Selenium, Benzeneselenol, and Selenophene Interaction with Cu(100

Article pubs.acs.org/JPCC

Selenium, Benzeneselenol, and Selenophene Interaction with Cu(100) Yongfeng Tong,†,‡ Tingming Jiang,† Azzedine Bendounan,‡ François Nicolas,‡ Stefan Kubsky,‡ and Vladimir A. Esaulov*,† †

Institut des Sciences Moléculaires d’Orsay (ISMO), UMR 8214, CNRS-Université-Paris Sud, Université Paris Saclay, Bâtiment 351, 91405 Orsay, France ‡ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France S Supporting Information *

ABSTRACT: A high-resolution synchrotron photoemission study of selenium, selenophene (Seph), and benzeneselenol (BSe) adsorption on a Cu(100) surface was performed, to investigate the film structure, chemical adsorption properties, and molecular orientation. Complementary measurements for selenophene adsorption on Au(111) were also performed. The investigation is based on determination of core level binding energies (CLBEs) by X-ray photoelectron spectroscopy (XPS) and near-edge adsorption fine structure (NEXAFS). Se chemisorption was studied as a complement to the work on molecules in order to obtain data on the CLBEs for the atomic case. Se adsorption was performed from a Na2Se solution and thereafter annealing in vacuum. Annealing leads to appearance of ordered structures in LEED. Se 3d CLBEs were determined for the different cases. Fits of Se 3d XPS spectra show existence of several components attributable to different adsorption configurations. Also coadsorption with S was studied, which leads to changes in adsorption configurations depending upon the coadsorption procedure: simultaneous or preadsorption of S. Annealing after simultaneous adsorption leads to S elimination, while it is retained after preadsorption suggesting a higher reactivity of Se. In the case of both BSe and selenophene, XPS spectra indicate that Se−C bond scission occurs, which leads to appearance of some atomic Se on the surface and coadsorption of molecules. This is accompanied by attenuation of characteristic aromatic ring peaks in NEXAFS. In the case of selenophene single Se−C bond scission leads to appearance of alkenechain selenol molecules bound with the Se atom to the surface. A similar situation is encountered for selenophene on Au(111). NEXAFS did not show dichroism possibly partly because of dissociation processes.

1. INTRODUCTION

Various semiconducting and metal surfaces have been employed, with gold being the most extensively studied, partly because of the ease of preparation of a pristine substrate and its use as a contact in electronics. The case of SAMs on copper52−59 has attracted attention as a means of oxidation protection and in an effort to explore their usefulness in protecting ultrathin copper contacts. It has been shown that well ordered alkane and some aromatic thiol SAMs can be formed on Cu surfaces.52−55 At the same time there is evidence that in some cases there may appear dissociative, S−C bond breaking,57,58 processes that lead to formation of a sulfidic S− Cu interface, like this happens in the case of some transition metals.60−62 Much effort has gone into investigation of thiophene derivatives on surfaces because of interesting characteristics of these films for molecular electronics.25 It has been noted that

The study of self-assembled monolayers (SAMs) has been an attractive field since SAMs allow tailoring of surface properties1−9 such as adhesion, corrosion, and biocompatibility, and furthermore, these have been used in research on molecular electronic devices.2−5 Historically work on SAMs involved much work on thiols1,6,8,9 with the S atom serving as linker to a substrate. Commonly investigated systems involve various thiols with alkane and phenyl groups1,6,8−23 and also thiophenes.24−33 The interaction between the headgroup and substrate affects the characteristics of the SAMs such as orientation, structural configurations, and charge transfer properties and, to a large extent, determines the performance of SAMs. In recent years, therefore, interest in other headgroups as linkers, with possibly better characteristics for a given application, engendered research into molecules bearing heavier chalcogens34−48 (Se, Te) as well as other atoms such as N49 and C.50,51 Several investigations have thus been carried out on selenol SAMs and selenophene adsorption.34−48 © 2016 American Chemical Society

Received: June 20, 2016 Revised: August 29, 2016 Published: September 6, 2016 21486

DOI: 10.1021/acs.jpcc.6b06217 J. Phys. Chem. C 2016, 120, 21486−21495

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The Journal of Physical Chemistry C thiophene adsorption on transition metals63−65 and also on Au25,27−33 is accompanied by S−C bond scission. Evaporation of coinage metals, including Cu, onto polythiophenes shows existence of dissociation processes as well.25,66,67 S−C bond breaking processes lead to changes in the characteristics of the molecular films and interface characteristics that may not be desirable in a given application, although it has been noted in some cases that existence of the sulfidic interface was actually beneficial in, e.g., corrosion protection.60 Studies of adsorption of Se headgroup SAMs on copper surfaces are still very limited. There are, however, reports that alkaneselenol SAMs are formed on Cu.46 In this work our objective was to explore the interaction of some other molecules of a type that can be of interest in molecular electronics: benzeneselenol and selenophene on Cu, to ascertain the characteristics of the films formed and if Se−C bond breaking processes play a role. The investigation is based on determination of core level binding energies (CLBE) by Xray photoelectron spectroscopy (XPS) and near-edge adsorption fine structure (NEXAFS). Since information on CLBEs for the case of Se chemisorption on Cu surface appears to be unavailable, as a complement to the work on molecules, we also studied in some detail atomic Se adsorption, as was also done for Se on Au,68 and S adsorption.69−71 These results are of interest in their own right because of the current focus on metal chalcogenides for various applications.72−77 Results of these studies will be presented in the following.

5000). Several points on the surface were selected during the measurement to avoid the damage caused by the beam. The damage aspect was addressed, and corresponding results are given in the Supporting Information. All the core level spectra were calibrated to Au 4f at 84 eV taken at each photon energy with a clean Au(111) sample with an accuracy of ±50 meV. The near-edge X-ray absorption fine structure (NEXAFS) spectra were taken at the C 1s K-edge by varying the angle between the normal of the surface and the incidence beam. The angle varied from θ = 20° (with the electric field vector near perpendicular to the scattering plane) to θ = 90° (parallel to it). The energy resolution at the C edge was of 60 meV and polarization 100%. The spectrum was normalized with a reference adsorption spectrum of carbon-free clean Au(111) taken under the same experimental conditions. All the measurements were taken at room temperature.

3. RESULTS AND DISCUSSION 3.1. Selenium Adsorption on Cu(100). In the first section below we describe results of selenium adsorption from a sodium selenide solution, which leads to strong selenization of the surface. Annealing of this surface leads to ordered surface structures. We also studied the effect of coadsorption of sulfur and selenium in order to see how adsorption characteristics may be affected and what would be the CLBEs since the adsorption sites and chemical shifts could be different. While this may not be directly pertinent for molecular adsorption studies, such a study could also provide some information on the differences in interaction of sulfur and selenium, which are frequently mentioned in the literature in relation with headgroup interactions in chalcogenide SAMs.39,41,42 These results well be described in the second part of this section. This investigation follows an earlier study of sulfur adsorption on Cu surfaces.71 Most features of Se adsorption are quite similar to those for the sulfur case, and therefore, in this section we will mainly focus on the XPS spectra for the Se 3d and Se 3p regions. Overview XPS spectra for the clean Cu(100) surface, after initial selenization in the Na2Se solution, after heating to 300 °C and then further to 500 °C are given in the Supporting Information (Figure S1). Figure S2 similarly summarizes the spectra for the Cu 2p, Cu LMM Auger, and Cu 3p regions. Briefly, initial selenization leads to strong shifts to higher energies for the Cu 2p peak and in the Auger peak region, like this is observed for Cu selenide compounds.77 The initial changes in the Cu spectra are compatible with appearance of Cu+ species. Annealing appears to decrease the amount of Se on the surface and the Cu 2p CLBE shifts decrease. Further details can be found in the Supporting Information. The XPS spectrum of Se 3p and Se 3d are reported in Figure 1 and Figure 2 upon initial selenization and after heating. Figure 1 shows the XPS spectra in the Se 3p region. After initial selenization one observes a broad multicomponent spectrum. The central component at about 162 eV decreases strongly after annealing leaving a somewhat broad doublet with Se 3p3/2 at 160.5 eV after heating to 300 °C, which then shifts to 160 eV after further annealing to 500 °C. The Se 3d peaks (Figure 2a) are also broad and displayed a continuous shift toward lower energy upon heating. This effect is similar to the observation for S on Cu.71 Shifts from the bulk Cu2Se position at 54 eV to higher energies have been reported for various molecular cluster copper selenide compounds.77 The peak positions (in the following we shall always refer to the

2. EXPERIMENTAL SECTION Sample Preparation. A monocrystalline Cu(100) sample was used. The sample was prepared by cycles of sputtering and annealing, and the cleanliness was checked by the XPS and LEED. All the chemicals were purchased from Sigma-Aldrich and used without any further purification. Selenization was performed by immersing the Cu sample into a freshly prepared 1 mM (millimolar) solution of Na2Se in 0.1 molar NaOH for about 30 s.68 For sulfidation Na2S was used as described previously.61,71 This provides an easy method for creating sulfidized or selenized layers in realistic (ambient) conditions and has been used frequently by other authors.79 SAMs of selenophene were prepared by immersion into pristine selenophene, whereas a 1 mM solution of benzeneselenol in ethanol was used. The samples well thoroughly rinsed with Milli-Q water in the case of selenization (sulfidation) and ethanol in the case of the SAMs and dried with N2. Na2Se (Na2S), selenophene, and the benzeneselenide solution were stored in an Ar-filled glovebox without light exposure. The immersion process was performed in a glovebox in N2 atmosphere, and samples were rinsed and dried with N2 before introducing into the UHV chamber. Measurements on Se adsorption and coadsorption with S involved successive annealing in UHV in the temperature range between 300 and 600 °C, as indicated later in this paper. Photoemission and LEED measurements on these were performed after cooling the samples to room temperature. Photoemission. High-resolution X-ray photoemission was performed at the TEMPO beamline (Synchrotron Soleil, France) equipped with an undulator.68,71 Photoemission data were acquired with a high resolution in the 60 to 1150 eV range using a Scienta SES2002 electron spectrometer. The measurements were made with an energy resolution of about 50 meV at 260 eV (the resolving power of the beamline (E/δE) is about 21487


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Table 1. Se 3d5/2 Peak Position for Se on Cu, Simultaneous Adsorption (Se+S)_Cu, and Successive Adsorption Se_(S_Cu)a Se_Cu

S

RT 300 °C 500 °C

53.5 eV (29%) (Se + S)_Cu S

Figure 1. Se 3p spectrum after initial selenization and after annealing to the indicated temperatures.

A

B

C

D

53.8 eV (14%) 53.8 eV (27%) 53.8 eV (65%) A

54.2 eV (50%) 54.2 eV (56%)

54.5 eV (26%) 54.5 eV (17%) 54.5 eV (6%) C

55.0 eV (11%)

54.5 eV (27%) 54.5 eV (9%)

54.9 eV (10%)

RT 300 °C

Se 3d5/2 component) are given in Table 1. One should note that no structure due to Se oxidation was observed at 57 eV. All the Se 3d spectra, after a Shirley background subtraction, were fitted using a Voigt contour with a spin orbit splitting of 0.86 eV and an intensity ratio I(Se 3d5/2)/I(Se 3d3/2) of 3:2. To reduce uncertainties in fitting, we first fitted the narrowest Se 3d spectrum, which presented the least number of components and then used the Voigt parameters from this fit for all other spectra. The spectrum used for this appears in the next section on coadsorption (Figure 4b, 600 °C, and in Figure S6f). We used as few components as possible. In this manner the accuracy of the positioning of the main components is about ±50 meV, while for the small components it is about ±100 meV. After initial selenization, the Se 3d peak in Figure 2b, can be fitted with two predominant components: B and C, as well as two smaller peaks A and D. The peak positions are listed in Table 1 along with the relative intensities of the components. After heating to 300 °C, the C component decreases while the A component increases somewhat. The D component disappears. After heating to 500 °C the spectrum shifts to lower energies. It is dominated by the A component. The B and C components decrease leaving a small peak at 54.4 eV, and a

500 °C Se_(S_Cu) RT 300 °C 600 °C

53.4 eV (5%) 53.5 eV (19%) A 53.8 eV (13%) 53.8 eV (25%) 53.8 eV (89%)

B

53.8 eV (14%) 53.8 eV (58%) 53.8 eV (72%)

54.2 eV (49%) 54.2 eV (28%) 54.3 eV (9%)

D

B

C

D

54.2 eV (5%) 54.2 eV ( 58%) 54.2 eV (11%)

54.5 eV (29%) 54.5 eV (17%)

54.9 eV (5%)

a

The most intense component is indicated in bold letters. The numbers in parentheses are percentage contributions of the peak components to the corresponding spectra.

new S component needs to be added. These different peaks correspond to different adsorption configurations of Se on Cu(100), which result in changes in core level binding energies. The changes as a function of temperature suggest structural changes in the Se layer and are reminiscent of the ones observed for sulfur interaction with Cu(100).71 In a previous study of Se adsorption on Au68 at room temperature using the same method, we had also observed complex Se 3d spectra in which, by analogy with other studies

Figure 2. (a) XPS spectra of Se 3d core levels for Cu(100) taken at a 260 eV photon energy after initial Se adsorption and after annealing. (b−d) Fits of the background subtracted spectra at different temperatures. 21488


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Figure 3. XPS spectra in the S 2p and Se 3p regions at RT, after annealing at 300 and 500 °C (600 °C). (a) Sample from mixed Na2S and Na2Se solution. (b) Sample prepared by successive adsorption of Se after S (see text). (c) Fit of the 600 °C spectrum for the successive adsorption case.

narrowed down the possibilities to just a few main possibilities, but to date there is still no completely conclusive evidence70 in favor of one of the possible structures. One point worth noting was that the models indicate the existence of different types of coordinated S atoms that would then have different core level binding energies, as was indeed observed experimentally by us.71 Given the complex characteristics of S adsorption on Cu, it is not surprising that for Se we observe some restructuring, differently coordinated Se atoms and that the dominant CLBEs change as a function of temperature. We will not go into these interesting aspects of the Se on Cu structures, although obviously it is important to be able to assign the CLBEs to given Se atoms. Further studies are necessary before attempting such identifications. STM investigations similar to the ones performed for the sulfur case, would help in elucidating the chemisorbed Se surface structures and the changes that occur as temperature and coverage change. Ab initio calculations would also be required to identify these, and inclusion of calculations of CLBEs could, along with our data, be instrumental in resolving possible ambiguities such as those that persist for sulfur. 3.2. Selenium Coadsorption with Sulfur. Simultaneous S and Se Adsorption. As a complement to our work on S and Se adsorption we performed a study of coadsorption of these atoms. We first performed coadsorption from a Na2S/Na2Se equal proportion aqueous solution. Figure S5a in the Supporting Information shows the overview spectra using a 260 eV photon energy. The spectrum in the Se 3p region is different from the one observed for pure Se adsorption. Note that the S 2p CLBE lies between the doublet components of Se 3p. One of the main observations is that both S and Se are initially adsorbed, but with increasing temperature the sulfur component is progressively eliminated. This is clearly observed in Figure 3, where one initially observes a very intense peak at about 161.9 eV, which we ascribe to S 2p. In Figure 1 we saw that there exists also a Se 3p component at about 162 eV, but here the spectrum

of S and Se adsorption on Au, different sets of doublets were attributed to Se chemisorbed on (i) gold atomically (at 53.54 eV), (ii) polymeric Se8 (at 54.25 eV) ring structures,78 seen for sulfur,79,80 and (iii) smaller components (at 55.1 and 55.73 eV) possibly due to Se atoms in molecular and bulk-like components. The temperature stability and evolution of these was not investigated. It is possible that similarly to gold the initial adsorption in the Cu case leads to appearance of Se components corresponding to copper selenide, chemisorbed Se atoms, and also some ring or bulk-like components. The latter could be the case of the D component only observed at room temperature. Because of the very low melting point of bulk Se (tabulated value of 221 °C) the lower energy components, which remain after heating, must be related to strongly bonded Se: copper selenide or surface chemisorbed Se atoms. The latter corresponds to Se in the ordered structures observed in LEED discussed later. In order to ascertain what types of structural changes take place, we made some LEED measurements given in the Supporting Information (Figure S3). Upon heating the sample, these show the appearance of well-defined structures, that evolved with increasing annealing temperature. The structures observed here are different, from the ones observed for the case of S on Cu.69−72 Sulfur on Cu has been investigated in considerable detail, but since the system is complicated, there is still lack of complete clarity. In our own investigation71 at similar annealing temperatures, respectively high and low S coverage structures were observed in agreement with earlier works.69,70 Briefly, at 300 °C the observed LEED pattern, was assigned to a 0.47 ML (√17 × √17) R14° structure,69,70 whereas at higher temperatures, upon further heating a 0.25 ML, coverage p(2 × 2) structure separated by local c(4 × 2) domain boundaries is observed.72 The initial experimental and recent theoretical studies suggest for the (√17 × √17) R14° S_Cu structure a surface with significant reconstruction, involving appearance of groups of four Cu adatoms, with S occupying different adsorption sites. A number of possible surface structures have been considered. This quest 21489


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Figure 4. XPS of Se 3d of the (Se + S)_Cu (a) and Se_(S_Cu) (b) as a function of temperature. (a) Sample obtained by immersing the Cu into mixed Na2S and Na2Se solution. (b) Sample prepared by successive adsorption of Se after S (see text).

Figure 5. (a) Comparison of XPS Se 3d peaks for emission into 90° and 20° with respect to the surface of the sample. (b,c) Fits of background subtracted spectra at different emission angle. (d) Carbon K-edge NEXAFS as a function of incident irradiation angle.

well-defined S 2p peak with a CLBE at 161.9 eV. This would exclude existence of bulk-like S. This energy is close to that observed for copper sulfides but is different from S 2p CLBEs in the monolayer range.71 This difference may be because of S adsorption on a selenized Cu surface. The Se 3d peak for different temperatures is shown in Figure 4. The spectra turn out to be quite similar to the case of only Se adsorption as described above. A decomposition into different components was performed, as for Se adsorption, by fitting the spectra using Voigt contours. Results of these fits are given in the Supporting Information (Figure S6). The CLBEs of the different Se 3d5/2 components are given in Table 1. Upon heating the predominant Se binding site at 300 °C is different from the one of only Se adsorption. In this coadsorption case, LEED imaging reveals appearance of quite complex structures. These are given in the Supporting Information (see Figure S7). To see if one could obtain a more stable S layer in coadsorption we performed a successive adsorption study as outlined in the next section. Successive Adsorption of Se after S. Sulfur was chemisorbed on Cu as described in a previous paper from a Na2S aqueous solution.71 The sulfidized sample was annealed in vacuum to 300 °C and checked by LEED. This annealing

is quite sharp with a doublet splitting corresponding to sulfur, which argues in favor of our assignment. While this S 2p related peak is dominant here, it should be noted that at the 260 eV photon energy the photoionization cross section for S 2p is about 4 times higher than for Se 3p (see also below). The changes in the S 2p and Se 3p peak intensities as a function of temperature suggest that Se reacts more strongly and has a higher reaction rate than S and hence a Se−Cu interface is initially formed. Sulfur adsorption takes place on top of this layer and the elimination of sulfur at higher energies indicates that S adsorption energies are not very high. The Se in the selenized Cu interface appears more stable. Adsorption of S on top of the Se_Cu layer should lead to some attenuation of the Se signal, depending upon the amount of S present, and hence, the S signal from the top of the layer is strong. As mentioned above, earlier studies of S adsorption on Au,79 following the same protocol we use here for Se, have shown that there may appear several components due to atomic S (161.1 eV), ring S8 (polymeric type, 162.1 eV), and more loosely bound bulk-like (163−164 eV range) components. One could assume that on Cu the bulk-like components would have similarly large CLBEs, while the atomic ones would be lower, as actually was shown by us earlier.71 It is also possible that some Se−S bonded species could exist. Here we observe one strong 21490


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The Journal of Physical Chemistry C results in the formation of the 0.47 ML S coverage (√17 × √17)R14° structure, which was shown to have S 2p3/2 CLBE of 161.2, 161.6, and 162.1 eV.71 The first two components were of almost equal intensity and the third one was less intense. The sample was then placed into a Na2Se solution for 30s and then reintroduced into the analysis chamber after rinsing and drying. LEED and XPS measurements were then performed immediately and again after heating the sample. XPS spectra in the S 2p−Se 3p region are shown in Figure 3b. The initial spectrum is now rather broad with several structures attributable to both S 2p and Se 3p. There appears to be one main S 2p related feature at about 162 eV, which is less intense than in the coadsorption case. The S 2p peak shape is somewhat different from what was reported71 for only S adsorption in similar conditions. In general it is possible that presence of Se could affect the CLBEs of preadsorbed S. One can also not exclude some broad contribution from Se at this energy as observed in Figure 1. However, at higher photon energy this contribution decreases, and hence, the assignment to sulfur appears reasonable (see Figure S5b). Upon heating to 300 °C, however, the Se 3p component becomes less intense, while a more intense and narrow peak with a CLBE of 162 eV is observed, which we attribute to S 2p. The initial S adsorbed structure was clearly affected by Se. Further heating to 600 °C leads to a relative increase of the Se 3p component with respect to S 2p. The S 2p peak shifts to lower energy and now the S 2p3/2 CLBE is 161.3 eV and is close to what is observed in the case of only S adsorption after heating to this temperature.71 A fit of the spectrum using one component for the Se peak is given in Figure 3c. The S peak does not disappear, and in this case, one thus sees that a more stable coadsorbed sulfoselenide phase is formed. The evolution of the Se 3d peak is shown in Figure S6 along with fits. As opposed to the case of simultaneous S and Se adsorption the Se 3d peak shifts to the lower CLBE only at high temperature (see Table 1). LEED imaging reveals again complex structures, which change as a function of annealing temperatures, clearly indicating restructuring in the Se_Cu layer. Details are shown in Figure S8 in the Supporting Information. 3.3. Benzeneselenol Adsorption. Let us now turn to the case of benzeneselenol adsorption. Results for measurements in the Cu 2p, Cu 3p, and Cu Auger regions shown in Figure S9 are similar to those observed in Figure S2 for Se on Cu after strong heating and also close to those for clean Cu. Data for C 1s and the valence band region are also shown in the Supporting Information. Here we describe the Se 3d and NEXAFS results. Figure 5a shows the Se 3d spectrum of benzeneselenol on Cu for different emission angles with respect to the surface of the sample. The spectrum is broad and fairly complicated. There is a shoulder at around 54.0 eV, whose intensity increases with the emission angle with respect to the sample surface, indicating a component that is located close to the molecule/ Cu interface. Figure 5b,c show spectra fitted with Voigt contours, where one can see this evolution more clearly. This observation indicates that the corresponding Se atoms are located lower, i.e., closer to the Cu surface. The main fitted components have the Se 3d5/2 CLBEs as given in Table 2. Earlier works on phenyl and alkane selenide SAMs suggest that the A and B peaks can be assigned to atomic Se and the BSe molecule, respectively,43−45,48 while C could correspond to the physisorption of extra BSe molecules on the

Table 2. Fitted Se 3d5/2 Peak Positions for the Different Molecule−Substrate Combinationsa

a

System

A

B

BSe_Cu(100) Seph_Cu(100) Seph_Au(111) Seph_Cu(111)28

54.0 eV 54.2 eV 53.5 eV 54.2 eV

54.5 eV 54.8 eV 54.2 eV 54.8 eV

C 55.0 55.8 55.1 55.5

D eV eV eV eV

55.6 eV 55.5 eV

The Seph_Cu(111) Se 3d data is from an earlier work.28

BSe layer and not removed completely in the rinsing procedures. A comparison of the XPS spectra for benzeneselenol in Figure 5 with those for atomic Se on the surface (Figure 2) does show that the lowest binding energy peak A can correspond to atomic Se, which lies close to the surface. It can come from C−Se bond scission in molecular dissociation. This would then lead to formation of a molecular BSe layer on top of a selenized Cu surface. This is akin to our earlier observation of dissociation processes in BDMT adsorption on Cu57,58 in which S−C bond scission occurred in the initial stages of adsorption. Thereafter the surface is passivated, leading to molecular adsorption on the sulfidized copper. There also exists another possibility. There could be molecules adsorbed on alternative adsorption sites contributing to the peak at this energy, as this has been discussed for alkanethiols.18,57 In the former case, however, it is easier to explain the angular dependence reported in Figure 5. Concerning the above spectra we do not believe that the results are affected by beam damage. This aspect was addressed by monitoring changes in the spectra scan by scan and the corresponding results are given in the Supporting Information (Figure S13). One can gain further information by studying the NEXAFS spectra for different angles, as shown in Figure 5d. The spectra show several resonances, with the main π*1 resonance located at 285.0 eV and smaller structures corresponding to π*2/ σ*(C−Se), σ*(C−H), and σ*(C−C) resonances.11,81−83 The intensity of the π*1 peak varies only slightly with angle, being somewhat more intense at 90°. This weak dependence on angle would suggest that either a sizable fraction of molecules in the layer is disordered or that they are tilted strongly from the surface normal. The presence of a selenized interface layer may lead to disorder in the molecular layer. 3.4. Selenophene Adsorption on Cu(100) and Au(111). In an earlier work we studied28 selenophene adsorption on Cu(111), and the present work extends that investigation to a different Cu surface. We only show the XPS Se 3d and NEXAFS of selenophene Cu(100) here, and the rest of the data can be found in Supporting Information Figure S12. We also made some measurements for Au(111) for which there existed previous measurements82 concerning NEXAFS and the CLBE for Se 3p level, but not for Se 3d. We stop first briefly on results of this work on Au. Selenophene on Au. Experiments of Kondoh et al.82 revealed differences in selenophene adsorption on Au(111) from solution and vacuum evaporative adsorption, similar to the ones observed for thiophene.30 The NEXAFS and XPS data led to the conclusion that in UHV adsorption selenophene adsorbed undissociatively at 150 K, but in liquid phase adsorption, Se−C bond dissociation occurred leading to appearance of alkene chain molecules with a selenolate bond on Au (Au−Se−R).82 Indeed their NEXAFS spectra show an 21491


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Figure 6. Se 3d XPS spectra of selenophene on (a) Au(111), (b) Cu(100), and (c) the corresponding C K-edge NEXAFS for the Cu(100) surface.

intense peak at 285 eV corresponding to the π* resonance in vacuum deposition at low temperature, but it was strongly attenuated in case of liquid phase adsorption. Their experiments show also that the Se 3p CLBE is 160.5 eV for adsorption in air like for CH3Se and 162 eV for adsorption in UHV at low temperature. Results of our measurements for Se 3p and Se 3d CLBEs on Au(111) and NEXAFS spectra are given in Figure 6a and in the Supporting Information Figure S14. Briefly, for the case of Se 3p we observe a peak at 160.5 eV (Figure S14a), which, based on the results of Kondoh et al.,70 suggests dissociation. The Se 3d spectrum (Figure 6a) was fitted using Voigt contours with three main doublet components: A, B, and C. The very weak D component appears to improve the fit. The corresponding CLBEs are given in Table 2. The exact assignments are somewhat difficult. Checks as a function of scanning did not reveal any beam damage effects as shown by the color map in Figure S14c. Peak B could be due to an alkeneselenide chain adsorption following Se−C bond scission. This CLBE is close to that of alkane and aromatic selenide on gold36,44 reported to be at 54.2 and 54.44 eV, respectively. Component A could be assigned to atomic Se adsorption resulting from the complete dissociation of molecules. A CLBE of 53.54 eV was observed in previous experiments on atomic Se adsorption.68 It is also possible, however, that peak A corresponds in fact to an alternative adsorption site for such an alkenechain. Peak C could be due to a remaining undissociated selenophene molecule and the very weak D structure to the weak chemisorption of molecules sticking on top of the SAM as this has been seen to occur in the case of alkanethiol SAMs. These could arise from inadequate rinsing of the sample. Presence of some Se atoms could possibly contribute to survival of intact selenophene. This possibility was mentioned above for BSe. The NEXAFS data in Figure S14b, as the results of Kondoh et al.,82 do show a peak at 285 eV, which could be interpreted as being due to either some small amount of remaining selenophene, or else due to metallocycle formation or another

type of C−Au bond. This is discussed in more detail in the Supporting Information. Selenophene on Cu. Our XPS results for selenophene on Cu(100) (Figure S12) are similar to the ones we obtained for the Cu(111) surface28 and indicate existence of dissociation. The Auger spectrum is significantly different from the one observed for BSe and has a much stronger component at about 916.5 eV indicating stronger interaction with Cu. Also the valence band spectrum bears a stronger similarity with selenized Cu at the lower temperatures, with more strongly pronounced features above the d band. For further details see Supporting Information. Coming to the characteristics of the Se 3d spectra, first no significant exit angle dependence in electron emission was observed. The Se 3d core level of selenophene can be fitted with several components (Figure 6b) as this was done for the Cu(111) surface.28 We would tentatively assign them in the same manner as proposed for selenophene on Cu(111) as the CLBEs are similar. The A component could be related to the Cu selenolate (Cu−Se−R) with an alkene chain resulting from the Se−C bond scission, peak B due to undissociated selenophene and peak C at higher binding energy could be related to the weak physical adsorption (e.g., on top of the rest of the molecular layer). Notice that there is no significant intensity at the atomic selenium position at 53.7 eV as observed in the Se adsorption and in the BSe case above, though we can not entirely rule out a small contribution there. There could also exist an atomic Se contribution at higher CLBEs as given in Table 1, e.g., at 54.2 eV contributing to peak A. The existence of atomic Se could contribute to the special features in the Auger and VB spectra mentioned above. Initial reactions, leading to formation of a selenized layer, could passivate the surface with respect to further dissociation processes and possibly contribute to survival of intact selenophene. This possibility was mentioned above for BSe. As for other systems, beam damage was monitored in the spectra scan by scan, and the corresponding results are given in the Supporting Information (Figure S13). 21492


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The Journal of Physical Chemistry C Further information is provided by NEXAFS data. In the corresponding NEXAFS spectrum in Figure 6c, the π*1 resonance at 285 eV is not intense, and this would concord with presence of dissociation. Other resonances are identified according to other works.29,30,82 Resonance σ*(C−H) also seen in selenophene-Au NEXAFS (Figure S14) can be related to an alkyl chain. The weak resonance π*1 and the other higher lying structures can be related to the remaining selenophene (related peak B in the XPS spectrum). An alternative assignment for the resonance π*1 could be the aforementioned possibility of existence of metallocycles corresponding to complete or partial deselenization. This, as pointed out by Kondoh et al.82 and Sako et al.,30 could be Cu−C4H4−Cu or else Cu−Se−C4H4− Cu type structures that involve Cu adatoms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.T. and T.J. thank the Chinese Scholarship council for their Ph.D. scholarship. We thank Karine Chaouchi and Stéfanie Blachandin for their help in the Soleil Chemistry laboratory.

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4. GENERAL CONCLUSIONS Interaction of Cu(100) with Se initially leads to formation of a selenide layer, which upon heating results in the appearance of various ordered chemisorbed Se structures on the possibly reconstructed Cu surface. Coadsorption of S and Se was investigated. Interestingly it revealed that, while adsorption from a mixed equal proportion Na2S and Na2Se solution led to a mixed Cu−S−Se surface layer, upon heating, sulfur was lost. This suggested that Se reacted more strongly with Cu, and the binding of S on the selenized surface was not strong. When S, however, was preadsorbed and Se adsorption was performed on a chemisorbed S layer, then heating did not remove S. The CLBEs for S and Se for these different cases were reported. Identification of ordered structures are pending and would require further investigation including both other experimental methods like STM and theory. Results on benzenselenide adsorption suggest that part of the molecules dissociate losing Se, which remains as a surface selenide layer. Possibly this occurs in early stages of adsorption and thereafter passivation of the surface occurs and molecular adsorption takes place, as this has been observed for some thiol molecules on Cu. NEXAFS measurements suggest that the molecular layer is either poorly organized or molecules adopt a strongly inclined orientation on the surface. In case of selenophene Se−C bond scission occurs predominantly, leading to adsorption of alkeneselenide molecules and possibly appearance of significant atomic Se. In the latter case some passivation of the Cu surface could promote appearance of undissociated molecules as mentioned for BSe. There could also exist some metallocycle formation as suggested in the literature. In both cases of molecular adsorption, further studies involving progressive adsorption starting from small doses would help to better identify reactive processes and appearance of atomic Se on the surface as this has been done in evaporative adsorption for 1,4-benzenedimethanethiol on Cu.57,58 Finally ab initio calculations on adsorption characteristics including in particular calculations of core level binding energies and investigation of reaction barriers and pathways will play an essential role in unraveling many complex situations encountered here.

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XPS and LEED for Se on Cu; X-ray beam damage assessment for Cu; XPS and NEXAFS for benzeneselenol and selenophene on Au(111) and Cu(100) (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06217. 21493


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