L/O/G/O Surface Analysis 化學工程系李玉郎
Text Book: Surface Analysis The Principal Techniques (2nd Ed.) Edited by John C. Vickerman, and Ian S. Gilmore, John Wiley & Sons, 2009 Reference Book: 1. Surface Analysis Methods in Material Science Edited by D.J. O Connor, B.A. Sexton, R.St.C. Smart (Springer, 1992) 2. An Introduction to Surface Analysis by XPS and AES John F. Watts and John Wolstenholme (John Wiley & Sons, 2003)
Syllabus 1. Introduction- definition of the surface. 2. Vacuum technology for applied surface science. 3. Electron spectroscopy for chemical analysis (ESCA). 4. Auger electron spectroscopy (AES). 5. Secondary ion mass spectrometry (SIMS). 6. Vibrational spectroscopy from surfaces (FTIR-ATR). 7. Scanning tunneling microscopy and atomic force microscopy (STM, AFM). 8. Surface properties determination by surface wettability.
Surface Structure concentration outmost bulk phase surface region 5-20 nm Why should surface be so important? depth 1. The surface atoms are different from those in the bulk. 2. The surface atoms are the first to be encountered regarding the interaction with another phase.
Applications of surface materials: Chapter 1. Introduction corrosion protection - surface treatments modification of optical behavior - surface coatings or changing surface composition surface chemistry of polymers: cling for packing, non-stick for cooking, implanted into body - drug delivering, replace body component catalyst - chemical reaction. waste gas removing of combustion engine
It is the surface which interfaces with its environment, and the surface reactivity will determine how well the material behavior in its intended function.
How do we define the surface The surface can be considered in terms of three regimes: 1.The top surface monolayer:the top layer of surface atoms are those which are immediate interface with the other phases impinging on it. (surface tension, contact angle, adsorption..) 2.The top 2-10 atomic layers ( 0.5 ~ 3 nm ): The structure and chemistry of the top layer will be significantly determined by the atoms immediately below. (LB film, lipid membrane) 3.Surface films in the range 10 100 nm. Apply these films to the devices and components, to protect, lubricate, change the surface optical properties. Techniques are required to distinguish the properties of these three regimes.
How many atoms in a surface? 1 atom 3Å = 3x10-10 m = 3 x 10-8 cm Number of atoms in 1 cm 1/ (3 x 10-8 ) = 3 x 10 7 Consider a metal of 1 cm cube 10 23 of atoms in 1 the cube 1 cm 2 surface has roughtly 10 15 atoms ratio of surface to bulk atoms: s / b 10 15 /10 23 = 10-8 = 10-6 % Consider the surface analysis The area that a surface analysis technique probes is about 1 mm 2, contains 10 13 atoms in the top monolayer. If top ten layers can be detected, the atoms detected are 10 14 atoms ( 10-10 mole )
Consider the composition of species Chemical species which play an important role in influencing surface reactivity may be present in very low concentration, about 10-3 - 10-6 (1000ppm- 1ppm) on the top layer, the atoms can be detected: 10 10 ~ 10 7 atoms (10-14 - 10-17 mole) within 1 mm 2 For the technique to map variations in chemistry across a surface (monitor the homogeneity of an optical or a protective coating ) Spatial relation: 1μm or even less area: 1μm 2 10 7 atoms in the top monolayer If the species to be determined has 10-3 atom fraction level, 10 4 atoms can be detected
Principles for Surface Analysis The surface analysis techniques are based on bombarding the surface to be studied with electrons, photons, neutrons, or ions, and detecting the emitted electrons, photons, neutrons, or ions.
Information required physical topography chemical composition chemical structure atomic structure electronic state bounding of molecules at the surface No one technique can provide all these different pieces of information.
EXAFS: Extended X-ray Absorption Fine Structure EELS: Energy electron loss spectroscopy LEED: low energy electron diffraction
Vacuum required Most techniques of surface analysis are carried out in vacuum because electrons and ions are scattered by molecules in the gas phase. Vacuum based methods:allow one to control the influence of the ambient on the surface under study. A surface uncontaminated by any adsorbate operate in ultra-high vacuum(uhv, < 10-9 mmhg) At 10-6 torr:one monolayer of adsorbed species formed within 1 second(if sticking coefficient = 1)
Surface Sensitivity Ion Scattering Spectrometry (ISS): derives almost all its information from the top monolayer. Electron Spectroscopy for Chemical Analysis (ESCA):samples in the top 10 layers of the surface X-ray can penetrate deep into the solid, the resultant emitted electrons which can be detected without loss of energy can only arise from 1~ 8 nm of the surface. Infra-red(IR)spectroscopy: not surface sensitive, sample deep into the solid, (unless reflection mode)
Surface Sensitivity Secondary Ion Mass Spectrometry(SIMS): The energy of bombarding ions can penetrate down to 30 ~ 40 nm. However, 95 % of the secondary ions which are knocked out(sputtered)of the solid, arise from the top two layers(0.6 nm).
Information depth: Information Depth a specified depth defined as the average distance normal to the surface from which a specified percentage(90, 95, 99 % )of the detected single originates. Sampling depth:95 % of information depth 3 times of inelastic mean free path (λ) 2 atomic layers (ca. 0.6 nm) --- for metal, inorganic material Inelastic mean free path(imfp, λ)of an electron: the mean distance that an electron travels before undergoing an inelastic event, i.e, some interacting whereby it loses energy.
IMFP Inelastic mean free path(imfp, λ) thickness of a material through which 63% of the traveling electrons will loss energy. d N/N o =e d/λ d=λ, N/N o =0.365 d=2λ, N/N o =0.135 d= 3λ, N/N o =0.05 N o N N o : number of electron before collision N :number of electron without suffering collision after traveling distance d
Radiation Effects Radiation Effects surface damage Most of the techniques require the surface to be bombarded with photon, electron or ions. They will affect the chemical and physical state of the surface being analyzed. For a particle with energy of 1000 ev, the penetration depth are: Particle Energy (ev) Depths (A) Photon 1000 10 000 (1 um) Electron 1000 20 (2 nm) Ions 1000 10 (1 nm)
Extent of surface damage: photons < electrons < ions Low damage technique: ESCA/XPS High damage technique: SIMS a technique based on the damage of surface. However, it can be operated in a low damage mode Power input: SIMS < LEED <XPS
The sample can be fired under the bombardment of photons or electrons if: (1) the material is delicate (organic materials); (2) power input is too high; (3) analyzed time is too long. It is consequently very difficult to analyze the surface of organic materials using any technique which relies on electron bombardment.
L/O/G/O Thank You! 化學工程系李玉郎