2.Scanning electron microscopy (SEM)

ESEM Applications

-Dissolution and precipitation of halite -intergranular microporosity - Na2SO4 (sodium sulfate) salt study -showing potential damaging effect of salt crystallization -formation of thenardite crystals Thenardite: (Na2SO4) -anhydrous sodium sulfate mineral -occurs in arid evaporite environments

Differences between EDS & WDS

-EDS first used for element identification, for fast analysis of the entire spectrum -WDS is then used to solve various spectral problems EDS: -element analysis -Resolution: Energy-dependent -Min useful probe size: ~5nm -Typical data-collection time: minutes -Spectral artifacts: escape peaks, electron-beam scattering, peak overlap,... WDS: -quantitative analysis (not suited for element identification) -Resolution: Crystal-dependent -Min useful probe size: ~200nm -Typical data-collection time: tens of minutes -Spectral artifacts: rare

Factors that affect SE emission b) Beam energy and beam current Example: diatomen

Low accelerating voltage (5kV): (A) -fine detail imaged Higher accelerating voltage (20kV): (B) -decrease in resolution and contrast -Bar is 1μm -Magnification = x 4000 -Working Distance = 8mm

Factors that affect SE emission a) Workind distance WD

Working distance (=the distance between the final condenser lens and the specimen): -short WD -> high-resolution -influence on the spherical aberration Aberration: =the failure of the lens system to image central and peripheral electrons at the same focal point Spherical aberration present: -beam will scan with enlarged, unsharp spot (A) Spherical aberration absent: -beam will scan with sharpest spot possible (B)

Factors that affect SE emission

a) Working distance b) Beam energy and beam current c) Atomic number (Z) d) Local curvature of surface

ESEM

=new type of SEM, allows the examination of specimens in the presence of gases -wet and dry, insulating or conducting -all specimens can be viewed with no or minimal preparation, in their natural state, or in vacuum -pressures as high as 50Torr -temperatures as high as 1500°C

spectral lines in EDS vs WDS Example2

• When compared to EDS, WDS exhibits superior peak resolution of elements and sensitivity of trace elements • Comparison of EDS (left) and WDS (right) detection of trace Si: The presence of a trace amount of silicon is questionable in the EDS spectrum, but certain in the WDS spectrum.

Factors that affect SE emission b) Beam energy and beam current

-Electron yield goes through a max at low incident electron energy/kV and then decreases with increasing incident electron energy/kV -a higher accelerating voltage will increase secondary yield due to a greater beam penetration => reduction of the edge effect and therefore a diminishing effect on contrast

Secondary Electrons (SE)

-Generated from the collision between the incoming electrons and the loosely bonded outer electrons -low energy electrons (~10-50eV) -only SE genearted close to surface escape (topographic information is obtained) -we differentiate between SE1 & SE2 SE1: -generated by the incoming electron beam as they enter the surface -high resolution signal with a resolution which is only limited by the electron beam diameter SE2: -generated by the backscattered electrons that have returned to the surface after several inelastic scattering events -SE2 come from a surface area that is bigger than the spot from the incoming electrons -> resolution is poorer than for SE1

SEM types/technologies

-Low vacuum scanning electron microscopy (LVSEM) -Environmental scanning electron microscope (ESEM) -Using freezing equipment on a SEM (Cryo-SEM) -Focused ion beam (FIB) technology

X-rays2

-Photons not electrons -each element has a fingerprint X-ray signal -poorer spatial resolution than BSE & SE -relat. long signal collecting times -most common spectrometer: EDS (energy-dispersive sp.) -signal overlap can be a problem -we can analyze our sample in different modes (spot analysis, line scan, chemical concentration map=elemental mapping) -wavelength typ. <10 nm -Energy E=h*c/λ -can penetrate matter (unlike visible light) -Suited for the internal inspection of objects -Contrast depends on density and atomic number Z of the materials

BSE images

-Polished surfaces are ideal for backscattered electron images (BEI) -BEI give information based on the atomic number of the elements in the sample -White structures contain a high atomic number, dark structures contain a low atomic number -Intensity of BSE signal proportional to the average atomic number (AAN) of the specimen e.g.: Gold: -has relatively high AAN of 79.0 -appears bright Quartz (SiO2): -low AAN of 10.8 -appears dark

What is a SEM?

-Scanning Electron Microscope (SEM) -generates a beam of electrons in a vacuum which is scanned in a raster pattern over a specific area (called a frame) -BSE reflected off the minerals in each frame are collected on a screen to produce an image -Electrons that interact with minerals generate X-rays with measured energies characteristic of the elements present

Backscattered electrons (BSE)

-a fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180° the electron can escape from the surface

SEM: technical details The interaction volume; depending on electronic number

-a shape that ranges from a tear-drop to a semi circle within the specimen -its depth and diameter depends on: kV and density of specimen

Cathodoluminiscence (CL)

-is the emission of photons of characteristic wavelengths from a material that is under high energy electron bombardment -The nature of CL in a material is a complex function of composition, lattice structure and superimposed strain or damage on the structure of the material Provide: -general info on trace elements contained in minerals -production of mechanically induced defects in the crystals -inside in processes as crystal growth, replacement, deformation and provenance These applications include: - investigations of cementation and diagenesis processes in sedimentary rocks - provenance of clastic material in sedimentary and metasedimentary rocks - details of internal structures of fossils - growth/dissolution features in igneous and metamorphic minerals - deformation mechanisms in metamorphic rocks

Factors that affect SE emission c) Atomic number (Z)

-more SE2 are created with increasing Z -the Z-dependence is more pronounced at lower beam energies

Factors that affect SE emission d) Local curvature of surface

-most important factor

Scan Unit

-moves the beam in a raster pattern over the specimen area -the signal detected from the sample is synchronised with the unit producing the on screen image -Magnification is controlled by the size of area scanned by the unit

EDX images Example

-point analysis -line scanning -point mapping -> determine the mineralogy of the stone

Wavelength-Dispersive X-Ray Spectroscopy (WDS)

-quantitative analyses (down to trace element levels) -measured at spot sizes as small as a few µm -can create element X-ray compositional maps over a broader area by means of rastering the beam

Limitation of SEM?

-requiring a high vacuum sample environment -samples need to be clean, dry and electrically conductive -> coating with metal or carbon Drawback: -Beam-related contamination: refers to the deposition of material (e.g. C)

Factors that affect SE emission a) Workind distance WD Example: diatomen

At WD=48mm: -spherical aberration is present decreasing resolution (A). WD = 8mm: -effect of spherical aberration is less resulting in an image with improved resolution (B) -Bar is 5μm -Magnification = x 3300 -Acceleration Voltage = 5kV

X-rays; continuous vs. characteristic spectrum

Bombardment of material by electrons produces X-rays by 2 mechanisms: -continuous spectrum (=bremsstrahlung): produced by electrons interacting with atomic nuclei -characteristic spectrum: contains lines that result from electron transitions between energy levels that are specific to each element

EDX images

EDX analysis (energy dispersive X-rays): -element analysis -each element has a fingerprint X-ray signal -EDX cannot tell us which hydrated phase formed (cannot characterize the number of H2O-molecules in the crystal)

SEI vs BEI Example2

Fossil inside a tertiary limestone

Electron Beam - Sample Interaction

Incident beam collides with electrons of the elements inside the sample and creates SE, BSE and X-rays -detector units pick up these signals and convert them to an amplified electrical signal -signal is synchronised to the Scan Unit and displayed on the screen to produce an image -Energy-dispersive X-ray (EDX) detectors collect the spectrum of energies of X-ray photons released

Why is coating necessary?

Picture: white areas are charging artefacts -prevents charging of the specimen -reduces thermal damage -improves SEI -Insufficient coating causes build-up of electrons in the observed area -> charging artefacts

spectral lines in EDS vs WDS Example1

Resolution of Mo and S spectral lines in: EDS (yellow): -Mo & S lines are overlapped WDS (blue): -overlap resolved

SEM vs LM

SEM has 3 main advantages over LM: a)Resolution at high magnification: -Best resolution LM = about 400nm -SEM has a resolution of better than 10nm (typ. 5nm) b)Depth of field: =height of a specimen that appears in focus in an image ->300x depth of field compared to the LM => great topographical detail can be obtained c)Microanalysis: -information about chemical composition, crystallographic, magnetic & electrical characteristics

SEM - Main components

Vacuum system: - coherent electron beam Electron Gun: - Tungsten filament - Field emission source Lens system: - Electromagnetic lenses Scan Unit: - Imaging Detector Units: - BSE, SE, X-ray

Factors that affect BSE emssion

a)Direction of the irritated surface -more electrons will hit the BSE detector when surface is aligned towards the BSE detector b)Average atomic number When you want to study differences in atomic numbers the sample should be as levelled as possible (sample preperation is an issue)