Q. G.
Jiang
a,
J. F.
Zhang
*a,
Z. M.
Ao
*b and
Y. P.
Wu
a
aCollege of Mechanics and Materials, Hohai University, Nanjing 210098, China. E-mail: jfzhang@hhu.edu.cn
bCentre for Clean Energy Technology, School of Chemistry and Forensic Science, University of Technology Sydney, PO Box 123, Broadway, Sydney, NSW 2007, Australia. E-mail: zhimin.ao@uts.edu.au
First published on 3rd March 2015
The thermal stability of silicene/silicane nanoribbons (SSNRs) has been investigated by using density functional theory calculations, where silicane is the fully hydrogenated silicene. It was found that the minimum energy barriers for the diffusion of hydrogen atoms at the zigzag and armchair interfaces of SSNRs are 1.54 and 1.47 eV, respectively, while the diffusion of H atoms at both interfaces is always endothermic. Meanwhile, the minimum diffusion energy barriers of one H atom and two H atoms on pristine silicene are 0.73 and 0.87 eV, respectively. Therefore, the thermal stability of SSNRs can be significantly enhanced by increasing the hydrogen diffusion barriers through silicene/silicane interface engineering. In addition, the zigzag SSNR remains metallic, whereas the armchair SSNR is semiconducting. However, the silicene nanoribbons part-determine the metallic or semiconducting behaviour in the SSNRs. This work provides fundamental insights for the applications of SSNRs in electronic devices.
Alternatively, high quality SNRs might be fabricated by selectively hydrogenating silicene, as proposed for the graphene system.24,25 A band gap opening in graphene with the patterned absorption of atomic hydrogen was recently found experimentally,26 which indicates that this may happen in the silicene system as well. Although experiments on hydrogenating silicene are absent, hybrid silicene/silicane nanoribbons (SSNRs) were studied by ab initio calculations.27 It was shown that the electronic and magnetic properties of SSNRs strongly depend on the degree of hydrogenation of the interface.27 However, the hydrogen diffusion associated with high mobility of the isolated H atoms on pristine silicene strongly affects the stability of the silicene/silicane interface, which needs to be clarified.
In this work, we study the stability of the silicene/silicane interface in hybrid nanoribbons by calculating the diffusion barrier of H atoms located at the silicene/silicane interface using density functional theory (DFT). All the possible diffusion pathways are analysed to find the minimum diffusion barrier, and therefore to provide a reference for designing viable silicene electronic devices that possess high thermal stability at standard operating conditions.
The stability of two hydrogen atoms on pristine silicene was also considered, as shown in Fig. 2. After careful examination, we found that the most favourable configuration is that two H atoms adsorbed on the two Si atoms next to each other on opposite sides of the silicene, as shown in Fig. 2(a). There are four possible diffusion pathways for the H atom at the 0 site, i.e., from site 0 to site 1 (path 1), from site 0 to site 2 (path 2), from site 0 to site 3 (path 3) and from site 0 to site 4 (path 4). The diffusion barriers for the H atom at site 0 are 0.97, 1.06, 1.37 and 0.87 eV for pathways 1–4, respectively, where the minimum diffusion barrier is 0.87 eV along path 4. Therefore, with the presence of another H atom nearby, the diffusion barrier increases from 0.73 to 0.87 eV, but the barrier is still lower than the critical barrier of Ecbar = 0.91 eV,37 indicating the possible high mobility of H atoms on silicene at room temperature.
Inspired by the stability enhancement of H atoms at the graphene/graphane interface,38 we considered the diffusion of H atoms at the silicene/silicane interface. The supercells used for the zigzag and armchair SSNRs are shown in Fig. 3(a) and (b), respectively. We minimized the interlayer interaction by allowing a vacuum width of 20 Å normal to the layer. For both types of nanoribbons, the Si atoms are displaced from the Si plane by about 0.25 Å due to the bonded H atoms. This value is similar to the shift of 0.30 Å that Si atoms experience when a H2 molecule undergoes dissociative adsorption on silicene.12 In both cases, this is a consequence of the change in the hybridization of the Si atoms from sp2 in silicene to sp3 in silicane. In addition, for the zigzag SSNR, both the silicene and the silicane nanoribbons are flat [see Fig. 3(a)]. However, the silicene and silicane layers are not in the same plane; they are connected with an angle of about 153° at the interface, which is similar to the previous reports for zigzag graphene/graphane nanoribbons.38 For the armchair SSNR [see Fig. 3(b)], the silicene and silicane regions are almost in the same plane with little curvature in the silicene nanoribbon.
We then analysed the stability of the two types of interfaces by calculating the diffusion barriers for hydrogen atoms at the interfaces. For the case of a zigzag interface, there are two types of Si and H atoms, denoted as sites A and B in Fig. 3(a). For the diffusion of the H atom at site A, there are two possible diffusion pathways labelled as 1 and 2 in Fig. 3(a). At site B, there are three possible diffusion pathways for the H atom that are labelled as 3, 4, and 5. In the case of an armchair interface, all the Si atoms at the interface are equivalent from a diffusion point of view. So there are five different diffusion pathways that we label as 6–10 in Fig. 3(b). When analysing the diffusion paths, we find that all of the diffusion is along linear pathways and also that the H atom is free without directly binding to any Si atom in the transition state (TS). The PDOSs of Si and H at sites A, B and C are also plotted in Fig. 3(c)–(e), which shows that the strength of the Si–H bond is the largest at site A, while that of the Si–H bond at sites B and C is similar but weaker from the weight of the overlap of the Si and H bands. It indicates that H atoms at sites B and C are more active.
The diffusion barriers of both types of silicene/silicane interfaces with different paths are summarized in Table 1. For the zigzag interface, the diffusion barriers at site A are 1.75 and 2.05 eV for pathways 1 and 2, respectively, where the former is the minimum diffusion barrier. The diffusion barriers at site B are 1.54, 1.56 and 2.25 eV for pathways 3–5, respectively, where the minimum diffusion barrier at site B is 1.54 eV along path 3. As a result, site A is more stable than site B. After LST/QST and NEB calculations, the detailed reaction pathway and the energy barrier for hydrogen diffusion along path 3 is shown in Fig. 4(a) where the initial state (IS), the final state (FS), and the atomic structure of the transition state (TS) are given. For the armchair interface, the energy barriers at site C are 1.47, 1.69, 2.15, 1.61 and 2.11 eV for diffusion pathways 6–10, respectively. Thus, the H diffusion path on armchair interfaces with the minimum energy barrier of 1.47 eV from site C to the second nearest Si atom is path 6. The corresponding reaction pathway and energy barrier are present in Fig. 4(b). Since the occurrence of the surface reaction needs Ebar > Ecbar = 0.91 eV at ambient temperature,37 both the zigzag and armchair interfaces are stable at room temperature.
Diffusion pathway | E bar (eV) | E r (eV) | E bar′ (eV) | ||
---|---|---|---|---|---|
Zigzag interface | A | 1 | 1.75 | 1.00 | 0.75 |
2 | 2.05 | 1.68 | 0.37 | ||
3 | 1.54 | 0.88 | 0.66 | ||
B | 4 | 1.56 | 0.92 | 0.64 | |
5 | 2.25 | 0.59 | 1.66 | ||
Armchair interface | C | 6 | 1.47 | 0.70 | 0.77 |
7 | 1.69 | 1.19 | 0.50 | ||
8 | 2.15 | 0.79 | 1.36 | ||
9 | 1.61 | 1.26 | 0.35 | ||
10 | 2.11 | 0.43 | 1.68 |
In light of the above analysis, we can see that the minimum diffusion barriers for both the armchair and zigzag interfaces are almost 2 times larger than the energy barrier for H diffusion on pristine silicene. From the diffusion energy in Table 1, the total energy increases ∼1 eV after diffusion for all the cases. At the same time, the reverse diffusion energy barrier Ebar′ in Table 1 is much lower than the corresponding diffusion barrier Ebar. Therefore, the exothermic reverse diffusion is energy preferred with a lower diffusing energy barrier, which confirms the enhanced stability of H atoms at silicene/silicane interfaces from another side. Note that the backward barriers (Ebar′) for H diffusion are defined as the energy difference between the final state and the TS state, and can be obtained from Table 1 as the difference between the diffusion barrier Ebar and the reaction energy Er.
Such stability enhancement can be understood by calculating the binding energy of the H atoms under different conditions, which are proportional to the strength of the Si–H bonds. The binding energies (Eb) were calculated by Eb = Ei − (Ef + EH), where Ei is the initial energy of the system, Ef is the energy of the system after removing the H atom, and EH is the energy of an isolated H atom. For the zigzag interface, we found that the binding energy of the Si–H bond at sites A and B are −3.80 and −3.34 eV, respectively. While for a H atom at site C of the armchair interface, this value is −3.23 eV. All of these values are larger than the binding energy of an isolated H atom on a silicene supercell containing 18 Si atoms (−2.31 eV). This indicates the stability enhancement of H atoms at silicene/silicane interfaces. The results of the binding energies also explain why it is easier to move the H atoms from site B (Eb = −3.34 eV) than from site A (Eb = −3.80 eV) in the zigzag interface, and why moving the H atoms from site C (Eb = −3.23 eV) in the armchair interface is slightly easier than that from site B (Eb = −3.34 eV) in the zigzag interface. As a result, the zigzag interface is slightly more stable than the armchair interface.
To further understand the higher stability of the H atom at site A, we analyse the Mulliken atomic charges of Si and H atoms at different sites. Table 2 gives the atomic charges of atoms near the interfaces. We can see that at both interfaces (i.e., at sites A, B, and C) Si atoms are less positive and the corresponding H atoms are less negative than those in silicane away from the interfaces. Furthermore, it also shows that both interfaces mainly affect the charge distribution of the first row of atoms at the interfaces. This result agrees with the fact that an interface influences mainly the atoms of the first two rows.38,39 It is known that the atomic charge is mostly affected by the atoms belonging to the same silicon ring, especially the nearest atoms. For the silicon and hydrogen atoms at site A, they have similar nearest atoms as sites in the silicane region far apart from the interface, where the three nearest Si atoms are bonded by sp3 orbitals. For the Si and H atoms at site B, only two nearest Si atoms are bonded by sp3 orbitals; the third on its right hand side at site 3 is bonded by sp2 orbitals. Therefore, the effect of the interface on site B is stronger than that on site A. On the other hand, for both sites A and B, there are three Si atoms bonded by sp2 orbitals in the silicon ring. Thus, the charge distribution of the atoms on both sites is changed by the interface. A similar reasoning can be applied to the charge difference on the atoms at site C of the armchair interface. Therefore, the H atom at site B (−0.053e) and that at site C (−0.050e) are more chemically active than that at site A (−0.062e) due to the weak Si–H bond strength. In addition, one H atom at site 0 (−0.035e) on pristine silicene is less charged than that at the interface of SSNRs, indicating the lower diffusion barrier for the hydrogen atom on pristine silicene.
Atom site | Si atom | H atom | |
---|---|---|---|
Zigzag interface | A | 0.066 | −0.062 |
B | 0.064 | −0.053 | |
d | 0.071 | −0.065 | |
e | 0.067 | −0.067 | |
f | 0.068 | −0.067 | |
1 | −0.006 | ||
2 | −0.007 | ||
3 | −0.006 | ||
4 | −0.007 | ||
5 | −0.006 | ||
Armchair interface | C | 0.064 | −0.050 |
g | 0.065 | −0.064 | |
h | 0.067 | −0.066 | |
i | 0.068 | −0.067 | |
6 | −0.011 | ||
7 | 0.001 | ||
8 | 0.001 | ||
9 | −0.001 | ||
10 | −0.001 | ||
One H on pristine silicene | 0 | 0.064 | −0.035 |
Two H on pristine silicene | 0 | 0.062 | −0.046 |
To understand the electronic properties of SSNRs, the band structures of zigzag and armchair SSNRs are calculated and the results are shown in Fig. 5. For the zigzag SSNR in Fig. 5(b), the Fermi level crosses over the conduction band due to the electron inefficiency, indicating n-type doping. From the charge distribution of the LUMO state, we can see that the conduction band across the Fermi level is mainly contributed to by the silicene nanoribbon. The PDOS of the Si atom [site d in Fig. 3(a)] in the silicane part, and Si atom [site 2 in Fig. 3(a)] in the silicene part are shown in Fig. 5(a). Clearly, the conduction band around the Fermi level is mostly contributed to by the Si-2p states in the silicene part, consistent with the charge distribution of the LUMO state. The silicene nanoribbon has high mobility due to its delocalized nature, which implies that the zigzag SSNR should be highly conductive.
For armchair SSNRs [see Fig. 5(d)], the Fermi level moves down to the exact top of the valence band and the armchair SSNR changes to become a pristine semiconductor with a band gap of 0.30 eV. The charge distribution of the HOMO and LUMO states indicate that the band gap is mainly determined by the silicene nanoribbon. The PDOS of the Si atom [site h in Fig. 3(b)] in the silicane part, and the Si atom [site 7 in Fig. 3(b)] in the silicene part is shown in Fig. 5(c), where the conduction band minimum (CBM) and valence band maximum (VBM) are mostly contributed to by Si-2p states in the silicene part. This is consistent with the charge distribution of the HOMO and LUMO states. Therefore, the zigzag SSNRs maintain a metallic character while the armchair SSNRs turn out to be semiconducting.
This journal is © The Royal Society of Chemistry 2015 |