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DOI: 10.1039/C4NR05111D
ARTICLE Novel Micro-Rings of Molybdenum Disulfide (MoS2) †
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Chao Fan,a Tao Li,b Zhongming Wei,*b Nengjie Huo,a Fangyuan Lu,a Juehan Yang,a Renxiong Li,a Shengxue Yang,a Bo Li,a Wenping Hu,*c and Jingbo Li*a Recently, molybdenum disulfide (MoS2) has become a hot-spot material due to its unique electrical, chemical properties as a potential substitute for graphene. Herein, we report a new two-step method by utilizing thermal evaporation-sulfurization to synthesize MoS2 which possesses an innovative micro-ring structure. The average statistical values of height, width and external diameter were 69 nm, 0.3 μm and 5.0 μm, respectively. Then the mechanism for the growth of such MoS2 micro-rings was proposed. A device based on the MoS2 micro-ring was prepared by electron beam lithography, and the electrical transport property was detected at different temperatures.
Introduction Since the breakthrough in the preparation of monolayered graphene, it has attracted extensive interest due to its excellent physical and chemical properties.1 However, graphene does not naturally possess a band gap. With similar layered structure, two dimensional transition metal dichalcogenides (TMDCs) have the natural band gap and are promising as suitable substitutes for graphene.2,3,4 Inside the layered structure of TMDCs, a sheet of transition metal element is sandwiched between two sheets of chalcogens with covalent interaction. Atoms between the layers interact with each other by weak interaction, normally the Vander Waals (VdW) forces. Electrical, optical and chemical properties of TMDCs are varied obviously with thickness ranging from bulk to single layer.5,6 For example, MoS2, MoSe2 and WSe2 are indirect semiconductors in a bulk form, but become direct semiconductors when they are monolayered.3,7,8 Due to their excellent properties, TMDCs have enabled wide applications, such as being used in field-effect transistors, chemical sensors and photodetectors.4,5,9 MoS2 is a hot-spot two dimensional material in the TMDCs’ family, which could be synthesized by hydrothermal method, mechanical exfoliation, chemical vapour deposition and chemical exfoliation10-14. Considering the structure dependence of 2D nanostructures of graphene, such as that zigzag-edge nanoribbons have zero energy states and always showed matallic15-18, this structure dependence is also explored into TMDCs. However as one of TMDCs, less research on different structures of MoS2 were proposed. Herein, we report the synthesis of an innovative micro-ring structure of MoS2. MoS2 micro-rings and MoO3 film (which was the precursor) were
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characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), optical microscopy (OM) and Raman spectroscopy. Meanwhile the mechanism for the growth of MoS2 micro-rings was investigated. Devices based on the MoS2 micro-rings were fabricated by electron beam lithography (EBL) and their electrical properties at different temperatures were also investigated.
Experimental Synthesis of MoS2 micro-rings MoS2 micro-rings were synthesized in two steps: thermal evaporation and sulfurization. The flow chart of whole synthesis process is shown in Fig. 1a. The thermal evaporation technique was applied in an evaporating chamber. MoO3 nanopowder (99.999% purity, Aldrich) was chosen as evaporation source. 0.001 g MoO3 powder was placed in a Molybdenum boat. The vertical distance between SiO2/Si substrate and the Molybdenum boat was kept constant at 11 cm. The system in the evaporating chamber was pumped to a final vapor pressure of 2.25×10-5 Torr, and then heated with direct current (DC) by Joule effect. Meanwhile film thickness measuring instrument was used to monitor the growth of MoO3 films. Finally, MoO3 films on SiO2/Si substrate were prepared with their thickness around 150 nm. The as-prepared MoO3 films were placed on the porcelain boat at the center of a quartz tube, and S powder (99.99% purity, Aldrich) was poured into a small quartz vessel as sulfur resource. The horizontal distance between S powder and MoO3 films was 20 cm. Initially, S powder vessel was close to the edge of the furnace, and MoO3 films were placed close to the center. The quartz tube was heated up to 680 oC at a rate of 30
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C//min. The flow w of Ar gas waas firstly very large (300 scccm) to blo ow away residu ual air in the quartz q tube wh hen temperaturre was bellow 150 oC, an nd then the flow of Ar gas was w reduced to about 21 sccm. As soo on as the centeer of the furnaace reached 6880 oC, thee quartz tube w was pushed in, placing MoO3 films at the ccenter of furnace. f At the same time th he flow of Ar gas was enlargged to 100 0 sccm. The teemperature waas remained att 680 oC for 200 min. Thiis procedure aallowed MoO3 to react with sulfur vapor. After 20 min the temp perature was raamped down and the quartzz tube was pulled out. Then ethanoll was sprayed d on the hot qquartz tub be outside the furnace to co ool down the quartz q tube quuickly, wh hich minimizeed the oxidattion of the product. p Wheen the furrnace was coolled down to ro oom temperatu ure, the producct was tak ken out of the q quartz tube. Characterization n and device fab brication Thee products aftter thermal ev vaporation and d sulfurizationn were all characterized by TEM (Hitachi H--9000NAR), AFM pping mode A AFM, Veeco Dimension D Ico on), OM (Olyympus (tap BX X51WI) and R Raman spectrroscopy (Ren nishaw). The TEM sam mples were preepared by usin ng a copper grrid to transfer the as preepared MoS2. Because on nly van der Walls W force exists bettween MoS2 an nd SiO2/Si sub bstrate, the as prepared MoS S2 can be transferred o onto Cu TEM M grids. Thee TEM data were k with a poiint-toobttained at an accceleration voltage of 200 kV, poiint resolution of 0.18 nm. The Raman measuremennt was perrformed over an accumulation time of 10 s with a 5332 nm laseer source, and d spot size off the laser is 1 μm. The deevices were fabricated w with standard electron e beam lithography (E EBL). y thermal evaaporation (5 nnm Cr Eleectrodes were deposited by folllowed by 40 n nm Au). The electrical prop perty measurem ments of the t devices weere performed d in a vacuum chamber c pumpped to abo out 7.5×10-4 Torr by a mechanical m fuel pump. Annd the cry yogenic systeem of Sum mitomo was used to make meeasurements att low temperatu ures from 293 to 13 K.
Fig g.1 (a) Diagrram of the whole w synthes sis procedure e; (b) OM M and (c) AF FM images off MoS2 micro o-rings on SiiO2/Si sub bstrate, Insett of (b) is a zoom-in OM image of th he aspre epared MoS2; TEM image es of the as-p prepared MoS S2 (d)
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at high resolution. The d100 at low magnificattion and (e) a et of (e) is the is 0..27 nm and d110 is 0. 16 nm. Inse View Article Online corre esponding SA AED pattern. DOI: 10.1039/C4NR05111D
Resu ults and disccussion OM image i of the as-prepared a prroduct is show wn in Fig. 1b. The as-prrepared MoS2 shows a miicro-ring struccture distribu uting comp pactly on SiO2/Si substrate. Due to the diffference in heiight, MoS2 micro-rings display differe rent colors. Insset of Fig. 1b is a zoom m-in image of the marked arrea in Fig. 1b,, and it obviou usly depiccts that MoS2 micro-ringss are constitu uted by different colorred homocentrric loops from m blue to gold d. The purple part repreesents SiO2 sub bstrate. Fig. 1 d and 1e show w TEM imagees at and high resollution, respecttively. Note th low magnification m hat it is no ormal MoS2 micro-rings m crrack during the t TEM sam mple prepaaration process, due to its llarge size. Th he high resolu ution TEM M image of Fiig. 1e and thhe corresponding selected area a electrron diffraction n (SAED) patttern with [100 0] zone axis (in nset of Fig. 1e) reveal the hexagonal lattice structu ure with the latttice spaciing of 0.27 an nd 0.16 nm asssigned to thee (100) and (1 110) planees. The SAED pattern also ssuggests that MoS M 2 micro-riings are polycrystalline.. AFM A image an nd Raman specctrum of the th hermal evaporaated MoO O3 film are prresented in F Fig. 2a and 2c. MoO3 has the ortho orhombic struc cture which is different from m that of MoS S2, It has 16 1 atoms per primitive p cell aand belongs to o the space grroup Pbnm m D162h. The Raman R spectrum m of MoO3 is in fine agreem ment 19, 20 with the previous articles. a T The Vibration Modes M of Ag and Bg are a identified, and four mainn peaks are laabeled in Fig. 2c. The Raman spectrrum reveals thhat the as-preepared producct is pure MoO3. As in the t 3D-AFM iimage of the as-prepared a MoO3 film shown in Fig g. 2a, MoO3 ppyramids with h different height distriibuted random mly and comppactly on the relatively ro ough surfaace. MoO3 pyrramids were stteep and contiinually conneccted with each other at the t bottom.
Fig.2 2 3D-AFM im mage of (a) th he thermal ev vaporated Mo oO3 film and (b) Mo oS2 micro-rin ng; Raman spectrum s of (c) MoO O3 film and (d) MoS2 at ttwo marked positions in (b) using g excitation of 532 nm. Inset of (d d) is a Zoom m-in imag ge of the A1g and E12g mod des.
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Jou urnal Name Fig. 2b show ws 3D-AFM im mage of a Mo oS2 micro-ringg, and Ram man spectrum m (Fig. 2d) at two positionss marked in Fiig. 2b were detected. T Two positions are different: one is on the fringe of the t MoS2 micrro-ring, and th he other is in th he center. As ccan be seeen from Fig. 2 2b, there are seeveral MoS2 dots d distribute at the cen nter of the Mo oS2 micro-ring g. The Raman spectrums usiing an exccitation waveleength of 532 nm n at two positions are pressented in Fig. 2d, and both of them m refer to the pure MoS2 w which ind dicate the com mplete sulfurizzation of MoO O3 precursor. There aree six peaks in b both Raman sp pectrums of th he MoS2 microo-ring at 177, 226, 381, 406, 455 and d 630 cm-1, in ndicating the ccrystal stru ucture of such h two parts arre monolithic. The bands off 226, 381 1 and 406 cm m-1 belong to first-order Raaman active m modes, wh hich refer to LA (M), E12g and a A1g vibratiion modes. LA A (M) preesents a longitu udinal acousticc mode.21 Thee vibration moddes of E12g e due to the vibration v of th he adjoining llayers. 2 and A1g are Thee bands of 17 77, 455 and 630 cm-1 are seecond-order R Raman acttive modes. Th he 177 and 63 30 cm-1 band can c be attribuuted to sub btraction and ccombination of o the LA (M)) frequency annd the A1gg mode. Each vibration mod de of the first-- and second- order ban nds are labeled d in Fig. 2d. The T relatively larger l peak wi dth of -1 E12g mode and A1gg (~22 cm-1) mode m suggesteed that 2 (~11 cm ) m cry ystallinity of th he MoS2 micrro-rings was sttill not perfectt. The peaaks at the fringe of the MoS S2 micro-ringss were strongeer and steeeper than thatt at the center of the MoS2 micro-rings, w which dem monstrated thaat MoS2 rings had better crystallinity thaan that of dots d at the cen nter of the Mo oS2 micro-rings. It is clear thhat the frin nge of the M MoS2 micro-rin ngs is thickerr than those MoS2 residual dots at tthe center from m Fig. 2b. Th he distance bettween E12g ge of the MoS S2 micro-rings (26.4 2 and A1g mo de at the fring cm m-1) was longerr than that at the center (24 4.5 cm-1), accoording to different thicckness of the MoS2 micro o-ring, which is in agrreement to the previous repo orts.5, 6 Statistical an nalysis for sizee parameters of MoS2 microo-rings were made based d on OM imag ges and AFM images in thee ESI. Thee values of siize parameterss were counted (Table S1, E ESI†), and d histograms o of statistical reesults for exterrnal diameter, width and d height weree given in thee ESI (Fig. S2 S †). The lenggth of extternal diameteer mainly distrributed in thee range from 44.0 to 5.5 5 μm. Statisticcal numbers specifically s co oncentrated in three ran nges: 4.9-5.0, 5.1-5.2 and 4.5-4.6 4 μm. The T width of MoS2 miccro-rings (defiined as the sub btraction of ex xternal diameteer and inteernal diameterr) distributed in the range off 0.3-0.4 μm. H Height meeasured by AF FM mainly disstributed in th he range from 50 to 100 0 nm. Accordiing to the Statiistical results of o three param meters, typ pical external d diameter, widtth and height of the MoS2 m microring g could be estiimated as 5.0 μm, 0.3 μm an nd 69 nm. In order to geet the full und derstanding off its growth pro rocess, diffferent experim mental param meters such as a time, flow w and tem mperature weree changed to investigate i thee mechanism of the Mo oS2 micro-rin ngs growth. Temperature is an impportant parrameter for thee growth of MoS M 2 micro-rin ngs, and the ggrowth tem mperature wass properly 680 0±10ºC. Wh hen the tempeerature was in the rangee, MoS2 micro o-rings were fabricated f andd have littlle change w with temperatu ure. But belo ow or abovee this tem mperature the M MoS2 micro-riings cannot bee fabricated (F Fig. S3,
Thiss journal is © Th e Royal Society o of Chemistry 201 12
ARTIICLE ) Flow of Ar gas is the vittal parameter for the growth h of ESI†). MoS2 micro-rings,, and the M MoS2 micro-rin ngs only can n be View Article Online succeessfully fabric cated when floow of ArDOI: gass10.1039/C4NR05111D was little larrger than or equal to 100 1 sccm (Figg. S3, ESI†). Reaction time is anoth her important factor in thee growth process to obtain the MoS2 micro-rings. The MoS2 micro-rings would w be burrned away y or over-reacted when thhe reaction tiime is too lo ong. However, if reactio on time is shorrt, the MoS2 micro-rings m wo ould not grew g enough.
Fig.3 3 (a) The sch hematic diag ram of the proposed p growth mechanism, with h six stages included: 1. original Mo oO3 film; 2. nucleation of pyramid ds; 3. growth h of nucleus; 4. further growth of nucleus; 5. Intersection;; 6. figuration n of micro o-rings.; (b-f)) OM images responding to each stage e of the growth g proce ess from 2 to o 6 at different reaction time (3, 5, 10, 15 and 20 min). Mechanisms M off the MoS2 mi micro-rings gro owth are propo osed after investigating the products uunder differentt reaction timee (3, 5, 10 0, 15 and 20 min), and a diaggram of the gro owth mechanisms is sho own in Fig. 3. When the vappor of S was blown b passing g the surfaace of MoO3 fiilm, pyramids in the surfacee areas with larrger heigh ht were preferred to react w with S to synth hesize MoS2 (Fig. S5, ESI E †). When the reaction w was carried out in about 3 min, m somee of MoO3 pyrramids formedd crystal nucleeus, just as sho own in Fig. 3b, where light dots in tthe OM image represent MoS M 2 and rest r parts repre esent MoO3 fillm. The reactiion was descriibed in equation1 and eq quation 2.12,22 2MoO3 + S → 2MoO 2 + SO2 (1) MoO2 + 3S MoS2 + SO2 (2) Crystal C nucleu uses distributeed randomly and the adjaccent crystal nucleuses didn’t connecct with each other in Fig. 3b.
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Wiith S vapor co ontinuing to flo ow pass the su urface of the M MoO3 film m, the crystal n nucleus began n to grow quick kly. Because M MoO3, Mo oO2 and MoS S2 belong to different spacce group and have diffferent unit-celll volume, afteer reaction of MoO M 3 with S vapor thee crystal latticees shrunk. Wiith reaction tim me increased, peaks neaar crystal nuclleus also begaan to react with S vapor raapidly duee to strong flow w of S vapor. Adjacent pyraamids cracked and it forrmed a gap bettween the crysstal nucleus an nd peaks arounnd the nuccleus. This is the key stage that it formed d original circcles of Mo oS2 micro-ring gs. It can be observed obvio ously in Fig. 33c that theere are light ccrystal nucleu uses in the center of light MoS2 ring gs. Because o of large contacct surface with h S vapor, thee sides of MoO3 pyramiids around the crystal nuclleus reacted w with S vap por faster than n other parts of o MoO3 film. And when reaaction tim me increased, several circless around som me crystal nuclleuses forrmed and greew up. Crysstal nucleusess were distriibuted ran ndomly. With the reaction was w carried ou ut at 10 min, more and d more MoS2 ccircles grew ass shown in of Fig. F 3d. With large fflow of Ar gas, g MoS2 greew rapidly annd the con ntact part to S SiO2/Si substrrate became loose l due to phase tran nsition from M MoO3 to MoS S2. The edge of o the MoS2 ccircles was blown and b burned away because b of strong S vapor’ss flow folllowing the reaaction equation n 3.22, 23 (3) Thus the gap between the crystal c nucleuss and circles aaround was enlarged. A And MoS2 circcles nearby grew g and interracted witth each other. At the same time t original crystal nucleuus was also o burned awaay and vanish hed. MoS2 ciircles had diffferent gro owth rate and some MoS2 ciircles stayed at a the junctionn at 15 min n, as shown iin Fig. 3e. In Fig. 3e the dark d part repreesents Mo oO3, the gray part at the cen nter of circless represents SiiO2/Si sub bstrate and thee light part rep presents MoS2. Fig. 3f show ws the lastt step of the ssulfurization, when reaction n time was 200 min. Enttire MoS2 circcles continued d growing and d intercrossedd with eacch other, finallly fragments of intersectio on and microo-rings were left. Fragm ments of interrsection showeed different sshapes and d were then bu urned away by y strong S vapo or’s flow. Finaally, it onlly remained M MoS2 micro-rin ngs on the SiO2/Si substrate.
Journal Naame devic ce at different temperatu ures from 29 93 to 13 K. (d) Expe erimental res sults of the cconductance e for the dev vice View Article Online unde er a bias volta age of 1 V. DOI: 10.1039/C4NR05111D A device based d on the MoS S2 micro-ring was w fabricated d by EBL,, OM image an nd AFM imagge of the device are presented in Fig. 4a and 4b. Two T gold elecctrodes were deposited d on two sides of the MoS2 micro-ring. Fiield-effect tran nsistor and photothe respo onsive characteristics weree detected. Unfortunately, U devicce didn’t display obvious fieeld-effect and photo-responsive perfo ormance. It is due to distincct property of the MoS2 miccroring in about tw wo aspects. T The main reaason is that the crystalline of the MoS M 2 micro-ri ring is not perrfect according g to the TEM, T SEAD and Raman sp spectrum discu ussed above. The otherr fiddling one is that the dim mension of wid dth and heightt are closee, and it also afffects the moddulation of the electrons’ drifft in field--effect measurrement. Eleectrical prope erty of MoS2 micro-ring was w detected in i a vacuu um chamberr, and elecctrical condu uctance at low temperatures from 293 to 13 K was detected.. The temperaature was reduced from 293 to 13 K with 10 K intervals. Fig. 4c show ws the I-V curv ves at low tem mperatures, and d Fig. 4d presents experrimental results of the curreent under a bias voltage of 1V. All I-V I curves are e linear even aat low temperrature of 13 K. K It reveaals fine Ohmic c contact of thhe device. Th he conductancee of such MoS2 micro-rring at 293 K is 3.51 nS, which shows the semicconducting trransport propeerty. The sheeet resistancee Rs (Rs=R R/S, R is the resistance annd S is the arrea of surfacee) is estim mated as 2×1 108 Ω/μm2, wh which is higheer than reportss in previious literature es.6, 11, 22 Thiss might be resulted r from the relatiive larger thic ckness of MoS S2 micro-ringss here (about 50100 nm) n comparing g to MoS2 layeer in other literatures.12, 24 Th he conductance of MoS2 m micro-rings can n match a sim mple poweer exponent eq quation G(T)= G0eAT, where G0 is the orig ginal condu uctance, A is the exponnent constantt and T is the temperature. From the fitting off curve as show wn in Fig. 4d, the results yields G0=2.734×10-122 S and A= =0.02449. When W temperature was above 223 K, current decreased steeply with w temperatures decre easing. When ttemperatures were w below 22 23 K, curreent changed slowly with relatively low values. The poten ntial property and applicatioon of the MoS S2 micro-rings are still under u investiga ation.
Con nclusions
Fig g.4 (a) OM an nd (b) AFM images of the e device base ed on a MoS M 2 micro-rring. (c) I-V curves c of the e MoS2 micro o-ring
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In su ummary, we synthesized ann innovative structure s of MoS M 2 micro o-ring through h a two-step m method. The as-prepared a M 2 MoS rings showed the average valuee of height, width w and external diam meter at 69 nm m, 0.3 μm and 5.0 μm. Ram man spectrum was meassured and the results show wed that the micro-rings were w consiisted of pure polycrystallinne MoS2. Meechanism for the grow wth of such MoS M 2 micro-rinngs was prop posed, and it was found d that the gas flow played aan important role r in the gro owth proceess. A device based on the MoS2 micro-rring was prepaared by EBL, E and its electrical e propperty was detected at different temperatures from 293 to 13 K. W When temperaatures were ab bove
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Journal Name 223 K, current decreased steeply with temperatures decreasing. The work here reveals an innovative structure of MoS2, and it develops a potential method to synthesis special structures of TMDC materials for their intentional application in optoelectronics.
ARTICLE 10 G. G. Tang, J. R. Sum, C. Wei, K. Q. Wu, X. R. Ji, S. S. Liu, H. Tang and C. S. Li, Mater. Lett., 2012, 86, 9.
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11 K. K. Liu, W. J. Zhang, Y. H. Lee, Y. C. Lin, M.10.1039/C4NR05111D T. Chang, C. Y. Su, DOI: C. S. Chang, H. Li, Y. M. Shi, H. Zhang, C. S. Lai and L. J. Li, Nano Lett., 2012, 12, 1538. 12 Y. H. Lee, X. Q. Zhang, W. J. Zhang, M. T. Chang, C. T. Lin, K. D.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No.91233120) and the National Basic Research Program of China (Grant No.2011CB921901).
Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li and T. W. Lin, Adv. Mater., 2012, 24, 2320. 13 Y. C. Lin, W. J. Zhang, J. K. Huang, K. K. Liu, C. T. Liang, C. W. Chu and L. J. Li, Nanoscale, 2012, 4, 6637. 14 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. W. Chen and M.
a
State Key Laboratory for Superlattices and Microstructures, Institute of
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Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China. b
Nano-Science Center & Department of Chemistry, University of
Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark. c
Beijing National Laboratory for Molecular Sciences, Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected], [email protected] and [email protected] † Electronic Supplementary Information (ESI) available: [OM images of MoS2 micro-rings; statistical results and histogram of external diameter, width and height of MoS2 micro-rings; OM images of MoS2 at 665 and 675 ºC, OM images of MoS2 when the flow of Ar gas was 50 sccm; OM images of the scratches parts of MoO3 films after reaction]. See DOI: 10.1039/b000000x/
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Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 10, 666. 2 K. S. Novoselov, D. Jiang, F. schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov and A. K. Geim, PNAS, 2005, 102, 10451. 3 Q. H. Wang, K. K. Zadeh, A. Kls, J. N. Coleman and M. S. Strano, Nature Nanotech., 2012, 7, 699. 4 R. M. Balleste, C. G. Navarro, J. G. Herrero and F. Zamora, Nanoscale, 2011, 3, 20. 5 H. S. Lee, S. W. Min, Y. G. Chang, M. K. Park, T. Nam, H. Kim, J. H. Kim, S. Ryu and S. Im, Nano Lett., 2012, 12, 3695. 6 K. Lee, R. Gatensby, N. Mcevoy, T. Hallam and G. S. Duesberg, Adv. Mater., 2013, 25, 6699. 7 A. Splendiani, L. Sun, Y. B. Zhang, T. S. Li, J. Kim, C. Y. Chim, G. Galli and F. Wang, Nano Lett., 2010, 10, 1271. 8 J. K. Huang, J. Pu, C. L. Hsu, M. H. Chiu, Z. Y. Juang, Y. H. Chang, W. H. Chang, Y. Iwasa, T. Takenobu and L. J. Li, ACS Nano, 2014, 8, 923. 9 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nature Nanotech., 2011, 6, 147.
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Notes and references
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DOI: 10.1039/C4NR05111D
ARTICLE A Graphica G al abstra act Novel Micro-R M Rings of M Molybd denum Disulfide D e (MoS2)
A novel nanostrructure of molybdenum m disulfide d (MoS S2) was fabriicated through h a two-step method by utilizing therrmal evaaporation and sulfurization.
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Ch hao Fan, Taoo Li, Zhongm ming Wei,* Nengjie N Huoo, Fangyuan Lu, Juehan Yang, Renxxiong Li, Sheengxue Yang, Bo o Li, Wenpinng Hu,* and Jingbo Li*
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DOI: 10.1039/C4NR05111D
ARTICLE Novel Micro-Rings of Molybdenum Disulfide (MoS2) †
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
Chao Fan,a Tao Li,b Zhongming Wei,*b Nengjie Huo,a Fangyuan Lu,a Juehan Yang,a Renxiong Li,a Shengxue Yang,a Bo Li,a Wenping Hu,*c and Jingbo Li*a Recently, molybdenum disulfide (MoS2) has become a hot-spot material due to its unique electrical, chemical properties as a potential substitute for graphene. Herein, we report a new two-step method by utilizing thermal evaporation-sulfurization to synthesize MoS2 which possesses an innovative micro-ring structure. The average statistical values of height, width and external diameter were 69 nm, 0.3 μm and 5.0 μm, respectively. Then the mechanism for the growth of such MoS2 micro-rings was proposed. A device based on the MoS2 micro-ring was prepared by electron beam lithography, and the electrical transport property was detected at different temperatures.
Introduction Since the breakthrough in the preparation of monolayered graphene, it has attracted extensive interest due to its excellent physical and chemical properties.1 However, graphene does not naturally possess a band gap. With similar layered structure, two dimensional transition metal dichalcogenides (TMDCs) have the natural band gap and are promising as suitable substitutes for graphene.2,3,4 Inside the layered structure of TMDCs, a sheet of transition metal element is sandwiched between two sheets of chalcogens with covalent interaction. Atoms between the layers interact with each other by weak interaction, normally the Vander Waals (VdW) forces. Electrical, optical and chemical properties of TMDCs are varied obviously with thickness ranging from bulk to single layer.5,6 For example, MoS2, MoSe2 and WSe2 are indirect semiconductors in a bulk form, but become direct semiconductors when they are monolayered.3,7,8 Due to their excellent properties, TMDCs have enabled wide applications, such as being used in field-effect transistors, chemical sensors and photodetectors.4,5,9 MoS2 is a hot-spot two dimensional material in the TMDCs’ family, which could be synthesized by hydrothermal method, mechanical exfoliation, chemical vapour deposition and chemical exfoliation10-14. Considering the structure dependence of 2D nanostructures of graphene, such as that zigzag-edge nanoribbons have zero energy states and always showed matallic15-18, this structure dependence is also explored into TMDCs. However as one of TMDCs, less research on different structures of MoS2 were proposed. Herein, we report the synthesis of an innovative micro-ring structure of MoS2. MoS2 micro-rings and MoO3 film (which was the precursor) were
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characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), optical microscopy (OM) and Raman spectroscopy. Meanwhile the mechanism for the growth of MoS2 micro-rings was investigated. Devices based on the MoS2 micro-rings were fabricated by electron beam lithography (EBL) and their electrical properties at different temperatures were also investigated.
Experimental Synthesis of MoS2 micro-rings MoS2 micro-rings were synthesized in two steps: thermal evaporation and sulfurization. The flow chart of whole synthesis process is shown in Fig. 1a. The thermal evaporation technique was applied in an evaporating chamber. MoO3 nanopowder (99.999% purity, Aldrich) was chosen as evaporation source. 0.001 g MoO3 powder was placed in a Molybdenum boat. The vertical distance between SiO2/Si substrate and the Molybdenum boat was kept constant at 11 cm. The system in the evaporating chamber was pumped to a final vapor pressure of 2.25×10-5 Torr, and then heated with direct current (DC) by Joule effect. Meanwhile film thickness measuring instrument was used to monitor the growth of MoO3 films. Finally, MoO3 films on SiO2/Si substrate were prepared with their thickness around 150 nm. The as-prepared MoO3 films were placed on the porcelain boat at the center of a quartz tube, and S powder (99.99% purity, Aldrich) was poured into a small quartz vessel as sulfur resource. The horizontal distance between S powder and MoO3 films was 20 cm. Initially, S powder vessel was close to the edge of the furnace, and MoO3 films were placed close to the center. The quartz tube was heated up to 680 oC at a rate of 30
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C//min. The flow w of Ar gas waas firstly very large (300 scccm) to blo ow away residu ual air in the quartz q tube wh hen temperaturre was bellow 150 oC, an nd then the flow of Ar gas was w reduced to about 21 sccm. As soo on as the centeer of the furnaace reached 6880 oC, thee quartz tube w was pushed in, placing MoO3 films at the ccenter of furnace. f At the same time th he flow of Ar gas was enlargged to 100 0 sccm. The teemperature waas remained att 680 oC for 200 min. Thiis procedure aallowed MoO3 to react with sulfur vapor. After 20 min the temp perature was raamped down and the quartzz tube was pulled out. Then ethanoll was sprayed d on the hot qquartz tub be outside the furnace to co ool down the quartz q tube quuickly, wh hich minimizeed the oxidattion of the product. p Wheen the furrnace was coolled down to ro oom temperatu ure, the producct was tak ken out of the q quartz tube. Characterization n and device fab brication Thee products aftter thermal ev vaporation and d sulfurizationn were all characterized by TEM (Hitachi H--9000NAR), AFM pping mode A AFM, Veeco Dimension D Ico on), OM (Olyympus (tap BX X51WI) and R Raman spectrroscopy (Ren nishaw). The TEM sam mples were preepared by usin ng a copper grrid to transfer the as preepared MoS2. Because on nly van der Walls W force exists bettween MoS2 an nd SiO2/Si sub bstrate, the as prepared MoS S2 can be transferred o onto Cu TEM M grids. Thee TEM data were k with a poiint-toobttained at an accceleration voltage of 200 kV, poiint resolution of 0.18 nm. The Raman measuremennt was perrformed over an accumulation time of 10 s with a 5332 nm laseer source, and d spot size off the laser is 1 μm. The deevices were fabricated w with standard electron e beam lithography (E EBL). y thermal evaaporation (5 nnm Cr Eleectrodes were deposited by folllowed by 40 n nm Au). The electrical prop perty measurem ments of the t devices weere performed d in a vacuum chamber c pumpped to abo out 7.5×10-4 Torr by a mechanical m fuel pump. Annd the cry yogenic systeem of Sum mitomo was used to make meeasurements att low temperatu ures from 293 to 13 K.
Fig g.1 (a) Diagrram of the whole w synthes sis procedure e; (b) OM M and (c) AF FM images off MoS2 micro o-rings on SiiO2/Si sub bstrate, Insett of (b) is a zoom-in OM image of th he aspre epared MoS2; TEM image es of the as-p prepared MoS S2 (d)
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at high resolution. The d100 at low magnificattion and (e) a et of (e) is the is 0..27 nm and d110 is 0. 16 nm. Inse View Article Online corre esponding SA AED pattern. DOI: 10.1039/C4NR05111D
Resu ults and disccussion OM image i of the as-prepared a prroduct is show wn in Fig. 1b. The as-prrepared MoS2 shows a miicro-ring struccture distribu uting comp pactly on SiO2/Si substrate. Due to the diffference in heiight, MoS2 micro-rings display differe rent colors. Insset of Fig. 1b is a zoom m-in image of the marked arrea in Fig. 1b,, and it obviou usly depiccts that MoS2 micro-ringss are constitu uted by different colorred homocentrric loops from m blue to gold d. The purple part repreesents SiO2 sub bstrate. Fig. 1 d and 1e show w TEM imagees at and high resollution, respecttively. Note th low magnification m hat it is no ormal MoS2 micro-rings m crrack during the t TEM sam mple prepaaration process, due to its llarge size. Th he high resolu ution TEM M image of Fiig. 1e and thhe corresponding selected area a electrron diffraction n (SAED) patttern with [100 0] zone axis (in nset of Fig. 1e) reveal the hexagonal lattice structu ure with the latttice spaciing of 0.27 an nd 0.16 nm asssigned to thee (100) and (1 110) planees. The SAED pattern also ssuggests that MoS M 2 micro-riings are polycrystalline.. AFM A image an nd Raman specctrum of the th hermal evaporaated MoO O3 film are prresented in F Fig. 2a and 2c. MoO3 has the ortho orhombic struc cture which is different from m that of MoS S2, It has 16 1 atoms per primitive p cell aand belongs to o the space grroup Pbnm m D162h. The Raman R spectrum m of MoO3 is in fine agreem ment 19, 20 with the previous articles. a T The Vibration Modes M of Ag and Bg are a identified, and four mainn peaks are laabeled in Fig. 2c. The Raman spectrrum reveals thhat the as-preepared producct is pure MoO3. As in the t 3D-AFM iimage of the as-prepared a MoO3 film shown in Fig g. 2a, MoO3 ppyramids with h different height distriibuted random mly and comppactly on the relatively ro ough surfaace. MoO3 pyrramids were stteep and contiinually conneccted with each other at the t bottom.
Fig.2 2 3D-AFM im mage of (a) th he thermal ev vaporated Mo oO3 film and (b) Mo oS2 micro-rin ng; Raman spectrum s of (c) MoO O3 film and (d) MoS2 at ttwo marked positions in (b) using g excitation of 532 nm. Inset of (d d) is a Zoom m-in imag ge of the A1g and E12g mod des.
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Jou urnal Name Fig. 2b show ws 3D-AFM im mage of a Mo oS2 micro-ringg, and Ram man spectrum m (Fig. 2d) at two positionss marked in Fiig. 2b were detected. T Two positions are different: one is on the fringe of the t MoS2 micrro-ring, and th he other is in th he center. As ccan be seeen from Fig. 2 2b, there are seeveral MoS2 dots d distribute at the cen nter of the Mo oS2 micro-ring g. The Raman spectrums usiing an exccitation waveleength of 532 nm n at two positions are pressented in Fig. 2d, and both of them m refer to the pure MoS2 w which ind dicate the com mplete sulfurizzation of MoO O3 precursor. There aree six peaks in b both Raman sp pectrums of th he MoS2 microo-ring at 177, 226, 381, 406, 455 and d 630 cm-1, in ndicating the ccrystal stru ucture of such h two parts arre monolithic. The bands off 226, 381 1 and 406 cm m-1 belong to first-order Raaman active m modes, wh hich refer to LA (M), E12g and a A1g vibratiion modes. LA A (M) preesents a longitu udinal acousticc mode.21 Thee vibration moddes of E12g e due to the vibration v of th he adjoining llayers. 2 and A1g are Thee bands of 17 77, 455 and 630 cm-1 are seecond-order R Raman acttive modes. Th he 177 and 63 30 cm-1 band can c be attribuuted to sub btraction and ccombination of o the LA (M)) frequency annd the A1gg mode. Each vibration mod de of the first-- and second- order ban nds are labeled d in Fig. 2d. The T relatively larger l peak wi dth of -1 E12g mode and A1gg (~22 cm-1) mode m suggesteed that 2 (~11 cm ) m cry ystallinity of th he MoS2 micrro-rings was sttill not perfectt. The peaaks at the fringe of the MoS S2 micro-ringss were strongeer and steeeper than thatt at the center of the MoS2 micro-rings, w which dem monstrated thaat MoS2 rings had better crystallinity thaan that of dots d at the cen nter of the Mo oS2 micro-rings. It is clear thhat the frin nge of the M MoS2 micro-rin ngs is thickerr than those MoS2 residual dots at tthe center from m Fig. 2b. Th he distance bettween E12g ge of the MoS S2 micro-rings (26.4 2 and A1g mo de at the fring cm m-1) was longerr than that at the center (24 4.5 cm-1), accoording to different thicckness of the MoS2 micro o-ring, which is in agrreement to the previous repo orts.5, 6 Statistical an nalysis for sizee parameters of MoS2 microo-rings were made based d on OM imag ges and AFM images in thee ESI. Thee values of siize parameterss were counted (Table S1, E ESI†), and d histograms o of statistical reesults for exterrnal diameter, width and d height weree given in thee ESI (Fig. S2 S †). The lenggth of extternal diameteer mainly distrributed in thee range from 44.0 to 5.5 5 μm. Statisticcal numbers specifically s co oncentrated in three ran nges: 4.9-5.0, 5.1-5.2 and 4.5-4.6 4 μm. The T width of MoS2 miccro-rings (defiined as the sub btraction of ex xternal diameteer and inteernal diameterr) distributed in the range off 0.3-0.4 μm. H Height meeasured by AF FM mainly disstributed in th he range from 50 to 100 0 nm. Accordiing to the Statiistical results of o three param meters, typ pical external d diameter, widtth and height of the MoS2 m microring g could be estiimated as 5.0 μm, 0.3 μm an nd 69 nm. In order to geet the full und derstanding off its growth pro rocess, diffferent experim mental param meters such as a time, flow w and tem mperature weree changed to investigate i thee mechanism of the Mo oS2 micro-rin ngs growth. Temperature is an impportant parrameter for thee growth of MoS M 2 micro-rin ngs, and the ggrowth tem mperature wass properly 680 0±10ºC. Wh hen the tempeerature was in the rangee, MoS2 micro o-rings were fabricated f andd have littlle change w with temperatu ure. But belo ow or abovee this tem mperature the M MoS2 micro-riings cannot bee fabricated (F Fig. S3,
Thiss journal is © Th e Royal Society o of Chemistry 201 12
ARTIICLE ) Flow of Ar gas is the vittal parameter for the growth h of ESI†). MoS2 micro-rings,, and the M MoS2 micro-rin ngs only can n be View Article Online succeessfully fabric cated when floow of ArDOI: gass10.1039/C4NR05111D was little larrger than or equal to 100 1 sccm (Figg. S3, ESI†). Reaction time is anoth her important factor in thee growth process to obtain the MoS2 micro-rings. The MoS2 micro-rings would w be burrned away y or over-reacted when thhe reaction tiime is too lo ong. However, if reactio on time is shorrt, the MoS2 micro-rings m wo ould not grew g enough.
Fig.3 3 (a) The sch hematic diag ram of the proposed p growth mechanism, with h six stages included: 1. original Mo oO3 film; 2. nucleation of pyramid ds; 3. growth h of nucleus; 4. further growth of nucleus; 5. Intersection;; 6. figuration n of micro o-rings.; (b-f)) OM images responding to each stage e of the growth g proce ess from 2 to o 6 at different reaction time (3, 5, 10, 15 and 20 min). Mechanisms M off the MoS2 mi micro-rings gro owth are propo osed after investigating the products uunder differentt reaction timee (3, 5, 10 0, 15 and 20 min), and a diaggram of the gro owth mechanisms is sho own in Fig. 3. When the vappor of S was blown b passing g the surfaace of MoO3 fiilm, pyramids in the surfacee areas with larrger heigh ht were preferred to react w with S to synth hesize MoS2 (Fig. S5, ESI E †). When the reaction w was carried out in about 3 min, m somee of MoO3 pyrramids formedd crystal nucleeus, just as sho own in Fig. 3b, where light dots in tthe OM image represent MoS M 2 and rest r parts repre esent MoO3 fillm. The reactiion was descriibed in equation1 and eq quation 2.12,22 2MoO3 + S → 2MoO 2 + SO2 (1) MoO2 + 3S MoS2 + SO2 (2) Crystal C nucleu uses distributeed randomly and the adjaccent crystal nucleuses didn’t connecct with each other in Fig. 3b.
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Wiith S vapor co ontinuing to flo ow pass the su urface of the M MoO3 film m, the crystal n nucleus began n to grow quick kly. Because M MoO3, Mo oO2 and MoS S2 belong to different spacce group and have diffferent unit-celll volume, afteer reaction of MoO M 3 with S vapor thee crystal latticees shrunk. Wiith reaction tim me increased, peaks neaar crystal nuclleus also begaan to react with S vapor raapidly duee to strong flow w of S vapor. Adjacent pyraamids cracked and it forrmed a gap bettween the crysstal nucleus an nd peaks arounnd the nuccleus. This is the key stage that it formed d original circcles of Mo oS2 micro-ring gs. It can be observed obvio ously in Fig. 33c that theere are light ccrystal nucleu uses in the center of light MoS2 ring gs. Because o of large contacct surface with h S vapor, thee sides of MoO3 pyramiids around the crystal nuclleus reacted w with S vap por faster than n other parts of o MoO3 film. And when reaaction tim me increased, several circless around som me crystal nuclleuses forrmed and greew up. Crysstal nucleusess were distriibuted ran ndomly. With the reaction was w carried ou ut at 10 min, more and d more MoS2 ccircles grew ass shown in of Fig. F 3d. With large fflow of Ar gas, g MoS2 greew rapidly annd the con ntact part to S SiO2/Si substrrate became loose l due to phase tran nsition from M MoO3 to MoS S2. The edge of o the MoS2 ccircles was blown and b burned away because b of strong S vapor’ss flow folllowing the reaaction equation n 3.22, 23 (3) Thus the gap between the crystal c nucleuss and circles aaround was enlarged. A And MoS2 circcles nearby grew g and interracted witth each other. At the same time t original crystal nucleuus was also o burned awaay and vanish hed. MoS2 ciircles had diffferent gro owth rate and some MoS2 ciircles stayed at a the junctionn at 15 min n, as shown iin Fig. 3e. In Fig. 3e the dark d part repreesents Mo oO3, the gray part at the cen nter of circless represents SiiO2/Si sub bstrate and thee light part rep presents MoS2. Fig. 3f show ws the lastt step of the ssulfurization, when reaction n time was 200 min. Enttire MoS2 circcles continued d growing and d intercrossedd with eacch other, finallly fragments of intersectio on and microo-rings were left. Fragm ments of interrsection showeed different sshapes and d were then bu urned away by y strong S vapo or’s flow. Finaally, it onlly remained M MoS2 micro-rin ngs on the SiO2/Si substrate.
Journal Naame devic ce at different temperatu ures from 29 93 to 13 K. (d) Expe erimental res sults of the cconductance e for the dev vice View Article Online unde er a bias volta age of 1 V. DOI: 10.1039/C4NR05111D A device based d on the MoS S2 micro-ring was w fabricated d by EBL,, OM image an nd AFM imagge of the device are presented in Fig. 4a and 4b. Two T gold elecctrodes were deposited d on two sides of the MoS2 micro-ring. Fiield-effect tran nsistor and photothe respo onsive characteristics weree detected. Unfortunately, U devicce didn’t display obvious fieeld-effect and photo-responsive perfo ormance. It is due to distincct property of the MoS2 miccroring in about tw wo aspects. T The main reaason is that the crystalline of the MoS M 2 micro-ri ring is not perrfect according g to the TEM, T SEAD and Raman sp spectrum discu ussed above. The otherr fiddling one is that the dim mension of wid dth and heightt are closee, and it also afffects the moddulation of the electrons’ drifft in field--effect measurrement. Eleectrical prope erty of MoS2 micro-ring was w detected in i a vacuu um chamberr, and elecctrical condu uctance at low temperatures from 293 to 13 K was detected.. The temperaature was reduced from 293 to 13 K with 10 K intervals. Fig. 4c show ws the I-V curv ves at low tem mperatures, and d Fig. 4d presents experrimental results of the curreent under a bias voltage of 1V. All I-V I curves are e linear even aat low temperrature of 13 K. K It reveaals fine Ohmic c contact of thhe device. Th he conductancee of such MoS2 micro-rring at 293 K is 3.51 nS, which shows the semicconducting trransport propeerty. The sheeet resistancee Rs (Rs=R R/S, R is the resistance annd S is the arrea of surfacee) is estim mated as 2×1 108 Ω/μm2, wh which is higheer than reportss in previious literature es.6, 11, 22 Thiss might be resulted r from the relatiive larger thic ckness of MoS S2 micro-ringss here (about 50100 nm) n comparing g to MoS2 layeer in other literatures.12, 24 Th he conductance of MoS2 m micro-rings can n match a sim mple poweer exponent eq quation G(T)= G0eAT, where G0 is the orig ginal condu uctance, A is the exponnent constantt and T is the temperature. From the fitting off curve as show wn in Fig. 4d, the results yields G0=2.734×10-122 S and A= =0.02449. When W temperature was above 223 K, current decreased steeply with w temperatures decre easing. When ttemperatures were w below 22 23 K, curreent changed slowly with relatively low values. The poten ntial property and applicatioon of the MoS S2 micro-rings are still under u investiga ation.
Con nclusions
Fig g.4 (a) OM an nd (b) AFM images of the e device base ed on a MoS M 2 micro-rring. (c) I-V curves c of the e MoS2 micro o-ring
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In su ummary, we synthesized ann innovative structure s of MoS M 2 micro o-ring through h a two-step m method. The as-prepared a M 2 MoS rings showed the average valuee of height, width w and external diam meter at 69 nm m, 0.3 μm and 5.0 μm. Ram man spectrum was meassured and the results show wed that the micro-rings were w consiisted of pure polycrystallinne MoS2. Meechanism for the grow wth of such MoS M 2 micro-rinngs was prop posed, and it was found d that the gas flow played aan important role r in the gro owth proceess. A device based on the MoS2 micro-rring was prepaared by EBL, E and its electrical e propperty was detected at different temperatures from 293 to 13 K. W When temperaatures were ab bove
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Journal Name 223 K, current decreased steeply with temperatures decreasing. The work here reveals an innovative structure of MoS2, and it develops a potential method to synthesis special structures of TMDC materials for their intentional application in optoelectronics.
ARTICLE 10 G. G. Tang, J. R. Sum, C. Wei, K. Q. Wu, X. R. Ji, S. S. Liu, H. Tang and C. S. Li, Mater. Lett., 2012, 86, 9.
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11 K. K. Liu, W. J. Zhang, Y. H. Lee, Y. C. Lin, M.10.1039/C4NR05111D T. Chang, C. Y. Su, DOI: C. S. Chang, H. Li, Y. M. Shi, H. Zhang, C. S. Lai and L. J. Li, Nano Lett., 2012, 12, 1538. 12 Y. H. Lee, X. Q. Zhang, W. J. Zhang, M. T. Chang, C. T. Lin, K. D.
Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant No.91233120) and the National Basic Research Program of China (Grant No.2011CB921901).
Chang, Y. C. Yu, J. T. W. Wang, C. S. Chang, L. J. Li and T. W. Lin, Adv. Mater., 2012, 24, 2320. 13 Y. C. Lin, W. J. Zhang, J. K. Huang, K. K. Liu, C. T. Liang, C. W. Chu and L. J. Li, Nanoscale, 2012, 4, 6637. 14 G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. W. Chen and M.
a
State Key Laboratory for Superlattices and Microstructures, Institute of
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Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China. b
Nano-Science Center & Department of Chemistry, University of
Copenhagen, Universitetsparken 5, DK-2100 Copenhagen Ø, Denmark. c
Beijing National Laboratory for Molecular Sciences, Key Laboratory of
Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected], [email protected] and [email protected] † Electronic Supplementary Information (ESI) available: [OM images of MoS2 micro-rings; statistical results and histogram of external diameter, width and height of MoS2 micro-rings; OM images of MoS2 at 665 and 675 ºC, OM images of MoS2 when the flow of Ar gas was 50 sccm; OM images of the scratches parts of MoO3 films after reaction]. See DOI: 10.1039/b000000x/
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This journal is © The Royal Society of Chemistry 2012
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DOI: 10.1039/C4NR05111D
ARTICLE A Graphica G al abstra act Novel Micro-R M Rings of M Molybd denum Disulfide D e (MoS2)
A novel nanostrructure of molybdenum m disulfide d (MoS S2) was fabriicated through h a two-step method by utilizing therrmal evaaporation and sulfurization.
Thiss journal is © Th e Royal Society o of Chemistry 201 13
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Published on 13 October 2014. Downloaded by Temple University on 30/10/2014 14:19:31.
Ch hao Fan, Taoo Li, Zhongm ming Wei,* Nengjie N Huoo, Fangyuan Lu, Juehan Yang, Renxxiong Li, Sheengxue Yang, Bo o Li, Wenpinng Hu,* and Jingbo Li*
