Chapter 4 - Preparation of Naomaterials Flashcards
How can synthesis routes be classified and what are some examples?
Physical or chemical (or biological)
physical - often gas phase (PVD, ball milling, electrospraying)
chemical - liquid or solid phase (sol-gel, thermal decomposition)
What are dimensionality and classes of synthetic routes?
dimensionality = how many dimensions of the nanoscale does it create? (0D, 1D, 2D)
classes - does is create:
1 - discrete nano-objects
2 - surface nano-featured materials
3 - bulk nano-structured materials
*all methods have 1 of each (if not more)
Explain physical vapor deposition (PVD)
A vapor is created in a vacuum chamber by direct heating or electron beam heating of the metal, from which it condenses onto the cold substrate
- synthesis of 1D or 2D nanomaterials
What are the different ways vapour can be generated for PVD?
- evaporation
- sputtering
- laser ablation
- wire explosion
PVD: evaporation (how does it work?)
- atoms are removed from the sample through heating the sample above its melting point with thermal ionization (heated with electricity). the atoms then condense onto the target (substrate) which is being cooled (can be liquid nitrogen)
NPs are remove with a scraper from the substrate
When does PVD: evaporation work well? when does it not?
works well: for metals and metal oxide NPs with well defined size and narrow size distribution
does not work well: for bimetallic NPs due to difference in melting point
What are the 3 fundamental parameters that size, morphology, and yield of cluster for PVD: evaporation?
- rate of supply of atoms to the substrate (supersaturated region)
- rate of energy removal from the hot atoms via condensation
- rate of removal of cluster once nucleated from substrate
PVD: sputtering (how does it work?)
Argon ions (plasma) are accelerated by an electric filed onto a metal target, ejecting atoms to the substrate surface
- reactive gas can be added for compound formation
- used extensively in Si technology
When is PVD: sputtering best?
- ideal for high melting point materials such as refractory metals and ceramics - resulting in better stoichiometric control of the film
In what ways is sputtering better than evaporation for PVD?
- creates films of higher density since sputtered atoms have more energy
- almost any metal or chemical that doesn’t decompose chemically can be sputtered, making this a very flexible though expensive process
- sputtering is more prone to contamination than evaporation due to lower purity of the sputtering target materials
What is PVD: laser ablation?
Uses an intense pulsed laser beam to irradiate the target, thereby vaporizing atoms and clusters from the target
- atoms ablated collide with He atoms, losing their kinetic energy rapidly and condense to form a cloud consisting of fine nanocrystalline clusters
- ideal to control the evaporation process by congruent evaporation of constituent elements of multicomponent materials in a very short period of time
PVD: laser ablation - what is is good for?
- production rate can be varied with He gas pressure and laser pulse energy
- can be used for the preparation of metals, metal oxides, and metal carbides
What is chemical vapor deposition?
a process where one or more gaseous adsorption species react or decompose on a hot surface to form stable solid products
What is CVD ideal for?
- pure dense films
- fine particles
- coating complex-shared components
*can be used for metals, ceramics, and semiconductors
How does CDV work?
A reactant gas mixture is brought into contact with the surface to be coated. When it decomposes, it deposits a dense pure layer of metal or compound. The product gas, or unreacted reactant gas are then pumped out of the system
The deposit can be formed by a reaction between the precursor gases in the vapor phase, or by a reaction between the vapor and then the surface of the substrate itself.
It frequently requires high temperatures, 800C or more
What are the activation sources for CVD?
how is energy added?
- thermally activated: dissociation of gaseous species takes place by heating the reactor above the reaction temperature (substrate must have a melting point above this T)
- plasma-enhanced CVD: lowers deposition temperature and often uses microwave plasma generation.
- laser CDV: deposition of chemical vapours takes place using a laser beam. This process yields good quality films at lower temperatures with better control on composition
What is electrodeposition?
- electrodepsition of metallic layers from aqueous solution is based ont he discharge of metal ions present in the electrolyte at a cathode surface
- is a bottom up approach
- mass of material is produced is found from Faraday’s law
- can make 3D, 2D, 1D, and even 0D materials (3D is not NP)
What is electroplating?
The coating of an electrically conductive object with a layer of metal using electric current. The result is a thin, smooth, even coat of metal on the object
-anode is connected tot he positive terminal and cathode at negative (material is plated to the cathode)
- metal at the anode is oxidized to form cations with a positive charge - these cations dissolve into the electrolyte
- the cations are reduced at the cathode to deposit the metallic NPs
ex. in acid (H2SO4), Cu is oxidized to form Cu at the cathode
*could also use NaCl as the electrolyte
What is Faraday’s law?
dictates the number of moles of a substance produced or consumed during an electrode process is proportional to the electric charge passed through the electrode, Q (assuming no parallel processes)
n = Q/zF = It/zF
What is atomic layer deposition?
It is a form of CVD but uses two or more chemical precursors (often 2 - an active material and a reducing agent). The precursors react with the substrate one at a time in a sequential, self-limiting manner to grow a thin film
- performed with a laser (in shown setup) to vaporize the precursors
What are the advantages and disadvantages of atomic layer deposition?
advantages:
- high quality deposits
- deposition in 3D nanostructures
- precise control of the thickness of the deposit
disadvantages:
- slow technique
What are the steps of atomic layer deposition?
4 steps per cycle:
1. reaction products - create precursor A and absorb it on the substrate
2. pumping - remove all non-absorbed precursor A
3. add precursor B - B chemically reacts with precursor A
4. remove all non-absorbed B and side products
*repeat (A will then react with the AB product) to slowly build layer it layer by layer
What is anodization?
The electrochemical anodization process allows the formation of 3D morphology formation. In this process, the anodization occurs on the anode surface after applying a high voltage (>30 or up to >100V)
It creates self-ordered oxides and many other morphologies
What is sol-gel?
A chemical synthesis method ideal for creating ultrafine particles (3D), nanofilms (2D) and nanoporous membranes.
-suitable for ceramic and metal oxide powders
The process involves generation of a colloidal suspension (‘sol’), which is subsequently converted to a viscous gel and solid material
What is the process of sol-gel?
- add precursors (usually inorganic metal salts or metal organic compounds)
- hydrolysis & condensation occur to create the ‘sol’
- extraction:
a) can coat, gel, precipitate, or spin the solution to produce different forms (dense film, dense ceramic, aerogel, uniform powder, or fibers)
What are factors that influence the hydrolysis reaction for sol-gel?
- nature of the alkyl group
- nature of the solvent
- concentration of each species in the solvent
- temperature
- water to alkoxide molar ratio
- presence of acid or base catalysts
What are the pros and cons of sol-gel?
advantages:
- ideal synthesis of metal oxides or ceramics
- formation of high purity powders as a result of homogeneous mixing
disadvantages:
- alkoxides are expensive
- hydrolysis and condensation of molecular precursors have very high reaction rates. The aqueous chemistry of transition metal ions can be rather complicated because of the formation of a large number of oligomeric species depending on the oxidation state, the pH, or the concentration (you need to know what you’re doing)
What is wet chemical synthesis?
Routes used to synthesize nanoparticles based on the use of chemical precursors
these processes are attractive due to their simplicity, versatility, and availability of low cost precursors
*the inorganic salts used in wet chemical synthesis routes are more versatile and economic than the alkoxides employed in the sol-gel process
**this class focuses on polyol synthesis
How can you stabilize nanoparticles to prevent agglomeration or aggregation?
stabilize by:
- electrostatic mode - electrostatic repulsion prevents particles from agglomerating (add a bilayer of anions and a second layer of cations to prevent high electrostatic repulsion as they near each other)
-steric stabilization - attach a single bulky molecule to the surface (usually a P, N or S donor, alkyl thiols are common)
What are the steps of wet chemical synthesis?
- start with precursor salt (ex. PtCl4)
- dissolve salt in medium
- apply a reducing agent (ex. heat, solvent, other reagent)
- nucleation
- crystal growth (could be amorphous growth)
- precipitation (try to avoid this one with stabilization techniques) - end up with nanoparticles after stabilizing
What is polyol method for nanoparticle synthesis?
- a form of wet chemical synthesis
- polyols are reducing agents (an alcohol containing multiple hydroxyl groups - ex. glycerin, ethylene glycol)
**ethylene glycol is often used for the synthesis of NPs in colloidal solution
How is polyol performed?
- add salts + polyol agent + solvent
- heating/stirring/sonication (or a combination of 2 or all)
- nucleation
- crystal growth (or amorphous growth)
What are advantages of Polyol method?
- easy to scale-up
- particle size can be controlled (define size between 1-10 nm)
- very versatile (monometallics and bimetallics - may require additional step to form the bimetallic (add precursors for each metal stepwise not simultaneously))
- occurs at low temperatures
What is the 1 danger of ethylene glycol use in polyol method?
- need to control the pH since ethylene glycol has competing reaction pathways
pH < 7: creates protonated glycolic acid
pH > 7: creates glycolate (this is the desired stabilizing agent)
How does pH change polyol synthesis?
- higher pH will form smaller NP due to an increased amount of glycolate
- thus particle size can be controlled by varying concentration of base (NaOH)
How can NPs be transformed into heterogeneous catalysts?
- colloidal NPs can be deposited on supports and transformed into real heterogeneous catalysts (mixed with support, filtered, then washed with water)
- the support can be added during synthesis - one pot reaction. Filtering and washing also occur as in method 1
What is self-assembly?
- a good method for making 2D NPs
- relies on the self-organization of organic molecules (crystallization of sugar or salt when cooling a saturated solution)
- often used in biology for the development of complex functional structures
- require that the target structures be thermodynamically stable, thus they tend to produce structures that are relatively defect-free and self-healing
How does self-assembly work?
aggregation occurs either:
1. spontaneously
2. chemically - complementary binding of organic molecules and supramolecules
Substrate is often water - so polar part locks onto it while the non-polar part rejects it (causing the system to arrange itself such that the polar part sticks to the substrate)
- when the substrate is removed, the process has created a single monolayer of material which can be thickened by repeatedly dipping and withdrawing the sample
What is mechanical alloying-milling
- an extensively used method to produce nanocrystalline powders
- physical top-down approach
- put material in a drum which rotates and break up particles (can be done in 2 ways)
How does mechanical alloying-milling work?
2 methods:
1. stationary vertical drum with a vertical shaft that rotates with a series of horizontal impellers attached to it
- head-energy rotating drum in which 2 metals (A and B) are spun. Heavy steel or carbide balls are thrown against each other, trapping the squashing the metal particles. The particles flatten, weld, and then break up
What is cathodic contact glow discharge electrolysis?
- a top down approach where cathode material electro-erosion is caused due to high temperature spots at its surface
What is micromachining?
Using a cutting tool against a rotating work piece. Produces shreds, chips, and tangles when machining material has suffered extreme deformation.
*works best with a diamond tip for a cutting tool
What is focused ion-beam machining?
- a top down approach
- fires a beam of gallium ions from a liquid metal ion source. The ion is accelerated, filtered, and focused with electromagnetic lenses to give a spot size of 5-8 nm
- high-energy ions blast atoms from the surface, allowing simple cutting of slots and channels or the creation of more elaborate 3D shapes
- very high precision, but an extraordinarily slow process
What is lithography?
- a technique used to manufacture integrated circuits and microchips
- able to create patterns with a feature size down to a few nanometers
Can be done in 2 ways:
1. masked - use of a mask or mold to transfer patterns over a large area simultaneously (allows high throughput)
2. create patterns in a serial manner. Allows ultrahigh resolution but very slow (not used in mass production)
What is photolithography?
- a masked lithography method
- a light-sensitive polymer (photo-resist) is exposed to UV light to define a desired pattern. photo mask is set across the photo-resist and contains opaque features to prevent UV from passing through.
- the UV passes through exposed areas and breaks down the photo-resist. The broken down photo-resist is washed away after the photo-mask is removed to create areas void of photo-resist
What are the different methods of photolithography?
a) contact printing (photomask in contact with photo-resist)
b) proximity printing (photomask close to the photoresist)
c) projection printing (a lens is used to enhance the light after it passes through the photomask and onto the photo-resist - this allows for high-resolution patterns
*a) and b) are capable of making patterns as small as a few micrometers. c) is used for down to 40nm precision
What is electron beam lithograph and focused ion beam lithography?
- uses an electron beam or ion beam to serially hit the resist and break away the desired parts
- high precision (down to a couple of nm precision)
- but highly specialized and small-scale only
**unmasked lithography method
What is dip-pen nanolithography?
- uses a scanning probe in an atomic force microscope to heat, scratch, oxidize, or transfer substances tot he surface of a substrate for patterning nanoscale features
