Effects of ZnO Quantum Dots Decoration on the Field Emission

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Effects of ZnO Quantum Dots Decoration on the Field Emission Behavior of Graphene Lei Sun, Xiongtu Zhou, Zhixian Lin, Tailiang Guo, Yongai Zhang, and Yongzhi Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10454 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Effects of ZnO Quantum Dots Decoration on the Field Emission Behavior of Graphene Lei Sun,a,b Xiongtu Zhou,a Zhixian Lin,a Tailiang Guo,a Yongai Zhang,*aand Yongzhi Zeng*a

a

National & Local United Engineer Laboratory of Flat Panel Display Technology, Fuzhou

University, 350002 Fuzhou, China b

Zhicheng College, Fuzhou University, 350002 Fuzhou, China

*Corresponding authors: Tel: +86 591 87893299, Fax: +86 591 87892643 E-mail: [email protected], [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: ZnO quantum dots (QDs) have been decorated on graphene deposited on the patterned Ag electrodes as a field emission cathode by solution process. Effects of ZnO QDs on the field emission behavior of graphene are studied by the experiments and first-principle calculations. The results indicate that the attachment of ZnO QDs with C atom leads to the enhancement of electron emission from graphene, which mainly attributes to the reduction of work function, ionization potential and the increasement of Fermi level of graphene after the decoration. The change of local density distribution and density of states near the Fermi level may also account for the reason. Our study may help to develop new field emission composites and expand ZnO QDs in the application for electron emission devices as well.

KEYWORDS: ZnO quantum dots, graphene, field emission, hybrid, first-principle density functional calculations

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1. INTRODUCTION Until now, the unique structural characteristic and outstanding electrical properties have rendered graphene as an attractive material in cold cathodes, as well as in the photonic and electric field.1-3 However, the conventional deposition processes, which yield graphene sheets lying on the surface of substrate, are not beneficial for the field emission properties. The electrophoretic deposition (EPD) methods, which facilitate the vertically standing of graphene, were performed in our experiment.4 On the other hand, Zinc oxide,5-7 with wide band gap and low work function, have aroused wide concerns in nanodevice applications of field emission. In order to extend the application of the pristine materials, extensive attentions have been paid to combine graphene with ZnO recently, where found that there existed subtle nanocontacts between ZnO and graphene during recombination by first-principle density functional calculations (DFC).8 DFC theoretical calculations have been widley utilized to investigate the electrical structures of these nanomaterials. For instance, Huang et al. have studied the field emission of a single In-doped and pure ZnO nanowire by first principle calculations.9 Driscoll et al. have used DFT to study electron field emission from graphene nanoribbons.10 Guo et al. have investigated the change of internal electrical structures of the nanocomposite of graphene sheets and ZnO structures by DFT.11 Actually, the improved field emission (FE) performance of graphene-ZnO hybrid structure, due to large surface area and effective surface passivation, have been reported in several researches.12,13 For example, graphene films have been transferred 3



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onto the ZnO nanowires,14 which not only descends the turn-on field but also improves the enhancement factor15, where the factor of field emitters is the relationship between the local and the applied electric field when the electron tunneling occurs and it is mainly related to the high aspect ratio. ZnO structures have been also grown on graphene sheets, which leads to higher aspect ratio and better thermal stability, thus improves the field emission properties.16 Among all of these efforts, most investigations have focused on the composites of graphene with one-dimensional ZnO structure. However, the graphene sheets are combined with ZnO QDs hybrid structure in a mild solution process in our works, while the first-principle density functional calculations are also performed to account for the experiment results. Effects of ZnO QDs on the electronic configulations and electron tunneling probability of graphene is emulated by Vienna ab initio simulation package (VASP) software package17. The purpose of present study is expected to break new ground of quantum dots in the aspect of electron field emission devices, as well as utilize the density functional theory to investigate the electronic structures for the design of novel nanocomposites. 2. EXPERIMENTAL DETAILS Graphene powders were synthesized by modified Hummers’ method. Firstly, 1 g of natural graphite, together with the certain proportion 9:1 of H2SO4 and H3PO4 was mixed into three-necked flask. Then, 6 g of potassium permanganate was added gradually under vigorous stirring about 1 h in an ice-water bath. Secondly, the temperature was increased to 50 ◦C, then the reaction was kept about 12 h. Thirdly, the 4


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synthesized product was poured into ice water, and an appropriate amount of H2O2 was added while stirring until the solution became golden. After the filtration, the suspension was washed with 5% HCl and deionized water until no sulphate was detected in the filtrate. Finally, the graphite oxide were reduced to graphene by hydrazine hydrate. The synthesized graphene powders together with the identical weight of Mg(NO3)2·6H2O were mixed in pure isopropyl alcohol to prepare the electrophoretic deposition (EPD) suspensions under ultrasonication. Along with the graphite plate as the anode, the Ag electrode with a width of 200 µm which is printed on glass substrate as the cathode, was kept in the above suspension at room temperature. The distance between two electrodes was 3 mm. The deposition process was performed with a steady direct voltage at 40 V for 5 min. After cleaning and annealing, the pattern electrodes with graphene were decorated by ZnO QDs as following. The cathode was put into the 0.25 mol/L Zn2+ ethanol solution, where Zn2+ ions were generated by certain amount of [Zn(CH3COO)2·2H2O]. 0.5 mol/L LiOH of ethanol solution was then droped into the Zn2+ ethanol solution under vigorous stirring. After being ketp in ice-bath for one hour, the sample was cleaned by DI water and one hour of annealing treatment at 150 ◦C in vacuum was subsequently applied. To illustrate the effects of ZnO QDs decoration on the FE behaviors, the measurement of FE properties were proceeded under high vacuum of 1.0×10-5 Pa at room temperature. The cathode of as-prepared graphene-coated Ag electrode was isolated from the anode of phosphor-based ITO glass by the 300 µm-height separant. The emission current was tested at the anode during DC voltage appling to the 5



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electrodes. Fig. 1(a) depicts the system diagram of FE testing configuration for the emitters and Fig. 1(b) presents the micrograph of the graghene-based cathode. 3. RESULTS AND DISCUSSION 3.1 Morphological Characteristics Fig. 2(a) shows the atomic force microscope (AFM) image of graphene sheets on Ag electrodes. The cross-sectional analysis of the selected region between the two arrows, as presented in the inset of Fig. 2(a), indicated the thickness of the graphene sheets is about 4 nm. Scanning electron microscopy (SEM, Hitachi S-3000N) images in Fig. 2(b) were analyzed to characterize the morphologic features of the samples, where the graphene sheets can be seen covering perpendicular to electrode surface. The wrinkled edges are able to facilitate electron field emission due to high aspect ratio. From the image of high-resolution transmission electronic microscope (TEM, Tecnai G2 F20 S-TWIN)) in Fig. 2(c), one can observe that most graphene sheets were uniformly decorated with ZnO QDs with the average diameter around 5 nm. The interlayer spacing of 0.29 nm is clear from the close-up in the inset, which is consistent with the lattice spacing of ZnO QDs.18 Fig. 2(d) shows the XRD spectrum of hybrid graphene sample. The wide peak of the graphene appears at 2θ value of 24.6◦, while the rest sharp peaks fit well to the hexagonal lattice pattern of wurzite type ZnO. Raman spectra in Fig. 2(e) indicates both the disordered structure at D-band (1339 cm-1) and the crystallized structure at G-band around (1596 cm-1) as well as the defects-related 2D band at 2680 cm-1. The higher D band intensity may be attributed to more structural defects during ZnO QDs combination. Three low 6


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intensities at around 330 cm-1, 437 cm-1 and 580 cm-1 reflect from the characteristic bands from ZnO QDs. The I(D)/I(G) band intensity ratio is calculated to be around 1.10. 3.2 Field emission property Fig. 3(a) shows the relationship between the field emission current density and the applied electric field (J-E), while the apparent left-shift of the values reflects the improvement of the field emission behaviors upon ZnO QDs-decoration. The turn-on field (Eto) and the threshold field (Ethr) are usually defined as the electric fields required to produce a current density of 10 µA/cm2 and 1 mA/cm2, respectively. As shown in Fig. 3(a), the turn-on field Eto and threshold field Ethr of ZnO QDs-decorated graphene are approximate values of 0.9 V/µm and 2.6 V/µm, which is lower than that of pristine graphene (Eto=1.5 V/µm and Ethr=3.5 V/µm). The linearity of the ln(J/E2)-(1/E) curves of the inset in Fig. 3(a) indicates that the electron emission is in accordance with field emission mechanism and will be formulated by the Fowler-Nordheim (F-N) law.19,20 The emission current versus time (I-t) curves at the current density of 5 mA/cm2 during 80 minutes, which describe the stability of emission current, are plotted in Fig. 3(b). The current density of the as-prepared graphene-ZnO QDs system remains stable, while for the pure graphene, the larger current fluctuations are easily observed. The corresponding field emission microgaphs of the pure graphene and graphene-ZnO QDs hybrid are given in the insets, indicating the better field emission stability ascribed to the modification of surface topography of graphene by ZnO QDs 7



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decoration. 3.3 Models and calculating methods To investigate the mechanism of the electronic structure change, the Vienna ab initio simulation package (VASP) software package was utilized, which is based on the density function theory (DFT) and the plane-wave basis and the projector augmented wave (PAW) representation.21,22 The graphene surface with 6 × 6 honeycomb structure of repeated slabs containing 72 carbon atoms was modeled and separated by 15 Å vacuum region. ZnO QD is consisted of 8 six-atom ring and 6 four-atom ring (Fig. 4) and adsorbed to graphene sheet to form the grapheme-ZnO nanocomposite by additional six carbon atoms ring attaching to graphene surface (named as C-ZnO). Such a supercell model, which is considered the most stable structure upon the grapheme-ZnO nanocomposites, can well simulate the adsorption of ZnO QDs on the graphene surface. The BZ integrations in supercell calculations are performed by using a 3 × 3 × 3 Monkhorst-Pack k-point mesh,23 containing 14 special points in the irreducible Brillouin zone (BZ). In order to obtain high accuracy, periodic boundary is applied with 450 eV of the plane wave kinetic energy cutoff and the vacuum is set to 15 Å between slabs. The conjugate-gradient technique was employed

to

optimize

the

atomic

structures

by

using

the

calculated

Hellmann-Feynman forces.22 As the force on every atom was converged up to 0.01 eV/Å, the convergence condition is obtained and the atomic geometries can be completely

relaxed.

The

generalized

gradient-corrected

exchange-correlation

functional (GGA), proposed by Perdew and Wang, is processed to deal with the 8


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exchange-correlation effect.22,25 The method of artificial planar dipole layer is adopted in the Hamiltonian for the simulation of external electric field.26 3.4 Analyses based on the first principles calculations 3.4.1 Ionization Potentials and Work Functions The calculations for ionization potential (IP) of the ZnO QDs-graphene composite, along with the work functions (WF), are presented in Fig. 5. The ionization potential represents the energy difference between neutral (Q=0) and charged (Q=+1) nanocomposite, while the work function refers to the energy difference between the vacuum level and the Fermi level. The emulations are based on the following formulas:27,28

IP = E graphene-ZnO (Q = +1) − E graphene-ZnO (Q = 0) WF=Φ-EF

(2) (3)

where Egraphene-ZnO(Q=+1) and Egraphene-ZnO(Q=0) is the total energy of the as-prepared composite with charge and without charge, respectively. Φ is the energy of vacuum level, EF represents the Fermi level inside material. The calculated WF and IP are shown in Fig. 5(a). As is known, low ionization potential or work function is beneficial for electrons to overcome the surface potential barrier, thus the decreased threshold field or increased field emission current can be obtained.29,30 Obviously, without electric field, the ionization potential of the graphene decorated by ZnO QDs is 4.75 eV, which is lower either than that of pure graphene (5.21 eV) or pure ZnO (7.52 eV). The reduction of IP with increasing electric field can be also observed in Fig. 5(a), where IP decreased to 0.95 eV with the electric field of 0.6 VÅ-1. As the 9



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external electric field increased, a similar downward trend of WF is appeared in Fig. 5(a). The calculated results indicate that WF of graphene and ZnO QDs is estimated to be 4.50 eV and 4.97 eV, respectively. The WF of graphene is consistent with the experimental measurements of graphene at 4.53 eV by UPS31 or the calculation results at 4.5 eV reported by Giovannetti32. The WF of ZnO QDs is also favorably close to ZnO nanomaterials ranged from 4.5 to 5.3 eV.33,34 The graphene decorated ZnO QDs has a estimated value of work function (3.66 eV), which is much lower than that of ZnO (4.97 eV) and graphene (4.50 eV) without electric field. On one hand, when ZnO connected to C atoms of graphene, the polarized oxygen lone pairs35,36 of ZnO and the quantum entrapment by π orbitals of graphene cause the local electron aggregations, thus more impurity states were introduced in the band gap, leading to the ascended Fermi level and the reduced work function. On the other hand, the combination of ZnO QDs with graphene induced stable oxygen groups in the form of cyclic edge ethers, more C-O-C chains were found in the ZnO-graphene hybrid, which is caused by the reduction of the potential barrier and the variation of the electronic density distribution of the states close to the Fermi level.31,37 The WF of the composite reduced to 0.35 eV when the electric field is 0.5 VÅ-1, indicating that only a small energy can lead to electronic excitation. The value of WF turned to be negative when the electric field goes up to 0.55 VÅ-1, in other words, this means that the electrons can be transferred from other parts of the material to the surface without any more energy under high electric field above 0.55 VÅ-1. From the WF and IP calculation, one can infer that the graphene-ZnO QDs composites are more sensitive 10


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to the electric field and have enhanced field emission performance. It is worth nothing that the reduction of IP and WF is also related to the Fermi level, as depicted in Fig. 5(b). The increasement of the Fermi level of graphene plays a key role in the depression of the surface potential barrier due to ZnO QDs-attaching, 3.4.2 Electronic Density Distribution To visualize how the interactions between the adsorbed ZnO QDs and the graphene sheet affect the field emission behaviors, the electronic density distribution is presented in Fig. 6. The charge density differences between the ZnO QDs-graphene hybrid systems and the accumulation of atomic charge densities in Fig. 6(a), help to characterize the bonding. The depicted plane is perpendicular to the graphene sheet and crossing two atoms of C6 ring and the lowest O and Zn atoms. As both the charge depletion and superposition regions respectively plotted in dashed and solid lines in Fig 6(a), strong covalent bonding for C atoms in the graphene sheet and C6 ring were visualized, as well as the bonding between the C6 ring and ZnO cluster. The charge density differences in Fig. 6(b), which represent the difference between the graphene-ZnO QDs hybird systems and the pure ZnO nanostructure and clean graphene sheet, reveal the charge transfers from the graphene sheets to the adsorbed ZnO QDs. After ZnO decoration, the charges redistribute in the vicinity of C atom right under ZnO cluster. The remarkable interaction between the graphene and ZnO QDs, attributed to the strong covalent bonding between atoms, can be implied from the curves of charge redistribution and charge transfers in Fig. 6(b). Furthermore, the even stronger covalent bonds inside ZnO QDs, as suggested from Fig. 6(a), 11



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demonstrate the stability of the graphene-Zno hybrid structure. The decorated ZnO QDs are closely attached on the surface of the substrates and easily to set apart. In the inset of Fig. 6(b), the plots of local electrons density help to understand why the graphene-ZnO QDs composite by attaching ZnO to the graphene sheet by six additionnal carbon atoms can increase the emission current. The electrons transfer from C atom to O atom, due to the smaller electronegativity of C (2.5) comparing with that of O (3.5), and result in the concentration of electrons around the O-termination. Consequently, spontaneous charge transfers between O atom and Zn atom come into being due to the polar field. The C-C homonuclear bonds in the connecting region can induce some new orbitals around the Fermi level under applied electric field, leading to higher tunneling probability for occupied orbitals. Either the large electron-emitting region or the high electronic density from these occupied orbitals ultimately results in a larger emission current from the top region. 3.4.3 Density of states The density of states (DOS) of C atom for the original graphene and graphene-ZnO composite and O atom for ZnO QDs were calculated, as plotted in Fig. 7. The energy zero is set at Fermi level. The Fermi level is close to the bottom of conduction band, and the density of states moves towards lower energy in an integral manner. The C-2p orbital electrons contributes to both the valence band and the conduction band. The peak near the Fermi level is induced by hybridizaiton of C-2p and C-2s orbital electrons, which reveals the existence of C-C covalent interaction. The interaction of peaks of C-2p and O-2p between -11 eV and -1 eV indicates the 12


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ZnO QDs absorbed on the graphene sheets. The changes of the electronic structures of graphene may facilitate the field emission of graphene. On the other hand, the band gap of original graphene is zero due to its semi-metallic properties. The ZnO decorations on graphene lead to the increasement of the band gap of graphene. The C-2p orbital electrons move to the Fermi level after adsorption due to the strong interaction between the O-2p orbital electrons in ZnO and the C-2p orbital electrons in graphene, which result in more electrons gathering around the Fermi level. The C-2P orbital electrons in the conduction band above the Fermi level transfer to a higher level, so that the conduction band near the Fermi level can accept more electrons. Therefore, the electrons in the valence band below the Fermi level may easily emit into the conduction band under the external electric field after absorption, thus improved the transmission capacity. On the other hand, one can inspect from Fig. 7 that the O-2p and Zn-3d peaks moved closer to Fermi energy level in the hybird graphene, which implied that the electrons can be more likely to emit from the conduction band of ZnO QDs to vacuum. In other words, after ZnO QDs combination, besides the edges of the graphene, the ZnO QDs on the graphene surface might also be the emission sites. When the graphene attached ZnO with C atom, the energy level of C atom is incorporated into the band gap, these enable more electrons to tunnel across the barriers from graphene sheet surface under an external electric field. The lowest point of the minimum value of DOS above Fermi level, as marked by arrow in Fig. 7, moves towards low-energy ranges below the Fermi level, meaning that the 13



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connections of ZnO QDs arouse Fermi level shift to the conduction band and increase the value of DOS at the Fermi level. 4. CONCLUSIONS In conclusion, graphene-ZnO QDs composite has been prepared on patterned Ag as a field emission cathode by a mild solution method. The electron field emission property of graphene is substantially improved upon ZnO QDs decoration. The first-principles calculation for graphene-ZnO QDs is performed to account for the reason. The results, which is consistent with the experiments, show that the adsorption of ZnO QDs not only increase the tunneling probability and electron density but also effectively decrease the work function、the ionization potential and the field emission potential barrier of graphene, thus facilitate the emission of electrons to vacuum. The enhanced emission current may be attractive to apply this hybrid structure in the electron emission devices and open new possibilities of using quantum dots decoration in the vacuum microelectronics field.

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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 11404060 and 61474024), the National Science and Technology 863 Major Project (Grant No. 2013AA030601), the Natural Science Foundation of Fujian Province, China (Grant No. JK2014002), the Program for New Century Excellent Talents and the Outstanding Youth Project in Fujian Province.

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Figure captions: Fig. 1 (a) Configuration for field emission measurements; (b) Micrograph of printed graphene-based Ag substrate. Fig. 2 (a) AFM images of graphene-ZnO QDs hybrid; (b) SEM images of the hybrid structures; (c) TEM image of composites; (d) XRD spectrum of composites; (e) Raman spectrum of composites.

Fig. 3 (a) Emission current density versus electric field (J-E) plots of pure graphene and as-prepared hybrid emitters, where the corresponding F–N curves are given in the inset; (b) Emission current versus time (I-t) curves, where the insets show the field emission micrographs of the pure graphene (left) and as-prepared hybird sample (right). Fig. 4 Supercell used for calculation, which is named as C-ZnO, where ZnO QD is adsorbed to graphene substrate by C6 six carbon atoms ring attaching. The red, gray and black balls represents O, Zn, C atoms, respectively. Fig. 5 (a) Ionization potential and work function of hybrid graphene-ZnO structure; (b) Fermi level of hybrid graphene-ZnO emitters and original graphene, respectively. Fig. 6 Contour plots of the charge density differences, where the vertical section is perpendicular to the graphene substrate (a) for the differences between the hybrid emitter and the accumulation of atomic charge densities ∆ρ1; (b) the same as (a), but for ∆ρ2 defined as the difference between graphene-ZnO composites and the ZnO nanostructure and clean graphene sheet. Solid and dashed lines correspond to ∆ρ > 0 and ∆ρ < 0, respectively. The inset is the plot of local electrons density, where the 21



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green balls are the electrons and the bottom layer represents the connected C atoms, while the middle layer represents the O atoms. Fig. 7 The density of states(DOS) of C atom for (a) the clean graphene sheet; (b) right under ZnO cluster on the graphene sheet; (c) C6 ring; (d) O atom of ZnO cluster;(e) Zn atom of ZnO cluster. The Fermi level is set to 0eV.

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Fig. 1

Fig. 2

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Fig. 3

Fig. 4

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Fig. 5

Fig. 6

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Fig. 7

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Table of Contents Graphic

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Effects of ZnO Quantum Dots Decoration on the Field Emission

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