Probing Organization and Structural Characteristics of Alkanethiols

Hyunwook Song , Youngsang Kim , Heejun Jeong , Mark A. Reed , and Takhee Lee. The Journal of Physical Chemistry C 2010 114 (48), 20431-20435...
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J. Phys. Chem. B 2000, 104, 9029-9037

9029

Probing Organization and Structural Characteristics of Alkanethiols Adsorbed on Gold and of Model Alkane Compounds through Their Valence Electronic Structure: An Ultraviolet Photoelectron Spectroscopy Study A.-S. Duwez,*,† G. Pfister-Guillouzo,‡ J. Delhalle,† and J. Riga† Laboratoire Interdisciplinaire de Spectroscopie Electronique, Faculte´ s UniVersitaires Notre-Dame de la Paix, rue de Bruxelles 61, 5000 Namur, Belgium, and Laboratoire de Physico-Chimie Mole´ culaire, Centre UniVersitaire de Recherche Scientifique, UMR CNRS 5624, aVenue de l’uniVersite´ , 64000 Pau, France ReceiVed: April 21, 2000; In Final Form: July 7, 2000

In this paper we report an ultraviolet photoelectron spectroscopy (UPS) study of saturated alkane chains in various configurations and conformations. The dependence of the valence spectra on molecular structure characteristics has been assessed by comparing the results obtained from n-alkanethiol, R,ω-alkanedithiol, and R-cycloalkyl-ω-alkanethiol monolayers adsorbed on gold, and from gas-phase model alkane compounds. The differences between the spectra reflect directly the structural changes induced in the electronic structure of the alkane chains. We have determined the type of folding sequences adopted in a 1,12-dodecanedithiol monolayer. Angular dependent measurements and investigations on the photoelectron attenuation length have evidenced very fine structural differences between films obtained on deposited gold films, annealed gold films, and gold single crystals.

1. Introduction Optimization and control of the properties of technologically useful organic layers require the sound knowledge of their surface molecular structure. The primary and secondary structure of organic materials are indeed crucial in many fields such as adhesion of polymer surfaces, recognition of biological materials, molecular electronics and photonics, etc. Owing to their very small sampling depth, electron spectroscopies enable direct and highly specific insight into the quantitative elemental composition and chemical bonding at the extreme surface of polymer materials and within organic thin films. Photoelectron spectroscopies are indeed routinely used to obtain information on the chemical composition and homogeneity of organic surfaces, but more rarely to extract information on their surface molecular structure. Combined experimental and theoretical photoemission studies1-9 have, however, built over the years evidences of direct relationships between the electronic structure and the molecular architecture. UPS studies on various monolayers of alkanethiols on gold and model alkane compounds are reported here with the aim of investigating the configurational and conformational characteristics of these materials, and to evaluate the potential of photoelectron spectroscopies in the elucidation of the molecular structure of organic surfaces. Ultraviolet photoelectron spectroscopy (UPS) is a powerful tool for studying the electronic structure of organic materials. X-ray photoelectron spectroscopy (XPS) offers information on the whole valence band, but UPS usually surpasses XPS in resolution, and gives rise to larger * Author to whom correspondence should be addressed. Present address: Unite´ de Physique et de Chimie des Hauts Polyme`res, Universite´ catholique de Louvain, Place Croix du Sud, 1, B-1348 Louvain-la-Neuve, Belgium. E-mail: [email protected]. † Laboratoire Interdisciplinaire de Spectroscopie Electronique, Faculte ´s Universitaires Notre-Dame de la Paix. ‡ Laboratoire de Physico-Chimie Mole ´ culaire, Centre Universitaire de Recherche Scientifique, UMR CNRS 5624.

photoionization cross sections of p-type orbitals which constitute the highest valence bands of organic materials. Previous works on thick films and gas-phase samples of saturated hydrocarbon chains have shown that XPS valence spectra contain useful information on their primary and secondary structure.2,10-12 Valence XPS measurements were used to study the changes induced in the electronic structure by different molecular conformations and configurations. In particular, several theoretical investigations on large n-alkane chains13-15 or cycloalkane compounds16 gave evidences for striking conformational signatures at the top of the inner valence region and bottom of the outer valence region. XPS measurements on thin films and microcrystalline lamellae of polyethylene11 have confirmed these findings, and gave the impulse to further investigations on polypropylene,17 or gaseous samples of n-pentane, n-hexane, cyclopentane, and cyclohexane.12 The potential of ultraviolet photoemission spectroscopy in disclosing information on structural details has been illustrated in pioneering works on gas-, liquid-, and solid-phase systems. For instance, measurements on compounds with lone pair atoms developing sufficiently strong nonbonding interactions have definitely pointed out the possibility to identify individual conformations with gas-phase UPS.18-20 Ballard et al.21 have also successfully used UPS on liquids to study phenomena such as surface composition of binary liquid mixtures, band shifts due to hydrogen bonding, surface activity of solutes, changes in surface structure, etc. Furthermore, a correlation between UPS spectral features and molecular structure and orientation has been assessed in the outer valence region of organic thin films.22-26 In continuation of the above-mentioned works, the present study aims at clarifying what type of structural information can be obtained from an analysis of the UPS valence spectra of organic surfaces. We therefore consider here self-assembled monolayers (SAMs) of alkanethiols, which from their welldefined composition, thickness, and molecular structure,27 provide particularly well-suited reference samples. In this study,

10.1021/jp001528t CCC: $19.00 © 2000 American Chemical Society Published on Web 08/31/2000

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Duwez et al. 2. Experimental Details

Figure 1. Schematic representation of octadecanethiol, 1,12-dodecanedithiol, 1-cyclohexyl-12-dodecanethiol, 1-cyclopentyl-12-dodecanethiol, and two models of folds of n-nonane (type A and type B).

specifically, we consider the monolayers obtained by grafting on gold the following compounds: linear alkanethiols (CnH2n+1SH with n ) 2, 4, 6, 8, 10, 12, 16, 18), 1,12-dodecanedithiol (HS(CH2)12SH), 1-cyclohexyl-12-dodecanethiol (C6H11(CH2)12SH), and 1-cyclopentyl-12-dodecanethiol (C5H9(CH2)12SH) (Figure 1). These molecules are chemically very similarsthey are all saturated hydrocarbon chainssbut differ in their primary (configuration) and secondary structure (conformation). By comparing the UPS spectra of the self-assembled monolayers derived from these compounds, we intend to assess the extent to which variations in the connectivity and conformation of chains manifest themselves through their electronic structure. Significant modifications of the electronic structure were actually observed in our UPS preliminary study of alkanethiol monolayers,28 and the possibility of identifying molecular configurations and conformations in the C2p valence band was suggested and briefly reported. The identification of the folding structure from the C2p band in dithiol films was hampered, however, because of the too short alkanedithiol chain (9 carbon atoms). In the present paper, we will present a more comprehensive study, including more details about the organization and the molecular structure of alkanethiol monolayers according to the quality of the gold substrate. For comparison purpose, gas-phase measurements on alkane model molecules have also been considered. The motivation for such measurements and comparison with the electronic structure of the alkanethiol molecules reside not only in the higher resolution achieved in gas-phase UPS, but also in the distinction of intramolecular characteristics (chemical bonding, contribution of the sulfur atom in the valence structure, vibrational influences on the line profiles...) from intermolecular interactions (polarization of neighbor molecules, structural disorder...), and solid-state related effects. In section 2, we describe some experimental details such as the procedure used for the preparation of the alkanethiol monolayers, as well as the conditions in which the UPS spectra have been obtained. In section 3, we undertake a systematic structural investigation of the molecules in various configurations and conformations by studying the solid-phase UPS spectral features of the alkanethiol monolayers and the gasphase UPS spectral features of model alkane chains. Concluding remarks are given in section 4.

Materials. Gold substrates were prepared by evaporating gold (∼1000 Å) onto silicon wafers (100) following the procedure described elsewhere.29 The obtained substrates are polycrystalline with a (111) preferential orientation, and a typical grain size of about 300 Å. Some of these gold-coated slides have been exposed to a post-deposition thermal treatment (annealing), carried out in air, at 250 °C, for 180 min. The heating rate was 10 °C/min. After annealing, the slides were left to cool to ambient temperature. The annealed gold substrates show an increase in grain size from 300 to 1200 Å. Gold (111) and (100) single crystals were lent by R. Feidenhans’l (Risø National Laboratory, Denmark). n-Alkanethiols (Aldrich) have been used as received. 1,12-Dodecanedithiol was received from B. Liedberg (Laboratory of Applied Physics, Linko¨ping University, Sweden). The noncommercially available R-cycloalkyl-ω-alkanethiols (CnH2n-1(CH2)12SH, with n ) 6 or 5) were prepared from the coupling of the Grignard reagent CnH2n-1MgBr with 1,12-dibromododecane.30 Combining the product of this reaction with thiourea lead to the corresponding R-cycloalkyl-ω-alkanethiol.31 1-Cyclohexylmethane, 1-cyclohexylethane, 1-cyclohexylpropane, 1-cyclohexylbutane, 1-cyclopentylmethane, and 1-cyclopentylethane were high-purity commercial compounds (Aldrich). Preparation of Monolayers. Prior to their use, the gold substrates were cleaned to ensure a good adsorption of the thiols. The Au/Si slides were freed from contaminants by oxygen plasma treatment and the treated Au slides were then immersed in ethanol to remove the gold oxide before monolayer assembly.32 The gold single crystals were cleaned in UHV by repeated cycles of argon sputtering and annealing until a characteristic LEED pattern was obtained. Adsorption proceeded then by immersing the gold substrates for 18 h in 10 -3 M solutions of alkanethiols, using absolute ethanol as a solvent. Upon removal from the solution, the samples were rinsed twice with n-hexane and absolute ethanol, and dried in an argon stream. Quality of Monolayers. The atomic C/S ratios, obtained from the area of the photoemission peaks corresponding to the C1s and S2p levels, were very close to the expected theoretical values. These ratios have been obtained from X-ray photoemission spectra (XPS) recorded with a Scienta-ESCA300 spectrometer using the (Al KR1,2) monochromatized radiation (1486.6 eV). The takeoff angle (TOA) was 90° (normal to the sample surface). No oxygen contamination could be found in these spectra. The monolayers have also been characterized by contact angles measurements, ellipsometry, IRAS, electrochemical measurements, photoemission with synchrotron radiation, and HREELS, the results of which are presented elsewhere.28,33,34 UPS Measurements. Solid-phase UPS spectra of the alkanethiol monolayers were obtained with a Scienta-ESCA300 spectrometer using a HeI light (21.21 eV) (Laboratoire Interdisciplinaire de Spectroscopie Electronique, Namur, Belgium). The takeoff angle was modified by rotation of the sample. The angle between the detector and the incident direction of the photons and the angle between the axis of rotation of the sample and the incident beam have both been fixed at 45°. A pass energy of 10 eV and a slit width of 0.1 mm were chosen, giving a theoretical resolution of about 0.15 ( 0.01 eV. The binding energies of the valence band lines in the UPS solid-phase spectra were referenced to the Fermi level (EF ) 0) of the gold substrate (measured at 4.65 eV with EV ) 0). The gas-phase UPS spectra of the model molecules were recorded with a Perkin-Elmer P.

UPS Study of Saturated Alkane Chains

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Figure 2. UPS (HeI) spectra recorded from n-alkanethiol monolayers (CnH2n+1SH) adsorbed on a Au(111) single crystal with a TOA of 90° (a) and 20° (b).

S. 18 spectrophotometer using the p1/2 and p3/2 doublets of argon and xenon as references (Laboratoire de Physico-Chimie Mole´culaire, Pau, France). A UV lamp (HeI radiation) was used as the source of ionizing radiation (21.21 eV). The valence band lines in the gas-phase UPS spectra were referenced to the vacuum level (EV ) 0). 3. Results and Discussion Influence of the Chain Length of Alkanethiols on the UPS Spectral Features. One of the problems that has to be dealt with when analyzing the UPS spectra of the alkanethiol films is the presence of the Au5d signal superimposed to the valence band of the organic material. We have shown in a previous paper28 that working with synchrotron radiation with an incident energy close to the Cooper minimum of the Au5d levels enables to reduce considerably the contribution of the gold substrate in the valence spectra. We have tried here to determine the chain length needed so that the gold substrate does not interfere anymore in the UPS valence spectra of the SAMs. The UPS spectra of n-alkanethiol monolayers (CnH2n+1SH) with n ) 2 to n ) 18, obtained on a Au(111) single crystal, have been recorded with takeoff angles of 90° and 20° (Figure 2). For n ) 2, i.e., ethanethiol, the spectrum is completely dominated by the gold signal. As a comparison, the spectrum of a bare Au(111) single crystal recorded under the same conditions is shown in Figure 3. There are, however, obvious changes to the shape of the bare Au(111) spectrum even after the adsorption of very short alkanethiol molecules. The intensity of the surface state35 (at 2.7 eV) immediately below the Fermi level is attenuated with increasing chain length, in agreement with the formation of an overlayer on the surface.36 There is also a striking modification to the whole Au5d band structure evidenced by a general reduction in intensity. The bands are however not homogeneously attenuated. The intensity of the peak situated at 6.1 eV decreases slightly after the adsorption, whereas the intensities of the structures at 4.5 and 2.7 eV are drastically lowered. These modifications of the valence band

Figure 3. UPS (HeI) spectra recorded from a bare Au(111) single crystal with a TOA of 90° and 20°.

structure of gold show that there is a chemical bonding between the gold substrate and the alkanethiol molecules. The bond formation occurs through a rearrangement of the 5d orbitals of the gold substrate and of the orbitals of the adsorbed molecules. In the spectrum of the bare gold substrate, recorded with a TOA of 90° (Figure 3), there is a ratio of 28 between the intensity of the Fermi level and that of the peak at 6.1 eV. We have measured the intensity of the Au Fermi level in the spectrum of each monolayer, and we have multiplied this intensity by 28. We have thus obtained an estimation of the

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Duwez et al.

TABLE 1: Contribution (%) of the Au5d Levels to the Total Intensity Measured at 6.1 eV (EF ) 0) in the Spectra of Alkanethiol Monolayers (Figure 2) contribution of Au5d (%) CnH2n+1SH

TOA ) 90°

TOA ) 20°

n)2 n)4 n)6 n)8 n ) 10 n ) 12 n ) 16 n ) 18

87 72 50 23 15 12 5 4

55 45 39 16 12 11 5 3

intensity of the Au peaks at 6.1 eV. By comparing this intensity with the total intensity measured at 6.1 eV in the monolayer spectrum, we can evaluate the contribution of the gold peak at 6.1 eV in the monolayer spectral features. For n ) 2, the intensity of the gold structure at 6.1 eV is 87% of the total intensity measured at 6.1 eV in the monolayer spectrum. For n ) 4, gold peaks are slightly attenuated, and carbon structures (for example at 7.4 eV) become more intense. The intensity of Au5d levels at 6.1 eV is 72% (n ) 4) of the total intensity. For n ) 6, this intensity is still about 50%. It decreases to 23% for n ) 8, 15% for n ) 10, 12% for n ) 12, 5% for n ) 16, and 4% for n ) 18 (Table 1). For a TOA of 20°, the ratio between the intensity of the Fermi level and that of the peak at 6.1 eV is about 23. The gold contribution to the intensity measured at 6.1 eV is thus about 55% for n ) 2 and decreases to 3% for n ) 18 (Table 1). The gold contribution is more important for a takeoff angle of 90°, due to the higher sampling depth. It appears that this contribution decreases in a significant way between n ) 12 and n ) 16 for both TOA. From n ) 16, the gold structures do not disturb anymore the spectral features of the monolayer as the overall shape of the spectrum becomes independent of the chain length. In connection with the above discussion on the gold signal, it is interesting to point out that the almost complete attenuation of the signal from the gold substrate by the alkyl chain shows the very short electron mean free path of the electrons from the 5d levels of gold in the monolayer. Related information has also been reported by Seki et al. in the study of Cd arachidate films22 where they evidenced that the valence spectra are not affected by the Cd part of the chains. These authors have suggested an electron mean free path e10 Å. It is possible to calculate approximately the attenuation length in the alkanethiol monolayers from the following equation:

I(Au) ) I°(Au) e-d/λsinθ

(1)

where I(Au) is the intensity from the gold photoelectrons attenuated by the monolayer, I°(Au) is the intensity from clean gold substrate, d is the thickness of the film, and θ is the takeoff angle. The film thickness, d, can be approximated by nl, where n is the number of carbon atoms in the chain, and l is the height equivalent to one C-C bond perpendicular to the surface.37 A graph of ln I(Au) versus n should yield a straight line with slope of (-l/λ sinθ), from which λ can be obtained (Figure 4):

ln I(Au) ) const + (-nl/λ sinθ)

(2)

As for SAMs of alkanethiols on gold, the alkyl chains are believed to tilt at an angle of 30° to the surface normal,27 we have used a value of 1.1 Å () 1.27 cos30°) for l, where 1.27 Å is the incremental chain length per CH2 unit group. The

Figure 4. Intensities from Au5d levels (at 6.1 eV) as a function of the number of carbon atoms (n) estimated from CnH2n+1SH monolayers adsorbed on a Au(111) single crystal.

attenuation length of photoelectrons with energies in the range 50-1400 eV in SAMs has been reported in the literature.37 We have found a value of about 5 Å for λ, in excellent agreement with the value predicted by the universal curve of electron mean free path38 for electrons with a kinetic energy of about 15 eV () 21.21 eV - 6.1 eV), although the data are not very well fitted when one uses only one straight line (Figure 4). It is indeed interesting to note that the data are better fitted when one uses two straight lines with different slopes for n e 6 and for n g 8 (Figure 4). This difference could come from structural changes in the layer. Infrared spectra39 indicate that the packing of the alkanethiol monolayers with n < 10 is different from the one obtained from longer chains. The shorter thiols could plausibly have a lower surface coverage than longer thiols. The measured gold signal would thus be enhanced by photoelectrons from these bare patches, giving rise to a longer electron mean free path. This is evidenced by the linear regression (Figure 4) obtained for n e 6 which gives a slope of -0.14, corresponding to λ ) 8 Å. The linear regression for n g 8 gives a slope of -0.18, corresponding to λ ) 6 Å. When using X-ray incident radiation (Al KR), the photoelectrons from the 5d levels of gold at 6.1 eV have a kinetic energy of 1480 eV. It corresponds to an electron mean free path of about 100 Å, as deduced from a study on n-C36H7440 films. XPS spectroscopy is thus not appropriate to study the valence band structure of ultrathin organic films grafted on a metallic substrate, whereas UPS spectroscopy is very favorable. Moreover, the ratio between the photoionization cross sections of C2p and Au5d levels is much higher in UPS than in XPS, further reducing the hope of using XPS for valence band study. Influence of the Crystallinity of the Substrate on Spectral Features. We have compared the UPS spectra of an octadecanethiol monolayer obtained on Au/Si substrates and on Au single crystals. In contrast with the results obtained by Seki et al. on oriented films of n-C36H74 and Cd arachidate LangmuirBlodgett films,22,23 the features of the spectrum of the octadecanethiol monolayer adsorbed on a Au/Si substrate remain essentially independent of the TOA value.33 The polycrystallinity and the residual surface roughness of the Au/Si substrates used in this work probably prevent our samples from organizing in large crystalline domains and likely account for this independence versus the TOA. Similar measurements on gold singlecrystal surfaces are thus interesting from that perspective. We have tried to obtain larger crystalline domains in our films by working with gold single crystals, Au(111) and Au(100).

UPS Study of Saturated Alkane Chains

Figure 5. UPS (HeI) spectra from the octadecanethiol monolayer adsorbed on a Au(111) single crystal, recorded with a TOA of 90° and 20°.

The UPS spectra of octadecanethiol deposited onto a Au(111) single crystal are presented in Figure 5. They are rigorously similar for the Au(100) substrate (not shown). A change of the features is observed between the spectra recorded at a TOA of 20° and a TOA of 90°. For example, the maximum intensity in the spectrum recorded at a takeoff angle of 90° is situated at about 6.6 eV, and this maximum is shifted to 7.6 eV in the case of grazing angle. This is corroborated by UPS results obtained by Seki et al. on alkane chains in the extended and random coil states which point to the disappearance of the peak at 7.4 eV (with EF ) 0) in the coiled form.22-24 Such TOA dependence reflects the good orientation of the molecules. On the contrary, the lack of difference noted for the Au/Si substrates is an indication of an average tilt angle (random orientation). The octadecanethiol monolayer is thus better-oriented on gold single crystals than on the Au/Si substrates. This improvement of the film organization on gold single crystal and further support for higher degree of orientation have been extensively evidenced by HREELS studies.41 It is also very interesting to note the differences between the relative intensity of the Au Fermi level in the octadecanethiol spectrum recorded from the layer adsorbed on a Au/Si substrate or on a Au(111) single crystal (Figure 6). The Fermi level is about 4 times more attenuated in the UPS spectrum of the film adsorbed on Au/Si than in the spectra corresponding to the film on Au(111) single-crystal recorded in the same conditions (TOA ) 90°). However, when the Au/Si slides are annealed, the intensity of the Fermi level measured in the octadecanethiol spectrum is very close to that corresponding to the film on Au(111) single crystal. A shorter immersion time (2 h) contributes also to decrease the Fermi level intensity. It means that the electron mean free path in the monolayer is lower when the film is adsorbed on a Au/Si slide and for a short immersion time. This is an evidence of reduced order of the monolayer obtained on deposited gold films or obtained after a short immersion time. Electron transmission through organic thin films depends indeed on the electronic structure of the film,

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Figure 6. UPS (HeI) spectra in the region of the Fermi level of gold recorded with a TOA of 90° from an octadecanethiol monolayer adsorbed on a Au/Si substrate (a) and on a Au(111) single crystal (b).

and thus on details of the conformation of the chains. Haran et al.42 have shown that not only the structure of the film in the direction of propagation of the electron but also its structure in the perpendicular plane are important in determining the transmission probability. Each chain considered independently is a periodic structure which could support one-dimensional band conduction. When the chains are in all-trans conformation, the monolayer is ordered and the electronic wave functions in the band are delocalized. The formation of gauche bonds results in introducing disorder, which increases scattering and reflection. This picture explains the observation that electrons are better conducted through all-trans chains than through chains containing some gauche bonds42,43 and also explains the high conductance through organic layers as measured by scanning tunneling microscopy.44 The very low intensity of the Fermi level in the case of the Au/Si substrate indicates that the electron transmission through the monolayer is lower and that the chains are thus less ordered and contain more gauche defects than on annealed gold films and on Au(111) single crystal. Solid-Phase UPS Spectra of Monolayers. HeI spectra of the monolayers, obtained on annealed Au/Si slides after 18 h of dipping time, are shown in Figure 7. These UPS spectra are not disturbed by the gold valence peaks in the case of the octadecanethiol and R-cycloalkyl-ω-alkanethiol monolayers. As for the 1,12-dodecanedithiol chains, the gold contribution is not totally negligible, and appears as a small structure at about 2.8 eV in Figure 7b. In the case of a 1,9-nonanedithiol monolayer, the UPS valence spectrum, presented in a previous work,28 is totally dominated by the gold features and it is thus impossible to extract information about the C2p band. The outer valence spectrum of the octadecanethiol sample (a) is composed of four peaks (6.6, 7.4, 9.1, and 10.6 eV) of decreasing intensity with increasing binding energies. The spectrum of 1,12-dodecanedithiol (b) contains also four peaks, situated at 5.9, 7.2, 8.6, and 10.2 eV. The last peak at 5.9 eV is shifted by 0.7 eV toward lower binding energies in comparison to that of octadecanethiol measured at 6.6 eV. The 1-cyclohexyl-

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Duwez et al. TABLE 2: Experimental Binding Energies (eV) from the Valence Spectra of the Alkanethiol Monolayers (Figure 7) and of Polyethylene (from ref 11) in the C2p Region (EF ) 0) peak molecule octadecanethiol 1,12-dodecanedithiol 1-cyclohexyl-12-dodecanethiol 1-cyclopentyl-12-dodecanethiol crystalline lamellae of PE11 PE film11 C13H28 gas-phase11

(1)

4.8

(2)

(3)

(4)

(5)

6.6 5.9 6.6 6.2 5.8 6.5 5.9a

7.4 7.2 7.7

9.1 8.6 9.5 8.8 9.2 8.5

10.6 10.2 10.8 10.5

7.8

a The 0 of the binding energy scale was set at the Fermi level by subtracting the work function of 4.8 eV,22 since the original energy scale of the XPS gas-phase spectrum is relative to the vacuum level.

Figure 7. UPS (HeI) spectra recorded with a TOA of 20° from the octadecanethiol (a), 1,12-dodecanedithiol (b), 1-cyclohexyl-12-dodecanethiol (c), and 1-cyclopentyl-12-dodecanethiol (d) monolayers adsorbed on annealed Au/Si substrates.

12-dodecanethiol spectrum (c) contains a shoulder around 4.8 eV, and four well-resolved peaks at 6.6, 7.7, 9.5, and 10.8 eV. For the 1-cyclopentyl-12-dodecanethiol monolayer (d), an intense and dominating structure is found at 6.2 eV, together with two less intense peaks at 8.8 and 10.5 eV. As noted from the comparison of these spectra, the valence bands of these molecules are notoriously different, notwithstanding their closely related chemical nature. Binding energies from the UPS spectra of Figure 7 are summarized in Table 2. The spectra recorded from octadecanethiol, 1-cyclohexyl-12-dodecanethiol, and 1-cyclopentyl-12-dodecanethiol SAMs are very similar to those recorded from the monolayers obtained on deposited gold films after only 2 h of dipping time and reported in ref 28. It means that the presence of defects in monolayers obtained on deposited gold films, as discussed above, does not affect the C2p spectral features. The peak assignment has thus been directly adopted from ref 28. Accordingly, the relationship between the spectral features obtained from the R-cycloalkyl-ω-alkanethiols monolayers and the molecular structure will not be discussed here. We will exploit in detail the structural characteristics of 1,12dodecanedithiol layer and compare it with theoretical simulations and with the octadecanethiol layer. Already published theoretical simulations28 based on 1p-GF calculations in the so-called diagonal two-particle-hole TammDancoff approximation (2ph-TDA)16 are used in the form of convoluted Densities of States (DOS) as a support to the

Figure 8. Convoluted density of states calculations for zigzag planar n-nonane (a), fold A (bA), and fold B (bB) of n-nonane. The spread function used to convolute these spectra has been taken as a linear combination of one Gaussian and one Lorentzian with equal weight and width (fwhm ) 1.5 eV).28

interpretation of the experimental spectra. The latter consists of a renormalized treatment of ionization energies, which is correct up to the second-order in correlation only. Since it accounts for the leading correlation and relaxation effects, it affords nonetheless a rather satisfactory (i.e., semiquantitative) description of the ionization spectra of saturated hydrocarbons.12,15,16 No cross section effects are accounted for in the calculations. The n-nonane in its zigzag planar and folded conformations [2 types of folds (Figure 1)] has been considered as model of the dithiol chains. The spread function used to convolute these spectra has been taken as a linear combination of one Gaussian and one Lorentzian with equal weight and width (fwhm ) 1.5 eV). These simulated spectra (Figure 8) exhibit significant differences from one compound to the other, in relationship with disturbances of the methylenic hyperconjugation pattern. Please note that the binding energies are referenced to the vacuum level, whereas the binding energies

UPS Study of Saturated Alkane Chains in the experimental spectra are referenced to the Fermi level of gold as the work function of the alkanethiol layer is not known. Theoretical simulations of the photoemission spectra of large saturated hydrocarbon chains13-15 indicate that the normally prevailing zigzag planar conformation is specifically fingerprinted by a high and narrow peak at the top of the inner valence band (C2s). This is in excellent agreement with the C2s band recorded in photoemission with synchrotron radiation from an octadecanethiol monolayer.28 This signature reflects a much denser accumulation of one-electron energy levels around 18 eV (Ev ) 0). By contrast with a zigzag planar chain, methylenic hyperconjugation in a folded structure is prevented because of the relative disposition of a few CH2 groups in left conformation. Any disruption of the long-range hyperconjugation pattern tends to result in a more homogeneous distribution of energy levels at the top of the inner valence band. As the extra-stabilization due to hyperconjugation is ruled out in this case, the highest levels in the inner valence band shift by about 1 eV toward lower binding energies.14 Compared with zigzag planar structures, the final outcome of a fold in the hydrocarbon chain is therefore a net broadening of the peak at the top of the inner valence band. Changes due to this conformation difference (zigzag planar or folded chain) should also be noted in the outer valence band (C2p), but the large number of overlapping levels make these changes difficult to exploit. We can however evidence a shift of about 0.7 eV between the last line in the outer valence spectrum of octadecanethiol (6.6 eV) and the last line in the spectrum of 1,12-dodecanedithiol (5.9 eV) (Figure 7a, b and Table 2). These results complement thus previous photoemission with synchrotron radiation studies28 that had evidenced the zigzag planar and folded conformation of octadecanethiol and 1,9-nonanedithiol monolayers, respectively, in their C2s valence spectra. A close examination of the C2p band recorded in XPS on polyethylene crystalline lamellae11 reveals that the position of the last line of the outer valence region (5.8 eV) is close to that recorded from 1,12-dodecanedithiol (5.9 eV), whereas that measured from a polyethylene film (6.5 eV) is close to the one observed in the octadecanethiol spectrum (6.6 eV) (Table 2). The extreme surface of a crystalline lamellae is indeed rich in folded chains, whereas the surface of a film is composed mainly of zigzag planar chains. In the n-C13H28 XPS valence spectrum recorded in the gas-phase,11 this line is situated at about 5.9 eV as in the folded structures. In the gas-phase, the molecules are indeed continuously twisting and folding, and are certainly not in the form of rigid rods, due to the thermal agitation and molecular collisions. The fold A (Figure 1) induces a well-marked peak at the bottom of the C2p region (14.8 eV) (Figure 8bA) which corresponds to the peak observed at 10.2 eV in the experimental spectrum (Figure 7b, peak number 5). On the contrary, the absence of this peak is the signature of a fold of type B (Figure 1). The 1,12-dodecanedithiol monolayer consists thus predominantly of folds of type A, as opposed to the folds of type B found in the (110) growth sector of crystalline lamellae of polyethylene.14 The fold A conformation is characterized by the parallel orientation of the linear part of the segments, whereas within the fold B, these segments are twisted and fold back along planes approximately perpendicular to each other. Torsional angles in the carbon backbone given in Table 3 help to compare both folded structures. Gas-Phase UPS Spectra of Model Alkane Compounds. As expected from UPS HeI results on small saturated hydrocarbon molecules,45 the UPS spectra of the R-cycloalkyl-ω-alkanethiols

J. Phys. Chem. B, Vol. 104, No. 38, 2000 9035

Figure 9. Gas-phase UPS (HeI) spectra of 1-cyclohexylmethane (a), 1-cyclohexylethane (b), 1-cyclohexylpropane (c), and 1-cyclohexylbutane (d).

TABLE 3: Successive Torsional Angles in the Models of n-Nonane14 torsional angles (°) C1-C2-C3-C4 C2-C3-C4-C5 C3-C4-C5-C6 C4-C5-C6-C7 C5-C6-C7-C8 C6-C7-C8-C9

fold A

fold B

ZZ chain

-176.088 55.615 56.494 178.012 69.329 58.663

174.783 68.421 91.614 -58.415 -64.390 174.425

180.000 180.000 180.000 180.000 180.000 180.000

cannot be directly compared with the spectra of isolated moieties, e.g., cycloalkyl and n-alkane, since none of these structures includes a tertiary carbon atom. The difference in connectivity and the cyclic structure grafted to a n-alkane chain impart important alterations in the electronic structure of these molecules compared to a n-alkanethiol. We have thus considered the UPS spectra of cycloalkyl-alkane compounds to go further in the analysis of the R-cycloalkyl-ω-alkanethiols spectra. The UPS gas-phase spectra of four cyclohexylalkane compounds are shown in Figure 9. They are composed of 5 main peaks situated at 10.0, 11.7, 12.8, 14.8, and 16.0 eV. The wellmarked peak centered on 14.8 eV results from methylenic hyperconjugation effects and is thus the signature of the ring in chair conformation which is the more stable one.16 When considering cyclohexane, no torsional degree of freedom remains in the chair conformation, and the more stable conformation is thus rigid. The long-range methylenic hyperconjugation can play a significant role, leading to an obvious spectral feature: a rather intense peak at about 14 eV (Ev ) 0). It is interesting to note that the ring inversion to the boat or twisted-boat forms (about, respectively, 28 and 24 kJ mol-1 less stable than the chair conformation), implying a complete disruption of the methylenic hyperconjugation around the cycle, results in the disappearance of the peak at 14 eV. This peak was observed at about 9.5 eV (EF ) 0) in the UPS spectrum of the 1-cyclohexyl-12-

9036 J. Phys. Chem. B, Vol. 104, No. 38, 2000

Duwez et al. 4. Concluding Remarks

Figure 10. Gas-phase UPS (HeI) spectra of 1-cyclopentylmethane (a) and 1-cyclopentylethane (b).

dodecanethiol monolayer (Figure. 7c, peak number 4), indicating that the rings remain in chair conformation in the film. The increase of the chain length grafted to the ring (as in the 1-cyclohexyl-12-dodecanethiol film for which the chain contains 12 atoms) results in a progressive smoothing of the features from the ring, notably the structure around 10 eV (Figure 9). The 1-cyclohexylbutane spectrum (Figure 9d) is in excellent agreement with the UPS spectrum of the 1-cyclohexyl-12dodecanethiol monolayer (Figure 7c). Figure 10 presents the experimental UPS spectra of 1-cyclopentylmethane and 1-cyclopentylethane. We observe 3 main peaks centered on 11, 14, and 16 eV. The peak at the top of the outer valence band (around 11 eV) of 1-cyclopentylmethane is much more intense and narrower (fwhm ) 2.3 eV) than the one observed in the XPS gas-phase spectrum of cyclopentane.12 For a structure of D5h symmetry, the six outermost ionization lines group in three degenerate pairs within an energy interval centered on 10 eV and smaller than 0.3 eV. The degeneracy of these levels is removed only by a few hundredths of an electronvolt when cyclopentane resides in its envelope or halfchair conformation. This compound is thus a good candidate for a Jahn-Teller distortion after the removal of one electron from its highest occupied level. As the ring is highly flexible, strong vibronic coupling may then yield fast rotating vibrations in the radical cation, which explains the broadening of the peak at about 11 eV in the spectrum of cyclopentane.12 These rotating vibrations are prevented in the 1-cyclopentyl-12-dodecanethiol monolayer because of the grafting on gold, and it appears that the presence of a methyl group, as in the case of 1-cyclopentylmethane, is probably sufficient by itself to prevent these vibrations. The spectrum of 1-cyclopentylethane (Figure 10b) is in excellent agreement with the UPS spectrum of the 1-cyclopentyl-12-dodecanethiol monolayer (Figure 7d). Since the spectra of the R-cycloalkyl-ω-alkanethiol monolayers are very similar to the UPS-gas-phase spectra of the cycloalkylalkane model chains, we can conclude that the sulfur atom (S3p at 8 eV with EF ) 0) does not contribute significantly to the UPS spectral features of the films. Furthermore, this similarity shows that the spectra of the R-cycloalkyl-ωalkanethiol monolayers recorded at a TOA of 20° are dominated by the cyclic structure.

This work has investigated the electronic structure of various alkane chains with the aim of evaluating the potential of photoelectron spectroscopies in elucidating the molecular structure of ultrathin organic films. The molecules that have been chosen for this purpose are chemically very similar, but differ in their configurational and conformational structures. We have shown that UPS is highly sensitive to the primary and secondary structure of the alkanethiol chains, each molecule (n-alkanethiols, R,ω-alkanedithiols, R-cycloalkyl-ω-alkanethiols) giving rise to a typical fingerprint in the valence spectra. The differences between the spectra reflect directly the structural changes induced by the folding and by the cyclic structure at the end of the grafted hydrocarbon chains. The shape of the C2p band of the octadecanethiol film reflects the extended alltrans conformation of the chains. The spectral features of the 1,12-dodecanedithiol monolayer reflects, however, the folded conformation of the chains. We have determined that the layer consists dominantly of folds of type A. This detailed analysis of the UPS spectra of octadecanethiol and 1,12-dodecanedithiol SAMs and their striking similarity with outer valence spectra from polyethylene surfaces show that SAMs could be used as models of polymer surfaces. Angular dependent spectra obtained from the octadecanethiol monolayer have evidenced the poorly ordered structure of the films prepared on Au/Si substrates, probably because of the substrate polycrystallinity and roughness. Using a Au single-crystal substrate, or even an annealed Au/Si substrate, improves considerably the order in the monolayers. These differences of the organization of the films versus the nature of the substrate have also been evidenced by investigations on the attenuation length of photoelectrons in the organic layer. UPS spectroscopy appears to be extremely sensitive to the organization of the SAMs and is thus a powerful tool to study the order and the molecular structure of organic ultrathin films. Thus, the present measurements show that UPS spectroscopy is of potential use to extract information on the secondary structure of linear alkane chains and to check the quality of linear alkanethiol monolayers. The comparison of the valence spectra with gas-phase UPS spectra of some model molecules has shown that self-assembled alkanethiol monolayers are good reference systems to study typical molecular structures. Such systems have allowed to evidence the significant changes appearing in the photoemission spectra of alkane chains with different configuration and conformation and thus to specify the potential of UPS concerning the determination of the molecular structure at the extreme surface of organic materials. Acknowledgment. A.-S.D. expresses her gratitude to FRIA (Fonds pour la formation a` la Recherche dans l’Industrie et l’Agriculture) for the financial support, to B. Liedberg (Laboratory of Applied Physics, Linko¨ping University, Sweden) for providing the 1,12-dodecanedithiol and to R. Feidenhans’l (Risø National Laboratory, Condensed Matter Physics and Chemistry Department, Denmark) for lending the gold single crystals. This work is supported in part by the Interuniversity Research Program in Reduced Dimensionality Systems (PAI/IUAP 4/10). We thank Prof. Jean-Jacques Pireaux, Dr. Petra Rudolf, and Dr. Jacques Ghijsen for their interest, suggestions, and many useful discussions. References and Notes (1) Delhalle, J.; Andre´, J.-M.; Delhalle, S.; Pireaux, J.-J.; Caudano, R.; Verbist, J. J. Chem. Phys. 1974, 60, 595.

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