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. 2020 Feb 28;6(9):eaay8913.
doi: 10.1126/sciadv.aay8913. eCollection 2020 Feb.

Three-dimensional Graphene Nanoribbons as a Framework for Molecular Assembly and Local Probe Chemistry

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Free PMC article

Three-dimensional Graphene Nanoribbons as a Framework for Molecular Assembly and Local Probe Chemistry

Shigeki Kawai et al. Sci Adv. .
Free PMC article

Abstract

Recent advances in state-of-the-art probe microscopy allow us to conduct single molecular chemistry via tip-induced reactions and direct imaging of the inner structure of the products. Here, we synthesize three-dimensional graphene nanoribbons by on-surface chemical reaction and take advantage of tip-induced assembly to demonstrate their capability as a playground for local probe chemistry. We show that the radical caused by tip-induced debromination can be reversibly terminated by either a bromine atom or a fullerene molecule. The experimental results combined with theoretical calculations pave the way for sequential reactions, particularly addition reactions, by a local probe at the single-molecule level decoupled from the surface.

Figures

Fig. 1
Fig. 1. Synthesis of 3D-GNR.
(A) Ullmann-type on-surface chemical reaction. (B) Chemical structure of 6Br-TNP. Scanning tunneling microscopy (STM) topographies after annealing at (C) 180° and (D) 400°C. Inset in (C) shows the dissociated bromines on Au(111). (E and F) Corresponding close-up STM topographies. Green arrows indicate the thermally disrobed bromine sites. Sketch of the on-surface chemical reaction with 6Br-TNP: (G) schematic drawings of 6Br-TNP, (H) 3D-OMC, and (I) 3D-GNR. Measurement parameters: sample bias voltage, V = 200 mV. Tunneling current: I = 0.8 pA in (C) and (D), I = 1 pA in (E), and I = 0.8 pA in (F).
Fig. 2
Fig. 2. Local probe chemical reaction.
(A to E) A series of STM topographies during the stepwise debromination and the corresponding I-V curves during the debromination. (F) AFM image taken after further debromination in (E). (G to I) A series of tip-induced debromination and bromination. (J) Corresponding frequency shift, (K) force, and (L) energy curves during the tip-induced bromination. Measurement parameters: V = 200 mV and I = 1 pA in (A) to (I), and V = −2 mV and oscillation amplitude A = 60 pm in (J).
Fig. 3
Fig. 3. Synthesis of C60-propellane complex.
(A) STM topography of a C60 island and 3D-OMC, taken with a Au tip. Green arrows indicate sites of the debromination. (B) STM topography after twice of the debromination, taken with a Br tip. A black arrow indicates the C60 to be picked up from the island. A green arrow indicates the site to implement the C60 from the tip. (C) STM topography after the synthesis of C60-propellane complex, taken with a Au tip. Insets show the schematic drawings of the tip apex. (D) Schematic drawing of C60-propellane complex. Measurement parameters: V = 200 mV and I = 2 pA in (A) and (B), and V = 1500 mV and I = 2 pA in (C).
Fig. 4
Fig. 4. Theoretical explanation of debromination and (re)bromination.
(A) Geometry of a single unit of the OMC on the Au(111) surface with four gold adatoms (depicted in turquoise) and Au[111] tip set 4.5 Å above the reacting Br atom (blue). (B) Energy paths for NEB calculations for sample debromination (left to right) or bromination (right to left) for two tip heights, 3.5 and 4.5 Å. The calculated mechanical barrier for the debromination is 1 to 1.5 eV. The bromination needs only about 0.4 eV for the reaction to proceed. (C) Product of debromination with reacted Br atom (blue) attached to the tip. (D) Schematics showing presumed mechanism for the debromination—injection of electrons into antibonding state(s)—for (i) metal tip, (ii) a Br tip with gap around the Fermi level, and (iii) Br tip whose electrostatics are shifting the antibonding state to the higher energies. Both mechanisms (ii) and (iii) leads to larger bias for reaching the antibonding state(s). (E) 3D representation of electron density for unpopulated π* and σ* states with Au tip set 3.5 Å above the bromine atom. (F) Atom projected density of states (PDOS) for both Br atoms of the OMC for two cases when the tip is close (3.5 Å above the Br 1 atom) or far (10.5 Å). The inset shows, in detail, the density of the unpopulated antibonding states. The PDOS shows that the π* state has the same energy for both Br atoms without any tip distance dependency, while the σ* state is at a slightly higher energy and appears only for a very low Br-Au distance. a.u., arbitrary units.

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