ARTICLE

Reproducible flaws unveil electrostatic aspects ofsemiconductor electrochemistryYan B. Vogel1, Long Zhang1,2, Nadim Darwish1, Vinicius R. Gonçales3, Anton Le Brun4, J. Justin Gooding 3,

Angela Molina5, Gordon G. Wallace2, Michelle L. Coote 6, Joaquin Gonzalez5 & Simone Ciampi1

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular

electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the

electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering

non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here

we show that non-idealities of redox probes confined at silicon electrodes, namely full width

at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be

reproduced and are not flawed data. These are the manifestation of electrostatic interactions

between dynamic molecular charges and the semiconductor’s space-charge barrier. We

highlight the interplay between dynamic charges and semiconductor by developing a model

to decouple effects on barrier from changes to activities of surface-bound molecules. These

findings have immediate general implications for a correct kinetic analysis of charge-transfer

at semiconductors as well as aiding the study of electrostatics on chemical reactivity.

DOI: 10.1038/s41467-017-02091-1 OPEN

1 Department of Chemistry, Curtin Institute of Functional Molecules and Interfaces, Curtin University, Bentley, WA 6102, Australia. 2 ARC Centre ofExcellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Wollongong, NSW 2500, Australia. 3 School ofChemistry, Australian Centre for NanoMedicine and ARC Centre of Excellence for Convergent Bio-Nano Science and Technology, The University of NewSouth Wales, Sydney, NSW 2052, Australia. 4 Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organization (ANSTO),Lucas Heights, NSW 2234, Australia. 5 Departamento de Quimica Fisica, Universidad de Murcia, 30003 Murcia, Spain. 6 ARC Centre of Excellence forElectromaterials Science, Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia. Correspondence and requests formaterials should be addressed to J.G. (email: josquin@um.es) or to S.C. (email: simone.ciampi@curtin.edu.au)

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In 1876 Ferdinand Braun presented to a Natural Societymeeting the first deviations from Ohm’s law he had observedin crystals of galena, a natural form of lead sulfide. In the

following century his discovery revolutionized our civilization.From galena to silicon, materials that can turn from conductorsto insulators are at the basis of all our digitized technology1.Understanding the full spectrum of factors at play, when chargesare transferred across a semiconductor interface is crucial; itunderpins the design of devices whose function span from con-verting light into electricity, to sensing their environment2, cul-minating in the very recent report of single-molecule rectifiers onSi(111)3. Chemically modified electrodes are therefore a veryimportant laboratory model system4, however, a search of theliterature will indicate that in contrast to metallic electrodes thekinetic parameters for electron transfer at semiconductors aredifficult to reproduce from laboratory to laboratory5, 6. Widelyaccepted approaches that are used to gain insights on electrodekinetics have clearly failed to reproduce the complex energeticlandscape that determines the redox behavior observed. Byhighlighting the participation of dynamic electrostatic factors oncharge-transfer, we are implicitly demonstrating that theelectrostatic landscape of a silicon/molecular layer/electrolyteinterface should either be accounted for or eliminated whenthe focus is on extracting kinetic data at semiconductor orphotoconductor electrodes. This knowledge also opens up asemiconductor-based platform to aid the study of electrostaticson chemical reactivity7–11, and molecular electronics3, 12, 13.

Here, we study how electrostatic interactions manifest onsurface-confined redox molecules and how to gauge these inter-actions by a general form of electrochemical spectroscopy14, i.e.,cyclic voltammetry. The electrostatic landscape of silicon is par-ticularly rich and the poor Debye screening of the material resultsin a fraction of the applied bias appearing inside the semi-conductor phase itself. This opens up the possibility of interplaybetween this charged penetration zone within the semiconductor(hereafter referred to as the space-charge region) and a chargedlayer of surface dipoles15. Opposite to the more common situa-tion of a metal-semiconductor contact16, in this report the excesscharge is not localized at step edges or defect sites, but in a redoxmonolayer outside the semiconductor phase. We have built aphenomenological model to account for electrostatic effects onsurface molecules in the presence of faradaic currents at semi-conductor/electrolyte systems. This model merges a modifiedFrumkin isotherm that accounts for both the electrostatic inter-actions sensed by a molecular layer confined on the electrodesurface17, 18 as well as for the current–potential relationship of theunderlying semiconductor photodiode19 under realistic finitekinetics (Butler–Volmer). Hence, the value of this study is tojustify the experimental evidences for electroactive monolayers atSi(111) and to present a general framework to conceptualizecharging and current dynamics of semiconductor electrodes. Weremark however that the model cannot be directly connected withfield values; precise field numbers (V nm−1) experienced by sur-face molecules cannot yet be generated until additional assump-tions are validated independently20.

ResultsReproducible electrochemical flaws of a surface model system.Native silicon always carries a dipole layer of charges on its silicasurface and these charges are reflected by a space-charge potentialbarrier. This barrier, which is electrostatic in nature, is however avery sensitive function of surface conditions, and in actualpractice it has no unique reproducible value for a given system21.Hence, the first step we took was to limit the possibility ofadventitious surface charges by chemically passivating an oxide-

free silicon electrode by the hydrosilylation of 1,8-nonadiyne (1,Fig. 1)22–25. The acetylene-terminated surface (S-1) was thenmodified by the covalent attachment of a reversible redox species,azidomethylferrocene (2)26, with the goal of (i) having control onthe surface density of positively charged ferricenium tethers (S-2samples) and (ii) analyzing their electrochemical traces todecouple electrostatic effects due to molecule—molecule inter-actions27 from molecule—space-charge ones. Figure 2a showsrepresentative cyclic voltammograms obtained for the ferrocene-functionalized Si(111) electrodes (S-2 on n-type, 7–13 Ω cm,hereafter lowly doped in short hand). The voltammetry wasperformed under light as the semiconductor electrode is operat-ing in depletion and thereby it requires illumination to carry acurrent28 (see Supplementary Fig. 1). This is the conceptualequivalent of the reverse bias in a solid junction, where the cur-rent is mainly carried by holes flowing under the barrier (Fig. 2b).The ferrocene-confined probes exhibit broad voltammetry waves(black symbols in Fig. 2a), with the observed full width at halfmaximum (fwhm hereafter) being on average 142± 6 mV.

The ideal fwhm from the Langmuir isotherm of a nernstianprocess is 90.6 mV (model 1, Supplementary Note 1), andtheoretical models exist to relate the non-ideal behavior of avoltammetric peak to the balance between attractive and repulsiveinteractions experienced by a strongly-adsorbed molecule27. Theincrease over the ideal fwhm’s is often observed in the literature29

and can be attributed to a predominance of repulsive interactionsbetween the electroactive species. The apparent kinetics is fast(ket = 200 s−1, see Supplementary Fig. 2 and Supplementary Note 1(finite kinetics limit) for details on the calculations of charge-transfer rates) and a quantitative model that takes into accountthese interactions can be implemented by assuming a nernstianbehavior (i.e., very fast electrode kinetics) and Frumkin isotherm,as reported by Laviron in his seminal works (model 2,Supplementary Note 2)17, 18. The simulated curves are shownin Fig. 2a as a solid black line. Only the Frumkin parameter G wasadjusted in the model; changes to G account for an imbalance inthe electrostatic push/pull, forcing the voltammetric wave tobroaden or narrow when the activities of the reduced andoxidized species do not follow their surface concentrations. Apredominance of repulsive forces in S-2 samples leads to negativeG values of −0.93± 0.10 in average. This is expected due to thehigh density of ferrocenes (Γ= 2.89± 0.24 × 10–10 mol cm–2, ca.65% of a full monolayer) and in fact similar observations

Native SiOx

Si(111) or a-Si

CuAAC

NH4F (aq)

Si Si

N

N

N–

+

N

NN

S-1 S-2

1 2

Fe Fe2

SiSi

SiSi

Si

Si

H H

Si365 nm

Fig. 1 The surface model system. Light-assisted hydrosilylation of 1,8-nonadiyne 1 is used to chemically passivate a Si–H electrode and generatean acetylene-terminated monolayer (S-1). Azidomethylferrocene 2 isgrafted on the electrode via CuAAC click reactions to yield a redox-activefilm (S-2)

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(i.e., large fwhm’s and negative G values) are observed for S-2samples of a similar coverage prepared on substrates of differentconductivity type and level (Supplementary Fig. 3). It is thereforeapparent from the broad shape of the voltammograms in Fig. 2a(black traces) that detecting evidence of an electrostatic effect bythe space-charge would first require limiting the extent of in-plane ferrocene—ferrocene repulsions and/or decreasing thedielectric screening between the space-charge and the ferrocene(Supplementary Note 3). By means of shortening the clickreaction time of azide 2 on samples of S-1 to a few seconds, theferrocene coverage can be lowered to 5.9 × 10–11 mol cm–2 (ca.13% of a full monolayer), however, this has no significant impacton the attraction/repulsion balance (Supplementary Fig. 4) andfwhm’s remain above the ideal 90.6 mV.

Very surprisingly, the experimental fwhm’s drops reproduciblybelow the ideal value when the monolayer coverage is being lostby deliberately introducing an oxidative damage (Fig. 2a, bluesymbols). Applying a large anodic bias to samples of S-2 results inthe partial loss of the monolayer, with the density of ferrocenesdropping below 1.7 × 10–10 mol cm–2 (i.e., ca. 35% of a fullmonolayer) and fwhm’s unexpectedly dropping to 75–55 mV(Fig. 2a, blue traces, and Supplementary Fig. 5). The experimentalvoltammograms in Fig. 2a are fitted to yield a G value of +0.52(solid blue line, model 2), which is diagnostic of attractive forcesdominating on the surface tethers. Several lines of evidence relatethis remarkable increase in Frumkin G to a space-charge effect.Firstly, to the best of our knowledge this has never been observedon a metallic electrode. Secondly, we discarded that theseinteractions could involve ionized silicon dioxide (SiO2); theeffect is pH-independent and narrow waves are observed at bothpH values of 0.5 and 6.0 (Supplementary Fig. 5). At these valuesof pH, and neglecting for simplicity fields effects30, the chemicalequilibrium between ionizable surface groups should lead toeither positive (SiOH2

+) or negative (SiO‒) surface groups,respectively31. Furthermore, a deliberate oxidative damage tosamples of S-2 prepared on highly doped electrodes led to nonoticeable effects on G (Supplementary Fig. 6) and thereby it isunlikely that, besides the space-charge of a lowly doped substrate,other obvious sources of electrostatic interactions, such as anionsfrom the electrolyte27, are contributing toward such a largeincrease in the self-interaction parameter. AFM images and XPSnarrow scans of the Si 2p region in Fig. 3 show the increase inSiO2 presence and the evolution of surface topography upon thedeliberate anodic damaging of S-2 samples. Voltammogramsappear as insets to the AFM micrographs to illustrate that as soonas the high-energy XPS emissions from silicon oxides (Si+‒Si4+,SiOx:Si 2p peak area ratio of 0.05) become measurable in Fig. 3e,the voltametric traces narrow below 90.6 mV. The parallelbetween the spectroscopic appearance of oxides and topographi-cal changes (Fig. 3a–c) is also noteworthy. Figure 3a shows anAFM image taken for an as-prepared S-2 sample. The surfaceroughness is extremely low which is an indication of the highquality of the surface samples (ca. 0.2 nm peak-to-valley rough-ness measured by AFM on individual terraces, details in Fig. 3, ora ca. 0.3 nm overall roughness determined independently byX-ray reflectometry, Supplementary Note 4)32–34. The width ofthe individual surface terraces amounts to ca. 70–100 nm, withthe height of the steps being ca. 0.5 nm, which is only slightlyhigher than a single atomic step between the adjacentlattice planes (0.3 nm). The ⟨111⟩ terraces are also remarkablysmooth, with an Rq value of 0.08± 0.02 nm (Rtm = 0.15 nm),however a number of small protrusions can be observed (ca. 200protrusions µm–2), which are on average 0.4 nm in height and 5nm in radius. An extensive anodic treatment (Fig. 3c, f, SiOx:Si 2pof ca. 0.1) leads to a rougher surface (Rq of 0.11± 0.01 nm(Rtm = 0.23 nm), Supplementary Figs. 7 and 8) and the number

IL Drop in IL (θ↑)

–+

h+

e–

+

+

+

+

+

+

+

+

+

+

+

+

–+

E v

s. r

efer

ence

–0.30 –0.15 0.00

–6

–3

0

3

6 63 mV (fwhm)+0.52 (G)

I (μA

)

E (V vs. Ag/AgCl, KClsat)

142 mV (fwhm)–0.93 (G)

h+

e– e–

h+

+

–

–0.30 –0.15 0.00–1.0

–0.5

0.0

0.5

1.0

I (µA

)

E (V vs. Ag/AgCl, KClsat)

Ep,a – Ep,c = – 15 mV

a

b

c

Fig. 2 Reproducible electrochemical flaws. a Representative background-subtracted voltammograms (100mV s−1, 1.0M HClO4) for S-2 samples onn-type Si(111). Simulated traces (solid black line) for as-prepared samples(symbols) indicate repulsive forces dominate the electrostatic balance ofthe monolayer system. Applying a potential step of 0.3 V for 140 s shift thebalance in favor of attractive interactions (blue line and symbols). Averagedvalues from cathodic and anodic peaks of experimental fwhm’s and refinedFrumkin G values appear as labels in figure. b Distortion of thesemiconductor side of the barrier for a photoanode due to the presence ofan electrochemically induced dipole layer of surface charges. The depictionof the photodiode used for the simulations indicates downward flip of thebands due to the positive ferriceniums units and the changes to thephotogenerated anodic current. c Inverted voltammograms for S-2 samples(25mV s−1, n-type). Ep,c is 15 mV higher than Ep,a. The hydrosilylationreaction time (S-1) is 10min

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and size of protrusions increases (ca. 900 protrusions µm–2, 0.5nm in height and 8 nm of radius). However, if the anodictreatment is milder (SiOx:Si 2p of ca. 0.05), just to suffice innarrowing peaks below 90.6 mV (inset to Fig. 3b), there are nomeasurable changes to surface Rq and Rtm, (Fig. 3b; Supplemen-tary Fig. 7) but only an increase to the number of nanometer-sizesprotrusions (ca. 300 protrusions µm–2, Fig. 3b; SupplementaryFig. 7). It has been suggested by Allongue and co-workers32 thatthe number of these rounded features is possibly related to silicaislands that can be measured by AFM even when the XPS SiOx

emission is below the detection limit (Fig. 3a).

Inversion of peak potentials. As for the current-potential datashown in Fig. 2a, the near-surface barrier of the electrode can betransiently lifted by illumination in the visible28, 35. Even a strongillumination will only minimize without nulling the semi-conductor barrier, hence illumination—which is required to get acurrent output—may not totally compromise the semi-conductor’s ability to sense or exert electrostatic interactions onnearby molecules. Figure 2b shows the conventional energy-bandrepresentation that describes a plausible overall effect of anexternally applied electric field and a dipole layer of surfacecharges on the semiconductor barrier. The convention is to drawenergy-levels diagrams such that the energy of the system islowered as electrons fall down toward the source of positivepotential (i.e., drawn at the bottom of the diagram). Theexperimental observation of voltammetric fwhm’s below 90.6 mVand a refined positive G indicates the presence of attractive forceson the positive ferricenium tethers, which can be conceptualizedas electrons redistributing over the semiconductor side of thebarrier. We should therefore analyze this hypothesis against its

most obvious consequence. According to the convention onenergy-level diagrams, the higher the semiconductor side is raisedby the static positive charges of the ferricenium ions, that is to saymoving from the left to the right panel of Fig. 2b, the more(photo)electrons there would be at an energy above the top of thebarrier (i.e., increasing reduction rates). At the same time, thisdistortion of the bands, induced by the presence of the dipolelayer of surface charges, should lift the opposition current carriedby holes (i.e., decreasing oxidation rates). The direct consequencewould be a very intriguing inversion of peak positions (Supple-mentary Note 5), with the potential for the cathodic peak shiftinganodic of the anodic maxima (Ep,c> Ep,a)27. The narrowing of thepeaks below 90.6 mV and the inversion of the peak potentials aretherefore the anticipated manifestation of the same electrostaticprocess. Interestingly, only narrow but not inverted peak poten-tials are observed consistently. In our lowly doped n-type system,S-2 samples systematically showed inverted peak potentials (5–55mV, Fig. 2c; Supplementary Fig. 9) only if the hydrosilylationreaction time to prepare S-1 samples was lowered from 2 h toeither 10 or 2 min (Supplementary Figs. 9–11). Notably, shorttimes for the passivation step also lead to the inversion andnarrow waves, and with no requirements of an oxidative pre-treatment, but the phenomena is however short-lived and doesnot allow for systematic changes to the voltage sweep rate andhence rigorous data modeling (Supplementary Fig. 9c). Wespeculate that a low coverage of diyne 1 molecules after a short 2or 10 min reaction suffices to reduce the dielectric screening ofthe carbonaceous film (Supplementary Fig. 12)36, thereby allow-ing for inversion, but on the other hand the poor passivationleads to the growth of SiO2 which masks the inversion by redu-cing the electron transfer kinetics (vide infra). It is in fact veryplausible that the manifestation of peak potential inversion is

a

SiOx:Si 2p = 0.05 SiOx:Si 2p = 0.09

110 105 100 95 90

Inte

nsity

(a.

u)

Binding energy (eV)

110 105 100 95 90

Inte

nsity

(a.

u)

Binding energy (eV)

110 105 100 95 90

Inte

nsity

(a.

u)

Binding energy (eV)

b c

d e f

1.4 nm

Fig. 3 Anodic monolayer stripping. Tapping mode AFM images (2 × 2 μm) and XPS Si 2p narrow scans for S-2 samples on Si(111). Cyclic voltammogramsare shown as insets to the AFM images. Data for as-prepared samples are in a and d, and data after the short potential step (0.3 V, 1 min) that leadreproducibly to narrow voltammetric waves (ca. 60mV fwhm) are in b and e. Owing to the kinetic factors, the narrow waves are lost upon more extensiveoxidation of the electrodes (c, f). High-energy emissions from silicon oxides (Si+−Si4+) are evident in e and f, and the evolution of the SiOx:Si 2p peak arearatios appear as labels in the XPS panels. a Small protrusions are observed (222 protrusions µm−2, which are on average 0.4 nm in height and 5 nm inradius) over the staircase structure of the ⟨111⟩ surface (Rq of 0.08± 0.02 nm (Rtm=0.15 nm) on the terraces and Rq=0.20± 0.15 nm (Rtm=0.63 nm) overthe whole area). The number of protrusions increases upon the anodic treatment (327 protrusions µm−2 in b and 945 protrusions µm−2 in c) and tracksXPS SiOx emissions. For clarity and comparison purposes all the AFM images are normalized to 1.4 nm. The scale bar is 500 nm

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masked to some degree by kinetic factors, which forces a pro-gressive peak-to-peak separation (Ep,a> Ep,c). To quantitativelyreinforce this argument, we have developed a model of a systemthat assumes finite kinetics (Butler–Volmer), accounts forFrumkin’s interactions of the surface tether and includes dynamicparameters describing diode effects for the potential and currentsacross the space-charge (model 3, Supplementary Note 6).Parameters for the diode element can be determined in thereversible region and can yield quantitative information on themagnitude of reverse saturation current and photocurrent.Changes to these two values, or more precisely how the value of θdecreases in response to an increase in the surface concentrationof positively charged ferricenium units is crucial to the under-standing of the inversion effect, where θ is given by Equation (1):

θ ¼ Ipeak=ðIL þ I0Þ ð1Þ

where Ipeak is the peak current height in the voltammograms, IL isthe diode photocurrent and I0 is the diode reverse saturationcurrent, see Supplementary Fig. 13.

Simulations to illustrate how changes in θ are reflected in thepeak splitting when the kinetics of electron transfer is either fastor slow are shown in Fig. 4a and b, respectively. For fast kinetics(ket of 100 s–1) that is coupled to a Frumkin G of +0.5 our modelpredicts the inversion to be clearly evident in voltammograms if θincreases from 0.04 in the anodic segment to 1.95 in the cathodicone. This change in θ is the result of bands flattening toaccommodate for the electrostatics in the system (Fig. 2b), itindicates a ca. 50-fold drop in the (photo)hole opposition current(IL, model 3), and it manifests as inverted peak potentials(Fig. 4a). For the same G value and the same change in θ butunder a slower kinetics, for instance with ket dropping to 1.0 s–1,the peak order would revert to an apparent normal situation (i.e.,Ep,a> Ep,c, Fig. 4b). The apparent ket for the narrow blue trace inFig. 2a is ca. 80 s–1 (see also Supplementary Fig. 2), but uponmore extensive oxidative damage of the sample (Fig. 3c, f;Supplementary Fig. 2), this value declines further towards theirreversible region (ket = 17 s–1). At this point both experimentaldata and simulations (Supplementary Figs. 2, 14 and 15) indicatethat the electrostatic pull between the space-charge and surface-bound molecules, which leads to narrow waves and inversion, iscompletely masked by kinetics. As a consequence of the positivecharges on the surface tethers, bands in the semiconductor aredistorted downward (Fig. 2b). Using the model 3 (SupplementaryNote 6, finite kinetics limit), we have accounted for the invertedpeak potentials in n-type S-2 samples and rationalized this as anelectrostatics-induced decrease in the anodic photocurrent in thereverse scan (Fig. 2c; Supplementary Figs. 9 and 13).

Electrostatic effects on diode and kinetic analysis. If our rea-soning is correct, and electrostatics is indeed a major factor atplay, it is thereby easy to see that a similar inversion effect (Ep,c>Ep,a) would also hold for p-silicon photocathodes (Fig. 5a, IL hereincreases in the reverse scan, vide infra). The technical problemhere is that the chemical system (i.e., S-2) we are using to sensethe electrostatics of the solid/liquid interface has a relativelyanodic formal potential, and it would therefore fall in the accu-mulation regime of a p-type electrode. To get around this pro-blem we prepared control S-2 samples on thin films (ca. 4 μm) ofamorphous silicon (a-Si hereafter) that are grown on highlydoped p-type substrates. The a-Si electrodes used in this study arenear-intrinsic photoconductors, but the nature of the p-typeback-contact forces the interface to act, to some degree, as aphotocathode (Supplementary Fig. 16). Figure 5b and Supple-mentary Figs. 17 and 18 show cyclic voltammograms for S-2samples on a-Si. It is apparent that on the a-Si surface the redoxtether experiences analogous attractive electrostatic forces asobserved for the n-type Si(111) samples. The interactions betweencharged ferricenium cations and the semiconductor side of thebarrier (Fig. 5a) consistently leads to narrow waves and invertedpeak potentials, but unlike the crystalline Si(111) system, theeffect is already apparent in as-prepared samples (Fig. 5b, blacktrace), it does not require a short anodic pulse (Fig. 2a) and it islong-lived (Supplementary Fig. 18). A rigorous explanation onthese aspects—as why the manifestation of electrostatic is morepronounced and more robust in rough amorphous samples(Supplementary Fig. 19)—is beyond the scope of this work, but atthis stage it is possible to speculate on a relationship betweendisorder in the film and kinetics factors37. The electrochemicalnon-idealities (i.e., narrow and inverted waves) in a-Si samplesare sustained over a prolonged analysis; hence, it is possible toapply model 3 (Supplementary Note 6) to its full extent. Atvoltage sweep rates of about 100 mV s−1 we can observe a driftfrom the inverted region (Fig. 5b, black traces, and Fig. 6) to anormal region, where the shift caused by redox kinetics masks the

1.0

0.5

0.0��

–0.5

–1.0

–0.8

1.0

–0.6 –0.4 –0.2

–0.6 –0.4 –0.2

0.5

0.0

–0.5

–1.0

E –E 0′ (V)

E –E 0′ (V)

Ep,a – Ep,c = 48 mV

Ep,a – Ep,c = –56 mV

a

b

Fig. 4 Kinetic factors mask the inversion of peak potentials and the truepeak dispersion. Simulated voltammograms (25mV s−1) showing themasking of the system’s electrostatic (i.e., diode and Frumkin effects) byslow kinetics. Curves are calculated using model 3 and by setting ket toeither 100 s−1 (a) or 1 s−1 (b). Frumkin G was + 0.5 and θ is adjusted from0.04 in the anodic segment to 1.95 in the cathodic one. This increase in θreflects the ca. 50-fold drop in the (photo)hole opposition current (IL)generally required to fit experimental data. The y-axis current is expressedas ψ ¼ I=Ip;rev

� �´ RT= QFFvð Þð Þ, with Ip;rev being the peak current obtained

for a fast charge-transfer process (see Supplementary Note 6 andSupplementary Fig. 14 for details)

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electrostatic effect (Fig. 5b, blue and red traces). In Fig. 6, we havetentatively marked two regions relative to the scan rate (R as shorthand for reversible and NR for non-reversible, SupplementaryNote 7) and it is clear that the reverse peak suffers the strongestinfluences of both diode and kinetics. In the a-Si system thephotodiode element is expected to point towards the electrolyteand the photocurrent is cathodic (Fig. 5a; Supplementary Fig. 20).

Model 3 combines the effects of diode (through photocurrentand reverse saturation current) plus finite kinetics for the charge-transfer reaction at the monolayer side (through the correspond-ing rate constant and charge-transfer coefficient in the B–Vformalism), and presence of interactions (through the Frumkininteraction parameters). The model appears therefore to be rathercomplex, but nonetheless the manifestation of the electrostaticeffect remains simple to diagnose in terms of narrow and invertedwaves. The parameters used to refine theoretical curves (Fig. 5b,solid lines) against experimental data (symbols) are given inTable 1. From the data in table it can be concluded that for smallpotential sweep rates (e.g., 20 mV s–1), there is no significantperturbation caused by the redox kinetics on the electrostatic ofthe system. As a consequence of the electrostatics experienced bythe space-charge of the electrode the photocurrent value for thereverse scan is one order of magnitude greater that in forward

segment (IL (reverse scan) = 10 × IL (direct scan)), hence theinversion of the current peak on the potential. An increase in thevoltage sweep rate, ν, leads to a decrease of the effective rateconstant (ketν–1) and therefore to an enhancement of theinfluence of kinetics on the response, causing a shift of thedirect/reverse curves towards more positive/negative potentials,respectively, which in turn masks the potential inversion.

– +

h+

+

+

+

+

+

+

+

+

+

+

+

+

+

–

0.2 0.3 0.4 0.5

–2

–1

0

1

2

3

I / v

olta

ge s

wee

p ra

te (

μC)

E (V vs. Ag/AgCl, KClsat)

20 mV s–1100 mV s–1

500 mV s–1

e–

– +

Photocurrent

a

b

E v

s. r

efer

ence

h+

e–

e–

h+

Fig. 5 Validation of the model and its implications for the correct kineticanalysis of charge-transfer reactions at semiconductors. a Energy-leveldiagram depicting the distortion of the semiconductor side of the barrier fora-Si photocathodes due to electrochemically induced changes to surfacecharges. b Experimental (symbols) and simulated voltammograms (solidlines, model 3, Supplementary Note 6) for S-2 samples over a wide range ofvoltage sweep rates (20–500mV s−1). Simulation parameters are listed inTable 1

0.0 0.2 0.4 0.6 0.8 1.0–10

0

10

20

30

40

50

60

70

80

I pea

k (μ

A)

� (V s–1)

R NR

R NR

Inversion region(Epeak, c > Epeak ,a)

Mas

ked

inve

rsio

n re

gion

(Epe

ak, c

< E

peak

,a)

–3.0 –2.5 –2.0 –1.5 –1.0 –0.5 0.0

0.30

0.32

0.34

0.36

0.38

0.40

0.42

0.44

Epe

ak (

V v

s A

g/A

gCl,

KC

l sat

)

log � (V s–1)

a

b

Fig. 6 Kinetic regions for the voltammetry as a function of the scan rate.Evolution of the experimental peak potentials (Epeak, a) and currents (Ipeak,b) as a function of voltage sweep rate (ν) in the cyclic voltammetry of S-2samples on a-Si. Black and white symbols correspond to the anodic andcathodic data, respectively. Reversible (fast kinetics, R), and non-reversible(finite kinetics, NR) zones are qualitatively indicated in figure. Lines are aguide to the eye only

Table 1 Interaction parameters obtained from current-potential curves in Fig. 5b

Scan rate(mV s–1)

Direct scan Reverse scan

QF G s y QF G s y

20 6.9 1.0 0.15 0.2 6.1 1.0 –0.9 0.2100 7.5 1.0 –0.2 0.2 6.5 0.9 –0.8 0.2500 6.7 1.2 –0.7 0.1 6.7 0.8 –0.6 0.2

Refined parameters are ket= 80 s−1, α= 0.5, I0= 10−5 μA, IL (direct scan) = 10 μA, IL (reversescan)= 100 μA. QF is in μC and defined as the product FAΓT . The interaction parameters can becalculated as aor = (y–G)/2, ar= (s + y)/2, ao= −(s–y)/2

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Figure 6a and b (and Supplementary Fig. 21) are plots of theexperimental peak potentials and currents values obtained over abroader range of voltage sweep rates. For small values of ν, thepeak potential inversion is obvious, and both the anodic andcathodic current maxima depend linearly on ν (R regioncorresponding to a fast kinetics, with ket ¼ ket= Fv=RTð Þ � 20).An increase in the voltage sweep rate leads to a progressivenearing of the direct and reverse curves arising from a kineticdistortion of the response for 20< ket< 5 with Ep,a ≈ Ep,c and lossof the linear relationship between peak currents and ν. For higherscan rates, ket < 5 (NR region), kinetics dominate the response butit is noteworthy that to some extent diode effects are still present.Although peak currents show a linear dependence with ν, theanodic and cathodic slopes are different and the reverse peak ismuch more affected than the forward one. This aspect is alsoreflected in the experimental fwhm’s, which do not coincide withthose predicted by considering kinetics only (SupplementaryFig. 15).

DiscussionThis study highlights the previously overlooked importance of theelectrostatic interactions between a molecular layer and excesscharges in the space-charge of a semiconductor electrode. Weshow how these interactions do manifest at a Si(111)/liquidelectrolyte interface and under what circumstances they dominatedynamic currents to and from a surface tether. We demonstratethat electrochemical non-idealities cannot be overlooked anddiscarded as flaws (fwhm’s< 90.6 mV and inverted peak poten-tials, Epeak cathodic> Epeak anodic). A model is developed to accountfor these observations based on the relative weight of diodecurrents, the balance between static attractions and repulsions,and kinetic factors for the electron transfer reaction. It expandson previous work devoted to account for molecular charges onthe potential profile at metallic electrodes38, showing that theinclusion of dynamic changes to the local energy of boundmolecules (i.e., changes to activation free energy of the reactionlinked to the presence of intermolecular interactions as describedby Frumkin isotherm) and dynamic changes to space-chargeeffects (i.e., changes to diode parameters) are crucial for a suitabledescription of semiconductor electrodes. It has immediateimplications for the study of electrode kinetics at semiconductorsand photoconductors since the electrostatics of the space-chargeon surface-tethered molecules can either be prevented orenhanced at will by modifying, for instance, the dielectric of thesurrounding environment, here exemplified by changes to thesurface coverage of the organic monolayer upon electrochemicalcleavage. The cautionary note is that the established kineticmodels based on the analysis of peak positions in current-potential traces must either be revised to take into account elec-trostatics, or the experimentalist must take precautions to limitthis type of effects.

The models described are also intended to guide the develop-ment of the experimental platforms for the study of how chargedgroups or externally applied electric fields can influence chemicalbonding and reactivity, an area that is beginning to attract sig-nificant interest in chemical catalysis7, 10, 11, 39–41. This electro-static aspect of chemistry has been long suggested bytheoreticians, with models developed and refined by Shaik42–45

and others9, 46 starting from 1981, but until recently this hasremained mainly a theoretical exercise7. Some progress has beenmade on insulators, where Kanan and co-workers have demon-strated the effect of static electricity on carbene reactions andepoxide rearrangements at insulators/electrolyte interfaces usingelegant surface chemistry on insulating Al2O3 films47, 48. The useof an electrical insulator (Al2O3) blocks the flow of faradaic

currents, and, at least in part49, it removes complications fromredox side reactions. There are however two caveats with insu-lators. Firstly, insulators can indeed gain excess surface char-ging30, 50, but in a material that by its very nature does notconduct electricity it is hard to define, control and measure theseeffects systematically. These tasks can also be further complicatedby surface ionization and adventitious adsorption reactions51.Secondly, and most importantly, the use of an insulator com-promises a priori the exploration of electrostatic effects overchemical processes that involve a mixed sequence of redox andnon-redox steps52, or the possible use of redox switches53, 54

together with external electric fields55 to control plasmonicresonances56. A key example of the former case, still awaiting anexperimental scrutiny, is cytochrome P450. Intriguing theoreticalpredictions by Shaik suggest an external oriented field could beused to promote both the non-redox gating as well as the tworeduction steps in the cycle of P450; overall increasing at will theenzyme’s efficiency52. The experimental insights reported in thispaper will help to decouple the electrostatic from the dynamicelectrochemical process; most importantly allowing one to detectthe presence of residual static charges while redox currents areallowed to flow.

MethodsChemicals and materials. Unless stated otherwise all chemicals were of analyticalgrade and used as received. Hydrogen peroxide (30 wt% in water), sulfuric acid(PuranalTM, 95–97%), ammonium fluoride (PuranalTM, 40 wt% in water),ammonium sulfite monohydrate ((NH4)2SO3, 92%), and 1,8-nonadiyne (1, 98%)used in wafers cleaning, etching, and silicon modification procedures were obtainedfrom Sigma-Aldrich. Redistilled solvents and Milli-Q water (>18 MΩ cm) wereused for substrate cleaning, surface modification procedures, and to prepare elec-trolytic solutions. Azidomethylferrocene (2) was prepared from (hydroxymethyl)ferrocene through published methods26. Prime grade 111-oriented (⟨111⟩± 0.5°)silicon wafers were obtained from Siltronix, S.A.S. (Archamps, France). The waferswere polished on one side only, were 180–220 μm in thickness and were either n-type (phosphorous) or p-type (boron) doped. Two different grades of phosphorousdoping were used, with the resistivity specified by the manufacturer being either7–13 Ω cm (hereafter referred to as lowly doped Si(111)), or 0.003–0.007 Ω cm(referred to as highly doped). Boron-doped samples were 500 µm thick, 0.03–0.07Ω cm, and were referred to as highly doped p-Si. Amorphous hydrogenated siliconfilms were ~4 μm in thickness and 4.2 nm in roughness (see Supplementary Fig. 19)and were prepared by decomposition over a hydrogenated p-type silicon substrate(boron-doped, 100-oriented, (⟨100⟩± 0.5°), 0.001–0.003 Ω cm resistivity) of silanegas (SiH4) in an AC plasma (13.56 MHz, 300W). The amorphous samples arereferred to as a-Si.

Electrode preparation. Silicon samples were mechanically cut into ~1 cm2 pieces,cleaned under a stream of nitrogen gas, rinsed several times with small portions ofdichloromethane and water, cleaned in hot Piranha solution for 20 min (100 °C, a3:1 (v/v) mixture of concentrated sulfuric acid to 30% hydrogen peroxide), rinsedthoroughly with water and immediately etched with an argon-saturated 40%aqueous ammonium fluoride solution for 10–12 min under an argon atmosphere.The etching bath was added with a small amount (ca. 5 mg) of ammonium sulfite.The freshly etched samples were washed sequentially with water and dichlor-omethane and blown dry in argon before the dropping of a small deoxygenatedsample of diyne 1 (ca. 50 μl) on the wafer. The liquid sample was contacted with aquartz slide to limit evaporation, rapidly transferred to an air-tight and light-proofreaction chamber, and kept under positive argon pressure. A collimated LEDsource (λ = 365 nm, nominal power output >190 mW, Thorlabs part M365L2coupled to a SM1P25-A collimator adapter) was fixed over the sample at a distanceof ca. 10 cm. After illumination for a 2 h period (unless stated otherwise, seeSupplementary Figs. 9, 10 and 12) the acetylene-functionalized samples (S-1, Fig. 1)were removed from the reaction chamber, rinsed several times with dichlor-omethane and rested in a sealed vial under dichloromethane at 4 °C for 8 h. Theacetylene-terminated samples (S-1) were then rinsed several times with smallamounts of 2-propanol and transferred to a reaction tube containing (i) the fer-rocene molecule (2, 0.5 mM, 2-propanol/water, 1:1), copper(II) sulfate pentahy-drate (20 mol % relative to 2) and sodium ascorbate (ca. 100 mol % relative to 2).Unless stated otherwise (see Supplementary Fig. 4), the click copper-catalyzedalkyne−azide cycloaddition reaction (CuAAC57 in short hand) was carried out inthe dark, at room temperature under air and stopped by removing the ferrocene-functionalized samples (S-2) from the tube after a reaction time of 30 min. Samples(S-2) were rinsed sequentially with copious amounts of 2-propanol, water, 0.5 Maqueous hydrochloric acid, water, 2-propanol, dichloromethane, and blown dryunder nitrogen before being analyzed.

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Surface characterization. X-ray photoelectron spectroscopy. X-ray photoelectronspectroscopy measurements were performed on a Kratos Axis Ultra DLD spec-trometer using a monochromatic Al-Kα (1486.6 eV) irradiation source operating at150W. Spectra of Si 2p (90–110 eV), C 1s (277–300 eV), N 1s (390–410 eV), and Fe2p (690–740 eV) were taken in normal emission at or below 7 × 10–9 Torr. Datafiles were processed using CasaXPS© software and the reported XPS energies arebinding energies expressed in eV. After background subtraction (Shirley), spectrawere fitted with Voigt functions. To correct for energy shifts caused by adventitiouscharging all peak energies were corrected with a rigid shift to bring the C 1semission to 285.0 eV. See Supplementary Methods for a detailed analysis of theXPS measurements.

Atomic force microscopy. Surface topography was imaged with a BrukerDimension atomic force microscope. All images were obtained in air at roomtemperature and using silicon nitride cantilevers (TESPA-Bruker AFM probes,with spring constant of 20 Nm–1). The scan area was set to between 3 × 3 and 2 × 2µm with a resolution of 512 points/line. The surface roughness was determinedusing Bruker’s Nanoscope Software by measuring the average root mean square,Rq, deviation in height of at least five areas on each sample. Values of peak-to-valley roughness, Rtm are also reported. The 95% confidence limit of the mean (x)on the Rq is reported as x± tn–1s/n1/258, where tn –1 depends on the number ofrepeats and was varied between 2.78 and 2.26, s is the standard deviation, and n isthe number of measurements (n was between 5 and 10).

Specular X-ray reflectometry. Specular X-ray reflectometry was measured underambient conditions at the solid—air interface using a Panalytical X’Pert Pro X-rayreflectometer. The X-ray beam was focused and collimated using a Göbel mirrorand a 0.1 mm wide pre-sample slit. Specular reflectometry (angle of incidence =angle of reflection) was collected at glancing angles with an incident angle range of0.05° to 5.00° using a step size of 0.01°. The counting time for each step was 7 s. Thedata were reduced by normalizing the raw data so that the critical edge was unityand was presented as reflectivity ( = reflected intensity/incident intensity) vs.momentum transfer (Q) which is equal to 4π sinθ/λ, where θ is the angle ofincidence and λ is the X-ray wavelength (1.54 Å). Structural parameters for themonolayer were refined in MOTOFIT reflectometry analysis software59.

Electrochemical characterization. Electrochemical experiments were performedin a single-compartment, three-electrode PTFE cell with the modified siliconsurface (S-2) as the working electrode, a platinum mesh as the counter and a silver/silver chloride in saturated potassium chloride as the reference electrode. Allpotentials are reported vs. the reference electrode. All aqueous solutions for elec-trochemical experiments contained 1.0 M of either NaClO4 or HClO4 (with a pH of6.0 and 0.5, respectively). The surface coverage, Γ, expressed in mol cm–2, wascalculated from the faradaic charge taken as the background-subtracted integratedcurrent from the anodic scan of the voltammograms. Unless specified otherwise, allelectrochemical experiments were performed in air at room temperature (22± 2 °C) under the illumination provided by a collimated 625 nm LED source (nominalpower output > 770 mW, Thorlabs part M625L3, coupled to a SM1P25-A colli-mator adapter). The incident light was pointed on the top-side (i.e., electrolyte side)of the working electrode. Ambient light was used for the highly doped n-type andfor p-type samples. A rectilinear cross-sectional Viton gasket defined the geometricarea of the working electrode to 0.28 cm2. Ohmic contact to the working electrodewas ensured by gently grinding with emery paper a thin layer of gallium-indiumeutectic over the back side of the wafer. A copper plate was pressed against theeutectic. Electrochemical measurements were performed using a CHI650D elec-trochemical workstation (CH Instruments, Austin, TX). The 95% confidence limitof the mean of experimentally determined quantities, such as Γ and fwhm’s isreported as tn–1sn–(1/2), where tn–1 was varied between 4.30 and 2.45, and n wasbetween 3 and 7.

Simulation methods. The experimental cyclic voltammograms of S-2 samplesshowed non-idealities that were explained by comparison of the data againstsimulated current-potential responses. Three simulation models were used in thiswork. These are described only briefly below, and in more detail in the Supple-mentary Notes 1, 2 and 6. The simulations for models 1 and 2 were based onpublished works17, 18, 55, 60, 61 and were written and performed in MATLAB®.Model 3 accounts for finite kinetics and for the presence of attractive and repulsiveelectrostatic forces on space-charge diode and it was programmed in Mathcad®14.0. Model 1—The Langmuir model for a single charge-transfer (SupplementaryNote 1)61. This model describes the voltammetric responses for electroactive sur-face adsorbed molecules on metallic electrodes when either fast or finite kinetics(Supplementary Note 1 and 2, respectively). Model 2—The interaction model for asingle charge-transfer (Supplementary Note 2). The model includes parametersdescribing interactions between the adsorbed molecules, which are reflected inchanges to fwhm’s values and peak positions (Supplementary Fig. 22). Model 3—The diode model with Frumkin interactions (Supplementary Note 6). We devel-oped a phenomenological model to accommodate for electrostatic forces involvingelectroactive molecules, space-charge diode effects and fast or finite kinetics(Butler–Volmer treatment, B–V in short hand notation, Supplementary Fig. 14).We note that in this work the schematics on band-diagrams and diode elements(space-charge) are drawn with the arrows showing the flow of electrons (IUPAC

convention). Tilting of the energy-level diagram downward toward the positive endof the crystal/electrolyte system is neglected for clarity.

Code availability. Scripts are available from the authors upon request.

Data availability. The data supporting the findings of this study are available fromthe corresponding authors upon request.

Received: 12 February 2017 Accepted: 6 November 2017

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AcknowledgementsThis work was supported by grants from the Australian Research Council (ARC,DE160100732 (S.C.), DE160101101 (N.D.)). J.G.S. and A.M. greatly appreciate thefinancial support provided by the Fundación Séneca de la Región de Murcia (Projects19887/GERM/15 and 18968/JLI/13) and by the Ministerio de Economía y Competiti-vidad (projects CTQ-2015-65243-P and CTQ-2015-71955-REDT Network of excellence“Sensors and Biosensors”). L.Z., M.L.C., and G.G.W. acknowledge funding from the ARCCentre of Excellence Scheme (Project No. CE 140100012). J.J.G. and G.G.W. are underARC Laureate Fellowships (FL150100060 and FL110100196). We thank Dr. Jean-PierreVeder from the John de Laeter Centre for the assistance with XPS measurements.

Author contributionsAll authors contributed to conceiving the work and designing and discussing theexperiments. Y.B.V. conducted most of the surface and electrochemical experiments withassistance from S.C. and L.Z. N.D. and Y.B.V. carried out the AFM experiments andanalyzed the data. J.G. developed the theoretical models and codes and performed thetreatment of the electrochemical data with comments and suggestions from A.M. Y.B.V.and S.C. collected and analyzed the XPS data. A.L.B. collected and analyzed the XRRdata. S.C. wrote the manuscript with significant contributions from all the other authors.

Additional informationSupplementary Information accompanies this paper at https://doi.org/10.1038/s41467-017-02091-1.

Competing interests: The authors declare no competing financial interests.

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