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Stable Silicene in Graphene Silicene Van der Waals
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www advancedsciencenews com www advmat de,higher Si dosage multilayer silicene forms. between the graphene and the substrate,The as prepared graphene silicene hetero. structures have been exposed in ambient,conditions for two weeks and no observable. damage was found The vertical graphene,multilayer silicene Ru heterostructure shows. rectification behavior with an ideality factor,of 1 5 This work ushers the development of.
stable silicene based devices,Figure 1 illustrates the sequence of fabri. cating a graphene silicene heterostructure,First a graphene monolayer is epitaxially. grown on a Ru 0001 substrate Silicon atoms,are then deposited on top and annealed at. 900 K This result in Si intercalation and the,formation of silicene nanoflakes monolayers. and multilayers depending on the Si dosage,the amount of time the Si source is turned.
on At each Si dosage the samples are,subsequently cooled down to 5 K for STM. characterization, Figure 1 Schematic diagram of the formation of silicene structures at the graphene Ru 0001. Figure 2a shows a typical STM topog, interface The deposited Si atoms intercalate between graphene and the Ru substrate during. annealing process With a small Si dosage Si atoms form honeycomb silicene nanoflakes raphy of a Si intercalated sample for which. below the atop regions With more Si intercalation silicene monolayers and multilayers form the Si source was on for 5 min 26 28 This. small dosage of Si enables us to study the,initial structure of the Si atoms intercalated. and silicene layers are confirmed by density functional theory between graphene and Ru 0001 The periodic pattern can be. DFT calculations indicating that the fabricated structures assigned to the moir structure of graphene on Ru 0001 26. are graphene silicene van der Waals heterostructures At even However compared with the moir structure of graphene on Ru. Figure 2 Formation of silicene nanoflakes a STM topography showing the graphene Ru 0001 structure after Si intercalation Inset zoom in image. of a b c Atomic resolution images taken at the same area under different sample bias voltages 0 5 V for b and 0 1 V for c d Proposed. atomic model showing 26 Si atoms intercalated below the atop region e f Simulated STM images of the configuration in d at different sample bias. voltages 0 5 V for e and 0 1 V for f, Adv Mater 2018 1804650 1804650 2 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim.
www advancedsciencenews com www advmat de, without Si intercalation Figure S1a Supporting Information with the hexagonal Ru surface leading to pseudomorphic hon. expansion and distortion of the atop regions 29 can be clearly eycomb growth Moreover the calculations suggest that the. identified The line profile analysis Figure S1 Supporting preferential intercalation indeed results in an enhanced corru. Information shows that the rippling of the moir structure is gation of 0 85 of the graphene film Figure S5 Supporting. 0 2 larger after Si intercalation indicating a further corruga Information This corrugation is larger than the experimental. tion of the graphene film along the direction perpendicular to value 0 2 since the latter actually reflects the variation of. the substrate surface In addition a zoom in image of the inter the local density of states rather than the real height. calated sample Figure 2a inset reveals patches of a new honey In order to reproduce the STM images in Figure 2b c we. comb lattice with a nearest neighbor distance of 2 67 0 07 performed STM simulations for the model in Figure 2d The. which is significantly larger than the carbon carbon distance results with a sample bias of 0 5 V Figure 2e and 0 1 V. in the graphene lattice This new honeycomb lattice is never Figure 2f agree well with the experimental data which fur. observed on a graphene Ru 0001 sample without Si intercala ther verifies the honeycomb arrangement of the intercalated Si. tion and has not been reported in previous works 26 30 We can atoms We note that ruthenium silicide is known to form under. therefore safely conclude that the formation of the new honey certain experimental conditions The formation of ruthenium. comb lattice patches at the atop regions is related to the inter silicide gives a shoulder like feature in the X ray photoelectron. facial Si atoms The periodic formation of the patches in effect spectroscopy XPS spectrum of the Si 2p peak due to the exist. results in a novel form of intrinsically patterned 2D materials ence of Ru Si bonds 31 32 We performed XPS measurements. in the sense of ref 25 and observed a single peak of the Si 2p spectrum indicating. Figure 2b c shows the bias dependent STM images acquired no ruthenium silicide formation in this experiment Figure S6. at the same area but under different sample bias voltages Supporting Information The shape and position of the. 0 5 V for Figure 2b and 0 1 V for Figure 2c Such bias Si 2p peak suggest that the intercalated silicon atoms in a G Ru. dependent images are fully reproducible with different tips and interface are in the elemental or zero valence unoxidized. samples Examination of Figure 2b c clearly reveals the exist state 27 32 Therefore we can safely conclude that the interfacial. ence of two different periodicities in 30 rotation with respect Si honeycomb structures observed here are silicene nanoflakes. to each other The smaller honeycomb feature in Figure 2c Since both the silicene and graphene lattice can be resolved. exhibits a nearest neighbor distance of 1 50 which can be under different bias voltages we found that nearest Si Si. attributed to the intrinsic graphene lattice This feature sug distance 2 76 is larger than the previously reported values. gests that the graphene lattice remains intact in spite of the 2 20 as well as the value in bulk silicon 2 35 which. geometric change at the atop regions during the intercalation means that the silicene nanoflakes are stretched The reason is. process However the larger honeycomb feature in Figure 2b that the Si atoms prefer to reside on top of the hollow sites of. shows a nearest neighbor distance of 2 67 which is larger the Ru 0001 substrate leading to the pseudomorphic growth. than the C C bonding length and can be attributed to the of silicene nanoflakes The pseudomorphic silicene nanoflake. underlying silicon atoms A Fourier transformed image of arrays can grow up to micrometer size Figure S3 Supporting. Figure 2c clearly reveals the existence of the two sets of honey Information. comb lattices Figure S2 Supporting Information We have further observed that under high Si dosage the. An intuitive way of thinking about the emerging honeycomb silicene nanoflake features disappear After all the atop sites. pattern is that the Si atoms are confined below the atop regions have been intercalated by Si atoms further incoming Si atoms. packing into honeycomb nanoflakes Moreover by employing sequentially occupy the fcc and hcp sites as has been demon. the long range periodicity of the moir pattern of graphene as strated in a previous study 30 and an intercalation diffusion. a template a unique array of interfacial Si structures can be mechanism leads to formation of monolayer silicene Figure 3a. formed Figure S3 Supporting Information The lateral sizes shows a typical sample obtained by supplying Si for 20 min A. of these flakes are uniformly distributed 15 due to the con new structure with a periodicity of 0 7 nm is clearly imaged. finement of the overlying strained graphene film and is attributed to the structure of the interfacial Si atoms. To validate our interpretation of the experimental observa Figure 3b displays an atomic resolution image of the carbon. tions we have performed DFT calculations A supercell with lattice indicating that the graphene layer is intact The nearest. 12 12 graphene on 11 11 Ru 0001 is used and the silicon carbon carbon distance is measured to be 1 41 similar to the. atoms are placed between graphene and Ru 0001 We found value in freestanding graphene 2 indicating the decoupling of. that after six silicon atoms are intercalated they preferentially graphene from the Ru substrate 26. stay below the atop site 29 and form a hexagon at the center In order to determine the structure of the interfacial Si layer. Figure S4 Supporting Information Further incoming silicon we performed DFT calculations Indeed it is more compli. atoms extend the hexagon into a honeycomb structure with a cated to experimentally resolve the Si atoms in monolayers For. rotation angle of 30 with respect to graphene Figure S4 Sup example in other epitaxial silicene systems due to the buckling. porting Information which is consistent with the experimental and probably reconstruction of silicene on the substrates only. observations A typical relaxed structure of the atop region a fraction of the Si atoms can be clearly identified by STM 3 7 33. intercalated by 24 silicon atoms with the geometry shown in The silicene structure was determined by combining the STM. Figure S4c Supporting Information reveals a stretched hon images with DFT calculations which is a well established tech. eycomb structure with Si Si distances of 2 62 2 87 which nique Our DFT calculations show that the most stable structure. matches well the STM results 2 67 The DFT results also show of the interfacial Si is a full buckled silicene layer with a 3 3. that at low coverage these Si atoms tend to settle in registry superstructure on 7 7 Ru 0001 as shown in Figure 3c. Adv Mater 2018 1804650 1804650 3 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim. www advancedsciencenews com www advmat de,atoms beneath the atop site is lower com. pared with the fcc and hcp sites 30 In the,case of monolayers Si atoms form the com. mensurate 3 3 superstructure on 7 7,Ru 0001 Figure 3c Therefore in both. cases the graphene layer essentially plays no,role in defining the silicene symmetry which.
is consistent with the fact that graphene and,silicene monolayers interact weakly. If the Si dosage is further increased bilayer,or even multilayer silicene intercalated hetero. structures form When the number of layers of,the intercalated silicene becomes larger than. two the graphene layer becomes flat and,the moir structure disappears 26 which. makes it difficult to analyze the underlying,silicene structure by STM We performed low.
energy electron diffraction LEED analysis,to determine the structure of the multilayer. silicene as shown in Figure S7 Supporting,Information We found that as the thick. ness of silicene increases an attenuation in,the intensity of the graphene Ru 0001 moir. spots is observed which is attributed to the,decoupling of the graphene from the Ru sub. strate by the intercalated silicene layer On,the other hand the 7 7 spots of silicene.
get brighter and sharper indicating construc, Figure 3 Formation of silicene monolayers a STM image of monolayer silicene encapsulated tive addition of intensity from each layer. between graphene and Ru 0001 b Atomic resolution image showing intact carbon lattice Therefore the single layer silicene is taken as. A hexagon is used to outline the honeycomb feature The nearest carbon carbon distance is a seed for successive overlayer growth sim. measured to be 1 41 Scale bar 1 nm c Top and side views of the relaxed atomic model of ilar to the observation by Grazianetti et al on. the 7 7 Ru 0001 21 21 silicene 8 8 graphene configuration supercell is marked multilayer silicene grown on Ag 111 mica. by a red rhombus d Simulated STM image for the configuration in c. substrate 34 We performed pertinent DFT,calculations The structures of the bilayer. one supercell is marked by a dashed rhombus A slightly dis and multilayer silicene intercalated between graphene and Ru. torted honeycomb Si lattice can be clearly recognized with are shown in Figure S8 Supporting Information Both struc. Si Si distances ranging from 2 32 to 2 75 While the hon tures show a flat top Si layer and a graphene silicene distance. eycomb Si lattice cannot be directly differentiated in the STM of 3 5 suggesting a van der Waals vertical stacking We also. image due to its buckled structure bottom panel in Figure 3c calculated the projected density of states of multilayer silicene. the 7 7 structure agrees well with the 0 7 nm periodicity Figure S9 Supporting Information and found that for more. whereby each bright spot in Figure 3a corresponds to the loca than five layers except for the top two and bottom two layers. tion of a Si atom highlighted in orange in Figure 3c The large which are metallic the middle layers exhibit a gap of only. distance 2 94 between graphene and silicene suggests 0 2 eV compared with the calculated gap of 0 61 eV for bulk Si. relatively weak interactions In order to confirm our identifi the calculated values are underestimated because of the use of. cation of the silicene structure and compare with the experi an approximate exchange functional We conclude that multi. mental images an atomic model containing 7 7 Ru 0001 layer silicene is distinct from bulk Si. 21 21 silicene 8 8 graphene supercell is marked by We have found that the silicene structures in Figures 2 and. a red rhombus is constructed and simulated Figure 3d dis 3 are very stable No observable damage or change of structure. plays the STM simulation of the model which reproduces the was observed even after exposure in air for extended periods. experimental data well up to two weeks Figure S10 Supporting Information This. DFT calculations show that both the nanoflake and result supports the notion that graphene in the fabricated het. monolayer silicene structures are grown in registry with the erostructure acts as a natural protection layer of silicene against. Ru substrate Figure 2d Figure S4 Supporting Information air exposure We note that capping with a single atomic layer. and Figure 3c respectively In the case of the nanoflakes enables ex situ characterization of silicene by means of surface. intercalating Si atoms stay preferentially below the atop sites analysis tools such as STM. The reason for this preferential adsorption behavior is because The interactions between graphene and silicene were inves. the spacing between the Ru substrate and the graphene layer tigated by calculating the electron localization function ELF. at the atop site is larger and the energy required to insert Si The ELF has been demonstrated to be a useful tool to identify. Adv Mater 2018 1804650 1804650 4 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim. www advancedsciencenews com www advmat de,of silicene is estimated to be 10 by taking. account of both the Si dosage and interca,lating cycles Rectification is not observed in. the case of graphene monolayer silicene Ru,heterostructure most likely due to the strong.
bonding between monolayer silicene and the,Ru substrate The device structure multilayer. silicene intercalated between graphene and a,Ru substrate and its measurement setup are. schematically shown in the inset of Figure 4c,Linearly fitting the logarithmic plot of the. current voltage curve based on Schockley s,model an ideality factor of 1 5 is extracted. Figure 4d suggesting formation of a good,interface between graphene and silicene.
Measurements on the heterostructure at,increasing temperatures show a trivial linear. transport behavior since the elevated tem,peratures suppress the Schottky junction It. is worth noting that multilayer silicene FET,device exhibits a characteristic ambipolar. charge carrier transport behavior 34 We also,note that rectifying behavior at low tempera. ture was previously predicted in a graphene,silicene bilayer heterostructure despite the.
metallic nature of the two materials 14 For,practical applications of this unique vertical. heterostructure further work such as opti,mized device fabrication process and thick. ness control of silicene needs to be achieved,In conclusion by carefully controlling the. intercalation process between Si and gra, Figure 4 ELF calculation and transport measurement of the graphene silicene heterostruc phene we have successfully achieved dif. tures a b ELF maps of the pseudomorphic silicene nanoflake and monolayer at the plane of ferent types of silicon based nanostructures. the Si atoms Both the top view upper panels and side view lower panels images are shown pseudomorphic silicene nanoflake arrays and. The unit cell of the monolayer structure is outlined by the rhombus c Current voltage curve of continuous silicene monolayers and multi. a graphene silicene Ru vertical heterostructure measured at a temperature of 105 K showing. layers Both experimental STM characteriza, typical Schottky like rectification behavior Inset is a schematic diagram of the device structure.
and measurement setup d Logarithmic plot of the current voltage curve An ideality factor tion and DFT simulations unambiguously. of 1 5 can be extracted by fitting it with Schockley s model support the identification of the observed. structures The interaction between graphene,and silicene is similar to van der Waals layer. the chemical bonding strength from charge redistribution heterostructures as evidenced by an ELF study The demon. among atoms 7 Figure 4a b shows the ELF results for the pseu strated air stability of these structures would be useful in future. domorphic silicene nanoflake and the monolayer at the plane silicene device fabrication Measurements on the vertical het. of the Si atoms The ELF values between neighboring Si atoms erostructure indeed show a well defined Schottky rectification. are relatively large 0 63 0 75 for a nanoflake and 0 73 0 90 for behavior suggesting that the as grown graphene silicene hetero. monolayer silicene suggesting the existence of covalent bonds structures represent an emerging class of stable and functional. between Si atoms Cross section images are also provided in 2D heterostructures We note that the fabricated heterostruc. the lower panels of Figure 4a b where we can see that the ELF tures are still bonded to the metallic Ru substrate which limit. values are nearly zero between graphene and silicene The low practical applications Future research on transferring the heter. ELF values again verify the weak graphene silicene interac ostructures onto insulating substrates is necessary. tions This result suggests that the fabricated graphene silicene. structure is effectively a van der Waals heterostructure. To demonstrate the potential application of this unique Experimental Section. graphene silicene Ru heterostructure we have measured. Sample Preparation All experiments were performed in an Omicron. the current voltage characteristics of the graphene multi. low temperature STM system equipped with a sample preparation. layer silicene Ru heterostructure along the vertical direction chamber under a base pressure better than 1 0 10 10 mbar Monolayer. at a temperature of 105 K and found a typical Schottky like graphene was prepared by a well developed technique through exposure. rectification behavior as shown in Figure 4c The layer number of a Ru 0001 single crystal surface to ethylene at 1300 K 26 After that. Adv Mater 2018 1804650 1804650 5 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim. www advancedsciencenews com www advmat de, Si atoms were deposited onto the surface at room temperature by 1 K S Novoselov A K Geim S V Morozov D Jiang Y Zhang. running a current through a thin Si slice followed by annealing at 900 K S V Dubonos I V Grigorieva A A Firsov Science 2004 306. for the Si atoms to sufficiently intercalate In order to characterize the 666. growth process step by step different amounts of Si were intercalated 2 A H Castro Neto F Guinea N M R Peres K S Novoselov. between graphene and the Ru 0001 substrate by varying the deposition A K Geim Rev Mod Phys 2009 81 109. time The as grown sample was transferred to a low temperature STM 3 P Vogt P De Padova C Quaresima J Avila E Frantzeskakis. chamber for characterization All the STM images were acquired at 5 K M C Asensio A Resta B Ealet G Le Lay Phys Rev Lett 2012. DFT Calculation Theoretical calculations were performed by using 108 155501. density functional theory as implemented in the Vienna ab initio. 4 L Chen C C Liu B J Feng X Y He P Cheng Z J Ding S Meng. simulation package VASP 35 with the projector augmented wave. Y G Yao K H Wu Phys Rev Lett 2012 109 056804, PAW 36 method Local density approximation LDA 37 in the form of. 5 S X Sheng J B Wu X Cong W B Li J Gou Q Zhong, Perdew Zunger was adopted for the exchange correlation potential In. the calculation a vacuum layer of 15 was used and all Si atoms were P Cheng P H Tan L Chen K H Wu Phys Rev Lett 2017 119. relaxed until the net force on every atom is smaller than 0 01 eV 1 196803. The energy cutoff of the plane wave basis set was 400 eV and a single 6 A Fleurence R Friedlein T Ozaki H Kawai Y Wang. point was employed for Brillouin zone integrations due to computational Y Yamada Takamura Phys Rev Lett 2012 108 245501. limitations The LDA method gives an upper limit in evaluating the 7 L Meng Y L Wang L Z Zhang S X Du R T Wu L F Li. interactions between graphene and silicene Y Zhang G Li H T Zhou W A Hofer H J Gao Nano Lett 2013. Device Fabrication Graphene silicene Ru vertical heterostructure 13 685. devices were fabricated using a standard e beam lithography process 8 Z Y Ni Q H Liu K C Tang J X Zheng J Zhou R Qin. and a metal stack of Cr Au 5 50 nm as contact electrodes The I V Z X Gao D P Yu J Lu Nano Lett 2012 12 113. characteristics of the vertical heterostructures were measured in a home 9 N D Drummond V Zolyomi V I Fal ko Phys Rev B 2012 85. designed four probe UHV STM system with a cryostat using continuous 075423. liquid N2 or He flow as cooling media All electrical parameters were 10 R G Quhe R X Fei Q H Liu J X Zheng H Li C Y Xu Z Y Ni. collected using a Keithley 4200 SCS system Y Y Wang D P Yu Z X Gao J Lu Sci Rep 2012 2 853. 11 C C Liu W X Feng Y G Yao Phys Rev Lett 2011 107 076802. 12 M Ezawa Phys Rev Lett 2012 109 055502,13 A Molle C Grazianetti L Tao D Taneja M H Alam.
Supporting Information D Akinwande Chem Soc Rev 2018 47 6370. Supporting Information is available from the Wiley Online Library or 14 X F Qian Y Y Wang W B Li J Lu J Li 2D Mater 2015 2. from the author 032003, 15 L Shi T S Zhao A Xu J B Xu J Mater Chem A 2016 4 16377. 16 F Peymanirad M Neek Amal J Beheshtian F M Peeters. Acknowledgements Phys Rev B 2015 92 155113, 17 H B Shu Y L Tong J Y Guo Phys Chem Chem Phys 2017 19. G L and L Z contributed equally to this work The authors gratefully. acknowledge discussions with W D Xiao and H M Guo This work. 18 A K Geim I V Grigorieva Nature 2013 499 419, was supported by grants from National 973 projects of China. Grants No 2013CBA01600 National Key Research Development 19 G Brumfiel Nature 2013 495 152. Projects of China Nos 2016YFA0202300 and 2018FYA0305800 20 L Tao E Cinquanta D Chiappe C Grazianetti M Fanciulli. National Natural Science Foundation of China Grant Nos 61390501 M Dubey A Molle D Akinwande Nat Nanotechnol 2015 10. 61474141 and 11604373 the CAS Pioneer Hundred Talents 227. Program the Strategic Priority Research Program of Chinese 21 Y Du J C Zhuang J O Wang Z Li H Liu J Zhao X Xu. Academy of Sciences No XDB28000000 and Beijing Nova Program H Feng L Chen K Wu X Wang S X Dou Sci Adv 2016 2. No Z181100006218023 A portion of this research was performed in e1600067. CAS Key Laboratory of Vacuum Physics Work at Vanderbilt S T P and 22 A Molle C Grazianetti D Chiappe E Cinquanta E Cianci. Y Y Z was supported by the U S Department of Energy under Grant G Tallarida M Fanciulli Adv Funct Mater 2013 23 4340. No DE FG02 09ER46554 and by the McMinn Endowment Y Y Z and 23 B Kiraly A J Mannix M C Hersam N P Guisinger Chem Mater. S T P acknowledge the National Energy Research Scientific Computing 2015 27 6085. Center NERSC a DOE Office of Science User Facility supported by 24 M De Crescenzi I Berbezier M Scarselli P Castrucci. the Office of Science of the U S Department of Energy under Contract M Abbarchi A Ronda F Jardali J Park H Vach ACS Nano 2016. No DE AC02 05CH11231 10 11163, 25 X Lin J C Lu Y Shao Y Y Zhang X Wu J B Pan L Gao. S Y Zhu K Qian Y F Zhang D L Bao L F Li Y Q Wang, Conflict of Interest Z L Liu J T Sun T Lei C Liu J O Wang K Ibrahim.
D N Leonard W Zhou H M Guo Y L Wang S X Du, The authors declare no conflict of interest S T Pantelides H J Gao Nat Mater 2017 16 1209. 26 J H Mao L Huang Y Pan M Gao J F He H T Zhou,H M Guo Y Tian Q Zou L Z Zhang H G Zhang Y L Wang. S X Du X J Zhou A H Castro Neto H J Gao Appl Phys Lett. Keywords 2012 100 093101, density functional theory graphene scanning tunneling microscopy 27 Y Cui J F Gao L Jin J J Zhao D L Tan Q Fu. silicene van der Waals heterostructures X H Bao Nano Res 2012 5 352. 28 C Xia S Watcharinyanon A A Zakharov R Yakimova L Hultman. Received July 19 2018 L I Johansson C Virojanadara Phys Rev B 2012 85 045418. Revised September 4 2018 29 Y Pan H G Zhang D X Shi J T Sun S X Du F Liu H J Gao. Published online Adv Mater 2009 21 2777, Adv Mater 2018 1804650 1804650 6 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim. www advancedsciencenews com www advmat de, 30 G Li H T Zhou L D Pan Y Zhang L Huang W Y Xu S X Du 33 L Huang Y F Zhang Y Y Zhang W Y Xu Y D Que E Li.
M Ouyang A C Ferrari H J Gao J Am Chem Soc 2015 137 J B Pan Y L Wang Y Q Liu S X Du S T Pantelides H J Gao. 7099 Nano Lett 2017 17 1161, 31 S Lizzit R Larciprete P Lacovig M Dalmiglio F Orlando 34 C Grazianetti E Cinquanta L Tao P De Padova C Quaresima. A Baraldi L Gammelgaard L Barreto M Bianchi E Perkins C Ottaviani D Akinwande A Molle ACS Nano 2017 11 3376. P Hofmann Nano Lett 2012 12 4503 35 G Kresse J Furthmuller Phys Rev B 1996 54 11169. 32 L Pasquali N Mahne M Montecchi V Mattarello S Nannarone 36 P E Blochl Phys Rev B 1994 50 17953. J Appl Phys 2009 105 044304 37 J P Perdew A Zunger Phys Rev B 1981 23 5048. Adv Mater 2018 1804650 1804650 7 of 7 2018 WILEY VCH Verlag GmbH Co KGaA Weinheim.

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