Carbon Nanomembranes

1 Molecular Mechanisms of Electron-Induced Cross-Linking

It has been shown that electron, ion and photon beams induce specific chemical reactions, i.e., bond dissociation, oxidation/reduction or polymerization, in SAMs that depend strongly on their building blocks – the molecules. The response of a SAM to low-energy electron irradiation depends on the chemical structure of the ordered molecules. In general the aliphatic monolayers are heavily destroyed and thus broadly utilized as positive tone resists [27], but the phenyl-based SAMs are stabilized through the formation of intermolecular cross-links and can be used as negative resists. In aliphatic SAMs a cleavage of C–H bonds through resonant and nonresonant processes is induced, which leads to an orientational and conformational disorder of the chains, desorption of material and formation of C=C double bonds in the fragments, remaining on the surface, whereas in aromatic SAMs, it was found that upon irradiation with electrons, C–H cleavage occurs, which is then followed by cross-linking between neighboring phenyl units. During this process, cross-linked molecules maintain their preferred orientation and almost no material desorbs. Hence the electrons generate a molecularly dense monolayer that could be used as a negative tone electron resist [23, 27, 28].

In addition, electron irradiation also changes the surface chemistry of the nanomembrane. Several groups have investigated the chemical transformation of nitro-, cyano-, trifluoromethyl- para-substituents upon electron irradiation [29, 30]. While the underlying aromatic rings are partially dehydrogenated, the hydrogen atoms liberated from the aromatic cores locally reduce the nitro and cyano groups to amino and aminomethyl groups, which can be further chemically modified by electrophilic agents [26a, 26b, 31]. By employing a local electron exposure, one can define surface regions that can be used as a template for a site-selective molecular immobilization. This process is named chemical lithography, as the lithographic exposure directly affects the surface chemistry of the SAM [29a, 32].

Resolving the molecular mechanisms of the e-beam-induced polymerization in aromatic SAMs is a complicated task, since the long-range order in the cross-linked layers is distorted and a broad spectrum of molecular species can be formed. Turchanin et al. [33] employed complementary spectroscopic techniques such as X-ray and UV photoelectron spectroscopy (XPS and UPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and thermal desorption spectroscopy (TDS) to investigate the mechanisms of cross-linking in biphenyl-4-thiol (BPT) SAMs on gold (Figure 5). The experimental data are completed by quantum chemical calculations that allow the derivation of a model for the e-beam-induced chemical and structural transformations in aromatic thiol SAMs and agree well with the response to electron irradiation of other oligophenylene-based SAMs [29b, 29c, 34].

Figure 5:

Schematic presentation of cross-linking of biphenylthiol-based monolayers: (a) low-energy electron irradiation and emission of secondary electrons, (b) dissociation of C–H bonds, (c) cross-linked molecules.

When a molecular layer is exposed to low-energy electrons, the electrons are effectively captured by the ordered molecular film, followed by a partial decomposition, desorption of hydrogen and molecular fragments, orientational and conformational disordering, and intermolecular cross-linking. For alkanethiol SAMs on gold [35], it was found that the electron energies, resonant with the electron orbitals of the aliphatic components, have a maximum at ~10 eV with a cross section of 10^–16 cm². Therefore, dissociative electron attachment (DEA) is most likely the dominating process. DEA [36] is observed when a free electron interacts with an atom or molecule and is temporally captured to form a negative ion resonance (also called transition negative ion). This electronically excited anionic state, in which the attached electron resides on one of the unoccupied molecular orbitals, decays in a release of hydrogen anion and formation of carbon radical centers within the film or a release of hydrogen atom and formation of molecular anionic species. These reactive centers further react with the adjacent phenyl rings to form intermolecular C–C covalent bonds as the delocalization of π-electrons over the δ-framework of the aromatic ring retains its integrity through the irradiation process. Desorption of hydrogen molecules is generated through scattering of the formed hydride/hydrogen atom within the dense molecular layer.

For BPT SAMs ~650 primary electrons per molecule with energy of 50 eV are necessary to create the cross-linked molecular network [33]. It has been shown that secondary electrons (SEs) generated in the substrate [37] also contribute to the cross-linking along with the primary e-beam (Figure 5) [38]. Taking into account the SE yield [39], the cross section for the cross-linking of BPT was estimated to be ~10^–18 cm². This number is an average value without specifying an exact number of the involved chemical bonds and their chemical nature.

Electron-induced processes in polycyclic aromatic hydrocarbons were studied in the gas phase [40]. Resonances, characteristic for DEA, were observed at energies of 7–8 eV with effective cross sections of 10^–17–10^–16 cm². As these cross sections are close in magnitude to the one found for biphenyl cross-linking, it is likely that DEA contributes to the C–H cleavage in BPT SAMs.

In Figure 6, XPS, UPS and NEXAFS data for a BPT SAM before and after irradiation with 50 eV electrons and a dose of 45 mC/cm² are presented. A comparison of these data reveals specific transformations upon cross-linking of the carbonaceous part of the monolayer and of the sulfur/gold interface. While the XP C1s signal (cf. Figure 6(a)) mostly preserves its initial intensity, demonstrating small reduction in the amount of carbon atoms after electron irradiation, the corresponding data of the C1s X-ray absorption show a reduction of the π* resonance intensity while at the same time the δ* resonances become more intense (cf. Figure 6(d)). The width of all resonances increases; however, their positions remain unchanged. These observations are attributed to a partial loss of aromaticity within the monolayer due to the interconnection of adjacent molecules via C–C bonds or even due to their partial decomposition. Measurements of angular dependencies of the resonance intensities of the pristine and e-beam-irradiated SAMs revealed an increase of the averaged molecular tilt within the monolayer upon cross-linking from ϕ = 31° to 41°.

Figure 6:

Spectroscopic characterization of the e-beam-induced cross-linking of a biphenyl-4-thiol SAM (e-beam energy of 50 eV, electron dose of 45 mC/cm²): (a) X-ray photoelectron spectroscopy (XPS), (monochromatic Al Ka, 1,486.7 eV), (b) temperature desorption spectroscopy (TDS), (c) ultraviolet photoelectron spectroscopy (UPS), (He I, 21.22 eV), (d) near-edge X-ray absorption fine structure spectroscopy (NEXAFS) (schematic representation of a pristine BPT SAM), (e) an e-beam cross-linked BPT SAM. Reprinted from Ref. [1]. Copyright (2012) with permission from Elsevier.

The UPS data show photoemission bands at 2.7 and 4.4 eV as well as a loss of fine structures around 7–10 eV upon cross-linking (Figure 6(c)). Based on quantum chemical calculations of the electronic structure, which were carried out for numerous possible cross-linked BPT dimers, the features in the UPS can be attributed to electronic excitations and thus allow the identification of individual molecular species such as BBDS, B1BPT, B4BPT, BBPDT and BBPDTn (Table 1). For the analysis of the photoemission lines at first the molecular energy levels were calculated and then the allowed dipole transitions were considered [41]. The comparison of calculated and measured UPS suggests a prevailing formation of BBPDT and BBPDTn species upon e-beam irradiation.

Table 1:

Molecular species whose formation upon cross-linking of a BPT SAM was considered and for which UPS and HOMO–LUMO gaps were calculated.

^[1]

This conclusion is consistent with the thermal desorption of biphenylene fragments (m/z = 151) in a partially cross-linked BPT SAM, which was observed by TDS (Figure 6(b)). The appearance of dimers is also consistent with a time of flight secondary ion mass spectrometry study of irradiated 4ʹ-methyl-1,1ʹ-biphenyl-4-thiol (MBPT) SAMs on gold by Cyganik et al. [34a].

Distinct changes upon e-beam-induced cross-linking take place at the sulfur/gold interface of a BPT SAM. Figure 6(a) shows that besides thiolate SAM species [42] with a S2p_3/2 binding energy (BE) of 162.0 eV, new sulfur species with a BE of 163.5 eV form in a cross-linked SAM. Although its BE coincides with that of thioethers (R–S–R) or organodisulfides (R–S–S–R) [43], the presence of other species cannot be unequivocally excluded. It has been demonstrated that metal surfaces are not that rigid and actually reveal a substantial rearrangement upon formation of thiol-based SAMs [44]. In addition to vacancy depressions or ad-islands, which were attributed to a stress release [45], formation of thiolate-dimers (RS–Au–SR) also takes place [46, 47] and the measured S2p doublet at 162.0 eV is attributed to these sulfur species. It was shown that some molecules within thiol-SAMs form pairs, stabilized by an additional gold atom between neighboring sulfur atoms as depicted schematically in Figure 7(a). The density functional theory (DFT) - based geometry optimization (carried out for the related bisbiphenyl-disulfanyl species (cf. Table 1)) [33] revealed a substantial outward tilting of the aromatic backbones (see middle of Figure 7(a)) due to steric repulsion of the lower phenyl rings, which is in a qualitative agreement with the observed by NEXAFS increase in the tilt of the aromatic backbone upon cross-linking from ϕ = 31° to 41° [33]. Since cross-linked BPT molecules with multiple cross-links are less separated, they may provide sufficient space for formation of thiolate-Au-thiolate pairs with a higher downward tilt. These pairs can be stabilized by additional links at the upper phenyl rings as shown in Figure 7(a), middle.

Figure 7:

Schematic summary of the structural properties of pristine and cross-linked BPT SAMs: (a) molecular species formed upon e-beam irradiation, (b) conversion of a pristine BPT SAM into cross-linked BPT SAM. Reprinted with permission from Ref. [1]. Copyright (2012) with permission from Elsevier.

To realize a cross-linking in two dimensions and the formation of CNMs, it is important to consider the molecular packing motifs adopted in the pristine BPT SAMs (Figure 7(b), left) [48, 49]. Isolated BPT molecules reveal a characteristic torsional angle between the upper and lower phenyl rings. Within the crystalline SAM both phenyl rings may not be coplanar but still have a substantial twisting. Theoretical studies indicate that this molecular degree of freedom is of key importance for the formation of ordered films of oligophenylene-based SAMs [50]. Moreover, in view of the herringbone arrangement, which is adopted in the crystalline phase of oligophenylenes [51], a lateral cross-linking occurring upon BBPDT (Table 1) formation comprises a molecular rearrangement, such as a rotation. Therefore, the formation of extended chains exhibiting the double Ph–Ph bonding motif (Figure 7(a), left) is sterically unfavorable and a 2D network can propagate via molecules that are linked either at the upper or lower ring as schematically shown in Figure 7(a) middle.

A further consequence of the cross-linking is a reduction of the spatial separation of neighboring BPT molecules. While the intermolecular distances within the pristine film are essentially given by the van der Waals dimensions of the molecules, the irradiation-induced additional carbon–carbon links also enable shorter distances. The density of the cross-linked films increases locally and may lead to a formation of “nano-voids,” containing isolated and noncross-linked molecules (denoted as dark molecules in the scheme in Figure 7(b), right).

Due to the multiple cross-links the CNM exhibits a thermal stability up to 1,000 K [24]. Pristine biphenylthiol SAMs desorb above 400 K (~130°C) (cf. Figure 6(b)), whereas e-beam-irradiated SAMs become more stable with an increasing irradiation dose, i.e., with an increasing degree of cross-linking. This process saturates at a dose of ~50 mC/cm² for 50 eV electrons. For such samples only a slight (~10 %) reduction of their carbon XP signal is observed even upon annealing at ~1,000 K. But the data show that the sulfur atoms initially present in the monolayer, and in the CNM, continuously desorb upon heating through cleavage of C–S bonds until they completely vanish at temperatures above 800 K (cf. Figure 6(b)). The absence of any sulfur signal in the cross-linked monolayer after annealing at elevated temperatures demonstrates that the remaining carbonaceous film is not anchored by thiolate but is solely stabilized by covalent bonds within the aromatic network, which is directly coupled to the gold surface via van der Waals interactions. This conclusion is well corroborated by the temperature-dependent NEXAFS measurements [33], revealing a downward tilting of the aromatic rings and a substantial broadening of the π* resonances at elevated temperatures. As demonstrated by quantum chemical calculations, the saturation of the cross-linking process at a dose of ~50 mC/cm² can be attributed to the electronic quenching of the anionic molecular states by means of tunneling or hopping (Figure 7).

Upon electron irradiation of an aromatic monolayer, a dense molecularly thick network is formed, which is robust enough to be further removed from its initial substrate (Au [20], Si₃N₄ [30], Cu [52]) and placed onto solid or perforated support (Figure 8) as the thickness of the resulting CNM is adopted by the thickness of its parenting SAMs. Motivated by these findings, we applied similar protocols to a variety of other polyaromatic molecules, aiming to examine the change in structural features and functionality of the membrane. Figure 9 shows schematic drawing of the molecular route, applied to different types of monolayers.

Figure 8:

(a) Interference contrast of transferred CNM onto Si substrate with 300 nm silicon oxide layer, (b) helium ion microscopy image of a freestanding CNM (in blue color), transferred onto copper TEM hexagonal mesh grid.

Figure 9:

Schematic for the formation of CNMs and graphene from various molecular precursors: (a–c) schematic illustration of the fabrication route for CNMs and graphene. SAMs are prepared on a substrate (i), then cross-linked by electron irradiation to form CNM of a monomolecular thickness (ii). The CNM can be released from the underlying substrate (iii) and transferred onto a new substrate as a freestanding or supported material. Annealing to 900°C transforms CNM into graphene. (a) Fabrication of atomically thin CNMs and graphene from precursors 1a–c in (d), (b) fabrication of thicker CNMs and few layer graphene sheets from precursors 2a–f in (d), (c) fabrication of CNMs and graphene sheets with nanopores from precursors 3a–c in (d), (d) chemical structures of the molecular precursors used for preparation of CNMs and graphene. Reprinted with permission from Ref. [6]. Copyright (2013) American Chemical Society.

2 Tuning of CNM’s Properties on a Molecular Level

Various types of thiol-based precursors were studied on Au (111) polycrystalline substrates: nonfused oligophenyl derivatives (Figure 9(d), structures 1a–c), which possess linear molecular backbones providing an improved structural ordering of the formed SAMs; condensed polycyclic precursors like naphthalene (NPTH), anthracene (ANTH) and pyrene (MP) derivatives (Figure 9(d), structures 2a–f), which are more rigid and should result in a higher stability and an increased carbon density of the monolayers; “bulky” molecules, like the noncondensed hexaphenylbenzene derivative with a propeller-like structure (structure 3a) and extended disk-type polycyclic aromatic hydrocarbons such as HBC derivatives (Figure 9(d), structures 3b–c) [53]. The former are equipped with long alkyl chains and a surface-active group, which is attached to the π-conjugated backbone through a flexible methylene linker. This molecular structure enables a control over the thickness and packing density of the SAMs by varying the conditions of preparation. In Figure 9(a) oligophenyls with a linear molecular backbone form well-ordered monolayers that can be cross-linked into homogeneous CNMs. After pyrolysis, the CNMs transform into graphene, whose thickness depends on the density of carbon atoms in the monolayers. In Figure 9(b), condensed polycyclic precursors also form monolayers that are cross-linked into CNMs. After pyrolysis, the CNMs transform into graphene, whose thickness is higher than that in structure 1a, Figure 9(d), even if the carbon density is the same as in Figure 9(a). Figure 9(c) shows bulky aromatic hydrocarbons that assemble in a less-ordered monolayer and polymerize into CNMs with pores. After annealing, these nanomembranes transform into thicker graphene sheets with pores. Hence, the produced graphene adopts features from the preceding CNM, which itself adopts features from the monolayer, i.e., from molecules and surface.

SAMs form due to formation of strong bonds between the sulfur and the gold atoms, which is accompanied by van der Waals interactions between the carbon atoms. To obtain SAMs with a desired molecular packing, one can adjust parameters like immersion time, temperature, concentration and polarity of the solvents. XPS data of aromatic SAMs, representing diverse types of precursors, are shown in Figure 10 (left). For each SAM, its chemical composition and thickness can be derived from the BE and intensity of the C1s, S2p and Au4f photoelectron signals. The sulfur signal consists of a doublet with a S2p_3/2 BE of 162.0 eV, which unambiguously demonstrates the formation of sulfur–gold bonds [54].

Figure 10:

XPS data of pristine SAMs and CNMs: (a–f) XP spectra of C1s and S2p signals of the pristine (in left) and electron-irradiated monolayers (in right). In the insets are presented the chemical structures of the monolayer precursors. Reprinted with permission from Ref. [6]. Copyright (2013) American Chemical Society.

Only for the HBC derivatives (structures 3b–c), the presence of a second sulfur species with the BE of the S2p_3/2 signal at 163.6 eV is observed. This signal originates from physisorbed molecules and/or of disulfides [54], which may be stabilized by π–π interactions between the large aromatic cores. Aromatic and aliphatic carbons contribute to the C1s signal at BEs of ~284.2 and ~285.0 eV, respectively [55]. Stoichiometry and thickness as obtained from XPS correspond to the composition of the precursor molecules and indicate the formation of SAMs with an “upright” molecular orientation. By varying the precursors, the thickness of the aromatic monolayers can be adjusted from ~6 Å for NPHT (Figure 9(d): structure 2a) to ~24 Å for HBC-CN (Figure 9(d): structure 3c), which directly correlates with their molecular lengths (Figure 10 and Table 2).

In addition, the temperature- and solvent-dependent intermolecular interactions of the HBC derivatives [56] allow one to tune the final SAM thickness by varying the preparation conditions (Table 2).

Scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) data of three molecular precursors, containing different numbers of carbon atoms per molecule – 2-anthracene (C14), 3-biphenylpropane (C15) and terphenyl (C18) thiols – are shown in Figure 11. The structure and surface density of the SAMs were investigated by STM and LEED. They showed that precursor molecules 1b, 1c, 2a, 2b, 2c and 2e (Figure 9(d)) form well-ordered SAMs with the densely packed (√3 × √3) [57] unit cell of the adsorption places and with the (2√3 × √3) superstructure of the molecular backbones. These structures correspond to a surface area of 26 Ų per molecule. Note that for these SAMs the surface density of carbon atoms in the monolayer can be precisely tuned by the carbon content of the respective molecular precursors. Short biphenylthiols (Figure 9(d), 1a) exhibit a (2 × 2) arrangement of the adsorption places, which corresponds to a less densely packed monolayer of 28.7 Ų per molecule [49a]. The formation of LEED patterns and well-ordered SAMs by STM for precursor molecules 2d and 2f and 3a–c (Figure 9(d)) was not verified. As XPS indicates a formation of sulfur–gold bonds for all precursors, we conclude that monolayers of the “bulky” and polycyclic molecules (structures 2d, 2f, 3a–c from Figure 9(d)) are less ordered and probably less densely packed than the monolayers of oligophenyls (1a–c) and the small fused-ring systems (2a–c, 2e).

Figure 11:

Structure of pristine SAMs. STM micrographs and LEED patterns (insets) of SAMs from molecular precursor: (a) ANTH, 2b; LEED pattern at 116 eV, (b) BP3, 1b; LEED pattern at 127 eV, (c) TPT, 1c; LEED pattern at 129 eV. For molecular structures see Figure 9(d). Reprinted with permission from Ref. [6]. Copyright (2013) American Chemical Society.

The irradiation of these SAMs with low-energy electrons (50 or 100 eV), using typical doses of ~60 mC/cm², corresponding to ~3,500 electrons per 1 nm², leads to a loss of order, as observed in LEED and STM. Figure 10 shows XPS data before and after electron irradiation in the left and right parts, respectively. As seen from the intensities of the C1s and Au4f (not shown), for purely aromatic SAMs the irradiation reduces the carbon content and the monolayer thickness by 5–10 %; in the SAMs that also contain aliphatic chains (structures 1b and 2f from Figure 9(d)) the carbon loss is up to ~16 %.

The obtained freestanding (self-supported) CNMs from the above-mentioned molecular precursors can be seen in Figure 12. It shows HIM micrographs of CNMs from three different types of aromatic molecular precursors (Figure 9(d): molecular structures 1–3). Simply the fact that one can take these images demonstrates that the SAMs of all these molecules have been cross-linked into mechanically stable CNMs. Figure 12(a–c) shows the freestanding CNMs from precursors 1c, 2c and 2d. These HIM micrographs were acquired at different magnifications, demonstrating the successful fabrication of CNMs of various sizes. The field of view in Figure 12(a) is 15 × 15 μm², which allows to observe some folds in the freestanding 1.2-nm-thick CNM. In the lower left corner of Figure 12(b), the boundary between the freestanding or supported CNM and substrate can clearly be seen. Figure 12(c) shows the field of view of 300 × 300 μm² with a large and homogeneous CNM of a thickness of 0.8 nm. Macroscopic defects in these nanomembranes are practically negligible on the length scale of these images.

Figure 12:

HIM micrographs of freestanding CNMs. After cross-linking the nanomembranes were transferred onto TEM grids. CNMs prepared from (a) TPT, structure 1c in Figure 9(d), suspended over gold TEM grid, (b) MP1, 2d, (c) 1MP, 2c, (d) BP3, 2b, (e) HBC-CN, 3c, (f), HBC-CN, 3c, (g) TPT, 1c, (h) HPB, 3a, in Figure 9(d) the inset shows the histogram of the pore size distribution over the displayed area, (i) freestanding nanocrystalline graphene after pyrolysis of NPHT (2a) CNM, (b–f) CNMs suspended over copper TEM grids, (g–i) CNMs and graphene are suspended over a grid with thin carbon film on Cu grid. Reprinted with permission from Ref. [6]. Copyright (2013) American Chemical Society.

Since the thickness of CNMs is determined by the precursor molecules and their packing density in SAMs, it can be controlled by tailoring these parameters. Figure 12(e) and (f) displays an example of the thickest CNM (2.2 nm) from precursor 3c, Figure 9(d) at two scales that has about four times higher thickness than the thinnest nanomembrane, prepared from precursor 2a, Figure 9(d) with a thickness of only 0.6 nm (Table 2). An image of an annealed NPHT CNM, transformed into nanocrystalline graphene sheet, can be seen in Figure 12(i). The opportunity to flexibly tune the thickness of CNMs opens broad avenues for the engineering of nanomembranes. A thorough investigation of the surface and structural features of different CNMs by HIM revealed the relation between properties of the precursor molecule, its SAMs and the appearance of the ensuing CNM. If the molecule forms a densely packed SAM (1a–c, 2a–c, 2e in Figure 9(d)), the following CNM is continuous and free of holes. Figure 12(g) shows a high magnification HIM image of a homogeneous CNM made from terphenylthiol. Conversely, CNMs made from HBC (3b–c in Figure 9(d)) or HPB (3a in Figure 9(d)) precursors, two molecules that possess larger sizes and form less well-ordered SAMs, exhibit pores (cf. the HIM images in Figures 12(h) and 13(a–f)). The dark spots in these images are pores that have a very small diameter and a narrow size distribution, as shown in the respective histograms (shown in the insets).

In case of the HBC precursor the mean size of the nanopores is ~6 nm with the surface density of 9.1 × 10¹⁴ pores/m²; the more compact HPB precursor shows a size of ~2.4 nm with a surface density of 1.3 × 10¹⁵ pores/m². The formation of nanopores in these CNMs is thus attributed to the large van der Waals radii of HBC and HPB structures and in the case of HBCs to the propensity of the disk-like molecules for intermolecular stacking, which competes with the molecule–substrate interactions and lowers the homogeneous coverage in the respective SAMs. We also observe that the average pore diameter of the HBC-based CNMs decreases from 6.4 to 2.5 nm when the SAM thickness increases from 1 to 2 nm (Figure 13).

Figure 13:

HIM images of freestanding CNMs: (a–c) prepared from HBC-Br (Figure 9(d), 3b) monolayers of different thickness, (d–f) prepared from HBC-CN (3c) monolayers of different thickness. The thickness of the membranes is written at the top right corner of each image. The histograms in the insets show the distribution of the pore diameter with a step of 0.5 nm. The membranes are placed onto Quantifoil TEM grids type Multi A. Reprinted with permission from Ref. [6]. Copyright (2013) American Chemical Society.

Further studies demonstrated that the mechanical stiffness and the resonance frequency of a monolayer membrane can also be finely tuned via varying the precursor molecules. To investigate the mechanics of such ultrathin 2D sheets, mechanical characterization methods must be adapted to the nanoscale by combining them with nanoanalytical tools. Bulge testing was used to characterize the mechanical properties of freestanding films. The technique involves clamping of a freestanding membrane over an orifice and application of an overpressure to one side. The deflection of this 1-nm-thin carbon nanosheet was monitored by atomic force microscopy (AFM). The Young’s modulus and prestress were calculated from the obtained pressure–deflection relationship. Figure 14(a) shows a schematic diagram of the AFM bulge test setup. Freestanding CNMs with a thickness between 0.6 and 1.7 nm from diverse polyaromatic precursors were investigated by Zhang et al. [58, 59]. A correlation between the rigidity of the precursor molecules and the macroscopic mechanical stiffness of CNM was found. The data show that CNMs from rigid and condensed precursors like NPTH and pyrene thiols prove to exhibit higher Young’s moduli of 15−19 GPa, while CNMs from nonfused oligophenyls possess lower Young’s moduli of ~10 GPa. Figure 15 shows the elastic performance of CNM among other material classes. Materials with similar stiffness are porous ceramics and composites, whereas the class of polymers exhibits lower mechanical stability than the CNM.

Figure 14:

(a) Schematic diagram of the AFM bulge test setup where a gas pressure is applied with compressed nitrogen, and the corresponding membrane deflection is recorded with an AFM. (b) Plot of Young’s moduli of CNMs prepared from different precursor molecules. (c) Residual stress of CNMs, where CNMs from bulky molecules are separately presented. Reprinted with permission from Ref. [59]. Copyright (2014) from American Chemical Society.

Figure 15:

Young’s modulus plot of different materials [60].

The interpretation of Zhang et al.’s data relates the Young’s modulus to the properties of single molecules and to the structure of the pristine SAMs. The membranes with the highest stiffness are formed from molecules that exhibit higher molecular rigidity (less conformational freedom), whereas the terphenyl-based CNM is more mechanically stable than biphenylthiol-based CNM due to the denser packing of molecules within the SAM and to the larger number of cross-linking sites in the molecular skeleton. The degree of cross-linking of the SAM proved to be also an important parameter that has a strong impact on the membrane prestress or tensile stress. Although van der Waals interactions between the sheet and the substrate are the main factors that determine the constant tension of a self-supported monolayer, different prestresses for CNMs prepared from different precursor molecules, in spite of their similar thickness, were estimated. This implies that electron irradiation plays a key role, as a higher number of conformational degrees of freedom of the precursor molecules enable more ways to cross-link the molecules and thus achieve a lower prestress in the CNM, as in the case of TPT-CNMs. The ultimate tensile strength of CNM was also determined by means of bulge tests, measuring the ultimate pressure at which the membrane ruptures [58]. The tensile strength of biphenyl-based CNMs ranges from 400 to 700 MPa. In this range materials like graphite, iron and silicon nitride break. Of course, the tensile strength of CNM is orders of magnitude lower than that of graphene and carbon nanotubes, but compared to other nanomembranes, such as polymer membranes and the organic–inorganic IPNs, exhibiting a tensile strength of 105 MPa, the ultimate tensile strength of CNMs is five to six times higher.

Zhang et al. [61] also studied the mechanical motion of the CNM and showed that they can be applied as mechanical resonators. Their vibrational mode shapes were characterized and visualized by optical interferometry (Figure 16). The freestanding CNM was transferred onto Si substrates with square or rectangular orifices. A vibration of the membrane was actuated by applying a sinusoidal voltage to a piezoelectric disk on which the sample was glued. The dynamic behavior of the CNM’s vibrational modes is described by linear response theory of a membrane with negligible bending rigidity. A phase velocity of 61.6 ± 10 m/s and 55.06 ± 9 m/s was obtained for 4ʹ-nitro-1,1ʹ-biphenyl-4-thiol (NBPT)-based CNM and TPT–CNM, respectively. By comparing the dispersion relation to an analytical model, the static stress of the membranes of ~5 MPa was determined and found to be caused by the fabrication process. The extremely low mass and bending stiffness ensures that CNM resonators are in the membrane regime where the pretension of the CNM dominates over its elastic stiffness.

Figure 16:

Several representative vibration modes of a 4ʹ-nitro-1,1ʹ-biphenylthiol CNM with dimensions of 50 × 50 μm² at (a) 700 kHz, (b) 1,660 kHz, (c) 1,810 kHz. Reprinted with permission from Ref. [61]. Copyright (2015) from AIP Publishing LLC.

Stimulated by the development of polymer and asymmetric hollow fiber membranes, membrane separation processes gained rapid progress not only in medical applications (artificial kidneys and lungs) and in the field of environmental protection (wastewater treatment, desalination, purification of flue gases), but even in energy renewable technologies. Since the end of the twentieth century, carbon materials, such as carbon molecular sieves, fullerenes, single and multiwall nanotubes, were extensively studied for their implementation in membrane technology. Two-dimensional nanomembranes with a thickness below ~5 nm and pores tuned to act as molecular sieves are predicted to be ideal separation membranes with many advantages over bulk membranes [62]. Especially, such extremely thin and dense carbon-based nanomembranes are considered to be effective and superior separation media for chemical and gas purification [63], offering the advantage of fast flow and lack of aging. Ai et al. [64] investigated the gas permeation performance of CNMs, built up from biphenylthiol (BPT) and p-nitrobiphenylthiol (NBPT) SAMs. Their permeance was measured onto a polydimethylsiloxane-supported membrane (PDMS-TFC, 150 nm PDMS/PAN/nonwoven) [65, 66] in order to enhance the mechanical stability and minimize roughness-induced strain. Considering the geometry and composition of the measured system – CNM/PDMS/PAN/nonwoven – it is intuitive to consider that the gas transport properties of PDMS determine and contribute significantly to the measured permeance values of CNM-PDMS. Therefore, it is necessary to consider the change of the permeance of bare PDMS due to the deposition of CNMs. A number of gases, such as hydrogen, helium, carbon dioxide, oxygen, nitrogen, argon, methane and ethane, have been tested and their corresponding permeance and ideal gas selectivity were obtained. The data are presented in relation to the penetrant kinetic diameters of the gases in Figure 17. They clearly show that the gas permeation of PDMS membranes reduces by the deposition of CNMs. This reduction is more pronounced for gas species with larger kinetic diameters. A simple model to extract the permeance of CNMs from the measured values of CNM-PDMS membranes revealed a different gas permeation behavior for single-layer and three-layer CNMs. In both cases, He and H₂ showed substantially higher permeance values in comparison to gases with larger kinetic diameters with the exception of CO₂, which displayed high permeances for single-layer CNMs and low values for three-layer CNMs. The established relationship of permeance and molecular size determines the molecular sieving mechanism of permeation through this ultrathin sheet. It can be understood by assuming that molecular-sized channels in the CNM dominate the permeation properties. These molecular-sized channels may form during the cross-linking process due to its statistical nature, which is expected to result in a random variation of intermolecular distances. Thus, even in a defect-free CNM, a high density of small free volumes (or pores) may form at locations of relatively large intermolecular distances, which act as the proposed molecular-sized channels.

Figure 17:

(a) Permeance of the CNM part of the composite membranes that accounts for the finite permeance of the PDMS support as well as a partial coverage of the PDMS surface by CNM. round symbol – single layer of NBPT-CNM, square symbol – single layer of BPT-CNM, triangle, pointing up – three-layer BPT-CNM, triangle, pointing down – three-layer NBPT-CNM. (b) Schematic depiction of the proposed gas transport mechanism in single-layer CNMs. Molecular-sized channels (highlighted by bright regions) favor the permeation of CO₂ and smaller gas molecules. (c) Schematic depiction of the proposed gas transport mechanism in multilayer CNMs. A low diffusion of CO₂ in between the individual CNMs hinders its permeation through multilayer CNMs. Reprinted with permission from Ref. [64]. Copyright (2014) from Wiley-VCH.

The transport through a three-layer CNM is consistent with lateral diffusion that is high for He and H₂ but hindered for the larger gases including CO₂. Lateral diffusion of gas molecules may be described by Knudsen-like diffusion or by condensation and surface flow of gas molecules in between the CNMs. Both possible mechanisms result in a permeance characteristic, which prefers the transport of smaller gases. The three-layer CNMs exhibit very small and constant values for all gases except the two gas species with the smallest kinetic diameters, which show more than one order of magnitude higher permeances. These higher permeance values cannot be explained by considering defects in the membrane as they would increase the permeance for all gas species that implies that this behavior is an intrinsic property of CNMs, resembling the permeation mechanism through glassy polymers with stiff polymer backbones, which also act like molecular sieves [67].

In addition, these experiments demonstrate that the gas permeation of PDMS membranes can be modified by the deposition of CNMs in a controlled way. The flexibility in the assembly of CNMs, by employing different precursor molecules as well as functionalizing CNMs, opens a path to molecular designed membranes also with respect to desired gas permeation characteristics.

This chapter describes a universal scheme to fabricate tailored CNM. CNMs of different size, thickness, elasticity and perforation have been generated. Their properties can be tailored by the choice of the precursor molecule, the structure of the SAM, the cross-linking and the transfer process. As the procedures that fabricate the CNMs are to a large extent compatible with nanofabrication protocols in industry, CNMs can be easily incorporated into electronic devices [68], into TEM grids as ultrathin carbon supports for high-contrast imaging [69], in separation technologies [64] or as protective coatings on chemically and mechanically sensitive technological components. This gives the material scientist a tool to produce membranes for technological applications. Here, a focus on material separation seems logical, as the manifold of diverse and specific separation problems is best addressed by a modular construction system of tailored CNMs. Beyond that the “similarity” of the CNM with biomembranes opens new opportunities for research. We can think of designing hybrid structures where biological membrane proteins are inserted into CNMs, creating an artificial 2D system with functional biological units. This could provide reference structures for the incorporation of single molecules into biotechnological sensors or devices. The future will show whether this will become a reality.