Peer review of: Acoustic phonon recycling for photocarrier generation in graphene-WS2 heterostructures (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This is an experimental work which demonstrates the existence of acoustic phonon recycling in a 
graphene-WS_2 heterostructure. The authors show that acoustic phonons transmitted from the 
semimetal graphene into the insulator WS_2 can generate conduction electrons in that insulator 
before they are transmitted into the substrate. 
The authors show that a simple thermal transport model is capable of predicting this effect. The 
theoretical foundations are solid and the arguments and evidence used to show that this is indeed 
phonon recycling and not some other phenomena, are convincing. Despite the potential impact, 
there are the following major concerns. 
1. To make sure that the phononic excitation is because of graphene phonons, it will need more 
information on phonon transport (spectral interfacial transmission, etc.) and photonic excitation in 
WS_2. Since WS_2 is a 2-D material, its properties (excitation, phonon, etc.) are significantly 
influenced by the spacer or interfacing materials. 

2. The graphene and WS_2 contact may not be ideal planar, especially with the inserted h-BN or 
other spacers [Figure 2(a)]. Acoustic phonons generated in graphene may not be transported if 
the contact is not sufficiently good. 

3.Several figures seem to be missing and some figure captions do not look accurate either. 
Figure 1. (d) is missing although the caption is provided and cited in line 93 on page 5. Figure 1 
caption title ‘Experimental realization and results’ are not correct (since ‘results’ are not provided’) 
Figure 2- 2(b) and 2(e) are missing. The title of Figure 2 caption contains, ‘modeling’, but the 
presented figures are not about modeling. 
Figure 4(a) is incomplete [x- and y- axis labels are missing, and it seems to have more figure 
pane(s)] 
Their writing needs improvement: 
p2. Line 36: in ‘… hot carriers to electrical energy…’ => I think, ‘electrical energy’ should be 
phonon energy, 
p.8, line 164: the general exfoliated monolayer WS_2 is n doped’ -> with what elements is WS2 
doped? 
p. 17: clarify -> clarification 

4. It would be more beneficial to provide more guidance how the presented phonon recycling can 
be applied for high-efficiency electronic and optoelectronic applications. 

5. There have been a number of theoretical studies focused on acoustic and optical phonon 
recycling for the reduction in heat generation or increase in device efficiencies, but very little 
experimental evidence supporting these efforts, making this a notable and important work. 
However, It would increase the impact of the study if more connections were drawn to the many 
theoretical efforts to realize phonon recycling, emphasizing the importance of this work. For 
Example, Applied Physics Reviews, 6, 021305, 2019; Phys. Rev. B, 96, 205444, 2017; Physical 
Review B, 91, 85301, 2015; Journal of Applied Physics, 114, 083710-1-9, 2013; Applied Physics 
Letters, 95, 074103-1-3, 2009. 

6. The writing is generally poor, making it difficult to follow to authors in many places. Some 
Figures also appear to be missing. With minor revision, primarily to the quality of writing, I would 
recommend that it is published. 

Based on the above, the manuscript and its results have merit but not acceptable in the current 
form. Major revisions are needed and the manuscript should be reviewed again and assessed after 
revision. 


Reviewer #2 (Remarks to the Author): 

The authors report a new acoustic phonon recycling (APR) process in graphene-WS2 
heterostructures using transient absorption microscopy. APR can couple the lattice heat generated 
in graphene back into the carrier density in WS2. The APR mechanism is found to have long 
characteristic time (>100 ps) and broadband response (from around 1.8 to 0.5 eV; below the 
energy gap of WS2). The authors also find that the APR process can be well described by a 
thermal transport model at the graphene-WS2 interface. The observation of the TA signal after a 
long period of excitation in this work is interesting and novel in graphene-WS2 heterostructures. 
The APR process and relevant heat management are important to the 2D materials community as 
well as optoelectronic device applications. However, the authors haven’t considered the crystal 
lattice misalignment and energy band mismatch between layers in the heterostructure. The 
mismatch can affect the heat and charge transport between layers. Some of the experiment 
details are not clear. Provided the authors address several questions and concerns below, I will 
consider the manuscript to be published in Nature Communication after revision. 

1. The pump photon energy (i.e. 0.48 eV) used in this work is in the sub-bandgap regime, which is 
far from the WS2 bandgap of 2 eV. Does the selected excitation energy depend on the twist angle 
between graphene and WS2? Energy band misalignment between graphene and WS2? What are 
the criteria the authors apply to determine the pump photon energy? 

2. The twist angle between layers (graphene-WS2, graphene-hBN, hBN-WS2, WSe2-WS2) should 
alter the thermal transport between layers, thereby the phononic excitation mentioned in this work. 
How do the authors manage to account for the effect of layer misalignment on the observation in 
the manuscript (i.e. Fig. 3)? 

3. The authors chose graphene-WS2 heterostructures in this work. However, the APR process 
seems to work in a different heterostructure consisting of graphene and other transition metal 
dichalcogenides (TMDs such as WSe2, MoS2). Could the authors explain the motivation of picking 
up WS2 but not other TMDs? 

4. Following Q.3, does bilayer (2L) or even trilayer (3L)-WS2 affect APR in their heterostructures 
with graphene? It has been reported that the characteristic time of electronic excitation in 
graphene-2L-WS2 heterostructures is in the same order as in graphene-1L-WS2 heterostructures. 

5. As presented in Fig. 2(a), there are many green spots in the heterostructure regions (graphene-
WS2 and graphene-hBN-WS2), suggesting poor interfaces between layers. The interface quality in 
heterostructures should affect the thermal contact resistance and charge transfer between layers. 
How do the authors rule out the effect of interface quality on the TA feature >> 20 ps? 



Reviewer #3 (Remarks to the Author): 

The manuscript describes an experimental study on the transient absorption of van der Waals 
heterostructures made of graphene/WS2 (without and with hBN or WSe2 spacer layers). The 
authors discuss the charge, energy and heat transfers in this layered system. 
In particular, they describe an original signal that appears on a longer timescale than the well 
studied direct charge/energy transfer of carriers at the interface. They claim that this optical signal 
is related to a mechanism they refer as “phonon recycling”. 
Although I support their discussion and modeling for the prior steps of this mechanism: electron 
thermalization in graphene, thermal transfer to the graphene lattice and subsequently to the WS2 
lattice, I am not convinced by the final step: “re-excite the electron-hole pairs in WS2”, which is 
basically the “phonon recycling” at the heart of the paper. 
My claim is that the observed signal can be explained considering only thermal effect in WS2. 
At stake is the discussion at pages 6-7 when the direct lattice heating effect on the transient 
absorption is ruled out. As it is stated, the experimental signal is very similar to a derivative signal 
(fig S8) that would originate from spectral shift induced by the increase in temperature only. If it 
were to be the creation of excitons, the transient would be similar to the signal at short timescale 
(fig2d or figS5, naming a bleaching at A0 peak). Therefore, I am not convinced by the qualitative 
and vague arguments used to quickly rule out the direct temperature effect. 
I strongly advise the authors to be quantitative here: their model allows to extract the lattice 
temperature of WS2 for the experimental fluence, therefore they could simulate the expected 
change in absorption induced by such heating (using for example data from ref 44, naming the 
absorption spectra as function of temperature). 
I would be happy to see a revised version which includes such quantitative discussion. 
At the moment, I would recommend NOT to publish the manuscript in Nature Communications 
unless this previous point is correctly answered. 
If no better evidence for a “phonon recycling” rather than simple lattice heating can be shown, 
then the manuscript should be heavily modified to present this narrower claim. It would eventually 
transform into an interesting, well documented, study on thermal management in 2d 
heterostructures, which might however not be up to the Nature Communications standards.