SURFACE ANALYSIS

Nota:

• use all the techniques to characterize the atoms of the surface

1. INTRO
   1. WHY?
      1. 1. Coatings

         Nota:

         • want to see how the surface looks like
      2. 2. CIP process (adsorption/ desorption)

         Nota:

         • carbon import process wan to see how porous the surface is - microscopically, the surface is not a straight line (more porous - more reactive)
      3. 3. corrosion
      4. 4. friction
      5. 5. heterogeneous catalysis
      6. 6. Environmental applications --> wastewater/gas treatments

         Nota:

         • high porosity absorbs more pollutant (e.g. water tap)
   2. BASIC CONCEPTS
      1. Discriminate for the (small amount) of ‘surface’ against the (large amount) of the ‘bulk’
      2. 'Surface’ --> direct contact (interface) with other phases, e.g. gas or liquid.
      3. why is it important to analyse surface?

         Nota:

         • in Carbon Import Process (CIP) - need to analyse activity carbon - to absorb gold complex from ores. analyse the surface of the activity carbon to see the changes before & after the process of absorbing the complex (porous?)
      4. describe the concept

         Nota:

         • Fire a particle from a source (can be a thermionic gun) - heat up the source. - emits particles (e.g. electrons/ions etc) - travel through a path of vaccum (no obstruction) - hit the surface of the sample (results in emission of some of the electron from the surface) (some particles bounce back) - measure the emitted particles/energy
   3. What is a surface?
      1. Depends on the technique (& does not usually mean only the first layer of atoms)
      2. 0.1 nm (AES; STM) --> 100 nm (XRD)
   4. In situ vs. ex situ techniques
      1. In situ - some atomic force microscopy (STM/AFM) surface enhanced Raman spectroscopy (SERS)

         Nota:

         • e.g. want to see how the virus work under certain environment (virus can't get out of the body- die)
      2. ex situ- more broadly applicable

         Nota:

         • samples dry and operate in a vacuum
   5. Surface physics techniques

      Nota:

      • ‘fire’ something (photon, electron, ion) at surface & measure what comes off (emitted photon, electron, ion)
      1. Photon --> a quantum of light or other electromagnetic radiation.
      2. Ion --> Atom/molecule with net electric charge due to the loss or gain of one or more electrons.
      3. Electron --> subatomic particle (negative charge).
      4. All require high vacuum (HV ~10^–9 atm) or ultra-high vacuum (UHV ≲10^–12 atm) due to:
         1. scattering effect – gas molecules.

            Nota:

            • the path of the fire should be clear to avoid collision (from the source and the surface)
         2. contamination – fast generation of monolayer

            Nota:

            • if the surface is reactive and the env is not clean, it will react with the surface. e.g. gases bind on the surface, hence when fire the surface, the results come from the reflection of the contaminates
   6. Required surface data
      1. Morphological images

         Nota:

         • is rough? smooth?
      2. Topographical data
      3. Chemical composition & structure
      4. Electronic state

         Nota:

         • e.g. Pb2+?
      5. Bonding descriptions
   7. Surface sensitivity

      Nota:

      • E.g. XPS is construed to be a ‘surface sensitive’ technique. However, radiation can be derived from few atomic layers of the ‘surface’!
      1. Surface sensitive technique  more sensitive to atoms near ‘surface’ than atoms in the ‘bulk’.
      2. should only detect radiation due to atoms in the ‘surface’.
   8. Origin of Surface Sensitivity
      1. arises from short escape depth of the emitted ‘particle’

         Nota:

         • Thus, detector only ‘sees’ particles being emitted from the surface and not from the bulk
2. ELECTRON MICROSCOPY (EM)
   1. Resolution depends on lambda of the ‘light’ being used to form the image:

      Nota:

      • microscope - imaging device
      1. (b) EM (5-30 keV --> lambda ~5 pm) --> ~1 nm (or better!)
      2. (a) Optical microscope (visible light, lambda ~ 500 nm) --> ~1 um
   2. Types
      1. Scanning (SEMs)

         Nota:

         • (range --> usually micron)
         1. Image formation
            1. processes occur when electrons hit a surface

               Nota:

               • (eV) is a unit of energy ≈1.6×10^−19 joule
            2. Energy focused by one or two condenser lenses to a spot

               Nota:

               • (ca 0.4 nm to 5 nm in diameter).
         2. operation

            Nota:

            • can be used for non-conductive samples --> resolution enhanced significantly by sputtering them with conductive metals (e.g. platinum or gold). -Usually for determining the morphologies of surface coatings, solid state samples, ores etc.
            1. electron beam - focussed -contact w the sample - electrons lose energy - energy exchange -> primary backscattering & X-rays - detectors

               Nota:

               • •An electron gun emits a beam which is focused by a series of condenser lens. •Once beam is in contact with sample, the electrons lose energy •Energy exchange  resulting in primary back-scattering of electrons. •X-ray is also produced. •Detectors then collect the secondary (electrons emitted by the surface) or backscattered electrons (originated from the beam), and convert them to a signal.
               1. secondary electron - other radiation (morphology & topography)
               2. primary back-scattered electron - info on composition

                  Nota:

                  • what is contained inside the material
         3. Typical configuration

            Nota:

            • 1.Magnetic lenses rather than optical lenses = more control of magnification. 2.Typically narrow electron beam  good depth of field  good clarity. 3.10X to 100,000X, maybe even more.
         4. Can be coupled with: (EDS or EDX)

            Nota:

            • Energy-dispersive X-ray spectroscopy
            1. Difference in energy between higher/lower energy shell --> X-ray --> detected by spectrometer

               Nota:

               • Different element has different atomic structure = different set of peaks on its X-ray spectrum. •Beam ‘excites’ and ‘ejects’ electron in inner shell. •Electrons from higher energy levels to the ‘hole’ until minimum-energy state is regained.
            2. - give a micrograph - give % atoms/elements
         5. Sample prep
            1. dry
            2. •‘Sputtering’ for non-conductive samples (e.g. biological samples) --> e.g. thin gold coating

               Nota:

               • e.g. coating a spider with gold (have to have coat - for interaction w the beam)
            3. Specimen ‘stub’
         6. Examples of output and applications

            Nota:

            • •Porous materials •Metal surfaces •Bio-organic material - fungi •Ceramic surfaces •Nanoparticles/nanowires
      2. Transmission (TEMs)

         Nota:

         • (range --> usually nano) can look at the atom of the samples- atomic arrangement - the arrays on the surface
         1. Electrons transmitted through sample --> magnified image via a camera.
         2. Main components
            1. (1) vacuum chamber (2) emission source (production of electrons); (3) Electromagnetic lenses and electrostatic plates.
         3. Bio samples / nanotech / crystallography
         4. advantage - higher resolution - can see atoms

            Nota:

            • Small wavelengths of high-energy electrons to probe solids at the atomic scale
         5. disadvantages : 1- selectivity(result reasonable?) 2- sample damage under the beam 3- x wet/live sample
         6. operation

            Nota:

            • 1.Top of the TEM column  electron gun  source of electrons. 2.Electrons are accelerated to high energies (ca 100-400 keV) and then focused towards the sample via set of condenser lenses and apertures. 3.Thermionic emission. Thermal energy is added to a material  electrons may overcome the energy barrier of the work function and escape. 4.The material used must either have a very high melting point (e.g. W).
            1. electron beam - focussed - thermionic emission - (passes through elements and scatter)

               Nota:

               • 1.Electrons pass through the specimen and scattered. 2.The information --> converted into an image. 3.One conventional imaging way is to magnify diffraction pattern until it is of the required size for analysis.
            2. 4.Another way --> fine beam of electrons is rastered across the sample --> quantity of scattering from each point may be measured separately and successively.

               Nota:

               • raster - scan the beam across the sample conventional TEM - spot the beam on the sample
         7. e.g. output
            1. metal nanoparticles (Pt) on nanoporous material support

               Nota:

               • What we see in the image is variations in the amount of scattering at different locations in the sample.
         8. Other TEM-based/related techniques
            1. SAED

               Nota:

               • Selected-area electron diffraction
               1. crystallographic information (selected region)

                  Nota:

                  • if the sample is crystal, it difracts
               2. Diffraction patterns - electrons scatter
               3. Crystalline samples --> planes of atoms that scatter (diffract) the electrons at specific angles.

                  Nota:

                  • what type of crystal is the sample? single crystalline, highly crystalline, polycrystalline amorphous, etc
                  1. •Polycrystalline samples (or samples with small particles) = ring patterns.
                  2. •Single crystal samples = patterns (e.g. spots).

                     Nota:

                     • no spots in non-crystalline particles
                  3. •From the ring radii or spot positions, - determine the planar spacings.
               4. Diffraction patterns --> measure the distances between the atom planes.
               5. Basic diffraction pattern interpretation

                  Nota:

                  • e.g. is the sample crystalline or amorphous?
            2. High resolution (crystal lattice) imaging
            3. Element distribution imaging (EFTEM)

               Nota:

               • (via energy-filtered TEM, EFTEM)
               1. + with EELS
               2. generate qualitative or quantitative elemental maps indicating the location of specific elements in the sample.
               3. Create images showing distribution of selected element and thickness variation in sample.
            4. Composition analysis - EELS

               Nota:

               • electron energy-loss spectroscopy,
               1. amt of energy lost by electrons after they have passed through a sample is analysed.
               2. characterise ENERGY LOSS - identify the existing element
               3. EELS spectrum => # of electrons (intensity) as a function of the amount of energy they lose
               4. measure the loss of energy - converts into a curve - info on valence states and bonding
            5. STEM

               Nota:

               • Scanning TEM imaging
      3. Scanning probe microscopy

         Nota:

         • can produce a topograph of the sample (rough?smooth?spiky?) -use mechanical probe instead of electron beam to scan the sample
         1. Surface topography
         2. (STM)

            Nota:

            • Scanning tunneling microscopy
            1. general operation
               1. sample must be conductive

                  Nota:

                  • When the tip is close to surface, a tunnelling current can flow. Relies on the quantum mechanical electron tunneling through an energy barrier larger than its kinetic energy.
                  1. Constant height, d.
                     1. current varies
               2. Measures surface electron density.

                  Nota:

                  • when the probe is close to sample,  w electric current - sucks up the electron from the sample - when look on the screen, can see the atoms in an array (arranged)
               3. Constant current, It,
                  1. Tips move up and down;
                  2. Measure tip position (x,y,z) by piezoelectric device

                     Nota:

                     • (Measurement of expansion/contraction (nm) of ceramic material and converting them to an electrical charge) *(1nm ≈ 1V)
            2. QMT

               Nota:

               • Quantum mechanical tunnelling
               1. - predicts a finite probability of electron “tunnelling” through barrier.
               2. It ≈ V e^–kd

                  Nota:

                  • d = tip/surface separation; k = tunnelling probability (constant) Two modes, namely, (1) constant current, It, (2) constant height, d.
         3. (AFM)

            Nota:

            • Atomic force microscopy
            1. diff from STM ( AFM probe relies on forces (electrostatic, VDW forces)) --> spring action (Hooke's Law)
            2. laser diodes measure the variation of the spring actions (cantilever)
            3. Basic

               Nota:

               • When tip is close to sample surface, forces induced  deflection of the cantilever (Hooke's law). Contours of the surface are measured directly using the deflection of the cantilever
               1. Cantilever = probe (tip)
               2. Tip radius of curvature = nm
            4. Imaging Modes
               1. contact mode - tip touches and exerts forces (0.1 – 1 nN) on the sample.
               2. tapping mode - the cantilever tip vibrates near the resonance frequency at 300kHz. Less destructive than contact mode
               3. non-contact mode - the cantilever oscillates at slightly above the resonant frequency near the surface of the sample, but does not contact it.
         4. Physical probe scans specimen by moving sharp probe in a raster
   3. diff btwn (AFM & SEM) - probes and electron beam
   4. similarity (AFM &SEM) - relies on rastering while TEM (1 electron beam on the sample)
3. AES
   1. Identification and quantification of elements on materials surfaces.
   2. Based upon the measurement of the kinetic energies of the emitted electrons
   3. 3 STEPS
   4. FEATURES
      1. photon OR electron beam
      2. E(e-a) depends only on E(orbital) - provides atomic (elemental) composition

         Nota:

         • independent of the beam E
      3. accuracy ~ +-10%
      4. True surface technique ~ 0.1 – 1 nm, E(eA–) ~ 20 – 1000 eV (low!) --> v. short escape depth

         Nota:

         • very surface sensitive compared to TEM. bcoz the E used is quite low.
      5. recorded in differentiated mode
      6. Little matrix (chemical shift) effect
      7. Auger ‘spot’ size 1–500 um (good 2-D spatial resolution)
   5. APPLICATIONS
      1. e.g. metallurgy, electronics
      2. Only limitations are those of sample
         1. stable to vacuum (non-volatile)
         2. stable to beam (localized T v. high)
   6. Exploiting the spatial resolution of AES & XPS
4. XPS

   Nota:

   • a.k.a ESCA - Electron Spectroscopy for Chemical Analysis
   1. QUANTITATIVE spectroscopy

      Nota:

      • Spectroscopy = absorption, emission or scattering of light and other radiation by matter, as function of wavelength
   2. Determines elemental composition and chemical state of sample
   3. Bombards sample w X-ray -> measures the kinetic energy + # of electrons from surface (< 10 nm). - RELATIVELY SURFACE SENSITIVE

      Nota:

      • X-ray usually comes from aluminium or magnesium (monochromatic) SURFACE SENSITIVE - things can emerge - photoelectron from K/L shell - eject into the vacuum
   4. irradiated w PHOTON

      Nota:

      • E(X-ray or hV) is such that ep– (the ejected ‘photo-electron’) is usually from inner (K or L) shell.
      1. x-ray less strong than electron beam - relatively non-destructive - can analyse sensitive sample e.g. polymers
   5. Features
      1. E(e-p) ~ Ehv - Eb

         Nota:

         • kinetic energy of emitted electron - E(photoelectron) depends on E(X-ray) &amp; E(binding)-binding energy of electron in atom
      2. Sensitivity ~0.1 at.% (< AES); accuracy ~ +-10 %
      3. Simpler line shape; peaks identified by element & orbital of origin, eg, C(1s), S(2p), etc
      4. Larger spot size
      5. less surface sensitive (~1–5 nm)
      6. Better for non-conducting samples
      7. bigger chemical shift.

         Nota:

         • Atoms of a higher positive oxidation state --&gt; higher binding energy --&gt; additional coulombic interaction between the photo-emitted electron and nucleus
      8. UHV
      9. x detect hydrogen or helium- orbital diameter small
   6. Chemical Shifts
      1. Peak positions sensitive to : a) chemi env b) oxidation state

         Nota:

         • contamination in  the air- volatile carbon
      2. chem shift : CH. IN BINDING E of a core electron of an element due to a CH IN THE CHEM BONDING of that element.
      3. Core binding energies are determined by:
         1. 1) electrostatic interaction
         2. 1 REDUCED BY: 2 ) electrostatic shielding by other electrons
         3. 3) removal or addition of electronic charge
      4. Withdrawal of valence electron charge increase in BE (oxidation)
      5. Addition of valence electron charge decrease in BE
      6. Chemical shifts in XPS and AES

         Nota:

         • because: 1- better resolution- finer peak 2- single electron process (AES - 3 energy levels/3 steps)
         1. XPS better at detecting chemical shifts
   7. Output
      1. wide scan and narrowed down spectra(gives %)
5. SECONDARY ION MASS SPECTROSCOPY
   1. sputters surface of sample with a focused PRIMARY ION BEAM and collecting and analyzing ejected SECONDARY IONS.

      Nota:

      • sputter - shoot something to remove the surface (ions) (some atoms ejected are neutral, but some are ionized) MEASURED SECONDARY IONS BY SPECTROMETER
   2. determination of elemental, or molecular composition
   3. BASIC
      1. high energy ion beam (5-20 keV)

         Nota:

         • non reactive ions - He+/Ar+
      2. Ion beam produced by bombarding gas with e–
      3. Focus - Spot size ~ 1–5000 um
   4. ADVANTAGES/FEATURES
      1. Can determine all elements
      2. Sensitivity extremely high ~10^–15 g (NB small volume so only ~ppm level)
      3. Microprobe versions especially powerful (spatial resolution ~0.5 to 5 um)
      4. Sampled surface <1 nm (true surface technique)
      5. determine +ve & –ve secondary ions
   5. DISADVANTAGES
      1. Incompatibility between spatial resolution and sensitivity
      2. Problems with differential sputtering

         Nota:

         • can be uneven spatttering
      3. expensive