Preparation of Carbon Nanomembranes without Chemically Active

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Preparation of Carbon Nanomembranes without Chemically Active Groups Christof Neumann, Monika Szwed, Martha Frey, Zian Tang, Krzysztof Kozieł, Piotr Cyganik, and Andrey Turchanin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09603 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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ACS Applied Materials & Interfaces

Preparation of Carbon Nanomembranes without Chemically Active Groups Christof Neumann†, Monika Szwed‡, Martha Frey†, Zian Tang†, Krzysztof Kozieł§, Piotr Cyganik‡* and Andrey Turchanin†,*. †Institute

of Physical Chemistry, Friedrich Schiller University Jena, 07743 Jena, Germany Institute of Physics, Jagiellonian University, 30-348 Krakow, Poland §Faculty of Chemistry, Jagiellonian University, 30-387 Krakow, Poland ‡Smoluchowski

Jena Center for Soft Matter (JSCM), 07743 Jena, Germany



KEYWORDS carbon nanomembranes (CNM), 2D materials, self-assembled monolayers, carboxylic acid, low-energy electron irradiation induced crosslinking ABSTRACT: The electron irradiation induced synthesis of Carbon Nanomembranes (CNMs) from aromatic thiol-based selfassembled monolayers (SAMs) on gold substrate is a well-established method to form molecular thin nanosheets. These molecular 2D materials can be prepared with tunable properties, and therefore, they find a variety of applications in nanotechnology ranging from ultrafiltration to nanobiosensors. However, no chemically inert CNM was fabricated up to now, as the reactive thiol group is present on the membrane surface even after transferring it to other substrates. Here we study electron irradiation of carboxylic acidbased SAMs on silver substrate as an alternative route for CNMs formation. Our analysis, based on combination of X-ray photoelectron spectroscopy and scanning electron microscopy, demonstrates that for this type of SAMs purely carbonaceous CNMs with tunable porosity can be obtained.

Figure 1: Schematic fabrication route of the CNMs prepared from SAMs with thiol-based (a) and carboxylic (b) bonding groups. After selfassembly (i) the monolayers are irradiated with low-energy electrons leading to the crosslinking of molecules and modification of the bonding groups (ii). Importantly, whereas in the case of carboxylic group the oxygen is efficiently desorbed from the sample, in the case of thiols most of the sulfur remains after irradiation (iii). Finally, the CNMs formed by the crosslinking are transferred onto TEM grids forming mechanically stable nanosheets exhibiting sulfur components in the case of thiol-based and all carbon composition for the carboxylic-based analogue system, respectively (iv).

1. INTRODUCTION The synthesis of new types of two-dimensional (2D) materials beyond graphene attracted a large research attention within the last several years.1-4 Beside the discovery of different transition

metal dichalcogenides,5, 6 also organic monolayers came more into focus7-10 including covalently bonded 2D polymers synthesized via UV irradiation11 or 2D metal-organic frameworks.12 Among these organic 2D materials an important class is Carbon Nanomembranes (CNMs), which are prepared

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(iv) after cleaning the sample in ultrapure water the CNM/PMMA sandwich is transferred onto the desired substrate and dried at 50°C for 1h on a hotplate, (v) to remove the PMMA layer after transfer, the CNM on a SiO2/Si wafer is immersed in acetone followed by rinsing with isopropanol. To minimize damage of the freestanding sheets, critical point drying (Autosamdri-815, Tousimis) was performed for the CNMs on TEM grids.31 2.2 Optical microscopy. The optical microscope image was obtained with a Zeiss Axio Imager Z1.m microscope equipped with a 5 megapixel CCD camera (AxioCam ICc5) in bright field operation. 2.3 Scanning electron microscopy (SEM). The SEM images were obtained with a Zeiss Sigma VP at a beam energy of 10 kV using the in-lens detector of the system. 2.4 X-ray photoelectron spectroscopy (XPS). XPS was performed using a Multiprobe system (Scienta Omicron) with a monochromatic X-ray source (Al Kα) and an electron analyzer (Argus CU) with 0.6 eV spectral energy resolution. The spectra were fitted using Voigt functions (30:70) after Shirley (C 1s) or

by low-energy electron induced crosslinking of thiol-based aromatic self-assembled monolayers (SAMs) typically on gold substrate.10 The properties of the CNMs such as porosity13 and stiffness14 can be tuned by choosing suitable precursor molecules and preparation conditions depending on the targeted application. Chemically active functional groups on CNMs makes them a promising platform for biosensing.15 Furthermore, CNMs can be used for synthesis of lateral16 and van der Waals17 heterostructures or as a precursor for the conversion into graphene.18, 19 However, no chemically inert CNMs were reported up to now, as the reactive sulfide groups remain in the CNMs even after transferring them to other substrates.20 To remove this limitation, important for applications such as ultrafiltration,21, 22 here we demonstrate formation of all-carbonaceous CNMs by using carboxylic acid based hybrid aromatic-aliphatic SAMs prepared on silver substrates.23-26 The experiments were performed mainly for two analogous hybrid aliphatic-aromatic SAMs based on either thiol (BP2S, CH3-(C6H4)2-(CH2)2-SH) or carboxylic (BP2COOH, H(C6H4)2-(CH2)2-COOH) bonding groups. Recent spectroscopic and microscopic analysis27 has revealed that both SAMs form identical structures on the Ag(111) substrate enabling thus identification of the role of the bonding group in the CNMs formation process. The synthetic route including the respective structural changes are schematically shown in Figure 1. In the first step, carboxylic based SAMs are formed from a solution on freshly prepared Ag(111) substrates (Figure 1b (i)). Next, monolayers are crosslinked into CNMs by low-energy electron irradiation (Figure 1b (ii)-(iii)). The changes in the chemical composition and structure were analyzed stepwise by the X-ray photoelectron spectroscopy (XPS) with an increasing irradiation dose. Finally, the successful synthesis of freestanding CNMs was confirmed by scanning electron microscopy (SEM) (Figure 1b (iv)). For comparison, the CNM formation was analyzed for an analogues thiol system, which is schematically presented in Figure 1a. 2. EXPERIMENTAL SECTION 2.1 Sample preparation. The Ag(111) substrates were prepared by evaporation of 100 nm of silver (rate 0.1nm/s) on freshly prepared mica sheets at 533 K. The BP2COOH molecules were purchased from Alfa Aesar and used without further purification. The synthesis of BP6COOH and BP2SH molecules was conducted according to the procedure described in the literature.28, 29 All analyzed SAMs were prepared by immersion of freshly evaporated Ag(111) substrate into absolute ethanol (99.8%, POCH-Poland) solution (1mM) of the respective compound. After incubation of the BP2COO/Ag (5 min.), BP6COO/Ag (5 min.) or BP2S/Ag (24 h), samples were rinsed with pure ethanol and dried under nitrogen stream. The electron irradiation was performed in the same UHV system (base pressure < 10-9 mbar) as used for the XPS analysis with an electron beam of 50 eV generated by an NEK 150 (Staib) electron gun. The CNMs were transferred onto silicon wafers with dry thermal oxide surface layer (300 nm, Sil’tronix) and TEM grids (Quantifoil R 2/2 on Cu, 400 mesh) using the following protocol: (i) the PMMA (50K, AR-P 631.04) layer is spin-casted on the CNM/Ag sample and baked on a hotplate at 90 °C for 5 min, (ii) second PMMA (950K, AR-P 671.04) layer is spin-casted and baked using the same parameters, (iii) the CNM is detached from the Ag substrate by electro-chemical delamination in NaOH (0.2 Mol/L) using a voltage of 2-3 V,30

Figure 2: XPS data of monolayers prepared from (a) BP2SH and (b) BP2COOH molecules on Ag(111) as a function of the electron irradiation dose using an electron energy of 50 eV. In the bottom part of (a) and (b) the data for CNMs after transfer on SiO2/Si substrates are presented. For better representation, intensities of the S 2p and O 1s spectra are multiplied by the factor presented in the figure.

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Figure 3: Summary of the XPS data for BP2S/Ag (left column) and BP2COO/Ag (right column) samples as a function of the electron irradiation dose using an electron energy of 50 eV. In (a) and (b) comparison of the binding energy and total peak area modification of the C 1s signal are shown. In (c) and (d) peak area analysis of different components of the S 2p and O 1s signals are shown, respectively.

gradually converted into silver sulfide and disulfides, already for doses above 0.3 mC/cm². After an electron dose of 150 mC/cm² the thiolate species (green) and the silver sulfides (red) have almost the same peak intensity corresponding to ~38 % of the total signal each, whereas the remaining ~24 % corresponds to disulfides (blue) (Figure 2a). The formation of silver sulfides upon low-energy electron irradiation of SAMs was also reported in Ref.33, which is due to a partial cleavage of the sulfur head group from the CNM. Nevertheless most of the sulfur remain covalently bonded to this 2D molecular sheet (see bottom panel Figure 2a and further text). Similar to the sulfur signal, the total amount of carbon for the BP2S/Ag SAMs is reduced by 5% during the entire irradiation process (Figure 3a), which results in reduction of the thickness (see SI for details) from 1.7 ± 0.1 nm to 1.6 ± 0.1 nm. The crosslinking process, associated with new C-C bonds formation, leads to an increase in the FWHM of the C 1s peak from 0.8 to 1.1 eV. The saturation of the spectral modifications of the C 1s peak above an electron dose of 10 mC/cm² is indicative for the crosslinking of the BP2S/Ag SAM. This result correlates with the previous studies of BPnS/Au SAMs34 as well as purely aromatic thiol-based SAMs on the Au(111)13, 32 and Cu(111)18 substrates. Next, we analyze the XPS results for the BP2COO/Ag analogue. Similar as for the BP2S/Ag system, the C 1s spectrum of the SAM is dominated by the main component associated with the biphenyl moiety at 283.6 eV (red, Figure 2b) which is accompanied by a small shoulder at 284.3 eV due to the aliphatic linker (green, Figure 2b). In comparison with the BP2S/Ag system, however, both components are shifted by 0.8 eV towards lower BE due to the interfacial charge rearrangement associated with the modification of the bonding group, as was reported earlier.27, 35 The presence of the COO-

linear (O 1s, S 2p) background subtraction, respectively. The spectra recorded on SiO2/Si were calibrated using the Si 2p peak (SiO2, 103.6 eV). 3. RESULTS AND DISCUSSION A summary of the XPS study of the electron-irradiation process is presented in Figure 2 and Figure 3. First, we start with the analysis of a SAM with the thiol binding group. For the BP2S/Ag SAMs the S 2p spectrum shows a doublet at the binding energy (BE) of 161.8 eV (S 2p3/2) and 163.0 eV (S 2p1/2), due to the formation of thiolates, which confirms the successful SAM formation (Figure 2a, green).27 The C 1s spectrum of the BP2S/Ag SAM consist of a main peak at 284.2 eV (red) with a full width at half maximum (FWHM) of 0.8 eV corresponding to the biphenyl moiety. This peak is accompanied by a shoulder at 285.0 eV which is attributed to the aliphatic linker and the C-S bond (green). In order to crosslink the SAMs into CNMs, low-energy electron irradiation (50 eV) was employed, similar to biphenyl-thiol (H-(C6H4)2SH) based CNMs on Au substrates.32 To study the conversion in detail, the SAM was irradiated stepwise followed by an XPS measurement after each step. The irradiation doses were chosen to result in equally spaced data points on a logarithmic scale, Figure 3. The modification of the S 2p and C 1s signals of selected characteristic spectra after an irradiation dose of 1 mC/cm², 15 mC/cm² and 150 mC/cm² is presented in Figure 2a and Table S1. The full analysis of the gradual changes in peak position and intensity of the C 1s signal as well as the modifications in the S 2p spectra are presented in Figure 3a and 3c. Due to the electron irradiation, two new sulfur species are formed, which can be attributed to silver sulfides (BE (S 2p3/2) = 161.0 eV) and disulfides (BE (S 2p3/2) = 163.7 eV). It is shown in Figure 3c that the total amount of sulfur is only slightly reduced (10 %), whereas the original thiolate species are



a

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BP6COO

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Figure 4: SEM images of freestanding CNMs transferred onto TEM grids. The crosslinking was achieved using an electron energy of 50 eV and a dose of 150 mC/cm². (a-b) CNM prepared from BP2S/Ag SAMs form a continuous nanomembrane. (c-d) CNM prepared from BP2COO/Ag SAMs shows nanopores due to the material loss upon crosslinking. (e-f) CNM prepared from BP6COO/Ag SAMs with a higher carbon content forms a continuous nanomembrane. desorption of molecules from this type of monolayer compared to the thiol-based analogue. As a consequence, the electron irradiation causes about five times higher relative reduction of the film thickness for the BP2COO/Ag SAM (25%) compared to the BP2S/Ag (5%) SAM. Taking an onset of the C 1s signal saturation with an increasing electron irradiation dose as a measure for the crosslinking process, we expect that the crosslinking is efficient for the BP2S/Ag already at a dose of 10 mC/cm2, whereas for the BP2COO/Ag a higher dose of 40 mC/cm2 is required (see Fig. 3a and 3b). We relate the latter to a higher desorption rate of the aromatic cores upon the crosslinking. Finally, after prolonged electron irradiation up to the dose of 150 mC/cm², elimination of oxygen leads to complete removal of the binding group from the crosslinked film, which enables formation of all-carbonaceous CNMs (Figure 1b). To exclude that reported here structural modifications result from prolonged exposition of samples to the X-rays during XPS data collection, we analyzed the BP2COO/Ag sample after 3 hours of continuous XPS analysis (equivalent to the total time of XPS analysis shown in Figure 2) and noticed only minor modification of the XP spectra (Figure S1). A detailed analysis of the XP spectra of the BP2COO/Ag samples is presented in Table S2. The CNMs obtained after irradiation of the BP2S/Ag and BP2COO/Ag samples were transferred using the PMMA-based transfer protocol onto the SiO2/Si substrates (Figure S2). The C 1s spectra, presented in Figure 2, confirm successful transfer and show similar characteristics as observed before with, however, some additional contribution of the C-C and C-O signals due to the PMMA residuals. As expected in the case of the CNM obtained from the BP2S/Ag samples different sulfur species are still visible in the S 2p spectrum. Beside thiolates (green) and disulfide (blue) mainly oxidized sulfur (purple, 53 %) at a BE of 167.7 eV (S 2p3/2) can be found as a result of conducting the CNMs transfer process under ambient conditions. As expected, the XP analysis of the identically transferred BP2COO/Ag CNMs does not show any sulfur traces

bonding group in the C 1s spectrum can be identified via small peak at 287.1 eV. The signature of the bidentate carboxylicmetal bond formation can be found in the O 1s spectrum characterized by a single, and symmetric, component at 530.4 eV (dark blue).27, 35 The estimated thickness of the BP2COO/Ag monolayer (1.4 ± 0.1 nm) is slightly lower from the BP2S/Ag analogue, similar as it was reported earlier.27 To compare the influence of the binding group on the crosslinking process, the conversion into CNMs was studied using the same electron irradiation conditions as for the BP2S/Ag samples. As shown in Figure 3b, the intensity of the C 1s signal drops gradually with increasing irradiation dose. After an irradiation dose of 150 mC/cm² the reduction of the C 1s intensity by 28% corresponds to a reduction of the film thickness to 1.1 ± 0.1 nm. In parallel, the position of the C 1s main component shifts towards higher BE. At electron doses above 15 mC/cm² a value of ~284.4 eV is reached, which is essentially the same as reported before for the BP2S/Ag CNM. This effect is associated with vanishing the C 1s signal from the COO-group (dark blue), as seen in Figure 2b. The changes in the C 1s spectra correlate with the O 1s signal, which exhibits gradual vanishing of the main COO-component at 530.4 eV accompanied with appearance of two new components at 531.4 eV and 533.0 eV, which correspond to the C=O (turquoise) and C-O/C-OH groups (green), respectively (Figure 2b). Such behavior of the O 1s and C 1s spectra indicates that electron irradiation causes complete elimination of the original bidentate carboxylatemetal bond and intermediate formation of new carbon oxide species which desorb at doses above ~5 mC/cm² (see Figure 3b and 3d). This scenario is also consistent with the above reported electron irradiation induced shift in BE of the C 1s spectra for BP2COO/Ag SAMs caused by the interfacial charge rearrangement upon elimination of the original carboxylicmetal bonding. The elimination of oxygen from the BP2COO/Ag has few important consequences. First, high efficiency of this process at the early stage of the irradiation, when molecules are not sufficiently crosslinked, leads to much more efficient


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ASSOCIATED CONTENT

(Figure S3) confirming the formation of an all-carbonaceous CNM without chemically active groups. Successful preparation of freestanding CNMs is further confirmed by their transfer on holey TEM grids. Our SEM analysis demonstrates, that for a complete crosslinking of the thiol- and carboxylic acid-based SAMs an irradiation dose above 50 mC/cm² is sufficient. Figure 4a shows a large-area SEM micrograph of a suspended CNM prepared from the BP2S/Ag SAMs. A magnified image demonstrates high quality of the fabricated nanomembrane without visible defects or nanopores, Figure 4b. This result is in good agreement with the CNMs prepared from linear aromatic thiol-based molecules on gold substrates.13 Similarly, CNMs prepared from the BP2COO/Ag SAMs also form suspended membranes (Figure 4c). Nevertheless, a closer look to the magnified image in Figure 4d reveals formation of pores with diameter below 50 nm, which results from a much higher carbon loss for BP2COO/Ag compared to BP2S/Ag upon the cross-linking (see Figure 3). In order to compensate higher material loss during electron irradiation of carboxylic SAMs, we used an analogue system with a longer alkyl spacer (six unites), i.e., the BP6COO/Ag. Both SAMs show similar packing density and orientations of the biphenyl units as reported earlier.36 Therefore, by this modification we can clearly determine the influence of the spacer length on the efficiency of crosslinking process. The analysis of the C 1s and O 1s spectra of the BP6COO/Ag upon the irradiation (Figure S4, Table S3) reveals a similar behavior as for the BP2COO/Ag (Figure 2b). However, as expected, the effective thickness of the BP6COO/Ag monolayer (1.8 ± 0.1 nm) and the respective CNM (1.4 ± 0.1 nm) are in this case higher compared to the BP2COO/Ag system (see the SI for details). The increased length of the aliphatic linker for BP6COO/Ag system increases intermolecular interactions, which should somewhat slow down the desorption process, and thus enhance efficiency of the crosslinking. Following this argumentation application of the longer homolog of the BPnCOO/Ag series leads to formation of CNMs without any observable defects or nanopores as shown in Figure 4e-f, confirming thus that fabrication of continuous, suspended CNMs from SAMs with carboxylic bonding groups is achieved.

Supporting Information. Experimental methods, supporting optical microscope image, supporting XPS analysis and supporting references. This material is available free of charge via the Internet at XXX

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources National Science Centre Poland (grant UMO2015/19/B/ST5/ 01636). Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Projektnummer 364549901 – TRR 234.

ACKNOWLEDGMENT We thank Stephanie Höppener and Ulrich S. Schubert for the access to the scanning electron microscope. The SEM facilities of the Jena Center for Soft Matter (JCSM) were established with a grant from the German Research Council (DFG). CN and AT thank the DFG for support within the research grant TU149/8-2 and the research infrastructure grant INST 275/25 7-1 FUGG.

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4. CONCLUSION In summary, we have demonstrated that low-energy electron irradiation of hybrid aromatic-aliphatic SAMs of BPnCOO/Ag with carboxylic bonding group leads to the crosslinking process similarly as for the aromatic SAMs based on thiols. In this case, however, the crosslinking is associated with the complete removal of the bonding group unlike in the CNMs prepared from thiols. Thus a method to prepare all-carbonaceous CNMs without chemically active groups with about ~1 nm thickness is proposed and realized. Due to more efficient electron-induced desorption process compared to thiol-based systems, the resulting CNMs can have different level of porosity which depends on the length of the aliphatic part of these hybrid SAMs, and provides a simple approach to control porosity of the nanomembrane. We expect that such kind of completely carbonaceous nanomembranes are of great interest for application in ultrafiltration, where the surface charges in nanomembrane, due to the presence of chemically active groups, have to be avoided.



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Preparation of Carbon Nanomembranes without Chemically Active

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