First-principles study on the electronic and magnetic properties
of armchair graphane/graphene heterostructure nanoribbons
W.X.Zhang a,n,C.He b,n,T.Li a,S.B.Gong a,L.Zhao a,J.Y.Tao a
a School of Materials Science and Engineering,Chang’an University,Xi’an710064,China
b State Key Laboratory for Mechanical Behavior of Materials,School of Materials Science and Engineering,Xi’an Jiaotong University,Xi’an710049,China
a r t i c l e i n f o
Article history:
Received11February2015
Received in revised form
16March2015
Accepted17March2015
Communicated by Ralph Gebauer
Available online26March2015
Keywords:
A.Graphene nanoribbon
D.Electronic properties
E.First-principles
a b s t r a c t
In this paper,the electronic and magnetic properties of electronic and magnetic properties of armchair
graphane/graphene heterostructure nanoribbons(AGA/GNRs)have been systematically investigated by
first-principles calculations based on density functional theory.The calculated results indicate that13-
armchair graphane nanoribbon(13-AGANR),13-armchair graphene nanoribbons(13-AGNR)and hybrid
armchair graphane/graphene nanoribbons(AGA13Àx/G x NRs)are all direct semiconductors(13,13Àx and
x are denoted as the nanoribbons’widths).The band structures near the Fermi level of AGA13Àx/G x NRs
are mainly determined by the graphene section and the atomic charge transfers in the interface of
AGA13Àx/G x NRs are stronger.AGA7/G6NR with DB defects at AGANR edge obviously affect the magnetic
properties.These diverse and tunable electronic and magnetic properties can be a theoretical guidance
for the design of novel nanoelectronic devices.
&2015Elsevier Ltd.All rights reserved.
1.Introduction
Graphene consists of a hexagonal monolayer network of sp2-hybri-
dized carbon atoms.Since graphene wasfirst experimentally fabricated
in2004[1],graphene monolayers has motivated considerable interest in
variety of one-atom-thick two-dimensional(2D)crystals[2–4],especially
after large-scale synthesis methods like chemical vapor-deposition[6]
and epitaxial growth[7]on metal and SiC substrates are developed.The
quasi-one-dimensional(1D)graphene ribbons(GNRs)with armchair or
zigzag edges have attracted much attention because of their electronic
[8–11],magnetic[12,13],and quantum-transport properties[14,15].A
graphene nanoribbons(GNRs)can be realized by cutting mechanically
exfoliated graphene or patterning epitaxially grown graphene structures
[16,17].Recently,the fascinating electronic properties associated with
one-dimensional(1D)fully and partially hydrogenated graphene[18,19],
graphane[20],BN[21],ZnO[22],SiC[23],GaN[24],and AlN[24,25]
nanoribbons derived from either monolayer or multilayer sheets heavily
depend on the ribbons’width,thickness and edge modification.The fully
and partially hydrogenated nanoribbons exhibit completely distinct
properties from their pristine forms.The high quality graphane nanor-
ibbons GANRs can be fabricated by selectively hydrogenating graphene or
by carving GNRs on a graphane sheet[26,27].Furthermore,hybrid
graphane/graphene nanoribbons(GA/GNRs)could exhibit unique elec-
tronic properties that differ from the pristine armchair(AGNRs)and
zigzag GNRs(ZGNRs)[28,29].These suggest such ribbon-hybridized
graphene-like materials as promising candidates for applications in future
electronic and optoelectronic nanodevices.Therefore,serious efforts are
highly warranted to explore the physical properties of GNR for various
technological applications.
Yet a systematically theoretical understanding of electronic prop-
erties of these functionalized armchair graphane/graphene nanorib-
bons(AGA/GNRs)remains unclear.How and to what extent does the
ratio of GA affect the electronic properties of different GA/GE NRs
systems?Because the interaction of hydrogen with graphene is of
great technological interest,compared with the unsaturated nanor-
ibbon,how would its band gap change in a hydrogenated AGNRs?
Therefore,the above questions are discussed in this paper.The
structural,electronic properties of hybrid AGA/GNRs with the varia-
tion of the proportion are extensive carried out based onfirst-
principles calculations with density functional theory(DFT).Mean-
while,Band structure distribution(BS),the Density of States(DOS),
atom Mulliken charges and the population analysis are performed to
determine changes of atomic and electronic structures of hybrid AGA/
GNRs.These studies provide us a deep understanding of the novel
properties of AGA/GNRs,which is essential to employ them as
building blocks for future nanodevices.
2.Computational methods
The simulation is calculated byfirst-principles DFT,which is
provided by DMOL3[30–32].The generalized gradient approxima-
tion(GGA)with the Perdew–Burke–Ernzerhof scheme(PBE)[33]is
Contents lists available at ScienceDirect
journal homepage:www.elsevier/locate/ssc
Solid State Communications
/10.1016/j.ssc.2015.03.014
0038-1098/&2015Elsevier Ltd.All rights
reserved.
n Corresponding authors.
E-mail addresses:wxzhang@chd.edu(W.X.Zhang),
hecheng@mail.xjtu.edu(C.He).
Solid State Communications211(2015)23–28
employed to optimize geometrical structures and calculate proper-ties.The all-electron relativistic Kohn–Sham wave functions are expanded in the local atomic orbital basis set for DMOL3[30]. Pseudopotentials with C–2s22p2,and H–1s1valence electron con-figurations are used for C and H atoms.Similar functional have been successfully used to study the structural and electronic properties of water,Si and Cu nanowires[34,35].The nearest distance between for edge–edge and layer–layer in neighboring cells is greater than 15Åto ensure no interactions.For geometry optimization,both the cell and the atomic positions are allowed to fully relax.The
Brillouin Fig.1.Schematic illustration of supercell13-AGANR(a)and13-AGNR(b)arrangements,where the gray and white spheres are C and H atoms,
respectively.
Fig.2.Band structure,Partial DOS and charge density isosurfaces of LUMO and HOMO at Gamma point of13-AGANR(a)and13-AGNR(b).The E f is set to zero.Blue and yellow denote the positive and negative wave function contours,respectively,and the value of the isosurfaces is0.025e/Å3.(For interpretation of the references to color in thisfigure legend,the reader is referred to the web version of this article.)
W.X.Zhang et al./Solid State Communications211(2015)23–28
24
zone is sampled by1Â8Â1k-points for all structures in the geometry optimization calculations,which brings out the conver-gence tolerance of energy of  1.0Â10À5Ha(1Ha¼27.2114eV), maximum force of0.002Ha/Å,and maximum displacement of 0.005Å[34,35].The electronic distributions of AGA/GENRs are carried out by Mulliken charge analysis,which is performed using a projection of a Linear Combination of Atomic Orbitals(LCAO)basis and to specify quantities such as atomic charge,bond population, charge transfer etc.LCAO supplies better information regarding the localization of the electrons in different atomic layers than a plane wave basis set does[36].The obtained relative values
of the charge e,but not the absolute magnitude,display a high degree of sensitivity to the atomic basis set and a relative distribution of charge[37,38].
electronic去掉ic是什么To evaluate the structural stability of the hybrid AGA/GNRs,we calculated the formation energies for different systems.The for-mation energy(E f)is expressed as E f¼E totÀ(χCμCþχHμH),where E tot is the cohesive energy per atom of hybrid AGA/GNRs,μC is the
cohesive energy per atom of the graphene single layer,μH is half of the binding energy of H2,andχC(χH)is molar fraction of the atom in the nanoribbons(H atom)[20,39].3.Results and discussion
In our simulation,the host AGANRs with W¼13is represented by a supercell of C52H60,where the ribbon width(W)is defined by the number the total number of C–C chains across the ribbon width.In the armchair graphane nanoribbon(13-AGANR)configuration,every two adjacent C atoms are hydrogenated from the opposite sides of the graphene plane.The obtained C–C and C–H bond lengths of graphane are1.529and1.109Å,respectively,which are in good agreement with the previous reported data[4].The supercells used for the13-AGANR and13-AGNR are shown in Fig.1a and b,respectively.In addition,the edge C atoms of the considered structures are passivated with H atoms. After relaxation,1
3-AGNR is completelyflat.In this case,the geometry change is obvious:the atoms converge to just one planar layer, accompanied by a little C–C(1.450Å)and C–H(1.087Å)contraction.
The electronic structures of13-AGANR and13-AGNR are also investigated here.Fig.2shows BS,Partial DOS and the corresponding charge density isosurfaces of the lowest unoccupied molecular orbital (LUMO)and the highest occupied molecular orbital(HOMO)at Gamma point of the two structures.Both of the13-AGANR and13-AGNR are nonmagnetic semiconductors,and the direct band gaps
are
Fig.3.Schematic illustration of AGA13Àx/G x NRs(x¼2,4,6,8,10and12)with H terminated at both edges.The x and13Àx are the number of armchair graphane and graphene chains,respectively.
W.X.Zhang et al./Solid State Communications211(2015)23–2825
4.68,and0.79eV,respectively,which are corresponding to other theoretical values of3.84and0.83eV by DFT studies[20,29,40].
In comparison,some differences in HOMO and LUMO can be observed between13-AGANR and13-AGNR.In13-AGANR(Fig.2a), HOMO and LUMO orbitals distribute mainly along the C–C bonds. However,the densities are inhomogeneous over the whole region, much weak in the edge area.While in13-AGNR(Fig.2b),the charge densities are concentrated homogenously along the C–C bonds over the whole framework.Moreover,the charge densities of LUMO orbitals are mainly centered at single C atoms,which seems strong in the interior area.In addition,HOMO orbitals distribute mainly along the periodical direction.
Similarly,the armchair graphane/graphene heterostructure nanor-ibbons(AGA13Àx/G x NRs)are also c
onstructed by removing H atoms from the13-AGANR.The x and13Àx are the number of armchair graphane and graphene chains,respectively.Thus,the geometry structures of AGA13Àx/G x NRs(x¼2,4,6,8,10and12)with H terminated at both edges are shown in Fig.3a–e.After relaxation, the graphene regions for each systems are completelyflat.However, since the hybridization of the C atoms at the interfaces changes from sp3in graphane to sp2in graphene,the interfaces are notflat.At the interface of graphane–graphene heterostructure,the adjacent C atoms bonded to the hydrogen move out of the plane in opposite directions,which is similar to the case of triangular graphene nanoflakes embedded in graphane[4].The average C–C bond length at the interface are1.509Å,between the C–C bond length of13-AGANR (1.529Å)and13-AGNR(1.450Å).
It is also investigated in the present work by performing spin-polarized and non spin-polarized calculations for AGA13Àx/G x NRs shown in Fig.4a–f.The obtained results show that almost no energy difference can be found for them between these two methods,indicating that all AGA13Àx/G x NRs exhibit the nonmag-netic characteristics.The band structures of AGA13Àx/G x NRs indi-cate that they are all direct semiconductors since both LUMO orbitals and HOMO orbitals at Gamma point.E g values vary with the composition and the corresponding E g values of each AGA x/ G13Àx NRs are1.02,2.41,1.39,0.51,1.05and0.81eV,respectively.
Moreover,in order to deeply describe the electronic structure of AGA13Àx/G x NRs,the corresponding electronic distributions at the Gamma point have been explored.For all the semiconducting systems AGA13Àx/G x NRs,the electronic distributions of bands display similar behaviors.Thus,taken the system AGA7/G6NR as an example(in Fig.4c),HOMO and LUMO orbitals are primarily localized on the right part of the structure,which mean that E g values of AGA13Àx/G x NRs is dominated by the graphene rather than graphane.Moreover,both of the electronic distribution
for Fig.4.Band Structures of AGA13Àx/G x NRs(x¼2,4,6,8,10and12)with H terminated at both edges.HOMO and LUMO orbitals of AGA7/G6NR.Fermi level is set to zero.
W.X.Zhang et al./Solid State Communications211(2015)23–28
26
HOMO and LUMO orbitals also distribute along C–C bonds.How-ever,the densities of HOMO orbitals are symmetric along the periodic direction,while LUMO are mainly centered at single C atoms in width direction of nanoribbon.
In Fig.4g,we show the width dependence of band gap of AGA7/ G6NR.It is clear that like as AGNRs[27,28],the variation is also separated into three different groups(3p,3pþ1,3pþ2),where p is a non-negative integer.The results are corresponding to the electronic distributions of HOMO and LUMO orbitals at the Gamma point and the band structures near the Fermi level of AGA13Àx/G x NRs are thus mainly determined by the graphene section.
The atomic charge transfers in13-AGANR and13-AGNR are analyzed by the Mulliken charge analysis[35]and the corresponding results are shown in Table1.The location of the sites is shown in Fig.
1.The results indicate that the same atoms at the edge(inner)are equivalent and e C¼À0.138(À0.061)and e H¼0.084(0.063)for AGANR.Meanwhile,for AGNR,at the edge,e C¼À0.096and e H¼0.075.With the C atoms away from the edge,the changes of C atoms at sites I1,I2and I3(I is short for Inner)are0.011,0.08,and0.03, respectively.Therefore,the C atom at the edge of AGANR(À0.138)is more chemically active than the one AGANR(À0.096)because it has more electrons.
Similarly,the atomic charges of AGA13Àx/G x NRs are also calculated by the Mulliken charge analysis[35].Taken the system AGA7/G6NR as an example(in Fig.3c),the atomic charges of atoms near the interface are shown in Table1.The location of the sites is shown in Fig.3.The interface atoms at site D(À0.134)are more charged than other atoms. At the interface,C atoms are more negative and the corresponding H atoms are more positive.While the sites away from the interface,the charge distributions are weaker.Thus,an interface influences mainly the atoms at the conjunction of AGANR and AGNR.It is known that the atomic charge is mostly affected by the atoms belonging to the same carbon ring,especially the nearest atoms.For the carbon and hydrogen atoms at site E,they have similar nearest atoms as sites in graphane region far apart from the interface,where the three nearest C atoms are bonded by sp3orbitals.For the C and H atoms at site D, only two nearest C atoms are bon
ded by sp3orbitals;the other one on its right hand side at site C is bonded by sp2orbitals.Therefore,the effect of the interface on site D is stronger than that on site    E. Therefore,the C atom at site D(À0.134)is more chemically active than the one at site E(À0.080).
It is well-known that DB(dangling band)defects around the vacancy sites or at the tips significantly influences their electronic properties[41].Therefore,in present work,we investigate the spin-polarized band structures of AGA7/G6NR with DB defects at AGANR and AGNR edges.Both of them are still semiconductors with direct band gaps,the direct band gaps of AGA7/G6NR with DB defects at AGANR and AGNR edges are  1.306eV and0.93eV,respectively.When AGA7/G6NR with DB defects at AGNR edge, the up-spin and down-spin are fully superimposed.While in AGA7/G6NR with DB defects at AGANR edge,the asymmetry of BS between the up and down spins is slightly enlarged at the Fermi level,which is shown in Fig.5a.
To deeply study the electronic and magnetic properties of AGA7/ G6NR with DB defects at AGANR edge,the spin density distribution (Δρ¼ρupÀρdown)of AGA7/G6NR with DB defects at AGANR edge are shown in Fig.5b.A significant fact can be seen that the net
spin-up
Fig.  5.(a)Band structure,(b)DOS and(c)isosurfaces of the spin density distribution(Δρ¼ρupÀρdown)of AGA7/G6NR with DB defects at AGANR edge.The black lines and the red lines represent the spin up and down bands,respectively. Fermi level is set to zero.Dark blue and light orange surfaces correspond to the isosurfaces of up(positive)and down(negative)spin density.The isosurfaces are set to be70.001electrons/au3.(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this
article.)
Fig.6.The formation energies of13-AGANR,AGA13Àx/G x NRs,and13-AGNR.(a–f) Denote the structures of AGA13Àx/G x NRs(x¼2,4,6,8,10and12),respectively.The solid line serves as a guide to the eye.
Table1
Atom Mulliken charges of13-AGANR,13-AGNR,and AGA7/G6NR.The location of the
sites is shown in Figs.1and3.The unit of charge is e.
Structure Atom site C atom H atom
AGANR EdgeÀ0.1380.084
InnerÀ0.0610.063
AGNR EdgeÀ0.0960.075
I10.011
I20.008
I30.003
AGA7/G6NR A0.007
B0.007
C0.029
DÀ0.1340.102
EÀ0.0800.074
FÀ0.0620.067
GÀ0.0620.065
W.X.Zhang et al./Solid State Communications211(2015)23–2827

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