Part 5 - Fabrication 3 Flashcards
(42 cards)
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
