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MS3 > Introduction To Nanomaterials > Flashcards

Introduction To Nanomaterials Flashcards

1
Q

Describe pyrolysis:

A

Also known as injection CVD, works by aerosol or gas being injected (carbon containing compounds) and passing through a furnace with laminar flow over a catalyst. CNTs are then built up layer by layer by either tip growth or root growth.

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2
Q

Describe laser ablation:

A

Ablation of graphite forms CNTs. High power laser vaporises carbon. Then reforms as CNTs and condenses of wall of flow tube. Furnace ≈1200ºC

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3
Q

Describe arc discharge:

A

Between two graphite electrodes causes CNTs to form. Electrodes in deionised water to prevent oxygen from reacting and too cool. Electrodes connected by DC. Carbon sublimes from anode and is deposited at cathode as CNTs. No catalyst makes MWCNTs.

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4
Q

Different growth parameters for CNTS

A 
Presence of metal catalyst (MW in any process except arc discharge)
Catalyst size.
Catalyst interaction.
Heat
Time
Catalyst spacing
Thin films for carpet growth.
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5
Q

What are Stone-Wales transformations?

A

A defect in CNTs where the regular lattice of hexagons broken two heptagons and pentagons forming.
The bonding between two hexagons rotes 90º to form pentagons and heptagons.

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6
Q

How is CVD graphene formed?

A

Carbon rich input.
Ni or Cu catalyst
Lamianr floe over catalyst in furnace.
As cools cyrtsallises on substrate.

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7
Q

How is graphene produced by chemical exfoliation?

A

Graphite mixed with peroxyacetic acid and sulphuric acid at RT. On standing acids break layers apart. Wash and dry to remove sheets of graphene.

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8
Q

What is La Mer theory?

A

The theory that nucleation and growth are separate. A homogeneous nucleation occurs when free energy is in excess, then the growth occurs by diffusion in the surrounding solution adding layers.

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9
Q

What are the limits of La Mer model?

A

Only describes nucleation process, cannot predict size or characteristics of nanoparticles.
Limited by solubility of precursor.

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10
Q

Can a C60 in BN exist?

A

No as penatgons are needed for positive curvature in C60 which would result in N-N and B-B bonds which are not stable.

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11
Q

If m=n then what are the properties of a SWCNT?

A

It is metallic.

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12
Q

If n-m is a multiple of three what are the SWCNT properties?

A

It is quasi metallic.

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13
Q

What are all other SWCNTs?

A

Semiconducting.

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14
Q

How to calculate diameter of CNTs?

A

d=a/π√(n^2+nm+m^2)

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15
Q

What types of defects exist in 2D carbon nano materials?

A

Valency defects (one or more C missing)
Line defects (where different orientations grow on a plate).
Out of plane atoms (lie outside of plane from Stone-Wales transformation).
Foreign atoms
Substitutional impurities (dopants).

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16
Q

What information can be extracted from Ramen Spectroscopy?

A

Chemical structure, phases, polymorphs, crystallinity and molecular interactions.

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17
Q

RBM appears where in Ramen?

A

For SWCNTs due to change in diameter.

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18
Q

G band in Ramen

A

Sp2 hybridised C atoms and is from tangential vibrations of C atoms. Dependent on chirality.

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19
Q

D band in Ramen

A

Caused by sp3 hybridised atoms, intensity is inversely proportional to quality.

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20
Q

G’ Band (2D) in Ramen

A

Caused by existence of double electron phonon mechanism. Intensity means defects.

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21
Q

What do some (n,m) indices cause peaks on photoluminescence while others do not?

A

Some are metallic so do not show peak as no band gap, whereas semiconducting show peak due to band gap.

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22
Q

Difference between thermal transport on macro and nano scales.

A

On macro scale conduction is propagated by diffusion, generally nano materials are more conductive.
On macro scale conductivity dependent on microstructure, whereas nanoscale dependent on geometry.

23
Q

What kind of MWCNTs (structure) form from pyrolysis and arc discharge?

A

Pyrolysis - spaghetti

Arc Discharge - straight CNTs

24
Q

What curvature do heptagons and pentagons cause in a graphene layer?

A

Pentagons cause positive curvature.

Heptagons cause negative curvature.

25
Q

Describe MBE or VLS Epitxay.

A

1D crystal growth assisted by a metal catalyst, resulting in whiskers, rods and wires.

26
Q

What are the three proposed mechanisms for MBE?

A

Oberlin - tip growth
Baker - C diffusion inside
Baker & Waite - root mechanism.

27
Q

What kind of CNTs result from using a metal-organic catalyst?

A

A large amount of clean MWCNT arrays.

28
Q

What kind of CNTs result from using a boron catalyst in Arc discharge?

A

Longer CNTs

Selected chiralities

29
Q

What kind of CNTs result from using a boron catalyst in CVD?

A

B containing fibres, no CNTs

Increased oxidation resistance

30
Q

What kind of CNTs result from using an oxide nanoparticle as a catalyst?

A

Spiral growth of MWCNTs

Exhibit 5 and 7 member rings.

31
Q

What kind of CNTs result from using an N-doped system catalyst?

A

Arrays of MWCNTs
Bamboo structure
An amorphous coating on catalyst tip.

32
Q

How can growth of CNTs be controlled by substrate?

A

Laser patterned substrates yield nanotubes of uniform length and diameter.

33
Q

Different dopants change what about CNTs

A

Can change the morphology even within the same CNT

Can change oxidation resistance of CNTs

34
Q

How can thermogravimetric analysis be used to compare oxidation resistance of doped CNTs

A

TGA measures the mass change over time. As materials are heated in air they combust and turn to ash so there is a reduced mass of combustion as they are oxidised..

35
Q

Examples of destructive characterisation techniques.

A

TGA, atom probe.

36
Q

Examples of non-destructive characterisation techniques.

A

Raman, IR, BET

37
Q

Advantages of CVD graphene.

A

Relatively cheap
Large area sheets
Scalable

38
Q

How can pressure vary CVD graphene polycrystallinity?

A

Low pressure strongly influenced by the crystallography of the substrate.

39
Q

Where is the sweet spot in the CVD furnace for high-quality graphene?

A

Centre of the furnace.

40
Q

Why does graphene quality vary along the length of the furnace in CVD?

A

Impossible to have uniform T along the length of the furnace and T plays a big role in CVD of graphene.

41
Q

What is the quality of graphene like when it is CVD grown on a liquid copper substrate?

A

Very uniform with little mismatching.

42
Q

What steps are involved with graphene transfer off of a substrate?

A 
Bubble
Separate
Transfer
Remove PMMA
Hot acetone
Transferred
43
Q

Problem with the common graphene transfer method?

A

Positive protrusions on the graphene surface are created.

44
Q

What does epitaxial growth mean?

A

Crystal growth/material deposition whereby new crystalline layers are formed in a well-defined orientation, with respect to the crystalline substrate.

45
Q

Describe the structure of c-BN

A

Diamond-like structure with B connected to 4 N and N connected to 4 B.

46
Q

Describe h-BN structure.

A

Graphene-like structure with B connected to 3 N and N connected to 3 B.

47
Q

What property does h-BN have that is different to graphene?

A

It is insulating.

48
Q

Is it possible to make h-BN nanotubes?

A

Yes

49
Q

What kind of structures do h-BN nanotubes have?

A

The same as carbon nanotubes.

50
Q

What form can defects take in h-BN?

A

Must be square cannot be pentagons due to bonding of B and N.

51
Q

Give an example of a chalcogenide nanomaterial

A

WS2 can form nanotubes.

52
Q

Is it possible to build layered structures with layers being different single layered materials?

A

Yes if VdW interactions between layers, stacking can occur.

53
Q

Why do nanoparticles have a lower melting point with a decreasing radius?

A

Surface atoms have lower surface energy so lower melting point due to greater SA to volume ratio

54
Q

How can the photoluminescence (PL) of SWCNTs be used to determine chirality?

A

Metallic chirality has no PL as no separation between conduction band and valence band. This means holes are immediately filled so no emissions.

MS3 (5 decks)

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