{"id":"175763df-afaa-4aa8-9c51-799a4047743a","arxiv_id":"2411.09392","paper_version":1,"verdict":"CONDITIONAL","confidence":"HIGH","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":3,"one_line_summary":"Fluorine adatoms on silver appear as depressions or sombreros in STM because oxidation contracts the silver 5s orbitals while fluorine 2p orbitals protrude at negative bias.","lead":"This paper uses computer simulations to show why fluorine atoms on silver surfaces appear as holes, bumps, or 'sombreros' in scanning tunneling microscope images. The pattern is governed by how fluorine oxidizes nearby silver atoms, suggesting that such images could reveal chemical states at surfaces.","discovery_kind":"new_method","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The depression-depth mechanism in Eq. (24)-(25) is not uniquely tied to oxidation: it is driven by an ad hoc change in effective charge ΔZ_eff and a re-tuned Zeff, so the oxidation-state link is not yet established.","rationale":"The paper is honest and explicitly flags its own discrepancies (AT(C), overestimated depth, underestimated width, the unresolved long-bridge absence). The reader's CONDITIONAL verdict is reasonable and I do not see grounds to move it. My concern is slightly different from the reader's weakest_assumption: the reader focuses on neglected effects (PDOS redistribution, wave-function deformation, tip polarization, off-diagonal Wannier terms) as possibly dominating. I agree those are the right risk categories, but I would sharpen the issue: the model's map from topography to oxidation state is not identifiable. Eq. (25) contains ΔZ_eff as an adjustable Slater-rule input, and the quantitative agreement is achieved only after re-tuning Zeff to the work function, which the authors themselves note puts the tip in the oscillatory region of the wave function. So the analytical model does not independently confirm the DFT result; it is a post-hoc rationalization. The DFT PDOS/charge-transfer analysis is independent evidence, but the connection from DFT charge transfer (≤0.05 e on Ag 5s, Table II and Sec. III D) to a ~10-30 pm depression is not directly demonstrated: Table II reports total Bader/sphere charges, and the text says the 5s transfer is larger than the total, but no direct calculation shows that contracting the 5s Wannier function by the computed δ reproduces the DFT topography. A neutral-atom or point-charge control would discriminate. I also note the citation issue flagged by the reader (Ref. [53] appears to be a cuprate transport paper, not an Ag(110) adsorption study), which is real and should be corrected, but it is peripheral to the central physical claim. The kinetic vs thermodynamic point (Sec. III B) is argued from a phase diagram and the absence of bulk fluorides in experiment; it is plausible but also depends on the PBEsol dissociation-energy error, which the authors acknowledge. That is a secondary concern. Overall: CONDITIONAL is the right verdict, and the concrete test above would settle whether the oxidation-state interpretation is uniquely supported.","tokens_in":25088,"tokens_out":2117,"duration_ms":19018,"concrete_test":"Recompute the constant-current topography for a model system in which the F adatom is replaced by a neutral closed-shell atom (or a negative point charge with no covalent charge transfer, e.g., a noble-gas atom at the same height) using the same DFT + Tersoff-Hamann pipeline. If a comparably deep topographic depression appears without any Ag oxidation (δ≈0), then the depression is not a specific signature of oxidation state and the central claim fails. If no significant depression appears, the oxidation-state mechanism is supported. A second, cheaper check: in the DFT-calculated constant-height LDOS, decompose the change at the tip position into (i) the change due to Ag 5s Wannier-density contraction and (ii) the change due to the F 2p and off-diagonal cross terms; Eq. (16) is only valid if (ii) is below ~10% of (i).","verdict_should_be":"CONDITIONAL","load_bearing_attack":"The central claim — that the apparent STM depression tracks the oxidation state of nearby Ag atoms — rests on the orbital model of Sec. III D, specifically Eq. (24) and Eq. (25). In Eq. (25), the depression depth is proportional to ΔZ_eff, and ΔZ_eff = 0.35δ/ζ is inserted by Slater rules, not computed or derived from the DFT electronic structure. The model then separates the density effect from the PDOS effect by asserting from Fig. 8 that ΔG5s/G∞5s is negligible; but the quantitative comparison is made only after adjusting Zeff so that the ionization energy equals the silver work function, and the resulting wave function is said to put the tip in the oscillatory region. That admission means the analytical prediction is not a robust derivation of the depression depth. More importantly, the depression can be reproduced without invoking oxidation: any mechanism that contracts or depresses the Ag 5s density near the F adatom (wave-function deformation, tip-induced polarization, off-diagonal Wannier terms dropped in Eq. (16)) would produce the same constant-current topography. The paper does not provide a discriminating test that the topographic depression is specifically due to the Slater-rule contraction from charge transfer rather than to the electrostatic potential of the F- ion or other perturbations. Without that discrimination, the headline assertion that apparent topographies encode local oxidation states is under-supported, even though the qualitative association between deeper wells and bridge sites (ζ=2) vs hollow (ζ=4) is suggestive.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper combines DFT calculations, Tersoff–Hamann STM simulations, and a simplified hydrogenic orbital model to analyze the early stages of fluorine adsorption on Ag(100) and Ag(110). The authors compute adsorption energies for several high-symmetry sites, construct a thermodynamic coverage phase diagram, and argue that under typical experimental conditions the surface state is controlled by kinetics rather than equilibrium thermodynamics. They then simulate STM apparent topographies, propose assignments of the experimental features to specific adsorption sites, and introduce a model in which the STM depression is attributed to oxidation-induced contraction of Ag 5s orbitals while the protrusion is attributed to filled F 2p orbitals. The central claim is that apparent STM heights can encode the local oxidation state of metal atoms near the adatom.","tokens_in":25296,"tokens_out":7220,"duration_ms":68334,"significance":"If the central claim holds, the paper would provide a broadly applicable interpretive framework for STM of atomic adsorbates, potentially allowing local valence information to be extracted from topographic images. The DFT calculations are carefully specified and the authors are commendably transparent about discrepancies: depression depth overestimated, width underestimated by about a factor of 0.5, the AT(C) assignment questioned, and the short-bridge sombrero mismatch at -1.5 V. The orbital model is simple, falsifiable, and yields qualitative trends that are consistent with both DFT and experiment. However, the quantitative link between depression depth and oxidation state is not yet established, for the reasons detailed in the major comments.","major_comments":[{"comment":"The numerical estimate in Eq. (25) is obtained with a full-electron oxidation state that is inconsistent with the DFT charge transfer reported in the same paper. The text states that 'we assume that 1/ζ of an electron is transferred to the fluorine' and the numerical check after Eq. (25) uses Zeff = 0.35/ζ, i.e., δ = 1, yielding |Δz| ≈ 15–30 pm. Yet Table II reports δn ≈ 0.16–0.20 e on F, and Sec. III A 1 explicitly uses the smallness of δn to argue that dipole–dipole interactions are weak. Since Eq. (25) is linear in δ, using the tabulated charge transfer would give depression depths of roughly 3–6 pm, an order of magnitude smaller than the DFT and experimental values. The authors must reconcile the effective charge transfer used in the orbital model with the DFT electronic structure, or show through an explicit calculation that the depression depth is insensitive to δ because of a compensating change in the calibrated Zeff.","section":"Sec. III D, Eq. (25), and Table II"},{"comment":"The derivation does not establish that the depression is specifically caused by Slater-rule contraction of Ag 5s orbitals. The only oxidation-dependent input is the ad hoc ΔZeff = 0.35δ/ζ, and the quantitative comparison is performed only after re-tuning Zeff so that the Ag 5s ionization energy coincides with the silver work function, with the resulting tip position falling in the oscillatory region of the hydrogenic density (text after Eq. (25) and Fig. 10(a)). Under those conditions the asymptotic expression (A4) is not valid, as the authors acknowledge. Other mechanisms, such as the electrostatic potential of the F− ion, tip-induced polarization, or the off-diagonal Wannier products discarded in Eq. (16), would also reduce the Ag 5s density at the tip height. A discriminating test is needed: for example, computing from the DFT Kohn–Sham states the constant-height changes Δρ5s and ΔG5s at the hollow site and comparing their magnitude and radial dependence with the Slater-contracted hydrogenic prediction, rather than only comparing final apparent heights.","section":"Sec. III D, Eqs. (24)–(25)"},{"comment":"The neglect of ΔG5s/G∞5s is asserted but not quantified. The text says that from Fig. 8 the integrated 5s PDOS is 'practically unchanged' and that 'ΔG5s/G∞5s ≪ 1', but Fig. 8 shows PDOS curves for one Ag neighbor and does not display the bias-integrated differences that enter Eq. (24). Because G05s and G∞5s appear in Eq. (24) with the same weight as Δρ5s, a numerical bound on ΔG5s/G∞5s over the full bias range is required before Eq. (24) can be reduced to Eq. (25). If the integrated PDOS change is not small, the depression depth depends on bias and on the cumulative DOS, which would undermine the claimed bias independence of the depression depth (point i after Eq. (25)).","section":"Sec. III D, Fig. 8 and Eq. (24)"}],"minor_comments":[{"comment":"Ref. [53] appears to be a mis-citation: the listed paper (Wang et al., Phys. Rev. B 64, 224519 (2001)) is on cuprate superconductors, not on fluorine adsorption on Ag(110). Please cite the correct DFT study with which the 0.5 ML adsorption energies are compared.","section":"References"},{"comment":"The phrase 'the energy of a F 2 molecule' should read 'the energy of an F2 molecule'.","section":"Sec. II B"},{"comment":"The quantity G2p(VB) is used without being defined; please define it as the cumulative F 2p PDOS, analogous to G5s(VB).","section":"Eq. (26)"},{"comment":"The sentence 'the 4 s charge transfer is smaller in magnitude' appears to be a typo for '5s charge transfer', since the surrounding discussion concerns the Ag 5s channel and the 4s orbital is not otherwise considered.","section":"Sec. III D"},{"comment":"The notation is inconsistent: both kbT and kBT appear, as do ¯h and h. Please standardize the symbols for the Boltzmann constant and Planck constant.","section":"Eq. (13)"}],"recommendation":"major_revision","confidential_remarks":"The most serious issue is the quantitative inconsistency between the full-electron oxidation state used in the orbital model and the DFT charge transfer of about 0.18 e reported in Table II. As written, the claimed 15–30 pm agreement appears to be an artifact of using δ = 1 rather than the computed charge transfer. If a revised analysis with the DFT charge transfer cannot reproduce the depression depths, the conclusion that STM topography encodes local oxidation states should be downgraded to a qualitative hypothesis. The paper is otherwise within the journal's scope, and the DFT/STM simulations are carefully documented; the authors' explicit discussion of limitations is a strength."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Colleague,\n\nThe paper is a genuine attempt to explain STM apparent topographies of F on Ag(100) and Ag(110) with a simple orbital picture. The new part is the hydrogenic-orbital model that attributes the depression to oxidation-induced contraction of Ag 5s orbitals and the protrusion to filled F 2p states at negative bias. That goes beyond the jellium and tight-binding literature, and it is a useful way to think about these images.\n\nThe DFT work is solid: careful slab calculations, energetics for multiple sites and coverages, and thermodynamics showing kinetics dominates under typical fluorination conditions. The authors are refreshingly honest about where theory and experiment disagree — depth overestimated, width underestimated, AT(C) unresolved, short-bridge sombrero missing at -1.5 V. They also provide analytical expressions (Eqs. 24-26) that make the model testable.\n\nThe soft spot is the load-bearing claim. The depression depth in Eq. 25 is proportional to a Slater-rule ΔZ_eff inserted by hand, not extracted from the DFT. The quantitative match needs Z_eff tuned to the silver work function and a Gaussian FWHM of 4.7 Å that is effectively fit. The stress-test note is right that any mechanism contracting the Ag 5s density near the adatom — wave-function deformation, the electrostatic potential of F-, tip effects — would produce a similar depression. The paper does not provide a discriminating test that it is specifically the oxidation-driven Slater contraction. So the conclusion's phrasing \"The depression is attributed to the oxidation...\" is stronger than the evidence supports. The authors themselves qualify it later as a suggestion, so softening the language would help.\n\nAlso, the citation to Wang et al. [53] looks wrong — that reference appears to be a superconductivity paper, not Ag(110) fluorination. It should be corrected. Minor: the diagonal approximation in Eq. 16 drops off-diagonal Wannier terms; at the 5-6 Å tip distance those could matter, so the model's quantitative reach is limited.\n\nOverall, this is a thoughtful paper with a genuinely new interpretive framework, even if the central claim is not uniquely established. It deserves peer review. I would send it to a referee who knows STM theory and ask them to press on whether the oxidation link is distinguishable from other density-contracting mechanisms. If the authors provide input files, justify the smoothing and effective-charge choices, and correct the citation, the claim would be much stronger.","headline":"Fresh orbital model for F/Ag STM topographies, but the oxidation-state link is a plausible rationalization, not a proven mechanism.","tokens_in":26024,"tokens_out":3795,"would_cite":true,"duration_ms":35838,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"STM depressions over fluorine on silver reveal oxidized silver neighbors","keywords":["scanning tunneling microscopy","apparent topography","fluorine adsorption","silver surfaces","density functional theory","oxidation state","effective nuclear charge screening","s-wave tip approximation"],"falsifier":"Measure the apparent depression depth of a fluorine adatom on Ag(100) as a function of tip–sample distance and of an independent local-oxidation probe such as X-ray photoemission core-level shifts, and compare with Eq. (25): the depth should grow linearly with tip height and with the per-atom charge loss. A clear deviation, or a depression that does not scale with per-atom charge loss across different adsorption sites, would falsify the screening-contraction mechanism.","tokens_in":24797,"feed_emoji":"🔬","tokens_out":9836,"duration_ms":88432,"temperature":0.7,"pith_summary":"This paper combines density functional theory with simulated scanning tunneling microscopy to work out where fluorine atoms sit on silver (100) and (110) surfaces and why those atoms look the way they do. Its central proposal is that an adatom's apparent shape is set by two opposite orbital effects: oxidation of the neighboring silver atoms contracts their $5s$ orbitals, producing the topographic depression seen at positive bias, while the filled $2p$ orbitals of the fluoride ion protrude above that depression at negative bias, producing the sombrero shape. The authors reduce the depression depth to a simple analytic expression proportional to the charge removed from each neighboring silver atom, with matrix elements and coordination numbers cancelling out. If the model is right, STM apparent topographies during halogenation carry local oxidation-state information, not just geometry. The paper also argues that under typical fluorination conditions the observed surface is a kinetic state, since thermodynamics would favor bulk silver fluoride.","feed_headline":"STM depressions over fluorine on silver reveal oxidized neighbors","feed_subtitle":"A minimal orbital model reads the local oxidation state of silver atoms straight from the microscope's apparent topography.","key_machinery":"The load-bearing object is a minimal orbital model of the tunneling current built from atom-centered orbitals with hydrogenic radial shapes and screened effective charges. Starting from the s-wave tip approximation, the model keeps only diagonal orbital contributions, so the local DOS at the tip is a sum over shells of the projected DOS times the shell density (Eq. (18)); at tip distances of 5–6 Å the silver $4d$ states, despite their large projected DOS, are too short-ranged to matter, leaving Ag $5s$ and F $2p$ as the active channels. Oxidation of a silver neighbor is represented by a screening-rule increase in its effective nuclear charge, $\\Delta Z^{5s}_{\\rm eff}=0.35\\,\\delta/\\zeta$, which contracts the $5s$ orbital and lowers the constant-current height; the analytic depression formula (Eq. (25)) cancels matrix elements and coordination factors, leaving a depth that depends only on geometry, effective charge, and cumulative DOS. The protrusion is controlled by the competing F $2p$ and Ag $5s$ densities and cumulative DOS (Eq. (26)), which is why it appears only at negative bias where F $2p$ is occupied.","core_discovery":"The paper's central claim is that the apparent topography of a fluorine adatom on Ag(100) and Ag(110) is governed by two orbital channels with opposite signs. At positive bias the tunneling current is carried by silver $5s$ states; the fluorine removes roughly $1/\\zeta$ of an electron from each of its $\\zeta$ nearest silver neighbors, and by the standard screening rule this raises the effective nuclear charge of those silvers, contracts their $5s$ orbitals, and produces a topographic depression whose depth is approximately independent of bias and proportional to the per-atom charge depletion (Eqs. (24)–(25)). At negative bias the filled F $2p$ states contribute strongly, and because the fluorine sits above the surface the $2p$ evanescent density can rise above the depressed silver contour, producing the central protrusion that turns the feature into a sombrero (Eq. (26)). The same model assigns the experimental topographies to specific adsorption sites: the hollow site on Ag(100), and the short bridge, long bridge, and hollow sites on Ag(110), with the rarest observed feature still unresolved. The paper further argues that under realistic fluorination conditions bulk silver fluorides are thermodynamically stable, so the adatom configurations seen in experiments are set by kinetic barriers and sticking rather than by equilibrium.","pith_inferences":["Beyond the paper: the same two-channel mechanism should apply to other electronegative adsorbates such as oxygen, sulfur, or chlorine on silver; any adsorbate that oxidizes its nearest metal neighbors should create a positive-bias depression whose depth tracks the per-atom charge loss, which would unify the sombrero shapes reported for S/Ag and O/Ag.","Beyond the paper: a direct test would compare STM depression depths with core-level shifts from X-ray photoemission on the same surface; agreement would confirm the oxidation link, while disagreement would point to wave-function deformation or $4d$ screening as the controlling factor.","Beyond the paper: because Eq. (25) predicts depression depth grows roughly linearly with tip height, systematic constant-current measurements over a wide tip–sample distance range should expose where the diagonal-orbital approximation breaks down and off-diagonal channels or tip-induced polarization take over.","Beyond the paper: the unresolved rarest feature (AT(C)) might be settled by searching off-symmetry adsorption positions or mixed F/H adsorbates; the model's volcano-shaped fingerprint for a vacancy site is a specific prediction that could be tested by deliberately creating single Ag vacancies."],"forward_implications":["After subtracting the central protrusion, the depth of the STM depression over a fluorine adatom is a direct measure of the oxidation state of the neighboring silver atoms: depth grows with the fraction of an electron removed per neighbor and is essentially independent of bias.","The sombrero protrusion appears only at negative bias, where the filled F $2p$ states contribute to the current, and vanishes wherever the Ag $5s$ channel dominates the tunneling.","On Ag(100) the stable hollow-site adatom, and on Ag(110) the three near-degenerate short-bridge, long-bridge, and hollow-site adatoms, can be matched to the experimental topographies by combining adsorption energetics, simulated images, and the orbital model.","Under the pressures and temperatures of typical fluorination experiments, the clean-to-fluoride equilibrium would favor bulk AgF or AgF$_2$, so the adatom coverages observed in experiments are kinetic states controlled by exposure time and sticking coefficient.","The same reasoning implies STM can supply local-valence information on metal surfaces during reactions, not merely geometric corrugation."],"supporting_citations":[{"why":"provides the experimental STM topographies and adsorption-site assignments on Ag(100) and Ag(110) that the paper reproduces and refines.","marker":"[25]"},{"why":"supplies the s-wave tip approximation that relates the tunneling current to the sample local density of states in all simulated images.","marker":"[29]"},{"why":"implements the STM topography computation from the DFT densities.","marker":"[30, 31]"},{"why":"gives the classical screening rule (0.35 per transferred electron) that turns oxidation of neighboring silver into $5s$ orbital contraction.","marker":"[61]"},{"why":"supplies the screened effective charges used to estimate the Ag $4d$ and $5s$ orbital radii.","marker":"[62]"},{"why":"provides the fluoride ionic radius used to set the effective charge of the F $2p$ orbitals.","marker":"[65]"},{"why":"provides the ab initio thermodynamics framework used to translate adsorption energies into a coverage–chemical-potential phase diagram.","marker":"[32]"},{"why":"is the earlier tight-binding theory of adatom apparent topographies whose assumptions this orbital model extends.","marker":"[28]"},{"why":"supports decomposing the tunneling current into through-surface and through-adsorbate channels for the adatom feature.","marker":"[66]"}],"fun_headline_variants":["Bias flips between depression and sombrero for F on Ag","STM contrast of F on Ag tracks orbital channels","Fluorine's fingerprint in STM reveals oxidized neighbors","Kinetic control sets F adatom sites on Ag surfaces","Orbital model decodes bias-dependent F/Ag STM"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The argument stands on the assumption that the dominant effect of the fluorine on neighboring silver is the classical screening rule: every fraction of an electron removed shrinks the Ag $5s$ orbital by the textbook 0.35 factor, while wave-function deformation, $4d$ screening, off-diagonal orbital overlap, and tip-induced polarization are all small enough to ignore.","fun_headline_variants_meta":{"raw":{"variants":["Bias flips between depression and sombrero for F on Ag","STM contrast of F on Ag tracks orbital channels","Fluorine's fingerprint in STM reveals oxidized neighbors","Kinetic control sets F adatom sites on Ag surfaces","Orbital model decodes bias-dependent F/Ag STM"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000902,"raw_usage":{"total_tokens":3918,"prompt_tokens":1015,"completion_tokens":2903,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":631,"completion_tokens_details":{"reasoning_tokens":2819}},"tokens_in":631,"tokens_out":2903,"duration_ms":23239,"temperature":1.0,"reasoning_tokens":2819,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-12T20:41:19.191249+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure the apparent depression depth of a fluorine adatom on Ag(100) as a function of tip–sample distance and of an independent local-oxidation probe such as X-ray photoemission core-level shifts, and compare with Eq. (25): the depth should grow linearly with tip height and with the per-atom charge loss. A clear deviation, or a depression that does not scale with per-atom charge loss across different adsorption sites, would falsify the screening-contraction mechanism.","supporting_citations":[{"cited_title":"The initial stages of silver fluorination: a scanning tunneling microscopy investigation","cited_arxiv_id":"2410.04858","evidence_quote":"provides the experimental STM topographies and adsorption-site assignments on Ag(100) and Ag(110) that the paper reproduces and refines."},{"cited_title":"Lee, Y.-J","cited_arxiv_id":null,"evidence_quote":"supplies the s-wave tip approximation that relates the tunneling current to the sample local density of states in all simulated images."},{"cited_title":"Scheffler and C","cited_arxiv_id":null,"evidence_quote":"gives the classical screening rule (0.35 per transferred electron) that turns oxidation of neighboring silver into $5s$ orbital contraction."},{"cited_title":"Linstrom and W","cited_arxiv_id":null,"evidence_quote":"supplies the screened effective charges used to estimate the Ag $4d$ and $5s$ orbital radii."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"provides the fluoride ionic radius used to set the effective charge of the F $2p$ orbitals."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"provides the ab initio thermodynamics framework used to translate adsorption energies into a coverage–chemical-potential phase diagram."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"is the earlier tight-binding theory of adatom apparent topographies whose assumptions this orbital model extends."}],"review_version":1}