Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag 2 Te thin films

Transcription

1 Synergism in inry nnorystl superltties leds to enhned p-type ondutivity in self-ssemled PTe/Ag 2 Te thin films JEFFREY J. URBAN 1 *,DMITRIV.TALAPIN 2, ELENA V. SHEVCHENKO 2, CHERIE R. KAGAN 1 AND CHRISTOPHER B. MURRAY 1 1 I.B.M. T. J. Wtson Reserh Center, Nnosle Mterils nd Devies Group, 111 Kithwn Rod, Yorktown Heights, New York 1598, USA 2 The Moleulr Foundry, Lwrene Berkeley Ntionl Lortory, Berkeley, Cliforni 9472, USA *e-mil: urn@post.hrvrd.edu Pulished online: 21 Jnury 27; doi:1.138/nmt1826 The ordered orystlliztion of nnoprtiles into inry superltties enles lose ontt of nnorystls with distint physil properties, providing route to metmterils design. Here we present the first eletroni mesurements of multiomponent nnorystl solids omposed of PTe nd Ag 2 Te, demonstrting synergisti effets leding to enhned p-type ondutivity. First, syntheses of size-tunele PTe nd Ag 2 Te nnorystls re presented, long with deposition s thin-film nnorystl solids, whose eletroni trnsport properties re hrterized. Next, ssemly of PTe nd Ag 2 Te nnorystls into AB inry nnorystl superltties is demonstrted. Furthermore, inry omposites of vrying PTe Ag 2 Te stoihiometry (1:1 nd 5:1) re prepred nd eletronilly hrterized. These omposites show strongly enhned (ondutne 1-fold inresed in 1:1 omposites over the sum of individul ondutnes of single-omponent PTe nd Ag 2 Te films) p-type eletroni ondutivity. This oservtion, onsistent with the role of Ag 2 Te s p-type dopnt in ulk PTe, demonstrtes tht nnorystls n ehve s dopnts in nnostrutured ssemlies. The modulr design of multiomponent solids using miro 1 - or nnometre 2,3 -sle prtiles provides ess to unique omintions of stoihiometry nd symmetry not ville in single-omponent ulk solids 4. Preise ssemly of nnorystls with desired optil, eletroni nd mgneti properties into inry nnorystl superltties provides route to metmterils with potentilly novel hemil or physil properties 2.Adiverse olletion of these strutures hs een reently reported; however, it remins unknown whether these omplex struturl properties will trnslte into modified physil properties. This mnusript reports the first eletroni studies of these inry nnorystl systems, demonstrting synergisti effets in whih the p-type ondutne of the omposite system exeeds the sum of the individul nnoprtile omponents. One lssi exmple of intentionlly introduing nnometresle prtiles to modify the physil properties of solid is semiondutor doping. This onept, entrl to solid-stte devie physis, hs een exeptionlly diffiult to relize in nnorystlline mterils 5,6. Despite gret interest in nnorystl doping, there hve een few suessful reports to dte, whih led to the hypothesis tht self-purifition phenomen intrinsilly limit the introdution of impurities into nnorystls 5,6. Reent theoretil nd experimentl works orroorte this view, showing tht selfpurifition mehnisms re energetilly fvourle over dopnt inorportion, prtiulrly for smll nnorystls 6. Therefore, there exists need for lterntive pprohes to nnorystl doping. This mnusript develops n lterntive to trditionl nnorystl doping the use of nnoprtiles themselves s dopnts in multiomponent nnorystlline solid. Binry nnorystl superltties offer the potentil to design these nnostrutured solids with progrmmle hemil nd struturl fetures. Furthermore, the presene of lile orgni shells retes n dditionl opportunity the evolution of mteril properties s funtion of nnorystl oupling n e investigted 7. With hemil tretment, heting or other methods, we n study the properties of nnorystl solids with wekly interting elements nd then densify the solid to explore the strong-oupling limit. Here we study the influene of nnorystl nnorystl intertions y prepring inry nnorystl superltties of PTe nd Ag 2 Te nd mesuring how the eletroni properties of these multiomponent solids develop fter heting nd hemil tretment. These results represent the first steps in the rtionl design of nnorystl superlttie for trgeted physil pplition. The hoie of nnorystl omponents, PTe nd Ag 2 Te, ws motivted y the gol of onstruting highly ondutive, grnulr, p-type nnorystl omposites. This omintion of mteril properties ws intentionlly seleted to yield mteril with high p-type eletroni ondutivity nd low therml ondutivity, desirle trits for the p-type leg of thermoeletri devie. PTe hs the highest thermoeletri figure of merit (ZT) of ny single-phse ulk solid from 55 to 7 K (ref. 8), the trget rnge for mny wste-het-reovery pplitions. The omplementry omponent (Ag 2 Te) ws hosen euse it provides Ag +,whihisneffetive p-type dopnt for ulk PTe (ref. 9), with muh more rpid diffusion rtes thn other ommon hoies suh s I 2 or K + (ref. 1). Additionl dvntges of this design re the sustntil inreses in phonon sttering nture mterils VOL 6 FEBRUARY

2 Intensity (.u.) 25nm Angle (2 θ ) Asorne (.u.) 1,35 1,7 2,5 2,4 Wvelength (nm) Figure 1 Synthesis nd hrteriztion of PTe nnorystls., TEM mirogrph of 5. nm PTe nnorystls in lose-pked rry., Series of optil sorption spetr otined on 4.2, 5.5, 6.6, 7.5, 8.2 nd 8.8 nm PTe nnorystls (from ottom to top). These spetr demonstrte the exellent mid-infrred tuneility of these PTe nnorystls., Series of XRD ptterns otined from 5.5, 6.2, 8.8, 1.5, 16.8 nd 32.1 nm PTe nnorystls (from top to ottom). Anlysis of pek width in eh of these XRD ptterns mthes losely the dimeters otined for these smples from TEM. inherent to nnorystlline mterils with high interfe density 11,12 nd known enhnements to the Seeek oeffiient in zerodimensionl mterils 13. Indeed, historil nlysis of reported vlues of ZT illustrtes tht nnosle ptterning is essentil to produing high-performne thermoeletri mterils tht exeed the ZT = 1 eiling 14, s reent work on ulk solids with nnorystl inlusions 15, Strnski Krstnov dots in ulk mtrix of PTe (ref. 13) nd thin-film superlttie 16 mterils hs shown. The PTe nnorystls synthesized for the inry omposites (Fig. 1) re size tunele ( 4 12 nm) nd monodisperse (stndrd devition 5 6%) nd form stle olloidl solutions s previously reported 17. Chnges in lignd onentrtion re used to ontrol nnorystl size y mnipulting retion kinetis vi ltertions in preursor stility. Struturl informtion out nnorystl omposition, sizes nd size distriutions were otined using severl omplementry tehniques tht ess different length sles. Produt omposition, rystl struture nd verge rystllite size were otined y nlysing X-ry diffrtion (XRD) ptterns s shown in Fig. 1. Nnorystl dimeters otined from trnsmission eletron mirosopy (TEM) imges (Fig. 1) mth those otined from nlysis of XRD ptterns. Optil sorption of PTe nnorystl smples is lso studied, s shown in Fig. 1. This series of spetr shows tht nnorystlline PTe smples possess tunele, quntum-onfined trnsitions (ulk PTe hs ndgp (mesured t the nd edge).23 ev t 3 K) in the mid-infrred region. Additionl detils on the synthesis nd hrteriztion re ville in the originl referene 17. Preprtion of the Ag 2 Te nnorystls neessry for the inry omposites proeeds through proess of nnorystl growth nd susequent ripening, phenomenologilly similr to the digestive ripening of Au nnorystls 18,19. The evolution in Ag 2 Te nnorystl size nd shpe is followed over period of 9 dys y omintion of struturl nlysis (TEM, XRD nd grzing-inidene smll-ngle X-ry sttering (GISAXS)) nd optil spetr (ner-infrred sorption), nd is summrized hronologilly in Fig. 2 f. Figure 2,d shows TEM nd optil spetrum orresponding to retion produts isolted 1 5 h fter injetion of triotylphosphine tellurium omplex (TOPTe). Here, the retion ontins mixture of lrge (12 15 nm in dimeter) nd smll (3 4 nm) Ag 2 Te nnorystls, s shown in Fig. 2. XRD nlysis of retion isoltes onfirms the orthorhomi Ag 2 Te struture (see Supplementry Informtion, Fig. S1). At this stge, there re two distint fetures visile in the optil spetrum one rod pek round 1.15 μm nd shrp pek round 1.4 μm. As this retion is mintined t 85 C it evolves s shown in Fig. 2,e, whih shows TEM nd optil spetrum otined from retion liquots tken fter 3 dys. The initil retion hs now evolved into mixture of lrge, prolte rystls nd smll, spheril nnorystls. The orresponding optil spetrum (Fig. 2e) shows n inrese in the reltive intensity nd shrpness of the pek t 1.15 μm. Susequent liquots from dys 3 7 show derese in the mount of nisotropi speies in solution, with onomitnt enrihment in the numer of smll, spheril Ag 2 Te nnorystls. Whether this ours owing to digestive ripening or the preipittion of insolule, mrosopi hins of Ag 2 Te rods is now eing explored. After mintining the retion for t lest 8 dys t 85 C the solution onsists entirely of monodisperse, spheril Ag 2 Te nnorystls, s shown in Fig. 2,f. Figure 2 demonstrtes the exellent uniformity nd monodispersity of the smples isolted t this stge. Optil sorption spetr now onsist of single, shrp feture t 1.15 μm. Although these optil fetures re useful for monitoring retion progress, their eletroni origins re unknown t present, s Ag 2 Te is reltively unexplored mteril. We note tht the 1.15 μm pek is lue-shifted in mnner onsistent with quntum-onfined exitoni fetures (the ulk nd-gp of Ag 2 Te is.64 ev (ref. 2)), lthough we hve een unle to lote relile reports of effetive msses nd dieletri onstnts of Ag 2 Te tht would enle omprison of this ssignment with simple Brus effetive-mss lultions 21. The evolution in prtile shpe nd hining of nisotropi Ag 2 Te prtiles re intriguing results we re now trying to etter omprehend; however, for the ssemly of inry nnorystl superltties, monodisperse, spheril prtiles re preferred, so we hose to fous on this element of the synthesis. Finlly, the smll Ag 2 Te rystls otined y this method re monodisperse, nd re used without further purifition. Formtion of well-ordered nnorystl solids requires uilding loks with exellent uniformity. Therefore, we ssemle thin nnorystl films, not only ntiipting future devie studies, ut lso to onfirm the high qulity of the synthesis. Well-known self-orgniztion proesses ssoited with ontrolled solvent evportion re exploited to produe highly ordered (strutured over more thn 1 prtile dimeters) nnorystl thin solid films for hrteriztion of their eletroni properties. Owing to the hydrophoi hrter of the nnorystl solutions, SiO 2 sustrtes re first pretreted with hexmethyldisilzne to ensure uniform wetting of the surfe. After tretment, the nnorystls re suspended in hexne otne solution ( 8:1 y volume) nd susequently drop st onto the SiO 2 surfe. Anlysis of the GISAXS, snning eletron mirosopy (Supplementry Informtion, Fig. S2) nd TEM imges indites the exellent sptil order (>1 μm per superlttie rystl in plne) of these polyrystlline superlttie films. Hving estlished preprtive ontrol over oth PTe nd Ag 2 Te nnorystl systems, we re now ple of designing rtifiil solids in whih ritrry nnorystlline uilding loks my e rtionlly orystllized into inry superltties of desired 116 nture mterils VOL 6 FEBRUARY 27

3 5 nm 2 nm 5 nm 2 nm 5 nm 1 nm d e f Asorne (.u.) Asorne (.u.) Asorne (.u.) 1, 1,2 1,4 1,6 1, 1,2 1,4 1,6 1, 1,2 1,4 1,6 Wvelength (nm) Wvelength (nm) Wvelength (nm) Figure 2 Evolution of prtile size nd shpe in Ag 2 Te nnorystls., TEM mirogrph of Ag 2 Te nnorystls isolted fter 5 h of TOPTe injetion. The inset shows tht the retion mixture ontins oth lrge (12 15 nm) nd smll (3 4 nm) nnorystls t this stge., TEM mirogrph of retion produts fter 3dyst85 C. The retion mixture hs evolved to ontin oth hins of lrge (>2 nm long the long xis) nisotropi prtiles nd smll ( 3 nm) monodisperse rystls. The inset is mgnified portion of TEM mirogrph, highlighting the hining proess., TEM mirogrph otined from smples fter 8 dys reflux t 85 C. The inset demonstrtes the monodispersity of the rystls fter more thn 7 dys. d, Optil sorption spetrum tken from 5 h retion liquot showing oth rod feture t 1.15 μm nd shrp feture t 1.4 μm. e, Optil sorption spetrum otined from smple fter 3 dys reflux. Here the 1.15 μm pek hs inresed in intensity nd shrpness reltive to smples from the retion mixture from less thn 1 dy. f, Optil sorption spetrum otined from more-thn-7-dy smples. Only the 1.15 μm sorption pek is present in the smples, orresponding to smll, monodisperse nnorystls. stoihiometry nd symmetry 2,4. Here we report the ssemly of inry AB superltties 22 omposed of PTe nd Ag 2 Te nnorystls. Superlttie formtion ws pursued s method to ensure the ontrolled ptterning of oth lsses of nnorystls, design feture we wnted to use for the eventul design of ulk nnorystlline solids with progrmmed stoihiometry vi sintering of these superlttie phses. Assemly of inry nnorystl superltties y severl groups 1 3 hs highlighted three ritil fetures neessry for formtion: the reltive size of eh nnoprtile omponent, high monodispersity of eh nnoprtile omponent nd the ility to mnipulte nnoprtile hrge sttes. Both PTe nd Ag 2 Te re size tunele nd yield highly monodisperse nnorystls, whose stndrd devition is less thn 5.5%. Also, reproduile superlttie smples (s judged y extent of superlttie formtion, size of superlttie domins nd lk of defets) were only hieved in smples in whih smll mounts ( 1% y volume of 1:1 dilute lignd/solvent mixture) of hrged mphiphili lignd were dded. These dditives were inluded in n effort to nrrow the hrge-stte distriution of individul nnorystl popultions, s disussed previously 2, lthough the hrge distriutions were not independently verified in this work. Ordered inry nnorystl superltties (shown in Fig. 3,) re oserved to form on slow evportion of the solvent. Figure 3 highlights two different AB superlttie strutures ommonly oserved: superltties isostruturl with NCl (Fig. 3) nd CuAu (Fig. 3). The ssignment of these strutures is sed on nlysis of two-dimensionl Fourier trnsformtion power spetr of rel-spe imges (tilted long t lest two prinipl xes) nd onsisteny with three-dimensionl lttie models uilt from the 18 most ommon rystllogrphi spe groups using Aelrys MS Modelling 3.1 softwre. Also, owing to the size-tuneility of oth nnorystl systems, we hve prepred oth hemil isomers of these AB superltties (tht is, oth PTe (ig)/ag 2 Te (smll) nd PTe (smll)/ag 2 Te (ig) AB superltties hve een studied). Previous work on nnorystl trnsistors hs demonstrted tht the n-type trnsistor hrteristis re fr superior to the p-type hrteristis, lthough less stle over time 23. As suh, our ojetive with the inry solids ws to develop highly ondutive p-type nnorystl devies to omplement this work. We trgeted multiomponent omposites omposed of PTe nd Ag 2 Te, euse Ag + is n estlished p-type dopnt for ulk PTe 9,1. In order to ssess the doping effiy of Ag 2 Te in the PTe Ag 2 Te omposites, it is first neessry to fully hrterize the eletroni trnsport properties of PTe nd Ag 2 Te individully, nd then ompre these results to those otined from omposites. For this reson, we hose to first mesure the eletroni properties of eh of the individul nnorystl omponents in thin-film trnsistor (TFT) mesurement geometry. Cpturing the properties of nnorystls in tive solidstte devies is n ppeling gol; however, there exist severl hllenges 23,24 : (1) the presene of ulky, insulting, lignd shells mintins lrge interprtile spings nd impedes eletron trnsport, (2) surfe dngling onds rete mid-gp trp sttes nd (3) low dieletri onstnts result in hrging energies greter thn k B T, where k B is the Boltzmnn onstnt nd T the nture mterils VOL 6 FEBRUARY

4 5 nm Intensity (.u.) 2 nm Angle (2 θ ) Figure 3 Assemly of inry nnorystl superltties onsisting of PTe nd Ag 2 Te., Superlttie ssemled from 3.2 nm Ag 2 Te nd 6.5 nm PTe. The struture is onsistent with [1] projetion of NCl lttie (inset)., Superlttie ssemled from 1.1 nm Ag 2 Te nd 6.5 nm PTe. This struture is onsistent with [1] projetion of CuAu lttie (inset)., XRD of the solution used to mke superlttie 3 deposited on qurtz plte. This XRD onfirms the presene of oth lrge Ag 2 Te nd smll PTe omponents; the literture vlues for the Ag 2 Te nd PTe refletions re shown in mgent nd lue, respetively. temperture. Here we use the pproh of hemil tivtion of nnorystl solids y tretment with hydrzine 23, hemil whih dereses interprtile sping y removing lignd shells while simultneously pssivting trp sttes. This tivtion proess is rried out on oth the PTe nd Ag 2 Te films, nd inreses their ondutne drmtilly ( 1 11 orders of mgnitude inrese in ondutne in PTe; 5 6 orders of mgnitude enhnement in Ag 2 Te). Thin-film nnorystl devies for eletroni studies re prepred y depositing PTe (Ag 2 Te) nnorystls from hexne otne solution onto highly doped silion wfers with 1 nm SiO 2 therml gte oxide nd preptterned soure nd drin Ti Au (1/4 Å) eletrodes (whose sping ws vried from 6 to 5 μm), s depited in Fig. 4d. Although there re slight differenes in proessing etween the TEM smples nd the hips for trnsport mesurements, the eventul removl of the lignds nd evportion of the solvent renders these hnges insignifint. These films re hrterized y GISAXS nd snning eletron mirosopy s disussed previously. Eletroni mesurements show tht sdeposited films (PTe or Ag 2 Te) re insulting, demonstrte no gte effet nd possess very low ondutivities (G 1 13 Sm 1 ), s shown in Supplementry Informtion, Fig. S3. At this stge, without hydrzine tretment, short (<3 h) therml tretments up to 2 C (under mient pressure) do not drmtilly enhne the ondutne of the film (the est smples hve G 1 12 Sm 1 ). Agin, we ttriute this poor ondutivity to the smll exhnge oupling etween nnoprtiles rising from the lrge interprtile spings mintined y the orgni lignds. This perspetive is supported y ompring GISAXS mesurements of verge interprtile spings in single-omponent nnorystl films efore nd fter hemil tivtion (see Supplementry Informtion, Fig. S4). The dt show tht lrge enhnements in film ondutivity (oth PTe nd Ag 2 Te) orrelte diretly to sustntil dereses in verge interprtile sping (on verge from 1.7 to.3 nm). Owing to the drmti derese in interprtile sping, rking often develops in the nnoprtile films fter lignd removl, whih neessittes multiple rounds of film deposition (on verge three) to ensure ontinuous film for trnsport mesurements. The hemil tivtion proess involves tretment of the films with 1. M solution of hydrzine in etonitrile for 2 5 h, nd susequent wshing with etonitrile. After tretment, film ondutivity is drmtilly enhned in oth PTe nd Ag 2 Te films. For PTe, initil hydrzine tretment results in order-of-mgnitude enhnements in ondutivity nd gtele n-type ehviour s shown in Supplementry Informtion, Fig. S3. Immeditely fter hydrzine tretment, rief (<3 h) therml tretments up to 15 C (under mient pressure) do not hnge the film ondutne. However, s reported previously for PSe trnsistors, prolonged het tretment t higher tempertures desors hydrzine nd ultimtely swithes devie ehviour to p type, s shown in Fig. 4,. This proess, nd its effets on film trnsport, re reversile. Oserving onventionl TFT properties in the PTe trnsistors enled the lultion of liner-regime moilities from these devies (n-type moilities re.95 m 2 V 1 s 1 ; p-type moilities re.15 m 2 V 1 s 1 ); representtive p-type gte sn is shown in Fig. 5. All of the relted dt for the n-type PTe trnsistors my e found in the Supplementry Informtion. In ontrst, lthough hydrzine tretment inreses the ondutivity of Ag 2 Te nnorystlline films y 5 6 orders of mgnitude, it remins y omprison very poor ondutor nd shows no disernile gte modultion of the urrent. The lk of onventionl TFT devie ehviour (either n or p type) in the Ag 2 Te films preludes the lultion of field-effet moilities. Assemly of PTe Ag 2 Te superltties ws pursued s method of onstruting highly ondutive p-type nnoomposites of ontrolled omposition. Ultimtely, fter sintering nd lignd removl, these nnorystl solids would ontin ontrolled quntities of eletroni dopnts homogeneously distriuted throughout the solid, providing method to irumvent ommon hllenges ssoited with nnorystl doping 5,6. This onept motivted the inorportion of Ag 2 Te nnorystls into the omposites, s Ag + is n estlished p-type dopnt for ulk PTe with exellent moility nd low tivtion rriers for diffusion 9,1. This lterntive doping method ws ppeling, s trditionl, diret sustitutionl pprohes towrds nnorystl doping often result in exeptionlly low, or zero, stle inorportion of dopnts 5,6. We report here the first eletroni mesurements on inry nnorystl solids omposed of PTe nd Ag 2 Te. These initil mesurements highlight the gret potentil for the nnoprtile s dopnt ide, showing sustntil enhnements in ondutivity ompred to single-omponent PTe or Ag 2 Te films. We present eletroni mesurements on nnorystl films ssemled with onditions nd ompositions onsistent with formtion of AB nd AB 5 inry nnorystl superltties, referred to s 1:1 nd 5:1 films, respetively. Ultimtely, this proedure my llow exquisite ontrol over oth dopnt onentrtion nd doping profile y depositing films of inry nnorystl superlttie rystls with desired stoihiometry nd symmetry. Preprtion of the PTe Ag 2 Te nnoomposite films for mesurement proeeds identilly to the pproh disussed for ssemly of inry nnorystl superltties, exept tht preptterned silion wfer is sustituted for TEM grid. These films re then immeditely trnsferred to nitrogenfilled dry-ox nd the eletril properties re hrterized. Asdeposited films re poorly onduting nd show no gte effet (see Supplementry Informtion, Fig. S5), trnsmitting only pa or fa urrents over 5 4 μm long hnnels (G 1 13 Sm 1 ). As in the single-omponent film studies, we ttriute these low ondutivities to the ulky lignd shells. Without hydrzine tretment, short (<3 h) therml tretments up to 2 C (under mient pressure) do not drmtilly enhne the ondutne of the film (G 1 12 Sm 1 ). This is euse lrge interprtile 118 nture mterils VOL 6 FEBRUARY 27

5 I sd (μa) 6 I sd (μa) V g = V, 1 V, 2 V, 3 V V sd = 2 V V g (V) d.5 S D I sd (μa) 1. G (Si 2+ ) SiO V g = V, 1 V, 2 V, 3 V 1 5 Figure 4 Eletroni hrteriztion of single-omponent nnorystl films. All mesurements were reorded in forwrd nd reverse to ssess devie hysteresis. Gte urrents (mgent) re lso plotted to demonstrte tht no urrent lekge ours through the gte oxide. These gte urrents (I g ) versus soure drin voltge (V sd )re reorded in prllel for eh soure drin urrent (I sd )versusv sd mesurement t every gte voltge; however, owing to the low I g vlues they seem olliner., I V mesurement of p-type PTe nnorystl film in the liner regime., I V g mesurement of PTe nnorystl film reorded with V sd = 2 V, lerly demonstrting p-type ehviour., I V mesurement of Ag 2 Te nnorystl film. Even fter hemil tretment, these films were poorly onduting nd non-gtele, s shown y omprison of the 1 V g nd 2 V g sns. d, Nnorystl films were mesured y deposition onto Si/SiO 2 wfers with preptterned Ti Au eletrodes. spings re still mintined fter heting, s thermogrvimetri nlysis shows tht lignds re not deomposed until tempertures in exess of 4 C (see Supplementry Informtion, Fig. S6). These high-temperture tretments re potentilly destrutive to the devies, nd were not pursued. As estlished previously, hydrzine tretment reples the ulky pping groups nd gretly dereses interprtile sping. We used this pproh here to ring the Ag 2 Te nd PTe nnorystls into lose ontt. Immeditely fter tretment, lrge enhnements in eletron trnsport were oserved with verge vlues of G 1 1 Sm 1 for the most ondutive 1:1 films. Owing to the lrger lttie onstnt of inry nnorystl superlttie systems reltive to single-omponent ltties, we elieve this enhned ondutne nnot e ttriuted only to dereses in interprtile sping. Furthermore, the mesured p-type ehviour in these systems, present even immeditely fter hydrzine tretment, indites tht the hydrzine itself is not the only opertive mehnism. Intriguingly, the est results were otined y heting PTe Ag 2 Te nnoomposite smples previously treted with hydrzine (Fig. 5,) to 15 C. As depited in Fig. 5, this mild heting does not seem to destroy superlttie order or use nnoprtile melting. These smples susequently show exellent hole trnsport with n verged low-field ondutne of G.35Sm 1 nd ondutivities s gret s G 4.8Sm 1 for the most ondutive 1:1 smples. As shown in Fig. 5,, the hydrzinetreted nd heted omposites show little gte modultion of their urrent, result onsistent with high rrier densities or homogeneous distriution of dopnts. However, owing to the lk of n identifile threshold voltge, stndrd nlysis of the moility nd rrier onentrtion ould not e exeuted here. We re now prepring smples for Hll effet mesurements to quntittively ssess these prmeters. For thermoeletri nd power genertion pplitions, however, we note tht gting is irrelevnt ut tht high eletril ondutivity is impertive. Additionlly, omprison of Fig. 5, with Fig. 4, revels tht the I V hrteristis of the omposite systems show nonliner ehviour. These nonliner fetures re onsistent with eletrifield-enhned therml tivtion of rriers from oulomi trps, s ommonly seen in grnulr semionduting systems 25.However, despite these ovious similrities, further study is neessry to verify this hypothesis. Although detiled studies of the moility of eh smple ould not e rried out t this time, we did ssess the effiy of doping nture mterils VOL 6 FEBRUARY

6 V g = V, 2 V, 3 V V g = V, 15 V 1 2 I sd (ma) 2 4 1:1 film 5:1 film I sd (ma) 3 S D 6 4 G (Si 2+ ) SiO G (Si 2+ ) SiO d Low-field G vg (S m 1 ) nm PTe in nnorystl film (%) Figure 5 Chrteriztion of inry PTe Ag 2 Te nnorystl films. All mesurements were reorded in forwrd nd reverse to ssess devie hysteresis. Gte urrents (mgent) re lso plotted to demonstrte tht no urrent lekge ours through the gte oxide. These gte urrents (I g versus V sd ) re reorded in prllel for eh I sd versus V sd mesurement t every gte voltge; however, owing to the low I g vlues they seem olliner., I V mesurement of 1:1 PTe Ag 2 Te inry nnorystl film demonstrting high p-type ondutivity nd no gte response., I V mesurement of 5:1 PTe Ag 2 Te inry nnorystl film, gin demonstrting p-type trnsport nd no gte response., TEM mirogrph of 1:1 PTe Ag 2 Te superlttie fter het tretment, demonstrting tht even 2 C therml tretments under vuum do not rogte superlttie order. d, Comprison of verge low-field ondutnes (not exeeding 1 V in soure drin or 15 V in gte voltge) lulted over t lest 12 devies for pure Ag 2 Te films, pure PTe films, inry 1:1 PTe Ag 2 Te films nd inry 5:1 PTe Ag 2 Te films. The verge ondutnes for these ompositions re Sm 1, Sm 1,.346 S m 1 nd.21 S m 1, respetively. The error rs re the stndrd devitions in ondutne vlues for eh of the ompositions. These dt demonstrte the synergisti enhnement of ondutivity mnifest in the inry nnorystl solids. y ompring the results for four different doping levels (Fig. 5d): pure PTe, 5:1 (PTe Ag 2 Te), 1:1 (PTe Ag 2 Te) nd pure Ag 2 Te. These low-field ondutnes re ompred under onsistent set of soure drin nd gte voltges verged over no fewer thn dozen devies. All other experimentl prmeters (oxide thikness, mesurement geometry, ontt mterils) remined onstnt from smple to smple. The rtios of 1:1 nd 5:1 were hosen euse these re prtile onentrtion rtios used to prepre AB nd AB 5 inry nnorystl superltties. We note tht, owing to the size of the devies reted here (5, μm hnnel widths nd 6 5 μm hnnel lengths) in omprison to verge superlttie domin sizes ( 2 4 μm 2 ), these mesurements re reorded on polyrystlline inry nnoprtile systems. Although it is hllenging to provide diret orrespondene etween the formtion of inry nnorystl superltties explored y mirosopy nd the eletroni mesurements provided here, this set of onditions ws used s guide for future experiments on oordinted struturl nd eletroni mesurements (see Supplementry Informtion, Fig. S7). Furthermore, struturl studies indite (Fig. 5) tht neither hydrzine nor therml tretments mterilly disrupt the pking order present in oth single-omponent nd inry nnorystl superltties. As summrized in Fig. 5d, inresed onentrtions of Ag 2 Te in the inry omposites result in enhned ondutivities, wheres the limiting ses of 1% PTe nd 1% Ag 2 Te possess the lowest ondutivities. Bsed on previous experimentl reports of onditions required for diffusion of Ag + into PTe (refs 9, 1) nd the estlished diffiulty of liovlent doping of nnostrutures 5,6, we elieve nother mehnism is opertive. Although distint from onventionl semiondutor doping proesses, this pproh hs produed inry solids whose omposite ehviour is enhned over either of the individul omponents. Further studies re neessry to fully eluidte the mehnism of ondutne enhnement, ut this system ers similrity to reports of surfe trnsfer doping 26 demonstrted in dimond 27 nd silion 28. METHODS NANOCRYSTAL SYNTHESIS AND ISOLATION Synthesis of led telluride nnorystls proeeds ording to the protools developed in ref nture mterils VOL 6 FEBRUARY 27

7 Silver telluride nnorystls re synthesized s follows:.91 g AgNO 3 (.53 mmol) nd 2 ml H 2 O re dded to flsk ontining.32 ml dodenethiol (1.33 mmol) nd 2 ml toluene. This mixture is stirred for 2 3 h to intermix the phses, fter whih stirring is stopped nd the queous phse removed. The orgni phse (ontining oth silver regents nd lignds) is then stirred nd heted to 85 C. Next, 3 ml of.75 M TOPTe is injeted into the retion mixture, whih is susequently mintined t 85 C. After one dy t 85 C, mixture of lrge (12 15 nm), prolte nnorystls nd smll (4 5 nm), spheril nnorystls is oserved. Mintined t this temperture, the lrge, nisotropi nnorystls stk together into hins (3 4 dys) wheres the smll nnorystls develop into islnds of monodisperse, spheril nnorystls (7+ dys). This synthesis hs een rried out t severl extended intervls from 7 to 12 dys, nd the retion produts do not hnge drmtilly fter 9 dys. Isoltion nd purifition of either PTe or Ag 2 Te nnorystls proeeds s outlined in ref. 17. Both lsses of resulting nnorystls re le to e resuspended in hloroform, hexne, tetrhloroethylene or severl other solvents to form stle olloidl solutions. ASSEMBLY OF SINGLE-COMPONENT NANOCRYSTAL FILMS Assemly of nnorystl solids proeeds vi ontrolled evportion of onentrted nnorystl solution onto sustrte. Typil sustrtes re either Si SiO 2 hips treted with hexmethyldisilzne t 15 C for 3 min efore nnorystl deposition, silion nitride memrnes or morphous-ron-oted TEM grids. For eletroni mesurements of single-omponent nnorystl solids, films were st from solution of PTe (Ag 2 Te) nnorystls suspended in hexne otne ( 8:1 volumetri rtio). This produed films of exellent uniformity in oth plnr dimensions nd thikness. Informtion on interprtile sping nd, more glolly, the degree of ordering of the nnorystlline films ws otined y GISAXS. ASSEMBLY OF BINARY NANOCRYSTAL SUPERLATTICES Sustrtes re pled in glss vil ontining olloidl solutions of PTe nd Ag 2 Te nnorystls ( 1 5 μl of eh omponent). The vil is tilted to 6 7 ngle nd pled inside low-pressure hmer mintined t 5 C, s disussed previously 4. Ordered inry ssemlies form on ontrolled evportion of the solvent. Preferred solvents re trihloroethylene, toluene or mixtures of oth. Optiml superlttie formtion (judged y length sle of ordering nd defet density) is otined y dding smll mounts of either olei id or triotylphosphine oxide solutions ( 4 μl of 1:1 olei id trihloroethylene y volume) to nrrow the hrge distriutions of nnoprtile popultions s disussed in ref. 2. SAMPLE CHARACTERIZATION TEM, powder XRD, energy-dispersive X-ry spetrosopy nd ner-infrred sorption spetrosopy re used to hrterizethesize, shpe, struture, omposition nd optil properties of the PTe nnorystls. Chrteriztion of prtile ordering nd interprtile sping in the PTe films is rried out vi GISAXS t grzing inidene using D8 Disover series II diffrtometer (Bruker) with two-dimensionl re detetor. A opper node ws used s the rdition soure (elertion voltge 4 kv, flux 4 ma) nd the signl integrtion time vried from 1 to 6 min. The ngle of inidene ws typilly.83, slightly greter thn the ritil ngle of the nnorystl film. TEM nd high-resolution TEM imges were otined using Philips CM-12 mirosope operting t 12 kv. Smples for TEM nlysis were prepred y depositing drop of dilute nnorystl solution in hloroform or pentne on 4 mesh ron-oted opper grid nd llowing the solvent to evporte t room temperture. Sttistil nlysis of the size distriutions of the nnorystls is otined using Sion Imge dt-proessing softwre; for further detils onsult ref. 17. Ner-infrred sorption spetr were olleted on trihloroethylene solutions of PTe nnorystls using QulitySpe Pro infrred spetrometer (Anlytil Spetrl Devies). Wide-ngle powder XRD mesurements were rried out on Bruker D5 diffrtometer operting in the Brgg Brentno onfigurtion with Co Kα rdition (l = 1.79 Å) with stter nd diffrtion slits of 1 nd.15 mm olletion slit. Smples for wide-ngle XRD mesurements re prepred y depositing onentrted PTe nnorystl solutions in hexne onto glss plte. To study the eletroni properties of nnorystl ssemlies (oth single-omponent nd inry ssemlies) we deposited 4±1 nm thik films of nnorystls on 1 nm thik SiO 2 gte oxides thermlly grown on hevily doped Si wfers used s the k gte. Soure nd drin Ti Au (1/4 Å) eletrodes were ptterned on the SiO 2 surfe y lithogrphy efore depositing the nnorystl film. Sping etween the soure nd drin eletrodes ws typilly 1 μm, tht is, the typil pth of hrge rriers from soure to drin eletrode involved 1 3 or more individul nnorystls. Field-effet devies re tested using n Agilent 4156B semiondutor-prmeter nlyser. The soureeletrodewsgrounded. Allroom-temperture eletril mesurements re rried out under dry nitrogen tmosphere. Reeived 12 Septemer 26; epted 12 Deemer 26; pulished 21 Jnury 27. Referenes 1. Leunissen, M. E. et l. Ioni olloidl rystls of oppositely hrged prtiles. Nture 437, (25). 2. Shevhenko, E. V., Tlpin, D. V., Kotov, N. A., O Brien, S. & Murry, C. B. Struturl diversity in inry nnoprtile superltties. Nture 439, (26). 3. Klsin, A. M. et l. Eletrostti self-ssemly of inry nnoprtile rystls with dimond-like lttie.siene 312, (26). 4. Shevhenko, E. V., Tlpin, D. V., Murry, C. B. & O Brien, S. Struturl hrteriztion of self-ssemled multifuntionl inry nnoprtile superltties. J. Am. Chem. So. 128, (26). 5. Erwin, S. C. et l. Doping semiondutor nnorystls. Nture 436, (25). 6. Dlpin, G. M. & Chelikowsky, J. R. Self-purifition in semiondutor nnorystls. Phys.Rev.Lett. 96, (26). 7. Koole, R., Liljeroth, P., de Mello Doneg, C., Vnmekelergh, D. & Meijerink, A. Eletroni oupling nd exiton energy trnsfer in CdTe quntum-dot moleules. J. Am. Chem. So. 128, (26). 8. Rowe, D. M. (ed.) incrc Hndook of Thermoeletris (CRC Press, New York, 1995). 9. Orihshi, M., Nod, Y., Kie, T. H. & Nishid, I. A. Evlution of thermoeletri properties of impurity-doped PTe. J. Jpn Inst. Met. 61, (1997). 1. Nod, Y., Orihshi, M. & Nishid, A. Thermoeletri properties of p-type led telluride doped with silver or potssium. J. Jpn Inst. Met. 61, (1997). 11. Costesu, R. M., Chill, D. G., Freguette, F. H., Sehrist, Z. A. & George, S. M. Ultr-low therml ondutivity in W/Al 2O 3 nnolmintes. Siene 33, (24). 12. Kim, W. et l. Therml ondutivity redution nd thermoeletri figure of merit inrese y emedding nnoprtiles in rystlline semiondutors. Phys.Rev.Lett.96, 4591 (26). 13. Hrmn, T. C., Tylor, P. J., Wlsh, M. P. & LForge, B. E. Quntum dot superlttie thermoeletri mterils nd devies. Siene 297, (22). 14. Mjumdr, A. Thermoeletriity in semiondutor nnostrutures.siene 33, (24). 15. Hsu, K. F. et l. Cui AgP mste 2+m ulk thermoeletri mterils with high figure of merit. Siene 33, (24). 16. Venktsurmnin, R., Siivol, E., Colpitts, T. & O Quinn, B. Thin-film thermoeletri devies with high room-temperture figures of merit. Nture 413, (21). 17. Urn, J. J., Tlpin, D. V., Shevhenko, E. V. & Murry, C. B. Self-ssemly of PTe quntum dots into nnorystl superltties nd glssy films.j. Am. Chem. So. 128, (26). 18. Stoev, S., Klunde, K. J., Sorensen, C. M. & Drgiev, I. Grm-sle synthesis of monodisperse gold olloids y the solvted metl tom dispersion method nd digestive ripening nd their orgniztion into two- nd three-dimensionl strutures. J. Am. Chem. So. 124, (22). 19. Lin, X. M., Jeger, H. M., Sorensen, C. M. & Klunde, K. J. Formtion of long-rnge-ordered nnorystl superltties on silion nitride sustrtes. J. Phys. Chem. B 15, (21). 2. Dlven, R. Fundmentl optil sorption in B-silver telluride. Phys.Rev.Lett.16, (1966). 21. Brus, L. E. Eletron eletron nd eletron hole intertions in smll semiondutor rystllites: The size dependene of the lowest exited eletroni stte. J. Chem. Phys. 8, (1984). 22. Sunders, A. E. & Korgel, B. A. Oservtion of n AB phse in idisperse nnorystl superltties. ChemPhysChem 6, (25). 23. Tlpin, D. V. & Murry, C. B. PSe nnorystl solids for n- nd p- hnnel thin film field-effet trnsistors. Siene 31, (25). 24. Yu, D., Wng, C., Wehrenerg, B. L. & Guyot-Sionnest, P. Vrile rnge hopping mehnism in semiondutor nnorystl solids. Phys.Rev.Lett.92, (24). 25. Ben-Chorin, M., Moeller, F. & Koh, F. Nonliner eletril trnsport in porous silion. Phys.Rev.B 49, (1994). 26. Ristein, J. Surfe trnsfer doping of semiondutors. Siene 313, (26). 27. Stroel, P., Riedel, M., Ristein, J. & Ley, L. Surfe trnsfer doping of dimond. Nture 43, (24). 28. Zhng, P. et l. Eletroni trnsport in nnometre-sle silion-on-insultor memrnes. Nture 439, (26). Aknowledgements We grtefully thnk the ONR (N ) for funding nd support. Correspondene nd requests for mterils should e ddressed to J.J.U. Supplementry Informtion ompnies this pper on Author ontriutions J.J.U. exeuted ll of the mterils syntheses, superlttie ssemly, trnsport mesurements nd dt nlysis presented here. D.V.T. nd E.V.S. provided generl ssistne nd projet suggestions. C.R.K. provided trnsport equipment nd tehnil dvie. C.B.M. provided generl ssistne, dvie nd projet plnning. Competing finnil interests The uthors delre tht they hve no ompeting finnil interests. Reprints nd permission informtion is ville online t nture mterils VOL 6 FEBRUARY