High-resolution electrohydrodynamic jet printing

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2 High-resolution electrohydrodynmic jet printing JANG-UNG PARK 1, MATT HARDY 1, SEONG JUN KANG 1,2, KIRA BARTON 3, KURT ADAIR 3, DEEP KISHORE MUKHOPADHYAY 3, CHANG YOUNG LEE 4, MICHAEL S. STRANO 4, ANDREW G. ALLEYNE 3, JOHN G. GEORGIADIS 3, PLACID M. FERREIRA 3 AND JOHN A. ROGERS 1,3 * 1 Deprtment of Mterils Science nd Engineering, Beckmn Institute, nd Frederick Seitz Mterils Reserch Lortory, University of Illinois t Urn-Chmpign, 134 West Green Street, Urn, Illinois 6181, USA 2 Division of Advnced Technology, Kore Reserch Institute of Stndrds nd Science, 1 Doryong-Dong, Yuseong-Gu, Dejon 35-34, South Kore 3 Deprtment of Mechnicl Science nd Engineering, University of Illinois t Urn-Chmpign, Urn, Illinois 6181, USA 4 Deprtment of Chemicl & Biomoleculr Engineering, nd Beckmn Institute, University of Illinois t Urn-Chmpign, Urn, Illinois 6181, USA *e-mil: jrogers@uiuc.edu Pulished online: 5 August 27; doi:1.138/nmt1974 Efforts to dpt nd extend grphic rts printing techniques for demnding device pplictions in electronics, iotechnology nd microelectromechnicl systems hve grown rpidly in recent yers. Here, we descrie the use of electrohydrodynmiclly induced fluid flows through fine microcpillry nozzles for jet printing of ptterns nd functionl devices with sumicrometre resolution. Key spects of the physics of this pproch, which hs some fetures in common with relted ut comprtively low-resolution techniques for grphic rts, re reveled through direct high-speed imging of the droplet formtion processes. Printing of complex ptterns of inks, rnging from insulting nd conducting polymers, to solution suspensions of silicon nnoprticles nd rods, to singlewlled cron nnotues, using integrted computer-controlled printer systems illustrtes some of the cpilities. High-resolution printed metl interconnects, electrodes nd proing pds for representtive circuit ptterns nd functionl trnsistors with criticl dimensions s smll s 1 µm demonstrte potentil pplictions in printed electronics. Printing pproches used in the grphic rts, prticulrly those sed on ink-jet techniques, re of interest for pplictions in high-resolution mnufcturing owing to ttrctive fetures tht include (1) the possiility for purely dditive opertion, in which functionl inks re deposited only where they re needed, (2) the ility to pttern directly clsses of mterils such s frgile orgnics or iologicl mterils tht re incomptile with estlished ptterning methods such s photolithogrphy, (3) the flexiility in choice of structure designs, where chnges cn e mde rpidly through softwre-sed printer-control systems, (4) comptiility with lrge-re sustrtes nd (5) the potentil for low-cost opertion 1 3. Conventionl devices for ink-jet printing rely on therml or coustic formtion nd ejection of liquid droplets through nozzle pertures 3. A growing numer of reports descrie dpttions of these devices with specilized mterils in ink formts for pplictions in electronics 4 9, informtion disply 1 12, drug discovery 13,14, micromechnicl devices 15,16 nd other res 3,17. The functionl resolution in these pplictions, s defined y the nrrowest continuous lines or smllest gps tht cn e creted relily, is 2 3 µm (refs 9,18 2). This corse resolution results from the comined effects of droplet dimeters tht re usully no smller thn 1 2 µm (2 1 pl volumes) nd plcement errors tht re typiclly ±1 µm t stndoff distnces of 1 mm (refs 5,21,22). Some methods cn void these limittions, for certin clsses of fetures. For exmple, lithogrphiclly predefined ssist fetures 5,23,24 or surfce functionliztion of preprinted inks 25 in the form of ptterns of wettility or surfce relief cn confine nd guide the flow of the droplets s they lnd on the sustrte. In this mnner, gps etween printed droplets, for exmple, cn e controlled t the sumicrometre level This cpility is importnt for pplictions in electronics when such gps define trnsistor chnnel lengths. These methods do not, however, offer generl pproch to high resolution. In ddition, they require seprte ptterning systems nd processing steps to define the ssist fetures. Electrohydrodynmic jet (e-jet) printing is technique tht uses electric fields, rther thn therml or coustic energy, to crete the fluid flows necessry for delivering inks to sustrte. This pproch hs een explored for modest-resolution pplictions (dot dimeters 2 µm using nozzle dimeters 5 µm) in the grphic rts To our knowledge, it is unexmined for its potentil to provide high-resolution (tht is, <1 µm) ptterning or to fricte devices in electronics or other res of technology y use of functionl or scrificil inks. Here, we introduce methods nd mterils for e-jet printing with resolution well within the sumicrometre rnge. Ptterning of wide rnging clsses of inks in diverse geometries illustrtes some of the cpilities. Printed electrodes for functionl trnsistors nd representtive circuit designs demonstrte potentil pplictions in electronics. These results define some dvntges nd disdvntges of this pproch, in its current form, compred with other ink printing techniques. Figure 1 shows schemtic digrm of our e-jet printing system. A syringe pump (flow rtes 3 pl s 1 ) or pneumtic pressure controller (pplied pressure 5 psi) connected to 782 nture mterils VOL 6 OCTOBER 27

3 Print hed 5 nm 11 9 V ~1 µm.2 1 µm V 5 µm 2 µm c Pump Print hed (Nozzle dimeter:.3 3 µm) Power supply x z y Sustrte Conductive support Trnsltion nd tilting stge Computer control Figure 1 Nozzle structures nd schemtic digrms of high-resolution e-jet printer., SEM imges of gold-coted glss microcpillry nozzle (2 µm internl dimeter). A thin film of surfce-functionlized Au cots the entire outer surfce of the nozzle s well s the interior ner the tip. The insets on the right show views of this tip region., Nozzle nd sustrte configurtion for printing. Ink ejects from the pex of the conicl ink meniscus tht forms t the tip of the nozzle owing to the ction of voltge pplied etween the tip nd ink, nd the underlying sustrte. These droplets eject onto moving sustrte to produce printed ptterns. In this digrm, the sustrte motion is to the right. Printed lines with widths s smll s 7 nm cn e chieved in this fshion. c, Printer set-up. A gold-coted nozzle (internl dimeter: 1, 2 or 3 µm) is locted ove sustrte tht rests on grounded electrode with seprtion (H) of 1 µm. A power supply connects to the nozzle nd the electrode under the sustrte. The sustrte/electrode comintion mounts on five-xis (x, y, z xes nd two tilting xes in the x y plne) stge for printing. glss cpillry (internl dimeter of etween.3 nd 3 µm nd outer dimeter of etween.5 nd 45 µm) delivers fluid inks to the cleved end of the cpillry, which serves s nozzle. The nozzle friction process is descried in the Methods section. Figure 1 shows scnning electron microscope (SEM) imges of the nozzle nd the nozzle opening. A thin film of sputter-deposited gold cots the entire outside of the microcpillry s well s the re round the nozzle nd the inner surfces ner the tip. A hydrophoic self-ssemled monolyer (1H,1H,2H,2H-perfluorodecne-1-thiol) formed on the gold limits the extent to which the inks wet the regions ner the nozzle, therey minimizing the proility for clogging nd/or errtic printing ehviour (see Supplementry Informtion, Tle S1). We refer to this functionlized gold-coted cpillry mounted on mechnicl support fixture nd connected to the pump s the e-jet print hed. The nozzles used in these print heds hve internl dimeters tht re much smller thn those used in previous work on e-jet printing 26 29, where the focus ws on reltively low-resolution pplictions in grphic rts. The smll nozzle dimensions re criticlly importnt to chieving high-resolution performnce for device friction, for resons descried elow. A voltge pplied etween the nozzle nd conducting support sustrte cretes electrohydrodynmic phenomen tht drive flow of fluid inks out of the nozzle nd onto trget sustrte. This sustrte rests on metl plte tht provides n electriclly grounded conducting support. The plte, in turn, rests on plstic vcuum chuck tht connects to computer-controlled x, y nd z xes trnsltion stge. A two-xis tilting mount on top of the trnsltion stge provides djustments to ensure tht motion in the x nd y directions does not chnge the seprtion (H, typiclly 1 µm) etween the nozzle tip nd the trget sustrte. A d.c. voltge (V) pplied etween the nozzle nd the metl plte with nture mterils VOL 6 OCTOBER

4 Pulsting jet mode Pulsting jet mode Pulsting jet mode t = ms 5 µm t = 2.31 ms 5 µm 5 µm t = 2.74 ms Pulsting jet mode Pulsting jet mode Stle jet mode t = 3.15 ms 5 µm 5 µm 5 µm t = 3.55 ms Figure 2 Time-lpse imges of the pulsting liquid meniscus in one cycle t V/H = 3.5 V µm 1. V is the pplied voltge etween the nozzle nd the sustrte nd H is the distnce etween the nozzle tip nd the sustrte. The ottom right imge corresponds to the stle jet mode, which is chieved t V/H 9 V µm 1 for this system. These imges were cptured t frme rte of 66, fps nd exposure time of 11 µs, using high-speed cmer. The reference time (t = ) corresponds to the time t which the meniscus first reches its fully retrcted stte. computer-controlled power supply genertes n electric field tht cuses moile ions in the ink to ccumulte ner the surfce of the pendent meniscus t the nozzle. The mutul coulomic repulsion etween these ions induces tngentil stress on the liquid surfce, therey deforming the meniscus into conicl shpe, known s Tylor cone 3. At sufficiently high electric fields, this electrosttic (Mxwell) stress overcomes the cpillry tension t the pex of the liquid cone; droplets eject from the pex to expel some portion of the surfce chrge (Ryleigh limit). Even very smll ion concentrtions re sufficient to enle this ejection process. For exmple, in uncontrolled spry modes, ejection is possile with liquids tht hve electricl conductivities tht spn ten decdes 31, from 1 13 to 1 3 S m 1. Coordinting the opertion of the power supply with the system of trnsltion stges enles direct-write e-jet printing of inks in ritrry geometries (see Fig. 1,c). To understnd the fundmentl dynmics of this electric-field-driven jetting ehviour, high-speed cmer (Phntom 63, 66, fps) ws used to imge the process of Tylor-cone deformtion nd droplet ejection directly t the nozzle. For these experiments, n queous ink of lend of poly(3,4-ethylenedioxythiophene) nd poly(styrenesulphonte) (PEDOT/PSS) ws used. The imges, shown in Fig. 2, show tht the meniscus t the nozzle expnds nd contrcts periodiclly owing to the electric field. A complete cycle, which occurs in roughly 3 1 ms for the systems investigted here, consists of stges of liquid ccumultion, cone formtion, droplet ejection nd relxtion 32. The initil sphericl meniscus t the nozzle tip chnges grdully into conicl form owing to the ccumultion of surfce chrges. The rdius of curvture t the pex of the cone decreses until the Mxwell stress mtches the mximum cpillry stress, resulting in chrged fluid jet ejection. This ejection decreses the cone volume nd chrges, therey reducing the electrosttic stress to vlues less thn the cpillry tension. The ejection then stops nd the meniscus retrcts to its originl sphericl shpe. The pex of the cone cn oscillte, leding to the ejection of multiple droplets in short ursts. The frequency of this oscilltion, which is in the khz frequency rnge, increses in nonliner fshion with the electric field 33,34. After period of ejection in the form of multiple pulstions similr to the cycle shown in Fig. 2, the retrcted sphericl meniscus remins stle nd lrgely unpertured until the next period of ejection. This ccumultion time depends on the flow rte imposed y the pump nd on the electricl chrging times ssocited with the resistnce nd cpcitnce of the system 33,34. At sufficiently high fields, stle jet mode (s opposed to the pulsting mode descried ove) cn e chieved. In this sitution, continuous strem of liquid emerges from the nozzle, s shown in Fig. 2. At even higher fields, multiple jets cn form, culminting ultimtely in n tomiztion mode (e-spry mode) of the type used in mss spectroscopy nd other well-estlished fields of ppliction 35,36. For controlled high-resolution printing of the type introduced here, this mode must e voided. Either the stle jet or the pulsting modes cn e used. The sensitivity of the stle jet mode to pplied fields (too high results in uncontrolled spry, nd too low results in pulstion) fvours, in prcticl sense, the pulsting opertion. A key to chieving high resolution, from the stndpoint of print-hed design, is the use of fine nozzles with shrp tips. Such nozzles led directly to smll droplets/strems. In ddition, the low V nd H vlues tht result from electric-fieldline focusing t the shrp tips of such nozzles nd the distriution of the electric field lines themselves comine to minimize lterl vritions in the plcement of the droplets/strems on the printed sustrte (see Supplementry Informtion, Fig. S1). A wide rnge of functionl orgnic nd inorgnic inks, including suspensions of solid ojects, cn e printed using this pproch, with resolutions extending to the sumicrometre rnge. Figure 3, shows dot-mtrix text ptterns formed using solution ink of conducting polymer PEDOT/PSS nd photocurle polyurethne prepolymer (NOA 74, Norlnd Products) printed 784 nture mterils VOL 6 OCTOBER 27

5 f 2 µm c 1 µm 5 µm d e Alignment direction 1 µm 5 µm 1 µm Figure 3 Opticl microgrphs nd SEM imges of vrious imges formed with different inks., Letters printed with the conducting polymer PEDOT/PSS. The verge dot dimeter is 1 µm., Letters printed with photocurle polyurethne polymer with dot dimeters of 1 µm. c, Fluorescence opticl microgrph (emission t 68 nm) of Si nnoprticles (verge dimeter of 3 nm) printed from suspension in 1-octnol. The dimeter of the printed dots is 4 µm. d, Opticl microgrph of single-crystl Si rods (thickness: 3 µm, length: 5 µm, width: 2 µm) printed from suspension in 1-octnol. e, SEM imge of ligned SWNTs grown y CVD on qurtz using printed ptterns of ferritin s ctlyst. f, Imge of flower formed with printed dots ( 8 µm dimeters) of SWNTs from n queous solution. In ll cses, nozzles with internl dimeters of 3 µm were used. onto SiO 2 (3 nm)/si sustrte. Figure 3c,d shows exmples of printed inks tht consist of suspensions of Si nnoprticles (verge dimeter: 3 nm) 37 nd single-crystl Si rods (length: 5 µm, width: 2 µm, thickness: 3 µm) 38 dispersed in 1-octnol. The Si nnoprticles emit fluorescent light t 68 nm wvelength, s shown in Fig. 3c. Suspensions of ferritin nnoprticles cn lso e printed nd then used s ctlytic seeds for the chemicl vpour deposition (CVD) growth of single-wlled cron nnotues (SWNTs). Figure 3e shows the results, in which the printing nd growth occurred on n nneled ST (stle temperture)- cut qurtz sustrte 39, to yield well-ligned individul SWNTs. For the structures printed onto SiO 2 /Si, the silicon formed the conducting support for printing. In the cse of qurtz, metl supporting plte ws used. Computer-coordinted control of the power supply nd the stges enles printing of complex ptterns, such s digitized grphic imges or circuit lyouts. Figure 3f shows printed imge of flower formed with n ink consisting of surfctnt-stilized SWNTs in wter 4. The verge dot dimeter is 8 ±.3 µm, nd the uniformity in the sizes is shown in the Supplementry Informtion, Fig. S2. For the results in Fig. 3, the nozzle internl dimeter ws 3 µm nd the sustrtes moved t speeds of 1 µm s 1 (1 mm s 1 for Fig. 3,). These conditions yielded dot-mtrix versions of the imges with 1 µm dot dimeters. These dots re ssocited with the ccumultion of multiple micro/nnodroplets ejected t the khz level frequency in the pulsting mode; the seprtion etween these dots corresponds to the ccumultion time mentioned previously. (For Fig. 3d, owing to the low concentrtion of Si rods ( 5 rods nl 1 ), reltively lrge drop dimeter of 1 µm ws selected y pplying the voltge for 1 ms with the nozzle held fixed.) Although the 1 µm feture sizes shown in Fig. 3 re suitle for vrious pplictions, the resolution cn e improved y using smller nozzles. Supplementry Informtion, Fig. S3 shows portrit imge composed of 2 µm dots printed with 2-µm-internl-dimeter nozzle nd printing speed of 2 µm s 1. The printing resolution cn e extended much further into the sumicrometre regime. Figure 4 shows n imge of the ncient scholr, Hypti, printed using polyurethne ink. Dots 49 nm in dimeter were chieved with 5-nm-internl-dimeter nozzle for this cse. Further reducing the internl dimeter to 3 nm reduces the dot size to 24 ± 5 nm, s shown in Fig. 4. Ptterns of continuous lines nd other shpes cn e chieved y printing t stge trnsltion speeds tht llow the dots to merge. Figure 4c shows ptterns of lines printed onto SiO 2 /Si sustrte using the 2-µm-internl-dimeter nozzle nd printing speed of 1 µm s 1 ; the line widths, for single-pss printing, re 3 µm. With 1-µm-internl-dimeter nozzle, line widths of 7 nm cn e chieved using polyethyleneglycol methyl ether solution (Aldrich), s shown in Fig. 4d. These results represent resolution tht significntly exceeds tht of conventionl unssisted therml- or piezoelectric-type ink-jet systems. The slight wviness in the sumicrometre dots or lines in Fig. 4,,d is due to the comined effects of mechnicl resonnces in the long cpillry used in the print hed nd slight fluctutions ssocited with the e-jet process. Printed electronics represents n importnt ppliction re tht cn tke dvntge of oth the extremely high-resolution cpilities of e-jet printing s well s its comptiility with rnge of functionl inks. To demonstrte the suitility of e-jet printing for fricting key device elements in printed electronics, we ptterned complex electrode geometries for ring oscilltors, source/drin electrodes for trnsistors, nd we uilt working trnsistors. In these exmples, photocurle polyurethne precursor provided printle resist lyer for ptterning metl electrodes y chemicl etching. The print hed in this cse used nture mterils VOL 6 OCTOBER

6 6 nm 5 µm 2 µm 4 µm 55 µm c 4 µm Twice Once printed 1 µm d 1 µm 2 µm 1 µm Figure 4 High-resolution e-jet printing with printed feture sizes in the rnge from 24 nm to 5 µm., Opticl microgrph of portrit of the ncient scholr, Hypti, printed using polyurethne ink nd 5-nm-internl-dimeter nozzle. The dimeters of the dots re 49 nm. The inset shows n AFM imge of the printed dots., Three-dimensionl AFM imge of ligned rrys of dots with dimeters of 24±5 nm, formed using the polyurethne nd 3-nm-internl-dimeter nozzle. The lue dshed lines show the scn direction of the nozzle, nd the top right inset shows mgnified AFM imge of the printed dot rry. c, Continuous lines printed using the SWNT ink nd 2-µm-internl-dimeter nozzle. The horizontl lines (widths: 3 µm) were printed in single pss, wheres the verticl lines (width: 5 µm) were formed y printing in two psses. d, Opticl microgrph of printed line of polyethyleneglycol (width: 7 8 nm) formed using 1-µm-internl-dimeter nozzle. 1-µm-internl-dimeter nozzle; the printing speed ws 1 µm s 1. The sustrte consisted of SiO 2 (3 nm)/si coted uniformly with Au (13 nm) nd Cr (2 nm). Figure 5 shows pttern of printed polyurethne fter curing y exposure to ultrviolet light ( 1 J cm 2 ). The resolution ws 2 ±.4 µm, s defined y the minimum line widths. Much lrger fetures, shown here in the form of electrode pds with dimensions up to 1 mm, re possile y overlpping the fine lines. Wet etching the printed sustrte (Au etchnt: trifluorocetic cid, Trnsene; Cr etchnt: Cr msk etchnt, Trnsene) removed the Au/Cr ilyer in regions not protected y the polyurethne. Removing the polyurethne y soking in methylene chloride nd, in some cses, y oxygen plsm etching (plsmtherm rective ion etch system, 2 s.c.c.m. O 2 flow with chmer se pressure of 15 mtorr, 15 W, nd rdiofrequency power for 5 min), completed the friction or prepred the sustrte for deposition of the next functionl mteril. Figure 5 e shows vrious ptterns of Au/Cr electrodes formed in this mnner. Figure 5d shows n rry of printed source/drin electrodes with different spcings (tht is, chnnel lengths, L). As shown in the inset of Fig. 5d, chnnel lengths s smll s 1 ±.2 µm cn e chieved with chnnel widths of up to hundreds of micrometres ( 17 µm in this cse). An tomic force microscopy (AFM) imge of prt of the chnnel re shows shrp, well-defined edges (Fig. 5e). The ility to print chnnel lengths with sizes in the micrometre rnge in direct fshion, without the use of sustrte wetting or relief ssist fetures, is importnt owing to the key role of this dimension in determining the switching speeds nd the output currents of the trnsistors. As demonstrtion of device friction y e-jet printing, thin-film trnsistors (TFTs) tht use perfectly ligned rrys of SWNTs 41 s the semiconductor nd e-jet-printed electrodes for source nd drin were fricted on flexile plstic sustrtes. The friction process egn with e-em evportion of uniform gte electrode (Cr: 2 nm/au: 7 nm/ti: 1 nm) onto sheet of polyimide (thickness: 25 µm). A lyer of SiO 2 (thickness: 3 nm) deposited y plsm-enhnced CVD t 25 C nd spin-cst film of epoxy (SU-8, thickness: 2 nm) formed ilyer gte dielectric. The epoxy lso served s n dhesive for the dry trnsfer of SWNT rrys grown y CVD on qurtz wfers using ptterned stripes of iron ctlyst 41. Evporting uniform lyers of Cr (2 nm)/au (1 nm) onto the trnsferred SWNT rrys, followed y e-jet printing nd photocuring of polyurethne nd then etching of the exposed prts of the Cr/Au to define source/drin electrodes completed the friction of devices with different chnnel lengths, L. SWNTs outside the chnnel res were removed y rective ion etching (15 mtorr, 2 s.c.c.m. O 2, 15 W, 3 s) to isolte these devices. Figure 6, shows schemtic digrms of the device lyouts nd n SEM imge of the ligned SWNTs with the e-jet-printed source/drin electrodes. The rrys consist of 2.5 SWNTs/1 µm. Figure 6c shows typicl trnsfer chrcteristics tht indicte the expected p-chnnel ehviour 42. The current outputs increse pproximtely linerly with 1/L, with rtios of the on to the off 786 nture mterils VOL 6 OCTOBER 27

7 1 µm 1 µm 1 µm 1 µm c d 1 µm 1 µm e 3 µm 13 nm 1 µm 5 µm 91 nm Figure 5 Ptterns of electrode structures for ring oscilltor nd isolted trnsistors formed y e-jet printing of photocurle polyurethne ink tht cts s n etch resist for uniform underlying lyer of metl (Au/Cr)., E-jet-printed polyurethne etch resist for ring oscilltor circuit efore etching the metl lyers., Ptterned Au electrode lines with 2 µm width fter etching nd stripping the resist. The insets show mgnified imges. c, Au electrode lines (widths 2 µm). d, Arry of source/drin electrode pirs formed y e-jet printing of the resist lyer, etching of metl nd then stripping the resist. The inset shows n electrode pir seprted y 1 µm. e, AFM imge nd depth profile of portion of this pir. currents tht re etween 1.5 nd 4.5 (inset of Fig. 6c), s expected owing to the popultion of metllic tues in the rrys. Figure 6d (lck circles) shows pproximte device moilities evluted in the liner regime, clculted from the physicl widths of source/drin electrodes (W = 8 µm), prllel-plte model for cpcitnce (C), nd the trnsfer curves, ccording to µ dev = (L/W CV D ) ( I D / V G ). These moilities re etween 7 nd 42 cm 2 V 1 s 1 with L in the rnge of 1 42 µm, nd decrese with L owing to the contct resistnce The nture of electrosttic cpcitnce coupling of the gte to the SWNTs is importnt to the ehviour of these devices. Clcultions of this gte cpcitnce re given in the Supplementry Informtion. This ccurte cpcitnce model yields moilities of cm 2 V 1 s 1, s shown in Fig. 6d (red squres). We speculte tht exposing the tues to etchnts for Cr/Au cn induce defects, therey resulting in lower moilities thn those reported previously with devices fricted y other mens 41. The on/off rtios cn e enhnced y n electricl rekdown process 41. Trnsfer curves evluted efore nd fter this process re compred in Fig. 6e, for the cse of trnsistor with L = 22 µm. The on/off rtio improves to >1, without sustntil reduction in moility (28 to 21 cm 2 V 1 s 1 ). Figure 6f shows full current voltge chrcteristics efore (inset) nd fter rekdown. Figure 6g shows n opticl microgrph of set of devices on flexile sheet of polyimide, nd Fig. 6h shows the normlized moility nd on/off rtio s function of ending-induced strin 44 (ε). No significnt chnge in the moility or on/off rtio occurs for ending to rdii of curvture (R C ) s smll s 2 mm. The dvntges of this high-resolution form of e-jet printing over conventionl ink-jet printing lie minly in the high levels of resolution tht cn e otined. We expect, in fct, tht further reductions in the nozzle dimensions will enle resolution even deeper into the nnoscle. Achieving resolution t this level represents topic of current work. The speeds for printing, using the prticulr systems descried here, re reltively slow, lthough multiple-nozzle implementtions, conceptully similr to those used in conventionl ink-jet print heds, cn reduce the printing durtion. Key findings from nlysis of throughput with conventionl ink-jet printing systems lso pply to the e-jet pproch 45. A min disdvntge of the e-jet pproch is tht the printed droplets hve sustntil chrge tht might led to unwnted consequences in resolution nd in device performnce, prticulrly when used with electriclly importnt lyers such s gte dielectrics nd semiconductor films. The nture mterils VOL 6 OCTOBER

8 Source Drin Aligned SWNTs Dielectric Gte Flexile plstic 2 μm c d 16 e l D (μa) V G (V) 1 1 l D (μa) L (μm) 1 2 μ dev (cm2 V 1 s 1 ) L (μm) l D (na) V G (V) f g h l D (μa) 3 l D (μa) V D (V) 1 Vrition 1.65 μdev on/off rtio ε (%) V D (V) 3 mm R c (mm) 14 Figure 6 Friction of perfectly ligned SWNT-TFTs on plstic sustrte with e-jet printing for the criticl fetures, tht is, the source nd drin electrodes., Schemtic digrm of the trnsistor lyout, where the source/drin electrodes re ptterned y e-jet printing., SEM imge of the ligned SWNTs connected y e-jet-printed source/drin electrodes. The tue density is 2.5 SWNTs/1 µm. c, Trnsfer curves mesured from trnsistors with chnnel lengths, L = 1, 6, 12, 22 nd 42 µm, from top to ottom, nd chnnel widths, W = 8 µm t source/drin voltge, V D =.5 V. The inset shows on nd off currents (lck nd red lines, respectively) s function of L. d, Liner-regime device moilities (µ dev ) clculted from the prllel-plte (lck circles) nd rigorous (red squres) cpcitnce models, s function of L. e, Trnsfer curves from trnsistor with L = 22 µm efore (lck line) nd fter (red) n electricl rekdown process. This rekdown reduces the off current to less thn 1 na to yield n on/off rtio of 1,. f, Current voltge chrcteristics recorded fter the electricl rekdown process. The gte voltge vries etween 2 nd 1 V in steps of 1 V, from top to ottom. The inset shows the current voltge curve efore the rekdown with the sme gte voltges for comprison. g, Photogrph of n rry of flexile SWNT-TFTs. h, Vrition of the normlized moility (lck squres) nd on/off rtio (red circles) of SWNT-TFT s function of ending-induced strin (ε) nd the rdii to curvture (R C ). effects of this chrge might e minimized y using high-frequency lternting driving voltges for the e-jet process. These nd other process improvements, together with explortion of pplictions in iotechnology nd other res, represent promising res for future work. METHODS PREPARATION OF NOZZLES Au/Pd (7 nm thickness) nd Au (5 nm) lyers were coted onto glss pipettes with tip internl dimeters of etween.3 nd 3 µm (World Precision Instruments) using sputter coter (Denton, Desk II TSC). Dipping the tip of the metl-coted pipette into 1H, 1H, 2H, 2H-perfluorodecne-1-thiol (Fluorous Technologies) solution (.1 wt% in dimethylformmide) for 1 min formed hydrophoic self-ssemled lyer on the gold surfce of the nozzle tip. The cpillry ws connected to syringe pump (Hrvrd Apprtus, Picoplus) or pneumtic pressure ctutor through polyethylene tue. SYNTHESIS OF FUNCTIONAL INKS PEDOT/PSS ink: PEDOT/PSS (Bytron P, H.C. Strck) ws diluted with H 2 O (5 wt%), nd then mixed with polyethyleneglycol methyl ether (Aldrich, 15 wt%) to reduce the surfce tension (to lower the voltge needed to initite printing) nd the drying rte t the nozzle. Single-crystl Si rods: Ptterning the top Si lyer (thickness: 3 µm) of silicon-on-insultor wfer y rective ion etching, nd then etching the underlying SiO 2 with n queous etchnt of HF (49%) 38 with.1% of surfctnt (Triton X-1, Aldrich) formed the rods. These rods were suspended 788 nture mterils VOL 6 OCTOBER 27

9 in H 2 O nd then filtered through filter pper (pore size: 3 nm). The rods were then suspended in 1-octnol. After printing this ink, the surfctnt residue ws thermlly removed y heting to 4 C in ir for 5 h. Ferritin: First, ferritin (Sigm) ws diluted in H 2 O with volume rtio of 1(ferritin):2(H 2 O). Then 1 wt% of surfctnt (Triton X-1) ws dded to this solution to reduce the surfce tension (to lower the voltge needed to initite printing). The surfctnt residue ws removed t 5 C efore CVD growth of SWNTs. SWNT solution: Single-wlled cron nnotues produced y the electric rc method (P2-SWNT, Cron Solution) were suspended in queous octyl-phenoxy-polyethoxyethnol (Triton X-45, 2 wt%). The concentrtion ws 6.9 mg l 1. PREPARATION OF SUBSTRATES Si wfers with 3-nm-thick lyers of therml SiO 2 (Process Specilties) were used s sustrtes. The underlying Si ws electriclly grounded during printing. A glss slide (thickness: 1 µm) ws used for fluorescence opticl microgrphy (Fig. 3c), nd n ST-cut qurtz wfer ws used fter nneling t 9 C for guided growth of SWNTs (Fig. 3e). Here, the glss/qurtz sustrtes were plced on n electriclly grounded metl plte during printing. For printing of complex imges (Figs 3f, 4,), the Si wfers were exposed to perfluorosilne vpour efore e-jet printing to produce hydrophoic self-ssemled monolyer. Received 23 Ferury 27; ccepted 21 June 27; pulished 5 August 27. References 1. Forrest, S. R. The pth to uiquitous nd low-cost orgnic electronic pplictions on plstics. Nture 428, (24). 2. Gns, B. J., Duineveld, P. C. & Schuert, U. S. Inkjet printing of polymers: Stte of the rt nd future development. Adv. Mter. 16, (24). 3. Prshkov, R., Becker, E., Riedl, T., Johnnes, H. & Kowlsky, W. Lrge re electronics using printing method. Proc. IEEE 93, (25). 4. Chng, P. C. et l. Film morphology nd thin film trnsistor performnce of solution-processed oligothiophenes. Chem. Mter. 16, (24). 5. Sirringhus, H. et l. High-resolution inkjet printing of ll-polymer trnsistor circuits. Science 29, (2). 6. Shimod, T. et l. Solution-processed silicon films nd trnsistors. Nture 44, (26). 7. Burns, S. E., Cin, P., Mills, J., Wng, J. & Sirringhus, H. Inkjet printing of polymer thin-film trnsistor circuits. Mter. Res. Soc. Bull. 28, (23). 8. Wong, W. S., Redy, S. E., Lu, J. P. & Street, R. A. Hydrogented morphous silicon thin-film trnsistor rrys fricted y digitl lithogrphy. IEEE Electron Device Lett. 24, (23). 9. Szczech, J. B., Megridis, C. M., Gmot, D. R. & Zhng, J. Fine-line conductor mnufcturing using drop-on-demnd PZT printing technology. IEEE Trns. Electron. Pckg. Mnufctur. 25, (22). 1. Shimod, T., Morii, K., Seki, S. & Kiguchi, H. Inkjet printing of light-emitting polymer displys. Mter. Res. Soc. Bull. 28, (23). 11. Chng, S. C. et l. Multicolor orgnic light-emitting diodes processed y hyrid inkjet printing. Adv. Mter. 11, (1999). 12. Hener, T. R. & Sturm, J. C. Locl tuning of orgnic light-emitting diode color y dye droplet ppliction. Appl. Phys. Lett. 73, (1998). 13. Lemmo, A. V., Rose, D. J. & Tisone, T. C. Inkjet dispensing technology: Appliction in drug discovery. Curr. Opin. Biotechnol. 9, (1998). 14. Heller, M. J. DNA microrry technology: Devices, systems, nd pplictions. Annu. Rev. Biomed. Eng. 4, (22). 15. Nllni, A., Chen, T., Lee, J. B., Hyes, D. & Wllce, D. Wfer level optoelectronic device pckging using MEMS. Proc. SPIE: Smrt Sensors Actutors MEMS II 5836, (25). 16. Bietsch, A., Zhng, J., Hegner, M., Lng, H. P. & Gerer, C. Rpid functionliztion of cntilever rry sensors y inkjet printing. Nnotechnology 15, (24). 17. Hiller, J., Mendelsohn, J. D. & Runer, M. F. Reversily ersle nnoporous nti-reflection cotings from polyelectrolyte multilyers. Nture Mter. 1, (22). 18. Ling, M. M. & Bo, Z. Thin film deposition, ptterning, nd printing in orgnic thin film trnsistors. Chem. Mter. 16, (24). 19. Clvert, P. Inkjet printing for mterils nd devices. Chem. Mter. 13, (21). 2. Snur, S., Whlley, A., Almeddine, B., Crnes, M. & Nuckolls, C. Jet-printed electrodes nd semiconducting oligomers for elortion of orgnic thin-film trnsistors. Org. Electron. 7, (26). 21. Cheng, K. et l. Inkjet printing, self-ssemled polyelectrolytes, nd electroless plting: Low cost friction of circuits on flexile sustrte t room temperture. Mcromol. Rpid Commun. 26, (25). 22. Cregh, L. T. & McDonld, M. Design nd performnce of inkjet printheds for non grphic rts pplictions. Mter. Res. Soc. Bull. 28, 87 (23). 23. Wng, J. Z., Gu, J., Zenhusern, F. & Sirringhus, H. Low-cost friction of sumicron ll polymer field effect trnsistors. Appl. Phys. Lett. 88, (26). 24. Stutzmnn, N., Friend, R. H. & Sirringhus, H. Self-ligned, verticl chnnel, polymer field effect trnsistors. Science 299, (23). 25. Sele, C. W., Werne, T., Friend, R. H. & Sirringhus, H. Lithogrphy-free, self-ligned inkjet printing with su-hundred nnometer resolution. 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Acknowledgements The uthors thnk L. Jng nd M. Nyfeh for supplying Si nnoprticle solutions, R. Shepherd nd J. Lewis for the use of their high-speed cmer, nd R. Lin for ssistnce with setting initil experimentl conditions. In ddition, the uthors cknowledge the Center for Nnoscle Chemicl Electricl Mechnicl Mnufcturing Systems in the University of Illinois, which is funded y the Ntionl Science Foundtion under grnt DMI , nd the Center for Micronlysis of Mterils in University of Illinois, which is prtilly supported y the US Deprtment of Energy under grnt DEFG2-91-ER Correspondence nd requests for mterils should e ddressed to J.A.R. Supplementry Informtion ccompnies this pper on Author contriutions J.-U.P. nd J.A.R. designed the experiments nd wrote the pper. J.-U.P. crried out the nozzle friction, ink design, printing nd chrcteriztion. S.J.K. nd J.-U.P. contriuted to device friction. K.B., K.A., D.K.M., A.G.A. nd P.M.F. designed the printing mchine nd contriuted to project plnning. J.G.G. ws responsile for hydrodynmics nlysis nd project plnning. C.Y.L. nd M.S.S. synthesized SWNT solutions. M.H. developed the softwre lgorithm nd mesured contct ngles. Competing finncil interests The uthors declre no competing finncil interests. Reprints nd permission informtion is ville online t nture mterils VOL 6 OCTOBER