MBN 403

Nanoscale Characterization Methods

Prof. Dr. Fatih Buyukserin
MBN 403
MBN 403
 GRADUATE STUDENTS

• Grad students has to submit a 1002 proposal (10 pages, 12 arial character, double space, no budget)
that has a characterization method involved.

• It may be related to their thesis.

• They will have only report no presentation.

• They will need to approve to take this class which will be presented in English.
 Characterization

• It is the collection of signals and their

variations from a material through detectors
in response to the exposed probes to reveal
its properties
• Generally the asistance of standards and
statistic methods are utilized
 CHARACTERIZATION
• The Question • The Method
• What does the sample look like (sample • Optical microscopy, Electron Mic. Force
morphology) ? Lambda /2 Mic. Electron Tunneling Mic.
• What is the sample made of? • XPS, EDAX, AES, NMR, SIMS
• Elemental composition, impurities
• Chemical states (sp2, sp3)
• What is the structure of the sample? • XRD, RHEED, LEED techniques, TEM
• Density, Grain size, crystal forms
• What are the optical properties of the • Ellipsometry, UV-Vis, flourescence
material? spectroscopy
• Refractive index, dielectric properties
• What are the electrical properties of the
material? • 4 point probe, PPMS
• What are the mechanical properties of
the sample? • Compression, shear tests, adhesion test
 Available Probes for Characterization

• EM Radiation
• Electrons
• Ions
• Neutrals
• Forces
Available Probes for Characterization

Demo case SIMS

You have ultrasensitive detectors

that weighs the mass of extracted
ions from sample surface

Detector measures 40,078 g

 Ca+

Mapping can be done….

 PES
• Measures the ionization energy of • When a photon ionizes a sample, the
molecules (typically on a surface) energy of the ejected photoelectron is
when electrons are ejected from
different orbitals as a result of
impacting light.
• Orbital energies
• Core vs. outer
• S p d f order
• Change of ionization energy on the same
period
• Radii

•
 PES
• Wavelength requirement • When core electrons are being
• Typically the wavelentgh of the PES studied, photons of even higher
instruments are less than 200 nm energy are needed to expel them
• Xrays are required
• Ionization energy of molecules are several
electronvolts for even valence Shell electrons
• Essential to work at or below UV region • Modern PES instruments use both
xrays and UV radiation
• Primary radiation source in UPS (ultraviolet
photoelectron spec) is He I line.
• 1s1 2s1 converts to 1s2
• Lambda is 58 nm, 21 eV
PES Setup

• Speed energy correlation

• The deflection of the electron path
depends on the speed at which
they are ejected from the sample
• vary the field, electrons of
different speed reach the detector

• Hv is known, you can plot Ek or

Ionization energy vs. counts.
HBr PES Spectrum

• Distinguish the origin of ejected

electrons

• Bonding vs. nonbonding etc.

 XPS

• Electron ejection from inner core of • E.g.: K Shell ionization energies of

atoms by large energy Xray photons second row elements
• Li 50 ev
• Characteristic XPS lines for elements • N 400 ev
present in your • F 690 ev
compound/alloy/composite • Detection of these values indicate the presence
• Core ionization energies are almost of the corresponding element
insentisive to bonds between atoms • ESCA, electron spectroscopy for chemical
because they are too tightly bound to be analysis
affected
• Atom property rather than molecule
 XPS

• Limited to surface analysis • XPS is very useful for studying the surface
state of several material applications from
• Xrays are highly energetic and can catalysis to sensors to SC but be careful
penetrate microns into the sample with SCs
• Ejected electrons can not escape except • Material surfaces are affected from impurities
to a much greater extend
from a few nanometers of the surface • If not careful or vacuum conditions are not
appropriate you only see dirt (C)

• Difference between surface and bulk

structures via etching
• Depth profiling
• Line profiling and mapping is possible via
scanning XPS
XPS

• General approximation of bonding

insensitivity is true however small
distinguishable differences can be
detected
• For azide the XPS spectrum should lie
around 400 eV
• Number of electrons/p, e repulsion

• Information about chemically

inequivalent atoms of the same
element
• Si peak different than SiO2, highly
important in SC industry
• Chemical state info
 XPS

• Depth profiling typically used in

combination with etching in SC
• Composition as well as thickness
info can be gathered

• Quantitative elemental
information through atomic
percent results
• Eg: SiO2, if x percent Si than 2x
percent O
 XPS

• Typically instead of a single sharp peak, broad peaks with

shoulder are obtained
• Peak resolving to find the different states of the present
element
• Eg sp2 C vs. sp3 C
XPS
 XPS

• Binding energy is used in XPS hv = Ek + EB + φ

instead of ionization energy as φ = work function = Evac – Efermi (tabulated values for each
solid samples are used. element)

• IE is for gaseous samples EB = │Eorb – Efermi │

Efermi = energy of the highest occupied electronic state

Ek

Evac
φ
Efermi ?
EB
 XPS Concerns

• Importance of high vacuum

• Typically 10E-9 torr
• Surface atoms tendency to bind with gaseous species
• Requires waiting times

• Importance of sample handling

• Beam damage is possible

• Remember sputtering caused X ray generation from target
• Certain semiconductor materials requires special attention
 Xray Sources

• λ around 1 angström
• They are produced when electrons of several
keV are decelerated by metals
• Electrons can be ejected fom different orbitals
• If core electrons are ejected (eg 1s), an outer +
electron (2p, 3s) can fall down to occupy 1s
• Emit characteristic Xray
• If from 2p  1s the Xray line is called Kα
• If from 3s  1s the Xray line is called Kβ
• Normally 2p1/2 1s ile called Kα1, 2p3/2  Kα2
• Mg and Al Kα lines are popular Xray sources used in electron beam
XPS instruments
 Xray Sources

• When electrons are stopped at anode , only a cathode anode

fraction (1 %) of total energy is converted to
Xrays, remaining causes anode heating
• Typically anode is cooled

• The EM radiation from anode is not a peak

spectrum but instead it is peaks in a continues Xray tube
broad emission
• The reason is electron decelaration and energy loss
near a nuclei (+)
• No collision but energy loss in the beam due to the void
structure of atoms
• The resultant profile is Bremsstrahlung radiation
 Xray Sources

filters

• The resultant profile is Bremsstrahlung radiation E0

 Xray Sources

• Access to valence level electron can be satisfied by deep UV sources

• Access to core (medium) electrons by soft Xrays (100 eV)

• Access to deeper core electrons of heavy metals by hard Xrays (1keV)

• Can be damaging, hence, mid orbital energy electrons of heavy/transition metals can be
preferred for several applications (see deep profiling scenario)

• The idea is same, these electrons do not get influenced from the bondings too much,
• Ek is measured, remaning energy is related to their binding energy (and fermi level)
• Libraries are present to 3rd decimal, eg Hf 4f Eb is 18,9 eV
• Hf 1s electron is very hard to expel
 XPS

• He and H is not visible in XPS!

• He does nt form solid compounds

• H shares its only electron when making monds, the e energy varies from compound to compound
They both have 1s orbitals with ultratiny crosssections for photoemission to occur
 AUGER Electron Spectroscopy

• The Auger electron arises as a consequence of

photoemission (conventional description
discussion of flourescence)

• When the core hole is produced, it is filled with

higher lying electron
• Atom is at excited state
• Excess energy can be dissipated by X ray generation
• Excess energy is dissipated by the emission of another
electron from a higher level
• Auger electrons are specific to material
• Instead of light electron beam does the same job
• Because it can brought to a narrow size, illumination of
microscopic areas is possible
• This allows scanning Auger microscopy
 AUGER Electron Spectroscopy

• Instead of light, electron beam does the

same job
• Because it can brought to a narrow size, illumination
of microscopic areas is possible
• This allows scanning Auger microscopy
• Analyzes Auger intensity of a peak as the e beam is
scanned to produce the image of the sample
• Composition on morphology with SEM instruments
• $$
 Xray Fluorescence

• By definition, this fluorescence is caused as a

consequence of incoming EM radiation
• Fluorescence is a subgroup of luminescence
• Chemiluminescence
• Bioluminescene
• Photoluminescence

• Xrays can expel electrons from K,L,M levels and

create vacancies
• Higher level electron occupies these levels and the Front. Plant Sci., 14 November 2018 |
energy difference is released by emitted photons https://doi.org/10.3389/fpls.2018.01588

• Remember previous slide, so they compete with Auger

emission
 Xray Fluorescence

• Eg: When Pb L level (E2) electron is expelled and

M level electron (E3) occupies the vacancy, hv=
lE2-E3l is released corresponding to the Lα line of
Pb which is detected at 10,2 eV.

• XRF can be generated by other sources (against

fluorescence definition) involving radioisotopes,
charged particles like electrons
• Energy dispersive analysis of Xrays (EDX) uses e beam
of SEM/TEM to create such spectra
• The difference between energy levels  energy of the Front. Plant Sci., 14 November 2018 |
https://doi.org/10.3389/fpls.2018.01588
emittted photon is characteristic for elements present
• Compositional analysis
 EDX

Boshnakova I, et al., Investigation of montmorillonite as carrier for OER, International Journal of

Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.01.012
 Auger vs. XRF

• As the nuclei being hit by incoming radiation (or e beam) gets

heavier, the difference between energy levels goes up
• E of K and L goes down drastically

• Probability of Auger electron creation is α e-ΔE accorrding to quantum

mechanical selection rules

• For Z > 29 Auger electron intensities goes down and XRF becomes
dominant
• Cu, Mo and W commonly used as Xray sources all with this spec
 Auger Electron Energy

• Suppose that EM expelled K level electron from Tungsten,

L level electron occupied this hole and an M level electron
is emitted.
• This electron is called M level Auger electron for W
• It has a characteristic energy: 2,7 eV
= l (EK-EL) l – l (EM-Ef) l – φ
W
would be hv EB
K
M (-100) – (-50)  energy release
L
• For each element, K,L,M,N level energies can be calculated and Ef as well as φ is
tabulated
• Thus, before experiment, you can calculate accurately the expected Auger electron
energies
• Notice the minute magnitude
 Xray Fluorescence-PQ 1

• Eg: When Pb L level (E2) electron is expelled

and M level electron (E3) occupies the
vacancy, hv= lE2-E3l is released corresponding
to the …… line of Pb which is detected at 10,2
eV.

Front. Plant Sci., 14 November 2018 |

https://doi.org/10.3389/fpls.2018.01588
 Xray Diffraction (XRD)

• What is the % of crystalline solid materials?

• 95 % of solids are crystalline
• When Xrays interact with a crystalline substance, one
gets a diffraction pattern
• The same substance always gives the same pattern
• Mixture of substances each produces its pattern
independent of others
• Fingerprint of substance
• Identification of different phases in a polycrystalline
powder
 Xray Diffraction (XRD)

• Steel and all common metals are polycrstalline in nature

• Crystal grains
• Ordered orientation within the grains
• If no ordering even within grains 
• Amorphous
• Semicrystalline materials
• Several common plastics

• Today for about 50000 inorganic and 25000 organic single

component crystalline phase, the diffraction pattern has
been collected in the libraries

• To identify components in a sample by search/match procedure

• The areas under the peak are related to the amounts of each
phase
 Solid Material
• Amorphous: Atoms are arranged • Crystalline: Atoms are arranged
randomly like a liquid in a regular pattern
• Glass • There is a smallest volume
element that by repetition in 3D
describes to whole crystal
• Unit cell
• Bcc
• Fcc (Fe, NaCl)
• Hcp
• Tetragonal
• ccp
 Bragg’s Law
• When an Xray beam hits atoms of
crystalline material, we will have
constructive interference in a very
few directions

• A diffracted beam is a beam

composed of a large number of
scattered rays muturally reinforcing d is lattice
one another spacing

• Water waves
• In an amorphous material since there
is no ordering, the reflective light is
present in all directions
• No certain angle is present at which
constructive interference exists
• White board