Fabrication, characterization, and kinetic study of vertical single-crystalline CuO nanowires on Si substrates
© Cheng and Chen; licensee Springer. 2012
Received: 30 November 2011
Accepted: 13 February 2012
Published: 13 February 2012
We report here on the first study of the growth kinetics of high-yield, vertical CuO nanowires on silicon substrates produced by the process of thermal oxidation. The length of the CuO nanowires could be tuned from several to tens of micrometers by adjusting the oxidation temperature and time. The grown CuO nanowires were determined to be single-crystalline with different axial crystallographic orientations. After a series of scanning electron microscopy examinations, the average length of CuO nanowires produced at each temperature was found to follow a parabolic relationship with the oxidation time. The parabolic growth rate at different oxidation temperatures was measured. The activation energy for the growth of CuO nanowires calculated from an Arrhenius plot was found to be about 174.2 kJ/mole. In addition, the current-voltage characterization indicated that the sample with high-density CuO nanowires exhibited ohmic behavior, and its resistance was found to significantly decrease with increasing environmental temperature. The result can be attributed to an increase in the number of carriers at higher temperatures.
In recent years, one-dimensional [1D] metal-oxide semiconductor nanostructures, such as whiskers and nanowires, have attracted increasing interest due to their unique properties and variety of potential applications [1–3]. Among the metal oxides, cupric oxide [CuO] has been extensively studied as a p-type metal-oxide semiconductor. It has a direct bandgap of about 1.2 to 2.0 eV and exhibits many excellent physical and chemical properties [4–6]. Nanostructured CuO materials, especially 1D CuO nanowires, have received much more attention, having already been applied in field-effect transistors, photovoltaic cells, field emission nanodevices, and chemical and gas sensors [7–10]. Various growth techniques, such as the hydrothermal method , the thermal decomposition method , and the templating sol-gel method , have been developed to produce large-scale CuO nanowires. Recently, a simpler and more convenient route for the fabrication of CuO nanowires by directly oxidizing copper substrates has been proposed [14–19].
Using this versatile method, large-scale, vertically aligned CuO nanowires have been successfully produced on various types of copper substrates (sheets, foils, and grids) by directly heating the substrates at 350°C to 700°C in air for several hours. Although numerous studies have been performed on the fabrication of CuO nanowires by the thermal oxidation method, issues related to the growth kinetics and electrical characteristics of CuO nanowires produced under different oxidation conditions have not been extensively explored. Since the growth kinetic data can provide crucial information leading to an understanding of the formation process of CuO nanowires, it is of both fundamental and scientific interests to investigate them further under different experimental conditions.
In the present study, we show the successful fabrication of length-tunable, vertically aligned CuO nanowires on Cu film-coated Si substrates by thermal oxidation in air. The results of a systematic investigation of the growth kinetics, surface morphologies, crystal structures, chemical compositions, and electrical properties of CuO nanowires produced at different oxidation temperatures and times are reported.
Square pieces (8 × 8 mm2) were cut from single-crystalline, 10- to 20-Ω cm, p-type (001)Si wafers for use as the deposition substrates in this study. All of these Si substrates were cleaned chemically following a standard procedure and then dipped in a dilute HF solution (HF/H2O = 1:20), before being loaded into an electron gun evaporation chamber. A 30-nm-thick Cu thin film with a 30-nm-thick Ti adhesion layer was deposited on the Si substrates to act as the electrode during the subsequent Cu electrodeposition process. The base pressure in the evaporation chamber was better than 1 × 10-5 Pa. The Cu electroplating electrolyte was composed of 0.3 M copper sulfate (CuSO4·5H2O) and 2.7 M sulfuric acid (H2SO4). The electrodeposition of pure Cu films on the Cu/Ti film-coated Si substrates was carried out at room temperature using a DC power supply with an applied voltage of 0.2 V. After the Cu electroplating process, the obtained samples were cleaned in a 1 M hydrochloric acid (HCl) solution to remove any surface contaminants and the native oxide layer, then washed with deionized water and blown dry with N2 gas. Subsequently, the cleaned Cu-plated Si substrates were oxidized isothermally in an air oven at 400°C to 500°C for 30 to 420 min to grow CuO nanowires. To control the growth process, the air oven temperature was monitored continuously by placing a thermocouple in the vicinity of the Cu film sample during annealing. After the Cu film samples were oxidized, the samples were slowly cooled down in the air oven to room temperature to avoid thermal shocks and achieve crack-free nanowire samples.
The lengths and surface morphologies of the nanowire arrays fabricated on (001)Si substrates at various thermal oxidation temperatures and for various lengths of time were examined by scanning electron microscopy [SEM]. X-ray diffraction [XRD] (Cu Kα radiation, λ = 0.154 nm), transmission electron microscopy [TEM], high-resolution TEM [HRTEM], and selected-area electron diffraction [SAED] analysis were carried out for phase identification, atomic structure examination, and crystallography determination. Link ISIS energy-dispersive spectrometers [EDS] attached to the scanning electron microscope and the high-resolution transmission electron microscope were utilized to determine the chemical composition of the produced samples. For TEM and EDS investigations, some of the as-grown nanowires were scratched from the Si substrates and transferred onto carbon film-coated molybdenum mesh grids. The sample was prepared for current-voltage [I-V] measurement by peeling off a piece of black sheet with CuO nanowires from the surface of an oxidized Cu-plated Si sample and then fixing it to a glass substrate using epoxy glue. Ag paste was then placed at the two ends of the prepared black sheet to act as contact electrodes. The electrical I-V characteristics of the produced nanowire samples were measured at different temperatures using a Keithley 4200 semiconductor parameter analyzer (Hsinchu, Taiwan).
Results and discussion
Summary and conclusions
In summary, the present study demonstrates that by controlling the thermal oxidation temperatures and time, length-tunable, single-crystalline CuO nanowires can be successfully produced on silicon substrates. The growth kinetics, surface morphologies, crystal structures, chemical compositions, and electrical properties of the CuO nanowires produced were investigated.
From the results of TEM, SAED, HRTEM, and EDS analyses, it can be concluded that all the produced nanowires were single-crystalline CuO nanowires with a monoclinic crystal structure, but their axial crystallographic orientations were quite different. Cross-sectional SEM illustrated that for samples oxidized at 400°C to 500°C, the average length of the grown CuO nanowires increased parabolically with the oxidation time. The observed results indicate that the growth of CuO nanowires by thermal oxidation is a diffusion-controlled process. The parabolic growth rate at different oxidation temperatures was also measured. The activation energy for the growth of CuO nanowires was readily derived from an Arrhenius plot to be about 174.2 kJ/mole. On the other hand, the I-V measurements revealed that the resistance of CuO nanowires decreased significantly from 370.4 to 79.4 kΩ with an increase in the environmental temperature from 20°C to 120°C. The temperature-dependent conductive behavior of the CuO nanowire sample suggests its potential for applications in chemical and gas sensors.
The research was supported by the National Science Council of Taiwan, Republic of China.
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