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Nanomaterials Preparation Module2 Module 2 Classification of nanomaterials – based on dimensions (quantum dot, Q wires, Q wells) Synthesis approach (topdown, bottom up, mention with examples which will come later) Issues – can be covered briefly as mentioned in slides, (but can also be discussed more ) Synthesis method according to the syllabus Nano fabrication – given in slides Classification of nanomaterials Nanomaterials synthesis approach 1.Top down approach: Breaking of bulk material 2.Bottom approach: Build up of material Atommoleculecluster Synthesis issues • Size , shape and composition control – The monodispersity should be very high for their uniform physical properties. ( Several synthesis strategies were made for this control- changing the rate of nanomaterials formation, changing the solvent, the reducing agents, stabilizing agents etc.,) • Stability of nanomaterials are important for their application. – Due to higher surface energy and surface area nanomaterials tries to always agglomerate. Several synthetic approaches were made for stability of nanomaterials. (eg . Surface encapsulation with ligands, dispersion in different matrixes, template approach etc.,) • Scaling up the methods, reproducibility, building up complex nanostructures are some of issues in prepartion of nanomaterials Nanomaterials growth mechanism • Nucleation • Growth • Nucleation without and with seeds Homogeneous nucleation without seed :In gas phase system – The size of the nuclei is same as the size of gas molecules. Heterogeneous nucleation with seed : Seeds of definite size dispersed into liquid, gas or on solid surface Definition: Nucleation is the process where droplets of liquid can condense from a vapor, or bubbles of gas can form in a boiling liquid. Nucleation can also occur in crystal solution to grow new crystals. Example: Dust and pollutants provide nucleation sites for water vapor in the atmosphere to form clouds. Seed crystals provide nucleation sites for crystal growing. Nanomaterials growth mechanism Nucleation • A solution with solute exceeding the solubility or supersaturated possesses a high Gibbs free energy; the overall energy of the system would be reduced by segregating solute from the solution. • The reduction of the overall Gibbs free energy of a supersaturated solution by forming a solid phase and maintaining an equilibrium concentration in the solution. • This reduction of Gibbs free energy is the driving force for both nucleation and growth. • The change in free energy dependents on the solute concentration Where C- conc. of solute, Co – equilibrium conc. , - atomic volume • When Gv = 0 no nucleation • When C > Co then Gv = negative, spontaneous nucleation Nanomaterials growth mechanism Nucleation • For the synthesis of nanoparticles with uniform size distribution, it will be the best if all nuclei form at the same time with the same size. • In addition, all the nuclei will have the same subsequent growth. Consequently, monosized nanoparticles can be obtained. • So it is obvious that it is highly desirable to have nucleation occur in a very short period of time. Nanomaterials growth mechanism Growth • When the concentration of growth species reduces below the minimum concentration for nucleation, nucleation stops, whereas the growth continues • The size distribution of nanoparticles is dependent on the subsequent growth process of the nuclei. The growth process of the nuclei involves multi-steps and the major steps are (1) Generation of growth species, (2) Diffusion of the growth species from bulk to the growth surface (3) Adsorption of the growth species onto the growth surface (4) surface growth through irreversible incorporation of growth species onto the solid surface. Nanomaterials growth mechanism Growth • The Growth steps can also be further grouped into two processes. (1) Supplying the growth species to the growth surface is termed as diffusion, which includes the generation, diffusion, and adsorption of growth species onto the growth surface (2) whereas incorporation of growth species adsorbed on the growth surface into solid structure is denoted as growth. • A diffusion-limited growth would result a different size distribution of nanoparticles as compared with that by growth-limited process. Nanomaterials growth mechanism Preparation methods Nanomaterials preparation Physical Methods Ball milling Gas condensation processing (GPC) Laser ablation Ion beam Electron beam Nanolithography Chemical Methods Sol-gel synthesis Wet chemical synthesis Precipitation method Chemical vapour condensation Catalytic chemical vapour deposition Template assisted CVD Electrochemical method Reverse micelles Preparation Any Preparation technique should provide: 1. Identical size of all particles (mono sized or uniform size distribution). 2. Identical shape or morphology. 3 Identical chemical composition and crystal structure. 4 Individually dispersed or mono dispersed i.e., no agglomeration. Preparation – Physical method High-Energy ball milling (Top down approach) : *Interest in the mineral, ceramic processing, and powder metallurgy industry. * Involves milling process include particle size reduction (Fig.3). * Restricted to relatively hard, brittle materials which fracture and/or deform during the milling operation. * Different purposes including; tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills, etc. Violent or agitation, ~50 m nm Schematic representation of the principle of mechanical milling. *Hardened steel or tungsten carbide (WC) coated balls the basic process of mechanical attrition (rubbing away) (Fig.3). 15 Preparation – physical method • Limitation of Ball milling: (Even though high production rates) 1. Severe plastic deformation associated with mechanical attrition due to generation of high temperature in the interphase, 100 to 200º C. Thermal decomposition or evaporation of materials 2. Difficulty in broken down to the required particle size. 3. Contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2, in rare gases) can be a problem. (inert condition necessary) Preparation – Physical method (B) Gas Condensation Processing (GPC)-Bottom-up approach: Thermal or electric or e- beam evaporation (like PVD) Metal in crucible Cooling (Rotating cylinder) Liquid N2 (-80oC) Metal cluster (gaseous state) Homogenous nucleation in gas phase Nanoparticles deposits (2-50nm) scrapping Collection of the nanoparticles General Scheme for GPC for the nanoparticle synthesis 17 •Major advantage over conventional gas flow is the improved control of the particle sizes. Fig. 4 Schematic representation of typical set-up for gas condensation synthesis of nanomaterials followed by consolidation in a mechanical press or collection in an appropriate solvent media. •These methods allow for the continuous operation of the collection device and are better suited for larger scale synthesis of nanopowders. •However, these methods can only be used in a system designed for gas flow, i.e. a dynamic vacuum is generated by means of both continuous pumping and gas inlet via mass flow controller. Limitation:1.Control of the composition of the elements has been difficult and reproducibility is poor. 2.Oxide impurities are often formed. The method is extremely slow. 18 Preparation (C) Chemical Vapour Condensation (CVC) (Bottom-up approach): •Involves pyrolysis (heat treatment) of vapors of metal organic precursors (starting materials) like Hexamethyldisilazane (CH3)3Si-NHSi-(CH3)3 to produce SiCxNyOz. •Evaporate source in the GPC is replaced by a hot wall reactor in the CVC process (Fig.5). Fig. 5 A schematic of a typical CVC reactor •Precursor residence time is the key parameter to control the size of nanoparticle here (gas flow rate, pressure, heating temperature can be controlled). •Other procedure similar to GPC. Production capabilities are much larger than in the GPC processing. 19 Nanowire synthesis - CVD Tubular furnace for synthesis of nanomaterials, nanowires by Chemical vapour deposition(CVD) The precursor materials were converted to desired product and deposited on the substrate The precursor material, temperature, the carrier gas are important in product formation When catalyst is used it is known as catalytic chemical vapour deposition (CCVD) Plasma-enhanced chemical vapor deposition (PECVD) is a process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after creation of a plasma of the reacting gases. The plasma is generally created by RF (AC) frequency or DC discharge between two electrodes, the space between which is filled with the reacting gases. [edit] Film examples & Applications Plasma deposition is often used in semiconductor manufacturing to deposit films conformally (covering sidewalls) and onto wafers containing metal layers or other temperature-sensitive structures. PECVD also yields some of the fastest deposition rates while maintaining film quality (such as roughness, defects/voids), as compared with sputter deposition and thermal/electron-beam evaporation, often at Film examples & Applications Silicon dioxide can be deposited using a combination of silicon precursor gasses like dichlorosilane or silane and oxygen precursors, such as oxygen and nitrous oxide, typically at pressures from a few millitorr to a few torr. Plasma-deposited silicon nitride, formed from silane and ammonia or nitrogen, is also widely used, although it is important to note that it is not possible to deposit a pure nitride in this fashion. Plasma nitrides always contain a large amount of hydrogen, which can be bonded to silicon (Si-H) or nitrogen (Si-NH);[1] this hydrogen has an important influence on IR and UV absorption,[2] stability, mechanical stress, and electrical conductivity.[3] PECVD • • • The reactor has two electrodes of different area Use of radio frequency creates highly reactive species in low temperature plasma Substrate can be kept low temperature ( 423-623 K) Plasma deposition Plasma decomposition of hydrocarbons ( acetylene) Two electrodes with different area Higher mobility of electrons than ions create a sheath next to electrode with excess of ions For good thin film deposition the plasma has to operated at lowest possible pressure This will increase the fraction of ions to radical of the plasma In pressure plasma, use of magnetic field, increase the path length of electron and the ionisation efficiency Preparation - Chemical Methods (Bottom-up approachs): Wet Chemical Synthesis of nanomaterials (Sol-gel Process) 1. Very popular & widely employed to prepare oxide materials (SiOx). 2. The sol-gel process: formation of a colloidal suspension (sol) gelation of the sol to form a network in a continuous liquid phase (gel) solid. 3. Metal or metalloid element surrounded by various reactive ligands (Si(OCH3)4, tetramethoxy silane, TMOS, alkoxide) is the reactant. 4. The starting material is processed to form a sol in contact with water or dilute acid. Removal of the liquid from the sol yields the gel, and the sol/gel transition controls the particle size and shape. Calcination of the gel produces the product (eg. Oxide). 25 Sol – Gel synthesis Sol-Gel • • • • Sol-gel processing refers to the hydrolysis and condensation of alkoxide-based precursors such as Si(OEt)4 (tetraethyl orthosilicate, or TEOS). The reactions involved in the sol-gel chemistry based on the hydrolysis and condensation of metal alkoxides can be described as follows: Classic sol-gel reaction scheme • • Over all Steps: Step 1: Formation of different stable solutions of the alkoxide (the sol). • Step 2: Gelation resulting from the formation of an oxide- or alcohol- bridged network (the gel) by a polycondensation or polyesterification reaction • Step 3: Aging of the gel, during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. • Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. – If isolated by thermal evaporation, the resulting monolith is termed a xerogel. – If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel. • Step 5: Dehydration, during which surface- bound M-OH groups are removed, there by stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 8000C. • Step 6: Densification and decomposition of the gels at high temperatures (T>8000C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The typical steps that are involved in sol-gel processing are shown in the schematic diagram below. 28 Wet chemical synthesis • Use of chemical stabilizing agents • Preparation of different types of nanostructures • Stabilizing agents - eg., Citrate, Thiols, Amines, Carboxylates, Phosphine, Phosphine oxide, Surfactants, coordinating polymer • Use of different reducing agents • Coordination of stabilizing agents with the nanostructures • Stability of the nanostructures depends on the chemical nature of the stabilizing agents too. • Control on the composition, size, shape of the nanostructures • Larger control on the reaction rates during preparation Wet chemical synthesis • Shape and size depends on starting materials and reaction conditions Wet chemical synthesis • Preparation nanoparticles • HAuCl4 + Stabilizing agent + NaBH4 Au nanoparticles AgNO3 + Stabilizing agent + NaBH4 Ag nanoparticles HAuCl4 + AgNO3 + Stabilizing agent + NaBH4 AuAg alloy NPs Stabilizing agents – Sodium citrate, Alkanethiols, alkylammonium salts, R- amines, R-COOH, surfactants etc • Citrate method of Au nanoparticle preparation • HAuCl4 + Trisodium citrate + NaBH4 Au nanoparticles • Ratio of gold to citrate is important in size control Wet chemical synthesis of Au NP • 1 – AuCl4- is transferred into toluene from aqueous phase using tetraoctylammonium salts as phase transfer agent • 2. it is reduced in presence of thiol to give the NPs 1 2 • These NPs canbe ligand exchanged • Alloy nanoparticles with large control in their composition can be prepared by this method Wet chemical synthesis • Silver nanoparticles also can be prepared using citrate, thiol methods • Silver nanocubes synthesis using AgNO3 and Ethylene glycol at 160oC Precipitation method Precipitation reactions involve the simultaneous occurrence of nucleation, growth, coarsening, and/or agglomeration processes. Pprecipitation reactions exhibit the following characteristics: (i) The products are generally insoluble species formed under conditions of high supersaturation. (ii) Nucleation is a key step, and a large number of small particles will be formed. (iii)Secondary processes, such as Ostwald ripening and aggregation, dramatically affect the size, morphology, and properties of the products. (iv) The supersaturation conditions necessary to induce precipitation are usually the result of a chemical reaction. Precipitation method • Nanomaterials are produced by precipitation from a solution. • The method involves high degree of homogenization and low processing temperature. • ZnS powders were produced by reaction of aqueous zinc salt solutions with thioacetamide (TAA). TAA • Precursor zinc salts were chloride, nitric acid solutions, or zinc salts with ligands (i.e., acetylacetonate, trifluorocarbonsulfonate, and dithiocarbamate). 0.05 M Zn2+/ 70oC/pH2 Eg.1: The 0.05 M cation solution was heated in a thermal bath maintained at 70° or 80 °C in batches of 100 or 250 ml. Acid was added dropwise to bring it to a pH of 2. The reaction was started by adding the TAA to the zinc salt solution, with the molar ratio of TAA and zinc ions being set to an initial value of either 4 or 8. 35 Template synthesis • Template technique - Catalyst free formation of CNT Fabrication of nanostructures on surfaces Electrochemical method • • • • Large control on the rate of electrodeposition Substrate dependent Product depends on the electrolytic composition The size and shapes can be controlled by the method of deposition • Methods of electrochemical preparation. ( Galvanostatic, potentiostatic, potentiodynamic, pulse method). Method have control on the composition, size and shape. • Nucleation and growth of nanostructured can be studied by electrochemical method • Different types of metal, alloy, metaloxiide nanomaterials can be prepared by this method. Electrodeposited nanostructures • Au nanoclusters electrodeposited on glassy carbon electrode by electroreduction of Au3+ solution • Deposition was done using cyclic voltammetric method • Nanowires of Au were deposited using polycarbonates template Au nanocluster Fe nanocubes CNT with Ag NPs Micro/Nano - Lithography • Lithography is the technique used to transfer a computer-generated pattern onto a substrate. • This pattern is subsequently used to etch the underlying tin film ( oxide, nitride). • It is used for micro, Nano electronic fabrication (MEMS, NEMS) Micro/Nano lithography Nanolithography Nanofabrication techniques • Scanning probe microscopy systems (STM, AFM) are capable of controlling the movement of an atomically sharp tip in close proximity to or in contact with a surface with subnanometer accuracy • Nanofabrication using microscopic probes have control on the surface architectures and size control • STM (scanning Tunneling), AFM( Atomic force) probes were used to pattern or engrave the substrate surface • Following that etching of resist surface is done to obtain nanopatterned surfaces Nanolithography • The substrate is cleaned • Substrate spin coated with organic resist (PMMA) • Electron emitted from thee biased tip is used to pattern the resist similiar to electron beam lithography • Development is done using standard solutions Nan- Dip Pen lithography • AFM tip is inked with chemical of interest and brought near to the surface • Ink molecules flow from the pen to the surface • Water meniscus between tip and substrate help in diffusion and transport of molecules • Conducting polymers, organic dyes, DNA, antibodies etc can be fabricated • Solid organic ink can fabricated using heated AFM cantilever