On-Surface Synthesis of Chlorinated Narrow Graphene Nanoribbon Organometallic Hybrids

Graphene nanoribbons (GNRs) and their derivatives attract growing attention due to their excellent electronic and magnetic properties as well as the fine-tuning of such properties that can be obtained by heteroatom substitution and/or edge morphology modification. Here, we introduce graphene nanoribbon derivatives—organometallic hybrids with gold atoms incorporated between the carbon skeleton and side Cl atoms. We show that narrow chlorinated 5-AGNROHs (armchair graphene nanoribbon organometallic hybrids) can be fabricated by on-surface polymerization with omission of the cyclodehydrogenation reaction by a proper choice of tailored molecular precursors. Finally, we describe a route to exchange chlorine atoms connected through gold atoms to the carbon skeleton by hydrogen atom treatment. This is achieved directly on the surface, resulting in perfect unsubstituted hydrogen-terminated GNRs. This will be beneficial in the molecule on-surface processing when the preparation of final unsubstituted hydrocarbon structure is desired.

G raphene nanoribbons (GNRs) and nanographenes are regarded as promising candidates for the next generation of nano-and optoelectronic applications. 1−6 While this new class of materials retains outstanding transport properties characteristic of graphene (e.g., high mobility of charge carriers), the lateral confinement of armchair graphene nanoribbons (AGNRs) opens a band gap. This is crucial in overcoming the most important obstacle of graphene-based transistors to applications in digital electronics and highfrequency circuitry, 7 that is, the inability to completely turn them off at room temperature. On one hand, GNR electronic and magnetic properties strongly depend on the edge morphology; for example, the zigzag edge structure is predicted to result in spin-polarized edge states with potential applications in spintronic devices. 8,9 On the other hand, armchair GNRs are characterized by a band gap dependent on their width, 10−13 and the researcher's attention is also drawn to topological modifications by inclusion of nonhexagonal rings. 14 The electronic and magnetic properties of GNRs could also be influenced and tuned by intentional substitutional doping. 15−26 The preparation of these well-defined narrow GNRs has been made possible by advances in on-surface synthesis in ultrahigh-vacuum conditions. Most often the on-surface synthesis of GNRs proceeds in two sequential steps: (a) the halogen-equipped molecular precursors are activated thermally by carbon−halogen bond cleavage followed by Ullmann coupling to form polymers 27,28 and (b) at higher temperature the oligomers are transformed into GNR by surface-assisted cyclodehydrogenation. 29 However, a single-step synthesis avoiding the high-temperature on-surface cyclodehydrogenation 30−32 would be instrumental for the fabrication of nanostructured ribbons which cannot tolerate high-temperature treatment. Among different molecular nanostructures created through surface-assisted synthesis, organometallic architectures attract in recent years growing attention as a promising class of materials. The family of fused, electronically delocalized organometallic polymers has been widely studied in solution. They potentially allow rational fine-tuning of the electronic, electrochemical, optical, and magnetic properties by controlling both the chemical structures of the organic ligands and the metallic centers. For instance, Osuka and collaborators have developed long molecular ladders comprising up to 24 porphyrin subunits. 33,34 However, the stepwise synthesis of such systems in solution is complex and time-consuming, and thus it is tempting to explore on-surface polymerization as a rapid and efficient route to fused organometallic complexes. 35−51 Here we demonstrate the on-surface synthesis of specific GNR derivatives, that is, stable narrow chlorinated graphene nanoribbon organometallic hybrids (GNROHs). They are synthesized without the cyclodehydrogenation reaction by onsurface polymerization associated with organometallic coupling of specially designed tailored molecular precursors. In order to fabricate GNROHs in a single Ullmann coupling step, 1,6,7,12tetrachloro-3,4,9,10-tetrabromoperyleneequipped with four Br and four Cl side substituentswas synthesized. 52 After onsurface activation by C−Br bond cleavage, the precursor allowed for simultaneous C−C bond formation and aromatic ring closure associated with complex formation with Au incorporated between two pairs of C−Cl bonds. This leads to chlorinated 5-AGNROHs synthesized directly on the Au(111) surface (see Figure 1) with 4-fold-coordinated Au atoms, similarly as in gold−organic hybrids reported by Zhang et al. 44 and Liu et al. 53 Our approach creates also perspectives for the synthesis of GNRs and their derivatives directly on semiconducting and insulating substrates, where the controlled surface-assisted cyclodehydrogenation reaction does not occur. 54,55 Moreover, following our recent report, 30 we demonstrate that by exposing the fabricated GNROHs to atomic hydrogen, we induce reduction of the gold complex to yield atomically perfect unsubstituted 5-AGNRs ( Figure 1).
The precursor monomers synthesized following Zagryanski et al. 52 were sublimed on the Au(111) surface kept at room temperature, where they self-assemble into islands (see Figure  S1 of the Supporting Information). Further slow annealing up to 160−190°C induces C−Br bond cleavage that activates the molecular building blocks. Subsequently, the activated molecules undergo polymerization reaction, and each two neighboring precursors are fused, forming two new covalent bonds, while within the same step the C−Cl bonds transform into organometallic hybrids with Au atoms inserted between carbon skeleton and side Cl atoms (Figure 1). Such a transformation resembles the process observed for Br4-PTCDA precursors by Liu et al. 50 The chlorinated 5-AGNROHs are formed at an annealing temperature significantly lower compared to standard procedures involving the cyclodehydrogenation process. 29 Figure 2a,c shows a scanning tunneling microscopy (STM) overview image of the synthesized structures. There are two particular assemblies that could be discerned, namely, the islands comprised of linearly extended objects and dotted structures characterized by a quasi-hexagonal ordering. We attribute the first to the islandtype assemblies of chlorinated 5-AGNROHs. To further support the assignment, we analyze in detail the highresolution STM topography displayed in Figure 2b. The STM image contains the linear backbone complemented by large lobes located sideways. The linear structure could be assigned to the aromatic 5-AGNR core of the hybrid. The  The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter measured spacing between neighboring tailored building block units of 0.87 ± 0.03 nm matches perfectly the anticipated periodicity of the aromatic 5-AGNR structure as indicated in Figure 2b,d. The large side lobes could be attributed to the presence of Cl side substituents. The spacing of these lobes across the molecular structure reaching 1.25 ± 0.03 nm (see Figure 2b) is too large for a direct C−Cl connection. This suggests incorporation of Au adatoms between the carbon skeleton and side Cl atoms, which would lead to on-surface synthesis of the chlorinated complex. Altogether this suggests attachment of Cl atoms to the aromatic skeleton through 4fold-coordinated Au adatoms after annealing to 160−190°C.
In the STM image visualized in Figure 2b, each lobe corresponds to one of two neighboring Cl side atoms. The dotted structures surrounding the islands with chlorinated 5-AGNROHs are formed by Br, which gathers on the surface after C−Br bond cleavage and precursor activation, similarly as it was recently reported by Zuzak et al. 30 and Sun et al. 32 We emphasize at this point that the GNROHs are formed at temperature below 200°C, which is lower than the desorption temperature of bromine atoms. 31,56 This results in the clearly discernible appearance of Br on the surface as the reaction byproduct. Moreover, as coupling reactions yield four bromine atoms per building block, a large number of bromine atoms are released and form the quasi-hexagonal assemblies identical with the structures reported by Gottardi 57 and also recently by Sun et al. 32 Although the synthesized chlorinated 5-AGNROHs gather into islands, we note that unsubstituted 5-AGNRs do not exhibit at all any tendency to aggregate and stay well separated in the absence of bromine. 31,58 Contrarily, similar self-assembly into islands of 5-AGNRs surrounded by bromine has recently also been reported. 32 Furthermore, it is interesting to note that the assembly of the chlorinated 5-AGNROHs is dependent on their length. In Figure 2c, we mark the longer ribbons, which evidently assemble parallel to each other. The shorter molecular species containing no more than 3 molecular units do not exhibit such tendency of higher order self-assembly as can be seen in the surrounding areas. These findings suggest that the ordering of longer ribbon assemblies emerges from the presence of chlorinated long edges, whose role in the intermolecular coupling is enhanced when the length of the ribbon increases. To analyze the hypothesis, we prepared a sample with reduced density of precursors, which may enable the creation of longer GNR hybrids, for example, due to a decreased amount of residual bromine, which is expected to influence the precursor mobility. This is documented in Figure 3a,b showing the coexistence of chlorinated GNROHs with increased length and assembled into ordered islands and less ordered groups of shorter ones. It is interesting to note that the amount of surrounding Br structures is greatly reduced due to the diminished density of tailored building blocks. From the high-resolution images shown in Figure 3c,d, it is evident that the long GNROHs tend to gather parallel to each other.
The STM images clearly reveal that if the number of sublimed molecules is reduced, the Au(111) surface herringbone reconstruction is absent in the surface region covered by the Br structures (see Figure 2a,c) and appear only in the clean area in-between the self-assembled pattern, as shown in Figure 3a,b. It is evident that Br atoms present on the surface may also significantly influence the GNR mobility. Consequently, the removal of Br atoms from the surface could be beneficial. One way to remove Br, usually applied when GNRs are formed by cyclodehydrogenation, is thermal desorption. It is reported that the process takes place at temperatures above 250°C, 56 higher than the chlorine 59 or fluorine 60 equipped aromatic platforms can survive. Therefore, to analyze the role of surface Br atoms on the molecule behavior, we have applied an alternative procedure that has been proposed for postpolymerization Br removal. 61,62 This is done by exposure of the sample to molecular H 2 flux. We kept the sample heated to 180°C with the H 2 partial pressure at 5 × 10 −7 mbar. The heating temperature was selected to ensure efficient Br removal while maintaining the chlorinated GNROH intact. Low annealing temperatures did not result in surface cleaning from Br structures; for example, annealing at 150°C for 2 h had no impact on the observed Br structures. The experiments showed that almost complete Br removal from the surface requires at least 2.5 h treatment at 180°C as shown in the STM image in Figure 4. The image shows that    Figure 4 by the red dotted circles, red arrow, and red dashed rectangle. We can also expect that some Br atoms might still be present between the parallel oriented 5-AGNROHs. The surface herringbone pattern is restored after Br extraction in the area surrounding the GNROH islands, whereas it is clear that underneath the GNROHs it does not appear. Thus, our observation points out to either noticeable interaction between the GNROHs and the surface, most likely arising from the edge functionalization with Cl and formation of organometallic complex, or the still important role of the Br adatoms buried within the assemblies of the 5-AGNROHs. As described above, the application of molecular hydrogen is not capable of complete residual bromine removal. Alternatively, bromine atoms may be removed from the surface either by deposition of silicon atoms 32 or by atomic hydrogen dosing, 30,63 the application of which has recently been introduced in on-surface synthesis approach for intermolecular fusion. 64 It is likely that application of atomic hydrogen shall be much more efficient in removing residual halogen atoms; 30 however, it may also modify the generated gold complexes. Indeed, in the following we show that the application of atomic hydrogen to GNROHs leads to organometallic state quenching and passivation of the carbon skeleton providing perfect 5-AGNRs. In Figure 5a, a typical overview image of the sample exposed to the atomic hydrogen source is presented. It is apparent that the application of the atomic hydrogen treatment strongly influences the GNROHs. During the process the sample is kept at elevated temperature of 100°C with a partial pressure of 1 × 10 −7 mbar for 15 min. STM images show that previously generated GNROHs become separated, and their appearance closely resembles the unsubstituted 5-AGNRs observed by Kimouche et al., 58,66 as can be also observed in a high-resolution image of the GNR containing seven molecular precursors in Figure 5b. Remarkably, the herringbone surface pattern is no longer affected by the presence of the GNRs, strongly indicating that the Cl side atoms and the previously residual Br adatoms are successfully removed by the applied procedure. This supports the efficiency of halogen atom removal from noble metal surfaces by atomic hydrogen 30,64 also in cases where the halogen atom is a part of an organometallic gold complex.
However, we note here that the analysis of the molecular structure solely based on STM visualization might be misleading due the complicated influence of topography and electronic structure on the resulting image. To perform a doubtless identification of the product of the reaction, we apply the noncontact atomic force microscopy (nc-AFM) technique with a CO functionalized tip. It has been shown that such functionalization allows not only for the detailed imaging of the internal structure of organic species 65,66 but also for the precise analysis of the molecular edges. 67,68 The image shown in Figure 5c indicates the presence of the 5-AGNR aromatic core without Cl atoms but with hydrogen-terminated edges. This proves that the applied procedure leads to the pristine 5-AGNRs containing sp 2 -hybridized C atoms without superhydrogenated edges. 64 In contrast, an edge with radical character or saturated with two H atoms would lead to a darker or brighter appearance, respectively. 69,70 Therefore, we conclude that the exposure to atomic H source not only cleans the surface completely from Br adatoms but also leads to the full quenching of the organometallic complex leading to perfect unsubstituted 5-AGNRs.
The atomic hydrogen treatment gives also a chance for unambiguous identification of the chlorinated 5-AGNROHs. Whereas there are reports showing successful measurements with CO functionalized tips in the presence of Br on Ag(111), 71 we did not manage to functionalize the nc-AFM tip with a CO molecule on Au(111) containing Br contamination 30 due to noticeable interaction between Br adatoms with sublimed CO molecules. To overcome these difficulties, we can apply a shorter lasting exposure of the sample containing 5-AGNROHs to atomic hydrogen, which allows to effectively get rid of the remaining Br adatoms 30 and removes only a part of Cl side substituents within AGNROHs, transforming them into 5-AGNR/5-AGNROH heterojunctions. This is documented in Figure 6a,b, which allows for the doubtless identification of the 5AGNR/5AGNROH junctions and for the precise description of their atomic composition based on their nc-AFM appearance. The nc-AFM image in Figure 6a corroborates presence of 4-fold coordinated Au atoms located between the carbon-based GNR skeleton and side Cl substituents as schematically drawn for clarity in Figure  6b. An additional STM image of partially dechlorinated AGNROHs can be found in Figure S2.
To support the existence and stability of chlorinated 5-AGNROHs on Au(111), we have performed density functional theory (DFT)-based calculations of their structural arrangement (see the Supporting Information for calculation details). 72 The optimized structure of the chlorinated 5-AGNROH complex on Au(111) is shown in Figure 6c,d.
The calculations indicate a slightly bent geometry of the chlorinated AGNROH with both sides comprising Cl atoms facing toward the Au(111) surface. Between Cl and C atoms, 4-fold-coordinated Au atoms are incorporated forming a complex, in a similar way as reported by Liu et al. 57 The   Figure 6d. On the basis of the obtained geometrical architecture, we have performed nc-AFM image simulations following the approach introduced by Hapala et al. 73,74 The resulting simulated nc-AFM image presented in Figure 6e matches well with the experimentally captured organometallic units in Figure 6a,b, providing confirmation of the successful fabrication of the chlorinated 5-AGNROHs.
In conclusion, we have fabricated a chlorinated organometallic 5-AGNR complex on the Au(111) surface, which could be regarded as representatives of a special class of compounds: GNR organometallic derivatives. The nanostructures have been generated in a single-step procedure involving only the on-surface polymerization process accompanied by the formation of the organometallic state with omission of the cyclodehydrogenation. Consequently, the GNROHs are formed at significantly lower temperature than following a standard GNR fabrication procedure, which altogether enables the incorporation of Au atoms between two C−Cl bonds and the synthesis of organometallic complex. Our approach opens up new prospects for the fine-tuning of the electronic properties of GNRs and their derivatives as well as for the enhancement of their processability and the fabrication of nanocomposites. The on-surface synthesis is achieved by application of specially designed molecular precursors, each equipped with four Br and four Cl atoms. This allows for simultaneous C−C bond formation and aromatic ring closure. Such a procedure is expected to be highly beneficial when applied on other surfaces, which do not promote the metal catalyzed on-surface cyclodehydrogenation, and gives perspectives for direct fabrication of GNRs on nonmetallic substrates by homolytic cleavage of C−X bonds. 75 Finally, it is demonstrated that chlorinated GNROHs could be transformed into unsubstituted, H-terminated perfect GNRs by the onsurface side atom exchange using an atomic H source at elevated temperature. This is expected to be beneficial in the synthesis of a wide range of molecular moieties giving a powerful tool to end up with purely hydrocarbon structures or heterojunctions.
Experimental methods, computational details and additional STM data, calculated electronic structure of AGNROHs (PDF) Astronomy and Applied Computer Science, Jagiellonian