- Email: [email protected]
Influence of Cu dopant on the electronic and optical properties of graphene-like ZnO monolayer
Physica E 115 (2020) 113702
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Influence of Cu dopant on the electronic and optical properties of graphene-like ZnO monolayer Zhidan Wang a, Hongjuan Wang b, Lirui Wang a, Huifeng Zhao a, Muhammad Adnan Kamboh a, Lei Hao a, Qili Chen a, Kaihua He a, Qingbo Wang a, * a b
School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan, 430074, People’s Republic of China College of Mechanical and Electrical Engineering, Nanyang Normal University, Nanyang, 473061, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: First principles ZnO monolayer Cu doping Optical property Intrinsic vacancy
We have performed first principles to study the electronic and optical properties of Cu-doped ZnO monolayer. ZnO monolayer has both the properties of ZnO and 2D material, which is a promising material for construction of next generational electronic and optical devices. Cu doping is a method to get a p-type semiconductor and tune the optical properties, simultaneously. Our calculated formation energy shows the formation of an oxygen va cancy is easier than zinc vacancy during preparation. The band and density of states do not show Cu is a p-type doping in ZnO monolayer though Cu-doped bulk ZnO is p-type. Cu and intrinsic vacancy induce band levels between the band gap. Cu also induced a peak around the visible, near ultraviolet region. On the other hand, oxygen vacancy enhances the peak in the ultraviolet region and zinc vacancy enhances the absorption in the visible region. The colorful optical properties extend the applications of Cu-doped ZnO monolayer. We use photocatalytic applications as an example to demonstrate an application of (Zn,Cu)O monolayer. The possible reaction mechanism of Cu-doped ZnO monolayer was analyzed. Our studies not only stimulate future studies but also pave a road for broad applications of Cu-doped ZnO monolayer.
1. Introduction ZnO is an important n-type wideband semiconductor and optical material, which have large exciton binding energy, high electron mobility, lower cost, nontoxic and stable properties [1–3]. ZnO has been widely used in electronic devices, antibacterial devices, sensors, light emitting diodes (LEDs), a flat panel display, phosphorescence, UV laser, solar cell and photocatalysts [4–8]. Since the discovery of graphene [9], monolayer has drawn much attention because of its high surface area and electronic motion [10–12]. Monolayer can be used in catalysts and next-generation electronic, optical devices [13–15]. Since ZnO mono layer has been synthesized recently [16], ZnO monolayer draws much attention because ZnO monolayer contain the properties of ZnO and 2D material simultaneously. For example, Ma et al. have studied the O2 adsorption on Al-doped ZnO (1010) surface and CO oxidation on Al-doped ZnO monolayer [17,18]. Experts also have studied the elec tronic and optical properties of ZnO monolayer [19,20], and their studies show the band gap of ZnO is 4.48eV [21]. We can extend the applications of ZnO monolayer if we tune the band gap to a suitable
region. We can use solar energy more efficiently if we tune the optical properties of ZnO to the visible region because the solar energy is concentrated on the visible region [22]. At the same time, the applications of ZnO have been limited by ptype doping [23,24]. The valence electrons of Cu and Zn are 3d104s1 and 3d104s2. The valence electrons of Cu are less than those of Zn. Cu doping is a method to induce hole and get a p-type bulk ZnO semiconductor, which can tune the optical properties simultaneously [25–28]. Simi larly, Cu adsorption on the ZnO monolayer can affect the properties. The effect of Cu adsorption is also an important topic and can be consider in future studies while this paper focus mainly on the effects of Cu doping on ZnO monolayer. Oxygen and zinc vacancy appeared during ZnO monolayer preparation [29]. Cu dopant and intrinsic vacancy can tune the band gap and optical properties of ZnO monolayer, which is important and an urgent need for applications. We can manipulate the properties by introducing proper Cu and vacancy, intentionally. How ever, there are few studies on Cu-doped ZnO monolayer up to date. On the other hand, ZnO is a transition metal oxide. We need to use hybrid function [30,31] or GGA þ U [32–34] methods to study the electronic
* Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.physe.2019.113702 Received 18 May 2019; Received in revised form 14 July 2019; Accepted 3 September 2019 Available online 6 September 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
properties. The hybrid function method needs more time and computer resources, where GGA þ U can provide sufficient information in reasonable time and by using limited computer resources. Considering our conditions, we used the GGA þ U method in our calculations after careful tests.
Table 1 The relative total energy of Zn15CuO16, Zn14Cu2O16 with and without intrinsic vacancy. Relative energy (eV) of Zn14CuO16 (zinc vacancy) Relative energy (eV) of Zn15CuO15 (oxygen vacancy) Relative energy (eV) of Zn14Cu2O16 (two Cu doping) Relative energy (eV) of Zn13Cu2O16 (zinc vacancy) Relative energy (eV) of Zn14Cu2O15 (oxygen vacancy)
2. Computational details Our studies were performed by the CASTEP program in materials studio software [35], which is based on the density functional theory (DFT) [36]. Spin polarization, ultrasoft pseudopotentials [37] and Broyden Fletcher Goldfarb Shanno (BFGS) algorithm [38] have been used in our calculations. Calculations were carried out in a reciprocal space. We sampled the Brillouin zone with a 2 � 2 � 1 Monkhorst-pack grids [39]. The cutoff energy is 600 eV after the test. The used converge criterion is 5.0 � 10 5 eV/atom. Maximum force, stress and displace ment are 0.01 eV/Å, 0.02 GPa and 5.0 � 10 4 Å, respectively. We used Hubbard U [40] in our calculations are UZn,d ¼ 10.5 eV, UCu,d ¼ 4.0 eV, UO,p ¼ 7.0 eV [41,42]. The used valence electrons of Zn, Cu and O are 3d104s2, 3d104s1 and 2s22p4. A ZnO monolayer has been built and a 15 Å vacuum slab was added to avoid the interaction (Fig. 1). One and two Cu atoms were used to replace two Zn to get a 6.25 and 12.5% consent (Fig. 1). We first optimized the geometry and then calculated the elec tronic and optical properties.
1 (10 )
2 (20 )
3 (30 )
4 (40 )
5 (50 )
6 (60 )
0.4138
0.2954
0.5288
0.4206
0
0.5647
0.1106
0.0694
0
0.1154
0.0760
–
0.3622
0.5825
0.4555
0.5873
0
0.5411
0
0.1636
1.4185
0.5261
0.3386
–
0
0.1554
0.4457
0.1934
0.0463
0.1204
by Ef(Zn16-xCuxO16-y) ¼ Etotal-16EZnO þ xμmetalZn-xμmetalCu þ yμO2/2. Ef of Zn15-xCuxO16 (zinc vacancy) can be calculated by Ef(Zn15metal xCuxO16) ¼ Etotal-16EZnOþ(xþ1)μZn-xμ Cu, μ is the chemical poten tial. In the oxygen poor and oxygen rich condition, chemical potentials of O and Zn are determined by O2 gas and Zn metal. At the same time, oxygen and zinc satisfy the relation μmetalZnþμgasO2/2 ¼ EZnO. Positive and negative Eform means the endothermic and exothermic reaction. The exothermic reaction can be performed easier than endothermic reaction. The calculated Ef are shown in Fig. 2. The formation energy (Ef) of Zn15CuO16, Zn14Cu2O16 are 1.43 and 1.38 eV, which means that the high-content Cu doping is difficult. And Cu doping is easy to be formed in an oxygen atmosphere. Fig. 2 shows the Ef of Zn15CuO16, Zn14Cu2O16 with zinc or oxygen vacancy. We can see the Ef of oxygen vacancy is almost negative, while the Ef of zinc vacancy is positive. The value of Ef means the oxygen vacancy can be formed spontaneously, while zinc vacancy can be formed in a specific condition. The vacancy can be formed easier in two Cu-doped ZnO monolayer. We can select our experimental condition and get a proper
3. Results and discussions 3.1. Geometry optimization and formation energy As shown in Fig. 1, one Cu doping in ZnO monolayer has one equivalent site. There are 6 equivalent sites for another Cu doping or zinc vacancy to occupy. On the other hand, there are 5 equivalent sites for O vacancy. The configurations with the lowest energy is the most stable. The calculated energies are shown in Table 1. We can see in Table 1 zinc and oxygen vacancy prefer to 5 and 30 sites of Zn15CuO16 (Fig. 1a). Another Cu inclines to occupy site 5 in Fig. 1a. Similarly, we determined the intrinsic vacancy sites in Fig. 1b. Zinc and oxygen va cancy of Zn14Cu2O16 inclines to stay at site 1 and 10 . Our following calculations are based on the most stable configurations of Zn15CuO16 and Zn14Cu2O16 with and without intrinsic vacancy. Formation energy (Eform) is an important parameter in experimental preparation. Ef of Zn16-xCuxO16 with oxygen vacancy can be calculated
Fig. 1. The scheme of Cu doped ZnO monolayer with and without intrinsic vacancy: (a) Zn15CuO16 and (b) Zn14Cu2O16. The number of the atoms is the possible sites for doping or vacancy. 2
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
vacancy, which is in line with proper electronic and optical properties. 3.2. Band gap and electronic density of state The calculated band is shown in Fig. 3. The horizontal dash line represents the Fermi level. Conduction band minimum (CBM) and valence band maximum (VBM) are at high symmetry point Γ, which means the band gap of ZnO monolayer is direct. And the direct band gap is a merit for applications. The calculated band of the pristine ZnO monolayer is 4.03 eV, which is in line with the experiment (4.48 eV). Cu induces a band level to the band gap of ZnO monolayer, which can induce colorful optical properties. Accordingly, the band gap decreases from 4.03 to 1.897 eV (Fig. 3a). Two Cu decrease the band gap to 1.800 eV (Fig. 3a). Oxygen vacancy increases electrons while zinc vacancy decreases electrons. The vacancy decreases the localizations of the band level in the band gap. Zinc va cancy releases holes and an acceptor level appeared above the VBM. Zinc vacancy further decreases the band gap to 1.1 and 0.53 eV (Fig. 3b). Oxygen vacancy induces donor level below CBM. The band gap of Zn15CuO16 and Zn14Cu2O16 with oxygen vacancy decrease from around 1.8 to 1.016, 1.640 eV (Fig. 3c). The induced levels provide a spring board for electrons to transfer and induce colorful optical properties. The changes in the band gap of ZnO monolayer reflect the inside pro cesses of increase or decrease in electrons.
Fig. 2. The formation energy of Zn15CuO16, Zn14Cu2O16 with zinc or oxy gen vacancy.
Fig. 3. The band structures of (a) pristine ZnO monolayer, with one and two Cu dopant; (b) and (c) Zn15CuO16, Zn14Cu2O16 monolayer with zinc or oxygen vacancy. 3
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 4. The density of states (DOS) and partial DOS (PDOS) of (a) pristine ZnO monolayer, with one and two Cu dopant; (b) and (c) Zn15CuO16, Zn14Cu2O16 monolayer with zinc or oxygen vacancy. Right shows the enlarged DOS around the Fermi level. 4
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 4. (continued).
To study the band and optical properties in detail, we calculated the density of states (DOS) of Zn15CuO16, Zn15CuO16 monolayer with and without oxygen, zinc vacancy (Fig. 4). The vertical dash line does not across the Fermi level. The DOS do not show Cu-doped ZnO monolayer is p-type though the Cu-doped bulk ZnO is p-type. Fig. 4a shows the DOS of Zn15CuO16, Zn14Cu2O16 without vacancy. The DOS of pristine ZnO monolayer can be divided into four parts (one part for above the Fermi level and three parts for below the Fermi level). There are three parts of the DOS of Zn16O16, Zn15CuO16, Zn14Cu2O16 below the Fermi level (around 15.16~-14.02, 9.14~-6.08 and 5.18–0 eV, around 15.39~-14.18, 9.32~-6.37 and 5.94–0 eV, around 15.56~14.26, 9.49~-6.51 and 6.28–0 eV corresponding to Zn16O16, Zn15CuO16, Zn14Cu2O16, respectively). We can use the partial DOS (PDOS) to analyze the contribution of the different orbitals. The part above the Fermi level mainly comes from Zn 4s and the part 6.28–0 eV comes from the O 2p. On the other hand, the part around 9.46~-5.92 eV mainly comes from O 2p and Zn 3d elec trons, which means O 2p and Zn 3d hybridizes each other. The deep DOS around 15.0 eV mainly comes from the electrons of O 2s. The DOS is according to the source of the band at the same energy. Cu induces DOS around the Fermi level and interact with O 2p orbital, which means Cu mainly affect the surrounding O atoms. Cu extends the valence band and decreases the band gap, which can induce colorful optical properties. Fig. 4 b and c shows the DOS of Zn15CuO16, Zn14Cu2O16 with zinc or oxygen vacancy. Zinc vacancy decreases electrons while oxygen vacancy increases the electrons in the system. Vacancy extends the DOS around the Fermi level, which come from the PDOS of Cu 3d and O 2p. For a Cudoped ZnO monolayer with oxygen vacancies around Fermi level have 4 or 5 parts, the region of Zn15CuO15 are 16.57~-15.34, 10.69~-7.53, 7.12~-1.21, 0.98–0 eV, while the region of Zn14Cu2O15 are 17.78~-15.53, 11.83~-10.95, 10.95~-7.98, 7.29~-2.17, 2.17–0 eV. For two Cu-doped ZnO monolayer with zinc vacancies around Fermi level have 3 parts, the region of Zn14CuO16 are 17.37~-
14.74, 12.06~-6.82, 6.82–0 eV, while the region of Zn13Cu2O16 are 17.49~-14.49, 11.22~-6.54, 6.54–0 eV. The extended DOS induce colorful optical properties. 3.3. Optical properties 3.3.1. Dielectric function The optical properties can be described by a dielectric function ε(ω) ¼ ε1(ω)þiε2(ω). The imaginary part ε2(ω) is more important than real part ε1(ω). Firstly, we used equation (1) to calculate ε2(ω) and then Kramers–Kronig relation (eq. 2) to calculate the ε1(ω) [43]. The prop erties of Cu-doped ZnO monolayer is anisotropic, we used a polarized approach in our calculations (x and y are equal and parallel to the plane, while z is vertical to the plane).
ε2 ðωÞ ¼
� 2e2 π X�� c ⟨ψ k jb u ⋅rjψ vk ⟩�δ Eck Ωε0 k;v;c 2
ε1 ðωÞ ¼ 1 þ P π
Z
∞ 0
ω’ε2 ðω’Þ d ω’ ω’2 ω2
Evk
E
�
(1)
(2)
In the equation, Ω, k and Ψ are the volume, wave vector and wave function. Superscript c and v represent the conduction and valence band. Р is the principal value of integration. The calculated ε(ω) are shown in Fig. 5. We can see the effects of Cu and vacancy mainly on the lower energy region. The ε2(ω) is a pandect of optical properties. We will discuss ε2(ω) in this paragraph and ε1(ω) in the next paragraph. Fig. 5 shows there are five main peaks of imaginary part ε2(ω) of Zn16O16, Zn15CuO16, Zn14Cu2O16 in z direction. The posi tions of these peaks appearing on Zn16O16 monolayer without vacancy are 8.24, 12.98, 15.17, 18.28, 23.79 eV. For Zn15CuO16 monolayer without vacancy, these peaks appear at 8.32, 13.06, 14.94, 17.64, 23.82 eV. These peaks of Zn14Cu2O16 appearance are like Zn15CuO16. The ε2(ω) of Zn16O16 has four peaks in the x direction (4.58, 9.65, 13.87, 5
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 5. The imaginary and real part of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen of zinc vacancy in x and z directions.
15.01 eV). On the other hand, the imaginary part of the complex dielectric function of Zn15CuO16 and Zn14Cu2O16 has only three peaks in the x direction (4.58, 9.60, 13.98 eV and 4.63, 9.55, 14.04 eV) of Zn15CuO16, Zn14Cu2O16 monolayer without vacancy. The peaks of ε2(ω) come from the electronic transition, which obey the selection rules. The broad peak below 4.6 eV comes from the intra-shell transition of Cu 3d orbital. The peaks between 8.3–15.0 eV come from the transition be tween Zn 3d and O 2p orbital. The peaks around 17.5 eV come from the electronic transition between Zn 3d and O 2s orbital. The peaks around 23.8 eV come from the deep orbital electronic transition. For the real part ε1(ω), the static ε1(0) and high frequency limitation ε1(∞) are important parameters in applications. The ε1(0) of Zn16O16 is 1.21 and 1.12 in x and z direction. The ε1(0) of Zn15CuO16 and Zn14Cu2O16 are similar in the z direction (1.11). In the x direction, the ε1(0) of Zn15CuO16 and Zn14Cu2O16 are 1.22 and 1.24 respectively. On the other hand, high frequency limitation ε1(∞) approaches 0.98 and 0.97 for Zn16O16, Zn15CuO16, Zn14Cu2O16 in x and z direction. For the (Zn,Cu)O monolayer with oxygen vacancy, the ε1(0) of Zn15CuO15 changes to 1.14, while the Zn14Cu2O15 changes to 1.12 (z direction). The similar ε1(0) of Zn15CuO15 and Zn14Cu2O15 is 1.30 and 1.26 (x direc tion). The ε1(0) of Zn14CuO16 and Zn13Cu2O16 change to 1.11 (z
direction). In the x direction, the real part of Zn14CuO16 and Zn13Cu2O16 with zinc vacancy change the ε1(0) to 1.24 and 1.35. Th1e oxygen and zinc vacancy change the ε1(∞) to 0.98 (x and z direction). We can use these values to identify the Cu doping and vacancy. 3.3.2. Optical constants The optical constants are important in applications. We used equa tions (3)–(7) [44,45] to calculate the optical constants (absorption co efficient (α(ω)), loss function (L(ω)), reflectivity (R(ω)), real part of refraction index (n(ω)), and imaginary part of refraction index (k(ω))):
αðωÞ ¼
LðωÞ ¼
� pffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ω ε21 ðωÞ þ ε22 ðωÞ
ε2 ðωÞ ε21 ðωÞ þ ε22 ðωÞ
�pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � � ε ðωÞ þ jε ðωÞ 1�2 � 1 � 2 RðωÞ ¼ �pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � � ε1 ðωÞ þ jε2 ðωÞ þ 1�
6
�1=2
ε1 ðωÞ
(3) (4)
(5)
Z. Wang et al.
nðωÞ ¼
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
�qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi �1=2 ε21 ðωÞ þ ε22 ðωÞ þ ε1 ðωÞ
�qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kðωÞ ¼ ε21 ðωÞ þ ε22 ðωÞ
,
pffiffi 2
(6)
�1=2 , pffiffi 2 ε1 ðωÞ
(7)
vacancy of Zn15CuO16 and Zn14Cu2O16 enhances the absorption in the visible and near ultraviolet region. 3.3.3. Loss function The loss function (L(ω)) (Fig. 7) reflects the energy loss when a photon passes through the monolayer. There are 5 peaks (8.42, 13.28, 15.25, 17.72 and 23.88 eV) in Fig. 7a and 3 peaks (4.70, 10.00 and 15.44 eV) in Fig. 7b, which reflect the changes due to energy loss. The energy loss in z direction concentrates on higher energy than that in x direction. Cu induces energy loss at low energy. Oxygen enhances the energy loss at lower energy while the changes of that with zinc vacancy are little, which means vacancy can boost the performance of Cu-doped ZnO monolayer in the visible and ultra-violet region.
Since α(ω) and L(ω) are more important than the other constants in solar energy usage, firstly we discuss α(ω) in this section and L(ω) in the next section. Other optical constants are in the supplementary data (SD). Due to eq. (3), the profile of α(ω) is like ε2(ω). There are mainly five peaks in Fig. 6a (around 8, 13, 15, 18 and 24 eV). Fig. 6a shows the Cu changes the α(ω) in z direction little. On the other hand, Fig. 6b shows Cu induces and absorption in the visible region appeared in x direction which can extend the applications of ZnO monolayer as optical devices and catalysts. Fig. 6c shows oxygen vacancy shifts the α(ω) of Zn14Cu2O16 mono layer to lower energy mostly in z direction. The effects of vacancy mainly at lower energy. The other vacancy changes the α(ω) little. To the x direction aspect (Fig. 6d), oxygen vacancy of Zn15CuO16 monolayer induces a broad peak in the near ultraviolet region (3.73 eV). Zinc
3.3.4. Photocatalytic application mechanism The tuned band gap and optical properties provide flexible applica tions of (Zn,Cu)O monolayer. In this paragraph, photocatalytic appli cations are discussed to demonstrate an application. Fig. 8 shows the photocatalytic mechanism of Cu-doped ZnO monolayer with and without a vacancy. Cu induces a band level between the band gap.
Fig. 6. The absorption coefficient (α(ω)) of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen or zinc vacancy in x and z directions. The Vis and NUV mean the visible and near ultraviolet region. 7
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 7. The loss function (L(ω)) of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen or zinc vacancy in x and z directions.
Oxygen vacancy induces a donor level while zinc vacancy induces an acceptor level. The appeared band level serves as a trap to constrain the recombination between photo-generated electrons and holes. The chemical reaction is in eq. (8)–(11): ðZn; CuÞO þ h→ðZn; CuÞOðe þhþ Þ
(8)
e þhþ →heat
(9)
O2þe →(Zn,Cu)Oþ�O2 þ
h þOH → OH �
can react with pollution, directly. 4. Conclusions In this paper, we calculated the formation energy, electronic and optical properties of Cu-doped ZnO monolayer by first principles. Our calculations do not show Cu is a p-type doping in ZnO monolayer though the Cu-doped bulk ZnO is p-type. Cu induces a band level and colorful optical properties. The Ef shows high-content Cu doping is more difficult than low-content Cu doping. Oxygen vacancy can be created easily than zinc vacancy. Cu induces absorption around the visible and ultraviolet region. Oxygen vacancy enhances the absorption more than zinc va cancy. We also discuss the photocatalytic mechanism where the Cu doping and vacancy play an important role in photocatalytic process. We can tune the electronic and optical properties by the proper exper imental condition. Our studies explored the properties of Cu-doped ZnO monolayer and pave a road for its flexible applications.
(10) (11)
(Zn,Cu)O monolayer absorbs the solar energy and produces electrons (e ). Electrons transfer to the conduction band and leave holes (hþ) around the valence band (eq. 8). The e and hþ can recombine and release heat (eq. 9). Cu and intrinsic vacancy can serve as a trap to restrain the recombination between e and hþ. Electrons respond to free O2 and produce superoxide radicals (�O2 ) (eq. 10). On the other hand, OH from the moisture react with holes and produce hydroxyl radicals (�OH) (eq. 11), which is a strong oxidant. The �O2 and �OH can decompose pollution to CO2 and H2O. Holes in the conduction band also
Conflicts of interest We declare that we have no financial and personal relationships with 8
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 8. The photocatalytic mechanism of Cu doped ZnO monolayer with and without intrinsic vacancy.
other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Influence of Cu dopant on the electronic and optical properties of graphenelike ZnO monolayer”.
[14] A. Favron, E. Gaufres, F. Fossard, A.-L. Phaneuf-L’Heureux, N.Y.W. Tang, P. L. Levesque, A. Loiseau, R. Leonelli, S. Francoeur, R. Martel, Nat. Mater. 14 (2015), 826-þ. [15] M.-Y. Li, Y. Shi, C.-C. Cheng, L.-S. Lu, Y.-C. Lin, H.-L. Tang, M.-L. Tsai, C.-W. Chu, K.-H. Wei, J.-H. He, W.-H. Chang, K. Suenaga, L.-J. Li, Science 349 (2015) 524–528. [16] C. Tusche, H.L. Meyerheim, J. Kirschner, Phys. Rev. Lett. 99 (2007), 026102. [17] D. Ma, Q. Wang, T. Li, Z. Tang, G. Yang, C. He, Z. Lu, J. Mater. Chem. C 3 (2015) 9964–9972. [18] D. Ma, Z. Wang, H. Cui, J. Zeng, C. He, Z. Lu, Sens. Actuators B 224 (2016) 372–380. [19] H. Chen, C. Tan, D. Sun, W. Zhao, X. Tian, Y. Huang, RSC Adv. 8 (2018) 1392–1397. [20] T. Sahoo, S.K. Nayak, P. Chelliah, M.K. Rath, B. Parida, Mater. Res. Bull. 75 (2016) 134–138. [21] J. Lee, D.C. Sorescu, X. Deng, J. Phys. Chem. Lett. 7 (2016) 1335–1340. [22] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243–2245. [23] H.D. Weldekirstos, M.-C. Kuo, S.-R. Li, W.-L. Su, M.A. Desta, W.-T. Wu, C.-H. Kuo, S.-S. Sun, Dyes Pigments 163 (2019) 761–774. [24] W. Huang, C. Harnagea, X. Tong, D. Benetti, S. Sun, M. Chaker, F. Rosei, R. Nechache, ACS Appl. Mater. Interfaces (2019) 13185–13193. [25] S. Zhuang, M. Lu, N. Zhou, L. Zhou, D. Lin, Z. Peng, Q. Wu, Electrochim. Acta 294 (2019) 28–37. [26] P.K.R. Boppidi, P.M.P. Raj, S. Challagulla, S.R. Gollu, S. Roy, S. Banerjee, S. Kundu, J. Appl. Phys. 124 (2018) 214901. [27] Z.N. Kayani, S. Iram, R. Rafi, S. Riaz, S. Naseem, Appl. Phys. A 124 (2018) 468. [28] Z. Ma, F. Ren, X. Ming, Y. Long, A.A. Volinsky, Materials 12 (2019). [29] T. Hussain, M. Hankel, D.J. Searles, J. Phys. Chem. C 121 (2017) 24365–24375. [30] X. Han, N. Amrane, Z. Zhang, M. Benkraouda, J. Phys. Chem. C 123 (2019) 2037–2047. [31] W.J. Kim, M.H. Han, S. Lebegue, E.K. Lee, H. Kim, Front. Chem. 7 (2019) 47. [32] R. Liu, F. Yang, Y. Xie, Y. Yu, Appl. Surf. Sci. 466 (2019) 568–577. [33] A. Bentouaf, R. Mebsout, B. Aissa, J. Alloy. Comp. 771 (2019) 1062–1071. [34] Y.-J. Li, M. Wang, M.-y. Tang, X. Tian, S. Gao, Z. He, Y. Li, T.-G. Zhou, Physica E 75 (2016) 169–173. [35] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M. C. Payne, J. Phys. 14 (2002) 2717–2744. [36] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868. [37] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758–1775. [38] Z.W. Geem, J. Irrig. Drain. Eng. 132 (2006) 474–478. [39] P. Wisesa, K.A. McGill, T. Mueller, Phys. Rev. B 93 (2016). [40] V.I. Anisimov, J. Zaanen, O.K. Andersen, Phys. Rev. B 44 (1991) 943–954. [41] R.M. Sheetz, I. Ponomareva, E. Richter, A.N. Andriotis, M. Menon, Phys. Rev. B 80 (2009) 195314. [42] L. Wang, T. Maxisch, G. Ceder, Phys. Rev. B 73 (2006) 195107. [43] G.E. Jellison, F.A. Modine, Appl. Phys. Lett. 69 (1996) 371–373. [44] M. Roknuzzaman, M.A. Hadi, M.J. Abden, M.T. Nasir, A.K.M.A. Islam, M.S. Ali, K. Ostrikov, S.H. Naqib, Comput. Mater. Sci. 113 (2016) 148–153. [45] A. Chowdhury, M.A. Ali, M.M. Hossain, M.M. Uddin, S.H. Naqib, A.K.M.A. Islam, Phys. Status Solidi B 255 (2018) 1700235.
Acknowledgements The Project was supported by the National Natural Science Foun dation of China (41402034 and 41474067), China Postdoctoral Science Foundation (2018T110815 and 2016M600622) and the Fundamental Research Funds for the Central Universities, China University of Geo sciences (Wuhan) (CUGL150418). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.physe.2019.113702. References [1] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005), 041301. [2] Z.L. Wang, J. Phys. 16 (2004) R829–R858. [3] P. Dash, A. Manna, N.C. Mishra, S. Varma, Physica E 107 (2019) 38–46. [4] W. Yu, D. Xu, T. Peng, J. Mater. Chem. 3 (2015) 19936–19947. [5] Y.K. Mishra, G. Modi, V. Cretu, V. Postica, O. Lupan, T. Reimer, I. Paulowicz, V. Hrkac, W. Benecke, L. Kienle, R. Adelung, ACS Appl. Mater. Interfaces 7 (2015) 14303–14316. [6] W. Kuang, Q. Zhong, X. Ye, Y. Yan, Y. Yang, J. Zhang, L. Huang, S. Tan, Q. Shi, J. Nanosci. Nanotechnol. 19 (2019) 3982–3990. [7] Y. Al-Hadeethi, A. Umar, K. Singh, A.A. Ibrahim, S.H. Al-Heniti, B.M. Raffah, J. Nanosci. Nanotechnol. 19 (2019) 4199–4204. [8] S. Li, J. Pan, H. Li, Y. Liu, W. Ou, J. Wang, C. Song, W. Zhao, Y. Zheng, C. Li, Chem. Eng. J. 366 (2019) 305–312. [9] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [10] C. Yu, X. Cheng, C. Wang, Z. Wang, Physica E 110 (2019) 148–152. [11] A.J. Mannix, X.-F. Zhou, B. Kiraly, J.D. Wood, D. Alducin, B.D. Myers, X. Liu, B. L. Fisher, U. Santiago, J.R. Guest, M.J. Yacaman, A. Ponce, A.R. Oganov, M. C. Hersam, N.P. Guisinger, Science 350 (2015) 1513–1516. [12] S. Wu, S. Buckley, J.R. Schaibley, L. Feng, J. Yan, D.G. Mandrus, F. Hatami, W. Yao, J. Vuckovic, A. Majumdar, X. Xu, Nature 520 (2015) 69–U142. [13] X. Wang, A.M. Jones, K.L. Seyler, T. Vy, Y. Jia, H. Zhao, H. Wang, L. Yang, X. Xu, F. Xia, Nat. Nanotechnol. 10 (2015) 517–521.
9
Contents lists available at ScienceDirect
Physica E: Low-dimensional Systems and Nanostructures journal homepage: http://www.elsevier.com/locate/physe
Influence of Cu dopant on the electronic and optical properties of graphene-like ZnO monolayer Zhidan Wang a, Hongjuan Wang b, Lirui Wang a, Huifeng Zhao a, Muhammad Adnan Kamboh a, Lei Hao a, Qili Chen a, Kaihua He a, Qingbo Wang a, * a b
School of Mathematics and Physics, China University of Geosciences (Wuhan), Wuhan, 430074, People’s Republic of China College of Mechanical and Electrical Engineering, Nanyang Normal University, Nanyang, 473061, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: First principles ZnO monolayer Cu doping Optical property Intrinsic vacancy
We have performed first principles to study the electronic and optical properties of Cu-doped ZnO monolayer. ZnO monolayer has both the properties of ZnO and 2D material, which is a promising material for construction of next generational electronic and optical devices. Cu doping is a method to get a p-type semiconductor and tune the optical properties, simultaneously. Our calculated formation energy shows the formation of an oxygen va cancy is easier than zinc vacancy during preparation. The band and density of states do not show Cu is a p-type doping in ZnO monolayer though Cu-doped bulk ZnO is p-type. Cu and intrinsic vacancy induce band levels between the band gap. Cu also induced a peak around the visible, near ultraviolet region. On the other hand, oxygen vacancy enhances the peak in the ultraviolet region and zinc vacancy enhances the absorption in the visible region. The colorful optical properties extend the applications of Cu-doped ZnO monolayer. We use photocatalytic applications as an example to demonstrate an application of (Zn,Cu)O monolayer. The possible reaction mechanism of Cu-doped ZnO monolayer was analyzed. Our studies not only stimulate future studies but also pave a road for broad applications of Cu-doped ZnO monolayer.
1. Introduction ZnO is an important n-type wideband semiconductor and optical material, which have large exciton binding energy, high electron mobility, lower cost, nontoxic and stable properties [1–3]. ZnO has been widely used in electronic devices, antibacterial devices, sensors, light emitting diodes (LEDs), a flat panel display, phosphorescence, UV laser, solar cell and photocatalysts [4–8]. Since the discovery of graphene [9], monolayer has drawn much attention because of its high surface area and electronic motion [10–12]. Monolayer can be used in catalysts and next-generation electronic, optical devices [13–15]. Since ZnO mono layer has been synthesized recently [16], ZnO monolayer draws much attention because ZnO monolayer contain the properties of ZnO and 2D material simultaneously. For example, Ma et al. have studied the O2 adsorption on Al-doped ZnO (1010) surface and CO oxidation on Al-doped ZnO monolayer [17,18]. Experts also have studied the elec tronic and optical properties of ZnO monolayer [19,20], and their studies show the band gap of ZnO is 4.48eV [21]. We can extend the applications of ZnO monolayer if we tune the band gap to a suitable
region. We can use solar energy more efficiently if we tune the optical properties of ZnO to the visible region because the solar energy is concentrated on the visible region [22]. At the same time, the applications of ZnO have been limited by ptype doping [23,24]. The valence electrons of Cu and Zn are 3d104s1 and 3d104s2. The valence electrons of Cu are less than those of Zn. Cu doping is a method to induce hole and get a p-type bulk ZnO semiconductor, which can tune the optical properties simultaneously [25–28]. Simi larly, Cu adsorption on the ZnO monolayer can affect the properties. The effect of Cu adsorption is also an important topic and can be consider in future studies while this paper focus mainly on the effects of Cu doping on ZnO monolayer. Oxygen and zinc vacancy appeared during ZnO monolayer preparation [29]. Cu dopant and intrinsic vacancy can tune the band gap and optical properties of ZnO monolayer, which is important and an urgent need for applications. We can manipulate the properties by introducing proper Cu and vacancy, intentionally. How ever, there are few studies on Cu-doped ZnO monolayer up to date. On the other hand, ZnO is a transition metal oxide. We need to use hybrid function [30,31] or GGA þ U [32–34] methods to study the electronic
* Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.physe.2019.113702 Received 18 May 2019; Received in revised form 14 July 2019; Accepted 3 September 2019 Available online 6 September 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
properties. The hybrid function method needs more time and computer resources, where GGA þ U can provide sufficient information in reasonable time and by using limited computer resources. Considering our conditions, we used the GGA þ U method in our calculations after careful tests.
Table 1 The relative total energy of Zn15CuO16, Zn14Cu2O16 with and without intrinsic vacancy. Relative energy (eV) of Zn14CuO16 (zinc vacancy) Relative energy (eV) of Zn15CuO15 (oxygen vacancy) Relative energy (eV) of Zn14Cu2O16 (two Cu doping) Relative energy (eV) of Zn13Cu2O16 (zinc vacancy) Relative energy (eV) of Zn14Cu2O15 (oxygen vacancy)
2. Computational details Our studies were performed by the CASTEP program in materials studio software [35], which is based on the density functional theory (DFT) [36]. Spin polarization, ultrasoft pseudopotentials [37] and Broyden Fletcher Goldfarb Shanno (BFGS) algorithm [38] have been used in our calculations. Calculations were carried out in a reciprocal space. We sampled the Brillouin zone with a 2 � 2 � 1 Monkhorst-pack grids [39]. The cutoff energy is 600 eV after the test. The used converge criterion is 5.0 � 10 5 eV/atom. Maximum force, stress and displace ment are 0.01 eV/Å, 0.02 GPa and 5.0 � 10 4 Å, respectively. We used Hubbard U [40] in our calculations are UZn,d ¼ 10.5 eV, UCu,d ¼ 4.0 eV, UO,p ¼ 7.0 eV [41,42]. The used valence electrons of Zn, Cu and O are 3d104s2, 3d104s1 and 2s22p4. A ZnO monolayer has been built and a 15 Å vacuum slab was added to avoid the interaction (Fig. 1). One and two Cu atoms were used to replace two Zn to get a 6.25 and 12.5% consent (Fig. 1). We first optimized the geometry and then calculated the elec tronic and optical properties.
1 (10 )
2 (20 )
3 (30 )
4 (40 )
5 (50 )
6 (60 )
0.4138
0.2954
0.5288
0.4206
0
0.5647
0.1106
0.0694
0
0.1154
0.0760
–
0.3622
0.5825
0.4555
0.5873
0
0.5411
0
0.1636
1.4185
0.5261
0.3386
–
0
0.1554
0.4457
0.1934
0.0463
0.1204
by Ef(Zn16-xCuxO16-y) ¼ Etotal-16EZnO þ xμmetalZn-xμmetalCu þ yμO2/2. Ef of Zn15-xCuxO16 (zinc vacancy) can be calculated by Ef(Zn15metal xCuxO16) ¼ Etotal-16EZnOþ(xþ1)μZn-xμ Cu, μ is the chemical poten tial. In the oxygen poor and oxygen rich condition, chemical potentials of O and Zn are determined by O2 gas and Zn metal. At the same time, oxygen and zinc satisfy the relation μmetalZnþμgasO2/2 ¼ EZnO. Positive and negative Eform means the endothermic and exothermic reaction. The exothermic reaction can be performed easier than endothermic reaction. The calculated Ef are shown in Fig. 2. The formation energy (Ef) of Zn15CuO16, Zn14Cu2O16 are 1.43 and 1.38 eV, which means that the high-content Cu doping is difficult. And Cu doping is easy to be formed in an oxygen atmosphere. Fig. 2 shows the Ef of Zn15CuO16, Zn14Cu2O16 with zinc or oxygen vacancy. We can see the Ef of oxygen vacancy is almost negative, while the Ef of zinc vacancy is positive. The value of Ef means the oxygen vacancy can be formed spontaneously, while zinc vacancy can be formed in a specific condition. The vacancy can be formed easier in two Cu-doped ZnO monolayer. We can select our experimental condition and get a proper
3. Results and discussions 3.1. Geometry optimization and formation energy As shown in Fig. 1, one Cu doping in ZnO monolayer has one equivalent site. There are 6 equivalent sites for another Cu doping or zinc vacancy to occupy. On the other hand, there are 5 equivalent sites for O vacancy. The configurations with the lowest energy is the most stable. The calculated energies are shown in Table 1. We can see in Table 1 zinc and oxygen vacancy prefer to 5 and 30 sites of Zn15CuO16 (Fig. 1a). Another Cu inclines to occupy site 5 in Fig. 1a. Similarly, we determined the intrinsic vacancy sites in Fig. 1b. Zinc and oxygen va cancy of Zn14Cu2O16 inclines to stay at site 1 and 10 . Our following calculations are based on the most stable configurations of Zn15CuO16 and Zn14Cu2O16 with and without intrinsic vacancy. Formation energy (Eform) is an important parameter in experimental preparation. Ef of Zn16-xCuxO16 with oxygen vacancy can be calculated
Fig. 1. The scheme of Cu doped ZnO monolayer with and without intrinsic vacancy: (a) Zn15CuO16 and (b) Zn14Cu2O16. The number of the atoms is the possible sites for doping or vacancy. 2
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
vacancy, which is in line with proper electronic and optical properties. 3.2. Band gap and electronic density of state The calculated band is shown in Fig. 3. The horizontal dash line represents the Fermi level. Conduction band minimum (CBM) and valence band maximum (VBM) are at high symmetry point Γ, which means the band gap of ZnO monolayer is direct. And the direct band gap is a merit for applications. The calculated band of the pristine ZnO monolayer is 4.03 eV, which is in line with the experiment (4.48 eV). Cu induces a band level to the band gap of ZnO monolayer, which can induce colorful optical properties. Accordingly, the band gap decreases from 4.03 to 1.897 eV (Fig. 3a). Two Cu decrease the band gap to 1.800 eV (Fig. 3a). Oxygen vacancy increases electrons while zinc vacancy decreases electrons. The vacancy decreases the localizations of the band level in the band gap. Zinc va cancy releases holes and an acceptor level appeared above the VBM. Zinc vacancy further decreases the band gap to 1.1 and 0.53 eV (Fig. 3b). Oxygen vacancy induces donor level below CBM. The band gap of Zn15CuO16 and Zn14Cu2O16 with oxygen vacancy decrease from around 1.8 to 1.016, 1.640 eV (Fig. 3c). The induced levels provide a spring board for electrons to transfer and induce colorful optical properties. The changes in the band gap of ZnO monolayer reflect the inside pro cesses of increase or decrease in electrons.
Fig. 2. The formation energy of Zn15CuO16, Zn14Cu2O16 with zinc or oxy gen vacancy.
Fig. 3. The band structures of (a) pristine ZnO monolayer, with one and two Cu dopant; (b) and (c) Zn15CuO16, Zn14Cu2O16 monolayer with zinc or oxygen vacancy. 3
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 4. The density of states (DOS) and partial DOS (PDOS) of (a) pristine ZnO monolayer, with one and two Cu dopant; (b) and (c) Zn15CuO16, Zn14Cu2O16 monolayer with zinc or oxygen vacancy. Right shows the enlarged DOS around the Fermi level. 4
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 4. (continued).
To study the band and optical properties in detail, we calculated the density of states (DOS) of Zn15CuO16, Zn15CuO16 monolayer with and without oxygen, zinc vacancy (Fig. 4). The vertical dash line does not across the Fermi level. The DOS do not show Cu-doped ZnO monolayer is p-type though the Cu-doped bulk ZnO is p-type. Fig. 4a shows the DOS of Zn15CuO16, Zn14Cu2O16 without vacancy. The DOS of pristine ZnO monolayer can be divided into four parts (one part for above the Fermi level and three parts for below the Fermi level). There are three parts of the DOS of Zn16O16, Zn15CuO16, Zn14Cu2O16 below the Fermi level (around 15.16~-14.02, 9.14~-6.08 and 5.18–0 eV, around 15.39~-14.18, 9.32~-6.37 and 5.94–0 eV, around 15.56~14.26, 9.49~-6.51 and 6.28–0 eV corresponding to Zn16O16, Zn15CuO16, Zn14Cu2O16, respectively). We can use the partial DOS (PDOS) to analyze the contribution of the different orbitals. The part above the Fermi level mainly comes from Zn 4s and the part 6.28–0 eV comes from the O 2p. On the other hand, the part around 9.46~-5.92 eV mainly comes from O 2p and Zn 3d elec trons, which means O 2p and Zn 3d hybridizes each other. The deep DOS around 15.0 eV mainly comes from the electrons of O 2s. The DOS is according to the source of the band at the same energy. Cu induces DOS around the Fermi level and interact with O 2p orbital, which means Cu mainly affect the surrounding O atoms. Cu extends the valence band and decreases the band gap, which can induce colorful optical properties. Fig. 4 b and c shows the DOS of Zn15CuO16, Zn14Cu2O16 with zinc or oxygen vacancy. Zinc vacancy decreases electrons while oxygen vacancy increases the electrons in the system. Vacancy extends the DOS around the Fermi level, which come from the PDOS of Cu 3d and O 2p. For a Cudoped ZnO monolayer with oxygen vacancies around Fermi level have 4 or 5 parts, the region of Zn15CuO15 are 16.57~-15.34, 10.69~-7.53, 7.12~-1.21, 0.98–0 eV, while the region of Zn14Cu2O15 are 17.78~-15.53, 11.83~-10.95, 10.95~-7.98, 7.29~-2.17, 2.17–0 eV. For two Cu-doped ZnO monolayer with zinc vacancies around Fermi level have 3 parts, the region of Zn14CuO16 are 17.37~-
14.74, 12.06~-6.82, 6.82–0 eV, while the region of Zn13Cu2O16 are 17.49~-14.49, 11.22~-6.54, 6.54–0 eV. The extended DOS induce colorful optical properties. 3.3. Optical properties 3.3.1. Dielectric function The optical properties can be described by a dielectric function ε(ω) ¼ ε1(ω)þiε2(ω). The imaginary part ε2(ω) is more important than real part ε1(ω). Firstly, we used equation (1) to calculate ε2(ω) and then Kramers–Kronig relation (eq. 2) to calculate the ε1(ω) [43]. The prop erties of Cu-doped ZnO monolayer is anisotropic, we used a polarized approach in our calculations (x and y are equal and parallel to the plane, while z is vertical to the plane).
ε2 ðωÞ ¼
� 2e2 π X�� c ⟨ψ k jb u ⋅rjψ vk ⟩�δ Eck Ωε0 k;v;c 2
ε1 ðωÞ ¼ 1 þ P π
Z
∞ 0
ω’ε2 ðω’Þ d ω’ ω’2 ω2
Evk
E
�
(1)
(2)
In the equation, Ω, k and Ψ are the volume, wave vector and wave function. Superscript c and v represent the conduction and valence band. Р is the principal value of integration. The calculated ε(ω) are shown in Fig. 5. We can see the effects of Cu and vacancy mainly on the lower energy region. The ε2(ω) is a pandect of optical properties. We will discuss ε2(ω) in this paragraph and ε1(ω) in the next paragraph. Fig. 5 shows there are five main peaks of imaginary part ε2(ω) of Zn16O16, Zn15CuO16, Zn14Cu2O16 in z direction. The posi tions of these peaks appearing on Zn16O16 monolayer without vacancy are 8.24, 12.98, 15.17, 18.28, 23.79 eV. For Zn15CuO16 monolayer without vacancy, these peaks appear at 8.32, 13.06, 14.94, 17.64, 23.82 eV. These peaks of Zn14Cu2O16 appearance are like Zn15CuO16. The ε2(ω) of Zn16O16 has four peaks in the x direction (4.58, 9.65, 13.87, 5
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 5. The imaginary and real part of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen of zinc vacancy in x and z directions.
15.01 eV). On the other hand, the imaginary part of the complex dielectric function of Zn15CuO16 and Zn14Cu2O16 has only three peaks in the x direction (4.58, 9.60, 13.98 eV and 4.63, 9.55, 14.04 eV) of Zn15CuO16, Zn14Cu2O16 monolayer without vacancy. The peaks of ε2(ω) come from the electronic transition, which obey the selection rules. The broad peak below 4.6 eV comes from the intra-shell transition of Cu 3d orbital. The peaks between 8.3–15.0 eV come from the transition be tween Zn 3d and O 2p orbital. The peaks around 17.5 eV come from the electronic transition between Zn 3d and O 2s orbital. The peaks around 23.8 eV come from the deep orbital electronic transition. For the real part ε1(ω), the static ε1(0) and high frequency limitation ε1(∞) are important parameters in applications. The ε1(0) of Zn16O16 is 1.21 and 1.12 in x and z direction. The ε1(0) of Zn15CuO16 and Zn14Cu2O16 are similar in the z direction (1.11). In the x direction, the ε1(0) of Zn15CuO16 and Zn14Cu2O16 are 1.22 and 1.24 respectively. On the other hand, high frequency limitation ε1(∞) approaches 0.98 and 0.97 for Zn16O16, Zn15CuO16, Zn14Cu2O16 in x and z direction. For the (Zn,Cu)O monolayer with oxygen vacancy, the ε1(0) of Zn15CuO15 changes to 1.14, while the Zn14Cu2O15 changes to 1.12 (z direction). The similar ε1(0) of Zn15CuO15 and Zn14Cu2O15 is 1.30 and 1.26 (x direc tion). The ε1(0) of Zn14CuO16 and Zn13Cu2O16 change to 1.11 (z
direction). In the x direction, the real part of Zn14CuO16 and Zn13Cu2O16 with zinc vacancy change the ε1(0) to 1.24 and 1.35. Th1e oxygen and zinc vacancy change the ε1(∞) to 0.98 (x and z direction). We can use these values to identify the Cu doping and vacancy. 3.3.2. Optical constants The optical constants are important in applications. We used equa tions (3)–(7) [44,45] to calculate the optical constants (absorption co efficient (α(ω)), loss function (L(ω)), reflectivity (R(ω)), real part of refraction index (n(ω)), and imaginary part of refraction index (k(ω))):
αðωÞ ¼
LðωÞ ¼
� pffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2ω ε21 ðωÞ þ ε22 ðωÞ
ε2 ðωÞ ε21 ðωÞ þ ε22 ðωÞ
�pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � � ε ðωÞ þ jε ðωÞ 1�2 � 1 � 2 RðωÞ ¼ �pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � � ε1 ðωÞ þ jε2 ðωÞ þ 1�
6
�1=2
ε1 ðωÞ
(3) (4)
(5)
Z. Wang et al.
nðωÞ ¼
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
�qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi �1=2 ε21 ðωÞ þ ε22 ðωÞ þ ε1 ðωÞ
�qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi kðωÞ ¼ ε21 ðωÞ þ ε22 ðωÞ
,
pffiffi 2
(6)
�1=2 , pffiffi 2 ε1 ðωÞ
(7)
vacancy of Zn15CuO16 and Zn14Cu2O16 enhances the absorption in the visible and near ultraviolet region. 3.3.3. Loss function The loss function (L(ω)) (Fig. 7) reflects the energy loss when a photon passes through the monolayer. There are 5 peaks (8.42, 13.28, 15.25, 17.72 and 23.88 eV) in Fig. 7a and 3 peaks (4.70, 10.00 and 15.44 eV) in Fig. 7b, which reflect the changes due to energy loss. The energy loss in z direction concentrates on higher energy than that in x direction. Cu induces energy loss at low energy. Oxygen enhances the energy loss at lower energy while the changes of that with zinc vacancy are little, which means vacancy can boost the performance of Cu-doped ZnO monolayer in the visible and ultra-violet region.
Since α(ω) and L(ω) are more important than the other constants in solar energy usage, firstly we discuss α(ω) in this section and L(ω) in the next section. Other optical constants are in the supplementary data (SD). Due to eq. (3), the profile of α(ω) is like ε2(ω). There are mainly five peaks in Fig. 6a (around 8, 13, 15, 18 and 24 eV). Fig. 6a shows the Cu changes the α(ω) in z direction little. On the other hand, Fig. 6b shows Cu induces and absorption in the visible region appeared in x direction which can extend the applications of ZnO monolayer as optical devices and catalysts. Fig. 6c shows oxygen vacancy shifts the α(ω) of Zn14Cu2O16 mono layer to lower energy mostly in z direction. The effects of vacancy mainly at lower energy. The other vacancy changes the α(ω) little. To the x direction aspect (Fig. 6d), oxygen vacancy of Zn15CuO16 monolayer induces a broad peak in the near ultraviolet region (3.73 eV). Zinc
3.3.4. Photocatalytic application mechanism The tuned band gap and optical properties provide flexible applica tions of (Zn,Cu)O monolayer. In this paragraph, photocatalytic appli cations are discussed to demonstrate an application. Fig. 8 shows the photocatalytic mechanism of Cu-doped ZnO monolayer with and without a vacancy. Cu induces a band level between the band gap.
Fig. 6. The absorption coefficient (α(ω)) of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen or zinc vacancy in x and z directions. The Vis and NUV mean the visible and near ultraviolet region. 7
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 7. The loss function (L(ω)) of Zn15CuO16, Zn14Cu2O16 monolayer with and without oxygen or zinc vacancy in x and z directions.
Oxygen vacancy induces a donor level while zinc vacancy induces an acceptor level. The appeared band level serves as a trap to constrain the recombination between photo-generated electrons and holes. The chemical reaction is in eq. (8)–(11): ðZn; CuÞO þ h→ðZn; CuÞOðe þhþ Þ
(8)
e þhþ →heat
(9)
O2þe →(Zn,Cu)Oþ�O2 þ
h þOH → OH �
can react with pollution, directly. 4. Conclusions In this paper, we calculated the formation energy, electronic and optical properties of Cu-doped ZnO monolayer by first principles. Our calculations do not show Cu is a p-type doping in ZnO monolayer though the Cu-doped bulk ZnO is p-type. Cu induces a band level and colorful optical properties. The Ef shows high-content Cu doping is more difficult than low-content Cu doping. Oxygen vacancy can be created easily than zinc vacancy. Cu induces absorption around the visible and ultraviolet region. Oxygen vacancy enhances the absorption more than zinc va cancy. We also discuss the photocatalytic mechanism where the Cu doping and vacancy play an important role in photocatalytic process. We can tune the electronic and optical properties by the proper exper imental condition. Our studies explored the properties of Cu-doped ZnO monolayer and pave a road for its flexible applications.
(10) (11)
(Zn,Cu)O monolayer absorbs the solar energy and produces electrons (e ). Electrons transfer to the conduction band and leave holes (hþ) around the valence band (eq. 8). The e and hþ can recombine and release heat (eq. 9). Cu and intrinsic vacancy can serve as a trap to restrain the recombination between e and hþ. Electrons respond to free O2 and produce superoxide radicals (�O2 ) (eq. 10). On the other hand, OH from the moisture react with holes and produce hydroxyl radicals (�OH) (eq. 11), which is a strong oxidant. The �O2 and �OH can decompose pollution to CO2 and H2O. Holes in the conduction band also
Conflicts of interest We declare that we have no financial and personal relationships with 8
Z. Wang et al.
Physica E: Low-dimensional Systems and Nanostructures 115 (2020) 113702
Fig. 8. The photocatalytic mechanism of Cu doped ZnO monolayer with and without intrinsic vacancy.
other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Influence of Cu dopant on the electronic and optical properties of graphenelike ZnO monolayer”.
[14] A. Favron, E. Gaufres, F. Fossard, A.-L. Phaneuf-L’Heureux, N.Y.W. Tang, P. L. Levesque, A. Loiseau, R. Leonelli, S. Francoeur, R. Martel, Nat. Mater. 14 (2015), 826-þ. [15] M.-Y. Li, Y. Shi, C.-C. Cheng, L.-S. Lu, Y.-C. Lin, H.-L. Tang, M.-L. Tsai, C.-W. Chu, K.-H. Wei, J.-H. He, W.-H. Chang, K. Suenaga, L.-J. Li, Science 349 (2015) 524–528. [16] C. Tusche, H.L. Meyerheim, J. Kirschner, Phys. Rev. Lett. 99 (2007), 026102. [17] D. Ma, Q. Wang, T. Li, Z. Tang, G. Yang, C. He, Z. Lu, J. Mater. Chem. C 3 (2015) 9964–9972. [18] D. Ma, Z. Wang, H. Cui, J. Zeng, C. He, Z. Lu, Sens. Actuators B 224 (2016) 372–380. [19] H. Chen, C. Tan, D. Sun, W. Zhao, X. Tian, Y. Huang, RSC Adv. 8 (2018) 1392–1397. [20] T. Sahoo, S.K. Nayak, P. Chelliah, M.K. Rath, B. Parida, Mater. Res. Bull. 75 (2016) 134–138. [21] J. Lee, D.C. Sorescu, X. Deng, J. Phys. Chem. Lett. 7 (2016) 1335–1340. [22] S.U.M. Khan, M. Al-Shahry, W.B. Ingler, Science 297 (2002) 2243–2245. [23] H.D. Weldekirstos, M.-C. Kuo, S.-R. Li, W.-L. Su, M.A. Desta, W.-T. Wu, C.-H. Kuo, S.-S. Sun, Dyes Pigments 163 (2019) 761–774. [24] W. Huang, C. Harnagea, X. Tong, D. Benetti, S. Sun, M. Chaker, F. Rosei, R. Nechache, ACS Appl. Mater. Interfaces (2019) 13185–13193. [25] S. Zhuang, M. Lu, N. Zhou, L. Zhou, D. Lin, Z. Peng, Q. Wu, Electrochim. Acta 294 (2019) 28–37. [26] P.K.R. Boppidi, P.M.P. Raj, S. Challagulla, S.R. Gollu, S. Roy, S. Banerjee, S. Kundu, J. Appl. Phys. 124 (2018) 214901. [27] Z.N. Kayani, S. Iram, R. Rafi, S. Riaz, S. Naseem, Appl. Phys. A 124 (2018) 468. [28] Z. Ma, F. Ren, X. Ming, Y. Long, A.A. Volinsky, Materials 12 (2019). [29] T. Hussain, M. Hankel, D.J. Searles, J. Phys. Chem. C 121 (2017) 24365–24375. [30] X. Han, N. Amrane, Z. Zhang, M. Benkraouda, J. Phys. Chem. C 123 (2019) 2037–2047. [31] W.J. Kim, M.H. Han, S. Lebegue, E.K. Lee, H. Kim, Front. Chem. 7 (2019) 47. [32] R. Liu, F. Yang, Y. Xie, Y. Yu, Appl. Surf. Sci. 466 (2019) 568–577. [33] A. Bentouaf, R. Mebsout, B. Aissa, J. Alloy. Comp. 771 (2019) 1062–1071. [34] Y.-J. Li, M. Wang, M.-y. Tang, X. Tian, S. Gao, Z. He, Y. Li, T.-G. Zhou, Physica E 75 (2016) 169–173. [35] M.D. Segall, P.J.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, M. C. Payne, J. Phys. 14 (2002) 2717–2744. [36] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865–3868. [37] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758–1775. [38] Z.W. Geem, J. Irrig. Drain. Eng. 132 (2006) 474–478. [39] P. Wisesa, K.A. McGill, T. Mueller, Phys. Rev. B 93 (2016). [40] V.I. Anisimov, J. Zaanen, O.K. Andersen, Phys. Rev. B 44 (1991) 943–954. [41] R.M. Sheetz, I. Ponomareva, E. Richter, A.N. Andriotis, M. Menon, Phys. Rev. B 80 (2009) 195314. [42] L. Wang, T. Maxisch, G. Ceder, Phys. Rev. B 73 (2006) 195107. [43] G.E. Jellison, F.A. Modine, Appl. Phys. Lett. 69 (1996) 371–373. [44] M. Roknuzzaman, M.A. Hadi, M.J. Abden, M.T. Nasir, A.K.M.A. Islam, M.S. Ali, K. Ostrikov, S.H. Naqib, Comput. Mater. Sci. 113 (2016) 148–153. [45] A. Chowdhury, M.A. Ali, M.M. Hossain, M.M. Uddin, S.H. Naqib, A.K.M.A. Islam, Phys. Status Solidi B 255 (2018) 1700235.
Acknowledgements The Project was supported by the National Natural Science Foun dation of China (41402034 and 41474067), China Postdoctoral Science Foundation (2018T110815 and 2016M600622) and the Fundamental Research Funds for the Central Universities, China University of Geo sciences (Wuhan) (CUGL150418). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.physe.2019.113702. References [1] U. Ozgur, Y.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, H. Morkoc, J. Appl. Phys. 98 (2005), 041301. [2] Z.L. Wang, J. Phys. 16 (2004) R829–R858. [3] P. Dash, A. Manna, N.C. Mishra, S. Varma, Physica E 107 (2019) 38–46. [4] W. Yu, D. Xu, T. Peng, J. Mater. Chem. 3 (2015) 19936–19947. [5] Y.K. Mishra, G. Modi, V. Cretu, V. Postica, O. Lupan, T. Reimer, I. Paulowicz, V. Hrkac, W. Benecke, L. Kienle, R. Adelung, ACS Appl. Mater. Interfaces 7 (2015) 14303–14316. [6] W. Kuang, Q. Zhong, X. Ye, Y. Yan, Y. Yang, J. Zhang, L. Huang, S. Tan, Q. Shi, J. Nanosci. Nanotechnol. 19 (2019) 3982–3990. [7] Y. Al-Hadeethi, A. Umar, K. Singh, A.A. Ibrahim, S.H. Al-Heniti, B.M. Raffah, J. Nanosci. Nanotechnol. 19 (2019) 4199–4204. [8] S. Li, J. Pan, H. Li, Y. Liu, W. Ou, J. Wang, C. Song, W. Zhao, Y. Zheng, C. Li, Chem. Eng. J. 366 (2019) 305–312. [9] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183–191. [10] C. Yu, X. Cheng, C. Wang, Z. Wang, Physica E 110 (2019) 148–152. [11] A.J. Mannix, X.-F. Zhou, B. Kiraly, J.D. Wood, D. Alducin, B.D. Myers, X. Liu, B. L. Fisher, U. Santiago, J.R. Guest, M.J. Yacaman, A. Ponce, A.R. Oganov, M. C. Hersam, N.P. Guisinger, Science 350 (2015) 1513–1516. [12] S. Wu, S. Buckley, J.R. Schaibley, L. Feng, J. Yan, D.G. Mandrus, F. Hatami, W. Yao, J. Vuckovic, A. Majumdar, X. Xu, Nature 520 (2015) 69–U142. [13] X. Wang, A.M. Jones, K.L. Seyler, T. Vy, Y. Jia, H. Zhao, H. Wang, L. Yang, X. Xu, F. Xia, Nat. Nanotechnol. 10 (2015) 517–521.
9