REVIEW 3 major objections 5 minor 8 references
Two-dimensional covalent crystals by chemical conversion of thin van der Waals materials
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Fluorinating few-layer InSe converts it into atomically thin covalent crystals of InF3.
desk verdict A credible bulk conversion of InSe to Se-doped InF3 with solid structural evidence, but the few-layer claim rests on AFM alone and the novelty is overstated relative to prior ion-exchange work. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the chemical reaction of exfoliated InSe with gaseous XeF2, which replaces Se by F and rearranges the remaining atoms into the non-layered R-3c InF3 lattice while the overall flake outline is retained. The argument is carried by an identity chain: bulk fluorinated InSe matches InF3 in XRD, selected-area diffraction, HRTEM lattice spacings, and XPS; the same Raman signature appears in flakes as thin as 1.5 nm; and DFT says fully fluorinated mono- and bilayer InSe are dynamically unstable, fixing the observed three-layer threshold. That calculation changes the reaction from a one-off observation into a mechanism with a predicted thickness floor.
What would settle it
Take a fluorinated flake that started as three layers (final AFM thickness 1.5 nm) and obtain a plan-view HRTEM image with a fast Fourier transform: if the pattern does not index to the R-3c InF3 structure, or if Raman shows In-Se modes alongside the 250 cm-1 feature, then the thinnest product is not the claimed crystalline Se-doped InF3.
Extended reading notes
Core claim
The central claim is that direct fluorination of InSe with XeF2 converts the layered crystal into the non-layered, rhombohedral covalent compound InF3, and that this conversion works for flakes as thin as three layers while preserving the original 2D shape. In bulk samples the product is identified as InF3 by XRD, selected-area electron diffraction, HRTEM lattice spacings, and XPS; a residual selenium content of about 2% accounts for the difference from commercial InF3 in the Raman spectrum. For thin flakes the same Raman signature and a drop in AFM thickness from 2.4 nm to 1.5 nm are used to assign the same product. Tauc analysis of optical absorption gives a direct bandgap of $E_g \approx 2.2$ eV, and DFT with Se impurities reproduces that gap and predicts that fully fluorinated mono- and bilayer InSe are dynamically unstable, matching the observed three-layer threshold.
Load-bearing premise
The load-bearing premise is that a 1.5 nm fluorinated flake, identified mainly by Raman similarity to bulk fluorinated InSe and by AFM thickness, is actually crystalline Se-doped InF3 and not a partially fluorinated or amorphous residue.
Editorial extensions
If this is right
- Thin covalent crystals that cannot be exfoliated can be reached from cleavable precursors, so the set of accessible 2D materials is no longer limited to layered bulk compounds.
- The conversion works on liquid-exfoliated InSe laminates, producing free-standing Se-doped InF3 films with a 2.2 eV direct gap, so the route is scalable to large areas.
- Se-doped InF3 transmits visible and infrared light above 600 nm with a refractive index near 2, making it a candidate for transparent-semiconductor and optical-coating applications.
- The three-layer minimum set by DFT means the method preserves a predictable lower size limit: attempts with one- or two-layer InSe will not produce InF3.
Reading between the lines
- The authors do not test other anion-replacement pairs, but the same logic suggests thin GaSe or SnS2 exposed to the right halogen or chalcogen source could give non-layered 2D GaF3 or similar covalent products; that is an extrapolation beyond this paper.
- A direct atomic-resolution image of a flake converted from exactly three layers is absent; if taken, it would either confirm or disprove the extrapolation of the bulk InF3 assignment to the thinnest product.
- Since the optical gap (2.2 eV) exceeds twice the transport activation energy (1.4 eV), the paper's own data imply mid-gap states dominate conduction; a natural follow-up is to vary the XeF2 dose and measure how transport changes with the measured 2-9% Se content.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports a chemical conversion route from exfoliated and bulk InSe to Se-doped InF3 by direct fluorination with XeF2. The authors claim that the conversion works for InSe flakes as thin as three layers (2.4 nm InSe converting to ~1.5 nm product), that the resulting covalent InF3 is doped with selenium, and that it is a direct-gap semiconductor with Eg ≈ 2.2 eV, high transparency above ~600 nm, and thermal stability. Supporting evidence includes Raman, HRTEM/FFT, SAED, XRD, XPS, electrical resistivity, optical transmission and Tauc analysis, and DFT calculations including phonon stability and Raman spectra. The paper also demonstrates scalability by fluorinating liquid-exfoliated InSe laminates into free-standing films.
Significance. If substantiated, this work would demonstrate a general and scalable route to atomically thin covalent solids that have no layered bulk analogues, going beyond exfoliation of van der Waals crystals and high-vacuum surface growth. The bulk conversion of InSe to InF3 is well supported by multiple phase-sensitive techniques: XRD, SAED, HRTEM/FFT, XPS, and the disappearance of InSe Raman modes. The DFT calculations add mechanistic insight, including the prediction that three layers are the minimum stable thickness and a calculated Raman spectrum consistent with the core modes. The main significance of the paper, however, lies in the few-layer limit, and at that thickness the phase identification is the weakest point of the experimental evidence.
major comments (3)
- [§3, first paragraph and Figure S1] The central claim of converting InSe down to three layers rests on AFM thickness alone for the thinnest product. The 1.5 nm flakes shown in Figure S1 are not characterized by Raman, electron diffraction, or XPS; the Raman spectrum in Figure 1g is from a ~10 nm flake, and the HRTEM/FFT, SAED, and XRD data are obtained from bulk or polycrystalline material (Methods: TEM samples were prepared by rubbing bulk InF3 crystals against a grid). AFM height cannot exclude a partially fluorinated InSeF_x residue, an amorphous product, or an etched remnant, especially because mono- and bilayer InSe flakes disappear upon fluorination. Direct micro-diffraction, nanoscale Raman, or composition mapping of the ~1.5 nm product is needed to support the headline few-layer claim.
- [§4, Eq. (2) and Figure 2b inset] The direct-gap assignment is based on selecting n = 1/2 as the Tauc exponent that gives the 'best linear regime'; no quantitative criterion, uncertainty, or comparison with n = 2 (indirect) or n = 3/2 (direct forbidden) is shown. Since the 'direct bandgap of 2.2 eV' is a central result of the paper, please provide the alternative Tauc fits or a more objective linearity metric to support the choice.
- [Methods, DFT-methodology] The DFT+U calculations use Ueff = 7 eV on In d orbitals without justification or sensitivity analysis, and the Se doping concentration is set to 2.1% based on XPS. The computed bandgap and Raman spectra are then compared with experiment, but with U as a free parameter the agreement is not a parameter-free validation. Please justify the choice of Ueff or show that the main conclusions are robust to variations in U.
minor comments (5)
- [Figure S7 caption] The caption states that panel b is 'fully fluorinated bulk InSe', while the text discusses fully fluorinated bilayer InSe and concludes that three-layer InSe is the minimum stable thickness; the caption and text should be aligned.
- [Page 6, after Eg ≈ 2.2 eV] There is a stray '3.' in the sentence '... have a direct bandgap with Eg ≈ 2.2 eV 3. This value is significantly larger...' that should be removed.
- [Page 3, first paragraph] The phrase 'thicker layers of InSe preserved their anisotropic structure after fluorination' is ambiguous; it should clarify whether this refers to flake morphology or to the crystal symmetry of the product.
- [Methods, XPS and Figure S5] The main text reports 'approximately 2%' selenium doping, while Figure S5 presents 6% and 9% doped samples; the relationship between the XPS quantification and the doping levels discussed in the main text should be stated more explicitly.
- [Supplementary text 1] The main text refers to 'supplementary text 1', but the supplementary text is not numbered; please add a label so the cross-reference is unambiguous.
Circularity Check
No significant circularity: central claims rely on independent diffraction, spectroscopy, and XPS, not on self-citation or fitted predictions.
full rationale
The paper's derivation chain is not circular under the required standard. The core phase assignment for bulk and roughly 10 nm flakes rests on independent XRD, SAED, HRTEM/FFT, and XPS identifications of InF3, not on the DFT calculation or on the Raman/bandgap comparisons. The DFT section states U_eff = 7 eV and uses an Se concentration (about 2.1%) taken from XPS, then compares the resulting bandgap and Raman spectra with experiment; there is no quotation or equation showing those parameters were fitted to the 2.2 eV bandgap or to the measured Raman peaks, so this is parameterization, not a prediction derived from the output. Self-citations (refs 6, 29, 30) are methodological (fluorination procedure and Raman-activity calculation) and do not carry the central claim. The Tauc analysis selects n = 1/2 by best linear fit, which is a fitting convention rather than a smuggled input; even if it were debated, the direct-gap claim is not the paper's load-bearing novelty. The only notable weakness, that the thinnest 1.5 nm flakes are not directly phase-characterized by diffraction or Raman, is an evidentiary gap at the boundary of the claim, not a circular reduction: the bulk and roughly 10 nm identifications do not presuppose the few-layer conclusion. No step reduces an output to an input, so score 0.
Assumptions & free parameters
free parameters (3)
- DFT+U Hubbard Ueff on In d orbitals =
7 eV
- Se doping concentration in DFT supercell =
2.1%
- Tauc exponent n =
n=1/2 (direct allowed)
assumptions (4)
- domain assumption PBE+U DFT with Ueff=7 eV gives reliable electronic, phonon, and Raman properties for InF3 and Se-doped InF3.
- domain assumption XeF2 fully fluorinates InSe to InF3 with no residual InSe or intermediate phases in the reacted flakes.
- domain assumption The Tauc relation and extrapolation of the linear region give the true optical bandgap.
- domain assumption The reference InF3 crystal structure (R-3c space group, lattice spacings 3.93 Å and 2.854 Å) is correct.
Cite this review
Pith. "Pith review of Two-dimensional covalent crystals by chemical conversion of thin van der Waals materials." pith.science (2026). https://pith.science/paper/Q2ZVH2HV
@misc{pith2026190807357,
author = {Pith},
title = {Pith review of: Two-dimensional covalent crystals by chemical conversion of thin van der Waals materials},
year = {2026},
howpublished = {\url{https://pith.science/paper/Q2ZVH2HV}},
note = {Machine review of arXiv:1908.07357}
}
read the original abstract
Most of the studied two-dimensional (2D) materials have been obtained by exfoliation of van der Waals crystals. Recently, there has been growing interest in fabricating synthetic 2D crystals which have no layered bulk analogues. These efforts have been focused mainly on the surface growth of molecules in high vacuum. Here, we report an approach to making 2D crystals of covalent solids by chemical conversion of van der Waals layers. As an example, we use 2D indium selenide (InSe) obtained by exfoliation and converted it by direct fluorination into indium fluoride (InF3), which has a non-layered, rhombohedral structure and therefore cannot be possibly obtained by exfoliation. The conversion of InSe into InF3 is found to be feasible for thicknesses down to three layers of InSe, and the obtained stable InF3 layers are doped with selenium. We study this new 2D material by optical, electron transport and Raman measurements and show that it is a semiconductor with a direct bandgap of 2.2 eV, exhibiting high optical transparency across the visible and infrared spectral ranges. We also demonstrate the scalability of our approach by chemical conversion of large-area, thin InSe laminates obtained by liquid exfoliation into InF3 films. The concept of chemical conversion of cleavable thin van der Waals crystals into covalently-bonded non-cleavable ones opens exciting prospects for synthesizing a wide variety of novel atomically thin covalent crystals.
Figures
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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