Thiophene Derivatives on Gold and Molecular Dissociation Processes

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Thiophene Derivatives on Gold and Molecular Dissociation Processes Tingming Jiang,† Walter Malone,‡ Yongfeng Tong,†,§ Diana Dragoe,∥ Azzedine Bendounan,§ Abdelkader Kara,*,‡ and Vladimir A. Esaulov*,† †

Institut des Sciences Moléculaires d’Orsay, UMR 8214 CNRS, Université Paris Sud, Université Paris-Saclay, bât 520, F-91405 Orsay, France ‡ Department of Physics, University of Central Florida, Orlando, Florida 32816-2385, United States § Synchrotron Soleil, L’Orme des Merisiers, Saint-Aubin, BP 48, Gif-sur-Yvette CEDEX F-91192, France ∥ Institut de Chimie Moléculaire et Matériaux d’Orsay, CNRS, Université Paris Sud, Orsay France S Supporting Information *

ABSTRACT: We report a systematic study of thiophene derivatives on gold surfaces. These molecules are of interest in molecular electronics, and the characracteristics of the thiophene− electrode interface in devices needs to be understood as it affects electron transport characteristics. Some experiments indicated S− C bond scission in contact with metals resulting in disruption of the π-electron system that affects charge transport, which would also be affected by presence of split-off chemisorbed sulfur. We explored this dissociation aspect by photoemission for the case of monocrystalline Au(111) surfaces and Au films grown on mica for a series of polythiophenes molecules (nT, n = 1−4, 6) as well as for α,ω-diquaterthiophene (DH4T) and dihexylsexithiophene (DH6T). The S 2p X-ray photoelectron spectroscopy peaks are found to have complex line shapes corresponding to S atoms with different core level binding energies (CLBE). Density functional theory calculations of adsorption energies and CLBEs were performed for various adsorption configurations of thiophene on a perfect Au(111) plane and for comparison, calculations were also performed for bithiophene, terthiophene, alkenethiol, alkenethiol chain, and a broken thiophene related metallocycle, incorporating an Au adatom and an S atom. On the basis of these results we relate the different contributions to the S 2p peak to intact molecules on different adsorption sites and broken molecules. Calculations in particular show that the CLBEs for intact thiophene (1T) can be the same as for the alkene and alkanethiol cases as opposed to usual assumptions in the literature. The existence of intact thiophenes is confirmed by the presence of clear π resonance peaks in the near edge X-ray fine structure (NEXAFS) spectra. Spontaneous dissociation appears to a variable extent in different samples, and we tentatively relate this to the presence of a more or less large number of steps and defects sites. X-ray induced beam damage was investigated for 1T and 3T using an intense synchrotron beam of 260 eV photons, and showed changes in the S 2p spectra related to S−C bond scission. occur on transition metal surfaces,34−40 which is not very surprising given their high reactivity. However, there are indications in recent literature10,11,21,22,25−30 that this may also occur on the less reactive coinage metal surfaces including gold, which is an important electrode material. This S−C bond scission leads to an opening up of the ring and observation of alkene thiol species. The loss of aromaticity disrupts the πelectron system; thus, charge transport along the chains will be hindered, which is not desirable in molecular electronics. There are indications of possible appearance of atomic S on the surface,31−33 resulting from complete desulfurization for these thiophenes. Consequently, the undissociated molecules of the adsorbed molecular layer are placed on a sulfurized metal

1. INTRODUCTION Thiophene (C4H4S) derivative π-conjugated systems are of much interest in molecular electronic applications1−13 for the fabrication of field-effect transistors, light-emitting diodes, and solar cells because of a number of interesting properties. These include charge transport behavior with high carrier mobilities, light-harvesting efficiency, and structural versatility. Quite promising results have been obtained from a molecular electronics point of view. Many studies have therefore been devoted to the formation of multilayer and monolayer films of these molecules on various surfaces of interest as electrodes.10,14−33 A number of problems subsist however and among these, there appeared one, relating to possible dissociation of these molecules at the electrode surface that involves S−C bond scission.10,21−40 The existence of such dissociation processes leading to dehydrogenation and desulfurization is known to © XXXX American Chemical Society

Received: August 12, 2017 Revised: November 27, 2017 Published: November 28, 2017 A

DOI: 10.1021/acs.jpcc.7b08006 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

observation of the two CLBEs mentioned above.23 In their more recent work25 they concluded that although they do not observe a peak in NEXAFS at 285.6 eV the 162 eV CLBE corresponds to the bonding of an undissociated thiophene molecule. They affirm this on the basis of observation of ordered structures in STM for thiophene and tetrahydrothiophene (C4H8S), which has an aliphatic cyclic ring without a π conjugated system. Concerning deposition in liquid phase, it is also interesting to point out that electrochemical STM measurements of thiophene assembly in 0.1 M HClO4 solution24 showed molecular adsorption both in a standing up configuration with a (√3 × √3) structure and also a lying down configuration with a 3 × 3 structure depending upon the potential in the cell. Possible dissociation was not discussed, and no XPS measurements were reported. Liu et al.31 reported intact molecular adsorption at low temperature, with however a S 2p3/2 CLBE at 163.4 eV, unlike the 163.9 eV result of Sako et al.26 Some small structures at lower energies (162 and 161 eV) were also observed. At room temperature, their results suggest either a desulfurization or else some completely different bonding on gold of thiophene molecules or partly dissociated thiophene with S 2p3/2 CLBE at 162 and 161 eV. We note here that the attribution of CLBEs for extensively studied alkanethiols, which serve as a guideline here, is sometimes controversial. Indeed, while for well-ordered SAMs the thiolate S CLBE is unambiguously determined to be close to 162 eV, there sometimes appears a structure at 161 eV, which has been ascribed to atomic S in some works. However, a number of other studies attribute the 161 eV peak to an alternative adsorption site of the alkanethiols46,47,52,53 because of its only transient appearance in some cases. A detailed study of some of these aspects has been recently given by Jia et al.50 A comparison of thiophene and bithiophene adsorption from liquid phase onto Au(111),29 showed rather different results. Interestingly, in case of bithiophene adsorption the S 2p3/2 CLBE was found to be 163.4 eV, i.e., different from thiophene adsorption with S 2p3/2 at 162 eV. It is the same as that reported in monolayer vacuum evaporative adsorption at low temperature by Liu et al.31 but different from the 163.8 eV one of Sako et al.26 Studies of 3,4-ethylenedioxythiophene (EDOT) and its derivatives33 on polycrystalline Au, Au(111), and Au nanoparticles surfaces from vapor phase and solution revealed complex S 2p spectra with components corresponding to molecular adsorption and appearance of alkene thiolate and possibly atomic sulfur. These components were observed for all the studied molecules, albeit with varying intensities. A study10 of α,ω-dihexylquaterthiophene (DH4T) molecular layers on Au has also been performed and showed that the metal molecule interface region is characterized by appearance of S 2p3/2 CLBE at about 163.4, 162, and 161 eV, of which the lowest two were ascribed to existence of dissociation processes. 1.2. X-ray Induced Effects. In relation to the questions of bond breaking in thiophene systems, the effect of radiation damage was also investigated. Nambu et al.58 deposited a thick layer of 1T at low temperature, which was irradiated and then heated to 250 K, resulting in desorption of the multilayer. The resulting spectra of the monolayer film showed the appearance of the 162 eV peak alongside the thiophene 163.9 eV peak. They noted that in irradiated 1T samples the 285.5 eV peak in NEXAFS became stronger and attributed this to metallocycle

surface, whose electron transport properties would also be different. The situation is quite unlike the one for the common case of self-assembled monolayers of thiols on gold.41−54 This aspect has not been studied in detail and led us to conduct a systematic study of a series of thiophene derivatives. Before presenting our results, it is instructive to summarize succinctly some key observations concerning adsorption of such molecules on gold. 1.1. Background on Adsorption of Thiophene Derivatives. A number of X-ray photoemission studies of thiophene adsorption on Au have been performed and focus on analysis of characteristic core level binding energies (CLBEs) for mainly sulfur (S 2p) and near-edge X-ray adsorption fine structure (NEXAFS) at the C 1s edge. There also exist scanning tunneling microscopy (STM) studies. Some discrepancies appear in the conclusions of these works, which we will try to delineate here. In low-temperature evaporative adsorption, well-organized multilayer thick films have been formed on various surfaces including gold, with “standing up” molecules tilted ca. 17° away from the surface normal.26,31,57,58 Monolayer thiophene films are formed on gold at low temperatures, while most molecules desorb at room temperature.26,31,56,57 However, adsorption from liquid phase at room temperature does appear to occur. Thus, early STM studies of adsorption from ethanolic solutions by Dishner et al.27 show formation of a dilute structure of thiophene (2√19 × √3)R30° on Au. It was later reported that adsorption characteristics actually depend on the mode of molecular deposition.23 In evaporative assembly of thiophene onto Au(111) at low temperatures (around 120 K) initial adsorption occurred in a lying down configuration followed by a more standing up configuration,26 until at high exposures a multilayer was formed. In liquid-phase adsorption, in an ethanolic solution, on the other hand,26 X-ray photoelectron spectroscopy (XPS) experiments led to the conclusion that S−C bond scission occurred with appearance of alkene chains. This is based on differences in S 2p core level binding energies: the S 2p3/2 peak is found at 163.9 eV for vacuum phase adsorption and at 162 eV in liquid phase adsorption, an energy similar to that of the thiolate S for alkanethiol SAMs. This was supported by NEXAFS measurements, in which, for molecular adsorption in vacuum, a sharp peak at 285.6 eV related to the π*1 orbital of thiophene is observed, which almost disappears for liquid-phase adsorption indicating breaking of a S−C bond of the thiophene molecule. In the latter case, there appear structures due to σ*(C−H) at 288.5 eV and at 293 eV related to the σ*(C−C) resonance, as in the case of alkanethiolates adsorbed on metal surfaces. The observed dichroism indicated a vertical orientation of the alkene chains tilted at 30° from the normal, and LEED showed a (√3 × √3)R30° pattern. A very small residual structure at 285.5 eV subsisted but was attributed to the presence of unsaturated CC bonds. Similar XPS and NEXAFS results for thiophene reported by Noh et al.25,28,29 showed an S 2p3/2 peak at 162 eV with a smaller structure at about 161 eV. They concluded that thiophene SAMs form through chemical interaction between the sulfur headgroups in thiophene and the Au(111) surface, but we do not clearly interpret this as due to C−S bond breaking at room temperature. This is indicated to occur at higher temperatures. At room temperature, STM imaging reveals interesting paired row ordered structures, suggesting differently coordinated molecules, which could explain the B


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Figure 1. Thiophene derivatives studied in this work.

and Au thin films on mica, which are generally known to have large Au(111) terraces. The study is based on photoemission: an XPS analysis of core level binding energies, complemented in some cases by near-edge X-ray adsorption fine structure (NEXAFS) at C 1s edge. These results which do show existence of spontaneous dissociation processes in some cases will be discussed below. We also investigated X-ray beam damage effects, and these are described here along with a brief summary of earlier works on this aspect. Finally, density functional theory (DFT) calculations were performed in order to gain more insight into adsorption and core level binding energies of thiophene, bithiophene, and terthiophene and some dissociation products for different adsorption configurations.

formation. They suggest that this metallocycle could include sulfur, i.e., corresponding to (Au)−CHCH−CHCH−S− (Au) type molecular bound species, involving S and C bonded to a Au atom. It is not known if this structure is stable and what would be the S 2p CLBE in this case. For thiophenes on other metal surfaces, the possibility of polymerization by irradiation has been discussed.56,57 It is noteworthy that the conclusion that polymerization occurs is based on the observation of a much higher desorption temperature (above 500 K) compared to that for unirradiated thiophene films. This affirmation however does not take into account that alkanethiols desorb from Au in this temperature range, and the authors do report appearance of S 2p3/2 CLBE at about 162 eV. For what follows, it is also useful to recall here results on Xray induced damage of alkane and aromatic thiols.59−61 The XPS spectra for ordered thiols show a S 2p3/2 peak at ca. 162 eV. It has been shown that irradiation of alkanethiols leads to the appearance of a structure in the vicinity of 163.4 eV attributed60 to S−Au bond cleavage and trapping of the freed species within the molecular layer. Appearance of a 161 eV peak, frequently attributed to atomic sulfur,53 was not reported in these studies.58 In the case of aromatic thiols (biphenyl and terphenyl thiols), no significant changes in the spectra were observed, but cross-linking61 in the organic layer due to C−C bond formation occurs. It is in general thought that much of the damage arises from secondary electrons induced by the Xrays. Nambu et al.58 have shown that for 1T the damage effect is stronger for X-ray energies above the 1s carbon threshold and ascribe part of the damage to core hole creation with associated Auger electron emission and cation formation. The presented summary reveals a number of apparently contradictory results and conclusions that given the importance of the thiophene related compounds clearly warrants further investigations. We therefore performed a study of adsorption of a series of polythiophenes with increasing number of thiophene units (nT) and also for the case of α,ω-dihexylthiophenes (DHnT) as shown in Figure 1. The latter molecules attracted particular attention5,6,8−12 in recent molecular electronics oriented research. In our work, adsorption was performed from the liquid phase onto monocrystalline Au(111) samples

2. EXPERIMENTAL SECTION The experimental procedures we use have been outlined in detail in previous publications,53−55,62 and we only give essential points about experiments relevant to this report. 2.1. Sample Preparation. The thiophenes were purchased from Sigma-Aldrich with the exception of terthiophene and α,ω-dihexylquaterthiophene (DH4T) which was purchased from SYNCOM. These were used as supplied. Adsorption of thiophene, bithiophene, and terthiophene was performed from a 1 mM ethanolic solution, whereas for adsorption of quaterthiophene, sexithiophene, DH4T, and DH6T a 1 mM solution in dicholoromethane was used. In order to reproduce earlier reported results,26−29 we mainly used a 12−24 h immersion time into the thiophene solution. We did not observe any differences related to incubation times. Ethanol and dichloromethane were purchased from Sigma-Aldrich with purities of ≥99.5 and 99.8%. We used a Au(111) monocrystal and gold on mica films. The Au samples were prepared by evaporation onto hot mica that had been degassed for 3 h at 300 °C. Au deposition was done at this temperature; then a brief 5 min heating to 550 °C was performed. This procedure is known to yield large micrometersized terraces of Au(111),48,62 and such samples were used by us earlier to prepare high-quality alkanethiol and other SAMs.43,49,53,63 The samples were used immediately after preparation. The Au(111) monocrystal was purchased, oriented, and polished from Surface Preparation Laboratories. C


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Figure 2. XPS S 2p spectra along with fits (see text) for (a) 1T and 2T for two samples showing some variation; (b) 3T, 4T, and 6T.

A study of beam damage due to X-ray irradiation was performed by varying the X-ray beam spot size and recording XPS spectra scan by scan. Under what we shall call “normal” beam conditions, the photon flux at 260 eV is about 2.5 × 1016 photons/sec/cm2. By focusing down the beam into a smaller spot the photon flux is increased by 50 times. Note that during NEXAFS measurements the beam intensity is a factor of 5 less than under the “normal” conditions.

In situ surface preparation of the monocrystal consisted in cycles of sputtering and annealing, and surface cleanliness and crystallinity was checked by XPS and low-energy electron diffraction (LEED). The prepared samples were extracted from the ultrahigh vacuum preparation chamber under N2 flow and immediately immersed into the solutions. Thereafter, they were rinsed in the corresponding solvents and dried by N2 gas. The samples were then transferred immediately into the analysis chamber. 2.2. Photoemission. The photoemission XPS experiments for the gold on mica samples were performed using a VG Microtech K-Alpha spectrometer using a monochromatic X-ray source with an Al anode at the Orsay campus using 1486.6 eV photons. In this case, the energy resolution was 500 meV. The measurements on the Au(111) monocrystal were performed on the TEMPO beamline at Soleil Synchrotron (France)64 and experimental procedures have been described in the indicated references. The TEMPO photon beam resolution E/dE is about 5000. The energy resolution in our measurements is thus estimated to be about 50 and 75 meV at TEMPO for 260 and 380 eV photons, respectively. With some exceptions, we used photon energies so that the final kinetic energy would be around 100 eV to enhance the surface contributions. The binding energies in the XPS spectra were calibrated with respect to the Au 4f7/2 peak, set at 84 eV. The experimental binding energy calibration error is estimated to be of ±50 meV on TEMPO and about ±100 meV for the K-Alpha spectrometer. In the following, we refer to peak energies derived from fits of TEMPO data. To get a better idea of possible variability, we generally performed XPS measurements for the same thiophene compound on several mica samples from the same solution and repeated this using newly remade solutions. We also used different batches of the supplied molecules. In all these cases, the sample preparation procedure was kept the same. NEXAFS spectra were recorded in partial yield mode by measuring the carbon Auger signal. We have used synchrotron light with 100% linear horizontal polarization. To probe the molecule orientation on the surface, we varied the polar angle by rotating the sample as indicated later in the inset of the NEXAFS figures (the polarization is parallel to the surface plane for Θ = 90°).

3. THEORETICAL APPROACH In a recent paper we presented results of a study of thiophene adsorption on Cu and Ni surfaces.67 We use a similar approach for the present investigation. For the periodic DFT calculations, we employed the Vienna ab initio Simulation Package68−70 (VASP) version 5.3.5, using the projector augmented wave (PAW) method.71,72 For all calculations, we used a 400 eV plane wave energy cutoff, a 4 × 4 × 1 k-point mesh to sample the Brilouin zone, and a 0.02 eV/Å force criterion for structural optimization. For the exchange-correlation functional we chose the van der Waals (vdW) inclusive optB88-vdW73 functional. The choice of the optB88-vdW functional was dictated by previous computational studies favoring this functional74−77 For the bulk lattice constant we used the calculated value, 4.178 Å. Before a molecule was placed on the surface, both the surface and the molecule were separately relaxed. We explored the adsorption characteristics of a thiophene molecule on Au(111) for different configurations. We determined the adsorption energy and CLBE for three variations of thiophene: regular thiophene, thiophene with an adatom, and thiophene with a broken C−S bond bonded to an adatom. For each of these three cases, we tried two different unit cells: a 6 layer √7 × √7 unit cell, 7 Au atoms per layer, and 6 layer 4 × 4 unit cell, 16 atoms per layer. For both cells, the bottom two layers were held at their bulk values, while the other atoms in the molecule/substrate system were allowed to relax. We also relaxed two thiophene derived chains, SC4H5 and SC4H7 (alkanechain), in these unit cells. Moreover, in the 4 × 4 unit cell we tried both intact and broken bithiophene with a severed C−S bond. Finally, in a four layer 5 × 5 and a 2 × 6 unit cell we relaxed terthiophene. For terthiophene, the bottom two layers of the substrate were held at their relaxed values, while the top two layers were allowed to structurally relax. For terthiophene the molecule’s z coordinates were only allowed to relax on the surface while the x and y coordinates were fixed at D


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Figure 3. (a) C 1s spectra for 1T and 4T and (b) O 1s spectra for 4T. The 4T spectra correspond to the sample with rather large A and B components (see Figure S5).

Figure 4. S 2p spectra for some thiophene molecules adsorbed on Au(111).

their relaxed values. For regular thiophene, thiophene with an adatom, and terthiophene in the 5 × 5 unit cell, we tried adsorption sites with the molecule’s plane both parallel and perpendicular to the surface. For all other molecules, we only explored perpendicular configurations.

2p3/2 peaks lying at about 161.10 eV (A), 162.05 eV (B), 163.75 eV (C), 163.35 eV(C′), and 164.05 eV (D). We first describe general characteristics of the spectra, while the peak assignments will be discussed later. For 1T we observed dominantly components A and B, with in general a higher intensity of the A component (Figures 2a and S5). There also appears a small C′ component. In the case of 2T, the component C′ is much larger and sometimes includes a higher lying component (peak D). For both molecules, we quite systematically obtained spectra, with intense A and B components. Some variability was noted, as may be seen in Figures 2a and S5, but in general, these components were intense as opposed to earlier reports.28,29 It is known that X-ray irradiation induces damage, leading to the appearance of additional features in the S 2p spectrum,58 as this occurs for other organic compounds.59−61 Careful checks were therefore performed in our measurements. None were observed here that could lead us to attribute any of the observed components in the spectra to beam damage. These effects were studied in greater detail using high-intensity

4. RESULTS AND DISCUSSION We performed several sets of measurements on the adsorption of thiophenes (1T, 2T, 3T, 4T, and 6T) and dihexyl thiophenes (DH4T and DH6T) on gold evaporated on mica and some on monocrystalline Au(111). These are discussed below starting with data on mica substrates. Some additional results including some overview spectra are given in Figures S1−S5. 4.1. Gold on Mica Substrates. The S 2p XPS spectra are shown in Figures 2. After a Shirley background subtraction, along with fits using Voigt contours, a 1.2 eV spin orbit splitting and a 2:1 branching ratio between the 3/2 and 1/2 S 2p doublet components were used. In general, the spectra could be decomposed into multiple doublet components with the S E


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Figure 5. NEXAFS spectra at the C 1s K edge for (a) 1T, (b) 2T, and (c) 3T on Au(111) for different angles

and 6T along with fits. We again observe that the spectra for 1T and 2T are a superposition of several components. The A and B components still remain more intense than those reported previously.28,29 In the case of the 3T and 6T thiophenes, the spectra displayed very small A and B components. The main peak for the latter can be reasonably well-fitted with just one doublet, although there remains a slight difference in the highenergy tail. We will return to this point later. As for the Au/ mica samples some variability was noted as illustrated by another 3T sample that has larger A and B components (Figure S3). 4.3. NEXAFS Measurements. NEXAFS spectra for the 1T, 3T, and 6T samples are displayed in Figure 5. For the latter two, XPS S 2p spectra do not display significant contributions from A and B components in Figure 3 above. In all cases, a resonance peak appears close to 285 eV associated with the thiophene π1* orbital and other structures assignable to σ* (C−S) and π2*(CC and C−H) at about 287.0 and 288.4 eV and σ* (C−C) at 293 eV.26 There is a clear intensity variation of the π*1 resonance with angle, consistent with a strongly inclined molecule. Thus, for 3T, a tilt angle of 50° from the surface normal can be deduced, assuming a homogeneous layer. There are significant differences between the spectra for 1T and the other molecules. The π*1 resonance peak at 285 eV and the smaller π*2 resonance peak have a smaller relative intensity, whereas the 288.5 eV peak is quite intense. We recall that in the case of alkanethiolates adsorbed on metal surfaces, there appears the σ*(C−H) peak at 287.5 eV and the σ*(C− C) resonance at 293.5 eV.51 This would suggest that the molecular layer is composed partly of lying down 1T molecules and partly to dissociated molecules with C−S bond breaking leading to the appearance of alkene chains. An interesting point is that for the case of a 3T sample for which the A and B components were much more prominent (Figure S3), the π* resonance peaks were much less intense, and there was no clear angular variation. This again suggests partial dissociation of the thiophene ring.

synchrotron radiation on TEMPO and will be discussed later in the following. For the other thiophenes from 3T to 6T and dihexyl thiophenes, the spectra (Figure 2b) in general also show the appearance of A−D components. However, for these longer thiophene chains, components A and B can be very small (Figure 2b), whereas component C is dominant. As for the case of 1T and 2T, in spite of using the same protocol of preparation we obtained different results with varying intensity of the A and B components (see examples in Figure S5). XPS spectra in the O 1s and C 1s region are shown in Figure 3 for some cases. The C 1s XPS spectra for some of these systems are shown in Figure 3a. It was found that in some cases the XPS spectrum is fairly narrow with a peak at 284.2 eV and a higher energy tail, whereas in other cases it has a pronounced shoulder at higher energy around 286 eV. The 284.1 eV peak position was reported by Sako et al.26 for undissociated 1T molecules assembled on Au in a standing configuration. For 3T-DH6T, it was noted that the 286 eV peak was larger whenever the low-energy structures were intense in the S 2p spectra. It is possibly related to oxidation or hydroxylation of the carbon atom after C−S bond splitting on the surface, while in the thiophene solution. The fairly broad O 1s peak observed in this case covers the known energy range for −COH and −CO for some functional polymers.66 Note in this context that the S 2p spectra in Figure 2 however do not show any significant oxidized sulfur component at 168 eV. The thickness of these organic layers was estimated to be about 0.4 ±0.2 nm based on the attenuation of the Au 4f7/2 signal, assuming a homogeneous layer and a 2.5 nm49 value of the attenuation length derived from alkane SAM studies. This estimate should be taken with caution because of the uncertainty in the accurate knowledge of the attenuation length and the assumption of the homogeneity of the molecular layer. 4.2. Thiophenes on Monocrystalline Au(111). The high-resolution XPS spectra in the S 2p region for SAMs on the Au(111) monocrystal are shown in Figure 4 for 1T, 2T, 3T, F


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Figure 6. Effect of X-ray irradiation with 260 eV photons on 1T and 3T samples. Comparison of spectra from the first three and last three scans for a total of 77 scans (1T) and 72 scans (3T). Fitted spectra showing the evolution of the spectral components are given for the first and last scans.

Table 1. Adsorption Energies, 2p Energies, and Adsorption Heights for Intact and Broken Thiophene and Alkane Chain As Discussed in the Text for the 4 × 4 and √ 7 × √7 Unit Cellsa

a

4 × 4 (down) refers to a lying down thiophene molecule.

5. X-RAY INDUCED EFFECTS We investigated X-ray damage by irradiating the 1T and 3T samples prepared at room temperature. Spectra were acquired scan by scan under “normal” beam conditions and by increasing the photon flux by 50 times as mentioned in section 2. Under “normal” beam irradiation, significant changes are not observed during typical acquisition times corresponding to 5 to 10 scans. However, when the intense beam is used, a strong modification is observed as shown in Figure 6a,b for 1T and 3T where the first and last scans (see figure caption) spectra are compared. A more visual view of scan by scan evolution is provided by a color intensity map shown in Figure S4. The main peak C in the 3T spectrum broadens (Figure 6b) and appears shifted to lower CLBEs. A fit of the spectrum shows that an extra C′ component at 163.3 eV is required which grows in intensity with increasing irradiation time. Both A and B components increase slightly in intensity. Changes in case of the 1T component are shown in Figure 6a. The intensity of peak A decreased steadily with increasing irradiation time. The 163.3 eV peak became quite intense.

adsorption configurations. We also performed similar calculations for the “gas-phase” free molecules placed alone with no surface in the unit cell. It should be mentioned that in a recent work, a theoretical study of CLBEs for atomic sulfur adsorption on Au(111) was conducted. These were found to be 162.61 (atop), 163.65 (bridge), 164.06 eV (hollow), and similarly 162.68, 164.07, and 164.35 eV for the (√3 × √3) and (√7 × √7) adsorption configurations. Experimentally, the CLBEs for atomic S have been reported by Rodriguez et al.78 to be 160.8 and 161.6 eV. Note that the absolute values of the DFT CLBEs cannot be compared directly to experiment and in the following we rely only on binding energy differences. In the following, we give a brief account of the main theoretical results necessary to comment on the experimental data. A more detailed description will be given in a forthcoming paper. 6.1. Thiophene. Calculations were performed for the 4 × 4 and √7 × √7 unit cells. We considered the following: (i) Thiophene adsorbed lying down or standing up on the Au surface on atop, bridge and hollow sites, in presence and absence of an adatom. For the lying down configuration, calculations were done only for the 4 × 4 unit cell. (ii) Broken thiophene alkene chain in an upright configuration (CH2(CH)3-SH). (iii) Following a suggestion58 of possible metallocycle formation incorporating a sulfur atom and a surface atom, upon S−C bond scission, calculations were performed also for this case. (iv) For comparison, we

6. DISCUSSION AND DFT CALCULATIONS In order to gain more insight into the adsorption characteristics and S 2p core level binding energies of these thiophene derivatives and their dissociation fragments adsorbed on the Au(111) surface, we performed DFT calculations for thiophene, bithiophene, and terthiophene for different G


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Figure 7. Summary of CLBEs for (from bottom up as illustrated by the cartoons): (a) intact thiophene with and without adatom, broken thiophene metallocycle, broken thiophene chain and alkane chain (C4); (b) broken and intact bithiophene; (c) broken (B) terthiophene and intact thiophene with the central sulfur pointing up and down for the 5 × 5 unit cell. For thiophene the 4 × 4 and √7 × √7 configurations are designated by 4 and √7 and 4L refers to a lying down thiophene. For the terthiophene case the inset scale corresponds to the shifted one used to compare with experiment.

Table 2. S 2p Core Level Binding Energies for the Gas-Phase Molecule, S 2p CLBEs, and Adsorption Heights for the Adsorbed 2T Molecule and for the Broken Molecule (First Sulfur Carbon Bond) first sulfur molecule 2T gas phase 2T broken 2T

adsorption energy (eV) 0.60−0.71 −4.73 to −3.06

second sulfur

S−Au (Å)

2p (eV)

2.89−3.29 2.41−2.52

−185.64 −182.99 to −183.39 −181.76 to −182.88

performed calculations for a short chain alkanethiol (CH3(CH2)3-SH; C4) placed vertically on the surface. The results regarding adsorption energies, adsorption heights, and S 2p CLBEs are given in Table 1, and Figure 7a summarizes the data on CLBEs for the different cases. The adsorption height is defined as the distance from the sulfur atom to the nearest substrate atom. Positive adsorption energies refer to stable configurations. For the intact thiophene molecule, the general trends are as follows: (i) The CLBEs are higher in the presence of an adatom. (ii) They are lower for the less dense 4 × 4 unit cell by on the average almost 1 eV. (iii) The adsorption energies are higher for the lying down configuration and for the 4 × 4 unit cell. They vary with adsorption site. This variation for a given configuration is of the order of 0.3−0.5 eV. (iv) The adsorption energies are of the same order of magnitude as those for Cu but significantly lower than those found for Ni for which they can attain 2 eV.67 For the broken thiophene metallocycle, incorporating an Au adatom, the adsorption energies for some configurations were negative and for the stable configurations were lower than that for the intact molecule. For the standing up broken thiophene alkene chain, the adsorption energies are much higher but somewhat lower than for the calculated alkanethiol. Insofar as the CLBEs are concerned, they vary significantly with packing density and for different adsorption sites as may be seen in Table 1 and Figure 7. What is most striking is that with the exception of the case of the standing up thiophene molecule with an adatom, the groups of CLBEs for the broken thiophene, intact thiophene, and alkanethiol overlap. For the sake of comparison with experiment, we rely below on a 20 eV

S−Au (Å)

2p (eV)

2.89−3.29 2.65−3.01

−185.64 −182.99 to −183.39 −182.56 to −183.28

shift to lower energies so that 182 eV corresponds to the experimental 162 eV energy. With this in mind, clearly previous assignment of the A and B peaks at 162 and 161 eV (S 2p3/2) to only formation of an alkene chain following S−C bond scission is not justified. Rather, these must be due to overlapping contributions from both intact and broken thiophene molecules. This is consistent with our observation of a π resonance peak in NEXAFS. Possibly a contribution from a metallocycle associated with a broken thiophene, as suggested by Nambu et al.,58 could be present, but the calculations suggest that this system is less stable than thiophene. In the case of radiation damage, the appearance of the higher energy C′ component could be due to different reasons. It could, like for alkanethiol damage mentioned in the introduction, be due to freeing of a sulfur with the broken molecule remaining59,60 in the layer. Alternatively, it could be that some degree of cross-linking57,61 occurs following cleavage of C−H or C−C bonds, which would then lead to appearance of components observed for the larger thiophenes with this energy as we shall see below. 6.2. Bithiophene. Calculations were performed for a bithiophene molecule placed on a 4 × 4, 5-layer Au(111) slab. For the intact bithiophene, we observe adsorption heights ranging from 2.89 to3.29 Å and CLBEs from −183.39 to −182.99 eV, i.e., somewhat higher than for thiophene. As in the gas-phase calculations results for the two S atoms are similar (Table 2). Finally, the adsorption energy for this molecule ranged from 0.60 to 0.71 eV. The more interesting results occur in the broken bithiophene case, for which we notice large buckling of the first layer of the surface, attaining 1.0−1.6 Å for certain configurations. As H


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expect a moderate shift to lower energy from the intact molecule position and possibly appearance of a significantly lower energy component (Figure 7b). However, if the bithiophene broke up completely into a thiophene and thiophene fragments, then it would lead to appearance of the same components at low energies as for thiophene. Concerning peak D, a study of adsorption of terthiophene thiol (3T-SH),65 which adsorbs in a configuration with the thiol S bound to Au and the thiophene rings away from the surface, S 2p peaks at 164 and 163.6 eV were observed and attributed to thiophene ring sulfurs not in contact with the surface. In our case, the D peak could be compatible with a partly dissociated 2T molecule with a “dangling” thiophene unit. We return to this in the following sections. 6.3. Terthiophene. 6.3.1. “Horizontally” Placed Molecule. We constructed a standing up terthiophene molecule (C12H8S3) and an inverted terthiophene molecule as shown by the cartoons in Figure 7c, on a 4-layer 5 × 5 Au (111) surface. The broken thiophene calculation corresponds to breaking the S−C bond for one side of the molecule (see Figure 7). The results of the calculations are summarized in Figure 7 and Table 3. A noteworthy point is that in the case of the gas phase molecule the CLBEs of the central and side sulfurs differ by 0.35 eV. Similar calculations were done for a lying down 3T molecule. We also did calculations for a 2 × 6 unit cell corresponding to close-packed standing 3T molecules. For the unbroken terthiophene, we observe a straightforward trend. For the central sulfur atom, we observed CLBEs ranging from −182.68 to −182.62 eV (Table 3). For the sulfur atoms, on either the right or the left side of the molecule we observed a core energy ranging from −182.30 to −182.14 eV. The energy difference between the central and side sulfurs varies from 0.3 to 0.5 eV. This also agrees with the results for “dangling” terthiophene mentioned above. For the adsorption height (sulfur−nearest-gold distance) for all three sulfurs, we observe a value in the range of 3.37−3.91 Å or 5.46−5.77 Å depending if the sulfur belonged to a thio pointing to the surface or away from the surface. Altogether, we notice that inverting the terthiophene does not noticeably affect the CLBEs or adsorption heights. The sulfur was much more sensitive to their position in the terthiophene than to the overall orientation of the terthiophene on the surface.

shown in Figure 8, it appears as if the broken carbon atom tends to extract a gold atom from the surface. Along with this

Figure 8. Broken bithiophene adsorbed on Au(111) 4 × 4 and √7 × √7 unit cell for (A, C) atop and (B, D) hollow metallocycle configuration. Each row shows the top view looking down the z axis and the side view looking along the x axis and on the right the side view of the final configuration with an extracted Au atom.

buckling we observe a lower, by ∼0.3 Å, adsorption height of the broken sulfur atom. For the sulfur with only one carbon attached, one C−S bond broken, the core level energies shift is lower, ranging from −182.88 eV down to −181.76 eV. For the unmodified sulfur, the CLBEs also tend to move a little lower ranging from −183.28 to −182.56 eV. For comparison with experiment, we again rely on a shift of these CLBEs close to 20 eV. We must mention, however, that the adsorption energies for broken bithiophene are negative suggesting that broken bithiophene are not stable configurations over the Au(111) surface. This state may be an intermediate step in the process of bithiophene breaking apart into a thiophene and thiophene fragments on the surface. As mentioned in the introduction for bithiophene, Ito et al.29 reported mainly a single doublet with S 2p3/2 at 163.4 eV with only traces of lower lying A and B peaks. We observe a 163.3 eV peak with a smaller higher lying component at 164 eV and generally quite large A and B peaks. It seems reasonable to assign peak C to intact bithiophene and the lower A and B peaks to broken thiophene components. In view of the results of the calculations for the single S−C bond scission, one would

Table 3. S 2p Core Level Binding Energies for the Gas Phase Molecule; 2p Energies and Adsorption Heights for the Adsorbed 3T Molecule with the Middle Sulfur Pointing up or down for the 5 × 5 and 2 × 6 Unit Cells, and Results for the Broken Molecule (Third Sulfur Carbon Bond) with the Middle Sulfur Pointing up for the 5 × 5 Unit Cell as Well as the S−Au Distance first sulfur molecule

adsorption energies (eV)

S−Au (Å)

3T gas phase 3T 5 × 5 (up)

0.58−0.64

3.38−3.61

3T 5 × 5 (down)

0.21−0.55

5.56−5.77

broken 3T 5 × 5 (up)

−1.91 to −1.86

3.46−3.85

3T 5 × 5 L

1.68−1.84

3.6−3.89

3T 2 × 6 (up)

1.06−1.07

3.52−3.86

3T 2 × 6 (down)

1.10−1.13

5.8−5.99

second (middle) sulfur

2p (eV) −184.66 −182.20 to −182.30 −182.14 to −182.27 −182.23 to −182.41 −182.52 to −182.71 −184.25 to −184.27 −184.27 to −184.28

S−Au (Å) 5.5−5.69 3.53−3.91 5.55−5.70 3.32−3.89 5.63−5.81 3.82−4.18

I

2p (eV) −185.01 −182.62 to −182.64 −182.64 to −182.68 −182.56 to −182.60 −182.99 to −183.19 −184.82 −184.86 to −184.87

third sulfur S−Au (Å) 3.37−3.57 5.46−5.77 3.32−3.72 3.6−3.89 3.58−3.83 5.87−6.00

2p (eV) −184.66 −182.20 to −182.30 −182.18 to −182.27 −181.98 to −182.19 −182.52 to −182.71 −184.25 to −184.27 −184.27


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The Journal of Physical Chemistry C In case of the lying down terthiophene, the CLBEs are slightly larger than for the standing up case, which could be related to the slightly larger sulfur Au distance. In the case of the 2 × 6 unit cell and standing molecule, the difference between the side and middle sulfur CLBEs are slightly higher. For the broken terthiophene, we observed different core energies for the broken sulfur, the central sulfur, and the unbroken sulfur (Table 3). For the central sulfur, the core energy ranges from −182.60 to −182.56 eV, close to the core energy of the central sulfur atom of the unbroken tethiophene. For unbroken sulfur, we note core energies from −182.41 to −182.23 eV, i.e., they can be higher than those for the intact molecule. For the broken sulfur, the CLBEs are in some cases lower than that for the unbroken case. The adsorption heights were very similar to the intact terthiophene except of course for the broken sulfur. Overall, we see that breaking the terthiophene has a noticeable effect on the results as opposed to the terthiophene’s position on the surface, which has a much smaller effect on the results. In order to compare with experiment, a shifted scale is shown in the Figure 7. Since the NEXAFS data suggests an inclined but not flat-lying molecule, the scale is positioned with respect to the standing up 3T calculation for the 5 × 5 unit cell. Conerning adsorption energy, we note energies comparable to those of bithiophene when the moleucle is placed standing up. When the central sulfur atom is pointed away from the surface, we note adsorption energies ranging from 0.58 to 0.64 eV. When the central sulfur atom is pointed toward the surface, we observe a decrease in adsorption energy with energies now ranging from 0.21 to 0.55 eV. This decrease in adsorption energy may be expected, because when the central sulfur atom is pointing away from the surface the two side sulfur atoms are close to the surface. However, when the central sulfur is pointing toward the surface, we only have one, i.e., the central sulfur atom, close to the surface. Moving to a smaller unit cell, 2 × 6, increases adsorption energy and reduces the adsorption energy difference between the up and down configurations. We can attribute these changes to the fact that intermolecular forces play a larger role in the smaller, as compared to a large unit cell. Moreover, we obsevre that laying the molecule flat on the surface results in the highest adsorption energies which range from 1.68 to 1.84 eV, indicating that at least at low coverages terthiophene perfers to adsorb flat on Au(111) surface. Finally, similar to bithiophene, terthiophene exhibits a strong perference to remain intact on the surface. We observe only large negative adsorption energies for broken terthiophene. This could mean, as mentioned with the bithiophene, that this state with a single C−S bond maybe an intermediate state to a completely broken apart terthiophene molecule. 6.3.2. “Vertically” Placed Molecule. Finally, it has been suggested that terthiophene may adsorb with its long axis perpendicular to the surface as this happens in the case of the dihexyl-quater (sexi) thiophenes. We therefore performed calculations with the molecule long axis placed perpendicular to the surface (Figure 9) so that none of the sulfur atoms are close to the surface as in the preceding case. As with the other vertical configurations, we performed calculations in both the 2 × 6 and 5 × 5 unit cells. Unlike the other vertical configurations, we only tried one adsorption site as our previous calculations demonstrated the core level of the sulfur atoms changes little when the entire molecule is rotated or translated on the surface. In Table 4, we have listed the corresponding results. We note smaller adsorption energies,

Figure 9. Side view of terthiophene adsorbed on Au(111) in the 5 × 5 unit cell.

consistent with our previous observations that moving the sulfur atoms away from the surface decreases adsorption energy. We should mention again that calculated absolute core level binding energies are sensitive to the calculation set up. Hence, only energy differences are meaningful. From Table 4, we note that the CLBE for the sulfur atoms at the two ends of the molecules are the same, regardless of the coverage. The difference in CLBE between the middle sulfur atom and those at the ends is about 1 eV for the most compact 2 × 6 structure and about 1.8 eV for the larger one. Again, the molecule− molecule interaction, and hence the coverage, substantially influences the calculated CLBE. 6.3.3. Chatracteristics of S 2p Spectra. In Figures 2 and 4 above, the main peak in the 3T spectrum was fitted initially with one doublet only. Following the theoretically predicted difference in CLBEs between the central and side sulfurs, we fitted the 3T peak using two narrower components. The best fit was obtained as shown in Figure 10 with CLBEs of 163.67 and 164.03 eV and with a less intense higher energy component. The fit turns out to slightly better reproduce the high-energy tail of the spectrum than that in Figure 3. The slightly lower fitted peak intensity ratio than the expected 1:2 could be due to attenuation effects. In some cases we observed the appearance of pronounced A and B components and a broadening of the central peak. In case of the irradiated 3T sample (Figure 5), this shift was large and was taken into account in the fit by adding the 163.3 eV peak. The broadening and ensuing added extra component must correspond to the broken 3T sulfur end with the low energies in Figure 7. One can attribute the A and B components to either a completely broken 3T, leading to the same features as those for thiophene, or possibly to the presence of atomic sulfur from desulfurization.

7. CONCLUDING REMARKS The results on adsorption of thiophene derivative molecules presented here show that when these are deposited on gold surfaces in a number of cases one observes dissociation processes that correspond to S−C bond scission. This can affect charge transport along the chains: a point of importance for molecular electronics. The DFT calculations indicate a diversity of core level binding energies for intact and broken thiophene depending upon packing density and adsorption sites. They also show an important point, that as opposed to previous ideas, the CLBEs for intact thiophene (1T) and dissociated thiophene overlap and hence the previous assignment of peaks in XPS spectra at J


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Table 4. Core Energies (2p), Sulfur Adsorption Heights (S−M), and Sulfur−Carbon Bond Lengths (S−C) for Terthiophene Adsorbed on Au(111) with the Long Axis Perpendicular to the Surface in the 2 × 6 and 5 × 5 Unit Cell first sulfur molecule 3T gas phase 3T 2 × 6 3T 5 × 5

adsorption energies (eV) 0.49 0.30

second (middle) sulfur

S−Au (Å)

2p (eV)

12.18 12.18

−184.66 −183.07 −181.40

■

S−Au (Å)

2p (eV)

8.34 8.35

−185.01 −184.82 −182.34

third sulfur S−Au (Å)

2p (eV)

4.45 4.47

−184.66 −183.02 −181.34

Au 4f spectra for clean Au(111) and after thiophene adsorption, overview spectra, damage evolution for 1T and 3T, 3T S 2p XPS spectrum and NEXAFS data for a case with dissociation, variability: spectra for several samples for the same molecule (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 10. Fits of the 3T XPS spectra taking into account the difference in energy between the end and middle sulfur CLBEs.

Azzedine Bendounan: 0000-0001-7557-4322 Abdelkader Kara: 0000-0003-1445-1315 Vladimir A. Esaulov: 0000-0002-7263-9685

161 and 162 eV as a fingerprint of thiophene dissociation may not be correct. For terthiophene calculations indicate that at least at low coverages, the highest adsorption energy of all calculations corresponds to the molecule lying flat on Au(111) surface. A configuration with the long axis perpendcular to the surface leads to the lowest adsrption energies. Calculations have not yet been performed for the longer thiophenes, but the situation for these appears to be similar to that of terthiophene. In the case of terthiophene, it is theoretically found that the CLBEs for middle and side sulfurs are different, which allows to better explain the reported observation of two peaks in earlier work on terthiophenethiol and is consistent with the fitting of the spectra performed here. We underline the variability in adsorption results, where we see that under seemingly similar preparation conditions quite different results are obtained with significant dissociation occurring in some cases, even though the preparation procedures follow the same protocols. We would relate this at least partly to surface morphology, since reactivity can be large at low coordination sites and depends on the density of steps and different kinds of surface defects79,80 and adatoms. It is important to delineate this from the point of view of creation of metal contacts in organic electronic devices by various methods including the case of evaporative deposition or contact printing11,20,81−84 by transfer from a stamp. Finally, the X-ray damage analysis shows that in nT SAMs irradiation leads to appearance of the 162 and 163.4 eV S 2p3/2 CLBEs due to S−C bond scission. In the case of thiophene, the latter could be due to secondary reactions in the irradiated layer due to cross-linking. We hope that this report will stimulate further investigations of these reactive processes and be useful to researchers dealing with these systems in various applications mentioned here and in particular to those who use XPS for characterizing oligothiophene layers and interfaces.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.J. and Y.T. thank the Chinese Scholarship Council for their Ph.D. scholarships. We thank Karine Chaouchi for help in the Soleil chemistry laboratory. A.K. acknowledges support from the U.S. Department of Energy Basic Energy Science under Contract No. DE-FG02-11ER16243. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-FG0211ER16243.

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REFERENCES

(1) Zhang, F.; Wu, D. Q.; Xu, Y.; Feng, X. L. Thiophene-based Conjugated Oligomers for Organic Solar Cells. J. Mater. Chem. 2011, 21, 17590−17600. (2) Meerheim, R.; Körner, C.; Leo, K. Highly Efficient Organic Multi-junction Solar Cells with a Thiophene Based Donor Material. Appl. Phys. Lett. 2014, 105, 063306. (3) Mishra, A.; Ma, C. Q.; Bäuerle, P. Functional Oligothiophenes: Molecular Design for Multidimensional Nanoarchitectures and Their Applications. Chem. Rev. 2009, 109, 1141−1276. (4) Swathi, S. K.; Ranjith, K.; Kumar, P.; Ramamurthy, P. C. Novel Thiophene Derivative Hybrid Composite Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 96, 101−107. (5) Serban, D. A.; Kilchytska, V.; Vlad, A.; Martin-Hoyas, A.; Nysten, B.; Jonas, A. M.; Geerts, Y. H.; Lazzaroni, R.; Bayot, V.; Flandre, D.; et al. Low-power Dihexylquaterthiophene-based Thin Film Transistors for Analog Applications. Appl. Phys. Lett. 2008, 92, 143503. (6) Leydecker, T.; Trong Duong, D.; Salleo, A.; Orgiu, E.; Samori, P. Solution-processed Field-Effect Transistors Based on Dihexylquaterthiophene Films with Performances Exceeding Those of VacuumSublimed Films. ACS Appl. Mater. Interfaces 2014, 6, 21248−21255. (7) Generali, G.; Dinelli, F.; Capelli, R.; Toffanin, S.; Di Maria, S.; Gazzano, M.; Barbarella, G.; Muccini, M. Correlation among Morphology, Crystallinity, and Charge Mobility in OFETs Made of Quaterthiophene Alkyl Derivatives on a Transparent Substrate Platform. J. Phys. Chem. C 2011, 115, 23164−23169.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08006. K


Article

The Journal of Physical Chemistry C (8) Generali, G.; Capelli, R.; Toffanin, S.; Facchetti, A.; Muccini, M. Ambipolar Field-effect Transistor Based on a,x-Dihexylquaterthiophene and a,x-Diperfluoroquaterthiophene Vertical Heterojunction. Microelectron. Microelectron. Reliab. 2010, 50, 1861−1865. (9) Mannebach, E. M.; Spalenka, J. M.; Johnson, P. S.; Cai, Z. H.; Himpsel, F. J.; Evans, P. G. High Hole Mobility and ThicknessDependent Crystal Structure in α, ω-Dihexylsexithiophene Singlemonolayer Field-effect Transistors. Adv. Funct. Mater. 2013, 23, 554− 564. (10) Capelli, R.; Dinelli, F.; Gazzano, M.; D’Alpaos, R.; Stefani, A.; Generali, G.; Riva, M.; Montecchi, M.; Giglia, A.; Pasquali, L. Interface Functionalities in Multilayer Stack Organic Light Emitting Transistors (OLETs). Adv. Funct. Mater. 2014, 24, 5603−5613. (11) Reeja-Jayan, B.; Manthiram, A. Influence of Polymer−metal Interface on the Photovoltaic Properties and Long-term Stability of ncTiO2-P3HT Hybrid Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 907−914. (12) Garnier, F.; Hajlaoui, R.; El Kassmi, A.; Horowitz, G.; Laigre, L.; Porzio, W.; Armanini, M.; Provasoli, F. Dihexylquaterthiophene, a Two-Dimensional Liquid Crystal-like Organic Semiconductor with High Transport Properties. Chem. Mater. 1998, 10, 3334−3339. (13) Leydecker, T.; Trong Duong, T.; Salleo, A.; Orgiu, E.; Samori, P. Solution-processed Field-effect Transistors Based on Dihexylquaterthiophene Films with Performances Exceeding those of Vacuumsublimed Films. ACS Appl. Mater. Interfaces 2014, 6, 21248−21255. (14) Buimaga-Iarinca, L.; Morari, C. Adsorption of Small Aromatic Molecules on Gold: DFT Localized Basis Set Study Including Van der Waals Effects. Theor. Chem. Acc. 2014, 133, 1502. (15) Matsuura, T.; Nakajima, M.; Shimoyama, Y. Growth of Selfassembled Monolayer of Thiophene on Gold Surface: An Infrared Spectroscopic Study. Jpn. J. Appl. Phys. 2001, 40, 6945−6950. (16) Moret, M.; Campione, M.; Borghesi, A.; Miozzo, L.; Sassella, A.; Trabattoni, S.; Lotz, B.; Thierry, A. Structural Characterisation of Single Crystals and Thin Films of α,ω-Dihexylquaterthiophene. J. Mater. Chem. 2005, 15, 2444−2449. (17) Campione, M.; Borghesi, A.; Moret, M.; Sassella, A. Growth Dynamics of Quaterthiophene Thin Films. J. Mater. Chem. 2003, 13, 1669−1675. (18) Zotti, G.; Vercelli, B.; Berlin, A. Gold Nanoparticles Linked by Pyrrole and Thiophene-Based Thiols. Electrochemical, Optical, and Conductive Properties. Chem. Mater. 2008, 20, 397−412. (19) Milligan, P. K.; Murphy, B.; Lennon, D.; Cowie, B. C. C.; Kadodwala, M. A. Complete Structural Study of the Coverage Dependence of the Bonding of Thiophene on Cu(111). J. Phys. Chem. B 2001, 105, 140−148. (20) Lachkar, A.; Selmani, A.; Sacher, E.; Leclerc, M.; Mokhliss, R. Metallization of Polythiophenes I. Interaction of Vapor-deposited Cu, Ag and Au with Poly(3-hexylthiophene) (P3HT). Synth. Met. 1994, 66, 209−215. (21) Guarnaccio, A.; D’Auria, M.; Racioppi, R.; Mattioli, G.; Bonapasta, A. A.; De Bonis, A.; Teghil, R.; Prince, K. C.; Acres, R. G.; Santagata, A. Thiophene-based Oligomers Interacting with Silver Surfaces and the Role of a Condensed Benzene Ring. J. Phys. Chem. C 2016, 120, 252−264. (22) Elfeninat, F.; Fredriksson, C.; Sacher, E.; Selmani, A. A Theoretical Investigation of the Interactions between Thiophene and Vanadium, Chromium, Copper, and Gold. J. Chem. Phys. 1995, 102, 6153−6158. (23) Noh, J.; Ito, E.; Nakajima, K.; Kim, J.; Lee, H.; Hara, M. Highresolution STM and XPS Studies of Thiophene Self-assembled Monolayers on Au(111). J. Phys. Chem. B 2002, 106, 7139−7141. (24) Su, G. J.; Zhang, H. M.; Wan, L. J.; Bai, C. L. Phase Transition of Thiophene Molecules on Au(111) in Solution. Surf. Sci. 2003, 531, L363−L368. (25) Ito, E.; Hara, M.; Kanai, K.; Ouchi, Y.; Seki, K.; Noh, J. Comparative Study of Tetrahydrothiophene and Thiophene Selfassembled Monolayers on Au(111): Structure and Molecular Orientation. Bull. Korean Chem. Soc. 2009, 30, 1755−1759.

(26) Sako, E. O.; Kondoh, H.; Nakai, I.; Nambu, A.; Nakamura, T.; Ohta, T. Reactive Adsorption of Thiophene on Au(111) from Solution. Chem. Phys. Lett. 2005, 413, 267−271. (27) Dishner, M. H.; Hemminger, J. C.; Feher, F. J. Formation of a Self-assembled Monolayer by Adsorption of Thiophene on Au(111) and its Photooxidation. Langmuir 1996, 12, 6176−6178. (28) Noh, J.; Ito, E.; Araki, T.; Hara, M. Adsorption of Thiophene and 2,5-Dimethylthiophene on Au (111) from Ethanol Solutions. Surf. Sci. 2003, 532-535, 1116−1120. (29) Ito, E.; Noh, J.; Hara, M. Different Adsorption States between Thiophene and α-Bithiophene Thin Films Prepared by Self-assembly Method. Jpn. J. Appl. Phys. 2003, 42, L852−L855. (30) Terzi, F.; Seeber, R.; Pigani, L.; Zanardi, C.; Pasquali, L.; Nannarone, S.; Fabrizio, M.; Daolio, S. J. Phys. Chem. B 2005, 109, 19397−19402. (31) Liu, G.; Rodriguez, J. A.; Dvorak, J.; Hrbek, J.; Jirsak, T. Chemistry of Sulfur-containing Molecules on Au(111): Thiophene, Sulfur Dioxide, and Methanethiol Adsorption. Surf. Sci. 2002, 505, 295−307. (32) Pasquali, L.; Terzi, F.; Doyle, B. P.; Seeber, R. Photoemission and X-ray Absorption Study of the Interface between 3,4-Ethylenedioxythiophene-Related Derivatives and Gold. J. Phys. Chem. C 2012, 116, 15010−15018. (33) Terzi, F.; Pasquali, L.; Montecchi, M.; Nannarone, S.; Viinikanoja, A.; Aäritalo, T.; Salomaki, M.; Lukkari, J.; Doyle, B. P.; Seeber, R. New Insights on the Interaction between Thiophene Derivatives and Au Surfaces. The Case of 3,4-Ethylenedioxythiophene and the Relevant Polymer. J. Phys. Chem. C 2011, 115, 17836−17844. (34) Zaera, F.; Kollin, E. B.; Gland, J. L. Thiophene Chemisorption and Thermal Decomposition on Nickel (100) Single - Crystal Surfaces. Langmuir 1987, 3, 555−557. (35) Stohr, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Desulfurization and Structural Transformation of Thiophene on the Pt(111) Surface. Phys. Rev. Lett. 1984, 53, 2161. (36) Caldwell, T. E.; Abdelrehim, I. M.; Land, D. P. Thiophene Decomposition on Pd(111) Forms S and C4 Species: a Laser-induced Thermal Desorption/Fourier Transform Mass Spectrometry Study. Surf. Sci. 1996, 367, L26−L31. (37) Heise, W. H.; Tatarchuk, B. J. Thiophene Adsorption on Clean and Sulfur Precovered Ru(OOO1). Surf. Sci. 1989, 207, 297−322. (38) Roberts, J. T.; Friend, C. M. The Reactions of Thiophene on Mo(1l0) and Mo(ll0)-p(2 × 2)-S. Surf. Sci. 1987, 186, 201−218. (39) Lang, J. F.; Masel, R. I. The Adsorption of Thiophene and Tetrahydrothiophene on Several Faces of Platinum. Surf. Sci. 1987, 183, 44−66. (40) Zaera, F.; Kollin, E. B.; Gland, J. L. Vibrational Characterization of Thiophene Decomposition on the Mo (100). Surf. Sci. 1987, 184, 75−89. (41) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. Spontaneously Organized Molecular Assemblies. 3. Preparation and Properties of Solution Adsorbed Monolayers of Organic Disulfides on Gold Surfaces. J. Am. Chem. Soc. 1987, 109, 2358−2368. (42) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Self-assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103−1170. (43) Hamoudi, H.; Guo, Z. A.; Prato, M.; Dablemont, C.; Zheng, W. Q.; Bourguignon, B.; Canepa, M.; Esaulov, V. A. On the Self-assembly of Short Chain Alkanedithiols. Phys. Chem. Chem. Phys. 2008, 10, 6836−6841. (44) Castner, D. G.; Hinds, K.; Grainger, D. W. X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12, 5083−5086. (45) Guo, Z.; Zheng, W.; Hamoudi, H.; Dablemont, C.; Esaulov, V. A.; Bourguignon, B. On the Chain Length Dependence of CH3 Vibrational Mode Relative Intensities in Sum Frequency Generation Spectra of Self Assembled Alkanethiols. Surf. Sci. 2008, 602, 3551− 3559. L


Article


Combined NEXAFS and XPS Study. J. Electron Spectrosc. Relat. Phenom. 2005, 144-147, 433−436. (66) Briggs, D.; Beamson, G. XPS Studies of the Oxygen 1s and 2s Levels in a Wide Range of Functional Polymers. Anal. Chem. 1993, 65, 1517−1523. (67) Malone, W.; Yildirim, H.; Matos, J.; Kara, A. A van der Waals Inclusive Density Functional Theory Study of the Nature of Bonding for Thiophene Adsorption on Ni(100) and Cu(100) Surfaces. J. Phys. Chem. C 2017, 121, 6090−6103. (68) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (69) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (70) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (71) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (72) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (73) Klimeš, J.; Bowler, D. R.; Michaelides, A. Chemical Accuracy for the Van Der Waals Density Functional. J. Phys.: Condens. Matter 2010, 22, 022201. (74) Yildirim, H.; Kara, A. Effect of Van der Waals Interactions on the Adsorption of Olympicene Radical on Cu(111): Characteristics of Weak Physisorption versus Strong Chemisorption. J. Phys. Chem. C 2013, 117, 2893−2902. (75) Yildirim, H.; Greber, T.; Kara, A. Trends in Adsorption Characteristics of Benzene Transition Metal Surfaces: Role of Surface Chemistry and Van Der Waals Interactions. J. Phys. Chem. C 2013, 117, 20572−20583. (76) Yildirim, H.; Matos, J.; Kara, A. Role of Long-Range Interactions for the Structure and Energetics of Olympicene Radical Adsorbed on Au(111) and Pt(111) Surfaces. J. Phys. Chem. C 2015, 119, 25408− 25419. (77) Matos, J.; Yildirim, H.; Kara, A. Insight into the Effect of Long Range Interactions for the Adsorption of Benzene on Transition Metal (110) Surfaces. J. Phys. Chem. C 2015, 119, 1886−1897. (78) Rodriguez, J. A.; Dvorak, J.; Jirsak, T.; Liu, G.; Hrbek, J.; Aray, Y.; Gonzalez, C. Coverage Effects and the Nature of the Metal-Sulfur Bond in S/Au(111): High-resolution Photoemission and Densityfunctional Studies. J. Am. Chem. Soc. 2003, 125, 276−285. (79) Caprile, L.; Cossaro, A.; Falletta, E.; Della Pina, C.; Cavalleri, O.; Rolandi, R.; Terreni, S.; Ferrando, R.; Rossi, M.; Floreano, L.; et al. Interaction of l-Cysteine with Naked Gold Nanoparticles Supported on HOPG: A High Resolution XPS Investigation. Nanoscale 2012, 4, 7727−7734. (80) Yim, W. L.; Nowitzki, T.; Necke, M.; Schnars, H.; Nickut, P.; Biener, J.; Biener, M. M.; Zielasek, V.; Al-Shamery, K.; Klüner, T.; et al. Universal Phenomena of CO Adsorption on Gold Surfaces with Lowcoordinated Sites. J. Phys. Chem. C 2007, 111, 445−451. (81) Liu, C.; Xu, Y.; Noh, Y. Y. Contact Engineering in Organic Field-effect Transistors. Mater. Today 2015, 18, 79−96. (82) Fahlman, M.; Salaneck, W. R. Surfaces and Interfaces in Polymer-based Electronics. Surf. Sci. 2002, 500, 904−922. (83) Loo, Y. L.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Electrical Contacts to Molecular Layers by Nanotransfer Printing. Nano Lett. 2003, 3, 913−917. (84) Carlson, A.; Bowen, A. M.; Huang, Y.; Nuzzo, R. G.; Rogers, J. A. Transfer Printing Techniques for Materials Assembly and Micro/ Nanodevice Fabrication. Adv. Mater. 2012, 24, 5284−5318.

(46) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. High Resolution X-ray Photoelectron Spectroscopy Measurements of Octadecanethiol Self-assembled Monolayers on Au(111). Langmuir 1998, 14, 2092−2096. (47) Ito, E.; Kang, H.; Lee, D.; Park, J. B.; Hara, M.; Noh, J. Spontaneous Desorption and Phase Transitions of Self-assembled Alkanethiol and Alicyclic Thiol Monolayers Chemisorbed on Au(111) in Ultrahigh Vacuum at Room Temperature. J. Colloid Interface Sci. 2013, 394, 522−529. (48) Prato, M.; Moroni, R.; Bisio, F.; Rolandi, R.; Mattera, L.; Cavalleri, O.; Canepa, M. Optical Characterization of Thiolate Selfassembled Monolayers on Au(111). J. Phys. Chem. C 2008, 112, 3899−3906. (49) Hamoudi, H.; Prato, M.; Dablemont, C.; Cavalleri, O.; Canepa, M.; Esaulov, V. A. Self-assembly of 1,4-Benzenedimethanethiol Selfassembled Monolayers on Gold. Langmuir 2010, 26, 7242−7247. (50) Pasquali, L.; Terzi, F.; Seeber, R.; Nannarone, S.; Datta, D.; Dablemont, C.; Hamoudi, H.; Canepa, M.; Esaulov, V. A. UPS, XPS, and NEXAFS Study of Self-assembly of Standing 1,4-Benzenedimethanethiol SAMs on Gold. Langmuir 2011, 27, 4713−4720. (51) Grunze, M. Preparation and Characterization of Self-assembled Organic Films on Solid Substrates. Phys. Scr. 1993, T49B, 711−717. (52) Jia, J.; Kara, A.; Pasquali, L.; Bendounan, A.; Sirotti, F.; Esaulov, V. A. On Sulfur Core Level Binding Energies in Thiol Self-assembly and Alternative Adsorption Sites: An Experimental and Theoretical Study. J. Chem. Phys. 2015, 143, 104702. (53) Hamoudi, H.; Esaulov, V. A. Self-assembly of α,ω-Dithiols on Surfaces and Metal Dithiol Heterostructures. Ann. Phys. 2016, 528, 242−263. (54) Hamoudi, H.; Ariga, K.; Uosaki, K.; Esaulov, V. A. Going beyond the Self-assembled Monolayer: Metal Intercalated Dithiol Multilayers and Their Conductance. RSC Adv. 2014, 4, 39657−39666. (55) Jia, J.; Bendounan, A.; Chaouchi, K.; Kubsky, S.; Sirotti, F.; Pasquali, L.; Esaulov, V. A. Chalcogen Atom Interaction with Palladium and the Complex Molecule−Metal Interface in Thiol Self Assembly. J. Phys. Chem. C 2014, 118, 24983−24994. (56) Salaneck, W. R.; Wu, C. R.; Nilsson, J. O.; Brédas, J. L. Synth. Met. 1987, 21, 57−61. (57) Baumgartner, K. M.; Volmer-Uebing, M.; Taborski, J.; Bauerle, P.; Umbach, E. Adsorption and Polymerization of Thiophene on a Ag(ll1) Surface. Ber. Buns. Phys. Chem. 1991, 95, 1488−1495. (58) Nambu, A.; Kondoh, H.; Nakai, I.; Amemiya, K.; Ohta, T. Film Growth and X-ray Induced Chemical Reactions of Thiophene Adsorbed on Au(111). Surf. Sci. 2003, 530, 101−110. (59) Zerulla, D.; Chassé, T. X-ray Induced Damage of Self-assembled Alkanethiols on Gold and Indium Phosphide. Langmuir 1999, 15, 5285−5294. (60) Hamoudi, H.; Chesneau, F.; Patze, C.; Zharnikov, M. ChainLength-Dependent Branching of Irradiation-Induced Processes in Alkanethiolate Self-assembled Monolayers. J. Phys. Chem. C 2011, 115, 534−541. (61) Turchanin, A.; Gölzhäuser, A. Carbon Nanomembranes From Self-assembled Monolayers: Functional Surfaces without Bulk. Prog. Surf. Sci. 2012, 87, 108−162. (62) Jia, J.; Bendounan, A.; Kotresh, H. M. N.; Chaouchi, K.; Sirotti, F.; Sampath, S.; Esaulov, V. A. Selenium Adsorption on Au(111) and Ag(111) Surfaces: Adsorbed Selenium and Selenide Films. J. Phys. Chem. C 2013, 117, 9835−9842. (63) O’Rourke, B. E.; Flores, M.; Esaulov, V. A.; Yamazaki, Y. Modification of Self-assembled Monolayers by Highly Charged Ions. Nucl. Instrum. Methods Phys. Res., Sect. B 2013, 299, 68−70. (64) Polack, F.; Silly, M.; Chauvet, C.; Lagarde, B.; Bergeard, N.; Izquierdo, M.; Chubar, O.; Krizmancic, D.; Ribbens, M.; Duval, J.-P.; et al. TEMPO: A New Insertion Device Beamline at SOLEIL for Time Resolved Photoelectron Spectroscopy Experiments on Solids and Interfaces. AIP Conf. Proc. 2009, 1234, 185−188. (65) Fan, L. J.; Yang, Y. W.; Tao, Y. T. Molecular Orientation and Bonding of Terthiophene-thiol Self-assembled on Au(111): A M


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