Scanning probe microscopy in probing low-dimensional carbon-based nanostructures and nanomaterials

Carbon, as an indispensable chemical element on Earth, has diverse covalent bonding ability, which enables construction of extensive pivotal carbon-based structures in multiple scientific fields. The extraordinary physicochemical properties presented by pioneering synthetic carbon allotropes, typically including fullerenes, carbon nanotubes, and graphene, have stimulated broad interest in fabrication of carbon-based nanostructures and nanomaterials. Accurate regulation of topology, size, and shape, as well as controllably embedding target sp n -hybridized carbons in molecular skeletons, is significant for tailoring their structures and consequent properties and requires atomic precision in their preparation. Scanning probe microscopy (SPM), combined with on-surface synthesis strategy, has demonstrated its capabilities in fabrication of various carbon-based nanostructures and nanomaterials with atomic precision, which has long been elusive for conventional solution-phase synthesis due to realistic obstacles in solubility, isolation, purification, etc. More intriguingly, atom manipulation via an SPM tip allows unique access to local production of highly reactive carbon-based nanostructures. In addition, SPM provides topographic information of carbon-based nanostructures as well as their characteristic electronic structures with unprecedented submolecular resolution in real space. In this review, we overview recent exciting progress in the delicate application of SPM in probing low-dimensional carbon-based nanostructures and nanomaterials, which will open an avenue for the exploration and development of elusive and undiscovered carbon-based nanomaterials.


Introduction
Carbon is an indispensable and ubiquitous chemical element on Earth. All life is dependent on carbon element and inevitably involved in the chemistry of carbon. Carbon atoms generally form bonds with different types of hybridization (i.e. sp 3 , sp 2 , and sp hybridization), leading to various bond angles and geometries and accordingly, structural diversity. The diverse covalent bonding ability of carbon element gives rise to extensive pivotal carbon-based structures in multiple scientific fields, ranging from chemistry and materials to biology and pharmacy. Among others, organic chemistry is entirely based on carbon element, which constructs carbon-based compounds via bonding to a wide variety of chemical elements. For instance, hydrocarbon is a class of chemical compounds made up of carbon and hydrogen exclusively, including alkanes, alkenes, alkynes, and aromatic hydrocarbons as representative ones. Moreover, the diverse bonding ability of carbon also enables itself to bond together constructing carbon allotropes, such as diamond (comprising sp 3 -hybridized carbon atoms, shortened as C(sp 3 )) and graphite (with C(sp 2 )) in nature [1]. The emergence of zero-dimensional (0D) fullerenes (containing C(sp 2 )) [2] further brings the new era of carbon allotropes and has stimulated intense interest and indepth exploration of synthetic carbon allotropes thereafter [1], typically including one-dimensional (1D) carbon nanotubes [3] and two-dimensional (2D) graphene [4]. These C(sp 2 )containing synthetic carbon allotropes, featured with delocalization of π-electrons, have been demonstrated to share extraordinary physicochemical properties (such as outstanding electrical transport and redox activity [1,5]) while having their respective unique properties, which are directly determined by their chemical structures (topology, size, shape, etc). In addition, depending on the different types of sp n (n = 1, 2, 3) hybridization involved in the synthetic carbon allotropes, the corresponding structures and properties are distinct. Therefore, controllably embedding the target C(sp n ) in the molecular skeletons, along with accurate regulation of topology, size, and shape, is significant for tailoring the structures as well as outcoming physicochemical properties, inspiring a broad range of synthetic carbon allotropes and other carbon-based materials. A well-known example is the 2D graphyne family (including graphyne, graphdiyne, and graphyne-n) [6][7][8] with acetylenic (sp) linkages embedded in graphene (sp 2 ) structures, which is promising for nanoelectronics, photovoltaics, and catalysis. Another interesting example is the 1D carbyne [9], which features ideal finite sp-hybridized carbon chain and is theoretically predicted to be a mechanically strong and electronically prominent material [10,11].
Despite lots of remarkable achievements in the discovery, prediction, and preparation of carbon-based materials, many of them are still in their very early infancy. It has long been of great challenge to synthesize or prepare uniform monodisperse carbon-based structures and materials [1] by traditional solution-phase synthesis. Difficulties lie in many key processes, such as dissolution in solvents, stabilization as dispersions, and purification [5], which are also closely related to the high reaction activity and poor reaction selectivity of unsaturated carbon skeletons (C(sp) skeletons, in particular). Consequently, it turns out to be elusive to precisely fabricate such carbon-based structures and materials, which is a realistic obstacle for their detailed structural characterization, property measurement, and further full exploitation (e.g. application in devices). On-surface synthesis strategy, as an emerging synthetic strategy generally combined with scanning probe microscopy (SPM) techniques, takes full advantage of the templating and catalytic effects provided by the substrates involved [12][13][14][15] and has proved its capabilities in synthesizing numerous carbon-based nanostructures and nanomaterials with atomic precision. Monodispersed carbon-based nanostructures, such as isolated 0D motifs, 1D chains, and 2D single layers, are accessible based on this versatile protocol, which overcomes the limitations of conventional synthetic methods, and more significantly, enables novel or unexpected reaction pathways and products [15][16][17][18][19]. Standing on the shoulder of this strategy, SPM techniques [20], which are mainly based on scanning tunneling microscopy (STM) and atomic force microscopy (AFM), provide easy access to topographic information of carbonbased nanostructures in real space as well as their characteristic electronic structures on surfaces (cf figure 1). More intriguingly, atom manipulation [21] by applying controlled voltage pulses from an SPM tip to the nanostructures beneath is able to trigger chemical reactions locally, producing unique carbon-based nanostructures [22,23] with extreme reactivity which are generally considered to be inaccessible (figure 1). These powerful aspects have been pushing the development of many elusive and undiscovered low-dimensional carbon allotropes and relevant hydrocarbons with embedding of various sp n -hybridized carbons.
In this review, we summarize recent exciting progress in the delicate application of SPM in probing low-dimensional carbon-based nanostructures and nanomaterials mainly from the aspects of on-surface synthesis, topographic characterization, and electronic structure detection. Some typical carbonbased nanostructures and nanomaterials are discussed, which cover different dimensions ranging from 0D to 2D (see figure 1 for more details), and the main focus is the 0D and 1D ones as elementary components or segments for the extended 2D ones. In the 0D carbon-based nanostructures and nanomaterials, carbon allotropes involving sp 2 -hybridized fullerenes and pure sp-hybridized cyclo[n]carbons are introduced as a starting point with limited lateral sizes. As an extension, a series of C(sp 2 )-constructed nanographenes fabricated by virtue of on-surface synthesis strategy are displayed, including small rectangular graphene segments, cycloarenes, segments embedded with non-benzenoid n-membered rings, and more complicated graphene-related oligomers and macrocycles (such as triangulene-based structures). Regarding 1D structures, we describe the transformations and reactions involved in the synthesis of polyethylene (PE) (C(sp 3 )), polyacetylene (PA) (C(sp 2 )), and (organometallic) polyyne (C(sp)) and their skeleton visualization and electronic property measurement. In addition, studies on graphene nanoribbons (GNRs) (C(sp 2 )) and other π-conjugated polymer chains (with dominantly C(sp 2 ) and integration of C(sp)) are summarized. By extending the lateral width from 1D structures, several typical 2D carbon-based nanostructures and nanomaterials that are highly related to graphene, have been achieved and are also briefly reviewed herein. Moreover, recent advances in tip-functionalized STM (also known as bond-resolved STM (BRSTM)) and non-contact AFM (nc-AFM) offer possibilities of identification and assignment of unknown structures with submolecular or even atomic resolution, which also enable bond-order analysis of carbon-based nanostructures. Additionally, scanning tunneling spectroscopy (STS) provides complementary spectroscopic information, including but not limited to determination of band gaps and spin configurations and visualization of molecular frontier orbitals, which describe semi-conducting and metallic features of carbon-based nanostructures and nanomaterials as well as carbon-based magnetism and spintronics. These important applications of SPM techniques are also discussed in this review, which are expected to provide a brief overview of the connection between SPM techniques and carbon-based materials.

0D carbon-based nanostructures and nanomaterials
The discovery of C 60 (buckminsterfullerene, with all carbon atoms in sp 2 hybridization) in 1985 [2] opened the era of carbon allotropes and has stimulated intense interest and indepth exploration of synthetic carbon allotropes [1]. Thereafter, the successful fabrication of 1D carbon nanotubes [3] and 2D graphene [4] further enriched the family of sp 2hybridized synthetic carbon allotropes, and brought about new opportunities and challenges for the development of many elusive and undiscovered allotropes as well as related hydrocarbons with embedding of various sp n -hybridized carbons. The combination of on-surface synthesis strategy and SPM techniques has been demonstrated to be versatile in breaking through numerous limitations of conventional solutionphase synthesis (e.g. poor stability, difficulties in purification) and facilitating unique on-surface reactions. In this part, we focus on some 0D carbon-based nanostructures and nanomaterials which have been precisely synthesized and finely characterized by virtue of on-surface synthesis strategy and SPM techniques.

Fullerenes, cyclocarbons, and nanographenes in small size
In 2008, Otero et al reported the synthesis of fullerenes (i.e. C 60 and triazafullerene C 57 N 3 ) from aromatic precursors on Pt (111) by a surface-catalyzed cyclodehydrogenation process (figure 2(a)) [24], which provided an amazing example revealing the great potentials of surface-facilitated reactions. Since then, fullerene-related studies have attracted great interest, such as the synthesis of higher fullerenes (C 84 ) and buckybowls [25], the reaction selectivity between triazafullerene C 57 N 3 and 2D polyaromatic architectures [26], the sequential synthesis of nanohelicene, nanographene and nanodome [27], the fabrication and mechanistic exploration of graphene quantum dots from C 60 via a metal-catalyzed cageopening process [28], etc. Due to the relatively large apparent height of fullerenes (in the range of 0.3-0.4 nm) on surfaces [24] compared to planar molecules, they appeared as bright round protrusions in STM images [24]. Such an obvious corrugation of surface structures along z-axis leads to the great challenge in the intramolecular structure characterization. As well-demonstrated by Gross et al [29], nc-AFM based on a qPlus sensor [30], which is further functionalized with a single CO molecule at the tip apex, enables unprecedented atomic resolution by probing short-range chemical forces involved. With such a CO-terminated tip, nc-AFM could also be applied to discriminate different bond orders involved in the carbon-based nanostructures (such as fullerenes), which were demonstrated to be closely related to the electron density in bonds [31]. Interestingly, due to the tilting of the CO molecule positioned at the tip apex, only a hexagonal tile was exhibited in the nc-AFM images of fullerenes, and the atomic contrast stemmed from Pauli repulsion. Thus, it is generally difficult yet highly desirable to develop new methodologies to characterize 3D or nonplanar molecular skeletons as well as surface structures. Remarkably, Moreno et al developed a multipass method to image such nonplanar molecules (including C 60 ) with intramolecular resolution based on the application of AFM with silicon cantilevers as force sensors (figure 2(b)) [32]. Two consecutive line scans were conducted as schematically shown in the top left panel of figure 2(b). The first line scan of a single C 60 adsorbed on a (101) anatase surface (top right) provided the topography resolved as a bright dot, while the ∆ƒ and ∆ƒ 2 images (bottom) obtained during the second line scan (with an offset along z-axis of −0.21 nm in this case) revealed the skeletal features comprising hexagonal and pentagonal rings. This intramolecular bond-resolved capability indicates a promising way to study 3D systems such as nonplanar carbon nanostructures, which would also be an important supplement to that acquired from CO-functionalization.
In 2019, Kaiser et al, for the first time, fabricated an isolated cyclo [18]carbon (C 18 , an sp-hybridized molecular carbon allotrope) by atom manipulation on a C 24 O 6 molecule adsorbed on bilayer NaCl/Cu(111) (figure 2(c)) [22]. By applying voltage pulses from an STM/AFM tip, the CO moieties were sequentially cleaved, resulting in the generation of C 18 with a yield of 13%. The nc-AFM images recorded at different tip offsets ∆z with respect to the setpoint clearly displayed the differences in bond order, which helped to experimentally discriminate between the two possible chemical structures of C 18 (i.e. polyynic and cumulenic forms). Consequently, a nine-fold symmetry was directly observed from the AFM contrast, and a polyynic structure was revealed. Apart from the decarbonylation pathway, the authors further demonstrated that an alternative strategy, dehalogenation from a C 18 Br 6 precursor via voltage pulses, provided a higher yield of 64% [33]. These results open up the possibility to synthesize carbon-rich structures with high reactivity followed by structural characterization based on SPM techniques, which would otherwise be inaccessible by conventional synthetic methods due to the difficulties not only in synthesis but also in isolation.
Aside from C(sp 2 )-containing fullerenes, nanographenes, which are well-defined graphene segments and extended polycyclic aromatic hydrocarbons (PAHs) [34], have great potential in molecular electronics and have been a topic of broad interest. Experimental and theoretical investigations have revealed a close correlation between electronic properties and various edge geometries [35] and sizes [36] of nanographenes. A series of progress concerning nanographenes based on the application of STM was reviewed by Müllen and Rabe [34], and they described the structures and electronic properties (e.g. rectification) probed by STM/STS. Recent years have witnessed the booming on-surface synthesis strategy for nanographenes as well as the development of real-space fine characterization. Among all the synthetic strategies, reaction pathways via C-H [37] and/or C-X [38] (X: halogen) activation have shown their superiority in producing products with a high reaction selectivity. Based on the surface-assisted C-H activation, periacenes, such as peripentacene [39] and oxygenboron-oxygen-doped perihexacene [40], were synthesized via On-surface synthesis and characterization of 0D nanostructures, including fullerenes and cyclo [18]carbons. (a) Synthesis of triazafullerene C 57 N 3 on Pt(111) and corresponding STM characterization. Reproduced from [24], with permission from Springer Nature. (b) AFM characterization of C 60 on a (101) anatase surface with intramolecular resolution using a multipass method. Reprinted with permission from [32]. Copyright (2015) American Chemical Society. (c) In-situ generation of cyclo [18]carbon by atom manipulation and subsequent structural characterization. From [22]. Reprinted with permission from AAAS. cyclodehydrogenation reactions on Au (111). In addition, a rationally designed synthetic route via the dehydrogenative coupling of adjacent methyl groups led to the ultrahigh-yield fabrication of circumcoronene on Cu(111) [41]. Furthermore, by a combination of STM/nc-AFM imaging and density functional theory (DFT) calculations, Zhong et al reported the onsurface synthesis of dibenzoperihexacenes (with and without tert-butyl (tBu) groups) and dibenzoperioctacene (with tBu groups, as shown in figure 3(a)) on Au(111) starting from tetranaphthyl-p-terphenyl and tetra-anthryl-p-terphenyl precursors, respectively [42]. They clearly revealed the differences between reactions occurring on surfaces and those in solution [43] in regioselectivity, reaction pathways, and products. Interestingly, the energy gaps were measured to be 2.1 eV and 1.3 eV for dibenzoperihexacenes and dibenzoperioctacene on Au(111) by STS, respectively, indicating the influence of the nanographene core size in the electronic properties. The critical dependence of electronic structure on the topologies was also systematically demonstrated by Fasel et al in the open-shell nanographene systems [38,[44][45][46], which host unpaired electrons and remain challenging for solution synthesis. Open-shell nanographenes were generated on surfaces, and bond topologies [44] and topological defects [38] involving non-benzenoid rings were demonstrated to be capable of inducing magnetism by STM/STS. Moreover, it is worthy to remark that in the system of Clar's goblet (C 38 H 18 ) [45], a combination of STM (with a magnetic field) and spin excitation spectroscopy directly provided electronic and magnetic characterization and revealed the unconventional magnetism resulting from the topological frustration, which would be significant for carbon-based spintronics. Recently, rhombus-shaped nanographenes with zigzag peripheries including [4]-and [5]-rhombenes [46] were synthesized, and a magnetic spin singlet ground state emerged with increasing nanographene size.
In addition, a special type of PAHs featuring defined central cavities, that is cycloarenes, are regarded as models for porous graphene (i.e. unique nanographenes) and have been intriguing researchers due to their singular molecular and electronic structures [47]. As restricted by the challenges in synthesizing such macrocyclic molecular systems, the precise synthesis and in-depth characterization were quite  [52]. The nc-AFM characterization unambiguously visualized the armchair edges at both inner and outer sides. In addition to such an edge geometry, remarkably, C168 and C140 cycloarenes (namely [6]-and [5]coronoids) featuring inner zigzag edges and outer armchair edges were later synthesized on Au(111) (figure 3(c)) [53] by Di Giovannantonio et al. The well-designed dibrominated Ushaped precursor (cf figure 3(c)) underwent dehalogenative coupling and subsequent cyclodehydrogenation, which was similar to the situation in the previous case [52], leading to the fabrication of planar [6]coronoids and strain-driven nonplanar [5]coronoids as elucidated by the ultra-high-resolution STM/nc-AFM images shown below (figure 3(c)). The tipheight (z)-dependent nc-AFM images shown in figure 3(d) recorded the frequency shift when the CO-tip approached a [5]coronoid at different ∆z values with respect to the setpoint, and a height difference of 180 ± 10 pm was thus extracted. The dI/dV spectra directly detected the HOMO-LUMO (shortened for highest occupied molecular orbital to lowest unoccupied molecular orbital) gaps, that is ∼2.97 eV for the C108 cycloarene on Au(111) [52], and ∼2.2 eV and ∼2.5 eV for [6]-and [5]coronoids on Au(111) [53], respectively. Besides, extended from the single nanopore structure described above, a well-designed triple-porous C78 nanographene with a nonplanar conformation was also realized on Au (111) with an electronic gap of ∼3.0 eV [54], which broadened the diversity of nanographenes.
Moreover, due to the surface confinement and facilitation, some unique 0D carbon-based nanostructures, such as those embedded with non-benzenoid n-membered rings, have been demonstrated to be accessible by on-surface reactions. Simultaneously, SPM techniques play a significant role in the detection of intermediates and revelation of reaction pathways [55,56], visualization of bond structures and topographies [55,[57][58][59][60][61][62][63][64], characterization of electronic properties [58,59,65], tip-induced structural transformations [59,66], etc. Notably, Zeng et al applied a tetrabromobiphenyl molecular precursor with dibromo groups functionalized at the bay region of both sides and selectively fabricated biphenylene dimers containing four-, six-, and eight-membered rings on Ag(111) (figure 3(e)) [55]. Intriguingly, a unique reaction pathway was determined based on STM and synchrotron radiation photoemission spectroscopy. It involved a key step of intramolecular annulation, resulting in a surface-stabilized biradical biphenylene monomer, which was followed by the generation of an organometallic intermediate state and a biphenylene dimer (lower panels of figure 3(e)) with the increasing temperature. A detailed STM-based structural detection and analysis suggested antiaromaticities for four-and eightmembered rings. The controlled integration of n-membered rings and attendant novel properties shown in the above studies indicates a bright prospect in the subtle regulation of nanographenes alternatively.

Nanographene-related oligomers and macrocycles
In addition to the above-mentioned nanographenes in relatively small sizes, some larger nanographene-related structures have also been successfully fabricated on surfaces. Although their solution synthesis usually encounters size and shape limitations, the on-surface synthesis protocol takes full advantage of key molecular building blocks stemming from solution chemistry (namely, molecular precursors) as well as surface-facilitated chemical reactions, and thus enables a precise bottom-up preparation of these covalently coupled carbon nanostructures. In this regard, a cascade of wellpredesigned on-surface intermolecular and intramolecular reactions is a reaction strategy of choice, which generally involves Ullmann-type coupling reactions (via C-X activation and C-C coupling) and cyclodehydrogenation reactions (as a surface-assisted planarization process via C-H activation).
An amazing example of such exquisite design was shown in figures 4(a)-(c) [67], as reported by Hieulle et al. A 12ring dibromo polycyclic aromatic compound (C 62 H 38 Br 2 ) was applied as the precursor and underwent Ullmann coupling and subsequent cyclodehydrogenation processes on Au (111), forming a trimer-like structure. Owing to the high steric hinderance (which is often the case for cyclodehydrogenation steps [42,49,50]) at the conjoined cove regions (indicated by red arrows in the inset of figure 4(a)), cyclodehydrogenation was spontaneously triggered, and planar nanographenes (C 186 H 60 ) with the incorporation of six azulene moieties was consequently synthesized (figures 4(a) and (b)). Among all the trimers, a C3-symmetric one was shown in figure 4(b), and the intramolecular skeletal structure was clearly resolved from a constant-height dI/dV map using a CO-functionalized STM tip. About 21 rings (including two azulene moieties) were resolved for each blade with a large [18]annulene one located at the center of the trimer, indicating that the trimer was composed of 64 carbon rings. A highly localized electronic state at the [18]annulene pore was visualized by dI/dV mapping at an energy of 2.4 V (figure 4(c)), displaying a special electronic state related to super-atom molecular orbitals. Besides, carbon nanostructures with regularly fused azulenes were further synthesized through a skeletal rearrangement by Hou et al [68].
Triangulene nanographenes, known as Clar's hydrocarbon, serve as appealing model systems for the exploration of carbon-based magnetism and spintronics, and thus have been extensively studied on surfaces in the recent five years, typically including [n]triangulenes and trangulene-based nanographenes [69] (e.g. Clar's goblet [45], C 38 H 18 , as discussed above). Notably, the extreme reactivity of unsubstituted [n]triangulenes originated from their special chemical structures with unpaired electrons brings about a huge challenge in their synthesis. Consequently, the synthesis had long been elusive until the landmark work concerning the generation of [3] triangulene on surface via STM/AFM tip-induced dehydrogenation was reported by Pavliček et al [70] in 2017. The subsequent successful synthesis and characterization of [4]-, [5]-, and [7]-triangulenes [71][72][73] and [7]triangulene quantum ring [74] by SPM-based techniques have greatly expanded the fundamental understanding and further stimulated intensive exploration of triangulene family [69], such as triangulene dimer [75], trimer [76], chains [77], and aza-triangulene [78]. Moreover, by subtly combining solution synthesis and one-step on-surface cyclodehydrogenation, Hieulle et al reported the synthesis of a triangulene-based nanostar, which was a macrocycle constructed by six [3] triangulenes, and its spin excitations on Au(111) (figures 4(d)-(g)) [79]. It is noteworthy that a cascade of Ullmann coupling and cyclodehydrogenation of compound 2 (cf figure 4(d)) on Au (111) led to the formation of triangulene-based oligomers [77] instead of a nanostar structure. Alternatively, the macrocycle 1 served as the precursor and was directly sublimated onto Au(111) by flash-annealing, followed by a surface-mediated cyclodehydrogenation upon further annealing. The resulting planarized triangulene-based nanostar was confirmed based on the BRSTM image obtained by a CO-terminated STM tip ( figure 4(e)). Besides, its spin configuration was probed by the dI/dV spectra conducted at the edge of a triangulene unit (marked by the blue dot in figure 4(e)), which indicated bias-symmetric stepped features as a fingerprint of inelastic electron tunneling induced spin excitation ( figure 4(f)). The stacked dI/dV plot along the line AB displayed the distribution of the inelastic signals, which were localized at the edges and dissipated at the unit center (figure 4(g)). Three steps located at ±14(1) mV, ±42(2) mV, and ±80(2) mV were clearly resolved, presenting a direct evidence for the unique collective spin state for such a triangulene hexamer, which corresponded to the antiferromagnetic ordering of six S = 1 sites. Besides, detailed exploration of length-dependent magnetic excitations in triangulene-extended chains and macrocycles was sophisticatedly performed by Mishra et al [77], and gapped spin excitations and fractional edge states were directly observed based on the STM/STS. More details and insights into the characterization of spin-polarized electronic properties of graphene nanostructures using SPM techniques have been comprehensively covered by the review paper by Song et al [80] and are skipped herein.
Moreover, SPM techniques have also shown the remarkable potential in the bottom-up synthesis of sizeable covalently linked organic quantum corrals and direct visualization of the quantum resonance states residing inside [81] as reported by Peng et al (figures 4(h)-(k)). A perfect 12-unit circular organic ring (with a pore of 3.86 nm in diameter) was prepared following the on-surface synthetic route shown in figure 4(h), along with the formation of chains and ring segments. BRSTM imaging (figure 4(i)) visualized each unit involved, which was made up of one five-membered ring and four six-membered rings resulting from dehydrogenative cyclization. Electronic characterization by dI/dV spectra revealed the valence band and conduction band of the organic quantum corral to be located at −1.05 eV and +1.3 eV with respect to the Fermi level, respectively. The corresponding characteristic frontier orbitals were experimentally visualized by dI/dV mapping at these energies, showing a localized strong intensity at the edges. Interestingly, the authors further probed quantum resonance states inside the organic corrals based on dI/dV spectra and mapping (figures 4(j) and (k)). Five electronic states (i.e. P 1 -P 5 ) were detected from the dI/dV spectra at different positions with respect to the pore center (figure 4(j)), and the corresponding spatial distribution was probed by mapping showing distinct characteristic features (figure 4(k)). These quantum resonance states were also shown to be related to the topographies of organic corrals, which opened up a new avenue to engineer desired quantum states.

1D carbon-based nanostructures and nanomaterials
When the dimension of carbon-based nanostructures extends to 1D, many intriguing nanomaterials emerge with fascinating physicochemical properties and phenomena. In this part, we will mainly focus on 1D carbon-based nanostructures and nanomaterials probed by SPM. Carbon-based chains with negligible widths will be discussed first, which typically include PE, PA, and (organometallic) polyyne. Thereafter, with the increasing width to several atoms, GNRs and other π-conjugated polymer chains will be introduced herein, which would also be important model systems for the corresponding 2D extended networks.

PE, PA, and (organometallic) polyyne
Alkanes, known as single-bonded saturated hydrocarbons with a molecular formula of C n H 2n+2 , are generally applied as natural gas (methane and ethane), fuel, and feedstock in chemical production. With each carbon atom in sp 3 hybridization forming strong and localized C-C and C-H bonds [82], alkanes are usually considered to be less reactive and are accessible from alkenes (C n H 2n ) and alkynes (C n H 2n−2 ) by addition reactions with molecular hydrogen. Meanwhile, the selective hydrogenation of alkynes to alkenes without further formation of alkanes has long been an important yet challenging topic in heterogeneous catalysis. The transformation from acetylene (C 2 H 2 ) to ethylene (C 2 H 4 ) or ethane (C 2 H 6 ) has been systematically explored on single-atom alloys (SAAs) and metal substrates using ultrahigh vacuum (UHV) techniques [83,84] by Trenary et al as model systems to monitor such a selective H 2 addition process and unravel catalytic mechanisms. In addition, due to the relative inertness and abundance of alkanes, controllable and efficient C-H bond activation in alkanes is of great importance in exploiting alkane resources for fine chemical synthesis [82,85]. The combination of surface science methodologies and catalysis studies has successfully opened the door to the mechanistic understanding of C-H bond activation at an atomic scale, including catalytically active sites, reaction pathways, reaction intermediates, etc. For example, the dehydrogenation of alkane was investigated by Marcinkowski et al on Pt/Cu SAAs [86], revealing the evolution of methyl groups and the promising catalytic role of SAAs in C-H bond activation.
Aside from these great efforts, the transformations and reactions of alkane, PE, and PA have been intriguing in the field of on-surface chemistry. Dated as far back as the early nineties, seminal works on self-assembled alkanes were carried out at the liquid-solid interface by Rabe et al [87,88], providing indications of molecular structures and dynamics. In 2011, Zhong et al reported the polymerization of linear alkanes on Au(110) [89], where the C-H activation selectively took place at the sites of terminal CH 3 or penultimate CH 2 groups. In such a pioneering work, Au(110) surface served as both a platform and a catalyst for the alkyl C-H bond activation, and provided 1D constraint as well. A long-chain alkane, C 32 H 66 , was deposited and the sample was annealed to 440 K, leading to polymerization with formation of PE chains. The dehydrogenative polymerization process was further confirmed based on a control experiment of alkane chains with phenylene connections. High selectivity in the reaction sites of C(sp 3 )-H activation was thus determined by a combination of STM imaging and DFT calculations. This work opened the door of C-H activation on surfaces [90][91][92][93][94] and inspired on-surface activation of relatively inert functional groups typically like methyl and aryl groups. Recently, concentrating on the C(sp 3 )-H activation, Li et al discovered a novel phenomenon of direct transformation from n-alkane into trans-polyene [95] with the assistance of complementary microscopic and spectroscopic characterization techniques (figure 5(a)). By applying the same molecular precursor C 32 H 66 yet different substrate, Cu (110), they showed the close-packed lamellar structure (left panel) upon molecular deposition at room temperature (RT). A low-flux and hot deposition strategy with a substrate temperature of 453 K, interestingly, led to the formation of monodispersed molecular chains as shown in the middle panel. The molecular topography and skeleton characterized by STM and nc-AFM displayed characteristic line features of in-plane C-H bonds (right panel). Furthermore, a cascade alkane-to-polyene dehydrogenation scenario was clarified based on experimental and theoretical investigations. Thereafter, the authors reported the coupling of polyenes on Cu(110) forming PA after thermal treatment at 470 K ( figure 5(b)) [96]. Three types of connections were unambiguously distinguished from STM/ nc-AFM images, including α, β, and γ types, which stemmed from different coupling ways of two terminal alkenyl groups. Among them, α-type appeared in an all-trans configuration with a seamless morphology, while the other two were in cis configurations at the linkage with features of obvious protrusions. Note that the proportion of α-type turned out to be related to the initial molecular coverage and could be effectively increased at a higher coverage. The series of studies presented the possibility of C-H activation of alkanes (C(sp 3 )-H) as well as the transformation from alkanes to PAs (i.e. from C(sp 3 ) to C(sp 2 )), which would shed light on the exploitation of alkane feedstocks in an efficient and clean way. PE, with a chemical formula of (C 2 H 4 ) n , is a mixture of polymers of ethylene (i.e. with various values of n) and widely used as plastics. The catalytic production of PE is of utmost importance in the chemical industry and attracts great interest from chemists. With such a motivation, Guo et al applied it as a model system and successfully visualized the activator-free polymerization process of ethylene forming PE on a carburized Fe(110) surface using STM [97]. They captured the insitu chain growing process during continuous RT-STM scanning in a C 2 H 4 atmosphere of 1 × 10 −8 mbar ( figure 5(c)). Based on low temperature (LT) STM imaging, C 2 H 4 (left) and CHCH 3 (right, as a chain initiator) were found in the initiation of polymer growth ( figure 5(d)). Short PE chains (e.g. C 8 and C 10 chains) with triangularly shaped ends were revealed to grow by ethylene insertion (figure 5(e)), and a specific triangular Fe site at the domain boundary was the key for the polymerization. Such a real-space single-molecule-level investigation enriched the mechanistic understandings on the PE polymerization and should be significant for both chemistry and materials science.
In addition to the applications in the exploration of 1D PA and PE (involving C(sp 2 ) and C(sp 3 ), respectively), SPM techniques have also exhibited availability and versatility in the precise synthesis and fine characterization of 1D C(sp)-containing chain structures as well as interconversions between sp 2 -and sp-hybridized carbon skeletons. The beginning of this century witnessed the precise control over initiation and termination of linear propagation of diacetylenes on graphite, which was stimulated by either an STM tip or UV light irradiation [98][99][100], forming extensive conjugated nanowires under ambient conditions. Recently, a series of organometallic or intrinsic polyynes have been fabricated on surfaces with atomic precision [23,[101][102][103][104] starting from various molecular precursors, as typically shown in figure 6. Based on the acetylene (C 2 H 2 ) precursors, Xu et al have successfully synthesized cis-PAs, trans-PAs [105], and organometallic Cu-polyynes [101] (i.e. metalated carbynes, with a repeating unit of -C≡C-Cu-) on Cu(110) by applying different substrate temperatures ( figure 6(a)). Upon deposition of C 2 H 2 at 300 K and mild annealing, a mixture of cis-and trans-PA segments formed, and a cis-to-trans isomerization could be triggered by further thermal treatment. High-resolution STM and nc-AFM images provided both morphology and skeleton information, and accordingly, determined the corresponding  [102]. Copyright (2020) American Chemical Society. (j) Triacetylenic Ag-carbyne on Ag (111). Reprinted with permission from [103]. Copyright (2022) American Chemical Society. (k) In-situ synthesis of polyynes on NaCl/Cu (111). Reproduced from [23], with permission from Springer Nature. structures (figures 6(b) and (c)). Interestingly, by constructing oxides on the substrate, the hybridization between trans-PA chains and underlying layer was highly reduced compared to that in the case of bare surface. An interface state (middle, indicated in blue) was discovered between the metal-(left, indicated in black) and oxide-supported trans-PA segments (right, indicated in red) from both STM image and dI/dV spectra ( figure 6(d)), revealing the metallic-to-semiconducting transformation. Moreover, metalated carbynes were synthesized after deposition of C 2 H 2 onto Cu(110) held at 450 K. Notably, the C-C triple bonds appeared as bright features in the nc-AFM image, while the incorporated Cu atoms were only visible in the STM image as bright protrusions (figure 6(e)). Apart from the strategy of C(sp)-H activation, Yu et al further applied C(sp 2 )-Br 2 activation based on cumulene moiety and synthesized the 1D diacetylenic Au-carbyne (-C 4 -Au-) as another organometallic polyyne on Au(111) (figure 6(f)) [102]. By applying a voltage pulse locally on the molecular precursors at LT, they observed the cleavage of two C-Br bonds and a simultaneous topographic transformation from a shape of dog bone to a linear one (figure 6(g)). A combination of STM and nc-AFM imaging further corroborated the skeleton rearrangement from a cumulene moiety to a diyne one (Br-C≡C-C≡C-Br). In contrast, deposition of precursors at RT directly led to the fabrication of Au-carbyne ( figure 6(h)). The band gap was experimentally determined to be ∼2.0 eV on Au(111) by STS, indicating a semiconducting feature (figure 6(i)). Recently, the family of organometallic polyynes was further extended to triacetylenic Ag-carbyne by the same group [103]. Starting from C 6 Br 6 molecules, an unexpected ring-opening process of C 6 rings was triggered along with a complete debromination on Ag(111) at 300 K, leading to the formation of Ag-carbyne chains with a repeating unit of -(C≡C) 3 -Ag-(figure 6(j)). The characteristic features were clearly shown in the STM/nc-AFM images and nicely reproduced by DFT calculations. Furthermore, the transformation between metallic carbynes with different periodicities was investigated, from -C≡C-Ag-to -(C≡C) 2 -Ag-on Ag (110) and from -(C≡C) 2 -Cu-to -C≡C-Cu-on Cu(110) [104]. Such a skeleton reconstruction was revealed to originate from the corresponding thermodynamic stability by extensive DFT calculations. It was also theoretically predicted that the band gap of organometallic polyynes would be highly related to the number of C-C triple bonds involved (acetylenic > diacetylenic > triacetylenic Ag/Cu/Au-carbyne) as well as the incorporated metal elements (Ag-carbyne > Cucarbyne > Au-carbyne).
Moreover, intrinsic polyyne, with consecutive sp-carbon atoms, is promising for applications as molecular wires for charge transport in electronic circuitry [106]. Due to the instability and high reactivity of multiple alkynyl groups (-C≡C-) involved in polyene (which is also the case of 0D cyclocarbon), the synthesis of intrinsic polyyne has long been a challenging topic. By functionalization of bulky terminal groups as end-caps to stabilize the long -(C≡C) n -chain structures, the electrical conductance of oligoynes (n = 1, 2, 4) has been directly detected by STM-molecular break junction techniques [106]. Significantly, by virtue of atomic manipulation (that is, applying voltage pulses on the target position of molecular precursors), Pavliček et al fabricated a long polyyne, Ph-(C≡C) 8 -Ph, on NaCl/Cu(111) via skeleton rearrangement and monitored the whole process by nc-AFM and STM (figure 6(k)) [23]. They applied a molecule containing 1,1-dibromo alkenes as precursor and precisely cleaved the C-Br bonds in situ by a qPlus-based STM/AFM tip, which was followed by the rearrangement of remaining carbon skeleton, forming various polyenes (n = 3, 4, 6, 8). STS indicated that the LUMO energies of polyyne family decreased as the value of n increased, resulting in a reduced band gap (in good agreement with that in the case of organometallic polyynes as discussed above). This study thus demonstrates the versatility of SPM techniques in the aspects, including but not limited to bond-resolved topographic and skeletal characterization [70,[107][108][109][110], orbital imaging [111][112][113][114], triggering reactions in situ [115][116][117], visualization of reaction pathways [22,32,117], and detection of properties [70,81,[118][119][120][121] (e.g. electronic and magnetic).
The applications of SPM in the 1D nanostructures and nanomaterials typically like PE, PA, and (organometallic) polyyne thus unraveled an interesting interplay among C(sp 3 ), C(sp 2 ), and C(sp), as well as the interconversions among alkyl, alkenyl, and alkynyl groups and the corresponding halo-substituted functional groups. The methodologies and on-surface synthesis strategies open a new window on novel carbon-based materials, especially on the atomically precise fabrication of highly reactive and unstable carbon-based chains, which would otherwise be hindered in conventional solution chemistry due to obstacles such as cross-linking, cycloaddition, solubility, isolation, and purification.

GNRs and other π-conjugated polymer chains
Graphene is known as a 2D single-atom-thin carbon layer [4] embedded with C(sp 2 ) hexagonal rings, which is a semimetal with a zero band gap. When it is laterally confined to the nanometer scale, for instance, 1D ribbons, the band gap opens. GNRs thus emerge as a promising 1D carbon nanostructure as well as an attractive research topic due to their outstanding electronic properties. As the band gap is crucially dependent on the topologies of GNRs (such as width of ribbons [122] and their edge geometries [123]), preparation of GNRs with atomic precision is a prerequisite. Since the pioneering bottom-up fabrication of armchair GNR (7-AGNR) reported by Cai et al in 2010 [124], on-surface synthesis strategy has displayed its capability in the subtle regulation of the topologies and consequently precise steering of the electronic properties. So far, GNRs with AGNRs and zigzag edges [124][125][126], chiral GNRs [121,127,128], heteroatom-doped GNRs [120,[129][130][131], and extended nanoporous graphenes [132,133] have been precisely synthesized on surfaces and extensively investigated by virtue of SPM techniques. Several strategies turn out to be effective in tuning the band gaps. A common method is increasing the width by the rational selection of molecular precursors, for example, 13-AGNRs and 9-AGNRs have a similar band gap of ∼1.4 eV on Au (111) [134,135] while ∼2.3-2.5 eV for 7-AGNRs as experimentally determined by STS [134,136]. Moreover, topological band engineering by forming heterojunctions [137,138], for instance, by the integration of alternating topologically trivial 7-AGNR and topologically nontrivial 9-AGNR components [137,139,140] forming 7/9-AGNR heterojunctions, was also reported, which resulted in the formation of interface states within 1D ribbons and new bulk frontier bands. Profiting from this remarkable effort, semi-conducting GNRs have been prepared with the band gaps reduced to ∼0.74 eV for the 7/9-AGNR [137] and ∼0.65 eV for the 7-AGNR-S(1,3) [138].
Interestingly, Rizzo et al reported the formation of metallic GNRs for the first time by introducing a symmetric superlattice of zero-energy modes into GNRs [118] as shown in figure 7. Starting from the delicately designed molecular precursor 1 with dibromo-substitution and a methyl group,  (111). (e) dI/dV spectra and mapping of 5-sGNR on Au (111). State 1, state 3, and state 2 showed the valence band, conduction band, and zero-mode band, respectively. From [118]. Reprinted with permission from AAAS. they followed the routine recipe of Ullmann coupling and cyclodehydrogenation and synthesized sawtooth-GNRs (sGNRs) on Au(111) at 300 • C (figures 7(a) and (b)). Surprisingly, further annealing at 300 • C resulted in the transformation to the 5-sGNRs with the fusion of five-membered rings along the cove edges. The superposition of two H atoms at the cove regions of sGNRs caused the nonplanar configuration and thus appeared as periodic bright dots at both edges in the STM image (figure 7(c)), which disappeared in the 5-sGNRs with a planar configuration (figure 7(d)). BRSTM imaging by mapping the dI/dV signal at a low bias further unambiguously proved the structures involved (figures 7(c) and (d)). The sGNR was revealed to have a sharp peak near Fermi level showing a metallic feature on Au(111) by dI/dV spectra, which was theoretically calculated to be induced by the underlying substrate. Nevertheless, the construction of pentagonal rings brought about a robust metallicity with a wider bandwidth, shown as a broad density of states feature that spans Fermi level (state 2) in the dI/dV spectra (upper panel of figure 7(e)), which was further calculated to be independent of the substrate. The dI/dV maps displayed the local density of states (LDOS) at the edges of valence band, zero-mode band, and conduction band, respectively (cf lower panel of figure 7(e)). The modification of zero-mode superlattices accompanied by the STM/STS evidences revealed a feasible methodology to induce and explore the metallicity in not only GNRs but also other carbon-based nanostructures. More relevant studies in the regard of on-surface synthesis of GNRs and their outstanding electronic properties have been nicely reviewed by Talirz et al [123], and the correlation between topological structures and electronic states of graphene could be found in the review paper by Li et al [141]. In addition, some GNRlike ribbons with nonhexagonal carbon rings [142][143][144][145][146] have also been exploited with the purpose of fine tuning of their electronic properties.
Additionally, numerous 1D π-conjugated polymers have been explored as an extension from GNRs and GNR-like nanostructures. One promising way is the incorporation of n-membered rings with π conjugation based on C(sp 2 ), for example, preparation of 1D π-conjugated polymers with indenofluorene units where five-membered rings were incorporated [147]. Alternatively, C(sp)-containing skeletons have been successfully embedded in the sp 2 -hybridized GNRs and GNR-like nanostructures for better electronic tailoring, such as ethynylene-bridged [148][149][150][151], diacetylene-bridged [152,153], and cumulene-bridged [154,155] polymers. Besides, 1D cumulene-linked polymers with the involvement of five-and seven-membered rings (based on a tribenzoazulene building block) [156] was also achieved. Notably, Cirera et al explored the topological order involved in such π-conjugated polymers [157] as shown in figure 8. 1D C(sp)-bridged acene-containing polymers were synthesized on Au(111) using the same strategy ( figure 8(a)), which was based on precursors with various acene units (including anthracene, pentacene, and bisanthene). The nc-AFM characterization ( figure 8(b)) provided clear features of bisanthene units and linear bridges, which were attributed to cumulene (=C=C=) links instead of ethynylene (-C≡C-) ones. It is worth noting that the topological phase transition between ethynylene-linked acene and cumulene-linked quinoid structure strongly depends on the size of acene unit involved. Owing to the formation of quinoid-cumulene resonant form, a narrow band gap of ∼0.3 eV was detected from the dI/dV spectra ( figure 8(c)). The LDOS at the edges of valence band (∼−75 meV) and conduction band (∼200 meV) was visualized by dI/dV mapping (figure 8(d)), showing an obvious electronic swap compared to the case involving anthracene units. Furthermore, the in-gap zero-energy edge state was directly observed to fade away from the termination to the center in the dI/dV spectra (figure 8(e), from top to bottom) and STM image ( figure 8(f)). Such a study revealed the interplay between resonant forms and topological quantum phases, and successfully engineered the 1D π-conjugated polymers to have narrow band gaps and zero-energy edge states, which would be inspirational for fabricating intrinsic metallic polymers.

2D carbon-based nanostructures and nanomaterials
By further extending the lateral width from 1D GNRs, GNRlike ribbons, and other π-conjugated polymer chains, some 2D novel carbon-based nanostructures have been obtained, such as extended nanoporous graphenes [132,133] as mentioned above. Among them, one recent exciting example is the synthesis of biphenylene network [119], a nonbenzenoid carbon allotrope composed of 4-6-8 membered rings of C(sp 2 ), which was achieved by on-surface interpolymer hydrogen-fluorine-zipping reaction on Au (111). The utilization of SPM techniques provided real-space characterization of the detailed structure and metallicity, indicating that it would be up-and-coming for conductive applications in carbon-based circuitry. Despite these achievements, it is actually quite challenging to perfectly extend the dimension of nanostructures from 1D to 2D with controlled covalent bonding based on on-surface synthesis. One of the difficulties lies in the synthetic strategy, which is dominated by a routine recipe of Ullmann-type-based (C-X) coupling independently [158] or followed by cyclodehydrogenation [132], generally resulting in the poor selectivity in the multiple reaction sites, generation of metal-incorporated reaction intermediates, appearance of excessive halogen byproducts in the vicinity, etc. These unavoidable factors or phenomena prohibit researchers from processing toward 2D large-scale and uniform carbon-based nanostructures and nanomaterials with designed structures and patterns. To overcome these aspects, high-dilution strategy [159] on hot substrates would be a good option. In addition, developing novel on-surface reaction types with higher selectivity and less byproducts is in desperate need. In this regard, H-F zipping [119,160] as mentioned above shows high efficiency. More recently, a programable hierarchical synthetic strategy was reported by Tenorio et al based on a subtle combination of Ullmann coupling, cyclodehydrogenation, and H-Cl zipping [133], leading to the fabrication of 2D hybrid nanoporous graphene. Interestingly, GNRs prepared in the first step (using the first component) was designed to serve as a dynamic template providing nanochannels for fusing with the second component, and consequently, the 1D GNRs laterally extended to 2D. Nevertheless, further efforts in developing synthetic strategy and preparing 2D carbon-based nanostructures with atomic precision would be prerequisite for fine characterization and precise measurement. Leaving aside these challenges in synthesis, carbon-based nanostructures and nanomaterials characterized by SPM share similarity in features at different dimensions, and the related studies will not be focused herein.
In addition to the UHV-SPM combined with on-surface synthesis strategy as mainly discussed above, which highly relies on the catalytic property and 2D confinement of the underlying metal substrates, the application of SPM under ambient conditions or in direct connection with solution chemistry has also shown its great contribution in understanding carbon-based nanostructures and nanomaterials and the corresponding chemical processes. In ambient conditions, highly oriented pyrolytic graphite (HOPG), which has weak hybridization with adsorbed structures and is generally inert, provides an alternative platform for such investigations, untangling the intrinsic chemical activities. For example, nucleation-elongation processes are approachable and could be directly monitored on HOPG at the molecular scale under ambient conditions [161] based on SPM. As reported by Zhan et al recently, STM revealed time-dependent evolution of the dynamic polymerization and crystallization processes at the solid-liquid interface in real space [161], which provided mechanistic and kinetic insights into the construction of 2D covalent polymers. Besides, highly ordered 2D covalent organic frameworks (COFs) could be directly fabricated using various strategies and explored at interfaces [162], such as aldehyde-amine condensation via a solid-vapor interface reaction [163] and a self-condensation of diboronic acid at the liquid-solid interface [164]. More features about these graphene-like single-layered COF structures and their STM studies could be found in the review paper [165]. Moreover, in-situ observation and detection offered by SPM under ambient conditions provides possibilities to bridge the pressure gap between UHV-based experiments and real world for the exploitation of carbon-based nanomaterials.

Summary and outlook
STM and AFM become more and more indispensable and extremely versatile in the atomic-scale exploration of lowdimensional carbon-based nanostructures and nanomaterials. Dependent on the UHV conditions, UHV-SPM has shown its great contribution to a series of sophisticated procedures on surfaces in combination with on-surface synthesis strategy, including induction of localized chemical reactions with simultaneous monitoring, determination of unknown structures with unambiguous skeletal characterization, visualization of reaction pathways with detection of intermediate states, identification of electronic and magnetic configurations, revelation of structure-activity relationship, etc. In this review, we mainly discussed the applications of STM and AFM in probing several typical carbon-based nanostructures and nanomaterials at different dimensions concerning the above aspects, ranging from novel carbon allotropes to hydrocarbons and carbonbased organometallic structures. Besides, intriguing interconversions among C(sp 3 ), C(sp 2 ), and C(sp) were also displayed and corroborated in real space, indicating the regulation rules of sp n -hybridized carbons involved in the corresponding nanostructures and nanomaterials. Therefore, STM/AFM opens up new frontiers in chemical synthesis, structural characterization, and property measurement with atomic precision.
Nevertheless, some improvements are expected to promote broader application of these techniques as concisely illustrated in figure 9. The first point would be visualizing the chemical arrangement of carbon-based nanostructures (including both chemical element and spatial arrangement), in other words, the combination of chemical sensitivity and skeletal information. Although the current advance in STM/AFM has made it possible to discriminate different bond orders as well as skeletons (spatial arrangements) involved in the relatively planar structures as extensively displayed above, STM/AFM is still suffering from the lack of chemical sensitivity that requires characteristic fingerprints related to specific chemical bonds or groups. The combination of complementary spectroscopic information and topographic one provided by STM/AFM has shown its feasibility in this aspect (cf upper left panel of figure 9). For instance, application of tip-enhanced Raman spectroscopy (TERS) [166,167] and STM/AFM has been demonstrated to be capable of determining structural and chemical heterogeneities in the dehydrogenation processes at the single-bond limit [168] and identifying π-skeletons in coupling reactions [155], where the characteristic vibrational motions originated from specific bonds are the key to such chemical sensitivity. Moreover, TERS measurements have been shown to provide chemical information both laterally and vertically [167,168], which offer access to nonplanar stereo structures [167] and even more general 3D nanostructures. Meanwhile, it is also highly desirable to develop imaging techniques or methodologies of STM/AFM itself to characterize general 3D molecular skeletons.
In addition, on-surface synthesis of carbon-based nanostructures and nanomaterials strongly relies on either thermal excitation or atom manipulation (by means of injecting tunneling electrons) so far, while other extensive excitation sources (such as photo-excitation [169][170][171][172][173] and localized surface plasmons [174][175][176][177] as illustrated in the bottom panel of figure 9) have been much less reported or applied in synthesis. Given that thermal excitation would activate several chemical groups simultaneously, poor reaction selectivity has long been a realistic obstacle. Besides, for pericyclic reactions, selection rules (also known as Woodward Hoffmann rules) indicate that some reactions would be ground state (thermally) allowed, while others would be excited state (photochemically) allowed yet thermally forbidden. Notably, plasmoninduced reactions by confining light at the SPM junction can significantly reduce the energy requirements and facilitate efficient energy conversion with a strong enhancement of the electric field [174,176,177]. Thus, integration of a broader range Moreover, intact deposition of carbon-based molecular precursors or structures is not only prerequisite for on-surface synthesis protocol, but also essential for direct investigation under UHV conditions (cf upper right panel of figure 9). Traditional thermal sublimation is generally available for relatively small organic molecules, while it may not be applicable to those with larger molecular mass due to side reactions before or during evaporation (that is, fragility of structures), for example, the macrocycle 1 shown in figure 4(d). In this regard, methods typically like flash-annealing a silicon wafer loaded with large compounds [79] and electrospray deposition (ESD) under UHV conditions (UHV-ESD) [178][179][180] have been demonstrated to be effective and powerful. Nonetheless, a high ratio of polymeric fragments could still be found (e.g. ∼92% [79] in the study of figure 4(d)) coexisting with intact ones in the former case, while a mixture of solvent and solute would be present on surfaces based on UHV-ESD method [179,180]. Recently, a hybrid bottom-up approach toward GNRs called matrix-assisted direct transfer technique [181] by a win-win combination of solution-based polymerization and on-surface synthesis was reported. Carbon-based polymer sample dispersed in an inert matrix was loaded into a fiberglass applicator under ambient conditions and was further transferred onto a substrate in UHV. Thereafter, the bulk matrix was easily removed from surface by annealing, followed by on-surface cyclodehydrogenation reactions of polymers forming GNRs. Such a technique would be promising not only in directly bridging the gaps between solution chemistry and UHV conditions, but also in the synergy between both bottom-up approaches (i.e. solution-based and on-surface synthesis). It is also eagerly expected to develop similar techniques or expand the database of polymer-matrix association to further extend this concept to the synthesis and exploration of more general carbon-based nanostructures and nanomaterials, which would greatly suppress the limitations of either synthetic method and bring the superiority of SPM into full play.
The understanding of carbon-based nanostructures and nanomaterials will be further enriched by comprehensive consideration on the above several aspects, which will also open an avenue for the exploration and development of elusive and undiscovered carbon-based nanomaterials.