Selective Thermochemical Growth of Hierarchical ZnO Nanowire

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Selective Thermochemical Growth of Hierarchical ZnO Nanowire Branches on Silver Nanowire Backbone Percolation Network Heaters Habeom Lee, Junyeob Yeo, Jinhwan Lee, Hyunmin Cho, Jinhyeong Kwon, Seungyong Han, Sangwan Kim, Sukjoon Hong, and Seung Hwan Ko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08129 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry

Selective Thermochemical Growth of Hierarchical ZnO Nanowire Branches on Silver Nanowire Backbone Percolation Network Heaters By Habeom Lee†, Junyeob Yeo†, Jinhwan Lee, Hyunmin Cho, Jinhyeong Kwon, Seungyong Han, Sangwan Kim, Sukjoon Hong*, and Seung Hwan Ko*

H. Lee, H. Cho, Dr. J. Kwon, Prof. S.H. Ko, Applied Nano and Thermal Science Lab, Department of Mechanical Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea and Department of Mechanical Engineering / Institute of Advanced Machinery and Design (SNU-IAMD), Seoul National University, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea Prof. J. Yeo, Novel Applied Nano Optics Lab, Department of Physics, Kyungpook National University, 80 Daehak-ro, Pook-gu, Daegu 41566, Korea Dr. J. Lee, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, USA Prof. S. Han, Department of Mechanical Engineering, Ajou University, San 5, Woncheon-Dong, Yeongtong-Gu, Suwon 16499, Korea Prof. S. Kim Department of Electrical and Computer Engineering, Ajou University, San 5, Woncheon-Dong, Yeongtong-Gu, Suwon 16499, Korea Prof. S. Hong, Department of Mechanical Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do, 15588, Korea

[*]

To whom correspondence should be addressed. E-mail: [email protected], [email protected]

[†]

Habeom Lee and Junyeob Yeo contributed equally to this work.

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Abstract The development of hydrothermal growth enabled facile growth of ZnO nanowire over large area at mild environments, yet its usage in actual electronic devices has been restricted due to poor spatial selectivity in the growth and a difficulty in the integration to other components. We introduce a local thermochemical growth of ZnO nanowire branches on Ag nanowire backbone percolation networks by electrical current induced local resistive heating in liquid environment for easy fabrication of metal-semiconductor hierarchical nanowire structure. Through vacuum filtration transfer and laser ablation process, patterned conductive network composed of ZnO nanoparticle functionalized Ag nanowire percolation network was prepared as a conductive network for localized resistive heating. Upon the application of proper bias voltage, highly localized temperature field was generated in the vicinity of the patterned Ag nanowire network heater to induce the selective growth of ZnO nanowire from the Ag nanowire backbone. As the temperature rise is related to the electrical current flow, ZnO nanowire was selectively synthesized at the area subject to the maximum current density, which was defined by the laser ablation technique. It was further confirmed that the characteristics of grown ZnO nanowires could be controlled by changing the growth condition.

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Introduction ZnO has been extensively studied up to date due to its interesting material properties1 and the ease in synthesizing various nanostructures.2 Among numerous relevant nanostructures,3 ZnO nanowire (NW) has been successfully applied to diverse applications such as optoelectronics,4-5 sensors,6-7 energy conversion8 and harvesting devices,9 whereas the development in low temperature hydrothermal growth10 expands its compatibility and the range of application even further. For a conventional hydrothermal growth of ZnO NW, target substrate is firstly covered with ZnO seed11 and immersed in precursor solution at elevated temperature to induce further NW growth. As a result, densely packed ZnO NW is grown on the substrate with high uniformity even at wafer scale.10 However, conventional hydrothermal growth of ZnO NW has several drawbacks in terms of device integration. For the implementation of ZnO NW as a useful component in electronic devices, it is often necessary to form metal-semiconductor junction that requires photolithography based techniques in general.12-13 These techniques not only increase the complexity of the overall process, but also lessen the merit of hydrothermal growth by employing harsh processing conditions such as vacuum environment and high temperature. Another limitation of the hydrothermal process comes from poor selectivity of the growth, as ZnO NW is grown everywhere on the seeded substrate. Selective hydrothermal growth of ZnO NW has been attempted with diverse techniques such as photolithography,14 inkjet printing15 and microcontact printing16 by patterning the seed layer prior to the growth, but they either require a pre-made photo-mask or exhibit relatively slow patterning speed. Instead of patterning the seed layer, the growth area can be confined by using local heating scheme.17 Recently, laser induced hydrothermal growth18-19 and single Ag NW joule heater based hydrothermal growth20 have been introduced to create a localized temperature rise at the desired spot to synthesize ZnO NW in a confined area. Although these techniques effectively synthesizes ZnO NW array only at the targeted spot, the laser induced hydrothermal growth method needs a laser and specific absorbing layer. Also, the single Ag NW joule heater based hydrothermal growth method not only has a 3



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limitation in scalability which is essential for practical application and but also requires additional auxiliary electrical pads for the wiring of the heater to the external power supply. In this study, we report a selective thermochemical growth of hierarchical branched ZnO NW on Ag NW backbone (h-BZAB) percolation network in liquid environment for the selective preparation of metal-semiconductor hierarchical nanowire network without any photolithographic techniques. Metal NW percolation network can be employed as metallic backbone as well as the selective resistive heating source for the selective hydrothermal growth of the secondary nanostructure. Uniform conductive percolation network of ZnO nanoparticle (NP) functionalized Ag NW was firstly prepared on the target substrate through vacuum filtration transfer. By applying a constant bias voltage across the Ag NW percolation network in the precursor solution, resistive heating and subsequent localized temperature rising in the vicinity of the network triggered the selective hydrothermal growth of branched ZnO NW from the ZnO NP functionalized Ag NW backbone percolation network to realize h-BZAB metal-semiconductor hetero-nanostructure. Meanwhile, in order to achieve the spatial selectivity in the growth, electrical current density was spatially manipulated by patterning the Ag NW backbone conductive network with selective laser ablation method. As a result, ZnO NW was synthesized only selectively at the area subject to the maximum current density and temperature which were determined by the resultant Ag NW percolation network heater pattern. The proposed method thus enables not only facile integration of two heterogeneous NWs, but also selective growth of secondary NW with low power consumption.

Experimental Backbone Ag NW Synthesis: Backbone Ag NW percolation network is synthesized by following the method reported by Lee et al.21 In a typical synthesis, 50 mL of ethylene glycol (EG) in a flask is preheated in an oil bath at 160 ˚C for 1 h with mild stirring at 200~300 rpm, while the reagent 4


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solutions of CuCl2, PVP, and AgNO3 are separately prepared in EG during the preheating step. 400 µL of 4 mM CuCl2 solution is firstly added to the preheated flask, and 15 mL of 0.147 M PVP reagent solution is injected into the flask after 15 mins. Finally, 15 mL of 0.094 M AgNO3 solution is added to the flask with the injection time of 15−30 min. The reaction is halted after two hours by cooling the reaction chamber to room temperature. For the cleaning, acetone and ethanol are added to the solution with 1:1:8 volume fraction followed by centrifugation at 5,000 rpm for 15 minutes to remove remaining chemicals including EG and PVP. This cleaning step is repeated 5 times, and the resultant Ag NWs are dispersed to pure ethanol. Seed ZnO NP Synthesis: ZnO NP is synthesized from zinc acetate dehydrate (Zn(OAc)2) and sodium hydroxide (NaOH). In detail, 10mM zinc acetate dehydrate (Zn(OAc)2) is dissolved in ethanol at 60 ˚C. 30mM sodium hydroxide (NaOH) in ethanol is prepared separately and slowly added to the zinc acetate dehydrate solution. The resultant seed solution is obtained by gently stirring for another 2 hours at 60 ˚C. The resultant seed solution consists of crystalline ZnO NP whose size is 5-10 nm in diameter. ZnO NP functionalized Ag NW network electrode: A vacuum pump (KODIVAC GHP-240) is connected to the glass microanalysis holder (300 mL, STERLITECH) covered by Nylon (WATMAN) and PTFE (STERLITECH) filters. ZnO NP functionalized Ag NW solution is poured on the PTFE filter. Once the vacuum pump is turned on, the solution passes through the filter and randomly dispersed NW percolation network remains on the PTFE filter. The target substrate is then placed on the PTFE filter for several tens of seconds. After the pump is turned off, the filters are gently peeled off from the target substrate, leaving the NW percolation network on the target substrate. Branched ZnO NW Synthesis: Zinc nitrate hexahydrate (25 mM Zn(NO3)2⋅6H2O) and hexamethylenetetramine (25 mM HMTA, C6H12N4) are added to DI water and stirred until they dissolve completely. For the synthesis of branched ZnO NW with high aspect ratio, polyethylenimine (5~7 mM PEI, C2H5N) is injected in addition to suppress the lateral growth further. The mixture 5



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solution is heated up to 95 ˚C and cooled down while the precipitates are filtered out to obtain transparent solution.

Result and discussion For the conventional hydrothermal template growth of ZnO NW, either ZnO thin film or ZnO NP seed layer22 is required at the surface of the target substrate in order to induce the anisotropic growth towards c-axis with minimized homogeneous nucleation in the precursor solution. The target substrate therefore normally undergoes a coating step with the solution that contains ZnO NP seed through a wet process such as drop casting, dip coating or spin coating.23 In this study, Ag NW and ZnO NP seed, which are both in solution form, are simply mixed together under a stirrer for ~2 hours in order to attach ZnO NP on Ag NW surface by Van der Waals force.24 The resultant ZnO NP functionalized Ag NW percolation network is then cleaned repeatedly with extra ethanol to remove excessive ZnO NPs from the solution. Upon the conventional hydrothermal process, ZnO NW branches are expected to grow radially from the ZnO NP functionalized Ag NW surface to form h-ZBAB percolation network structure as schematically shown in Figure 1a. The selective thermochemical ZnO NW growth method proposed in this research is described in Figure 1b. Before the application of selective thermochemical growth by localized Joule heating, ZnO NP functionalized Ag NW percolation network goes through a number of distinct steps for efficient manipulation of electrical current density. ZnO NP functionalized Ag NW percolation network is firstly transferred on a cleaned glass substrate by vacuum filtration method to ensure random dispersion on the target substrate for uniformly interconnected conduction network. In this structure, ZnO NP on the surface of Ag NW does not significantly affect the electrical conductivity of the Ag NW percolation network as confirmed from high electrical conductivity of the resultant film. Laser ablation is subsequently applied on the ZnO NP functionalized Ag NW percolation network to 6


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for Ag NW network patterning,25-26 so that the electrical current is concentrated at a specific point for selective Joule heating and subsequent selective thermochemical growth of ZnO NW. Commercial copper tapes are then attached to each end of patterned ZnO NP functionalized Ag NW network and the resultant sample is immersed in ZnO precursor solution. As a constant bias voltage (Vb) is applied at the patterned ZnO NP functionalized Ag NW percolation network, electrical current flows through the Ag NW percolation network to induce resistive Joule heating that raises the temperature nearby.20 When the temperature reaches the threshold temperature, ZnO NW branches start to grow from the Ag NW surfaces which are functionalized with ZnO NP seeds to create h-BZAB heterogeneous nanostructure. At this time, the selective growth of the ZnO NW branches can be achieved by adjusting the Vb so that only the temperature of the target area exceeds the threshold temperature. Figure 2a shows the photograph of ZnO NP functionalized Ag NW percolation network immediately after the vacuum filtration transfer process. Macroscopically, the resultant ZnO NP functionalized Ag NW network is very much similar to the one prepared with pristine Ag NW percolation network, and its overall optical transmittance and electrical conductivity can be also tuned by changing the concertation during the vacuum filtration process. The difference between pristine Ag NW network and ZnO NP functionalized Ag NW network becomes distinctive when a hydrothermal growth process is applied to the network. Different from the ZnO NP functionalized Ag NW, negligible amount of ZnO NW is grown on the prist

ine Ag NW surface after a hydrothermal growth

as can be seen in Figure S1. The glass substrate with ZnO NP functionalize Ag NW network is firstly cut in half and one of the two undergoes the hydrothermal growth at 90 °C for 5 hours. The other part of the network sample was placed close together in Figure 2b for comparison, and it is confirmed that the ZnO NP functionalized Ag NW network after the hydrothermal growth shows different optical characteristics due to the increased light scattering by the branched ZnO NW growth on the backbone Ag NW surface. The difference can be more closely observed through scanning electron microscope (SEM) images in Figure 2c. As shown in the inset of Figure 2c, the ZnO NP functionalized Ag NW network before the hydrothermal growth is composed of Ag NW having a length of several tens of 7



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micrometer with the thickness of 100 nanometer and ZnO NPs are attached on the surface of the Ag NWs. The transmission electron microscope image (TEM) in Figure S2 shows that the attached seed ZnO NP has a diameter of ~10 nm. It is noticeable that the Ag NW is no longer visible after the hydrothermal growth as ZnO NW branches are densely grown normal to the surface of backbone Ag NW. The presumed locations of the underlying Ag NW are denoted as red lines in Figure 2c. Although the surface of Ag NW is not exposed after the growth, x-ray diffraction (XRD) measurement in Figure 2d reveals that the underlying backbone Ag NWs remain in the resultant hierarchical structure, showing distinct peaks that correspond to both ZnO NW and Ag NW. The density of the surrounding ZnO NW branches can be easily adjusted by the ratio of ZnO NP solution to the Ag NW solution at the mixing stage. Identical conventional hydrothermal growth is applied to three different samples with ZnO NP functionalized Ag NW networks prepared at different mixing conditions, and their optical microscope images are shown in Figure 2e. When the volume ratio of ZnO NP solution and Ag NW solution is 1:1, excessive ZnO NP remains at the final Ag NW network to produce unintended ZnO NW all over the surface even distant from the network. By reducing the relative amount of ZnO NP solution, no ZnO NW is grown at the void and h-BZAB heterogeneous structure is solely attained as a result. As the optimum mixing ratio, ZnO NP solution and Ag NW solution mixed at 1:4 has been used for the preparation of ZnO NP functionalized Ag NW network throughout the study. When a sufficient amount of electrical current flows through the Ag NW backbone percolation network in the Zn precursor solution, the localized temperature field is generated in the vicinity of the network to trigger the ZnO NW growth, following the same growing mechanism as the conventional hydrothermal growth.10, 19, 22 As the temperature rise is known to be a function of electrical current density, the laser ablation scheme has been employed to manipulate the current density profile for selective thermochemical growth. The optical setup for laser ablation process is illustrated in Figure S3a.27 Pulsed UV laser operating at 355 nm wavelength with ns pulse width is used as the ablation source, while the laser beam is focused with telecentric f-theta lens at f=100 mm for the efficient use 8


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of the laser energy. The laser power, or the energy per laser pulse, is controlled by a half-wave plate and a polarized beam splitter before the beam enters the focusing optics. The ablation feature created by a single laser pulse at the focus should be circular at micron size, but a series of laser pulses can generate a continuous line at a sufficiently high repetition rate as the pulses spatially overlap. Figure 3a shows the optical image of typical ablation line patterns created on the ZnO NP functionalized Ag NW percolation network by moving the stage at 10 mm/s with the laser repetition rate of 20 kHz. It is noticeable that the Ag NW networks are clearly removed from the glass without any observable damage on the underlying substrate. The beam path is further altered by 2D electrically driven galvano-mirror controlled by a computer aided design (CAD) program so that the ZnO NP functionalized Ag NW network is easily patterned at a desired shape over a large area. As an example, ablation features are formed on highly dense ZnO NP functionalized Ag NW network along specific letters as shown in Figure S3b. Its high magnification image proves that the ablation features are consistent throughout the scanning path. In addition, to approve that the removal of the target material by laser ablation is complete, electrical resistance of the sample is measured in real time during the ablation procedure as plotted in Figure 3b. The initial resistance of the sample across the ZnO NP functionalized Ag NW network is as low as ~1 Ω, but rapidly increases by the laser ablation to reach infinity as the laser beam crosses the network completely. The patterned ZnO NP functionalized Ag NW percolation network can be utilized as a platform for further thermochemical reaction by applying a bias voltage, yet the as-prepared network cannot assign spatial selectivity to the growth. For the selective thermochemical growth, Ag NW network is firstly patterned at a ribbon shape with the narrow waist so that the electrical current density and the local temperature are maximized at the center. The heating characteristics of the electrodes before and after laser patterning are shown in Figure 3c with an IR camera, where the voltage applied across each electrode is fixed at 1 V for both cases. In the case of the as-prepared electrode, the entire area of the electrode is uniformly heated, while the temperature distribution varies greatly for the patterned Ag NW electrode. In particular, the highest temperature is observed at the waist as expected, and these 9



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results are directly interrelated to the current density. The graphs inserted in the figures are the line profiles of temperature measured along the green dotted line in each case. It shows that, for the patterned electrode, the heating and so is the growth of ZnO NW can be efficiently concentrated at the waste. Since the thermochemical growth of ZnO NW has to be performed above the threshold temperature for ZnO NW growth (~60 ˚C),28 the electrode must be capable of generating sufficient temperature and maintaining the temperature stably for a certain duration of time. To confirm this, the heating characteristics of the ribbon shaped electrode is measured for every 10 minutes under various voltage conditions and the results are plotted in Figure 3d, showing that the final electrode is sufficiently robust at high temperature. The overall experimental schematics for the thermochemical growth is depicted in Figure S4a. The sample after the preparation is immersed in the precursor solution and a voltage bias is applied through two probe tips. The voltage has been fixed at 1 V in order to prevent any unexpected electrochemical reaction within the precursor solution. The resultant power consumption during the growth is less than 1 W, which is minute compared to the power required for the equipment used in conventional hydrothermal growth such as convection oven or hot plate. In addition, as the temperature rise is highly confined at the surface of ZnO NP functionalized Ag NW network, other equipment can be directly integrated to the system without considering any thermal damage. Based on this advantage, entire thermochemical growth is conducted under a vision system with zoom lens and objective lens for real-time monitoring of the growth. The photographic image of the overall setup prepared in a dark faraday cage is attached Figure S4b. Figure S4c shows some snapshots captured during the growth which confirm that the hydrothermal reaction is more likely initiated from the center where the maximum temperature happens. The resultant branched ZnO nanostructures grown on Ag NW backbone are more closely investigated in terms of optical and SEM images. Figure 4a shows the microscope image of the patterned ZnO NP functionalized Ag NW after 10 mins of thermochemical growth. It is noticeable that the resultant network can be separated in several regions with dissimilar optical properties. We presume that these distinctions are caused by the difference in 10


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the temperature, as their boundaries are similar to the constant temperature contours generated at a constant bias voltage. Close examination by SEM proves that the morphology of the thermochemically grown nanostructure is different at each point as shown in Figure 4b. At the outermost point (Figure 4a,b(iv)), no secondary nanostructure is created as the temperature rise caused by electrical current is largely insignificant. As the inspecting point approaches more towards the center, the thermochemical growth initiates as grass-like nanostructure (Figure 4a,b(ii)) and becomes more like conventional NW (Figure 4a,b(i)) reported elsewhere.19-20 Meanwhile, it is noteworthy that the hierarchical branched growth of ZnO NW on Ag NW backbone changes into faceted growth (Figure 4b(i)) from disordered growth (Figure 4b(ii)) as it reaches more to the center. The growth mode change can be shown more effectively by increasing the growth time of the ZnO NW to 30 mins as shown in Figure 4c. In order to grow ZnO NWs with only one morphology in target area by keeping a uniform temperature, the ZnO NP functionalized Ag NW is redesigned as Figure S5b to have a sharply defined growth zone with a flat temperature profile. Since the difference in widths between the electrical pad (~1 cm) and the growth zone (150 µm) is sufficiently large, the transition between Th and Tc occurs sharply at the boundary to yield ZnO NW with a single morphology within the growth zone. We further demonstrated that the morphology of the ZnO NW grown in the growth zone can be controlled by manipulating Th. Figure S5c and S5d are the SEM images taken from the growth zone after grown for 10 mins at Th=75˚C and 90˚C respectively. These images explain that the higher growth temperature leads to more ordered growth, which is in accordance to the result from the ribbon shaped electrode. Also, ZnO NW growth can be assisted by electric field as demonstrated on a coplanar Cr/Au electrode pair.29 Since the current configuration is different from the previous work, we attempted to investigate the effect solely from the electric field in our study by decreasing the density of the Ag NW percolation network with the same bias voltage. Since the resistance of the overall Ag NW percolation network becomes higher than the ones from the above experiments, the amount of Joule heating is decreased and we confirmed that the maximum temperature at the growth zone does not exceed 35 ˚C. Figure S6 is the SEM image of the ZnO NP 11



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functionalized Ag NW percolation network after applying 1 V bias voltage for 10 mins, showing that negligible amount of ZnO NW is grown. This result suggests that the temperature is still dominant factor in the growth when the heating is induced by the electrical current. This phenomenon suggests that the growth mode – faceted ((Figure 4c(i))) or disordered (Figure 4c(ii)) – can be controlled solely by changing the growth conditions including the temperature without reference to the conditions for ZnO NP seeding. In our opinion, disordered or faceted ZnO NW on Ag NW may find different applications according to their morphologies. For general chemical sensing applications that operate through surface adsorption and desorption of the target species, disordered ZnO NW appears to be suitable in order to maximize the surface-to-volume ratio for higher sensitivity.30 However, the directionality of ZnO NW may play a critical role in specific sensors or optoelectronic applications such as ZnO NW piezoelectric sensor31 and lasing.32 that utilizes two faceted hexagonal end faces as reflecting mirrors of a resonance cavity.

Conclusion We demonstrate selective synthesis of hierarchical branched ZnO NW on Ag NW backbone percolation network structure by employing ZnO NP functionalized Ag NW network as a localized Joule heating source. Through vacuum filtration transfer, uniform conductive Ag NW network is created on the target substrate, while the current density is spatially manipulated by laser ablation process to ensure the selectivity of the h-BZAB heterogeneous growth. As a result, branched ZnO NW on Ag NW backbone hierarchical nanostructure is only attained at the desired area upon the application of a fixed bias voltage. This process is expected to be useful for direct integration of metallic component with hydrothermally grown semiconductor NW at a very low power consumption.

Supporting Information SEM images of the samples (Figure S1, S5, and S6); TEM image of the sample (Figure S2); Optical 12


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microscope images of the sample (Figure S3, S4); Instrument setup (Figure S3, S4)

Acknowledgements This work is supported by the National Research Foundation of Korea (NRF) Grant funded through Basic Science Research

Program

(2017R1A2B3005706, NRF-2016R1A5A1938472, NRF-

2016M3D1A1900035, NRF-2016R1C1B1014729, NRF-2017R1C1B1008847) and Institute for Information & communications Technology Promotion(IITP) grant funded by the Korea government(MSIP) (No. 2017000910001100, Development of multi-material 3D printing technologies for flexible motion detection and control sensor modules)

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References 1. Özgür, Ü.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doğan, S.; Avrutin, V.; Cho, S.-J.; Morkoç, H., A Comprehensive Review of Zno Materials and Devices. J. Appl. Phys. 2005, 98, 041301 2. Cho, S.; Jung, S.-H.; Lee, K.-H., Morphology-Controlled Growth of Zno Nanostructures Using Microwave Irradiation: From Basic to Complex Structures. J. Phys. Chem. C 2008, 112, 12769-12776. 3. Xu, S.; Wang, Z., One-Dimensional Zno Nanostructures: Solution Growth and Functional Properties. Nano Res. 2011, 4, 1013-1098. 4. Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D., Room-Temperature Ultraviolet Nanowire Nanolasers. Science 2001, 292, 1897-1899. 5. Nadarajah, A.; Word, R. C.; Meiss, J.; Konenkamp, R., Flexible Inorganic Nanowire Light-Emitting Diode. Nano Lett. 2008, 8, 534-537. 6. Menzel, A.; Subannajui, K.; Güder, F.; Moser, D.; Paul, O.; Zacharias, M., Multifunctional ZnoNanowire-Based Sensor. Adv. Funct. Mater. 2011, 21, 4342-4348. 7. Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D., Zno Nanowire Uv Photodetectors with High Internal Gain. Nano Lett. 2007, 7, 1003-1009. 8. Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P., Nanowire Dye-Sensitized Solar Cells. Nat. Mater. 2005, 4, 455-459. 9. Xu, S.; Qin, Y.; Xu, C.; Wei, Y.; Yang, R.; Wang, Z. L., Self-Powered Nanowire Devices. Nat. Nanotechnol. 2010, 5, 366-373. 10. Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P., Low-Temperature Wafer-Scale Production of Zno Nanowire Arrays. Angew. Chem. 2003, 115, 3139-3142. 11. Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P., General Route to Vertical Zno Nanowire Arrays Using Textured Zno Seeds. Nano Lett. 2005, 5, 1231-1236. 12. Harnack, O.; Pacholski, C.; Weller, H.; Yasuda, A.; Wessels, J. M., Rectifying Behavior of Electrically Aligned Zno Nanorods. Nano Lett. 2003, 3, 1097-1101. 13. Lao, C. S.; Liu, J.; Gao, P.; Zhang, L.; Davidovic, D.; Tummala, R.; Wang, Z. L., Zno Nanobelt/Nanowire Schottky Diodes Formed by Dielectrophoresis Alignment across Au Electrodes. Nano Lett. 2006, 6, 263-266. 14. Kim, Y.-J.; Lee, C.-H.; Hong, Y. J.; Yi, G.-C.; Kim, S. S.; Cheong, H., Controlled Selective Growth of Zno Nanorod and Microrod Arrays on Si Substrates by a Wet Chemical Method. Appl. Phys. Lett. 2006, 89, 163128. 15. Ko, S. H.; Lee, D.; Hotz, N.; Yeo, J.; Hong, S.; Nam, K. H.; Grigoropoulos, C. P., Digital Selective Growth of Zno Nanowire Arrays from Inkjet-Printed Nanoparticle Seeds on a Flexible Substrate. Langmuir 2011, 28, 4787-4792. 16. Kang, H. W., et al., Simple Zno Nanowires Patterned Growth by Microcontact Printing for High Performance Field Emission Device. J. Phys. Chem. C 2011, 115, 11435-11441. 17. Zeng, Z.-C.; Wang, H.; Johns, P.; Hartland, G. V.; Schultz, Z. D., Photothermal Microscopy of Coupled Nanostructures and the Impact of Nanoscale Heating in Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. C 2017. 121, 11623-11631. 18. Hong, S.; Yeo, J.; Manorotkul, W.; Kim, G.; Kwon, J.; An, K.; Ko, S. H., Low-Temperature Rapid Fabrication of Zno Nanowire Uv Sensor Array by Laser-Induced Local Hydrothermal Growth. J. Nanomater. 2013, 2013, 7. 19. Yeo, J.; Hong, S.; Wanit, M.; Kang, H. W.; Lee, D.; Grigoropoulos, C. P.; Sung, H. J.; Ko, S. H., Rapid, One-Step, Digital Selective Growth of Zno Nanowires on 3d Structures Using Laser Induced Hydrothermal Growth. Adv. Funct. Mater. 2013, 23, 3316-3323. 20. Yeo, J., et al., Single Nanowire Resistive Nano-Heater for Highly Localized Thermo-Chemical Reactions: Localized Hierarchical Heterojunction Nanowire Growth. Small 2014, 10, 5015-5022. 21. Lee, J. H.; Lee, P.; Lee, D.; Lee, S. S.; Ko, S. H., Large-Scale Synthesis and Characterization of Very Long Silver Nanowires Via Successive Multistep Growth. Cryst. Growth Des. 2012, 12, 5598-5605. 22. Hong, S.; Yeo, J.; Manorotkul, W.; Kang, H. W.; Lee, J.; Han, S.; Rho, Y.; Suh, Y. D.; Sung, H. J.; Ko, S. H., Digital Selective Growth of a Zno Nanowire Array by Large Scale Laser Decomposition of Zinc Acetate. Nanoscale 2013, 5, 3698-3703. 14


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Figures

Figure 1. Schematic illustration of (a) synthesis of ZnO NW-Ag NW hierarchical structure and (b) relevant processes. Ag NW percolation network was used as a localized joule heater and backbone nanostructure for subsequent ZnO nanowire growth.

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Figure 2. (a) ZnO NP functionalized Ag NW percolation network immediately after the vacuum filtration transfer. (b) Photograph image of the ZnO NP functionalized Ag NW percolation network showing different optical transmittance before and after the sample undergoes conventional hydrothermal growth. (c) SEM image of ZnO NW - Ag NW hierarchical structure synthesized with conventional hydrothermal growth. Inset is a high magnification SEM image of ZnO NP-Ag NW. (d) XRD data of ZnO NP-Ag NW network before and after the growth. (e) Optical microscope image of ZnO NW-Ag NW hierarchical nanostructure prepared at different mixing ratio (Ag NW solution : ZnO NP solution=1:1, 2:1, 4:1 from the top).

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Figure 3. (a) Optical microscope image of the ZnO NP functionalized Ag NW percolation network before and after the laser ablation patterning. (b) Real-time measurement of the resistance across the ZnO NP functionalized Ag NW network during the laser ablation process. (c) Joule heating characteristic of the ZnO NP functionalized Ag NW network before (left) and after (right) the laser pattering. (d) Stepwise heating performance of the ribbon shaped ZnO NP functionalized Ag NW network at various applied voltage.

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Figure 4. (a) Dark-field microscope image of the resultant hierarchical ZnO NW grown on Ag NW network after 10 mins at 1 V bias voltage. (b) SEM images taken at different locations showing various nanostructures with distinct morphology; (i) Faceted NW, (ii) Disordered NW, (iii) Grass-like nanostructure, and (iv) Insignificant growth. (c) SEM images taken at location of (i) and (ii) after 30 mins growth.

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TOC Graphic

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Selective Thermochemical Growth of Hierarchical ZnO Nanowire

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