Tuning the electronic properties of graphene oxide nanoribbons with armchair edges through lithium doping

Abstract

The electronic properties of armchair graphene oxide nanoribbons (AGONRs) doped with lithium atoms at one-edge, centre and both-edges, in comparison with the H-terminated cases are investigated using the local density approximation based on density functional theory. The results indicate that H-terminated AGONRs are a direct band gap semiconductor with an energy gap of 0.225 eV involving hybridization and charge transfer among C-2p and O-2s, 2p electrons near the Fermi level in the conduction band. The AGONRs for centre Li-doping is an indirect band gap semiconductor with an energy gap of 0.164 eV involving hybridization among C-2p, Li-2s, 2p and O-2s, 2p electrons and electrical conductivity of lithium atoms. The one-edge and both-edge Li-doped AGONRs is metallic with contributions from Li-2p, O-2p and H-1s electrons. Their band gaps decrease when doping with oxygen and lithium atoms. In addition, charge density difference calculations show charge transfer from C and Li atoms to O atoms. Thus, doping atoms and different doping configurations can be used to tune the band gap of H-terminated AGONRs.

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1. Introduction

Graphene nanoribbons (GNRs) are a promising material for electronic and optoelectronic applications, due to their remarkable novel electronic, transport and optical properties.^1–3 Field-effect transistors and micromechanical resonators based on GNRs have already been fabricated.¹ The different edge configurations of GNRs determine their electronic properties: while zigzag edge graphene nanoribbons (ZGNRs) have peculiar localized electronic states at each edge under a magnetic field,^4,5 AGNRs are non-magnetic semiconductors with band gaps.⁵ The electronic properties of GNRs, including ZGNRs and armchair edge graphene nanoribbons (AGNRs), have been explored by Ezawa.⁶ GNRs present very rich edge chemistry and show distinct electronic properties. The impacts of incorporation of impurities, defects and different edges on GNRs electronic properties have been widely explored.^5,7 The adsorption of gas molecules on GNRs have been studied using a first principles method⁸ and the doping atoms and adsorption atoms have been found to play an important role.

Graphene oxide nanoribbons (GONRs) have interesting properties and unique structure, and call for further studies. Field-effect transistors based on single sheets of GONRs have been studied by Wang et al.⁹ and it has been revealed that devices with different thicknesses show semiconducting behaviours and ambipolar field effect transistor properties. GONRs have oxygen atoms substituting carbon atoms in their surface. GONRs can be reduced to the functional groups using a chemical reduction approach. They were prepared using electrospun polymer-based nanowires as the etching mask by He et al.¹⁰ Experimental studies show that GONRs are wide band gap semiconductors that can be reduced to metallic graphene nanoribbons.¹¹ Liu et al.¹² found that the GNRs are covered by a nanocrystalline lithium (Li) oxide layer attached to the surfaces and edges of the GNRs.

Recently, Li-doped AGNRs have been investigated by our group.¹³ We found that the AGNRs would become metallic with lithium-doping. The size-dependent electronic structure of oxygen-terminated zigzag graphene nanoribbons was studied using standard density function theory by Ramasubramaniam¹⁴ who predicted the semiconducting behaviours of oxygen-terminated zigzag graphene nanoribbons. The interactions between boron and nitrogen dopant atoms in graphene were studied by Al-Aqtash et al.¹⁵ It was shown that the B–B and N–N interactions were repulsive and the B–N interaction was attractive. The charge transfer between the molecules and the graphene surface was studied using first-principles calculations by Leenaert et al.¹⁶ However, both Li doped GONRs and armchair graphene oxide nanoribbons (AGONRs) still need further researches.

In this work, we investigate the electronic properties of Li-doped AGONRs in different configurations, using first principles calculations, following the definition of the charge density difference and Li-doped AGNRs in different configurations in the previous work.¹³ In particular, we examine the stability of Li-doped AGONRs in three different configurations and the tunability of their electronic structures.

2. Modeling and computational methods

The charge density difference, electronic energy band and projected density of states of Li-doped AGONRs in different doping configurations were calculated using density function theory (DFT) under the local density approximation (LDA) with norm-conserving pseudopotential for exchange-correlation functional. The periodic supercell calculations were performed using the QUANTUM ESPRESSO package.¹⁷ The plane-wave cutoff energy was set to 125 Ry, with 500 Ry for the charge density to achieve satisfactory calculation efficiency and accuracy. The sampling of the Brillouin zone (BZ) was done using a 1 × 1 × 21 Monkhorst–Pack grid with testing to give converged results. In order to avoid interaction between the nanoribbon and its periodic image, the AGONRs were modeled to maintain the periodic boundary condition along the z-direction and set at 6.8 Å and 18 Å along the x and y axis, respectively.¹³ The AGONRs produced by substituting carbon atoms in the supercell by oxygen atoms with H-termination edges are shown in Fig. 1(a). For AGONRs, the predicted average equilibrium C–C bond length is 1.403 Å, which agrees with the experimental value in graphene. Lithium-doped AGONRs with three different configurations (one-edge, centre and both-edge), by substituting carbon atoms in the supercell by Li atoms and oxygen atoms, are shown in Fig. 1(b)–(d). The optimization of the Li-doped AGONRs is carried out for all the internal structural parameters by the quasi-newtonian algorithm and minimization. The valence electronic states C-2s, 2p, Li-2s, 2p, O-2s, 2p and H-1s are considered in our calculations.

Fig. 1 Schematic diagram of H-terminated AGONRs and Li-doped AGONRs with different configurations: (a) H-terminated, (b) one-edge doped, (c) centre doped, and (d) both-edge doped.

3. Results and discussion

3.1 Stability analysis

The equilibrium geometries of Li-doped AGONRs are examined. For both-edges Li-doped AGONRs, an equilibrium Li–C bond length of 1.944 Å, which is close to the experimental value of 2.13 Å,¹⁸ is obtained. The bond length between C and Li atoms is different for centre doping due to neighbouring O atoms. Our calculations determine the equilibrium C–O bond lengths for one-edge, centre and both-edges doping to be 1.297, 1.360, and 1.309 Å respectively. The bond length of Li–O equals to 1.568 Å, 1.565 Å in one-edge and both-edges Li-doped AGONRs, respectively. The centre Li-doping results in a larger bond length with its neighbouring C atoms. These results are close to previous theoretical work on Li absorbed graphene.¹⁹ It is shown that the edge bond lengths depend strongly on the doping configuration.

3.2 Band structures

The calculated charge density differences and their plots for different Li-doping configurations are shown in Fig. 2 and 3. The charge density difference of H-terminated AGONRs is obtained by subtracting the total charge density of H-terminated AGNRs.^20,21 The charge plots indicate that the charge density is reduced around C and H atoms and increased around O atoms, as shown in Fig. 2(a). The analysis of charge density shows that there is a charge transfer of about 0.8428e⁻/ų from C and Li atoms to O atoms. Fig. 2(b)–(d) shows that the charge density is higher near O atoms and lower near Li atoms. It is shown that strong hybridization between the Li and O atoms is induced. Chemical bonds are formed between Li and C, Li and O due to charge density transfer among O, C and Li atoms. The bonding interaction between the Li and H atoms are reduced due to strong localization, as shown in Fig. 2(b) and (d). Thus, the electronic properties of the system are tuned due to the charge transfer by doping atoms.

Fig. 2 (a) The isosurface plots of the difference charge density for (a) H-terminated AGONRs and (b) one-edge, (c) centre, (d) both-edge Li-doped AGONRs. Isovalues: 0.005 and −0.005 a.u.

Fig. 3 Quantity plots of the difference charge density for (a) one-edge, (b) centre, (c) both-edge Li-doped AGONRs.

The band structures of H-terminated AGONRs and all the Li-doped ribbons are presented in Fig. 4(a)–(d). Fig. 4(a) shows that the H-terminated AGONRs is a direct band gap semiconductor with an energy gap of 0.225 eV. The results for H-terminated AGONRs are in excellent agreement with experimental studies.¹¹ Due to the presence of oxygen atoms, six extra bands compared with the bare AGNRs¹³ are induced near the Fermi level which is marked by the red dashed line in Fig. 4(a). Fig. 4(c) shows that centre Li-doped AGONRs is an indirect band gap semiconductor with an energy gap of 0.164 eV. In centre Li-doped, the valence band near Fermi level of H-terminated AGONRs crosses into the conduction band, different from that of the H-terminated AGONRs. The result shows that the electronic properties of H-terminated AGONRs are tuned. The Li-doped AGONRs with one-edge and both-edge become metallic with their Fermi level crossing the conduction band, as shown in Fig. 4(b) and (d). Notably the band gap of H-terminated AGONRs decreases with doping atoms and different doping configurations and eventually becomes metallic. Since additional p_z electrons of the edge C atoms participate in π* bonds with Li and O atoms, the energy bands appear near the Fermi level. In H-terminated and Li-doped AGONRs, additional electronic states appear in the valence band and in the conduction band, which are attributed to Li-2p, O-2p and H-1s electron states, reduce the band gap in H-terminated AGONRs and possibly give rise to metallic in Li-doping AGONRs. Near the Fermi level the valence band is disturbed due to Li and O doping atoms for Li-doping AGONRs. The both-edge Li-doped ribbons have more conducting channels due to having the most extra conductive bands, as shown in Fig. 4(d). Among all the considered ones, the H-terminated and Li-doped AGONRs exhibits smaller band gap, metallic and better electronic properties.

Fig. 4 Calculated band structures of (a) H-terminated AGONRs and (b) one-edge, (c) centre, (d) both-edge Li-doped AGONRs.

3.3 Analysis of density of states

The calculated electronic projected density of states (PDOS) near the Fermi level for H-terminated and Li-doped AGONRs are shown in Fig. 5. The C-2p and O-2p electrons contribute to the upper valence band and lower conduction band near the Fermi level in the H-terminated AGONRs as shown in the Fig. 5(a). The H-terminated AGONRs present a localization state which is caused by the O-2p, 2s electronic states at −2.14 eV as shown in Fig. 5(a). Strong hybridization is produced among C-2p and O-2s, 2p electrons, enhancing conductivity of the ribbons, reducing band gap and tuning electronic properties near the Fermi level. These results are explained in term of charge transfer between C and O atoms. The electronic properties of the H-terminated AGONRs are therefore superior to the bare AGNRs.¹³ The PDOS of centre Li-doped AGONRs shows a peak in the valence band due to C-2p and O-2p electrons, whereas Li-2s, 2p electrons contribute to electronic states near the Fermi level in Fig. 5(c). Furthermore, strong hybridization is induced among C-2p Li-2s, 2p and O-2s, 2p electrons near the Fermi level. Upon centre Li-doping, the contribution from C-2p electrons increases near Fermi level.

Fig. 5 Projected density of states of (a) H-terminated AGONRs and (b) one-edge, (c) centre, (d) both-edge Li-doped AGONRs.

As shown in Fig. 5(b) and (d), the density contribution of energy bands of Li-doped AGONRs are mainly induced by C-2p, O-2p, H-1s and Li-2s, 2p states. A number of additional electronic states are located near the Fermi level due to hybridization among C-2p, O-2p and Li-2s, 2p electrons in the doping structures. This is due to the charge transfer between the C, O and Li atoms. These energy bands cross the Fermi level and make the Li-doped AGONRs metallic. In the one-edge Li-doped ribbons, the sharp peak in the conduction band is induced by electronic states of C-2p, while the Li-2s, 2p, and H-1s electronic states induce the sharp peak and promote the conductive properties for the conduction band of the both-edge Li-doped AGONRs. The sharp peaks corresponding to localization states at −2.48 eV and −5.77 eV in the valence band are mainly induced by the C-2p electron state for both-edge Li-doped AGONRs. The other peaks for electric conduction are produced by the O-2p state at −3.53 eV. These occur because Li, O and H atoms provide an extra electron to saturate the dangling σ electron of carbon atoms at the armchair edge. A quantum confinement effect results in due to the impurities. The other localization state is induced by Li and H atoms at −8.56 eV and −8.53 eV for the one-edge and both-edge doped AGONRs, respectively, and is weaker compared with that in our previous work¹³ due to the O-2s, 2p electronic states. The PDOS of centre Li-doped AGONRs shows a peak in the valence band due to C-2p and O-2p electrons, whereas Li-2s, 2p electrons only contribute to electronic states near the Fermi level.

4. Conclusions

In summary, the effects of Li atoms on the structural and electronic properties of H-terminated AGONRs have been examined. The charge density is reduced around C and H atoms and increased around O atoms. Our results show strong hybridization in the conduction band and valence band, and many additional electronic states are induced by the C-2p, O-2s and Li-2s, 2p electrons of doping Li and O atoms near the Fermi level. The one-edge and both-edge Li-doped AGONRs become metallic because of Fermi level crossing in the conduction band. The O-2s, 2p electronic states cause localization state to be induced by the Li and H atoms. It can be seen that AGONRs and Li-doped AGONRs provide a wide range of possible electronic properties based on the same ribbon structure but with different doping configurations.

Acknowledgements

This work is supported by National Natural Science Foundation of China No. 11464034 and Doctoral Scientific Research Foundation of Inner Mongolia University for Nationalities No. BS319. It is also supported by National Natural Science Foundation of China No. 11304143 and Natural Science Foundation of Inner Mongolia No. 2013MS0807.

Notes and references

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