UNIT 5 CHARACTERIZATION OF NANO MATERIALS

Introduction to Nanometrology

Nanometrology is a subfield of metrology, concerned with the science of measurement at

thenanoscale level. Nanometrology has a crucial role in order to produce nanomaterials and

devices with a high degree of accuracy and reliability innanomanufacturing.

A challenge in this field is to develop or create new measurement techniques and standards to

meet the needs of next-generation advanced manufacturing, which will rely on nanometer

scale materials and technologies.

The needs for measurement and characterization of new sample structures and characteristics

far exceed the capabilities of current measurement science.

Control of the critical dimensions are the most important factors in anotechnology.

Nanometrology today, is to a large extent based on the development

insemiconductortechnology. Nanometrology is the science ofmeasurementat the nanoscale

level.

In Nanotechnology accurate control of dimensions of objects is important. Typical dimensions

of nanosystems vary from 10nm to a few hundred nm and while fabricating such systems

measurement up to 0.1nm is required.

In the last 70 years various techniques for measuring at nanoscale have been developed most

of them based on some physical phenomena observed on particle interactions or forces at

nanoscale. Some of the most commonly used techniques are

Atomic Force Microscopy, X-Ray Diffraction, Scanning Electron Microscopy,

Transmission Electron Microscopy, High Resolution Transmission Electron Microscopy, and

Field Emission Scanning Electron Microscopy.

The full list of nanomeasuring systems are:

(i) Atomic Force Microscopy

(ii) X-ray absorption Spectroscopy

(iii) X- Ray Diffraction

(iv) Small Angle X-Ray Scattering

(v) Scanning Tunneling Microscopy

(vi) Transmission Electron Microscopy

(vii) Scanning Tunneling Microscopy

(viii) Capacitance Spectroscopy

(ix) Polarization Spectroscopy

(x) Auger Electron Spectroscopy

(xi) Raman Spectroscopy

(xii) Small Angle Neutron Scattering

(xiii) Scanning Electron Microscopy

(xiv) Cyclic Voltammetry

(xv) Linear Sweep Voltammetry

(xvi) Nuclear Magnetic Resonance

(xvii) Mössbauer Spectroscopy

(xviii) Fouier Transform Infrared Spectroscopy

(xix) Photoluminescence Spectroscopy

(xx) Electroluminescence Spectroscopy

(xxi) Differential Scanning Calorimetry

(xxii) Secondary Ion Mass Spectrometry

(xxiii) Cathodoluminescence Spectroscopy

(xxiv) Electron Energy Loss Spectroscopy

(xxv) EnergyDispersiveX-RaySpectroscopy

(xxvi) Four point probe and I-V technique

(xxvii) X-Ray Photoelectron Spectroscopy

(xxviii) Scanning Near-field Optical Microscopy

(xxix) Single-molecule Spectroscopy

(xxx) Neutron Diffraction

Analytical Imaging

The fundamental of nanotechnology lies in the fact that properties of materials change

dramatically when their size is reduced to the nanometer range. But measuring this nano

dimension is not a very easy task.

Although research is going on to synthesise nanostructured and nanophasic materials,

characterizing these nano sized materials is also an emerging field posing lot of challenges

to scientists and technonologists.

Thus, nanotechnology has motivated the upsurge in research activities on the discovery and

invention of sophisticated nano characterization techniques to allow a better control of

morphology, size and dimensions of materials in nano range.

Nanomaterials to be thoroughly characterized as much as possible using a combination of

experimental techniques due to the intrinsic complex nature of nanomaterials. For example,

two of the most basic characteristics are size and shape of nanostructures.

Another important property is the surface of nanomaterials due to their extremely large

surface-to-volume ratio. Furthermore, it is often important to know the crystal structures of

the nanomaterials. To study these different properties requires many other experimental

methods, besides optical spectroscopy.

Nanotechnology was mostly a dream until the invention of the Scanning Tunneling

Microscope and the Atomic Force Microscope. In addition, all other existing facilities are

used to characterize nanomaterials. List of nanocharacterisation techniques

Image Analysis

Surface Analysis

Structural Analysis

Organic

Optical microscopy

Confocal microscopy

SEM/EDX – Scanning Electron Microscopy with Energy Dispersive

X-Ray detector

SPM – Scanning Probe Microscopy

AFM – Atomic Force Microscopy

TEM – Transmission Electron Microscopy

AES – Auger Electron Spectroscopy

XPS – X-Ray Photoelectron Spectroscopy

TOFSSIMS-Time of Flight Static Secondary Ion Mass Spectra LEED – Low Energy Electron

Diffraction

XRD – X-Ray Diffraction

XAX/EXAFS - X-ray Absorption Spectroscopy and Extended

X-Ray Absorption Fine Structure Raman spectroscopy

TEM – Transmission Electron Microscopy

EELS – Electron Energy Loss Spectroscopy (typically combined with TEM)

FTIR – Fourier Transform Infrared Spectroscopy

Analysis

Elemental Analysis

GC/MS – Gas Chromatography with Mass Spectroscopy (detector) HPLC – High Performance

Liquid Chromatography Raman

spectroscopy (structural organic)

ICP – Inductively Coupled Plasma

XRF – X-Ray Fluorescence

PIXE - Particle-Induced X-ray Emission

Optical atomic spectroscopy - CHN (Carbon / Hydrogen / Nitrogen)

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Microscopy

Structural determination is essential for nanomaterials research. Since the nanostructures are usually

too small to be visualized with conventional optical microscopes, it is important to use appropriate

tools to adequately characterize their structure and surface in detail at the molecular or atomic level.

This is important not only for understanding their fundamental properties but also for exploring

their functional and technical performance in technological applications.

There are several experimental techniques that can be used to characterize structural and surface

properties of nanomaterials either directly or indirectly, e.g. XRD, STM (scanning tunneling

microscopy), AFM (atomic force microscopy), SEM (scanning electron microscopy), TEM

(transmission electron microscopy), XAS (X-ray absorption spectroscopy), EXAFS, XANES (X-

ray absorption near edge structure), EDX (energy dispersive X-ray), XPS, IR (infrared), Raman,

and DLS (dynamic light scattering).

Some of these techniques are more surface sensitive than others. Some techniques are directly

element-specific while others are not. The choice of techniques depends strongly on the information

being sought about the material.

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Electron microscopy

Optical microscopes have limited spatial resolution, usually on the order of a few hundred nm in the

best case scenario due to the diffraction limit of light. Higher resolution, a few nm or even sub nm,

is needed for many applications, especially in the study of nanomaterials.

The trend toward nanoscience and nanotechnology is mainly motivated by the fact that the

underlying physical laws change from macroscopic to microscopic.

As we move to smaller and smaller length scales, new characterization techniques have to be

developed to probe the properties of novel nanostructures. There is a continuing demand for new

measurement methods that will be positioned to meet emerging measurement challenges.

Electon microscopes help to see nanocrystals and atomic arrangement in nanocrystals.

DIFFRACTION TECHNIQUE

Introduction

X-rays are electromagnetic radiation similar to light, but with a much shorter wavelength

(few Angstrom). They are produced when electrically charged particles of sufficient energy are

decelerated. In an X-ray tube, the high voltage maintained across the electrodes draws electrons

toward a metal target (the anode). X-rays are produced at the point of impact, and radiate in all

directions

Basic Principle

If an incident X-ray beam encounters a crystal lattice, general scattering occurs. Although most

scattering interferes with itself and is eliminated (destructive interference), diffraction occurs when

scattering in a certain direction is in phase with scattered rays from other atomic planes. Under this

condition the reflections combine to form new enhanced wave fronts that mutually reinforce each

other (constructive interference). The relation by which diffraction occurs is known as the Bragg’s

law or equation

nλ = d sinθ

where n is order of reflection, λ is wavelength of the x-ray, d is interplanar spscing and θ is Bragg’s

angle.

The method of X-ray diffraction is quantitative, in general, but it is also used for qualitative

analysis. This form of analysis extends to all crystalline solids including ceramics, metals,

insulators, organics, polymers, thin films, powders, etc.

X-ray diffractometers can be used either for single crystals or for powders, with both using

significantly different infrastructure.

While single crystal diffractometers are used for the study of molecular structure, powder

diffractometers are used for the analysis of phases, though the latter can also be used to

derive molecu lar information. X-rays corresponds to electromagnetic radiation in the

wavelength range of 1 ?. The wavelength range is below that of ultraviolet light and above

that of gamma rays.

This radiation is produced when charged particles are decelerated by metals, thus producing

a continuum called Bremsstrauhlung radiation.

X-rays are generally produced when electrons of several thousands of electron volts are

decelerated or stopped by metals. This will produce a white radiation up to a threshold

frequency corresponding to the kinetic energy of the particle. This threshold corresponds to

a wavelength (in angstroms), λ = 12399/V where V is the accelerating voltage of the

electrons.

When particles such as electrons fall on matter with high energy, electrons can be ejected

from various energy levels. Electron ejection from the core orbital is also accompanied by

the emission of characteristic X-rays. In the case of electron from the 1s orbital an outer

electron from the 2 p or 3p orbital can fall down to occupy the vacant 1s orbital. This 2 p →

1s transitions leads to the emission of Kα radiation.

A similar transition is possible from the 3 p level resulting in Kβ. The Kα is a doublet with

Kα1 and Kα2 corresponding to electronic transition from the two possible spin states of the

2p electron (2p3/2 and 2p1/2, respectively).

In most of the diffraction experiments Kα1 and Kα2 are not separated and the statistically

weighted average of the two wavelengths is taken. The wavelength of a given X-ray line

depends on the atomic numberMonochromatization by diffraction can also be done to

improve the optical purity of the radiation.

In a typical X-ray tube used to generate X-rays, high energy electrons are accelerated to a

target in an evacuated tube. Only a fraction of the incident electron energy is converted into

X- rays.

Most of it is converted to heat and efficient cooling of the anode is necessary to avoid it

from melting. Constructive interference is shown to occur in two directions, marked by

lines.

In the direction represented by the bottom arrow, the waves are in phase though each wave

is shifted by one wavelength from the other. Between these two directions, in all the other

directions interference occurs reducing the intensity.

In the case of several line sources, as would be present in the case of a grating, interference

occurs over several waves and no intensity can be seen between the directions shown

In the case of a grating, the condition of constructive interference is that the path length

between the beams should be an integral multiple of the wavelength.

This can be written as nλ = d sinθ , where d is the distance between the grooves and is θ the

angle of observation.

For the first order diffraction, λ = d sinθ . As the maximum value of sinθ = 1 and θ = 90°, the

first order diffraction will be observed at this angle.

Three kinds of radiations are generally used for diffraction: X-rays, electrons and neutrons.

Commonly, the characteristic X-ray used for diffraction is the copper Kα radiation at 1.5418

? wavelength.

Two approaches are generally used for the analysis of X-ray diffraction data.

These are the Laue equations and the Bragg’s law.In the Laue equations, diffraction from

a one-dimensional crystal may be treated in the same way as the diffraction by an optical

grating.

Upon projection, the grating is like an array of points similar to a crystal. The diffraction

condition is again, nλ = d sinθ. In a crystal arrangement of atoms is periodic in all the three

directions and three independent Laue equations can be written.

The three equations have to be satisfied simultaneously for diffraction to occur.

In Bragg’s law , a crystal is viewed as a plane containing several lattice points. The

reflection of X-rays will take place from these planes with the angle of reflection being

equal to the angle of incidence as shown in figure. The reflected beams are in phase when

the path length between the beams is an integral multiple of the wavelength.

The planes of light travelling after reflection will be in phase only when this condition is

satisfied.

This would mean that the distance, ABC = nλ or 2d sinθ = nλ (shown in the inset).

For all angles other than θ, destructive interference will occur leading to cancellation of the

intensity.

For crystals containing thousands of such planes, Bragg’s law imposes severe restrictions on

θ and the cancellation of intensities is usually complete. The X-ray diffraction experiment

requires the following: a radiation, a sample and a detector for the reflected radiation.

In each of these cases, there can be several variations. For example, the radiation can be of

many kinds, a single monochromatic source or of variable frequency.

The sample can be powder, single crystal, solid piece or a thin film. The detector can be of

several kinds, ranging from a simple photographic plate to a sophisticated counter or an area

detector. In a powder diffraction experiment, there are crystals arranged in all possible

orientations in a finely powdered sample.

The various lattice planes are also arranged in all possible orientations. For each crystal

plane, there will be a number of orientations. The reflected X-rays may be collected on a

photographic plate or by using a counter that is suitably connected to a recorder.

In the Debye-Scherrer method of diffraction, we use a monochromatic X-ray and a sample

with every possible set of lattice planes exposed to the radiation.

The diffracted radiation gives rise to a cone. The condition of diffraction is that the radiation

is at an angle θ to the incident beam. The cone arises because there are several angular

positions of the crystals. The cone is a result of several closely separated spots (inset).

In the case of a finely ground sample, the spots will be replaced by a continuous line. Each

(hkl) results in one core.

The detector is moved in a circle to collect all the reflections corresponding to various ( hkl).

In the modern diffraction method called diffractometry, a convergent beam strikes the

sample and the intensity as a function of diffraction angle is measured.

The position of the diffraction peak and the intensity at this point are the two factors used in

the determination.

Both these can be measured accurately and compared with standards in the literature. In fact,

this is what one normally does in the phase identification work.

SCANNING ELECTRON MICROSCOPY

The scanning electron microscope is an electron microscope that images the sample surface byscanning it with a high energy beam of electrons. Conventional light microscopes use a series ofglass lenses to bend light waves and create a magnified image while the scanning electronmicroscope creates the magnified images by using electrons instead of light waves.

Basic Principle

• When the beam of electrons strikes the surface of the specimen and interacts with the atoms of

the sample, signals in the form of secondary electrons, back scattered electrons and

characteristic X-rays are generated that contain information about the sample's surface

topography, composition, etc .

• The SEM can produce very high-resolution images of a sample surface, revealing details

about 1-5nm in size in its primary detection mode i.e. secondary electron imaging.

Characteristic X-rays are the second most common imaging mode for an SEM.

• These characteristic X-rays are used to identify the elemental composition of the sample by a

technique known as energy dispersive X-ray (EDX). Back-scattered electrons (BSE) that

comefrom the sample may also be used to form an image. BSE images are often used in

analytical SEM along with the spectra made from the characteristic X-rays as clues to the

elemental composition of the sample.

Electron Gun - The electron gun provides a stable beam of electrons. The most common form of an

electron gun is a thermionic emitter, wherein the work function of the metal is overcome by the

surface temperature of the filament.

Electron Lenses - This consists of a coil of wire generating a magnetic field enclosed in an iron

casing. A magnetic field is generated between the polepieces by applying a current through the

coils.

Scan Coils - The next part of the SEM consists of the scan coils. In SEM, the scanned image is

formed point by point and the scan is achieved by the scan coils. There are two pairs of coils, one

each for the X and Y axes. The scan coils lie within the column and move the electron beam as per

the requirement across the specimen. They are electromagnetic coils and are energized by the scan

generator.

Electron Detector - There are two kinds of electrons coming out from the sample in an SEM; the

backscattered electrons, with high energies being larger or lower than the primary beam energy, and

the secondary electrons which have energies of a few eV, or less than a few tens of eV. The most

common kind of detector used in SEM is the Everhart-Thornley (E-T) detector.

In a typical SEM, the beam passes through pairs of scanning coils or pairs of deflector plates

in the electron column to the final lens, which deflect the beam horizontally and vertically so that it

scans in a raster fashion over a rectangular area of the sample surface.

• Electronic devices are used to detect and amplify the signals and display them as an image

on a cathode ray tube in which the raster scanning is synchronized with that of the

microscope. The image displayed is therefore a distribution map of the intensity of the

signal being emitted from the scanned area of the specimen.

• SEM requires that the specimens should be conductive for the electron beam to scan the

surface and that the electrons have a path to ground for conventional imaging. Non-

conductive solid specimens are generally coated with a layer of conductive material by low

vacuum sputter coating or high vacuum evaporation. This is done to prevent the

accumulation of static electric charge on the specimen during electron irradiation. Non-

conducting specimens may also be imaged uncoated using specialized SEM instrumentation

such as the "Environmental SEM" (ESEM) or in field emission gun (FEG) SEM operated at

low voltage, high vacuum or at low vacuum, high voltage.

.

Applications

• The SEM shows very detailed three dimensional images at much high magnifications (up to

×300000) as compared to light microscope (up to × 10000). But as the images are created

without light waves, they are black and white.

• The surface structure of polymer nanocomposites, fracture surfaces, nanofibres, nanoparticles

and nanocoating can be imaged through SEM with great clarity. As very high resolution

images of the dimension 1-5nm can be obtained, SEM is the most suitable process to study

the surface of nanostrutures.

• Electrospun nanofibres are extensively studied in biomedical, environmental and other

technical textile applications for their huge surface area. Electrospun nylon 6 nanofibres

decorated with surface bound silver nanoparticles used for antibacterial air purifier can be

characterized using SEM (Figure 39.3-39.7).

• In tissue engineering or cell culture applications, the SEM image is the prime characterization

technique for scaffold construction, cell development and growth. SEM technique is used to

observe the plied CNT yarns in 3D braided structures.

• The SEM technique can also be used to view dispersion of nanoparticles such as carbon

nanotubes, nanoclays and nanofillers in the bulk and on the surface of nanocomposite

samples.

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TRANSMISSION ELECTRON MICROSCOPY

• Transmission electron microscopy is a microscopy technique whereby a beam of electrons is

transmitted through an ultra thin specimen and interacts as passes through the sample. An

image is formed from the electrons transmitted through the specimen, magnified and

focused by an objective lens and appears on an imaging screen.

• In TEM, the transmitted electrons are used to create an image of the sample. Scattering

occurs when the electron beam interacts with matter.

• Scattering can be elastic (no energy change) or inelastic (energy change). Elastic scattering

can be both coherent and incoherent (with and without phase relationship). Elastic scattering

occurring from well-ordered arrangements of atoms as in a crystal, results in coherent

scattering, giving spot patterns.

This can be in the form of rings in the case of a polycrystalline material. However, inelastic

scattering also occurs, which also gives regular patterns as in the case of Kikuchi patterns.

Inelastic processes give characteristic absorption or emission, specific to the compound or element

or chemical structure.

There are two main mechanisms of contrast in an image . The transmitted and scattered beams

can be recombined at the image plane, thus preserving their amplitudes and phases. This results in

the phase contrast image of the object.

An amplitude contrast image can be obtained by eliminating the diffracted beams. This is

achieved by placing suitable apertures below the back focal plane of the objective lens. This image

is called the bright field image.

One can also exclude all other beams except the particular diffracted beam of interest. The image

using this is called the dark field image.

➢ In bright field, the transmitted beam is used for imaging.

➢ In dark field, the diffracted beam is used for imaging.

The contrast in a TEM image is not like the contrast in a light microscope image. In TEM, the

crystalline sample interacts with the electron beam mostly by diffraction rather than by absorption.

• The intensity of the diffraction depends on the orientation of the planes of atoms in a crystal

relative to the electron beam; at certain angles the electron beam is diffracted strongly from

the axis of the incoming beam, while at other angles the beam is largely transmitted.

• Modern TEMs are equipped with specimen holders that allow to tilt the specimen to a range

of angles in order to obtain specific diffraction conditions. Therefore, a high contrast image

can be formed by blocking electrons deflected away from the optical axis of the microscope

by placing the aperture to allow only unscattered electrons through.

• This produces a variation in the electron intensity that reveals information on the crystal

structure. This technique, particularly sensitive to extended crystal lattice defects, is known

as ‘bright field’ or ‘light field’.

• It is also possible to produce an image from electrons deflected by a particular crystal plane

which is known as a dark field image.

• The specimens must be prepared as a thin foil so that the electron beam can penetrate.

Materials that have dimensions small enough to be electron transparent, such as powders or

nanotubes, can be quickly produced by the deposition of a dilute sample containing the

specimen onto support grids.

• As polymeric nanocomposites are not as hard as metals, they are cut into thin films (<

100nm) using ultra-microtome with diamond knife at cryogenic condition (in liquid

nitrogen).

Applications

1.The TEM is used widely both in material science/metallurgy and biological sciences. In both

cases the specimens must be very thin and able to withstand the high vacuum present inside

the instrument.

2.For biological specimens, the maximum specimen thickness is roughly 1micrometer. To

withstand the instrument vacuum, biological specimens are typically held at liquid nitrogen

temperatures after embedding in vitreous ice, or fixated using a negative staining material

such as uranyl acetate or by plastic embedding.

3.The properties of nanocomposites depend to a large extent on successful nanolevel dispersion

or intercalation/exfoliation of nanoclays, therefore monitoring their morphology and

dispersion is very crucial.

4.TEM images reveal the distribution and dispersion of nanoparticles in polymer matrices of

nanocomposite fibres, nanocoatings, etc. The extent of exfoliation, intercalation and

orientation of nanoparticles can also be visualized using the TEM micrograph.

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ATOMIC FORCE MICROSCOPE

Atomic force microscopy, provides a way to image conducting and non-conducting surfaces.

In AFM, the tip is attached to a flexible cantilever and is brought into contact with the

surface

ATOMIC FORCE MICROSCOPE- Two Modes

Repulsive (contact)

At short probe-sample distances, the forces are repulsive

Attractive Force (non-contact)

At large probe-sample distances, the forces are attractive

The AFM cantilever can be used to measure both attractive force mode and repulsive forces.

Non-Contact Type

1. Uses attractive forces to interact surface with tip

2. Operates within the van der Waal radii of the atoms

3. Oscillates cantilever near its resonant frequency (~ 200 kHz) to improve sensitivity

4. Advantages over contact: no lateral forces, non-destructive/no contamination to sample, etc.

Contact Type

1. Contact mode operates in the repulsive regime of the van der Waals curve

2. Tip attached to cantilever with low spring constant.

3. In ambient conditions there is also a capillary force exerted by the thin water layer present

(2-50 nm thick).

The force between the tip and the surface is detected by sensing the cantilever deflection. A

topographic image of the surface is obtained by plotting the deflection as a function of the X-Y

position. In a more common mode of operation, a feedback loop is used to maintain a constant

deflection while the topographic information is obtained from the vertical displacement of the

cantilever.

Some scanning probe systems use a combination of AFM and STM modes: the tip is mounted in a

cantilever with electrical connection so that both the surface force and the tunneling current are

controlled or monitored.

1. STM systems can be operated in ultrahigh vacuum (UHV STM) or in air, whereas AFM

systems are typically operated in air.

2. When a scanning probe system is operated in air, water adsorbed onto the sample surface

accumulates underneath the tip, forming a meniscus between the tip and the surface.

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Scanning tunneling microscopy

The principle of operation of Scanning Tunneling Microscope is based on marvelous quantum

phenomenon known as the tunnel effect.

Scanning tunneling microscopy (Figure 41. 1) is an instrument for producing surface images with atomic-scale lateral resolution, in which a fine probe tip is scanned over the surface of a conducting specimen, with the help of a piezoelectric crystal at a distance of 0.5–1nm, and the resulting tunneling current or the position of the tip required to maintain a constant tunneling current is monitored.

• The principle of STM is based on the concept of quantum tunneling. When a conducting tip

is brought very near to a metallic or semi-conducting surface, a bias between the two can

allow electrons to tunnel through the vacuum between them.

• For low voltages, this tunneling current is a function of the local density of states at the

Fermi level of the sample. Variations in current as the probe passes over the surface are

translated into an image. For STM, good resolution is considered to be 0.1 nm lateral

resolution and 0.01nm depth resolution.

• They normally generate images by holding the current between the tip of the electrode and

the specimen at some constant value by using a piezoelectric crystal to adjust the distance

between the tip and the specimen surface, while the tip is piezoelectrically scanned in a

raster pattern over the region of specimen surface being imaged by holding the force, rather

than the electric current, between tip and specimen at a set-point value.

• Atomic force microscopes similarly allow the exploration of nonconducting specimens. In

either case, when the height of the tip is plotted as a function of its lateral position over the

specimen, an image that looks very much like the surface topography results.

• The STM can be used not only in ultra high vacuum but also in air and various other liquid

or gas, at ambient and wide range of temperatures. STM can be a challenging technique, as

it requires extremely clean surfaces and sharp tips.

The formation of the STM images depends on the operating mode. The basics modes are

constant current mode and

constant height mode.

In Constant Current Mode (CCM) tunneling current is kept constant by feedback circuit during

the scanning process. This is achieved by moving the tip up and down accordingly to the surface

topography, so that the current is constant during the whole scanning process.

In this case, vertical displacement of the scanner (feedback signal) reflects surface topography (fig.

135). The scanning speed in constant current mode is lower than that in constant height mode, but

in CCM it is possible to investigate the samples with developed relief.

In Constant Height Mode (CHM) the tip movement is implemented only in X-Y plane and the

vertical (Z) position is kept constant, so the current changes according to the surface topography

(fig. 136). It allows to increase the speed of scanning, but only provided that the surface is very flat.

Otherwise the tip will be damaged.

Other important modes include Local Barrier High (LBH) imaging, Local Density of States

(LDOS), I(z) Spectroscopy and I(V) Spectroscopy (or Current Imaging Tunneling Spectroscopy).

Application field of STM is quite wide. In addition to mentioned above, the STM can be used to

study charge transport mechanisms in molecules or other small structures.

The other important application of STM is the investigations of crystal growth. Furthermore, STM

can be used as a tool for the surface modification. It can be done by indenting the surface with the

tip or by the electron emission. At low temperatures it is even possible to move single atoms with

high accuracy using STM tip.

Due to exponential dependence of the tunnel current on the distance, the resolution of STM in the

direction normal to the surface is very high and reaches fractions of Angstrom even at ambient

conditions. The lateral resolution strongly depends on the atomic geometry of the tip apex. The best

STM tips have a single atom or small cluster of atoms on their apex, so the tunnel current flows just

between these atoms on the apex and particular surface atoms. In this case, provided that tip is

clean, the atomic resolution can be achieved.

Despite an excellent resolution, STM technique has a serious disadvantage – the choice of material

for investigation with STM is limited by the requirement for them to be conducting.

Another important thing is the surface condition, primarily its cleanliness. Any insulator particle on

the investigated surface will cause the tunnel current to decrease and, as a result, will look on the

STM image the same as cavity.

For the same reason the surface areas with low conductivity (nonconductive impurities) also make

the inter- pretation of STM image more difficult. For high-resolution of metals and semi-

conductors, the STM is usually operated in ultrahigh vacuum to avoid contamination or oxidation of

the surface.

Raman Spectroscopy

Introduction

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, or Raman scattering of monochromatic laser light. The laser light interacts with phonons

or other excitations 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.

Basic Principle

The Raman effect occurs when light impinges upon a molecule, interacts with the electron cloud of

the bonds of that molecule and 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 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.

When light is scattered from a molecule or crystal, most photons are elastically scattered. The

scattered photons have the same energy (frequency) and, therefore, wavelength, as the incident

photons. However, a small fraction of light (approximately 1 in 107 photons) is scattered at opti- cal

frequencies different from, and usually lower than, the frequency of the incident photons. The

process leading to this inelastic scatter is termed the Raman effect. Raman scattering can occur with

a change in vibrational, rotational or electronic energy of a molecule. If the scattering is elastic, the

process is called Rayleigh scattering. If it’s not elastic, the process is called Raman scattering.

If the substance being studied is illuminated by monochromatic light, for example from a laser, the

spectrum of the scattered light consists of a strong line (the exciting line) of the same frequency as

the incident illumination together with weaker lines on either side shifted from the strong line by

frequencies ranging from a few to about 3500 cm-1. The lines of frequency less than the exciting

lines are called Stokes lines, the others anti-Stokes lines.

The Raman effect arises when a photon is incident on a molecule and interacts with the electric

dipole of the molecule. In quantum mechanics the scattering is described as an ex- citation to a

virtual state lower in energy than a real electronic transition with nearly coinci- dent de-excitation

and a change in vibrational energy. The scattering event occurs in 10-14 seconds or less. The virtual

state description of the scattering is shown in Figure 1.

The energy difference between the incident and scattered photons is represented by the arrows of

different lengths in Figure 1. Numerically, the Raman shift in wave numbers (cm-1), is calculated

through Eq. 1,

V = 1/ λincident -- 1/ λscattered

in which the λ’s are the wavelengths (in cm) of the incident and Raman scattered photons, respec-

tively.

At room temperature the thermal population of vibrational excited states is low, although not zero.

Therefore, the initial state is the ground state, and the scattered photon will have lower energy

(longer wavelength) than the exciting photon. This Stokes shifted scatter is what is usually ob-

served in Raman spectroscopy. Figure 1(a) depicts Raman Stokes scattering.

A small fraction of the molecules are in vibrationally excited states. Raman scattering from vibra-

tionally excited molecules leaves the molecule in the ground state. The scattered photon appears at

higher energy, as shown in Figure 1(b). This anti-Stokes-shifted Raman spectrum is always weaker

than the Stokes-shifted spectrum, but at room temperature it is strong enough to be useful for

vibrational frequencies less than about 1500 cm-1. The Stokes and anti-Stokes spectra contain the

same frequency information. The anti-Stokes spectrum can be used when the Stokes spectrum is not

directly observable, for example, because of poor detector response at lower frequencies.

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3-D Surface Analysis

Surface metrology methods can be divided into “contact” and “noncontact”.

Noncontact (Optical) Methods

Microscopically, there is little ambiguity about the notion of contact but in the nanoscale one

is aware that the Born repulsion prevents atoms from moving arbitrarily close to each other.

In the atomic force microscope (AFM), nanoscale stylus scans over a sample.

The repulsion between the atoms of the AFM tip and sample asperity is actually action at a

distance, hence the method could equally well be classified as noncontact.

Noncontact (Optical) Methods

Optical profilers are analogous to the mechanical stylus instruments but use focused beams to

detect the location of the surface.

They are therefore unlikely to have the resolution required to characterize nanotexture.