Chapter 2 Formation of Nanostructures PDF

Chapter 2 Formation of Nanostructures Section: B...

Chapter 2 Formation of Nanostructures Section: B 1 Originally by : Prof. Rachid Sbiaa and Dr. Fatma Al Ma’Mari, Physics Department, Sultan Qaboos University, Modified by Dr. Afsal  This chapter 2 includes.. Nanomaterial fabrication is divided into two types: top-down and bottom-up methods. Bottom-up Approach  Physical Methods 1. High energy ball milling 2. Arc discharge 3. Laser ablation 4. Inert gas condensation 5. Ion sputtering  Chemical Methods 1. Chemical vapor deposition (CVD)  Self-assembly Top-down Approach 2  Learning Outcomes Upon completion of todays session, students will be able to: 1. Understand and Explain the workingprinciples of key fabrication methods, including o Ion-gas condensation methods o Ion sputtering methods o Chemical vapor deposition methods 2. Assess the advantages and disadvantages of these fabrication methods. 3  4. Inert-gas Inert Gas condensation Condensation (IGC) is a method for making nanoparticles by evaporating a material (like metal or ceramic) in a vacuum, then cooling it with an inert gas (like argon or helium). 1. Evaporation of Material: The bulk source material (metal, ceramic, or alloy) is heated and evaporated in a vacuum chamber using methods like resistive heating, electron beam, or laser ablation. The vacuum environment ensures minimal contamination, and the material vaporizes into free atoms or molecules. 2. Inert Gas Introduction: An inert gas (argon or helium) is introduced at a controlled pressure, acting as a non- reactive cooling medium. Collisions between the vaporized atoms and gas reduce the kinetic energy, allowing atoms  4. Inert-gas condensation 3. Nucleation and Growth : As atoms cool, they cluster to form small nuclei. These nuclei grow into nanoparticles, with their size and distribution influenced by factors like gas pressure, evaporation rate, and temperature. 4. Collection on Cooled Substrate: The nanoparticles are transported by the inert gas to a cold substrate (cooled by liquid nitrogen or helium). This surface captures the particles, preventing them from re- evaporating, resulting in uniform deposition for further use. More than one crucibles can be used. O2 or N2 gases can be added to the inert gas for forming oxides and nitrides.  4. Inert-gas condensation Key Advantages: o Size Control: By carefully controlling the evaporation rate, inert gas pressure, and condensation time, the size of the nanoparticles can be controlled. o Purity: Because the process uses an inert atmosphere, there is minimal contamination, resulting in highly pure nanoparticles. o Multiple sources Applications: o Catalysis: Nanoparticles produced by inert-gas condensation are often used as catalysts due to their high surface area. o Materials Science: These nanoparticles are used in research and development of new materials, including nanocomposites and nanostructured coatings.  5. Ion Sputtering Ion sputtering is a physical process used to eject atoms or molecules from a solid target material by bombarding it with high-energy ions. This technique is commonly used in material analysis, surface cleaning, and thin-film deposition, allowing precise control over surface composition and structure. Catho Substra de te Sputtering Gas + (Inert gas Pow er - Ar) Ar + Vacuum pump Anod Targ e et 7 https://www.youtube.com/watch?v=bkvvnbFYE0w  5. Ion Sputtering Substra te Ejecte Ar ion + d s atom s Target The energies required for sputtering are much higher than lattice bonding or vibrational energies, therefore sputtering collisions can be considered elastic. Coating - How the PVD sputtering process works https://www.y 8 outube.com/watch?v=8mVK5dwyoEY  5. Ion Sputtering Working Principle: 1. Ion Generation: o An ion source generates a stream of high-energy ions, usually argon ions (Ar⁺), in a high-vacuum chamber. The ions are accelerated using an electric field to achieve high velocities. 2. Ion Bombardment: o The ions are directed toward the target material (typically a solid sample) at high energies. Upon impact, they transfer momentum and kinetic energy to the surface atoms of the target.  5. Ion Sputtering 3. Ejection of Atoms (Sputtering): o The collision between the ions and surface atoms results in a cascade of collisions beneath the target's surface. If the energy transferred is higher than the surface binding energy of the atoms, the surface atoms are ejected from the target, a process called sputtering. o Multiple ejected atoms can leave the surface as neutral atoms, ions, or clusters. 4. Deposition of nanostructures: o The ejected atoms can either be collected onto a substrate (in the case of thin-film deposition). After deposition, these samples can be analyzed.  5. Ion Sputtering Details: Sputter Yield: The efficiency of ion sputtering is measured by the sputter yield, which is defined as the number of atoms ejected per incident ion. It determines the deposition rate (deposited material thickness / time ) This yield depends on several factors, including ion energy, angle of incidence, target material, ion species, and pressure. Substra Substra te te ion ion Ejecte Eject Ar Ar + d + ed atom atom s s Targ Targ et et  5. Types of Ion There are various Sputtering sputtering techniques, including RF Sputtering, DC Sputtering, Magnetron Sputtering, and Reactive Sputtering. Each has a unique application depending on the materials and desired thin-film properties. 1. RF Sputtering (Radio Frequency) o Description: Uses alternating high-frequency (13.56 MHz) electrical fields. o Application: Suitable for insulating and non-conductive materials. o Key Advantage: Prevents charge build-up on the target. 2. DC Sputtering (Direct Current) o Description: Uses a constant DC voltage. o Application: Ideal for metallic and conductive targets. o Key Limitation: Cannot be used for insulating materials due to charge accumulation. 3. Magnetron Sputtering o Description: Utilizes magnetic fields to confine plasma close to the target. o Application: Commonly used for high-quality thin films in electronics and optics. o Key Advantage: Increases sputtering rate and improves deposition uniformity. 4. Reactive Sputtering o Description: Introduces a reactive gas (e.g., oxygen or nitrogen) into the chamber during  Ion Sputtering : Advantages 1.Versatility: o Can be used for a wide range of applications, including thin- film deposition, surface cleaning, and material analysis. o Various materials can be deposited using sputtering o Even materials with very high melting temperatures can be easily sputtered. o Alloys and nanocomposites can be deposited (Multi-targets) 2.High Precision, Uniform, large Deposition: o In sputtering deposition, the film thickness can be controlled accurately, o Large area target can be used: gives uniform growth over a large substrate. Advanced ion sputtering system 1 6  Chemical Methods Chemical vapor deposition (CVD) Chemical Vapor Deposition (CVD) is a process used to produce thin films and nanomaterials onto a substrate through chemical reactions of precursors. The precursors are introduced into a reaction chamber where they decompose or react on the heated substrate surface, forming a thin solid layer. This technique is widely used in semiconductor manufacturing, nanotechnology, and material science to create high-purity, high- performance coatings and structures. Reaction Reactant Chamber MCl2 + Waste gages gases H2  M + 2HCL Vacuu Substrate m Temperature :200 0C – Chemical vapor deposition (CVD) The product (solid material) is obtained as a coating, a powder or as a single crystal. The substrate is exposed to one or more volatile precursors which react and decompose on the surface of the substrate to produce the desired nanomaterial. High temperature: high surface diffusion During the process, volatile-by products are also produced. They are removed by gas flow thorough the reaction chamber 1 8  Chemical Methods Three main components of CVD equipment: 1. Chemical vapor precursor supply system (Reactant gases). 2. CVD reactors. 3. Gas handling 1 system (Waste 2 gases) 3 Monomer: Molecule that can be bonded (react Building blocks chemically) to other identical molecules to form a larger of the desired molecule or a new substance called polymer. nanomaterial Example of monomers: Amino acids are the monomers that make up protei 1 ns 9 Schematic representation of chemical vapor deposition (CVD) process. a Simplified scheme of a CVD reactor for CNTs synthesys. 20 https://www.youtube.com/watch?v=hkYb35e5JGo  CVD The Chemical Vapor Deposition (CVD) mechanism involves a series of mechanism steps that lead to the formation of a thin film or coating on a substrate. These steps can be summarized as follows: 1.Precursor Gas Transport: o Gas Phase Precursor Delivery: The precursors (reactant gases or vapors) are introduced into the reaction chamber using mass flow controllers. o Flow Control: The flow rate, composition, and ratio of the gas mixture are carefully controlled to ensure uniform deposition. 2.Gas Phase Reactions: o The precursor gases react or decompose in the gas phase as they are transported toward the heated substrate. o This reaction may produce intermediate species or by- products depending on the type of CVD process. 3.Surface Adsorption: o The reactive species (atoms, molecules, or radicals) adsorb onto the substrate surface. o The adsorption process is influenced by substrate temperature, surface energy, and the reactant concentration. 2 1  CVD mechanism 4. Surface Chemical Reactions: o The adsorbed species undergo chemical reactions on the surface, leading to the formation of a solid film. o This reaction may involve decomposition, reduction, or oxidation depending on the desired film material (e.g., Si from SiH₄). 5. Nucleation and Film Growth: o Nucleation begins as the first few layers of atoms or molecules form islands or clusters on the surface. o The clusters grow and coalesce, eventually covering the surface uniformly, forming a continuous film. 6. Desorption of By-Products: o By-products generated during the surface reactions (e.g., H₂, CO₂) desorb from the surface and are transported away by the carrier gas stream. 7. Gas Phase Exhaust: o The residual gases, unreacted precursors, and by-products are 2 removed from the reaction chamber through an exhaust 2  2 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 3  Some CVD reactions Trimethyl Ga (TMG) (CH3)3 Ga + H2 → Ga (s) + reduction 3CH4 GaA Arsen Methane s e 2AsH3 → 2As (s) + 3H2 Arsene Si-nitride compound formation 3 SiCl2H2 (g) + 4NH3 (g) → Si3N4 (s) + 6H2(g) + 6HCl (g) Hydrogen Ammonia (may explode when highly chloride (750° C) heated) Dibora B2H6 (g) →2B (s) + ne 3H2 (g) Diborane 2 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 4  Experimental-ZnO/SiO2 coreshell 800 850 1100 Pump Gases Pressure : 1 Torr Ar : 30 sccm O2 : 10 sccm Cooling Source Materials : ZnO - 6.6g, C - 2.22g Water Substrate : Si with Au Seed (15nm & 4nm Au) Time : 1 hour Nano Structures and Dynamics Laboratory, MSE, NTHU  CVD Advantages CVD films are generally uniform (conformal) even on topographically complex substrate Any element or compound can be deposited High density materials can be obtained Economical process CVD disadvantages Flammable, toxic and hazardous precursors can be harmful. High temperature (often more than 600 C) are not suitable for some substrates. 2 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 6