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Electrochimica Acta
jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta
a and As competition for thiolate formation at p-GaAs(1 1 1) surfaces
oredana Predaa, Catalin Negrilab, Mihail F. Lazarescub, Mirela Enachea,ihai Anastasescua, Ana M. Toadera, Sorana Ionescuc, 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, RomaniaUniversity of Bucharest, Blvd Elisabeta 4-12, RO-70346 Bucharest, Romania
a r t i c l e i n f o
rticle history:eceived 29 August 2012eceived in revised form 9 April 2013ccepted 10 April 2013
a b s t r a c t
Self-assembled layers of 4,4′-thio-bis-benzene-thiolate spontaneously adsorbed on p-doped GaAs(1 1 1)Aand GaAs(1 1 1)B electrodes were examined by EIS, XPS, and AFM investigations. XPS data provideevidence that (i) both As and Ga atoms are involved in the thiolate formation no matter which oneis prevailing on the semiconducting surface and (ii) only one of the two thiol groups participates in
the chemisorption bond. EIS and AFM results point to a more stable thiolate layer formed on the As-terminated surface than that formed on the Ga-terminated surface due to stronger self-assembling effectsdeveloped between the adsorbed species.
Passivation effects of the self-assembled monolayers spon-aneously formed by thiols adsorption at GaAs surfaces, firstlyeported 20 years ago [1], promoted a steady increasing inter-st for understanding the nature of the chemical bonding andhe influence exerted on the electronic properties of the semi-onductor [2–5]. The main motivation of this particular attentions undoubtedly related with the possibility to control the chemi-al and the electronic properties of the GaAs surface for specificpplications, since its poor chemical stability and high densityf surface/interface states are considered major impediments forunctional performances of the GaAs-based devices. Given thatithiols are excellent linkers for grafting ions, molecules or metallicanoparticles to the surface, forming bilayers or building complexeterostructures [6,7], studying the interaction of the conjugatedhenyl dithiols, such as 4,4′-thio-bis-benzene-thiol (TBBT), withaAs surfaces could provide useful information in many potentialpplications. Our previous work revealed that both the self-ssembled layers of TBBT [8] and the self-assembled TBBT protected
u-nanoparticles [9] onto a p-GaAs(1 0 0) electrode bring abouttrong adsorbate–substrate interactions which affect the semicon-uctor surface state population as well as the field effects operating
in the interfacial region. These results suggest that electronic inter-action between thiolate and semiconductor can be understood interms of changes in the local density of the electronic states atthe surface caused by the particular electronic interaction of thethiol with the surface atoms [8]. The aim of the present work isto investigate further the influence exerted by the chemical natureof the surface atoms on the 4,4′-thio-bis-benzene-thiolate formedat p-GaAs(1 1 1) surfaces. With this view in mind, we chose X-rayphotoelectron spectroscopy (XPS) to monitor the changes in chem-ical composition of the surface, atomic force microscopy (AFM) toexamine the surface morphology and electrochemical impedancespectroscopy (EIS) to explore the electrochemical behavior of thethiolate films and their effects on the electronic properties of thesemiconductor electrodes.
2. Experimental
The p-GaAs(1 1 1) electrodes used in this study were preparedfrom Zn doped (p = 1–2 × 1019 cm−3) wafers (supplied by GEOSemiconductor (UK) Ltd.) mounted on Teflon holders with therear part and the edges sealed by epoxy resin. Ohmic contacts tothe sample were made by alloying with Ti–Pt–Au (18.5:7.5:74)using magnetron sputtering deposition and thermal annealingtechniques.
The 4,4′-thio-bis-benzene-thiolate (TBBT) self-assembled lay-ers were formed from anhydrous ethanol solutions, usually of2 mM concentration. Before preparing the thiol solution, solventwas purged with Ar to remove the oxygen traces. The cleaning
rocedures used to prepare the bare semiconductor substratesor either electrochemical measurements or thiolation steponsisted in their treating with the selective etching solutionsH2SO4:H2O2:H2O (1:8:1) for GaAs(1 1 1)-A and 1 N HCl foraAs(1 1 1)-B) that allowed Yao et al. [10] to obtain atomic STM
mages revealing stable (1 × 1) structures for both GaAs(1 1 1)A andaAs(1 1 1)B electrodes in contact with H2SO4 solution. Freshlytched GaAs substrates, rinsed thoroughly with Milli-Q gradeater and ethanol, were immediately (without drying) immersed
nto the dithiol solution for 20 h, then rinsed with ethanol andried in air. TBBT as well as the other chemicals (all of reagentrade) were purchased from Sigma–Aldrich (Netherlands) andsed as received. The samples used in these experiments wereot recycled in order to avoid the inherent changes in the surfaceorphology and/or the surface state population.The electrochemical measurements were performed in the dark,
nd at room temperature, in a three-compartment electrochemi-al cell of conventional design, and with well deaerated 1 N H2SO4olutions. Pure Ar (99.999% Linde) was bubbled for 1 h before anduring the measurements in order to remove the dissolved oxygennd thereby to avoid the gallium and arsenic oxide film forma-ion on the substrate. All potentials refer to the saturated calomellectrode (SCE). The impedance spectra taken with an IM-6 Zahnerrequency analyzer between 0.3 Hz and 300 kHz were fitted using-View software (Scribner Associates Inc., Southern Pines, NC).
XPS spectra were obtained with a SPECS spectrometer equippedith a monochromatized Al K�-anode radiation (h� = 1486.6 eV)
ource operated at 400 W. The base pressure during the mea-urements was better than 2 × 10−9 mbar. The survey and regionpectra were acquired at pass energy of 100 eV and 20 eV, respec-ively. The spectra were charge corrected assuming the C-1s (C C)eak occurs at 285.0 eV. Peaks were resolved by a least-squaresurve fit routine self-consistently performed over the entire dataet and peak assignments have been done by considering reliableiterature reports. The spectra were fitted using Voigt peak profilesnd either linear or Shirley background depending on the back-round shape.
The AFM experiments were carried out in the intermittentontact mode, using an EasyScan2 model from Nanosurf® AGwitzerland, having a X–Y (10 �m × 10 �m) high resolution scan-er (vertical range (Z) of 2 �m, z-axis resolution of 0.027 nm and–Y linearity mean error of less than 0.6%) equipped with sharpips from NanosensorsTM.
. Results
.1. EIS
The impedance investigations on the bare and TBBT-covered-GaAs(1 1 1) electrodes were carried out in 1 N H2SO4 solution
n a potential range where no Faradaic process takes place. Thempedance spectra were analyzed by considering the equivalentircuit in Fig. 1 which gave the best fit of the experimental data.esides the capacitive contribution of the semiconductor space
ig. 1. Equivalent circuit of the chemically modified semiconductor/solution junction (SS/RSS – surface states capacitive/resistive contributions; Cads/Rads – adsorption capacitsol – sample and electrolyte combined resistance).
a Acta 104 (2013) 1– 11
charge region, CSC, and the capacitive/resistive contributions of thesurface/interface states localized within the band gap (CSS/RSS),the model takes also into account the changes induced by theadsorption bonds on the charge transfer across the semiconduc-tor/solution interface, and the direct electrical contributions (Ct/Rt)of the thiolate film. The electrical perturbation of the interfacialcharge transfer caused by the adsorption bonds is represented ina conventional way [11–13], by inserting the capacitive, Cads, andthe resistive, Rads, elements which contain the contributions of theadsorption/desorption rate and the surface concentration of theadsorbed species, respectively [11].
Under dark conditions and in indifferent electrolytes,GaAs/solution interface has been usually represented eitherby the conventional five elements circuit proposed by Dare-Edwards et al. [14] and used by many other groups [15–19] or thesimplified one, R(R/C), composed of a serial resistance correspond-ing to Ohmic contributions from the electrolyte and contacts, aparallel resistance attributed to Ohmic behavior of the interface,and a capacitor to represent the interface capacitance [20–24].We demonstrated in two earlier reports [25,26] that EIS data canbe fitted much better by taking into account the surface/interfacestates contributions, at least in our work systems. Later, we found[27] that the quality of the fit was significantly improved byadding an impedance component accounting for the capacitiveand resistive contributions of the adsorbed species in series to thecharge transfer resistance as seen in Fig. 1, because an incompletemonolayer adsorption leading to either a 2D or 3D insulatingphase may prevent the uniform accessibility of the real electrodeinterfaces [28] and be associated with delayed reaction processes[11–13,29]. This model circuit has been previously successfullyused by us [8,30] to examine the impedance spectra recorded forsimilar systems.
As seen in Fig. 2, there are no major differences between theelectrochemical behavior of the Ga-terminated and As-terminatedsurfaces. Both types of electrodes exhibit slight deviations fromlinearity of the Mott–Schottky (CSC
−2 vs. E) plot around −0.2 V(Fig. 2a) due to the competitive charging of the surface/interfacestates within the semiconductor band gap (Fig. 2b) as observed atother GaAs(h k l) surfaces [8,26,27,31–33] as well as other semicon-ductor electrodes [34,35].
The coincidence of the slope change in the Mott–Schottky plot(Fig. 2a) and the increase of the surface/interface state capacitance,CSS (Fig. 2b) shows that these surface/interface states are in anelectronic equilibrium with the semiconductor bulk and undergopotential-induced changes in their population in the detrimentof the space-charge layer [36]. The only distinction between theelectrical responses of the two types of electrodes consists in theadditional group of surface/interface states which becomes elec-trically active above 0 V at p-GaAs(1 1 1)A, as previously reported[27].
The similar shaped peaks of the CSS/E (Fig. 2b) and Cads/E (Fig. 2c)centered at the same potential value (−0.15 V) for both types ofelectrodes suggest that the observed surface/interface states eitheroriginate in or are closely related to the adsorbed (most probably
CSC – semiconductor space-charge capacitance; Rct – charge transfer resistance;ive/resistive contributions; Ct/Rt – thiolate film capacitive/resistive contributions;
L. Preda et al. / Electrochimica Acta 104 (2013) 1– 11 3
Fig. 2. Potential dependence of Mott–Schottky plot (a), surface/interface statecapacitance, CSS, (b) and adsorption capacitance, Cads, (c) calculated from impedancese
OmttdosepAcaoGbvwes
E
wNvtbppsot
a limited validity at the thiolate-coated semiconductor electrodes,
pectra recorded for the bare p-GaAs(1 1 1)A (open symbols) and p-GaAs(1 1 1)Blectrodes (filled symbols).
H) species. Ion species adsorbed on semiconductor surfacesay induce surface/interface states within the bandgap due to
he bonds formed with the surface atoms of the substrate [37],he resulting bonding and antibonding levels being identified asonor- and acceptor-like surface states, respectively [38,39]. Thebserved surface/interface states may, however, represent as wellurface defects (point or linear defects, such as kinks and stepdges) existing on the electrode surface or induced by chemicalolishing since they are known to act as active sites for adsorption.llongue et al. [40,41] identified two such surface/interface states,aused either by chemical attack or by photo-corrosion, locatedt 0.12 and 0.37 eV above the valence band edge. The groupf surface/interface state involved in the Fermi level pining ataAs(1 1 1) electrodes has a similar position within the bandgap,eing electrically active at about 0.4 eV above the semiconductoralence band edge. The position of the surface/interface statesithin the semiconductor band gap could be estimated by consid-
ring the position of the valence band edge at the semiconductorurface, EV,S given by the flat band potential, EFB, [42]:
V,S = −eEFB + kT · ln(
p
NV
)(1)
here p represents the hole concentration in the valence band andV = 9.5 × 1018 cm−3 [43] is the effective density of states in thealence band. Flat-band potential, EFB, estimated by extrapolatinghe first linear segment of the Mott–Schottky has close values foroth types of electrodes, 0.28 V for p-GaAs(1 1 1)A and 0.26 V for-GaAs(1 1 1)B. Unlike Rajeshwar and Mraz [44] who reported aositive shift of 200 mV for the E observed at n-GaAs(1 1 1)A
FBurface in comparison with n-GaAs(1 1 1)B, we found a differencef approximately 20 mV. Although Rajeshwar and Mraz suggestedhat the observed difference cannot be rationalized on the basis of
Fig. 3. Current/potential profiles recorded during the EIS measurements at the bare(dotted lines) and TBBT-covered p-GaAs(1 1 1) (solid lines) electrodes.
the variation of one order of magnitude of the donor densities, noother explanation was given.
The electrochemical behavior of the two types of electrodesbecomes, however, quite distinct after their chemical modificationwith self-assembled layers of TBBT. The current density increaseswith the applied bias at TBBT/p-GaAs(1 1 1)A but keeps a con-stant and very low value over the entire potential window atTBBT/p-GaAs(1 1 1)B, as seen in the j/E profiles recorded duringthe EIS measurements shown in Fig. 3 (solid lines). Although thebare p-GaAs(1 1 1)A surface is obviously more stable electrochem-ically than the bare p-GaAs(1 1 1)B surface (see the higher cathodiccurrents below −0.2 V), the TBBT-covered p-GaAs(1 1 1)A electrodeexhibits significantly higher currents than the TBBT-covered p-GaAs(1 1 1)B electrode, under the same experimental conditions.The thiolate film exerts clearly passivation effects only at the As-terminated surface whereas at the Ga-terminated surface it ratherfacilitates the interfacial charge transfer.
The comparison of the EIS spectra recorded at the beginning ofthe series of impedance measurements (−0.6 V), shown as Bode(impedance magnitude and phase angle vs. frequency) plots inFig. 4a, reveals similar electrical responses of the thiolate-coveredp-GaAs(1 1 1)A and p-GaAs(1 1 1)B electrodes. The magnitude ofthe impedance, |Z|, acquires a constant (modulus) value higherthat 105 � cm−2 at frequencies lower than 1 Hz while the phaseangle, ϕ, exhibits comparable frequency-dependence profiles forboth types of electrodes. |Z| and ϕ have, however, lower valuesat Ga-terminated surface than at As-terminated surface and therather small variation observed initially becomes more and moresignificant during the potential sweep. As seen in Fig. 4b, both theimpedance modulus and the phase shift are much lower at 0.1 V forTBBT/p-GaAs(1 1 1)A than for TBBT/p-GaAs(1 1 1)B over the entirefrequency range. This difference is more pronounced in the fre-quency window (0.3–300 Hz) where the dielectric properties of thethiolate layer are supposed to prevail [45,46].
Boubour and Lennox [47,48] reported that phase angle is ahighly sensitive indicator of the ionic insulating properties of thethiolate layer formed at gold substrates, by assuming that ϕ = 90◦
means no current leakage through ion penetration at defect or pin-holes. Based on the assumption that the electrochemical responseis entirely due to the thiolate layer (which behaves as an idealcapacitor having the metal surface as one capacitor plate and theelectrolyte ions adsorbed at the thiolate/solution interface acting asthe other capacitive plate [47]) such considerations have certainly
where the electrical contributions of the semiconductor space-charge layer and the surface/interface states are significant. How-ever, taking into account the similar electrochemical behavior of
4 L. Preda et al. / Electrochimic
Fs
toltGfcS
tocaaitMuTobiaoectcac
ig. 4. Bode plots of the impedance spectra measured for TBBT/p-GaAs(1 1 1)A (openymbols) and TBBT/p-GaAs(1 1 1)B (filled symbols) at −0.6 V (a) and 0.1 V (b).
he two bare p-GaAs(1 1 1) electrodes, the lower values of |Z| and ϕbserved at the TBBT-coated p-GaAs(1 1 1)A electrode and particu-arly their further decrease during the potential sweep clearly pointo a lower electrochemical stability of the thiolate layer formed ata-terminated surface. Similar potential-induced changes in the
requency response of the phase shift observed at other thiolate-overed electrodes were assigned to formation of defects within theAM allowing for ion penetration into the thiolate layer [24,46,49].
The analysis of the impedance spectra revealed other impor-ant differences between the electrical responses of the two typesf electrodes. The electrical charging of the thiolate layer stronglyompetes with that of the semiconductor space charge regionnd brings about upward shifts of the Mott–Schottky plots, whichre more pronounced at the As-terminated surface (Fig. 5a). Thenfluence exerted by the thiolate layer on the semiconductor deple-ion region on varying the electrical bias is also different: the
ott–Schottky plot becomes linear at TBBT/p-GaAs(1 1 1)B butndergoes a shift to more positive potentials around −0.2 V atBBT/p-GaAs(1 1 1)A. The linearization of the Mott–Schottky plotbserved at the thiolate-covered p-GaAs(1 1 1)B electrode shoulde related to the strong diminution of the surface/interface capac-
tive contributions of the surface/interface states (Fig. 5b) broughtbout by the inhibition of the anion adsorption. The positive shiftf the Mott–Schottky plot observed at the TBBT/p-GaAs(1 1 1)Alectrode may result in a potential-induced change in the surfacehemistry suggested by the instability of the capacitive contribu-
ion of the thiolate layer, Ct, observed above −0.15 V (Fig. 5c). Theoncomitant changes of all capacitive contributions, CSC, CSS, Cadsnd Ct, also point to significant changes in the thiolate–substratehemical bond.
a Acta 104 (2013) 1– 11
The quite similar electrochemical behavior of bare GaAs(1 1 1)Aand GaAs(1 1 1)B electrodes (Fig. 2 and dotted lines Fig. 3), provesthat the differences observed after their chemical modification(Fig. 5 and solid lines in Fig. 3) could not be due to the possibleoxygen contamination during the sample transfer steps as the elec-trodes undergo the same etching treatment before their immersioninto electrolyte or thiol solutions. The cause should then lie in theproperties of the thiolate layer itself.
From the mean Ct values of 4.71 �F cm−2 and 2.89 �F cm−2,evaluated for TBBT/p-GaAs(1 1 1)A below −0.25 V, and for TBBT/p-GaAs(1 1 1)B in the whole potential range (Fig. 5c), one mayestimate [50] that thiolate layer thickness, d = εε0A/Ct (where,ε = 4.5, is the common aromatic thiolate film dielectric constant[51,52], ε0 = 8.854 × 10−12 F m−1, the permittivity of free space, andA, the electrode area) is 8.5 A at Ga-terminated surface and 13.8 A atAs-terminated surface. These estimations agree very well with theresults of the calculations performed with the Gaussian 09 package[53], using DFT with the B3LYP functional set [54] and the LANL2DZbasis set [55], and adding d polarization functions on Ga and As.Clusters of the molecular formula Ga16As10H25 and Ga10As16H25were used in order to simulate the unreconstructed (1 1 1)A and(1 1 1)B surfaces, respectively. As mentioned before, Yao et al. [10]reported STM images revealing stable (1 × 1) structures for bothGaAs(1 1 1)A and GaAs(1 1 1)B electrodes in contact with H2SO4solution after their chemical etching. All the marginal bonds, exceptthe ones corresponding to the surface, were saturated with H atomsas seen in Fig. 6. The initial Ga As distance was taken as 2.45 A andcorresponds to the GaAs crystal structure previously reported in theliterature [43]. On the top layer a TBBT dissociated fragment wasadsorbed. The geometry was allowed to fully optimize. The thick-ness of the monolayer was estimated as the distance between theatom from the top layer of the cluster and the most distant atomof the TBBT fragment (nucleus-to-nucleus distance), to which thevan der Waals radius of the most distant atom was added. Thesedistances are 8.6 A on the (1 1 1)A and 13.2 A on the (1 1 1)B sur-face, which are very close to the values estimated from impedancemeasurements. One may also observe that the adsorbed moleculeis more inclined at (1 1 1)A surface than at (1 1 1)B. Jun and Zhu[56] observed that short-chain alkanethiolate films having thick-nesses much less than their corresponding molecular length donot form a dense monolayer on GaAs. The lower film thicknessresulted in the larger inclination of the adsorbed molecule againstthe substrate surface suggests thus a less dense packing of the SAMconstituents at the Ga-terminated surface. This assumption is ingood agreement with the poorer organization and larger chain tiltangles observed at octadecanethiolate (ODT) formed at n-dopedGaAs(1 1 1)A compared to that formed at n-doped GaAs(1 1 1)B[57]. Under such circumstances, one may infer that the weaker pas-sivation effects observed at TBBT/p-GaAs(1 1 1)A electrode than atTBBT/p-GaAs(1 1 1)B pointed out by the larger cathodic and anodiccurrents (Fig. 3), the lower values of |Z| and phase shift (Fig. 4), andthe higher density of surface/interface states (Fig. 5b) are due to theeasier access of the electrolyte ions to the less covered surface ofthe semiconductor substrate.
3.2. AFM
The surface morphology of the TBBT-covered p-GaAs(1 1 1) sur-faces was examined in the AFM investigations. Fig. 7 exhibitstypical 2D-AFM images of a 2 �m × 2 �m area for the TBBT-coveredGaAs(1 1 1) substrates, where a line fit correction has been applied
for display purpose and subsequent roughness calculations.
The surface of thiolate film looks compact and uniform but gran-ular, exhibiting a surface roughness of 1.91 nm at Ga- and 0.92 nmAs-terminated surfaces, respectively, as found in other similar
L. Preda et al. / Electrochimica Acta 104 (2013) 1– 11 5
F itancef ols) an
eS
S
TtGpsq
ig. 5. Potential dependence of Mott–Schottky plot (a), surface/interface state capacrom impedance spectra recorded for the TBBT-covered p-GaAs(1 1 1)A (open symb
xperimental reports [5,58,59]. The surface roughness parameter,q, was calculated as:
q =
√√√√ 1MN
M−1∑k=0
N−1∑l=0
(z(xk, yl))2 (2)
he lower value of Sq observed at TBBT/p-GaAs(1 1 1)B suggestshat thiolate species are more compact packed than at TBBT/p-
aAs(1 1 1)A, giving rise to a more uniform and smoother overlayer,ossibly due to the different chemical bondings of the thiolatepecies on the Ga-terminated and As-terminated surfaces as theuantum mechanical calculations presented above revealed.
Fig. 6. Optimized geometry of the TBBT adsorbed on GaAs clus
Changes in the composition of the p-GaAs(1 1 1) surfaces causedby their chemical modification with self-assembled layers of TBBTwere monitored by XPS investigations before and after the elec-trochemical investigations. Fig. 8 shows that the bare GaAs(1 1 1)surfaces (solid lines) exposed to air after etching are covered withmixed oxides, no matter if the terminal atoms are Ga or As. Thelower weight of the Ga O and As O species observed at the freshlyetched As-terminated surface (Fig. 8c and d) than at freshly etched
Ga-terminated surface (Fig. 8a and b) indicates a better stabilityagainst the air oxidation in the first case. This result is in goodagreement with both thermodynamic [60] and ab initio densityfunctional theory (DFT) estimations [61].
ters simulating the (1 1 1)A (a) and (1 1 1)B (b) surfaces.
6 L. Preda et al. / Electrochimica Acta 104 (2013) 1– 11
1 1)A
TetiA
ideGd
fwltarss
Fe
Fig. 7. AFM images of the TBBT-covered p-GaAs(1
The amount of Ga O and As O species decreases at theBBT-coated p-GaAs(1 1 1) surfaces (dashed lines in Fig. 8), asxpected. The effect is, however, much more pronounced athe TBBT/pGaAs(1 1 1)B than at the TBBT/p-GaAs(1 1 1)A, mean-ng that thiolate layer inhibits better the oxide formation at thes-terminated surface.
The significant changes brought about by the EIS experimentsn Ga-3d and As-3d core-level regions (dotted lines in Fig. 8)emonstrate that not only the passivation effects but also thelectrochemical stability of the thiolate layer are weaker at TBBT/p-aAs(1 1 1)A (Fig. 8a and b) than at TBBT/p-GaAs(1 1 1)B (Fig. 8c and).
In order to obtain more accurate information for such dif-erences, the As-2p3/2 and Ga-2p3/2 core level regions (Fig. 9)ere also explored since the lower kinetic energy of the 2p
ines considerably increases their surface sensitivity as comparedo the 3d lines. The Ga-2p3/2 spectrum is split into two peaks
ssigned to metal and oxide at 1117.6 ± 0.2 eV and 1118.9 ± 0.2 eV,espectively (Fig. 9a and c). The As-2p3/2 spectra (Fig. 9b and d)uggest the presence of As S species (BE = 1324.9 ± 0.2 eV), As Opecies (BE = 1326.2 ± 0.2 eV) as well as the substrate As atoms
ig. 8. As-3d and Ga-3d core-level regions in the XPS spectra recorded for the bare-GaAs(1 1lectrodes before (dashed lines) and after the EIS experiments (dotted lines).
(a) and TBBT-covered p-GaAs(1 1 1)B (b) samples.
(BE = 1323.2 ± 0.2 eV). These assignments are in good agreementwith the literature data [62–65].
The presence of Ga O and As O species suggests that thiolatecannot bind to all As and/or Ga atoms in the surface, as previouslypointed out [66], leaving a significant fraction of surface atomsuncovered and thus available for oxygen bonding. It is important tonote that TBBT formed at As-terminated surface completely blocksthe further oxidation of As atoms (Fig. 9d) and significantly inhibitsthat of the Ga atoms (Fig. 9c), whereas thiolate formed at the Ga-terminated surface allows the further oxidation of the Ga atoms(Fig. 9a) and As atoms (Fig. 9b), although to a lower extent than thebare surface. Ga atoms are obviously more easily oxidized than Asatoms due to the significantly higher heat of formation of Ga2O3(−258 kcal mol−1) than that of As2O3 (−156 kcal mol−1) [67]. Theweight of the oxidized Ga species decreases, however, after the EISexperiments, slightly at TBBT/p-GaAs(1 1 1)B (Fig. 9c) and morepronounced at TBBT/p-GaAs(1 1 1)A (Fig. 9a), whereas the oxidized
As species, absent at TBBT/p-GaAs(1 1 1)B (Fig. 9d) becomes preva-lent at TBBT/p-GaAs(1 1 1)A (Fig. 9b) after the electrochemicalinvestigations. The significant change in the As S/As O ratio foundat TBBT/p-GaAs(1 1 1)A electrode (Fig. 9b) contrasting with the
1) surfaces exposed to air after etching (solid lines) and TBBT covered-p-GaAs(1 1 1)
L. Preda et al. / Electrochimica Acta 104 (2013) 1– 11 7
F aAs(1b y: As
sas
Gctmsotbtafli
FG
ig. 9. As-2p and Ga-2p core-level regions in the XPS spectra recorded for the bare-Gefore and after the EIS experiments (light gray: GaAs; gray: Ga2O3/As2O3; dark gra
tability of that observed at TBBT/p-GaAs(1 1 1)B electrode (Fig. 9d)fter the EIS experiments points to a different electrochemicaltability of the As S species on the two crystallographic faces.
The lower stability of the thiolate layer self-assembled ataAs(1 1 1)A surface is also evidenced by the potential-inducedhanges in the S-2p core level region illustrated in Fig. 10. Althoughhe partial overlap of the Ga-3s (BE = 160 eV) and the As bulk plas-
on loss peak (BE ≈ 157 eV) bring some difficulty in fitting the S-2ppectra [65], two distinct doublets could be resolved: a dominantne with binding energies of 163.5/164.7 ± 0.2 eV correspondingo the unbound thiol group [4,56,66,68] and a smaller one, withinding energies of 162.3/163.5 ± 0.2 eV, which is characteristic forhiolate species [56,57,62] at TBBT/p-GaAs(1 1 1)A (Fig. 10a) as wells at TBBT/p-GaAs(1 1 1)B (Fig. 10b). To extract the S-2p doublets
rom the spectrum envelope, the shape of the Ga-3s/As plasmonoss peak was investigated on the clean surface and the correspond-ng constraint was subsequently used for the curve fitting.
ig. 10. S-p core-level regions in the XPS spectra recorded for the TBBT covered-p-aAs(1 1 1) electrodes before and after the EIS experiments.
1 1) surfaces exposed to air after etching and TBBT covered-p-GaAs(1 1 1) electrodesS).
The Snot-bound/Sbound peak area ratio is approximately 2 at bothtypes of TBBT-covered p-GaAs(1 1 1) surfaces before the EIS exper-iments, suggesting that only one of the thiol heads is involved inthe thiolate bonding as found for other dithiols [56]. The higherweight of Snot-bound species is due to the sulfur atom connect-ing the two benzene rings with a bonding state similar to the
C S C in thiophene and BE close to that of the unbound thiolgroup (163.6 eV) [69]. This ratio keeps constant only at the elec-trochemically biased TBBT/p-GaAs(1 1 1)B sample (Fig. 10b) butincreases at the TBBT/p-GaAs(1 1 1)A electrode after the potentialsweep (Fig. 10a). In the latter case, the decrease of the Sbound species(Fig. 10a) is obviously caused by its transformation into the newspecies observed at higher binding energy. The binding energiesof this new doublet (BE = 168.3/169.5 ± 0.2 eV) are characteristicfor SOx compounds [64] and point to chemical changes in the As-bound thiolate under the applied potential control. These chemicalchanges in the surface chemistry are in good agreement with theshift of the Mott–Schottky plot (Fig. 5a) and the decrease of thio-late layer capacitance, Ct (Fig. 5c) observed at TBBT/p-GaAs(1 1 1)Aabove −0.2 V, when the current density changes its sign (Fig. 3a).
Although XPS spectra cannot provide direct or irrefutable evi-dence for Ga S bonding (mainly because of its proximity to theGa O bonding [57,66]), the decreases of the Ga oxide weight atthe TBBT-covered surfaces (Fig. 9a and c) as well as the fact thatthe Ga/As atomic ratio estimated from As-3d/Ga-3d peak area is 1at both the TBBT/GaAs(1 1 1)A and the TBBT/GaAs(1 1 1)B samplescertifies the involvement of the Ga atoms in the thiolate forma-tion. The broadening of the Ga-2p3/2 (significantly higher at (1 1 1)Asurface) well observed in the normalized superimposed overlayspectra recorded at the bare and the TBBT-covered p-GaAs(1 1 1)surfaces shown in Fig. 11 brings additional proof in this respect[70].
4. Discussion
The nature of the thiolate-GaAs bonding has remained a sourceof continual debate. There were reports supporting either Ga S
8 L. Preda et al. / Electrochimica Acta 104 (2013) 1– 11
F line)
(
[c[neiGsAbtmnsittAtGb[taaGwi
atrG(mftaS[iabs
ie
ig. 11. Normalized superimposed overlay Ga-2p spectra recorded at bare (soliddashed line) substrates.
2,71] or As S bonding [65,72–76] and sometimes, only thehanges in S-2p core-level region indicated thiolate formation74,77,78]. The discrepancy between the experimental data origi-ates in the difficulty to discern between species with close bindingnergies. Neither Ga S nor As S could be actually unambiguouslydentified in the XPS spectra due to their partial overlapping witha O and As0 species, respectively [79]. So, since the chemicalhift observed for As0 ranges from 0.6 to 0.9 eV [80,81] and fors S, varies between 0.8 and 1.7 eV [72,82,83], one of them maye easily overlooked. Therefore, the particular attention paid tohe elemental As excess due to its presumed role in passivation
echanisms, might be the reason for which Lunt et al. [78] couldot detect AsxSy phase in the thiolate layer as they found on GaAsurfaces passivated with inorganic sulfides. In the latter case, chem-cal shift is, however, much larger, 1.9–2.4 eV [78] and thus easiero be discerned. In the high-resolution XPS measurements usinghe photon energy of 130 eV, all the three (bulk GaAs, As0 ands S) possible doublets in the As-3d region could be resolved at
he thiolate covered GaAs(h k l) substrates [57,66,72,84] but still noa S species were identified. Ga S formation was evidenced onlyy using high resolution surface-sensitive (5–6 A escape depth)83,85] and angle-resolved XPS [86] measurements, able to resolvehe Ga-3d peak in Ga S and Ga O components shifted at 0.55 eVnd 0.9 eV from the bulk GaAs species, respectively. Besides, therere reports [2,66] mentioning that XPS did not provide evidence fora S and/or As S species but GaxSy
− [2,66] and AsxSy− [2] species
ere found in the negative ion mass spectra taken in the ToF-SIMSnvestigations.
Not only experimental results but also theoretical approachesre lacking agreement. From a thermochemical point of view,he thiolate formation should preferentially involve gallium sitesather than arsenic sites, given the higher enthalpy of formation ofa2S3 (�Hf (Ga2S3) = −572 kJ mol−1) compared with that of As2S3
�Hf (As2S3) = −169 kJ mol−1) [2,87]. However, semiempirical PM6ethod led Saavedra et al. [88] to the conclusion that the most
avorable adsorption site for the thiol radical molecule is the Asop site. Density functional calculations reported by Luo et al. [61]lso predicted that the thiol–GaAs interaction is dominated by the
As bond while those reported by Tang and Cao [89], Gao et al.90], Voznyy and Dubowski [91] showed that thiolate prefers bind-ng to Ga sites due to an optimal overlap of the sulfur lone pair withn empty Ga dangling bond. Moreover, S Ga bond was found toe stronger than S As bond for a single thiolate adsorbed on GaAs
ubstrate [3].
Whether the different XPS results may be related to thenstrumental resolution, the various conclusions of the theoreticalstimations are certainly connected with the diverse modeling
and TBBT-covered p-GaAs(1 1 1)A (dotted line) and TBBT-covered p-GaAs(1 1 1)B
approaches taken into account. However, despite the varying view-points, it appears clear that thiolate may form chemical bonds withGa as well as with As atoms. The presence of both types of bonding,As S and Ga S, at GaAs(1 1 1) surfaces makes no doubt that Asand Ga atoms do compete for thiolate formation, no matter thenature of the terminal atom. The involvement of the As atoms intothe thiolate formation is clearly prevailing at the As-terminatedsurface, where the As oxidation is completely prevented (Fig. 8d).Arsenic atoms are also strong competitors for thiolate formation atthe Ga-terminated surface (Fig. 8b), where the As S bonds couldappear either by the replacement of the top Ga atoms with Asatoms in the second atomic layer (in view of the high mobilityof As atoms evidenced by the low energy barrier and activationenergy found in the molecular dynamics simulations [92]) or bythe direct involvement of the As in the second atomic layer inthe TBBT bonding. Even if the pronounced inhibition of the Gaoxidation observed at TBBT/p-GaAs(1 1 1)B (Fig. 9c) might be dueto simple steric effects, the significant diminution of Ga O speciesat TBBT/p-GaAs(1 1 1)A must be entailed by the Ga participationto the thiolate bond. Furthermore, one may state that Ga S bondis stronger than As S bond at GaAs(1 1 1)A surface, because theweight of Ga O species is lowered and that of As O is increasedafter the electrochemical bias, the latter one in the detriment ofAs S chemical bond. This agrees well with the thermodynamicestimation [93] that GaS forms a chemically stable interface withGaAs, unlike AsS, which tends to react with GaAs resulting inelemental As segregation at the GaAs/native sulfide interface. Thedifferent chemical and electrochemical stabilities of the thiolateformed at GaAs(1 1 1)A and GaAs(1 1 1)B surfaces cannot be thusassigned to the different Ga S and As S bond strengths but ratherto the different self-assembling effects.
The higher roughness values observed in the AFM image (Fig. 7)as well the poor passivation against the oxidation in air found inthe XPS analysis (Fig. 9) and the lower electrochemical stabilityevidenced in the EIS spectra (Fig. 4), the potential dependence ofall the capacitive interfacial contributions (Fig. 5) and XPS anal-ysis (Figs. 8, 9 and 10) clearly point to weaker self-assemblingeffects at TBBT/p-GaAs(1 1 1)A than at TBBT/p-GaAs(1 1 1)B. Thenature of the terminal atoms plays obviously a key role in drivingthe self-assembling forces. Similar differences were also found atODT/n-GaAs(1 1 1)A and ODT/n-GaAs(1 1 1)B [57] even if in othercases, the dopant nature turned out to be decisive for the orga-nization of the self-assembled monolayers of thiols [24,94]. The
higher tilt of the TBBT molecule adsorbed on Ga-sites revealed byour calculations in good agreement with both our EIS-based esti-mations and literature data [57] has certainly a large contribution toa poorer self-organization as found for other systems [56]. Besides,
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iven that As and Ga have different electron affinities, the chargeistribution within the thiolate monolayers adsorbed on differentite types is different and has thus different effects on the packingperating between the adsorbed molecules. As McGuiness et al.57] have recently pointed out, self-organization of the adsorbed
onolayer is driven by the intermolecular packing forces operatings a strongly cooperative process with substrate–adsorbate inter-ctions. Such cooperative effects are common to dithiolate filmsormed at GaAs substrates. Due to these effects, the thickness ofhe 1,8-octanedithiolate is close to its molecular length [56], unlikehat of the alkanethiolate, which exhibits a non-linear variation ofhe film thickness with the molecular length.
The different self-assembling effects at the thiolate formednto the p-GaAs(1 1 1)A and p-GaAs(1 1 1)B substrates should thenesult in the influence exerted by the terminal atoms on theoint action of the chemisorption forces and the packing forces.he poorer self-organization of the thiolate layer formed at Ga-erminated substrate is responsible for both the lower protectiongainst the oxidation in air and the potential-induced defectsithin the thiolate layer.
. Conclusions
Thiolate bonding with GaAs substrates entailed an ample debateoncerning the role played by As and Ga atoms. Built on controver-ial experimental results and different theoretical approaches, thisispute seems to draw to a close nowadays, when both the instru-ental performances and modeling methods are significantly
mproved. The XPS investigations coupled with EIS measurementsarried out at p-GaAs(1 1 1)A and p-GaAs(1 1 1)B electrodes modi-ed with 4,4′-thio-bis-benzene-thiolate plead for the involvementf both As and Ga atoms in the chemisorption bond, no matterhe nature of the terminal atom, in good agreement with otherecent reports [79,95]. The thiolate layer formed at the two (1 1 1)urfaces exhibits, however, distinct chemical and electrochemicaltability due to the balance between the chemical and packingorces operating within the ad-layer, that should be sensitive to theerminal atom nature. From our EIS and XPS studies one may con-lude that TBBT monolayer is better organized on GaAs(1 1 1) Ashan on GaAs(1 1 1)-Ga. Given that TBBT/GaAs(1 1 1) As electroderovides an uniform binding interface, stable chemically and elec-rochemically, it may successfully candidate as adsorption platformor metallic ions [96] as an alternative to the self-assembled
onolayers of mercaptoalkanoic acids [97] in order to build newssemblies of high potential technological interest for optoelec-ronic and sensor applications.
cknowledgements
This work was supported by a grant of the Romanian Nationaluthority for Scientific Research, CNCS-UEFISCDI, project numberN-II-ID-PCE-2011-3-0304. The theoretical calculations were per-ormed on the HPC Infrastructure created with the financial supportf the National Authority for Scientific Research, Romania throughhe Grant Capacities 84/2007.
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