Facet–doping coupling governs ferrocene monolayer redox on silicon surfaces

Abstract

Semiconductor electrodes offer powerful routes to engineer electrochemical function, yet predicting surface confined charge transfer remains challenging because crystallography and doping reshape interfacial structure, band bending, and potential distribution. Here we map these coupled effects using ferrocene (Fc) monolayers grafted onto hydrogen-terminated p- and n-type Si(100), Si(110), and Si(111). Successful functionalization was confirmed by X-ray photoelectron spectroscopy and cyclic voltammetry. The Fc surface coverage (Γ) is strongly facet dependent and, in particular, doping reverses the facet selectivity: p-type follows (100) > (110) > (111), whereas n-type follows (111) > (110) > (100). In contrast, the Fc/Fc⁺ mid-point potential shows a consistent orientation hierarchy for both dopings ((100) > (110) > (111)) with an additional ~20–40 mV positive shift on n-type relative to p-type, indicating robust redox energetics with doping-controlled offsets. Peak widths exceed the ideal surface-confined limit and, together with impedance responses, point to non-ideal behavior dominated by interfacial electrostatics rather than ohmic artifacts. This facet-by-doping map clarifies how the silicon surface structure and electronic boundary conditions partition their influence across the monolayer formation and redox energetics, providing guidance for silicon-based molecular electrochemical interfaces in sensing and molecular electronics.

Graphical abstract

Keywords

semiconductor electrochemistry / surface-confined redox / ferrocene monolayer / crystal orientation / doping effect

References

[1] Zhang Z, Yates J T . Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces.Chemical Reviews, 2012, 112(10): 5520–5551

[2] Hofmann O T, Rinke P . Band bending engineering at organic/inorganic interfaces using organic self-assembled monolayers.Advanced Electronic Materials, 2017, 3(6): 1600373

[3] Harilal S, Sadhujan S, Zhang K F, et al. Uniform tendency of surface dipoles across silicon doping levels and types of H-terminated surfaces.Advanced Electronic Materials, 2024, 10(10): 2300873

[4] Lichterman M F, Hu S, Richter M H, et al. Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy.Energy & Environmental Science, 2015, 8(8): 2409–2416

[5] Zhang S, Ciampi S . Facet-resolved electrochemistry: from single particles to macroscopic crystals.Current Opinion in Electrochemistry, 2022, 35: 10185

[6] Zhang S, Ferrie S, Lyu X, et al. Absence of a relationship between surface conductivity and electrochemical rates: redox-active monolayers on Si(211), Si(111), and Si(110).The Journal of Physical Chemistry C, 2021, 125(33): 18197–18203

[7] Kavan L . Electrochemistry and band structure of semiconductors (TiO₂, SnO₂, ZnO): avoiding pitfalls and textbook errors.Journal of Solid State Electrochemistry, 2024, 28(3-4): 829–845

[8] Moradi K, Melander M M . Electronic structure methods for simulating the applied potential in semiconductor electrochemistry.Current Opinion in Electrochemistry, 2025, 49: 101615

[9] Kim Y J, Kwon Y, Yoo Y, et al. Demonstration of CMOS-compatible memristor-based electrochemical biosensor transducer with threshold-sensing functionality.Nature Communications, 2025, 16(1): 10851

[10] Song J, Li L, Wong W Y, et al. Organic mixed conductors in electrochemical transistors for bioelectronic applications.Accounts of Materials Research, 2024, 5(9): 1036–1047

[11] Tropp J, Meli D, Rivnay J . Organic mixed conductors for electrochemical transistors.Matter, 2023, 6(10): 3132–3164

[12] Paulsen B D, Tybrandt K, Stavrinidou E, et al. Organic mixed ionic–electronic conductors.Nature Materials, 2020, 19(1): 13–26

[13] Xi Z Y, Liu M Z . Advancing photoelectrochemical systems for sustainable energy and chemical production: challenges and opportunities.NPJ Materials Sustainability, 2025, 3: 18

[14] Yu J M, Lee J, Kim Y S, et al. High-performance and stable photoelectrochemical water splitting cell with organic-photoactive-layer-based photoanode.Nature Communications, 2020, 11(1): 5509

[15] Wang S H, Cao S, Wang L, et al. Enhanced photoelectrochemistry for energy conversion, environmental remediation, detection, and sensing through single-atom catalysts modified photoelectrodes: a comprehensive review.Materials Today: Energy, 2024, 43: 101582

[16] Li T, Dief E M, Lyu X, et al. Nanoscale silicon oxide reduces electron transfer kinetics of surface-bound ferrocene monolayers on silicon.The Journal of Physical Chemistry C, 2021, 125(50): 27763–27770

[17] Mancini K M, Khatib Y, Shahine L, et al. Photoelectrochemistry of redox-active self-assembled monolayers formed on n-Si/Au nanoparticle photoelectrodes.Langmuir, 2024, 40(33): 17536–17546

[18] Ciampi S, Harper J B, Gooding J J . Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si−C bonds: surface preparation, passivation and functionalization.Chemical Society Reviews, 2010, 39(6): 2158–2183

[19] Gooding J J, Ciampi S . The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies.Chemical Society Reviews, 2011, 40(5): 2704–2718

[20] Fabre B, Pujari S P, Scheres L, et al. Micropatterned ferrocenyl monolayers covalently bound to hydrogen-terminated silicon surfaces: effects of pattern size on the cyclic voltammetry and capacitance characteristics.Langmuir, 2014, 30(24): 7235–7243

[21] Fabre B . Ferrocene-terminated monolayers covalently bound to hydrogen-terminated silicon surfaces.Toward the development of charge storage and communication devices. Accounts of Chemical Research, 2010, 43(12): 1509–1518

[22] Yaffe O, Ely T, Har-Lavan R, et al. Effect of molecule–surface reaction mechanism on the electronic characteristics and photovoltaic performance of molecularly modified Si.The Journal of Physical Chemistry C, 2013, 117(43): 22351–22361

[23] Ravishankar S, Bisquert J, Kirchartz T . Interpretation of Mott–Schottky plots of photoanodes for water splitting.Chemical Science, 2022, 13(17): 4828–4837

[24] Fontanesi C, Como E D, Vanossi D, et al. Redox-active ferrocene grafted on H-terminated Si(111): electrochemical characterization of the charge transport mechanism and dynamics.Scientific Reports, 2019, 9(1): 8735

[25] Niwano M, Takeda Y, Ishibashi Y, et al. Morphology of hydrofluoric acid and ammonium fluoride-treated silicon surfaces studied by surface infrared spectroscopy.Journal of Applied Physics, 1992, 71(11): 5646–5649

[26] Niwano M, Kurita K, Miyamoto N . Hydrogen termination of the NH₄F-treated Si(111) surface studied by photoemission and surface infrared spectroscopy.Proceedings of the Materials Research Society, 1993, 315(1): 505–510

[27] Gräf D, Bauer-Mayer S, Schnegg A . Reaction of NH₄F/HF solutions on Si(100) and Si(111) surfaces.Journal of Vacuum Science & Technology A, 1993, 11(4): 940–944

[28] Houbertz R, Memmert U, Behm R J . Morphology of anodically etched Si(111) surfaces: a structural comparison of NH₄F versus HF etching.Journal of Vacuum Science & Technology B, 1994, 12(6): 3145–3148

[29] Tung J, Khung Y L . Influences of doping and crystal orientation on surface roughening upon alcohol grafting onto silicon hydride.Applied Sciences, 2017, 7(8): 859

[30] Rao C C, Olsen B C, Luber E J, et al. Kinetics of plasmon-driven hydrosilylation of silicon surfaces: photogenerated charges drive silicon–carbon bond formation.The Journal of Physical Chemistry C, 2021, 125(32): 17983–17992

[31] Higashi G S, Becker R S, Chabal Y J, et al. Comparison of Si(111) surfaces prepared using aqueous solutions of NH₄F versus HF.Applied Physics Letters, 1991, 58(15): 1656–1658

[32] Niwano M, Kageyama J I, Kinashi K, et al. Infrared spectroscopic study of initial stages of ultraviolet ozone oxidation of Si(100) and (111) surfaces.Journal of Vacuum Science & Technology A, 1994, 12(2): 465–470

[33] Allongue P, Henry-de-Villeneuve C, Morin S, et al. The preparation of flat H−Si(111) surfaces in 40% NH₄F revisited.Electrochimica Acta, 2000, 45(28): 4591–4598

[34] Lewis N S . Progress in understanding electron-transfer reactions at semiconductor/liquid interfaces.The Journal of Physical Chemistry B, 1998, 102(25): 4843–4855

[35] Vogel Y B, Molina A, Gonzalez J, et al. Quantitative analysis of cyclic voltammetry of redox monolayers adsorbed on semiconductors: isolating electrode kinetics, lateral interactions, and diode currents.Analytical Chemistry, 2019, 91(9): 5929–5937

[36] Gonzalez J, Sequí-Castellano J A . Electrochemical determination of kinetic parameters of surface confined redox probes in presence of intermolecular interactions by means of cyclic voltammetry.Application to TEMPO monolayers in gold and platinum electrodes. Electrochimica Acta, 2021, 365: 137331

[37] Levey K J, Edwards M A, White H S, et al. Finite element modeling of the combined faradaic and electrostatic contributions to the voltammetric response of monolayer redox films.Analytical Chemistry, 2022, 94(37): 12673–12682

[38] Lloyd-Laney H O, Robinson M J, Bond A M, et al. A spotter’s guide to dispersion in non-catalytic surface-confined voltammetry experiments.Journal of Electroanalytical Chemistry, 2021, 894: 115204

[39] Lazanas A C, Prodromidis M I . Electrochemical impedance spectroscopy — a tutorial.ACS Measurement Science Au, 2023, 3(3): 162–193

[40] Reimers J R, Yang J H, Darwish N, et al. Silicon–single molecule–silicon circuits.Chemical Science, 2021, 12(48): 15870–15881

[41] Liu Y R, Qiu X K, Soni S, et al. Charge transport through molecular ensembles: recent progress in molecular electronics.Chemical Physics Reviews, 2021, 2(2): 021303

[42] Yang Q Y, Liao J M, Feng L Y, et al. One-step construction of multiplexed enzymatic biosensors using light-addressable electrochemistry on a single silicon photoelectrode.Biosensors & Bioelectronics, 2024, 253: 116194

[43] Awawdeh K, Jiang X, Dahan L, et al. Porous silicon biosensors meet zwitterionic peptides: tackling biofouling from proteins to cells.Nanoscale Horizons, 2025, 10(11): 3072–3084

[44] Vogel Y B, Gooding J J, Ciampi S . Light-addressable electrochemistry at semiconductor electrodes: redox imaging, mask-free lithography and spatially resolved chemical and biological sensing.Chemical Society Reviews, 2019, 48(14): 3723–3739

[45] Oh I, Pence M A, Lukhanin N G, et al. The electrolab: an open-source, modular platform for automated characterization of redox-active electrolytes.Device, 2023, 1(5): 100103

RIGHTS & PERMISSIONS

Higher Education Press