{"id":"b432ccbe-85bb-45ed-aad7-cfec563be82c","arxiv_id":"2608.03347","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":4.0,"correctness_risk":"low","formal_verification":"none","parameter_count":7,"one_line_summary":"In MoS2xSe2(1-x) monolayers, increasing sulfur content raises the optical gap, shrinks the B-A exciton splitting, increases phonon energy, and raises circular polarization from ~0% to ~15%.","lead":"This paper maps how mixing sulfur and selenium in single-layer MoS2/MoSe2 alloys shifts the optical gap, the spacing between A and B excitons, the average phonon energy, and the valley polarization across the full composition range. It shows that chalcogen alloying can continuously tune these excitonic properties, and that the polarization gain with sulfur content tracks a predicted change in conduction-band spin splitting.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"Polarization mechanism rests on single-configuration DFT; configurational averaging could change the conduction-band splitting trend, weakening the central spin-valley claim.","rationale":"The reader identified the same weakest assumption: the polarization mechanism depends on a single-configuration DFT representation of alloy disorder and on the inferred bright-dark splitting. My analysis confirms that this is the most load-bearing concern. The central empirical claims — continuous gap tuning, reduction of B-A splitting, and phonon-energy scaling — are direct measurements with straightforward fits and are not seriously threatened. The polarization mechanism, by contrast, is an interpretation that leans on DFT calculations the authors themselves flag as configuration-sensitive, plus literature values for endpoint Δbd and a roughly constant exchange contribution. If the DFT trend is not robust to configurational averaging, the monotonic polarization increase could still stand as an empirical result but the mechanistic attribution to conduction-band spin splitting and Rashba-assisted depolarization would be unsupported. The proposed concrete test — multi-configuration SQS averaging with spread reporting — directly settles whether the trend survives. This does not change the reader's CONDITIONAL verdict; it reinforces it. No new objection beyond the reader's is raised, and an honest non-finding for the other parts of the paper is appropriate.","tokens_in":14472,"tokens_out":4062,"duration_ms":46145,"concrete_test":"Generate 8–10 independent SQS 5×5 supercells at x=0.3, 0.52, and 0.7 using the same VASP/PBE/D3 parameters as in Methods; unfold each band structure and extract the K-point conduction-band spin splitting and Δv+Δc. Report mean ± standard deviation over configurations on Fig. 2(d)/4(b). If the ensemble-averaged CB splitting is monotonic in x and the per-composition spread is small relative to the ~20 meV endpoint difference, the single-configuration concern is resolved. If the spread is comparable to the trend, the polarization mechanism must be revised.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The empirical trends are solid: the optical-gap shift (Fig. 1), the B-A splitting (Fig. 2), and the phonon reduced-mass scaling (Fig. 3) are well supported by the data. The load-bearing weakness is the mechanistic explanation of Fig. 4(a): the monotonic increase of circular polarization is attributed to a composition-dependent bright-dark exciton splitting Δbd, whose evolution is inferred from the DFT-computed conduction-band spin splitting (Fig. 4(b)). This inference stacks two assumptions: (i) each intermediate composition is represented by a single SQS configuration — the Methods section explicitly states SQS supercells for x=0.3 and x=0.7, and x=0.52 is not described — and the authors concede that intermediate splittings are sensitive to the Se/S arrangement (Discussion after Fig. 2(d)); (ii) Δbd is not computed directly. Instead, exchange is assumed approximately composition-independent (~20 meV from Ref. [67]) and binding-energy differences are assumed small, so the CB-splitting trend is taken to govern Δbd. If configurational averaging changed the CB-splitting trend — e.g., large configuration-to-configuration spread at intermediate x — or if exchange/binding contributions vary with composition, the proposed link between composition and polarization would be unsupported. The polarization data show no error bars and each composition corresponds to one sample, so this is the least secure pillar of the central claim.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports an optical-spectroscopy and DFT study of monolayer MoS2xSe2(1-x) alloys at x = 0, 0.3, 0.52, 0.7, and 1, using hBN-encapsulated samples. The experimental results show a continuous optical-gap blueshift of about 0.35 eV with small bowing parameters, a monotonic decrease of the B-A exciton splitting from about 200 to 150 meV, an increase of the average phonon energy from about 17 to 22.5 meV described by a one-parameter reduced-mass scaling model, and an increase in circular polarization from near zero in MoSe2 to about 15% in MoS2 under fixed detuning at 78 K. DFT (PBE+SOC with SQS supercells for x = 0.3 and 0.7) is used to support the B-A splitting trend and to attribute the polarization increase to composition-dependent conduction-band spin splitting that changes the bright-dark exciton splitting and Rashba-assisted valley depolarization.","tokens_in":14905,"tokens_out":8931,"duration_ms":99833,"significance":"If the trends hold, the paper offers a valuable unified dataset for alloy engineering of TMD monolayers, combining excitonic, vibrational, and polarization properties in a single, well-characterized sample set. The strengths include the fixed-detuning polarization protocol, transparent fits with quoted parameters, the use of hBN encapsulation, and the authors' candid caveats about single-configuration DFT. The empirical trends—the gap shift, B-A splitting, and phonon scaling—are convincing and reproducible from the presented figures. The polarization mechanism is plausible but less conclusive, and it is the main limitation of the manuscript.","major_comments":[{"comment":"The DFT bridge for the polarization mechanism is single-configuration SQS at x = 0.3 and x = 0.7, with no calculation at x = 0.52. The authors concede that intermediate splittings are sensitive to the Se/S arrangement; configurational averaging could therefore change the conduction-band splitting trend used to link composition to bright-dark splitting Δbd and polarization. In addition, Δbd is not computed directly; the argument assumes composition-independent exchange (~20 meV) and small binding-energy differences. The polarization data have no error bars and one sample per composition. I recommend either adding configurational averaging (and ideally direct Δbd estimates) or explicitly presenting the mechanism as a hypothesis rather than a conclusion. This is load-bearing because the spin-valley tuning claim rests on it.","section":"Results and Discussion, Eq. (2), Figs. 2(d) and 4(b), Methods"}],"minor_comments":[{"comment":"The caption states the A and B exciton data were extracted from PL spectra at 78 K, while the main text says they were extracted from PL spectra at 8 K (Fig. S2). Please reconcile.","section":"Fig. 2(c) and main text"},{"comment":"The extracted average phonon energies are shown without error bars. Reporting fit uncertainties would let the reader judge the quality of the reduced-mass scaling and the quoted prefactor A = (111.8 ± 0.6) meV·sqrt(amu).","section":"Fig. 3(f)"},{"comment":"The text specifies SQS supercells for x = 0.3 and x = 0.7 only. The figures and text should make clear that no DFT point is available for x = 0.52, and that the phrase 'full composition range' applies strictly to the experimental data.","section":"Methods, Calculations"},{"comment":"Equation (3) has three fitted parameters (Eg(0), S, and <ħω>), while the 'single-parameter' language later refers to the scaling model A/sqrt(μ). Please clarify this distinction to avoid confusion.","section":"Eq. (3) and phonon discussion"},{"comment":"The statement that the B-A splitting is 'in agreement with density functional theory calculations' is stronger than the explicitly qualitative comparison made in the text. Add 'qualitatively' in the abstract and conclusions, or provide quantitative uncertainty estimates.","section":"Abstract and Conclusions"},{"comment":"The annealing temperature '1500C' should read '150 °C'.","section":"Methods, Sample fabrication"}],"recommendation":"major_revision","confidential_remarks":"The empirical core of the paper is solid and appropriate for the journal. The main risk is overclaiming the polarization mechanism, which currently rests on a single-configuration DFT assumption that the authors themselves identify as uncertain. The self-citations are appropriate and relevant."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"The useful thing here is the unified dataset: five hBN-encapsulated MoS2xSe2(1−x) monolayers spanning the full composition range, all measured with the same protocols, including a fixed 93 meV detuning for the polarization runs. The optical-gap shift, the near-Vegard behavior with tiny bowing, the monotonic B–A splitting drop, and the phonon energy increase from ~17 to ~22.5 meV are all cleanly presented. The reduced-mass scaling with one fitted prefactor is a nice, transparent way to tie the phonon trend to chalcogen mass; it is a description rather than a prediction, but it fits the data well and the authors do not oversell it. The DFT calculation of the band-edge splitting tracks the B–A exciton splitting reasonably, and the authors are careful to call the comparison qualitative because of the single-configuration issue. That honesty matters.\n\nThe soft spot is exactly where the stress-test note points: Fig. 4. The monotonic increase in circular polarization is real as reported, but the mechanism runs through the conduction-band spin-splitting trend computed from one SQS configuration per composition, and the link to the bright–dark exciton splitting is inferred, not computed. Exchange is assumed roughly composition-independent and binding-energy differences are assumed small. Those are reasonable working assumptions, but the polarization data have no error bars and each composition is a single sample, so the mechanistic conclusion is the least secure pillar. The authors explicitly admit the intermediate splittings are sensitive to the Se/S arrangement, so this is not a hidden flaw; it is a stated limitation. The paper would be stronger with configurational averaging and/or direct dark-exciton measurements, but the empirical trends do not depend on that mechanism.\n\nWho is this for? Anyone working on TMD alloys, exciton physics, or valleytronics. It consolidates known trends into one coherent picture and adds the phonon-composition relationship. It deserves a serious referee. The citation pattern is self-heavy in places but the cited works are directly relevant, and the paper engages with prior reports on bandgap, spin-orbit, and polarization. I would send it to review and ask for a clearer statement that the polarization mechanism is a hypothesis supported by indirect evidence, plus error bars or at least a acknowledgement that each composition is one sample.","headline":"Solid full-composition dataset with a fresh phonon trend; the polarization mechanism is plausible but rests on single-configuration DFT, which the authors openly acknowledge.","tokens_in":15377,"tokens_out":1259,"would_cite":true,"duration_ms":15409,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":["78.67.-n","71.35.-y","71.70.Ej"],"model":"deepseek-v4-flash","headline":"Chalcogen alloying in monolayer MoS2xSe2(1-x) continuously tunes the optical gap, the spin-orbit exciton splitting, the phonon energy, and the valley polarization, attributing the polarization trend to alloy-modified bright-dark exciton mix","keywords":["excitons","transition metal dichalcogenides","alloy monolayers","optical spectroscopy","phonons","polarization","spin-valley","density functional theory"],"falsifier":"Measure the circular polarization of the A exciton at fixed detuning and temperature across several independently prepared monolayers of the same nominal alloy composition; if sample-to-sample spread is comparable to the reported monotonic increase from about 0% to 15%, the proposed composition-polarization link is not supported. Alternatively, compute the conduction-band spin splitting averaged over many random alloy configurations: the mechanism fails if the averaged splitting does not decrease monotonically with sulfur content.","tokens_in":14455,"feed_emoji":"🔬","tokens_out":11072,"duration_ms":97880,"temperature":0.7,"pith_summary":"The paper reports that the full alloy series MoS2xSe2(1-x) provides a way to continuously tune the optical gap, the spin-orbit-related exciton splitting, the average phonon energy, and the exciton circular polarization of a monolayer semiconductor by adjusting the sulfur-to-selenium ratio. It claims the optical gap shifts by about 0.35 eV, the B-A exciton splitting falls from roughly 200 to 150 meV, the average phonon energy rises from about 17 to 22.5 meV according to a reduced-mass scaling law with a single fitted constant, and the circular polarization increases from near zero to about 15% at 78 K. The polarization trend is attributed to alloy-induced changes in the conduction-band spin splitting that modify bright-dark exciton mixing and the efficiency of valley depolarization. A sympathetic reader would care because this makes alloying a synthesis-compatible dial for electronic, vibrational, and spin-valley properties in one two-dimensional material system.","feed_headline":"Alloying tunes monolayer gap by 0.35 eV; valley polarization hits 15%","feed_subtitle":"From MoSe2 to MoS2, the gap shifts 0.35 eV, phonons stiffen, and valley polarization climbs to 15%.","key_machinery":"The central object is the alloy composition axis x in MoS2xSe2(1-x), the sulfur fraction, used as a continuous tuning parameter. The argument is carried by three quantitative mechanisms: a Vegard-like linear interpolation of the A and B exciton energies with small bowing parameters, so the B-A splitting tracks the spin-orbit splitting of the band edges; the reduced-mass scaling of the average phonon energy, <hbar omega>(x) = A / sqrt(mu_eff(x)), where mu_eff is the Mo-chalcogen reduced mass and A is a single fitted constant; and the bright-dark exciton splitting Delta_bd = E_bright - E_dark, which controls Rashba-assisted valley depolarization and is linked through DFT calculations to the al","core_discovery":"The central claim is that chalcogen alloying in monolayer MoS2xSe2(1-x) is a powerful platform for tuning the electronic, vibrational, and spin-valley properties of two-dimensional semiconductors. Across compositions from MoSe2 (x=0) to MoS2 (x=1), the A-exciton photoluminescence shifts continuously from 1.63 to 1.93 eV at 8 K; the B-A exciton splitting decreases monotonically from about 200 to about 150 meV, in agreement with DFT-calculated spin-orbit splittings; the average phonon energy extracted from temperature-dependent PL increases from about 17 to 22.5 meV and follows a reduced-mass scaling with one fitted prefactor; and at fixed 93 meV detuning and 78 K, the circular polarization of","pith_inferences":["Editorial extension: if the reduced-mass phonon scaling is as clean as reported, the same one-parameter fit should predict the average phonon energy in related alloy series with the same MoX2 skeleton, such as MoS2xTe2(1-x) or MoSe2xTe2(1-x), unless the effective force constant changes enough to break the assumed constancy.","Editorial extension: the proposed bright-dark mixing mechanism implies a testable temperature dependence: the polarization gain from sulfur alloying should shrink at higher temperatures if depolarization is thermally assisted, whereas a purely disorder-driven intervalley scattering channel would show a different trend.","Editorial extension: because the DFT trend was computed for one special quasi-random structure per composition, averaging band structures over many random configurations would sharpen the claim; if the averaged conduction-band splitting does not decrease monotonically with sulfur content, the attribution of the polarization rise to this splitting would be weakened."],"forward_implications":["The same hBN-encapsulated monolayer platform can provide any optical gap between about 1.63 and 1.93 eV by choosing the sulfur fraction, without needing external strain or electrostatic doping.","The linear composition dependence of the B-A splitting means the spin-orbit energy scale can be dialed in over a roughly 50 meV range across the alloy series.","Because a single reduced-mass prefactor describes the average phonon energy for all five compositions, the model predicts the phonon scale for any intermediate x and offers a fast composition diagnostic from temperature-dependent PL.","The monotonic rise of circular polarization from near zero to about 15% at 78 K under fixed detuning implies that valley initialization becomes more robust as sulfur content increases, making sulfur-rich alloys preferable for applications requiring spin-valley contrast.","The near-zero bowing parameters imply the alloy behaves nearly as a virtual crystal for exciton energies despite the mixed chalcogen sublattice, so alloying introduces limited electronic perturbation beyond the intended band-edge shifts."],"supporting_citations":[{"why":"Supplies the prior demonstration of bandgap tuning in the same MoS2xSe2(1-x) alloy series that this work extends to exciton, phonon, and polarization properties.","marker":"[30]"},{"why":"Gives the nearly equal exciton binding energies of encapsulated MoS2 and MoSe2, used to read the optical gap shift as a band-edge shift.","marker":"[44]"},{"why":"Provides the empirical coth model used to fit temperature-dependent PL data and extract average phonon energies.","marker":"[55]"},{"why":"Supplies the mass-spring relation omega proportional to sqrt(k/mu) that underlies the reduced-mass phonon scaling model.","marker":"[60]"},{"why":"Provides the bright-dark exciton mixing framework invoked to explain valley depolarization and its composition dependence.","marker":"[63]"},{"why":"Explains the near-zero polarization in MoSe2 through near-degenerate bright and dark exciton states enabling Rashba-assisted mixing.","marker":"[64]"},{"why":"Reports the about -1.5 meV bright-dark splitting in MoSe2, anchoring it in the regime of strongest depolarization.","marker":"[65]"},{"why":"Reports the about +14 meV bright-dark splitting in MoS2, showing why mixing is suppressed as sulfur content increases.","marker":"[66]"},{"why":"Establishes that the exchange contribution to the bright-dark splitting stays near 20 meV across the MoX2 family, making conduction-band spin splitting the likely controlling factor.","marker":"[67]"}],"fun_headline_variants":["Alloying TMD monolayers tunes optical gap by 0.35 eV","From MoSe2 to MoS2: gap shifts 0.35 eV, polarization to 15%","Alloy engineering boosts valley polarization to 15% in TMDs","0.35 eV exciton gap tuning in TMD alloy monolayers","Alloying tunes gap, phonons, and valley polarization in monolayers"],"cache_read_input_tokens":2816,"weakest_assumption_plain":"The load-bearing premise is that a single calculated atomic arrangement per alloy composition captures how real alloy disorder changes the conduction-band spin splitting; the paper itself notes that intermediate splittings depend on which Se/S arrangement is used.","fun_headline_variants_meta":{"raw":{"variants":["Alloying TMD monolayers tunes optical gap by 0.35 eV","From MoSe2 to MoS2: gap shifts 0.35 eV, polarization to 15%","Alloy engineering boosts valley polarization to 15% in TMDs","0.35 eV exciton gap tuning in TMD alloy monolayers","Alloying tunes gap, phonons, and valley polarization in monolayers"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000855,"raw_usage":{"total_tokens":3548,"prompt_tokens":735,"completion_tokens":2813,"prompt_tokens_details":{"cached_tokens":256},"prompt_cache_hit_tokens":256,"prompt_cache_miss_tokens":479,"completion_tokens_details":{"reasoning_tokens":2706}},"tokens_in":479,"tokens_out":2813,"duration_ms":19323,"temperature":1.0,"reasoning_tokens":2706,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-05T20:30:26.290586+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the circular polarization of the A exciton at fixed detuning and temperature across several independently prepared monolayers of the same nominal alloy composition; if sample-to-sample spread is comparable to the reported monotonic increase from about 0% to 15%, the proposed composition-polarization link is not supported. Alternatively, compute the conduction-band spin splitting averaged over many random alloy configurations: the mechanism fails if the averaged splitting does not decrease monotonically with sulfur content.","supporting_citations":[{"cited_title":"S.; Preciado, E.; Klee, V.; Bobek, S.; Yamaguchi, K.; Li, E.; Oden- thal, P","cited_arxiv_id":null,"evidence_quote":"Supplies the prior demonstration of bandgap tuning in the same MoS2xSe2(1-x) alloy series that this work extends to exciton, phonon, and polarization properties."},{"cited_title":"Monolayer semicon- ducting transition metal dichalcogenide alloys: Stability and band bowing.Journal of Applied Physics2013,113","cited_arxiv_id":null,"evidence_quote":"Gives the nearly equal exciton binding energies of encapsulated MoS2 and MoSe2, used to read the optical gap shift as a band-edge shift."},{"cited_title":"Sensitivity of excitonic transitions to tem- perature in monolayers of TMD alloys.The Journal of Physical Chemistry C2026,130, 1014–1022","cited_arxiv_id":null,"evidence_quote":"Provides the empirical coth model used to fit temperature-dependent PL data and extract average phonon energies."},{"cited_title":"K.; Plo- chocka, P","cited_arxiv_id":null,"evidence_quote":"Supplies the mass-spring relation omega proportional to sqrt(k/mu) that underlies the reduced-mass phonon scaling model."},{"cited_title":"Spin orientation of electrons as- sociated with the interband absorption of light in semi- conductors.Soviet Journal of Experimental and Theoret- ical Physics1971,33, 1053","cited_arxiv_id":null,"evidence_quote":"Provides the bright-dark exciton mixing framework invoked to explain valley depolarization and its composition dependence."},{"cited_title":"Exciton valley depolarization in monolayer transition-metal dichalcogenides.Physical Re- view B2020,101, 115307","cited_arxiv_id":null,"evidence_quote":"Explains the near-zero polarization in MoSe2 through near-degenerate bright and dark exciton states enabling Rashba-assisted mixing."},{"cited_title":"Polarization analysis of excitons in monolayer and bilayer transition-metal dichalcogenides","cited_arxiv_id":null,"evidence_quote":"Reports the about -1.5 meV bright-dark splitting in MoSe2, anchoring it in the regime of strongest depolarization."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Reports the about +14 meV bright-dark splitting in MoS2, showing why mixing is suppressed as sulfur content increases."},{"cited_title":"R.; Bartos, M.; Watanabe, K.; Taniguchi, T.; others Measurement of the spin-forbidden dark excitons in MoS2 and MoSe2 mono- layers.Nature Communications2020,11, 4037","cited_arxiv_id":null,"evidence_quote":"Establishes that the exchange contribution to the bright-dark splitting stays near 20 meV across the MoX2 family, making conduction-band spin splitting the likely controlling factor."}],"review_version":1}