Nanotechnology (Unit IV) Notes PDF

# UNIT IV - NANO-TECHNOLOGY ## Introduction Nanotechnology is the study and use of structures between 1 nanometer and 100 nanometers in size. Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced. In its or...

# UNIT IV - NANO-TECHNOLOGY ## Introduction Nanotechnology is the study and use of structures between 1 nanometer and 100 nanometers in size. Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced. In its original sense, nanotechnology refers to the projected ability to construct items from the bottom up, using techniques and tools being developed today to make complete, high-performance products. ## Nanoscience It deals with the study of properties of materials at nanoscales where properties differ significantly than those at larger scales. The applications of nanoscience emerged as nanotechnology. Nanoscience deals with the synthesis, manipulation, and the characterization of materials at atomic and molecular levels and to study the various properties like electrical, magnetic, optical, mechanical, and chemical etc. ## Nanoscale 1nm = 10⁻⁹m = 10⁻⁷cm Nano means 10⁻⁹m i.e. a billionth part of a meter. Atoms are extremely small, and the diameter of a single atom can vary from 0.1 to 0.5nm depending on the type of the element. ### Dimensions of few nanomaterials: | Nanomaterial | Dimensions | |---|---| | Carbon atom | 0.15 in diameter | | Water molecule | 0.3nm | | Red blood cell | 7,000nm | | Human hair | 80,000nm wide | | White blood cell | 10,000nm | | Virus | 100nm | | Hydrogen atom | 0.1nm | | Bacteria range | 1,000 to 10,000nm | | Proteins | 5 to 50nm | | DNA | 2nm Width | | Quantum dots | 8nm | ## Nanotechnology It deals with the design, characterization, production, and application of nanostructures, nano-devices, and nano-systems. ## Nanomaterials The materials in which the atoms are arranged in the order of 1 to 100nm in any one of the dimensions, and these atoms will not move away from each other, called as nano materials. (All materials are composed of grains, which in turn comprise many atoms. The visibility of these grains depends on their size. The materials possessing grains of size ranging from 1 to 100nm. known as nonmaterial's, can be produced with different dimensionalities). For example: C, Zno, Cu - Fe alloys, Ni, Pd, Pt etc. ### Zero-dimensional nanomaterials Materials wherein all the dimensions are measured within the nanoscale (no dimensions, or 0-D, are larger than 100 nm). The most common representation of zero-dimensional nanomaterials are nanoparticles. ### One-dimensional nanomaterial Materials that are nanoscale in one dimension called as one dimensional nano materials (nano layers). For example: Nano tubes and nano wires. ### Two-dimensional nanomaterials Materials having two of its dimensions in nano scale is called two dimensional nano materials. For example: Nano thin films, nano plates. ### Three-dimensional nanomaterials Materials having three of its dimensions in nano scale is called three dimensional nano materials. For example: 3D particles of precipitates, Colloids, quantum dots, tiny particles of semi-conductor materials. ## Basic principles/properties of Nanomaterials When the material size of the object is reduced to nanoscale, then it exhibits different properties than the same material in bulk form. The factors that differentiate the nanomaterials from bulk material are: - Increase in surface area to volume ratio - Quantum confinement ### Quantum Confinement The properties of materials can be studied based on the energy levels. When atoms are isolated, their energy levels are discrete. When very large no of atoms are closely packed to form a solid, the energy levels split & form bands. Nano materials represent intermediate stage. As a result, the energy levels change. When we apply the problems of particles in a potential well as well as in a potential box, the dimensions of such wells or boxes are of the order of deBroglie wavelength of electrons. The energy levels of electrons change. This effect is called Quantum confinement. This affects the optical, electrical, magnetic properties of nanomaterials. When the electrons are confined, the particles will have more oscillations and this will result in colour change of the materials. For example, nano gold colloids are dispersed in ruby glass, the ruby glass exhibits red hue. ## Synthesis (or) Fabrication of Nanomaterials The production of nano materials or nano crystalline materials requires precise methods. There are various techniques that are capable of creating nano structures. In general there are two approaches that are classified as: - Top-down approach (or) technique - Bottom-up approach (or) technique ### Top-down approach Top-down techniques involve starting with a block of individual material, etching (removing the surface by dissolution) or convert it down to the desired size. The challenge here is to produce smaller and smaller structures. Nano material particle can be made through this method. In this method, the nanomaterials are synthesized by assembling or arranging the bulk materials into nanosizes. Top-down processing has been and will be the dominant process in semiconductor manufacturing. Examples: (i) Sol-gel method. (ii) Ball-milling method. (iii) Lithography. (iv) Mechanical grinding. ### Bottom-up approach Bottom-up technique involves the assembly of smaller sub unit (atoms or molecules) to make larger structure. In this method, the nano materials are synthesized by assembling (or) arranging the atoms or molecules together to form the nano materials. Examples: (i) physical vapor deposition method (ii) Chemical vapor deposition method (iii) Plasma arcing and (iv) Electro deposition ## Sol-Gel Method The sol-gel method is a wet chemical method or chemical solution deposition method. This technique is used to generate nanoparticles & nanopowders. A given material converted into colloids & dissolved in water or in acids, then forms a solution (Sol). A colloid suspended in a liquid is called as “Sol”. A suspension that keeps its shape is called Gelatin or “Gel”. The sol-gel formation occurs in different stages: - Hydrolysis of precursors. - Condensation and polymerization of monomers to form particles. - Growth of particles and development of particles. (Agglomeration) The schematic representation of the synthesis of nano particles using the Sol-Gel method is shown in fig. This method takes place in following steps: - Take a material & convert it into liquid precursors (inorganic salts or organic species such as metal -alkoxide) & dissolved in water or in other solvents. It forms colloidal suspensions known as "Sol". - This solution is kept at a suitable temperature and some amounts of gelling agents are added to it. This will produce a gel.( By dehydration reaction with Sol forms Gel) - Rapid drying of the gel, under super critical conditions an aero-gel. - Drying of the “Gel” i.e. water & other liquids are removed from the gel forms a Xerogel. By calcination xerogel forms ceramics. - The solution further proceed through spinning & finally by calcination forms thin films & nanopowder respectively. ### Advantages - This method is used to prepare thin films, nanopowder, glasses, glass ceramics etc at very low temperatures. - To prepare mono-sized nanoparticles. - Very high purity in synthesized materials can be obtained. ### Disadvantages - The raw materials are very costly. - The synthesis reaction requires relatively longer time. - Organic solvents used are harmful to the environment. ## PVD (Physical vapor deposition technique) Physical Vapour Deposition (PVD) is a collective set of processes used to deposit thin layers of material, typically in the range of few nanometers to several micrometers. PVD processes are environmentally friendly vacuum deposition techniques consisting of three fundamental steps: - Vaporization of the material from a solid source assisted by high temperature vacuum or gaseous plasma. - Transportation of the vapor in vacuum or partial vacuum to the substrate surface. - Condensation onto the substrate to generate thin films. Different PVD technologies utilize the same three fundamental steps but differ in the methods used to generate and deposit material. The two most common PVD processes are thermal evaporation and sputtering. ### Thermal evaporation A deposition technique that relies on vaporization of source material by heating the material using appropriate methods in vacuum. ### Sputtering A plasma-assisted technique that creates a vapor from the source target through bombardment with accelerated gaseous ions (typically Argon). In both evaporation and sputtering, the resulting vapor phase is subsequently deposited onto the desired substrate through a condensation mechanism to give nanofilms(thin-films). ### Applications PVD is used in a variety of applications & used in: - including fabrication of microelectronic devices, - interconnects, battery and fuel cell electrodes, - diffusion barriers, - optical and conductive coatings, - surface modifications. ### Advantages - This method consists good strength and durability - It is environment friendly vapor deposition technique. ### Disadvantages - Cooling systems are required, to get nanomaterials. - Mostly high temperature and vacuum control needs skill and experience. ## Characterization of Nano-particles Characterization refers to the study of material features such as its composition, structure and its properties like physical, electrical, magnetic etc. For characterization of nano particles both X-ray diffraction (XRD) & electron microscope are the most widely used techniques. They are: - XRD - Electron microscope ### Electron microscope It is an instrument by using we can study & analysis of small particles & crystal structures. It’s magnification is high i.e. 10⁶ times greater than the size of given particle (or) object. In electron microscopes, current carrying coils produce magnetic fields that act as lenses to focus an electron beam on a specimen. They are two types of electron microscope: - SEM (Scanning electron microscope) - TEM (Transmission electron microscope) ### SEM (Scanning electron microscope) An electron microscope that images the sample surface by it with a high energy beam of electrons. #### Principle The surface of a sample is scanned using a high energy beam of electrons. This gives rise to secondary electrons, back scattered electrons, and characteristic X-rays. Conventional light microscopes use a series of glass lenses to bend light waves and create a magnified image. while the SEM Creates magnified images by using electrons instead of light waves. SEM consists of: - Electron gun. - Anode. - Magnetic lens (consists of two condensed lens) - Scanning coils. - TV scanner. - Detectors. - Specimen stage. #### Construction and working 1. The virtual source at the top represents the electron-gun which produces a stream of high energy monochromatic electrons. 2. Electrons are attracted and travel through anode there by attains directionality. 3. Two magnetic lenses are used as condenser lenses to convert the diverging electron beam into a fine pencil beam and condenser lens eliminated the high angled electrons from the beam so electron-beam becomes thin and coherent. 4. A scanning coil is used to deflect the electron beam to scan the sample. 5. The objective lens is used to focus the scanning beam on a desired spot on the sample. 6. When the high energy electron beam strikes the sample, some electrons scattered due to elastic scattering (due to back scattering) called back scattered electrons, some electrons are knocked off from the surface called secondary electrons and some electrons penetrate deep into the inner shells of the sample atoms to knockoff inner shell electrons due to which X-rays (wavelength matches)are produced. 7. The intensities of secondary electrons, back scattered electrons and X-rays recorded using detectors and the signals are amplified and the images are then displayed on a TV scanner (monitor). 8. This process is repeated several times.i.e.30 times/sec to get accurate results. #### Applications - Topography: To study the surface features of an object and its texture. - Morphology: To study the shape, size, arrangement of particles. - Composition: To study the elements and compound ratio in a sample. - Crystallography: Arrangement of atoms, and their order in the crystal. - SEM shows very detailed 3D images at much high magnifications as compared to light microscopes. - The surface structure of polymer nano composites, fracture surfaces, nano fibres, nano particles and nano coating can be imaged through SEM with great clarity. ## Applications of Nano materials or Nano technology Nano materials are 'small materials with big future' because of their extremely small size, they have many applications and advantages. ### Material technology - Nano materials used in cutting tools made up of nano crystalline materials which are much harder, much more wear-resistant and last longer. - Nano materials used as sensors. They are used as smoke detectors, ice-detectors on air craft wings. ### Information-technology - Nanoparticles are used for information storage. - Nan photonic crystals are used in chemical/l computers. - Nano thickness -controlled coating are used in optoelectronic devices. - Nanoscale-fabricated magnetic materials are used in data storage. - Used in opto electronic devices, mobiles and laptops etc. ### Electronic industry - Nano materials used in: Glass fibres - Used to prepare laser diodes. - Optical switches - data memory ### Medical field - Nano materials used in drug delivery systems. - Used as agents in cancer therapy. - Used as active agents. - Used to reproduce or repair damaged tissues. # UNIT -3 - NANOTECHNOLOGY ## Synthesis of Nanomaterials There are a large number of techniques available to synthesize different types of nanomaterials in the form of colloids, clusters, powders, tubes, rods, wires, thin films etc. There are various physical, chemical, biological and hybrid techniques available to synthesize nanomaterials. The technique to be used depends upon the material of interest, type of nanostructure viz., zero dimensional, one dimensional, or two dimensional material size, quantity etc. - Physical methods: (a) mechanical: ball milling, melt mixing (b) Vapor: physical vapor deposition, laser ablation, sputter deposition, electric arc deposition, ion implantation - Chemical methods: colloids, sol-gel, L-B films, inverse micelles. - Biological methods: biomembranes, DNA, enzymes, microorganisms. ### Physical methods - **Ball milling**: It is used in making of nanoparticles of some metals and alloys in the form of powder. Usually the mill contains one or more containers are used at a time to make fine particles. Size of container depends upon the quantity of interest. Hardened steel or tungsten carbide balls are put in containers along with powder or flakes (<50 um) of a material of interest. Initial material can be of arbitrary size and shape. Container is closed with tight lids. The containers are rotated at high speed (a few hundreds of rpm) around their own axis. Additionally they may rotate around some central axis and are therefore called as 'planetary ball mill'. When the containers are rotating around the central axis, the material is forced to the walls and is pressed against the walls. But due to the motion of the containers around their own axis, the material is forced to other region of the container. By controlling the speed of rotation of the central axis and container as well as duration of milling, it is possible to ground the material to fine powder whose size can be quite uniform. Some of the materials like Co, Cr, W, Ni-Ti, Al-Fe, Ag-Fe etc. are made nanocrystalline using ball mill. Large balls, used for milling, produce smaller grain size and larger defects in the particles. The process may add some impurities from balls. The container may be filled with air or inert gas. However, this can be an additional source of impurity. A temperature rise in the range of 100 to 1100 C is expected to take place during the collisions. Cryo-cooling is used to dissipate the generated heat. - **Melt Mixing**: It is possible to form or arrest the nanoparticles in glass. Structurally, glass is an amorphous solid, lacking long range periodic arrangement as well as symmetry arrangement of atoms/molecules. When a liquid is cooled below certain temperature, it forms either a crystalline or amorphous solid (glass). Nuclei are formed spontaneously with homogenous (in the melt) or inhomogeneous (on the surface of other materials) nucleation, which can grow to form ordered, crystalline solid. Usually, metals form crystalline solids but, if cooled at very high cooling rate, they can form amorphous solids. Such solids are known as metallic glasses. Even in such cases the atoms try to reorganize themselves into crystalline solids. Addition of elements like B, P, Si etc. helps to keep the metallic glasses in amorphous state. It is possible to form nanocrystals within metallic glasses. It is also possible to form some nanoparticles by mixing the molten streams of metals at high velocity with turbulence. On mixing thoroughly, nanoparticles are formed. - **Physical Vapor Deposition**: It involves material for evaporation, an inert gas or reactive gas for collosion of material vapor, a cold finger on which clusters or nanoparticles can condense, a scraper to scrape the nanoparticles and piston - anvil (an arrangement in which nanoparticle powder can be compacted). All the processes are carried out in a vacuum chamber so that the desired purity of the end product can be obtained. Metals or high vapor pressure metal oxides are evaporated or sublimated from filaments or boats of refractory metals like W, Ta, Mo in which materials to be evaporated are held. Size, shape and even the phase of evaporated material can depend upon the gas pressure in deposition chamber. Clusters or nanoparticles condensed on the cold finger (water or liquid nitrogen cooled) can be scraped off inside the vacuum system. The process of evaporation and condensation can be repeated several times until enough quantity of material falls through a funnel in which a piston-anvil arrangement has been provided. - **Ionized Cluster Beam Deposition**: It is useful to obtain adherent and high quality single crystalline thin films. The set up consists of a source of evaporation, a nozzle through which material can expand into the chamber, an electron beam to ionize the clusters, an arrangement to accelerate the clusters and a substrate on which nanoparticle film can be deposited, all housed in a suitable vacuum chamber. Small clusters from molten material are expanded through the fine nozzle. The vapor pressure, ~10 torr to 10-2 torr needs to be created in the source and the nozzle needs to have a diameter larger than the mean free path of atoms or molecules in vapor form in the source to form the clusters. On collision with electron beam clusters get ionized. Due to applied accelerating voltage, the clusters are directed towards the substrate. By controlling the accelerating voltage, it is possible to control the energy with which the clusters hit the substrate. Thus it is possible to obtain the films of nanocrystalline material using ionized cluster beam. - **Laser Vaporization**: In this method, vaporization of the material is effected using pulses of laser beam of high power. The set up is a ultra high vacuum or high vacuum system equipped with inert or reactive gas introduction facility, laser beam, solid target and cooled substrate. Clusters of any material of which solid target can be made are possible to synthesize. Usually laser giving UV wavelength such as excimer laser is necessary because other wavelengths like IR or visible are often reflected by some of the metal surface. A powerful beam of laser evaporates the atoms from a solid source, atoms collide with inert gas atoms (or reactive gases) and cool on them forming clusters. They condense on the cooled substrate. The method is often known as laser ablation. Gas pressure is very critical in determining the particle size and distribution. Simultaneous evaporation of another material and mixing the two evaporated materials in inert gas leads to the formation of alloys or compounds. - **Laser Pyrolysis or Laser Assisted Depositon**: Here a mixture of reactant gases is decomposed using a powerful laser beam in presence of some inert gas like helium or argon. Atoms or molecules of decomposed reactant gases collide with inert gas atoms and interact with each other, grow and are then get deposited on cooled substrate. Many materials like Al2O3, WC, Si3Ni4 etc. are synthesized in nanocrystalline form by this method. Here too, gas pressure plays an important role in deciding the particle size and their distribution. - **Sputter Deposition**: In sputter deposition, some inert gas ions like Ar are incident on a target at a high energy. The ions become neutral at the surface but due to their energy, incident ions may get implanted, get bounded back, create collision cascades in target atoms, displace some of the atoms in the target creating vacancies, interstitials and other defects, desorb some adsorbents, create photons while loosing energy to target atoms or even sputter out some target atoms/molecules, clusters, ions and secondary electrons. Sputter deposition is a widely used thin film deposition technique, specially to obtain stoichiometric thin films from target material. Target material may be some alloy, ceramic or compound. It is a very good technique to deposit multilayer films for mirrors or magnetic films for spintronic applications. Sputter deposition can be carried out using Direct Current (DC) sputtering, Radio Frequency (RF) sputtering or magnetron sputtering. In all these methods, one uses discharge or plasma of some inert gas atoms or reactive gases. The deposition is carried out in a required gas pressurized high vacuum or ultra high vacuum system equipped with electrodes, one of which is a sputter target and the other is a substrate, gas introduction facility etc. In DC sputtering, the target is held at high negative voltage and substrate may be at positive, ground or floating potential. Substrates may be simultaneously heated or cooled depending upon the requirement. Once the required base pressure is attained in the vacuum system, usually argon gas introduced at a low pressure. A visible glow is observed and current flows between anode and cathode indicating the deposition onset. When sufficiently high voltage is applied between anode and cathode with a gas in it, a glow discharge is set up with different regions as cathode glow, Crooke's dark space, negative glow, Faraday dark space, positive column, anode dark space and anode glow. These regions are the result of plasma. Plasma is a mixture of free electrons, ions and photons. Plasma is overall neutral but there can be regions, which are predominantly of positive or negative charge. The density of various particles and the length over which they are spread and distributed depends upon the gas pressure. In RF sputtering 5-30 MHz frequency is used and the electrodes can be insulating. However, 13.56 MHz is a commonly used frequency for deposition. Target itself biases to negative potential becoming cathode. RF and DC sputtering efficiency can be further increased using magnetic field. When both electric and magnetic fields act simultaneously on a charged particle, force is acted upon it. Electrons moves in a helical path and is able to ionize more atoms in the gas. In practice, both parallel and magnetic fields to the direction of electric field are used to further increase the ionization of the gas, increasing the efficiency of sputtering. By introducing gases like O2, N2, NH3, CH4, H2S etc. while metal targets are sputtered, one can obtain metal oxides like Al2O3, nitrides, carbides etc., This is known as reactive sputtering. The plasma density can be further enhanced using microwave frequency and coupling the resonance frequency of electrons in magnetic field. Ionization density using Electron Cyclotron Resonance plasma is about 2-3 orders of magnitude larger. Thin films and nanoparticles of Si2O3, SiN, GaN etc. have been obtained using this technique. - **Chemical Vapour Deposition (CVD)**: It is a hybrid method using chemicals in vapour phase. Basic CVD process can be considered as a transport of reactant vapour or reactant gas towards the substrate kept at some high temperature where the reactant cracks into different products which diffuse on the surface, undergo some chemical reaction at appropriate site, nucleate and grow to form the desired material film. The by-products created on the substrate have to be transported back to the gaseous phase removing them from the substrate. Vapours of desired material may be often pumped into reaction chamber using some carrier gas. In some cases the reactions may occur through aerosol formation in gas phase. There are various processes such as reduction of gas, chemical reaction between different source gases, oxidation or some disproportionate reaction by which CVD can proceed. However, it is preferable that the reaction occurs at the substrate rather than in the gas phase. Usually temperature ~ 300 to 1200 C is used at the substrate. There are two ways viz., hot wall and cold wall by which substrates are heated. In hot wall set up the deposition can take place even on reactor walls. This is avoided in cold wall design. Besides this, the reaction can take place in gas phase with hot wall design,which is suppressed in cold wall set up. Further, coupling of plasma with chemical reaction in cold wall set up is feasible. Usually gas pressures in the range of 0.1 torr to 1.0 torr are used. Growth rate and film quality depend upon the gas pressure and the substrate temperature. When the growth takes place at low temperature, it is limited by the kinetics of surface tension. CVD is widely used in industry because of relatively simple instrumentation, ease of processing, possibility of depositing different types of materials and economic viability. Under certain deposition conditions nanocrystalline films or single crystalline films are possible. There are many variants of CVD like metallo organic CVD (MOCVD), atomic layer epitaxy (ALE), vapor phase epitaxy (VPE), plasma enhanced CVD (PECVD) etc. They differ in source gas pressure, geometrical layout, temperature used etc. - **Electric Arc Deposition**: This is one of the simplest and useful methods, which leads to mass scale production of fullerenes and carbon nanotubes. It requires water cooled vacuum chamber and electrodes to strike an arc between them. The positive electrode itself acts as the source of material. If some catalyst are to be used, there can be some additional thermal source of evaporation. Inert gas or reactive gas introduction is necessary. Usually the gap between the electrodes is ~1mm and high current ~50 to 100 amperes is passed from a low voltage power supply (~12-15 volts). Inert gas pressure is maintained in the vacuum system. When an arc is set up, anode material evaporates. This is possible as along as the discharge can be maintained. By striking the arc between the two graphite electrodes, it is possible to get fullerenes in large quantity. In case of fullerenes, the formation occurs at low helium pressure as compared to that used for nanotube formation. Also, fullerenes are obtained by purification of soot collected from inner walls of vacuum chamber, whereas nanotubes are found to be formed only at high He gas pressure and in the central portion of the cathode. No carbon nanotubes are found on the chamber walls. - **Ion Implantation**: In this method high energy (few keV to hundereds of keV) or low energy (<200 eV) ions are used to obtain nanoparticles. lons of interest are usually formed using an ion gun specially designed to produce metal ions, which are accelerated to high or low energy towards the substrate heated to few hundered of C. Depending upon the energy f the incident ions, various other processes like sputtering and generation of electromagnetic radiation may take place. It is possible to obtain single element nanoparticles or compounds and alloys of more than one element. In some experiments it has been possible to even obtain doped nanoparticles using ion implantation. There is possibility of making nanoparticles using swift heavy ions (few MeV energy) employing ion accelerators like a pelletron. - **Molecular beam epitaxy (MBE)**: This technique of deposition can be used to deposit elemental or compound quantum dots, quantum wells, quantum wires in a very controlled manner. High degree of purity in materials is achievable using ultra high vacuum (better than torr ). Special sources of deposition known as Kundsen cell (K-cell) or effusion cell are employed to obtain molecular beams of the constituent elements. The rate of deposition is kept very low and substrate temperature is rather high in order to achieve sufficient mobility of the elements on the substrate and layer by layer growth to obtain nanostructures. - **Thermolysis**: Nanoparticles can be made by decomposing solids at high temperature having metal cations, and molecular anions or metal organic compounds. The process is called thermolysis. For example, small lithium particles can be made by decomposing lithium azide, LiN3. The material is placed in an evacuated quartz tube and heated to 400 C. At but 370 C LiN3 decomposes, releasing N2 gas, which is observed by an increase in the pressure on the vacuum gauge. In a few minutes the pressure drops back to its original low value, indicating that all the N2 has been removed. The remaining lithium atoms coalesce to form small colloidal metal particles. Particles less than 5nm can be made by this method. Passivation can be achieved by introducing an appropriate gas. - **Pulsed laser method**: Pulsed lasers have been used in the synthesis of nanoparticles of silver. Silver nitrate solution and a reducing agent are flowed through a blenderlike device. In the blender there is a solid disk, which rotates in the solution. The solid disk is subjected to pulses from a laser beam creating hot spots on the surface of the disk. Silver nitrate and the reducing agent react at these hot spots, resulting in the formation of small silver particles, which can be separated from the solution using a centrifuge. The size of particles is controlled by the energy of the laser and rotation speed of the disk. This method is capable of a high rate of production. ### Chemical Methods (Wet Chemical route) There are numerous advantages of using chemical methods, which are – - Inexpensive, less instrumentation compared to many physical methods - Low temperature (< 350 C) synthesis - Doping of foreign atoms (ions) possible during synthesis - Variety of size and shapes are possible - Self assembly or patterning is possible - **Colloids and Colloids in solutions**: A class of materials in which two or more phases (solid, liquid, gas) of same or different materials co-exist with at least one dimension less than a micrometer is known as colloids. Colloids may be particles, plates, or fibers. Nanomaterials are a sub-class of colloids, in which one of the dimensions of colloids is in about 1 to 100 nm range. Colloids are the particles suspended in some host matrix. **Interactions**: Colloids are particles with large surface to volume ratio. Therefore atoms on the surface are in a highly reactive state, which easily interact to form bigger particles or tend to coagulate. It is thus necessary to understand the stability of colloids i.e., how the colloids dispersed in a medium can remain suspended particles. In general there are a number of interactions involved. There are two types of interactions: attractive and repulsive. Repulsive interaction involves short distance of Born repulsive interaction and long range attractive interaction van der Waals attraction. Repulsive part arises due to repulsion between electron clouds in each atom and attractive part is due to interaction between fluctuating or permanent dipoles of atoms/molecules. The attractive forces between colloidal particles reduced in colloids in a liquid medium. Colloids in liquid may be positively charged, negatively charged or even neutral. But in most cases they are charged. As there are some charges on particles, ions of opposite charges accumulate around them. Oppositely charged ions are known as counter ions. This accumulation of counter ions leads to formation of an electric double layer. Stability of colloids can be increased by stearic hinderance or repulsion. By adsorbing some layers of a different material on colloidal particles eg. polymer it is possible to reduce the attractive forces between them.. **Syntheis**: Chemical reactions in which colloidal particles are obtained are carried out in glass reactor of suitable size. Glass reactor usually has a provision to introduce some precursors, gases as well as measure temperature, pH etc. during the reaction. It is usually possible to remove the products at suitable time intervals. Reaction is usually carried out under inert atmosphere like argon or nitrogen gas so as to avoid any uncontrolled oxidation of the products. There is also provision made to stir the reactants during the reaction by using Teflon coated magnetic needle. Although chemical synthesis of nanoparticles is a complex process, by understanding how nucleation and growth of particles takes place, it is possible to control the various steps and try to achieve monodispersed nanoparticles. This can be done with the help of LaMer diagram. As we keep on increasing the concentration of the reactants in the solution, at certain concentration, say Co, the formation of nuclei begins. There is no precipitate at this concentration. Further increase in concentration increases nuclei formation up to a concentration CN, above which there is 'super saturation' between CN and Cs. Concentration CN denotes the maximum rate of nuclei formation. When nuclei formation reduces, again Co the minimum concentration for nucleation is reached. No new nuclei can be formed and crystal growth reduces the concentration. At this concentration Cs, an equilibrium is obtained. If new nuclei are formed during the growth of particles, particle with large size distribution are obtained. Therefore it is very important that concentration of solute and its diffusion to dissolve species be adjusted properly in order that no fresh nuclei are formed once the concentration of solute and its diffusion to dissolve species be adjusted properly in order that no fresh nuclei are formed once the concentration has reached CN. Particles can grow even at the expense of smaller particles. Larger particles are more stable and grow at the expense of smaller particles. This growth mode is known as Ostwald ripening. The driving force for large particles is the reduction in surface free energy. - **Colloidal metal nanoparticles** are often synthesized by reduction of some metal salt or acid. For example highly stable gold particles can be obtained by reducing choloroauric acid (HAuCI4) with tri sodium citrate (Na3C6H5O7). The reaction takes place as follows: HAUCI4+ Na3C6H5O7 → Au++C6H5O-+ HCI + 3NaCl. Au atoms are formed by nucleation and condensation. They grow bigger in size by reduction of more Au+ ions on the surface. These atoms are stabilized by oppositely charged citrate ions. Metal gold nanoparticles exhibit intense red, magenta etc., colours, depending upon the particle size. Gold nanoparticles are stabilized by repulsive Coulomb interaction. It is also possible to stabilize gold nanoparticles using thiol or some other capping molecules. In a similar manner, silver, palladiurn, copper and other metal nanoparticles can be synthesized using appropriate precursors, temperature, pH, duration of synthesis etc., Particle size, size distribution and shape strongly depend on the reaction parameters and can be controlled to achieve desired results. It is also possible to synthesize alloy nanoparticles using appropriate precursors. - **Compound semiconductor nanoparticles** can be synthesized by wet chemical route using appropriate salts. Sulphide semiconductors like CdS and ZnS can be synthesized easily by what is known as co- precipitation. For example to obtain ZnS nanoparticles any zinc salt like Zinc sulphate (ZnSO4), zinc chloride (ZnCl2) can be dissolved in aqeous (or nonaqeous) liquid and Na2S is added to the solution. Following simple reaction results to give particles of ZnS. ZnCl2 + Na2S → ZnS + 2NaCl To obtain zinc oxide particles one can use following reactions: ZnCl2 + 2NaOH → Zn(OH)2 + 2NaCl Zn(OH)2→ ZnO + H2O Selenide particles can be obtained using appropriate selenium giving salt. However, all these nanoparticles need to be surface passivated as colloids formed in liquids have a tendency to coagulate or ripen due to attractive forces existing between them. The electrostatic and other repulsive forces may not be sufficient to keep them apart. However, stearic hindrance can be created by appropriately coating the particles to keep them apart. This is often known as 'chemical capping' and has become a widely used method in the synthesis of nanoparticles. Advantage with this chemical route is that, one can get stable particles of variety of aterials not only in the solution, but even after drying off the liquid. Coatings may be part of post-treatment or a part of the synthesis reactions to obtain nanoparticles. If it is a part of the synthesis reaction, the concentration of capping molecules can be used in two ways, to control the size as well as to protect the particles from coagulation. Chemical capping can be carried out at high or low temperature depending on the reactants. In high temperature reactions, cold organometallic reactants are injected in some solvent like triocylphosphineoxide held at temperature >300 C. - **Langmuir-Blodgett (L-B) method**: This technique to transfer organic layers at air-liquid interface onto solid substrates is known for nearly 70 years. The technique was developed by the two scientists Langmuir and Blodgett. In this technique one uses amphiphilic long chain molecules like that in fatty acids. An amphiphilic molecule has a hydrophilic group (water loving) at one end and a hydrophobic group (water hating) at the other end. As an example consider the molecule of arachidic acid, which ahs a chemical formula [CH3(CH2)16 COOH]. There are many such long chain organic chains with general chemical formula [CH3(CH2)nCOOH], where n is a positive integer. In this case, -CH3 is hydrophobic and -COOH is hydrophilic in nature. Usually molecules with n>14 are candidates to form L-B films. This is necessary in order to keep hydrophobic and hydrophilic ends well separated from each other. When such molecules are put in water, the molecules spread themselves on surface of water in such a way that their hydrophilic ends. often called as heads, are immersed in water, whereas the hydrophobic ends called as tails remain in air. They are also surface active agents or surfactants. Surfactants are amphiphilic molecules ie, an organic chain molecule in which at one end there is polar, hydrophilic (water loving) and at the other a nonpolar, hydrophobic (water hating) group of atoms. Using a movable barrier, it is possible to compress these molecules to come close together to form a monolayer and align the tails. It is however necessary that hydrophilic and hydrophobic ends are well separated. Such a monolayer is two dimensionally ordered and can be transferred on some suitable solid substrates like glass, silicon etc. This is done by dipping the solid substrate in the liquid, in which ordered organic molecular monolayer is already formed. Deposition of L-B films is done by following steps: (1) A monolayer of amphiphilic molecules is formed (2) A substrate is dipped in the liquid (3) The substrate is pulled out, during which ordered molecules get attached to the substrate (