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All issues Volume 98 (2023) Eur. Phys. J. Appl. Phys., 98 (2023) 7 Full HTML
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Eur. Phys. J. Appl. Phys.
Volume 98, 2023
Special Issue on ‘Advances in Renewable Energies, Materials and Technology’, edited by Laurene Tetard, Hamid Oughaddou, Abdelkader Kara, Yannick Dappe and Nabil Rochdi
Article Number 7
Number of page(s) 8
Section Surfaces and Interfaces
DOI https://doi.org/10.1051/epjap/2022220273
Published online 18 January 2023

© EDP Sciences, 2023

1 Introduction

Phthalocyanine (Pc) molecules are considered among the potential materials for the development of future organic electronics. Their chemical, and thermal stability, as well as their affordability make them attractive [1,2]. Due to the importance of metal-Pc molecules in contemporary environmental technology applications, research into their adsorption is still ongoing. For instance, several reports explored the photovoltaic characteristics of FePc/TiO2 interface [3–5]. On the other hand, silicon-based substrates are commonly utilized in solar-driven applications. On SiB surface, CrPc, MnPc, FePc and CoPc molecules are reported to form chemical bonds with the substrate. Whereas ZnPc, NiPc and CuPc molecules are bound by vdW interactions [6]. On Si(110) surface, FePc molecule's electronic structure exhibits a rehybridization near the interface [7]. Silica thin films are commonly utilized for support in catalysis and transistor construction. The adsorption of CuPc molecules on Si(111)/RCA–SiO2, Si(111)-native SiO2, n-Si(100) and SiO2/n-Si substrates exhibits an interface dipole which orientation depends on the substrate's conductivity type [8–10]. However, significantly less information has been published about the adsorption of Pc molecules on silica films. SiO2 films can be grown on several surfaces such as Mo [11], Ru [12], Pt [13] and Si [14] to name a few. Small molecules such as CO and D2 on these substrates were found to chemisorb on the Pd(111) by penetrating through the silica film [15]. Similarly, single metal atoms: Cu, Ag, Au and Pd energetically prefer to adsorb at the silica/metal interface whereas Pt atom prefers the bottom ring of silica bilayer [16]. Moreover, Fe's preferred site is below a [ī10]-oriented Si-O-Si bridge [17]. It was reported that the strength of the metal-oxygen bond is essential for determining the atomic structure of silica overlayers on metals [13]. In addition, it was observed that Ru(0001) surface has a greater oxygen affinity than of Pt(111) and Pd(111) making the Ru(0001) substrate more suitable for accommodating crystalline structure through monolayer and bilayer layers [15,18]. In this context, we modeled our system choosing both monolayer and bilayer silica supported by O(2 × 2)-Ru(0001) surface to better understand the geometric and electronic structure of interfacial FePc/silica systems. The focus of this investigation is a wide numerical and theoretical study of the bonding between the organic molecule and the Si4O10-2O/Ru(0001) and Si8O16-2O/Ru(0001) substrates.

2 Computational details

To perform a full computational investigation of the silica-based systems. We employ DFT calculations using VASP package version 5.4.1 [19–21]. The calculations are performed using GGA+ vdW in order to include the vdW interactions that it has been shown to be key to reliable results for the adsorption characteristics of small [22,23] and large molecules [24,25]. We used specifically the optB88-vdW functional [26]. The adsorption consists of first placing the silica monolayer or bilayer on oxygenated Ru(0001) surface. To do that, we followed the instructions given by Yang et al. [18]. Second, we place the FePc molecule on the composite and we relax the system until the forces fall below 0.02 eV/Å. The relaxation is performed using gamma Kpoints due to the large number of atoms in the systems. The electronic structure is calculated using 4 × 4 × 1 K points. After relaxation, we calculate the charge transfer using Bader Charge analysis as well as the PDOS of the atoms that are in interplay.

3 Results and discussion

It is necessary to first investigate the structural characteristics of these systems to understand their adsorption properties. In both systems, monolayer and bilayer, we place the center of the FePc molecule (Fe atom) in specific sites and perform a series of calculations to extract the adsorption energy as well as the adsorption height for each configuration. Once the most stable configuration is identified, we perform the electronic study for that configuration, and we compare the obtained results to those of the free FePc molecule shown in Figure 1. In fact, several studies have given a full discussion of the molecule's properties in gas phase. All findings show a C4 rotational symmetry of the FePc in gas phase. In addition, the results show S = 1 spin state where the spin densities of the N atoms are distributed (as shown in Fig. 1) antiparallel to that Fe atom, whereas the spin densities of the C atoms are aligned parallel to it. The results also show a distribution of charge within the molecule where N atoms gain electrons from the neighboring C atoms and Fe atom due to their electronegativity.

thumbnail Fig. 1

(Left) Schematic structure of FePc in gas phase. (Right) Spin density distribution of the free FePc molecule. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).

3.1 FePc/Si4O10-2O/Ru(0001)

As displayed in Table 1, the FePc molecule is adsorbed at hollow, Si-top and O-top sites. The adsorption energies of these configurations are very close only ∼50 meV difference between the three configurations. However, the findings also suggest that have different properties than stable one when adsorbed on surfaces as shown in the Charge transfer in Table 1. Therefore, the most stable configuration is at O-top site and the metastable ones are at hollow and Si-top sites. The adsorption energy is relatively small which was also the case for the metal-free phthalocyanine molecules on the 6H-SiC(0001)3 × 3 substrate [27]. Geometrically, the FePc molecule shows slight bending when adsorbed on monolayer silica displayed in Figure 2. Fe–O distance is found to be 2.81 Å which is larger than Fe–O distance reported in systems where FePc is adsorbed on oxygenated surfaces. The tilt found in the molecule suggests that the Fe atom was pushed towards the substrate by 0.10 Å from the average height of the molecule. In addition, the organic part of the molecule showed various geometry patterns. The C atoms were at the average height of the molecule whereas N and H atoms were also slightly pushed towards the substrate creating an umbrella like shape. Similarly, the substrate, especially the silica monolayer, exhibited structural changes. The O atom under Fe atom was pulled towards the molecule by 0.36 Å from the average surface height, therefore pulling the 2 neighboring Si atoms. However, the distance between O and these Si atoms was elongated by almost 0.03 Å suggesting a stronger interaction between O and Fe atoms. Baffou et al. observed that in H2Pc molecule, the 2 Nb atoms of opposite sides are close to Si atoms of the topmost layer. The distance between these 2 Nb atoms was extended pulling eventually the Si atoms towards the molecule to form a Si–Nb bonds upon adsorption [27]. Our findings, suggest that the opposite Nb–Nb distance contracted to minimize the distances with the corresponding Si atoms. These observed deformations of both the molecule and the substrate fall in accordance with the results of Baffou et al. [27].

Moreover, according to Table 1 results, the Bader Charge analysis [28] showed that the charge transfer occurring in the system goes from the molecule to the substrate. This direction of the electron transfer was observed in the experimental results of CuPc molecule adsorbed on various Si based substrates: Si(111)(√3 ×√3)R30°Ag substrate [29], Si(111) p-type/native SiO2 substrates [8] and the n-type Si(100)/SiO2 [10]. We observed that the loss of 0.59e–was mainly from the organic part of the molecule whereas 0.16e–was lost from Fe atom. This charge was transferred to both the silica monolayer and the oxygenated surface. We believe that these transfers highlight specifically the chemical interactions at the organic/inorganic interface. The silica monolayer gained 0.56e–where around 0.54e–was transferred to O atoms. In addition, 2O-Ru(0001) surface gained 0.19e–.

We proceed our investigation by studying the magnetic moment and its distribution in FePc/Si4O10-2O/Ru(0001) system. The total magnetic moment is 2.28 µB, as shown in Figure 2, the overall spin density distribution of the FePc molecule resembles that of gas phase; Ca and Fe atoms show parallel spin density distributions whereas Na atoms show antiparallel distributions. The FePc molecule shows almost identical distribution between the left and right ligand parts. However, we report that only 4 neighboring Ca atoms show spin density distribution which suggests that the FePc molecule exhibits a symmetry reduction of its C4 rotational symmetry. The top view of the FePc/Si4O10-2O/Ru(0001) shows that each opposite Nb atoms and Ca atoms attached to them (encircled in purple) are deposited on 2 different sites. In addition, the side view of Figure 2 shows a small additional spin moment localized at O atom under Fe atom. We believe that the spin state of the system is unchanged S = 1 and that the adsorption on silica monolayer and especially at the O-top site induced the coupling strength between Fe atom and its neighboring atoms, including O atom underneath. Fe and O atoms are found to interact via σ bond (see inset of Fig. 3), the PDOS of the Fe and O orbitals reveals that pz orbital of O atom and orbital of Fe are overlapping. It also shows the splitting of the pz orbital that follows that of orbital (see arrows) which confirms the magnetization of the O atom upon adsorption. It is worth noting that several reports have stated that the direct contact between Fe and O helps to preserve the spin state of the FePc molecule [30–32]. Figure 3 also demonstrates a breakup of the degenerate dxz–dyz orbitals which was more important around −3.57 eV and −3.43 eV for dyz and dxz, respectively. This breakup was also found in Nb atoms px and py orbitals at the same energies (not shown). We investigated the distances and angles of these atoms to shed light into the symmetry reduction of the rotational C4 of FePc. Table 2 shows the obtained results for before and after adsorption in comparison to FePc molecule in gas phase experiment. The results first show agreement between the calculated distances and angles of the isolated molecule and experiment findings of Kirner et al. [33]. This suggests that computational parameters used for this work could correctly describe the interactions between the molecule and the substrates. Table 2 also confirms that the rotational symmetry becomes two-fold due to a change in angles, although there are negligeable distance differences between the free molecule and after adsorption.

The interaction between FePc molecule and the monolayer silica leads to a symmetry reduction to the rotational C2. This interaction is strong enough to bind the central Fe of the FePc molecule chemically to the O atom of the silica layer which also results in a strong coupling displayed in the increase in the magnetic moment and in the σ bond. It is also strong enough to bind the organic ring of the molecule to the inorganic substrates via chemical and vdW interactions. We adopt the same computational scheme for FePc on bilayer silica to identify the important interactions involved.

Table 1

Binding energies and charge transfers of the FePc/Si4O10-2O/Ru(0001) configurations.

thumbnail Fig. 2

Top and side view of: (a) the final geometry and (b) spin density distribution of FePc on Si4O10-2O/Ru(0001) at O-top site. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).
thumbnail Fig. 3

PDOS of pz orbital of O atom and , dyz and dxz, orbitals of Fe of FePc/Si4O10-2O/Ru(0001) system. Inset shows zoomed-in of pz orbital of O atom and orbital of Fe.
Table 2

Bond lengths and angles between FePc atoms of free configuration in comparison to experiment [33] and upon adsorption on Si4O10-2O/Ru(0001).

3.2 FePc/Si8O16 -2O/Ru(0001)

Similarly to FePc/Si4O10-2O/Ru(0001) interface's investigation, we first start by searching for the stable configuration with the lowest binding energy between the 3 chosen configurations: Hollow, O-top and Si-top. The results show that all configurations are metastable, and their binding energies are very close. We used Nudged Elastic Band (NEB) [34,35] to distinguish the stable configuration using the O-top “a” and Hollow “d” configurations at initial and final sites of the trajectory, respectively (as shown in Fig. 5). The FePc molecule moves in two images Si-top “b” site and another Hollow site “c” later referred to as Hc site. The NEB method helps find the lowest energy possible for the specified images. The calculation gives a set of results shown in Figure 4. The figure shows the energy in reference to the initial position for each image. The findings suggest that the Hc site is the most stable configuration with the lowest binding energy of −2.40 eV. The geometry of the FePc molecule when adsorbed on bilayer silica remains somewhat flat, as shown in Figure 5b, at a binding height of 3.33 Å. However, the Si and O atoms of the substrate that are in direct contact with the molecule were pushed down towards the surface by 0.02 Å in reference to the average height of the upmost layer of silica. We also noticed that the molecule placement on the substrate showed only a left/right symmetry which suggests that the rotational symmetry of the molecule is reduced to below the C2 rotational symmetry found when adsorbed on the monolayer silica substrate. Therefore, we investigated the distances and angles of the FePc's atoms. Table 3 shows larger differences in these parameters compared to FePc/Si4O10-2O/Ru(0001) system. The distances between atoms were preserved to those of gas phase which could be explained by the flat geometry of the molecule upon adsorption. In contrast, the distortion of the molecule was lateral and was mainly observed in the angles of FePc atoms. The angle between Fe and the neighboring Na atoms is a right angle at gas phase and was found to slightly increase to 90.24 in both left and right parts of the molecule, but slightly decrease in the top and bottom parts of the molecule. Similar results were found for Ca1NbCa2 angle, it became wider compared to the molecule at gas phase for left and right angles and smaller for the bottom part. In agreement with monolayer results and Baffou et al. ‘s findings [27], the molecule on bilayer silica shows both an elongation and contraction on the opposite Nb–Nb distances in order to minimize the distance between each Nb and its closest Si atom.

In the case of bilayer, the direction of the charge transfer was similar to Si4O10-2O/Ru(0001) from the molecule to the substrate. However, it becomes weaker, around 0.28e–and half of this charge was transferred from the central atom Fe which we noticed to be a similar amount of electron transfer in FePc/Si4O10-2O/Ru(0001) system. Jerratsch et al. also reported that single Fe atoms become positively charged upon adsorption on an ultrathin silica/Mo(112) film [17]. Our results suggest that the interaction between the organic ligand of FePc and the silica substrate becomes weaker which is translated in a smaller electron transfer and therefore a lower binding energy. In addition, the bilayer Silica only gained around 0.11 e–, whereas the rest was transferred to the O-Ru surface.

Moreover, we calculated the corresponding total magnetic moment and observed that the spin state of FePc molecule remains the same S = 1 when the molecule is adsorbed on Si8O16-2O/Ru(0001). The spin densities distribution within the molecule is similar to when adsorbed on monolayer, spin density of C atoms are aligned parallel to Fe's and N's are aligned antiparallel. However, dividing the molecule into top and bottom parts shows that the distribution is not uniform, especially at N atoms and benzene-like cycles as shown in Figure 5. Unlike, on the monolayer silica, the O atoms underneath Fe showed no additional spin density. The side view of Figure 5 illustrates that the spin density's symmetry of Fe is also affected by the adsorption. Therefore, a PDOS investigation is necessary to fully understand the electronic change of FePc upon adsorption. Figure 6 shows that the orbitals of Fe atom after adsorption loses electrons to the substrate. In addition, the breakup of dxz–dyz's degeneracy is quite visible around the fermi level and at lower energies. In this case, the Fe atom hybridizes with the two O atoms underneath it at Hc site (Fig. 5a). All the results point out that the molecule not only physisorbs on the bilayer silica films, but also chemisorbs through its central Fe atom.

thumbnail Fig. 4

The total energy in reference to the energy of the initial position a as a function of the configurations.
thumbnail Fig. 5

(a) Top view of bilayer silica substrate. (a–d) Denotes the site configurations of the adsorption images used for NEB calculations. (b) Final geometry of top and side view of FePc adsorbed at hollow “c” site on Si8O16 -2O/Ru(0001) substrate. (c) Spin density distribution of FePc on Si8O16-2O/Ru(0001)) at hollow2 site. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).
Table 3

Bond lengths and angles between FePc's atoms upon adsorption on Si8O16-2O/Ru(0001).

thumbnail Fig. 6

PDOS of , dyz and dxz orbitals of Fe of FePc/Si8O16-2O/Ru(0001) system. Inset shows zoomed-in of pz orbital of O atom and orbital of Fe.

4 Conclusion

To sum up, this systematic investigation of the adsorption properties of FePc on monolayer and bilayer silica supported by O(2 × 2)-Ru(0001) focused mainly on three major points: The geometric structure FePc of the molecule which undergoes a symmetry reduction when adsorbed on both substrates. The origin of this behavior was expressed in both structural and electronic by changes in the geometry of the molecules as well as the break-up of the Fe's degenerate dyz and dxz orbitals and in the spin density distribution of the molecules too. Moreover, our results report that the charge is transferred from the organic molecules to the corresponding inorganic substrates. In the case of bilayer silica, the contact between the organic ligand of FePc and the Si8O16-2O/Ru(0001) substrate weakens, resulting in a smaller electron transfer and hence a lower binding energy. Lastly, the nature of adsorption that FePc/silica thin film systems exhibits is both physisorption and chemisorption simultaneously.

This research used computational resources at the National Energy Research Scientific Computing Center (NERSC) and the University of Central Florida High Performance Computing STOKES.

Author contribution statement

MJ, MS and AK contributed to the calculations and analysis of the results. MJ, MS, ME, MYE and AK contributed in discussions and writing of the manuscript.

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Cite this article as: Meysoun Jabrane, Mohamed El Hafidi, Mohamed Youssef El Hafidi, Muhammad Sajid, and Abdelkader Kara, Effects of monolayer and bilayer silica films on Fe-Phthalocyanine adsorption properties, Eur. Phys. J. Appl. Phys. 98, 7 (2023)

All Tables

Table 1

Binding energies and charge transfers of the FePc/Si4O10-2O/Ru(0001) configurations.

In the text
Table 2

Bond lengths and angles between FePc atoms of free configuration in comparison to experiment [33] and upon adsorption on Si4O10-2O/Ru(0001).

In the text
Table 3

Bond lengths and angles between FePc's atoms upon adsorption on Si8O16-2O/Ru(0001).

In the text

All Figures

thumbnail Fig. 1

(Left) Schematic structure of FePc in gas phase. (Right) Spin density distribution of the free FePc molecule. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).
In the text
thumbnail Fig. 2

Top and side view of: (a) the final geometry and (b) spin density distribution of FePc on Si4O10-2O/Ru(0001) at O-top site. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).
In the text
thumbnail Fig. 3

PDOS of pz orbital of O atom and , dyz and dxz, orbitals of Fe of FePc/Si4O10-2O/Ru(0001) system. Inset shows zoomed-in of pz orbital of O atom and orbital of Fe.
In the text
thumbnail Fig. 4

The total energy in reference to the energy of the initial position a as a function of the configurations.
In the text
thumbnail Fig. 5

(a) Top view of bilayer silica substrate. (a–d) Denotes the site configurations of the adsorption images used for NEB calculations. (b) Final geometry of top and side view of FePc adsorbed at hollow “c” site on Si8O16 -2O/Ru(0001) substrate. (c) Spin density distribution of FePc on Si8O16-2O/Ru(0001)) at hollow2 site. Isosurface is set to ±0.01 (e/Å3). Green (yellow) region corresponds to spin down (up).
In the text
thumbnail Fig. 6

PDOS of , dyz and dxz orbitals of Fe of FePc/Si8O16-2O/Ru(0001) system. Inset shows zoomed-in of pz orbital of O atom and orbital of Fe.
In the text

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