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SB.Nano > Part 5 - Fabrication 3 > Flashcards

Part 5 - Fabrication 3 Flashcards

1
Q

What is the fundamental problem in producing colloidal most materials?

A

The thermodynamically stable state of metals, semiconductors and polymers is bulk material, not colloidal particles.

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2
Q

What can be done to improve the stabilisation of colloids?

A

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.

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3
Q

What is the stabiliser for gold nanoparticles and how does it act as a stabiliser?

A

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.

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4
Q

What is a stabiliser?

A

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.

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5
Q

How do you stabilise polystyrene latex?

A

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.

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6
Q

What reaction synthesises gold nanoparticles?

A

Reduction reaction.

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7
Q

What controls nanoparticle nucleation and growth?

A

Melt salt, solvent, REDUCING AGENT, stabiliser

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8
Q

What controls the nanoparticle shape/size?

A

Metal/stabiliser ratio (need good M/S ratio to have good control over the size of the product), growth temperature, growth time.

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9
Q

Give advantages for mono dispersed QD synthesis

A

High yield
Size control
Monodispersed
Shape control

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10
Q

Detail QD synthesis by hot injection

A

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
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11
Q

Why do QDs change colour with size?

A

Additional energy is required to “confine” the semiconductor excitation to a smaller volume.

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12
Q

What is electrohydrodynamic atomisation (EHDA)?

A

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

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13
Q

Give details of EHDA deposition method

A

Uses a high voltage to produce NPs from clear solutions, polymeric solutions and melts.

SEE DIAGRAM

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14
Q

What is coulomb fission?

A

Original droplet explodes creating many smaller, more stable droplets.

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15
Q

What is 3-D electrospinning?

A

Uses a high voltage to draw nanofibers.

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16
Q

What is Plasma Assisted synthesis?

A

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.

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17
Q

Give the 3 basic steps of lithography

A

Coat Si with photoresist
Shine light on the photoresist
The resist has changed

18
Q

How do you form nanoparticles by atomisation of solution ?

A

Usually with an orifice with high pressure drop

19
Q

Give details of EHDA

A

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.

20
Q

What is electro spray ionisation?

A

Produces charged droplets, when they shrink below a size, they undergo coulomb fission, leaving nanoparticles

21
Q

Give the modes of electro spray and the causes of them

A

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.

22
Q

Give applications for Electrospray

A

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.

23
Q

Give advantages/disadvantages of electro spray for DEPOSITION OF NANOPARTICLES

A

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

24
Q

Give advantages/disadvantages for Electrospray in mass spectrometry

A

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

25
Q

What is electrospinning

A

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

26
Q

Give the four general formation mechanisms for 3D electrospinning

A

Multilayering, Folding/stacking, 3-D template, self-assembly.

27
Q

Give advantages/disadvantages to electrospinning

A 
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

28
Q

What is microfluidic nanoparticle synthesis?

A

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)

29
Q

Give the main key advantage for electrospinning and microfluidic synthesis techniques

A

They have the ability to achieve scale up, which is a big problem in the nano field industry.

30
Q

Give the methods of biological synthesis of nanoparticles

A

Reduction (bacteria)
Biosorption (fungi)
Enzyme mediation (plants)

31
Q

Give the disadvantage of using natural processes for NP synthesis

A

natural process, very slow.

32
Q

What are the methods of carbon nanotube (CNT) synthesis?

A

Arc vaporisation
Laser vaporisation
CVD

33
Q

Give details about arc vaporisation for CNT synthesis

A

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.

34
Q

Give advantages for arc vaporisation for CNT synthesis

A

Advantages
Good yield
Produces both single and double multi walled nanotubes

Disadvantages
CNTs are very short (~50 um)

35
Q

What are CNTs hollow?

A

Because of their synthesis methods: extrusion and tip growth.

SEE DIAGRAM

36
Q

What are methods for graphene synthesis?

A

Scotch tape
CVD
Epitaxial growth
Electrochemical exfoliation

37
Q

What are methods for nanowire synthesis?

A

Electrodeposition
Vapour-liquid-solid (VLS) CVD
Solution phase

38
Q

Give details about nanowire synthesis

A

Anisotropic growth of nanowire (not equally in every direction)
Using nanoparticle catalysts and gas-phase precursors.

39
Q

Define SAMs

A

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.

40
Q

Give the different interactions between adsorbate and substrate

A

Physiosorption - weak bonds (Van der Waals forces)
Chemisorption - Strong, covalent bonds
Other - e.g. hydrogen bonding.

41
Q

What does SAM stand for?

A

Self-assembled monolayer

42
Q

Give SAM applications

A

Prepare functional films

  • lubricants for hard discs
  • corrosion protection
  • photo patterning
  • electronic devices

SB.Nano (8 decks)

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