fabrication of micro patterns via near field electrospray

Fabrication of micro-patterns via near-field electrospray Wenwang Li, Gaofeng Zheng, Lei Xu, and Xiang Wang Citation: AIP Advances 6, 115002 (2016); doi: 10.1063/1.4967200 View online: http://dx.doi.org/10.1063/1.4967200 View Table of Contents: http://aip.scitation.org/toc/adv/6/11 Published by the American Institute of Physics Articles you may be interested in Electrospinning jet behaviors under the constraints of a sheath gas AIP Advances 6, 115022115022 (2016); 10.1063/1.4968603 Thin film zinc oxide gas sensor fabricated using near-field electrospray AIP Advances 6, 125306125306 (2016); 10.1063/1.4971273 Electrohydrodynamic direct-writing orderly pattern with sheath gas focusing AIP Advances 6, 115304115304 (2016); 10.1063/1.4967342 AIP ADVANCES 6, 115002 (2016) Fabrication of micro-patterns via near-field electrospray Wenwang Li,1 Gaofeng Zheng,2 Lei Xu,3 and Xiang Wang1,a School of Mechanical and Automotive Engineering, Xiamen University of Technology, Xiamen 361024, China Department of Instrumental and Electrical Engineering, Xiamen University, Xiamen 361005, China School of Mechanical and Electric Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China (Received 10 August 2016; accepted 24 October 2016; published online 31 October 2016) A near-field electrospray process is developed to deposited micro-patterns Compared with conventional electrospray, near field electrospray uses a steel probe instead of capillary nozzle, and its nozzle-to-substrate distance is shortened to several millimeters to realize micro-scale deposition area The liquid is supplied by discretely dipping the probe into solution in advance so that electrospray process maintains until the consumption of liquid adhered at the probe tip The influence of solution conductivity and applied voltage on deposition are investigated, as increasing solution conductivity and high applied voltage may promote the electrospray process and enlarge the line width In addition, micro-patterns with various materials are directly electrosprayed © 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4967200] INTRODUCTION Many techniques are currently used to fabricate nano/micro particles and thin films, including plasma spraying, magnetron sputtering, pulsed laser deposition, and so on Compared with these techniques, electrospray, also called electrohydrodynamic atomization, wins an extensive attention due to its simple equipment, low cost, and good material compatibility Technically, electrospray utilizes an applied electric field to generate micro-/nano-meter-scale droplets, particles and thin films.1 The Columbic interaction of charges atomizes the liquid drop under such electric field The electrosprayed droplets and particles can range from hundreds micrometers down to several tens of nanometers with nearly monodisperse distribution Electrospray process can be controlled to some extent by the flow rate and electric field These atomized monodisperse droplets and particles may be applied to biological skeleton,2 micro ceramic structures,3,4 biomedicine,5 and energy storage.6 The electrospraying apparatus is composed of high voltage generator, fluid supply system, nozzle, and substrate High voltage is applied between nozzle and substrate to produce electric field and stretch the viscoelastic liquid Typically, the liquid is injected through a capillary nozzle and forms a meniscus at the end of the nozzle A fine jet issues from the meniscus and then break into nanoparticles when the electric force overcomes the surface tension Traditionally, the nozzle-to-substrate distance is larger than tens of centimeters for the preparation of large area of atomized particles and films To fabricate micro-/nano-patterns, various template/molding strategies are utilized in the electrospraying process Commonly, the patterns form on the substrate after removing the special designed template, which covers the substrate For example, Nithyanandan et al.7 adopted the template-assisted electrostatic spray to deposit the line array with a width of about 50µm Zhu et al.8 used the templateassisted method to fabricate micro-patterns of nano-hydroxyapatite/silk fibroin composite However, a Author to whom correspondence should be addressed Electronic mail: wx@xmut.edu.cn 2158-3226/2016/6(11)/115002/6 6, 115002-1 © Author(s) 2016 115002-2 Li et al AIP Advances 6, 115002 (2016) these methods require elaborate operations and expensive apparatus Wang et al.9 proposed an electrohydrodynamic printing instrument to pattern centimeter-scale three-dimensional structures by using a shortened nozzle-to-substrate distance They also illustrate the feasibility of downscaled nozzleto-substrate distance in eliminating scattering of droplets Hashimdeen et al.10 demonstrated the significant effect of printing head designs on the deposition and resolution of electrohydrodynamic printed patterns In this paper, a low-cost near-field electrospray technology is demonstrated and investigated Compared with traditional electrospray, the nozzle-to-substrate distance is shortened to several millimeters and a probe is used as electrospray nozzle instead of capillary tube Electrospraying of micro-/nano-particles in small area is achieved, and various micro-patterns are directly deposited without assistance of any templates EXPERIMENTAL DETAILS The experimental setup is presented in Fig 1(a), where a stainless steel probe is connected to the anode of high-voltage generator and a grounded silicon substrate to the cathode The silicon substrate was fixed on a XY motion stage The probe-to-substrate distance was adjustable according to experimental requirements The solution adhered to the probe tip by dipping into the solution in advance The quantity of solution was enough to be electrosprayed for about 10∼30 seconds The near-field electrospray directly deposited orderly patterns by controlling the motion of substrate within a short probe-to-substrate distance Glycerol was used to investigate the influences of solution conductivity and applied voltage on the electrospray process Various amounts of sodium chloride (NaCl) was added to the glycerol to change the conductivity On the other hand, the 0.5 wt% polyethylene oxide (PEO, M w = 300,000 g/mol) solution with mixed solvent of water and alcohol (v:v = 1:1), commercial silver ink (Cabot CSD-32), and wt% zinc acetate (Zn(CH3 COO)2 ) aqueous solution were also used to deposit micro-patterns The viscosity and surface tension of these solution are listed in Table I The as-prepared samples were characterized by an optical microscope and a scanning electron microscope (SEM) FIG (a) Schematic setup of near-field electrospray Scale bar: µm (b) Stable glycerol jet is ejected from the apex of Taylor cone to the substrate by using a capillary nozzle The inner and outer diameter of nozzle are 0.06 mm and 0.19 mm The nozzle-to-substrate distance is mm and the applied voltage is 2.0 kV Scale bar: 0.2 mm (c) Glycerol is electrosprayed by utilizing a probe as spinneret Atomized particles deposit on the substrate The probe-to-substrate distance is mm and the applied voltage is 0.8 kV Scale bar: 20 µm TABLE I The viscosity and surface tension of solution.a Viscosity (mPa·s) Surface tension (mN/m) a Adding Glycerol PEO Cabot CSD-32 Zn(CH3 COO)2 945 63 34 100 48 68 of traces of NaCl to glycerol does not significantly change the viscosity and surface tension 115002-3 Li et al AIP Advances 6, 115002 (2016) RESULTS AND DISCUSSIONS Conventional electrospray process has the nozzle-to-substrate distance of tens centimeters so that deposition area is too large to collect micro-pattern In order to minimize the deposition area, one effective way is to shorten the nozzle-to-substrate distance However, short nozzle-to-substrate distance tends to result in a jet rather than droplets in orderly deposition of microscale patterns Our experimental results demonstrate this tendency when a capillary nozzle (O.D = 0.19 mm, I.D = 0.06 mm) is used to electrospray glycerol as shown in Fig 1(b) The nozzle-to-substrate distance is mm, and the applied voltage reaches a critical value of 2.0 kV for jetting A straight glycerol jet issues from the tip of liquid pendant and travels to the substrate, as the charged jet does not have enough time to atomize under short nozzle-to-substrate distance This process to deposit a straight jet is usually classified as electrohydrodynamic printing/direct-writing.9–12 The surface tension and viscosity of fluid affect the electrospray process They are the resistances to electrospray The surface tension opposes the electric field applied at the fluid interface and affects the ability of a liquid to realize electrospray The viscosity opposes the fluid to atomize High surface tension increases the threshold electric field for an ejection, while high viscosity of liquid increases the size of deposited droplets Therefore, high viscous liquid favors electrohydrodynamic printing/directwriting rather than electrospray A key strategy to improve the electrospray is to increase the Coulomb repulsion inside the charged jet Theoretically, the Coulomb force increases with increasing applied voltage However, high applied voltage would result in atmospherical discharge and even interrupt the deposition process The critical electrical field for liquid ejection under such a short nozzle-tosubstrate distance is on the order of 106 ∼ 107 V/m, which is an order of magnitude larger than that of conventional electrospray In our previous research,13 there is no atomization for glycerol at a nozzle-to-substrate distance of mm while the applied voltage is increased to the breakdown value It has been demonstrated that sharp tip needles may make intense electrostatic stresses on the liquid meniscus due to its small radius.10 Thus, a probe rather than a capillary nozzle is applied to near-field electrospray Free charges are aggregated at the probe tip to increase the charge density on solution surface The Coulomb repulsion and electric force increase with the increasing surface charge density, which contributes to the atomization of charged jet Fig 1(c) illustrates that the glycerol electrosprays under a probe-to-substrate distance of mm and an applied voltage of 0.8 kV by using the probe as spinneret Despite the electrospraying process is not visible, a large number of atomized droplets are deposited on the substrate The near-field electrospray utilizes a discrete liquid supplement instead of traditionally continuous way The solution adhered to the probe tip is consumed and the volume shrinks during electrospraying Figs shows the morphology of electrosprayed glycerol patterns The conductivity of solution, applied voltage, probe-to-substrate distance, and substrate speed are 2.67 µS/cm, 0.8 kV, 500 µm, and mm/s, respectively Initially, the solution at the tip is sufficient and the short probe-to-substrate distance results in more atomized particles The atomized particles in the center of deposition area are assembled into large principal drops due to surface tension as more particles cluster together as shown in Fig 2(a), and the remained atomized particles behave as satellite droplets surrounding the principal droplets The diameter of principal drops is about µm while the size of satellite droplets is far less than µm With persistent electrospraying, the shrink of solution volume at the tip reduces the amount of atomized particles so that principal drops on substrate is scaled down as shown in Fig 2(b) Finally, the depletion of solution leads to small principal drops and the decrease of line width as shown in Fig 2(c) The advantage of near-field electrospray over conventional electrospray is to control the deposition location of droplets for the fabrication of incontinuous and small-area patterns Fig 2(d) shows the deposited Arabic numbers of “2” and “3” The substrate speed is decreased to 0.2 mm/s such that atomized particles overlap into larger droplets There is a variety of line width for both characters to the extent, especially at the beginning, ending, and corner Furthermore, quick substrate speed may block the emergence of large droplets as shown in Fig 2(e) The line patterns are generated at a substrate speed of mm/s and the atomized particles may be observed clearly The conductivity of solution impacts the amount of charges within the liquid jet and subsequent electrospray process Fig 3(a) shows the morphology of deposited patterns under various solution 115002-4 Li et al AIP Advances 6, 115002 (2016) FIG Electrosprayed patterns of glycerol solution (a) Patterns generated under sufficient liquid supply at initial period of electrospray process (b) Patterns generated at middle period when the solution shrinks (c) Patterns generated at final period when the solution is insufficient The substrate speed is mm/s for (a) - (c) (d) Arabic number ‘2’ and ‘3’generated at a substrate speed of 0.2 mm/s (e) Line patterns deposited under a substrate speed of mm/s conductivities The applied voltage, probe-to-substrate distance and substrate speed are kV, mm and mm/s, respectively The conductivities of applied solution are 2.67, 3.88, 5.27, and 9.14 µS/cm The width of deposited line composed of atomized particles increases with increasing solution FIG (a) Deposited lines of glycerol generated under various solution conductivity Scale bar: 100 µm (b) The line width of electrosprayed patterns versus the conductivity of solution (c) The line width of electrosprayed patterns versus the applied voltage under the solution conductivity of 3.88 µS/cm The probe-to-substrate distance and substrate speed are mm and mm/s, respectively 115002-5 Li et al AIP Advances 6, 115002 (2016) conductivity as depicted in Fig 3(b) These nano-scale droplets are highly related to the atomization area, amount of charges, and Coulomb repulsion of droplets The applied voltage also impacts the near-field electrospray process High applied voltage results in strong electric field, more free charges, and large deposition area The average width of deposited lines increases from 55 µm to 88 µm as applied voltage increases from kV to 1.4 kV, as shown in Fig 3(c) The conductivity of glycerol, probe-to-substrate distance, and substrate speed are 3.88 µS/cm, mm, and mm/s, respectively Near-field electrospray is able to deposit fine particles for specific patterns It may generate cost-effective micro-patterns consisted of nanoscale particles using a variety of materials Fig 4(a) shows an electrosprayed line pattern using PEO solution These near-field electrosprayed particles have similar morphology as the conventional electrospray does The applied voltage is 1.5 kV and the probe-to-substrate distance is mm The density of PEO particles in middle region is much higher than that at the edge More uniform distribution results from higher voltage or longer probeto-substrate distance The controllability of deposition is demonstrated by moving the substrate to generate complex patterns The characters “XMU” are made up of discrete silver particles on the heated substrate at 473 K as shown in Fig 4(b) The applied voltage is 0.8 kV and the probe-tosubstrate distance is 0.5 mm Fig 4(c) demonstrates a more continuous silver track under an applied voltage of 1.2 kV and a probe-to-substrate distance of 0.5 mm Fig 4(d) depicts a ZnO film, which is the calcined Zn(CH3 COO)2 pattern by electrospraying in air at 673 K for 10 minutes The applied voltage is 1.5 kV and the probe-to-substrate distance is mm Furthermore, the near-field electrospray may be combined with the template-assisted method to generate more complex and regular patterns Fig 4(e) illustrates the PEO particles electrosprayed on a triangle template under an applied voltage of 1.5 kV and probe-to-substrate distance of mm All the PEO particles are intensively focused inside FIG Various near-field electrosprayed patterns (a) SEM images of the electrosprayed PEO line under the applied voltage of 1.5 kV and probe-to-substrate of mm (b) Characters “XMU” made up of discrete silver particles under the applied voltage of 0.8 kV and probe-to-substrate distance of 0.5 mm (c) A continuous silver track generated under an applied voltage of 1.2 kV and a probe-to-substrate distance of 0.5 mm (d) SEM image of a ZnO film under the applied voltage of 1.5 kV and probe-to-substrate distance of mm (e) PEO particles electrosprayed on a triangle template under an applied voltage of 1.5 kV and probe-to-substrate distance of mm 115002-6 Li et al AIP Advances 6, 115002 (2016) the template and the resulting pattern is regular, as the template is made of polyethylene terephthalate (PET), which may prevent the deposition of PEO particles Near-field electrospray is a direct-write, maskless material deposition method for micro-/nanofabrication On-demand traces and patterns may be deposited layer-by-layer onto substrate by controlling the substrate motion Recently, many techniques have been used to deposit micro-/nano-scale patterns For example, drop-on-demand inkjet printing14 may deposit discrete micro-scale droplets The size of droplet is usually larger than that of nozzle and the resolution nearly reaches the limit of the droplet generation process Aerosol jet deposition,15 on the other hand, may deposit sub-10-µm material by using a focused aerosol particles spray, but the monodispersion of particles and stability of aerosol are still challenges However, the near-field electrospray is the controllable one for micro/nano-scale fabrication, including particle deposition and thin-film coating It is a simple, low-cost and versatile tool for various liquids atomization Near-field electrospray is suitable for composing small and complex planar patterns rather than three-dimensional (3D) structures due to its discontinuous liquid feeding It is further applicable for 3D shape if a continuous solution feed is realized A possible method is to insert the probe into an insulated capillary nozzle.16 CONCLUSIONS Near-field electrospray process is developed to deposit micro-scale electrosprayed patterns under a shortened nozzle-to-substrate distance by using a steel probe instead of capillary nozzle Electrosprayed particles have similar morphology as the conventional electrospray does The near-field electrospray utilizes a discrete liquid supplement, the process is stable and continuous until the liquid adhered to the probe tip depletes It may generate orderly patterns due to its slight variety The line width of deposited patterns are of the order of tens micrometers Increasing solution conductivity and high applied voltage may promote the electrospray process and enlarge the line width Various complex micro-patterns are deposited by controlling the movement of substrate This study indicates a simple and potential route for on-demand deposition of micro-/nano-patterns at high resolution These patterns may be applied to electronic devices and biological systems ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (No 51575464), the Foundation for High-level Talents of Xiamen University of Technology (No YKJ15005R and No YKJ14041R), and Xiamen Municipal Science and Technology Project (No 3502Z20143033) A Jaworek and A T Sobczyk, J Electrostat 66, 197 (2008) S Thian, X Li, J Huang, M J Edirisinghe, W Bonfield, and S M Best, Thin Solid Films 519, 2328 (2011) S A Rocks, D Wang, D Sun, S Jayasinghe, M Edirisinghe, and R A Dorey, J Electroceram 19, 287 (2007) X Li, J Huang, and M Edirisinghe, J R Soc Interface 5, 253 (2008) W Kim and S S Kim, Polymer 52, 3325 (2011) S E Park, S Kim, K Kim, H.-E Joe, B Jung, E Kim, W Kim, B K Min, and J Hwang, Nanoscale 4, 7773 (2012) A Nithyanandan, S Mahalingam, J 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(2016) Fabrication of micro- patterns via near- field electrospray Wenwang Li,1 Gaofeng Zheng,2 Lei Xu,3 and Xiang Wang1,a School of Mechanical and Automotive Engineering, Xiamen University of Technology,... October 2016) A near- field electrospray process is developed to deposited micro- patterns Compared with conventional electrospray, near field electrospray uses a steel probe instead of capillary... used as electrospray nozzle instead of capillary tube Electrospraying of micro- /nano-particles in small area is achieved, and various micro- patterns are directly deposited without assistance of any