Low temperature and large-scale growth of ZnO nanoneedle arrays with enhanced optical and surface-enhanced Raman scattering properties
Graphical abstract
Introduction
Zinc Oxide (ZnO) is one of most promising oxide materials, which has attracted considerable interest due to its unique physical properties, such as its direct and wide band gap (3.37 eV), n-type semiconductor, large exciton binding energy (60 meV), high electron mobility (100 cm2 V−1 s−1), and piezoelectricity [1], [2], [3]. It is an important functional oxide, exhibiting high photoreactivity, near-UV emission, visible light transparency, biosafety, and biocompatibility [4], [5]. ZnO nanostructures have great potential for application in ultraviolet (UV) lasers [6], [7], light-emitting diodes [8], [9], [10], thin-film transistors [11], [12], field emission (FE) devices [13], [14], solar cells [15], photocatalysis [4], [16], and piezo-nanogenerators [17], [18].
ZnO nanostructures have be synthesized by various methods, such as metal organic chemical vapor deposition [19], molecular beam epitaxy (MBE) [20], physical vapor deposition (PVD) [21], [22], pulsed laser deposition [23], and thermal evaporation [24], [25]. However, these methods generally require high temperature, involve complicated equipment, and have a low yield [5]. The aqueous chemical growth (ACG) method is more attractive attributed to its low cost, low temperature and feasibility for industrial-scale fabrication [4], [16], [26]. Previous works on ACG processes have been controlled to grow different morphologies by adjusting reaction conditions, including pH [27], [28], precursor concentrations [29], temperature [30], and surfactants [31], [32], [33], etc. Recently, ACG methods have been developed for fabrication of ZnO nanostructures in various geometrical morphologies, including nanowires [34], [35], nanorods [29], [36], nanotubes [37], nanopagodas [4], [26], nanoneedles [30], nanoplates [4], [38], nanoparticles [39], and nanoflowers [27], etc. Among them, ZnO nanoneedles can provide for sharp curvatures of tips, which are expected to be of particular importance in field emissions [40], photocatalysis [41], and optical properties [30], [42]. However, there are fewer reports about low temperature and large-scale growth of ZnO nanoneedle arrays with high performance antireflective, photocatalytic, and surface-enhanced Raman scattering properties.
The present work has synthesized well-aligned ZnO nanoneedle arrays by the ACG method on four-inch silicon wafers with ZnO seed film. The appropriate volumes of 1,3-diaminopropane (DAP) can be used to grow the highest aspect ratio of ZnO nanoneedle arrays at a relatively low growth temperature of 80 °C for 1.5 h. Hexamethylenetetramine plays an important role in inhibiting the influence of high pH values with large dimensions. The ZnO nanoneedle arrays exhibit a very weak UV emission and very strong green emission from a defect in the cathodoluminescence spectrum. The ZnO nanoneedle arrays have good geometric structures for antireflection coatings, which display broadband reflection suppression from 400–1950 nm. ZnO nanoneedle arrays can provide a higher surface-to-volume ratio and better stability against aggregation, resulting in greater photocatalytic activity. In addition, ZnO nanoneedle arrays have good geometric structures for deposition three-dimensional Ag nanoparticles, which lead to high performance surface-enhanced Raman scattering (SERS) detection. The present work can provide insight into further structural design for nanostructured optical and SERS applications.
Section snippets
Synthesis
A Si (0 0 1) wafer was cleaned ultrasonically for 10 min in ethanol. A thin film of zinc acetate was then coated on the substrate by spinning a layer of solution of 5 mM zinc acetate dihydrate (98% Aldrich) in ethanol and repeating for ten times. 5–10 nm thick ZnO seed film was produced after annealing at 300 °C in air for 20 min [4], [43]. The ZnO nanoneedle arrays were grown by an ACG method in 100 mL of aqueous solution containing 10 mM equimolar zinc nitrate hexahydrate (98% Aldrich) and
Evaluation of ZnO nanoneedle arrays
Fig. 1a–d show the cross-sectional SEM images depicting the vertical ZnO nanoneedle arrays grown from equimolar (10 mM) zinc nitrate and HMTA, and the different volumes of DAP solution by an ACG method at the growth temperature of 80 °C for 1.5 h. The volumes of DAP were 0.25, 0.5, 0.75, and 1 mL, respectively. The lengths of ZnO nanoneedles gradually increased with an increase in DAP volumes, as shown in Fig. 1e. The average lengths of ZnO nanoneedles were 0.36, 1.29, 1.99, and 5.07 μm,
Conclusions
Vertically aligned ZnO nanoneedle arrays have been synthesized on a Si wafer with ZnO seed film using a facile ACG method with a low reaction temperature and a short reaction time. The dimensions and optical emission properties of ZnO nanoneedle arrays are effectively controlled by the volumes of DAP. The ZnO nanoneedle arrays exhibit very strong and broad green emission from defect in the cathodoluminescence spectrum. The sharp tips of ZnO nanoneedle arrays provide excellent impedance matching
Acknowledgments
This study was supported financially by the Ministry of Science and Technology of Taiwan (MOST 104-2221-E-035-018-MY3).
Yu-Cheng Chang received his PhD degrees in materials at National Tsing Hua University in 2007. He is presently an associate professor in School of Materials of Feng Chia University. His current fields of interest are solar cell, photocatalysis, biosensor, and nanomaterials.
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Yu-Cheng Chang received his PhD degrees in materials at National Tsing Hua University in 2007. He is presently an associate professor in School of Materials of Feng Chia University. His current fields of interest are solar cell, photocatalysis, biosensor, and nanomaterials.