Interplay between Structural and Electronic Properties in 1,4,5,8

Article pubs.acs.org/JPCC

Interplay between Structural and Electronic Properties in 1,4,5,8Naphthalenetetracarboxylic Dianhydride Films on Cu(100) Yongfeng Tong,† Francois Nicolas,† Stefan Kubsky,† Hamid Oughaddou,‡ Fausto Sirotti,† Vladimir Esaulov,‡ and Azzedine Bendounan*,† †

Synchrotron SOLEIL, L’Orme des Merisiers, BP 48, F-91192 Gif-sur-Yvette Cedex, Saint-Aubin, France Institut des Sciences Moléculaires d’Orsay, Université Paris-Sud, Bâtiment 351, 91405 Orsay Cedex, Orsay, France

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ABSTRACT: Using various surface science techniques, we have studied the properties of 1,4,5,8naphthalenetetracarboxylic dianhydride (NTCDA) thin films on a Cu(100) surface. STM investigations suggest the high mobility of the NTCDA molecules over the surface, leading to the formation of large and well-ordered islands. In line with LEED results, two typical domains with different molecular orientations and brick-wall-like structure are revealed in the STM images. On the other hand, a fingerprint of covalent bonding at the NTCDA/Cu interface is obtained from the core level shifts measured by high-resolution XPS. As a consequence, a charge transfer process develops at the interface and enables a total filling of the π*-like lowest unoccupied orbital, which is the signature of the semiconductor character of the NTCDA layer. The carbon K-edge NEXAFS data confirm the chemisorption nature of the interaction between NTCDA and the Cu(100) surface and indicate an evolution from a flat-lying orientation of NTCDA at 1 ML to a straight-up orientation in a thick layer.

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substrates.19 In this context, large aromatic molecules have proven to be promising materials for application due to their high electron mobility resulting from the intermolecular πbond.20,21 In particular, molecules such as 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA),22−27 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA),28−30 naphthalenetetracarboxylic diimide (NTCDI),31−34 and 3,4,9,10-perylenetetracarboxylic diimide (PTCDI)35−37 have received much interest. NTCDA is a prototypical π-conjugated molecule and its interaction with different substrates has been largely investigated experimentally and theoretically38−43 due to its promising application in molecular devices. Various techniques, like scanning tunneling microscopy (STM), low energy electron diffraction (LEED), X-ray photoemmission spectroscopy (XPS), and near edge X-ray adsorption find structure (NEXAFS), have been used to study the characteristics of NTCDA films on different substrates. These studies reveal that the characteristics of the substrates play an important role in determining the orientation of NTCDA monolayers. In some studies, NTCDA thin films are deposited on weakly interacting substrates, like silicon oxide (SiO2),44 sulfites (MoS2),45 and graphite (HOPG),45 and even on Au substrates, for which the bonding mechanism was compared to the case of rare gases on noble-metal surfaces.46−48 In addition, preadsorption of alkali atoms on metal surfaces can lead to a weakening of the bonding of NTCDA to such surfaces.49 Two different phases were often observed: a flat-lying structure stabilized by the lateral interactions of the polar groups of the molecules and a bulklike

INTRODUCTION Organic materials are being widely investigated for the purpose of observing useful optical and electrical properties. Most of these materials exhibit semiconducting properties with direct band gap transitions, large visible extinction coefficients, photostability, and low cost of fabrication (see ref 1 and references therein). In particular, thin films of organic molecules deposited on metal surfaces have demonstrated great performance as components in electronic and photoelectronic devices, such as organic field emission transistors (OFETs),2,3 organic light emission diodes (OLEDs),4,5 lithium ion batteries (LIBs),6 and solar cells.7,8 One expects also a strong dependence of the charge transport on the crystalline order and crystal packing of these deposited thin films.9 From a technological point of view, the ordering of the first layer is also decisive for the success of a true epitaxial growth of organic films on inorganic substrates. In fact, in the organic/metal hybrid system, the performance of these devices depends strongly on the film morphology.10 The interaction at the interface, between the organic molecules and the metal surface, influences directly the molecular structural order, the band bending, and the interface dipole characteristics.11 Therefore, this interaction may have an impact on the subsequent growth behavior of the following layers and thus determine the structural and functional properties of the entire thin film and, hence, the performance of the whole device. Accordingly, investigation of organic molecule/substrate interactions, ranging from weak van der Waals forces and hydrogen bonding to strong π−π bonding or covalent bonding,12−18 is of prime importance. It is moreover of fundamental interest in basic research, since it offers a significant extension of knowledge on the adsorption behavior of organic molecules on different © XXXX American Chemical Society

Received: November 30, 2016 Revised: February 16, 2017 Published: February 21, 2017 A

DOI: 10.1021/acs.jpcc.6b12050 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

evaporator. The film thickness was accurately defined by finely controlling the deposition rate at 0.5 Å/min by using a quartz microbalance. Once the sample preparation was completed, high-resolution photoemission measurements were conducted using a Scienta SES2002 electron spectrometer at different incident photon energies of the synchrotron radiation. The energy resolution depends essentially on the chosen photon energy and can be estimated from the resolving power, which amounts to 5 × 103 for the TEMPO beamline. All the core level spectra were calibrated with respect to Au4f spectra taken on clean Au(111). The NEXAFS experiments were carried out in Auger electron yield mode at the carbon K-edge using linearlike light polarization. To probe the molecule orientation over the surface, we varied the incidence angle of the photon beam from θ = 15° (grazing geometry with the electric field vector nearly perpendicular to the surface plane) to θ = 90° (electric field vector parallel to the surface plane). The NEXAFS spectra were normalized with a reference adsorption spectrum measured on clean Au(111) taken under the same experimental conditions, in order to eliminate the contribution from the carbon contamination that is known to be present on the optical mirrors of the beamline. On the other hand, the beam damage that can be induced by the synchrotron radiation in the organic layer, in particular for the thick-layer system, was deeply reduced by defocusing the photon beam using dedicated optical mirrors. The STM imaging is done in the constant current mode, with −0.34 V bias on the tip, and 0.18 nA of tunneling current. The images shown in this paper represent unfiltered raw data after drift correction and without any further computer processing. All the measurements reported here have been conducted at room temperature. The fitting procedure of the photoemission spectra was conducted after a proper Shirley background subtraction, and Voigt profiles were used for both C 1s and O 1s core levels. The only constraints used in the fitting process were applied to the peak line width of the main features and of the satellite structures. The imposed values were chosen by referring to a published work on similar systems.52,55

phase with its molecular plane perpendicular to the substrate. The scanning tunneling spectroscopy (STS) revealed two molecular states corresponding to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Moreover, a large number of research works have been devoted to investigate the adsorption of NTCDA on reactive metal substrates like Ag, Cu, and Ni, since the understanding of the interaction at the organic/metal interface plays an important role for various devices.50−52 Compared with the inert substrates, there is a chemisorption type bonding between the organic molecules and the reactive metallic substrate, leading to a hybridization of the molecular and metal states and to a charge transfer from the substrate to molecules, since the NTCDA acts as an electron-acceptor-type molecule. Kilian et al.53 investigated the behavior of NTCDA on Ag(111) and found that the monolayer of NTCDA displayed a sequence of three well-ordered structures with increasing coverage due to the adsorption of further molecules in the first layers. A commensurate-to-incommensurate phase transition develops upon increasing coverage within the submonolayer regime. Another investigation by Braatz et al.54 indicates that by squeezing NTCDA into the empty space of the relaxed first layer, the initially flat-lying molecules adopt a highly inclined adsorption structure, leaving a perpendicular orientation of all the NTCDA molecules through the bonding of carboxyl oxygen atoms to the Ag substrate. In summary, the NTCDA film commonly displays a flat-lying structure with respect to the surface at monolayer coverage, where the molecule is in direct contact to the substrate. However, in the multilayer system, the intermolecular interaction exceeds the molecule−substrate interaction, and thus, NTCDA tends to display a standing-up orientation. We note that no STM data on NTCDA/Cu(100) have yet been reported. Therefore, the aim of our present combined STM, LEED, PES, and NEXAFS investigations is 2-fold: on the one hand, to clarify the superstructure of NTCDA on Cu(100) and, on the other hand, to gain new insight into the mobility and orientation of NTCDA at the submonolayer regime and as a function of the film thickness. The present paper is organized as follows: first, we present STM images with high lateral resolution obtained on NTCDA monolayer deposited on Cu(100). One observes well-ordered and long-range two-domain structures, which is in good agreement with the LEED patterns. Afterward, a detailed fit analysis of high-resolution XPS core level spectra is given in order to probe the chemical bonding of NTCDA with the substrate and its evolution as a function of the film coverage. In the following section, we examine the properties of the valence band, before discussing the NEXAFS data, which provide information about the change in the NTCDA orientation depending on the film thickness.

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RESULTS AND DISCUSSION LEED and STM Investigations. Figure 1 displays LEED and STM results obtained on a submonolayer of NTCDA

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EXPERIMENTAL SECTION The spectroscopic experiments reported here have been performed at the TEMPO beamline of Synchrotron SOLEIL, while the LEED and STM measurements have been done at the SOLEIL Surface Laboratory using an Omicron VT-STM microscope. The sample preparation and the measurements were made under ultrahigh vacuum with a basic vacuum in the low of 10−10 mbar range. The Cu(100) crystal was cleaned by repeated cycles of Ar+ sputtering and annealing at about 800 K. Then, the cleanliness and the quality of the resulting surface were checked by XPS and LEED. The NTCDA molecules were evaporated by thermal in situ sublimation using a homemade

Figure 1. (a) LEED pattern of the NTCDA monolayer on Cu(100) taken at 50 eV after annealing at 120 °C for 10 min and (b) the corresponding simulated pattern. (c) Representative 35 × 35 nm STM images of the NTCDA deposited on Cu(100) by thermal evaporation; two different molecular domains are indicated in panels d and e. Straightforward molecular representations are on top of the STM images in order to illustrate the in-plane orientation of the NTCDA molecule and the hexagonal unit cell of the monolayer, as marked by the blue-colored arrows. B


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The Journal of Physical Chemistry C deposited on Cu(100) at room temperature. As shown in panel a, one observes a diffraction pattern associated with a superstructure from a commensurate packed layer. This superstructure has been described by a 22 −33 matrix with base vectors a = 9.2 Å, b = 9.2 Å, and θ = 112°.56 Thus, the area of the unit cell amounts to 76 Å2. The LEED pattern suggests the presence of two molecular domains with different orientations, as indicated by different colors in the simulation pattern shown in panel b. In both orientations, the unit cell contains only one NTCDA molecule and all the molecules are aligned parallel to each other.57 For further details about this structure, we report here a high-resolution STM image, shown in Figure 1c, where two molecular domains with compressed− packed and ordered arrangements are observed. The molecules appear bright on a dark background and are apparently lying flat on the surface and are arranged in rows shifted from each other, giving rise to a brick-wall-like structure. One may notice that the image displays also surface regions with poor lateral resolution, which can correspond either to free Cu terraces or to areas where the molecules are constantly moving. Within each covered region, there exists a network with high longrange order and only few molecular defects. Close inspections of the two domains are shown in Figure 1d,e, where it was possible to determine accurately the location of the NTCDA molecule and obtain the unit cell of each domain, as indicated by straightforward representations on top of the STM images. The blue-colored arrows mark the hexagonal unit cell of the ordered NTCDA layer. The observation of such domains by STM permits an explanation of the diffraction pattern obtained by LEED. In Figure 2, we present a sequence of STM images recorded successively, where site-to-site molecular motion is clearly illustrated by appearing and disappearing NTCDA molecules at the level of the domain edges, as it is shown for example in the regions marked by blue circles. Such an effect indicates that the

(

NTCDA molecules are highly mobile, although they are considered to be chemisorbed on the Cu(100) surface. In fact, the ordering of organic molecules at the surface is governed essentially by parameters such as the surface mobility and the competition between intermolecular interaction and the molecule−substrate interaction, as well as the thermal energy. The balance of those factors is ultimately responsible for molecular self-assembly. In our case, it seems that the thermal energy at room temperature is sufficiently high to cause a continuous displacement of free molecules over the surface. At the same time, strong bonding develops at the NTCDA/Cu interface and enables the formation of large islands with flatlying orientation of the molecule over the surface. Analysis of the Core Level Photoemission Spectra. The carbon C 1s and oxygen O 1s core level XPS spectra are displayed in Figure 3. As can be seen in panel a, the C 1s

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Figure 3. (a) C 1s and (b) O 1s core level XPS spectra of NTCDA on Cu(100) at different thicknesses, from a monolayer to a thick film. Panels c and d show detailed fitting analyses of these spectra. Two photon energies were used: 380 eV for C 1s and 700 eV for O 1s. The fitting peaks of the monolayer thickness are shown with the dashed line, those of the 2 ML with the solid line, and those of the thick layer with the dotted line.

spectrum of the NTCDA monolayer on Cu(100) shows mainly two distinct peaks located around 287 and 284.5 eV, which can be assigned to the naphthalene carbon atoms in the ring backbone and the carbonyl carbon atoms, respectively. By increasing the coverage to 1.5 ML, a third feature appears at 289 eV and a shoulder develops at 285 eV, which leads to an apparent shift of the whole C 1s structures toward higher binding energy. This new feature is obviously associated with NTCDA molecules in the second monolayer. The change becomes more significant in the multilayer system, since the peaks shift further respectively to around 290 and 286 eV,

Figure 2. Sequence of in situ STM images for NTCDA submonolayer deposited on Cu(100). The measurements were performed at room temperature and the acquisition time for every single image is 8 min. Observation is focused on the region marked by the blue-colored circles, where NTCDA molecules are appearing and disappearing. Such an effect indicates that the molecules are highly mobile, although they are considered to be chemisorbed on this surface. C


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and three components assigned to the naphthalene core, in order to simulate the fitting of the 1.5 ML spectrum. Note that an energy shift is systematically observed for the different carbon photoemission components associated with the 2 ML thickness with respect to those of the 1 ML system. However, the shift is larger for the anahydride peak C1 (∼1.95 eV) compared to that of the naphthalene group peaks C2, C3, C4 (∼1 eV). This effect represents clear evidence of the strong involvement of the anhydride group in the bonding and stabilization of the monolayer over the surface. It is also consistent with the molecular distortion behavior observed by the normal incidence X-ray standing wave (NIXSW) technique.30 On the other hand, from the C 1s shift one can conclude that the bonding of NTCDA within the second monolayer becomes weaker. Now, one has to examine whether this bonding is already similar to the intermolecular interaction in the multilayer system. In this respect, a detailed fitting of the multilayer system indicates that the photoemission peaks move further toward high energies. The C 1s binding energies of the anahydride group C1 and of the naphtalene core C2, C3, and C4 are reported in Table 1. By comparison between 2 ML and thick layer systems, we found that a significant energy shift exists, which occurs relatively similar for the different carbon atoms within the molecule. This energy shift can be explained as due to a band-bending behavior. A similar conclusion can be drawn in the O 1s spectra shown in panel d of Figure 3: the O 1s spectrum at 1 ML consists of two main peaks, O1 and O2, at 530.8 and 533.1 eV; two intense satellites, S1 and S2, at 531.9 and 533.75 eV; and a small tail, S3, at 535.1 eV. The O1 and O2 features can be assigned to the oxygen atoms at the bridging O and carbonyl O, respectively, which are substantially affected by the hybridization process between the π-orbitals of the NTCDA molecules and the metal state. Consequently, a strong bonding develops at the interface and a large lowering of binding energy takes place due to a charge-screening mechanism at the monolayer coverage.58 The satellite features S1 and S2 have been attributed in the literature to poorly screened O 1s final states, and S3 was interpreted as a shake-up satellite of the poorly screened O 1s state (S1).52,59 Such a kind of weak interaction exists typically in the multilayer system, which may explain the similarity between the main peaks in the multilayer film and the S1, S2 peaks existing in the monolayer system. The binding energies of the O 1s features of the 2 ML thickness appear very close to that of the thick layer. This effect can originate also from the screening process, which seems to be considerably important in this system. Valence Band Properties. Figure 4 displays valence band spectra measured on a Cu(100) surface covered respectively by 1 ML, 2 ML, and a thick layer of NTCDA. The measurements have been done at hν = 60 eV. In the monolayer regime, the spectrum depicts a very high intensity in the energy range from 1.5 to 5 eV, which corresponds to the 3d-density of states of Cu. However, the spectroscopic features originating from the organic monolayer occur at high binding energies within the energy window from 5 to 14 eV and also in the low-energy range close to the Fermi level. The most important feature occurs at 0.7 eV and represents a hybridization orbital resulting from filling of the NTCDA LUMO orbital through a charge transfer from the Cu states.24,61 This structure, commonly named ”former-LUMO” or ”F-LUMO”, was analyzed in many organic/metal systems because it provides valuable information on the monolayer character. Here, this orbital is entirely filled,

which can reasonably be assigned to carbon atoms of the weakly bound NTCDA molecules. The O 1s core level spectrum is presented in panel b of Figure 3, where similar behavior to that of C 1s is observed. For the monolayer system, one sees roughly two peaks at 530 and 533 eV, due to the presence of different chemical environments indicated by O1 and O2 in Figure 3. Upon increasing the film thickness to 1.5 ML, only a slight downward shift, on the order of 0.1 eV, is obtained. This shift becomes stronger for the thick layer, since the O features are located at 532.5 eV and 534 eV, respectively. The energy separation between the main peaks also changes from 2 eV in the monolayer to 1.2 eV for the multilayer. In order to better understand these changes in the electronic structure and their origins, a detailed peak-fitting analysis was performed for C 1s (panel c) and O 1s (panel d) photoemission levels. Starting from the C 1s spectrum of the monolayer system, the feature associated with the dianhydride carbon atom can be fitted with only one peak (labeled C1) at a binding energy of 287.1 eV; however, in the naphthalene core, there are three types of C atoms in different chemical environments, and accordingly, the C 1s structure around 284.5 eV is composed of three components (indicated by C2, C3, and C4). The binding energies of those peaks are given in Table 1, where we summarize the entire results of the fitting Table 1. Binding Energy Values of the C 1s and O 1s Core Levels As Inferred from Fits of the Photoemission Spectra Measured on 1 ML, 1.5 ML, and multilayer NTCDA Deposited Respectively on the Cu(100) Surfacea C 1s C1 C2 C3 C4 SC C1 C2 C3 C4 C1 C2 C3 C4

EB (eV)

O 1s

Monolayer System 287.1 O1 284.65 O2 283.9 S1 284.3 S2 285.5 S3 2 ML System 289.05 O1 285.65 O2 284.95 285.3 Multilayer System 289.75 O1 286.35 O2 285.65 S3 286.0

EB (eV) 530.8 533.1 531.9 533.75 535.1 532.5 534.3

532.7 534.4 535.5

a

Various features are obtained that correspond to C and O atoms in different chemical environments. Binding energy values of peaks from shake-up process are also provided.

analysis. Upon increasing the NTCDA coverage to 1.5 ML, the surface should in principle present regions with 2 ML thickness and the rest with only 1 ML, assuming at this stage a layer-bylayer growth. In this case, the fitting of the associated C 1s spectrum may be complicated. In addition to the peaks associated with the monolayer thickness (discussed above), the spectrum should also include 2 ML spectroscopic features. Therefore, we have assigned a set of new peaks (shown as a solid line in Figure 3c) to the different carbon atoms of the NTCDA molecules present within the second monolayer. To this end, it was necessary to add one single peak at 289.05 (C12ML) associated with the carbon atom of the anhydride group D


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Figure 4. Valence band data obtained by photoemission on different thicknesses of NTCDA grown on Cu(100) using a photon energy of 60 eV. The right panel displays an expanded view of the low-energystates region close to the Fermi level. The binding energy here was referred to the Fermi level. The measurements were taken at room temperature with an emission angle of 30° from the normal to the surface in order to increase the spectral weight of the molecular orbitals, as was demonstrated in ref 60.

Figure 5. (a) The C K-edge NEXAFS spectra of the NTCDA multilayer in comparison with NEXAFS data from monolayer system taken in the photon energy range from 282 to 292 eV. The spectra were measured by varying the incidence angle from θ = 15° (grazing geometry with the electric field vector nearly perpendicular to the surface plane) to θ = 90° (electric field vector parallel to the surface plane), as indicated in panel b. In panel c, the evolution of the π* resonance intensity is plotted as a function of the incidence angle measured on the multilayer system.

which is a signature of semiconductor character of the monolayer system. The 2 ML spectrum is very similar to that of the 1 ML system, except a slight attenuation of the Cu dband structures at the expense of the molecular orbitals of NTCDA. At multilayer coverage, the spectrum shows several resonances, indicated by a−g, that occur at higher binding energy, roughly from 4 to 12 eV, corresponding to the orbitals localized within the molecules; however, the F-LUMO feature at low energy is no longer observed. Due to the high thickness of the NTCDA film, the Cu d-bands are significantly reduced and the Fermi level has almost vanished, as shown in panel b of Figure 4, where one can still distinguish a tiny step due to the fact that the growth mode is not layer-by-layer. The two features f and g at 5.1 and 6.0 eV are assigned to the oxygen atoms of the anhydride group, and the rest are localized close to the naphthalene carbon atoms.62,63 NEXAFS Investigations. NEXAFS is very sensitive to both the bonding environment and the local structural configuration. Figure 5 shows a comparison of the C K-edge adsorption spectra of NTCDA on Cu(100) measured respectively at normal and grazing incidence geometries and this for both the monolayer and the multilayer systems. The π* adsorption resonance features appear in the range from 284 to 292 eV, while the σ* resonance lies at higher photon energy and is not shown here. Typically, there are four pronounced resonance structures associated with the carbon atoms within the molecule. According to previous studies, the π* adsorption resonances located at 284.1 and 285.7 eV are assigned to the excitations of C 1s electrons of the naphthalene core into different unoccupied molecular orbitals, whereas resonances at 287.8 and 288.9 eV are attributed to the excitations of the carbon atoms of the anhydride groups.61,64 An additional feature at 290.4 eV is systematically observed on the multilayer film. Such a feature can originate from the carbon linked to the hydrogen atoms. As illustrated in panel a of Figure 5, the π* resonance features are significantly broader in the monolayer system compared to those on the multilayer film. This broadening is due to the effect of the hybridization of substrate and adsorbate valence states, which would influence the unoccupied LUMO orbitals. Accordingly, such orbitals exhibit a band-type character having charge carriers relatively free and thus show large spectral widths. Regarding the molecule

orientation over the surface, it was inferred by a comparison between the normal and grazing photon incidence geometries. Upon varying the θ angle from 15° to 90°, all the π* resonances undergo great attenuation, which means in this case that the NTCDA molecules are orientated flat with π-orbitals pointing up. In contrast, the situation is different for the multilayer system, where the opposite behavior is observed: the intensity of the π* resonances is significantly enhanced at normal geometry and decreases sensitively when the photon beam becomes grazing with respect to the surface plane. However, the signal does not vanish completely, which indicates that the NTCDA molecules tend to orient in standing-up configuration. By analogy to earlier work, it can be stated that the interaction between the naphthalene core and the Cu substrate represents the determining parameter in adopting the f lat-lying orientation of the NTCDA monolayer. However, in the multilayer system the intermolecular interaction becomes dominant and leads to a significant change in the NTCDA arrangements, resulting in a standing-up configuration. From the angular dependence of the π* adsorption intensity, one can get an estimation of the average orientation of the molecules with respect to the surface plane. The molecular orientation within a layer can easily be derived from the relationship between the resonant intensities and incidence angle of light. In fact, it was well established that the resonant intensity is enhanced when the electric field vector (E) of synchrotron light polarization is parallel to the direction of the molecular orbital, and the intensity of the resonance is suppressed if E is perpendicular to the orbital direction. This approach involves a fitting of one resonance structure in order to obtain the peak area as a function of the photon beam angle. This method is well-established and has already been utilized very successfully in numerous studies.65−67 According to those studies, the adsorption intensity can be written as I(θ ) = CP(2 cos 2 θ cos 2 α + sin 2 θ sin 2 α) + C(1 − P) sin 2 α E

(1) DOI: 10.1021/acs.jpcc.6b12050 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C where C is a normalization constant, P is the polarization factor of the X-ray beam (in our case the degree of linear polarization is 100%), and α is the average tilt angle of the π* resonance orbital vector with respect to the surface normal. The best fitting suggests that the average angle α is about 68.8° ± 10°. This value indicates that the average orientation of the NTCDA molecules within a thick layer is slightly inclined from the surface normal. However, in the valence band spectra of a thick NTCDA layer, we still observe spectroscopic contributions from the first monolayer (the presence of a Fermi step in the spectrum). For that reason, it is most likely that the obtained tilt angle is due to the 3D-type growth mode of the organic film rather than an inclination of the NTCDA molecules. This means that the NEXAFS intensity is an average of two contributions: one from the first monolayer, with a flat-lying molecule orientation, and the second from the upper monolayers, where the NTCDA is standing up.

thank the Chinese Scholarship Council (CSC) for the Ph.D. financial support (scholarship). We thank also Karine Chaouchi for the technical support at the chemistry laboratory of SOLEIL.

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CONCLUSIONS To summarize, we provided here the first STM images of NTCDA submonolayer deposited in situ on a Cu(100) substrate, where highly packed structures with two different molecular domains were observed. In addition, the large mobility of the NTCDA molecules was obtained at room temperature despite the strong molecule−Cu bonding, which involves a charge transfer process from the metal to the organic monolayer, leading to complete filling of the LUMO orbital. The binding energy of this filled-LUMO orbital suggests a semiconductor character of the monolayer system. Upon increasing the coverage, electronic and structural changes were observed by XPS and NEXAFS. Quantitative fitting of the C 1s and O 1s XPS core levels demonstrates clearly significant shifts of the different components corresponding to the anhydride carbons and the naphthalene cores, indicating an evolution from covalent bonding of the NTCDA molecules with the Cu substrate to physisorption-type weak intermolecular interaction in the thick layer. Similar information can also be obtained from NEXAFS, where the π* resonance from both the anhydride and the naphthalene orbitals displays a dramatic modification from monolayer to multilayers. Moreover, an angular-dependent NEXAFS approach was used to probe the molecular orientation. The NEXAFS spectra show that the intensity of the π* resonance varies significantly with the incidence angle of the photon beam and reaches minimum and maximum values at θ = 15° and θ = 90°, respectively. From a quantitative analysis, we have found that the NTCDA molecules of the thick layer are ordered in a standing-up configuration with an average tilt angle of 68.8° from the substrate surface.

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REFERENCES

(1) Forrest, S. R. Ultrathin Organic Films Grown by Organic Molecular Beam Deposition and Related Techniques. Chem. Rev. 1997, 97, 1793−1896. (2) Bao, Z.; Lovinger, A. J.; Dodabalapur, A. Organic Field-Effect Transistors with High Mobility Based on Copper Phthalocyanine. Appl. Phys. Lett. 1996, 69, 3066−3068. (3) Greenham, N. C.; Friend, R. H. Semiconductor Device Physics of Conjugated Polymers. Solid State Phys. 1996, 49, 1−149. (4) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos-Santos, D. A.; Brédas, J. L.; Lögdlund, M.; et al. Electroluminescence in Conjugated Polymers. Nature 1999, 397, 121−128. (5) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Light-Emitting Diodes Based on Conjugated Polymers. Nature 1990, 347, 539−541. (6) Wang, S.; Wang, L.; Zhang, K.; Zhu, Z.; Tao, Z.; Chen, J. Organic Li4C8H2O6 Nanosheets for Lithium-Ion Batteries. Nano Lett. 2013, 13, 4404−4409. (7) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (8) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868−5923. (9) Hirose, Y.; Kahn, A.; Aristov, V.; Soukiassian, P.; Bulovic, V.; Forrest, S. R. Chemistry and Electronic Properties of Metal-Organic Semiconductor Interfaces: Al, Ti, In, Sn, Ag, and Au on PTCDA. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 13748−13758. (10) Moons, E. Conjugated Polymer Blends: Linking Film Morphology to Performance of Light Emitting Diodes and Photodiodes. J. Phys.: Condens. Matter 2002, 14, 12235−12260. (11) Hill, I. G.; Rajagopal, A.; Kahn, A.; Hu, Y. Molecular Level Alignment at Organic Semiconductor-Metal Interfaces. Appl. Phys. Lett. 1998, 73, 662−664. (12) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Controlling Molecular Deposition and Layer Structure with Supramolecular Surface Assemblies. Nature 2003, 424, 1029− 1031. (13) Schenning, A. P. H. J.; Meijer, E. W. Supramolecular Electronics; Nanowires from Self-Assembled π-Conjugated Systems. Chem. Commun. 2005, 26, 3245−3258. (14) Zwaneveld, N. A. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. Organized Formation of 2D Extended Covalent Organic Frameworks at Surfaces. J. Am. Chem. Soc. 2008, 130 (21), 6678−6679. (15) Abel, M.; Clair, S.; Ourdjini, O.; Mossoyan, M.; Porte, L. Single Layer of Polymeric Fe-Phthalocyanine: An Organometallic Sheet on Metal and Thin Insulating Film. J. Am. Chem. Soc. 2011, 133, 1203− 1205. (16) Mannsfeld, S. C. B.; Virkar, A.; Reese, C.; Toney, M. F.; Bao, Z. Precise Structure of Pentacene Monolayers on Amorphous Silicon Oxide and Relation to Charge Transport. Adv. Mater. 2009, 21, 2294− 2298. (17) Scheybal, A.; Mü ller, K.; Bertschinger, R.; Wahl, M.; Bendounan, A.; Aebi, P.; Jung, T. A. Modification of the Cu(110) Shockley Surface State by an Adsorbed Pentacene Monolayer. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 115406. (18) Käfer, D.; Ruppel, L.; Witte, G. Growth of Pentacene on Clean and Modified Gold Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 085309. (19) Barlow, S. M.; Raval, R. Complex Organic Molecules at Metal Surfaces: Bonding, Organisation and Chirality. Surf. Sci. Rep. 2003, 50, 201−341.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 (0)169 359799. ORCID

Azzedine Bendounan: 0000-0001-7557-4322 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the TEMPO beamline staff of Synchrotron SOLEIL for their assistance. Y.T. would like to F


Article

The Journal of Physical Chemistry C (20) Ueno, N.; Kera, S. Electron Spectroscopy of Functional Organic Thin Films: Deep Insights into Valence Electronic Structure in Relation to Charge Transport Property. Prog. Surf. Sci. 2008, 83, 490− 557. (21) Kakuta, H.; Hirahara, T.; Matsuda, I.; Nagao, T.; Hasegawa, S.; Ueno, N.; Sakamoto, K. Electronic Structures of the Highest Occupied Molecular Orbital Bands of a Pentacene Ultrathin Film. Phys. Rev. Lett. 2007, 98, 247601. (22) Cao, L.; Wang, Y. Z.; Zhong, J. Q.; Han, Y. Y.; Zhang, W. H.; Yu, X. J.; Xu, F. Q.; Qi, D. C.; Wee, A. T. S. Molecular Orientation and Site Dependent Charge Transfer Dynamics at PTCDA/TiO2(110) Interface Revealed by Resonant Photoemission Spectroscopy. J. Phys. Chem. C 2014, 118, 4160−4166. (23) Godlewski, S.; Tekiel, A.; Piskorz, W.; Zasada, F.; PrauznerBechcicki, J. S.; Sojka, Z.; Szymonski, M. Supramolecular Ordering of PTCDA Molecules: The Key Role of Dispersion Forces in an Unusual Transition from Physisorbed into Chemisorbed State. ACS Nano 2012, 6, 8536−8545. (24) Duhm, S.; Gerlach, A.; Salzmann, I.; Bröker, B.; Johnson, R. L.; Schreiber, F.; Koch, N. PTCDA on Au(111), Ag(111) and Cu(111): Correlation of Interface Charge Transfer to Bonding Distance. Org. Electron. 2008, 9, 111−118. (25) Kilian, L.; Hauschild, A.; Temirov, R.; Soubatch, S.; Schöll, A.; Bendounan, A.; Reinert, F.; Lee, T. L.; Tautz, F. S.; Sokolowski, M.; et al. Role of Intermolecular Interactions on the Electronic and Geometric Structure of a Large π-Conjugated Molecule Adsorbed on a Metal Surface. Phys. Rev. Lett. 2008, 100, 136103−136106. (26) Gustafsson, J. B.; Zhang, H. M.; Johansson, L. S. O. STM Studies of Thin PTCDA Films on Ag/Si(111)-√3 × √3. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 155414. (27) Stadtmüller, B.; Schröder, S.; Bocquet, F. C.; Henneke, C.; Kleimann, C.; Soubatch, S.; Willenbockel, M.; Detlefs, B.; Zegenhagen, J.; Lee, T. L.; et al. Adsorption Height Alignment at Heteromolecular Hybrid Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 161407R. (28) Schöll, A.; Kilian, L.; Zou, Y.; Ziroff, J.; Hame, S.; Reinert, F.; Umbach, E.; Fink, R. H. Disordering of an Organic Overlayer on a Metal Surface Upon Cooling. Science 2010, 329, 303−305. (29) Pshenichnyuk, S. A.; Komolov, A. S. Relation between Electron Scattering Resonances of Isolated NTCDA Molecules and Maxima in the Density of Unoccupied States of Condensed NTCDA Layers. J. Phys. Chem. A 2012, 116, 761−766. (30) Stadler, C.; Hansen, S.; Schöll, A.; Lee, T. L.; Zegenhagen, J.; Kumpf, C.; Umbach, E. Molecular Distortion of NTCDA upon Adsorption on Ag(111): A Normal Incidence X-ray Standing Wave Study. New J. Phys. 2007, 9, 50−63. (31) Teyssandier, J.; Battaglini, N.; Seydou, M.; Anquetin, G.; Diawara, B.; Sun, X.; Maurel, F.; Lang, P. Elaboration of HydrogenBonded 2D Supramolecular Assemblies on Au(111) From Solutions: Toward Naphthalene Tetracarboxylic Diimide-Melamine Nanoporous Networks. J. Phys. Chem. C 2013, 117, 8737−8745. (32) Seydou, M.; Teyssandier, J.; Battaglini, N.; Kenfack, G. T.; Lang, P.; Tielens, F.; Maurel, F.; Diawara, B. Characterization of NTCDI Supra-Molecular Networks on Au(111); Combining STM, IR and DFT Calculations. RSC Adv. 2014, 4, 25698−25708. (33) Perdigao, L. M. A.; Fontes, G. N.; Rogers, B. L.; Oxtoby, N. S.; Goretzki, G.; Champness, N. R.; Beton, P. H. Coadsorbed NTCDIMelamine Mixed Phases on Ag-Si (111). Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76 (24), 245402. (34) Lee, Y. L.; Hsu, H. L.; Chen, S. Y.; Yew, T. R. SolutionProcessed Naphthalene Diimide Derivatives as n-Type Semiconductor Materials. J. Phys. Chem. C 2008, 112, 1694−1699. (35) Nowakowski, R.; Seidel, C.; Fuchs, H. In situ” STM Studies of the Ordering Process of a Nonplanar Derivative of PTCDI on Ag(1 1 0). Surf. Sci. 2004, 562, 53−64. (36) Guillermet, O.; Glachant, A.; Hoarau, J. Y.; Mossoyan, J. C.; Mossoyan, M. Perylene Tetracarboxylic Diimide Ultrathin Film Deposition on Pt(1 0 0): A LEED, AES, REELS and STM Study. Surf. Sci. 2004, 548, 129−137.

(37) Seidel, C.; Schäfer, A. H.; Fuchs, H. Oriented growth of DMePTCDI on Ag(110); an LEED, XPS, SFM and STM Characterisation. Surf. Sci. 2000, 459, 310−322. (38) Ziroff, J.; Gold, P.; Bendounan, A.; Forster, F.; Reinert, F. Adsorption Energy and Geometry of Physisorbed Organic Molecules on Au(111) Probed by Surface-State Photoemission. Surf. Sci. 2009, 603, 354−358. (39) Tachikawa, H.; Kawabata, H. Electronic States of Alkali-MetalNTCDA Complexes: A DFT Study. Solid State Sci. 2015, 48, 141− 146. (40) Tsirkin, S. S.; Zaitsev, N. L.; Nechaev, I. A.; Tonner, R.; Höfer, U.; Chulkov, E. V. Inelastic Decay of Electrons in Shockley-Type Metal-Organic Interface States. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 235434. (41) Mura, M.; Gulans, A.; Thonhauser, T.; Kantorovich, L. Role of Van der Waals Interaction in Forming Molecule-Metal Junctions: Flat Organic Molecules on the Au(111) Surface. Phys. Chem. Chem. Phys. 2010, 12, 4759−4767. (42) Ziroff, J.; Hame, S.; Kochler, M.; Bendounan, A.; Schöll, A.; Reinert, F. Low-Energy Scale Excitations in the Spectral Function of Organic Monolayer Systems. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85, 161404R. (43) Jia, J.; Esaulov, V. A.; Bendounan, A. Adsorption and Desorption Kinetics of NTCDA Molecules on Ag(111)and Au(111) Surfaces Studied by Ion Scattering. Radiat. Eff. Defects Solids 2017, accepted. (44) Ai, Y.; Liu, Y.; Cui, T.; Varahramyan, K. Thin Film Deposition of n-Type Organic Semiconductor by Inkjet Printing Technique. Thin Solid Films 2004, 450, 312−315. (45) Strohmaier, R.; Ludwig, C.; Petersen, J.; Gompf, B.; Eisenmenger, W. STM Investigations of NTCDA on Weakly Interacting Substrates. Surf. Sci. 1996, 351, 292−302. (46) Forster, F.; Bendounan, A.; Reinert, F.; Grigoryan, V. G.; Springborg, M. The Shockley-Type Surface State on Ar Covered Au(111): High Resolution Photoemission Results and the Description by Slab-Layer DFT Calculations. Surf. Sci. 2007, 601, 5595−5604. (47) Bendounan, A. Adsorption Properties of Xe on Ag/Cu(111) System: Real-Time Photoemission Investigation. J. Phys. Chem. C 2016, 120, 24279−24286. (48) Forster, F.; Bendounan, A.; Ziroff, J.; Reinert, F. Importance of Surface States on the Adsorption Properties of Noble Metal Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 161408R. (49) Bendounan, A.; Aït-Ouazzou, S. Role of the Shockley State in Doping of Organic Molecule Monolayer. J. Phys. Chem. C 2016, 120, 11456−11464. (50) Gador, D.; Zou, Y.; Buchberger, C.; Bertram, M.; Fink, R.; Umbach, E. NEXAFS Investigations of NTCDA Monolayers on Different Metal Substrates. J. Electron Spectrosc. Relat. Phenom. 1999, 101−103, 523−528. (51) Ludwig, C.; Gompf, B.; Petersen, J.; Strohmaier, R.; Eisenmenger, W. STM Investigations of PTCDA and PTCDI on Graphite and MoS2: A Systematic Study of Epitaxy and STM Image Contrast. Z. Phys. B: Condens. Matter 1994, 93, 365−373. (52) Schöll, A.; Zou, Y.; Schmidt, T.; Fink, R.; Umbach, E. HighResolution Photoemission Study of Different NTCDA Monolayers on Ag(111): Bonding and Screening Influences on the Line Shapes. J. Phys. Chem. B 2004, 108, 14741−14748. (53) Kilian, L.; Stahl, U.; Kossev, I.; Sokolowski, M.; Fink, R.; Umbach, E. The Commensurate-to-Incommensurate Phase Transition of an Organic Monolayer: A High Resolution LEED Analysis of the Superstructures of NTCDA on Ag(111). Surf. Sci. 2008, 602, 2427− 2434. (54) Braatz, C. R.; Esat, T.; Wagner, C.; Temirov, R.; Tautz, F. S.; Jakob, P. Switching Orientation of Adsorbed Molecules: Reverse Domino on a Metal Surface. Surf. Sci. 2016, 643, 98−107. (55) Schöll, A.; Zou, Y.; Jung, M.; Schmidt, T.; Fink, R.; Umbach, E. Line Shapes and Satellites in High-Resolution X-Ray Photoelectron Spectra of the π-Conjugated Organic Molecules. J. Chem. Phys. 2004, 121, 10260−10261. G


Article

The Journal of Physical Chemistry C (56) Weißner, M.; Kübert, J.; Feyer, V.; Puschnig, P.; Schöll, A.; Reinert, F. Lateral Band Formation and Hybridization in Molecular Monolayers: NTCDA on Ag(110) and Cu(100). Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 075437. (57) Fink, R.; Gador, D.; Stahl, U.; Zou, Y.; Umbach, E. SubstrateDependent Lateral Order in Naphthalene-Tetracarboxylic-Dianhydride Monolayers. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 2818−2826. (58) Zou, Y.; Kilian, L.; Schöll, A.; Schmidt, T.; Fink, R.; Umbach, E. Chemical Bonding of PTCDA on Ag Surfaces and the Formation of Interface States. Surf. Sci. 2006, 600, 1240−1251. (59) Cho, S. W.; Newby, D., Jr.; DeMasi, A.; Smith, K. E.; Piper, L. F. J.; Jones, T. S. Determination of the Individual Atomic Site Contribution to the Electronic Structure of 3,4,9,10-Perylene Tetracarboxylic-Dianhydride (PTCDA). J. Chem. Phys. 2013, 139, 184711−184717. (60) Ziroff, J.; Forster, F.; Schöll, A.; Puschnig, P.; Reinert, F. Hybridization of Organic Molecular Orbitals with Substrate States at Interfaces: PTCDA on Silver. Phys. Rev. Lett. 2010, 104, 233004. (61) Bendounan, A.; Forster, F.; Schöll, A.; Batchelor, D.; Ziroff, J.; Umbach, E.; Reinert, F. Electronic Structure of 1 ML NTCDA/ Ag(111) Studied by Photoemission Spectroscopy. Surf. Sci. 2007, 601, 4013−4017. (62) Gador, D.; Buchberger, C.; Fink, R.; Umbach, E. Characterization of High-Quality NTCDA Films on Metal Substrates. J. Electron Spectrosc. Relat. Phenom. 1998, 96, 11−17. (63) Baston, U.; Jung, M.; Umbach, E. Different Bonding Configurations of the Aromatic Anhydride NDCA on Ni(111) and ONi(111): A Photoemission and TDS Study. J. Electron Spectrosc. Relat. Phenom. 1996, 77, 75−89. (64) Schöll, A.; Zou, Y.; Huebner, D.; Urquhart, S. G.; Schmidt, T.; Fink, R.; Umbach, E. A Comparison of Fine Structures in HighResolution X-Ray-Absorption Spectra of Various Condensed Organic Molecules. J. Chem. Phys. 2005, 123, 044509−044523. (65) Sing, M.; Meyer, J.; Hoinkis, M.; Glawion, S.; Blaha, P.; Gavrila, G.; Jacobsen, C. S.; Claessen, R. Structural vs Electronic Origin of Renormalized Band Widths in TTF-TCNQ: An Angular Dependent NEXAFS Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 245119. (66) de Jong, M. P.; Friedlein, R.; Sorensen, S. L.; Ö hrwall, G.; Osikowicz, W.; Tengsted, C.; Jönsson, S. K. M.; Fahlman, M.; Salaneck, W. R. Orbital-Specific Dynamic Charge Transfer from Fe(II)-Tetraphenylporphyrin Molecules to Molybdenum Disulfide Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 035448. (67) Cerruti, M.; Rhodes, C.; Losego, M.; Efremenko, A.; Maria, J. P.; Fischer, D.; Franzen, S.; Genzer, J. Influence of Indium-Tin Oxide Surface Structure on the Ordering and Coverage of Carboxylic Acid and Thiol Monolayers. J. Phys. D: Appl. Phys. 2007, 40, 4212−4221.

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