Direct Synthesis of ZnO Nanorod Field Emitters on Metal Electrodes

Article pubs.acs.org/crystal

Direct Synthesis of ZnO Nanorod Field Emitters on Metal Electrodes Gregory Wrobel,† Martin Piech,‡ Pu-Xian Gao,*,† and Sameh Dardona‡ †

Department of Chemical, Materials and Biomolecular Engineering & Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269-3136, United States ‡ Department of Physical Sciences, United Technologies Research Center, East Hartford, Connecticut 06108, United States S Supporting Information *

ABSTRACT: Tapered zinc oxide (ZnO) nanorods have been grown directly on Fe and Cu electrode surfaces using a facile, one-step seedless (catalyst-free) solution processing method. Diaminopropane (DAP) is used to facilitate the growth of tapered nanorods. Fe electrodes remain nearly intact for the duration of synthesis, yielding low areal density ZnO nanorod arrays, while Cu electrodes were continuously etched and grew relatively high areal density arrays with a strong interfacial connection. However, despite the presence of dissolved metal ions within the synthesis solution, Auger analysis reveals dopant-free ZnO nanorods. The fabricated nanorod arrays were demonstrated as a field emission source.

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temperature. The technique produces nanorods with a tip diameter of ∼10 nm capable of relatively high field enhancement for electron field emission. Such direct synthesis of semiconductor nanostructures onto metal electrodes provides important technological merits, including the simplification of nanostructure integration into devices, and low interfacial resistance, intended to enhance electrical conduction in devices such as field emitters, and photovoltaics, thus improving efficiency.

o date, much of the one-dimensional (1D) nanomaterial research is directed toward development of the “nextgeneration” electronic and optoelectronic devices, including field effect transistors,1 photovoltaic cells,2 field emission sources,3,4 lasers,5 sensors,6−8 and nanogenerators.9 For these purposes, the nanostructures must be interfaced with conducting or semiconducting electrodes such as metals, transparent conducting oxides, or doped silicon. Zinc oxide remains as one of the chief materials for electronic and optoelectronic studies. In particular, 1D ZnO nanostructures have been extensively researched, with many different fabrication methods identified for synthesis of different nanostructure architectures,10 and they have successfully found applications in photovoltaics,11,12 ultraviolet light sources,13,14 piezoelectric devices,15 RRAM,16 photodetection,17 antireflection coating,18 and chemical sensors.19,20 As it turns out, most fabrication techniques require surface pretreatment, including ZnO seeding via sputtering,21,22 zincacetate decomposition,23 spin-casting,24 dip coating,25 pulsed laser deposition,26 deposition of a metal catalyst layer,27−30 lithography,31,32 or self-assembled monolayer (SAM) deposition.33 ZnO nanorod FE sources have been synthesized via vapor deposition methods. Tseng et al.34 created tightly packed ZnO nanorods with tip diameters of 20−30 nm with a 550 °C vapordeposition method on a Ga-doped ZnO/Si3N4/SiO2/Si substrate. Lee et al.35 have made ZnO high-aspect ratio nanorod arrays with tip diameters of ∼50 nm by vapor deposition at 550 °C on n-doped Si substrates. Zhu et al.36 have generated ZnO nanorod arrays with tip diameters of ∼7 nm using a 500 °C vapor phase deposition approach on p-doped Si substrates without seeding or a catalyst layer. Here, we report a one-step solution processing method for growing tapered ZnO nanorod arrays directly on metal electrodes without the need for surface pretreatment or high © 2012 American Chemical Society

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EXPERIMENTAL SECTION

High purity Cu (99.99 atom %, Alfa Aesar) and Fe (99.95 atom %, Goodfellow) metal rods were cut, ground, and polished to 6 μm, sonicated in hexanes, and argon plasma cleaned for 2 min prior to synthesis. The nanorod growth solution is comprised of zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ZNH; 20 mM), hexamethylenetetramine (C6H12N6, HMT; 20 mM), and 1,3-diaminopropane (C3H10N2, DAP; 140 mM). The substrates were secured within the reaction vessel with the polished surface facing downward to prevent ZnO precipitate deposition due to gravity. The field emission samples were kept in the growth solution for 17 h at 60 °C. Nanorod structure and composition were characterized under a FEI Tecnai 12 transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectrometer (EDXS). Substrate surface dissolution was studied via a JEOL 6335F field emission scanning electron microscope (SEM), and the aqueous substrate metal ion concentration was monitored via the change in sample light attenuation using an ultraviolet−visible absorption spectrometer (UVS; Agilent Cary 5000). Substrates for cross-sectional microscopy were made via electronbeam assisted metal evaporation on silicon wafers starting with a 0.2 μm titanium binding layer, followed by a 1.5 μm copper layer and a 12 Received: July 18, 2012 Revised: September 7, 2012 Published: September 12, 2012 5051

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mm2. The difference here may be attributed to surface etching via surface oxidation and dissolution.22,40 To visualize this effect, substrates were polished to 0.25 μm and immersed in an aqueous solution of 20 mM HMT and 140 mM DAP for 17 h at 60 °C. Results before and after this treatment in Figures 2a and b clearly display heavy pitting of the Cu substrate while Fe surface appears unchanged in Figures 2c and d.

h synthesis. The substrate was fractured in half and mounted at 90° in the SEM for imaging. The ZnO nanorod field emission behavior was evaluated at a vacuum base pressure of 5 × 10−8 Torr. Emission area was maintained at 0.317 cm2 with a cathode-to-anode separation of ∼60 μm using an electrically insulating spacer. A high-voltage source (Keithly 240) and a high sensitivity pico-ammeter (Keithley 6485) were connected through a 10 MΩ resistor for the emission current measurements.

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RESULTS AND DISCUSSION Randomly oriented ZnO nanorods grow directly on Fe and Cu substrates, without a seeding or catalyst layer, as illustrated in parts a and b, respectively, of Figure 1. The TEM and EDXS

Figure 1. SEM micrographs of (a) Fe- and (b) Cu-grown ZnO nanorods after 17 h synthesis at 60 °C. Figure 2. SEM images of Cu (a, b) and Fe (c, d) substrates polished to 250 nm before and after 17 h in a solution of 20 mM HMT and 140 mM DAP at 60 °C. (e) Cu2+ concentration as a function of time.

analyses confirmed the nanorods to be individually single crystalline ZnO with wurtzite structure (see Supporting Information, Figure S1). The tapered nanorod profile is a result of reversible DAP adsorption onto the (0001) crystal facet during the nanorod growth, which retards Ostwald ripening and preserves minute (0001) steps.37 Morphology similar to that of the Cu-grown nanorods was previously obtained with nanoparticle seeded glass and silicon surfaces.37,38 Improvement to nanorod orientation was attempted by altering the final surface polish in a range from 0.25 to 25 μm, but no obvious improvement was achieved. Similar to the Fe substrate, sparsely packed and randomly oriented nanorods are generally a result of low areal density (AD).30,39 Alignment of densely packed nanorod arrays is attributed to the space confinement effect, where the growth of initially misaligned nanorods is physically impinged by neighbors, as the better aligned nanorods are confined to grow upward.39 Despite the relatively high AD on Cu, it may still be too low to support the alignment confinement mechanism. Without changing precursor concentrations, growth time, or temperature, Figures 1 a and b clearly show that the choice of substrate influences the ZnO nanorod AD.30 The Fe substrate yields a relatively low AD of ∼5 × 103 nanorods/mm2 compared to the Cu-substrate, with ∼4 × 105 nanorods/

A steady time-dependent increase in Cu2+ cation concentration within growth solution is depicted in Figure 2e, as estimated from UVS. An initial linear etch rate of ∼50 nm/h was observed during the first 3 h of synthesis and then slowed to ∼20 nm/h in the remaining 14 h, resulting in a ∼430 nm thick layer of surface etched away. The etch rate of Fe was below the sensitivity limit of the UVS technique (16.7 V/μm to 13.5 V/μm, respectively, for Cu-grown nanorods; and from 2.52 mA/cm2 to 11.26 mA/cm2; 8.1 V/μm to 6.5 V/μm; and 14.6 V/μm to 10.9 V/μm for Fe-grown nanorods. CC resulted in higher β values with 930 and 610 for nanorods grown on Cu and Fe substrates, respectively. A comparison of parts a and b of Figure 5 reveals significantly different field emission behavior between the Cu and Fe samples. Cu-grown nanorods exhibit lower emission current and higher Eto and Eth, which may be attributed to the electrostatic screening effect and the misalignment of nanorods.47 The tight nanorod packing on the Cu surface results in a high surface charge density and diminished field enhancement perpendicular to the nanorod surfaces. Given the large number of variables effecting the FE measurements (such as anode and emitter geometry, emission area, cathode−anode separation, vacuum pressure, and chosen values of ϕ, etc.), equivalent comparison of FE results between one setup and another is tricky, yet vital. Demonstrations of FE from carbon nanotube (CNT) arrays have been shown to exhibit typical properties of Eto = 0.9−9.8 V/μm and Eth = 1.5− 15 V/μm, with Jmax values ranging from 0.1 to 10 A/cm2, with most typical values in the low-mid range.48−55 ZnO FE arrays typically exhibit β values of ∼372−1680 with Eto of 1.92−18 V/ μm, Eth values of 4.0−11 V/μm, and Jmax of 0.002−880 mA/ cm2.56−63 FE extracted from CdS arrays show β values of ∼129−820, Eto at 10 μA/cm2 ranging from 12.2 to 21.9 V/μm, and an Eth (estimated from presented data) of ∼20.9−28 V/ μm.64,65 In summary, Cu and Fe electrodes support growth of tapered ZnO nanorods, using a facile, one-step seedless hydrothermal process. DAP has been used to facilitate the synthesis process as a growth modifier. Cu substrates yield relatively high areal density as compared to the growth on Fe substrates, which may

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the United Technologies Research Center, the University of Connecticut New Faculty start-up funds, and the Department of Energy. G.W. is also grateful for financial support through the NSF GK12 fellowship program for sustainable energy. Thanks also go to Mr. Paresh Shimpi for his help on TEM characterization and analysis.

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