Atomic-scale Reconstructions on Metal and Semiconductor SurfacesAndrew WeeSurface Science Laboratory Department of Physics, NUSIMS Workshop, 27 Nov 04

Surface Science Lab NUSVT-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 SIMSVG ESCA MkII/SIMSLAB[EXAFS endstation]

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

Scope of PresentationStructure of SurfacesA rule for structures of open (high index) metal surfacesA high index surface: Cu(210)SiC(0001)-63x63 honeycomb reconstructionAdsorbate-induced ReconstructionsSiC(0001)-OCu(210)-O; Cu(210)-BrSurface as TemplateMonodispersed Co nanoparticles on SiC(0001) honeycomb templateCo ring clusters on Si(111)-(77)

1. Structure of SurfacesA rule for structures of open (high index) metal surfacesA high index surface: Cu(210)Adsorbate-induced reconstructions: Cu(210)-O; Cu(210)-BrSiC(0001)-63x63 honeycomb reconstruction

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 slab in 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 SlabBulkContractsExpands

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. LDAGGA

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

A rule for structures of open metal surfacesFirst-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.

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

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).

Low Energy Electron Diffraction (LEED):Quantitative Determination of Surface StructureLEED diffraction pattern

A high index surface: Cu(210)Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B 68 (2003) 115420Clean 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.

A high index surface: Cu(210)Sun YY, Xu H, Zheng JC, Zhou JY, Feng YP, Huan ACH, Wee ATS, Phys. Rev. B 68 (2003) 115420Multilayer relaxation of Cu(210) surface: IV-LEED vs DFT

LEEDDFTd12 (%)-11.1-16.7d23 (%)-5.0-4.3d34 (%)+3.7+6.8r12 (%)-1.9-1.0r23 (%)-1.9-0.6r34 (%)+0.6+1.9

cf. A rule for structures of open metal surfaces

Structure of 6H-SiCWide 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.

Monodispersed Co nanoparticles on SiC(0001) honeycomb template 6H-SiC(0001) surface reconstruction(1x1) (3x3) (3x3) R30 (63 x 63 )R301170K1230K1250K30 nm x 20 nm

Photoelectron spectroscopy data of SiC(0001) surface reconstructionsThe 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.

C 1s of the carbon nanomesh surfaceThe 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.

Building the SiC(0001) honeycomb modelone-layer thick nanomesh; identical honeycomb cells topmost Si atoms desorball the outermost surface atoms are C atoms C atoms collapse, can substitute Si atoms below

Building modelClass IIIIII-12

Building modelClass IIIIII-13b

Unit cell parameters: a=b= 18.450, c= 20.0. ~ 300 atoms; CPU time ~ 3 weeksDFT-LDA Calculation results

STM images calculated to compare with experimental imagesPartial charge density calculatedSmoothing techniquesSTM imagesDFT-LDA Calculation results

Model III-12DFT-LDA Calculation resultsModel III-13b

Relaxed structure of model III-12, 2x2x1 cellDFT-LDA Calculation resultsRelaxed structure of model III-13b, 2x2x1 cell

Simulated STM imagesModel III-12, V=1.6 eVModel III-13b, V=1.6 eV

Explanation of PES PeaksRelaxed 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)

2. Adsorbate-induced ReconstructionsSiC(0001)-OCu(210)-O; Cu(210)-Br

6H-SiC(0001)-33

6H-SiC (0001) 33 twisted reconstructed modelU. Starke et.al, PRL, 80, 758 (1998); PRB, 62, 10335 (2000).

Initial oxidation mechanism

Initial oxidation mechanism

Explanation

DFT simulations (Using CASTEP codes)

Models where O2 reacts with the third Si-layer

Models where O2 reacts with Si-adatoms

Chen W, Xie XN, Xu H, Wee ATS, Loh KP Atomic scale oxidation of silicon nanoclusters on silicon carbide surfaces J PHYS CHEM B 107 (42): 11597-11603 OCT 23 2003

Oxygen coverage: C (ML)Surface modelsChemisorption energy: E (eV/unit cell)x=1, C = 2/933:2O surfaceC1C2C3C4A1A2A3A4-4.10-4.32-5.61-6.93-3.52-3.48-3.50-3.51

Adsorbate-induced Surface ReconstructionsSuperstructure formation in the Cu(210)-O system1000 x 1000 2 image of (2x1) reconstructionWee ATS, Foord JS, Egdell RG, Pethica JB, Phys. Rev. B 58 (1998) R7548.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. 859-863O-Cu(210) adsorbate induced reconstructions

Adsorbate-induced Surface ReconstructionsLEED study of oxygen-induced reconstructions on Cu(210)Buckled (3x1) reconstruction 2/3 MLGuo YP, Tan KC, Wang HQ, Huan CHA, Wee ATS, Phys. Rev. B 66 (2002) 165410.

Adsorbate-induced Surface Reconstructions(a) 2000 x 2000 2 (VB = -1.0 V, IT = 2.5 nA),(b) 300 x 300 2 (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.Cu(210)-O superstructures

Adsorbate-induced Surface ReconstructionsCu(210)-Br system200 x 200 2 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)Cu(100)-Br systemT.W. Fishlock, J.B. Pethica and R.G. Egdell, Surf. Sci. 445, L47 (2000)

Adsorbate-induced Surface ReconstructionsCu(100)-N systemAdsorbate induced nanostructures also observed in Cu(110), Cu(111)-N systemsF. M. Leibsle, Surf. Sci. 514, 33 (2002)

3. Surface as TemplateMonodispersed Co nanoparticles on SiC(0001) honeycomb templateCo ring clusters on Si(111)-(77)

Self-assembly in a Honeycomb template?

Monodispersed Co nanoparticles on SiC(0001) honeycomb template1616nm2 STM filled state images for the carbon nanomesh with:0.1 Co coverage 0.2 Co coverageline profile 1 for (a) and line 2 for clean surface. VT=2.5V

Monodispersed Co nanoparticles on SiC(0001) honeycomb templateAt the lower coverage (0.1 Co), the clusters will adsorb on these active sites, with a diameter of 1.40.2nm and a height of 1.70.1. At the higher coverage (2.0 Co), neighbouring Co clusters will coalesce to form big clusters, 3.40.2 nm in diameter and 3.30.1 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.8nm8nm 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.

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.

Co ring clusters on Si(111)-(77)

Co ring clusters on Si(111)-(77)STM simulationM.A.K. Zilani, Y.Y. Sun et al., in preparation

Published work on other nanotemplates

AcknowledgementsCurrent students:Md. Abdul Kader ZilaniQi Dongchen

Past students:Ong Wei Jie Tan Kian ChuanWang HuiqiongDr Zheng JinchengDr Sun Yiyang*Dr Chen Wei*

* Currently Research FellowResearch Fellows:Dr Xu HaiDr Liu LeiDr Guo Yong PingDr Xie Xianning Dr Gao Xingyu

Collaborators:Dr Loh Kian PingDr Tok Eng SoonDr Wang Xuesen A/P Alfred HuanA/P Feng Yuan Ping