Peer review of: Heterostructures formed through abraded van der Waals materials (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this manuscript, the authors stated that to fully exploit van der Waals materials and their 
vertically stacked heterostructures, a new mass-scalable production route, through the mechanical 
abrasion of bulk powders, was introduced. They successfully achieved significant performance 
improvements with the abraded heterostructures compared to ink-jet printing of nanocrystal 
dispersions prepared layers. Different functional heterostructures such as resistors, capacitors, 
photovoltaics, the creation of energy devices, and enhanced trioboelectric nanogenerator have 
been demonstrated. The results are exciting, but some fundamental problems should be stated 
clearer to make the work repeatable and convincible before the acceptance for publication. 

As many devices have been shown in the manuscript, I do not doubt that there is indeed some van 
der Waals material film on different substrates and functions very well if the experimental results 
are correct. However, as the title of this manuscript shows, the novelty of this work is the 
fabrication of heterostructures through abraded van der Waals materials. The mechanical abrasion 
is the key protocol to achieve all those high performances of various devices listed in the 
manuscript. Therefore, it is very important to have a good characterization and explanation of this 
abrasion process. 

1. As abrasion is a type of friction process, therefore, the whole procedure involves the substrate, 
the van der Waals material, the upper object used to carry out the abrasion, and the working 
conditions like the environment, load, sliding speed. All those parameters should be given in the 
MS or the supplementary information. It is too vague only state that “The PDMS pad is oscillated 
back and forth against the substrate with various 2D”. 
2. Since abrasion is so important to give a high performance of the various devices, the interface 
between the van der Waals materials and various substrates should be at a very good contact 
condition. How did this happen? The abrasion mechanism, friction process and mechanism should 
be scientifically characterized. Such as under different load, oscillating speed (amplitude and 
frequency?), how did this van der Waals material film attached well to the substrate in a controlled 
manner? Because different substrates have been used in different devices, the interaction at the 
interface is always different. What is the size of the PDMS pad, and what kind of PDMS materials is 
used? What is the elastic modulus or viscoelasticity of the PDMS? 
3. PDMS is coated in a bulk 2D material powder and used as a writing pad. How is it coated, what 
is the state of those 2D material powder stay on the PDMS substrate? Film thickness, conformation 
etc. ? 
4. As state in the manuscript, the thickness of the abraded films can be controlled by the abrasion 
time and the force applied to the pad used to write the materials on the substrate, adjustment of 
the film thickness via back-peeling with specialist tapes is also possible in order to tailor the 
transparency/resistance of a particular film. The mechanism of those abrasion related fabrications 
should be reasonably revealed as it’s the major contribution to the area of the nanofabrication of 
heterostructures with van der Waals materials. 



Reviewer #2 (Remarks to the Author): 

Freddie Withers and coworkers in their work “Heterostructures formed through abraded van der 
Waals materials” reported an simple and scalable approach to realise a variety of functional 
heterostructures based on van der Waals nanocrystal films produced through the mechanical 
abrasion of bulk powders, which appears to be interesting. The following are some remarks which 
need to be carefully considered: 

Comment 1. The authors claimed that the performance improvements can be obtained in abraded 
heterostructures compared to those fabricated through ink-jet printing of nanocrystal dispersions. 
However, the test condition should be considered, otherwise, it is meaningless. Thus, it is better to 
contrast the key performance parameter (e.g. photocurrent, photoresponsivity, response time, 
EQE etc for the photodetector) of heterostucture device with that of reported in literatures in a 
table which including the test condition (e.g. bias, light intensity etc for the photodetector). 

Comment 2. It was hard to find that the thickness of hBN in Figure S10 was 5 ± 1μm. Is it 
possible to give out a cross-sectional line profiles from the AFM top-views to display the thickness? 
or a SEM images? In addition, the measuring scale was blurry for the AFM maps in Figure S10. 

Comment 3. Please double check their manuscript carefully to avoid some typical mistakes and 
mistypes, just as example all authors’ names were given out in some references while others not. 

Comment 4. There are some grammatical and spelling mistakes, such as “whilst” and “at the same 
time” in line 6 on page 2 was the same meaning, “achiee” in line 11 on page 2, “works” in line 10 
on page 3, and so on, please revise them and check the whole text. 




Reviewer #3 (Remarks to the Author): 

Review of: Heterostructures formed through abraded van der Waals materials 

• General Remarks 

The authors present a deposition method for creating networks composed of layered crystal 
particulates. These networks are created by rubbing bulk crystals onto a substrate whereby the 
friction presumably shears off the smaller particles. The materials can be layered upon one 
another to create vertically stacked heterostructures with surprisingly clean interfaces. The authors 
create several different device types from such heterostructures to demonstrate the versatility of 
the technique. The TENG in particular is a very good demo. This is an interesting piece of work but 
the following needs to be addressed before publication here can be considered. 

• Primary Issues 

The main issue I have here is that no explanation of how the technique works is given anywhere 
and “abraded” isn’t defined anywhere in the text. Can the authors explain what the abrasion 
process is? Is the crystal shearing apart, basal fracture, etc.? What is the average size of the 
shorn/fractured crystallites (average length and thickness)? Are these comparable to exfoliated 
nanosheets? How or how not? What does the size distribution of the crystallites look like? 

There is no discussion of why the networks are so good, it’s implied that the reader will infer they 
are through the device demonstrations. At the very least, the conductivities of the various abraded 
materials could be given (it appears the dimensions are all known?) as a proxy for network quality. 
This would then give a ballpark to compare actually compare the networks to other exfoliation 
methods. And how do the in- and out-of-plane conductivities compare? The data appears to be 
already available from the in- and out-of-plane photodetectors. 

The deposition itself is described as a function of abrasion time and force. How is force quantified 
here? Is it just hand pressure? As these networks are binder-free, one can easily imagine that the 
networks could be wiped off the substrate by finger pressure, is that right? How much/little 
pressure is required to not affect the lower networks when building up a heterostructure? This 
seems critical. 

Am I right in saying that the thickness is controlled by making the networks thick and then using 
tape to peel material from the top? How is the quantity of peeled material controlled? Is the tape 
lightly lain on the network? Is it pressed down? How would one go about making a 100 nm 
network? What’s the average thickness for a single abraded line? 

What’s the problem with the graphite? The authors state that “We find direct abrasion of graphite 
onto TMDC’s and hBN frequently damages the barrier material, likely due to the different materials 
mechanical properties. While the reverse combination, e.g. TMDs on graphite are non-damaging.” 
The graphite also cannot be abraded on SiO2 while all other materials can. I know this is probably 
a question that requires a study to answer but could this be illuminated with a discussion of bulk 
crystal properties (interlayer binding energies, etc)? 

How is the bulk crystal applied to the PDMS writing pad? How are the “in-house” PDMS pads 
created? Do all crystals adhere similarly or uniformly? 

The structure of the planar photodetector in Figure 2 is not clear at all. What am I looking at in the 
optical image inset? Is the graphite layer split into a source and drain contact in the lower 
schematic? Unclear. In fact, the figures are heavily overloaded in general, in particular Figures 1 
and 2. I’d recommend moving some data to the SI. The inset images are frequently too small to 
make out anything. 

In Figure 2D, there is a ~ 1.5 order of magnitude difference between D3 and D1 & D2. Is this sort 
of device-to-device variability common across these devices? What’s the typical yield? Is the 
variation related to doing things by hand? 

The authors state “The first devices were found to yield responsivities up to 24 mA/W for WS2, 
constituting a ~103-104 improvement compared fully printed 2D-ink photodetectors.30” Hardly a 
fair comparison given the reference is a fully inkjet printed device and the structure presented 
here is completely different. Are there not more appropriate references with lithographically 
deposited contacts? 

The authors state “Moreover, residual solvent in the printed films has been shown to degrade the 
electrical properties of the devices by further reducing the quality of the interface between 
neighbouring nanocrystals11”. Does Ref 11 contain data showing the effect of residual solvent? I 
can’t see it in the main or supplementary figures. In fact, I’m unaware of such data in the 
literature so I’d be very interested in a reference showing such. 

It’s probably not worth referring to these networks as 2D films since all the properties are bulk 
crystal properties and not mono- or few-layer properties. Alsop6, for example, refers to “few-layer 
thick abraded graphitic channels”. What does few-layer mean in this context? Is this implying few-
layer graphene or a few abraded layers? How does one distinguish the number of “abradings” for 
the same material. Same for “bilayer” used throughout the text. These adjectives already have 
specific meanings in the context of these materials. 

Figure 1D, the dependent variable is resistance by units but the symbol for resistivity is used. 
“Resistivity” is used in the caption and “resistance” is used in the main text. Please clarify.