Structural transition of silicene on Ag(111)

Surface Science 608 (2013) 297–300

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Surface Science

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Structural transition of silicene on Ag(111)

Ryuichi Arafune a,⁎, Chun-Liang Lin b, Kazuaki Kawahara b, Noriyuki Tsukahara b, Emi Minamitani c,Yousoo Kim c, Noriaki Takagi d, Maki Kawai d

a International Center for Materials Nanoarchitectonics, National Institute for Materials, Science, 1-1 Namiki, Ibaraki 304-0044, Japanb Department of Advanced Materials Science, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japanc RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japand Department of Advanced Materials Science, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan

⁎ Corresponding author.E-mail address: [email protected] (R. Ar

0039-6028/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2012.10.022

a b s t r a c t
a r t i c l e i n f o
Article history:Received 1 September 2012Accepted 26 October 2012Available online 6 November 2012

Keywords:SiliceneLEEDSTMSurface structure

Low energy electron diffraction (LEED), low temperature scanning tunneling microscopy (STM) and densityfunctional theory (DFT) based calculations were used to determine the evolution of the silicene structure on aAg(111) surface. The phase diagram of the structure was obtained using LEED patterns. The correspondingatomic arrangements were confirmed using STM observations. Results show that the structure of silicene iscontrolled by the substrate temperature during deposition. Finally, we succeeded in synthesizing siliceneon silicene/Ag(111), i.e. bilayer silicene.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

A two-dimensional (2D) honeycomb lattice of Si is known assilicene [1–3]. Although silicene does not exist in nature, theoreticalcalculations [4–7] have shown that a free standing silicene is thermo-dynamically stable and acquires fascinating electronic properties suchas massless Dirac fermions and quantum spin Hall effects. In addition,silicene should be more compatible with current silicon-based tech-nology in comparison with graphene. Thus, silicene is one of themost promising materials for next generation devices.

Although conceptually attractive, freestanding silicene itself can-not be used for the fabrication of a device and must be placed on asolid substrate. Recently, Lalmi et al. reported on the synthesis ofsilicene on Ag(111) [8]. They described how a 2

ffiffiffi3

p� 2

ffiffiffi3

pR30∘

phase of silicene forms on the Ag(111) surface. This finding demon-strated that silicene can form on solid substrates.

Nevertheless, the geometric structure of silicene on a Ag(111)substrate remains puzzling. Recently, we reported two phases ofsilicene on Ag(111), namely, the 4×4 and

ffiffiffiffiffiffi13

p�


pR13:9∘ phases

and pointed out that the 2ffiffiffi3

p� 2

ffiffiffi3

pR30∘ phase observed by Lalmi

et al. could not be the correct phase of silicene on Ag(111) [9]. Vogtet al. [10] also argued that the result of Lalmi et al. [8] is quite unrea-sonable, and that the 4×4 structure must be one of the correct phasesof silicene on Ag(111). The controversy surrounding the correctstructural phase originates from the intrinsic flexibility of silicene.

afune).

rights reserved.

As silicene takes a buckled structure, it can exist in various phasesin contrast to graphene.

We have systematically investigated the structural evolution ofsilicene on Ag(111) using low energy electron diffraction (LEED), scan-ning tunneling microscopy (STM), and calculations based on densityfunctional theory (DFT). In addition to the 4×4 and


p�


pR13:9∘

phases already reported [9], a 4=ffiffiffi3

p� 4=

ffiffiffi3

pphase corresponding to

bilayer silicene on Ag(111) has been found. Furthermore, we pointout that the 2

ffiffiffi3

p� 2

ffiffiffi3

pR30∘ phase must instead correspond to the

3.5×3.5R26° phase obtained in this study.

2. Experimental

The synthesis of silicene on Ag(111) and geometric structural anal-ysis were performed in two independent ultra-high vacuum(UHV) sys-tems, equipped with a low temperature STM and room temperatureLEED, respectively. Ag(111) samples were cleaned through several cy-cles of Ar ion sputtering (600 eV, 15 min) and annealing (730 K,30 min) until clean Ag(111) STM images or sharp LEED p(1×1) spotswere obtained. The deposition of Si was carried out by direct heatingof a piece of Si wafer. The electric power for the heatingwas kept at con-stant during the deposition in order to maintain a constant evaporationrate. As described below, the Ag(111) surface was almost fully coveredwith 4×4 phase after the 40 minute deposition. Assuming that thesticking probability is unity, we can determine the evaporation rateto be 0.02–0.03 ML/min (1 ML=1×1015 atoms/cm2). The tempera-ture of the Ag(111) substrate during the deposition was heldconstant.

http://dx.doi.org/10.1016/j.susc.2012.10.022

mailto:[email protected]

http://dx.doi.org/10.1016/j.susc.2012.10.022

http://www.sciencedirect.com/science/journal/00396028

0.06

0.04

0.02

0.00

Hei

gh

t (

nm

)

12840Distance ( nm )

0.3

0.2

0.1

0.0

Hei

gh

t (

nm

)

20151050Distance ( nm )

0.236 nm

0.06

0.04

0.02

0.00

Hei

gh

t (

nm

)

a

b

c

4x4

4x4

√13x√13 R13.9

4x4

298 R. Arafune et al. / Surface Science 608 (2013) 297–300

3. Results and discussion

Fig. 1 summarizes the typical LEED patterns obtained, showing thestructural evolution of silicene on Ag(111) as a function of substratetemperature during Si deposition and deposition time. In the low cover-age region (for the deposition time of less than 40 min), the LEED patternshows a single 4×4 phase [Fig. 1(a)]. When the amount of evaporated Siwas increasedwith the sample temperature controlled at 250–270 °C, anadditional phase appeared, namely,


p�


pR13:9∘ [Fig. 1(b)]. Similar-

ly, the LEED pattern also changes by increasing the substrate temperatureduring the Si deposition while maintaining the amount of deposited Siatoms constant. However, in this case, the

ffiffiffi3

p�

ffiffiffi3

pR30∘ and


p�ffiffiffiffiffiffi

19p

R23:4∘ phases appear instead of theffiffiffiffiffiffi13

p�


pR13:9∘ phase.

Additionally, 3.5×3.5R26° spots appeared in the mixture of 4×4,ffiffiffi3

p�

ffiffiffi3

pR30∘, and


p�


pR23:4∘ LEED patterns [Fig. 1(c)]. The

4×4 phase of silicene is free of other phases, while other phases coexistwith each other.

We successfully observed STM images of the 4×4 andffiffiffiffiffiffi13


13p

R13:9∘ phases [9]. The formation of the 4×4 single-phase surfaceindicates that the silicene sheet extends on the Ag(111) surface uni-formly. Fig. 2(a) shows a wide-area STM image of silicene onAg(111). Under optimal conditions, silicene spreads over at least1000 nm2 without line defects. Fig. 2(b) shows an STM image of amixture of the 4×4 and


p�


pR13:9∘ phases corresponding to

Fig. 1(b). Fig. 3 shows the structure models of 4×4 andffiffiffiffiffiffi13


13p

R13:9∘ silicene on Ag(111) calculated with DFT, which reason-ably reproduce the STM results [9].

The coupling of silicene with the Ag substrate is required to deter-mine the proper structure model. We found that the exclusion of thesubstrate misleads one into unreasonable structure of the silicene onAg(111). For example, Feng et al. [11] proposed a structure model ofthe 4×4 phase based on the DFT calculations without the substrate.They assigned the “corner holes” observed in the STM image[Fig. 2(a)] to missing Si atoms, and demonstrated the structuremodel where hydrogen atoms terminate the dangling bonds causedby the absence of Si atoms. The introduction of H atoms is not realistic

150

40

0290270250

Substrate temperature ( C )

Dep

ositi

on ti

me

( m

in )

(a)

cb

ed

E=55 eV E=55 eV

E=60 eV E=60 eV

E=48 eV

Ag 4x4 √13x√13 R13.9o √3x√3 R30o

√19x√19 R23.4o 3.5x3.5 4/√3x4/√3

Fig. 1. LEED pattern of phase evolution of silicene on Ag(111). (a) The pure 4×4 phaseand (b) a mixture of the 4×4 and


p�


pR13:9∘ phases. (c) A mixture of a 4×4,ffiffiffi

3p

�ffiffiffi3

pR30∘ ,


p�


pR23:4∘ and a 3.5×3.5 phase. (d) and (e) 4=

ffiffiffi3

p� 4=

ffiffiffi3

p

phase. From (d) to (e) a halo pattern grows, which implies that the second silicenelayer is incommensurate with the Ag substrate (see text).

151050Distance ( nm )3.5x3.5 R26

Fig. 2. STM image (left) and cross sectional view (right) of phase evolution of siliceneon Ag(111). The cross section is plotted along the blue solid line. (a) Wide-area scan of4×4 phase silicene on Ag(111). The scan area is 34×20.5 nm2. The silicene single layeroverlays the atomic step of Ag(111). (b) A mixture of the 4×4 and


p�


pR13:9∘

phases. The scan area is 17×10.2 nm2. (c) A mixture of the 4×4 and 3.5×3.5R26°phases. The scan area is 17×10.2 nm2.

because H2 molecule does not adsorb on Ag(111) dissociatively atroom temperature [12,13]. The alternative model for the 4×4 phasepresented by Feng et al. is quite unreasonable. Taking into accountthe substrate, the DFT calculations provide the buckled honeycombstructure model of 4×4 phase shown in Fig. 3(a), which well repro-duces the measured STM image [9,10].

Building up the structure model of silicene from the observedLEED patterns is not easy because of structural flexibility. It is veryhelpful to evaluate whether the silicene overlayer with a reasonableSi–Si bond length is commensurate with the substrate or not. Consid-ering the 2D hexagonal lattice on the triangular lattice as shown inFig. 4, we derived two requirements that silicene characterized byn×nRφS LEED pattern is commensurate with the Ag substrate (seeAppendix A). One requirement is that the lattice constant of hexago-nal lattice aH must satisfy the following formula:

aH ¼ naTffiffiffiffiffiffiKij

q ; ð1Þ

Fig. 3. Structure model of silicene on Ag(111) (Ref. [9]). (a) 4×4 phase and (b)ffiffiffiffiffiffi13

p�


pR13:9∘ phase. The green and blue spheres represent the upper and lower Si atoms, re-

spectively. The rhombus represents the unitcell of the superstructure.

299R. Arafune et al. / Surface Science 608 (2013) 297–300

where aT is the lattice constant of the substrate triangular lattice(aT=0.289 nm for the Ag(111) surface). In Eq. (1), the double seriesKijis defined as:

Kij ¼ i2 þ j2 þ ij i; j : integer; i2 þ j2≠0� �

¼ 1;3;4;7;9;12;13;16;19; ::::ð2Þ

The other requirement is that n2 must be an integer, which iscoprime to Kij. Furthermore, the angle (φu) of the unitcell of the hex-agonal lattice with respect to the superstructure (Fig. 4) is describedas

φu ¼ tan−1ffiffiffi3

pj

2iþ j

��

��: ð3Þ

The length of the normal Si–Si double bond ranges over 0.214–0.229 nm [14] and that of the triple bond is 0.206 nm [15]. For thebulk Si crystal, the Si–Si bond length is 0.235 nm. Thus, the Si–Sibond length in silicene reasonably falls into the range of 0.21–0.23 nm. Eq. (1) together with these values yields a value for aH

Fig. 4. Two-dimensional hexagonal lattice (blue) on surface triangular lattice (gray).The solid rhombus is the unitcell of the hexagonal lattice. The dotted rhombus is thesuperstructure unitcell. a

→T ; a

→H and a

→S are unit vectors of the triangular lattice, hexag-

onal lattice, and the lattice for the superstructure, respectively (See Appendix A). φu isthe angle between the unitcell of hexagonal lattice and the superstructure. φS is theangle relative to the surface triangular lattice, which is determined from the LEEDpattern.

suitable for the LEED pattern. In addition, assuming the regularlybuckled silicene, one can determine the angle (θSi) of the Si–Si bondwith respect to the substrate plane. As an example, it is not straight-forward whether the


p�


pR13:9∘ structure is commensurate to

the Ag(111) substrate. Applying Eq. (1) to theffiffiffiffiffiffi13

p�


pR13:9∘

phase, one finds that theffiffiffiffiffiffi13

p�


pR13:9∘ phase can take commen-

surate structure with the Si–Si bond length of 0.227 nm. Startingwith this structure, we determined the structure model as shown inFig. 3(b) by DFT calculations. The


p�


pR23:4∘ phase can also

take commensurate structures with a reasonable Si–Si bond length.If we had observed a STM image, we could discuss the atomic ar-rangement of


p�


pR23:4∘ in detail.

We presume that theffiffiffi3

p�

ffiffiffi3

pR30∘ phase is highly buckled (HB)

silicene. The value of aH calculated for theffiffiffi3

p�

ffiffiffi3

pR30∘ phase with

Eq. (1) is too short. However, assuming the buckled structure, wefound the HB silicene with the bond length of 0.225 nm and θSi of50°. This structure is similar to the freestanding HB (FHB) silicenetheoretically predicted in Ref. [4]. The FHB structure is one of the sta-ble structures for the freestanding silicene. The DFT calculationsshowed that the 2×2 supercell of the FHB structure was not stable,indicating that the FHB structure can only occur under the constraintof the 1×1 hexagonal lattice [4]. However, this discussion assumesfreestanding silicene so that interfacial interactions may maintain anunstable HB structure for the

ffiffiffi3

p�

ffiffiffi3

pR30∘ phase.

One sees that the 3.5×3.5R26° phase is incommensurate with theAg(111) surface. This phase should not correspond to the silicene struc-ture. Thus, these LEED spots would originate from a Si film not formingsilicene, or may reflect a Moiré structure, as observed in graphene onthe Rh(111) substrate [16]. Fig. 2(c) shows an STM image of the3.5×3.5R26° phase. Althoughwe have not a reached the definitive con-clusion, we have surmised that the 3.5×3.5R26° phase belongs to a Sifilm not forming silicene, because the corrugation is significantly largerthan that for the 4×4 and


p�


pR13:9∘ phases [See Fig. 2(b) and

(c)]. Here, we remark that the 3.5×3.5R26° LEED pattern is very similarto that of the 2

ffiffiffi3

p� 2

ffiffiffi3

pR30∘ reported as one of the silicene struc-

tures [8,17]. If the LEED spots were diffused, these two patterns cannotbe distinguished. We judged that the 2

ffiffiffi3

p� 2

ffiffiffi3

pR30∘ phase should in-

stead correspond to the 3.5×3.5R26° phase.Recently, Jamgotchian et al. [17]. reported on the structure of

silicene on Ag(111) investigated with LEED and STM. The LEED andSTM results for the 4×4 phase are essentially identical with our re-sults. On the other hand, two types of STM images for the



13p

R13:9∘ phase were observed, while we have observed the singlephase for the


p�


pR13:9∘ phase. Moreover, they claimed that

the LEED pattern and STM images corresponding to the 2ffiffiffi3

p�

2ffiffiffi3

pR30∘ phase, which has not appeared in our work, were observed

in addition to the 4×4 andffiffiffiffiffiffi13

p�


pR13:9∘ phases. In the present

stage, we do not have a definitive explanation on the possible causesfor the inconsistency of the STM results. (As noted above, we believedthat their LEED spots corresponding to 2

ffiffiffi3

p� 2

ffiffiffi3

pR30∘ phase should

be attributed to diffused LEED spots corresponding to the 3.5×3.5phase.) Since their STM images contain many impurities, we guessthat such impurity may affect the formation of silicene on Ag(111).

image of Fig.�4

0.63 nm

0.22 nm

a b0.3

0.2

0.1

0.00 2 4 6 8 10

Distance ( nm )

Hei

ght (

nm

)

Fig. 5. (a) STM image of bilayer silicene. The first layer formsffiffiffiffiffiffi13

p�


pR13:9∘ , while

the second layer forms a 4=ffiffiffi3

p� 4=

ffiffiffi3

pphase. (b) The cross section along the arrow

indicated in (a). The periodicity in the second layer is 0.63 nm, which indicates thatthe second layer shown in (a) is consistent with the 4=

ffiffiffi3

p� 4=

ffiffiffi3

pphase.

300 R. Arafune et al. / Surface Science 608 (2013) 297–300

It would be interesting to identify the effect of the impurity or defecton the growth of silicene.

Further evaporation of Si changes the silicene structure [Fig. 1(d)and (e)]. The LEED pattern shows a 4=

ffiffiffi3

p� 4=

ffiffiffi3

pphase. By increasing

the substrate temperature or deposition time, the LEED pattern changesinto a halopattern. This indicates that the4=

ffiffiffi3

p� 4=

ffiffiffi3

pphase is incom-

mensurate with the substrate. Indeed, from the second requirement,the4=

ffiffiffi3

p� 4=

ffiffiffi3

pphase is incommensuratewith the Ag(111) substrate,

as the single layer silicene. Fig. 5(a) shows an STM image of the 4=ffiffiffi3

p�

4=ffiffiffi3

pphase. The darker region in the STM image is the



13p

R13:9∘ phase and the brighter region shows a different periodicityof 0.63 nmmatched with the 4=

ffiffiffi3

p� 4=

ffiffiffi3

punit cell. The height differ-

ence between the darker and brighter regions is 0.22 nm as shown inFig. 5(b), which is slightly different from the step height of themonoatomic step onAg(111) (0.236 nm). Although theoretical supportis necessary, the layer thickness and the six-fold symmetry of the sur-face structure revealed by the STM image strongly indicate that the4=

ffiffiffi3

p� 4=

ffiffiffi3

pphase represents a bilayer silicene structure on the

Ag(111) surface.

4. Summary

In summary, using LEED, STM, and DFT calculations, the evolutionof the silicene structure grown on the Ag(111) substrate was investi-gated. Various structural phases of silicene on Ag(111) were ob-served, the presence of which strongly depends on the substratetemperature during evaporation. We found that a second layer ofsilicene forms a two-dimensional lattice, which strongly suggeststhe formation of bilayer silicene.

Acknowledgment

This workwas partially supported by theMinistry of Education, Cul-ture, Sports, Science and Technology (MEXT) through a Grants-in-Aidfor Scientific Research (No.24241040), World Premier InternationalResearch Center Initiative (WPI), MEXT, Japan, and the National ScienceCouncil, Taiwan(No. 100-2917-I-564-022). Computation in this workwas performed using the facilities of the Supercomputer Center, theInstitute for Solid State Physics, University of Tokyo.

Appendix A

In this appendix, we derive Eq. (3). We consider three unit vector

sets of 2D lattices,a 1ð Þ→

T ; 2ð Þ,a 1ð Þ→

H ; 2ð Þ, anda 1ð Þ→

S ; 2ð Þ as shown in Fig. 4.a 1ð Þ→

T and

a 2ð Þ→

T are the unit vectors for the triangular lattice; a 1ð Þ→

H and a 2ð Þ→

H for the

triangular lattice; a 1ð Þ→

S and a 2ð Þ→

S for the lattice of the superstructure.When the hexagonal lattice is commensurate with the triangular lat-tice characterized by n×nRφS, the following equation can be written:

a 1ð Þ→

S ¼ ia 1ð Þ→

H þ ja 2ð Þ→

H ; ðA:1Þ

where i and j are integers, and i2+ j2≠0. Obviously,

a 1ð Þ→

S ¼ nR φSð Þa 1ð Þ→

T ; ðA:2Þ

where R(φ) is a matrix to describe the rotation of coordinates throughthe angle φ. Then,

a 1ð Þ→

S

��2

¼ nR ϕSð Þa 1ð Þ→

T

��2

¼ naTð Þ2

¼ ia 1ð ÞH þ ja 2ð Þ

H

��2 ¼ ia 1ð Þ

H þ jR 60∘� �a 1ð ÞH

��2

¼ i2 þ j2 þ ij� �

a2H

≡Kija2H :

ðA:3Þ

Thus, we obtain Eqs. (1) and (2). Since the angle ϕu is identical

with the angle between the two vectors, a 1ð Þ→

H and a 1ð Þ→

S , one can deduceEq. (3) using Eq. (A.3) as well.

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Structural transition of silicene on Ag(111)