Synthesis and investigation of optical properties of ZnS nanostructures

Transcription

1 Bull. Mter. Si., Vol. 34, No. 2, April 2011, pp Indin Ademy of Sienes. Synthesis nd investigtion of optil properties of ZnS nnostrutures NESLIHAN ÜZAR nd M ÇETIN ARIKAN Physis Deprtment, Siene Fulty, Istnul University, Vezneiler, Istnul, Turkey MS reeived 27 July 2010; revised 20 Septemer 2010 Astrt. Struturl hrteriztions of wurtzite zin sulfide (ZnS) nnostrutures synthesized y vpour liquid solid tehnique (VLS) were rried out y snning eletron mirosopy (SEM), energy dispersive spetrosopy (EDS) nd X-ry diffrtion (XRD) nlyses. Spetrl dependene of photoluminesene (PL) ws lso rried out for optil hrteriztion. PL results indite tht the ndgp energy of ulk ZnS whih is 3 68 ev t room temperture hnges from 3 7 evto3 72 ev depending on the size of the strutures. We lso supported these results y lulting the ndgp energies theoretilly with using the infinite potentil well pproximtion for 1D strutures. Keywords. ZnS nnowires; VLS; SEM; EDS; XRD; PL. 1. Introdution Semiondutor nnostrutures suh s nnowires, nnoelts nd nnorions hve ttrted gret ttention due to very high surfe-to-volume rtio nd their size-relted quntum onfinement properties, i.e. onfinement of rriers y the oundries of mteril when the mteril size is in nnosle region. In this size regime s the mteril size eomes smller, the semiondutor energy levels eome more seprted from eh other nd hene the effetive ndgp inreses. In ddition, the eletroni density of sttes in 3-, 2- nd 1-dimensionl systems vry s E 1/2, E 0 nd E 1/2, respetively. These effets led to physil nd hemil ehviours tht re different from the ulk ounterprts (Dun nd Lieer 0). They n e tuned nd mnipulted in fsinting wys y ontrolling the dimension. Therefore, these mterils hve wide rnge of potentil pplitions inluding hemistry, physis, eletronis, optis, mterils siene, iomedil siene nd nnodevies (Sun et l 1999; Kishimoto et l 2). ZnS, s typil II VI semiondutor ompound with wide diret ndgp energy of 3 68 ev t room temperture, exhiits wide optil trnspreny from the ultrviolet (UV) to the infrred (IR) region. This optil trnspreny omined with hemil nd therml stility mkes ZnS one of the most widely used mterils for optil windows. Disrete energy levels formed in the ndgp of ZnS y impurities nd vnies n e exited y illumintion to produe PL ehviour tht is utilized in eletro- Author for orrespondene (neslihnuzr@gmil.om, neslihnuzr@yhoo.om) opti devies, suh s flt pnel displys. Its inding energy is 40 mev, whih mkes it n exellent ndidte for exploring the intrinsi reomintion proesses in dense exiton systems. Moreover, ZnS is lso widely used for lser, sensor (Jing et l 1), infrred windows, the thode ry tue nd eletroluminesene devie pplitions (Bredol nd Merihi 1998; Clndr et l 1999). ZnS nnostrutures n e synthesized using vrious tehniques, suh s solvotherml (Chen et l 3), lser ltion (Jing et l 3), liquid templte (Xie et l 0), sol gel (Krumeih et l 1999) nd VLS (Wng et l 2; Li et l 4), et. In this study, ZnS nnostrutures were synthesized y the simple, rpid nd low-ost VLS tehnique sed on therml evportion of ZnS powders onto Si sustrtes overed y Au s tlyst. In this tehnique, the sustrte temperture, onentrtion of ZnS vpour nd flow rte of rrier gs re the ritil experimentl prmeters to otin ZnS nnostrutures of different morphologies. The optil hrteriztion of ZnS nnostrutures ws rried out using PL mesurement. 2. Experimentl The onventionl VLS mehnism is sed on evportion of soure mteril nd trnsfer of vpour to lower temperture regions y using n inert gs s rrier. The growth prmeters re strongly dependent on the soure temperture, sustrte tempertures, the thikness of tlyst lyer nd the flow rte of rrier gs. Our experimentl setup onsists of horizontl tue furne (dimeter 6 m, length 78 m) with temperture ontrol unit, horizontl qurtz tue (dimeter 4 m, length 150 m), qurtz ot, rotry vuum pump, Au-oted Si sustrtes nd Ar gs s the rrier gs with gs flow ontrolling system, s shown in figure 1(). 287

2 288 Neslihn Üzr nd M Çetin Arikn Temperture ( o C) Distne of tue (m) Figure 1.. Experimentl set up,. temperture mp of the furne nd. shemtil set up. Before the experimentl proess strted, the qurtz tue previously pled in the furne, heted up to 1100 C, elow the sulimtion point of ulk ZnS (sulimtion point of ZnS, 1180 C) nd the temperture distriution of the tue ws otined using movle thermoouple (figure 1()) for the purpose of the sustrtes nd the soure mteril lotions determintion. One terminl of the qurtz tue ws onneted to vuum pump nd the gs flow system. The qurtz ot ontining the soure mteril (dimeter 10 μm, 99 9% pure ZnS powder) nd the Au-oted (50 Å) Si sustrtes were pled t the other edge of the qurtz tue (figure 1()). Then the rotry vuum pump evuted the tue for 3 h. After losing the rotry pump, the tue ws filled with high-purity Ar gs to eliminte ny O 2 in the tue nd the furne ws strted to het. When the furne rehed the proess temperture (1100 C), the terminl opposite to the gs flow system opened nd ws kept open during the proess for the gs to flow out. Then the qurtz ot ontining ZnS powder ws trnsferred from edge of the tue to 1100 C temperture pled in the tue. The Au-oted Si sustrtes were pled t the pre-determined ples hving 850 C, 750 C nd 600 C tempertures. Evported ZnS vpour ws rried y 500 sm of Ar gs nd ondensed on the sustrtes. After 90 min proess, the qurtz ot ws tken off from the qurtz tue rpidly. The Au-oted Si sustrtes were seen to e overed y white wool lyers. SEM nd EDS nlyses determine the surfe morphologies nd the hemil ompositions of the synthesized strutures, respetively. The rystl struture nd the fine hemil ompositions of the synthesized strutures were investigted y XRD nlysis (with Cu Kα 1 line). The PL mesurement ws rried out for optil hrteriztion t room temperture. A merury lmp with 253 nm filter ws used s n exittion light, the luminesene from the surfe of smple ws direted through the monohromtor nd the dispersed light ws olleted using photomultiplier tue (PMT). 3. Results nd disussion Figure 2() shows typil SEM imge of the nnostrutures in the form of nnorions nd nnowires mixture, synthesized t the 850 C temperture zone of the horizontl tue (herefter lled s ZnS_850). Our SEM oservtion demonstrtes tht the widths of the nnorions rnge from 250 nm to 450 nm nd the dimeters of the nnowires rnge from 80 nm to 150 nm. The lengths of oth the strutures n reh to pproximtely hundreds of mirometers

3 Synthesis nd investigtion of optil properties of ZnS nnostrutures 289 Element Weight% Atomi% S K Zn K Totls Element Weight% Atomi% S K Zn K Totls (103) (1 0 0) (0 0 2) (1 0 1) (1 1 0) (1 0 3) (1 1 2) (1 0 0) (1 0 5) (1 0 6) Au(111) (0 0 12) (1 1 0) (1 0 15) (1 1 10) (2 0 5) Thet (degree) Figure 3.. SEM imge,. EDS spetrum nd. XRD pttern of ZnS_750. Figure 2. ZnS_ Thet (degree). SEM imge,. EDS spetrum nd. XRD pttern of while the verge thikness of the nnostrutures is out 10 nm. Au nnodots re lerly seen t the tip of the nnostrutures in SEM. The EDS nlysis shows tht the smple is omposed of Zn nd S elements s illustrted in figure 2(). Atomi rtio of S in the smple is lower thn tht of Zn element (S:Zn=41 49%:58 51%). The pek whih is ner the S pek elongs to Si element. Si pek in the EDS spetrum omes from the sustrte, owing to very thin ZnS lyer whih formed on the sustrte t high temperture point. Figure 2() shows the XRD pttern of ZnS_850. Diffrtion peks in the XRD pttern n e indexed to hexgonl (wurtzite) 2H rystl strutured ZnS with lttie prmeters of = nm nd = nm, whih mthes well with the JCPDS rd ( ). Figure 3() shows the typil SEM imge of the deposition lyer on the surfe of the Au-oted Si wfer pled t

4 290 Neslihn Üzr nd M Çetin Arikn the 750 C temperture zone in the tue (herefter lled s ZnS_750). SEM oservtion demonstrtes tht the sustrte ws densely overed with nnowires. The dimeters of the nnowires re out nm. The lengths of nnostrutures n reh to severl tens of mirometres nd the verge thikness of nnostrutures is out 8 nm. The EDS spetrum in figure 3() shows tht the tomi rtio of Zn nd S is 46 37%:53 63% with low devition from stoihiometry. In figure 3(), the diffrtion peks in XRD pttern re indexed to hexgonl-10h strutured ZnS with lttie prmeters of = 3 82 nm nd = 31 2 nm, whih mthes well with the JCPDS rd ( ). Au pek whih ws seen in the XRD pttern is evident in VLS growth mehnism. Figure 4() presents the SEM imge of the struture synthesized t 600 C (herefter lled s ZnS_600). SEM oservtion demonstrtes tht only wire-like nnostrutures formed nd their lengths n reh to severl tens of mirometers nd their dimeters hnge from 50 to 70 nm. The verge thikness of nnowires is round 5 nm. The EDS nlysis of ZnS_600 shows tht the smple is omposed of Zn nd S elements, s illustrted in figure 4(). The tomi rtio of Zn nd S is nerly 47:53%. This elementry nlysis shows tht the synthesized struture is slightly zin defiient. The XRD pttern in figure 4() indites tht the nnostruture ould e formed of wurtzite-2h strutured ZnS with lttie prmeters of = nm nd = nm, whih mthes well with the JCPDS rd ( ). We point out tht the sustrte temperture is the most importnt prmeter to ontrol the sizes nd morphology of the produts. The ove-mentioned results indite tht low growth tempertures resulted in smll dimeters of ZnS nnostrutures. Nnostrutures strt to eome 2D, typilly in the form of rion-like strutures t high tempertures, i.e. t higher temperture ZnS rystl grew long the two optimized diretions. At the strt of ondenstion the lower energy side surfes re formed. At high tempertures, the moility of ZnS vpour inreses nd the side surfe re expnds. At slightly lower temperture, ZnS rystl strutures generlly grow long single diretion to hve the lowest energy sttes. In ddition, thikness of tlyst n determine the size of nnostrutures. The used Si sustrtes were Au-oted. Hene, the mismth etween the lttie onstnts of Au nd Si n use the formtion of nnometer sle Au islnds with temperture. At high tempertures, the sizes of islnds n e lrger thn those of tht formed t lower tempertures due to the inrese in the kineti energy of Au toms nd these smll islnds n olese with eh other. This is reson why rion-like strutures form. At the lower tempertures, strutures generlly grow long single diretion to hve the lowest energy sttes. We oserved tht the smple prepred t high temperture (850 C) hs higher zin or lower sulfur ontent ompred to smples prepred t lower tempertures. The ZnS soure mteril evportes t 1100 C s Zn nd S vpours. Zn nd S vpours rry long with rrier gs to the low temperture zone, where they depose on the Au-oted sustrte. Firstly, (100) (002) Element Weight% Atomi% S K Zn K Totls Au (111) (102) Thet (degree) Au () (110) (103) (112) (201) Figure 4..SEM imge,. EDS nd. XRD pttern of ZnS _600. Au (Zn) liquid droplets form nd these droplets would ontinuously sor more Zn nd S vpours from the rrier gs nd inside the droplet Zn nd S ret gin. As the onentrtion of ZnS rehes super sturtion t the sustrte temperture, ZnS phse would then seprte out from droplet nd form ZnS nnostrutures. ZnS lyer whih ws deposited on the sustrte t 850 C ws thinner thn the smples prepred t lower temperture zone. The ondenstion whih ws on the sustrte ws very low t 850 C due to the high temperture. S is more voltile element thn Zn, i.e. sulimtion point nd pressure of Zn is higher thn S nd 850 C

5 Synthesis nd investigtion of optil properties of ZnS nnostrutures 291 is very high temperture for S element. Hene, ondenstion of S element on the sustrte my not e esy nd the mount of S toms is lower thn the Zn toms. On the other hnd, hemil ond etween the Zn nd S whih re inside the droplet my rek esily nd S toms my evporte t high temperture zone. Due to these resons, it is more possile to form struturl defets t high temperture zone ompred to lower temperture zone. Au peks re seen in XRD ptterns for smples prepred t 750 nd 600 Cut there is no tre of Au oserved in EDS spetr due to different detetion limits of XRD nd EDS, i.e. dt of XRD nd EDS re tken from sme smple ut different regions of the smple. Aording to VLS mehnism, Au prtiles re t the tip of nnowires. We lso investigted the optil properties of ZnS_850, ZnS_750 nd ZnS_600 with PL tehnique in ir tmosphere nd t room temperture. PL mesurements then yielded informtion out ndgp energies of nnostrutures tht n e tuned y vrying their sizes nd the energeti positions of the eletroni sttes in the ndgp. On the other hnd, wide ndgp semiondutors re idel mterils for studies on trp sttes. Suh lolized sttes n e due to vrious types of imperfetions like vnies, interstitil toms nd dngling onds. Bulk ZnS hs wide diret ndgp of 3 68 ev. The lrge exiton inding energy (40 mev) is muh higher thn therml energy t room temperture ( 26 mev), the nd edge PL of ZnS t room temperture is oserved in high qulity rystl. The typil PL spetr of these synthesized nnostrutures re given in figure 5. The lk line in figure 5 shows the PL spetrum of ZnS_850. When only the green luminesene ws oserved from the smple with the nked eyes, the PL mesurement shows three emission peks t round 335 nm, 380 nm nd 538 nm. PL emission pek t 335 nm (3 7 ev) orresponds to the ndgp energy of ZnS nnostrutures (Jing et l 3). The emission pek t 380 nm (3 26 ev) ould e ssigned to free-exiton reomintion. The green emission t out 538 nm (2 3 ev) orresponds to the trnsition from the ondution nd (3 7 ev) to the sulfur vnies (V S ) level (1 4 ev) (Kurtov et l 9). Low onentrtion of S ions in ZnS whih ws given y EDS s shown in figure 2() uses lrge numer of V S. Sulfur vnies n t s douly ionized donor entres. The red line in figure 5 shows the PL spetrum of ZnS_750. Three emission peks re oserved. The stronger two peks re t round 334 nm, 373 nm nd the weker pek is t out 430 nm. The UV emission nd, entred t 334 nm (3 71 ev) my show the nd-to-nd trnsition of ZnS nnowires. The intensive pek of PL spetrum t 373 nm (3 32 ev) n proly relte to the UV exitoni emission depending on exiton inding energy of 40 mev (Jing et l 3; Li et l 8). The lue emission t 431 nm (nerly 2 9 ev) is very wek nd n e sried to either zin vnies (V Zn ) (Suih et l 7) or surfe sttes (SS). 1,0 0,8 0,6 0,4 0,2 ZnS_850 ZnS_750 ZnS_600 0, Wvelength (nm) Figure 5. PL spetr of ZnS_850, ZnS_750 nd ZnS_600. The lue line in figure 5 shows the PL spetrum of ZnS_600. Three emission peks re lerly oserved t round 333 nm (3 72 ev), 367 nm (3 38 ev) nd 491 nm (2 5 ev). Aording to the EDS nlysis shown in figure 4(), the tomi rtio of Zn is lower thn tht of S so tht emission t 491 nm my e used y the trnsition from the ondution nd to the V Zn level (this lolized vny level is ove the vlene nd t 1 1 ev). The pek t 368 nm (3 37 ev) n indite exiton reomintion. The pek t 333 nm (3 72 ev) in the UV region lose to the ndgp energy of ulk ZnS (3 68 ev) my e the ndgp energy of ZnS nnostrutures (Meng et l 3). The ndgp energies of ZnS_850, ZnS_750 nd ZnS_600 re lrger thn tht of ulk ZnS ndgp energy. Both exitoni emission energies nd ndgp energies of otined ZnS nnostrutures inrese with deresing dimeters of nnostrutures. Beside trnsition nd-to-nd, the possile emission entres in ZnS nnostruture n e relted to surfe/lttie defets (stoihiometri defets nd interstitil impurities). Inresing of ndgp energies of ZnS nnostrutures ould e n indition of the quntum onfinement effet due to deresing size of strutures. In order to prove this ide, we lulted the effetive ndgp energies using the infinitive quntum well pproximtion for 1D struture. The ndgps of nnostrutures n then e otined y dding the ulk gp to the two onfinement energies. The solution of this pproximtion is very simple. The effetive ndgp energy is given s Eg effetive = Eg ulk(zns) + E e1 + E hhl, (1) where E e1 = E hh1 = h 2 8m e L2 x h 2 8m h L2 x + h2, 8m e L2 y + h2 8m, (2) h L2 y

6 292 Neslihn Üzr nd M Çetin Arikn Tle 1. Clultion prmeters with lulted nd experimentl ndgp energy of ZnS nnostrutures. Effetive mss of Effetive mss of Averge thikness Averge dimeter Clulted Experimentl Smple eletron for ZnS (m e ) hole for ZnS (m h ) of strutures ( L x) of strutures ( L y ) Eg effetive Eg effetive ZnS_ m m 0 10 nm 150 nm 3 7 ev 3 7eV ZnS_ m m 0 8nm 90nm 3 71 ev 3 71 ev ZnS_ m m 0 5nm 60nm 3 74 ev 3 72 ev where m e nd m h re the effetive mss of eletron nd hole, respetively, h is the Plnk onstnt, nd L x nd L y re the verge thikness nd dimeter of the smples. The vlue of prmeters used in the lultions nd the otined results re given in tle 1. The lulted effetive ndgp energies using the infinite potentil well pproximtion for one dimension is well mthed with the experimentl effetive ndgp energies. Aording to these results, the ndgp energy of the synthesized ZnS nnostruture is lrger thn tht of ulk ZnS nd energy (3 68 ev) due to the quntum onfinement effet. Sine the smples re not very thin their ndgp energy re lose to tht of ulk ZnS struture. In ddition, vnies nd defets sttes in the ndgp of ZnS nnostrutures my use the emission in visile region. 4. Conlusions In onlusion, semiondutor ZnS nnostrutures hve een suessfully synthesized in ulk quntities y the simple low-ost proess sed on therml evportion of ZnS powders onto silion sustrtes in the presene of Au tlyst. We found tht the synthesized strutures re rion nd wire like, ll hving different sizes nd dimeters depending on sustrte temperture. The UV emissions t 333 nm, 334 nm nd 335 nm orrespond to ndgp energies of ZnS_850, ZnS_750 nd ZnS_600, respetively. The effetive ndgp energies were lulted using the infinite quntum well pproximtion. The lulted ndgp energies re well mthed with the experimentl effetive ndgp energies. Aording to oth experimentl nd theoretil results, the ndgp energies of the synthesized ZnS nnostruture is lrger thn ulk ZnS nd energy due to quntum onfinement effet. Aknowledgement This study ws finnilly supported y the Reserh Fund of the Istnul University in Turkey (Projet numer T-676). Referenes Bredol M nd Merihi J 1998 J. Mter Si Clndr P, Goffredi M nd Liveri V T 1999 Colloids Surf. A9 160 Chen X, Xu H, Xu N, Zho F, Lin W, Lin G, Fu Y, Hung Z, Wng H nd Wu M 3 Inorg. Chem Dun X F nd Lieer C M 0 Adv. Mter Jing X, Xie Y, Lu J, Zhu L Y, He W nd Qin Y T 1 Chem. Mter Jing Y, Meng X M, Lee C S nd Lee S T 3 Adv. Mter Jing Y, Meng X M, Lui J, Xie Z Y, Lee C S nd Lee S T 3 Adv. Mter Kishimoto S, Kto A, Ntio A, Skmot Y nd Tid S 2 Phys. Sttus Solidi B1 391 Krumeih F, Muhr H J, Niedererger M, Bieri F, Shyder B nd Nesper R 1999 J. Am. Chem. So Kurtov D, Kosyk V, Opnsyuk A nd Melnik V 9 Physi B Li J et l 8 Appl. Phys. A Li M, Sudhirnjn T, Boothryd C nd Loh K P 4 Chem. Phys. Lett Meng X M, Lui J, Ling Y, Chen W W, Lee C S, Bello I nd Lee S T 3 Chem. Phys. Lett Suih Y P V, Prthp P, Reddy K T R nd Miles R W 7 J. Phys. D: Appl. Phys Sun L, Liu C, Lio C nd Yn C 1999 J. Mter. Chem Wng Y, Zhng L, Ling C, Wng G nd Peng X 2 Chem. Phys. Lett Xie Y, Hung J X, Li B nd Qin Y T 0 Adv. Mter