Identification of active atomic defects in a monolayered tungsten disulphide nanoribbon

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1 Reeived 1 Nov 2010 Aepted 2 Fe 2011 Pulished 1 Mr 2011 DOI: /nomms1224 Identifition of tive tomi defets in monolyered tungsten disulphide nnorion Zheng Liu 1, Kzu Sueng 1, Zhiyong Wng 2, Zujin Shi 2, Eiji Okunishi 3 & Sumio Iijim 1 Edge strutures nd tomi defets n signifintly ffet the physil nd hemil properties of low-dimensionl mterils, suh s nnorions, nd therefore merit thorough investigtion t the tomi sle. Here, we suessfully disriminte single toms on monolyered tungsten disulphide nnorion y mens of time-resolved nnulr drk-field imging nd sptilly resolved eletron energy-loss spetrosopy. We unmiguously identify nd suessfully visulize in motion tomi defets, suh s vnies nd edge toms, using snning trnsmission eletron mirosopy. We lso report diret oservtion of slip deformtion in the nnorions nd present evidene demonstrting tht the deformtion proess involves the migrtion of vnies nd rerrngement of tungsten toms. Single-tom defets re suessfully oserved for the first time during plsti deformtion. 1 Nnotue Reserh Center, Ntionl Institute of Advned Industril Siene nd Tehnology (AIST), Centrl 5, Higshi 1-1-1, Tsuku, Irki , Jpn. 2 Beijing Ntionl Lortory for Moleulr Sienes, Stte Key L of Rre Erth Mterils Chemistry nd Applitions, College of Chemistry nd Moleulr Engineering, Peking University, Beijing , PR Chin. 3 Eletron Optis Division, JEOL Ltd, Tokyo , Jpn. Correspondene nd requests for mterils should e ddressed to Z.L. (emil: liu-z@ist.go.jp) or to K.S. (emil: sueng-kzu@ist.go.jp).

2 nture ommunitions DOI: /nomms1224 Nnorions, whih re qusi, one-dimensionl strutures typified y the grphene nnorion, hve ttrted n extrordinry mount of ttention for their intriguing eletroni properties. Other mterils of interest for reting nnorions inlude lyered trnsition-metl dihlogenides, suh s tungsten disulphide (WS 2 ), whih re omposed of sndwih-type si uilding loks onsisting of sheet of hexgonl losepked trnsition-metl toms etween two sheets of hexgonl lose-pked hlogen toms 1 4. Suh sndwihed lok is lwys onsidered to e monolyer, lthough it tully onsists of three sheets of toms. There re three polytypes distinguished y different stking sequenes of the lyers: trigonl symmetry, hexgonl symmetry nd rhomohedrl symmetry with single lyer, two lyers nd three lyers in the unit ell, respetively. Atoms within sheet re strongly onded euse of ovlent onds, wheres the individul sheets re stked euse of the weker vn der Wls intertion. Compred with ulk WS 2, WS 2 nnostrutures, omprising smll numer of lyers, ould exhiit unique physil properties euse of quntum onfinement effets, prtiulrly t the edges. Reently, monolyered MoS 2 nd WS 2 nnorions enpsulted in single-wlled ron nnotues (SWNTs) or supported on SWNTs were suessfully synthesized 5,6. In ddition to the edges, tomi defets n lso signifintly ffet the physil nd hemil properties of suh low-dimensionl mterils nd therefore merit thorough investigtion t the tomi sle. The reent development of diret imging nd nlytil tehniques using snning trnsmission eletron mirosope (STEM) with proe-forming errtion orretors hs provided diret ess to informtion on the lol tomi struture nd the hemil omposition t the tomi sle In prtiulr, nnulr drk-field (ADF) imging with n STEM, whih ws introdued y Crewe nd o-workers 17, is powerful tehnique for tom-y-tom identifition t high sptil resolution. ADF ws reently used to investigte the tomi strutures of oron nitride monolyer nd to determine the tomi numer of toms diretly on the sis of the intensity of their ADF signl 16. Also, exit wve phse reonstrution of thin MoS 2 lyers ws performed using n errtionorreted trnsmission eletron mirosopy (TEM) 18. However, to unmiguously identify single toms, n uthenti tom-y-tom spetrosopi tehnique, suh s eletron energy-loss spetrosopy (EELS), tht results in element-speifi signls is required. Thus, EELS hemil nlysis hs een performed on nno-sle mterils with tomi resolution 11,16 nd tomi sensitivity In this study, we report suessful EELS hemil mpping on n STEM to disriminte the W sites in monolyered WS 2 nnorion nd disply n ADF-sed visuliztion of the dynmi ehviours of single-tomi defets, llowing interprettion of the tomi proess involved in slip deformtion. Beuse of the nture of the monolyered struture, rel, single-tom defets during deformtion were oserved suessfully for the first time. Results Edge strutures of monolyered WS 2 nnorion. The nnorions used in the experiments were onfirmed to onsist of WS 2 enpsulted in SWNTs fter performing preliminry experiment using TEM nd energy dispersive X-ry (EDX) nlysis, s shown in Supplementry Figures S1 nd S2. The ross-setionl view of the nnorion onfirmed the monolyered struture: one sheet of W toms sndwihed etween two sheets of S toms, whih often exhiits twisted struture (Supplementry Fig. S2). The ADF imge of Figure 1 illustrtes the trigonl symmetry of WS 2 nnorion enpsulted inside SWNT, whih onfirmed the monolyered struture. The upper prt of Figure 1 shows n enlrged view of the retngulr region shown in Figure 1, nd the lower prt shows the orresponding simulted imge. Figure 1 displys model of the monolyered WS 2 struture with trigonl Figure 1 STEM ADF imge of monolyered WS 2 nnorion enpsulted inside SWNT. () The monolyered WS 2 nnorion imge tken in the [001] diretion. () An enlrgement of the retngle region shown in nd the orresponding simulted imge on the lower side. () Model of the monolyered WS 2 struture (view from the [001] nd [010] diretions). Sle r, 0.5 nm. symmetry out the [001] diretion. In the ADF imge, the W toms pper righter thn the S toms, s n e verified from the simulted imge (Fig. 1). The ontrst of the S toms is extremely sensitive to the nnorion orienttion euse the two S toms nnot overlp nd projet t the sme position if the nnorion is not perfetly perpendiulr to the inident em. The vrile S-tom ontrsts were systemtilly simulted in Supplementry Figure S3. Beuse the S ontrst in Figure 1 grees well with the simultion, resulting in hrdly ny mislignment, one n resonly onlude tht ll the edge toms on the upper side onsisted of S toms (s indited y the lue rrow). Notly, the edges of this monolyered WS 2 nnorion re S-terminted on one side (upper side in Fig. 1) nd W-terminted on the other side (lower side in Fig. 1), nd this struture does not hnge with the width of the monolyered WS 2 nnorion. It is interesting tht only the zigzg edges were oserved in this study. The two types of edge strutures (W- nd S-terminted edges) ontrst with the edge strutures of BN nnosheets, whih lso hve trigonl symmetry, ut only N toms pper t the edges 22. This onfigurtion of WS 2 nnorions, with oth S nd W toms t the edges, my e the reson for their peulir nd polymorphi eletroni nd mgneti hrteristis nd therefore merits thorough investigtion. STEM EELS nlysis of monolyered WS 2 nnorion. Beuse the ADF imges reorded in this study devited from the typil high-ngle nnulr drk-field (HAADF) ondition, toms ould not e simply ssigned ording to the ontrsts. To orroorte the ADF imges, EELS hemil nlysis ws neessry to expliitly disriminte the elements. Figure 2 shows set of results from the STEM EELS nlysis of monolyered WS 2 nnorion. Here, 340 EELS spetr (20 17 pixels) were reorded with onstnt step of 0.03 nm in the red retngulr region from Figure 2. Another re, mrked y the white retngle, ws used for drift ompenstion. Figure 2 shows n ADF imge tht ws reorded simultneously x y W z S x

3 nture ommunitions DOI: /nomms1224 ARTICLE W S d Figure 2 EELS hemil mp of W toms. () ADF imge of monolyered WS 2 nnorion supported y SWNT showing the spetrum imge re; the referened re mrked y the white retngle ws used for drift ompenstion. () ADF imge of the spetrum imge re. () Chemil mp of the W N-edge. Sle r, 0.5 nm. t eh pixel point, refleting preisely the tomi positions; it n e orrelted with the EELS mps. Eh spetrum ws sujeted to stndrd kground stripping, nd the edge intensities for the S L-edge nd W N-edge were independently isolted (Supplementry Fig. S4). Thus, the orresponding EELS hemil mps for these elements were onstruted. Although the signl-to-noise rtios for oth edges were suffiiently high to detet the presene of toms, the hemil mp does show onsiderle deloliztion effet. A omprison of the signl rodening in the line profiles etween the ADF imge nd the EELS mp is shown in Supplementry Figure S5. Despite the deloliztion effet, we suessfully extrted the lol mxim from the W N-edge mp to otin the W-tom positions (Fig. 2) tht perfetly oinided with the righter ontrsts in the ADF imges (Fig. 2). Exept for the speimen/em instilities, the deloliztion effet is the gretest ostle in identifying the tomi position in n EELS hemil mp 23,24. As seen in Figure 2, we were le to resolve the W W distne (0.32 nm) for the W mp despite the onsiderle deloliztion effet (Supplementry Fig. S5). However, the S hemil mp does pper lurrier, mking it rther diffiult to identify the positions of the S toms (Supplementry Fig. S6). Although lower elerting voltge (60 kv ws used) would help to redue the deloliztion of the EELS signls 21,25,26, the deloliztion effet ws estimted nd found to e more ruil for the S L-edge thn the W N-edge. However, there might e nother reson for the unertinty of the S-tom positions in the EELS hemil mp. EDX quntittive nlyses lerly identified extr S toms permeted inside the SWNTs, s shown in Supplementry Figure S7. The extr S toms were proly residul leftovers from the synthesis proedure nd oexisted in the SWNTs, whih oviously resulted in the lurry S hemil mp. Vrious tomi defets in monolyered WS 2 nnorion. Atomi defets in low-dimensionl mterils, suh s nnorions, n strongly ffet the physil properties, suh s mehnil or trnsport properties, nd hemil properties muh more thn those in ulk mterils. Therefore, n investigtion into the nture of tomi defets t the tomi sle is essentil. Vrious point defets n e indued in low-dimensionl mteril when the em urrent is intentionlly inresed. In this study, we reorded the dynmi ehviours of the tomi defets tht were in ontinuous motion during Figure 3 Single tomi defet in monolyered WS 2 nnorion. (, ) Dynmi proess of the tomi defets reted t S sites. () Edgetom loss of S nd () vny retion. () Simple W monovny nd (d) W vny with lrge distortion in the surrounding lttie. The open lue rrows indite the loss of S toms nd the red open rrows indite W vnies. Sle r, 0.2 nm. the oservtion. Figure 3 shows the tomi proess of vny retion in monolyered WS 2 nnorion. Figures 3, show two different proesses involved in knoking out S toms. Figure 3 shows the sttes efore nd fter the loss of n S tom t n edge, whih ourred within 1 s (the typil time intervl for one ADF imge sn). A monovny of n S tom ws lso reted, s shown in Figure 3, in whih the S tom nd the S vny re indited y the solid nd empty rrows, respetively. Note tht the S-vny retion does not indue lrge distortion in the WS 2 nnorion. In ontrst, the retion of W vny indues drsti struturl deformtion in the nnorions. Nnorions ontining two different W vnies re shown in Figures 3,d (mrked y red rrows). Note tht the vnies were reorded in different regions on the sme rion. The W vny in Figure 3 is simple monovny; tht is, ll the lttie points remined unhnged exept for the missing W tom. However, one n notie onsiderle elsti

4 nture ommunitions DOI: /nomms1224 Figure 4 Two sequentil ADF imges for the edge-tom migrtion. () Migrtion of W tom long the S-terminted edge. () Migrtion of W tom/vny long the W-terminted edge. The yn rrows indite single S toms. Sle r, 0.2 nm. deformtion of the nnorion, seen s smll end in the middle. The other W vny, shown in Figure 3d, is more interesting euse it indued lrge struturl deformtion. In the W vny region (indited y the open rrow), the tomi rows eme mismthed, nd dislotion ws indued, s shown y the yellow lines. Therefore, ll the lttie points surrounding the W vny eme relxed to ommodte the dislotion ore. We ll this dislotion euse in the projetion diretion, it is similr to dislotion in ordinry three-dimensionl rystls. However, it should e noted tht the dislotion here indites point defet. An S vny my lso e ssoited with this dislotion to rete more relxed struture euse one of the S lttie points seems to hve disppered. Another interesting phenomenon onerning the tomi defets in motion is the migrtion of edge toms. Two sets of ADF imges showing the migrtion of W tom nd/or vny re shown in Figure 4. This W-tom migrtion exhiited different ehviour depending on whether the edge ws W-terminted or S-terminted. Figure 4 shows n extr W-tom migrting on n S-terminted edge. In the two sequentil ADF imges reorded t 1-s intervl, W tom instntly jumped from n edge position to the next site (indited y red rrows). This W tom ws lmost re, in whih only single neighouring S site (likely two S toms) ws ville to ond. The seond exmple shows W tom jumping to n djent vny long W-terminted edge nd leving ehind nother vny s if the W tom nd vny exhnged positions (solid nd open rrows in Fig. 4). In this se, the W tom hd t lest two neighouring S sites nd would hve required greter energy to migrte thn in the previous se. Finlly, we investigted the tomi proess involved in slip deformtion, whih ws frequently oserved during the STEM oservtions of the nnorions. Generlly, slip deformtion ours in rystls tht re sujet to tensile stress lrger thn the ritil sher stress. Although most types of plsti deformtions in rystlline mterils re elieved to originte from the sliding of dislotions, the tomi proesses involved hve never een diretly oserved thus fr. The six sequentil ADF imges in Figure 5 show nnorion exhiiting slip deformtion. Only the W toms n e seen euse the S-tom positions ould not e properly identified given the short snning time (1 s per frme, wheres the snning time ws 6 s per frme in Fig. 3 nd 1.5 s per frme in Fig. 4) nd the possile defletion from the [001] diretion. In Figure 5, W vny (indited y n open rrow) nd n extr W tom t the edge ppered in the nnorion, with four tomi rows of W (indited y the yellow lines). A relxtion suddenly ourred round the vny, together with reorgniztion of the tomi rows (Fig. 5). Then, the W vny ws pushed to the edge (indited y the open rrows in Fig. 5) nd prtil slip, indited y the green nd yellow lines, ppered t the sme time. The dynmi reovery of the tomi rows my our instntly due to the slip k of the lttie mismth (Fig. 5d). Another prtil slip emerged just fter the reovery (Fig. 5e), nd the slip deformtion ws omplete when extr tomi rows ppered, indited y white rrows (Fig. 5f). Disussion Notly, there were dditionl tom-like fetures showing wek ontrsts t the edge of the WS 2 rion, s shown in Figures 3,d nd 4 nd indited y the yn rrows. Suh ontrsts were weker thn those of the two S toms. Aording to the EDX nlyses, there were no other elements inside the SWNT exept for C, O, S nd W. Contrst intensity rtios of the unknown toms to the doule S toms were mesured from the experimentl ADF imges nd ompred with the ontrst rtios of single S tom to doule S toms (S(1)/S(2)), single oxygen tom to doule S toms (O/S(2)) nd single ron tom to doule S toms (C/S(2)) lulted from the simulted ADF imge, s shown in Supplementry Figure S8. The ontrst rtio of the unknown toms to the doule S toms fits well with tht of the S(1)/S(2) from the lultion. Therefore, we n resonly onlude tht these tom-like fetures with weker ontrst re due to single S toms. The slip deformtion investigted here does not represent norml deformtion proess used y n externl sher fore, ut ws indeed generted y the dynmi movements of the tomi defets due to the inident eletron em. Suh n eletron em-indued deformtion hs lso een oserved in i-rystl of gold 27. Nevertheless, this oservtion provides deeper insights into the ehviour of tive tomi defets during deformtion proesses nd ould eventully explin how they n indue mrosopi deformtion. The low-elerting voltge used here (60 kv) ws enefiil euse it redued the proility of point defet eing indued y knok-on dmge, mking suh n in situ oservtion t the tomi level possile. We ntiipte tht further deresing the elerting voltge will lso e dvntgeous in reduing the EELS deloliztion effet, whih is urrently mjor ostle to single-tomi elementl nlysis. If this ostle n e overome, dynmi nlysis of tive point defets my eome more ommon. Methods Synthesis nd hrteriztion. SWNTs produed y the r-dishrge method were used s templtes for the frition of the WS 2 nnorions. Typilly, 5 mg

5 nture ommunitions DOI: /nomms1224 ARTICLE d e f Figure 5 A sequentil set of ADF imges depiting the tomi proess during slip deformtion. () W vny reted (0 s). () Slip deformtion (1 s) with n edge-tom migrtion. () Vny migrtion nd emergene of the slip-line (or the lttie mismth; 11 s). (d) Dynmi reovery (12 s) nd the mismth disppers. (e) Another slip-line ppers (25 s) nd then (f) the slip deformtion proess ended (26 s). Sle r, 0.2 nm. of ron nnotues (CNTs) were dded to 10 ml of sturted, queous solution of H 3 PW 12 O 40. Then, the solution ws sonited for 1 h nd refluxed for 2 h. The WS 2 nnorions were produed y heting the CNTs filled with H 3 PW 12 O 40 t 800 C in H 2 S/H 2 tmosphere for 2 h. STEM nd EELS oservtions. A JEOL 2100F (JEOL) with old field-emission gun equipped with newly designed errtion orretor (the DELTA-orretor) ws operted t 60 kv. Typilly, 0.11-nm resolution ws hieved for the STEM ADF imging. A Gtn GIF Quntum (Gtn) ws used for the EELS hemil nlyses. The inner nd outer olletion ngles for the ADF imge (β 1 nd β 2 ) were 45 mrd nd 100 mrd, respetively. The em urrent ws 10 pa for the ADF imging nd 20 pa for the EELS hemil nlysis. Referenes 1. Dikinson, R. G. & Puling, L. The rystl struture of molydenite. J. Am. Chem. So. 45, (1923). 2. Wykoff, R. G. W. Crystl Strutures (Intersiene, 1963). 3. Strnerg, H. I., Bruer, H. E. & Hughes, H. P. Eletron Spetrosopies Applied to Low-Dimensionl Mterils 41 (Springer, 2001). 4. Tenne, R., Mrguils, L., Genut, M. & Hodes, G. Polyhedrl nd ylindril strutures of tungsten disulphide. Nture 360, (1992). 5. Wng, Z. et l. Mixed low-dimensionl nnomteril: 2D ultrnrrow MoS 2 inorgni nnorions enpsulted in qusi-1d ron nnotues. J. Am. Chem. So. 132, (2010). 6. Wng, Z. et l. Ultr-nrrow WS 2 nnorions enpsulted in ron nnotues. J. Mter. Chem. 21, (2011). 7. Shit, N. et l. Oservtion of rre-erth segregtion in silion nitride ermis t sunnometre dimensions. Nture 428, (2004). 8. Bun, J. P. et l. Grin oundry strengthening in lumin y rre erth impurities. Siene 311, (2006). 9. Sueng, K. et l. Visulizing nd identifying single toms using eletron energyloss spetrosopy with low elerting voltge. Nt. Chem. 1, (2009). 10. Muller, D. A. et l. Atomi-sle hemil imging of omposition nd onding y errtion-orreted mirosopy. Siene 319, (2008). 11. Kimoto, K. et l. Element-seletive imging of tomi olumns in rystl using STEM nd EELS. Nture 450, (2007). 12. Krivnek, O. L. et l. Advnes in errtion-orreted snning trnsmission eletron mirosopy nd eletron energy-loss spetrosopy. Adv. Img. Eletron Phys. 153, (2008). 13. Pennyook, S. J. et l. Aerrtion-orreted snning trnsmission eletron mirosopy: from tomi imging nd nlysis to solving energy prolems. Phil. Trns. R. So A 367, (2009). 14. Molin, S. I. et l. Column-y-olumn ompositionl mpping y Z-ontrst imging. Ultrmirosopy 109, (2009). 15. Bosmn, M. et l. Two-dimensionl mpping of hemil informtion t tomi resolution. Phys. Rev. Lett. 99, (2007). 16. Krivnek, O. L. et l. Atom-y-tom struturl nd hemil nlysis y nnulr drk field eletron mirosopy. Nture 464, (2010). 17. Crewe, A. V., Wll, J. & Lngmore, J. Visiility of single tom. Siene 168, (1970). 18. Kisielowski, C. et l. Imging MoS 2 nnotlysts with single-tom sensitivity. Angew. Chem. Int. Ed. 49, (2010). 19. Sueng, K. et l. Element-seletive single tom imging. Siene 290, (2000). 20. Vrel, M. et l. Spetrosopi imging of single toms within ulk solid. Phys. Rev. Lett. 92, (2004). 21. Sueng, K. & Koshino, M. Atom-y-tom spetrosopy t grphene edge. Nture 468, (2010). 22. Jin, C., Lin, F., Sueng, K. & Iijim, S. Frition of freestnding oron nitride single lyer nd its defet ssignments. Phys. Rev. Lett. 102, (2009). 23. Allen, L. J., Findly, S. D., Oxley, M. P., Witte, C. & Zluze, N. J. Modelling high-resolution eletron mirosopy sed on ore-loss spetrosopy. Ultrmirosopy 106, (2006). 24. Cosgriff, E. C., Oxley, M. P., Allen, L. J. & Pennyook, S. J. The sptil resolution of imging using ore-loss spetrosopy in the snning trnsmission eletron mirosope. Ultrmirosopy 102, (2005). 25. Egerton, R. F. Eletron Energy-Loss Spetrosopy in the Eletron Mirosope (Plenum Press, 1996). 26. Muller, D. A. & Silox, J. Deloliztion in inelsti sttering. Ultrmirosopy 59, (1995). 27. Mrtin, A. V., Ishizuk, K., Kisielowski, C. & Allen, L. J. Phse imging nd the evolution of gold-vuum interfe t tomi resolution. Phys. Rev. B 74, (2006). Aknowledgments This work is supported y CREST nd Grnt-in-Aid from MEXT ( ). Z.L knowledges the prtil support y Hyshi Memoril Foundtion for Femle Nturl Sientists. Z.S nd Z.W knowledge the support y NSFC ( ) nd Ntionl 973 Projets (2006CB932701, MOST of Chin). Author ontriutions Z.W. nd Z.S. synthesized the mterils. Z.L. performed the experiments. Z.L. nd K.S. nlysed the dt. E.O. performed EDX experiment. S.I. oneived experiments. Z.L. nd K.S o-wrote the pper. All uthors disussed the results nd ommented on the mnusript. Additionl informtion Supplementry Informtion ompnies this pper t ntureommunitions Competing finnil interests: The uthors delre no ompeting finnil interests. Reprints nd permission informtion is ville online t reprintsndpermissions/ How to ite this rtile: Liu, Z. et l. Identifition of tive tomi defets in monolyered tungsten disulphide nnorion. Nt. Commun. 2:213 doi: /nomms1224 (2011).