Nanotechnology

Synthesis to Applications

Sunipa Roy, Chandan Kumar Ghosh, Chandan Kumar Sarkar, Sunipa Roy, Chandan Kumar Ghosh, Chandan Kumar Sarkar

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351642828
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Mikroelektronik

Nano particles have created a high interest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties and find wide applications in all fields of engineering. This edited volume aims to present the latest trends and updates in nanogenerators, thin film solar cells and green synthesis of metallic nanoparticles with a focus on nanostructured semiconductor devices. Exclusive chapter on electrical transport of nanostructure explains device physics for material properties for reduced dimensions. Additionally, the text describes the functionality of metallic nanoparticles and their application in molecular imaging and optical metamaterials. Piezoelectric nanogenerators has been touched upon from the energy perspective as well.

Key Features:

• Organized contents on Nanogenerators, VOC sensing, nanoelectronics, and NEMS.

• Discusses eco-friendly green synthesis methods for metallic nanoparticles.

• Touches upon low power nano devices (e.g. nanogenerators) for energy harvesting with quantum mechanical study.

• Thin film/heterojunction based high efficiency solar cell addressed aimed at reducing global energy consumption.

1
Introduction: Motivation for Nanotechnology

Sunipa Roy, Chandan Kumar Ghosh, and Chandan Kumar Sarkar

Nanotechnology is a completely new branch of science and engineering that hit the market a few years ago. Nanotechnology is a revolution where even a 0.1 nm variation in particle size plays a significant role. Fundamentally, nanoscale implies a range from 1 to 100 nm. Nanotechnology can be best explained as “the development, synthesis, characterization, and application of materials and devices by tailoring their shape and size at the nanoscale.” Surprisingly, each permutation of shape and sizes produces a unique property with essentially new characteristics and potentiality.

The goal of this book is to provide some ideas about the optical and electrical properties of semiconducting and metallic materials at the nanoscale, and then to discuss some real-life applicational opportunities for fabricating devices. To start, we discuss basic quantum mechanics to understand the behavior of microscopic particles such as electrons and holes. We have also introduced the concept of quantum theory of radiation. It has been discussed that earlier experiments predicted the wave nature of radiation, and phenomena like interference and diffraction could be well established by Maxwell’s theory. In contrast to the particle nature of matter, the wave nature of matter is discussed conceptually. We then move on to a discussion of electronic behavior in metallic or semiconducting systems where the Sommerfeld free electron theory is addressed followed by Bloch theory. We have also qualitatively discussed the band structure and density of states for the bulk system, followed by nanoparticles. This energy band theory explains the electrical transport phenomenon at nanoscale.

Nanotechnology starts with quantum dots (QDs), defined as nanoparticles exhibiting three-dimensional quantum confinements, which leads to the development of many unique optical and transport properties depending on their shape and size. QDs could be prepared either from metal or from semiconductors. The reduction in the number of atoms in QDs results in the confinement of normally delocalized energy states when the diameter of QDs approaches the de Broglie wavelength of electrons in the conduction band or hole in the valence band. The result is that the energy difference between energy bands is increased with decreasing particle size.

Here, it is worth mentioning that the effect of van der Waals force is very significant in nanomaterials. This is the force between two atoms with a closed electronic shell, as in the inert gases, when no overlap in their wave function is observed. Due to van der Waals force, the binding energy associated with individual atoms is quite small (0.1 eV per atom), though the binding energies for ionic and covalent bonds are 100 times greater than the van der Waal bond. The origin of this force is polarization (mutual polarization) mediated and is the result of temporary transient dipoles on molecules leading to localized charge fluctuations. In this context, the concept of mutual polarization arises due to the localization of the electron charge cloud at any instant of time around the nucleus and generates instantaneous fluctuation of a dipole moment even when atoms have a zero averaged dipole moment. This instantaneous dipole moment on an atom generates an electric field, which in turn induces a dipole moment on other atoms or molecules, thus polarizing any nearby neutral atom. The resulting polarization of the two nearest atoms gives rise to an instantaneous attractive force between these two atoms. van der Waal forces are always active between two atoms or molecules, which could be stretched up to 10 nm and possibly 100 nm in the case of two surfaces. It is pertinent to mention that in between two surfaces, interaction is proportional to 1/r² where r is the separation between the two surfaces.

There is some confusion among newcomers about the difference between nanoscience and nanotechnology. To make it understandable, one can state that nanoscience deals with the arrangement of atoms and understanding their fundamental properties at the nanoscale, whereas nanotechnology is the controlling of matter at atomic scale while synthesizing a new material with different exotic properties.

Nanotechnology is already receiving attention across all branches of engineering as it is an interdisciplinary area of research. The general population isn’t aware of its presence in daily life but it is emerging in medicine, energy and the environment, defense and security, and electronics and materials.

Research in this field mainly depends on two concepts: positional assembly and self-replication. Positional assembly is a technique to move molecular pieces into their proper places and maintain their position throughout the process. Molecular robots are one of the examples that carry out positional assembly. On the other hand, self-replication occurs by multiplying the positional arrangements in some habitual way. The applications of MEMS and nanotechnology are overlapping everywhere. An ideal example of this is the development by researchers at the Technical University of Munich of carbon nanotube–based small sensors that can be sprayed over the packet. These lilliput sensors detect the concentrations of volatile organic compound emitted by the product at very low concentrations. The output of the sensors is interfaced with a wireless device that alerts authorities to the infection of food and thus prevents damage.

This emerging technology is also a breakthrough in the domain of highly powerful computers and communication devices. According to Moore’s law, there is a limit to the number of components that can be fabricated onto a silicon wafer. Conventionally, circuits have been made on the wafer by removing the unwanted portion of the material in the region. In view of the upcoming emerging trend of nanotechnology, scientists suggest that it is possible to build chips with a single atom to make the devices smaller than ever, which is not possible using traditional methods of etching. If this becomes possible, there will be no extra atoms, implying that each atom bears its own meaning and a particular purpose. Conductors like nanowire would be only one atom thick. It would be remarkable if a data bit could be represented by the presence or absence of a single electron.

Nanotechnology is the study of phenomena and fine-tuning of materials at atomic level, where a significantly different property is obtained compared to a larger scale. Very recently, individuals and groups have been working on different aspects of nanotechnology such as renewable energy harvesting and converting it into useful electrical energy. Thermal, nuclear, wind, hydrolytic, and solar energy scavenging have ushered in a new area of research, “nanopiezotronics,” whose fundamental principle utilizes the coupled piezoelectric and semiconducting properties of nanowires and nanorods for fabricating electronic devices or systems such as field-effect transistors and diodes. The term nanopiezotronics was coined by Professor Zhong Lin Wang at Georgia Tech and is included in this book.

The physics of nanopiezotronics is based on the principle of a nanogenerator that converts mechanical energy into electric energy. When a piezoelectric material is twisted, electric charges collect on its surfaces. Further, bending the structures creates a charge separation, positive on one side and negative on the other. The output in the form of charge creation taken from the device, can be used to produce measurable electrical currents in a nanogenerator when an array of nanowires is bent and then released subsequently. As the structures that are responsible for the generation of electric current have a dimension at nanoscale, the term nanogenerator is most suitable.

The basic principle of a solar cell is the conversion of solar energy to chemical energy of electron-hole pairs followed by the conversion of chemical energy to electrical energy. Among all the heterojunction methods, solar cells have the greatest potential, highest efficiency, and greatest stability under light and thermal exposure due to the tunneling of electrons.

The use of graphene in solar cell technology has improved its efficiency tremendously. Graphene/Si heterojunction solar cells are a very recent area of research which have replaced dye-sensitized solar cells due to their high cost, and they have been included in this volume. Graphene absorbs only 2% of light and it is a very good conductor because it has only three covalent bonds per atom, compared to the full four in diamond. This makes it possible for electrons to move freely over a sheet of graphene to conduct electricity. Like metals, this means it will absorb or reflect light because the free electrons can absorb the small amount of energy in the photon. Graphene/Si heterojunction solar cells can be assembled by transferring as-synthesized graphene films onto n-type Si.

With the advancement of technology, new industries are being formed, but the main concern is that they are polluting the environment and are hazardous to the health of every one of us. Here nanoparticles play a crucial role in minimizing pollution. For example, dye industries are leaving different azo dyes in the environment. Here the photocatalytic ability of nanoparticles is used to degrade these dyes into less harmful materials. In this context, it should be mentioned that the photocatalytic activity significantly depends on the shape and size of the nanoparticles since catalytic activity originates from surface atoms. It is a triumph of nanoscience and nontechnology that the fundamental relation between the properties of surface atoms and catalytic property is being examined under nanoscience where the search for new materials with superior activity is illustrated in the field of nanotechnology. Nanoparticles also exhibit a few environmental applications like antibacterial, anticancer, and antifungal activity.

This book is an amalgamation of nanoscale engineering, fundamental concepts, and novel nanodevices to prove where nano is these days, and what we can anticipate from it in the future. The chapters will highlight the fundamental ideas as well as ground-breaking applications of nano which will amaze the whole world. The authors have also provided images to make these concepts, the objectives, and the lab facilities more understandable for research students.

In the near future, nanotechnology will control the way we live, work, and communicate.

2
Introductory Quantum Mechanics for Nanoscience

Chandan Kumar Ghosh

CONTENTS

2.1Introduction

2.2Quantum Theory of Radiation

2.3Quantum Theory of Matter

2.3.1Development of Schrödinger’s Wave Mechanics

2.3.2Time-Independent Schrödinger’s Equation: One-Dimensional Consideration

2.3.2.1Solution of Schrödinger’s Equation: One-Dimensional System—Particle in ...