1、 25 2 2010 M 2 null Journal of InorganicM aterialsVo.l 25, No. 2null Feb. , 2010Article ID: 1000null324X( 2010) 02null0216null05 DO I: 10. 3724 /SP. J. 1077. 2009. 09563Received date: null 2009null08null10, M odified date: null 2009null09null17, Published online: null 2009null10null14Foundation item
2、: null NationalN atural Science Foundation of Ch ina ( 50772125, 50732002); the S cience and Technology C omm ission of Sh anghaiM un icipa lity ( 08JC1420700); N ationa lH igh Technology Research and Developm en t Program of Ch ina ( 2008AA03Z303)Biography: null LIU Zh en ( 1982 - ), m ale, cand id
3、ate ofm aster. Enullm ai:l jack. lew m sn. comCorrespond ing author: null ZHU Y ingnullChun, p rofessor. Enullm ai:l yingchunzhu yahoo. comPreparation and Characterization of ZnSe Tetrapodswith SphaleriteNucleusLIU Zhen1, 2, ZHU Y ingnullChun2, YUAN JiannullH ui2, RUAN Q inullChao2, MA GuonullH ong1
4、( 1. Departm ent of Physics, Shanghai Un iversity, Shanghai 200444, Ch ina; 2. The Key Laboratory of Inorgan ic CoatingM aterials, Shanghai Institute of C eram ics, Chinese Academ y of Sciences, Shangha,i 200050, Ch ina)Abstract: M an ipu lation ofm aterials w ith nanoscale control is the key of nan
5、oscience and nanotechnology, because the intrinsicproperties of nanoscalem aterials are closely determ ined by their structures. In this work, a tetrapodnullshaped ZnSe nanostructurewas synthesized successfu lly via therm al evaporation and their crystallographic structures were characterized by h i
6、ghnullresolutiontransm ission electron m icroscope. Them orphology and construction of ZnSe nanostructure are d iscussed in detai.l The asnullgrownZnSe tetrapods include one zinc blende nucleus and four arm s w ith un iform wurtz ite structure grow ing along the 001 direcnulltion. And the conullexis
7、tence of two crystal structures in different dom ains of one ZnSe nanocrystal is experim entally dem onstrated.Accord ing to the crystal phase stability in d ifferen t tem perature regions and crystallograph ic property of ZnSe, an acceptablegrow th m odel is em ployed to exp lain the form ation pro
8、cess of th is new ly discovered ZnSe tetrapods nanocrysta.lKeywords: therm al evaporation method; ZnSe tetrapods; crysta;l nanoscience; nanotechnologynull null Com plex sem iconductor nanocrystals have drawnmuch attention, since further app lications and new funcnulltionalm aterials m ight em erge i
9、f nanocrystals can be synnullthesized w ith com plex shapes and w ell defined three dinullm ensional ( 3D ) architectures 1null2 . The fabrication ofnullnull fam ily tetrapod nanocrystals w as attem pted w ithd ifferent techn iques since A livisatos and cow orkers sucnullceeded in the synthesis of t
10、etrapodnullshaped CdSe nanonullcrystals 3null6 . It has been also dem onstrated that these tetnullrapod nanocrystals have m any potential applications inw aveguides, photodiodes, transistors 7 , field em issiondevices, gas sensors, and biom olecule delivery 8 . W iththe developm en t of research abo
11、ut tetrapodnullshaped m atenullrials, several form ation m odels on the branched tetrapod3D nullnull nanostructures have been proposed. Onem odnullel for the tetrapod form ation is that the in itial nucleus isform ed by eightw urtzite dom ains connected to one anothnuller via tw in boundaries, w hic
12、h is proposed as octahedralmu ltiplenulltw in m odel 9 . Another is that the tetrapod isform ed by a sphalerite tetrahedral nucleus, onto w hichwurtzite arm s ( branches) are developed by continuationof grow th from four equivalent ( 111 ) facets 4 . Th ism odelw as supported by experim ental eviden
13、ces revealingthe existence of a ZnO zinc blende core in the tetrapodnanostructures 10 . As for ZnSe tetrapods, it w as previnullously reported that in itial nucleus w as form ed by eightw urtzite dom ains connected to one another via tw inboundaries using Sn as catalyst 6 . In th is w ork, it isdem
14、onstrated that the tetrapods are form ed from sphalernullite tetrahedral nucle,i onto wh ich w urtzite arm s are denullveloped via subsequent deposition.1null ExperimentThe threenulld im ensional nanostructure m aterials w erefabricated w ith careful control of reaction cond itions,such as tem perat
15、ure, concentration, etc. The ZnSe tetranullpods w ere synthesized through therm al evaporation ofZnSe raw powders ( 99. 99%, w ith a m ean size of45nullm ) in a horizontal highnulltem perature resistance furnullnace. A graph ite crucib le con taining a m ixture of ZnSe( 2. 00g) and activated carbon
16、( 0. 12g) pow ders w as innullserted into the central zone of a quartz tube. A ll thecom ponen ts w ere enclosed in to th is tube. The furnacew as heated to 1150! at a rate of 10! /m in, and held at1150! for 4h. The whole process was carried out undera constant flow of pure N2 at a rate of 500mL /m
17、in. A fterthe furnace cooled to room tem perature, a yellownullcoloredproduct w as collected from the inner w all of the tube atthe dow nstream end, w here the deposition temperaturesare in the range of 250- 350! . The asnullprepared prodnulluct w as characterized w ith an Xnullray diffractom eter 2
18、 LIU Zhen, etal: Preparation and Characterization of ZnSe Tetrapods w ith Sphalerite Nucleus( R INT 2200), a field em ission scann ing electron m icronullscope ( SEM, JSM null6700F ) and a field em ission highresolution transm ission electron m icroscope ( HRTEM,JEM null3000F, equ ipped w ith an Xnu
19、llray Energynulldispersivespectrom eter ( EDS) ).2null Results and discussionA lownullm agn ification scanning electron m icroscope( SEM ) im age ( F ig. 1( a) ) reveals thatm ost of the obnulltained product consists of short rodnulllike nanocrystals,wh ich are com plex tetrapod branched un iform na
20、norodarchitectures, as shown in the highnullm agnification SEMim ages ( F ig. 1 ( b ), ( c) ). Each branch w ith in thesearchitectures has a hexagonal cross section and w ellnulldefined crystallograph ic facets, form ing a regular hexanullgonal prism atic structure. Typ ically, the lengths and thed
21、iam eters of the arm s are ca 70 and ca 110nm, respecnulltively. An Xnullray d iffraction pattern ( Fig. 2 ) confirm sthat these nanostructures possess both w urtzite phase andcubic phase ZnSe structures w ith the hexagonal latticeconstants ofa = 0. 3996 and c= 0. 655nm and cubic latnulltice constan
22、ts of a = 0. 5699nm, respectively.F igure 3 ( a) d isplays the lownullm agnification transnullm ission electron m icroscope ( TEM ) im age of ZnSe. Thetetrapod displays a quasinullhexagonal cross section that isform ed by the in tersection of three arm s as shown inFig. 3( b). In F ig. 3( c), the te
23、trapod appears a crossF ig. 1null ( a) Lownullm agnification SEM im age of the asnullgrown ZnSenanostructures; ( b, c) H ighnullm agn ification SEM im ages show ingtetrapodnullshaped arch itecturesF ig. 2null XRD pattern of the asnullgrown ZnSe nanostructuresarch itecture. The rodnulllike arm s have
24、 a sm ooth surface,and the four arm s are found to be structurally un iform.F igure 4( a) d isplays the in terface betw een two branchesof a ZnSe tetrapodnullshaped crysta.l The tw o shadow zonesin Fig. 4( a) are revealed in Fig. 4( b) and ( c) in denulltai.l The two branches d isplay the sam e char
25、acteristiclattice im ages w h ich stretching out along 001 direcnulltions w ith a tw in relationship ( F ig. 4 ( b ) ), show ingclearly that the lattice fringesw ith interplanar spacings of0. 65 and 0. 34nm correspond ing to the ( 001 ) and( 100) characteristic p lanes of ZnSe w urtzite structure.Th
26、e angle betw een the correspond ing p lanes of ( 001) upand ( 001) down m eeting at the tw in boundary is 70. 5,w hich is consisted w ith the angle between the ( 001)planes w ith in the # bicrystal in the SAED pattern ( insetof F ig. 4( b) ). The SAED pattern taken from the boundnullary area consist
27、s of tw o sets ( up branch and dow nbranch) of diffraction spots, each displaying the 100zone axis. In HRTEM im age Fig. 4( c), it suggests thatthe zinc b lende phase of ZnSe is present as the core ofthe tetrapod arch itectures, and show s the juncture benulltw een the zinc b lende core and w urtzit
28、e branches. TheSAED pattern recorded from the central area of these tetnullrapods ( inset of F ig. 4 ( d) ) exhibits diffraction spotsthat are indexed to the 001 zone axis pattern ofw urtznullite ZnSe branch, w hich is also indexed to the 111zone axis pattern of zinc b lende nucle.i In this directio
29、n,the diffraction spots of cubic phase appear the sam e ashexagonal phase.It is known that the angle betw een two neighboringfacets of regu lar tetrahedral zinc b lende nucleus is70. 5. Considering the sam e angle between the ( 001)planes in wurtzite branches ( Fig. 4 ( b ) ) and theHRTEM observatio
30、n in Fig. 4( c), it is ind icated that the217 25 F ig. 3null ( a) TEM im age of the tetrapodnullbranched ZnSe nanorodarch itectures; ( b, c) H ighnullm agn ification TEM images of the sameZnSe nanorod architectureZnSe is stacking along ( 111) planes of tetrahedral zincb lende core, w hich then becom
31、 e ( 001) planes ofw urtznullite branches. The trigonal im age of the nucleus throughthe section of a branch can be seen in F ig. 4( d). It canbe concluded that there is a tetrahedral zinc b lende nunullcleus inside these tetrapods. It is obviously d ifferen t fromprevious research 6 , wh ich is sin
32、gle w urtzite crystalphasew ith a octahedron core in a ZnSe tetrapod nanonullcrysta.lNucleation and grow th are usually considered as twobasic stages of the form ation of nanostructures. The renullsearchers utilized a seednullep itaxial tw onullstep route to fabrinullcate these ZnSe tetrapods. The f
33、orm ation of crystal seedsis the first step of ZnSe tetrapods nanocrystals grow th. InF ig. 4( c), the ( 111) planes of the zinc b lende nuclei isshown obviously, w hosednullspace ( 0. 33nm ) is signed inth is H RTEM im age. This suggests that the nuclei is atetrahedron term inated in ( 111) surface
34、s. The form ationof four w urtzite arm s is another important stage for th isbranchednullshaped nanostructure 11 . A fter the tetrahedralZnSe # seeds are first form ed in h igh tem perature regionnear the heating area, they are transferred downstream tothe low tem perature region by carrier gas N2.
35、As thetherm al evaporation continu ing, the new ly arrived ZnSevapor deposits on the zinc blende seeds. The form edZnSe nanocrystals shou ld bew urtzite phase in th is low ertemperature area, i. e. , epitaxial grow th of w urtzitenullFig. 4null ( a) HRTEM im age of the core and neighboring branches
36、in one ZnSe tetrapod; ( b) HRTEM im age of ZnSe tetrapodw urtzite branches ( The inset show s the correspond ing ED pattern); ( c) HRTEM image of ZnSe tetrapod sphalerite core;( d) TEM image of ZnSe tetrapod, wh ich is taken along one branch ax is d irection, i. e. the 001 or 001 d irections( The lo
37、wer right inset ED pattern recorded from the central areas of the tetrapods, the tetrahedral core can be seen fromthe section of the central branch and its illustration is shown top right)218 2 LIU Zhen, etal: Preparation and Characterization of ZnSe Tetrapods w ith Sphalerite NucleusF ig. 5null 2D
38、representation show ing the relationship betw een thezinc blende and w urtzite structures in ZnSestructured arm s w ill occur on the seeds. It is generallybelieved that the zinc blende structure is the high temnullperature stable phase wh ile wurtzite structure is the lowtem perature stab le phase f
39、or ZnSe. This explainsw hy theZnSe nuclei are form ed w ith zinc blende phase and brannullches are form ed w ith another phase in this research. A fnullter these new ly form ed tetrapods w ere deposited in lowtem perature region, four w urtzite arm s kept grow ingalong each own c axis equ ivalently
40、out of the four ( 111)equivalent faces of a tetrahedral zinc b lende nucleus, atetrapodnullshaped ZnSe is finally obtained around 300! .This form ation m echan ism of asnullgrown ZnSe tetrapod iscom parab le to the reported CdSe tetrapodnullshaped nanonullcrystals 12 .The nanostructure also has a cl
41、ose connection w ithcrystallography 13null14 . A s the ( 001) p lanes of the wurtznullite structure, the ( 111) p lanes of the zinc b lende strucnullture contain layers alternately com posed of either Zn orSe as can be seen in Fig. 5. W urtzite has ABAB stacnullk ing, w h ile zinc blende has ABCABC
42、stack ing. The tw ostructures are related by a stack ing fault ( illustrated bythe heavy black line in Fig. 5), w hich also can be obnullserved experim entally from the boundary betw een thesphalerite nuclei and w urzite branches in Fig. 4( c). Ina w ord, it is the low er tem perature and the potent
43、ial ofphase transform ation in crystallography, w h ich drives theZnSe crystal phase change from zinc b lende to w urtzite.So each branch prefers to be wurtzite rather than zincb lende w hen they grow from the sphalerite nuclei in lowtem perature region.3null ConclusionsTetrapodnullbranched ZnSe nan
44、orod arch itectures w ithsphalerite nucleus w ere synthesized by evaporationprocess. Four branches w ith w ellnulldefined regu lar hexanullgonal prism atic structure w ere obtained successfully. Todate, these tetrapods w ith sphalerite cores and wurtzitebranches has been discovered and d iscussed in
45、 m ostcomm on null null sem iconductor exceptZnSe. Based on thediscussion of the ZnSe nanocrystals m entioned above,the researchers have am p le evidences to prove that theseZnSe tetrapods have tetrahedral sphalerite nuclei andw urtzite branches. Th is work cou ld be a com plem ent forthe nullnull t
46、etrapodnullshaped sem iconductor nanostrucnulltures. The phase dependence on tem perature and thecrystallograph ic relationsh ip betw een the zinc b lende andw urtzite structures are provided to explain the grow thprocess and k inetics. These architectures are intrigu ingob jects for further exp lor
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