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Atomic-scale Reconstructions on Metal and Semiconductor Surfaces

Atomic-scale Reconstructions on Metal and Semiconductor Surfaces. Andrew Wee Surface Science Laboratory Department of Physics, NUS. IMS Workshop, 27 Nov 04. Surface Science Lab NUS. VT-STM/XPS/LEED system + growth chamber with molecular beam & reactive atom sources [+ cryogenic STM]

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Atomic-scale Reconstructions on Metal and Semiconductor Surfaces

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  1. Atomic-scale Reconstructions on Metal and Semiconductor Surfaces Andrew Wee Surface Science Laboratory Department of Physics, NUS IMS Workshop, 27 Nov 04

  2. Surface Science Lab NUS • VT-STM/XPS/LEED system + growth chamber with molecular beam & reactive atom sources [+ cryogenic STM] • Soft X-ray synchrotron end station on SINS beamline [+ growth chamber + STM/AFM] • Cameca IMS 6f Magnetic sector SIMS • VG ESCA MkII/SIMSLAB • [EXAFS endstation]

  3. Grand Challenge: Self assembly of single molecule devices W Ho et al., Science, published online Sept. 4, 2003. H Park et al., Nature, 417 (2002)722

  4. Scope of Presentation • Structure of Surfaces • A rule for structures of open (high index) metal surfaces • A high index surface: Cu(210) • SiC(0001)-63x63 “honeycomb” reconstruction • Adsorbate-induced Reconstructions • SiC(0001)-O • Cu(210)-O; Cu(210)-Br • Surface as Template • Monodispersed Co nanoparticles on SiC(0001) honeycomb template • Co ring clusters on Si(111)-(7×7)

  5. 1. Structure of Surfaces A rule for structures of open (high index) metal surfaces A high index surface: Cu(210) Adsorbate-induced reconstructions: Cu(210)-O; Cu(210)-Br SiC(0001)-63x63 “honeycomb” reconstruction

  6. A rule for structures of open metal surfacesRef: YY Sun, YP Feng, CHA Huan, ATS Wee, Phys. Rev. Lett. 93 (2004) 136102. • Open metal surfaces: The coordination of the atoms in at least two layers is reduced when creating the surface; hence, more than one atomic layer is “exposed” to the vacuum. • Rule: “At bulk-truncated configuration, define a surface slabin which the nearest neighbors of all atoms are fewer than those in the bulk; in the process of relaxation, the interlayer spacing between each pair of atomic layers within this slab contracts, while the spacing between this slab and the substrate expands.” Surface Slab Contracts Expands Bulk

  7. Density Functional Theory (DFT) • Kohn-Sham equation: where the last term (the exchange-correlation) is not known exactly. Various approximations are available. Among others, the LDA and GGA are most widely used. LDA GGA

  8. Methodology • Advantages: • Simple mathematical formulism • Independency of basis set on ion positions • Availability of fast Fourier transform (FFT) between direct and reciprocal spaces • Plane Wave Expansion: • Pseudopotentials: • Keep the eigenvalues and scattering properties unchanged compared with those of the real potential. • Softer in the core regions, hence fewer PW’s are needed for the expansion above. • Vienna Ab-initio Simulation Package (VASP) is a very efficient implementation of the pseudopotential plane-wave package.

  9. A rule for structures of open metal surfaces First-principles calculations: Based on density functional theory with either LDA or GGA approximation for the exchange-correlation functional Ref: Sun YY, Phys. Rev. Lett. 93 (2004) 136102.

  10. A rule for structures of open metal surfaces Physical picture:For more open surfaces, electrons from the deeper layers contribute to the smoothing, hence more layers relax.

  11. Further evaluation of the rule • All fcc(311) surfaces have relaxation sequence (- + …) • All fcc(331) and fcc(210) surfaces have relaxation sequence (- - + …) • All these surfaces obey the rule. Reference: Sun YY, Xu H, Feng YP, Huan ACH, Wee ATS, Surf. Sci. 548, 309 (2004).

  12. I-V data collection Q-LEED analysis Guess a structure Multiple scattering calculations Adjust structure Reliability factor Bad Good Stop Low Energy Electron Diffraction (LEED):Quantitative Determination of Surface Structure LEED diffraction pattern

  13. A high index surface: Cu(210) • Clean Cu(210): I-V LEED • Studied by layer-doubling LEED analysis and pseudopotential DFT calculations. • Excellent agreement between the calculated and measured I-V curves as judged by small Pendry R factor of 0.12. Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B68 (2003) 115420

  14. A high index surface: Cu(210) Multilayer relaxation of Cu(210) surface: IV-LEED vs DFT Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B68 (2003) 115420

  15. cf. A rule for structures of open metal surfaces

  16. A A A A B B B B A C C C A C A C A B B B C A A C atom C B Si atom C A A B A 15.11Å 7.55Å 3C-SiC(111) 4H-SiC(0001) 6H-SiC(0001) 10.05Å Structure of 6H-SiC • Wide band gap semiconductor, very hard, good thermal conductor, chemical inert. • Structure: Si-C sp3 configuration, different Si-C bilayer stacking sequence and orientation, ≥200 polytypes, determine the physical property.

  17. Monodispersed Co nanoparticles on SiC(0001) honeycomb template • 6H-SiC(0001) surface reconstruction 30 nm x 20 nm (1x1) (3x3) (√3x√3) R30 (6√3 x 6√3 )R30 1170K 1230K 1250K

  18. Photoelectron spectroscopy data of SiC(0001) surface reconstructions The C1s binding energy of carbon nanomesh is at 285.1 ev; graphite (HOPG) is at 284.4 eV. The well-developed carbon nanomesh surface is formed before the graphitization of the SiC surface. Therefore, the carbon nanomesh surface is not due one monolayer graphite.

  19. C 1s of the carbon nanomesh surface The carbon nanomesh is a honeycomb superstructure formed by the self-assembly of carbon atoms at high temperature. Two surface-related components for the carbon nanomesh surface have been identified with a binding energy of 283.8 eV and 285.1 eV, respectively.

  20. Building the SiC(0001) honeycomb model • one-layer thick nanomesh; identical honeycomb cells • topmost Si atoms desorb • all the outermost surface atoms are C atoms • C atoms collapse, can substitute Si atoms below

  21. Building model • Class III III-12

  22. Building model • Class III III-13b

  23. Structure Optimization: force on ion < 10 meV/Å C atom Si atom H atom fixed DFT-LDA Calculation results Unit cell parameters: a=b= 18.450Å, c= 20.0Å. ~ 300 atoms; CPU time ~ 3 weeks

  24. DFT-LDA Calculation results STM images calculated to compare with experimental images Partial charge density calculated Smoothing techniques STM images

  25. DFT-LDA Calculation results Model III-12 Model III-13b

  26. DFT-LDA Calculation results Relaxed structure of model III-13b, 2x2x1 cell Relaxed structure of model III-12, 2x2x1 cell

  27. (a) VT = 1.5V (b) VT = 1.8V (c) VT = 2.0V (d) VT = 2.5V 5nm Simulated STM images Model III-12, V=1.6 eV Model III-13b, V=1.6 eV

  28. Explanation of PES Peaks Relaxed nanomesh structure consists of graphene-like superstructure bonded to Si atoms below. • C1s spectrum can be understood accordingly by: • a graphite-like C-C peak (S1) • an asymmetric low energy tail due to the boundary C atoms which have both C-C bonds and C-Si bonds (S2) • bulk SiC substrate with Si-C bonds (B)

  29. 2. Adsorbate-induced Reconstructions SiC(0001)-O Cu(210)-O; Cu(210)-Br

  30. LEED, E=70eV 60 × 60 nm2 and detailed 9 ×7 nm2 (insert). (I = 0.30 nA VT = 2.2 V) 6H-SiC(0001)-3×3

  31. T T2 T0 T3 T [1010] [1010] 1 T1 T 3 4 [1210] T T2 T T2 2 6 [2110] Top view A B [1120] [1120] B T0 T T T0 T3 T T T3 1 2 2 5 6 30º 1 T T1 T1 T 60º 60º 3 4 T T2 2 5 4 4 3 3 3 T0 T T T3 T1 T A Tetra- cluster 1st-layer 2nd-layer T0 T0 [0001] Side view T0 T2 T3 T2 T1 T3 T1 T2 T3 T1 4 1 2 A 6 2 1 3 B 4 5 Si adlayer Bulk Si Bulk layer 6H-SiC (0001) 3×3 twisted reconstructed model U. Starke et.al, PRL, 80, 758 (1998); PRB, 62, 10335 (2000).

  32. F. Amy, et. al., Phys. Rev. Lett. 86, 4342 (2001) O2 O2 reacts with the third Si-layers. Dangling bond O2 Si-adatom is much more active. Si-adatom sites or the third Si-layers? Initial oxidation mechanism

  33. Clean Surface 0.2 L O2 1.0 L O2 2.0 L O2 * * * * * * * I = 0.10 nA, VT = 2.2 V 2 nm In-situ oxidation with low tunneling current to minimize the inelastic tunneling electron scattering induced reactions. Darksites appear initially, saturated after 1.0L O2 exposure. Bright sites appear after O2 exposure, and keep increasing. Initial oxidation mechanism

  34. Or O2, initial Si Si Si Si Si adatom+trimer O2 attach on the dangling bond of Si adatom. Dark sites, O2 depletes the DOS of Si atom O2 inserts into the back bonds of Si adatom. Bright sites, Si atom is lifted by 0.5 Å. Thermal stable sites. More O2 Explanation

  35. O Top view 1 2.299 2.295 Models where O2 reacts with the third Si-layer 2.340 1.662 2.322 2.283 1.594 2 1.642 1.634 1.650 2.302 2.801 1.623 2.229 1.611 1.601 2.502 2.529 1 1 1 O O O 2 2 2 6 6 2.321 2.373 2.331 2.229 2.479 2.582 2.342 2.342 2.367 2.338 5 5 5 5 Side view 4 4 4 4 3 3 3 3 A-O-6 =120.8o 2-O-A =119.5o 2-O-1 =121.2o 2-O-A =128.4o 2.430 2.428 A A A A 6 6 A1 A2 T0 T0 T0 T1 T2 T3 T3 T1 T2 T2 T2 T2 T3 T3 T3 T1 T1 T1 DFT simulations(Using CASTEP codes)

  36. 3 1 1 4 2 2 2 6 6 5 5 4 4 3 3 1 3 4 B B Top view O 2.378 2.246 2.341 1.622 1.638 2.349 1.645 1.641 6 O 6 O 1.604 1.633 B B 2.355 2.342 T0 T0 T0 T0 5 5 2.442 1.607 1.636 1 2.522 2.385 O T0 T1 T3 T2 T3 T2 T0 T3 T3 T0 T3 T1 T0 T3 T1 T0 T1 T0 T2 T2 T2 T2 2.476 T3 T3 T3 T3 T1 T1 T1 T1 2 Side view B-O-3 =117.7o B-O-3 =120.1o 3-O-4 =118.5o B-O-1 =119.4o 2.429 2.432 A3 A4 Models where O2 reacts with the third Si-layer

  37. 1 1 1 1 1 1 2 2 2 2 2 2 6 6 6 6 6 6 5 5 5 5 5 5 4 4 4 4 4 4 3 3 3 3 3 T0 T3 T0 T2 T0 T1 T3 T2 T2 T3 T1 T1 T0 T1 T2 T3 T1 T0 T0 T2 T3 T1 T2 O2 1.213 1.343 + Tetra- cluster o o o o 175.9 123.1 123.3 128.5 1.566 2.428 2.428 2.370 2.408 2.423 2.432 T3 o 124.5 1.505 1.648 1.644 1.656 C1 2.401 1.642 1.641 o 122.0 1.637 2.531 2.654 1.213 1.373 + 1.638 1.631 C3 C4 2.428 2.380 2.428 2.413 2.432 2.398 C2 Models where O2 reacts with Si-adatoms

  38. The model where O2 insets into the back bonds of the Si-adatoms is thermally most stable! Chen W, Xie XN, Xu H, Wee ATS, Loh KPAtomic scale oxidation of silicon nanoclusters on silicon carbide surfacesJ PHYS CHEM B 107 (42): 11597-11603 OCT 23 2003

  39. Superstructure formation in the Cu(210)-O system 1000 x 1000 Å2 image of (2x1) reconstruction Wee ATS, Foord JS, Egdell RG, Pethica JB, Phys. Rev. B 58 (1998) R7548. (a) (2x1) FHS-MR LBS-BR FHS-BR d03 d02 d12 d01 [120] L02 [120] L01 L03 Definition of parameters for LBS-MR [001] LBS-MR Adsorbate-induced Surface Reconstructions O-Cu(210) adsorbate induced reconstructions • LBS-MR (oxygen at long bridge site with missing row), LBS-BR (long bridge site with inward buckled row), FHS-MR (four-fold hollow site with missing row) and FHS-BR (four-fold hollow site with inward buckled row) • Tan K. C., Guo Y. P., Wee A. T. S. and Huan C. H. A., Surf. Rev. Lett.6 (1999) pp. 859-863

  40. LEED study of oxygen-induced reconstructions on Cu(210) Buckled (3x1) reconstruction – 2/3 ML Guo YP, Tan KC, Wang HQ, Huan CHA, Wee ATS, Phys. Rev. B 66 (2002) 165410. 1st Cu-O row (side view) 2nd Cu-O row (side view) d01=0.12Å [001] D22=0.17Å d01’=-0.17Å d12 0.99Å(+22.6%) d12’ 1.03Å(+27.6%) D33 D22=0.17Å d23’ 0.78Å (-3.4%) D44 d23 0.70Å (-13.3%) [120] D33 d34 d34’ D44 L01’=0.54Å L01=0.25Å Top view L00=4.84Å(+19.9%) Adsorbate-induced Surface Reconstructions

  41. (a) 2000 x 2000 Å2 (VB = -1.0 V, IT = 2.5 nA), (b) 300 x 300 Å2 (VB = -1.0 V, IT = 0.30 nA) images after 500 L RT oxygen exposure and subsequent annealing to 620 K for a few minutes. Analysis of corrugation profiles shows that A and C are at the same height, whereas B is one unit cell below and D one above. (a) [001] (b) A B C D [001] Adsorbate-induced Surface Reconstructions Cu(210)-O superstructures

  42. Cu(210)-Br system 200 x 200 Å2 images of the triangular checkerboard recorded at VB = -1.0 V, IT = 0.1 nA, showing an inversion of the triangles during different scans but using the same tunnel current and sample bias. Wee ATS, Fishlock TW, Dixon RA, Foord JS, Egdell RG, Pethica JB, Chem. Phys. Lett. 298, 146 (1998) [001] Adsorbate-induced Surface Reconstructions • Cu(100)-Br system • T.W. Fishlock, J.B. Pethica and R.G. Egdell, Surf. Sci. 445, L47 (2000)

  43. Adsorbate-induced Surface Reconstructions • Cu(100)-N system • Adsorbate induced nanostructures also observed in Cu(110), Cu(111)-N systems • F. M. Leibsle, Surf. Sci. 514, 33 (2002)

  44. 3. Surface as Template Monodispersed Co nanoparticles on SiC(0001) honeycomb template Co ring clusters on Si(111)-(7×7)

  45. Self-assembly in a Honeycomb template?

  46. (b) (a) (c) 0.1ÅCo 1.7Å (1) nanomesh (2) 4nm Monodispersed Co nanoparticles on SiC(0001) honeycomb template • 16×16nm2 STM filled state images for the carbon nanomesh with: • 0.1Å Co coverage • 0.2Å Co coverage • line profile 1 for (a) and line 2 for clean surface. VT=2.5V

  47. Monodispersed Co nanoparticles on SiC(0001) honeycomb template • At the lower coverage (0.1Å Co), the clusters will adsorb on these active sites, with a diameter of 1.4±0.2nm and a height of 1.7±0.1Å. • At the higher coverage (2.0Å Co), neighbouring Co clusters will coalesce to form big clusters, 3.4±0.2 nm in diameter and 3.3±0.1Å in height. • Monodisperse Co nanoclusters can be fabricated on SiC honeycomb template under submonolayer condition. • Boundaries of honeycomb structures serve as active sites for Co cluster growth. 8nm×8nm STM image: blue circles highlight the Co cluster adsorption sites. • References: • W Chen, KP Loh, H Xu, ATS Wee, Appl. Phys. Lett. 84 (2004) 281 • W Chen, KP Loh, H Xu, ATS Wee, to appear in Langmuir.

  48. The BN nanomesh was formed by deposition of B3N3H6 on Rh(111). cf. Boron Nitride NanomeshM. Corso et al., Science, 303 217 (2004) • Hole formation is likely driven by the lattice mismatch of the film and the rhodium substrate. • This regular nanostructure is thermally very stable and can serve as a template to organize molecules, e.g. C60 molecules.

  49. C C′ U B F A Co ring clusters on Si(111)-(7×7)

  50. Empty state: 1.9 V , 0.1 nA Co ring clusters on Si(111)-(7×7) M.A.K. Zilani, Y.Y. Sun et al., in preparation STM simulation

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