The (110) surface is obtained by cutting the fcc unit cell in a manner that intersects the x and y axes but not the z-axis - this exposes a surface with an atomic arrangement of 2-fold symmetry i.e. a rectangular symmetry of the surface layer atoms. In one direction (along the rows) the atoms are in contact i.e. the distance between atoms is equal to twice the metallic(atomic) radius, but in the orthogonal direction there is a substantial gap between

2 The pictures of figure 10 left and 11 left have been made from NIST SSD output and processed with BALSAC by K. Hermann. http://www.fhi-berlin.mpg.de/~hermann/Balsac/SSDpictures.html SSD is the NIST Standard Reference Database no. 42 by P. R. Watson, M. A. Van Hove, and K. Hermann

In case of the [111] planes noble metal (Cu, Ag, Au and Pt) have a close packed structure, while the surfaces having [110] orientation are more "open" and a (2x1) reconstruction can

Fig. 9. Left: Structure bulk terminated fcc (111) 2. Right: side view of the Au(111) herringbone

The (111) surface is obtained by cutting the fcc metal in such a way that the surface plane intersects the x-, y- and z- axes at the same value - this exposes a surface with an atomic arrangement of 3-fold (apparently 6-fold, hexagonal) symmetry. This layer of surface atoms actually corresponds to one of the close-packed layers on which the fcc structure is based.

In a fcc (111) surface all surface atoms are equivalent and have a relatively high CN and the

The surface offers the following adsorption sites: On-top sites, Bridging sites, between two

The effect of surface formation induces relaxation of the topmost metal layer. For example the clean Pt(111) surface does not show any reconstruction, however a S-XRD analysis of its CTRs (see section 2.1) shows that the surface top layer is slightly relaxed with respect to the nominal bulk termination and their fitting allows to determine an expansion of 4*.*5 ± 0*.*5 pm *(*+2*.*0%*)* with respect to the ideal bulk termination (Felici&Pedio 2009). Au(111) clean surface shows a Au (111) 22 *√*3 "herringbone" reconstructed surface (van Hove 1881, Sandy 1991), that involves a great number of surface atoms (Fig. 9 left , Herringbone side view, STM

The (110) surface is obtained by cutting the fcc unit cell in a manner that intersects the x and y axes but not the z-axis - this exposes a surface with an atomic arrangement of 2-fold symmetry i.e. a rectangular symmetry of the surface layer atoms. In one direction (along the rows) the atoms are in contact i.e. the distance between atoms is equal to twice the metallic(atomic) radius, but in the orthogonal direction there is a substantial gap between

2 The pictures of figure 10 left and 11 left have been made from NIST SSD output and processed with BALSAC by K. Hermann. http://www.fhi-berlin.mpg.de/~hermann/Balsac/SSDpictures.html SSD is the NIST Standard Reference Database no. 42 by P. R. Watson, M. A. Van Hove, and K. Hermann

**3.1 Two extremes: Comparison between (111) and (110) surfaces**

This is the most efficient way of packing atoms within a single layer.

surface is almost smooth at the atomic scale.

atoms, Hollow sites, between three atoms.

take place at RT.

reconstruction

image Fig. 1 right).

the rows. This means that the atoms in the underlying second layer are also, to some extent, exposed at the surface. Normally the bulk terminated (110) plane is energetically not favourable. The (110) surfaces of Ag and Au metals are characterized by the strong surface diffusion anisotropy energies. Typically this surface can be unstable and shows reconstructions ranging from the 1x2 to the 1x5 unit cell.

The clean Au(110) presents the missing row 1x2 reconstruction, while Cu(110) remains unreconstructed (Africh, 2010) as shown in Figure 10.

Fig. 10. a) bulk terminated fcc (110) picture and STM image of Cu(110) surface (C. Africh). B) (110) missing row 2x1 reconstruction picture and topographical image of Au(110) 2x1 reconstruction ref. http://www.redespm. org.br/grupo10.html

The (110) substrates have higher mobility and reactivity. They offer a wide variety of possible adsorption sites, including : On-top sites, Short bridging sites between two atoms in a single row, Long bridging sites between two atoms in adjacent rows. Moreover higher coordination sites are possible (in the troughs that are facets with (111) orientation). The Au(110)-(1×2) surface shows an order-disorder transition with critical temperatures, Tc, between 650 K and 695 K depending on finite-size effects (Clark, 1986).

A large number of experimental and theoretical works have been envisaged to single out the basic aspects of molecules interacting with noble metal. In particular, the Au(111) surface is widely used as support of SAM providing an almost uniform reserve of electron charge. Differently from the (111) case, in a (110) 2x1 reconstructed fcc surface, the surface is atomically rough, and highly anisotropic. The surface atoms on a more open ("rougher") surface have a lower CN - this has important implications when it comes to the chemical reactivity of surfaces, as we discuss in the next section.

#### **4. Survey of ordered molecular layers: From weak interaction to anchoring**

Organic molecules can provide complex functionalities at a metal surface. The engineering of the functionalities of organic-inorganic interfaces has important technological applications, as cathalysis, sensors, optoelectronics, molecular spintronics, photovoltaic and so on. The development in these fields implies a deep understanding of the bonding and the lateral interactions that govern the organization of the molecular thin films and ultimately the design for the optimization of their functionalities. In order to accomplish this goal a rigorous surface science approach is necessary: dosing of known molecular fluxes on defined surfaces under ultra high vacuum conditions and fine characterization of these systems, including sophisticated data analysis, theoretical modelling and simulation.

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 267

transfer from the substrate and/or from dopant atoms (influenced by the substrate work function, the molecule-substrate interaction and possible screening effects), on the geometry of the molecular film which depends on the substrate surface chosen and on the thermal history of the film) , on possible adlayer-induced substrate reconstruction and on defects. In

Fig. 11. Structures of different molecules. Left: Fullerene C60, Center: Pentacene C22H14,

C22H14

Fullerenes (C60) can be considered as model systems of large molecules. This kind of soccerball shaped molecule with its three dimensional character along with the strong modulation of the valence charge distribution over the molecular cage. This unique electronic structure leads to several possible interaction mechanisms with the host surface. The geometrical constraints imposed by the three dimensional cage allow only a few carbon atoms per molecule to be in contact with the substrate. The distribution of electronic charge (interface region) and the electronic properties of the first C60 ML can be tuned. When deposited on a single crystal surface, fullerene molecules are proving useful to encapsulate surface reconstructions and/or are functioning as templates for the formation of novel structures. They are widely used to create complex architectures on surfaces for their stability, electronic properties (doping), functionalization, and capability of forming reconstructed

Monolayer (ML) and sub-monolayer films based on fullerene molecules deposited on top of metal or semiconductor surfaces, therefore, form a class of systems that could exhibit a wide range of physical behaviours. An increasing number of works is aimed at studying the interaction of single layers of C60 with nobel metal surfaces. Due to its large work function (7.6 eV) and electron affinity (2.7 eV), C60 has the propensity to accept electrons from many metallic surfaces. However, charge transfer from the metallic substrate to C60 is not always observed and the character of the interaction depends mainly on the metal, almost independently of its work function. Thin films of C60 adsorbed on crystalline substrates usually tend to form hexagonal or quasi-hexagonal ordered structures with a large variety of the substrate-adsorbate interactions, ranging from covalent to ionic character. An important issue is the investigation of the consequences of the C60 adsorption on the underlying substrate structure. According Maxwell et al. (Maxwell 1994) the bonding interaction of C60 in direct contact with a metal surface can be divided in three categories:

the next section we will describe the research methods with some detail.

Right: Ni Octaethylporphyrin NiOEP.

surfaces (Rudolf 1996 as cited in Gavioli &Cepek, 2008).

**4.1 C60 on noble metals**

The molecular ordening onto surfaces strongly depends on the delicate balance of the molecule-molecule and molecule-surface interactions (see discussions in ref. Rosei et al. 2003). This is reflected in the complexity of the phase diagrams of these systems. Small differences in the properties of the noble metal substrates (discussed previously) can lead to marked differences in the molecular deposited layers.

Depending on their structure the molecular semiconductor ordered films have important properties for microelectronics: integrability with inorganic semiconductor, low cost, large area bulk processing, tailoring for specific electronic and optical properties, systems more tolerant of defects (no electronically active), no dangling bonds that leads to great flexibility on the choice of substrates, charged carrier mobility increases with increasing pz-pz orbital, anisottropic mobility (for axample Pentacene *µ*~*1 cm2/Vs direction in-plane, µ* ~*10*-*4 cm2/Vs stacking).*

The disadvantages concern mainly the fact that a high degree of molecular ordering is necessary, to avoid the problems related to low carrier mobility.

The growth procedures are relevant in the bond, ordening and structure of the monolayer of organic semiconductors. All the cases discussed in the following are related to vapor deposition in UHV conditions.

We compare different single organic layers deposited onto ordered noble metal surfaces. The interest and the strong effort in the characterization of the first MONOLAYER3 of organic molecules on single crystal surfaces adsorption is due to the general accepted result that structural and chemical properties of the monolayer are keys for controlled multilayer growth. The nucleation, growth, structure and interaction molecules strongly depend on the substrate nature and on the particular experimental protocol used during the deposition. The geometry of the first organic layer is exploited to drive the orientation of the next growing layers. From the electronic point of view, the first layer acts as a buffer layer, enabling the following layers to be exploited for the intrinsic electronic properties of the free molecule. Case studies of molecules that differ in size, symmetry and morphology, deposited on different low index noble metal substrates are here presented, on the basis of structural and electronic properties determined by the above techniques ( section 2). Figure 11 shows the structural formulae of the molecules discussed in the following. Due to the extension of the literature on SAM deposited onto noble metal surfaces, the list of systems treated in this chapter cannot be complete. We focus our attention on our results obtained on ordered molecular layers mainly deposited onto Au(111) and Au(110) substrates, considering gold as the prototypical noble metals. Other specific systems involving other metal surfaces will be taken into account and discussed.

The large dimension of these molecules (scale 10 Å) with respect to the unit cell of metal surfaces (Table 2), implies that the local molecule-substrate interaction involves several unit cells of the substrate. The properties of these low dimensional systems depend, for example, on the nature of the chemical bond (ionic *versus* covalent *versus* Van der Waals), on the partial filling of the lowest unoccupied molecular orbital (LUMO) as a result of a charge

3 For example 1 ML on Au(111) is defined as the packing density compared to the Au(111) plane,

1.391015 atoms/cm2, which is equivalent to an effective thickness of 2.35 A at the density of bulk gold.

transfer from the substrate and/or from dopant atoms (influenced by the substrate work function, the molecule-substrate interaction and possible screening effects), on the geometry of the molecular film which depends on the substrate surface chosen and on the thermal history of the film) , on possible adlayer-induced substrate reconstruction and on defects. In the next section we will describe the research methods with some detail.

Fig. 11. Structures of different molecules. Left: Fullerene C60, Center: Pentacene C22H14, Right: Ni Octaethylporphyrin NiOEP.

#### **4.1 C60 on noble metals**

266 Noble Metals

The molecular ordening onto surfaces strongly depends on the delicate balance of the molecule-molecule and molecule-surface interactions (see discussions in ref. Rosei et al. 2003). This is reflected in the complexity of the phase diagrams of these systems. Small differences in the properties of the noble metal substrates (discussed previously) can lead to

Depending on their structure the molecular semiconductor ordered films have important properties for microelectronics: integrability with inorganic semiconductor, low cost, large area bulk processing, tailoring for specific electronic and optical properties, systems more tolerant of defects (no electronically active), no dangling bonds that leads to great flexibility on the choice of substrates, charged carrier mobility increases with increasing pz-pz orbital, anisottropic mobility (for axample Pentacene *µ*~*1 cm2/Vs direction in-plane, µ*

The disadvantages concern mainly the fact that a high degree of molecular ordering is

The growth procedures are relevant in the bond, ordening and structure of the monolayer of organic semiconductors. All the cases discussed in the following are related to vapor

We compare different single organic layers deposited onto ordered noble metal surfaces. The interest and the strong effort in the characterization of the first MONOLAYER3 of organic molecules on single crystal surfaces adsorption is due to the general accepted result that structural and chemical properties of the monolayer are keys for controlled multilayer growth. The nucleation, growth, structure and interaction molecules strongly depend on the substrate nature and on the particular experimental protocol used during the deposition. The geometry of the first organic layer is exploited to drive the orientation of the next growing layers. From the electronic point of view, the first layer acts as a buffer layer, enabling the following layers to be exploited for the intrinsic electronic properties of the free molecule. Case studies of molecules that differ in size, symmetry and morphology, deposited on different low index noble metal substrates are here presented, on the basis of structural and electronic properties determined by the above techniques ( section 2). Figure 11 shows the structural formulae of the molecules discussed in the following. Due to the extension of the literature on SAM deposited onto noble metal surfaces, the list of systems treated in this chapter cannot be complete. We focus our attention on our results obtained on ordered molecular layers mainly deposited onto Au(111) and Au(110) substrates, considering gold as the prototypical noble metals. Other specific systems involving other

The large dimension of these molecules (scale 10 Å) with respect to the unit cell of metal surfaces (Table 2), implies that the local molecule-substrate interaction involves several unit cells of the substrate. The properties of these low dimensional systems depend, for example, on the nature of the chemical bond (ionic *versus* covalent *versus* Van der Waals), on the partial filling of the lowest unoccupied molecular orbital (LUMO) as a result of a charge

3 For example 1 ML on Au(111) is defined as the packing density compared to the Au(111) plane, 1.391015 atoms/cm2, which is equivalent to an effective thickness of 2.35 A at the density of bulk gold.

marked differences in the molecular deposited layers.

necessary, to avoid the problems related to low carrier mobility.

metal surfaces will be taken into account and discussed.

~*10*-*4 cm2/Vs stacking).*

deposition in UHV conditions.

Fullerenes (C60) can be considered as model systems of large molecules. This kind of soccerball shaped molecule with its three dimensional character along with the strong modulation of the valence charge distribution over the molecular cage. This unique electronic structure leads to several possible interaction mechanisms with the host surface. The geometrical constraints imposed by the three dimensional cage allow only a few carbon atoms per molecule to be in contact with the substrate. The distribution of electronic charge (interface region) and the electronic properties of the first C60 ML can be tuned. When deposited on a single crystal surface, fullerene molecules are proving useful to encapsulate surface reconstructions and/or are functioning as templates for the formation of novel structures. They are widely used to create complex architectures on surfaces for their stability, electronic properties (doping), functionalization, and capability of forming reconstructed surfaces (Rudolf 1996 as cited in Gavioli &Cepek, 2008).

Monolayer (ML) and sub-monolayer films based on fullerene molecules deposited on top of metal or semiconductor surfaces, therefore, form a class of systems that could exhibit a wide range of physical behaviours. An increasing number of works is aimed at studying the interaction of single layers of C60 with nobel metal surfaces. Due to its large work function (7.6 eV) and electron affinity (2.7 eV), C60 has the propensity to accept electrons from many metallic surfaces. However, charge transfer from the metallic substrate to C60 is not always observed and the character of the interaction depends mainly on the metal, almost independently of its work function. Thin films of C60 adsorbed on crystalline substrates usually tend to form hexagonal or quasi-hexagonal ordered structures with a large variety of the substrate-adsorbate interactions, ranging from covalent to ionic character. An important issue is the investigation of the consequences of the C60 adsorption on the underlying substrate structure. According Maxwell et al. (Maxwell 1994) the bonding interaction of C60 in direct contact with a metal surface can be divided in three categories:

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 269

The most direct experimental technique, giving detailed information regarding interaction between the fullerene molecules and the substrate, is PES valence band (VB). Fig. 14 left compares the VB spectrum of the ordered monolayer with the clean substrate and the C60 multilayer. The spectra were measured at room temperature (RT) (Cepek, 2000 a). The C60 multilayer spectrum is the typical molecular-like spectrum of this compound. The fivefold degenerate highest occupied molecular orbital (HOMO) derived band lies at energy of ≈2.2eV, while the HOMO-1 derived band has energy of ≈3.6eV. The clean Ag(100) spectrum does not show any features in the HOMO and HOMO-1 energy range, so all the structures observed in the ML spectrum in this region are only due to the C60 adsorbed. We note that the ML spectrum shows an extra emission at the Fermi level compared to the multilayer spectrum, indicating a metallic behaviour. We want to remark that this extra emission cannot be due to the metallic Ag substrate, if the strong attenuation by the C60 layer is considered. So it results that the presence of the peak at the Fermi level is the typical fingerprint of a partially filling of the C60 lowest unoccupied molecular orbital (LUMO) derived band and it means that charge transfer from the Ag substrate to the C60 molecules takes place. By K doping, it has have found that the molecules adsorbed on the monolayer are in a charge state ≈-2 (Cepek, 2000 b)). Finally we want to note that the bond

Fig. 13. STM images of different C60 coverages deposited on Ag(100). See text for discussion.

Fig. 14, right shows the VB photoemission spectra of 0.7 ML of C60 adsorbed on Ag(001) at 150 K and subsequently annealed to different temperatures (RT and 600 K for ≈ 5 minutes, compared to a thick C60 multilayer (Cepek, 2000 a). The 150 K spectrum has been measured at 150 K, all the others at RT. The deposition of the same quantity of C60 molecules at RT produces a photoemission spectrum (not shown) identical to that of the 150 K deposited film annealed at RT. The same is true for the spectrum of a 600K deposited or annealed film.

Also in this case, the adsorption of C60 induces an increase of the photoemission intensity at the FL with respect to the clean surface thereby indicating that in all samples the bond is characterized by charge transfer from silver atoms to C60 molecules. The 150 K and RT VB photoemission spectra show that the HOMO and HOMO-1 derived bands consist of two components: one centered at the same BE of the multilayer spectrum, the other at the BE of one ordered C60 ML. This, combined with the STM images of Fig. 13 a, allows us to attribute

is activated by temperature.

(Cepek et al., 2000 a)

strong predominantly covalent bonding, intermediate predominantly covalent bonding and intermediate predominantly ionic bonding. Strong predominantly covalent bonding has been observed for a single layer of C60 on Ni(110), Pt(111), Rh(111) and Ta(110) where the strength of the interaction is able to catalyze the fullerene cage decomposition prior to the desorption of the monolayer. Interaction of intermediate strength with bonds of mixed ionic character has been observed for the C60 monolayer on Au(110), Au(111), Ag(110), Ag(111) and polycrystalline Ag Cu(110) and Cu(111),. Charge transfer has also been observed for C60 monolayers deposited on polycrystalline Au and Cu samples, but the stability of the C60 monolayer and the strength of the interaction onto these surfaces was not investigated in detail. The trend of these bond strengths reflects the Hammer- Nørskov model (par. 3).

A particular case has been observed on Ag(100). By using the appropriate sample preparation conditions, this system shows the reversible opening of a gap at the Fermi level at temperatures 25 ≤ T< 300K (see Fig. 12), whose origin is still under debate (Cepek et al. 2001, Goldoni et al., 1998).

Fig. 12. Fermi Level photoemission spectra of the C60/Ag(100) system at varying the temperature.

The C60 layer may not present long-range order, but in any case the quasi-hexagonal geometry is locally preserved. The C60 molecules adsorbed on Ag(100) follow this general trend. The growth mode and morphology of the C60 molecules adsorbed on Ag(100) at room temperature (RT) or above has been widely studied (Goldoni, 1998, Giudice,1998), Cepek,2000a), Lu, 2003). It has been observed that the C60 molecules form a strong bond on the Ag(100) surface at any growing temperature investigated (for STM, ranging from 100K to ≈670K). When deposited at room temperature or below, and by using an evaporation rate of ≈0.2ML/min.), the growth mode is not layer by layer, but the second layer start to grow before the first is completed (Fig. 13, a, b) (Cepek, 2000 a). When the C60 molecules are deposited at a temperature higher than the C60 sublimation temperature, or by annealing a C60 multilayer film, only one single layer of molecules are left on the surface. When the annealing or growing temperature is higher that ≈600K the molecules form a well ordered overlayer, characterized by long range order and by a sharp quasi-hexagonal LEED pattern, where the C60 - C60 distance is ≈10.4Å (see Fig. 13 c).

The most direct experimental technique, giving detailed information regarding interaction between the fullerene molecules and the substrate, is PES valence band (VB). Fig. 14 left compares the VB spectrum of the ordered monolayer with the clean substrate and the C60 multilayer. The spectra were measured at room temperature (RT) (Cepek, 2000 a). The C60 multilayer spectrum is the typical molecular-like spectrum of this compound. The fivefold degenerate highest occupied molecular orbital (HOMO) derived band lies at energy of ≈2.2eV, while the HOMO-1 derived band has energy of ≈3.6eV. The clean Ag(100) spectrum does not show any features in the HOMO and HOMO-1 energy range, so all the structures observed in the ML spectrum in this region are only due to the C60 adsorbed. We note that the ML spectrum shows an extra emission at the Fermi level compared to the multilayer spectrum, indicating a metallic behaviour. We want to remark that this extra emission cannot be due to the metallic Ag substrate, if the strong attenuation by the C60 layer is considered. So it results that the presence of the peak at the Fermi level is the typical fingerprint of a partially filling of the C60 lowest unoccupied molecular orbital (LUMO) derived band and it means that charge transfer from the Ag substrate to the C60 molecules takes place. By K doping, it has have found that the molecules adsorbed on the monolayer are in a charge state ≈-2 (Cepek, 2000 b)). Finally we want to note that the bond is activated by temperature.

(Cepek et al., 2000 a)

268 Noble Metals

strong predominantly covalent bonding, intermediate predominantly covalent bonding and intermediate predominantly ionic bonding. Strong predominantly covalent bonding has been observed for a single layer of C60 on Ni(110), Pt(111), Rh(111) and Ta(110) where the strength of the interaction is able to catalyze the fullerene cage decomposition prior to the desorption of the monolayer. Interaction of intermediate strength with bonds of mixed ionic character has been observed for the C60 monolayer on Au(110), Au(111), Ag(110), Ag(111) and polycrystalline Ag Cu(110) and Cu(111),. Charge transfer has also been observed for C60 monolayers deposited on polycrystalline Au and Cu samples, but the stability of the C60 monolayer and the strength of the interaction onto these surfaces was not investigated in detail. The trend of these bond strengths reflects the Hammer- Nørskov model (par. 3).

A particular case has been observed on Ag(100). By using the appropriate sample preparation conditions, this system shows the reversible opening of a gap at the Fermi level at temperatures 25 ≤ T< 300K (see Fig. 12), whose origin is still under debate (Cepek et al.

Fig. 12. Fermi Level photoemission spectra of the C60/Ag(100) system at varying the

where the C60 - C60 distance is ≈10.4Å (see Fig. 13 c).

The C60 layer may not present long-range order, but in any case the quasi-hexagonal geometry is locally preserved. The C60 molecules adsorbed on Ag(100) follow this general trend. The growth mode and morphology of the C60 molecules adsorbed on Ag(100) at room temperature (RT) or above has been widely studied (Goldoni, 1998, Giudice,1998), Cepek,2000a), Lu, 2003). It has been observed that the C60 molecules form a strong bond on the Ag(100) surface at any growing temperature investigated (for STM, ranging from 100K to ≈670K). When deposited at room temperature or below, and by using an evaporation rate of ≈0.2ML/min.), the growth mode is not layer by layer, but the second layer start to grow before the first is completed (Fig. 13, a, b) (Cepek, 2000 a). When the C60 molecules are deposited at a temperature higher than the C60 sublimation temperature, or by annealing a C60 multilayer film, only one single layer of molecules are left on the surface. When the annealing or growing temperature is higher that ≈600K the molecules form a well ordered overlayer, characterized by long range order and by a sharp quasi-hexagonal LEED pattern,

2001, Goldoni et al., 1998).

temperature.

Fig. 13. STM images of different C60 coverages deposited on Ag(100). See text for discussion.

Fig. 14, right shows the VB photoemission spectra of 0.7 ML of C60 adsorbed on Ag(001) at 150 K and subsequently annealed to different temperatures (RT and 600 K for ≈ 5 minutes, compared to a thick C60 multilayer (Cepek, 2000 a). The 150 K spectrum has been measured at 150 K, all the others at RT. The deposition of the same quantity of C60 molecules at RT produces a photoemission spectrum (not shown) identical to that of the 150 K deposited film annealed at RT. The same is true for the spectrum of a 600K deposited or annealed film.

Also in this case, the adsorption of C60 induces an increase of the photoemission intensity at the FL with respect to the clean surface thereby indicating that in all samples the bond is characterized by charge transfer from silver atoms to C60 molecules. The 150 K and RT VB photoemission spectra show that the HOMO and HOMO-1 derived bands consist of two components: one centered at the same BE of the multilayer spectrum, the other at the BE of one ordered C60 ML. This, combined with the STM images of Fig. 13 a, allows us to attribute

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 271

We discuss two cases systems: the "covalent" C60/Pt(111) (Felici, 2005) with the partially

Fig. 15 shows the phase diagram of the C60 deposited onto Pt(111) surface. The C60 deposited on Pt(111) at room temperature weakly interacts with the substrate atoms and molecular features measured by electron spectroscopies are similar in intensity and energy separation to the isolated molecule or to C60 multilayer. After annealing the fullerene bond with Pt(111) substrate become strongly covalent. The deposition of one monolayer of C60 on the Pt(111) surface at T < 580°C leads to an ordered double domain hexagonal reconstruction, that is present also for coverages exceeding 1 ML (Pedio et al.,

a) d)

Fig. 15. a) Phase diagram of the C60/Pt(111) systems obtained with C60 flux of 0.05 ML/min and slow temperature variations (1 K/min)*.* The peaks belong to the two hexagonal domains separated of 27.8°, b) LEED pattern of the single layer of √13 √ 13*R*13.9° , c) LEED pattern of the hexagonal 3-domain reconstruction. d) Azimuthal scan of the x-ray scattered intensity for an exchanged vector modulus corresponding to the first order of the √13 √

The long range molecular ordening formation is a kinetically controlled process. The double hexagonal pattern was observed even for 1 ML adsorbed at 100 K and annealed above 770 K (Cepek et al., 1996). When the sample is heated above 800 K or if less than a complete monolayer is adsorbed at T>670 K, a third domain becomes visible in the LEED pattern.

About 10 MLs of C60 were deposited at room temperature. Then the system was slowly annealed while monitoring with the x-rays the appearance of the √13 √ 13*R*13.9° reconstruction. At about 420 K sharps peaks corresponding to this reconstruction appear. This temperature is much lower than the multilayer desorption of the fullerenes and, in fact, x-ray reflectivity scans show the presence of a well ordered C60 multilayer. In Fig.15 d) we show an azimuthal plot of the intensity of the fist order reconstruction. The two P6 domains separated of 27.8° with a practical equal intensity are clearly observable. Further annealing at 650 K removes the multilayer and leaves only one layer bonded to the surface. The analysis of the CTRs provides the following information: a) the vertical expansion of the topmost substrate layer reduces to 1.1 +/- 0.4 pm, b) the increase in surface roughness is compatible with the

presence of one vacancy per reconstruction unit cell in the substrate surface.

"ionic" C60/Au(110) (Pedio et al., 2000, Hinterstein et el., 2008) systems.

1999, Felici et al., 2005].

13*R*13.9° reconstruction.

the former component to C60 cages in the second layer and the latter to C60 molecules in direct contact with the substrate. Data fittings (continuous line of Fig. 14, right. See Cepek2000 a, b for more detail) confirms this hypothesis, showing that the percent of the molecules of the second layer is *x*  24% for the 150K spectrum and *x*  20% for the RT spectrum. The very good agreement of these results with the value of 17% obtained by STM, confirms that the molecules in direct contact with the substrate present the same bond at 150 K, RT, and 600 K while the second layer molecules do not interact with the substrate, as already observed for different systems (Cepek et al., 2000a). Taking into account that core level spectra indicate that no C60 molecules desorbs passing from 150 K to RT, this indicates that the increased thermal energy enables a fraction of the second layer molecules to descend the step edge and to incorporate in the first layer islands. From a careful inspection of the fittings in Fig. 14 right) (continuous line), one can notice that the 150 K deposited film spectrum is slightly sharper than the simulation result. This is due to the specific phonon broadening of the experimental data (measured at 150 K), which is lower than that used in the simulation, performed using spectra measured at RT. However it presents also lower photoemission intensity at the Fermi level with respect to the simulation, which can not be due to the different temperatures and may indicate a lower C60 charge state, or a higher disorder. In spite of these minor differences, passing from 150 K to RT, there is no evidence for the appearance of two different electronic states corresponding to the two different kinds of molecules (bright and dim) observed in STM images, which may be easily explained with the occurrence of the two molecular orientation revealed by the high resolution STM images (Lu, 2003) and by XPD.

Fig. 14. Left: Valence Band spectra of C60 deposited onto Ag(100) . Clean Ag(100) and C60 Multilayer are compared. Right: Valence Band spectra for different Temperatures. See text for details.

the former component to C60 cages in the second layer and the latter to C60 molecules in direct contact with the substrate. Data fittings (continuous line of Fig. 14, right. See Cepek2000 a, b for more detail) confirms this hypothesis, showing that the percent of the molecules of the second layer is *x*  24% for the 150K spectrum and *x*  20% for the RT spectrum. The very good agreement of these results with the value of 17% obtained by STM, confirms that the molecules in direct contact with the substrate present the same bond at 150 K, RT, and 600 K while the second layer molecules do not interact with the substrate, as already observed for different systems (Cepek et al., 2000a). Taking into account that core level spectra indicate that no C60 molecules desorbs passing from 150 K to RT, this indicates that the increased thermal energy enables a fraction of the second layer molecules to descend the step edge and to incorporate in the first layer islands. From a careful inspection of the fittings in Fig. 14 right) (continuous line), one can notice that the 150 K deposited film spectrum is slightly sharper than the simulation result. This is due to the specific phonon broadening of the experimental data (measured at 150 K), which is lower than that used in the simulation, performed using spectra measured at RT. However it presents also lower photoemission intensity at the Fermi level with respect to the simulation, which can not be due to the different temperatures and may indicate a lower C60 charge state, or a higher disorder. In spite of these minor differences, passing from 150 K to RT, there is no evidence for the appearance of two different electronic states corresponding to the two different kinds of molecules (bright and dim) observed in STM images, which may be easily explained with the occurrence of the two molecular orientation

revealed by the high resolution STM images (Lu, 2003) and by XPD.

Fig. 14. Left: Valence Band spectra of C60 deposited onto Ag(100) . Clean Ag(100) and C60 Multilayer are compared. Right: Valence Band spectra for different Temperatures. See text

for details.

We discuss two cases systems: the "covalent" C60/Pt(111) (Felici, 2005) with the partially "ionic" C60/Au(110) (Pedio et al., 2000, Hinterstein et el., 2008) systems.

Fig. 15 shows the phase diagram of the C60 deposited onto Pt(111) surface. The C60 deposited on Pt(111) at room temperature weakly interacts with the substrate atoms and molecular features measured by electron spectroscopies are similar in intensity and energy separation to the isolated molecule or to C60 multilayer. After annealing the fullerene bond with Pt(111) substrate become strongly covalent. The deposition of one monolayer of C60 on the Pt(111) surface at T < 580°C leads to an ordered double domain hexagonal reconstruction, that is present also for coverages exceeding 1 ML (Pedio et al., 1999, Felici et al., 2005].

Fig. 15. a) Phase diagram of the C60/Pt(111) systems obtained with C60 flux of 0.05 ML/min and slow temperature variations (1 K/min)*.* The peaks belong to the two hexagonal domains separated of 27.8°, b) LEED pattern of the single layer of √13 √ 13*R*13.9° , c) LEED pattern of the hexagonal 3-domain reconstruction. d) Azimuthal scan of the x-ray scattered intensity for an exchanged vector modulus corresponding to the first order of the √13 √ 13*R*13.9° reconstruction.

The long range molecular ordening formation is a kinetically controlled process. The double hexagonal pattern was observed even for 1 ML adsorbed at 100 K and annealed above 770 K (Cepek et al., 1996). When the sample is heated above 800 K or if less than a complete monolayer is adsorbed at T>670 K, a third domain becomes visible in the LEED pattern.

About 10 MLs of C60 were deposited at room temperature. Then the system was slowly annealed while monitoring with the x-rays the appearance of the √13 √ 13*R*13.9° reconstruction. At about 420 K sharps peaks corresponding to this reconstruction appear. This temperature is much lower than the multilayer desorption of the fullerenes and, in fact, x-ray reflectivity scans show the presence of a well ordered C60 multilayer. In Fig.15 d) we show an azimuthal plot of the intensity of the fist order reconstruction. The two P6 domains separated of 27.8° with a practical equal intensity are clearly observable. Further annealing at 650 K removes the multilayer and leaves only one layer bonded to the surface. The analysis of the CTRs provides the following information: a) the vertical expansion of the topmost substrate layer reduces to 1.1 +/- 0.4 pm, b) the increase in surface roughness is compatible with the presence of one vacancy per reconstruction unit cell in the substrate surface.

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 273

phase. The experimental width in all monolayers is about 1 eV, a value sensibly higher than that of C60 pristine. This implies that the C atoms in the single layer have different chemical configurations (C-metal bonds) with respect to the C60. Figure 17 left shows a set of IPES spectra for low coverages of C60 deposited on Pt(111) at RT and the single monolayer evolution after annealing at different temperatures. The bottom curves, related to the multilayer and the clean Pt(111), are shown for comparison. The top most spectrum is related to the chemisorbed (6x5) C60/Au(110) that will be discussed in the next section. IPES spectra of the hexagonal phases of a single monolayer are substantially different from the C60-Au(110) where was estimated by electron spectroscopies and an ionic character dominates interaction a charge transfer of 11 (see for discussion Hinterstein et al. 2008, Felici et al. 2009) electron. When the surface is annealed a 2-domain structure is formed rather dramatic changes in the spectrum IPES spectrum occur: the LUMO-derived feature appears centered at 0.5 eV and the spectral weight near EF is greatly increased, indicating that there is a redistribution of the empty states. This fact has been interpreted as a strong interaction between the substrate and the C60 overlayer when the covalent bond takes place. This result nicely confirms the formation of 12 Pt-C bonds, suggested by the SXRD analysis.

Au(111)

Pt(111)

Ag(111)

C60

Fig. 17. a) C1s photoemission spectra of C60 deposited onto Ag(111), Pt(111), Au(110) and pristine C60. b) Example of IPES spectra taken at normal incidence for different C60

overlayers on Pt(111). The spectra for the chemisorbed 1 ML C60/Au(110) (top) and for the multilayer of C60 on the same Au( 110) (bottom) are plotted for comparison (Pedio1999).

For the two-domain structure on Pt(111), vibrational studies have indicated no evidence for charge transfer. Actually in case of Pt substrate a further annealing of the system leads to a 3-domain structure and to fragmentation of the fullerene. The corresponding LEED pattern shows graphitic rings. Higher T and longer annealing produces changes in the IPES spectra. At this stage it has been suggested that the fragmentation products consist in pentagon-

Au(110)

**C1s XPS**

**282 283 284 285 286 287**

**Binding Energy (eV)**

From the x-ray diffraction analysis this vacancy helps enormously the structural solution of the system. In our case the vacancy can be considered as a heavy atom with a negative number of electrons and the Patterson must then be interpreted by looking at the negative peaks (Fig.16). The dashed rhombus in the figure shows the surface unit cell of the Pt(111) substrate while the large continuous line rhombus is the surface unit cell of the √13 √13*R*13.9° reconstruction. The dashed contour lines represent the negative values of the map and the deepest minima are localized around the vacancies and at the Pt surface atom positions. A fitting of the data, assuming only 13 structural parameters, leads to a very simple solution of this complicated system. The surface Pt atoms displace slightly from their bulk terminated positions while the C60 lies on top of the vacancies pointing to the substrate surface with one of its hexagonal surfaces. The fullerene enters as much as possible in the vacancy trying to maximize the number of bonds which are formed with the Pt atoms and about 12 Carbon atoms form directs bonds with the Pt substrate. The C60 lies with one of its hexagonal faces on top of the vacancies at a height slightly larger than in the case of the Pt substrate. Similar results have been also observed in the case of the C60 adsorbed onto the Ag(111) (Li etal. 2009) and the Au(111) surface. Also in these cases the dominant effect is the formation of vacancies at the substrate surface which stabilize the position of the fullerene molecules. In case of Au(111) substrate the number of C-Au bonds is 6, and the herringbone substrate reconstruction is disappeared indicating the release of the stress of the topmost layer, induced by the C60 monolayer.

Fig. 16. a) the Patterson map of the √13 √13*R*13.9° reconstruction. Because the pattern is dominated by the signal due to a vacancy in the surface unit cell this map has an inverted sign usual. The negative peaks (dashed lines) correspond to the positions of atoms participating in the reconstruction. The dashed rhombus is the Pt(111) surface unit cell. In (b) we show a top view of the best model fit and in (c) a detail on the C atoms forming bonds with the substrate.

Figure 17 shows the spectroscopy results of C60 monolayers deposited onto different noble metal surfaces. On the left the photoemission of the C1s core levels of the ordered monolayers (Maxwell et al 1994, Pedio et al. 1999 private communication) are compared (bottom curve) with that relative to the pristine C60. Note that the bulk C60 is a molecular solid tìwith the fullerene that interact by weak, Van der Waals interactions and the spectra are comparable (even though broader) with those obtained on the free molecule in gas

From the x-ray diffraction analysis this vacancy helps enormously the structural solution of the system. In our case the vacancy can be considered as a heavy atom with a negative number of electrons and the Patterson must then be interpreted by looking at the negative peaks (Fig.16). The dashed rhombus in the figure shows the surface unit cell of the Pt(111) substrate while the large continuous line rhombus is the surface unit cell of the √13 √13*R*13.9° reconstruction. The dashed contour lines represent the negative values of the map and the deepest minima are localized around the vacancies and at the Pt surface atom positions. A fitting of the data, assuming only 13 structural parameters, leads to a very simple solution of this complicated system. The surface Pt atoms displace slightly from their bulk terminated positions while the C60 lies on top of the vacancies pointing to the substrate surface with one of its hexagonal surfaces. The fullerene enters as much as possible in the vacancy trying to maximize the number of bonds which are formed with the Pt atoms and about 12 Carbon atoms form directs bonds with the Pt substrate. The C60 lies with one of its hexagonal faces on top of the vacancies at a height slightly larger than in the case of the Pt substrate. Similar results have been also observed in the case of the C60 adsorbed onto the Ag(111) (Li etal. 2009) and the Au(111) surface. Also in these cases the dominant effect is the formation of vacancies at the substrate surface which stabilize the position of the fullerene molecules. In case of Au(111) substrate the number of C-Au bonds is 6, and the herringbone substrate reconstruction is disappeared indicating the release of the stress of the topmost

Fig. 16. a) the Patterson map of the √13 √13*R*13.9° reconstruction. Because the pattern is dominated by the signal due to a vacancy in the surface unit cell this map has an inverted sign usual. The negative peaks (dashed lines) correspond to the positions of atoms participating in the reconstruction. The dashed rhombus is the Pt(111) surface unit cell. In (b) we show a top view of the best model fit and in (c) a detail on the C atoms forming

Figure 17 shows the spectroscopy results of C60 monolayers deposited onto different noble metal surfaces. On the left the photoemission of the C1s core levels of the ordered monolayers (Maxwell et al 1994, Pedio et al. 1999 private communication) are compared (bottom curve) with that relative to the pristine C60. Note that the bulk C60 is a molecular solid tìwith the fullerene that interact by weak, Van der Waals interactions and the spectra are comparable (even though broader) with those obtained on the free molecule in gas

layer, induced by the C60 monolayer.

bonds with the substrate.

phase. The experimental width in all monolayers is about 1 eV, a value sensibly higher than that of C60 pristine. This implies that the C atoms in the single layer have different chemical configurations (C-metal bonds) with respect to the C60. Figure 17 left shows a set of IPES spectra for low coverages of C60 deposited on Pt(111) at RT and the single monolayer evolution after annealing at different temperatures. The bottom curves, related to the multilayer and the clean Pt(111), are shown for comparison. The top most spectrum is related to the chemisorbed (6x5) C60/Au(110) that will be discussed in the next section. IPES spectra of the hexagonal phases of a single monolayer are substantially different from the C60-Au(110) where was estimated by electron spectroscopies and an ionic character dominates interaction a charge transfer of 11 (see for discussion Hinterstein et al. 2008, Felici et al. 2009) electron. When the surface is annealed a 2-domain structure is formed rather dramatic changes in the spectrum IPES spectrum occur: the LUMO-derived feature appears centered at 0.5 eV and the spectral weight near EF is greatly increased, indicating that there is a redistribution of the empty states. This fact has been interpreted as a strong interaction between the substrate and the C60 overlayer when the covalent bond takes place. This result nicely confirms the formation of 12 Pt-C bonds, suggested by the SXRD analysis.

Fig. 17. a) C1s photoemission spectra of C60 deposited onto Ag(111), Pt(111), Au(110) and pristine C60. b) Example of IPES spectra taken at normal incidence for different C60 overlayers on Pt(111). The spectra for the chemisorbed 1 ML C60/Au(110) (top) and for the multilayer of C60 on the same Au( 110) (bottom) are plotted for comparison (Pedio1999).

For the two-domain structure on Pt(111), vibrational studies have indicated no evidence for charge transfer. Actually in case of Pt substrate a further annealing of the system leads to a 3-domain structure and to fragmentation of the fullerene. The corresponding LEED pattern shows graphitic rings. Higher T and longer annealing produces changes in the IPES spectra. At this stage it has been suggested that the fragmentation products consist in pentagon-

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 275

degree of metal surface deformation. These dimples formation is a thermally activated process that produce an anchoring of the molecule. The molecular states are results

 Fig. 18. Left: Comparison between experimental and calculated in-plane data corresponding to the C60/Au(110)-p(6x5) surface reconstruction. The measured values and their associated

semicircles are proportional to the calculated values using the final structure Right: Lateral views of the C60/Au(110)-p(6x5) surface final structure. The observed corrugation has a

Large acenes, C4n+2 H2n+4 (Figure 11 b) are conjugated organic molecules composed by *n* hexagonal rings, with H atoms, which saturate the frontier C orbitals. The molecular states orientation is perpendicular to the molecular plane. Acenes form molecular crystals with triclinic structure showing polymorphism in the stacking d(001)-spacing (Mattheus 2001). In these crystals the electronic mobility is comparable with silicon (Schöhn 2000a in

Thin films of tetracene (Tc, C18H10) and pentacene (Pn, C22H14), appear most promising candidates for the building of molecular organic semiconductors. Their transport properties in case of well-ordered thin films is highly anisotropic and is strongly influenced by the first layer structural, optical and electronic properties (Lukas, 2001, Söhnchen, 2004 Danişman,

The recent literature is quite rich on this topic. The bond of acenes on metals, have been revealed weaker than that of C60 on the same substrates. The presence of the H atoms at the molecular borders and their morphology reduce the possibility of hybridization with metal states. Different ordered phases are found for Tc and Pn adsorption on noble metals Pn/Au(111) (Witte 2004) (Schroder2003), Pn/Au(110) (Floreano2006, Badvek2008), Pn/Au(111) (Kafer 2007a, Duhm 2008), Pn/Ag(111),(Danişman 2005, Eremtchenko 2005, Pedio 2007, Kaefer2007b), Tc/Ag(111) (Langner et al. 2005) and in some cases the phase

uncertainties are proportional to the radii of the two empty semicircles. The filled

strongly hybridised.

height of one atomic level

Mattheus 2001).

diagram appears complex.

**4.2 Acenes on noble metals**

2005, Käfer, 2007, Witte, 2004 and references therein).

hexagon groups, still arranged in a pseudo-order related to graphitic-like features in diffraction, as discussed in ref. Pedio 1999.

Upon deposition of 1 ML of C60 on Au(110) it is possible to observe by STM a nice hexagonal pattern of the fullerene layer showing alternating brighter and darker lines. The uncovered regions of Au modify their reconstruction from 1x2 to a 1x5 surface cell. This 1x5 reconstruction is in perfect register with the brighter and darker lines in the STM images leading the authors to conclude that this kind of reconstruction was extending under the fullerene layer (Modesti et al. 1996 cited in Pedio et al. 2000). On clean 2x1- Au(110) deposition of few monolayers of C60 followed by thermal annealing leads to the formation of large terraces showing a (6x5) reconstruction. The evolution of the IPES spectra of C60 deposited on Au(110) for different molecular depositions at room temperature (RT) (Pedio et al. 1995 cited in Pedio et al. 2000) shows, for coverages < 1 ML, that all the molecular structures are broader and their energy separation is different with respect to the multilayer spectrum.

The first structure (LUMO) is practically not detectable and a clearly Fermi level emission is present. As soon as we reach the second layer, the LUMO feature appears clearly, though the peaks are still broader than in the multilayer system. For 1 ML deposition at about 450 °C we observed, in the LEED pattern, a (6x5) superstructure. The energy position of the features in the IPES spectra corresponds to the peaks of the 1ML C60 grown at RT, while their intensities resulr much more pronounced. The LUMO emission is confused with the Fermi. The topmost spectrum of Fig. 17 b) shows the 6x5 C60/Au(110). During the formation of the first monolayer, when the system is annealed (either during C60 deposition or after) there is a strong deformation of *all* the molecular C60 localized states due to a charge transfer from the substrate to the C60 indicating a chemisorbed phase. SXRD data confirmed a strong redistribution of the substrate Au atoms induced by the formation of the 6x5 reconstruction after thermal annealing. When analyzed with xray diffraction the structure of the interface appeared to be much more complicated. The substrate reconstruction was not a "simple" 1x5 but a complex 6x5 reconstruction with no obvious symmetries. This makes the data analysis an almost impossible task. For solving this system we have applied direct methods which are able to provide hints on the electronic density maps of the surface unit cell (Pedio, 2000).

Starting from the model proposed by the direct methods, we have been able to refine the analysis and obtain a full 3D solution of the system (Hinterstein 2008). The peculiarity of this solution is the proof that fullerene adsorption induces a strong mass transport involving several layers of the substrate. The gold surface modifies itself in order to form a kind of calyx structure where the fullerene molecules fit. Using this proposed structure we have been able to calculate the electronic properties of the interface which can then be compared with the PES data (Fig. 18). The larger contact area between the C60 and the metal, resulting from the substrate rearrangement, allows for the formation of strong directional C-Au bonds. The theoretical simulations based on the SXRD model can single out the contributions of HOMO and LUMO of the spectra in PES and IPS (see Fig. 18), and clarify the LUMO contribution and the "ionicity" character of the bond.

Summarizing, in C60 deposited on (111) and (110) substrates the formation of directional bonds between C and substrate reconstructed atoms is the important factor of the high degree of metal surface deformation. These dimples formation is a thermally activated process that produce an anchoring of the molecule. The molecular states are results strongly hybridised.

Fig. 18. Left: Comparison between experimental and calculated in-plane data corresponding to the C60/Au(110)-p(6x5) surface reconstruction. The measured values and their associated uncertainties are proportional to the radii of the two empty semicircles. The filled semicircles are proportional to the calculated values using the final structure Right: Lateral views of the C60/Au(110)-p(6x5) surface final structure. The observed corrugation has a height of one atomic level

#### **4.2 Acenes on noble metals**

274 Noble Metals

hexagon groups, still arranged in a pseudo-order related to graphitic-like features in

Upon deposition of 1 ML of C60 on Au(110) it is possible to observe by STM a nice hexagonal pattern of the fullerene layer showing alternating brighter and darker lines. The uncovered regions of Au modify their reconstruction from 1x2 to a 1x5 surface cell. This 1x5 reconstruction is in perfect register with the brighter and darker lines in the STM images leading the authors to conclude that this kind of reconstruction was extending under the fullerene layer (Modesti et al. 1996 cited in Pedio et al. 2000). On clean 2x1- Au(110) deposition of few monolayers of C60 followed by thermal annealing leads to the formation of large terraces showing a (6x5) reconstruction. The evolution of the IPES spectra of C60 deposited on Au(110) for different molecular depositions at room temperature (RT) (Pedio et al. 1995 cited in Pedio et al. 2000) shows, for coverages < 1 ML, that all the molecular structures are broader and their energy separation is different with

The first structure (LUMO) is practically not detectable and a clearly Fermi level emission is present. As soon as we reach the second layer, the LUMO feature appears clearly, though the peaks are still broader than in the multilayer system. For 1 ML deposition at about 450 °C we observed, in the LEED pattern, a (6x5) superstructure. The energy position of the features in the IPES spectra corresponds to the peaks of the 1ML C60 grown at RT, while their intensities resulr much more pronounced. The LUMO emission is confused with the Fermi. The topmost spectrum of Fig. 17 b) shows the 6x5 C60/Au(110). During the formation of the first monolayer, when the system is annealed (either during C60 deposition or after) there is a strong deformation of *all* the molecular C60 localized states due to a charge transfer from the substrate to the C60 indicating a chemisorbed phase. SXRD data confirmed a strong redistribution of the substrate Au atoms induced by the formation of the 6x5 reconstruction after thermal annealing. When analyzed with xray diffraction the structure of the interface appeared to be much more complicated. The substrate reconstruction was not a "simple" 1x5 but a complex 6x5 reconstruction with no obvious symmetries. This makes the data analysis an almost impossible task. For solving this system we have applied direct methods which are able to provide hints on the

Starting from the model proposed by the direct methods, we have been able to refine the analysis and obtain a full 3D solution of the system (Hinterstein 2008). The peculiarity of this solution is the proof that fullerene adsorption induces a strong mass transport involving several layers of the substrate. The gold surface modifies itself in order to form a kind of calyx structure where the fullerene molecules fit. Using this proposed structure we have been able to calculate the electronic properties of the interface which can then be compared with the PES data (Fig. 18). The larger contact area between the C60 and the metal, resulting from the substrate rearrangement, allows for the formation of strong directional C-Au bonds. The theoretical simulations based on the SXRD model can single out the contributions of HOMO and LUMO of the spectra in PES and IPS (see Fig. 18), and clarify

Summarizing, in C60 deposited on (111) and (110) substrates the formation of directional bonds between C and substrate reconstructed atoms is the important factor of the high

diffraction, as discussed in ref. Pedio 1999.

respect to the multilayer spectrum.

electronic density maps of the surface unit cell (Pedio, 2000).

the LUMO contribution and the "ionicity" character of the bond.

Large acenes, C4n+2 H2n+4 (Figure 11 b) are conjugated organic molecules composed by *n* hexagonal rings, with H atoms, which saturate the frontier C orbitals. The molecular states orientation is perpendicular to the molecular plane. Acenes form molecular crystals with triclinic structure showing polymorphism in the stacking d(001)-spacing (Mattheus 2001). In these crystals the electronic mobility is comparable with silicon (Schöhn 2000a in Mattheus 2001).

Thin films of tetracene (Tc, C18H10) and pentacene (Pn, C22H14), appear most promising candidates for the building of molecular organic semiconductors. Their transport properties in case of well-ordered thin films is highly anisotropic and is strongly influenced by the first layer structural, optical and electronic properties (Lukas, 2001, Söhnchen, 2004 Danişman, 2005, Käfer, 2007, Witte, 2004 and references therein).

The recent literature is quite rich on this topic. The bond of acenes on metals, have been revealed weaker than that of C60 on the same substrates. The presence of the H atoms at the molecular borders and their morphology reduce the possibility of hybridization with metal states. Different ordered phases are found for Tc and Pn adsorption on noble metals Pn/Au(111) (Witte 2004) (Schroder2003), Pn/Au(110) (Floreano2006, Badvek2008), Pn/Au(111) (Kafer 2007a, Duhm 2008), Pn/Ag(111),(Danişman 2005, Eremtchenko 2005, Pedio 2007, Kaefer2007b), Tc/Ag(111) (Langner et al. 2005) and in some cases the phase diagram appears complex.

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 277

modelled by an electrostatic model. The existence of a surface dipole built into molecular layers is conceptually different from the surface dipole at metal Surfaces. Its origin lies in the details of the molecular electronic structure and its magnitude depends on the orientation of

The analysis of the dependence by incident light polarization of C K-edge NEXAFS of the 1 ML Pn on Ag(111) indicate that the Pn molecule is chemisorbed with a tilt angle of 10°+/- 5°. Higher tilt angle 25°+/- 5° were found for submonolayer coverages. This deformation of the LUMO with respect to the LUMO transitions in gas phase Pn (Alagia, 2005) is present for coverages close to the single layer (Figure 19, Pedio2007). This framework in the Pn/Ag(111) is also suggested by (Kaefer 2007b). The electronic molecular states changes across this orientation transition. XAS spectra show a redistribution of the oscillator strength in the C 1s LUMO excitations as a function of the Pn coverage, together with modifications

Valence band UPS and NEXAFS spectroscopies applied to Tc and Pn adsorption on Ag(111) and Au(111), from subML to 1 ML coverages, show differences in the HOMO and LUMO states related to a redistribution of the molecular states (Pedio, 2008). From NEXAFS, LUMO results strongly modified in the cases of Tc and Pn deposited on Ag(111) substrate and less

> 284 286 288 290 292 294 296 298 **Photon Energy (eV)**

Fig. 19. C K edge XAS of Pentacene on Ag(111) at different molecular coverages in monolayer taken at the magic angle (Pedio 2007). Gas phase spectrum is shown (top); the bars indicate the calculated contributions from different inequivalent C atoms (see numbers

**0.3 ML**

**0.5 ML**

**1 ML**

**10°**

**27°**

**25°**

**2 ML**

**3 ML**

**Multilayer**

**3 1 gas phase 5**

**C K-edge**

molecules relative to the surface of an ordered assembly.

of the density of states in the filled HOMO region.

**1 5 3**

in the sketch of pentacene molecule).

**LUMO**

**6 4 2**

 **resonances**

**6 4 2**

**LUMO+1**

perturbed on Au(111).

As in other cases the growth mode of acenes is controlled by the nature of the substrate surface and the molecular orientation is determined by a competition of molecule-molecule vs molecule-substrate interactions. On noble metal surfaces acenes have a nearly flat orientation likely to allow the conjugated molecular orbital to overlap at different degree with the metal electronic states, while on semiconductors acenes has the tendency to standing up. The orientation of pentacene molecules is controlled by the electronic structure of the substrate, namely the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene *π* orbitals. A reduction of this density, as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5×2)Au surfaces, results in pentacene standing up (Thayer 2005).

The interaction of pentacene with metal surfaces has been studied extensively (Toyoda, 2010 and references therein) Pn/Cu(111) and Cu(110) (Lukas 2002, Lukas 2004). The intermolecular orientation onto noble metal surfaces, generally, adopts the preferred closedpacked side-by-side orientation Lukas 2002, Lukas 2004, but particular geometry of the substrate can induce a head-to head configuration (see for example Floreano, 2006). The film defects formation versus growth protocols and their effect on the subsequent grown layers, are widely discussed in the literature as well as the role of surface defects and stepped surfaces (Lukas 2001, Danişman 2005).

A recent DFT theoretical work relates the distance of the molecular plane of Pn from fcc (111) surfaces, with work function changes and the with hybridization of molecular states with substrate electronic properties. Calculation found that the Au(111) is rather "inert" to Pn layer, while pentacene interacting with Ag(111) and Cu(111) induces hybridization and a weak chemical bond. The sequence of the bond strength follows the Hammer Nørskov reactivity model (section 3). Similarly for the PTCDA molecule on (111) the strength of the bond follows the same trend (Duhm et al. 2008 b). On the other hand the interaction has been interpreted as mainly due to a redistribution of the metal substrate charge ("cushion" effect Witte 2005, Bredas 2010, Li 2009) drawn back by the deposition of weak adsorbates. This effect could lead to work function variations after acenes deposition, compatible with the experimental values on noble metals discussed by Baldacchini et al., 2006.

High Resolution photoemission of the Pn/Cu(119) and theoretical DFT results indicate that pentacene adsorbed on noble metals has the tendency to redistribute states upon a weak "chemisorption" (Ferretti 2007).

For single layer of Pn/Cu(111) and Pn/Cu(110) the broadening of the NEXAFS features (Lukas, 2004) to was attributed to the electron coupling of the the molecular orbital. Based on PES measurements, some kind of hybridization has been proposed for Pn/Cu(111) and Pn/Ag(111) (Jaeckel, 2008), while the molecule-substrate interaction for Pn/Au(111) is considered weaker.

Pn on Au(111) is an example of physisorptive organic/metal interface (Koch 2007). The interaction Pn\_Au is weak, non-covalent and resulting from different contributions (Duhm2008). In case of Au(111) the Pn forms a variety of ordered phases, up to the completion of the single monolayer. The Au(111) herringbone reconstruction is still detectable by STM images (Schöder2003, Pong et al. 2009). The flat orientation (Käfer2007) of the first layer is related to a W of 0.95 eV and to the formation of an intermolecular dipole

As in other cases the growth mode of acenes is controlled by the nature of the substrate surface and the molecular orientation is determined by a competition of molecule-molecule vs molecule-substrate interactions. On noble metal surfaces acenes have a nearly flat orientation likely to allow the conjugated molecular orbital to overlap at different degree with the metal electronic states, while on semiconductors acenes has the tendency to standing up. The orientation of pentacene molecules is controlled by the electronic structure of the substrate, namely the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene *π* orbitals. A reduction of this density, as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5×2)Au surfaces,

The interaction of pentacene with metal surfaces has been studied extensively (Toyoda, 2010 and references therein) Pn/Cu(111) and Cu(110) (Lukas 2002, Lukas 2004). The intermolecular orientation onto noble metal surfaces, generally, adopts the preferred closedpacked side-by-side orientation Lukas 2002, Lukas 2004, but particular geometry of the substrate can induce a head-to head configuration (see for example Floreano, 2006). The film defects formation versus growth protocols and their effect on the subsequent grown layers, are widely discussed in the literature as well as the role of surface defects and stepped

A recent DFT theoretical work relates the distance of the molecular plane of Pn from fcc (111) surfaces, with work function changes and the with hybridization of molecular states with substrate electronic properties. Calculation found that the Au(111) is rather "inert" to Pn layer, while pentacene interacting with Ag(111) and Cu(111) induces hybridization and a weak chemical bond. The sequence of the bond strength follows the Hammer Nørskov reactivity model (section 3). Similarly for the PTCDA molecule on (111) the strength of the bond follows the same trend (Duhm et al. 2008 b). On the other hand the interaction has been interpreted as mainly due to a redistribution of the metal substrate charge ("cushion" effect Witte 2005, Bredas 2010, Li 2009) drawn back by the deposition of weak adsorbates. This effect could lead to work function variations after acenes deposition, compatible with the experimental values on noble metals discussed by

High Resolution photoemission of the Pn/Cu(119) and theoretical DFT results indicate that pentacene adsorbed on noble metals has the tendency to redistribute states upon a weak

For single layer of Pn/Cu(111) and Pn/Cu(110) the broadening of the NEXAFS features (Lukas, 2004) to was attributed to the electron coupling of the the molecular orbital. Based on PES measurements, some kind of hybridization has been proposed for Pn/Cu(111) and Pn/Ag(111) (Jaeckel, 2008), while the molecule-substrate interaction for Pn/Au(111) is

Pn on Au(111) is an example of physisorptive organic/metal interface (Koch 2007). The interaction Pn\_Au is weak, non-covalent and resulting from different contributions (Duhm2008). In case of Au(111) the Pn forms a variety of ordered phases, up to the completion of the single monolayer. The Au(111) herringbone reconstruction is still detectable by STM images (Schöder2003, Pong et al. 2009). The flat orientation (Käfer2007) of the first layer is related to a W of 0.95 eV and to the formation of an intermolecular dipole

results in pentacene standing up (Thayer 2005).

surfaces (Lukas 2001, Danişman 2005).

Baldacchini et al., 2006.

considered weaker.

"chemisorption" (Ferretti 2007).

modelled by an electrostatic model. The existence of a surface dipole built into molecular layers is conceptually different from the surface dipole at metal Surfaces. Its origin lies in the details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly.

The analysis of the dependence by incident light polarization of C K-edge NEXAFS of the 1 ML Pn on Ag(111) indicate that the Pn molecule is chemisorbed with a tilt angle of 10°+/- 5°. Higher tilt angle 25°+/- 5° were found for submonolayer coverages. This deformation of the LUMO with respect to the LUMO transitions in gas phase Pn (Alagia, 2005) is present for coverages close to the single layer (Figure 19, Pedio2007). This framework in the Pn/Ag(111) is also suggested by (Kaefer 2007b). The electronic molecular states changes across this orientation transition. XAS spectra show a redistribution of the oscillator strength in the C 1s LUMO excitations as a function of the Pn coverage, together with modifications of the density of states in the filled HOMO region.

Valence band UPS and NEXAFS spectroscopies applied to Tc and Pn adsorption on Ag(111) and Au(111), from subML to 1 ML coverages, show differences in the HOMO and LUMO states related to a redistribution of the molecular states (Pedio, 2008). From NEXAFS, LUMO results strongly modified in the cases of Tc and Pn deposited on Ag(111) substrate and less perturbed on Au(111).

Fig. 19. C K edge XAS of Pentacene on Ag(111) at different molecular coverages in monolayer taken at the magic angle (Pedio 2007). Gas phase spectrum is shown (top); the bars indicate the calculated contributions from different inequivalent C atoms (see numbers in the sketch of pentacene molecule).

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 279

phase, but they display a wider separation since new chains are accommodated in between, with the additional molecules aligned head-to-tail along the 110 direction and tilted by approximately 90 around their molecular axis. This structure, formed by three molecules

In fig. 20 the HAS measurements shows the disappearing of the ½ peak of the gold reconstruction indicating a deconstruction of the Au(110) missing row caused by the

Fig. 21. Models of the 3x6 and 6x8 Pn/Au(110) reconstruction, based on He scattering, STM and NEXAFS measurements. The directional pai states of the Pn are presented in red.

These results enlighten the local rearrangement of the charge at the interface. A possible depletion of electron charge in correspondence of the new chains, inducing an upstanding

Porphyrine derivatives constitute the essential functional units for many processes and Nature demonstrates their great functionality, variability and chemical stability in many cases. Metalloporphyrin are base components of biological molecules as chlorophyll (Mn

They are formed by a macrocycle and surrounding groups (Fig. 11 right). The central porphine ring is composed by 4 Pyrrole rings linked by methine bridges (porphyrins) or N (Phtalocyanins). By varying the choice of the metal center, the bonding and characteristics of the metalloporphyrin or the phtalocyanine may be dramatically affected. This is reflected in

A clear description of the electronic properties of porphirinoids is still under debate in the literature, due to the high degree of variation in the simulations. The energy sequence of the valence and conduction electronic states show discrepancies, indicating that the energy ordering in metal-organic molecules crucially depends on the electronic solution employed

Porphyrins films grown by various methods (UHV vapour deposition, solutions etc) received a paramount interest and the literature is extremely rich. New interesting results have been achieved recently, as for example a Kondo effect related to the spin of the central atom in Co and Fe phtalocyanines on Au(111), the induced magnetic order of Mn

porphyrin), emoglobin (Fe porphyrin) and are efficient in the Oxygen exchange.

and the functional theory adopted (Stenuit et al., 2010, Maron et al. 2008).

STM

molecular deposition. This has been also confirmed by preliminary SXRD results4.

per unit cell (two flat and one tilted), forms large domains of nanorails.

orientation of Pn, can be inferred.

**4.3 Porphyrine derivatives on noble metals**

spectroscopic measurements as XPS and XAS.

4 L.Floreano privat communication

Further work is in progress to understand the hybridization degree and the dispersion of the molecular states of the various presented systems. In any case we can indicate a progression of the bond from the stronger Pn/Ag(111) to the weaker Tc/Au(111) (Pedio, 2008). The modification of the HOMO and LUMO molecular states confirms the trend found by DFT (Toyoda 2010) and supporting the modification of the molecular states of adsorbed acenes on the Ag(111) substrate and the weaker interaction with Au(111).

A peculiar case is the Pn/Au(110). Even in this case the phase diagram is complex (Figures 20 and 21). A multitechnique approach permitted to determine the molecular orientation of the 6x3, 3x6 and 6x8 phases.

The phase diagram indicates that that two-dimensional commensurate growth only occurs in the monolayer range for a substrate temperature, *T*s, higher than about 370 K. The highest coverage ordered phase displays a (6 x 8) symmetry and corresponds to the saturation coverage at *T*s = 420 K. The (3 x 6) phase corresponds to the saturation coverage of the first layer at *T*s =470 K. Helium scattering (HAS) experiments provide information similar to the S-XRD. Figure 20 shows HAS along the [001], Y, direction collected at RT after deposition at *T*s = 420 K for the (6 x8) phase (filled triangles) and *T*s = 470 K for the (6 x 3) and (3 x 6) phases (open circles and filled squares, respectively). The 2-fold symmetry pattern of the clean missing row reconstructed substrate is also reported at the bottom of the graphic (full line). As a guide to the eye, fractional peaks of the eighth order are indicated at the bottom *x*  axis, while fractional peaks of the sixth order are reported at the top *x* axis.

Fig. 20. a) Phase diagram of pentacene films on Au(110). The (6 x 3) phase is quoted since it has not a well defined equilibrium coverage. The completion of the first monolayer is conventionally assigned between the (3 x 6) and (6 x 8) phases. In figure b are shown the peaks of the different ordered phase

The models of the systems were obtained by means of HAS (Floreano, 2006), NEXAFS and STM (Badvek, 2008). In the monolayer range, pentacene forms two commensurate phases with (3 x 6) and (6 x 8) symmetry. The (3 x 6) phase only contains equivalent, perfectly flat molecules that are aligned side-by-side along [001], thus forming crosstie chains extending along the 110 direction. These chains are also preserved in the high coverage (6 x 8)

Further work is in progress to understand the hybridization degree and the dispersion of the molecular states of the various presented systems. In any case we can indicate a progression of the bond from the stronger Pn/Ag(111) to the weaker Tc/Au(111) (Pedio, 2008). The modification of the HOMO and LUMO molecular states confirms the trend found by DFT (Toyoda 2010) and supporting the modification of the molecular states of

A peculiar case is the Pn/Au(110). Even in this case the phase diagram is complex (Figures 20 and 21). A multitechnique approach permitted to determine the molecular orientation of

The phase diagram indicates that that two-dimensional commensurate growth only occurs in the monolayer range for a substrate temperature, *T*s, higher than about 370 K. The highest coverage ordered phase displays a (6 x 8) symmetry and corresponds to the saturation coverage at *T*s = 420 K. The (3 x 6) phase corresponds to the saturation coverage of the first layer at *T*s =470 K. Helium scattering (HAS) experiments provide information similar to the S-XRD. Figure 20 shows HAS along the [001], Y, direction collected at RT after deposition at *T*s = 420 K for the (6 x8) phase (filled triangles) and *T*s = 470 K for the (6 x 3) and (3 x 6) phases (open circles and filled squares, respectively). The 2-fold symmetry pattern of the clean missing row reconstructed substrate is also reported at the bottom of the graphic (full line). As a guide to the eye, fractional peaks of the eighth order are indicated at the bottom *x*

 (a) (b) Fig. 20. a) Phase diagram of pentacene films on Au(110). The (6 x 3) phase is quoted since it has not a well defined equilibrium coverage. The completion of the first monolayer is conventionally assigned between the (3 x 6) and (6 x 8) phases. In figure b are shown the

The models of the systems were obtained by means of HAS (Floreano, 2006), NEXAFS and STM (Badvek, 2008). In the monolayer range, pentacene forms two commensurate phases with (3 x 6) and (6 x 8) symmetry. The (3 x 6) phase only contains equivalent, perfectly flat molecules that are aligned side-by-side along [001], thus forming crosstie chains extending along the 110 direction. These chains are also preserved in the high coverage (6 x 8)

1.5

HAS **Y**

AuY : 4.078 Å

1.0

He diffraction scaled to the (0,0) peak

0.5

0.0

adsorbed acenes on the Ag(111) substrate and the weaker interaction with Au(111).

axis, while fractional peaks of the sixth order are reported at the top *x* axis.

the 6x3, 3x6 and 6x8 phases.

peaks of the different ordered phase

phase, but they display a wider separation since new chains are accommodated in between, with the additional molecules aligned head-to-tail along the 110 direction and tilted by approximately 90 around their molecular axis. This structure, formed by three molecules per unit cell (two flat and one tilted), forms large domains of nanorails.

In fig. 20 the HAS measurements shows the disappearing of the ½ peak of the gold reconstruction indicating a deconstruction of the Au(110) missing row caused by the molecular deposition. This has been also confirmed by preliminary SXRD results4.

Fig. 21. Models of the 3x6 and 6x8 Pn/Au(110) reconstruction, based on He scattering, STM and NEXAFS measurements. The directional pai states of the Pn are presented in red.

These results enlighten the local rearrangement of the charge at the interface. A possible depletion of electron charge in correspondence of the new chains, inducing an upstanding orientation of Pn, can be inferred.

#### **4.3 Porphyrine derivatives on noble metals**

Porphyrine derivatives constitute the essential functional units for many processes and Nature demonstrates their great functionality, variability and chemical stability in many cases. Metalloporphyrin are base components of biological molecules as chlorophyll (Mn porphyrin), emoglobin (Fe porphyrin) and are efficient in the Oxygen exchange.

They are formed by a macrocycle and surrounding groups (Fig. 11 right). The central porphine ring is composed by 4 Pyrrole rings linked by methine bridges (porphyrins) or N (Phtalocyanins). By varying the choice of the metal center, the bonding and characteristics of the metalloporphyrin or the phtalocyanine may be dramatically affected. This is reflected in spectroscopic measurements as XPS and XAS.

A clear description of the electronic properties of porphirinoids is still under debate in the literature, due to the high degree of variation in the simulations. The energy sequence of the valence and conduction electronic states show discrepancies, indicating that the energy ordering in metal-organic molecules crucially depends on the electronic solution employed and the functional theory adopted (Stenuit et al., 2010, Maron et al. 2008).

Porphyrins films grown by various methods (UHV vapour deposition, solutions etc) received a paramount interest and the literature is extremely rich. New interesting results have been achieved recently, as for example a Kondo effect related to the spin of the central atom in Co and Fe phtalocyanines on Au(111), the induced magnetic order of Mn

 4 L.Floreano privat communication

Organic Molecules on Noble Metal Surfaces: The Role of the Interface 281

density guides the interaction, from charge transfer to directional bonds, between molecules

Noble metal surfaces are valuable substrates for building self assembled organic thin layers

 Low chemical reactivity together with high atomic surface mobility resulting in the capability of reordering after molecular deposition. Often order is thermally activated.

 Guide for the molecular ordering through the symmetry of the surface. If commensurable the overlayer must follow the substrate symmetry. In case the intermolecular interactions prevail, in strength, compared to the molecular-structural

 High electron density at the crystal surface (in case of (111) in Au, Ag and Cu the electrons form a quasi-two-dimensional gas at surface) providing a charge reservoir.

The anchoring of the molecules is due to a combination of effects (Rosei 2005): i) Adaptation of the surface geometry Larger interaction area, that allows to the molecule to fit into the local structure of substrates ; ii) Higher reactivity of the substrate (stronger bond) facilitates the creation of nanodimple sites underneath the molecules. A simple correlation exists between the bonding strength of a molecule and the metal coordination number of the adsorption site *d* -band model by Hammer and Nørskov. iii) The adsorption inducing restructuring is typically thermally activated: it provides an Energy gain in the adsorption energy *E*ad of the molecules on the reconstructed surface, balancing the energy cost *E*metal

The formation of directional bonds seems the base mechanism for the nanopit formation, at least for strong chemisorbed species (as fullerene). C60 induced reconstruction of substrates is thermally activated, even on the close packed fcc (111) surfaces. The molecule results locally perturbed with the electronic properties (particularly the LUMO orbitals), that are modified by the interaction. The structural characterizationof the molecules onto the (110) substrate demonstrates the formation of NANOPITS and/or

In case of "flat" and conjugated molecules, as acenes and prophyrin derivatives, the interrelations of the various aspects (morphology, dipoles-electrostatic configuration at interfaces, distribution of the charge of the metal substrate etc…) are particularly evident. Molecular orientation, long range ordening, surface reconstruction and charge (re)distribution (interface dipole, molecular dipole, "cushion" effect) are all present at different but never negligible degree, depending on the chosen molecule and on the

Africh C. 2011. Surface Structure and Reactivity laboratory CNR-IOM, TASC Available

Alagia, M.; Baldacchini, C. Betti, M. G. Bussolotti, F. Carravetta, V. Ekström, U.; Mariani, C.

& Stranges S. (2005). Core-shell photoabsorption and photoelectron spectra of gas-

The details of interaction are crucial for the functionalization of the molecular films.

Good degree of morphological order of the crystal surfaces

the layer can by uncommensurate.

required to break the metal bonds

FACETTING of the metal surface.

**6. References**

substrate morphology/surface energy/reactivity.

from: www.tasc.infm.it

and substrate.

because of their:

porphyrins on Co films as well as magnetic coupling effects (Wende et al. 2007 cited in Fanetti et al. 2011). Recently in the CoEOP deposited onto Ag(110) (Fanetti et al 2011), a charge transfer to the LUMO orbital has been found. DFT calculations indicate that the molecular interaction with the substrate leads to the electron transfer from the Ag substrate to the molecule. This can be responsible of the steric adsorption of the CoOEP, that results titled of 15°, at variance with other similar systems.

Porphyrins can be deposited in highly ordered single monolayers on metal surfaces, leading to ordered structures. Covalent interaction between the central metal atom and substrates, the "Cushion" effect induced at the noble metal surface, and the out-of-plane central atom dipole moments plays a role in the different cases. No clear trend, as a function of external group, central metal atom element and substrate element and structure, is still available.

On the noble metal (111) surfaces, and often on the (100) planes, both porphyrins and phtalocyanines are found to lay in a planar-like configuration presenting large terraces domains. In case of Au(111) substrates, the herringbone reconstruction remains detectable. The long range ordening of these "floating" molecular layers show dependency on the substrate element and on the kind of porphyrinoid and central metal atom. For example differences between single layers Cu and Ni octaethylporphyrins deposited on Au(111) have been detected in the diffraction patterns and in the electronic properties (Resta et al. 2010).

The interaction with (110) substrates appears different. Restructuring has been indicated by STM in case of Cu-phtalocyanine (CuPc) deposited on Ag(110) (ref. Bohringer et al. 1997 cited in Rosei et al. 2003). CuPc induces a transition from thermodynamically controlled 3 dimensional faceting at submonolayer coverage, to a kinetically dominated 2-dimensional step faceting process, when coverage approaches a full monolayer. Transition metalphtalocyanine on Au(110) surfaces result regularly spaced. High Resolution PES show interface electronic states close to the Fermi level. Their energies are different from CuPc to FePc and CoPc molecules. Structurally CuPc on Au(110) present a complex phase diagram with strong mass transport of the substrate as a function of the molecular coverage from submonolayer to the 1 ML (Floreano 2008). Again the molecular deposition deconstructs the Au missing row and a set of higher periodicities (x5, x3, x7) are progressively detected along the [001] direction. In conclusion coverage-dependent transformation on fcc 110 substrates seems to be driven by the molecule−substrate interaction. This interaction seems to take place in other porphyrinoid deposited onto fcc (111) substrates but, in any case it appears such that no mass transport of these substrate takes place.

Due to the complexity of the roles played, in the balance among interactions, by the central metal atom in the porphyn ring, by the external groups and frontier orbitals, and by the interaction with the metal substrates (polarization, dipoles etc.), a complete understanding of these systems deserve further study.

Note, finally, that in some cases the molecular morphology results altered by STM (Donovan et al. 2010).

#### **5. Conclusions**

Noble metal surfaces offer a variety of structural/electronic moieties which can be used in the organic-inorganic engineering. The comparisons of the similarities and differences of the (111) and (110) surfaces show how the charge reservoir of isotropic or anisotropic electron

porphyrins on Co films as well as magnetic coupling effects (Wende et al. 2007 cited in Fanetti et al. 2011). Recently in the CoEOP deposited onto Ag(110) (Fanetti et al 2011), a charge transfer to the LUMO orbital has been found. DFT calculations indicate that the molecular interaction with the substrate leads to the electron transfer from the Ag substrate to the molecule. This can be responsible of the steric adsorption of the CoOEP, that results

Porphyrins can be deposited in highly ordered single monolayers on metal surfaces, leading to ordered structures. Covalent interaction between the central metal atom and substrates, the "Cushion" effect induced at the noble metal surface, and the out-of-plane central atom dipole moments plays a role in the different cases. No clear trend, as a function of external group, central metal atom element and substrate element and structure, is still available.

On the noble metal (111) surfaces, and often on the (100) planes, both porphyrins and phtalocyanines are found to lay in a planar-like configuration presenting large terraces domains. In case of Au(111) substrates, the herringbone reconstruction remains detectable. The long range ordening of these "floating" molecular layers show dependency on the substrate element and on the kind of porphyrinoid and central metal atom. For example differences between single layers Cu and Ni octaethylporphyrins deposited on Au(111) have been detected in the diffraction patterns and in the electronic properties (Resta et al. 2010). The interaction with (110) substrates appears different. Restructuring has been indicated by STM in case of Cu-phtalocyanine (CuPc) deposited on Ag(110) (ref. Bohringer et al. 1997 cited in Rosei et al. 2003). CuPc induces a transition from thermodynamically controlled 3 dimensional faceting at submonolayer coverage, to a kinetically dominated 2-dimensional step faceting process, when coverage approaches a full monolayer. Transition metalphtalocyanine on Au(110) surfaces result regularly spaced. High Resolution PES show interface electronic states close to the Fermi level. Their energies are different from CuPc to FePc and CoPc molecules. Structurally CuPc on Au(110) present a complex phase diagram with strong mass transport of the substrate as a function of the molecular coverage from submonolayer to the 1 ML (Floreano 2008). Again the molecular deposition deconstructs the Au missing row and a set of higher periodicities (x5, x3, x7) are progressively detected along the [001] direction. In conclusion coverage-dependent transformation on fcc 110 substrates seems to be driven by the molecule−substrate interaction. This interaction seems to take place in other porphyrinoid deposited onto fcc (111) substrates but, in any case it appears

Due to the complexity of the roles played, in the balance among interactions, by the central metal atom in the porphyn ring, by the external groups and frontier orbitals, and by the interaction with the metal substrates (polarization, dipoles etc.), a complete understanding

Note, finally, that in some cases the molecular morphology results altered by STM

Noble metal surfaces offer a variety of structural/electronic moieties which can be used in the organic-inorganic engineering. The comparisons of the similarities and differences of the (111) and (110) surfaces show how the charge reservoir of isotropic or anisotropic electron

titled of 15°, at variance with other similar systems.

such that no mass transport of these substrate takes place.

of these systems deserve further study.

(Donovan et al. 2010).

**5. Conclusions**

density guides the interaction, from charge transfer to directional bonds, between molecules and substrate.

Noble metal surfaces are valuable substrates for building self assembled organic thin layers because of their:

The details of interaction are crucial for the functionalization of the molecular films.

The anchoring of the molecules is due to a combination of effects (Rosei 2005): i) Adaptation of the surface geometry Larger interaction area, that allows to the molecule to fit into the local structure of substrates ; ii) Higher reactivity of the substrate (stronger bond) facilitates the creation of nanodimple sites underneath the molecules. A simple correlation exists between the bonding strength of a molecule and the metal coordination number of the adsorption site *d* -band model by Hammer and Nørskov. iii) The adsorption inducing restructuring is typically thermally activated: it provides an Energy gain in the adsorption energy *E*ad of the molecules on the reconstructed surface, balancing the energy cost *E*metal required to break the metal bonds

The formation of directional bonds seems the base mechanism for the nanopit formation, at least for strong chemisorbed species (as fullerene). C60 induced reconstruction of substrates is thermally activated, even on the close packed fcc (111) surfaces. The molecule results locally perturbed with the electronic properties (particularly the LUMO orbitals), that are modified by the interaction. The structural characterizationof the molecules onto the (110) substrate demonstrates the formation of NANOPITS and/or FACETTING of the metal surface.

In case of "flat" and conjugated molecules, as acenes and prophyrin derivatives, the interrelations of the various aspects (morphology, dipoles-electrostatic configuration at interfaces, distribution of the charge of the metal substrate etc…) are particularly evident. Molecular orientation, long range ordening, surface reconstruction and charge (re)distribution (interface dipole, molecular dipole, "cushion" effect) are all present at different but never negligible degree, depending on the chosen molecule and on the substrate morphology/surface energy/reactivity.

#### **6. References**

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**13**

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**The Effect of Addition of ppm-Order-Pd to Fe-K**

**Catalyst on Dehydrogenation of Ethylbenzene**

Ryo Watanabe, Yasushi Sekine, Masahiko Matsukata and Eiichi Kikuchi

Styrene, used for polymeric materials such as polystyrene resin, acrylonitrile–butadiene– styrene resin, and styrene–butadiene rubber, is an important monomer in the petrochemistry (Meima et al., 2001). The styrene production volume is about 23 million tons annually; more than 90% of styrene is produced by dehydrogenation of ethylbenzene, as in

 C8H10 → C8H8 + H2 (1) Dehydrogenation of ethylbenzene is an endothermic reaction. High temperatures are necessary for high ethylbenzene conversion because of its thermodynamic limitation. Potassium promoted iron oxide (Fe–K) has been applied for dehydrogenation of ethylbenzene as an industrial catalyst (Lee, 1974). The Fe–K catalyst can be further modified by the addition of other promoters such oxides as MgO, Cr2O3, CeO2 and MoO3 (Eggertsen

Generally, radial-type adiabatic reactors have been applied in commercial processes for dehydrogenation of ethylbenzene (Addiego, 2001). Superheated steam is supplied to the catalyst bed with ethylbenzene as a heating source for control of the reaction temperature. Steam also acts as a diluent for shifting the equilibrium conversion higher and suppresses over-reduction of the catalyst in the dehydrogenation atmosphere. Additionally, steam has the role of elimination of carbon deposition on the catalyst surface. The removal of coke from the catalyst surface using steam treatment improved the catalytic activity and that a low H2O/ethylbenzene ratio caused coke deposition on the catalyst (Herzog et al., 1984; Devoldere et al., 1999). It was also reported that the increasing the feed molar-ratio of

The reaction mechanism of dehydrogenation of ethylbenzene on the Fe–K catalyst has been investigated widely. Langmuir–Hinshelwood (L–H) type including chemisorption of ethylbenzene and styrene proceeded on the Fe-K catalyst according to kinetic studies (Carrà et al., 1965). Analysis of infrared spectroscopy indicated that ethylbenzene adsorbed with the phenyl ring oriented to α-Fe2O3 (Busca et al., 1984). The styrene adsorption state on α-Fe2O3 catalyst was reported that styrene adsorbed via the vinyl group on the catalyst. (Addiego et al., 1994). The result of isotope exchange reaction revealed that the dissociation

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### **The Effect of Addition of ppm-Order-Pd to Fe-K Catalyst on Dehydrogenation of Ethylbenzene**

Ryo Watanabe, Yasushi Sekine, Masahiko Matsukata and Eiichi Kikuchi *Department of Applied Chemistry, Waseda University Japan*

#### **1. Introduction**

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Styrene, used for polymeric materials such as polystyrene resin, acrylonitrile–butadiene– styrene resin, and styrene–butadiene rubber, is an important monomer in the petrochemistry (Meima et al., 2001). The styrene production volume is about 23 million tons annually; more than 90% of styrene is produced by dehydrogenation of ethylbenzene, as in the following chemical equation (1) (Shekhah et al., 2004).

$$\rm{C}\_{8}H\_{10} \rightarrow \rm{C}\_{8}H\_{8} + H\_{2} \tag{1}$$

Dehydrogenation of ethylbenzene is an endothermic reaction. High temperatures are necessary for high ethylbenzene conversion because of its thermodynamic limitation. Potassium promoted iron oxide (Fe–K) has been applied for dehydrogenation of ethylbenzene as an industrial catalyst (Lee, 1974). The Fe–K catalyst can be further modified by the addition of other promoters such oxides as MgO, Cr2O3, CeO2 and MoO3 (Eggertsen et al., 1947; Kearby et al., 1945; O'Hara, 1975; Pitzer, 1957; Riesser, 1979).

Generally, radial-type adiabatic reactors have been applied in commercial processes for dehydrogenation of ethylbenzene (Addiego, 2001). Superheated steam is supplied to the catalyst bed with ethylbenzene as a heating source for control of the reaction temperature. Steam also acts as a diluent for shifting the equilibrium conversion higher and suppresses over-reduction of the catalyst in the dehydrogenation atmosphere. Additionally, steam has the role of elimination of carbon deposition on the catalyst surface. The removal of coke from the catalyst surface using steam treatment improved the catalytic activity and that a low H2O/ethylbenzene ratio caused coke deposition on the catalyst (Herzog et al., 1984; Devoldere et al., 1999). It was also reported that the increasing the feed molar-ratio of H2O/ethylbenzene decreased the carbon deposition (Coulter et al., 1995).

The reaction mechanism of dehydrogenation of ethylbenzene on the Fe–K catalyst has been investigated widely. Langmuir–Hinshelwood (L–H) type including chemisorption of ethylbenzene and styrene proceeded on the Fe-K catalyst according to kinetic studies (Carrà et al., 1965). Analysis of infrared spectroscopy indicated that ethylbenzene adsorbed with the phenyl ring oriented to α-Fe2O3 (Busca et al., 1984). The styrene adsorption state on α-Fe2O3 catalyst was reported that styrene adsorbed via the vinyl group on the catalyst. (Addiego et al., 1994). The result of isotope exchange reaction revealed that the dissociation

The Effect of Addition of ppm-Order-Pd to Fe-K Catalyst on Dehydrogenation of Ethylbenzene 289

atmospheric pressure in the presence of steam; the molar ratio of steam to ethylbenzene was 12 and WHSV was 1.2 g h-1 g-cat-1. Ethylbenzene and H2O were fed separately using a micro feeder and a micro pump, respectively. Liquid products such as ethylbenzene, benzene, toluene and styrene were analysed using off-line flame ionization detection (FID) gas chromatography (GC8A; Shimadzu Corp.) with a capillary column of DB210 (0.25 mm×30 m) using N2 as a carrier gas at 373 K. Tetralin was used as an internal standard substance. Gaseous products such as H2, CO and CH4 were analyzed using an off-line thermal conductivity (TCD) gas chromatography (GC8A; Shimadzu Corp.) with a 3 mm×2 m stainless

steel column packed with active carbon (60/80 mesh) using Ar as a carrier gas at 393 K.

Ethylbenzene conversion = ([Sty] + [Bz] + [Tol])/([EB] + [Sty] + [Bz] + [Tol])×100 (2)

In these equations, [EB], [Sty], [Bz], and [Tol], respectively represent the yield of ethylbenzene, styrene, benzene, and toluene. Carbon balances of these investigations

For investigating the effect of Pd addition to the Fe–K based catalyst on dehydrogenation activity and selectivity, activity tests of the Fe–K and 10 ppm-Pd/Fe–K catalysts were

0123456

Fig. 1. Ethylbenzene conversion and styrene selectivity of (,) Fe-K catalyst and (,)

Time on stream / h

amounted to ≥95%.

0

10

20

30

Ethylbenzene conversion / %

Pd/Fe-K catalyst.

40

50

carried out.

Styrene yield = [Sty]/([EB] + [Sty] + [Bz] + [Tol])×100 (3)

Styrene selectivity = [Sty]/([Sty] + [Bz] + [Tol])×100 (4)

80

85

90

Styrene selectivity / %

95

100

of α-hydrogen of ethylbenzene was the first step of ethylbenzene dehydrogenation, followed by the dissociation of β-hydrogen, which was the rate-determining step or post rate-determining step. (Miura et al., 1994)

Some researchers investigated the active phase of the Fe-K catalyst and showed KFeO2 phase as the active site for dehydrogenation of ethylbenzene using a combination of XPS/UPS surface analysis (Hirano et al., 1986; Muhler et al., 1989, 1992). The active state of the Fe–K catalyst was analysed by transient response experiments and reported that initially high activity was correlated with Fe3+ state and the reduction from Fe3+ to Fe2+ state caused deactivation of the catalytic activity (Zhu et al., 2004). The reaction cycle was proposed that ethylbenzene adsorbed onto a terrace Fe3+ site; then C–H in the ethyl group was deprotonated by lattice oxygen at a step site. H2O containing a lattice oxygen was postulated to desorb on the catalyst instead of H2, meaning that oxidative dehydrogenation proceeded on the Fe-K catalyst (Weiss et al., 2000).

Recently, it was reported that the addition of small amounts of Pd or Pt enhanced the activity of Fe–K based catalysts (Williams et al., 2002). Previously we investigated hydrogen production by the steam–iron reaction using iron oxide modified with very small amounts of Pd at the temperature of 723 K and under atmospheric pressure. The addition of ppmorder Pd to iron oxide enhanced rates of the reduction by H2 and the oxidation by H2O (Urasaki et al., 2005). This work is intended to clarify the role of added precious metals such as Pd and Pt in relation to the reaction mechanism of dehydrogenation of ethylbenzene on the Fe–K catalyst (Sekine et al., 2008).

#### **2. The activity for dehydrogenation of ethylbenzene over ppm-order Pd/Fe-K catalyst**

For investigating the effect of Pd addition to the Fe-K catalyst on dehydrogenation activity and selectivity to styrene, we compared the result of activity tests on the Fe-K catalyst and Pd-promoted Fe-K catalyst. (Section 2.1) And also, the effect of Pd loading on dehydrogenation activity was investigated in the section 2.2.

#### **2.1 Comparison of the activity and selectivity over Pd/Fe-K catalyst to those over bare Fe-K catalyst**

Catalysts used in this study were commercially available Fe-K catalyst and Pd/Fe-K catalyst prepared by an impregnation method on the Fe-K catalyst. Fe-K mixed oxide which contained some promoters such as 10 wt%K2O, 10 wt%CeO2 and trace amounts of MoO3, MgO and CaO immersed in an aqueous solution of the metal precursor salt of Pd(NO3)2. The amount of Pd was 10, 30, 300 and 1000 ppm. Then, the resulting slurry was dried on a hot plate under continuous stirring with subsequent calcination at 873 K for 6 h. Pt catalyst was prepared using the same impregnation method with an acetone solution of Pt(CH3COO)2.

The reactor used in this study comprised of a quartz tube (12-mm i.d.) containing the catalyst bed, which was fixed by quartz wool. A type K thermocouple enclosed in a quartz thermowell of 3-mm outer diameter was positioned inside the catalyst bed for accurate measurement of the catalyst temperature.

Catalytic activities, selectivities and stabilities were examined in a conventional fixed bed flow reactor. The weight of charged catalyst was 1.0 g. Reactions were carried out at 813 K at

of α-hydrogen of ethylbenzene was the first step of ethylbenzene dehydrogenation, followed by the dissociation of β-hydrogen, which was the rate-determining step or post

Some researchers investigated the active phase of the Fe-K catalyst and showed KFeO2 phase as the active site for dehydrogenation of ethylbenzene using a combination of XPS/UPS surface analysis (Hirano et al., 1986; Muhler et al., 1989, 1992). The active state of the Fe–K catalyst was analysed by transient response experiments and reported that initially high activity was correlated with Fe3+ state and the reduction from Fe3+ to Fe2+ state caused deactivation of the catalytic activity (Zhu et al., 2004). The reaction cycle was proposed that ethylbenzene adsorbed onto a terrace Fe3+ site; then C–H in the ethyl group was deprotonated by lattice oxygen at a step site. H2O containing a lattice oxygen was postulated to desorb on the catalyst instead of H2, meaning that oxidative dehydrogenation proceeded

Recently, it was reported that the addition of small amounts of Pd or Pt enhanced the activity of Fe–K based catalysts (Williams et al., 2002). Previously we investigated hydrogen production by the steam–iron reaction using iron oxide modified with very small amounts of Pd at the temperature of 723 K and under atmospheric pressure. The addition of ppmorder Pd to iron oxide enhanced rates of the reduction by H2 and the oxidation by H2O (Urasaki et al., 2005). This work is intended to clarify the role of added precious metals such as Pd and Pt in relation to the reaction mechanism of dehydrogenation of ethylbenzene on

**2. The activity for dehydrogenation of ethylbenzene over ppm-order Pd/Fe-K**

For investigating the effect of Pd addition to the Fe-K catalyst on dehydrogenation activity and selectivity to styrene, we compared the result of activity tests on the Fe-K catalyst and Pd-promoted Fe-K catalyst. (Section 2.1) And also, the effect of Pd loading on

**2.1 Comparison of the activity and selectivity over Pd/Fe-K catalyst to those over bare**

Catalysts used in this study were commercially available Fe-K catalyst and Pd/Fe-K catalyst prepared by an impregnation method on the Fe-K catalyst. Fe-K mixed oxide which contained some promoters such as 10 wt%K2O, 10 wt%CeO2 and trace amounts of MoO3, MgO and CaO immersed in an aqueous solution of the metal precursor salt of Pd(NO3)2. The amount of Pd was 10, 30, 300 and 1000 ppm. Then, the resulting slurry was dried on a hot plate under continuous stirring with subsequent calcination at 873 K for 6 h. Pt catalyst was prepared

The reactor used in this study comprised of a quartz tube (12-mm i.d.) containing the catalyst bed, which was fixed by quartz wool. A type K thermocouple enclosed in a quartz thermowell of 3-mm outer diameter was positioned inside the catalyst bed for accurate

Catalytic activities, selectivities and stabilities were examined in a conventional fixed bed flow reactor. The weight of charged catalyst was 1.0 g. Reactions were carried out at 813 K at

using the same impregnation method with an acetone solution of Pt(CH3COO)2.

rate-determining step. (Miura et al., 1994)

on the Fe-K catalyst (Weiss et al., 2000).

the Fe–K catalyst (Sekine et al., 2008).

measurement of the catalyst temperature.

dehydrogenation activity was investigated in the section 2.2.

**catalyst**

**Fe-K catalyst**

atmospheric pressure in the presence of steam; the molar ratio of steam to ethylbenzene was 12 and WHSV was 1.2 g h-1 g-cat-1. Ethylbenzene and H2O were fed separately using a micro feeder and a micro pump, respectively. Liquid products such as ethylbenzene, benzene, toluene and styrene were analysed using off-line flame ionization detection (FID) gas chromatography (GC8A; Shimadzu Corp.) with a capillary column of DB210 (0.25 mm×30 m) using N2 as a carrier gas at 373 K. Tetralin was used as an internal standard substance. Gaseous products such as H2, CO and CH4 were analyzed using an off-line thermal conductivity (TCD) gas chromatography (GC8A; Shimadzu Corp.) with a 3 mm×2 m stainless steel column packed with active carbon (60/80 mesh) using Ar as a carrier gas at 393 K.