X-ray photoelectron spectroscopy studies of novel Π-conjugated ethynyl thiophenecontaining Pd(II) complexes and of their interaction with chromiumG. Iucci, G. Polzonetti, P. Altamura, G. Paolucci, A. Goldoni, and M. V. Russo Citation: Journal of Vacuum Science & Technology A 18, 248 (2000); doi: 10.1116/1.582142 View online: http://dx.doi.org/10.1116/1.582142 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/18/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Time-resolved x-ray photoelectron spectroscopy techniques for real-time studies of interfacial charge transferdynamics AIP Conf. Proc. 1525, 475 (2013); 10.1063/1.4802374 X-ray photoelectron spectroscopy study of the chemical interaction at the Pd/SiC interface J. Appl. Phys. 108, 093702 (2010); 10.1063/1.3500374 X-ray photoelectron spectroscopy study of polyimide thin films with Ar cluster ion depth profiling J. Vac. Sci. Technol. A 28, L1 (2010); 10.1116/1.3336242 Chromium deposition on ordered alumina films: An x-ray photoelectron spectroscopy study of the interaction withoxygen J. Chem. Phys. 116, 3870 (2002); 10.1063/1.1434954 X-ray photoelectron spectroscopy and scanning electron microscopy characterization of novelpoly(monosubstituted) acetylenes containing doping species J. Vac. Sci. Technol. A 16, 35 (1998); 10.1116/1.581006

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X-ray photoelectron spectroscopy studies of novel P-conjugated ethynylthiophene containing Pd „II… complexes and of their interactionwith chromium

G. Iucci,a) G. Polzonetti, and P. AltamuraDepartment of Physics ‘‘E. Amaldi’’ and INFM, University «Roma Tre», Via della Vasca Navale 84,00146 Roma, Italy

G. Paolucci and A. GoldoniSincrotrone Trieste, Padriciano 99, I-34012, Trieste, Italy

M. V. RussoDepartment of Chemistry, University ‘‘La Sapienza,’’ P.le Aldo Moro 5, 00185, Roma, Italy

~Received 18 March 1999; accepted 22 October 1999!

The structure of some novel oligomeric thiophene-containingp-conjugated Pd~II ! complexes,recently prepared in our laboratory, Cl–Pd~PBu3!2-C[C–C4H2S-C[C–C4H2S–C[C–C4H2S–C[C-Pd~PBu3!2Cl or Pd–TRI and Cl@–Pd~PBu3!2–C[C–C4H2S–C[C–#3–Pd~PBu3!2Cl or Pd–DET, were studied by means of x-ray photoelectron spectroscopy. The interface formation betweenchromium and the organometallicp-conjugated polymer films was also investigated. Chromiumwas evaporated stepwisein situonto the surface of the polymer films and the XPS core-level spectraof the polymer and of the metal overlayer were studied as a function of Cr coverage. Acharge-transfer reaction from the deposited metal atoms to the substrate material was evidenced bythe modifications taking place in the polymer spectra upon Cr deposition. A comparison with thesurface reactivity towards chromium of@ – Pd~PBu3!2–C[C–C6H4–C6H4–C[C–#n, Pd–DEBP, arelated polymer where benzene groups replace thiophene in the organic conjugated moiety, will bediscussed. ©2000 American Vacuum Society.@S0734-2101~00!03801-X#

I. INTRODUCTION

Poly-yne polymers containing transition metals in themain chain are a new class ofp-conjugated materials ofgreat interest for molecular electronics,1,2 due to their pecu-liar physical properties, such as third-order nonlinearity3 ca-pability of alignment in a magnetic and electric field4 andphotoluminescence;5 all these properties are related to theextended delocalization of thep-electrons system that, inrigid-rod organometallic polymers, involves the metalorbitals.6

Some of us recently reported the synthesis of Pd–DEBP and Pt–DEBP, respectively, poly@palladium-bis~tributylphosphine! diethynylbiphenyl# and poly@platinum bis~tributyl-phosphine! diethynylbiphenyl#,$@ – Me~PBu3!2–C[C–C6H4–C6H4–C[C–#n ,Me5Pd,Pt%;7

the central metal atom is present in square-planar trans con-figuration and the polymer structure is assumed to be rod-like, although some studies performed by wide angle x-raydiffraction techniques, presently in progress in our group,seem to give evidence for a chain distortion leading to acoiled structure for the Pt–DEBP polymers. The PBu3

groups do not take part in thep-conjugated system but havebeen introduced in order to increase the polymer solubility.The high stereoregularity of these materials, when spin de-posited in the form of thick films, was recently proved bypolarization-dependent near-edge x-ray absorption spectros-

copy measurements,8 that evidenced the unexpected align-ment of the polymer chains, despite the film thickness in themultilayer region. An ordered structure of the polymericmembrane is expected to induce nonlinear optical propertiesand to improve the acoustic phase velocity variations inpolymer-based surface acoustic wave~SAW! sensors; Pd–DEBP, for instance, was successfully employed as a sensi-tive membrane in a SAW device.9 The use of organometallicpolymers for applications in electrical or electronic deviceswith metallic electrodes has dramatically increased the studyof polymer/metal interfaces.

In previous articles, we reported a synchrotron radiation-induced photoelectron spectroscopy study of the interfaceformation between, respectively, Pd–DEBP and Pt–DEBPand vapor-deposited chromium;10,11 the formation ofcarbide-like and phosphide-like species at the metal/polymerinterface was detected, as a result of an electron-transfer re-action from the evaporated chromium atoms to the aromaticcarbons in the DEBP moiety and to the phosphorus atoms inthe phosphine groups, respectively.

More recently, we have been interested in the modifica-tions induced in the chemical structure and in the chemicaland physical properties of the organometallic polymers bychanging the conjugated organic moiety. In this framework,a series of organometallic polymers containing thiophene or-ganic spacers of the type–C[C–~C4H2S–C[C!n-(n51, 3) were prepared, as described in a preliminary note,12

by reacting the corresponding terminal alkynes with@PdCl2~PBu3)2] and @PtCl2~PBu3)2]. The thiophene spacerswere chosen because they can work as an electron-donating

a!Author to whom all correspondence should be addressed; Tel:39 06 55173401; FAX: 39 06 55173390; electronic mail:iucci@uniroma3.it

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group and enhance the third order nonlinear optical proper-ties of macromolecular structures.13 In addition, the insertionof thienyl units in the metal polyynes determines a betterdelocalization along the chain and a reduction of the bandgap of the polymer.14,15

In this article we present a photoelectron spectroscopystudy of the organometallic oligomers obtained in the reac-tion of @PdCl2~PBu3!2# with bis~ethynylthiophenes! havingdifferent extent ofp conjugation, i.e., diethynylthiophene orDET ~HC[C–C4H2S–C[CH! and the trimeric alkynylthiophene or TRI ~HC[C–C4H2S–C[C–C4H2S–C[C–C4H2S–C[CH!. The x-ray photoelectron spectroscopy~XPS! investigation yielded useful results for determiningthe chemical structure of the investigated materials, as shownin Fig. 1. We have also investigated the interface formationbetween chromium and, respectively, Pd–DET and Pd–TRI,by depositing chromium stepwisein situ onto the polymersurface and studying the evolution of the core-level spectraof the substrate polymer and of the metal overlayer as afunction of metal thickness. The comparison of the reactivitytowards chromium of the thiophene Pd~II ! polymeric com-plexes with the results obtained in the Pd–DEBP interfacecharacterization yields some insight into the influence of theorganic spacers on the electronic structure and the reactivityof the investigated materials.

II. EXPERIMENT

Pd–TRI and Pd–DET were synthesized by reacting@PdCl2~PBu3!2# (Bu5n-butyl) in diethylamine CuI with thecorresponding dialkyne (HC[C–C4H2S–C[CH, diethy-nylthiophene, DET, HC[C–C4H2S–C[C–C4H2S–C[C–C4H2S–C[CH, TRI! and characterized by means ofstandard chemical techniques~IR, NMR!, as described inRef. 12. Thin films, less than 1mm thick, were prepared byspinning CHCl3 solutions~2 mg/ml at 2000 rpm onto pol-ished stainless steel substrates. The chemical structures ofPd–DET and Pd–TRI are reported in Fig. 1.

The XPS spectra of Pd–TRI and Pd–DET were recordedby means of both conventional x-ray sources and of synchro-tron radiation. Conventional XPS measurements were per-

formed in a UHV VG electron spectroscopy for chemicalanalysis~ESCA! 3 MK1 instrument using nonmonochroma-tised Al Ka radiation. The experimental apparatus consistsof separated preparation and analysis chambers; an electro-static analyser operating in the fixed analyzer transmissionmode with an instrumental resolution of 1.7 eV was em-ployed. Synchrotron radiation~SR!-induced XPS measure-ments were carried out at the third generation storage ringELETTRA ~Trieste! on the super ESCA beamline. The ex-perimental station is built on three levels, one for the mea-surements, one for sample transfer and one for the low-energy electron diffraction. The experimental chamber isequipped with a 16 channel electron energy analyzer~VSWclass 150!. Before the measurements, polymer samples wereintroduced into the chamber and left outgassing in UHV(10210Torr! overnight. During the photoemission measure-ments, the sample surface was positioned at an angleu510° with respect to the incident radiation~grazing inci-dence!, the angle between the photon beam and the analyzerbeing 40°. 494 eV photons were used for recording all theanalyzed core level spectra (C 1s, Pd 3d, P 2p, S 2p, andC 2p). The high photon flux measured at the third generationstorage ring ELETTRA~Calculated flux 1015–1014 photonss/0.1% bw/200 mA! allows us to obtain XPS spectra havingat the same time high intensity and high resolution, even forcore-level signals of low cross section and corresponding toelements present in low concentration in the investigatedsamples, such as phosphorus (P 2p) and sulphur (S 2p).

All the spectra were energy referenced to the main C 1speak positioned at a binding energy~BE! of 285.0 eV. Theatomic ratios were determined from the measured peak areasby using the proper cross sections. For the spectra acquiredfrom the conventional source spectrometer the cross sectionss determined by Scofield16 have been used; the cross sec-tions determined by Yeh and Lindau17 have been used for thespectra recorded at ELETTRA.

The Cr/Pd–TRI interface formation was studied by meansof conventional XPS, evaporating high-purity chromium~99.997%! stepwisein situ onto the polymer film surface

FIG. 1. Chemical structures of Pd–DET and Pd–TRI.

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from the submonolayer region up to a nominal thickness of20 Å; metal thickness was measured by a quartz crystal mi-crobalance. The core-level spectra of the polymer film and ofthe metal overlayer (Cr 2p) were recorded after each Crdeposition. Some representative high-resolution spectra ofthe Cr/Pd–TRI interface were also recorded at ELETTRAafter the deposition of a 5-Å-thick Cr layer; 643 eV photonswere used for recording the Cr 2p spectra.

III. RESULTS AND DISCUSSION

A. Pristine materials spectra

High-resolution XPS core-level spectra of Pd–TRI andPd–DET were recorded using synchrotron radiation inducedphotoemission; some representative spectra of Pd–TRI(Pd 3d,Cl 2p,P 2p,S 2p) are shown in Fig. 2. The experi-mental conditions allow the resolution of the two spin orbitcomponents 2p3/2 and 2p1/2 in the Cl 2p, S 2p, and P 2pspectra; the measured peak area ratios are in good agreementwith the expected values (2p3/2:2p1/252:1).

A summary of the measured BE, full widths at half maxi-mum ~FWHM!, spin-orbit coupling constants~D! for theanalyzed core-level spectra of Pd–TRI and Pd–DET is pre-sented in Table I; the measured atomic ratios (Natom/NatomPd

ratio between the number of atoms of the analyzed elementand the number of palladium atoms! are also shown andcompared with the calculated values for the molecular struc-tures displayed in Fig. 1. The measured FWHMs are slightly

higher for Pd–DET than for Pd–TRI, due to a weak chargingeffect of the film. The BE values measured for Pd 3d5/2 inboth complexes are in agreement with those expected for aPd~II ! complex and also the P 2p3/2 BEs correspond to thevalue of phosphine ligands in a Pd~II ! complex.18 TheS 2p3/2 BEs are in good agreement with the value found inpolythiophenes.19–21 The presence of Cl 2p signals in theXPS spectra of both investigated materials implies that Pd–DET and Pd–TRI are actually oligomers, with–Pd~PBu3!2Clmoieties as terminal groups. This hypothesis is confirmed bythe BE value measured for Cl 2p3/2, that is typical for achlorine atom directly bonded to palladium in a Pd~II !complex.18

The measured atomic Cl/Pd ratios yield information onthe number of terminal chlorines per monomer unit i.e., onthe chain length. The S/Pd ratio reveals the number ofthiophene units per Pd~PBu3!2 group. The two data togetherallowed the elucidation of the molecular structures shown inFig. 1. Pd–DET consists on average of four Pd~PBu3!2

groups linked by three dialkynylthiophene moieties; Pd–TRIis actually a palladium-capped trimer of alkynyl thiophene.In fact, the P/Pd ratio is about 2:1 in both materials, as ex-pected for the proposed structures and the measured C/Pdratio is also in fairly good agreement with the calculatedvalues.

The C 1s spectra of Pd–DET and Pd–TRI result fromsignals due to three different types of carbon: the C 1s sig-nals of thesp carbons directly bonded to palladium~C–Pd!is expected to occur at;283.3 eV according to literaturereports,10 the sp2 thiophene carbons bonded to sulphur~C–S! yield a signal at about 285.8 eV;20,21 the remainingspandsp2 carbon atoms of the thiophene moiety bonded onlyto carbon and thesp3 carbons of PBu3 cannot be distin-guished and produce the main photoemission peak at 285.0eV ~C–C!. A summary of the different carbon atoms in Pd–DET and Pd–TRI and of the expected BEs for the corre-sponding C 1s peaks is shown in Table II. In the Pd–DETmolecule, on the basis of the chemical structure shown inFig. 1 there are 96sp3 C–C carbon atoms resulting from 8PBu3 groups, 6sp2 C–C carbons, and 6sp C–C carbons inthe alkynylthiophene moieties, resulting in 108 C–C carbonsaltogether, 6 C–S carbons, and 6 C–Pd carbons. In the

FIG. 2. Core-level spectra of Pd–TRI: Pd 3d,Cl 2p,P 2p,S 2p.

TABLE I. Summary of the results of the Pd–DET and Pd–TRI XPS spectra.

Polymer SignalBE~FWHM!

~eV!D

~eV!

Natom/NatomPd

Experimental Calculated

Pd–DET C 1s 285.0~1.7! 43 32Pd 3d5/2 338.0~1.6! 5.2 1 1P 2p3/2 130.7~1.4! 0.8 2.4 2S 2p3/2 163.8~1.4! 1.3 0.86 0.75Cl 2p3/2 197.7~1.4! 1.6 0.52 0.50

Pd–trim C 1s 285.0~1.2! 42 34Pd 3d5/2 338.2~1.2! 5.2 1 1P 2p3/2 130.8~1.1! 0.85 1.8 2S 2p3/2 164.2~1.1! 1.2 1.3 1.5Cl 2p3/2 198.0~1.2! 1.6 1.1 1.0

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Pd–TRI molecule there are 60 C–C carbon atoms 48 (sp3

carbons in 4 PBu3 units, 6sp carbons and 6sp2 carbons inthe alkynylthiophene moiety!, 6 C–S carbons and 2 C–Pdcarbons. Therefore, the intensity ratio of the C–Pd, C–C,and C–S peaks is expected to be 1:18:1 and 1:34:3 for Pd–DET and Pd–TRI, respectively. The curve-fitting deconvo-lution of the C 1s spectra of Pd–DET and Pd–TRI is shownin Fig. 3; the measured peak areas for the three componentsare consistent with the atomic ratios calculated for the pro-posed structures. The weak and broad feature at about 288.0eV is probably a shake-up satellite due top→p* transitionof the thiophene molecule, as suggested for similar structuresby Elfeninatet al.21

The proposed chemical structures have been confirmed byother techniques, mainly31P and13CNMR, and details aboutthese results will be discussed in the forthcoming articleabout the synthesis and characterization of these and otherrelated Pt-containing materials. It is important to point outthat it is difficult to get information about the chain lengthfrom the elemental analysis because of the small differencein the atomic percentages in different oligomers. Gel Perme-ation Chromatography~GPC! data are rough; indeed, GPC is

not an absolute method to carry out molecular weights oforganometallic materials, because it needs a calibration withpolystyrene standards that have a quite different conforma-tion in solution. However, this technique has been useful togive evidence of the low polydispersion of these materials.

XPS has been a useful tool for the study of the chemicalstructure of these organometallic oligomers.

B. Pd–TRI/Cr interface formation

The results obtained in the Pd–DET/Cr and Pd–TRI/Crinterfaces characterization are very similar; therefore, wewill examine in detail only the Pd–TRI/Cr interface. Theevolution of the XPS core-level spectra as a function of Crthickness was studied first by means of conventional XPSspectroscopy from the submonolayer region up to a nominalthickness of 20 Å. In order to obtain a better resolution of thedifferent features appearing in the core-level spectra uponinteraction with Cr, high-resolution XPS spectra were alsorecorded by using SR-induced photoemission, as describedin the experimental section, after the deposition of a 5-Å-thick layer of chromium onto the surface of a Pd–TRI thinfilm. Due to the lower photon energy~491 eV!, the photo-emission experiment performed at ELETTRA is much moresurface sensitive; therefore, the spectra recorded at the samemetal overlayer thickness cannot be directly compared.However, taking into account that the intensity of the signalsdue to surface-localized species is expected to be higherwhen lower energy photons are used, the information yieldedby the high-resolution spectra have been very useful in theinterpretation of the experimental data. When Cr is evapo-rated onto the surface of Pd–TRI, modifications can be seenin the C 1s, S 2p, Pd 3d, and P 2p spectra, while the Cl 2pspectrum is unaffected. New low BE components appear incore-level spectra, as a result of a charge transfer reactionfrom the deposited Cr atoms to the Pd–TRI film substrate,the intensity increasing upon increasing Cr thickness.

The C 1s spectra of Pd–TRI, pristine~bottom curve! andafter increasing Cr deposition, are shown in Fig. 4; the cor-

TABLE II. Summary of the different types of carbon atoms in Pd–DET andPd–TRI and of expected BEs for the corresponding C 1s peaks.

Polymer Carbon Hybridization

Atom number

BE ~eV!Total

sp3 96

C–C sp2 6 108 285.0Pd–DET sp 6

C–S sp2 6 6 285.8C–Pd sp 6 6 283.2

sp3 48C–C sp2 6 60 285.0

Pd–trim sp 6C–S sp2 6 6 285.8C–Pd sp 2 2 283.2

FIG. 3. C 1s spectra of Pd–DET and Pd–TRI.

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responding thickness of the metal overlayer is indicated neareach spectrum; the spectra have been normalized so that theintensity of the main photoemission peak at 285.0 eV is keptconstant. In the displayed spectra a shoulder can be seenappearing at about 283.2 eV on the low BE side of the mainC 1s photoemission peak, the intensity increasing with metalthickness, as evidenced also by the curve-fitting deconvolu-tion of the experimental spectra shown in the figure. Figure 5shows the high-resolution spectrum recorded at ELETTRAafter the deposition of a 5-Å-thick Cr overlayer. As shown inthe picture, the experimental spectrum can be resolved bycurve-fitting deconvolution into two components positioned,respectively, at 285.0 eV and labeled ‘‘pristine’’ in Fig. 5and 283 eV, labeled ‘‘reduced.’’ The first component corre-sponds to the unmodified Pd–TRI C 1s signal, while the lowBE component is due to electron-rich species created by acharge-transfer reaction from the deposited Cr atoms to thePd–TRI film. By comparison with literature data,22 the BE ofthe new component can be accounted for in terms of carbide-like species formation at the Pd–TRI/Cr interface. A similarresult was found in the XPS characterization of the Pd–DEBP/Cr interface10 and in other polymer/Cr interfaces.23–25

Figure 6 shows the modifications of the normalized P 2p

FIG. 4. Evolution of the C 1s spectra of Pd–TRI upon increasing Cr depo-sition; the corresponding metal thickness is indicated nearby each spectrum.Markers represent the experimental points, lines the fitting components. Thespectra have been normalized so that the intensity of the main peak at 285.0eV is kept constant.

FIG. 5. Curve-fitting deconvolution of the high-resolution C 1s spectrum ofPd–TRI after the deposition of a 5-Å-thick layer of chromium.

FIG. 6. Evolution of the P 2p spectra of Pd–TRI upon increasing Cr depo-sition; the corresponding metal thickness is indicated nearby each spectrum.The spectra have been normalized so that the intensity of the main peak at130.8 eV is kept constant. Markers represent the experimental points, linesthe fitting components.

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spectrum of Pd–TRI as a consequence of Cr deposition; theP 2p signal becomes gradually broader upon Cr deposition,mainly due to a shoulder appearing on the low BE side of thepristine polymer peak. The SR-radiation induced high-resolution spectrum recorded at 5 Å Cr thickness, whosecurve-fitting deconvolution is shown in Fig. 7, clearly showsthat the broadening of the overall P 2p signal is due to newpeaks~labeled C and D in the figure! appearing 2 eV fromthe pristine Pd–TRI signals~A and B!, with the 2p3/2 com-ponent at;129.0 eV, produced by the interaction with Cr. Asimilar experimental result was detected also in the study ofthe Pd–DEBP/Cr interface10 and interpreted in terms of re-duction of the phosphorus atoms of PBu3 to phosphide-likespecies, by comparison with the measured BE of chromiumphosphide.22 Curve-fitting deconvolution of the spectra re-corded by traditional x-ray radiation has also been per-formed, as shown in Fig. 6, by using the same componentpeaks, labeled A and B for the pristine polymer, C and D forthe reduced species. The fit results are in agreement with theexperimental data.

Major modifications take place as a consequence of Crdeposition in the Pd 3d spectra, as shown in Fig. 8. Uponincreasing Cr thickness, the two spin orbit components be-come broader on the low BE side, and gradually shift tolower BE. The evidenced modifications can be interpreted,by curve-fitting deconvolution of the experimental spectra,also shown in Fig. 8, in terms of new low BE components~Cand D! appearing at about 2 eV from the pristine polymer

peaks~A and B!. Curve-fitting deconvolution of high resolu-tion SR-radiation induced Pd 3d spectra~Fig. 9! was alsoperformed by using four peaks generated by the two spinorbit components 3d5/2 and 3d3/2, separated by a spin orbitcoupling constantD55.2 eV, and by two different types ofpalladium, corresponding to pristine Pd–TRI, signal (BE5338.2 eV for Pd 3d5/2) and to a new reduced species (BE5336.2 eV for Pd 3d5/2) created by the interaction withchromium. The BE of the new peak can be compared to thevalues found for Pd~0! compounds;22,26 moreover, at thisphoton energy~491 eV!, the signal due to the new reducedspecies is more intense than the pristine polymer signal. Theappearance of a new component in the Pd 3d spectrum is anunexpected result. In fact, when Cr was deposited on thesurface of Pd–DEBP no change was detected in the Pd 3dspectra. The experimental data seem to indicate an influenceof the conjugated organic moiety on the reactivity of the Pdatom. On the other hand, when Al was deposited onto a thinfilm of Pd–DEBP27 the charge transfer towards the palla-dium atom was the only evidence of the interaction betweenthe deposited metal and the polymer film. Anyway, the ex-tent of the charge transfer in that case was less pronounced,leading to a binding energy shift of about 0.7 eV. For theinteraction between Cr and Pd–DET or Pd–TRI we find ashift of about 2.0 eV. The final charge density around themetal is typical of Pd~0! complexes. These results suggest

FIG. 7. Curve-fitting deconvolution of the high-resolution P 2p spectrum ofPd–TRI after the deposition of a 5-Å-thick layer of chromium.

FIG. 8. Evolution of the Pd 3d spectra of Pd–TRI upon increasing Cr depo-sition; the corresponding metal thickness is indicated nearby each spectrum.The spectra have been normalized so that the intensity of the main peak at338.0 eV is kept constant. Markers represent the experimental points, linesthe fitting components.

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that the charge-transfer reaction depends on the metals in-volved in the interface formation, as well as on the nature ofthe organic spacer.

The sulphur atoms of the thiophene groups are also af-fected by Cr deposition in the early stages of interface for-mation, as shown in Fig. 10. Also in this case, we see ashoulder appearing on the low BE side of the pristine poly-mer signal, the intensity increasing with Cr thickness, sug-gesting a charge-transfer reaction from chromium to thethiophene groups, and involving mainly sulphur; at highchromium thickness~.6 Å, not shown in the figure!, thesignal is extremely broad due to the presence of sulphur at-oms in different chemical environments. Curve-fitting decon-volution of the SR-radiation induced high resolution S 2pspectrum recorded at 5 Å Cr thickness is shown in Fig. 11.The four peaks correspond to the two spin orbit components(S 2p3/2,S 2p1/2, labeled A and B, respectively! of the unre-acted sulphur (S 2p3/25164.2 eV) and of the sulphur atomsinteracting with chromium (S 2p3/25161.5 eV, labeled Cand D!. The traditional XPS spectra, shown in Fig. 10, havealso been deconvoluted using the line position of the corre-sponding S 2p high-resolution spectra; the fit results are inagreement with the experimental spectra. Selmaniet al.20,21

studied the interface formation between poly~3-hexylthiophene! and various metals including chromium. Atthe Cr/poly~3-hexylthiophene! interface the authors detectedthe growth of new low BE 2p3/2,2p1/2 components in theS 2p spectrum, positioned at about 2 eV form the corre-sponding peaks of the pristine polymer;21 at the same time,the appearance of a low BE component was detected at about

283.0 eV in the C 1s spectrum. The experimental resultswere interpreted, by comparison with quantum chemical cal-culations, in terms of formation of a covalent bond betweenchromium and the thiophene moiety inducing loss of aroma-ticity and planarity of the thiophene ring. Previous theoreti-cal, XPS and ultraviolet photoelectron spectroscopy studiesby Dannetunet al. about the interaction between Al andpolythiophene19 showed that Al interacts with the surface ofthiophene-based conjugated systems by the formation of Althiophene complexes characterized by new Al–Ca bondswhere thea carbons aresp3 hybridized. Modifications werefound in the S 2p and C 1s core-level signals; new compo-nents at lower BE than the main photolines have been seen.

Concerning the metal overlayer, the Cr 2p spectra werealso studied as a function of Cr thickness. No significantmodification upon increasing metal deposition were detected,apart from the expected intensity increase. A similar resultwas evidenced in the XPS investigation of similar metal/polymer interfaces.10

The analysis of Figs. 4, 6, 8, and 10 allows one to deter-mine the temporal sequence of the interaction. It seems evi-dent that the palladium atoms and the thiophene groups arethe first centres of reaction with the incoming chromium: thepresence of a second component at lower BE in the Pd 3d

FIG. 9. Curve-fitting deconvolution of the high-resolution Pd 3d spectrum ofPd–TRI after the deposition of a 5-Å-thick layer of chromium.

FIG. 10. Evolution of the S 2p spectra of Pd–TRI upon increasing Cr depo-sition; the corresponding metal thickness is indicated nearby each spectrum.The spectra have been normalized so that the intensity of the main peak at164.2 eV is kept constant. Markers represent the experimental points, linesthe fitting components.

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and S 2p signals can be recognized at the first stages ofcoverage~1.5 Å!. According to the results obtained by Elf-eninatet al.,21 the interaction between Cr and the thiophenerings should produce modifications both in the S 2p and inthe C 1s spectra. However, the C 1s signal of Pd–TRI re-sults from contributions due to both the phosphine groupsand the thiophene moieties. The low intensity of the carbide-like peak detected in the C 1s spectra in the early stages ofinterface formation suggests that initially only the thiophenecarbons interact considerably with chromium while the PBu3

carbons are less affected. This hypothesis is confirmed by thelow intensity of the phosphide-like peak in the P 2p spectrarecorded at low Cr coverage. By increasing the Cr coverage,the interaction between the deposited metal atoms and thePBu3 groups becomes more effective, as evidenced by therapid increase in the intensity of the phosphide-like peak inthe P 2p spectra and of the carbide-like peak in the C 1sspectra. A difference in the reactivity with chromium be-tween the aromatic carbon of the conjugated DEBP moietyand the aliphatic carbons of the PBu3 groups was evidencedalso in the XPS study of the Cr/Pd–DEBP interfaceformation.28

IV. CONCLUSIONS

The chemical structure of two novel organometallic ma-terials, Pd–TRI and Pd–DET, containing alkynylthiophenemoieties intercalated between Pd~II ! centers, has been inves-tigated by means of conventional and SR-induced XPS. Themeasured ratio between terminal chlorines, Pd atoms andthiophene moieties was fundamental for determining thechemical structure and the chain length.

The interface formation between chromium and the inves-tigated materials was also studied. Since Pd–DET and Pd–TRI show similar features of the XPS spectra upon interfaceformation with Cr, only the results concerning Pd–TRI arereported. The interaction between chromium and the sub-strate material results in an electron transfer from the depos-ited metal atoms to the organometallic materials. New lowBE signals have been detected on the low BE side of thepristine polymer peak in the C 1s, P 2p, S 2p, Pd 3dspectra, the intensity increasing with chromium thickness;the new peaks correspond to negatively charged reduced spe-cies created by the Cr/Pd–TRI interaction.

The interface formation starts with a considerable chargetransfer from the deposited chromium atoms mainly towardsthe palladium atoms and the thiophene moieties. The phe-nomenon is proved by the immediate appearance of newcomponents at lower binding energy in the Pd 3d and S 2pspectra, due to the reduction of these atoms. At higher metalthickness, the PBu3 groups are also involved in the reactionwith chromium, as indicated by the relevant changes de-tected in the P 2p and C 1s spectra at high metal coverage.The final result is a strong interaction between chromium andthe surface of these new materials.

ACKNOWLEDGMENTS

Some of us~G. Polzonetti, G. Iucci, M. V. Russo! wouldlike to thank the MURST~Nat. Res. Prog. On the structureand reactivity of surfaces! and CNR~PFMSTAII! Italy forfinancial support.

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