CH250 Intermediate Analysis – Part 3Materials & Nanotechnology

Dr Raymond WhitbyC407

Overview

1. Defining nano

2. Formation of nanocarbon

3. Viewing the nanoscale; direct analysis

4. Indirect analysis of the nanoscale

5. Adsorption experiment

3. Viewing the nanoscale

Various interactions of electrons with a sold target

© P.J. Grundy, Electron Microscopy in the Study of Materials, 1976

DATE NAME EVENT1897 J. J. Thompson Discovers the electron

1924 Louis deBroglie Identifies a wavelength to moving electrons λ=h/mv where λ = wavelength h = Planck's constant (6.626 x 10-34 J s)m = mass v = velocity (For an electron at 60kV λ = 0.005 nm)

1926 H. Busch Magnetic or electric fields act as lenses for electrons

1929 E. Ruska Ph.D thesis on magnetic lenses 1931 Knoll & Ruska First electron microscope built 1931 Davisson & Calbrick Properties of electrostatic lenses

1934 Driest & Muller Surpass resolution of the LM 1938 von Borries & Ruska First practical EM (Siemens) - 10 nm resolution

1940 RCA Commercial EM with 2.4 nm resolution 1945   1.0 nm resolution

p = momentum

Viewing the nanoscale

Abberation

Spherical aberration occurs when electrons travelling further from the optic axis, which are more likely to be deviated, are focussed at different points along the optic axis. Each point in the object is therefore imaged a disc of radius, Δrs

3ss Cr

Spherical aberration constant usually Cs 1-2mm

(1.1)

Abberation

Chromatic aberration arises because of a spread in energy acting on the electrons, leading to a small divergence of the electron beam, Δrc

E

ECr cc

Astigmatism occurs when electrons go through different focal lengths, depending on the plane of the ray paths, ΔrA

aa fr

Resolution

4/1

sC

Giving a maximum resolution:4/14/3

min sCr

(1.3)

(1.4)

Increasing aperture size increases resolution according to (1.2) but decreases according to (1.1), an optimum aperture size minimising their sum:

© J. W. Edington, Practical Electron Microscopy in Materials Science, Macmillan Press, 1974

The Rayleigh criterion specifies the minimum separation between two light sources that may be resolved into distinct objects

Point to point resolution limit:

Δrd = 0.61λ / β (1.2)

Interference between scattered wave and undeviated wave causes amplitude variation

Electron waves

λ = h / m.v

de Broglie

An electron’s momentum is usually defined by falling through a potential difference:

eV = ½ m.v2

e is the electronic charge in volts (1 electron volt = 1.602 x 10-19 J), therefore:

λ = h / (2.m.eV)½

When V is large electrons approach speed of light, therefore electron mass must be accounted for through the relativistic accelerating voltage Vr, but usually when V ≥ 105 volts:

Vr = V.[1 + eV / 2.m0.c2]

So, when accelerating voltage is 100 kV, the wavelength is 0.0037nm and the optimum resolving power is 0.23nm, less than lattice spacing of graphite!

Resolution limits:

Light Microscope = 500nm(air/water)

Scanning ElectronMicroscope = 2nm

Transmission Electron Microscope = 0.1 nm(both vacuum/dry)

Viewing the nanoscale

© J. W. Edington, Practical Electron Microscopy in Materials Science, Macmillan Press, 1974

EM imagingContrast effects are generated by small phase variations among the electrons passing through the sample. By combining all or most of the beams transmitted and scattered by the sample in the formation of the image, a phase contrast image is produced.

A model of a nanotube interacting with an electron beam is shown above. Part of the electron beam scatters at the top and bottom parts of the tube (point H). The contrast of this scatter is relatively weak, due to a low number of atoms in this (hk0) plane aligned to the electron beam, but structural detail here is often observed on precisely tuned TEMs. Part of the electron beam also scatters at the sides of the tube (point V). The contrast is relatively stronger as there are a greater number of atoms in the (00l) plane aligned to the electron beam. This gives rise to the appearance of the nanotube “walls”.

When the electron beam passes through a sample, a diffraction pattern is formed at the back focal plane of the objective lens, which contains all of the information present in the specimen.

The diffracted beams forming this pattern can be recombined with the transmitted electrons in the image plane to form a lattice image. By collecting the transmitted electrons only, achieved by placing an aperture to obstruct the diffracted electrons, a bright field image is obtained.

Conversely, by only collecting the diffracted beams and obstructing the transmitted beams, a dark field image can be obtained.

EM imaging

TEM gallery

© M. Monthioux, et al., Carbon, 39, 1261 (2001) & Carbon (2003)

Carbon nanotube growth

© M. Terrones, et al., Top. Curr. Chem., 199, 189-234 (1999)

Electron diffraction (TEM cont.) The crystal lattice of a sample causes diffraction of the electron beam. When the crystal planes are aligned with an hkl plane at an angle θ to the beam axis, commonly scattered beams interfere constructively. Since the Bragg angle for high-velocity electron beams is very small, sin θ approximates to θ, and for n = 1 the Bragg equation (2dhkl sin θ = nλ) is simplified:

2hkld

2Lr

In the electron microscope, the film is placed at a distance L (the camera length) from the sample. The distance r from the centre spot O to the diffracted spot H is expressed:

sin2 hkldn Bragg Equation

Standard x-ray diffraction

Electron diffraction (TEM cont.)

By combining these equations, it is possible to determine an unknown spacing dhkl, when r is measured from the diffraction pattern and Lλ is known. The relationship between the distance of the diffracted spot, from the centre spot, r and the interplanar spacing dhkl of the sample from which it originated is:

hkld

Lr

© M. Monthioux, et al., Carbon, 39, 1261 (2001) & Carbon (2003)

Single-walled carbon nanotube with a rolling vector of 12.5 ±0.5o

When the electron beam strikes the sample, an incident electron may collide with an electron in the K shell and ejects it, resulting in ionisation of the atom. Another electron, from say a higher shell, can fall into this vacancy and emits its excess energy as an X-ray photon. Such a transition from the L to K shell results in a Kα line and from the M to K shell gives a Kβ line, etc. These X-rays are detected by energy dispersive techniques.

Energy Dispersive X-ray (TEM cont.)

Electron Energy Loss Spectroscopy (TEM cont.)EELS is concerned with the study of the electron excitation processes, each of which results in a fast electron losing a characteristic amount of energy. After interacting with the sample, the transmitted electrons are directed into a high-resolution electron spectrometer which separates the electrons according to their kinetic energy and produces an EELS spectrum showing the scattered intensity as a function of the decrease in kinetic energy of the fast electron:

Inelastic scattering from outer-shell electrons is visible as a peak in the 5-50 eV region of the spectrum. Inner-shell excitations appear in the form of edges, which represent the ionisation threshold. Since inner-shell binding energies depend on the atomic number of the scattering atom, the ionisation edges present in an EELS spectra indicate which elements occur within the sample. By monitoring particular energy losses within a selected range, it is possible to produce energy-filtered images that reveal the elemental mappings of desired elements.

Electron Energy Loss Spectroscopy (TEM cont.)

The K-shell electrons in the carbon atoms, of MWCNTs, interact with the high-energy electrons to be excited into the vacant π* and σ* levels. Through this process, the transmitted electrons lose characteristic energy of ca. 285 keV, similar to that of graphite:

© K. Yase, et al., Thin Solid Films, 273, 222 (1996).

EELS & TEM

Scanning Electron Microscopy

© P.J. Grundy, Electron Microscopy in the Study of Materials, 1976

SEM involves the production of back-scattering and emission of electrons from the near surface of a sample. Larger samples provide longer path lengths increasing the probability of inelastic collisions as well as scattering. Incident electrons are dispersed into a ‘pear’ shaped volume; if secondary electrons that are created possess sufficient energy to escape to the surface, then they will be sampled. Overall, the interaction volume will be much larger than the sampled volume, therefore, SEM is better suited for larger scale materials.

Scanning Electron Microscopy

© N. Liu et al. Diamond & Related Materials 18 (2009) 1375–1380

Patterned carbon nanotube (CNT) arrays on Si substrate fabricated using a two layer Sn/Ni catalyst in a diffusion ethanol flame.

SEM of buckycolumns

Frit

Membrane disk

Frit

Carbon nanotube

suspension

Force

Force

R.L.D. Whitby, T. Fukuda, T. Maekawa, accepted Nanotechnology, 2010

SEM of buckycolumns

Self-contained variable filtration

Petrochemical & biochemical screening

Topographical Microscopy

Scanning Tunnelling Microscopy (STM) is a process of mapping a surface with an ultrafine point in a sample contact situation, giving a resolution down to individual atoms. The tip has a bias voltage applied and passed over a conducting surface. The tunnelling current is subsequently measured yielding sample topology.

Atomic Force Microscopy (AFM) is a process of passing a tip over a surface in a non-contact situation, allowing measurement of the tip-surface forces. In addition to topology, the friction and adhesion properties can also be measured.

200nm

3nm

© Google images & IBM

Iron atoms on copper surface Silica spheres

STM

The ultrafine tip is manipulated through a piezoelectric tube. The low tunnelling currents (ca. 10-

10A) are amplified and measured as a function of piezoelectric position of the tip.

The tunnelling current measured is a function of the gap (W) between the tip and substrate:

I(W) = Ce(-W√f)

C is a constant, f is the sample’s work function. Work function is equal to the energy barrier of electron transfer and can be controlled by changing the potential on the tip or substrate.

© www.bath.ac.uk, CH20016 Lecture 10 notes

STM

Two operation modes:

Maintain a constant tip to surface height, but a variable tunnelling current

Maintain a constant current, but varying the tip to surface height

© Jeroen Wildoër, Delft University of Technology

STM image of a chiral, single walled carbon nanotube

STM requires ultrahigh vacuums and conductive samples only!

AFM

A sharp tip on a cantilever is drawn across the surface of a sample. The changes in forces between the tip and the sample are measured as a function of the position of the tip to the surface.

The tip position is measured by imagining the reflection of a laser from the rear side of the cantilever. A 4 quadrant photodiode is used to record movement in the x, y, z planes. Similar to STM, the tip forces or tip height can be kept constant depending on the mode of operation.

© xintek.com & Google images

AFMThe forces that are exerted on the tip will vary according to:

• the substrate under examination,• the environment around the tip (e.g. water, air or vacuum)• the chemical functionality of the surface (e.g. hydrophilic / hydrophobic)• softness / hardness of the sample’s surface• Tip raster velocity

“In contact mode, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection. However, close to the surface of the sample, attractive forces can be strong, causing the tip to 'snap-in' to the surface. Thus static mode AFM is almost always done in contact where the overall force is repulsive.

Tapping mode occurs when the tip is oscillated near its resonance frequency by a small piezoelectric element mounted in the AFM tip holder. Due to the interaction of forces acting on the cantilever when the tip comes close to the surface, Van der Waals force or dipole-dipole interaction, electrostatic forces, etc cause the amplitude of this oscillation to decrease as the tip gets closer to the sample. A tapping AFM image is therefore produced by imaging the force of the oscillating contacts of the tip with the sample surface.”

© Wikipedia 2010

J-P. Salvetat, et al., Phys. Rev. Lett., 82, 944 (1999)

Individual arc-MWCNT, Young’s modulus = 270 to 950 GpaIndividual cvd-MWCNT = 15- 50 Gpa

Carbon nanotube strength + AFM

“The superposition principle implies that the total deflection, σ, is the sum of the deflections due to bending, σB, and to shear, σS. If we use the unit-load method for a concentrated load F, the deflection at the middle becomes σ = σB + σS = F L3/192.E.I + fs.F.L /4.G.Awhere L is the suspended length, E is the elastic modulus, fs is the shape factor (equal to 10/9 for a cylindrical beam), G is the shear modulus, I is the second moment of area of the beam (I = πD4/64 for a filled cylinder), and A is the cross-sectional area. The ratio σB / σS increases with the ratio of beam length to diameter.”

AFM revolution

© http://i.livescience.com/images/090827-pentacene-image-02.jpg© http://www.newscientist.com/article/dn17699-microscopes-zoom-in-on-molecules-at-last.html

“The molecule is very fragile, but the researchers were able to capture the details of the hexagonal carbon rings and deduce the positions of the surrounding hydrogen atoms. One key breakthrough was finding a way to stop the microscope's tip from sticking to the fragile pentacene molecule because of attraction due to electrostatic and van der Waals forces... The team achieved this by fixing a single carbon monoxide molecule to the end of the probe so that only one atom of relatively inactive oxygen came into contact with the pentacene.”

Questions on microscopy

1. Why does electron microscopy have a better resolution than light microscopy?

2. Briefly describe the three aberrations that exist within electron microscopy.

3. What is the difference between bright field and dark field imaging?

4. What are the differences between transmission electron microscopy and scanning electron microscopy?

5. What are the differences between atomic force microscopy and scanning tunnelling microscopy?

All material under copyright was scanned under a CLA licence