Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Physical and Chemical Control of Interface Stability in Porous Si− Eumelanin Hybrids Aleandro Antidormi,† Giulia Aprile,‡ Giancarlo Cappellini,† Eleonora Cara,‡ Roberto Cardia,† Luciano Colombo,†,§ Roberta Farris,† Marco d’Ischia,∥ Mehran Mehrabanian,† Claudio Melis,†,§ Guido Mula,*,†,§ Alessandro Pezzella,∥ Elisa Pinna,†,§ and Eugenio Redolfi Riva†,§
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Dipartimento di Fisica, Università degli Studi di Cagliari, Cittadella Universitaria di Monserrato, S.P. 8 km 0.700, 09042 Monserrato, CA, Italy ‡ Quantum Research Labs & Nanofacility Piemonte, Nanoscience & Materials Division, Istituto Nazionale di Ricerca Metrologica (INRIM), Strada delle Cacce 91, 10135 Turin, Italy § Istituto Officina dei Materiali CNR-IOM, Unità di Cagliari SLACS, Cittadella Universitaria di Monserrato, S.P. 8 km 0.700, 09042 Monserrato, Italy ∥ Department of Chemical Sciences, University of Naples, “Federico II” via Cintia 4, 80126 Naples, Italy ABSTRACT: The organic/inorganic interface in thin nanosized porous structures has a key role in determining the final properties of the composite materials. By use of the porous silicon/eumelanin hybrids as a case study, the role of this interface was investigated by experimental and computational methods. Our results show that an increased polymer density close to the hybrid interface strongly modifies the diffusion of the chemical species within the polymer molecule, affecting then the oxidation level of the pores’ inner Si surface. We observed a greater stability induced by increased pore diameter, a behavior that with computational and chemical arguments we attributed to a modified diffusion of the hydrogen peroxide toward the Si/eumelanin interface. Our results show that the overall behavior of a polymer when inserted in a tiny nanoscale structure must be taken into account for a correct understanding and control of the hybrids properties and that the formation of the interface alone may not be sufficient.
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INTRODUCTION Porous materials have recently become the focus of increasing interest for various applications, including energy harvesting and production,1,2 due to their low fabrication cost and their large surface-to-volume ratio. This feature allows the fabrication of large interfaces in small volumes with efficient photocurrent production and energy storage, as well as the functionalization of the developed surface with molecules of interest.3 Relevant examples include a Pt/N-doped TiO2 with a leaf-shaped hierarchical structure for photocatalytic applications inspired to natural photosynthesis4 and the use of titanium dioxide templates for photovoltaic applications because of their interesting conductive properties.5−7 The role of the interface in such structures is extremely complex to study experimentally, since none of the available characterization techniques are suitable for a precise understanding of the hybrid structures at the nanoscale. Surface-sensitive techniques such as electrochemical impedance spectroscopy (EIS) are used instead for porous materials,8 while significant help may come in the near future from secondary ion mass spectrometry (SIMS)9,10 or highly sophisticated techniques such as atom probe tomography.11 An interesting material for © XXXX American Chemical Society
technological application is porous silicon (PSi), which is increasingly investigated for photovoltaic, energy harvesting, and biomedical applications due to its high versatility and biodegradability.12−17 The electrochemical etching primarily used for the PSi formation allows the fabrication of highly controllable porous matrices, starting from bulk silicon and using a hydrofluoric acid (HF) aqueous solution and an electrochemical cell.18 By tuning HF concentration, current intensity, etching time, and silicon doping level, it is possible to control the porous architecture within the silicon matrix.19−21 In this frame, the possibility of interfacing an inorganic matrix with an organic material, such as a conductive polymer in organic/inorganic nanocomposites, has greatly expanded the range of application of porous structures as energy materials.22,23 More specifically, pore impregnation with conducting polymers in PSi was shown to increase the absorption spectral region, i.e., the spectral region where photocurrent can be generated with respect to empty PSi.24 Received: October 5, 2018 Revised: November 15, 2018 Published: November 21, 2018 A
DOI: 10.1021/acs.jpcc.8b09728 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
M NaOH solution to obtain a nanostructured surface whose holes serve as seeds for the pores nucleation in the third layer. Pore structures were characterized by scanning electron microscopy (SEM), while photoconductivity and its time stability over days were determined by photocurrent measurements. Furthermore, an atomistic model has been presented to explain eumelanin polymerization process and to predict the possible arrangement inside the structure of pores and its dependence on environmental conditions. Additional information on the polymers’ properties has also been obtained from the calculation of the absorption spectra of selected tetramers, whose shape has been determined by the atomistic modeling just described, with and without the presence of the Si surface.
However, the large developed surface of PSi is at the same time a powerful advantage when a large interface is needed, and a source of complexity for its nanoscale dendritic nature. Given the demanding challenges arising from the fabrication of hybrid materials with dendritic pores with nanoscale diameter, we chose PSi/eumelanin, whose temporal stability was demonstrated to be a significant issue,25 as a case study for the understanding of the role of the interface in hybrid mesoporous structures. A range of materials have been used to functionalize PSi in order to enhance its photoconductivity.24,26,27 An interesting example includes a version of a PSi/PEDOT:PSS heterojunction as low-cost solar cell. The uniform SiOx passivation layer obtained by the oxidation process allows better interaction between silicon and PEDOT:PSS polymer, thus resulting in improved photoconducting performances.28 Another interesting polymer interfaced with PSi is polyaniline (PANI).24,27 Spin coating of PANI onto the PSi surface to deposit a homogeneous surface layer of the polymer24 has resulted in an overall enhancement of the electrical properties. PANI electropolymerization inside the porous structure led to an increase of the overall photoconductance with respect to pristine PSi substrates;27 however, the use of PANI may be problematic for environmental safety.29 We recently explored the potential of the synthetic biopolymer eumelanin as a conductive soft biocompatible material for the heterojunction for PSi functionalization.30 Synthetic polymers mimicking the black pigment of human skin and hair have recently been the focus of keen interest because of their unique optoelectronic properties, as well as their biocompatibility and biodegradability. The choice of a “green” polymer like eumelanin for photocurrent enhancement provides a promising option for the design and fabrication of bionanocomposites for sustainable photovoltaic applications.29 In our previous proof-of-concept investigations we reported PSi/eumelanin nanocomposites for photovoltaic applications and demonstrated the ability of eumelanin functionalization to enhance substrate photoconductivity.25,31 However, despite the improved photoconductivity produced by eumelanin functionalization, the reduced photocurrent stability over days limits the ability to accomplish steps forward toward technology transfer.31 Apparently, the stability issues stem from the characteristics of the PSi/eumelanin hybrid, including chiefly the buildup of the polymer within the pores in relation to the nature and shape of the inorganic material (irregular dendritic pores are present in the case of PSi). In this framework, we found that modification of the PSi/eumelanin interface leads to an improved, although still not sufficient, stability.25 The peculiar temporal decay of this hybrid,25 and its relatively low sensitivity to the chemical modifications at the interface, strongly suggests that there is a combination of physical and chemical factors contributing to the observed behavior that have to be better understood. We then report here our combined computational and experimental study on the effect of the pore size, a factor that is tightly connected to the thickness of the interface in the polymer side, on the temporal stability of the PSi/eumelanin hybrid. A three-step approach termed electrochemical nanolithography (ENL) has been used as an etching method of silicon bulk matrix to allow for a better control of the opening, surface density, and diameter of pores.32 The process consists of the fabrication of a porous double layer that is then etched away using aqueous 1
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MATERIALS AND METHODS Porous Silicon/Eumelanin Hybrids. PSi layers have been fabricated by electrochemical etching in the dark of n+ Si wafers (15−18 mΩ/cm, from Sil’tronix, France). The etching was performed using several HF concentration in a HF/H2O/ EtOH solution. HF is a highly corrosive acid that may be lethal if manipulated without the necessary personal protection equipment and the correct chemical laboratory tools. For reference see the the University of North Carolina Web pages dedicated to the HF safety procedures.33 Before the formation of the porous layer, the samples underwent the electrochemical nanolithography (ENL) process,32 a procedure that allows control of the pores density and surface ordering by using an electrochemical approach that separates these two characteristics from the electrochemical parameters used for the fabrication of the final porous layer. The advantage of ENL is that it is possible to choose the pores formation parameters for the layer while their distribution and density are predetermined, different from the standard procedure where pores density and distribution are tightly related to the chosen formation parameters. The pores diameter dependence of the samples stability has been tested by using a diluted NaOH aqueous solution (0.1 M) to enlarge the pores after the fabrication of the porous layer and reduce the dendritic structure. The pores enlarging process durations were 0 (untreated), 30 and 60 s. After the formation of the porous layer, the samples were impregnated with eumelanin following the procedure described in previous publications.25 Different 5,6-dihydroxyindole (DHI) concentrations (8, 16, and 24 mg/mL in EtOH) have been used for the impregnation process. The polymerization of DHI to eumelanin was obtained through ammonia-induced solid-state polymerization.34 Prior to the measure of the performance of photocurrent measurements, a semitransparent gold contact has been deposited onto the surface of all samples using an Emitech K450 sputter coater. All samples have been placed in the sputter chamber and treated for 6 min using a sputtering current of 20 mA. The gold layer diameter is 2 mm smaller (0.7 mm) than the PSi area diameter (0.9 mm) to ensure that no contact between the semitransparent gold layer and the bulk Si region is made. After sputtering, a first silver paint drop has been placed onto the gold layer and a second one has been put on a freshly scratched area of the bulk Si substrate to ensure a contact through the PSi layer alone.25 Photoconductivity has been measured using a PM8 analytical prober and a Keithley 2450 SMU. The samples were illuminated using a tungsten halogen lamp focused on a 0.125 cm2 circular spot. The amount of photocurrent versus different values of B
DOI: 10.1021/acs.jpcc.8b09728 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. SEM characterization of PSi samples. Each sample has been fabricated using ENL and NaOH pore-enlarging solution. Top view and cross-section view of PSi_0 (a, α), PSi_30 (b, β), and PSi_60 (c, γ). All scale bars are 500 nm.
wavelength has been assessed using glass low-pass filters following the procedure described in a previous publication31 in order to characterize samples photoconductivity. The SEM images were collected using a FEG Inspect-F from FEI at 20 kV, with a spot of 3.5 and a working distance of 10 mm. Computational Methods: MPMD. Modeling hybrids by means of MPMD is a challenging task since a fully reliable model potential for our system is actually missing. On the contrary, reliable potentials for silicon and melanin separately are available. Here we combine such existing force fields (by adding long-range Coulomb and dispersive interactions) to describe the silicon−polymer interface. Specifically, the silicon−silicon interactions are described by the Tersoff model potential which has proved to correctly reproduce both mechanical and morphological properties of silicon-based systems. 35 To describe the inter- and intramolecular interactions in eumelanin protomolecules, we used the AMBER force field36 which includes both bonding (bonds, bending, torsional) and nonbonding (van der Waals plus Coulomb) contributions. The parameters in the bonding and vdW terms are taken from the GAFF database.37 As for the atomic partial charges, they are estimated with the restrained electrostatic potential (RESP) method38 using an HF/6-31G* QM calculation to generate the electrostatic potential as implemented in the Gaussian package.39 Finally, the eumelanin−silicon interaction is given by a combination of nonbonding and bonding terms. In particular, the covalent silicon−oxygen interaction, which has been proved to occur at the interface,40 is modeled via a reactive Tersoff potential;41 all the remaining nonbonding interactions are described by Lennard-Jones and Coulomb potentials. The Lennard-Jones parameters for silicon−eumelanin were obtained by usual AMBER mixing rules (with the Si−Si parameters taken from Emami et al.42). As for the Coulomb interactions, we used the Mulliken partial charges calculated by density functional theory (DFT) using the Quantum Espresso package.43 MPMD simulations were performed using the LAMMPS44 package implementing the velocity-Verlet algorithm with a
time step of 0.5 fs to solve the equations of motion. A particle− particle mesh solver is used for describing the long-range electrostatic forces, and the van der Waals interactions are cut off at 0.1 nm. The Nosé−Hoover thermostat and barostat with corresponding relaxation time equal to 50 fs and 0.5 ps, respectively, are used to control the simulation temperature and pressure. Computational Methods: DFT and TD-DFT. We have performed all the DFT45,46 and TD-DFT47 calculations using the NWChem48,49 and Gaussian 09/1650 packages. The structural optimizations are obtained using the Becke three-parameters Lee−Yang−Parr (B3LYP) hybrid exchange− correlation (XC) functional51,52 which presents a stable behavior, albeit with some well-known limitations.53 All the structural optimizations of each tetramer were performed without the use of symmetry constraints and are obtained using tight convergence criteria, specified by maximum and root-mean-square gradient thresholds of 1.5 × 10−5 and 1.0 × 10−5 atomic units, respectively, and maximum and root-meansquare thresholds of the Cartesian step respectively of 6.0 × 10−5 and 4.0 × 10−5 atomic units. As basis set to approximate the electronic wave functions, we used the 6-31+G* basis set, a valence double-ζ set augmented with d polarization functions and extended by addition of diffuse functions. To obtain the singlet−singlet excitation energies and the electronic absorption spectrum in the visible/UV region for each tetramer, the TD-DFT calculations were performed at the same level B3LYP/6-31+G* employed for its electronic ground-state using the implementation of the TD-DFT in frequency space based on the linear response of the density matrix. In this computational approach, the poles of the linear response function are associated with vertical excitation energies and the pole strengths with the corresponding oscillator strengths.54 For each molecule, we take into account only the first 120 singlet−singlet roots, which in all cases were enough to cover the energy range considered. The same computational scheme has been successfully applied on several other families of organic molecules and to C
DOI: 10.1021/acs.jpcc.8b09728 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C another class of molecules that are the basic constituents of eumelanin.25,55−58
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RESULTS AND DISCUSSION Fabrication and Characterization of the PSi/Eumelanin Hybrids. Pristine PSi samples were initially formed following the procedure described in the “Materials and Methods” section. After formation, part of the samples were treated with a diluted NaOH pores-enlarging solution. The treated samples have been named PSi_0, PSi_30, PSi_60, and PSi_90 for simplicity, where the numbers after “PSi_” correspond to the duration of the chemical etching (in seconds). For the samples that were also impregnated with eumelanin, the suffix “_eu” has been added. Figure 1 shows SEM images of a series of PSi samples subjected to varying immersion times in the pore enlargement solution: PSi_0 (Figure 1, images a and α), PSi_30 (Figure 1, images b and β), PSi_60 (Figure 1, images c and γ) for top views and cross sections, respectively. The images show that the pore openings are regularly distributed and homogeneous in size and shape, an effect that comes from the use of the ENL fabrication process.32 The squared shape is due to the use of the NaOH solution before the formation of the final porous layer, as a result of silicon etching in crystalline direction ⟨111⟩ typical of alkaline solutions.59,60 To obtain a reference parameter for the pores opening size, we chose to approximate the pores openings shape to a square and use the length of its side as parameter. Within this approximation, the NaOH solution treatment caused an overall increase of average pores size: 25 nm for PSi_0, 36 nm for PSi_30, and 41 nm for PSi_60. Cross-section images as well show an increase of average pores dimensions, ranging from 34 nm for PSi_0 to 54 and 57 nm for PSi_30 and PSi_60, respectively. Porous layer depth was about 1.5 μm and cross-section images show the dendritic structure of the pores. NaOH treatment also led to a reduction in dendrites length resulting in a flattening of the pores’ longitudinal surface with respect to our previous study.25,27,31 No detectable dependence of the photoconductive properties on the NaOH dipping time was observed. Since PSi_60 samples displayed a less pronounced dendritic structure and larger pores with respect to the other samples, these samples were selected for determination of photocurrent stability with time. The impregnation process with melanin is obtained starting from a solution of DHI monomer61 that is inserted into the pores following the procedure described by Pinna et al.34 Since the impregnation process may also depend on pore diameter, given that larger pores and then higher mobility of the DHI solution in the pores length with respect to small pores samples may require a different DHI concentration for best results, an assessment of the best DHI concentration with changing pore diameter seemed mandatory. For this reason, the data in Figure 2 and Figure 3 are reported for three different DHI concentrations. The lowest concentration used (8 mg/mL) equaled that used in our previous study with thinner pores samples. Figure 2 shows white-light photocurrent intensities for samples impregnated with different DHI concentrations (8, 16, and 24 mg/mL). These results show a consistently enhanced photocurrent generation for the impregnated samples compared to empty PSi matrices, as was the case for the samples with thinner pores,25,27 in agreement with previous results.25,62 Both time stability and absolute photocurrent
Figure 2. Time evolution of white-light photocurrent for PSi_60 samples filled using different DHI concentration: (blue line) 8 mg/ mL; (red line) 16 mg/mL; (green line) 24 mg/mL; (dark yellow) empty PSi_60 sample; (magenta line) reference PSi sample impregnated with [DHI] = 8 mg/mL and having 10 nm diameter pores.
Figure 3. Time evolution of the spectral dependence of the photocurrent as a function of the DHI concentration: (a) PSi_60 filled using 8 mg/mL; (b) PSi_60 with 16 mg/mL; (c) PSi_60 with 24 mg/mL. A comparison with empty PSi matrix is also shown for reference in each panel. The green arrows roughly indicate the evolution of the photocurrent behavior. D
DOI: 10.1021/acs.jpcc.8b09728 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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pore dimensions affect the stability of the eumelanin/silicon interface, with larger pores improving the overall photocurrent stability. In this respect, we argued that the molecular interactions with the PSi substrate represent a key factor in affecting the resulting polymer morphology and consequently transport properties and the diffusion of the H2O2 generated during the polymerization process. It is then crucial to elucidate how the interface affects polymer structural features. In this perspective, atomistic simulations represent a powerful tool to investigate this problem with a resolution hardly obtainable in experiments. In particular, model potential molecular dynamics (MPMD) allows unveiling of local adhesion properties in realistic systems with dimensions of tens of nanometers. In this section, we employ MPMD to the aim of analyzing the adhesion properties of eumelanin on silicon. Specifically, we will show that (i) the resulting hybrid interface is stable in time at finite temperature and (ii) the presence of the substrate modifies the local density of the polymer with respect to its bulk value. The main factor hindering a straightforward study of the properties mentioned above is represented by the lack of detailed knowledge of DHI melanin structure.65 Several possible protomolecular structures have been recently proposed with the aim of elucidating the broad optical absorption spectrum in biological systems.66,67 However, experimental evidence68,69 suggests that the protomolecules of the most common forms of eumelanin produced in the laboratory have small molecular weight, supporting tetramers as the most appropriate protomolecular candidates. We recently40 analyzed the process of formation and adhesion of single tetrameric protomolecules on silicon with different oxidation state and chemical configuration. Starting from these results, we will concentrate on the two tetrameric protomolecules with the best formation and adhesion energies, whose structure is shown in Figure 4. The two structures will
intensity reproducibility were improved by the use of larger pores, while no sizable difference was noticed as a function of the DHI concentration in the impregnating solution. In marked contrast with the previous results, an 80% overall decrease of the current intensity was measured after about 20 days, revealing a significant improvement with respect to the “PSi_eu reference” samples. A still measurable photocurrent was recorded after as long as 50 days. Another noteworthy difference with previous data was the reproducibility of the photocurrent intensity from sample to sample. While for thin pores there was a low reproducibility and huge variations spanning over orders of magnitude (100 nA to 700 μA), for the large pore samples the range of variation was markedly reduced to a factor of 2−3, independent of the details of the fabrication process. The variations in the spectral behavior of the samples shown in Figure 2 caused by impregnation with eumelanin were then measured and compared with the previous results using a set of low-pass filters.25 The results are summarized in Figure 3, using normalized photocurrents, to allow for comparison of spectral features for the different DHI concentrations: 8 mg/mL (Figure 3a), 16 mg/mL (Figure 3b), 24 mg/mL (Figure 3c). For all samples, increased spectral absorption was induced by eumelanin, confirming polymerization of DHI within the pores. Interestingly, for all concentrations studied, eumelanin absorption contribution increased with time. As shown in Figure 3, the photocurrent generation range increased with time until about 20 days, a behavior that was not observed in the thin pore samples. This is roughly indicated by the green arrows in each panel. Although some fluctuation with respect to the indication of the arrows is observed for the early days measurements, the overall tendency is well represented. The origin of this phenomenon was then examined. More specifically, we observed a correlation between the number of days in which this spectral evolution was observed and the number of days before photocurrent intensity stabilizes (Figure 2). As apparent from Figure 3, no detectable increase of eumelanin-dependent absorption was determined after 20 days. At the same time, the data in Figure 2 indicate that after 20 days the decrease of the white-light photocurrent intensity is strongly reduced. We argue that these results may come from the variations on the polymerization behavior of DHI, leading to eumelanin, that are induced by a pores diameter that has been increased with respect to the previous results.25 It is well-known63,64 that polymerization of DHI induced by oxygen leads to the production of hydrogen peroxide, a strong oxidant for the Si surface. In our samples, the H2O2 produced by the polymerization process may be envisaged to interact with the PSi pore surface and transform this latter in an insulating surface by oxidative processes, gradually decreasing and eventually stopping the photocurrent generation. Since the H2O2 formation should in principle occur also in the previously reported thin-pore samples, in which by contrast the evolution of the melanin signature was not observed, there is a need to better understand the effect of the interface on polymer formation. One of the main differences between thin and large pores is the volume-to-surface ratio, which is significantly larger for larger pores. To further investigate the polymer properties and gain more insight on these results, a computational approach was then used to model the polymer behavior at the interface. Melanin at PSi Surface: Atomistic Modeling. In the previous section, we experimentally demonstrated that the
Figure 4. Eumelanin tetramers considered in this work.
be hereafter referred to as model 1 (Figure 4, left) and model 2 (Figure 4, right), respectively. It should be noticed that model 1 is an experimentally characterized molecule from Panzella et al.,70 while model 2 is a structure from Kaxiras et al.71 and has never been isolated experimentally.72 This latter is included as a representative example of possible flat oligomer components in the eumelanin polymer, as opposed to the linear and more conformationally flexible model 1. E
DOI: 10.1021/acs.jpcc.8b09728 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C As a necessary benchmark for the hybrid model potential described above, we calculated the binding energy between the two protomolecular models of Figure 4 and a planar slab of silicon (100) having dimension of 5 × 5 replica of the surface unit cell, containing 400 atoms. Figure 5 shows the
Figure 6. Schematic of eumelanin−Si interface and its time evolution: (left) schematic representation of the eumelanin−silicon interface created upon the deposition process described in the text; (right, top) time evolution of the root-mean-square displacements for the two systems considered; (right, bottom) polymer mass density profile as a function of the distance perpendicular to the interface where z < 1.2 nm corresponds to the silicon substrate.
Figure 5. (Left) Attraction basin of the protomolecular structures with the silicon substrate. The two lines correspond to the DFT calculations. (Right) Stick and ball representation of the systems in the most energetically stable configurations.
value is quickly reached in both cases demonstrating the overall stability of the hybrid interfaces at T = 300 K for at least 500 ps. We argue that such a stability is due to the combination of the strong covalent bonding between the single molecules and the substrate and the strong dispersive intermolecular attractive interaction. In particular we estimated a surface coverage larger than 75% in both cases and an average distance between adjacent molecules as small as 0.32 nm. Once the stability of the system was proved, we investigated the possible effect of the substrate on the resulting density of the polymeric film. This allows us to understand to which extent the presence of the substrate is able to influence the polymer structure and the corresponding transport and absorption properties. The mass density profile along the direction perpendicular to the interface is shown in Figure 6 (bottom right) for the two systems (z < 1.2 nm corresponds to the substrate). For the sake of comparison, we also show the mass density equilibrium values without the substrate (dashed lines). Two main features can be observed: (i) in the proximity of the substrate (z < 1.3 nm) a dramatic increase (more than 100%) is found with respect to the bulk value as due to the strong molecule−substrate interaction. (ii) For distances between 1.3 and 5 nm the density is generally larger than bulk. These findings provide a clear evidence that the presence of the silicon substrate unavoidably affects the local properties of the grown eumelanin film in a non-negligible spatial region as large as 4−5 nm beyond the interface. Although the investigated system (chosen for computational convenience) is rather different with respect to experimental samples, it is nevertheless very useful to understand the effects we experimentally observed. Moreover, Si(100) surface is the Si surface with the lowest surface atoms density. We therefore expect that further simulation on denser Si surfaces will give an enhanced polymer densification effect near the polymer/Si interface. The modification of the structural properties of the polymer film for a distance as large as 5 nm from the Si surface strongly suggests that when dealing with the thin pores, whose average diameter was in the 10−15 nm range, we were working with a eumelanin almost entirely affected by the inner interface of the Si pores. This no longer holds for the larger pores, where
corresponding binding energy as a function of the molecule− substrate distance (Figure 5, left), calculated as the difference between the energies of the composite system and the sum of the energies of the components (noninteracting surface and molecule) and a stick-and-ball representation of the systems in the most energetically stable configurations for model 1 (Figure 5, top right) and model 2 (Figure 5, bottom right) molecules. In order to validate the present model potential, we compared these data with the results obtained by ab initio density functional theory (DFT) calculations performed using the Quantum Espresso package (under the PBE73 exchange− correlation functional with the Grimme corrections to include dispersive forces). The dots in Figure 5 correspond to the binding energy of the most stable configuration obtained by the present DFT calculations. These values are in good agreement with firstprinciples calculations with an error bar of
