(Nano)materials characterization

Lecture 8

(Nano)materials

characterization

Lecture 8

OUTLINE

-What SEM and AFM are good for?

- What is the Atomic Force Microscopes

Contribution to Nanotechnology?

- What is Spectroscopy?

MTX9100

Nanomaterials

Atomic Force Microscopes

(AFM)

The Atomic Force Microscope was

developed to overcome a basic

drawback with STM - that it can

only image conducting or

only image conducting or

semiconducting surfaces.

The AFM, however, has the

advantage of imaging almost any

type of surface, including

polymers, ceramics, composites,

glass, and biological samples.

Why AFM?

An atomic force microscope (AFM) creates a

highly magnified three dimensional image of a

surface. The magnified image is generated by

monitoring the motion of an atomically sharp

probe as it is scanned across a surface. With

probe as it is scanned across a surface. With

the AFM it is possible to directly view

features on a surface having a few

nanometer-sized dimensions including single

atoms and molecules on a surface. This gives

scientists and engineers an ability to directly

visualize nanometer-sized objects and to

measure the dimensions of the surface

features.

Atomic Force Microscopes

• Monitors the forces of attraction and repulsion between a probe and a sample surface

• The tip is attached to a cantilever which moves up and down in response to forces of attraction or

STM

forces of attraction or repulsion with the sample surface

– Movement of the cantilever is detected by a laser and

photodetector

AFM

Shading shows interaction strength

ZnO

Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer, where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. AFM tips and cantilevers are microfabricated from Si or Si₃N₄. Typical tip radius is from a few to 10s of nm.

Schematic of the AFM

operation

Because the atomic

force microscope relies

on the forces between

the tip and sample,

knowing these forces is

important for proper

imaging. The force is not

measured directly, but

measured directly, but

calculated by measuring

the deflection of the

lever, and knowing the

stiffness of the

cantilever. Hook’s law

gives F = -kz, where F is

the force, k is the

stiffness of the lever,

and z is the distance the

lever is bent.

Measuring forces

The fundamental interaction at short distances is the van der Waals interactions, which are responsible for

the formation of solids, wetting, etc. At distances of a few nm, van der Waals

forces are sufficiently strong to move macroscopic objects such as AFM

cantilevers.

Van derWaals interactions consist of Van derWaals interactions consist of

three components: polarization, induction, and

dispersion.

Polarization refers to permanent dipole moments such as exist in water

molecules.

Induction refers to the contribution of induced dipoles.

Dispersion is due to instantaneous fluctuations of electrons, which occur at

the frequency of light.

Modes of operation

- Used for Contact Mode, Non-contact and TappingMode AFM -Laser light from a solid state diode is reflected off the back of the cantilever and collected by a position sensitive detector (PSD). This consists of two closely spaced photodiodes. The output is then collected by a differential amplifier

- Angular displacement of the cantilever results in one

photodiode collecting more light than the other. The resulting output signal is proportional to the deflection of the cantilever. output signal is proportional to the deflection of the cantilever. - Detects cantilever deflection <1A

Contact mode

Constant force is applied to the surface while scanning

Potential diagram showing the region of the probe while scanning in contact mode.

In contact mode the probe glides over the surface.

Contact mode is typically used for scanning hard samples and when a resolution of greater than 50 nanometers is required. The cantilevers used for contact mode may be constructed from silicon or silicon nitride. Resonant frequencies of contact mode cantilevers are typically around 50 KHz and the force constants

are below 1 N/m.

Contact mode images

Left: Bits on a compact disk. Center: Image of a

metal surface. Right: Nano-particles on a

surface

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- A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force)

- The tip contacts the surface through the adsorbed fluid layer. - Forces range from nano to micro N in ambient conditions and even

lower (0.1 nN or less) in liquids.

Tapping mode

The probe is vibrated in and out of surface potential. The modulated signal can then be processed with a phase or amplitude demodulator.

-A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface.

- A constant oscillation amplitude (and thus a constant tip-sample

interaction) are maintained during scanning. Typical amplitudes are 20-100nm.

- Forces can be 200 pN or less

- The amplitude of the oscillations changes when the tip scans over bumps or depressions on a surface.

Tapping mode images

Vibrating mode AFM images. Left: Silicon

wafer. Center: Cancer cells. Right: Proteins.

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AFM cantilever

Non – contact mode

-The cantilever is oscillated slightly above its resonant frequency.

- Oscillations <10nm

-The tip does not touch the sample. Instead, it oscillates above the

adsorbed fluid layer.

- A constant oscillation amplitude is

⇒ In contact mode, the tip is usually maintained at a constant force by

moving the cantilever up and down as it scans. ⇒ In non-contact mode or tapping mode the tip is driven up and down by an oscillator. Especially soft materials may be imaged by a magnetically-driven cantilever (MAC ModeTM). ⇒ In non-contact mode, the bottom-most point - A constant oscillation amplitude is

maintained.

-The resonant frequency of the cantilever is decreased by the van

derWaals forces which extend from 1-10nm above the adsorbed

fluid layer. This in turn changes the amplitude of oscillation.

⇒ In non-contact mode, the bottom-most point of each probe cycle is in the attractive region of the force-distance curve.

In tapping and contact mode the bottom-most point is in the repulsive region. Variations in the measured oscillation amplitude and phase in relation to the driver frequency are indicators of the surface-probe interaction.

Advantages of the main modes of

AFM

Contact

Mode

– High scan speeds

– The only mode that can obtain

“atomic resolution” images

– Rough samples with extreme

changes in topography can be

– Higher lateral resolution on most

samples (1 to 5nm)

– Lower forces and less damage to

soft samples imaged in air

– Lateral forces are virtually

eliminated so there is no scraping

changes in topography can be

scanned more easily

Tapping

Mode

Disadvantages of the main modes

of AFM

Contact

Mode

– Lateral (shear) forces can distort features

in the image

– The forces normal to the tip-sample

interaction can be high in air due to capillary

forces from the adsorbed fluid layer on the

sample surface.

sample surface.

– The combination of lateral forces and high

normal forces can result in reduced spatial

resolution and may damage soft samples (i.e.

biological samples, polymers, silicon)

Tapping

Mode

– Slightly lower scan speed than contact

mode AFM

SPM family

SPM – AFM tips

There are several types of Scanning Probe Microscopes that

are used for measuring surface topography and physical

properties. The primary types of SPM's are the AFM, STM

and NSOM. AFM's account for about 80% of the total number

of scanning probe microscopes.

What SEM and AFM are good for?

Both the AFM and SEM measure topography. However, both types of microscopes can measure other surface physical properties. The SEM is good for measuring chemical composition and the AFM is good for

measuring mechanical properties of surfaces.

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Manipulating materials at

the atomic level

A tip is used to pick up atoms by attracting them with an attracting them with an electrostatic field. This tip is moved to the predefined

position and the atom released by turning off the electric field.

Phase imaging

Accessible via Tapping Mode

Oscillate the cantilever at its resonant

frequency. The

amplitude is used as a feedback signal. The feedback signal. The phase lag is dependent on several things,

including composition, adhesion, friction and viscoelastic properties. Identify two-phase structure of polymer blends

Identify surface contaminants that are not seen in height images Less damaging to soft samples than lateral force microscopy

Phase Imaging - examples

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Nanoindentation and Scratching

» A diamond tip is mounted on a metal cantilever

and scanned either with contact or Tapping Mode.

» Indenting mode presses the tip into the sample

» Scratch mode drags the tip across the sample

at a specific rate and with a specified force.

» The use of Tapping Mode makes it possible to

at a specific rate and with a specified force.

» The use of Tapping Mode makes it possible to

simultaneously image soft samples

Nanoindenter

Indentation cantilevers are thicker, wider, and longer than standard AFM cantilevers and are diamonds, as compared with silicon or silicon nitride.

The typical ranges for spring constants of contact mode, Tapping Mode, and indentation cantilevers are 0.01-1.0N/m, 20-100N/m, and 100-300N/m

respectively.

The resonant frequency for indentation cantilevers is generally in the range of 35-60kHz, depending on the dimensions of the cantilever and the size of the diamond. For comparison, the resonant frequency for standard Tapping the diamond. For comparison, the resonant frequency for standard Tapping Mode cantilevers is about 300kHz.

dP

Nanoindentation - basic

dh

dP

S =

For Berkovich tip:

A = 24.5h

_,

where

h

is the contact depth

h

= h

- 0.75(r

/s).

Analytical model –

Basic concept

Deformation upon unloading is purely elastic

The compliance of the sample and of the indenter

tip can be combined as springs in series

'

1

1

1

−

υ

−

υ

Hardness

2

5

.

24

p

h

P

H =

'

'

1

1

1

E

E

E

υ

υ

−

+

−

=

Comparison of AFM and other imaging

Comparison of AFM and other imaging

Comparison of AFM and other imaging

Comparison of AFM and other imaging

techniques

techniques

techniques

techniques

1. AFM versus STM:

In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is much more complex when characteristics such as tip shape and contact force are considered. STM is generally applicable only to conducting samples while AFM is applied to both conductors and insulators. In terms of versatility, the AFM wins. Furthermore, the AFM offers the advantage that the writing voltage and tip-to-substrate spacing can be controlled independently, whereas with STM the two parameters are integrally linked.

parameters are integrally linked. 2. AFM versus SEM:

Compared with Scanning Electron Microscope, AFM provides extraordinary topographic contrast direct height measurements and unobscured views of surface features (no

coating is necessary). 3. AFM versus TEM:

Three dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available from

cross-sectioned samples.

4. AFM versus Optical Microscope:

The AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials.

Comparison of imaging techniques

Comparison of imaging techniques

Comparison of imaging techniques

Comparison of imaging techniques

An AFM is capable of resolving features in the dimensions of a few nanometers with scan ranges up to a hundred microns.

Summary of tools

Comparison of the time for measurements and instrumentation cost of optical, AFM, and SEM/TEM microscopes.

Spectroscopy

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength (λ).

The sample is irradiated with an electron probe. The incident

electron beam causes ionization of electrons belonging to the inner electrons belonging to the inner shells of the atoms composing the material.

What is light?

Light consists of oscillating electric and

magnetic fields.

Because nuclei and electrons are charged

particles, their motions in atoms and molecules

generate oscillating electric fields.

Light – matter interaction

E

_total

= E

_spin

+ E

_translation

+ E

_rotation

+

E

_vibration

+ E

_electrons

+ E

_nucleus

The total energy of a molecule

1913 - Bohr postulated that a quantum of light of angular

frequency ω is absorbed or emitted whenever an atom

frequency ω is absorbed or emitted whenever an atom

jumps between two quantized energy levels E

and E

E

2

− E

= ħω

1916–7 Einstein introduced the Einstein coefficients to

quantify the rate at which the absorption and emission of

quanta occur

What happens when light interacts

with a molecule?

Absorption

Absorption

Emission

Emission

A transition from a lower level to a higher level

with transfer of energy from the radiation field

to an absorber, atom, molecule, or solid.

A transition from a higher level to a lower level

with transfer of energy from the emitter to the

radiation field. If no radiation is emitted, the

Emission

Emission

Scattering

Scattering

radiation field. If no radiation is emitted, the

transition from higher to lower energy levels is

called nonradiative decay.

Redirection of light due to its interaction with

matter. Scattering might or might not occur with

a transfer of energy, i.e., the scattered radiation

might or might not have a slightly different

wavelength compared to the light incident on the

sample.

Radiation

Type of Radiation

Frequency Range (Hz) Wavelength Range

Type of Transition

gamma-rays

10

_-10

_{<1 pm}

_nuclear

X-rays

10

_-10

_{1 nm-1 pm}

_{inner electron}

ultraviolet

10

_-10

_{400 nm-1 nm}

_{outer electron}

visible

4-7.5x10

_{750 nm-400 nm}

_{outer electron}

visible

4-7.5x10

750 nm-400 nm

outer electron

near-infrared

1x10

_-4x10

_{2.5 µm-750 nm}

outer electron

molecular vibrations

infrared

10

_-10

_{25 µm-2.5 µm}

_{molecular vibrations}

microwaves

3x10

_-10

_{1 mm-25 µm}

molecular rotations,

electron spin flips*

Classification of methods

The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an intensity, either of energy absorbed or produced.

Electromagnetic spectroscopy involves interactions of matter with electromagnetic radiation, such as light.

Electron spectroscopy involves interactions with electron beams.

Auger spectroscopy involves inducing the Auger effect with an electron beam. In this case the measurement typically involves the kinetic energy of the electron as variable.

the electron as variable.

Mass spectrometry involves the interaction of charged species with

magnetic and/or electric fields, giving rise to a mass spectrum. The term "mass spectroscopy" is deprecated, for the technique is primarily a form of measurement, though it does produce a spectrum for observation. This

spectrum has the mass m as variable, but the measurement is essentially one of the kinetic energy of the particle.

Acoustic spectroscopy involves the frequency of sound.

Dielectric spectroscopy involves the frequency of an external electrical field

Mechanical spectroscopy involves the frequency of an external mechanical stress, e.g. a torsion applied to a piece of material.

Absorption spectroscopy

Absorption spectroscopy

refers to a

range of techniques employing the

interaction of electromagnetic

radiation with matter.

The intensity of a beam of light

measured before and after

interaction with a sample is compared.

By measuring the frequencies of light absorbed by an atom or molecule, we can determine the frequencies of the various transition motions that the atom or

molecule can have. Thus we can use light absorption to probe the dynamics of atoms and molecules!

interaction with a sample is compared.

An atom or molecule can absorb energy from light if the frequency of the light oscillation and the frequency of the electron or molecular

"transition motion" match. Unless these frequencies match, light absorption cannot occur. The "transition motion" frequency is related

to the frequencies of motion in the higher and lower energy states.

Infrared spectroscopy

IR spectroscopy allows to examine the vibrational

motions of molecules.

E

_translation

, E

_rotation

, E

_vibration

The quantum energy of infrared photons is in the range 0.001 to 1.7 eV which is in the range of energies separating the quantum states of

molecular vibrations. Infrared is absorbed more strongly than microwaves, but less strongly than visible light. The result of infrared absorption is heating of the tissue since it increases

molecular vibrational activity.

Raman spectroscopy

When electromagnetic radiation passes through matter, most of the radiation continues in its original direction but a small fraction is scattered in other directions. Light that is scattered at the same wavelength as the incoming light is called Rayleigh scattering. Light that is scattered in transparent solids due to vibrations (phonons) is called Brillouin scattering. Brillouin scattering is typically shifted by 0.1 to 1 cm-1 _{from the incident light.}

Light that is scattered due to vibrations in molecules or optical

phonons in solids is called Raman scattering. Raman scattered light is Light that is scattered due to vibrations in molecules or optical

phonons in solids is called Raman scattering. Raman scattered light is shifted by as much as 4000 cm-1 _{from the incident light.}

Raman spectroscopy is the measurement of the wavelength and intensity of inelastically scattered light from molecules. The

Raman scattered light occurs at wavelengths that are shifted from the incident light by the energies of molecular vibrations. The

mechanism of Raman scattering is different from that of infrared absorption, and Raman and IR spectra provide complementary

information. Typical applications are in structure determination, multicomponent qualitative analysis, and quantitative analysis.

Raman spectroscopy - basic

Raman spectroscopy is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other

low-frequency modes in a system. It relies on inelastic scattering of

monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.

The laser light interacts with phonons or other excitations in the system, resulting in the in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives

information about the phonon modes in the system. IR

spectroscopy yields similar, but complementary, information.

Basic theory of RS

The Raman effect occurs when light impinges upon a molecule

and interacts with the electron cloud of the bonds of that

molecule. The incident photon excites one of the electrons into

a virtual state. For the spontaneous Raman effect, the

molecule will be excited from the ground state to a virtual

energy state, and relax into a vibrational excited state, which

generates Stokes Raman scattering. If the molecule was

generates Stokes Raman scattering. If the molecule was

already in an elevated vibrational energy state, the Raman

scattering is then called anti-Stokes Raman scattering.

A molecular polarizability change, or amount of deformation of

the electron cloud, with respect to the vibrational coordinate

is required for the molecule to exhibit the Raman effect. The

amount of the polarizability change will determine the Raman

scattering intensity, whereas the Raman shift is equal to the

vibrational level that is involved.

Raman microspectroscopy

Raman spectroscopy offers several advantages for microscopic analysis. Since it is a scattering technique, specimens do not need to be fixed or sectioned. Raman spectra can be collected from a very small volume (< 1 µm in diameter); these spectra allow the identification of species

present in that volume. Water does not interfere very strongly. Raman spectroscopy is suitable for

the microscopic examination of minerals, materials such as

minerals, materials such as

polymers and ceramics, cells and proteins. A Raman microscope begins with a standard optical

microscope, and adds an excitation laser, a monochromator, and a

sensitive detector (such as a charge-coupled device (CCD), or photomultiplier tube (PMT)). FT-Raman has also been used with microscopes.

Characterization techniques