Formation of Nanostructures PDF

Chapter 2 Formation of Nanostructures Section: A...

Chapter 2 Formation of Nanostructures Section: A 1 Originally by : Prof. Rachid Sbiaa and Dr. Fatma Al Ma’Mari, Physics Department, Sultan Qaboos University, Modified by Dr. Afsal Manekkathodi, Copyright ©  This chapter 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 2  Learning Outcomes Upon completion of todays session, students will be able to: 1. Classify various fabrication methods of nanomaterials: Top-down and Bottom-up 2. Explain the working principles of key fabrication methods, including o High-energy ball milling o Arc discharge o Laser ablation. 3. Assess the advantages and disadvantages of these fabrication methods. 3  Top-down and Bottom-up Fabrication Method Nanomaterials include nanostructured surfaces, nanoparticles, nanowires, nonporous materials, etc. Fabrication methods of nanomaterials can be generally subdivided into two groups: top-down methods and bottom-up methods. Micromet Bulk Material Top Down Thin Film er Heterostrctures Nanocrystals Nanomet Lithographic Wires Molecular Wires Quantum Dots er Proteins Molecules Angstro Atoms m Bottom 4 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos Up  Top-down and Bottom-up Fabrication Top-Down Method Approach: Starts with Bottom-Up bulk material, reduces size Approach: Builds structures atom by atom. Top Down Micromet Bulk er Powd er Nanomet Nanopartic Bottom er les Up Cluste Angstro rs m Atom s 5  Top-down versus Bottom-up Top-Down approach Bottom-Up approach These approaches use larger These approaches include the (macroscopic) initial structures. miniaturization of materials After the processing, components up to atomic level. nanostructures can be made. During the process, the physical These methods are forces operating at nanoscale are collectively used to combine basic units into called lithography larger stable structures. Nanomaterial is obtained starting from the atomic or molecular precursors and gradually assembling them until the desired structure is obtained. Top- Down Bottom- 6 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos  Top-down versus Bottom-up Top-Down approach Bottom-Up approach Nanostructures uniformity is Top-down methods are generally limited to small area. not cheap and very slow in manufacturing. This method allows smaller features than top-down method These are very precise method and More economical and can obtain more uniform patterns with controlled Certain nanostructures can be dimensions of nanostructures. done only by bottom-up methods. Example organic semiconductor, carbon nanotubes. 7 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos  Bottom Up Approach General methods of making nanomaterials (nanoparticles, nanostructures …) can be divided into two categories: physical and chemical methods. Building nanostructures by stacking atoms or molecules one by one (from a smaller building blocks). They arrange themselves when a physical or chemical trigger is applied. Physical method includes the following techniques: 1. High energy ball milling 2. Arc discharge 3. Laser ablation 8 4. Inert gas condensation Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos University,  What is a vacuum? Condition in which there is no matter or very little matter. An enclosed space, such as the space inside a container, in which there are far fewer gas molecules than in an equal volume of the air outside it. 9 How to get a vacuum? Why do we need a vacuum? 1 0 What is the meaning of Plasma? Blood plasma, by the way, is something completely different. It is the liquid portion of blood. It is 92 percent water and constitutes 55 percent of blood volume. Ga plasmas are ionized liberating s gases formed by atomsusingexternal electrons energy from gas molecules and sources such as lasers or high electrical voltages. 1 1 1. High-energy ball milling Mill: cut or shape with a rotating tool. Machine designed to cut or shape metal using a rotating tool. High-energy ball milling: Nanoscale powders are produced by milling bulk materials. A ball milling process where a powder mixture placed in the ball mill is subjected to high- 1 energy collision from the balls 2 The impact energy of the milling balls in the normal direction attains a value of up to 40 times higher than that due to gravitational acceleration. The motions of the balls and the powder https:// www.youtube.com/watch?v=5ShOAS3EGG U 1 3  Correct speed with Advantages cascading Nano-powders of 2-20 nm in be produces. size can The size High speed with powder of nano depends upon the Low speed with sliding centrifuging speed of the rotation and size of the balls. It is not expensive and easy process. Disadvantage s The shape of the nanomaterials is irregular There may be contaminants This method producesinserted crystal defects. from the ball and milling additives. SEM images of the powders after ball milling for 1 6 different times at 500 rpm 2. Plasma Arc discharge method 1 5 2. Arc discharge Arc discharge: An arc is a continuous electrical discharge between two electrodes with the visible discharge of light. These highly conductive plasma generate heats to evaporate the samples. Alternating current (AC) or direct current (DC) arcs are used to evaporate materials. This method uses an electric current between two electrodes under an inert gas (helium or argon). Plasma is produced https:// 1 www.youtube.com/watch?v=1kboMCaGuM 6  Arc discharge The first electrode (anode) vaporizes as electrons are taken from it by the potential difference. The arc heats and eventually sublimates the anode material. These positively charged ions pass to the other electrode, pick up electrons and are deposited to form nanotubes. Advantages: rapid production of nanoparticles, nanotubes. Disadvantage: Poor controllability of the synthesis. 1 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 7 3. Laser Ablation 1 8  3. Laser Ablation It is based on high-energy laser to induce evaporation (atomization in gaseous phase). Laser ablation also referred to pulsed laser deposition uses a short laser pulses to rapidly vaporize material from the surface of a target. The material is then collected unto a substrate with a material thickness per pulse of about 0.1 nm. Target: Just about anything! (metals, semiconductors…) Laser: few nanosecond pulses Vacuum: Atmospheres to ultrahigh vacuum. Substrate usually maintained at elevated temperature. 2 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 3  Why an inert gas atmosphere such as argon is introduced to the system? Prevents Oxidation: Protects materials from unwanted reactions. Controls Deposition Rate: Adjusts how fast material reaches the substrate. Modifies nanostructure Morphology: Affects size and surface characteristics. 2 0  Why an inert gas atmosphere such as argon is introduced to the system? Prevent Oxidation: Inert gases, being non-reactive, prevent unwanted reactions like oxidation, keeping the target material and thin films free from contamination. Control Deposition Rate: The inert gas pressure helps control how fast ablated material reaches the substrate, allowing for adjustments in the deposition rate and nanoparticle formation. Modify Film Morphology: The interaction between ablated material and inert gas affects the size, distribution, and surface characteristics of the deposited thin film. Promote Nanoparticle Formation: The inert gas enables ablated material to condense into nanoparticles before reaching the substrate, aiding in the creation of nanostructured surfaces. 2 1  Laser Ablation Roughly: Intensity  104 – 105 W/cm2 : Heating Intensity  105 – 107 W/cm2 : Melting Intensity  107 – 1010 W/cm2 : Evaporization and Plasma formation Process: Melting (tens of ns) Evaporization Plasma formation (s) Resolidification Laser pulse: few nanoseconds (ns) to few hundreds 2 microseconds (s) 6 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos  Laser Ablation Advantages:  Flexible and relatively easy to implement  Growth in any environment (atmospheric to high vacuum)  Variable growth rate  Atoms arrive in bunches, allowing for much more controlled deposition  Greater control of growth (e.g., by varying laser parameters) Disadvantages: Uneven coverage High defect or particulate concentration 2 Prof. Rachid Sbiaa, Physics Department, Sultan Qaboos 8