Part 5 - Fabrication 3 Flashcards
What is the fundamental problem in producing colloidal most materials?
The thermodynamically stable state of metals, semiconductors and polymers is bulk material, not colloidal particles.
What can be done to improve the stabilisation of colloids?
Stable colloidal dispersions require an interfacial stabiliser, which is a chemical that reduces the interfacial free energy between the particle and the solvent and makes short range forces between the particles repulsive.
What is the stabiliser for gold nanoparticles and how does it act as a stabiliser?
Citrate ion. Its negative charge is opposite to that of positive gold ions on the particle surface. The excess negative charge due to adsorption of citrate on the surface of the particles makes the particles repel one another.
What is a stabiliser?
It is often a surfactant, which is often a chemical compound such as sodium dodecyl sulphate (SDS) whose structure has one end that is chemically attracted to the particle and the other end chemically attracted to the solvent. NO surfactants in our gold NP and polystyrene latex preparations.
How do you stabilise polystyrene latex?
Charge stabilised. Dissociation of a fraction of the sodium ions of the sodium 4-styrenesulfonate units of the polymer leave the particles with a negative charge.
What reaction synthesises gold nanoparticles?
Reduction reaction.
What controls nanoparticle nucleation and growth?
Melt salt, solvent, REDUCING AGENT, stabiliser
What controls the nanoparticle shape/size?
Metal/stabiliser ratio (need good M/S ratio to have good control over the size of the product), growth temperature, growth time.
Give advantages for mono dispersed QD synthesis
High yield
Size control
Monodispersed
Shape control
Detail QD synthesis by hot injection
The energy barrier has to be overcome initially, Large injection of sample. Following this, nucleation growth with Ostwald ripening (smaller crystals dissolve whilst larger ones will grow).
- The requisite supersaturation and subsequent nucleation can be triggered by rapid injection of metal-organic precursors into a vigorously stirring flask containing hot (150-300 degC) solvent stabiliser
Why do QDs change colour with size?
Additional energy is required to “confine” the semiconductor excitation to a smaller volume.
What is electrohydrodynamic atomisation (EHDA)?
At a critical point, the so-called Taylor cone develops, with a fine stable jet issuing from its tip. The formation of such cones and jets is essential for the production of an electrohydrodynamic spray
Give details of EHDA deposition method
Uses a high voltage to produce NPs from clear solutions, polymeric solutions and melts.
SEE DIAGRAM
What is coulomb fission?
Original droplet explodes creating many smaller, more stable droplets.
What is 3-D electrospinning?
Uses a high voltage to draw nanofibers.
What is Plasma Assisted synthesis?
Vacuum arc deposition is well-established process for production of thin films and nanoparticles.
Initiation of an arc by contacting a cathode made of a target material. An igniter is attached to an anode in order to generate a low-voltage, high current self sustaining arc.
The arc ejects ions and material droplets from a small area on the cathode. Further, the ions are accelerated towards a substrate while any large droplets are filtered out before.
Plasma does 2 things: heats and charges.
Give the 3 basic steps of lithography
Coat Si with photoresist
Shine light on the photoresist
The resist has changed
How do you form nanoparticles by atomisation of solution ?
Usually with an orifice with high pressure drop
Give details of EHDA
Electrohydrodynamic atomisation (EHDA) uses a high voltage, high pressure drop and orifice for atomisation. Because of the high voltage, electrostatic forces overcome the surface tension of tiny liquid droplets.
What is electro spray ionisation?
Produces charged droplets, when they shrink below a size, they undergo coulomb fission, leaving nanoparticles
Give the modes of electro spray and the causes of them
No flow rate
Dripping - Low flow rate
Unstable cone-jet - Either flow rate or voltage unstable
Multi-jet - too high electric charge
Irregular instabilities - Either flow rate or voltage unstable
Stable cone-jet - THIS IS WHAT WE WANT.
Give applications for Electrospray
Formation of organic nanoparticles (e.g. drugs)
Space - thruster
Mass spectrometry
As long as voltage is high enough to overcome the surface tension of the liquid.
Give advantages/disadvantages of electro spray for DEPOSITION OF NANOPARTICLES
Advantages
Inexpensive (Maria made system in the lab)
Operation at atmospheric conditions
Uniform coatings
Disadvantages
Hard to scale up - many charged nozzles next to each other leads to instabilities
Clogging of orifice - Require very small diameters to work.
Small choice of systems
Slow production
Give advantages/disadvantages for Electrospray in mass spectrometry
Advantages
Good sensitivity
Easily adopted for liquid chromatography
safest ionisation method
MAIN POINT: IT WORKS WELL FOR MASS SPECTROMETRY
Disadvantages
Presence of salts can reduce sensitivity
Complex mixtures can reduce sensitivity
What is electrospinning
The only technique that allows the fabrication of continuous fibers with diameters down to a few nanometers. In polymeric solutions, the polymer concentration is high enough that instead of making droplets, you make fibers.
By moving the x,y,z axis, you can create 3D patterns
Give the four general formation mechanisms for 3D electrospinning
Multilayering, Folding/stacking, 3-D template, self-assembly.
Give advantages/disadvantages to electrospinning
Advantages Inexpensive Operation in atmospheric conditions Possibility to control fibre morphology Practically all kinds of polymers with high enough MW can be processed by electrospinning (just need the correct viscosity)
Disadvantages
Used solvents can be toxic
Many variables
What is microfluidic nanoparticle synthesis?
Microfluidic is a science of designing and manufacturing devices in a microscale
Usually dealing with nanoliters or picoliters
Low reagents and energy consumption
Laminar flow
Surface forces are dominant over volume
Capillarity and interface phenomena are present over inertial and gravitational phenomena
Mass transfer is dominated by diffusion
(makes colloidal particles)
Give the main key advantage for electrospinning and microfluidic synthesis techniques
They have the ability to achieve scale up, which is a big problem in the nano field industry.
Give the methods of biological synthesis of nanoparticles
Reduction (bacteria)
Biosorption (fungi)
Enzyme mediation (plants)
Give the disadvantage of using natural processes for NP synthesis
natural process, very slow.
What are the methods of carbon nanotube (CNT) synthesis?
Arc vaporisation
Laser vaporisation
CVD
Give details about arc vaporisation for CNT synthesis
During this process, the carbon contained in the negative electrode sublimates because of the high temperature caused by the discharge. Because nanotubes were initially discovered using this technique, this is the most widely used method.
Can make single or double walled CNTs.
If you have a low voltage, high current, you can heat up your material and start evaporating.
Give advantages for arc vaporisation for CNT synthesis
Advantages
Good yield
Produces both single and double multi walled nanotubes
Disadvantages
CNTs are very short (~50 um)
What are CNTs hollow?
Because of their synthesis methods: extrusion and tip growth.
SEE DIAGRAM
What are methods for graphene synthesis?
Scotch tape
CVD
Epitaxial growth
Electrochemical exfoliation
What are methods for nanowire synthesis?
Electrodeposition
Vapour-liquid-solid (VLS) CVD
Solution phase
Give details about nanowire synthesis
Anisotropic growth of nanowire (not equally in every direction)
Using nanoparticle catalysts and gas-phase precursors.
Define SAMs
Organic assemblies that are formed spontaneously by the adsorption of molecular constituents from solution or gas phase onto a substrate with a specific affinity of its heat group.
Give the different interactions between adsorbate and substrate
Physiosorption - weak bonds (Van der Waals forces)
Chemisorption - Strong, covalent bonds
Other - e.g. hydrogen bonding.
What does SAM stand for?
Self-assembled monolayer
Give SAM applications
Prepare functional films
- lubricants for hard discs
- corrosion protection
- photo patterning
- electronic devices
