29,31-H Phthalocyanine Covalently Bonded Directly to a Si(111

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29,31‑H Phthalocyanine Covalently Bonded Directly to a Si(111) Surface Retains Its Metalation Ability Chuan He and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, University of Delaware, Newark DE 19716, United States

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S Supporting Information *

ABSTRACT: The reaction of metal-free phthalocyanine molecules with a chlorine-terminated Si(111) surface is investigated to produce a phthalocyanine functionality directly attached to a semiconductor surface, without additional linkers or layers. The carefully prepared Cl−Si(111) surface provides an oxygen-free substrate that is reacted with 29,31-H phthalocyanine (H2Pc) in a wet-chemistry process resulting in HCl elimination. The in situ metalation of this H2Pc-modified silicon surface with cobalt is confirmed, suggesting that the produced functionality is chemically active. These processes are investigated by X-ray photoelectron spectroscopy, Fouriertransform infrared spectroscopy, and time-of-flight secondary ion mass spectrometry supplemented by density functional theory calculations. The morphology of the surface is monitored by atomic force microscopy. The combined spectroscopic, microscopic, and theoretical investigations demonstrate that additional linkers are not required for phthalocyanine attachment to occur, as the direct attachment can take place by forming Si−N bonds, and that the resulting surface species can participate in a metalation process.

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INTRODUCTION Functionalization of semiconductor surfaces with dye molecules is important for a number of applications, including catalysis, sensing, electronics, solar energy conversion, and many others1−4 because of the versatile optical and electronic properties introduced by the produced interfaces.5−7 As one of the most common and simple dye molecules, phthalocyanine (H2Pc) [as well as corresponding metallophthalocyanines (MPc)] has been widely studied and utilized for sensitization of various semiconductor surfaces, including silicon,8 TiO2,9 ZnO,10 and others.11 In order to achieve reproducible and robust attachment of phthalocyanine to these solid substrates, several approaches have been utilized, including spin coating,12 vapor deposition,13 and Langmuir−Blodgett deposition.14 However, the most stable and well-defined interfaces have been produced by making covalent bonds between phthalocyanine and solid substrates by different methods, such as chemical vapor deposition in ultra-high vacuum (UHV),15 ligand exchange reactions,16 and hydrosilylation.17−19 Among these approaches, hydrosilylation schemes are commonly used for producing stable dye-containing organic monolayers on silicon substrates based on Si−C bonds.20−22 Furthermore, the physical and chemical properties of the resulting dye functionality can be altered by using various M-Pc (Cu, Co, Fe, Zn, etc.) with differently modified silicon surfaces, thus making a wide range of applications possible by design.11,23 Even self-metalation processes following the deposition of modified H2Pc on metal and semiconductor surfaces have been reported.24,25 However, the intrinsic limitation of this general © XXXX American Chemical Society

approach is the common necessity to use an organic linker to attach the dye molecule to the surface, which may alter chemical and physical properties of the resulting interface.1 To have a better control and understanding of the interface between the H2Pc molecules and silicon surface, a direct covalent attachment without the use of additional linkers is desired. The question is how to achieve a direct covalent attachment of Pc to silicon and at the same time to keep the properties of the dye functionality intact. In order to address this issue, an alternative to the hydrosilylation process should be considered. For example, Baffou et al.26 reported the density functional theory (DFT) study of anchoring phthalocyanine molecules on the 6H−SiC(0001) 3 × 3 surface through the Si−N bond, which was also confirmed by scanning tunneling microscopy. A number of recent studies have targeted the formation of stable organic layers on silicon via the formation of chemically stable and versatile Si−N bonds.27 The formation of these bonds has been initially investigated in UHV by reacting ammonia,28,29 amines,30,31 and azides32−34 with clean silicon surfaces. However, even more recently, the wet chemistry method has been confirmed to produce the same type of bonding by reacting ammonia,35 amines,36 or hydrazine37 with the Cl-terminated Si(111) surface. Therefore, it is expected that the protonated nitrogen atoms from the Received: July 4, 2018 Revised: August 16, 2018 Published: August 23, 2018 A

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Reaction of Phthalocyanine with the Chlorine-Terminated Si(111) Surface. The freshly prepared Cl−Si(111) sample was immediately transferred into a clean Schlenk flask containing 10 mL of 10% (w/v) phthalocyanine solution prepared by dissolving 10 mg of phthalocyanine in 10 mL of anhydrous mesitylene under N2 atmosphere. The entire setup was then placed in an oil bath at 190 °C for different reaction times ranging from 0.5 to 5.0 h to optimize the reaction conditions, as indicated in the text. All of the samples were washed and sonicated with mesitylene and toluene three times to remove any physisorbed H2Pc and then kept under freshly distilled THF for further investigations. Cobalt Metalation of the Phthalocyanine-Modified Si(111) Surface. The freshly prepared Pc-Si(111) wafer was immersed in a flask containing a cobalt solution and then refluxed under N2 at 160 °C for 8 h. The solution was prepared by dissolving 30 mg of CoCl2 in 10 mL of anhydrous diglyme. Following the reaction, the substrates were washed several times with diglyme and sonicated first in CH2Cl2 and then in EtOH for 5 min to eliminate any unreacted salt. Surface Characterization Techniques. X-ray Photoelectron Spectroscopy. XPS studies were performed on a K-Alpha+ X-ray photoelectron spectrometer instrument from Thermo Scientific. The Al Kα X-ray source with an energy of 1486.6 eV was used at a 35.3° takeoff angle with respect to the analyzer. The survey spectra were collected over the energy range of 0−1000 eV. A passing energy of 58.5 eV was used in collecting the high-resolution spectra for Si 2p, C 1s, N 1s, O 1s, and Co 2p over the range of 20 eV at the 0.1 eV/step. CasaXPS software (version 2.3.16)41 was used for all data analysis. All peak positions were calibrated to the C 1s peak at 284.6 eV.40,42,43 Fourier-Transform Infrared Spectroscopy. All infrared spectra were collected on a Nicolet Magna-IR 560 spectrometer with a liquid nitrogen-cooled mercury cadmium telluride detector in the range of 4000−650 cm−1. The incident angle of the incoming beam was set to 60°. A total of 512 scans per spectrum and a resolution of 8 cm−1 were used to collect all the spectra. The native oxide-covered and hydrogen-terminated Si(111) wafers were used as backgrounds, as indicated below. Time-of-Flight Secondary Ion Mass Spectrometry. All the ToFSIMS analysis was performed on a TOF-SIMS V spectrometer (IONTOF, Münster, Germany), equipped with a 25 keV monoisotopic Bi3+ ion beam. All static negative- and positive-ion spectra were collected with a mass resolution of m/Δm = 9000 at an area of 100 × 100 μm2 over 50 scans. The fluence Bi3+ primary ion beam fluences were controlled below the static SIMS limit of 1 × 1013 ions/ cm2 with a target current of 0.27 pA. At least three samples with two spots on each sample were analyzed for each set of the experiment. All spectra were collected within the range of m/z 0−650 for both positive-ion and negative-ion spectra, but only the most informative negative-ion spectra are provided. ION-TOF measurement explorer software (version 6.3) was used for the calibration of ToF-SIMS data. The masses of H+, H2+, H3+, C+, CH+, CH2+, CH3+, C2H3+, C3H5+, C4H7+, C5H5+, C6H5+, C7H7+, and C32H17N8+ in the positive-ion mode and those of H−, H2−, C−, CH−, CH2−, CH3−, C2−, C2H−, C3−, C4−, C5−, C6−, C7−, C8−, and C32H17N8− in the negative-ion mode were used for calibration. Atomic Force Microscopy. AFM studies were performed on a Jscanner scanning probe microscope (MultiMode, NanoScope V) under the tapping mode. All the AFM images were collected utilizing Tap300Al-G tips (Budget Sensor) with a force constant of 40 N/m and a resonant frequency of 300 kHz. All the images were analyzed using Gwyddion software. For the ex situ characterization methods, the freshly prepared samples were transported under nitrogen to the experimental setups with minimal exposure to ambient conditions. Computational Details. DFT calculations were performed using Gaussian 09 suite of programs44 with the B3LYP functional and the 6311+G(d,p) basis set.45−48 For comparison with XPS measurements and to explore the possible surface reaction mechanisms and species formed, a Si17H22Cl2 model including two surface Si−Cl sites was built to represent the Cl-terminated Si(111) surface. All the core-level energies of N 1s in the models were predicted using Koopmans’

pyrrole structure of H2Pc could react with the Cl−Si(111) surface to produce stable Si−N linkages upon eliminating HCl. In this work, we report a novel approach to achieve the covalent attachment of phthalocyanine to the silicon surface directly, without additional linkers, based on its reaction with a Cl-terminated Si(111) surface. Metalation of the resulting surface-bound dye molecule with Co2+ is used to confirm that the chemical properties of the Pc functionality are preserved following this surface modification. To confirm the Si−N bond formation and to identify the surface species following metalation, X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) are utilized to monitor each modification step. Atomic-force microscopy (AFM) is performed to follow the morphology change following phthalocyanine modification. DFT computations are used to predict spectroscopic observables including vibrational spectra and core-level energies and to evaluate the formation of feasible surface species.

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EXPERIMENTAL SECTION

Surface Modification Details. Materials. Si(111) wafers (Ntype, double-side polished, >0.1 Ω·cm resistivity, 500 μm thickness) purchased from Virginia Semiconductor were used as substrates. All chemicals were purchased as indicated: phthalocyanine (SigmaAldrich, ≥98.0%), nitrogen (Praxair, research purity), ammonium hydroxide (Fisher Scientific, 29% certified ACS Plus), hydrogen peroxide (Fisher Scientific, 30% certified ACS), hydrochloric acid (Fisher Scientific, 37% certified ACS), HF improved buffer (Transene Company, Inc.), phosphorus pentachloride (Sigma-Aldrich, ≥98.0%), benzoyl peroxide (Acros, ≥98.0%), chlorobenzene (Acros, ≥99%), mesitylene (Sigma-Aldrich, ≥99.8%), diglyme (Sigma-Aldrich, anhydrous, ≥99.5%). The first-generation Milli-Q water system (Millipore) was used to produce deionized water with 18 MΩ·cm resistivity for all experimental processes. Preparation of the Hydrogen-Terminated Si(111) Surface. A modified RCA cleaning procedure38,39 was utilized to prepare the hydrogen-terminated Si(111) surface. Two solutions were freshly prepared for this etching procedure. SC1 solution consisted of Milli-Q water, hydrogen peroxide, and ammonium hydroxide in a ratio of 4:1:1 (v/v). SC2 was made by mixing 4:1:1 (v/v) Milli-Q water, hydrogen peroxide, and hydrochloric acid. The freshly prepared SC1 solution was used to clean the Teflon beakers and Si(111) wafers in an 80 °C water bath for 30 and 10 min, respectively. After rinsing with Milli-Q water, the precleaned Si(111) wafer was immersed into HF buffer solution for 2 min and rinsed again with Milli-Q water. Following this step, the Si(111) wafer was placed in freshly prepared SC2 solution in the 80 °C water bath for another 10 min etching to form a silicon oxide layer. After rinsing, the Si(111) wafer was immersed in HF buffer solution again for 1 min etching, and another 6 min etching in 40% ammonium fluoride solution was immediately followed to establish a well-ordered hydrogen-terminated Si(111) surface. A sharp absorption band at 2083 cm−1 was observed in the corresponding infrared spectra, confirming the formation of wellordered monohydride Si−H.38,39 Preparation of the Chlorine-Terminated Si(111) Surface. A wellestablished procedure40 was utilized to produce the chlorineterminated Si(111) surface. The reaction solution was prepared by dissolving phosphorus pentachloride in chlorobenzene solvent with a trace amount of benzoyl peroxide as a reaction initiator. After purging with N2 for at least 30 min, the gaseous impurities were removed from the solution, and a freshly prepared H-terminated Si(111) wafer was immediately transferred into this solution. The reaction was set up in an oil bath at 110 °C for 1 h to produce the Cl−Si(111) surface. Freshly distilled tetrahydrofuran (THF) was used to store the prepared Cl−Si(111) samples for further reactions and experimental measurements. B

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Langmuir theorem and calibrated by using the correction factor of 8.76 eV based on the previous investigations from our group.49,50

The summary of the N 1s XPS investigations following the reaction of phthalocyanine with a Cl−Si(111) substrate is presented in Figure 2. It clearly shows that no surface nitrogen

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RESULTS AND DISCUSSION Confirmation of Covalently-Bonded Phthalocyanine on the Cl−Si(111) Surface. Covalent attachment of H2Pc molecules to a Cl−Si(111) surface has been confirmed and investigated by XPS following each modification step. Washing and sonication with mesitylene and toluene were performed before any further characterization. C 1s, N 1s, Cl 2p, and Si 2p spectral regions were monitored to help identify surface species formed and to quantify the elemental contributions to the signal during silicon surface modification. Because C 1s spectra can be easily affected by the presence of ubiquitous adventitious carbon,51,52 the signal of N 1s would be more reliable to evaluate the success of direct covalent attachment of H2Pc. Nevertheless, high-resolution C 1s spectra of the key surfaces investigated are summarized in Figure 1,

Figure 2. High-resolution N 1s XPS spectra of the Si(111) surface before and after phthalocyanine treatment: (a) H−Si(111) surface that is the starting point and (b) freshly prepared Cl−Si(111) surface were compared with the (c) Cl−Si(111) surface reacted with phthalocyanine for 2.0 h, and (d) N 1s spectrum of phthalocyanine powder. Two possible computational models are shown schematically in the figure with the positions of computationally predicted XPS features shown as solid bars underneath the experimental spectra. Computationally predicted positions of the features in the model species are shown as solid bars directly underneath the experimentally recorded spectrum (d).

Figure 1. High-resolution C 1s XPS spectra of the Si(111) surface before and after phthalocyanine modification: (a) H−Si(111) surface that is the starting point is compared with (b) freshly prepared Cl− Si(111) surface and (c) the Si(111) surface reacted with phthalocyanine for 2.0 h.

is observed on H−Si(111) (shown in Figure 2a) or Cl− Si(111) (shown in Figure 2b), respectively. Figure 2c clearly shows three peaks of N 1s at binding energies (B.E.) of 399.5, 400.7, and 402.1 eV on the phthalocyanine-modified surface [Pc-Si(111)]. This result is compared to the selected results of the DFT calculations for the models schematically presented in Figure 2. The single Si−N bonded model is predicted to exhibit three peaks within the N 1s binding region at energies of 399.0, 399.8, and 400.7 eV. Another possible model has two Si−N bonds with predicted B.E. for N 1s at 398.5, 399.2, and 400.1 eV. A more detailed discussion of the model systems will be provided later. For the moment, it is important to note that the presence of three different features for the computational models is consistent with the experimental results. Another useful comparison can be made with the N 1s spectra of H2Pc powder shown in Figure 2d, which has two features at B.E. of 398.8 and 400.3 eV. The first signal at 398.8 eV is due to six nonprotonated nitrogen atoms from the pyrrole rings and iminic bridges, while the other signal at higher B.E. of 400.3 eV is due to the two protonated nitrogen atoms.54 This is perfectly consistent with the DFT prediction of the N 1s core-level B.E. of H2Pc. However, the experimental intensity ratio between 398.8 and 400.3 eV features is 1:2, which is different from the

which reveals the presence of surface carbon following each modification step. Despite the presence of the C 1s signal from possible contamination during sample transfer, the differences between H−Si(111), Cl−Si(111), and Pc-Si(111) surfaces support the fact that H2Pc molecules are adsorbed on the Cl− Si(111) surface. As shown in Figure 1a,b, two major components at 284.6 and 286.5 eV are observed before the H2Pc modification due to the ambient carbon. Therefore, these two features can be treated as a background; however, a new feature appeared in Figure 1c at 288.4 eV, which has been reported to be associated to the shake-up transition from the pyrrole carbon atoms in phthalocyanine.13,53 In addition, compared with the H−Si(111) and Cl−Si(111) surfaces, the intensity of the C 1s signal on the phthalocyanine-modified Si(111) surface is substantially higher, as would be expected. At the same time, cleaning and sonication following the surface modification procedure are expected to remove all the physisorbed H2Pc molecules, as confirmed by additional control experiments (Figure S1, vide infra). C

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modification. Substantial chlorine removal occurred by 1.0 h of the reaction time; meanwhile, a significant increase of surface nitrogen is observed within the first 0.5 h of the reaction time and a nitrogen coverage of 23 ± 3% is achieved following 2.0 h of reaction time. The quantitative evaluation of the coverage for the phthalocyanine-modified Si(111) surface is based on the previous surface adsorbate coverage studies by Lewis et al.35,37,40,57 On the basis of the ∼99% coverage of Cl on the Cl−Si(111) surface, the ratio of Si 2p and Cl 2p features was used to calibrate the ratio of other elemental (N 1s) concentrations on the phthalocyanine-modified surface. The relative surface coverages of N atoms and Cl atoms on the Si(111) surface were quantitatively calculated and are summarized in Figure 4. It reveals that the apparent coverage

predicted value (1:3) for 29,31-H phthalocyanine. The higher intensity of the 398.8 eV signal can be explained by the intermolecular interactions because the signal we measured is from the surface of a powder sample instead of a single H2Pc molecule. This observation is also consistent with previously reported explanations of these spectral features.54−56 Finally, the presence of a low-intensity peak at approximately 402.1 eV can be explained by the formation of the Si−N bond following the deprotonation of the pyrrole nitrogen atoms.24,54 Although the models considered show a shift of the observed N 1s features toward higher B.E. compared to the molecular species, which is consistent with experimental observation, neither of the proposed structures could be identified based solely on the XPS spectra. To rule out the influence from physisorbed H2Pc molecules, a strict cleaning and sonication process has been used to prepare the PcSi(111) samples before any characterization. To confirm successful removal of physisorbed species, control experiments were performed by dispensing 10% (w/v) phthalocyanine solution onto freshly prepared Cl−Si(111) wafer and following 30 min exposure at room temperature, without additional heating, giving us a sample covered with physisorbed H2Pc molecules. The sample was then washed and sonicated with mesitylene and toluene as well. Figure S1 in the Supporting Information section shows the summary of XPS N 1s spectra of the control samples; only a very small signal of nitrogen can be observed after dispensing the solution and drying it. This signal disappears completely after the full cleaning and sonication treatment, which clearly indicates that physisorbed H2Pc molecules can be removed by careful cleaning and sonication treatment. Therefore, the covalent Si−N bond is confirmed to form based on the XPS results presented for PcSi(111) samples in Figure 2. In order to understand the efficiency of the reaction of phthalocyanine with the Cl−Si(111) and to optimize the reaction time, the experimental investigations of the N 1s and Cl 2p spectral regions were performed following the reaction between phthalocyanine and Cl−Si(111) as a function of the reaction time, as shown in Figure 3. As expected, at the start, the Cl-terminated Si(111) exhibits a very strong Cl signal and has no surface nitrogen observed before the phthalocyanine

Figure 4. Summary of the relative surface coverages of N and Cl as a function of the reaction time (0.0−2.5 h) between phthalocyanine and the Cl−Si(111) surface based on the XPS studies.

of N atoms increases up to 2.0 h of the reaction time and decreases at longer exposure. The decrease is likely caused by surface oxidation and surface reconstruction observed at very long reaction times even in the controlled conditions used in this work. The apparent coverage of Cl decreases quickly and approaches minimum for reaction times higher than 2.0 h. However, the surface chlorine coverage during phthalocyanine modification is not reaching zero within the reaction times studied, which can be explained by blocking some of the sites on a Si(111) surface by large phthalocyanine molecules preventing complete chlorine removal. In other words, it appears that phthalocyanine moieties attached to the surface preserve the chlorine atoms from further chemical reactions on the Pc-modified Si(111) surface. In order to support the XPS study and confirm the covalent modification of phthalocyanine on the Cl−Si(111) surface, AFM is used to show that the starting H−Si(111) surface is clean and atomically flat and also to identify possible morphology changes following the phthalocyanine modification. Figure S3 in the Supporting Information section presents the AFM images of the H-terminated Si(111) surface and the Pc-modified Si(111). In Figure S3a, an atomically flat surface is shown as the atomic steps on the H−Si(111) surface can be observed.39 Figure S3b reveals an evenly distributed change in roughness on the Si(111) surface after the phthalocyanine modification. The root mean square of the surface changes from 0.718 to 1.513 nm, which is consistent with a single layer of phthalocyanine molecules. Also, it consistently shows no distinctive multiple layer formation or polymerization on the Si(111) surface after phthalocyanine modification. However, it

Figure 3. Time-dependent high-resolution N 1s (left) and Cl 2p (right) XPS spectra of the Cl−Si(111) surface following the reaction with phthalocyanine from 0.0 h reaction time as the freshly prepared Cl−Si(111) surface up to 2.5 h reaction time. D

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sharp peak at 2083 cm−1 is corresponding to Si−H stretching, which is a well-known signature of a clean and well-ordered H−Si(111) surface.58−60 After chlorination, this peak fully disappears and remains absent following the phthalocyanine modification. Because the Si−H stretching mode is very sensitive to the purity and quality of the ordering of the monohydride termination on Si(111), the summarized IR results confirm that a clean and well-ordered monohydride layer is fully replaced by Cl, with the resulting surface reacted with H2Pc. The right panel of Figure 5 presents the spectrum of the N− H and C−H stretching regions between 2800 and 3500 cm−1 for the phthalocyanine-modified Si(111) surface. On spectra in Figure 5a,b, no absorption features corresponding to N−H or C−H are recorded within this signal-to-noise ratio, which suggests that there are no carbon or nitrogen-containing species on the surface before the reaction with H2Pc. Figure 5c clearly shows that there is a signal corresponding to the CH stretching of aromatic rings observed at 3070 cm−1. Another peak at 3290 cm−1 is also present corresponding to N−H stretching, which is consistent with the −NH feature from pyrrole nitrogen. Furthermore, the positions of CH, N−H peaks are consistent with the predicted vibrational result from the DFT calculation of the phthalocyanine molecule shown as solid bars underneath the experimentally recorded spectrum of Pc-Si(111). The presence of N−H stretching also reveals that the Pc-Si(111) surface still has these functionality following modification. There is no signal observed around 1600 cm−1, which would be associated with −NH2 bending. Thus, consistent with the XPS results, we can confirm that the phthalocyanine molecules have been covalently attached to the Cl−Si(111) surface and that in the species formed, only some of the nitrogen atoms are protonated, suggesting a possibility of metalation reactions. The nature of the resulting species needs to be investigated further, as additional information can be obtained from ToF-SIMS measurements. ToF-SIMS Identification of Surface Species on the Phthalocyanine-Modified Si(111) Surface. The XPS and infrared spectroscopy investigations reported above help confirm the covalent attachment and the presence of certain surface functionalities on the Pc-modified Si(111) surface. However, on the basis of these studies alone, it is practically impossible to identify the phthalocyanine as a complete molecule present on a surface without decomposition. A

is impossible to observe individual phthalocyanine molecules on the Si(111) surface using AFM in this set of studies. Therefore, more spectroscopic characterization needs to be applied for further analysis on the surface species. Infrared Spectroscopy Studies of Surface Species on the Phthalocyanine-Modified Si(111) Surface. To better understand the formation of the surface species formed on the phthalocyanine-modified Si(111) surface, infrared spectra were recorded to monitor each modification step. Figure 5

Figure 5. Infrared spectroscopy studies of phthalocyanine modification on the Si(111) surface. The spectral ranges for the Si−H stretching region (left panel), N−H stretching region, and CH stretching region (right panel) are compared between (a) H−Si(111) surface, (b) freshly prepared Cl−Si(111) surface, and (c) phthalocyanine-modified Si(111) surface after 2.0 h reaction followed by washing and sonication treatment. DFT predicted vibrational frequencies for molecular species (scaled by 0.965) are shown as solid bars underneath the experimental spectra of Pc−Si(111) (right panel).

summarizes the most informative spectral regions: Si−H stretching (left panel) and N−H and C−H stretching (right panel). In both panels, spectrum (a) corresponds to the H− Si(111) surface with a clean silicon wafer covered with a thermal oxide used as a background, and spectrum (b) and spectrum (c) are the spectra of Cl−Si(111) and Pc-Si(111), respectively, using H−Si(111) as a background. In Figure 5a, a

Figure 6. ToF-SIMS spectra (negative ion) of m/z ranges of 512.5−514.5 (left) and 540.5−542.0 (right) on the phthalocyanine-modified Si(111) surface (red solid line) compared with the phthalocyanine powder sample (blue solid line). E

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Langmuir sensitive and effective technique that can address this ambiguity is ToF-SIMS. Negative-ion ToF-SIMS spectra are used to follow the comparison of the phthalocyanine powder sample and the Si(111) surface following phthalocyanine modification. Figure 6 provides a summary of the most relevant spectral ranges studied by this surface analytical technique. Specifically, negative-ion m/z ranges of 512.5−514.5 and 540.5−542.0 are summarized in Figure 6. The examination of these spectral regions reveals that the phthalocyanine structure is preserved on the Si(111) surface following phthalocyanine modification as the exact same signal of [C32H17N8]− as the phthalocyanine powder sample can be observed on Pc-Si(111). The signal of the molecular ion [C32H18N8]− at 514.165 is also observed on the Pc-Si(111) surface, possibly because of surface recombination and the overlapping peaks caused by atomic isotopes and ionic fragments bonded with silicon atoms because [12C3113CH17N8]− at 514.167, [C32H17N628Si]− at 514.159, [C32H16N629Si]− at 514.152, [C28H10N8Si2]− at 514.169, [C26H6N8Si3]− at 514.61, and many other complex species which have very close mass-to-charge-ratios will exist on the Pc-modified silicon surface. More importantly, the right panel shows that the Pc-modified Si(111) has a signal of [C32H17N8−Si]−, which is not observed in the phthalocyanine powder sample. This ion can only be formed on a Pc-modified Si(111) surface by a phthalocyanine fragment bonded directly to the surface. Therefore, a direct covalent attachment of phthalocyanine molecules on the Si(111) surface can be confirmed by the combination of XPS, infrared spectroscopy, and ToF-SIMS analysis. Computational Investigation. On the basis of spectroscopic and microscopic studies discussed above, the covalent attachment of the H2Pc on Cl−Si(111) surface through the Si−N bond has been confirmed. However, the configuration and stability of the resulting surface species to some degree still remain unclear. On the basis of the previous knowledge of reactions of various primary amines with a Cl−Si(111) surface, a number of possible models can be proposed. Two of the possible models are shown in Figure 7. These two models

instability can be compensated by the interaction of the HCl released during the reaction with the excess amines present in the solution, which will affect the overall energetics quite substantially.37 Overall, the relative comparison of the proposed models could be more informative, as it suggests that the single Si−N bond configuration has a distinctly higher stability compared to the model that has two Si−N bonds. This difference is 148.2 kJ/mol. This comparison is also consistent with the geometry of the proposed models, as the relative orientation of the two sp3-like silicon orbitals and nitrogen lone pairs simultaneously when the H2Pc molecule lies parallel to the surface is clearly unfavorable.26 Because the exact arrangement of the surface Pc−N−Si species is still unknown, a more complex surface reconstruction is very likely to occur to stabilize the resulting modified surface further. Overall, the DFT calculations are consistent with the experimental observations to suggest the formation of Si−N bonds during the H2Pc modification of the Cl−Si(111) surface, with a single-bonded model being preferred thermodynamically. In Situ Cobalt Metalation of the PhthalocyanineModified Si(111) Surface. Because metal-free phthalocyanine was successfully bonded to the Si(111) surface through the Si−N bond, it is important to further test the hypothesis that it can provide a platform for in situ metalation by different metal ions (Co2+, Zn2+, Cu2+, etc.). To test the chemical properties of the obtained Pc-modified Si(111) surface, this surface was subjected to cobalt metalation. For comparison, Cl−Si(111) was treated with a solution of CoCl2 following the same procedure to rule out a possibility that the cobalt salt is the main species present on a surface following metalation. XPS studies of the Co 2p spectral region for key samples studied are summarized in Figure 8. There is clearly no

Figure 7. DFT models and the comparison of relative energies of two different bonding configurations on the Cl−Si(111) surface.

Figure 8. Co 2p XPS spectral region of (a) CoCl2 powder, (b) CoCl2treated Cl−Si(111) surface, and (c) cobalt metalation of the PcSi(111) surface.

represent the surface configurations formed through two Si−N bonds and one single Si−N bond, respectively. Overall, the energetic analysis given in Figure 7 suggests that the proposed surface structures are predicted to be thermodynamically unstable, which is a similar conclusion to that reached in the previous computational investigations of reactions of amines with a Cl−Si(111) surface.35,37 Partially, this thermodynamic

substantial presence of cobalt on the Cl−Si(111) surface following CoCl2 solution treatment shown in Figure 8b, indicating that the washing procedure was sufficiently efficient to remove physisorbed cobalt salt. Thus, the cobalt signal observed in Figure 8c can be confirmed as a strong evidence of the successful cobalt metalation. Overall, the XPS spectra are F

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Figure 9. Negative-ion ToF-SIMS spectral region of (a) Pc-modified Si(111) and (b) cobalt metalation of Pc-Si(111).

increase of surface roughness following the reaction of H2Pc with a Cl−Si(111) surface. DFT calculations were performed to predict spectroscopic observables and to propose possible models of the species formed. On the basis of the observation of −NH stretching in IR and on DFT calculations, it is proposed that the surface species with a single Si−N bond attachment is preferred. The in situ cobalt metalation was tested to investigate chemical properties of the produced phthalocyanine-modified surface and confirmed by XPS and ToF-SIMS. On the basis of these results, a promising modification of a silicon surface and its sensitization for potential electrochemical and photoelectrochemical applications is established.

simply consistent with the presence of Co(II). However, the clear satellite peak position shift to lower B.E. (>2 eV) and its lower intensity indicate that the cobalt signal after metalation is different from that for CoCl2. In fact, the sharper bands, much lower intensity satellite peak, and lower B.E. shift than that typical of Co(II) salts such as CoCl2 shown in Figure 8a, are fully consistent with the formation of Co-Pc species on the PcSi(111) surface, as was also demonstrated previously by XPS for the molecular cobalt−phthalocyanine species.61,62 Thus, this observation confirms that the Pc-modified Si(111) retained the chemical properties of the phthalocyanine functionality toward cobalt metalation. Additional proof of cobalt metalation of the Pc-Si(111) surface was obtained from the ToF-SIMS analysis. Figure 9 presents a summary of the negative-ion m/z ranges of 86.8− 87.2 and 599.04−599.36 on the Pc-Si(111) surface before and after cobalt metalation, which reveals the presence of [CoN2]− and [CoC32H16N8−Si]− ion species on the Co−Pc−Si(111) surface following cobalt metalation, which is not observed in the Pc-Si(111) sample. It is also important to confirm that the signal corresponding to [C32H17N8−Si]− is still observed following cobalt metalation. This is indeed the case; however, this observation cannot be used as a quantitative characteristic of the process because (1) the efficiency of the metalation process is not likely to be 100% and (2) the introduction of CoCl2 during metalation onto the surface will lead to the increased complexity of all the surface species and ions despite the fact that cobalt itself only has one stable isotope. Nevertheless, the in situ cobalt metalation on the phthalocyanine-modified Si(111) surface can be confirmed by the combination of XPS and ToF-SIMS analysis. It can be added that the AFM images following cobalt metalation do not change substantially compared to the PC-modified Si(111) samples. They do help to rule out the multilayer formation or cobalt salt deposition as the contributing factors in this set of studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02259. Additional XPS studies, summary of AFM investigations, computational models used, and complete ref 44 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone.: (302) 831-1969. Fax: (302) 831-6335. ORCID

Andrew V. Teplyakov: 0000-0002-6646-3310 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was also partially supported by the National Science Foundation [CHE1057374 and DMR1609973 (GOALI)]. The authors acknowledge the NSF (9724307; 1428149) and the NIH NIGMS COBRE program (P30-GM110758) for partial support of activities in the University of Delaware Surface Analysis Facility.

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CONCLUSIONS The work presented here investigates the direct covalent attachment of phthalocyanine molecules to the chlorineterminated Si(111) surface without any additional linkers. This covalent linkage through Si−N bonds is confirmed by the combined XPS, FT-IR, and ToF-SIMS analysis. XPS spectra suggested the formation of Si−N bonds. Infrared spectra and ToF-SIMS confirmed the integrity of phthalocyanine moieties attached to the Si(111) surface. AFM revealed the uniform

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REFERENCES

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