Peer review of: Exciton-driven change of phonon modes causes strong temperature dependent bandgap shift in (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Referee report on the manuscript “Exciton-driven shift of phonon modes causes giant temperature 
dependence of the bandgap in nanoclusters” by Muckel et al. 

The authors measure and calculate the temperature dependence of the band gap of magic sized CdSe 
nanoclusters. They find a strong temperature dependence, increased by a factor of two compared to 
bulk. They compare their experimental results with theory and obtain excellent agreement. They 
conclude that the strong increased temperature dependence originates from a excitonic effect. 

The manuscript is generally well written and kept rather short and focussed, which is appropriate for 
the journal. I have the impression that the experimental work is solid and the theory is defendable, 
although a few questions arise summarized bellow. I enjoyed reading the manuscript and I truly 
appreciate the topic which is simple and fundamental: temperature dependence of the band gap. 

My main concern with the work is about the conclusions driven and the big claims made. A very small 
CdSe NC has a stronger (by a factor of 2) temperature dependence than bulk CdSe. Which is as such 
not astonishing, at which point the word “giant” claimed by the authors may be questioned. The more 
important and more intricate point is about the excitonic origin of the effect. I would challenge this 
statement as well. If I would calculate the temperature dependent band gap in these small cluster 
using an approach that uses no exciton. Using for instance the AHC approach or a frozen phonon 
method, i.e., the change of the electronic eigenvalues resulting from the phonon (instead of the 
change of frequency resulting from the changed electronic occupation, as the authors do), I would 
presumably get the same results, if we all believe in Brook’s theorem which makes this connection. 
Now Brooks theorem is for bulk and the authors use it for a very small NC and change the occupation 
drastically, which may seem in contradiction to the original Brook work. I could be convinced by the 
importance of the excitonic effect if the results of AHC or frozen phonon would be significantly 
different (and in disagreement with experiment) than the results of this work (in agreement with 
experiment). This would be very important and fundamental and would deserve a Nature 
Communication publication. At the moment I cannot see a convincing evidence for it. 

A few more technical points: 

1) The title should reflect that the results are for CdSe and not for “nanoclusters”. 
2) Line 45, in which temperature range? 
3) Line 59, how can you have 2% Mn in a 26 Atom NC? 
4) Is it maybe interesting to talk about the fact that the HOMO-LUMO transition is forbidden? Or is this 
known already? 
5) Line 133. Please add a short note why MD overestimates the occupation of higher energy phonon 
modes. 
6) Why is in Fig.3b the phonon DOS starting at 0? For such a small NC, there is a phonon gap and no 
very low frequency phonons. Also in the supplementary. It looks to me as there are some problems 
with the phonon-DOS? 



Reviewer #2 (Remarks to the Author): 

In the manuscript presented here, Bacher and coworkers investigate the temperature dependent 
bandgap of (CdSe)13 clusters. 
Combining theoretical and experimental approaches, they show the importance of phonon modes on 
the temperature dependent bandgap of such magic sized clusters. The conclusions are sound and the 
calculations are novel, and thus, this work will be impactful to the scientific community. 
The manuscript is well written, and the conclusions are well supported by data and theory, and I 
believe the manuscript can be published with only minor changes. The manuscript could benefit from a 
brief introduction into the field, at present the authors dive right into the field and it may be beneficial 
to have an overview of what has been done in this direction previously.