Peer review of: Integrated near-field thermo-photovoltaics for heat recycling (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
This manuscript reports measurements of heat-to-electricity conversion via integrated near-field TPVs, 
where the high-temperature emitter and low-temperature photodetector are separated by vacuum 
gaps from ~500 nm down to ~100 nm. The contribution of evanescent modes is quantified by 
comparing experimental data against theoretical predictions based on fluctuational electrodynamics. It 
is shown that the power generated is enhanced due to evanescent wave coupling at sub-wavelength 
vacuum gap distances. 
 
The topic of this paper is interesting and timely. While a number of papers have experimentally 
demonstrated the enhancement of the radiative flux beyond the blackbody limit in the near field, 
experimental demonstration of TPV power generation capitalizing on thermally generated evanescent 
modes is very limited. The approach used in the paper (i.e., integrated near-field TPV platform) is 
novel, and different from the current state-of-the-art. Fig. 1a suggests that the proposed platform is 
scalable to real-world application, which is in contrast with current laboratory-scale near-field TPV 
experiments. I therefore support publication of this manuscript in Nature Communications after the 
authors address the following questions/comments: 
 
1. Through simulations, Fig. 1c shows that the photodetector remains at low temperature (~300 K) 
even if the emitter is at high 
temperature (~900 K). I am confused by the temperature of the photodetector, both in the 
simulations and experiments: 
 
a) In the simulations, what are the boundary conditions? Are you assuming that the back of the 
photodetector is at constant temperature of ~300 K? If not, are you using a convective boundary 
condition? 
 
b) In the experiments, are you actively cooling the photodetector? If not, can you describe how the 
near-field TPV platform is heat sinked? 
 
2. What is the surface roughness of the Cr and Al2O3 layers? Is the roughness negligible for the 
theoretical calculations? 
 
3. Fig. 2 (and Fig. S2 in supplementary section 2) shows near-field radiative heat transfer simulations 
for gap distances of 50 nm and 50 um. For consistency with the experimental results, I suggest to 
replace the 50 nm results by theoretical predictions at a gap distance of 100 nm. 
 
4. The absorption bandgap of the Ge photodetector (~0.67 eV) is, as mentioned in the paper, lower 
than conventional Si cells (~1.1 eV). Yet, for an emitter temperature of ~900 K, only a small portion 
of the thermal spectrum is above the Ge photodetector bandgap. Can you comment on this? How the 
spectral mismatch between the emitter and photodetector affects near-field TPV performance? 
 
5. In Fig. 4b, why do you have theoretical data for 2 values of TH? My understanding is that 
TH,average is ~ 880 K, with an uncertainty of +- 50 K. Then, why not plotting a colored band of 
theoretical prediction instead of 2 distinct curves? 
 
6. Is the efficiency provided in line 192 in %? Do you think that the low efficiency of near-field TPVs 
can be explained (partially at least) by the spectral mismatch between the emitter and photodetector? 
 

Minor comments: 
 
7. Lines 28-30. The first sentence of the paper is not clear. For example, what do you mean by 
"dynamic temperature profiles"? 
 
8. Line 31. Evanescent modes are emitted, regardless of the regime at which radiative transfer takes 
place (i.e., far field versus near field). As such, I would change "be emitted" by "propagate". 
 
9. Line 39. What do you mean by "non-configurable passive platforms"? 
 
10. Line 104. Why do you call propagating waves "propagating guided modes"? More specifically, why 
do you use the word "guided"? 
 
11. It is mentioned twice in the manuscript (lines 78 and 150) that the equivalent thermal circuit is 
provided in supplementary section 5. Only one reference to supplementary section 5 is sufficient. 
 
12. Line 54 of the supplementary material. Change "estimate" by "estimated". 
 
 
 
 
Reviewer #2 (Remarks to the Author): 
 
The work described in the manuscript by Bhatt et al. (entitled “Integrated near-field thermo-
photovoltaics for on-demand heat recycling”) is, broadly speaking, directed towards the development 
of near-field thermo-photovoltaic energy conversion devices. Recent theoretical and experimental 
work (mostly) by others in the community has demonstrated the benefits of leveraging near-field 
effects in this energy conversion process to dramatically increase the heat-to-electricity conversion 
efficiency (some of the relevant papers are cited by the authors) 
The authors assert that “The challenge lies in designing a platform that can separate two surfaces by a 
small and tunable gap while maintaining a large temperature differential.” They furthermore imply 
that a reconfigurable platform for on-demand energy conversion is essential and finally claim that they 
have developed a “fully integrated, reconfigurable and scalable platform” that is suitable for nearfield-
based thermoelectric energy conversion. 
While nearfield thermophotovoltaics (TPV) has (re-)emerged as an interesting concept, the central 
challenge in the field is to develop the fundamental understanding and implement systems (nearfield 
TPV cells) that enable heat-to-electricity conversion that is at least comparable in efficiency to existing 
(commercial) thermoelectric devices. The proposal by the authors that reconfigurable platforms and 
on-demand heat recycling are central to TPV is completely misguided and wrong. Using NEMS actuator 
to readjust the distance between the emitter and photodetector is far too complex and expensive, and 
it is of no interest for controlling the power output from a TPV platform (as implied by on-demand in 
the title). To “turn the system off” one should (simply) remove the flow of heat to the TPV device (the 
only real option to avoid any additional inefficiency). Additionally, the generated electricity is ideally 
suited for switching on and off applications. 
In the study of nearfield radiation and TPV (that means in fundamental science and engineering 
studies) continuous adjustments of the distance between the emitter and photodetector have (and will 
continue) to play a central role in the detailed analysis of the processes under investigation. However, 
the platform described by Bhatt et al. falls far short of providing a suitable and precise measured 
range of distances to critically evaluate device function. In fact, the distance data reported (Fig 3 and 
4) lack the independent confirmation and accuracy of previous studies (see for example ref. 32). 
Is the work novel and of interest to the community? 

The work is original in that is applies NEMS actuation to the design of a TPV system, but, as I outlined 
above, this is an unnecessary and misguided use that will not benefit the development of efficient, 
successful TPV systems. In addition, electrostatic NEMS actuation as described in this paper is not 
novel and has been leveraged in the senior author’s lab in the study of nearfield radiation previously 
(ref. 24). The reported platform is, even by the standards of this young field, inefficient (~0.3 10-4) 
as compared to recently published work (for example ref. 32), and the assertion by that authors that 
the electrostatic control of the NEMS actuator consumes little power is not reassuring, because the 
actuation is not desirable or improving practical TPV platforms. 
With regard to the writing and presentation quality of the manuscript, it is difficult to fully comprehend 
the design of the NEMS system in relation to the photodetector and critically evaluated important 
aspects of the platform, including the thermal design and nanometer-level actuation of the system. 
Critical data, for example, on the cleanliness (particle contamination) and flatness of the devises are 
missing, and I am not at all convinced that the device it truly scalable to large areas. Figure 1 is not 
clear, there should be a schematic drawing. In addition, the background information (relative to 
nearfield radiation, TPV and electrostatic NEMS actuation) lacks balance in the presentation of past 
work by other groups, and the manuscript falls short in critically discussing the accomplishments made 
in this work. Also, the use of the term reconfigurable appears inappropriate in the context of what past 
work on reconfigurable engineering systems is referring to. In this manuscript the authors 
demonstrated limited adjustability of a single parameter – the emitter to photodetector distance. 
In summary, the paper offers no meaningful progress in near-field TPV. Consequently, it will not 
impact the thinking in the field and I cannot support publication of the manuscript. 
 
 
 
Reviewer #3 (Remarks to the Author): 
 
The topic of the manuscript, namely that of heat recycling using controlled near-field thermo-
photovoltaics, is very important and relevant, enabled by the recent advances in precision 
nanofabrication technologies. Indeed, the nanostructure complexity and precision in the control of the 
distance between the heater and the photodetector at the nanoscale, demonstrated in this manuscript, 
are impressive. The main results, reported in Fig 4b, show a superlinear increase in the generated 
power versus the decreasing gap distance. The measurement is carried out in 6 points only with rather 
large error bars both in the abscissa and ordinate axes. I apologize for my possible omission, but I 
couldn't see how were the error bars considered in the report of the measurement? In particular, why 
does the error bar grow with the increase in the signal amplitude (generated power)? Furthermore, 
two graphs based on the calculation (dotted red and blue lines) are reported to give the 'confidence 
interval' on the temperature of the hot side. However, I see that neither of the calculated curves fits 
the entire data set in a satisfactory manner (even including large error bars). Does this indicate 
systematic issues with the measurement of the generated power or the estimate of the hot 
temperature of the metal or both? I believe authors should offer some explanations for the large 
discrepancy between the measurement and the theory, as it relates directly to the main presented 
dataset. 
 
The interpretation of the data strongly relies on the knowledge of the hot (T_hot) and cold (T_cold) 
reservoir temperatures. To the best of my knowledge, I did not see a reported measurement of the 
T_hot in the manuscript nor supplemental information (only an estimate based on the dissipated 
power into the W strip). The cold side is estimated in temperature from model fits of the I-V curves. I 
do not know what instrumentation is available to the authors, but given how important is the 
knowledge of these temperatures, I would like to see a more elaborated discussion of the 
measurement(s)/estimate(s)/fit(s) of these quantities, together with the confidence intervals 
(especially, in connection to the discussion of Fig 4b above). Given large error bars in the data, I 
believe that the claimed factor of 11 enhancement in the generated electrical power (in the near-field), 
should be specified with the experimental certainty bounds as well. 
 
On a similar note, I found it quite astonishing that the temperature difference between a ~ 900K and 
300K body can be maintained so well when the bodies are in a perfect mechanical contact with each 
other over few-micron distances. As authors point out, this should only be possible by a "thin-film 
metallic emitter with high thermal resistance" and I think this is a discovery in itself that the authors 
found such a highly efficient thermal insulation. For clarity, I would like to understand about the 
details of the measurements. Are these steady-state measurements or obtained via modulation 
techniques (e.g. lock-in amplification)? Also, on what timescales are these measurements taken? 
 
I would like to request mandatory revisions addressing the points raised. I believe the manuscript is 
otherwise very well written, clear and addresses an interesting topic, worthy of the attention of the 
broad readership of Nature Communications.