Physical and Electrical Performance of Vapor–Solid Grown ZnO Straight Nanowires
© to the authors 2008
Received: 14 October 2008
Accepted: 11 November 2008
Published: 3 December 2008
Physical and electrical properties of wurtzitic ZnO straight nanowires grown via a vapor–solid mechanism were investigated. Raman spectrum shows four first-order phonon frequencies and a second-order Raman frequency of the ZnO nanowires. Electrical and photoconductive performance of individual ZnO straight nanowire devices was studied. The results indicate that the nanowires reported here are n-type semi-conductors and UV light sensitive, and a desirable candidate for fabricating UV light nanosensors and other applications.
KeywordsNanostructures Nanosensors ZnO
Wurtzite structure zinc oxide (ZnO) is a very important II–VI group semiconductor. It has a direct wide bandgap of 3.37 eV, higher exciton binding energy (60 meV for ZnO vs. 28 meV for GaN), and higher optical gain (300 cm−1 for ZnO versus 100 cm−1 for GaN) at room temperature [1–4]. Recently, ZnO has attracted extensive interest for its applications in numerous fields. It is of interest for low-voltage and short wavelength (green or green/blue) electro-optical devices such as light emitting diodes and laser diodes. It also can be widely used as transparent ultraviolet (UV)-protection films, transparent conducting oxide materials, piezoelectric materials, electron-transport medium for solar cells, chemical sensors, photo-catalysts, and so on [1–4].
In the past few years, extensive reports regarding the study of ZnO nanowires continue at a dizzying pace for their great prospects in fundamental physical science, novel nano-technological applications, and significant potential for nano-optoelectronics, and nano-ZnO was suggested to be the next most important nano-material after carbon nanotubes [5–14].
Herein, we report the physical and electrical properties of vapor–solid grown ZnO straight nanowires. The ZnO straight nanowires were grown via a facile catalyst-free method on amorphous fused quartz surfaces on a large-scale. Raman spectra of the ZnO nanowires were investigated. Individual ZnO straight nanowire devices were fabricated, and photoconductive and electrical studies were carried out with the single ZnO nanowire devices.
As a precursor, high pure metal zinc powders were loaded into the center of the tube. The vapor H2O was carried into the quartz tube through the flow of high pure argon. At high temperature (700–800 °C) zinc reacted with vapor H2O and produced ZnO nanowires. The nanowires were deposited on amorphous fused quartz substrates.
Results and Discussion
The vapor–liquid–solid (VLS) mechanism is common for the catalyzed growth of one-dimensional materials, and catalyst particles are typically detected at tips of the VLS grown one-dimensional materials . In the present case, however, the VLS mechanism is not responsible for explaining the growth of the ZnO nanowires. This is because there is no catalyst used in the growing process, and no catalyst particles are detected at the ZnO nanowire tips. It appears the growth of the ZnO straight nanowires reported here occurs via a vapor–solid (VS) mechanism . The vapor H2O was carried into the quartz tube and transported downstream by the flow of argon. At the center of the tube furnace, metal zinc vaporized and reacted with vapor H2O to generate ZnO vapor. Then the ZnO vapor deposited on the substrates and nucleation of wurtzite structure ZnO nanoparticles (nanorods) took place. Through a VS growth mechanism, the ZnO nanoparticles (nanorods) prolonged and the solid ZnO long nanowires were formed.
Raman scattering spectrum is a very useful tool for the structure characterization of nanomaterials. Wurtzite structure ZnO possesses four atoms per primitive cell and the space group of wurtzite structure is C6v4 (P6 3 mc) with all atoms occupying the C3v sites. According to the factor group analysis, single crystal wurtzite structure ZnO possesses eight sets of optical phonon modes near the zone center. The modes are classified into Raman allowed (A1 + E1 + 2E2), infrared allowed (A1 + E1), and both Raman and infrared silent (2B1). The A1 and E1 modes, corresponding to optical phonons, will split into a longitudinal-optical (LO) and a transverse-optical (TO) component due to the macroscopic electrical field associated with the longitudinal vibration [18–20]. The high frequency E2 mode involves only the oxygen atoms, and the low frequency E2 mode is associated with the vibration of the heavy Zn sub-lattice .
It can be also found from Fig. 2 that the Raman spectrum of the ZnO nanowires is slightly asymmetrical with respect to those of ZnO crystals . This asymmetrical spectrum shape is mainly attributed to the confinement effects of phonons in nanowire samples. The phonons can be confined within the nanosized systems and thus phonons other than those of Brillouin-zone center can also contribute to the Raman spectrum, which leads to the asymmetrical shapes of the Raman spectrum. Such an asymmetrical shape of the Raman modes associated with the phonon confinement effects in ZnO nano-sized systems are also reported on ZnO nanoparticles .
In conclusion, the physical and electrical properties of vapor–solid grown hexagonal ZnO straight nanowires were investigated. The Raman spectrum shows four first-order phonon frequencies ofE2(high) = 440 cm−1,A1(TO) = 380 cm−1,E1(TO) = 410 cm−1, andE1(LO) = 586 cm−1; and a second-order Raman spectrum at 334 cm−1which arises from zone-boundary phonons of hexagonal ZnO. Individual ZnO straight nanowire devices were fabricated and the electrical and photoconductive studies were carried out with the single nanowire devices. The result indicates that the ZnO straight nanowire is a fine n-type semiconductor and a desirable candidate for fabricating UV light nano-sensors and other applications.
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