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ZnO nanowire lines and bundles: Template-deformation-guided alignment for patterned field-electron emitters
Current Applied Physics 15 (2015) 1296e1302
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
ZnO nanowire lines and bundles: Template-deformation-guided alignment for patterned field-electron emitters Jizhong Song a, b, Xue Ning a, Haibo Zeng a, b, * a
State Key Laboratory of Mechanics and Control of Mechanical Structures & College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 15 April 2015 Accepted 16 April 2015 Available online 17 April 2015
One-dimensional ZnO materials have been promising for field-emission (FE) application, but how to facially control the alignment of ZnO emitters is still a great challenge especially for patterned display application. Here, we report the fabrication of novel ZnO nanowire (NW) line and bundle arrays for patterned field-electron emitters. The effects of PS template size and heating time on the resulted ZnO nanoarrays were systematically studied. The deformation degree of PS templates was controlled and hence utilized to adjust the alignment of electrochemically deposited ZnO arrays. It was found that the length of NW lines and the density of NW bundles can effectively tuned by the PS template heating time. The optimal FE performance with turn-on electric field as low as of 4.4 V mm1 and the fieldenhancement factor as high as of 1450 were achieved through decreasing the screening effect among the patterned field-electron emitters. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Nanowire lines and bundles Patterns Field-emission Template deformation
1. Introduction Among various ZnO nanostructures [1e7], vertically aligned nanowires/nanorods (NWs/NRs) have a variety of promising applications ranging from solar cells [8,9], UV lasers [10,11], lightemitting diodes [12,13], and piezo-nanogenerators [14,15], to field-emission (FE) devices [16,17]. Vertically aligned NWs/NRs with high aspect ratios have been considered as excellent candidates for electron field emitters as they can act as electron transporting channel. The FE performance of such materials is highly affected by their intrinsic physical and structural parameters, such as density [18], uniformity [19] and tapering [20]. After being stimulated by an applied electric field and before reaching the counter electrode, electrons have to pass through the entire ZnO nanostructure. Thus, the FE performance is closely related to the local emitting intensity and the interaction among emitters, which can be extremely affected by the density of aligned NWs/NRs. Both the high density aligned NWs/NRs due to the screening effect of the emitters and low density aligned NWs/NRs due to the small electron emitting density can deteriorate the FE performance. Recently,
* Corresponding author. E-mail address: [email protected] (H. Zeng). http://dx.doi.org/10.1016/j.cap.2015.04.011 1567-1739/© 2015 Elsevier B.V. All rights reserved.
many efforts have been made to adjust the density of aligned NWs/ NRs for the improvement of emitting efficiency [18,20e23]. For example, Benjamin et al. [24] proposed a simple, scalable, and costeffective technique for controlling the growth density of ZnO arrays based on a layer-by-layer polyelectrolyte polymer film. The FE results revealed that an emitter density of 7 nanorods mm2 and a tapered nanorod morphology generated a high field enhancement factor (b) of 5884. Lee et al. prepared different density arrays of ZnO nanorods by electrodeposition assisted by process nanosphere lithography technique [25]. Compared with the control of the whole density of arrays, control of local NW/NR density, uniformity and patterns were rather rarely addressed. This could significantly affect the local screening effect of the emitters. Remarkably, the patterned NWs/NRs field emitter demands the integrated control of the density of 1D vertical NWs. However, such integrated control on vertical NW/NR arrays remains to be a challenge till now. Recently, our group [26] has observed that vertical ZnO NW/NR arrays in centimeter-scale areas can be fully controlled by a facile and effective route of the deformation of colloidal crystal monolayer templates. The NR/NW density and uniformity were all adjusted through selection of template size and deformation. In line with the adjustments, the FE performance of the arrays can be tuned. On the other hand, patterned field-electron emitters are highly demanded
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in the practical display applications, and usually achieved by photography technique, which is expensive and hence greatly affect the current development. Therefore, how to facially obtain patterned ZnO field emitters is still a great challenge up to now. Here, the novel vertical ZnO NW lines and bundles were obtained and tuned by the deformation of polystyrene (PS) template size and heating time. The length and wire density of patterned ZnO arrays can be effectively tuned by the heating time and template size. When the 200 nm PS sphere acts as template, uniformly vertical ZnO NWs transit from coalescent vacancy bundles to lines along the increase of heating time due to PS diffusion, and the length of line-shaped ZnO NWs can increase along the increase of heating time. Meantime, the increase of heating time makes the density of bundle-shaped ZnO NW/NR array decrease by using 500 nm PS template. Such density control of ZnO bundles can optimize the performance of field emitters. The FE performance results imply that the template deformation heating time of 3 min can effectively avoid the screening effect and obtain optimal FE efficiency. Such findings and results are beneficial for potential patterned field electron emitter applications in the cold-cathodebased electronics. 2. Experiments 2.1. Self-assembly of colloidal monolayer PS template The uniformly and large-scale monolayer PS template was assembled by our previous method [26]. Briefly, a 2.5 wt% suspension of monodispersed PS spheres with different diameter was purchased from Alfa Aesar. A droplet of PS sphere suspension was placed on a cleaned heavily doped Si substrate ((100)-oriented highly doped silicon) with an area of 15 15 mm2 fixed on a spin coater at a rotating speed of 600 rpm for 1 min. A colloidal monolayer with an area of about 2 cm2 was formed on the substrate by a self-assembling process. In the same manner, the PS colloidal monolayers were assembled on the Si substrate for ZnO NW growth. 2.2. Electrochemical deposition growth of ZnO The colloidal monolayers on substrates were heated at 115 C for different times (1e10 min) to adjust the deformation. Subsequently, the galvanostatic cathodic deposition was employed on the deformed colloidal layer-Si substrates (as the cathodes) at a current density of 0.5 mA cm2. Zinc sheets (99.99% purity) acted as the anodes, and zinc nitrate aqueous solution (0.05 M) as the electrolyte. The deposition temperature was fixed at 70 C by a water bath and the deposition time was changed from 10 min to 1 h for getting different wire lengths. The hexamethylenetetramine (HMTA, C6H12N4) was added into the electrolyte with a concentration of 0.1 M for the growth of tapered nanorods. 2.3. Characterizations and FE measurement The samples were characterized by FE scanning electron microscopy (SEM, JEOL JSM-6700F). The crystal graphic interpretations were performed on an X'Pert Pro XRD system with Xray Mirror PFX at an operation voltage of 40 kV and a current of 40 mA, in which Cu Ka (l ¼ 0.1540 nm) was used and scanned in a 2q range from 20 to 80 . The FE properties of different samples on Si substrates were studied at room temperature in a high vacuum chamber (106 Pa) using a 1 mm2 cross-sectional area Al anode. A dc voltage sweeping from 200 to 1100 V was applied to a sample at a step of 50 V. The distance between the electrodes was 200 mm.
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3. Results and discussion 3.1. Line-shaped patterning of ZnO NWs Colloidal templates have a potential application in controlling the morphology of nanostructures. We have found the ZnO NW/NR arrays can be effectively tuned by PS template size and short heating time. The deformation gaps among PS templates are different along the template size and deformation heating time. Thus, the ZnO NW/NR arrays assemble structures can be obtained by the vacancy among the PS templates. As reported before, the alignment of the PSs is relatively loose and the spatial gaps surrounding PS spheres are large and continuous at the pristine state. When PS templates heated for a specific time (short time ~ 1.5 min), the spheres gradually approach each other and the gaps become smaller. Yet the vacancies among the PS sphere gradually evolve to linear patterns along the continuously of heating time with small size templates. Fig. 1 shows the typical pattern vacancies of 200 nm PS sphere template deforming with heating time of 0, 3, 5 min, respectively. The gaps among PS sphere are continuous and relatively large without heating anneal shown in Fig. 1a. The gaps among PS sphere decrease and emerge some vacancies with the increase of heating time due to the diffusion of colloidal template. The transition of interspace from separated cavities to line-like, linear pattern grooves was induced by the heating-driven deformation of PSs given in Fig. 1bec yellow label. Such line-shaped grooves can almost exit entire monolayer PS template in large scale area (Fig. S1). Thus, the vacancy patterns can used as template for the growth of patterned ZnO NW/NR arrays on these sites by controlling the heating time. ZnO NW pattern assembles were prepared by above mentioned vacancy patterns obtained by 200 nm PS sphere template deformation of heating time shown in Fig. 2. The uniform large area ZnO NW arrays were grown on Si substrates by PS template without heating treatment (Fig. 2a). While the PS sphere was heated for 3, 5 min as template, uniformly vertical ZnO NWs transit to coalescent vacancy bundles and line-shaped NWs growth on the Si substrate due to PS diffusion (Fig. 2b,c). Meanwhile, the entire density of ZnO NWs on the substrates has a decrease tendency. Such small size PS sphere of 200 nm acts as template with a short heating time (<5 min), the line-shaped ZnO NWs cannot be obtained due to insufficient formation of ZnO NW line-like growth sites. When the heating time is longer (~5 min), the line patterns can be easily obtained through the heating-driven deformation of PS template. Thus, line-shaped ZnO NWs assemble on the vacancy coming from the PS sphere deformation is a unique phenomenon for patterning ZnO NWs. The length of line-shaped ZnO NWs can be tuned by the heating-driven deformation of PSs. The ZnO NW lines can be prolonged as time goes by increasing the heating time. The length of ZnO NW lines is >10 mm (Fig. 3a) with heating time of 10 min relative to short line of 5 mm (Fig. 2c). Meanwhile, the PS template deformation can be observed with increasing the heating time shown in Fig. 3b. The longer line space easily exists on the Si substrate with longer heating time due to fully diffusion of PS sphere. The line-shaped ZnO NWs obtained by controlling the deformation of 200 nm with heating treatment are not common for other size of PS sphere template. If the larger size PS sphere acts as template with heating time of 5 min, the migration of entire large PS sphere may be difficult and the slightly diffusion make the PS lay over the substrates resulting in seldom space for NW lines formation. When the larger PS sphere acts as template with heating, only ZnO NW bundles were obtained and the density of growth ZnO NW arrays could also be effectively confined and adjusted by the space between the PSs.
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Fig. 1. FESEM images of 200 nm PS sphere monolayer templates after heating at 115 C for 0 (a), 3 (b), and 5 (c) min. Labels in yellow clearly present the transition of interspace from separated cavities (b) to line-shaped grooves (c) by the heating-driven transfer of PSs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. FESEM images of ZnO NWs electrochemically deposited on 200 nm PS sphere monolayer templates with heating time of 0 (a), 3 (b), and 5 (c) min. The occupation of vatical NWs transits from uniformly separated small interstitials (a), to coalescent vacancy clusters (b), and to line-shaped grooves (c).
Patterned NW/NR line and bundle nanostructures can be easily prepared by the electrochemical deposition via the template of 200 nm colloidal PS deformation with different heating time. The density of NW density decreases along the increase of heating time. The regarded phenomena demonstrate the possible control on the patterns of arrays using deformation-tailored templates.
using 200 nm PS sphere as template. Here, the bundle-shaped patterning of ZnO NWs were obtained by using a larger template size and the density of NW bundles can be optimized for efficient patterned field emitters. Fig. 4 shows the typical 500 nm PS template annealed at 115 C for 3, 6 min, respectively. The NW array densities decrease corresponding to the increase PS sphere deformation heating time, which is attributed to the gaps among PS sphere decrease arising from the PS deformation along with the increase of heating time. However, more NWs deviate from the normal direction of the substrate along with the increase of heating time (6 min) due to large growth site and small surrounding swaddling clothes. The whole density determines the density of the
3.2. Bundle-shaped patterning of ZnO NWs As above results, the line-shape and the density of ZnO NW arrays can be effectively tuned by the deformation of heating time
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Fig. 3. FESEM images of ZnO NW-lines (a) fabricated on deformed 200 nm PS template (b) with heating time of 10 min. The high similarity of them demonstrates the effectiveness utilizing template deformation to control the alignment of vertical NWs.
emitting sites and also affects the screening effects, and hence is an important factor for the resultant emitting efficiency. The above mentioned adjustments of orientation and density endow us a chance to accurately regulate the arrays, to minimize the screening effects and to preserve enough emitting sites. The typical XRD patterns of bundle-shaped ZnO NWs with different deformation time were used to study the NW orientation shown in Fig. 5. The XRD confirms that the products are wellcrystallized ZnO structure with wurtzite phase. The XRD patterns are dominated by a sharp XRD reflection peak at ~34.4 , which corresponds to the lattice plane (002). There are also other diffraction peaks with much lower intensity mainly at 31.7, 36.2 , and 47.5 corresponding to the following lattice planes of (100), (101), and (102), respectively. The sharp (002) peak shows that the wires are grown along their c-axis and the deposit is highly textured with the c-axis perpendicular to the substrate. This is in good agreement with the SEM views, which show that the wires have a hexagonal section and along the normal direction of the substrates. To analyze the high texturing of the samples and the evolution of this parameter with heating time, we have estimated the texture coefficient (TC) from Fig. 5b. The TC parameter for the (002) orientation was calculated from the following relation [27,28]:
TC002 ¼
. 0 I002 I002
. . 0 0 I002 I002 þ I101 I101
(1)
where TC002 is the relative texture coefficient of diffraction peaks (002) over (101), I002 and I101 are the measured diffraction
Fig. 4. Low resolution (a) and high resolution (b) FESEM images of ZnO NW bundles fabricated on deformed 500 nm PS templates with heating time of 3 min, and (c) high resolution FESEM images with heating time of 6 min, showing well control on bundle density.
0 0 intensities due to (002) and (101) planes, respectively, I002 and I101 are the corresponding values of standard PDF NO. 036-1451 measured from randomly oriented powder samples. The calculated TC002 of bundle-shaped ZnO NWs are shown in Fig. 5b. The very high values of the (002) TC for all the samples confirm that the wires are textured and well-aligned with the c-axis normal to the substrate surface. An interesting result is that TC002 decreases slightly with the increasing heating deformation time. It indirectly confirms the SEM observation that the growth of the vertically oriented wires deviate from the normal direction of the substrate. Such density and orientation controlled NWs have a potential application in optimizing patterned FE performance.
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Fig. 5. (a) Typical XRD of ZnO NW bundles grown on Si substrates for 1 h with different heating time. (b) TC002 of ZnO NWs with different heating time.
3.3. Performances of patterned ZnO field-electron emitters In principle, one-dimensional ZnO nanostructures are potentially good field-emitters because of their favorable aspect ratios, effective electron transport channel and suitable work functions. In reality, the FE properties are affected by many parameters, e.g. the curvature radius of a single emitter, uniformity and density of the emitters [8e18], and so forth. It is remarkable that these factors could be simultaneously controlled by the presently developed method for the ultimate improvement. Meanwhile, the bundleshaped and line-shaped ZnO NWs prepared by PS sphere as template have a potential application in pattern field-electron emitters. The density dependent FE of bundle-shaped ZnO NWs with 500 nm PS as template is illustrated in Fig. 6. All the NW bundles under tests were electrochemically grown for 1 h. The bundleshaped ZnO NWs have sharp top when HMTA of 0.05 M as electrolyte was added [29,30]. The ZnO NWs have a length of ~3 mm for the growth of 1 h. When the heating time is changed from 0 to 9 min, the turn-on emitting fields, i.e., the field required to produce an emission current density of 0.1 mA cm2, exhibits strong dependence on the PS sphere heating time. As shown in Fig. 6a, the turn-on fields first decrease slightly from 4.8 to 4.4 V mm1 with the PS diameter deformation time increasing from 0 to 3 min, and then sharply increase as further deformation with the increase of heating time. The high density NW bundles prepared without the heating time have a slightly increase in turn on field due to the screening effect among the NW emitters. When the PS spheres
Fig. 6. (a) J-E curves of ZnO NW bundles with different density controlled by different heating time of 0, 3, 6, 9 min. (b) Corresponding FowlereNordheim plot and a linear fit. (c) FE current density of ZnO NWs bundles prepared with deformation heating time of 3 min recorded over 1000 min under applied electric field of 5 V mm1.
deformation heating time are relatively long (e.g. more than 5 min), the whole density of NWs is relatively low, which leads to an insufficient number of the emitting sites and depresses the emitting efficiency. Therefore, the whole density has a drastic decrease with the increase of heating time. Such colloidal template can effectively control the overall emitting efficiency of bundle-shaped ZnO NWs. The FE efficiency comes to a maximum with a PS template deformation time of ~3 min. In addition, the oblique of NW was also found to obviously affect the FE performance. According to the above analysis of NW orientation, when the PS template deformation heating time is longer, the NW bundles are obviously away from the normal position to the Si substrates, and such lean of NWs can deteriorate the FE performance. Meanwhile, the excessive
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heating induces the absence of NWs in some places and decreases the NW density (Fig. 4). The former increases the screening effect in the local areas and the later reduces the whole density of arrays, both reducing the emitting efficiency. Therefore, the turn on field has a sharp increase after 6 min deformation heating time. The dependence of turn-on electric field on heating time is summarized in Fig. 6a. It can be seen that a 500 nm PS template and 3 min heating time are optimal conditions for the pattern FE of bundleshaped ZnO NWs. An emission current density of 1 mA cm2 (the minimum required to produce the luminescence of 300 cd m2 for a video graphics array FE display with a typical high-voltage phosphor screen efficiency of 9 lm W1) has already obtained at 5.0 V mm1, which is relatively low compared to other reported values [31,32]. The acquired FE currentevoltage characteristics were further analyzed by means of the FowlereNordheim (FeN) equation:
ln J E2 ¼ ln Ab2 F BF3=2 bE
(2)
where A and B are constants with the values of 1.54 106 A eV V2 and 6.83 103 V mm1 eV3/2, J is the current density, b is the field enhancement factor, E is the applied field, and Ф is the work function of the emitting material, which is 5.3 eV for ZnO [33]. Fig. 6b presents the FeN plots, ln(J/E2) versus 1/E, for the corresponding samples as shown in Fig. 6a. The FeN plots in Fig. 6b are seen to have approximately linear relationships within the measurement range, which confirms that the electron emission from all the ZnO NW bundles follows the FeN behavior. The bundle-shaped ZnO NW grown by deformation time of 3 min demonstrates a very attractive b value of 1450, having a huge potential in patterned FE applications. The patterned FE performance of ZnO NW lines with 200 nm PS sphere as template was studied shown in the Fig. S2. The turn-on fields decrease with heating time increasing from 0 to 3, 5, 10 min, respectively. Such results are due to the decrease of electron emission originating from the reduce of NW density (Fig. 2). The turn-on fields of ZnO NWs prepared with 200 nm PS sphere template without heating is almost the same as the large template size, but has a gradually decrease along with the increase of heating time without an optimal value due to the decrease of NW density. Such controllable and patterned field emitters have a huge potential in display applications. Meanwhile, such patterned NW has attracted intensive attention in recent years due to their potential applications in photoelectronic devices. For example, the brightnessenhancing property of NW arrays could be used to improve LED light efficiency via passing incident light through the nanolens array [34,35]. The method proposed here utilizes deformationtailored colloidal PS monolayers and electrochemical deposition at a temperature as low as of 70 C without catalysts or pre-formed seed layers [36e38]. Such simple, facile method is very costeffective and able to produce large scale patterned NW arrays for promising future practical applications, such as flexible optoelectronic devices, nanolens and photonic crystal [39e42]. Stability of the patterned field emitters is another important issue closely related to their potential applications. Fig. 6c shows the variation of emission current density of bundle-shaped ZnO NW grown by deformation time of 3 min measured over a period of ~1000 min at an applied electric field of 5 V mm1. The emission current density was observed to fluctuate only within 7.8%, proving a high stability of the emitters. The stable FE performance observed for such NW bundle structures is beneficial to potential applications in cold-cathode-based electronics. The approach used here demonstrates the effectiveness of the PS template adjustment for the patterned and density-dependent FE performance. This allows one to synchronously control the growth of ZnO array patterns and
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patterned field emitters. 4. Conclusion In summary, we demonstrate a facile and effective route toward a multiple control of the vertical ZnO NW patterns with uniformity over the centimeter-scale area. The deformation of a colloidal PS sphere template was controllably introduced during the ZnO NW pattern electrodeposition growth. Novel line-shaped and bundleshaped ZnO NWs can be easily obtained by controlling the deformation of template size and heating time for the first time. Under the moderate heating time, the spatial voids in the templates (between PS spheres) gradually become separated, guaranteeing uniformly structured NW lines, and the length of NW lines can be tuned by the heating time. The density of bundle-shaped ZnO NWs can be effectively controlled by deformation heating time. As a consequence of all these adjustments, the optimal FE performance of the NW bundles was obtained. A low turn-on electric field of 4.4 V mm1 and the field-enhancement factor of up to 1450 were ultimately obtained. Such results are very attractive for the present NW structure patterns and patterned field electron emitters in practical applications of the cold-cathode-based electronics. Acknowledgments This work was supported by the National 973 project from National Basic Research Program of China (2014CB931700), the National Natural Science Foundation of China (61222403), the Doctoral Program Foundation from the Ministry of Education of China (20123218110030), the Fundamental Research Funds for the Central Universities (30920130111017 and NE2012004), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2012KF06) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.cap.2015.04.011. References [1] Z.L. Wang, Splendid one-dimensional nanostructures of zinc oxide: a new nanomaterial family for nanotechnology, ACS Nano 2 (2008) 1987e1992. [2] H. Zeng, W. Cai, P. Liu, X. Xu, H. Zhou, C. Klingshirn, H. Kalt, ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metalsemiconductor interface, improvement of blue emission and photocatalysis, ACS Nano 2 (2008) 1661e1670. [3] J. Song, S.A. Kulinich, J. Yan, Z. Li, J. He, C. Kan, H. Zeng, Epitaxial ZnO nanowire-on-nanoplate structures as efficient and transferable field emitters, Adv. Mater. 25 (2013) 5750e5755. [4] J. Song, S.A. Kulinich, J. Li, Y. Liu, H. Zeng, A general one-pot strategy for the synthesis of high-performance transparent-conducting-oxide nanocrystal inks for all-solution-processed devices, Angew. Chem. Int. Ed. 54 (2015) 462e466. [5] J. Song, S.A. Kulinich, J. Li, Y. Liu, H. Zeng, A general one-pot strategy for the synthesis of high-performance transparent-conducting-oxide nanocrystal inks for all-solution-processed devices, Angew. Chem. 127 (2015) 472e476. [6] D. Gedamu, I. Paulowicz, S. Kaps, O. Lupan, S. Wille, G. Haidarschin, Y.K. Mishra, R. Adelung, Rapid fabrication technique for interpenetrated ZnO nanotetrapod networks for fast UV sensors, Adv. Mater. 26 (2014) 1541e1550. € der, S. Kaps, O. Lupan, S. Chemnitz, W. Benecke, [7] T. Reimer, I. Paulowicz, R. Ro C. Ronning, R. Adelung, Y.K. Mishra, Single step integration of ZnO nano- and microneedles in Si trenches by novel flame transport approach: whispering gallery modes and photocatalytic properties, ACS Appl. Mater. Inter. 6 (2014) 7806e7815. [8] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nanowire dyesensitized solar cells, Nat. Mater. 4 (2005) 455e459. [9] Y. Yang, W.X. Guo, Y. Zhang, Y. Ding, X. Wang, Z.L. Wang, Piezotronic effect on the output voltage of P3HT/ZnO Micro/Nanowire heterojunction solar cells,
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Current Applied Physics journal homepage: www.elsevier.com/locate/cap
ZnO nanowire lines and bundles: Template-deformation-guided alignment for patterned field-electron emitters Jizhong Song a, b, Xue Ning a, Haibo Zeng a, b, * a
State Key Laboratory of Mechanics and Control of Mechanical Structures & College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China b Institute of Optoelectronics & Nanomaterials, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 March 2015 Received in revised form 15 April 2015 Accepted 16 April 2015 Available online 17 April 2015
One-dimensional ZnO materials have been promising for field-emission (FE) application, but how to facially control the alignment of ZnO emitters is still a great challenge especially for patterned display application. Here, we report the fabrication of novel ZnO nanowire (NW) line and bundle arrays for patterned field-electron emitters. The effects of PS template size and heating time on the resulted ZnO nanoarrays were systematically studied. The deformation degree of PS templates was controlled and hence utilized to adjust the alignment of electrochemically deposited ZnO arrays. It was found that the length of NW lines and the density of NW bundles can effectively tuned by the PS template heating time. The optimal FE performance with turn-on electric field as low as of 4.4 V mm1 and the fieldenhancement factor as high as of 1450 were achieved through decreasing the screening effect among the patterned field-electron emitters. © 2015 Elsevier B.V. All rights reserved.
Keywords: ZnO Nanowire lines and bundles Patterns Field-emission Template deformation
1. Introduction Among various ZnO nanostructures [1e7], vertically aligned nanowires/nanorods (NWs/NRs) have a variety of promising applications ranging from solar cells [8,9], UV lasers [10,11], lightemitting diodes [12,13], and piezo-nanogenerators [14,15], to field-emission (FE) devices [16,17]. Vertically aligned NWs/NRs with high aspect ratios have been considered as excellent candidates for electron field emitters as they can act as electron transporting channel. The FE performance of such materials is highly affected by their intrinsic physical and structural parameters, such as density [18], uniformity [19] and tapering [20]. After being stimulated by an applied electric field and before reaching the counter electrode, electrons have to pass through the entire ZnO nanostructure. Thus, the FE performance is closely related to the local emitting intensity and the interaction among emitters, which can be extremely affected by the density of aligned NWs/NRs. Both the high density aligned NWs/NRs due to the screening effect of the emitters and low density aligned NWs/NRs due to the small electron emitting density can deteriorate the FE performance. Recently,
* Corresponding author. E-mail address: [email protected] (H. Zeng). http://dx.doi.org/10.1016/j.cap.2015.04.011 1567-1739/© 2015 Elsevier B.V. All rights reserved.
many efforts have been made to adjust the density of aligned NWs/ NRs for the improvement of emitting efficiency [18,20e23]. For example, Benjamin et al. [24] proposed a simple, scalable, and costeffective technique for controlling the growth density of ZnO arrays based on a layer-by-layer polyelectrolyte polymer film. The FE results revealed that an emitter density of 7 nanorods mm2 and a tapered nanorod morphology generated a high field enhancement factor (b) of 5884. Lee et al. prepared different density arrays of ZnO nanorods by electrodeposition assisted by process nanosphere lithography technique [25]. Compared with the control of the whole density of arrays, control of local NW/NR density, uniformity and patterns were rather rarely addressed. This could significantly affect the local screening effect of the emitters. Remarkably, the patterned NWs/NRs field emitter demands the integrated control of the density of 1D vertical NWs. However, such integrated control on vertical NW/NR arrays remains to be a challenge till now. Recently, our group [26] has observed that vertical ZnO NW/NR arrays in centimeter-scale areas can be fully controlled by a facile and effective route of the deformation of colloidal crystal monolayer templates. The NR/NW density and uniformity were all adjusted through selection of template size and deformation. In line with the adjustments, the FE performance of the arrays can be tuned. On the other hand, patterned field-electron emitters are highly demanded
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in the practical display applications, and usually achieved by photography technique, which is expensive and hence greatly affect the current development. Therefore, how to facially obtain patterned ZnO field emitters is still a great challenge up to now. Here, the novel vertical ZnO NW lines and bundles were obtained and tuned by the deformation of polystyrene (PS) template size and heating time. The length and wire density of patterned ZnO arrays can be effectively tuned by the heating time and template size. When the 200 nm PS sphere acts as template, uniformly vertical ZnO NWs transit from coalescent vacancy bundles to lines along the increase of heating time due to PS diffusion, and the length of line-shaped ZnO NWs can increase along the increase of heating time. Meantime, the increase of heating time makes the density of bundle-shaped ZnO NW/NR array decrease by using 500 nm PS template. Such density control of ZnO bundles can optimize the performance of field emitters. The FE performance results imply that the template deformation heating time of 3 min can effectively avoid the screening effect and obtain optimal FE efficiency. Such findings and results are beneficial for potential patterned field electron emitter applications in the cold-cathodebased electronics. 2. Experiments 2.1. Self-assembly of colloidal monolayer PS template The uniformly and large-scale monolayer PS template was assembled by our previous method [26]. Briefly, a 2.5 wt% suspension of monodispersed PS spheres with different diameter was purchased from Alfa Aesar. A droplet of PS sphere suspension was placed on a cleaned heavily doped Si substrate ((100)-oriented highly doped silicon) with an area of 15 15 mm2 fixed on a spin coater at a rotating speed of 600 rpm for 1 min. A colloidal monolayer with an area of about 2 cm2 was formed on the substrate by a self-assembling process. In the same manner, the PS colloidal monolayers were assembled on the Si substrate for ZnO NW growth. 2.2. Electrochemical deposition growth of ZnO The colloidal monolayers on substrates were heated at 115 C for different times (1e10 min) to adjust the deformation. Subsequently, the galvanostatic cathodic deposition was employed on the deformed colloidal layer-Si substrates (as the cathodes) at a current density of 0.5 mA cm2. Zinc sheets (99.99% purity) acted as the anodes, and zinc nitrate aqueous solution (0.05 M) as the electrolyte. The deposition temperature was fixed at 70 C by a water bath and the deposition time was changed from 10 min to 1 h for getting different wire lengths. The hexamethylenetetramine (HMTA, C6H12N4) was added into the electrolyte with a concentration of 0.1 M for the growth of tapered nanorods. 2.3. Characterizations and FE measurement The samples were characterized by FE scanning electron microscopy (SEM, JEOL JSM-6700F). The crystal graphic interpretations were performed on an X'Pert Pro XRD system with Xray Mirror PFX at an operation voltage of 40 kV and a current of 40 mA, in which Cu Ka (l ¼ 0.1540 nm) was used and scanned in a 2q range from 20 to 80 . The FE properties of different samples on Si substrates were studied at room temperature in a high vacuum chamber (106 Pa) using a 1 mm2 cross-sectional area Al anode. A dc voltage sweeping from 200 to 1100 V was applied to a sample at a step of 50 V. The distance between the electrodes was 200 mm.
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3. Results and discussion 3.1. Line-shaped patterning of ZnO NWs Colloidal templates have a potential application in controlling the morphology of nanostructures. We have found the ZnO NW/NR arrays can be effectively tuned by PS template size and short heating time. The deformation gaps among PS templates are different along the template size and deformation heating time. Thus, the ZnO NW/NR arrays assemble structures can be obtained by the vacancy among the PS templates. As reported before, the alignment of the PSs is relatively loose and the spatial gaps surrounding PS spheres are large and continuous at the pristine state. When PS templates heated for a specific time (short time ~ 1.5 min), the spheres gradually approach each other and the gaps become smaller. Yet the vacancies among the PS sphere gradually evolve to linear patterns along the continuously of heating time with small size templates. Fig. 1 shows the typical pattern vacancies of 200 nm PS sphere template deforming with heating time of 0, 3, 5 min, respectively. The gaps among PS sphere are continuous and relatively large without heating anneal shown in Fig. 1a. The gaps among PS sphere decrease and emerge some vacancies with the increase of heating time due to the diffusion of colloidal template. The transition of interspace from separated cavities to line-like, linear pattern grooves was induced by the heating-driven deformation of PSs given in Fig. 1bec yellow label. Such line-shaped grooves can almost exit entire monolayer PS template in large scale area (Fig. S1). Thus, the vacancy patterns can used as template for the growth of patterned ZnO NW/NR arrays on these sites by controlling the heating time. ZnO NW pattern assembles were prepared by above mentioned vacancy patterns obtained by 200 nm PS sphere template deformation of heating time shown in Fig. 2. The uniform large area ZnO NW arrays were grown on Si substrates by PS template without heating treatment (Fig. 2a). While the PS sphere was heated for 3, 5 min as template, uniformly vertical ZnO NWs transit to coalescent vacancy bundles and line-shaped NWs growth on the Si substrate due to PS diffusion (Fig. 2b,c). Meanwhile, the entire density of ZnO NWs on the substrates has a decrease tendency. Such small size PS sphere of 200 nm acts as template with a short heating time (<5 min), the line-shaped ZnO NWs cannot be obtained due to insufficient formation of ZnO NW line-like growth sites. When the heating time is longer (~5 min), the line patterns can be easily obtained through the heating-driven deformation of PS template. Thus, line-shaped ZnO NWs assemble on the vacancy coming from the PS sphere deformation is a unique phenomenon for patterning ZnO NWs. The length of line-shaped ZnO NWs can be tuned by the heating-driven deformation of PSs. The ZnO NW lines can be prolonged as time goes by increasing the heating time. The length of ZnO NW lines is >10 mm (Fig. 3a) with heating time of 10 min relative to short line of 5 mm (Fig. 2c). Meanwhile, the PS template deformation can be observed with increasing the heating time shown in Fig. 3b. The longer line space easily exists on the Si substrate with longer heating time due to fully diffusion of PS sphere. The line-shaped ZnO NWs obtained by controlling the deformation of 200 nm with heating treatment are not common for other size of PS sphere template. If the larger size PS sphere acts as template with heating time of 5 min, the migration of entire large PS sphere may be difficult and the slightly diffusion make the PS lay over the substrates resulting in seldom space for NW lines formation. When the larger PS sphere acts as template with heating, only ZnO NW bundles were obtained and the density of growth ZnO NW arrays could also be effectively confined and adjusted by the space between the PSs.
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Fig. 1. FESEM images of 200 nm PS sphere monolayer templates after heating at 115 C for 0 (a), 3 (b), and 5 (c) min. Labels in yellow clearly present the transition of interspace from separated cavities (b) to line-shaped grooves (c) by the heating-driven transfer of PSs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. FESEM images of ZnO NWs electrochemically deposited on 200 nm PS sphere monolayer templates with heating time of 0 (a), 3 (b), and 5 (c) min. The occupation of vatical NWs transits from uniformly separated small interstitials (a), to coalescent vacancy clusters (b), and to line-shaped grooves (c).
Patterned NW/NR line and bundle nanostructures can be easily prepared by the electrochemical deposition via the template of 200 nm colloidal PS deformation with different heating time. The density of NW density decreases along the increase of heating time. The regarded phenomena demonstrate the possible control on the patterns of arrays using deformation-tailored templates.
using 200 nm PS sphere as template. Here, the bundle-shaped patterning of ZnO NWs were obtained by using a larger template size and the density of NW bundles can be optimized for efficient patterned field emitters. Fig. 4 shows the typical 500 nm PS template annealed at 115 C for 3, 6 min, respectively. The NW array densities decrease corresponding to the increase PS sphere deformation heating time, which is attributed to the gaps among PS sphere decrease arising from the PS deformation along with the increase of heating time. However, more NWs deviate from the normal direction of the substrate along with the increase of heating time (6 min) due to large growth site and small surrounding swaddling clothes. The whole density determines the density of the
3.2. Bundle-shaped patterning of ZnO NWs As above results, the line-shape and the density of ZnO NW arrays can be effectively tuned by the deformation of heating time
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Fig. 3. FESEM images of ZnO NW-lines (a) fabricated on deformed 200 nm PS template (b) with heating time of 10 min. The high similarity of them demonstrates the effectiveness utilizing template deformation to control the alignment of vertical NWs.
emitting sites and also affects the screening effects, and hence is an important factor for the resultant emitting efficiency. The above mentioned adjustments of orientation and density endow us a chance to accurately regulate the arrays, to minimize the screening effects and to preserve enough emitting sites. The typical XRD patterns of bundle-shaped ZnO NWs with different deformation time were used to study the NW orientation shown in Fig. 5. The XRD confirms that the products are wellcrystallized ZnO structure with wurtzite phase. The XRD patterns are dominated by a sharp XRD reflection peak at ~34.4 , which corresponds to the lattice plane (002). There are also other diffraction peaks with much lower intensity mainly at 31.7, 36.2 , and 47.5 corresponding to the following lattice planes of (100), (101), and (102), respectively. The sharp (002) peak shows that the wires are grown along their c-axis and the deposit is highly textured with the c-axis perpendicular to the substrate. This is in good agreement with the SEM views, which show that the wires have a hexagonal section and along the normal direction of the substrates. To analyze the high texturing of the samples and the evolution of this parameter with heating time, we have estimated the texture coefficient (TC) from Fig. 5b. The TC parameter for the (002) orientation was calculated from the following relation [27,28]:
TC002 ¼
. 0 I002 I002
. . 0 0 I002 I002 þ I101 I101
(1)
where TC002 is the relative texture coefficient of diffraction peaks (002) over (101), I002 and I101 are the measured diffraction
Fig. 4. Low resolution (a) and high resolution (b) FESEM images of ZnO NW bundles fabricated on deformed 500 nm PS templates with heating time of 3 min, and (c) high resolution FESEM images with heating time of 6 min, showing well control on bundle density.
0 0 intensities due to (002) and (101) planes, respectively, I002 and I101 are the corresponding values of standard PDF NO. 036-1451 measured from randomly oriented powder samples. The calculated TC002 of bundle-shaped ZnO NWs are shown in Fig. 5b. The very high values of the (002) TC for all the samples confirm that the wires are textured and well-aligned with the c-axis normal to the substrate surface. An interesting result is that TC002 decreases slightly with the increasing heating deformation time. It indirectly confirms the SEM observation that the growth of the vertically oriented wires deviate from the normal direction of the substrate. Such density and orientation controlled NWs have a potential application in optimizing patterned FE performance.
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Fig. 5. (a) Typical XRD of ZnO NW bundles grown on Si substrates for 1 h with different heating time. (b) TC002 of ZnO NWs with different heating time.
3.3. Performances of patterned ZnO field-electron emitters In principle, one-dimensional ZnO nanostructures are potentially good field-emitters because of their favorable aspect ratios, effective electron transport channel and suitable work functions. In reality, the FE properties are affected by many parameters, e.g. the curvature radius of a single emitter, uniformity and density of the emitters [8e18], and so forth. It is remarkable that these factors could be simultaneously controlled by the presently developed method for the ultimate improvement. Meanwhile, the bundleshaped and line-shaped ZnO NWs prepared by PS sphere as template have a potential application in pattern field-electron emitters. The density dependent FE of bundle-shaped ZnO NWs with 500 nm PS as template is illustrated in Fig. 6. All the NW bundles under tests were electrochemically grown for 1 h. The bundleshaped ZnO NWs have sharp top when HMTA of 0.05 M as electrolyte was added [29,30]. The ZnO NWs have a length of ~3 mm for the growth of 1 h. When the heating time is changed from 0 to 9 min, the turn-on emitting fields, i.e., the field required to produce an emission current density of 0.1 mA cm2, exhibits strong dependence on the PS sphere heating time. As shown in Fig. 6a, the turn-on fields first decrease slightly from 4.8 to 4.4 V mm1 with the PS diameter deformation time increasing from 0 to 3 min, and then sharply increase as further deformation with the increase of heating time. The high density NW bundles prepared without the heating time have a slightly increase in turn on field due to the screening effect among the NW emitters. When the PS spheres
Fig. 6. (a) J-E curves of ZnO NW bundles with different density controlled by different heating time of 0, 3, 6, 9 min. (b) Corresponding FowlereNordheim plot and a linear fit. (c) FE current density of ZnO NWs bundles prepared with deformation heating time of 3 min recorded over 1000 min under applied electric field of 5 V mm1.
deformation heating time are relatively long (e.g. more than 5 min), the whole density of NWs is relatively low, which leads to an insufficient number of the emitting sites and depresses the emitting efficiency. Therefore, the whole density has a drastic decrease with the increase of heating time. Such colloidal template can effectively control the overall emitting efficiency of bundle-shaped ZnO NWs. The FE efficiency comes to a maximum with a PS template deformation time of ~3 min. In addition, the oblique of NW was also found to obviously affect the FE performance. According to the above analysis of NW orientation, when the PS template deformation heating time is longer, the NW bundles are obviously away from the normal position to the Si substrates, and such lean of NWs can deteriorate the FE performance. Meanwhile, the excessive
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heating induces the absence of NWs in some places and decreases the NW density (Fig. 4). The former increases the screening effect in the local areas and the later reduces the whole density of arrays, both reducing the emitting efficiency. Therefore, the turn on field has a sharp increase after 6 min deformation heating time. The dependence of turn-on electric field on heating time is summarized in Fig. 6a. It can be seen that a 500 nm PS template and 3 min heating time are optimal conditions for the pattern FE of bundleshaped ZnO NWs. An emission current density of 1 mA cm2 (the minimum required to produce the luminescence of 300 cd m2 for a video graphics array FE display with a typical high-voltage phosphor screen efficiency of 9 lm W1) has already obtained at 5.0 V mm1, which is relatively low compared to other reported values [31,32]. The acquired FE currentevoltage characteristics were further analyzed by means of the FowlereNordheim (FeN) equation:
ln J E2 ¼ ln Ab2 F BF3=2 bE
(2)
where A and B are constants with the values of 1.54 106 A eV V2 and 6.83 103 V mm1 eV3/2, J is the current density, b is the field enhancement factor, E is the applied field, and Ф is the work function of the emitting material, which is 5.3 eV for ZnO [33]. Fig. 6b presents the FeN plots, ln(J/E2) versus 1/E, for the corresponding samples as shown in Fig. 6a. The FeN plots in Fig. 6b are seen to have approximately linear relationships within the measurement range, which confirms that the electron emission from all the ZnO NW bundles follows the FeN behavior. The bundle-shaped ZnO NW grown by deformation time of 3 min demonstrates a very attractive b value of 1450, having a huge potential in patterned FE applications. The patterned FE performance of ZnO NW lines with 200 nm PS sphere as template was studied shown in the Fig. S2. The turn-on fields decrease with heating time increasing from 0 to 3, 5, 10 min, respectively. Such results are due to the decrease of electron emission originating from the reduce of NW density (Fig. 2). The turn-on fields of ZnO NWs prepared with 200 nm PS sphere template without heating is almost the same as the large template size, but has a gradually decrease along with the increase of heating time without an optimal value due to the decrease of NW density. Such controllable and patterned field emitters have a huge potential in display applications. Meanwhile, such patterned NW has attracted intensive attention in recent years due to their potential applications in photoelectronic devices. For example, the brightnessenhancing property of NW arrays could be used to improve LED light efficiency via passing incident light through the nanolens array [34,35]. The method proposed here utilizes deformationtailored colloidal PS monolayers and electrochemical deposition at a temperature as low as of 70 C without catalysts or pre-formed seed layers [36e38]. Such simple, facile method is very costeffective and able to produce large scale patterned NW arrays for promising future practical applications, such as flexible optoelectronic devices, nanolens and photonic crystal [39e42]. Stability of the patterned field emitters is another important issue closely related to their potential applications. Fig. 6c shows the variation of emission current density of bundle-shaped ZnO NW grown by deformation time of 3 min measured over a period of ~1000 min at an applied electric field of 5 V mm1. The emission current density was observed to fluctuate only within 7.8%, proving a high stability of the emitters. The stable FE performance observed for such NW bundle structures is beneficial to potential applications in cold-cathode-based electronics. The approach used here demonstrates the effectiveness of the PS template adjustment for the patterned and density-dependent FE performance. This allows one to synchronously control the growth of ZnO array patterns and
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patterned field emitters. 4. Conclusion In summary, we demonstrate a facile and effective route toward a multiple control of the vertical ZnO NW patterns with uniformity over the centimeter-scale area. The deformation of a colloidal PS sphere template was controllably introduced during the ZnO NW pattern electrodeposition growth. Novel line-shaped and bundleshaped ZnO NWs can be easily obtained by controlling the deformation of template size and heating time for the first time. Under the moderate heating time, the spatial voids in the templates (between PS spheres) gradually become separated, guaranteeing uniformly structured NW lines, and the length of NW lines can be tuned by the heating time. The density of bundle-shaped ZnO NWs can be effectively controlled by deformation heating time. As a consequence of all these adjustments, the optimal FE performance of the NW bundles was obtained. A low turn-on electric field of 4.4 V mm1 and the field-enhancement factor of up to 1450 were ultimately obtained. Such results are very attractive for the present NW structure patterns and patterned field electron emitters in practical applications of the cold-cathode-based electronics. Acknowledgments This work was supported by the National 973 project from National Basic Research Program of China (2014CB931700), the National Natural Science Foundation of China (61222403), the Doctoral Program Foundation from the Ministry of Education of China (20123218110030), the Fundamental Research Funds for the Central Universities (30920130111017 and NE2012004), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2012KF06) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.cap.2015.04.011. References [1] Z.L. Wang, Splendid one-dimensional nanostructures of zinc oxide: a new nanomaterial family for nanotechnology, ACS Nano 2 (2008) 1987e1992. [2] H. Zeng, W. Cai, P. Liu, X. Xu, H. Zhou, C. Klingshirn, H. Kalt, ZnO-based hollow nanoparticles by selective etching: elimination and reconstruction of metalsemiconductor interface, improvement of blue emission and photocatalysis, ACS Nano 2 (2008) 1661e1670. [3] J. Song, S.A. Kulinich, J. Yan, Z. Li, J. He, C. Kan, H. Zeng, Epitaxial ZnO nanowire-on-nanoplate structures as efficient and transferable field emitters, Adv. Mater. 25 (2013) 5750e5755. [4] J. Song, S.A. Kulinich, J. Li, Y. Liu, H. Zeng, A general one-pot strategy for the synthesis of high-performance transparent-conducting-oxide nanocrystal inks for all-solution-processed devices, Angew. Chem. Int. Ed. 54 (2015) 462e466. [5] J. Song, S.A. Kulinich, J. Li, Y. Liu, H. Zeng, A general one-pot strategy for the synthesis of high-performance transparent-conducting-oxide nanocrystal inks for all-solution-processed devices, Angew. Chem. 127 (2015) 472e476. [6] D. Gedamu, I. Paulowicz, S. Kaps, O. Lupan, S. Wille, G. Haidarschin, Y.K. Mishra, R. Adelung, Rapid fabrication technique for interpenetrated ZnO nanotetrapod networks for fast UV sensors, Adv. Mater. 26 (2014) 1541e1550. € der, S. Kaps, O. Lupan, S. Chemnitz, W. Benecke, [7] T. Reimer, I. Paulowicz, R. Ro C. Ronning, R. Adelung, Y.K. Mishra, Single step integration of ZnO nano- and microneedles in Si trenches by novel flame transport approach: whispering gallery modes and photocatalytic properties, ACS Appl. Mater. Inter. 6 (2014) 7806e7815. [8] M. Law, L.E. Greene, J.C. Johnson, R. Saykally, P.D. Yang, Nanowire dyesensitized solar cells, Nat. Mater. 4 (2005) 455e459. [9] Y. Yang, W.X. Guo, Y. Zhang, Y. Ding, X. Wang, Z.L. Wang, Piezotronic effect on the output voltage of P3HT/ZnO Micro/Nanowire heterojunction solar cells,
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