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REVIEW 3 major objections 6 minor 72 references

Revisiting THz absorption in GO and rGO liquid crystalline films

T0 review · 3 major / 6 minor · reviewed 2026-08-06 · deepseek-v4-flash

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Pith's one-line read Ultrathin liquid-crystalline graphene-oxide films can absorb broadband terahertz radiation, with a 1.68-micron reduced film taking in about half of the incident 0.4-1.6 THz light.

desk verdict Useful first THz-TDS dataset on GO/rGO liquid crystalline films, but the headline absorption numbers rest on an absorption-extraction equation that ignores reflection and scattering; a fixable flaw, not a fatal one. read the letter →

arxiv 2507.21535 v1 pith:4JCV5VDL submitted 2025-07-29 cond-mat.mtrl-sci

A. Vasilev , M. Zhezhu , H. Parsamyan , G. Baghdasaryan , M. Sargsyan , D.A. Ghazaryan , H. Gharagulyan This is my paper
classification cond-mat.mtrl-sci
keywords grapheneoxideliquidcrystalsreducedterahertzabsorberstime-domainspectroscopyvacuumfiltrationelectrochemicalexfoliationbroadbandabsorption
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Using only vacuum filtration, the paper makes thin films from liquid-crystalline dispersions of electrochemically exfoliated graphene oxide and its hydrazine-reduced form, and reports that these films absorb terahertz radiation strongly across 0.4-1.6 THz. The graphene-oxide film (2.12 μm thick) absorbs on average about 37% of the incident radiation, and the reduced film (1.68 μm) absorbs about 50%, with thicknesses 221 and 279 times smaller than the central wavelength respectively. The paper argues that the liquid-crystalline ordering packs the flakes more densely and uniformly than random GO films, while chemical reduction adds absorbing defect sites and restores conductive sp2 domains. If correct, this is a simple, scalable route to broadband THz absorbers without the lithographic patterning required by metasurface designs.

What carries the argument

The load-bearing object is the liquid-crystalline phase of graphene oxide, whose self-aligned, densely packed flake structure is the proposed source of uniformity and absorption. The quantitative argument rests on Equation (6), $A = (I_{\mathrm{ref}} - I_{\mathrm{sample}})/(2 I_{\mathrm{ref}})$, which turns a metal-backed reflection measurement into an absorbance by assuming the beam passes through the film twice, reflects once off the metal, and that all loss between reference and sample is absorption. The LC phase is established through a uniaxial anisotropic ellipsometric model (Equations 1-2), and Raman-derived defect distances and densities (Equations 3-4) support the defect-absorption mechanism.

What would settle it

Measure the unbacked film's THz reflectance and transmittance at the same 35-degree geometry and compute $1 - T - R$; if that value is substantially smaller than the absorbance from Equation (6), or if the unbacked film alone reflects a non-negligible fraction of the THz beam, the paper's absorption percentages and wavelength-to-thickness ratios overstate true absorption.

Watch

Extended reading notes

Core claim

The paper claims that electrochemically exfoliated GO forms a nematic liquid-crystalline phase (confirmed by birefringence and a uniaxial anisotropic ellipsometric model) and that vacuum-filtered films of this phase, before and after hydrazine reduction, are efficient broadband terahertz absorbers. In metal-backed reflection measurements at 35 degrees incidence, the GOLC film of 2.12 μm thickness absorbs 27-41% of incident THz power over 0.4-1.6 THz (about 37% average), while the rGOLC film of 1.68 μm absorbs 48-50% (about 50% average), corresponding to central-wavelength-to-thickness ratios of 221 and 279. The paper also reasons that stacking two rGOLC layers would absorb about 75% with a ratio near 140, and attributes the high performance to compact liquid-crystalline stacking and to reduction-induced defects and restored sp2 regions.

Load-bearing premise

The reported absorbance derived from Equation (6) assumes that every drop in metal-backed reflected intensity comes from absorption during two identical passes through the film, with no front-surface reflection, scattering, or interference; the paper measures only the metal-backed reflectance and never isolates these losses.

Editorial extensions

If this is right

  • A 0.4-1.6 THz absorber can be made by vacuum filtration alone, with no metasurface patterning, at thicknesses near two microns or less.
  • The rGOLC film's roughly 50% single-layer absorption implies a double-stacked rGOLC absorber would reach about 75% absorption with a central wavelength/thickness ratio near 140.
  • Because absorption scales with film thickness and reduction state in the measured series, the same fabrication route should allow tunable absorbers with selected absorbances.
  • If the reported mechanism holds, defect engineering (e.g., controlled reduction) is a direct handle for maximizing THz absorption in carbon films.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The paper's absorbance values would be straightforward to test independently by measuring an unbacked film's transmittance and reflectance and checking whether $1 - T - R$ matches the metal-backed double-pass result; the manuscript does not report such a standalone reflectance measurement.
  • If front-surface Fresnel reflection or scattering contributes to the metal-backed intensity drop, the reported absorption percentages and the 221/279 wavelength ratios would be overestimates; this is a caveat the authors address only by asserting low reflectance from the transmission geometry.
  • One could test the proposed defect-absorption mechanism by measuring a series of films reduced to intermediate degrees: the Raman-based defect density predicts a monotonic rise in THz absorption.
  • The same vacuum-filtration method may extend to other two-dimensional liquid crystals (e.g., MXene or boron nitride) if their LC phases pack densely enough, though the paper does not explore this.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 6 minor

Summary. The manuscript reports the fabrication of graphene oxide (GO) and reduced graphene oxide (rGO) liquid crystalline (LC) films by vacuum filtration and their characterization using XRD, Raman, FTIR, SEM, AFM, and spectroscopic ellipsometry, together with THz time-domain spectroscopy in transmission and metal-backed reflection configurations. The authors claim average THz absorption of ~37% for a 2.12 μm GOLC film and ~50% for a 1.68 μm rGOLC film in the 0.4–1.6 THz range, with central-wavelength-to-thickness ratios of 221 and 279, and propose these as ultra-thin broadband THz absorbers.

Significance. If the quantitative absorption values were correct, the work would demonstrate a simple, solution-processed route to ultra-thin broadband THz absorbers with record subwavelength thickness, which is of practical interest. The materials characterization is thorough and the qualitative trend of increasing THz attenuation with thickness and with reduction is supported by the transmission spectra and temporal waveforms. However, the central quantitative claim rests on a single data-reduction formula that does not separate absorption from front-surface reflection and scattering, so the significance is contingent on a substantial re-analysis.

major comments (3)
  1. [Section 2.3, Eq. (6)] The absorption extraction in Eq. (6) assumes that the entire reduction of metal-backed reflected intensity is due to two equal passes of absorption. This neglects Fresnel reflection at the air-film interface and scattering from the rough film surface, which the authors themselves report to be up to 33.4% RMS roughness for GOLC-6 and 84.9% for rGOLC-6 relative to film thickness. Since no standalone reflectance or scattering measurement is provided, the reported absorbance values conflate absorption with reflection and scattering losses. This is load-bearing because the headline absorption percentages and the λ0/thickness ratios are all derived from Eq. (6).
  2. [Section 2.3, Eq. (6)] The formula is also internally inconsistent with standard definitions of absorption. If A is the single-pass absorbance, the double-pass reflected intensity is (1 − A)^2 I_ref, so the correct expression would be A = 1 − sqrt(I_sample/I_ref), not (I_ref − I_sample)/(2 I_ref). If A is meant to be the total absorption of the metal-backed structure, then A = 1 − I_sample/I_ref, without the factor 1/2. Under the current formula, A = 0.5 would imply I_sample = 0, i.e., zero reflected power. Please clarify the intended definition and use the appropriate formula, or better, report a directly measured or modeled reflectance to compute A = 1 − T − R.
  3. [Abstract and Section 2.3] The abstract claims 'low reflectance and transmittance' as the basis for strong absorption, but standalone reflectance is never measured. The metal-backed reflection measurement only yields a relative reduction compared to the mirror; it cannot separate intrinsic Fresnel reflection (which is expected to be non-negligible for conductive rGO) from absorption. Without a direct measurement of R or a transfer-matrix model that fits both transmission and reflection data, the 'low reflectance' narrative is unsupported, and the absorption values are not quantitatively established.
minor comments (6)
  1. [Section 2.3, Eq. (7)] Equation (7) is dimensionally incorrect as written: T(dB) should be 10 log10(Pt/Pi), not 10 × (Pt/Pi). Please correct this expression and its relationship to shielding efficiency.
  2. [Section 2.1] The ellipsometric confirmation of the LC phase would be strengthened by a quantitative comparison of the anisotropic model against an isotropic model on the same GOLC film. The reported RMSE of 13.79 for the anisotropic fit alone does not establish that the anisotropy is significant relative to an isotropic description.
  3. [Figure 5 insets] The insets of Fig. 5(a,b) show 'absorption coefficient' dependencies, but the method of extracting the absorption coefficient from the measured data is not defined anywhere in the text. Please provide the equation and the assumptions used.
  4. [References] References [43] and [46] appear to be the same paper (Shen et al., Appl. Phys. Lett. 99, 141911 (2011)) and should be merged or disambiguated.
  5. [Table 1] The units of defect density nD are given as 10^11 cm^-1 in the table header, but defect density should have units cm^-2. Please check the units against Eq. (4) and the source reference.
  6. [Throughout] There are several typographical and grammatical issues, including 'Notabl y' in the Introduction, inconsistent decimal notation in Fig. 4 (e.g., '049 μm' instead of '0.49 μm' and '1․68' with a middle dot), and awkward phrasing such as 'graphene oxide liquid crystalline (GOLC) films with multiple layers is discussed'. A careful language and formatting pass is needed.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the THz absorption values are measured, not fitted, and the self-citations are method-only.

full rationale

The paper reports measured THz absorption of GO/rGO liquid crystalline films; it does not present a predictive model into which parameters are fitted. Equation (6), A = (I_ref - I_sample)/(2 I_ref), is a measurement formula for the metal-backed reflection configuration, not a fitted relation, and it is not equivalent to any claimed prediction by construction. The headline absorption percentages and the lambda0/thickness ratios are arithmetic transforms of the measured reflected intensities and measured thicknesses; they are not derived from fitting a parameter to a subset of data and then predicting a closely related quantity. The self-citations [34,35] are used only for the electrochemical exfoliation and LC phase formation procedures; the LC phase is independently confirmed in this work by ellipsometric characterization, and the THz absorption claim does not rest on those citations. The extrapolation that two stacked rGOLC layers would give 75% absorption is a simple additive estimate from the measured single-layer value, not a forced or circular derivation. The concern that Eq. (6) may conflate absorption with front-surface reflection and scattering is a measurement-validity issue, not circularity. No load-bearing step reduces to its own input, so the circularity score is 0.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central quantitative claims rest on an ad hoc 'central wavelength' metric and an absorption extraction model that ignores reflection and scattering. The LC-enhancement mechanism is asserted without a matched control. These are modeling and interpretive choices, not invented physical entities.

free parameters (1)
  • central wavelength λ0 = ~468 μm (implied by λ0/thickness = 221 and thickness 2.12 μm)
    Used to compute the headline λ0/thickness ratios in the abstract and Table 2. The paper never defines how λ0 is obtained from the 0.4-1.6 THz band; the arithmetic or geometric mean would give a different value, so this is an ad hoc metric.
assumptions (5)
  • domain assumption The reduction in metal-backed reflected intensity is entirely due to absorption, with a factor 2 for double pass, and front-surface reflection, scattering, and interference are negligible.
    Section 2.3, Eq. (6): A = (I_ref - I_sample)/(2 I_ref). This ignores Fresnel reflection at the air-film interface, THz scattering by rough films, and multiple reflection interference, all of which are likely significant for films with RMS roughness up to 84.9% of thickness.
  • domain assumption The LC phase persists in rGOLC films after reduction.
    Section 2.1 confirms LC for GOLC via anisotropic ellipsometric fits, but rGO films were fitted with an isotropic model (Supplementary Fig. S3). Yet the paper calls them rGOLC and attributes enhanced absorption to LC ordering, without direct evidence of anisotropy after reduction.
  • domain assumption The enhanced absorption is caused by LC ordering rather than by thickness, defect density, or measurement artifacts.
    The discussion attributes the improved absorption of rGOLC films to compact stacking and defect formation, but no direct comparison with non-LC GO/rGO films of matched thickness is made. Literature comparisons in Table 2 differ in thickness, measurement geometry, and film preparation, so the LC enhancement claim is not isolated.
  • standard math Standard literature formulas for defect distance and density (Eqs. 3 and 4) apply to these films.
    Used in Section 2.2 to estimate LD and nD from Raman ID/IG ratios, following ref [65]. These formulas carry their own assumptions about laser wavelength and defect type.
  • standard math Bragg's law and the Debye-Scherrer equation are applicable for the XRD analysis.
    Used in Section 2.2 to compute interlayer spacing from the 2θ peak and crystallite size from the FWHM, following standard practice.

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Pith. "Pith review of Revisiting THz absorption in GO and rGO liquid crystalline films." pith.science (2026). https://pith.science/paper/4JCV5VDL

@misc{pith2026250721535,
  author       = {Pith},
  title        = {Pith review of: Revisiting THz absorption in GO and rGO liquid crystalline films},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4JCV5VDL}},
  note         = {Machine review of arXiv:2507.21535}
}
read the original abstract

With a swift progress in modern high-throughput communication systems, security, sensing and medicine utilizing THz range technologies, the demand for easy-to-fabricate, lightweight and high-performance absorbing materials has increased drastically. Notably, traditional approaches of eliminating unwanted radiation based on metasurfaces often face fabrication challenges limiting their practicality. In this study, we propose a straightforward approach for fabricating graphene oxide (GO) and reduced graphene oxide (rGO) liquid crystalline (LC) films via the vacuum filtration method and investigate their THz absorption characteristics. Here, the presence of LC phase in our electrochemically exfoliated GO and rGO LC films was confirmed by ellipsometric characterization. THz time-domain spectroscopy (TDS) measurements reveal that these films possess a low reflectance and transmittance confirming their strong absorptive properties within 0.4 - 1.6 THz frequency range for 2 micrometer thick GO and rGO LC films. Particularly, the GOLC film shows 37 % average absorption at a thickness of 2.12 micrometer, which is 221 times smaller than the central wavelength. Similarly, the rGOLC film reaches 50 % absorption with a 1.68 micrometer thickness, 279 times smaller than the central wavelength. These findings provide valuable insights for development of GO- and rGO-based LC THz absorbers with highly tunable properties due to the ordering of GO flakes. Specifically, the LC phase of GO contributes to the formation of more uniform films with enhanced absorption due to the compact stacking and denser packing, compared to conventional GO films with randomly oriented GO flakes.

Figures

Figures reproduced from arXiv: 2507.21535 by the authors.

Figure 1
Figure 1. Schematic representation of the experimental setup of the THz [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Schematic representation of the GO synthesi [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Optical constants of GO and GOLC films obtained [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: Structural characterization of the GOLC and rGOLC films. (a) Pictures of GOLC samples with thicknesses: 2.12 μm (GOLC-1); 1.71μm (GOLC-2); 1.22μm (GOLC-3); 0.72μm (GOLC-4); 049 μm (GOLC-5); and 0.28 μm (GOLC-6), and rGOLC samples with thicknesses: 1․68 (rGOLC-1); 1․04 …
Figure 5
Figure 5. Figure 5: THz response of GOLC and rGOLC films. Absorption spectra of (a) GOLC and (b) rGOLC films with different thicknesses. Insets show absorption coefficient dependencies on the film thickness. (c) Temporal profiles of a THz pulse for GOLC-1 and rGOLC-1 films and the referen…

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Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.