Hydrogenations and electric field induced magnetic behaviors in armchair silicene nanoribbons
Hydrogenations and electric field induced magnetic behaviors in armchair silicene nanoribbons
OPEN Using the first-principles calculations, we investigate the geometric, electronic and magnetic properties of armchair silicene nanoribbons with different edge hydrogenations. Our results show that the interesting magnetic behaviors such as the bipolar magnetic semiconductor can be found. Moreover, the addition of the transverse electric field can modulate the bipolar magnetic semiconductor to halfmetal or spin-splitting metal. And the spin-up electrons are localized at one edge, the spin-down holes localized at the opposite edge under the external electric field. These results may present a new avenue for band engineering of silicene nanoribbons and benefit the design of silicon-based nano-spin-devices in nanoelectronics.
In the past decade, the successful preparation of one-atom-thick two-dimensional honeycomb graphene1 opens
an approach for the nano-sized electronic devices since it possesses many excellent and unusual
characteristics, such as massless Dirac fermions2, high carrier mobility3, magnetic zigzag edges4, long phase coherence
lengths5, quantum Hall effect6, unique transport properties7 and so on8?10. However, graphene is a semi-metal
with zero band gap, which limits its utilization in electronic devices. Furthermore, it?s incompatibility with
current silicon-based electronic technology also places obstructions on the road to the brave-new-world of
graphene-based devices. Hence, great efforts have been devoted to either opening an appropriate band gap in
graphene11 or searching for other two-dimensional materials with favorable carrier mobility and opportune band
gap12. Silicene, a material isostructural to graphene but with atomic bonds that are buckled rather than flat, has
recently been synthesized and attracts enormous attentions of researchers13?17. It features a Dirac-like electron
dispersion at the K points of the Brillouin zone and exhibits exciting properties beyond those present in graphene.
For example, the quantum spin Hall effect induced by spin-orbit interaction18, a small band gap opening in
Recently, the silicene nanoribbons (SiNRs) have been synthesized on Ag(100) and Ag(110) surfaces20,21,
respectively. Similar to graphene nanoribbons (GNRs), SiNRs can also be mainly classified into two types: zigzag
silicene nanoribbons (ZSiNRs) and armchair silicene nanoribbons (ASiNRs). As to the ZSiNRs, they have stable
antiferromagnetic (AFM) configuration, and the band structures can be tuned to half-metallic property under
an external transverse electric field22, which have attracted numerous attentions. And a lot of other methods have
been used to tune the electronic properties23, such as doping24 and edge decoration25. For ASiNRs, the intrinsic
electronic structures show semiconductor with no magnetism and the band gap oscillates with a period of three
when the ribbon width increases26. Presently, a few researches pointed out that, the magnetism and spin-splitting
band in ASiNRs can be realized by doping. For example, Bezanilla et al.27 found nitrogen (N) atoms doping can
make the ASiNRs exhibit a net magnetic moment. Zhang et al.28 studied the electronic and magnetic properties
of both ASiNRs and ZSiNRs doped with a single phosphorus (P) atom, get the conclusion that the ASiNRs with
P doping at edge site change into ferromagnetic (FM) semiconductor.
Moreover, the edge hydrogen saturation is also an effective way to modulate the magnetism and electronic
structures of nanoribbons25,29,30. So, in this letter, we modulate the magnetism and electronic properties of
ASiNRs by different edge hydrogenations and the bipolar magnetic semiconductor (BMS) have been found. In
addition, it can also be changed to half-metal or spin-splitting metal under different external electric field. More
interestingly, the charge carriers are not only spin polarized in energy space but also spatially separated at
different edges under the external electric field.
Results and Discussion
ASiNRs can be classified by the number of silicon (Si) atoms along the ribbon width, noted as n-ASiNRs, as
shown in Fig.?1, the width of n = 7 has been chosen in here (1.41 nm). The bare edged SiNRs is extremely reactive
due to the dangling bonds of edged Si atoms and will be undergo reconstruction to lower the total energy31 and
hydrogen (H) is the most common element to saturate the edged Si atom. What?s more, edge functionalization
can also improve the performance of SiNRs and thus extend their applications in electronics. There are two kinds
of saturated manners: monohydrogenation (i.e. sp2 edge) and dihydrogenation (i.e. sp3 edge). So the following
we have a study about the electronic properties of different saturated manners of the ASiNRs unit cell. Fig.?1(a)
shows the structure of single H saturated 7-ASiNRs, the red dashed rectangle represents the unit cell (marked as
M1(7A)). The pink and white balls denote Si and H atoms, respectively. The side view of 7-ASiNRs is present in
Fig.?1(b), which can be found that the Si atoms are in different planes, corresponding to two different sublattices.
The Si atoms at the bottom plane are labeled as A kind sublattice and those at the top plane are labeled as B kind
sublattice, as shown in Fig.?1(b). According to the percentage of sp3-like edged bonds and the kinds of saturated
sublattice, there are other six models. The relaxed structures are shown in Fig.?1(c?h), which named as M2(7A),
M3(7A), M4(7A), M5(7A), M6(7A), and M7(7A) for short, respectively.
In order to have an understanding about the stabilities of the ASiNRs with different edge hydrogenations, we
calculate their formation energies, which are based on the following formula25,
E form = Ertioblbon+H ? Ertioblbon ?
N HE H2
here the Ertioblbon+H, Ertioblbon, NH and E H2 are represent the total energy of the different edge hydrogenated 7-ASiNRs,
the total energy of the bare edged 7-ASiNRs, the number of H atoms, and the energy of the isolated H2 molecule,
The formation energies of different systems are shown in Table?1. It is shown that all the formation energies are
negative, which implies that the formation of 7-ASiNRs with edge hydrogenation is an exothermic reaction from
the bare edged 7-ASiNRs and H2, thus the edge hydrogenation can effectively enhance the stability of ASiNRs.
Comparing different H saturated 7-ASiNRs, it is clear that the stability is enhanced with the number of edge
dihydrogenated Si atoms (sp3 bonds) increasing. For the edge monohydrogenated structure, the bond length
between the edged Si atoms is 2.23 ?. When one of the adjacent edged Si atoms is saturated by a single H atom
and the other is saturated by double H atoms, the bond length increases to 2.33 ?. Once the edged Si atoms are
dihydrogenated, the bond length becomes 2.35 ?.
Furthermore, we consider three different spin-polarized manners: nonmagnetic (NM), ferromagnetic
coupling in each edge and between two edges (FM), and ferromagnetic ordering along each edge and antiparallel
spin orientation between the two edges (AFM) for M1(7A)?M7(7A). After having a comparison about the energy
difference between three spin-polarized states, we get the ground states of each structure, as listed in Table?1. The
results show M1(7A), M3(7A), M4(7A), and M7(7A) are NM, while M2(7A), M5(7A) and M6(7A) systems are FM with the
magnetic moment of 1.00 ?B , 2.00 ?B and 1.00 ?B , respectively. Addition, the effect of the nanoribbon width have
also been considered, the ground states and corresponding magnetic moments for different edge hydrogenated
6-ASiNR and 8-ASiNR are calculated, and the similar conclusions can also be gotten.
To explore their electronic properties, we then present the band structures of M1(7A)?M7(7A) under their
ground states, as shown in Fig.?2. For M1(7A), the four edged Si atoms are saturated by one H atom, we can find
the band structure presents semiconductive properties with an energy gap of 0.59 eV, which agrees well with the
previous study32. When one of the edged Si atoms is saturated by two H atoms, as to M2(7A), the band structure
is changed a lot. The spin-up bands (black solid line) and spin-down bands (red dashed line) are completely
separated from each other, meanwhile, around the Fermi level (FL), we can find the highest valence band (VB1)
and the lowest conduction band (CB1) are spin-up and spin-down states, respectively, which indicates M2(7A)
is a BMS33. When two of the edged Si atoms are separately saturated by two H atoms, and the other two are
saturated by one H atom, there are three kinds of manners, namely M3(7A), M4(7A) and M5(7A). The corresponding
band structures are presented in Fig.?2(c?e), respectively. One can see that M3(7A) and M4(7A) are non-magnetic
systems with semiconductive properties. Differently, M3(7A) is a semiconductor with a direct band gap of 0.43 eV,
while M4(7A) is a semiconductor with an indirect band gap of 0.27 eV. When it comes to M5(7A), one can find it
show the spin-density distributions of M2(7A) and M5(7A), from which we can see the spin-density are mainly
concentrated on the Si atoms adjacent to the dihydrogenated ones, and the spin-up density are much greater than
that of the spin-down.
Furthermore, we also consider the effects of the external transverse electric field to the magnetism and
electronic properties of M5(7A). The external transverse electric field is applied through two metallic regions on both
edges of the ribbon and tuned by the electrostatic potential of the metallic regions, the orientation of electric field
is along x axis, as shown in Fig.?4(a). Our calculations show the addition of external transverse electric field can
influence the magnetic moment of M5(7A) observably. When the external transverse electric field is smaller than
1.00 V/nm, the magnetic moment of M5(7A) keeps 2.00 ?B. When the external transverse electric field is in the
region of [1.00, 2.50] V/nm, the magnetic moment decreases rapidly from 2.00 ?B to 1.17 ?B with the electric field
increases. For the band structures of M5(7A) under external transverse electric field, we can find the two
intersected spin-up (spin-down) subbands below (above) the FL are split away from each other, and the spin band gap
between VB1 and CB1 decreases with the electric field increases, as presented in Fig.?4(d,f,h). When the applied
electric field increases to 2.00 V/nm (Fig.?4(f)), the CB1 goes across the FL, while the VB1 still below the FL, so
the M5(7A) presents half-metallic property. And the M5(7A) then converts to spin-split metal when the electric field
is 2.25 V/nm, as plotted in Fig.?4(h), both the VB1 and CB1 go across the FL.
We also give the Bloch states of VB1 and CB1 for M5(7A) at X point with Eext = 0.00, 0.50, 2.00, 2.25 V/nm, as
shown in Fig.?4(c,e,g,i), respectively. When the Eext = 0.00 V/nm, we can find the Bloch states of VB1 and CB1 are
mainly distributed on the A kind of sublattices. And the states of VB1 are mainly localized in the left part, while
that of CB1 are concentrated on the right part of M5(7A). Nevertheless, this situation is broken by the applied
electric field, as shown in Fig.?4(e,g,i), we can find the Bloch states are separated spatially, and independent of the
strength of the electric field. The Bloch states of VB1 are mainly in one of the edges; meanwhile, those of CB1 are
distributed on the other edge of M5(7A). As a result, the charge carriers are not only spin polarized in energy space,
but also spatially separated at different edges under the external electric field, which would supply a new method
to separate electrons and holes with different spins.
In conclusion, the stability, magnetic and electronic properties of ASiNRs with different H terminations have
been studied by using the first-principles calculations. The results show that the stability of ASiNRs increases
with the increase of the sp3-like edged bonds. Meanwhile, the ASiNRs with different edge hydrogenated
manners present different electronic properties, and the BMS can be found. What?s more, the BMS can be changed
to half-metal and then to spin-splitting metal as the addition external transverse electric field increase. More
interesting, the states of the spin-up electrons and those of the spin-down holes are localized at the opposite edges
under the external electric field. These results may be helpful in the fields of band gap tuning engineering and the
designing of ASiNRs-based spin devices with control over the spin in spintronics.
The geometry optimization and electronic properties calculation are performed within the density functional
theory by using the Atomistix ToolKit (ATK)39,40 package. The wave function is expanded with the double-? plus
polarized basis set. The Generalized gradient approximation with the Perdew-Burke-Ernzerh exchange
correlation functional is adopted to describe the exchange correlation interaction. All self-consistent calculations are
performed with the plane-wave cutoff energy of 200 Ry on a 1 ? 21 ? 1 Monkhorst-Pack k-point mesh. A vacuum
layer of 20 ? is added along x and z axis to eliminate the interaction between the adjacent slabs. The geometrical
structures are fully optimized until the force tolerance on each atom is smaller than 0.01 eV/? before other
quantities are calculated.
D.Z. carried out the first-principles calculations, prepared all figures and wrote the manuscript. M.L. directed this
work and revised the manuscript. F.X., J.O., H.X. and Y.G. involved in discussion. All authors analyzed the results
and reviewed the manuscript.
Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Zhang, D. et al. Hydrogenations and electric field induced magnetic behaviors in
armchair silicene nanoribbons. Sci. Rep. 6, 23677; doi: 10.1038/srep23677 (2016).
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This work is supported by the National Natural Science Foundation of China (Grant Nos 61306149 and 11334014) and Shenghua Lieying Scholarship by the Central South University .