Lithographically patterned nanowire electrodeposition

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1 Lithogrphilly ptterned nnowire eletrodeposition E. J. MENKE, M. A. THOMPSON*, C. XIANG*, L. C. YANG AND R. M. PENNER Institute For Surfe nd Interfe Siene nd Deprtment of Chemistry, University of Cliforni, Irvine, Cliforni , USA *These uthors ontriuted eqully to this work e-mil: Pulished online: 22 Otoer 26; doi:1.138/nmt1759 Nnowire frition methods n e lssified either s top down, involving photo- or eletron-em lithogrphy, or ottom up, involving the synthesis of nnowires from moleulr preursors. Lithogrphilly ptterned nnowire eletrodeposition (LPNE) omines ttriutes of photolithogrphy with the verstility of ottom-up eletrohemil synthesis. Photolithogrphy defines the position of srifiil nikel nnond eletrode, whih is reessed into horizontl trenh. This trenh ts s nnoform to define the thikness of n inipient nnowire during its eletrodeposition. The eletrodeposition durtion determines the width of the nnowire. Removl of the photoresist nd nikel exposes polyrystlline nnowire omposed of gold, pltinum or plldium hrterized y thikness nd width tht n e independently ontrolled down to 18 nd 4 nm, respetively. Metl nnowires prepred y LPNE my hve pplitions in hemil sensing nd optil signl proessing, nd s interonnets in nnoeletroni devies. 914 letron-em lithogrphy (EBL), invented in the erly 197s1,2, provides mens for ptterning polyrystlline metl nnowires s smll s 2 nm in dimeter onto surfes3. The ppliility of EBL, however, hs een limited to reserh nd development pplitions, euse it is seril ptterning tehnology. In 199, prllel version of EBL ws desried4,5, ut spe-hrge lurring 6 hs prevented this tehnique from pprohing the resolution of diret-write EBL. By using, s templte, semiondutor surfes with tomilly defined grooves nd troughs, su-1 nm metl nnowires hve een prepred using vpour deposition7 9. A vrint of this pproh ws used to rete high-density rrys of liner, 1-nm-dimeter Pt nnowires1. We hve demonstrted11 tht ensemles of 3 nm ntimony nnowires n e prepred y eletrohemil step-edge deortion on grphite surfes oupled with ething, ut no ontrol of nnowire position on the surfe or inter-wire pith is possile using this method. Lithogrphilly ptterned nnowire eletrodeposition (LPNE) is new method for synthesizing nole-metl nnowires on glss or oxidized silion surfes. LPNE (Fig. 1) involves the preprtion of srifiil nikel nnond eletrode using optil lithogrphy nd the susequent eletrodeposition of nole-metl (, Pt or Pd) nnowire t this eletrode. However there is n importnt wrinkle: the nikel nnond eletrode is reessed into horizontl trenh defined y the insultor surfe nd photoresist. The eletrodeposition of nnowire into this nnoform produes wire with retngulr ross-setion, where the thikness nd width re independently ontrollle with 5 nm preision, down to 18 nm in height nd 4 nm in width. Beuse we use optil lithogrphy to define the position of the nnowires on the surfe, the optil diffrtion limit onstrins the sping etween nnowires. The position of nikel nnond eletrodes prepred in the first four steps of the LPNE proess shown in Fig. 1 determines the position of nnowires on glss surfe. Fritsh nd oworkers12 prepred gold nnond eletrodes using essentilly the sme first four steps s shown here, ut our method deprts in n importnt wy from tht of Fritsh in step 4: we remove exposed nikel eletrohemilly y pplying n oxidizing potentil in the rnge from.85 to.1 V versus sturted lomel referene E nture mterils VOL 5 NOVEMBER Nture Pulishing Group

2 1 2 Flot glss PVD nikel 3 (+) 4 Contt pds Figure 1 Seven-step proess flow for metl-nnowire frition using LPNE nm 4 nm.3.5 After Before Oxidtion time (s) 1. After 1.5 Reessed nikel nnond eletrode Current (ma).6 thikness: 9 nm nm 7 nm 4 nm E ( V ) versus SCE E ( V ) versus SCE Figure 2 Overething of the nikel lyer produes horizontl trenh originting t the edge of the photoresist., Oxidtion urrent versus time during the removl of nikel from lithogrphilly ptterned surfes y potentiostti eletro-oxidtion (retion: e ) t.1 V versus SCE in queous.1 M KCl t ph = in queous Three trnsients re shown for nikel lyers with pproximte thiknesses of 4, 7 nd 9 nm, s shown.,, Cyli voltmmogrm for Ru(NH3 ) 3+ 6 t 5 mv s.1 M NCl efore the removl of nikel () nd fter the removl of nikel () y eletro-oxidtion, resulting in the formtion of horizontl trenh 2 nm in depth, terminted y nikel nnond 4 nm in thikness. eletrode (SCE) (Fig. 2). We ontinue nikel oxidtion until we eth nikel t the exposed edges of this pttern, underutting the photoresist y 1 3 nm (Fig. 2). For nikel films of three thiknesses (Fig. 2), the ething urrent is pproximtely proportionl to the nikel film thikness s this eletrohemil underutting ours. kel overething produes horizontl trenh into whih we eletrodeposit nole-metl nnowire in step 5 (Fig. 1). Is this reessed nikel nnond oth eletrohemilly retive nd essile to redox speies? To nswer this question, we investigted the yli voltmmetry of Ru(NH3 )36+, metl omplex tht undergoes fst, reversile one-eletron redution. Before nikel dissolution (Fig. 2), the yli voltmmogrm shows urrent peks nd hysteresis of the forwrd nd reverse sns: ll onsistent with the plnr diffusion of this moleule to the reltively lrge nikel res exposed y the dissolution of photoresist fter lithogrphy. After eletro-oxidtion of the nikel nd the formtion of the horizontl trenh (Fig. 2), the Ru(NH3 )36+ redution urrent ws redued y ftor of more thn 1 nd stedy-stte, sigmoidl CV showing no hysteresis ws oserved, qulittively s expeted for nnond eletrode13. This result demonstrtes tht the reessed nikel eletrode is oth eletrohemilly tive nd essile to the dissolved redox speies Ru(NH3 )36+, s required for nnowire eletrodeposition. The growth of this nnowire into this horizontl trenh ours in three phses (Fig. 3): First, immeditely fter the pplition of plting voltge for plldium, the urrent inreses s plldium nuletes long the entire length of the nikel nnond. When this nuletion proess is omplete, we oserve qusi-onstnt deposition urrent during growth of the onfined metl nnowire euse the wetted surfe re of the onfined wire is onstnt, nd euse for suh kinetilly ontrolled retion the deposition urrent per unit re of the metl is onstnt13. Terminting growth during this phse results in nnowire hving retngulr ross setion, suh s seen in Fig. 3. If we ontinue wire growth, deposited metl fills the trenh nd egins to emerge from it, nd the totl urrent inreses euse the wetted surfe re inreses. This looming of the nnowire s it emerges from the trenh is undesirle, euse it produes distriution of dendrites long one edge of the nnowire (Fig. 3). nture mterils VOL 5 NOVEMBER Nture Pulishing Group 915

3 w(t ) = Jdep tdep Vm, nf (1) where tdep is the deposition time, Vm is the molr volume of the deposited metl, n is the numer of eletrons required to redue eh metl omplex ion nd F is the Frdy onstnt (96,485 C eq. 1 ). Plots of w versus tdep for gold, plldium nd pltinum (Fig. 5) re ll pproximtely liner in ordne with eqution (1). The slopes of these plots re proportionl to Jdep, the mgnitude of whih depends on the onentrtion of metl omplex present in the plting solution nd on the eletrodeposition potentil. We hve not yet estlished how Jdep influenes the width uniformity of LPNE nnowires. Bsed on our prior work14, there re good resons to elieve tht slower deposition rte my redue the wire width vriility, mking possile the preprtion of nnowires tht re nrrower thn the minimum of 4 nm demonstrted here. Previous mesurements of the eletril properties of metl nnowires hve used wires rnging in length from 5 nm (refs 15,16) to 5 μm (refs 16 18). Using four-point proe shown in Fig. 6 (inset), we mesured the eletril ehviour of 4 μm long setions of two gold nnowires with ross-setions of 2 nm 233 nm nd 1 nm 166 nm. Two hrteristis of metlli ondution re seen in these nnowires: first, the urrent voltge ehviour ws ohmi (Fig. 6), nd seond, we oserved positive temperture oeffiient of resistivity, α (Fig. 6). However, we lso see ler quntittive differenes in the ehviour of these nnowires ssoited with their diminutive size. The mesured room-temperture eletril resistivity of these nnowires ws 916 Growth in Overgrowth trenh 2. Nuletion The nnowires produed y LPNE hve n pproximtely retngulr ross-setion (Fig. 5), whih is enfored y the dimensions of the horizontl trenh. The minimum dimensions for the thikness nd width of these nnowires re 18 nd 4 nm, respetively. For exmple, we show pltinum nnowire with thikness of 39 nm, width of 46 nm nd length of 1 m in the SEM imge of Fig. 4. We n lithogrphilly pttern lrge sustrte res exeeding 1 m2 with nnowires (for exmple Fig. 4 d). The nnowires produed show remrkle dimensionl uniformity over the entire smple surfe. The reltive stndrd devitions of the nnowire thikness nd width re <5% nd 1 12%, respetively, for the ptterns shown in Fig. 4. We tke prtiulr note of the ft tht orners, suh s those present in the pttern of Fig. 4, do not use either nrrowing or thikening of the nnowire the ltter expeted for diffusion-ontrolled deposition proess. We expet this urvture invrine of the wire width if the wire eletrodeposition retion ours under onditions of kineti, not diffusion, ontrol. We n independently ontrol the thikness nd width of these nnowires: the nikel lyer thikness fixes the thikness of the nnowires, wheres the width is proportionl to the eletrodeposition durtion. We doument this ontrol y the dt shown in Fig. 5,. The AFM-mesured nnowire thikness inreses linerly with the nikel lyer thikness for nnowires omposed of gold, plldium nd pltinum (Fig. 5). The slope of these dt points is.98, inditing tht the nnowire thikness is equl to the thikness of the evported nikel lyer. The smllest ttinle nnowire thiknesses were in the 18 nm rnge for nd Pt nd somewht lrger for Pd. Nnowire thiknesses for ll three metls in the 15 nm rnge will proly e possile with some dditionl refinement of the nikel evportion onditions to further improve the uniformity of this film. For metleletrodeposition proess ourring within retngulr trenh t time-invrint urrent density Jdep, the width of the deposited nnowire, w, is given y Time (s) nm 87 nm 1 μm 5 nm Figure 3 The filling nd the eventul overfilling of the trenh with gold, pltinum or plldium produes distintive wire eletrodeposition-urrent trnsient., Eletrodeposition urrent versus time for the potentiostti growth, nd overgrowth, of plldium nnowire t V versus SCE. This queous solution ontined 2 mm PdCl2, 2 mm shrin nd.1 M KCl. We oserve the qusi-onstnt urrent seen from 2 to 5 s euse, for this kinetilly ontrolled deposition proess, urrent is proportionl to the wetted surfe re, whih remins onstnt while growth of the nnowire is onfined to the horizontl trenh., nnowire with retngulr ross-setion otined for trenh-onfined growth for 1 s., nnowire deposited for 1, s, showing looming from the edge of the photoresist. signifintly higher thn expeted for ulk gold (Tle 1), nd this disprity inresed with deresing temperture, down to 1 K (Fig. 6), just s reported for nnowires in severl previous studies How n these disprities e understood? Two ftors ontriute to the resistivity of polyrystlline nnowires: (1) inelsti sttering of ondution eletrons t wire surfes2,21 nd (2) the refletion of eletrons t grin oundries22. Steinhogl et l.23 lulted the extr resistivity ontriuted y these two mehnisms for nnowires with retngulr rosssetion. Their eqution (our eqution (2)) uses Fuhs Sondheimer theory2,21 to ount for surfe sttering nd Myds nd Shtzkes theory22 to ount for grin-oundry sttering23 : α 2 3 ρ = ρ + α α ln α AR l + C 1 p 8 AR w with α = l R, d 1 R (2) where AR is the rtio of wire height to wire width nd C is geometril prmeter, whih is equl to 1.2 for nnowires with nture mterils VOL 5 NOVEMBER Nture Pulishing Group

4 46 nm 5 μm 5 nm d 1 μm 5 μm Figure 4 Exmples of ptterned nnowires prepred using LPNE., A pltinum nnowire with width of 46 nm nd thikness of 39 nm. This nnowire ws ontinuous, s mesured y snning eletron mirosopy (SEM), for more thn 1 m., Prllel gold nnowires, 1 m in length nd sped y 9 μm., A oiled nnowire with totl length of 2.7 m. d, Nnowire loops. 1 Height (nm) 6 Nnowire width (nm) 5 nm Wire thikness (nm) Pd Pt Pd Pt 2 Δ = 65.4 Å Distne (μm) Thikness of nikel lyer (nm) 1 Underut Time (s) 8 1, 1,2 Figure 5 Both the thikness of nnowire produed y LPNE nd its width my e independently ontrolled., Atomi fore mirosope (AFM) imge of gold nnowire prepred using the LPNE method on glss, with the flt wire profile shown elow., Nnowire thikness, mesured y AFM, versus the thikness of the nikel lyer deposited in step 1., Nnowire width, mesured y SEM, s funtion of the eletrodeposition time. For oth nd, error rs represent ±1σ for t lest ten mesurements of eh nnowire. retngulr ross-setion. w is the width of the nnowire nd d is the men grin size, whih is in the 1 2 nm rnge. We ssume other prmeters speifi to gold to e s follows: the eletron men free pth t T = 3 K, l = 5 nm, the refletion oeffiient for nture mterils VOL 5 NOVEMBER Nture Pulishing Group 917

5 4 Tle 1 Experimentlly mesured resistivities t 3 K, ρ 3 K, for two gold nnowires, nd omprison with lulted resistivities nd ulk gold Ω m - nnowire Wire thikness (nm) 4 μm Ω m Bulk gold ρ = Ω m t = 1 nm 4 ρ 3 K ( m) Clulted t = 2 nm Grin dimeter (nm) Experimentl 2.5 E (V) Bulk gold Estimted from snning eletron mirogrphs. Clulted using equtions (2) nd (3) ρ /ρ3 K t = 2 nm.9 Eqution (2).8 Bulk gold α =.3715 t = 1 nm T (K) Figure 6 A high degree of wire uniformity results in nnowires tht re eletrilly ontinuous for up to 1. m., Current-versus-voltge urves quired using four evported eletrodes (inset). Dt for two gold nnowires prepred y LPNE re shown. These nnowires hd dimensions of 2 nm (thikness) 233 nm (width), nd 1 nm (thikness) 166 nm (width) the isolted wire length in oth ses ws 4 μm., Temperture dependene of the wire resistivity, ρ, normlized to the resistivity t 3 K, ρ 3, from 1 to 35 K, for the sme two nnowires s shown in. We lso plot the resistivity of ulk gold nd the temperture-dependent resistivities (dshed lines) lulted using equtions (2) nd (3), using the prmeters indited in the text. In this lultion, we took the temperture dependene of the eletron men free pth to e inversely proportionl to the resistivity for ulk gold. grin oundries, R =.9, nd the frtion of eletrons inident on wire surfes tht re speulrly sttered, p =.5. We n predit the temperture-dependent ρ in eqution (2) using the Bloh Gru neisen eqution: ρ(t ) = C T5 Θ6 Θ /T x 5 dx (ex 1)(1 e x ) (3) where the Deye temperture for gold, Θ, is 225 K (refs 16,24,25). How well do these equtions model the mesured resistivity of our LPNE gold nnowires? Fousing first on the resistivity dt t 918 Wire width (nm) 3 K (Tle 1), eqution (2) predits resistivity tht is too high for oth nnowires when grin oundry dimeter of 2 nm is ssumed. This disprity is tully lrger, pproximtely one order of mgnitude, for the lrger of the two nnowires, suggesting tht the ssumed R vlue of.9 is too high23. A seond disrepny with eqution (3) involves the temperture dependene of ρ: eqution (3) predits nerly liner ρ versus T ehviour, wheres we see signifint urvture to higher ρ vlues with diminishing temperture for oth nnowires (Fig. 6). We nnot explin this effet within the ontext of equtions (2) nd (3). Two dditionl ontriuting ftors to the resistivity re impurities in the eletrodeposited gold, nd mismth in the oeffiients of therml expnsion for gold (κ = K 1, 3 K) nd the sod-lime glss support (κ = K 1, 3 K). This mismth, whih inreses with deresing temperture, mens tht the length of the gold nnowire shrinks more rpidly with deresing temperture thn the glss surfe on whih it is supported, pling it in tension. The totl strin tht umultes in ooling from 3 to 8 K, for exmple, is signifint: pproximtely.7 μm for the 4 μm nnowires investigted here. Investigtions of nnowires on surfes hving different κ vlues will e neessry to determine the influene of this effet on the wire resistivity. METHODS We implemented the seven-step LPNE proedure shown in Fig. 1 s follows. We lened sod-lime glss mirosope slides in queous Nohromix solution, ir dried them nd died them into 1 1 squres. Onto eh squre, we deposited nikel film (ESPI, 5N purity), 2 1 nm in thikness, y hot-filment evportion t rte of A s 1 (step 1). We monitored the film thikness nd evportion rte using qurtz-rystl mirolne (Sigm Instruments). We then oted nikel-overed glss squres with positive photoresist lyer (Shipley 188) y spin oting (step 2). This involved the deposition of 1 ml liquot of photoresist onto eh squre nd the rottion of the squre t 2,5 r.p.m. for 4 s. This produed photoresist thikness (fter soft king) of 1 μm. We soft-ked freshly oted squres t 9 C for 3 min. After ooling to room temperture, we pressed trnsprent ontt msk onto the photoresist with qurtz plte nd exposed this msked surfe to hnd-held ultrviolet lmp, with wvelength of 365 nm nd n output power of.5 mw m2 for 2 min. We then soked the slide first in developer wter solution (one prt Shipley MF-351 to four prts wter) for 2 s, nd then in pure wter for 1 min, efore drying in strem of ultr-high-purity (UHP) N2 nture mterils VOL 5 NOVEMBER Nture Pulishing Group

6 (step 3). We then eletrohemilly removed exposed nikel (step 4) nd eletrodeposited plldium, pltinum or gold (step 5) onto the nikel nnond eletrodes produed y this proess. We rried out the eletrohemil stripping of the nikel nd the eletrodeposition of these metls in 5 ml, one-omprtment, three-eletrode ell. We rried out nikel dissolution in queous.1 M KCl ontining.1 ml of onentrted HCl. We eletroplted plldium from n queous solution ontining 2 mm PdCl2 nd.1 M KCl t.225 V versus SCE. We eletroplted pltinum from solution ontining.1 M KCl nd 1. mm K2 PtCl6 t.25 V versus SCE. We eletroplted gold from queous ommeril gold-plting solution (Clen Erth Solutions 6 mm Cl3 solution), with dded 1. M KCl t.9 V versus SCE. We prepred ll queous solutions using Millipore MilliQ wter (ρ > 18. M m). We lso used n SCE nd 2 m2 Pt foil ounter-eletrode. We rried out oth the stripping nd deposition on omputer-ontrolled EG&G 273A potentiostt/glvnostt. We hieved nikel stripping y seuring the photoresist-overed nikel film with self-losing metl tweezers, nd pling prt of the exposed nikel film in the nikel-stripping solution. We then held the film t potentil of.85 V versus SCE for 1, s. This removed ll of the exposed nikel nd produed n underut eneth the photoresist t eh nikel edge produed y photolithogrphy on the surfe. We then rinsed the slide with Nnopure wter, nd pled it in the plldium-deposition solution. We eletrodeposited plldium metl y holding the nikel film t potentil of V versus SCE for times rnging from 25 to 4 s. We then rinsed the glss slide with Nnopure wter nd dried it with UHP N2. We then removed the photoresist (step 6) y rinsing the slide with eletroni grde etone (Aros), methnol (Fisher) nd Nnopure wter, respetively, efore drying with UHP N2. We then removed the exess nikel film y wshing with dilute HNO3 (step 7). Complete removl of nikel required exposure to 5 7 M HNO3 for t lest 8 h. We quired SEM imges using Philips model XL-3FEG operting t 1 kv. We sputter oted thin gold film onto the glss slides efore imging to prevent hrging in the SEM. We otined tomi fore mirosopy imges on Prk Sientifi Instruments model CP tomi fore mirosope. We olleted the 256 pixels 256 pixels imges in non-ontt mode, using Mikromsh AFM tips with sn rte of.5 lines s 1. After olletion, we flttened imges using line-y-line seond-order fit to remove ny urvture imprted y the piezoeletri snning tue. Reeived 14 July 26; epted 6 Septemer 26; pulished 22 Otoer Vrnell, G. L., Spier, D. F. & Rodger, A. C. E-Bem writing tehniques for semiondutor-devie frition. J. V. Si. Tehnol. 1, (1973). 3. Vieu, C. et l. Eletron em lithogrphy: Resolution limits nd pplitions. Appl. Surf. Si. 164, (2). 4. Berger, S. D. & Gison, J. M. New pproh to projetion-eletron lithogrphy with demonstrted.1 μm linewidth. Appl. Phys. Lett. 57, (199). 5. Berger, S. D. et l. Projetion eletron-em lithogrphy A new pproh. J. V. Si. Tehnol. B 9, (1991). 6. Liddle, J. A. et l. Spe-hrge effets in projetion eletron-em lithogrphy: Results from the SCALPEL proof-of-lithogrphy system. J. V. Si. Tehnol. B 19, (21). 7. Jorritsm, J., Gijs, M. A. M., Kerkhof, J. M. & Stienen, J. G. H. Generl tehnique for friting lrge rrys of nnowires. Nnotehnology 7, (1996). 8. Ntelson, D., Willett, R. L., West, K. W. & Pfeiffer, L. N. Frition of extremely nrrow metl wires. Appl. Phys. Lett. 77, (2). 9. Ntelson, D., Willett, R. L., West, K. W. & Pfeiffer, L. N. Geometry-dependent dephsing in smll metlli wires. Phys. Rev. Lett. 86, (21). 1. Melosh, N. A. et l. Ultrhigh-density nnowire ltties nd iruits. Siene 3, (23). 11. Thompson, M. A., Menke, E. J., Mrtens, C. C. & Penner, R. M. Shrinking nnowires y kinetilly ontrolled eletrooxidtion. J. Phys. Chem. B 11, (26). 12. Ngle, M. P. & Fritsh, I. Individully ddressle, sumirometer nd eletrode rrys. 1. Frition from multilyered mterils. Anl. Chem. 7, (1998). 13. Brd, A. J. Eletrohemil Methods: Fundmentls nd Applitions (Wiley, New York, 21). 14. Wlter, E. C. et l. Nole nd oinge metl nnowires y eletrohemil step edge deortion. J. Phys. Chem. B 16, (22). 15. Durkn, C. & Wellnd, M. E. Size effets in the eletril resistivity of polyrystlline nnowires. Phys. Rev. B 61, (2). 16. Mrzi, G. D., Iopino, D., Quinn, A. J. & Redmond, G. Proing intrinsi trnsport properties of single metl nnowires: Diret-write ontt formtion using foused ion em. J. Appl. Phys. 96, (24). 17. Yng, F. Y. et l. Lrge mgnetoresistne of eletrodeposited single-rystl ismuth thin films. Siene 284, (1999). 18. Yng, F. Y. et l. Lrge mgnetoresistne nd finite-size effet in eletrodeposited ismuth lines. J. Appl. Phys. 89, (21). 19. Chiu, P. & Shih, I. A study of the size effet on the temperture-dependent resistivity of ismuth nnowires with retngulr ross-setions. Nnotehnology 15, (24). 2. Fuhs, K. The ondutivity of thin metlli films ording to the eletron theory of metls. Pro. Cm. Phil. So. 34, 1 18 (1938). 21. Sondheimer, E. H. The men free pth of eletrons in metls. Adv. Phys. 1, 1 42 (1952). 22. Myds, A. F. & Shtzkes, M. Eletril-resistivity model for polyrystlline films se of ritrry refletion t externl surfes. Phys. Rev. B 1, (197). 23. Steinhogl, W., Shindler, G., Steinleserger, G., Trving, M. & Engelhrdt, M. Comprehensive study of the resistivity of opper wires with lterl dimensions of 1 nm nd smller. J. Appl. Phys. 97, 2376 (25). 24. Mrom, H. & Eizenerg, M. The temperture dependene of resistivity in thin metl films. J. Appl. Phys. 96, (24). 25. Steinhogl, W., Shindler, G., Steinleserger, G. & Engelhrdt, M. Size-dependent resistivity of metlli wires in the mesosopi rnge. Phys. Rev. B 66, (22). Aknowledgements The Ntionl Siene Foundtion funded this work through grnt CHE Correspondene nd requests for mterils should e ddressed to R.M.P. Competing finnil interests Referenes The uthors delre tht they hve no ompeting finnil interests. 1. Spier, D. F., Rodger, A. C. & Vrnell, G. L. Computer-ontrolled pttern generting system for use with eletron-em writing instruments. J. V. Si. Tehnol. 1, (1973). Reprints nd permission informtion is ville online t nture mterils VOL 5 NOVEMBER Nture Pulishing Group 919