{"id":"f9e41ace-a4cc-4001-a11f-5e33a484a8da","arxiv_id":"2501.08742","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":6.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"In a 2D copper-benzenehexathiol coordination polymer, hot carriers reach a mobility of ~2,000 cm2/Vs and traverse grain boundaries over hundreds of nanometers within a picosecond.","lead":"High-energy 'hot' charge carriers in a copper-organic 2D film move with a mobility of about 2,000 cm2/Vs, roughly five times the value for relaxed carriers. The finding suggests 2D coordination polymers could support organic hot-carrier electronics, a regime previously thought impractical.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"If the early-time THz decay reflects carrier-density loss rather than mobility loss, the headline ~2,000 cm2/Vs hot-carrier mobility is an overestimate; the linear offset-vs-fluence test does not exclude a fluence-independent fast trapping channel.","rationale":"The reader's weakest assumption identifies the same load-bearing step: the interpretation of the normalized THz transient as a pure mobility evolution requires n(t) to be constant in the first 10 ps. My reading of the manuscript finds this to be the least secure link in the quantitative headline claim. The linear Δσ_offset vs Nabs argument in Fig. 2b is necessary but not sufficient, since a linear surviving fraction under fast trap-assisted loss is fully compatible with that observation. The paper's own assignment of the long-lived decay to trap-assisted recombination shows that trapping is active, making the constant-n assumption particularly fragile. At the same time, the TAM data provide independent evidence for ultrafast spatial spreading, so the existence of highly mobile carriers is reasonably supported; the concern is the specific magnitude of 2,000 cm2/Vs and the absence of an uncertainty estimate for it. The proposed TA-integrated-population comparison is a direct test that can be carried out with data already in the paper and would settle whether the fast THz component is mobility decay or density decay. Since the reader already recommended a conditional verdict, my analysis does not change that recommendation; it sharpens the condition that should be met before the 2,000 cm2/Vs number is quoted as a quantitative result.","tokens_in":14402,"tokens_out":7171,"duration_ms":83561,"concrete_test":"Re-analyze the raw TA data behind Fig. 4a/4c: from the global-fit components, compute the total PIA population (sum of hot and band-edge component amplitudes) as a function of delay over 0-10 ps and compare it with Δσ(t)/Nabs from Fig. 2c under the same excitation conditions. If the TA population decays with the same ~500 fs component as the THz transient, then n is not constant and the scaling to ~2,000 cm2/Vs overestimates hot-carrier mobility; if the TA population stays flat while Δσ/Nabs drops, the hot-carrier mobility interpretation survives.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Fig. 2c normalizes Δσ by Nabs and states, \"Within the first 10 ps time where n can be considered constant,\" thereby converting the transient directly into μ(t). The fast ~500 fs component is then identified with hot-carrier cooling, and μ_hot ≈ 2,000 cm2/Vs is obtained by scaling the Drude band-edge mobility of 405 cm2/Vs by Δσ_peak/Δσ_offset. The only support for n = const is the linear Δσ_offset vs Nabs dependence in Fig. 2b. That linearity cannot distinguish constant n from a fast first-order trapping or recombination channel that removes a fixed fraction of carriers within the first picosecond: the surviving population at 6-8 ps would still scale linearly with Nabs. The paper itself assigns t2 ~ 1.2 ns to trap-assisted recombination, so traps are demonstrably present, and nothing independently measures n(t) on the sub-picosecond timescale. The TA blue shift is an energetic/spectral signature, not a direct population measurement. Additionally, the peak/offset ratio is recorded at the THz probe frequency while the band-edge mobility is the Drude DC limit; the early-time Drude-Smith spectrum has suppressed DC conductivity, so the ratio-to-DC-mobility conversion is not automatically valid without a reported c parameter and finite-frequency correction. TAM's early spatial expansion does independently indicate a highly mobile species, so the concern is quantitative — the specific 2,000 cm2/Vs value and its uncertainty — rather than the existence of hot-carrier transport.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript presents a combined time-resolved terahertz spectroscopy (TRTS), transient absorption spectroscopy (TAS), and transient absorption microscopy (TAM) study of photogenerated carriers in thin films of the two-dimensional conjugated coordination polymer Cu3BHT. The authors identify two transport regimes following above-gap photoexcitation: a non-equilibrium regime in which a fast (~500 fs) decay of the THz photoconductivity is assigned to hot-carrier cooling with an inferred hot-carrier mobility of ~2,000 cm2 V-1 s-1, and a quasi-equilibrium regime in which the photoconductivity spectra are Drude-like with a band-edge mobility of ~400 cm2 V-1 s-1 and an intrinsic diffusion length exceeding 1 um. The interpretation is supported by the photon-energy dependence of the THz transients, the temperature dependence of the Drude scattering rate, the blue shift of the transient absorption band, and direct real-space imaging of an ultrafast expansion of the photoexcited population.","tokens_in":14755,"tokens_out":8899,"duration_ms":83865,"significance":"If correct, this would be the first observation of hot-carrier transport in a two-dimensional conjugated coordination polymer, with record mobility values for organic-based systems and potential relevance to hot-carrier photovoltaics and transistors. The paper has notable strengths: the critical photon energy extrapolated from the THz ratio (~560 meV) agrees with the independent Tauc absorption edge (~0.5 eV); the TAM measurements provide a real-space, technique-independent signature of ultrafast carrier spreading; and the temperature-dependent Drude scattering rate shows a positive temperature coefficient consistent with band-like transport. These complementary elements make the qualitative picture of high-mobility hot carriers credible, but the quantitative claims require additional scrutiny.","major_comments":[{"comment":"The assertion that the carrier density n is constant during the first 10 ps is not established by the data shown. The linear dependence of Δσ_offset on Nabs (Fig. 2b) demonstrates only that the surviving population at 6–8 ps scales linearly with absorbed photon number; a fluence-independent fast trapping or recombination channel that removes a fixed fraction of carriers within the first picosecond would produce the same linear dependence. Because the paper itself assigns t2 ≈ 1.2 ns to trap-assisted recombination, traps are demonstrably present, and no independent sub-picosecond measurement of n(t) is provided, the conversion of Δσ(t)/Nabs into μ(t) is not uniquely justified. This directly affects the headline value of ~2,000 cm2 V−1 s−1 for the hot-carrier mobility.","section":"Observation of highly mobile hot carriers and hot phonon bottleneck in Cu3BHT (Fig. 2c)"},{"comment":"The estimate μ_hot ≈ 2,000 cm2 V−1 s−1 is obtained by scaling the Drude DC mobility (405 ± 30 cm2 V−1 s−1) by the ratio Δσ_peak/Δσ_offset. This scaling is not quantitatively valid without a finite-frequency correction: at Δσ_peak the conductivity spectrum is described by the Drude-Smith model, with a negative imaginary component and a suppressed low-frequency real part (Fig. 3b), while 405 cm2 V−1 s−1 is the Drude DC limit. The Drude-Smith parameter c is not reported in the main text, so the reader cannot evaluate whether the measured ratio over- or underestimates the true mobility ratio. The authors should either report the frequency-resolved ratio at the probe frequency with the corresponding c value or provide the corrected mobility.","section":"Crossover from non-equilibrium to quasi-equilibrium transport regime (Fig. 3b) and Methods"},{"comment":"The TAM-derived hot-carrier diffusion coefficients of 677 ± 59 cm2 s−1 (at 1.77 eV) and 1224 ± 94 cm2 s−1 (at 2.59 eV) imply, through the Einstein relation at room temperature, ambipolar mobilities of roughly 26,000–47,000 cm2 V−1 s−1 if the carriers are in quasi-equilibrium with the lattice. The paper does not reconcile these values with the claimed hot-carrier mobility of ~2,000 cm2 V−1 s−1; if the discrepancy is meant to be resolved by a high effective carrier temperature, that temperature and its uncertainty should be stated and propagated into the TAM-based propagation lengths.","section":"Spatiotemporal and energetic evolution of non-equilibrium photoexcitation (Fig. 4g)"}],"minor_comments":[{"comment":"In the sentence 'the Cu valence state can sever as an effective knob', 'sever' should be 'serve'.","section":"Synthesis and characterization of Cu3BHT films"},{"comment":"'Owning to the low optical phonon energy' should read 'Owing to the low optical phonon energy'.","section":"Main text, introductory paragraphs"},{"comment":"The triclinic lattice parameters are reported as 'a = β = 99.94°, and g = 60.12°'; these should presumably be α = β = 99.94° and γ = 60.12°.","section":"Synthesis and characterization of Cu3BHT films"},{"comment":"'D of hot carriers observed in the synthesized Cu3BHT film is superior that of hot carriers' should be 'superior to that of hot carriers'.","section":"Spatiotemporal and energetic evolution of non-equilibrium photoexcitation"},{"comment":"The abstract states 'traversing grain boundaries up to 300 nm within a picosecond' while the text says the propagation length ranges from 200 to 320 nm; please ensure consistency.","section":"Abstract and main text"}],"recommendation":"major_revision","confidential_remarks":"The manuscript is within scope and of significant interest. The main risk is that the fast THz decay is interpreted as a mobility decrease without a direct population measurement; the TAM data mitigate but do not eliminate this concern. I recommend major revision rather than rejection because the qualitative finding is likely robust, but the quantitative mobility claims need additional support or caveats."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Here's the short version: this is the first paper I know of that makes a credible case for hot-carrier transport in a 2D conjugated coordination polymer, and the multi-technique attack is genuinely good. But the headline number—~2,000 cm²/Vs for hot carriers—is softer than the abstract implies, and the paper would be stronger if the authors said so.\n\nWhat's new: the synthesis and structural work is careful (AC-HRTEM, GIWAXS, XANES), and the TRTS data show a clean Drude-like response at late times with a scattering time of ~41 fs and a band-edge mobility of ~400 cm²/Vs, with an Arrhenius temperature dependence pointing to a ~8 meV phonon mode. The photon-energy dependence of the early THz transient, together with the Tauc edge at ~0.5 eV and the extrapolated critical photon energy of ~560 meV, is a nice internal check. The TAM experiment is the most convincing independent evidence: the initial spatial expansion is fast, with diffusion coefficients of ~700–1200 cm²/s, and it increases with pump photon energy. That's a real observation of a highly mobile non-equilibrium species.\n\nWhere I'd push back: the conversion of the early-time THz decay into a hot-carrier mobility relies on n being constant over the first ~10 ps. The linear ∆σ_offset vs Nabs check doesn't rule out a fluence-independent fast trapping channel that removes a fixed fraction of carriers within the first picosecond—and the paper itself invokes trap-assisted recombination (t2 ≈ 1.2 ns), so traps are clearly present. I don't see a direct sub-picosecond population measurement anywhere. Second, the early-time THz spectrum is fit with Drude-Smith, not Drude, so the ratio of photoconductivity at the THz probe frequency to the Drude DC mobility is not automatically valid; without reporting c and τ_DS and applying the finite-frequency correction, the ~2,000 cm²/Vs value is an estimate, not a measurement. The authors should give an uncertainty budget for that number, or at minimum present it as an order-of-magnitude inference.\n\nThat said, the stress-test concern doesn't sink the paper. The TAM data independently establish that something fast and mobile exists, and the qualitative picture—hot carriers with higher mobility than band-edge carriers—survives. The quantitative value needs work.\n\nWho this is for: anyone working on 2D c-CPs, hot-carrier physics in organic or hybrid systems, or ultrafast THz spectroscopy. It deserves a serious referee. I'd recommend sending it out, but with a request for the Drude-Smith parameters, error bars on the 2,000 cm²/Vs, and a more careful treatment of the n=const assumption.","headline":"First credible case for hot-carrier transport in a 2D coordination polymer; the 2,000 cm²/Vs headline is an order-of-magnitude estimate, not a measured value.","tokens_in":15341,"tokens_out":2958,"would_cite":true,"duration_ms":28693,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Ultrafast terahertz and microscopy measurements show that hot carriers in the two-dimensional coordination polymer Cu3BHT move with ~2,000 cm² V⁻¹ s⁻¹ mobility and cross grain boundaries up to ~300 nm within a picosecond.","keywords":["hot carriers","terahertz spectroscopy","two-dimensional conjugated coordination polymers","Cu3BHT","charge mobility","Drude transport","transient absorption microscopy","hot phonon bottleneck"],"falsifier":"Measure the carrier density directly in the first picosecond, for example by time-resolved photoemission or by comparing the early-time spectrally integrated transient absorption amplitude with the THz photoconductivity, and check whether the density is constant while the THz signal decays. If the density decays on the same ~500 fs timescale as the fast component, the fast decay is carrier loss, not hot-carrier cooling, and the ~2,000 cm² V⁻¹ s⁻¹ estimate collapses.","tokens_in":14238,"feed_emoji":"⚡","tokens_out":12306,"duration_ms":98831,"temperature":0.7,"pith_summary":"This paper claims that in films of the two-dimensional conjugated coordination polymer Cu3BHT, photoexcited hot carriers keep their excess kinetic energy for hundreds of femtoseconds and move with a mobility near 2,000 cm² V⁻¹ s⁻¹, crossing grain boundaries up to 300 nm within a picosecond. Once the carriers cool to the band edge, they continue to move by Drude-like band transport with a mobility around 400 cm² V⁻¹ s⁻¹ and an intrinsic diffusion length beyond one micrometre. The authors separate the two regimes with time-resolved terahertz spectroscopy, transient absorption spectroscopy, and transient absorption microscopy, and they identify a hot-phonon bottleneck that slows cooling at high excitation densities. If the interpretation holds, it overturns the assumption that organic-based systems cool too quickly and transport too poorly to host hot-carrier phenomena, opening a route to hot-carrier devices built from solution-processed two-dimensional coordination polymers.","feed_headline":"Hot carriers in an organic 2D film reach 2,000 cm²/Vs mobility","feed_subtitle":"Terahertz and microscopy reveal hot carriers in a 2D coordination polymer outrunning band-edge carriers.","key_machinery":"The central object is the time-resolved THz photoconductivity transient $\\Delta\\sigma(t) = n e \\mu$, normalized by the absorbed photon number $N_\\mathrm{abs}$. Under the assumption that the carrier density $n$ is constant over the first ~10 ps, $\\Delta\\sigma/N_\\mathrm{abs}$ directly tracks the temporal evolution of carrier mobility, and the paper's 'fast decay is a mobility drop' interpretation is carried by the photon-energy dependence of $\\Delta\\sigma_\\mathrm{peak}/\\Delta\\sigma_\\mathrm{offset}$ together with Drude and Drude-Smith fits to the frequency-resolved complex photoconductivity. The Drude-Smith model, with its backscattering parameter $c$ between 0 and $-1$, supplies the description of hot carriers whose long excursions across grain boundaries make them susceptible to backscattering, while the Drude model describes the delocalized band-edge carriers. A secondary mechanism is the hot-phonon bottleneck: low-energy optical phonons near 100 cm⁻¹ and acoustic-optical phonon up-conversion slow carrier cooling at high excitation densities.","core_discovery":"The central claim is that the fast-decaying component of the THz photoconductivity transient in Cu3BHT films is a mobility drop, not a carrier-density drop. Under photoexcitation above the ~0.5 eV band edge, the normalized photoconductivity $\\Delta\\sigma/N_\\mathrm{abs}$ rises to a peak and decays within about one picosecond to a plateau that persists beyond a nanosecond; the ratio $\\Delta\\sigma_\\mathrm{peak}/\\Delta\\sigma_\\mathrm{offset}$ grows with pump photon energy and extrapolates to unity at ~560 meV, matching the absorption edge. The authors therefore assign the peak to hot carriers with a mobility of ~2,000 cm² V⁻¹ s⁻¹, obtained by scaling the Drude mobility of band-edge carriers (~405 cm² V⁻¹ s⁻¹, from fits with a scattering time of ~41 fs and a reduced mass of 0.187 $m_0$) by the measured peak-to-offset ratio. Hot-carrier cooling times of ~500 fs, rising to ~750 fs above a fluence threshold, plus a blue shift in the transient absorption band that tracks the THz decay, support the assignment. Real-space imaging by transient absorption microscopy shows an ultrafast expansion with diffusion coefficients from ~680 to ~1,200 cm² s⁻¹, with the larger value at higher photon energy.","pith_inferences":["A testable extension the authors do not report: vary the grain size of Cu3BHT films and check whether the hot-carrier propagation length stays near 300 nm; if it scales with grain size, the cross-boundary claim is directly confirmed, and if it does not, grain boundaries are not the main scatterers.","The extrapolated critical photon energy of ~560 meV matching the Tauc edge suggests the same peak-to-offset ratio could become a quick spectroscopic estimate of the band gap in other coordination polymers, without transport modelling.","The hot-phonon bottleneck picture implies the fluence threshold for cooling slowdown should shift if the phonon spectrum is altered by isotope substitution or by exchanging copper for another metal; measuring the threshold in a series of substituted films would probe the mechanism directly."],"forward_implications":["If the interpretation is right, Cu3BHT becomes a candidate for hot-carrier photovoltaic and hot-electron transistor devices that require charge extraction before the carriers cool.","The ~750 fs hot-carrier cooling time, comparable to lead-halide perovskites, means solution-processable two-dimensional coordination polymers could work as organic hot-carrier platforms despite their normally strong electron-phonon coupling.","The intrinsic diffusion length of band-edge carriers exceeding one micrometre implies that Cu3BHT films can transport carriers across typical device feature sizes without recombination losses.","Because two-dimensional conjugated coordination polymers are tunable through metal substitution and ligand design, the same transport regimes should be searchable across the wider family, making the result a proof-of-principle rather than a single-material curiosity."],"supporting_citations":[{"why":"Supplies the time-resolved THz spectroscopy method and the photoconductivity analysis used to extract carrier mobility and dynamics.","marker":"[49]"},{"why":"Provides the precedent that a fast-decaying THz photoconductivity component can stem from highly mobile hot carriers, the interpretive template applied to Cu3BHT.","marker":"[52]"},{"why":"Sets the benchmark long-range hot-carrier diffusion in hybrid perovskites against which the Cu3BHT diffusion coefficients are compared.","marker":"[10]"},{"why":"Supplies the experimental signature and interpretation of the hot-phonon bottleneck used for the fluence-dependent cooling time.","marker":"[53]"},{"why":"Provides the precedent for acoustic-optical phonon up-conversion as the origin of the hot-phonon bottleneck, applied here to Cu3BHT.","marker":"[54]"},{"why":"Defines the Drude-Smith model whose backscattering parameter quantifies the confined hot-carrier transport in the non-equilibrium regime.","marker":"[55,56]"},{"why":"Supplies the liquid-liquid interfacial synthesis protocol for Cu3BHT films, the material platform of the study.","marker":"[32]"},{"why":"Provides the calculated phonon dispersion with low-energy optical branches near 100 cm⁻¹ that underpins the slow-cooling and hot-phonon-bottleneck argument.","marker":"[41]"}],"fun_headline_variants":["Hot carriers in organic 2D film hit 2,000 cm²/Vs","Organic 2D polymer hot carriers sprint to 2,000 cm²/Vs","Hot carriers outrun band-edge in 2D organic polymer","2D organic coordination polymer: hot carriers at 2,000 cm²/Vs"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The claim rests on the assumption that the photogenerated carrier density stays constant during the first ~10 picoseconds, so the fast drop in the normalized THz signal is a drop in mobility rather than a drop in the number of carriers; if trapping or recombination removes a significant fraction of carriers within the first picosecond, the inferred hot-carrier mobility would be too high.","fun_headline_variants_meta":{"raw":{"variants":["Hot carriers in organic 2D film hit 2,000 cm²/Vs","Organic 2D polymer hot carriers sprint to 2,000 cm²/Vs","Hot carriers outrun band-edge in 2D organic polymer","2D organic coordination polymer: hot carriers at 2,000 cm²/Vs"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000903,"raw_usage":{"total_tokens":3953,"prompt_tokens":1079,"completion_tokens":2874,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":695,"completion_tokens_details":{"reasoning_tokens":2788}},"tokens_in":695,"tokens_out":2874,"duration_ms":21089,"temperature":1.0,"reasoning_tokens":2788,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:19:23.677384+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the carrier density directly in the first picosecond, for example by time-resolved photoemission or by comparing the early-time spectrally integrated transient absorption amplitude with the THz photoconductivity, and check whether the density is constant while the THz signal decays. If the density decays on the same ~500 fs timescale as the fast component, the fast decay is carrier loss, not hot-carrier cooling, and the ~2,000 cm² V⁻¹ s⁻¹ estimate collapses.","supporting_citations":[],"review_version":1}