Full Length ArticlePrecise micropatterning of silver nanoparticles on plastic substrates
Graphical abstract
Introduction
Silver nanoparticles patterning on various substrates have a broad range of applications in advanced biomedical devices [1], electronic circuits [2], [3], and optics [4], [5] because of the unique physicochemical, electrical, optical, and biological properties of silver nanoparticles. Meanwhile, a new generation of the miniaturized devices meets important requirements such as flexibility and portability, and hence, light weight and shock resistance of the substrate material. At this point, plastic substrates fulfil those requirements compared to the hard and brittle conventional silicon and glass substrate, however, less attention has been paid in designing and patterning on plastic substrates. Conventional photolithography based metal patterning methods [6], [7], [8], [9] usually are performed at high temperatures which is incompatible with plastics or require complex fabrication steps, such as additional photomask fabrication or selective substrate modification.
Fortunately, maskless direct patterning techniques based on the nanoparticle ink transferring such as inkjet printing [10] and an atomic force microscope (AFM) tip based liquid depositions [11], [12] are simple and inexpensive methods to obtain metallic patterns. The AFM tip transfers the ink directly onto the substrate owing to capillary bridging between the tip and the substrate [13], [14], [15], [16]. The contact mode of the AFM tip based deposition provides precise control of the deposited ink. On the contrary, in drop-on-demand (DOD) inkjet printing [17] the liquid is in a non-contact mode with the substrate, where the individual drops of metal precursor are ejected from the nozzle aperture, falling directly onto polymer substrate. Hence, the trajectory changes of depositing drops arising from the asymmetric nozzle wetting and air motion, can cause the micropattern dislocations [18].
Further, DOD inkjet printing has become popular for printing nanoparticles not only on planar but also within three-dimensional (3D) geometries [19], [20], [21], [22]. Mogalicherla et al. deposited metal oxide nanoparticles suspensions inside the stainless steel 200 μm channels using 100 μm sized nozzle [23]. Nevertheless, the smaller the targeted points within the 3D object, the smaller sized nozzles should be employed to have a precise deposition, especially in complex topographies with the pattern size restrictions. However, deposition of non-Newtonian fluids and other solutions, may cause aggregation or agglomeration and give extra restrictions to the nozzle size reduction [24]. Therefore, the precise deposition of the nanoparticle inks is even more challenging when compared to Newtonian fluids or polymer solutions.
Recently, a microworking robot based dip painting process with different sized needles has been shown to overcome the pattern size restrictions of the nozzle based techniques. The needles, sizes ranging from 5 μm to 500 μm, are able to scan surface profiles and subsequently deposit with a precise control [25]. Apart from the size restrictions, the microworking robot deposition technique overcomes challenges when the deposition on multileveled surfaces is needed. The additional z-axis control and the contact mode of the method allows to deposit the ink precisely on any desired surface locations.
Regardless of different ink deposition methods, the substrate-ink interaction dominates the quality and uniformity of the micropatterns. Therefore, selective modification of the substrate wettability may facilitate the targeted control of ink deposition. Previously, selective three-dimensional chemical anisotropies on hydrophobic polymer substrates have been created and demonstrated to be effective in the selective filling of polar liquids in a confined microenvironment [26], [27], [28]. The polypropylene (PP) was chosen as a substrate material for modifications because of its wide industrialization in a global market, low cost, and easy processing. Moreover, the flexibility, thermal resistance, and shock resistance make PP ideal candidate for potential applications.
In this study, we explore the possibility to precisely pattern silver nanoparticle microarrays, either on planar or within 3D topography of PP substrates, with additional z-axis control. The precise microworking robot dip painting enables the selectively deposit nanoparticle ink within the 3D topography, while the subsequent low temperature sintering generates solid metal micropatterns. While the conventional methods require complex fabrication processes and high temperature to create metal micropatterns, the advantage of the method is to deposit metal nanoparticle ink at ambient conditions and at low sintering temperature. Moreover, we probe the influence of surface wettability changes, deposition cycles, and sintering temperatures on the size of the ink microarray and the distribution of the sintered silver nanoparticles.
Section snippets
Materials
The polypropylene (PP) homopolymer (HD 120 MO) was purchased from Borealis Polymer Ltd., (Porvoo, Finland). Ethanol was purchased from Altia Oyj, (Rajamäki, Finland), ammonium persulfate (CAS: 71310-21-9), acetone (CAS: 67-64-1), polyethylene glycol 400 (PEG 400, CAS 25322-68-3), and silver nitrate (AgNO3, CAS 7761-88-8 047-001-00-2) were obtained from Sigma-Aldrich and were used as received.
Polymer substrates fabrication
Flat PP polymer discs with thickness of 1500 μm were manufactured by injection molding with a HAAKE®
Silver nanoparticle ink properties
The polyol method used for preparation of silver nanoparticles reduces the silver cations into metallic silver [32]. The absence of other organic molecules, such as stabilizers and surfactants, ensures the purity of silver nanoparticles. Most importantly, the removal of the organic molecules, conventionally, requires high temperature, normally above 200 °C, for chemical decomposition [33]. Hence, the limited chemical use in the proposed processes is of great importance and enables a low
Conclusions
In this study, silver nanoparticle precursor was selectively deposited on injection molded smooth, topographically prestructured, and multileveled, PP substrates. The use of PEG in the ink preparation not only increased the water viscosity to facilitate the direct painting, but also ensured the formation of metallic silver. The contact mode of the computer controlled robotic technique and z-axis control enabled the precise and superimposable deposition both on planar and on the bottom of the
Acknowledgements
The University of Eastern Finland (Tailored Materials, Organometallic Glow and NAMBER projects) are gratefully acknowledged for their financial support. In addition, the authors owe their gratitude to Mr. Anish Philip for practical advice.
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