Direct patterning of silver particles on porous silicon by inkjet printing of a silver salt via in-situ reduction
© Chiolerio et al.; licensee Springer. 2012
Received: 27 April 2012
Accepted: 4 August 2012
Published: 6 September 2012
We have developed a method for obtaining a direct pattern of silver nanoparticles (NPs) on porous silicon (p-Si) by means of inkjet printing (IjP) of a silver salt. Silver NPs were obtained by p-Si mediated in-situ reduction of Ag+ cations using solutions based on AgNO3 which were directly printed on p-Si according to specific geometries and process parameters. The main difference with respect to existing literature is that normally, inkjet printing is applied to silver (metal) NP suspensions, while in our experiment the NPs are formed after jetting the solution on the reactive substrate. We performed both optical and scanning electron microscopes on the NPs traces, correlating the morphology features with the IjP parameters, giving an insight on the synthesis kinetics. The patterned NPs show good performances as SERS substrates.
KeywordsInkjet printing technology Ag nanoparticles Porous silicon SERS
Inkjet printing (IjP) technology has gained a great interest for the information technology compartment due to its high throughput and low costs . Basically, it consists of a microelectromechanical system or a capillary where a piezoelectric actuator is able to produce a controlled droplet ejection. A servo-assisted head positioning makes it possible to additively draw complex structures and geometries in a way pretty similar to pattern printing by commercial printers. As a matter of fact, the fabrication of quite high-resolution patterns either on flexible substrates or on rigid ones, by means of this simple additive process, is highly attractive. Recent works report IjP as a new promising manufacturing technique for the fabrication of many different devices . In the recent past, some electronic applications of IjP based on Ag NPs have been explored [3, 4].
In the framework of useful applications, silver NPs synthesized directly on p-Si showed a good behavior as surface enhanced Raman scattering (SERS) substrates , which represent a high-sensitive label-free detection method in materials science, biophysics, medical diagnostics, and molecular biology . In this specific application, no sintering is necessary. The performances of these types of substrates, dictated by their plasmonic response, depend on the morphology of the synthesized NPs that can be adjusted, controlling the concentration of silver nitrate, the process temperature, and the solvent typology impregnating the p-Si surface.
Herein is reported a study related to Ag NPs obtained by merging IjP technology with the redox reaction of AgNO3 solutions with p-Si surfaces. The morphology of the Ag NPs is correlated to the synthesis parameters. Such process opens new possibilities of simple fabrication concerning with metal-dielectric nanostructures aimed to sensing applications based on amplified Raman spectroscopy.
Porous silicon substrates were obtained by electrochemical etching of boron-doped silicon wafer (34 m(Ω)/cm) in HF solution (20:20:60 HF:H2O:CH3CH2OH) with a current density of 80 mA/cm2. We have obtained three solutions of AgNO3 having a concentration of 10−2 M using water (W), ethylene glycol (EG), and a solution composed of 50% water and 50% ethylene glycol (W:EG 1:1).
The so-prepared ink was printed by means of a piezoelectric Jetlab® 4 printer from MicroFab Technologies Inc. (Plano, TX, USA) equipped with a 60-μm nozzle diameter MJ-AT-01 dispenser. The print head was heated at 60°C, and the substrate was kept at room temperature during the deposition process to improve the quality of the deposit. The waveform used to expel a single droplet was a 35V pulse lasting 18 μs, followed by a −13V pulse lasting 36 μs as echo dwell with the rise, fall, and final rise time of 25, 5, and 2 μs, respectively. The step size was set to 100 μm, and 6 layers were printed on top of each other.
Concerning the drop on position (DOP) experiment, we used as ink a solution composed of 50% water and 50% ethanol with a AgNO3 concentration of 10−2 M; for the IjP parameters, we applied a waveform consisting of a 22V pulse lasting 20 μs, followed by a −15V pulse lasting 40 μs as echo dwell with the rise, fall, and final rise time of 5, 5, and 5 μs, respectively. During this process, the printing head is placed in a specific position of the plane and delivers a certain number of droplets in the same place, ejecting them at a specific frequency ranging within 250 Hz to 3 kHz.
Scanning electron microscopy (SEM) images of Ag NPs on p-Si samples were obtained as secondary and backscattered electron images with 5- to 10-keV electrons using a Zeiss SUPRA 40 (Zeiss SMT, Germany) field emission electron microscope. Specular reflection spectra were obtained using an Agilent Cary 5000 (Agilent California, USA) UV-visible-NIR spectrophotometer equipped with a 12.5° reflectance unit, in the range of 200 to 2000 nm.
Raman spectra were obtained by means of a Renishaw inVia Reflex (Renishaw PLC, United Kingdom) micro-Raman spectrophotometer equipped with a cooled charge-coupled device camera. Samples were excited with an Ar-Kr laser source (514.5 nm), providing a photon flux lower than 60 W/cm2. The spectral resolution and integration time were 3 cm−1 and 10 s, respectively.
Results and discussion
Direct patterning of stripes
IjP stripe sample features
Number of passes
Average NP diameter (nm)
DOP sample IjP-ejection parameters and correlated features numerically determined from FESEM images
Drop number (nd); Piezo actuation frequency (f, kHz)
2D-percolation factor (PF) and filling fraction (FF)
Drop number; (ND) frequency (F) (kHz)
2-D percolation factor (PF)
Filling fraction (FF)
Drop number; (ND) frequency (F) (kHz)
Filling fraction (FF)
Drop number; (ND) frequency (F) (kHz)
Filling fraction (FF)
Drop number; frequency (kHz)
Filling fraction (FF)
Drop num; (ND) frequency (F) (kHz)
Filling fraction (FF)
FFs and PFs show a common trend versus the droplet number at several ejection frequencies. Actually, in order to quantify the correlation between PF and FF, we computed a 2-D cross-correlation factor (for a reference see ), obtaining a remarkable value of 0.7317, resulting from the numerical analysis of 40 different samples.
In order to demonstrate the potential application of the ink-jet printed Ag NPs as active SERS substrates, we analyzed the optical response of a selected sample obtained using the same IjP parameters used for the above-mentioned stripes and an AgNO3 solution in pure water with a concentration of 10−1 M for the ink.
It is widely proved that the main contribution to the SERS effect arises from an enhancement of the local electromagnetic field close to the metallic surface because of the excitation of LSPs . Figure 6b shows the SERS spectra (excitation at 514.5 nm that is within the wavelength of the plasmonic dip shown in Figure 6a of a Cy3 dye adsorbed from aqueous solutions with several molar concentrations). The characteristic vibrational peak at 1,370 cm−1, ascribed to the −CH3 deformation mode (methyl groups are present as substituents on the Cy3 aromatic rings), is clearly detectable up to a concentration as low as 10−9 M, while as-anodized porous silicon substrates without Ag NPs do not show any Raman signal even for concentration of 10−6 M.
Ag NPs on p-Si were obtained by means of IjP technology after in-situ reduction by means of chemical reaction between AgNO3 contained in the ink and p-Si substrate. Morphological characterizations and numerical calculation of some geometrical parameters were performed, showing that the pattern uniformity is strictly dependent on the ink viscosity (i.e., the type of solvent). In particular, for the same printing condition, higher ink viscosity facilitates the synthesis of particles and their coalescence. The DOP technique allowed the exploration of how the in-situ reduction leading to Ag NP formation is influenced by the number of droplets provided for each fixed position on the substrate and by the ejection frequency which modulate the reaction time and reactant flow rate. The correlation between such process parameters and NP population features was given in terms of 2-D percolation, numerically determined from FESEM images. It has been evidenced that for ejection frequency lower than 1 kHz, it is possible to control the nanostructure morphology by tuning the drop amount.
Finally, Ag NPs synthesized by IjP technique showed good performances as Raman enhancers. Thus, plasmonic patterns featuring a controlled nanostructure morphology could be exploited to produce arrayed elements active for SERS-based detection. Several potential applications in biomedicine can be foreseen such as microRNA SERS detection, where checking and quantifying different non-coding RNAs are important, and in a multiplexing label-free framework with the aim to actuate an early-cancer diagnosis .
AC received his degree in Materials Engineering at Politecnico di Torino in 2005 and his Ph.D. in Electron Devices in 2009. He joined the χ-Lab Materials and Microsystems Laboratory research group, developing spintronic devices, MEMS and nanocomposite materials for electronic applications. AC is currently coordinating the technologies in support of the Smart Materials platform at Istituto Italiano di Tecnologia, with the aim of realizing a smart sensing skin. He is the co-author of more than 30 scientific papers, 3 national patents, 1 international patent, 7 book chapters and editor of 1 book. AV received his degree in Chemical Engineering at Politecnico di Torino in 2005 and his Ph.D. in Physics in 2011. The main field of interest is the production of silver and gold nanostructures, prepared through either nanosphere lithography or deposition, for SERS (surface enhanced Raman scattering) substrates. PP is an R&D engineer at Politronica S.r.L., a spin-off company of Politecnico di Torino (Italy), where he is the responsible of the Print Lab. In 2008, he obtained his masters degree in Materials Engineering. From 2008 to 2010, he worked at Politecnico di Torino as a fellow researcher in the Physics and Materials Science Department dealing with the deposition of thin films by means of vacuum techniques. Currently, his research activities are focused on printing technologies such as inkjet, R2R and pad printing of materials for electronic applications. He is a co-author of several scientific papers published on international journals. PM obtained her degree in Chemistry in 2001 at the University of Turin and her Ph.D. in Chemical Science in 2006. As post-doc, she joined the Spintronic research group at the Department of Physics (Politecnico di Torino), studying and developing magnetic devices, tunnel magnetoresistance and magnetic tunnel junctions (MTJs). At present, she works at Politronica S.r.L., a spin-off company of Politecnico di Torino, where she is responsible for the chemical laboratory. Her current area of research concerns with the synthesis and characterization of conductive, magnetic and dielectric inks for inkjet printing of electronic devices. She is the author and co-author of several scientific papers published on international journals. PR is an assistant researcher at the Applied Science and Technology Department of Politecnico di Torino (Italy), where she operates in the Materials and Microsystems Laboratory, a laboratory of Politecnico di Torino. In 1999, she obtained her masters degree in Chemistry. In 2003, she took her Ph.D. degree in Materials Science and Technology. Her research activities are focused on surface chemical modification of several materials (also porous and nanostructured), even by means of plasma-assisted techniques, in order to obtain hybrid organic–inorganic materials to be applied as optical and electromechanical sensors mainly for biological diagnostics. FrG is an associate professor at the Applied Science and Technology Department of Politecnico di Torino. He obtained his masters degree in Industrial Chemistry in 1990 and his Ph.D. in Chemical Science in 1995. His current area of research includes the synthesis and characterization of materials with special attention devoted to porous silicon for several applications. He is the author and co-author of more than 110 scientific papers published on peer-reviewed international journals. FaG is an associate professor of Physics of Condensed Matter at the Applied Science and Technology Department of Politecnico di Torino where he is in charge as research delegate. He obtained his masters degree in Physics in 1990 and his Ph.D. in Solid State Physics in 1995. He is a senior researcher at the χ-Lab Materials and Microsystems Laboratory of Politecnico di Torino and at the Istituto Italiano di Tecnologia - Center for Space Human Robotics. His current area of research concerns with the synthesis and characterization of silicon-based thin films in amorphous, micro-crystalline and porous phase with applications in photonic and plasmonic nanostructures. He is the author and co-author of more than 140 scientific papers published on peer-reviewed international journals.
drop on position
field emission scanning electron microscopy
localized surface plasmon
surface enhanced Raman scattering
Financial support from Projects NANOMAX (Progetto Bandiera MIUR PNR 2011–2013) and NEWTON (MIUR FIRB 2011–2014) is gratefully acknowledged.
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