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First-Principles Study of Field Emission Properties of Graphene-ZnO Nanocomposite Shengli Zhang,† Yonghong Zhang,*,‡ Shiping Huang,*,† Hui Liu,§ Peng Wang,| and Huiping Tian| DiVision of Molecule and Materials Simulation, Key Lab for Nanomaterials, Ministry of Education, and State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, Department of Physics, Tianjin Polytechnic UniVersity, Tianjin 300160, China, and Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China ReceiVed: August 17, 2010; ReVised Manuscript ReceiVed: October 15, 2010
The electronic structures and field emission properties of hybrid graphene-ZnO are investigated by density functional theory. The binding energies, energy levels, and the corresponding local electron density distributions of neutral and charged graphene-ZnO in electric fields are analyzed, which show that the electronic structures of graphene-ZnO are modified significantly by the applied electric fields. The reduction of the energy gaps and the change of the electron density distributions of the states in the vicinity of the Fermi level are observed. The work functions of graphene-ZnO decrease linearly with increasing electric fields, which indicate the enhancement of field emission properties. In addition, the ionization potentials decrease drastically with increasing electric fields, which further indicate the improvement of field emission properties of grapheneZnO. The Hirshfeld charges and the electrostatic potential derived charges are also explored, and it is found that the electron accumulation becomes obvious on the topmost six-membered ring with increasing electric field. Our results show that the graphene-ZnO is a promising candidate for a field emission electron source. 1. Introduction The high surface-to-volume ratio, thermal stability, and oxidation resistance render ZnO nanomaterials as one of the most promising candidates for field emission devices.1,2 Up to now, several groups have investigated the fabrication of various ZnO nanostructures, such as nanopins, nanopencils, nanotubes, nanoscrews, nanorods, needlelike nanowires, and tetrapod-like nanostructures.3,4 These investigations reveal the high field emission efficiency of the ZnO nanostructures. Although ZnO nanostructure has these advantages, it is a lack of the capability of sustaining current densities.5 On the other hand, twodimensional graphene, the mother of all graphitic materials, can be wrapped up into zero-dimensional fullerenes, rolled into onedimensional nanotubes or stacked into three-dimensional graphite.6 Especially, the free-standing paperlike materials have wondrous properties that have potential applications in touch screens, solar cells, energy storage devices, cell phones, and high-speed computer chips.7 Graphene sheets are currently of great interest in efficient field emission sources due to their unique electronic properties, large surface areas, sharp edges, and sustaining current densities.8 However, the existing deposition methods, which lead to the planar morphological features of graphene sheets along entire substrates, limit the field enhancement.9 Therefore, the design of novel nanoarchitectures by connecting ZnO with graphene is expected both to reinforce the pristine properties of materials and to extend the application of simplex ZnO and graphene materials. * To whom correspondence should be addressed. Fax: +86-10-64427616. E-mail address: (S.H.)
[email protected]; (Y.Z.) yonghongzhang@ tjpu.edu.cn. † Division of Molecule and Materials Simulation, Beijing University of Chemical Technology. ‡ Tianjin Polytechnic University. § State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology. | Research Institute of Petroleum Processing.
In fact, graphene-ZnO nanocomposites have been synthesized recently.9,10 Graphene-ZnO has a lower turn-on field, 1.3 V/µm, compared to that, 2.5 V/µm, of pure ZnO at a current density of 1 µA/cm2.9 Vertically aligned ZnO nanostructures grown on graphene layers are good candidates for field emission emitters due to their sharp tips, high aspect ratio, high thermal stability, and high mechanical stability.10 In view of these experimental efforts, we believe that the mechanism of the field emission of graphene-ZnO nanocomposites is important and valuable to be conducted urgently by theoretical studies. Searching for stable configurations of graphene-ZnO nanocontacts and studying their field emission properties are both highly desirable by density functional theory (DFT) calculations. The aim of the present study is to establish a basic physical picture of graphene-ZnO nanojunction by interconnecting ZnO with graphene, and to study its field emission properties. To investigate the electronic structures of graphene-ZnO nanocomposites in the presence of an external electric field, the DFT calculations are performed. First, the total binding energies of neutral and charged graphene-ZnO as a function of electric field are calculated. Then, the energy levels of the neutral grapheneZnO in different electric fields are obtained. The electron density distributions of the states in the vicinity of the Fermi level with and without an electric field are also investigated. By the studies, the accurate values of work function (WF) and ionization potential (IP) of graphene-ZnO are provided, and the field emission performance is evaluated by the values of WF and IP. Finally, we present the changes of the Hirshfeld charges and the electrostatic potential derived charges (ESP) in different electric fields. 2. Models and Methods 2.1. Model. Graphene is made of carbon atoms arranged on a honeycomb structure with hexagons and can be considered as composed of benzene rings stripped out from their hydrogen
10.1021/jp107780q 2010 American Chemical Society Published on Web 10/28/2010
Field Emission Properties of Graphene-ZnO Nanocomposite
Figure 1. (a) The supercell of graphene monolayer and four different graphene nanodisks can be obtained from the graphene sheet, which are indicated by white, pink, green, and yellow circles, respectively. The biggest graphene nanodisk is used in this study. (b) The optimized graphene-ZnO, a uniform electric field is applied along graphene-ZnO axis. The red, gray, black, and white balls represent O, Zn, C, and H atoms, respectively.
atoms. A zero-dimensional graphene nanodisk can be obtained from two-dimensional graphene, which is shown in Figure 1. It is well-known that the Zn3O3 six-membered ring is a stable and frequent structural motif in ZnO nanocrystals and nanoclusters.11,12 In this study, the graphene-ZnO nanocontact is formed by attaching a (Zn3O3)3 cluster to a graphene sheet by way of six additional carbon atoms. Edge region of grapheneZnO is terminated by hydrogen atoms to keep the stability of the structure and emulate infinite grapheme.13 The optimized graphene-ZnO in Figure 1b forms a cone structure with a height of 1.668 Å and an angle of 156.9°. To study the field emission properties of graphene-ZnO, a uniform electric field is applied along the structure axis, as shown in Figure 1b. 2.2. Methods. All calculations are performed by DFT within the generalized-gradient approximation (GGA) implemented in the DMol3 software.14 The Perdew, Burke, and Ernzerhof (PBE) exchange-correlation functional15 is used, and the structure of graphene-ZnO nanocontacts is optimized without any symmetry constraints. Double numerical basis sets including polarization functions on all atoms (DNP) are used in the calculations.16,17 The DNP basis set corresponds to a double-ζ quality basis set with a p-type polarization function added to hydrogen and d-type polarization functions added to heavier atoms. The DNP basis set is comparable to 6-31G** Gaussian basis sets with a better accuracy for a similar basis set size.18,19 The global cutoff radius is set to be 4.4 Å. The convergence criteria applied during geometry optimization is 1.0 × 10-6 Hartree for energy. In addition, we perform the harmonic vibrational frequency computation to ensure the graphene-ZnO to be true minima instead of being saddle points on the potential energy surface of the nanostructure. 3. Results and Discussion 3.1. Binding Energies for Neutral and Charged GrapheneZnO. The electrons of graphene-ZnO could be extracted by an external electric field. Therefore, we calculate the total binding energies as a function of electric fields for neutral and charged graphene-ZnO, which are shown in Figure 2. We have defined the total binding energy (Eb) as the energy gained in assembling the graphene-ZnO from its isolated constituent atoms. Hence, the total binding energies of graphene-ZnO are calculated by the equation
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Figure 2. The total binding energy (Eb) of neutral and charged graphene-ZnO changes with the applied electric field.
Eb ) (Ei - Et)
(1)
where Ei is the sum of the energies of individual atoms and Et is the total energy of the system. The -1, 0, and +1 |e| charged graphene-ZnO nanostructures are investigated in this research. The model is supposed to have perfect atomic structure. Extra electrons might cause a structural relaxation. Yet calculations of Kim et al.20 showed that the capped tubes withstood electric fields up to 2 V/Å. Luo et al.21 used perfect carbon nanotube clusters to study the electronic properties in strong electric fields with maximum local electric field up to 2 V/Å. The results show that there is rather small variation of the total electronic density after adding the extra electrons. In the calculation, the number of extra electrons is small compared with the total electron number of the neutral model (978 electrons). At the same time, as the maximum of the applied electric field (0.5 V/Å) is smaller than 2 V/Å, the effects of the structural relaxation could be ignored, and the atomic structures are considered not to vary with extra electrons in the applied electric field. Without an external electric field, the graphene-ZnO with one negative charge is more stable than others, as shown in Figure 2. This phenomenon is similar to those of C60 and capped (5, 5) armchair nanotube and is attributed to the strong electron affinity of carbon species.22 Also, we calculate the electron affinity of graphene-ZnO. The electron affinity is the separation between the vacuum level and the bottom edge conduction band. Without any external electric field, the graphene-ZnO has a strong electron affinity of 2.463 eV. 3.2. Energy Levels and Local Electron Density Distributions. To obtain the detailed information about the field emission properties of graphene-ZnO, the energy levels of graphene-ZnO in different electric fields for a neutral system are calculated as shown in Figure 3. As the states in the vicinity of the Fermi level are mainly responsible for the electron field emission, we only focused on the electronic states on both sides of the Fermi level. It should be mentioned that the Fermi level is defined in the midgap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which is in agreement with the experimental measurements.23 The Fermi levels are presented by the dashed lines in Figure 3. According to our calculations, the smallest energy gap between the HOMO and LUMO is 0.312 eV in an electric field of 0.5 V/Å. It indicates that the field emission is no longer to be explained by the metal model due to the existence of the energy band gap. Therefore, the field emission currents of grapheneZnO could not be fully explained by the Fowler-Nordheim
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Figure 3. The energy levels of graphene-ZnO for a neutral state with 0 and 0.5 V/Å electric fields. The Fermi levels are drawn by the dashed lines.
equation.20 With increasing electric field, the HOMO and LUMO levels shift toward the vacuum level, as shown in Figure 3. In other words, the Fermi level shifts toward the vacuum level, which lowers the effective height of the potential barrier of graphene-ZnO to cause the electrons to be emitted easily. Figures 3 and 4 show the corresponding local electron density distributions of graphene-ZnO with electric fields 0 and 0.5 V/Å respectively. The local electron density distributions with different electric fields are similar and not presented here. Without an electric field, the electron densities of the HOMO, HOMO-1, and HOMO-2 states are mainly localized at the surface of the graphene and at the contacts between graphene and ZnO. The electron densities of the HOMO-1 and HOMO-2 states with electric fields 0 and 0.5 V/Å are presented in the Supporting Information Figure S1 and Figure S2. A fraction of electron density of the HOMO-2 without an electric field is localized at the side of the ZnO structure, which is shown in the Supporting Information Figure S1b. The electron densities of the LUMO are localized mostly at the surface of the graphene. When a electric field 0.5 V/Å is applied along graphene-ZnO axis, a significant change in the electron density distribution of LUMO is observed. The electron density distribution of LUMO of the graphene-ZnO shifts from the surface of graphene to the top of the ZnO cage when a electric field 0.5 V/Å is applied. It makes LUMO levels shift toward the vacuum level, as seen from the energy levels in Figure 3. The result is similar to the studies of capped nanotubes reported by Kim et al.22 Local electron density distributions of charged graphene-ZnO have been shown in Figure 4. For negatively charged states (Q ) -1 |e|), only the electron densities of the HOMO is localized at the top of the ZnO nanocage in electric field, and the electron densities of the LUMO is mainly localized at the surface of the graphene. On the contrary, for the charged state (Q ) +1 |e|), the LUMO is localized at the top of the ZnO nanocage in electric field, but the electron densities of the HOMO is not affected by the external electric field, as seen in Figure 4. The electron densities of the HOMO and LUMO of charged graphene-ZnO (Q ) -1 and +1 |e|) are localized at the top of the ZnO cage, which can contribute directly to the field emission currents. Kim et al. have reported that the electrons of a capped carbon nanotube emitted easily in electric field, because the local field was enhanced easily at the cap by the local field enhancement factor.20,22 In fact, on the surface of graphene-ZnO composite, the small ZnO nanocage plays as a sharp emission site and gives rise to the increase of the field enhancement factor. Therefore,
Figure 4. Side views of the local electron density distributions for neutral and charged graphene-ZnO with 0 and 0.5 V/Å electric fields (EF). It refers to HOMO and LUMO. The red, gray, black, and white balls represent O, Zn, C, and H atoms, respectively.
the design of graphene-ZnO by connecting ZnO nanocage with graphene is expected to improve the field emission performance. 3.3. Work Functions and Ionization Potentials. Actually, the combination of ZnO and graphene makes use of the curvature effect and the oxidation that induce charge enhancement. The charge enhancement is ascribed to the polarization by oxygen lone pairs24,25 and the graphene π-bond electrons by the graphene edge induced local quantum entrapment and densification of the core and bonding electrons. These electrons generate impurity states in the gap and raise the Fermi energy. High-efficiency electron emission is anticipated. To further understand the field emission properties of graphene-ZnO, the work function and ionization potential have been calculated. The WF and IP are quantities for understanding the field
Field Emission Properties of Graphene-ZnO Nanocomposite
Figure 5. The work function and ionization potential of grapheneZnO changes with applied electric field.
emission properties of graphene-ZnO. A qualitative correlation has been observed between the field emission and the WF or IP: the lower the WF or IP, the easier for an electron to be extracted from the emitter, and the higher field-emission current or the lower threshold voltage.26-28 The calculated WFs and IPs of graphene-ZnO with and without electric field are shown in Figure 5. In semiconductor materials, the emitted electrons originate from the top of the valence band or from the bottom of the conduction band. In fact, graphene-ZnO exhibits semiconducting property with a finite value of energy gap between LUMO and HOMO as mentioned above. The work function is defined as the energy difference between the midgap and the vacuum level.22 From the Figure 3, the gap of HOMO-LUMO decreases from 0.948 to 0.312 eV with increasing electric field, indicating a significant change of WFs. Using the same calculation strategy, we first calculated the WFs of (ZnO)12 nanocage and graphene nanodisk due to the availability of their experimental data for comparison. Without electric field, the WF of (ZnO)12 nanocage is 4.970 eV, which compares favorably with the experimental measurements mentioned above (4.5-5.3 eV).29-31 The WF of graphene nanodisk is 4.022 eV. It should be mentioned that the graphene nanodisk is obtained from a graphene sheet, which is shown in Figure 1a. The dangling bonds of the graphene nanodisk are saturated by hydrogen atoms. Chan et al.32 and Giovannetti et al.33 calculated the WF to be 4.260 and 4.500 eV for a single graphene sheet by DFT. The small deviation between the reported two WFs and our calculated WF may be due to different graphene structures. Our graphene nanodisk has 24 hydrogen atoms to be free from dangling bonds. In fact, Qiao et al. have used a similar calculation method to calculate the WF of a pristine carbon nanotube and it showed great agreement with experiments.28 In our calculations, without the applied electric field the WF of graphene-ZnO is 3.623 eV. The result suggests that grapheneZnO nanocomposite has an enhanced field emission comparing with pristine ZnO and graphene materials. On the other hand, the WF of graphene-ZnO decreases linearly when the electric field is applied, which is clearly observed in Figure 5. At a higher electric field, the WF is less than 0.669 eV, and electrons can be emitted more easily. Theoretically, the ionization potential is defined as the energy difference between the cationic graphene-ZnO (+1 |e|) and the neutral one. In addition, the cationic graphene-ZnO should be frozen at the relaxed geometry for the neutrally charged system, because the “hole” left by emitting electron recombines almost instantaneously with an incoming electron from the whole
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Figure 6. Hirshfeld and ESP population analysis of the topmost sixmembered ring in different electric fields. The topmost six-membered ring is indicated by the black circle.
system.34 The ionization potentials of (ZnO)12 and the graphene nanodisk are 7.472 and 5.195 eV. However, the graphene-ZnO has a low ionization potential of 4.835 eV. From Figure 5, the IPs of graphene-ZnO follows the same trend as the WFs of graphene-ZnO, decreasing linearly with increasing electric field. These results indicate that graphene-ZnO nanocomposite has an enhanced field emission, which can be considered as a good candidate for a field-emission electron source. 3.4. Charge Analysis. The charge accumulation in the tip of the carbon nanotubes can significantly modify the tip-vacuum barrier and thereby leads to a change of the field emission properties.22,28 The Mulliken method for electron population analysis has well-known shortcomings.35 The Hirshfeld and ESP Charges are recommended because they yield chemically meaningful charges.36,37 Hence, we have calculated the Hirshfeld and ESP charges of graphene-ZnO in different electric fields. The charges are the sum of the topmost six-membered ring, which can be seen in Figure 6. Although the Hirshfeld charge is sensitive to the increase of electric field, the two curves of the Hirshfeld and ESP charges are the same in variation trend. From Figure 6, we can find that the electron accumulation becomes obvious on the topmost six-membered ring with the increase of the applied electric field. The negative charges on the topmost six-membered ring show that the electrons move from the graphene-ZnO body to the topmost six-membered ring. The accumulation electrons are localized and contribute to the field emission. In fact, the extra electrons on the topmost sixmembered ring of graphene-ZnO are concentrated in the LUMO level, and consequently the Fermi level would shift toward the vacuum level. The result is consistent with the changes of energy levels as mentioned above. Therefore, the accumulated electrons will reduce the potential barrier of graphene-ZnO and enhance the field emission properties. 4. Conclusions A novel structural model of graphene-ZnO nanocomposite has been proposed. We have performed first-principles density functional calculations of electric structures and field emission properties of the hybrid graphene-ZnO. Effects of the applied electric field on the electronic structures of graphene-ZnO have been investigated. With increasing electric field, the HOMO and LUMO levels shift toward the vacuum level. As a result, it leads to a decrease in the potential barrier of graphene-ZnO and causes electrons to be emitted more easily. Both the work function
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and the ionization potential decrease linearly with increasing electric field, which confirm that graphene-ZnO has fairly good field emission properties. Without electric field, graphene-ZnO has a smaller work function of 3.623 eV, comparing with the work functions of pristine ZnO nanocage 4.970 eV and graphene nanodisk 4.022 eV. Therefore, graphene-ZnO may be a good field emission electron source material. We suggest future experiments to give more attention to the graphene-ZnO nanocomposite and examine their potential as field-emission electron sources. Acknowledgment. This work is supported by the National Natural Science Foundation of China (20876005) and the National Basic Research Program of China (2010CB732301). Supporting Information Available: The local electron density distributions of HOMO-1 and HOMO-2 states for neutral graphene-ZnO with 0 and 0.5 V/Å electric fields. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tseng, Y. K.; Huang, C. J.; Cheng, H. M.; Lin, I. N.; Liu, K. S.; Chen, I. C. AdV. Funct. Mater. 2003, 13, 811. (2) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y.; Ruh, H.; Lee, H. J. Appl. Phys. Lett. 2002, 81, 3648. ¨ zgu¨r, U ¨ .; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; (3) O Dogan, S.; Avrutin, V.; Cho, S. J.; Morkoc¸, H. J. Appl. Phys. 2005, 98, 041301. (4) Wang, Z. L. Mater. Today 2004, 7, 23. (5) Heo, Y. W.; Tien, L. C.; Kwon, Y.; Norton, D. P.; Pearton, S. J.; Kang, B. S.; Ren, F. Appl. Phys. Lett. 2004, 85, 2274. (6) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. ReV. Mod. Phys. 2009, 81, 109. (7) Park, S. J.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217. (8) Geim, A. K. Science 2009, 324, 1530. (9) Zheng, W. T.; Ho, Y. M.; Tian, H. W.; Wen, M.; Qi, J. L.; Li, Y. A. J. Phys. Chem. C 2009, 113, 9164. (10) Kim, Y. J.; Lee, J. H.; Yi, G. C. Appl. Phys. Lett. 2009, 95, 213101. (11) Matxain, J. M.; Mercero, J. M.; Fowler, J. E.; Ugalde, J. M. J. Am. Chem. Soc. 2003, 125, 9494.
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