Interior-architectured ZnO nanostructure for enhanced electrical conductivity via stepwise fabrication process
© Chong et al.; licensee Springer. 2014
Received: 11 May 2014
Accepted: 26 July 2014
Published: 24 August 2014
Fabrication of ZnO nanostructure via direct patterning based on sol-gel process has advantages of low-cost, vacuum-free, and rapid process and producibility on flexible or non-uniform substrates. Recently, it has been applied in light-emitting devices and advanced nanopatterning. However, application as an electrically conducting layer processed at low temperature has been limited by its high resistivity due to interior structure. In this paper, we report interior-architecturing of sol-gel-based ZnO nanostructure for the enhanced electrical conductivity. Stepwise fabrication process combining the nanoimprint lithography (NIL) process with an additional growth process was newly applied. Changes in morphology, interior structure, and electrical characteristics of the fabricated ZnO nanolines were analyzed. It was shown that filling structural voids in ZnO nanolines with nanocrystalline ZnO contributed to reducing electrical resistivity. Both rigid and flexible substrates were adopted for the device implementation, and the robustness of ZnO nanostructure on flexible substrate was verified. Interior-architecturing of ZnO nanostructure lends itself well to the tunability of morphological, electrical, and optical characteristics of nanopatterned inorganic materials with the large-area, low-cost, and low-temperature producibility.
KeywordsElectrical conductivity Interior-architecturing ZnO nanostructure Nanoimprint Lithography(NIL) Zinc oxide (ZnO) Hydrothermal growth
Zinc oxide (ZnO) has been widely pursued due to its electronic and optoelectronic characteristics arising from a direct wide bandgap (Eg ~ 3.37 eV) and an isoelectronic point with a large excitation binding energy (60 meV) at room temperature –. These are sought-after features in a number of electric devices, such as sensor/detectors, light-emitting diodes, solar cells, field-effect transistors (FETs), and nanogenerators –. Recently, much research has been carried out in the field of nanostructured ZnO, investigating one hole-one nanorods, micro/nanodots, patterned seed layers, and nanoparticles. In-depth research results have confirmed that nanostructured ZnO is a promising material in the fields of photonics and electronics.
Increasing the ease of fabricating ZnO nanostructure arrays on almost any substrate would enable large-scale production for a wide range of applications. However, the current commonly applied methods for ZnO deposition, such as chemical vapor deposition (CVD) [8, 9], pulsed laser deposition (PLD) , metal-organic chemical vapor deposition (MOCVD) , and atomic layer deposition (ALD) –, require high-cost equipment and high-vacuum conditions, and/or are subject to substrate limitations due to high process temperatures. For the latter reason, hydrothermal synthesis of ZnO has been welcomed due to its low process temperature. However, the resulting structures are generally short and are difficult to integrate to form large arrays. As for fabricating micro/nanosized arrays or nanopatterns, the main methods utilized are electron-beam lithography (EBL) [15, 16], photo- lithography (PL) [17, 18], and laser interference lithography (LIL) . These processes have their share of limitations too: some are costly and require a controlled environment, while others are difficult to perform on flexible or large substrates.
For these reasons, direct nanopatterning of metal oxides using the sol-gel process such as nanoimprint lithography (NIL) has garnered much attention as a simple, low-cost, and rapid technique potentially suited to large-area fabrication and producibility on flexible or non-uniform substrates. Directly nanoimprinted ZnO layers have been applied in light-emitting devices and advanced nanostructuring –. However, its application as an electrically conducting medium has been limited by its low conductivity, which has been attributed to organic residues and a low degree of crystallinity after low-temperature calcination, or increased porosity after high-temperature calcination. Improving the electrical conductivity of directly nanoimprinted ZnO nanostructures at low process temperatures would open opportunities for their application in low-temperature electronic devices, including flexible devices and allow for large-area mass fabrication.
Chemicals: ZnO precursor-containing resin for nanoimprint
ZnO precursor resin was prepared by dissolving 0.5 mol zinc acetate dihydrate (Zn(CH3COO)22H2O, Aldrich, Wyoming, IL, USA, 99.5%), 2-nitrobenzaldehyde (Aldrich UV-linker), and the molar equivalent of monoethanolamine (MEA, (NH2CH2CH2OH, Aldrich, 99.5%) in 2-methoxyethanol (2ME (CH3OCH2CH2OH, Aldrich, 99.5%). The resulting solution was stirred at 25°C for 3 h and 75°C for 24 h to yield a homogeneous and stable colloid solution.
Solution for additional growth of ZnO
The solution for hydrothermal synthesis of ZnO nanocrystals was prepared as Zn(NO3)2 ∙ 6H2O (25 mM)-zinc nitrate hexahydrate (Zn(NO3)2 ∙ 6H2O, Aldrich, 98%) in deionized water with HMTA (25 mM)-hexamethylenetetramine (C6H12N4, Aldrich, 99.5%) and PEI (0.834 mM)-polyethylenimine (PEI, Aldrich, molecular weight 1,300 g mol - 1LS).
Step 1. Nanopatterning of ZnO film
Nanopatterning of ZnO was conducted via UV-NIL as follows. First, ZnO precursor-resin was spin-coated onto substrates at 3,500 rpm for 1 min to yield a thickness of 200 nm, then prebaked at 80°C on a hot plate. Si wafer with a 300-nm-thick SiO2 layer and polyimide (PI) film were used as the substrates. Next, a nanopatterned polyurethane acrylate (PUA) mold with a regular line pattern array with 200-nm line width and 1-μm period was prepared with the similar method to that previously reported . The mold-attached substrate was then illuminated with 365-nm wavelength light for 3 min to cure the resin, under an applied air pressure of 0.02 MPa. The illumination time of 3 min was set for the resin to be partially cured for the facile detachment of the mold. And then, the PUA mold was detached from the substrate and line patterns were formed on the ZnO resin. After the de-molding, the film was illuminated with 365-nm light for the additional cure of the film and the resultant preservation of the pattern shape during the next thermal annealing. The film was annealed at 350°C for 60 min in a furnace for calcination to take place and cause crystallization. A thermally annealed line-nanopatterned ZnO film resulted, with a residual layer left under the ZnO nanopattern.
Step 2. Wet etching for removal of residual layer
To fabricate individually separated ZnO nanoline, the residual layer was removed by wet etching using 0.25% HNO3 solution. During removal of the residual layer, some organic components in the ZnO nanopattern originated from ZnO precursor resin were also removed.
Step 3. Additional growth of ZnO
Additional ZnO was grown on the ZnO nanostructures by hydrothermal synthesis. The substrate with ZnO nanostructures was kept in the prepared solution for 30 min at 75°C or 90°C in a convention oven. The ZnO nanostructure-deposited surface was positioned so as to be face-down in the solution. After the growth step, the substrate was thoroughly rinsed with deionized water and dried in air.
For the electrical characterization of ZnO nanolines, a single nanoline was connected to Au electrodes via deposition of Pt using a focused ion beam (FIB) system. The Au electrodes were patterned so as to be separated by 2 μm on the SiO2/Si substrate prior to deposition of the ZnO resin using lift-off process. For characterization of the ZnO nanostructure arrays, two top electrodes of silver (Ag) were painted through a shadow mask onto the ZnO nanostructure using Ag paste (ELCOAT, P-100, CANS, Japan). The width and length (W/L) of the gap between the two Ag electrode arrays were 2 and 1 mm, respectively.
The morphology of the ZnO nanolies was investigated using field emission scanning electron microscopy (FE-SEM, FEI co., Hillsboro, OR, USA). X-ray diffraction (XRD, Rigaku, Shibuya-ku, Japan, D/MAX-2500) analysis was used for inspection of the nanostructured ZnO film crystal structure. Diffraction patterns were taken in the 2θ range of 10 to 60° using Cu-Kα radiation (λ = 0.15405 nm), with a scanning rate of 2°/min, step of 0.02°, and incident angle of 3° to the surface. X-ray photoelectron electroscopy measurement was performed in a thermo spectrometer (MultiLab 2000, Thermo Scientific, Waltham, MA, USA; a base pressure of 1 × 10-9 Torr) using monochromatized Al-Kά radiation. The electrical characteristics of the fabricated ZnO nanostructure and the response to UV illumination were measured with a semiconductor parameter analyzer (Keithley 4200-SCS, Cleveland, OH, USA) at room temperature in a dark room. UV light with 365-nm wavelength was used for illumination. In order to test the mechanical stability of the devices, bending tests were conducted using a single-axis linear stage. The flexible devices were mounted on a carrier substrate and then attached to the linear stage. A bending test with curvature radii from ρ = 10 mm to ρ = 86 mm was performed over 1,000 cycles while the current was measured at a bias of 5 V using a potentiostat (CHI601D, CH Instruments, Austin, TX, USA).
Results and discussion
The chemical binding states measured using XPS are depicted in Figure 4b. The sample surface was pre-cleaned with argon plasma to remove surface contaminants. The presence of C1s peaks at 286 eV for all samples is due to contamination by exposure to the atmosphere. The O1s peaks showed an asymmetric shape and were deconvoluted into two peaks by Lorentzian-Gaussian spectral fitting [24, 25]. Those O1s peaks can be assigned to oxide lattices with metal-oxygen bonding (Zn-O), and to oxygen vacancies (Zn-V O ), or other hydroxide groups (Zn-OHs) as depicted in Figure 4b. For the TA line-patterned film, the O1s peaks were convoluted to 529.8 and 531.9 eV. Following the wet-etching process, the intensity ratio of the Zn-O-related O1s peak to the Zn-V O -related O1s peak decreased, due to the increased surface area and porosity. The same ratio increased for both the NB and NR films following the additional growth step, implying that the ratio of crystal defects in the ZnO crystal structure were reduced during hydrothermal growth. It is also supported from Figure 4c, which shows that the binding energy of Zn2p3 was shifted from Zn2+ to ZnO binding energy . The higher value of the ratio for the NB film than the NR film can be attributed to the lower surface area of NB film.
The photoresponse of various single-ZnO nanolines was also measured with illumination by UV light and is depicted in Figure 5b,c. Electrical resistance and resistivity both decreased upon illumination with UV light. The mechanism of photodetection is well known . Oxygen molecules are adsorbed onto the surface of ZnO while capturing free electrons, forming a low-conductivity depletion layer near the surface. When illuminated by UV light, oxygen molecules are desorbed; electron-hole pairs are generated inside of ZnO; and the holes migrate to the surface and are trapped, leaving behind unpaired electrons to contribute to the photocurrent. When UV light is not present, oxygen molecules are re-adsorbed on the surface, retrapping free electrons, and the resistance is recovered.
Responsivity, defined as the ratio of the change of electrical current to the illuminated UV power, was calculated to be 1.66 × 104, 4.67 × 105, and 4.41 × 104(A/W) for the single TA nanoline, NB nanoline, and NR nanoline, respectively. The responsivity of the NB nanoline was superior to the TA and NR nanolines. This result is noteworthy because both the electrical conductivity and responsivity of the ZnO nanoline were enhanced in the NB nanoline by the interior-architecturing, without the use of a high-temperature process of over 500°C . We believe that fewer voids and interfacical trap states in the NBs nanoline led to an increase in the number of electron-hole pairs generated during UV illumination, to a greater extent than in the TA nanoline. In the case of the NR nanoline, its responsivity was almost as low as the TA nanoline. This result can be attributed to increased carrier scattering by the complex path in NR nanolines as well as increased grain boundaries due to augmented junctions between nanorods .
In this paper, interior-architecturing of ZnO nanostructure was demonstrated to improve its electrical characteristic processed at low temperature by using stepwise fabrication process. Investigation of the structural morphology and crystallographic orientation confirmed that interior structure of ZnO nanostructure was changed as filling of voids with newly growing nanocrystalline ZnO during the fabrication process and the change contributed to the improvement in electrical characteristics. The fabricated ZnO nanolines also showed good electrical responses to illumination by UV light.
The arrays of ZnO nanoline were easily implemented on both rigid and flexible substrates and functioned as UV-sensitive devices with good sensitivity. Interior-architecturing of ZnO nanostructure lends itself well to tunability of morphological, electrical, and optical characteristics of nanopatterned inorganic materials with the large-area, low-cost, and low-temperature producibility. Furthermore, all experimental nano arrays on PI films showed outstanding robustness under prolonged bending tests and, in combination with printing technology, could be widely applied to next-generation flexible applications.
This research was supported by the National Research Foundation of Korea under Grant Nos., 2009-0082527 and Global Frontier R&D Program (2011-0031563) and also funded by the grant (SC0890) from the Korea Resarch Council for Industrial Science and Technology. The authors would like to thank Mr. Yun-Chang Park at the National Nanofab Center (NNFC) for valuable discussion.
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