Controlled insulator-to-metal transformation in printable polymer composites with nanometal clusters

Transcription

1 Controlled insultor-to-metl trnsformtion in printle polymer composites with nnometl clusters ARTICLE ANKARAN IVARAMAKRIHNAN 1, PERQ-JN CHIA 1,2, YEE-CHIA YE 2, LAY-LAY CHUA 1 * AND PETER K.-H. H 1 * 1 Deprtment of Physics, Ntionl University of ingpore, Lower Kent Ridge Rod , ingpore 2 Deprtment of Electricl nd Computer Engineering, Ntionl University of ingpore, 4 Engineering Drive 3, , ingpore *e-mil: phycll@nus.edu.sg; phyhop@nus.edu.sg Pulished online: 17 Decemer 26; doi:1.138/nmt186 Although orgnic semiconductors hve received the most ttention, the development of comptile pssive elements, such s interconnects nd electrodes, is lso centrl to plstic electronics. For this, lignd-protected metl-cluster films hve een shown to nnel t low tempertures elow 25 C to highly conductive metl films, ut they suffer from crcking nd indequte sustrte dhesion. Here, we report printle metl-cluster polymer nnocomposites tht nnel to controlled-percoltion nnostructure without complete sintering of the metl clusters. This overcomes the previous chllenges while still retining the desired low trnsformtion tempertures. Highly wter- nd lcohol-solule gold clusters (75 mg ml 1 ) were synthesized nd homogeneously dispersed into poly(3,4-ethylenedioxythiophene) to give mteril with nneled d.c. conductivity tunele etween 1 4 nd 1 5 cm 1. These composites cn inject holes efficiently into ll-printed polymer orgnic trnsistors. The insultor metl trnsformtion cn lso e electriclly induced t 1 MV cm 1, suggesting possile memory pplictions. For current-crrying interconnects in orgnic semiconductor integrted circuits 1 4, the minimum d.c. conductivity (σ d.c. ) required is of the order of 1 4 cm 1 to void severe resistive losses. Conductive polymers hve mximum reported σ d.c. 1 1 cm 1 nd re thus not suitle. Conventionl metl nd grphite pints re lso unsuitle s they redisperse during the solvent processing to uild the orgnic circuits. Metl clusters (or nnometls), on the other hnd, hve depressed melting points 5 owing to surfce effects nd hence cn e nneled to conductive films t tempertures comptile with plstic electronics. These clusters re protected y n orgnic lignd shell during synthesis to ensure tht they do not premturely colesce in the dispersed stte in solution 6. Au nd Ag clusters protected this wy hve een used in lyer-y-lyer ssemly for lnket film deposition 7,8, which my e useful for the friction of lrge-re sensor films. These clusters hve een used more recently s nnometl inks for inkjet nd other printing methods, which re more prcticl pproches to electrode nd interconnect depositions 9,1.However, numer of chllenges remin. Ultrthin films of these mterils hve tendency to crck during nneling ecuse of the lrge intrinsic volume reduction tht ccompnies complete sintering of the clusters, nd the wek sustrte dhesion nd poor film cohesion in the nnometls reported so fr. Furthermore, they require orgnic solvents such s toluene which cn potentilly hrm underlying orgnic semiconductor lyers. To deposit 1-nm-thick film y single-pss printing, for exmple, the concentrtion of the Au clusters needs to e >.25% y volume, which corresponds to concentrtion of >5 mg ml 1 owing to the high Au density of 19 g cm 3. uch high concentrtions hve een chieved for orgnic-solule Au clusters 11, ut not yet for wter- or lcohol-solule ones tht re importnt for orthogonl solvent processing. Despite the recent discovery of severl routes to mke hydrophilic Au clusters for iochemicl pplictions 12 19, none hve yet reported dequte concentrtions in wter or lcohols for prcticl printing of electrodes nd interconnects in orgnic electronics. We report here tht y simple modifiction of the Brust process 11, we cn produce Au clusters (2 5 Å dimeter, protected y 5 Å monolyer lignd shell) tht re exceptionlly dispersile up to 7 mg ml 1 in wter nd lcohols, including methnol, ethnol nd iso-propnol, without needing ultrsoniction, nd tht re stle indefinitely. A 1 mm pth length through these dispersions is prcticlly opque to light (Fig. 1). The pproch used here is n extension of the recognized role tht the terminl group plys in determining the soluility chrcteristics of nnoprticles (Fig. 1). The Au cores were produced using sodium orohydride (NBH 4 ) reduction of tetrchlorourte(iii) (AuCl 4 ) in twophse toluene wter system with tetr-n-ocylmmonium romide ((C 8 H 17 ) 4 NBr or c 4 N + Br ) s phse-trnsfer gent nd ω- croxyl-lkylthiol (or mixture with ω-hydroxyl-lkylthiol) s protecting lignds (see upplementry Informtion I nd Tle 1). The monolyer-protected Au clusters thus otined re in the form of slt with surfce C chrges compensted y c 4 N + ions, which cn e purified to give protected Au clusters tht re exceedingly solule in the lower lcohols (for exmple, for 22 ÅAu cluster,75mgml 1 in methnol) nd cn e repetedly isolted in dry wxy form nd redispersed without ultrsoniction. We found tht these c 4 N + surfce ions cn further e ion-exchnged with N + (or other lkli ions) to give free-flowing powder tht is exceedingly solule in wter (for the sme clusters, 7 mg ml 1 nture mterils VL 6 FEBRUARY

2 olule in toluene CH 3 H 4 mg ml 1 nno-au in wter olule in methnol 1 mm Au cluster 7 mg ml 1 in wter olule in methnol N N 6 mg ml 1 in methnol olule in wter Figure 1 Highly wter- nd lcohol-solule nnogold dispersions., Photogrph of 33-Å-dimeter Au dispersions in 1.-mm-dimeter glss cpillries with cklight illumintion. The dispersion shows the chrcteristic pink colourtion of high-qulity non-ggregting clusters t concentrtion of 4 mg ml 1, which cn e concentrted to highly stle opque dispersion, with >5 mg ml 1, which is required for prcticl pplictions in orgnic semiconductor circuits., chemtic outline of the wide tuneility of the soluility chrcteristics of the Au clusters depending on the terminl group of the protection monolyer, from toluene (ref. 11) to lcohols nd wter Film conductivity ( cm 1 ) Bulk Au Au (33 Å) Au (42 Å) Film conductivity ( cm 1 ) :3 Au(22 Å) PEDT 7:3 Au(33 Å) PEDT 7:3 Au(42 Å) PEDT 1:1 Au(22 Å) PEDT 1:1 Au(22 Å) PHT r.t r.t Annel temperture (ºC) Annel temperture (ºC) c d 2. i L L w i 1.5 r d V d/r V f Figure 2 Film d.c. conductivity s function of nnel temperture (hotplte: rmp rte 2 Cmin 1 ) in nitrogen., Pure Au (33 nd 42 Å dimeter) cluster films. r.t. represents room temperture., 7:3 vol/vol Au PEDT films with 22-, 33- nd 42-Å-dimeter Au, nd of 5:5 vol/vol Au PEDT nd Au PHT films with 22-Å-dimeter Au. Hotplte temperture ws clirted y melting-point references. c, For these mesurements,.25 1-μm-thick films were deposited onto lithogrphiclly ptterned four-in-line sustrtes (L = 5 nd 25 μm; w = 2.4 nd 4.8 mm). Current ws injected into the outer proes nd voltge ws mesured etween the inner proes. σ d.c. ws extrcted from the ohmic slope of the current voltge chrcteristics (Keithley 42 semiconductor prmeter nlyser). d, Computed men intercluster spcing normlized to cluster rdius (d/r) s function of cluster volume frction (V f ). This is otined from the rithmetic men of the intersphere spcing to the first coordintion lyer of fce-centred-cuic nd ody-centred-cuic lttices (see upplementry Informtion II), excluding the lignd shell thickness. It is not possile to increse the cluster volume frction eyond pproximtely 7 vol% without creting internl voids in the film. 15 nture mterils VL 6 FEBRUARY 27

3 c d 5 μm 5 μm 5 μm 5 μm e f g h 5 μm 5 μm 5 nm 5 nm Figure 3 urfce morphology of nnogold films with nd without the polymer mtrix. f, pticl microgrphs on hydrophilic fused-silic sustrtes:.5-μm-thick Au(33-Å) film t room temperture (), nd fter nneling (5 min, nitrogen) t 18 C(), 24 C(c) nd 28 C(d); nd.5-μm-thick 7:3 Au(33 Å) PEDT films t room temperture (e), nd fter nneling t 28 C(f). g,h, Atomic force microscopy imges of the sme film fter nneling t 24 C: Au (33 Å) film (g); 7:3 Au(33 Å) PEDT film (h). The initilly lck films turn to gold lustre ove 2 C for Au, nd 21 C for Au PEDT film. Microscopic crcks re found in Au film even t reltively low nnel tempertures, ut not in the Au PEDT film. At elevted tempertures ove 25 C, gigntic crcks pper s result of volume shrinkge. The Au PEDT films, on the other hnd, remin smooth nd continuous up to t lest 3 C. in H 2 ). These concentrted dispersions hve een stle for more thn one yer t room temperture in our l. These dispersions cn e mixed with corresponding solutions of, for exmple, poly(3,4-ethylenedioxythiophene):poly(styrenesulphonte) (PEDT) or poly(4-hydroxystyrene) (PHT), which cn e printed, drop-cst or spin-coted onto vrious sustrtes, to give films with wide composition rnge of Au of up to 7 vol% without phse seprtion (see elow), nd which cn lso e nneled to high σ d.c.. The dispersions re lso stle for more thn one yer t room temperture. Figure 2, shows the film σ d.c. with nnel temperture in nitrogen. The nno-au films s-deposited give σ d.c cm 1, nd re in the insultor regime 16,2,21, owing to the required tunnelling or chrge-hopping 22 cross the lkyl lignd shell etween the clusters efore nneling. n nneling to 16 2 C, σ d.c. increses to cm 1, nd then more shrply t Cto>3 1 5 cm 1, independent of cluster size in the Å rnge. This temperture (T p ) corresponds to the onset of the percoltive insultor-to-metl trnsformtion to form interconnected conducting pths etween the Au clusters s result of cluster cluster colescence. The T p found here is similr to tht reported for toluene-solule 5 Å Au clusters protected with long lkylthiols (C 8 C 12 ) (ref. 1). The Au polymer films hve slightly higher initil σ d.c cm 1, owing to the PEDT mtrix (σ d.c. 1 6 cm 1 ). PEDT is roust conducting polymer tht cn tolerte up to 3 C in nitrogen. σ d.c. rises to cm 1 t 16 2 C, depending on film composition nd cluster dimeter. The 7:3 v/v Au PEDT film with Å-dimeter Au trnsforms etween C to the highly conductive stte with σ d.c cm 1. After the trnsformtion, the conductivity is expected to e determined y the length nd density of the percolted metllic pths. These re sufficiently high for lrge σ d.c. toerechedevenin the presence of the PEDT polymer mtrix. The mtrix thus seems to retrd T p mrginlly y only 15 2 C, nd hs no detrimentl effect on the finl σ d.c. for films with high Au volume frction of.7 nd smll Au size of 33 Å or less. This lso proves tht the Au polymer films re homogeneous, otherwise the percolted pths would rek in the mcroscopic regions tht hve low cluster density. This homogeneity contrsts with the usul phse seprtion in mny nnoprticle polymer systems, nd hs een directly proven y trnsmission electron microscopy for films with reltively low Au cluster density (<3 vol% Au, upplementry Informtion, Fig. 1). Films with higher cluster density re too opque to the electron em its homogeneity hs een estlished indirectly from the oserved conductive percoltion s well s from opticl trnsmission spectroscopy. No phse seprtion occurs here even in the solid stte, presumly owing to fvourle interction (oth ionic nd hydrogen-onding) etween the polymer nd the lignd shell. Fourier-trnsform infrred spectroscopy confirms tht the terminl groups in the protection monolyer re in the ionic C form (see upplementry Informtion, Fig. 2), which should fford fvourle ionic interctions with the PEDT polyelectrolyte complex. For the lrger Au clusters, T p is further retrded, nd the ttinle σ d.c. is decresed, s confirmed y dt tken t higher tempertures (not shown). This is not unexpected, ecuse the films with lrger Au clusters hve proportionlly lrger men intercluster spcing, which dversely ffects the development of the percoltion pths. In these homogeneous composites, the men intercluster spcing is set y the mtrix volume frction spnning the intercluster spce together with the cluster dimeter (see upplementry Informtion II nd Fig. 2d), in ddition to the lignd shell thickness. The volume frction together with cluster dimeter therefore provide sensitive mens to djust the overll conductive properties of the nnocomposite. We hve determined tht for films with lower Au content down to 3:7 v/v (dt not shown), the percoltive trnsformtion nture mterils VL 6 FEBRUARY

4 log(trnsmittnce) Annel T (ºC) log(trnsmittnce) Annel T (ºC) , , Wvelength (nm) Wvelength (nm) c 2. d (i) (ii) log(trnsmittnce) (i) Nnostructure (viii) (vii) (vi) (v) (iv) (iii) (ii) , Wvelength (nm) As-deposited Relxtion of Au core (iii) (iv) (v) (vii) Loss of lignd shell Development of percolted pths Figure 4 Thermlly induced insultor-to-metl trnsformtion. Evidence for the controlled percoltion morphology for high electricl conductivity, without complete sintering of the Au clusters., pticl trnsmission spectr of 25-nm-thick Au(33 Å) film t vrious stges of nneling (5 min, nitrogen) deposited on fused silic. The rek in the spectr is due to our deletion of the irrdince spike in the deuterium lmp., me s in for 6-nm-thick 7:3 Au(33 Å) PEDT film. c, Clculted trnsmission spectr of the film in using effective-medium theory nd Fresnel trnsfer mtrix. Dielectric functions nd ll input prmeters (film thickness nd composition) re known seprtely. The nno-au core dimeter nd lignd shell thickness re 33 Å nd 5Å respectively. The free prmeters give the film nnostructure s shown in d. d, chemtic outline of the evolution of the film nno-morphology: (i) s-deposited film hs severe electron scttering in the Au clusters (C 2) (see text); followed y (ii) perfection of order in the Au clusters (C.75); nd then (iii) (iv) progressive elimintion of lignd shell, from 4 Å to.2 Å, whilst retining the distinct core shell structure; nd finlly (v) (vii) core core colescence nd trnsformtion of frction of the guest host nnostructure to the percolted nnostructure (with 15, 2 nd 25 vol% of percolted domins, respectively), which is sufficient to provide high conductivity. (viii) is computed for hypotheticl structure in which the Au domins chieve complete percoltion. cn still occur, ut the mximum σ d.c. is gretly reduced (to cm 1 for 5:5 v/v,1 1 3 for 4:6 nd 4 for 3:7). This suggests tht the volume percoltion threshold (v c ) (ref. 23) is just elow.3. It seems rodly consistent with the rndomlttice percoltion model, which predicts v c =.3, nd the simplecuic lttice model for ond percoltion, which predicts v c =.25 (ref. 23). Just ove percoltion, σ d.c. is expected to scle s (v v c ) p,wherep is the criticl exponent 23. From the limited dt, we deduced p 6 nd v c.24. Even with dielectric polymer s mtrix (PHT), the films still show the conductive percoltion trnsition. Therefore, these mterils provide mens to regulte the finl film conductivity etween 1 4 nd few 1 5 cm 1 depending on volume frction nd cluster dimeter. This is the orgnic nlogue of the well-known inorgnic cermets, nd llows prcticl resistors of to e fricted y printing. Without the polymer mtrix, this is not redily possile ecuse it is difficult to rrest the colescence of the nno-au t the desired conductivity. Another key distinction etween films fricted from pure Au clusters nd from the Au polymer nnocomposites lies in the greter therml stility of the nnocomposites. Figure 3 d shows, for comprison, the opticl imges of the pure nno- Au film with nnel temperture. Microcrcks re formed in the Au film even t reltively low tempertures, owing to severe volume reduction during elimintion of the protecting lignd shell, which cn mount to s much s 5% for 33 Å Au cores protected y 5 Å lignd shell. Furthermore, the ultrthin Au films otined (<1 nm) re not thermlly stle nd strt to show signs of reking up (owing to high surfce moility of Au toms) ove 25 C. The ultrthin Au polymer nnocomposite films, on the other hnd, mintin their integrity up to 3 C. The polymer mtrix in our Au polymer films thus cts s inder to significntly improve the film cohesion oth during deposition nd the susequent nneling, therey preventing microcrcks nd dewetting. The nno-au nd in prticulr the Au polymer films reported here pss the cotch-tpe peel test on ppropritely treted sustrtes (for exmple, HCCH 2 CH 2 i 3 -treted silicon sustrtes), in contrst to those otined from lkylthiolte-protected nno-au. This dequte sustrte dhesion is essentil for electrode nd device integrity on flexile sustrtes to e chieved. The role of the polymer mtrix s moleculr filler in limiting the colescence of the neighouring Au clusters nd hence in controlling the percoltion of the Au pths is confirmed y opticl spectroscopy. If the Au clusters were llowed to colesce completely s in net films of the clusters, the films suffered huge volume reduction nd lrge lterl stresses. As descried erlier, direct imging y trnsmission electron microscopy is not possile for films with high Au cluster volume frctions, ut opticl spectroscopy turns out to e still usle to chrcterize their internl morphology for films up to 1 nm thick. The opticl trnsmission spectr of nno-au film nd Au PEDT film re 152 nture mterils VL 6 FEBRUARY 27

5 c i d (A) Nno-Au source electrode X id (na) Chnnel PEDT gte Nno-Au drin electrode X' 3 μm V gs (V) 8 V 6 V 4 V 2 V V ds (V) V ds (V) 8 V 6 V 4 V 2 V V gs (V) Glss sustrte Nno-Au electrodes 5 nm TFB 13 nm BCB PEDT gte Figure 5 All-printed polymer FET., pticl microgrph of the ll-printed polymer orgnic p-fet with current-crrying nno-au PEDT s source drin electrodes nd PEDT s gte electrode, TFB semiconductor nd crosslinked BCB gte dielectric. The chnnel (L = 3 μm;w = 4 μm) is formed y stylus micro-cutting., Cross-sectionl view of the lyers cross X X. c, Logrithmic output chrcteristics of the device. Inset: Trnsfer chrcteristics of the device. Mesured hole moility is cm 2 V 1 s 1, which is similr to conventionl dignostic devices fricted on lithogrphiclly ptterned Au source drin rrys. shown for comprison in Fig. 4 nd respectively. Below T p, the nno-au film shows the distinct plsmon nd 24,25 tht increses in oscilltor strength nd red-shifts from 54 to 575 nm. At T p, the plsmon feture disppers ruptly into Drude til tht extends into the infrred nd ner-infrred, indictive of the formtion of 3D-percolted structure. The film is however still slightly nnostructured nd distinct from ulk Au, s evidenced y the retention of wek plsmon residul t 65 nm. imilr chrcteristics re repeted lso in the Au PEDT film, except for the dely in the T p y 15 2 C nd decrese in the shrpness of the trnsformtion. This mkes it possile to cross the insultor metl oundry in incrementl steps to otin more detiled informtion. From opticl spectroscopy, the emergence of the Drude til indictes tht the insultor-to-metl trnsformtion occurs over C, which is fully consistent with σ d.c. mesurements. However, the plsmon nd remins distinct even for films nneled to 255 C, which indictes retention of the cluster chrcter in these nnocomposite films lthough this is nerly completely oliterted y 195 C in the pure Au cluster film. To chrcterize the internl film morphology using opticl spectroscopy, we first developed models for the Au PEDT composite nnostructure t different stges of nnel, nd clculted their opticl trnsmission spectr. The models (Fig. 4d) nd clcultions re descried in detil in upplementry Informtion III. Briefly, the two possile limiting nnostructures were considered: (1) guest host morphology of isolted Au clusters (with or without their lignd shell s pproprite to the nneling temperture) dispersed in PEDT, nd (2) fully percolted morphology of i-continuous Au nd PEDT. The complex dielectric function for the Au clusters ws first evluted using stndrd methods Then the dielectric function of the guest host nnostructure ws evluted y the Mxwell Grnett 29 nd tht of the fully percolted nnostructure y the Bruggemn 29 effective medium pproximtions. The initil nnostructure ws expected to e of the guest host type, which evolves to mixture of the guest host nd the fully percolted domins fter T p. The trnsmission spectr were then computed from the opticl complex dielectric functions using stndrd Fresnel trnsfer mtrix methods 3, which tke into ccount reflection losses nd multipleem interference. All other input prmeters (such s opticl constnts of the components, film thickness nd volume frction) re mesured or known priori. The computed spectr re thus relile, with no free prmeters other thn the nnostructure prmeters (lignd shell thickness nd volume frction of the percolted domins) tht we wish to extrct. These prmeters re given in upplementry Informtion, Tle 2. The sequence of events schemticlly illustrted in Fig. 4d provides excellent greement etween the clculted (Fig. 4c) nd the oserved (Fig. 4) trnsmission spectr. The key step in the trnsformtion is the colescence of neighouring Au clusters to give controlled percoltion nnostructure following elimintion of the lignd shell. The volume frction of the percolted domins reched is pproximtely fter sintering. In this wy, the high σ d.c. is chieved without complete collpse of the Au clusters nd the ttendnt lrge volume losses. To demonstrte the viility of these mterils, we hve fricted polymer orgnic p-type field-effect trnsistor (FET) y first printing the interconnect nd source drin electrodes from Au(33-Å) PEDT solution using picolitre pipette, followed y micro-cutting the chnnel (length, L = 3 μm nd width w =4 μm) nd nneling the Au PEDT to the conductive stte. We then deposited the semiconductor (poly(9,9-dioctylfluorene-co- (phenylene-(n-4-sec-utylphenyl)iminophenylene) (TFB) (5 nm thick) followed y the gte-dielectric divinyltetrmethyldisiloxneis(enzocycloutene) (BCB) (13 nm thick) which ws then crosslinked y rpid therml nnel, nd finlly we deposited surfctnt-ion-exchnged 31 PEDT gte electrode. Figure 5 nd show n opticl microgrph nd cross-sectionl schemtic digrm of the device, respectively. Figure 5c shows the trnsfer nd output chrcteristics otined. The moderte gte voltge, V gs, threshold is due to the semiconductor. Good sturtion nd on off rtio is otined even for such smll chnnel width. The liner fieldeffect hole moility is determined to e cm 2 V 1 s 1, which is slightly higher thn corresponding devices fricted on lithogrphiclly ptterned Au source drin rrys 31. This shows tht the Au PEDT electrode descried here is le to inject holes well into TFB nd does not hve detrimentl effects on trnsistor performnce. nture mterils VL 6 FEBRUARY

6 Current (ma) 5 Current (ma) Current (ma) Cell voltge (V) Cell voltge (V) Cell voltge (V) Figure 6 Electriclly induced insultor-to-metl trnsformtion., Current voltge (IV ) chrcteristics of pristine sndwiched cross-wire cells showing n electriclly induced insultor-to-metl trnsformtion t 4 V corresponding to 1.6MVcm 1 (R initil = 2 k,r finl = 3 ). Inset: IV chrcteristic t lower fields efore trnsformtion., IV chrcteristics of cells pre-nneled to 22 C(2min,N 2 ) showing tht the trnsformtion now occurs t lower field of.9mvcm 1 s result of the development of nscent percoltion pths during the nnel (R initil = 65,R finl = 4 ). Cell structure: glss/5 nm Cr/5 nm Au/6 nm PEDT/25 nm 5:5 Au(33 Å) PEDT/6 nm PEDT/3 nm Cr/1 nm Au. Are, cm 2. Current complince ws set to 15 ma for nd 4 ma for. Prsitic resistnce due to the line nd proe ws less thn 1. Finlly, the trnsformtion cn lso e electriclly induced, s shown in Fig. 6,. Verticl cross-wire cell structures were fricted with 1:1 Au(33 Å) PEDT. At low voltges, the cell gives the expected non-ohmic current voltge (IV) chrcteristics of the Au PEDT, nd the expected low-field resistnce of 2 k, corresponding to σ d.c cm 1. As the pplied voltge is rmped to 4 V, the cell resistnce drops ruptly to 3 nd the IV chrcteristic turns ohmic (σ d.c. 1 3 cm 1 ). It is cler tht the insultor-to-metl trnsformtion does not pervde the entire ulk of the Au PEDT, ut is limited possily to filmentry pths. n nneling of nother cell to 22 C to just rech the onset of thermlly induced trnsformtion, the cell resistnce drops from 2 k to 65 (σ d.c cm 1 ) s expected. For this cell, during the voltge rmp, the cell resistnce drops ruptly to 4 (σ d.c. 1 2 cm 1 ) when V reches only 25 V. The writing voltge required is now lower thn efore. Consistent with the presence of nscent Au conducting pths tht hve een induced thermlly, the electric field required to completely percolte these pths is smller. nce formed, the current conduction pths re stle nd irreversile, in contrst to the known reversile formtion of electricl shorts in dielectric films 32, or with Au chrge-trnsfer complexes 33. This suggests possile non-voltile memory pplictions 34 for these printle cermets. As the current filment psses through Au clusters, their properties (volume frction, size nd nnel temperture) provide mens to tilor the chrcteristics of the cell. Received 3 April 26; ccepted 1 Novemer 26; pulished 17 Decemer 26. References 1. Crone, B. et l. Lrge-scle complementry integrted circuits sed on orgnic trnsistors. Nture 43, (2). 2. irringhus, H. et l. High-resolution inkjet printing of ll-polymer trnsistor circuits. cience 29, (2). 3. Huitem, H. E. A. et l. Plstic trnsistors in ctive mtrix displys. Nture 414, 599 (21). 4. Kwse, T., irringhus, H., Friend, R. H. & himod, T. Inkjet printed vi-hole interconnections nd resistors for ll-polymer trnsistor circuits. Adv. Mter. 13, (21). 5. Bufft,P.&Borel,J.P.ize effect on the melting point of gold nnoprticles. Phys.Rev.A13, (1976). 6. Dniel, M. & Astruc, D. Gold nnoprticles: ssemly, suprmoleculr chemistry, quntum-size-relted properties, nd pplictions towrd iology, ctlysis, nd nnotechnology. Chem. Rev. 14, (24). 7. Myy, K.. & stry, M. A new technique for spontneous growth of colloidl nnoprticle superlttices. Lngmuir 15, (1999). 8. Musick, M. D. et l. Metl films prepred y stepwise ssemly. 2. Construction nd chrcteriztion of colloidl Au nd Ag multilyers. Chem. Mter. 12, (2). 9. Fuller,. B., Wilhelm, E. J. & Jcoson, J. M. Ink-jet printed nnoprticle microelectromechnicl systems. J. Microelectromech. yst. 11, 54 6 (22). 1. Hung, D., Lio, F., Moles,., Redinger, D. & urmnin, V. Plstic-comptile low resistnce printle gold nnoprticle conductors for flexile electronics. J. Electrochem. oc. 15, (23). 11. Brust, M., Wlker, M., Bethell, D., chiffrin, D. & Whymn, R. ynthesis of thiol derivtised gold nnoprticles in two phse liquid-liquid system. J. Chem. oc., Chem. Commun (1994). 12. Myy, K.., Ptil, V. & stry, M. n the stility of croxylic cid derivtized gold colloidl prticles: the role of colloidl solution ph studied y opticl sorption spectroscopy. Lngmuir 13, (1997). 13. Johnson,. R., Evns,. D. & Brydson, R. Influence of terminl functionlity on the physicl properties of surfctnt-stilized gold nnoprticles. Lngmuir 14, (1998). 14. Chen,. & Kimur, K. ynthesis nd chrcteriztion of croxylte-modified gold nnoprticle powders dispersile in wter. Lngmuir 15, (1999). 15. Templeton, A. C., Chen,., Gross,. M. & Murry, R. W. Wter-solule, isolle gold clusters protected y tiopronin nd coenzyme A monolyers. Lngmuir 15, (1999). 16. Cliffel, D. E., Zmorini, F. P., Gross,. M. & Murry, R. W. Mercptommonium-monolyerprotected, wter-solule gold, silver, nd plldium clusters. Lngmuir 16, (2). 17. Gittins, D. I. & Cruso, F. pontneous phse trnsfer of nnoprticulte metls from orgnic to queous medi. Angew. Chem. Int. Edn 4, (21). 18. Knrs, A. G., Kmounh, F.., chumurg, K., Kiely, C. J. & Brust, M. Thiolkylted tetrethylene glycol: A new lignd for wter solule monolyer protected gold clusters. Chem. Commun (22). 19. Hong, R., Emrick, T. & Rotello, V. M. Monolyer-controlled sustrte selectivity using noncovlent enzyme-nnoprticle conjugtes. J. Am. Chem. oc. 126, (24). 2. Musick, M. D., Keting, C. D., Keefe, M. H. & Ntn, M. J. tepwise construction of conductive Au colloid multilyers from solution. Chem. Mter. 9, (1997). 21. Brust, M., Kiely, C. J., chiifrin, D. & Bethell, D. elf-ssemled gold nnoprticle thin films with nonmetllic opticl nd electronic properties. Lngmuir 14, (1998). 22. Aeles, B., heng, P., Coutts, M. D. & Arif, Y. tructurl nd electricl properties of grnulr metl films. Adv. Phys. 24, (1975). 23. Pike, G. E. & eger, C. H. Percoltion nd conductivity: computer study I. Phys.Rev.B1, (1974). 24. Quinten, M. & Kreiig, U. pticl properties of ggregtes of smll metl prticles. urf. ci. 172, (1986). 154 nture mterils VL 6 FEBRUARY 27

7 25. El-yed, M. A. & Link,. ize nd temperture dependence of the plsmon sorption of colloidl gold nnoprticles. J. Phys. Chem. B 13, (1999). 26. Vn de Hulst, H. C. Light cttering y mll Prticles (Wiley, New York, 1957). 27. Grnqvist, C. G. & Hunderi,. pticl properties of ultrfine gold prticles. Phys.Rev.B16, (1977). 28. cffrdi, L. B., Pellegri, N., de nctis,. & Tocho, J.. izing gold nnoprticles y opticl extinction spectroscopy. Nnotechnology 16, (25). 29. Aspnes, D. E. pticl properties of thin films. Thin olid Films 89, (1982). 3. Born, M. & Wolf, E. Principles of ptics: Electromgnetic Theory of Propgtion, Interference nd Diffrction of Light (Cmridge Univ. Press, Cmridge, 1999). 31. Chu, L. L., Ho, P. K. H., irringhus, H. & Friend, R. H. High stility ultrthin spin-on enzocycloutene gte dielectric for polymer field-effect trnsistors. Appl. Phys. Lett. 84, (24). 32. Crchno, H., Lcoste, R. & egui, Y. Bistle electricl switching in polymer thin films. Appl. Phys. Lett. 19, (1971). 33. uyng, J. Y., Chu, C. W., zmnd, C. R., M, L. P. & Yng, Y. Progrmmle polymer thin film nd non-voltile memory device. Nture Mter. 3, (24). 34. Ner, R. C. G., de Boer, B., Blom, P. W. M. & de Leeuw, D. M. Low-voltge polymer field-effect trnsistors for nonvoltile memories. Appl. Phys. Lett. 87, 2359 (25). Acknowledgements We thnk NU (Project ) nd A*TAR (Project ) for funding. Correspondence nd requests for mterils should e ddressed to L.-L.C. or P.K.-H.H. upplementry Informtion ccompnies this pper on Competing finncil interests The uthors declre tht they hve no competing finncil interests. Reprints nd permission informtion is ville online t nture mterils VL 6 FEBRUARY