EEW508

1. EEW508 Structure of Surfaces Surface structure Rice terrace

2. EEW508 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 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 Structure of Surfaces Terrace – flat surface Stepped surface Kinked surface

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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