Hybrid ZnO NR/graphene structures as advanced optoelectronic devices with high transmittance
© Chung et al.; licensee Springer. 2013
Received: 9 June 2013
Accepted: 3 August 2013
Published: 10 August 2013
A hybrid structure (HS) made of one-dimensional ZnO nanorods (NRs) and a two-dimensional synthesized graphene sheet was successfully constructed in this study. The uniform ZnO NRs were obtained by hydrothermal method and grown on a graphene surface that had been transferred to a polyethylene terephthalate substrate. The HS exhibited high transmittance (approximately 75%) over the visible wavelength range, even after cyclic bending with a small radius of curvature. Raman spectroscopy and Hall measurement were carried out to verify the chemical composition and electrical properties of the structure. Stable electrical conductance of the ZnO NR/graphene HS was achieved, and increase in carrier mobility decreased the resistance of the ZnO-with-graphene sheet in comparison with bare ZnO NRs.
Various investigations have concentrated on the development of promising materials with multifunctionality for emerging electronic and optoelectronic systems [1, 2]. For example, interest has been growing in combining the specific properties of different dimensional structures on a flexible and transparent substrate. Recently, the composition of hybrid structures (HS) using one-dimensional (1D) and two-dimensional (2D) components to synthesize novel materials has been gaining much attention, especially for 1D ZnO nanorods (NRs) and 2D graphene sheets. Graphene, which consists of monolayers of sp2 hybrid carbon atoms, has been the most attractive carbon material in recent years [3–6]. Because of carbon-carbon covalent bonds, a graphene sheet exhibits extraordinary electrical and mechanical properties, including high intrinsic mobility (15,000~20,000 cm2/Vs) , a stretchable nature, and high thermal conductivity (approximately 5,300 W/mK) . Moreover, with high optical transmittance and high chemical stability, graphene is a promising building block of window material for optoelectronic devices. In addition to graphene, transparent ZnO NRs with a wide bandgap are good candidates for use in next-generation electronics and optoelectronics [9–13]. Many methods have been developed to prepare ZnO NRs, including chemical vapor deposition (CVD) , vapor–liquid-solid epitaxy , and pulsed laser deposition . However, these techniques are only applicable to limited substrate sizes and require high process temperatures, which are prohibitive for many practical applications. On the other hand, the hydrothermal process is regarded as a promising technique for the synthesis of ZnO NRs because it has several advantages, including the fact that it is a low-cost and low-temperature process that provides wafer-scale uniformity and high growth rates. Therefore, the hybridization of 2D graphene with 1D ZnO NRs has recently been reported for multifunctional applications, such as gas sensors , light-emitting diodes , solar cells , and piezoelectric nanogenerators . In addition, ZnO is an excitingly attractive material for use as a transparent conducting oxide (TCO), but ZnO cannot solitarily exist as a TCO because of its intrinsic point defects [21, 22]. To overcome this problem, increasing the carrier concentration or carrier mobility is effectively equivalent to decreasing the sheet resistance. In our opinion, ZnO NR/graphene HSs have characteristics that are of particular interest to the development of such structures for use as TCOs.
In this work, 1D ZnO NRs were synthesized by hydrothermal method onto a 2D graphene sheet to form an HS. High transmittance over the visible light region was obtained after synthesizing the ZnO NRs, and the sample displayed excellent mechanical properties after bending with a small radius. Notably, we essayed a Hall measurement of the HS, which consisted of ZnO NRs/graphene on a polyethylene terephthalate (PET) substrate.
Each graphene sheet was prepared on a Cu foil by CVD and then spin-coated with a protective layer of poly(methyl methacrylate) (PMMA). The PMMA/graphene/Cu foil sample was immersed into an FeCl3/HCl solution for etching to strip the Cu foil. After etching, we retrieved the sample from the FeCl3/HCl solution, transferred it onto the PET substrate, and cleaned it with deionized water. Finally, the PMMA/graphene was soaked in an acetone solution to remove the PMMA and then washed with isopropyl alcohol . A 100-nm ZnO seed layer was coated onto the graphene sheet with an E-gun evaporation system. Following this step, the ZnO NRs were grown in an equal molar aqueous solution of hexamethylenetetramine (HMTA) and zinc nitrate hexahydrate at 95°C for 2 h. The sample was cleaned with acetone and deionized water and then dried at room temperature. After the growth process, a morphological study of the ZnO nanostructures was performed with a JEOL JSM-6500 (Tokyo, Japan) field-emission scanning electron microscope (FE-SEM). Optical transmittance measurements were collected for nearly normal light incidence covering the spectral region from 400 to 800 nm with a standard UV-Visible spectrometer (ARN-733, JASCO, Easton, MD, USA). In this measurement, the noise level was approximately 0.002%. Raman spectrum was measured with a triple spectrometer (T64000, HORIBA Jobin Yvon SAS, Canal, France) equipped with a charge-coupled device cooled to 160 K. Hall measurement was performed with an Ecopia Hall effect measurement system (HMS-3000 ver 3.51.4).
Results and discussion
The results of Hall measurements of ZnO and ZnO NRs/graphene on PET substrate
ZnO NRs/graphene after bending
Uniform ZnO NRs were obtained by hydrothermal method and grown on a graphene surface that had been transferred to a PET substrate. The ZnO NR/graphene HS exhibited high transmittance (approximately 75%) over the visible wavelength range, even after cyclic bending with a small radius of curvature. Stable electrical conductance of the ZnO NR/graphene HS was achieved, and the improvement of the ZnO sheet resistance by the incorporation of the graphene sheet can be attributed to the resultant increase in carrier mobility.
The authors are grateful to the part sponsor of this research, the National Science Council of the Republic of China, grants NSC 101-2622-E-027-026-CC3 and NSC 102-2221-E-027-009.
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