Strain-induced tunability of optical and photocatalytic properties of ZnO mono-layer nanosheet

Computational Materials Science 91 (2014) 38–42

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Strain-induced tunability of optical and photocatalytic properties of ZnO mono-layer nanosheet Thanayut Kaewmaraya a, Abir De Sarkar b,⇑, Baisheng Sa c, Z. Sun c, Rajeev Ahuja a,d a

Condensed Matter Theory Group, Department of Physics and Astronomy, Uppsala University, Box 516, Uppsala S-75120, Sweden Institute of Nano Science and Technology, Habitat Centre, Phase X, Sector-64, Mohali, Punjab 160 062, India c Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China d Department of Materials Science and Engineering, Applied Materials Physics, Royal Institute of Technology (KTH), Stockholm S-100 44, Sweden b

a r t i c l e

i n f o

Article history: Received 9 January 2014 Received in revised form 10 April 2014 Accepted 20 April 2014 Available online 13 May 2014 Keywords: Strain ZnO Monolayer Nanosheet Band structure

a b s t r a c t Strain-induced tunability of several properties of ZnO monolayer nanosheet has been systematically studied using density functional theory. The band gap of the sheet varies almost linearly with uniaxial strain, while it shows a parabola-like behavior under homogeneous biaxial strain. Tensile strain reduces ionicity of Zn–O bonds, while compressive strain increases it. This provides ample implications for the photocatalytic dissociation of water molecules and the scission of polar molecules on ZnO nanosheet. The dynamical stability of the sheet assessed by the calculation of its vibrational frequencies has shown the sheet to be unstable for 10% and 7.5% compressive biaxial homogeneous strain. Ó 2014 Elsevier B.V. All rights reserved.

ZnO is an attractive wide-band gap (3.4 eV) semiconductor for its numerous potential applications in gas sensing, solar cells, ultraviolet laser, etc. [1–3]. The bulk ZnO is thermodynamically stable in the wurtzite structure under ambient conditions. However, ZnO thin films comprising a few layer thicknesses have been theoretically predicted to undergo structural transformation from the wurtzite structure to graphitic-like ZnO layers [4,5]. The interaction between adjacent ZnO layers weakens in the ZnO thin films, causing ZnO to transform into a structure with a shorter Zn–O bond length and a bigger Zn–O–Zn angle, as compared to its bulk [4,5]. Topsakal et al. [6] and Tu [7] have theoretically scrutinized the factors responsible for the stability of the ZnO ML nanosheet. Two monolayer-thick ZnO(0 0 0 1) films have been synthesized experimentally on Ag(1 1 1) substrate, where X-ray diffraction and scanning tunneling microscopy reveal the flat graphitic-like morphology of the sheets [8]. The structure gradually transforms into its bulk wurtzite structure when the number of layers is increased [8]. In addition, ZnO sheets have recently been grown on Pd(1 1 1) substrate [9], further confirming the existence of graphene-like ZnO nanosheets. Although the synthesis of one monolayer has not been experimentally achieved, there have been several studies on graphitic ZnO consisting of both a monolayer

⇑ Corresponding author. Tel.: +91 9023548853. E-mail address: [email protected] (A. De Sarkar). http://dx.doi.org/10.1016/j.commatsci.2014.04.038 0927-0256/Ó 2014 Elsevier B.V. All rights reserved.

(ML) and a few layer thicknesses [5–7]. ZnO monolayer (ZnO– ML) is reported to be a direct wide band gap non-magnetic semiconductor. However, ZnO shows different properties when it is subject to different chemical treatments. For instance, Wang et al. have reported that a semi-fluorinated ZnO sheet exhibits half-metallic behavior due to a decrease of charge transfer from Zn to O atoms. On the other hand, the fully fluorinated one is non-magnetic [10]. In addition, ZnO nanoribbons have also been predicted to be magnetic depending on the terminations (either zig–zag or arm-chair) [11] and magnetism in the zigzag nanoribbons has been demonstrated to be effectively tuned by transverse electric fields [12]. Therefore, exploration of the interesting aspects in ZnO nanosheet may potentially lead to several technological applications. Mechanical strain is known to play a crucial role in the manipulation of electronic structures e.g., band gaps and carrier mobility. Homogeneous strain applied to graphene is considered as equivalent to the effect of a high uniform magnetic field affecting Landau levels and a zero-field quantum Hall effect [13]. It has been recently demonstrated that mechanically applied strain enhances mobility in metal–oxide-semiconductor field effect transistors [14]. Tensile strain applied along the armchair direction of graphene nanoribbons with line-defect has also been found to affect the magnetic properties [15]. In experiments, strain in graphene has been found to be tunable by depositing it on a flexible substrate e.g., polyethylene terephthalate (PET) and stretching it along

T. Kaewmaraya et al. / Computational Materials Science 91 (2014) 38–42

one direction [16]. For graphitic ZnO, Si and Pan [17] have found that strain can induce semiconductor-to-metal transition in ZnO zigzag nanoribbons. Behera and Mukhopadhyay [18] have used first-principles calculations to study the variation in the band gap of ZnO–ML with mechanically applied strain and they have found a parabola-like relation. Band gap is gradually reduced with both compressive and tensile strain and the strain-free ZnO–ML shows the maximum band gap. However, their work considers only the effect of homogeneous biaxial strain on the band gap and the underlying microscopic mechanism in terms of changes in the polarity of the ML nanosheet and electronic structure has not been revealed. Although both compressive and tensile narrow the band gap of ZnO nanosheet, they may induce opposite effects in modulating the ionicity in the Zn–O bonds of the nanosheet. This may affect the catalytic, photocatalytic and even the gas sensory functionality of the sheets. Even for the same magnitude of strain, compressive or tensile, may not narrow the band gap equally. Furthermore, in their studies, they have used only the Perdew Zunger variant of LDA approximation for the exchange–correlation functional, which is known to overbind, giving smaller lattice constants. Therefore, there are several issues which have not been addressed in their studies. In this work, we have performed firstprinciples calculations based on the density functional theory (DFT) to investigate effects of strain on a ZnO–ML nanosheet. We have considered both biaxial strain and uniaxial strain along the zig–zag and the arm-chair directions. In order to investigate electronic structures, band structures of the strained sheets have been calculated. Additionally, vibrational frequencies have also been calculated to check on the stability of the strained sheet in the range of strain studied in our work. Furthermore, Bader analysis of atomic charges has been invoked to account for the interatomic charge transfer and the changes in the ionicity of the Zn–O bonds in the ZnO–ML nanosheet under strain. Density functional theory (DFT) [19,20] based first principles calculations have been performed. The Perdew–Burke–Ernzerhof (PBE) [21] variant of the generalized gradient approximation (GGA) for the exchange correlation functional and the projectoraugmented wave (PAW) potentials [22], as implemented in the Vienna Ab-initio Simulation Package (VASP) [23,24] was used in our theoretical investigations. Twelve electrons of Zn (3d104s2) and six electrons of O (2s22p4) were considered as valence electrons. The cut off energy for a basis set and k-point sampling for Brillouin zone integration were set to 600 eV and 16  16  8 respectively. Structural optimizations based on the conjugate gradient scheme were allowed until Hellman–Feymann forces on each

(a) Top View O Zn

(b) Side View Fig. 1. The optimized geometries of the ZnO nanosheet: (a) top view and (b) side view. The bounding box containing 1 Zn and 1 O atom constitutes the unit cell. Grey and red spheres represent Zn and O atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

39

atom reached less than 0.005 eV/Å. We modeled an infinite ZnO monolayer nanosheet in a hexagonal honeycomb lattice, as shown in Fig. 1 [6]. The unit cell consists of 2 atoms and two equivalent lattice vectors are both aligned along the zig–zag direction. A vacuum thickness of 15 has been found to be large enough to decouple the periodic images of the nanosheets repeated along the z direction. Strain application was simulated by scaling the lattice constants along the two lattice vectors. The atomic positions were relaxed, while the cell shape was held. This approach has been successfully used in the previous studies [25–27]. We have considered both compressive strain and tensile strain in our calculations. Strain was varied from 10% to +10% in steps of 2.5%. In addition, we have also investigated the effect of strain on the bonding characteristic in ZnO nanosheets by using Bader charge analysis [28,29]. This approach provides us the amount of electronic charges allocated for each atom in the system. The zero flux surface of the electronic charge density is used to determine the amount of charge on a particular atom. We have first performed geometry optimization calculations and the optimized geometry of ZnO ML nanosheet is shown in Fig. 1. The optimized sheet exhibits a planar structure. The equilibrium lattice constants given by the two exchange correlation functionals, local density approximation (LDA) and generalized gradient approximation (GGA–PBE), are found to be 3.20 Å and 3.29 Å respectively, while the Zn–O bond lengths are found to be 1.85 Å and 1.90 Å respectively. The experimental value for the Zn–O bond length has been reported to be 1.92 Å [8]. This Zn–O bond length is slightly shorter than that of bulk ZnO, which is around 2.01 Å. Actually, the atoms in the sheet have a lower coordination number (i.e., less number of neighbors) than the bulk. As a result, the sheet atoms reinforce their bonding amongst themselves, resulting in a smaller Zn–O bond length, as compared to the bulk. The calculated O–Zn–O bond angle is found to be around 120 for both functionals. Overall, the optimized structure and the Zn–O bond length of the sheet from this work are found to be in good agreement with previous theoretical results [6,7,30–32]. We also investigated magnetism in the sheet but no magnetic moments are found in our spin polarized calculations, confirming that graphitic ZnO sheet is non-magnetic [6]. In addition, the calculated cohesive energy of the sheet per Zn–O pair is 8.98 (8.64) eV by LDA (PBE) functional, in good agreement with the previous calculated value of 8.419 eV and the sheet cohesive energy can be compared to the bulk cohesive energy of 8.934 eV calculated by Topsakal et al. [6]. It can be seen that PBE functional predicts Zn–O bond length closer to experiment than LDA. Therefore, PBE was selected as a reliable exchange correlation functional for the rest of the calculations in studying the effects of strain on ZnO nanosheet. After having optimized the sheet, strain ranging from 10% to +10% was applied to the sheet in steps of 2.5%. For completeness, we have considered both uniaxial and homogeneous biaxial strain in our studies. The electronic structures of the sheets at each magnitude of applied strain were determined. Fig. 2 shows the band structures of the sheet under different degrees and types of strain, (a) 5% homogenous biaxial compressive strain, (b) strain-free, (c) 10% homogenous biaxial compressive strain, (d) 10% uniaxial compressive strain and (e) 10% uniaxial tensile strain, respectively. It should be first mentioned that the unstrained sheet has a direct band gap of 1.65 eV as shown in Fig. 2(b), which displays good agreement with previous calculated values [6,7,30–32]. Although no experiments have been reported till date, Tu have recently reported the band gap of two-dimensional ZnO nanosheet to be 3.57 eV by using GW approximation [7]. It can be seen that uniaxial strain of both kinds (compressive as well as tensile) is found to lift the degeneracy of two bands at the valence band edge at the C point. On the other hand, biaxial strain of both types is found to

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(b) Strain-free

Energy (eV)

Energy (eV)

(a) 5% biaxial compressive 14 12 10 8 6 4 2 0 -2 -4 -6 -8

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M

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E G = 1.65 eV

K

K

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M

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(f) Band gap as a function of strain

(e) 10% uniaxial tensile 14 12 10 8 6 4 2 0 -2 -4 -6 -8

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14 12 10 8 6 4 2 0 -2 -4 -6 -8

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Biaxial Uniaxial

1.4 1.3 1.2

K

M

K

-10 -7.5

-5

-2.5

0

2.5

5

7.5

10

Applied strain (%) Fig. 2. Electronic band structures of the ZnO nanosheet under (a) 5% homogenous biaxial compressive strain, (b) no strain, (c) 10% homogenous biaxial compressive strain, (d) 10% uniaxial compressive strain and (e) 10% uniaxial tensile strain. In addition, (f) shows the band gap as a function of applied strain.

retain this degeneracy. The degeneracy owes its origin to the structural symmetry, which is broken upon the application of uniaxial strain and is maintained otherwise in the rest of the cases studied in our work. Strain of all kinds is observed to lift the degeneracy of two bands at Gamma point around 6 eV. The band widths and dispersions at energies higher than 6 eV are found to decrease with all types of strain. Moreover, strain is found to pull these bands down in energy. The band structure of the deeper level bands (i.e., bands lower than the valence band edge) is also found to change with strain. It should be also noted that the sheet remains a direct band gap semiconductor under the application of mechanical strain. In addition, Fig. 2(f) shows the band gap as a function of applied strain. Although the PBE functional is well known for band gap underestimation, it has been found to predict the correct trend in the band gap variation with respect to applied strain in several Zn-based II–VI and wurtzite III–V semiconductors as compared to the more accurate GW approaches [33,34]. For symmetric biaxial strain, the band gap is found to change non-linearly with applied strain, exhibiting a parabolic-like variation with respect to applied strain. There is a monotonic increase in the band gap from 1.32 eV at 10% to its maximum value at 1.65 eV at the strain-free geometry. After that, it drops steadily to the value of 1.26 eV at +10%. The band gaps for the same magnitude of tensile and compressive strain are not found to be equal, implying the differences in the effects induced by tensile and compressive strain. On the other hand, it can be observed that application of uniaxial strain evokes a linear response in the band gap and the band gap decreases both with compressive and tensile strain. Moreover, uniaxial strain can reduce the band gap more sharply, as compared to the homogeneous biaxial strain. The band gap of the sheet at 10% is 1.35 eV. The band gap increases with decreasing compressive strain and reaches its maximum at the unstrained condition, which is 1.65 eV. The band gap decreases steadily under the application of tensile strain until it reaches 1.24 eV at +10% strain. It should be mentioned that the band gaps induced by the application of uniaxial strain of the same magnitude are lower than the one caused by the application of homogeneous biaxial strain. The uniaxial strain is found to be more effective in tuning the band gap than homogeneous biaxial strain, as the former lowers the band gap. The reason

is that the application of homogeneous biaxial strain still conserves the hexagonal symmetry. On the other hand, this symmetry disappears under the application of uniaxial strain. However, it should be noted that the band gap of the sheet at 10% of homogeneous biaxial strain is lower than that at 10% of uniaxial strain. This is due to the lack of crystal symmetry because the sheet might become unstable in this condition. The deformation potential can be used to account for the changes in band gap with the unit cell volume [35]. The positive deformation potential implies the reduction of band gap with respect to lattice expansion while the negative one signifies the increase in band gap with lattice expansion. For ZnO sheet, it can be seen that both compressive and tensile strains reduce the band gap in ZnO sheet. In this case, the unusual behaviors of deformation potentials should be expected. Another reason is that ZnO sheet has the mixture of p–p and r–r interactions. They affect electronic properties differently with respect to interatomic distances [36]. It differs from that of graphitic BN sheet where p–p interactions play an important role in modifying the band gap, leading to the reduction of band gap with respect to lattice expansion. In addition, its deformation potential is calculated to be negative [36]. Our results show that strain is an effective approach in reducing the band gap of ZnO nanosheet. Provided that the band positions of ZnO are well aligned with respect to the redox chemical potentials of H2 and O2, it has significant implications for its photocatalytic applications where the band gap of ZnO needs to be adjusted in order to enhance the light absorption efficiency [1]. Fig. 3 shows the density of states (DOS) of the sheet under different set of conditions. For the unstrained sheet, it can be seen that the top of the valence band is mainly formed by O-2p orbitals, while the lower part of the valence band is dominantly composed of Zn-3d states. If the sheet is subject to both compressive and tensile strain, charge localization is found to occur on the Zn-3d and O2p states at the edge of the valence band near the Fermi level, EF. The localized states around EF destabilize the sheet under the application of strain. The charge localization on Zn-3d and O-2p states near EF is found to be equal for the biaxial strain of both kinds. This is due to the fact that the biaxial strain retains the hexagonal structural symmetry. The top of the valence band is predominantly contributed by the p–p interactions arising from

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T. Kaewmaraya et al. / Computational Materials Science 91 (2014) 38–42 2

Zn-s Zn-p Zn-d O-s O-p

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(b) Strain-free DOS (state/eV)

(a) 5% biaxial compressive

DOS (states/eV)

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Fig. 3. Density of states (DOS) of the ZnO nanosheet under (a) 5% homogenous biaxial compressive strain, (b) no strain, (c) 10% homogenous biaxial compressive strain, (d) 10% uniaxial compressive strain and (e) 10% uniaxial tensile strain. The Fermi level has been set at zero.

the hybridization of Zn-3d and O-2p states, while the bottom of the conduction band is constituted primarily by the r–r interactions resulting from the overlap or hybridization between Zn-4s and O-2s states. Localization of states at the conduction band edge is also found to appear under the application of strain. In order to probe into the bonding mechanism in the ZnO nanosheet, we have performed Bader charge analysis [28,29] to quantify the electronic charge partitioned for each atom. The strain-free sheet exhibits the covalently bonded behavior, where each Zn atom loses 1.25 electronic charge (e) while each O atom gains +1.25e. In comparison, Topsakal et al. [6] employed another approach based on Mulliken population of orbital charge analysis to calculate charge transfer in a monolayer graphene-like nanosheet. They have found that 1.18e is transferred between Zn and O atoms, which is close to our results. Subsequently, the strain-free sheet was used as a reference for determining the induced charge transfer when compressive and tensile strain was applied. Here, we have considered 10% and +10% of strain. Symmetric biaxial compressive strain induces each Zn atom to lose 1.36e to each O atom whereas symmetric biaxial tensile strain results in the transfer of 1.22e from Zn atoms to the O atoms. On the other hand, uniaxial compressive strain induces each Zn atom to lose 1.28e to each O atom whereas uniaxial tensile strain results in the transfer of 1.24e from Zn atoms to the O atoms. It can be seen that compressive strain comparatively causes a greater magnitude of charge transfer between Zn and O atoms than the tensile strain. Furthermore, homogenous biaxial compressive strain induces more charge transfer than uniaxial compressive strain, while homogeneous biaxial tensile strain induces less charge transfer than uniaxial compressive strain. Compressive strain is found to introduce more charge delocalization, while tensile strain leads to charge localization. As compared to the unstrained sheet, it should be noted that compressive strain causes more charge transfer than the tensile strain. Therefore, compressive strain is found to enhance the polarity of the Zn–O bond, while tensile strain reduces the same. This will have profound implications for modulating the catalytic activity of the ZnO nanosheet, e.g., in adsorbing and dissociating polar molecules like H2O on its surface.

(a)

(b)

(d)

(e)

(c)

Max. (2.90 e)

Min. (0.00 e) Fig. 4. Isosurfaces of electronic charge density at (a) 5% homogenous biaxial compressive strain, (b) no strain, (c) 10% homogenous biaxial compressive strain, (d) 10% uniaxial compressive strain and (e) 10% uniaxial tensile strain.

The isosurface of electronic charge density shown in Fig. 4 substantiate the previous arguments. Although both types of strain narrow the band gap, they are found to induce opposite effects in terms of electronic charge transfer between Zn and O atoms, as revealed by the Bader analysis of atomic charges. The charge density plots support the Bader analysis. It may be noted here that the colors for the isosurfaces on Zn and O atoms swap as one switches from compressive to tensile strain. Tensile strain causes charge localization on the atoms, i.e., fractional electronic charges of equal magnitude are found to shift from the bonding region to the atomic regions. It reduces the ionicity and strength of the Zn–O bonds.

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30

Transverse-1 Transverse-2 Longitudinal

Vibrational freq. (THz)

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Fig. 5. The vibrational frequencies (in THz) of the ZnO monolayer nanosheet in longitudinal and transverse modes under (a) homogeneous biaxial strain and (b) uniaxial strain.

On the contrary, compressive strain contributes electronic charges to the bonding region, as shown by the charge density plots. It enhances the polarity of the Zn–O bonds. To ascertain the stability of the sheet for the range of strain studied in our work, we have calculated its vibrational frequencies at the C point, as shown in Fig. 5. The sheet has not been found to be stable for all the cases studied in our work. The longitudinal modes in the acoustic branch show negative frequencies at 10% and 7.5% biaxial strain, as can be seen in Fig. 5a. The sheet is found to be stable for the rest of the cases considered in our studies. The two transverse modes in the optical branch show the same variation with biaxial strain, as the hexagonal structural symmetry is retained in biaxially strained sheet unlike the uniaxially strained sheet. However, the range of uniaxial strain, 10% to 10%, is not found to cause any instabilities or negative frequencies in Fig. 5b. These results provide useful pointers to the degree and type of strain which are practically realizable. Strain is found to be very efficacious in manipulating the band gap of ZnO monolayer nanosheet. Homogeneous biaxial and uniaxial strain ranging from 10% to +10% has been studied in our work. Uniaxial strain is more effective in tuning the band gap as compared to biaxial strain due to the steeper variation in band gap with uniaxial strain. Although both tensile and compressive strain reduce band gap, still they affect the charge transfer between Zn and O atoms differently. Tensile strain reduces the ionicity of Zn–O bonds, while compressive strain enhances the ionicity of the same. The photocatalytic cleavage of water molecules and catalytic scission of polar molecules on ZnO sheet can be effectively tuned by strain-induced alteration in the ionicity in the Zn–O bonds. Vibrational frequencies calculated at the C point show the sheet to be unstable for 10% and 7.5% homogeneous biaxial strain. The sheet is found to be stable for all the other cases studied. Our findings provide valuable pointers and guidance for an optimal utilization of strain induced tunability in the optical, catalytic and photocatalytic properties of ZnO monolayer nanosheet in actual applications e.g., in nano-electromechanical systems (NEMS), nano-optomechanical systems (NOMS) and nano-electronic devices. Acknowledgements We would like to thank FORMAS (Sweden) and SWECO (Sweden) for financial support. T.K. would like to acknowledge

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