Dewetting of onuting polymer inkjet roplets on ptterne surfes J. Z. WANG 1,Z.H.ZHENG 1,H.W.LI 2,W.T.S. HUCK 2 AND H. SIRRINGHAUS 1* 1 Cvenish Lortory,University of Cmrige,Mingley Ro,Cmrige,CB3 0HE,UK 2 Deprtment of Chemistry,University of Cmrige,Lensfiel Ro,Cmrige,CB2 1EW,UK *e-mil: hs220@phy.m..uk Pulishe online: 8 Ferury 2004; oi:10.1038/nmt1073 The mnufture of high-performne eletroni evies with mirometre or even sumirometre imensions y solution proessing n iret printing, requires the ility to ontrol urtely the flow n spre of funtionl liqui inks on surfes. This n e hieve with the help of surfe-energy ptterns using inks to e repelle n ewette from pre-efine regions of the sustrte. To exploit this priniple for the frition of sumirometre evie strutures, etile unerstning of the ftors using ink roplets to ewet on ptterne surfes is require. Here, we use hyrophoi surfe-energy rriers of ifferent geometries to stuy the influene of solution visosity, ink volume, n ontt ngle on the proess of ewetting of inkjet-printe roplets of wter-se onuting polymer. We emonstrte polymer fiel-effet trnsistor evies with hnnel length of 500 nm frite y surfe-energy-ssiste inkjet printing. The pplition of solution-se iret-printing tehniques to the eposition n iret-write ptterning of funtionl mterils is proviing new opportunities for the mnufture of eletroni evies, suh s orgni fiel-effet trnsistors (FET) for pplitions in low-ost,lrge-re eletronis on flexile sustrtes 1 4.A rnge of iret printing tehniques, suh s sreen printing 1,2 or inkjet printing 3,4 hs een use. However, the ility of most iret-printing tehniques to efine mirometre-size ptterns is limite to typilly 20 ue to the iffiulties of ontrolling the flow n spre of liqui inks on surfes. One pproh to overome these resolution limittions is to eposit the funtionl ink onto sustrte ontining preefine surfe-energy pttern tht is le to steer the eposite ink roplets into ple. This onept hs een use suessfully for ptterning soure rin eletroes of polymer FETs with hnnel lengths of 5 µm y inkjet printing 4.Dewetting y ip oting hs lso een use to pttern the tive semionuting lyer in trnsistor frition 5,6. The performne of FET evies woul gretly enefit from further reution of hnnel length to sumirometre imensions.however,to hieve this, etile unerstning of the vrious ftors tht govern the intertion of roplets ontining solute of funtionl mteril with ptterne surfe is require. Intertions etween non-soluteontining liquis n struture, flt, soli surfes ompose of hyrophili n hyrophoi res hve een extensively stuie oth theoretilly n experimentlly 7 14.However,no etile investigtion hs een one on the ewetting of solute-ontining inks where the proess of rying les to n inrese of ink visosity, n whih limits the ility of the ink to ewet from nrrow hyrophoi strips. Here,we investigte the ewetting of the wter-se onuting polymer poly(3,4-ethyleneioxythiophene)/poly(4-styrenesulphonte) (PEDOT/PSS) inks on ptterne SiO 2 surfes moifie with the fluorinte self-ssemle monolyer (SAM) 1H, 1H, 2H, 2Hperfluoroeyltrihlorosilne (FDTS, C 10 F 17 H 4 SiCl 3 ). Vrious hyrophoi SAMs hve een wiely investigte n use for their hyrophoiity 15 18.Sumirometre hyrophoi lines with withs vrying from 250 nm to 20 µm were efine y eletron-em lithogrphy (EBL) (250 nm 1µm, home-uilt set-up) n optil lithogrphy (2 20 µm, Krl Suss, MJB3, Germny), respetively. The smllest line-with of 250 nm, whih we were le to efine y EBL, ws limite y hrging of the insulting sustrte uring nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils 171
S D Ink-jette PEDOT/PSS wter roplet 1:1, 700 nm 1:1, 500 nm O 2 plsm trete G S D SiO 2 / n + - Si sustrte 1:1, 300 nm 1:3, 250 nm Evporte SiO 2 FDTS SAM Semionutor lyer F8T2 Insultor lyer PMMA e f Figure 1 The ewetting proess.,fdts SAM is elevte y SiO 2 mes ptterne on SiO 2 /n + -Si sustrte.,pedot/pss wter solution is ink-jette on top of FDTS SAM for oserving ewetting phenomenon.,pedot/pss is ewette y FDTS /SiO 2 mes.,struture of polymer trnsistor frite y ewetting. 1:1, 500 nm, 30 nm mes g 1:3, 5 µm, ip-ote h 3 mm 100 µm 100 µm eletron-em exposure. For EBL ptterning, lines were written into 250-nm resist lyer of polymethylmethrylte (PMMA) on SiO 2 /n + -Si sustrte. The line with ws ontrolle y vrying the exposure ose. (We use SiO 2 /n + -Si sustrte to reue hrging effets.) After the evelopment of the expose PMMA resist, the sustrte surfe in the regions expose to the eletron em ws moifie with monolyer of FDTS eposite from the vpour phse. Alterntively, lyer of SiO 2 30 80 nm thik ws sputter-eposite into the nrrow wells efine y the eletron em, followe y FDTS SAM eposition. In oth ses, efore the FDTS eposition, the sustrte surfe ws lene n onitione y short two-minute oxygen plsm exposure. This llowe efining mes-type strutures in whih the surfe-energy rriers hve finite thikness. Susequently, the resist ws issolve in etone, lifting off the lyer of FDTS/SiO 2 on top of the PMMA, n unovering the unerlying hyrophili re of the sustrte (Fig. 1). Dewetting is then investigte y ink-jetting PEDOT/PSS wter roplets of ifferent onentrtions with roplet volume of 65 pl per rop on top of the ptterne surfe (Fig. 1). (A 1:1 (1:3) PEDOT/PSS ink enotes 1:1 (1:3) mixture of Bytron P PEDOT/PSS solution, from Byer, n pure wter.) Drying of the ink roplets ourre in len-room ir t 50% reltive humiity. Experiments uner higher humiity onitions showe signifint inrese in rying time, ut little effet on the ility of the roplets to ewet. Droplets tht ln on top of the nrrow FDTS-moifie lines split into two uring the rying of the ink, so efining the soure n rin eletroes of the FET (Fig. 1). Top-gte polymer FETs were frite using ewette PEDOT/PSS soure n rin eletroes y spin oting 50-nm polymer semionutor lyer of poly(9,9 -iotyl-fluorene-oithiophene) (F8T2) from xylene solution n 1-µm insulting lyer of PMMA from n-utyl ette solution, n inkjet printing PEDOT/PSS top-gte eletroe (Fig. 1). 0.3 mm s 1 0.1 mm s 1 1:1, 500 nm, 30 nm mes 0.3 mm s 1 0.1 mm s 1 1:1, 500 nm, 80 nm mes Figure 2 Photogrphs of vrious ewette PEDOT/PSS illustrting ftors ffeting ewetting., PEDOT 1:1, 700-nm FDTS SAM., PEDOT 1:1, 500-nm FDTS SAM, oxygen-plsm-trete surfe., PEDOT 1:1, 300-nm FDTS SAM., PEDOT 1:3, 250-nm FDTS SAM. e, PEDOT 1:1, 500-nm FDTS SAM, 30-nm SiO 2 mes. f,5-µm FDTS SAM gp, ip-ote y PEDOT 1:3. g, PEDOT 1:1 lines printe with jet frequeny of 4 Hz, printing spee of 0.1 n 0.3 mm s 1, 500-nm FDTS SAM, 30-nm mes. h, PEDOT 1:1 lines printe with jet frequeny of 4 Hz, printing spee of 0.1 n 0.3 mm s 1, 500-nm FDTS SAM, 80-nm mes. Severl ftors hve een foun to e importnt to hieve splitting of roplets y sumirometre hyrophoi lines (Fig. 2). Figure 2, ompres ewetting of 1:1 PEDOT/PSS ink on top of sustrtes with ifferent egree of hyrophiliity in the re SiO 2 regions. On sustrte lene y oxygen plsm efore eposition of the PMMA resist, ewetting from 500-nm-wie line is oserve (Fig. 2). In ontrst, if the sustrte is only lene y wshing in etone n isopropnol, resulting in higher ontt ngle n smller roplet imeter (Fig. 2), even on 700-nm-wie line, no omplete ewetting is oserve. This inites tht ewetting is fvoure y low ontt ngle in the wetting region of the sustrte. Dewetting lso epens on the reltive position of the entre of the roplet with respet to the 172 nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils
hyrophoi rrier. If the hyrophoi line is lose to the ege of the roplet, ewetting is possile even from very nrrow hyrophoi lines (ompre Fig. 2 n ).Another key ftor is the ink onentrtion n ink visosity. Lower PEDOT/PSS onentrtions enle ewetting on very nrrow (250 nm) lines from whih ewetting of more onentrte solutions is not possile (ompre Fig. 2,).Finlly,the use of mes of finite thikness lso improves the ility to ewet signifintly. On top of 30-nm-thik mes onentrte 1:1 PEDOT/PSS solution is ple of ewetting from signifintly nrrower lines thn on top of monolyer surfe-energy rriers (ompre Fig. 2,e). We hve lso investigte the influene of the totl mount of liqui eposite y printing ontinuous lines of PEDOT/PSS ross n rry of hyrophoi FDTS stripes. The totl eposite liqui volume per unit length of the line ws ontrolle y the spee of the smple stge, while keeping the roplet ejetion frequeny (4 Hz) the sme. Figure 2g shows the ewetting result on 30-nm FDTS/SiO 2 mes.when the stge moves with spee of 0.3 mm s 1,PEDOT/PSS solution is ewette, wheres for stge spee of 0.1 mm s 1,omplete ewetting i not our. Note tht lso here, thiker mes strutures promote ewetting (ompre lines printe with 0.1 mm s 1 in Fig. 2g n h). The flui-ynmil proesses ehin these oservtions n e rtionlize y simple moel. Figure 3 shows shemti igrm of the ewetting proess (for simpliity, two-imensionl moel is use). The whole surfe is overe y thin liqui film of thikness H on top of hyrophoi strip of length. After ewetting, the liqui vpour interfe re is inrese y n mount (2 S L), where 2 S is the inrese of the surfe re in the hyrophili regions ue to the urve eges on oth sies of the hyrophoi strip. The liqui soli interfe re ereses y L, n the soli vpour interfe re inreses y L. For the totl surfe/interfe energy fter ewetting to e less thn the surfe/interfe energy efore ewetting, the following reltionship must e stisfie: (2 S L)E LV + ( LE LS ) + LE SV 0 (1) E LV, E LS, E SV re the liqui vpour, liqui soli, soli vpour interfe tension respetively. Two onitions re ssume in our moel: (i) The liqui volume efore n fter ewetting is ssume onstnt. (ii) Grvity is neglete. Bse on eqution (1), omplete ewetting ours if: where L S os β = 2 (2) 1 os β E SV E LS E LV where β is the ontt ngle of the liqui on the hyrophoi surfe. As seen from eqution (2), ewetting is fvoure for hyrophoi surfes with lrge ontt ngle, suh s FDTS. The simple moel suggests tht for given imension of the hyrophoi stripe,ewetting ours if the thikness of the film is reue elow ritil thikness ( S ereses with eresing thikness). This is onsistent with etile moelling of the equilirium shpe of liqui roplets on heterogeneous surfes 9, s well s kineti moelling of ewetting inue y spinol instility 10.The simple two-imensionl moel neglets ege effets ner the originl ontt line from whih the ewetting proess strts. It is possile tht uner ertin onitions, higher visosity ner the rying ege might inhiit the ewetting proess. Moelling of suh three-imensionl proesses goes eyon the sope of the present pper. In the experiment, wter is ontinuously evporting uring the rying proess. Therefore, the onentrtion of PEDOT/PSS, n therey the solution visosity is inresing. If uring the rying proess H L Hyrophili surfe Hyrophili stripe of length L Liqui of thikness H Figure 3 The ewetting moel.see text for isussion. the visosity exees ritil vlue η ritil efore the film rehes its ritil thikness H ritil for ewetting, the roplet nnot split ompletely on top of the hyrophoi lines.this simple moel provies n explntion for the experimentl oservtions esrie ove. As the thikness of the roplet is eresing from entre to ege, ewetting ours more esily when the hyrophoi line is lote ner the ege of the roplet. This is lso the reson why uner ertin onitions (Fig. 1,g) ewetting strts from the thin ege of the roplet, ut stops efore rehing the thiker entre of the roplet. On very hyrophili surfes, the more pronoune spreing of the roplet les to erese of its thikness ove the hyrophoi strip. Finlly, for higher onentrtion inks, the ritil visosity is rehe t lrger liqui thiknesses, wheres low onentrtion ink is le to ry to thinner film efore it rehes η ritil. Within the moel, the enefiil effet of mes surfe-energy rrier, 30 80 nm thik, on the ewetting proess n lso e unerstoo.the effet of the mes is to erese the liqui film thikness on top of the hyrophoi stripe. When the liqui film thikness ereses, y wter evportion, to vlue omprle to the mes height, this reution in effetive thikness promotes the ewetting proess. Therefore, solute-ontining ink strts to ewet on mes-shpe rrier t n erlier time uring the rying proess,tht is, t lower visosity, ompre with monolyer rrier. Dewetting is the esier to hieve, the lrger the thikness of the mes rrier (see Fig. 2g,h). The use of hyrophili mes hs nother importnt vntge for using split-onuting polymer ink roplets s soure rin eletroes of FET evies. Figure 4,,e shows tomi fore (AFM) topogrphy, phse n ross-setionl imges, respetively, of ewette 1:3 PEDOT/PSS roplets split on top of 250-nm FDTS SAM without mes. Figure 4,,f gives the orresponing ewetting results of 1:1 PEDOT/PSS roplets on FDTS SAM,500 nm wie,with 30-nm mes. It n e seen in Fig. 4,,e tht the PEDOT/PSS ontt line is not in ontt with the ege of the FDTS line, n the istne etween the ontt lines of the two split hlves of the PEDOT roplets is signifintly lrger (out 500 nm) thn the with of the FDTS line (250 nm). This implies tht fter ewetting from the FDTS nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils 173
0 10 µm 0 10 µm 0 20 µm 0 20 µm e Height (nm) 8 6 4 2 0 2 4 6 8 10 12 30 20 10 0 10 20 30 nm mes 0 2 4 6 8 10 5 10 15 Length (µm) f Height (nm) Length (µm) Figure 4 AFM pitures of ewette PEDOT/PSS.,,e show AFM topogrphy (),phse () n ross-setionl imge (e) of ewette 1:3 PEDOT/PSS roplets split on top of 250-nm FDTS SAM without mes.,,f give the orresponing ewetting results of 1:1 PEDOT/PSS roplets on 500-nm-wie FDTS SAM with 30-nm mes. line,the ontt line of the liqui PEDOT roplets move wy from the FDTS line, efore it eomes pinne on the sustrte. In ontrst, in the se of the mes struture (Fig. 4,,f), the PEDOT/PSS ontt line remins pinne to the ege of the mes struture, n the thikness of the PEDOT/PSS eposit immeitely next to the mes rrier is finite. This n e lerly seen lso in Fig. 4f ompring the height of the mes in the PEDOT-free regions (30 nm, re line) of the sustrte with the height in the region of the split roplets (25 nm, lk line). We elieve tht this is use y the wetting nture of the hyrophili sie-wlls of the SiO 2 mes, whih re not moifie y FDTS using the ontt line of the rying PEDOT/PSS roplets to remin pinne. In the se of the mes struture, the PEDOT/PSS thikness profile in the viinity of the surfe-energy rrier les to shorter FET hnnel length, n smller soure rin ontt resistne thn in the se of monolyer SAM rriers. The ontt resistne is relte to the finite onutivity of the PEDOT/PSS, n is minimize y thiker PEDOT films in the viinity of the injeting soure rin eges.in the se of monolyer FDTS SAMs,the hnnel length tens to e lrger, ontt resistne is higher, n lower solution onentrtion nees to e use to hieve ewetting, whih further inreses ontt resistne. The use of mes struture for ewetting is ruil in hieving short sumirometre hnnel evies y surfe-energy-ssiste inkjet printing. By inresing the mes height, the thikness of PEDOT/PSS n e inrese, n ontt resistne n e lowere (see Fig. 2h y ompring the two PEDOT/PSS lines printe with ifferent spee). The pinning of the ontt line y the hyrophili mes sie-wlls lso ensures tht the hnnel length of the evie is efine onsistently y the with of the mes.in the sene of the mes,the movement of the ontt line wy from the hyrophoi SAM uses unesirle vritions of the hnnel length epening on lol wetting onitions of the sustrte. Top-gte trnsistors with moilities vrying from 0.001 to 0.003 m 2 V 1 s 1 etween evies on the sme sustrte n ON-OFF urrent rtio of 10 4 were frite using mes strutures 30 nm thik. The yiel of ewette evies on sustrte, tht is, the numer of evies without eletril short etween soure n rin, is typilly higher thn 80%. Figure 5 n respetively shows the trnsfer n output hrteristis of top-gte trnsistor with hnnel length of 500 nm n with of 80 µm. The moility (0.003 m 2 V 1 s 1 ) is slightly smller thn tht of F8T2 trnsistors with mirometre hnnel length, whih we frite in the sme wy using optil lithogrphy. The reltively low OFF urrent is similr to those of referene evies with onventionlly frite gol soure rin eletroes (L = 500 nm), whih inites tht the ewetting of the FDTS terminte mes is omplete, tht is, no eletrilly onutive PEDOT resiue is etetle in the hnnel region. Figure 5, gives the orresponing results of top-gte trnsistor with hnnel length of 2 µm n with of 80 µm (moility is 0.005 m 2 V 1 s 1 ). The lower moility in the 500-nm evie might result from ontt resistne effet, whih might e reue y improving onutivity of PEDOT/PSS (0.6 S m 1 in our se), n short hnnel effets, whih n e seen lerly from the omprison with the 2 µm evie in Fig. 5 (threshol-voltge shift, high off-urrent n sene of sturtion). In ontrst,in the se of monolyer SAM rriers,the extrte fiel-effet moility (µ = 0.0005 0.001 m 2 V 1 s 1 ) ws foun to e signifintly lower, whih is strong inition tht in this se ontt resistne 174 nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils
I (A) 10 6 10 7 10 8 10 9 10 10 10 11 50 V 30 V V s = 10 V I (A) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 V g = 60 V 40 V 20 V 0 V 20 0 20 40 60 V g (V) 0 10 20 30 40 50 V s (V) I (A) 10 6 10 7 10 8 10 9 I (A) 0.5 0.4 0.3 0.2 V g = 60 V 40 V 20 V 0 V 10 10 10 11 60 V 0.1 40 V V s = 20 V 0.0 20 0 20 40 60 V g (V) 0 10 20 30 40 50 60 V s (V) Figure 5 Trnsfer (,) n output (,) hrteristis of F8T2 top-gte trnsistors with with of 80 µm n 30-nm mes.,,re for trnsistor of 500-nm hnnel length; n,,for trnsistor of 2-µm hnnel length.i rin urrent; V rin voltge; V g gte voltge; V s soure-rin voltge. effets re more severe n prevent the FET urrent from sling in the esire wy.as the swithing spee of trnsistor (with fixe gte line with) is proportionl to µ/l,trnsistors with sumirometre hnnel length n lose moility result in signifintly higher swithing spee thn orresponing mirometre-sle evies. Furthermore, sumirometre-sle evies offer the possiility of proing the hrgetrnsport properties of polymer semionutors on the length sle of the persistene length of the polymer hins, n the size of mirorystlline omins, whih is reporte 19 21 to e of the orer of 50 to 100 nm. At present, only few methos hve een emonstrte to hieve sumirometre hnnel length,n inorgni nole metls were use s eletroes in these reporte tehniques 22 26.A few methos hve een reporte to pttern polymer soure n rin eletroes 27 30,ut sumirometre-sle hnnel length nnot e efine y these tehniques. The present proof-of-onept stuy shows tht the metho of surfe-energy-ssiste inkjet printing 4 provies suffiient ontrol over the flow of liqui ink roplets to efine sumirometre ritil fetures. Of ourse, EBL might not e the tehnique of hoie for lowost proution of surfe-energy ptterns, ut lterntive tehniques suh s iret lser ptterning, soft lithogrphi stmping or emossing might e use. An interesting question is whether simple ip oting, rther thn inkjet printing, n lso e use for high-resolution ptterning of funtionl inks y ptterning the sustrte surfe into hyrophili res where liquis re esigne to oupy, n hyrophoi res where liquis re esigne not to oupy. Figure 2f shows photogrph of photo-lithogrphilly ptterne SiO 2 /n + -Si sustrte ip-ote with 1:3 PEDOT/PSS solution. The sustrte ontins rrys of two retngulr hyrophili res seprte long one sie y nrrow 5-µm hyrophoi FDTS SAM rrier, n oun on the other three sies y wie hyrophoi FDTS SAM regions. In this ip-oting (or spin-oting) proess,ewetting ours s two-step proess.in the first step, the liqui ewets onto the two hyrophoi regions while still overing the nrrow hnnel in etween. Then in seon step, the liqui ewets from the nrrow hnnel region. This proess hppens very muh in the sme wy s in the se of inkjet printing,provie tht the two hyrophili regions re suffiiently smll tht the mount of onfine liqui is omprle to the liqui eposite in the inkjet se. By reful esign of the size of the hyrophili surfe regions, whih etermines the mount of onfine liqui, n justment of the solution onentrtion, the proess of ip oting is ple of hieving similr resolution to the inkjet proess esrie ove. In onlusion, we hve investigte the ritil ftors tht etermine the ewetting of solute-ontining inks of nrrow, sumirometre repulsive surfe-energy rrier. We foun tht ewetting is etermine y the ompeting proesses of erese liqui thikness n inrese liqui visosity use y wter evportion. By refully ontrolling ink onentrtion n visosity, surfe-energy ontrst n the mount of eposite liqui,sumirometre strutures of onuting polymer were efine y surfe-energy-ssiste inkjet printing. The use of surfe-energy rrier with finite thikness ws nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils 175
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Appl. Phys. Lett. 82, 463 465 (2003). Aknowlegements: We grtefully knowlege the Dow Chemil Compny for supply of semionuting polymers, n the Engineering n Physil Sienes Reserh Counil (EPSRC) for finnil support. Corresponene n requests for mterils shoul e resse to H.S. Competing finnil interests The uthors elre tht they hve no ompeting finnil interests. 176 nture mterils VOL 3 MARCH 2004 www.nture.om/nturemterils