Download Lecture 13 - Analytical techniques

Survey
yes no Was this document useful for you?
   Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Transcript
Synchrotrons
• A synchrotron is a ring which uses magnets
and electrodes to accelerate x-rays or light to
nearly the speed of light
• These extremely bright sources have
widened the range of information which we
can use traditional spectroscopy, diffraction,
and even microscopy techniques for
National
Synchrotron
Light
Source
(NSLS)
XANES and EXAFS
• X-ray adsorption near-edge spectroscopy
and Extended X-ray adsorption Fine
Structure, commonly done with
synchrotron radiation because the higher
energy X-ray yields more precise data
• X-ray techniques which look at the fine
details of X-ray interactions with minerals
• Sensitive to oxidation states and specific
bonding environments
Other synchrotron-based
techniques
• X-ray microprobe, spectromicroscopy –
use synchrotron X-rays to image and do
elemental mapping of a sample (including
trace elements)
• X-ray standing wave – analyses very close
to the surface – ‘see’ steps, adsorbed ions
• Techniques applicable to the water/mineral
interface – probe chemistry, mineral
changes
Magnetic Techniques
• Electron Paramagnetic Spectroscopy
(EPR) utilizes the magnetic properties of
minerals and the ions that may be
impurities in an overall mineral structure
Fe3+
Zn2+
Nuclear Magnetic Resonance
(NMR)
• A technique based on analysis of isotopes
with a spin character (related to edistribution) which can be affected by a
magnet
• This spin is used to characterize the
structure and bonding of thise elements
• NMR ‘active’ materials include 29Si, 31P,
14N, 13C, 1H
Electron Microscopy/ Spectroscopy
• Interaction of electrons with a sample
Secondary e-
e- scattering off bulk sample
•
Backscattered Electrons: Caused by an incident electron colliding with an atom
in the specimen which is nearly normal to the incident's path. The incident
electron is then scattered "backward" 180 degrees.
– Utilization -The production of backscattered electrons varies directly with the
specimen's atomic number. This differing production rates causes higher atomic
number elements to appear brighter than lower atomic number elements.
•
Secondary Electrons: Caused by an incident electron passing "near" an atom in
the specimen, near enough to impart some of its energy to a lower energy
electron. This causes a slight energy loss and path change in the incident
electron and the ionization of the electron in the specimen atom. This ionized
electron then leaves the atom with a very small kinetic energy (5eV) and is then
termed a "secondary electron". Each incident electron can produce several
secondary electrons.
– Utilization - Production of secondary electrons is very topography related. Due to their
low energy, 5eV, only secondaries that are very near the surface (<10nm,
seeInteraction Volume) can exit the sample and be examined. Any changes in
topography in the sample that are larger than this sampling depth will change the yield
of secondaries due to collection efficiencies.
•
Auger Electrons and X-rays –these emmissions are caused by the formation of
secondary electrons and rearrangements of electrons as a result of emission.
– Utilization – Auger electrons or X-rays emitted from the atom will have a characteristic
energy which is unique to the element from which it originated.
e- penetration into a sample
• Details dependent on mineral composition and
accelerating voltage of e- beam, but for SEM
applications:
•
•
•
•
•
•
•
The electron gun, produces a stream of
electrons.
A series of condensers and apertures (i.e.
lenses and holes) restricts the electron flow
to a nice, tight beam
A set of coils then "scan" or "sweep" the
beam in a grid fashion (like a television),
dwelling on points for a period of time
determined by the scan speed (usually in
the microsecond range)
The final lens, the Objective, focuses the
scanning beam onto the part of the
specimen desired.
When the beam strikes the sample (and
dwells for a few microseconds) interactions
occur inside the sample and are detected
with various instruments
Before the beam moves to its next dwell
point these instruments count the number
of interactions and display a pixel on a CRT
whose intensity is determined by this
number (the more reactions the brighter the
pixel).
This process is repeated until the grid scan
is finished and then repeated, the entire
pattern can be scanned 30 times per
second.
Scanning electron
microscope (SEM)
SEM – what do we get?
• Topography (surface picture) – commonly
enhanced by ‘sputtering’ (coating) the
sample with gold or carbon
But wait – that’s not all!
• SEM also detects electrons and x-rays which
identify electron density distributions (BackScattered Electrons) and chemical identity (Auger
and X-rays) using specific detectors (BSE, EDX,
Auger)
e- that go through a sample
•
Unscattered Electrons - Incident electrons which are transmitted through the thin
specimen without any interaction occurring inside the specimen.
– Utilization - The transmission of unscattered electrons is inversely proportional to the
specimen thickness.
•
•
•
•
Elasticity Scattered electrons - Incident electrons that are scattered (deflected
from their original path) by atoms in the specimen in an elastic fashion (no loss of
energy). These scattered electrons are then transmitted through the remaining
portions of the specimen.
Utilization - All electrons follow Bragg's Law and thus are scattered according
spacing between planes. All incidents that are scattered by the same atomic
spacing will be scattered by the same angle. These "similar angle" scattered
electrons can be collated using magnetic lenses to form a pattern of spots; each
spot corresponding to a specific atomic spacing (a plane). This pattern can then
yield information about the orientation, atomic arrangements and phases present
in the area being examined. Very small particles will yield rings instead of spots
(nanometer-sized xstals)
Inelastically Scattered Electrons - Incident electrons that interact with specimen
atoms in a inelastic fashion, loosing energy during the interaction. These electrons
are then transmitted trough the rest of the specimen
Utilization :Electron Energy Loss Spectroscopy: The inelastic loss of energy by
the incident electrons is characteristic of the elements that were interacted with.
These energies are unique to each bonding state of each element and thus can
be used to extract both compositional and bonding (i.e. oxidation state)
information on the specimen region being examined.
Electron Microscopy/ Spectroscopy
•
Interaction of electrons with a sample – can make the sample thinner and
‘shoot’ e- with higher energy, more of them go through the sample.
Secondary e-
Transmission Electron Microscopy (TEM)
•
•
•
•
•
Electron beam transmitted just as in the
SEM
This transmitted portion is focused by the
objective lens into an image
Optional Objective and Selected Area metal
apertures can restrict the beam; the
Objective aperture enhancing contrast by
blocking out high-angle diffracted electrons,
the Selected Area aperture enabling the
user to examine the periodic diffraction of
electrons by ordered arrangements of atoms
in the sample
The image is passed down the column
through the intermediate and projector
lenses, being enlarged all the way
The image strikes the phosphor image
screen and light is generated, allowing the
user to see the image. The darker areas of
the image represent those areas of the
sample that fewer electrons were
transmitted through (they are thicker or
denser). The lighter areas of the image
represent those areas of the sample that
more electrons were transmitted through
(they are thinner or less dense)
TEM (+ HRSTEM) – What do we get?
• ‘See’ smallest features with this – sub-nm!
• Morphology – size, shape, arrangement of
particles on scale of atomic diameters
• Crystallographic information – from diffracted
electons, get arrangement and order of atoms as
well as detection of atomic-scale defects
• Compositional
information – Chemical
identity, including redox
speciation (distinguish Fe2+
and Fe3+ for instance)
Electron Microprobe
• Very similar to SEM and TEM in many respects, but
utilizes ‘thick sections’ and a set of detectors which
measure the emitted X-Rays from e- bombardment
and excitation more accurately than the detectors
used in SEM or TEM analyses
• These detectors are wavelength dispersive
spectrometry (WDS) detectors, there are usually an
array of 3-5 which record over some range of
wavelength much more accurately than the EDX
detector available with SEM and TEM instruments
Scanned proximity probes
• Use a probe or ‘tip’ which is placed very close to the
sample
• Measures either the attractive or repulsive forces
between the sample and probe to image the
property in question
• This property can be physical height, absorption of
light (or other parts of EM spectrum), electrical
‘flow’, or magnetic/electrostatic
attraction/ repulsion (+- or ++ charges
coming together)
Atomic Force Microscopy (AFM)
• Can be done in water or air (unlike
SEM/TEM which requires a vacuum)
• The probe is attached to a cantilever spring,
in which the force ‘sensed’ is measured
• Get topograpgic information at an atomic
scale
Scanning tunneling
microscopy (STM) is
the precursor to this
technique, and is
still used to yield
similar information
2.5 nm2 rendering of
a surface – what are
the bumps??