Microscopy and Surface Analysis 1 doc

Microscopy and Surface Analysis– Optical microscopy – Confocal microscopy – Electron microscopy SEM and TEM, related methods – Scanning probe microscopy STM and AFM, related methods – X-

Lecture Date: March 11 th , 2008

Microscopy and Surface Analysis 1

Reading Assignments for Microscopy and

Surface Analysis

Characterization by Spectroscopy and Microscopy”

Microscopies in Chemistry,” J Phys Chem., 1996, 100,

13103-13120

Microscopy and Surface Analysis

– Optical microscopy

– Confocal microscopy

– Electron microscopy (SEM and TEM, related methods)

– Scanning probe microscopy (STM and AFM, related methods)

– X-ray fluorescence (from electron microscopy)

– Auger electron spectrometry

– X-ray photoelectron spectrometry (XPS/UPS/ESCA)

– Other techniques:

 Secondary-ion mass spectrometry (SIMS)

 Ion-scattering spectrometry (ISS)

 IR/Raman methods

Why Study Surfaces?

system, and the techniques available are extremely

powerful

reactions

X-ray wavelengths/energies, have been combined with

microscopy to create some of the most powerful analytical

tools available…

Imaging Resolution and Magnification

(x)

-Optical Microscopy

Electron Microscopy

Probe Microscopy

Optical Microscopy - History

thousands of years Chinese tapestries dating from 1000

B.C depict eyeglasses

the first theoretical study of the lens.

telescope.

possible John and Zaccharias Jannsen (Holland) made

the first commercial and first compound microscopes.

many great discoveries….

Modern Optical Microscopy in Chemistry

developed, the compound

microscope was applied to

the study of chemical

crystals.

(1880): can see boundaries

between materials with

different refractive indices,

while also detecting

isotropic and anisotropic

materials

http://www.microscopyu.com/articles/polarized/polarizedintro.html

Optical Microscope Design

characterized NA

(numerical aperature)

– The numerical aperture of a

microscope objective is a

measure of its ability to

gather light and resolve fine

specimen detail at a fixed

object distance

– Large NA = finer detail =

better light gathering http://www.microscopyu.com/articles/polarized/polarizedintro.html

Diagram from Wikipedia (public domain)

not changed much in

300 years

– But the lenses are more

perfect – free of

abberations

The Diffraction Limit

small point of light is not a

point – it is an “Airy” disk

with concentric bright/dark

conventional optical microscope is limited by Fraunhofer diffraction

at the entrance aperture of the objective lens

Airy disk

Resolved Not resolved

The Diffraction Limit

microscopy Its lateral resolution is limited to ~200 nm.

– The need for the light-gathering objective lens and its aperture in

a conventional microscope leads to a diffraction limit

overcome the diffraction limit A fiber tip with an aperture <100

nm is scanned over a sample (forming the basis of techniques

like NSOM/SNOM – the scanning near-field optical

microscope).

– NSOM is now being using in conjunction with AFM – to study

“nano-phototonics”

– Resolution now down to ~30 nm

Confocal Scanning Microscopy

CSM) was first proposed by Marvin Minsky in 1957.

point is illuminated and focused at a time The light

reflected (or produced e.g by fluorescence) is detected

for just that point Light from out-of-focus areas is

suppressed A complete image is formed by scanning.

– Greater depth of field from images

– Images are free from out-of-focus blur

– Greater signal-to-noise ratio (for a spot – but images take time!)

– Better effective resolution (diffraction limit)

M Minsky, “Memoir on inventing the confocal scanning microscope” Scanning, 1988, 10, 128-138.

Confocal Scanning Microscopy: Imaging Types

scanning:

– disks with staggered holes – block all but a certain lateral portion of

the sample beam

Laser Confocal Scanning Microscopy

fluorescent dye to stain

biological samples

Diagram from http://www.cs.ubc.ca/spider/ladic/images/system.gif

Laser Confocal Scanning Microscopy

Diagram from http://www.cs.ubc.ca/spider/ladic/images/system.gif

Laser Confocal Scanning Microscopy

Diagram from http://www.cs.ubc.ca/spider/ladic/images/system.gif

IR Microscopy and Spectroscopy

– However, this is at the expense of resolution (limited to ca 10 um),

in contrast with scanning techniques Resolution in FTIR imaging is

of course limited by the diffration limit, which is even worse for IR

wavelengths.

Figure from J L Koenig, S Q Wang, and R Bhargava, Anal Chem., 73, 361A-369A (2001)

IR Microscopy: Image Analysis

color-coding peaks based on their IR frequency (a)

chosen via a scatter plot (c) of

every point in the image vs two

(or more) frequencies, followed

by location of the center-of-gravity

and possible statistical analysis

constructed (b)

Figure from J L Koenig, S Q Wang, and R Bhargava, Anal Chem., 73, 361A-369A (2001)

IR Microscopy: Polymer Chemistry Applications

material science, chemical and biochemical applications

dissolution of a polymer by a solvent

Figure from J L Koenig, S Q Wang, and R Bhargava, Anal Chem., 73, 361A-369A (2001)

IR Microscopy: Polymer Chemistry Applications

molecular motion process is observed for polymers (e.g

polymethylstyrene) above their entanglement mwt:

Figure from J L Koenig, S Q Wang, and R Bhargava, Anal Chem., 73, 361A-369A (2001)

Raman Microscopy

inherent resolution than IR

(uses lasers at shorter

optical wavelengths)

(must still scan the sample)

– this does have its

advantages though

systems for combined 3D

visualization and

spectroscopy

Raman Microscopy: Forensic Applications

one that is not so obvious is for forensic analysis of

colored fibers

fingerprint, and the complex spectra obtained from dye

mixtures can be used to determine if two fibers are

from the same origin

– The individual dyes used in fabics are varied, and their ratios

are especially varied (even from batch to batch!)

LC or ESI MS on dye-containing extracts from fibers

For more about forensic Raman microscopy, see: T A Brettell, N Rudin and R Saferstein, Anal Chem., 75, 2877-2890 (2003).

Electron Microscopy (EM)

microscopy (SEM) – an

electron beam is scanned

in a raster pattern and

“reflected” effects are

different manner in EM

Bottom photo - http://www.mos.org/sln/sem/velcro.html Top photo - http://emu.arsusda.gov/snowsite/default.html

Velcro (x35) Ice crystals

optical SEM

Electron Microscopy: Basic Design

Electrongun (1-30 keV)

Magneticlenses andscanning coils

Sample

Detectors

Detectors

electronsphotons

electrons

Computer

Electron Microscopy: Resolution

are impossible to discern with optical microscopy?

accelerated by a 10 kV potential:

nm 0.0123 m

10 23 1

) V C)(10 10

60 1 )(

kg 10 2(9.11

s J 10 63 6

2 2

2

11

4 19 - 31

-34

2 2 1

m m h

m

eV v

eV mv

EM can see >10000x more detail than visible light!

Note: Resolution

is limited by lens aberations!

Electron Microscopy: Resolution

EM can in some cases be moving pretty close to the

speed of light.

nm 10 7 3

) 1

)(

V C)(10 10

60 1 )(

kg 10 2(9.11

s J 10 63 6

) 1 ( 2 2

3

) 10 3 ( kg) 10 9.11 ( 2

) V C)(10 10 60 1 ( 4

19 - 31

-34 2

2 8 -31 4 19 - 2

mc eV

meV

h eV

m m h

At high potentials, EM can see atomic dimensions

Using the

relativistically

corrected form of the

previous equation:

Electron Microscopy: Sample-Beam Interactions

control how both SEM and

TEM (i.e STEM) operate:

– Formation of images

– Spectroscopic/diffractometric

analysis

eight) types of sample-beam

interactions (which can be

confusing and hard to

remember!)

sample-beam interactions:

– bulk specimen interactions (bounce off sample – “reflected”)

– thin specimen interactions (travel through sample- “transmitted”)

SEM: Sample-Beam Interactions

Backscattered Electrons (~30 keV)

 Caused by an incident electron colliding with an

atom in the specimen which is almost normal to

the incident electron’s path The electron is then

scattered "backward" 180 degrees

 Backscattered electron intensity 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 This creates contrast

in the image of the specimen based on different

average atomic numbers

 Backscattered electrons can come from a wide

area around the beam impact point (see pg 552

of Skoog) – this also limits the resolution of a

SEM (along with abberations in the EM lenses)

SEM: Sample-Beam Interactions

Secondary Electrons (~5 eV)

 Caused by an incident electron passing "near"

an atom in the specimen, close enough to

impart some of its energy to a lower energy

electron (usually in the K-shell) This causes a

slight energy loss, a change in the path of the

incident electron and ionization of the electron in

the specimen atom The ionized electron then

leaves the atom with a very small kinetic energy

(~5 eV) One incident electron can produce

several secondary electrons

 Production of secondary electrons is closely

linked to sample topography Their low energy

(~5 eV) means that only electrons very near to

the surface (<10 nm) are detected They also

don’t suffer from the backscattered electron

lateral resolution problem depicted in Fig 21-16

of Skoog Changes in topography in the sample

that are larger than this sampling depth can

change the yield of secondary electrons via

indirect effects (called collection efficiencies)

Electron Microscope: Image Formation

From JEOL Application Note, V E Robertson, ca 1985.

SEM: Sample-Beam Interactions

Auger Electrons (10 eV – 2 keV)

 Caused by relaxation of an ionized atom after a

secondary electron is produced The lower

(usually K-shell) electron that was emitted from

the atom during the secondary electron process

has left a vacancy A higher energy electron from

the same atom can drop to a lower energy, filling

the vacancy This leaves extra energy in the atom

which can be corrected by emitting a

weakly-bound outer electron; an Auger electron

 Auger electrons have a characteristic energy,

which is unique and depends on the emitting

element Auger electrons have relatively low

energy and are only emitted from the bulk

specimen from a depth of several angstroms

SEM: Sample-Beam Interactions

X-ray Emission

 Caused by relaxation of an ionized atom after a

secondary electron is produced Since a lower

(usually K-shell) electron was emitted from the

atom during the secondary electron process an

inner (lower energy) shell now has a vacancy A

higher energy electron can "fall" into the lower

energy shell, filling the vacancy As the electron

"falls" it emits energy in the form of X-rays to

balance the total energy of the atom

 X-rays emitted from the atom will have a

characteristic energy which is unique to the

element from which it originated

 X-ray (elemental) mapping of sample surfaces is

a common applications and a very powerful

analytical approach

SEM: Sample-Beam Interactions – X-rays

SEM: Sample-Beam Interactions

Cathodoluminescence (CL)

 Caused by electron hole pairs, which are created

by the electron beam in certain kinds of materials

When the pairs recombine, cathodoluminescence

(CL) can result CL is the emission of

UV-Visible-IR light by the recombination effect CL is usually

very weak and covers a wide range of

wavelengths, and requires high beam currents,

lowering resolution and challenging detector

systems!

 CL signals typically result from small impurities in

an otherwise homogeneous material, or lattice

defects in a crystal

 CL can be used effectively for some analytical

problems Some “random” examples:

– Differentiation of anatase and rutile

– Studying ferroelectric domains in sodium niobate

– Location of subsurface crazing in ceramics

– Forensic analysis of glasses

TEM: Sample-Beam Interactions (Thin Sample)

Unscattered Electrons

 Incident electrons which are transmitted

through the thin specimen without any

interaction occurring inside the specimen

 Used to image - the transmission of

unscattered electrons is inversely proportional

to the specimen thickness Areas of the

specimen that are thicker will have fewer

transmitted unscattered electrons and so will

appear darker, conversely the thinner areas

will have more transmitted and thus will

appear lighter

TEM: Sample-Beam Interactions (Thin Sample)

Elastically-Scattered Electrons

 Incident electrons that are scattered (deflected

from their original path) by atoms in the

specimen in an elastic fashion (without loss of

energy) These scattered electrons are then

transmitted through the remaining portions of

the specimen

 Electrons follow Bragg's Law and are

diffracted All incident electrons have the

same energy (and wavelength) and enter the

specimen normal to its surface So all incident

electrons 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,

This pattern can then yield information about

the orientation, atomic arrangements and

phases present in the area being examined

TEM: Sample-Beam Interactions (Thin Sample)

Inelastically-Scattered Electrons

 Incident electrons that interact with sample atoms

inelastically (losing energy during the interaction)

These scattered electrons are then transmitted

through the rest of the sample

Inelastically scattered electrons have two uses:

1 Electron Energy Loss Spectroscopy (EELS): The

amount of inelastic loss of energy by the incident

electrons can be used to study the sample

These energy losses are unique to the bonding

state of each element and can be used to extract

both compositional and bonding (i.e oxidation

state) information on the sample region being

examined

2 Kakuchi bands: Bands of alternating light and dark

lines caused by inelastic scattering, which are

related to interatomic spacing in the sample

These bands can be either measured (their width

is inversely proportional to atomic spacing) or

used to help study the elasticity-scattered electron

pattern

Electron Optics: Electron Source (Gun)

 Positive electrical potential applied to the anode

 The filament (cathode) is heated until a stream of

electrons is produced

 The electrons are then accelerated by the positive

potential down the column (can be up to 30 kV)

 A negative electrical potential (~500 V) is applied

to the Wehnelt cap

 Electrons are forced toward the column axis by

the Wehnelt cap

 Electrons collect in the space between the

filament tip and Wehnelt cap (a space charge or

“pool”)

 Those electrons at the bottom of the space

charge (nearest to the anode) can exit the gun

area through the small (<1 mm) hole in the

Wehnelt cap

 These electrons then move down the column

towards the EM lens and scanning systems

Figure from http://www.unl.edu/CMRAcfem/interact.htm

Thermionic electron gun – how it works:

The results:

- Electrons are emitted from

a nearly perfect point source (the space charge)

- The electrons all have similar energies (monchromatic)

- The electrons will travel parallel to the column axis