Chapter 2 - Fabrication I: Top-Down vs. Bottom-Up

Question 1

Name the three common top-down approaches to fabrication of 2D materials.

Answer 1

Mechanical exfoliation (‘Scotch tape method’), liquid exfoliation and chemical delamination.

Question 2

How can the colors of 2D material solutions tell us something about the band gap of the material?

Answer 2

Dark colors means smaller band gap, due to higher rate of absorption, while clear colors mean high band gap as almost nothing is absorbed.

Question 3

What should the difference in surface energy of solvent and crystal be in chemical exfoliation, in order for the exfoliation to proceed as quickly as possible?

Answer 3

They should be as similar as possible. The change in enthalpy is related to the square of this difference, and the higher change in enthalpy, the higher the energy of exfoliation is, that is the harder it is to separate the layers.

Question 4

Which two methods can be used to intercalate layered structures with ions such as Li-ions?

Answer 4

Chemically (material is submersed in ionic solution, left typically for days for the intercalation to complete) and electrochemically (using the material as a cathode in a battery test system, usually takes a few hours).

Question 5

What is a potential consequence of intercalation, that can have unwanted effects on our 2D material?

Answer 5

The lattice structure can change.

Question 6

Which family of 2D materials exhibit ‘many-particle’ behavior, such as magnetism and superconductivity, and why?

Answer 6

Transition metal dichalcogenides (TMDC). Due to the interactions of the d-orbitals.

Question 7

What is epitaxial growth?

Answer 7

Crystal growth that grows the same way as the structure of the underlying substrate.

Question 8

What is van der Waals epitaxy?

Answer 8

Epitaxial growth where there are no chemical bonds to the substrate.

Question 9

How can one create a 2D material using a 3D growth process?

Answer 9

By using regular growth techniques, then disconnecting the 2D material from the substrate and transferring it to a supporting substrate where only van der Waals interactions exist between the material and the substrate.

Question 10

Which processes occur on top of the substrate during thin film growth?

Answer 10

Deposition, desorption, diffusion on surface, nucleation, cluster formation, edge absorption, diffusion along edge, diffusion across step.

Question 11

What are the steps in thin film growth? What variables are the steps dependent on?

Answer 11

1. Physisorption of molecules on substrate (flux, sticking coefficient, trapping)
2. Surface diffusion (surface temperature, surface energy)
3. Chemisorption
4. Nucleation (substrate temperature, deposition rate)

Question 12

Why would we want to heat the substrate upon thin film growth?

Answer 12

In order to ensure higher surface diffusion rates, so that we rather get few, but large, nucleation sites instead of many small, which would be the case when the surface diffusion is hindered.

Question 13

What different types of growth modes exist?

Answer 13

1. Frank van der Merwe (Layer-by-layer growth)
2. Volmer-Weber (Island growth)
3. Stranski-Krastanov (Strain induced 3D-growth)
4. van der Waals epitaxy (Only vdW-forces between layers)

Question 14

What is the requirements for the substrate material for vdW epitaxy on layered materials?

Answer 14

That they are inert (without dangling bonds) and stability at high temperatures.

Question 15

How is the material supplied during vdW epitaxy?

Answer 15

Through evaporation of solid or as gaseous precursors.

Question 16

How is the issue of high sublimation temperature of graphite at low pressures resolved in graphene growth?

Answer 16

We use a glassy carbon filament, which is a very poor conductor, and heat it through current heating.

Question 17

Describe the process of graphitization of silicon carbide (SiC).

Answer 17

SiC is heated in Ar or C atmosphere. The Si is evaporated, and a layer of graphene is formed.

Question 18

In CVD of graphene on metals, what are the typical catalysts? Name a few differences.

Answer 18

The typical catalysts are Cu and Ni. C atoms have bad solubility in Cu, which means that the growth is self-limiting. Ni on the other hand solves C pretty well, and depending on the amount of exposure graphite can be produced. The C floats to the top when cooled down.

Question 19

What are the carrier gases in CVD of graphene on metals, and what is the carbon precursor?

Answer 19

The carrier gases are typically Ar and H2. The carbon precursor is CH4 (methane).

Question 20

Name the process steps of graphene growth via CVD.

Answer 20

i) Catalyst substrate and gases are heated.
ii) Annealing step to clean/reduce the catalyst surface.
iii) Introducing precursor for graphene growth.
iv) Cooling down.

Question 21

Name four methods of CVD-like growth of TMDCs (especially MoS2).

Answer 21

i) Dip coating and sulfurisation: substrate is dipcoated in solution containing (NH4)2MoS4. Annealed in presence of sulfur-containing gas.
ii) Sulfurization on substrate with thin deposited Mo film: Mo samples are placed in chamber with a S-source. Heated so the S evaporates, and carried by inert gas so the Mo-films can be sulfurized.
iii) Hot Ar-gas carries evaporated S and Mo: sources of S and Mo (MoO3) are evaporated, deposited on a substrate.
iv) Metal organic CVD: using organic metal precursors (W(CO)6, Mo(CO)6 etc.) or metal chlorides (MoCl5, WCl5 etc.) with chalcogenide precursor (eg. dimethylselenide, (CH3)2Se).