Direct Printing of Nanostructures by Electrostatic Autofocussing of Ink Nanodroplets

8/19/2019 Direct Printing of Nanostructures by Electrostatic Autofocussing of Ink Nanodroplets

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NATURE COMMUNICATIONS | 3:890 | DOI: 10.1038/ncomms1891 | www.nature.com/naturecommunications

© 2012 Macmillan Publishers Limited. All rights reserved.

Received 7 Dec 2011 | Accepted 8 May 2012 | Published 12 Jun 2012 DOI: 10.1038/ncomms1891

Nanotechnology, with its broad impact on societally relevant applications, relies heavily on theavailability of accessible nanofabrication methods. Even though a host of such techniques exists,

the flexible, inexpensive, on-demand and scalable fabrication of functional nanostructures

remains largely elusive. Here we present a method involving nanoscale electrohydrodynamic

ink-jet printing that may significantly contribute in this direction. A combination of nanoscopic

placement precision, soft-landing fluid dynamics, rapid solvent vapourization, and subsequent

self-assembly of the ink colloidal content leads to the formation of scaffolds with base diameters

equal to that of a single ejected nanodroplet. The virtually material-independent growth of

nanostructures into the third dimension is then governed by an autofocussing phenomenon

caused by local electrostatic field enhancement, resulting in large aspect ratio. We demonstrate

the capabilities of our electrohydrodynamic printing technique with several examples, including

the fabrication of plasmonic nanoantennas with features sizes down to 50 nm.

1 Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, Department of Mechanical and Process Engineering,

ETH Zürich, CH-8092 Zürich, Switzerland. 2 Laboratory of Physical Chemistry, ETH Zürich, CH-8092 Zürich, Switzerland. † Present address:

Max Planck Institute for the Science of Light, 91058 Erlangen, Germany. Correspondence and requests for materials should be addressed to

D.P. (email: [email protected]).

Direct printing of nanostructures by electrostatic

autofocussing of ink nanodroplets

P. Galliker1, J. Schneider1, H. Eghlidi1,2, S. Kress1, V. Sandoghdar2,† & D. Poulikakos1


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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms1891



Alarge pallet o techniques has been explored in the past ewdecades or nanoabrication, including powerul approachessuch as photo- and electron-beam lithography, ocussed-ion

beam milling as well as nanoparticle-based sel-assembly 1–7. A par-ticularly attractive approach is printing by ink jet4, electrohydro-dynamics1,5,7,8 and related techniques2,3, in which a nanoparticle-laden ink is dispensed rom a scanning nozzle. Although thesestrategies provide flexible and inexpensive abrication platorms, various physical and experimental processes have limited their res-

olution. Electrohydrodynamic (EHD) liquid e jection, well knownrom its use in electro-spinning9 and -spraying10, has recently beenemployed in microabrication or the production o defined planarentities o the order o a micrometre1,7.

EHD ejection o liquid rom a pipette nozzle is induced by therelaxation o ions to the liquid surace, on activation by an electricfield. Te ions are naturally present in the liquid (same amount opositive and negative species) and are its electrically conductivespecies. Te induced electric stresses at the liquid–air interace canultimately overcome the resisting effect o surace tension, causingliquid ejection. Depending on the direction o the electric field,either an excess o positive or negative ions will drive the ejection oliquid and attribute to its non-zero electric charge. Whereas differ-ent EHD ejection modes have been identified11, most related appli-

cations, including those related to microabrication1,7, are based onthe so-called cone-jet ejection mode in which a thin jet is releasedrom the tip o a larger sharply converging liquid cone at high vol-ume flow rates10–12. Another mode o ejection, called microdrip-ping5,11,12, results in the periodic ejection o single, micron-sizedspherical droplets rom the apex o a larger steady liquid meniscus.

Here we downscale this mode to the regime o nanodripping,while avoiding clogging o the dispenser tip by a sufficient reductiono the nanoparticle concentration. We will show that afer dropletimpact and solvent vapourization, nanoparticles are uniormly dis-tributed in an area essentially equivalent to the diameter o a singledroplet. In addition, we take advantage o an electrostatic nanodro-plet autoocussing (ENA) effect to generate high aspect ratio outo plane nanostructures, built rom the nanoparticles contained

in the individual nanodroplets. Working together, the capabilitieso this process allow the realization o nanostructures o differentgeometries and materials, which have homogeneous lateral sizeequal to that o a single nanodroplet.

ResultsWorking principle and set-up. Figure 1a displays the essentialsteps in the temporal sequence o ENA NanoDrip printing. In thefirst step, a voltage is applied between the pipette/ink and a counterelectrode, resulting in the generation o a meniscus and periodicejection o ink nanodroplets rom its lowest point. Once ejected,the nanodroplet is accelerated by the electric field towards thesubstrate where viscous effects pronounced by the nanodropletsize acilitate a sof landing and, owing to wetting-enhanced rapid

vapourization, the droplet can dry during an ejection period τ e.Once the solvent has vapourized, the nanoparticles, ormerlydispensed in the ink droplet, are ound to be uniormly spread in anarea corresponding to the projection o the spherical nanodroplet(Fig. 1b). Periodic occurrence o these events leads to a narrowaccumulation o nanoparticles on the substrate. As this structuregrows in height (Fig. 1c,d), its tip acts as a sharp electrode, creatingstrong electric-field gradients that ocus the incoming dropletstowards it and allow the structure to grow at a homogeneousdiameter. Te interval between voltage initiation and termination isdenoted as pulse, as it has a defined duration and a sharp rise and all(Fig. 1a). However, the pulse length does not influence the ejectionrequency, nor the ejection process in general. Te only purposeo using pulses is to precisely control the number o depositeddroplets, that is, nanoparticles, and therewith the extent o printed

nanostructures. It is worth stressing that our terminology o a pulseshould not be conused w ith that o pulsed EHD jet-type processesreported by other authors7,8. In these studies, the pulse duration isrelated to the amount o liquid accumulated on the substrate, andis thereore an important process parameter affecting the lateralsize o printed structures, afer evaporation o the accumulatedliquid. In contrast, the remarkable eatures o the nanodrippingmode attained here, allow the solvent o each ejected and depositeddroplet to be removed beore the impact o the next one. Te extent

o accumulated liquid is thereore independent o the duration othe applied pulse.

Te experimental set-up is shown schematically in Fig.1e anddiscussed in more detail in the Methods. Te essential requirementsand related physical mechanisms needed to achieve the nanodrip-ping mode are also commented on in the Methods. I not specifi-cally mentioned, the ink in all experiments consisted o a dispersiono 3–7 nm gold nanoparticles in n-tetradecane. Te nozzle was fixedat a separation o 3–4µm rom the substrate, and a piezo-stage wasused to position the substrate with nanometer accuracy at will. Teprinted structures were detected and imaged during deposition byan in-house-built microscope (Fig. 1e and Methods). We remark,however, that the nanodroplets could not be detected during flightbecause o their large velocity magnitude o up to the order o

100 m s − 1 (Supplementary Methods and Supplementary Fig. S1).We used scanning electron microscopy (SEM) or a quantitativeanalysis o the nanoparticle patterns o a single droplet afer deposi-tion and solvent vapourization (Fig. 1b). For brevity, these patternswill be denoted as ootprints in the ollowing. Footprints are ocrucial importance to ENA NanoDrip printing, because they allowthe determination o the droplet size and ejection requency. In addi-tion, ootprints help us understand the physical mechanisms behindthe emergence o the resulting nanostructure geometries. Singleootprints were obtained by fixing the nozzle in space while separat-ing the sequential droplet impact positions through a sufficientlyrapid movement o the substrate by the piezoelectric stage. Withthe knowledge o the substrate velocity and the spatial separation oootprints on the substrate, it is possible to evaluate an exact value

or the ejection requency (Fig. 2a). Te lateral extent o ootprintswas ound to approximately represent the droplet diameter. Tisunexpected result is based on an analysis o the volume o printednanostructures (Supplementary Fig. S2) in relation to the appliedpulse length. Dividing the resulting ejection flow rate (ejected volume per pulse length) by the experimentally obtained ejectionrequency yielded droplet sizes approximately matching the sizeo ootprints (Supplementary Fig. S3 and Supplementary Methods).It is also ound that the largest ootprint diameters match the outerdiameter o the employed pipette. As no other orces than the elec-tric and surace tension orces are present (gravitation is negligible),the largest possible droplets are indeed expected to be similarly sizedas the outer pipette diameter. Further reasoning that strengthens theequality argument o droplet and ootprint sizes is provided later.

aking into account that ootprints approximately represent thesize o an ejected spherical droplet, their spatial extent could beemployed to determine the size o droplets as a unction o applied voltage (Fig. 2a). Combining requency and size, the ejection flowrate could be determined (Fig. 2b). We find that with a nozzle oouter diameter ~1,200 nm, ootprints as small as 80 nm were pro-duced at an ejection requency o several tens o kilohertz. o illus-trate the homogeneous nature o requency and droplet size in thenanodripping mode, we show in Supplementary Fig. S4 a series oseparated ootprints. Tese ootprints are practically equidistantand o equal size. Tese and all other experiments were perormedwithout active eedback control o the nozzle–substrate distance.Tat active eedback control is not a necessary requirement or areproducible printing process is also confirmed by additional con-siderations and experiments on the sensitivity o the results on the


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distance between nozzle and substrate (Supplementary Fig. S5 andSupplementary Methods).

Numerical and experimental analysis of the ejection process . oestimate the diameter o the ejected droplets theoretically, we use asemi-numerical approach where we employ a simple geometricalmodel in which the small droplet to be ejected is pendant at thelower end o a hemispherical meniscus (Fig. 2c). On the basis othis model, we then calculate the orces acting onto the small drop-let. Although the hemispherical meniscus shape has been observedbeore5,12,13, we experimentally verified that it develops with asimilar shape also or the small nozzle sizes employed in our study(Supplementary Fig. S6). Along this line, the size o the drop-lets can be predicted by a balance o electric and surace tensionorces, whereby the electric orce is calculated by the finite elementmethod (Fig. 2c shows the resulting electric field distribution) withthe assumption that nozzle, meniscus and pendant droplet are all

equipotential (see Methods). Green symbols in Fig. 2a show thecalculated droplet diameters as a unction o the applied voltages.

Te agreement with the experimental data is very good up to theregime marked by the dashed arrow, where the liquid charge relaxa-tion time τ ε (ratio o liquid permittivity and electrical conductivity)equals the ejection period, τ e, rendering the equipotential assump-tion invalid. We find that this regime change also corresponds to aminimum in the flow rate. For larger applied voltages, intensifiedelectric stresses are produced at the meniscus, whereas charges can-not relax to the pendant droplet surace ast enough. As a result,a large liquid flow cannot be counteracted by a urther reductiono the pendant droplet diameter, and the flow rate strongly increasesor τ e below ~τ ε. Tat we can reproduce both the evolution o thedroplet diameter or τ e >τ ε and the divergence o experimental andtheoretical data or τ e


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is not to reproduce the ejection dynamics (flow rate estimates arenot possible). Tis task is highly complex and deserves a separatededicated study, beyond the purpose o the present experimentalwork. Here we are only interested in a simple prediction o the sizeo ejected droplets, as direct observation was not possible.

In Fig. 3a, we show, in addition to the ootprint sizes o Fig. 2a,a second dataset where a nozzle with a diameter o 600 nm (hal othat used beore) was employed. It is ound that the minimal ejec-tion voltage (the voltage needed or ejecting a droplet with a diam-eter equal to that o the meniscus) decreases with decreasing nozzlediameter. In the Supplementary Methods, we illustrate theoreticallythe generality o this behaviour. Also plotted in Fig. 3a are numeri-cal estimates o the droplet size. Evidently, the ootprint diameters

agree well with the theoretical estimates o the droplet size, confirm-ing reproducibility o the numerical results. Te presented data werethen nondimensionalized by dividing the actual ootprint diameterby the diameter o the meniscus, and by dividing the applied voltageby the minimal ejection voltage (see Supplementary Methods orreasoning). Figure 3b shows that the diameters o the nondimen-sional ootprints are practically identical. In both cases, the smallestdroplet size is ~1/15 o the nozzle size. Te merging o the dimen-sionless datasets as well as the agreement o our numerical estimatewith the experimental results substantially underpins the claimedequality o ootprint and droplet diameter. In Fig. 3a, we also plotthe dimensional ejection requency as a unction o nondimen-sional voltage. Te ejection requency seems to be nearly unaffectedby the nozzle size variation, which implies that the flow rate at thesame nondimensional voltage approximately scales with the droplet

1,000a

b

c

100

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100 150 200 250

Applied voltage (V)

Applied voltage (V)

E

j e c t i o n f r e q u e n c y ( k H z )

D i a m e t e r ( n m )

F l o w

r a t e ( µ m

3 s – 1 )

300 350

100 150 200 250 300

0 V m–1

5e7

1e8

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2e8

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Figure 2 | Summary of measured and numerically derived key quantities

for a 1.2m nozzle. (a) Experimentally obtained footprint diameter (full

black squares) and ejection frequency (open circles) and numerically

derived droplet diameter (full green triangles connected by straight green

lines) as a function of applied voltage. The dashed arrow indicates the

voltage threshold at which the charge relaxation time (τ ε ) equals the

ejection period (τ e). Data points are based on up to 50 averaged values

with standard errors below 5% (diameter) and 10% (frequency). The

highest errors reach 10% (diameter) and 30% (frequency). (b) Flow rate

of ejected fluid calculated from experimental footprint and frequency data.

(c) Nozzle geometry used for numerical calculation of the electric force

acting onto the liquid. The surface plot shows the calculated z-component

of the electric field around the modelled nozzle region during droplet

ejection. The nozzle is depicted with a green line and the meniscus with

a blue line. The image depicts a state in which the surface tension of

the pendant droplet is matched by the electric stress and the droplet is

therefore ready to be ejected.

1,000

100

a

b 100

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1.0 1.5 2.0

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N o n d i m e n s i o n a l d r o p l e t

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Figure 3 | Reproducibility with respect to different nozzle diameters.

(a) Experimental (black symbols) and numerically (green symbols)

derived footprint/droplet diameter for a 1.2-µm nozzle (squares, reprinted

from Fig. 2) and a 600-nm nozzle (triangles). The minimal ejection

voltage for the smaller nozzle was found to be ~20 V lower than that of

the larger one. (b) The same experimental footprint data (squares) after

nondimensionalizing (droplet size divided by nozzle size, applied voltage

divided by minimal ejection voltage), with full symbols representing

the large and open symbols representing the small nozzle. The fact that

the datasets merge suggests that footprint sizes do indeed represent

the droplet size, which is elucidated by additional experiments in the

Supplementary Methods. We also plot the ejection frequency in absolute

values (circles). At the same nondimensional voltage, frequencies

observed for both nozzles, are about the same. This suggests an almost

constant ejection frequency independent of the nozzle diameter.


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volume. At least or a droplet o the size o the meniscus, this maybe understood by scaling the flow rate with the help o the classicalPoiseuille equation (Supplementary Methods).

Nanostructure growth initiation. Te cyclic release and deposition

o spherical droplets, one at a time, in nanodripping allows enoughtime or the efficient removal o solvent by vapourization during τ e.Tis lets nanoparticles o a multitude o ootprints accumulate intoa tight scaffold, which only increases in height while maintainingconstant lateral size. Tis mechanism is in great contrast to that ocone-jet printing in which the ink and the accompanying nanopar-ticles continuously spread on the substrate owing to the high imme-diate volume flow rates7,8.

Fast vapourization o a droplet implies that to avoid clog-ging at the nozzle, the volumetric rate o vapourization at a singledeposited droplet must be higher than that at the meniscus itsel(Supplementary Methods). Our experiments show that this is ul-filled, i the ink is able to sufficiently wet the substrate surace14.Indeed, the employed glass slips exhibited excellent wetting proper-

ties with contact angles


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between the actual and aspired positions is in the range o only a ewnanometers. Te spacing between structures can be urther reducedas shown by a pair o sequentially printed nanopillars o diam-eter 120 nm, separated by a gap o 80 nm (Fig. 5c,d). Tis exampleemphasizes that moving the central impact region by an amount assmall as the structure diameter is sufficient to prevent droplets rombeing attracted to the previously printed nanopillar.

Te attainable positioning accuracy and resolution are ur-ther demonstrated in Fig. 5e with an example o printed in-plane

tracks. Tese were created by introducing a constant relative move-ment between the nozzle and the substrate during droplet ejection.Figure 5e displays tracks at pitch sizes ranging rom 75 nm to250 nm, where ull separation is achieved or track distances downto 100 nm. Atomic orce microscopy profiles urther reveal repro-ducible track heights o about 40 nm (Fig. 5e, inset). Such tracks maybe employed as electrical conductors. Supplementary Figure S11provides an example o a printed track afer an annealing treatment,proving that those structures do not loose integrity when beingsintered but instead orm one entity. Quantitative results onthe electrical behaviour o printed tracks in relation to differentannealing treatments will be dedicated to uture studies.

Flexibility towards growth direction and material. Interestingly,ENA also allows abrication o nanopillars at a variety o tilting

angles, paving the way towards the ormation o a wide palette oout-o-plane nanostructures. Tese tilted structures are created bya slow lateral movement o the piezo-stage during the depositionprocess. Te resulting shif o the projected droplet impact posi-tion with respect to the apex o a growing nanostructure will induceENA to bend the trajectory o the approaching droplet towards thenanostructure extremity. Te direction at which a droplet impactsat the structure apex depends on the velocity o the stage move-ment and ultimately on the velocity at which the structure grows.

For example, i the piezo-stage moves at hal the structure growth velocity, the nanostructure will grow at a 45° tilting angle. Tis couldbe reproduced by growing tilted nanopillars at varying piezo-stage velocities, where the ti lting angle was ound to be proportional tothe velocity o the substrate movement (Fig. 6).

Finally, we point out that ENA NanoDrip printing can be easilyadopted or growth rom materials other than gold or even non-metals as long as their dielectric constants are higher than that othe surrounding medium. o demonstrate this, we have successullyprinted nanopillars made o silver and zinc oxide (SupplementaryFig. S12).

ENA NanoDrip printing for plasmonics. ENA NanoDrip print-ing paves the way or easy and versatile creation o a large varietyo complex nanostructures. An application o particular interest in

100 200

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300 400

Profile width (nm)

500 600 700 800 9000

0

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Figure 5 | SEM micrographs of printed nanostructures with dimensionalities from 0D to 2D. (a) Gold nanopillar of diameter ~50 nm and aspect ratio

of ~17 (Scale bar, 200 mm). (b) Top and (c) side view of nanopillars printed subsequently at 200 nm center-to-center distance (scale bar, 200 nm).

(d) 80-nm wide dots printed into a 1-µm lattice constant array (1µm scale bar). (e) Printed tracks with pitch sizes of 250, 200, 150, 100 and 75 nm(scale bar, 2 µm). The inset shows AFM (full black lines) and SEM (red dashed lines) profiles of 150-nm pitch size. The height of AFM profiles is

given in nanometers. The SEM profiles are in arbitrary units. Tracks have reproducible heights of ~40 nm and are well separated.


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the past hal-a-decade concerns metamaterials20 and optical nano-antennas21 made o plasmonic nanostructures in various shapessuch as split ring, split-rod, bow-tie and so on. Currently, thesestructures can only be abricated using e-beam lithography (EBL) orocussed-ion-beam (FIB) milling, both o which are very expensiveand thus not available to many researchers. ENA NanoDrip printingpromises to deliver the same perormance or optical antennas andmetamaterials. As an example, we have printed a so-called Yagi-Udaantenna (optical analogue o the old V antennas). Figure 7a dis-plays a SEM image o this structure. An antenna using the samedesign parameters (structure size and separations) was recently real-

ized using EBL and demonstrated a directional emission pattern22

.Having mastered the required geometrical parameters, one mightwonder whether the structures resulting rom the assembly o smallcolloidal gold nanoparticle have proper plasmonic response. oinvestigate this, we have printed a nanopillar with a base diametero 50 nm and height o 200 nm and annealed it at 260°C. In Fig. 7b,we plot the plasmon spectra recorded under illumination polarizedperpendicular (transverse) and parallel (longitudinal) to the nano-pillar long axis. Resonances at 528 nm and 660 nm are in reasonableagreement with the theoretical predictions o 525 nm and 700 nm,respectively 23. Te spectra beore the annealing process deviaterom these values and are ound in the Supplementary Fig. S13.We remark that recent works have shown that, in act, structuresas simple as single plasmonic nanospheres24, nanodisks25 or nano-

rods26

can act as optical antennas with strong scattering proper-ties and large optical near-field enhancement20,21,24. Printing suchplasmonic nanostructures may also be employed or applicationsin gas sensing27, graphene pholtovoltaics28 or low-energy photondetection29.

DiscussionIn this work, we have presented eature sizes down to 50 nm by usingnozzle diameters in the range o 1µm. Downscaling these by onlya actor o 2–5 would make our method ully competitive with thestate-o-the-art perormances o EBL or FIB. Observed ootprints,originating rom a 600-nm nozzle, have already reached ~35 nm.Besides reducing the size o employed nozzles, even smaller drop-lets could be achieved by pinning the meniscus not at the outer,but at the inner opening30, or example, by treating the nozzle

surace with a solvent-repelling coating1,30. Tis may allow drop-lets as small as 20 nm rom a 600 nm nozzle at a manageable effort.Further downscaling may be achieved by employing ever smallernozzles. Along this line, we believe that a lower limit to the processwill mainly be induced by difficulties o urther nozzle downscaling(also the involved clogging issues) or size restrictions imposed bythe nanoparticles (as particles should still be substantially smallerthan the structures they build). O great importance or any techni-cal advancement will also be a better understanding o the differentphysical aspects. We have identified several stringent conditions,

which have to be ulfilled or proper unctioning o ENA NanoDripprinting. Most important is a proper adjustment o the diverse flowrates, namely o ejection flow rate and vapourization flow rate atboth, droplet and meniscus (see also the Supplementary Methods,concerning this topic). Te implementation o strongly deviatingsolvents (or example, with respect to surace tension, viscosity, vapour pressure and so on) will not be successul at the same con-ditions used here. In this respect, it will be o importance to alsoinclude external actors that we have not yet investigated (or exam-ple, temperature). We believe that both downscaling and urtherprocess versatility will be accompanied with a better understand-ing o the process. In contrast to techniques like EBL and FIB, ENANanoDrip printing is orders o magnitude less expensive, does notrequire vacuum, and can easily create out-o-plane structures. Weanticipate that availability o reproducible microabricated print

0.75 µm s–1

65°

1.23 µm s–1

50°

1.71 µm s–1

34°

2.95 µm s–1

expected 0°

Figure 6 | Structures printed at varying substrate movement velocity.

Tilted nanopillars generated by ejecting nanodroplets during relativenozzle-substrate movement at linearly increasing substrate velocities.

Pillars have a diameter of ~55 nm (scale bar is 500 nm). The velocities

and immediate pillar-tilting angles at the positions marked with an arrow

are displayed in the SEM micrograph. It is found that the tilting angle

is directly proportional to the employed substrate velocity. From this

linear relationship, the structure growth velocity can be deduced, which

is ~3µm s − 1. If the substrate movement is above 3µm s − 1, the pillars will

merge with the substrate and generate a track. That the pillar already

merges with the substrate at lower velocity is due to the non-zero pillar

diameter and the still increasing velocity, which prohibit the pillar to

elevate above the substrate. The separation between pillars is not induced

manually, by briefly interrupting ejection, but occurs spontaneously during

the process. This phenomenon may be related to the inhomogeneous

nature of the electric vector field (Supplementary Discussion).

1.0

0.8

0.6

0.4

I n t e n s i t y

( a . u .

)

0.2

0.0

500 550 600 650

Wavelength (nm)

700 750 800

Figure 7 | ENA NanoDrip printing for plasmonic applications. (a) SEM

micrograph of the ENA NanoDrip-printed geometry of a recently demons-

trated Yagi Uda antenna22 (scale bar, 200 nm). (b) The measured scat-

tering spectrum of a printed and subsequently annealed gold nanopillar

with a diameter of 50 nm and aspect ratio of ~4. The graph shows spectra

for longitudinal (black line) and transverse (red line) excitation. The small

peak at 660 nm for transverse polarization is due to slight coupling of theexcitation into the longitudinal mode.


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heads31 in the uture will introduce urther controllability and par-allelization o the process with a multitude o nozzles, providing anon-demand, bottom-up and scalable nanoabrication technique.

MethodsPrinting process. Te set-up shown in Fig. 1a consists o an in-house built micro-scope equipped with a 3D piezo-stage (MadCityLabs), electrical equipment orpulse generation and a central control unit. A glass substrate was placed on topo an IO coated glass slide mounted at the piezo-stage. Te piezo-stage had aworking distance o 300µm. In the respective area, structures can be well aligned

at the inherent precision o the piezo-stage (generally


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AcknowledgementsFinancial support or the work reported in this paper provided by the Swiss National

Science Foundation through grant number 2-77485-09 is grateully acknowledged. Te

authors would also like to thank Dr. M. iwari o the Laboratory o Termodynamics in

Emerging echnologies o EH Zurich and Dr. Mario Agio o the Nano-Optics group o

EH Zurich or useul discussions.

Author contributionsP.G. and J.S. built the set-up, designed and analysed experiments. P.G. interpreted data

and perormed FEM simulations. P.G. and S.K. designed the model or the simulation

and perormed experiments. P.G., D.P. and V.S. wrote the manuscript, which was edited

by all authors. H.E. designed and perormed the optical measurements and interpreted

optical spectra. D.P. supervised all aspects o the project, designed the research and gave

scientific and conceptual advice. V.S. supervised the optical measurements and gave

scientific advice.

Additional informationSupplementary Information accompanies this paper at http://www.nature.com/

naturecommunications

Competing financial interests: Te authors declare no competing financial interests.

Reprints and permission inormation is available online at http://npg.nature.com/

reprintsandpermissions/

How to cite this article: Galliker, P. et al. Direct printing o nanostructures

by electrostatic autoocussing o ink nanodroplets. Nat. Commun. 3:890

doi: 10.1038/ncomms1891 (2012).

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Direct Printing of Nanostructures by Electrostatic Autofocussing of Ink Nanodroplets