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Patrick Vogt Epitaxial Silicene: Silicon in the 2D World

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Patrick Vogt Epitaxial Silicene: Silicon in the 2D World Epitaxial silicene: Silicon in the 2D world Patrick Vogt Technische Universität Chemnitz, Germany Technische Universität Berlin, Germany Winter school: New Frontiers in 2D Materials Villard-de-Lans January 15 - 20, 2017 Chemnitz ∙ 03. January 2017 ∙ Dmytro Solonenko M.Sc. www.tu-chemnitz.de/physik/HLPH/ Location | Saxony – City of Chemnitz Villard-de-Lans January 2017 ∙ Patrick Vogt 2 Impressions | City of Chemnitz Villard-de-Lans January 2017 ∙ Patrick Vogt 3 A new research centre in Chemnitz Center for Materials, Architecture and Integration oif Nano-Membranes (MAIN) • 43 million € funding • 4000 m2 lab and office space • for ~100 scientists •starting at the end of 2017 • nano-membranes • 2D materials For technological applications • 2D organic layers and flexible electronics • hetero-structures Villard-de-Lans January 2017 ∙ Patrick Vogt 4 Motivation: carbon-structures graphite 1564 γράφειν = to write Novoselov, Geim Nobel Prize in Physics 2010 graphene 2004 Curl, Kroto, Smalley Nobel Prize in Chemistry 1996 fullerene nanotube 1985 Iijima 1991-1993 Villard-de-Lans January 2017 ∙ Patrick Vogt 5 Motivation: graphene Honeycomb structure Energy dispersion relation E(k) “Dirac-cone” semiconductor * k-points Ek = nF k 2 2 * 3D energy surface in k space. Ek = h k /2m • linear dispersion of the and * bands close to the K points • resembling properties of relativistic Dirac fermions 6 -1 • Fermi velocity nF = 1.0 ˣ 10 ms But: • Graphene is semi-metallic and has no bandgap! • Graphene is not (easily) applicable in logic and photonic devices. Are there other similar elemental 2D materials? Villard-de-Lans January 2017 ∙ Patrick Vogt 6 Motivation: 2D materials beyond graphene? Other elemental 2D materials? Schematic energy diagram sp3 109.5° 3 Diamond (sp ) C Silicene (sp2) energy sp2 120° Graphite (sp2) C Bulk-Silicon (sp3) Graphite (Graphene) is the most stable form of carbon sp 180° But: C sp2 hybridized silicene, germanene,... are less stable than the sp3 hybridized bulk forms Acetylene Solution: a mixed sp2/sp3 hybridization state Villard-de-Lans January 2017 ∙ Patrick Vogt 7 Silicene: sp2/sp3 hybridization N. Takagi, et al., Prog. Surf. Sci. 90, 1 (2015). Analogy to the dangling bond state on Si (111) where q = h/d – “buckling ratio” (buckling distance/Si-Si bond length) Varying the buckling ratio q: q = 0 DB=Ep; BB=Es+2Ep sp2 hybridization q ~ 1/√3 sp2/sp3 hybridization q = 1/3 DB=BB=Es+3Ep sp3 hybridization Villard-de-Lans January 2017 ∙ Patrick Vogt 8 Silicene: geometric structure K. Takeda, K. Shiraishi, PRB 50, 14916 (1994) G. G. Guzmán-Verri, et al., PRB 76, 075131 (2007) S. Cahangirov et al.,PRL 102, 236804 (2009) Si-Si bond diamond Si: 2.35 Å Si atoms Buckling (Δ) mixed sp2/sp3 diamond Si (111) hybridization plane: 0.29 Å Villard-de-Lans January 2017 ∙ Patrick Vogt 9 Silicene: electronic structure S. Cahangirov et al.,PRL 102, 236804 (2009) Low-buckled (LB) Planar (PL) N. Takagi, et al., Prog. Surf. Sci. 90, 1 (2015). q = h/d: q = 0 (θ = 90°) sp2 hybr. The bonds have a q ~ 1/√3 sp2/sp3 hybr. mixed sp2 and sp3 character q = 1/3 (θ = 109.5°) sp3 hybr. Villard-de-Lans January 2017 ∙ Patrick Vogt 10 Silicene: vibrational structure Diamond-like Silicon TO LO TO LA TA S. Cahangirov, et al., Phys. Rev. Lett. 102, 236804 (2009) P. Gori, et al. Energy Proc. 45, 512 (2014). X. Li, et al. Phys. Rev. B 87, 115418 (2013). TA Characteristics: • Higher energy of the L(T)O phonons • Out-off-plane ZO phonon R. Tubino, et al., J. Chem. Phys. 56, 1022 (1972). Villard-de-Lans January 2017 ∙ Patrick Vogt 11 Raman mode symmetries S. Cahangirov, et al., Phys. Rev. Lett. 102, 236804 (2009) • A-symmetry phonons: out-of-plane motion • E-symmetry phonons: in-plane motion J. Ribeiro-Soares, et al., Phys. Rev. B 91, 205421 (2015). Villard-de-Lans January 2017 ∙ Patrick Vogt 12 Expected properties of silicene • Silicene could be superconducting - possibility of high Tc by B doping • Because of the large spin-orbit coupling in Si compared to C, it is predicted to exhibit a quantum spin Hall-effect in an accessible temperature regime (10-20K) • Silicene could be integrated easily in silicon-based nanotechnology (maybe) • Predicted high carrier mobilities (~2.6*105 cm2V-1s-1) • Band gap opening by an electric field or functionalization • Novel silicene-based devices: DualHydrogenatedSilicene-gated-based silicene bilayer Spintronic FET- silicene applications for optical applications HydrogenationBy tuning of bilayer the A vertical E-field0 is Gated silicene: silicene opensspin ofa directthe current tunable able to open a band 2 gapped Dirac bandgap between 1.5 – 2.9 eV cons with ~ 100% gapcan in besingle selected-layer spin-polarization buckledbetween silicene “up” and “down” Z. Ni et al., Nano Lett. 12, 113(2012) B.W. Huang-F.Tsai et, et al, al, Phys. Nat. CommunRev. X. 4., 4021029, 1500 ((2014)2013) Villard-de-Lans January 2017 ∙ Patrick Vogt 13 How to synthesize silicene..... Contrary to graphene, silicene does not exist in nature! Roald Hoffmann 1981 “Silicene exists and will be made only on a support of some sort, metal or semiconductor” Angew. Chem. Int. Ed. 52, 93 (2013) Synthesis on a nonreactive substrate preventing 3D growth Synthesis of 2D silicene on a templates Preparation under vacuum conditions (UHV) Villard-de-Lans January 2017 ∙ Patrick Vogt 14 Synthesis of silicene on Ag surfaces 1D Si nano-ribbons on Ag(110) STM structure model cones at the Dirac points Ronci et al., pss c 7, 2716 (2010) Sahaf et al., APL 90, 263110 (2007) M.E. Davila et al., Nanotechnology 23, 385703 (2012) Le Lay et al., Appl. Surf. Sci. 256, 524 (2009) De Padova et al., APL 96, 261905 (2010) No silicide formation! Ag(111) templates for the synthesis of 2D silicene Villard-de-Lans January 2017 ∙ Patrick Vogt 15 Synthesis of silicene on Ag(111) in 2012 Phys. Rev. Lett. 108, 155501 (2012) C.-L. Lin et al., Appl. Phys. Express, 5 (2012) 045802 [1-10] A. B. Feng et al., Nano Letters 12, 3507 (2012) Villard-de-Lans January 2017 ∙ Patrick Vogt 16 Outline • part 1: - epitaxial growth of silicene on Ag(111) • part 2: - Vibrational properties of epitaxial silicene / Ag(111) - Other elemental 2D materials Villard-de-Lans January 2017 ∙ Patrick Vogt 17 Synthesis of 2D silicene on Ag(111) STM Preparation of Ag(111) • sputtering for 1h (1.5kV, 5x10-5 mbar Ar+) • annealing at 560°C for 30 minutes LEED: (1x1) Ubias = - 0.2 V 100nm 600nm x 600nm,Ubias = - 0.18 V • flat surface with big terraces Si deposition: Si source: UHV system • directly heated Si-wafer piece Growth parameters: • sample temperature: RT – 450°C • deposition rate and coverage Si source Ag sample Villard-de-Lans January 2017 ∙ Patrick Vogt 18 Growth of 2D Si-layers on Ag(111) Si-evaporator Ag(11) 2D silicene Ag(111) RT-500°C Preparation of a clean Si deposition at a certain Formation of a Ag(111)(11) surface sample temperature 2D silicene layer Growth Conditions: UHV The sample temperature is Growth parameters: crucial for the formation of Si- • sample temperature: RT – 500°C phases on Ag(111) • deposition rate and coverage Villard-de-Lans January 2017 ∙ Patrick Vogt 19 Si deposition on Ag(111) at different temperatures RT <180°C ~220°C >220°C >250°C Si-clusters “disordered ” (33)/(44) multi-phase “(2323)” ~1/40 ML ~1/10 ML ~1 ML ~1 ML ~1 ML 50nm x 50nm 50nm x 50nm 30nm x 30nm 30nm x 30nm Si-clusters (1313)-α (1313)-β The formation 2D Si-layers / Ag(111) depends crucially on the temperature Villard-de-Lans January 2017 ∙ Patrick Vogt 20 Si deposition on Ag(111) at 220°C Growth mode LEED STM 6 1 ML/h Ag(111) (4x4) 4 2 ¼ spots Si 2p / Ag 4d area ratio 0 integer spots U = - 1.4 V 0 20 40 60 0,4 deposition time (minutes) honeycomb-like ~ 0.1nm structure 0,2 ~ 0.3 nm hight (nm) 0,0 linear growth mode clear coincidence 0 5 10 15 (4x4) symmetry distance (nm) 2D layer 2D growth of a 2D Si layer with a (4x4) periodicity (with respect to Ag 1x1) Villard-de-Lans January 2017 ∙ Patrick Vogt 21 STM of the Si “(4×4)” structure line profile [1-10] 0,04 1.14 nm 0,03 0,02 hight (nm) 0,01 0.7nm 0,00 0 2 4 6 8 10 12 0.4nm distance (nm) 5 nm 1.14nm ~ 4x Ag-Ag distance U = - 1.1V distorted area “flower” structure 1.14 nm The structure has a (4x4) periodicity with respect to Ag(111) In agreement with LEED Phys. Rev. Lett. 108, 155501 (2012) Villard-de-Lans January 2017 ∙ Patrick Vogt 22 Comparison of STM and nc-AFM STM nc-AFM Atomically resolved AFM show the same “flower”-like structure for the (3x3) silicene line profile Both images are clearly dominated by geometric factors A. Resta et al., Sci. Rep. 3, 2399 (2013) Villard-de-Lans January 2017 ∙ Patrick Vogt 23 (3×3)/(4×4) silicene on Ag(111): structure model Structure model DFT results • TE calculation: structure is energetically stable STM: [1-10] Ag(111) ~ 110° • sp2/sp3-character STM: ~ 112°-118° Si layer • Si-Si distance: 0.23nm • simulated STM image agrees Model well with the experimental ones DFT results support the model The (3×3)/(4×4) structure indicates a significant Ag-silicene interaction! Phys. Rev. Lett. 108, 155501 (2012) Villard-de-Lans January 2017 ∙ Patrick Vogt 24 Confirmation of the model by diffraction methods Positron diffraction (RHEPD) Y. Fukuya et al.,Phys.
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