mse 4003 final

Solid probe scans a surface

STM = scanning tunneling microscopy
SFM/AFM = scanning/atomic force microscopy

- Ambient, liquid, vacuum
- Medium depth of field
- Small depth of focus
- XY Resolution: 0.1-3.0 nm
- Z Resolution: 0.01 nm
- Mag: 500x-100,000,000x
- no sample prep necessary; no excessive height variations

Scanning system - piezoelectric actuators (tube configuration)
Probe - atomically sharp tip
Detector - detects the spacing between probe and sample
Electronics - interfaces the control and scanning systems (rapid feedback)
Vibration isolation needed

Bias voltage applied between tip and sample
- Tunneling current arises when tip is close to sample
- DECAYS RAPIDLY WITH SAMPLE-TIP DISTANCE
- PROPORTIONAL TO LDOS OF SAMPLE AT THE FERMI LEVEL
- Very sensitive (atomic) resolution

W or Pt-Ir alloy wires
- Oxidation-resistance and featureless LDOS
- Size and sharpness determine lateral resolution

STM is done in ultra-high vacuum
(Cryo STM) = reduces thermal drift in piezo actuators; static surface image
(Elevated T STM) = allows for dynamic processes to be studied

(1) Constant-current
- Most common mode
- Bias V held fixed, HEIGHT ADJUSTED TO KEEP CURRENT CONSTANT

(2) Constant-height
- Probe scanned at fixed z and bias V; VARIATION IN CURRENT MEASURED
- Faster (no feedback), but risky on rough surface

(3) Spectroscopy mode
- Probe held at fixed position; BIAS V RAMPED UP AND CURRENT IS MEASURED

(4) Manipulation mode
- Probe is pulsed to pick up/deposit atoms from/at specific locations

Image is the SURFACE OF EQUAL LDOS (light and dark spots affected by varying bias V)

Short-range forces
van der Waals forces
Electrostatic forces
Capillary forces

Interatomic forces due to orbital overlap (~0.5-1 nN)
- Attractive when overlap reduces total energy
- Repulsive when overlap increases total energy
- Near-field forces are used to form topographic atomic images

F = HR/6d^2 (~2 nN)
* Interactive forces between dipole of molecules

- Interactive forces between charges on tip and sample
- Charges can accumulate if both tip and sample are insulators

F = π(eps0)R/d*(biasV-cpdV)^2 (~0.5 nN)

* cpdV = contact potential difference (difference in work functions)

Forces from water condensed between tip and sample

F = 4πR(gamma)cos(theta)/(1+d/(R-Rcos(phi)))
(~ 90 nN)

gamma = surface tension of water
phi = angle subtended by tip radius

Au-coated cantilever
- Usually beams of Si or Si3N4
- Elastic bending deflects the beam
- Monitored and recorded by 4-quadrant photodiodes

** Sample scanning required due to sensitive geometry of detection system

(1) Contact mode AFM
- Tip brought into contact with sample
- Cantilever deflects continuously during scanning
- Resolution depends on contact area at tip apex
(2) Lateral/Friction force microscopy
- Torsion on cantilever registers signal in photodiodes
- Measure surface roughness and friction coefficient

**IDEAL FOR HIGH-RES AND SOFT SAMPLES/NO DAMAGE

(1) Dynamic Non-Contact Mode
- Atomic resolution possible
- Cantilever oscillates at or near resonant frequency
- Tip senses the weak attractive force between tip and sample
**Interactions shift the resonant frequency
**Feedback used to adjust tip height to return to set-point resonant frequency

(2) Tapping Mode
- Intermittent contact with sample
- Lifting tip off sample avoids sticking and surface damage
- Force of tip on sample is set to maintain a constant amplitude of oscillation
--- At protrusions, amplitude decreases; tip raises
--- At depressions, amplitude increases; tip lowers

(3) Phase Imaging
- Phase lag of cantilever oscillation relative to signal sent from piezo actuators
** Very sensitive to variations in material properties like adhesion and viscoelasticity

(4) Force modulation imaging
- Identifies and maps differences in surface stiffness/elasticity
- Low frequency oscillation induced when the tip; constant force on sample is applied
** Stiffer areas on the sample = less elastic deformation and less bending of the cantilever (lower oscillation amplitude)

1) Contact mode
- detect nanoscale roughness
- simplest mode for basic topographic information

2) Lateral force microscopy
- generates contrast from geometrical features and changes in surface properties

3) Dynamic non-contact mode
- atomic resolution in UHV environment

4) Tapping mode
- very high resolution

5) Force modulation mode
- reveals regions of different elastic modulus as well as imaging

Due to near-field interactions between tip and sample

*SCANNER ARTIFACTS
- Nonlinear piezoelectricity
- Hysteresis
- Creep

*TIP ARTIFACTS
- Probe-size issues (protrusions or divots)
- Tip fracture

Detecting them:
- Repeat scanning in same area
- Scan in different direction or speed

Use of characteristic X-rays to measure chemical composition

Types:
1) Add-on spectrometers = found in SEM, FIB, and TEM
2) Dedicated instruments (XRF, EPMA)
-- XRF = X-Ray Fluorescence spectroscopy
-- EPMA = Electron probe micro-analyzer

(wavelength of L->K) proportional to 1/(Z-const)^2
Z = atomic number

1) Change in "n" must be ≥ 1
2) Change in "l" must be ±1
3) Change in J = L + S must be 0 or ±1
i.e. L3->K, L2->K = valid; L1->K = forbidden

Probability of emitting a characteristic X-ray rather than Auger electron due to the creation of a core-level vacancy

Low Z = low fluorescent yield
Lower n_final in transition = HIGHER fluorescent yield

- Electrons bombard target to produce primary X-ray beam of continuous and characteristic radiation
- Targets = Cr, Rh, W, Ag, Au, Mo

1) Wavelength-dispersive (WDXS)
- Better resolution
- Wider range of elements detected
(!) More complicated and expensive apparatus
** Analyzer crystal + detector/scintillation counter
- Needs appropriate d-spacing to detect each wavelength during Bragg diffraction
- Large d is needed for light elements
- Though, small d = higher angular dispersion
*** Serial data collection

2) Energy-dispersive (EDXS)
(!) Poorer resolution
(!) Cannot analyze elements below O
- Cheaper, faster, much easier
** Si-Li detector (PIN photodiode)
-- P-type Si
-- Lithium
-- Cooled with liquid N2
-- X-ray photons generate electron-hole pairs
* Higher energies = higher # of e-h pairs
-- Signal amplified and voltage amplitude is proportional to characteristic X-ray energy
* BERYLLIUM WINDOW = low Z metal window that allows X-rays through but holds vacuum inside detector
*** Parallel data collection

For examining small samples/thin films
- Problem = signals weak compared to background
- Principle = reflectivity of a flat target strongly increases below a critical glancing angle

Primary X-rays radiating from substrate materials mostly eliminated via placement of detector

More compact than WDXS

Good collimation is necessary to ensure characteristic X-rays are from sample, not from elsewhere in chamber

*Ability to focus electron beam as a probe allows for chemical analysis in a microscopic area

Spatial resolution of signal defined by excitation volume (larger than SEs and BSEs due to greater escape depth of char. X-rays)

- Reduce accel. voltage to obtain better resolution

Scanning modes:
1) Stationary mode = probe is static; X-rays collected
2) Scanning modes = probe is scanned and X-rays are collected at each position
- Counting statistics are poorer

Pixel intensity is proportional to X-ray counts in a preset ∆E or ∆(wavelength) window

- Useful for analyzing complicated structures
- Spatial resolution and compositional accuracy are necessarily poor due to low counts-per-pixel for reasonable counting time

Background noise and primary char. X-ray peaks should be removed from spectra

Use uncommon elements like Rh for X-ray tube targets

Intensities of data peaks related to element concentrations
C = MK(relative intensity)

M = matrix factor
K = instrument factor (source, geometry, detector)

(1) Internal standard method
- Standard element of known concentration compared to sample (similar in Z and concentration)
Cx/C_stand. = Ix/I_stand.
- Low conc. analysis only

(2) Fundamental Parameter Method
- Matches theoretical (I_t) and measured X-ray intensities (I_x)
(!) Requires detailed physical parameters, accurate instrumentation, and lots of computation
- Estimate composition from measured Ix
- Calculate I_t using method
- Convert I_t to Ix scale
- Use calculated intensity as new estimate and iterate

1) Primary absorption = radiation is absorbed prior to reaching the atoms to be excited

2) Secondary absorption = fluorescent X-ray absorbed from atoms on its path inside the specimen towards the detector

3) Secondary fluorescence = energy from primary fluorescence from heavier atoms triggers secondary fluorescence from lighter atoms

ZAF Method
- Separate standards for each element being analyzed
- Spectra from standards acquired in identical instrumentational conditions as sample
- For heavy and medium-weight elements
- Matrix effects cannot be ignored

C/C_stand. = (ZAF)*(I/I_stand.)

Z = atomic # effect
** Accounts for ∆(char. X-ray) generation due to ∆Z; arises from:
- Electron backscattering = some e are backscattered; no X-rays generated (higher with high Z)
- Electron retardation = continuous e energy loss due to inelastic scattering (lower with high Z)

A = X-ray absorption effect
** Accounts for reduced X-ray density from emitting location to detector

F = X-ray fluorescence
** Char. X-rays from matrix with higher energy cause fluorescence of X-rays from atoms of element to be analyzed
- Enhanced signal for lighter elements

PROBLEMS WITH ZAF METHOD
- errors in analysis for Z<10 elements
- assumption is that specimen is in bulk and surface is microscopically flat

Observing low-energy characteristic electrons (Auger and photoelectrons) from surface for elemental analysis
- From upper-most 10 nm only

FOR ALL ELEMENTS EXCEPT HYDROGEN AND HELIUM

XPS
- X-rays in, electrons out
- photoelectron ejected completely via X-ray photon

- monochromatic X-ray source
- photoelectrons emitted tell about composition, chemical states, charge densities, etc.
* kinetic energy of photoelectron recorded
* input photon energy known
*** Characteristic binding energy calculated:
E_B = hv - E_k - (work function)
(KINETIC ENERGY DEPENDS ON PHOTON ENERGY)

SPECTRA (PLOTTED AGAINST E_B)
- Peaks labeled with element and shell from which photoelectron came
- Those from subshells p, d, and f marked with total quantum angular momentum # too

Poor chemical mapping: poor collimation of X-ray beam

AES
- high-E electrons in, Auger electrons out
--- Core-level vacancy created
--- Higher E electron relaxes down and emits photon
--- Emitted photon does NOT escape atom, but ejects an Auger electron from another level ≥ the relaxation electron

** Most common E_KLL
- Knock out K-shell electron
- An L1 electron fills K vacancy
- L2,3 electron ejected as Auger electron
E_KLL = |E_B_K| - |E_B_L1| - |E_B_L2,3|
(KINETIC ENERGY OF AUGER e ONLY DEPENDS ON BINDING ENERGIES OF LEVELS)

SPECTRA: (PLOTTED AGAINST E_B)
- Derivative of intensity vs. E_B may be better; not for quantitative analysis

Scanning Auger Microscopy (SAM)
- resolution is similar to beam diameter due to low Auger escape depth

Changing the energy of the input X-rays will shift the AES peaks!!
- XPS peaks remain stationary since E_B = hv - E_k - phi does NOT change
- AES peaks move since E_B_apparent = hv - E_k, where E_k is NOT dependent on the photon energy

- (High flux) Argon ion gun and soft X-ray guns (mono or not; Mg or Al targets) eject electrons from sample
- They're guided downetarded with electrostatic lenses
- ANALYZER IN ULTRA HIGH VACUUM CHAMBER
--- Necessary for avoiding monolayer molecular adsorption on surfaces and chances of electron scattering
- Channel electron multiplier, amplifier, and display
(!) Must be magnetically shielded from surroundings

Most common = CHA/HSA
(Concentric hemispherical analyzer; hemispherical sector analyzer)

Potential difference established between hemispherical shells
- Median equipotential surface at median radius defines the PASS ENERGY of the CHA
- CHA allows ONLY electrons with energy eV_0 to reach the detector

Method 1:
- Constant Analyzer Energy (CAE)
- for XPS
- Keeps the CHA pass energy constant
* Electron energies analyzed by recording ∆(electron retardation)

Method 2:
- Constant retarding ratio (CRR)
- for AES
* Electron energies analyzed by recording the ∆(pass energy)
- AES requires suppression of electron signals at low E spectrum (they cause background noise)

XPS Satellite Peaks
1) Shake-up (energy loss) peaks
- Electrons slightly deeper in surface zone of sample may lose energy due to exciting valence electrons nearby, prior to ejection and detection
** Useful; can detect oxidation state differences sometimes

2) Multiplet splitting
- Core-level peak in a compound with unpaired valence electrons
* Useful; complicated QM explanation

3) Plasmon Loss
- Collective vibrations of conduction electrons in metal dissipate photoelectron energy
* Not useful

Direct mode = I vs. E_B (useful for quantitative analysis)
Derivative mode = dI/dE_B vs. E_B (peaks more obvious to pick out)

Low Z = KLL
Medium Z = LMM
High Z = MNN

AES Spectra:
- straightforward
- peaks identified via comparison with standards in databases

XPS Spectra:
- two different X-ray sources to distinguish XPS peaks from mixed-in AES peaks

Shifts in E_B caused by surrounding chemical state
Useful for identifying chemical compounds and bonding states (especially for polymers)
- Shifts decrease with increasing Z

Significantly larger than those in XPS
- Peak shape may be affected by chemical state

XPS = electron depletion (+ve charge buildup)
AES = electron accumulation (-ve charge buildup)

* Change of surface potential changes measured E_K and E_B from a standard value in published table
** Neutralized charge with low-flux electron flood gun (for XPS)

Charge control for AES:
1) Reduce primary electron beam energy
2) Conductive coating

Sensitivity factors:
X_i = (I_i/S_i)/(sum of I_i/S_i)

AES sensitivity factors
- Values relative to Cu_KLL line (S=1)

XPS sensitivity factors
- Values relative to F 1s peak (S=1)

Ion-gun sputtering allows for depth profiling

Distortions can occur due to different etching rates for different types of atoms

Concentration of elements with higher etching rates will be lower than the actual concentration in the sample

- Detects all elements (includes H) which cannot be detected by AES and XPS
- Elements and concentrations as low as 10^-6
- Excellent depth resolution due to escape depth of < 1 nm
- Ability to distinguish between different isotopes

Energized primary ion particles
- Ar+, Ga+, Cs+ bombard solid surface to induce sputtering
1) Sputtering
2) Ionization (only charged ions are useful)

(1) Direct knock-on collision
- extremely fast process (10^-15 to 10^-14 s)

(2) Collision cascade
- slower collision process; atoms in solid transfer impact energy to surface atoms after a series of collisions (10^-14 to 10^-12 s)

(3) Thermal Sputtering
- vaporization of the area impacted by primary ions (10^-13 to 10^-10 s)

Only about 1% of total secondary particles are ionized
- Yield of secondary ion current strongly affected by primary ion beam and electronic state of specimen
= (primary particle flux)*sputter yield)*(probability of positive ion formation)*(fractional conc.)*(transmission in analysis system)

Effect of Z on Ion Yield
- Elements further "left" on periodic table = higher ion yields

Effect of Chemical State on Ion Yield
- Yield varies dramatically for different chemical states of same element

Dynamic:
- High flux bombardment
- Able to remove many layers of atoms
- Depth profiling analysis

Static:
- Extremely low primary ion flux
- < 1% of atoms at surface are bombarded
(~10^13 ions/cm^2 to be bombarded)
- Surface chemistry measurements

Primary ion system:
- Source
- Wein filter
- Deflectors

Sample
Ion extractor filter
Mass Analyzer
Detector

Noble gases: Ar+, Xe+
O2+ (commonly used in dynamic SIMS due to ability to increase positive secondary ion yield)
Ga+
Cs+ (enhances negative secondary ion yield)
Bi+ (modern static SIMS)

GASEOUS ION SOURCES (Ar+, Xe+, O2+)
- electron bombardment source
- gas injected in grid with potential difference accelerating free electrons

DUOPLASMATRON SOURCE (Ar+, Xe+, O2+)
- hollow cathode containing high pressure gas
- current generates arcs to create plasma ion beam

LMIS (Liquid Metal Ion Sources) (Ga+, Cs+, Bi+)
- Metal coated onto a reservoir and needled
- Liquid metal flows over the tip and extends to form a Taylor cone under high tension
- dynamic eq between extraction force and surface tension
** generates high-current-density ion beam

Primary beam ideally contains ions with identical mass
Hall-effect-style filter allows passage of a specific q/m ratio for ions

(1) Magnetic Sector Analyzer
- Only ions with specific m/Z can travel through exit slit in magnetic sector
- Suitable for dynamic SIMS where a few specific ions are measured
(!) only for sequential adjustment of m/Z ratio

(2) Quadrupole Analyzer
- Ion selection based on generating unstable oscillations for all ions EXCEPT a certain m/Z
- Quadrupole with DC and RF voltages generate fields that cause ion oscillations
- Pass ion beam has a direct trajectory
* resolution of each m/Z is the same

***(3) Time of Flight Analyzer
- Mirror = series of rings with gradually increasing applied E field
- Ions with different KE penetrate further/shallower before being reflected to detector
*Ions with same m/Z will arrive at detector at same time
*Must know when the ions left, and must have time for them to reach detector
--- Requires pulsed primary ion beam so as not to flood chamber
(!) Not efficient for chemical analysis in depth profiles

HIGH MASS ions increase yield of secondary ions
LOW KINETIC ENERGY IONS avoids fragmentation and makes spectra easier to analyze

Flood gun of electrons helps to avoid surface charging of insulating samples
- Positive charge accumulation from primary ion bombardment
- Unaccounted electric potential increases +ve ion KE and retards -ve ion emission

Clean sample from contaminants:
1) Sputter cleaning for inorganics
2) Solvent rinse for organics (e.g. hexane)

Sample Geometry:
- Flat as possible (ToF measurements and electric field uniformity highly sensitive to height variations)

Sample Positioning:
- Mass resolution is sensitive to this
- Spectral intensity is not
- Tilting specimen causes serious distortions via curved secondary ion trajectories

Can be positive or negative ion spectra

Elemental Identification
- Most electropositive elements give intense positive ion peaks
- Most electronegative elements give intense negative ion peaks
- Hydrogen gives a high yield of both H+ and H- ions

Polymer Spectra
- Sub-monomer: mass values < monomer mass (gives information on the basic chemical units of monomers)
- N-mer: mass = n(monomer) + fragments (contains info about macromolecular structure)
- Oligomer regions: high (m/Z) peaks generated by cationization with metals (e.g. Ag+)

Focused primary ion beam over an area of the sample surface

For each pixel in a SIMS image, collect signals from a few (m/Z) in dynamic SIMS or from the whole mass spectrum in TOF-SIMS

Image Quality
- Problem in TOF-SIMS = too low of ion flux to generate significant pixel signal
- Solution
--- Choose heavier primary ions to increase ionization probability
--- Reduce noise by digital image processing such as FFT

Topographical features can be identified by comparing SIMS images from several types of secondary ions

E = hf(n+1/2), n = vibrational quantum #
Harmonic oscillator approximation
***These are not phonons since the center of gravity does not shift

∆n = +1 for vibrations

- Illuminate sample with BROAD IR SPECTRUM
- Measure transmittance as a function of radiation wavenumber
- Valleys represent molecular vibration frequencies since those wavenumbers don't get transmitted

INELASTIC SCATTERING OF LIGHT
- Stokes scattering = energy transferred to the SAMPLE (scattered photons have lower frequency)
--- Higher intensity in spectrum
- Anti-Stokes scattering = energy transferred to the PHOTON (scattered photons have higher frequency)
--- Low intensity in spectrum

Illuminate sample with MONOCHROMATIC IR LASER (to be a reference for measuring Raman shifts)

Measure intensity of scattered IR vs. corresponding scattered wavenumber (translate x-axis to Raman wavenumber shift)

Peaks represent molecular vibration frequencies

Two characteristics:
1) Nuclei in a molecule vibrate at same frequency and in the same phase
2) Nuclear motion gives no net molecular translation or rotation

# of normal modes = 3N-6 (6 degrees of freedom subtracted from 3 translation and 3 rotation)
-- some exceptions like CO2

1) Stretching = 1 bond minimum
2) In-plane bending vibrations = 2 bonds minimum
3) Out-of-plane bending vibrations = 3 bonds minimum
4) Torsion vibrations = 3 bonds minimum

Frequency of vibration DECREASES with INCREASING number of bonds involved

IR Active:
- Vibrations which cause DIPOLE ALTERNATION (ASYMMETRIC ABOUT EQUILIBRIUM)

Raman Active:
- Vibrations which case POLARIZABILITY ALTERNATION (ASYMMETRIC ABOUT EQUILIBRIUM)

Broad IR source
More efficient than dispersive spectroscopy
*Based on Michelson interferometer with moveable mirror
-- ∆(OPL) arises from moveable mirror
-- Causes variations in light intensity (at different periodicity) for different wavenumbers as mirror moves
-- All of these interferogram waveforms are superimposed (in time domain, given constant mirror velocity)

***Superposed interferogram of IR radiation incident on sample --> some are removed due to molecular vibration absorption

* Fourier transform of output interferogram shows missing wavenumbers which were present in source

IR light source
- wide-band IR from heating of rare-earth oxides
- SiC

Beam splitter
- semi-transparent to IR light
- KBr/Ge/KBr sandwich structure
- Ge splits IR light
- KBr is transparent to IR and a good substrate
- Drawback is hygroscopic nature of material
**Requires low humidity (purged with N2 or dry air)

IR detector
- pyroelectric crystal as IR thermal detector
TYPES:
1) Deuterated Triglycine sulfate (DTGS)
--- simple, inexpensive
--- temperature change caused by IR radiation
--- changes dielectric constant
--- capacitance of DTGS changes; voltage pulse results proportional to IR energy

2) Mercury Cadmium Telluride (MCT)
--- 10x more sensitive than DTGS
--- Requires cooling to liquid N2 and tends to saturate
--- IR absorption promotes electrons into CB from VB; generate current signal

Collect spectra without sample and subtract this background from with-sample spectra

Absorbance spectrum should be used for quantitative analysis since Transmission peaks are NOT linearly proportional to concentration

Most common approach due to:
- High SNR
- Versatile (solid, liquid, powder, gas)
(!) Thickness limitation on samples (< 20 µm)
(!) Too thin yields too weak of absorption (> 1 µm)

Solid samples = usually thin films or powders
Polymers as thin films
Powders usually diluted in IR-inert KBr (into pellet or mull/slurry)
Liquid/Gas samples confined in capillary KBr plates or insoluble ZnSe or AgCl cells

Less common approach because:
- Limited penetration depth
- Challenging collection geometries
- More noise

Reflection types:
Specular = for flat samples
Absorption = for IR-transparent coatings
Diffuse = for rough samples, powders, etc.

Optical microscopy + FTIR spectroscopy
- Requires mirror-based cassegrain lenses
- No chromatic aberration
- No accidental IR absorption from glass lenses
(!) Low objective power though (x10)

Applications
- Smaller volumes of material than conventional FTIR
- 10µm x 10µm areas
- FTIR mapping achieved with motorized sample stages

Laser source
- continuous wavelength lasers (Ar+, Kr+, He-Ne, etc.)
- monochromatic light

Microscope system
- illuminate only a small area; large NA needed for efficient collection
- pinhole filters to eliminate Airy rings

Pre-filters
- remove elastically scattered light, so that weaker than Raman signal is visible

Diffraction Gratings
- separate the Raman scattered light by wavenumber
- Bragg diffraction angle depends on wavelength

Detector
- photoelectric materials convert absorbed photons to electric signals (CCDs)

Colored samples or impurities can absorb laser radiation and fluoresce at intensities up to 10,000x times that of Raman signal!!

Solutions:
1) Bleaching = use higher powered lasers for long times to bleach out impurity fluorescence
2) Longer wavelengths = low energy, near-IR light minimizes probability of fluorescence
3) Pulsed illumination = Raman-scattering has a much shorter lifetime than fluorescence

- Used for ceramic and polymeric samples
- Micro-Raman for small areas
- Phase ID in polymorphic solids - via localization and intensity of Raman peaks
- Polymer identification - layered structure cross-sections
- Composition determination
- Determination of residual strain - altered equilibrium bond lengths
- Determination of crystallographic orientation

Polar bonds give WEAK Raman but STRONG IR signals

Quantitative Analysis:
Beer's Law:
A = -log(I_0/I) = abc

Measure material properties as a function of temperature

Solid-phase transformations
Glass transition
Melting
Sublimation (mass change)
Thermal Decomposition (mass change)

Heat flowing in or out of a system at constant pressure

Furnace with controllable environment
Transducer to measure property changes

Experimental Parameters: data strongly affected by:
- Sample dimensions and mass (< 10 mg)
- Heating and cooling rates (slow rate is better)
- Atmosphere around sample (fixed flow rate, inert gas)
- Thermal and mechanical history of the sample

GOAL: stable partial pressures, approximate thermal equilibrium in sample during experiment

Both objectives = examine thermal events without mass exchange with surroundings

DTA
- Qualitative (only where the thermal events occur on T-axis)
- Operates on wider range of T (up to 1500˚C)

DSC
- Quantitative (the enthalpy involved with the thermal events as well)
- Restricted up to 750˚C

- Measures ∆T between sample and reference material
- Reference must:
--- Have no thermal events over T range
--- Not react with instrument
--- Have a heat capacity and thermal conductivity similar to that of the sample

∆T > 0: event is EXOTHERMIC (sample T > reference T)
∆T < 0: event is ENDOTHERMIC (sample T < reference T)

DTA Curves:
∆T vs. reference T (or time if heating rate is constant)
(inverted ∆T axis)

- Measure ∆H between sample and reference
Type 1) Heat Flux DSC (quantitative DTA)
--- Measures ∆T directly, converts into ∆H
Type 2) Power-compensated DSC
--- Directly measures ∆H by maintaining ∆T = 0
--- Separate heaters for sample and reference
--- Heater power changes are equal to amount of ∆H for thermal event

Plot = Heat flow vs. T (endothermic = up on ∆H/mass axis)

- Constant heating rate + oscillatory heating rate
** Main advantage = able to separate reversible from irreversible thermal events

Sample in dense powder or small discs to yield thermal equilibrium
- DTA holders = Pt or Au crucibles
- DSC holders = Al pans; sealed to avoid mass change

Instrument baseline (zero line)
- DSC curve recorded without sample over T range
- should ideally be a straight line

Sample baseline
- Line connecting the DSC curve before and after a peak
**∆H measurement relies on correctly constructing the sample baseline from the curve

- Change the curve shape and the peak or trough T

- Intersection of two tangents to sample baseline curves before and after transition

∆H/mass = Kc*Ap
Ap = peak area
Kc = calibration factor; based on measuring well-defined standards' melting temperatures

- Measure DSC curve with pan only
- Measure DSC curve with sample + pan
- Vertical displacement between curves = heat capacity
* Can also use a standard to determine calibration factor

Determining Phase Transitions
Determining Phase Diagrams
Physicochemical changes in polymers
Curing of polymers
Polymer ID in mixtures

** Measures ∆(sample mass) with T
- Sample in furnace on MICROBALANCE
- Analyzes material decomposition and thermal stability through ∆(mass) as a function of:
--- T (scanning mode)
--- time (isothermal mode)

Plots: % ∆(mass) vs. T or time

Microbalance
- null-point type
- keeps sample at same height while mass changes

Furnace
- thermocouple located under and close to sample
- tight tube; little excess space

- Sample mass, volume, and form are important for recording accurate and reproducible TGA curves
- ∆T between sample and programmed T must be minimized

Atmosphere
- Interactive atmosphere or non-interactive atmospheres
- Gas flow around sample can cause thermal shielding and/or buoyancy effects

Temperature calibration
- Good TGA results need careful temperature calibration
*** CURIE POINT METHOD OF CALIBRATION (APPARENT ∆m FOR FERROMAGNETICS THROUGH CURIE POINT NEAR B-FIELD)

- Monitoring chemical reactions
- Monitoring mass change
- Assessing thermal stability
- Derivative plots sometimes more easily interpreted
- Compositional identification for thermally decomposable components

Charge-coupled device

Continuous wavelength

Diamond-like carbon

Deuterated Triglycine Sulfate (IR detector)

Ethylene-vinyl-acetate

Fourier Transform InfraRed

Mercury Cadmium Telluride (IR detector)

Auger Electron Spectroscopy

Constant Analyzer Energy

Concentric Hemispherical Analyzer

Constant Retarding Ratio

Hemispherical Sector Analyzer (same as CHA)

Scanning Auger Microscopy

Ultra-High Vacuum

X-Ray Photoelectron Spectroscopy

Energy-Dispersive X-Ray Spectrometer

Electron Probe Microanalyzer

Layered Synthetic Microstructure

Total Reflection X-Ray Fluorescence Spectrometer

Thallium Acid Phthalate

Wavelength-Dispersive X-Ray Spectrometer

X-Ray Diffraction

ZAF-Method

Atomic Force Microscopy

Local Density of States

Lateral Force Microscopy

Metal-Organic Chemical Vapor Deposition

Scanning Force Microscopy

Scanning Probe Microscopy

Scanning Tunneling Microscopy