Structural and Compositional Characterisation of Materials Flashcards
State the 3 kinds of microscopy.
Projection image
Optical image
Scanning image
How does projection image microscopy work?
An object is placed in from of a point source of illumination.
How does optical image microscopy work?
Object is magnified using conventional lenses
How does scanning image microscopy work?
Each point of an object is presented serially.
How does a field ion microscope work?
High potential is applied between a very fine tip of radius r and a scene at distance R (magnification R/r)
A low pressure inert gas is maintained. Gas atoms are ionised in the very high field near the surface of the specimen.
Each ion is accelerated towards the screen which contributes a speck if light to the image (phosphor screen)
Ionisation occurs preferentially above the atoms which protrude the specimen surface.
What are the limitations of field ion microscopes?
Only conducting samples can be studied (conductor or semi-conductor)
Only the surface can be studied which may not be representative of the bulk.
Explain how time-of-flight mass spectrometry works.
Aim to determine the elements by mass number and isotopes.
Vaporised sample injected at low pressure. Electron gun fires and kooks an outer electron out of orbit forming positive ions.
Positive ions accelerated by electric field to a constant KE.
Positive ions with smaller m/z will take longer to move through drift area.
Ions distinguished by different flight times by generating a small current when reaching a sensor.
Explain how positive sense atom probe works.
Aim to map the atom concentration within a given volume.
High potential or short laser pulse us applied to the tip, a layer of ionised atoms is stripped from the surface.
Ionised atoms will accelerate towards an imaging screen which is specifically orientated so that the ion falls over a hole in the fluorescent screen which leads to a TOF mass spectrometer.
The mass spectrometer can then identify the element within the sample.
What are the limitations of atom probe?
Only conducting samples can be studied.
Material needs to be needle shaped specimen
Only a volume of 20x20x50nm can be studied at a time.
Describe Abbe’s theory of image formation.
Interaction of illuminating beam and the sample leads to diffraction.
Objective lens corrects the diffraction and a pattern is formed at the back focal plane.
Waves propagate to the image plane and re-interfere to form a magnified image of the sample.
Finite size limits the maximum angle of diffraction collected which limits the smallest features to be seen.
Explain the Rayleigh criterion.
Rayleigh’s criterion specifies the minimum separation between two light sources that may be resolved into distinct objects.
This is when the maximum of one diffraction pattern caused by the circular aperture coincides with the minimum of the other diffraction pattern.
See notion answers
Explain numerical aperture.
In a microscope, NA is important as it relates to the resolving power if a lens.
A lens with a large NA will be able to resolve finer details.
Lenses with larger NA will also be able to collect more light and so give brighter images. Another way to describe this situation is that the larger the NA the larger the cone of light that can be brought into the lens and so the more diffraction modes will be collected.
Thus the microscope has more information to form a clear image, hence its resolving power will be brighter.
What is the role of a condenser lens?
It is the lens that focuses the illuminating beam into the specimen.
What is the role of the objective lens?
The lens that leads to the diffraction pattern in a real image.
What is the role of the projection lens?
The lens that is used to magnify the image to give a final upright (virtual) image.
Describe an optical lens.
Glad with spherical or near spherical polished surfaces, coated to minimise reflection so that light refracts from the surface.
Used in combinations to minimise spherical and chromatic aberrations.
Describe a magnetic lens.
A magnetic field (solenoid) is created in which electrons are focus by having a decreasing radius to spiral about the optical lens.
Lenses always convergent
Electron lenses have both spherical and chromatic aberrations
The aberrations of the lenses scale with focal length. Objective lens focal length is just a few mm are used and sample sits in the magnetic field of the lens
Describe how spherical aberration arises.
Lenses are made spherical and is not made in the ideal shape for focusing.
Arises because of different path lengths of different rays from an object point to the image point.
Rays furthest from optical axis brings rays to focus near the lens (marginal focus) than does the central portion of the lens (axial focus)
A disk of least confusion exists at the best compromise position of focus.
Check notion to see diagram.
Explain how chromatic aberration arises.
Arises due to different wavelengths in photons or the different energies of electrons.
The faster an electron travel, the less it is diffracted by a magnetic lens. The higher the wavelength, the less it is refracted by an optical lens, thus different focal lengths.
A disk of least confusion exists at the compromise position of focus.
How can chromatic aberration be removed?
By using monochromatic light (filters) or multiple lenses of different shapes.
Explain how astigmatism arises.
Arises because focus for rays travelling in the horizontal plane are at a different position from the focus for rays travelling in the vertical plane because of different path lengths.
A disk of least confusion exists at the compromise position of focus.
How can astigmatism be removed?
Using stigmators (correcting lenses) by imposing a weak electric or magnetic quadrupole field on the electron beam.
List the different ways that photons can be produced for microscopy.
Heated filaments (cheap, white light, low intensity) Arc discharge (small, white light, high intensity) Gas discharge (monochromatic) Laser (monochromatic, intense)
Explain how an electron gun works.
Requires:
-Stable voltage supply
-Controllable filament heater
-controlled bias voltage
Steps:
-Cathode is heated which emits a stream of electrons.
-Electrons are accelerated using an electric field in a vacuum.
How can an electron gun be improved?
Pointed filaments to reduce are in which electrons are emitted.
Lanthanum Boron filaments which is a brighter source due to lower work function.
High electric field to increase intensity.
Explain how critical and Koehler illumination works in microscopy.
Light source is focused on specimens using condenser lens.
Critical Illumination:
-Light from the source is focused on the image plane.
Koehler Illumination:
-An extra field diagram (Koehler lens in diagram on notion) is placed between the source and the condenser lens.
-The rays of the source are then instead focus on the aperture planes (including the eyepiece) and defocus on the sample (illumination).
State the advantages of Koehler illumination over Critical illumination.
More illumination on the sample (no glare)
Removes the image of the light source on the final image.
How are virtual images detected?
With an eyepiece
How are real images detected?
Ground glass, fluorescent phosphor
How are photons and electrons detected?
Photographic film
CCD
TV cameras
State the properties of an aperture.
Opaque to radiation. Thin in the axial direction Zero reflectivity Adjustable in size Adjustable in position
Explain how phases contrast microscopy works.
A condenser annulus limits the number of rays entering the illuminated specimen.
Condenser focuses the illuminating light on the specimen
The specimens then scatters and refracted the light causing a phase shift.The thicker the sample, the large the phase contrast.
An objective lens focuses the scattered and illuminating light on to a detector.
A phase plate is placed between the detector and the objective lens. The plate is thicker in the region where background light hits it, thus creating a larger phase change.
The background light is dimmed by a grey filter ring.
The resultant waves interfere to give rise to contrast in the final intensity in regions of the field of view that contain the specimen.
Explain how polarising microscopy works.
The incident light becomes plan polarised using a polarising filter.
The light is shone on a specimen which causes the polarisation to change depending on the angle of incidence (normally causes elliptical polarisation)
The reflected light is then analyses by the polarising unit which is perpendicular to the polariser of the incident light.
The reliant intensities are dependent on the angles of polarisation.
Explain how interference microscopy works.
Light from the specimen only differs in phase and has the same amplitudes.
A reference wave is added to the system which superposes with the existing waves
A contrast is seen as the waves have different amplitudes.
Explain the difference between 2-beam and multiple beam TEM.
2-beam:
-Reciprocal lattice is rotated by double slit only (x-y coordinates)
-g(hkl) satisfying the Bragg condition will have a diffraction spot just as intense as the central (000) spot
-Only one reciprocal lattice vector (with small indices) is seen due to a decrease in intensity of higher integer vectors
Multiple beam:
-Many g(hkl) satisfies the Bragg condition and there are many diffraction spots but less intensity than the 000 spot.
Many reciprocal lattice vectors are seen.
Why do electron diffraction patterns of thin crystals have many spots?
Wavelength of electron is much less than the spacing between the planes. Bragg angle is very small thus the large Ewald sphere intersects many reciprocal lattice points.
Bragg condition in thin samples is less rigorous as there are less scattering centres which contribute to the diffraction beam (consider double slit vs multiple slits)
Explain what happens to the electron diffraction pattern when the sample thickness increases.
As sample thickness increases, the diffraction spots strait to blur out and disappear due to the stricter Bragg angle as many scattering centres contribute to the diffracted beam.
Explain how selective area diffraction (SAD) works in electron microscopy.
Objective lens forms a diffraction pattern at the back focal plane.
An intermediate lens is to focus on the back focal plane instead of the specimen.
An aperture is place where the intermediate image forms (selected area).
The diffraction pattern is projected by the projection lens.
