Molecular Stacking on Graphene.

The sequential vertical polyfunctionalization of 2D addend-patterned graphene is still elusive. Here, we report a practical realization of this goal via a "molecular building blocks" approach, which is based on a combination of a lithography-assisted reductive functionalization approach and a post-functionalization step to sequentially and controllably link the molecular building blocks ethylpyridine, cis-dichlorobis(2,2'-bipyridyl)ruthenium, and triphenylphosphine (4-methylbenzenethiol, respectively) on selected lattice regions of a graphene matrix. The assembled 2D hetero-architectures are unambiguously characterized by various spectroscopic and microscopic measurements, revealing the stepwise stacking of the molecular building blocks on the graphene surface. Our method overcomes the current limitation of a one-layer-only binding to the graphene surface and opens the door for a vertical growth in the z-direction.

Introduction

Well‐defined graphene architectures exhibiting structural hierarchies towards the z‐direction providing an increased complexity and widespread functional properties of extended 2D materials are challenging synthetic targets. Such multifunctional patterned structures with covalently attached functional entities, yielding highly sophisticated nano‐molecular architectures are of utmost importance for potential applications in fields such as electronic devices, biosensors, and catalysis.[ , , , , , ] So far, a broad variety of 2D‐engineered graphene architectures have been fabricated based on a combination of conventional patterning techniques (e.g. e‐beam lithography (EBL) and laser writing) and mature wet‐chemical strategies.[ , , , , , , , , , , , ] In this context, we and others have refined and significantly propelled forward the 2D patterning of single layer graphene from the binding of just one specific type of chemical functionality to the more advanced tailor‐made attachment of multiple functionalities.[ , , , ] However, even in these more recently synthesized multiply patterned structures the respective addends are just defined in two dimensions and confined laterally. To our knowledge, no post‐functionalization strategies towards a vertical scaffolding of the specific graphene lattices have been developed up to now and previous attempts were mainly limited to non‐patterned/random polyfunctionalization.[ , ] This largely limits the potential complexity and functional versatility of the corresponding graphene patterns. The realization of this challenging task relies on the development of flexible and still elusive protocols that allow for a z‐direction stacking of functional building blocks in the patterned lattice regions of a graphene matrix in a sequential and controlled manner. This approach bears the striking analogy with the vertical on‐demand scaffolding of macro‐pieces in the macroscopic world. Therefore, once the nano‐molecular bricks are well designed and controlled, a properly designed “molecular building blocks” concept could, in principle, provide a feasible way toward this goal. Key to this approach, on the one hand, is to stack molecular bricks on the graphene plane in a spatially controlled fashion, and on the other hand, to provide a sequential connection in the vertical direction (z‐axis) by equipping the molecular building blocks with suitable anchoring groups. We have previously demonstrated that lithography‐assisted reductive 2D chemistry can be used as an elegant method for the 2D engineering of graphene.[ , ] The combination of this approach with suitable post‐functionalization reactions can be expected to allow for the facile realization of a nano‐molecular building blocks stacking on graphene.

Here, we report, for the first time, on the spatially resolved stepwise stacking of different molecular building blocks on a pre‐patterned graphene matrix. In a first step, a diazonium salt involving a terminal pyridine unit is used to pre‐pattern graphene by a lithography‐assisted reductive functionalization technique. Subsequently, as a second molecular building block cis‐dichlorobis(2,2′‐bipyridyl)ruthenium is grafted to the first addend layer via a ligand‐to‐metal coordination. Finally, as a third type of molecular bricks—in their function as an end‐cap—triphenylphosphine (4‐methylbenzenethiol, respectively) is added to finalize the molecular architecture. As a consequence, in the vertical dimension, the sequential fusion of three functionalities in a spatially defined manner is accomplished.

Results and Discussion

We started our stepwise stacking approach of nano‐molecular bricks with a tailor‐made 2D patterning of a graphene monolayer with ethylpyridine, based on a mask‐assisted lithography step coupled with a reductive‐based diazonium functionalization sequence (Figure 1). Such a reliable first step patterning is essential for the subsequent accomplishment of a spatially resolved molecular stacking as it localizes the “base brick” for the attachment of the following building components by providing distinct binding positions. For this purpose, we employed EBL to generate periodic patterns (FAU) in a PMMA mask covered single layer graphene flake to selectively expose the underlying extended π‐surface for the first addend binding step (for details see Supporting Information). In order to improve the degree of addition, prior to reaction, the exposed graphene regions were reductively activated by a treatment with a Na/K alloy, leading to their increased reactivity towards electrophiles.[ , , ] Once the Na/K alloy was removed, the efficient 2D stacking of the first nano‐molecular “base brick” was accomplished by an addition of pyridine‐3‐diazonium tetrafluoroborate, yielding the 2D‐functionalized sample denoted as G. After removal of the protective PMMA mask by rinsing with acetone, the second set of molecular bricks (construction in z‐direction), namely cis‐dichlorobis(2,2′‐bipyridyl)ruthenium was anchored to this 2D architecture via a ligand to metal coordination approach, leading to the formation of the assembly G. During this procedure, the cis‐dichlorobis(2,2′‐bipyridyl)ruthenium bricks were directly coordinated to the initially introduced ethylpyridine. As a consequence, in the vertical direction, the cis‐dichlorobis(2,2′‐bipyridyl)ruthenium moieties were positioned exclusively in the patterned regions carrying the ethylpyridine anchor units. In a similar way, the third molecular building block triphenylphosphine (4‐methylbenzenethiol, respectively) was grafted exclusively to the cis‐dichlorobis(2,2′‐bipyridyl)ruthenium connector, resulting in the formation of the target structures denoted as G and G, respectively.

Figure 1

A) Schematic illustration of the stepwise nano‐molecular stacking of functional entities on the graphene matrix. The grey FAU logo are the EBL‐exposed regions of the PMMA‐covered graphene. The other regions remain covered by a green PMMA protective layer after the lithographic step. The black, purple, red, and blue FAU pattern represent the exposed graphene regions that undergo a stepwise reaction to bind with different nano‐molecular bricks. B) A typical macro‐bricks stacking is comparable with that of our z‐direction nano‐molecular building blocks stacking technique. Please note that the building blocks involved might not be perfectly vertical to the graphene lattice as shown here.

This entire process of a vertical stacking of functional building blocks on a highly defined and precisely patterned graphene monolayer was monitored by Raman spectroscopy and the corresponding Raman spectra are illustrated in Figure 2. Clearly, after the initial lithography‐assisted 2D functionalization process, the Raman spectrum of the PMMA‐protected regions of G resembles to that of pristine graphene featuring a high‐intensity G band at 1582 cm−1 (assigned to the sp2 carbon lattice), a pronounced 2D band at 2680 cm−1, and a negligible defect‐induced D band at 1350 cm−1. This demonstrates that the PMMA‐covered graphene regions remain intact after the reductive functionalization step. In contrast, a high‐intense D band is observed in the patterned regions, indicative of a conversion of sp2 lattice carbon atoms into their corresponding sp3 configuration due to the covalent binding of the pyridine‐based addends. Owing to the binding of these slightly electron‐donating species, the Raman G band upshifts to 1588 cm−1, while the 2D band downshifts to 2675 cm−1, compared to the pristine graphene (1582 cm−1 for the G band and 2682 cm−1 for the 2D band).[ , , ] All these results clearly prove that only the e‐beam exposed regions are selectively grafted with the ethylpyridine bricks. To visualize the imprinted patterns, a large‐scale Raman mapping (50×40 μm2) was carried out. The corresponding pattern of “FAU” can be clearly distinguished in the Raman I D/I G plot (Figure 2B), which correlates well with the optical image of the e‐beam defined pattern (Figure 2C). On the other hand, a significant increase in the ID/IG ratio from 0.1 (for the pristine graphene) to 2.8 is observed, indicative of a highly efficient 2D functionalization (being located in the high‐functionalization regime of the Cançado curve ) within the patterned regions of sample G. This highly efficient binding of the molecular anchor units is beneficial for the coordinative attachment of the second layer consisting of cis‐dichlorobis(2,2′‐bipyridyl)ruthenium. The successful binding is clearly confirmed by the observation of a further upshift of the G band to 1592 cm−1 along with a downshift of the 2D band to 2670 cm−1 as cis‐dichlorobis(2,2′‐bipyridyl)ruthenium represents an electron‐donating binding entitiy.[ , , ] This was further corroborated by the detection of one of the characteristic Raman signatures of cis‐dichlorobis(2,2′‐bipyridyl)ruthenium in sample G at 1487 cm−1. The respective peak is upshifted relative to the pristine unit at 1478 cm−1, underlying the intramolecular interaction between the introduced molecular building block and the graphene matrix. No intensity alteration of the D band in G is observed as expected as the cis‐dichlorobis(2,2′‐bipyridyl)ruthenium binding bricks are directly coordinated to the first layer of molecular anchor units instead of to the graphene matrix, which does not introduce additional sp3 centers in the respective patterned graphene region. The phenomenon that the intensity of the D band remains unchanged is also found in the sample G and G, which is reasonable given the fact that the involved binding reaction only occurs between the second and third molecular layer (triphenylphosphine and 4‐methylbenzenethiol, respectively). Nevertheless, similar shifts of the position of the G band and the 2D band, detected in samples G and G, corroborate the successful vertical grafting of the third electron‐donating building unit.

Figure 2

A) Raman spectra of a) the pristine graphene, b) PMMA‐protected regions, c) G, d) G, e) G, f) G, and g) cis‐dichlorobi(2,2′‐bipyridyl)ruthenium. B) Raman I D/I G mapping image of G and C) the corresponding optical image. Scale bar: 3 μm. λ exc=532 nm.

A further solid proof for these hierarchical 2D hetero‐architectures is provided by KPFM and SEM‐EDS measurements. Owing to its high potential sensitivity and spatial resolution, KPFM has been routinely used to investigate the surface potential change of the graphene surface by measuring the local contact potential difference (CPD) between the graphene and the employed AFM tip.[ , ] In general, the CPD is correlated to the work function of the graphene, which in turn is related to the electron‐withdrawing and electron‐donating properties of surface‐bound species. As a consequence, KPFM can easily reflect the functionality‐induced variations in the graphene surface potential. Figure 3 displays the AFM and KPFM results obtained for G. The AFM topography shows negligible height variations between the patterned regions and unpatterned regions, which is most likely attributed to the introduction of the small sized ethylpyridine groups (less than 0.5 nm in height), consistent with previous studies.[ , , ] However, the pattern exemplified by the letter of “F” can be clearly visualized by KPFM due to the alteration of the surface potential of the patterned regions stemming from the chemical binding of the ethylpyridine addends. Owing to the electron‐donating nature of the ethylpyridine moiety and the resulting n‐doping effect, the patterned regions exhibit a higher surface potential of ca. 30 mV compared to the unpatterned areas (Figure 3C), in perfect accordance with previous findings.[ , , ] Moreover, in going from G only equipped with the ethylpyridine building block to G containing also cis‐dichlorobis(2,2′‐bipyridyl)ruthenium connecting units, an increased n‐doping effect can be expected. On the other hand, the increase of the length of the functional groups will lengthen the arm of the dipole moment, thereby accentuating the dipole effect. As such, a more pronounced surface potential change should, in principle, be observed. Indeed, compared to G, the patterned regions in G show a more significant surface potential variation, which is about 60 mV higher than that of the corresponding unpatterned regions (Figure 4), solidifying the successful attachment of cis‐dichlorobis(2,2′‐bipyridyl)ruthenium units in G. This pronounced surface potential difference yields a clear pattern image with sharp contrast, in which a very bright F‐pattern is displayed (Figure 4B). Furthermore, the introduction of the larger cis‐dichlorobis(2,2′‐bipyridyl)ruthenium building blocks yields an obvious height variation (ca. 2 nm) of the patterned regions within G as proved by the corresponding AFM topography and the respective cross‐sectional line profile (Figure 4A and C). Compared to G, the patterned regions of G/G carrying even longer species exhibit higher surface potential changes relative to unpatterned regions (64 mV and 70 mV for G and G, respectively, Figure 5 and 6). However, the magnitude of variation from G to G/G is much moderate than that from G to G. This mild potential change trend can be rationalized considering the leveling of dipole effect that occurs once the functional group length reaches a certain level. Nevertheless, these results distinctly demonstrate not only the success of the spatial stacking (in the z‐direction) of nano‐molecular building blocks attached to the graphene matrix, but also the effective modulation of the graphene surface characteristics.

Figure 3

A) AFM topography image. B) KPFM image, and C) cross‐sectional height profile (top) and surface potential (bottom) of G. Scale‐bar: 3 μm.

Figure 4

A) AFM topography image. B) KPFM image, C) cross‐sectional height profile (top) and surface potential (bottom) of G . Scale bar: 3 μm.

Figure 5

A) AFM topography image. B) KPFM image, and C) cross‐sectional height profile (top) and surface potential (bottom) of G. Scale‐bar: 3 μm.

Figure 6

A) AFM topography image. B) KPFM image, and C) cross‐sectional height profile (top) and surface potential (bottom) of G. Scale‐bar: 3 μm.

Given the merits of chemical sensitivity and high spatial‐resolution, SEM‐EDX was applied to probe the chemical nature and elemental distribution of the generated 2D hetero‐architectures (Figure 7). The SEM‐EDS measurement of the G shows discernable N (Figure 7B) and Cl signals (Figure S3), in which the element N exhibits a clear pattern‐dependent distribution, consolidating the successful 2D patterning of the cis‐dichlorobis(2,2′‐bipyridyl)ruthenium units. On the other hand, the distribution of nitrogen, localized only in the patterned regions, confirms the covalent nature of the addend binding in good agreement with the Raman results. Moreover, the respective EDS mapping of the elements S and P exhibits a similar pattern‐related distribution (Figure 7C and D), corroborating the fact that the third nan‐molecular layer of the “end cap” bricks 4‐methylbenzenethiol (triphenylphosphine, respectively) are chemically bound in relation to the patterned areas within the sample G and G, respectively. The covalent binding of different building blocks within G/G was further consolidated by XPS measurements of non‐patterned counterparts (for details see Supporting Information), where the detected binding energy of Ru 3d (281.3/281.5 eV) and N 1s (400(±0.8)/400.6 eV) agree well with previous findings (Figure S4 and S5). Additionally, the first‐ and second‐step reactions were easily quantified based on the XPS results. Unfortunately, the rather small photoionization cross section of P and S coupled with the charging effect prevent us from reliably identifying their spectra and thus unable to quantify the third‐step reaction. Altogether, the Raman spectroscopic results in combination with the AFM/KPFM and SEM‐EDS characterization explicitly prove that based on our proposed “molecular building blocks” concept, the spatially resolved covalent stacking of different molecular bricks along the z‐direction of the underlying graphene matrix has been successfully realized.

Figure 7

A) SEM and B) N elemental mapping of sample G. C) P elemental mapping of sample G, and D) S elemental mapping of sample G. Scale bar: 3 μm.

In addition, temperature‐dependent Raman spectroscopy was performed to explore the thermal stabilities of the individually bound components in G and G. The corresponding Raman spectra at different temperatures are depicted in Figure 8. Clearly, the Raman spectra of both G and G remain unchanged to temperatures up to 200 °C, suggesting a relatively high thermal stability of the samples in this temperature regime. A further increase of the temperature to 250 °C results in the decline of the intensity of the D band, indicative of the initiation of a defunctionalization reaction. This thermal‐induced decrease of the intensity of the D band is directly correlated with a reversible sp3–sp2 re‐hybridization of basal carbon atoms due to the cleavage of the covalently bound molecular addends. A further increase of the temperature boosts this defunctionalization reaction, resulting in a sharp drop in the intensity of the D band in the samples G and G. The D band has almost disappeared at around 400 °C, indicating a complete restauration of the intact sp2 basal carbon lattice of graphene. The full recovery of the intact graphene structure is further confirmed by the observation of the re‐establishment of the respective initial G and 2D bands, where the I D/I G ratio is suppressed to <0.2 and the I 2D/I G ratio is enhanced to value of about 1.9. Another favorable pillar for this conclusion is the return of the G and 2D bands to their initial positions of 1582 cm−1 and 2680 cm−1, respectively, as a result of the detachment of the electron‐donating species. This excellent reversibility of the addend binding provides an opportunity to precisely tune the degree of functionalization in the molecular assemblies of G and G by a simple adjustment of the temperature in a subsequent annealing step.

Figure 8

Temperature‐dependent Raman spectra of A) G and C) G. Mean Raman I D/I G ratios and I 2D/I G ratios of B) G and D) G extracted from the temperature‐dependent Raman spectra. λ exc=532 nm.

For instance, by annealing G and G at 250 °C, we can adjust their I D/I G ratios to ca. 1.5. As a result, the spectroscopically imaged pattern obtained by a large‐scale Raman I D/I G mapping is changed from the initial red color to a predominant green color, as shown in Figure 9A and 9C. A subsequent increase of the temperature to 400 °C leads to a complete defunctionalization of G and G reflected by their decreased I D/I G ratios to <0.2, as revealed by the corresponding large‐scale Raman I D/I G mappings, where the chemical patterns have almost completely vanished (Figure 7B and D). Considering the influence of the surface‐bound species on the graphene properties, this subtle thermal adjustment enables an accurate property manipulation of our graphene nano‐assemblies.

Figure 9

Raman I D/I G mapping of G after annealing at A) 250 °C and B) 400 °C, and of G after annealing at C) 250 °C and D) 400 °C. λ exc=532 nm. Scale bar: 3 μm.

Conclusion

In conclusion, we have accomplished the first prototype of spatially defined and highly assembled graphene nano‐systems that exhibit structural hierarchies across multiple height scales and controlled properties. These unprecedented 2D hetero‐architectures were constructed in such a “nano‐molecular building blocks” approach, implemented on the basis of a combination of an EBL‐assisted reductive functionalization and a post‐functionalization step to chemically link different molecular building units including diethylpyridine, cis‐dichlorobis(2,2′‐bipyridyl)ruthenium, and triphenylphosphine (4‐methylbenzenethiol, respectively) to a graphene matrix in a sequential and controlled manner, utilizing the z‐direction. The resulting hetero‐architectures were unambiguously characterized by a combination of statistical Raman spectroscopy, AFM, KPFM, and SEM‐EDS. Moreover, the temperature‐dependent Raman analysis clearly prove the reversible nature of the building block binding, which provides a facile route for the adjustment of the degree of addition and the manipulation of the respective surface properties of the nano‐structured architectures. Strikingly, beyond the examples shown here, other molecular bricks such as polymers, dendrons, porphyrins, peptides, proteins, and DNA helixes, and many others can, in principle, be chemically stacked on graphene in a controlled fashion, as exemplified. Our approach beaks the bottleneck that graphene chemical patterning can only be proceeded along its basal plane (lateral direction) and inspires the vertical chemical patterning of graphene. This lays the foundation for exploring the more promising structures such as qusi‐3D patterned architectures.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

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