On-Surface Synthesis of NBN-Doped Zigzag-Edged Graphene Nanoribbons

Abstract

We report the first bottom-up synthesis of NBN-doped zigzag-edged GNRs (NBN-ZGNR1 and NBN-ZGNR2) through surface-assisted polymerization and cyclodehydrogenation based on two U-shaped molecular precursors with an NBN unit preinstalled at the zigzag edge. The resultant zigzag-edge topologies of GNRs are elucidated by high-resolution scanning tunneling microscopy (STM) in combination with noncontact atomic force microscopy (nc-AFM). Scanning tunneling spectroscopy (STS) measurements and density functional theory (DFT) calculations reveal that the electronic structures of NBN-ZGNR1 and NBN-ZGNR2 are significantly different from those of their corresponding pristine fully-carbon-based ZGNRs. Additionally, DFT calculations predict that the electronic structures of NBN-ZGNRs can be further tailored to be gapless and metallic through one-electron oxidation of each NBN unit into the corresponding radical cations. This work reported herein provides a feasible strategy for the synthesis of GNRs with stable zigzag edges yet tunable electronic properties.

Keywords: NBN doping; graphene nanoribbons; on-surface synthesis; radical cations; zigzag edges.

Figure 1

a) Structures of dibromo‐dimethyl‐biphenylbenzo[m]tetraphene (DBBT) and 6‐ZGNR.4b b) Structures of pristine fully‐carbon‐based ZGNR1 (PCZGNR1) and NBN‐ZGNR1. c) Structures of PCZGNR2 and NBN‐ZGNR2. d) NBN‐dibenzophenalene (NBN‐DBP) and its radical cation as well as its isoelectronic structures.10 e) On‐surface synthetic routes toward NBN‐ZGNR1 and NBN‐ZGNR2.

Scheme 1

Synthetic routes toward M1 and M2.

Figure 2

a) HR MALDI‐TOF mass spectrum of M1. b) ¹H NMR spectrum (300 MHz, 273 K, C₂D₂Cl₄) of M1, insert: assignment for each proton. c) HR MALDI‐TOF mass spectrum of M2. d) ¹H NMR spectrum (300 MHz, 373 K, C₂D₂Cl₄) of M2, insert: assignment for each proton.

Figure 3

a) STM image of M1 as sublimed on Au(111). b) STM image of M1 after annealing at 200 °C on Au(111), inducing deiodination and polymerization. c) STM image of M1 after annealing at 450 °C on Au(111). d) nc‐AFM image of NBN‐ZGNR1. e) STM image of M2 after annealing at 450 °C on Au(111), showing the formation of NBN‐ZGNR2. f) nc‐AFM image of NBN‐ZGNR2. Scanning parameters: (a)–(c) and (e) V=−500 mV and I=30 pA. (d,f) amplitude=100 pm.

Figure 4

STS and calculated band structures of NBN‐ZGNR1 and NBN‐ZGNR2. a) Differential conductance (dI/dV) spectra taken on NBN‐ZGNR1, CB: conduction band, VB: valence band. b) DFT‐calculated band structure and density of states (DOS) results of NBN‐ZGNR1. c) Differential conductance (dI/dV) spectra taken on NBN‐ZGNR2. d) DFT‐calculated band structure and DOS results of NBN‐ZGNR2.

Figure 5

a), b) DFT‐calculated band structures of pristine PCZGNR1 and PCZGNR2. The bands of different spins are degenerated. c), d) Chemical structures of NBN‐ZGNR1 and NBN‐ZGNR2 radical cations. e), f) DFT‐calculated band structures of NBN‐ZGNR1 radical cations and NBN‐ZGNR2 radical cations in which each NBN unit loses one electron. The bands of different spins are degenerated.