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Electrochimica Acta 77 (2012) 8– 16

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

lectronic effects at self-assembled 4,4′-thio-bis-benzenethiolate protected Auanoparticles on p-GaAs (1 0 0) electrodes

irela Enachea, Loredana Predaa, Catalin Negrilab, Mihail F. Lazarescub, Ionel Mercioniub,lizabeth Santosc,d, Mihai Anastasescua, Gianina Dobrescua, Valentina Lazarescua,∗,1

Institute of Physical Chemistry “Ilie Murgulescu”, Splaiul Independentei 202, P.O. Box 12-194, RO-060041 Bucharest, RomaniaNational Institute of Material Physics, P.O. Box MG7, RO-77125 Bucharest, RomaniaFaculdad de Matemática, Astronomía y Física, Instituto de Física Enrique Gaviola (IFEG-CONICET), Universidad Nacional de Córdoba, 5000 Córdoba, ArgentinaInstitute of Theoretical Chemistry, Ulm University, D-89069 Ulm, Germany

r t i c l e i n f o

rticle history:eceived 18 November 2011eceived in revised form 12 April 2012ccepted 14 April 2012vailable online 31 May 2012

a b s t r a c t

The effects of the self-assembled 4,4′-thio-bis-benzenethiolate protected gold nanoclusters onto a p-GaAs (1 0 0) electrode were examined by AFM, XPS, SHG and EIS investigations. The AFM and XPS resultsrevealed a well-ordered overlayer exhibiting a bi-modal highly correlated fractal behavior which, how-ever, cannot fully protect the semiconducting surface against the oxidation in air. The EIS data pointed outthe influence exerted by the gold-monolayer protected clusters (Au-MPCs) over the charging/discharging

eywords:-GaAs (1 0 0)u-MPCsISPSFMHG

processes observed at p-GaAs (1 0 0) electrode. Although the applied potential is varied linearly, thepotential drop within the semiconductor space charge region as well as that across the Au-MPCs layerundergoes stepped changes supposed to result in the discrete charging of the Au-MPCs. These effectspoint to an electronic equilibrium between the Au-MPCs and the semiconducting substrate. Fermi levelpinning and enhancement of the SHG response in the potential range where the surface/interface statesin the semiconductor band gap become electrically active bring further proof in this respect.

© 2012 Elsevier Ltd. All rights reserved.

. Introduction

Self-assembling techniques are of particular interest for elec-ronic device and materials applications since they provide meanso control the semiconductor electronic properties [1,2], and/or touild novel structures [3,4]. Due to their relative stability and easef forming well-ordered monolayers on metal and semiconductorurfaces, thiols received extensive attention during the last twoecades. Since the most potential devices require connecting by theoth ends of a molecule (or molecular film) to electrodes, dithiolseserve special consideration. Dithiols are, however, more diffi-ult to handle experimentally than monothiols. Common reportedroblems are formation of multilayers at surface, large tilt angles,nd absence of order [5]. Still, because in the most cases, dithi-ls have been found to attach to substrate only through one sulfur6] they provide a convenient tool for attaching active materials

r inserting particular species (ions, molecules or metal particles)nto a surface. Among them there are the so-called Au-monolayerrotected clusters (Au-MPCs), which are very small clusters of

∗ Corresponding author. Tel.: +40 723705463; fax: +40 21 312 11 47.E-mail address: vlazarescu@icf.ro (V. Lazarescu).

1 ISE member.

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.04.161

gold atoms (5 nm average core diameter), coated with thiolatemonolayers. After Brust et al. [7] developed a synthetic routefor their preparation, Au-MPCs gave birth to a rapidly emergingfield of interest due to their remarkable chemical, electronic andoptical properties, attractive for both academic and technologicalreasons [8,9]. Like the better known nanoparticles called quan-tum dots, Au-MPCs exhibit size-dependent -quantized double-layercapacitance [10] -band gaps [11] and tunable solid-state conduc-tivity [12], properties which recommend them as highly promisingcandidates for molecular electronic devices. Within this context,semiconductor substrates are of particular interest because theyprovide the most direct path to useful electronic functionality[13–15].

Our previous studies revealed that 4,4′-thio-bis-benzenethiolate film spontaneously formed on p-GaAs (1 0 0)surfaces brings about chemical passivation, both in air and insolution as well as strong adsorbate–substrate interactions whichaffect both the semiconductor surface state population and thefield effects operating in the interfacial region [16]. In this paper wereport the self-assembling effects of Au nanoparticles protected by

4,4′-thio-bis-benzenethiolate at p-GaAs (1 0 0) electrodes. Atomicforce microscopy (AFM) and X-ray photoelectron spectroscopy(XPS) were used to explore the changes in the surface morphologyand chemical composition whereas electrochemical impedance

chimica Acta 77 (2012) 8– 16 9

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EIS) and second harmonic generation (SHG) investigationsrovided information on the electrochemical behavior of theu-MPCs films and their effects on the electronic properties of theemiconductor.

. Experimental

The p-GaAs (1 0 0) electrodes used in this study were preparedrom Zn doped (n = 2.3 × 1018 cm−3) wafers supplied by AXT Com-any (GEO Semiconductor (UK) Ltd.) mounted on Teflon holdersith the rear part and the edges sealed by epoxy resin. Back ohmic

ontacts to the sample were made by alloying with Au–Zn alloysing thermal evaporation technique. The self assembled layersf 4,4′-thio-bis-benzenethiolate-protected gold nanoparticles (Au-PCs) were built from ultrasonic dispersed Au-MPCs powder in

nhydrous ethanol solutions. Freshly etched GaAs substrate (in aixture H2SO4:H2O:H2O2 (5:1:1)) rinsed with miliQ water and

thanol were immersed into the Au-MPCs solution for 20 h, theninsed with ethanol and dried in air.

Au-MPCs were prepared by the two-phase synthesizing methodeported by Brust et al. [7] by using 4,4′-thio-bis-benzenethiolTBBT) instead of dodecanethiol. The microstructure of the Au-

PCs samples was investigated using a JEOL 200 CX TEM operatingt an accelerating voltage of 200 kV. Particle sizes were measuredrom bright and dark field images. Samples were prepared by dip-ing some drops of ultrasonic dispersed powder in ethanol on a

mm holey carbon grid.AFM experiments were carried out in air by using the

ynamic Force Module of an EasyScan2 model from Nanosurf®

G Switzerland operating in the intermittent contact mode. Thiss equipped with a high resolution scanner (10 �m × 10 �m in–Y surface plane) with a vertical range of 2 �m, Z-axis resolution.027 nm and an X–Y linearity mean error of less then 0.6%. The scanate was around 1–2 Hz. Sharp tips (NCLR from NanosensorsTM)ere employed for all measurements, with a radius of curvature of

ess than 10 nm (typically 7 nm).The fractal analysis of the AFM images taken for the bare-,

BBT- and Au-MPCs-covered p-GaAs (1 0 0) substrates was carriedut after the elimination of the background electronic noise bysing the image histograms and removing the noise peaks since

t is known that the presence of electronic noise in the image canncrease the fractal dimension [17].

XPS spectra were obtained with a SPECS spectrometer equippedith monochromatized Al K�-anode radiation source operated at

00 W. The base pressure during the measurements was betterhan 2 × 10−9 mbar. The wide survey and detail spectra were takent pass energy at 100 eV and 20 eV, respectively. Binding energiesere referenced to the C-1s peak at 285.00 eV. Peaks were resolved

y a least-squares curve fit routine self-consistently performedver the entire data set and peak assignments have been doney considering reliable literature reports. The spectra were fittedsing Voigt peak profiles and either linear or Shirley backgroundepending on the background shape.

The electrochemical measurements were performed in 0.1 Mhosphate buffer pH 7.5 with an IM-6 Zahner frequency analyser

n the range of 0.3 Hz–300 kHz. The impedance spectra were fit-ed using Zview software (Scribner Associates Inc., Southern Pines,.C.). All potentials refer to saturated calomel electrode (SCE). The

mpedance data were analyzed by using the serial connection ofhe electrical contributions of the semiconducting electrode (giveny the conventional five-component equivalent circuit of the bareemiconductor electrode [18]) and the organic overlayer (repre-

ented by its capacitive/resistive elements, CFILM/RFILM) shown inig. 1, which gave the best fit of the experimental data. This equiv-lent circuit has been previously successfully used by us [18] tonalyze similar systems. A detailed description has been given

Fig. 1. Equivalent circuit for the chemically modified semiconductor/solution junc-tion (see the text for details).

there. Briefly, beside the combined resistance of the sample andthe electrolyte (RSOL), the charge transfer resistance, RCT, and thespace charge capacitance, CSC, the model circuit also considers theappropriate elements for the electrical contributions of the surfacestates, a resistor RSS and a capacitor CSS in parallel connection withCSC. This is the simplest way to describe the response of the sur-face/interface states having time constants within the frequencyrange as Dare-Edwards et al. [19] and Batchelor and Hamnett [20]thoroughly discussed.

The SHG set-up is essentially the same as described previ-ously [18]. All the measurements were performed in the p-in/p-outconfiguration by using the fundamental output (1064 nm) from aQ-switched Nd:YAG laser operating at 20 Hz with 9 ns pulse width,incident at an angle of 45◦ on the sample. The p polarized SHradiation from the surface was detected after proper filtering bya photomultiplier in conjunction with a SRS-gated integrator anda boxcar averager system (Stanford Research SR250) at the sameangle of 45◦.

3. Results and discussion

3.1. TEM

The size distribution and the particle size obtained from the TEManalysis clearly show isolated metal nanoparticles without aggre-gation. Gold nanocrystals are distributed in zones, forming high andlow density regions, and in some areas they may also aggregate inchains. The particle shape of many Au nanoparticles is nearly spher-ical but cubic, cuboctahedral and icosahedral morphology can alsobe observed, similar to that reported by Brust et al. [7]. As seenin Fig. 2, Au particles have diameters between 2 and 7 nm (meanvalue 3.5 nm). Since the effects of the metal cluster dimensionswere beyond the aim of these investigations, we used unfractioned(i.e., as-prepared) Au-MPCs. A magnified TEM image of Au particlesis presented in Fig. 2a and associated histogram is given in Fig. 2b.

3.2. AFM

Fig. 3 illustrates typical 2D-AFM images of a 2 �m × 2 �m areafor bare-, TBBT- and Au-MPCs covered GaAs (1 0 0) substrates. Theas-received GaAs (1 0 0) substrate exhibits a very smooth surface(with root-mean-square roughness, rms = 0.19 nm), morphologi-cally featureless, similar to other reports [21,22]. After the surfacemodification with TBBT and Au-MPCs, a homogeneous overlayerconfirmed by the line-scans along X-axis was observed and the rmsincreased at 0.47 nm and 0.87 nm, respectively.

3.3. Fractal analysis

The fractal theory plays an important role in understanding sur-faces and structures with irregular geometries since it provides amethod to compute a number (the fractal dimension) that describeshow irregular, porous, agglomerate they are as well as if they areself-similar or not. Self-similarity, i.e., the property of an object tolook the same at different magnifiers, is usually obeyed only at finite

scale range, known as the self-similarity domain. At molecular-sizerange, surfaces of the most materials are self-affine, i.e., geometricirregularities and defects obey scaling laws in X–Y directions, butnot in Z-direction.

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Fig. 2. Magnified TEM image (a) and size histogram (b) of Au-MPCs particles.

The fractal dimension of the samples under discussion wasomputed by using two methods: the height correlation functionethod and the variable length scale method. Both methods use

he self-similarity property of the fractal object [23,24], i.e., theroperty of an object to look the same when zooming it:(

r

R

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r

R

)−D

(1)

here D is the fractal dimension and N(r, R) is the number of boxesf size r which cover the object of linear size R.

Fractal dimension of a rough surface can be computed from theeight correlation function [25]:

(r) ≡< C(�x, r)>x (2)

here the symbol <. . .> denotes an average over x, and C(x, r) isefined as:

(�x, r) = [h(�x) − h(�x + �r)]2

(3)

urface being described by the function h(x) which gives the maxi-

um height of the interface at a position given by x. Thus the height

orrelation function G(r) obeys the following scaling relation [26]:

(r)∼r2˛, r << L, (4)

ca Acta 77 (2012) 8– 16

where

˛ = 3 − D (5)

for a surface embedded in a 3-dimensional Euclidean space.The scaling range in which Eq. (4) is obeyed defines the self-

affinity domain (the self-similarity of surfaces) and it indicates therange where there are correlations between surface points. The firstmethod [25,26] uses Eqs. (2)–(5) to compute fractal dimension. Thesecond method was proposed by Chauvy et al. [27] and consistsin computing the root mean square deviation of the surface. Thealgorithm is the following:

(i) an interval of length ε, in case of a profile (or a box of size ε × ε,in case of a surface) is defined;

ii) a linear (or planar) least square fit on the data within the intervalis performed and the roughness is calculated;

iii) the interval (box) is moved along the profile (surface) and step(ii) is repeated;

iv) the rms deviation for multiple intervals is computed, and(v) steps (ii)–(iv) are repeated for increasing lengths (box sizes).

Rms deviation Rqε, averaged over nε, the number of intervals oflength ε, is defined by:

Rqε = 1nε

nε∑i=1

√√√√ 1pε

pε∑j=1

z2j

(6)

where zj is the jth height variation from the best fit line within theinterval i, and pε is the number of points in the interval ε.

For a self-similar structure (self-affine), the rms deviationdepends of the interval length ε as a power function. Thus, thelog–log plot of Rqε vs. ε gives the Hurst or roughening exponentH, and the fractal dimension D, can be calculated as:

D = DT − H (7)

where DT is the topological dimension of the embedding Euclideanspace (DT = 2 for profiles and DT = 3 for surfaces).

As seen in Table 1, the as-received p-GaAs (1 0 0) is a rough,disordered and low-correlated surface, with fractal behavior onlywithin narrow self-similarity domains. At lower values of cut-offlimits, the fractal dimension is 2.25 close to the value describing aplane (2), and at higher values of cut-off limits the fractal dimen-sion is 2.76. The two methods of computing fractal dimensions leadto different values (on different self-similarity domains) showingthat the structure is low-correlated and have weak fractal proper-ties. In contrast to it, the Au-MPCs covered p-GaAs (1 0 0) sampleis a bi-modal highly correlated fractal surface, characterized bytwo fractal dimensions: D = 2.69–2.78, the fractal dimension of thehigher peaks with long-range correlations and D = 2.40–2.46, thefractal dimension of the lower peaks with short-range correlations.The higher fractal dimension is similar to that found at the TBBT-covered p-GaAs (1 0 0) sample, D = 2.65, with a large self-similaritydomain indicating strong fractal behavior and highly correlatedpeaks.

3.4. XPS

XPS was used to examine the surface chemical composition ofthe bare-, TBBT- and Au-MPCs covered GaAs substrates exposedto air. The analysis of the XPS spectra brought useful details con-cerning the species present in the organic overlayer formed on the

semiconducting substrate.

The 4f core level data provided valuable information on thechemical nature of the gold nanoparticles. The binding energiesof 83.8 eV and 87.5 eV found for the doublet Au-4f7/2 and Au-4f5/2

M. Enache et al. / Electrochimica Acta 77 (2012) 8– 16 11

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ig. 3. 2D topographic view and height profile measured along the X-axis for bare p-

Fig. 4) certifying [7] that Au is zero-valent in these compounds [28]long with the narrow peak width (FWHM = 1.31 eV) [29] clearlyevealed the metallic character of gold nanoparticles.

Evidence on sulfur species was found in the S-2p core levelegion. The binding energy of a covalent bond between the sulfurnd the surface is expected to be between 161.5 and 162.5 eV30]. Two components can be resolved in the S-2p region (Fig. 5).

(1 0 0) (top), TBBT-covered (middle) and Au-MPCs-covered p-GaAs (1 0 0) (bottom).

The doublet with the lower binding energies, 162.0 eV/163.2 eV(As-2p3/2/As-2p1/2) is characteristic for thiolate species [31] andthe doublet with the higher binding energies, 163.0 eV/164.2 eV

(As-2p3/3/As-2p1/2) is consistent with S in free thiols [32]. Theweight of the Snot-bound should be higher than that of the Sboundspecies due to both the sulfur atom connecting the two ben-zene rings, with a bonding state similar to the C S C in

12 M. Enache et al. / Electrochimica Acta 77 (2012) 8– 16

Table 1Fractal dimensions of the bare p-GaAs (1 0 0), TBBT- and Au-MPCs-covered p-GaAs (1 0 0) samples.

Sample Method Fractaldimension

Linear correlationcoefficient

Self-similarityrange (nm)

p-GaAs (1 0 0) Correlation function 2.25 ± 0.01 0.996 4–27Variable scale 2.76 ± 0.01 0.977 123–289

TBBT/p-GaAs (1 0 0) Correlation function 2.65 ± 0.01 0.995 7–42Variable scale 2.81 ± 0.01 0.995 103–207

Au-MPCs/p-GaAs (1 0 0) Correlation function 2.40 ± 0.012.69 ± 0.01

0.9880.965

4–3232–60

Variable scale 2.46 ± 0.022.78 ± 0.01

0.9920.960

62–144145–476

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Fig. 4. Au-4f core-level region for Au-MPCs-covered p-GaAs (1 0 0).

hiophene (BE = 163.6 eV [33]), and the free-end thiol groups. Asxpected, the peak area ratio Snot-bound/Sbound is much higher foru-MPCs/p-GaAs (1 0 0) than that that found at TBBT/p-GaAs (1 0 0),bviously because there are much more free-end thiol groupsounded only to the Au nanoparticles. Although the gold-thiol bondoes not have the character of gold sulfide [7], the Sbound speciesight represent as well not only the chemical bonding between

u-MPCs and GaAs but also that between TBBT and Au nanopar-

icles. Murray and co-workers [28,34] reported binding energies

ig. 5. S-2p core-level region for p-GaAs (1 0 0) (a), TBBT-covered p-GaAs (1 0 0) (b)nd Au-MPCs-covered p-GaAs (1 0 0) (c) samples.

Fig. 6. As-3d and Ga-3d core-level regions for Au-MPCs-covered p-GaAs (1 0 0).

ranging from 162.1 eV to 162.3 eV/163.2 eV to 163.5 eV for the S-2p3/2/S-2p1/2 in case of Au-MPCs with different cluster dimensions.

The examination of the As-3d and Ga-3d core level regions forthe Au-MPCs-covered p-GaAs (1 0 0) surfaces shows other impor-tant differences with respect to the TBBT-covered p-GaAs (1 0 0)surfaces. As seen in Fig. 6, the Au-MPCs film brings only a par-tial protection against the surface oxidation instead of the totalinhibition caused by the simple thiolate film. Although signifi-cantly diminished, the two main native oxide peaks correspondingto Ga2O3 (BE = 20.7 ± 0.1 eV) [35,36] and As2O3 (BE = 43.9 ± 0.1 eV)[36,37] on the bare semiconducting substrate exposed to air(Fig. 5a), could be still observed on the Au-MPCs/GaAs surface butnot on TBBT/GaAs surface. The strong bonding of the TBBT to GaAsevidenced by both the S-2p doublet at BE of 162.0 eV/163.2 eV(Fig. 4b) and the As–S species (BE = 43.0 ± 0.1 eV) [38–41](Fig. 5b) clearly prevent the further oxidation the semiconductorsurface.

The weaker protection against the oxidation observed at the Au-MPCs/GaAs samples exposed in air is most probably due to thelower percent of the surface As atoms involved in the chemicalbonding as the lower peak area of As–S core-level in Fig. 6c suggests.The higher dimension of Au-MPCs particles allows the formationof a thicker but non-homogeneous overlayer, being hence less effi-cient in the surface covering. The fractal dimension correspondingto the lower peaks with short-range correlations, D = 2.40–2.46,observed at Au-MPCs/p-GaAs (1 0 0) samples is, therefore, sup-posed to represent the oxidized surface species. Since the amountof Ga2O3 is significantly higher than that of the As2O3 on Au-

MPCs/GaAs (Fig. 5c), one may conclude that As sites are stillpreferred for the chemical bonding of Au-MPCs as does for TBBT[16], even if the changes in the surface stoichiometry are differ-ent. The XPS analysis of surface chemical composition revealed

M. Enache et al. / Electrochimica Acta 77 (2012) 8– 16 13

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Fig. 8. Mott–Schottky plots (a), and potential dependence of the overlayer capaci-tance (CFILM), (b), and the surface state capacitance (CSS), (c) for p-GaAs (1 0 0) (solidlines), TBBT/p-GaAs (1 0 0) (open symbols) and Au-MPCs-covered p-GaAs (1 0 0)

AuMPC

ig. 7. Cyclic voltammograms for p-GaAs (1 0 0) (dash-line), TBBT- (dot-line) andu-MPCs-covered p-GaAs (1 0 0) (solid line) electrodes.

hat the initial surface excess of gallium atoms, (Ga/As)GaAs = 1.24,ound on the air exposed p-GaAs (1 0 0), as in other experi-

ental reports [16,42] becomes 1.07 at TBBT/GaAs and 1.31 atu-MPCs/GaAs.

.5. Electrochemical investigations

The Au-MPCs overlayer formed on p-GaAs (1 0 0) substrateignificantly hinders the electron transfer at the semiconduc-or/solution interface, as one may see in the cyclic voltammogramshown in Fig. 7. However, the passivation effects of the Au-MPCsre less effective in preventing the anodic oxidation of the semicon-uctor than those of the simple thiolate layer. The reason shoulde the access of the solvent to the electrode surface in the freepace existing between the TBBT-embedded Au nanoparticles, asiscussed above.

The impedance data revealed that surface/interface states playn important role at both the bare and the chemically modi-ed p-GaAs (1 0 0) electrodes. The Mott–Schottky plot of the bareemiconducting substrate (Fig. 8a) exhibits a slope change below0.55 V vs. SCE due to the preferential charging of the midgap sur-

ace/interface states located at about 0.65 eV above the valenceand edge (Fig. 8c), which most probably represent the Ga-antisite

attice defects, GaAs (i.e., Ga atom occupying an As site) pointedut by the surface excess of gallium atoms observed in the XPSesults. The self-assembled layers of TBBT formed on the semi-onductor substrate brings forth significant diminution both in theurface/interface state population (Fig. 8c) and the deviation frominearity of the Mott–Schottky plot (Fig. 8a). Similar effects previ-usly observed at roughly polished p-GaAs (1 0 0) electrodes wereonsidered the result of an electronic compensation of the acceptortates associated with the initial surface excess of Ga atoms by theonor states associated with the surface excess of As atoms entailedy the thiolate formation [16]. Such a mechanism, proposed bypindt and Spicer [43] to explain the sulfide passivation at GaAsurface, is supposed to be valid in this case too. The weaker effectsf electronic passivation observed at the smooth polished GaAs sur-ace, where the deviation from linearity of the Mott–Schottky plots only diminished and not removed as in case of the rough polished

aAs surface, are in good agreement with lower decrease of the sur-

ace excess of Ga brought about by the thiolate bonding. Unlike theough GaAs surface, where the surface excess of Ga changes from

(solid filled symbols) electrodes; black/gray symbols represent the experimentaldata in forward/backward scan, respectively.

an over unit value to a subunit value, this decreases only from 1.24to 1.07.

The analysis of the impedance data also showed that the effectsof the 4,4′-thio-bis-benzenethiolated encapsulated Au clusterson the electronic properties of the semiconducting substrateare quite different to that of the 4,4′-thio-bis-benzenethiolate.As seen in Fig. 8a, the Au-MPCs self-assembled layer formed onp-GaAs substrate caused a series of slight shifts to less negativepotentials of the Mott–Schottky plot, which are supposed to berelated with the potential induced changes in the Au-MPCs-filmcapacitance, CAuMPC, shown in Fig. 8c. Although not as obviousas the successive jumps found by Chen and Murray [44] in theirimpedance-derived capacitance values for Au-MPCs monolayersformed on Au electrodes, these effects suggest that the potentialdrop within the semiconductor space charge region as well asthat across the Au-MPCs layer undergoes stepped changes despitethe linear variation of the applied potential. Above −0.4 V vs. SCE,CAuMPC experiences a significant decrease which is concomitantwith the sudden increase of the surface state capacitance, CSS(Fig. 8c). Since the Mott–Schottky plot points to a pronouncedFermi level pinning by the group of surface/interface states elec-trically active in this potential region, this decrease of C

should be due to the inherent changes in potential drop over theHelmholtz double layer [45] entailed by the preferential chargingof theses surface/interface states. Unlike the simple TBBT layer

1 chimica Acta 77 (2012) 8– 16

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hich suppresses the semiconductor midgap surface/interfacetates, the TBBT-protected Au nanoparticles develop obviously aore complex electronic interaction with the GaAs surface which

oes not only let these deep surface/interface electrically activeut also results in new high density surface/interface states. Theirror-like behavior of CSS/E and CFILM/E profiles strongly suggests

hat these surface/interface surface states are in an electronicquilibrium with both the semiconductor bulk and the Au-MPCsverlayer. One may state that although higher by an order ofagnitude than CSC, CAuMPC has actually the control over the

otential drop distribution in the interfacial region. It is worth tootice that all these charging/discharging effects are reversible, theame trend being observed for all the potential induced capacitivehanges by reversing the potential scan (gray symbols in Fig. 8).

.6. SHG investigations

Optical surface SHG has been successfully used in studyingoble-metal nanoparticles in diverse environmental conditions46–49] proving that this nonlinear optical technique is very sen-itive to the collective excitation of the conduction band electronsnown as surface plasmon excitation. The second harmonic signals resonantly enhanced when the harmonic wavelength is tunedn the vicinity of the plasmon resonance. For gold nanoparticlesispersed in an aqueous solution, this surface plasmon resonance

s located at 520 nm and leads to an increase of the absorptionross-section and the hyperpolarizability. Interfaces supporting theetal nanoparticles bring, however, an environmental asymmetry

n the medium in which the particles are present and it is thus stillnknown whether the dominant contribution to the SHG signalrises from the particles themselves or from a polarization inducedy their interaction with the interface [47].

Fig. 9 shows the contour plots of the second harmonic signal as function of the azimuthal angle and the applied potential for theare p-GaAs (1 0 0) electrode and the p-GaAs (1 0 0) electrode cov-red by an Au-MPCs self-assembled layer. The C4v symmetry of theubstrate is evident in both cases, although the presence of the goldanoparticles seems to produce a screening effect on the signal cor-esponding to p-GaAs (1 0 0). We did not observe any enhancementt 520 nm which is expected as a consequence of the plasmon reso-ance of the nanoparticles. This should be due to the fact that, in ourase, the nanoparticles are not dispersed in solution, but attachedo the surface of the electrode. Besides, the sensitivity of the tech-ique can be lost altogether if the diameter of the particle is muchmaller than the wavelength of light as it is in this case. Becauset is always possible to find two infinitely small surface elementsestructively interfering at the surface of the sphere under suchircumstances, the nonlinear response is expected to be weak [49].

Fig. 9 shows a clear screening effect exerted by the Au-MPCsverlayer on the second harmonic response coming from semicon-ucting substrate. This effect is dependent on the applied potential,eing weaker in the potential region where the surface statesssociated to the Au-MPCs bonding to the GaAs surface becomelectrically active (see Fig. 8c). The stronger decrease of the signaln some regions than in others (e.g., the decrease of the maximumt the azimuthal angle of 135◦ is less pronounced than that of theaximum at 225◦) suggests that nanoparticles are aligned along

he preferential direction of the crystal substrate in good agreementith the AFM results. The potential induced changes in the contri-

utions of the Fourier coefficients shown in Fig. 10 bring furtherroof in this respect.

The Fourier analysis of the SH signal described in a previous

ork [50] is very useful to distinguish the contributions of the dif-

erent symmetry elements of the system. One may observe thathile the magnitude and the relative phase of the F0 and F4

oefficients, which are predominant, feel no significant influence

Fig. 9. Contour plots of the second harmonic signal as a function of the appliedpotential and the azimuthal angle for bare p-GaAs (1 0 0) electrode (top) and p-GaAs(1 0 0) electrode covered by an Au-MPCs self-assembled layer (bottom).

of the applied potential at the bare p-GaAs (1 0 0) electrodesurface (Fig. 10a, b), they experience significant changes between−0.4 V and −0.5 V at the p-GaAs (1 0 0) electrode covered by aAu-MPCs self-assembled layer. In this narrow potential range, theF0 coefficient (which contains both isotropic and anisotropic con-tributions and it is sensitive to the applied potential influencebecause of the susceptibility element �zzz) and the F4 coeffi-cient (which contains only the anisotropic contributions of the(1 0 0) crystal) undergo a sudden increase (Fig. 10c). Simultane-ously, their relative phase, which exhibits a remarkable linearvariation with the applied potential, changes the sign (Fig. 10d).The one- and three-fold coefficients, F1 and F3, which are mainlyresponsible for the surface defects contributions, show also anoticeable increase within the same potential region. Since sur-face defects are the preferred adsorption sites, coefficients F1 andF3 are more sensitive to the applied potential effects in the pres-ence of the adsorbed species [50]. Therefore, the observed variationof the Fourier coefficients within the potential range where thesemiconductor midgap surface/interface states become electrically

active (see Fig. 8c) suggests a coupling between the substrate andthose electronic surface states. These results advise that thesesurface states are participating in the chemical bonding of theAu-MPCs too.

M. Enache et al. / Electrochimica Acta 77 (2012) 8– 16 15

Fig. 10. Fourier analysis of the second harmonic response as a function of the applied potential for bare p-GaAs (1 0 0) electrode (top) and p-GaAs (1 0 0) electrode coveredby a Au-MPCs self-assembled layer (bottom). The modules of the different coefficients are shown at left and the phases of the F4 coefficient relative to the F0 are shown atright.

1 chimi

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6 M. Enache et al. / Electro

. Conclusions

Au nanoparticles with diameters between 2 and 7 nm embed-ed in self-assembled monolayers of 4,4′-thio-bis-benzenethiolateorm a well-ordered overlayer on p-GaAs (1 0 0) substrates whichxhibits a bi-modal highly correlated fractal behavior, character-zed by two fractal dimensions: the fractal dimension of the highereaks, D = 2.69–2.78 with long-range correlations, similar to thatound at the TBBT-covered p-GaAs (1 0 0) sample, and the frac-al dimension of the lower peaks, D = 2.40–2.46, with short-rangeorrelations. The XPS investigations revealed weaker passivationffects against the surface oxidation in air than that of the sim-le 4,4′-thio-bis-benzenethiolate layer self-assembled on p-GaAs1 0 0) substrates, most probably due to the less compact struc-ure of the Au-MPCs overlayer. The impedance investigationsointed out the influence exerted by Au-MPCs layer over the charg-

ng/discharging processes observed at p-GaAs (1 0 0) electrode.lthough the applied potential is varied linearly, the potential dropithin the semiconductor space charge region as well as that across

he Au-MPCs layer undergoes stepped changes that might be theesult of the discrete charging of the Au-MPCs. The EIS and SHGesults suggest that surface/interface states responsible for thebserved Fermi level pining and involved in driving the surfaceensitivity of the non-linear optical properties are in an electronicquilibrium with both the semiconductor bulk and the Au-MPCsverlayer.

cknowledgements

This work was financially supported by CNCSIS–UEFISCDI,roject no. PNII–IDEI 500/2009. The additional support of Deutscheorschungsgemeinschaft for performing the SHG investigationst Ulm University through the German–Romanian Cooperationroject no. 436Rum 113/33/0-1 is also gratefully acknowledged.

E.S. acknowledges financial support by the Deutscheorschungsgemeinschaft FOR1376 and PIP-CONICET 112-201001-0411.

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