Direct fabrication and morphology of metallic micropatternsby pulsed jet nanoelectrospraying of silver nano-ink

Ke Wang, John P. W. Stark

� ACA and OCCA 2011

Abstract A pulsed jet nanoelectrospray techniquewas applied to direct fabrication of silver micropat-terns. The deposition of a commercial organic silvernano-ink was performed in a fully voltage-controlledfashion by voltage pulses ranging from 550 to 800 Vwith variable durations. By using 15 lm nozzles,patterns with 100-lm-sized features were locally free-formed on a silicon substrate with a spraying distanceof 250 lm. An energy-dispersive X-ray spectrum con-firmed metallic silver was developed in all the patternsafter heat treatment at 220�C. The size and micro-structural evolution of silver films was observed tostrongly depend on the deposition volume and materialflow over substrate surface. A good linear relationshipbetween the deposition volume and pulse duration wasexhibited over the applied voltage range in the cone-jetmode, demonstrating a drop-on-demand capability. Byfitting, the deposition volume rate was estimated to bein the range of 0.38–0.59 pL/ms and was shown toincrease with the applied voltage.

Keywords Nanoelectrospray technique, Flexible jetprinting, Freeform prototyping, Microfabrication,Surface encapsulation

Introduction

Recently, the electrospray (ES) technique has beenrapidly developing as a novel rapid prototyping tool

due to its wide applications in nanoscience, drugmicroencapsulation, and printed electronics.1–8 In theES process, a conductive liquid fed by a capillary issubjected to an electrical field. The electric stressesdeform the liquid meniscus into a conical shape, knownas a Taylor cone.9 When the electrical field is increasedto overcome the surface tension of the liquid, a jet isemitted from the apex of the cone and droplets aresubsequently ejected.10 Consequently, functional mate-rial contained in the droplets can be delivered orfreeformed locally on the desired position without aselective masking and complicated wet-chemical pro-cessing. Different ES modes such as dripping, spindle,pulsation, cone-jet, and multijets might be induced byvarying the applied voltage and flow rate.11 Amongthem, spray modes including microdripping, pulsation,and cone-jet have been demonstrated to be used fordeposition.5,7,12,13 Structured patterns can be createdusing a continuous liquid stream driven by a DC highvoltage while moving the substrate.14,15

Compared with its continuous ES counterpart, thepulsed jet ES technique has attracted particular atten-tion due to the application in mass spectrometer (MS)analysis for its high sensitivity and good signal-to-noiseratios.16 A stream or spray of uniform droplets can beproduced by switching on/off a driving voltage. This‘‘drop-on-demand’’ mode provides excellent controlover the placement of individual droplets, rather thanthe deposition using continuous stream. By using apulsed jet, Yogi et al. demonstrated drop-on-demanddeposition ranging from picoliter to femtoliter vol-ume.17 Chen et al. reported pulsed ES of de-ionizedwater with a 50-lm Teflon nozzle.18 Uniformed trackswith excellent conductivity and truly electronic deviceswere fabricated on Si substrates.6,19,20 Deposition dotsize below 10 lm was also achieved by pulsed elec-trospraying of aqueous gold colloids.21 Our ES config-uration was performed in a full voltage-controlledfashion without the additional assistance of pump or

K. Wang (&), J. P. W. StarkSchool of Engineering and Materials Science, Queen Mary,University of London, London E1 4NS, UKe-mail: K.Wang@bath.ac.uk; wangke1997@hotmail.com

K. WangDepartment of Physics, University of Bath,Bath BA2 7AY, UK

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DOI 10.1007/s11998-011-9370-x

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gas pressure. This unforced feature makes the systemrelatively simple.6,13,22 Meanwhile, by using a micron-sized nozzle instead of a millimeter-sized one, sprayingwas operated in a nano-ES manner.23 Very recently, anequivalent circuit method was proposed to analyze thespraying properties, which can well capture the current–voltage characteristics in this unforced nano-ES.24

In this article, a commercial organic silver ink wasdeposited on Si substrates by using a pulsed cone-jet ina fully voltage-controlled configuration. While previ-ous works have investigated the pulsed ES character-istics such as jet stability, charge relaxation time, dropgeneration frequency, and driving schemes,18,25–29 thiswork focuses on the study of microstructural evolutionsof the material deposited by pulsed voltage and abetter quantitative controlling of material delivery inelectrosprayed thin film materials.

Experimental details

Figure 1 illustrates the experimental configuration forunforced e-jet printing, similar to our previouslypublished apparatus.6,19 The nozzle used for thedeposition was produced by pulling borosilicate capil-laries to form an outlet with a 15 lm exit, which wasconfirmed by SEM. The ink for spraying was fed intothe nozzle and a stainless steel wire was submergedinto the ink material; this wire was held at groundpotential. A 2 kV high-voltage supply from F.u.G.Electronik was connected to a fast voltage switch(PVX4130, DEI) to generate voltage pulses. Thepulsed voltage was applied to an aluminum electrode,on top of which an Si substrate for collection wasattached. The aluminum electrode was fixed to a PC-controlled movable translation stage. The distance

between the Si substrate and the nozzle was adjusted to250 lm. The shape of the liquid meniscus at the nozzletip during the spray was monitored and recorded by anoptical microscope coupled with a high-resolutionCCD video camera (V500, Sony). For the microscopean infinity corrected objective lens (109, Mitatoyu) ona variable zoom (12.59, Thales Optem) was used, anda �2 lm resolution was obtained. For illumination acold light source was used.

A commercial organic silver ink (TEC-IJ-020) with20 wt% silver contents for the spray was purchasedfrom InkTec Elec., South Korea. The surface tensionand viscosity of the ink were specified to be in therange of 30–32 dyne/cm and 9–15 cps, respectively. Themorphology and the chemical composition of thedeposits were examined by a scanning electron micro-scope (SEM, FEI Inspect F) and an energy-dispersiveX-ray spectrometer (EDX, Oxford INCA x-act),respectively. The topography of the patterns wasmeasured by an atomic force microscope (AFM,NTEGRA NT-MDT).

Results and discussion

Scanning electron microscope (SEM) images in Fig. 2show ink patterns on an Si substrate created by usingpulsed voltage at 550 V with varying durations. Afterdeposition, the ink patterns on an Si substrate wereimmediately cured at 220�C to form metallic silver. Nopattern was found on the substrate when pulse dura-tion was applied for less than 50 ms. When pulsedurations from 50 ms to 1 s were applied regular dotswith mean diameters in the range of 118–227 lm werecreated. In the case of the longer pulses applied, thesize of deposits is clearly shown to increase with the

High voltage

Nozzle

Coldpower supply source

light

Steelwire

Ink

CCD

JettingSubstrate

Fast voltage

switch

PCcontroller

Movable

stage

Electrode

250 µm

Fig. 1: Schematic configuration of a nanoelectrospray setup in a fully pulsed-voltage controlled form. Insets show anoptical image (top) of an electrified jet from the nozzle and an SEM image (bottom) of the exit of a 15 lm nozzle used in thespray experiment, respectively

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voltage duration. At a high magnification of 50 k9,SEM revealed that the pattern created by a pulse with50 ms mainly consists of particles, shown in Fig. 2a.The ‘‘dewetting’’ surface observed can be attributed toelectrostatic repulsion in the charged droplets andinsufficient ink materials at such a small depositionvolume. No pronounced increase in the coverage of thedeposition was exhibited until a pulse of 200 ms wasapplied. The coverage of the surface was graduallyimproved with increasing voltage duration, revealed bySEM at a high magnification. The inset image revealedthat the pattern created by a pulse of 200 ms formed aninterconnected structure, indicating sufficient ink wasdeposited and that flow occurred. On increasing thepulse duration to 500 ms a characteristic ‘‘coffee stain’’rim appeared in the pattern produced, shown inFig. 2d. This typical feature is commonly observed inthe jet deposition of materials in a liquid form. Itclearly shows evidence of a long-ranged outwardmaterial flowing over the surface at this mediumdeposition volume, which carries the materials fromthe center to the edge of the pattern due to a rapidevaporation rate. However, a long pulse of 800 msformed a crack in the central regime of the deposit inFig. 2e. The formation of the crack can be ascribed to alarge deposition volume generated by the long pulseand an excess of accumulation of material in thecentral position, which cannot spread fast enough overthe substrate surface during the evaporative drying ofwet material. During curing for thermal decomposition

of the ink, the stress build-up from the releasing ofgaseous byproducts can cause a fracturing of the film,particularly in the thick regime. Indeed, a large amountof materials accumulating in the central regime with anappearance of a dome was observed by an opticalmicroscope after evaporative drying of ink. Furtherincreasing pulse duration to 1 s makes both patternsize and crack regime larger, shown in Fig. 2f.

Figure 3 illustrates the chemical composition deter-mined from the surface of the deposits after curing.Here, the spectra from the deposits created by thepulse voltage at 550 V with 50 ms and 1 s durationwere chosen for a comparison. The EDX spectrumconfirms that the main constituent in both relics issilver. The additional evidence that it is an Si elementcomes from the substrate. The ratio change of silverpeak intensity to Si peak in the spectra is attributed tothe different coverage of the two patterns. Excellentconductivity in the range of 2�4 9 107 S m�1, which isclose to the theoretical value of bulk silver, wasobtained in the printed tracks that were fabricated ina drop-on-demand manner by this method.6,19

Scanning electron microscopy images in Fig. 4 showink patterns created by the voltage pulse of 800 V withan on-time variable from 10 ms to 1 s. When voltagesof 800 V with duration from 50 ms to 1 s were applied,circular deposits with diameters in the range of 138–290 lm were created. When the pulse duration of10 ms was applied, cloud-like patterns with irregularshapes were found to be formed. At the higher

Fig. 2: SEM images of silver patterns formed by pulsed voltage of 550 V with duration of (a) 50 ms, (b) 100 ms, (c) 200 ms,(d) 500 ms, (e) 800 ms, and (f) 1 s, respectively. Insets show local regime at a higher magnification. After deposition, thepatterns were subsequently cured at 220�C for 10 min for the development of metallic silver. A ‘‘coffee staining’’ rimappeared in the pattern created with a pulse of 500 ms and cracks formed when a pulse up to 800 ms was used

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magnification of 50 k9, the inset revealed that ultrafineparticles were formed in the sprayed area. The forma-tion of ultrafine particles was attributed to dropletbreak-up. When the pulse duration reached 50 ms,a relatively dense film was formed in the depositions.A clear ‘‘coffee stain’’ rim appears with the appliedpulse up to 200 ms. Film cracks were formed when thepulse duration reached 400 ms.

After curing, high-resolution SEM revealed that thepatterns created by pulses longer than 100 ms consistedof densely packed silver nanocrystals. The thickness ofthe patterns was measured to be quite constant,�200 nm in all the depositions performed at the250 lm spray distance. Figure 5 exemplifies an SEMimage of the pattern surface created by a pulse of700 V and 200 ms. It revealed that silver nanocrystalswere arranged in a sheet-like structure in the films. Thediameter and mean thickness of the pattern weredetermined to be around 135 lm and 200 nm, respec-tively, by AFM.

The size and the thickness of the deposited silvermaterials were used to estimate the total volume of inkejected by a pulse. Roughly assuming an ideal cylindershape, the volume of the silver pattern can be estimatedby the equation VAg ¼ p � d2 � h=4, where d and h arethe diameter and the thickness of the pattern, respec-tively. The volume of silver materials created by a pulseof 700 V and 200 ms is calculated to be VAg = 2.9 910�15 m3 (�3 pL) by using the value of d = 135 lm andh = 200 nm of the pattern. Considering the density of

1

1 2 3 4 5 6 7 8

2 3 4 5 6 7 8keV

keV

Full scale 7500 cts cursor: 7.983 (254 cts)

Full scale 59155 cts cursor: 8.261 (464 cts)

Si

Si

Ag

Ag

Ag

Ag

(a)

(b)

Fig. 3: Energy-dispersive X-ray (EDX) spectra of two curedpatterns. Spectrum (a) and spectrum (b) were obtainedfrom deposits produced by voltage pulse at 550 V with50 ms and 1 s durations, respectively

Fig. 4: SEM images of silver patterns formed by pulse voltages at 800 V with a duration of (a)10 ms, (b) 50 ms, (c) 200 ms,(d) 400 ms, (e) 500 ms, and (f) 1 s, respectively

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the ink qInk = 1.07 g/cm3 and the silver qAg = 10.5g/cm3 and the weight content of silver in the ink is 20%,the total volume of the ink ejected by one pulse is givenby V ¼ 5qAg � VAg=qInk. The total volume of the inkdeposited in 200 ms at 700 V was estimated to be140.3 9 10�15 m3, i.e., �140 pL.

The deposition volume of ink was plotted againstpulse duration at different voltages in Fig. 6. The totaldeposition volume at each duration was estimated asexplained above. Five depositions were performed foreach condition and an average of the five volumes wasmade. A good linear fitting with a regression coefficientbetter than 99.2% between the deposition volume andpulse duration was obtained, confirming that thevolume of a collected drop is proportional to the pulseduration assuming that the time delay (� several ms)to form a Taylor cone can be ignored.27 The poor fit ofpoints at short pulse times is thought to arise from

incomplete coverage that leads to an over-estimationof deposition volume. The slopes of the plots are ratesof volume deposition in units of pL/ms. The ratesincrease from 0.38 to 0.59 pL/ms over a range of 550 to800 V in the cone-jet mode.

Conclusions

In brief, drop-on-demand delivery of silver nano-inkwas demonstrated by fully voltage-controlled nano-ESin pulsed cone-jet mode. The morphology of thedeposits was observed to change pronouncedly withincreasing deposition volume. A good linear relation-ship between the deposition volume and the voltageduration was demonstrated. The deposition volumerate, determined in the order of pL/ms in magnitude,was shown to increase with the pulsed voltage.

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