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Studies of the adsorption of tetraphenylporphyrin molecules on graphite
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Surface Science 601 (2007) 5526–5532 www.elsevier.com/locate/susc
Studies of the adsorption of tetraphenylporphyrin molecules on graphite M. Scarselli
a,*
, P. Castrucci a, D. Monti b, M. De Crescenzi
a
a
b
Dipartimento di Fisica and CNISM, Universita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1-00133, Roma, Italy Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1-00133, Roma, Italy Received 23 April 2007; accepted for publication 14 September 2007 Available online 22 September 2007
Abstract Combined scanning tunneling microscopy, reflection electron energy loss spectroscopy and X-ray photoelectron studies have been performed in situ under ultra high vacuum condition, on tetraphenylporphyrin molecules (H2TPP) vacuum sublimated on highly oriented pyrolitic graphite. The experimental studies were performed at room temperature, as a function of the amount of deposited porphyrins. The propensity of H2TPP to self-assembly on the graphite surface could be detected after a threshold of deposited material. In this case tetraphenylporphyrin molecules arranged according to a quasi-hexagonal lattice separated from their nearest neighbours by a minimum distance of about 1 nm. The formation of an additional incomplete layer, at a slightly higher coverage, was also detected where the quasi-hexagonal symmetry is retained. Finally, subsequent tetraphenylporphyrins depositions gave molecular aggregates randomly distributed on the graphite surface with subsequent loss of order. 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption; Scanning tunneling microscopy; Electron energy loss spectroscopy; X-ray photoelectron spectroscopy
1. Introduction The quest for morphologically defined molecular assemblies, with controlled size and shape, onto solid surfaces, is a mandatory issue not only in materials science [1] but also in the development of molecule-based nano-devices [2]. In this respect porphyrin derivatives and related congeners are playing an increasingly important role as molecular components of optical devices [3], chemical sensors [4] and functional supramolecular materials [5]. The deposition of ordered porphyrins layers on solid surfaces with controlled features can be obtained by following different methods. Mostly important are self-assembly of monolayers on gold [6], Langmuir–Blodgett or Langmuir–Schaefer techniques [7–9], liquid-phase deposition [10–15] and vacuum sublimation [16–18]. Porphyrin derivatives have been deposited and characterized in air [10–13], in ultra high vacuum condition *
Corresponding author. Tel.: +39 06 72594116; fax: +39 06 2023507. E-mail address: [email protected] (M. Scarselli).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.09.024
(UHV) [14–18] and under liquid [13,20–22], after deposition on different single crystal surfaces including metals [13–22] and highly oriented pyrolitic graphite (HOPG) [10–15]. The majority of the studies on ordered molecular adlayers on HOPG, mainly deal with either simple metalloporphyrins or free-base porphyrin derivatives bearing ad hoc functional groups, such as peripheral alkyl chains, or carboxylic substituents, which exert stabilization and ordering by Van der Waals forces or hydrogen bonding, respectively [10–12]. Recent STM studies on adlayers from simple octaethylporphyrin derivatives on HOPG in dichlorobenzene solution showed assembling geometries significantly influenced by subtle differences in the molecular structures [13]. It appears that moderately functionalized porphyrins tend to give unpredictable and intriguing arrangements that deserve a thoroughly investigation to figure out the role played by molecular shapes, sizes, and interactions on adlayer patterns. Accordingly, we focused on the study of the simple 5,10,15,20-tetraphenyl-21H, 23H-porphine (i.e. tetraphenylporphyrin, H2TPP hereafter), sublimated
M. Scarselli et al. / Surface Science 601 (2007) 5526–5532
under UHV conditions on HOPG, through in situ STM, reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS). 2. Experimental The synthesis of the title porphyrin was accomplished by following published methods [23]. Analytical and spectroscopic characterizations (UV–visible, 1H NMR, FABMS) were in full agreement to those reported in the literature. Pyrrole (14.5 g; 0.216 mol) was added over the course of 30 min to a refluxing solution of benzaldehyde (23 g, 0.216 mol) in acetic acid. The resulting mixture was refluxed for additional 3 h, then cooled to room temperature and left standing overnight. The bright-purple crystals separated from the bulk solution were collected, washed (MeOH) and chromatographed (SiO2, CHCl3, then to give the desired product (11,2 g, 0.18 mol; 32% yield). A sketch of the molecule is reported in Fig. 1a together with the space filling model Fig. 1b, [24]. The tetraphenylporphyrin consists of a flat tetrapyrrole monocycle, having four phenyl substituents in the meso position. These substituents lie in an orthogonal plane with respect to the main plane of the monocycle. However, they are free to rotate upon energetic or structural constraints (e.g. steric hindrance). Freshly cleaved highly oriented pyrolitic graphite (HOPG) was introduced in the preparation chamber (base pressure low 1010 Torr) and vacuum sublimation was performed by heating a tungsten filament previously dipped in a 1% H2TPP chloroform solution. The source was outgassed and stabilized before each sublimation cycle. The temperature reached as a function of the current flowing in the W filament was calibrated in preliminary experiments. During each sublimation cycle the temperature was kept constant at about 560 K while the base pressure was kept as low as 107 Torr. The quantity of sublimated molecules was 0.1 ML min1 at a temperature of 560 K. One monolayer (1 ML) of tetraphenylpoprhyrin (H2TPP) has been defined as one molecule per 9.46 nm2 from the density of the molecular arrangement calibrated form largescale STM images. After the sublimation process the sample
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was transferred into the main chamber without breaking the vacuum. Several samples differing in the amount of deposited H2TPP molecules have been prepared. Each sample was first studied in situ at room temperature, by scanning tunneling microscopy (STM) and subsequently by X-ray photoelectron spectrosocopy (XPS) and reflection electron energy loss spectroscopy (REELS). Scanning tunneling microscopy imaging was performed at room temperature using an Omicron-STM system. Electrochemically etched tungsten tips were employed for the experiments. Generally, tips required a UHV cleaning procedure (performed in with Ar+ ion bombardment) before use in order to improve the resolution. STM calibration was done by comparing images of molecular adsorbates with atomically resolved ones of graphite. All images were unfiltered apart from rigid plane subtraction. X-ray photoelectron spectrosocopy data were measured with a semi-imaging analyser MAC 2 (Riber Instruments) operating in the constant pass energy mode (with a total energy resolution of 1.4 eV), using non-monochromatized Mg Ka (1253.6 eV) radiation source (9 kV, 700 W). The distance between the sample and the anode is about 40 mm, the illumination area was about 5 · 5 mm2 and the takeoff angle between the sample surface and the photoelectron energy analyser was kept fixed at 45. No angular dependence was detected. This could be due to an inhomogeneity in the coverage of the graphite surface and also to the low thickness of the assembled molecules on graphite. The energy scale was calibrated with reference to the binding energy of the C1s from cleaved graphite, measured to be 284.7 eV with respect to the Fermi level. Extended energy distribution spectra were first recorded, followed by high-resolution scans over N1s photoelectron binding energy regions. XPS spectra were analysed using a standard Gaussian curve fit routine with a Shirley background subtraction [25], the quality of the fit was evaluated by using v2 minimization test. Reflection electron energy loss spectroscopy measurements were carried out in the constant pass energy mode (with a total energy resolution of 0.8 eV) with the same semi-imaging analyser MAC 2 (Riber Instruments) used for XPS experiments. Incident electrons were emitted by a tungsten filament of an electron gun (VG Instruments) set at a constant kinetic primary energy (Ep) of 300 eV. The incidence angle between the beam and the sample was kept fixed at less than 30 to ensure that the incident electrons have little probability to penetrate into the sample surface. Energy loss data were recorded on graphite soon after cleavage and after porphyrin deposition. Loss spectra were analysed using a standard Gaussian curve fit routine checked using v2 minimization test.
3. Results and discussion Fig. 1. (a) Chemical structure of H2TPP, the four phenyl groups are substituted at meso positions of the central free-base porphyrin and (b) correspondent space filling model.
Porphyrin molecules deposited on HOPG by UHV sublimation at room temperature, were initially studied by
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STM in situ to investigate on their spatial arrangement. At very low exposure time, only isolated molecules randomly distributed on the graphite surface were imaged. As an example we report in Fig. 2a, an STM image of two isolated molecules on HOPG. The molecules arrange on the substrate at a mean distance of about 1.8 nm. We deduce from several STM images collected, that at low coverages the interaction is rather weak and not sufficient to generate a stable chemical bond between them. In addition, the stable interaction with the HOPG substrate has been tested from the reproducibility of the images obtained after repeated scans on the same area. In Fig. 2b, we report an STM image obtained on a single molecule in which the choice of the colour palette makes the single H2TPP molecule appear in darker colour with respect to the HOPG atomic substrate. The molecule maintains its four-fold symmetry dominated by the central empty porphyrin core and the four phenyl substituents groups, which allows to identify the conformation and orientation. It is well known that molecule–substrate interaction can alter the ground state conformation of the molecule forcing the phenylporphiryn single bonds to be rotated [22]. In this case, the dimensions obtained from the profile plot (from such image) suggest that the molecule lay completely flat on the HOPG substrate with the phenyl groups oriented approximately parallel to the molecular core. The orientation of the molecule with respect to the atomic HOPG substrate evaluated from this image results to be about 30. By increasing the amount of deposited H2TPP molecules (0.3–0.5 ML), areas on the graphite surface turned out to
Fig. 2. (a) STM image of two H2TPP molecules on the HOPG substrate (6 nm · 6 nm). The molecules on the substrate lay apart at a mean distance, of about 1.8 nm, comparable to that obtained in the case of selfassembly and (b) STM image of a single H2TPP molecules on the HOPG (2.5 nm · 2.5 nm). The choice of the colour palette makes the single H2TPP molecule appear in darker colour with respect to the HOPG atomic substrate. The molecule maintains its four-fold symmetry dominated by the central empty porphyrin core and the four phenyl substituents groups, which allows to identify the conformation and orientation with respect to the substrate, which results to be planar with the phenyl substituents oriented almost parallel to the HOPG. The dimensions obtained from such image suggest that the H2TPP lay completely flat on the HOPG substrate with the phenyl groups oriented approximately parallel to the molecular core. (I = 400 pA, Vsample = 400 mV).
be covered by ordered molecular adlayers. In particular, it was possible to observe, from several images collected on different sample areas, that the ordered regions extend over hundreds of square nanometers, as shown in Fig. 3a. Images obtained on a smaller area of the adlayer, as the one reported in Fig. 3b, show bright emerging structures, mainly circular in shape, arranged (within the experimental error) in a quasi-hexagonal pattern, reminiscent of that of the graphite substrate with a measured periodicity of about 3.2 nm along the molecular rows. The two dimensional (2D) unit cell, sketched in the figure, is nearly hexagonal (cell vectors: a = 3.18 ± 0.10 nm; b = 3.12 ± 0.10 nm; a = 61 ± 2, a is the angle between the two main vectors of the unit cell). Numerous STM images were collected on similarly ordered areas by changing the bias voltage and set-point current, without revealing an appreciable variation of the mean height and lateral dimension of the molecules in the assembly. In addition, no dependence of image contrast on the polarity of the applied sample voltage was detected. We also compared the dimensions of the H2TPP molecules as reported in the literature data with those evaluated on the average from profile plots like that reported in Fig. 1. The mean diameter (1.80 ± 0.10 nm) obtained, matches the distance separating the hydrogens in the para positions of opposite phenyl rings of the H2TPP structure. This distance turns out to be 1.77 nm, as obtained by hybrid density-functional theory geometry optimization carried out at the B3LYP/6-31G(d) level of theory [26,27]. In addition, the mean height of molecules in the layer evaluated from the same profiles (0.35 ± 0.10 nm) supports the hypothesis that H2TPP molecules assume a planar geometry on HOPG surface [26]. It appears that even under the best STM experimental conditions adopted, the inner structure of the H2TPP molecules remains unresolved. High-resolution has been observed in the case of simple porphyrins deposited on HOPG mainly when imaged under liquid [13,20]. It has been also observed that the addition of ad-hoc functional groups to free-base porphyrin such as carboxylic substituents or hydrocarbon chains, exerts stabilization and ordering and improves the molecular resolution [10–12,15]. In the case of metal-porphyrins the molecular conformation was clearly determined by STM [10]. In fact, the insertion of metal atoms with their specific and quite localized structure can determine a significant adjustment of the electronic density of states, thus making it possible to increase the contrast for optimal molecular resolution. In order to get additional information on the H2TPP arrangement on the HOPG surface, we acquired STM data at appropriate values of the sample bias and tunneling current so that the molecular film together with the atomic HOPG substrate could be resolved, as reported in Fig. 4a. In this image, the molecular film covers part of a HOPG plane up to the step edge. Once the lattice of the HOPG is well localized together with the molecular structure we have performed the Fourier transform of the image and this evidences the co-existence of the two lattices (one
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Fig. 3. (a) STM image of the HOPG partly covered with H2TPP molecules (150 nm · 150 nm), (b) STM image obtained on a smaller area (25 nm · 25 nm). Unit cell: a = 3.18 ± 0.10 nm; b = 3.12 ± 0.10 nm; a = 61 ± 2. (I = 500 pA, Vsample = 250 mV), and (c) profile plot obtained along the direction shown in (b).
Fig. 4. (a) STM image in which the H2TPP molecular film extending on a HOPG plane together with the atomic HOPG substrate (25 nm · 25 nm) is visible (I = 550 pA, Vsample = 250 mV) and (b) Fourier transform evaluated from the image in which two misoriented hexagonal lattices appear correspondent to 0.24 nm distance between second nearest neighbours in the HOPG basal plane (outer hexagon), compared to 3.2 nm of the molecular lattice in the plane (inner hexagon).
of the HOPG and the other to the H2TPP film) with nearly hexagonal symmetry and allows the assessment of their relative orientation, as reported in Fig. 4b. In order to check the registry of the H2TPP overlayer with the graphite substrate we also evaluate the epitaxy matrix C that relates the overlayer unit cell vectors (b1, b2) with those of the substrate lattice (a1, a2) [28]. The
lattice parameters of the H2TPP overlayer and the HOPG substrate, in which only ß atoms contribute to the STM image [29], have been evaluated by averaging the lattice values obtained from several images. b1 a1 9:4 5:7 a1 ¼C ¼ : b2 a2 a2 5:6 14:8
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Within the experimental error, no element of the C matrix is integer, so we conclude that the H2TPP overlayer is not commensurate with the underlying graphite substrate. Although this result could suggest that molecules in the adlayer have no preferential adsorption site on the substrate, the huge intermolecular distance observed in the adlayer together with the quasi-hexagonal symmetry in the cell cannot exclude an interaction of the molecules with the graphite substrate. In this regard, the information obtained from STM images relative to a self-assembly arrangement on graphite has been combined with that obtained from EELS and X-Ray spectroscopy, in order to point out the presence of the molecules, investigate about their integrity and interaction with the HOPG surface. Fig. 5 the results of the reflection energy loss spectra obtained first on graphite after cleavage (a) and after a deposition of tetraphenylporphyrin correspondent to an observed self-assembly (b) are reported. The most significant p–p* and r–r* transitions of graphitic carbon [30] were found to be significantly modified after molecule deposition. In particular the original energy position of the two main transitions located at about 5.8 eV and 25.9 eV in the HOPG spectrum (a) are shifted at about 6.9 eV and 24 eV, respectively (b). We suggest that these transitions maintain their electronic nature when occurring in the basal plane and in the perpendicular direction of HOPG. However, they turn out to be significantly influenced by the p–p interactions between the adsorbed porphyrins and the substrate [31]. The REELS spectrum of the graphite after H2TPP deposition also shows an additional p–p* feature located around 3.0 eV. This new contribution is related to the transition involving HOMO and LUMO molecular states of the porphyrins and can be associated to their optical absorption, usually referred as Soret band which, for these systems, occurs at around 2.8 eV [32–34].
Fig. 5. REELS (Ep = 300 eV) spectra obtained on clean HOPG (a) and after H2TPP deposition (b).
The XPS study was performed on graphite after cleavage and then after tetraphenylporphyrin sublimation. Extended energy distribution spectra were first recorded as reported in Fig. 6a–b. Line spectrum (a), has been obtained on cleaved graphite, while the line spectrum (b), has been obtained after a deposition of tetraphenylporphyrin corresponding to a self-assembly arrangement on graphite, as observed by STM (see Fig. 3). From this data we detected an additional contribution from nitrogen N1s, while both C1s and O1s signals increased. Therefore, XPS investigation after H2TPP sublimation, was mainly focused on the binding energy of 1 s core level of nitrogen, as reported in Fig. 7.
Fig. 6. Extended XPS (hm = 1253.6 eV) spectra obtained on HOPG (a), after H2TPP deposition (b).
Fig. 7. XPS (hm = 1253.6 eV) N1s core level spectrum obtained on HOPG, after H2TPP deposition. Empty circles are experimental data (s). Solid lines represent the overlapping components obtained by nonlinear curve fitting. Small dots (d) are fitting data calculated by summing up the three gaussian components.
M. Scarselli et al. / Surface Science 601 (2007) 5526–5532
The nonlinear curve fitting of the observed intensities evidences two main features at 399.5 eV and 397.6 eV. These correspond to the pyrrole-type nitrogen and the imine-type ones, respectively, as previously reported for similar free-base porphyrin derivatives. The energy difference between the N1s pyrrole-type and imine-type energies is about 1.9 eV [35]. An additional feature is also detected at 401.6 eV; this might be associated to the interaction between carbon atoms of the graphite surface and the H2 TTP molecules. The same satellite structure has already been observed in both the free-base porphyrin and in its related Zn complex [35]. A careful comparison of the energy position of this core peak with the kinetic energy of the EELS structure associated to the HOMO–LUMO transition for the H2TPP molecule also supports this hypothesis. Both the resulting assembling geometry and the indication given by the spectroscopy data, suggest that, although the HOPG surface is almost inert, it interacts significantly with molecules. At higher H2TPP coverages (greater than 1 ML), the graphite surface is completely modified by porphyrin molecules as shown in Fig. 8, where the first complete adlayer
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is partially covered with additional structures on top. In particular, along the two graphite step edges in the upper right part of the figure, we observed the formation of new rod-like structures which suggest a three-dimensional arrangement of the molecules. Although the additional layer is incomplete, from the inset of Fig. 8 and the profile plot reported, it is possible to see that molecules have a preferential binding site on top of those of the layer underneath. Nevertheless, it must be underlined that the molecular height registered is about 1–2 nm. This value is the result of a modification of the electronic properties associated to a morphological rearrangement of the porphyrins in the adlayer. Therefore, we cannot exclude that the observed molecular height originates from a columnar structure made of porphyrins emerging from the ordered layer underneath. The formation of columnar structures has already been observed in the case of porphyrin aggregates [11,36]. In any case, the conclusion drawn from the profile plot about the position of the additional molecules in the adlayer remains valid. As a consequence, the ordering tendency driven by the graphite substrate is reduced as the additional porphyrin molecules bind to those in the layer underneath and experience a weaker interaction with the graphite substrate. Finally, STM images obtained after subsequent H2TPP depositions (greater than 1.3 ML), show a highly disordered surface (data not shown) covered with a uniform background made of molecular aggregates mostly spherical in shape randomly distributed on the surface. From the results obtained it appears that a layer-bylayer growth is difficult to obtain for this system while in the literature has been observed for more complex porphyrin molecules [19]. 4. Conclusions
Fig. 8. STM image of the HOPG covered with H2TPP molecules (200 nm · 200 nm). Inset: STM image obtained on a smaller area (20 nm · 20 nm). The profile plot helps to pin point the position occupied by the additional molecule in the new layer. (I = 400 pA, Vsample = 50 mV).
Ultra high vacuum sublimation showed to be an easy method to deposit simple tetraphenylporphyrin on highly oriented pyrolitic graphite. It has been possible to monitor that up to critical degree of coverage H2TPP molecules selfassembly on the surface and a well-ordered layer structure is formed. In particular STM showed that the constituent H2TPP molecules arranged according to a quasi-hexagonal lattice with a large periodicity. The information obtained from STM data has been linked with XPS and REELS measurements to investigate about the orientation and conformation of the molecules when adsorbed on the surface. We conclude that the packing of the molecular adlayer is controlled by intermolecular interactions while the orientation of the layer is induced by the coupling to the HOPG substrate. This surface-mediated symmetry has been observed for one complete layer and an additional partial second one. In the second layer the symmetry is mainly driven by the preferential adaptation of the additional molecules with the underlying ordered first layer.
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The results obtained indicate that the design and construction of thin film of tetraphenylporphyrin molecules with controlled features can be obtained within a limited quantity of molecules. Acknowledgement We thank Prof. G. Ercolani of the Dipartimento di Scienze e Tecnologie Chimiche of the University of Rome Tor Vergata, for the calculation of the geometry of the molecular structure and useful discussion. References [1] C.F.J. Faul, M. Antonietti, Adv. Mater. 15 (2003) 673. [2] J.M. Tour, Acc. Chem. Res. 33 (2000) 791; C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541. [3] M.P. Debreczeny, W.A. Svec, M.R. Wasielewski, Science 274 (1996) 584. [4] R. Paolesse, F. Mandoj, A. Marini, C. Di Natale, in: H.S. Nalwa (Ed.), Porphyrin-Based Chemical Sensors in Encyclopedia of Nanoscience and Nanotechnology, vol. 9, American Scientific Publishers CA, 2004, p. 21. [5] C.F. Van Nostrum, R.J.M. Nolte, Chem. Commun (1996) 2385. [6] D.T. Gryko, C. Clausen, J.S. Lindsey, J. Org. Chem. 64 (1999) 8635; S. Chiang, Chem. Rev. 97 (1997) 1083; Z. Zhang, S. Hou, Z. Zhu, Z. Liu, Langmuir 16 (2000) 537. [7] A. Ulmann, An Introduction of Ultrathin Organic Films, from Langmuir–Blodgett Films to Self-Assembly, Academic Press, San Diego, CA, 1991. [8] L. Zhang, J. Yuan, M. Liu, J. Phys. Chem. B 107 (2003) 12768. [9] Y. Ni, R.R. Puthenkovilakom, Q. Huo, Langmuir 20 (2004) 2765. [10] H. Wang, C. Wang, Q. Zeng, S. Xu, S. Yin, B. Xu, C. Bai, Surf. Interf. Anal. 32 (2001) 266; X. Qiu, C. Wang, Q. Zeng, B. Xu, S.X. Yin, H. Wang, S. Xu, C. Bai, J. Am. Chem. Soc. 122 (2000) 5550. [11] Y. Zhou, B. Wang, M. Zhu, J.G. Hou, Chem. Phys. Lett. 403 (2005) 140. [12] S.B. Lei, C. Wang, S.X. Yin, H.N. Wang, F. Xi, H. Liu, W.B. Xu, L.J. Wan, C.L. Bai, J. Phys. Chem. B 105 (2001) 10838. [13] Z.-Q. Zou, L. Wei, F. Chen, Z. Liu, P. Thamyongkit, R.S. Loewe, J.S. Lindsey, U. Mohideen, D.F. Bocian, J. Porphy. Phthalocyan. 9 (2005) 387. [14] A. Ogunrinde, K.W. Hipps, L. Scudiero, Langmuir 22 (2006) 5697.
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Surface Science 601 (2007) 5526–5532 www.elsevier.com/locate/susc
Studies of the adsorption of tetraphenylporphyrin molecules on graphite M. Scarselli
a,*
, P. Castrucci a, D. Monti b, M. De Crescenzi
a
a
b
Dipartimento di Fisica and CNISM, Universita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1-00133, Roma, Italy Dipartimento di Scienze e Tecnologie Chimiche, Universita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica 1-00133, Roma, Italy Received 23 April 2007; accepted for publication 14 September 2007 Available online 22 September 2007
Abstract Combined scanning tunneling microscopy, reflection electron energy loss spectroscopy and X-ray photoelectron studies have been performed in situ under ultra high vacuum condition, on tetraphenylporphyrin molecules (H2TPP) vacuum sublimated on highly oriented pyrolitic graphite. The experimental studies were performed at room temperature, as a function of the amount of deposited porphyrins. The propensity of H2TPP to self-assembly on the graphite surface could be detected after a threshold of deposited material. In this case tetraphenylporphyrin molecules arranged according to a quasi-hexagonal lattice separated from their nearest neighbours by a minimum distance of about 1 nm. The formation of an additional incomplete layer, at a slightly higher coverage, was also detected where the quasi-hexagonal symmetry is retained. Finally, subsequent tetraphenylporphyrins depositions gave molecular aggregates randomly distributed on the graphite surface with subsequent loss of order. 2007 Elsevier B.V. All rights reserved. Keywords: Adsorption; Scanning tunneling microscopy; Electron energy loss spectroscopy; X-ray photoelectron spectroscopy
1. Introduction The quest for morphologically defined molecular assemblies, with controlled size and shape, onto solid surfaces, is a mandatory issue not only in materials science [1] but also in the development of molecule-based nano-devices [2]. In this respect porphyrin derivatives and related congeners are playing an increasingly important role as molecular components of optical devices [3], chemical sensors [4] and functional supramolecular materials [5]. The deposition of ordered porphyrins layers on solid surfaces with controlled features can be obtained by following different methods. Mostly important are self-assembly of monolayers on gold [6], Langmuir–Blodgett or Langmuir–Schaefer techniques [7–9], liquid-phase deposition [10–15] and vacuum sublimation [16–18]. Porphyrin derivatives have been deposited and characterized in air [10–13], in ultra high vacuum condition *
Corresponding author. Tel.: +39 06 72594116; fax: +39 06 2023507. E-mail address: [email protected] (M. Scarselli).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.09.024
(UHV) [14–18] and under liquid [13,20–22], after deposition on different single crystal surfaces including metals [13–22] and highly oriented pyrolitic graphite (HOPG) [10–15]. The majority of the studies on ordered molecular adlayers on HOPG, mainly deal with either simple metalloporphyrins or free-base porphyrin derivatives bearing ad hoc functional groups, such as peripheral alkyl chains, or carboxylic substituents, which exert stabilization and ordering by Van der Waals forces or hydrogen bonding, respectively [10–12]. Recent STM studies on adlayers from simple octaethylporphyrin derivatives on HOPG in dichlorobenzene solution showed assembling geometries significantly influenced by subtle differences in the molecular structures [13]. It appears that moderately functionalized porphyrins tend to give unpredictable and intriguing arrangements that deserve a thoroughly investigation to figure out the role played by molecular shapes, sizes, and interactions on adlayer patterns. Accordingly, we focused on the study of the simple 5,10,15,20-tetraphenyl-21H, 23H-porphine (i.e. tetraphenylporphyrin, H2TPP hereafter), sublimated
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under UHV conditions on HOPG, through in situ STM, reflection electron energy loss spectroscopy (REELS) and X-ray photoelectron spectroscopy (XPS). 2. Experimental The synthesis of the title porphyrin was accomplished by following published methods [23]. Analytical and spectroscopic characterizations (UV–visible, 1H NMR, FABMS) were in full agreement to those reported in the literature. Pyrrole (14.5 g; 0.216 mol) was added over the course of 30 min to a refluxing solution of benzaldehyde (23 g, 0.216 mol) in acetic acid. The resulting mixture was refluxed for additional 3 h, then cooled to room temperature and left standing overnight. The bright-purple crystals separated from the bulk solution were collected, washed (MeOH) and chromatographed (SiO2, CHCl3, then to give the desired product (11,2 g, 0.18 mol; 32% yield). A sketch of the molecule is reported in Fig. 1a together with the space filling model Fig. 1b, [24]. The tetraphenylporphyrin consists of a flat tetrapyrrole monocycle, having four phenyl substituents in the meso position. These substituents lie in an orthogonal plane with respect to the main plane of the monocycle. However, they are free to rotate upon energetic or structural constraints (e.g. steric hindrance). Freshly cleaved highly oriented pyrolitic graphite (HOPG) was introduced in the preparation chamber (base pressure low 1010 Torr) and vacuum sublimation was performed by heating a tungsten filament previously dipped in a 1% H2TPP chloroform solution. The source was outgassed and stabilized before each sublimation cycle. The temperature reached as a function of the current flowing in the W filament was calibrated in preliminary experiments. During each sublimation cycle the temperature was kept constant at about 560 K while the base pressure was kept as low as 107 Torr. The quantity of sublimated molecules was 0.1 ML min1 at a temperature of 560 K. One monolayer (1 ML) of tetraphenylpoprhyrin (H2TPP) has been defined as one molecule per 9.46 nm2 from the density of the molecular arrangement calibrated form largescale STM images. After the sublimation process the sample
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was transferred into the main chamber without breaking the vacuum. Several samples differing in the amount of deposited H2TPP molecules have been prepared. Each sample was first studied in situ at room temperature, by scanning tunneling microscopy (STM) and subsequently by X-ray photoelectron spectrosocopy (XPS) and reflection electron energy loss spectroscopy (REELS). Scanning tunneling microscopy imaging was performed at room temperature using an Omicron-STM system. Electrochemically etched tungsten tips were employed for the experiments. Generally, tips required a UHV cleaning procedure (performed in with Ar+ ion bombardment) before use in order to improve the resolution. STM calibration was done by comparing images of molecular adsorbates with atomically resolved ones of graphite. All images were unfiltered apart from rigid plane subtraction. X-ray photoelectron spectrosocopy data were measured with a semi-imaging analyser MAC 2 (Riber Instruments) operating in the constant pass energy mode (with a total energy resolution of 1.4 eV), using non-monochromatized Mg Ka (1253.6 eV) radiation source (9 kV, 700 W). The distance between the sample and the anode is about 40 mm, the illumination area was about 5 · 5 mm2 and the takeoff angle between the sample surface and the photoelectron energy analyser was kept fixed at 45. No angular dependence was detected. This could be due to an inhomogeneity in the coverage of the graphite surface and also to the low thickness of the assembled molecules on graphite. The energy scale was calibrated with reference to the binding energy of the C1s from cleaved graphite, measured to be 284.7 eV with respect to the Fermi level. Extended energy distribution spectra were first recorded, followed by high-resolution scans over N1s photoelectron binding energy regions. XPS spectra were analysed using a standard Gaussian curve fit routine with a Shirley background subtraction [25], the quality of the fit was evaluated by using v2 minimization test. Reflection electron energy loss spectroscopy measurements were carried out in the constant pass energy mode (with a total energy resolution of 0.8 eV) with the same semi-imaging analyser MAC 2 (Riber Instruments) used for XPS experiments. Incident electrons were emitted by a tungsten filament of an electron gun (VG Instruments) set at a constant kinetic primary energy (Ep) of 300 eV. The incidence angle between the beam and the sample was kept fixed at less than 30 to ensure that the incident electrons have little probability to penetrate into the sample surface. Energy loss data were recorded on graphite soon after cleavage and after porphyrin deposition. Loss spectra were analysed using a standard Gaussian curve fit routine checked using v2 minimization test.
3. Results and discussion Fig. 1. (a) Chemical structure of H2TPP, the four phenyl groups are substituted at meso positions of the central free-base porphyrin and (b) correspondent space filling model.
Porphyrin molecules deposited on HOPG by UHV sublimation at room temperature, were initially studied by
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STM in situ to investigate on their spatial arrangement. At very low exposure time, only isolated molecules randomly distributed on the graphite surface were imaged. As an example we report in Fig. 2a, an STM image of two isolated molecules on HOPG. The molecules arrange on the substrate at a mean distance of about 1.8 nm. We deduce from several STM images collected, that at low coverages the interaction is rather weak and not sufficient to generate a stable chemical bond between them. In addition, the stable interaction with the HOPG substrate has been tested from the reproducibility of the images obtained after repeated scans on the same area. In Fig. 2b, we report an STM image obtained on a single molecule in which the choice of the colour palette makes the single H2TPP molecule appear in darker colour with respect to the HOPG atomic substrate. The molecule maintains its four-fold symmetry dominated by the central empty porphyrin core and the four phenyl substituents groups, which allows to identify the conformation and orientation. It is well known that molecule–substrate interaction can alter the ground state conformation of the molecule forcing the phenylporphiryn single bonds to be rotated [22]. In this case, the dimensions obtained from the profile plot (from such image) suggest that the molecule lay completely flat on the HOPG substrate with the phenyl groups oriented approximately parallel to the molecular core. The orientation of the molecule with respect to the atomic HOPG substrate evaluated from this image results to be about 30. By increasing the amount of deposited H2TPP molecules (0.3–0.5 ML), areas on the graphite surface turned out to
Fig. 2. (a) STM image of two H2TPP molecules on the HOPG substrate (6 nm · 6 nm). The molecules on the substrate lay apart at a mean distance, of about 1.8 nm, comparable to that obtained in the case of selfassembly and (b) STM image of a single H2TPP molecules on the HOPG (2.5 nm · 2.5 nm). The choice of the colour palette makes the single H2TPP molecule appear in darker colour with respect to the HOPG atomic substrate. The molecule maintains its four-fold symmetry dominated by the central empty porphyrin core and the four phenyl substituents groups, which allows to identify the conformation and orientation with respect to the substrate, which results to be planar with the phenyl substituents oriented almost parallel to the HOPG. The dimensions obtained from such image suggest that the H2TPP lay completely flat on the HOPG substrate with the phenyl groups oriented approximately parallel to the molecular core. (I = 400 pA, Vsample = 400 mV).
be covered by ordered molecular adlayers. In particular, it was possible to observe, from several images collected on different sample areas, that the ordered regions extend over hundreds of square nanometers, as shown in Fig. 3a. Images obtained on a smaller area of the adlayer, as the one reported in Fig. 3b, show bright emerging structures, mainly circular in shape, arranged (within the experimental error) in a quasi-hexagonal pattern, reminiscent of that of the graphite substrate with a measured periodicity of about 3.2 nm along the molecular rows. The two dimensional (2D) unit cell, sketched in the figure, is nearly hexagonal (cell vectors: a = 3.18 ± 0.10 nm; b = 3.12 ± 0.10 nm; a = 61 ± 2, a is the angle between the two main vectors of the unit cell). Numerous STM images were collected on similarly ordered areas by changing the bias voltage and set-point current, without revealing an appreciable variation of the mean height and lateral dimension of the molecules in the assembly. In addition, no dependence of image contrast on the polarity of the applied sample voltage was detected. We also compared the dimensions of the H2TPP molecules as reported in the literature data with those evaluated on the average from profile plots like that reported in Fig. 1. The mean diameter (1.80 ± 0.10 nm) obtained, matches the distance separating the hydrogens in the para positions of opposite phenyl rings of the H2TPP structure. This distance turns out to be 1.77 nm, as obtained by hybrid density-functional theory geometry optimization carried out at the B3LYP/6-31G(d) level of theory [26,27]. In addition, the mean height of molecules in the layer evaluated from the same profiles (0.35 ± 0.10 nm) supports the hypothesis that H2TPP molecules assume a planar geometry on HOPG surface [26]. It appears that even under the best STM experimental conditions adopted, the inner structure of the H2TPP molecules remains unresolved. High-resolution has been observed in the case of simple porphyrins deposited on HOPG mainly when imaged under liquid [13,20]. It has been also observed that the addition of ad-hoc functional groups to free-base porphyrin such as carboxylic substituents or hydrocarbon chains, exerts stabilization and ordering and improves the molecular resolution [10–12,15]. In the case of metal-porphyrins the molecular conformation was clearly determined by STM [10]. In fact, the insertion of metal atoms with their specific and quite localized structure can determine a significant adjustment of the electronic density of states, thus making it possible to increase the contrast for optimal molecular resolution. In order to get additional information on the H2TPP arrangement on the HOPG surface, we acquired STM data at appropriate values of the sample bias and tunneling current so that the molecular film together with the atomic HOPG substrate could be resolved, as reported in Fig. 4a. In this image, the molecular film covers part of a HOPG plane up to the step edge. Once the lattice of the HOPG is well localized together with the molecular structure we have performed the Fourier transform of the image and this evidences the co-existence of the two lattices (one
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Fig. 3. (a) STM image of the HOPG partly covered with H2TPP molecules (150 nm · 150 nm), (b) STM image obtained on a smaller area (25 nm · 25 nm). Unit cell: a = 3.18 ± 0.10 nm; b = 3.12 ± 0.10 nm; a = 61 ± 2. (I = 500 pA, Vsample = 250 mV), and (c) profile plot obtained along the direction shown in (b).
Fig. 4. (a) STM image in which the H2TPP molecular film extending on a HOPG plane together with the atomic HOPG substrate (25 nm · 25 nm) is visible (I = 550 pA, Vsample = 250 mV) and (b) Fourier transform evaluated from the image in which two misoriented hexagonal lattices appear correspondent to 0.24 nm distance between second nearest neighbours in the HOPG basal plane (outer hexagon), compared to 3.2 nm of the molecular lattice in the plane (inner hexagon).
of the HOPG and the other to the H2TPP film) with nearly hexagonal symmetry and allows the assessment of their relative orientation, as reported in Fig. 4b. In order to check the registry of the H2TPP overlayer with the graphite substrate we also evaluate the epitaxy matrix C that relates the overlayer unit cell vectors (b1, b2) with those of the substrate lattice (a1, a2) [28]. The
lattice parameters of the H2TPP overlayer and the HOPG substrate, in which only ß atoms contribute to the STM image [29], have been evaluated by averaging the lattice values obtained from several images. b1 a1 9:4 5:7 a1 ¼C ¼ : b2 a2 a2 5:6 14:8
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Within the experimental error, no element of the C matrix is integer, so we conclude that the H2TPP overlayer is not commensurate with the underlying graphite substrate. Although this result could suggest that molecules in the adlayer have no preferential adsorption site on the substrate, the huge intermolecular distance observed in the adlayer together with the quasi-hexagonal symmetry in the cell cannot exclude an interaction of the molecules with the graphite substrate. In this regard, the information obtained from STM images relative to a self-assembly arrangement on graphite has been combined with that obtained from EELS and X-Ray spectroscopy, in order to point out the presence of the molecules, investigate about their integrity and interaction with the HOPG surface. Fig. 5 the results of the reflection energy loss spectra obtained first on graphite after cleavage (a) and after a deposition of tetraphenylporphyrin correspondent to an observed self-assembly (b) are reported. The most significant p–p* and r–r* transitions of graphitic carbon [30] were found to be significantly modified after molecule deposition. In particular the original energy position of the two main transitions located at about 5.8 eV and 25.9 eV in the HOPG spectrum (a) are shifted at about 6.9 eV and 24 eV, respectively (b). We suggest that these transitions maintain their electronic nature when occurring in the basal plane and in the perpendicular direction of HOPG. However, they turn out to be significantly influenced by the p–p interactions between the adsorbed porphyrins and the substrate [31]. The REELS spectrum of the graphite after H2TPP deposition also shows an additional p–p* feature located around 3.0 eV. This new contribution is related to the transition involving HOMO and LUMO molecular states of the porphyrins and can be associated to their optical absorption, usually referred as Soret band which, for these systems, occurs at around 2.8 eV [32–34].
Fig. 5. REELS (Ep = 300 eV) spectra obtained on clean HOPG (a) and after H2TPP deposition (b).
The XPS study was performed on graphite after cleavage and then after tetraphenylporphyrin sublimation. Extended energy distribution spectra were first recorded as reported in Fig. 6a–b. Line spectrum (a), has been obtained on cleaved graphite, while the line spectrum (b), has been obtained after a deposition of tetraphenylporphyrin corresponding to a self-assembly arrangement on graphite, as observed by STM (see Fig. 3). From this data we detected an additional contribution from nitrogen N1s, while both C1s and O1s signals increased. Therefore, XPS investigation after H2TPP sublimation, was mainly focused on the binding energy of 1 s core level of nitrogen, as reported in Fig. 7.
Fig. 6. Extended XPS (hm = 1253.6 eV) spectra obtained on HOPG (a), after H2TPP deposition (b).
Fig. 7. XPS (hm = 1253.6 eV) N1s core level spectrum obtained on HOPG, after H2TPP deposition. Empty circles are experimental data (s). Solid lines represent the overlapping components obtained by nonlinear curve fitting. Small dots (d) are fitting data calculated by summing up the three gaussian components.
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The nonlinear curve fitting of the observed intensities evidences two main features at 399.5 eV and 397.6 eV. These correspond to the pyrrole-type nitrogen and the imine-type ones, respectively, as previously reported for similar free-base porphyrin derivatives. The energy difference between the N1s pyrrole-type and imine-type energies is about 1.9 eV [35]. An additional feature is also detected at 401.6 eV; this might be associated to the interaction between carbon atoms of the graphite surface and the H2 TTP molecules. The same satellite structure has already been observed in both the free-base porphyrin and in its related Zn complex [35]. A careful comparison of the energy position of this core peak with the kinetic energy of the EELS structure associated to the HOMO–LUMO transition for the H2TPP molecule also supports this hypothesis. Both the resulting assembling geometry and the indication given by the spectroscopy data, suggest that, although the HOPG surface is almost inert, it interacts significantly with molecules. At higher H2TPP coverages (greater than 1 ML), the graphite surface is completely modified by porphyrin molecules as shown in Fig. 8, where the first complete adlayer
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is partially covered with additional structures on top. In particular, along the two graphite step edges in the upper right part of the figure, we observed the formation of new rod-like structures which suggest a three-dimensional arrangement of the molecules. Although the additional layer is incomplete, from the inset of Fig. 8 and the profile plot reported, it is possible to see that molecules have a preferential binding site on top of those of the layer underneath. Nevertheless, it must be underlined that the molecular height registered is about 1–2 nm. This value is the result of a modification of the electronic properties associated to a morphological rearrangement of the porphyrins in the adlayer. Therefore, we cannot exclude that the observed molecular height originates from a columnar structure made of porphyrins emerging from the ordered layer underneath. The formation of columnar structures has already been observed in the case of porphyrin aggregates [11,36]. In any case, the conclusion drawn from the profile plot about the position of the additional molecules in the adlayer remains valid. As a consequence, the ordering tendency driven by the graphite substrate is reduced as the additional porphyrin molecules bind to those in the layer underneath and experience a weaker interaction with the graphite substrate. Finally, STM images obtained after subsequent H2TPP depositions (greater than 1.3 ML), show a highly disordered surface (data not shown) covered with a uniform background made of molecular aggregates mostly spherical in shape randomly distributed on the surface. From the results obtained it appears that a layer-bylayer growth is difficult to obtain for this system while in the literature has been observed for more complex porphyrin molecules [19]. 4. Conclusions
Fig. 8. STM image of the HOPG covered with H2TPP molecules (200 nm · 200 nm). Inset: STM image obtained on a smaller area (20 nm · 20 nm). The profile plot helps to pin point the position occupied by the additional molecule in the new layer. (I = 400 pA, Vsample = 50 mV).
Ultra high vacuum sublimation showed to be an easy method to deposit simple tetraphenylporphyrin on highly oriented pyrolitic graphite. It has been possible to monitor that up to critical degree of coverage H2TPP molecules selfassembly on the surface and a well-ordered layer structure is formed. In particular STM showed that the constituent H2TPP molecules arranged according to a quasi-hexagonal lattice with a large periodicity. The information obtained from STM data has been linked with XPS and REELS measurements to investigate about the orientation and conformation of the molecules when adsorbed on the surface. We conclude that the packing of the molecular adlayer is controlled by intermolecular interactions while the orientation of the layer is induced by the coupling to the HOPG substrate. This surface-mediated symmetry has been observed for one complete layer and an additional partial second one. In the second layer the symmetry is mainly driven by the preferential adaptation of the additional molecules with the underlying ordered first layer.
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