Surface Structure and Determination Techniques in Surface Chemistry

1. EEW508 II. Structure of Surfaces Surface structure Rice terrace

2. EEW508 II. Structure of Surfaces Surface structure revealed by SEM and STM Surface Chemistry and Catalysis, second edition G. A. Somorjai and Y. Li (2010) Using STM (Scanning tunneling microscopy) or other techniques such as field ion microscopy (FIM) or LEED (low energy electron diffraction), atomic model of surface structure can be determined.

3. EEW508 II. Structure of Surfaces Terrace-step-kink model Steps and kinks are line defects to distinguish them from atomic vacancies or adatoms, which are called point defects. Relative concentration of atoms in terraces, in line defects, or in point defects can be altered, depending the methods of sample preparation.

4. EEW508 II. Structure of Surfaces Terrace – flat surface Stepped surface Kinked surface

5. EEW508 II. Structure of Surfaces Dislocations creat surface defects such as steps and kinks Surface Chemistry and Catalysis, second edition G. A. Somorjai and Y. Li (2010) On heterogeneous solid surface, atoms in terraces are surrounded by the largest number of nearest neighbors. Atoms in steps have fewer, and atoms in kinks have even fewer. In a rough surface, 10-20% of atoms are often step sites, with about 5% of kink sites.

6. EEW508 II. Structure of Surfaces Limitation of Terrace-step-kink model Terrace-step-kink model has the assumption of a rigid lattice where every surface atom is located in its bulk-like equilibrium position and can be located by the projection of the bulk structure to that surface. The vertical position of surface atoms is shifted from the atomic positions in the bulk– exhibiting a significant contraction or ‘relaxation’ of the interlayer distance between the first and the second layer. As the surface structure with less packing density, the contraction perpendicular to the surface becomes larger. Not only the vertical direction, but the relocation of surface atoms along the surface takes place. Also, the adsorption of molecules or atoms lead to relocation of surface atoms to optimize the strength of the adsorption-substrate bond.

7. EEW508 II. Structure of Surfaces Determination of surface structure – Low energy electron diffraction (LEED) LEED produce the quantitative data on bond distance and angles as well as on location of surface atoms and of adsorbed molecules.

8. EEW508 II. Structure of Surfaces Surface Diffraction – LEED, X-ray diffraction, and atom diffraction The de Broglie wavelength of a particle is given by Where h is Planck’s constant, m is the mass of the particle, and E is the kinetic energy of the particle For electron, and He atoms For X-ray

9. EEW508 II. Structure of Surfaces Surface Diffraction – LEED, X-ray diffraction, and atom diffraction Electrons with energies in the range of 10-200 eV and helium atoms with thermal energy (~0.026 eV at 300K) has the atomic diffraction condition ( < 1A) Glazing angle X-ray diffraction is used for surface and interface structure studies X-ray bombardment induced emission of electron  photoelectron diffraction

10. EEW508 II. Structure of Surfaces Principle of Low energy electron diffraction (LEED) The single crystal surfaces are used in LEED studies. After chemical or ion-bombardment cleaning in UHV, the crystal is heated to permit the ordering of surface atoms by diffusion to their equilibrium positions. The electron beam (in the range of 10-200 eV) is backscattered. The elastic electrons that retain their incident kinetic energy are separated from the inelastically scattered electron by applying the reverse potential to the retarding grids. These elastic electrons are accelerated to strike a fluorescent screen and LEED pattern can be obtained. Types of LEED Video LEED : LEED patterns can be visualized on a fluorescent screen. Dynamic LEED or called I-V curve: the intensity I of the diffracted beam is measured as a function of the kinetic energy.

11. EEW508 II. Structure of Surfaces LEED pattern of a Si(100) reconstructed surface. The underlying lattice is a square lattice while the surface reconstruction has a 2x1 periodicity. The diffraction spots are generated by acceleration of elastically scattered electrons onto a hemispherical fluorescent screen. Also seen is the electron gun which generates the primary electron beam. It covers up parts of the screen.

12. EEW508 II. Structure of Surfaces Example – Si(111)- (7x7) DAS structure: dimer, adatom, and stacking fault

13. EEW508 II. Structure of Surfaces Scanning Tunneling Microscopy – brief description

14. EEW508 II. Structure of Surfaces Example – Si(111)- (7x7) Gerd Binnig and Heinrich Rohrer Nobel prize in Physics (1986)

15. EEW508 II. Structure of Surfaces If the surface unit-cell vector and that are different from and obtained from the bulk projection, then the surface unit vector can be related to the bulk unit vectors mij defines a matrix On unreconstructed surface

17. EEW508 II. Structure of Surfaces Unreconstructed surface of the face-centered crystal structure

18. EEW508 II. Structure of Surfaces Unreconstructed surface of the body-centered crystal structure

19. EEW508 II. Structure of Surfaces Unreconstructed surface of the diamond crystal structure

20. EEW508 II. Structure of Surfaces For example, fcc (100) – (2x2)

21. EEW508 II. Structure of Surfaces For example, fcc (111) – (2x2)

22. EEW508 II. Structure of Surfaces For example, fcc (110) – (2x2)

23. EEW508 II. Structure of Surfaces Abbreviated and Matrix Notation for a variety of superlattices

24. EEW508 II. Structure of Surfaces Abbreviated and Matrix Notation for a variety of superlattices

25. EEW508 II. Structure of Surfaces Notation of High-Miller-Index Stepped Surface

26. EEW508 II. Structure of Surfaces Notation of High-Miller-Index Stepped Surface

27. EEW508 II. Structure of Surfaces Notation of High-Miller-Index Stepped Surface stepped surface kinked surface 6(111) x (100) 4(111) x (100)

28. EEW508 II. Structure of Surfaces Bond-Length Contraction or Relaxation close-packed less close-packed

29. EEW508 II. Structure of Surfaces Chemical bonds and surface reconstruction

30. EEW508 II. Structure of Surfaces Strong chemical bonds Ionic bonds: Na+ (cation) - Cl-(anion) These oppositely charged cations and anions are attracted to one another because of their opposite charges. That attraction is called an ionic bond.

31. EEW508 II. Structure of Surfaces Strong chemical bonds Covalent bonds: H –F both atoms are trying to attract electrons that are shared tightly between the atoms. The force of attraction that each atom exerts on the shared electrons is what holds the two atoms together.

32. EEW508 II. Structure of Surfaces Strong chemical bonds Metallic bonds : Metal consists of metal ions floating in a sea of electrons. The mutual attraction between all these positive and negative charges bonds them all together. the sharing of "free" electrons among a lattice of positively-charged ions (cations),

33. EEW508 II. Structure of Surfaces Dangling bonds

34. EEW508 II. Structure of Surfaces Reconstruction – (2x1) Reconstruction of Si(100) The (2x1) reconstruction of Si (100) crystal structure as obtained by LEED crystallography. Note that the surface relaxation extends to three atomic layer into the bulk

35. EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) DAS structure: dimer, adatom, and stacking fault LEED and STM image of (7x7) reconstructed structure of Si (111) The total number of dangling bonds is reduced from 49 to 19 through this reconstruction.

36. EEW508 II. Structure of Surfaces (7x7) Reconstruction of Si (111) 19 dangling bonds of (7x7) reconstructed surface (12 adatom, 6 rest atom, 1 corner hole)

37. EEW508 II. Structure of Surfaces Reconstruction on metallic surface – Ir(100) (5x1) reconstruction Bulk structure:the square lattice Surface structure: hexagonally close packed layer

38. EEW508 II. Structure of Surfaces Reconstruction on metallic surface –Ir (110) missing dimer row (2x1) reconstruction structure

39. EEW508 II. Structure of Surfaces Reconstruction – Ionic crystal Ionic crystal consists of charged spheres stacked in a lattice.

40. EEW508 II. Structure of Surfaces Surfaces with strong chemical bonds exhibits more drastic rearrangement of surface atoms Generally speaking, surfaces with weak chemical bonds (van der Waals, hydrogen, dipole-dipole and ion-dipole) exhibits less pronounced reconstructed structure -- for example, Graphite (0001) surface

41. EEW508 II. Structure of Surfaces Reconstruction of high-Miller-index surfaces Reconstruction at Cu(410) stepped surface. Atoms in the first row at the each step become adatoms which are pointed out in the side view of the reconstructed surface. Roughening transition: If the surface is heated near the melting temperature, the steps become curved and break up into small islands

42. III. Molecular and Atomic Process on Surfaces

43. EEW508 III. Molecular and Atomic Process on Surfaces Structure of ordered monolayer When atoms or molecules adsorb on ordered crystal surface, they usually form ordered surface structure over a wide range of temperature and surface coverages. Two factors which decide the surface ordering of adsorbates are Adsorbate-adsorbate(AA) interaction and adsorbate-substrate(AS) interaction Chemisorption – adsorbate-substrate interaction is stronger than adsorbate-adsorbate interaction, so the adsorbate locations are determined by the optimum adsorbate-substrate bonding, while adsorbate-adsorbate interaction decides the long-range ordering of the overlayer. Physisorption or physical adsorption – AA interaction dominates the AS interaction –the surface could exhibit incommensurate structures.

44. EEW508 III. Molecular and Atomic Process on Surfaces Coverage of adsorbate molecules Definition of coverage: one monolayer corresponds to one adsorbate atom or molecules for each unit cell of the clean, unreconstructed substrate surface. For example, the surface coverage of atom on fcc(100) is one-half a monolayer.

45. EEW508 III. Molecular and Atomic Process on Surfaces Ordering of adsorbate molecules Atomic oxygen on Ni (100) Up to one quarter of the coverage: Ni(100)-(2x2)-O Between one quarter and one half Ni(100)-c(2x2)-O

46. EEW508 III. Molecular and Atomic Process on Surfaces Epitaxial Growth With metallic adsorbates, very close packed overlayers can form because of attractive force among adsorbed metal atoms. When the atomic sizes of the overlayer and substrate metals are nearly the same, we can observe a one-monolayer (1x1) surface. This is called epitaxial growth.

47. EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring – Ni (100) – c(2x2) - C Carbon chemisorption induced restructuring of the Ni (100) surface. Four Ni atoms surrounding each carbon atom rotate to form reconstructed substrate.

48. EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring – Fe (110) – (2x2)-S S-Fe (110), Sulfur-chemisorption-induced restructuring of the Fe(110) surface.

49. EEW508 III. Molecular and Atomic Process on Surfaces Adsorbate induced restructuring of steps to multiple-height step – terrace configuration

50. EEW508 III. Molecular and Atomic Process on Surfaces Sulfur-chemisorption-induced restructuring of the Ir (110) surface Open and rough surfaces reconstruct more readily upon chemisorption. For example, fcc(111) surface restructure more frequently upon chemisorption than do the closer-packed crystal faces.