Letter pubs.acs.org/NanoLett
Multilayer Silicene Nanoribbons Paola De Padova,*,† Osamu Kubo,‡ Bruno Olivieri,§ Claudio Quaresima,† Tomonobu Nakayama,‡,∥ Masakazu Aono,‡ and Guy Le Lay*,⊥ †
Consiglio Nazionale delle Ricerche -ISM, via Fosso del Cavaliere, 00133 Roma, Italy International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § Consiglio Nazionale delle Ricerche-ISAC, via Fosso del Cavaliere, 00133 Roma, Italy ∥ Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ⊥ Aix-Marseille University, CNRS-CINaM, Campus de Luminy, Case 913, 13288 Marseille Cedex 9, France ‡
S Supporting Information *
ABSTRACT: The synthesis of silicene, graphene-like silicon, has generated very strong interest. Here, we reveal the growth of high aspect ratio, perfectly straight, and aligned silicon nanoribbons, exhibiting pyramidal cross section. They are multistacks of silicene and show in angle-resolved photoemission cone-like dispersion of their π and π* bands, at the X̅ point of their one-dimensional Brillouin zone, with Fermi velocity of ∼1.3 × 106 m sec−1, which is very promising for potential applications.
KEYWORDS: Silicene multi stacks, Dirac fermions, STM, ARPES
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the X̅ points for the SiNRs. This implies that the quasiparticles in either 1D or 2D epitaxial silicene behave at room temperature as massless relativistic Dirac fermions. This is, indeed, of key importance for potential applications, and, nowadays, silicene is recognized as a potential device alternative to graphene.14 Even before graphene was conjectured, theoretical calculations15,16 demonstrated the possible existence of silicon multilayers stacked in a graphite-like structure. Here we show such a realization upon growing and characterizing multilayer-thick, high aspect ratio, straight, all parallel, silicon nanoribbons formed on the Ag(110) surface; as we will show, they are stacks of atom-thin silicene nanoribbons piled up as graphite with a pyramidal cross-section. We followed their growth by reflection high-energy electron diffraction (RHEED), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). We observed these multilayer-thick SiNRs by scanning tunnelling microscopy and measured by ARPES π and π* bands yielding Dirac cones at the 1D Brillouin zone extremities. RHEED and STM observations were carried out at the International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),
ilicene was recently highlighted as graphene’s silicon cousin.1 The discovery of this new allotrope of silicon, a one-atom thin silicon sheet arranged in a honeycomb lattice similar to graphene,2 has generated a strong interest. The signatures of the synthesis of silicene on Ag(111) surfaces have been given by STM observations revealing the silicon honeycomb structure as in graphene and angle-resolved photoemission spectroscopy measurements demonstrating the presence of cones related to π-orbitals at the K̅ points of the silicene Brillouin zone, pointing to the presence of Dirac fermions near the Fermi level.2 For silicon, usually sp3 hybridized, this implies to adopt an unusual and rare sp2-like hybridization, which could provide important physical properties. The possible existence of silicene was first theoretically conjectured.3 Subsequently a silicene-type arrangement was synthetized4,5 and calculated6 as a possible candidate for the atomic structure of one-dimensional (1D) silicon nanoribbons (SiNRs) grown in a massively parallel array on the anisotropic Ag(110) surface.7−10 The very low reactivity to oxygen11 of the 5 × 2/5 × 4 SiNR grating further compares favorably with graphene and the sp2-like hybridization12 of the Si−Si bonds in this dense array of SiNRs. In both cases, whether silicene nanoribbons or twodimensional (2D) sheets, angle-resolved photoemission spectroscopy (ARPES),2,13 has shown the presence of π (and eventually π*) bands with conelike dispersions at the corners of the Brillouin zone, i.e., at the K̅ silicene points for 2D silicene or at © 2012 American Chemical Society
Received: June 13, 2012 Revised: October 10, 2012 Published: October 11, 2012 5500
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Figure 1. (a) STM image of an ensemble of multilayer-thick SiNRs on Ag(110) (400 × 400 nm2 ; V = −0.6 V; I = 1 nA); (b) STM 3D enlarged view of multilayer-thick SiNRs (30 × 20 nm2 ; V = 0.2 V; I = 0.1 nA); (c) P1 line profile along the [100] Ag direction.
the growth mode of Stranski-Krastanov type derived from AES measurements13,18 (see Supporting Information). These multilayer-thick SiNRs, as shown in Figure 1b,c, are regular stacks of silicon layers with a pyramidal cross section. All successive layers correspond to step heights of h ∼ 0.29 nm. This value is about 50% larger than the height of the atom-thin SiNRs on Ag(110) (∼0.2 nm), which is in reasonable agreement with that theoretically predicted for the optimized “graphitic” Si structure with lattice parameters a0 = 3.86 Å and c = 6.68 Å.16 Remarkably, the Si atoms on the top layer of these multilayer-thick SiNRs are organized in the same fashion, as those within the atom-thin SiNRs located at their bottom: the dashed ovals evidence the same 5 × 4 internal structure. This key observation is in direct line with the LEED and RHEED patterns, shown in Figure 2a,b; the coherence lengths in LEED/RHEED ensure that the 5 × 4 structure is not a local reconstruction but that it extends to the mesoscopic scale all over the sample surface. ARPES measurements from these multilayer-thick 5 × 4 SiNRs taken along their lengths, the Ag(110) [1̅10] direction (Γ̅ → X̅ direction), have given the E(k//) band dispersions along the SiNRs, as shown in Figure 3 a. It highlights two types of linearly dispersing bands, one with a Λ-like shape (bottom) and one with a V-like shape crossing the Fermi level (top), separated by a small gap of about 0.14 eV around 0.72 eV below EF. These bands located in a gap of the Ag(110) surface and centered at k// = 1.087 Å−1, that is, exactly at the X̅ point of the 1D Brillouin zone for atom-thin SiNRs, correspond to the 1D projection of π* (upper bands) and π (lower bands) Dirac cones in silicene.13 To visualize this small gap better, Figure 3b,c shows an enlargement of the dispersion image reported in Figure 3a and the energy distribution curve
(Tsukuba, Japan). The STM images were recorded at RT in constant-current mode at a bias voltage from ±0.03 to ±3 V and a tunneling current from 0.03 to 3 nA. ARPES data have been taken at the VUV beamline of the Italian synchrotron radiation facility (ELETTRA, Trieste, Italy) with energy resolution better than 50 meV. The photoemission spectra were acquired at RT by using the angular resolved mode of the electron energy analyzer with an acceptance of ∼30° (SCIENTA R4000). The photon energy was set to 126 eV. Auger spectroscopy and LEED have been performed at CNRISM (Roma, Italy).17 In these laboratories, the Si growth on Ag(110) was followed by LEED/Auger spectroscopy, Si2p core level and Ag4d valence states, LEED/ARPES and RHEED/ STM measurements, after a careful calibration of the Si sources. The Ag(110) substrate was cleaned in the UHV chamber (base pressure: 7.0 × 10−11 mbar) by repeatedly sputtering with Ar+ ions and annealing the substrate at 750 K, while keeping the pressure below 1 × 10−10 mbar during heating. An infrared pyrometer and a thermocouple were used to monitor the sample temperature. Si was evaporated at a rate of ∼0.03 ML/ min from a Si source up to about 4 monolayers (MLs), while the Ag substrate was kept at T ∼ 470 K to produce the multilayer-thick of SiNRs. Figure 1a displays a filled-states STM image showing multilayer-thick SiNRs grown at ∼470 K on the Ag(110) surface. They are massively parallel, all aligned along the [11̅0] direction of the Ag(110) surface. They have a very high aspect ratio; typically, their widths and heights vary from 10 to 30 nm and from 2 to 5 nm, while their lengths can reach more than several hundred nanometers, for instance, the arrow L in Figure 1a marks a length of ∼500 nm. In the regions in-between those multilayer-thick SiNRs the entire Ag(110) surface is covered by a single layer of atom-thin silicene SiNRs, as also confirmed by 5501
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excellent accord with the mean Si−Si distance found for atomthin SiNRs.3,6,13,15,16 As a matter of fact, one could possibly object that we just measured the atom-thin SiNRs in-between the multilayer thick ones. However, for such atom-thin SiNRs we have found, instead, a gap of ∼0.56 eV, centered at 0.62 eV below EF; this clearly distinguishes the multilayer-thick SiNRs from their precursors, the atom-thin ones.13 Hence, the narrowing of the gap is a clear signature of the multilayer-thick SiNRs. However, still, the atom-thin precursors might partly contribute to the ARPES structures. Along this view, the π* bands, normally empty, would be populated here due to charge transfer from the silver substrate to the Si NRs; this explains the metallic character of the Si nanoribbons. We further attribute the upward shift and the closing of the gap to the increasing separation from the Ag(110) substrate of the silicene layers, piled as in graphite, within the multilayer-thick SiNRs. This modifies the charge transfer to the upper silicene layers, as in the case of graphene grown on different surfaces, where the charge transfer pulls down the π* band and the interaction with the substrate opens a gap.19 Typically, when graphene is epitaxially grown on SiC substrates, a 0.26 eV gap is produced. This value decreases as the sample thickness increases and eventually approaches to zero when the number of layers exceeds four.20 The geometry of these multilayer-thick epitaxial SiNRs results from that of their atom-thin precursors, namely they are zigzag silicene nanoribbons with hexagon apexes aligned along their lengths (i.e., the silver [1̅10] direction), with the following match: 4 Ag−Ag nearest neighbor distances along the [1̅10] direction coincide precisely with 3 Si−Si second neighbor distances; this is in perfect accord with the ×4 symmetry found in STM real space images as well as in both LEED and RHEED patterns. We stress that this ×4 periodicity along the zig-zags of the SiNRs, that is the [1̅10] direction of silver, is the same of that obtained on epitaxial silicene sheets showing a 4 × 4 coincidence cell on Ag(111),2 evidencing in these two different situations that the silicene layers arrange in a similar configuration. For these multilayer-thick SiNRs, we derive a very high Fermi velocity of ∼1.3 × 106 m sec−1 comparable with those reported for free-standing graphene (1.1 × 106 m sec−1),20 or in graphite (1.0 × 106 m sec−1).21 Indeed, this opens fascinating prospects for potential devices based on such multilayer-thick silicon nanoribbons. To summarize, we have grown straight, massively parallel, high aspect ratio multilayer-thick silicene nanoribbons on the Ag(110) surface with a pyramidal cross section. Dirac cones derived from π and π* bands are measured at the onedimensional Brillouin zone extremities, yielding a very high Fermi velocity of ∼1.3 × 106 m sec−1. This confers to these unique, graphite-like, silicon nanostructures built-up by stacks of silicene, exceptional electronic properties opening exciting perspectives for future nanometric devices suitable for nanoscale applications.
Figure 2. (a) A 5 × 4 LEED pattern collected at 64 eV from multilayer-thick SiNRs grown on Ag(110) at 470 K. The red circles indicate silver integer order spots and the blue one and 1/5 fractional order one. On the RHEED patterns (b), the large arrows indicate the Ag(110) integer streaks and the small ones the 1/4 (left) and 1/5 (right) order streaks stemming from the ×4 and ×5 periodicities, respectively. V-shaped features around the silver integer streaks are typical of 3D growth.
Figure 3. (a) ARPES intensities (hν = 126 eV) from multilayer-thick 5 × 4 SiNRs recorded along their lengths at ky = 0.4 Å−1, the (Γ̅ → X̅ ) direction: details of the Dirac cones at the X̅ point, between 0.67 and 1.5 Å−1. (b) Zoom-in of the dispersion shown in (a) to better visualize the small gap between the two Si bands, the Λ-like one (bottom) and the V-like one (top), which crosses the Fermi level. (c) EDC cut taken at k// = 1.087 Å−1 along the direction indicated by the arrow in (b) placed at the silver X̅ point.
(EDC) cut taken at k// = 1.087 Å−1 along the direction indicated by the arrow in (b), centered on the silver X̅ point. Assuming a honeycomb silicene lattice for each layer within the stacks (the distance of about 0.3 nm between successive layers, similar to that anticipated for graphite-like silicon, supports this assumption), this value yields a Si−Si nearest neighbor distance within the hexagons of 2.24 Å, which is in
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ASSOCIATED CONTENT
S Supporting Information *
Additional information and figures. This material is available free of charge via the Internet at http://pubs.acs.org. 5502
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(14) Zeyuan, N.; Qihang, L.; Kechao, T.; Jiaxin, Z.; Jing, Z.; Rui, Q.; Zhengxiang, Q; Dapeng, Y.; Jing, L. Nano Lett. 2012, 12, 113. (15) Takeda, K.; Shiraishi, K. Phys. Rev. B 1994, 50, 14916. (16) Wang, Y. C.; Scheerschmidt, K.; Gösele, U. Phys. Rev. B 2000, 61, 12864. (17) We specify, here, how these multilayer-thick silicene nanoribbons were obtained and characterized. Their growth was followed by LEED and Auger spectroscopy before the photoemission measurements were performed at the VUV beamline at the ELETTRA facility under proposal no. 20105043, entitled: “Silicite: Multilayers of Silicene nano-ribbons grown on Ag(110)” and before the STM observations were carried out in Japan. We have abandoned the Silicite denomination, given previously in analogy to graphite, because it already exists to name the Labradorite mineral. (18) Sahaf, H.; Masson, L.; Léandri, C.; Aufray, B.; Le Lay, G.; Ronci, F. Appl. Phys. Lett. 2007, 90, 263110. (19) Grüneis, A.; Kummer, K.; Vyalikh, D. V. New J. Phys. 2009, 11, 073050. (20) Zhou, S. Y.; Gweon, G. -H.; Graf, J.; Fedorov, A. V.; First, P. N.; De Heer, W. A.; Lee, D. -H.; Guinea, F.; Castro Neto, A. H.; Lanzara, A. Nat. Mater. 2007, 6, 770. (21) Zhou, S. Y.; Gweon, G.-H.; Graf, J.; Fedorov, A. V.; Spataru, C. D.; Diehl, R. D.; Kelevichop, Y.; Lee, D. -H.; Steven, G. L.; Lanzara, A. Nat. Phys. 2006, 2, 595.
AUTHOR INFORMATION
Corresponding Author
*(P.D.P.) Tel:+39-06-49934144. Fax:+39-06-49934153. Email:
[email protected]. (G.L.L.) E-mail: guy.lelay@ univ-provence.fr. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the World Premier International Research Center (WPI) Initiative on Materials Nanoarchitectonics, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan and the staffs of ELETTRA and of the VUV beamline. This research was supported by the Project for International Scientific Cooperation short-term mobility 2011: Italy/Japan: “Two STM Probes Resistance Measurements on Silicene”; the Project for International Scientific Cooperation (PICS) Italy/France under Grant No. 40237: “Self-assembled silicene nano-ribbons: controlling and engineering their structural, electronic and magnetic properties”; the GRANTin-AID for young Scientist No. 22710108 from the MEXT, Japan and the “2D-NANOLATTICES” project of the Future and Emerging Technologies (FET) program within the seventh framework program for research of the European Commission, under FET Grant 270749.
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