Characterization of Gold-Sputtered Zinc Oxide Nanorods—a Potential Hybrid Material
© Perumal et al. 2016
Received: 6 October 2015
Accepted: 6 January 2016
Published: 19 January 2016
Generation of hybrid nanostructures has been attested as a promising approach to develop high-performance sensing substrates. Herein, hybrid zinc oxide (ZnO) nanorod dopants with different gold (Au) thicknesses were grown on silicon wafer and studied for their impact on physical, optical and electrical characteristics. Structural patterns displayed that ZnO crystal lattice is in preferred c-axis orientation and proved the higher purities. Observations under field emission scanning electron microscopy revealed the coverage of ZnO nanorods by Au-spots having diameters in the average ranges of 5–10 nm, as determined under transmission electron microscopy. Impedance spectroscopic analysis of Au-sputtered ZnO nanorods was carried out in the frequency range of 1 to 100 MHz with applied AC amplitude of 1 V RMS. The obtained results showed significant changes in the electrical properties (conductance and dielectric constant) with nanostructures. A clear demonstration with 30-nm thickness of Au-sputtering was apparent to be ideal for downstream applications, due to the lowest variation in resistance value of grain boundary, which has dynamic and superior characteristics.
Advances in nanotechnological approaches have provided an insight to nanocreations and manipulation of various nanomaterials to yield unique metal nanostructures having interesting properties and functions [1–3]. In the past, special attentions have been paid to make nanostructures for fine-tuning their properties towards the development of sensing substrates [4–6]. Among the different nanohybrid structures, involvements of metal oxides have elevated a step ahead in different applications [7–10]. Nanoparticle made from metal oxides proved for their participation in sensing applications, especially for bio-recognition [11, 12]. Oxide groups reside in the nanomaterials/nanoparticle prepared by metal oxide impart improvement in sensitivity of biosensors . With the explored metal oxides, zinc oxide (ZnO)-based nanostructures have recently been aroused much interest due to its unique optical and electrical properties [14, 15].
ZnO has been widely used as material for semiconductor, because of its appealing characteristics, such as large exciton binding energy (60 meV). Special interests with ZnO usage have been taken to be used for electro-optical devices. Moreover, due to low cost, simplicity in fabrication and high electron mobility, optoelectronic devices rely on ZnO nanostructures (nanorods, nanowires and nanoflowers) [16–18]. Additionally, ZnO is stable at low and higher pH extremes and an ideal material for functionalization with biological and chemical compounds [11, 15]. With a property of excellent surface-to-volume ratio, ZnO nanohybrid has considered for improved catalytic activity. On the other hand, metal particles such as gold nanoparticle (AuNP) are shown to have high electron affinity; between AuNP and metal oxides, high Schottky barrier can be produced [19, 20]. Similar to ZnO, Au has been accepted as suitable material for biocompatibility, high conductivity and surface chemical functionalization [21–23]. Considering all these vital things, it is a wise approach to make ZnO and Au hybrid for creating nanostructures to be used for a wide range of applications.
In general, there are two major techniques that have been in nanofabrication of metal oxide nanostructures: “top-down” and “bottom-up”. Top-down approach is not a promising method because it contains some limitations such as low yield assembly, large-scale uniformity and repeatability issues, whereas bottom-up approach owns its superiority compared to top-down approach in terms of photolithography and is capable of producing various nanostructures with high yield, less defect and better range ordering . ZnO nanostructures prepared by bottom-up approach are catalytically synthesized by chemical vapour deposition (CVD) and vapour liquid solid (VLS) methods, where structures are assembled from their atomic level [24, 25]. Hence, ZnO nanostructure from bottom-up fabrication approaches has been preferred as it possesses unique physical, optical and electrical properties, which are highly suitable for downstream applications.
In the present study, we demonstrated a simple and facile route to synthesis Au-sputtered ZnO nanorods, and the thickness of Au-sputtering is tuned to investigate physical, optical and electrical properties of ZnO nanorods on the interdigitated electrode (IDE). Currently, there are only limited numbers of research articles highlighting the impedance spectroscopic analyses on hybrid materials. Herein, impedance spectroscopy tool was employed to investigate on Au-sputtered ZnO nanohybrid. By the addition of AuNPs to ZnO nanostructures, it leads changes in conduction and polarization mechanism, which are clearly addressed.
Au-Interdigitated Electrode Fabrication
ZnO Nanorods Synthesis
ZnO nanorods (ZnO-NRs) were prepared as described in our previous report . Briefly, 8.78 g of Zn(CH3COO)2·2H2O (98 %; Sigma-Aldrich) was dissolved in 200 ml of ethanol solvent (EtOH; 99.99 %; J.T. Baker) (ZnO seed solution solgel). The concentration of ZnO was kept constant as 0.2 M. The mixed solution was then vigorously stirred with a magnetic stirrer at 60 °C for 30 min. The stabilizer, monoethanolamine (MEA; 99 %; Merck), was added drop by drop to the ZnO solution with constant stirring for 2 h. Finally, the transparent and homogenous solution was stored for aging at room temperature. The aged ZnO solgel was deposited on the IDE device by using a spin coating technique at a speed of 3000 rpm for 20 s. The deposition process of the seed layer was repeated for three times to get a thicker ZnO thin film. For each deposition process, the coated ZnO thin films were dried at 150 °C for 20 min to remove the organic residuals that might exist on the ZnO thin films. The coated ZnO thin films were then annealed in a furnace under ambient air at 300 °C for 2 h to get highly crystallized ZnO. For the hydrothermal growth of ZnO nanofilm, the prepared substrate with the coated seed layer was submerged backward inside the growth solution using a Teflon sample holder. Equal concentration (25 mM) growth solution was prepared by mixing both zinc nitrate hexahydrate (99 %; Sigma-Aldrich) and hexamethylenetetramine (99 %; Merck) in deionized water. The growth process was completed inside an oven at 93 °C for 5 h. The prepared hydrothermally grown ZnO nanofilm was cleaned with isopropanol and deionized water to remove residual salts prior to annealing in a furnace under ambient air at 300 °C for 2 h.
Au-Decorated ZnO Nanorods Preparation
ZnO-NR-Au nanohybrids were prepared using a sputtering method. To form the ZnO-NR-Au nanohybrids, 10, 20, 30 and 40 nm Au wetting layers were physically deposited by a sputter coater (EMS550X) with Au target and a rotating stage. The detailed experimental conditions were as follows: electric current was maintained at 25 mA for 2–8 min with vacuum pressure of argon process level at 10−2 mbar. This process allowed us to obtain Au-decorated ZnO-NR forming ZnO-NR-Au nanohybrids. Figure 1a–d shows the schematic illustration of steps involved in the synthesis of Au IDE coated with Au-sputtered ZnO-NRs.
ZnO-NR-Au Hybrids Material Characterization
The morphology and structural properties of ZnO-NR-Au nanohybrid samples were investigated under field emission scanning electron microscopy (FESEM; Carl Zeiss AG ULTRA55, Gemini). High-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction pattern (SAED) of ZnO-NR-Au nanohybrids were acquired using PHILIPS, CM-200 TWIN with an incident energy 200 keV. X-ray diffraction (XRD; Bruker D8, Bruker AXS, Inc., Madison, WI, USA) with a Cu Kα radiation (λ = 1.54 Ǻ) was used to study the crystallization and structural properties of the ZnO-NR-Au nanohybrids. The material composition was analysed using X-ray photoelectron spectroscopy (XPS) (Omicron Dar400, Omicron, Germany). The chamber pressure was maintained at 2.4e−10 Torr throughout the measurement. The obtained peak was deconvolution using CasaXPS software. In addition, the optical and luminescence properties of ZnO-NR-Au nanohybrids were studied through photoluminescence (PL; Horiba Fluorolog-3, HORIBA Jobin Yvon Inc., USA). The PL spectra of the sample were recorded at different angles and positions to assure the result is not influenced by sample non-homogeneity. The impedance spectroscopy measurements were taken with applied AC amplitude of 1 V RMS in the frequency range of 1 Hz to 100 MHz using Novocontrol Alpha high-frequency analyser (Hundsangen, Germany). All the measurements were performed at room temperature.
Results and Discussion
Morphological Features of Au-Sputtered ZnO Thin Film
Parameters for ZnO nanorods sputtered with different Au thicknesses
ZnO/Au 10 nm
ZnO/Au 20 nm
ZnO/Au 30 nm
ZnO/Au 40 nm
ZnO-Au hybrid nanorods were grown successfully using hydrothermal method sputtered with different thicknesses of Au and have superior structural, optical and electrical characteristics compared to bare ZnO-NRs. Complete characterization of this structure was clearly demonstrated for their ultimate high-performance sensing. To investigate the effect of sputtered AuNP layers on the conduction mechanism, AC impedance spectroscopic analyses were performed. The results showed that the impedance and dielectric constant were decreased with the thickness of AuNP seeding increased. These variations were attributed to the grain sizes and dipole dynamics . A clear demonstration was shown with 30 nm thickness of Au, has a lowest variation in resistance value of grain boundary compared to other sizes fabricated, to be an optimal material for sensor. This study has shown an optimized ZnO/Au nanohybrid with complete characterizations, a tailored nanomaterial for downstream applications.
The authors would like to thank the Ministry of Higher Education Malaysia for the financial support through MTUN-COE grant 9016-00004 to conduct the research. The authors thank the Universiti Malaysia Perlis (UniMAP) for the opportunities to conduct the research in the Nano Biochip Lab, Failure Analysis Lab and Microfabrication Cleanroom. The authors thank Yaleeni Kanna Dasan from the Universiti Teknologi Petronas for the technical support. Appreciation is also due to all the team members and staff in the Department of Biotechnology; the Asian Institute of Medicine, Science and Technology University (AIMST); and the Institute of Nano Electronic Engineering (INEE) and School of Microelectronic Engineering (SoME), UniMAP.
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