collection of nt by sravan

7/31/2019 Collection of Nt by Sravan

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UNIT-V

1.AFM

HOW DOES THE AFM WORK?

AFM provides a 3D profile of the surface on a nanoscale, by measuring forces between a sharp

probe (


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mode the deflection of the cantilever is fixed and the motion of the scanner in z-direction is

recorded. By using contact-mode AFM, even atomic resolution images are

obtained. For contact mode AFM imaging, it is necessary to have a cantilever which is soft

enough to be deflected by very small forces and has a high enough resonant frequency to not be

susceptible to vibrational instabilities. Silicon Nitride tips are used for contact mode. In these

tips, there are 4 cantilever with different geometries attached to each substrate, resulting in 4

different spring constants (Figure 6).

Figure 6. Probe with four different cantilevers with different spring constants (N/m .

To avoid problems caused by capillary forces which are

generated by a liquid contamination layer usually present on surfaces in air, the sample can be

studied while immersed in a liquid. This procedure is especially beneficial for biological

samples.

2.Non Contact Mode

In this mode, the probe operates in the attractive force region and the tip-sample interaction is

minimized. The use of non-contact mode allowed scanning without influencing the shape of the

sample by the tip-sample forces. In most cases, the cantilever of choice for this mode is the one

having high spring constant of 20- 100 N/m so that it does not stick to the sample surface at

small amplitudes. The tips mainly used for this mode are silicon probes.

Tapping Mode (intermittent contact Mode)

The force measured by AFM can be classified into long-range forces and shortrange forces. The

first class dominates when we scan at large distances from the surface and they can be Van der

Waals force, capillary forces (due to the water layer often present in an ambient environment).

When the scanning is in contact with the surface the short range forces are very important, in

particular the quantum mechanical forces (Pauli Exclusion Principle forces). In tapping mode-

AFM the cantilever is oscillating close to its resonance frequency. An electronic feedback loop

ensures that the oscillation amplitude remains constant, such that a constant tip-sample

interaction is maintained during scanning. Forces that act between the sample and the tip will not


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only cause a change in the oscillation amplitude, but also change in the resonant frequency and

phase of the cantilever. The amplitude is used for the feedback and the vertical adjustments of

the piezoscanner are recorded as a height image. Simultaneously, the phase changes are

presented in the phase image (topography). The advantages of the tapping mode are the

elimination of a large part of permanent shearing forces and the causing of less damage to the

sample surface, even with stiffer probes. Different components of the sample which exhibit

difference adhesive and mechanical properties will show a phase contrast and therefore even

allow a compositional analysis. For a good phase contrast, larger tip forces are of advantage,

while minimization of this force reduces the contact area and facilitates high-resolution imaging.

So in applications it is necessary to choose the right values matching the objectives. Silicon

probes are used primarily for Tapping Mode applications. Table 1 is a summary of the main

characteristics of the three modes explained before. In these modes we can work in different

environments: air, liquid and vacuum. In contact mode the tip touches the sample surface, which

leads to a high force and allows manipulation of the sample. The disadvantage is that the AFM

tip may be contaminated by the sample. The opposite happens in the noncontact mode, where the

tip stays at a distance above the sample. In tapping mode the tip touches the surface periodically

therefore manipulation of the sample, as well as contamination of the tip is possible.

Operation mode Contact mode Non-contact

mode

Tapping

mode

tip loading force low high low low

contact with sample

surface

yes no periodical

manipulation of

sample

yes no yes

contamination of AFM

tip

yes no yes

Table 1. Properties of the different operation modes in AFM.

4.4. Advantages and Disadvantages of AFM Modes


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Contact Mode AFM

Advantages:

- High scan speeds.

- Atomic resolution is possible.

- Easier scanning of rough samples with extreme changes in vertical

topography.

Disadvantages:

- Lateral forces can distort the image.

- Capillary forces from a fluid layer can cause large forces normal to the tipsample interaction.

- Combination of these forces reduces spatial resolution and can cause damage to soft samples.

Non-contact Mode AFM

Advantage:

- Low force is exerted on the sample surface and no damage is caused to soft samples

Disadvantages:

- Lower lateral resolution, limited by tip-sample separation.

- Slower scan speed to avoid contact with fluid layer.

- Usually only applicable in extremely hydrophobic samples with a minimal fluid layer.

Tappping Mode AFM

Advantages:

- Higher lateral resolution (1 nm to 5 nm).

- Lower forces and less damage to soft samples in air.

- Almost no lateral forces.

Disadvantage:

- Slower scan speed than in contact mode.

LIMITATIONS OF AFM

The AFM can be used to study a wide variety of samples (i.e. plastic, metals, glasses,

semiconductors, and biological samples such as the walls of cells and bacteria). Unlike STM or

scanning electron microscopy it does not require a conductive sample. However there are

limitations in achieving atomic resolution. The physical probe used in AFM imaging is not

ideally sharp. As a consequence, an AFM image does not reflect the true sample topography, but


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rather represents the interaction of the probe with the sample surface. This is called tip

convolution.

Scanning Electron Microscopy (SEM)

Susan Swapp, University of Wyoming

What is Scanning Electron Microscopy (SEM)

A typical SEM instrument, showing the electron column, sample chamber, EDS detector, electronics console, and visual display monitors.

The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a

variety of signals at the surface of solid specimens. The signals that derive from electron-sample

interactions reveal information about the sample including external morphology (texture), chemical

composition, and crystalline structure and orientation of materials making up the sample. In most

applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional

image is generated that displays spatial variations in these properties. Areas ranging from

approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM

techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100

nm). The SEM is also capable of performing analyses of selected point locations on the sample; this

approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions

(using EDS), crystalline structure, and crystal orientations (usingEBSD). The design and function of

the SEM is very similar to the EPMAand considerable overlap in capabilities exists between the two

instruments.

Fundamental Principles of Scanning Electron Microscopy (SEM)

Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is

dissipated as a variety of signals produced byelectron-sample interactionswhen the incident electrons

are decelerated in the solid sample. These signals include secondary electrons (that produce SEM

images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD that are used to

determine crystal structures and orientations of minerals), photons (characteristic X-rays that are

used for elemental analysis and continuum X-rays), visible light (cathodoluminescence--CL), and heat.
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Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary

electrons are most valuable for showing morphology and topography on samples and backscattered

electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e. for

rapid phase discrimination). X-ray generation is produced by inelastic collisions of the incident

electrons with electrons in discrete ortitals (shells) of atoms in the sample. As the excited electrons

return to lower energy states, they yield X-rays that are of a fixed wavelength (that is related to the

difference in energy levels of electrons in different shells for a given element). Thus, characteristic X-

rays are produced for each element in a mineral that is "excited" by the electron beam. SEM analysis

is considered to be "non-destructive"; that is, x-rays generated by electron interactions do not lead to

volume loss of the sample, so it is possible to analyze the same materials repeatedly.

Scanning Electron Microscopy (SEM) Instrumentation - How Does It Work?

Essential components of all SEMs include the following:

Electron Source ("Gun")

Electron Lenses

Sample Stage

Detectors for all signals of interestDisplay / Data output devices

Infrastructure Requirements:

o Power Supply

o Vacuum System

o Cooling system

o Vibration-free floor
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o Room free of ambient magnetic and electric fields

SEMs always have at least one detector (usually a secondary electron detector), and most

have additional detectors. The specific capabilities of a particular instrument are critically

dependent on which detectors it accommodates.

Applications

The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and

to show spatial variations in chemical compositions: 1) acquiring elemental maps or spot

chemical analyses using EDS, 2)discrimination of phases based on mean atomic number

(commonly related to relative density) using BSE, and 3) compositional maps based on

differences in trace element "activitors" (typically transition metal and Rare Earth elements)

using CL. The SEM is also widely used to identify phases based on qualitative chemical

analysis and/or crystalline structure. Precise measurement of very small features and objectsdown to 50 nm in size is also accomplished using the SEM. Backescattered electron images

(BSE) can be used for rapid discrimination of phases in multiphase samples. SEMs equipped

with diffracted backscattered electron detectors (EBSD) can be used to examine microfabric

and crystallographic orientation in many materials.

Strengths and Limitations of Scanning Electron Microscopy (SEM)?

Strengths

There is arguably no other instrument with the breadth of applications in the study of solid

materials that compares with the SEM. The SEM is critical in all fields that require

characterization of solid materials. While this contribution is most concerned with geological

applications, it is important to note that these applications are a very small subset of the

scientific and industrial applications that exist for this instrumentation. Most SEM's are

comparatively easy to operate, with user-friendly "intuitive" interfaces. Many applications

require minimal sample preparation. For many applications, data acquisition is rapid (less than
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5 minutes/image for SEI, BSE, spot EDS analyses.) Modern SEMs generate data in digital

formats, which are highly portable.

Limitations

Samples must be solid and they must fit into the microscope chamber. Maximum size in

horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much

more limited and rarely exceed 40 mm. For most instruments samples must be stable in a

vacuum on the order of 10-5 - 10-6 torr. Samples likely to outgas at low pressures (rocks

saturated with hydrocarbons, "wet" samples such as coal, organic materials or swelling clays,

and samples likely to decrepitate at low pressure) are unsuitable for examination in

conventional SEM's. However, "low vacuum" and "environmental" SEMs also exist, and many

of these types of samples can be successfully examined in these specialized instruments.EDS

detectorson SEM's cannot detect very light elements (H, He, and Li), and many instruments

cannot detect elements with atomic numbers less than 11 (Na). Most SEMs use a solid state x-

ray detector (EDS), and while these detectors are very fast and easy to utilize, they have

relatively poor energy resolution and sensitivity to elements present in low abundances when

compared to wavelength dispersive x-ray detectors (WDS) on most electron probe

microanalyzers (EPMA). An electrically conductive coating must be applied to electrically

insulating samples for study in conventional SEM's, unless the instrument is capable of

operation in a low vacuum mode.

X-ray Powder Diffraction (XRD)
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,

What is X-ray Powder Diffraction (XRD)

X-ray powder diffraction (XRD) is a rapid analytical technique primarily used for phase identification of

a crystalline material and can provide information on unit cell dimensions. The analyzed material is

finely ground, homogenized, and average bulk composition is determined.

Fundamental Principles of X-ray Powder Diffraction (XRD)

Constructive interference

occurs only when

n l = AB + BC

AB=BC

n l = 2AB

Sinq=AB/d


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AB=dsinq

n l =2dsinq

l = 2dhklsinqhkl

Max von Laue, in 1912, discovered that crystalline substances act as three-dimensional diffraction

gratings for X-ray wavelengths similar to the spacing of planes in a crystal lattice. X-ray diffraction is

now a common technique for the study of crystal structures and atomic spacing.

X-ray diffraction is based on constructive interference of monochromatic X-rays and a crystalline

sample. These X-rays are generated by a cathode ray tube, filtered to produce monochromatic

radiation, collimated to concentrate, and directed toward the sample. The interaction of the incident

rays with the sample produces constructive interference (and a diffracted ray) when conditions satisfy

Bragg's Law (n=2d sin ). This law relates the wavelength of electromagnetic radiation to the

diffraction angle and the lattice spacing in a crystalline sample. These diffracted X-rays are then

detected, processed and counted. By scanning the sample through a range of 2angles, all possible

diffraction directions of the lattice should be attained due to the random orientation of the powdered

material. Conversion of the diffraction peaks to d-spacings allows identification of the mineral because

each mineral has a set of unique d-spacings. Typically, this is achieved by comparison of d-spacings

with standard reference patterns.

All diffraction methods are based on generation of X-rays in an X-ray tube. These X-rays are directed

at the sample, and the diffracted rays are collected. A key component of all diffraction is the angle

between the incident and diffracted rays. Powder and single crystal diffraction vary in instrumentation

beyond this.

X-ray Powder Diffraction (XRD) Instrumentation - How Does It Work?

X-ray diffractometers consist of three basic elements: an X-ray tube, a sample holder, and an X-ray

detector. X-rays are generated in a cathode ray tube by heating a filament to produce electrons,

accelerating the electrons toward a target by applying a voltage, and bombarding the target material

with electrons. When electrons have sufficient energy to dislodge inner shell electrons of the target

material, characteristic X-ray spectra are produced. These spectra consist of several components, the

most common being K and K. K consists, in part, of K1 and K2. K1 has a slightly shorter

wavelength and twice the intensity as K2. The specific wavelengths are characteristic of the target

material (Cu, Fe, Mo, Cr). Filtering, by foils or crystal monochrometers, is required to produce

monochromatic X-rays needed for diffraction. K1and K2 are sufficiently close in wavelength such that

a weighted average of the two is used. Copper is the most common target material for single-crystal

diffraction, with CuK radiation = 1.5418. These X-rays are collimated and directed onto the sample.

As the sample and detector are rotated, the intensity of the reflected X-rays is recorded. When the

geometry of the incident X-rays impinging the sample satisfies the Bragg Equation, constructive

interference occurs and a peak in intensity occurs. A detector records and processes this X-ray signal
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and converts the signal to a count rate which is then output to a device such as a printer or computer

monitor.

X-ray powder diffractogram. Peak positions occur where the X-ray beam has been diffracted by the

crystal lattice. The unique set of d-spacings derived from this patter can be used to 'fingerprint' the

mineral. Details

The geometry of an X-ray diffractometer is such that the sample rotates in the path of the collimated

X-ray beam at an angle while the X-ray detector is mounted on an arm to collect the diffracted X-

rays and rotates at an angle of 2. The instrument used to maintain the angle and rotate the sample is

termed a goniometer. For typical powder patterns, data is collected at 2 from ~5 to 70, angles that

are preset in the X-ray scan.

ApplicationsX-ray powder diffraction is most widely used for the identification of unknown crystalline materials

(e.g. minerals, inorganic compounds). Determination of unknown solids is critical to studies in

geology, environmental science, material science, engineering and biology.

Other applications include:

characterization of crystalline materials

identification of fine-grained minerals such as clays and mixed layer clays that are difficult

to determine optically

determination of unit cell dimensions

measurement of sample purity

With specialized techniques, XRD can be used to:

determine crystal structures using Rietveld refinement

determine of modal amounts of minerals (quantitative analysis)

characterize thin films samples by:
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o determining lattice mismatch between film and substrate and to

inferring stress and strain

o determining dislocation density and quality of the film by rocking curve

measurements

o measuring superlattices in multilayered epitaxial structures

o determining the thickness, roughness and density of the film using

glancing incidence X-ray reflectivity measurements

make textural measurements, such as the orientation of grains, in a

polycrystalline sample

Strengths and Limitations of X-ray Powder Diffraction (XRD)?

Strengths

Powerful and rapid (< 20 min) technique for identification of an unknown

mineral

In most cases, it provides an unambiguous mineral determination

Minimal sample preparation is required

XRD units are widely available

Data interpretation is relatively straight forward

Limitations

Homogeneous and single phase material is best for identification

of an unknown

Must have access to a standard reference file of inorganic

compounds (d-spacings, hkls)

Requires tenths of a gram of material which must be ground into

a powder

For mixed materials, detection limit is ~ 2% of sample

For unit cell determinations, indexing of patterns for non-

isometric crystal systems is complicated

Peak overlay may occur and worsens for high angle 'reflections'

TEM Transmission Electron Microscope

In Transmission Electron Microscope (TEM), a thin specimen is irradiated with an

electron


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beam of uniform current density : the electron energy is in the range of 60 -150 KeV

(usually, 100 keV),

or 200 KeV-1 MeV in case of the high voltage electron microscope (HVEM) or high

resolution

transmission electron microscope (HRTEM).

The electrons are emitted in the electron gun by the 'thermionic emission' from tungsten cathodes

or LaB6 rods or by the field emission from the pointed tungsten filaments. The latter are used

when high gun brightness is needed. A two-stage condenser-lens system permits the variation of


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the illuminated aperture, and the area of the specimen is imaged with a three- or four-stage lens

system onto a fluorescent screen. The image can be recorded in emulsion inside the vacuum.

The lens aberrations of the objective lens are so great that it is necessary to work with very small

objective apertures, of the order of 10-25 mrad, to achieve a resolution of the order of 0.2 nm -

0.5 nm. The bright-field contrast is produced either by the adsorption of the electrons scattered

through theangles, which are larger than the objective aperture (i.e. scattering contrast), or by

the interferencebetween the scattered wave and the incident wave at the image point (i.e. phase

contrast). The phase of the electron waves behind the specimen is modified by the wave

aberration of the objective lens. Thisaberration, and the energy spread of the electron gun, which

is of the order of 1-2 eV, limits the contrast transfer (i.e. Fourier transform) of high spatial

frequencies.The electrons interact strongly with the atoms by elastic and inelastic scattering. The

specimen must therefore be very thin, typically of the order of 5 nm - 0.5 m for 100 KeV

electrons, depending on the density and the elemental composition of the object, and the

resolution desired. The specialpreparation techniques are needed for this purpose.The TEM can

provide high resolution, because the elastic scattering is an interaction process that is highly

localized to the region occupied by the screened Coulomb potential of an atomic nucleus,

whereas the inelastic scattering is more diffuse. It spreads out over about a nanometer.A further

capability of the modern TEM is the formation of very small electron probes, 2 nm - 5 nm in

diameter, by means of a three-stage condenser-lens system, the last lens field of which is the

objective pre-field in front of the specimen. This enables the instrument to operate in a scanning

transmission mode with a resolution determined by the electron probe-diameter. This has the

advantage for imaging thick or crystalline specimens, and for recording secondary electrons and

back-scattered electrons, cathode-luminescence and electron-beam-induced currents. The main

advantage of equipping a TEM with a STEM attachment is the formation of a very small electron

probe, with which the elemental analysis and micro-diffraction can be performed on extremely

small areas. The X-ray production in thin foils is confined to small volumes excited by the

electron probe, which is only slightly broadened by the multiple scattering. Therefore, a better

spatialresolution is obtainable for the segregation effects at crystal interfaces or precipitates,

for example, than in an X-ray micro-analyser with the bulk specimens, where the spatial

resolution is limited to 0.1- 1 mm by the diameter of the electron-diffusion cloud.


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2.6.4. Sample Preparation for TEM Study

For TEM study, the cylindrical specimen of 3 mm diameter and 1 mm high, which is a suitable

size for the fabrication of TEM specimens, is cut directly from the bulk sintered pellets of alpha

silicon carbide. The specimens are prepared from these samples by mechanical thinning to 75

m, which is followed by dimpling and subsequent low-energy (5 to 6 kV) and low angle (15)

Ar+ ion beam milling. The films are then examined in a transmission electron microscope

(Model - TEM-400CX, JEOL,Japan), which was operated at an accelerating voltage of 100 KeV.

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