Nanotechnology: Principles and Practices

From the first edition (2006), there have been continuous attempts to cover contemporary ideas in the field. The beginning of the twenty-first century has witnessed a tremendous increase in scientific activity in the field of 'nanoscience' and 'nanotechnology', the seeds of which were sown in the last century.

Introduction

The blackbody spectrum spans a wide range of wavelengths and has the greatest intensity. As early as 1900, he discovered what is now known as "Planck's Law of Blackbody Radiation".

Fig. 1.1 Spectra of Black body radiation. Note that as the temperature increases, the spectral intensity increases and maximum intensity shifts to shorter wavelength

Matter Waves

He spent 7 years there and was a guest lecturer at the University of Bern for 1 year. But these two systems of physics could not remain separate from each other: they had to be united by the formulation of the theory of energy exchange between matter and radiation. were actually achieved which were neither correct nor permissible.

Heisenberg’s Uncertainty Principle

The question is how the electrons know to reach the places where the intensity maxima occur. Can we now imagine an experiment to determine which slit the electrons pass through to produce the diffraction pattern.

Fig. 1.6 (a) Schematic diagram to obtain diffraction pattern (slits S1 and S2) on the photographic plate P (one may use some counter also to detect the intensity)

Schrödinger Equation

Let me say at the outset that in this discourse I am not opposing some special theorems of quantum mechanics that are held today (1950s). If the potential energy of a particle does not change with time (as in the case of energy states of electrons in an atom), we can write for one dimension.

Electron Confinement

Particle in a Box

Since the particle exists only inside the box, the wave function should not exist outside the box and should be zero at the boundaries. 1.8 (a) Quantized energy states, (b) corresponding wave functions, and (c) probability of finding a particle at various locations between '0' and 'a' in the field.

Fig. 1.8 (a) Quantized energy states, (b) corresponding wavefunctions and (c) probability of finding the particle at different locations between ‘0’ and ‘a’ in the box

Density of States

It can be shown that the density of states (D(E)) in the two-dimensional solid, which is nothing but the case of thin films, is given as. It can therefore be shown that the density of states is constant in the two-dimensional case (see Fig.1.11).

Particle in a Coulomb Potential

It is the spherically symmetric form of the potential that enables us to simplify the procedure. The energy state of the system is described by three quantum numbers: n (principal quantum number), `(orbital quantum number) and m (magnetic quantum number).

Tunnelling of a Particle Through Potential Barrier

Introduction

Later, Wien showed that as the temperature of the blackbody increased, the maximum in the blackbody spectrum shifted to shorter wavelength or higher energy. Thus, neither equation was satisfactory to explain the blackbody radiation over the entire region.

Arrangement of Atoms

A circle is a perfectly symmetrical object that can be rotated by any angle about an axis through its center. It can be shown that there are only and 2/6 possible rotations corresponding to translational symmetry that can leave the crystal structure invariant.

Fig. 2.4 A lattice C atom makes a solid or crystal

Two Dimensional Crystal Structures

Three Dimensional Crystal Structures

Some Examples of Three Dimensional Crystals

Body Centred Cube (bcc)

Planes in the Crystals

Crystallographic Directions

Reciprocal Lattice

Therefore, it is often convenient to use the reciprocal lattice concept to deal with crystals.

Fig. 2.12 Schematic diagram illustrating CsCl, NaCl, diamond and graphite crystal structures

Quasi Crystals

Now we can see (Fig.2.18) how they fit together and fill a large area without leaving spaces in between or overlapping with each other.

Fig. 2.17 Penrose tiles (first line) and various motifs generated using them

Liquid Crystals

In the cholesteric (also called chiral nematic) the molecules are oriented in the same direction as in a nematic crystal in a single plane, but there is rotation of molecules from plane to plane like turning. They are widely used in the display screens of televisions, computers, clocks and many other applications.

Bonding in Solids

Covalent Bond
Ionic Bond
Metallic Bond
Mixed Bonds
Secondary Bonds

The energy difference between energy of free atoms and that of crystal is known as cohesive energy. Such electrons form what is known as an electron gas (of free electrons) and are responsible for the metallic bond which is an electrostatic interaction between the positive ions of atoms that have lost the electrons and are unable to move themselves (however, ions vibrate about their average positions) in the crystal.

Electronic Structure of Solids

Free Electron Motion

Bloch’s Theorem

Origin of Band Structure

This gives rise to energy gaps in the free electron curve of Figure 2.24, as illustrated in Figure 2.26. Thus, a band image of solids makes it possible to understand differences between metals, semiconductors and insulators.

Fig. 2.25 Electron probability distribution in periodic potential

Introduction

Mechanical Methods

High Energy Ball Milling

However, this can be an additional source of impurities if proper precautions are not taken to use high purity gases. By controlling the rotation speed of the central axis and the bowl and the grinding time, it is possible to grind the material into a fine powder (a few nm to a few tens of nm), the size of which can be quite uniform.

Melt Mixing

It is also possible to form some nanoparticles by mixing the molten streams of metals at high speed with turbulence. Such silicate units, which do not share the edges but are connected only through the corners, are randomly distributed in 3-D to form glassy structure.

Fig. 3.3 Cooling pattern of glass forming melt

Methods Based on Evaporation

Physical Vapour Deposition with Consolidation
Ionized Cluster Beam Deposition
Laser Vapourization (Ablation)
Laser Pyrolysis

Usually the sources are heated electrically so that enough vapors are formed of the material to be deposited. The electron beam focuses on the material to be deposited, stored in the crucible, as it is generated from a filament that is not near the evaporating material.

Fig. 3.5 Different shapes of filaments, canoe and baskets used for holding the materials for evaporation

Sputter Deposition

DC Sputtering
RF Sputtering
Magnetron Sputtering
ECR Plasma Deposition

Depending on the energy of ions, the ratio of ion mass to the mass of the target atoms, the ion-target interaction can be a complex phenomenon. Sputter deposition can be performed using Direct Current (DC) sputtering, Radio Frequency (RF) sputtering or magnetron sputtering.

Fig. 3.10 Interaction of an ion with target

Chemical Vapour Deposition (CVD)

When growth takes place at low temperature, it is limited by the kinetics of the surface reaction. When two types of sayPandQ atoms or molecules are incorporated into the desired film, there are two ways to grow.

Electric Arc Deposition

In principle, other nanocrystals or tubes should also be possible to obtain by this method. However, this method is mainly found to be suitable for the deposition of fullerenes or carbon nanotubes.

Ion Beam Techniques (Ion Implantation)

Molecular Beam Epitaxy (MBE)

Introduction

Interestingly, by introducing a small amount of dopant (atoms different from those in the host material), we can introduce some localized energy states into the energy gap. All procedures are carried out in a vacuum chamber, so that the desired purity of the final product can be obtained.

Fig. 1.2 (a) Circuit diagram to observe photoelectric effect. (b) Variation of current due to photoelectrons

Colloids and Colloids in Solutions

Interactions of Colloids and Medium
Colloids in Vacuum
Colloids in a Medium
Effect of Charges on Colloids
Stearic Repulsion
Synthesis of Colloids

The surface free energy per unit volume will increase as shown in Fig.4.5 with separation between two particles. The concentration of electrolyte will strongly influence the electric potential curve and is shown schematically in Fig.4.10.

Fig. 4.2 Brownian motion of colloidal particles

Nucleation and Growth of Nanoparticles

Homogeneous nucleation is said to occur when it involves nucleation around the constituent atoms or molecules of the resulting particles. It should be remembered that the concentration of solutes and the temperature of the solution would greatly affect the growth.

Fig. 4.13 A typical chemical reactor to synthesize nanoparticles

Synthesis of Metal Nanoparticles by Colloidal Route

Nanoparticles of the metal gold display intense red, magenta and other colors, depending on the size of the particles. It is also possible to stabilize gold nanoparticles using thiols or some other capping molecules.

Synthesis of Semiconductor Nanoparticles by Colloidal Route

If it is a part of the synthesis reaction, the concentration of cap molecules can be used in two ways, i.e. Chemical capping can be carried out at high or low temperature depending on the reactants.

Fig. 4.18 Capping of nanoparticles by different molecules

Langmuir-Blodgett (LB) Method

Therefore, while extracted from the liquid, the head groups can be easily attached to the glass surface. The substrate dipping-pulling process can be repeated several times to obtain ordered multilayer molecules.

Fig. 4.20 A variety of organic molecules used for L-B thin film deposition

Microemulsions

As shown in Figure 4.26, micelles are formed with an excess of water, and inverse micelles are formed with an excess of organic liquid or oil. You can determine the composition by drawing lines parallel to all three sides of the triangle, as shown in Figure 4.28.

Fig. 4.24 Amphiphilic molecules in aqueous solutions

Sol-Gel Method

Sol-gel synthesis generally involves hydrolysis of precursors, condensation followed by polycondensation to form particles, gelation, and a drying process through various routes as shown in Figure 4.31. We will also see that there are recent methods for combining the microemulsion method with the sol-gel method to produce some new materials.

Hydrothermal Synthesis

Sonochemical Synthesis

Microwave Synthesis

Synthesis Using Micro-reactor or Lab-On-Chip

The fluids may be aqueous or non-aqueous, and an appropriate reactor must be selected according to the reactions to be performed. However, with the help of parallel processing, one can increase the amount of the product to be obtained in microreactors.

Introduction

They are all synthesized in the bioenvironment and can self-assemble or dissociate; they can recognize some specific molecules or bodies because of their charges or shapes, bind weakly or strongly to other molecules. They are genetically encoded and are therefore most important as nature's messengers of life from one generation to the next.

Table 5.1 Some of the biominerals produced by the biological world

Synthesis Using Microorganisms

When silver metal salt with another fungus Verticillium sp. treated, the nanoparticles can be produced intracellularly. Silver nanoparticles can be synthesized using MKY3 yeast strain isolated from garden soil.

Fig. 5.3 (a) Prokaryotic and (b) Eukaryotic cells

Synthesis Using Plant Extracts

The flask must be shaken for eight hours at 30 ºC, after which the silver nitrate solution can be poured into it. The cell-free medium contains silver nanoparticles that can be recovered by a freeze-thaw technique that exploits the different thawing temperatures of the different ingredients.

Use of Proteins, Templates Like DNA, S-Layers etc

We will now discuss the procedure to convert ferritin into apoferritin and how to use it in the synthesis of CdS nanoparticles. Process of CdS formation is stepwise (see Fig.5.6) with Cd loading of 55 atoms per apoferritin colloid occurring in each step.

Fig. 5.6 Synthesis of CdS nanoparticles using ferritin

Synthesis of Nanoparticles Using DNA

Introduction

In case there is stress induced in nucleation, the additional term43 r3© can be added to Eq. The reaction can be carried out in water using the setup shown in Fig.4.13.

Mechanism of Self Assembly

The static and dynamic assemblies can be further divided into 'hierarchical self-assembly', directed self-assembly' and 'co-assembly' as illustrated schematically in Fig.6.2. Hierarchical self-assembly is characterized by short-range, medium-range, and long-range interactions of one type of building block.

Some Examples of Self Assembly

Self Assembly of Nanoparticles Using

Self Assembly in Biological Systems

For example, ferritin solution in NaCl and phosphate at 5.8 pH can be filled in a trough. Chloroform containing dichloroacetic acid can be used to dissolve poly-1-benzal-L-histidine (PBLH) and spread over ferritin solution in the trough.

Fig. 6.5 Assembly of CdS nanoparticles using S-layer

Self Assembly in Inorganic Materials

If complex self-assembly arrangements can be made as in biological processes, compact devices can also be realized through self-assembly. It is able to store information, process and also has control over different parts of the body, all through self-assembly or organization.

Fig. 6.7 SEM image of self assembled silica (SiO 2

Introduction

Microscopes

Optical Microscopes

As illustrated in Fig.7.1a, the smaller the distance from the eye, the larger the image of the object in the eye. It is the ratio of the size of the image to the size of the object.

Fig. 7.1 (a) Size of the image depends on the distance from the eye. (b) By keeping a convex lens close to the eye, image of an object can be magnified

Confocal Microscope

7.3 (a) On a flat surface, the intense reflected beam will pass through the detector. b) On a rough surface, due to scattering in different directions, the intensity of the reflected beam passed to the detector would be smaller. However, if we consider a light beam falling on a rough surface, as illustrated in Fig.7.3b, then depending on the roughness or morphology of the surface, the intensity of the reflected beam can change in different directions (Snell's law -it is respected, but it is necessary to consider the local normal on the surface) or the reflected ray would diverge.

Electron Microscopes

Scanning Electron Microscope

The metal film is usually sputter-coated onto the sample to be investigated prior to insertion into the electron microscope. When comparing the analyzes of the energies and intensities of such characteristic X-rays, the analysis of the composition of the sample under investigation can be obtained (Fig. 7.7).

Fig. 7.7 Scanning electron microscopy images of ZnO with flower-like morphology and belt- belt-like morphology

Transmission Electron Microscope (TEM)

In the case of TEM, because enough electrons must be passed through the sample, the thickness of the sample has a limit. Since both SEM and TEM require a vacuum environment for their operation, degassing of the sample must be avoided.

Fig. 7.9 SiO 2 nanoparticles image and diffraction pattern

Scanning Probe Microscopes (SPM)

Scanning Tunnelling Microscope

If the Fermi level of the sample is at a higher level, electrons below the Fermi level flow to the tip. The advantage of the constant height mode over the constant current mode is that the tip can move faster over the surface of the sample as there is no need for a feedback circuit.

Fig. 7.11 Tunnelling of electrons from one metal to other. (a) Metals are at small distance, but not less than 10 nm

Atomic Force Microscope

Thus, the image is the copy of the current change as the tip scans the desired area of the sample surface. The tip oscillates near the surface at a distance of 50 nm so that it almost touches the sample during its oscillation cycle.

Fig. 7.13 Photograph of a JSPM-5200 scanning probe microscope

Scanning Near-Field Optical Microscope (SNOM)

The diameter of the opening in the fiber as well as the distance between the opening and the sample must be smaller than the wavelength of the light. It can be easily understood now that the resolution achieved in a SNOM will depend on the size of the aperture and the distance at which the probe can be placed.

Fig. 7.16 Field of an object is shown. The electron currents and charge densities inside the object induce an electromagnetic field radiating from the surface

Diffraction Techniques

X-Ray Diffraction (XRD)
Atomic Scattering Factor
Bragg’s Law of Diffraction
Diffraction from Different Types of Samples
Crystal Structure Factor
Diffraction from Nanoparticles
X-ray Diffractometer
Dynamic Light Scattering

However, in the case of a polycrystalline sample, the peaks are broadened due to the size of the grains. Therefore, the width of the diffraction peaks can be considered as the effect of convolution of several peaks giving the mean grain size.

Fig. 7.18 Thomson’s explanation of scattering of X-rays by electrons located at origin

Spectroscopies

Optical (Ultraviolet-Visible-Near Infra
UV-Vis-NIR Spectrometer
Infra Red Spectrometers
Dispersive Infra Red Spectrometer
Fourier Transform Infra Red Spectrometer
Raman Spectroscopy
Luminescence
X-Ray and Ultra Violet Photoelectron
Auger Electron Spectroscopy

Fourier Transform Infra Red (FTIR) spectrometer uses the Michelson interferometer for recording the spectra. As shown in Fig.7.36, a parallel beam of infrared rays is incident on the beam splitter BS.

Fig. 7.30 Schematic optical absorption spectrum (semiconductor)

Magnetic Measurements

Vibrating Sample Magnetometer (VSM)

Mechanical Measurements

Some Common Terminologies Related

Introduction

A tip is mounted on cantilever as shown in Fig.7.14 which can be brought close to sample surface. By measuring g1(t) at one scattering angle, diffusion coefficient (thus particle radiusR) of particles can be obtained.