First-principle study on Ag-2N heavy codoped of p-type graphene-like ZnO nanosheet

First-principle study on Ag-2N heavy codoped of p-type graphene-like ZnO nanosheet

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First-principle study on Ag-2N heavy codoped of p-type graphene-like ZnO nanosheet W.X. Zhang a,n, T. Li a, C. He b,n, X.L. Wu b, L. Duan a, H. Li a, L. Xu a, S.B. Gong a a b

School of Materials Science and Engineering, Chang’an University, Xi’an 710064, China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China

art ic l e i nf o

a b s t r a c t

Article history: Received 8 October 2014 Received in revised form 12 December 2014 Accepted 13 December 2014

In this article, two different Ag-2N heavy codoped of graphene-like ZnO nanosheets have been investigated based on first-principles density-functional theory. The geometric optimization, Density of States (DOS) and Band structure (BS) for all models are calculated, respectively. The results indicate that Ag substituted on the cation site (AgZn) exhibit a strong attractive interaction with a nitrogen acceptor located at the nearest-neighbor oxygen site, forming passive Ag-N complex. This study can be a theoretical guidance to improve the electrical conductivity of p-type graphene-like ZnO nanosheet by heavy codoping. & 2014 Published by Elsevier Ltd.

Keywords: ZnO nanosheet p-type conductivity First-principles

1. Introduction Zinc oxide is a promising optoelectronic material, which can be utilized for blue and ultraviolet light emitting diodes, laser diodes, and solar cells due to its wide band gap of 3.2 eV [1,2]. It has been suggested that undoped ZnO is n type due to a large number of intrinsic defects such as oxygen vacancies (VO) and Zn interstitials (Zni) [3]. p-type doping of ZnO has attracted great attention, both theoretically and experimentally, because of its potential application for next-generation short-wavelength optoelectronic devices [4–9]. The obstacle that prevents full utilization of ZnO as a novel optoelectronic material is the inability of obtaining p-type conductivity with high hole concentration and low resistively. Therefore, many efforts have been made to achieve p-type ZnO, with using many techniques and dopants [10]. At the nanoscale, ZnO brings us more sweet surprises [4–11]. The versatile chemical bonding of ZnO leads to probably the richest family of nanostructures among all materials, which have been successfully synthesized through a variety of experimental techniques. Compared with the bulk crystalline ZnO, these low-dimensional structures have demonstrated extraordinary electrical and optical performances and are promising candidates for many novel applications in transparent electronics, gas sensors, transducers, solar cells, and biomedical devices [12]. Particularly, two-dimensional (2D) systems show peculiar properties, which are different from their counterparts bulk phases. The formation of planar nanosheets of ZnO have been predicted to occur as the number of ZnO layers are reduced [13], n

Corresponding authors. E-mail addresses: [email protected] (W.X. Zhang), [email protected] (C. He).

and one monolayer of ZnO has been shown to be stable [14]. ZnO in nanoscale may have even more interesting properties [15–19]. It has been experimentally observed that planar ZnO nanosheets are stable and the transition to wurtzite structure occurs in the 3–4 monolayers for ZnO on Ag substrates through surface X-ray diffraction and scanning tunneling microscopy, which provided a direct evidence for the presence of planar ZnO nanosheets [20]. Because the interaction of Ag and N codoped of graphene-like ZnO (Gr-ZnO) is of great technological interest, what properties distinguishable from unsaturated bare nanosheets? Yet a theoretical understanding of electronic properties of these functionalized ZnO nanosheets remains unclear. In this article, Density functional theory (DFT) computations are performed to systematically investigate the structural, electronic and magnetic properties of a low dimensional system composed by a flat monolayer of ZnO atoms with a graphenelike structure. Two different graphene-like Ag, N codoped Gr-ZnO nanosheet models, which the proportion of Ag:N equals to 1:2, have been constructed. Meanwhile, Density of States (DOS), Band structure (BS), atom Mulliken charges and the populations analysis are performed to determine changes of atomic and electronic structures of doped Gr-ZnO nanosheet. These studies provide us a deep understanding of the novel properties of doped ZnO nanosheets, which is essential to employ them as building blocks for future nanodevices.

2. Computational methods The simulation is calculated by first-principles DFT, which is provided by DMOL3 [21–23]. The generalized gradient approximation (GGA) of Perdew and Wang (GGA-PW91) is employed to optimize geometrical structures and calculate properties [24]. The

http://dx.doi.org/10.1016/j.ssc.2014.12.014 0038-1098/& 2014 Published by Elsevier Ltd.

Please cite this article as: W.X. Zhang, et al., Solid State Commun (2014), http://dx.doi.org/10.1016/j.ssc.2014.12.014i

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all-electron relativistic Kohn-Sham wave functions are expanded in the local atomic orbital basis set for DMOL3 [21]. The valenceelectron configurations are: Zn-3d104s2, O-2s22p4, N-2s22p3, Ag4d105s1. Similar functional have been successfully used to study the structural and electronic properties of water, Si and Cu nanowires [25,26]. The nearest distance between nanosheets in neighboring cells is greater than 15 Å to ensure no interactions. For geometry optimization, both the cell and the atomic positions are allowed to fully relax. The k-point is set to 6  6  1 for all structures, which brings out the convergence tolerance of energy of 1.0  10  5 Ha (1 Ha ¼27.2114 eV), maximum force of 0.002 Ha/ Å, and maximum displacement of 0.005 Å. The electronic distributions of Gr-ZnO 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 [27]. 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[10,28,29]. The structure of Gr-ZnO nanosheet is originally constructed from ZnO wurtzite crystal, in which all atoms are in sp3 hybridization with each Zn (O) atom surrounded by four neighboring O (Zn) atoms at the corners of a tetrahedron. The Zn-O bond length in ZnO nanosheet is calculated to be 1.876 Å. Moreover, the bond angle within the newly formed planar layer increases from the wurtzite tetrahedral, 108.0441 to plane trigonal, 120.0171. Similar to the hexagonal wurtzite ZnO, the Gr-ZnO is also a semiconductor with a direct band gap of 1.649 eV. Based on the unit cell, a 4  4  1 supercell of Gr-ZnO containing 32 atoms is constructed. The formation energies (Ef) of a defect (NO or AgZn) are defined as follows: [30] Ef ðDÞ ¼ Etot ðDÞ  Etot ðZnOÞ  ∑ ni μi

ð1Þ

i

where Etot (D) is the total energy of the supercell containing the defect and Etot(ZnO) is the total energy of the equivalent supercell containing only Gr-ZnO. ni and μi are the number and the chemical potential of the atoms added to (positive ni), or taken from (negative ni) the reference supercell in order to create the defect, respectively. The chemical potentials of O and N are measured according to the energy of an oxygen atom in an oxygen molecule and a N atom in a nitrogen molecule. The chemical potentials of Zn and Ag are taken as the energy per atom of bulk metallic Zn and Ag, respectively. To determine whether it is energetically preferred that two dopants bind, for example, AgZn and NO, in the neutral charge state, we calculate the binding energy which is defined as: Eb ðZnO : Ag  2NÞ ¼ Etot ðZnO : Ag  2NÞ þ Etot ðZnOÞ Etot ðZnO : N Þ  Etot ðZnO : AgÞ

ð2Þ

where Etot(ZnO:Ag-2N), Etot(ZnO:N), and Etot(ZnO:Ag) are the total energies for supercells containing defects AgZnNO, NO, and AgZn, respectively. A negative value of Eb corresponds to a metastable or a stable dopant pair when both are present in the system. According the equation, in the above case, the system energy is the lowest, and the structure is the most stable [30].

3. Results and discussion For substitutional N on the O site forming the defect NO, and substitutional Ag on a Zn site forming the defect AgZn, when a second N atom is added to the defect (AgZn-NO), there are two possible locations: the defects (AgZn-NO) and NO in the near and far apart arrangements, respectively. The co-doping concentration of two different heavy codoped Gr-ZnO are both 9.375at%. The corresponding structures are shown in Fig. 1. In addition, considering the symmetry, stability, and computing speed, the substitution location is as close as possible to the center position. According to Eq. (1), we find the total energies of the configurations in the near arrangement is 41 meV lower than the energy of the complex defect (AgZn-NO) and NO far apart. Thus, the second N atom slightly prefers to occupy nearestneighbor sites of Ag to form the complex defect (AgZn -2NO). The bond distance of undoped and doped Gr-ZnO are shown in Table 1. N1, N2 mean that N atom is in the far away site and nearest-neighbor site, respectively. As for the (AgZn-2NO) complex defect system, the distance between Ag and the N atom is 1.976 Å. The bond length of O-Ag is 2.053 Å on average, and the average distance between N and Zn atom is 1.854 Å. While for the (AgZnNO þNO) defect system, the distance between Ag and the N atoms is 2.036 Å. The bond length of O-Ag is 2.013 Å on average, and the average distance between N and Zn atom is 1.851  1.854 Å. In order to clarify the impurity mechanism of p-type doping of GrZnO, the populations analysis and atom Mulliken charge transfers are analyzed. By analyzing the results of populations, the distribution of charge transfer and chemical properties of atoms in the solid could be understood. The populations analysis undoped and doped Gr-ZnO are also shown in Table 1. It is a complement to the Table 1 The populations analysis and bond distance of doped Gr-ZnO. N1, N2 mean that N atom is in the far away site and nearest-neighbor site, respectively. Structure

Bond type

Population

Bond length (Å)

Undoped-ZnO (AgZn-2NO) in ZnO

O-Zn N-Zn N-Ag O-Ag N1-Zn N2-Zn N-Ag O-Ag

0.48 0.70 0.33 0.32 0.60 0.75 0.26 0.28

1.876 1.854 1.976 2.053 1.854 1.851 2.036 2.013

(AgZn-NO þ NO) in ZnO

Fig. 1. Schematic illustration of 4  4  1 supercell Gr-ZnO nanosheet with defects (AgZn-NO) and NO in the near (a) and far (b) arrangements, where larger (gray) balls, small (red) balls, blue balls and light blue balls stand for Zn, O, dopant N and Ag, respectively.

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electron charge density maps of Fig. 6, and can be used to quantify the bond overlap as well as the charges transfer from Ag, N to O, Zn atoms, respectively. The NO defect could decrease the bond distance. The ionic radius of N [31] (1.46 Å) and covalent radii of N (0.75 Å) are both longer than those of O2  (1.32 Å) and O (0.66 Å), respectively. But the proportion of the covalent bond to the chemical bond is larger for the N-Zn bond than for the O-Zn bond according to population analysis, which is consistent with experimental results [32]. The radius of Ag þ (1.26 Å) is longer than that of Zn2 þ (0.74 Å) as Ag is on the Zn site, which leads to the increasing of the distance between Ag and O atom in Gr-ZnO. The electronic distributions charge transfer in undoped Gr-ZnO is analyzed by the Mulliken charge analysis [27] and the corresponding results are shown in Table 2. The results indicate that the same atoms are equivalent and eZn ¼ 0.94 and eO ¼ 0.94 in charge transfer. When Ag-2 N heavy codoped of Gr-ZnO, the charge transfers of Ag and surrounding Zn atoms all decrease, which means that the ionic bonding feature is weakened. While the bond population increases, which indicates the degree of the bond overlapping increases and forms a covalent bond with weak ionic nature. These results are quantitative descriptions of the above electron charge density plots, and also show that the N-Zn bond has a combination of covalent and ionic bonding feature. Band structures and Partial DOS of Gr-ZnO nanosheet with (AgZn-2NO) complex defect are shown in Figs. 2 and 3. The (AgZn2NO) complex defect yields a singlet hole defect state above the new VBM, which is above the VBM of Gr-ZnO, hence acting as an acceptor. The calculated ionization energy of the (AgZn-2NO) complex defect is 0.296 eV, which is shallower than the single N acceptor. Compared with the Band structures and Partial DOS of

3

(AgZn-NO þNO) defect system in Figs. 4 and 5, it can be seen that the asymmetry between the N-2p state of the up and down spins is significantly enlarged at the Fermi level and there lies more localized unoccupied bands above the Fermi level in the downspin bands. In addition, the difference occurs in the up-spin state

Fig. 3. The total DOS for of Gr-ZnO nanosheet doped with (AgZn-2NO) complex defect is shown in (a). (b) shows the partial DOS of N atoms and (c) shows the partial DOS for the Ag atom, respectively. Fermi level is set to zero.

Table 2 Atom Mulliken charges of doped Gr-ZnO. N1, N2 mean that N atom is in the far away site and nearest-neighbor site, respectively. Structure Undoped-ZnO

Atom s

O Zn (AgZn-2NO) in ZnO N Ag Zn (AgZn-NO þNO) in ZnO N1 N2 Ag Zn

1.85 0.57 1.78 0.56 0.61 1.75 1.77 0.55 0.59

p

d

Total

Charge (e) Spin (hbar)

0 0.54 4.09 0.19 0.59 4.18 4.13 0.16 0.57

0 9.96 0 9.43 9.93 0 0 9.48 9.95

6.94 11.06 5.87 10.18 11.12 5.93 5.90 10.19 11.11

 0.94 0.94  0.87 0.82 0.88  0.93  0.90 0.81 0.89

0 0 0.50 0.04 0 0.76 0.04 0 0 Fig. 4. Band structures and Partial DOS of Gr-ZnO nanosheet doped with (AgZnNO þ NO) defect: the up spin (a), the down spin (b). Fermi level is set to zero.

Fig. 2. Band structures and Partial DOS of Gr-ZnO nanosheet doped with (AgZn2NO) complex defect: the up spin (a), the down spin (b). Fermi level is set to zero.

Fig. 5. The total DOS for of Gr-ZnO nanosheet doped with (AgZn-NO þNO) defect is shown in (a). (b) shows the partial DOS of N atoms and (c) shows the partial DOS for the Ag atom, respectively. Fermi level is set to zero.

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Fig. 6. The density difference of Gr-ZnO nanosheet with Ag, N codoped in the near (a) and far (b) arrangements. The position of each atom is the same with Fig. 1. The value of the isosurfaces is 0.02 e/Å3.

and the partial DOS of N-2 s states in Fig. 5 at approximately – 13 eV. The Partial DOS [Fig. 2(b)] suggests the magnetic properties of (AgZn-2NO) complex defective system, resulting in a magnetic moment of 1.24 μB per supercell. The local moment at every N atom is about 0.5 μB and the rest magnetic moment mainly comes from the spin-polarized Ag and O atoms. This clearly indicates that the magnetic moment is mainly contributed by spin-polarized N and Ag atoms. The unpaired N-2p electrons and Ag-4d electrons are responsible for the spin density of N and Ag atoms. But the total magnetic moment is lower than the (AgZn-NO þNO) defect system in Gr-ZnO, that the magnetic moment of 1.85 μB per supercell. The magnetic of (AgZn-NO þ NO) defect system is original from the N atom (0.76 μB) in the far away site. 4. Conclusions In summary, we have performed a full first-principle study on the atomic structures, electronic properties, the populations analysis and atom Mulliken charges of Ag-2 N heavy codoped of GrZnO nanosheet by using first-principles calculations based on DFT. The calculation results reveal that the substitutional N atoms on O sites slightly prefer to occupy nearest-neighbor sites of Ag to form the complex (AgZn-2NO) defect, introducing a fully occupied singlet state above the VBM. The charge transfers of Ag and surrounding Zn atoms decrease, the ionic bonding feature is weakened. Overall results of the DOS, electron charge density and Mulliken population conclude that the bonding nature of Zn-N is a combination of covalent and ionic. Ag-2N heavy codoped of Gr-ZnO forms a shallow acceptor level results in a magnetic moment of 1.24 μB per supercell, which is more favorable for ptype Gr-ZnO. Acknowledgments The authors acknowledge supports by National Key Basic Research and Development Program (2012CB619400), National Natural Science Foundation of China (NSFC, Nos. 51301020 and 51471124), Natural Science Foundation of Shaanxi province, China (2014JQ6196 and 2013JM8017), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20110201120002), the special fund for basic scientific research of central colleges of Chang’an University (No. 2013G1311053) and State Key Laboratory for Mechanical Behavior of Materials.

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