Silver nanocolloid generation using dynamic Laser Ablation Synthesis in Solution system and drop-casting
Graphical abstract
Introduction
Conductive inks have been extensively investigated in recent years due to their popularity in flexible electronics and printing technologies [1]. There has been a growing demand for highly conductive printable tracks in various fields, including flexible displays, wearable devices, smart packaging, thin-film transistors, smart textiles and solar cells [1], [2]. Printed electronics (PEs) offer several attractive characteristics, including lower costs, optical transparency, light weight and increased production of devices on different flexible substrates, including polymers, paper, glass etc., at large scales [1]. Due to their application in flexible PEs, the global market for conductive inks has grown significantly in recent years. Estimates predict that the conductive ink market will reach 3.7 billion USD by 2025, up from 3.0 billion USD in 2020 [3]. The main element of conductive inks are the functional materials, for example, metal nanoparticles (NPs) [4], organic/metallic compounds [5], carbon [6], conductive polymers [7], and metal precursors [8]. Metal NPs (e.g., copper (Cu) [9], gold (Au) [10] and silver (Ag) [11], [12], [13]) are most commonly used in conductive inks since their resistivity is comparable to that of the bulk material. These conductive particles are suspended in a liquid medium, allowing them to act as an ink, being a liquid at room temperature. This provides ease of handling and printing compared with molten metal, and the inks can form conductive patterns when deposited and dried.
Chemical and physical methods are commonly used to generate NPs. Evaporation condensation and laser ablation are two important physical processes that are implemented. Laser Ablation Synthesis in Solution (LASiS) is a physical technique that synthesizes NPs in liquid media [15], [16], [17], [18], [19], [20], [21], [22] using laser energy, unlike wet-chemical synthesis methods which use environmentally harmful reducing agents. The process involves a pulsed laser focused on the surface of a solid target in a liquid environment, ablating material from the target and producing ligand free NPs. In 1987, Patil et al. first explored material synthesis at a solid–liquid interface using a pulsed laser (ruby laser with a pulse width of 30 ns) by generating metastable iron oxide from an iron substrate in water [23]. Later in 1993, Neddersen et al. reported the generation of organic solvent and water based stable colloids, without any ionic or organic species, from metal targets such as Cu, Pt, Pd, Au and Ag via laser ablation [24]. The LASiS technique has been used for the generation of highly pure nanostructures in aqueous solutions at room temperature and normal pressure conditions since 2000 [14], [25], [26], [27], [28] and several NP colloids of metals (e.g., Ag [29], Cu [29], [30] and Au [31]), polymers [32], semiconductors (e.g., silicon carbide (SiC) [33], silicon (Si) [34] and zinc oxide (ZnO) [35]) and carbon (C) [36] have been synthesized using this method.
NP-based inks enable the development of novel applications due to the different possible material combinations and properties. Among the conductive inks, Ag NP based ink is widely used in various applications owing to its high thermal conductivity and printability. This material conducts well in both its oxide and metallic states, so the technological risk associated with its use is low. Due to the high conductivity of the ink, lower volumes of ink will be needed to produce good quality PE devices. During the past few decades, LASiS has been widely investigated for nanomaterial production in liquid, with a view to enabling cost-effective PE conductive ink formulation. In this simple method, several experimental parameters, including laser energy, repetition rate, laser wavelength, solvent, etc. can easily be adjusted to determine size of the NPs, their stability in liquids and the productivity of the NPs [37]. The properties of colloidal solutions have a strong influence on the size, shape, production rate and polydispersity of NPs [38]. In PEs, the NP productivity plays a crucial role, since the NP concentration and size in the colloids heavily influence the conductivity of the printed track and thereby the quality of the device. Although the commonly used static LASiS technique is simple, its low productivity hinders acceptance of this method within manufacturing industries [39]. In this study, the use of a high production rate, dynamic flow-based LASiS system to produce Ag NP colloids for conductive inks and their drop-cast pattern characterization is reported. The resistance properties of the functional inks were studied with the drop-cast technique prior to using them in inkjet or aerosol jet printing applications. The volume of liquids required for flow-based production may lead to low concentrations, despite the high production rates. After the production of the Ag nanocolloids via LASiS, they were centrifuged to increase their concentration, as ink conductivity is highly dependent on the ink concentration. The as-produced and centrifuged Ag nanocolloids were deposited on glass slides by drop-casting and the resistance was measured in-situ during heating, in order to determine the as-deposited resistance, the temperature required for heat-treatment and the reduced resistance achievable after heating. This achieves the objective of establishing the suitability of the LASiS method for ink production for PE application, whether concentration and the correct parameters and effect for annealing are necessary.
Section snippets
Materials
Ag discs (99.99+% metals basis, sourced from Goodfellow Cambridge Ltd) were used as the target material in the LASiS process. The DI water for nanocolloid formation was purchased from Merck (LC-MS Grade LiChrosolv).
Method: Laser ablation synthesis in solution and Ag NP generation
A schematic of the experimental LASiS dynamic flow based set-up used for the generation of Ag NPs is shown in Fig. 1a [14]. The laser ablation of the Ag target (8 mm diameter) was carried out using a micro-machining low-power picosecond Nd: YAG laser (WEDGE HF 1064, BrightSolutions,
Ag nanocolloid characterisation characterization
The UV–VIS absorption spectrum of the as-produced LASiS Ag colloid (Fig. 3a) shows the presence of an absorption peak in the 400 nm wavelength region, confirming the Ag NP formation in DI water. The optical characteristics of the Ag NPs are strongly influenced by their diameter, i.e. with an increase in the NP size, the absorption peak will move towards a higher wavelength region. For larger particles, particularly above 80 nm in size, a secondary absorption peak appears at a lower wavelength
Conclusions
Conductive inks are a low-cost way of applying conductive tracks and layers, and drop casting allows for simple characterization of the inks and their behaviour. In this work, the generation of Ag NP colloids using a dynamic flow based LASiS system capable of high production rates and the heat-treatment behaviour of drop-cast layers of concentrated Ag NP based ink are reported. Metallic Ag NP colloids were successfully produced with a concentration of 0.9 mg mL −1 and an average particle size
CRediT authorship contribution statement
Éanna McCarthy: Conceptualization, Methodology. Sithara Pavithran Sreenilayam: Conceptualization, Methodology, Writing – original draft. Oskar Ronan: Experiment and reviewing. Hasan Ayub: Experiment. Ronan McCann: Experiment and reviewing. Lorcan McKeon: Experiment and reviewing. Karsten Fleischer: Conceptualization. Valeria Nicolosi: Writing – review & editing. Dermot Brabazon: Conceptualization, Supervision, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported in part by a research grant from Science Foundation Ireland (SFI) under Grant Numbers 16/1571 RC/3872 and 19/US-C2C/3579, and is co-funded under the European Regional Development Fund and by I-Form industry partners, Ireland and from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 862100. Authors wish to acknowledge the Advanced Microscopy Laboratory, Trinity College Dublin, Ireland for the use of their facilities.
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