ZnS has been extensively investigated as an important wideband-gap

Transcription

1 RTILES Morphology-tuned wurtzite-type ZnS nnoelts ZHONGWU WNG 1 *,LUKEL.DEMEN 1, YUSHENG ZHO 1,.S.ZH 2, ROERT T. DOWNS 3, XUDONG WNG 4, ZHONG LIN WNG 4 ND RUSSELL J. HEMLEY 5 1 Los lmos Ntionl Lortory, Los lmos, New Mexio 87545, US 2 HESS, Wilson Lortory, ornell University, Ith, New York 14853, US 3 Deprtment of Geosienes, University of rizon, Tuson, rizon 85721, US 4 Shool of Mterils nd Engineering, Georgi Institute of Tehnology, tlnt, Georgi 30332, US 5 Geophysil Lortory, rnegie Institution of Wshington, Wshington D 20015, US *e-mil: z wng@lnl.gov Pulished online: 13 Novemer 2005; doi: /nmt1522 Nnometre-sized inorgni dots, wires nd elts hve wide rnge of eletril nd optil properties, nd vrile mehnil stility nd phse-trnsition mehnisms tht show sensitive dependeny on size, shpe nd struture. The optil properties of the semiondutor ZnS in wurtzite strutures re onsiderly enhned, ut the lk of struturl stility limits tehnologil pplitions. Here, we demonstrte tht morphology-tuned wurtzite ZnS nnoelts show prtiulr low-energy surfe struture dominted y the ±{2 10} surfe fets. Experiments nd lultions show tht the morphology of ZnS nnoelts leds to very high mehnil stility to 6.8 GP, nd lso results in n explosive mehnism for the wurtzite-to-sphlerite phse trnsformtion together with in situ frture of the nnoelts. ZnS wurtzite nnoelts provide model tht is useful not only for understnding the morphology-tuned stility nd trnsformtion mehnism, ut lso for improving synthesis of metstle nnoelts with quntum effets for eletroni nd optil devies. ZnS hs een extensively investigted s n importnt widend-gp semiondutor (3.6 ev). It is one of the oldest nd proly one of the most importnt mterils in the eletronis industry with wide rnge of pplitions inluding light-emitting diodes nd effiient phosphors in fltpnel displys 1. n ssortment of luminesent properties exited y ultrviolet, X-ry, thode ry or eletril urrents hve een oserved y doping ZnS with vrious metls 2. Exellent light trnsmission with high index of refrtion (2.27 t 1 μm) lso mkes ZnS useful in photoni rystl devies tht operte in the region from visile to ner-infrred 3. t mient onditions, ZnS shows two struturl polymorphs: hexgonl wurtzite nd ui sphlerite 4. Surprisingly, wurtzite ZnS is muh more desirle for its optil properties thn the sphlerite phse 5. For instne, phosphors of ZnS re synthesized in the wurtzite phse to optimize luminesene t temperture ner 1,000 (ref. 5), euse ulk wurtzite rystllizes t tempertures ove 1,035. However, the quenhed wurtzite esily trnsforms to the sphlerite form t mient onditions 5,6. Deresing the prtile size lowers the temperture oundry nd nnosized wurtzite n e synthesized t tempertures 400 lowerthntheulk 7. Thus, it seems tht it my e possile to synthesizetheznswurtzitephsendkeepitstletroom onditions y djusting the surfe energy through prtilesize tuning. Synthesis of nnosle semiondutors demonstrtes tht the quntum-onfinement effet n mrkedly vry optil nd eletril properties, mostly resulting in their improvement reltive to those of their ulk ounterprts 8,9. These enhned properties hve lso een oserved in three-dimensionl wurtzite ZnSnnorystlsotinedinvrietyofwys 10, ut the struturl instility remins nlogous to its ulk ounterprt 5. Here, we report the experimentl disovery of ultrstle one-dimensionl wurtzite ZnS nnoelts, the morphology-tuned explosive trnsformtion mehnism to the sphlerite form, nd model for the kineti ehviour. High-pressure synhrotron X-ry diffrtion demonstrtes tht wurtzite ZnS nnoelts hve 922 nture mterils VOL 4 DEEMER

2 RTILES Tle 1 omprison of the trnsition pressures oserved in different wurtzite ZnS forms. Wurtzite Trnsition pressure Referene form (GP) ulk 0 Ref. 6 Nnorystl <0.5 Ref. 5 Nnoelt 6.8 This study wide field of struturl stility up to 6.8 GP, remrkly different from the ulk nd three-dimensionl monodisperse spheril nnoprtiles tht trnsform to the sphlerite struture slowly t mient onditions nd esily with even slightly pplied ompression (Tle 1). The trnsformtion mehnism is explosive, not the generl sluggish kinetis oserved in the ulk form nd nnoprtiles. Theoretil lultions show tht the nnoelt morphology produes prtiulr low-energy stte tht is the min reson for the improved stility of this metstle struture, in whih the ±{2 10} surfefetsdomintethesurfe struture of wurtzite ZnS nnoelts. High-resolution trnsmission eletron mirosopy (HRTEM) further shows tht twinning nd stking fults provide further stiliztion onstrints. This study not only provides n interprettion of the stility of the metstle wurtzite ZnS nnoelts, ut lso suggests tht the ontrolled synthesis of stle nnoelts n e optimized to produe elts, wires or other funtionl units required for iologil lelling or the frition of nno-optoeletroni devies. Wurtzite-struture ZnS nnoelts were synthesized y solid vpour phse therml-sulimtion tehnique 11. snning eletron mirosopy (SEM) imge indites tht the s-synthesized nnoelts hve uniform ross-setion long their length, with typil width in the su-mirometre rnge, extending to over 100 μm in length (Fig. 1). HRTEM shows tht prtil nnoelts hve sw-like edge (Fig. 1). X-ry nd eletron diffrtion demonstrte tht the ZnS nnoelts dopted the hexgonl wurtzite struture (Fig. 1 e). Nnoelts re 10 nm thik on verge nd hve speifi growth diretion long [120], with side surfes (001) nd top surfes (010) (Fig. 1f). The mehnil properties nd struturl stility of wurtzitetypeznsnnoeltswerestudiedindimondnvilellyin situ high-pressure synhrotron X-ry diffrtion 12. High-pressure X-ry-diffrtion ptterns olleted under hydrostti (or qusihydrostti t higher pressure) onditions re shown in Fig. 2. t 1 tm pressure, oth X-ry-diffrtion (Fig. 2) nd eletrondiffrtion ptterns (Fig. 1) indite tht the ZnS nnoelts hve hexgonl symmetry, nd unit ell prmeters omputed from the X-ry-diffrtion pttern re 0 = (2) Å, 0 = (5) Å nd V 0 = 78.15(8) Å 3, where 0, 0 nd V 0 re the unit ell prmeters lulted t mient onditions, in greement with the reported vlues of the ulk hexgonl ZnS phse 6. nlysis of these dt does not support size-indued ontrtion s ws oserved for sphlerite with slight tetrgonl distortion from ui symmetry 7. Severl wek X-ry-diffrtion peks ourintheoservedptternswell.spressurepprohes 6.8 GP, severl new peks of the high-pressure phse pper, wheres the hrteristi X-ry-diffrtion peks of the strting hexgonl ZnS phse drstilly weken. The newly oserved peks re onsistent with the sphlerite struture. Wurtzite (w) nd sphlerite (s) show signifint overlpping of w(002) nd s(111), nd of w(110) nd s(220). On the sis of the onstnt intensity rtios etween the three hrteristi peks of w(100), w(002) nd w(101), it is ovious tht the wurtzite struture remins stle to 6.8 GP. The full-width t hlf-mximum (FWHM) of wurtzite peks lso does not hnge, so the nnoelt seems e [100] (002) [001] (010) [120] d Intensity f (100) (002) 3 μm (101) Energy (kev) [100] [120] [001] 100 nm 100 nm Figure 1 The synthesized wurtzite-struture ZnS nnoelts., Low-resolution SEM imge;, HRTEM imge showing sw-like edge;, eletron-diffrtion imge; d, X-ry-diffrtion pttern; e, shemti representtion of the wurtzite struture; nd f, the rystllogrphi dimensions of nnoelts. to mintin onstnt shpe elow the trnsition pressure of 6.8 GP. ove this pressure, the two peks of w(100) nd w(101) redue signifintly in intensity nd eome extremely wek; in ontrst, the intensity of s(002) inreses. Simultneously, the new peks show n rupt nd very notiele rodening. These hnges re interpreted s rising from ollpse of the nnoelt ourring on the wurtzite-to-sphlerite phse trnsformtion. Suh phenomenon ws previously oserved in the nnowire form of the semiondutor dse, where the pressure-indued wurtzite-toroksltphsetrnsformtiontkesple 13. lthough the phse oundry ws not preisely determined, it is resonle to ssume tht the hexgonl phse is stle to 6.8 GP, nd thus quikly trnsforms to the sphlerite struture. Orgni oil pressure medium nture mterils VOL 4 DEEMER

3 RTILES S Zn = = 11.4 GP 9.6 GP 8.1 GP Sphlerite (ui) Intensity 6.8 GP s(111) s(220) s(311) = < 4.5 GP Wurtzite (hexgonl) 1.3 GP w(100) w(002) w(101) Ruy u(111) w(102) u(200) Ruy w(110) w(103) w(112) 0 GP Energy (kev) Figure 2 High-pressure X-ry-diffrtion ptterns of the wurtzite ZnS nnoelts. The upwrd nd downwrd rrows ( nd ) represent the ourrene nd dispperne of X-ry-diffrtion peks of the new ui phse nd hexgonl phse. Their rystllogrphi hrteristis re shemtilly shown in the two inset figures: ottom for hexgonl wurtzite nd top for ui sphlerite. Weker X-ry-diffrtion peks re due to the smll mount of gold nnoprtiles used s tlyst during synthesis nd y the ruy used s pressure lirnt. only enles hydrostti stte to e mintined t pressures elow 5 GP; ove 5 GP, qusi-hydrostti ondition ould generte the devitori stress nd slight pressure grdient ross the smple hmer.thismywellexplintheexisteneofverysmllrtio of wurtzite phse nd its extremely wek X-ry-diffrtion peks. quik omplishment of the phse trnsformtion in nnoelts pprently differs from the sluggish kinetis oserved in ulk nd nnoprtile ZnS forms tht show wide trnsition-pressure rnge of 5 8 GP (refs 5,7). This implies tht the trnsformtion mehnism is explosive. The resulting sphlerite phse remins stle to the pek pressure of 11.4 GP, nd is quenhle on relese of pressure. With deresing prtile size, surfe energy plys signifint role in the optil nd eletroni properties nd struturl stility, in prtiulr with extension to mient onditions for tehnologil pplitions. It is found tht ZnS nnorystls show enhned optil nd eletroni properties, ut the struturl stility in wurtzite does not improve in fshion similr to ulk wurtzite 6,7. However, euse the nnorystl is tuned to eltlike morphology, wurtzite ZnS nnoelts retin their morphology with hexgonl rystllogrphi symmetry to pressures s high s 6.8 GP.It is diffiult to understnd this remrkle oservtion from simple onsidertions of either the thermodynmis pplied in ulk mterils or the nnosize-indued enhnement of surfe energy 6,14,15.Wesuggestthttheprtiulrmorphologyofthe wurtzite ZnS nnoelts is responsile for the exeptionl struturl stility of the mterils. Speifilly, we show tht the enhned stility is onsequene of the ft tht nnoelts re regulr extended solids long the length nd width of the struture, ut hve nnometre-sle thiknesses. diret onsequene is the very different surfe-energy density on the top nd ottom surfes of nnoelt ompred with the surfe-energy density 924 nture mterils VOL 4 DEEMER

4 RTILES forthefes(the sides ofthennoelt),whihrennometresized in one dimension. To explore this effet of the nnoelt morphology, we lulted the struturl stility field of wurtzite ZnS nnoelt s funtion of the thikness of the nnoelt y minimizing the totl free energy of the wurtzite nd sphlerite tking into onsidertion the extr ontriutions ssoited with surfe, size, morphology, shpe, twinning nd stking fults, nd volumetri ontrtion. t298knd1tm,ulkznspolymorphsofwurtzitend sphlerite hve Gis free-energy differene of kj mol 1, whih reflets the greter stility of sphlerite ompred with wurtzite 16. With deresing prtile size, surfe energy strts to ply n inresingly dominnt role in determining struturl stility 8,14,15. Simultions indite tht eh fet ssoited with well-defined diffrtion plne possesses different surfe energy In wurtzite, (001) hs the highest surfe energy of J m 2, wheres the (110) fe hs the lowest surfeenergyof Jm 2, nd the (100) fe hs n intermedite energy of Jm 2. In sphlerite, oth (111) nd (100) fes hve the two highest surfe energies of 1.84 nd 2.56 J m 2, respetively, wheres the other fets hve lower surfeenergy(<1.0 Jm 2 ).Thetotlsurfeenergyofthreedimensionl spheril nnoprtile of wurtzite or sphlerite is then the men surfe energy, omputed from the surfe energy of lloftheexistingrystlfetsthtrevergelyndrndomly exposed on the prtile surfe. ordingly, three-dimensionl spheril nnoprtiles of sphlerite hve men surfe energy of 0.79 J m 2, greter thn the surfe energy of 0.57 J m 2 in wurtzite 20. The oservtion tht the trnsformtion temperture of sphlerite to wurtzite redues with deresing prtile size strongly supports this surfe-energy estimtion 20. The wurtzite ZnS nnoelts in this study hve speifi growth diretion long [120], with ±(2 10) plnes defining the two dominnt surfes (see Fig. 3 inset: top nd ottom fes nd Fig. 1d). Suh ±{2 10}-dominnt surfe struture is different fromtheommonsurfestrutureofnnorystlsththve high perentge of ui-, tetrhedrl- nd othedrl-like shpes, dominntly ounded y the omined fets of {111}, {110}, {001} nd {100}; ref.21.thesurfe-energydifferene ssoited with different rystllogrphi plnes holds generl sequene s γ {111} <γ {100} = γ {001} <γ {110} in the ui symmetry nnorystls. The high-energy {110} surfeismostlyoservedinnnorods, ut its instility is often oserved y the formtion of spheril lusters in terms of the tom sulimtion, suh s u nnorods 21. However, s prtiulr se, the surfe energies in ZnS with hexgonl symmetry hve reverse sequene nd the (hk0) fets, inluding six equl plnes of ±(110), ±(2 10) nd ±(1 20), hve the lowest surfe energy nd so fvour the formtion of this type of low-energy nnoelt growing long the [120] diretion with the lowest energy. s result, the front ross-setion ±(010) nd side ±(001) surfe fes n e negleted in the estimtion ofthetotlsurferendsurfeenergy.slightvolumetri redution of 1.25% tkes ple on the wurtzite-to-sphlerite phse trnsformtion t 6.8 GP. It is well known tht the nuleting sphlerite rystls show predominntly the (111) plne tht results diretly from the (001) plne of the wurtzite phse 4. In ddition, (111) nd (100) represent the two lrgest d-sping plnes, so oth of them unmiguously dominte the surfe re of the sphlerite when no speifi rystl diretion growth otherwise exists 21.Thisisdemonstrtedytheoserveduindtetrhedrl morphologies of reovered nnoprtiles (Fig. 3 inset: HRTEM imge). Therefore, it is resonle to ssume tht the newly formed sphlerite represents struture with higher surfe energy thn wurtzite. In omintion with the ulk Gis energy ( G 298 K,1tm ), surfe-energy differene ( γ) nd the volumetri ollpse (P V; Trnsition pressure (GP) (010) 6.2 nm 20 nm 7.4 nm (001) (210) S Zn D (nm) Figure 3 orreltion etween the nno-thikness (D ) nd trnsition pressure of one-dimensionl wurtzite nnoelts. Inset figures show the three hrteristi surfes nd the orresponding tomi rrngements in reiprol lttie demonstrtions. Inset HRTEM figure shows tht the reovered smples hve the ui nd tetrgonl shpes with n verge size of 10 nm. see Methods), we lulted the struturl stility field of wurtzite ZnS nnoelts (the wurtzite-to-sphlerite trnsition pressure) s funtion of nnoelt thikness. elow the ritil thikness of 7.4nm,wurtzitennoeltismorestlethnsphlerite(Fig.3); on reduing the thikness, the stility seems to e enhned. In our wurtzite ZnS nnoelt, the oserved trnsition pressure of 6.8 GP orresponds to lulted thikness of 6.2 nm. Trnsmission eletron mirosopy (TEM) oservtion indites tht the reovered smples hve n verge prtile size of 10 nm (Fig. 3 inset: HRTEM imge), in resonle greement with the prtile size of 12 nm lulted from the rod X-ry-diffrtion peks (Fig. 4). However, these vlues re greter thn either the lulted nnoelt thikness of 6.2 nm, whih orresponds to the oserved trnsformtion pressure of 6.8 GP, or the ritil thikness of 7.4 nm. Thus, resonle explntion my lso require the inorportion of further effets indued y rystllogrphi defets, twins, stking fults nd volumetri ontrtion generted in the newly formed sphlerite on phse trnsformtion. oth the Zn nd S toms in wurtzite nd sphlerite re four-oordinted 4,23, so the phse trnsformtion only requires prtil tomi rerrngement. Wurtzite hs the simple hexgonllose-pked stking order of long the [001] diretion with resultnt se of (001) nd sphlerite hs the ui-losepked stking order of long the [111] diretion with the hrteristi rystllogrphi fet of (111); ref. 23. On trnsformtion from wurtzite to sphlerite, the (001) plne of wurtzite onverts diretly to the (111) plne of sphlerite 22,24.On the sis of suh fundmentl geneti reltion nd omined with the prtiulr rystllogrphi hrteristis of the two ZnS polymorphs, two types of trnsformtion mehnism n e suggested(fig.5).thefirstinvolvesthererrngementofthree {ZnS} lyers (Fig. 5), leding to (111) twinning in sphlerite nture mterils VOL 4 DEEMER

5 RTILES s(111) Wurtzite s(220) Sphlerite w(112) (1) e 14.8 Twin w(110) w(002) s(311) f 30 1 nm Energy (kev) g Figure 4 omprison of the three hrteristi X-ry-diffrtion peks etween wurtzite nd sphlerite polymorphs t 1 tm pressure. Here w nd s represent the wurtzite nd sphlerite, respetively. d Stking fults relted y 180 rottion (Fig. 5). The seond involves the rerrngement of four {ZnS} lyers (Fig. 5), resulting in the development of stking fults (Fig. 5d). It seems tht the trnsformtion initited y the first type of tomi rerrngement requires higher energy thn the seond type of trnsition mehnism, euse the rottion in the first type of mehnism is produed s onsequene of fulted stking of perfet rystls, nd the onding onfigurtion t the stking fults nd the twin oundries remins lose to tht of the perfet struture24. The two types of mehnism hve een oserved in the HRTEM imges tken from the reovered sphlerite (Fig. 5e,f) tht show the formtion of the oundry twins () (Fig. 5e) nd stking fults (for exmple, the doule stking fults shown in Fig. 5f). From thermodynmi viewpoint, the resultnt stking fults nd twins imply high-energy sphlerite struture. lthough numerous types of energy ontriution, s suggested ove, re dded to the totl energy of sphlerite, the nnoelt thikness (lso inluding the ritil thikness) ould e signifintly greter for the wurtzite nnoelts tht trnsform to sphlerite t modelled pressure with thinner thikness. It hs een oserved tht sphlerite nnoprtiles show size-indued ontrtion y undergoing tetrgonl distortion from the ui struture, whih leds to volumetri derese of the order of 2% ompred with ulk sphlerite with ui symmetry7. t the trnsition pressure of 6.8 GP, this ould result in n energy hnge s lrge s 12.5 kj mol 1 etween sphlerite nd wurtzite, responsile for trnsition pressure jump of 2.4 GP. omining the ontriutions from the ove-mentioned ftors provides n understnding of the high trnsition pressure of 6.8 GP in the wurtzite ZnS nnoelts synthesized here. In summry, we hve oserved n enhned stility field nd the explosive phse-trnsformtion mehnism for wurtzite ZnS nnoelts tht re understood y onentrting on the prtiulr morphology nd shpe using the omined tehniques of in situ high-pressure synhrotron X-ry-diffrtion mesurements, theoretil lultions nd HRTEM investigtion. The enhned stility field shows tht the speifi morphology of wurtzite ZnS nnoelts represents the most-fvourle low-energy surfe struture, whih hs dominnt effet in ontrolling the formtion nd struturl stility of nnoelts. Owing to the similrities etween semiondutors, pplition of this mehnism to the retion of further metstle phses ould open up the possiility for mnipulting the surfe energetis of different semiondutor 926 (2) Figure 5 Shemti representtion of the wurtzite-to-sphlerite phse trnsformtion.,, The first (1) involves the rerrngement of three {ZnS} lyers (), leding to (111) twinning in sphlerite relted y 180 rottion ().,d, The seond (2) involves the rerrngement of four {ZnS} lyers (), resulting in the development of stking fults (d). e g, Filtered HRTEM imges of the smples: strting wurtzite phse (e); reovered sphlerite phse (f,g). nnoforms in ontrolled mnner tht ould hve prtil onsequenes8,9,25,26. ordingly, the synthesis of stle nd relile nnoelts, quntum wells or wires with speifi sizes nd thikness (<10 nm) n e otined y djusting the surfe energy, morphology nd intrinsi struture with ner-tomi preision tht my prove to e pplile in eletronis, optis nd sensors. METHODS Wurtzite-struture ZnS nnoelts were synthesized y heting ulk ZnS in flowing rgon with the ssistne of u prtiles ting s tlyst11. The resulting ZnS nnoelts were deposited on n lumin sustrte. The s-deposited mterils ws hrterized nd nlysed y SEM (LEO 1530 FEG), TEM (Hithi HF-2000 FEG t 200 kv, JEOL 4000EX t 400 kv) nd energy-dispersive X-ry spetrosopy. The wurtzite ZnS nnoelt undles were removed from the sustrte nd loded without ny further preprtion into dimond nvil ell for in situ pressure mesurements. stinless-steel gsket ws pre-indented to 50 μm in thikness y two opposing dimond nvils with 450 μm ulets. 250-μm-dimeter hole ws mde s smple hmer. Orgni oil nd smll ruy hip served s the hydrostti pressure medium nd pressure mrker, respetively. The mss of ZnS nnoelt smples is estimted to e pproximtely 0.01 mg from the volume of the smple hmer nd the density of ZnS nnoelts with onsidertion of the existene of the pressure medium. High-pressure X-ry-diffrtion mesurements were performed t room temperture with energy-dispersive synhrotron rdition t HESS12. Energy lirtions were mde using well-known rdition soures (55 Fe nd 133 ), nd ngle lirtions were mde t fixed ngle of 15 using the six rgg peks of u powder stndrd. X-ry-diffrtion ptterns olleted up to nture mterils VOL 4 DEEMER Nture Pulishing Group

6 RTILES pressures of 11.4 GP were used for struture nlysis nd ell-prmeter refinement. The reovered smples were lso exmined y SEM nd TEM. Prtile sizes re estimted y TEM nd the FWHM of the oserved X-ry-diffrtion peks. The stndrd Sherrer s eqution for energy dispersive X-ry diffrtionismodifieds τ = (0.94 Ed)/ E, where τ is the prtile size in ångströms, E nd E re the energy nd FWHM of the oserved rgg pek in kev nd d is the inter-plnr sping in ångströms. Theoretil lultions re sed on the totl Gis energy differene etween the two ZnS polymorphs of wurtzite nd sphlerite in omintion with the size-indued surfe-energy ontriution. t 1 tm, the Gis thermodynmi eqution is modified s G = H T S+ γs,where G, H, S nd γ represent the differene of the Gis free energy, enthlpy, entropy nd the unit surfe energy etween wurtzite nd sphlerite, respetively, nd s is the surfe re. To evlute the reliility for the simulted surfe energies of the surfe plnes used in this study, the temperture t G = 0 for eh prtile size is lulted, in greement with the synthesized tempertures 10,20. In wurtzite ZnS nnoelts, the surfe energy isdomintedythe±(2 10) fets, nd in the newly formed sphlerite nnorystls, it is dominted y (111) nd (100) fets. The orresponding ui nd tetrgonl morphologies re hrterized in the reovered smples (Fig. 3 inset: HRTEM imge). t ertin thikness of nnoelt, the Gis free-energy differene ( G 1tm 298 K ) t 298 K nd 1 tm pressure n e lulted. On ompression, lthough the energy gp of G 1tm 298 K is overome y newly dded energy term of P V, wurtzite theoretilly trnsforms to the sphlerite phse. Here, P nd V represent the trnsition pressure of the wurtzite-to-sphlerite phse trnsformtion nd the volumetri differene etween the two ZnS polymorphs t P, respetively. Reeived 26 pril 2005; epted 28 Septemer 2005; pulished 13 Novemer Referenes 1. Monroy, E., Omnes, F. & lle, F. Wide-nd gp semiondutor ultrviolet photodetetors. Semiond. Si. Tehnol. 18, R33 R51 (2003). 2. hrgv, R. N., Gllgher, D., Hong, X. & Nurmikko, D. Optil properties of mngnese-doped nnorystls of ZnS. Phys.Rev.Lett.72, (1994). 3. Prk,W.,King,J.S.,Neff,. W., Liddell,. & Summers,. ZnS-sed photoni rystls. Phys. Sttus Solidi 229, (2002). 4. Gilert,. et l. X-ry sorption spetrosopy of the ui nd hexgonl polytypes of zin sulfide. Phys. Rev. 66, (2002). 5. Qdri, S.. et l.theeffetofprtilesizeonthestruturltrnsitionsinzinsulfide.j. ppl. Phys. 89, (2001). 6. Desgreniers, S., eulieu, L. & Lepge, I. Pressure indued struturl hnges in ZnS. Phys.Rev.61, (2000). 7. Qdri, S.. et l. Size-indued trnsition-temperture redution in nnoprtiles of ZnS. Phys.Rev. 60, (1999). 8. livistos,. P. Semiondutor lusters, nnorystls, nd quntum dots. Siene 271, (1996). 9. Lieer,. M. One-dimensionl nnostrutures: hemistry, physis pplitions. Solid Stte ommun. 107, (1998). 10. Zho, Y. W. et l. Low-temperture synthesis of hexgonl (wurtzite) ZnS nnorystls. J. m. hem. So. 126, (2004). 11. M,., Moore, D., Li, J. & Wng, Z. L. Nnoelts, nnooms, nd nnowindmills of wurtzite ZnS. dv. Mter. 15, (2003). 12. Wng, Z. W. et l. quenhle superhrd ron phse synthesized y old ompression of ron nnotues. Pro. Ntl d. Si. US 101, (2004). 13. Zziski, D. et l. ritil size for frture during solid-solid phse trnsformtions. Nno Lett. 4, (2004). 14. hen,.., Herhold,.., Johnson,. S. & livistos,. P. Size dependene of struturl metstility in semiondutor nnorystls. Siene 276, (1997). 15. Tolert, S. H. & livistos,. P. The wurtzite to rok slt struturl trnsformtion in dse nnorystls under high pressure. J. hem. Phys. 102, (1995). 16. rin, I., Knke, O. & Kushewski, O. Thermohemil Properties of Inorgni Sustnes (Springer, erlin, 1977). 17. Hmd, S., ristol, S. & tlow,. R. Surfe strutures nd rystl morphology of ZnS: omputtionl study. J. Phys. hem. 106, (2002). 18. Wright, K., Wtson, G. W., Prker, S.. & Vughn, D. J. Simultion of the struture nd stility of sphlerite (ZnS) surfes. m. Minerl. 83, (1998). 19. Wright, K. & Gle, J. D. Intertomi potentils for the simultion of the zin-lende nd wurtzite forms of ZnS nd ds: ulk struture, properties, nd phse stility. Phys.Rev.70, (2004). 20. Zhng, H. Z., Hung, F., Gilert,. & nfield, J. F. Moleulr dynmis simultions, thermodynmi nlysis, nd experimentl study of phse stility of zin sulfide nnoprtiles. J. Phys. hem. 107, (2003). 21. Wng, Z. L. Trnsmission eletron mirosopy of shpe-ontrolled nnorystls nd their ssemlies. J. Phys. hem. 104, (2000). 22. Willims, V.. Diffusionofsomeimpuritiesinzinsulfidesinglerystls.J. Mter. Si. 7, (1972). 23. irmn, J. L. Simplified LO method for zinlende, wurtzite, nd mixed rystl strutures. Phys. Rev. 115, (1959). 24. Posfi, M. & usek, P. R. in Modulr spets of Minerls Vol. 1 (ed. Merlino, S.) (EMU NotesinMinerlogy,Eötvös Univ. Press, udpest, 1997). 25. livistos, P. The use of nnorystls in iologil detetion. Nture iotehnol. 22, (2004). 26. Ding, Y., Wng, X. D. & Wng, Z. L. Phse ontrolled synthesis of ZnS nnoelts: zin lende vs wurtzite. hem. Phys. Lett. 398, (2004). knowledgements We ppreite finnil support from the Diretor s Funded Postdotorl Fellowship t Los lmos Ntionl Lortory. We lso knowledge grtefully the stff t HESS, Wilson Lortory of ornell University for ssistne with experimentl mtters. X.D.W. nd Z.L.W. re grteful for support from NSF. Speil ppreition goes to the rnegie/doe lline enter (D) for signifint support. orrespondene nd requests for mterils should e ddressed to Z.W. ompeting finnil interests Theuthorsdelrethttheyhvenoompetingfinnilinterests. Reprints nd permission informtion is ville online t nture mterils VOL 4 DEEMER