Materials for Electronic Applications Chapter 2 PDF

## Chapter 2: Materials for Electronic Applications ### 2.1 Nanomaterials **Introduction** Nanomaterials are materials whose characteristic length scale lies within the nanometric range (1-100 nm) at least in one dimension. The word 'nano' has the origin from the Greek meaning 'Dwarf'. It is one...

## Chapter 2: Materials for Electronic Applications ### 2.1 Nanomaterials **Introduction** Nanomaterials are materials whose characteristic length scale lies within the nanometric range (1-100 nm) at least in one dimension. The word 'nano' has the origin from the Greek meaning 'Dwarf'. It is one billionth of a metre (1/10⁹ m). Nanomaterials are objects with any one of the three external dimensions at the nanoscale. Nanoparticles that are naturally occurring (eg; volcanic ash, soot from forest fires) or are the incidental byproducts of combustion processes (eg; welding, diesel engines) are usually physically and chemically heterogeneous and often termed ultrafine particles. Engineered nanomaterials are intentionally produced and designed with very specific properties related to shape, size, surface properties and chemistry. These properties are reflected in aerosols, colloids, or powders. The behavior of nanomaterials may depend more on surface area than particle composition itself. Two principal factors cause the properties of nanomaterials to differ significantly from other materials: increased relative surface area, and quantum effects. These factors can change or enhance properties such as reactivity, strength and electrical characteristics. As a particle decreases in size, a greater proportion of atoms are found at the surface compared to those inside. For example, a particle of size 30 nm has 5 percentage of its atoms on its surface, at 10 nm 20 percentage of its atoms, and at 3 nm 50 percentage of its atoms. Thus nanomaterials have a much greater surface area per unit mass compared with larger particles. As growth and catalytic chemical reactions occur at surfaces, this means that a given mass of material in nanoparticulate form will be much more reactive than the same mass of material made up of larger particles. #### 2.1.1 Classification There are several ways of classification of nanomaterials. Here we discuss two types. 1. **Classification based on dimension:** This is the classification based on the number of dimensions, which are not confined to the nanoscale range (< 100 nm). - **Zero dimension (0-D):** Here all the three dimensions are in the nanometric range eg, Nano particles. - **One dimension (1-D):** Here one of the dimensions is outside the nanometric range and the other two are within the range eg, Nano wires, fibers and tubes. - **Two dimension (2-D):** Here two of the dimensions are outside the nanometric range and one is within the range eg, Nano films, layers and coat. - **Three dimension (3-D):** Here all the dimensions are outside the nanometric range (> 100 nm) eg, Bundles of nano wires and tubes, multinano layers. 2. **Classification based on materials** - **Carbon based nanomaterials:** These are defined as materials in which the nanocomponent is pure carbon eg, Carbon nano tubes (CNT), wires, spheres (fullerenes) and grephene. - **Metal based nanomaterials:** Metal-based nanomaterials are materials made of metallic nanoparticles like gold, silver, metal oxides, etc. For example, titanium dioxide (TiO₂). - **Nanocomposites:** Composite nanomaterials contain a mixture of simple nanoparticles or compounds such as nanosized clays within a bulk material. The nanoparticles give better physical, mechanical, and/or chemical properties to the initial bulk material. - **Nano polymers or Dendrimers:** Dendrimers are nanosized polymers built from branched units. These are tree-like molecules with defined cavities. They can be functionalized at the surface and can hide molecules in their cavities. A direct application of dendrimers is for drug delivery. - **Biological nanomaterials:** These nanomaterials are of biological origin and are used for nanotechnological applications. The important features of these particles are i) self assembly properties and ii) specific molecular recognition eg; DNA nano particles, nanostructured peptides. Various self assembled peptide structures can be designed to release compounds under specific conditions and are used in drug delivery systems. #### 2.1.2 Synthesis of nanoparticles There are many methods available for synthesis of nano-particles. Physical methods include Laser ablation, spluttering techniques etc. Some chemical methods are discussed below **Chemical synthesis of nano particles** 1. **Hydrolysis:** Nano particles of metal oxides can be prepared by the hydrolysis of their alkoxide solutions under controlled conditions. Commercially important nano particles of silica (SiO₂), titania (TiO₂), alumina (Al₂O₃) are prepared by this method. Hydrothermal method and sol-gel method come under this category. $Ti(OR)₄+2H₂O → TiO₂ + 4ROH$ **Sol-Gel method:** The sol-gel method is based on the phase transformation of a sol into a gel. A sol is colloidal system of nano-solid particles dispersed in a liquid. A gel is colloidal system in which liquid droplets are dispersed in a network of solid nanoparticles. Hydrolysis of metallic alkoxides or metal salts can give a sol at a suitable temperature and pH. The sol contains many other impurities. In order to remove impurities sol is transformed into a gel by changing the pH or other factors. The gel can be purified by filtration and washing with suitable solvents. The purified gel on drying give solid nanoparticle. For example aluminium oxide nano particles are obtained by hydrolysis of aluminium alkoxide by sol-gel technique. $Al(OR)₃ + 3H₂O → Al(OH)₃ + 3ROH$ 2. **Reduction:** Nano particles of gold and silver can be prepared by the reduction of their respective solutions using reducing agents, such as sodium borohydride, ascorbic acid, glucose etc. along with a protective agent like thyol, glucose etc. This method can be divided into two, reduction using reducing agents and electro reduction. $Ag⁺ + e⁻ → Ag$ **Reduction using reducing agents:** Silver nano particles can be prepared by the following method. 60 mL of 1 mM AgNO₃ solution is taken in a beaker, covered with a watch glass and heated in hot plate. The solution is then stirred using a magnetic stirrer. On boiling the solution, 6mL of 10 mM of trisodium citrate is added dropwise, about one drop per second. The beaker is then closed and kept for some time till the colour of the solution changed to a light golden colour. Then it is allowed to cool. The solvent can be removed by freeze-drying. ### Properties of Nanomaterials 1. **Physical Properties:** Crystal structure of nanoparticles is same as bulk structure with different lattice parameters. The inter-atomic spacing decreases with size and this is due to long range electrostatic forces and the short range core-core repulsion. The melting point of nanoparticles decreases with size. 2. **Chemical Properties:** A large fraction of the atoms are located at the surface of the nanomaterial which increase its reactivity and catalytic activity. Properties of materials with nanometer dimensions are significantly different from those of atoms and bulks materials. This is mainly due to the nanometer size of the materials which render them: (i) large fraction of surface atoms; (ii) high surface energy; (iii) spatial confinement; (iv) reduced imperfections; which do not exist in the corresponding bulk materials. These materials have created a high interest in recent years by virtue of their unusual mechanical, electrical, optical and magnetic properties. Nanophase ceramics are of particular interest because they are more ductile at elevated temperatures as compared to the coarse-grained ceramics. Nanostructured semiconductors are known to show various non linear optical properties. Semiconductor Q-particles also show quantum confinement effects which may lead to special properties, like the luminescence. ### Applications: 1. Single nanosized magnetic particles are mono-domains. Magnetic nano-composites have been used for mechanical force transfer (ferrofluids), for high density information storage and magnetic refrigeration. 2. Nanostructured metal-oxide thin films are receiving a growing attention for the realization of gas sensors (NO, CO, CO₂, CH₄ and aromatic hydrocarbons). 3. Nanostructured semiconductors are used as window layers in solar cells. 4. Nano sized metallic powders have been used for the production of gas tight materials, dense parts and porous coatings. 5. Carbon nanotube based transistors are used for miniaturizing electronic devices. 6. Carbon nanotube are used for making paper batteries. 7. A mixture of carbon nanotubes and fullerenes is used for making solar cells. 8. Microcrystalline TiO₂ is an insulator, whereas nano-crystalline TiO₂ is a semiconductor, Its Band gap can be tuned by controlling the size of nano-particle. Nano TiO₂ can be use for making dye sensitised solar cells. 9. Nanoparticles are used in the fight against tumours, nanostructured and functionalised surfaces and membranes improved diagnosis and more targeted use of active agents; neuro active implants. 10. Nano Cadmium telluride exhibit different colour depending upon its size. It can be used for dyeing fabrics, such nano colourants never fades. ### 2.1.3 Graphene Graphene is an allotrope of carbon consisting of mono layer of carbon atoms arranged in a two-dimensional honey comb lattice (fig 2.1). It can be visualized as a single layer extracted from the layered structure of graphite. Graphite is three-dimensional whereas graphene is two dimensional with one atom thickness. Graphene is defined as "the two-dimensional monolayer of carbon atoms, which is the basic building block of graphitic materials (i.e. fullerene, nanotube, graphite)". It is considered to be one of the most promising materials of the century and gained worldwide attention due to its extraordinary charge transport, thermal, optical, and mechanical properties. It is considered to the lightest, thinnest, strongest material that conducts heat and electricity. It is stronger than diamond and ten times more conducting than copper. Nobel Prize in Physics 2010 was awarded to Andre Geim and Konstantin Novoselov for the ground breaking experiments regarding graphene. There are various methods available for the synthesis of graphene, which includes the following. 1. **Mechanical exfoliation method (such as Scotch tape method):** Encompasses the repeated peeling off layers of graphite using adhesive tape. This was the method employed by Geim and Novoselov. 2. **Chemical vapor deposition:** Involves the deposition of carbon atoms onto a substrate (like copper) in the presence of a carbon-containing precursor gas such as methane. 3. **Thermal decomposition on SiC:** Silicon carbide substrate is heated under ultra-high vacuum which results in the sublimation of silicon atoms and deposition of carbon atoms to form graphene layers on the SiC surface. 4. **Graphene oxide reduction method:** Graphite oxide, obtained from graphite, is chemically treated to exfoliate into graphene oxide (GO), which is then reduced to graphene. Different techniques such as optical microscope, atomic force microscopy (AFM), electron microscopy (SEM & TEM) are used for the characterization of graphene.  In graphene the carbon atoms are sp² hybridized and are arranged in hexagonal fashion. Each hexagonal ring comprises of three strong in-plane sigma bonds, with a bond length of 0.142 nm, and a Pz orbitals perpendicular to the plane. These orbitals hybridize together to form two half-filled bands of free-moving electrons, π and π*, which are responsible for most of graphene's notable electronic properties. #### 2.1.3.1 Properties of graphene **Electrical conductivity:** Graphene has high electrical conductivity. Graphene, being twodimensional material shows Quantum Hall effect. They behave as massless relativistic particles (Dirac fermions) which allows the electron speed comparable to light. They have high electron mobility compared to metals. **Mechanical strength and elasticity:** They possess high elastic modulus and strength. It is 200 times stronger than steel. Graphene is highly is flexible and can be stretched by up to 20% of its original length without undergoing structural damage. **Thermal Conductivity:** Graphene exhibits excellent thermal conductivity and is highly efficient for heat dissipation and thermal management applications. **Optical:** Graphene absorbs only 2.3% of incident light over a broad wavelength range and hence makes it suitable for applications in transparent electrodes for displays, solar cells, touchscreens etc. **Surface area: ** It has high surface area due to its single-atom thickness and hence useful for applications in energy storage devices. Graphene is chemically inert, stable and biocompatible also. #### 2.1.3.2 Applications of graphene 1. **Electronics:** - Due to its lower resistance and higher transparency, graphene-based thin film can be used in touchscreens, which is found to be superior than indium tin oxide. - Smaller size transistors can be developed using graphene which shows better performance. 2. **Energy storage:** - Graphene incorporated lithium-ion batteries have longer life span, faster charging time and higher capacity. - It can improve the efficiency of hydrogen fuel cell by lowering fuel cross over (fuel permeating through the electrolyte or membrane to the opposite side of the fuel cell). - Graphene is used in supercapacitors to provide high energy density. 3. **Biomedical:** - Suitably functionalized graphene can be used to carry chemotherapy drugs to cancer cells. - Graphene-based biosensors are highly sensitive when detecting DNA, ATP, dopamine etc. 4. **Composites and coating:** - By combining graphene with paint, a unique graphene coating is formed which will effectively prevent rusting. - Graphene in the carbon-fibre coating of aircraft's wing resists impact better and consumes less fuel. - Graphene-based composites and coatings could play a significant role in improving sports equipment for skiing, cycling etc. 5. **Environmental:** - Graphene based membranes can purify water in a more efficient, cheaper and environmental friendly way. - It can be used in filters and coatings to remove pollutants and toxins from the air. ### 2.1.4 Carbon Nanotubes Carbon nanotubes (CNTs) are allotropes of carbon with cylindrical nanostructures. They are conceptually graphene sheet rolled into a tube. CNTs were discovered in 1991 by the Japanese electron microscopist Sumio Iijima who was studying the material deposited on cathode during the arc-evaporation synthesis of fullerenes. There are different methods available for the synthesis of CNTs with different structure and morphology. Arc-discharge method, laser ablation/evaporation method, chemical vapor deposition method etc. are the commonly used methods for the synthesis of CNTs. Carbon nanotubes are one-dimensional nano particles with diameter measuring 1-50 nm and are generally only a few micrometres in length. They are molecular scale tubes and are nanoscopic hollow fibres of pure carbon. It is 10⁵ times thinner than human hair. They are characterised by high tensile strength and according to structure are conductive or semi conductive. CNTs are at least 100 times stronger than steel, but only one-sixth as dense. In addition, they conduct heat and electricity far better than copper. #### 2.1.4.1 Classification of CNTs There are two types of CNTs namely single-walled (SWCNT, one tube) and multi-walled (MWCNT, several concentric tubes). Both of these are typically a few nanometres in diameter and several micrometres to centimetres long. SWCNT can be visualized as rolled-up tubular shell of graphene sheet. Based on the variations arising from the specific orientation and rolling of graphene sheets, SWCNTs are classified into three viz, armchair, zigzag, and chiral. Armchair nanotubes exhibit a symmetrical arrangement where the rolling axis aligns with the hexagonal lattice of graphene, resulting in equal edge lengths. Zigzag nanotubes, on the other hand, are formed when the rolling axis is along the zigzag pattern of the graphene lattice, leading to edges that consist solely of zigzag lines. Chiral nanotubes are characterized by a rolling axis that is oriented at an angle to the graphene lattice, resulting in unequal edge lengths and a helical structure. The optoelectronic properties of carbon nanotubes vary significantly with molecular structure and diameter of the tube. MWCNT is a stack of graphene sheets rolled up into concentric cylinders. There are three different models proposed for MWCNT namely Russian doll model, Parchment model and mixed model. In Russian doll model, sheets of graphene are arranged into concentric cylinders whereas in Parchment model single graphene sheet is rolled around itself, similar to a scroll of parchment. Mixed model is a mixture of both Russian doll and Parchment model. Classification of CNTs is shown in figure 2.2.  #### 2.1.4.2 Properties of Carbon Nanotubes: **Strength and hardness:** They are the strongest and stiffest materials. Single walled nanotubes (SWNTs) are used for synthesizing super-hard material, by compressing it at room temperature. High strength could be attributed to the covalent sp² bonds formed between the individual carbon atoms. They can withstand a pressure upto 24 GPa without deformation. Multi walled nanotubes (MWNT) without inter connected inner shells exhibit telescoping property. **Electrical conductivity:** Their electrical conductivity is better than metals. Electron travelling through a CNT behaves like a wave travelling through smooth channel- Ballistic Transport. MWNTs with interconnected inner shells show superconductivity with relatively high temperatures. **Thermal conductivity:** Their thermal conductivity also is better than metals. All nanotubes are expected to be very good thermal conductors along the tube but good insulators laterally to the tube axis-Ballistic Conduction. The thermal stability of carbon nanotubes is found to be upto 2800°C in vacuum and about 750°C in air. #### 2.1.4.3 Application of Carbon Nanotubes: 1. **Electrical circuits:** Nanotube based transistors have been made that operates at room temperature. Carbon nanotubes are used for miniaturizing electronic devices. 2. **Energy storage:** Due to high surface area, optimised electrical and thermal properties they are widely employed in energy storage applications. - **Paper Batteries:** A paper battery is a battery engineered to use a paper-thin sheet of cellulose infused with aligned carbon nanotubes. It gives a steady power output. This battery also functions as a super-capacitor which give a quick explode of high energy. - **Solar Cells:** CNT-fullerene hybrid solar cell is formed by a mixture of carbon nanotubes and fullerenes. Electrons trapped inside fullerenes are excited by sunlight leading to the flow of this electrons, which produce the current. CNT acts as the conductive pathway. - **They are used for making ultra-capacitors which provide a large surface to store electrical charge** 3. **Aerospace and automotive industry:** Due to easier molding, high strength, stiffness, and reduced weight, CNT polymer composites are preferred for aerospace and automotive applications. 4. Research is also being carried out to assess the ability of CNTs for the storage of hydrogen. ### 2.1.5 Carbon Quantum Dots (CQDs) Carbon quantum dots are zero-dimensional small carbon nano particle. It consists of ultrafine, distributed, quasi-spherical carbon nanoparticles with size less than 10 nm. Recently they have attained much attention due to their good solubility and strong luminescence properties. CQDs are found to be much more superior than traditional semiconductor quantum dots due to its lower molecular weight, reduced toxicity, ease of surface functionalization, cost-effectiveness, exceptional fluorescence stability, tunable emission wavelengths, strong biocompatibility etc. Carbon quantum dot was first discovered by Xu et all in 2004 accidentally during the purification of single-walled carbon nanotubes via electrophoresis. It can be synthesised using two main methods: 'top-down' and 'bottom-up'. The top-down approach involves breaking down large carbon structures into small CQDs using techniques like chemical oxidation, laser ablation, arc discharge, and electrochemical synthesis. The bottom-up method involves building CQDs from small carbon molecules through processes like hydrothermal synthesis, microwaveassisted synthesis, and pyrolysis, which allows control over their size and shape. Synthesised particles can be purified using electrophoresis, centrifugation, dialysis, column chromatography etc. #### 2.1.5.1 Properties **Optical:** CQDs shows absorption mainly in the UV region which can extent to the visible region also. They exhibit strong fluorescence, photoluminescence and chemiluminescence which makes them useful in imaging and sensing applications. They can be excited by various wavelengths of light and often display tunable emission spectra. **Stability and biocompatibility:** CQDs show excellent photostability and biocompatible with less toxicity than other quantum dots and hence suitable for medicinal and biological applications. #### 2.1.5.2 Applications 1. **Biomedical:** - Due to its fluorescence property, better biocompatibility and low biotoxicity they are employed in fluorescent bioimaging. - CQDs are used as biosensor carriers because of their solubility in water, tunable excitation property, excellent biocompatibility and photostability. CQDs-based biosensors can visually monitor various substances including glucose, copper, phosphate, iron, potassium, and nucleic acids. - They are used in drug delivery systems also. 2. **Optoelectronics:** - CQDs have shown potential in enhancing the performance of DSCs due to their stable light absorption properties, photostability and low cost. By creating CQD-bridged dye/semiconductor complex systems, CQDs improve the photoelectric conversion efficiency significantly. - CQD-based hybrids have been identified as excellent materials for supercapacitors. CQD-RuO₂ hybrid show remarkable electrochemical performance. 3. **Lighting:** Due to their stable light emitting, low cost and eco-friendliness they are used as LED materials. Switchable electroluminescence behaviour makes it useful for developing colourful LEDs. 4. **Environmental:** CQDs can be used to improve the photocatalytic degradation of organic pollutants and dyes in water. Their ability to absorb light and facilitate charge separation enhances the efficiency of photocatalysts like TiO₂ in breaking down harmful organic compounds. 5. **Catalysis:** CQDs-modified P25 TiO₂ composites (CQDs/P25) enhance photocatalytic hydrogen evolution. ### 2.1.6 Fullerenes Fullerenes are zero-dimensional, hollow, closed cage nanoparticles made of carbon atoms. The discovery of fullerenes in 1985 by Curl, Kroto, and Smalley culminated in their Nobel Prize in 1996. Fullerenes, or Buckminster fullerene (fig. 2.3), are named after Richard Buckminster Fuller the architect and designer of the geodesic dome and are sometimes called bucky balls.  Fullerene molecules are denoted based on the number of carbon atoms in the spherical carbon ball. The smallest fullerene was reported with twenty carbon atoms as C20 followed by C24 and C28 analogues. Each fullerene generally contains 12 pentagonal and (n/2-10) hexagonal rings, where n ≥ 20. C60 fullerene, known as the Buckminster fullerene is the primarily discovered and most widely studied form of fullerene. In 1990, a technique to produce larger quantities of C60 was developed by heating graphite rods in a helium atmosphere. The sooty material formed by condensation of vaporized carbon consists of mainly C60 with smaller quantity of C70 (rugby ball shape) sand traces of fullerenes consisting of even number of carbon atoms up to 350 or above. It is composed of fused pentagonal and hexagonal carbon rings. It contains 12 five membered rings (12 x 5= 60 atoms) and 20 six membered rings, possess a perfect icosahedral symmetry. The geometry is same as that of soccer football. Fullerenes are stable, but not totally unreactive. The sp² hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp² hybridized carbons into sp³ hybridized ones. The change in hybridized orbitals causes the bond angles to decrease from about 120° in. the sp² orbitals to about 109.5° in the sp³ orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable. #### 2.1.6.1 Properties **Physical:** Fullerenes are extremely strong molecules, able to resist great pressures - they will bounce back to their original shape after being subject to over 3,000 atmospheres. This property makes fullerenes become harder than steel and diamond. An interesting experiment shows that Fullerenes can withstand collisions of up to 15,000 mph against stainless steel, merely bouncing back and keeping their shapes. This experiment demonstrates the high stability of the molecule. **Solubility:** Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include aromatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminster fullerene have a deep purple color. Solutions of C70 are a reddish brown. The higher fullerenes C76 to C84 have a variety of colors. **Electrical:** Fullerenes are normally electrical insulators, but when crystallized with alkali metals, the resultant compound can be conducting or even superconducting. **Light absorption:** Fullerenes absorb strongly in the UV and moderately in the visible regions of the spectrum. **Chemical:** The carbon atoms within a Fullerene molecule are sp² and sp³ hybridized, of which the sp² carbons are responsible for the considerably angle strain presented within the molecule. C60 and C70 exhibit the capacity to be reversibly reduced with up to six electrons. #### 2.1.6.2 Applications 1. **Coatings and lubricant:** - Fullerenes are used for making durable, high-performance coatings that resist wear and corrosion. - Fullerenes are used as miniature 'ball bearings' to lubricate surfaces. C60 can be used used as excellent microscopic ball bearings, lubricant and catalyst. 2. **Biomedical:** - Fullerenes can be used to deliver drugs to specific cells or tissues due to their ability to form stable complexes with various molecules. - Fullerenes can be used as contrast agents in imaging techniques such as MRI or ultrasound. 3. **Energy storage:** - Fullerenes and their derivatives are employed energy storage devices due to their electrical conductivity and stability. - They can be used to store hydrogen. 4. **Environmental:** - Used in water treatment for the removal of pollutants and contaminants. - Used in sensors for detecting environmental toxins and pollutants. ### 2.2 Polymers Polymers are molecules with large molecular masses obtained by the covalent linkage of several small repeating chemical units called monomers. The repeating chemical unit may be same or different. If there is only one type of monomer in a polymer then it is known as homopolymer, eg. Polyethylene, Poly propylene, PVC, etc. The number of repeating units is called the degree of polymerization. A co-polymer is made of two or more different monomeric species, eg. butadiene-styrene (BS) Acrylonitrile butadiene-styrene (ABS), etc. Based on the types of polymerizations, polymers can be broadly divided into addition (eg; PVC, neoprene etc.) and condensation polymers (PET, bakelite etc). Two special classes of polymers viz., fire-retardant polymers and conducting polymers will be discussed in this chapter. #### 2.2.1 Fire-retardant Polymers ![Fire Retardant Polymers](https://www.google.com/search?q=examples+of+non-halogenated+fire-retardant+polymers&tbm=isch&ved=2ahUKEwjB0rG9_L3AAxV9gQIHHX-C7AQ2-cCegQIABAA&oq=examples+of+non-halogenated+fire-retardant+poly