Efficient Carrier Separation in Graphitic Zinc Oxide and Blue

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Efficient Carrier Separation in Graphitic Zinc Oxide and Blue Phosphorus van der Waals Heterostructure Xianghong Niu, Yunhai Li, Huabing Shu, Xiaojing Yao, and Jinlan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12613 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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The Journal of Physical Chemistry

Efficient Carrier Separation in Graphitic Zinc Oxide and Blue Phosphorus van der Waals Heterostructure Xianghong Niu, Yunhai Li, Huabing Shu, Xiaojing Yao, and Jinlan Wang* School of Physics, Southeast University, Nanjing, 211189, China

ABSTRACT: Efficient carrier separation is the key to the application of photoelectric device. However, photo-generated electron-hole pairs in simplex semiconductors generally occupy the same regions spatially and are easy to recombine. Here, we design a graphitic zinc oxide (g-ZnO) based intrinsic type-II heterostructure, g-ZnO/blue phosphorus (BP), based on first-principles calculations. The type-II band offsets and large built-in electric field ensure the photogenerated electrons easily migrating from g-ZnO to BP, which significantly enhances the separation of electron-hole pairs. Improved optical absorption is also observed in the heterostructure. Furthermore, the perpendicular external electric field can greatly modulate band edges and achieve a direct band gap at Г point, which provides further promotion in the separation of carriers.

Email: [email protected]

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INTRODUCTION Efficient carrier separation of electrons and holes after photoexcitation is the key to renewable energy applications, including solar cells,1 photoelectrochemical water splitting and photo-degradation of environmentally hazardous compounds.2 However, photo-generated electron-hole pairs in a pure semiconductor typically stay in the same regions spatially, and the large wave-function overlap of electrons and holes leads to a high rate of recombination.3 Nanoscale semiconductors can partly suppress the electron-hole recombination due to relatively large excitation binding energy.4 Most importantly, the use of nanocomposite, formed by stacking two-dimensional (2D) atomic monolayers, with type II conduction band offsets and built-in electric field at interfaces can greatly facilitate the separation of photo-generated electron-hole pairs.5, 6

Zinc oxide (ZnO) possesses a host of useful properties including piezoelectric,7 optical,8 catalytic activity

9, 10

and biocompatibility.11 Meanwhile, ZnO is also a

chemically stable, cheap, and environmentally friendly material.11 These unique features make it a great candidate in solar cells,8 photo-catalysts,10 photovoltaics, gas sensors and piezoelectricity.12, 13 Recently, the stable existence of graphitic zinc oxide (g-ZnO) has been predicted by previous theoretical studies14-16 and confirmed by experiments,

17-19

which provides possibility for design of g-ZnO based

nanocomposites. In fact, previous studies have revealed that g-ZnO/MoS2 forms matched band offset.20 Nevertheless, the mobility of MoS2 is too low (80 cm2V-1s-1),21, 22

and the built-in electrical potential has opposite direction at interface (from g-ZnO


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to MoS2) which is not good to the separation of electron-hole pairs.20 Other 2D materials, such as black phosphorus, because of their mismatched band gap and band edge, g-ZnO and black phosphorus form a type I vdW heterojunction.23-25 Most recently, blue phosphorus (BP), a layered allotrope of black phosphorus, has been successfully synthesized and owns high mobility and wide bandgap, which may form a better photoelectric heterostructure with g-ZnO.26-30 In this work, we explore the possibility of g-ZnO/BP heterostructure as efficient carrier separation of photoelectric material by using first-principles calculations. Our calculations show that the intrinsic electronic properties of g-ZnO and BP can be preserved in heterostructure due to weak vdW interactions. The heterojunction exhibits intrinsic type-II band alignment and forms large built-in electrical potential from BP to g-ZnO, which facilitate the separation of photo-generated electron-hole pairs in g-ZnO and BP layer. The g-ZnO/BP heterojunction also exhibits better UV and visible light absorption than the simplex g-ZnO and BP monolayer. More interestingly, the heterojunction shows tunable band offsets and direct band gap at Γ point upon perpendicular electric field, which enhances the spatial separation of carriers. THEORETICAL METHODS The calculations are performed by using the plane-wave basis set and norm-conserving Troullier Martins pseudopotential within the framework of DFT in Quantum Espresso.31 The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE)

32

and Heyd-Scuseria-Ernzerhof (HSE06)


33

are


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employed for electron exchange and correlation functionals. A vacuum of 25 Å along the z direction is constructed to eliminate the interaction between adjacent images. The kinetic energy cutoff is set as 100 Ry for wave-function. A k-point grid of 12×12×1 is adopted for the Brillouin zone integrations. Structure relaxation is carried out using PBE functional until the force on each atom is less than 0.01eV/Å. Dipole correction is employed to cancel the errors of atomic force, total energy, and electrostatic potential, caused by periodic boundary condition.34 Van der Waals interactions are included by the vdW-DF level.35-37 Electronic band structures are calculated by HSE06 hybrid functional since it yields reasonably accurate band gaps in semiconductors. An external sawtooth potential is used to simulate the effect of external electric field in the z direction. RESULTS AND DISCUSSION The structures of monolayer g-ZnO and BP are presented in Figure 1a and b. The lattice parameters are 3.36 and 3.33 Å for g-ZnO and BP, respectively, consistent with previous theoretical and experimental results.17,

26, 28

The well matched lattice

parameters allow us to simulate the g-ZnO/BP heterostructure using unit cell. Figure 1c depicts the optimized structure with the lattice constant of 3.35 Å. The binding energy is calculated to evaluate the coupling interaction between the g-ZnO and BP layer, which is defined as,

Eb = Eg−ZnO/ BP − Eg−ZnO − EBP , where Eg−ZnO/ BP , Eg − ZnO and

EBP represent the total energy of g-ZnO/BP

heterostructure, isolated g-ZnO, and BP monolayer, respectively. The DFT-DF


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calculation gives a typical vdW vertical separation of 3.21 Å with the binding energy of -125 meV per unit cell. This indicates that the interfacial interaction between g-ZnO and BP layers is physical adsorption and the electronic and optical properties of constituted layer will not be greatly affected.

Figure 1．Monolayer band structures of (a) g-ZnO and (b) BP. The Fermi level is set as zero. The inset is unit cell. (c) The top and side view of lattice structure of g-ZnO/BP vdW heterostructure. Brillouin zone with high symmetry points labeled. (d) The band structure of g-ZnO/BP heterostructure, the bands plotted in blue and red indicate the bands dominated by BP and g-ZnO layer, respectively. The black circles indicate the highest valence-band and lowest conduction-band edges.

Figure 1c shows the Brillouin zone of unit cell labeled with high symmetry k-points. The band structures of g-ZnO, BP and g-ZnO/BP are displayed in Figures 1a, b and d.



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Clearly, the g-ZnO monolayer is a semiconductor with a direct band gap of 3.17 eV, while monolayer BP shows an indirect band gap of 2.64 eV, in accordance with previous HSE06 results (g-ZnO: 3.25 eV,

33, 38

BP: 2.73 eV 39). The band structure of

heterojunction clearly demonstrates that the electronic structures of BP and g-ZnO layers are well-preserved. The band gap of g-ZnO/BP is reduced to about 2.25 eV. Moreover, the valence band maximum (VBM) of g-ZnO/BP heterojunction is localized on g-ZnO, while the conduction band minimum (CBM) is distributed on BP, suggesting the formation of type-II heterojunction as shown in Figure 2a. This is the first key to achieve the efficient separation of photo-generated electron-hole pairs. The photo-generated electrons in g-ZnO can be easily shifted to the CB of BP layer due to the CBM offset (CBO: the CBM difference between g-ZnO and BP layers). On the other hand, the VBM offset (VBO: the VBM difference between g-ZnO and BP layers) results in the countermovement of photo-generated holes. To gain further insight, we plot the band decomposed charge density of VBM and CBM in g-ZnO/BP heterostructure as shown in Figure 2b. The electrons and holes of frontier orbital are obviously localized on BP layer and g-ZnO layer, respectively. As a result, the energy-wasted electron-hole recombination could be greatly reduced.40 This is totally different from g-ZnO/black phosphorus, which the band gap and band edge of their constitutions are significantly mismatched and form type I heterojunctions. 23, 24


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Figure 2. (a) Band alignment of g-ZnO/BP heterostructure. (b) The band decomposed charge density of VBM and CBM in the g-ZnO/BP. The iso-value is 0.003 e Å-3 (c) The in-plane average electrostatic potential of g-ZnO/BP. (d) The integrated charge density difference between g-ZnO/BP and its components. The inset in (d) is the corresponding iso-surfaces of charge density difference. The iso-value is 0.002 e Å-3. Yellow and blue areas denote electron accumulation and depletion, respectively. As a good photoelectric heterostructure material, large built-in electric field is another prerequisite to drive charge carriers and separate the electrons and holes efficiently. As illustrated in Figure 2c, the heterostructure forms a significant in-plane averaged electrostatic potential difference at interface region, which results in the large built-in electric field from BP to g-ZnO layer. The intrinsic built-in electric field greatly increases the interlayer coupling and promotes the electron transition from g-ZnO to BP layer. Compared with g-ZnO/BP, the built-in electrical field has opposite direction at g-ZnO/MoS2 heterostructure interface (from g-ZnO to MoS2), which



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accelerates the recombination of the electron and hole pairs.20 The integrated charge density differences between g-ZnO/BP and its component monolayers are further calculated, ∆ρ ( z ) = ∫ ρ S ( x, y , z ) dxdy − ∫ ρ g − ZnO ( x, y , z ) dxdy − ∫ ρ BP ( x, y , z ) dxdy ,

where ρ S ( x, y, z ) , ρg −ZnO ( x, y, z ) and ρBP ( x, y, z ) are the charge density in heterostructure, g-ZnO and BP unit cell at the (x, y, z) point, respectively. As shown in Figure 2d, evident charge transfers from g-ZnO to BP surface, implying efficient hole accumulation at g-ZnO region. On account of this well separation, the lifetime of photo-generated carriers will be effectively prolonged, which would greatly improve the performance of optoelectronic devices.

Figure 3 ． Absorption coefficients of g-ZnO, BP monolayer and g-ZnO/BP heterostructure. The stronger optical absorption and wider absorption range of heterostructure are marked by blue arrows. To achieve efficient application of photoelectric devices, it is naturally expected


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that the g-ZnO/BP heterostructure can absorb as much as UV/ visible light since the interlayer interaction in g-ZnO/BP has opportunity to induce new optical transitions.41 Here, we investigate the optical absorption via the energy-dependent dielectric functions. The imaginary part of dielectric matrix ε2(ω) is determined from the following equation: 42

ε 2 (ω ) =

4π 2 e 2 1 lim 2 q → 0 Ω q

∑ 2ω δ (Ε k

ck

− E vk − ω ) × 〈 µ ck + eα q | µvk 〉〈 µ ck + e β q | µ vk 〉 ∗

c ,v ,k

Where Ω is the volume of the primitive cell, q is the electron momentum operator, c and v are the conduction and valence band states, respectively, ωk is the k point weight, Eck, Evk and µck, µvk are the eigenvalues and wave-functions at the k point, respectively, and eα, eβ are the unit vectors for the three Cartesian directions. The real part of dielectric function ε1(ω) is obtained from the imaginary part ε2(ω) using the Kramers-Krönig transformation. Then, the optical absorption coefficient α(ω) is calculated from the following relation: 43 α (ω ) = 2

1/ 2 ω 2 ε 1 (ω ) + ε 22 (ω ) − ε 1 (ω ) 

c 



As illustrated in Figure 3, the hybrid g-ZnO/BP heterostructure exhibits a wider absorption range and stronger visible light and UV absorption compared with g-ZnO and BP monolayer, especially in energy range of 2.0 to 4.5 eV. This can be understood by the fact that the charge transfer

44

and interlayer coupling41 induce the overlap of

electronic states and new optical transitions.



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Figure 4. External perpendicular electric field modulates the electronic property of g-ZnO/BP heterostructure. (a)-(c) The band structures of g-ZnO/BP nanocomposite. The Fermi level is set as zero. The black circles indicate the highest valence-band and lowest conduction-band edges. The bands plotted in blue and red indicate the bands dominated by BP and g-ZnO layer, respectively. (d) Band alignment of g-ZnO/BP heterostructure. (e) The integrated charge density difference between the g-ZnO/BP nanocomposite with and without external electric field. (f) The iso-surfaces of charge density difference between the g-ZnO/BP heterostructure with and without external electric field. Yellow and blue areas denote electron accumulation and depletion, respectively, and the iso-value are 0.002 e Å-3. Furthermore, applying an external electric field is an effective way to tune the electronic gap and enhance the performance of materials. For this purpose, we explore the effect of an external perpendicular electric field applied in direction from BP to g-ZnO layer, which is similar with a gate voltage in FET’s. It is found that the


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external electric field does not evidently affect the geometric structure, even if the strength of the electric field is increased up to 1.0 V/Å. While the band structure changes remarkably, particularly for the band structure of BP under a strong electric field (see Figure 4a-c). Interestingly, the most distinct change takes place in the Γ point of conduction band edge shifting toward the VBM with the increase of electric field, which is similar with isolate BP layer.39 When the external electric field is increased to 0.8 V/Å, the Γ point of the conduction band edge transforms as the CBM of g-ZnO/blue-P nanocomposite, forming the direct band gap. The direct band gap is greatly beneficial to the transition of photo-generated electrons from the CBM of g-ZnO layer to CBM of BP layer as the momentum is preserved in this process, i.e., it does not need to gain additional momentum. Simultaneously, the external electric field increases the CBO and VBO (see Figure 4d), which enhances the band offsets to promote the separation of electron-hole pairs. When the electric field is increased to 1.0 V/Å, the CBM and VBM of heterostructure shift toward each other closely, which sharply reduces the band gap and eventually turns the heterostructure into a metal. Moreover, the external electric field can also reinforce the built-in electrostatic potential from BP to g-ZnO layer since they have the same potential direction. The enhanced interlayer coupling induced by the external electric field can be well characterized by the charge transfer in the nanocomposite. The integrated charge density difference under different external fields is calculated according to the formula, ∆ρ E ⊥ ( z ) = ∫ ρ E ⊥ ( x, y , z ) dxdy − ∫ ρ S ( x, y , z ) dxdy



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where ρE⊥ ( x, y, z ) and ρ S ( x, y, z ) are the charge density in the unit cell of g-ZnO/BP nanocomposite at (x, y, z) point with and without external electric field, respectively. As clearly seen in Figure 4e and 4f, the transfer charge between g-ZnO and BP layer becomes more prominent with the increase of the electric field strength. The enhanced interaction at the interface can offer improved performance of carrier separation in g-ZnO/BP heterojunction. CONCLUSIONS We have performed systemically DFT calculations to explore the possibility of g-ZnO/BP heterostructure as an efficient photoelectric device. The intrinsic type-II band alignment and large built-in electrostatic potential ensure the photo-generated electrons transferred from g-ZnO to BP layer easily, which results in high separation efficiency. Meanwhile, the hybrid g-ZnO/BP heterostructure exhibits enhanced optical absorption compared to g-ZnO and BP monolayer. Furthermore, the band gap of g-ZnO/BP heterostructure can be effectively modulated by external electric field and the indirect-to-direct band gap transition is observed under an electric field at 0.8 V/Å. Simultaneously, the external electric field increases the band edge offsets and built-in electrostatic potential, which further promotes the separation of electron-hole pair. The type-II interface band alignment, large built-in electrostatic potential and improved optical absorption, enable g-ZnO/BP heterostructure to be promising candidate for application in nano- and optoelectronics device.

Notes


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The authors declare no competing financial interest. Acknowledgments This work is supported by the NSFC (21525311, 21373045) and SRFDP (20130092110029) in China, the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1620) and Jiangsu Innovation Projects for Graduate Student (KYZZ16_0117) in China. The authors thank the computational resources provided by Southeast University. References (1) Zhang, Y.; Wang, L. W.; Mascarenhas, A. “Quantum Coaxial Cables” for Solar Energy Harvesting. Nano Lett. 2007, 7, 1264-1239. (2) Long, R.; English, N. J.; Prezhdo, O. V. Photo-induced Charge Separation Across the Graphene-TiO2 Interface Is Faster Than Energy Losses: A Time-Domain Ab Initio Analysis. J. Am. Chem. Soc. 2012, 134, 14238-14248. (3) Wu, Z.; Neaton, J. B.; Grossman, J. C. Charge Separation via Strain in Silicon Nanowires. Nano Lett. 2009, 9, 2418-2422. (4) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829-R858. (5) Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huang, Y.; Duan, X. Van der Waals Heterostructures and Devices. Nat. Rev. Mater 2016, 1, 16042. (6) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A. H. 2D Materials and van der Waals Heterostructures. Science 2016, 353, aac9439. (7) Nadarajah, A.; Word, R. C.; Meiss, J.; Könenkamp, R. Flexible Inorganic Nanowire Light-Emitting Diode. Nano Lett. 2008, 8, 534-537. (8) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nanowire Dye-sensitized Solar Cells. Nature materials 2005, 4, 455-459. (9) Look, D. C. Recent Advances in ZnO Materials and Devices. Mater. Sci. Eng., B 2001, 80, 383-387. (10) Maeda, K.; Takata, T.; Hara, M.; Saito, N.; Inoue, Y.; Kobayashi, H.; Domen, K. GaN:ZnO Solid Solution as a Photocatalyst for Visible-Light-Driven Overall Water Splitting. J. Am. Chem. Soc. 2005, 127, 8286-8287. (11) Wang, Z. L. ZnO Nanowire and Nanobelt Platform for Nanotechnology. Mater. Sci. Eng., R 2009, 64, 33-71. (12) Chang, S. J.; Hsueh, T. J.; Chen, I. C.; Huang, B. R. Highly Sensitive ZnO Nanowire CO Sensors with the Adsorption of Au Nanoparticles. Nanotechnology 2008, 19, 175502. (13) Chen, Q.; Zhu, L.; Wang, J. Edge-passivation Induced Half-metallicity of Zigzag Zinc Oxide Nanoribbons. Appl. Phys. Lett. 2009, 95, 133116.



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Efficient Carrier Separation in Graphitic Zinc Oxide and Blue

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