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[Shigeki Kawai](https://orcid.org/0000-0003-2128-0120)

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[Nanocarbon materials synthesized by local probe chemistry](https://mdr.nims.go.jp/datasets/2150cf58-4002-4cba-95f0-c92cedbeac71)

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Nanocarbon materials synthesized by local probe chemistryResearch ReportNanocarbon materials synthesized by local probe chemistryShigeki KawaiNational Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, JapanE-mail: KAWAI.Shigeki@nims.go.jpScanning probe microscopy has been used to characterize structures and electronic properties of surfaces as well as toconstruct nanostructures via atom-by atom manipulation. Recent advances in tip-functionalized scanning probe microscopyallow us to measure the inner structures of molecules. This bond-resolved imaging technique is of particular importance in theinvestigation of precursors and products during on-surface reactions. In this article, tip-induced structural isomerization andadditional reactions will be discussed.Received January 13, 2022; Accepted February 16, 2022Translated from Oyo Buturi 91, 356 (2022) DOI: https://doi.org/10.11470/oubutsu.91.6_356Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.1. IntroductionIt has already been many years since the development ofscanning tunneling microscopy (STM) [1] and atomic forcemicroscopy (AFM) [2], and they are now deployed not onlyin surface science but also in various other fields, such aschemistry and biology. It is no exaggeration to say that theyare one of the most important measurement methods thatsupport today’s nanoscience. These techniques, which usethe tunneling current flowing between a probe and a surfaceand the generated force to produce images, can elucidate thestructures and states of solid surfaces and adsorbates inreal space and with high resolution. In addition, atoms andadsorbates on a surface can also be directly manipulatedusing a probe [3,4], resulting in the realization of thefabrication of nanoscale structures [5–7].With the development of STM, research on moleculesadsorbed onto solid-liquid interfaces and surfaces in ultra-high vacuum has been actively conducted [8,9]. In particular,experiments at the single-molecule level are possible atextremely low temperatures, and it became possible toobserve molecular orbitals [10] and to measure their statesusing the inelastic tunneling currents [11] caused by thevibrational excitations of bonds. In recent years, a fusion withtip optical measurements [14,15] that combines probe-enhanced Raman spectroscopy [12,13], terahertz light, etc.has been developing [16]. Research on structural changes atthe single-molecule level is also being actively conducted.Carbon nanostructures, such as novel compounds [17] andgraphene nanoribbons (GNR) [18,19] can now be synthe-sized by chemical reactions of molecules deposited onto ametal surface by heating. In addition, highly reactive radicalspecies were also synthesized by elimination reactions, whichcuts off specific bonds in a molecule using the tunnelingcurrent flowing from a probe [20–23]. Isomerization usinginstabilities [24,25] and single-molecule synthesis by addi-tion reactions that stabilize atoms and molecules have alsobeen realized [26]. The measurement method for identifyingthe internal structure of a molecule using a probe terminatedwith a carbon monoxide (CO) molecule can be cited as thereason for the rapid development of this “local probechemistry” field [27,28]. Syntheses that were difficult tocarry out using conventional organic chemistry methodsbecame possible by directly looking at the structure of onemolecule and changing it using the tunneling currents with aCO probe. We have been engaged in research on surfacechemistry using this ultra-high resolution probe microscopefor 10 years [29–31]. In this paper, we will introduce thecreation of direct nanomaterials by “local probe chemistry”recently carried out using a probe, and elemental technology.2. Elemental technology2.1 Molecular and atomic manipulationAtomic-level pointed STM and AFM probes can be posi-tioned horizontally ðX; YÞ and vertically ðZÞ with picometer-level accuracy for moving atoms and molecules mechanicallyor electrically [3]. There is a pulling mode and a pushingmode for mechanical movements, and they can be modulatedand judged based on the tunneling current and the modulationof the signal in the Z direction [32]. In addition, not onlyhorizontal movements are possible but this method can alsobe used to pick up and drop atoms and molecules adsorbedonto a surface. In contrast, if an object consists only ofmolecules, it is possible to move it using the excitations ofbonds and the tunneling currents and the interactions betweendipoles of the molecule and the electric field from the probe[33].In addition, intramolecular bonds can also be broken bythe tunneling current flowing between the probe and thesample and applying a bias voltage greater than a certainvalue [20]. So far, cleavages of C–H bonds, C–Br bonds,C=O bonds, etc. have been realized. Breaking a bond yieldshighly reactive radicals with unpaired electrons [21]. Mo-lecular manipulation by isomerization using this instability isalso possible [24,25].By making full use of these atomic and molecularmanipulations, “local probe chemistry,” such as the rear-rangement of single molecules, the cleavage of bonds andisomerization can be performed.2.2 Intramolecular structure observationIn non-contact AFM, the resonance frequency that changesdepending on the force acting between the probe of the forcesensor and the sample is used as a signal [34]. In additionto long-range forces, such as the van der Waals force andelectrostatic forces, the interactions forces include a chemicalbond force, which is a short-range force that provides atomicresolution. Short-range forces, as expressed by the Lennard-Jones potential, decrease with the 6th power for the force ofattraction and with the 12th power for the force of repulsionJSAP ReviewJSAP Rev. 2022, 220410https://doi.org/10.11470/jsaprev.220410220410-1 © 2022 The Author(s)https://doi.org/10.11470/oubutsu.91.6_356https://creativecommons.org/licenses/by/4.0/https://doi.org/10.11470/jsaprev.220410with respect to distance. The distance dependence of arepulsive force is stronger than that of an attractive force,which leads to a higher resolution. In contrast, as a probe isterminated by atoms of the substrate in normal observations,a probe with a sharp tip is more active than a substrate with astable crystal plane. Due to this high reactivity, the atoms atthe tip of the probe move irregularly with a large attractiveforce before reaching the repulsive force region, makingstable imaging difficult. Especially in the case of observingmolecules adsorbed onto the surface of a substrate, theadsorption energy to the probe is higher than the adsorptionenergy to the substrate, and unintended molecular manipu-lation occurs frequently [35].In 2009, a measurement method using a probe terminatedwith a CO molecule was proposed [27]. The reactivity of aCO probe is low, and stable imaging is possible in therepulsive region. In addition, by tilting the CO probe in theXY direction with the interaction force, the total chargedensity derived from the bond is very localized and can bedetected. As a result, intramolecular bonds could be clearlyimaged similar to as if drawing a structural formula [36].Even in STM, it has been reported that a similar contrast canbe obtained by terminating the probe with a CO molecule orsomething similar [28].3. Local chemical reactions using a probeIn this section, we will introduce research on structureidentification by high-resolution observations of compoundsobtained by the radicalization of molecules and subsequentisomerization. We can do this by making full use of theabove-mentioned manipulation of atoms and molecules, andaddition reactions, in which different elements and moleculesare joined to radicals with a probe.3.1 Isomerization by elimination reactionIn this study, we aimed to perform the single-moleculesynthesis of Sondheimer–Wong diyne, which was organ-ically synthesized by Wong et al. in 1974 [37]. The diynemolecule has a distorted triple bond (Fig. 1(a)), which is whyit is highly reactive. In organic synthesis, it is used tosynthesize a more stable pentalene molecule fused with afive-membered ring. In this study, the reverse reaction wasperformed by a probe.With pentalene as the nucleus and naphthalene at bothends thereof, a precursor molecule substituted with bromineatoms at positions 6 and 13 of the five-membered ring wasused (Fig. 1(b)). High-purity molecules obtained by organicsynthesis were placed in a crucible and adsorbed onto aclean Cu(111) substrate by heat sublimation under ultra-highvacuum. When the temperature of the substrate reached roomtemperature (20 °C), the C–Br bond was broken due tothe catalytic effect of Cu. Therefore, the temperature of thesubstrate was maintained at about 20K for the adsorption ofthe molecules. Figure 2(a) is an STM topographic image ofthe precursor molecule adsorbed onto the substrate. Themolecule indicated by the arrow is the reactant, and whenmagnified, an asymmetric contrast derived from bromine wasobserved (Fig. 2(b)). Next, cleavage of the C–Br bond usingthe tunneling current was attempted. After positioning theprobe on the red mark, the control between the probe and thesample was turned off, and further, the bias voltage wasswept from 0V to a positive value while measuring thetunneling current. Above 2V, the tunneling current suddenlydecreased (Fig. 2(c)). After that, when STM observation wasperformed, the bromine atom was desorbed, and a dentedcontrast was observed instead (Fig. 2(d)). Furthermore,another bromine atom was also removed similarly (Figs. 2(e)and 2(f)). Synthesis of radicals with unpaired electrons canbe expected from this elimination reaction. However, whenthe structure was examined using an STM simulation imagebased on DFT calculations, it was found that the carbonbonded to bromine is stabilized by bonding with the Cu atomon the surface of the substrate. Therefore, it was found thatdiyne, the target compound, could not be synthesized onCu(111).Next, molecules were deposited onto a two-layer NaCl thinfilm to prevent stabilization with the Cu surface. The white4BrBr3XXX2(X = Br, I)12on-surfacereactionNaCl/Cu(a)(b)Fig. 1. (a) Sondheimer-Wong diyne 1 and pentalene derivative 2.(b) Precursor molecule 3 for local probe chemistry and its product 4.(c)(e)Sample Bias (V)2 nm0 143ΔZ (pm)(b)(a)0 140ΔZ (pm)(d)0 135ΔZ (pm)(f)0 136ΔZ (pm)I (nA)I (nA)0.500 2101(g) (h) (i)1 nm 1 nm 1 nmFig. 2. (a) STM topography of the precursor molecules adsorbed onCu(111) and (b) the close-up view. (c) I–V curve measured during thedebromination reaction. (d) STM topography after the debromination. (e) I–Vcurves measured during the second debromination reaction. (f) STMtopography after the second debromination. (g–i) Simulated STMtopographies and their adsorption geometries.JSAP ReviewJSAP Rev. 2022, 220410S. Kawai220410-2 © 2022 The Author(s)circles in Fig. 3(a) indicate single molecules. In order todirectly observe the structure, the tip of a probe wasterminated with a CO molecule, and an dI/dV image wasacquired at a constant height. As shown in Figs. 3(b) and3(c), the contrast reflected in the structure of the precursormolecule could be obtained. Next, the probe was moved overthe C–Br bond, and the bias voltage was gradually increased,similar as shown in Figs. 2(c) and 2(e). When the tunnelingcurrent decreased, the voltage returned and STM observationwas performed (Fig. 3(d)). At that point, precursor mole-cules, which used to have a cross-shaped contrast, became abent line. The probe was terminated again with a COmolecule and the structure was observed (Fig. 3(e)). As aresult, it was found that the two C–Br bonds had beenreduced to one (Fig. 3(f)). The remaining C–Br bonds couldalso be broken by a similar sweeping of the voltage.Subsequent observations confirmed two types of molecules.One is a slightly bent structure (Fig. 3(g)) and the other is acompletely straight-line structure (Fig. 3( j)). High-resolutionobservations of these structures revealed that both bromineatoms were detached. Analysis focusing on the axis of thenaphthalene site revealed that the original pentalene skeletonremained in the bent molecule (Figs. 3(h) and 3(i)). Incontrast, in the compound with a linear structure, the centralbond collapsed, and isomerization occurred, and the targetdiyne with two triple bonds was generated (Figs. 3(k) and3(l)). When DFT calculations were performed, it was foundthat these sequential reactions are endergonic reactionscaused by injecting energy (tunneling current) from theoutside. It was also found that the reaction barrier issignificantly reduced by temporarily charging the moleculeso that it is monovalent or divalent with electrons from theprobe. That is, in order to isomerize flat molecules witha probe, a NaCl thin film, which is a reaction field, isimportant.3.2 Addition reaction by probeNext, we will introduce an addition reaction, in which atomsand molecules are joined by a probe [22]. In this study, athree-dimensional GNR was synthesized, and “local probechemistry” was performed to manipulate the C–Br bondprotruding from the substrate with a probe. So far, GNRswith various width and edge states, and even differentintroduced elements have been synthesized [19]. However,all GNRs synthesized so far have a flat shape. Therefore, inorder to extend this to three dimensions, a propellanemolecule with a three-dimensional structure was used asprecursor (Fig. 4(a)). By vapor deposition onto Au(111) andAg(111) and further heating, we succeeded in synthesizinga three-dimensional organometallic compound (OMC) andGNR (Figs. 4(b) and 4(c)). Two bromine atoms, both pro-truding vertically from each unit, were observed. In addition,in 3D-GNR generated by heating to a high temperature of(a)0 1.02ΔZ (nm)10 nm(d)0 218ΔZ (pm)1 nmCOdip(radical)DissociatedBr(g)0 201ΔZ (pm)2 nmCO(j)0 196ΔZ (pm)3 nm(e)missing300 pmLow HighdI/dV(k)300 pmLow HighdI/dV300 pmLow HighdI/dV(b)300 pmLow HighdI/dV(h) Dissociated Br(c) (f) (i) (l)BrBrBr3 3a 3b 4Dissociated BrFig. 3. (a) STM topography of the molecule adsorbed on a thin NaCl film. (b) High-resolution image taken with a CO terminated tip and (c) thecorresponding chemical structure. (d) STM topography of the molecule taken after the first denomination. (e) High-resolution image and (f) the correspondingchemical structure. (g) STM topography of the molecule after the second debromination. (h) High-resolution image and (i) the corresponding chemicalstructure. ( j) STM topography of another product, (k) the corresponding high-resolution image, and (l) the chemical structure.(b) (c)0 895(81)ΔZ (pm)10 nm 0 887ΔZ (pm)5 nm(a)Fig. 4. (a) Precursor molecule, the intermediate, and product of the three-dimensional graphene nanoribbon (3D-GNR). (b) 3D-organometalliccompound (OMC) and (c) GNR.JSAP ReviewJSAP Rev. 2022, 220410S. Kawai220410-3 © 2022 The Author(s)400 °C, some C–Br bonds collapsed due to heat wereobserved.An addition reaction using this three-dimensional carbonnanostructure is described. First, an Au probe was positionedon the bromine atom, and a bias voltage was swept whilemeasuring the tunneling current (Fig. 5(a)). The currentdecreased at a voltage of 2.6V (Fig. 5(b)). Subsequently,when STM imaging was performed, the bromine atomdirectly under the probe was desorbed and became a radicalunit (Fig. 5(c)). Moreover, as the obtained contrast becameclearer, it was found that the probe was terminated by theremoved bromine atom. This elimination reaction is ex-tremely reproducible, and it is possible to sequentiallyremove bromine from a targeted site. It was also found thatthe voltage necessary for causing the elimination reaction isalmost constant. From the DFT calculations, it was foundthat an excitation of the π+ orbital of the molecule cleavesthe C–Br bond. Furthermore, it was also found that in thisthree-dimensional structure, the six-membered ring fromwhich bromine was desorbed was separated from the metalsubstrate, so that radicals can be maintained even without aNaCl thin film.Next, an addition reaction using a bromine probe isdescribed. First, an Au probe was brought into contact toadsorb a single atom of gold, which is a marker, onto theAu(111) surface (Fig. 5(d)). Subsequently, two bromineatoms were removed by an elimination reaction. Using thisprocess, a probe terminated with a bromine atom wasobtained (Fig. 5(e)). Next, while detecting the resonancefrequency, which is the signal of AFM, the Br probe wasgradually brought closer to the target radical in the verticaldirection. A sudden change was detected at Z = −380 pm(Fig. 5(g)). Subsequently, when 3D-OMC was observed, itwas found that the bromine atom had bonded (Fig. 5(f)). Inaddition, as the bromine atom at the tip of the probe hadmoved, it changed the contrast of the STM topographicimage obtained with the Au probe. By integrating themeasured frequency shift, the force and potential could beobtained. It was found that just before the addition reaction,the probe was already approaching the region in which therepulsive force is stronger than the maximum value of theattractive force (Fig. 5(h)). Furthermore, it was found that thepotential of about 100meV changes before and after theaddition reaction (Fig. 5(i)).Finally, there was a union with a fullerene molecule, whichresulted in a different molecule. After surface synthesis of3D-OMC, a fullerene molecule was adsorbed onto a surface(Fig. 6(a)). As this structure is larger than a bromine atom,both bromine atoms in the unit were removed (Fig. 6(b)). Forthe process, the probe was terminated with a bromine atom.As the fullerene molecule could not be picked up as it was, it(g)(h)(i)0debrominationdebromination0-4000-20-2001. Br tip2. Au tip1. Br tip2. Au tip1. Br tip2. Au tipF (pN)0-4000U (meV)0-300Z distance (pm)Z distance (pm)Z distance (pm)-400 -200-400 -200(a)0 6021 nm0 6291 nm(c)1. Au tip2. Br tip(b)I (pA)050Sample bias (V)320 1(f)0 6121 nmmarkerAuAu(e)0 6211 nmmarkerbrominationBrAu(d)0 6291 nmmarkerdebrominationBrAuBrAuBrominationDebrominationΔZ (pm)ΔZ (pm) ΔZ (pm)ΔZ (pm)ΔZ (pm)Δf (Hz)Fig. 5. (a) STM topography taken before the debromination. (b) I–V curve taken during the debromination. (c) STM topography after the debromination.(d–f) Bromination reaction by the local probe. (g) Frequency shift curve measured during the addition reaction, (h) the extracted force and ( j) potential curves.0 6261 nm 0 6631 nm0 8241 nm2. pick up3.implement1. debromination x 2diradical(a) (b)(c) (d)Au AuAuBrAuBrAuC60AuC60 islandMissing3D compoundsC60-propellaneΔZ (pm) ΔZ (pm)ΔZ (pm)Fig. 6. (a) STM topography of the 3D-OMCs and fullerene molecules.(b) Synthesis of the diradical species and (c) the complex of 3D-OMC andfullerene. (d) Schematic drawing of the product.JSAP ReviewJSAP Rev. 2022, 220410S. Kawai220410-4 © 2022 The Author(s)was once removed from Au (111) and then returned to theAu probe. Subsequently, we succeeded in synthesizing acomplex with a fullerene molecule and 3D-OMC by pickingup the fullerene molecule in the part indicated by the arrowand bringing it closer to the radical obtained by the earlierelimination reaction (Figs. 6(c) and 6(d)). This bond isconsidered to be due to the [2+2] addition reaction. In thisway, by making full use of “local probe chemistry”, werealized the introduction of different atoms and molecules tothe intended molecular site.4. ConclusionWe think it is no exaggeration to say that the “local probechemistry” introduced in this paper, which is capable ofmanipulating structures at the single-molecule level, is anew research area that has been pioneered along with thedevelopment of scanning probe microscopy. In the future,developments such as the synthesis of various novel com-pounds and their functional development, and further, themanufacture of novel devices using these can be expected.AcknowledgmentsThis research is the result obtained by joint research withProfessor Takashi Kubo of Osaka University, a researcher insynthetic organic chemistry, Professor Kyoko Nozaki of TheUniversity of Tokyo, Dr. Shinko Ito of Nanyang Techno-logical University, Singapore, Professor Adam Foster ofAalto University, Finland, a theoretical computationalscientist, and Professor Lev Kantorovich of Kings CollegeLondon, UK. I would like to express my deep gratitude toDr. Kewei Sun of our laboratory and Professor Ernst Meyerof the University of Basel, Switzerland, who is the groupleader of the author’s predecessor. Part of this research wasconducted with the support of the Grant-in-Aid for ScientificResearch (19H00856, 21K18885, 21F21058) of the (Inde-pendent) Japan Society for the Promotion of Science (JSPS).References[1] G. Binnig, H. Rohrer, Ch. Gerber, and E. Weibel, Phys. Rev. Lett. 49, 57(1982).[2] G. Binnig, C. F. Quate, and Ch. Gerber, Phys. Rev. Lett. 56, 930 (1986).[3] D. M. Eigler and E. Schweizer, Nature 344, 524 (1990).[4] D. M. Eigler, C. P. Lutz, and W. E. Rudge, Nature 352, 600 (1991).[5] M. F. Crommie, C. P. Lutz, and D. M. Eigler, Science 262, 218 (1993).[6] Y. Sugimoto, M. Abe, S. Hirayama, N. Oyabu, Ó. Custance, and S. Morita,Nat. Mater. 4, 156 (2005).[7] F. E. Kalff, M. P. Rebergen, E. Fahrenfort, J. Girovsky, R. Toskovic, J. L.Lado, J. Fernández-Rossier, and A. F. Otte, Nat. Nanotechnol. 11, 926(2016).[8] D. P. E. Smith, J. K. H. Hörber, G. Binnig, and H. Nejoh, Nature 344, 641(1990).[9] T. A. Jung, R. R. Schlittler, J. K. Gimzewski, H. Tang, and C. 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Kichin, C. Wagner, F. S. Tautz, R. Temirov, and P. Jelínek,Phys. Rev. B 90, 085421 (2014).[37] S. Kawai, H. Sang, L. Kantorovich, K. Takahashi, K. Nozaki, and S. Ito,Angew. Chem., Int. Ed. 59, 10842 (2020).Technical termsGraphene nanoribbonA one-atom layer carbon material with the width of a nanometer. It isattracting attention as a next-generation device element because can have avariety of electrical characteristics depending on its width and edge structure.Organometallic compoundGeneral term for a compound with a bond between a carbon and a metal. Theterm metal includes alkali metals, alkaline earth metals, transition metals,main group elements, and further, it also includes metalloids, such as silicon.[2+2] Addition reactionA cycloaddition reaction in which two different Π electron systems form afour-membered ring and fuse. It also occurs when fullerene moleculesdimerize.Sondheimer–Wong diyneA molecule organically synthesized by Wong, Garratt, and Sondheimer et al.in 1976.ProfileShigeki Kawai received his doctor degree inEngineering from the University of Tokyo in 2005.After he worked as a postdoc in EMPA, Switzerlandand a senior researcher in University of Basel,Switzerland, he became a principal researcher inNational Institute for Materials Science in 2016. In2020, he became a group leader as well as anassociate professor in University of Tsukuba.JSAP ReviewJSAP Rev. 2022, 220410S. 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