Direct fabrication and morphology of metallic micropatterns by pulsed jet nanoelectrospraying of silver nano-ink.

Abstract A pulsed jet nanoelectrospray technique was applied to direct fabrication of silver micropatterns. The deposition of a commercial organic silver nano-ink was performed in a fully voltage-controlled fashion by voltage pulses ranging from 550 to 800 V with variable durations. By using 15 [micro]m nozzles, patterns with 100-[mu]m-sized features were locally freeformed on a silicon substrate with a spraying distance of 250 [micro]m. An energy-dispersive X-ray spectrum confirmed metallic silver was developed in all the patterns after heat treatment at 220[degrees]C The size and micro-structural evolution of silver films was observed to strongly depend on the deposition volume and material flow over substrate surface. A good linear relationship between the deposition volume and pulse duration was exhibited over the applied voltage range in the cone-jet mode, demonstrating a drop-on-demand capability. By fitting, the deposition volume rate was estimated to be in the range of 0.38-0.59 pL/ms and was shown to increase with the applied voltage.

Keywords Nanoelectrospray technique, Flexible jet printing, Free form prototyping, Microfabrication, Surface encapsulation

Introduction

Recently, the electrospray (ES) technique has been rapidly developing as a novel rapid prototyping tool due to its wide applications in nanoscience, drug microencapsulation, and printed electronics. (1-8) In the ES process, a conductive liquid fed by a capillary is subjected to an electrical field. The electric stresses deform the liquid meniscus into a conical shape, known as a Taylor cone. (9) When the electrical field is increased to overcome the surface tension of the liquid, a jet is emitted from the apex of the cone and droplets are subsequently ejected. (10) Consequently, functional material contained in the droplets can be delivered or freeformed locally on the desired position without a selective masking and complicated wet-chemical processing. Different ES modes such as dripping, spindle, pulsation, cone-jet, and multijets might be induced by varying the applied voltage and flow rate. (11) Among them, spray modes including microdripping, pulsation, and cone-jet have been demonstrated to be used for deposition. (5), (7), (12), (13) Structured patterns can be created using a continuous liquid stream driven by a DC high voltage while moving the substrate. (14), (15)

Compared with its continuous ES counterpart, the pulsed jet ES technique has attracted particular attention due to the application in mass spectrometer (MS) analysis for its high sensitivity and good signal-to-noise ratios. (16) A stream or spray of uniform droplets can be produced by switching on/off a driving voltage. This "drop-on-demand" mode provides excellent control over the placement of individual droplets, rather than the deposition using continuous stream. By using a pulsed jet, Yogi et al. demonstrated drop-on-demand deposition ranging from picoliter to femtoliter volume. (17) Chen et al. reported pulsed ES of de-ionized water with a 50-[mu]m Teflon nozzle. (18) Uniformed tracks with excellent conductivity and truly electronic devices were fabricated on Si substrates. (6), (19), (20) Deposition dot size below 10 [micro]m was also achieved by pulsed electrospraying of aqueous gold colloids. (21) Our ES configuration was performed in a full voltage-controlled fashion without the additional assistance of pump or gas pressure. This unforced feature makes the system relatively simple. (6), (13), (22) Meanwhile, by using a micron-sized nozzle instead of a millimeter-sized one, spraying was operated in a nano-ES manner. (23) Very recently, an equivalent circuit method was proposed to analyze the spraying properties, which can well capture the current-voltage characteristics in this unforced nano-ES. (24)

In this article, a commercial organic silver ink was deposited on Si substrates by using a pulsed cone-jet in a fully voltage-controlled configuration. While previous works have investigated the pulsed ES characteristics such as jet stability, charge relaxation time, drop generation frequency, and driving schemes, (18), (25-29) this work focuses on the study of microstructural evolutions of the material deposited by pulsed voltage and a better quantitative controlling of material delivery in electrosprayed thin film materials.

Experimental details

Figure 1 illustrates the experimental configuration for unforced e-jet printing, similar to our previously published apparatus. (6), (19) The nozzle used for the deposition was produced by pulling borosilicate capillaries to form an outlet with a 15 [micro]m exit, which was confirmed by SBM. The ink for spraying was fed into the nozzle and a stainless steel wire was submerged into the ink material; this wire was held at ground potential. A 2 kV high-voltage supply from F.u.G. Electronik was connected to a fast voltage switch (PVX4130, DEI) to generate voltage pulses. The pulsed voltage was applied to an aluminum electrode, on top of which an Si substrate for collection was attached. The aluminum electrode was fixed to a PC-controlled movable translation stage. The distance between the Si substrate and the nozzle was adjusted to 250 [micro]m. The shape of the liquid meniscus at the nozzle tip during the spray was monitored and recorded by an optical microscope coupled with a high-resolution CCD video camera (V500, Sony). For the microscope an infinity corrected objective lens (10x, Mitatoyu) on a variable zoom (12.5 x, Thales Optem) was used, and a ~2 [micro]m resolution was obtained. For illumination a cold light source was used.

A commercial organic silver ink (TEC-IJ-020) with 20 wt% silver contents for the spray was purchased from InkTec Elec., South Korea. The surface tension and viscosity of the ink were specified to be in the range of 30-32 dyne/cm and 9-15 cps, respectively. The morphology and the chemical composition of the deposits were examined by a scanning electron microscope (SEM, FEI Inspect F) and an energy-dispersive X-ray spectrometer (EDX, Oxford INCA x-act), respectively. The topography of the patterns was measured by an atomic force microscope (AFML NTEGRA NT-MDT).

[FIGURE 1 OMITTED]

Results and discussion

Scanning electron microscope (SEM) images in Fig. 2 show ink patterns on an Si substrate created by using pulsed voltage at 550 V with varying durations. After deposition, the ink patterns on an Si substrate were immediately cured at 220[degrees]C to form metallic silver. No pattern was found on the substrate when pulse duration was applied for less than 50 ms. When pulse durations from 50 ms to 1 s were applied regular dots with mean diameters in the range of 118-227 jam were created. In the case of the longer pulses applied, the size of deposits is clearly shown to increase with the voltage duration. At a high magnification of 50 kx, SEM revealed that the pattern created by a pulse with 50 ms mainly consists of particles, shown in Fig. 2a. The "dewetting" surface observed can be attributed to electrostatic repulsion in the charged droplets and insufficient ink materials at such a small deposition volume. No pronounced increase in the coverage of the deposition was exhibited until a pulse of 200 ms was applied. The coverage of the surface was gradually improved with increasing voltage duration, revealed by SEM at a high magnification. The inset image revealed that the pattern created by a pulse of 200 ms formed an interconnected structure, indicating sufficient ink was deposited and that flow occurred. On increasing the pulse duration to 500 ms a characteristic "coffee stain" rim appeared in the pattern produced, shown in Fig. 2d. This typical feature is commonly observed in the jet deposition of materials in a liquid form. It clearly shows evidence of a long-ranged outward material flowing over the surface at this medium deposition volume, which carries the materials from the center to the edge of the pattern due to a rapid evaporation rate. However, a long pulse of 800 ms formed a crack in the central regime of the deposit in Fig. 2e. The formation of the crack can be ascribed to a large deposition volume generated by the long pulse and an excess of accumulation of material in the central position, which cannot spread fast enough over the substrate surface during the evaporative drying of wet material. During curing for thermal decomposition of the ink, the stress build-up from the releasing of gaseous byproducts can cause a fracturing of the film, particularly in the thick regime. Indeed, a large amount of materials accumulating in the central regime with an appearance of a dome was observed by an optical microscope after evaporative drying of ink. Further increasing pulse duration to 1 s makes both pattern size and crack regime larger, shown in Fig. 2f.

[FIGURE 2 OMITTED]

Figure 3 illustrates the chemical composition determined from the surface of the deposits after curing. Here, the spectra from the deposits created by the pulse voltage at 550 V with 50 ms and 1 s duration were chosen for a comparison. The EDX spectrum confirms that the main constituent in both relics is silver. The additional evidence that it is an Si element comes from the substrate. The ratio change of silver peak intensity to Si peak in the spectra is attributed to the different coverage of the two patterns. Excellent conductivity in the range of 2~4 x [10.sup.7] S [m.sup.-1] which is close to the theoretical value of bulk silver, was obtained in the printed tracks that were fabricated in a drop-on-demand manner by this method. (6), (19)

Scanning electron microscopy images in Fig. 4 show ink patterns created by the voltage pulse of 800 V with an on-time variable from 10 ms to 1 s. When voltages of 800 V with duration from 50 ms to 1 s were applied, circular deposits with diameters in the range of 138-290 [micro]m were created. When the pulse duration of 10 ms was applied, cloud-like patterns with irregular shapes were found to be formed. At the higher magnification of 50 kx, the inset revealed that ultrafine particles were formed in the sprayed area. The formation of ultrafine particles was attributed to droplet break-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 applied pulse up to 200 ms. Film cracks were formed when the pulse duration reached 400 ms.

After curing, high-resolution SEM revealed that the patterns created by pulses longer than 100 ms consisted of densely packed silver nanocrystals. The thickness of the patterns was measured to be quite constant, ~200 nm in all the depositions performed at the 250 [micro]m spray distance. Figure 5 exemplifies an SEM image of the pattern surface created by a pulse of 700 V and 200 ms. It revealed that silver nanocrystals were arranged in a sheet-like structure in the films. The diameter and mean thickness of the pattern were determined to be around 135 [micro]m and 200 nm, respectively, by AFM.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The size and the thickness of the deposited silver materials were used to estimate the total volume of ink ejected by a pulse. Roughly assuming an ideal cylinder shape, the volume of the silver pattern can be estimated by the equation [V.sub.Ag] = [pi] * [d.sup.2] * h/4, where d and h are the diameter and the thickness of the pattern, respectively. The volume of silver materials created by a pulse of 700 V and 200 ms is calculated to be [V.sub.Ag] = 2.9 x [10.sup.-15] [m.sup.3] (~3 pL) by using the value of d = 135 [micro]m and h = 200 nm of the pattern. Considering the density of the ink [[pho].sub.Ink] = 1.07 g/c[m.sup.3] and the silver [[pho].sub.Ag] = 10.5 g/c[m.sup.3] and the weight content of silver in the ink is 20%, the total volume of the ink ejected by one pulse is given by V = 5[[pho].sub.Ag] * [V.sub.Ag]/ [[pho].sub.Ink]. The total volume of the ink deposited in 200 ms at 700 V was estimated to be 140.3 x [10.sup. -15] [m.sup.3], i.e., ~140 pL.

The deposition volume of ink was plotted against pulse duration at different voltages in Fig. 6. The total deposition volume at each duration was estimated as explained above. Five depositions were performed for each condition and an average of the five volumes was made. A good linear fitting with a regression coefficient better than 99.2% between the deposition volume and pulse duration was obtained, confirming that the volume of a collected drop is proportional to the pulse duration assuming that the time delay several ms) to form a Taylor cone can be ignored. (27) The poor fit of points at short pulse times is thought to arise from incomplete coverage that leads to an over-estimation of deposition volume. The slopes of the plots are rates of volume deposition in units of pL/ms. The rates increase from 0.38 to 0.59 pL/ms over a range of 550 to 800 V in the cone-jet mode.

Conclusions

In brief, drop-on-demand delivery of silver nano-ink was demonstrated by fully voltage-controlled nano-ES in pulsed cone-jet mode. The morphology of the deposits was observed to change pronouncedly with increasing deposition volume. A good linear relationship between the deposition volume and the voltage duration was demonstrated. The deposition volume rate, determined in the order of pL/ms in magnitude, was shown to increase with the pulsed voltage.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

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K. Wang (*), J. P. W. Stark

School of Engineering and Materials Science, Queen Mary, University of London, London El 4NS, UK

e-mail: [e-mail]; [e-mail]

K. Wang

Department of Physics, University of Bath, Bath BA2 7AY, UK

DOI 10.1007/s11998-011-9370-x