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.