Oxidization stability of atomically precise graphene nanoribbons

The stability of graphene nanoribbons (GNRs) against oxidation is critical for their practical applications. Here we study both the thermal stability and the oxidation process of the ambient-exposed armchair GNRs with a width of seven carbon atoms (7-aGNR), grown on an Au(111) surface. The atomic scale evolution of the armchair edges and the zigzag ends of the aGNRs after annealing at different temperatures are revealed by using scanning tunneling microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. We observe evidence that the zigzag ends start to be oxidized and decomposed at 180 °C, while the armchair edges are intact at 430 °C but become oxidized at 520 °C. Two different oxygen species are identified at the armchair edges, namely the hydroxyl pair and the epoxy bonding motif with one oxygen bonded to two edge carbons. These oxidization species modify the electronic properties of the pristine 7-aGNRs, with a band-gap reduction from 2.6 to 2.3 eV and 1.9 eV for the hydroxyl pair- and epoxy-terminated edges, respectively. These findings demonstrate the oxidation stability of both the zigzag and armchair edges of GNRs, and they provide an opportunity to harness the high density of edge atoms in applications such as GNR-based high-temperature oxygen sensors.

Figure 1

STM images revealing the stability of the 7-aGNRs with annealing after ex situ air exposure. (a) Large-area STM image of a low-coverage as-grown 7-aGNR sample (sample bias $V_{\mathrm{s}}=-2.0\mathrm{V}$, tunneling current $I_{\mathrm{t}}=100\mathrm{pA}$). Inset: high-resolution STM image of the 7-aGNR, showing a typical end structure ($V_{\mathrm{s}}=-0.6\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$), superimposed with the atomic structural model. (b) STM image of the as-grown highly dense 7-aGNR sample ($V_{\mathrm{s}}=+1.0\mathrm{V}$, $I_{\mathrm{t}}=40\mathrm{pA}$). Inset: Two profiles acquired along the black (i) and red (ii) lines marked in (b) and (d), respectively. (c) STM image of the highly dense 7-aGNR sample in (b) after air exposure for ∼0.5 h and then 180 °C annealing for 2 h ($V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=20\mathrm{pA}$). The red arrows mark possible oxygen-containing molecules at the GNR and second-layer polymer edges. (d) STM image of the sample in (c) after 520 °C further annealing for 0.5 h ($V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=20\mathrm{pA}$). Inset: magnified STM image of the black boxed region in (d). The white and red boxes mark single ribbons, showing two kinds of defect structures at the GNR edges, respectively.

Figure 2

Raman spectroscopy study of the thermal stability of the 7-aGNRs after exposure to air. Raman spectra of the 7-aGNRs in the as-grown sample, and samples annealed to 180, 350, 430, and 520 °C, after air exposure for about 0.5 h for each measurement. The key Raman peaks are labeled. Vibrational patterns of three dominant Raman modes below $1000{\mathrm{cm}}^{-1}$ at $262{\mathrm{cm}}^{-1}$ (SLM), $396{\mathrm{cm}}^{-1}$ (BLM), and $955\mathrm{c}{\mathrm{m}}^{-1}$ (BLM3) are shown at the top as insets, labeled yellow for carbon and blue for hydrogen.

Figure 3

XPS spectra of the air exposed and sequentially annealed 7-aGNRs. (a) C $1s$ emission spectra, and (b) O $1s$ emission spectra of the 7-aGNRs, after air exposure (i.e., as inserted) and after 180 and 520 °C annealing. In (b), the double-peak features are fitted for the O $1s$ spectra. The corresponding oxygen species after 520 °C annealing are schematically illustrated as insets with yellow for carbon, blue for hydrogen, and red for oxygen.

Figure 4

STM study on the oxidization of the 7-aGNRs after in situ ${\mathrm{O}}_2$ exposure. (a,b) Large-area and magnified STM images of the pristine 7-aGNRs, (c,d) after ${\mathrm{O}}_2$ exposure at room temperature and 180 °C annealing, and (e,f) after 520 °C annealing. In (d), white arrows mark the partially decomposed ribbon ends. Setpoint: (a) $V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=20\mathrm{pA}$; (b) $V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$; (c) $V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=50\mathrm{pA}$; (d) $V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$; (e) $V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$; and (f) $V_{\mathrm{s}}=+1.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$.

Figure 5

Characterization of the structural and electronic properties of the oxidized 7-aGNRs. (a) STM image showing two kinds of defects, enlarged in the insets, at the oxidized 7-aGNR edges, with the white and red boxes marking the respective defect structures ($V_{\mathrm{s}}=+1.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$). (b) dI/dV spectra acquired at the cross-marked sites (sites i–iv) in (a), with a spectrum from the clean Au(111) surface as reference ($V_{\mathrm{s}}=-2.0\mathrm{V}$, $I_{\mathrm{t}}=100\mathrm{pA}$). The dI/dV curves are equally vertically separated for clarity, with a short solid line marking zero for each curve. Five peaks are marked as states 1–5. (c)–(f) Relaxed structural models and simulated STM images of the epoxy oxygen, ${\mathrm{O}}_{22}$, ${\mathrm{O}}_{12}$, ${\mathrm{O}}_{11}$, and the hydroxyl pair (2OH) at the armchair edge, respectively, where yellow, blue, and red represent carbon, hydrogen, and oxygen, respectively. (g) Simulated LDOSs of the epoxy oxygen and the hydroxyl pair, with the blue curves from the bulk 7-aGNRs containing the corresponding defect as reference.

Figure 6

Possible reaction paths for oxidation of armchair and zigzag edges. (a) Reaction paths for three possible configurations, 1, 2, and 3, of the adsorbed oxygen at the armchair edge. $\mathrm{T}{\mathrm{S}}_1$, $\mathrm{T}{\mathrm{S}}_2$, and $\mathrm{T}{\mathrm{S}}_3$ are the transition states for the reactions from 1, 2, and 3, respectively. (b) Reaction paths for two possible configurations, 4 and 5, of the adsorbed oxygen at the zigzag edge. $\mathrm{T}{\mathrm{S}}_4$ and $\mathrm{T}{\mathrm{S}}_5$ are the transition states for the reactions from 4 and 5, respectively.

Figure 7

Stability of the oxidization species. Calculated energies of the stable oxidation species in relation to the oxygen chemical potential. ${\mathrm{O}}_{11}$, ${\mathrm{O}}_{12}$, ${\mathrm{O}}_{22}$, and 2OH are considered (solid lines) for the armchair edge of the 7-aGNR, and ${\mathrm{O}}_{11}$, ${\mathrm{O}}_{12}$, and 2OH are considered (dashed lines) for the zigzag edge of the 4-zGNR (a zigzag GNR that is four carbon zigzag lines wide), as illustrated in Fig. 6. The chemical potential is measured relative to that of molecular oxygen. The positions of the oxygen chemical potentials in ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{O}}_2$, and CO are also marked on top of the diagrams. Inset: energies of two OH groups with different spacing distances at the aGNR edge. The nearest OH pair, located on the two neighboring carbon edge atoms, has the lowest energy.