REVIEW Carbon Nanostructures www.advmat.de On-Surface Synthesis of Carbon Nanostructures Qiang Sun, Renyuan Zhang, Jun Qiu, Rui Liu, and Wei Xu* Furthermore, their properties have been Novel carbon nanomaterials have aroused significant interest owing to proved to be promising in numerous sci- their prospects in various technological applications. The recently developed entific and industrial applications.[4–8] on-surface synthesis strategy provides a route toward atomically precise Formation of nanostructures through fabrication of nanostructures, which paves the way to functional molecular on-surface chemical reactions is particu- larly attractive because of its following nanostructures in a controlled fashion. A plethora of low-dimensional nano- characteristics: (i) it allows the formation structures, challenging to traditional solution chemistry, have been recently of covalently interlinked nanostructures, fabricated. Within the last few decades, an increasing interest and flourishing which possess high thermal and chemical studies on the fabrication of novel low-dimensional carbon nanostructures stability with respect to noncovalent self- using on-surface synthesis strategies have been witnessed. In particular, assembled nanostructures; (ii) the reduced degree of freedom of molecular precursors carbon materials, including fullerene, carbon nanotubes, and graphene on surfaces (i.e., confinement effect) as well nanoribbons, are synthesized with atomic precision by such bottom-up as the interactions between molecular pre- methods. Herein, starting from the basic concepts and progress made in the cursors and surfaces may favor some spe- field of on-surface synthesis, the recent developments of atomically precise cific molecular adsorption configurations, fabrication of low-dimensional carbon nanostructures are reviewed. resulting in unexpected chemical reactions and formation of nanostructures, which could hardly obtained by conventional solu- tion chemistry; (iii) it prohibits the influence 1. Introduction of solvents and other contaminants under the well-controlled ultrahigh vacuum (UHV) conditions; (iv) it allows the synthesis From the life and organic chemistry, we know how unique of macromolecules or nanostructures with high molecular weight the binding properties of carbon element are. Those proper- that could not be synthesized in solutions due to the solubility ties are rooted in its ability to form linear (sp1), planar (sp2), or issues; and (v) it is feasible to be in situ characterized by surface tetragonal (sp3) bond configurations. Its unique properties have sensitive techniques, e.g., scanning tunneling microscopy (STM), spanned an incredibly rich structural phase diagram of pure noncontact atomic force microscopy (nc-AFM), and X-ray photo- carbon nanomaterials due to the sp2 bond configuration. Over electron spectroscopy (XPS), which allows us to get both the the course of the last 30 years, a continuous series of discov- structural and chemical information with high resolution. eries and scientific breakthroughs (e.g., fullerenes, multi- and Here, we will summarize the recent work regarding on-surface single-walled carbon nanotubes (CNTs), isolation of graphene, synthesis of atomically precise low-dimensional carbon nanostruc- synthesis of porous graphene, and graphene nanoribbons tures, including graphene-derived porous graphene, graphene (GNRs)) have made many of those materials envisioned in this nanoribbons, and single chirality carbon nanotubes, as well as the carbon structure diagram physically available.[1–3] emerging carbon nanostructures with acetylenic scaffolding and Recently, on-surface synthesis has emerged as a prom- organometallic carbon chains (i.e., metalated carbyne). ising approach to construct novel nanostructures, which are still of significant challenge to traditional solution-based chemistry. Special interest is paid to low-dimensional carbon 2. On-Surface Synthesis of 0D Carbon nanostructures, owing to the successive discoveries of carbon Nanostructures allotropes, including fullerene, carbon nanotubes, and graphene. One of the most famous 0D carbon nanomaterials is fullerene, which represents a family of cage molecules with a variety of sizes and shapes. The C molecule has gained most atten- Dr. Q. Sun, Prof. R. Zhang, Prof. J. Qiu, Prof. R. Liu, Prof. W. Xu 60 Key Laboratory for Advanced Civil Engineering tion because of its superconducting property, along with high [9] Materials (Ministry of Education) mechanical and thermal resistance. Despite numerous College of Materials Science and Engineering research efforts devoted in this field, the controllable synthesis Tongji University of the fullerenes with reliable yield and purity still remains as a Shanghai 201804, P. R. China great challenge. E-mail: [email protected] On-surface synthesis involving dehydrogenation and cycliza- The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201705630. tion of planar molecular precursors has proved to be a promising choice. Otero et al. have designed two nearly planar polycyclic [10] DOI: 10.1002/adma.201705630 aromatic hydrocarbons, C60H30 and its analogue C57H33N3. Adv. Mater. 2018, 30, 1705630 1705630 (1 of 12) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de As shown in Figure 1a, they found that after the deposition and thermal heating of the molecular precursor on Pt(111) surface, Wei Xu received his Ph.D. the molecular precursors dehydrogenated, performed a surface degree in surface physical catalyzed cyclodehydrogenation process and eventually formed chemistry from Aarhus the corresponding fullerene and triazafullerene molecules on University, Denmark, in the Pt(111) surface. Surprisingly, the efficiency of this cyclode- 2008. Following this, he was hydrogenation process is almost 100% along with no indication a postdoctoral fellow at the of desorption of the molecules on the surface suggesting that all Interdisciplinary Nanoscience of molecular precursors are transformed into fullerenes. In the Center (iNANO), Aarhus lower panel of Figure 1a, it shows the STM image after deposition University, Denmark, and of C57H33N3 on a platinum (111) surface. The size and topology at the Departments of of the STM image, with three wings characterizing the molecular Chemistry and Physics, shape, confirm that the molecules do not fragment but retain their Penn State University, USA. planar structure upon adsorption. Upon annealing the sample Currently, he is a Professor at Tongji University, P. R. at 750 K, a surface reaction takes place and the triangular mol- China, where he has been since 2009. His main research ecules transform into round molecules with an apparent height of interests include molecular self-assembly and reaction on about 0.38 nm and a diameter of 1.5 nm, in analogy to the STM surfaces, and novel carbon nanostructures like graphyne/ appearance of a commercial C60 adsorbed on a platinum surface. graphdiyne and carbyne. Moreover, further evidence for the thermally induced cyclization of C57H33N3 is provided by XPS spectra. Here, the strong covalent interaction between the precursor molecules and the platinum the choice of the surface is also critical in determining the on- surface are considered to play a key role in this reaction. To fur- surface reaction. The different interactions between the mole- ther test the catalytic effect of the Pt(111) surface, they have also cules and substrates could rule the competitive reaction path- performed similar experiments on a relatively inert Au(111) sur- ways, that is, cyclodehydrogenation versus dehydrogenative face, which contrarily did not exhibit the same reaction. polymerization. Pinardi et al. reported that the aromatic precur- This surface-catalyzed cyclodehydrogenation method could sors tend to carry out intramolecular cyclodehydrogenation on also be applied to the synthesis of other cage molecules. The Pt(111), while intermolecular dehydrogenative polymerization synthesis of fullerenes with higher molecular weight than dominates on Au(111), Cu(110), or Cu(111) surface.[13] the most familiar C60 and C70 remains a challenge because of Nanographenes or graphene quantum dots (GQDs) have the low yield of the evaporation technique and the complicated attracted widespread attention due to their potentials in the purification issues. Amsharov et al. used the surface-catalyzed field of molecular electronics. However, the atomically pre- cyclodehydrogenation of planar precursors on Pt(111) to achieve cise fabrication of nanographenes has still remained as a [11] the synthesis of C84. They demonstrated that this reaction is challenge. Surface-assisted cyclodehydrogenation provides a quite efficient and controllable. Moreover, they have designed synthetic route to the fabrication of tailored nanographenes a modified 60C H30 isomer, which finally formed an open-cage from polyphenylene precursors. As shown in Figure 1b, Treier structure, highlighting the importance of the precursor design. et al. designed a polyphenylene precursor, a kind of nanogra- In addition, Abdurakhmanova et al. found that two molecular phene, that could be thermally induced by intramolecular cyclo- [14] precursors C54H24 and C60H30 could yield (9, 0) zigzag and dehydrogenation following several intermediate steps. They (6, 6) armchair carbon nanotube end caps using the surface- have also observed two intermediates stabilized on the sur- assisted cyclodehydrogenation