Surface-enhanced Raman spectroscopy of carbon nanomembranes

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Surface-enhanced Raman spectroscopy of carbon nanomembranes from aromatic self-assembled monolayers Xianghui Zhang, Marcel Mainka, Florian Paneff, Henning Hachmeister, André Beyer, Armin Gölzhäuser, and Thomas Huser Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03956 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Surface-enhanced Raman spectroscopy of carbon nanomembranes from aromatic self-assembled monolayers Xianghui Zhang, 1,* Marcel Mainka,1,† Florian Paneff,1 Henning Hachmeister,2André Beyer,1 Armin Gölzhäuser1, Thomas Huser2,* 1

Physics of Supramolecular Systems and Surfaces, Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany. 2

Biomolecular Photonics, Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany.

Abstract Surface-enhanced Raman scattering spectroscopy (SERS) was employed to investigate the formation of self-assembled monolayers (SAMs) of biphenylthiol (BPT), 4'-nitro-1,1'-biphenyl4-thiol (NBPT) and p-terphenylthiol (TPT) on Au surfaces, and their structural transformations into carbon nanomembranes (CNMs) induced by electron irradiation. The high sensitivity of SERS allows us to identify two types of Raman-scattering in electron-irradiated SAMs: (1) Raman-active sites exhibit similar bands as those of pristine SAMs in the fingerprint spectral region, but with indications of an amorphization process; (2) Raman-inactive sites show almost no Raman-scattering signals, except a very weak and broad D band, indicating a lack of structural-order but for the presence of graphitic domains. Statistical analysis showed that the ratio of the number of Raman-active sites to the total number of measurement sites decreases exponentially with increasing the electron irradiation dose. The maximum degree of crosslinking ranged from 97% to 99% for the three SAMs. Proof-of-concept experiments were conducted to demonstrate potential applications of Raman-inactive CNMs as a supporting membrane for Raman analysis.

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INTRODUCTION Carbon possesses the most unique bonding properties, which enable it to form different allotropes, such as diamond, fullerene and carbon nanotubes, graphite and graphene. Due to the different anisotropy of sp2 and sp3 hybridized orbitals, graphite exhibits very different physical properties compared to diamond. It is intuitive to bridge these two allotropes by preparing new carbon materials with mixed sp2 and sp3 phases with controlled spatial distributions. Indeed, there are a variety of carbon materials consisting of both sp2 and sp3 phases, such as tetrahedral amorphous carbon (or diamond-like carbon) and nanocrystalline diamond.1, 2 Local and precise transformation of the sp3 hybridized carbon matrix into a sp2 phase can be achieved by utilizing an ion- or an electron-beam, and the properties of such a transformed carbon matrix have been investigated by several surface sensitive analytical techniques.3,

4

A bottom-up approach to

fabricate carbon nanomaterials with mixed sp2 and sp3 phases is to form self-assembled monolayers (SAMs) on surfaces and then to polymerize them via ultraviolet (UV) irradiation.5, 6 Aliphatic SAMs usually undergo fragmentation and structural disordering when being exposed to electron-beam irradiation,7 whereas aromatic SAMs become mechanically stabilized via covalent cross-linking of adjacent molecules and turn into carbon nanosheets .8, 9, 10 The cross-linked aromatic SAMs, also known as carbon nanomembranes (CNMs), possess monomolecular thickness and represent a new type of functional quasi-two-dimensional material.11 A variety of aromatic molecules have been used to prepare CNMs with well-defined thicknesses ranging from 0.6 nm to 3 nm.12 As the CNMs are derived from the precursor molecules, there is a correlation between the rigidity of the precursor molecules and the mechanical stiffness of corresponding CNMs, which has been revealed by bulge testing in atomic force microscopy (AFM).13, 14 CNMs can be utilized as supporting films for transmission


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electron microscopy (TEM), 15, 16 as ultrathin membranes for gas separations,17 and potentially as a thin dielectric layer for molecular electronics.18, 19 The outstanding mechanical properties also make them suitable candidates for nanoelectromechanical systems (NEMS).20 The conversion of aromatic SAMs into CNMs has been investigated by using different complementary surface spectroscopic techniques and theoretical methods.21, 22, 23, 24, 25 Surface-enhanced Raman Scattering spectroscopy (SERS) is a powerful surface-sensitive analytical technique that allows for detecting chemical and structural information of molecules on metallic surfaces,26, 27, 28 as well as on other SERS-active substrates.29, 30, 31 The dominant contribution of the SERS effect arises from the localized excitation of surface plasmons, which leads to an electromagnetic field enhancement at nanoscopic surface features termed as hot spots.32 There is usually another enhancement mechanism called chemical enhancement due to the formation of charge-charge complexes.33 SERS has been used to characterize the bonding, structure and orientation of adsorbed molecules at Au and Ag surfaces.34, 35, 36, 37 On the other hand, SAMs have been employed as model systems to study the influence of nanoparticle shapes and their optical properties on the SERS enhancement factors.38 Structural transformation of both aliphatic SAMs and fullerene molecules into amorphous carbon by high-energy electrons has been recently investigated using SERS.39 In this paper, we use biphenylthiol (BPT), 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) and pterphenylthiol (TPT) as model molecules to form SAMs on Au(111) and prepare CNMs via electron irradiation. Surface enhanced Raman spectroscopy (SERS) assisted by Au nanoparticles is employed not only to confirm the chemisorption of precursor molecules on Au surfaces but also to monitor structural changes involved in the conversion of pristine SAMs to cross-linked monolayers induced by electron-beam irradiation. Further, the statistics are used to estimate the


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cross-linking degree in the three SAMs from their Raman-scattering spectra. Finally we present a proof-of-concept of how CNMs could be used as an ultrathin supporting layer for Raman analysis.

EXPERIMENTAL SECTION Molecules and Materials 1,1'-biphenyl-4-thiol was bought from Sigma-Aldrich. 4'-nitro-1,1'-biphenyl-4-thiol was purchased from Taros Chemicals (Dortmund, Germany). P-terphenylthiol was specially synthesized. 300 nm thermally evaporated Au film on mica supports (Georg Albert PVDCoatings) were used as substrates for the SAM preparation. Suspensions of Au NPs in water were acquired from G. Kisker GbR (Steinfurt, Germany). Preparation of SAMs and CNMs Cleaning of the Au surfaces was achieved with an UV/ozone-cleaner (FHR, UVOH 150 LAB) and subsequent immersing in absolute ethanol. The SAMs were prepared by immersion of the cleaned Au substrate into ~10 mM solutions of the precursor molecules in dried and degassed N,N-Dimethylformamide (DMF) under a nitrogen atmosphere in a sealed flask for 72 h. Crosslinking of the SAMs was conducted by exposing the SAMs to 100 eV electrons (SPECS flood gun) in a high vacuum (10-7 mbar) using doses of 5, 10, 20 and 60 mC·cm-2. The Au NP suspension was deposited as received onto SAMs or CNMs by drop casting method at room temperature. Preparation of a BPT–CNM carrying 80 nm Au NPs and transfer it onto a NBPT–SAM To achieve a system consisting of Au/NBPT–SAM/BPT–CNM/Au-NPs, a BPT–CNM was firstly transferred onto a Si3N4/Si substrate. To this end, two layers of PMMA (1st layer: AR-P


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631.04; 2nd layer: AR-P 671.04) with an overall thickness of ~400 nm, were spin-cast on the sample and baked on a hot plate at 90°C for 5 min. The underlying mica support was separated from the Au/BPT–CNM/PMMA structure by a slight dipping into water of one of the edges/corners of the sample. The separated BPT–CNM/PMMA structure was transferred from the water surface to an I2/KI/H2O etching bath (1:4:10) where the Au layer would be dissolved. After that, the Au/BPT–CNM/PMMA stack was transferred to a pure water bath and subsequently fished out with a Si3N4/Si substrate. The sample was immersed into acetone for ~45 min to dissolve the PMMA and blown dry in a stream of nitrogen. Secondly, 80 nm Au NPs were deposited on the BPT–CNM by drop casting method at room temperature. Again two protective layers of PMMA were spin-coated on the Si/Si3N4/BPT–CNM/Au-NPs wafer. The sample was brought onto the surface of HF (48 %) for 5 s to weaken the adhesive forces between wafer and CNM. Then the system BPT–CNM/Au-NPs/PMMA was separated from the Si3N4 and transferred onto a water surface. The BPT–CNM/Au-NPs/PMMA stack was fished out with a NBPT–SAM on a Au/mica substrate. Finally, the PMMA was dissolved in acetone to achieve a system consisting of Mica/Au/NBPT–SAM/BPT–CNM/Au-NPs. Raman Spectroscopy Raman measurements were conducted using a custom-built Raman microspectroscopy system using a 785 nm wavelength semiconductor laser (Innovative Photonic Solutions) with a power of 10 mW in the focal point. For the acquisition of the spectra we used a 60 x objective with 1.2 NA (Olympus). For the testing of the sample surface for Raman activity (statistics) we used a 20 x objective with 0.4 NA. A spectrometer with a 600 lines per millimeter grid (ACTON SpectraPro 2300i) combined with a charge-coupled device (CCD) camera (ANDOR Newton) was used to acquire Raman spectra. The CCD camera was cooled to -75 °C and full vertical binning was


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chosen to minimize noise. For testing sample surfaces for Raman activity 260 sites with a 100 µm distance to the nearest neighbor were investigated using a micrometer screw-controlled translation stage to access the respective sites. By obtaining a minimum signal-to-noise ratio of 3 for the peaks at ~1080 cm-1 and ~1600 cm-1, a site was assessed to be Raman active. Since a minimum of 97 % of the sites showed Raman activity for a pristine BPT–SAM, the maximum random error is estimated to be ~3 % of all measured sites. Helium ion microscopy (HIM) A Carl Zeiss Orion Plus microscope was utilized to conduct HIM measurements. For the helium ion beam we used energies of 33 - 38 keV at beam currents of 0.2 -1.4 pA. A 10 µm aperture was used for all measurements. For the detection of secondary electrons an Everhart-Thornley detector with a grid voltage of 500 V was employed. The working distance ranged from 10 mm to 29 mm.


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RESULTS AND DISCUSSION

Figure 1 (a) Formation of a biphenylthiol self-assembled monolayer (BPT–SAM) on the Au substrate. Low-energy electron irradiation leads to the cross-linking of molecules and the conversion to a carbon nanomembrane (CNM). (b) Deposition of Au nanoparticles on a SAM or a CNM for surface-enhanced Raman spectroscopy analysis.

Figure 1a shows a schematic illustration of a pristine biphenylthiol self-assembled monolayer on a gold surface. When the SAM is exposed to low-energy electrons, the precursor molecules are dehydrogenated and cross-linked. For SERS measurements we adopted a sphere-plane configuration by depositing Au nanoparticles on pristine and cross-linked monolayers on Au(111) substrates, as schematically shown in Figure 1b.

Figure 2 Normal Raman spectra (the upper plots) of precursor molecules in powder form: (a) BPT, (b) NBPT and (c) TPT. The SER spectra (the lower plots) of their corresponding SAMs on Au(111) substrates, where 80 nm Au NPs were used to obtain the surface-enhanced Raman scattering.


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Normal Raman spectra of three precursor molecules in powder form have been compared with the SER spectra of their corresponding SAMs formed on Au(111) substrates. In the Raman spectrum (the upper plot of Figure 2a) of BPT molecules, the bands at 2560 cm-1 and 913 cm-1 are assigned to the stretching mode of the S–H bond and the deformation mode of the C–S–H vibration, respectively. The former band is not accessible by SERS and the latter one is absent in the SER spectrum (the lower plot of Figure 2a). The band at 759 cm-1 assigned to be the out-ofplane C–H bending with a contribution from the C–S–H bending mode, becomes significantly depressed in the SAM.35, 40 Notice that previous assignments of the band at to be the Au–S stretching mode can be excluded due to a lack of Au surfaces here.35, 41 The bands at 271 cm-1 and 320 cm-1 can be probably assigned to the out-of-plane shearing motion (B2g fundamental) and the in-plane ring-ring shearing motion (Ag fundamental), respectively.40,

42

The

disappearance of both bands with the emergence of a new band at 286 cm-1 occurs in the SER spectrum of the BPT–SAM. In the SER spectrum a new band appears at 479 cm-1 and is assigned to an out-of-plane phenyl vibration coupled with a torsion mode around the C–S bond.43 No significant shifts of the inter-ring C–C stretching mode at 1282 cm-1 was found in the SER spectrum of the BPT–SAM. The symmetric breathing mode of the phenyl ring that appeared at 1105 cm-1 in the bulk phase had a downshift of ~26 cm-1 and a substantial increase in intensity in the SER spectrum. An apparent Fermi resonance doublet that occurs at about 1595 cm-1 in the bulk phase is found at 1584 cm-1 and 1601 cm-1 in the SER spectrum, respectively. The significant downshifts of ring modes and the broadening of bands are characteristic of the chemisorbed SAM.34, 35 All these changes indicate the chemisorption of BPT molecules to the Au surface and the absence of physisorbed molecules on the surface. Detailed assignment of the Raman bands is given in Table S1.


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The Raman spectrum of NBPT molecules and the SER spectrum of the NBPT–SAM are shown in Figure 2b. In the Raman spectrum of NBPT molecules in powder form, the band at 2561 cm-1 correspond to the stretching vibration of the S–H bond. The symmetric stretching vibration of the NO2 group at 1339 cm-1 doesn’t show any shifts in the SER spectrum.44 The in-plane rocking mode (b1 vibration) of the NO2 group at 530 cm-1 becomes stronger in the SER spectrum. The inplane bending mode (a1 vibration) of the NO2 group at 856 cm-1 and the out-of-plane C–H angle deformation at 820 cm-1 (a2 fundamental, ν11) can be found in both spectra.45 The band at 763 cm-1 assigned to be the out-of-plane C–H bending with a contribution from the C–S–H bending mode, becomes absent in the SAM.35,

40

The C–C–C angle deformation at 625 cm-1 (b1

fundamental, ν18) is absent in the SER spectrum.45 The C–C–C angle deformation at 407 cm-1 (a1 fundamental, ν18´) and the NO2 out-of-plane rocking vibration at 420 cm-1 (b2 fundamental), which are resolved in the Raman spectrum of NBPT molecules, merge into one peak in the SER spectrum.45 NBPT molecules possess a symmetric ring breathing mode at 1110 cm-1. However, there was a pair of bands at 1081 cm-1 and 1114 cm-1 in the SER spectrum of the NBPT–SAM. The same spectral changes were also found in the p-nitrothiophenol adsorbed on a silver surface.44 No significant shifts of the inter-ring C–C stretching mode at 1284 cm-1 and of the inplane deformation mode of C–H bond at 1195 cm-1 were found in the SER spectrum of the NBPT–SAM. The ring mode at 1591 cm-1 appeared to have a downshift of 7 cm-1 and a broadening of the peak. These changes also suggest the chemisorption of NBPT molecules on the gold surface. The Raman spectrum of the TPT molecules and the SER spectrum of the TPT–SAM are also compared in Figure 2c. The splitting of the band at 1592 cm-1 into bands of 1584 cm-1 and 1601 cm-1 is pronounced, indicating a downshifting of the mode associated with lower phenyl rings


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due to the chemical adsorption of TPT molecules.46 The band at 1272 cm-1 in the spectrum of TPT molecules, which is assigned to the inter-ring C–C stretching mode, exhibits an upshift of 3 cm-1 in the spectrum of the TPT–SAM.46 The in-plane deformation mode of C–H bond at 1223 cm-1 is downshifted by ~4 cm-1. The symmetric breathing mode of the phenyl ring at 1107 cm-1 exhibit a downshift by ~26 cm-1 and an increase in intensity in the SER spectrum. The out-ofplane deformation of C–H bond and C–C bond at 1040 cm-1 appears weaker in the SER spectrum. The in-plane deformation of C–H bond (the middle phenyl ring) at 1003 cm-1 appears much stronger in the SER spectrum, whereas the out-of-plane deformation of C–C–H bond (the upper and lower phenyl rings) at 993 cm-1 appears largely suppressed. The intensity of the in-plane ring vibration mode at 778 cm-1 decreases dramatically. There are also a few new peaks below 750 cm-1 in the SER spectrum: (1) the out-of-plane deformation of C–C bond at 233 cm-1; (2) the inplane deformation of C–C bond at 370 cm-1 and 404 cm-1; (3) the inter-ring bending mode of C– C bonds at 503 cm-1 and 523 cm-1; (4) the intra-ring bending mode of C–C–C bonds at 615 cm-1 and 638 cm-1; (5) the out-of-plane deformation of C–C bond at 720 cm-1.46 All these changes indicate the chemisorption of the TPT molecules and the formation of a well-ordered molecular monolayer on the Au surface.


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Figure 3 Comparison of representative SER spectra taken on a pristine BPT–SAM (green), on a BPT–CNM (blue: Raman-active site, red: Raman-inactive site), and the Raman spectrum of a BPT–CNM that was annealed at 520 °C (black).

Figure 3 compares several selective SER spectra obtained from a pristine BPT–SAM, a BPT– CNM and an annealed BPT–CNM. We found two types of spectra from a BPT–CNM: the first spectrum (in blue) exhibits similar bands as those of a pristine SAM (in green), whereas the second one (in red) shows almost no Raman-scattering signal except a very broad D peak at 1360 cm-1. The D peak is due to the breathing modes of the phenyl rings, which arises from the preserved sp2 hybridized carbon.47 Actually sp3 hybridized carbon centers should be generated at the expense of the loss of sp2 hybridized carbon due to the electron irradiation, as confirmed from the electron energy loss spectroscopy (EELS) investigation.23,

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However, the Raman

scattering cross section of the sp2 phase is about 50–230 times higher than that of sp3 phase.3, 48 The SER spectrum of an irradiated SAM is very likely dominated by the non-cross-linked molecules and any structural transformation may only appear as superimposed weak signals on that. Nevertheless, a slight increase in the relative intensities of two bands at 408 cm-1 and 287 cm-1 can be noticed in the SER spectrum recorded on Raman-active sites. In the Raman spectrum of biphenyl the band at 410 cm-1 was assigned to the deformation vibration which corresponds to two nearly degenerate fundamentals, i.e., one of B3g symmetry and the other of Au symmetry.49 Ishii et al. reported that Raman bands at 273 cm-1, 316 cm-1 and 410 cm-1, were found in biphenyl films evaporated at 5 K in which biphenyl molecules twisted around the phenyl-phenyl bond.50 These characteristic bands disappear at temperatures above 140 K for the polycrystalline states in which biphenyl molecules have a flattened conformation.50 Tashiro et al. reported that twisted biphenyl structures in liquid-crystalline acrylate polymers also exhibited three characteristic Raman bands, i.e., 290 cm-1, 320 cm-1 and 420 cm-1, and the bands’ intensities


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decreased down to zero as the biphenyl group changed the conformation from a twisted form to a planar form.51 Therefore, the intensity of the band at 408 cm-1 could indicate the degree of the twisting biphenyl molecules in the system.50, 51 The formation of a network might also require an interlinking of the lower phenyls along one direction and of the upper ones along a different direction.24 The twist angle of a biphenylthiol SAM consisting of the CN functional group was determined to be 41°.52 But the twist angle of a pristine biphenylthiol SAM has not yet measured so far. As any perturbation of the electron delocalization upon electron irradiation would lead to higher twist angles between two phenyl rings, a slight increase in the intensity of the band at 408 cm-1 may be attributed to an enhanced dihedral twist as a result of the loss of aromaticity. The non-cross-linked molecules are surrounded by a network of cross-linked molecules and such a confinement may also have an effect on their molecular conformations. Moreover, as long-range structural vibrations spread throughout the entire cross-linked structure and the Raman bonds in the lattice-vibrational region of an amorphous biphenyl film were only observed below 120 cm1 50

,

it is also possible that the cross-linking results in a strong downshift of these modes below

the detectable range. In addition, Figure 3 shows that a Raman spectrum (in black) of a BPT– CNM (irradiated at 60 mC·cm-2) annealed at 520°C exhibits a very similar D peak at 1355 cm-1 and an additional broad G peak at 1604 cm-1, which indicates an absolute absence of non-crosslinked molecules and also a presence of nanocrystalline graphitic domains.


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Figure 4 The percentage of Raman-inactive spots increases drastically with increasing irradiation dose, where a fully-cross-linked monolayer appears to be Raman-inactive.

To explore the relationship between the degree of cross-linking and the corresponding Raman activity, we performed a statistic analysis on BPT–SAMs with different electron irradiation doses, i.e. 5, 10, 20 and 60 mC·cm-2. For this purpose, 260 surface sites were tested for Raman activity for each sample, where Au NPs with a diameter of 80 nm were used. Figure 4 shows that the ratio of Raman inactive sites increases exponentially with increasing irradiation dose, which correlates well with the degree of crosslinking: (1) for a BPT–SAM without any crosslinking nearly all sites were found to be Raman active; (2) for a partially cross-linked SAM with an irradiation dose of 20 mC·cm-2, the percentage of Raman-inactive sites increases rapidly to ~94%; (3) for a fully cross-linked SAM with an irradiation dose of 60 mC·cm-2, the ratio of Raman-inactive sites remains constant at 97%. The same tendency was found in SER spectra of the NBPT–SAM and the TPT–SAM. The ratio of Raman inactive sites for both SAMs irradiated at 60 mC·cm-2 was ~99%, which might be correlated with a slightly higher degree of conformational freedom of NBPT and TPT molecules. It has been reported that both NBPT– CNMs and TPT–CNMs possess lower residual stress of 4–5 MPa because of a more efficient cross-linking among adjacent molecules.14, 20 These results support the hypothesis that a fully cross-linked CNM is Raman inactive and only non-cross-linked molecules that reside in the


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hotspots contribute to the SER spectra. In addition, we also demonstrated the successful formation of BPT–SAMs and BPT–CNM shells on Au NPs via electron irradiation (see Figure S1 in the SI). The precise molecular structure of a cross-linked SAM remains unknown to some extent. It was proposed by Cabrera-Sanfelix et al. based on density functional theory (DFT) calculations that the dehydrogenated BPT molecules form “graphene-like” nanoflakes as building blocks.22 However, classical molecular dynamics investigations of the BPT–CNM suggest that the molecular system does not reach the ground state to form graphene fences, but instead “freezes” into a metastable irregular configuration described by a local energy minimum.24 X-ray photoelectron spectroscopy (XPS) has been employed to investigate the electron irradiation induced changes in a BPT–SAM.21 Although the C1s peak position is known to depend on the aromaticity of the molecules, no peak shift but a slight drop in the intensity was observed, and only a reduction of C1s intensity of the irradiated SAM down to ~90% was observed there.21 Also a complete absence of orientational long-range order in the irradiated NBPT–SAM was suggested by Meyerbroeker et al. based on the change in the C 1s K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra.53 Since a transition from a metastable state to a more stable state can be readily achieved by thermal annealing, pristine amorphous CNMs can be crystallized with short-range structural order, where the existence of nm-scale graphene domains was revealed by transmission electron microscopy.54, 55 Figure S2 shows another Raman spectrum of a BPT–CNM annealed at 930 °C that exhibits the D peak from a disordered sp2 phase and the in-plane bond-stretching vibrations of sp2 C-C bonds (the G peak at 1605 cm-1).


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Figure 5 (a) Au NPs were carried by a BPT–CNM and placed onto a pristine NBPT–SAM on Au. HIM micrograph showing the Au(111)/NBPT–SAM/BPT–CNM/Au-NPs (from bottom to top) system. (b) SER spectrum (in red) obtained from the Au(111)/NBPT–SAM/BPT–CNM/Au-NPs, where it was overlaid with the spectrum (in grey) of the Au(111)/NBPT–SAM/Au-NPs. Inset: a schematic of 1-nm-thick CNM that behaves as a supporting layer for transferring Au NPs and as a well-defined spacer between the Au NPs and the Au film.

The finding that 1-nm-thick CNMs are Raman-inactive potentially makes them promising for applications where ultrathin and flexible organic films are needed in Raman spectroscopy, as the outstanding mechanical properties allow CNMs to be used as a freestanding support for microscopic objects such as Au dots56 and nanoparticles.57 Firstly, as a proof of concept, we deposited polystyrene (PS) beads with a diameter of 1.1 µm on freestanding CNM supported by a copper grid. Raman signals from three PS beads are clearly distinguishable from the fluorescence signals of the CNM (see Figure S3a in the SI). Secondly, we employed a BPT– CNM to carry 80 nm Au NPs and transferred the stack onto a NBPT–SAM on Au(111). On the one hand, Raman signals from individual BPT molecules should be absent due to the fact that non-cross-linked molecules cannot sustain a lift-off from their initial substrate during the transfer process. On the other hand, much lower intensities of Raman bands from the underlying NBPT– SAM were expected because the electric field enhancement factor decreases with increasing size of the interstices.58, 59 Figure 5a and 5b show a helium ion micrograph and the corresponding


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SER spectra of such an assembly. Even though the intensities of those characteristic bands become approximately one tenth of those of a pristine NBPT–SAM, the SER spectrum exhibits the same characteristic features of the underlying NBPT–SAM. These results prove the capability of CNMs to function as a flexible supporting material for Raman studies. CONCLUSION In summary, we have investigated the formation of three aromatic self-assembled monolayers on gold substrates and their conversion into carbon nanomembranes upon electron irradiation by using surface enhanced Raman spectroscopy. Although the SERS effect is very sensitive to the non-cross-linked molecules, the amorphization process can be identified. By applying statistical analysis over the whole monolayer, we found that Raman active sites decrease exponentially with the degree of cross-linking. The cross-linking induced disappearance of Raman bands arises from to a lack of long-range structural order in carbon nanomembranes. Proof-of-concept experiments were performed to use CNMs as a flexible Raman-inactive supporting material for Raman analysis.

ASSOCIATED CONTENT Supporting Information. Detailed assignment of Raman peaks; BPT–CNM on Au NPs; Raman spectrum of annealed CNMs; Raman spectrum of polystyrene beads on CNM and on BK7 glass. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (X.Z.); [email protected] (T.H.)


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Present Addresses †M.M.: Weidmueller Group, 32758 Detmold, Germany Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the German Ministry for Education and Research (BMBF) and Deutsche Forschungsgemeinschaft (SPP 1928). The authors thank Prof. Andreas Terfort for providing us with precursor molecules and Dr. Andreas Winter for the schematic pictures.

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Table of Contents/Abstract Graphic


Surface-enhanced Raman spectroscopy of carbon nanomembranes

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