Vanadium doping on magnetic properties of H-passivated ZnOnanowires

Pengfei Lu • Xianlong Zhang • Huawei Cao •

Zhongyuan Yu • Ningning Cai • Tao Gao •

Shumin Wang

Received: 30 October 2013 / Accepted: 6 January 2014 / Published online: 22 January 2014

� Springer Science+Business Media New York 2014

Abstract A comprehensive theoretical investigation on

the electronic and magnetic properties of V-doped and

H-passivated ZnO nanowires (NWs) was performed using

spin-polarized density functional theory. The magnetic

couplings of six configurations of V-doped ZnO NWs are

studied in detail and stable ferromagnetism (FM) ordering

is found in certain configurations. The FM mechanism

originated from the strong hybridization of V 3d and O

2p around the Fermi level. Our results show that the uni-

axial strain is an effective method to tune the magnetic

properties of this material system. Room temperature fer-

romagnetism in these V-doped ZnO NWs indicates that

these materials have a promising application in nanoscale

spintronics.

Introduction

Diluted magnetic semiconductors (DMS) with room tem-

perature ferromagnetism have attracted great attention in

the past decade due to their potential application in the new

generation of spintronic devices, which enable the utiliza-

tion of electron spin states rather than the motion of elec-

tron charges in conventional electronic devices. For

industrial application, spintronic devices must present a

certain density of states at the Fermi level for electrons that

are fully spin polarized and have a high Curie temperature

(TC). In general, DMS is usually produced by doping

semiconductors with transition metals (TMs). ZnO is a

wide-gap (3.4 eV) semiconductor with a large exciton

binding energy (60 meV) and has been theoretically pre-

dicted to be an ideal hosting candidate for high TC [1–4]

DMS. Extensive studies of bulk ZnO-based DMS have

been performed. TMs (V, Cr, Mn, Fe, Co, Ni, Cu)-doped

ZnO bulk system may result in diverse magnetic coupling

states [5–7]. For V-doped ZnO bulk system, both theoret-

ical and experimental investigations of the magnetic

properties have been studied in detail [8–11]. Wang et al.

[8] have calculated that different geometries with FM, and

antiferromagnetic (AFM) configurations are found to be

energetically nearly degenerate in Zn1-xVxO bulk. Rama-

chandran et al. [9] have reported that the Zn1-xVxO system

does not exhibit any signature of ferromagnetism. There

are some controversies about the magnetic coupling

mechanism, which is very sensitive to sample conditions

and defect distributions.

Recently, ZnO NWs have been reported for their

potential applications in DMS devices [12, 13] with the

advancement of experimental techniques. Therefore, it is

very important to have a theoretical understanding of the

electronic, magnetic and optical properties of spintronic

P. Lu (&) � X. Zhang � H. Cao � Z. Yu � N. Cai

State Key Laboratory of Information Photonics and Optical

Communications, Ministry of Education, Beijing University of

Posts and Telecommunications, P.O. Box 72, Beijing 100876,

China

e-mail: photon.bupt@gmail.com

T. Gao

Institute of Atomic and Molecular Physics, Sichuan University,

Chengdu 610065, China

S. Wang

State Key Laboratory of Functional Materials for Informatics,

Shanghai Institute of Microsystem and Information Technology,

Chinese Academy of Sciences, Shanghai 200050, China

S. Wang

Photonics Laboratory, Department of Microtechnology and

Nanoscience, Chalmers University of Technology,

41296 Goteborg, Sweden

123

J Mater Sci (2014) 49:3177–3182

DOI 10.1007/s10853-014-8020-y

materials based on low-dimensional ZnO. A systematic

research on 3d TM-doped ZnO NWs is performed and FM

properties at a wide range of temperatures have been found

[14, 15]. According to their conclusions, 3d TM doping is

an effective route to obtain room temperature ferromag-

netism. In particular, Cr ions in the ZnO NWs will favor

the ferromagnetic state and magnetic coupling is more

stable in the NWs than in bulk ZnO [15].

Although previous studies have focused on the V-doped

ZnO bulk system [8, 9], there are few theoretical investi-

gations in V-doped ZnO NWs. Therefore, the main moti-

vation of this paper is to systematically investigate

V-doped ZnO NWs, to find a proper high TC of ZnO NWs.

H-passivation and uniaxial strain effects are also intro-

duced. Our paper is organized as follows. In ‘‘Models and

methods’’ Section, first-principles calculations based on

density function theory (DFT) are described and theoretical

models are proposed. We present the results and discussion

in ‘‘Results and discussions’’ Section. To conclude, we give

a summary in ‘‘Conclusions’’ Section.

Models and methods

All the structural optimizations, electronic and magnetic

properties are performed based on DFT, as implemented in

the Vienna ab initio simulation package (VASP) [16]. The

PW91 approach is taken as the exchange-correlation

potential in the generalized gradient approximation (GGA)

[17]. The electron wave function is expanded in plane

waves with a cutoff energy of 380 eV. The ultra-soft

pseudo-potentials (USPP) are introduced for Zn, O, and V

atoms. The valence-electron configurations for these atoms

are employed as Zn (4s23d8), O (2s22p4) and V (4s23d3),

respectively. The geometries are fully relaxed until the

energy is less than 10-5 eV between two ionic steps, and

the force acting on each atom is less than 5 9 10-4 eV/A.

In the self-consistent calculations, the Brillouin zone is

sampled in the k space within Monkhorst–Pack scheme by

1 9 1 9 9 mesh points.

To reveal the V-doped effect, a supercell of ZnO NWs

consisting of 96 atoms is cut from the 7 9 7 9 2 supercell

of bulk Wurtzite (WZ) ZnO by cutting the outside part of

the circled area in Fig. 1a along the [0001] direction. To

prevent the electrostatic interaction between the neigh-

boring images of the NWs, those ZnO NWs are surrounded

by a vacuum space with a distance of about 10 A per-

pendicular to the NWs’ axis. The periodic part is shown in

Fig. 1b.

Generally, surface states will locate in the gap for un-

passivated ZnO NWs [18]. In our models, the dangling

bonds of Zn and O atoms on the surface of NWs are sat-

urated by H atoms to decrease the potential influence on

magnetic properties from surface states [15, 19]. Therefore,

the supercell will extend to 144 atoms with 48 Zn, 48 O,

and 48 H atoms, respectively. Two of V atoms will be

introduced to replace the Zn atoms. Six different configu-

rations have been considered. For all of these configura-

tions, the geometry of the supercell was fully optimized

without any symmetry constraint. Our models are listed in

Fig. 2.

Results and discussions

Structural optimization is firstly performed for pristine H-

passivated ZnO NWs. The Zn–O bond lengths for the outer

and inner surface along the [0001] direction are increased

to 1.999 and 2.005 A after relaxation. In addition, the Zn–

O–Zn bond angles h of the ZnO supercell along the

direction of [01�10] change from 110.90� to 118.98�. These

results indicate that the relaxation of the outer surface’s

atoms is much larger than those on the inner surface. To

determine the preferable sites for V dopants of different

configurations, we calculate the formation energy of the

NWs given by:

Fig. 1 a Top view of a

7 9 7 9 2 ZnO supercell with

wurtzite structure (the red and

gray spheres represent O and

Zn, respectively). b ZnO

supercell which produces a

nanowire of infinite length

along the [0001] direction

(Color figure online)

3178 J Mater Sci (2014) 49:3177–3182

123

Ef ¼ EðV� ZnOÞ � EðZnOÞ þX

nilðiÞ ð1Þ

where E(V-ZnO) and E(ZnO) are the total energy of the

doped supercell and the pure H-passivated ZnO NWs,

respectively; i is the atom doped or removed from the pure

H-passivated ZnO NWs and ni is the number of atoms

exchanged between a perfect ZnO NWs (ni is negative on

adding one atom and positive on removing one atom).

l(i) is the chemical potential of Zn, O, and V, respectively.

The calculated formation energies for V dopant in different

positions are listed in Table 1. It indicates that the forma-

tion energy of configuration I is extremely lower than that

of other configurations. The V dopants tend to show

clustering effect on the ZnO NW surface which is similar

to the Ag-doped ZnO NWs system [20].

DE is defined as the total energy difference between the

FM and AFM states (DE = EAFM-EFM) to determine the

magnetic coupling for the pair of V atoms. Negative DE

means that the AFM state is more stable, while the positive

value means that the FM state is more stable. The FM state

in configurations I and II lies 0.330 and 0.154 eV lower in

energy than the AFM state, respectively, while in config-

uration III and V the AFM state are lower in energy than

the FM state by 0.245 and 0.143 eV, respectively. Thus,

our calculated results show that V-doped H-passivated ZnO

NWs are magnetic and this could account for the diverse

Fig. 2 Six configurations of V-doped H-passivated ZnO (the red, gray, black and green spheres represent O, Zn, V and H atoms, respectively)

(Color figure online)

Table 1 The energy difference DE between AFM and FM states

(DE = EAFM-EFM in eV), relative total energy ER with respect to the

most stable configuration I, the optimized V–V and V1–O distances,

V2–O distance, and the magnetic moments at each V atom and the

nearest neighbor O atom for the configurations are also given

Configurations DE/eV Coupling ER dV–V/nm dV1–O/nm dV2–O/nm lV1/uB lV2/uB lO/uB

I 0.330 FM 0 0.3362 0.1970 0.1970 2.280 2.279 0.043

II 0.154 FM 0.213 0.3503 0.1987 0.1987 2.152 2.152 -0.037

III -0.245 AFM 0.078 0.3366 0.1936 0.2063 1.805 -2.603 -0.017

IV -0.0046 AFM 0.026 0.4541 0.2042 0.2500 2.291 -2.291 -0.005

V -0.143 AFM 0.356 0.3420 0.1961 0.2042 2.009 -2.311 -0.021

VI 0.0026 FM 0.463 0.3722 0.1998 0.2147 2.154 2.206 0.010

J Mater Sci (2014) 49:3177–3182 3179

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magnetic behaviors in the experiments [21–25]. Configu-

ration I is set to be the ground state with the lowest energy.

In the fourth column, we list the relative energies ER cor-

responding to all configurations with respect to configura-

tion I. In the last three columns, the local magnetic

moments of each V atoms and the nearest neighboring O

atom are presented.

The total electronic density of states (DOS) for the pure

H-passivated ZnO NWs is presented in Fig. 3a. The DOS

curves for spin-up and spin-down electrons are completely

symmetric, indicating that the pure H-passivated ZnO NWs

is nonmagnetic. To present an insight into the origin of

magnetic coupling in H-passivated V-doped ZnO NWs,

configuration II is taken as an example for discussion in

detail. The distance between V atoms changes from 3.249

to 3.503 A after structural relaxation, and the correspond-

ing bond angle h(V–O–V) is changed to 123.58�. The spin-

polarized total DOS is listed in Fig. 3b. The DOS figures

clearly show that V atoms incorporated into ZnO will

introduce new states either inside the valence band or

above the valence band maximum (VBM). For V-doped

ZnO, V atoms are in the crystalline field of the tetrahedron.

In Fig. 3c, the surrounding O ligands will split the V

3d states into doubly degenerate e states (dz2 and dx

2-y2 ) and

triply degenerate t2 states (dxy, dxz and dyz). The exchange

splitting is larger than the crystal field splitting between the

e and t2 states. This will result in empty spin-down state in

the range of conduction band states. Therefore, the Fermi

energy level passes through the gap in the spin-up DOS and

the system behaves as half metallic with an energy gap of

1.85 eV in the minority spin DOS. The corresponding

partial DOS of the V atoms and the mediating O atom are

plotted in Fig. 3c, which suggests much of the contribution

to the magnetic moment comes from the V 3d electrons.

The partial DOS of V 4s, 3p, 3d are also presented in

Fig. 3d, while V 3d plays a dominant role obviously. In

addition, the induced new states appearing in the gap

originate from the majority states of the V 3d and O

2p orbits in Fig. 3c. There is a clear overlap between V

3d and O 2p states from -0.15 to 0.15 eV leading to strong

p-d hybridization near Fermi level which passes right

through the gap in the spin-up DOS. The spontaneous

magnetization originates mostly from the V 3d states,

which suggests that the FM coupling in this system is most

likely mediated by the double exchange mechanism [26].

Besides, the total DOS around the Fermi level is attributed

Fig. 3 Calculated total and partial DOSs of pure and V-doped H-passivated ZnO NW

3180 J Mater Sci (2014) 49:3177–3182

123

to the spin-up channel; so the system shows a half-metallic

nature. Our conclusion is in good agreement with some

reports observed in other systems [27, 28].

Strain plays a key role to modulate the electronic and

optical properties of ZnO nanostructures [26]. However,

little is known about the uniaxial strain effect on the

magnetic properties of ZnO NWs. The modulation of

uniaxial strain on the magnetic coupling of V-doped ZnO

NW is presented here. With reference to previous reports

[29–31], the uniaxial strain is given in the following

manner. The V-doped ZnO NW is strained along [0001]

direction; the range of the strain e is chosen from -8 to

8 % corresponding to the compressive and tensile strains,

respectively. The purpose is to avoid unwanted structure

modulation. Variations of the energy of both FM and AFM

with the uniaxial strain are displayed in Fig. 4.

It is interesting that the lowest energy of the considered

configurations is found to be in the unstrained states. To

intuitively present the uniaxial strain on the magnetic

coupling of V-doped ZnO NWs,variations of DE with

strain are also listed in Fig. 4. For configuration I in

Fig. 4b, DE monotonously increases with the increase in

the compressive and tensile strains. However, DE is always

positive during the whole range, indicating stable FM

coupling. Contrary to configuration I as shown in Fig. 4d,

the compressive strain pulls down the DE of configuration

II, while the tensile strain increases it. In other words, the

magnetic coupling of V-doped ZnO is affected by the

uniaxial strain in different ways which offers us a new

approach to design high TC DMS devices.

Conclusion

In summary, extensive theoretical investigations of the

magnetic properties of V-doped H-passivated ZnO NWs

are presented by using spin-polarized DFT. A fierce

hybridization between V 3d and O 2p states appears at the

Fermi level, which is found to be responsible for the FM

coupling. A half-metallic nature is found in V-doped ZnO

NWs, which is ideal for injection of spin-polarized charge

carriers into the nonmagnetic system. The uniaxial strain is

taken into account and it is proved to have important

influence on the magnetic properties of H-passivated

V-doped ZnO NWs. Our theoretical results indicate that

Fig. 4 a and c The total energies as a function of strain in the FM and AFM states. b and d The energy difference DE between the AFM and FM

state as a function of strain

J Mater Sci (2014) 49:3177–3182 3181

123

V-doped ZnO NW has good magnetic properties which

display diverse magnetic couplings.

Acknowledgements This work was supported by the National

Natural Science Foundation of China (No. 61102024), the Funda-

mental Research Funds for the Central Universities (No.

2012RC0401), the National Basic Research Program of China (973

Program) under Grant No. 2014CB643900 and the Open Project

Program of State Key Laboratory of Functional Materials for

Informatics.

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