On-surface Synthesis of Graphene Nanoribbons

Citation: Yan-Ning TANG, Ke-Wei SUN, Xue-Chao LI, Hai-Ming ZHANG, Li-Feng CHI. On-surface Synthesis of Graphene Nanoribbons[J]. Chinese Journal of Structural Chemistry, 2020, 39(8): 1377-1384. doi: 10.14102/j.cnki.0254–5861.2011–2944 shu

On-surface Synthesis of Graphene Nanoribbons

通讯作者: 张海明, hmzhang@suda.edu.cn
迟力峰, chilf@suda.edu.cn

English

On-surface Synthesis of Graphene Nanoribbons

Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China

Corresponding author: Hai-Ming ZHANG, hmzhang@suda.edu.cn Li-Feng CHI, chilf@suda.edu.cn

Received Date: 22 June 2020
Accepted Date: 23 July 2020
Available Online: 01 August 2020

Abstract: On-surface synthesis never fails to fascinate chemists by producing new functional polymers which can hardly been prepared via traditional solution chemistry. Among those newly prepared polymers, graphene nanoribbons (GNRs), featured with tunable band gap, have attracted substantial attention because they are considered as promising candidates for next generation carbon-based semiconductors. Here, we summarize the recent advances of GNRs prepared on single crystal surfaces with emphasis on the structural tuning and electronic properties of GNRs. Moreover, critical developments toward the application of GNRs have also been reviewed including the mass fabrication and the performance of GNRs as field effect transistors.

Key words:

1. INTRODUCTION

Graphene nanoribbons (GNRs) have been intensively explored over decades, because they can exhibit tunable bandgaps closely related to geometric structures due to the quantum confinement. Although many types of GNRs are classified according to their edge geometry, e.g. armchair-edged GNRs (AGNRs)^[1-5], zigzag-edged GNRs (ZGNRs)^{[6, 7]} and cove-edged GNRs^[8], AGNRs and ZGNRs are two fundamental types of GNRs in terms of their unique properties in bandgap and spintronics^{[1, 7]}. While the top-down fabrication fails to obtain atomically uniform edge structure, produced mixtures with all kinds of edge geometry lead to the poor control of the electronic properties of GNRs^[9-11]. It is until the emergence of on-surface synthesis that building GNRs with atomic precision become possible. The width and edge structure of GNRs have been successfully tuned by simply changing the chemical structures of precursors^{[3, 4, 12]}. Not only pristine GNRs have been prepared with various width and edges, heteroatom doped^[13-20] and non-hexagonal ring embedded GNRs^[21-28] have also been realized through on-surface synthesis. Both doped atoms and embedded non-hexagonal ring can be varied by reasonable design of precursor molecules.

Prior to the applications in electronics, it's important to experimentally realize the electronic properties of GNRs. The band gap of GNRs can be not only opened by quantum confinement, but also tuned through width, especially for the well-studied AGNRs. Three subfamilies of 3p, 3p+1 and 3p+2 AGNRs (p represents a positive integer) can be defined according to the carbon number across the width of AGNRs. In consideration of image screen effect, the measured band-gap of (3p+1) AGNRs on Au(111) by scanning tunneling spectroscopy (STS) is in line with that of theoretical predictions^[9], but the consistence between experimental and theoretical results is missing in the (3p+2) subfamily of AGNRs^[4]. Possible electronic interactions between AGNRs and supporting mental substrate might impede the investigation of intrinsic properties of the GNRs. Fortunately, various intercalation layers have been attempted to inset between metal substrate and the GNRs to suppress the electronic interactions and charge transfer, such as silicon^[29], NaCl^[30], WSe₂^[31], and cyanuric acid and melamine (CA·M) supramolecular network^[32]. Therefore, a relatively more intrinsic band structure is able to be characterized. In this perspective, we will briefly summarize recent progress in the preparation, physical properties and potential applications of GNRs.

2. RECENT ADVANCES

Contrast to traditional organic synthesis in solution, a new concept on surface synthesis was proposed by Grill et al in 2007^[33], aiming at building covalently connected molecular structures on single crystal surfaces. This bottom-up construction facilitates numerous on-surface reactions. Among those early investigated reactions, Ullmann type reactions are one of the highly effective representatives utilized in C–C covalent connections. Molecular radicals formed upon the dissociation of bromo substituents can be coupled on the surface, producing new entities. Simply changing the position of bromo substituents, Grill et al prove the efficiency of on-surface synthesis in producing both one dimensional linear chains and two dimensional networks of covalently connected molecular nanostructures^[33]. Thanks to the success of on-surface synthesis, various types of GNRs have been prepared on single crystal surfaces, including armchair-edged GNRs (AGNRs), zigzag-edged GNRs (ZGNRs) and cove-edged GNRs. The pioneer work was reported by Fasel and Muellen in 2010. The synthesis of 7-AGNRs on Au (111) includes the initial aryl-aryl connections by using Ullmann-type coupling reactions and further intramolecular dehydrogenation to planarize the twistedly connected precursors, gaining GNRs with length longer than 30 nm (Fig. 1, a)^[1]. After that, a series of systematical investigations has been conducted concerning the geometry control of AGNRs to fill up all subfamilies, namely 3p, 3p+1 and 3p+2 (p represents a positive integer) according to the carbon number across the width (Fig. 1)^[12].

Figure 1

Figure 1. Representative set of reactions towards AGNRs on Au (111). (a) The synthesis of 3-, 5- and 7 AGNRs via Ullmann type coupling of single precursor molecules^{[1, 4, 35]}. (b) The preparation of 8- and 10-AGNRs through lateral fusing of AGNRs belong to various subfamilies^[3]

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Due to the limitation of the precursor, our group managed to synthesize more broaden GNRs by fusing two different width GNRs, obtaining 8- and 10-AGNRs with promising length (Fig. 1b)^[3]. Albeit AGNRs are well-studied, ZGNRs is lack of investigation due to the absence of suitable reaction pathways and precursors. Fasel and Muellen together with coworkers managed to synthesize n = 10 ZGNR with exquisitely designed precursors^{[6, 7]}. They also contrive to fabricate cove-edged GNR through similar Ullmann-type reactions^[8]. Along this way, other GNRs with fascinating structure were realized on-surface with well-designed precursors, containing chiral GNR^[34] and chevron-type GNRs^[15].

Furthermore, elements with high electronegativity such as N^[18], B^[13], and S^[36] functional groups like cyano and tert-butyl substituents etc, are able to be decorated on the precursors, forming heteroatom-doped GNRs. Among those heteroatom-doped GNRs, N-doped GNRs are mostly studied because of the photo- and electrocatalytic activity of nitrogen doped carbon materials. Gao's group firstly report on-surface synthesis and electronic properties of N-doped GNRs. The band gap of the N-doped GNRs is suggested to be 1.02 eV, which is narrower compared with the pristine GNR^[18]. Narita's group had thoroughly investigated the structural difference between various heteroatom-doped nanographene^[20]. Unlike planar graphene, heteroatom-doped graphene nanoribbons are slightly out of plane at certain doped position, which can be a challenge for high resolution characterization, e.g. qPlus noncontact atomic force microscopy (nc-AFM). As for those with planar geometry, bond length measurement with qPlus nc-AFM could provide more information about bond order. Moreover, nc-AFM plays an unreplaceable role in characterizing molecular backbones, especially in non-hexagonal graphene^{[21, 27, 28]}. Generally speaking, non-hexagonal membered rings can be embedded through two ways, 1) decorated precursors in advanced^{[25, 26]} and 2) through reactions^[24].

The electronic structure of GNRs should be comprehensively understood before GNRs can be used in electronics as carbon-based semiconductors. Scanning tunneling spectroscopy (STS), as a complementary characterization of STM, is capable to reveal the local electronic structure of the target objects, allowing us to realize the structure-property relationship of GNRs. Other characterization methods have also been used, e.g. Angle resolved photoemission (ARPES) and inverse photoemission (IPE)^[37-39]. Tunable properties of GNRs like band-gap^{[12, 40]}, spintronics^[41], and magnetism^[42] have been realized through the structural adjustment of fine edge. The band gap of GNRs was theoretically realized to strictly depend on the edge structure. As for the well-studied AGNRs, the band gaps of three AGNRs subfamilies are ranked as Eg(3p+1) > Eg(3p) > Eg(3p+2). In each sub-family, the band gap of narrower GNRs should be larger than that of other GNRs. Among band gaps of AGNRs determined in experiment^{[2-4, 43]}, most of values are in line with theoretical predictions^[9] considering the reduced value by image screen effect^[44], but the band gap of 5-AGNRs on Au(111) varies from 0.1 eV^[5] to 2.8 eV^[4] determined by different groups. The metal substrate is believed to remarkably affect the experimental results by electronic interactions with supported GNRs. To reveal the decoupled properties of GNRs, a spacer layer such as ultrathin films of graphene^[45], h-BN^[46], NaCl^{[30, 47]}, SiC^[29] and cyanuric supramolecular networks^[32] can be intercalated between the metal substrate and GNRs. It can provide thorough understanding of the intrinsic properties of GNRs.

Figure 2

Figure 2. Theoretical and experimental band gap of AGNRs. (a) Variation of band gaps with the width of AGNRs. The three families of AGNRs are represented by different symbols. The open symbols are LDA band gaps while the solid symbols are the corresponding quasiparticle band gaps^[9]. (b) STS spectra of 6-AGNR (red), 9-AGNR (green), 12-AGNR (pink), and 15-AGNR (blue) recorded on Au(111)^[44]. (c) Determination of band gaps of 3p+1 AGNRs. (d) The bandgap determined by STS of 3p+2 AGNRs on Au (111)

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While the precise control of on-surface synthesis requires an expensive and complex ultra-high vacuum system, a mass fabrication of GNRs should be developed in a mild and low-price preparation condition to meet the requirement for future applications. Chemical vapor deposition (CVD) is an alternative bottom-up approach and has been widely used in fabricating graphene and other 2D materials. To obtain GNRs with economical and efficient synthetic process, CVD method has been introduced to prepare large-scale, well-ordered GNRs by Sakaguchi and coworkers^[49]. The reported method, radical-polymerized chemical vapor deposition (RP-CVD) has been succeeded in fabricating large-scale GNRs on Au (111) surfaces in extremely low-vacuum conditions (1 Torr Argon atmosphere). This method originates from a two-zone furnace (Fig. 3): one is a temperature-controlled quartz tubular wall for controlling precursor spread path (zone 1) and the other one serves as a restricted area for reaction occurrence (zone 2). Similar researches have also been reported, such as the synthesis of chevron-type and acene-type GNRs under ambient-pressure by using CVD method^[50-52].

Figure 3

Figure 3. Experimental setup of RP-CVD with an illustration of the presumed GNR growth mechanism by using 10, 10'-dibromo-9, 9'-bianthryl as a precursor molecule^[49]. (b) Mechanism of homochiral polymerization in a chain. The rate of polymerization for conformers with the same chirality was presumed to be significantly greater than that for conformers with different 2D chirality^[51]. (c) STM image (20 × 20 nm²) of homochiral polymers grown on Au(111) at 250 ℃ by 2Z-CVD. (d) Schematic representation of the growth of acene-type GNRs

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The success in the preparation of GNRs with efficient amount allows further investigations on their performance in electronic devices. Bokor et al. successfully transferred 7-AGNR from Au film to a 50 nm thick SiO₂, the target substrate for further building of field effect transistors (FET)^[53]. Other than the breakthrough of GNR transferring, the ambipolar behavior in this FET was also observed as long as 60 nm. Muellen's group reported a high current on/off ratios up to 6000 in FET devices containing transferred GNR through CVD growth (Fig. 4, a)^[54]. A FET device from these GNRs built by Sakaguchi et al. exhibit an excellent on/off current ratio of 1.6±0.6×10³, and an average hole mobility of 3.6±1.4×10^-4 cm²·V^-1·s^-1[55]. Those existing achievements indicate GNRs can exhibit good semiconducting properties in FET devices, which shed the light on GNR applications in other functional devices.

Figure 4

Figure 4. (a) Photograph of a 25 × 75 mm² GNRs/Au/mica plate. Inset figure is 5 K UHV STM images depicting the chevron-type structure^[54]. (b) The GNR/HSQ/TR tape and SAMs/Au(111)/sapphire after the delamination (step v in (a))^[56]. (c) Comparative cycling performance of N-GNRs and nitric acid-treated commercial CNTs (A-CMCNTs) at a current density of 0.1 A·g^-1[57]. (d) RDE curves of ORR for GNR and N-GNR nanomaterials in the O₂-saturated 0.1 mol/L KOH solution at a scan rate of 10 mV·s^–1 with an electrode rotation rate of 1600 rpm^[58]

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Furthermore, N-doped GNRs have been applied to other areas, such as lithium ion battery^[57], electrocatalysis^[58], etc. For instance, Liu et al. demonstrate that due to the abundant nitrogen doping, N-doped GNRs, prepared by longitudinal cutting of N-doped carbon nanotubes, exhibit table capacity retention of 714 mA·hg^-1 after 100 cycles at a current density of 0.1 A·g^-1 (Fig. 4, c)^[57]. Liu's group reports an enhanced ORR catalytic activity of N-doped GNRs, obtained through unzipping a N-doped nanotube, which shows a 100 mV more positive cathodic peak compared to pristine GNRs (Fig. 4, d)^[58].

3. SUMMARY AND OUTLOOK

This mini-review has demonstrated more than ten years development of bottom-up fabrications in covalent frame-work construction, and the constant achievement of on-surface synthesis towards the preparation and property characterization of GNRs with atomically precise edge structure. It's clear that on-surface synthesis is growing into a mature method in structural control of GNRs. Both the edge structure and the width of GNR can be controlled precisely, obtaining GNRs with uniform edge structures much longer than 30 nm. Furthermore, on-surface synthesis also works effectively in the preparation of heteroatom-doped GNRs, e.g. N-, B- and O-doped GNRs. In combination with additional characterization tool of STS, the electronic structures of GNRs with various edge geometry, width and length have been realized on metal substrate. However, some non-negligible issues still remain in pursuing the intrinsic properties of GNRs. Inevitable charge transfer and electronic interactions between GNRs and supporting metal substrate may remarkably interfere the measured results from STS, especially in the characterization of AGNRs belonging to (3p + 2) subfamily. The method for electronic decoupling between GNRs and supporting metal substrate might be a promising approach for more comprehensive understanding of the electronic structure of GNRs, e.g. intercalating space layers of NaCl, Si and h-BN between the GNRs and substrate.

The applications of GNRs have also been discussed in several aspects, especially in the application of FET devices. Because the GNRs synthesized on single crystal surfaces have to be removed from the surface to assemble a device, several key issues remain to be resolved. 1) A simple but effective method should be developed in order to remove the GNRs from the metal substrate. Current method by dissolving Au (111) in royal water is too expensive and might result in the damage of GNRs. 2) The length of GNRs has to be long enough to increase the probability for successful assembly. The direct synthesis of GNRs on insulating surfaces as an alternative method should be more challenging, but recently reported results about the synthesis of GNRs on semiconductor metal oxide show promising prospects in this direction. Other than electronic devices, heteroatom doped GNRs are ideal model catalysts in carbon-based electrocatalytic system. As demonstrated before, through on-surface reactions, structural control of heteroatom doped GNRs is achievable at atomic precision. Both doping position and amount are also able to be tuned and determined by high-resolution structural characterization. Combined with electrochemistry characterization, structureactivity relationship of heteroatom doped GNRs can be therefore established.

Finally, on-surface synthesis offers an unusual way to prepare GNRs on single crystal surfaces and paves a solid way for its applications in 2D functional devices. Real applications of GNRs, as well as other functional carbon-based materials, e.g. graphynes nanoribbons are expected in future, constantly contributing the new chapter of semiconductor industry.
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Figure 1 Representative set of reactions towards AGNRs on Au (111). (a) The synthesis of 3-, 5- and 7 AGNRs via Ullmann type coupling of single precursor molecules^{[1, 4, 35]}. (b) The preparation of 8- and 10-AGNRs through lateral fusing of AGNRs belong to various subfamilies^[3]

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Figure 2 Theoretical and experimental band gap of AGNRs. (a) Variation of band gaps with the width of AGNRs. The three families of AGNRs are represented by different symbols. The open symbols are LDA band gaps while the solid symbols are the corresponding quasiparticle band gaps^[9]. (b) STS spectra of 6-AGNR (red), 9-AGNR (green), 12-AGNR (pink), and 15-AGNR (blue) recorded on Au(111)^[44]. (c) Determination of band gaps of 3p+1 AGNRs. (d) The bandgap determined by STS of 3p+2 AGNRs on Au (111)

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Figure 3 Experimental setup of RP-CVD with an illustration of the presumed GNR growth mechanism by using 10, 10'-dibromo-9, 9'-bianthryl as a precursor molecule^[49]. (b) Mechanism of homochiral polymerization in a chain. The rate of polymerization for conformers with the same chirality was presumed to be significantly greater than that for conformers with different 2D chirality^[51]. (c) STM image (20 × 20 nm²) of homochiral polymers grown on Au(111) at 250 ℃ by 2Z-CVD. (d) Schematic representation of the growth of acene-type GNRs

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Figure 4 (a) Photograph of a 25 × 75 mm² GNRs/Au/mica plate. Inset figure is 5 K UHV STM images depicting the chevron-type structure^[54]. (b) The GNR/HSQ/TR tape and SAMs/Au(111)/sapphire after the delamination (step v in (a))^[56]. (c) Comparative cycling performance of N-GNRs and nitric acid-treated commercial CNTs (A-CMCNTs) at a current density of 0.1 A·g^-1[57]. (d) RDE curves of ORR for GNR and N-GNR nanomaterials in the O₂-saturated 0.1 mol/L KOH solution at a scan rate of 10 mV·s^–1 with an electrode rotation rate of 1600 rpm^[58]

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