges and outlook considered for the market leaders: silver and copper, in comparison with the emerging field of aluminium inks.

Water-Soluble Copper Ink for the Inkjet Fabrication of Flexible Electronic Components

The aim of this work is preparation and investigation of copper conductive paths by printing with a different type of functional ink. The solutions based on copper-containing complex compounds were used as inks instead of dispersions of metal nanoparticles. Thermal characteristics of synthesized precursors were studied by thermogravimetry in an argon atmosphere. Based on the comparison of decomposition temperature, the dimethylamine complex of copper formate was found to be more suitable precursor for the formation of copper layers. Structure and performance of this compound was studied in detail by X-ray diffraction, test of wettability, printing on flexible substrate, and electrical measurements.

Comparative Determination of Cytotoxicity of Sub-10 nm Copper Nanoparticles to Prokaryotic and Eukaryotic Systems

Copper nanoparticles demonstrate antibacterial activity, but their toxicity to eukaryotic systems is less understood. Here, we carried out a comparative study to determine the biocompatibility and cytotoxicity of sub-10 nm copper nanoparticles to a variety of biological systems, including prokaryotic cells (Escherichia coli), yeast, mammalian cell lines (HEK293T, PC12), and zebrafish embryos. We determined the bearing threshold for the cell-death-inducing concentration of copper nanoparticles by probing cell growth, viability, as well as embryological features. To exclude the partial toxicity effect from the remnant reactants, we developed a purification approach using agarose gel electrophoresis. Purified CuONP solution inhibits bacterial growth and causes eukaryotic cell death at 170 and 122.5 ppm (w/w) during the 18 h of treatment, respectively. CuONP significantly reduces the pigmentation of retina pigmented epithelium of zebrafish embryos at 85 ppm. The cytotoxicity of CuONP in eukaryotic cells could arise from the oxidative stress induced by CuONP. This result suggests that small copper nanoparticles exert cytotoxicity in both prokaryotic and eukaryotic systems, and therefore, caution should be used to avoid direct contact of copper nanoparticles to human tissues considering the potential use of copper nanoparticles in the clinical setting.

Surface and Interface Designs in Copper-Based Conductive Inks for Printed/Flexible Electronics

Silver (Ag), gold (Au), and copper (Cu) have been utilized as metals for fabricating metal-based inks/pastes for printed/flexible electronics. Among them, Cu is the most promising candidate for metal-based inks/pastes. Cu has high intrinsic electrical/thermal conductivity, which is more cost-effective and abundant, as compared to Ag. Moreover, the migration tendency of Cu is less than that of Ag. Thus, recently, Cu-based inks/pastes have gained increasing attention as conductive inks/pastes for printed/flexible electronics. However, the disadvantages of Cu-based inks/pastes are their instability against oxidation under an ambient condition and tendency to form insulating layers of Cu oxide, such as cuprous oxide (Cu2O) and cupric oxide (CuO). The formation of the Cu oxidation causes a low conductivity in sintered Cu films and interferes with the sintering of Cu particles. In this review, we summarize the surface and interface designs for Cu-based conductive inks/pastes, in which the strategies for the oxidation resistance of Cu and low-temperature sintering are applied to produce highly conductive Cu patterns/electrodes on flexible substrates. First, we classify the Cu-based inks/pastes and briefly describe the surface oxidation behaviors of Cu. Next, we describe various surface control approaches for Cu-based inks/pastes to achieve both the oxidation resistance and low-temperature sintering to produce highly conductive Cu patterns/electrodes on flexible substrates. These surface control approaches include surface designs by polymers, small ligands, core-shell structures, and surface activation. Recently developed Cu-based mixed inks/pastes are also described, and the synergy effect in the mixed inks/pastes offers improved performances compared with the single use of each component. Finally, we offer our perspectives on Cu-based inks/pastes for future efforts.

Green synthesis of copper oxide nanoparticles using sinapic acid: an underpinning step towards antiangiogenic therapy for breast cancer

Synthesis of copper oxide nanoparticles without any chemical reductant is always a challenging methodology for biological studies. In this study, sinapic acid, a phytochemical, is used for the synthesis of stable copper oxide nanoparticles. The as-synthesized nanoparticles were characterized thoroughly using UV-Visible, IR spectroscopy, Transmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Nanoparticles collected during different time intervals of synthesis (60,120 and 180 min) were subjected for analysis, where the occurrence of copper oxide nanoparticles with substantial morphology was seen at 180 min. Further, nanoparticles synthesized at 120 and 180 min were studied for their potential biological applications. These copper oxide nanoparticles evinced potential cytotoxic effects on breast cancer cells, MCF7 and MDA-MB231. Supplementarily, it also exhibited anti-angiogenic effect on endothelial cells (EA.hy926), thus confirming its potential to inhibit angiogenesis in cancer.

Synthesis and characterization of DGEBA composites reinforced with Cu/Ag modified carbon nanotubes

Carbon nanotubes (CNTs) are among the strongest and stiffest contender to be used as filler to elevate the properties of epoxy. The aim of this research work is to evaluate the structural, thermal, and morphological properties of multiwalled carbon nanotubes (MWCNTs) hybridized with silver, copper and silver/copper nanoparticles (Ag/CuNP) obtained via chemical reduction of aqueous salts assisted with sodium dodecyl sulphate (SDS) as stabilizing agent. The MWCNTs/NP was further incorporated in DGEBA (epoxy) using ethyl cellulose as hardener. Scanning electron microscopy (SEM) reveals micro structural analysis of the MWCNTs/NP hybrids. The Fourier transform infrared (FTIR) spectra prove the interactions between the NP and MWCNTs. Thermogravimetric analysis (TGA) shows that the MWCNTs/NP hybrids decompose at a much faster rate and the weight loss decreased considerably due to the presence of NP. X-ray diffraction (XRD) confirms the formation of NP on the surface of MWCNTs and X-ray photoelectron spectroscopy (XPS) confirms the full covering of MWCNTs/NP hybrids with DGEBA.

Ambient Aqueous-Phase Synthesis of Copper Nanoparticles and Nanopastes with Low-Temperature Sintering and Ultra-High Bonding Abilities

Copper nanoparticles (NPs) with an average particle diameter of 50–60 nm were successfully obtained by reducing an aqueous solution of a copper(II)-nitrilotriacetic acid complex with an aqueous hydrazine solution at room temperature under an air atmosphere. Copper NP-based nanopastes were printed onto a glass substrate using a metal screen mask and pressureless sintered under a nitrogen atmosphere at 200 °C for 30 min. The electrical resistivity of the resulting copper electrode was 16 μΩ · cm. For a metal-to-metal bonding test, copper nanopaste was printed on an oxygen-free copper plate, another oxygen-free copper plate was placed on top, and the bonding strength between the copper plates when pressureless sintered under a nitrogen atmosphere at 200 °C for 30 min was 39 MPa. TEM observations confirmed that highly crystalline metal bonding occurred between the copper NPs and the copper plate to introduce the ultrahigh strength. The developed copper NPs could provide promising advances as nanopastes for sustainable fabrication of copper electrodes and die attachment materials for the production of next-generation power semiconductors.

Aqueous synthesis of protectant-free copper nanocubes by a disproportionation reaction of Cu2O on synthetic saponite

Here, we report a synthesis of Cu nanocubes by photoreduction of CuSO4. Because synthetic saponite (one of the layered clay minerals) was used as the adsorbent, the nanocubes contained no capping agents or protectants, and the disproportionation reaction of Cu2O with H2SO4 was found to be the key for morphological control.

Zinc Oxide-Supported Copper Clusters with High Biocidal Efficacy for Escherichia coli and Bacillus cereus

Cu clusters on ZnO have been prepared by a simple low-temperature solid-state reaction from their respective acetate precursors. The formation of metallic Cu along with a small quantity of CuO was influenced by the presence of the zinc acetate precursor. Although there is a lack of formation of any metallic Cu in the absence of zinc acetate, increase in the heating duration helps in the formation of increased metallic Cu. A mechanism for formation of the Cu@ZnO nanocomposite has been suggested. The prepared Cu@ZnO nanocomposite, with metallic Cu, was identified by X-ray diffraction studies followed by confirmation of clusters of the kind Cu9 and Cu18 by transmission electron microscopy and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The photoelectron spectroscopy is able to clearly distinguish the Cu from CuO, which is very well complimented by electron spin resonance analysis. The morphological feature of ZnO changes from flakes to rods on increasing the duration of heating, as shown by scanning electron microscopy (SEM) analysis. The observed Cu plasmonic band in UV-vis diffuse reflectance gets blue-shifted to 463 nm from its normally observed position of 550-580 nm possibly due to cluster formation and interaction with ZnO, the band gap of the latter getting red-shifted to 3.2-3.0 eV. The antibacterial activity of the synthesized Cu cluster-ZnO nanocomposites was investigated against Escherichia coli ATCC-25922 for Gram-negative and Bacillus cereus ATCC-10876 for Gram-positive bacteria. Tests were performed on a nutrient agar medium and liquid broth supplemented with different concentrations of nanoparticles. SEM analysis of the native and treated Gram-positive and Gram-negative bacteria established a high efficacy of biocide activity in 24 h, with 200 μg/mL of Cu@ZnO nanocomposites.

Fabrication of Conductive Copper Films on Flexible Polymer Substrates by Low-Temperature Sintering of Composite Cu Ink in Air

The development of a thermal sintering method for Cu-based inks under an air atmosphere could greatly expand their application for printed electronics. However, it is well-known that Cu-based inks cannot produce conductive Cu films when sintered at low temperatures in air because Cu readily oxidizes under such conditions. In this study, we have successfully demonstrated air atmosphere sintering at low temperatures (less than 150 °C) via a simple hot plate heat treatment for producing conductive Cu films on flexible polymer substrates, using a novel Cu-based composite ink with sub-10 nm Cu nanoparticles protected with 1-amino-2-propanol with micrometer-sized Cu particles and submicrometer-sized Cu particles; oxalic acid was also added to prevent the oxidation of the Cu during sintering. The Cu films showed a minimum resistivity of 5.5 × 10^-5 Ω·cm when sintered in air at 150 °C for a very short period of 10 s. To the best of our knowledge, this is the first report of sintering of Cu-based inks in air at less than 150 °C. Another novel property of the present Cu-based composite ink is the lowest reported resistivity at 80 °C under N₂ flow (5.3 × 10^-5 Ω·cm at 80 °C and 8.4 × 10^-6 Ω·cm at 120 °C). This fast, efficient, and inexpensive technology for thermal sintering in ambient air using composite inks could be a commercially viable method for fabricating printed electronics on flexible substrates.

Synergetic Effect of Blended Alkylamines for Copper Complex Ink To Form Conductive Copper Films

Cu(II) complex ink consisting of copper formate (Cuf) and a primary alkylamine could yield highly conductive copper films at low heating temperatures without a reducing atmosphere. A synergetic effect of the blended alkylamines on the formation of conductive films was observed. It was found that blending two types of amines with different alkyl chain lengths as ligands could improve the conductivity of copper films, compared with using one of these amines alone. The decomposition mechanism of the Cuf-amine complex and the role of amines with different alkyl chain lengths were investigated. It was found that the decrease in the decomposition temperature and the formation of copper films were attributed to the activating effect and capping effect of the amine, and these two effects were dependent on the alkyl chain length. The relative intensity of the dual effects determined the decomposition rate of the complex and the nucleation and growth of particles. The use of blended amines with different alkyl chain lengths as ligands could balance the two effects and lead to appropriate nucleation and growth rates, so that densely packed copper films with low resistivity could be obtained at low heating temperature in a short time. The Cuf-butylamine-octylamine (Cuf-butyl-octyl) ink with 1:1 molar ratio of the amines showed the best performance. The understanding of the synergetic effect could provide guidance to the design of copper complex inks to control the morphology of the films.

Low temperature sintering process of copper fine particles under nitrogen gas flow with Cu2+-alkanolamine metallacycle compounds for electrically conductive layer formation

A novel low cost sintering process of copper fine particles to a copper conductive layer with Cu²⁺-alkanolamine metallacycle complexes at as low as 100 °C without reductive gas flow.