Surface AnalysisMicroscopy and Spectroscopy

Fundamentals of Electrochemistry

CHEM*7234

CHEM 720

Lecture 11

Areas of Application

ELECTROCHEMISTRY

SYNTHESISANALYSIS

Microscopy

Spectroscopy

Current Microscopy Example

“Advanced Plating Chemistry for 65 nm Copper Interconnects”Semiconductor International May 2003

Mounding when filling trenches with electrochem deposited Cu using two component electrolyte system.

No mounding with three component system.

Utilized Fast Ion Beam (FIB) Microscope for these images.

Current Spectroscopy Example

“Structural Studies in Lithium Insertion into SnO-B2O3 Glasses and Their Applications for All-Solid-State Batteries”, Katada et al., JES 150, A582 (2003)

XRDNMR

Mössbauer

Ex Situ vs. In Situ

Ex Situ Experiment: An experiment performed on a sample after it has been removed from the location wherein it was formed.

• wider range of experimental techniques available.

In Situ Experiment: An experiment performed on a sample while it is still located in its native environment.

• less risk of altering the sample’s true properties.

Challenges Analyzing Electrochemical Samples Ex Situ

• loss of electrochemical control

• loss of solvent, ion atmosphere

• risk contamination, oxidation

Remove sample ?

Challenges Analyzing Electrochemical Samples In Situ

electronsions

photons• the electrolyte solution can strongly absorb the various probe particles which might be used to perform different analyses.

• cell design needs to account for refraction

Microscopy

• What is the structure of the surface of the sample?

• Resolution: Lateral, Vertical

• Contrast mechanism

• Ex situ or In situ

• Dynamic Range

1 cm1 mm1 µm1 nm1 Å

Atoms

Molecules

Viruses

ComputerCircuits

Red BloodCells

Hair

Lateral Resolution

1 cm1 mm1 µm1 nm1 Å

Here are some of the techniques we will examine and a comparison of their lateral resolution capabilities.

AFM

STM IMSEM

OMSAM

Vertical Resolution

1 cm1 mm1 µm1 nm1 Å

AFM

STMIM SEM

OMSAM

Here are the same techniques, comparing their vertical resolution.

Dynamic Range

Not only do we need image small things, but large things too. What is the largest field of view that the instrument can provide? What is the range of features, largest to smallest, that can be observed?

1 cm1 mm1 µm1 nm1 Å

AFM STM

OM

SEM SAM

IM

Diffraction vs. Scanning

Two approaches to image formation

Diffraction:

Incident wave scatters from surface features, interfering with itself and forming a diffraction pattern. When diffracted wave is refocused, it produces an image of the surface.

•Entire image formed simultaneously

•Resolution limited by wavelength

Scanning:

Incident wave focused to a small point and rastered across surface. Signal is acquired from each point on surface.

•Image formed sequentially

•Resolution determined by spot size

Vibration Isolation

Buildings vibrate (motors, air conditioners, walking, vehicles).

Resonances between 1 and 100 Hz. Amplitudes in micron range.

• build microscope rigid

• couple to building loosely

• provide multiple stages with alternate rigid/loose coupling

• shield acoustically for very precise measurements

Optical Microscopy

• a diffraction experiment

• basic lens components

• coarse/fine focus

• Mon/Bin/Tri ocular schemes

• working distance

• adjust interpupillary distance

• quantitation with reticle

• image recording

http://www.greatscopes.com/important.htm

A good web site for a brief introduction to optical microscopes can be found below.

http://www.olympusmicro.com/primer/opticalmicroscopy.html

Optical Microscopy continued

Select the correct combination of lenses for your task.

Optical Microscopy Resolution

• Rayleigh equation

d = 0.61 (/ N.A.)

d is distance between objects that can still be distinguished is wavelength of lightN.A. is numerical aperture of lens = n sin(vertex)

Scanning Electron Microscopy

Electron GunSecondaryElectronDetector

Vacuum Chamber

SEM Focusing Column

Sample

ThermalFieldEmitter

SuppressorAssembly

Extractor

BeamAcceptanceAperture

Lens 1

SteeringQuadrupole

1 SteeringQuadrupole

2

BeamBlankingPlates

Lens 2

DeflectionOctupole

SEM Experiment

Trochodiscus longispinus in OM and SEM. Note improved depth of field and resolving capability of the SEM experiment.

aa

Electron Reemission

e–Backscattered

Elastically scattered

Inelastically scattered

Secondary electron emission

Fraction of Incident Beam Energy

Rel

ativ

e In

tens

itySEM

SEM

BSE vs. 2° Detection

Both can be used, different information, different detection scheme.

BSE

Specular reflection

Higher energy

Encode some chemical information

2° Electrons

Isotropic emission

Very low energy

Better structural contrast

Excitation Depth Profile

Incident electron beam penetrates 100 - 200 m into sample.

Different emission mechanisms arise from different depths.

X-ray

SED

BSE

CL

Atomic Number Dependence

The probability of an incident electron being scattered varies as the square of the atomic number of the atom and inversely as the incident kinetic energy.

• Greater depth of penetration for low Z materials (e.g. Al vs. W)

• BSE emission branch increases with Z

Low Z High Z

Equation d prop to W V2/Z rho

SEM Example

Microstructural Development and Surface Characterization of Electrodeposited Nickel/Yttria Composite Coatings, Cunnane et al., JES 150, C356 (2003)

Changing the Y content in the Ni electrolyte bath from 1 to 5 g/L. Preferential growth directions are altered as the nucleation rates are changed by the co-depositing material.

Scanning Auger Microscopy

Auger process. Chemical maps. Hemispherical Electron Analyzer. Secondary scattering in samples.

Uses same e-beam source as SEM. Energy analyzes electrons emitted in 100 - 1000 eV range (higher than secondary, lower than backscattered).

Provides unique atomic identity information.

Very surface specific (10 nm)

Chemical maps of surfaces

The Auger Process

K

L

M

Vacuum Level

Eject core electron

Higher electronfalls into hole

Another higher level electron is ejected to carry away excess energy.

Measure kinetic energy of ejected electron.

IncidentElectronBeam (5 kV)

Ekinetic = E(K) - E(L) - E(L)

Auger Spectra

SAM Resolution

BS electrons are also scattered into the neighbouring regions of the sample with sufficient energy to further excite atoms not in the original excitation volume.

Spatial resolution degraded 2 to 5 times over that of the corresponding SEM resolution.

Scanning Tunneling Microscopy

a

Probe Tip

Sample

Tunneling Electron Current

Tunneling gap ~ 5 Å

Tunneling Current 10 pA - 10 nA

Tunneling Mechanism

Sample Tip

EF

EF

VBias

0 d

DOSDOS

IT exp(-2d)

Density of States

Every substance has a complex electronic structure. At every energy, there are a certain number of electronic states. The number is so large for bulk material, that one reports the number of states per unit energy – the Density of States or DOS.

Tunneling can occur between states of the same energy; the electron’s energy does not change during the tunneling event.

Control Electronics

—

Feedback Electronics

Set Point

Current Amplifier

Logarithmic Amplifier

Error Signal

Difference

Z-piezo

Sample

Resolution

VerticalLateral

R∆x

In Situ Electrochemical STM

There’s still a vacuum gap, even in water!

Shield tip to minimize faradaic processes. Melted wax or plastic to coat shank of tip. Expose last few nanometers only. Tunneling current must be large compared to faradaic current.

STM Example #1

Adlayer of 1,10-phenanthroline on Cu(111) in acidic solution

Itaya, et al. J.E.S. 150 E266 (2003).

Monitored molecular orientation on surface in real time

Scanning Electrochemical Microscope (SECM)

http://www.msstate.edu/dept/Chemistry/dow1/secm/secm.html

Create an ultramicroelectrode and use the faradaic current as the control signal.

Signal modulated by proximity to surface.

Scanning Force Microscopy

Depends on forces (repulsive or attractive) between atoms.

a

Sharpened Cantilevered Tip

Diode laserReflected lightTo PositionSensitive Detector

Position Sensitive Detector

4-Quadrant Photodiode(current in each quadrant changeswith light intensity)

1 2

3 4

1+2-(3+4) = 0

1+2-(3+4) < 0

1+2-(3+4) < 0and1+3-(2+4) > 0

Contact Mode SFM

a

Repulsive force between surface atoms and tip atoms, lead to cantilever deflection, altering of relected beam path.

Sample is rastered and moved vertically to maintain constant cantilever deflection.

Can damage delicate samples.

Lateral Force Mode SFM

Frictional force measurement. During scan, frictional forces on surface will tend to twist the cantilever.

Use Signal = 1+3 - (2+4) as feedback/imaging signal.

Chemically sensitive: –CH3 covered surface vs. –COOH covered surface

Non-Contact Mode SFM

Important when dealing with delicate samples.

Can achieve atomic resolution.

Vibrate tip at resonant frequency (100’s of kHz).

As tip approaches surface, the attractive forces between the substrate and the tip alter the resonance condition.

For feedback/imaging

• frequency shift

• phase shift

• damping

Cantilevers

For contact mode

For LFM and non-contact mode

SFM Example

The Electrochemical Reaction of Lithium with Tin Studied By In Situ AFM, Dahn et al., JES 150, A419 (2003).

Li is driven into Sn electrochemically which leads to a swelling of the Sn grains. SFM images were used to measure the grain sizes as the potential changed, contributing to a model rgarding Li incorporation in the Sn film.

Interference Microscopy

Instrument. Interference technique. Computational process. VSI mode. PSI mode. Angle of acceptance. Terraced surface vs. rough surfaces.

Visible wavelength optical microscope.

Also called Non-contact Profilometry.

Nanometer resolution vertical to surface.

Uses interferometry to measure surface profile.

Large dynamic range.

Interference Fringes

a

First reflecting surface

Structured reflecting surface

Top view

In-phase reflections are bright; out-of-phase are dark

Side view

Imaging Process

Recombined, reflected light is directed to image plane of CCD camera.

Points on surface that are separated from lens by an integer number of wavelengths is bright; those a half-integer are dark.

ObjectiveLens

Interferometer

Imaging Process continued

Interference is strong only when reflected light is in focus; the sample-lens distance is at the focal position.

Scan sample-lens distance around the focal length.

Each pixel will strongly modulate its intensity when the lens reaches the focal position corresponding to each point on the surface.

High resolution position information comes from a linear variable differential transformer (LVDT) connected to the lens scanning drive.

VSI and PSI Modes

Vertical Scanning Interferometry

1. Scan objective over range of µm.

2. Record image frames sequentially.

3. Search each pixel through frames and locate frame where intensity modulation is greatest.

4. Assign height information by correlating frame number to LVDT.

Phase Shifting Interferometry

1. Alter optical path length in series of steps.

2. This causes fringe pattern to shift laterally.

3. The series of shifted fringe patterns are combined to form interferograms from which height information is calculated

Rough vs. Terraced Surfaces

Interference can occur only if light is reflected back into objective lens. If surface angle is inclined beyond acceptance angle of lens, no interference is observed.Lens Angle

2.5x obj. 2°

10x obj. 10°

50x obj. 25°

O.K.

Missed data

Terraced surface

IM Example

Preparing Au substrates on mica for use in forming nanostructured electrodes from self-assembled monolayers. Heat treatment created mounds on surface.

Raman Imaging Microscopy

Raman spectroscopy is molecular vibrational spectroscopy. Microscope uses a focused laser beam as the excitation source. The detector can be tuned to look for a particular spectral peak and this can be used to produce a chemical map - now based on molecular and not just atomic features.

Raman Effect

Incident laser impinges on sample. Scattered light is shifted slightly to longer wavelengths; small amount of photon energy is left in molecules to excite vibrations. This scattered light, looking for loss of energy, correlates with molecular vibrational spectrum.

Mapping

Distribution of beclamethasone dipropionate (BDP) and salbutamol in an allergy medication. Particle size is important for effectiveness.

Imaging

Much faster than mapping. Uses bandpass filters instead of dispersive grating detection. Entire image passes through filter and exposed to CCD camera at once. Image keyed to the radiation intensity passing through the bandpass. This is selected for a particular molecular transition.

Raman image can pick out the 5 differfent layers very easily. From a forensics study of a car.

Raman Example

Fuel cell development. Troubled by contamination with NO+ in solid oxide fuel cell electrolyte, which poisoned process. IR is weak and overlapped by CO2.

Spectroscopy

What is on the surface? (Atoms or Molecules or Bulk)

What is the structure of the surface layer?

How are they oriented?

What is their oxidation state?

How do these properties change with potential? with time? with additional participants in the electrolyte solution?

Energy Dispersive X-ray Spectroscopy (EDX)

Done in conjunction with SEM. Name shifting to EDS.

Add an X-ray detector. Emitted X-rays identify atomic species in excitation volume.

Detector analyzes X-ray photons by energy, rather than wavelength.

Can be used to chemically map a surface.

Can also be done in wavelength dispersion mode. Higher resolution (10 eV compared to 100 eV), but more complex. Getting better. Also higher sensitivity. Order of magnitude better.

EDS Detector

Cooled in LN2 temps, Si crystal converts X-ray photon into charge by ionization. Charge is integrated through the FET and is proportional to X-ray energy.

WDS Detector

Concave mirror crystal is key to the process. Can be LiF, thallium acid phthalate, or multilayered structures such as W/C, W/Si, or Mo/B.

MoS2

EDX Example

A cast iron sample

SEM C map Si Map Fe map

X-ray Photoelectron Spectroscopy

Irradiate sample with monochromatic X-ray beam and energy analyze the photoelectrons which are ejected. (Kind of opposite of EDS).

High resolution (< 1eV) allows chemical state identification (Si, Si2+, Si4+, SiO2 compared to SiTe2.

Vacuum required to be able to detect the electrons.

New instruments can focus X-ray to a few µm in diameter. The beam can be scanned to do imaging XPS.

X-ray Source: Anode

Electron beam (15 kV) strikes an anode (Mg or Al). Emits x-rays. Tuned to maximize for narrow emission range (example, Mg K).

X-ray Source: Synchrotron

Electron Energy Analyzer

Hemispherical analyzer. Electron lens systems adjusts incoming electron energy to particular kinetic energy. Only specific energy passes through the hemispherical path to reach detector.

Detector is electron multiplier. Can be multichannel.

XPS Spectrum #1

Spectrum for Yttrium

XPS Spectrum #2

Ag spectrum showing the spin-orbit splitting of the 3d peaks. The instrumental linewidth of 0.82 eV is also shown.

IR Spectroscopy

Vibrational information about molecules. Valuable because of surface selection rules

Ep

Ep

Es

Es

Phase shifts 180° upon reflection

P-polarized: electric vector amplified at surface.

S-polarized: electric vector cancels at surface.

SNIFTIRS

Subtractively Normalized Interfacial Fourier Transform Infared Spectroscopy

Working ElectrodeThin Film Electrolyte (2 µm)ZnSe prism

)(

)()(

1

12

ER

ERER

R

R −=

Δ i d

d

PM FTIRRAS

Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy

BaF2 prism

Electrolyte (D2O) µm

Organic layer nm

Electrode surface

Kerr Cell

Electronically modulate polarization at 150 kHz.

AII

IIS

ps

ps 3.2)(2=

+−

=Δ

PM FTIRRAS Spectrum - Pyridine

Pyridine bound to Au(111) changes orientation with cell potential