Lecture 7
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what are the seven crystal systems ?
cubic tetragonal orthorhombic monoclinic triclinic trigonal hexagonal
what does polymorphism mean ?
elements can have a different crystal system depending on the temperature
what do the diffractograms of single crystals, polycrystalline and amorphous materials look like ?
single -> regularly spaced dots poly -> concentric circles amorphous -> faded circle
explain the formation of a diffraction pattern (several peaks)
the peaks are constructive interference between 2 reflective waves (x-ray beams) on 2 consecutive planes. Several peaks because multiple reflection planes in a lattice
explain the principle of an XRD measurement
we shine x-ray beams on a specimen at a certain angle -> we detect the reflected waves if they are coherent (constructive interference)
list the components (5) of an XRD system and explain function
• x-ray tube = source of x-rays • x-ray optics = condition the beam before it hits the sample • goniometer = platform that holds and moves the sample, optics, detector and/or tube • x-ray optics = condition the beam after it encountered the sample • detector = count the number of scattered x-rays
Bragg equation ?
2dsin(theta) = n*lambda d is the spacing between the planes n is the order of diffraction lambda is the wavelength
how is the diffractogram influenced by : lattice paramater, crystallite size, atomic number
• increase lattice parameter = angles decrease • increase crystallite size = same angles but higher intensity, if very small -> tends to amorphous (scherrer equation) • intensity is proportional to atomic number
describe the functionality of a SEM, what can we see ?
Interaction of fast electrons with a sample : - backscattered electrons : atomic number contrast (heavy = bright), a bit deeper than surface - secondary electrons : topography contrast, come from surface - photons : for EDX, the deepest
demonstrate usefulness of BSE and EDX detectors
we can combine the info from BSE and EDX to map a whole surface : see where the heavy / light elements are, see each individual elements
explain creation of an EDX mapping
when shooting electrons to a sample, there is a photon released -> creates energy peaks depending on the element. We can map a surface that way, to find out where each element is
what is the depth of info in SEM/EDX ?
the depth is around 0.5 to 2 micro meters.
explain the photoelectric effect, minimum frequency ?
shine light on a surface (= emits photons) -> if the photons have a high enough energy, they will kick out electrons from the sample. The minimum frequency / max wavelength is element dependent
explain the principle of UV and X-Ray photoelectron spectroscopy
we shine light of a sample, and depending on the kinetic energy of the resulting electrons, we can find their binding energy h*f = Ek + Eb (+work) UV -> electrons in valence band X-ray -> electrons in core levels
describe how XPS spectra are aquired and interpreted
We get the binding energy of the electrons from the surface of the material. It also gives info on what kind of bonds are present in the material (electron transfers change the bonding energy).
tell about the surface sensitivity of XPS
it only goes about 100 angstrom deep. The x-ray beam can penetrate further but the deep electrons won’t have the energy to emerge from the surface.
discuss the idea of angle resolved XPS measurement
the smaller the angle between the detector and sample, the smaller the information depth (because electrons have to travel a longer distance). Grazing angle -> only top layer.
in what condition is XPS perfomed ?
ultra high vacuum because electrons can’t travel in air
how does the applied voltage affect the current in photoelectric effect ?
A negative voltage will slow down the elctrons (lower current). If too negative, current is zero, then it increases until the saturation current. After saturation, an increase in voltage will not change the current.