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REVIEW 3 major objections 4 minor 21 references

Study of gold and bismuth electrical contacts to a MoS$_{2}$ monolayer

T0 review · 3 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read Bismuth is the better contact metal for MoS2 monolayers.

desk verdict A modest experimental report with an overreaching headline claim; the data are honest but the Au/Bi comparison is confounded by geometry and processing. read the letter →

arxiv 2501.07444 v1 pith:CYH47CVQ submitted 2025-01-13 cond-mat.mes-hall cond-mat.mtrl-sci

Agata Zielińska , Joanna Prażmowska-Czajka , Mateusz Dyksik , Jonathan Eroms , Dieter Weiss , Regina Paszkiewicz , Mariusz Ciorga This is my paper · ORCID
classification cond-mat.mes-hallcond-mat.mtrl-sci
keywords transition-metaldichalcogenidesohmiccontactsSchottkyMoS2monolayerbismuthAFMironingcontactresistancephotoluminescence
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

The paper asks whether semimetal bismuth can outperform conventional metallic gold as an electrical contact to a monolayer of the transition-metal dichalcogenide MoS2, a material of interest for future electronic devices. It reports that on bismuth-contacted devices the current-voltage characteristics stay linear (ohmic) from 280 K down to 100 K and carry substantially more current, whereas gold-contacted devices show Schottky-like, gate-dependent curves with much lower current. It also reports that a contact-mode atomic force microscopy step, called AFM ironing, raises the number of working gold contacts from two to six per sample and gives a Schottky barrier height of $0.1617(41)\,\mathrm{eV}$, slightly below the literature average of about $0.19\,\mathrm{eV}$. The conclusion is that bismuth is the better choice for contacting MoS2, although the measured bismuth contact resistance is still high.

What carries the argument

The named central object is the metal-induced gap state (MIGS): wavefunctions from the metal penetrating into the semiconductor bandgap, which pin the Fermi level and make the Schottky barrier nearly independent of the metal. Bismuth is used because, as a semimetal, its near-zero density of states at the Fermi level should suppress these gap states and yield ohmic contacts. The other load-bearing mechanism is AFM ironing, a contact-mode AFM scan that removes residues, bubbles, and wrinkles at the MoS2/metal interface; the paper credits it with raising the gold-contact yield and lowering the barrier. The barrier analysis is carried by the thermal transport (thermionic emission) model for two back-to-back Schottky diodes, using an Arrhenius plot of $\ln(I/T^{3/2})$ to extract the effective barrier and the flat-band voltage to obtain the true value.

What would settle it

Fabricate gold and bismuth contacts on the same MoS2 monolayer using identical geometry, metal thickness, lithography, and either ironing for both or none, then measure I-V curves at 100 K: if only the bismuth devices remain linear, the material-level claim survives, and if linearity follows the geometry or processing instead, the claim fails.

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Extended reading notes

Core claim

The central claim, stated in the Conclusions, is 'We can confirm that bismuth is better than gold for contacting MoS2.' The evidence is a comparison of two sample types: gold contacts made by placing an exfoliated MoS2 flake on top of pre-patterned Ti/Au electrodes and then ironing the flake with an AFM tip, versus bismuth contacts made by evaporating Bi/Au on top of an etched MoS2 flake without ironing. The gold devices display Schottky behavior, losing conduction at 190, 90, or 2 K depending on the contact pair, and permit extraction of a barrier of $0.1617(41)\,\mathrm{eV}$. The bismuth devices display linear I-V curves down to 100 K with substantially higher current, although the transfer-length method gives a contact resistance of $522(99)\,\mathrm{k\Omega\cdot\mu m}$, about three orders of magnitude above the best reported bismuth/MoS2 value. The paper also reports an extra photoluminescence peak near $1.86\,\mathrm{eV}$ only in bismuth-contacted samples, which it attributes tentatively to the bismuth evaporation process.

Load-bearing premise

The comparison assumes that gold and bismuth devices differ only in contact metal, but the two device types also differ in contact geometry, metal stack thickness, etching, and use of AFM ironing, so those processing differences could account for part or all of the electrical contrast.

Editorial extensions

If this is right

  • If bismuth contacts are genuinely ohmic on MoS2, semimetal contacting becomes a practical route to low-barrier TMDC devices at and above 100 K.
  • AFM ironing of gold contacts appears to turn a mostly dead device into a fully contacted one, suggesting a low-cost processing fix for existing metal/TMDC stacks.
  • The measured barrier of $0.1617(41)\,\mathrm{eV}$ implies that ironed gold contacts can sit slightly below the average literature barrier, making them more transparent than typical evaporated gold.
  • The linear resistance-versus-length data support a uniform sheet resistance and extract a bismuth contact resistance that future work needs to reduce by orders of magnitude before the ohmic behavior is technologically useful.

Reading between the lines

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

  • The comparison in the paper does not isolate the contact metal: the gold and bismuth devices differ in stack order, metal thickness, etching, and ironing, so the ohmic-versus-Schottky contrast could partly reflect geometry or processing rather than the intrinsic metal choice.
  • If the extra emission line near $1.86\,\mathrm{eV}$ is a signature of the Bi/MoS2 interface, photoluminescence could serve as a quick optical check for interfacial quality in future bismuth-contacted devices.
  • Because the bismuth contact resistance is still high, a natural next test is whether thicker or differently evaporated bismuth films reduce the resistance while preserving the ohmic character.
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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 / 4 minor

Summary. The manuscript compares two contacting schemes for monolayer MoS2: gold contacts (Ti/Au, with the flake transferred on top and AFM-ironed) and bismuth contacts (Bi/Au evaporated on top of an etched flake, without ironing). For the Au-contacted sample the authors report Schottky-type I-V characteristics, extract a Schottky barrier height of 0.1617(41) eV from one contact pair, and observe that AFM ironing improves the yield of working contacts (all six contacts on ironed samples versus two on non-ironed samples). For the Bi-contacted sample they report ohmic behavior from 280 K down to 100 K, with a contact resistance Rc = 522(99) kΩ·µm and sheet resistance Rs = 1065(34) kΩ/□. Photoluminescence spectra for both sample types are also presented, including an unusual additional emission peak in the Bi-contacted sample. The Conclusions state that 'bismuth is better than gold for contacting MoS2'.

Significance. The reported data are potentially useful as a case study of two contacting approaches, and the AFM-ironing yield improvement is a concrete, falsifiable observation. The Schottky barrier extraction follows a standard thermionic-emission analysis, and the authors correctly note the high contact and sheet resistances of the Bi devices. However, the central comparative claim is not established by the presented experiments: Sample A and Sample B differ in contact metal, contact placement (bottom contacts with flake on top versus top contacts on an etched flake), metal stack thickness and composition, and processing history (AFM ironing, etching), so the observed electrical contrast cannot be uniquely attributed to the choice of metal. The manuscript would be acceptable as a report on two specific contacting procedures if the comparison were reframed, but as written the headline conclusion overreaches the evidence.

major comments (3)
  1. [§2.1 and Conclusions] The central claim that bismuth is better than gold for contacting MoS2 is confounded by differences between the two sample types. Sample A (Au) has 5 nm Ti / 20 nm Au bottom contacts with the flake transferred on top and AFM-ironed, while Sample B (Bi) has 20 nm Bi / 80 nm Au contacts evaporated on top of an etched flake, with no ironing. These differences in contact geometry, metal stack, etching, and AFM ironing each affect carrier injection; the observed ohmic-versus-Schottky contrast cannot be uniquely assigned to the contact metal. A geometry- and processing-matched control (or a within-sample Au/Bi comparison) is needed to support the conclusion as stated.
  2. [§2.2, Fig. 3] The Schottky barrier height of 0.1617(41) eV is extracted from a single contact pair (contacts 3–4); the text states that for all other contacts the current was too low, leading to large uncertainties. In addition, the 'true' barrier is determined at an assumed flat-band backgate voltage of 6 V without an independent check of the flat-band condition. Thus the comparison with the literature average of about 0.19 eV and the attribution of the lower barrier to AFM ironing rest on one contact pair and an unverified assumption.
  3. [§2.2, Fig. 4] The claim that bismuth is better than gold is also weakened by the reported Bi contact resistance Rc = 522(99) kΩ·µm and sheet resistance Rs = 1065(34) kΩ/□, which are orders of magnitude above state-of-the-art values, as the authors themselves acknowledge. Moreover, the statement that ohmic behavior 'was typical for all our devices with bismuth contacts' is not supported by per-device statistics; only one representative device is shown. Please provide device-level data or explicitly limit the conclusion to the specific, unoptimized geometry studied here.
minor comments (4)
  1. [§2.1] There is a typo in 'electron-litoghaphy'; it should read 'electron-beam lithography'.
  2. [§2.2, Eq. (2)] The propagation of uncertainty from the ideality parameter n and the linear Arrhenius fit into the reported 0.1617(41) eV value is not described; please state how the uncertainty was obtained.
  3. [§2.2, Fig. 2] The figure caption states the backgate range as -8 to 16 V while the text mentions -10 V to 16 V; please reconcile these values.
  4. [§2.2, Fig. 4(c)] The numbers overlaid on the resistance-versus-length plot are hard to read and not explained in the caption; please clarify what these labels denote.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the analysis fits I-V data to a standard thermionic emission model and benchmarks results against external literature values.

full rationale

The paper's claims are derived from direct measurements rather than from definitions or self-citations. The Au Schottky barrier height is obtained by fitting measured I-V curves to a standard back-to-back Schottky diode model (eqs. 1-3), and is then compared to an independent literature average of ~0.19 eV; no fitted parameter is relabeled as a prediction. The Bi contact resistance is extracted with a standard transfer-length-method fit to eq. 4 and compared to external best values (Shen et al., ref. 11). The 'true' SB at 6 V backgate is an assumption about the flat-band condition, not a circular reduction, because the value is still fixed by the Arrhenius fit and ideality correction from the measured data. The central comparison between Au and Bi is an experimental contrast between differently fabricated devices; while this raises a validity/confounding concern, it is not an instance of circularity because the conclusion is not equivalent to the inputs by construction. No load-bearing self-citation is used to justify the core result.

Assumptions & free parameters 5 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new physical entities. The PL peak L is an observed spectral feature, not a postulated object. The main free parameters are standard extraction outputs from thermionic emission and TLM analyses. The central claim depends on the comparability assumption listed as an axiom, which is the most fragile input.

free parameters (5)
  • Ideality parameter n = 1.02993(61)
    Extracted from fitting the I-V curve of contacts 3-4 at zero backgate, 270 K, using eq. 3; used to compute the Schottky barrier height.
  • Effective Schottky barrier heights from Arrhenius fits = Varies with backgate; true value 0.1617(41) eV at 6 V
    Obtained by linear fits of ln(I/T^3/2) versus 1/T at V_DS = -1.9 V for a range of backgate voltages; the reported value corresponds to the assumed flat-band condition.
  • Contact resistance Rc = 522(99) kOhm*um
    Extracted from the intercept of a linear fit of total resistance versus channel length at 280 K, using the transfer length method.
  • Sheet resistance Rs = 1065(34) kOhm/square
    Extracted from the slope of the same TLM fit; two orders of magnitude higher than typical monolayer MoS2 values.
  • Flat-band backgate voltage = 6 V
    Assumed to be the flat-band condition based on the dependence of the effective barrier on backgate; determines the reported 'true' Schottky barrier height.
assumptions (4)
  • domain assumption Thermionic emission and back-to-back Schottky diode model describes the Au/MoS2 junctions.
    Used to fit I-V curves and extract ideality and Schottky barrier height (eqs. 1-3).
  • domain assumption The flat-band condition occurs at a backgate voltage of 6 V.
    The 'true' Schottky barrier is defined as the value at this voltage, without direct confirmation of flat-band.
  • domain assumption Transfer length method assumptions: uniform sheet resistance, ohmic contacts, and contact resistance independent of channel length.
    Used to extract Rc and Rs from the resistance versus channel length plot at 280 K.
  • domain assumption Comparability of the Au and Bi device geometries, so electrical differences are attributed to the contact material.
    The Au sample has MoS2 on top of Ti/Au contacts with AFM ironing, while the Bi sample has Bi/Au contacts evaporated on an etched MoS2 flake; the comparison assumes the contact material is the dominant variable.

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Cite this review

Pith. "Pith review of Study of gold and bismuth electrical contacts to a MoS$_{2}$ monolayer." pith.science (2026). https://pith.science/paper/CYH47CVQ

@misc{pith2026250107444,
  author       = {Pith},
  title        = {Pith review of: Study of gold and bismuth electrical contacts to a MoS$_2$ monolayer},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CYH47CVQ}},
  note         = {Machine review of arXiv:2501.07444}
}
abstract

Semiconducting transition metal dichalcogenides (TMDCs) present new possibilities for designing novel electronic devices. An efficient contacting scheme is required to take advantage of exceptional opto-electronic properties of TMDCs in future electronic devices. This is however challenging for TMDCs, mostly due to the typically high Schottky barrier formed between a metal and a semiconductor. Here we investigate different approaches for contacting MoS$_{2}$, utilizing both metallic gold and semimetallic bismuth as contact materials. The collected I-V characteristics of Bi-contacted devices are compared with the performance of traditional gold contacts. The method of AFM ironing, which we used to enhance the parameters of gold contacts, is also described. Additionally, we show preliminary results regarding an optical response for both types of samples.

Figures

Figures reproduced from arXiv: 2501.07444 by the authors.

Figure 1
Figure 1. (a) Optical image of sample A with gold contacts. All contacts are 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. (a)-(c): I-V curves for sample A for backgate voltages in range -8 to 16 V with 2 V step for 270 K. Each graph denotes for which contact [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. (a): I-V curve of contacts 3-4 for zero backgate voltage at 270 K (black dots) with fitted formula from eq. 3 (blue line). Ideality parameter [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figures from the paper (2 more)
Figure 4
Figure 4. Figure 4: (a): I-V curve for sample B for backgate voltages from 0 to 20 V at 280 K. The contacts have ohmic behaviour. (b): I-V curve for sample [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]
Figure 5
Figure 5. Figure 5: (a): Representative PL spectrum of a sample with gold contacts at 4.5 K compared with a PL spectrum of a bare MoS [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

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Reference graph

Works this paper leans on

21 extracted references · 21 canonical work pages

  1. [1]

    Y . Liu, Y . Gao, S. Zhang, J. He, J. Yu, Z. Liu, Valleytronics in transition metal dichalcogenides materials, Nano Research 12 (2019) 2695– 2711

    work page 2019

  2. [2]

    Avsar, H

    A. Avsar, H. Ochoa, F. Guinea, B. ¨Ozyilmaz, B. J. van Wees, I. J. Vera-Marun, Colloquium: Spintronics in graphene and other two- dimensional materials, Rev. Mod. Phys. 92 (2020) 021003

    work page 2020

  3. [3]

    Zollner, P

    K. Zollner, P. E. Faria Junior, J. Fabian, Proximity exchange e ffects in MoSe2 and WSe2 heterostructures with CrI3: Twist angle, layer, and gate dependence, Phys. Rev. B 100 (2019) 085128

    work page 2019

  4. [4]

    Z. M. Wang, MoSe 2, Springer Cham, 2013

    work page 2013

  5. [5]

    A. V . Kolobov, J. Tominaga, Two-Dimensional Transition-Metal Dichalcogenides, Springer, 2016

    work page 2016

  6. [6]

    A. T. Wee, D. Chi, K. Goh, 2D Semiconductor Materials and Devices, Elsevier Science, 2019

    work page 2019

  7. [7]

    D. S. Schulman, A. J. Arnold, S. Das, Contact engineering for 2D materials and devices, Chemical Society Reviews 47 (9) (2018) 3037–3058

    work page 2018

  8. [8]

    Allain, J

    A. Allain, J. Kang, K. Banerjee, A. Kis, Electrical contacts to two-dimensional semiconductors, Nature Materials 14 (2015) 1195–1205

    work page 2015

Show all 21 references
  1. [9]

    Zheng, J

    Y . Zheng, J. Gao, C. Han, W. Chen, Ohmic Contact Engineering for Two-Dimensional Materials, Cell Reports Physical Science 2 (1) (2021) 100298

    2021

  2. [10]

    Farmanbar, G

    M. Farmanbar, G. Brocks, Controlling the Schottky barrier at MoSe 2/metal contacts by inserting a BN monolayer, Physical Review B 91 (16) (2015) 161304(R)

    2015

  3. [11]

    P.-C. Shen, C. Su, Y . Lin, A.-S. Chou, C.-C. Cheng, J.-H. Park, M.-H. Chiu, A.-Y . Lu, H.-L. Tang, M. M. Tavakoli, G. Pitner, X. Ji, Z. Cai, N. Mao, J. Wang, V . Tung, J. Li, J. Bokor, A. Zettl, C.-I. Wu, T. Palacios, L.-J. Li, J. Kong, Ultralow contact resistance between sem...

    2021

  4. [12]

    S. J. Haigh, A. Gholinia, R. Jalil, S. Romani, L. Britnell, D. C. Elias, K. S. Novoselov, L. A. Ponomarenko, A. K. Geim, R. Gorbachev, Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices, Nature Materials 11 (...

    2012

  5. [13]

    M. R. Rosenberger, H.-J. Chuang, K. M. McCreary, A. T. Hanbicki, S. V . Sivaram, B. T. Jonker, Nano-”Squeegee” for the Creation of Clean 2D Material Interfaces, ACS Applied Materials & Interfaces 10 (12) (2018) 10379–10387

    2018

  6. [14]

    Y . Kim, P. Herlinger, T. Taniguchi, K. Watanabe, J. H. Smet, Reliable Postprocessing Improvement of van der Waals Heterostructures, ACS Nano 13 (12) (2019) 14182–14190

    2019

  7. [15]

    S. Chen, J. Son, S. Huang, K. Watanabe, T. Taniguchi, R. Bashir, A. M. van der Zande, W. P. King, Tip-Based Cleaning and Smoothing Improves Performance in Monolayer MoS2 Devices, ACS Omega 6 (5) (2021) 4013–4021

    2021

  8. [16]

    Somvanshi, S

    D. Somvanshi, S. Kallatt, C. Venkatesh, S. Nair, G. Gupta, J. K. Anthony, D. Karmakar, K. Majumdar, Nature of carrier injection in metal/2D- semiconductor interface and its implications for the limits of contact resistance, Phys. Rev. B 96 (2017) 205423

    2017

  9. [17]

    Scott, W

    D. Scott, W. Hunter, H. Shichijo, A transmission line model for silicided di ffusions: Impact on the performance of vlsi circuits, IEEE Transactions on Electron Devices 29 (4) (1982) 651–661. doi:10.1109/T-ED.1982.20758

    1982

  10. [18]

    K. S. Kim, J. Kwon, H. Ryu, C. Kim, H. Kim, E.-K. Lee, D. Lee, S. Seo, N. M. Han, J. M. Suh, J. Kim, M.-K. Song, S. Lee, M. Seol, J. Kim, The future of two-dimensional semiconductors beyond Moore’s law, Nature Nanotechnology 19 (2024) 895—-906

    2024

  11. [19]

    Yang, T.-Y

    M. Yang, T.-Y . Kim, T. Lee, S. Hong, Nanoscale enhancement of photoconductivity by localized charge traps in the grain structures of monolayer MoSe2, Scientific Reports 8 (15822) (2018)

    2018

  12. [20]

    Y . Zhan, Z. Liu, S. Najmaei, P. M. Ajayan, J. Lou, Large-area vapor-phase growth and characterization of MoSe 2 atomic layers on a SiO 2 substrate, Small 8 (7) (2012) 966–971

    2012

  13. [21]

    Jadczak, J

    J. Jadczak, J. Kutrowska-Girzycka, P. Kapu ´sci´nski, Y . S. Huang, A. W ´ojs, L. Bryja, Probing of free and localized excitons and trions in atomically thin WSe2, WS2, MoSe2 and MoS2 in photoluminescence and reflectivity experiments, Nanotechnology 28 (39) (2017) 395702 6

    2017

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