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Presented by Dr. Cao Yong

An Introduction to Surface Chemistry. Presented by Dr. Cao Yong. What you want to learn. general concepts tools for research how instruments work analytical process solve practical problems applications and new advances. Introduction (to Surface Chemistry).

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Presented by Dr. Cao Yong

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  1. An Introduction to Surface Chemistry Presented by Dr. Cao Yong

  2. What you want to learn • general concepts • tools for research • how instruments work • analytical process • solve practical problems • applications and new advances

  3. Introduction (to Surface Chemistry) • Vacuum Technology • Geometric and Electronic Structure of Surfaces • Surface Analytic Techniques • Adsorption and Reaction of molecules on Surfaces Course Outline

  4. G. A. Somorjai, Introduction to Surface Chemistry and Catalysis, John Wiley (1994). D. P. Woodruff and T. A. Delchar, Modern Techniques of Surface Science, 2nd ed.,Cambridge (1994). 陆家和，陈长修等, 表面分析技术， 电子工业出版社 (1987). 华中一等, 表面分析, 复旦大学出版社 (1989). R. M. Nix, An Introduction to Surface Chemistry, http://www.chem.qmw.ac.uk/surfaces/scc/ L.Karlsson,Electron Spectroscopy,http://skinner.fysik.uu.se/ ~andreik/www/Teaching/ElecSpec/ElecSpec.html Literature Resources

  5. Introduction • Importance of Surfaces • Development of Surface Chemistry • Surface Sensitivity and Specificity • TheSurface Science Approach • Surface Analytical Techniques • The Need for Ultra-high Vacuum (UHV) • Experimental Surface Science Systems

  6. Sensors Energy Conversion Catalysis Surface Processes Pollution Electronic Devices Corrosion Why are surfaces important

  7. Solar cells/Fuel cells A clean, efficient energy source !

  8. ULSI technology ! A typical MOSFET! Semiconductor/Electronics industry

  9. 霜惊日月，锋敌天下 Anti-corrosion layer Sulfide Protection layer/Bronze (Sn-Cu)

  10. Ammonia /F-T synthesis Chemical industry Modeling catalyst !

  11. Complexing Approach - Simplify the problem without losing relevance Designing a model catalyst system G.A. Somorjai et. Al. , "Surface Science Approach to Modeling Supported Catalysts" Catal. Rev. Sci. Eng. 39, 77-168, 1997. J.W. Niemantsverdriet et. al. , Eds. Modeling Supported Catalysts in Surface Science, Top. Catal. Vol. 13

  12. (111) (775) (100) (1087) prevent the hydrolysis by S doping to poison the kink sites The reaction path is influenced by the surface structure

  13. Surface Science Surface Science is the study of the natures of surfaces and their interactions with the surrounding environment.

  14. Surface chemistry One important branch of surface science, surface chemistry is concerned with the measurement of surface composition and the study of surface chemistry of solid samples.

  15. An representative surface chemistry system

  16. Development of Surface Chemistry Catalysis Electrochemistry Photography Tribology Surface Instrumentation Surface Thermodynamics Colloids Adsorption Science Electron Emission Microporous Solids Monolayer Science Surface Magnetics Optical Surfaces Polymer Surfaces Cluster Science 1800 1850 1900 1950 2000

  17. Typical single crystal surface of 1 cm2 area contains ~ 1015 atoms/10-9 moles Area probed ~ 1 mm2 and sensitivity ~ 1% monolayer  analytical techniques with picomolar (10-12 moles) detection limits necessary Surface Sensitivity and Specificity • Two major difficulties with surface science studies: (a) Absolute number of atoms in surface is small (problem of sensitivity)

  18. Typical surface: bulk atom ratio in solid ~ 10-7 - 10-8. depending on surface structure  need to be able to separate surface signal from bulk signal. To ensure that the bulk signal is small compared to the surface signal i.e. that the vast majority of detected signal comes from the surface region of the sample. To ensure that the surface signal is distinguishable (shifted ) from the comparable bulk signal, and that the detection system has sufficient dynamic range to detect very small signals in the presence of neighbor -ing large signals. (b) Ratio of surface atoms to bulk atoms is small (problem of specificity)

  19. Complexity of Real Surfaces

  20. vacancy screw dislocation terrace vacancy step kink adatom terrace edge dislocation adatom

  21. Chemistry and physical properties of surface depend on its electronic structure, which is in turn a function of nature of atoms comprising surface and their spatial distribution. • Real surface consist of a mixture of flat regions terra-ces) and defects (steps, kinks and point defects), with different distribution of atoms and hence electronic properties at these surface sites. • Each surface site exhibits its own chemistry and physic-al response  any information gathered from a real surface compositions  impossible to investigate real surface reliably/reproducibly at atomic level.

  22. The Surface Science Approach • Necessary to define precisely chemical and structural state of surface under investigation in order to extract atomic level information. • Most simplified systems: well defined single crystalsurfaces with high ratio of terrace to defect sites.

  23. Greater complexity can be introduced into system by adding controlled amounts of surface defects or coverages of chemically distinguishable adsorbate atomsand molecules. • Once model system has been established, a range of experimental surface science techniques can be used to analyze the surface.

  24. Understanding Surfaces • What is the structure of the surface? • How do molecules adsorb on it and how do they breakup and react? • What atoms, molecules, and fragment of molecules are present on the surface? • How do atoms and molecules move around on the surface?

  25. What is the effect of physically and / or chemically modifying the surface at the molecular scale and how can this be used to control its behaviour? • Fundamentally chemistry and physics at the atomic/molecular level. • Structure-activity relationships established through model studies. • Scientific principles used to engineer surfaces for specific applications.

  26. Surface Analytical Techniques • Modern surface analytical techniques are characterized by a beams in/beams out or probe/measure approach. • Incident beam/probe interacts with surface layer, while output beam carrying the measured surface information is analyzed. • Beams may consist of photons, electrons,ions,atoms or molecules.

  27. Electric (magnetic) field Heat Photons Ions Neutrals Electrons Surface Synchrotron radiation?

  28. Technique (Full Name) Acronym AES Auger Electron Spectroscopy AFM Atomic Force Microscopy HREELS High Resolution Electron Energy Loss Spectroscopy LEED Low Energy Electron Diffraction MBS Molecular Beam Scattering NEXAFS Near Edge X-ray Absorption Fine Structure RAIRS Reflection Absorption Infrared Spectroscopy SEXAFS Surface Extended X-ray Absorption Fine Structure SIMS Secondary Ion Mass Spectroscopy STM Scanning Tunneling Microscopy TDS Thermal Desorption Spectroscopy UPS Ultraviolet Photoelectron Spectroscopy XPD X-ray Photoelectron Diffraction XPS X -ray Photoelectron Spectroscopy

  29. Adsorption of Molecules on Surfaces • Atoms at surfaces are coordinatively unsaturated  high tendency to form bonds with molecules in another phase to which they are exposed  adsorption. • Substrate: solid surface onto which adsorption can occur. • Adsorbate: atoms or molecular species which are adsorbed (or are capable of being adsorbed ) onto the substrate.

  30. three generally accepted ways for coverage expression • Coverage  (monolayers/ML): • a measure of the extent of adsorption of a species onto a surface and can be specified in a number of ways

  31. actual surface coverage  = , 0    1 saturation surface coverage no. of adsorbed species per unit area of surface  = no. of surface substrate atoms per unit area (a) as the number of adsorbed species per unit area of surface atoms (e.g. in molecules cm-2). (b) as a fraction of the maximum attainable surface coverage: (c) relative to the atom density in the topmost atomic layer of the substrate:

  32. N P n (molecules m-3) = = V kT 8kT c (m s-1) =  m Adsorption at the Solid-Gas Interfaces • Gas density: no. of gas molecules per unit volume: • Average molecular speed: • Mean free path of molecule in gas phase: average distance travelled in gas phase between collisions

  33. 1 kT  (m) = = nd2 2 P2 d: molecular diameter,  : collision cross section P 1 nc F (molecules m-2 s-1) = = 4 2mkT • Incident molecular flux on surface: number of incident molecules per unit time per unit area of surface: • Sticking cofficient/probability S: fraction of incident molecules which adsorb upon the surface; depends on many factors including coverage, temperature and crystal surface.

  34. The Necessity of Ultrahigh Vacuum (UHV) • Consider a clean solid surface exposed to a flux of gas molecules F with sticking probability S(). • A monolayer of atoms/molecules has a number density of ~ 1015 cm-2 or ~ 1019 m-2. • The number of molecules dN adsorbed on the surface in time interval dt is given by dN = FS()  dt.

  35. Degree of Vacuum P (Torr) n (molecules m-3)  (m)  (s) Atmospheric 760 2  1019 10-7 10-9 Low 1 3  1016 10-4 10-6 Medium 10-3 3  1013 10-1 10-3 High 10-6 3  1010 102 1 Ultra-high 10-10 3  106 106 104 • Assuming that S = 1, time required for clean surface to become covered with a complete monolayer of adsorbate   1019/F. • The exposure of gas molecules is measured in units of Langmuirs (1L = 10-6 Torr s).

  36. Dimensions of experimental vacuum system ~ 1m  for collision free conditions of probe and detected particles (electrons, ions, atoms) in surface analysis, P < 10-4 Torr. • Time for experiment > 1 hour  for maintenance of a clean surface with minimal contamination from background gas adsorption, P < 10-9 Torr.

  37. Preparation of Clean Solid Surfaces • Solid surfaces under ambient conditions are usually covered with contaminants such as O, C and S, which have to be removed before reliable and reproducible surface science experiments can be performed. • Exposure to UHV to desorb weakly bound volatile contaminants (e.g. Mica, NaCl(100)).

  38. Heating in UHV to desorb strongly bound volatile contaminants or dissolve contaminants into bulk (e.g. O and C from Si(111)). • Heating in a gas atmosphere to react off contaminants as volatile species (e.g. C from Ni(111) with O2).

  39. Bombarding with noble gas ions (e.g. Ar+, Ne+) to physially knock contaminants off surface, following by annealing to reorder surface atoms. • Cleaving in UHV to expose new surface (e.g. LiF(100), CaF2(111)) • Evaporation onto suitable substrate to prepare thin films.

  40. Preparation of a clean surface by heating and bombardment cycles

  41. Ultrahigh Vacuum (UHV) System超高真空系统

  42. Experimental Surface Science Systems

  43. An UHV system with muti-techniques

  44. Stainless steel ultrahigh vacuum chamber. • Metal vacuum seals. (what material?) • Chamber assembly bakeable to 200 oC. • Combination of vacuum pumps. • In-situ surface analytical techniques.

  45. 真空系统的连接－－－法兰(flange)

  46. Copper gasket between two knife-edges

  47. UHV chamber containing MBE growth/analysis system

  48. UHV system for purpose of surface science studies

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