2D Transport Lab @TU/e

In the last part of this introduction to CDW, we look at various ways of detecting charge density waves.

In the second part of this introduction to CDW, we take a look at the Fermi Surface Nesting condition often used to describe the origin of charge density waves. This however doesn’t hold in 2D.

This is the first part on the introduction to CDW in 2D materials. We start off this episode with the introduction to charge density waves, its history and mean field derivation in 1D. 

This is the third part of the first episode of the series: “An Introduction to 2D materials”. In this video, I finally discuss the various transfer methods used to make van der Waals heterostructures. 

This is the second part of the first episode of the series: “An Introduction to 2D materials”. In this video, I start by introducing the exfoliation of 2D materials onto polymers. We finish off this section by exploring various ways of exfoliation. 

In the first episode of the series: “An Introduction to 2D materials”, I begin talking about the various exfoliation and transfer methods. 

This episode is separated into several parts. In this part, we discuss the field in general and how it all began. It is fitting to also introduce the original methods of exfoliating 2D materials, which is still being used to this day. 

Exfoliation and transfer of 2D materials using polydimethylsiloxane (PDMS). This is based on the original work by Andres Castellanos-Gomez, who built the original setup in TU Delft.

PDMS is a widely versatile material which can easily be moulded into any shape. Just like other resins, you mix the base to the curing agent (in a 10:1 ratio – the ratio influences the stickiness and the mechanical properties of the resulting PDMS film) and cure it. Depending on the model (i.e. Sylgard 184), the curing temperature and duration may be different. 

One can also find pre-made PDMS films from gelFilm. They come in three categories depending on what type of other films are adjacent to the PDMS, and each of these are further diversified by the thickness, and retention levels (tack).

As many other aspects in life, each method of 2D exfoliation&transfer have their pros and cons. PDMS residues are notoriously difficult to clean and getting monolayers is quite difficult (although not impossible with graphene) with this method, but if one can’t afford to use solvents or thermal annealing, this is perhaps the best way. Exfoliation of 2D materials by direct exfoliation onto SiO2/Si has been the go-to way to isolate ultra clean monolayers. However, this approach is not always applicable, depending on the situation. For example, if one needs to suspend the 2D material into a resonator geometry (as I have done for many of my PhD works), doing direct exfoliation – with low probability that the perfect flake lands on the desired spot – and polymer transfers – which require further solvent/thermal treatments – become quite challenging to implement. In this case, transferring with PDMS is the best way.  

On the other hand, if the cleanest and flattest (no wrinkles or topographic undulations) flakes are required – for example, graphene encapsulated in hBN for quantum transport – then this is perhaps not the best solution. The residues and wrinkles and pockets of air can cause not-so-ideal interfaces to form.

There are several factors to consider when doing the PDMS transfer in a clean way. In my opinion, I find that home-made PDMS (using Sylgard 184 in 10:1 ratio) is much cleaner than gelfilm. However, I had much better yield of monolayer graphene, MoS2, WSe2, etc. using gelFilm 4x because of its tack. In this video, I am using the home-made version of the PDMS. I made it many months ago by rigorously mixing the two components in a 10:1 ratio, degassing multiple times in a nalgene desiccator and letting it cure naturally by itself in ambient conditions (in a cleanroom) over 48 hours. By letting the liquidy PDMS solution settle, we get flat surfaces on the top and bottom.

To clean off the film of residues from the surface of PDMS, one can use a fresh tape and slowly peel it away. [Similar to this demonstration of a ball with strings on top and the bottom, if you slowly peel the tape away, it will pick up things with it. If you violently peel the tape away, then the 2D material will separate along the van der Waals interface, leaving thin flakes on the PDMS.] This method is just as good as sonication in solvents in my opinion, but much faster to implement. To further clean the PDMS residues, one can use UV Ozone cleaners. One might be tempted to substitute the ozone cleaner with a O2 plasma cleaner, but I do not recommend this, as it can create cracks to form on the surface of the PDMS. 

Perhaps in the next episode, I can give an overview of the various exfoliation and transfer methods.